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Molecular organic semiconductors for electronic devicesJurchescu, Oana Diana

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1Introduction

1.1 Organic conductors - general aspects

Organic semiconductors are a fascinating class of materials, with a wide range

of properties [1–7]. The interest in organic electronics is motivated by increasing

demands of supplementing Si-based electronics with materials that offer easier

processing methods completed with functionalization by chemical manipulation.

Typical deposition techniques for organics are readily available or not very difficult

to implement (spin-coating, drop-casting, direct printing via stamps or ink-jets).

Their compatibility with light-weight, mechanically flexible plastic substrates,

completed by new, innovative fabrication routes, make them possible candidates

for future electronic devices. Moreover, due to different molecular tailoring pos-

sibilities via chemical synthesis, organic materials present an infinite variety in

functionality. The design of the molecular structures can be engineered to enhance

particular properties (solubility in different solvents, color of light emission, crystal

packing). In particular, addition of polar groups in polymers (e.g. polyvinylidene

fluoride and its copolymers with trifluoroethylene and tetrafluoroethylene) leads

to rich systems for investigation of ferroelectricity [8]. By incorporating them in

transistors (ferroelectric field-effect transistors - FeFETs), combined functionality

can be achieved for non-volatile memory devices that are compatible with flex-

ible plastic substrates [9]. Modification of the chemical end-groups also allows

fabrication of large transistor arrays for organic sensors. Different methods are

developed to increase the sensitivity and selectivity of organic sensing transistors

to different chemical or biological species. Applications include environmental

issues (e.g. air pollution), pH indicators [10], food freshness, toxic compounds,

1

2 Chapter 1. Introduction

stress and pressure indication in clothes [11].

Rapid progress is being made in the industrial development of the organic

electronic devices. Organic devices can be implemented on large scale application

area when their operation offer high performance, reliability, stability, long life

time, good control and reproducibility. Impressive steps have been accomplished

towards incorporation of organic conductors in devices, as constituents of plastic

electronic components, by continued innovation in materials, processing methods

and device design. For example, Philips and Polymer Vision already presented

their prototype rollable display based on pentacene FETs that will be used for fab-

rication of mobile phones [12]. OLED1 technology has already been incorporated

in commercial applications such as small displays for mobile phones, car radios,

and digital cameras [13]. It was suggested that if ”the field continues to progress

at its current, rapid pace, electronics based on organic thin-films materials will

soon become a mainstay of our technological existence” (S. Forrest [14]).

In the academic community, the interest in electrical properties of organic ma-

terials has grown enormously in the last years [16–26]. Different materials and

structures are actively being investigated. On the one hand, the research is mo-

tivated by new applications for organic (plastic) electronics that can overcome

limitations of inorganic materials. On the other hand, the effort is driven by

the intellectual challenges that the investigations of new generation of functional

materials offer. This implies addressing new questions, concepts and models that

allow the manipulation of chemical, structural, physical properties and device

performance. The field is moving very fast; at the moment researchers are able to

control the material properties at the molecular level. They can impose different

molecular packing during growth, add side groups that enhance particular prop-

erties, as a result of the understanding of the various microscopic contributions to

the charge transport. Past investigations were hindered by material instabilities

(to environmental conditions and/or processing methods) and structural defects

that prevented the measurement and understanding of the intrinsic properties of

the organic materials. In the last years, by careful design of new methods that

enable the exploitation of the fascinating properties of carbon-based semiconduc-

tors, good control of the structural properties and charge density distribution in

organic materials is achieved (both by electrostatic and chemical doping). The

degradation is avoided as a consequence of a better understanding of the material

behavior in different conditions (temperature, moisture, light) and interactions

during processing.

1OLED - organic light emitting diodes

1.2. Charge transport in organic semiconductors 3

1.2 Charge transport in organic semiconductors

Organic semiconductors are wide-band-gap and small bandwidth semiconductors.

The HOMO2-LUMO3 gap is in the range of 1-4 eV [27]. With such a large gap,

it might be expected that the organic materials are insulators (an electron would

have to acquire a large thermal energy to make the jump from the valence band

to the conduction band). There are some effective methods that can generate

charge carriers in the organic semiconductors:

⋄ injection of carriers from metallic electrodes;

⋄ optical excitation (creation of electron-hole pairs);

⋄ electrostatic or chemical doping.

Different organic molecular structures are of current interest for the contin-

ued search for novel and useful properties. Carbon nanotubes (CNTs) are one-

dimensional, cylindrical systems that are attractive for nanometer-sized electron-

ics because they combine the high electrical and thermal conductivity with good

mechanical strength and flexibility [28]. The physical properties of nanotubes

are imposed by their diameter, length, and chirality, or twist. For example, they

can be both metallic and semiconducting, depending on the chirality [29]. Nan-

otubes are attractive also for the field emission properties [30] because they are

long and very thin, conductive, with high mechanical strength, and have a low

work-function. As the CNTs are generally accepted as being the bridge between

the nano- and microscopic length scales, a step further was done in coupling them

to biological systems aiming the fabrication of biosensors [31, 32]. More recently,

superconductivity has been observed in nanotubes [33].

