Rights / License: Research Collection In Copyright - Non ...41808/... · Trap states in organic...

Research Collection

Doctoral Thesis

Trap states in organic field-effect transistors: quantification,identification and elimination

Author(s): Kalb, Wolfgang L.

Publication Date: 2009

Permanent Link: https://doi.org/10.3929/ethz-a-005813523

Rights / License: In Copyright - Non-Commercial Use Permitted

This page was generated automatically upon download from the ETH Zurich Research Collection. For moreinformation please consult the Terms of use.

ETH Library

https://doi.org/10.3929/ethz-a-005813523

http://rightsstatements.org/page/InC-NC/1.0/

https://www.research-collection.ethz.ch

https://www.research-collection.ethz.ch/terms-of-use

Diss. ETH No. 18324

Trap states in organic field-effect transistors:Quantification, identification and elimination

A dissertation submitted to the

SWISS FEDERAL INSTITUTE OF TECHNOLOGY

ZURICH

for the degree of

Doctor of Natural Sciences

presented by

WOLFGANG L. KALB

Dipl.-Phys., RWTH Aachen

born on the 5th of December 1977

citizen of Germany

accepted on the recommendation of

Prof. Dr. B. Batlogg, examiner

Prof. Dr. G. Horowitz, co-examiner

2009

to Ludovica

We are all, at heart, gradualists,our expectations set by the steady passage of time.But the world of the Tipping Point is a placewhere the unexpected becomes expected,where radical change is more than possibility.It is - contrary to all our expectations -a certainty.

Malcolm Gladwell, The Tipping Point

Contents

Frequently used symbols vii

Frequently used acronyms ix

Abstract 1

Kurzfassung 3

1 Introduction 7

2 Oligomeric semiconductors: Electronic structure, charge transport and tran-

sistor operation 13

2.1 Intrinsic electronic states . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 Transport mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2.1 Band transport . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.2 Hopping transport . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2.3 Diffusive transport limited by thermal disorder . . . . . . . . . . 22

2.3 Trap-controlled transport . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.3.1 Trap-controlled band transport . . . . . . . . . . . . . . . . . . . 24

2.3.2 Trap-controlled hopping transport . . . . . . . . . . . . . . . . . 27

2.4 Causes of trap states in oligomeric semiconductors . . . . . . . . . . . . 29

2.4.1 Structural defects . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.4.2 Chemical impurities . . . . . . . . . . . . . . . . . . . . . . . . 33

2.4.3 Trap states due to the gate dielectric . . . . . . . . . . . . . . . . 37

2.5 Transistor operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.5.1 Qualitative description . . . . . . . . . . . . . . . . . . . . . . . 40

ii Contents

2.5.2 Analytical description of an ideal field-effect transistor . . . . . . 42

2.5.3 Deviations from the ideal transistor behaviour . . . . . . . . . . . 47

2.5.4 Electrical stability of organic field-effect transistors . . . . . . . . 49

3 Experimental details 51

3.1 Organic semiconductors investigated in this study . . . . . . . . . . . . . 51

3.2 Purification of pentacene and rubrene . . . . . . . . . . . . . . . . . . . 52

3.3 Preparation of the gate dielectric . . . . . . . . . . . . . . . . . . . . . . 52

3.3.1 Cleaning of Si/SiO2 substrates . . . . . . . . . . . . . . . . . . . 54

3.3.2 Surface modification with self-assembled monolayers of octade-

cyltrichlorosilane (OTS) . . . . . . . . . . . . . . . . . . . . . . 55

3.3.3 Polymeric gate insulators and polymeric buffer layers . . . . . . . 56

3.4 Growth of the semiconductor and electrical characterization . . . . . . . 58

3.4.1 Evaporation of organic films . . . . . . . . . . . . . . . . . . . . 58

3.4.2 Single crystal growth . . . . . . . . . . . . . . . . . . . . . . . . 59

3.4.3 Electrode deposition . . . . . . . . . . . . . . . . . . . . . . . . 59

3.4.4 Electrical characterization of transistors . . . . . . . . . . . . . . 60

3.5 Additional thin-film characterization . . . . . . . . . . . . . . . . . . . . 61

3.5.1 Static water contact angles . . . . . . . . . . . . . . . . . . . . . 61

3.5.2 Atomic force microscopy . . . . . . . . . . . . . . . . . . . . . . 63

3.5.3 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.5.4 Surface step profiling . . . . . . . . . . . . . . . . . . . . . . . . 64

3.5.5 Leakage current and capacitance measurements . . . . . . . . . . 64

3.6 Advanced fabrication and characterization of thin-film transistors . . . . . 64

3.6.1 Device fabrication and characterization system . . . . . . . . . . 65

3.6.2 Electrical characterization by gated four-terminal measurements . 67

3.6.3 Parameter extraction . . . . . . . . . . . . . . . . . . . . . . . . 69

4 Quinoid heteropentacenes as promising organic semiconductors for field-effect

transistor applications 73

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.2.1 Synthesis of 7,14-Diphenyl-chromeno[2,3-b]xanthene (DPCX) . . 75

4.2.2 Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . 76

4.2.3 Electrical characterization . . . . . . . . . . . . . . . . . . . . . 77

Contents iii

4.2.4 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 78


4.3.2 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . 81

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5 Organic small molecule field-effect transistors with fluoropolymer gate di-

electric: Eliminating gate bias stress effects 87

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

5.3.1 Properties of the gate dielectric . . . . . . . . . . . . . . . . . . . 90

5.3.2 Comparison of different devices . . . . . . . . . . . . . . . . . . 91

5.3.3 Reproducibility . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

5.3.4 Gate bias stress experiments . . . . . . . . . . . . . . . . . . . . 93

5.3.5 Contact effects in SC-FET’s . . . . . . . . . . . . . . . . . . . . 95

5.3.6 Temperature-dependent measurements . . . . . . . . . . . . . . . 96

5.3.7 Trap-controlled transport in TFT’s . . . . . . . . . . . . . . . . . 100

5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

6 Defect healing at room temperature in pentacene thin films and improved

transistor performance 105

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

6.2 Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

6.2.1 Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . 107


6.3 Parameter extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

6.3.1 Basic parameter extraction . . . . . . . . . . . . . . . . . . . . . 109

6.3.2 Advanced parameter extraction . . . . . . . . . . . . . . . . . . 112

6.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

6.4.1 Improvement of the device performance with time . . . . . . . . 116

6.4.2 Influence on the density of states function . . . . . . . . . . . . . 118

6.4.3 Comparison of several experiments . . . . . . . . . . . . . . . . 121

6.4.4 Influence of oxygen and nitrogen . . . . . . . . . . . . . . . . . 122

iv Contents

6.4.5 Annealing at slightly elevated temperatures . . . . . . . . . . . . 126

6.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

6.5.1 Defect healing at room temperature . . . . . . . . . . . . . . . . 127

6.5.2 Defects and contact resistance . . . . . . . . . . . . . . . . . . . 128

6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

7 Oxygen-related traps in pentacene thin films: Energetic position and impli-

cations for transistor performance 131

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

7.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

7.3 Charge transport parameters . . . . . . . . . . . . . . . . . . . . . . . . 134

7.3.1 Field-effect conductivity, field-effect mobility and contact resistance134

7.3.2 Spectral density of trap states and free hole density . . . . . . . . 135

7.3.3 Fraction of free holes and band mobility . . . . . . . . . . . . . . 138

7.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

7.4.1 Extraction method and the influence of the contact resistance . . . 139

7.4.2 Oxygen-related device degradation . . . . . . . . . . . . . . . . 141

7.4.3 Oxygen-related traps . . . . . . . . . . . . . . . . . . . . . . . . 144

7.4.4 Trap induced changes in the free hole density . . . . . . . . . . . 147

7.4.5 Stability of the oxygen-related defects . . . . . . . . . . . . . . . 148

7.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

7.5.1 Effect of oxygen on the trap DOS . . . . . . . . . . . . . . . . . 148

7.5.2 Influence of oxygen-related traps on the field-effect mobility . . . 150

7.5.3 Consistency check: trapped holes vs. traps . . . . . . . . . . . . 151

7.5.4 Deep traps and device performance . . . . . . . . . . . . . . . . 151

7.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

8 Summary, conclusions and outlook 153

Bibliography 159

Acknowledgments 173

Curriculum Vitae 177

Publication list 179

Contents v

Contributions at conferences 181

Frequently used symbols

Ci Capacitance per gate unit area

d Thickness of the organic semiconducting layer

EV Valence band edge / mobility edge

Ea Activation energy of the field-effect conductivity

E0 = kT0 Characteristic slope of the trap density of states

εi Dielectric constant of the gate insulator

εs Dielectric constant of the organic semiconductor

Id Drain current

L Channel length

L′ Distance between the voltage sensing electrodes

l Thickness of the gate insulator

µ0 Intrinsic mobility

µe f f Effective mobility

NV Effective density of valence band states

N(E) Density of states (DOS)

P Holes per gate unit area (Ptotal , P f ree, Ptrapped)

p Volume hole density (p, p f ree, ptrapped)

Rcontact Contact resistance

Rchannel Channel resistance

σ Field-effect conductivity

S Subthreshold swing

Ug = |Vg−VFB| Effective gate voltage

Vg Gate voltage

viii Frequently used symbols

Vd Drain voltage

V1,V2 In-channel potentials

Vt Threshold voltage

Von Onset voltage

VFB Flatband voltage

V0 Interface potential

W Channel width

Frequently used acronyms

AFM Atomic force microscopy

CB Conduction band

Cytop Cyclic transparent optical polymer

DOS Density of states

DPCX 7,14-Diphenyl-chromeno[2,3-b]xanthene

HOMO Highest occupied molecular orbital

ITO Indium tin oxide

LUMO Lowest occupied molecular orbital

MOSFET Metal-oxide-semiconductor field-effect transistor

MIS Metal-insulator-semiconductor

RMS Root mean square

SC-FET Single crystal field-effect transistor

TFT Thin-film transistor

VB Valence band

XRD X-ray diffraction

Abstract

This study contributes to the broad topic of quantification, identification and elimination

of electronic trap states in organic field-effect transistors based on oligomeric semicon-

ductors and to an understanding of the charge transport mechanism in organic semicon-

ductors.

Field-effect transistors with new oligomeric semiconductors were fabricated and

characterized in order to identify possible relations between the chemical structure and

the crystal structure of the oligomeric material on one hand, and the transistor perfor-

mance on the other hand. The transistor performance reflects the efficiency of the intrinsic

transport mechanism and the extent of charge carrier trapping. Field-effect mobilities of

µ = 0.16 cm2/Vs and µ = 0.01 cm2/Vs were achieved, respectively, in single crystal and

thin-film transistors with the new quinoid heteropentacene 7,14-Diphenyl-chromeno[2,3-

b]xanthene (DPCX). The transistors show favourable properties, such as a near zero on-

set/threshold voltage and a small current hysteresis. A comparison of DPCX thin films

on octadecyltrichlorosilane (OTS)-treated and bare SiO2 gate dielectrics provides clear

evidence that the frequently used OTS surface treatment leads to organic thin films with a

better structural order. The experiments confirm the crucial importance of structural order

of the organic semiconductor for charge transport and thus for the transistor performance.

Furthermore, we studied the influence of different polymeric gate dielectrics and

polymeric buffer layers on the performance of pentacene- and rubrene-based field-effect

transistors. The experiments identify two causes of electrical instability in organic field-

effect transistors: traps due to the surface of the gate dielectric and due to structural dis-

order of the semiconductor. Organic single crystal transistors with a highly hydrophobic

and low permittivity fluoropolymer gate dielectric are essentially unaffected by long-term

gate bias stress. This highlights the high intrinsic electrical stability of oligomeric semi-

conductors. Due to the low density of interface traps and structural defects in rubrene

2 Abstract

single crystal field-effect transistors with fluoropolymer gate dielectric, we have achieved

a subthreshold swing of 0.75 nFV/(dec cm2). This is only ≈ 4 times larger than the in-

trinsic MOSFET limit of 60 mV/dec at room temperature. In the context of this study

we also address the issue of parasitic contact resistances since they may limit current in-

jection/extraction in high quality organic single crystal transistors for their low channel

resistance.

Moreover, we report on measurements of pentacene-based thin-film transistors with

a new experimental setup. This setup allows for measurements under highly controlled

conditions, e.g. without exposing the samples to ambient air between the transistor fabri-

cation steps and the electrical characterization. Great care was taken in order to separate

parasitic contact effects from properties of the semiconducting layer. The transistors were

characterized by gated four-terminal measurements. A key finding is that, under high

vacuum conditions (base pressure of order 10−8 mbar), the device performance improves

with time. The effective field-effect mobility increases by as much as a factor of two

and we obtained mobilities up to 0.45 cm2/Vs. In addition, the contact resistance de-

creases by more than an order of magnitude. These effects are attributed to a healing

of structural defects within the pentacene film at room temperature. This peculiar effect

is a direct consequence of the weak intermolecular interaction which is characteristic of

organic semiconductors. We show that the relevant structural defects that anneal at room

temperature are associated with shallow traps≤ 0.15 eV from the mobility edge/transport

level.

Finally, we studied the influence of oxygen on the spectral density of trap states of

a pentacene thin film. This was done by carrying out gated four-terminal measurements

on pentacene-based thin-film transistors as a function of temperature and without ever ex-

posing the samples to ambient air. We developed a scheme which allows for a calculation

of the spectral density of trap states from the dependence of the drain current on gate volt-

age and on temperature in a straightforward and unambiguous fashion. Photo-oxidation

of pentacene is shown to lead to a broad peak of trap states centered at 0.28 eV from the

mobility edge, with trap densities of the order of 1018 cm−3. The experiments support the

assumption of a mobility edge for charge transport, and contribute to a detailed under-

standing of an important degradation mechanism of organic field-effect transistors. We

can conclude that deep traps in an organic field-effect transistor, in general, reduce the

effective field-effect mobility by reducing the number of free carriers and their mobility

above the mobility edge.

Kurzfassung

Diese Dissertation trägt zu dem breiten Themengebiet der Quantifizierung, Identifizierung

und Ellimination von elektronischen Defektzuständen in organischen Feldeffekttransis-

toren auf der Basis von Oligomeren bei, sowie zum Verständnis des Ladungstransportes

in organischen Halbleitern.

Es wurden Feldeffekt-Transistoren mit neuen Oligomeren hergestellt und charak-

erisiert mit dem Ziel, mögliche Zusammenhänge zu identifizieren zwischen der chemis-

chen Struktur und der Kristallstruktur der Oligomere auf der einen und der Leis-

tungsfähigkeit der Transistoren auf der anderen Seite. Die elektrische Qualität

der Transistoren spiegelt die Effizienz des intrinsischen Ladungstransportes und die

Dichte von Defektzuständen wieder. Mit dem neuen chinoiden Pentazenderivat 7,14-

Diphenyl-chromeno[2,3-b]xanthene (DPCX) wurden Ladungsträger-Beweglichkeiten

von µ = 0.16 cm2/Vs und µ = 0.01 cm2/Vs entsprechend in Einkristall-Transistoren und

Dünnfilm-Transistoren erreicht. Die Transistoren mit DPCX haben günstige Eigen-

schaften wie etwa eine nahezu bei Null liegende Schwellenspannung und eine kleine Hys-

terese in den Transistor-Kennlinien. Ein Vergleich von Dünnfilmen aus DPCX auf mit

Octadecyltrichlorosilan (OTS)-behandeltem und unbehandeltem Gate-Isolator aus SiO2

stellt einen klaren Beweis dafür dar, dass die häufig verwendete Oberflächenbehandlung

mit OTS zu organischen Dünnfilmen mit besserer struktureller Ordnung führt. Die Exper-

imente bestätigen die grosse Bedeutung von struktureller Ordnung im organischen Hal-

bleitermaterial für den Ladungstransport und somit für die Leistungsfähigkeit der Tran-

sistioren.

Desweiteren wurde der Einfluss verschiedener löslicher Polymere als Gate-Isolator

in Feldeffekt-Transistoren mit Pentazen und Rubren im Hinblick auf die Leistungs-

fähigkeit der Transistoren untersucht. Durch die Experimente konnten wir zwei Ursachen

4 Kurzfassung

von elektrischer Instabilität organischer Feldeffekt-Transistoren identifizieren: Defek-

tzustände aufgrund der Oberflächen des Gate-Dielektrikums und strukturelle Unordnung

des Halbleiters. Organische Einkristall-Transistoren mit einem stark wasserabweisenden

Fluorpolymer sehr niedriger Permeabilität sind im wesentlichen von langzeitigem Gate

Bias-Stress unbeeinflusst. Dieses Ergebnis unterstreicht die hohe intrinsische elektrische

Stabilität von Oligomeren. Aufgrund der niedrigen Dichte von Grenzflächenzustände

und strukturellen Defekten in Einkristall-Transistoren auf der Basis von Rubren mit dem

Fluorpolymer als Gate-Isolator haben wir einen sehr schmalen Einschaltbereich unter-

halb der Schwellenspannung von nur 0.75 nFV/(dec cm2) erreichen können. Dies ist

nur ≈ 4 mal grösser als das intrinsische Limit in MOSFETs (60 mV/dec bei Raumtem-

peratur). Im Zusammenhang mit dieser Studie wird auch auf parasitäre Kontaktwider-

stände hingewiesen, denn diese können die Injektion/Extrahierung der Ladungsträger

in hochqualitativen organischen Einkristall-Transistoren aufgrund des niedrigen Kanal-

widerstandes limitieren.

Ferner beschreiben wir Messungen an Pentazen Dünnfilm-Transistoren mit einem

neuen experimentellen Aufbau. Dieser Aufbau ermöglicht Messungen unter höchst kon-

trollierten Bedingungen, also z. B. ohne die Proben zwischen der Herstellung der Transis-

toren und der elektrischen Charakterisierung der Umgebungsluft auszusetzen. Mit grosser

Sorgfalt wurden zwischen parasitäre Kontakteffekten und Eigenschaften der Halbleiter-

schicht unterschieden. Die Transistoren wurden durch Vier-Punkt-Messungen mit zusät-

zlicher Gate-Elektrode charakterisiert. Ein zentrales Ergebnis ist, dass sich die Qualität

der Transistoren im Hochvakuum (Basisdruck in der Grössenordnung 10−8 mbar) mit der

Zeit verbessert. Die effektive Ladungsträgerbeweglichkeit nimmt um bis zu einem Faktor

zwei zu und es wurden Beweglichkeiten bis zu 0.45 cm2/Vs erreicht. Zusätzlich nimmt

der Kontaktwiderstand um mehr als eine Grössenordnung ab. Diese Effekte werden einem

Ausheilen von strukturellen Defekten in den Pentazen-Dünnfilmen bei Raumtemperatur

zugeschrieben. Dieser eigentümliche Effekt ist eine direkte Konsequenz der für organ-

ische Halbleiter charakteristischen schwachen intermolekularen Wechselwirkung. Wir

zeigen, dass die relevanten strukturellen Defekte, welche bei Raumtemperatur ausheilen,

flachen Defektzuständen mit Energien ≤ 0.15 eV von der Mobilitätskante (bzw. von dem

Transportlevel) zuzuordnen sind.

Schliesslich wurde der Einfluss von Sauerstoff auf die energieaufgelösten De-

fektzustandsdichten von Pentazen-Dünnfilmen untersucht. Dazu wurden Vier-Punkt-

Messungen mit Gate an Pentazen-Dünnfilmtransistoren als Funktion der Temperatur

Kurzfassung 5

durchgeführt, und zwar ohne die Proben jemals der Umgebungsluft auszusetzen. Wir

haben ein Verfahren entwickelt mit dem man die energieaufgelösten Defektzustands-

dichten aus der gemessenen Abhängigkeit des Drain-Stroms von der Gate-Spannung und

von der Temperatur und in direkter und eindeutiger Art und Weise ausrechnen kann.

Es wird gezeigt das die Fotooxidation von Pentazen zu einem breiten Peak von Defek-

tzuständen führt, der bei 0.28 eV zentriert ist. Die Dichte der Fallenzustände ist in der

Grössenordnung von 1018 cm−3. Die Experimente bestätigen die Richtigkeit der An-

nahme einer Mobilitätskante für den Ladungstransport und tragen zum detaillierten Ver-

ständnis eines wichtigen Degradationsmechanismuses von organischen Feldeffekttransi-

storen bei. Im Allgemeinen reduzieren tiefe Defektzustände in organischen Feldeffekt-

transistoren die effektive Ladungsträgerbeweglichkeit durch die sich verringernde Anzahl

freier Ladungsträger und durch die Reduzierung der Beweglichkeit oberhalb der Mobil-

itätskante.

1 Introduction

Organic semiconductors have been studied for about 100 years now. The discovery of

photoconduction in anthracene in 1906 can be referred to as the beginning of organic

semiconductor research. [1] Although the principle of the metal-oxide-semiconductor

field-effect transistor (MOSFET) was already proposed in 1930 [2] and although the first

silicon-based MOSFET’s were fabricated in 1960 [3], it took another two decades until

the first organic field-effect transistors were realized. [4]

Organic semiconductors can be classified as either oligomeric materials (also called

small molecules) or conjugated polymers. Fig. 1.1 and 1.2 show some frequently used

materials. Representatives from both classes have loosely bound π-electrons which are

the source of charge conduction. Oligomers, on the one hand, tend to be crystalline and

can be obtained in high purity. Polymers, on the other hand, tend to be amorphous and

are difficult to purify. Within the long polymeric chains, there is very good orbital overlap

that leads to efficient intrachain transport.

One of the first reports on organic field-effect transistors dates back to 1983, when

Ebisawa et al. reported on field-effect transistors with (insoluble) polyacetylene as the

active semiconductor (see Fig. 1.2 for the chemical structure of this material). [4] In the

following years, several reports on field-effect transistors with conjugated polymers were

published. For example, polyacetylene was used as the active semiconductor and the

films were prepared by a precursor route. [5, 6] Field-effect mobilities in these devices

ranged between 10−6 and 10−4 cm2/Vs. Other studies with (insoluble) polythiophene

were published in the 1980’s and the polymer films were electrochemically polymerized.

[7–9] In this way, field-effect mobilities up to 10−3 cm2/Vs were achieved. [9] One of the

first reports with a soluble organic semiconductor was published in 1988. [10] Poly(3-

hexylthiophene) (P3HT, Fig. 1.2) served as the semiconductor and was deposited by spin

coating. Field-effect mobilities up to 10−4 cm2/Vs were measured. [10] The first organic

8 Introduction

Figure 1.1: Common oligomeric semiconductors, also called small molecules. Sexithiophene(6T) was one of the first oligomers that was used in organic field-effect transistors. Pentacene andits soluble derivatives (e.g. TIPS pentacene) are very promising for transistor applications; field-effect mobilities in excess of 1 cm2/Vs have been achieved. Rubrene is often used in fundamentalresearch due to the outstanding field-effect mobilities > 20 cm2/Vs in the best devices. Most ofthe experimental work for this thesis was done with pentacene and rubrene.

field-effect transistors with (thermally evaporated) oligomers as the active semiconductor

emerged at the end of the 1980’s as well. [11, 12] Horowitz et al. reported on transistors

based on vacuum evaporated sexithiophene (6T, Fig. 1.1) and obtained mobilities up to

10−3 cm2/Vs. [12]

Research on organic field-effect transistors has intensified substantially in recent

years. The research efforts are fueled by the fact that organic semiconductors can be de-

posited by thermal evaporation or from solution at low cost on large areas. Virtually any

substrate (including flexible plastic substrates) can be used since all fabrication steps can

be carried out by keeping the substrate at room temperature or at relatively low elevated

temperatures. Consequently, organic semiconductors are promising candidates for future

9

Figure 1.2: Common polymeric semiconductors. The first transistors with a polymeric semicon-ductor employed the insoluble polymers polyacetylene or polythiophene. Nowadays, the solublematerials P3HT and PQT-12 are frequently used and mobilities in excess of 0.1 cm2/Vs have beenobtained with both materials.

flexible and/or low-cost electronics. Apart from the easy deposition of organic semicon-

ductors, an important advantage is that the properties of the organic semiconductor can

be easily adjusted by means of synthetic organic chemistry.

The performance of organic field-effect transistors has improved dramatically since

the 1980’s. Progress was often due to the discovery of a new material and the subse-

quent optimization of the device fabrication. [13] Currently, field-effect mobilities up

to 5 cm2/Vs have been achieved both with vacuum-evaporated pentacene films (“p-type

semiconductor”, Fig. 1.1) or C60 films (“n-type semiconductor”). [14,15] Other examples

of the substantial progress are field-effect mobilities ≥ 20 cm2/Vs measured with rubrene

single crystals field-effect transistors [16] and mobilities in excess of 1 cm2/Vs with the

solution processable pentacene derivative TIPS pentacene [17].1 Structural order is of

importance for oligomeric semiconductors and seems to be relevant also in the case of

conjugated polymers. Mobilities of 0.1 cm2/Vs have now been achieved with polycrys-

talline P3HT films. [19] Moreover, it has been demonstrated that polymers exhibiting

1 In an earlier report transistors with TIPS pentacene showed mobilities up to 0.4 cm2/Vs but the TIPSpentacene was deposited by vacuum evaporation. [18]

10 Introduction

liquid-crystalline behaviour reach mobilities of 0.6-0.7 cm2/Vs. [20] Consequently, or-

ganic semiconductors are outperforming hydrogenated amorphous silicon (a-Si:H, typi-

cal field-effect mobility: 1 cm2/Vs) as the active semiconductor in thin-film transistors in

terms of field-effect mobility.

Although field-effect mobilities in organic transistors are already adequate for many

applications, other requirements need to be met for a successful commercialization of

organic field-effect transistors. It is clear that, for low-cost electronics the organic tran-

sistors should be inexpensive. Apart from low-cost fabrication steps, low-cost synthesis

and purification of all involved materials are required. In addition to a high field-effect

mobility, useful organic transistors must have a near zero threshold voltage, a steep sub-

threshold swing and a low current at zero applied gate bias. Moreover, a high electrical

and environmental stability of the devices is mandatory. Any changes of the transistor

characteristics during the lifetime of a device are to be avoided.

The device parameters and stability of organic field-effect transistors are intimately

related to the efficiency of the charge transport mechanism and the extend of charge car-

rier trapping in extrinsic traps. The main scientific challenge thus is to clarify the nature of

the charge transport in organic semiconductors and the microscopic origin of charge car-

rier traps. Pioneering work in this matter was done by Karl and coworkers. [21] Organic

semiconductors are distinct in that they generally consist of neutral molecules which in-

teract by rather weak forces (predominantly Van der Waals forces). The energy associated

with a Van der Waals bond is 10−3-10−2 eV, i.e. orders of magnitude smaller than the en-

ergy of a covalent bond. [22] The weak interaction strength leads to narrow energy bands

and may render band transport an inconsistent description of charge transport in organic

semiconductors, at least at room temperature. [23, 24]

All of the experimental work we performed was done with oligomeric semiconduc-

tors grown from the vapour phase. In Chap. 4 we describe a new oligomeric semicon-

ductor that was synthesized by Ciba Speciality Chemicals Inc. Most of the experiments

(Chap. 5-7) were, however, conducted on the common organic semiconductors pentacene

and rubrene since high field-effect mobilities can readily be obtained with these materials.

The thesis is organized as follows: in Chap. 2 we describe concepts to rationalize

the charge transport in organic semiconductors and particularly in crystalline oligomers.

Moreover, we summarize our knowledge on the microscopic origin of traps in oligomeric

semiconductors. Particular attention is paid to structural defects in pentacene thin films,

as much of the experimental work was done with pentacene-based thin-film transistors.

11

In Chap. 2 we also describe possible causes of additional traps in the vicinity of the inter-

face between the organic semiconductor and the gate dielectric. These traps may become

important since charge transport in a field-effect transistor takes place in the first few

nanometers at the insulator-semiconductor interface. Chap. 2 concludes by describing the

operation of an ideal field-effect transistor followed by a discussion of deviations from the

ideal transistor behaviour. Chap. 3 is intended to give an overview over the materials and

methods that were used for the experiments. More specific experimental details can be

found in Chap. 4-7, where the respective experimental results are described. The order of

the results chapters 4-7 is chronological. This means that the results in Chap. 4 were ob-

tained in the beginning of the experimental work for this thesis and the results in Chap. 7

towards the end. In Chap. 4 we describe the study of a new quinoid heteropentacene which

is promising as organic semiconductor for field-effect transistor applications. Chap. 5 ad-

dresses the issue of electrical stability. We show that, if a suitable gate dielectric is used

and if the organic semiconductor has a high degree of structural order, organic field-effect

transistors can have a very high electrical stability and outperform a-Si:H also in that re-

spect. The high degree of structural order is reached by employing organic single crystals

as the active semiconductor. Chap. 6 and 7 contribute to an understanding of traps in

organic thin films. The experiments in Chap. 6 and 7 were done with a vacuum prober

station which was attached to an evaporation system. These studies were thus performed

under highly controlled conditions. We developed and used a scheme which allows for

the extraction of the spectral density of trap states from the measured transfer character-

istics with high accuracy in an unambiguous and straightforward fashion. The results in

Chap. 4-7 are published in scientific journals (see publication list on p. 179).

2 Oligomeric semiconductors:Electronic structure, charge transportand transistor operation

Here we introduce concepts to rationalize the charge transport in oligomeric semicon-

ductors. We begin by describing the nature of the intrinsic electronic states followed by

a discussion of possible transport mechanisms in organic crystals of high quality. Ex-

trinsic trap states, however, can dominate the charge conduction; in Sec. 2.3 we describe

trap-controlled band and trap-controlled hopping transport. Then, we give a summary of

knowledge on the microscopic origin of trap states in oligomeric semiconductors. Finally,

we describe the operation of an ideal organic field-effect transistor and discuss deviations

from the ideal transistor behaviour.

2.1 Intrinsic electronic states in oligomericsemiconductors

Estimating the energy levels of the isolated molecules that make up organic crystals can

already be a very complex task. An approach that has long been applied to aromatic

hydrocarbons is the separation of σ- and π-electronic systems. [25] The 2s-, 2px and

2py-orbitals of each carbon atom can be combined to form a sp2-hybrid orbital. The

sp2-orbitals are orthogonal to the remaining 2pz-orbitals and their interaction can thus be

neglected. [25] The sp2-orbitals form molecular σ-orbitals which are localized between

two adjacent carbon atoms. The σ-electrons are strongly bound in the molecule. The

π-orbitals of the molecule, on the contrary, result from the overlap of the remaining 2pz-

orbitals at each carbon atom. This is illustrated in Fig. 2.1 for benzene. The π-electrons

are delocalized over the whole molecule and are more loosely bound compared to the

14 Oligomeric semiconductors

ð2pz

X

Z

Y

Figure 2.1: The π-electronic system in an aromatic hydrocarbon (e.g. benzene) results from theoverlap of atomic 2pz-orbitals. The σ-electrons are localized between two adjacent carbon atomsand are more strongly bound. Adapted from [26, 27].

b

a

a

c

c´

Figure 2.2: Herringbone structure of pentacene. In a simplistic view, the structure can be seenas a result of the interplay of weak intermolecular forces, i.e. dispersion forces and weak hydro-gen bonds. The pentacene crystal structure has a space group P1, two molecules per unit cell,a = 7.93 Å, b = 6.14 Å, c = 16.03 Å, α = 101.9, β = 112.6, γ = 85.8 as determined by Camp-bell et al. [30] This structure is often called bulk phase. Several other polymorphs exist due to theweak intermolecular interaction. [31, 32] Adapted from [33].

σ-electrons. The highest occupied molecular orbital (HOMO) and the lowest unoccupied

molecular orbital (LUMO) are π-type orbitals.

In organic crystals, the molecules are closely packed and adopt different structures,

such as a herringbone structure in the case of pentacene (Fig. 2.2). The atoms in the

molecules are bound by strong covalent bonds, but the intermolecular forces are relatively

weak. We may distinguish dispersion forces and weak hydrogen bonds. Dispersion forces

are due to fluctuations in the π-electronic clouds of the molecules and contribute to the

attraction of the molecules. Polarizabilities are often anisotropic with different values

along the different crystallographic directions. In addition, we have an attraction caused

by the weak hydrogen bonds between hydrogen and carbon atoms of adjacent molecules.

This latter type of interaction promotes edge to plane orientation of the molecules as in

the case of the herringbone structure. [28, 29]

2.1 Intrinsic electronic states 15

When forming the condensed phase, the weak intermolecular forces produce much

smaller changes in the electronic structure of the isolated molecules compared to co-

valently bound inorganic materials. The molecules in the crystal essentially keep their

identity. [22] In the tight binding-model, the energy levels in the organic crystal are seen

to arise from the molecular energy levels. The interaction among the HOMO’s and the

LUMO’s of a large number of molecules leads to the formation of the valence and conduc-

tion band. The intermolecular transfer integral of a certain electronic state of the isolated

molecule determines the width of the resulting band in the crystal. For example, the elec-

tronic bandwidth of a one-dimensional infinite stack of molecules is simply four times

the transfer integral. [34] X-ray analysis of the electron density in aromatic hydrocarbons

yields a maximum electron density around the carbon atoms which practically drops to

zero between the molecules. [22] Consequently, all transfer integrals have a rather small

value and the valence and conduction bands are narrow.

Brédas et al. calculated the influence of the intermolecular distance on the electronic

splitting of the HOMO and LUMO levels. [34] Fig. 2.3 shows the splitting of the HOMO

and LUMO of a cofacial sexithienyl dimer as a function of the intermolecular distance.

For a larger number of molecules, this splitting translates into the width of respectively

the valence and conduction band. Clearly, the intermolecular transfer integral and the

transfer of charge are closely related. Consequently, the larger the splitting of the HOMO

and the LUMO in the organic crystal is, the higher the mobility of holes and electrons

results to be. Fig. 2.3 shows a larger splitting of the HOMO which predicts a higher

intrinsic mobility for holes in sexithienyl. The calculations confirm the crucial importance

of electronic overlap on the electronic splitting and thus on the intrinsic mobility. Since

organic crystals are often highly anisotropic, the degree of electronic overlap can be very

different along the different crystallographic directions. For pentacene, the overlap is best

in the ab-plane (Fig. 2.2), leading to a superior charge carrier mobility within this plane

than in the c-direction.

Puschnig et al. studied the electronic band structure of the oligomers of poly(para-

phenylene) by first-principles calculations within the framework of density functional the-

ory. [35] The calculations confirm that the main character of the molecular π-orbitals is

preserved in the crystal structure. The band splittings are found to be 0.1− 0.2 eV for

the highest valence bands and lowest conduction bands. [35] Similar calculations for pen-

tacene combined with experimental evidence indicate significant differences for the dif-

ferent polymorphs under investigation. For the pentacene polymorph reported in [30] by


Figure 2.3: Calculated electronic splittings of the HOMO and LUMO levels in a cofacial dimermade of two sexithienyl molecules as a function of the distance between the two molecules. From[34].

Figure 2.4: Characteristic energies in an isolated molecule and in the condensed phase. Theinteraction of the HOMO’s and the LUMO’s of a large number of molecules leads to the formationof a narrow valence band (VB) and conduction band (CB) in the condensed phase. The ionizationenergy I and electron affinity A are changed in the condensed phase since the charge carrier in thecrystal is stabilized by polarization. Adapted from [1], p. 203.

Campbell et al., which is also called bulk phase, the width of the valence and conduction

band respectively are 0.34 eV and 0.53 eV. [36] In evaporated films of pentacene one often

finds a particular thin-film phase (see also Sec. 2.4.1, Fig. 2.9). In the thin-film phase the

molecules only have a small tilting angle and are standing almost upright. This leads to

a better overlap of the π-orbitals in the thin-film phase and the width of the valence and

conduction band are determined to be 0.64 eV and 0.62 eV. [36] The width of the valence

band is almost twice as large as for the bulk phase. The dispersion of the top valence

band is a measure for the hole mobility [35] and the increased valence band width in the

2.2 Transport mechanisms 17

Table 2.1: Ionization energies and electron affinities of an isolated pentacene molecule (Ig andAg) and of a pentacene crystal (Ic and Ac). The bandgap of pentacene is Eg = 2.2 eV. Adaptedfrom [1], p. 204.

Ig (eV) Ag (eV) Ic (eV) Eg (eV) Ac = (Ic−Eg) (eV)

6.7 1.2 5.1 2.2 2.9

thin-film phase may thus contribute to an understanding of the outstanding mobilities in

pentacene thin-film transistors.

Fig. 2.4 shows the formation of the valence and conduction band when going from

an isolated molecule (gas phase) to the condensed phase. The electron affinity Ag is the

energy related to binding an electron to a neutral molecule in the gas phase. The ioniza-

tion energy Ig is the energy which is necessary to remove an electron from the neutral

molecule. As illustrated in Fig. 2.4, the electron affinity and the ionization energy in the

crystal are changed. This is mainly attributable to the polarizing effect of the positively

or negatively charged molecule in the crystal which leads to stabilization. [1] Table 2.1

summarizes measured electron affinities and ionization energies for pentacene. The po-

larization energies which can be estimated with Pe ≈ (Ac −Ag) and Ph ≈ (Ig − Ic) are

significant, i.e. Pe ≈ 1.7 eV and Ph ≈ 1.6 eV.

2.2 Transport mechanisms

Only the loosely bound π-electrons (and possibly loosely bound electron pairs at het-

eroatoms) are transferred from molecule to molecule and, therefore, are the source of

charge transport in an organic crystal. The nature of the charge transport is still contro-

versial. It has been suggested that the charge carrier moves as a delocalized wave, as in the

case of many inorganic materials. Scattering, e.g. at phonons, would be seen as a small

perturbation. In the case of band transport, the charge carrier mobility µ0 is expected to

increase as the temperature is decreased. This is because the scattering of electrons at

phonons becomes less pronounced at a reduced temperature. For scattering at acoustic

phonons (one-phonon process), for example, the mobility is expected to vary as T−3/2 for

a wide band (W > kT ) and as T−2 for a narrow band (W ≤ kT , Ref. [1], p. 347).

As we will see, translational symmetry is not sufficient to make band transport a

self-consistent description of charge transport. In the hopping model the charge carrier

is highly localized. The polarization of adjacent molecules and the relaxation of inter-


Figure 2.5: Effect of phonons on the charge transport in (a) the band model and (b) a hoppingmodel. A phonon is illustrated as a peak in the energy surface. The peak cannot be surmounted bythe moving electron. The mobility is expected to increase with decreasing temperature in the bandmodel and to decrease with decreasing temperature in the hopping model. Adapted from [1, 27].

and intramolecular vibrations contribute to the localization. The localized charge carrier

hops from one molecule to the next. This process is expected to be assisted by phonons.

Consequently, one would expect a charge carrier mobility µ0 that decreases as the tem-

perature is decreased. The effect of phonons on charge transport in the band model and in

the hopping model is illustrated in Fig. 2.5.

2.2.1 Band transport

Band transport in the one electron approximation considers a perfect crystal with fixed

molecules, causing a periodic potential in which the electron moves. Scattering, e.g.

by phonons, is seen as a small perturbation of the zeroth order Hamiltonian. The small

perturbation induces transitions between Bloch states. The macroscopic mobility can be

approximated as follows:


µ0 ≈ eτ0

kTW 2a2

h2 . (2.1)

W is a measure of the bandwidth, a is a length of the order of the lattice constant and

τ0 is the mean lifetime of the Bloch states. [1] We have already mentioned that, in the

frame of the band model, it is assumed that scattering is weak. Consequently, the energy

broadening related to the finite lifetime τ0 of the Bloch states through the uncertainty

principle has to be significantly smaller than the bandwidth W . By introducing W À h/τ0

into Eq. 2.1, we have

µ0 À ea2

hWkT

. (2.2)

This criterion has to be fulfilled in order for band transport to be a self-consistent de-

scription. By inserting typical values for W and a, we can conclude that the band the-

ory is self-consistent only if the mobility at room temperature is significantly larger than

1 cm2/Vs. [1] The mobilities in organic crystals at room temperature are around 1 cm2/Vs,

however. With the definition of a mean free path λ0 = τ0v (v≈Wa/h is the carrier veloc-

ity) we can deduce from Eq. 2.2 that the mean free path λ0 should be at least 3−4 times

the lattice constant. [1]

The temperature dependence of the charge carrier mobility was measured e.g. by

Karl et al. in ultrapure naphthalene crystals using the time-of-flight method. [23] Impor-

tantly, charge carriers are created within the crystal by illumination and are capacitively

measured. This means that the time-of-flight method does not depend so much on high

quality contacts to the organic crystals which are difficult to achieve. [37] In Fig. 2.6 we

show the temperature dependence of the electron and hole mobilities parallel to the crys-

tallographic a-direction. [23] The electron mobilities are lower than the hole mobilities.

