© 2017 Szuheng Houfdcimages.uflib.ufl.edu/UF/E0/05/08/42/00001/HO_S.pdftoward organic displays:...

TOWARD ORGANIC DISPLAYS: SOLUTION PROCESSED ORGANIC LIGHT EMITTING DIODES AND TRANSPARENT VERTICAL LIGHT EMITTING

TRANSISTORS

By

SZUHENG HO

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2017

To my family

4

ACKNOWLEDGMENTS

This dissertation was made possible by a tremendous help from so many people,

no matter intellectually or mentally. First of all, I would like to thank my advisor, Prof.

Franky So, for the opportunity you gave me to pursuit my graduate study. I really

appreciate your patient support and inspiring encouragement for me to be an adaptive

thinker. I also want to thank my committee members including Prof. Stephen Pearton,

Prof. Rajiv Singh, Prof. Jennifer Andrew and Prof. Jing Guo for your service.

I want to give thanks to all of my labmates for being willing to help me when I

couldn’t complete the task on my own. I couldn’t finish several research projects without

your insightful discussions, knowledgeable technical supports and warm inspiration for

standing up from the failure. I want to express my gratitude to Dr. Rui Liu, Dr. Ying

Chen, Dr. Dewei Zhao, Dr. Hyeonggeun Yu and Dr. Shuyi Liu for your research related

collaboration and the influence for me to be a better scientist. I thank Dr. Chaoyu Xiang,

Cheng Peng, Xiangyu Fu and Ryan Larrabee for the good times of working together. I

would also like to thank Prof. Chin-Lung Kuo for your guidance during my

undergraduate research and the support for me go to an overseas study.

I wouldn’t have survived the tough time in my graduate school without friends

outside of the lab. I owe my great thanks to my fellows in Gainesville, Dr. Chun-Chieh

Wang, Justin Hung, Sean Chien, Brian Hsieh, Charles Wu and Chin-Lun Tsung for our

continuous chatters about life and future careers as well as occasional visits even when

I moved from Gainesville to Raleigh. I also want to thank my friends in Christ. I thank

Larry and Shirley Ingram for the English Bible class in Gainesville. I thank Karl and Jo

Ann Kosobucki, Andy and Cheryl White, and Bob and Barb Block at Colonial Baptist

Church of Cary. I am very grateful for Howard and Joann Su for your continuous prayer

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for me and the chance to co-work in the fellowship at your home. I would like to show

my appreciation to Esther Wei for our spiritual talk. It is great to have someone to share

about life of the graduate study and faith in Christianity.

Finally, I thank my beloved parents and other family members. You have been

caring about me during a lot of down time in my life. Your ceaseless love and prayer is

my greatest support. Last but not least, I thank God for every circumstance You brought

to my life. You know better than I. Your ways are higher than my ways. (Isaiah 55:9)

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TABLE OF CONTENTS page

ACKNOWLEDGMENTS .................................................................................................. 4

LIST OF TABLES ............................................................................................................ 9

LIST OF FIGURES ........................................................................................................ 10

ABSTRACT ................................................................................................................... 13

CHAPTER

1 INTRODUCTION .................................................................................................... 15

1.1 Organic Semiconductors ................................................................................... 15

1.1.1 Molecular Orbitals.................................................................................... 15

1.1.2 Carrier Injection and Transport ................................................................ 17

1.1.3 Exciton and Excitonic Energy Transfer .................................................... 22

1.1.4 Photo-physical Properties ........................................................................ 24

1.1.4.1 Singlet and triplet ........................................................................... 24

1.1.4.2 Optical transitions and the Jablonski diagram ................................ 25

1.2 Organic Light Emitting Diodes .......................................................................... 27

1.2.1 Development and Applications ................................................................ 27

1.2.2 Operating Mechanisms and Functional Layers ....................................... 28

1.2.2.1 Electrodes and injection layers ...................................................... 29

1.2.2.2 Transport layers ............................................................................. 30

1.2.2.3 Blocking layers ............................................................................... 31

1.2.2.4 Emitting layers ............................................................................... 32

1.2.3 OLED Device Structures ......................................................................... 32

1.2.4 Fabrication Techniques ........................................................................... 33

1.2.4.1 Vacuum thermal evaporation ......................................................... 33

1.2.4.2 Solution process ............................................................................ 35

1.2.5 Device Characterization .......................................................................... 36

1.2.5.1 OLED efficiency ............................................................................. 36

1.2.5.2 OLED lifetime ................................................................................. 39

1.2.5.3 Time resolved luminescence .......................................................... 40

1.2.6 Optics in OLEDs ...................................................................................... 40

1.3 Vertical Organic Field Effect Transistors and Light Emitting Transistors ........... 42

1.3.1 History and Applications .......................................................................... 42

1.3.2 Device Architecture and Working Principles ............................................ 44

1.3.2.1 Organic permeable base transistors .............................................. 44

1.3.2.2 Vertical organic field effect transistors ............................................ 45

1.3.2.3 Vertical organic light emitting transistors ........................................ 46

1.3.3 Porous Electrode Fabrication .................................................................. 46

1.3.4 Characterization of VOFETs and VOLETs .............................................. 47

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1.4 Dissertation Organization of Organic Displays .................................................. 49

2 SOLUTION PROCESSED MULTILAYER OLEDS ................................................. 61

2.1 Background and Motivation .............................................................................. 61

2.2 The Approaches for Hole Injection/Transport Layers ........................................ 62

2.2.1 Hole Injection Materials ........................................................................... 62

2.2.1.1 Polymers ........................................................................................ 62

2.2.1.2 Small molecules ............................................................................. 66

2.2.2 Cross-linkable Materials for HTLs ........................................................... 66

2.2.2.1 Oxetane-based HTLs ..................................................................... 66

2.2.2.2 Styrene-based HTLs ...................................................................... 68

2.2.2.3 Perfluorocyclobutane-based and BCB-based HTLs ....................... 69

2.2.2.4 Other cross-linking chemistries ...................................................... 70

2.2.3 Metal Oxides for HILs/HTLs .................................................................... 71

2.2.3.1 N-type metal oxides for HILs .......................................................... 71

2.2.3.2 P-type metal oxides for HTLs ......................................................... 73

2.3 The Approaches for Emitting Layers ................................................................. 74

2.3.1 Cross-linkable EMLs ................................................................................ 74

2.3.2 Orthogonal Material-solvent Set for Combined EML/ETL ........................ 76

2.4 The Approaches for Electron Transport Layers ................................................ 77

2.5 Summary .......................................................................................................... 80

3 SOLUTION PROCESSED HOLE INJECTION AND TRANSPORT LAYERS ......... 85

3.1 An Aqueous Based Polymer HIL for Stable OLEDs .......................................... 85

3.1.1 Background and Motivation ..................................................................... 85

3.1.2 Results and Discussion ........................................................................... 86

3.1.2.1 Space charge limited dark injection characterization ..................... 86

3.1.2.2 Phosphorescent green OLEDs ...................................................... 88

3.1.2.3 Device stability ............................................................................... 88

3.1.3 Summary ................................................................................................. 90

3.1.4 Experimental Section ............................................................................... 91

3.2 A Cross-linkable HTL for Solution Processed Multilayer OLEDs ...................... 92

3.2.1 Background and Motivation ..................................................................... 92

3.2.2 Results and Discussion ........................................................................... 93

3.2.2.1 Hole mobility measurement ............................................................ 94

3.2.2.2 Morphology .................................................................................... 95

3.2.2.3 Phosphorescent OLEDs performance ............................................ 96

3.2.2.4 Device stability ............................................................................... 97

3.2.3 Summary ................................................................................................. 98

4 INTERFACE EFFECT OF EML IN SOLUTION PROCESSED MULTILAYER OLEDS .................................................................................................................. 106

4.1 Background and Motivation ............................................................................ 106

4.2 Results and Discussion ................................................................................... 108

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4.2.1 Efficiency Loss in Solution Processed OLEDs of Two Distinct ETLs ..... 108

4.2.2 Effect from The Bulk Film Packing Density ............................................ 109

4.2.3 Effect of Interface States by A Single Carrier Device Study .................. 110

4.2.4 Further Investigation of Interface States ................................................ 111

4.2.5 Proposed Scenarios .............................................................................. 113

4.3 Summary ........................................................................................................ 113

4.4 Experimental Section ...................................................................................... 114

4.4.1 The EML Preparation and Study ........................................................... 114

4.4.2 OLED Fabrication and Characterization ................................................ 115

5 SEMI-TRANSPARENT VERTICAL ORGANIC LIGHT EMITTING TRANSISTORS .................................................................................................... 123



5.2.1 The Porous ITO Electrode ..................................................................... 124

5.2.2 Device Operation Mechanism ............................................................... 125

5.2.3 VOLET Device Performance ................................................................. 126

5.2.4 Porous ITO Scattering Effect ................................................................. 127

5.2.5 Effect of Channel Layer Thickness ........................................................ 128

5.3 Summary ........................................................................................................ 129


5.4.1 VOLET Fabrication ................................................................................ 129

5.4.2 Device and Film Characterization .......................................................... 131

5.4.3 Optical Modeling and Simulation ........................................................... 131

6 INDIUM-TIN OXIDE/INDIUM-GALLIUM-ZINC OXIDE SCHOTTKY JUNCTION BY GRADIENT OXYGEN DOPING ...................................................................... 138



6.2.1 Contacts between a-IGZO and Electrodes ............................................ 139

6.2.2 ITO/a-IGZO Diodes and Permeable Metal-base Transistors ................. 141

6.3 Summary ........................................................................................................ 143


7 CONCLUDING REMARKS ................................................................................... 147

7.1 Summary ........................................................................................................ 147

7.2 Outlook ........................................................................................................... 149

LIST OF REFERENCES ............................................................................................. 151

BIOGRAPHICAL SKETCH .......................................................................................... 177

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LIST OF TABLES

Table page 1-1 The corresponding unit of photometric and radiometric. ......................................... 60

2-1 Properties of various polymer-based HIL materials. ................................................ 82

2-2 The device structure and performance of a OLED using X-HTLs. .......................... 83

2-3 The HyLEDs with a solution processed metal oxide HIL/HTL. ................................ 84

4-1 List of previous works on comparing solution processed and vacuum evaporated films. .................................................................................................. 122

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LIST OF FIGURES

Figure page 1-1 Electronic configurations of hybrid orbitals and the orientations in space of a

carbon atom. ........................................................................................................... 51

1-2 The sp2 hybridization of σ and π bonding. .............................................................. 51

1-3 The schematic illustration of molecular orbital splitting and formation of continuous bands. ................................................................................................... 52

1-4 Energy band diagram showing the carrier injection mechanisms at the interfaces between metal and organics................................................................... 52

1-5 Energy band diagram at the metal/organic interface illustrating the image force effect. ...................................................................................................................... 52

1-6 The current density versus electric field characteristics under various regimes of applied fields. .......................................................................................................... 53

1-7 The field dependent and thermally assisted hopping transport. .............................. 53

1-8 Wannier-Mott, CT and Frenkel excitons in terms of the degree of delocalization. .. 53

1-9 The schematic description of Förster and Dexter excitonic energy transfer. ........... 54

1-10 The HOMO-LUMO illustration of singlet and triplet energy states. ........................ 54

1-11 The Jablonski diagram to illustrate the relaxation processes. ............................... 54

1-12 The absorption and emission spectra illustrating Stokes shift and Franck-Condon principle. .................................................................................................. 55

1-13 The schematic band diagrams of an OLED operated at different bias conditions. ............................................................................................................ 55

1-14 The illustration of the blocking layers in an OLED. ................................................ 55

1-15 OLED device structures. ....................................................................................... 56

1-16 The procedure of spin-coating. ............................................................................. 56

1-17 The eye sensitivity function, V(λ), as a function of wavelength. ............................ 56

1-18 The light out-coupling paths and the loss channels. .............................................. 57

1-19 Device structures of organic transistors. ............................................................... 57

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1-20 The band diagram and device structure of OPBTs. .............................................. 58

1-21 The schematic device structure and band diagram between ITO source and C60 channel. .......................................................................................................... 58

1-22 The procedure of colloidal lithography. ................................................................. 59

1-23 The typical electrical J-V characteristics of a VOFET. ........................................... 59

2-1 Cross-linkable hole transport materials. .................................................................. 82

3-1 The chemical structure of AQ1200 and the device structures used in this work.. . 100

3-2 The hole injection properties of AQ1200. .............................................................. 100

3-3 Phosphorescent OLED J-V-L with HILs (AQ1200 and PEDOT:PSS) and without a HIL. .................................................................................................................... 101

3-4 The current efficiency versus brightness for devices with different HILs. .............. 101

3-5 The J-V characteristic variation with time under ambient condition. ...................... 102

3-6 The operation stability of AQ1200 based phosphorescent OLED. ........................ 102

3-7 Chemical structure and properties about the PLEXCORE® HTL........................... 103

3-8 The AFM images. .................................................................................................. 103

3-9 The phosphorescent OLED and its performance. ................................................. 104

3-10 Device operation stability. ................................................................................... 104

3-11 PL spectra prior to and after lifetime testing. ....................................................... 105

4-1 The materials used in this work and the energy band diagram of the OLEDs. ...... 117

4-2 The device performance. ...................................................................................... 117

4-3 The refractive index of solution processed EML films with different solute concentration. ....................................................................................................... 118

4-4 The J-E characteristics of hole only devices fabricated by solution process. ........ 118

4-5 The bulk film packing effect on efficiency of TPBi ETL devices. ........................... 119

4-6 The bulk film packing effect on efficiency of B3PYMPM ETL devices. .................. 119

4-7 The J-E characteristics of hole only devices (HOD). ............................................. 119

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4-8 The normalized PL spectra of TCTA/B3PYMPM bilayer. ...................................... 120

4-9 The temperature dependent zero field hole mobility of neat TCTA films from solution process and vacuum deposition. ............................................................. 120

4-10 The proposed scenario energy band diagrams. .................................................. 121

5-1 The VOLET with a porous ITO source electrode. ................................................. 132

5-2 The transmittance spectra of thin metal drain electrode, the stack of porous ITO/HfO2/ITO and the VOLET device. .................................................................. 132

5-3 The working mechanism of the transparent VOLETs. ........................................... 133

5-4 The performance of a VOLET. .............................................................................. 134

5-5 The transparent OLED with a planar ITO electrode. ............................................. 134

5-6 The transparent OLED with a porous ITO electrode. ............................................ 135

5-7 The luminance distribution of OLEDs. ................................................................... 135

5-8 The optical scattering effect from the porous ITO source electrode. ..................... 136

5-9 The effect of channel layer thickness. ................................................................... 137

6-1 Current-voltage characteristic of Al/a-IGZO/ITO and Al/a-IGZO/Au devices showing ohmic contacts at the a-IGZO junctions for both cases. ......................... 144

6-2 The effect of oxygen component on the electrical property. .................................. 144

6-3 Performance of the graded IGZO diodes. ............................................................. 145

6-4 Performance of all transparent IGZO devices. ...................................................... 145

6-5 Characteristic of all transparent PMBT. ................................................................. 146

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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

TOWARD ORGANIC DISPLAYS: SOLUTION PROCESSED ORGANIC LIGHT

EMITTING DIODES AND TRANSPARENT VERTICAL LIGHT EMITTING TRANSISTORS

By

Szuheng Ho

May 2017

Chair: Franky So Major: Materials Science and Engineering

Organic semiconductors have been used for practical display applications, such

as the organic light emitting diode (OLED). Because of the self-emission of the organic

material upon a current application, no backlight is required. With the development of

more efficient and stable OLEDs, thinner lightweight panels with a lower power

consumption compared to conventional backlight displays have been realized.

Currently, the manufacturing cost and device lifetime remain as the major challenges in

OLED technologies. In order to bring down the cost, an OLED fabricated by solution

process is a promising approach. However, the lack of appropriate solution processed

hole injection and transport layers (HIL and HTL) is a primary issue. In addition,

fabricating a multilayer OLED device by solution process is never trivial. The issues,

such as layer intermixing or solvent residues, introduced by using a solvent pose a

limitation in terms of device efficiency and stability.

The first part of this dissertation aims to study HIL, HTL and emitting layer (EML)

in solution processed OLEDs. First, the incorporation of a solution processed HIL and

the correlation between the hole injection property to OLED performance are discussed.

14

Second, the demonstration of a suitable HTL for a multilayer solution processed OLED

and its properties are presented. Third, interface effect of an EML made by solution

process and thermal evaporation is investigated and correlated to the device

performance. It is found that band tail states broadening along with an energy level shift

at the interfaces between the EML and the electron transport layer causes hole leakage

current and hence reduced OLED efficiency.

The second part of this dissertation is focused on an emerging display

technology: the vertical organic light emitting transistor (VOLET). The VOLET device

structure is a vertical integration of an OLED with the switching transistor. A semi-

transparent VOLET is demonstrated with the luminance and current efficiency of 500

cd/m2 and 8.8 cd/A on the bottom side, and 250 cd/m2 and 4.6 cd/A on the top side.

Additionally, by using an oxygen doping technique a transparent permeable based

transistor is demonstrated with ITO and indium-gallium-zinc oxide back-to-back

Schottky junctions.

15

CHAPTER 1 INTRODUCTION

1.1 Organic Semiconductors

1.1.1 Molecular Orbitals

The definition of organic materials is generally referred to compounds with

carbon atoms as the backbone. Before the discovery of photoconductivity of anthracene

crystals,1 this class of materials was initially considered as insulators. In 1977, an

insulator to metal transition has been demonstrated by chemical doping in conjugated

polymers, which opened the era of organic electronics.2 The electrical conductivity of

organic materials is largely influenced by the chemical bonding of carbon atoms. The

ground state electronic configuration of a carbon atom is 1s22s22p2. Only the outer four

electrons are valence electrons, which may participate in bonding with other atoms.

Based on traditional valence-bond (VB) theory, the bonding should only involve the two

unpaired 2p electrons. Therefore, the tetravalence carbon atom needs to be explained

by introducing the concept of promotion, in which an electron is excited to an orbital with

higher energy before forming hybrid orbitals for bonding. Depending on the number of

2p orbital participating in the mixture of orbitals, there are three types of hybridizations:

sp3, sp2 and sp. The electronic configuration of hybrid orbitals in carbon and their

orientation in space are shown in Figure 1-1.

In organic semiconductors, the sp2 hybrid orbitals and the remaining

unhybridized 2p orbital play a critical role. There are three sp2 hybrid orbitals (composed

of the 2s orbital and two 2p orbitals) and one 2p orbital in such materials. The sp2 hybrid

orbitals form trigonal coplanar bonding, whereas the unhybridized 2p orbital is in the

plan perpendicular to the sp2 plane. When two adjacent carbon atoms are brought

16

together, the σ bonds can be formed in a head-on geometry between sp2 - sp2 hybrid

orbitals, which are strongly localized. As the unhybridized 2p orbital overlaps another 2p

orbital from a neighboring atom in the edge-on direction, the π bond is formed. Figure 1-

2 depicts the σ and π bonds in a sp2 hybridization system. When the amount of sp2

hybridization carbons is more than four, the conjugated structure with alternating single

and double bonds can be formed. The single bond consists of a σ bond and the double

bond is a combination of σ and π bonds. In the conjugated structure, the π bond is

relatively weak, in which the electrons are delocalized and move relatively freely within

the molecule. The delocalized π electron cloud in organic semiconductors is similar to

the band state delocalization in inorganic semiconductors. In a long chain conjugated

molecule, the overlap of molecular orbitals leads to the splitting of bonding π orbital and

anti-bonding π* orbital, according to the Pauli exclusion principle. If the number of

carbon atoms is significant, the degenerate π and π* levels become continuous bands

(Figure 1-3). The highest energy level of π is termed highest occupied molecular orbital

(HOMO), while the lowest energy level of π* is called lowest unoccupied molecular

orbital (LUMO). HOMO and LUMO of organic semiconductors are in a sense the

analogy to valance band maximum (VB) and conduction band minimum (CB) of

inorganic counterparts, respectively. Normally, the intramolecular (within a molecule)

orbital overlap is larger than the intermolecular orbital overlap. As a result, the

delocalized electron might not be able to move between molecules as freely as the case

within a molecule. The transport mechanism of organic semiconductors is thereby quite

different from that in inorganic semiconductors. The carrier injection and transport will

be discussed in the next part. One should note that the aforementioned description is

17

based on the VB theory, which can help understand the semiconducting properties of

organic materials more intuitively. The molecular orbital (MO) theory is, however, more

widely used in modern computational analysis of organic materials (including small

molecules, polymers and biomolecules).3 MO theory accepts that electrons should be

treated as spreading throughout the entire molecular rather than belonging to a

particular bond. The deduction of MO theory involves the scope of quantum chemistry

and mathematical operations, which are beyond the discussion here.

1.1.2 Carrier Injection and Transport

The carrier injection at the interfaces between metal and organic semiconductor

can be described by the following mechanisms: thermionic emission, Fowler-Nordheim

(F-N) tunneling and thermos-activated hopping injection (as illustrated in Figure 1-4).4,5

In the case of thermionic emission (Figure 1-4A), it is assumed that (i) the barrier

height is much larger than thermal energy kT, (ii) thermal equilibrium is established at

the interface, and (iii) the current flow from metal to organic doesn’t influence the current

from organic to metal. The thermionic emission current density can be written as5

𝐽𝑇𝐸 = 𝐴∗𝑇2exp(−𝑞𝜙𝐵

𝑘𝑇) . (1-1)

The Richardson constant A* is given by

𝐴∗ =4𝜋𝑞𝑚∗𝑘2

ℎ3 , (1-2)

where k is the Boltzmann constant; h is the Planck’s constant and m* is the effective

mass of the carrier.

In most cases, the organic semiconductors are in an amorphous form, which is

filled with traps and disordered states. If some carriers (e.g. electrons) are injected from

the metal into the organics and trapped at a distance from the metal/organic interface,

18

the positive charge will be induced at the metal surface. There is an attractive force

between the left-behind positive charge and the trapped negative charge, which is

referred to as the image force. Under an external field, the effective barrier height (ϕB)

for electron injection is as follow.5

𝜙𝐵(𝑥) = 𝜙𝑚 − 𝑞𝐸𝑥 −𝑞2

16𝜋𝜀𝑥 , (1-3)

where ϕm is the metal work function (equivalent to the injection barrier without the image

force), E the external applied field, x the distance between the trapped charge and the

metal surface, ε the permittivity of organic semiconductors and q the unit charge of one

electron. From Eq (1-3), the real barrier height experienced by the electron is lowered

by external field (second term) and image force potential (third term). Therefore,

considering the image force potential, Eq (1-3) should be substituted into Eq (1-1). The

resulting relationship of JTE is expressed as

𝐽𝑇𝐸 ∝ 𝑇2exp(𝑞𝑉

𝑘𝑇) , (1-4)

where V is the applied bias.

At a relative low temperature and strong electric field, F-N tunneling process

might dominate the carrier injection. The process pictures the situations (i) the barrier

has a triangular shape and (ii) the current only tunnels through part of the depletion

thickness (Figure 1-4B). The injection current is given by5

𝐽𝐹𝑁 =𝑞2𝐸2

16𝜋2ℏ𝜙𝐵exp[

−4√2𝑚∗(𝑞𝜙𝐵)3/2

3ℏ𝑞𝐸] , (1-5)

where ϕB is the potential barrier, and E is the applied field. After further simplification,

the equation gives

19

𝐽𝐹𝑁 ∝ 𝑉2exp(−𝑎

𝑉) . (1-6)

The thermos-activated hopping injection is shown in Figure 1-4C. The carriers are

supplied by the thermal energy. The potential barrier is the depth of each trap potential

well.

Nevertheless, no single model can be universally applied to describe the carrier

injection from the electrode into the organic. Under different temperature and bias

conditions, the injection might be dominated by any one of the above processes and

each case should be treated independently.

In addition to carrier injection, carrier transport is of tantamount importance

regarding the supply (or extraction) of carriers into the active layer. In most cases, the

solid phase organic semiconductors are amorphous, in which the interaction between

molecules is weak van der Waals forces. The typical carrier mobility in inorganic

semiconductors ranges from 10 to 1000 cm2/V-s. In contrast, most organic

semiconductors, due mainly to the amorphous structure with only weakly van der Waals

bonding, have the mobility in between 10-6 to 10-3 cm2/V-s. Therefore, the treatment for

crystal or long range order structure in inorganic semiconductors, such as band

transport, might not be suitable. The transport in organics is more like the scenario that

the carriers are hopping between several localized states. The description of hopping

models can be divided to microscopic and macroscopic views.6 The microscopic view

takes the hopping as wavefunction overlap between the initial and final states, as

explained by Marcus theory.7 The hopping rate k can be written as7

𝑘 =4𝜋2

ℎ𝑉𝑎𝑏2 exp(−𝜆/4𝑘𝑇)

√4𝜋𝜆𝑘𝑇 , (1-7)

where Vab is the terms related to the wavefunction overlap, λ is the reorganization

20

energy. As usual, T represents the temperature (thermal energy term), k the Boltzmann

constant and h the Planck’s constant. Intuitively, the increase of wavefunction overlap

and the decrease of reorganization energy of a charge carrier can increase the hopping

rate. The wavefunction overlap term and the reorganization energy are normally

obtained by quantum chemistry calculation. The First Principles calculation for

amorphous materials is sometimes very time-consuming.

