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Design of High Performance Organic Light Emitting
Diodes
by
Zhibin Wang
A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy
Graduate Department of Materials Science and Engineering University of Toronto
© Copyright by Zhibin Wang 2012
ii
Design of High Performance Organic Light Emitting Diodes
Zhibin Wang
Doctor of Philosophy
Materials Science and Engineering
University of Toronto
2012
Abstract
Organic light emitting diodes (OLEDs) are being commercialized in display applications,
and will be potentially in lighting applications in the near future. This thesis is about the design
of high performance OLEDs, which includes both the electrical and optical design of OLEDs. In
particular, the following work is included in this thesis: i) Energy level alignment and charge
injection at metal/organic interfaces have been systematically studied. ii) Transition metal oxide
anodes have been developed to inject sufficient holes into the OLEDs due to their high work
function. The oxide anodes have also been used to systematically study the transport properties
in organic semiconductors. iii) Highly simplified OLED devices with unprecedentedly high
efficiency have been realized using both fluorescent and phosphorescent emitters. The high
performance was enabled by using a high work function metal oxide anode and a hole transport
material with very a deep highest occupied molecular orbital (HOMO). iv) An optical model has
been developed to describe the optical electric field across the OLED device. By using the
model, a high performance flexible OLED using metal anode was designed and realized.
iii
Table of Contents
TABLE OF CONTENTS ............................................................................................................................................ III
LIST OF TABLES ..................................................................................................................................................... V
LIST OF FIGURES.................................................................................................................................................. VI
ABBREVIATIONS AND SYMBOLS ........................................................................................................................... XI
CHAPTER 1 INTRODUCTION ............................................................................................................................ 1
1.1. BRIEF REVIEW ON OLED DEVELOPMENTS .................................................................................................................... 1
1.2. MOTIVATIONS ........................................................................................................................................................ 6
1.3. OUTLINE ............................................................................................................................................................... 7
CHAPTER 2 EXPERIMENTAL METHODS ............................................................................................................ 8
2.1. DEVICE DESIGNS ..................................................................................................................................................... 8
2.1.1. Single carrier devices .................................................................................................................................. 8
2.1.2. OLED devices ............................................................................................................................................ 10
2.2. DEVICE FABRICATION ............................................................................................................................................. 10
2.3. DEVICE CHARACTERIZATION .................................................................................................................................... 16
CHAPTER 3 ENERGY LEVEL ALIGNMENT AT METAL/ORGANIC INTERFACES .................................................... 18
3.1. INTRODUCTION .................................................................................................................................................... 18
3.2. RESULTS AND DISCUSSION ...................................................................................................................................... 20
3.3. SUMMARY .......................................................................................................................................................... 26
CHAPTER 4 ANALYSIS OF CHARGE INJECTION CHARACTERISTICS AT ELECTRODE-ORGANIC INTERFACES........ 27
4.1. INTRODUCTION .................................................................................................................................................... 27
4.2. THEORY .............................................................................................................................................................. 29
4.2.1. Space charge limited current ................................................................................................................... 29
4.2.2. Injection limited current ........................................................................................................................... 30
4.2.3. In between SCLC and ILC (quasi-Ohmic) ................................................................................................... 31
4.3. RESULTS AND DISCUSSION ...................................................................................................................................... 34
4.3.1. IV characteristics ...................................................................................................................................... 34
4.3.2. Fitting IV characteristics and transport parameters ................................................................................ 36
iv
4.3.3. Built-in potential and device thickness dependence ................................................................................ 38
4.3.4. Criterion for SCLC, quasi-Ohmic and ILC ................................................................................................... 40
4.4 SUMMARY ........................................................................................................................................................... 45
CHAPTER 5 ORGANIC/ORGANIC INTERFACE DESIGNS OF OLEDS ................................................................... 47
5.1. INTRODUCTION .................................................................................................................................................... 47
5.2. CBP INTERLAYER TO REDUCE EXCITON QUENCHING ...................................................................................................... 48
5.3. DEEP HOMO HTL: ENABLE SIMPLE STRUCTURE WITH HIGH EFFICIENCY .......................................................................... 54
5.4. DEEP HOMO HTL FOR PHOSPHORESCENT OLEDS .................................................................................................... 60
5.4. SUMMARY .......................................................................................................................................................... 67
CHAPTER 6 OPTICAL DESIGNS OF OLEDS ....................................................................................................... 68
6.1. INTRODUCTION .................................................................................................................................................... 68
6.2. OPTICAL MODEL OF OLEDS .................................................................................................................................... 69
6.2.1. Theory ...................................................................................................................................................... 69
6.2.2. Evaluation of the model ........................................................................................................................... 78
6.3. FLEXIBLE OLEDS .................................................................................................................................................. 79
6.4. SUMMARY .......................................................................................................................................................... 85
CHAPTER 7 SUMMARY AND FUTURE WORK .................................................................................................. 86
7.1. SUMMARY .......................................................................................................................................................... 86
7.2. FUTURE WORK ..................................................................................................................................................... 87
REFERENCES ................................................................................................................................................. 90
APPENDIX A. ERROR ANALYSIS ............................................................................................................... 100
APPENDIX B. LIST OF PUBLICATIONS RELATED TO THIS THESIS ................................................... 104
v
List of Tables
Table 1-1 Phosphorescent OLEDs performance from UDC (adapted from Ref. 15) .................................. 4
Table 2-1 Molecular structures of the organic molecules used in this study.............................................. 13
Table 5-1 Hole injection barrier heights (Bp ) and interfacial dipoles ( ) at different organic/organic
interfaces extracted from the UPS spectra. ........................................................................................ 52
vi
List of Figures
Figure 1.1 Schematic structure of the first efficient OLED as well as the molecular structures. Adapted
from Ref. 2. .......................................................................................................................................... 1
Figure 1.2 Schematic energy diagram of an OLED with an organic heterostructure. ................................. 2
Figure 1.3 Schematic energy diagram of single (fluorescent) and triplet (phosphorescent) emission. ........ 3
Figure 1.4 Photograph of (left) the Samsung Galaxy S cell phone using 4 inch OLED display and (right)
the LG 55 inch OLED television prototype. ........................................................................................ 4
Figure 2.1 Energy diagram of a single carrier hole-only device with a structure of metal/organic/metal. .. 8
Figure 2.2 Schematic device structure of a single carrier device. ................................................................ 9
Figure 2.3 Schematic device structure of a typical multi-layer OLED. ....................................................... 9
Figure 2.4 Layout of patterned ITO used in this study. ............................................................................. 11
Figure 2.5 Schematic structure of the devices (single carrier devices or OLED). ..................................... 11
Figure 2.6 Picture of Kurt J. Lesker LUMINOS® cluster tool. ................................................................. 12
Figure 2.7 Home-made closed-loop He3 low-temperature cryostat. .......................................................... 17
Figure 3.1 Energy diagrams of (a) metal and inorganic semiconductors and (b) metal and organic
semiconductors before making contact. ............................................................................................. 19
Figure 3.2 Current density with Au and Au/C60 anodes at room temperature as a function of electric field
for single carrier devices. The average electric field (F) is taken as F = V/d, where V is the applied
voltage and d is the device thickness. ................................................................................................ 21
Figure 3.3 He Iα (hν = 21.22 eV) spectra of (a) secondary electron cut-off (SEC) of sputter cleaned Au
and Ag with and without 3 nm of C60, and (b) SEC and valence band of Au, Au/α-NPD (3 nm) and
Au/C60 (3 nm)/α-NPD (3 nm); SEC for Au/C60 (dashed line) is also shown for reference. Interfacial
dipoles ( ) and hole injection barrier heights ( Bp ) are as indicated. ............................................. 22
vii
Figure 3.4 Effective work function ,m eff as a function of the pristine anode metal work function m .
The data shown with open points is obtained from Ref. 47,48, while the data shown with solid
points is from this work. The inset is the schematic energy level diagram of the band alignment at
Metal/C60 interface. The Fermi level of the metal is pinned by the adsorbed C60 molecules. ........... 23
Figure 3.5 Schematic energy-level diagram of the band alignment at (a) Au/organic, (b) Au/C60/organic,
(c) Ag/organic and (d) Ag/C60/organic interfaces. The symbol of d indicates the thickness of the C60
layer.................................................................................................................................................... 24
Figure 4.1 Energy Current density (J) as a function of average electric field (F = V/d) for single carrier
hole-only devices with different metal oxide anodes at room temperature (297 K). The structure of
the devices is anode/α-NPD (~ 500 nm)/Au. The solid line is the calculated SCLC from Eq. (4.3) for
α-NPD using the field dependent mobility measured by the TOF technique and reported previously
in Ref. 79. Notice that the different oxides fall into three different groups in terms of the
current-voltage (s) characteristics. ..................................................................................................... 35
Figure 4.2 Current density (J) as a function of average electric field (F = V/d) for ITO/α-NPD,
V2O5/α-NPD and Ni2O3/α-NPD plotted as ln(J/F2 ) vs. F
1/2. The organic layer thickness (d) in all
cases is ~ 500 nm. The linear fits are used to extract the apparent mobility from the IV
characteristics using Eq. (4.3). However, since a good fit is achieved for all three anodes, despite the
significant difference in injection properties (see Fig. 4.1), the goodness of fit cannot be used to
distinguish an Ohmic contact. What is more, the extracted mobility values are significantly different
for each anode, and deviates significantly from the value measured by time of flight (TOF). This
serves as an example that transport parameters cannot be extracted from IV characteristics, without
verifying that a true Ohmic contact has been made using another technique. ................................... 37
Figure 4.3 Current density (J) as a function of average electric field (F = V/d) for Ni2O3/α-NPD/Ag with
different organic layer thicknesses (d) of α-NPD, of which (a) is before the subtraction of built-in
potential (Vbi) and (b) is after subtraction of an estimated ~ 0.9 eV built-in potential. The inset of (a)
is the current density as a function of voltage in the case of (a). Ag was chosen as cathode so as to
increase the built-in potential. ............................................................................................................ 39
Figure 4.4 Current density (J) as a function of voltage (V) for different injection barrier heights ( Bp ).
The solid symbols correspond to the time-domain simulation results. The solid line is the calculated
SCLC by Eq. (4.3) using the field dependent mobility measured by the TOF technique and reported
previously in Ref. 79. The dashed line is the injection limited current (ILC) calculated using Eq.
viii
(4.4). The current density at 10 V (i.e., F = V/d = 0.2 MV/cm) for group A oxides, group B oxides
and ITO (see Fig. 4.1) is also shown as solid star symbols for comparison. ..................................... 41
Figure 4.5 The calculated electric field at the charger-injecting contact as a function of barrier height
( Bp ) for α-NPD with an organic layer thickness (d) of 100 nm and 1000 nm respectively. The
average electric field (F = V/d) for each case is 0.5 MV/cm. Notice that the electric field at the
interface converges to the average value for increasing barrier height, and tends to zero for
decreasing barrier height; the region in between defines a quasi-Ohmic contact. ............................. 43
Figure 4.6 The calculated injection “phase” diagram for α-NPD indicating the boundaries of the
quasi-Ohmic regime (i.e., the criterion for ILC and SCLC) as a function of the injection barrier
height ( ) and the organic layer thickness (d). The first region (left side) defines the criterion for
an Ohmic contact (SCLC), the second region (middle) is for a quasi-Ohmic contact and the third
region (right side) is for an injection limited contact (ILC). The boundaries for these regimes are
dependent on applied bias. The solid symbols correspond to an average electric field of 0.5 MV/cm
while the open symbols to an electric field of 0.1 MV/cm. The results for the group A and group B
oxides as well as ITO are also shown for comparison and are obtained from the experimental results
shown in Fig. 4.1................................................................................................................................ 44
Figure 5.1 Efficiency and (b) IV characteristics of the OLED devices with the following structures: (I)
α-NPD/Alq3 (standard reference); (II) α-NPD/CBP (3nm)/Alq3 and (III) α-NPD/TPBi (3nm)/Alq3.
........................................................................................................................................................... 49
Figure 5.2 Normalized EL spectra of the OLED devices with different thickness (0, 3, 10 nm) interlayer
of CBP and TPBi................................................................................................................................ 50
Figure 5.3 He Iα (hν = 21.22 eV) valence band spectra for: (a) CuPc/α-NPD/Alq3; (b) α-NPD/CBP/Alq3;
and (c) α-NPD/TPBi/Alq3. In (b) and (c) the CuPc/α-NPD interface is not shown for clarity since it
is identical to (a). ............................................................................................................................... 51
Figure 5.4 Schematic energy diagram for the device structure: (a) CuPc/α-NPD/Alq3; (b) CuPc / α-NPD /
CBP / Alq3; and (c) CuPc/α-NPD/TPBi/Alq3. The LUMOs are estimated from cyclic voltammetry
measurements.92
................................................................................................................................. 53
Figure 5.5 Device structure of (a) standard reference device, and (b) device with non-blocking exciton
formation zone. .................................................................................................................................. 54
Bp
ix
Figure 5.6 (a) Luminance-Voltage and Current-Voltage characteristics and (b) efficiency of the OLED
devices with the following structures: ITO/CuPc/α-NPD (square); ITO/WO3/CBP (circle); ITO/CBP
(triangle). ........................................................................................................................................... 55
Figure 5.7 He Iα (hν = 21.22 eV) valence band spectra of ITO and ITO/WO3 with a 3 nm thick layer of
CBP showing (a) the secondary electron cut-off, (b) the valence band, and (c) the HOMO of CBP. 57
Figure 5.8 (a) Current efficiency and (b) power efficiency of the OLED devices with the following
structures: ITO/MoO3, V2O5, WO3 (1 nm)/CBP(50 nm); ITO/MoO3, V2O5, WO3 (1 nm) /α-NPD(50
nm) and ITO/CuPc (25 nm)/α-NPD(45 nm). ..................................................................................... 58
Figure 5.9 Electroluminescence (EL) spectra of the devices shown in Fig. 5.9. ....................................... 59
Figure 5.10 Schematic energy level diagram for the device structure: (a) CuPc/α-NPD/Alq3; (b)
CBP/Alq3. The energy offsets were obtained from UPS measurements. ........................................... 59
Figure 5.11 Schematic device structures and energy-level diagrams of the devices in this study. The
HOMO and LUMO levels are obtained from Ref. 93,98,110,111. .................................................... 61
Figure 5.12 (a) IV and LV characteristics of device A as well as its (b) EQE and power efficiency as a
function of luminance. The upper inset is the molecular structure of the emitter Ir(ppy)2(acac). The
lower inset is the corresponding EL spectra measured at various current densities........................... 63
Figure 5.13 (a) IV and LV characteristics of device A, B and C as well as (b) the corresponding EL
spectra measured at 5 mA/cm2. .......................................................................................................... 64
Figure 5.14 Current efficiency of device A, B and C. The insets are the enlarged EL spectra (by 30 times
in the range of 400-490 nm) that are measured at 5 and 50 mA/cm2. ................................................ 66
Figure 6.1 Schematic diagram of the dipole plane with vertical and horizontal dipoles. .......................... 70
Figure 6.2 Schematic diagram of a multilayer structure with n+1 layers. ................................................. 72
Figure 6.3 Schematic diagram of a multilayer structure with embedded source plane. ............................. 75
Figure 6.4 Experimental EL spectra of OLEDs with different thickness of CBP measured normal to the
substrate (open symbols) as well as the corresponding theoretical calculations (solid lines). The PL
spectrum of C545T doped Alq3 used in the calculation is also shown for comparison as dashed line.
........................................................................................................................................................... 78
x
Figure 6.5 Schematic OLED device structure with flexible plastic substrate. ........................................... 80
Figure 6.6 Calculated enhancement ratio of the Ta2O5/Au/MoO3 electrode relative to ITO as a function of
the thickness of both Au and Ta2O5. .................................................................................................. 82
Figure 6.7 (a) External quantum efficiency (EQE) and (b) Power efficiency (PE) of the device structure
optimized for Ir(ppy)2 (acac) as a function of luminance. ................................................................. 83
Figure 6.8 Current density as a function of average electric field of CBP single carrier hole only device
using Au/MoO3, ITO/MoO3 and Au anodes. The anode modified by MoO3 enables good hole
injection into CBP. The inset is the same data for Au/MoO3 and ITO/MoO3 plotted on a log-linear
scale. Clearly, the injection from Au/MoO3 is better than from ITO/MoO3. ..................................... 84
Figure 6.9 Photograph of a flexible OLED (50 mm × 50 mm) at high luminance. ................................... 85
xi
Abbreviations and symbols
OLED Organic light emitting diodes
ITO Indium tin oxide
IV Current-voltage
LV Luminance-voltage
CV Capacitance-voltage
HTL Hole transport layer
ETL Electron transport layer
EML Emissive layer
HOMO Highest occupied molecular orbital
LUMO Lowest unoccupied molecular orbital
VL Vacuum level
QCM Quartz crystal microbalance
UHV Ultra-high vacuum
UV Ultraviolet
ILC Injection limited current
SCLC Space charge limited current
TOF Time of flight
EL Electroluminescence
PL Photoluminescence
CNL
Charge neutrality level
S Interface slope parameter.
EF Fermi energy level
m and S Work function of metal and semiconductor respectively
E Energy level
EC Conduction band level
EV Valence band level
Δ Interface dipole
Electron affinity
xii
Bp and Be Injection barrier for holes and electrons
J Current density
V Voltage
d Thickness of the film
F Electric field
Mobility
0 Permeability of free space
Dielectric constant
e Electron charge
f Reduced electric field
A function of the reduced electric field
T Temperature
Bk Boltzmann constant
0N Density of chargeable sites in the organic film
p Total density of holes
P Normalized power density for dipoles
( )P Photopic response
E Optical electric field
n̂ Complex refractive index
L Layer matrix (Chapter 6)
ijI Interface matrix at i/j interface
M Total system transfer matrix
t Transmission
r Reflection
Wavelength
A Dipole source term
I Irradiance (the power per unit projected area)
Radiant power
xiii
S Projected area
Solid angle
Angle
vI Luminous intensity
Chapter 1 Introduction
1
Chapter 1 Introduction
In this chapter, a brief review on the major milestones in OLEDs developments will be
presented rather than a comprehensive review of the organic electronics technology. The
motivation of this thesis will also be discussed at the end.
1.1. Brief review on OLED developments
Organic light emitting diodes (OLEDs) have attracted considerable research interest due to
their potential to be used in next generation flat panel displays and low-cost solid state lighting.
The first electroluminescence (EL) in organic materials was observed by Helfrich and Schneider
from anthracene in National Research Council, Canada in 1965.1 However no practical
application of this technology was seen to be possible due to its extremely high operation
voltage (~ 100 V). In 1987, the first efficient low-voltage organic EL was demonstrated by Tang
et al2 in Kodak® using an organic heterostructure, i.e., an organic light emitting diode, which is
generally recognized as the most significant step towards the practical applications of OLEDs
technology (see Fig. 1.1).
Figure 1.1 Schematic structure of the first efficient OLED as well as the molecular structures. Adapted from
Ref. 2.
Chapter 1 Introduction
2
HOMO
LUMO
Metal
Cathode
Anode
(ITO)Hole Transport
Layer (HTL)
Electron Transport
Layer (ETL)
Emission layer (EL)
hv
1
2
1 2
3
3
4
Figure 1.2 Schematic energy diagram of an OLED with an organic heterostructure.
Such a “heterostructure” has become a standard practice in OLED design. The injected
electrons and holes from the cathode and anode respectively are accumulated at the
heterojunction. The probability of electron-hole recombination at this organic/organic interface
will be increased, which result in the reasonably high efficiency that was achieved. This
breakthrough also attracted lots of chemists and engineers to focus on the further development
of high performance OLEDs. In the last two decades, significant effort has been devoted to
maximizing device efficiency through the design and synthesis of new materials3-7
as well as
device engineering.8-10
For example, in 1989, Tang et al further proposed a guest-host system,11
in which charge transport and luminescence were separated into two different materials, i.e. host
and guest respectively. By using an emitter with high efficiency in photoluminescence (PL), the
internal quantum efficiency of the OLED was tremendously increased. This work was also well
recognized as another milestone in the OLED development.
In terms of electrode design, Mg was used as the cathode in early device design. However,
Mg is very sensitive to moisture and oxygen and hence can be easily oxidized, resulting in a
very short life time. Another breakthrough in OLEDs was achieved by Tohoku Pioneer® using
a Li compound (e.g. Li2O) as an electron injection layer to enable a more stable metal, Al, as the
cathode. At present, LiF/Al bi-layer cathode is the most commonly used cathode to inject
Chapter 1 Introduction
3
electrons into the devices.12,13
Indium tin oxide (ITO) is the de facto standard anode due to its
high conductivity and transparency. In fact, almost all of the device designs and optimizations
were based upon ITO. For example, different hole injection layers were proposed to inject
sufficient holes from the ITO to enhance the device performance.
Another major breakthrough in OLEDs was the electrophosphorescent device accomplished
by Forrest et al.,3 which significantly increased the internal quantum efficiency as compared to
the fluorescent devices. It is believed that by using a phosphorescent dopant in the guest-host
system, a nearly 100 % internal quantum efficiency can be achieved.3,14
Figure 1.3 shows a
schematic energy diagram of single (fluorescent) and triplet (phosphorescent) emission. Excited
states can be formed by both optical (photoluminescence) and electrical (electroluminescence)
excitation. In the electrical excitation, if singlets (excited states with total angular momentum
equal to zero in a two-particle system) and triplets (excited states with total angular momentum
equal to 1) are formed with equal probability, 25% of the excitons would be singlets and 75%
would be triplets. Therefore, the quantum efficiency of fluorescence (transition from single
states to ground states) has an upper limit of 25%. Usually, according to quantum mechanics,
the radiative relaxation from triplet states to ground states is generally forbidden (i.e. happens in
a very slow time scale). However, it has been shown that with the introduction of a
phosphorescent emitter in a host-guest system,3 the transition from the triplet states to ground
states become much faster than the non-radiative relaxation. Moreover, through the inter-system
crossing, the 25% excitons formed as single states can also be utilized (see Fig. 1.3) and
therefore it is theoretically possible to achieve a 100% internal quantum efficiency in
phosphorescent OLEDs.
S1
T1
S0
Inter-system
crossing
Fluorescent
(25%) Phosphorescence
Figure 1.3 Schematic energy diagram of single (fluorescent) and triplet (phosphorescent) emission.
Chapter 1 Introduction
4
Universal Display Corporation (UDC) is a company dedicated to the development of
phosphorescent OLEDs. Table 1.1 summarizes their most recent phosphorescent device
performance, which can also be considered as the state of the art in device performance reported
from industry.