When the carbon sheet is not rolled into a tube, it forms a graphene sheet.

Graphene is a candidate for nanometer-scale electronic devices that combine the

properties of carbon nanotubes with the compatibility with established micro-

electronics manufacturing techniques. Low scattering yields mobilities up to 104

cm2/Vs [34]. This value gives ultra-fast-switching transistors for electronics. Also,

the quantum Hall effect was recently reported in graphene [35].

The most common 3D structure in the fullerene structural family is C60,

the spherical buckyball (Fig. 1.1(d)). Because of its particular electronic struc-

ture, C60 exhibits semiconducting properties, but it reveals one amazing prop-

erty after another upon doping [36]. It can be tuned to a metallic or insulating

2HOMO - highest occupied molecular orbital3LUMO - lowest unoccupied molecular orbital

4 Chapter 1. Introduction

state by transferring charge on the molecule through doping. At low temper-

ature this metallic state often exhibits superconductivity, with transition tem-

peratures only exceeded by the high-temperature superconducting copper ox-

ides [37]. This is not an unique type of organic superconductor. Several organic

superconductors have been identified, such as the quasi-one-dimensional Bech-

gaard salts ((TMTSF)2X4 and (TMTTF)2X

5, with X= PF6−, FSO3

−, ClO4−,

etc.), and quasi-two-dimensional salts derived from the donor group BEDT-TTF6

(Fig. 1.1(a)) [38]. Organic superconductors are charge transfer compounds, un-

conventional superconductors, in which the interchain transfer integral can be

changed with pressure, temperature, and composition (the nature of the anion

X controls the chemical pressure), leading to a delicate balance of interactions

that gives rise to a rich variety of physical phenomena. These include charge or-

dering, spin-Peierls ground state, antiferromagnetism, spin density waves (SDW),

metallicity and superconductivity [38].

In polymers, the electrical properties are dominated by disorder that localize

the charges at low temperature. Still, these materials present fascinating proper-

ties that make them attractive for organic electronic devices [1,2,6]. We presented

in Section 1.1 the potential of polymers, that were already incorporated in com-

mercial devices, as components of the OLEDs. Their semiconducting properties

are also used in FETs and solar cells [16–18]. In OLEDs and solar cells an exciton

(excited electron - hole pair) is generated in the organic material. In OLEDs

light is generated through radiative recombination of electrons and holes and it

is emitted through the transparent electrode. Two distinct processes, equally im-

portant, govern the operation of the device: charge transport and recombination.

The color of the emitted light is tuned by the band gap of the active material that

is used. In solar cells light is absorbed through the transparent electrode and the

photons absorbed in the semiconductor create mobile excited electron-hole pairs.

These excitons subsequently undergo dissociation yielding free electrons and holes

that are collected at the contacts. The efficiency of the solar cell is given by the

photon absorption efficiency, exciton dissociation efficiency and charge collection

efficiency. Organic photovoltaic devices are limited by exciton lifetime, and low

charge carrier mobilities; only a fraction of the light-induced excitons contribute

to the current generation. Tang et al. demonstrated that the efficiency can be in-

creased considerably if a heterojunction of a electron donor and electron acceptor

material is manufactured [39]. To enhance the quantum separation efficiency, a

4TMTSF stands for tetramethyltetraselenafulvalene5TMTTF stands for tetramethyltetrathiafulvalene6BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene

1.2. Charge transport in organic semiconductors 5

Figure 1.1: Chemical formulae of organic molecules with different functionalities.

6 Chapter 1. Introduction

network of internal heterojunctions, with a large contact area between donor and

acceptor species (referred to as ”bulk heterojunctions”) was proposed [40].

Despite the fast developments in the field of organic electronics and dramatic

improvement in knowledge and manipulation of the charge injection and trans-

port in organic semiconductors, a reliable relation between microscopic properties

and their effect on the physical properties is still under development. There is

significant work done in understanding the mechanism of conduction in organic

semiconductors. Correct correlations between morphology, molecular packing and

the resulting electronic properties are essential, in order to elucidate fundamental

questions regarding charge transport in these materials. Small molecule con-

ductors allow these type of studies. Because of a higher degree of order than

solution-processed polymers, they can be structurally characterized straightfor-

ward using diffraction, and microscopy techniques. Complementary to this, the

theoretical modelling can be easier implemented because of the lower complexity

of these systems.