On the other hand, the macroscopic view on charge carrier hopping provides an

acceptable approximation. Gaussian disorder model (GDM), introduced by Bässler,

expressed that the hopping transport takes place among Gaussian distributed density of

states.8 For organic semiconductor films, several different characteristics of current

density versus voltage (J-V) behavior occur depending on the range of applied field and

the resultant carrier density in the film. The ohmic regime typically occurs at low applied

bias, during which the sample is free of space charge. The dielectric relaxation time in

this stage is shorter than the carrier transit time. The J-V behavior shows as

𝐽 ∝ 𝑉exp(−𝑐

𝑇) . (1-8)

At a given temperature, current is linearly proportional to the applied voltage.

Assuming that the ohmic contact can ideally supply infinite numbers of charge

carriers and that the free carrier density is low in the bulk organic material, the higher

applied bias can inject more carriers than the number of carriers that the bulk organic

material can transport. The excess carrier pile-up breaks the constant electric field in

the ohmic regime. Therefore, the J-V shifts into the space charge limited current (SCLC)

regime. According to Mott-Gurney’s Law, the current density reads as9–11

𝐽 =9

8𝜇𝜀𝑟𝜀0

𝑉2

𝑑3 , (1-9)

21

where µ, εr, ε0, V, and d are the carrier mobility, the relative permittivity, the vacuum

permittivity, the applied voltage and the thickness of the sample, respectively. The

equation is based on ohmic injection and unipolar transport. The Eq (1-9) takes the

relationship of J ∝ V2, with the slope of 2 if plotting J-V in the logarithm scale. As the

further increase of the applied voltage, the trap states are gradually filled and eventually

the trap filled limited current (TFLC) regime is reached. The J-V characteristics can be

described as the following equation12,13

𝐽 = 𝑒𝜇𝑁𝐶 (2𝑙+1

𝑙+1)𝑙+1

(𝑙

𝑙+1

𝜀𝑟𝜀0

𝑒𝑁𝑡)𝑙

𝑉𝑙+1

𝑑2𝑙+1 , (1-10)

where NC is the density of state at the transport level, Nt the trap density, l the

characteristic distribution parameter related to the depth of traps. Sometimes, Eq (1-10)

is simplified using empirical expression: J ∝ Vm+1, where m is the factor of trap density

and distribution.14 At TFLC regime, m varies from 6 to 8. The J-V characteristics under

different regime is shown in Figure 1-6.

From the Mott-Gurney’s Law, the carrier mobility is treated as a constant. In fact,

the carrier mobility in organic materials is dependent on the applied field. Poole-Frenkel

model (PFM) considers the dependence of carrier mobility to the electric field, with the

form as15,16

𝜇(𝐹) = 𝜇(0)exp(0.89𝛽𝑃𝐹√𝐹) , (1-11)

where the Poole-Frenkel slope βPF is given by

𝛽𝑃𝐹 = (𝑞3

𝜋𝜀𝑟) (1-12)

and µ(0) represents the mobility at zero-field, F the applied field, q the unit charge of

one electron and εr the relative permittivity. Substituting Eq (1-11) into Eq (1-9), modified

22

field-dependent Mott-Gurney’s Law is

𝐽 =9

8𝜀𝑟𝜀0𝜇(0)exp(0.89𝛽𝑃𝐹√

𝑉

𝑑)𝑉2

𝑑3 . (1-13)

However, the model of field-dependent mobility is not sufficient in some cases. A

more accurate description taking into account the temperature effect is known as

Gaussian disorder model (GDM). In this model, the carrier hopping is correlated to both

applied field and thermal activation. According to GDM, the mobility is written as6,8

𝜇(𝐹, 𝑇) = 𝜇∞exp[− (2𝜎

3𝑘𝑇)2]exp(𝛽√𝐹) , (1-14)

and 𝛽 = 𝐶[(𝜎

𝑘𝑇)2− Σ2] , (1-15)

where T is the absolute temperature, µ∞ the mobility at infinite temperature, k the

Boltzmann constant and C a constant. σ is the energetic disorder, which can be viewed

as the distribution of transport sites, whereas Σ is the structure disorder contributed from

the defect states. The schematic illustration of thermally assisted and field dependent

carrier hopping of GDM is shown in Figure 1-7.

1.1.3 Exciton and Excitonic Energy Transfer

An exciton is a bound electron-hole pair by Coulomb force. In a light emitting

device, electrons and holes are injected into HOMO and LUMO of the emitting layer and

form excitons, which will eventually recombine and generate photons. The exciton

formation process includes trapping of a charge carrier, carrier of the opposite charge

migration into the radius of Coulomb force attraction and binding of electron-hole pair.17

In a light harvest device, incident photons excite the electron from lower energy state

and create excitons, which will be dissociated into free electrons and holes and

collected by the electrodes. Depending on the degree of delocalization and strength of

23

bounding force, excitons can be classified into three categories: Mott-Wannier exciton,

charge transfer exciton and Frenkel exciton, as illustrated in Figure 1-8. Mott-Wannier

excitons are the excitons in inorganic (crystalline) semiconductors. The dielectric

constant of crystalline semiconductors like Si, Ge or GaAs are usually larger than their

organic analogies, which influences the screening of electron and hole. Therefore, Mott-

Wannier exciton are loosely bounded, with a much larger radius and a low binding

energy, typically on the order of 10 meV. In contrast, in organic materials, the electron-

hole pair is strongly bounded by Coulomb force and the spatial distribution of exciton is

localized within a single molecule. This type of exciton is called Frenkel exciton. The

binding energy is on the order of 1eV.18 In addition to the Frenkel exciton localized in

the same molecule, there is another type of intermolecular electron hole pair, which is

termed charge transfer (CT) exciton. This happens when an electron is transferred to an

adjacent molecule. These excitons are still localized by Coulomb force but with a

reduced bounding energy due to the increase of spatial separation. The CT exciton is a

similar idea to the exciplex (abbreviation of the excited complex) state used in organic

light emitting diodes (OLEDs).

An exciton localized on one molecule can transfer its energy to another molecule

via radiative energy transfer, Förster resonance energy transfer (FRET) or Dexter

energy transfer (DET).9 The radiative excitation energy transfer occurs when the

radiative emission (photon) of a donor is absorbed by an acceptor. In this case, the

absorption wavelength of the acceptor must overlap the emission wavelength of the

donor. The FRET process takes place when the dipole moment in the donor molecule

induced the dipole in the acceptor molecule. This type of dipole interaction requires the

24

spin conservation for both donor and acceptor. It occurs between two singlet states

within the range of 10 nm in a time scale of nanosecond. The other non-radiative

excitonic energy transfer is DET. This type of energy transfer only requires the

conservation of total spin configuration. Thus, the interaction of singlet-triplet and triplet-

triplet are allowed. DET is a short range process, which strongly depends on the orbital

overlap. The interaction can proceed within the range of 1 nm. An efficient DET is the

foundation of highly efficient phosphorescent OLED since it governs the triplet energy

transition in the emitting layer system. The processes of FRET and DET are illustrated

in Figure 1-9.

1.1.4 Photo-physical Properties

1.1.4.1 Singlet and triplet

Before the discussion of the optical transitions of excited states (excitons), the

singlet and triplet excited states will be briefly covered as its important role in modern

highly efficient OLEDs. According to Fermi-Dirac statistics, the electron is a fermion,

with an electron spin of ½ . Hole, taking the idea of an electron deficient state, shares

the same quantum mechanical characteristics. In an excited state, the exciton can be

regarded as a combination of two fermions (a hole and an electron). Based on Pauli’s

exclusion principle, a system with two particles of ½ spin must be described by an

antisymmetric total wavefunction. The total wavefunction can be written as the product

of spatial and spin wavefunctions: (total wavefunction) = (spatial wavefunction) x (spin

wavefunction). Therefore, the antisymmetric wavefunction can be achieved by

(antisymmetric spatial wavefunction) x (symmetric spin wavefunction) or (symmetric

spatial wavefunction) x (antisymmetric spin wavefunction).19 In the case of (symmetric

spatial wavefunction) x (antisymmetric spin wavefunction, S = 0), the spin wavefunction

25

is

SingletΨ𝑠𝑝𝑖𝑛 =1

√2(|↑⟩|↓⟩ − |↓⟩|↑⟩) . (1-16)

In the case of (antisymmetric spatial wavefunction) x (symmetric spin wavefunction, S =

1), the spin wavefunction reads

TripletΨ𝑠𝑝𝑖𝑛 = {

1

√2(|↑⟩|↓⟩ + |↓⟩|↑⟩)

|↑⟩|↑⟩

|↓⟩|↓⟩

. (1-17)

Typically, there are about 25% of excitons in the singlet state and 75 % excitons

in the triplet state for a small molecule organic material. Figure 1-10 illustrates the

HOMO-LUMO excited energy states. However, this ratio might be slightly favored for

singlet exciton formation in polymers, which has been attributed to the greater spatial

overlapping of electron and hole wavefunctions for singlet excitons in the same polymer

chain.20,21

1.1.4.2 Optical transitions and the Jablonski diagram

The optical transition processes can be comprehensively described by the

Jablonski diagram (Figure 1-11), where the straight arrows indicate radiative processes

and the squiggly arrows indicate non-radiative processes. In an OLED, the exciton is

electrically generated by charge injection and exciton formation. In a light harvesting

device like the solar cell, the exciton is optically generated by photon absorption, which

is a fast process in the time scale of femtosecond. Upon formation, the exciton may

reside on higher singlet states (Sn, n>1). They can go through an internal relaxation,

which involves the internal conversion (between electronic states) as well as vibrational

relaxation (between vibrational levels). These two processes are non-radiative and in

the time scale of sub-picosecond to picosecond. Due to the internal relaxation, a red

26

shift in the emission spectrum with respect to the absorption spectrum is observed,

which is termed Stokes shift, as shown in Figure 1-12. This property prevents the re-

absorption of a large portion of light within the emission wavelength of an OLED. The

vibrational transition follows the Franck-Condon principle, which states that the most

likely electronic transition is between energy states with the highest vibrational

wavefunction overlap.22 For example, upon excitation from v = 0 at S0 to v = 2 at S1, a

geometrical shift in nuclear coordinates occurs. This means now the vibrational

wavefunctions between v = 0 at S1 and v = 2 at S0 have the highest overlap, i.e. minimal

changes in the nuclear coordinates is required for vibrational relaxation. Hence, the v =

0 of S1 to v = 2 of S0 transition is strongest. The results of this transition is a mirror

between absorption and emission spectra.

The radiative emission process from lowest singlet state (S1) to ground state (S0)

is called fluorescence, in the scale of nanosecond. The exciton can also transit non-

radiatively from singlet to triplet states, and the process is called intersystem crossing.

The transition from lowest triplet state (T1) to S0 is spin-forbidden since the initial and

final states have different spin numbers (S). Generally, the rate is slower for processes

involving transition between states of different spin numbers, such as intersystem

crossing or T1 to S0 transition. In most fluorescent materials, the energy in T1 state is

released to ground state non-radiatively at room temperature. In contrast, the radiative

process of T1 to S0 transition is called phosphorescence. For the organic molecules with

heavy metal at the core, the strong spin-orbit coupling increases the wavefunction

overlap between T1 and S0, allowing the transition between these “original spin-

27

forbidden” states. Thus, the probability of intersystem crossing and triplet decay rate

increase, enhancing the phosphorescence over non-radiative T1 to S0 decay.

According to spin statistics, fluorescence (S1 to S0) gives 25% exciton and

phosphorescence generates the other 75%. In principle, the phosphorescent organic

materials can have 100% harvesting of excitons into photons.23 On the other hand,

there is another way to harvest triplet excitons by having the triplet exciton transfer back

to singlet state.24 The process is called thermally activated delayed fluorescence

(TADF). The key transition is the reverse intersystem crossing, which requires the

overlap of wavefunctions between singlet and triplet states. In addition, the energy

difference between S1 and T1 state (ΔEST in Figure 1-11) should be small such that the

thermal energy is sufficient for the exciton to overcome potential barrier and transfer

back to singlet state for radiative emission. This method can theoretically provide 100%

exciton harvesting as well.

1.2 Organic Light Emitting Diodes

1.2.1 Development and Applications

The first observation of electroluminescence (EL) in organic materials was

reported in 1953 under an ac mode high bias.25 In 1963, Pope et al. demonstrated the

EL on a single crystal anthracene under dc bias of 400 volts.26 After being quiet for

almost two decades, the breakthrough in organic EL was made by Tang and Van Slyke

at Eastman Kodak in 1987.27 In this prototype EL device, indium tin oxide (ITO) and

magnesium silver alloy (Mg:Ag) were used as anode and cathode, respectively. Two

organic layers, aromatic diamine (TPD) and aluminum-tris(8-hydroxy-quinolate) (Alq3),

were sandwiched in between. In 1990, Burroughes et al. at Cambridge University

developed the first polymer based LED in parallel.28 These two works established the

28

foundation of modern OLED device structures. The progress of OLEDs has been rapidly

soaring ever since then.

In 1992, Gustafsson et al. made the first flexible OLED on a polyethylene

terephthalate (PET) substrate, which demonstrated the possibility of flexible display

devices.29 The first white OLED was reported by Kido et al. by a combination of blue

and orange emission in 1995.30 Three years later in 1998, Baldo et al. introduced the

phosphorescent emitters, which boosted the internal quantum efficiency of the OLED to

100% (as the background in Section 1.1.4).23 The significant achievement in OLED

history marked its potential for being commercialized.

In 1997, the first OLED product, a passive matrix OLED (PMOLED), was

released by Pioneer Corporation for an automobile audio display.31 From around 2000

till present, active matrix OLED (AMOLED) have been developed and modified by the

leading organic display companies such as Sony, Samsung and LG. The OLED

television from Sony had its debut to commercial market in 2008. Two years later in

2010, OSRAM Opto Semiconductors released the first commercial white OLED lighting

panel.32 The nascent (green) OLED by Tang and Van Slyke had an external quantum

efficiency (EQE) of ~ 1% and the maximum brightness of 1,000 cd/m2. After nearly three

decades of development, the state-of-the-art green OLED can simultaneously achieve

EQE > 20%, maximum luminance > 50,000 cd/m2 and most importantly the lifetime (LT

70) > 400,000 hr at 1,000 cd/m2.33 All of these have declared the advent of versatile

OLED applications in daily electronic devices.

1.2.2 Operating Mechanisms and Functional Layers

An OLED consists of two electrodes and multiple organic layers sandwiched in

between. These organic layers commonly include hole injection layer (HIL), hole

29

transport layer (HTL), emitting layer (EML), electron transport layer (ETL), electron

injection layer (EIL), as shown in Figure 1-13. Under no bias (open circuit), the anode

and cathode energy levels are aligned. The HOMO and LUMO of these organic layers

are tilted. Without field assistance, the carrier injection is not favorable. The small

portion of injected carriers by thermal energy cannot easily transport to EML due to the

lack of applied field. (Figure 1-13A). The energy levels become flat band at the bias

condition V = Vbi (Figure 1-13B). The built-in potential (Vbi) is correlated to the work

function difference between anode and cathode. When V > Vbi, the electric field

facilitates the carrier injection. The injected carrier will be drifted to EML under the

external applied field, and eventually recombine to form radiative emission (Figure 1-

13C). The physical models of carrier injection, carrier transport, exciton formation and

recombination have been discussed previously in Section 1.1.2 through 1.1.4. In order

to have an OLED operate as ideally as the mechanism in Figure 1-13, a number of

functional layers are incorporated to the OLED architecture.

1.2.2.1 Electrodes and injection layers

To couple out the light emission, at least one electrode of the OLED needs to be

transparent. The option of transparent electrode includes transparent conductive oxides

(TCO) like ITO, thin metal and novel materials like graphene or nanowires. TCO and

thin metal layer are more widely used due to the simple fabrication. However, the work

function of TCO and metal are only restricted to a certain range. There might not be

suitable electrode for some organic materials to form a contact without an injection

barrier. Therefore, the idea of work function modification has been developed to

overcome the issue. For example, with the UV ozone treatment, the work function of

30

ITO can become deeper.34 In contrast, the introduction of a thin (< 5 nm) interfacial

dipole layer can make the surface work function of a metallic electrode shallower.35

In addition to the surface energy level modification, the insertion of an injection

layer is another way to facilitate carrier injection. The mechanism of an injection layer

can serve as a step to reduce barrier height. For example, poly (3,4-

ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) commonly serves as a

HIL,36 whereas thin alkali metal compounds like lithium fluoride (LiF)37 or 8-

hydroxyquinolinolatolithium (Liq)38 can play the role of an EIL. Another type of HIL like

MoOx facilitate hole injection via a strong electron withdrawing process from the organic

material, which is equivalent to a hole injection.39 Either the utilization of surface

modification or the insertion of an injection layer plays the role to reduce the carrier

injection barrier and thereby enhance the device efficiency.

1.2.2.2 Transport layers

The intrinsic material properties, such as the energy levels or carrier mobility, of a

transport layer can be changed significantly by electrical doping. By mixing with strong

acceptor or donor materials, the transport layer can be n-doped or p-doped,

respectively.40 There are at least two distinctive advantages of using a doped transport

layer. First, the conductivity of doped transport layer is improved, which implies a low

voltage drop within the transport layers. The high conductivity also brings the flexibility

of layer thickness design since the voltage drop barely changes with the thickness

variation. This directly benefits the optimization of optical properties of the device

without compromising the OLED driving voltage. Second, the space charge region

between the electrode and the organic layer is narrowed due to the doping, which

31

enhances the possibility of carrier tunneling through the triangular potential. The

scenario is similar to the heavily doped Schottky contact being a quasi ohmic contact in

inorganic semiconductors. Thus, the doped transport layer can serve as an injection

layer itself at the junction with the electrode. Due to the limited material options being

transparent electrode, a highly doped transport layer is powerful to ensure facile carrier

injection.

Since the p or n dopants are usually strong quenchers to triplet excitons in EML,

the doped transport layer should not be placed in a direct contact with the EML. A

blocking layer or simply the no-doped transport layer is usually required. The thickness

of this layer should be thicker than the longest distance of exciton interaction (Förster

resonance energy transfer ~ 10 nm).

1.2.2.3 Blocking layers

In order to confine charge carriers and excitons in the EML, the blocking layers

are sometimes added to the OLED structure. The position of a blocking layer is in

between the emitting layer and transport layers (as Figure 1-14). The hole blocking

layer (HBL) is between EML and ETL. The HOMO of HBL should be deeper than that of

EML such that the hole leakage can be minimized. The LUMO of the HBL should stay in

between the LUMO of EML and the LUMO of ETL such that the electron injection is not

hindered. The EBL serves the similar function on the other side of EML. The mobility of

the blocking layers should not be too low; otherwise, it will affect the carrier transport. In

addition to charge carrier blocking, the blocking layer confines the triplet excitons. The

triplet exciton has a longer lifetime, which implies the higher chance to be quenched by

32

non-radiative mechanisms. The triplet energy of the blocking layer should be higher

than that of the emitter in order to effectively confine excitons.

1.2.2.4 Emitting layers

The organic materials generally have a relatively low glass transition

temperature. Under operation, the joule heating can induce material crystallization and

aggregation, leading to exciton quenching. Doping of one organic material into the other

can inhibit the crystallization process. A host-guest system is normally used in EML of

OLEDs. The other benefit from host-guest system is the reduced concentration

quenching. If the spatial separation between triplet excitons are increased by doping the

phosphorescent material into a host, the quenching process from high triplet exciton

density in the neat film can be avoided. Depending on the dominant energy transfer

process in each phosphorescent emitter (phosphor), the optimal doping concentration

varies but generally below 10%.41

1.2.3 OLED Device Structures

The OLED device architecture can be categorized based on the stacking

sequence and the direction of light out-coupled, as the configurations in Figure 1-15.

The conventional structure has the anode at the bottom, followed by HTL, EML, ETL

and cathode. In inverted structure, the positions of anode and cathode are swapped.

For information display products, the OLEDs are integrated with thin film transistors

(TFTs). Amorphous silicon (a-Si) TFTs are the common driving transistors for the

display panels. Due to the low hole mobility of a-Si, only n-channel a-Si TFTs are readily

available. In this case, the inverted structure OLED is more favorable for the integration

with driving circuit. Another reason for choosing different structures is based on the

processing limitation, especially for wet-processed devices. Considering the solubility of

33

preceding layers in the solvent used for the subsequent layer, it is vital to find the

compatible materials combination and processing sequence.

The device structure is also classified regarding the direction of light out-

coupling. The position (top or bottom) of the transparent electrode determines the

direction light emission. This is typically irrelevant to sequence of stacks. A conventional

device can be either bottom emission or top emission, depending on where the

transparent electrode is. The top emission OLED plays a critical role in modern active

matrix OLED (AMOLED) techniques. Since the a-Si TFT is opaque and mainly n-

channel, an inverted structure top emission OLED is highly desirable. This configuration

can provide a high aperture ratio and hence a high resolution display, because the light

emission is not blocked by the backplane TFTs.42 If both electrodes are transparent, the

OLED can be transparent and emit light from both directions, which may provide more

novel applications.

Other than using one cell only OLED, the tandem OLEDs were also developed

recently.43,44 Multiple light emitting units are vertically stacked in series in a tandem

OLED, which provides multiple photons emission from one injected carrier. The

incorporation of a charge generation layer can reduce the power efficiency loss due to

linearly increased driving voltage with the number of light emitting units.45

1.2.4 Fabrication Techniques

1.2.4.1 Vacuum thermal evaporation

The vacuum deposition of small molecule organic materials is carried out at a

base pressure of 10-6 Torr or lower. Through the resistive heating to ceramic crucibles

(e.g. boron nitride and alumina) or metal boats (e.g. tantalum and tungsten), the organic

material is evaporated at a rate of 0.1 to 2 Å /s. The evaporation of organic materials

34

takes place from the liquid or the solid phase, depending on the vapor pressure at the

material’s melting point. 4,4’-bis[N-(1-napthyl)-Nphenyl-amino] biphenyl (NPB), a

common HTL, is a case of evaporation from liquid phase.46 Most of the other organic

materials reach high vapor pressure at their melting point and therefore evaporate via

sublimation. The deposition rate is monitored through a quartz crystal microbalance

(QCM) operated at the frequency of ~6 MHz.47 As the increase of material deposited

onto the QCM, the resonance frequency of the QCM decreases. Knowing the density of

the material and using a geometrical tooling factor (for calibrating the distance between

QCM and the substrate), the nominal thickness of the deposited material can be

estimated. Usually, the geometrical tooling factor needs to be reverse-calibrated

according to the real film thickness measured by the profilometer or ellipsometer. A

proportional integral derivative (PID) controller can also be integrated with the reading

from QCM to regulate a stable deposition rate. The shadow mask is used to pattern the

electrodes.

To assure a uniform film over the entire substrate, the distance between

evaporation sources and substrates should be much larger than the dimension of the

substrate. The substrate holder usually rotates during the deposition to further eliminate

the inhomogeneity. Other vapor deposition techniques have been reported to enhance

film uniformity. Organic vapor phase deposition has been developed with the idea to

separate the evaporation and deposition.48 A carrier gas is used to transport the

material from source to substrate. On the other hand, linear evaporation source was

proposed to eliminate the geometrical factors with a shorter distance between source

and substrate, which can reduce the material consumption per deposition.49

35

Vacuum deposition offers the advantages of easy multilayer fabrication and

simple film patterning. However, it has the drawbacks like low material utilization rate,

high requirement on vacuum and low throughput for large area samples, all of which

increase the cost of manufacturing. Generally, the thermal vacuum deposition can only

be applied to small molecular organics since under heating polymers tend to

decompose before evaporation. Therefore, a lot of research source has also been

extended to solution process fabrications.

1.2.4.2 Solution process

Solution process has the advantage of high throughput large area fabrication with

reduced cost. The process is compatible with the OLED fabrication of polymers as well

as small molecules. In fact, the active layers in most polymer LEDs are fabricated by

solution process. Multiple materials doping can be readily achieved in solution process

by simply mixing the materials or precursor solutions. However, due to the difficulty of

controlling the polymer purity and molecular weight distribution, the major commercial

development and manufacturing are still focused on small molecule organics. For the

small molecule OLEDs, it is challenging to fabricate multilayer solution processed

OLEDs (due to the re-dissolution) and obtain very thick films (owing to the solubility limit

of a solute material in a solvent). The challenge and progress of multilayer solution

processed OLEDs will be addressed in Chapter 3.

Spin-coating is widely used in small scale laboratory research. The substrate is

attached firmly to a chuck back sealed by a vacuum pump. The chuck along with the

substrate can be accelerated to a high speed after the solution is dropped. The dropped

solution goes through four stages, as illustrated in Figure 1-16.50 The solution is

36

dispensed and flows out to cover the whole substrate. In the early stage, the film

thickness is determined by the viscous forces (flow dominated). As the viscous flow rate

and the evaporation rate are equal, the dominant process transits to evaporation.50 The

film thickness is determined on the solute concentration, spin speed, air flow rate and

solution /substrate temperature. The final film thickness is typically determined within

the first 30 seconds; however, a longer spin time is used to remove solvent residues.

The spin-coating is usually followed by a thermal annealing step.