Table 1-1 Phosphorescent OLEDs performance from UDC (adapted from Ref. 15)
Performance (at 1000 cd/m2)
1931 CIE color Coordinates
Luminous Efficiency (cd/A)
Operating Lifetime (Hrs, LT 95%)
DEEP RED (0.69, 0.31) 17 14,000
YELLOW (0.44, 0.54) 81 85,000
GREEN (0.31, 0.63) 85 18,000
LIGHT BLUE (0.18, 0.42) 50 700
Figure 1.4 Photograph of (left) the Samsung Galaxy S cell phone using 4 inch OLED display and (right) the
LG 55 inch OLED television prototype.
Chapter 1 Introduction
5
In terms of application, the first commercialization was realized by Pioneer® in 1997 -
producing the first generation OLED display. About 10 years later in 2008, SONY® launched
the first commercial OLED TV (11 inch). However, the retail price of this product was still too
high. Currently, the OLED display market is dominated by Samsung. More than 67 million 4”
OLED displays were sold by Samsung in 2010 according to DisplaySearch.16
Most recently, LG
Display launched the word’s first 55 inch OLED TV at Consumer Electronics Show 2012,
which may set the future direction of the OLED display market.
Another major potential application of OLED is lighting. In a recent issue of Nature17
,
editor Dr. Stefano Tonzani wrote an editorial commentary titled “Lighting technology: Time to
change the bulb”. As was identified by Tonzani, OLEDs would be the ultimate in lighting
technology. When mature, OLED lighting will offer: energy savings (less energy will be
converted into heat); non-toxicity (mercury free); adaptability to any location due to thinness
and flexibility; cost savings requiring no large semiconductor fab for epitaxy; no filaments to
break; complete controllability in color temperature; instant start; and operability in low
temperatures, etc..
The requirements for lighting are different from display applications as lighting mainly
focuses on efficiency at high luminance. For example, a luminance of > 5×103
cd/m2 for
lighting is generally needed as compared with ~ 102
cd/m2 for most display applications such as
liquid crystal display (LCD). The goal set by the U.S. Department of Energy (DOE) for OLED
lighting in 2015 is 125 lm/W with an illuminance of 10,000 lm/m2.18
To achieve this goal, a low
driving voltage and high power efficiency device is needed. The p-i-n structure, i.e. a structure
with p-type and n-type doped hole transport layer and electron transport layer respectively to
increase the carrier density, that was proposed by Karl Leo19
greatly reduced the driving voltage
of OLEDs, resulting in a much increased power efficiency. Recently, OLEDs with efficiencies
exceeding that of standard incandescent and fluorescent lighting have been demonstrated in the
p-i-n design.20
However, usually the p-i-n structure has a challenge in the lifetime, e.g. a lot of
the materials for the p-type and n-type doping are not thermally stable. However, for lighting a
thermally stable device is needed since considerable heat will be created at high luminance.
Chapter 1 Introduction
6
1.2. Motivations
Despite recent advances, the device physics behind these breakthroughs is still not yet clear.
A lot of the improvements are in fact based on a trial and error method which is time
consuming and cost intensive. Our ultimate goal is device design and engineering, which
requires a solid understanding of device physics, such as the energy level alignment and charge
carrier injection at electrode-organic interfaces. Most organic semiconductors contain almost no
intrinsic charge carrier due to their weak intermolecular coupling. Therefore proper interface
design will help us to increase the injection of carrier charge from the electrode which will result
in enhanced device performance. Another example is exciton formation. Nearly all existing
device designs are still based on the original OLED structure introduced by Tang et al.2; the
conventional bi-layer heterojunction design of the exciton formation zone has remained
relatively unchanged. There has therefore been recent interest in improving the conventional
design of the exciton formation zone, as any improvement has the potential to improve the
device performance of nearly all OLEDs. Moreover, the efficiency of the entire device depends
not only on the internal quantum efficiency, but also on how much light can be extracted out of
the substrate (external quantum efficiency). It has been shown that by using a high index glass
to match the refractive index of organic materials, more than two times amount of light can be
extracted.21
Therefore, by tuning the optical properties of the materials to reduce the total
reflection in the stratified structure can boost the efficiency significantly.
The overall object of this thesis is to advance the state of the art in high performance OLED
devices. This work focuses on both the electrical and optical design of OLEDs. In terms of the
electrical design, energy level alignment and charge injection at metal/organic interfaces was
systematically studied. High work function anodes were developed to inject sufficient amount of
charge carriers into the OLEDs. Studies on how to extract charge carrier mobility, one of the
most important parameters for organic materials, were accompanied with the study of charge
injection. The focus was on the design of simple device structure that was enabled by the high
work function anodes as well as the design of emission zone to reduce the exciton quenching at
organic/organic interfaces. In terms of the optical design, the focus was on understanding the
Chapter 1 Introduction
7
optical electric field distribution across the OLED device so as to reduce the light trapping in
different modes, e.g. substrate mode and glass mode.
1.3. Outline
This thesis is organized as follows: Chapter 2 is about the experimental methods. Chapter 3
discusses the energy level alignment at metal/organic interfaces. Chapter 4 details the analysis
of charge injection characteristics at electrode-organic interfaces. Chapter 5 presents
enhancements in OLED performance by redesigning the exciton formation zone. Chapter 6
discusses the development of an optical model based on dipole emission. Chapter 7 is the
summary of the thesis as well as the future work.
Chapter 2 Experimental methods
8
Chapter 2 Experimental methods
This chapter gives an overview of the experimental designs and fabrication processes in this
thesis which includes the designs of single carrier devices and OLED devices as well as the
device characterization.
2.1. Device designs
2.1.1. Single carrier devices
Experimentally, single carrier devices with a structure of Anode / Organic / Cathode are
commonly used to study both injection and transport properties. By blocking one type of
carriers (holes or electrons) from one side (anode or cathode) with a much larger injection
barrier, the other type of carrier is dominant and a single carrier device is achieved. Figure 2.1 is
the energy diagram for a hole only single carrier device, i.e., holes are the dominant carriers (at
least 2-3 orders of magnitude higher than electrons) and electrons are blocked from the cathode
due to its high injection barrier. The corresponding schematic device structure is shown in Fig.
2.2. To reduce the impact of the built-in potential on the current-voltage (IV) characteristics, a
relatively thick organic layer is needed (~500 nm).22
FmE
LU M O
VL
VL
Metal Organic Metal
Barrier too high to
overcome
H O M O
Figure 2.1 Energy diagram of a single carrier hole-only device with a structure of metal/organic/metal.
Chapter 2 Experimental methods
9
Glass
Anode
Organic layer
(~ 500 nm)
Cathode
Figure 2.2 Schematic device structure of a single carrier device.
Substrate
Anode
HTL
EL
ETL
Cathode
Figure 2.3 Schematic device structure of a typical multi-layer OLED.
Chapter 2 Experimental methods
10
2.1.2. OLED devices
OLED is the most important device in this study as the whole purpose here is to develop an
OLED structure of extremely high performance. Figure 2.3 shows the typical multi-layer OLED
structure, which consists of hole transport layer (HTL), electron transport layer (ETL) and
emissive layer (EML). Sometimes, some devices also require buffer layers to facilitate the
injection of holes or electrons and they are called injection layers. In this study, the most
commonly used standard singlet OLED structure (from Pioneer®) for reference is: ITO/ copper
phthalocyanine (CuPc) (25 nm)/ N,N'-diphenyl-N,N'-bis-(1-naphthyl)-1-1'-biphenyl
-4,4'-diamine (α-NPD) (45 nm)/ tris(8-hydroxy-quinolinato)aluminum (Alq3) (45 nm)/LiF (1
nm)/Al (100 nm).
2.2. Device fabrication
Sample preparation consists of substrate treatments and thin film deposition on substrates.
In this study, mainly two types of substrate were used: Corning 1737 glass substrates and
commercially patterned ITO coated glass with a sheet resistance less than 15 / (see Fig.
2.4). Substrates were ultrasonically cleaned with a standard regiment of Alconox®, acetone, and
methanol followed by Ultraviolet (UV) ozone treatment for 15 minutes. The size of the substrate
is 50 mm × 50 mm, which can fit as many as 32 devices (see Fig. 2.4). By using different
masks, up to eight different structures (4 devices each to eliminate possible run-to-run
variability) can be fabricated on one substrate. A schematic structure of a device is shown in
Fig. 2.5. Pixels are defined by the intersection of the top and bottom electrode. The active area
for all devices was 2 mm2.
All devices were fabricated in a Kurt J. Lesker LUMINOS® cluster tool (Fig. 2.6). Metal
electrodes were also used in the study with the same pattern as shown in Fig. 2.4 (as bottom
electrode), and were defined using a stainless shadow mask. Ni, Cu, Co, Ag and Au were
thermally deposited in a metallization chamber with a base pressure of ~ 10-8
Torr from alumina
coated molybdenum boats. Al was also deposited in the same metallization chamber from a
pyrolytic BN crucible. Mg was deposited in another dedicated magnesium deposition chamber
with a base pressure of ~ 10-8
Torr from a BN crucible in a high temperature Knudsen cell.
Chapter 2 Experimental methods
11
Figure 2.4 Layout of patterned ITO used in this study.
Figure 2.5 Schematic structure of the devices (single carrier devices or OLED).
Chapter 2 Experimental methods
12
Figure 2.6 Picture of Kurt J. Lesker LUMINOS® cluster tool.
Different oxides were deposited on the substrate as surface modification layer (~ 1 nm) on
top of different anode. Tungsten trioxide (WO3) and molybdenum trioxide (MoO3) were
thermally evaporated from tungsten boats and alumina coated molybdenum boats respectively.
Vanadium pentoxide (V2O5) was deposited using radio frequency (rf) magnetron sputtering in a
dedicated sputtering chamber with an rf power of 150 W in 1 mTorr Ar.
It is noted that different surface treatments were used for different oxides. WO3, MoO3 and
V2O5 were UV ozone treated for 30 minutes after the deposition from the respective oxide
powder on the substrates (ITO for example). In contrast, the NiOx, CoOx, and CuO films were
fabricated from pure Ni, Co and Cu films respectively by ex situ oxidation using UV ozone for
30 minutes. For the NiOx, CoOx, and CuO, similar results were obtained from in situ oxidized
films using O2 plasma (without breaking vacuum), which indicates that any possible
atmospheric contaminants play a negligible role in device performance.
Chapter 2 Experimental methods
13
Various organic molecules (see table 1.1) were deposited from alumina crucibles in an
organic chamber with a base pressure of ~ 10-8
Torr. The entire procedure was finished without
breaking the vacuum, i.e., the sample was transferred from one chamber to another through the
central distribution chamber with a base pressure of ~ 10-9
Torr. Film thicknesses were
monitored using a calibrated quartz crystal microbalance (QCM). The error of the QCM can be
up to 10% due to the variance over its life time. Since the thickness of the organic layers is
critical in the studies, it was further verified (for each device) using both a stylus profilometer
(KLA Tencor P-16+) and capacitance-voltage (CV) measurements (Agilent 4294A).
Table 2-1 Molecular structures of the organic molecules used in this study
Chemical name and abbreviation Formula Molecular
Weight Chemical structure
4,4’,4’’-tris(N-3-methylphenyl-N-p
henyl-amino) triphenylamine
(m-MTDATA)
C57H48N4 789.02
gm/mol
N,N’-diphenyl-N,N’-bis-(1-naphth
yl)-1-1’-biphenyl-4,4’-
Diamine
(α-NPD)
C44H32N2
588.74
gm/mol
Chapter 2 Experimental methods
14
4,4',4"
-Tris(N-(2-naphthyl)-N-phenyl-am
ino)triphenylamine
(2T-NATA)
C66H48N4 789.02
g/mole
4,4'-Bis(carbazol-9-yl)biphenyl
(CBP)
C54H36N4 740.89
g/mole
Tris(8-hydroxy-quinolinato)alumin
ium
(Alq3)
C27H18AlN3O
3
459.43
g/mole
N
O
N
O
N
O
Al
Phthalocyanine
(CuPc)
C32H16N8Cu 576.07
g/mole
Chapter 2 Experimental methods
15
2,2',2"
-(1,3,5-Benzinetriyl)-tris(1-phenyl-
1-H-benzimidazole)
(TPBi)
C45H30N6 654.76
g/mole
Tris(2,4,6-trimethyl-3-(pyridin-3-y
l)phenyl)borane
(3TPYMB)
C42H42N3B 599.61
g/mole
Tris(2-phenylpyridine)iridium(III)
[Ir(ppy)3]
C33H24IrN3 654.78
g/mole
Bis(2-phenylpyridine)(acetylaceton
ate)iridium(III)
[Ir(ppy)2(acac)]
C27H23IrN2O2 599.70
g/mole
N
Ir
2
O
O
Chapter 2 Experimental methods
16
2,3,6,7-Tetrahydro-1,1,7,7,-tetrame
thyl-1H,
5H,11H-10-(2-benzothiazolyl)quin
olizino-[9,9a,1gh]coumarin
(C545T)
C26H26N2O2S 430.56
g/mole
Fullerene
(C60)
C60 720.64
g/mole
2.3. Device characterization
IV and luminance-voltage (LV) characteristics for OLED were measured using an HP4140B
pA meter and a Minolta LS-110 Luminance meter respectively in ambient air. The luminous
flux for calculating the EQE and power efficiency were measured using an integrating sphere
with a silicon photodiode with NIST traceable calibration.23
Error analysis of the IV and LV
measurement can be found in Apendix A. EL spectra were measured by a USB4000 miniature
fiber optic spectrometer, which couples a linear CCD-array detector ranging from 350 nm from
1100 nm. For single carrier devices, the IV characteristics were measured in a home-made
closed-loop He3 cryostat (Fig. 2.7) with a base pressure of ~ 10-6
Torr at room temperature. The
operating pressure of the cryostat was significantly less at low temperature (i.e. ~ 10-8
Torr) due
to the cryo-pumping effect of the cryostat cold finger (i.e. residual gases condense onto the cold
finger). The glass substrates were mounted on the Cu block cold finger of the cryostat using
Apiezon N cryogenic grease for good thermal contact. The temperature of the substrates was
monitored using a calibrated chromel-alumel (Type K) thermocouple bonded directly to the top
Chapter 2 Experimental methods
17
of the substrate using the same cryogenic grease. The temperature was stable to within 0.1 K
over all temperature ranges.
Figure 2.7 Home-made closed-loop He3 low-temperature cryostat.
Ultraviolet photoelectron spectroscopy(UPS) characterization was performed using a PHI
5500 Multi-Technique system attached to a Kurt J. Lesker multi-access chamber ultra high
vacuum (UHV) cluster tool (base pressure of ~ 10-10
Torr). Organic molecules were deposited in
a dedicated organic chamber from a homemade transfer-arm evaporator cell (TAE-cell).24
The
spectrometer (hemispherical analyzer) was calibrated using monochromatic Al Kα (hν = 1486.7
eV) as per ISO 15472.25
Chapter 3 Energy level alignment at metal/organic interfaces
18
Chapter 3 Energy level alignment at metal/organic interfaces
In this chapter, energy level alignment at metal/organic interfaces will be discussed. The
interface dipole theory will be used to describe the energy level alignment at metal/organic
interfaces. As an example, the metal/ C60/organic interfaces will be shown to illustrate the Fermi
level pinning at the metal/organic interfaces. The content of this chapter was published as Appl.
Phys. Lett. 95, 043302 (2009).
3.1. Introduction
Figure 3.1 (a) shows an idealized energy diagram for a particular metal and intrinsic
inorganic semiconductor before making contact. The vacuum level (VL) is used as a reference
level (zero energy level). Above the VL the electron can escape the solid. The difference
between VL and Fermi level EF, is called the work function. Here m and S denote the work
functions of the metal and the semiconductor respectively. The conduction band minimum EC is
a distance χ below the VL, where χ is the electron affinity. The difference between the valence
band level EV and EC is the band gap of the semiconductor. For an intrinsic semiconductor the
Fermi level is located approximately at the middle of the band gap. Similar to EV and EC, in an
organic semiconductor (see Fig. 3.1 b) there is the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO). The major difference is that the states are
highly localized in an organic semiconductor while they are continuous in an inorganic
semiconductor. The Gaussian distribution model is commonly used to describe the energy
disorder in organic semiconductors.26
When the metal and organic form a contact, before any possible charge transfer takes place,
the energy levels are still aligned at the vacuum level. After the charge transfer takes place, to
balance the charge transfer, an interfacial dipole is formed within the first few mono-layers (see
Fig. 3.1 c). This is different from the traditional inorganic semiconductor since there is almost
no intrinsic charge carrier in the organic semiconductor, thus a depletion zone near the junction
is not expected. The formation of the interfacial dipole will result in a change of injection
Chapter 3 Energy level alignment at metal/organic interfaces
19
barrier, i.e. the injection barrier would be different from the difference between the work
function of metal and the LUMO (e.g. for the injection of electron) of organic.
Metal Semiconductors
VLVL
m
Interface
s
CE
VE
FE
FE
Metal Organic
Interface
FE
FE
LUMO
HOMO
(a)
(b)
gE
Interfacial
dipole
(c)
be
VL
FE
FE
Figure 3.1 Energy diagrams of (a) metal and inorganic semiconductors and (b) metal and organic
semiconductors before making contact.
Unlike the traditional inorganic semiconductor, a lot of properties of organic semiconductor
are still unknown and are needed to be investigated. For example, since there are no dopants in
many applications of organic semiconductors, the Fermi level is not easily determined. The
argument that how the Fermi level aligns is still under debate.27
Mönch28
proposed that interface
dipole theory,29
originally developed to describe the interfacial dipole at Schottky contacts,
Chapter 3 Energy level alignment at metal/organic interfaces
20
could also be applied to metal/organic interfaces. From interface dipole theory, the dipole at a
metal/organic interface is given by,29
1 m CNLS , (3.1)
where CNL is the charge neutrality level of the organic and S is the interface slope
parameter. S is commonly used to describe the degree of Fermi level ( FE ) pinning at a
metal/semiconductor contact30
,
F mS dE d . (3.2)
For vacuum level alignment S = 1 (Schottky-Mott limit) and for Fermi level pinning S = 0.
Here, metal/C60 is taken as an example to study the energy level alignment. Despite the
broad applications of C60, i.e. OLEDs,31-34
organic photovoltaics (OPVs),35,36
organic thin film
transistors (OTFTs)37
and organic memory devices (OMDs),38
C60 still remains poorly
understood in devices physics. Recently, there have been several reports of C60 as efficient hole
injection layer at the anode in OLEDs (e.g., Au/C60).31,32,34,39
Considering the fact that C60 is an
electron transporting molecule with a deep HOMO, it seems unlikely that C60 can enhance hole
injection, rather it should serve as hole blocking layer. In the following subsection, the
energy-level alignment and charge injection properties at metal/C60 interfaces will be studied
using UPS as well as IV measurements of metal/C60/organic/metal single carrier devices.
3.2. Results and discussion
In order to elucidate the unusual charge injection properties at metal/C60/organic interfaces,
single carrier devices with Au and Ag anodes and m-MTDATA, 2T-NATA and α-NPD organic
layers were fabricated. The device structure is: anode/C60(0, 3 nm)/organic (500 nm)
/Au(cathode). Figure 3.2 shows the room temperature (297 K) IV characteristics for the case of
Au. For all three organics the C60 interlayer dramatically improves hole injection by nearly 3
orders of magnitude (reduced injection barrier height). Similar improvements in the IV
Chapter 3 Energy level alignment at metal/organic interfaces
21
characteristics were also found for Ag (which is not shown here). The seemingly universal
improvement in the IV characteristics is extremely surprising, given that the HOMO of C60 (6.3
eV) is much deeper than that of m-MTDATA (5.1 eV), 2T-NATA (5.1 eV) or α-NPD (5.4 eV).
Indeed, the injection barrier height from Au and Ag to C60 is expected to be ~ 1 eV and ~ 2 eV
respectively (HOMO to Fermi level offset).
0.01 0.110
-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Au/C60
/m-MTDATA
Au/m-MTDATA
Au/C60
/-NPD
Au/-NPD
Au/C60
/2T-NATA
Au/2T-NATA
Cu
rren
t D
ensity (
A/c
m2)
Electric Field (MV/cm)
0.5
Figure 3.2 Current density with Au and Au/C60 anodes at room temperature as a function of electric field for
single carrier devices. The average electric field (F) is taken as F = V/d, where V is the applied voltage and d
is the device thickness.
Since the C60 interlayer is very thin (~ 3 nm) it can be treated as a surface modification
layer on the metal surface. This assumption is reasonable given that C60 monolayers tend to be
metalized (i.e., take on metallic character) by the interaction with clean metal surfaces.40-43
It is
also well known44
that molecular adsorbates can significantly change the work function of a
clean metal surface. It is therefore likely that the C60 interlayer reduces the hole injection barrier
height by modifying the work function of the underlying metal, resulting in a more favorable
energy-level alignment for hole injection. Therefore photoemission measurements were
conducted to confirm this hypothesis. Figure 3.3 (a) shows the He Iα (hν = 21.22 eV) secondary
electron cut-off spectra for sputter cleaned Au and Ag with and without a 3 nm thick layer of
C60 (in situ deposited).
Chapter 3 Energy level alignment at metal/organic interfaces
22
The extracted work function values from these spectra are summarized in Fig. 3.4 and are in
excellent agreement with other UPS and Kelvin probe measurements reported in literature
(shown as open points in Fig. 3.4). Surprisingly, the work function of both metals is found to
converge to ~ 4.7 eV, which indicates that there is a strong interfacial dipole ( ) between the
metals and C60 that acts to pin the Fermi level. The effective work function ( ,m eff ) of the C60
modified metals is then determined by the difference between the pristine metal work function
m( )and , as shown in the inset of Fig. 3.4. From Eq. (3.1) we can deduce that the Fermi
level is thus pinned to the charge neutrality level ( ) of C60, namely 4.7 eV.
17 16 15
17 16 15 2 1 0 -1
(b)
60Ag/C
60Au/C
Ag
Au
Ag/C60
Au/C60
No
rma
lize
d I
nte
nsity (
a.u
.)
(a)
60Au/C
Au/α-NPD
60Au/C /α-NPD
Au
Au/-NPD
Au/C60
/-NPD
Binding Energy (eV)
Bp
Bp
EF
HOMO
Figure 3.3 He Iα (hν = 21.22 eV) spectra of (a) secondary electron cut-off (SEC) of sputter cleaned Au and Ag
with and without 3 nm of C60, and (b) SEC and valence band of Au, Au/α-NPD (3 nm) and Au/C60 (3
nm)/α-NPD (3 nm); SEC for Au/C60 (dashed line) is also shown for reference. Interfacial dipoles ( ) and
hole injection barrier heights ( Bp ) are as indicated.