The temperature-dependent data were fitted to µ0 ∝ T n, and exponents of n = −1.4 and

n =−2.9 were obtained respectively for electrons and holes. Karl et al. argue that charge

transport cannot proceed via hopping in these ultrapure crystals. This is because hop-

ping transport is not consistent with the increase in mobility with decreasing temperature.

Moreover, especially the mobilities at low temperatures are too high to be compatible with

hopping transport. [23] The conclusion of the authors is that the standard band model is

consistent for the low-temperature mobilities. Around room temperature, however, the

mean free path λ0 was estimated to be smaller than the lattice constant. This is inconsis-

tent with band transport. A complete description of charge transport in organic crystals


Figure 2.6: Measured temperature dependence of the electron and hole mobility in the a-directionof an ultrapure naphtalene crystal. The mobility increases as temperature decreases. Hole mobil-ities are in excess of 100 cm2/Vs at low temperatures. Hole mobilities are always higher thanelectron mobilities. From [23].

would have to explain both the low-temperature and the high-temperature regime and

should link the regimes in a unified way. [23]

Since the molecules in the crystal have highly polarizable π-orbitals, polarization ef-

fects may not be negligible in a suitable description of charge transport. Holstein’s polaron

band model considers electron-electron interactions. [38,39] With increasing temperature,

the polaron mass increases. This effect is accompanied by a bandwidth narrowing and in-

evitably results in a localization of the charge carrier. Consequently, this model predicts

a transition from band transport at low temperature to phonon-assisted hopping transport

at higher temperatures (e.g. room temperature). Recently, Holstein’s model was extended

by Hannewald et al. [40] The calculations predict a large increase of the bandwidth with

decreasing temperature in crystals of naphtalene, anthracene and tetracene, as shown in

Fig. 2.7.


Figure 2.7: Effective polaron bandwidths as a function of temperature for tetracene (Tc), an-thracene (Ac) and naphthalene (Nph) as calculated with the extended Holstein model. From [40].

2.2.2 Hopping transport

If the intermolecular transfer integral is small compared to energies associated with

electron-phonon coupling or to energies associated with the polarization of adjacent

molecules due to the presence of a charge carrier, charge transport by polaron hopping

has to be considered. The time the charge carrier stays at a given site is relatively large,

since the electron is stabilized by the polarization of adjacent molecules and the relaxation

of intermolecular vibrations. There is a large number of hopping models, which generally

lead to a temperature dependence of the form

µ0 ∝1

T n exp(− E

kT

)(2.3)

with a value of n not exceeding 1.5. [23]

From diffusion theory and from Einstein’s relation between diffusivity D and mo-

bility,

µ0 =DekT

(2.4)

a hopping mobility

µ0 =ea2

kTP (2.5)


follows. P is the probability for charge transfer between two hopping sites and a is the

spacing between the sites. [41, 42]

Holstein et al. [39] and Yamashita et al. [43] propose different approaches to the

hopping probability P. In the high temperature limit both approaches lead to the same

thermally activated hopping probability. The expression for the hopping mobility in this

limit is

µ0 =ea2

kT1h

(π

2EbkT

)1/2

J2 exp(− Eb

2kT

)

︸︷︷︸P

. (2.6)

J is the intermolecular transfer integral and Eb is the polaron binding energy. [42]

Another important hopping model is the semiclassical Marcus electron transfer

theory. [44, 45] The transport of polarons is thought to be governed by the physics of

electron-transfer processes, as established by Marcus for chemical reactions and biologi-

cal electron-transfer processes. [44] The charge transfer rate kmarcus can be written as

kmarcus =t2

h

√π

λkTexp

(− λ

4kT

), (2.7)

where t is the electronic coupling and λ is the reorganization energy. [46] The reorga-

nization energy is the energy that is required for the structural adjustments which are

necessary in order for the electron transfer to occur between molecule A and D, i.e.

A+D A·−+D·+. (2.8)

The system has to reach a transition state in which the charge transfer occurs. [46] This

hopping model also leads to a thermally activated mobility which is not in agreement with

the results from measurements on high quality crystals (e.g. Fig. 2.6). Very recently it has

however been shown that a full quantum mechanical golden rule treatment would affect

the reorganization energy λ and would lead to a bandlike mobility behaviour. [46]

2.2.3 Diffusive transport limited by thermal disorder

The measured mobility in pure organic crystals decreases as a function of temperature up

to room temperature according to a power law µ0 ∝ T n (e.g. Fig. 2.6). This might be

consistent with band transport. However, the mobilities at room temperature are around


1 cm2/Vs and the estimated mean free path λ0 is comparable to the lattice constants. It has

often been noticed that this is not consistent with band transport (see Sec. 2.2.1). Troisi

et al. suggested a transport model for organic crystals that resolves this contradiction. As

described in Sec. 2.1, the intermolecular transfer integral is very sensitive to small dis-

placements of the molecules. Consequently, it is reasonable to expect that the fluctuations

of the intermolecular transfer integrals caused by the thermal motions of the molecules

are significant. Troisi et al. have shown that, at least for temperatures above 100 K, the

fluctuation of the transfer integral is of the same order of magnitude as the transfer inte-

gral itself in organic semiconductors such as pentacene, anthracene or rubrene. [47, 48]

As a consequence, the fluctuations do not introduce a small correction, but determine the

transport mechanism and limit the charge carrier mobility. [49] The translational symme-

try of the Hamiltonian is broken, the band transport breaks down and the charge carriers

become localized. [24, 47]

In this scenario, the temperature dependent charge carrier mobility can be de-

termined with Einstein’s relation (Eq. 2.4). The diffusion coefficient D is computed

with a semi-classical model Hamiltonian and a one-dimensional stack of planar conju-

gated molecules. [24] The calculations predict a mobility that decreases with increas-

ing temperature up to room temperature, according to a power law. This is in excellent

agreement with the measured temperature-dependence in pure organic crystals (e.g. in

Fig. 2.6). [24,48] The model predicts mobilities at room temperature in the range between

0.1 cm2/Vs and 50 cm2/Vs, which also is in good agreement with the experiments. [24,48]

Interestingly, the importance of thermal disorder is supported by recent tetrahertz transient

conductivity measurements on pentacene crystals. [50]

This transport model may however not be consistent with other experimental facts.

Due to the thermal motions of the molecules, the electronic structure of organic crys-

tals is expected to resemble the electronic structure of amorphous materials even in the

absence of defects. [47, 48] However, SCLC measurements in the temperature range of

100−200 K clearly show a sharp peak of trap states in rubrene crystals caused by chemi-

cal impurities that were deliberately introduced. [51] The question whether this is consis-

tent with the model by Troisi et al. arises, or should the thermal motions of the molecules

lead to a significant broadening of the peak?


2.3 Trap-controlled transport

In Sec. 2.2, we have described charge transport in ultrapure organic crystals which is lim-

ited by electron-phonon coupling, by polarization effects and possibly by thermal disor-

der. Chemical impurities or structural defects were assumed to have a very limited effect

on charge transport, e.g. by scattering the charge carriers. Organic thin films are grown

rather rapidly and generally are polycrystalline and sometimes even amorphous. The sit-

uation can arrive in which the large number of extrinsic defects completely dominate the

charge transport. The charge carriers are multiply trapped by and thermally released from

trap states associated with the defects. [52, 53] Even at relatively high charge carrier den-

sities, most of the charge may be trapped on a time average. This leads to an effective

mobility

µe f f =n f

n f +ntµ0 (2.9)

where n f and nt respectively are the density of the free and trapped electrons and µ0 is

the intrinsic mobility. In this scenario, the Fermi level would be below the transport level.

The main effect of decreasing the temperature would be a reduction in the number of

free electrons in the transport level due to the sensitive dependence of the Fermi function

on temperature. The temperature dependence of the “intrinsic” mobility µ0 can therefore

be a secondary effect. The measured effective mobility µe f f may decrease with decreas-

ing temperature even if µ0 increases upon cooling. Sec. 2.3.1 describes trap-controlled

band transport and the mobility edge picture. In Sec. 2.3.2 we rationalize trap-controlled

hopping transport. We will see that even in a completely disordered solid, the hopping

transport can be described by a transport level and a distribution of localized states below

the transport level.

2.3.1 Trap-controlled band transport

The simplest case is to consider transport in extended states and localized states with

a fixed trap depth, e.g. due to a specific chemical impurity. The extended states have

an effective density NC at an energy EC and the localized states have a specific energy

Et and concentration Nt . If EC and Et are well separated from the the Fermi level EF ,

Boltzmann’s approximation can be applied. In that case, the density of free electrons is

n f ≈ NC exp[−(EC−EF)

kT

](2.10)

2.3 Trap-controlled transport 25

and the density of trapped electrons is

nt ≈ Nt exp[−(Et −EF)

kT

]. (2.11)

By inserting Eq. 2.10 and Eq. 2.11 into Eq. 2.9 we obtain [54]

µe f f ≈ 1

1+ NtNC

exp(

EC−EtkT

)µ0, (2.12)

where µ0 is the mobility in the extended states. Provided that the trapped charge exceeds

the free charge (nt À n f , low-temperature approximation), we have [54]

µe f f ≈n f

ntµ0 =

NC

Ntexp

[−(EC−Et)

kT

]µ0. (2.13)

The temperature dependence of the exponential factor in Eq. 2.13 may dominate the tem-

perature dependence of the effective mobility µe f f . In this case, measuring the mobility

as a function of temperature reveals the trap depth EC−Et . [21, 54, 55]

Especially in the case of organic thin films, it is more realistic to assume trap states

with a continuous distribution of the trap depth. According to extensive studies in our

group, this description is also appropriate for organic single crystals. [51] The situation in

organic thin films may be described in analogy to charge transport in amorphous inorganic

semiconductors, where the mobility edge picture has been developed. [54, 56] Fig. 2.8(a)

shows the influence on the spectral density of trap states of an increasing degree of dis-

order in a solid: the discrete trap levels which may exist in a crystal are smeared out as

disorder increases. The mobility edge (line between hatched and white area in Fig. 2.8(a))

separates extended from localized states. As shown in Fig. 2.8(a), its position depends on

the degree of disorder. At high levels of disorder, localization extends well into the energy

bands. At the mobility edge, the mobility as a function of energy increases abruptly, as

shown in Fig. 2.8(b). This can be rationalized as follows. We have seen that the mobility

in extended states is at least 1 cm2/Vs. Charge carriers may also be transported by tunnel-

ing between localized states. Mott et al. have however demonstrated that, for tunneling


(a)

(b)

Figure 2.8: (a) Influence of increasing disorder on the band structure of a solid. The hatched areasmark localized states, i.e. the spectral density of trap states. As disorder increases, the width ofdiscrete trap levels (left) increases and, eventually, the discrete trap levels are completely smearedout (right). In addition, more and more states become localized. (b) Schematic representationof the mobility edge. The mobility edge separates localized states (hatched area) from extendedstates. At the mobility edge, the mobility as a function of energy abruptly rises. From [54].

between sites of the same energy, the mobility is

µ∼= eν0R2

6kTexp

(−2R

R0

)exp

(− E

kT

). (2.14)

2.3 Trap-controlled transport 27

ν0 is the frequency of the hopping attempts (1012− 1013 Hz), R is the mean separation

between the sites, R0 is the localization radius of the charge carrier1 and E is the activa-

tion energy associated with the tunneling process. [54, 57] Even in the case of very high

densities of localized states (small R) and even if we assume E = 0, we can conclude that

µ ≤ 10−2 cm2/Vs. Hopping in localized states is thus expected to be negligible if trans-

port in extended states exists and we therefore have an abrupt increase in mobility at the

mobility edge. Only the charge carriers that are thermally activated to states above the

mobility edge contribute to the transport of charge.

In the low-temperature approximation (nt À n f ), the mobility can be expressed as

µe f f ≈n f

ntµ0 =

(Z ∞

EC

N(E) f (E)dE)(Z EC

EF

N(E) f (E)dE)−1

µ0 (2.15)

in analogy to Eq. 2.13. EC and N(E) are respectively the energetic position of the mobility

edge and the density of states, and f (E) is the Fermi function. [54] Again, the tempera-

ture dependence of the Fermi function can dominate the temperature-dependence of the

measured mobility µe f f when cooling down a sample.

2.3.2 Trap-controlled hopping transport

Trap-controlled hopping transport is very similar to trap-controlled band transport. In

both scenarios, the essence is thermal activation to a transport level. For the simple case

of traps with a discrete energy Et and a density Nt , the low-temperature approximation of

the mobility is essentially given by Eq. 2.13. Clearly, the effective density of extended

states NC needs to be replaced by the density of hopping sites NH at the energy EH . We

assume that the intrinsic hopping mobility µ0 can be described by Eq. 2.6. The traps

reduce this mobility to an effective mobility which can be written as

µe f f ∝NH

Ntexp

[−(EH −Et)+Eb/2

kT

]. (2.16)

If the activation energy of the hopping process Eb/2 is known, Eq. 2.16 can be used to

estimate the trap depth EH −Et and vice versa. For example, this approach has been used

1 For a localized charge carrier, the localization radius may be approximated with the Bohr radius.


to estimate the depth of discrete traps in (inorganic) As4S4. [41] The authors estimate

a trap depth of 0.26 eV with an activation energy of the intrinsic hopping transport of

0.14 eV. [41] Thermal activation to a hopping level may also be suitable for crystals of

small molecule organic semiconductors. [21]

In amorphous inorganic semiconductors such as amorphous Si, the existence of ex-

tended electronic states is attributed to the similarity of the short-range configuration of

the atoms in the amorphous phase compared to the configuration of the atoms in the corre-

sponding crystalline phase. [54] In the case of a completely disordered material, however,

one would expect a complete disappearance of extended states. The density of states is

not high enough to allow for the occurrence of extended states and all electronic states

are hopping sites. This description may not be suitable for crystalline or polycrystalline

organic semiconductors, but should be applicable to highly disordered conjugated poly-

mers. Bässler and Arkhipov et al. suggest a model which is based on variable-range

hopping and on the Miller-Abrahams equation for the hopping rate. [58] They show that

the complicated situation can be substantially simplified by introducing an effective trans-

port level and a broad distribution of localized states below this transport level. [59] The

density of states of the completely disordered system is a Gaussian function of energy.

Carriers in localized states jump to a shallower state with a universal energy (the transport

energy) and are transported away from the starting site. It is important to note that the po-

sition of the transport level remains unchanged, if the charge density is increased. There

is, however, an essential difference to trap-controlled band transport, where the proba-

bility of the charge carrier to be captured by the same localized state after reaching the

transport level is negligibly small. In the present situation, the site in the transport level is

still a localized state and adjacent to the starting site. Therefore, the carrier might return

to the initial site (backward jump). Consequently, we have to distinguish between the

energy level onto which most carriers jump from deeper states and the “genuine transport

level”. [59] Jumps onto the genuine transport level lead to a transport of the carrier away

from the initial site. It can be shown that, for a Gaussian distribution of states

N(E) =Nt√2πσ

exp(− E2

2σ2

)(2.17)

and with some simplifying assumptions (high temperatures and low carrier concentra-

tions), the charge mobility is

2.4 Causes of trap states in oligomeric semiconductors 29

µ =eν0

kT N3/2t

exp

[−1.2

(6γ3

πNt

)1/3]

exp[− σ2

2(kT )2

]. (2.18)

γ is the inverse localization radius and ν0 is the attempt-to-jump frequency. [59] Eq. 2.18

is valid for relatively low carrier concentrations only and is not applicable to operating

field-effect transistors. However, the concept of the effective transport level has also been

applied to a situation with a high charge density. [60]

2.4 Causes of trap states in oligomeric semiconductors

In this section we deal with microscopic causes of trap states in organic field effect tran-

sistors. Charge carrier traps within the organic semiconductor are caused by structural

defects or chemical impurities. Chemical impurities may also cause a surrounding of

structural defects by distorting the host lattice. [21] On the other hand, chemical impuri-

ties tend to accumulate in regions with increased structural disorder. [1] Apart from traps

within the semiconductor, trap states caused by the gate dielectric can become important

in organic field-effect transistors. Trap states may be caused by certain chemical groups

on the surface of the gate dielectric, so that trapping is expected to depend on the specific

surface chemistry of the gate dielectric. The ability of chemical groups to cause traps will,

however, depend on the nature of the organic semiconductor. In addition, water adsorbed

on the gate dielectric may cause traps. Moreover, polar gate dielectrics may broaden

the density of states within the semiconductor at the interface, since randomly oriented

dipoles within the gate dielectric locally modify the polarization energy. [61, 62]

2.4.1 Structural defects

Structural defects are classified as point defects or extended defects. An example of a

point defect is a vacancy and examples of extended defects are edge dislocations, screw

dislocations, or (low-angle) grain boundaries. Structural defects modify the energy levels

in their vicinity and often lead to energy levels in the band gap, i.e. to charge carrier

traps. Such traps are electrically neutral when empty and become charged upon trapping

a charge carrier. Certain structural defects can result in scattering centers, but do not

cause traps (so called antitraps). A vacancy, for example, is a point defect that leads to

a decreased polarization energy, if a charge carrier occupies a molecule adjacent to the

vacancy. Consequently, the energy levels around the vacancy are not as low as in the rest


of the crystal and we have antitraps around the vacancy. Typical densities of vacancies in

anthracene or naphthalene crystals are of the order of 1014−1015 cm−3 (Ref. [1], p. 222).

Vacancies as an example of point defects are expected to be concentrated close to other

structural defects due to a reduced formation energy. [1] Consequently, most structural

defects in organic crystals are located close to each other in extended lines or areas. Ex-

tended structural defects can result in significant defect densities in organic crystals, e.g.

1019 cm−3 (Ref. [1], p.226). Therefore, extended structural defects are thought to be the

main source of traps in ultrapure organic crystals. [63] Structural defects are often thermo-

dynamically unstable. Annealing e.g. naphthalene or anthracene crystals for 100−300 h

reduces the density of structural defects by typically one order of magnitude (Ref. [1],

p.228). The density of structural defects in an organic crystal sensitively depends on the

preparation method and also on the handling of the crystal after growth. [64, 65]

Clearly, organic thin films have a higher density of structural defects than organic

crystals. In the following, we discuss structural defects in vacuum evaporated pentacene

films, which are of particular relevance for our work. Pentacene films are grown rather

rapidly. For example, 50 nm thick films are evaporated in 30 min or less. The films are

often polycrystalline and have a layered structure within the grains. The molecules in the

layers are almost perpendicular to the substrate. The structure of vacuum evaporated thin

films is the result of the thin-film growth which depends on properties of the substrate’s

surface, including surface roughness [66, 67], surface free energy [68, 69] and the pres-

ence or absence of sites for heterogenous nucleation [27]. A low surface free energy, for

example, is thought to favour 3D-growth (Volmer-Weber growth). [70] In the case of a

high surface free energy, the material tends to wet the surface and layer-by-layer growth

is more likely. [70] This means that we have the completion of a molecular layer prior to

the nucleation of the next layer. On the other hand, the growth kinetics is substantially

influenced by the choice of the deposition rate and the substrate temperature during the

deposition. [71, 72] In the case of homogenous nucleation, the increase of the substrate

temperature and the decrease of the deposition rate are both expected to lead to films with

an increased grain size. [70]

Since pentacene films are often polycrystalline, large angle grain boundaries pro-

duce additional structural defects. Another important cause of structural disorder in pen-

tacene films is polymorphism. Pentacene can crystallize in at least four different struc-

tures (phases). This is due to the weak interaction among the molecules. [31, 32] It is

quite common that at least two of these phases coexist in pentacene thin films. [71, 73]


Figure 2.9: Structure of a pentacene film evaporated on a SiO2 surface as suggested by [73],based on measurements with X-ray diffraction (XRD) and Raman spectroscopy. The films have alayered structure and contain several crystalline phases. The tilt of the molecules increases as thefilm thickness is increased. From [73].

Fig. 2.9 shows a pentacene thin film as a mixture of different pentacene polymorphs. [73]

According to this study by Cheng et al., an orthorhomic phase grows on a SiO2 substrate

which transforms to a thin-film phase and eventually to the triclinic bulk phase as the film

thickness increases. [73] The tilt angle of the molecules with respect to the surface normal

depends on the crystallographic phase. Close to the insulator-semiconductor interface, the

pentacene molecules are oriented almost perpendicular to the substrate.

Verlaak et al. theoretically studied structural defects in vacuum evaporated pen-

tacene films. [74] The study is restricted to defects, where the pentacene molecules are

translated within the molecular layers parallel to the substrate, or rotated along the long

molecular axis. It is argued that translation or rotation out of a molecular layer is not

favourable. [74] Moreover, any interaction with the substrate is neglected. The theoretical

study identifies an interesting effect which is shown in Fig. 2.10. Structural defects are

formed during the film growth. Upon addition of more and more “defective” molecules at

a given site, the ideal crystal structure becomes energetically more and more favourable.

The system eventually relaxes into the ideal crystal structure during the continuation of

the film growth. [74] The relaxation happens, provided that the evaporation rate is low

enough and that there is enough time for relaxation. Otherwise, the defective molecules

would become “locked” upon addition of more molecules. [74] It is suggested that struc-

tural defects within the grains of a pentacene film that resist relaxation cannot exceed

densities of 1016 cm−3, at typical growth conditions. A structural defect can, however,

influence the electronic levels of 10 surrounding molecules even if these molecules are in


Figure 2.10: Formation of in-grain structural defects during the growth of a pentacene film. Uponadding more “defective” molecules, relaxation into the ideal crystal structure becomes energeti-cally more favourable. However, the increased number of defective molecules not only reducesthe energy of the respective ideal crystal but also increases the energy barrier which needs tobe overcome. The height of the barrier is related to the time needed for the relaxation process.Most structural defects which exist during the growth of a film are expected to have relaxed intothe ideal crystal structure at the end of the film growth, as long as typical growth conditions arechosen. From [74].

the perfect crystal configuration. The authors conclude that grain boundaries are the most

prominent cause of structural defects in pentacene thin films. [74]

Although Verlaak et al. argue that translation out of a molecular layer is not

favourable, an experimental study identifies pentacene molecules that are displaced

slightly out of the molecular layers. [75] By means of high impedance scanning tunneling

microscopy (STM), specific defect islands in pentacene films with monolayer coverage

are detected. Within the defect islands, the pentacene molecules are displaced up to 2.5 Å

along the long molecular axis out of the pentacene layer with a broad distribution in the

magnitude of the displacements. However, the two-dimensional packing within the layer

is not disturbed. Electronic structural calculations show that the displaced molecules lead

to traps for both electrons and holes. These traps are located very close to the valence

or conduction band edge. The maximum displacement of the pentacene molecules as

seen by STM is 2.5 Å and this corresponds to a maximum trap depth of 0.1 eV. [75] The

situation is illustrated in Fig. 2.11.

The influence of annealing on the structure of pentacene films for field-effect tran-

sistor applications has been studied rather extensively. [76–81] Annealing after device


Figure 2.11: Pentacene molecules that are displaced out of the pentacene layers along the longmolecular axis cause trap states very close to the valence and conduction band edge. Calculationspredict trap depths in the range of 0−0.1 eV for the experimentally observed displacements up to2.5 Å. From [75].

fabrication has also been done with pentacene/C60-based photovoltaic cells. [82] Several

studies agree that annealing pentacene thin films at moderate temperatures (e.g. 50 C)

results in an improved crystallinity of the films as measured by XRD. [76,77,79] In some

cases, the annealing is found to result in an increased field-effect mobility. [79] The mo-

bility can, however, also remain unchanged, although structural changes were identified

with XRD. [76] The latter results (improved crystallinity and unchanged mobility) are

rationalized by an improved ordering within the grains in most of the molecular layers,

but an unchanged molecular packing in the first layer close to the interface where the

charge is transported in a field-effect transistor. The effect of annealing in this scenario is

illustrated in Fig. 2.12.

2.4.2 Chemical impurities

The best method to produce organic crystals includes a zone refinement step in the purifi-

cation procedure (Ref. [1], p. 224). Even such crystals still have a considerable content of

impurities. Anthracene, for example, still has an impurity content of 0.1 ppm in the best


Figure 2.12: Thermal annealing at 50 C for 1 h leads to a significantly improved crystallinity ofpentacene films as seen by XRD. The film morphology, as examined with atomic force microscopy(AFM), remains unaffected. Consequently, the annealing results in an improved ordering withinthe grains. From [76].

crystals, which corresponds to a volume density of ≈ 1014 cm−3 (Ref. [1], p. 224). Zone

refinement produces organic materials of much higher purity as compared to purification

by sublimation. [83] However, zone refinement can only be applied if the material can

be molten without a chemical reaction or a decomposition to occur. This is not possible

for many small molecule semiconductors. For the acenes, there is a crossover between

materials which can be purified by zone refinement and those which can only be purified

by sublimation. This crossover occurs between anthracene and tetracene, i.e. tetracene

or pentacene cannot be purified by zone refinement. Thus, much higher impurity concen-

trations are expected in tetracene or pentacene. [83] An experimental study indicates that

in tetracene single crystals the charge carrier mobility is limited by chemical impurities

rather than by structural defects. [83]

Clearly, chemical impurities have different energy levels than the host material. This

may lead to empty states in the bandgap, i.e. traps located at the impurity molecules. For

a given impurity and a given host crystal, a simplistic approach can be used to estimate

the trap depths ∆Eet and ∆Eh

t , respectively for electron and hole traps. The situation is

illustrated in Fig. 2.13. The trap depth for electrons is approximated by the difference in

the electron affinity of the guest crystal (Ac)guest and of the host crystal (Ac)host .2 The

difference in the crystal electron affinities can be approximated by the difference in the

gas phase electron affinities, and the trap depth for electrons can thus be written as [1]

∆Eet ≈ (Ac)guest − (Ac)host ≈ (Ag)guest − (Ag)host . (2.19)

2 The term guest crystal describes a crystal made of the impurity material.


Figure 2.13: Energy levels of a host material with chemical impurities. The impurities lead toelectron traps at Ee

t and hole traps at Eht . The trap depths ∆Ee

t for electrons can be estimated withthe difference in electron affinity of the host and guest material. The trap depth for holes ∆Eh

t canbe derived with the respective difference in ionization energy. [27]

In analogy, the depth of hole traps due to a specific impurity can be estimated with the

ionization energies according to

∆Eht ≈ (Ic)host − (Ic)guest ≈ (Ig)host − (Ig)guest . (2.20)

Fig. 2.14 compares measured trap depths in anthracene with the corresponding estimates.

The simplistic approach predicts the trap depths surprisingly well in several cases. [84]

If the electron affinity of the host is larger than the electron affinity of the guest, the

impurity itself cannot act as a trap for electrons. However, due to difference in size, the

impurity may deform the host lattice which causes a local change in the polarizability of

the lattice. If the impurity is larger than the host molecule, we have a compression of

the lattice, an increase in polarization energy and thus traps located on host molecules

adjacent to the chemical impurity. On the other hand, a smaller impurity results in a

reduced polarization energy. This cannot lead to an increase in stabilization in the vicinity

of the impurity but can generate a scattering of charge carriers in more polarizable regions

of the crystal, thus impeding the charge transport. [1]

Much of our experimental work was done with pentacene as the organic semicon-

ductor. Therefore, we now discuss chemical impurities in pentacene. The center ring of

the pentacene molecule is expected to be the most reactive, as illustrated in Fig. 2.15(a).

[85, 86] An important impurity in pentacene is thus thought to be the oxidized pentacene

species 6,13-pentacenequinone, where two oxygen atoms form double bonds with the


Figure 2.14: Measured trap levels in anthracene due to various impurities (solid lines). Thecorresponding trap levels, as estimated with the electron affinities and ionization energies of thehost and guest material (Eq. 2.19 and 2.20), are shown as dashed lines. Anthracenequinone (e), forexample, leads to electron traps, at approximately 0.6 eV from the conduction band. From [84].

(a) (b) (c)

Figure 2.15: The center ring of the pentacene molecule is expected to be the most reactive (a).Common impurities in pentacene are the oxidized species 6,13-pentacenequinone (b) and 6,13-dihydropentacene (c).

carbon atoms at the 6,13-positions (Fig. 2.15(b)). This disrupts the π-electron system

of the molecule. Pentacenequinone is expected to lead to trap states in the band gap of

pentacene. [86] Repeated purification of pentacene by vacuum sublimation can result in


very high mobilities in pentacene single crystals. [87] This effect is attributed to reduc-

ing the concentration of pentacenequinone which degrades the transport properties by

scattering the charge carriers. [87] Another common impurity in pentacene may be 6,13-

dihydropentacene (Fig. 2.15(c)), where additional hydrogen atoms are bound both at the

6- and at the 13-position. [85]

2.4.3 Trap states due to the gate dielectric

Properties of the gate dielectric’s surface such as surface roughness, surface free energy

and density of heterogenous nucleation sites are expected to play a key role for the growth

of organic films from the vapour phase and thus influence the quality of the films. Apart

from growth-related effects, the sole presence of the gate dielectric may influence the

charge transport in a field-effect transistor especially because the charge is transported

in the first few molecular layers at the insulator-semiconductor interface. The chemical

nature of the gate dielectric and, in particular, water adsorbed on its surface appear to

be aspects of particular importance. Moreover, the polarity of the gate dielectric may

influence the charge transport.

Chemical nature of the gate dielectric

The surface of the gate dielectric contains chemical groups that may act as charge carrier

traps. The trapping mechanism may be as simple as the one discussed above for chemical

impurities. It may also involve a reversible or irreversible electrochemical reaction driven

by the application of a gate voltage. Chemical groups on the surface of the gate dielectric

are certainly important for the transport of electrons in a field-effect transistor. [88–90]

Silanol groups (-Si-OH) and carbonyl groups (-CO) on the surface of the gate dielectric

are identified as electron traps. [89,90] Silanol groups are present on the surface of a SiO2

gate dielectric particularly if the substrate cleaning is carried out with piranha solution

(30 % hydrogen peroxide in 70 % sulfuric acid). The efficiency of electron trapping by

these groups is expected to depend on the choice of the organic semiconductor. [90] The

trapping of electrons by various chemical groups is illustrated in Fig. 2.16. The presence

of chemical groups at the insulator-semiconductor interface with a high potential to trap

electrons explains why most organic field-effect transistors show p-type, but no n-type op-

eration. [88, 89] The chemical species are thought to completely suppress the conduction

of electrons, while good hole transport is still possible. Pentacene-based transistors, for


Figure 2.16: Surface chemistry of polystyrene (PSn) and poly(vinyl alcohol) (PVA) gate di-electrics, of a hexamethyldisilazan (HMDS)-treated and a bare SiO2 gate dielectric and of a PSdielectric which is exposed to an oxygen plasma (PS-Ox). The efficiency of electron trapping atthese surfaces increases from left to right. From [90].

Figure 2.17: A high dielectric constant of the gate insulator leads to a polar surface. The randomlyoriented dipoles at the insulator-semiconductor interface modify the local polarization energy. Thenet effect is a broadening of the bulk density of states at the interface and thus a reduced mobilityof the charge carriers. The density of states in the bulk of the organic semiconductor is a Gaussiandensity of states which results from static disorder. The carrier population is also shown both forthe bulk density of states and the broader interface density of states. From [62].

example, generally show p-type but no n-type operation, although the intrinsic mobilities

for electrons and holes in pentacene are not expected to be very different.


Adsorbed water

Water adsorbed on the gate dielectric may dissociate and react with pentacene. One pos-

sible reaction product is 6,13-dihydropentacene (Fig. 2.15(c)). The number of the impuri-

ties that are formed may depend on the electrochemical potential and would thus increase

as the gate voltage is ramped up in a field-effect transistor. [86]

It has also been suggested that water causes traps by reacting with the surface of the

gate dielectric. Water on a SiO2 gate dielectric treated with piranha solution (i.e. with a

large number of silanol groups (-Si-OH)) causes the formation of SiO−-groups according

the the reaction [91]

SiOH+H2O SiO−+H3O+. (2.21)

In addition to chemical reactions involving water, this species may impede the

charge transport in a different way. Polar impurities such as water can act as traps them-

selves if either Eq. 2.19 or Eq. 2.20 result in a positive trap depth. In that case, the polar

character of the impurity results in an electric field dependent trap depth. [92] Even if

the polar impurity does not lead to positive trap depths, its dipole moment modifies the

local value of the polarization energy. This results in traps in the vicinity of the water

molecules. [92, 93] We always have the polarization energy caused by the interaction be-

tween the charge carrier and the induced dipoles on the neighboring molecules Pc. Now,

we also find contributions due to the interaction of the charge carrier with the permanent

dipoles of the water molecules Pcp and between the permanent dipole moments and the

induced dipole moments Ppi. The local polarization energy can thus be written as

Ploc = Pc +∆Ploc, (2.22)

∆Ploc = Pcp +Ppi. (2.23)

The net effect is a broadening of the bulk density of states function at the insulator-

semiconductor interface. [92]

Dielectric constant of the gate dielectric

We have seen that polar molecules such as water can lead to additional traps due to a

local change of the polarization energy. Veres et al. have suggested that the polarity of


the gate dielectric surface impedes the charge transport in the same way. [61, 62] A more

polar surface has randomly oriented dipoles which lead to a modification of the local

polarization energy within the semiconductor and thus to a change of the site energies. As

in the case of water, this brings a broadening of the density of states function. Fig. 2.17

illustrates the effect of a polar gate dielectric on a Gaussian distribution of hopping sites.

Therefore, gate dielectrics with a low polarity result in organic field-effect transistors with

the best performance. If the gate dielectric has a low dielectric constant, we expect the

formation of a nonpolar surface. In this context, it is important to realize that surfaces

with a low polarity have a low surface free energy and are expected to have a high water

repellency as well. The high water repellency would also lead to a a reduced amount of

water at the critical insulator-semiconductor interface. [62] A high static water contact

angle on the insulator surface thus indicates that the surface has a low surface free energy,

a high water repellency and a low polarity.

In this context, it is interesting to notice that a clear correlation between the hole

mobility in rubrene single crystals field-effect transistors and the dielectric constant of the

gate dielectric has been observed. [94] This study is particularly meaningful, because the

single crystals are grown separately and growth-related effects can be excluded. A recent

explanation is somewhat different from the interpretation by Veres et al.: the dependence

of the mobility on the dielectric constant of the gate dielectric may be accounted for by a

two-dimensional Fröhlich polaron model. [95]

2.5 Transistor operation

2.5.1 Qualitative description

The operation principle of an organic field effect transistor is illustrated in Fig. 2.18 for

p-type operation. If no voltages are applied to the device, the density of mobile holes in

the semiconductor is low and may depend on the concentration of impurities that act as

dopants (Fig. 2.18(a)). [96] If a drain voltage Vd is now applied between the grounded

source and the drain electrode, the resulting current is very low. In principle, it would be

given by

Io f f =WL

dσ0Vd (2.24)

for sufficiently low drain voltages (ohmic regime). L is the channel length, W is the chan-

nel width and d is the thickness of the semiconductor. However, in organic field-effect

2.5 Transistor operation 41

transistors with an oligomeric semiconductor of high purity the (off-state) conductivity σ0

is generally so low that the current Io f f as described by Eq. 2.24 is unmeasurably small.

We have an off-current of typically 10−12 A which is due to leakage currents/experimental

limitations. In any case, the off-state of an organic field-effect transistor is guaranteed by

the low conductivity of the organic semiconductor.

If we apply a gate voltage Vg between the source and the gate but no drain volt-

age, holes are injected from the source electrode and are accumulated at the insulator-

semiconductor interface as shown in Fig. 2.18(b). This is analogous to charging a plane

plate capacitor, but the screening length La in a semiconductor is somewhat larger than in

a metal.

The application of a small drain voltage would now produce a large current due to

the highly conducting interface region (Fig. 2.18(c)). At a fixed drain voltage Vd , the drain

current Id can thus be modulated by several orders of magnitude by simply ramping up

the gate voltage. This is shown in Fig. 2.19(b) and the transistor characteristic is called

transfer characteristic. For a low drain voltage, the term “linear regime” is used.

In Fig. 2.18(c), the drain voltage is much lower than the gate voltage and the charge

density in the active channel is, to a good approximation, uniform all along the transistor

channel. The situation is more complicated if the drain voltage is increased. The charge

density now depends on the distance from the source electrode. At the grounded source

electrode, the voltage drop is Vg but at the drain electrode, the voltage drop is Vg−Vd .

Consequently, the density of accumulated holes at the interface is lower at the drain than

in the region at the source (Fig. 2.18(d)). [96]

When Vg = Vd , the channel is said to pinch of. If the drain voltage is further in-

creased, a depletion zone grows from the drain electrode (Fig. 2.18(e)). Due to the ex-

panding depletion zone, the drain current no longer increases if the drain voltage is further

increased (at a constant gate voltage). The drain current saturates. This behaviour of the

transistor can be observed in the output characteristic at sufficiently high source-drain

voltages (Fig. 2.19(a)). The transfer characteristic corresponding to the saturation regime

is measured by applying a high drain voltage which is kept constant and by sweeping the

gate voltage. We call this transistor characteristic the saturation regime transfer charac-

teristic (Fig. 2.19(c)).


Figure 2.18: Operation principle of an organic field-effect transistor. The scheme describes a p-type device. In the off-state, the density of mobile holes in the organic semiconductor is very lowand can be caused by chemical impurities that act as dopants (a). By applying a gate voltage Vg

between the gate electrode and the grounded source electrode, holes are injected from the sourceand are accumulated at the insulator-semiconductor interface (b). The application of a low drainvoltage Vd results in a high drain current Id (c). The situation is more complicated when the drainvoltage is increased. Now, the interfacial charge density depends on the position in the channeland is higher at the source than at the drain (d). If the drain voltage is increased even further sothat |Vd| > |Vg|, the drain region is depleted. This leads to a saturation of the drain current; at afixed gate voltage, the drain current is constant even if the drain voltage is increased. Adaptedfrom [96].

2.5.2 Analytical description of an ideal field-effect transistor

In this section we present an analytical description of an organic field-effect transistor.

This description is expected to be valid for samples with a low trap density and negligible

contact resistances. In addition, all traps are assumed to be “fast” traps. This means that


Figure 2.19: Transistor characteristics of an ideal organic field-effect transistor: (a) output char-acteristic, (b) linear regime transfer characteristic and (c) saturation regime transfer characteristic.In the linear regime (low drain voltage), the drain current linearly depends on the gate voltage (b).In the saturation regime (high drain voltage), the square root of the drain current linearly dependson the gate voltage (c). From [97].

Figure 2.20: Definition of coordinates and variables to describe the organic field-effect transistor.The schematic shows the channel length L and the channel width W . The gate insulator has athickness l and a dielectric constant εi, while the semiconductor has a thickness d and a dielectricconstant εs.

the trapping and release times are much shorter than the time necessary to measure a

transistor characteristic (e.g. 1 min).

Transistor equations, mobility and threshold voltage

We begin with the derivation of two simple equations that describe the dependence of

the drain current on gate voltage both in the linear and in the saturation regime. These

equations can be used to extract the “trap-free” mobility µ0 and a threshold voltage Vt

from the measured transfer characteristics.

We use V for the electrical potential in the semiconductor and V (x = 0) = V0 is the

electrical potential at the insulator-semiconductor interface. Since the dielectric strength

D at the insulator-semiconductor interface must be continuous, we have

Dx = ε0εiFx(x = 0) = ε0εiVg−V0

l= Ci(Vg−V0) =−ε0εs

(dVdx

)

x=0, (2.25)


where F is the electric field and Ci = ε0εi/l is the capacitance of the gate dielectric per

unit area (see Fig. 2.20 for the definition of the variables). On the other hand, Gauss’s law

yields

(dVdx

)

x=0−

(dVdx

)

x=d=

(dVdx

)

x=0=

1ε0εs

Qtotal (2.26)

as long as the electric field on the backside of the semiconductor (x = d) vanishes and the

channel length L is much longer than the thickness of the gate insulator l (L À l). Qtotal

is the total charge per unit area (trapped and free). Combining Eq. 2.25 and Eq. 2.26, we

immediately find that

Qtotal =−Ci(Vg−V0). (2.27)

Note that V0 =V0(y) in Eq. 2.27 depends on the distance from the source if a drain voltage

is applied. The threshold voltage Vt is defined as the gate voltage above which essentially

all of the incrementally added gate-induced charge is free. The threshold voltage depends

on the density of charge carrier traps in the device and on the value of the flatband volt-

age VFB. The flatband voltage is the gate voltage which needs to be applied in order to

enforce flat bands at the insulator-semiconductor interface. A non-zero flatband voltage

can result from a difference of the Fermi level in the semiconductor and in the gate. More

importantly, the flatband voltage is influenced by charge that is permanently trapped at

the interface or within the gate dielectric. We replace the gate voltage Vg in Eq. 2.27 by

the effective gate voltage Vg−Vt and obtain an expression for the free charge per gate unit

area:

Q f ree =−Ci(Vg−Vt −V0). (2.28)

Eq. 2.28 will be inserted into an equation that is derived in the following.