Inkjet printing processing is another noticeable wet process method. It can be

easily carried out with an inexpensive inkjet printer. The main advantages include high

material utilization rate and no mask required. A resolution as high as 1200 dpi has

been reported.51 Inkjet printing also provides a definite pattern, which may not be easily

achieved by other wet process techniques. On the other hand, roll-to-roll (R2R)

processing is believed to be the most practical for high throughput mass fabrication.51 It

is actually an integration of coating/printing operation with other process steps like

drying and curing. R2R manufacturing has been demonstrated in OLED devices.52

1.2.5 Device Characterization

1.2.5.1 OLED efficiency

There are two approaches to evaluate the OLED efficiency. From the

engineering perspective, it is treated as a display device. The measurement of a

standard display device involves the factor of human eye response, provided by eye

sensitivity function V(λ) from a representative number of human subjects (Figure 1-17).

The standard function now in the US is referred to CIE 1931 V(λ) function, which

correlated the radiometric quantity (optical power P, in the unit of Watt) to the

photometric equivalent (luminous flux φlum, in the unit of lumen). It reads as the following

37

equation:

Φ𝑙𝑢𝑚 = 683∫𝑉(𝜆)𝑃(𝜆)𝑑𝜆 . (1-18)

The photopic luminous efficacy (or simply luminous efficacy) is a conversion efficiency

between optical power (P) and luminous flux (φlum), defined as

Luminousefficacy =Φ𝑙𝑢𝑚

𝑃= [683∫𝑉(𝜆)𝑃(𝜆)𝑑𝜆]/[∫𝑃(𝜆)𝑑𝜆] . (1-19)

One should note that the luminous efficacy is NOT an efficiency. It represents the

equivalent stimulating radiation that is perceived by the human eye. Table 1-1 compiles

the comparison of photometric and radiometric measures and their units.

Now, we go to the efficiencies extensively used in the OLED community. The

current efficiency is calculated as the ratio of the luminance (L) output in the forward

direction to the current density (J) input to the device:

Currentefficiency:𝜂𝐶 =𝐿

𝐽[𝑐𝑑/𝐴] . (1-20)

The current density (J) equals device current (I) over device area (A). The luminance

can be collected directly from a commercial luminance meter oriented perpendicularly to

the emitting surface. Next, the power efficiency is the ratio of the total luminous power

output to the electrical power input. It can be calculated from current efficiency:

Powerefficiency(from𝜂𝐶) ∶ 𝜂𝑃 =𝑓𝐷𝜋

𝑉𝜂𝐶 [𝑙𝑚/𝑊] , (1-21)

where V is the operating voltage and fD is the angular distribution factor of EL emission

from the forward half-sphere:53

𝑓𝐷 =1

𝜋𝐼0∫ ∫ 𝐼(𝜃, 𝜑)𝑠𝑖𝑛𝜃𝑑𝜑𝑑𝜃

2𝜋

0

𝜋/2

0 , (1-22)

where I0 is the luminous intensity measured in the forward direction, I(θ,ϕ) the angular

distribution of the emitted light as a function of zenith (θ) and azimuth (ϕ). Under the

38

Lambertian distribution, the factor fD equals unity. If the angular distribution of the light

source is not readily available, the power efficiency can be measured from a calibrated

photodiode as

PowerEfficiency(fromPD):𝜂𝑃 =683∫𝑉(𝜆)

𝐼𝑃𝐷(𝜆)

𝑅(𝜆)𝑑𝜆

𝐼𝑉[𝑙𝑚/𝑊] , (1-23)

where V(λ) is the eye sensitivity function, IPD(λ) is the photocurrent generated in the

photodiode, R(λ) is the responsivity of the photodiode. I and V are the OLED input

current and voltage, respectively.

From the perspective of device physics, the quantum efficiency gives the more

insight into the fraction of light that is coupled out. The definition of external quantum

efficiency is the ratio of the total number of photons emitted (in all directions) to the total

number of electrons injected. The measurement, however, is not as straightforward as

the definition. The common method is to place the OLED into an integrating sphere with

a calibrated photodiode attached and then measure the photons output. The integrating

sphere is coated with reflective materials internally to minimize photon absorption loss

during the measurement. Considering that the responsivity (R) of a photodiode is not

uniform over the spectrum, the integration over the spectrum is required. The external

quantum efficiency is given by54

Externalquantumefficiency:𝜂𝐸𝑄𝐸 =∫[𝐼𝑃𝐷(𝜆)/𝑅(𝜆)]

ℎ𝑐/𝜆𝑑𝜆

𝐼/𝑞=

𝑞 ∫𝜆𝐼𝑃𝐷(𝜆)𝑑𝜆

ℎ𝑐𝐼 ∫𝑅(𝜆)𝑑𝜆[%], (1-24)

where h is the Planck’s constant; c is the speed of light in vacuum; q is the charge of an

electron. All the other terms have the definition the same as Eq (1-23). The EQE can be

further described as the multiplication of four factors:

𝜂𝐸𝑄𝐸 = 𝜂𝑜𝑢𝑡𝜂𝐼𝑄𝐸 = 𝜂𝑜𝑢𝑡𝜂𝐶𝛾𝑆𝑇Φ𝑃𝐿, (1-25)

39

where ηout is the out-coupling efficiency, ηC the charge balance factor, γST the fraction of

radiative excitons and ΦPL the photoluminescence (PL) quantum yield of the emitter.

The internal quantum efficiency (ηIQE) is the combination of the latter three factors. The

ηout is largely dependent on the optical structure of the organic layers, i.e. the thickness

and refractive index. To enhance ηout, total internal reflection at all interfaces should be

minimized. The ηC can be optimized by varying the material and thickness of transport

layers or by changing the injection barrier of each carrier. The single carrier J-V curve is

a way to monitor the factor. The γST and ΦPL are correlated to the exciton ratios and

material design.

1.2.5.2 OLED lifetime

The degradation of an OLED is a critical parameter. Most organic materials are

unstable under ambient conditions with higher levels of moisture and oxygen. The

growth of dot spots and catastrophic failure are the early issues subject to the operation

environment.55 With proper encapsulation, the intrinsic degradation mechanisms from

the device architecture or stability of materials under conditions with abundant of

charges and excitons can be studied. The measurement is typically performed under a

constant current operation. The luminance and driving voltage are monitored with the

elapsed of time. As more and more stable OLEDs have been demonstrated, the

acceleration factor is used to estimate the long term operational stability of OLEDs.

𝐿𝑛×𝑡 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡𝑜𝑟𝑡1

𝑡2= (

𝐿1

𝐿2)𝑛, (1-26)

where the L (or L1, L2) is the starting luminance; t is the time of luminance drop to a

certain percentage (say 90%) of the initial luminance, termed LT90. n is the acceleration

factor, typically ranging in between 1.5 and 2.56

40

1.2.5.3 Time resolved luminescence

The time resolved (or transient) luminescence is a technique to monitor the

exciton dynamics over time after the generation either by electrical source (EL) or

optical source (PL). The excitation is carrier by applying a voltage pulse to the OLED

from a function generator in transient EL, whereas the pulse laser is used us the

excitation source in transient PL. The photomultiplier tube (PMT) is utilized to increase

detecting sensitivity over the thermal noise limit. The signal is finally collected by an

oscilloscope.

They time resolved EL/PL decay not only tells the emission mechanism (like

phosphorescence or TADF),57 but it also reveals the interaction of excitons with

adjacent molecules. The time resolved EL technique has been applied to the study of

triplet exciton energy transfer,58 exciton annihilation and quenching,59,60 and the reaction

of photon to exciton.61 All of aforementioned topics shed light on the physical

understanding of reduced efficiency, efficiency roll-off at high density of carriers and the

device degradation mechanisms.

1.2.6 Optics in OLEDs

From Eq 1-25, the photons generated in the EML cannot be completely coupled

out. The reason is that the refractive index of the organic films is higher than that of the

glass substrate and ambient air as well. From a simple ray optics model,62 the out-

coupling efficiency (ηout) is given as

𝜂𝑜𝑢𝑡 =1

2𝑛2 , (1-27)

with n being the refractive index of organic materials. From Eq 1-27, the maximum out-

41

coupling efficiency based on n = 1.6 is about 20%, which can serve as a quick but rough

estimation.

The wave optics considers the light extraction as the electromagnetic wave

propagation from the emitter through multiple interfaces. To simplify, the following

discussion is based on non-magnetic, linear responding, and isotropic media. The wave

is either reflected or refracted at the interface between media. For the reflected wave,

the reflection law holds true, 𝜃𝑖 = 𝜃𝑟. The refracted wave is actually transmitted into the

neighboring medium, following Snell’s law:

�̃�1𝑠𝑖𝑛𝜃1 = �̃�2𝑠𝑖𝑛𝜃2 , (1-28)

where ñ1 and ñ2 are the refractive indices of medium 1 and 2, respectively; θ1 is the

incident angle relative to normal direction, whereas θ2 is the refracted angle relative to

normal direction. The refractive indices of typical organic materials (n = 1.6-1.8) and

glass (n ~ 1.5) are higher than that of air (n = 1). As the incident angle θ1 increases,

there is a critical angle that the refracted angle is 90o, reaching the condition of total

internal reflection (TIR). The TIR can happen at any interface, causing the trapped

photons within organic layers (waveguided mode) and in the glass substrate (substrate

mode). Figure 1-18 show the wave light propagation and major loss modes.

The electromagnetic field can be differentiated between s- and p- polarized field,

depending on the oscillating field perpendicular or parallel to the plan of incident. The

reflectance of s-polarized and p-polarized waves can be written using Fresnel

coefficients

𝑅𝑆 = |𝑛1𝑐𝑜𝑠𝜃𝑖−𝑛2𝑐𝑜𝑠𝜃𝑡

𝑛1𝑐𝑜𝑠𝜃𝑖+𝑛2𝑐𝑜𝑠𝜃𝑡|2 , (1-29)

𝑅𝑃 = |𝑛1𝑐𝑜𝑠𝜃𝑡−𝑛2𝑐𝑜𝑠𝜃𝑖

𝑛1𝑐𝑜𝑠𝜃𝑡+𝑛2𝑐𝑜𝑠𝜃𝑖|2 . (1-30)

42

In case of non-absorption, 𝑇𝑆 = 1 − 𝑅𝑆 and 𝑇𝑃 = 1 − 𝑅𝑃. If n1 > n2, the reflectance RS

and RP approaches 1 at a certain angle, which is the critical angle in Snell’s law.

The electromagnetic wave can also be coupled to the electron gas in the metal,

finally dissipating power as evanescent decaying field of surface plasmon polaritons

(SPP mode). All reflectance and transmittance light paths in a multilayer structure can

be taken into account with the help of transfer matrix method,63 which enables the

OLED optical analysis for loss mechanisms. In order to suppress these loss channels, a

number of light extraction techniques have been used. The key approaches include

random scattering layers, photonic crystals, macroscopic attached structures, dipole

orientation of emitter molecules and refractive index modulation.64–66

1.3 Vertical Organic Field Effect Transistors and Light Emitting Transistors

1.3.1 History and Applications

The first concept of the transistor was proposed by Edgar in 1926,67 as in the

patent “Method and apparatus for controlling electric currents.” Subsequently in late

1940s, the first practical transistor and theoretical studies were developed by Shockley

et al. at Bell Labs.68,69 Due to the breakthrough invention, the Nobel prize in physics was

awarded to the researchers (including Shockley) on the transistor effect in 1956.70 Later

in 1960 also at Bell Labs, Kahng and Atalla demonstrated the first metal-oxide-

semiconductor field-effect transistor (MOSFET),71 which is the unit component of

modern integrated circuits (ICs). The next step forward was the building of ICs devices

like microprocessors and memories from transistors. One of the key researcher at

Texas Instruments, Kilby, received the Nobel prize in physics for his contribution to the

invention of ICs in 2000.72 With the significant advance in processing techniques and

device architecture, Taiwan Semiconductor Manufacturing Company (TSMC) has

43

projected a 7 nm (which can be roughly understood as the distance between gate

pitches of transistors) fabrication in 2017.73 In terms of a computer microprocessor

nowadays, there might be more than 6.4 billion transistors in it.74 By far, the most

reliable MOSFET devices are based on silicon, germanium or III-V semiconductors. The

device fabrication typically includes crystal growth, ion implementation, etching, etc.

As mentioned Section 1.1.1, in 1977 Heeger et al. demonstrated that the electron

conductivity in organic materials could be tunable up to 7 orders, leading to the

possibility of conductive polymers.75 The researchers discovering conductive polymers

were awarded the Nobel prize in Chemistry in 2000.76 The realization of the polymer

based field effect transistors was in 1980s.77–79 The research of organic electronics has

tremendously expanded due to the merits of low cost, easy and low temperature

processing, and the potential for flexible devices. The first vertical type organic

transistor, in which the charge carriers transport along the direction perpendicular to the

substrate plane, was reported by Yang and Heeger in 1994.80 Similar to the inorganic

transistors, there are several different categories of vertical organic transistors.

According to how the vertical current in the device is controlled, they can be classified

as organic permeable base transistors (OPBT) and vertical organic field effect

transistors (VOFET).81 In a vertical transistor, the short channel can be achieved without

complicated patterning techniques. The channel length is simply determined by the

thickness of the organic layer rather than the lateral distance between electrodes in the

horizontal counterpart.

One of the function of a transistor is to modulate the current, which can also be

used to drive a light emitting device. If the light emitting unit is directly integrated with a

44

transistor, it is called the light emitting transistor. The first planar82 and vertical83 organic

light emitting transistors were reported in 2003 and 2007, respectively.

1.3.2 Device Architecture and Working Principles

A vertical transistor is a type of transistor that the current flow is normal to the

substrate plane. The vertical transistors are divided into two major category based on

the mechanism of current modulation. Figure 1-19 illustrates the schematic device

architectures of vertical transistors (including the permeable base transistor and the

vertical field effect transistor) along with the conventional horizontal transistor.

1.3.2.1 Organic permeable base transistors

Organic permeable base transistors (OPBT) consist of three electrodes stacked

vertically and at least one organic semiconductor sandwiched in between any two

electrodes (Figure 1-19A and 1-19B). In a typical structure, the emitter and collector

(source and drain in some literature) are at the bottom and top, respectively. A thin base

(or called gate) electrode is inserted in the middle to modulate the carrier flow, rendering

the on and off states of the transistor. Two prevailing mechanisms have been used to

explain the control of carrier flow. As the band diagram shown in Figure 1-20A, the

material of the base is intentionally chosen to form a high carrier (electron here)

injection barrier. During operation, the emitter to base is under forward biased, whereas

the collector to base is under reverse bias. The electron from emitter has high energy

(hot electron) and is injected into the base over its Fermi level. If the base electrode is

very thin, the hot electron is able to overcome the energy offset between the Fermi level

of the base and the LUMO of the organic material on the collector side. The IEC in

Figure 1-20A represents the hot electron flow, and the IEB can be viewed as the electron

loss to base electrode due to scattering in the base. On the other hand, the second

45

mechanism used to explain the modulation is based on the perforated base electrode;

that is, the physical pinhole in the electrode. Metal has a short Debye length. Therefore,

a fully continuous base electrode will completely screen the electric field between

source to drain. If the base electrode is not completely covered, the perforated region

allows the electric field from collector potential to influence the carrier transport. With a

certain base bias, a layer of depletion region can be formed near the thin base, which

serves as the controller to carrier flow (Figure 1-20B). This type of device is thereby

called organic static induction transistor (OSIT) in some cases. In either case of hot

electron transmission or perforated electrode transmission, the thin base plays a critical

role: to modulate the carrier flow and to avoid undesired carrier transmission during the

off state. Several approaches, such as thin electrode annealing, air exposure or

interlayers, have been reported to fabricate base electrode in high performance OPBTs

or OSITs.84–87

1.3.2.2 Vertical organic field effect transistors

Vertical organic field effect transistor (VOFET) or organic Schottky base

transistor (OSBT) is the vertical transistor with the gate electrode outside of the source-

drain Schottky diode (Figure 1-19C). It can be viewed as an integration of a Schottky

diode with the control gate capacitor. The mechanism of carrier flow control is by the

variation of charge injection barrier from source electrode to the organic channel layer.

The source electrode in VOFET needs to be porous due to the same reason of metal

screening effect as in the OPBT. The Schottky barrier suppresses the carrier injection

without gate bias (Figure 1-21A). When the gate bias is applied, the carriers (electron in

this case) accumulate at the porous regions with the channel material filled, leading to

46

the band bending. As the band bending becomes stronger with increased gate bias, the

electron tunneling current from ITO increases and the device is in the on state (Figure

1-21B). The porous electrode is also the most crucial part in a VOFET. To date, three

different methods to obtain the perforated source electrode have been reported. The

Yang group fabricated thin porous Al source by controlling the deposition conditions and

partial oxidation of Al to alumina.88,89 The Tessler group obtained the opening electrode

by patterning the Au source layer with self-assembly block copolymers.90,91 On the other

hand, the Rinzler group employed carbon nanotube and graphene as the source

electrode.92,93 Due to the low density of states (DOS), the work function is tunable in

these materials. Thus, the current injection modulation is based on the tunable work

function (Schottky barrier height) in addition to the band bending (Schottky barrier width)

in the former cases.94

1.3.2.3 Vertical organic light emitting transistors

A vertical organic light emitting transistor (VOLET) is a device that the light

emission is controlled by the gate electrode. IN a VOLET, the light emitting unit (i.e.

OLED) is directed integrated between the source and drain electrodes of a VOFET. In

this case, the modulation of current injection is transformed into the modulation of

luminance on and off in the VOLET.

1.3.3 Porous Electrode Fabrication

The Langmuir–Blodgett (LB) method is used to fabricate the porous source

electrode, as in Figure 1-22A. The polystyrene (PS) nano-spheres are dispersed into

the suspension solution. The substrate with pre-deposited layers is held vertically in the

suspension of colloidal PS particles. While the substrate is withdrawn from the

47

suspension solution, the monolayer nano-spheres are pinned to the frontier that is

drying.

The deposited PS nano-spheres serve as a mask for the subsequent source

electrode (ITO) deposition. After the ITO deposition, the PS can be removed by

adhesive tape, leaving a source electrode with nanometer scaled openings. The

procedure is referred as colloidal lithography (Figure 1-22B). The organic material

deposited subsequently fills the pore area, which is the injection region.

1.3.4 Characterization of VOFETs and VOLETs

In order to quantitatively understand the performance of a VOFET, the following

parameters are defined. The transfer curve is measured by sweeping the gate voltage

(VGS) at a given source-drain bias (VDS), as in Figure 1-23A. The output curves refer to

the source-drain current density versus voltage (JDS -VDS) characteristics under an

increment (or decrement for the opposite type) range of VGS bias (Figure 1-23B).

As in the transfer curve of horizontal type transistors, the transconductance (gm)

refers to the switching capability, calculated by the slope of source-drain current to the

gate bias (𝑔𝑚 = 𝑑𝐼𝐷/𝑑𝑉𝐺𝑆). The sub-threshold swing is calculated as 𝑆 = 𝜕𝑙𝑜𝑔𝐼𝐷/𝜕𝑉𝐺𝑆.

The threshold voltage (Vth) is referred to the extrapolation of the square root of JDS to

zero current of the tangent at the maximum slope (Figure 1-23A).95 From the transfer

curve, the on/off ratio can be defined as the ratio of current in on state to that in off

state. It is easy to tell the order if the current scale is plotted as the logarithm scale.

When the device is off, the off current exhibits a contact limited behavior, which takes

the form of Schottky diode limited by injection91,96

𝐽𝑜𝑓𝑓 = 𝑞𝜇𝑁0exp[−𝑞

𝑘𝑇(𝜑𝑏0 −√

𝑞𝑉𝐷𝑆

4𝜋𝜀0𝜀𝑟𝐿)]×

𝑉𝐷𝑆

𝐿(1 − 𝐹𝐹) . (1-31)

48

In the equation, q is the charge of one electron, µ the carrier mobility, N0 the effective

density of state, k the Boltzmann constant, T the absolute temperature, εr the dielectric

constant, ε0 the vacuum permittivity, VDS the source drain bias, L the channel length, FF

the ratio of the sum of porous area to the total device area. On the other hand, in the on

state, the current behavior can be approximated as the SCLC current (Eq 1-9 of Section

1.1.2) multiplying the injection area

𝐽𝑜𝑛 =9

8𝜀0𝜀𝑟𝜇

𝑉𝐷𝑆2

𝐿3×𝐹𝐹 . (1-32)

Therefore, the on/off ratio can be written as

𝐽𝑜𝑛

𝐽𝑜𝑓𝑓=

9

8

𝜀0𝜀𝑟𝐹𝐹

𝑞𝑁0(1−𝐹𝐹)

𝑉𝐷𝑆

𝐿2exp [

𝑞

𝑘𝑇(𝜑𝑏0 −√

𝑞𝑉𝐷𝑆

4𝜋𝜀0𝜀𝑟𝐿)] . (1-33)

The direct extraction of on/off ratio from the transfer curve is normally slightly higher

than the model prediction due to the non-ideal source electrode surface conditions. In

order not to interfere the current flow between source and drain electrodes, the

suppressed gate leakage is preferred, with at least one order of magnitude lower than

the source-drain current. The field effect mobility is measured from the same organic

semiconductor in a horizontal type transistor or by other mobility measurement

technique like SCLC J-V behavior. The cut-off frequency (fT, or transient frequency, -

3dB frequency) is another important figure of merit. Based on small signal

approximation, the fT can be written81

𝑓𝑇 =𝑔𝑚

2𝜋𝐶𝑔 , (1-34)

with the Cg being the capacitance between gate and source terminals.

49

For a VOLET, the corresponding on/off ratio in luminance and maximum

luminance (delivered in on state) are the most important factors. The brightness

requirement for a display device is typically in between 300 to 500 cd/m2.

1.4 Dissertation Organization of Organic Displays

Among the present display technologies, liquid crystal display (LCD) still stands a

dominate position. The older technology like cathode ray tube (CRT) is vanishing.

OLED, being an emerging technology, is finding its niche as a high image quality, low

power consumption and potentially low cost display. It can be roughly told that OLED is

going right on the way since more and more mobile phone companies attempt to adopt

OLED panel as the smart phone display. To further reduce the cost, the solution

processed OLED is an attractive but intractable approach. On the other hand, as an

additional benefit from the development of vertical structure organic transistors, VOLET

offers a new display concept, which features an easy integration with switching circuit

and a high aperture ratio.

In this dissertation, the structure is organized as follows. The above introduction

in Chapter 1 gives an overview from organic semiconductors to the device applications

of OLEDs and VOLETs. In Chapter 2, a progress review of the multilayer processing

challenge and the current solutions is presented. In Chapter 3 and 4, we studied the

functional layers in terms of the processing sequence in a conventional structure

solution processed OLED. Chapter 3 is focused on hole injection and transport layers.

Specifically, we investigated the injection efficiency of an HIL and the carrier transport

mobility of an HTL, and then correlated the properties with the device operational

stability. In Chapter 4, we systematically compared the properties of a solution

processed EML and its thermally evaporated counterpart from the perspective of bulk

50

properties and interfacial energy levels. In Chapter 5, we switched to another type of

organic display device, the VOLET, and demonstrated a semi-transparent VOLET.

Chapter 6 shows a transparent all oxides transistor, which is suitable as the driving or

switching transistors in transparent displays. Finally, the conclusions are summarized in

Chapter 7.

51

Figure 1-1. Electronic configurations of hybrid orbitals and the orientations in space of a

carbon atom. A) sp3 hybrid orbital, B) sp2 hybrid orbital and C) sp hybrid orbital.

Figure 1-2. The sp2 hybridization of σ and π bonding.97

52

Figure 1-3. The schematic illustration of molecular orbital splitting and formation of

continuous bands.98

Figure 1-4. Energy band diagram showing the carrier injection mechanisms at the

interfaces between metal and organics. A) Thermionic emission, B) Fowler-Nordheim tunneling and C) thermos-activated hopping injection.5

Figure 1-5. Energy band diagram at the metal/organic interface illustrating the image

force effect.5

53

Figure 1-6. The current density versus electric field characteristics under various

regimes of applied fields.99

Figure 1-7. The field dependent and thermally assisted hopping transport.

Figure 1-8. Wannier-Mott, CT and Frenkel excitons in terms of the degree of

delocalization.100

54

Figure 1-9. The schematic description of Förster and Dexter excitonic energy transfer.

Figure 1-10. The HOMO-LUMO illustration of singlet and triplet energy states.101

Figure 1-11. The Jablonski diagram to illustrate the relaxation processes.102

55

Figure 1-12. The absorption and emission spectra illustrating Stokes shift and Franck-

Condon principle.103

Figure 1-13. The schematic band diagrams of an OLED operated at different bias

conditions. A) Open circuit: V = 0, B) flat band condition: V = Vbi and C) high forward bias: V > Vbi.

Figure 1-14. The illustration of the blocking layers in an OLED.

56

Figure 1-15. OLED device structures. A) Conventional and inverted based on stacking

sequence. B) The categories based on the direction of light emission.

Figure 1-16. The procedure of spin-coating. A) Dispensation, B) acceleration, C) flow

dominated and D) evaporation dominated.50

Figure 1-17. The eye sensitivity function, V(λ), as a function of wavelength.104

57

Figure 1-18. The light out-coupling paths and the loss channels. The right hand side

shows the light propagation with a light extraction lens.64

Figure 1-19. Device structures of organic transistors. A) The permeable base transistor

based on hot electron transmission. B) The permeable base transistor based on perforated base transmission. C) The vertical organic FET (VOFET). D) The horizontal type organic FET.