CNL
Chapter 3 Energy level alignment at metal/organic interfaces
23
However, if C60 pins the Fermi level near 4.7 eV, then Au, with a higher work function of ~
5.1 eV, should still perform better than Au/C60 as anode. From Fig. 3.2 this is clearly not the
case for any of the Au/C60/organic interfaces. This discrepancy is due to the interfacial dipole at
the metal/organic interface (e.g., Au/α-NPD) that drastically reduces the effective work function
of Au, same as for the Au/C60 interface. In the case of m-MTDATA, 2T-NATA and α-NPD the
charge neutrality level is significantly lower than that of C60 resulting in a lower net value of
,45,46
and hence a higher injection barrier height.
4.0 4.5 5.0 5.53.0
3.5
4.0
4.5
5.0
5.5
Metal C60
Vacuum Level
HOMO
,m eff LUMO
Bp
Kelvin Probe (from Ref.)
UPS (from Ref.)
UPS (this work)
metal/C60
/NPD (transport)
metal/C60
/m-MTDATA (transport)
m
,eff
(eV
)
m (eV)
S ~ 0
Metal / C60
Figure 3.4 Effective work function ,m eff as a function of the pristine anode metal work function m . The
data shown with open points is obtained from Ref. 47,48, while the data shown with solid points is from this
work. The inset is the schematic energy level diagram of the band alignment at Metal/C60 interface. The Fermi
level of the metal is pinned by the adsorbed C60 molecules.
To illustrate this point the difference in energy-level alignment between Au/α-NPD and
Au/C60/α-NPD was also measured using UPS. Figure 3.3 (b) shows the He Iα secondary
electron cut-off spectra (left panel) and the He Iα valence band spectra (right panel) for
Au/α-NPD (3 nm) and Au/C60 (3 nm)/α-NPD (3 nm). As indicated in the figure (left panel) the
interfacial dipole ( ) is much larger for Au/α-NPD than for Au/C60/α-NPD resulting in a lower
,m eff
Chapter 3 Energy level alignment at metal/organic interfaces
24
value of . From the valence band spectra (right panel), it is clear that there is also a larger
offset between the HOMO and Fermi level for Au/α-NPD resulting in a much higher injection
barrier height ( Bp 1.3 eV) than for Au/C60/α-NPD (Bp 0.9 eV). This finding is consistent
with the larger interfacial dipole for Au/α-NPD discussed above (i.e. /Au NPD >60/ /Au C NPD ).
Au
Au
/Au Organic
Vacuum Level
HOMO
(a)
LUMO
Organic Au Organic
HOMO
(b)
LUMO
Vacuum Level
Bp
60/
4.6 eV
eff
Metal C
C60
HOMO
LUMO
60/Au C
Bp
d
Ag
Ag
/Ag OrganicVacuum Level
HOMO
(c)
LUMO
Organic Ag Organic
HOMO
(d)
LUMO
Vacuum Level
Bp
C60
HOMO
LUMO
60/Ag C
Bp
d
60/ /Au C organic
60/ /Ag C organic
'
Bp
'
Bp
60/
4.6 eV
eff
Metal C
Figure 3.5 Schematic energy-level diagram of the band alignment at (a) Au/organic, (b) Au/C60/organic, (c)
Ag/organic and (d) Ag/C60/organic interfaces. The symbol of d indicates the thickness of the C60 layer.
,m eff
Chapter 3 Energy level alignment at metal/organic interfaces
25
This finding can also be confirmed by comparing the injection barrier extracted from the IV
characteristics. The details of how to extract injection barrier from different injection models
will be discussed in the next chapter. If the effective work function of C60 modified metal is
calculated using the relation,m eff BpHOMO , for example for Ag/C60(3 nm)/α-NPD, a value
of ~ 4.55 eV can be found. However, this simple calculation is based on assumption that the
dipole between the C60 modified metal and the next organic layer is zero. As shown previously
in Fig. 3.3 (b), there is a small dipole between C60 modified metal and α-NPD. By applying Eq.
(3.1) using the values of and for different molecules reported elsewhere45
the
extracted effective work functions can be further corrected for the interfacial dipole between the
C60 modified metal (e.g., Ag/C60) and the next organic layer (see Fig. 3.5). After applying this
correction the effective work function was found to be ~ 4.7 eV, in excellent agreement with the
UPS results (see the results summarized in Fig. 3.4 for the other cases).
The findings discussed above are summarized in the energy-level diagrams shown in Fig.
3.5 for Au and Ag with and without a surface modification layer of C60. These energy diagrams
summarize three major points, which are the key findings of this work. First, clean metals, such
as Au and Ag, have a strong interfacial dipole with commonly used hole transporting molecules,
resulting in unfavorable energy-level alignment for hole injection. Second, by inserting a thin
interlayer of C60 the net interfacial dipole is reduced due to pinning of the electrode Fermi level
to the charge neutrality level of C60 (i.e., ~ 4.7 eV). Since the C60 interlayer acts as a surface
modification layer, holes are directly injected into the next organic layer, bypassing the HOMO
of C60, i.e. the injection barrier in this case is '
Bp (see Fig. 3.5 b and d). However, when the C60
layer is too thick, i.e. larger than 5 nm, the current density of the metal/C60/organic interface
becomes too small to be measured (beyond the range of the equipment, i.e. near the noise floor
of 10-10
A/cm2). In this case holes must be injected into the C60 layer, resulting in a large
injection barrier height of ~1.7 eV (6.3 eV – 4.6 eV). The C60 thickness of 3 nm is the optimized
value, i.e. it is thick enough to pin the Fermi level, but is thin enough to permit charge tunneling.
Finally, the previous two points do not negate the fact that there is a small dipole between the
C60 modified metal (e.g., Au/C60) and the next organic layer, which will also modify the
injection barrier at the metal/C60(3 nm)/organic interface. The net dipole (60/ /metal C organic ) as
CNL S
Chapter 3 Energy level alignment at metal/organic interfaces
26
shown in Fig. 3.5 (b) and (d) is found to be less than the intrinsic dipole at the metal/organic
interface (without C60).
3.3. Summary
In summary, it was shown that interface dipole theory accurately describes the energy level
alignment at metal/C60/organic interfaces. The Fermi level at metal/C60/organic interfaces was
found to be pinned to the charge neutrality level of C60 (~ 4.7 eV). This phenomena was
attributed to the C60 interlayer disrupting the interfacial dipole at the metal/organic interfaces,
resulting in more favorable energy-level alignment for hole injection. As a result holes were
injected directly into the organic layer, bypassing the deep HOMO of C60.
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
27
Chapter 4 Analysis of charge injection characteristics at
electrode-organic interfaces
In this chapter, the hole injection from different metal oxides into α-NPD will be
systematically studied in single carrier hole-only devices. A criterion that defines Ohmic,
quasi-Ohmic, and injection limited contacts will be quantified based on a time-domain
simulation of charge transport across α-NPD single carrier devices. A barrier-thickness-voltage
“phase” diagram that defined the regions of SCLC, quasi-Ohmic and ILC for α-NPD will also
be presented. The content of this chapter was published as Phys. Rev. B 80, 235325 (2009).
4.1. Introduction
As mentioned previously, organic semiconductors contain almost no intrinsic charge
carriers due to their weak intermolecular coupling. In order to enhance the performance of
OLEDs, one has to increase the extrinsic carrier concentration. Therefore, improving the
injection of charge from the electrodes is of great significance to achieve highly efficient
OLEDs. A commonly used approach to reduce the barrier height for charge injection is to
increase (decrease) the work function of the anode (cathode), towards the ultimate goal of
achieving Ohmic contacts. In the previous chapter, the reason why a thin (3 nm) C60 interlayer
can enable Au as anode in green OLED32
has been shown, where the thin C60 interlayer pins the
work function of the anode to be ~4.7 eV,49
resulting a more favorable injection barrier than
pure metal anode. However, the effective work function of 4.7 eV still limits the selection of
hole transport layer (HTL), i.e. the effective work function is not yet high enough.
Recently, transition metal oxides, such as CuO, WO3, MoO3 and V2O5 have been shown to
be promising candidates to replace the previous generation of organic hole injection layers
(HILs) at the anode, due to their stability, low cost and their high work function.50-53
It is this
high work function (~ 5 - 6 eV) in particular that has attracted the most attention, since it
suggests the possibility of forming an Ohmic contact for holes with many of the commonly used
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
28
hole transport layers (HTLs). However, a high work function does not guarantee an Ohmic
contact, or even good hole injection. For example, as studied in the previous chapter, Au with a
high work function of ~ 5.1 eV will form a strong interfacial dipole at the metal/organic
interface, which significantly increases the injection barrier for holes. Therefore not only do we
have to consider the work function of the oxides, but we also have to take into account the
energy-level alignment, for example, interfacial dipole effects. In order to better understand the
physics that governs the injection process at such interfaces, particularly for the case of an
Ohmic contact, a systematic study of different oxides used in devices is needed. However, few
comparative studies have been conducted to study the injection properties of different oxides in
devices, i.e. single carrier hole-only devices. There may be a large variation in device
performance from study-to-study and different processing methods and the organic materials
used may introduce a large variation. It is therefore difficult to compare these results.
Several transition metal oxides have also been assumed to form an Ohmic contact with
commonly used hole transporting molecules, such as α-NPD54
. With the assumption of Ohmic
contacts, mobility of several organic semiconductors was conveniently extracted by modeling
the IV characteristics using the space charge limited current (SCLC).54,55
As will be shown in the
following text, however, without clear criteria to distinguish between injection limited current
(ILC) and SCLC, the data may easily be misinterpreted. For example the square law, i.e. the
Mott-Gurney law 2
0 3
9
8
VJ
d , has been commonly used to analyze the IV characteristics of
devices and has also been commonly cited as a criterion for SCLC.56-60
However, such a
criterion becomes questionable if the field dependent mobility is unknown, as one can simply
compare the IV characteristics to the quadratic relation. It is also well known that other effects
such as non-uniform emission from a “patchy” interface,61
the distribution of trap states in the
bulk of the organic and the built-in potential62
, will all change the shape of the IV characteristics.
It is therefore extremely difficult to distinguish ILC from SCLC in organic devices from the IV
characteristics alone. Thus, any IV data analysis to extract parameters, such as the energy
disorder and mobility of organics, are dubious. This work will show a comparative study
between the most commonly used transition metal oxides in organic electronic devices. These
diverse experimental results enabled a thorough analysis of the various models relating to the
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
29
injection properties at electrode/organic interfaces. To aid the analysis, time-domain simulations
were used to establish a criterion to distinguish ILC from SCLC for α-NPD. As will be shown
below there is no clear boundary between ILC and SCLC, but rather a large region in between,
which will be referred to as a “quasi-Ohmic” regime for the convenience of discussion. This
intermediate quasi-Ohmic regime requires special consideration in terms of device modeling.
4.2. Theory
4.2.1. Space charge limited current
The maximum current that an organic semiconductor can sustain in the bulk (i.e., the
amount of carriers in thermal equilibrium) is called the SCLC. An Ohmic contact is therefore an
interface capable of injecting enough charges to sustain SCLC. One significant feature of SCLC
is that the spatial distribution of electric field is 1/2( )F x x , where x is the distance from the
charge-injecting contact.63
Therefore the electric field at an Ohmic contact is equal to zero,
which as will be shown is an important criterion for distinguishing SCLC from ILC. Based on
traditional semiconductor device physics, the SCLC for uni-polar transport (i.e., single carrier
devices) in a perfect insulator (no intrinsic carriers) without traps is given by the Mott-Gurney
law,63
2
0 3
9
8
VJ
d , (4.1)
where V is the applied voltage, d is the thickness of the film, and μ is the field-independent
mobility. With a further consideration of an exponential tail of trap states, the IV characteristics
based on Eq. (4.1) follows,63
1
2 1
l
l
VJ
d
, (4.2)
where l is a parameter derived from the trap distribution. However, it is well known that the
mobility for most organic semiconductors is field-dependent. Also, in disordered organic
materials, it is believed that all electronic states are localized and participate in conduction
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
30
through thermally activated hopping, which yields a Poole-Frenkel like field dependence of the
mobility, i.e. 0( ) exp( )F F . Under the assumption of the Poole-Frenkel dependence, an
approximation to the SCLC for a field dependent mobility is given by,64
2
0 0 3
9exp 0.89
8SCLC
V VJ
d d
. (4.3)
Indeed, it has already been demonstrated that the above equation mathematically describes well
the IV characteristics of many organic semiconductors.65-67
4.2.2. Injection limited current
When the current in an organic semiconductor is limited by the injection of charge from the
electrode rather than the bulk properties of the material, it is called ILC. Under ILC conditions
the spatial electric field distribution is assumed to be uniform, i.e. ( ) /F x V d ; whereas for
SCLC the value of V/d only gives the average value of the electric field. This suggests that
SCLC can be distinguished from ILC by the electric field at the charge-injecting contact [i.e.,
( 0) 0F x for SCLC and ( 0) /F x V d for ILC].
The most commonly used injection models for ILC are derived based on
Richardson-Schottky (RS) emission,68
Fowler-Nordheim (FN) tunneling69
and the hopping
model.70
It has been pointed out that RS emission is not strictly correct71
for systems where the
electron mean free path is very short, such as in organic semiconductors. Although the hopping
model includes the discrete (molecular) nature of organic semiconductors, it has also been
pointed out that RS emission provides a solid basis for the analysis of the charge injection
characteristics of organic semiconductors since there is little quantitative difference between RS
emission and the hopping model.72,73
In this work the RS emission based model for ILC
proposed by J. C. Scott will be used. The model considers the scattering and diffusion effect by
solving the drift-diffusion equation in the depletion zone of an amorphous semiconductor,74
2 1/2
04 exp exp( )BILC
B
eJ N e F f
k T
. (4.4)
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
31
In Eq. (4.4), 0N is the density of chargeable sites in the organic film, B is the injection
barrier height, F is the electric field at the charge-injecting contact, is the electric field
dependent (Poole-Frenkel) carrier mobility, Bk is the Boltzmann constant, T is temperature, e
is the electron charge and is a function of the reduced electric field ( 3 2 2/ 4 Bf e F k T ),
2/12/112/11 )21( ffff . (4.5)
4.2.3. In between SCLC and ILC (quasi-Ohmic)
The two conditions discussed above, SCLC and ILC, correspond to the upper and lower
limits respectively. However, what about the case in between SCLC and ILC, when the electric
field at the electrode contact is 0 ( 0) /F x V d ? This indicates that there is in fact no clear
boundary between SCLC and ILC, but rather an intermediate regime in between. In other words,
just because an IV characteristic is not ILC does not guarantee that it is SCLC, and vice versa.
Since the intermediate regime exhibits characteristics of both SCLC and ILC, it is incorrect to
apply the models for either case [e.g., Eq. (4.1) – (4.5)]. This can easily be understood since all
models for SCLC require that ( 0) 0F x at the charge-injecting contact, while all models for
ILC require that ( 0) /F x V d at the charge-injecting contact. Hence, for the region in
between SCLC and ILC [i.e., 0 ( 0) /F x V d ] neither type of model can be applied since
the boundary conditions at the charge-injecting contact is not met. Therefore a new modeling
approach is required in order to properly deal with the case between SCLC and ILC. From now
on we will refer to this intermediate region as “quasi-Ohmic”, i.e. the current in the organic
semiconductor is limited by both the injection at the electrode and by the bulk properties of the
material.
Here, it is important to discuss the above definition of quasi-Ohmic. The term
“quasi-Ohmic” has been misused in literature to describe the injection from a contact that is
close enough to Ohmic, so that the IV characteristics might be approximated by SCLC.
However, this loose definition has often lead to physically meaningless analyses of IV
characteristics using Eq. (4.1) or Eq. (4.3). Therefore, it is necessary to clearly define injection
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
32
from a quasi-Ohmic contact in terms of the electric field at the charge-injecting contact [i.e.,
0 ( 0) /F x V d ]. Based on this clear definition, the quasi-Ohmic regime will be shown to
be much larger (i.e., covers a greater range of injection barrier height) than has previously been
expected. Moreover, it is important to note that the boundaries of the quasi-Ohmic regime are
dependent on the field-dependent carrier mobility μ, the injection barrier height B , the applied
voltage V and the device thickness d. As a result the same electrode contact (i.e., anode/organic
interface) may display characteristics of ILC, quasi-Ohmic and SCLC in different ranges of
applied bias. This has significant implications for device modeling since most metal/organic
interfaces used in real devices are found to fall into the quasi-Ohmic regime at typical operating
voltages.
As discussed above, the quasi-Ohmic regime in between SCLC and ILC cannot be
described using traditional models for SCLC or ILC. As a result a new approach that includes
both the injection at the interfaces and the transport in the bulk of the organic is required to deal
with this special case. Based on the transport models developed for inorganic semiconductors,
i.e. the drift-diffusion and Poisson equations, a time domain simulation can be conducted to
study the distribution of electric field and carriers at steady state. This method has been
employed by J. C. Scott et al.75,76
to study the charge injection and transport properties in
single-layer OLEDs. Here, based on a similar theoretical framework, a simulation of single
carrier hole-only devices22
was conducted to study the quasi-Ohmic injection and to define a
clear criterion to distinguishing SCLC from quasi-Ohmic and ILC.
In the simulation, space x and time t are discrete. The injection current density described by
Eq. (4.4) and (4.5) serves as a boundary condition at 0, 0x t (i.e., the charge-injecting
contact). The time-dependent continuity equation follows,
( , ) 1 ( , )p x t J x t
t e x
, (4.6)
where p is the total density of holes and J is the conduction current density. The relation
between the electric field and the charge density can be expressed by the Poisson equation as,
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
33
0 ( , )( , )r F x t
p x te x
. (4.7)
In Eq. (4.7) r and 0 are the dielectric constant of the organic and the dielectric permittivity
in vacuum respectively. The conduction current density can be calculated through the
drift-diffusion equation,
( , )( , ) ( , ) ( )
p x tJ x t ep x t F F eD
x
, (4.8)
where D is the diffusion coefficient, which can be obtained from Einstein’s relation (as a
function of the field-dependent mobility),
Bk TD
e
. (4.9)
The other boundary conditions for Eq. (4.6) – (4.9) are,
0
( , 0) /
( , 0) 0
( , 0) 0
d
V F dx
F x t V d
p x t
J x t
, (4.10)
where d is the thickness of the film and V is the applied voltage. It is noted that the transient
current density tJ is contributed by the displacement current and the response of the charge
carrier density as,
0
( , )( , ) ( , )t r
F x tJ x t J x t
t
. (4.11)
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
34
From Eq. (4.11) the total transient current at x d can be calculated until the steady state
is reached. Also, the spatial distribution of electric field at steady state can be obtained from the
simulation. From these simulation results the boundaries of the quasi-Ohmic regime (i.e., the
lower limit of SCLC and the upper limit of ILC) can then be defined in terms of the electric
field at the charge-injecting contact. Clearly, the boundaries of the quasi-Ohmic regime are
dependent on the field-dependent carrier mobility μ, the injection barrier height B , the applied
voltage V and the device thickness d.
Finally, it is noted that based on the results of Monte Carlo simulations, a criterion has been
previously proposed to distinguish SCLC from ILC in organic semiconductors.77
However this
criterion does not take the field dependent mobility into consideration, i.e. the criterion should
be dependent on the applied electric field and should be different for different organic
semiconductors. Moreover, the dependence of the film thickness has not been taken into account
either. It is well known that the thicker the organic layer, the easier the injection will reach the
SCLC since less charge can be sustained in the bulk. Here a criterion for defining the boundaries
of the quasi-Ohmic regime will be presented as a function of both the injection barrier height
and film thickness for α-NPD by conducting a time domain simulation under Eq. (4.4) – Eq.
(4.11).
4.3. Results and discussion
4.3.1. IV characteristics
Figure 4.1 compares the IV characteristics of α-NPD single carrier hole-only devices with
different oxide anodes and ITO at room temperature (297 K). The device structures are:
anode/α-NPD (~ 500 nm)/Au. Here Au is used as the cathode to block electron injection.22
In
order to eliminate any possible run-to-run difference in the organic layer thickness, the organic
layer thickness was measured for each device and the IV characteristics were plotted as a
function of electric field. Also, the device thickness was chosen as 500 nm to minimize any
effects of built-in potential at high electric field. The average electric field is taken as F = V/d,
where d is the device thickness. The results for a pure Ni metal anode with high injection barrier
height78
is also shown for comparison. The device performance can be divided into three distinct
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
35
groups: A) CuO, Co3O4, Ni2O3, MoO3; B) V2O5, WO3; and C) ITO, Cu2O. As can be seen from
the figure, the first group performs the best; the current density is almost one order of magnitude
higher than for the second group and nearly three orders of magnitude higher than the ITO
device.
0.01 0.110
-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
SCLC
CuO
Co3O
4
Ni2O
3
MoO3
V2O
5
WO3
Cu2O
ITO
Ni
C
B
C
urr
en
t D
ensity (
A/c
m2)
Electric Field (MV/cm)
A
Figure 4.1 Energy Current density (J) as a function of average electric field (F = V/d) for single carrier
hole-only devices with different metal oxide anodes at room temperature (297 K). The structure of the
devices is anode/α-NPD (~ 500 nm)/Au. The solid line is the calculated SCLC from Eq. (4.3) for α-NPD
using the field dependent mobility measured by the TOF technique and reported previously in Ref. 79. Notice
that the different oxides fall into three different groups in terms of the current-voltage (s) characteristics.
The theoretical SCLC is calculated using Eq. (4.3) and shown in Fig. 4.1 (solid black line)
as the upper limit of the IV characteristics, where the field dependent mobility was measured by
the time-of-flight (TOF) technique and reported previously in Ref. 79. Indeed, the group A
oxides approach this upper limit at high electric field, which would suggest that they might be
close to forming an Ohmic contact with α-NPD. As discussed in the Theory section, this case
has traditionally been dealt with by assuming that the injection is close enough to an Ohmic
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
36
contact that the current can be approximated by the SCLC. However, it will be shown that none
of the oxides studied in this work were found to form an Ohmic contact with α-NPD (over the
studied range of applied voltage), despite previous reports to the contrary. This discrepancy is
due to the erroneous application of the Mott-Gurney law [i.e., Eq. (4.1) or Eq. (4.3)] to model IV
characteristics in the quasi-Ohmic regime between SCLC and ILC. The quasi-Ohmic regime is
found to cover a significant range of injection barrier heights and is dependent on the applied
voltage, which will be discussed in details in Section 4.3.4.