The drain current Id is given by

Id =Z

jd(x,y)dxdz = WZ d

0jd(x,y)dx =

= WZ d

0µ0ep f ree(x,y)Fy(y)dx, (2.29)


where µ0 and p f ree are the mobility and density of free holes. Fy is the component of the

electric field in the direction of the current flow. We can write

ID =−WdV0(y)

dyµ0

Z d

0ep f ree(x,y)dx

︸︷︷︸Q f ree(y)

=

=−WdV0(y)

dyµ0Q f ree(y). (2.30)

Combining Eq. 2.30 with Eq. 2.28, we have the differential equation

dV0

dy=

Id

Wµ0

[1

Ci(Vg−Vt −V0)

]. (2.31)

Variable separation (variables V0 and y) and integration gives

Z Vd

0Ci(Vg−Vt −V0)dV0 =

Id

Wµ0

Z L

0dy (2.32)

and eventually results in

Id =WL

µ0Ci

(Vg−Vt − Vd

2

)Vd (2.33)

as long as |Vd| ≤ |Vg−Vt |. The term which is quadratic in Vd is often neglected and so, in

the linear regime, we have

Id =WL

µ0Ci(Vg−Vt)Vd |Vd| ¿ |Vg−Vt |. (2.34)

In the linear regime, Eq. 2.34 predicts a linear dependence of the drain current on the

effective gate voltage Id ∝ (Vg−Vt). This behaviour was already mentioned above and

is represented in Fig. 2.19(b). A linear regression of the measured transfer characteristic

would yield the trap-free mobility µ0 and the threshold voltage Vt .

At Vg−Vt = Vd the depletion zone at the drain electrode is about to form and the

drain current saturates. By introducing Vg−Vt = Vd in Eq. 2.33, we obtain

Id =WL

µ0Ci

2(Vg−Vt)2 |Vd| ≥ |Vg−Vt |. (2.35)


Eq. 2.33 was derived by assuming |Vd| ≤ |Vg−Vt |. Due to the saturation of the drain cur-

rent, Eq. 2.35 is valid not only for |Vd|= |Vg−Vt | but also for |Vd|> |Vg−Vt |. According

to Eq. 2.35, the drain current in the saturation regime quadratically depends on gate volt-

age, i.e. Id ∝ (Vg−Vt)2. Fitting the square root of the measured saturation regime drain

current to a straight line yields the mobility and the threshold voltage. This is illustrated

in Fig. 2.19(c).

Onset voltage, subthreshold swing and correlation with trap densities

In addition to the threshold voltage Vt , the onset voltage Von is a useful parameter. It

is defined as the gate voltage where the drain current exceeds the noise level which is

typically at 10−12 A. The onset voltage is indicated in Fig. 2.19(b). The onset voltage

Von and the flatband voltage VFB are approximately equal in many cases. Deviations may

however exist in certain situations. Consequently, the trapped charge per unit area is

approximately Ci|Vt −Von|. The total density of traps per unit area N2 (unit: cm−2) can

thus be estimated with

N2 ≈ Ci|Vt −Von|e

. (2.36)

In addition to the onset voltage, the subthreshold swing S is an important device

parameter. It is a measure of how easily a transistor can be switched from the off-state to

the on-state. The subthreshold swing3 is defined as [3]

S =dVg

d(logId). (2.37)

With the simplistic assumption that both the density of deep bulk traps Nbulk (in

cm−3eV−1) and the density of interface traps Nint (in cm−2eV−1) are independent of

energy, the subthreshold swing may be written as [98]

S =kT ln10

e

[1+

eCi

(√

εsNbulk + eNint)]. (2.38)

3 The subthreshold swing S is the inverse of the subthreshold slope.


This may be simplified as follows:

S =kT ln10

e

[1+

e2

CiN2

]. (2.39)

Both the deep bulk traps and the interface traps contribute to N2 (unit: cm−2eV−1).

[90, 99] Consequently, the subthreshold swing is a simple measure of the deep trap den-

sity. The lower limit of the subthreshold swing is obtained with N2 = 0 in Eq. 2.39:

S =kT ln10

e. (2.40)

At T = 300 K this lower limit is≈ 60 mV/dec. In other words, it is impossible to measure

a subthreshold swing steeper than 60 mV/dec in a field-effect transistor with the operation

principle described above.

2.5.3 Deviations from the ideal transistor behaviour

It is common practice in the field of organic transistors to use Eq. 2.34 and Eq. 2.35 in

order to estimate a mobility and a threshold voltage. If the experimental transfer charac-

teristics are linear in the linear regime and quadratic in the saturation regime, the approach

is self-consistent and the extracted mobility and threshold voltage have physical meaning.

The extensive experimental studies for this thesis have shown that organic field-

effect transistors exhibit ideal characteristics only in rare cases. For samples with a very

low trap density (steep subthreshold swing, high on-currents), the drain current often

increases sub-linearly in the linear regime. A typical example of such a transfer charac-

teristic is shown in Fig. 2.21(a). For this type of device, the transconductance (∂Id/∂Vg)Vd

does not saturate, but decreases even at high gate voltages.4 On the other hand, in sam-

ples with an increased trap density (thin-film transistors), the drain current in the linear

regime increases faster than linearly (Fig. 2.21(b)). The transconductance is a function

that monotonically increases with gate voltage. These deviations from the ideal transis-

tor behaviour can easily be understood. Eq. 2.34 and Eq. 2.35 rest on two simplifying

assumptions:

4 For an ideal transistor, the transconductance is expected to saturate above the threshold voltage at a valueof (∂Id/∂Vg)Vd = (W/L)µ0CiVd (Eq. 2.34).


(a) (b)

Contacts

Gate Voltage (V)

Tran

scon

duct

ance

Contacts

Dra

in C

urre

nt (A

)

Gate Voltage (V)

Traps

Tran

scon

duct

ance

Gate Voltage (V)

Traps

Dra

in C

urre

nt (A

)

Gate Voltage (V)

Figure 2.21: Deviations from the ideal transistor behaviour. The device in (a) is severely af-fected by parasitic contact resistances and the drain current increases less than lineraly in thelinear regime. An increased trap density is attributed to the device in (b). The traps lead to asuperlinear dependence of the drain current on gate voltage. The insets show the linear-regimetransconductance (∂Id/∂Vg)Vd .

1. The effect of parasitic contact resistances at the source and at the drain are negligi-

ble. All the drain voltage Vd drops across the transistor channel.

2. The trap density is very low and the threshold voltage is a rather small voltage.

Upon reaching the threshold voltage, essentially all the traps are filled. All the

charge which is accumulated above the threshold voltage is free and contributes to

the drain current.

We suggest that, for samples with a low trap density, the few traps are readily filled as the

gate voltage is ramped up, resulting in a high current density. The contacts cannot supply

and/or extract a sufficient amount of charge and the current is thus severely affected by the

contacts. This interpretation is supported by measurements of single crystal transistors in

Chap. 5. On the contrary, in samples with an increased trap density, the majority of the

charge carriers induced by the gate is trapped even at relatively high gate voltages. The

percentage of the gate-induced charge that is free increases with gate voltage and this leads

to a “superlinear” transfer characteristic (Fig. 2.21(b)). Strictly speaking, the threshold

voltage is not reached even if large gate voltages are applied. Eq. 2.34 and Eq. 2.35

are not suitable for these devices. [100, 101] This is as in the case of amorphous silicon

field-effect transistors, where the trap densities are substancial. [102] Pentacene thin-film

transistors with a fluoropolymer gate dielectric (Chap. 5) or with a SiO2 gate dielectric


(Chap. 6 and Chap. 7) exhibit transfer characteristics with a superlinear dependence on

gate voltage. Due to the reduced current densities, contact effects are less severe. We

have developed and used an analytical approach for organic thin-film transistors with an

increased trap density. The analytical approach is outlined in Chap. 3 and is described in

detail in Chap. 6 and Chap. 7.

2.5.4 Electrical stability of organic field-effect transistors

In Sec. 2.5.2 we have also assumed that all traps are “fast” traps. However, organic field-

effect transistors often exhibit a current hysteresis (difference between the forward and the

reverse sweeps). Moreover, we often have a persistent shift of the transfer characteristic

when a gate bias is applied for a prolonged time. These phenomena are known as elec-

trical instability or gate bias stress effects. Electrical instability is most likely caused by

the trapping of charge in long-lived trap states. The term “long-lived” refers to a trapping

and release time which is long compared to the time needed to measure a transfer charac-

teristic (e.g. 1 min.). The shift of the transistor characteristic (threshold voltage shift) ∆Vt

can be used to estimate the surface density of long-lived traps according to [103]

N2 =Ci∆Vt

e. (2.41)

The long-lived states may be extrinsic in nature (e.g. certain chemical groups on the sur-

face of the gate dielectric). [103] The long-lived trap states may also be located within the

semiconductor in regions with increased structural disorder, i.e. may result from structural

defects. [67,80,104] Furthermore, a reversible electrochemical reaction of adsorbed water

with the organic semiconductor or with chemical groups on the surface of the gate dielec-

tric may play a role as well. [86, 105–107] It has also been suggested that gate bias stress

effects are an intrinsic phenomenon and are due to the formation of hole bipolarons, i.e. a

tightly bound state with a very low mobility. [108] Impurity states might even catalyze the

formation of bipolarons. [109] Other more marginal causes of electrical instability of an

organic field-effect transistor include the transfer of charge from the semiconductor to the

gate dielectric [110], or the movement of charged ions within the gate dielectric [111].

3 Experimental details

We now present an overview of the materials and methods that were employed for our

experimental work. More specific experimental details can be found in Chap. 4-7, where

we describe the respective experimental results. Here we begin by introducing the or-

ganic semiconductors that were employed. We then describe the transistor fabrication

and characterization steps with the equipment that had been established before this thesis

was started. This equipment was used for the studies in Chap. 4 and Chap. 5. In Chap. 6

and Chap. 7 we present measurements that were carried out with a new “device fabrica-

tion and characterization system” which we set up as a part of this thesis. In the present

chapter we introduce this system.

3.1 Organic semiconductors investigated in this study

All the experimental work was done with oligomeric semiconductors. The semiconductor

was always grown from the vapour phase by vacuum evaporation or by physical vapour

transport. Most of the experimental work was performed using the common materials

pentacene and rubrene (Fig. 1.1). The first project, however, was to fabricate and char-

acterize field-effect transistors with new oligomeric semiconductors in order to identify

relations between the chemical structure and the crystal structure of the oligomeric ma-

terial, on the one hand, and the transistor performance, on the other hand. This project

was carried out in collaboration with Ciba Speciality Chemicals Inc., Basel, Switzerland.

Fig. 3.1 shows most of the new organic semiconductors that we have investigated. The

materials that were obtained from Ciba were used as received, without any further pu-

rification. We made both thin-film transistors (TFT’s) and single crystal field-effect tran-

sistors (SC-FET’s) with the new oligomeric semiconductors (see Sec. 3.3-3.4 for details

on the transistor fabrication and characterization). Fig. 3.2 reveals the best field-effect


mobilities that were measured respectively in SC-FET’s and TFT’s. General relations,

e.g. between the chemical structure of the molecules and the device performance, could

not be identified. However, we see that most of the new oligomers which were stud-

ied led to operating transistors with mobilities ranging from 10−7 cm2/Vs to 0.1 cm2/Vs.

Field-effect mobilities from single crystal transistors (SC-FET’s) were found to be always

higher than mobilities from thin-film transistors (TFT’s), as long as operating devices of

both types could be made (factor of 3− 670 depending on the material). This is likely

caused by a better structural order of the semiconductor in the SC-FET’s. The electrical

performance of the new organic semiconductor 7,14-Diphenyl-chromeno[2,3-b]xanthene

(DPCX, Fig. 3.1) was found to be the most promising, and this material was thus studied

extensively. The study of DPCX highlights the crucial importance of structural order of

the semiconductor and is described in Chap. 4.

3.2 Purification of pentacene and rubrene

The experimental work in Chap. 5-7 was done with pentacene and rubrene. Both materials

were purchased from Sigma-Aldrich (pentacene: purum, Prod. No.: 76440; rubrene:

Prod. No.: R2206). The materials were purified by recrystallization in vacuum. This

was done by placing the starting material at the closed end of a horizontal glass tube.

The glass tube was introduced into a home-made holder and was evacuated with a turbo

pump. The side with the starting material was heated to a temperature slightly above

the sublimation temperature at the given pressure (e.g. ≈ 290 C for pentacene). While

heating the starting material, the other side of the tube was water-cooled, thus establishing

a temperature gradient along the tube. The sublimed material condensed at the walls of the

glass tube, in a specific region of the tube corresponding to a specific temperature. Lighter

impurities condensed further along the tube and heavier impurities did not sublime. The

recrystallization process was repeated with the recrystallized material at least once. A

schematic drawing of the purification system can be found in [112].

3.3 Preparation of the gate dielectric

An organic field-effect transistor consists of a gate electrode, a gate insulator, the active

organic semiconductor and the source and drain electrodes. The most convenient way

to fabricate organic field-effect transistors is to purchase highly-doped Si/SiO2 wafers.

3.3 Preparation of the gate dielectric 53

Figure 3.1: Most of the new oligomeric semiconductors that were studied in organic field-effecttransistors. The figure contains materials that were synthesized by Ciba Speciality Chemicals Inc.7,14-Diphenyl-chromeno[2,3-b]xanthene (DPCX) was found to be a promising organic semicon-ductor and was extensively studied.


10-710-610-510-410-310-210-1100101

No SC-FET operation

Amorp.films

AGH2SOL3

SOL1SOL2

NBR1

5,12-B

TBRAGH1

DPCX

XPEN2

XPEN1

Fiel

d-ef

fect

mob

ility

(cm

2 /Vs) SC-FET

TFT

Figure 3.2: Field-effect mobilities measured in single-crystal field-effect transistors (SC-FET’s)and in thin-film transistors (TFT’s) with new oligomeric semiconductors. The mobilities in SC-FET’s are always higher than in TFT’s. In several cases, no suitable crystals could be grown.Operating TFT’s were obtained with most of the materials.

The highly doped Si acts as gate electrode and the amorphous SiO2 layer (typical thick-

ness: 300 nm) acts as gate insulator. We purchased Si/SiO2 wafers (n/phosphorus) from

SilChem, Germany. The wafers were cut into appropriate pieces with an automatic dic-

ing saw. The transistors were fabricated by cleaning these substrates and by depositing

the semiconductor and the source/drain electrodes. A thin-film transistor of this type is

represented in Fig. 3.3(a). In Chap. 2 we have already seen that the surface of the gate

dielectric might influence the device performance in various ways. Consequently, other

device structures with an ultrathin organic buffer layer on the SiO2 gate dielectric were

also studied (Fig. 3.3(b)). Finally, we made transistors where the gate dielectric entirely

consisted of a spin-coated polymeric layer (Fig. 3.3(c)).

3.3.1 Cleaning of Si/SiO2 substrates

If we use a bare SiO2 gate dielectric, the device performance critically depends on the de-

tails of the cleaning procedure. Initially, the Si/SiO2 samples were washed with ultrapure

water (18.2 MΩcm) and were then cleaned with hot acetone and hot isopropanol in an

ultrasonic bath. Subsequently, the samples were immersed in piranha solution (30 % hy-

drogen peroxide in 70 % sulfuric acid). Finally, the samples were thoroughly washed with

ultrapure water and dried with nitrogen. The piranha treatment is important if the SiO2

is to be modified with a self-assembled monolayer of octadecyltrichlorosilane (OTS, see


Figure 3.3: Organic field-effect transistors with three types of gate insulators were used, asschematically shown for the TFT’s. We used a bare SiO2 gate dielectric (a), a SiO2 gate dielectricwith an additional ultrathin organic layer as surface modification (b) or a spin-coated polymericgate dielectric on a glass/ITO substrate. The highly doped Si in (a) and (b) and the ITO layer in(c) act as the gate electrode. For the TFT’s, the contacts were formed after the semiconductor inall cases (top-contact devices).

Sec. 3.3.2), since it creates reactive silanol groups (-Si-OH). In the course of the experi-

mental work, we realized that for TFT’s with a bare SiO2 gate dielectric the device per-

formance is much better if the cleaning with piranha solution is omitted (see also [112]).

Consequently, for the projects described in Chap. 6 and 7, the SiO2 was cleaned solely

with hot acetone and hot isopropanol in an ultrasonic bath. This led to a rather passive

surface of the gate dielectric.

3.3.2 Surface modification with self-assembled monolayers of oc-tadecyltrichlorosilane (OTS)

For the study of the new organic semiconductor DPCX in Chap. 4, we have modified the

SiO2 of some substrates with a self-assembled monolayer of OTS prior to the semicon-

ductor deposition, as illustrated in Fig. 3.3(b). The OTS-treatment was initially applied to

organic field-effect transistors by Lin et al. in the year 1997. [113, 114]


More generally speaking, self-assembled monolayers (SAM’s) are used as a means

of surface modification for various applications. The molecules have a rather long alkyl

chain and a head and tail group at the respective ends of the alkyl chain. The head group

is chosen so that it can bond to the substrate’s surface. The head groups of the SAM-

molecules attach to the surface to be modified and the van der Waals interaction between

the long alkyl chains pulls the molecular chains up. This process results in highly ordered

monolayers. [115, 116] The alkyl chains are almost perpendicular to the surface of the

substrate and have a thickness of typically 2−3 nm, depending on the length of the alkyl

chain. Once formed, the monolayer is very stable. [117–119] The collectivity of the tail

groups forms the new surface. Consequently, the tail group can be chosen according to

the desired properties of the new surface. [116]

In the case of OTS, the chemical structure is CH3(CH2)17SiCl3. We have a silane

group which covalently binds to the substrate, and the methyl groups (-CH3) at the other

end of the alkyl chains form a surface with a low surface free energy. The surface mod-

ification with OTS can be done from solution or by a vapour process. We have used the

latter approach. OTS was purchased from Sigma-Aldrich (≥ 90 %, Prod. No.: 104817).

0.1-0.2 ml of OTS were placed at the closed end of a horizontal glass tube. The cleaned

Si/SiO2 substrates were placed into the glass tube, ≈ 10 cm from the OTS, and then

the glass tube was evacuated. Subsequently, the OTS and the samples were heated to

120−125 C for 1−3 h.

3.3.3 Polymeric gate insulators and polymeric buffer layers

Several polymeric insulators were deposited from solution by spin-coating (Fig. 3.4). The

solution of the polymer was spin-coated either on ITO-coated glass substrates to form a

polymeric gate insulator (Fig. 3.3(c)), or onto Si/SiO2 samples to form a polymeric buffer

layer (Fig. 3.3(b)). The concentration of the solution and the spinning speed were adjusted

to yield the appropriate film thickness. By decreasing the concentration of the solution

or by increasing the rotation speed, thinner films were obtained. Typical rotation speeds

were 500−3000 rpm and a typical spinning time was 30 s. The polymeric films were then

dried on a hotplate. Both the spin-coating and the drying were carried out in air. A typical

thickness was 500 nm for a polymeric gate dielectric and 10− 30 nm for a polymeric

buffer layer.

The nonfluorinated materials polystyrene (PS), poly(α-methylstyrene) (AMS) and

poly(vinyl alcohol) (PVA) are all commercially available from Sigma-Aldrich (PS: Prod.


Figure 3.4: Several solution-processable polymers were used as polymeric gate dielectrics orpolymeric buffer layers in organic field-effect transistors. We worked with polymers such as thenonfluorinated materials polystyrene (PS), poly(α-methylstyrene) (AMS) and poly(vinyl alcohol)(PVA) as well as the fluoropolymer CytopT M. All materials are commercially available.

No.: 182427, AMS: 81520 (mol wt. ∼ 100000), PVA: 324590). These materials had

been used to some extend as gate dielectrics or polymeric buffer layers in pentacene-

based thin-film transistors, before we investigated the materials (PS: [90], AMS: [14,66],

PVA: [111]). We were able to reproduce some results including high field-effect mobil-

ities in pentacene thin-film transistors with AMS buffer layers1 and electron transport in

pentacene films on PVA gate dielectrics. However, the devices with AMS buffer layers

or PVA gate dielectrics were unsatisfactory in terms of electrical stability. The devices

had a rather large current hysteresis. Field-effect transistors with a PS gate dielectric

only showed a small current hysteresis in accordance with [90]. The best results were

achieved with the amorphous fluoropolymer CytopT M (Cytop is a registered trademark

of Asahi Glass and is a short hand for Cyclic Transparent Optical Polymer). CytopT M

was purchased in the dissolved state as a liquid (Cytop CTL-809M, concentration: 9 %)

along with a suitable solvent (CT-Solv.180) for further dilution which is very convenient.

Initially, we purchased the CytopT M from Asahi Glass, Japan and later from Bellex In-

ternational Cooperation, USA. CytopT M had previously been utilized by Veres et al. in

1 Record mobilities around 5 cm2/Vs have been achieved with pentacene thin films on AMS buffer layersby Kelley et al. [14]


Figure 3.5: Structure of the fluoropolymer CytopT M (Cyclic Transparent Optical Polymer). [120]This material was extensively studied as polymeric gate insulator in organic field-effect transistors.

combination with solution-processed semiconductors. [61, 62] The structure of the mate-

rial is illustrated in Fig. 3.5. We used CytopT M as gate insulator with vacuum-evaporated

pentacene films and with pentacene and rubrene single crystals. Transistors fabricated

with CytopT M showed a very high electrical stability, combined with high field-effect

mobilities and an excellent subthreshold performance. Therefore, this material was stud-

ied most extensively in field-effect transistors and the results are described in Chap. 5.

3.4 Growth of the semiconductor, electrode depositionand electrical characterization

After having detailed the preparation of the gate dielectric, we now proceed by describing

the vacuum evaporation of organic films and the growth of organic crystals by physi-

cal vapour transport. We then introduce the electrode deposition. Finally, the electrical

characterization of the completed transistors is described.

3.4.1 Evaporation of organic films

The established approach to evaporate organic thin films was to use a home built evapora-

tion chamber. The system was turbo-pumped with a base pressure of 2−3×10−6 mbar.

The samples with the gate insulators were fixed on a sample holder and shadow masks

were fastened on top of the samples. The sample holder was attached to the top lid of

the evaporation chamber. The temperature of the samples during the thin-film deposition

could be adjusted (between≈ 0 C and≈ 140 C) by simply heating or cooling the top lid

with the sample holder from the outside. The organic semiconductor in a molybdenum

boat within the evaporation chamber was resistively heated to temperatures of typically

200− 300 C (pentacene: 230− 260 C) in order to sublime the semiconductor. The

deposition rate was measured with a water-cooled quartz crystal microbalance and was

typically held at 0.3−0.5 Å/s. The nominal film thickness was 50 nm in most cases.

3.4 Growth of the semiconductor and electrical characterization 59

3.4.2 Single crystal growth

Organic single crystals were grown by physical vapour transport. [121] The apparatus

essentially consisted of an inner and an outer horizontal glass tube. The inner glass tube

had a gas inlet on one side and a gas outlet on the other side. A heating wire was wound

around the outer glass tube with an increasing spacing between the windings and thus

produced a temperature gradient. High purity argon was used as the transport gas and was

constantly flowing at a rate of 7−20 ml/min through the inner glass tube during the crystal

growth. Applying the temperature gradient with the heating wire led to the sublimation

of the source material in the inner tube and to the growth of single crystals in a region of

lower temperature. Rubrene powder, for example, was typically heated up to ≈ 290 C,

in order to have a sufficient sublimation rate. The single crystal growth method results in

a spatial separation of the organic semiconductor from lighter and heavier impurities and

is thus expected to result in a further purification.

3.4.3 Electrode deposition

All transistors incorporated vacuum-evaporated gold electrodes. In the established

approach to device fabrication, a turbo-pumped Tectra evaporator (base pressure:

2× 10−6 mbar) was used for the gold deposition. Again, the deposition rate was mon-

itored with a water-cooled quartz crystal microbalance.

All thin-film transistors were fabricated as top-contact devices, i.e. the electrodes

were deposited after the deposition of the organic semiconductor. This is known to lead to

better transistors, compared to devices where the electrodes are evaporated first (bottom-

contact devices). In the latter case, the thin-film growth encounters a step at the gold

electrode resulting in structural disorder in the vicinity of the electrode. [122] For the

deposition of gold electrodes, the samples remained attached to the sample holder after

the deposition of the organic film. The shadow masks for the semiconductor deposition

were replaced by shadow masks for the gold evaporation. The mask exchange was carried

out in air. The deposition rate of the gold was 0.5−1 Å/s and the film thickness typically

was 50 nm for the TFT’s. Fig. 3.6 is a photograph of completed thin-film transistor test

structures.

For the SC-FET’s, the gold electrodes were directly evaporated onto the gate di-

electric through shadow masks. The thickness of the electrodes was 15− 30 nm for the

SC-FET’s and the deposition rate was 0.5− 1 Å/s. The single crystals were then placed


Figure 3.6: Top view of thin-film transistor test structures. The use of a shadow mask for thedeposition of the organic semiconductor results in a “masked” organic film. The photograph showsvarious test structures with a channel width of W = 500 µm and a channel lengths of L = 200 µm,L = 150 µm, L = 100 µm and L = 50 µm. Typically, TFT’s with a channel length of L = 100 µmwere measured.

Figure 3.7: Device structure of a SC-FET with a polymeric gate dielectric. First, the contactswere formed (bottom-contact devices) and, afterwards, the separately grown single crystals wereplaced on the electrodes.

onto the electrodes in air and were held in place by electrostatic forces. The bottom

contacts are thought to lead to a superior device performance in the case of SC-FET’s,

because the charge is injected close to the active channel. [123] In the case of top-contact

SC-FET’s, the charge would have to pass through the crystals (e.g. 500 nm thick) and this

might result in a large access resistance. [123] The structure of a SC-FET with a glass/ITO

substrate and an organic dielectric is shown in Fig. 3.7.

3.4.4 Electrical characterization of transistors

The completed samples were quickly transported through air to a prober station in a

glovebox with a dry He atmosphere. We used a HP 4155A semiconductor parameter

analyzer for all electrical measurements. The parameter analyzer is equipped with four

source/monitor units (SMU, ±100 V) and with a ground connection. For convenience,

3.5 Additional thin-film characterization 61

the HP 4155A was controlled by means of a computer program which also allows for the

automatic measurement of test sequences. [112]

The transistors were generally characterized by measuring transfer characteristics

both in the linear regime (with a low drain voltage Vd) and in the saturation regime (with

a high drain voltage). Additionally, output characteristics were measured. These gated

two-terminal measurements were done by connecting three of the SMU’s to the source,

drain and to the gate of the transistor. The SMU’s were operated in the “force voltage -

measure current” mode.

The transfer characteristic in the saturation regime was generally used to estimate

the mobility µ0 and the threshold voltage Vt with Eq. 2.35. Other device parameters

including the onset voltage Von and the subthreshold swing S were extracted from the

transfer characteristic as well (see Sec. 2.5.2). Output characteristics were measured in

order to confirm the transistor operation and to quantify to which extend the device is

influenced by parasitic contact resistances.

Typically, both the forward and the reverse sweeps were measured. The difference

between the forward and the reverse sweeps gives a first indication of the electrical sta-

bility of the transistor. In some cases, gate bias stress studies were carried out to further

elucidate the electrical stability. A gate bias was applied for an extended period of time

while the source was grounded and the drain potential was held at 0 V. The transfer char-

acteristic measured after the gate bias stress period is often shifted, compared to a trans-

fer characteristic measured prior to the stress period. The magnitude of the shift of the

transfer characteristic in gate bias stress experiments is a measure of the device stability

(Sec. 2.5.4).

3.5 Additional thin-film characterization

Occasionally, we used additional techniques to characterize the gate insulator and the

evaporated organic semiconductor film and these methods are described in the following.

3.5.1 Static water contact angles

Static water contact angles on the surface of the gate dielectric were measured by carefully

placing a small droplet of ultrapure water on the surface of the sample. A photograph of

the droplet was taken immediately after placing the droplet. As an example, Fig. 3.8

shows a photograph of a water droplet on a CytopT M gate insulator. The water contact


Figure 3.8: Photograph of a water droplet on a highly hydrophobic amorphous fluoropolymerlayer (CytopT M). The static water contact angle is α = 112. In general, the water contact angleα is a function of the interfacial tensions/energies of all involved interfaces, i.e. substrate-air (sa),substrate-water (sw) and water-air (wa).

angle α was estimated with the photograph. The contact angle results from the interplay of

three different interfacial tensions/energies, i.e. σsa (substrate-air), σsw (substrate-water)

and σwa (water-air). [124] From Fig. 3.8 we see that

cos(α) =σsa−σsw

σwa≈ σs−σsw

σw. (3.1)

The interfacial tension σsa (substrate-air) can be approximated with the surface tension of

the substrate σs and similarly σwa ≈ σw, where σw is the surface tension of water.2

On the one hand, we measured contact angles in order to check the quality of the

SAM’s of OTS. The quality of a SAM critically depends on many factors, including the

cleaning procedure of the substrate, and is reflected in the static water contact angle: the

larger the contact angle is, the better the SAM. Contact angles of α = 90−95 were gen-

erally achieved on OTS-treated SiO2 with the vapour process. Sometimes, significantly

lower contact angles were measured and such samples were discarded. On the other

hand, the method was used to characterize the surface of polymeric gate dielectrics and

polymeric buffer layers. The contact angles on polymeric gate dielectrics and polymeric

buffer layers were more reproducible than on OTS and showed only small deviations from

2 It should be kept in mind that, in practice, the measured contact angle does not only depend on interfacialtensions. It is also affected by the roughness of the surface. A decrease in roughness should cause adecrease in contact angle.

3.5 Additional thin-film characterization 63

Table 3.1: Typical static water contact angles, as measured on OTS and on polymeric layers thatwere used as gate insulators or polymeric buffer layers.

Surface Static water contact angle

OTS 90-95

AMS 95

PVA water-soluble

PS 95

CytopT M 112

a mean value. Typical water contact angles from our measurements are summarized in

Table 3.1. In Chap. 2 we have seen that a high water contact angle is indicative of a non-

polar and water repellent surface which reduces the density of traps in the interface region

of an organic field-effect transistor.

3.5.2 Atomic force microscopy

In Chap. 2 we also mentioned that the roughness of the gate dielectric influences the

thin-film growth and thus the structure and quality of evaporated films. The roughness

of the polymeric gate insulators was occasionally determined by tapping mode atomic

force microscopy (AFM). Tapping mode AFM is suitable for soft polymeric surfaces that

are easily damaged, because the tip is not dragged over the surface as in contact mode. In

order to measure the surface roughness, AFM images with a scan size of 4 µm×4 µm were

recorded and the root mean square (RMS) roughness was calculated from the images. An

Asylum Research AFM located in the cleanroom of the FIRST Center for Micro- and

Nanoscience was used in order to obtain high quality AFM images.

3.5.3 X-ray diffraction

The structure of evaporated organic films was sometimes investigated with X-ray diffrac-

tion in the Bragg-Brentano geometry (Θ− 2Θ-mode). This was done with an in-house

Stoe Stadi P diffractometer. In the Θ−2Θ-mode, the angle of the incident X-rays is equal

to the angle of the scattered and detected radiation. As a consequence, the scatter vec-

tor K is perpendicular to the surface of the sample and the Laue condition K = G can

only be fulfilled with a reciprocal lattice vector G that is perpendicular to the surface of

the sample. Therefore, measurements in the Bragg-Brentano geometry can only reveal


information about lattice planes that are parallel to the surface of the sample. We recall

that evaporated organic films, such as pentacene films, often have a polycrystalline and

layered structure. The molecules in the layers are almost perpendicular to the substrate

and the layers are parallel to the substrate. It should be kept in mind, however, that X-ray

diffraction measurements give information about the whole film (typically 50 nm thick).

On the contrary, the charge transport in a field-effect transistor takes place in the first few

molecular layers. For pentacene, the thickness of one molecular layer is ≈ 1.5 nm.

3.5.4 Surface step profiling

For samples with a polymeric gate insulator (Fig. 3.3), the thickness of the gate insulator

was measured with a Tencor Alpha-Step 500 at the FIRST Center. This was done by

scratching the soft polymer film with plastic tweezers. A profile of the scratch was mea-

sured and the measured thickness l was used to calculate the gate capacitance per unit area

Ci = ε0εi/l. This value of Ci was used for the calculation of the field-effect mobilities.

3.5.5 Leakage current and capacitance measurements

Leakage current and capacitance measurements were sometimes carried out to further

characterize the gate insulator. These measurements were done with an open-air prober

station and respectively an Agilent 4339B high resistance meter and an Agilent 4192A

impedance analyzer. For these measurements, circular gold electrodes were evaporated

onto the gate insulator and the area of the circular gold electrodes was 0.985 mm2.

3.6 Advanced fabrication and characterization of thin-film transistors

Previously, we have described the established approach to fabricate and characterize or-

ganic field-effect transistors. These experimental details are relevant for the studies in

Chap. 4 and Chap. 5. The results in Chap. 6 and Chap. 7 were obtained with a “device fab-

rication and characterization system” which allows for measurements of organic thin-film

transistors under highly controlled conditions, e.g. without exposing the samples to am-

bient air between the transistor fabrication and the electrical characterization. Instead of

the commonly employed gated two-terminal measurements, we used gated four-terminal

measurements in these studies to rule out the effect of parasitic contact resistances. In

3.6 Advanced fabrication and characterization of thin-film transistors 65

addition, the system contains a cryostat and allows for temperature-dependent measure-

ments of the transistors. Finally, we developed and used a scheme for the parameter

extraction that is suitable for samples with an increased trap density and sufficiently good

contacts.

3.6.1 Device fabrication and characterization system

At the beginning of our work for this thesis, the laboratory had been equipped with a

new evaporation chamber purchased from Kurt J. Lesker company, Great Britain. The

chamber had a base pressure of 10−8 mbar. It allowed for the thermal evaporation of or-

ganic semiconductors and metals. A mask-positioning mechanism had been incorporated

in the system, in order to deposit pentacene films and gold electrodes without breaking

the high vacuum in between the two deposition steps. We designed a cryogenic vacuum

prober station with the aim to attach this prober station to the existing evaporation cham-

ber and to eventually characterize organic thin-film transistors without any exposure to

air between the deposition of the semiconductor, the deposition of the contacts and the

electrical characterization. This project was very promising, because organic semicon-

ductors were thought to be sensitive to ambient gases including water vapour [105, 125]

and oxygen [126–130]. Organic field-effect transistors, in general, had most often been

characterized after a transfer of the samples to the measurement chamber through air.

Electrical characterization without air exposure is still very rare. [131, 132] More specifi-

cally, the goal of the experiments with the device fabrication and characterization system

was to learn about the microscopic origin of trap states in organic field-effect transistors

by exploiting the high degree of control this setup has to offer. The challenges in the

design of the prober station were:

1) the realization of the sample transfer from the evaporation chamber to the cryostat in

the prober station without breaking the high vacuum,

2) the development of a mechanism which assures that the samples reach sufficiently low

temperatures when cooling down the cryostat in the prober station and

3) the reliable measurement of the temperature of the samples without breaking the high

vacuum.

The prober station was built by CryoVac, Germany. Drawings of the completed system

are shown in Fig. 3.9. Fig. 3.10 is a photograph of the system.


(a)

Evaporation chamber

Gate valve 1

Transfer rod 2

Load lock

Transfer rod 1

Gate valve 2

Prober station

(b)

Load lock

Cryostat

Sample holder / samples

Micro-Prober

Microscope

Evaporation chamber

Prober station

Figure 3.9: Schematic drawings of the device fabrication and characterization system: top view(a) and side view (b). The system essentially consists of an evaporation chamber, a cryogenicprober station and a load lock. It allows for the fabrication and electrical characterization oforganic thin-film transistors without breaking the high vacuum of the order of 10−8 mbar.


Figure 3.10: Photograph of the device fabrication and characterization system.

For this thesis, the device fabrication and characterization system was exclusively

used with pentacene and gold. The substrates (e.g. highly doped Si/SiO2 samples) were

mounted on a sample holder (Fig. 3.11) and were introduced into the device fabrication

and characterization system via the load lock (Fig. 3.9(a)). The base pressure of the sys-

tem was of the order of 10−8 mbar. The substrates were introduced into the evaporation

chamber with transfer rod 1 (Fig. 3.9(a)) in order to evaporate pentacene and gold. After

the completion of the device fabrication, the samples were transported to the prober sta-

tion with transfer rod 1 and 2 (Fig. 3.9(a)). Transfer rod 2 was equipped with a threaded

bar and was screwed into the sample holder. The sample holder was then pushed under

the clamps on the cryostat in the prober station with transfer rod 2 (Fig. 3.12(a)). In the

prober station we carried out the characterization of the transistors.

3.6.2 Electrical characterization by gated four-terminal measure-ments

The prober station is equipped with five micro-probers for the electrical characteriza-

tion. By means of an electrical feedthrough to the cryostat, a gate bias could be applied

to the transistor test structures. In order to measure the temperature on the surface of

the samples, a thermocouple was attached to one of the five micro-probers as shown in


(a) (b)

Figure 3.11: Top view (a) and side view (b) of the sample holder with four samples.

Fig. 3.12(b). The thermocouple was carefully pressed against the surface of the sample

with the micro-prober at each temperature.

Gated four-terminal measurements have the advantage that the measurements can

be corrected for the influence of parasitic contact resistances at the source and at the

drain. The contact-corrected channel conductivity as a function of gate voltage σ(Vg) can

be used to calculate the contact-corrected field-effect mobility. The gated four-terminal

measurement also allows for an estimation of the device contact resistance. Four-terminal

measurements have previously been used to estimate the field-effect mobility and the

contact resistance in TFT’s [133, 134] and SC-FET’s [135–137].

The transistor test structure for the gated four-terminal measurements is schemati-

cally shown in Fig. 3.13. The transistors consisted of a well-defined stripe of pentacene (a

“masked” pentacene film) and had two voltage sensing electrodes with little overlap to the

pentacene film. For the electrical measurements we used a HP 4155A semiconductor pa-

rameter analyzer. In principle, the gated four-terminal measurement is the measurement

of a transfer characteristic and, in addition, the potentials V1(Vg) and V2(Vg) are measured.

For each applied gate voltage Vg, the drain current Id and the potentials V1 and V2 between

the grounded source electrode and the respective voltage sensing electrode were measured

while keeping the source-drain voltage constant. This was done by connecting the source

of the transistor to the ground connector of the HP 4155A and by measuring the channel

potentials V1 and V2 with two SMU’s in the “source current - measure voltage” mode with

a sourced current of 0 A. The other two SMU’s were connected to the gate and the drain,

and were used as for the gated two-terminal measurements. The drain voltage was gener-

ally held at Vd = −2 V. We should point out that the gated four-terminal measurement is

most useful if measured with a low drain voltage Vd . In that case we can assume a linear

voltage drop all along the transistor channel. [138]


(a)

(b)

Figure 3.12: (a) Top view of the opened prober station. The sample holder (left) was pushedunder the clamps on the cryostat with transfer rod 2. The prober station is equipped with fivemicro-probers. The prober arms were connected to the cryostat with thick copper braids and werethus cooled when the cryostat and the sample holder with the samples were cooled down. (b)Four micro-probers were used for the four-terminal measurements. A thermocouple was attachedto one of the micro-probers and could thus be pressed against the surface of the samples. Thisallowed for the reliable measurement of the temperature in temperature-dependent measurements.

3.6.3 Parameter extraction

Since transistor characteristics are expected to critically depend on trap states, the field-

effect transistor can be used as a tool to extract the underlying spectral density of “fast”

traps. This has been extensively done with thin-film transistors employing amorphous

semiconductors (e.g. [139–147]) or polycrystalline silicon as the semiconductor (e.g.