58

Figure 1-20. The band diagram and device structure of OPBTs. A) The band diagram

of hot electron transmission across the base electrode of the device in Figure 1-19A.81 B) The current transmission through base electrode by the control of depletion region width.105

Figure 1-21. The schematic device structure and band diagram between ITO source

and C60 channel. A) Under off state and B) under on state.

59

Figure 1-22. The procedure of colloidal lithography. A) The schematic illustration of

Langmuir–Blodgett (LB) procedure for fabricating polystyrene monolayer.106 B) The deposition of ITO source and the lift-off process.

Figure 1-23. The typical electrical J-V characteristics of a VOFET. A) Transfer curves

and B) output curves.107

60

Table 1-1. The corresponding unit of photometric and radiometric.

Photometric unit Visible light (380 nm to 720 nm)

Radiometric unit UV to IR (10 nm to 1 mm)

Luminous flux lm Radiant flux W = J/s Luminous intensity cd = lm/sr Radiant intensity W/sr Illuminance lux = lm/m2 Irradiance (power density) W/m2 Luminance cd/m2 = lm/sr-m2 Radiance W/sr-m2 Luminous power efficiency lm/W Power efficiency W/W Current efficiency cd/A Radiance efficiency W/sr-A

61

CHAPTER 2

SOLUTION PROCESSED MULTILAYER OLEDS

2.1 Background and Motivation

The first organic light-emitting diode (OLED) was demonstrated by Tang and

VanSlyke27 and the device was a small-molecule organic bilayer device sandwiched

between two electrodes. At the end of the 1990s, since the invention of phosphorescent

emitters by Baldo et al.108 and Adachi et al.,109 the structure of an OLED has become

more complicated, with multiple functional layers acting as charge injection, charge

transport, exciton blocking, and emitting layers (EMLs).

Multilayer devices can be made by vacuum deposition, and commercial OLEDs

are currently made with this process.110–117 However, vacuum thermal evaporation

bears the drawbacks of low material utilization rates, poor scalability, high capital cost,

and difficulty in patterning. Solution processing, in principle, provides a low-cost

approach to fabricate OLEDs. Early solution processed OLEDs were based on a simple

architecture with an emitting polymer layer and a hole injection polymer layer.28,118–120

Recently, more attempts have been made on solution processed small-molecule

OLEDs with mixed EML.121–125 However, without using a multilayer architecture, the

performance of solution processed OLEDs is inferior to evaporated OLEDs.

To fabricate solution processed multilayer OLEDs, intermixing of layers is a

major issue because the deposition of a layer may dissolve or intermix with the

preceding layer. Tremendous efforts have been made to circumvent this issue. One

Reprinted with permission from Ho, S.; Liu, S.; Chen, Y.; So, F. Review of Recent Progress in Multilayer Solution-Processed Organic Light-Emitting Diodes. J. photonics energy 2015, 5 (1), 576111–576127. http://dx.doi.org/10.1117/1.JPE.5.057611. Copyright © 2015 Society of Photo Optical Instrumentation Engineers.

http://dx.doi.org/10.1117/1.JPE.5.057611

62

approach is to use orthogonal solvent systems, where the difference of material

solubility in different solvents is employed to process adjacent layers without

intermixing. An example is the widely used water soluble poly(3,4-

ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole injection layer (HIL)

with a subsequent hole transport layer (HTL), which is usually soluble in organic

solvents. The second approach is to use photo or thermal cross-linkable organic

functional materials for OLEDs. Because the cross-linked functional layer is not soluble

in solvents, subsequent deposition of another layer should not interfere with the

underlying layer. The third approach is to introduce inorganic functional materials such

as metal oxides into OLEDs. Again, these metal oxides are not soluble in organic

solvents which enable processing of subsequent layers.

In this work, we describe these materials for solution processed OLEDs. The

objective of this review is to describe these approaches to address the problems

associated with multilayer device processing, especially for HIL/HTL, EML, and ETL. In

addition, an overview of solution processed multilayer device applications will be

presented, followed by the future prospects and direction for solution processed

multilayer OLEDs.

2.2 The Approaches for Hole Injection/Transport Layers

2.2.1 Hole Injection Materials

2.2.1.1 Polymers

Most polymer-based HILs/HTLs are aqueous-based and insoluble in organic

solvents, which make them suitable for orthogonal solvent processing. Poly(3,4-

ethylenedioxythiophene) polystyrene sulfonate, known as PEDOT:PSS, has been

widely used as an HIL in organic electronic devices in the past.126 Currently, the

63

PEDOT:PSS HIL for organic optoelectronic devices is commercially available from

Heraeus Holding GmbH under the brand of Clevios™.127,128 The use of PEDOT:PSS in

solution processed multilayer OLEDs have been previously reviewed by Zhong et al.129

and readers are referred to this work for the early development and application. The

easy processing by spin casting from aqueous solution makes PEDOT:PSS a popular

HIL/HTL. PEDOT:PSS bears the merits of high conductivity and high transparency

along with its good film forming property and its ability to planarize the indium tin oxide

(ITO) surface. However, there are device stability issues associated with PEDOT:PSS

as an HIL. First, PEDOT:PSS has a high acidity (with pH value ranging from 1.0 to 2.5),

and corrodes the ITO electrode, leading to device degradation. Second, PEDOT:PSS

absorbs moisture, which is another source of device degradation. Third, its work

function is 5.2 eV and the hole injection barrier from HIL into HTL or EML leads to the

accumulation of carriers at the HTL interface, resulting in device degradation. Extensive

research has been done on modifying or identifying alternatives for PEDOT:PSS as a

hole injection material.

Lee et al. introduced perfluorinated ionomer (PFI) dopant to modify PEDOT:PSS

to a self-organized gradient hole injection layer (GradHIL).130–137 The driving force for

self-organizing behavior is from the more hydrophobic nature of the fluorocarbon chains

in PFI, making it preferentially stay at the surface of the film. By tuning the ratio of

PEDOT/PSS/PFI, the work function (ϕ) can be tuned from 5.05 to 5.70 eV as the

content of PFI increases,130 which enables its value to match the highest occupied

molecular orbital level of the EML. Since PFI with fluorocarbon chains is more

hydrophobic than the polystyrene chain, PFI tends to reside away from ITO and form a

64

“self-organized” gradient, rendering a work function gradient in the HIL. In addition to

reducing the injection barrier, the PFI-doped PEDOT:PSS can inhibit diffusion of indium

and tin. Time-of-flight secondary ion mass spectroscopy data reveal that the fluorinated

surface from PFI can retard the out-diffusion of In or Sn from the ITO anode, which is

important to improve the device lifetime. The half lifetime from 1000 cd/m2 in

solutionprocessed green polymer LED is 2,680 h with GradHIL, compared to that of 52

h without PFI modification.131 Han et al.133 integrated the GradHIL with graphene and

successfully achieved the power efficiency of 85 lm/W in phosphorescent OLEDs

(PhOLEDs) and 24 lm/W in fluorescent OLEDs.

In addition to modifying PEDOT:PSS, polyaniline (PANI) and its blends serve as

alternative HILs. PANI is by nature insoluble in common solvents.138,139 The solution

processable PANI is a blend protonated by functionalized protonic acids such as

camphorsulfonic acid140 or copolymers of aniline and sulfonated aniline derivatives

(PANI:PSS).141 Jang et al.139 demonstrated that PANI:PSS has a higher transmittance

and a smoother surface than PEDOT:PSS for contacts with subsequent organic layers.

Fehse et al. fabricated multilayer OLEDs using D1033 PANI dispersion with efficient

carrier injection and high power efficiencies.142 Choi et al.132,143 also incorporated PFI

into PANI:PSS to increase its work function similar to that in PEDOT:PSS. With an

optimal ratio of PANI:PSS:PFI, a fluorescent OLED using Bis(10-hydroxybenzo[h]

quinolinato)beryllium (Bebq2) as an emitter shows a maximum current efficiency of 19

cd/A.

More recently, Choudhury et al.144 and Chen et al.145 demonstrated

polythienothiophene doped with poly(perfluoroethylene-perfluoroethersulfonic acid)

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(PTT:PFFSA) with enhanced hole injection efficiency. Its work function is tunable from

5.2 to 5.7 eV, which serves as a buffer step between ITO anode and HTL. From the

dark injection-space charge limited current (SCL-DI) measurement, the hole injection

efficiency of PTT:PFFSA is 1.5 times that of PEDOT:PSS, resulting in a device lifetime

enhancement compared with devices using PEDOT:PSS. The reasons for improved

lifetime is attributed to the deeper lying work function of PTT:PFFSA such that fewer

charges are trapped between the HIL and HTL interface, suppressing degradation from

the exciton quenching centers. Since HIL plays a crucial role in device degradation, a

stable HIL is desirable for state-of-the-art OLEDs.

Orselli et al.146 and Ho et al.147 along with Plextronics Incorporation reported

stable aqueous or nonaqueous HILs for PhOLEDs. Plexcore® OC AQ1200 (AQ1200)

has a reduced acidity (with a pH value from 2.6 to 3.4) with a better air stability. When

OLEDs are fabricated with AQ1200 HIL, the degradation due to moisture uptake and

acidity of HIL can be minimized. With a work function ranging from 5.3 to 5.7 eV, the

hole injection barrier is reduced. Green PhOLEDs with AQ1200 show a maximum

current efficiency of 68 cd/A at a luminance of 200 cd/m2 and the half lifetime (LT50) at

1,000 cd/m2 is 8,400 h.

The resistivity and the work function of the aforementioned polymer-based

HILs—including PEDOT:PSS:PFI, PANI:PSS(:PFI), and AQ1200—are tunable, and

they have the potential to substitute for ITO as conducting polymer electrodes on

flexible substrates. The properties of these HILs are summarized in Table 2-1. These

HILs have low solubility in common organic solvents, which makes them resistive to

solvent rinse during processing and suitable for multilayer solution process fabrication.

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2.2.1.2 Small molecules

1,4,5,8,9,11-Hexaazatriphenylene hexacarbonitrile (HAT-CN), an evaporated HIL

material which has been widely used in OLEDs, is also reported for fabricating

multilayer solutionprocessed OLEDs. Lin et al.148 recently adapted the compound for

solution processing using 2-propanone as a solvent. Red, green, and blue OLEDs

fabricated with HAT-CN HIL showed high power efficiencies of 15, 55, 16 lm/W,

respectively. Because of the enhanced efficiency and stability, small-molecule HILs are

widely used in OLED manufacturing today.

2.2.2 Cross-linkable Materials for HTLs

In most OLEDs, there is a large barrier for hole injection from a typical HIL

(PEDOT:PSS) into the EML, thereby limiting the device performance. In vacuum-

deposited OLEDs, this limitation has been overcome by inserting an HTL between HIL

and EML, providing an intermediate step for hole injection. For solution processed

devices, the dissolution of the preceding layer by the solvent of the subsequent layer

makes multilayer processing a difficult task. One solution is to chemically cross-link the

functional layer such that layer-by-layer stacking is feasible without intermixing between

the adjacent layers. The prevailing cross-linking chemistry is to attach functional cross-

linkers to the functional molecules.149,150 In this section, we review conventional HTLs

modified with the following cross-linking groups: oxetanes, styrenes, trifluorovinyl

ethers, and benzocyclobutene (BCB).

2.2.2.1 Oxetane-based HTLs

Under ultraviolet (UV) illumination, oxetane-based HTLs can initiate cross-linking

via cationic ring-opening polymerization (CROP) and form linear polyethers.151,152 Yang

et al.118 have reported a series of cross-linkable HTLs (X-HTLs) based on adding

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sidechains containing four-membered cyclic ethers to conventional HTLs. Cross-

linkable N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine (TPD)

derivatives (N,N’-bis(4-(6-((3-ethyloxetan-3-y)methoxy)-hexyloxy)penyl-N,N’-bis(4-

methoxyphenyl)biphenyl-4,4’-diamin (QUPD) and N,N’-bis(4-(6-((3-ethyloxetan-3-

y)methoxy))-hexylpenyl)-N,N’-diphenyl-4,4’-diamin (OTPD), as shown in Figure 2-1,

were synthesized and used in OLEDs by multilayer solution process. By using a

combination of two cross-linkable HTLs, the barrier height can be divided into two

smaller steps. The current efficiency showed a threefold enhancement from ∼20 to 67

cd/A.118,153,154 External quantum efficiencies (EQE) of 11%, 19%, and 6% were

achieved in red, green, and blue OLEDs, respectively. A further electro-modulation

study suggests the effect of X-HTLs not only creates facile hole injection but also

confines electrons at the EML/HTL interface, resulting in an efficiency

enhancement.155,156 Another series of oxetane functionalized X-HTLs with high triplet

energies and large bandgap energies were synthesized based on 1-bis[4-[N,N’-di(4-

tolyl)amino]phenyl]-cyclohexane, resulting in an improved efficiency (18 cd/A), and a

reduced efficiency roll-off was observed in blue devices.157,158 Generally, this type of

cross-linking reaction occurs at lower temperatures with a rapid reaction rate. However,

owing to the use of photoacids, it is inevitable to have residual side products or initiators

in the cross-linked X-HTLs which might impair the device stability. To initiate the cross-

linking reaction via a photoacid-free path, Köhnen et al.151 proposed a concept of layer

by layer cross-linking. The cross-linking reaction is activated by protons from the excess

PSS of the acid PEDOT:PSS layer. The reaction then moves away from the

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PEDOT:PSS interface and throughout the X-HTL. Additionally, introducing a post-

annealing after UV illumination can alleviate this issue.148

2.2.2.2 Styrene-based HTLs

Other than photo cross-linking, thermal cross-linking is another option. In this

case, neither photoacid nor initiator is required, thus eliminating one of the factors giving

rise to exciton quenching and stability problems. To form polymer networks, generally,

two styryl [or termed vinyl benzyl (VB)] groups are functionalized to the HTL molecules.

The typical curing temperature is higher than 150 oC, requiring the hole transport moiety

to be sustainable to the high curing temperature. Liu et al.159 and Niu et al.160 reported a

cross-linkable 4,4’,4″-tris-(N-carbazolyl)-triphenlyamine (TCTA) derivatives (VB-TCTA)

as an HTL. White OLEDs with VBTCTA as HTL have a current efficiency of 11 cd/A

(EQE of 6%). Another conventionally evaporated HTL, N,N’-bis(1-naphthyl)-N,N’-

diphenyl-1,1’-biphenyl-4,4’-diamine (NPD), was also reported with styrene

functionalized derivatives (1-NPD, 2-NPD) for the cross-linking reaction. These HTLs

can be cured at 230 oC for 30 min, and a green polymer LED (PLED) with

PEDOT:PSS/2-NPD (HIL/HTL) exhibits a current efficiency of 11 cd/A.161 Ma et al.162

incorporated VB ether to iridium-based 1-phenylpyrazole, (PPZ-VB)2IrPPZ. Green

OLEDs with this X-HTL show a power efficiency of 14 lm/W (EQE of 8.5%). Recently,

Jiang et al.163 reported a high efficiency small-molecule OLED with X-HTL based on

3,3’-bicarbazole (BCz) with two VB ether units (BCz-VB). BCz-VB has a high triplet

energy and relatively lower curing temperature of 146 oC and the resulting blue OLEDs

[iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate, FIrpic] show a current

efficiency of 25 cd/A and a turn-on voltage of 5.6 V, which makes styrene-functionalized

HTLs promising X-HTLs.

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The ratio of the insulating cross-linkable moiety to the transport one is critical to

UV/thermal curing process and also the device efficiency.164 It is anticipated that the

solution processed X-HTL can perform comparable to its evaporated counterparts.

Xiang et al.165 demonstrated a styrene functionalized NPD derivative in phosphorescent

orange OLEDs. With optimized orange EML and cesium carbonate (Cs2CO3)-doped

electron transport layer (ETL), the OLED efficiency and stability from this X-HTL are

comparable to those from vacuum-deposited 4,4’-bis[N-(1-naphthyl)-N-

phenylamino]biphenyl device.

2.2.2.3 Perfluorocyclobutane-based and BCB-based HTLs

Perfluorocyclobutane (PFCB)-based cross-linkable groups have been used for X-

HTLs. Niu et al.166 used PFCB functionalized TPD on polystyrene backbone (PS-TPD-

PFCB) and PFCB-modified TCTA (TriTCTA-PFCB) in blue OLEDs with a very low EQE

of 1%. On the other hand, a BCB group can also undergo thermal dimerization. Ma et

al.167 introduced the BCB-modified TPD derivative (TPD-BCB) and the PhOLEDs with

TPD-BCB as HTL exhibited a maximum EQE of 10%. However, the cross-linking

reaction requires 180 oC for 2 h, followed by another 4 h baking at 250 oC, which is

challenging for processing. Zuniga et al. reported a method to cross-link the HTL by

rapid thermal annealing (RTA) to prevent PFCB/BCB-based X-HTLs from being

exposed to high temperatures for long time.168 A curing condition with a higher

temperature but much shorter curing time was applied to 3,6-bis(carbazol-9-

yl)carbazole with BCB moiety (TCz II). The green PhOLEDs fabricated by the RTA

process have a high efficiency (48 cd/A) compared to the ones processed by

conventional long-time annealing on a hotplate (27 cd/A).

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2.2.2.4 Other cross-linking chemistries

There are other functional groups that can undergo polymerization, dimerization,

or condensation to create a robust layer impervious to solvents of the subsequent layer.

For example, siloxane derivatives can proceed cross-linking with the presence of

moisture169,170 and azide-based X-HTLs cross-link under UV irradiation. The

triphenylamine derivative (X-PTPA-5) bears the advantage of a short UV exposure time.

A current efficiency of 44 cd/A was demonstrated in green PhOLEDs.164 Photo cross-

linking also makes the micron-scaled patterning viable since the photolithography can

be applied directly. Lee et al.171 reported the thiolene reaction for photo crosslinking

allyl-TFB [poly(9,9- dioctylfluorene-co-N-(4-butylphenyl)diphenylamine)], making

solution processed Ir-based green devices with a current efficiency of 31 cd/A. A

summary of these cross-linkable HTLs are shown in Table 2-2.

The UV-initiated X-HTLs, such as oxetane-based HTLs, enables the potential for

easy pixel patterning. But the reaction usually proceeds with photoinitiators which pose

an adverse effect on device performance and stability. Another cross-linking chemistry

is to initiate the reaction by thermal treatment. For example, the styrene-based HTLs

form insoluble films under annealing temperatures at 150 to 180 oC, whereas the X-

HTLs with PFCB and BCB functional groups generally require a longer curing time and

higher temperature. The former case seems more promising for device application

because the annealing temperature is moderate for the carrier transport moieties. The

initiator-free thermal cross-linking HTLs also reduce the exciton quenching by

impurities. To date, there is still a lack of systematic study on the effect of various cross-

linking functional groups on material properties and device performance. Efforts on

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studying this issue will provide a deeper insight in the design of cross-linkable HTL

materials.

2.2.3 Metal Oxides for HILs/HTLs

As an alternative to small-molecule/polymer functional layers, solution processed

metal oxides provide an inorganic option to fabricate hybrid organic–inorganic light

emitting diodes (HyLEDs). The high transparency in the visible spectrum, robust film

forming property, and compatibility to solution process makes metal oxides a promising

candidate for carrier transport layers in solution processed multilayer HyLEDs. Based

on their charge transport properties and energy level alignment, they are used as an

HIL, HTL, ETL, and electron injection layers to fabricate a multilayer device. Compared

to their organic counterparts, metal oxides have favorable characteristics for devices

such as high carrier mobility, tunable energy-level alignment, and good stability.

Precursors with transition metal complexes are synthesized and dissolved into polar

solvents and then spin cast onto electrode substrates.172–174 An oxidation process in

ambient atmosphere is usually necessary to transform the precursors into metal oxides.

2.2.3.1 N-type metal oxides for HILs

Most transition metal oxides are n-type semiconductors. Several transition metal

oxides such as tungsten oxide (WO3), molybdenum oxide (MoO3), and vanadium

pentoxide (V2O5) have very deep valence band maxima and are strong electron

acceptors.39,175–177 When they are in contact with ITO, there is a strong vacuum level

shift resulting in the formation of an interface dipole at the ITO interface and reduction in

hole injection barriers. Below is the summary of these solution processed metal oxides

for HIL applications.

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MoO3. Höfle et al.178 prepared solution processed MoO3 films by spin casting a

molybdenum(V) ethoxide [Mo(OEt)5] ethanol solution, followed by annealing at 150 oC

under ambient conditions. The ionization potential (IP) and electron affinity (EA) vary

slightly with respect to precursors (nanoparticle or sol-gel) and processing environments

(in air or inert gas). Solution processed MoO3 generally has an IP of ∼8.0 eV and an EA

of ∼5.0 eV. Solution processed phosphorescent HyLEDs employing MoO3 HIL showed

enhanced hole injection as well as superior device performance compared to the

devices with PEDOT:PSS.178 Similarly, Jian et al.179 synthesized MoO3 with ammonium

molybdate [(NH4)Mo7O24 · 4H2O] precursors to deposit a thick (>100 nm) HIL for large

area tris(2-phenylpyridine)iridium [Ir(ppy)3] devices. Due to the good transport properties

of the MoO3 layer, the device performance is not significantly hindered by the thickness

of the MoO3 films. HyLEDs with solution processed MoO3 and EML show a current

efficiency of 51 cd/A.179 Fu et al.180 reported a room-temperature synthesis for MoO3.

The operational lifetime of the solution processed green phosphorescent HyLEDs

incorporating MoO3 HILs was improved by two times with respect to that of the

corresponding PEDOT:PSS devices.180

WO3. The WO3 HILs were fabricated from an ethanol diluted or tungsten(VI)

ethoxide [W(OEt)6] precursor solution at room temperatures with a ϕ of 6.7 eV.181–183

Solution processed blue phosphorescent HyLEDs incorporating WO3 HIL showed a

75% enhancement of current efficiency compared to that of the PEDOT:PSS device.182

Youn et al.184 also reported a substantially improved operational lifetime of 1.8 × 106 h

and a current efficiency of 10 cd/A at 1,000 cd/m2 for the super yellow devices by

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sandwiching PEDOT:PSS between two separate WO3 layers, which effectively

suppressed indium diffusion and acid damage on ITO by PEDOT:PSS.

V2O5. V2O5 precursor solutions can be prepared by vanadium pentoxide powder

or vanadium(V) oxitriisopropoxide dissolved in 2-propanol.185,186 Similar to MoO3, V2O5

also has a deep ϕ of ∼5.6 eV.187 Kim et al.185 and Lee et al.187 reported a low

temperature treatment for V2O5, which showed comparable device efficiencies and

improved operational stability. Kim et al.185 applied V2O5-doped PEDOT:PSS in PLED

and demonstrated over 20% efficiency improvement to 15 cd/A. However, all of the

above-mentioned metal oxides are n-type with a deep EA, thus lacking the appropriate

energy levels to block the electrons.

2.2.3.2 P-type metal oxides for HTLs

NiOx. Nickel oxide (NiOx) is one of the few p-type metal oxides. Its conduction

band minimum is 1.7 eV, which is effective for blocking electrons in an OLED. Solution

processed NiOx has been used as an HTL in organic photovoltaics (OPV) devices.188

Synthesis of NiOx films requires a high annealing temperature (> 500 oC) and UV-ozone

treatment. Solution processed NiOx has a high hole mobility of 0.14 cm2/V-s. The hole

injection efficiency measured by SCL-DI was 0.85, ∼70% higher than that of

PEDOT:PSS. Recently, Liu et al.189,190 demonstrated a solution processed green

phosphorescent HyLEDs incorporating NiOx HIL/HTL, with a high current efficiency of

70 cd/A and a power efficiency of 75 lm/W.

The development of solution processed metal oxides HTLs has been motivated

by the success of the evaporated counterparts. The precursors suitable for low

temperature processing are largely used in OPVs.172 However, there are relatively fewer

reports in HyLEDs. Table 2-3 summarizes the performance of HyLEDs using solution

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processed metal oxide HTLs. As shown in the table, the lifetime data are rather limited,

and more comprehensive studies are needed to understand the efficiency and lifetime

of HyLEDs using solution processed metal oxides.

2.3 The Approaches for Emitting Layers

Instead of a single-material based functional layer, most EMLs in most solution

processed OLEDs have a more complex host–guest system. The requirements for good

host materials are stable film morphology, high triplet energy, bipolar charge transport

properties, good solubility in solvents, and high glass transition temperature. Within the

host–guest system, the luminescence quantum yield is also of concern. For more

information about host–guest in EML, readers are referred to previous reviews focused

on host materials in solution processed small-molecule OLEDs.33,191 The situation is

different when another layer is subsequently processed on top of the small-molecule

EML. In this section, the progress on making multilayer beyond EML is discussed.

2.3.1 Cross-linkable EMLs

Similar to HIL/HTL, cross-linkable materials offer an option for multilayer

processing in EML. But the key issues associated with EMLs are exciton quenching by

the initiators or byproducts, color shift due to exciplex formation, and change of

recombination profiles. Following the early development of using CROP in X-HTLs,

Rehmann et al.192 turned the Ir-based emitter into a cross-linkable derivative (x-emitter).

