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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
© 2017 Szuheng Ho
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
5
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)
6
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
7
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
8
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
5.1 Background and Motivation ............................................................................ 123
5.2 Results and Discussion ................................................................................... 124
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 Experimental Section ...................................................................................... 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
6.1 Background and Motivation ............................................................................ 138
6.2 Results and Discussion ................................................................................... 139
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
6.4 Experimental Section ...................................................................................... 143
7 CONCLUDING REMARKS ................................................................................... 147
7.1 Summary ........................................................................................................ 147
7.2 Outlook ........................................................................................................... 149
LIST OF REFERENCES ............................................................................................. 151
BIOGRAPHICAL SKETCH .......................................................................................... 177
9
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
10
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
11
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
12
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
13
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.
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.
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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.
85
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.
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.
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
4.1 Background and Motivation
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.
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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
5.1 Background and Motivation
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 Results and Discussion
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 Experimental Section
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.
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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.
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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.
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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
6.1 Background and Motivation
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 Results and Discussion
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
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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.
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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.
6.4 Experimental Section
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.
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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.