In organic molecular solids, the intermolecular forces are weak (van der Waals

and electrostatic type). A detailed description of these issues will be developed

in Section 1.4. The bonding energies are considerably lower than in covalent and

ionic inorganic semiconductors [27,41]. For this reason, the mechanism of charge

transport is fundamentally different. In organic conductors, the charge carriers

interact strongly with the lattice environment leading to polarization effects and

tendency of charge carrier localization. The weak van der Waals interactions result

in a small electronic bandwidth, strong electron-lattice interaction, and polaron

formation. For example, calculations performed on organic molecular crystals

that are free of defects, yield bandwidths in the order of 0.1-0.5 eV [42–47]. This

is more that one order of magnitude lower than the bandwidth in silicon (∼ 10

eV, [48]). The small bandwidths is reflected in low charge carrier mobilities (10−5-

10 cm2/Vs [16–26] for organic semiconductors, compared to 50-500 cm2/Vs in

silicon [49]), and strong interactions between free charge carriers and the lattice.

This interaction facilitates the localization of the charge carriers and narrowing

of the bandwidths even further, and thus are expected to crucially affect the

transport properties. Considerable effort is involved in describing the polaron

dynamics, lifetime, binding energies, and diffusion in the lattice [50, 51].

The mobility of the charge carriers reflects the drift velocity of the charges in

the lattice, thus it is influenced by all interactions that it encounters:

µ =vd

E(1.1)

where µ is the drift mobility, vd is the drift velocity and E the electric field.

1.2. Charge transport in organic semiconductors 7

Scattering by defects, impurities and phonons (lattice vibrations) will decrease

the drift velocity of charges, thus the electronic mobility. The conductivity of the

material is given by charge carrier density n and charge carrier mobility µ.

σ = neµ (1.2)

where e is the elementary charge. The mobility is influenced by the scattering

events, so that [52]:

µ =eτ

m∗(1.3)

where τ is the time between two consecutive scattering events, and m∗ is the

effective mass. This provides a direct relation between the morphology of the

materials and their mobility. In polymers the conduction (µ = 10−8 − 10−4

cm2/Vs) [16, 17] is limited by disorder and hopping of charges between polymer

chains, and thus is lower than in oligomers (µ = 10−3 − 10 cm2/Vs). Even for

the class of oligomer devices, the mobility increases with the degree of structural

order. Structural defects and chemical impurities present in the crystal lattice

promote the localization of the charge carriers. They can either form new states

in the semiconductor band-gap, leading to electronic traps, or scatter the charges.

Both processes result in a decrease of the mobility.

The value of the mobility directly affects the performance of the material in

devices, as it is related with the switching speed. Dramatic improvement in con-

trol and understanding of the transport mechanism in organic materials, on the

molecular level, together with the knowledge on the influence of intrinsic and ex-

trinsic factors on a good performance, has been achieved lately (the value of the

mobility increased 5 orders of magnitude in the last 15 years, reaching the value

of amorphous silicon [49]). This is very important, as some of the possible ap-

plications, like switching devices for active-matrix flat-panel displays (AMFPDs)

based on OLED displays, active-matrix backplanes of OTFTs (organic thin film

transistors) for ”electronic paper” displays, or radiofrequency identification tags

(RFID) require mobilities greater than 1 cm2/Vs [53], values already exceeded in

devices built on organic single crystals [19]. Parallel to organic electronic devices,

organic-inorganic hybrids emerge as new applications by coupling high carrier

mobilities inorganic semiconductors with the flexibility of organic materials [54].

Most organic semiconductors behave as either p-type or n-type semiconduc-

tors (they have either holes, or electrons as majority carriers) (see Fig. 1.1 c, d).

The absence of ambipolar behavior is a severe limitation for the organic electronic

devices for the fabrication of CMOS7-like circuits. Different strategies were pro-

posed to overcome this problem. Most of them involve separate steps to fabricate

7CMOS - complementary metal oxide semiconductor

8 Chapter 1. Introduction

n-type and p-type transistors [17,55]. However, for many materials it is generally

accepted that this is not an intrinsic property, but rather a result of trapping of

one type of charge carriers [16] or due to a high energetic barrier for either elec-

tron or hole injection from the metal electrodes, which is caused by the relatively

large bandgap of organic semiconductors.

1.3 Organic electronic devices

1.3.1 Field effect transistors - operation principles

Organic semiconductors are active materials in different devices. Field effect tran-

sistors, light emitting diodes and solar cells are intensely studied. Organic mate-

rials are soft, fragile and relatively reactive, thus the conventional semiconductors

device fabrication technologies are not always compatible with these compounds.

For this reason the intrinsic electronic properties could not be reached in de-

vices for a long period of time. They were explored with different techniques

(e.g. time of flight [56]). Only lately, using innovative approaches, the fabrication

of organic electronic devices was successful and a good reproducibility between

research groups was achieved [19, 24, 25].