Figure 3.13: Transistor test structure for the gated four-terminal measurements. The alignmentof the electrodes with respect to the pentacene was achieved by means of a high precision maskpositioning mechanism. The channel length and width were L = 450 µm and W = 1000 µm. Thedistance between the voltage sensing electrodes was L′ = 300 µm. With the voltage sensing elec-trodes, the potentials V1 and V2 were measured.

[148–151]). The method is emerging in the field of organic transistors as well. On the

one hand, a density of states function can be postulated a priori and the corresponding

transistor characteristic can be calculated by means of a suitable device simulation pro-

gram. The density of states function is then iteratively refined until, after a number of

predictor-corrector loops, good agreement between the measured characteristic and the

simulated curve is achieved. [19, 152–157] On the other hand, the density of states func-

tion can be calculated from the linear regime transfer characteristics in a straightforward

fashion. [52, 53, 128, 158–162] The straightforward approach has the advantage of giv-

ing an unambiguous result but, depending on the complexity of the extraction scheme,

spurious errors may result from simplifying assumptions. We underline that, although

contact effects can be severe in organic field-effect transistors, the influence of the contact

resistance is almost always neglected when extracting the trap DOS.

In Chap. 6 we used an extraction scheme for the interpretation of experimen-

tal results which was originally developed for amorphous inorganic semiconductors by

Grünewald et al. [140, 142, 145] We adapted this scheme to gated four-terminal mea-

surements in order to correct for the contact resistance. The original scheme neglects

the influence of parasitic contact resistances and had been used with gated two-terminal

measurements only. The scheme allows for the calculation of the trap DOS from a trans-

fer characteristic/gated four-terminal measurement obtained at a single temperature (e.g.

room temperature). It is therefore particularly suitable for the experimental study in

Chap. 6, since metastable trap states are under investigation. The scheme rests on the

assumption of trap-controlled transport above a mobility edge or in a transport level (see


Sec. 2.3). For Chap. 7 we developed an improved scheme to extract the trap DOS from

temperature-dependent gated four-terminal measurements. This new scheme

1) rests on a minimal set of simplifying assumptions,

2) gives the spectral density of trap states relative to the mobility edge / transport level in

an unambiguous and straightforward fashion,

3) confirms the assumption of trap-controlled transport above a mobility edge or in a

transport level and

4) provides information about other important transport parameters including the “intrin-

sic mobility”.

This scheme was used to study the influence of oxygen on the trap DOS with the device

fabrication and characterization system (Chap. 7).

4 Quinoid heteropentacenes aspromising organic semiconductors forfield-effect transistor applications

In this chapter we describe experiments with a quinoid heteropentacene as p-type semi-

conductor in organic field-effect transistors. Both single crystal and thin-film transistors

were fabricated with 7,14-Diphenyl-chromeno[2,3-b]xanthene (DPCX). In this new small

molecule organic semiconductor the field-effect mobility is as high as 0.16 cm2/Vs in

single-crystal devices and 0.01 cm2/Vs in thin-film devices. In addition, the transistors

show favourable properties such as near zero onset/threshold voltages and a small current

hysteresis. X-ray diffraction experiments show the molecules to be arranged in slipped

stacks and to have a flat backbone in the crystals. For thin films of DPCX the situation

is complicated by the co-existence of a thin-film phase with the bulk phase. However, a

comparison of DPCX thin films on octadecyltrichlorosilane (OTS)-treated and bare SiO2

gate dielectrics provides clear evidence that the OTS surface treatment leads to organic

thin films with a better structural order. The low-cost synthesis and purification of DPCX

along with the improved processability and the good electrical characteristics suggest that

quinoid heteropentacenes are promising materials for organic field-effect transistors.1

1 The studies described in this chapter contributed to patent application WO/2007/118779and are published inW. L. Kalb, A. F. Stassen, B. Batlogg, U. Berens, B. Schmidhalter, F. Bienewald, A. Hafner, T. Wagner,J. Appl. Phys. 105, 043705 (2009).

74 Quinoid heteropentacenes as promising organic semiconductors

4.1 Introduction

Apart from a high field-effect mobility, other requirements such as a near zero threshold

voltage, a steep subthreshold swing, a low current at zero applied gate bias and a high

electrical and environmental stability are important for practical applications. Moreover,

the organic transistors should be inexpensive, i.e. low-cost synthesis, purification and

processing of all involved materials are required.

The semiconductor material plays an important role for the performance of an or-

ganic field-effect transistor. An advantage of organic semiconductors is that their prop-

erties can be adjusted by means of synthetic organic chemistry. Consequently, intense

research efforts are currently being undertaken to synthesize new organic semiconduc-

tors. [37, 163–165] Pentacene derivatives and heteropentacenes are promising classes of

materials and are synthesized with the aim to mimic the excellent transport properties of

the benchmark material pentacene and to also have improved properties, such as a better

stability or solubility. [18]

We have seen that the subthreshold swing and the threshold voltage of a field-effect

transistor are dominated by charge carrier traps close to the interface between the gate

dielectric and the organic semiconductor (Sec. 2.5.2). The effective field-effect mobil-

ity can be affected by many factors such as traps, thin-film morphology or polymor-

phism. [166,167] The overlap of the π-orbitals of adjacent molecules in the crystal struc-

ture certainly plays an important, intrinsic role as well (e.g. Fig. 2.3 or [168, 169]). In

Chap. 2 we have pointed out that reversible and irreversible oxidation of the organic semi-

conductor by water and oxygen radicals are thought to be major causes of electrical and

environmental instability. [106,107,170] The susceptibility of the organic semiconductor

to oxidation can thus lead to low quality device characteristics with a poor subthreshold

performance and a non-zero threshold voltage due to the creation of traps.

Most heteropentacenes synthesized up to now use thiophene units instead of some

of the benzene rings in the pentacene molecule. [164] Other heteropentacenes containing

nitrogen have recently been reported. [171,172] Here we report on the synthesis, transistor

performance and crystal structure of the new oxygen-containing heteropentacene 7,14-

Diphenyl-chromeno[2,3-b]xanthene (DPCX). [173] The material was investigated in both

single crystal and thin-film transistors. It is remarkable that the synthesis of DPCX is

described in a report that was published in the year 1934. [174] In that work, however, a

somewhat different synthetic approach was used.

4.2 Experimental details 75

OH

OH

OH

OH

O

O

(a)

(b)

DPCX

Figure 4.1: Chemical structure of the intermediate 2,5-Bis-(hydroxy-diphenyl-methyl)-benzene-1,4-diol (a) and the new organic semiconductor 7,14-Diphenyl-chromeno[2,3-b]xanthene (DPCX)(b).

4.2 Experimental details

4.2.1 Synthesis of 7,14-Diphenyl-chromeno[2,3-b]xanthene (DPCX)

The synthesis of the new organic semiconductor was done by Dr. Ulrich Berens together

with Dr. Beat Schmidhalter, Dr. Frank Bienewald and Dr. Andreas Hafner at Ciba Spe-

ciality Chemicals Inc. It rests on the synthesis of the intermediate 2,5-Bis-(hydroxy-

diphenyl-methyl)-benzene-1,4-diol (Fig. 4.1(a)). In order to synthesize this intermediate,

a Schlenk flask was charged with 1,4-bis-(1-ethoxy-ethoxy)-benzene (10.16 g, 40 mmol)

and was flushed with nitrogen. Then, diethyl ether (50 ml), n-BuLi (29.6 ml of a 2.7 N so-

lution in heptane, 80 mmol) and N,N,N’,N’-tetramethyl ethylenediamine (9.3 g, 80 mmol)

were added. After stirring over night, a solution of benzophenone (14.58 g, 80 mmol) in

diethyl ether (in total 50 ml) was added dropwise. A greenish blue solution formed and

was stirred for another 45 min. Solvents were removed and the residue was treated with

a mixture of ether (≈ 150 ml) and an ammonium chloride solution (ca. 100 ml of a 10 %

solution). The clear organic layer was separated and the solvent was removed, leaving

26.3 g of oil. After being re-dissolved in methanol (ca. 200 ml) and, after an addition of

≈ 0.5 ml of 36 % HCl, 10.55 g of 2,5-bis-(hydroxy-diphenyl-methyl)-benzene-1,4-diol, a

colourless solid, formed over night.

2,5-bis-(hydroxy-diphenyl-methyl)-benzene-1,4-diol (9.5 g) was added to 50 g of ni-

trobenzene. The obtained suspension was heated at reflux for ≈ 45 min, and was then

allowed to cool over night. The formed crystals were filtered off, washed several times


with ethanol and dried at 10−3 mbar/190 C for two hours, giving 4.47 g of greenish red

crystals. Elementary analysis found C: 87.62 % and 87.78 % (calculated: 88.05 %); H:

4.29 % and 4.65 % (calculated: 4.62 %). The chemical structure of the resulting 7,14-

Diphenyl-chromeno[2,3-b]xanthene is shown in Fig. 4.1(b).

4.2.2 Device fabrication

Single crystal field-effect transistors (SC-FET’s)

Single crystals of DPCX were grown by physical vapour transport in a horizontal oven

with argon as the inert carrier gas (see Sec. 3.4.2). A temperature gradient was applied,

resulting in the evaporation of DPCX at 295 C and crystallization between 270 C and

240 C.

Heavily-doped Si wafers with 300 nm thick thermally grown SiO2 were used as

gate electrode and gate insulator. The samples were cleaned with hot solvents and pi-

ranha solution (30 % hydrogen peroxide in 70 % sulfuric acid). Afterwards, the substrates

were thoroughly washed with ultra pure water (18.2 MΩcm) and, subsequently, the SiO2

was treated with octadecyltrichlorosilane (OTS). [113] This was done by exposing the

substrates to OTS vapour in vacuum at 120 C for 1 h (Sec. 3.3.2). Static water contact

angles on OTS-treated substrates were 90-95 (see Sec. 3.5.1). 18 nm thick gold source

and drain electrodes were evaporated through shadow masks in high vacuum and, finally,

the single crystal field-effect transistors (SC-FET’s) were completed by placing the DPCX

single crystals on the gold electrodes in air. [135, 136]

Thin-film transistors (TFT’s)

As for the SC-FET’s, heavily doped Si-wafers with a 300 nm thick SiO2 layer were used

for the thin-film transistors (TFT’s). The substrates were cleaned with solvents and pi-

ranha then and were eventually washed with ultra pure water. Subsequently, half of the

substrates were treated with OTS with the vapour process. DPCX was vacuum-evaporated

on both types of surfaces (bare SiO2 and OTS-treated SiO2) in the same deposition run.

Shadow masks were used and the base pressure was ≈ 2× 10−6 mbar. The temperature

of the substrates Tsub during the deposition was kept constant at various temperatures be-

tween 0 C and 135 C. The deposition rate and nominal film thickness were 0.5 Å/s and

50 nm, as measured with a water-cooled quartz crystal in the deposition chamber. Gold

4.2 Experimental details 77

source and drain electrodes were vacuum-evaporated onto the DPCX thin films in a sep-

arate high vacuum chamber resulting in multiple transistor test structures with a channel

length of L = 100 µm and a channel width of W = 500 µm. More details on the evapora-

tion of organic films and gold electrodes can be found in Sec. 3.4.1 and Sec. 3.4.3.

4.2.3 Electrical characterization

All electrical measurements were done with a HP 4155A semiconductor parameter ana-

lyzer in a dry He atmosphere. The SC-FET’s and TFT’s were characterized by measuring

both transfer and output characteristics.

According to common practice, the mobility µ0 was estimated with Eq. 2.35, i.e. by

a linear regression of the square root of the drain current Id measured in the saturation

regime (Vd = −50 V). Eq. 2.35 implicitly assumes that, above a threshold voltage Vt , all

of the incrementally added, gate-induced charge is free and moves with the “trap-free”

mobility µ0 (see Sec. 2.5.2). We have pointed out in Sec. 2.5.3 that this assumption is

often not valid. In this case, µ0 may be interpreted as an average effective mobility.

In addition to the threshold voltage Vt , we extracted the onset voltage Von, which

is defined as the gate voltage where the drain current exceeds the noise level (typically

10−12 A). Moreover, the on-off current ratio Ion/Io f f was extracted and is given as an

order of magnitude. Finally, the subthreshold swing was determined as a measure of how

easily a device can be turned from the off-state to the on-state. The subthreshold swing is

defined by Eq. 2.37. S depends on the gate capacitance Ci and the figure of merit is the

normalized subthreshold swing CiS.

4.2.4 X-ray diffraction

Single crystal diffraction data were collected at 100 K with a Bruker AXS SMART 6000

CCD detector on a three-circle platform goniometer with graphite-monochromatized

Cu(Kα) radiation (λ = 1.54178 Å) from a sealed-tube generator.

X-ray diffraction patterns of DPCX thin films were obtained in the Θ−2Θ geometry

with a Stoe Stadi P diffractometer and Cu(Kα) radiation.


Table 4.1: Mobility µ0, subthreshold swing S, normalized subthreshold swing CiS, onset voltageVon, threshold voltage Vt and on-off current ratio Ion/Io f f from the devices in Fig. 4.2. For theTFT’s, the OTS-treatment leads to a drastic increase in mobility (factor of ≈ 500). This largeeffect is due to a different growth of the vacuum-evaporated DPCX thin films on the differentsurfaces.

Device µ0 S CiS Von Vt Ion/Io f f

(cm2/Vs) (V/dec) (nF V/(dec cm2)) (V) (V)

SC-FET (OTS) 0.16 1.3 15.0 +1 +1 106

TFT (OTS) 0.01 1.9 21.9 -1 -4 105

TFT (bare SiO2) 2×10−5 20.7 238.3 -13 −12 103

4.3 Results and discussion


In Fig. 4.2 we show transfer characteristics from a DPCX SC-FET and TFT both with

OTS-treated SiO2 gate dielectric and from a TFT with a bare SiO2 gate dielectric. The de-

vice parameters, as extracted from these measurements, are summarized in Table 4.1. The

DPCX thin films in Fig. 4.2 were evaporated at a substrate temperature of Tsub = 0 C for

which the best mobilities were obtained. The field-effect mobilities are µ = 0.16 cm2/Vs

for the SC-FET and 0.01 cm2/Vs for the TFT with OTS. Moreover, the on-off ratios are

respectively 106 and 105 for the SC-FET and TFT. Fig. 4.2 and Table 4.1 reveal a signif-

icant advantage of DPCX over some other organic semiconductors: the material leads to

devices with near zero onset/threshold voltages and a small current hysteresis. The sub-

threshold swing S normalized by the gate capacitance Ci = 11.51 nF/cm2 is CiS = 15.0 nF

V/(dec cm2) and CiS = 21.9 nF V/(dec cm2) respectively for the SC-FET and TFT with

OTS. As a consequence of the near zero onset/threshold voltage and the good subthresh-

old swing, the devices are turned off when no gate bias is applied and can be easily turned

on. This enables a low power consumption if the devices are to be employed in com-

plementary circuits. The near zero onset voltages of DPCX-based devices are in contrast

to some other organic semiconductors which lead to shifted transfer characteristics, even

when organic buffer layers such as OTS [130] or poly(α-methylstyrene) (AMS, [175]) are

used.

The OTS-treatment has a drastic influence on the performance of the thin-

film transistors. The field-effect mobility increases from µ0 = 2× 10−5 cm2/Vs to

µ0 = 0.01 cm2/Vs, i.e. by a factor of ≈ 500. X-ray diffraction experiments show a better

4.3 Results and discussion 79

0 -20 -40 -60 -8010-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

Vd = -50 V

TFT (bare SiO2)

TFT (OTS)

SC-FET (OTS)

| Id |

(A)

Vg (V)

Figure 4.2: Transfer characteristic from a DPCX single-crystal transistor (SC-FET) with OTS-treated SiO2 (µ0 = 0.16 cm2/Vs). Also shown are the transfer characteristics from a DPCXthin-film transistor (TFT) with OTS (µ0 = 0.01 cm2/Vs) and with a bare SiO2 gate dielectric(µ0 = 2× 10−5 cm2/Vs). Remarkable is the near zero onset voltage Von in the case of the OTS-treated substrates and the small current hysteresis. Depending on the type of device, the field-effectmobility spans five orders of magnitude. This highlights the importance of structural order to ob-tain high field-effect mobilities.

structural order of the films grown on OTS (see Sec. 4.3.2 of this chapter), compared to

the films grown on bare SiO2. This information and the following discussion demonstrate

that the large effect of the OTS-treatment on the field-effect mobility is due to a better

structural order of the molecules in the films on OTS.

The OTS-treatment reduces the surface free energy of the SiO2 gate dielectric, which

is indicated by the increased water contact angle on the OTS surface (90-95). A low

surface free energy is equivalent with a non-polar surface. This may be favourable, since

it decreases the extend of dipolar disorder at the insulator-semiconductor interface. [61]

In addition, a low-energy surface is water repellent. Water may impede the charge trans-

port because of the dipolar character of the water molecules and/or by an electrochemical

reaction with the organic semiconductor. [61, 86] Moreover, the OTS-treatment might

passivate certain chemical groups on the SiO2 surface and this may play a role even in

the case of hole transport. We suggest that the low surface energy results in a reduced in-

teraction between the small molecules and the substrate during thin-film growth and that

this interaction is a cause of structural disorder. Consequently, a smooth surface with a

low surface energy leads to evaporated organic films with improved structural order. This


effect may be more or less important during the growth of an organic thin film, depending

on the strength of the interaction between the molecules, i.e. depending on the orbital

overlap/charge carrier mobility: for vacuum-deposited pentacene films (typical mobil-

ity 0.1− 1 cm2/Vs), the OTS treatment has a significant influence on the subthreshold

performance but typically leads to an improvement of less than 2 in mobility. [114] For

tetracene thin films (mobilities of ≈ 0.1 cm2/Vs with OTS), the effect of the OTS treat-

ment on the field-effect mobility is larger than for pentacene, and a factor of ≈ 10 has

been reported. [166] The present study (mobilities of ≈ 0.01 cm2/Vs with OTS) reveals a

∼ 500 times better mobility on OTS-treated surfaces than on the bare SiO2.

Single crystal transistors are expected to reveal the ultimate performance of a ma-

terial since they should have the lowest degree of structural disorder. Indeed, the DPCX

SC-FET has an even better subthreshold swing and a higher mobility than the TFT on the

same type of substrate.

Fig. 4.3 shows the output characteristic of the same devices with OTS as in Fig. 4.2.

The output characteristics show clear p-type operation, and deviations from the ideal field-

effect transistor behaviour are minor. Apparently, gold is a suitable material for the source

and drain contacts of a DPCX field-effect transistor.

The substrate temperature Tsub during the deposition of the DPCX thin films has

also a pronounced effect on the field-effect mobility. For a given substrate temperature,

the DPCX was deposited onto a sample with bare SiO2 and with OTS-treated SiO2. Typ-

ically, three devices were measured on each sample and the values for the mobility in

Fig. 4.4 are averages for each sample. The drastic increase in field-effect mobility caused

by the OTS-treatment was observed in all experiments. We measure the highest mo-

bilities on films deposited at the lowest temperatures, which is rather unusual. Heating

the substrates during the vacuum-deposition of small molecule organic semiconductors

often leads to improved mobilities due to a better growth/structural order of the result-

ing films. [71, 72, 167, 176] The unusual temperature dependence in the case of DPCX

is likely attributable to a better connectivity between grains in films deposited at lower

substrate temperatures. This is supported by the fact that, for a substrate temperature of

Tsub = 135 C, there is no film on either type of substrate. The surface is not covered

because all the molecules have re-evaporated from the substrate at Tsub = 135 C. How-

ever, high-quality characteristics can be obtained with DPCX deposited at low substrate

temperatures. This may turn out as a major advantage when flexible plastic substrates are

used.


0 -20 -40 -600.0

-0.2

-0.4

-0.6

0 -20 -40 -60 -800

-1

-2

-3

-4

-5

-30 V

-50 V

-50 V

I d (A

)

Vd (V)

(b)

(a)

TFT (OTS)

-40 V

Vg = -60 V

-30 V

-40 V

Vg = -60 V

SC-FET (OTS)

I d (A

)

Vd (V)

Figure 4.3: Output characteristics of a DPCX SC-FET (a) and a TFT (b) with OTS-treated gatedielectric (SC-FET: µ0 = 0.16 cm2/Vs, TFT: µ0 = 0.01 cm2/Vs). There are only minor deviationsfrom the ideal transistor behaviour. This indicates gold to be a suitable contact material for thesource and drain electrodes of DPCX devices.

4.3.2 Crystal structure

Crystallographic data (excluding structure factors) have been deposited with the Cam-

bridge Crystallographic Data Centre as supplementary publication number CCDC

696211.2 Fig. 4.5 shows the crystal structure of DPCX. In contrast to pentacene, which

is known to crystallize in a herringbone structure (e.g. Fig. 2.2 in Chap. 2), the molecules

form slipped stacks along the b-axis. The space group is C2/c with a = 18.961(5) Å,

b = 6.058(2) Å, c = 20.229(5) Å, β = 116.351(9) and Z = 4. The backbone of DPCX

2 Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, CambridgeCB2 1EZ, UK [fax (+44) 1223 336033, email: [email protected]].


0 20 40 60 80 100 120 1400

5x10-3

1x10-2

0 20 40 60 80 100 120 140

0

1x10-5

2x10-5

3x10-5

4x10-5

with OTS

bare SiO2

(cm

2 /Vs)

no filmno film

TSub

(°C) (cm

2 /Vs)

TSub (°C)

bare SiO2

no film

Figure 4.4: Influence of the substrate temperature Tsub during the deposition of DPCX on thefield-effect mobility. Red squares: TFT’s with OTS-treatment; blue circles: TFT’s with bare SiO2.The OTS treatment reproducibly leads to a drastic increase in field-effect mobility. Interestingly,the highest mobilities are obtained for the lowest substrate temperatures.

in the crystal is flat, and the dihedral angle between the phenyl ring and the backbone is

70.22.

The oxygen atoms in the DPCX molecules disrupt the π-electronic system; it is sur-

prising that mobilities in excess of 0.1 cm2/Vs can be achieved with DPCX. A theoretical

approach to the electronic structure of the material may help to understand whether the

π-electrons from the phenyl rings and possibly the lone electron pairs at the oxygen atoms

contribute to the charge conduction.

In Fig. 4.6 we show X-ray diffraction patterns from 50 nm thick DPCX films grown

in the same deposition run at a substrate temperature of Tsub = 0 C, both on OTS-treated

and bare SiO2. For the OTS-treated sample, two peaks can clearly be observed, indicating

a polycrystalline nature of the DPCX. For the untreated SiO2, no peaks can be discerned.

The film is amorphous.

Importantly, the X-rays are scattered from the whole film, while in field-effect tran-

sistors the current flow is located close to the insulator-semiconductor interface. However,

if no peaks are present in a diffraction pattern, it is reasonable to conclude that also the

semiconductor very close to the insulator-semiconductor interface is amorphous. The

X-ray diffraction scans therefore confirm a higher degree of structural disorder in the in-

terface region of DPCX films on bare SiO2 accounting for the low field-effect mobility

without OTS.


Figure 4.5: Crystal structure of DPCX. The compound crystallizes in the monoclinic space groupC2/c with a = 18.961(5) Å, b = 6.058(2) Å, c = 20.229(5) Å and β = 116.351(9), Z = 4. Themolecules are ordered in slipped stacks along the crystallographic b-axis. The backbone of DPCXin the crystal is flat, and the dihedral angle between the phenyl ring and the backbone is 70.22.

5 10 15 20 25 30 35

500

1000

1500

2000

2500 OTS bare SiO2

Subtrate

Single crystalphase (004)

Thin-filmphase

Single crystalphase (002)

Thin-filmphase

Cou

nts

2Theta (°)

Figure 4.6: X-ray diffraction patterns from DPCX thin films grown on OTS-treated SiO2 (redline) and on bare SiO2 (black line). The films were deposited in the same deposition run. Thefilm on OTS is polycrystalline, while the film on bare SiO2 is amorphous. For the OTS-treatedsamples, the single crystal phase coexists with a thin-film phase.


The interlayer spacings calculated from the two peaks at 2θ1 = 7.616 and

2θ2 = 9.649 are d = 11.6 Å and d = 9.16 Å. Upon close inspection, a higher order of

these two peaks can also be observed in Fig. 4.6. One of the peaks (and its higher order)

can be related to the single crystal structure. From the structure determination we have

ccos(β−90) = 18.13 Å ≈ 2×9.07 Å. The second peak in Fig. 4.6 therefore is the (002)

peak from the single crystal phase. The first peak cannot be related to the single crystal

structure. One would conclude that the single crystal phase coexists with a phase which

is exclusively present in DPCX thin films. The ratio of the two peaks indicates that the

thin-film phase even dominates. As a matter of fact, polymorphism is a common feature

of organic semiconductors due to the weak van der Waals type interaction between the

molecules. [31, 32, 167]

4.4 Conclusions

The quinoid heteropentacene 7,14-Diphenyl-chromeno[2,3-b]xanthene (DPCX) was syn-

thesized. Single crystal and thin-film transistors were fabricated with this material.

DPCX is a p-type organic semiconductor and field-effect mobilities of 0.16 cm2/Vs were

achieved so far. The transistor characteristics reveal near zero onset/threshold voltages,

low off-currents at zero applied gate bias and a small current hysteresis.

For the TFT’s, the treatment of the SiO2 surface with octadecyltrichlorosilane (OTS)

leads to a drastic improvement of the device characteristics. The self-assembled mono-

layer of OTS significantly reduces the growth-related structural disorder in thin films of

DPCX, as seen by X-ray diffraction. Eventually, the best subthreshold swing and field-

effect mobility were obtained with single crystal devices that have the lowest degree of

structural disorder. This highlights the crucial importance of structural defects on the

device performance. The situation is, however, complicated by a non-ideal connectivity

between grains in the case of the thin films. This is revealed by the unusual dependence

of the field-effect mobility on the substrate temperature during the thin-film deposition.

The conjugation of the π-electrons in quinoid heteroacenes is of the quinoid type,

as in oxidized aromatic molecules. Quinoid heteroacenes are therefore expected to be

less susceptible to oxidation. A somewhat similar strategy for the stabilization of organic

semiconductors has very recently been disclosed. [177] Furthermore, both the synthesis

and the purification of DPCX are inexpensive. The phenyl groups of the DPCX molecule

4.4 Conclusions 85

result in an increased solubility if compared to the virtually insoluble pentacene. Further

functionalization at the phenyl rings may open the way for low-cost solution processing.

5 Organic small molecule field-effecttransistors with fluoropolymer gatedielectric: Eliminating gate bias stresseffects

In this chapter we describe organic field-effect transistors with unprecedented resistance

against gate bias stress. The single crystal and thin-film transistors employ an organic

fluoropolymer gate dielectric (CytopTM). CytopT M is easy to be used and can be de-

posited in air from solution. The fluoropolymer is highly water repellent, has a very low

dielectric constant εi = 2.1−2.2 and shows a remarkable electrical breakdown strength.

The single crystal transistors are consistently of very high electrical quality: near zero

onset, very steep subthreshold swing (up to 0.75 nF V/(dec cm2)) and negligible current

hysteresis. Furthermore, extended gate bias stress only leads to marginal changes in the

transfer characteristics. The experimental work described in this chapter suggests that

there is no conceptual limitation for the stability of organic semiconductors in contrast to

hydrogenated amorphous silicon. We also address the issue of contact resistance, which

becomes crucial in high quality organic single crystal devices.1

1 Some of the studies described in this chapter are the basis for patent application WO/2008/077463.The results are published inW. L. Kalb, T. Mathis, S. Haas, A. F. Stassen, B. Batlogg, Appl. Phys. Lett. 90, 092104 (2007) andW. L. Kalb, T. Mathis, S. Haas, A. F. Stassen, B. Batlogg, Proc. of SPIE, 6658, 665807 (2007).

88 Eliminating gate bias stress effects

5.1 Introduction

For convenience, organic field-effect transistor studies often employ Si/SiO2 substrates

(Fig. 3.3(a)). As described in Sec. 3.3.2, the SiO2 surface can be rendered hydrophobic

with the self-assembling agent octadecyltrichlorosilane (OTS). [113,114] The experimen-

tal results in Chap. 4 have shown that the OTS-treatment can have a large effect on the per-

formance of organic thin-film transistors, due to a better growth of the vacuum evaporated

organic film on the low-energy surface. In addition, we have seen that the sole presence of

the gate dielectric is expected to influence the charge transport in a field-effect transistor.

In Sec. 2.4.3 we referred to the chemical structure of the gate dielectric, adsorbed water

and the polarity of the gate dielectric as possible causes of traps in the vicinity of the

insulator-semiconductor interface. Consequently, to fully exploit the potential of organic

semiconductors in field-effect transistors, it is of great importance to carefully choose and

prepare the gate dielectric. In addition, the use of easily processable (organic) gate insula-

tors is mandatory for the implementation of low-cost electronics. That is why the search

for suitable organic dielectrics has intensified in recent years. [61, 62, 90]

One of the last obstacles to be overcome for a commercialization of organic thin-film

transistors is gate bias stress effects. Switching the devices on for some time generally

leads to a reduction in current at a given gate voltage. Gate bias stress effects can result

in a significant difference between the forward and reverse sweep in the measurement of

a transfer/output characteristic. [100, 103, 133] Gate bias stress effects have often been

studied by applying a fixed gate voltage for an extended time, followed by a measurement

of the shift of the transfer characteristic. [152] Possible causes of gate bias stress effects

were discussed in Sec. 2.5.4. The effects are thought to be caused by the trapping and

release of charge carriers on a time scale comparable to the measurement time. [103,108]

Mounting evidence indicates that water in the dielectric-semiconductor interface region

can cause gate bias stress effects. [106,107,178] It has also been suggested that electrical

instability is an intrinsic phenomena caused by the formation of bipolarons. [108, 109]

In this chapter we describe combinations of small molecule organic semiconduc-

tors with an organic spin-on dielectric that yield field-effect transistors with exceptionally

high quality characteristics and stability. The transistors have a bottom gate structure with

an amorphous fluoropolymer (CytopTM) as gate dielectric. The fluoropolymer has a ring

structure in the repeat units (Fig. 3.4 and Fig. 3.5), in contrast to ordinary polytetrafluo-

roethylene (PTFE). This renders the material amorphous and leads to desirable properties

5.2 Experimental 89

such as solubility in some perfluorinated solvents and a high optical transparency. Its

dielectric constant is εi = 2.1− 2.2. [179] CytopTM has previously been used by Veres

et al. in field-effect transistors with a solution processable semiconductor in a top gate

structure. [61, 62] We demonstrate this favorable material in combination with two small

molecule semiconductors: rubrene and pentacene. The device stability was evaluated by

applying a gate bias for extended periods of time.

5.2 Experimental

Organic field-effect transistors were fabricated as follows. Glass slides with an indium tin

oxide coating served respectively as substrate and gate electrode. The fluoropolymer so-

lution was prepared by mixing 3 parts of Cytop CTL-809M with two parts of the solvent

CT-Solv.180. The glass slides were cleaned in hot acetone and hot isopropanol and the

fluoropolymer solution was spin-coated onto the substrates at 500 rpm for 10 s followed

by 1000 rpm for 20 s. Eventually, the samples were dried for 1 h at 90 C on a hotplate. All

the processing steps were carried out in air. The thickness of the layers was determined

with a surface step profiler (Tencor Alpha-Step 500) and the gate capacitance was calcu-

lated from these measurements, assuming a dielectric constant of εi = 2.15. In addition,

the dielectric films were characterized by atomic force microscopy (surface roughness),

static water contact angle measurements and leakage current measurements.

Single crystal field-effect transistors (SC-FET’s) were made by evaporating 30 nm

thick gold electrodes onto the fluoropolymer. Rubrene and pentacene single crystals were

grown by physical vapour transport with argon as the inert carrier gas. The single crystals

were then placed onto the gold electrodes in air. [135] The typical channel length of the

completed transistors was L = 50 µm. Pentacene thin-film transistors (TFT’s) were made

by evaporating pentacene onto the fluoropolymer through a shadow mask in high vacuum

and were completed by evaporating gold top contacts. The device structure of the TFT’s

and SC-FET’s is illustrated in Fig. 3.3(c) and Fig. 3.7.

The devices were characterized in a glovebox with a dry He atmosphere (H2O,

O2 < 0.5 ppm) using a HP 4155A semiconductor parameter analyzer. The voltage was

increased in steps of 0.5 V and the integration and delay time were respectively 20 ms

and 0 ms. The devices were characterized by measuring transfer characteristics both in

the linear regime (drain voltage Vd = −5 V) and in the saturation regime (Vd = −80 V).


Figure 5.1: AFM image (scan size 4 µm×4 µm) of a spin-coated CytopTM film. The surface isrelatively smooth: a root mean square roughness of 0.63 nm was calculated from this image.

Additionally, output characteristics were measured. Forward and reverse sweeps were

obtained in all cases.

We also measured the temperature dependence of the transfer characteristics in some

cases. This was done by slowly cooling a sample down from room temperature and by

measuring a transfer characteristic about every 10 K.

5.3 Results

5.3.1 Properties of the gate dielectric

The CytopT M films are from 430 to 700 nm thick, which results in a gate capacitance of

Ci = 4.4−2.7 nF/cm2. The films are highly hydrophobic: static water contact angles are

on average 112 with a very small standard deviation of about 2.2 In Fig. 3.8 we show a

water droplet on a CytopT M surface with a water contact angle of 112. The water contact

angles are comparable or even slightly better than the water contact angles on methyl-

group terminated high quality self-assembled monolayers. [69] Fig. 5.1 shows a typical

AFM topography image of a CytopT M film. The root mean square roughness from this

image and similar images is ∼ 0.6 nm which is rather smooth.

2 A water droplet was placed on six samples and the contact angle was measured on both “sides” of thedroplet. This gave contact angles of (111, 116), (109, 113.5), (110, 114), (111, 112), (112.5,110) and (114, 112). This results in an average contact angle of 112±2.

5.3 Results 91

0 100 200 300 400 50010-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

0 2 4 6 8 10

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Leak

age

curr

ent d

ensi

ty [A

/cm

2 ]

Applied field [MV/cm]

451V

Leak

age

curr

ent [

A]

Applied voltage [V]

Figure 5.2: The CytopT M fluoropolymer is an outstanding electrical insulator. This 457±10 nmthick film breaks down at 451 V, which corresponds to a dielectric breakdown field of 9.8 MV/cm.The experiment was carried out with an evaporated circular gold electrode, A = 0.985 mm2.

In Fig. 5.2 we show the leakage current through a 457±10 nm thick film as a func-

tion of applied voltage. The measurement was carried out with an evaporated, circular

gold electrode with an area of A = 0.985 mm2. The fluoropolymer is an outstanding elec-

trical insulator: the leakage current is below 6.5×10−11 A for an applied voltage ≤ 70 V

and below 1 µA up to 450 V. The dielectric breaks down at 451 V for this sample, which

corresponds to a dielectric breakdown field of 9.8 MV/cm. This breakdown strength is

remarkable for an organic dielectric and is better than the SiO2 that we generally use.

5.3.2 Comparison of different devices

The excellent performance of organic field-effect transistors with a CytopT M gate dielec-

tric is shown in Fig. 5.3. The transfer characteristics from a rubrene SC-FET, a pen-

tacene SC-FET and a pentacene TFT, measured in saturation with Vd =−80 V, are given

for the forward and the reverse sweep. Most remarkable is the absence of any hystere-

sis for the SC-FET’s. We emphasize that no additional steps, such as electrical aging

or pre-stressing, were taken to obtain these curves. A further mark of the high qual-

ity of the devices is the steep subthreshold swing S (Eq. 2.37). The subthreshold swing

is S = 0.50 V/dec for the rubrene SC-FET and 0.29 V/dec for the pentacene SC-FET.


20 0 -20 -40 -6010-13

10-11

10-9

10-7

10-5

10-3

Dielectric:amorphousfluoropolymer

Pentacene TFT

Pentacene SC-FET

RubreneSC-FET

|I d| [A

]

Vg [V]

Figure 5.3: Transfer characteristics in saturation (Vd =−80 V) for a rubrene SC-FET, a pentaceneSC-FET and a pentacene TFT. Forward and reverse sweeps are shown but are indistinguishableover the entire operating range for the SC-FET’s. The TFT shows only a small current hysteresisnear the onset.

This gives normalized subthreshold swings CiS of respectively 1.6 nF V/(dec cm2) and

1.3 nF V/(dec cm2). Higher subthreshold swings and a large current hysteresis are mea-

sured when we place nominally identical crystals on OTS surface-treated SiO2. [103,106]

Remarkable as well is the very small (slightly positive) onset voltage of the two single

crystal devices (+3 V for rubrene and +1 V for pentacene). The saturation field-effect mo-

bilities from the crystals as calculated with Eq. 2.35 are 5.7 cm2/Vs and 1.4 cm2/Vs. The

thin-film transistor shows a notable but small hysteresis in the transfer characteristic close

to the onset voltage, and the onset is more negative than in the case of the single crystals,

i.e. −13 V (Fig. 5.3). The field-effect mobility is 0.26 cm2/Vs.3

5.3.3 Reproducibility

We have produced 17 rubrene SC-FET’s. The single crystals were grown in four different

runs and were placed on seven different substrates. Apart from an obviously malfunction-

ing device, 16 transistors had a near zero onset voltage (average: +0.8 V), a very small

current hysteresis and a steep subthreshold swing as low as 0.75 nFV/(dec cm2). The val-

ues are summarized in Table 5.1. A steep subthreshold swing and a near zero onset are

3 The TFT’s were transported through air, but were kept in the dry He atmosphere for several days priorto electrical characterization. The storage led to a slight improvement in hysteresis.

5.3 Results 93

Table 5.1: Onset voltage, subthreshold swing S and normalized subthreshold swing CiS from 16rubrene-based SC-FET’s. The onset voltage is essentially zero in all cases and we achieved arecord subthreshold swing of 0.75 nF V/(dec cm2) with device 16. The values were extracted fromthe saturation regime characteristics.

Device Onset S CiS [nF V/ Device Onset S CiS [nF V/

No. [V] [V/dec] (dec cm2)] No. [V] [V/dec] (dec cm2)]

1 +0.5 0.26 1.1 9 +1.0 0.46 1.3

2 +0.5 0.26 1.1 10 +1.0 0.45 1.3

3 +0.5 0.39 1.7 11 +1.0 0.34 1.0

4 +0.5 0.35 1.5 12 +0.5 0.35 1.0

5 +3.0 0.50 1.6 13 +1.0 0.28 1.1

6 -0.5 0.42 1.3 14 +0.5 0.40 1.6

7 0.0 0.32 1.0 15 +1.5 0.64 2.6

8 +0.5 0.41 1.2 16 +1.0 0.28 0.75

Average value: +0.8 1.3

most favorable since they facilitate a low power operation of the device. In fact, the sub-

threshold swing of the SC-FET’s would be sufficiently steep for a low power operation

(e.g. |Vg| ≤ 10 V) even with the relatively thick fluoropolymer films. We recall that the

theoretical limit for the subthreshold swing is 0.06 V/dec at T = 300 K (Eq. 2.40). We

have achieved a subthreshold swing which is higher by a factor of ≈ 4 only (device 16 in

Table 5.1). The subthreshold swing is a measure of the density of deep traps. According

to Eq. 2.39, a subthreshold swing of S = 0.28 V/dec (device 16) is related to a trap density

of N2 ≈ 5×1010 cm−2eV−1.4 If we assume the thickness of the accumulation layer to be

1.5 nm (approximate thickness of one molecular layer) we have a volume trap density of

N = 3×1017 cm−3eV−1.

5.3.4 Gate bias stress experiments

The advantages of the material combinations become striking in gate bias stress studies.

We have applied a gate voltage to the three devices in Fig. 5.3 for a prolonged time. After

the initial transfer characteristic measurement, a gate bias of Vg =−70 V was applied for

4 The gate capacitance of device 16 is Ci = 2.7 nF/cm2.


20 0 -20 -40 -601x10-13

1x10-11

1x10-9

1x10-7

1x10-5

1x10-30

3x10-4

6x10-4

2,8 2,6 2,4 2,2

1x10-11

2.8 2.6 2.4 2.2

2x10-11

8x10-12

6x10-12

|I d| [A

]

Vg [V]

Initial Vg = -70 V, 2h Vg = +70 V, 2h

|I d| [A

]

Figure 5.4: The rubrene single crystal device is highly stable against gate bias stress. The figureshows the transfer characteristic measured at Vd = −80 V prior to the stress sequence (full blackline), after two hours of gate bias stress at Vg =−70 V (dashed red line) and after subsequent gatebias stress at Vg = +70 V for two hours (dotted green line) both on a linear scale (upper panel)and on a logarithmic scale (lower panel). The graph includes the forward and reverse sweep in allthree cases. The inset shows the drain currents close to the onset voltage.

two hours. After a two hour relaxation period, a gate bias of Vg = +70 V was applied.