4.3.2. Fitting IV characteristics and transport parameters
In traditional semiconductor physics the slope of the IV characteristics in the saturation
regime is often used to identify SCLC following the Mott-Gurney law (i.e., Eq. 4.1). This
criterion has also been commonly used (incorrectly albeit) for organic semiconductors. For
example, the device performance of MoO3 shown in Fig. 4.1 is consistent with the data reported
in Ref. 54. (i.e., the current density is equivalent at the same applied electric field), in which the
contact between MoO3 and α-NPD was reported to be Ohmic since a slope of 2 of the IV
characteristics was achieved. However, as discussed in the Theory section the Mott-Gurney law
given by Eq. (4.1) does not apply to organic semiconductors with a field dependent mobility,
such as α-NPD. Hence a slope of 2 of the IV characteristics does not indicate that SCLC has
been obtained.
Since the mobility of α-NPD is field dependent, the slope of the IV characteristics for SCLC
should not be exactly 2, but in fact larger [see Eq. (4.3)]. The theoretical SCLC for α-NPD was
calculated (black solid line in Fig. 4.1) based on the field dependent mobility data that have been
reported elsewhere measured by the TOF method79
; a slope of ~ 2.5 is obtained from the
calculated curve at high electric field. However, as will be shown below, even a slope of ~ 2.5
of the IV characteristics is still not sufficient to indicate that SCLC has been obtained. It should
be pointed out that the shape of the IV curve is influenced by many other effects such as traps,
the built-in potential and non-uniform “patchy” emission. The effects of built-in potential will
be discussed in detail below. In any event, due to the convolution of injection and transport
properties, combined with the other effects mentioned above, the IV characteristics are
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
37
insufficient to determine whether the current density is SCLC. In other words it is extremely
difficult if not impossible to identify an Ohmic contact from the IV characteristics alone.
200 400 600 800-36
-34
-32
-30
-28
~ 10-6 cm
2/Vs
Ni2O
3
V2O
5
ITO
~ 10-5 cm
2/Vs
~ 10-7 cm
2/Vs
ln[ J/F
2(A
/V2)
]
[F (V/cm)]1/2
Figure 4.2 Current density (J) as a function of average electric field (F = V/d) for ITO/α-NPD, V2O5/α-NPD
and Ni2O3/α-NPD plotted as ln(J/F2 ) vs. F
1/2. The organic layer thickness (d) in all cases is ~ 500 nm. The
linear fits are used to extract the apparent mobility from the IV characteristics using Eq. (4.3). However, since
a good fit is achieved for all three anodes, despite the significant difference in injection properties (see Fig.
4.1), the goodness of fit cannot be used to distinguish an Ohmic contact. What is more, the extracted mobility
values are significantly different for each anode, and deviates significantly from the value measured by time
of flight (TOF). This serves as an example that transport parameters cannot be extracted from IV
characteristics, without verifying that a true Ohmic contact has been made using another technique.
Also, fitting the IV characteristics with an analytical equation for SCLC is another common
mistake used to distinguish an Ohmic contact, and has also been inappropriately used to extract
transport parameters, such as mobility. However, as discussed above the Mott-Gurney law given
by Eq. (4.1) is not applicable to organic semiconductors, since most have field dependent
mobility. Moreover, even the form of Eq. (4.3), which considers the field dependent mobility, is
only a necessary but not a sufficient condition for SCLC. In other words, a good mathematically
fit to the data using Eq. (4.3) does not guarantee that SCLC has been achieved. As a result,
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
38
transport parameters, such as mobility, extracted from merely fitting the IV characteristics are
physically meaningless. A quantitative example to emphasize this point is shown in Fig. 4.2.
Figure 4.2 shows the IV characteristics for Ni2O3 (group A), V2O5 (group B) and ITO along
with the fitting results for SCLC using Eq. (4.3) following the same method as in Ref. 55. The
data are linearized by plotting the IV characteristics in the form ln(J/F2
) versus F1/2
. For all three
oxides an excellent linear fit is achieved, despite the significant difference in injection properties
(see Fig. 4.1). Clearly, the goodness of mathematical fit cannot be used to distinguish SCLC.
This point is further illustrated by the extracted mobility values, which are ~10-7
cm2/Vs, ~10
-6
cm2/Vs and ~10
-5 cm
2/Vs extracted from ITO, V2O5 and Ni2O3 device respectively. Clearly
theses values are all incorrect, since a different value is obtained for the same organic
semiconductor, and all of which are orders of magnitude lower than the value measured by
TOF.79
Since the experiments were all conducted under the same well controlled conditions, the
possible system-to-system variation is eliminated. Furthermore, the thickness of the devices
shown in Fig. 4.1 is almost the same, ~ 500 nm. As a result, the difference in injection current is
not due to the apparent thickness dependent mobility proposed by other works.54,55
In any event,
the results of Fig. 4.2 clearly demonstrate that a simple fit of the IV characteristics is insufficient
to claim an Ohmic contact (SCLC), and furthermore any transport parameters extracted using
this technique are in general incorrect. As discussed in the Theory section, the reason behind
this incorrect analysis is the inappropriate assumption that the electric field at the
charge-injecting contact is equals to zero, i.e. ( 0) 0F x . All models for SCLC require that
( 0) 0F x as a boundary condition. However, as will be shown, for all of the examples in
Fig. 4.2 the value of the electric field at the charge-injecting contact is much greater than zero.
Clearly, the IV characteristics alone cannot be used to determine which models can be applied
(e.g., SCLC, quasi-Ohmic or ILC).
4.3.3. Built-in potential and device thickness dependence
Another common criterion used to identify SCLC is the thickness dependence of the IV
characteristics. For ILC the voltage V is proportional to the device thickness d (i.e., V d ) at a
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
39
fixed current density. While on the other hand, if the injection is bulk limited (SCLC), the
thickness dependence becomes (1 1.5)xV d x .80
However, without knowledge of the exact
value of the built-in potential it is difficult to distinguish between these two cases since the
built-in potential also introduces an additional thickness dependence. This point is illustrated in
Fig. 4.3, for Ni2O3 single carrier hole-only devices with different organic layer thickness.
0.01 0.1
10-7
10-6
10-5
10-4
10-3
10-2
10-1
0.01 0.110
-8
10-7
10-6
10-5
10-4
10-3
10-2
(b)
d = 625 nm
d = 443 nm
d = 278 nm
d = 98 nm
Cu
rre
nt
De
nsity (
A/c
m2)
(a)
1 1010
-5
10-4
10-3
10-2
10-1
Cu
rre
nt
De
nsity (
A/c
m2)
Voltage (V)
d = 625 nm
d = 443 nm
d = 278 nm
d = 98 nm
Electric Field (MV/cm)
Figure 4.3 Current density (J) as a function of average electric field (F = V/d) for Ni2O3/α-NPD/Ag with
different organic layer thicknesses (d) of α-NPD, of which (a) is before the subtraction of built-in potential
(Vbi) and (b) is after subtraction of an estimated ~ 0.9 eV built-in potential. The inset of (a) is the current
density as a function of voltage in the case of (a). Ag was chosen as cathode so as to increase the built-in
potential.
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
40
Figure 4.3 (a) shows current density as a function of average applied electric field (i.e., F =
V/d); the inset shows current density as a function of voltage. Clearly, the IV characteristics are
different for each organic layer thickness, implying that the voltage is not proportional to the
thickness, and hence the current is not ILC. However, if an estimated built-in potential (Vbi) of ~
0.9 eV is subtracted and Fig. 4.3 (a) can be re-plotted as shown in Fig. 4.3 (b), where the IV
characteristics (i.e., J vs. (V-Vbi)/d) are in good agreement for each organic layer thickness (i.e.,
V d ). This would suggest that the current is in fact ILC, in contrast to the previous analysis.
It is well-known that the built-in potential cannot be simply calculated by the difference of
work function between anode and cathode,62
and it must be measured using other techniques,
such as electroabsorption81
and photovoltaic62
measurements. For example, even a symmetric
device with identical anode and cathode (e.g. Au/α-NPD/Au) does not necessarily have a zero
built-in potential due to difference in the energy-level alignment between organic deposited on
metal, and metal deposited on organic. Regardless, without directly measuring the value of the
built-in potential the thickness dependence of the IV characteristics is insufficient to claim an
Ohmic contact. It is also noted that even experimental determination of the exact value of the
built-in potential in organic devices remains controversial.
Alternatively, one strategy to reduce the influence of the built-in potential is to increase the
layer thickness of the organic, so as to increase the applied bias for a given electric field
strength. In this work all of the single carrier devices were fabricated with a relatively thick (~
500 nm) organic layer. As we have previously shown29
this thickness is sufficient for α-NPD to
negate any influence of the built-in potential at high electric field (the region where fitting is
performed).
4.3.4. Criterion for SCLC, quasi-Ohmic and ILC
As demonstrated in the previous sections, simple analytical equations cannot be directly
applied to describe the IV characteristics of organic semiconductors without careful
consideration. As of yet it is unclear what regime (i.e., SCLC, quasi-Ohmic or ILC) the data
shown in Fig. 4.1 falls into for each of the studied oxides. As discussed above the difficulty
arises from the treatment of the quasi-Ohmic regime in between SCLC and ILC. We will
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
41
therefore begin by defining the boundaries of the quasi-Ohmic regime (in terms of barrier
height, device thickness and applied voltage). In most cases, the quasi-Ohmic region is the most
elusive (and most mistreated) region as one cannot use simple analytical equations for either the
bulk limited current (SCLC) or ILC. For example, one cannot use the Mott-Gurney given by Eq.
(4.1) or Eq. (4.3) to analyze the IV characteristics since the electric field at the charge-injecting
contact does not equal to zero [i.e., the Mott-Gurney law requires that ( 0) 0F x ]. Also, the
data cannot be analyzed using the ILC models in this region either, since /F V d at the
interface, and the injection current has to be treated as one of the boundary conditions. A time
domain simulation that takes into account the dynamic nature of charge injection and transport
is thus needed.
1 10
10-4
10-3
10-2
10-1
100
101
Group B
ITO
ILC
quasi-Ohmic
Cu
rre
nt D
en
sity (
A/c
m2)
Voltage (V)
Simulation (0.55 eV)
Simulation (0.40 eV)
Simulation (0.25 eV)
ILC (0.55 ev)
SCLCGroup A
Figure 4.4 Current density (J) as a function of voltage (V) for different injection barrier heights ( Bp ). The
solid symbols correspond to the time-domain simulation results. The solid line is the calculated SCLC by Eq.
(4.3) using the field dependent mobility measured by the TOF technique and reported previously in Ref. 79.
The dashed line is the injection limited current (ILC) calculated using Eq. (4.4). The current density at 10 V
(i.e., F = V/d = 0.2 MV/cm) for group A oxides, group B oxides and ITO (see Fig. 4.1) is also shown as solid
star symbols for comparison.
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
42
Figure 4.4 shows the simulated current density (solid symbols) as a function of applied
voltage for a 500 nm thick device. The dashed line is the ILC calculated using Eq. (4.4). The
solid line is the calculated SCLC [from Eq. (4.2)] which defines a perfect Ohmic contact. As
shown in the figure, the upper boundary of the simulated quasi-Ohmic regime with a 0.25 eV
barrier height (solid triangles) converges with the Ohmic SCLC (black solid line). The lower
boundary (~ 0.55 eV) of the simulated current is taken as the convergence of the ILC (dashed
line) from Eq. (4.4) and the simulation results (solid squares). When the barrier height is larger
than ~ 0.55 eV, the current density becomes strictly ILC, i.e. the bulk can support all of the
injected charge, and hence the simulated current density and calculated ILC from Eq. (4.4)
(dashed line and solid squares, respectively) are in excellent agreement. Therefore models for
ILC can be directly applied to analyze the IV characteristics. It is clear that the quasi-Ohmic
regime covers a significant range of current densities, and in fact includes both the group A and
B oxides. The experimental results at an applied bias of 10 V (F = V/d = 0.2 MV/cm) for the
group A oxides, group B oxides and ITO are also indicated in the figure. Clearly, ITO is the
only example that yields ILC under these conditions.
As discussed above, the boundaries of the quasi-Ohmic regime were taken as the
convergence of the simulation results with the SCLC from Eq. (4.3) and the ILC from Eq. (4.4).
These boundaries represent the strict limits for SCLC and ILC in terms of the electric field at the
charge-injecting contact [i.e., ( 0) 0F x for SCLC and ( 0) /F x V d for ILC] as
discussed in the Theory section. Figure 4.5 shows the calculated electric field at the
charge-injecting contact (i.e., anode/α-NPD interface) as a function of the injection barrier
height for two α-NPD layer thicknesses (same average electric field F = V/d) . As shown the
figure, the interfacial electric field depends strongly on the barrier height in the quasi-Ohmic
regime. The interfacial electric field approaches zero as the barrier height is reduced to below ~
0.25 eV, as expected. For high barrier > 0.55 eV, the interfacial electric field reaches the
average value (i.e., F = V/d) indicating the ILC regime. Figure 4.5 also shows that the transition
amongst Ohmic, quasi-Ohmic, and ILC regimes depends on the device thickness. It is therefore
critical to evaluate the effect of device thickness for various barrier heights.
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
43
0.2 0.3 0.4 0.5 0.6 0.70.0
0.1
0.2
0.3
0.4
0.5
d = 100 nm
d = 1000 nm
F(x
= 0
) (M
V/c
m)
Injection Barrier (eV)
F = V/d = 0.5 MV/cm
Figure 4.5 The calculated electric field at the charger-injecting contact as a function of barrier height ( Bp )
for α-NPD with an organic layer thickness (d) of 100 nm and 1000 nm respectively. The average electric field
(F = V/d) for each case is 0.5 MV/cm. Notice that the electric field at the interface converges to the average
value for increasing barrier height, and tends to zero for decreasing barrier height; the region in between
defines a quasi-Ohmic contact.
Figure 4.6 is the calculated thickness-barrier height “phase” diagram for α-NPD devices at
two nominal applied electric fields (i.e., F = V/d). Three regions can be defined on the figure,
i.e. SCLC, quasi-Ohmic and ILC. The solid symbols correspond to an average electric field of
0.5 MV/cm (the typical working electric field for OLEDs) while the open symbols to an average
electric field of 0.1 MV/cm. Surprisingly, the quasi-Ohmic regime covers a significant portion
of the phase diagram, and in fact encompasses the typical working range of injection barrier
heights in real devices, such as OLEDs. It is also important to note that the boundaries of the
quasi-Ohmic regime are strongly dependent on the average electric field (i.e., applied bias) as
well. This is not surprising since the mobility and charge injection are dependent on electric
field. As shown in Fig. 4.6 when the electric field increases from 0.1 to 0.5 MV/cm, the
boundaries of the quasi-Ohmic regime significantly expand.
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
44
0.1 0.2 0.3 0.4 0.5 0.6 0.70
200
400
600
800
1000
B ITOA
quasi-OhmicSCLC
Film
Thic
kn
ess (
nm
)
Injection Barrier (eV)
ILC
-NPD
Figure 4.6 The calculated injection “phase” diagram for α-NPD indicating the boundaries of the quasi-Ohmic
regime (i.e., the criterion for ILC and SCLC) as a function of the injection barrier height ( ) and the organic
layer thickness (d). The first region (left side) defines the criterion for an Ohmic contact (SCLC), the second
region (middle) is for a quasi-Ohmic contact and the third region (right side) is for an injection limited contact
(ILC). The boundaries for these regimes are dependent on applied bias. The solid symbols correspond to an
average electric field of 0.5 MV/cm while the open symbols to an electric field of 0.1 MV/cm. The results for
the group A and group B oxides as well as ITO are also shown for comparison and are obtained from the
experimental results shown in Fig. 4.1.
Since both the group A and B oxides fall into the quasi-Ohmic regime (for the device
thickness and range of applied bias considered in this work) the time domain simulation must be
used to extract the injection barrier height (or mobility) by fitting the IV characteristics. The
barrier heights for the group A and group B oxides as well as ITO are indicated in Fig. 4.6.
Using this method the injection barrier height is estimated to be ~ 0.42 eV for the group A
oxides (such as Ni2O3), ~0.50 eV for group B oxide (such as WO3). On the other hand, the
injection from ITO into α-NPD is obviously injection limited; the barrier is estimated to be ~
0.56 eV using Eq. (4.4). These values agree well with values independently extracted from UPS
Bp
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
45
measurements, the details of which will be discussed elsewhere. Here it is also noted that the
exact values of the barrier height may vary slightly for different injection models used as the
boundary condition in the simulation. Also, although the time domain simulation can describe
the IV characteristics across all three regimes (i.e., SCLC, quasi-Ohmic and ILC), simple
analytic equations are preferable and convenient to describe SCLC and ILC due to the
computational complexity of the time domain simulation.
4.4 Summary
In summary, the hole injection from different metal oxides into α-NPD has been
systematically studied in single carrier hole-only devices yielding a IV database for variable
injection barrier heights. The device performance data was found to aggregate into three distinct
groups: A) CuO, Co3O4, Ni2O3, MoO3; B) V2O5, WO3; and C) ITO, Cu2O. Based on the
experimental results several key finds have been made.
First, it was found that none of the metal oxides studied in this work form a true Ohmic
contact (SCLC) to α-NPD (over the practical range of applied bias used in devices), despite
previous reports to the contrary. This discrepancy was attributed to incorrect data analysis in
previous studies as a result of merely applying simply analytical equations for SCLC (e.g.,
Mott-Gurney) to evaluate the IV characteristics. Without prior knowledge of the field-dependent
mobility, IV characteristics cannot be used to identify an Ohmic contact. A recent study also
confirms the finding in this chapter.82
Second, it was found that there was no clear boundary between SCLC and ILC conditions,
but rather a large intermediate regime, namely quasi-Ohmic, which includes characteristics of
both. As a result, the IV characteristics in the quasi-Ohmic regime cannot be simply analyzed by
either the bulk transport models (e.g., Mott-Gurney law) or the injection models. The boundaries
of the quasi-Ohmic regime were defined by the electric field at the electrode contact (i.e.,
0 ( 0) /F x V d ).
Third, it was found that the quasi-Ohmic regime is surprisingly large and covers a wide
range of barrier heights. Using a time-domain simulation of the transport of charge carriers
Chapter 4 Analysis of charge injection characteristics at electrode-organic interfaces
46
across an organic semiconductor the boundaries of the quasi-Ohmic regime were evaluated as a
criterion to distinguish SCLC from quasi-Ohmic and ILC. It was found that the IV
characteristics for most electrode/organic contacts fall into the quasi-Ohmic regime. It was also
found that the boundaries of the quasi-Ohmic regime have a strong dependence on the thickness
of the organic layer and the applied bias.
Fourth, it was found that the built-in potential can significantly distort the thickness
dependence of the IV characteristics, particularly for organic layer thicknesses < 100 nm.
Without measuring the exact value of the built-in potential, a thicker organic layer of > 500 nm
is required to minimize the effects of any built-in potential on the IV characteristics.
Finally, an injection “phase diagram” for α-NPD has been shown as a case study to clearly
demonstrate the above mentioned effects. The group A and group B oxides discussed above
were found to fall within the quasi-Ohmic regime, while ITO was found to be purely injection
limited. Using the time domain simulation the injection barrier height for the various oxides has
been deduced to be in the range of ~ 0.4 eV for group A oxides, ~ 0.5 eV for group B oxides,
and ~ 0.6 eV for ITO. For other organic semiconductors with different field-dependent mobility
a new phase diagram should be calculated.
Chapter 5 Organic/organic interface designs of OLEDs
47
Chapter 5 Organic/organic interface designs of OLEDs
In this chapter, it will be shown that the accumulated space charges at the exciton formation
interface have a negative impact on the device performance due to the exciton quenching. It will
be demonstrated that using a hole transport layer with very large HOMO level such as CBP can
reduce the exciton quenching and thus increase the device performance. Not only the device
efficiency of both fluorescent and phosphorescent OLEDs can be enhanced by this new device
design concept, the device structure is also highly simplified because no additional “injection
layer” and “blocking layer” are needed. The content of this chapter was published as Appl. Phys.
Lett. 96, 043303 (2010), J. Appl. Phys. 108, 024510 (2010) and Appl. Phys. Lett. 98, 073310
(2011).
5.1. Introduction
In the previous two chapters, the energy level alignment and charge carrier injection at
metal/organic interfaces were systematically studied. Although charge carrier injection into
organic semiconductors is of great significance for OLED, in an actual OLED, the carrier
transport through organic/organic interfaces is also critical to the device performance.
Despite significant advances in device performance since the first OLED was reported in
the 1980s,2 much of the device physics of even the simplest OLED structures is still not well
understood.13,49
For example, hole injection layers (HILs), such as m-MTDATA,83,84
have
traditionally been thought to provide an intermediate energy-level to facilitate hole injection
from the ITO anode to the hole transport layer (HTL), typically α-NPD. However, it has
recently been demonstrated that the most significant role of m-MTDATA in Alq3 OLEDs is to
reduce exciton quenching by limiting accumulated holes at the α-NPD/Alq3 interface; space
charges, such as α-NPD+ radical cations, accumulated near the emission layer (EML) are known
to quench excitons.83
This recent finding suggests that exciton quenching from accumulated
space charges may play a far more significant role in device performance than has previously
been thought. Eliminating exciton quenching from accumulated space charges remains a
Chapter 5 Organic/organic interface designs of OLEDs
48
significant challenge in existing device designs. For example, in the case of α-NPD/Alq3
OLEDs, the offset between the HOMOs creates an injection barrier that will always tend to
accumulate holes, regardless of what HILs are used. Thus, exciton quenching from α-NPD+
radical cations cannot be eliminated in traditional device designs. In this chapter a simple
method will be shown to control exciton quenching due to accumulated radical cations at the
exciton formation interface. Eventually, a highly simplified device structure that can enable
exceptionally high efficiency is demonstrated for both fluorescent and phosphorescent OLEDs.
5.2. CBP interlayer to reduce exciton quenching
The OLED structure is: ITO/CuPc (25 nm)/α-NPD (45 - x nm)/interlayer (x = 0, 3, 10
nm)/Alq3 (45 nm)/LiF (1 nm)/Al (100 nm). The two interlayers used were CBP and TPBi. To
reduce charge accumulation at the α-NPD/Alq3 interface, the injection barrier at the interface
should be eliminated, for example by inserting different interlayers between α-NPD and Alq3.