Figure 1.2: Structure of a field effect transistor (FET) with organic semiconductor

as active material. The source electrode is connected to the ground.

This convention is valid for all the devices presented in this thesis.

1.3. Organic electronic devices 9

The field effect transistor (FET) is a three terminal device. The three contacts

are referred to as gate (G), drain (D) and source (S, connected to ground). The

schematic picture of a a FET is drawn in Figure 1.2. The active channel forms at

the semiconductor-insulator interface. The gate insulator acts like a capacitor and

the electric field applied at the gate electrode determines the density of the charge

carriers accumulated at the interface. The current between source and drain is

modulated by the gate voltage (VG). The operation principle of a FET was

introduced by Lilienfeld [57], in 1930, and later developed by Shockley and Pearson

[58]. In 1947 Bardeen, Brattain and Shockley (Bell Laboratories) discovered the

transistor effect and fabricated the first device [59]. They were awarded the Nobel

Prize in physics in 1956 for their discovery. The first metal-oxide-semiconductor

field-effect transistor (MOSFET) was introduced in 1960 by Khang and Atalla

[60].

MOSFETs, built at the surface of inorganic semiconductors, were intensively

studied, and they are incorporated in integrated circuits [41]. Owing to similar

experimental I-V characteristics between organic field-effect transistors (OFETs)

and MOSFETs, the theory developed for MOSFETs is used as starting point

in modelling the OFET behavior. However, the electrical transport in organic

semiconductors is different than in the covalently bonded inorganic semiconduc-

tors. Some attempts to describe the operation principle in OFETs were per-

formed [61, 62].

The transistor channel is active only when the gate voltage (VG) value exceeds

the value of the threshold voltage (VT ). Below this point, the transistor is turned

off, and there is no conduction between drain and source (sub-threshold region).

In the operation of a FET, two distinct regimes can be distinguished (Fig. 1.3(a)).

In the linear regime (small drain-source VD voltages, VD ≪ VG−VT ), the current

between drain and source (ID) depends linearly on the applied voltage (Eq. 1.4).

The device acts as a gate voltage - controlled variable resistor. The value of the

drain current, ID, is given by:

ID =W

LµCi(VG − VT )VD (1.4)

where L is the channel length, W is the gate width, Ci is the capacitance per unit

area of the gate insulator. This model assumes constant velocity, electric field,

and inversion layer charge density between the source and the drain. A more

realistic approach accounts for the variation of the inversion layer charge between

source and drain, and yields the following expression for the current:

ID =W

LµCi

[

(VG − VT )VD −V 2

D

2

]

(1.5)

10 Chapter 1. Introduction

Figure 1.3: I−V curves for a FET operation. (a) Output characteristics (ID−VD)

for different applied gate voltages (VG). The curves are obtained from

simulating the I − V curves that correspond to expression 1.5. The

linear and saturation regimes are indicated. The dotted line points

the pinch-off that separates the linear region of operation on the left

from the saturation region on the right. (b) Transfer characteristics

(ID − VG) for different drain voltages. The curves correspond to ex-

perimental points for a pentacene transistor.

At higher VD voltages, the channel is not continuous, but a depletion area forms at

the drain contact. The onset of this region is called pinch-off (Fig. 1.3(a)). Beyond

this point, the operation regime is referred to as saturation regime. The drain

current is now relatively independent of the drain voltage, being only controlled

by the gate voltage, and varies quadratically with the field:

ID =W

2LµCi(VG − VT )2 (1.6)

Equations 1.4, 1.5, and 1.6 represent expressions that yield the value of the mo-

bility of the semiconductor. The mobility can also be estimated from the gate

voltage sweep (Fig. 1.3(b)), at low drain voltages VD. Here, the expression for

the transconductance (gm) is:

gm =∂ID

∂VG(1.7)

From this equation, the value of the mobility can be extracted:

µ =L

W

1

CiVD

( ∂ID

∂VG

)

VD→0(1.8)

1.3. Organic electronic devices 11

However, all these expressions assume a field independent mobility, and that all

the charges induced by the gate voltage are mobile.

Key parameters in the operation of a FET are the electronic mobility of the

charge carriers and the on/off ratio. The first determines the switching speed and

the maximum current, and the later impose the switching of the device from a

non-conducting (off ) to a conducting state (on).

Good performance organic field-effect transistors (OFETs) were fabricated at

the surface of the organic single crystals using deposition techniques that min-

imize damage at the interface [19, 24]. Fabrication of devices with competitive

characteristics remains an ambitious task for large-scale applications.