During the stress periods, the source was grounded and the drain potential was held at

0 V to ensure a homogenous gate stress. The drain current quadratically depends on the

effective gate voltage for an ideal transistor (Eq. 2.35) and is very sensitive to changes

induced by a two hour stress period. Bias stress experiments were carried out in the

dark.5

5.3 Results 95

For the rubrene SC-FET, Fig. 5.4 shows the initial characteristic, the characteris-

tic measured after 2 hours of negative bias and after 2 hours of positive bias. The de-

vice is hardly influenced by the long application of a gate bias. There are only marginal

changes in the transfer characteristic. After negative stress, there is a very small shift of

the onset voltage to more positive voltages, accompanied by a small increase in current

hysteresis and a small decrease in on-current. For the pentacene SC-FET, the observa-

tions are similar. When compared to the rubrene device, the shift of the onset voltage

due to bias stress is even smaller, but the decrease in on-current is somewhat more pro-

nounced (3.8 % at Vg =−70 V). In contrast, in similar experiments with single crystals of

rubrene or pentacene on OTS-treated SiO2, large shifts of the transfer characteristics are

observed. [103,106] For the pentacene TFT, a gate voltage of Vg =−70 V applied for two

hours leads to a rigid shift of the curve by −5.2 V to more negative voltages.

5.3.5 Contact effects in SC-FET’s

In Fig. 5.5 we show the transfer characteristics from the rubrene SC-FET of Fig. 5.3 in the

linear regime (Vd = −5 V, upper panel) and in the saturation regime (Vd = −80 V, lower

panel). Also in the linear regime and when plotted on a linear scale, there is no current

hysteresis. The normalized subthreshold swing in the linear regime is even steeper than in

the saturation regime. In the saturation regime, the drain current depends approximately

quadratically on gate voltage (see Fig. 5.5, lower panel, linear scale). This is in accordance

with Eq. 2.35 which is valid for a device with a low trap density and negligible contact

resistances. In the linear regime, however, there is a deviation from the expected linear

current-voltage relationship (Fig. 5.5, upper panel, linear scale). This finding becomes

more apparent in Fig. 5.6, where we plot the same data as in Fig. 5.5 but, for the saturation

regime, the square root of the drain current is shown. In addition, the same plot is given

for the pentacene/CytopTM SC-FET in the lower panel of the graph (saturation field-effect

mobility: 1.4 cm2/Vs). Clearly, the shape of the transfer characteristics is closer to ideal

as the drain current is increased. In the case of the pentacene SC-FET, the characteristic is

very close to ideal in the saturation regime. The output characteristic of the rubrene SC-

FET is shown in Fig. 5.7. The output characteristic further confirms that the deviations

from the ideal transistor behaviour are more pronounced at low Vd . The non-idealities are

5 Both the negative and the positive bias stress period were shortly interrupted (for 70 s) after 10, 20, 40,and 60 minutes in order to measure a transfer characteristic.


20 0 -20 -40 -60

10-12

10-10

10-8

10-6

10-4

0

2x10-4

4x10-4

6x10-4

8x10-4

10-12

10-10

10-8

10-6

10-4

0

2x10-5

4x10-5

6x10-5

Vg [V]

|I d| [A

]

Vd = -80V

Vd = -5V

|I d| [A

]

Figure 5.5: Transfer characteristics from the rubrene SC-FET. The upper panel shows the linearregime transfer characteristic (Vd =−5 V) and the lower panel the saturation regime characteristic(Vd = −80 V) both on a linear and on logarithmic scale. Also when measured at a low drainvoltage and plotted on a linear scale, there is no current hysteresis.

most likely caused by parasitic contact resistances and are not related to the high quality

gate dielectric.

5.3.6 Temperature-dependent measurements

We also measured the temperature dependence of the transfer characteristics for rubrene-

based SC-FET’s. The drain current is found to decrease with decreasing temperatures in

the whole range of gate voltages, as shown in Fig. 5.8 for a sample with a 594 nm thick

gate dielectric (Ci = 3.2 nF/cm2) and a channel length of L = 50 µm. In principle, mea-

suring the temperature dependence of the transfer characteristic can reveal information

about the trap DOS and other transport parameters including the intrinsic mobility (see

5.3 Results 97

0

1x10-2

2x10-2

3x10-2

0

3x10-5

6x10-5

9x10-5

0 -20 -40 -600

2x10-3

4x10-3

6x10-3

8x10-3

0

-2x10-6

-4x10-6

-6x10-6

RubreneSC-FET

|I d|1/2

Vd = - 80V

Vd = - 5V

|I d| [A

]

Vd = - 5V

Vd = - 80V

Vg [V]

|I d|1/2

|I d| [A

]

PentaceneSC-FET

Figure 5.6: Near-ideal transfer characteristics in the saturation regime (Vd =−80 V) of a rubreneSC-FET (upper panel) and of a pentacene/CytopTM SC-FET (lower panel). The square root of thedrain current is shown for the saturation regime. For the linear regime characteristics (Vd =−5 V)the drain current increases sub-linearly.

0 -20 -40 -60 -800

-200

-400

-600

-800

Vg = -30V

Vg = -50V

Vg = -70V

I d [A

]

Vd [V]

Figure 5.7: Output characteristic of a rubrene SC-FET. Forward and reverse sweeps are shown,but are indistinguishable due to the very high electrical stability of the device. At low Vd , thedeviations from the ideal transistor behaviour are more pronounced.


10 0 -10 -20 -30 -40 -50 -60 -700

2x10-5

4x10-5

10 0 -10 -20 -30 -40 -50 -60 -7010-12

10-10

10-8

10-6

10-4

Rubrene / CytopTM SC-FET 296 K

251 K

261 K

270 K276 K

288 K

|I d| [A

]

Vg [V]

Vd = -5V

|I d| [A

]

Vg [V]

Figure 5.8: Linear regime transfer characteristics (Vd = −5 V) of a rubrene/CytopT M SC-FETmeasured at various temperatures. The drain current decreases as the temperature is decreased.The gate capacitance of this transistor is Ci = 3.2 nF/cm2.

0.0036 0.0040

-22

-20

-18

-16

-14

Vg = -15 VVg = -20 VVg = -30 VVg = -40 VVg = -50 VVg = -60 VVg = -70 V

Vg = -10 V

Vg = -7 V

Vg = -5 V

ln

T -1 [K -1]

Figure 5.9: Arrhenius plots of the field-effect conductivity for various gate voltages from themeasurements in Fig. 5.8. The fit of the experimental data to straight lines is passable, but couldbe better.

Chap. 7). More specifically, we show in Chap. 7 that, for trap-controlled band transport,

the field-effect conductivity

5.3 Results 99

σ(Vg) =LW

Id

Vd(5.1)

depends on temperature according to

σ(Vg) = Aexp(−Ea(Vg)

kT

). (5.2)

Ea is approximately equal to the energetic difference between the Fermi level at the

insulator-semiconductor interface E ′F and the valence band edge EV , i.e.

Ea ≈ EV −EF − eV0 = EV −E ′F . (5.3)

According to Eq. 5.2, we can determine the activation energy Ea for a given gate voltage

Vg from a linear regression of the ln(σ) vs. 1/T data. Fig. 5.9 shows the corresponding

Arrhenius plots for the measurement in Fig. 5.8. The fit of the straight lines to the exper-

imental data is quite acceptable, but could be better. Fig 5.10 finally shows the function

Ea(Vg). At low gate voltages, Ea steeply rises as a function of Vg. This is to be expected

for a sample with a low trap density (see Chap. 7). The non-monotonic dependence of

Ea on Vg and the large and almost constant value of the activation energy for |Vg|> 20 V

is surprising at first glance. This is because the Fermi level must approach the valence

band for an increased gate voltage/charge density. We suggest that for |Vg| > 20 V, the

temperature dependence of the drain current is controlled by the contact resistance and

not by the rubrene crystal.

A rough estimate of the trap density can be obtained from the activation en-

ergies Ea at low gate voltages as follows. From Fig. 5.10 we see that a change

of the gate voltage by ∆Vg = (8 − 5)V = 3 V leads to a shift of the activation

energy Ea (i.e. the Fermi level E ′F at the insulator-semiconductor interface) by

∆E ′F ≈ ∆Ea = (0.53− 0.23)eV = 0.3 eV. The change in gate voltage ∆Vg corresponds

to a charge carrier density of P = Ci∆Vg/e = 6× 1010 cm−2. By taking the thickness of

the accumulation layer as one molecular layer (d = 1.5 nm), we have a volume density of

p = P/d = 4×1017 cm−3. For the moment we assume a trap density that does not depend

on energy and we can finally estimate the trap density to be

N = p/∆Ea =Ci

ed

(∆Ea

∆Vg

)−1

= 1.3×1018 cm−3eV−1. (5.4)


This is in good agreement with the value of N = 3×1017 cm−3eV−1, as estimated from the

excellent subthreshold swing of device 16 in Sec. 5.3.3. The latter estimation also rests on

the assumption that the trap densities do not depend on energy. The trap density, however,

is expected to critically depend on energy (see e.g. Chap. 6 and Chap. 7 of this thesis).

The simple estimates here have to be considered as averages. They are an overestimate

of the correct trap density at large EV −E ′F (i.e. further away from the valance band edge

EV ) and an underestimate at reduced EV −E ′F .

A first step towards improving the calculation of the trap density would be to ex-

change the difference quotient in Eq. 5.4 with the respective derivative [160] which gives

N(E) =Ci

ed

(dEa

dVg

)−1

, (5.5)

i.e. the trap densities N as a function of energy E = Ea. However, the accumulation

layer thickness d is not constant but decreases with increasing gate voltage, which would

still be a significant source of error. In Chap. 7 we will introduce a more sophisticated

concept in order to calculate the trap density as a function of energy from the gate voltage

dependence of the activation energy.

5.3.7 Trap-controlled transport in TFT’s

Fig. 5.11 shows the transfer characteristic of the pentacene TFT in the linear regime

(Vd = −5 V) and the square root of the drain current in the saturation regime

(Vd = −80 V). The current hysteresis is much less apparent on a linear scale. The drain

current increases faster than lineraly in the linear regime and more than quadratic in the

saturation regime (Fig. 5.11). In Fig. 5.12 we show the field-effect mobility as a function

of gate voltage as calculated from the linear-regime transconductance (∂Id/∂Vg)Vd with

µe f f =L

WVdCi

(∂Id

∂Vg

)

Vd

. (5.6)

Eq. 5.6 is discussed in the beginning of Chap. 6. From Fig. 5.12 we see that the effective

field-effect mobility increases with gate voltage. This is indicative of charge transport

in the TFT to be controlled by “fast” traps within the pentacene layer (Sec. 2.5.3 and

[100–102]).

5.3 Results 101

0 -10 -20 -30 -40 -50 -60 -700.6

0.5

0.4

0.3

0.2

0.1

Vg [V]

E a [eV

]

Figure 5.10: Activation energy Ea(Vg) as derived with linear regressions according to Eq. 5.2and as illustrated in Fig. 5.9. For negligible contact resistances, Ea is approximately equal to theenergetic difference between the Fermi level at the insulator-semiconductor interface E ′F and thevalence band edge/mobility edge. Ea steeply rises with gate voltage at low gate voltages, whichmay be seen to further confirm the low trap density in SC-FET’s with CytopT M gate dielectric.The non-monotonic dependence of the activation energy on gate voltage and the large and almostconstant value of Ea for sufficiently large gate voltages (> 0.2 eV) indicates that the tempera-ture dependence of the drain current is dominated by the temperature dependence of the contactresistance at sufficiently high gate voltages.

0 -20 -40 -600.0

4.0x10-4

8.0x10-4

1.2x10-3

1.6x10-3

0

2x10-7

4x10-7

6x10-7

8x10-7

|I d|1/2

Vg [V]

Vd = - 80V

Vd = - 5V

|I d| [A

]

Figure 5.11: Thin-film transistor operation of a pentacene/CytopTM TFT with a saturation field-effect mobility of 0.26 cm2/Vs. The square root of the drain current in the saturation regime anddrain current in the linear regime are shown.

In Fig. 5.13 we show the output characteristic of the pentacene TFT. Again, the trap-

controlled transport is reflected in the superlinear dependence of the drain current at low

Vd and in a more than quadratic dependence at high Vd . It is worthwhile to compare this

characteristic with the characteristic from the rubrene SC-FET in Fig. 5.7.


-30 -40 -50 -60 -700.0

0.1

0.2

0.3

0.4

Vg [V]

eff [

cm2 /V

s]

Figure 5.12: Effective field-effect mobility µe f f from a pentacene TFT with CytopT M gate dielec-tric. The increase in mobility with gate voltage indicates that the charge transport in the TFT iscontrolled by multiple trapping and release.

0 -20 -40 -60 -800

-1

-2

Vg = -50 V

Vg = -70 V

I d [A

]

Vd [V]

Figure 5.13: Output characteristic from a pentacene/CytopT M TFT with a field-effect mobility of0.26 cm2/Vs. There is no current hysteresis.

5.4 Discussion

The surface of the amorphous fluoropolymer proves to have a highly desirable quality:

essentially no electrically active trap states form in combination with the organic semi-

conductors. Bias stress effects in SC-FET’s are marginal and, thus, long-lived states for

holes are (almost) non-existent at the insulator surface. The absence of energetically deep

5.4 Discussion 103

insulator surface states can account for an improved subthreshold swing. It is remark-

able that the insulator works very well with two different semiconductors, i.e. rubrene

and pentacene. This may indicate that the absence of surface traps is due to the absence

(or low density) of a specific chemical species on the insulator surface. The highly hy-

drophobic CytopTM surface (water contact angle of∼ 112) leads to an (almost) complete

elimination of gate bias stress effects. Water is a known cause of gate bias stress effects

which can be eliminated by employing a highly hydrophobic CytopTM gate insulator.

The high reproducibility of the excellent device performance matches the reproducibly

high water contact angle of the CytopTM films. In Sec. 2.4.3 we have described how wa-

ter could lead to charge carrier trapping. Water molecules may act as traps themselves.

Localization of charge carriers may also be caused by the dipolar nature of the water

molecules. [92,93] Moreover, an electrochemical reaction of water with the organic semi-

conductor [86] and/or with the gate dielectric [91] may impede charge transport. In the

latter case, OH-groups on the surface of the gate dielectric are expected to play a key

role and the CytopT M gate dielectric does not contain these groups. It has also been sug-

gested that a polar gate dielectric leads to an unfavorable broadening of the density of

states function due to dipolar disorder caused by randomly oriented dipoles within the

gate dielectric (see Sec. 2.4.3 and in particular Fig. 2.17). [61, 62] CytopT M has a very

low dielectric constant εi = 2.1−2.2 which may suppress a broadening of the density of

states in the interface region.

For the pentacene SC-FET and TFT we have combined the same semiconductor and

insulator. However, we observe that the TFT is less stable against bias stress than the SC-

FET. The shape of the TFT transfer characteristics reflects the presence of a substantial

density of traps. The field-effect mobility monotonically increases with gate voltage and

also the onset voltage of the TFT is more negative. These effects cannot be attributed to

insulator surface states but should be caused by growth-related structural defects within

the pentacene layer close to the dielectric-semiconductor interface.

For high quality organic single crystals the channel is highly conducting and thus

the contact resistance also needs to be small. The non-idealities of the transistor char-

acteristics of the SC-FET’s are likely due to a contact resistance that decreases with an

increase in drain voltage and thus might be treated as a Schottky diode. [155, 180] We

suggest that the non-monotonic dependence of Ea on Vg and the large and almost constant

value of the activation energy for |Vg|> 20 V is due to the temperature dependence of the

drain current controlled by the contact resistance and not by the rubrene crystal. For the


TFT’s, the quality of the contacts is not as important as for the SC-FET’s because of the

increased channel resistance. While the dependence of the drain current on gate voltage

is dominated by the contacts for the rubrene SC-FET, it is dominated by trapping in “fast”

traps for the pentacene TFT.

5.5 Conclusions

Our experiments highlight the intrinsically high performance and high stability of

small molecule organic semiconductors when combined with a suitable gate dielectric.

CytopT M is easy to handle (i.e. deposition in air from solution), gives an interface with

very few electrically active defects and is a good electrical insulator. SC-FET’s with

rubrene and pentacene in combination with a fluoropolymer show excellent electrical

characteristics with a high field-effect mobility, and they are hardly affected by long-term

gate bias stress. It seems that there is no conceptual limitation to the stability of organic

semiconductors in contrast to a-Si:H, where the diffusion of hydrogen leads to gate bias-

induced metastable defects. [181] However, since gate bias stress effects are somewhat

more pronounced in pentacene thin-film transistors than in the highly stable pentacene

single crystal transistors with the same gate dielectric, we can conclude that structural

disorder within the semiconductor is a cause of electrical instability.

CytopT M works well with rubrene and pentacene. It seems that the nature of the

gate dielectric is much more important than the chemistry of the organic semiconduc-

tor in order to obtain transistors with a high electrical stability. It is very likely that the

fluoropolymer leads to outstanding transistors with many other small molecule semicon-

ductors.

We have seen that parasitic contact resistances can significantly affect the transfer

characteristics and the temperature dependence of the drain current. It is thus very im-

portant to correct for parasitic contact resistances when extracting the trap DOS and other

transport parameters from the characteristics of organic field-effect transistors. The study

highlights the need to produce electrical contacts that are matching the good semiconduc-

tor/dielectric combination, i.e. that are not limiting current injection and extraction.

6 Defect healing at room temperaturein pentacene thin films and improvedtransistor performance

We now describe a trap reduction process in the organic semiconductor pentacene which

we ascribe to the healing of structural defects at room temperature. This peculiar effect is

a direct consequence of the weak intermolecular interaction which is characteristic of or-

ganic semiconductors. Pentacene thin-film transistors were fabricated and characterized

by gated four-terminal measurements without any unintentional air exposure. Under high

vacuum conditions (base pressure of the order of 10−8 mbar), the device performance is

found to improve with time. The effective field-effect mobility increases by as much as

a factor of two and mobilities up to 0.45 cm2/Vs were achieved. In addition, the con-

tact resistance decreases by more than an order of magnitude and there is a significant

reduction in current hysteresis. Oxygen and nitrogen exposure, as well as annealing ex-

periments, show the improvement of the electronic parameters to be driven by a thermally

promoted process and not by chemical doping. In order to extract the spectral density of

trap states from the transistor characteristics, we have implemented a scheme which al-

lows for a calculation of the trap densities with high accuracy in a straightforward fashion.

We show the performance improvement to be due to a reduction in the density of shallow

traps ≤ 0.15 eV from the valence band edge, while the energetically deeper traps are es-

sentially unaffected. This chapter contributes to an understanding of the shallow traps in

organic semiconductors and identifies structural defects as a major cause.1

1 The results in this chapter are published inW. L. Kalb, F. Meier, K. Mattenberger, B. Batlogg, Phys. Rev. B 76, 184112 (2007).

106 Defect healing at room temperature in pentacene thin films

6.1 Introduction

In Chap. 2 we have seen that the charge transport in crystalline organic semiconductors

such as pentacene can be described by assuming a mobility edge which separates extended

and localized states. The charge carriers are transported in the extended states above the

mobility edge, but are trapped by and thermally released from localized trap states below

the mobility edge. The mobility edge may be identified with the valence or conduction

band edge. We have also seen that, in highly disordered organic semiconductors, a de-

scription of the charge transport by variable range hopping may be more appropriate. Im-

portantly, this situation can be described by trap-controlled transport in a transport level

with a distribution of localized states below the transport level. This description is thus

very similar to the mobility edge picture. From these models, the conduction of charge

in organic semiconductors and thus the transistor characteristics are expected to critically

depend on trap states. Trap states in organic semiconductors have been the subject of

extended investigations. [21] This topic is currently attracting a lot of attention because of

the crucial importance of trap states for the emerging applications of organic field-effect

transistors. Microscopic causes of trap states in organic semiconductors are described in

Chap. 2.

Organic field-effect transistors are excellent devices to study the charge trans-

port in organic semiconductors, since the position of the Fermi level at the dielectric-

semiconductor interface can be fine-tuned by applying a gate voltage. The density of

trap states as a function of energy can be derived from the transistor characteristics. On

the one hand, a density of states function can be postulated a priori and the correspond-

ing transistor characteristic can be simulated by means of a suitable computer program.

[152, 153, 155] On the other hand, the density of states function can be calculated from

the linear regime transfer characteristics in a straightforward fashion. [52, 53, 128, 160]

Organic thin-film transistors (TFT’s) are most often characterized after the samples

have been exposed to ambient air and electrical characterization without air exposure is

very rare. [131, 132] The organic semiconductors, however, are generally presumed to be

sensitive to water vapour and oxygen. Moreover, the transistors are often characterized by

two-terminal measurements which do not allow to distinguish between contact effects and

effects of the semiconducting layer. As we will see below, the gated four-terminal method

yields the field-effect conductivity and the effective field-effect mobility free from contact

effects. It also allows for an extraction of the device contact resistance.

6.2 Experimental methods 107

For the experimental work described in this chapter, pentacene served as a prototyp-

ical oligomeric semiconductor; we have investigated the performance of pentacene thin-

film transistors by gated four-terminal measurements without ever exposing the samples to

ambient air. The corresponding trap states functions were derived with a straightforward

extraction scheme which had been successfully used to study trap states in hydrogenated

amorphous silicon.

6.2 Experimental methods

6.2.1 Device fabrication

Pentacene from Aldrich (purum) was sublimation purified twice (see Sec. 3.2) and was

introduced into the evaporation chamber of the device fabrication and characterization

system immediately after the purification. As substrates we used heavily doped Si wafers

with a 260 nm thick SiO2 layer. The substrates were cleaned with hot acetone and hot

isopropanol (MOS grade) in an ultrasonic bath. Immediately after the cleaning, the sub-

strates were mounted on a sample holder and were introduced into the device fabrication

and characterization system via the load lock (Fig. 3.9(a)). The evaporation chamber

and the prober station were both separated from the load lock by a gate valve and were

constantly kept under vacuum (base pressure ≈ 3×10−8 mbar). The vacuum in the evap-

oration chamber and in the prober station was maintained respectively with a cryopump

and a turbo pump. The substrates were introduced into the evaporation chamber with

transfer rod 1 (Fig. 3.9(a)) and were placed on a shadow mask for the pentacene evapo-

ration. A high precision mask positioning mechanism allowed for a proper adjustment of

the mask with respect to the substrates.

Prior to the device fabrication, the substrates were kept in high vacuum for ap-

proximately 24 h. After that time, also the pressure in the turbo-pumped load lock was

≈ 3× 10−8 mbar. A 50 nm thick film of pentacene was evaporated through the shadow

mask onto the Si/SiO2 substrates, while the substrates were kept at room temperature.

After the pentacene evaporation, the sample holder was positioned on a shadow mask

for the gold evaporation with transfer rod 1 and the pentacene TFT’s were completed by

evaporating 40 nm thick gold electrodes.

The resulting transistor test structures are schematically shown in Fig. 3.13. The

transistors consisted of a well-defined stripe of pentacene and had voltage sensing elec-

trodes with little overlap to the pentacene film. It has been demonstrated that the use


of a “masked” pentacene film and a proper alignment of the electrodes is important for

the four-terminal measurement. [134] The channel length and width of the devices were

L = 450 µm and W = 1000 µm. The voltage sensing electrodes were situated at (1/6)L

and (5/6)L, such that the distance between these electrodes was L′ = 300 µm.

After the completion of the device fabrication, the samples were transported to the

prober station by means of transfer rod 1 and transfer rod 2 (Fig. 3.9(a)).


For the electrical measurements we used a HP 4155A semiconductor parameter analyzer

connected to five microprobers at the prober station. Transfer characteristics in the lin-

ear regime were measured in steps of 0.2 V (drain voltage Vd = −2 V). In all cases, the

forward and the reverse sweeps were measured. The integration time was 20 ms and the

delay time was 0 s. In addition to the drain current Id , the voltage drops V1 and V2 between

the grounded source and the two voltage sensing electrodes were measured at each gate

voltage Vg (gated four-terminal measurement). All electrical measurements were carried

out in the dark.

A device was initially measured ≈ 4 h after the completion of the pentacene evap-

oration. Subsequently, the same device was regularly measured (normally twice a

day) for approximately one week while being kept in the prober station in the dark at

≈ 3×10−8 mbar.

In some experiments, we investigated the effect of oxygen or nitrogen exposure on

the device characteristics. This was done by introducing a continuous flow of gas (purity

≥ 99.9999 Vol.-%) into the prober station through a leak valve, thus adjusting the partial

pressure of the gas within the prober station. A few days prior to the device fabrication for

the gas exposure experiments, the prober station was filled with the respective gas through

the leak valve in order to flush the gas supply line and the valve. The prober station was

then re-evacuated and the supply lines were held at an overpressure of 0.5 atm until the

leak valve was opened in the experiment.

In other experiments, the influence of thermal annealing was explored by means of

an electrical heating element at the cryostat in the prober station.

6.3 Parameter extraction 109

6.3 Parameter extraction

In this section, we describe the extraction of key parameters from the transistor char-

acteristics of trap-controlled devices. Our approach consists of measuring the transfer

characteristics at a low drain voltage (Vd = −2 V). We can therefore assume the “un-

perturbed” situation where charge is accumulated by a gate voltage in a metal-insulator-

semiconductor (MIS) structure but no drain voltage is applied in order to calculate the

charge distribution as a function of the distance from the insulator-semiconductor inter-

face. [101] We begin by specifying the basic parameters to be extracted from the transfer

characteristics.

6.3.1 Basic parameter extraction

In single crystalline inorganic MOSFET’s above a threshold voltage, essentially all the

trap states are filled and the charge induced by the gate appears in the valence/conduction

band. [102] Both the constant mobility in the above threshold regime and the threshold

voltage are important device parameters. This approach is described in detail in Sec. 2.5.2.

It may also be valid in the case of organic single crystal transistors, i.e. organic field-effect

transistors with a low trap density. However, we have already seen in Chap. 4 that contact

effects often severely affect the characteristics of these devices due to the low channel

resistance. This is in agreement with other studies. [180]

In Chap. 2 we have also mentioned that the equations developed for single crystalline

MOSFET’s are not suitable to describe organic TFT’s with an increased trap density.

[100,101] Depending on the density of trap states, the majority of charge carriers induced

by the gate may be trapped even at relatively high gate voltages. [155] Provided that

contact effects are negligible, the drain current of an organic TFT in the linear regime

may be written as

Id = (W/L)σVd. (6.1)

σ is the field-effect conductivity, which is the effective field-effect mobility µe f f multi-

plied by the total gate induced charge per unit area Ci(Vg−VFB), i.e

σ = µe f fCi(Vg−VFB). (6.2)


VFB is the flatband voltage and Ci the capacitance per unit area. In Sec. 2.5.2 we have dis-

cussed the origins of a non-zero flatband voltage. The flatband voltage is approximately

equal to the onset voltage of the device. The onset voltage is defined as the gate voltage,

where the drain current, as a function of gate voltage, rises sharply if plotted on a loga-

rithmic scale. In the present work we assume that the flatband voltage is equal to the onset

voltage. The effective field-effect mobility is one of the most important device parameters

and, for a p-type semiconductor such as pentacene, µe f f can be written as

µe f f =Pf ree

Pf ree +Ptrappedµ0, (6.3)

where Pf ree and Ptrapped are respectively the density of free and trapped holes per unit

area. µ0 is the extended state mobility. The effective field-effect mobility µe f f is ex-

pected to increase with gate voltage even at relatively high gate voltages, because the

ratio Pf ree/(Pf ree + Ptrapped) increases as the valence band is bent towards the Fermi en-

ergy. [100]

From a technological point of view, it may be useful to define a threshold voltage

which marks the transition between the regime below threshold and the regime above

threshold. In the above-threshold regime, the deep traps are filled and the field-effect mo-

bility is less strongly dependent on gate voltage. The above-threshold regime in an organic

TFT can be understood as being in between the below-threshold regime and the above-

threshold regime of a single crystalline MOSFET. [102] It can be demonstrated that the

field-effect mobility in the above-threshold regime follows a power law µe f f = κ(Vg−Vt)α

and this allows for a refined extraction of the field-effect mobility and the threshold volt-

age Vt . [101, 102, 159]

The field-effect conductivity can be calculated from

σ(Vg) =LW

Id

Vd. (6.4)

With Eq. 6.2 and Eq. 6.4, µe f f can be approximated as

µe f f (Vg) =1Ci

(∂σ∂Vg

)

Vd

=L

WVdCi

(∂Id

∂Vg

)

Vd

. (6.5)

Since this approach is frequently used, it has the advantage that the values of µe f f can

easily be compared. Moreover, the definition and extraction of a threshold voltage is not


necessary. However, small errors in the absolute value of the field-effect mobility and its

gate voltage dependence are to be expected since the derivation rests on the assumption

of a weak dependence of the field-effect mobility on gate voltage. [101]

In an organic field-effect transistor, a significant fraction of the drain voltage Vd may

drop at the contacts; this can introduce significant errors when extracting the field-effect

conductivity and the field-effect mobility. From gated-four terminal measurements, the

conductivity can be derived without error with Id = (W/L′)σV ′d . L′ is the distance between

the voltage sensing electrodes and V ′d = V1−V2 the voltage drop between these electrodes

(Fig. 3.13). The contact-corrected field-effect conductivity is then given by

σ(Vg) =L′

WId

(V1−V2). (6.6)

The effective field-effect mobility µe f f is not influenced by contact effects when calcu-

lated from

µe f f (Vg) =L′

W (V1−V2)Ci

(∂Id

∂Vg

)

Vd

. (6.7)

In the following, we use the expressions “two-terminal conductivity” and “two-

terminal mobility” as short hand for Eq. 6.4 and Eq. 6.5. We furthermore use the ab-

breviations “four-terminal conductivity” and “four-terminal mobility” for the quantities

defined in Eq. 6.6 and Eq. 6.7.

The device contact resistance Rcontact was extracted from the four-terminal measure-

ment and was compared to the device channel resistance Rchannel . We now assume a linear

voltage drop all along the channel, i.e. from the source to the drain. With this assumption,

the voltage drop across the transistor channel is (V1−V2)L/L′ and the voltage drop at the

contacts is Vd− (V1−V2)L/L′. The contact resistance is thus given by

Rcontact(Vg) =Vd− (V1−V2)L/L′

Id(6.8)

and the channel resistance by

Rchannel(Vg) =(V1−V2)L/L′

Id. (6.9)


6.3.2 Advanced parameter extraction

The gate voltage dependence of the field-effect mobility reflects the spectral density of

trap states close to the valence band. A number of direct extraction schemes has been

suggested to obtain the underlying density of states function. [52, 53, 128, 160] In these

approaches, the relevant energy scale is derived from the activation energy Ea(Vg) of

the current (i.e. the field-effect conductivity) that is obtained from temperature depen-

dent measurements. If, however, the electrical characteristics of a transistor change on a

timescale that is comparable to the time of a temperature dependent measurement (hours),

this approach is not suitable.

Grünewald et al. have suggested an extraction scheme of high accuracy for amor-

phous silicon thin-film transistors which allows to convert a single linear regime transfer

characteristic into the underlying density of states function. [140, 142, 145] It is based on

surprisingly few simplifying assumptions including:

1. the charge density is homogenous all along the transistor channel,

2. the semiconductor is homogenous perpendicular to the insulator-semiconductor in-

terface, and

3. insulator surface states only introduce an initial band bending without applied field,

i.e. contribute to a non-zero flatband voltage VFB.

Extraction schemes are often based on the abrupt approximation: all the charge is as-

sumed to reside in a region of depth λ(Vg) close to the dielectric-semiconductor interface.

Grünwald’s method, however, is not based on this simplification, but takes proper account

of the gate-induced band bending. In the following, we present the key equations of the

extraction scheme.

Within the Boltzmann approximation, the field-effect conductivity can be written as

σ(Ug) =σ0

d

Z d

0exp

(eV (x)

kT

)dx. (6.10)

Now

Ug = |Vg−VFB| (6.11)

is the gate voltage above the flatband voltage, d is the thickness of the pentacene film,

eV (x) is the band shift as a function of the distance x from the insulator-semiconductor


Figure 6.1: Potential drop across the gate insulator (thickness l, dielectric constant εi) and thepentacene thin film (thickness d, dielectric constant εs). Most of the gate voltage drops across thegate dielectric. At the insulator-semiconductor interface the potential is V0.

interface and σ0 = σ(Ug = 0) is the conductivity at flatband. σ0 can be approximated as

σ0 = eµ0d NV exp(− EV −EF

kT

)= eµ0 Pf ree. (6.12)

NV is an effective density of extended states and EV the energy of the valence band edge

far from the insulator-semiconductor interface. The situation is depicted in Fig. 6.1 and

Fig. 6.2. The complicated dependence of the band shift on the space coordinate x in

Eq. 6.10 can be eliminated and an equation can be derived, which implicitly contains the

interface potential V (x = 0) = V0 as a function of gate voltage:

exp(

eV0

kT

)− eV0

kT−1

=e

kTεid

εslσ0

[Ugσ(Ug)−

Z Ug

0σ(U ′

g)dU ′g

]. (6.13)

l is the thickness of the gate insulator and εi and εs are the dielectric constants of the

insulator and the semiconductor (Fig. 6.1). A derivation of Eq. 6.13 can be found in [140].

For each gate voltage, Eq. 6.13 can be numerically evaluated with the measured data

σ(Ug) and a value for the interface potential V0 is obtained. Eventually, we have the

complete function V0 = V0(Vg).

We now outline how a straightforward conversion of Poisson’s equation along with

boundary conditions eventually leads to the key equation which can be used to calculate

the trap DOS from the interface potential V0. We assume that the electrical potential V (x)

at the surface of the pentacene film of thickness d vanishes under all biasing conditions,


Figure 6.2: Sketch of the energetics near the SiO2/pentacene interface: the application of a gatevoltage Vg above the flatband voltage leads to a bending of the valence band (VB) and of theconduction band (CB). At the interface (x = 0), the band shift is eV (x = 0) = eV0. Under theseconditions, the energy E ′ of specific trap states (dashed line) is raised at the interface so that it coin-cides with the Fermi energy EF of the sample. The energy of these trap states relative to the energyof the mobility edge EV is EV −EF − eV0. In Chap. 7, the energy EV −EF − eV0 is approximatedby the experimentally determined activation energy Ea of the field-effect conductivity.

i.e.

V (x = d) = 0. (6.14)

The electric field F at this position is also assumed to drop to zero:

F =−(

dVdx

)

x=d= 0. (6.15)

This is reasonable as long as the pentacene film is thicker than the decay length of the po-

tential. [140] The situation is depicted in Fig. 6.1. The dielectric strength at the insulator-

semiconductor interface must be continuous, i.e. for a zero flatband voltage

εiVg−V0

l=−εs

dVdx x=0

. (6.16)

A conversion of Poisson’s equation with these boundary conditions eventually leads to an

expression for the total hole density p as a function of the interface potential V0. [140]

This expression is

p(V0) =ε0ε2

iεsl2e

Ug

(dV0

dUg

)−1

. (6.17)


p (lower case) denotes a volume density of holes. The volume density depends on the

distance x from the insulator-semiconductor interface, i.e. on the electrical potential V (x)

in the semiconductor. The volume density p and the area hole density P are related by

integrating over the depth of the whole film, i.e. P =R d

0 p(x)dx.

Eq. 6.17 yields the functional dependence of the volume density of holes on the

potential V0. Since the total hole density p can be written as

p(V ) =Z +∞

−∞N(E) [ f (E + eV )− f (E)]dE, (6.18)

its derivative is given by

1e

d p(V )dV

=Z +∞

−∞N(E)

∣∣∣∣d f (E + eV )d(E + eV )

∣∣∣∣dE. (6.19)

Eq. 6.19 is a convolution of the density of states function N(E) with the derivative of

the Fermi function. Several deconvolution methods exist to solve this type of equation

for N(E), e.g. with cubic spline functions. [51, 182] However, for a slowly varying den-

sity of states function (absence of monoenergetic states), the difference between N(E)

and d p/edV is expected to be relatively small on a logarithmic scale. [182] We approxi-

mate the Fermi function with a step function according to the common zero-temperature

approximation. [52,183,184] Its derivative then is a delta function and we eventually have

1e

d p(V0)dV0

≈ N(EF + eV0). (6.20)

Within this zero temperature approximation, the band shift at the interface eV0 is equal

to the energy of the respective traps relative to the Fermi energy EF of the sample, i.e.

eV0 = E ′−EF (Fig. 6.2).

We have used Grünewald’s method to interpret the current voltage characteris-

tics from pentacene TFT’s. We used the four-terminal conductivity as a starting point

which permits to extract a density of states function free from contact artifacts. A sim-

ple MATLABr code allowed for the calculation of the density of states function from

Eq. 6.13, 6.17 and 6.20.


0

2x10-7

4x10-7

6x10-7

10 0 -10 -20 -30 -40 -5010-12

10-11

10-10

10-9

10-8

10-7

10-6140 h

Vd = -2 V

140 h

4 h

| Id |

(A)

4 h

| Id |

(A)

Vg (V)

Figure 6.3: Transfer characteristic of a pentacene TFT measured 4 h and 140 h after the comple-tion of the pentacene evaporation. The storage under high vacuum conditions leads to an increasedon-current and to a reduced current hysteresis.

6.4 Results

6.4.1 Improvement of the device performance with time

Fig. 6.3 shows the transfer characteristic of a pentacene TFT measured 4 h and 140 h

after the completion of the pentacene evaporation. The device was always kept at

3× 10−8 mbar. After 140 h, the device shows an increased on-current. In addition, the

current hysteresis is reduced: at a current level of 10−10 A, the difference between the

forward and the reverse sweep is 3.8 V after 4 h and 1.2 V after 140 h. The subthreshold

swing is essentially unaffected by the high vacuum storage. There is a small shift of the

onset voltage to more positive voltages from -6.4 V after 4 h to -4.9 V after 140 h.

An increase in on-current can be caused by changes of the pentacene film and/or by

a reduction of the device contact resistance. The gated four-terminal method can disentan-

gle the field-effect conductivity and the device contact resistance. In Fig. 6.4 we show the

four-terminal conductivity after 4 h and after 140 h, as derived from the forward sweeps

6.4 Results 117

0 -10 -20 -30 -40 -500.00

0.05

0.10

0.15 Four-terminal Two-terminal

140 h

4 h

(S)

Vg (V)

Figure 6.4: The four-terminal conductivity increases with time. The graph shows the four-terminal conductivity (circles) after 4 h and after 140 h. The dashed lines indicate the two-terminalconductivity for comparison.

(Eq. 6.6). The two-terminal conductivity (Eq. 6.4) is given for comparison. The four-

terminal conductivity is increased after 140 h, which reveals changes of the pentacene

film. The difference between the four-terminal conductivity and the two-terminal con-

ductivity is reduced after 140 h, indicative of an additional contact resistance reduction.

Fig. 6.5 shows the four-terminal mobility derived with Eq. 6.7 and the two-terminal

mobility calculated from Eq. 6.5. As expected, the mobility monotonically increases

with gate voltage. When comparing both measurements, a significant improvement in

mobility can be ascertained. At Vg ≈−50 V, the mobility is µ = 0.22 cm2/Vs after 4 h and

µ = 0.45 cm2/Vs after 140 h, i.e. µ has increased by a factor of 2.2

In Fig. 6.6 we show the width-normalized contact resistance RcontactW according to

Eq. 6.8 and, for comparison, the width-normalized channel resistance RchannelW (Eq. 6.9).

There is a drastic reduction in contact resistance. At Vg = −50 V the contact resistance

decreases by a factor of ≈ 11 from 1.95× 105 Ωcm to 1.81× 104 Ωcm. The channel

resistance decreases by a factor of ≈ 2. Importantly, the channel resistance is always

higher than the contact resistance: at Vg = −50 V and after 4 h the channel resistance is

≈ 3 times larger than the contact resistance, and after 140 h it is ≈ 17 times the contact

resistance. Thus, the device is always dominated by the channel resistance.

2 We compare mobilities for comparable total charge densities. Since we only observe small onset voltageshifts, a correction of the gate voltage by the onset voltage is not necessary.