In multilayer solution processed OLEDs, the previously reported X-HTL, namely N,N’-

bis(4-(6-((3-ethyloxetan-3-y)methoxy)-hexyloxy)penyl-N,N’-bis(4-

methoxyphenyl)biphenyl-4,4’-diamin, and N,N’-bis(4-(6-((3-ethyloxetan-3-y)methoxy))-

hexylpenyl)-N,N’-diphenyl-4,4’-diamin (OTPD), was first spin cast as double HTLs.118,153

The EML consisted of OTPD as the host and x-emitter as the guest; subsequently, the

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ETL was also solution processed with a mixture of 25% poly(methyl methacrylate) and

75% 2-(4-tert-butylphenyl)-5-biphenylyl-1,3,4-oxadiazole. The optimized device showed

a maximum current efficiency of 18 cd/A. Ma et al.162 incorporated two VB ethers to Ir

emitter of red, green, and blue. The green and near-white multilayer OLED has an EQE

of 8% and 2%, respectively. Aizawa et al. synthesized carbazole derivatives containing

a VB group (DV-CBP) and demonstrated an EQE of 2.3% in a solution processed

fluorescent OLED.193 In contrast to adopting the chemistry from X-HTL to cross-linkable

EML (X-EML), Volz et al.194 demonstrated an autocatalyzed method to attach copper

complexes to a polymer backbone, forming a crosslinked Cu-based EML. This strategy

was used to enhance the electrochemical stability of the emitter materials.194 The

formation of the solvent-resistive cross-linking layer requires a high curing temperature

which might damage the emitters. Another problem is that these cross-linkable side

groups are generally insulating, resulting in low carrier mobilities in these cross-linked

hosts or emitters. For X-EML prepared by CROP reaction, the existence of

photoinitiators poses an additional problem of exciton quenching. Another approach to

fabricate cross-linkable EMLs makes use of the electrochemical polymerization. Gu et

al.195 used sequentially electrochemical cross-linking to fabricate white OLEDs with a

current efficiency of 5.5 cd/A. While this approach is interesting, there has not been

follow up work done, suggesting the viability of this approach is questionable.

There is a paucity of reports on device stability in cross-linkable EML solution

processed OLEDs. The purity of OLED materials is a key factor determining the device

operating stability. Among these cross-linkable EMLs, it is inevitable to introduce

initiators or generate byproducts due to the cross-linking reaction. Thus, there are

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chemical species in the EML where charge transport and exciton formation occur.

These impurities can serve as either charge traps or exciton quenching centers,

degrading the device performance.

2.3.2 Orthogonal Material-solvent Set for Combined EML/ETL

Several types of solvents can be employed to achieve orthogonal multilayer

processing. One of the methods is to use fluorinated polymers and solvents. With

sufficiently high fluorine content in the polymers, fluorinated polymers are soluble in

fluorinated solvents but insoluble in common organic and aqueous/alcohol solvents.

Fong et al.196 and Zakhidov et al.197 developed fluorinated light-emitting polymers and

demonstrated a multilayer solution processed PLED using red and green fluorinated

EML stacks. The device can even be operated under chloroform immersion, showing

the orthogonal solubility of fluorinated polymers.196,197 However, owing to the fluorescent

nature of polymer EMLs, the device efficiency of fluorinated PLEDs is significantly lower

than that in phosphorescent OLEDs.

To incorporate phosphorescent emitters in multilayer solution processed devices,

polymer-based hosts, such as poly(N-vinylcarbazole) (PVK), are used as an EML in

most solution processed multilayer PhOLEDs.33 During postannealing after solution

process, polymer hosts form chain entanglement which might be able to withstand

solvent wash from the subsequent layers. Most alcohol/water soluble ETLs can be

processed on top of the polymer-based EML without dissolution. Since PVK is a hole-

transport host, most efficient OLEDs with PVK-based EML are incorporated with

electron transport molecules, such as 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-

yl]benzene. Blue phosphorescent devices, which are solution processed from HIL up to

the ETL, were demonstrated by Earmme et al.198–201 with the ETLs processed via a

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mixture of formic acid and water (composition ratio 3:1). The highest EQE obtained was

19% for the FIrpic emitter and 16% for the Ir(ppy)3 emitter. Huang et al.202 reported a

multilayer WOLED with solution processed HTL, EML, and ETL. The ETL was prepared

and spin cast in water/methanol solution and the overall device efficiency reached 14%

EQE.202

One of the fundamental problems with the orthogonal solvent system approach is

that small molecule materials can be washed away by a solvent even though the small

molecule might not be soluble in that solvent. For example, a small molecule layer

insoluble in alcohol solvents can still be washed away by the solvent during deposition

of the subsequent layer even if an alcohol-based solvent is used. Recently, Aizawa et

al.203 demonstrated that if the molecular weight of the small molecule exceeds a certain

threshold value, the film will remain intact. To illustrate this approach, they

demonstrated that a higher molecular weight molecule (such as a dimer and trimer of a

carbazole) has sufficient solvent resistance to 2-propanol used for the ETLs, whereas a

carbazole monomer can easily be washed away by the same alcohol based solvent.

With both dimers and trimers of carbazole as the host, they fabricated blue, green, and

white OLEDs with a maximum EQEs of 20%, 22%, and 20%, respectively. This work

opens up a route to fabricate small molecule multilayer solution processed OLEDs.

2.4 The Approaches for Electron Transport Layers

Since ETL is the last solution processed layer before cathode deposition in an

OLED of conventional structure, solvent resistance is usually not an issue with a

thermally evaporated cathode. The requirements of solution processed ETLs are high

triplet energy for exciton confinement, good electron transport, proper energy level for

electron injection and hole blocking, high glass transition temperature, high solubility

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ensuring film uniformity, and minimum processing damage to solution processed

EML.110–112,204

One of the approaches is to use water/alcohol-based solvents where typical

organic EML materials show very low solubilities. Huang et al.205 has developed

water/alcohol soluble conjugated polyelectrolytes (CPEs) for device fabrication,205 and

these materials have highly delocalized pi-conjugated main chains and polar pendant

group substituted side chains.206 CPEs can effectively modify the interface energy level,

improve electron injection from the cathode and enable the use of air-stable metals with

large work functions.207,208 However, a delay between current switch-on and luminance

turn-on was also observed. This is primarily attributed to the slow electrochemical

nature of ionic transport. High-efficiency PLEDs with conjugated polyelectrolyte

poly[9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] as

ETL and poly[2-(4-(3’,7’-dimethyloctyloxy)-phenyl)-p-phenylenevinylene] as EML were

demonstrated with an EQE of 7.85%.207 PLEDs with 105 fold enhancement of response

time (to microseconds) was also reported by thermal and voltage treatments.209 To

date, there are only limited reports of high efficiency OLEDs using CPEs simultaneously

with rapid response time. No work of CPEs applied in phosphorescent small-molecule

OLEDs was reported. The major reason might be the lack of full-solvent orthogonality

for small-molecule EMLs (as discussed in Section 2.3.2).

In addition to the polymer approach, Earmme et al.198,199 have synthesized a

series of small molecule oligoquinolines ETLs that are compatible to formic acid/water-

based solutions. Formic acid/water mixed solvents were utilized to process commercial

ETLs, such as 4,7-diphenyl-1,10-phenanthroline and 1,3,5-tri(3-pyrid-3-yl-

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phenyl)benzene, and the devices showed an EQE of 19% in blue OLEDs.200,201 Jiang et

al.210 reported diphenylphosphine oxide derivative for alcohol soluble ETL and the

resulting multilayer white OLEDs yielded an EQE of 12%. Ye et al.211 demonstrated

mixed ETL in an alcohol/water solution, and a yellow OLED with such a mixed ETL

showed an EQE of 13%.

Although high efficiencies are shown in the aforementioned solution processed

ETL devices, the solvents used for most ETL processing typically include water, which

is problematic to device stability. Pu et al.212 demonstrated a multilayer OLED

employing zinc oxide (ZnO) as an ETL, thus minimizing the possibility of introducing

water into devices. Multilayer solution processed OLEDs using ZnO ETL showed a

current efficiency of ∼19 cd/A and a prolonged LT50 lifetime of more than 500 h at a

luminance of 1,200 cd/m2. Currently, the concern of using ZnO as an ETL is the large

electron injection barrier due to its deep EA (varying from 3.8 to 4.2 eV), which cannot

match the lowest unoccupied molecular orbital energy of typical small-molecule hosts or

emitters (ranging from 2.5 to 3.0 eV) in OLEDs, thereby limiting the device performance.

Zhou et al.35 used a polymer containing simple aliphatic amine groups, such as

polyethyleneimine (PEI) to form a polymeric dipole layer, which serve as an interlayer

modifying the work function of ZnO. Due to the presence of the interfacial dipole, the

work function of the ZnO/PEI layer significantly decreased from 4.1 eV to 3.4 eV, which

decreases the electron injection barrier into common organic EMLs. Therefore, this

approach offers an option for ETL processed from a water-free solution, which is

favorable for stable multilayer solution processed OLEDs.

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2.5 Summary

In summary, this chapter highlight the challenge and current approaches to

fabricate solution processed multilayer OLEDs, including cross-linkable functional

materials, orthogonal solvents, and inorganic functional materials. The use of cross-

linkable materials enables wet processing which can withstand the subsequent solvent

rinse. Common moieties for modifying HIL/HTL include oxetanes, styrenes, trifluorovinyl

ethers, and BCB. However, cross-linking processes might lead to some stability issues

such as exciton quenching or charge traps. Further development of cross-linkable

materials is needed to address the device stability issues.

Orthogonal solvent systems provide a means to process multilayer devices. Most

reports of orthogonal solvent processing are actually based on high molecular weight

polymers. There are few reports on small-molecule HTLs or EMLs compatible with

orthogonal solvent processing. Small molecules can actually withstand multilayer

processing as long as the molecular weight is higher than a critical value.203 Currently,

most HTL processing is based on organic solvents, whereas ETL processing (of either

conjugated polyelectrolytes or small-molecule materials) is based on alcohol/aqueous-

based solvents. The introduction of water during processing is known to have

detrimental effects on device stability and this is a fundamental issue that needs to be

addressed in processing OLEDs based on orthogonal solvent systems.

Solution processed metal oxide functional layers serve as an emerging

alternative due to their insolubility in common solvents. OLEDs with NiOx and MoO3

HIL/HTL have been reported with high efficiencies. Additionally, OLEDs with ZnO ETL

showed improved device stability. Because of the complexity of multilayer OLEDs, it is

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apparent that a combination of these approaches will be necessary to achieve high

efficiencies and good stability OLEDs.

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Figure 2-1. Cross-linkable hole transport materials.

Table 2-1. Properties of various polymer-based HIL materials.

Material ϕ (eV)a σ (S/cm) pH Note Reference

Baytron/Clevios PEDOT:PSS PVP AI 4083b 5.0 to 5.2 2 to 20 × 10-4 1.2 to 2.2 127 and 128 P CH 8000b - 3 to 10 × 10-4 1.0 to 2.0 127 and 128 PH 500b 4.8 to 5.0 500 1.5 to 2.5 127 and 128 PH 1000b 4.8 to 5.0 1000 1.5 to 2.5 127 and 128 PEDOT:PSS:PFI 5.3 to 5.7 - - Doping ratio

dependent

130

Plextronics/Plexcore OC AQ-1200c 5.3 to 5.7 3 to 20 × 10-4 2.6 to 3.4 149 RG-1100c 5.1 to 5.2 4 to 40 × 10-4 2.2 to 2.8 146 PTT:PFFSA 5.2 to 5.5 - 2.2 to 3.5 144

PANI:PSS 5.2 - - 132

PANI:PSS:PFI (GradHIL)

5.8 to 6.0 - - Doping ratio

dependent 132

PSS-g-PANI:PFI 5.8 to 6.1 - - Doping ratio

dependent 143

a) Value measured by ultraviolet photoelectron spectroscopy. b) Trade name of Heraeus Holding GmbH. c) Trade name of Plextronics Incorporation.

83

Table 2-2. The device structure and performance of a OLED using X-HTLs.

Material Device structure Efficiency Reference

QUPD/OTPD ITO/PEDOT:PSS/QUPD/OTPD/PVK:PBD:OXD-7:emitter/CsF/Al R: Ir(piq)2(acac); G: Ir(mppy)3; B: FIrpic

R: 10.8%; G: 10.8%; B:

5.7%

118

X-TAPC ITO/PEDOT:PSS/X-TAPC/PVK:OXD-7:FIrpic/CsF/Al

18.4 cd/A 157 and 158

VB-TCTA ITO/PEDOT:PSS/VB-TCTA/PVK:FIrpic:Ir(ppy)3:Os-R1/TPBi/CsF/Al

10.9 cd/A (5.9%)

159 and 160

2-NPD ITO/PEDOT:PSS/2-NPD/PFBT5/CsF/Al 10.8 cd/A 161 (PPZ-VB)2IrPPZ ITO/(PPZ-VB)2IrPPZ/TPA-b-

OXA:TPY2Iracac/Cs2CO3/Al 14.2 lm/W

(9.2%)

162

BCz-VB ITO/PEDOT:PSS/BCz-VB/PVK:OXD-7:FIrpic/TPBi/Cs2CO3/Al

24.5 cd/A 163

PS-TPD-PFCB, TriTCTA-PFCB

ITO/PS-TPD-PFCB/TriTCTA-PFCB/PVK:FIr6/TPBi/CsF/Al

2.4 cd/A (1.2%)

166

TPD-BCB ITO/TPD-BCB/TPA-OXA:TPY2Iracac/BCP/LiF/Al

10.4% 167

TCz II ITO/TCz II/P1:P2:Ir(pppy)3/BCP/LiF/Al/Ag

48.4 cd/A (13.6%)

168

X-PTPA-5 ITO/PEDOT:PSS/X-PTPA-5/PVK:PBD:Ir(ppy)3/LiF/Al

43.7 cd/A (11.8%)

164

Plexcore® HTL ITO/AQ1200/Plexcore® HTL/NPB:Ir(2-phq)3/BAlq/Bphen:Cs2CO3/Al

18.0 cd/A 165

Note: Full name of the materials: QUPD: N,N'-bis(4-(6-((3-ethyloxetan-3-y)methoxy)-hexyloxy)penyl-N,N'-bis(4-

methoxyphenyl)biphenyl-4,4'-diamin OTPD: N,N'-bis(4–(6–((3–ethyloxetan-3-y)methoxy))-hexylpenyl)-N,N'-diphenyl-4,4'-diamin PVK: poly(N-vinylcarbazole) PBD: 2-(4-tert-butylphenyl)-5-biphenylyl-1,3,4-oxadiazole OXD-7: 1,3-bis[2-(4-tert -butylphenyl)-1,3,4-oxadiazo-5-yl]benzene Ir(piq)2(acac): bis(1-phenylisoquinoline)(acetylacetonate) iridium(III) Ir(mppy)3: tris[2-(p-tolyl)pyridine] iridium(III) FIrpic: bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate iridium(III) TPBi: 2,2',2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)

PFBT5: poly[2,7-(9,9′-dihexylfluorene)-co-4,7-(2,1,3-benzothiadiazole)]

FIr6: bis(2,4-difluorophenylpyridinato)tetrakis(1-pyrazolyl) borate iridium(III) BCP: 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline NPB: 4,4’-bis[N-(1-naphthyl)-N-phenylamino]biphenyl Ir(2phq)3: tris(2-phenylquinoline) iridium(III) BAlq: bis(2-methyl-8-quinolinolate)-4-(phenylphenolato) aluminium Bphen: 4,7-diphenyl-1,10-phenanthroline X-TAPC, VB-TCTA, Os-R1, 2-NPD, (PPZ-VB)2IrPPZ, TPA-b-OXA, TPA-OXA, TPY2Iracac, BCz-

VB, PS-TPD-PFCB, TriTCTA-PFCB, TPD-BCB, TCz II, P1, P2, Ir(pppy)3, X-PTPA-5, Plexcore® HTL: full names are not shown in the original papers; only the chemical structures are given in the original references.

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Table 2-3. The HyLEDs with a solution processed metal oxide HIL/HTL.

Material Device structure Efficiency

(cd/A) Lifetime

(h) Reference

MoO3 ITO/MoO3/soln. TCTA:TAPC:Ir(ppy)3/ TPBi/LiF/Al

51.5 - 179

ITO/ MoO3/soln. TCTA:Ir(ppy)3/TPBi/LiF/Al 58.6 ~120 a 180 WO3 ITO/WO3/soln. TCTA:FIrpic/TPBi/LiF/Al 14.0 - 182 ITO/WOx/PEDOT:PSS/WOx/soln. PDY-

132/LiF/Al 9.9 1.8 × 106 b 184

V2O5 ITO/V2O5/TAPC/evap. CBP:Ir(ppy)2(acac)/ TmPyPB/LiF/Al

65.0 - 187

ITO/V2O5/soln. Super Yellow/LiF/Al ~7.0 >20 c 187 IZO/PEDOT:PSS:V2O5/soln. PDY-

132/LiF/Al 15.1 - 185

NiOx ITO/NiOx/soln. CBP:Ir(mppy)3/ 3TPYMB/LiF/Al

70.0 - 189

a) Half lifetime (LT50). b) Projected LT50 at 1,000 cd/m2. c) Lifetime to 75% of the initial luminance (LT75) at 1,000 cd/m2.

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CHAPTER 3 SOLUTION PROCESSED HOLE INJECTION AND TRANSPORT LAYERS

3.1 An Aqueous Based Polymer HIL for Stable OLEDs

3.1.1 Background and Motivation

In the past 20 years, significant progress has been made in OLEDs in terms of

device efficiency and lifetime,23,27,213 demonstrating their advantages for applications in

flat panel displays and solid state lighting. Generally, an OLED is a multilayer device30

consisting of different layers controlling the carrier injection, charge transport and

exciton confinement. Among these functional layers, hole injection layer (HIL) plays a

crucial role in determining the device performance214 and stability.215 The hole injection

layer not only controls the energy barrier at the interface between the transparent ITO

anode and the HTL, but it also planarizes the ITO surface and reduces the leakage

current.

Compared with evaporated hole injection materials such as copper

phthalocyanines (CuPc),216 molybdenum oxide (MoOx)217 and 1,4,5,8,9,11-

hexaazatriphenylene hexacarbonitrile (HAT-CN),218 solution processed hole injection

materials are more attractive due to its easy fabrication as well as the ability to planarize

the ITO surface. For example, polyethylene dioxythiophene: polystyrene sulfonate

(PEDOT:PSS) is one of the most widely used solution processed hole injection

materials in OLEDs. However, there are problems with PEDOT:PSS. First, the work

function of PEDOT:PSS is 5.1 eV, which is not sufficient for efficient hole injection in

some OLEDs. Second, due to the high acidity of PEDOT:PSS, it corrodes the ITO

Reprinted with permission from Ho, S.; Xiang, C.; Liu, R.; Chopra, N.; Mathai, M.; So, F. Stable Solution Processed Hole Injection Material for Organic Light-Emitting Diodes. Org. Electron. 2014, 15 (10), 2513–2517. http://dx.doi.org/10.1016/j.orgel.2014.07.022. Copyright © 2014 Elsevier B.V.

http://dx.doi.org/10.1016/j.orgel.2014.07.022

86

anode and degrades the hole injection efficiency.219 Attempts have been made to

reduce the acidity using different solvents.36 However, the addition of solvents results in

losses in electrical conductivity. With an increase of the pH value, the highly conductive

PEDOT:PSS is de-doped, leading to reduction in electrical conductivity as well as work

function.36 Plexcore® OC AQ1200 is another commercially available aqueous-based

HIL, which can be adapted to fabricate OLEDs by solution process. AQ1200 has

reduced solution acidity while still maintaining a deep work function and high

conductivity. Figure 3-1A shows the molecular formula for Plexcore® OC AQ1200. It is

a self-doping polymer poly(thiophene-3-[2[(2-methoxyethoxy)ethoxy]-2,5-diyl) which can

minimize the generation of free radicals during doping and eliminate side-reactions.220

The pH value of AQ1200 is in the range of 2.6 - 3.4, which is less acidic than

PEDOT:PSS. By controlling the dopant and solvent in the system, the work function of

AQ1200 can be tuned from 5.3 to 5.7 eV, enabling a wide range of hole transport

materials for efficient hole injection.221 With a resistivity of 100 - 10000 ohm-cm,

AQ1200 is a good hole transporter.

In this work, hole only devices were fabricated to compare the hole injection

properties and the air stability of AQ1200 and PEDOT:PSS. Furthermore,

phosphorescent OLEDs with these two HILs were also studied. Our results show that

the devices with AQ1200 yield a better efficiency and improved lifetime.

3.1.2 Results and Discussion

3.1.2.1 Space charge limited dark injection characterization

Figures 3-1B shows the hole only device structure for space charge limited dark

injection (SCL-DI) study. The SCL-DI current transient measurements were carried out

to analyze the nature of charge injection222 and carrier transport.218 For ohmic contacts,

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the peak of the transient current density in the SCL-DI measurements is equal to 1.2

times of the steady-state space charge-limited current density (JSCL) calculated by the

Mott-Gurney’s law. If the contact is injection limited, the transient peak current (JDI) will

be reduced. Therefore, the injection efficiency η defined as the ratio of JDI and

theoretical JSCL can be determined by the following equation:

η = 𝐽𝐷𝐼/1.2𝐽𝑆𝐶𝐿 , (3-1)

and JSCL is given by223

𝐽𝑆𝐶𝐿 =9

8𝜀𝑟𝜀0𝜇0exp(0.89𝛽𝑃𝐹√𝐸)

𝐸2

𝑑 , (3-2)

where εr is the relative dielectric constant of the material, ε0 is the vacuum permittivity,

and E = V/d is the applied electric field. Here, NPB was used as the hole transport

material due to its trap-free nature and good chemically stability.224 Its zero field mobility

µ0 = 2.7 × 10-4 (cm2/V-s) and the Poole-Frenkel field-dependent mobility coefficient βPF

= 1.3 × 10-3 (cm/V)-1/2 were obtained by time-of-flight (TOF) data reported in the

literature.225 Due to the different injection efficiency of PEDOT:PSS and AQ1200, the

corresponding current densities are different. Figure 3-2A shows the current density

versus voltage for hole only devices having the same NPB thickness with the two

different HILs. A higher current density was observed for the AQ1200 devices because

of the better alignment with the HOMO energy of NPB from the deeper work function of

AQ1200. The injection efficiency of HIL into NPB was further calculated from the SCL-

DI measurements to verify this observation. The results of the SCL-DI transient

measurements (Figure 3-2C and 3-2D) also show higher current densities at all applied

voltages. Figure 3-2B presents the plot of hole injection efficiency of AQ1200 and

PEDOT:PSS into NPB under a different electric field. At low electrical fields, similar

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injection efficiencies were observed for both AQ1200 and PEDOT:PSS devices, while

the hole injection of AQ1200 increased at a faster rate at higher fields. At fields higher

than 190 kV/cm, the hole injection efficiency is about 20% higher for the AQ1200

devices.

3.1.2.2 Phosphorescent green OLEDs

Phosphorescent green emitting OLED devices with the structure

ITO/HIL/TAPC/TCTA: 6% Ir(ppy)3/CBP: 6% Ir(ppy)3/Bphen/LiF/Al (Figure 3-1C) were

fabricated using AQ1200 and PEDOT:PSS as the HILs. The double-emitting structure of

OLEDs expands the exciton formation zone and thereby decreases the triplet exciton

losses in the region not doped with the phosphorescent emitter, leading to a higher

efficiency.213 Current density-voltage-luminance (J-V-L) characteristics are shown in

Figure 3-3. The lower dark current in AQ1200 device reduces the efficiency loss at low

current densities, as shown in the current efficiency plot in Figure 3-4. Due to the

excess amount of carriers, the PEDOT:PSS device shows a slightly lower efficiency at

lower luminances. The peak current efficiency of the AQ1200 devices reaches 68.7

cd/A, which is similar to the PEDOT:PSS devices at 66.8 cd/A. Even at a brightness of

10,000 cd/m2, both the AQ1200 and PEDOT:PSS devices exhibit similar efficiencies (62

cd/A for the AQ1200 devices and 61 cd/A for the PEDOT devices). The more efficient

hole injection in the AQ1200 devices also leads to a slightly lower efficiency roll-off in

these green emitting OLEDs.

3.1.2.3 Device stability

The operation stability of device is equally important to the device performance. It

is known the main degradation process of PEDOT:PSS comes from the absorption of

water. Nardes et al. studied the environmental stability of PEDOT:PSS and found out

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the weight gain of PEDOT:PSS films was more than 30% after the films were exposed

to an environment of 50% relative humidity and room temperature for only 1 hour.226 In

the presence of water, PEDOT:PSS will be de-doped, resulting in a decrease in

electrical conductivity.226 Compared with PEDOT:PSS, AQ1200 was found to be less

prone to moisture uptake. We measured the weight gain of AQ1200 from the water

uptake. For an AQ1200 film stored in the ambient condition at room temperature (23 oC)

and a relative humidity of 60%, it took more than 20 hours to observe a 3% weight gain,

which is significantly less than that for PEDOT:PSS.221 Based on the above observation,

it is expected that devices using AQ1200 as an HIL should show a better environmental

stability. Thus, we investigated and compared the stability of the hole-only devices with

the two different HILs at room temperature and a relative humidity of 60%. To evaluate

the environmental stability of the HIL, the current-voltage characteristics of the hole-only

devices were monitored over time. The results are shown in Figure 3-5. In order to

protect the devices, we set a compliance current density of 50 mA/cm2 for both devices.