1.3.2 Fabrication of OFETs

Owing to the fragility of organic materials, processing to incorporate them in

electronic devices represents a challenge at this early stage. However, they trigger

interest to develop revolutionary methods that are simple and efficient to be used

for large scale applications. In this section we will describe recent advances in

organic electronic devices, focusing on OFETs fabricated on molecular crystals.

Different device structures were proposed to study the electrical transport at the

surface of organic crystals, in order to measure a high intrinsic mobility, that

is not diminished by disorder introduced during processing. There are many

factors involved in the good performance of the device. In spite of the better

reproducibility that is archived lately, the results reported by different groups

are still not always consistent. This can be attributed in part to the fact that

the performance of the electronic devices depends critically on the quality of the

crystals and the interfaces, as well as on the extrinsic factors (like, for example

the environmental conditions in which the experiments were performed). Organic

semiconductor quality, dielectric properties, contacts, and interface properties are

equally important.

Deposition of the organic semiconductor

The most common method used for the deposition of small molecule conductor

films is vacuum sublimation. The macroscopic electronic properties of the films

are imposed by their crystallinity [53]. Better crystallinity and larger grain size

facilitates higher mobilities. Values of 1 cm2/Vs were reached in vacuum subli-

mated pentacene TFTs, after optimization of the fabrication process [85].

We mentioned in Section 1.1 that the highest impact that the organic elec-

tronics can produce over traditional Si-based technology, is the relatively easy

12 Chapter 1. Introduction

processing techniques that their deposition requires. Polymers are attractive be-

cause they are soluble in organic solvents, thus they can be spin coated or printed

on flexible substrates, forming amorphous or polycrystalline films. Still, the high-

est mobilities are achieved in devices build with small molecules. A drawback is

that their solubility is limited and they require deposition methods like vacuum

sublimation, or physical vapor deposition. These demands are not straightforward

to accomplish. Because a high electronic mobility is not sufficient for a material

to be competitive for large scale applications, scientists develop different methods

to facilitate the compatibility with cheap and easy solution processing techniques.

Herwig et al. proposed a synthetic concept for fabrication of a soluble pentacene

precursor [63]. The precursor is converted to pentacene via thermal [63] or ir-

radiative [64] treatment, and the obtained thin film transistors (TFTs) exhibit

mobilities of 0.2 cm2/Vs. A different route to increase the solubility of oligomers

is the attachment of flexible side groups. At the molecular design, careful atten-

tion is payed to the interplay between the degree of solubility and the molecular

stacking that the side group induces. Field-effect transistors with maximum mo-

bilities of 0.01 cm2/Vs were fabricated from quaterthiophene and hexathiophene

end-substituted with 3-butoxypropyl groups [65].

The above mentioned directions represent a compromise, a balance between

performance and cost, because the mobility of materials deposited from solution

remains lower than that of the thermal evaporated material [19, 20]. Moreover,

in polycrystalline films [20, 21, 23], the mobility is lower than in single crystals

[19, 24–26]. This is partially caused by a large grain boundary resistance.

Gate dielectric materials

In field-effect transistors the conduction takes place at the surface of the semicon-

ductor, thus the performance is limited by the quality of the interface between

organic and dielectric, and only in part by the bulk properties. This is evident

from experiments that demonstrate that the gate insulator can modify the charge

density at the interface, having a crucial effect on the operation of both poly-

mer [66] and small molecule [67] devices. Particularly important are the roughness

of the semiconductor/dielectric interface, and the density of defects and impuri-

ties present in this region. Different treatments of the dielectric were proposed in

order to decrease the trap density (e.g. OTS8 treatment [68]).

General requirements for a high quality dielectric include several parameters.

The introduction of a large capacitance, that governs the magnitude of charge

8OTS denotes octadecyltrichlorosilane

1.3. Organic electronic devices 13

Figure 1.4: Different geometries of the OFETs. Source (S), drain (D), and gate

electrodes are indicated. The transport takes place at the interface

between semiconductor layer and gate insulator layer. (a) bottom-

gate geometry; (b) top-gate geometry.

induced in the channel by gate effect, can be done using high-k dielectrics or by

varying their thickness. Besides this, a good insulator in FETs should account

for a large breakdown voltage and low leakage currents, excellent thermal and

chemical stability. There are three classes of dielectric materials incorporated in

OTFTs [69]: inorganic dielectric materials (e.g. SiO2, Ta2O5, Al2O3), organic

dielectric materials (e.g. parylene, PS9, PMMA10, Fig. 1.1(e)) and self-assembled

mono-and multilayers. In FETs with top-gate configuration (Fig. 1.4(a)), the

deposition of inorganic dielectrics partially damages the surface of the organic

crystals and introduces chemical and structural disorder, due to violent process-

ing methods (sputtering, e-beam, plasma-enhanced chemical vapor deposition).