0 -10 -20 -30 -40 -500.0

0.1

0.2

0.3

0.4

0.5 Four-terminal Two-terminal

Vg (V)

4 h

140 h

eff (

cm2 /V

s)

Figure 6.5: Four-terminal mobility (circles) as a function of gate voltage. There is a significantimprovement in mobility with time. At Vg ≈−50 V, the mobility increases from µ = 0.22 cm2/Vsafter 4 h to µ = 0.45 cm2/Vs after 140 h, i.e. by a factor of two. The dashed lines show the two-terminal mobilities for comparison.

-30 -35 -40 -45 -50104

105

106

140 h

4 h

Contact resistance

Channel resistance

4 h140 h

Vg (V)

R*W

(*c

m)

Figure 6.6: Width-normalized contact resistance (triangles) after 4 h and after 140 h. The contactresistance is drastically reduced: at Vg = −50 V it decreases by a factor of ≈ 11. The graph alsocontains the respective width-normalized channel resistances (circles) for comparison. The contactresistance is always lower than the channel resistance.

6.4.2 Influence on the density of states function

In Sec. 6.4.4 and 6.4.5 we show that the performance improvement is not due to doping by

a residual gas, but due to a healing of structural defects at room temperature. In order to

investigate the energetic position of these defects, we have applied the scheme described

in Sec. 6.3.2. The first step was to calculate the interface potential V0 as a function of

gate voltage with Eq. 6.13. We have assumed a dielectric constant of εi = 3.9 for SiO2

6.4 Results 119

and εs = 3.0 for pentacene. [185, 186] Fig. 6.7(a) shows the result of the extraction for

the measurement after 4 h and after 140 h. In a second step, the density of states was cal-

culated by two numerical differentiations of V0(Ug) according to Eq. 6.17 and Eq. 6.20.

Some degree of data smoothing was applied in order to obtain a reasonably smooth den-

sity of states function. Fig. 6.7(b) shows the final result for the measurement after 4 h and

after 140 h. A step width of 0.2 V in the gate voltage sweeps leads to a good resolution of

the deep states.

The Fermi energy of the sample coincides with the zero point of the energy scale

in Fig. 6.7(b). At high gate voltages, the valence band edge at the interface is close to

the Fermi level. Consequently, the Fermi level is approximately 0.45 eV from the valence

band edge at flatband. For comparison, the bandgap of pentacene is about 2.2 eV. [187]

The interface potential in Fig. 6.7(a) reflects the spectral density of trap states. At low gate

voltages, bands bend easily, and this implies a low trap density. At high gate voltages,

however, band bending is more difficult and this is indicative of a high trap density very

close to the valence band edge.

From Fig. 6.7(b) it is clear that it is the shallow traps with energies approximately

0.15 eV from the valence band edge which are reduced by the high vacuum storage. It

is the density of these states that influences the value of the field-effect mobility µe f f . A

relatively small reduction causes a significant improvement in field-effect mobility. The

traps which are deeper in energy are essentially unaffected, resulting in an almost identical

subthreshold swing of the transfer characteristics.

After 4 h as well as after 140 h, the density of states function can reasonably well be

approximated by a single exponential function

N(E) = N0 exp(

EE0

). (6.21)

It is however slightly steeper than exponential. Fitting the curves in Fig. 6.7(b) for

eV0 ≥ 0.25 eV to Eq. 6.21 yields the parameters E0 = 32 meV for the measurement af-

ter 4 h and E0 = 37 meV for the measurement after 140 h.


010

2030

4050

0.0

0.1

0.2

0.3

0.4

0.00.1

0.20.3

0.410

17

1018

1019

1020

1021

(b)(a)

4 h

140 h

V0 (V)

|Vg -V

FB | (V)

0.00.1

0.20.3

0.4

1x1021

2x1021

3x1021

4x1021

4 h140 h

dp/edV N(E) (cm-3eV-1)

4 h

140 h

eV0

E-EF (eV)

Figure6.7:

(a)Interface

potentialV0

asa

functionof

gatevoltage

aboveflatband|V

g −V

FB |for

them

easurementafter

4h

andafter

140h.

(b)D

ensityof

trapstates

asa

functionof

energy.T

hem

ainpanelshow

sthe

dp/edV

dataas

afunction

ofthe

bandshiftatthe

interfaceeV

0on

alogarithm

icscale.

The

bandshiftatthe

interfaceis

approximately

equaltothe

energyofthe

trapsabove

theFerm

ienergyofthe

sample,i.e.eV

0 ≈E−

EF .T

hequantity

dp/edV

isa

goodapproxim

ationofthe

densityoftrap

statesN

(E).T

hehigh

vacuumstorage

leadsto

areduced

densityoftraps

closeto

thevalence

bandedge.T

heinsetshow

sthe

trapdensities

ona

linearscale.

6.4 Results 121

Table 6.1: Four-terminal mobility µ1, at Vg ≈ −50 V, width-normalized contact resistance R1Wat Vg =−50 V and onset voltage Von from the initial measurements of different experiments. Themobility increased by as much as a factor of 2 in the course of an experiment, so that mobilitiesup to 0.45 cm2/Vs were achieved.

Run µ1 (cm2/Vs) R1W (Ωcm) Von (V)

1 0.22 1.95×105 -6.4

2 0.24 1.10×105 -4.1

3 0.14 3.03×105 -5.1

4 0.14 3.35×105 -6.4

Oxygen 0.12 3.98×105 -6.2

Nitrogen 0.10 4.70×105 -7.0

Annealing 0.10 4.55×105 -5.7

6.4.3 Comparison of several experiments

The effects described above, i.e. a significant increase in the four-terminal mobility, a

drastic reduction in the contact resistance and a reduction in the current hysteresis, have

been observed in all experiments. Fig. 6.8 shows the evolution of the four-terminal mo-

bility and the contact resistance with time for four different runs. The values are for

Vg ≈−50V and are normalized by the value obtained after ≈ 4 h.3 A time span of at least

4 h was allowed in between subsequent measurements. The absolute values for the mobil-

ity and the contact resistance from the initial measurement are summarized in Table 6.1.

Table 6.1 also contains the respective onset voltages. Initially, the onset voltage is

between −4.1 V and −6.4 V and shifts in all cases by less than 2.5 V during the course

of an experiment. The reduction of the current hysteresis takes place in the early stages

of the experiments. In all four runs, the current hysteresis is significantly reduced after

the first ≈ 24 h to 1.0−1.5 V at a current level of 10−10 A. Subsequently, there is only a

small further reduction of the current hysteresis.

3 The value for the contact resistance was taken at Vg =−50V . For the mobility, an average in the rangebetween Vg = −45 V and −50 V was taken, since in some cases the derivative of the drain current wasmore noisy.


1.0

1.2

1.4

1.6

1.8

2.0

0 30 60 90 120 150 180

0.1

1

4

3

2

/

1

R /

R1

time (h)Figure 6.8: Upper panel: four-terminal mobility at Vg ≈−50V normalized by the mobility µ1 ofthe initial measurement for four different runs. The lower panel shows, on a logarithmic scale,the respective values for the contact resistance R at Vg = −50V relative to the contact resistanceR1 from the initial measurement. The field-effect mobility increases with time and the contactresistance reduces with time in all experiments. There is some variation in the rate of the effects.

We now proceed by providing experimental evidence that the performance improve-

ment is not due to doping of the pentacene thin films by residual oxygen or nitrogen and

show that the performance improvement is a thermally promoted process.

6.4.4 Influence of oxygen and nitrogen

Even at a pressure of the order of 10−8 mbar, the time for the formation of a monolayer

of residual gas molecules is less than ten minutes. [188] In the case of semiconducting

polymers, experimental evidence indicates that doping leads to an increased field-effect

mobility. [96] This may be understood if we assume the charge transport to take place by

variable-range hopping and the doping to increase the density of states/hopping sites. [96]

In crystalline small molecule organic semiconductors, however, charge transport in ex-

tended states has to be considered. Doping raises the Fermi level of the sample. This

6.4 Results 123

Figure 6.9: Effect of oxygen exposure: the graph shows the four-terminal mobility at Vg ≈−50V(upper panel) and the contact resistance at Vg = −50V (lower panel) as a function of time. Thedashed red line is an estimate of the time-dependence of the contact resistance without oxygenexposure. The drastic increase in oxygen partial pressure neither leads to a more rapid increase inmobility nor to a more rapid decrease in contact resistance. On the contrary, the oxygen exposureslows down the decrease in contact resistance. In the inset we show the device onset voltage asa function of time. The increase in oxygen partial pressure does not cause a sudden shift of theonset voltage, which would be indicative of doping.

results in an increase of the (flatband) conductivity, since the density of free holes is in-

creased (Eq. 6.12). It would, however, not immediately be obvious how chemical doping

would increase the field-effect mobility in pentacene.

Both oxygen and nitrogen have been reported to have the capability of doping pen-

tacene. Gas exposure is found to lead to a shift of the transistor transfer characteristic

to more positive voltages, which corresponds to a shift of the Fermi level. [132, 189] In

other studies, an increase in conductivity of gas exposed pentacene thin-films or single

crystals was observed in two-terminal measurements without gate-field induced charge.

The increased conductivity is ascribed to an increased charge carrier density caused by

doping. [190–192] There seems to be no evidence that doping leads to an increased ef-

fective mobility in pentacene. Recent measurements on rubrene single crystals also show

doping not to increase the field-effect mobility. [193]


Figure 6.10: Effect of nitrogen exposure: a nitrogen partial pressure of 10−4 mbar leads to aslowing of the increase in mobility and decrease in contact resistance. The dashed red line is anestimate for the time-dependence of the contact resistance if the sample had not been exposed tonitrogen. The inset shows the device onset voltage. When the nitrogen partial pressure is increasedto 10−6 mbar, there is a sudden shift of the onset voltage which is indicative of doping.

The issue of reversibility and the importance of light for the doping is not com-

pletely clear. Doping is observed when pentacene thin films are exposed to relatively

high partial pressures (0.01 atm-1 atm) of oxygen or nitrogen in the presence of light.

[189, 190, 192] The doping effect by oxygen is reported to be negligible in the absence

of light. [189, 190] Ultraviolet photoelectron spectroscopy (UPS) before and after the ex-

posure to 5× 10−6 mbar oxygen could not detect a lasting effect on the position of the

energy levels. [194] Another study shows that the exposure of pentacene TFT’s to an

oxygen partial pressure of 10−5 mbar leads to a doping of the films. [132]

Although the mobility increase in crystalline/polycrystalline samples cannot be eas-

ily linked to chemical doping, a reduction of the contact resistance might be reconciled

with doping. From inorganic semiconductor physics it is well known that doping close to

a contact reduces the contact resistance. Moreover, it has been demonstrated with UPS

that oxygen, at a low partial pressure, can lead to a lowering of the injection barrier for

holes at an Au/pentacene interface. [194]

6.4 Results 125

The results of our studies concerning the effect of gas exposure on the field-effect

mobility and on the contact resistance are discussed in the following. If e.g. oxygen were

responsible for the increase in mobility and/or reduction in contact resistance, an increase

in the oxygen partial pressure should accelerate the rate of the respective process. Fig. 6.9

shows the influence of oxygen on the four-terminal mobility and on the contact resis-

tance (see Table. 6.1 for the initial parameters). After 41 h, the oxygen partial pressure

was raised from a total turbo-pumped background pressure of the order of 10−8 mbar to

10−6 mbar. After 69 h, the partial pressure was increased to 10−4 mbar and, eventually,

after 98 h, the chamber was re-evacuated. The increase in oxygen partial pressure does

not accelerate the gradual increase in mobility or the reduction in contact resistance. The

dashed red line in Fig. 6.9 is an estimate for the time-dependence of the contact resistance

if the sample had not been exposed to oxygen. Consequently, the increase in oxygen par-

tial pressure even causes a delay of the reduction in contact resistance. It is noteworthy

that neither the small current hysteresis nor the subthreshold swing of the devices are ef-

fected by the oxygen exposure. Since oxygen at a partial pressure of 10−6 mbar for 28 h

and at a partial pressure of 10−4 mbar for 29 h appears not to accelerate the development

of the device parameters with time, we can conclude that oxygen at the base pressure is

not responsible for the performance improvement.

The inset in Fig. 6.9 shows the onset voltage for each of the measurements. The

overall shift of the onset voltage is small and smooth (≈ 2.6 V over≈ 140 h). In particular,

there is no sudden shift of the transfer characteristic after an increase in oxygen partial

pressure. Therefore, there is no evidence that oxygen at partial pressures ≤ 10−4 mbar

leads to a doping of the films.

Fig. 6.10 presents the analogous experiment with nitrogen. After 50 h the nitrogen

partial pressure was increased to 10−6 mbar, followed by an increase to 10−4 mbar after

69.5 h. After 98 h, the system was pumped down to the base pressure of the order of

10−8 mbar. The increase in nitrogen partial pressure does not accelerate the increase in

mobility or the reduction in contact resistance. Similarly to the oxygen experiment, we

have estimated the time-dependence of the contact resistance if the sample had not been

exposed to nitrogen (dashed red line in Fig. 6.10). Clearly, the nitrogen exposure leads to

a slowing down of the decrease in contact resistance. The nitrogen exposure leaves the

small current hysteresis and the subthreshold swing unaffected. Residual nitrogen is not

responsible for the performance improvement.


Figure 6.11: Effect of annealing at 320 K: the increased temperature accelerates the increase inmobility and the decrease in contact resistance, as indicated by the dashed red lines.

The inset in Fig. 6.10 shows the device onset voltage as a function of time. When

the nitrogen partial pressure is increased to 10−6 mbar, there is a sudden shift of the onset

voltage to more positive voltages by 1.2 V. We take this as evidence for doping by nitrogen

at a partial pressure of 10−6 mbar. When the pressure is increased to 10−4 mbar, there is

no additional marked shift of the onset voltage.

6.4.5 Annealing at slightly elevated temperatures

As pointed out in Chap. 2, annealing pentacene thin films at moderate temperatures (e.g.

50 C) results in an improved crystallinity of the films, as seen by XRD. [76,77,79] When

concentrating on the channel region adjacent to the gate dielectric in a field-effect tran-

sistor, the annealing is found to leave the mobility unchanged or to lead to an increased

mobility. [76, 79] Since the interaction between the pentacene molecules is of the weak

Van der Waals type, it is not too surprising that annealing at moderate temperatures causes

an improved crystallinity of the films. Moreover, it seems plausible that structural defects

6.5 Discussion 127

can be “annealed” even at room temperature, which will account for the effects we ob-

serve. In this scenario we would expect an acceleration of the performance improvement

at higher temperatures. Fig. 6.11 shows the influence of such an annealing at 320 K (see

Table 6.1 for the initial parameters). After 44.5 h, the sample temperature was slowly

raised from RT to 320 K at a rate of 0.2 /min. Transfer characteristics were measured

after the temperature of 320 K had been reached. After 104.5 h, the heating was switched

off, followed by a slow cooling of the sample to room temperature. Two effects can be

discerned in the annealing process. First, the overall level of the mobility and the contact

resistance are affected by the increase in temperature since both quantities depend on tem-

perature. Second, and more significant in the present context, is the rate of change: after

raising the temperature, the increase in mobility and the decrease in contact resistance is

significantly accelerated. This is indicated by the dashed red lines in Fig. 6.11.

In an additional experiment, the sample temperature was raised from room temper-

ature to 310 K after 45.5 h and was increased to 320 K after 74 h. Even at 310 K, which

is not that much above room temperature (≈ 297K), the increase in mobility and the

decrease in contact resistance is noticeably accelerated.

All these results taken together clearly indicate an improvement of the electronic

parameters driven by a thermally promoted process, and not by chemical doping. We

suggest this process to be a healing of structural defects. We now discuss the microscopic

origin of the relevant traps and suggest how a reduced trap density can lead to a reduced

contact resistance.

6.5 Discussion

6.5.1 Defect healing at room temperature

We keep in mind that pentacene thin films on Si/SiO2 substrates are known to have a

layered structure and the layers are parallel to the substrate. Within these layers, the

molecules are arranged in a herringbone pattern and are oriented almost perpendicular

with respect to the substrate. It has been shown by high impedance STM that some of

the pentacene molecules in the layers are displaced along their long molecular axis, while

the two-dimensional packing is not disturbed. [75] With electronic structural calculations

it could be shown that the displaced molecules result in traps ≤ 0.1 eV from the valence

band edge. [75] Fig 2.11 in Sec. 2.4.1 illustrates the key result of this study. On the other

hand, calculations reveal that structural defects that form during the growth of the film


can relax into the ideal crystal structure at a later point in time during the completion of

the film growth. [74] This theoretical study is described in more detail in Sec. 2.4.1 and

one of the findings is depicted in Fig. 2.10.

In Sec. 6.4.2 we have shown that only the shallow traps ≤ 0.15 eV from the valence

band edge are reduced during the high vacuum storage at room temperature. We suggest

that a major cause of the shallow traps in pentacene thin films are pentacene molecules

that are slightly misplaced, i.e. various types of structural defects located within the

grains or at grain boundaries. Some of these defects are in a metastable state before

relaxation. They require only a small amount of energy in order to align, which can

be provided by the thermal energy at room temperature. This is a direct manifestation

of the weak intermolecular interaction which is characteristic of small molecule organic

semiconductors. The annealing of shallow traps at room temperature can easily explain

the increase in effective field-effect mobility.

6.5.2 Defects and contact resistance

In a simplistic view, the contact resistance is given by the energetic difference between

the work function of the metal and the ionization energy of the pentacene. In reality,

however, a clear correlation between the metal work function and the contact resistance

is often not observed. [187] Interface states can significantly affect the energetics at the

metal-pentacene interface. [133] In a top contact device, also the film resistance should

contribute to the contact resistance. The film resistance in pentacene devices with gold top

contacts has even been suggested to dominate the contact resistance. [134] The situation

is illustrated in Fig. 6.12: the hole injection at the source/pentacene interface is good,

but the holes must cross the pentacene film in order to reach the channel at the insulator-

semiconductor interface. The intrinsic mobility perpendicular to the molecular layers is

lower than parallel to the layers. Combined with high trap densities, this can result in

a large resistance between the gold electrodes and the transistor channel. If the contact

resistance is dominated by the film resistance, a reduction in the degree of structural

disorder is therefore expected to lead to a reduced contact resistance.

6.6 Conclusions

Pentacene thin-film transistors were made by thermal evaporation, employing a high pre-

cision mask alignment mechanism. The devices were characterized electrically by gated

6.6 Conclusions 129

Figure 6.12: Sketch of the different contributions to the total contact resistance. The holes mustovercome a barrier associated to the gold/pentacene interface. The resistance of the 50 nm thickpentacene film adds to the total contact resistance both at the source and at the drain.

four-terminal measurements without breaking the high vacuum (base pressure of the or-

der of 10−8 mbar). Under high vacuum conditions, the device performance is found to

improve with time. Within approximately one week, the contact-corrected field-effect

mobility improves by a factor of up to two and the device contact resistance typically

decreases by more than an order of magnitude. In addition, the current hysteresis signif-

icantly reduces. We have shown that an increased partial pressure of oxygen or nitrogen

does not accelerate the performance improvement. On the contrary, the gas exposure de-

lays the decrease in contact resistance. Annealing at a slightly elevated temperature (e.g.

320 K), on the contrary, leads to an acceleration of the performance improvement.

Some defects within the pentacene “anneal” even at room temperature. This is a

peculiarity of the physics of organic semiconductors, which is governed by the weak Van

der Waals type interaction between the molecules. We have derived the spectral density

of trap states from the four-terminal conductivity. The calculations show shallow traps

≤ 0.15 eV from the valence band edge to be significantly affected by the defect heal-

ing. We suggest these traps to originate from structural defects, i.e. slightly misaligned

molecules within the polycrystalline film. The effective field-effect mobility critically de-

pends on the number of these shallow traps and a relatively small reduction results in a

significant improvement of the mobility. The contact resistance is likely to be dominated

by the film resistance and also depends on the active traps within the film.

The method to calculate the spectral density of traps is a powerful tool to further elu-

cidate the origin of trap states in organic semiconductors, provided that contact effects are


properly taken into account. It is particularly suitable to study metastable defects in or-

ganic semiconductors, because the density of states function can be derived from a single

transfer characteristic in an unambiguous fashion and with a minimal set of simplifying

assumptions.

7 Oxygen-related traps in pentacenethin films: Energetic position andimplications for transistor performance

In this chapter we describe a detailed study of the influence of oxygen on the electronic

trap states in a pentacene thin film. The study was done by carrying out gated four-

terminal measurements on thin-film transistors as a function of temperature and without

ever exposing the samples to ambient air. Photo-oxidation of pentacene is shown to lead

to a peak of trap states centered at 0.28 eV from the mobility edge, with trap densities of

the order of 1018 cm−3. As the gate voltage is ramped up, these trap states need to be oc-

cupied at first and cause a reduction in the number of free carriers at a given gate voltage.

Moreover, the exposure to oxygen reduces the mobility of the charge carriers above the

mobility edge. We correlate the change of these transport parameters with the change of

the essential device parameters, i.e. subthreshold performance and effective field-effect

mobility. The experiments described below support the assumption of a mobility edge for

charge transport, and contribute to a detailed understanding of an important degradation

mechanism of organic field-effect transistors: deep traps in an organic field-effect transis-

tor reduce the effective field-effect mobility by reducing the number of free carriers and

their mobility above the mobility edge.1

1 The results in this chapter are published inW. L. Kalb, K. Mattenberger, B. Batlogg, Phys. Rev. B 78, 035334 (2008)

132 Oxygen-related traps in pentacene thin films

7.1 Introduction

Critical issues of organic field-effect transistors are electrical stability and environmen-

tal stability. The experimental results in Chap. 5 clearly show that the electrical stability

of organic field-effect transistors can be very high, if suitable gate dielectrics are used

and if the semiconductor has a high degree of structural order (see also e.g. [15]). The

environmental stability of the organic semiconductor, thus, is an urgent issue to be ad-

dressed. [195, 196] Studies of the degradation of organic field-effect transistors are rare

and indicate that, in the case of p-type organic semiconductors, atmospheric oxygen or

ozone is a major cause. [126–130] It is crucial to understand in detail the way in which an

oxidation of the organic semiconductor impedes the charge transport and thus degrades

the transistor characteristics.

We characterized pentacene thin-film transistors by temperature dependent measure-

ments without ever exposing the samples to ambient air and after controlled exposure to

oxygen and light. The effect of oxygen can only be clarified with pristine samples. More-

over, since the field-effect conductivity depends (approximately) exponentially on tem-

perature, temperature is a very sensitive parameter. Again, another distinct feature of our

approach are the gated four-terminal measurements instead of the commonly employed

gated two-terminal measurements.

We have seen that charge transport in organic semiconductors can be described with

a mobility edge or transport level concept. In the mobility edge picture, the parameters

dominating charge transport are the trap densities as a function of energy relative to the

mobility edge, the number of delocalized charge carriers above the mobility edge and the

mobility of the latter charge.

In order to assess the fundamental transport parameters, we have developed a scheme

for organic field-effect transistors that is easy to use. The approach readily reveals all

the key parameters with high accuracy in a straightforward and unambiguous fashion.

The scheme is based on the method developed by Grünewald et al. which we used in

Chap. 6. [140] Instead of estimating the interface potential from the transistor character-

istic measured at a single temperature as in the original scheme, we extract the interface

potential from the temperature-dependence of the field-effect conductivity.

7.2 Experimental 133

7.2 Experimental

As in Chap. 6 we used the device fabrication and characterization system which is de-

scribed in Sec. 3.6.1 (Fig. 3.9 and Fig. 3.10). The device fabrication was the same as for

the experiments in Chap. 6. Heavily doped Si wafers with a 260 nm thick SiO2 layer were

cut, cleaned with hot solvents, fixed on a sample holder and were then introduced into the

cryo-pumped evaporation chamber of the device fabrication and characterization system.

After approximately 24 h, two times sublimation purified pentacene was evaporated onto

the samples through a shadow mask at a base pressure of the order of 10−8 mbar. The sub-

strates were kept at room temperature during the evaporation and the final film thickness

was 50 nm. The sample holder with the samples was then placed on a shadow mask for

the gold evaporation without breaking the high vacuum and gold electrodes were evapo-

rated onto the pentacene. Again, the completed thin-film transistors had a channel length

of L = 450 µm and a channel width of W = 1000 µm. Moreover, two voltage sensing

electrodes were situated at (1/6)L and (5/6)L and had little overlap with the “masked”

pentacene film, as schematically shown in Fig. 3.13. The sample holder was then trans-

ferred to and clamped on the cryostat in the turbo-pumped prober station of the device

fabrication and characterization system without breaking the high vacuum (10−8 mbar,

Fig. 3.9(b)).

In Chap. 6 we have seen that the device performance improves with time when

pentacene thin-film transistors are kept in high vacuum. Therefore, the devices were kept

in the prober station at a pressure of ≈ 2× 10−8 mbar for approximately three weeks

before starting the study. After that time, the device characteristics were found to be

stable on the timescale of several days.

The prober station is equipped with five micro-probers (Fig. 3.9(a)). The prober

arms are connected to the cryostat with thick copper braids and are thus cooled when the

cryostat and the sample holder with the samples is cooled down. For the temperature-

dependent gated four-terminal measurements, the source, the drain and the voltage sens-

ing electrodes were contacted with thin gold wires attached to four of the micro-probers.

By means of an electrical feedthrough to the cryostat, a gate bias could be applied. In

order to measure the temperature on the surface of the samples, an AuFe/Chromel ther-

mocouple was attached on the 5th micro-prober and was carefully pressed against the

surface of the sample at each temperature.


As always, the electrical measurements were carried out with an HP 4155A semicon-

ductor parameter analyzer. Transfer characteristics were measured with a drain voltage of

Vd =−2 V in steps of 0.2 V with an integration time of 20 ms and a delay time of 100 ms.

For each gate voltage Vg, the drain current Id and the potentials V1 and V2 between the

grounded source electrode and the respective voltage sensing electrode were measured

(Fig. 3.13). All electrical measurements were done in the dark.

The devices were exposed to oxygen by introducing 1 bar of oxygen (purity

≥ 99.9999 vol%) into the prober station through a leak valve. The pressure of the oxygen

in the prober station was measured with a mechanical pressure gauge. In addition, the

samples were exposed to a combination of 1 bar of oxygen and white light from a cold

light source (colour temperature 3200 K) trough a quartz window.

7.3 Charge transport parameters

As described for the experiments in Chap. 6, the transfer characteristics were measured

at a low drain voltage (Vd = −2 V). This results in a drastic simplification of the device

physics. The two-dimensional problem is approximated by two one-dimensional equa-

tions, i.e. Poisson’s equation for the charge distribution perpendicular to the insulator-

semiconductor interface and a simple Ohmic current-voltage relationship.

7.3.1 Field-effect conductivity, field-effect mobility and contact resis-tance

Once more, we use the terms “two-terminal conductivity” and “four-terminal conductiv-

ity” for the expressions defined respectively in Eq. 6.4 and Eq. 6.6. Eq. 6.5 is the “two-

terminal mobility” and Eq. 6.7 is the “four-terminal mobility”. We already mentioned that

µe f f , as calculated with Eq. 6.5 or Eq. 6.7, is an effective mobility. For a p-type semicon-

ductor such as pentacene, it is a rough estimate of the ratio of the free surface hole density

Pf ree to the total surface hole density Ptotal = Pf ree + Ptrapped multiplied by the mobility

µ0 of the holes in the valence band, i.e.

µe f f ≈Pf ree

Ptotalµ0. (7.1)

The device contact resistance Rcontact is given by Eq. 6.8 and should be compared to the

channel resistance Rchannel as given by Eq. 6.9.

7.3 Charge transport parameters 135

In order to gain a deeper insight into the physics of the organic semiconductor in

a field-effect transistor, a more sophisticated description is developed in the following

sections.

7.3.2 Spectral density of trap states and free hole density

We treat the polycrystalline pentacene layer as uniform and assume that trap states on the

surface of the gate dielectric only contribute to a non-zero flatband voltage. Consequently,

the trap densities to be determined are an average over in-grain and grain boundary regions

and may also be influenced, to some extend, by trap states on the surface of the gate

dielectric. The same simplifications were used in Chap. 6.

In the previous chapter we have already seen that the total hole density p as a func-

tion of the interface potential V0 may be written as

p(V0) =ε0ε2

iεsl2e

Ug

(dV0

dUg

)−1

. (7.2)

εi and εs are the dielectric constants of the gate insulator and the semiconductor and l and

d are the thickness of the gate dielectric and semiconducting layer (Fig. 6.1). p (lower

case) denotes a volume density of holes. Moreover,

Ug = |Vg−VFB| (7.3)

in Eq. 7.2 is the gate voltage above the flatband voltage VFB. Again, we approximate the

Fermi function with a step function (zero-temperature approximation) and we have an

expression for the density of states N(E)

1e

d p(V0)dV0

≈ N(EF + eV0). (7.4)

From Eq. 7.2 and Eq. 7.4 we can see that the interface potential V0 as a function of

gate voltage is the key to the density of states function (DOS). Since the change of the

interface potential and the change of the drain current with gate voltage are linked (see

Eq. 6.13 in Chap. 6), it is possible to extract the interface potential from the transfer char-

acteristic measured at a single temperature. [140] Once the interface potential is known,


the trap DOS in the mobility gap can be estimated with Eq. 7.2 and Eq. 7.4. Thus, the

trap densities can be plotted as a function of the band shift eV0 at the interface, which is

the energy of the traps relative to the Fermi energy of the sample (Fig. 6.2). This method

was applied to four-terminal conductivity data in Chap. 6. Here we have advanced the

extraction scheme. As described in the following, we used gated four-terminal measure-

ments at various temperatures to estimate the interface potential. We then used Eq. 7.2

and Eq. 7.4 to calculate the DOS. The consistency of the assumption of charge transport

above a mobility edge with the temperature-dependent measurements provides a greater

degree of confidence to any conclusion. Moreover, the latter approach has the advantage

of eventually giving the DOS as a function of energy relative to the mobility edge.

We now show that the activation energy Ea(Vg) of the field-effect conductivity as

defined by

σ(Vg) = Aexp(−Ea

kT

)(7.5)

and as determined with Arrhenius plots is related to the band shift eV0 at the insulator-

semiconductor interface. Following Boltzmann’s approximation, the field-effect conduc-

tivity

σ(Vg) = eµ0Pf ree (7.6)

may be written as

σ(Vg) = eµ0

Z d

0p f reedx =

= eµ0NV exp(−EV −EF

kT

)×Z d

0exp

(eV (x)

kT

)dx. (7.7)

NV is the effective (volume) density of extended states, EV is the energetic position of the

mobility edge and EF is the Fermi energy. For the moment we consider an exponential

trap DOS

N(E) = N0 exp(

EkT0

)(7.8)

with a characteristic slope of kT0. If we assume that all the gate-induced charge is trapped,

the integration of the exponential trap DOS leads to a simple exponential dependence of

7.3 Charge transport parameters 137

the total hole density p on the potential V (x), which is

p ∝ exp(

eVkT0

). (7.9)

This approximation is not expected to cause serious errors as long as the majority of

the gate-induced charge is trapped. [139] With Eq. 7.9 it can be shown that, except for

small values of V , also the electric field F perpendicular to the insulator-semiconductor

interface exponentially depends on V , as

F ∝ exp(

eV2kT0

). (7.10)

The electric field as in Eq. 7.10 results in an expression for the free surface hole density

Pf ree in Eq. 7.6. [52] This expression is

Pf ree ≈ 2kT ε0εs

eCiUg

l2l−1

NV exp(−EV −EF

kT

)×

×[

exp(

eV0

kT

)− exp

(eV0

2lkT

)], (7.11)

where

l =T0

T. (7.12)

The second exponential term in Eq. 7.11 can safely be neglected and so we have

Pf ree ≈ Lal

2l−1NV exp

(−EV −EF − eV0

kT

), (7.13)

with

La =2kT ε0εs

eCiUg. (7.14)

The factor l/(2l− 1) is expected to be close to unity and La may be understood as the

effective thickness of the accumulation layer. [159] Deviations from an exponential trap

DOS might be considered with a variable parameter l. However, the variation of l can be


ignored when compared to the exponential term in Eq. 7.13. If we compare Eq. 7.13 with

Eq. 7.5 and Eq. 7.6, we eventually have

Ea ≈ EV −EF − eV0 = EV −E ′F . (7.15)

The measured activation energy of the field-effect conductivity Ea(Vg) is approximately

equal to the energetic difference between the Fermi level EF and the mobility edge at the

interface, as indicated in Fig. 6.2. E ′F , as defined in Eq. 7.15, is the Fermi level at the

insulator-semiconductor interface. By substituting dV0 =−dEa/e in Eq. 7.2 and Eq. 7.4,

we finally have the DOS

N(E)≈ ddEa

[ε0ε2

iεsl2 Ug

(dEa

dUg

)−1]

(7.16)

as a function of the energy E = EV −E ′F ≈ Ea(Vg) relative to the mobility edge.

7.3.3 Fraction of free holes and band mobility

The fraction of free holes Pf ree/Ptotal is of crucial importance since it is proportional to

the effective field-effect mobility as described by Eq. 7.1. It can readily be extracted from

temperature dependent measurements. From Eq. 7.13 and Eq. 7.15 and with the total

surface hole density

Ptotal = CiUg/e, (7.17)

we eventually have

Pf ree

Ptotal=

LaeCiUg

l2l−1

NV exp(−Ea

kT

). (7.18)

Finally, from Eq. 7.6, we see that the band mobility µ0 can be estimated with

µ0 = σ/(ePf ree). (7.19)

7.4 Results 139

7.4 Results

7.4.1 Extraction method and the influence of the contact resistance

In this section, we demonstrate the extraction of the DOS and the hole densities from a set

of gated four-terminal measurements. Moreover, we analyze the influence of the contact

resistance on these functions.

In a first step, we derived the activation energy Ea(Vg) of the four-terminal conduc-

tivity as a function of the gate voltage, according to Eq. 7.5. Fig. 7.1 shows Arrhenius

plots and the corresponding linear regression lines. We found that only currents equal to

or above ≈ 1 nA are usable in the sense that the corresponding four-terminal conductivity

follows a straight line in an Arrhenius plot. Therefore, we used all currents above 1 nA

for the extraction of the activation energy. At low gate voltages, only the measurements

at the highest temperatures were considered as a consequence of the 1 nA limit. The final

result is given in Fig. 7.2. The activation energy Ea(Vg) was then represented by a smooth

fit (red line in Fig. 7.2) in order to suppress the noise in the data.

Finally, the DOS was obtained with Eq. 7.16 and is plotted as a function of the ener-

getic distance to the mobility edge Ea(Vg)≈ EV −E ′F . For the calculations, the dielectric

constant of pentacene was assumed to be εs = 3. In order to determine Ug = |Vg−VFB|in Eq. 7.16, the flatband voltage VFB was assumed to be equal to the device onset volt-

age at room temperature. The same assumption was used in Chap. 6. The onset voltage

is the gate voltage where the drain current sharply rises when plotted on a logarithmic

scale, i.e. where the drain current becomes measurable. The flatband voltage marks the

onset of the accumulation regime and a small difference between the flatband voltage and

the onset voltage may thus exist. A scheme to extract the flatband voltage was devel-

oped for amorphous silicon-based transistors and this scheme involves the temperature-

dependence of the device off-current. [143] However, the scheme cannot be applied to our

devices because the off-currents are due to experimental limitations and are not related to

the conductivity of the pentacene film.

Fig. 7.3 (circles) shows the DOS as derived from the activation energy in Fig. 7.2.

The procedure was also applied to the same data without correcting for the contact re-

sistance, i.e. to the two-terminal conductivity. The dashed line in Fig. 7.3 is the re-

sult, highlighting the necessity to correct for the contacts. Even for long channel devices

(L = 450 µm), neglecting the contact resistance leads to significant errors in the shape and

magnitude of the DOS, even more so closer to the mobility edge.


0.0036 0.0040-22

-21

-20

-19

-18

-17

Vg = -45 V

Vg = -35 V

Vg = -25 V

Measured Linear regression

Vg = -50 V

Vg = -30 V

Vg = -40 V

Vg = -20 V

ln[

]

T -1 (K -1)Figure 7.1: Arrhenius plots of the four-terminal conductivity at various gate voltages Vg. Theactivation energy Ea(Vg) was derived from the slope of the linear regression lines.

-10 -20 -30 -40 -50

0.4

0.3

0.2

0.1 Measured Smooth fit

E a (eV

)

Vg (V)

Figure 7.2: Activation energy Ea(Vg) as determined with linear regressions according to Eq. 7.5and as verified with Arrhenius plots (Fig. 7.1). The graph also shows a smooth fit of the activationenergy (red line).

7.4 Results 141

0.4 0.3 0.2 0.1

1019

1020

1021 Four-terminal Two-terminal

DO

S (c

m-3eV

-1)

EV-E'F (eV)

Figure 7.3: Density of traps as a function of energy relative to the mobility edge (circles). Thedashed line was extracted from the same set of temperature dependent measurements, but thecontact resistance was neglected. The contact resistance can lead to significant errors in the densityof states function, especially closer to the mobility edge.

The free hole density, the total hole density and the fraction of free holes were re-

spectively obtained with Eq. 7.13, Eq. 7.17 and Eq. 7.18. We have assumed that the

effective density of extended states NV is equal to the density of the pentacene molecules,

i.e. NV = 3× 1021 cm−3 and Fig. 7.4 is the result. For the given sample at high gate

voltages, ≈ 10 % of the holes that are induced by the gate are free, i.e. only this fraction

actually contributes to the drain current.

7.4.2 Oxygen-related device degradation

This study correlates the oxygen-related degradation of the transistor characteristics with

the change of the fundamental transport parameters. We begin by presenting the char-

acteristic effects of oxygen on the pentacene transistor characteristics. The blue line in

Fig. 7.5 is a transfer characteristic measured as grown (after a high vacuum storage time

of approximately 3 weeks). The sample was then exposed to 1 bar of oxygen for 19 h

and, additionally, to 1 bar of oxygen and white light for 4 h. The red curve in Fig. 7.5 is a

measurement of the same device after the oxidation process and after an evacuation time

of 22 h at a base pressure of the order of 10−8 mbar. Fig. 7.5 contains the forward and the

reverse sweeps in both cases. Fig. 7.6 shows the corresponding four-terminal mobilities

and, for comparison, the respective two-terminal mobilities. The degradation effects are

the following: a significant degradation of the subthreshold performance, a decrease in


10 20 30 40 50107

108

109

1010

1011

1012

1013

0

3

6

9

12

15

P (c

m-2)

Ug (V)

Pfree / Ptotal

Pfree

Ptotal

P free /

Pto

tal (

%)

Figure 7.4: Total hole density Ptotal = CiUg/e, free hole density Pf ree and fraction of free holesPf ree/Ptotal at room temperature as derived from the temperature dependence of the four-terminalconductivity. A significant fraction of the total gate-induced charge is trapped even at high gatevoltages. The dashed line is the ratio Pf ree/Ptotal if the contact resistance is neglected.

on-current, a decrease in effective mobility and a shift of the transfer characteristic to-

wards more positive voltages. Also the contact resistance is increased after the oxygen

exposure. At room temperature and Vg = −50 V it increases from the as grown value of

RcontactW = 2.8×104 Ωcm to 9.2×104 Ωcm after the oxygen exposure, i.e. it increases

by a factor of 3.3. The increase in contact resistance may be due to the fact that the

contact resistance is dominated by the film resistance. In a top-contact device, the holes

must pass from the electrodes through the pentacene film to the channel at the insulator-

semiconductor interface (see Chap. 6, Fig. 6.12 and [134]). Importantly, before and after

the oxygen exposure we have a device that is not limited by the contact resistance. At

Vg = −50 V for example, the contact resistance Rcontact is ≈ 14 times smaller than the

channel resistance Rchannel prior to the oxygen exposure and ≈ 9 times smaller than the

channel resistance after the oxygen exposure.

The degradation effects can be observed when the transistor is subjected to oxygen

in the dark. It is, however, much accelerated when the oxygen exposure is carried out in

the presence of light, i.e. in the presence of activated oxygen and oxygen radicals.