Due to the higher hole injection efficiency, the current density of the AQ1200 device

measured right after the device fabrication showed a steeper rise in current density and

reached a value of 19.8 mA/cm2 at 2 V, which was significantly higher than that of the

PEDOT:PSS device. Within the next 44 hours, there was a small change in the current

density of the AQ1200 device, which still maintained a value of 15 mA/cm2 at 2 V. After

104 hours of storage in the ambient condition, the current density decreased to 2

mA/cm2 at 2 V. On the other hand, the decrease in current density of the PEDOT:PSS

device during the same operating time was significantly larger. PEDOT:PSS exhibited a

good injection for about a half hour. But after 3.5 hours of storage in air, the current

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density decreased by almost 4 orders of magnitude. This significant change in current

indicates there is a change of work function leading to the complete loss of ohmic

contact.

Further stability test was conducted on the device operating lifetime of

phosphorescent orange OLEDs using AQ1200 as a HIL. The OLED device has the

following structure: ITO/AQ1200/NPB/NPB:Ir(2-phq)3/Balq/Bphen:Cs2CO3/Al. (Figure 3-

1D) Here, NPB was used as the HTL and Balq was the hole blocker.227 The device was

electrically stressed under a constant current density of 72 mA/cm2. Figure 3-6 shows

the luminance and voltage change with operating time. The LT80 lifetime (luminance

reduction to 80% of the original value) for the AQ1200 phosphorescent OLED is more

than 85 hours. During this period, the increase of voltage is less than 0.1 V. The

projected LT80 lifetime at an initial brightness of 1,000 cd/m2 can be estimated with an

empirical relation:

𝐿𝑛×𝑡 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 , (3-3)

where L is the initial luminance, t is the time of luminance drop to 80% of the initial

value, and n is the acceleration factor, typically ranging between 1.5 and 2.56 For an

initial luminance of 1,000 cd/m2, the estimated LT80 and LT50 times are ~3,300 and

~8,400 hours, respectively. The device lifetime is comparable to other iridium-based

device in the literature,228 whose LT50 from 1,000 cd/m2 is 4,500 hours. This device

lifetime is comparable with that of evaporated small molecule OLEDs.229

3.1.3 Summary

We have demonstrated AQ1200 as a solution processed hole injection material

in OLEDs with a good hole injection efficiency and improved air stability. Due to the

better work function alignment with the HOMO energy of NPB, an enhanced hole

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injection was observed in devices with AQ1200 from the results of space charge limited

dark injection transient measurements. Phosphorescent OLED devices made with

AQ1200 reached a maximum luminance of 68 cd/A and a low efficiency roll-off (61

cd/A) up to a luminance of 10,000 cd/m2. Furthermore, AQ1200 based phosphorescent

OLED devices also demonstrated long lifetime and good operating voltage stability.

3.1.4 Experimental Section

To characterize the hole injection properties, hole-only devices were fabricated

for SCL-DI current transient measurements. The device architectures are as follows.

Pre-patterned ITO glass substrates were sonicated with DI water, acetone and

isopropanol sequentially followed by UV-ozone treatment for 20 minutes. A 35 nm thick

hole injection layer (AQ1200 or PEDOT:PSS) was spin-coated onto the ITO substrate

and annealed at 170 oC for 20 minutes. Subsequently, a hole transport layer of NPB

was thermally evaporated at a pressure of ~1 × 10-6 torr. For SCL-DI measurements, a

1.8 µm thick layer of NPB was used as a HTL; for air stability measurements, a 100 nm

thick layer of NPB was used. A 5 nm-thick MoOx and 100 nm-thick of aluminum were

sequentially evaporated as the counter electrode on these devices. All thermally

evaporated layers were deposited with a rate of 0.5 – 2 Å /s. Figures 3-1B through 3-1D

illustrate the schematic diagram of the device structures used in the study.

Phosphorescent OLED devices having structures of ITO/35 nm HIL (AQ1200 or

PEDOT:PSS)/50 nm TAPC/15 nm TCTA: 6% (Ir(ppy)3)/15 nm CBP: 6% Ir(ppy)3/55 nm

Bphen/2 nm LiF/100 nm Al and ITO/10 nm AQ1200/30 nm NPB/20 nm NPB:Ir(2-

phq)3/10 nm Balq/45 nm Bphen: CsCO3/100 nm Al were fabricated for device

characterization and lifetime study. The HILs were solution processed while the rest of

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the layers were deposited by thermal evaporation. OLEDs were encapsulated with cap

glass lids and UV-curable epoxy in a N2 glove box after device fabrication.

The current-voltage-luminance characteristics were measured using a Keithley

2400 source meter and Keithley Series 6485 picoammeter with a calibrated Newport

silicon photodiode. The luminance was calibrated using a Konica Minolta luminance

meter (LS-100). For SCL-DI measurements, a pulse function generator (HP model

214B) was used as the power source. The transient current density through the devices

was determined by measuring the voltage across a resistor in series with the device

using a digital oscilloscope.

3.2 A Cross-linkable HTL for Solution Processed Multilayer OLEDs

3.2.1 Background and Motivation

After two decades of research, a lot of scientific progress has been made in

OLEDs.23,27 With the recent development made in efficiency and lifetime, OLEDs have

been commercialized for display213,230 and solid state lighting231 applications. To further

advance the technology, solution processing of OLEDs with promises of low cost and

large area manufacturing is still a grand challenge.222 However in solution processed

OLEDs, typically devices have lower efficiency and shorter lifetime compared with

evaporated ones.149,232 In order to understand the factors limiting the device

performance, a systematic study of the functionalities of each solution processed layer

is deemed necessary. In OLEDs, the HTL plays an important role in determining the

device efficiency and lifetime. More importantly in solution processed OLEDs, HTL is

Reprinted with permission from Xiang, C.; Chopra, N.; Wang, J.; Brown, C.; Ho, S.; Mathai, M.; So, F. Phosphorescent Organic Light Emitting Diodes with a Cross-Linkable Hole Transporting Material. Org. Electron. 2014, 15 (7), 1702–1706. http://dx.doi.org/10.1016/j.orgel.2014.03.009. Copyright © 2014 Elsevier B.V.

http://dx.doi.org/10.1016/j.orgel.2014.03.009

93

the first layer deposited and it should have the chemical and mechanical robustness to

withstand further processing of subsequent layers in the device stack. Therefore, it is

essential to establish a performance baseline for the solution processed HTL and

compare that with the evaporated counterpart. Arylamine based HTLs are widely used

in multilayer devices because of its chemical and thermal stability as well as its ability to

transport holes.157,233 In addition to hole mobility, their proper HOMO and LUMO energy

levels should enable good hole injection and effective electron blocking.

In this work, we report on the fabrication of OLEDs with a solution processed

cross-linkable HTL and compare their performance with similar devices using an

evaporated HTL. Specifically, we used the PLEXCORE® HTL234 as the solution

processed HTL and NPB as the evaporated HTL in this study. Our results show that

both devices show comparable device efficiency and lifetime indicating that the

PLEXCORE® HTL is promising for solution processed OLEDs.

3.2.2 Results and Discussion

The PLEXCORE® HTL from Plextronics Inc.235 is a new vinyl based multi-

component cross-linkable hole transport material, which is designed for fully solution

processed OLEDs by using a functionalized core structure of N2,N7-di(naphthalen-1-yl)-

N2,N7-diphenyl-9H-fluorene-2,7-diamine. As shown in Figure 3-7A, the PLEXCORE®

HTL is a hole transport material that can be cross-linked upon heating. The HTL ink is

formulated in toluene and its HOMO energy is 5.4 eV which is similar to NPB. It should

be noted that there is no material loss when the PLEXCORE® HTL is exposed to

solvents such as toluene or o-xylene during deposition of the EML, indicating its

chemical robustness for solution processing (Figure 3-7B). It is known that the cross-

linkable functional groups might affect the conjugated π bond of arylamines, changing

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the hole transport properties.236 The simplest way to evaluate the transport ability of this

new cross-linkable hole transport material is to directly compare and evaluate its

performance with another commonly used thermally evaporated arylamine, NPB. By

incorporating this cross-linkable HTL into a phosphorescent OELD, we carried out

experiments to study its transport properties, device performance and lifetime, and

compared the results of devices with thermally evaporated NPB HTL.

3.2.2.1 Hole mobility measurement

The hole mobility of an HTL can be extracted from the fitting of Space Charge

Limited Current (SCLC). The current density (JSCL) of hole only devices follows the Mott-

Gurney’s Law:237

𝐽𝑆𝐶𝐿 =9

8𝜀𝑟𝜀0𝜇0exp(0.89𝛽𝑃𝐹√

𝑉

𝑑)𝑉2

𝑑3 , (3-4)

where the ε0 is the vacuum permittivity, εr the relative permittivity. µ0 is the mobility at

zero electrical field, V the applied voltage and d the thickness of measured materials.

βPF is the Poole–Frenkel factor. By fitting the J-V characteristics with Eq (3-4), the

values of µ0 and βPF were extracted. The Poole–Frenkel field-dependent mobility238 can

be determined as follows:

𝜇 = 𝜇0 exp(𝛽𝑃𝐹√𝑉

𝑑) . (3-5)

Hole only devices were used to extract the mobility. To fabricate the hole only device a

30-nm-thick AQ1200 (available from Sigma Aldrich Inc.) HIL was first spin-coated on the

UV ozone treated ITO glass substrate. Then a 150-nm-thick PLEXCORE® HTL was

spin-coated and subsequently annealed at 170 oC in a nitrogen glove box for 40 min.

Finally, a 4-nm-thick MoOx and a 100-nm-thick Al cathode layer were thermally

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evaporated. For comparison, the NPB hole only device has the same structure but with

a 150-nm-thick NPB (Lumtech, Corp.) layer evaporated. AQ1200 is a water based hole

injection polymer, which has a work function of 5.4 eV.239 With AQ1200, ohmic contact

was formed at the interface of HTL. MoOx/Al was used as a counter electrode to prevent

injection of electrons. From the SCLC measurements, the zero field mobility of NPB was

determined to be 1.01×10-4 cm2/V-s, which is consistent with values from

literatures.218,240 The zero field mobility of PLEXCORE® HTL was 1.46×10-6 cm2/V-s,

two orders of magnitude lower than that of NPB. The lower mobility of PLEXCORE®

HTL came from the non-conjugated side chain, which affected the conjugated π bond of

bone molecule and the reduced packing of molecule by solution process. Figure 3-7C

shows the calculated field dependent mobility of PLEXCORE® HTL and NPB. There is a

stronger field dependence of mobility observed in PLEXCORE® HTL compared with

NPB. As the electrical field increases, the difference between PLEXCORE® HTL and

NPB decreases.

3.2.2.2 Morphology

The surface morphologies of the HTLs were investigated by atomic force

microscopy (AFM) (Veeco Co.). Solution processed PLEXCORE® HTL and vacuum

deposited NPB were deposited on top of AQ1200, which had an average surface

roughness (root mean square) of <1 nm. Figure 3-8 shows the typical AFM images of

PLEXCORE® HTL and NPB films. Because the annealing temperature (170 oC) was

lower than the glass transition temperature (200 oC) of PLEXCORE® HTL, no

crystallization was observed. But clear materials aggregation due to the annealing was

detected. On the other hand, the evaporated NPB film was amorphous and showed a

smooth surface. The average RMS roughness of vacuum deposited and solution

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processed films were 2.3 and 2.8 nm, respectively, which indicated the solution

processed PLEXCORE® HTL film had a quality as good as evaporated NPB.

3.2.2.3 Phosphorescent OLEDs performance

To study the effect of HTLs on OLEDs, phosphorescent OLEDs were fabricated

with PLEXCORE® HTL and NPB. Figure 3-9A shows the device structure and the

corresponding energy diagram. Except for HIL AQ1200 and PLEXCORE® HTL which

were deposited by solution processing, all other layers were thermally evaporated.

Taking the advantage of excellent stability and good HOMO level alignment with HTL,

NPB was also used as the host for the emitting layer. Here, iridium(III) tris(2-

phenylquinoline) (Ir(2-phq)3) is used as the emitting dye with a triplet energy of 2.1

eV.241 Facile energy transfer from NPB, with a triplet energy of 2.3 eV, to Ir(2-phq)3 is

expected in this host–guest system. Due to the LUMO level matching with Ir(2-phq)3,

aluminum(III) bis(2-methyl-8-quinolinate)(4-phenylphenolate) (BAlq) served as an ETL,

which also prevents exciton quenching from the Cs2CO3 doped 4,7-diphenyl-1,10-

phenanthroline (Bphen) layer. Figure 3-9B shows the plots of the current density and

luminance vs. the applied voltage for the PLEXCORE® HTL and NPB devices. Both

devices show an electrical turn-on at 2.3 V, which indicates the same HOMO levels for

these two materials. However, the current density of the NPB device rises more rapidly

after turn-on, from 10-4 mA/cm2 to 1 mA/cm2 within 0.3 V, while it takes 0.8 V for the

PLEXCORE® HTL device to rise in the same current range. At high driving voltages

(over 4.5 V), there is no appreciable difference in current density between the two

devices. These changes of the J–V curves are consistent with the field dependent

mobility determined from the hole only devices. The luminance–voltage curves have the

same rising trend as the current density curves. At low voltages, the NPB device has a

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higher luminance than the PLEXCORE® HTL device. However, the PLEXCORE® HTL

device gives a higher luminance above 4 V. As a result, the PLEXCORE® HTL device

achieved a higher current efficiency than the NPB device. The plots of current efficiency

vs. device luminance are shown in Figure 3-9C. Even though the efficiency roll-off of

both devices is small, the roll-off is faster in the NPB device than the PLEXCORE® HTL

device at brightness higher than 1,000 cd/m2. Due to the lower mobility, the hole

concentration in the emitting layer is lower in the PLEXCORE® HTL device, which

reduced the possibility of triplet-polaron quenching inside the emitting zone. Figure 3-9D

shows the EL spectra of PLEXCORE® HTL and NPB devices. The EL spectra were

measured at the same current density. Because of the efficient energy transfer from

NPB to guest emitter, both devices showed emission only from Ir(2-phq)3.

3.2.2.4 Device stability

Accelerated lifetime tests were carried out on encapsulated PLEXCORE® HTL

and NPB devices. Initial luminance was set at 8,053 cd/m2 and 8,060 cd/m2 for

PLEXCORE® HTL and NPB devices, respectively. We measured the time that the

luminance decreased to 80% of its initial value. Figure 3-10A shows the luminance

decay with operation time. The two devices followed very different decay curves.

Overall the NPB device showed a stable but fast decay, with a quick initial drop during

the first few minutes. It took 86 h for NPB device to drop to 80% of its initial luminance

(LT80). For the PLEXCORE® HTL device, there was a clearly initial faster decay at first

20 h, followed by a much slower descent. The overall LT80 for the PLEXCORE® HTL

device was longer, reaching 103 h. The initial faster luminance drop corresponded to

the dramatic increase of operation voltage. Figure 3-10B shows the operation voltage

increase with the device operation time. The initial operation voltages of the two devices

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were different due to the efficiency difference as discussed before. Within the entire

operating time, the voltage of the NPB device only increased by 0.1 V and the quick

initial increase was observed more clearly in voltage. On the other hand, the operation

voltage of PLEXCORE® HTL device increased at a faster rate during the first 20 h then

followed by a slow increase. Overall, the PLEXCORE® HTL devices show very a similar

lifetime compared with the NPB devices, indicating that it is promising for solution

processed OLEDs.

The PL spectra of both devices before and after lifetime testing are shown in

Figure 3-11. The excitation wavelength was 350 nm so that both HTL and EML can be

excited. The change of PL peaks of HTL and EML can provide information about the

degradation mechanism. In both figures, the peaks in the blue region (~450 nm) are

from HTLs and the peaks in the orange region belong to the emission from the EMLs. In

both devices, we did not observe a significant change in PL intensity before and after

lifetime test. The increase of relative NPB PL peak (Figure 3-11A) was due to the

annealing effect from the Joule heat generated. The post annealing might reduce the

defects of NPB and improve the interface structure.242 In PLEXCORE® HTL device

(Figure 3-11B), the PL intensity of EML remains the same while the PL intensity of HTL

decreases slightly. After electrical stressed, solution processed PLEXCORE® HTL might

generate more charge-induced defects, which serve as quenching centers of

luminescence.

3.2.3 Summary

In conclusion, we demonstrated orange emitting phosphorescent OLEDs with a

novel cross-linkable hole transport material, PLEXCORE® HTL. Device performance

was directly compared with the thermally evaporated NPB OLEDs. Although the mobility

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of PLEXCORE® HTL is lower than that of NPB, the overall device performance was

slightly better. The PLEXCORE® HTL devices showed a current efficiency reaching 18

cd/A at 1,000 cd/m2 and a LT80 over 100 h starting at 8,000 cd/m2, indicating that the

PLEXCORE® HTL is promising for solution processed OLEDs.

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Figure 3-1. The chemical structure of AQ1200 and the device structures used in this

work. A) Plexcore® OC AQ1200: a self-doping polymer poly(thiophene-3-[2[(2-methoxyethoxy)ethoxy]-2,5-diyl).243 B) The hole only devices, C) green phosphorescent OLED (PhOLED) for device J-V-L characteristics and D) orange PhOLED for lifetime study. TAPC: 4,4’-Cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]; TCTA: Tris(4-carbazoyl-9-ylphenyl)amine; Ir(ppy)3: Tris[2-phenylpyridinato-C2,N]iridium(III); CBP: 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl; Bphen: Bathophenanthroline; LiF: lithium fluoride; Ir(2-phq)3: Tris(2-phenylquinoline)iridium(III); Balq: aluminum(III) bis(2-methyl-8-quinolinate)(4-phenylphenolate); CsCO3: cesium carbonate.

Figure 3-2. The hole injection properties of AQ1200. A) The current density versus

voltage for NPB hole only device with different HILs. B) Hole injection efficiency η as a function of applied electric field. C) and D) The dark injection transient current density of AQ1200 and PEDOT:PSS devices, respectively.

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Figure 3-3. Phosphorescent OLED J-V-L with HILs (AQ1200 and PEDOT:PSS) and

without a HIL.

Figure 3-4. The current efficiency versus brightness for devices with different HILs.

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Figure 3-5. The J-V characteristic variation with time under ambient condition. A)

AQ1200 HIL and B) PEDOT:PSS HIL.

Figure 3-6. The operation stability of AQ1200 based phosphorescent OLED.

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Figure 3-7. Chemical structure and properties about the PLEXCORE® HTL. A) The

chemical structure. B) The absorption curves of the cross-linked film prior to and after toluene solvent rinse. C) The field dependent hole mobility of both NPB and PLEXCORE® HTL.

Figure 3-8. The AFM images. A) PLEXCORE® HTL film and B) NPB film.

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Figure 3-9. The phosphorescent OLED and its performance. A) Device structure and

energy band diagram. B) J-V-L curves. C) The current efficiency as a function of luminance. D) EL spectra.

Figure 3-10. Device operation stability. A) The luminance decay versus the operation

time. B) The voltage increase versus the operation time.

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Figure 3-11. PL spectra prior to and after lifetime testing. A) NPB device. B)

PLEXCORE® HTL device.

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CHAPTER 4

INTERFACE EFFECT OF EML IN SOLUTION PROCESSED MULTILAYER OLEDS


Organic light emitting diodes (OLEDs) have great potentials for displays and

lighting applications. Although vacuum deposited OLEDs are being commercialized for

displays, low material utilization rate and the requirement for high vacuum processing

are the reasons for high manufacturing cost, and therefore solution processed OLEDs

are still preferred for manufacturing. With the ability to manufacture devices by roll-to-

roll processing, solution processed OLEDs are also more favorable for large area and

flexible applications.

However, compared to thermally evaporated OLEDs with internal quantum

efficiencies close to 100%,244–246 achieving high efficiencies is still one of the challenges

to overcome in solution processed OLEDs. A multilayer structure for exciton

confinement and charge blocking is still an ideal architecture for OLEDs to achieve high

external quantum efficiencies (EQEs). Unlike vacuum evaporated OLEDs where a

multilayer architecture is easily achieved with sequential deposition, the re-dissolution of

the preceding layer by solvents used in the subsequent layers is a challenge for solution

processed OLEDs during the multilayer fabrication process.247 To get around this

problem, cross-linkable hole transport layers (HTLs) are typically used such that

subsequent deposition of the emitting layer (EML) would not damage the underlying

HTL during processing.157,159,160,165,171,248,249 To finish the device fabrication, evaporated

Reprinted with permission from Ho, S.; Chen, Y.; Liu, S.; Peng, C.; Zhao, D.; So, F. Interface Effect on Efficiency Loss in Organic Light Emitting Diodes with Solution Processed Emitting Layers. Adv. Mater. Interfaces 2016, 3 (19), 1600320. http://dx.doi.org/10.1002/admi.201600320. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

http://dx.doi.org/10.1002/admi.201600320

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electron transport layers (ETLs) are used in OLED fabrication to avoid solvent damages

to the EML.

Even with the same multilayer architecture, most OLEDs with a solution

processed EML have lower efficiencies than their thermally evaporated

counterparts.33,250–254 One fundamental difference between evaporated and solution

processed films is the molecular packing. As summarized in Table 4-1,122,211,253–261 it is

generally accepted that solution processed organic films have a lower packing density

than the vacuum deposited films. Previous studies comparing solution processed and

evaporated OLEDs were mostly focused on HTLs, where the packing density and

hence carrier transport plays a critical role.255,256,260,261 On the other hand, there are only

a few reports directly comparing the performance of devices with solution processed

and evaporated phosphorescent EMLs,122,254 and the root cause for the difference in

device performance is not well understood. It is, therefore, important to identify and

study the factors determining the efficiency loss mechanism in devices with a solution

processed EML.

In this work, we study the differences in high efficiency OLEDs having solution

processed and thermally evaporated EMLs. Similar to other findings, we found that the

EQE of OLEDs with a solution processed EML is 22% lower than that of the devices

with an evaporated EML using 1,3,5-tris(N-phenylbenzimidazol-2,yl) benzene (TPBi) as

an ETL. Interestingly, this difference in efficiency became significantly smaller as TPBi

was replaced with bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine (B3PYMPM) as

an alternative ETL. Because of the deep HOMO energy of B3PYMPM, we attribute the

lower efficiency in OLEDs with solution processed EMLs to the inefficient hole blocking

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properties of the TPBi ETL as revealed by the single carrier device results. A

subsequent study on interfacial exciplex formation and energetic disorder revealed that

for devices with a solution processed EML, band tail states broadening along with an

energy level shift at the EML/ETL interfaces results in a higher hole leakage current

from the EML to the ETL, leading to a lower efficiency in OLEDs with a solution

processed EML. By replacing TPBi with B3PYMPM as the ETL, holes are more

efficiently blocked due to the deeper HOMO energy of B3PYMPM, and high efficiency

solution processed-OLEDs were thus realized with an EQE of 29%, which is

comparable to the efficiency of their thermally evaporated counterparts.

4.2 Results and Discussion

4.2.1 Efficiency Loss in Solution Processed OLEDs of Two Distinct ETLs

Figure 4-1 shows the energy band diagram of the device structure and materials

used for this study. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate

(PEDOT:PSS) was solution processed as the HIL,262 and PLEXCORE® HTL was

solution processed as the cross-linkable HTL/electron blocking layer.165 For the EML,

we chose tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and TPBi as the co-host to avoid

space charge build-up to tune the recombination zone by adjusting the ratio of hole and

electron dominant hosts and minimizing exciton quenching by adjacent transport

layer.33,263 For the emitter, we chose bis(2-phenylquinoline)(2,2,6,6-tetramethylheptane-

3,5-dionate)iridium(III) (PQ2Ir(dpm)) due to its high solubility in organic solvent and high

quantum yield.245,264 The devices were fully optimized to ensure the comparison is made

on the highest achievable efficiency. The EMLs consisted of TCTA: TPBi: PQ2Ir(dpm) at

a weight ratio of 0.77: 0.17: 0.06 were deposited by either solution processing or

thermal evaporation. TPBi or B3PYMPM was then thermally evaporated (e-TPBi or e-

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B3PYMPM) as the ETL. The device structures are as follows:

Device E1: thermally evaporated EML (e-EML) with e-TPBi ETL

Device S1: solution processed EML (s-EML) with e-TPBi ETL

Device E2: thermally evaporated EML (e-EML) with e-B3PYMPM ETL

Device S2: solution processed EML (s-EML) with e-B3PYMPM ETL

Figure 4-2A and 4-2B show the current density-voltage-luminance (J-V-L)

characteristics and EQEs for OLEDs with TPBi as the ETL (Device E1 and S1). The

turn-on voltage (defined at 1 cd/m2) for Devices E1 and S1 is similar (~3.0 V), while

Device S1 shows a higher current density compared to Device E1. The slightly higher

current density and luminance in Device S1 than in E1 can result from the increased

states at the EML/ETL interface of S1, which induce higher leakage current but not

proportionally reflect on the output photons. As shown in Figure 4-2B, the maximum

EQEs of Devices E1 and S1 are 31% and 24%, respectively. The efficiency curves

show a similar trend and roll-off for both devices. On the other hand, the OLEDs with

B3PYMPM as ETL show a lower turn-on voltage of 2.5 V (Figure 4-2C), which can be

understood from the lower-lying LUMO of B3PYMPM with a smaller electron injection

barrier. Device S2 also shows a higher current density compared to Device E2.

However, the maximum EQEs are 31% and 29% for Devices E2 and S2, respectively.