This is the reason why organic dielectrics are used only in bottom-gate devices

(Fig. 1.4(b)). The deposition of an organic dielectric requires lower temperatures

(typical deposition techniques are spin-coating, casting, and printing), are not

destructive, and yield remarkably high mobilities (8 cm2/Vs in rubrene single

crystals transistors with parylene gate dielectric [24]).

Contacts

Contact issues are amply discussed in the field of molecular electronics. Different

models are proposed, as it is believed that the charge transport is either limited by

injection problems due to poorly defined contacts, or by bulk conductivity. Baldo

9PS denotes polystyrene10PMMA denotes polymethylmethacrylate

14 Chapter 1. Introduction

et al. elaborated an interface injection model, that accounts for two distinct

injection steps [70]. During the first step, charges are injected from the contacts

into an interface region that contain a broad distribution of states induced by

interface dipoles. In the second, limiting step, the charges migrate from the

interface to the bulk region.

Two types of device geometries are used in the fabrication of the field-effect

transistors: top-contact and bottom-contact structures. In the first case, it was

observed that the deposition of contacts on the organic semiconductors inflicts

with their chemical and mechanical fragility, and introduce traps locally, at the

metal/organic interface [71]. Different deposition methods are used, depending

on the nature of the contact. Metal contacts are usually evaporated or sputtered

using shadow masks, or painted (the latter method is also applied for colloidal

graphite paste). The choice of the metal contact gives the value of the Schottky

barrier, thus the deviation from the ohmic regime, and also the type of the major-

ity carriers. Alternative contact materials, with specific deposition requirements

were proposed and successfully applied. Room temperature lamination of metal

coated elastomeric stamps (e.g. PDMS11 [19]) represents a non-destructive, high

resolution method to fabricate contacts. The polymer PANI12 and the copolymer

PEDOT:PSS13 (Fig. 1.1(b)) are currently widely used in organic devices, espe-

cially OLEDs and solar cells because of their excellent transparency in the visible

region, but also in FETs due to good electrical conductivity, and environmental

stability [72].

1.3.3 Outlook

Reviewing the recent advances in materials, methods and emerging applications,

two very important issues can be mentioned. Firstly, organic semiconductor re-

search is an exciting and interdisciplinary area of current research activity that

raised the interest of theorists, chemists, physicists, and device scientists and is

developing extremely fast. Secondly, organic materials impose a new way of think-

ing because they exhibit a wide range of electrical properties, being tunable from

insulator (in gate dielectrics) to semiconductor (as active layers in devices), to

metal, and even superconductor. A scheme with the wide range of functionalities

that the organic materials offer can be found in Figure 1.1(a-e). These diverse

properties are spectacular and give the opportunity of building ”all-organic de-

vices”.

11PDMS denotes polydimethylsiloxane12PANI denotes polyaniline13PEDOT:PSS denotes polyethylene(3,4-dioxythiophene)/polystyrene sulfonate

1.4. Organic molecular crystals 15

1.4 Organic molecular crystals

Characteristic of organic semiconductors are the intramolecular bonds. The

alternation of single (σ) and double (π) bonds, referred to as conjugation, is typ-

ical for organic conductors. The Carbon atoms involved in this type of bonding

are sp2 hybridized. The three hybrid sp2 orbitals form the σ bonds, with the

σ-electrons being highly localized. The remaining nonhybridized pz orbitals of

adjacent Carbons form the π- orbitals perpendicular to the σ bonds and delocal-

ized over the molecule. The σ bonds are very strong (they are positioned lower

in energy). The molecular orbitals that are filled (π-bonding orbitals) form the

valence states. The filled orbital with the highest energy is called the Highest

Occupied Molecular Orbital. The vacant orbitals (π∗-antibonding orbitals) form

the conduction band, with LUMO being the Lowest Unoccupied Molecular Or-

bital. The π-electrons are responsible for an important part of the intermolecular

conduction in these materials. Besides this, the molecular packing in the solid,

imposed by the physical interactions, determine the physical properties, e.g. the

electronic transport.

The intermolecular forces are weak, and result from the cooperation and

competition between π − σ and π − π interactions. The interaction energy be-

tween molecules originates from several contributions. The electrostatic inter-

action between permanent multipoles (usually quadrupoles), and the dispersion

(van der Waals) forces between induced dipoles represent the dominant force.

The interaction between the permanent multipoles and the induced multipoles,

called induction, is generally treated as a second order term. In a simple pictorial

model, the van der Waals forces between molecules originate from the instan-

taneous polarization of the neutral molecules into dipoles [27]. The temporary

generated dipoles promote the formation of new induced dipoles in neighboring

molecules and spread in the lattice. For interplanar separation that characterize

organic molecular crystals (d > 3.4 A [32]), the van der Waals forces are always

attractive. The quadrupolar interactions emerge from the coupling between the

permanent quadrupolar moments of the molecules [73, 74], and can be attractive

or repulsive, depending on the relative orientation of the quadrupoles.