The degradation of the subthreshold performance and of the field-effect mobility

is a result of oxygen-related defects that cause electrically active trap states within the

mobility gap. The shift of the transfer characteristic is due to a change of the flatband

7.4 Results 143

10 0 -10 -20 -30 -40 -5010-12

10-11

10-10

10-9

10-8

10-7

10-6

Vd = -2V

After oxygen

As grown

-I d (A)

Vg (V)

Figure 7.5: Linear regime transfer characteristic of a pentacene thin-film transistor measured asgrown (blue line) and after oxidation (red line). The graph shows the forward and the reversesweeps in both cases. The characteristic oxygen-related degradation effects are a decrease insubthreshold performance, a decrease in on-current and a shift of the transfer characteristic tomore positive voltages. The current hysteresis is essentially unaffected.

0 -10 -20 -30 -40 -500.0

0.1

0.2

0.3

0.4

2-t.4-t.

2-t.4-t.

After oxygen

As grown

eff (

cm2 /V

s)

Vg (V)

Figure 7.6: Four-terminal effective mobility (4-t.) from an as grown sample (blue pentagons) andafter the oxygen exposure (red circles). At Vg = −50 V for example, the contact-corrected field-effect mobility decreases from µe f f = 0.35 cm2/Vs to 0.17 cm2/Vs, i.e. it is reduced by a factorof 2.1. The dashed lines represent the respective two-terminal mobility (2-t.) where the contactresistance is neglected.

voltage. It is known that oxygen can cause changes of the flatband voltage in an organic

semiconductor device. [197, 198]


0 -10 -20 -30 -40 -500

1x10-7

2x10-7

3x10-7

4x10-7

5x10-7

0 -10 -20 -30 -40 -5010-12

10-11

10-10

10-9

10-8

10-7

0 -10 -20 -30 -40 -500

5x10-8

1x10-7

2x10-7

2x10-7

3x10-7

0 -10 -20 -30 -40 -5010-12

10-11

10-10

10-9

10-8

10-7

Vd = -2V

-I d (A

)

(b)

As grown

-I d (A

)

247 K257 K270 K278 K288 K297 K

-I d (A

)

Vg (V)

Vg (V)

(a)

248 K259 K270 K281 K289 K298 K

After oxygen

-I d (A

)

Vd = -2V

Vg (V)

Vg (V)

Figure 7.7: (a) Linear regime transfer characteristics at various temperatures from an as grownsample, i.e. prior to oxygen exposure. (b) Transfer characteristics at similar temperatures afterexposing the sample to 1 bar of oxygen for 23 h (19 h in the dark and 4 h in white light). Thetemperature dependence of the drain current in the subthreshold regime is much more pronouncedafter the oxygen exposure.

7.4.3 Oxygen-related traps

We now turn to the determination of the trap densities prior to and after the oxygen ex-

posure. For the temperature-dependent gated four-terminal measurements, we used a low

cooling rate (0.2− 0.25 /min) in order not to damage the sample. In Fig. 7.7 we show

the temperature dependence of the transfer characteristics prior to and after the oxygen

exposure. The temperature dependent measurements in Fig. 7.7 are from the same sample

as the measurements in Fig. 7.5 and Fig. 7.6 and were carried out shortly after these latter

measurements. The main difference after the oxygen exposure is that the temperature de-

pendence in the subthreshold regime (drain currents on a logarithmic scale) is much more

pronounced.

The DOS was extracted for both sets of measurements as described in Secs. 7.3.2

and 7.4.1. The main panel of Fig. 7.8 shows the final result on a logarithmic scale. The

difference between two adjacent data points in Fig. 7.8 corresponds to a change of 0.2 V in

the gate voltage. Some gate voltages Ug are indicated in the graph. The spacing between

7.4 Results 145

0.5 0.4 0.3 0.2 0.1

1018

1019

1020

1021

+O2

After oxygen

As grown 0.4 0.3 0.20

1x1020

2x1020

40 V

40 V

30 V

15 V

30 V

15 V

8 VUg = 8 V

DO

S (c

m-3eV

-1)

EV-E'F (eV)

Figure 7.8: Main panel: DOS as a function of energy relative to the mobility edge on a logarithmicscale. The blue pentagons are trap densities measured prior to the oxygen exposure and the redcircles are trap densities measured after the oxygen process. The oxidation of the pentacene filmleads to a significant increase in traps that are somewhat deeper in energy. The corresponding gatevoltage Ug above flatband is indicated. The inset shows the deeper traps on a linear scale.

the data points decreases as the gate voltage is increased, since at high gate voltages it is

increasingly difficult to shift the Fermi level E ′F at the insulator-semiconductor interface

towards the mobility edge due to the increased trap density.

We keep in mind that even in an ideal (trap-free) metal-insulator-semiconductor

(MIS) structure the interface potential increases with gate voltage more rapidly at low

gate voltages, than at high gate voltages. This is a screening effect. The screening de-

pends on the total charge in the device and it increases with gate voltage.

The oxygen exposure leads to a significant increase in the density of traps that are

somewhat deeper in energy (Fig. 7.8). The inset in Fig. 7.8 shows the deeper traps on

a linear scale. In Fig. 7.9 we show the difference of the trap densities prior to and after

the oxygen exposure on a linear scale (full and dash-dotted black line). We assume that

our method allows for a determination of the DOS to an accuracy of 5 %, and this is

indicated by the error bars in Fig. 7.9. At energies ≤ 0.25 eV from the mobility edge,

the difference in the DOS is comparable to or smaller than the estimated error and so, for

energies ≤ 0.25 eV, the DOS is essentially unaffected by the oxygen exposure. At larger

energies, however, the oxygen exposure leads to a broad peak of trap states. The dashed

red line in Fig. 7.9 is a Gaussian fit of the experimental data for energies ≥ 0.25 eV. Good

agreement is achieved with the Gaussian fit. The peak is centered at EC = 0.28 eV and


0.40 0.35 0.30 0.25 0.200

1x1019

2x1019

3x1019 Pentacene andoxygen

EC=0.28 eV

Diff

eren

ce in

DO

S (c

m-3eV

-1)

EV-E'F (eV)

Measured Gaussian fit

Figure 7.9: Difference of the DOS prior to and after the oxygen exposure (full and dash-dottedblack line). A relative error of 5 % is assumed for the determination of a trap DOS and this isindicated by the error bars. The oxygen exposure leads to a broad peak of trap states. A Gaussianfit for energies ≥ 0.25 eV (dashed red line) gives good agreement with the measured curve. Thepeak is centered at EC = 0.28 eV. At energies≤ 0.25 eV, the difference in the DOS is comparable toor smaller than the estimated error and the DOS is essentially unaffected by the oxygen exposure.

from the area under the peak we estimate a volume trap density of ≈ 4× 1018 cm−3.

With a density of the pentacene molecules of 3×1021 cm−3, this gives an oxygen-related

impurity concentration of ≈ 0.1 %, provided that each impurity results in one trap.

The DOS close to the mobility edge is well described by a single exponential func-

tion. A fit for energies ≤ 0.22 eV gives essentially identical characteristic slopes prior to

and after the oxygen exposure, i.e. respectively kT0 = 47 meV and kT0 = 48 meV. These

values are in good agreement with characteristic slopes from pentacene-based field-effect

transistors in the literature. A characteristic slope of kT0 = 40 meV is reported, as de-

termined by simulating the measured transfer characteristics of pentacene thin-film tran-

sistors. [152] Characteristic slopes of kT0 = 32− 37 meV were derived from pentacene

thin-film transistors with another device simulation program. [155,157] Yet, another pro-

gram gives a slope of kT0 = 100 meV for pentacene thin-film transistors. [156] However,

the band mobility µ0 needs to be fixed for the simulations and, depending on the choice

of the band mobility, slopes of up to 400 meV are also used. [156] In Chap. 6 we have

extracted characteristic slopes of kT0 = 32 meV shortly after the evaporation of the pen-

tacene and of kT0 = 37 meV in the aged thin film with reduced trap density with the initial

scheme by Grünewald et al. For a pentacene single-crystal device, a characteristic slope

of kT0 = 109 meV is reported. [160]

7.4 Results 147

0 10 20 30 40 500

4

8

12

16

0

4

8

12

16

2x1011

4x1011

6x1011

1.9

P fre

e / P

tota

l (%

)

Ug (V)

After oxygen

As grown

After oxygen

As grown

P free (

cm-2)

9 V

Figure 7.10: Upper panel: free hole density Pf ree prior to and after the oxygen exposure as derivedfrom the temperature-dependent gated four-terminal measurements. After the oxygen exposure thecurve is shifted by 9 V. The magnitude of the shift is closely linked to the density of the additionaltraps. Additional trapped holes with a density of 5× 1018 cm−3 can be estimated from the shift,which is highly consistent with the trap density estimated from an integration of the peak in Fig 7.9(4×1018 cm−3). Lower panel: corresponding fraction of free holes functions Pf ree/Ptotal prior toand after the oxygen exposure. At a given gate voltage Ug, the fraction of free holes is significantlyreduced after the oxygen exposure. At Ug = 40 V for example, the fraction of free holes drops from15 % to 8 %, i.e. it is reduced by a factor of 1.9.

7.4.4 Trap induced changes in the free hole density

The upper panel in Fig. 7.10 shows the free surface hole density (Eq. 7.13) prior to and

after the oxygen exposure from the two sets of temperature dependent measurements in

Fig. 7.7. The parameter l = T0/T was calculated with the characteristic slopes mentioned

above: for room temperature l = 1.9 (l/(2l− 1) = 0.68) prior to and after the oxygen

exposure. At sufficiently high gate voltages, the free hole density as a function of gate

voltage is shifted by 9 V towards higher gate voltages as a consequence of the oxygen

exposure.

The corresponding fractions of the free holes Pf ree/Ptotal were extracted according

to Eq. 7.18 and are shown in the lower panel of Fig. 7.10. The fraction of the free holes

changes significantly due to the oxygen exposure. At Ug = 40 V for example, 15 % of


all the induced holes are free prior to the oxygen exposure and this fraction drops to 8 %

after the oxygen exposure. It is reduced by a factor of 1.9. A fraction of free holes of

8− 15 % at 40 V is in good agreement with values for pentacene thin-film transistors

found in the literature. A fraction of free holes around 10 % is specified for comparable

total gate-induced charge densities. [155]

7.4.5 Stability of the oxygen-related defects

The DOS after the oxygen exposure was measured after a re-evacuation time of ≈ 22 h.

In order to elucidate the stability of the oxygen-related traps, the sample was kept in the

prober station at 10−8 mbar for an additional 7 days period. After that time, temperature

dependent gated four-terminal measurements were carried out and the DOS was extracted.

After these measurements, the sample was kept at 10−8 mbar for another 10 days. The

pentacene films were then slowly heated to 50 C at a rate of 0.2 /min with an electrical

heating element at the cryostat. The temperature was held for 2 h and the sample was

then left to cool down. The same procedure was repeated with a final temperature of

70 C. Because of the low heating and even lower cooling rates, the whole process took

3 days and the effective heating time was very long. Fig. 7.11 shows the DOS after

a re-evacuation time of ≈ 1 day (same as in Fig. 7.8), 8 days and 22 days, the latter time

including the heating procedure. The DOS functions are very similar and we can conclude

that the oxygen-related trap states are very stable.

7.5 Discussion

7.5.1 Effect of oxygen on the trap DOS

The DOS as extracted from the measurements of the as grown sample is of particular

interest, since the sample was kept at a pressure of the order of 10−8 mbar all along. The

trap densities are relatively high (1018 − 1021 cm−3eV−1) with a rather smooth depen-

dence on energy. In the case of the measurements on the as grown samples, the effect of

ambient gases can be excluded. It should be kept in mind that we used pentacene powder

that was re-crystallized in high vacuum twice. We conclude that the “amorphouslike”

trap DOS measured with an as grown sample is mainly due to structural defects within

the pentacene. Trap states on the surface of the gate dielectric, caused by certain chemical

groups for example, may also contribute to the states that are deeper in energy.

7.5 Discussion 149

0.5 0.4 0.3 0.2 0.1

1018

1019

1020

1021

DO

S (c

m-3eV

-1)

EV-E'F (eV)

1 day 8 days 22 days + heating

Figure 7.11: Trap densities after oxygen exposure. The re-evacuation time after the oxygen ex-posure is 1 day (full red line), 8 days (dashed black line) and 22 days (dash-dotted green line). Theoxygen-related traps are very stable, i.e. the DOS functions coincide. Prior to the last characteri-zation after 22 days, the sample was slowly heated to temperatures up to 70 C.

When pentacene is exposed to oxygen, the gas migrates into the pentacene film and

interacts with the pentacene molecules. This effect is expected to be accelerated if, in the

presence of light, oxygen is activated and its dissociation is aided. We observe signifi-

cant and irreversible changes in the transfer characteristics and in the DOS caused by the

oxygen exposure. It should be kept in mind, however, that several hours of exposure to

1 bar of oxygen are necessary in order to observe these changes. Consequently, pentacene

thin-films are not very sensitive towards oxidation.

The oxygen exposure leads to a broad peak of trap states centered at 0.28 eV, as

shown in Fig. 7.9. This suggests the degradation mechanism to be dominated by the cre-

ation of a specific oxygen-related defect. The large width of the peak (0.16 eV) is thought

to result from local structural disorder that modifies the on-site energy of the oxygen-

affected molecules. As a matter of fact, very similar arguments are used to explain the

smooth distribution of trap states in hydrogenated amorphous silicon. Even small devia-

tions in the local structure of a defect lead to a different electronic state (see Fig. 2.8(a) in

Chap. 2 and [199]).

Theoretical studies predict various types of oxygen-related defects in pentacene.

[86, 200] For example, one study predicts oxygen defects, in which a H atom of a pen-

tacene molecule is replaced by an oxygen atom to form a C22H13O molecule. [86] The

oxygen forms a double bond with the respective C atom and the pz orbital at this atom


no longer participates in the π-electron system of the pentacene molecule. The oxidation

at the middle ring is shown to be energetically most favourable. These oxygen defects

are expected to lead to trap states in the mobility gap. [86] Another theoretical study pre-

dicts other oxygen defects. [200] An example is a single oxygen intermolecular bridge

where a single oxygen atom is covalently bound to the carbon atoms on the center rings

of two neighbouring pentacene molecules. This defect, for instance, is calculated to lead

to electrically active traps at 0.33 and 0.4 eV above the valence band maximum. [200]

7.5.2 Influence of oxygen-related traps on the field-effect mobility

It is immediately plausible that the oxygen-related traps which are somewhat deeper in

energy result in a degradation of the subthreshold performance of the thin-film transistors.

We do, however, also observe a significantly decreased field-effect mobility after oxygen

exposure. This can be understood as follows. The deep traps created by the oxidation

need to be filled at first and the position of the Fermi level lags behind the position of

the Fermi level before the oxygen exposure. This is indicated in Fig. 7.8 by labeling the

corresponding gate voltages Ug. At the same gate voltage (which is proportional to the

total gate-induced hole density), the Fermi level is further away from the mobility edge.

The fraction of free holes, however, exponentially depends on the position of the Fermi

level E ′F at the insulator-semiconductor interface (Eq. 7.18). The field-effect mobility as

described by Eq. 7.1 is proportional to Pf ree/Ptotal and so a reduction in the fraction of free

holes readily affects the field-effect mobility. At Ug = 40 V, for example, the fraction of

free holes is reduced by a factor of 1.9 after the oxygen exposure (main panel of Fig. 7.10).

In addition, it is quite possible that the mobility µ0 of the delocalized charge above

the mobility edge is changed after the oxygen exposure. With Eq. 7.19, this mobility is

estimated to be µ0 = 1.2 cm2/Vs prior to the oxygen exposure and µ0 = 0.95 cm2/Vs after

the oxygen exposure. We have a reduction by a factor of 1.3 and a change of the “intrinsic”

charge transport. Conclusively, the major cause for the reduction of the effective field-

effect mobility is occupancy statistics, and a reduction of the mobility above the mobility

edge also plays a role.

The reduction in the mobility µ0 might be explained by a scattering of charge carriers

at the oxygen-related defects. Another indication that scattering plays a role in organic

field-effect transistors is the fact that the mobilities µ0 that we extract are lower than the

best field-effect mobilities (up to 5 cm2/Vs) from pentacene thin-film transistors. [14] As

already mentioned in Chap. 2, repeated purification of pentacene has been shown to lead

7.5 Discussion 151

to very high mobilities in pentacene single crystals. [87] This effect is attributable to the

reduction of the concentration of the oxidized pentacene species 6,13-pentacenequinone

(Fig. 2.15) which degrades the transport properties by scattering the charge carriers. [87]

7.5.3 Consistency check: trapped holes vs. traps

The upper panel in in Fig. 7.10 shows that the oxidation causes a shift of the curve for

the free hole density Pf ree by ∆Ug = 9 V. The same free hole density Pf ree is realized for

different total hole densities Ptotal . Clearly, for an identical number of free holes, the

difference in the number of total holes must be attributed to a difference in the number

of the trapped holes. Consequently, due to the oxygen-related traps, we have additional

holes that are trapped with a density of Ci∆Ug/e = 7.5× 1011 cm−2. Except at very low

gate voltages above the flatband voltage, the charge in an organic field-effect transistor is

concentrated at the insulator-semiconductor interface. As explained above, our extraction

scheme only considers currents above 1 nA. Therefore, it is reasonable to assume that the

holes are trapped in a region at the insulator-semiconductor interface with a thickness of

the order of one molecular layer (≈ 1.5 nm for pentacene). This gives a volume density

of trapped holes of ≈ 5× 1018 cm−3. From the area under the peak in the trap DOS, we

have derived a trap density of ≈ 4×1018 cm−3 which is in very good agreement with the

density of the trapped holes.

7.5.4 Deep traps and device performance

The results in this chapter reveal how an increase in the density of deeper traps can sig-

nificantly affect the field-effect mobility. The influence of deep traps on the device char-

acteristics is of most general concern because deep traps can have various origins. Trap

states caused by the surface of the gate dielectric, for example, are expected to be elec-

tronically deep traps. In Chap. 5 we have seen that the use of a polymeric gate dielectric

not only leads to an improved subthreshold swing but can also result in improved mobili-

ties. Importantly, in the case of the SC-FET’s, the semiconductor is grown separately, and

growth-related effects can be excluded. Similar observations are reported in [103,180]. In

the light of the present study, it seems possible that these effects can be solely understood

with transport in extended states above a mobility edge and a distribution of trap states:

a reduced number of deep traps leads to an increased number of free carriers above the

mobility edge and to a higher mobility of that charge.


7.6 Conclusions

Pentacene-based thin film transistors were characterized without exposing the samples to

ambient air (as grown) and after exposure to oxygen in combination with white light. The

exposure of the pentacene to the oxidizing agent causes a degradation of the subthreshold

performance, a decrease in field-effect mobility, a shift of the flatband voltage and an

increased contact resistance.

Contact-corrected trap state functions were extracted from temperature dependent

gated four-terminal measurements. We show that the exposure to oxygen leads to a broad

peak of trap states centered at 0.28 eV. The emergence of a peak indicates the process to

be dominated by the creation of a specific oxygen-related defect. The large width of the

peak is a result of the energetically different surroundings induced by structural disorder.

The oxygen defects are very stable and are likely to be caused by pentacene molecules

with covalently bound oxygen.

The decrease in field-effect mobility is caused by the oxygen-related deep traps.

These states are filled upon increasing the gate voltage and the Fermi level at the inter-

face lags behind the position it has in as-deposited samples. This leads to a significantly

smaller fraction of free holes. The magnitude of the shift in the free hole function is highly

consistent with the density of the oxygen-related traps (≈ 4× 1018 cm−3), as estimated

from the difference in the trap DOS prior to and after the oxygen exposure. In addition,

the oxygen exposure leads to a decrease of the mobility of the charge carriers above the

mobility edge.

The results can be seen from a more general point of view. At first, the tempera-

ture dependent measurements are self-consistent with the assumption of a mobility edge

or transport level, thus contributing to an understanding of charge transport in organic

semiconductors. Moreover, they are an example of the way in which deeper traps can

influence the effective field-effect mobility.

Theoretical studies may help to identify the oxygen defect and organic synthetic

chemistry may soon find a way to tailor organic semiconductors where the creation of

defects by oxidation is completely inhibited.

8 Summary, conclusions and outlook

We carried out experiments in order to contribute to a quantification, identification and

elimination of electronic trap states in organic field-effect transistors based on oligomeric

semiconductors and to a better understanding of the charge transport mechanism in these

materials.

In essence, the experimental findings and conclusions are the following: we

have seen that it is quite unlikely to obtain field-effect mobilities ≥ 1 cm2/Vs with a

new oligomeric semiconductor, even if single crystals are employed in the transistors

(Fig. 3.2). However, most new oligomers studied for this thesis led to operating transis-

tors with mobilities ranging from 10−7 cm2/Vs to 0.1 cm2/Vs. Mobilities in single crystal

field-effect transistors (SC-FET’s) were found to be always higher than in thin-film tran-

sistors (TFT’s) by a factor of 3− 670, depending on the material (as long as operating

devices of both types could be made). This is likely due to increased structural disorder in

the evaporated films, as compared to the single crystals. As a matter of fact, the detailed

study of 7,14-Diphenyl-chromeno[2,3-b]xanthene (DPCX) in Chap. 4 shows very clearly

that the effective field-effect mobility critically depends on structural order (Fig. 4.2). The

mobility is 0.16 cm2/Vs in SC-FET’s, 0.01 cm2/Vs in polycrystalline films grown on OTS

and 2×10−5 cm2/Vs in amorphous films grown on bare SiO2.

The few known oligomeric semiconductors that do lead to mobilities ≥ 1 cm2/Vs

(e.g. pentacene) may have a very strong tendency to crystallize and to form high quality

crystals and films with few structural defects. This interpretation would be consistent with

the X-ray diffraction measurements in Fig 4.6. For the polycrystalline film of DPCX, the

peaks can only be observed up to the second order and the intensity of the second order

peaks is rather weak. For pentacene films 4-5 orders of the diffraction peaks are readily

observed. [69, 201] The structural disorder would cause traps, thus reducing the effective

mobility. The intrinsic mobility as derived from the orbital overlap of adjacent molecules


in the crystal is also important for the field-effect mobility of a given material. It is not

unreasonable to suggest that the orbital overlap (interaction strength) is related not only

to the intrinsic mobility but also to the tendency to crystallize and to form high quality

crystals and films.

In Chap. 5 we have seen that SC-FET’s with rubrene or pentacene can have a very

high electrical stability if an amorphous fluoropolymer gate dielectric is used. From

these measurements we conclude that electrical instability in organic field-effect tran-

sistors is not an intrinsic phenomena and is not e.g. due to the formation of bipolarons.

The experimental work in Chap. 5 identifies two important causes of gate bias stress ef-

fects in organic field-effect transistors. Since gate bias stress effects are somewhat more

pronounced in pentacene thin-film transistors than in the highly stable pentacene single

crystal transistors with the same gate dielectric, we can conclude that certain structural

defects within the semiconductor are a cause of electrical instability. Moreover, since

SC-FET’s with comparable single crystals and other gate dielectrics are significantly less

stable [103, 106], the choice of an unsuitable gate dielectric is the second cause of gate

bias stress effects. The gate dielectric may lead to electrical instability because of the

presence of electrically active traps on the surface of the gate dielectric. These traps may

also be created by an electrochemical reaction which would be driven by applying a gate

voltage to the device. For example, adsorbed water may react with the organic semicon-

ductor or with the gate dielectric and thus form traps. Moreover, dipolar disorder caused

by a polar gate dielectric may also lead to long-lived traps that cause electrical instability.

The highly hydrophobic, low dielectric constant, amorphous fluoropolymer gate di-

electric CytopT M works extremely well in combination with two organic semiconductors

of different chemical nature, i.e. pentacene and rubrene. We arrive at the conclusion that

the nature of the gate dielectric is much more important than the chemistry of the organic

semiconductor in order to obtain transistors with a high electrical stability. The electrical

stability may to some extend depend on the chemical nature of the organic semiconductor.

For example, passivating the most reactive sites in the pentacene molecule by means of

synthetic chemistry may impeade the electrochemical reaction of the organic semicon-

ductor with water. This leads to better semiconductors in that respect (e.g. DPCX in

Chap. 4).

Another aspect of the study in Chap. 5 is the issue of contact resistance. We have

pointed out the substantial influence of parasitic contact resistances in the case of high

quality single crystal transistors. Parasitic contact resistances can significantly affect the

155

transfer characteristics and the temperature dependence of the drain current. It is therefore

very important to correct for parasitic contact resistances when extracting the trap DOS

and other transport parameters from the characteristics of organic field-effect transistors.

This correction significantly contributes to the quality of the studies in Chap. 6 and 7.

The new device fabrication and characterization system allows for measurements

with a high degree of control. For example, it allows for the electrical characterization

of organic field-effect transistors by temperature-dependent gated four-terminal measure-

ments without any unintentional air exposure between the transistor fabrication and char-

acterization. The experiments with the system in Chap. 6 reveal that the performance of

pentacene-based field-effect transistors improves with time. As for the causes of elec-

trical instability, it is certainly important to understand the origin of these changes for a

successful commercialization of organic field-effect transistors. We show that the perfor-

mance improvement is due to structural changes within the pentacene film. The relevant

structural defects that anneal at room temperature are related to shallow traps with an en-

ergy of less than 0.2 eV from the mobility edge/transport level. The effective field-effect

mobility critically depends on the density of these traps. The exact microscopic nature

of the relevant structural defects is not known, but it seems plausible that some of these

defects are sliding defects. Once more, we can confirm the importance of structural order

of the semiconducting layer.

For the study of oxygen-related traps in pentacene films described in Chap. 7 we

have developed an improved scheme to extract the spectral density of trap states and other

important transport parameters from temperature-dependent gated four-terminal measure-

ments in an unambiguous and straightforward fashion. The measurements on pentacene

thin-film transistors confirm the validity of assuming charge transport above a mobility

edge or in a transport level and a distribution of localized states below the mobility edge.

Moreover, we demonstrate that the trap DOS in pentacene thin films is indeed quite well

described by a single exponential function of energy. Most of the trap states within the

pentacene films originate from structural defects. We show that the exposure of pentacene

films to oxygen results in the formation of a peak of trap states centered at 0.28 eV from

the mobility edge. The traps are thought to result from a specific oxygen-induced impu-

rity where oxygen is covalently bound. The large width of the peak (0.16 eV) is likely

caused by local structural disorder that modifies the on-site energy of the oxygen-affected

molecules.


The oxygen-related deep traps reduce the effective mobility by decreasing the num-

ber of free carriers at a given gate voltage and their mobility above the mobility edge.

This may also be true for deep traps of other origin, e.g. certain chemical groups on the

surface of the gate dielectric. Provided that all other parameters remain constant, we ex-

pect a correlation of general validity between the subthreshold swing and the effective

field-effect mobility.

We would like to suggest several directions of possible studies in order to continue

the experimental work described in this thesis. The environmental and electrical stability

of DPCX could be further tested. This would best be done with DPCX/CytopT M single

crystal transistors and by controlled gas exposure in the prober station of the device fab-

rication and characterization system. It would be interesting to compare the stability of

DPCX with the stability of pentacene in a systematic fashion.

We believe that the extraction scheme that we have developed in Chap. 7 is a valu-

able tool to further elucidate the origin of trap states and the charge transport mechanism

in organic field-effect transistors, both with an oligomeric and with a polymeric semi-

conductor. Specifically, it should be rewarding to use the extraction scheme in order to

calculate the trap DOS and the “intrinsic” mobility of several different organic semicon-

ductors in field-effect transistors. It would be interesting to see to what extend the trap

DOS and the intrinsic mobility vary from material to material, also keeping in mind that

for an intrinsic mobility of e.g. µ0 = 0.01 cm2/Vs trap-controlled band transport would

certainly not be a self-consistent description anymore. The extraction scheme can also

be adapted to the bipolar case and temperature-dependent measurements on bipolar field-

effect transistors e.g. with a polymeric semiconductor [202,203] would thus allow for the

determination of the trap DOS in the whole bandgap. Moreover, it would be possible to

compare the “intrinsic” electron and hole mobilities. Furthermore, it may be worth a try to

correlate the spectral density of trap states from pentacene films with the film morphology

(e.g. grain size), as seen by AFM. Different films could be obtained by varying the depo-

sition conditions (e.g. substrate temperature and deposition rate). Since charge transport

takes place in the first few molecular layers of the organic films, we would however have

to work with ultrathin pentacene films (e.g. 5 nm thick).

Finally, it seems promising to improve the contacts of single crystal/CytopT M tran-

sistors. We have seen that the transistor characteristics from devices made with the “flip-

crystal approach” have serious contact problems. The contacts might be improved by

157

changing the device geometry and/or the contact material. Other metals than gold or

conducting polymers such as PEDOT:PSS [204] might be useful.

Bibliography

[1] M. Pope and C. E. Swenberg. Electronic processes in organic crystals and polymers. 2nd ed., Oxford University Press, New

York (1999)

[2] J. S. E. Lilienfeld. “Method and apparatus for controlling electric currents”. US Patent No. 1,745,175 (Jan. 28, 1930)

[3] S. M. Sze. Physics of semiconductor devices. Wiley, New York (1981)

[4] F. Ebisawa, T. Kurokawa, and S. Nara. “Electrical properties of polyacetylene/polysiloxane interface”. J. Appl. Phys. 54, 3255

(1983)

[5] J. H. Burroughes, C. A. Jones, and R. H. Friend. “New semiconductor device physics in polymer diodes and transistors”.

Nature 335, 137 (1988)

[6] J. H. Burroughes, R. H. Friend, and P. C. Allen. “Field-enhanced conductivity in polyacetylene - construction of a field-effect

transistor”. J. Phys. D 22, 956 (1989)

[7] A. Tsumura, H. Koezuka, and T. Ando. “Macromolecular electronic device: Field-effect transistor with a polythiophene thin

film”. Appl. Phys. Lett. 49, 1210 (1986)

[8] H. Koezuka, A. Tsumura, and T. Ando. “Field-effect transistor with polythiophene thin film”. Synth. Metal. 18, 699 (1987)

[9] A. Tsumura, H. Koezuka, and T. Ando. “Polythiophene field-effect transistor: Its characteristics and operation mechanism”.

Synth. Metal. 25, 11 (1988)

[10] A. Assadi, C. Svensson, M. Willander, and O. Inganäs. “Field-effect mobility of poly(3-hexylthiophene)”. Appl. Phys. Lett.

53, 195 (1988)

[11] R. Madru, G. Guillaud, M. Al Sadoun, M. Maitrot, J.-J. André, J. Simon, and R. Even. “A well-behaved field effect transistor

based on an intrinsic molecular semiconductor”. Chem. Phys. Lett. 145, 343 (1988)

[12] G. Horowitz, D. Fichou, X. Peng, Z. Xu, and F. Garnier. “A field-effect transistor based on conjugated alpha-sexithienyl”.

Solid State Commun. 72, 381 (1989)

[13] J. M. Shaw and P. F. Seidler. “Organic electronics: Introduction”. IBM J. Res. & Dev. 45, 3 (2001)

160 Bibliography

[14] T. W. Kelley, D. V. Muyres, P. F. Baude, T. P. Smith, and T. D. Jones. “High performance organic thin film transistors”. Mat.

Res. Soc. Symp. Proc. 771, L6.5.1 (2003)

[15] X.-H. Zhang, B. Domercq, and B. Kippelen. “High-performance and electrically stable C60 organic field-effect transistors”.

Appl. Phys. Lett. 91, 092114 (2007)

[16] J. Takeya, J. Kato, K. Hara, M. Yamagishi, R. Hirahara, K. Yamada, Y. Nakazawa, S. Ikehata, K. Tsukagoshi, Y. Aoyagi,

T. Takenobu, and Y. Iwasa. “In-crystal and surface charge transport of electric-field-induced carriers in organic single-crystal

semiconductors”. Phys. Rev. Lett. 98, 196804 (2007)

[17] S. K. Park, T. N. Jackson, J. E. Anthony, and D. A. Mourey. “High mobility solution processed 6,13-bis(triisopropyl-

silylethynyl) pentacene organic thin film transistors”. Appl. Phys. Lett. 91, 063514 (2007)

[18] C. D. Sheraw, T. N. Jackson, D. L. Eaton, and J. E. Anthony. “Functionalized pentacene active layer organic thin-film

transistor”. Adv. Mater. 15, 2009 (2003)

[19] A. Salleo, T. W. Chen, A. R. Völkel, Y. Wu, P. Liu, B. S. Ong, and R. A. Street. “Intrinsic hole mobility and trapping in a

regioregular poly(thiophene)”. Phys. Rev. B 70, 115311 (2004)

[20] I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. MacDonald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang,

M. L. Chabinyc, R. J. Kline, M. D. McGehee, and M. F. Toney. “Liquid-crystalline semiconducting polymers with high charge-

carrier mobility”. Nature Mater. 5, 328 (2006)

[21] K. H. Probst and N. Karl. “Energy levels of electron and hole traps in the bandgap of doped anthracene crystals”. Phys. Status

Solidi A 27, 499 (1975)

[22] E. A. Silinsh and V. Cápek. Organic molecular crystals. AIP Press, New York (1994)

[23] W. Warta and N. Karl. “Hot holes in naphthalene - high, electric-field-dependent mobilities”. Phys. Rev. B 32, 1172 (1985)

[24] A. Troisi and G. Orlandi. “Charge transport regime of crystalline organic semiconductors: diffusion limited by thermal off-

diagonal disorder”. Phys. Rev. Lett. 96, 086601 (2006)

[25] E. Heilbronner and H. Bock. Das HMO-Modell und seine Anwendung. Verl. Chemie, Weinheim (1970)

[26] C. Mortimer. Chemie. Georg Thieme Verlag, Stuttgart (1996)

[27] W. L. Kalb. Structure-performance relationship in pentacene-based thin-film transistors. Master’s thesis, RWTH Aachen

(2004)

[28] A. Gavezzotti and G. R. Desiraju. “A systematic analysis of packing energies and other packing parameters for fused-ring

aromatic hydrocarbons”. Acta Cryst. B 44, 427 (1988)

[29] G. R. Desiraju and A. Gavezzotti. “Crystal-structures of polynuclear aromatic-hydrocarbons - classification, rationalization

and prediction from molecular structure”. Acta Cryst. B 45, 473 (1989)

[30] R. B. Campbell, J. Trotter, and J. M. Robertson. “Crystal and molecular structure of pentacene”. Acta Cryst. 14, 705 (1961)

Bibliography 161

[31] C. C. Mattheus, A. B. Dros, J. Baas, A. Meetsma, J. L. de Boer, and T. T. M. Palstra. “Polymorphism in pentacene”. Acta

Crystallogr., Sect. C: Cryst. Struct. Commun. 57, 939 (2001)

[32] T. Siegrist, C. Besnard, S. Haas, M. Schiltz, P. Pattison, D. Chernyshov, B. Batlogg, and C. Kloc. “A polymorph lost and found:

the high-temperature crystal structure of pentacene”. Adv. Mater. 19, 2079 (2007)

[33] R. Ruiz, B. Nickel, N. Koch, L. C. Feldman, R. F. Haglund, A. Kahn, and G. Scoles. “Pentacene ultrathin film formation on

reduced and oxidized Si surfaces”. Phys. Rev. B 67, 125406 (2003)

[34] J. L. Brédas, J. P. Calbert, D. A. da Silva Filho, and J. Cornil. “Organic semiconductors: A theoretical characterization of the

basic parameters governing charge transport”. Proc. Natl. Acad. Sci. 99, 5804 (2002)

[35] P. Puschnig and C. Ambrosch-Draxl. “Density-functional study for the oligomers of poly(para-phenylene): Band structures

and dielectric tensors”. Phys. Rev. B 60, 7891 (1999)

[36] D. Nabok, P. Puschnig, C. Ambrosch-Draxl, O. Werzer, R. Resel, and D.-M. Smilgies. “Crystal and electronic structures of

pentacene thin films from grazing-incidence x-ray diffraction and first-principles calculations”. Phys. Rev. B 76, 235322 (2007)

[37] J. E. Anthony. “The larger acenes: Versatile organic semiconductors”. Angew. Chem. Int. Ed. 47, 452 (2008)

[38] T. Holstein. “Studies of polaron motion part 1. The molecular crystal model”. Ann. Phys. 8, 325 (1959)

[39] T. Holstein. “Studies of polaron motion part 2. The small polaron”. Ann. Phys. 8, 343 (1959)

[40] K. Hannewald, V. M. Stojanovic, J. M. T. Schellekens, and P. A. Bobbert. “Theory of polaron bandwidth narrowing in organic

molecular crystals”. Phys. Rev. B 69, 075211 (2004)

[41] G. B. Street and W. D. Gill. “Photoconductivity and drift mobilities in single crystal realgar (As4S4)”. Phys. Stat. Sol. 18, 601

(1966)

[42] D. J. Gibbons and W. E. Spear. “Electron hopping transport and trapping phenomena in orthorhombic sulphur crystals”. J.

Phys. Chem. Solids 27, 1917 (1966)

[43] J. Yamashita and T. Kurosawa. “On electronic current in NiO”. J. Phys. Chem. 5, 34 (1958)

[44] H. Sirringhaus. “Device physics of solution-processed organic field-effect transistors”. Adv. Mater. 17, 2411 (2005)

[45] V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R. Silbey, and J. L. Bredas. “Charge transport in organic

semiconductors”. Chem. Rev. 107, 926 (2007)

[46] G. Nan, L. Wang, X. Yang, Z. Shuai, and Y. Zhao. “Charge transfer rates in organic semiconductors beyond first-order

perturbation: From weak to strong coupling regimes”. J. Chem. Phys. 130, 024704 (2009)

[47] A. Troisi and G. Orlandi. “Dynamics of the intermolecular transfer integral in crystalline organic semiconductors”. J. Phys.

Chem. A 110, 4065 (2006)

[48] A. Troisi. “Prediction of the absolute charge mobility of molecular semiconductors: the case of rubrene”. Adv. Mater. 19, 2000

(2007)

162 Bibliography

[49] A. Troisi. “Charge dynamics through pi-stacked arrays of conjugated molecules: effect of dynamic disorder in different

transport/transfer regimes”. Molecular Simulation 32, 707 (2006)

[50] H. A. v. Laarhoven, C. F. J. Flipse, M. Koeberg, M. Bonn, E. Hendry, G. Orlandi, O. D. Jurchescu, T. T. M. Palstra, and

A. Troisi. “On the mechanism of charge transport in pentacene”. J. Chem. Phys. 129, 044704 (2008)

[51] C. Krellner, S. Haas, C. Goldmann, K. P. Pernstich, D. J. Gundlach, and B. Batlogg. “Density of bulk trap states in organic

semiconductor crystals: discrete levels induced by oxygen in rubrene”. Phys. Rev. B 75, 245115 (2007)

[52] G. Horowitz, R. Hajlaoui, and P. Delannoy. “Temperature dependence of the field-effect mobility of sexithiophene -

Determination of the density of traps”. J. Phys. III France 5, 355 (1995)

[53] F. Schauer. “Temperature dependent field effect in organic-based thin-film transistor and its spectroscopic character”. J. Appl.