Figure 5.1(a) shows the current efficiency and power efficiency as a function of luminance of
OLEDs with and without a thin (~ 3 nm) CBP and TPBi interlayer inserted between α-NPD and
Alq3. The efficiency of the TPBi device is significantly lower than the reference device.
However, the current efficiency and power efficiency are increased by ~ 20 % in the CBP
device, despite similar HOMO energy-levels to TPBi. What is more, the maximum current
efficiency of the CBP device is 5.2 cd/A, which is amongst the highest device performance for
Alq3 based OLEDs with n-type or p-type doping.19,85,86
Figure 5.1(b) shows the IV characteristics of the same OLEDs. The CBP interlayer has no
impact on the IV characteristics, contrary to traditional wisdom, which suggests CBP should act
as a hole blocking layer since it has a much deeper HOMO than Alq3. For the TPBi interlayer,
however, the driving voltage is increased by nearly 0.8 V at a given current density of 100
mA/cm2. This finding suggests that the TPBi interlayer acts as a traditional hole blocking layer,
which is interesting given that the HOMO energy-levels of CBP and TPBi are similar. If TPBi
indeed acts as a hole blocking layer, a shift in the emission zone towards the α-NPD/TPBi
interface and hence blue emission from α-NPD in the EL spectra should be expected.
Chapter 5 Organic/organic interface designs of OLEDs
49
102
103
104
0
1
2
3
4
5
0 2 4 6 8 100
50
100
150
200
(b)
(I) -NPD/ Alq3
(II) -NPD/ CBP (3nm)/ Alq3
(III) -NPD/ TPBi (3nm)/ Alq3
Pow
er
Effic
iency (
lm/W
)
Cu
rren
t E
ffic
ien
cy (
cd/A
)
Luminance (cd/m2)
(a)
0
1
2
3
4
5
6
Cu
rren
t D
ensity (
mA
/cm
2)
Voltage (V)
Figure 5.1 Efficiency and (b) IV characteristics of the OLED devices with the following structures: (I)
α-NPD/Alq3 (standard reference); (II) α-NPD/CBP (3nm)/Alq3 and (III) α-NPD/TPBi (3nm)/Alq3.
Figure 5.2 shows the EL spectra of OLEDs with and without a thin (3 nm) and thick (10
nm) CBP and TPBi interlayer. For the TPBi interlayer blue emission from α-NPD is observed in
the EL spectrum (~ 430 nm),87,88
which explains the lower device efficiency in Fig. 5.1 (i.e., due
to excitons loss to the α-NPD). For a 10 nm thick TPBi interlayer the EL spectrum is dominated
by emission from α-NPD. This finding suggests that TPBi acts as a traditional hole-blocking
layer, shifting the emission zone towards the α-NPD/TPBi interface. A similar effect has
previously been reported for 2,9-dimethyl-4,7-diphenyl-phenanthroline (BCP) interlayers.89-91
However, the EL spectrum of the α-NPD/CBP/Alq3 device is identical to the standard reference
Chapter 5 Organic/organic interface designs of OLEDs
50
device (i.e., EL emission only from Alq3), indicating that the emission zone is unaffected. Even
for a CBP interlayer thickness of 10 nm only EL emission from Alq3 is observed. This finding
contradicts the exciton dissociation theory, proposed by Song et al.91
to explain the role of wide
band gap interlayers at the α-NPD/Alq3 interface. The theory predicts that any wide band gap
interlayer inserted at the α-NPD/Alq3 interface should result in blue emission from α-NPD.
Clearly, this is not the case since no blue emission from α-NPD was observed, even for a thick
CBP interlayer.
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0 (a) -NPD/Alq
3
(b) -NPD/CBP (3nm)/Alq3
(c) -NPD/CBP (10nm)/Alq3
(d) -NPD/TPBi (3nm)/Alq3
(e) -NPD/TPBi (10nm)/Alq3
Inte
nsity (
a.u
.)
Wavelength (nm)
-NPD
Figure 5.2 Normalized EL spectra of the OLED devices with different thickness (0, 3, 10 nm) interlayer of
CBP and TPBi.
To elucidate the effect of CBP and TPBi on the hole accumulation at the various
organic/organic interfaces UPS measurements were employed to study the injections barrier.
Figure 5.3 shows the He Iα (hν = 21.22 eV) valence band spectra for the in situ subsequent
deposition of: (a) CuPc/α-NPD/Alq3; (b) α-NPD/CBP/Alq3; and (c) α-NPD/TPBi/Alq3. From the
UPS spectra, the measured HOMOs of the various molecules are: 4.8 eV (CuPc), 5.4 eV
(α-NPD), 5.75 eV (Alq3), 6.1 eV (CBP), 6.2 eV (TPBi). Figure 5.4 shows the energy-level
diagrams for the different device structures deduced from the UPS measurements; the injection
barriers and interfacial dipoles are summarized in Table I. In the reference device [see Fig.
Chapter 5 Organic/organic interface designs of OLEDs
51
5.4(a)] holes are accumulated at the CuPc/α-NPD (0.6 eV) and α-NPD/Alq3 (0.55 eV) interfaces
due to the hole injection barriers. When a thin CBP or TPBi interlayer is inserted between
α-NPD and Alq3, the hole injection barrier, and hence the accumulation of holes, is shifted from
the α-NPD/Alq3 interface to the α-NPD/CBP or α-NPD/TPBi interface, as shown in Fig. 5.4(b)
and (c) respectively. As mentioned previously, accumulated α-NPD+ radical cations at the
α-NPD/Alq3 interface will quench excitons. Since the HOMO of CBP is deeper than that of
Alq3, holes cannot accumulate at the CBP/Alq3 interface, thus reducing α-NPD+ radical cation
quenching.
CuPc
-NPD
Alq3
-NPD
CBP
Alq3
Inte
nsity (
a.u
.)
3 2 1 0
-NPD
TPBi
Alq3
Binding Energy (eV)
Figure 5.3 He Iα (hν = 21.22 eV) valence band spectra for: (a) CuPc/α-NPD/Alq3; (b) α-NPD/CBP/Alq3; and
(c) α-NPD/TPBi/Alq3. In (b) and (c) the CuPc/α-NPD interface is not shown for clarity since it is identical to
(a).
Chapter 5 Organic/organic interface designs of OLEDs
52
Table 5-1 Hole injection barrier heights (Bp ) and interfacial dipoles ( ) at different organic/organic
interfaces extracted from the UPS spectra.
CuPc/
α-NPD
α-NPD/
Alq3
α-NPD/
CBP
α-NPD/
TPBi
CBP/
Alq3
TPBi/
Alq3
Bp (eV) 0.60 0.55 0.70 1.00 -
-
(eV) 0 0.20 0 0.20 0.20 0
Based on the UPS measurements the hole injection barrier for TPBi (1.0 eV) is larger than
for CBP (0.7 eV), due to a combination of its relatively deep HOMO and the interfacial dipole
at the α-NPD/TPBi interface (see Table I). Therefore, although the TPBi interlayer is very thin
(~ 3 nm), the voltage drop across the interlayer is not negligible, which results in a higher
driving voltage [Fig. 5.1.(b)] . Also, the interfacial dipole at the α-NPD/TPBi interface shifts
down the LUMO level of TPBi [see Fig. 5.4 (c)] resulting in a more favorable LUMO
energy-level alignment for electron injection from Alq3 to TPBi. The high injection barrier for
holes, combined with the easier injection pathway for electrons into α-NPD results in formation
of exciton in and emission from α-NPD and thus explains the EL spectra in Fig. 5.2.
Arguably the higher injection barrier for TPBi than for CBP could account for the
difference in device performance shown in Fig. 5.1 (i.e., TPBi blocks more holes), as commonly
believed in literatures. However, this simple explanation is problematic. For example, the
barrier at the α-NPD/CBP interface is still larger than that at the α-NPD/Alq3, but no increase in
driving voltage or blue emission from α-NPD was observed. Since excitons will tend to form at
the interface between an HTL and electron transport layer (ETL),2 the impact of the carrier
transport characteristics of the interlayer and the location of excitons must also be considered.
An ETL interlayer (e.g., TPBi or BCP) will tend to shift the exciton formation zone towards the
α-NPD/interlayer interface, while an HTL interlayer (e.g., CBP) will shift the exciton formation
zone towards the interlayer/Alq3 interface. Since HTL molecules are typically donors and ETL
molecules acceptors, a natural donor-acceptor interface will tend to form at the exciton
formation zone (HTL/ETL interface). Indeed, the interfacial dipole measured at the various
organic/organic interfaces (indicative of a donor-acceptor interface)44
coincide with the
Chapter 5 Organic/organic interface designs of OLEDs
53
HTL/ETL interfaces (see Fig. 5.4), and are consistent with the position of the emission zone as
indicated by the EL spectra (see Fig. 5.2).
-NPD 3AlqCuPc -NPD 3
AlqCuPc CBP
-NPD 3AlqCuPc TPBi
electron accumulation
hole accumulation
interfacial dipole
(a) (b)
(c)
excition formation
Vacuum
level
Vacuum
level
Vacuum
level
LUMO
HOMO
LUMO
HOMO
LUMO
HOMO
Figure 5.4 Schematic energy diagram for the device structure: (a) CuPc/α-NPD/Alq3; (b) CuPc / α-NPD / CBP
/ Alq3; and (c) CuPc/α-NPD/TPBi/Alq3. The LUMOs are estimated from cyclic voltammetry measurements.92
Clearly the traditional notion of confining holes and electrons at the emission interface is
not an optimal device design concept. By controlling the energy-level alignment at the
HTL/EML interface, radical cation accumulation and the associated exciton quenching can be
prevented. To demonstrate the universality of this approach, singlet-emitter doped OLEDs with
fluorescent dye molecule, C545T were also fabricated, with a structure: ITO/CuPc (25
nm)/α-NPD (45 nm)/CBP(0, 3 nm)/Alq3:C545T (1 wt %, 30 nm)/Alq3 (15 nm)/LiF (1 nm)/Al
(100 nm). A consistent ~ 25% improvement at high luminance was achieved with the thin CBP
interlayer without sacrificing driving voltage, similar to the pure Alq3 devices discussed above.
Chapter 5 Organic/organic interface designs of OLEDs
54
The maximum current efficiency was increased from 15 cd/A to 19 cd/A at high luminance
(~104 cd/m
2).
5.3. Deep HOMO HTL: enable simple structure with high efficiency
Although the CBP interlayer prevents the accumulation of holes at the CBP/Alq3 interface
due to its deeper HOMO (6.1 eV) than that of Alq3 (5.75 eV), which results in an improved
device performance. However, using such an interlayer introduces additional complexity into
the device structure. Also it does not completely eliminate exciton quenching from radical
cations due to the numerous other organic/organic interfaces for holes to accumulate at (e.g., the
α-NPD/CBP interface). In this section, a simple method will be demonstrated to eliminate
exciton quenching due to accumulated radical cations by using deep HOMO HTL,CBP, as a
direct drop-in replacement of the most commonly used HTL, α-NPD. The fluorescent OLEDs
were all doped with fluorescent dye molecule C545T (1 wt.%) that gives a higher internal
quantum efficiency than the un-doped devices, which also aims at showing the universality of
this approach.
(a)
Glass
ITO
CuPc (25 nm)
α-NPD (45 nm)
Alq3 :C545T(1 wt.%, 30 nm)
Alq3 (15 nm)
LiF(1nm)/Al (100 nm)
(b)
Glass
ITO/Oxides (1 nm)
CBP (50 nm)
Alq3 :C545T(1 wt.%, 30 nm)
Alq3 (15 nm)
LiF(1nm)/Al (100 nm)
Figure 5.5 Device structure of (a) standard reference device, and (b) device with non-blocking exciton
formation zone.
Chapter 5 Organic/organic interface designs of OLEDs
55
Since the HOMO of CBP (6.1 eV) is much deeper than that of α-NPD (5.4 eV),93
direct
hole injection from the ITO anode into the deep HOMO of CBP requires an anode buffer layer
with high work function, such as a transition metal oxide (see Chapter 3). Figure 5.5 shows the
device structure of a non-blocking exciton formation zone with CBP as HTL in comparison to a
standard reference device. For simplicity we will refer to the device with a non-blocking exciton
formation zone (Fig. 5.5 b) as the CBP device. As we will show that removing the hole blocking
barrier in the traditional exciton formation zone design eliminates exciton quenching due to
accumulated radical cations, resulting in significantly improved device performance.
2 4 6 8 10 1210
-1
100
101
102
103
104
105
101
102
103
104
105
4
8
12
16
20
(b)
ITO / CuPc / a-NPD
ITO / WO3 / CBP
ITO / CBP
Lum
inance (
cd/m
2)
Voltage (V)
(a)0
50
100
150
200
250
300
Curr
ent D
ensity (
mA
/m2)
Curr
ent E
ffic
iency (
cd/A
)
Luminance (cd/m2)
0
4
8
12
16
20
24
28
Pow
er
Effic
iency (
lm/W
)
Figure 5.6 (a) Luminance-Voltage and Current-Voltage characteristics and (b) efficiency of the OLED devices
with the following structures: ITO/CuPc/α-NPD (square); ITO/WO3/CBP (circle); ITO/CBP (triangle).
Fig. 5.6 shows the device performance using CBP as HTL with and without a thin interlayer
(1 nm) of WO3, in comparison with a standard reference device using α-NPD as the HTL with a
25 nm thick CuPc HIL. Without the WO3 interlayer the driving voltage of the CBP device is
Chapter 5 Organic/organic interface designs of OLEDs
56
extremely high (and the efficiency very low) due to the poor hole injection from ITO into the
deep HOMO of CBP (6.1 eV).93
WO3 has previously been shown to improve hole injection into
other molecules with deep HOMOs, such as 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA)
with a HOMO of 5.9 eV.94
Remarkably, the driving voltage of the CBP device with WO3
interlayer is nearly identical to the standard reference device, indicating that WO3 might also
form a good injection contact with CBP. Moreover, the luminance of the WO3/CBP based
device is higher than the CuPc/α-NPD reference device at any given voltage, due to the
elimination of exciton quenching caused by radical cation accumulation near the emissive layer
(EML). As a result, the current efficiency of the WO3/CBP device is much higher than the ~ 15
cd/A for the CuPc/α-NPD based reference device, reaching ~ 20 cd/A, even at high luminance
(>105 cd/m
2). More importantly, the power efficiency is also dramatically increased for the
WO3/CBP device, which confirms that the improvement is due to the higher quantum
efficiency. This is the highest efficiencies reported for C545T doped bottom emitting OLED
with Alq3 as host.9 It is important to note that this significant improvement is achieved with a
simplified device structure (two layers) and without the use of an optical microcavity (e.g.,
using metal anode or top emission).
Clearly, from Fig. 5.6 in the device with CBP as HTL, the WO3 interlayer is necessary to
reduce the driving voltage due to the deep HOMO of CBP. Transition metal oxides HILs have
been extensively studied with α-NPD as HTL,50,95,96
but not for molecules with much deeper
HOMOs, such as CBP. To elucidate the low driving voltage of the WO3/CBP device the
energy-level alignment at the anode interface was measured using UPS. Fig. 5.7 shows the He
Iα (hν = 21.22 eV) valence band spectra and secondary electron cut-off of UV ozone treated
ITO and ITO/WO3 with and without a 3 nm layer of CBP. The measured work function of the
ITO and ITO/WO3 substrates were 5.5 eV and 5.8 eV, respectively. From the UPS spectra the
hole injection barrier are 0.69 eV for ITO and 0.52 eV for ITO/WO3, consistent with the work
function of the substrates (i.e., higher work function yields a lower barrier). Although the
injection barrier for ITO/WO3 is only ~ 0.2 eV lower than for ITO, the exponential dependence
of the injection current on the barrier height, makes even a ~ 0.2 eV difference in barrier height
significant in terms of device performance. Clearly, WO3 provides favorable energy-level
alignment for hole injection into CBP, despite its deep HOMO. Based on our previous work
Chapter 5 Organic/organic interface designs of OLEDs
57
with α-NPD,96
other transition metal oxides, such as V2O5 and MoO3, should also have good
injection into CBP. Indeed, as shown in Fig. 5.7 the barrier heights measured using UPS for
ITO/MoO3 and ITO/V2O5 are 0.50 and 0.54 eV respectively, very close to the value for
ITO/WO3. This suggests that the V2O5 and MoO3 should also work in an actual OLED device.
4 2 0
ITO/CBP
ITO
WO3
100
100
WO3
Binding Energy (eV)
ITO
HOMO
Anode/CBP
16 15
(c)(b)
No
rmaliz
ed Inte
nsity (
a.u
.)(a)
2 1 0
WO3/CBP
~ 0.2 eV
Figure 5.7 He Iα (hν = 21.22 eV) valence band spectra of ITO and ITO/WO3 with a 3 nm thick layer of CBP
showing (a) the secondary electron cut-off, (b) the valence band, and (c) the HOMO of CBP.
Figure 5.8 shows the device performance using CBP as HTL with and without a thin
interlayer (1 nm) of WO3, V2O5 and MoO3. Clearly, both the current efficiency and power
efficiency are nearly identical for the three different oxide interlayers. This demonstrates that the
significant improvement in efficiency for the device with CBP as HTL is not due to some
unique properties of the WO3 interlayer but due to the CBP (i.e., non-blocking exciton
formation zone). To further confirm this, another series of devices with the CBP replaced by
α-NPD with exactly the same thickness (50 nm) was also fabricated. Clearly all of the α-NPD
based devices (see Figure 5.8) have consistently lower efficiencies than the CBP devices. It is
well known that holes are already the dominant carrier in the α-NPD/Alq3 device structure. The
device performance is therefore not limited by the injection of holes, but rather by exciton
quenching due to accumulated excess holes at the α-NPD/Alq3 interface.83
Hence, improving the
injection of holes into α-NPD (using an oxide interlayer) should not improve the device
Chapter 5 Organic/organic interface designs of OLEDs
58
performance. In fact, all of the α-NPD devices perform worse than the CuPc/α-NPD reference
device (shown again in Figure 5.8 for clarity). The additional barrier at the CuPc/α-NPD
interface has been shown to reduce exciton quenching (and hence boost efficiency) by limiting
the number of excess holes that tend to accumulate at the α-NPD/Alq3 interface.83
Also, the
electroluminescence spectra of the various devices (see Figure 5.9) are all identical, which
suggests that the emission zone is unaffected by the CBP layer. These results provide strong
evidence that the significant improvement in device performance is due to the CBP layer (i.e.,
non-blocking exciton formation zone), and not due to a particular oxide interlayer or optical
effect.
8
12
16
20
24
100
101
102
103
104
105
8
12
16
20
24
(b)
Cu
rrent D
enstiy (
cd/A
)
(a)
ITO / MoO3 / CBP
ITO / V2O
5 / CBP
ITO / WO3 / CBP
ITO / MoO3 / -NPD
ITO / V2O
5 / -NPD
ITO / WO3 / -NPD
ITO / CuPc / -NPD
Pow
er
Effic
iency (
lm/W
)
Luminace (cd/m2)
Figure 5.8 (a) Current efficiency and (b) power efficiency of the OLED devices with the following structures:
ITO/MoO3, V2O5, WO3 (1 nm)/CBP(50 nm); ITO/MoO3, V2O5, WO3 (1 nm) /α-NPD(50 nm) and ITO/CuPc
(25 nm)/α-NPD(45 nm).
Chapter 5 Organic/organic interface designs of OLEDs
59
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength (nm)
ITO / MoO3 / CBP
ITO / V2O
5 / CBP
ITO / WO3 / CBP
ITO / MoO3 / -NPD
ITO / V2O
5 / -NPD
ITO / WO3 / -NPD
ITO / CuPc / -NPDE
L (
a.u
.)
Figure 5.9 Electroluminescence (EL) spectra of the devices shown in Fig. 5.9.
ITO
ITO
3
2 5
3
MoO
V O
WO
-
-LiF/Al
LiF/Al
NN
HIL
(CuPc)
HTL
(α-NPD)
ETL
(Alq3)
HTL
(CBP)
ETL
(Alq3)
(a)
(b)
Figure 5.10 Schematic energy level diagram for the device structure: (a) CuPc/α-NPD/Alq3; (b) CBP/Alq3.
The energy offsets were obtained from UPS measurements.
Chapter 5 Organic/organic interface designs of OLEDs
60
Figure 5.10 shows the schematic energy-level diagram for the different device structures
deduced from the UPS measurements. The energy-level alignment for the CuPc/α-NPD
reference device and CBP/Alq3 interface were taken from our previous study.93
As discussed
above, holes will accumulate in the reference device at the CuPc/α-NPD and α-NPD/Alq3
interfaces, resulting in exciton quenching in the Alq3 emissive layer (see Fig. 5.10 a). In the
previous section, a thin (~ 3 nm) interlayer of CBP inserted at the α-NPD/Alq3 interface (i.e.,
α-NPD/CBP/Alq3) is shown to reduce the hole accumulation near the Alq3 layer, but not
eliminate entirely due to the accumulation at the α-NPD/CBP interface.93
However, it has been
shown that by using an oxide interlayer at the anode interface to efficiently inject holes directly
into CBP, the additional CuPc and α-NPD transport layers can be eliminated. As shown in Fig.
5.10 (b), such a device has no hole blocking interfaces. Therefore, exciton quenching due to
accumulated radical cations (i.e., holes) is eliminated, dramatically boosting device efficacy.
These findings suggest that removing the electron blocking barrier in the exciton formation
zone, for example by designing new HTL molecules with deeper LUMO (and HOMO) than that
of the ETL, may provide a further boost in device efficacy.
5.4. Deep HOMO HTL for Phosphorescent OLEDs
Phosphorescent organic light emitting diodes (PHOLEDs) that have the potential to achieve
an internal quantum efficiency close to 100% have attracted considerable research interest.3,97-100
Although a high external quantum efficiency (EQE) has been realized for green PHOLEDs at
low luminance (e.g., 100 cd/m2), it is still a significant challenge to obtain a similarly high
efficiency at high luminance (e.g., 10,000 cd/m2) due to various non-radiative recombination
processes that quench the long lived triplet excitons.100,101
For example, at high exciton densities
the triplet excitons will self-quench through triplet-triplet annihilation, greatly reducing the
efficiency.102-105
At high current densities triplet excitons may also quench with accumulated
polarons (charged molecules) at the various organic heterojunctions in the device.102-104
There
has therefore been significant effort devoted towards reducing the effect of these various
quenching processes at high luminance.99,106-109
However, many of these strategies significantly
increase the device complexity, which is undesirable for manufacturing. As a result, it remains a
challenge to realize a simplified device structure which maintains high efficiency at high
Chapter 5 Organic/organic interface designs of OLEDs
61
luminance.