Compared to Devices E1 and S1, the difference of efficiency between Devices E2 and

S2 is significantly smaller.

4.2.2 Effect from The Bulk Film Packing Density

A noticeable difference in packing density (~0.07 difference in refractive index)

can be adjusted by solute concentrations (the refractive index in Figure 4-3). We

fabricated hole only devices using films with different packing density and found that he

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correlation of film packing density to carrier transport was not significant (Figure 4-4).

Furthermore, the OLEDs with solution processed films of different packing density show

negligible difference of EQE. There was only less than 0.5% difference of EQE in the

OLED with TPBi ETL (Figure 4-5) and less than 0.1% difference of EQE in device with

B3PYMPM ETL (Figure 4-6), which suggests the dominant factor here might not be the

packing density. In short, the device efficiency is not significantly affected by the

solution processed EMLs with different molecular packing densities. Therefore, the

lower efficiency in Device S1 cannot be solely explained by the difference in the bulk

properties of solution processed films since there is an increase in efficiency just by

changing the ETL from TPBi to B3PYMPM.

4.2.3 Effect of Interface States by A Single Carrier Device Study

Considering the deep HOMO level of B3PYMPM, which can more effectively

block holes, we fabricated hole only devices (HOD) to verify whether the difference in

device performance is related to the different hole blocking properties in devices with

solution processed and thermally evaporated EMLs. Figure 4-7 shows the current

density-electric field (J-E) characteristics of HOD with the following device structure:

ITO/PEDOT:PSS/PLEXCORE® HTL/s-EML or e-EML/evaporated ETL (TPBi or

B3PYMPM)/MoOx/Al. Here, the MoOx layer is used to prevent electron injection from Al.

From the J-E curves of the HODs with s-EML and e-EML using e-TPBi as the ETL, we

found a similar trend in OLED devices: the hole current density is higher in s-EML HOD.

On the contrary, in the HOD with e-B3PYMPM ETL, the hole current density is similar

between e-EML and s-EML. These results revealed that the hole current in the solution

processed HOD with TPBi as the ETL is significantly higher than that in the evaporated

device, indicating that a higher hole leakage current leads to a lower EQE in OLEDs

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with a s-EML. However, when B3PYMPM was used as the ETL, due to its deep HOMO

level, holes were more effectively confined in the s-EML, resulting in an EQE

comparable to OLEDs with an e-EML. While an ETL with a deep HOMO energy is not

necessary required for the e-EML, our results indicate that to ensure high efficiencies in

devices with a s-EML, an ETL with a deep HOMO level is preferred. This observation

reveals that the interfacial energy alignment between EML and ETL might be different

for devices with solution processed and thermally evaporated EMLs. Therefore, it is

imperative to study the energy level alignment at the EML/ETL interfaces for both e-

EML and s-EML. As TCTA is the dominant component used in both s-EML and e-EML

and it is also responsible for hole transport in the EMLs, the hole blocking properties

should be directly related to the HOMO energy level alignment at the TCTA/ETL

interface. Therefore, we study the HOMO alignment of TCTA with respect to the ETL.

4.2.4 Further Investigation of Interface States

To study the energetic alignment at the EML/ETL interface, we measure the

photoluminescence (PL) of the exciplex formed at EML/ETL interfaces.265,266 If there is

an energy level shift of the s-EML relative to the e-EML, we expect to see a shift in the

exciplex emission spectrum. We first attempted to measure the exciplex emission from

the TCTA/TPBi interface. However, the exciplex emission was too weak to be detected,

which might result from the small energy level offset between donor (TCTA) and

acceptor (TPBi) and hence insufficient charge carrier accumulation at the interface.265–

267 Therefore, to investigate if there is any change in the HOMO alignment between the

EML and ETL, the exciplex emission was measured using B3PYMPM as an ETL

instead of TPBi.267 Herein, neat TCTA films were prepared by either solution processing

(s-TCTA) or thermal evaporation (e-TCTA) followed by thermal evaporation of a

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B3PYMPM thin film. Figure 4-8 shows the exciplex emission spectra of the

TCTA/B3PYMPM samples excited at 360 nm. The e-TCTA/B3PYMPM spectrum shows

a peak at 510 nm, which is close to the energy level offset between the LUMO of

B3PYMPM and the HOMO of TCTA. On the other hand, the exciplex emission peak for

the s-TCTA/B3PYMPM bilayer sample is at 494 nm, which corresponds to a ~80 meV

deeper HOMO level for the s-TCTA layer. Furthermore, the full width at half maximum

(FWHM) is broadened from 80 nm in the e-TCTA sample to 100 nm in the s-TCTA

sample. UV-vis absorption spectra revealed that there is no difference in the bandgap

energy between s-TCTA and e-TCTA films (inset of Figure 4-8). Therefore, the shift of

the exciplex PL should be due to a vacuum level shift in the s-TCTA sample. The

broadened exciplex PL further suggests that there is a broadening of the band tail

states in s-TCTA compared to e-TCTA.

To further verify that there is a broadening of band tail states in s-TCTA, we

study the energetic disorder in both s-TCTA and e-TCTA by measuring the temperature

dependent zero-field carrier mobility using the Gaussian disorder model (GDM) as

follows:8,268

𝜇0(𝑇) = 𝜇∞exp[−(2𝜎

3𝑘𝑇)2], (4-1)

where μ0 is the zero-field carrier mobility, μ∞ is the high temperature limit of the mobility,

k is the Boltzmann constant, T is the temperature in Kelvin, and σ is the energetic

disorder parameter corresponding to the distribution of charge transport (charge carrier

hopping) sites. Figure 4-9 shows the temperature-dependent zero field mobility for e-

TCTA and s-TCTA. The energetic disorder is 56 meV for e-TCTA and 75 meV for s-

TCTA. The higher energetic disorder in solution processed film reflects the presence of

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broadening hopping manifold, which might be ascribed to trap states introduced from

wet processing. The increase of energy disorders in the s-TCTA film gives a further

proof of more band tail states in solution processed films that contribute to the

broadening in the exciplex emission spectrum.

4.2.5 Proposed Scenarios

Based on these results, we identified the efficiency loss mechanism in OLEDs

with a s-EML (Figure 4-10). Due to the energy level shift and broadened band tail states

of s-TCTA in s-EML, the energy barrier height for holes at the s-EML/ETL interface is

reduced. For OLEDs with TPBi ETLs, due to the small barrier height at the TCTA/TPBi

interface (~0.3 eV, Figure 4-10A), hole leakage current is observed in s-EMLs, resulting

in a lower efficiency. However, in the case of B3PYMPM ETL, holes are efficiently

blocked in both s-EML and e-EML due to the large energy offset at the

TCTA/B3PYMPM interface (~1.0 eV, Figure 4-10C), rendering comparable EQEs for

OLEDs with s-EML and e-EML. Therefore, to achieve highly efficient OLEDs with a s-

EML, a high hole barrier is required at the s-EML/ETL interface to effectively confine

holes within the s-EML due to the deepened and broadened HOMO levels in the s-EML.

4.3 Summary

In this study, we demonstrated a high efficiency multilayer solution processed

OLED and highlighted the critical role that interfacial energy level plays in the device

performance. The different hole blocking properties of thermally evaporated ETLs for

solution processed and thermally evaporated EMLs were studied. From the single

carrier devices, a higher hole leakage current was found in devices with a solution

processed EML when TPBi was used as the ETL. The PL measurements of exciplex

formed at the EML/ETL interface revealed that there is a shift in interface energy at the

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EML/ETL interface along with a broadening in band tail states in the solution processed

EML. The energetic disorder measurements further confirmed the broadened tail states

in the HOMO level, which impaired the hole confinement due to increased hopping

states. However, the efficiency loss in s-EML caused by inefficient hole blocking can be

circumvented by using ETL materials with a deep HOMO level. We demonstrated that

with a B3PYMPM ETL, the efficiency of OLEDs with s-EML could be significantly

enhanced, leading to a high EQE of 29.0% which is comparable to the devices with e-

EML.

4.4 Experimental Section

4.4.1 The EML Preparation and Study

The EML used in our devices is composed of TCTA/TPBi co-host doped with

PQ2Ir(dpm) (77wt %/17wt %/6wt %). The materials were purchased from LumTech

Corporation and used without further purification. All materials were weighed and

dissolved to chlorobenzene (Sigma-Aldrich) of 14 mg/mL solution in nitrogen-filled glove

box. The solution processed films were prepared by spin-casting, whereas the vacuum

deposited films were evaporated at a vacuum base pressure of 5×10-7 Torr. For the

study of variable film packing density, three different solute concentrations (10, 14 and

22 mg/mL of total solute in the solvent) are used. The corresponding spin speed is

adjusted to ensure the film thickness is similar for solution processed films with different

solute concentrations. This processing procedure is applied to the sample prepatation

for refractive index measurements and device fabrication. The organic films for

ellipsometry measurement were prepared on silicon wafer substrates with a known

thickness of SiO2. Variable-angle spectroscopic ellipsometry equipped with a Xenon arc

lamp source (VASE, M88, J. A. Woollam Co., Inc.) were performed over the wavelength

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from 280 to 763 nm in air. The incident angles were used at 55°, 65°and 75° relative to

the surface normal. The organic thin film samples were measured with various

thickness ranging from 70 to 100 nm, which yielded consistent results. All organic films

were measured right after the fabrication in order to minimize the effect from ambient

conditions. The ellipsometric parameters Ψ and Δ were analyzed using the J. A.

Woollam WVASE32 software package. With the parameters of Si and SiO2 layers as

the substrates, the Cauchy model was used to obtain the refractive index.

4.4.2 OLED Fabrication and Characterization

To fabricate the devices, ITO-patterned substrates were sonicated in deionized

water, acetone and 2-propanol sequentially, followed by UV-Ozone treatment for 30

minutes to adjust the work function and improve wetting. PEDOT:PSS (Clevious AI

4083) was filtered with a 0.22 μm PVDF filter. A 30 nm PEDOT:PSS film was spin-

casted and then annealed at 140 oC for 20 minutes in air and 20 minutes in a N2 glove

box. The cross-linkable hole transport layer PLEXCORE® HTL (Plextronics Inc.,

SOLVAY)165 was spin-casted for the thickness of 35 nm prior to 180 oC annealing for

cross-linking. The samples were loaded to an evaporator for EML deposition. On the

other hand, for the devices with a solution processed EML, the EML was spin-casted

onto the cross-linked HTL. The solution processed EML devices were then heated at

130 oC for 20 minutes. Afterward, the substrates were cooled down in a N2 glove box

prior to transferring into the thermal evaporator for ETL and cathode deposition. All

deposited layers were evaporated at a rate of 0.1-1 Å /s at a base pressure of 5 × 10-7

Torr. In hole only devices, the cathode injection material lithium fluoride (LiF) was

substituted with molybdenum oxide (MoOx) to prevent electron injection.147 The J-V-L

characteristics were measured by a Keithley 2400 source meter and Keithley Series

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6485 picoammeter with a calibrated Newport silicon photodiode. We calibrated the

luminance using a Konica Minolta luminance meter (LS-100). The EL spectra were

obtained using an Ocean Optics spectrometer. In the energetic disorder experiments,

single carrier devices were fabricated with a structure ITO/MoOx/solution processed or

vacuum deposited TCTA/MoOx/Al. For temperature dependence mobility

measurements, devices were placed inside a Janis VPF-100 cryostat with a LakeShore

321 temperature controller.

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Figure 4-1. The materials used in this work and the energy band diagram of the

OLEDs.

Figure 4-2. The device performance. A) The J-V-L characteristics and B) EQE versus

luminance characteristics of TPBi devices with evaporated (E1) and solution processed (S1) EML. C) The J-V-L and D) EQE vs luminance characteristics of B3PYMPM OLEDs with vacuum deposited (E2) and solution processed (S2) EML. The inset shows the EL spectra.

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Figure 4-3. The refractive index of solution processed EML films with different solute

concentration. The original data is obtained and calculated by spectroscopic ellipsometer.

Figure 4-4. The J-E characteristics of hole only devices fabricated by solution process.

The hole only devices have similar structure to the OLED in this study, with the ETL and cathode substituted by MoOx/Al to suppress electron injection. The inset shows the hole only device structure.

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Figure 4-5. The bulk film packing effect on efficiency of TPBi ETL devices.

Figure 4-6. The bulk film packing effect on efficiency of B3PYMPM ETL devices.

Figure 4-7. The J-E characteristics of hole only devices (HOD).

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Figure 4-8. The normalized PL spectra of TCTA/B3PYMPM bilayer. The PL peak is the

exciplex emission between TCTA and B3PYMPM interface. The peak lies at 495 nm for s-TCTA/B3PYMPM bilayer, while it shifts to 510 nm for e-TCTA/B3PYMPM bilayer. Inset is the absorption spectra of the neat film TCTA by solution process (s-TCTA) or vacuum thermal evaporation (e-TCTA). No appreciable shift is observed at the wavelength where the absorption occurs, which implies the energy gap of TCTA is unaffected by the processing methods. The thin films for UV-vis absorption measurement were prepared on quartz substrate to avoid the absorption peak of glass.

Figure 4-9. The temperature dependent zero field hole mobility of neat TCTA films from

solution process and vacuum deposition.

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Figure 4-10. The proposed scenario energy band diagrams. A).Device S1, B) device

E1, C) device S2 and D) device E2.

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Table 4-1. List of previous works on comparing solution processed and vacuum evaporated films.

Materials Refractive indexa Transportb OLED devicec Year

NPB: TPBi: Irpiq3 (EML)

Evap > Soln N/A Evap > Soln 2007122

TBADN (EML) Evap > Soln Soln > Evap Evap > Soln 2009253 NPB: DPVBi Evap > Soln N/A N/A 2010258 TPBi (ETL) N/A N/A Evap ≈ Soln 2011211 TPD (HTL) Soln > Evap Soln > Evap Soln > Evap 2011255 TCTA (HTL) Evap > Soln Evap > Soln Evap > Soln 2013256 TPCz: FIrpic (EML) Solvent

dependentd N/A Evap > Soln 2014254

CBP: 4CzIPN (EML) Evap > Solne N/A Evap ≈ Soln 2015259 TCTA: OXD-7 Evap > Soln Evap > Soln N/A 2015260 TPD, NPB, CBP,

Alq3, etc. Evap > Soln N/A N/A 2015261

TCTA: TPBi: PQ2Ir(dpm) (EML)

Evap > Soln Evap ≈ Soln Evap > Solnf This work

a) The packing density were generally associated with the refractive index. The larger the refractive index, the denser the film.

b) Transport properties: The comparisons were performed through single carrier devices. “Evap > Soln” means evaporated films show better transport property (i.e. higher current density) than solution processed films, and vice versa.

c) Device Performance: “Evap > Soln” means higher efficiency (in cd/A, lm/W or %) was reported in the OLEDs.

d) In Ref. 8, the isopropanol film shows higher refractive index than that of evaporated film. But the chlorobenzene and n-butanol films show lower refractive index than its evaporated counterpart.

e) The film density was determined by the film thickness and weight but not from refractive index.

f) In this work, the device efficiency between evaporated and solution processed OLEDs is not directly related to the packing density or transport property.

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CHAPTER 5 SEMI-TRANSPARENT VERTICAL ORGANIC LIGHT EMITTING TRANSISTORS


The interests in transparent displays have rapidly increased with the

development of smartphones, televisions and wearable devices.269–271 This type of

devices can be driven as a self-emitting display while allowing users to look through it.

Due to the requirement of all electronic elements being transparent, it is challenging for

current display technologies employing a backlight module (e.g., liquid crystal displays)

or opaque driving transistors (e.g., amorphous silicon transistors) to serve as a

transparent panel.

Vertical organic light emitting transistors (VOLET) that directly combine an

organic light-emitting diode (OLED) with a vertical field effect transistor (VFET) offer a

number of advantages for transparent panel displays.107,272 First, due to the use of a

transparent gate, stacking of an OLED on top of it would readily enable a transparent

VOLET. Second, the combined device architecture saves the space originally allocated

for the driving transistor, leading to a light emission with high pixel aperture ratio and

high display resolution.273 Third, the short vertical channel can be readily achieved

without any complicated patterning process, leading to a lower power consumption.

However, a transparent VOLET has not been demonstrated yet because of the

difficulty in fabricating a transparent, perforated source electrode where the charge

injection from the source electrode to the channel layer is modulated via the porous

electrode region. Previously, the porous source electrode has been made by a thin

metal layer, such as Al83,88,89,274,275 or Au.90,276–278 However, the porosity was controlled

by natural pinholes in the thin metal film and hence control of the charge injection from

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the porous electrode was difficult. Alternatively, a lift-off process was employed to make

a perforation in a thin Au electrode. Due to the use of a block copolymer279 as a mask

for the lift-off, however, the porosity largely relied on the local phase separation of each

polymer, which is not homogeneous in a long-range order. Most importantly, the optical

transparency of these metals (Al and Au) drop significantly as the thickness of the metal

layers is over 5 nm,280 which rules out the possibility of making transparent displays.

In this work, we report a semi-transparent VOLET by fabricating a transparent,

porous indium-tin oxide (ITO) source electrode and top Mg:Ag drain electrode. With a

suitable capping layer, the transmittance of the Mg:Ag electrode can reach over 70% in

the spectrum of visible wavelengths (see Figure S1). The high transmittance allows the

device to exhibit a high luminance of 500 cd/m2 toward the substrate direction and 250

cd/m2 toward the top electrode direction. The current efficiency reaches 8.8 cd/A for the

light emission toward the substrate direction and 4.6 cd/A toward the top direction. We

discovered that the light extraction to both sides was enhanced due to the nano-

textured ITO source electrode. Furthermore, the luminance on/off ratio can be improved

by two orders of magnitude by increasing the channel layer thickness.


5.2.1 The Porous ITO Electrode

The schematic architecture of a semi-transparent VOLET is shown in Figure 5-

1A. The porosity of ITO source is important since it determines the number of electrons

injected into the C60 channel under a positive gate bias (VGS > 0).91 A continuous

electrode will screen the field effect from the gate completely. In contrast, the

disconnected ITO electrode can lead to poor conductivity and high local electric field,

which leads to an electrical short. To fabricate a porous ITO electrode, polystyrene

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sphere monolayer was deposited by Langmuir-Blodgett method, being the patterning

mask. To find the optimal porosity, the reactive ion etching (RIE) was employed to

control the size of polystyrene spheres and thereby the ITO pore size. Figure 5-1B

displays the scanning electron microscope (SEM) images, which confirms a closely-

packed porous ITO electrode with a center to center distance of about 1.0 µm. A

solution processed phenyl-C61-butyric acid methyl ester (PC60BM) was deposited prior

to the evaporated C60 layer. The PC60BM can passivate the roughness from porous ITO

surface and avoid the short problem. A transparent inverted OLED was evaporated on

top of the C60 channel layer. The VOLET can be seen through when switching off, as

the photos of the off and on states in the inset of Figure 5-1A and the transmittance in

Figure 5-2.

5.2.2 Device Operation Mechanism

The energy band diagrams for the VOLET are illustrated in Figure 5-3A to 3D.

The potential barrier between the lowest unoccupied molecular orbital (LUMO) of C60

and the work function of UV ozone treated ITO forms a Schottky junction,281 which

suppresses the source-drain current under off state. The vertical current flow between

source and drain electrodes is modulated by the applied gate field. A constant source-

drain bias (VDS) is applied during the operation. Thus, the hole carriers are injected from

the drain electrode to the OLED and confined at the interface of the hole blocking layer.

Without the application of a gate-source bias (VGS = 0 V), the Schottky barrier between

ITO and channel materials (PC60BM/C60) inhibits the electron injection (Figure 5-3A and

3B). When a gate-source bias is applied (VGS > 0 V), the electron carriers accumulate at

the porous ITO source electrode, raising the Fermi level of ITO closer to the LUMO of

C60. As a result, the depletion width is reduced, which allows the electron injection by

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tunneling through the ITO/C60 interface. The injected electron carriers are transported

across the C60 channel and injected into the electron transport layer of the OLED.

Eventually, the electrons recombine with holes in the emitting layer and generate

photons to both sides (Figure 5-3C and 3D).

5.2.3 VOLET Device Performance

The luminance transfer characteristics are plotted in Figure 5-4A. The VOLET

was operated under a source-drain voltage (VDS) of 18 V. The luminance shows a turn-

on at gate bias (VGS) of 2 V and the saturation at VGS > 6.5 V. Negligible luminance was

observed under a low VGS of the off state. As the VGS is increased, the maximum

luminance reaches about 500 and 250 cd/m2 on the bottom ITO side and top Mg:Ag

side, respectively. A luminance on/off ratio of > 103 was shown. We calculated the

current efficiency for light emission to both sides, as in Figure 5-4B. The current

efficiency is nearly unchanged among the VGS range of 6 to 10 V, giving 8.8 ± 0.1 cd/A

to the bottom substrate and 4.6± 0.1 cd/A to the top electrode. The combined maximum

efficiency from both sides is 13.6 cd/A, which is more than one order of magnitude

higher than reported transparent light emitting transistors.270,282 The EL spectra of the

VOLET is shown in Figure 5-4C. The slight shift of 520 nm peak is due to a weak

microcavity effect. The difference on the shoulder results from the slight absorption from

C60 channel layer. The cut-off frequency is another important figure of merit for a

display device, which corresponds to the refresh rate of a display panel. The temporal

response of our VOLET is presented in Figure 5-4D. The cut-off frequency is ~ 94 Hz,

which is slightly higher than that (76 Hz) of other planar structure organic light emitting

transistors.283

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5.2.4 Porous ITO Scattering Effect

The textured electrode is known to enhance light extraction of a light emitting

device.284 Thus, it is important to understand light extraction resulting from the

scattering porous ITO source electrode. In order to study the light emission behavior,

we fabricated a transparent OLED with the same layer structure as that of the emitting

unit of VOLET (see Supporting Information, Figure 5-5 and 5-6 for more details). We

compared efficiency and angular dependent EL of the transparent OLEDs with planar

and porous ITO electrodes. The peak efficiency was 29 cd/A and 31 cd/A from the ITO

side for planar and porous electrodes, respectively (Figure 5-5D and 5-6D). On the

other hand, a 55% enhancement from 9 cd/A of planar ITO to 14 cd/A of porous ITO

was measured toward the top electrode (Figure 5-5D and 5-6D), implying the increased

light extraction toward Mg:Ag side due to the scattering porous ITO electrode. In terms

of the EL spectra, the transparent OLED and the VOLET showed similar behavior

toward both sides (Figure 5-4C, 5-5C and 5-6C), with a smaller shoulder at longer

wavelength for Mg:Ag side. The angular dependent EL measurement for the

transparent OLEDs in Figure 5-5 and 5-6 was carried out to characterize the radiation

profile (Figure 5-7 and 5-8). The angular-dependent profile of a planar ITO device

showed a more intensive radiance in the normal direction (within the emission cone of

30 degrees) from either ITO or Mg:Ag side (Figure 5-8A and 5-8B). At a higher viewing

angle, a pronounced sub-Lambertian profile was observed on planar ITO devices, which

has been reported in other transparent OLEDs.285–287 In contrast, the emission at higher

viewing angles is enhanced in the porous ITO device compared to a planar device. We

further performed a finite-difference time-domain (FDTD) simulation to investigate the

scattering effect from the porosity in the electrode. The far-field distribution from ITO

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and Mg:Ag sides for both the planar and porous electrode at the emitting wavelength

(520 nm) is shown in Figure 5-8C and 5-8D. For the planar device, the light extraction

decreases above 20 degrees from the center (Figure 5-8C). On the contrary, the porous

electrode device shows broader light extraction to more than 30 degrees from the

center (Figure 5-8D). Also, the far-field broadening effect is observed from Mg:Ag side

for porous ITO compared to planar device (Figure 5-8E and 5-8F). Specifically, the

porosity of ITO gives a stronger and more uniform intensity from the center to 20

degrees toward the top Mg:Ag side. The overall emission behavior of porous ITO device

is closer to the Lambertian pattern, as observed from the experimental angular

dependent emission profile measurements (Figure 5-8A and 5-8B). Since the difference

of light extraction intensity comes majorly from side emission, we further look at the

scattering feature of the pore region.

5.2.5 Effect of Channel Layer Thickness

To understand the effect of channel layer on the emission behavior, we vary the

C60 thickness of the VOLET. The total luminance on/off ratio as a function of C60

thickness is shown in Figure 5-9A. The on state luminance of the VOLET is of the same

order regardless of the C60 thickness since the on state current is determined mainly by

the space charge limited current within the channel.96 However, at a thinner C60

thickness, the higher leakage current under the off state leads to a decreased

luminance on/off ratio. In our VOLET, it is discovered that the luminance ratio of ITO

side to Mg:Ag side (LITO/LMg:Ag) is in the range of 2.0 to 2.5 with the C60 thickness below

500 nm. The value is consistent with the reported values in other transparent

OLEDs,270,286,288–290 indicating a one-side preferred emission due to the relatively lower

transmittance of the top electrode. However, we observed that the LITO/LMg:Ag drops to

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about 1.0 with a 700 nm C60 layer. It is reasonable since the transmittance of the C60

layer drops with the increase of thickness, which reduces the light emission toward the

bottom substrate. Figure 5-9B demonstrates the luminance ratio as a function of source-

drain bias (VDS) for VOLETs with the C60 thickness of 320 nm and 700 nm. The

luminance ratio remains nearly unchanged with the increase of VDS, which is due to that

the shift of recombination zone is small compared to the cavity length of the entire light

emitting transistor. Therefore, the luminance ratio is a constant with respect to the

increase of brightness in a VOLET.