Phys. 86, 524 (1999)

[54] J. M. Marshall. “Carrier diffusion in amorphous semiconductors”. Rep. Prog. Phys. 46, 1235 (1983)

[55] R. J. Chesterfield, J. C. McKeen, C. R. Newman, P. C. Ewbank, D. A. da Silva Filho, J.-L. Brédas, L. L. Miller, K. R. Mann,

and C. D. Frisbie. “Organic thin film transistors based on n-alkyl perylene diimides: Charge transport kinetics as a function of

gate voltage and temperature”. J. Phys. Chem. B 108, 19281 (2004)

[56] P. W. Anderson. “The size of localized states near the mobility edge”. Proc. Nat. Acad. Sci. USA 69 (5), 1097 (1972)

[57] N. F. Mott and E. A. Davis. Electronic processes in non-crystalline materials. Clarendon Press, Oxford (1971)

[58] A. Miller and E. Abrahams. “Impurity conduction at low concentrations”. Phys. Rev. 120, 745 (1960)

[59] V. I. Arkhipov, E. V. Emelianova, and G. J. Adriaenssens. “Effective transport energy versus the energy of most probable jumps

in disordered hopping systems”. Phys. Rev. B 64, 125125 (2001)

[60] V. I. Arkhipov, P. Heremans, E. V. Emelianova, G. J. Adriaenssens, and H. Bässler. “Charge carrier mobility in doped

semiconducting polymers”. Appl. Phys. Lett. 82, 3245 (2003)

[61] J. Veres, S. D. Ogier, S. W. Leeming, D. C. Cupertino, and S. M. Khaffaf. “Low-k insulators as the choice of dielectrics in

organic field-effect transistors”. Adv. Funct. Mater. 13, 199 (2003)

[62] J. Veres, S. Ogier, G. Lloyd, and D. de Leeuw. “Gate insulators in organic field-effect transistors”. Chem. Mater. 16, 4543

(2004)

[63] J. Owen, G. P. Sworakowski, J. M. Thomas, D. F. Williams, and J. O. Williams. “Carrier traps in ultrahigh purity single-crystals

of anthracene”. J. Chem. Soc. Far. II 70, 853 (1974)

[64] J. O. Williams and J. M. Thomas. “Lattice imperfections in organic solids .1. anthracene”. Trans. Far. Soc. 63, 1720 (1967)

[65] J. N. Sherwood. “Lattice defects in organic crystals”. Mol. Cryst. Liq. Cryst. 9, 37 (1969)

[66] S. E. Fritz, T. W. Kelley, and C. D. Frisbie. “Effect of dielectric roughness on performance of pentacene TFTs and restoration

of performance with a polymeric smoothing layer”. J. Phys. Chem. B 109, 10574 (2005)

Bibliography 163

[67] D. Knipp, R. A. Street, A. Völkel, and J. Ho. “Pentacene thin film transistors on inorganic dielectrics: morphology, structural

properties, and electronic transport”. J. Appl. Phys. 93, 347 (2003)

[68] F.-J. Meyer zu Heringdorf, M. C. Reuter, and R. M. Tromp. “Growth dynamics of pentacene thin films”. Nature 412, 517

(2001)

[69] W. Kalb, P. Lang, M. Mottaghi, H. Aubin, G. Horowitz, and M. Wuttig. “Structure-performance relationship in

pentacene/Al2O3 thin-film transistors”. Synth. Metal. 146, 279 (2004)

[70] J. A. Venables, G. D. T. Spiller, and M. Hanbucken. “Nucleation and growth of thin-films”. Rep. Prog. Phys. 47, 399 (1984)

[71] C. D. Dimitrakopoulos and R. L. Malenfant. “Organic thin film transistors for large area electronics”. Adv. Mater. 14, 99

(2002)

[72] K. P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D. J. Gundlach, B. Batlogg, A. N. Rashid, and G. Schitter. “Threshold

voltage shift in organic field effect transistors by dipole monolayers on the gate insulator”. J. Appl. Phys. 96, 6431 (2004)

[73] H.-L. Cheng, Y.-S. Mai, W.-Y. Chou, L.-R. Chang, and X.-W. Liang. “Thickness-dependent structural evolutions and growth

models in relation to carrier transport properties in polycrystalline pentacene thin films”. Adv. Funct. Mater. 17, 3639 (2007)

[74] S. Verlaak, C. Rolin, and P. Heremans. “Microscopic description of elementary growth processes and classification of structural

defects in pentacene thin films”. J. Phys. Chem. B 111, 139 (2007)

[75] J. H. Kang, D. da Silva, J. L. Bredas, and X. Y. Zhu. “Shallow trap states in pentacene thin films from molecular sliding”.

Appl. Phys. Lett. 86, 152115 (2005)

[76] T. Komoda, Y. Endo, K. Kyuno, and A. Toriumi. “Field-dependent mobility of highly oriented pentacene thin-film transistors”.

Jpn. J. Appl. Phys. 41, 2767 (2002)

[77] R. B. Ye, M. Baba, K. Suzuki, Y. Ohishi, and K. Mori. “Effect of thermal annealing on morphology of pentacene thin films”.

Jpn. J. Appl. Phys. 42, 4473 (2003)

[78] F. Dinelli, M. Murgia, F. Biscarini, and D. M. De Leeuw. “Thermal annealing effects on morphology and electrical response

in ultrathin film organic transistors”. Synth. Metal. 146, 373 (2004)

[79] S. J. Kang, M. Noh, D. S. Park, H. J. Kim, C. N. Whang, and C. H. Chang. “Influence of postannealing on polycrystalline

pentacene thin film transistor”. J. Appl. Phys. 95, 2293 (2004)

[80] T. Sekitani, S. Iba, Y. Kato, Y. Noguchi, T. Someya, and T. Sakurai. “Suppression of DC bias stress-induced degradation of

organic field-effect transistors using postannealing effects”. Appl. Phys. Lett. 87, 073505 (2005)

[81] D. Guo, S. Ikeda, K. Saiki, H. Miyazoe, and K. Terashima. “Effect of annealing on the mobility and morphology of thermally

activated pentacene thin film transistors”. J. Appl. Phys. 99, 094502 (2006)

[82] A. C. Mayer, M. T. Lloyd, D. J. Herman, T. G. Kasen, and G. G. Malliaras. “Postfabrication annealing of pentacene-based

photovoltaic cells”. Appl. Phys. Lett. 85, 6272 (2004)

164 Bibliography

[83] J. Pflaum, J. Niemax, and A. K. Tripathi. “Chemical and structural effects on the electronic transport in organic single crystals”.

Chem. Phys. 325, 152 (2006)

[84] N. Karl. Festkörperprobleme 14, 261 (1974)

[85] C. C. Mattheus, J. Baas, A. Meetsma, J. L. de Boer, C. Kloc, T. Siegrist, and T. T. M. Palstra. “A 2:1 cocrystal of 6,13-

dihydropentacene and pentacene”. Acta Cryst. E58, o1229 (2002)

[86] J. E. Northrup and M. L. Chabinyc. “Gap states in organic semiconductors: Hydrogen- and oxygen-induced states in

pentacene”. Phys. Rev. B 68, 041202(R) (2003)

[87] O. D. Jurchescu, J. Baas, and T. T. M. Palstra. “Effect of impurities on the mobility of single crystal pentacene”. Appl. Phys.

Lett. 84, 3061 (2004)

[88] M. Ahles, R. Schmechel, and H. von Seggern. “N-type organic field-effect transistors based on interface-doped pentacene”.

Appl. Phys. Lett. 85, 4499 (2004)

[89] L.-L. Chua, J. Zaumseil, J.-F. Chang, E. C.-W. Ou, P. K.-H. Ho, H. Sirringhaus, and R. H. Friend. “General observation of

n-type field-effect behaviour in organic semiconductors”. Nature 434, 194 (2005)

[90] M. H. Yoon, C. Kim, A. Facchetti, and T. J. Marks. “Gate dielectric chemical structure-organic field-effect transistor

performance correlations for electron, hole, and ambipolar organic semiconductors”. J. Am. Chem. Soc. 128, 12851 (2006)

[91] D. Kumaki, T. Umeda, and S. Tokito. “Influence of H2O and O2 on threshold voltage shift in organic thin-film transistors:

Deprotonation of SiOH on SiO2 gate-insulator surface”. Appl. Phys. Lett. 92, 093309 (2008)

[92] J. Sworakowski. “Effect of polar molecules on the transport and localization of charge carriers in molecular crystals”. Brazilian

Journal of Physics 29, 318 (1999)

[93] S. Verlaak and P. Heremans. “Molecular microelectostatic view on electronic states near pentacene grain boundaries”. Phys.

Rev. B 75, 115127 (2007)

[94] A. F. Stassen, R. W. I. de Boer, N. N. Iosad, and A. F. Morpurgo. “Influence of the gate dielectric on the mobility of rubrene

single-crystal field-effect transistors”. Appl. Phys. Lett. 85, 3899 (2004)

[95] I. N. Hulea, S. Fratini, H. Xie, C. L. Mulder, N. N. Iossad, G. Rastelli, S. Ciuchi, and A. F. Morpurgo. “Tunable Fröhlich

polarons in organic single-crystal transistors”. Nature Mater. 5, 982 (2006)

[96] A. R. Brown, C. P. Jarrett, D. M. de Leeuw, and M. Matters. “Field-effect transistors made from solution-processed organic

semiconductors”. Synth. Metal. 88, 37 (1997)

[97] J. Zaumseil and H. Sirringhaus. “Electron and ambipolar transport in organic field-effect transistors”. Chem. Rev. 107, 1296

(2007)

[98] A. Rolland, J. Richard, J. P. Kleider, and D. Mencaraglia. “Electrical properties of amorphous silicon transistors and MIS-

devices: comparative study of top nitride and bottom nitride configurations”. J. Electrochem. Soc. 140, 3679 (1993)

Bibliography 165

[99] M. McDowell, I. G. Hill, J. E. McDermott, S. L. Bernasek, and J. Schwartz. “Improved organic thin-film transistor performance

using novel self-assembled monolayers”. Appl. Phys. Lett. 88, 073505 (2006)

[100] G. Horowitz, R. Hajlaoui, D. Fichou, and A. El Kassmi. “Gate voltage dependent mobility of oligothiophene field-effect

transistors”. J. Appl. Phys. 85, 3202 (1999)

[101] G. Horowitz, P. Lang, M. Mottaghi, and H. Aubin. “Extracting parameters from the current-voltage characteristics of organic

field-effect transistors”. Adv. Funct. Mater. 14, 1069 (2004)

[102] M. Shur and M. Hack. “Physics of amorphous silicon based alloy field-effect transistors”. J. Appl. Phys. 55, 3831 (1984)

[103] C. Goldmann, C. Krellner, K. P. Pernstich, S. Haas, D. J. Gundlach, and B. Batlogg. “Determination of the interface trap

density of rubrene single-crystal field-effect transistors and comparison to the bulk trap density”. J. Appl. Phys 99, 034507

(2006)

[104] A. Salleo, F. Endicott, and R. A. Street. “Reversible and irreversible trapping at room temperature in poly(thiophene) thin-film

transistors”. Appl. Phys. Lett. 86, 263505 (2005)

[105] M. L. Chabinyc, F. Endicott, B. D. Vogt, D. M. DeLongchamp, E. K. Lin, Y. L. Wu, P. Liu, and B. S. Ong. “Effects of humidity

on unencapsulated poly(thiophene) thin-film transistors”. Appl. Phys. Lett. 88, 113514 (2006)

[106] C. Goldmann, D. J. Gundlach, and B. Batlogg. “Evidence of water-related discrete trap state formation in pentacene single-

crystal field-effect transistors”. Appl. Phys. Lett. 88, 063501 (2006)

[107] H. L. Gomes, P. Stallinga, M. Cölle, D. M. de Leeuw, and F. Biscarini. “Electrical instabilities in organic semiconductors

caused by trapped supercooled water”. Appl. Phys. Lett. 88, 082101 (2006)

[108] R. A. Street, A. Salleo, and M. L. Chabinyc. “Bipolaron mechanism for bias-stress effects in polymer transistors”. Phys. Rev.

B 68, 085316 (2003)

[109] R. A. Street, M. L. Chabinyc, and F. Endicott. “Chemical impurity effects on transport in polymer transistors”. Phys. Rev. B

76, 045208 (2007)

[110] M. J. Powell. “The physics of amorphous-silicon thin-film transistors”. IEEE Trans. Electron Devices 36, 2753 (1989)

[111] T. B. Singh, F. Meghdadi, S. Günes, N. Marjanovic, G. Horowitz, P. Lang, S. Bauer, and N. S. Sariciftci. “High-performance

ambipolar pentacene organic field-effect transistors on poly(vinyl alcohol) organic gate dielectric”. Adv. Mater. 17, 2315 (2005)

[112] K. P. Pernstich. The influence of trap states on charge transport in organic transistors. Ph.D. thesis, ETH Zurich, Nr. 17245

(2007)

[113] Y. Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson. “Stacked pentacene layer organic thin-film transistors with improved

characteristics”. IEEE Electron Device Lett. 18, 606 (1997)

[114] Y.-Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson. “Pentacene-based organic thin-film transistors”. IEEE Trans.

Electron Devices 44, 1325 (1997)

166 Bibliography

[115] W. R. Thompson and J. E. Pemberton. “Characterization of octadecylsilane and stearic-acid layers on Al2O3 surfaces by

raman-spectroscopy”. Langmuir 11, 1720 (1995)

[116] A. Ulman. “Formation and structure of self-assembled monolayers”. Chem. Rev. 96, 1533 (1996)

[117] S. H. Chen and C. W. Frank. “Infrared and fluorescence spectroscopic studies of self-assembled n-alkanoic acid monolayers”.

Langmuir 5, 978 (1989)

[118] Y.-T. Tao. “Structural comparison of self-assembled monolayers of n-alkanoic acids on the surfaces of silver, copper and

aluminum”. J. Am. Chem. Soc. 115, 4350 (1993)

[119] Y.-T. Tao, G. D. Hietpas, and D. L. Allara. “HCl vapor-induced structural rearrangements of n-alkanoate self-assembled

monolayers on ambient silver, copper, and aluminum surfaces”. J. Am. Chem. Soc. 118, 6724 (1996)

[120] POF Atlas, Fachhochschule Nürnberg, http://www.pofac.de/pofac/en/information/glossary.php?term=CYTOP&glossarylang=en.

[121] R. A. Laudise, C. Kloc, P. G. Simpkins, and T. Siegrist. “Physical vapor growth of organic semiconductors”. J. Crystal Growth

187, 449 (1998)

[122] J. Wang, D. J. Gundlach, C. C. Kuo, and T. N. Jackson. “Improved contacts for organic electronic devices using self-assembled

charge transfer materials”. 41st Electron. Mater. Conf. Dig. p. 16 (1999)

[123] G. Horowitz, F. Garnier, A. Yassar, R. Hajlaoui, and F. Kouki. “Field-effect transistor made with a sexithiophene single crystal”.

Adv. Mater. 8, 52 (1996)

[124] J. N. Israelachvili. Intermolecular and surface forces. Academic Press, London (1997)

[125] Z.-T. Zhu, J. T. Mason, R. Dieckmann, and G. G. Malliaras. “Humidity sensors based on pentacene thin-film transistors”.

Appl. Phys. Lett. 81, 4643 (2002)

[126] C. Pannemann, T. Diekmann, and U. Hilleringmann. “Degradation of organic field-effect transistors made of pentacene”. J.

Mat. Res. 19, 1999 (2004)

[127] A. Maliakal, K. Raghavachari, H. Katz, E. Chandross, and T. Siegrist. “Photochemical stability of pentacene and a substituted

pentacene in solution and in thin films”. Chem. Mater. 16, 4980 (2004)

[128] F. De Angelis, S. Cipolloni, L. Mariucci, and G. Fortunato. “Aging effects in pentacene thin-film transistors: Analysis of the

density of states modification”. Appl. Phys. Lett. 88, 193508 (2006)

[129] M. L. Chabinyc, R. A. Street, and J. E. Northrup. “Effects of molecular oxygen and ozone on polythiophene-based thin-film


[130] H. Klauk, U. Zschieschang, R. T. Weitz, H. Meng, F. Sun, G. Nunes, D. E. Keys, C. R. Fincher, and Z. Xiang. “Organic

transistors based on di(phenylvinyl)anthracene: Performance and stability”. Adv. Mater. 19, 3882 (2007)

[131] M. Kiguchi, M. Nakayama, T. Shimada, and K. Saiki. “Electric-field-induced charge injection or exhaustion in organic thin

film transistor”. Phys. Rev. B 71, 035332 (2005)

Bibliography 167

[132] D. Knipp, A. Benor, V. Wagner, and T. Muck. “Influence of impurities and structural properties on the device stability of

pentacene thin film transistors”. J. Appl. Phys. 101, 044504 (2007)

[133] P. V. Pesavento, R. J. Chesterfield, C. R. Newman, and C. D. Frisbie. “Gated four-probe measurements on pentacene thin-film

transistors: Contact resistance as a function of gate voltage and temperature”. J. Appl. Phys. 96, 7312 (2004)

[134] P. V. Pesavento, K. P. Puntambekar, C. D. Frisbie, J. C. McKeen, and P. P. Ruden. “Film and contact resistance in pentacene

thin-film transistors: Dependence on film thickness, electrode geometry, and correlation with hole mobility”. J. Appl. Phys.

99, 094504 (2006)

[135] J. Takeya, C. Goldmann, S. Haas, K. P. Pernstich, B. Ketterer, and B. Batlogg. “Field-induced charge transport at the surface

of pentacene single crystals: A method to study charge dynamics of two-dimensional electron systems in organic crystals”. J.

Appl. Phys. 94, 5800 (2003)

[136] C. Goldmann, S. Haas, C. Krellner, K. P. Pernstich, D. J. Gundlach, and B. Batlogg. “Hole mobility in organic single crystals

measured by a “flip-crystal” field-effect technique”. J. Appl. Phys. 96, 2080 (2004)

[137] V. Podzorov, E. Menard, A. Borissov, V. Kiryukhin, J. A. Rogers, and M. E. Gershenson. “Intrinsic charge transport on the

surface of organic semiconductors”. Phys. Rev. Lett. 93, 086602 (2004)

[138] L. Bürgi, T. J. Richards, R. H. Friend, and H. Sirringhaus. “Close look at charge carrier injection in polymer field-effect

transistors”. J. Appl. Phys. 94, 6129 (2003)

[139] W. E. Spear and P. G. Le Comber. “Investigation of the localized state distribution in amorphous Si films”. J. Non-Cryst. Solids

8-10, 727 (1972)

[140] M. Grünewald, P. Thomas, and D. Würtz. “A simple scheme for evaluating field effect data”. Phys. Status Solidi B 100, K139

(1980)

[141] W. E. Spear, D. Allan, P. Le Comber, and A. Ghaith. “A new approach to the interpretation of transport results in a-Si”. Philos.

Mag. B 41, 419 (1980)

[142] M. Grünewald, K. Weber, W. Fuhs, and P. Thomas. “Field effect studies on a-Si:H films”. J. Phys. 42, 523 (1981)

[143] R. L. Weisfield and D. A. Anderson. “An improved field-effect analysis for the determination of the pseudogap-state density in

amorphous semiconductors”. Philos. Mag. B 44, 83 (1981)

[144] M. J. Powell. “Analysis of field-effect-conductance measurements on amorphous semiconductors”. Philos. Mag. B 43, 93

(1981)

[145] K. Weber, M. Grünewald, W. Fuhs, and P. Thomas. “Field effect in a-Si:H films”. Phys. Status Solidi B 110, 133 (1982)

[146] F. Djamdji and P. G. Le Comber. “An investigation of the conductivity prefactor in a-Si as a function of fermi level position

using the field-effect experiment”. Philos. Mag. B 56, 31 (1987)

[147] R. Schumacher, P. Thomas, K. Weber, W. Fuhs, F. Djamdji, P. G. Le Comber, and R. E. I. Schropp. “Temperature-dependent

effects in field-effect measurements on hydrogenated amorphous silicon thin-film transistors”. Philos. Mag. B 58, 389 (1988)

168 Bibliography

[148] G. Fortunato and P. Migliorato. “Determination of gap state density in polycrystalline silicon by field-effect conductance”.

Appl. Phys. Lett. 49, 1025 (1986)

[149] P. Migliorato and D. B. Meakin. “Material properties and characteristics of polysilicon transistors for large area electronics”.

Appl. Surface Si. 30, 353 (1987)

[150] G. Fortunato, D. B. Meakin, P. Migliorato, and P. G. Le Comber. “Field-effect analysis for the determination of gap-state

density and fermi-level temperature dependence in polycrystalline silicon”. Philos. Mag. B 57, 573 (1988)

[151] V. Foglietti, L. Mariucci, and G. Fortunato. “Temperature dependence of the transfer characteristics of polysilicon thin film

transistors fabricated by excimer laser crystallization”. J. Appl. Phys. 85, 616 (1999)

[152] A. R. Völkel, R. A. Street, and D. Knipp. “Carrier transport and density of state distributions in pentacene transistors”. Phys.

Rev. B 66, 195336 (2002)

[153] S. Scheinert, G. Paasch, M. Schrödner, H. K. Roth, S. Sensfuss, and T. Doll. “Subthreshold characteristics of field effect

transistors based on poly(3-dodecylthiophene) and an organic insulator”. J. Appl. Phys. 92, 330 (2002)

[154] D. Knipp, P. Kumar, A. R. Völkel, and R. A. Street. “Influence of organic gate dielectrics on the performance of pentacene thin

film transistors”. Synth. Metal. 155, 485 (2005)

[155] D. Oberhoff, K. P. Pernstich, D. J. Gundlach, and B. Batlogg. “Arbitary density of states in an organic thin-film field-effect

transistor model and application to pentacene devices”. IEEE Trans. Electron Devices 54, 17 (2007)

[156] S. Scheinert, K. P. Pernstich, B. Batlogg, and G. Paasch. “Determination of trap distributions from current characteristics of

pentacene field-effect transistors with surface modified gate oxide”. J. Appl. Phys. 102, 104503 (2007)

[157] K. P. Pernstich, B. Rössner, and B. Batlogg. “Field-effect-modulated Seebeck coefficient in organic semiconductors”. Nature

Mater. 7, 321 (2008)

[158] I. Zhivkov, S. Nešpurek, and F. Schauer. “Influence of oxygen on the parameters of a thin film copper phthalocyanine field

effect transistor”. Adv. Mater. Opt. Electron. 9, 175 (1999)

[159] G. Horowitz, M. E. Hajlaoui, and R. Hajlaoui. “Temperature and gate voltage dependence of hole mobility in polycrystalline

oligothiophene thin film transistors”. J. Appl. Phys. 87, 4456 (2000)

[160] D. V. Lang, X. Chi, T. Siegrist, A. M. Sergent, and A. P. Ramirez. “Amorphouslike density of gap states in single-crystal

pentacene”. Phys. Rev. Lett. 93, 086802 (2004)

[161] F. De Angelis, L. Mariucci, S. Cipolloni, and G. Fortunato. “Analysis of electrical characteristics of high performance pentacene

thin-film transistors with PMMA buffer layer”. J. Non-Cryst. Solids 352, 1765 (2006)

[162] N. Kawasaki, T. Nagano, Y. Kubozono, Y. Sako, Y. Morimoto, Y. Takaguchi, A. Fujiwara, C. C. Chu, and T. Imae. “Transport

properties of field-effect transistors with Langmuir-Blodgett films of C-60 dendrimer and estimation of impurity levels”. Appl.

Phys. Lett. 91, 243515 (2007)

Bibliography 169

[163] J. A. Merlo, C. R. Newman, C. P. Gerlach, T. W. Kelley, D. V. Muyres, S. E. Fritz, M. F. Toney, and C. D. Frisbie. “p-channel

organic semiconductors based on hybrid acene-thiophene molecules for thin-film transistor applications”. J. Am. Chem. Soc.

127, 3997 (2005)

[164] J. E. Anthony. “Functionalized acenes and heteroacenes for organic electronics”. Chem. Rev. 106, 5028 (2006)

[165] M. Mas-Torrent and C. Rovira. “Novel small molecules for organic field-effect transistors: towards processability and high

performance”. Chem. Soc. Rev. 37, 827 (2008)

[166] D. J. Gundlach, J. A. Nichols, L. Zhou, and T. N. Jackson. “Thin-film transistors based on well-ordered thermally evaporated

naphthacene films”. Appl. Phys. Lett. 80, 2925 (2002)

[167] R. T. Weitz, K. Amsharov, U. Zschieschang, E. Barrena Villas, D. K. Goswami, M. Burghard, H. Dosch, M. Jansen, K. Kern,

and H. Klauk. “Organic n-channel transistors based on core-cyanated perylene carboxylic diimide derivatives”. J. Am. Chem.

Soc. 130, 4637 (2008)

[168] J. E. Anthony, D. L. Eaton, and S. R. Parkin. “A road map to stable, soluble, easily crystallized pentacene derivatives”. Org.

Lett. 4, 15 (2002)

[169] H. Moon, R. Zeis, E.-J. Borkent, C. Besnard, A. J. Lovinger, T. Siegrist, C. Kloc, and Z. Bao. “Synthesis, crystal structure, and

transistor performance of tetracene derivatives”. J. Am. Chem. Soc. 126, 15322 (2004)

[170] W. L. Kalb, T. Mathis, S. Haas, A. F. Stassen, and B. Batlogg. “Organic small molecule field-effect transistors with CytopTM

gate dielectric: Eliminating gate bias stress effects”. Appl. Phys. Lett. 90, 092104 (2007)

[171] Y. Ma, Y. Sun, Y. Liu, J. Gao, S. Chen, X. Sun, W. Qiu, G. Yu, G. Cui, W. Hu, and D. Zhu. “Organic thin film transistors based

on stable amorphous ladder tetraazapentacenes semiconductors”. J. Mater. Chem. 15, 4894 (2005)

[172] P.-T. T. Pham, Y. Xia, C. D. Frisbie, and M. M. Bader. “Single crystal field effect transistor of a y-shaped ladder-type oligomer”.

J. Phys. Chem. C 112, 7968 (2008)

[173] U. Berens, A. Stassen, B. Schmidhalter, W. Kalb, and F. Bienewald. “Quinoid systems as organic semiconductors”. Patent

application WO 2007/118799 A1

[174] H. Liebermann and J. Barrollier. “über umformungen einiger 1,4-dioxy-tetraaryl-2,5-xylylenglycole”. Liebigs Ann. Chem.

509, 38 (1934)

[175] C. P. Gerlach. “Bis(2-acenyl)acetylene semiconductors”. US Patent No. 7,109,519, Sep. 19, 2006

[176] T. B. Singh, N. S. Sariciftci, H. Yang, L. Yang, B. Plochberger, and H. Sitter. “Correlation of crystalline and structural properties

of c60 thin films grown at various temperature with charge carrier mobility”. Appl. Phys. Lett. 90, 213512 (2007)

[177] N. Kobayashi, M. Sasaki, and K. Nomoto. “Stable peri-xanthenoxanthene thin-film transistors with efficient carrier injection”.

Chem. Mater. 21, 552 (2009)

[178] K. P. Pernstich, D. Oberhoff, C. Goldmann, and B. Batlogg. “Modeling the water related trap state created in pentacene


170 Bibliography

[179] Asahi Glass Company. “Cytop amorphous fluoropolymer technical data sheet”

[180] V. Podzorov, S. E. Sysoev, E. Loginova, V. M. Pudalov, and M. E. Gershenson. “Single crystal organic field effect transistors

with the hole mobility ∼ 8 cm2/Vs”. Appl. Phys. Lett. 83, 3504 (2003)

[181] W. B. Jackson, J. M. Marshall, and D. M. Moyer. “Role of hydrogen in the formation of metastable defects in hydrogenated

amorphous silicon”. Phys. Rev. B 39, 1164 (1989)

[182] F. Schauer, S. Nešpurek, and O. Zmeškal. “The bulk trap spectroscopy of solids by temperature-modulated space-charge-

limited currents (TMSCLC): Application to real crystalline and amorphous semiconductors”. J. Phys. C 19, 7231 (1986)

[183] W. L. Kalb, F. Meier, K. Mattenberger, and B. Batlogg. “Defect healing at room temperature in pentacene thin films and

improved transistor performance”. Phys. Rev. B 76, 184112 (2007)

[184] D. Braga, N. Battaglini, A. Yassar, G. Horowitz, M. Campione, A. Sassella, and A. Borghesi. “Bulk electrical properties of

rubrene single crystals: Measurement and analysis”. Phys. Rev. B 77, 115205 (2008)

[185] M. Matters, D. M. de Leeuw, M. J. C. M. Vissenberg, C. M. Hart, P. T. Herwig, T. Geuns, C. M. J. Mutsaers, and C. J. Drury.

“Organic field-effect transistors and all-polymer integrated circuits”. Optical Materials 12, 189 (1999)

[186] M. Schubert, C. Bundesmann, G. Jacopic, H. Maresch, and H. Arwin. “Infrared dielectric function and vibrational modes of

pentacene thin films”. Appl. Phys. Lett. 84, 200 (2004)

[187] D. J. Gundlach, L. Zhou, J. A. Nichols, T. N. Jackson, P. V. Necliudov, and M. S. Shur. “An experimental study of contact

effects in organic thin film transistors”. J. Appl. Phys. 100, 024509 (2006)

[188] A. Roth. Vacuum Technology. Elsevier Science B. V., Amsterdam (1990)

[189] S. Ogawa, T. Naijo, Y. Kimura, H. Ishii, and M. Niwano. “Photoinduced doping effect of pentacene field effect transistor on

oxygen atmosphere studied by displacement current measurement”. Appl. Phys. Lett. 86, 252104 (2005)

[190] H. Kuroda and E. A. Flood. “Effect of ambient oxygen on electrical properties of an evaporated film of pentacene”. Can. J.

Chem. 39, 1981 (1961)

[191] O. D. Jurchescu, J. Baas, and T. T. M. Palstra. “Electronic transport properties of pentacene single crystals upon exposure to

air”. Appl. Phys. Lett. 87, 052102 (2005)

[192] P. Parisse, M. Passacantando, S. Picozzi, and L. Ottaviano. “Conductivity of the thin film phase of pentacene”. Org. Electron.

7, 403 (2006)

[193] W.-Y. So, J. M. Wikberg, D. V. Lang, O. Mitrofanov, C. L. Kloc, T. Siegrist, A. M. Sergent, and A. P. Ramirez. “Mobility-

independent doping in crystalline rubrene field-effect transistors”. Solid State Commun. 142, 483 (2007)

[194] A. Vollmer, O. D. Jurchescu, I. Arfaoui, I. Salzmann, T. T. M. Palstra, P. Rudolf, J. Niemax, J. Pflaum, J. P. Rabe, and N. Koch.

“The effect of oxygen exposure on pentacene electronic structure”. Eur. Phys. J. E 17, 339 (2005)

[195] D. M. de Leeuw, M. M. J. Simenon, A. R. Brown, and R. E. F. Einerhand. “Stability of n-type doped conducting polymers and

consequences for polymeric microelectronic devices”. Synth. Metal. 87, 53 (1997)

Bibliography 171

[196] T. D. Anthopoulos, G. C. Anyfantis, G. C. Papavassiliou, and D. M. de Leeuw. “Air-stable ambipolar organic transistors”.

Appl. Phys. Lett. 90, 122105 (2007)

[197] E. J. Meijer, A. V. G. Mangnus, B.-H. Huisman, G. W. ‘t Hooft, D. M. de Leeuw, and T. M. Klapwijk. “Photoimpedance

spectroscopy of poly(3-hexyl thiophene) metal-insulator-semiconductor diodes”. Synth. Metal. 142, 53 (2004)

[198] A. Wang, I. Kymissis, V. Bulovic, and A. I. Akinwande. “Tunable threshold voltage and flatband voltage in pentacene field

effect transistors”. Appl. Phys. Lett. 89, 112109 (2006)

[199] R. A. Street. Hydrogenated amorphous silicon. Cambridge University Press, Cambridge (1991)

[200] L. Tsetseris and S. T. Pantelides. “Intercalation of oxygen and water molecules in pentacene crystals: First-principles

calculations”. Phys. Rev. B 75, 153202 (2007)

[201] D. J. Gundlach, T. N. Jackson, D. G. Schlom, and S. F. Nelson. “Solvent-induced phase transition in thermally evaporated

pentacene films”. Appl. Phys. Lett. 74, 3302 (1999)

[202] J. Zaumseil, R. H. Friend, and H. Sirringhaus. “Spatial control of the recombination zone in an ambipolar light-emitting organic

transistor”. Nat. Mater. 5, 69 (2006)

[203] L. Bürgi, M. Turbiez, R. Pfeiffer, F. Bienewald, H. J. Kirner, and C. Winnewisser. “High-mobility ambipolar near-infrared

light-emitting polymer field-effect transistors”. Adv. Mater. 20, 2217 (2008)

[204] K. Hong, S. Y. Yang, C. Yang, S. H. Kim, D. Choi, and C. E. Park. “Reducing the contact resistance in organic thin-film

transistors by introducing a PEDOT : PSS hole-injection layer”. Org. Electron. 9, 864 (2008)

Acknowledgments

The experiments for this thesis were done in the group of Prof. Dr. Bertram Batlogg at the

Laboratory for Solid State Physics at ETH Zurich. I would like to thank Bertram Batlogg

for the freedom to choose research projects and to take measurement-related decisions.

I would like to thank Bertram for sharing his ideas which has improved the quality of

this work substantially. His constant support has carried this work on a higher level of

quality. Moreover, I am thankful for having had the opportunity to participate in numerous

international conferences and in the collaboration with Ciba Speciality Chemicals. This

has enriched the whole PhD experience significantly.

I would like to thank Prof. Dr. Gilles Horowitz for accepting the responsibility of being

a co-examiner of this thesis. It is a great honor to have one of the pioneers of organic

field-effect transistors as co-examiner. I was influenced and inspired a lot by working

on organic thin-film transistors with Dr. Gilles Horowitz and Dr. Philippe Lang back in

the year 2004 during a 4-month internship at ITODYS, Paris in the frame of my diploma

work at RWTH Aachen.

I owe thanks to my colleagues from the early days Dr. Kurt Pernstich, Dr. Simon Haas and

Dr. Claudia Goldmann who also worked on organic semiconductors. I am thankful for

helping me with adjusting to being a PhD student and for introducing me to the equipment

in the lab. Later on they always found time to pass on their experience in the lab while

finishing up their PhD work.

I thank Dr. David Gundlach who was a PostDoc in the group when I started this the-

sis. Although he left the group only a few month after I started my research I benefited

greatly from meeting this experienced scientist. He helped me to find the right track in

the beginning and I am grateful for that.

Dr. Arno Stassen was a PostDoc in the group in the first year of my PhD project. Arno

Stassen is a chemist and specialist in organic single crystal field-effect transistors. We

worked together on the project with Ciba and made field-effect transistors with all kinds

of new organic semiconductors. I enjoyed his good humor and profited from working

with an experienced scientist in the lab.

Thomas Mathis started in the group as a summer intern. We worked together on Cytop

as gate dielectric in the early days. Later on he became one of my fellow PhD students

working on organic semiconductors. He contributed significantly to a good atmosphere

in the group with his humorous, optimistic and communicative spirit.

I owe many thanks to our technicians Kurt Mattenberger and Hans-Peter Staub for the

excellent support. Having profited from their decade-long experience in a lab has been

most fortunate and essential for the experimental work. The quality of their solutions and

in particular their efficiency never stopped surprising me. Kurt Mattenberger was an in-

dispensable part of setting up and maintaining the device fabrication and characterization

system.

I would like to thank our secretary Gaby Strahm who was most helpful with numerous

administrative concerns. She organized many Apéros and group excursions which were

an integral part of the group’s social life.

I thank Dr. Frank Bienewald, Dr. Ulrich Berens and Dr. Andreas Hafner at Ciba for

the fruitful collaboration on organic field-effect transistors with new organic semiconduc-

tors. It was most fortunate to work with these outstanding chemists and to have access

to organic semiconductors no one ever even thought about making a transistor with. In

addition to that it was interesting to understand the perspective of a scientist working

in industry in general. Specifically, it was enriching to understand the challenges to be

overcome for a commercialization of organic electronics.

I would like to thank Prof. Dr. Yoshihiro Kubozono, our energetic friend and collaborator

from Japan. It was a great pleasure to work with him in the lab during his visits and to

profit from his experience with organic semiconductors.

Matthias Walser, Fabian Meier, Thomas Brenner, Sieghard Seyrling and Pierre Joris did

their diploma thesis on organic field-effect transistors in the group. Their experimental

work is well appreciated and has contributed to the progress of this thesis.

I am thankful for the company of my fellow PhD students Dr. Benjamin Rössner, Dr.

Markus Brühwiler, Jakob Kanter, Philip Moll, Florian Pfuner, Andrey Belousov and

Michela Lavagnini. It was a pleasure to spend time together.

My very special thanks goes to my parents Gisela and Wilhelm Kalb who have supported

me all along the way.

Finally, I would like to dedicate this thesis to my partner in life Ludovica Stortini. The

extent of her support and patience is beyond words.

Curriculum Vitae

5.12.1977 Born in Neuss, Germany

1984-1988 Primary School in Dormagen, Germany

1988-1997 Gymnasium in Dormagen

June 1997 Abitur (general qualification for university entrance)

1997-1998 Civil service in the county hospital in Dormagen

1998-2004 Study of Physics at RWTH Aachen, Germany

and at the University of Manchester, Great Britain

2003-2004 Diploma work at the

III. Physikalischen Insitut, RWTH Aachen

and at ITODYS, Universite Paris 7, France

Diploma thesis: Structure-performance relationship

in pentacene-based thin-film transistors

August 2004 Diploma in physics from RWTH Aachen

2005-2009 Research and teaching assistant in the research group of

Prof. Dr. B. Batlogg at the

Laboratory for Solid State Physics, ETH Zurich, Switzerland

Ph.D. thesis: Trap states in organic field-effect transistors:

Quantification, identification and elimination

Publication list

Publications in journals

1. W. L. Kalb, A. F. Stassen, B. Batlogg, U. Berens, B. Schmidhalter, F. Bienewald, A.

Hafner, and T. Wagner. Quinoid heteropentacenes as promising organic semicon-

ductors for field-effect transistor applications. J. Appl. Phys. 105, 043705 (2009)

2. M. P. Walser, W. L. Kalb, T. Mathis, T. J. Brenner, and B. Batlogg. Stable comple-

mentary inverters with organic field-effect transistors on Cytop fluoropolymer gate

dielectric. Appl. Phys. Lett. 94, 053303 (2009)

3. W. L. Kalb, K. Mattenberger, and B. Batlogg. Oxygen-related traps in pentacene

thin films: Energetic position and implications for transistor performance. Phys.

Rev. B 78, 035334 (2008)

4. Y. Kubozono, S. Haas, W. L. Kalb, P. Joris, F. Meng, A. Fujiwara, and B. Batlogg.

High-performance C60 thin-film field-effect transistors with parylene gate insulator.

Appl. Phys. Lett. 93, 033316 (2008)

5. W. L. Kalb, F. Meier, K. Mattenberger, and B. Batlogg. Defect healing at room

temperature in pentacene thin films and improved transistor performance. Phys.

Rev. B 76, 184112 (2007)

highlighted in New Scientist, Vol. 196 No. 2632, p. 27 (1. Dec. 2007) and

Nanomaterials News, Vol. 3, Issue 21, p. 7 (29. Jan 2008)

6. W. L. Kalb, T. Mathis, S. Haas, A. F. Stassen, and B. Batlogg. High performance

organic field-effect transistors with fluoropolymer gate dielectric. Proc. of SPIE

6658, 665807 (2007)

7. W. L. Kalb, T. Mathis, S. Haas, A. F. Stassen, and B. Batlogg. Organic small

molecule field-effect transistors with CytopT M gate dielectric: Eliminating gate bias

stress effects. Appl. Phys. Lett. 90, 092104 (2007)

Patent applications

1. W. Kalb and T. Mathis. Organic field-effect transistors with polymeric gate dielec-

tric and method for making same. Patent application WO/2008/077463

2. U. Berens, A. Stassen, B. Schmidhalter, W. Kalb, and F. Bienewald. Quinoid sys-

tems as organic semiconductors. Patent application WO/2007/118779

Contributions at conferences

Talks as presenting author

1. Oxygen-related traps in pentacene thin films: Energetic position and implications

for transistor performance. APS March Meeting, March 2009, Pittsburgh, USA

2. A close look at oxygen-related traps in pentacene thin films: Energetic position

and implications for transistor performance. MRS Fall Meeting, December 2008,

Boston, USA

3. Defect healing at room temperature in pentacene thin films and improved transistor

performance. MSE 08, September 2008, Nürnberg, Germany

4. Room temperature defect healing in pentacene based thin-film transistors. E-MRS

Spring Meeting, May 2008, Strasbourg, France


performance. APS March Meeting, March 2008, New Orleans, USA

6. High performance organic field-effect transistors with a cyclic fluoropolymer gate

dielectric. ECOER’07, October 2007, Varenna, Italy

7. Eliminating gate bias stress effects in organic field-effect transistors. APS March

Meeting, March 2007, Denver, USA

8. New organic semiconductors for flexible electronics. MRC Graduate Symposium,

June 2006, Zürich, Switzerland

9. New organic semiconductors for flexible electronics. EUROFET Meeting, June

2006, Berlin, Germany

Poster presentations

1. Highly stable organic field-effect transistors with a fluoropolymer gate dielectric.

MRC Graduate Symposium, May 2008, Zürich, Switzerland


performance. Alpine Workshop, December 2007, Braunwald, Switzerland

3. Highly stable organic field-effect transistors with a fluoropolymer gate dielectric.

Alpine Workshop, December 2007, Braunwald, Switzerland

4. Field-effect in new small molecule organic semiconductors. ECOF 10, August

2006, Riga, Latvia

Rights / License: Research Collection In Copyright - Non ...41808/... · Trap states in organic...

Documents

Transcript of Rights / License: Research Collection In Copyright - Non ...41808/... · Trap states in organic...