CBP (3 nm)
CBP (35 nm)
α - NPD (35 nm)
CBP:Ir(ppy)2(acac) (15 nm)
TPBi (65 nm)
LiF/Al (100 nm)
Device B
CBP:Ir(ppy)2(acac) (15 nm)
TPBi (65 nm)
LiF/Al (100 nm)
Device A
Glass substrate
ITO/MoO3
α - NPD (32 nm)
CBP:Ir(ppy)2(acac) (15 nm)
TPBi (65 nm)
LiF/Al (100 nm)
Device C
α-NPD TPBi
Ir(ppy)2(acac)
6.1 eV 6.2 eV
5.6 eV
2.4 eV2.7 eV
3.0 eV
CBP
α-NPD TPBi
Ir(ppy)2(acac)
6.1 eV 6.2 eV
5.6 eV
2.4 eV2.7 eV
3.0 eV
CBP
2.8 eV
5.4 eV
2.8 eV
5.4 eV
TPBi
Ir(ppy)2(acac)
6.1 eV 6.2 eV
5.6 eV
2.7 eV
3.0 eV
CBP
2.8 eV
Glass substrate
ITO/MoO3
Glass substrate
ITO/MoO3
Figure 5.11 Schematic device structures and energy-level diagrams of the devices in this study. The HOMO
and LUMO levels are obtained from Ref. 93,98,110,111.
Chapter 5 Organic/organic interface designs of OLEDs
62
In this section, it will be shown that the approach that has been applied to achieve a simple
structure and high efficiency fluorescent OLED (see Section 5.3) can also be used to enable high
performance in phosphorescent OLEDs. In particular, a simplified bi-layer PHOLED with a
high EQE of >20% over a wide luminance range from 10 cd/m2 to >10,000 cd/m
2 is realized.
The simplified device structure is also shown to have comparable power efficiency to state of
the art p-i-n PHOLEDs.
The detailed device structures are depicted in Fig. 5.11, where the name of different
molecules have been summarized in Table 2-1. Ir(ppy)2(acac) was used as the phosphorescent
emitter and it was purified by gradient sublimation prior to use. The doping concentration of
Ir(ppy)2(acac) in CBP is 8 wt.%. Similarly (see Section 5.3), 1 nm MoO3 was thermally
evaporated on ITO to achieve a high work function for direct hole injection into CBP.
Figure 5.12(a) shows the IV and LV characteristics of the simplified bi-layer OLED
proposed in this work (device A). The EL spectra as a function of current density are shown in
the inset. The IV and LV curves increase rapidly after the onset, indicating efficient carrier
injection and transport in the CBP and TPBi layers. The device exhibits a low operating voltage
of 3.65 V at 100 cd/m2 and 4.55 V at 1,000 cd/m
2. Since there is no change in the EL spectra
with increasing current density, it can be concluded that the triplet excitons are well confined to
the EML.
Figure 5.12(b) shows the EQE and power efficiency of the same PHOLED (device A). The
EQE reaches 23.4% (81 cd/A) at 100 cd/m2, 24.5% (85 cd/A) at 1,000 cd/m
2, and 21.9% (76
cd/A) at 10,000 cd/m2. Even at an ultra-high luminance of 100,000 cd/m
2 the EQE is still as
high as 13% (45 cd/A). Due to the high EQE of our device over a broad range of luminance, the
power efficiency is also quite high and is equivalent to that of state of the art p-i-n
PHOLEDs,99,112,113
reaching 78.0 lm/W at 100 cd/m2, 65.0 lm/W at 1,000 cd/m
2, and 42.8 lm/W
at 10,000 cd/m2. The power efficiency can be further enhanced if the mobility of both the
electron and hole transport layer (e.g. CBP and TPBi herein) is increased.
Chapter 5 Organic/organic interface designs of OLEDs
63
0 2 4 6 810
-5
10-4
10-3
10-2
10-1
100
101
101
102
103
104
0
20
40
60
80
100
Voltage (V)
Cu
rren
t D
ensity (
mA
/cm
2)
100
101
102
103
104
N
Ir
O
O
CH 3
CH 32
Ir(ppy)2(acac)
(b)
Lu
min
an
ce (
cd/m
2)
(a)
Pow
er
Effic
iency (
lm/W
)
Luminance (cd/m2)
0
10
20
30
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
EL (
a.u
.)
Wavelength (nm)
0.1 mA/cm2
1 mA/cm2
10 mA/cm2
100 mA/cm2
23.4% @ 100 cd/m2
23.2% @ 5000 cd/m
21.9% @ 10000 cd/m
EQ
E (
%)
Figure 5.12 (a) IV and LV characteristics of device A as well as its (b) EQE and power efficiency as a function
of luminance. The upper inset is the molecular structure of the emitter Ir(ppy)2(acac). The lower inset is the
corresponding EL spectra measured at various current densities.
The high efficiency of our simplified PHOLED over a broad luminance range suggests that
the triplet exciton quenching processes at high luminance can be suppressed. Due to the unique
design of device A there are no energetic barriers in the device for charge carriers to accumulate
at (see Fig. 5.11). Using CBP as both the HTL and host for the phosphorescent emitter
eliminates any barrier at the HTL/EML interface. Furthermore, the energy-levels of CBP and the
TPBi electron transport layer (ETL) are nearly identical, resulting in almost no barrier at the
EML/ETL interface. Suppression of charge carrier accumulation may therefore account for the
low efficiency roll-off of our simplified PHOLED design (device A). To further investigate the
Chapter 5 Organic/organic interface designs of OLEDs
64
physics behind the low efficiency roll-off, PHOLEDs with a traditional device structure using
α-NPD as the HTL was fabricated for comparison (see device B in Fig. 5.11).
2 3 4 5 6 70
10
20
30
40
50
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
(b)
Voltage (V)
Curr
ent D
ensity (
mA
/cm
2) Device A
Device B
Device C
(a)
100
101
102
103
104
Lum
inance (
cd/m
2)
Device A
Device B
Device C
EL
(a
.u.)
Wavelength (nm)
Figure 5.13 (a) IV and LV characteristics of device A, B and C as well as (b) the corresponding EL spectra
measured at 5 mA/cm2.
In device B α-NPD is used as the hole transport layer (HTL) and CBP is used as the host for
the phosphorescent emitter. Since the HOMO of α-NPD (5.4 eV) is significantly shallower than
that of CBP (6.1 eV) a significant barrier should exist at the α-NPD/CBP interface. However, the
phosphorescent dopant can contribute to charge transport in the EML,98,114
which would greatly
reduce the barrier at the HTL/EML interface; the HOMO of Ir(ppy)2(acac) (5.6 eV) is very close
Chapter 5 Organic/organic interface designs of OLEDs
65
to that of α-NPD. To isolate the influence of direct charge injection from α-NPD into the
Ir(ppy)2(acac) dopant, a device with a thin un-doped layer of CBP (3 nm) inserted between the
α-NPD layer and the EML (see device C in Fig. 5.11) was also fabricated as a control device.
Figure 5.13(a) shows the IV and LV characteristics of the three devices. The operating
voltage of device C is significantly higher than that of device B, which confirms that the thin
un-doped layer of CBP does indeed limit direct charge injection from α-NPD into the
Ir(ppy)2(acac) dopant (i.e., there is a much larger energetic barrier at the α-NPD/CBP interface
in device C). It is worth noted that this is different from the fluorescent device as discussed in
the Section 5.2 (see Fig. 5.1), where the driving voltage of the device with the 3 nm CBP
interlayer does not change. The major difference is that in the fluorescent device, there is
already a very large hole injection barrier at the α-NPD/Alq3 interface (without CBP interlayer)
and therefore the introduced barrier at α-NPD/CBP interface in the device with CBP interlayer
has little impact on the driving voltage.
Figure 5.13(b) shows the EL spectra at 5 mA/cm2
of the three devices. Although the EL
spectra are nearly identical the efficiency roll-off of the three devices is markedly different (see
Fig. 5.14). Figure 5.14 shows the current efficiency of the three devices as a function of current
density. The EL spectra at different current densities are shown as the inset; the spectra are
normalized to the Ir(ppy)2(acac) peak at ~ 521 nm and have been enlarged by 30 times in the
region of 400-490 nm to highlight the fluorescence from α-NPD. Although the efficiency of the
three devices is similar at low current density (< 0.5 mA/cm2) the efficiency roll-off of devices
with α-NPD as the HTL (device B and C) is markedly worse than device A. Although this trend
might indicate reduced triplet-triplet annihilation and triplet-polaron quenching in device A, it
has been shown that these quenching processes take effect only at much higher current densities
(~1000 mA/cm2).
104
Also, the fluorescence from α-NPD observed from B and C at 50 mA/cm2 (see the inset of
Fig. 5.14) may suggest that the roll-off is due to the exciton “loss” to α-NPD, as the electrons
“leak” through the EML. However, no fluorescence from α-NPD is observed at 5 mA/cm2
where significant efficiency roll-off already takes place in B and C, which rules out exciton
“loss” to α-NPD as the major cause of the efficiency roll-off at high luminance. Recently, it has
Chapter 5 Organic/organic interface designs of OLEDs
66
been shown that loss of charge balance at high current density results in significant efficiency
roll-off.104
The fluorescence from α-NPD supports this hypothesis as it indicates excess
electrons “leaking” through the EML, a surefire indication of electron-hole imbalance. The
origin of this imbalance is most likely due to the blocking of holes at the α-NPD/CBP interface
in device B and C with a large energetic barrier. The efficiency roll-off is therefore less in device
A since there is no barrier between the doped and un-doped CBP regions for holes to be blocked
at. In fact, in the case of device A, the EQE increases from 0.5 mA/cm2 to 2.0 mA/cm
2
suggesting that the charge balance is actually improving over this range. This improvement in
charge balance therefore helps to suppress the efficiency roll-off.
10-3
10-2
10-1
100
101
102
0
20
40
60
80
100
120
140
160
400 440 480
EL
(a
.u.)
Wavelength (nm)
@ 50 mA/cm2
400 440 480
Wavelength (nm)
EL
(a.u
.)
Device A
Device B
Device C
Cu
rre
nt E
ffic
ien
cy (
cd
/A)
Current Density (mA/cm2)
@ 5 mA/cm2
Figure 5.14 Current efficiency of device A, B and C. The insets are the enlarged EL spectra (by 30 times in the
range of 400-490 nm) that are measured at 5 and 50 mA/cm2.
In summary, a simplified high efficiency bi-layer green PHOLED with EQE of > 20% up to
a high luminance of > 10,000 cd/m2 is demonstrated. This result was achieved without using any
Chapter 5 Organic/organic interface designs of OLEDs
67
discrete blocking layers or electrical doping, which greatly simplifies the device design. The
simplified devices are also shown to have comparable power efficiency to state of the art p-i-n
PHOLEDs. The high efficiency and low roll-off of the devices are attributed to the reduced
charge carrier accumulation and thus improved electron-hole balance, which is a result of using
CBP as both the HTL and host for the phosphorescent emitter. This simplified device design
strategy represents a pathway towards high efficacy OLEDs and should be applicable to other
phosphorescent emitters as well as white OLEDs.
5.4. Summary
In summary, it was demonstrated that exciton quenching due to accumulated space charges
at the exciton formation interface remains a significant loss of efficiency in existing device
designs. It was found that exciton quenching from α-NPD+ radical cations can be reduced by
using a wide band gap hole-transporting interlayer at the α-NPD/EML interface, resulting in
increased device efficacy. It was also found that an interfacial dipole at the interface between an
HTL and ETL molecule correlates well with the position of the exciton formation zone. The
findings of this work should have broad implications in materials selections in the design of
EML structures. It was further demonstrated that using CBP as a single layer HTL eliminates
the hole blocking barriers, preventing hole accumulation anywhere in the device. Exciton
quenching due to accumulated radical cations is thus eliminated, resulting in significantly
improved device performance in both fluorescent and phosphorescent OLEDs. It is envisioned
that this simple non-blocking design of the exciton formation zone represents a simple pathway
to achieve high performance OLEDs.
Chapter 6 Optical designs of OLEDs
68
Chapter 6 Optical designs of OLEDs
In this chapter, an optical model describing the optical electric field of an OLED will be
presented. The developed model will be used to design high efficiency OLEDs with state of the
art performance. The key feature of the design is the replacement of ITO anode with an
oxide/metal/oxide electrode stack, which enables the use of low cost flexible plastic substrates.
The content of this chapter was published as J. Appl. Phys. 109, 053107 (2011) and Nature
Photon. 5, 753 (2011).
6.1. Introduction
It is generally believed that only ~20% of the light can be out-coupled from a standard bottom
emission OLED, resulting in a much smaller external quantum efficiency compared with the
internal efficiency. Most of the light in an OLED is suppressed by the wave guide modes in the
substrate and organic layers. Recently, various approaches have been proposed to increase the
out-coupling factor, e.g. using high-index substrates,21
top emission OLEDs taking advantage of
the micro-cavity effect,115-117
introducing distributed-Bragg-reflectors (DBR),118
and using micro
lenses or grids119,120
and photonic crystals.121
In order to optimize the optical out-coupling of light
to achieve maximum efficiency it is important to get an in-depth understanding of the optics
occurring in OLEDs. In this chapter a model using a dipole emission source term to describe the
optical electric field distribution inside and out of the stratified device structure is shown in detail.
The optical electric field in a stratified structure of homogeneous and isotropic media is described
by 2 × 2 matrices. Parameters such as reflectivity, transmittance and Poynting vector flux can be
obtained numerically, such that the emission pattern and optical out-coupling factor can be
calculated as a function of the refractive index and the thickness of different layers. Furthermore,
OLEDs with a weak micro-cavity are used to evaluate the model. A flexible OLED with very
high efficiency will be also shown as a case study of the optical model at the end of this chapter.
Chapter 6 Optical designs of OLEDs
69
6.2. Optical model of OLEDs
Typically, an OLED is made up of a number of organic thin films sandwiched between two
electrodes, e.g. ITO and Al. An OLED may therefore be considered as an emission source
embedded in a Fabry-Perot microcavity. The theoretical spectrum for light emission normal to
the plane of the device has previously been approximated following Deppe’s approach.118,122
Since this model only describes emission normal to the plane of the device it cannot be used to
calculate the angular dependence. Also, this method is over simplified for OLEDs as it does not
consider the dipole emission characteristic of organic emitters. Although dipole emission can be
well described by both semi-classical and quantum mechanical approaches, for example using
the Hertz vector and Sommerfeld integrals,123,124
these methods are too complicated. A more
convenient method to describe the source term for dipole emission was proposed by Benisty et
al, 125
based on the earlier work by Lukosz126-128
. In this secton, a simple but accurate model
using a dipole emission source term combined with the transfer matrix method will be presented
to address the exciton distribution in OLEDs. Also, details on the application of this theory will
also be discussed for OLEDs with an example (experimental data).
6.2.1. Theory
a. Source term for dipole emission
The spontaneous emission of an OLED is modeled by vertical and horizontal dipoles
embedded in a homogeneous and isotropic media. Lukosz proposed that the normalized power
density of the s- and p- polarized light emitted in direction k, for the vertical and horizontal
dipoles respectively, is,128
23sin
8
pP
, (6.1)
2
/ /
3cos
16
pP
, (6.2)
0sP , (6.3)
Chapter 6 Optical designs of OLEDs
70
/ /
3
16
sP
. (6.4)
In Eq. (6.1) – (6.4), is the emission angle, i.e. angle between the surface normal and the
wave vector k. Figure 6.1 shows the schematic diagram of a plane of dipoles embedded in a
homogeneous and isotropic media. With these power densities we can construct the source term
and use the standard matrix transfer method to describe the optical electric field distribution in
the stratified structure.
z
x
yz
x
y
Plane of dipolesPlane of dipoles
Figure 6.1 Schematic diagram of the dipole plane with vertical and horizontal dipoles.
b. Optical electric field distribution
Figure 6.2 shows the simplest case without an embedded source. A plane wave is incident
from layer j = 0 into the stratified structure. The thickness of layer j (j=1,2,…n) is dj. The
complex refractive index in each layer is j j jn n i . With the complex refractive index we
can unify the expression for an evanescent wave (e.g., propagation in the metal electrode with
strong damping). The optical electric field that is incident in and out of the system can be
written by using the total system transfer matrix as,
Chapter 6 Optical designs of OLEDs
71
0 1
1
00
n
n
E EM
E
, (6.5)
where the + and – superscripts denote the right- and left- going waves respectively, and M is the
product of the layer matrix Li (phase change) and interface matrix Iij (refraction),
1 01 1 12 2 1k k kM I L I L L I
. (6.6)
In Eq. (6.6), the interface matrix can be obtained from the reflection (ij
r ) and transmission
(ij
t ) coefficients at interface i/j,
1/ /
/ 1/
ij ij ij
ij
ij ij ij
t r tI
r t t
. (6.7)
The reflection and transmission for s- and p- polarized light can be calculated using the Fresnel
equations as,
2 2
2 2
ˆ ˆ ˆ ˆcos cos
ˆ ˆ ˆ ˆcos cos
j i i i j jp
ij
j i i i j j
n n n nr
n n n n
, (6.8)
2
2 2
ˆ ˆ2 cos cos
ˆ ˆ ˆ ˆcos cos
i j i jp
ij
j i i i j j
n nt
n n n n
, (6.9)
ˆ ˆcos cos
ˆ ˆcos cos
j j i is
ij
j j i i
n nr
n n
, (6.10)
ˆ2 cos
ˆ ˆcos cos
s i i
ij
j j i i
nt
n n
. (6.11)
Chapter 6 Optical designs of OLEDs
72
0 1 j n
0 0( , )E E
0k
jk
jk
0 j
1n
1d jd nd
'x
( '), ( ')j jE x E x
n+1
n
Index:
0k
1( ,0)nE
1nk
x
Figure 6.2 Schematic diagram of a multilayer structure with n+1 layers.
The phase matrix in Eq. (6.6) can be written as,
2ˆ cos
2ˆ cos
0
0
i i i
i i i
i n d
ii n d
eL
e
, (6.12)
where is the wavelength of the light. The optical electric field distribution inside the
stratified structure is generally of greater interest and can also be obtained by the matrix transfer
method. For example, as shown in Fig. 6.2, when the layer j is considered, the electric field at x
can be expressed as follows,
2ˆ cos
0
2ˆ cos
0
( )0
( )0
j j
j j
i n x
j
ji n x
j
E xE eM
E xEe
. (6.13)
Now that we have addressed the optical electric field distribution with the transfer matrix
method, we must now consider how to deal with the embedded emission source. We firstly
assume that the emission is confined to be at one interface, i.e. at the hole transport layer
(HTL)/electron transport layer (ETL) interface, and ignore the exciton distribution. Therefore at
the emission interface, the optical electric field is discontinuous (see Fig. 6.3 a) and can be
Chapter 6 Optical designs of OLEDs
73
obtained by adding the source term as,
a b
a b
E EA
E A E
. (6.14)
In Eq. (6.14) ,
aE
and ,
bE
can be obtained from,
( )
0
0aa
a
EM
E E
, (6.15a)
1( )
0
b nb
b
E EM
E
, (6.15b)
where ( )aM and
( )bM are the system matrices on the left and right side of the emission
interface (see Fig. 6.3), i.e.,
( ) ( )
11 12( )
( ) ( )
21 22
a a
a
a a
m mM
m m
, (6.16a)
( ) ( )
11 12( )
( ) ( )
21 22
b b
b
b b
m mM
m m
. (6.16b)
From Eq. (6.14) to (6.16), with simple algebra, we can derive,
( ) ( ) ( ) ( )
( ) 22 11 11 21
0 21 ( )( )( ) ( ) 1112
21 11 ( )
12
a a b b
a
aab b
b
m m m A m AE m
mmm m
m
, (6.17)
where the source term ,A
can be expressed by the normalized power density P shown in Eq.
(6.1)-(6.4) (for s- or p- polarized) as,
,/ / ,/ /A P
, (6.18a)
Chapter 6 Optical designs of OLEDs
74
,/ / ,/ /A P
. (6.18b)
In an actual OLED device, however, there will be an exciton distribution and the
distribution can be incorporated by using superposition. To model the optical electric field, we
discretize the source distribution into H blocks with a thickness of dx=demission/H (see Fig. 6.3
b), where demission is the total thickness of the exciton distribution region. At the bh sub-layer
( 1,2,...,h H ), the source term can be substituted by
, ,
( ) ,/ / ,/ /( ' )
hA A D x hdx
, (6.18)
where ( ')D x is the exciton distribution function. To reflect the discrete nature of organic
materials, we use 1dx nm, the typical size of a small molecule (or sites of a polymer).
Similar to Eq. (6.16a) and (6.16b), the transfer matrix on the left and right side of the bh/bh+1
interface becomes respectively
( ) ( )
11 12( )
( ) ( )
21 22
a a
h ha
h a a
h h
m mM
m m
, (6.19a)
( ) ( )
11 12( )
( ) ( )
21 22
b b
h hb
h b b
h h
m mM
m m
. (6.19b)
Similar to Eq.(6.17), we have,
( ) ( ) ( ) ( )
( ) 22 11 11 21
( )0 21 ( )( )( ) ( ) 1112
21 11 ( )
12
a a b b
a h h h h
h h aab b hh
h h b
h
m m m A m AE m
mmm m
m
, (6.20)
Therefore, with Eq.(6.20) we can calculate the total emission from the contribution of
excitons at different location with different emission strength. It is noted that the one important
assumption behind the superposition is that the sources are not coherent. However it does not
negate the ability of coherent interference between the source and its reflections.129
Moreover,
the study of the location and distribution of the emission zone is another interesting scientific
Chapter 6 Optical designs of OLEDs
75
issue, which is highly dependent on the carrier concentration, i.e. the distribution of holes
injected from the anode and electrons injected from the cathode, and how they recombine, and
how the excitons diffuse. A simple uniform distribution130,131
of exciton across the doping region
will be employed in the calculation to describe the experimental EL spectra in the next section.
Since in a guest-host system, the dopants (guest), for example C545T in this work, may also
serve as hole traps,132,133
it is still a reasonable approximation to assume the uniform
distribution, which also gives a reasonably good description of the experimental EL spectra.