5.3 Summary

In summary, we have demonstrated a semi-transparent VOLET by directly

integrating a transparent OLED into a VFET, which possesses the advantage of a

higher aperture ratio and the lower power consumption. By using three electrodes with a

high transmittance, the VOLET exhibits a current efficiency of 8.8 cd/A at the bottom

and 4.6 cd/A at the top, and the luminance of 500 cd/m2 and 250 cd/m2 to the bottom

ITO and top Mg:Ag, respectively. Due to the usage of porous ITO as the source

electrode, the scattering effect leads to more out-coupled light on both sides, specifically

at higher viewing angles. The enhanced light extraction is further confirmed by FDTD

simulations. It is also found that with a thicker channel thickness, the leakage current is

suppressed and the luminance on/off ratio can reach close to 104.


5.4.1 VOLET Fabrication

The commercial ITO substrates were scrubbed with soap water, followed by the

ultrasonic bath in de-ionized water, acetone and isopropanol for 15 minutes

sequentially. A 10 minutes UV-ozone cleaning was performed to clean surface

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thoroughly. Next, the samples were loaded into atomic layer deposition (Ultratech Inc.,

Savannah200) chamber for HfO2 dielectric film deposition. Alternate pulse of

tetrakis(dimethylamino)hafnium precursor and ozone were applied for 500 cycles, giving

a HfO2 film of 50 nm. The porous ITO was fabricated by colloidal lithography where

polystyrene (1.1 µm & 800 nm, LB11 & LB8, Sigma-Aldrich) spheres were deposited by

Langmuir-Blodgett to form a monolayer. The size of close-packed PS particle was

controlled by a reactive ion etching (Trion RIE Phantom II, RIE power = 100 W,

chamber pressure = 40 mTorr, oxygen flow = 40 sccm, processing time = 110 sec).

After the RIE, the substrates were loaded into a sputter chamber (Kurt J. Lesker

PVD75) for ITO deposition. A 100 nm ITO was deposited by RF sputtering at the

conditions of 130 W power, 55 sccm Ar flow, chamber pressure ~2 mTorr for 34

minutes. The PS monolayer was lift-off by a 3M scotch tape, leaving a ITO with pore

center to center distance of ~ 1.1 µm. Afterwards, the porous ITO/HfO2/ITO/glass

substrate was under UV-ozone treated for 20 minutes to adjust the porous ITO surface

with a deeper work function. Phenyl-C61-butyric acid methyl ester (PC60BM) of a

thickness ~50 nm was spin-coated from chlorobenzene solution (~34 mg/mL) to

passivate the roughness of porous ITO, followed by a 90 oC annealing for 30 minutes in

a nitrogen filled glove box. The layer stack of PC60BM/porous ITO/HfO2/ITO/glass was

moved to an evaporation chamber for various thickness (150-700 nm) of C60 deposition

without the exposure to air. Following the C60 channel deposition, the samples were

shifted to another evaporation chamber for the deposition of the rest layers without air

exposure. The light emitting unit consists of a 35 nm of 4,7-diphenyl-1,10-

phenanthroline (Bphen) and a 10 nm T2T as electron transport layer/ hole blocking

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layer, a 25 nm 4,4'-bis(carbazol-9-yl)biphenyl (CBP) doped with 8% bis(2-

phenylpyridine)(acetylacetonate)iridium(III) (Ir(ppy)2acac) as the emitting layer, a 50 nm

1,1-bis[(di-4-tolyamino)phenyl]cyclohexane (TAPC) as the hole transport layer, a 10 nm

MoOx as the hole injection layer. The drain electrode was made by a 8 nm Mg:Ag (10:1)

alloy and a 5 nm pure Ag. The capping layer of 45 nm N,N'-diphenyl-N,N'-bis(1-

naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB) was deposited to enhance the optical

transparency.

5.4.2 Device and Film Characterization

The electrical characteristics were measured in air using Keithley 4200. The

photocurrent was obtained with a calibrated Newport silicon photodiode. We calibrated

the luminance from the corresponding photocurrent with a Konica Minolta luminance

meter (LS-100). The electroluminescence (EL) spectrum was obtained from Ocean

Optics spectrometer. The angular dependent EL intensity was measured on a rotation

stage with a sample holder. The intensity was obtained by integrating the absolute

photon count over the spectrum, which was collected by Ocean Optics spectrometer.

The scanning electron microscope (SEM) images of the porous ITO were taken by a

field emission SEM (FEI Verios 460L).

5.4.3 Optical Modeling and Simulation

The simulation was performed by FDTD modeling. The 3D simulation structure of

volume of 10 µm x 10 µm x 2 µm with perfectly match layer (PML) boundary condition is

used. The incoherence is achieved using multiple simulation for single dipole source

with x, y and z orientations. Mesh of 15 nm in x, y direction and 10 nm in z direction is

used for more accurate results. Far-field distribution and vertical light propagation for

scattering pattern is recorded for both the devices.

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Figure 5-1. The VOLET with a porous ITO source electrode. A) Schematic illustration of

device architecture. The inset displays the photos of VOLET under off (left) and on (right) conditions. The pixel emission area is 3.99 mm2. B) The SEM image of closely-packed porosity in the ITO source electrode.

Figure 5-2. The transmittance spectra of thin metal drain electrode, the stack of porous

ITO/HfO2/ITO and the VOLET device.

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Figure 5-3. The working mechanism of the transparent VOLETs. Under the off state of

a VOLET, the band diagrams are shown in A) for the entire device where a positive source-drain bias (VDS > 0) and no gate-source bias (VGS = 0) is applied, and in B) at the interface of ITO/C60. Under the on state of a VOLET, the band diagrams are shown in C) for the whole device where there biasing conditions are VDS > 0 and VGS > 0, and in D) at the interface of ITO/C60 where the band bending facilitates the electron injection from ITO source into C60 channel.

134

Figure 5-4. The performance of a VOLET. A) The luminance and current transfer

characteristics. B) The current efficiency. C) EL spectra of the VOLET from bottom ITO and top Mg:Ag sides. D) The temporal response of the VOLET.

Figure 5-5. The transparent OLED with a planar ITO electrode. A) The OLED device

structure and the corresponding VOLET architecture. B) The current density-voltage luminance (J-V-L) characteristics. C) The EL spectra of measured from both sides. D) The current efficiency from ITO and Mg:Ag sides.

135

Figure 5-6. The transparent OLED with a porous ITO electrode. A) The transparent

OLED with a porous ITO electrode is illustrated, and all the other layer structure remains the same as that of the planar ITO transparent OLED in Figure 5-4. B) The J-V-L characteristics. C) The EL spectra of measured from both sides. D) The current efficiency from ITO and Mg:Ag sides.

Figure 5-7. The luminance distribution of OLEDs. A) With a planar ITO electrode

(structure of Figure 5-4A). B) With a porous ITO electrode (structure of Figure 5-5A). The blue contour represents the bottom emission toward the Mg:Ag side, while the red contour shows the emission profile to the ITO side.

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Figure 5-8. The optical scattering effect from the porous ITO source electrode. A) The

calculated radiance from ITO side as a function of the viewing angle. B) The radiance from Mg:Ag side as a function of viewing angle. The far-field distribution from ITO side for C) planar D) porous device, from Mg:Ag side E) for planar F) for porous device.

137

Figure 5-9. The effect of channel layer thickness. A) The total luminance on/off ratio as

a function of C60 thickness. B) The luminance ratio between ITO and Mg:Ag sides (LITO/LMg:Ag) as a function of source-drain bias (VDS).

138

CHAPTER 6 INDIUM-TIN OXIDE/INDIUM-GALLIUM-ZINC OXIDE SCHOTTKY JUNCTION BY

GRADIENT OXYGEN DOPING


Amorphous InGaZnO (a-IGZO) has been extensively studied as an active

semiconductor material for the next generation electronics due to many advantages

such as optical transparency, high mobility, low temperature processability, and air

stability.291–296 Thus far, most of the applications are for channel materials used in thin-

film transistor (TFT) backplanes for display applications.297

Compared to TFT applications, little research has been performed on IGZO

diode in spite of the potential for high speed rectifiers used in modern electronics. Most

of metals form an ohmic contact with a-IGZO because the oxygen vacancies at the

interface induce Fermi-level pinning.298,299 Only a few metals such as Pt or Pd have

been shown to form Schottky contacts with a-IGZO under special oxidizing treatments.

Zhang et al. reported that an oxygen treatment during the sputtering of Pt electrode

reduced the oxygen vacancy concentration at the Pt/a-IGZO interface resulting in a high

Schottky barrier height of 0.92 eV.299 Yan et al. reported the similar effect on Pd/a-IGZO

junction after an ultraviolet ozone treatment of the metal contact leading to 107 of

rectification ratio.300 Post annealing of the Schottky junctions at 150 ~ 200 °C were also

reported to enhance the rectification ratio of a-IGZO diodes.298,301

For transparent electronics, the approaches described above are not applicable

to transparent indium-tin oxide (ITO) electrodes because the charge carrier density in

oxide semiconductors is proportional to the oxygen vacancy concentration302 and hence

an oxidizing treatment of the electrodes would significantly decrease the electrical

139

conductivity.303 To the best of our knowledge, transparent conducting oxide/a-IGZO

Schottky junction has never been reported.

In this work, we first report the formation of Schottky junctions formed at the ITO

and a-IGZO interface by applying a gradient doping of oxygen into the a-IGZO layer.

Using this approach, we can engineer the ITO/a-IGZO interface and control the

Schottky junction resulting in a device exhibiting a rectification ratio of 103 with a cut-off

frequency of 6.2 MHz. Finally, by using a-IGZO/ITO/a-IGZO back-to-back Schottky

junctions, all transparent permeable-base transistor with a common-base gain close to

unity is demonstrated.


6.2.1 Contacts between a-IGZO and Electrodes

Amorphous IGZO film typically exhibits n-type characteristic with high carrier

concentrations,298,300,304–306 which was confirmed by Hall effect measurements on as-

sputtered a-IGZO film where the Hall mobility and the carrier concentration were

measured to be 6.3 cm2/V-s and 4.4×1016 cm-3 respectively. As shown in Figure 6-1, we

observed ohmic contacts of a-IGZO with Au or ITO (ultraviolet ozone-treated)

electrodes in spite of the large work-function differences between the electrodes and

IGZO.

It is generally accepted that an oxygen doping into a-IGZO would decrease the

oxygen vacancy and reduce the carrier concentration of a-IGZO.307,308 Chasin et al.

reported that an oxygen flow during the sputtering process not only decreases the

carrier concentration of the a-IGZO film but it also changes the junction behavior from

ohmic to Schottky for Pd/a-IGZO and Pt/a-IGZO junctions.298 As such, we investigated

140

the effect of oxygen doping into a-IGZO films for an Al/a-IGZO/ITO device. Figure 6-2A

represents the current-voltage characteristics of the device with different oxygen flow

during the a-IGZO deposition. Here, the oxygen concentration was measured as a

percentage of oxygen flow in sccm relative to the Ar flow. As seen in the figure, the

overall current density drops by several orders of magnitude as the oxygen flow

increases from 0 % to 37.5 %, possibly due to a decreased carrier concentration within

the a-IGZO layer. We also observed a gradual increase in the current rectification ratio

as the oxygen flow increases as shown in Figure 6-2B. Although the rectification was

very small, this suggests that the oxygen flow not only decreases the conductivity of the

a-IGZO film but it also changes the ITO/a-IGZO junction resulting in a higher electron

injection barrier than that in an Al/a-IGZO junction.

However, when an oxygen flow is larger than 37.5 %, it would sacrifice too much

current density of the a-IGZO device, which depreciates the usage of a-IGZO with high

mobility. As such, we applied a gradual oxygen doping into the a-IGZO layer so that the

ITO/a-IGZO junction side is highly oxygen-doped and the film close to the Al side is less

oxygen-doped. With this approach, the Al/a-IGZO junction is ohmic whereas the ITO/a-

IGZO junction has an electron injection barrier. Figure 6-3A describes two different Al/a-

IGZO/ITO devices where the a-IGZO layer of Device A consists of 1 nm of 0 % oxygen

doping, 220 nm of 5 % oxygen doping, and 5 nm of 37.5% oxygen doping layers. On

the other hand, the a-IGZO layer of Device B consists of 1 nm of 0 % oxygen doping,

180 nm of 5 % oxygen doping, and 45 nm of 37.5% oxygen doping layers. Figure 6-3B

shows the resulting diode characteristics of the two devices (red and black lines)

compared to the reference device with no oxygen-treatment (blue line). As observed,

141

Device A shows a slight rectification after the gradient doping. The rectification ratio at ±

3 V is only 10 because the thickness of the 37.5 % doping layer is 5 nm. As the

thickness of 37.5 % oxygen-doping layer is increased to 45 nm while the total thickness

of the a-IGZO layer is kept at 226 nm, a higher rectification of 3×103 was observed. As

shown in the band diagrams in Figure 6-3B, the 37.5 % oxygen doped a-IGZO layer

should have a lower carrier concentration and marked as an ‘i-IGZO’ layer, resulting in

its Fermi-level far from the conduction band. This would induce an up-ward band

bending at the ITO/i-IGZO junction. However, the thickness of i-IGZO layer is only 5 nm,

leading to the high reverse current of the device A. The i-IGZO layer of Device B, on the

other hand, has a much thicker i-IGZO layer of 45 nm and hence the reverse current

decreases by 105 times. Although the forward current of the device B also drops by 300

times, the significant suppression of the electron injection from the ITO electrode to the

i-IGZO layer under reverse bias leads to much higher rectification of 103.

6.2.2 ITO/a-IGZO Diodes and Permeable Metal-base Transistors

Next, we fabricated all transparent a-IGZO diode on glass. The Inset in Figure 6-

4A shows the ITO/a-IGZO/ITO diode where the a-IGZO layer consists of 60 nm of

37.5% oxygen-doped layer and 180 nm of 10 % oxygen-doped layer. As observed in

Figure 6-4A, the all transparent diode exhibits 102 of rectification ratio due to the effect

of gradient oxygen doping. Figure 6-4B presents the speed of the all transparent diode

having a cut-off frequency of 6.2 MHz which is similar to the previous reported

values.292,299 Given that a maximum GHz operation was reported with a-IGZO,306,309 the

lower bandwidth may be due to the effect of the insulating IGZO layer at the Schottky

junction. Further optimization in terms of the oxygen doping concentration and the

142

doping layer thickness would be necessary to enhance the rectification ratio as well as

the dynamic response.

One application of the ITO/a-IGZO Schottky diode is for permeable metal-base

transistor (PMBT) fabrication where a permeable metal-base is sandwiched by two

semiconductor layers. PMBT has been highlighted for applications in high speed current

amplifier310,311 and ITO/a-IGZO junction would be an excellent platform for the

fabrication of all transparent PMBTs. In PMBTs, the base electrode must form Schottky

back-to-back junctions with the two adjacent semiconductor layers. In addition, the base

electrode must be thin enough for the carriers injected from the emitter electrode to

effectively transport through the base electrode. As such, a-IGZO PMBT was fabricated

using ITO emitter/graded IGZO/ITO base (10 nm)/graded IGZO/ITO collector structure

as shown in the inset of Figure 6-5A. PMBTs made from the back-to-back stack of two

ITO/a-IGZO Schottky diodes exhibit a transmittance greater than 85% over the

wavelength range from 415 nm to 800 nm as shown in Figure 6-5A. Figure 6-5B shows

the common-base characteristic of the PMBT. In the common-base measurements, the

collector voltage was swept while the emitter current was fixed with the base voltage

grounded. One of the figures of merit for PMBTs is the common-base gain (α) which is a

measure of its charge collection efficiency from the emitter.310 α is the ratio of the

collector current to the emitter current and typically measured at VCB = 0 V. As shown in

Figure 6-5C, the collected current is linearly proportional to the emitter current and the

common-base gain is close to unity which indicates that nearly all emitted charges can

be collected.

143

6.3 Summary

In conclusion, we demonstrated a transparent electrode/a-IGZO Schottky

junction by gradual doping of oxygen into the a-IGZO layer. The resulting diode

exhibited a rectification ratio of 103 and 6.2 MHz of cut-off frequency. Using the Schottky

diode, all transparent PMBT could be fabricated with common-base gain close to unity.

The present work would contribute to the development of future transparent electronics

based on a-IGZO semiconductor.


Commercial ITO/glass substrate was cleaned by acetone and isopropanol in an

ultrasonic bath for 15 minutes respectively. In case of Au bottom electrode, 5 nm of Cr

and 30 nm-thick Au electrode were thermal-evaporated. Next, a-IGZO was deposited by

radio-frequency sputtering at room temperature (Kurt J. Lesker, PVD). For 0, 5, 10, or

37.5% of O2/Ar flows during the a-IGZO film deposition, O2/Ar flows are set to be 0

sccm/80 sccm, 4 sccm/80 sccm, 8 sccm/80 sccm, or 30 sccm/80 sccm respectively.

The sputtering power was fixed at 130 watts for all conditions. After the a-IGZO film

deposition, the 100 nm-thick top Al electrode was deposited by thermal evaporation at 2

Å /s deposition rate. For the all transparent diode, ITO was deposited on top of the a-

IGZO layer at room temperature by DC sputtering at power of 130 Watts and an Ar flow

of 60 sccm. For the fabrication of all transparent PMBT, the same process condition

was used for the emitter-base diode and for the base-collector diode. All current-voltage

characteristics of the diode as well as the PMBT were measured in the air without

encapsulation by Keithley 4200. The speed of the diode was measured by a Tektronix

MDO3014 oscilloscope and a Keysight Agilent 33220A function generator.

144

Figure 6-1. Current-voltage characteristic of Al/a-IGZO/ITO and Al/a-IGZO/Au devices

showing ohmic contacts at the a-IGZO junctions for both cases.

Figure 6-2. The effect of oxygen component on the electrical property. A) Current-

voltage characteristic of Al/a-IGZO/ITO with different oxygen doping ratios into the a-IGZO layer. B) Current density plots at forward (+3V) and reverse (-3V) bias conditions as function of the oxygen flow.

145

Figure 6-3. Performance of the graded IGZO diodes. A) Two different device structures

where the device A has a thinner oxygen-doped layer and the device B has a thicker oxygen-doped layer. B) Current-voltage characteristics of the device A and B compared to the non-graded IGZO device. C) and D) Band diagrams of the device A and the device B, respectively.

Figure 6-4. Performance of all transparent IGZO devices. A) Current density-voltage

characteristic of the ITO/a-IGZO (graded)/ITO device. B) Dynamic response of the transparent diode having a bandwidth of 6 MHz.

146

Figure 6-5. Characteristic of all transparent PMBT. A) transmittance of the PMBT layers

referenced by the glass substrate. B) Common-base plot of the transparent permeable metal-base transistor where the emitter current is step-fixed from 0 to 0.25 mA/cm2, the base is grounded, and the collector voltage is swept. C) Linearity of the collector current as function of the emitter current. The calculated common-base gain is close to unity.

147

CHAPTER 7 CONCLUDING REMARKS

7.1 Summary

This dissertation is focused on two types of organic display devices: OLEDs and

VOLETs. For the former one, the issues of a low cost solution processed device are

investigated using electrical and optical characterization techniques. For the latter one,

a semi-transparent VOLET is demonstrated and the important figures of merit in such a

device are analyzed systematically. In Chapter 1, a comprehensive introduction from

the fundamental concepts of organic semiconductors to the principles of device

applications is given.

With nearly 30 years of research and development, vacuum thermal evaporation

has successfully made OLEDs to be commercialized. In contrast, there are still some

hurdles for solution processed OLEDs even though they offer the attractive advantages

of a higher material utilization rate and lower equipment costs. There is a dearth of

understanding on multilayer wet processing. Based on the introduction in Chapter 1,

Chapter 2 identifies the present challenges of solution process in OLEDs and provides

possible solutions in terms of each functional layer.

In former part of Chapter 3, the effect of incorporating a new HIL, AQ1200, into

an OLED is studied. The commonly used HIL PEDOT:PSS has a shallower work

function, which leaves a large hole injection gap from HIL to HTL or EML. With a work

function of 5.7 eV, AQ1200 enables the use of a wide range of HTLs. From SCL-DI

technique, it was found that the injection efficiency is enhanced in this material. On the

other hand, compared to PEDOT:PSS, AQ1200 has a reduced acidity, which

significantly inhibits the degradation from water absorption. The single carrier device J-V

148

characteristics with the elapsed of time showed a huge difference between AQ1200 and

PEDOT:PSS. Due to the degradation of moisture uptake from the environment,

PEDOT:PSS devices only lasted for less than 30 minutes and became an insulating

species, which can be very detrimental to OLED operation stability. The AQ1200

devices, on the contrary, last for more than 104 hours under the same conditions. The

latter part of Chapter 3 investigated a cross-linkable HTL, which will be a critical building

block in the multilayer solution processed OLEDs. It is discovered that despite the lower

hole mobility of this solution processed HTL compared to conventional evaporated HTL

NPB, it doesn’t necessarily lead to a lower efficiency or shorter lifetime.

As a continuous study from Chapter 3, Chapter 4 aim to understand the inferior

performance in solution processed multilayer OLEDs. An OLED with the state-of-the-art

architecture and efficiency (close to the current theoretical limit) is utilized to ensure the

study is based on a fair and meaningful baseline. With exactly the same device

structure except the processing method of EML, it is revealed that the solution process

can have an influence on interface energy states. From the results of exciplex

photoluminescence at the EML/ETL interface and energetic disorder measurements, it

is found that there is an energy level shift and a band tail broadening in the solution

processed EML compared to an evaporated EML. The observed phenomenon is

reasonable since the solvent processing can induce an increase of disorder in organic

films. The finding correlates the hole leakage current to the efficiency loss in an OLED

with a solution processed EML. The use of an ETL with a low-lying HOMO level can

ameliorate this problem due to the improved carrier confinement. It is believed that this

principle can be universally applied to any solution processed multilayer OLED.

149

Chapter 5 demonstrates an emerging device, the VOLET. The VOLET combines

the functions of an OLED and the switching transistor. This architecture can reduce the

number of elements and increase the aperture ratio in a display pixel. Also, a short

channel transistor is easily achievable without complicated photolithography process. In

this chapter, a top transparent electrode is adopted into the device, making the entire

device stack with a high transparency. The maximum luminance and current efficiency

are 500 cd/m2 and 8.8 cd/A on the bottom side, and 250 cd/m2 and 4.6 cd/A on the top

side, which are the highest among the reported transparent organic light emitting

transistors. The device on and off is controlled by a porous ITO source electrode, which

forms a Schottky contact with the channel organic material. With that mechanism, a

high luminance on/off ratio of 2,600 is demonstrated. The luminance ratio between the

bottom and top is an important figure of merit for novel application like mounted mirror in

a vehicle or head mounted googles. In this device, the ratio can be tunable by the

device optical structure from one side preferred to equivalent bi-direction emission. In

Chapter 6, a gradual oxygen doping method is used to make a Schottky junction at

ITO/a-IGZO contact, which has been found to be ohmic due to Fermi level pinning

effect. The transparent ITO/graded IGZO/ITO diode exhibits a rectification ratio of 103

and 6.2 MHz of cut-off frequency. Using the Schottky diode, a transparent PMBT has

been fabricated with a common-base gain close to unity, which opens a possibility to

make thin film transistors for transparent displays.

7.2 Outlook

The economic advantage of solution processed OLEDs makes them still under

research attention. However, the most critical challenge of device stability needs to be

addressed before a solution-based process first appears in the market. Before moving

150

forward, several questions have to be answered. Is the stability problem really coming

from the solvent traps and residues? If so, is there a processing condition that can make

the film from solution process as close in properties as that from vacuum evaporation?

Or is there any device architecture design that can circumvent the intrinsic limitation

posed by solution-based process? The investigation on material-solvent interaction and

the study on film drying procedure might shed light on the first two questions. For the

last one, the fundamental studies about charge and exciton distribution within the

solution processed device during operation may provide insight into it. Solution-based

printing fabrication still holds the benefits in the future flexible devices. With the

improved understanding in degradation mechanism, the strategic material and structure

design might overcome this stability barrier.

Other than OLEDs, people are still looking for new types of devices for displays.

The VOLET can find its niche in small size head up devices with the main advantage in

making short channel through simple and scalable method. The top priority in the

development of a VOLET is to fabricate a porous source electrode with robustness and

high yield. Although photolithography is not necessary in the present device, the high

throughput fabrication method can be adopted into the fabrication of source electrode

for mass fabrication in the future.

151

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BIOGRAPHICAL SKETCH

Szuheng Ho was born in Taichung, a city in central Taiwan. In 2010, he received

a Bachelor of Science degree in materials science and engineering from National

Taiwan University. After graduation, he served his country as the Second Lieutenant in

an army base for one year. In 2012, he joined the group of Professor Franky So at the

University of Florida and started his graduate research in organic electronics. During his

Ph.D. career, he studied the processing issues and the mechanisms behind for organic

display devices like organic light emitting diodes and organic light emitting transistors.

He has 7 peer-reviewed journal articles, 1 filed US patent and 4 conference

presentations. In the spring of 2017, he graduated with a Doctor of Philosophy degree

from the University of Florida.

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