The crystal structure is determined by an interplay between van der Waals

forces and quadrupolar interactions. The two competing interactions promote

different molecular stacking that minimize the energy. The van der Waals inter-

actions are optimized for ”face-to-face” orientation of the molecules, that result

in the maximum π− overlap. On the other hand, the presence of quadrupolar

moments favor the ”edge-to-face” stacking, in which the hydrogen on one ring

16 Chapter 1. Introduction

Figure 1.5: Crystal structure of pentacene single crystal. Left panel: view along

the [100] axis (the layered structure can be seen). Right panel: the

herringbone arrangement within the layer, view along the long axis of

the molecule. The unit cell orientation is also drawn.

encounter the π− network of the adjacent molecule. The most common crystal

packing that results from the afore mentioned mechanisms is the herringbone-

arrangement (Fig. 1.5). Oligomers often crystallize in a layered fashion, and

the molecular arrangement within the layers is imposed by the van der Waals

and quadrupolar interactions. Gavezzotti et al. distinguished four possible her-

ringbone modes in which polynuclear aromatic hydrocarbons pack: herringbone

structure (naphthalene, anthracene, pentacene ), sandwich herringbone structure

(pyrene, perylene), γ structure (benzopyrene, coronene), β structure (trybenzopy-

rene, tetrabenzoperylene) [75]. Cofacial π-stacking in molecular crystals is hardly

ever found. Anthony et al. synthesized these types of structure by substitution

of different groups to pentacene backbone [76]. They showed that different type

and amount of π-overlap can be controlled by the nature, size and position of the

substituent, leading to various stacking motifs. Although seldom encountered,

cofacial configurations are particularly attractive because they accommodate the

largest electronic splitting in the HOMO and LUMO levels, promoting the high-

est theoretical predicted charge carrier mobilities [77]. In the local picture (as

opposed to the band picture) of the charge transport in organic conjugated ma-

terials, the efficiency of this process reflects how easy the charges are transferred

1.4. Organic molecular crystals 17

between neighboring sites, and, as expected, is very sensitive to orientation of the

molecules with respect to each other. The electronic coupling between adjacent

molecules, quantified by the transfer integral t, is modulated by the molecular

arrangement and directly associated with the electronic mobility [45, 46, 77]. In

the framework of these calculations, the amplitude of the electronic coupling is

influenced by the intermolecular separation distance, the molecular overlap, the

length of the molecule, and, in the case of herringbone structures, the rotation

of molecular planes [77]. Additionally, the relevance of thermal motions in the

modulation of the electronic coupling between molecules in an an organic solid

was demonstrated [45].

Owing to the weak interaction forces between molecules in the solid, small

variations in the crystal packing can be present, leading to polymorphism. The

term polymorphism refers to the existence of more than one crystal structure

for a particular compound. This phenomenon is frequently displayed by organic

crystals and is driven by the growth conditions and/or subsequent treatment. In

rubrene, different polymorphs can be obtained, depending on the pressure in the

system in which the starting material is sublimed (see Chapter. 6 in this thesis

and the references therein). In quaterthiophene (α−4T) [79] and sexithiophene

(α−6T) [80,81] different molecular arrangements are induced by the source tem-

perature. The general picture is even more complex in pentacene films, where

four polymorphs were detected [78, 82]. The obtained polymorph is dictated by

the substrate type, the substrate temperature and the thickness of the film. For

pentacene single crystals, only one polymorph exists [78].

The existence of different polymorphs for some molecular crystals provides

a unique opportunity to understand the influence of the crystal packing on the

electronic properties, since these systems only differ with the orientation of the

constituent building blocks with respect to each other, the molecule being the

same [42, 46, 81]. For example, Troisi and Orlandi performed band structure

calculations on the four pentacene polymorphs and found that the mobility tensor

is highly anisotropic for three of the four considered polymorphs [83]. This result

is relevant for understanding the fundamental mechanisms of charge transport in

organic crystals.

18 Chapter 1. Introduction

1.5 Single crystals - model systems for investiga-

tion of intrinsic properties

The progress in the field of organic electronics requires a good fundamental under-

standing of factors that influence the electronic behavior. This leads to a better

control of the microscopic properties that determine the conduction of the mate-

rials used in the devices. A correct insight on the interplay between the effects of

chemical structure and molecular orientation on the transport process can only

be accomplished when single crystals are used.

Single crystals are not meant to be incorporated in applications, but to provide

a well defined structure, in which the intrinsic electronic properties of the material

can be measured [19,24–26,84]. They serve as model systems, for which structure-

properties correlations can be explored.

Although thin films (polycrystalline or amorphous) are more attractive for

organic electronic devices, here the intrinsic properties are generally masked by

grain boundaries where structural defects localize and trap the charge carriers.