However, consideration of different exciton distributions can be easily accomodated by
changing the distribution function ( ')D x .
source
0
0
E
1
0
nE
a
a
E
E
b
b
E
E
( )aM( )bM
(a)
source
0
0
E
1
0
nE
( )aM ( )cM
a
a
E
E
c
c
E
E
1b 2b
1
1
b
b
E
E
1
1
'
'
b
b
E
E
2
2
b
b
E
E
2
2
'
'
b
b
E
E
(b)
hb
bh
bh
E
E
'
'
bh
bh
E
E
x
x
Figure 6.3 Schematic diagram of a multilayer structure with embedded source plane.
Chapter 6 Optical designs of OLEDs
76
c. EL spectrum and device efficiency
Experimentally, we do not directly measure the optical electric field but the power instead.
Therefore, in order to relate the emission pattern and the efficiency of an OLED with the
calculations, we have to express the output power in the form of the dipole source terms. The
irradiance (the power per unit projected area) can be calculated by the Poynting vector flux as,
2~ | |d
I n EdS
, (6.21)
where is the power and S is the projected area.
The EL spectrum is proportional to the radiant intensity R
I (the power per solid angle).
Both the projected area and the solid angle will change when the light is propagating through the
stratified structure. Therefore, to extract the emission pattern, we have to consider all these
changes,
2
,
2
,
ˆ | | cos
ˆ | | cos
out
R out out out out active out out out active
activeR active active active out active active active out
active
d
I d I dS d n E d
dI I dS d n E d
d
, (6.22)
where the subscript “active” and “out” correspond to the active layer (i.e., emission zone) and
outside environment respectively. The change of solid angle has been derived in Ref. 127 as,
2
2
ˆ cos
ˆ cos
active out out
out active active
d n
d n
. (6.23)
Substituting Eq. (6.23) into (6.22), we can obtain,
3 2 2
,
3 2 2
,
ˆ cos | |
ˆ cos | |
R out out out out
R active active active active
I n E
I n E
. (6.24)
The emission power has contributions from both the vertical and horizontal dipoles. For an
isotropic source,125,128
Chapter 6 Optical designs of OLEDs
77
/ /2
3 3total
P PP . (6.25)
This ratio should be applied to Eq. (6.24) when the normalized emission pattern is calculated.
Also, as mentioned previously, when a detailed exciton distribution is considered, the
contribution from excitons at different locations and different intensity should also be summed
up. The ,R active
I in Eq. (6.24) can be considered as the PL spectrum of the emitter in the OLED. It
is noted that the assumption behind such calculation is that the electrical properties do not
change when bias is applied, e.g. the band gap does not change at different bias. Also, the
typical width of OLEDs emission is ~ 75 nm which corresponds to a coherence length of ~ 103
nm.129
Given that the thickness of the substrate, e.g. glass, is ~106 nm, orders of magnitude
larger than the coherence length, the calculation here ignores the far field interference in the
glass, i.e. assumes that the electric field exiting the glass/organic interface is the same as the one
incident at the glass/air interface.
d. Normalized OLED efficiency
The last step is to consider the photopic response of the human eye, i.e., to convert the
radiance to luminance, which can be simply accomplished by integrating the photopic response
over wavelength,
0
( )v e
L C L P d
, (6.26)
where P is the photopic response function, e
L and v
L denotes the radiance and luminance
respectively and0
C is a constant (683 lm/W). Therefore, with Eq. (6.26) and (6.24) the
normalized current efficiency of an OLED can be calculated. In other words, the optical
out-coupling of different device structures can be calculated. Also, to calculate the normalized
power efficiency, the luminance should be integrated over emission angle as,
/2
02 sin
vW I d
, (6.27)
where Iv is the luminoux intensity (luminous flux per unit solid angle).
Chapter 6 Optical designs of OLEDs
78
6.2.2. Evaluation of the model
Here, OLEDs with weak micro-cavity are used to evaluate the theoretical model, and the
structure of which is: Au (21 nm) / MoOx (1 nm) / CBP (x nm) / Alq3:C545T (33 nm) / Alq3 (33
nm) / LiF (1 nm) / Al (100 nm). Figure 6.4 shows the EL spectrum of the OLED devices with
different thicknesses of CBP HTL. The symbols are the experimental measurements while the
lines are the corresponding theoretical calculations using the present model. The calculation is
conducted by assuming that the excitons are uniformly distributed in the C545T doped region
(thickness of 33 nm). The calculation shows excellent agreement with the experiment results.
From the figure, we can see that when the CBP thickness increases, the EL spectrum is
broadened and red shifted.
400 500 600 700
CBP Thickness
25 nm
33 nm
44 nm
57 nm
68 nm
EL (
a.u
.)
Wavelength (nm)
Figure 6.4 Experimental EL spectra of OLEDs with different thickness of CBP measured normal to the
substrate (open symbols) as well as the corresponding theoretical calculations (solid lines). The PL spectrum
of C545T doped Alq3 used in the calculation is also shown for comparison as dashed line.
Although it is generally believed that the thickness of the ETL is much more sensitive than
that of the HTL to the device performance. However, it is only true when the microcavity effect
Chapter 6 Optical designs of OLEDs
79
is very weak, e.g., in devices with ITO anode. In this study, the reflectivity of Au anode is much
larger than ITO and the device with Au anode exhibits a much stronger cavity effect (although it
is still considered a weak microcavity9). Therefore, the thickness of CBP is also very sensitive to
the EL spectrum. This also implies that the out-coupling factor of the OLED with weak
microcavity is also highly sensitive to the device thickness, i.e. at some wavelengths it may be
enhanced while at others it may be suppressed due to the wavelength dependent interference. It
is believed118
with a strong microcavity, such as a DBR mirror, that along the substrate normal
the emission spectrum may be much narrowed and the emission can be enhanced by several
times. However, at different observation angle, the enhancement may be reduced.
6.3. Flexible OLEDs
The research community has focused on using high refractive index (n ≥ 1.8) substrates in
place of standard glass to enhance the out-coupling of this trapped light.20,21,134
However, one of
the major advantages of OLEDs is the use of lightweight flexible plastic substrates,135
which
unfortunately have a low refractive index (n ≤ 1.6) similar to that of standard glass. In this
section, a novel out-coupling enhancement approach that does not rely on high-index substrates
and is fully compatible with low cost flexible plastic substrates as well as regular glass will be
shown as an example on how the optical model developed can be used to design OLED devices.
Using this out-coupling enhancement a flexible green OLED on plastic with an exceptionally
high efficiency is demonstrated.
The key feature of this out-coupling strategy is the replacement of de facto standard ITO
anode with an oxide/metal/oxide anode stack, which altogether eliminates the need for a high
refractive index substrate and hence enables the use of flexible plastic substrates (see Fig 6.5).
The electrode stack consists of a semi-transparent Au thin film sandwiched between a layer of
Ta2O5 and MoO3 (i.e., Ta2O5/Au/MoO3). In the electrode stack the Au layer serves as the
conductive channel for charge transport. Although metal thin films have been extensively
studied as alternative anodes in OLEDs, most research has focused on top-emission
architectures, which incorporate a strong optical micro-cavity between the highly reflective
metal anode and Al cathode.136
Although a strong optical micro-cavity can be used to enhance
Chapter 6 Optical designs of OLEDs
80
the out-coupling of light in the forward direction, the EL emission spectrum is typically
narrowed and also exhibits a strong dependence on viewing angle,118
which is undesirable for
many applications. Therefore, in this electrode stack a thin semi-transparent layer of Au is
adopted, which forms only a weak optical micro-cavity with the Al cathode, and hence does not
narrow the EL spectrum or contribute to an angle dependent EL emission.9
Figure 6.5 Schematic OLED device structure with flexible plastic substrate.
Although semi-transparent metal thin films, such as Au, are well recognized as ideal
candidates to replace ITO, particularly for flexible devices on plastic substrates,9 there remain
two key challenges to overcome: i) injecting charge into organic materials from metals is
problematic due to a strong interfacial dipole formed at the metal/organic interface, and ii)
optimizing the weak optical micro-cavity formed between the semi-transport metal anode and
highly reflective Al cathode is extremely difficult without altering the electrical properties of the
device. In our electrode stack the MoO3 and Ta2O5 layers are employed to solve these two key
challenges, respectively.
It is well known that a strong interfacial dipole is formed at most metal/organic
interfaces.44
This dipole typically hinders the injection of charge from the metal into the organic,
resulting in very high operating voltages in devices. Many different strategies have been
proposed to reduce the interfacial dipole between Au and commonly used hole transport layers
(HTLs), such as N,N'-diphenyl-N,N'-bis-(1-naphthyl)-1-1'-biphenyl-4,4'-diamine (α-NPD), for
Chapter 6 Optical designs of OLEDs
81
example by introducing a layer of C60 at the interface,39
(i.e., Au/C60) or by oxidizing the surface
of the metal9 (i.e., Au/AuOx). However, these techniques are only effective for molecules like
α-NPD with a very shallow HOMO of only 5.4 eV. In Chapter 5, it has been demonstrated that
using a deep HOMO HTL such as CBP can significantly enhance the performance of OLEDs.
Clearly, a new strategy to enhance the injection of charge from Au into the deep HOMO of CBP
is thus required in order to make use of this state of the art device structure (i.e., to maximize the
IQE). In Chapter 4 and Chapter 5, it has been shown to use a thin layer of a transition metal
oxide, such as MoO3, to enhance the work function of anode. The knowledge gained in Chapter
4 and Chapter 5 is used here in the electrode stack to overcome this challenge, i.e. directly
injecting charge into CBP.
The traditional method of tuning the micro-cavity in an OLED is to change the thickness
of the organic layers. However, such tuning is problematic since changing the thickness of the
organic layers will also simultaneously impact the electrical characteristics of the device (e.g.,
the electron-hole balance reaching the emissive layer which directly influences the IQE). The
use of p- and n-doped transport layers has been shown as an effective method of decoupling the
optical and electric performance of OLEDs,20,21
allowing fine tuning of the optical modes
without significantly influencing the electrical characteristics of the device. However, using
doped transport layers significantly complicates the device fabrication and requires numerous
additional blocking layers. We thus elected to use a high refractive index layer Ta2O5 in the
electrode stack to tune the optical micro-cavity (i.e., to maximize the out-coupling) without
having to alter the structure of the organic layers or influence the electrical characteristics of the
device.
The device structure is then: Substrate / Anode / CBP (40 nm) / CBP: Ir(ppy)2(acac) (8
wt%, 15 nm) / TPBi (10 nm) / 3TPYMB (60 nm) / LiF (1 nm) / Al (100 nm). The normalized
luminance using the Ta2O5/Au/MoO3 stacked electrode was simulated based the optical model
presented in Section 6.2. Figure 6.6 shows the enhancement in the out-coupling of light from a
device utilizing the Ta2O5/Au/MoO3 electrode, relative to a reference device with MoO3
modified ITO electrode. A peak enhancement in the out-coupling of ~2 times is found for a Au
thickness of d Au = 18 nm, which implies that the device with the Ta2O5/Au/MoO3 electrode has
Chapter 6 Optical designs of OLEDs
82
the potential to be nearly twice as efficient as a standard ITO based device. Note that the organic
layer thicknesses for both devices were identical and were based on the optimized structure for
the ITO device.
Figure 6.6 Calculated enhancement ratio of the Ta2O5/Au/MoO3 electrode relative to ITO as a function of the
thickness of both Au and Ta2O5.
An OLED device was fabricated on a flexible polycarbonate substrate according to
optimization from the calculation (i.e., 70 nm Ta2O5 and 18 nm Au). Figure 6.7 a and b
summarize the measured EQE and power efficiency of the device fabricated in comparison to a
reference device fabricated on ITO coated glass. Remarkably the Ta2O5/Au/MoO3 device on
flexible plastic reaches a high EQE of ~40% EQE without the use of a high refractive index
substrate. However, the out-coupling enhancement is lower than that predicted by the optical
model, which is likely due to the slightly different hole injection from the Au/MoO3 electrode
than from ITO/MoO3, which is clearly shown in the different IV characteristics in the single
carrier hole only device as shown in Fig. 6.8. A similar phenomenon has also been reported for
En
ha
nce
nm
en
t ra
tio
dAu
(nm)d
Ta2O5 (nm)
Chapter 6 Optical designs of OLEDs
83
other metal/oxide/organic interfaces that the work function of the underneath material (for
example the ITO and Au here) has an impact on the effective work function of the electrode.45,95
In other words, the discrepancy between the simulation and experimental results is most likely
due to a subtle difference in the electrical properties (i.e., the IQE) of the devices, which is not
accounted for in the optical simulation.
Figure 6.7 (a) External quantum efficiency (EQE) and (b) Power efficiency (PE) of the device structure
optimized for Ir(ppy)2 (acac) as a function of luminance.
1 10 100 1000 10000
20
40
60
80
100
Glass/ITO
Plastic/Ta2O
5/Au
Plastic/Ta2O
5/Au (out-coupling)
E
QE
(%
)
Luminance (cd/m2)
(a)
1 10 100 1000 1000010
100
Pow
er
Effic
iency (
lm/W
)
Luminance (cd/m2)
Glass/ITO
Plastic/Ta2O
5/Au
Plastic/Ta2O
5/Au
(out-coupling)(b)
Chapter 6 Optical designs of OLEDs
84
Figure 6.8 Current density as a function of average electric field of CBP single carrier hole only device using
Au/MoO3, ITO/MoO3 and Au anodes. The anode modified by MoO3 enables good hole injection into CBP.
The inset is the same data for Au/MoO3 and ITO/MoO3 plotted on a log-linear scale. Clearly, the injection
from Au/MoO3 is better than from ITO/MoO3.
Not only does the novel Ta2O5/Au/MoO3 electrode design increases the efficiency, it also
enables the unique flexible form factor in OLEDs. Figure 6.9 shows a photograph of a large area
(50 mm × 50 mm) working device on flexible plastic at high luminance. Although the electrode
enhances the out-coupling of light, a significant fraction of the emitted photons are still trapped
in the substrate modes (i.e., the plastic substrate). To further improve the EQE of the device a
lens-based structure can be used to out-coupling the light trapped in the substrate modes,
resulting in a further enhancement of the efficiency by a factor of 1.55. As a result, the highest
EQE and power efficiency reaches 63% and 290 lm/W respectively (see Fig. 6.7), which is
equivalent to an enhancement of ~2.5 times over the ITO reference device. Although the further
extraction of light from the substrate modes using a lens is not fully compatible with flexible
devices, this issue can be easily overcome by using a plastic substrates with pre-molded half
sphere micro-lenses.3,27
0.2 0.4 0.6
0
10
20
30
40
0.2 0.3 0.4 0.5 0.6
10-1
100
101
Cu
rre
nt
De
nsity (
mA
/cm2)
Electric Field (MV/cm)
Au/MoO3
ITO/MoO3
Au
Cu
rre
nt D
ensity (
mA
/cm
2)
Electric Field (MV/cm)
Chapter 6 Optical designs of OLEDs
85
Figure 6.9 Photograph of a flexible OLED (50 mm × 50 mm) at high luminance.
6.4. Summary
In summary, an optical model describing the optical electric field of an OLED has been
developed based on the dipole emission model and the matrix transfer method. Experimentally,
OLEDs with weak microcavity were fabricated. The theoretical calculation was in good
agreement with experimental results. At the end, by using the model, high efficiency OLEDs
with state of the art performance were designed and achieved on flexible plastic substrates. The
unique out-coupling strategy developed here may help to unlock the full potential of OLEDs on
flexible plastic and to enable the low cost mass-production of flexible OLEDs using roll-to-roll
processing for next generation flexible displays and solid state lighting.
Chapter 7 Summary and future work
86
Chapter 7 Summary and future work
7.1. Summary
In Chapter 3, energy level alignment at metal/organic interfaces was systematically studied.
It was shown that interface dipole theory can accurately describe the energy level alignment at
metal/organic interfaces. Furthermore, examples at metal/ C60/organic interfaces were shown
and the Fermi level at metal/C60/organic interfaces was found to be pinned to the charge
neutrality level of C60 (~ 4.7 eV). This phenomena was attributed to the C60 interlayer disrupting
the interfacial dipole at the metal/organic interfaces, resulting in more favorable energy-level
alignment for hole injection. As a result holes were injected directly into the organic layer,
bypassing the deep HOMO of C60.
In Chapter 4, hole injection from different metal oxides into α-NPD was systematically
studied in single carrier hole-only devices yielding a IV database for variable injection barrier
heights. It was found that the quasi-Ohmic regime is much larger (i.e., covers a greater range of
injection barrier height) than was previously expected. A criterion that defined Ohmic,
quasi-Ohmic, and injection limited contacts was quantified based on a time-domain simulation
of charge transport across α-NPD single carrier devices. This criterion included the effects of the
electric field dependent mobility, organic layer thickness and charge injection barrier height.
The effects of the built-in potential on the IV characteristics were also evaluated. A
barrier-thickness-voltage “phase” diagram that defined the regions of SCLC, quasi-Ohmic and
ILC for α-NPD was presented.
In Chapter 5, it was demonstrated that exciton quenching due to accumulated space charges
at the exciton formation interface was the cause of the significant loss of efficiency in existing
device designs. It was found that exciton quenching from α-NPD+ radical cations can be reduced
by using a wide band gap hole-transporting interlayer at the α-NPD/EML interface, resulting in
increased device efficacy. It was further demonstrated that using CBP as a single layer HTL
eliminates the hole blocking barriers, preventing hole accumulation anywhere in the device.
Exciton quenching due to accumulated radical cations is thus eliminated, resulting in
Chapter 7 Summary and future work
87
significantly improved device performance in both fluorescent and phosphorescent OLEDs. The
essence of the design concept proposed in this chapter was the elimination of any redundant
“injection layer” and “blocking layer”, contrary to the traditional device design concepts, which
significantly simplified the device structure and enhance the device performance.
Finally, in Chapter 6, an optical model describing the optical electric field of an OLED was
developed. Theoretical calculations from the developed model were compared with the device
performance and were in good agreement with experimental results. By using the model, high
efficiency OLEDs with state of the art performance were designed and realized. The key feature
of the design was the replacement of an ITO anode with an oxide/metal/oxide electrode stack,
which enabled the use of low cost flexible plastic substrates.
7.2. Future work
Various findings of this thesis have attracted considerable interest from major flat-panel
display manufacturers and are being commercialized by a spin-off company, OTI Lumionics Inc.
Although the findings have been seen to be useful in display application, continuous work has to
be conducted for real device applications, not only in flat-panel display, but also solid state
lighting (SSL), another important application of OLEDs.
The most significant contribution of my thesis is the achievement of a highly simplified
OLED structure with an exceptionally high EQE at high luminance. However, only a single
color (green) has been demonstrated to achieve such high efficiency. OLEDs of other colors
such as white with similarly high efficiency are yet to be demonstrated. In fact, white OLEDs
are of greater significance for both display (i.e. white light plus color filter) and lighting
applications. More investigations have to be done to test if the same device design concept in
this thesis can be used in white OLEDs. For example, the triplet energy level of CBP is smaller
than most phosphorescent blue emitters, i.e., the CBP host may not be capable to confine the
excitons formed in the blue emitters, resulting in a lower efficiency in both blue and white
OLEDs. There are two possible ways to solve this problem. One is to use another host material
that is capable to transport holes with even deeper HOMO (larger triplet energy level) to replace
CBP as both host and HTL. The challenge is that an anode with even higher work function is
Chapter 7 Summary and future work
88
needed to inject sufficient holes into this host material. The second is to recycle the “unconfined
excitons”. For example, the device can be designed to transfer the energy to the lower energy
emitters such as green and red, so that the overall efficiency of the white OLEDs can remain
high.
Another direction of the future work would be the further development of semitransparent
metal anodes. Although Au anode has a relatively low absorption window for green emission,
the absorption in both red and blue is much larger, i.e. it is challenging to achieve a similarly
high efficiency in red and blue by using Au anode. Moreover, generally the adhesion of Au on
glass and other plastic substrate is poor. Some interlayer such as a thin layer of Ni may be
needed to increase the adhesion. Such interlayer will change the optical properties of the Au
anode and the device structure may have to be re-optimized.
References
90
References
1 W. Helfrich and W. G. Schneider, Phys. Rev. Lett. 14, 229 (1965).
2 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987).
3 M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, and S. R.
Forrest, Nature 395, 151 (1998).
4 S.-J. Su, Y. Takahashi, T. Chiba, T. Takeda, and J. Kido, Advanced Functional Materials 19,
1260 (2009).
5 B. X. Mi, P. F. Wang, Z. Q. Gao, C. S. Lee, S. T. Lee, H. L. Hong, X. M. Chen, M. S. Wong,
P. F. Xia, K. W. Cheah, C. H. Chen, and W. Huang, Adv. Mater. 21, 339 (2009).
6 S.-J. Su, T. Chiba, T. Takeda, and J. Kido, Adv. Mater. 20, 2125 (2008).
7 Z. Q. Gao, M. Luo, X. H. Sun, H. L. Tam, M. S. Wong, B. X. Mi, P. F. Xia, K. W. Cheah,
and C. H. Chen, Adv. Mater. 21, 688 (2009).
8 K. R. Choudhury, J. Lee, N. Chopra, A. Gupta, X. Jiang, F. Amy, and F. So, Advanced
Functional Materials 19, 491 (2009).
9 M. G. Helander, Z.-B. Wang, M. T. Greiner, Z.-W. Liu, J. Qiu, and Z.-H. Lu, Adv. Mater.
22, 2037 (2010).
10 Z. W. Liu, M. G. Helander, Z. B. Wang, and Z. H. Lu, Appl. Phys. Lett. 94, 113305 (2009).
11 C. W. Tang, S. A. VanSlyke, and C. H. Chen, J. Appl. Phys. 65, 3610 (1989).
12 L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett. 70, 152 (1997).
13 M. G. Helander, Z. B. Wang, L. Mordoukhovski, and Z. H. Lu, J. Appl. Phys. 104, 094510
References
91
(2008).
14 C. Adachi, M. A. Baldo, S. R. Forrest, and M. E. Thompson, Appl. Phys. Lett. 77, 904
(2000).
15 http://www.universaldisplay.com/default.asp?contentID=604
16 http://www.osa-direct.com/osad-news/474.html
17 T. Stefano, Nature 459, 312 (2009).
18 Bardsley Consulting, Navigant Consulting Inc., Radcliffe Advisors Inc., SB Consulting,
Solid State Lighting Services Inc., Solid-State Lighting Research and Development:
Multi Year Program Plan (2011)
19 J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, K. Leo, and S. Liu, Appl. Phys. Lett. 80,
139 (2002).
20 S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, and K. Leo, Nature
459, 234 (2009).
21 S. Mladenovski, K. Neyts, D. Pavicic, A. Werner, and C. Rothe, Opt. Express 17, 7562
(2009).