The major limitation of thin-film transistors (TFTs) comes from the fact that their

performance is severely dependent on the fabrication conditions. Moreover, even

films that are grown in an identical manner can exhibit very different electrical

properties [85].

The electrical properties of organic single crystals have been successfully probed

by time-of-flight (TOF) [56] and space-charge-limited current (SCLC) methods

[26], as well as field-effect transistor (FET) experiments [19, 24, 25].

1.6 Aim of the present research

Motivated by the various properties that organic semiconductors reveal, we eval-

uate in this thesis a critical analysis of the different factors that determine the

electrical conduction in these materials. Although the progress is fast, both in

performance and reliability, there are still numerous questions that lack an answer,

or for which inconsistent explanations were given.

The questions we address in this thesis concern intrinsic and extrinsic factors

that determine the charge transport in organic conductors. In order to answer

these questions we focus our work on single crystals (the advantages of working

with single crystals are outlined in Section 1.5). We systematically study the effect

of structural defects and impurities (Chapter 2, Chapter 3), geometrical factors

(Chapter 4), molecular stacking (Chapter 6) and exposure to ambient conditions

1.7. Outline of the thesis 19

(Chapter 5).

While the organic field-effect transistor characteristics were improved in the

past years by implementation of novel processing techniques, the charge carrier

mobility remains limited. In this context, our efforts focus on the understanding

of the microscopic processes that determine the value of the mobility in molecular

crystals, in particular the generation and migration of defects. The work aims

not only at improving the performance of organic materials for possible incor-

poration in devices, but also at satisfying the pressing need for insight into the

relevant physical processes that govern the electrical conduction in these materi-

als. We show that even small defect densities are critical and act as charge traps,

consequently decreasing the mobility significantly. By systematic measurements,

we demonstrate that the origin of defects is of dual nature. The trapping sites

such as crystal defects and chemical impurities are created during crystal growth.

Additional traps are introduced during processing or exposure to ambient con-

ditions. Moreover, we propose successful methods that allow the study of high

quality materials with desired properties.

Furthermore, we incorporate the investigated materials into field-effect de-

vices. Here, the conduction channel is limited to the vicinity of the interface. We

show that the conductivity is strongly influenced by disorder and charge traps

near the interface, and propose a reliable method to overcome this limitation.

Hence, our efforts can broadly be defined as contributions in the area of fun-

damental and applied studies on the organic molecular crystals properties. By

means of correlations between electrical measurements and structural properties,

determined via X-ray diffraction, or thermodynamic response via thermogravi-

metric experiments, we wish to cover important features of the chemical, physical

and technological problems that arise in these materials.

1.7 Outline of the thesis

This thesis is organized as follows:

Chapter 2 emphasizes the importance of the control of defects and impurity

states in organic molecular crystals in order to obtain a high electronic mobil-

ity. We measure record electronic mobilities as a result of the reduction of the

number of traps by careful crystal growth and subsequent handling. The temper-

ature dependence of the mobility is consistent with the band model for electronic

transport.

In recent studies, little attention is paid to the concentration, distribution

of impurities and their consequences on electronic properties. Thus far, it has

20 Chapter 1. Introduction

been generally assumed that the impurities are evenly distributed throughout

the lattice. In Chapter 3 we will show that this is not the case for pentacene

single crystals, where the impurities are are located preferentially at the surface.

Moreover, we describe a new, reliable method, and demonstrate that in our field-

effect transistor devices the impurity states at the interface can be made inactive

by incorporating them into the dielectric gate barrier.

In Chapter 4 we give a geometrical description of the electric field distribution

in organic crystals. We report the cross-over from 1D to 2D - space charge limited

conduction in pentacene single crystals with planar contacts. Furthermore, we

establish the parameters for which the conduction is dominated by bulk charges

and show that the transition to a surface-governed transport takes place gradually.

These results incorporate corrections for the anisotropic resistivity.

Chapter 5 is dedicated to the effect of air exposure on the electronic prop-

erties of pentacene single crystals. We show experimentally that air can diffuse

reversibly in and out of the crystals. This process is reflected in the electrical

properties. We are able to distinguish two competing mechanisms that modulate

the electronic transport: the doping effect of oxygen and the trapping caused by

the presence of water vapor. By combining the gravimetric and electric measure-

ments, we can describe quantitatively these effects.

The investigations that yield the results presented in Chapters 2, 3, 4, and 5

focused on the study of pentacene single crystals, the first organic crystalline

material already incorporated in prototypes of commercial devices [20]. The mea-

surements discussed in Chapter 6 were motivated by intriguing changes observed

in the values of electronic mobility of rubrene single crystals at low temperatures.

We relate this changes with the structural transformations in the material.

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