22 M. G. Helander, Z. B. Wang, M. T. Greiner, J. Qiu, and Z. H. Lu, Rev. Sci. Instrum. 80,
033901 (2009).
23 S. Forrest, D. Bradley, and M. Thompson, Adv. Mater. 15, 1043 (2003).
24 M. T. Greiner, M. G. Helander, Z. B. Wang, and Z. H. Lu, Rev. Sci. Instrum. 80, 125101
(2009).
25 M. P. Seah, Surf. Interface Anal. 31, 721 (2001).
26 H. Bässler, Phys. Status Solidi B 175, 15 (1993).
References
92
27 J. C. Scott, J. Vac. Sci. Technol. A 21, 521 (2003).
28 W. Mönch, Appl. Phys. Lett. 88, 112116 (2006).
29 A. M. Cowley and S. M. Sze, J. Appl. Phys. 36, 3212 (1965).
30 J. Tersoff, Phys. Rev. B 32, 6968 (1985).
31 M. G. Helander, Z. B. Wang, and Z. H. Lu, Appl. Phys. Lett. 93, 083311 (2008).
32 Y. Y. Yuan, S. Han, D. Grozea, and Z. H. Lu, Appl. Phys. Lett. 88, 093503 (2006).
33 X. D. Feng, C. J. Huang, V. Lui, R. S. Khangura, and Z. H. Lu, Appl. Phys. Lett. 86,
143511 (2005).
34 J. Y. Lee, Appl. Phys. Lett. 88, 073512 (2006).
35 P. Peumans and S. R. Forrest, Appl. Phys. Lett. 79, 126 (2001).
36 S. Uchida, J. Xue, B. P. Rand, and S. R. Forrest, Appl. Phys. Lett. 84, 4218 (2004).
37 G. J. Matt, T. B. Singh, N. S. Sariciftci, A. M. Ramil, and H. Sitter, Appl. Phys. Lett. 88,
263516 (2006).
38 A. Kanwal and M. Chhowalla, Appl. Phys. Lett. 89, 203103 (2006).
39 S. Han, Y. Yuan, and Z. H. Lu, J. Appl. Phys. 100, 074504 (2006).
40 S. J. Chase, W. S. Bacsa, M. G. Mitch, L. J. Pilione, and J. S. Lannin, Phys. Rev. B 46,
7873 (1992).
41 A. Tamai, A. P. Seitsonen, F. Baumberger, M. Hengsberger, Z. X. Shen, T. Greber, and J.
Osterwalder, Phys. Rev. B 77, 075134 (2008).
42 B. W. Hoogenboom, R. Hesper, L. H. Tjeng, and G. A. Sawatzky, Phys. Rev. B 57, 11939
(1998).
References
93
43 R. Nouchi and I. Kanno, J. Appl. Phys. 97, 103716 (2005).
44 H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater. 11, 605 (1999).
45 M. G. Helander, Z. B. Wang, J. Qiu, and Z. H. Lu, Appl. Phys. Lett. 93, 193310 (2008).
46 H. Vazquez, W. Gao, F. Flores, and A. Kahn, Phys. Rev. B 71, 041306 (2005).
47 N. Hayashi, H. Ishii, Y. Ouchi, and K. Seki, Journal of Applied Physics 92, 3784 (2002).
48 S. C. Veenstra, A. Heeres, G. Hadziioannou, G. A. Sawatzky, and H. T. Jonkman, Applied
Physics A: Materials Science & Processing 75, 661 (2002).
49 Z. B. Wang, M. G. Helander, M. T. Greiner, J. Qiu, and Z. H. Lu, Appl. Phys. Lett. 95,
043302 (2009).
50 G. B. Murdoch, M. Greiner, M. G. Helander, Z. B. Wang, and Z. H. Lu, Appl. Phys. Lett.
93, 083309 (2008).
51 C.-W. Chu, S.-H. Li, C.-W. Chen, V. Shrotriya, and Y. Yang, Appl. Phys. Lett. 87, 193508
(2005).
52 H. Kanno, R. J. Holmes, Y. Sun, S. Kena-Cohen, and S. R. Forrest, Adv. Mater. 18, 339
(2006).
53 Y. Han, D. Yanfeng, Z. Zhiqiang, and M. Dongge, J. Appl. Phys. 101, 026105 (2007).
54 M. Toshinori, K. Yoshiki, and M. Hideyuki, Appl. Phys. Lett. 91, 253504 (2007).
55 T.-Y. Chu and O.-K. Song, Appl. Phys. Lett. 90, 203512 (2007).
56 C. Ganzorig, M. Sakomura, K. Ueda, and M. Fujihira, Appl. Phys. Lett. 89, 263501 (2006).
57 N. Huby, L. Hirsch, G. Wantz, L. Vignau, A. S. Barriere, J. P. Parneix, L. Aubouy, and P.
Gerbier, J. Appl. Phys. 99, 084907 (2006).
References
94
58 A. Ruchi, K. Pramod, G. Subhasis, and M. A. Kumar, Appl. Phys. Lett. 93, 073311 (2008).
59 P. W. M. Blom, C. Tanase, D. M. de Leeuw, and R. Coehoorn, Appl. Phys. Lett. 86, 092105
(2005).
60 M. Kiy, P. Losio, I. Biaggio, M. Koehler, A. Tapponnier, and P. Gunter, Appl. Phys. Lett. 80,
1198 (2002).
61 C. Herring and M. H. Nichols, Rev. Mod. Phys. 21, 185 (1949).
62 G. G. Malliaras, J. R. Salem, P. J. Brock, and J. C. Scott, J. Appl. Phys. 84, 1583 (1998).
63 M. A.Lampert and P. Mark, Current Injection in Solids. (Academic Press, 1970).
64 P. N. Murgatroyd, J. Phys. D: Appl. Phys. 3, 151 (1970).
65 S. W. Tsang, S. K. So, and J. B. Xu, J. Appl. Phys. 99, 013706 (2006).
66 C. H. Cheung, K. C. Kwok, S. C. Tse, and S. K. So, J. Appl. Phys. 103, 093705 (2008).
67 S. K. So, S. C. Tse, and K. L. Tong, J. Display Technol. 3, 225 (2007).
68 S. M. Sze, Physics of semiconductor devices. (Wiley-Interscience, 1981).
69 R. H. Fowler and L. Nordheim, Proc. R. Soc. Lond. A 119, 173 (1928).
70 V. I. Arkhipov, E. V. Emelianova, Y. H. Tak, and H. Bässler, J. Appl. Phys. 84, 848 (1998).
71 J. G. Simmons, Phys. Rev. Lett. 15, 967 (1965).
72 V. I. Arkhipov and H. Bassler, Appl. Phys. Lett. 77, 2758 (2000).
73 U. Wolf, V. I. Arkhipov, and H. Bässler, Phys. Rev. B 59, 7507 (1999).
74 J. C. Scott and G. G. Malliaras, Chem. Phys. Lett. 299, 115 (1999).
75 G. G. Malliaras and J. C. Scott, J. Appl. Phys. 85, 7426 (1999).
References
95
76 G. G. Malliaras and J. C. Scott, J. Appl. Phys. 83, 5399 (1998).
77 U. Wolf, S. Barth, and H. Bassler, Appl. Phys. Lett. 75, 2035 (1999).
78 Z. B. Wang, M. G. Helander, S. W. Tsang, and Z. H. Lu, Phys. Rev. B 78, 193303 (2008).
79 S. W. Tsang, M. W. Denhoff, Y. Tao, and Z. H. Lu, Phys. Rev. B 78, 081301 (2008).
80 M. A. Baldo and S. R. Forrest, Phys. Rev. B 64, 085201 (2001).
81 I. H. Campbell, T. W. Hagler, D. L. Smith, and J. P. Ferraris, Phys. Rev. Lett. 76, 1900
(1996).
82 C. E. Small, S.-W. Tsang, J. Kido, S. K. So, and F. So, Advanced Functional Materials, n/a
(2012).
83 H. Wang, K. P. Klubek, and C. W. Tang, Appl. Phys. Lett. 93, 093306 (2008).
84 S.-F. Chen and C.-W. Wang, Appl. Phys. Lett. 85, 765 (2004).
85 J. Kido and T. Matsumoto, Appl. Phys. Lett. 73, 2866 (1998).
86 K. R. Choudhury, J.-h. Yoon, and F. So, Adv. Mater. 20, 1456 (2008).
87 Y. T. Tao, E. Balasubramaniam, A. Danel, B. Jarosz, and P. Tomasik, Appl. Phys. Lett. 77,
1575 (2000).
88 L. C. Palilis, A. J. Makinen, M. Uchida, and Z. H. Kafafi, Appl. Phys. Lett. 82, 2209
(2003).
89 Y. Divayana, X. W. Sun, B. J. Chen, G. Q. Lo, C. Y. Jiang, and K. R. Sarma, Appl. Phys.
Lett. 89, 173511 (2006).
90 H.-T. Lu, C.-C. Tsou, and M. Yokoyama, Journal of Crystal Growth 277, 388 (2005).
91 Q. L. Song, C. M. Li, M. B. Chan-Park, M. Lu, H. Yang, and X. Y. Hou, Phys. Rev. Lett. 98,
References
96
176403 (2007).
92 P. I. Djurovich, E. I. Mayo, S. R. Forrest, and M. E. Thompson, Organic Electronics 10,
515 (2009).
93 Z. B. Wang, M. G. Helander, Z. W. Liu, M. T. Greiner, J. Qiu, and Z. H. Lu, Appl. Phys.
Lett. 96, 043303 (2010).
94 J. Meyer, S. Hamwi, T. Bulow, H. H. Johannes, T. Riedl, and W. Kowalsky, Appl. Phys.
Lett. 91, 113506 (2007).
95 M. G. Helander, Z. B. Wang, M. T. Greiner, J. Qiu, and Z. H. Lu, Appl. Phys. Lett. 95,
083301 (2009).
96 Z. B. Wang, M. G. Helander, M. T. Greiner, J. Qiu, and Z. H. Lu, Phys. Rev. B 80, 235325
(2009).
97 Y. Kawamura, K. Goushi, J. Brooks, J. J. Brown, H. Sasabe, and C. Adachi, Appl. Phys.
Lett. 86, 071104 (2005).
98 C. Adachi, M. A. Baldo, M. E. Thompson, and S. R. Forrest, J. Appl. Phys. 90, 5048
(2001).
99 S. Reineke, T. C. Rosenow, B. Lüssem, and K. Leo, Adv. Mater. 22, 3189 (2010).
100 S. J. Su, H. Sasabe, Y. J. Pu, K. i. Nakayama, and J. Kido, Adv. Mater. 22, 3311 (2010).
101 D. Tanaka, H. Sasabe, Y.-J. Li, S.-J. Su, T. Takeda, and J. Kido, Jpn. J. Appl. Phys. 46, L10.
102 M. A. Baldo, C. Adachi, and S. R. Forrest, Phys. Rev. B 62, 10967 (2000).
103 S. Reineke, K. Walzer, and K. Leo, Phys. Rev. B 75, 125328 (2007).
104 N. C. Giebink and S. R. Forrest, Phys. Rev. B 77, 235215 (2008).
References
97
105 Z. D. Popovic and H. Aziz, J. Appl. Phys. 98, 013510 (2005).
106 F. X. Zang, T. C. Sum, A. C. H. Huan, T. L. Li, W. L. Li, and F. Zhu, Appl. Phys. Lett. 93,
023309 (2008).
107 S. Y. Kim, W. S. Jeon, T. J. Park, R. Pode, J. Jang, and J. H. Kwon, Appl. Phys. Lett. 94,
133303 (2009).
108 S. Reineke, G. Schwartz, K. Walzer, and K. Leo, Appl. Phys. Lett. 91, 123508 (2007).
109 W. S. Jeon, T. J. Park, S. Y. Kim, R. Pode, J. Jang, and J. H. Kwon, Appl. Phys. Lett. 93,
063303 (2008).
110 V. I. Adamovich, S. R. Cordero, P. I. Djurovich, A. Tamayo, M. E. Thompson, B. W.
D'Andrade, and S. R. Forrest, Organic Electronics 4, 77 (2003).
111 Z. Gao, C. S. Lee, I. Bello, S. T. Lee, R.-M. Chen, T.-Y. Luh, J. Shi, and C. W. Tang, Appl.
Phys. Lett. 74, 865 (1999).
112 G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, and J. Salbeck, Appl.
Phys. Lett. 85, 3911 (2004).
113 S. Watanabe, N. Ide, and J. Kido, Jpn. J. Appl. Phys. 46, 1186 (2007).
114 W. H. Choi, C. H. Cheung, and S. K. So, Organic Electronics 11, 872 (2010).
115 L. H. Smith, J. A. E. Wasey, and W. L. Barnes, Appl. Phys. Lett. 84, 2986 (2004).
116 Q. Wang, Z. Deng, J. Chen, and D. Ma, Opt. Lett. 35, 462 (2010).
117 Q. Wang, Z. Deng, and D. Ma, Opt. Express 17, 17269 (2009).
118 A. Dodabalapur, L. J. Rothberg, R. H. Jordan, T. M. Miller, R. E. Slusher, and J. M.
Phillips, J. Appl. Phys. 80, 6954 (1996).
References
98
119 S. Moller and S. R. Forrest, J. Appl. Phys. 91, 3324 (2002).
120 Y. Sun and S. R. Forrest, Nature Photon. 2, 483 (2008).
121 Y.-C. Kim, S.-H. Cho, Y.-W. Song, Y.-J. Lee, Y.-H. Lee, and Y. R. Do, Appl. Phys. Lett. 89,
173502 (2006).
122 D. Deppe, C. Lei, C. Lin, and D. Huffaker, Journal of Modern Optics 41, 325 (1994).
123 R. R. Chance, A. Prock, and R. Silbey, The Journal of Chemical Physics 60, 2744 (1974).
124 G. W. Ford and W. H. Weber, Physics Reports 113, 195 (1984).
125 H. Benisty, R. Stanley, and M. Mayer, J. Opt. Soc. Am. A 15, 1192 (1998).
126 W. Lukosz and R. E. Kunz, J. Opt. Soc. Am. 67, 1607 (1977).
127 W. Lukosz and R. E. Kunz, J. Opt. Soc. Am. 67, 1615 (1977).
128 W. Lukosz, J. Opt. Soc. Am. 69, 1495 (1979).
129 A. Epstein, N. Tessler, and P. D. Einziger, Quantum Electronics, IEEE Journal of 46, 1388
(2010).
130 V. Bulovicacute, V. B. Khalfin, G. Gu, P. E. Burrows, D. Z. Garbuzov, and S. R. Forrest,
Phys. Rev. B 58, 3730 (1998).
131 H. Riel, S. Karg, T. Beierlein, W. Riess, and K. Neyts, J. Appl. Phys. 94, 5290 (2003).
132 Y. Luo and H. Aziz, Advanced Functional Materials 20, 1285 (2010).
133 E. W. Forsythe, D. C. Morton, Q. T. Le, L. Yan, F. Nuesch, C. W. Tang, and Y. Gao, San
Jose, CA, USA, 1999 (unpublished).
134 A. Mikami and T. Koyanagi, SID Symposium Digest of Technical Papers 40, 907 (2009).
References
99
135 S. R. Forrest, Nature 428, 911 (2004).
136 M. Thomschke, R. Nitsche, M. Furno, and K. Leo, Appl. Phys. Lett. 94, 083303 (2009).
Appendix A. Error analysis
100
Appendix A. Error analysis
The major cause of device-to-device performance variation on the same substrate is
non-uniformities in the device structure. For example, the thicknesses of the organic layers may
vary with position on the substrate. Therefore the substrate was rotated during the deposition
process to average out the non-uniformities. However, the thickness of each organic layer may
still have a subtle difference across a very large substrate (2 inch by 2 inch). Figure A1 shows
the device performance of different devices, with the same device structure, fabricated on the
same substrate. The current efficiency of these devices at 100 cd/m2 is 83.0 0.7 cd/A, which
implies that the variation is within a 1 % range. Errors due to other processing steps during the
device fabrication are also within this range (< 1%).
Film thicknesses were monitored using a calibrated quartz crystal microbalance (QCM).
The error in the film thickness reading from the QCM can be up to 10% due to the build-up of
material on the QCM (i.e., change of its lifetime). Therefore, to reduce the run-to-run variation,
the QCMs were replaced when 4% of the maximum lifetime was reached. Note that the QCM
was changed considerably more often than the manufacturer's recommendation.
The IV curves shown in this thesis represent one measurement cycle (i.e., measuring the
device only once). The standard deviation for many measurement cycles is < 1%. Since the
organic materials used in this study are very sensitive to moisture and oxygen, proper
encapsulation is needed to increase the device stability. For example, if a device is measured
without encapsulation in ambient air, the variation of different measurement cycles of the same
device may be much larger (up to 5 %) depending on the nature of the materials (i.e., some
molecules are more sensitive to ambient air than others). An extreme example is OLED devices
with phosphorescent dopants, which are notorious for their poor ambient stability. Also due to
the unstable nature of phosphorescent emitters, the measured luminance of the phosphorescent
devices in this study (even with proper encapsulation or in situ measurement in vacuum)
dropped significantly after one full measurement cycle. Each measurement cycle typically spans
Appendix A. Error analysis
101
a device brightness from zero up to > 20,000 cd/m2. For example, the measured luminance of a
standard Ir(ppy)2(acac) based OLED during the second measurement cycle was already reduced
by more than 10% compared to the first cycle. Therefore, the I-V-L measurements reported in
this thesis are for the first measurement cycle only. However, as shown in Fig. A2, the results
are still highly reproducible since the device-to-device variation and the run-to-run variation are
small.
1 ------> 16
Device
Al
ITO
0 1 2 3 4 5 6 710
-6
10-5
10-4
10-3
10-2
10-1
100
101
Cu
rre
nt D
en
sity (
mA
/cm
2)
Voltage (V)
9
10
11
12
13
14
15
16
1
2
3
4
5
6
7
8
(B)
100
101
102
103
104
0
20
40
60
80
100
(C)
9
10
11
12
13
14
15
16
Cu
rre
nt E
ffic
ien
cy(c
d/A
)
Luminance (cd/m2)
1
2
3
4
5
6
7
8
(A)
Figure A1. (A) Picture of real substrate with 32 devices. (B)Current density and (C) current efficiency of
device 1 to 16 as labeled in (A).
Appendix A. Error analysis
102
In terms of the error in the efficiency measurement, one of the major systematic errors
comes from the luminance meter that was used to measure the efficiency. Luminance is a
parameter describing the power of light corrected for the spectral response of the human eye
(photopic response). Usually, a photopic filter is used in regular luminance meters to correct this
response. However, the spectral response of most photopic filters differs slightly from the actual
photopic response of the human eye. Figure A2 shows the photopic response of the luminance
meter used in this study (Minolta Luminance Meter LS-110) in comparison with the actual CIE
photopic luminosity response curve.[1] Clearly from Fig. A2, the error is dependent on the
wavelength, and will therefore vary depending on the emission spectrum of different emitters.
For example, the emission peak of an Ir(ppy)2(acac) green OLED is in the range of 500-540 nm.
The reading of the luminance (and calculated efficiency) from the LS-110 may be only 1%
over-estimated in this spectral range. However, for a blue OLED device with emission in the
range of 450 nm, the luminance and corresponding efficiency might be up to ~ 30%
under-estimated.
Figure A2. Dash line: CIE photopic luminosity response curve; solid line: spectral response of Minolta
Luminance Meter LS-110.[1]
Appendix A. Error analysis
103
In addition to the LS-110, the efficiency was also measured using an integrating sphere with
a silicon photodiode with NIST traceable calibration.[2] Compared to the luminance meter
efficiency measurements, this method does not require the assumption that the OLED emission
is Lambertian, since the total luminous flux can be measured directly. However, the
measurement setup used in this study has a low sensitivity in the low luminance range (< 100
cd/m2) resulting in an error in the measured total luminous flux of up to 5 – 10%. Another
source error of the integrating sphere with photodiode may arise if the emission spectrum of the
OLED changes as a function of voltage, i.e., the color is not stable with driving voltage. For
example, many white OLEDs are not color stable. If the quantum efficiency is calculated based
on one typical spectrum (e.g., at 1000 cd/m2) for a white OLED, there may be a very large error
in the calculated efficiency at other voltages due to significant differences in the spectrum. The
best solution is to replace the photodiode with a spectrometer with high sensitivity.
Detector
Baffle
SubstrateOLED
Integrating Sphere
Figure A3. Schematic diagram of the measurement geometry using an integrating sphere.
Reference
[1] Manual of Minolta Luminance Meter LS-110.
[2] Tanaka, I. & Tokito, S. Precise Measurement of External Quantum Efficiency of Organic
Light-Emitting Devices. Jpn. J. Appl. Phys. 43, 7733.
Appendix B. List of publications related to this thesis
104
Appendix B. List of publications related to this thesis
1. Z.B. Wang, M.G. Helander, M.T. Greiner, and Z.H. Lu, "Energy-level alignment and
charge injection at metal/C60/organic interfaces", Appl. Phys. Lett. 95, 043302 (2009).
2. Z.B. Wang, M.G. Helander, M.T. Greiner, J. Qiu, and Z.H. Lu, "Analysis of
charge-injection characteristics at electrode-organic interfaces: Case study of
transition-metal oxides",Phys. Rev. B 80, 235325 (2009).
3. Z.B. Wang, M.G. Helander, Z.W. Liu, M.T. Greiner, J. Qiu, and Z.H. Lu, "Controlling
carrier accumulation and exciton formation in organic light emitting diodes", Appl. Phys.
Lett. 96, 043303 (2010).
4. Z.B. Wang, M.G. Helander, J. Qiu, Z.W. Liu, M.T. Greiner, and Z.H. Lu, "Direct hole
injection in to 4,4'-N,N'-dicarbazole-biphenyl: A simple pathway to achieve efficient
organic light emitting diodes", J. Appl. Phys. 108, 024510 (2010).
5. Z.B. Wang, M.G. Helander, J. Qiu, D.P. Puzzo, M.T. Greiner, Z.W. Liu, and Z.H. Lu,
"Highly simplified phosphorescent organic light emitting diode with >20% external
quantum efficiency at >10,000 cd/m2", Appl. Phys. Lett. 98, 073310 (2011).
6. Z.B. Wang, M.G. Helander, X.F. Xu, D.P. Puzzo, J. Qiu, M.T. Greiner, and Z.H. Lu,
"Optical design of organic light emitting diodes", J. Appl. Phys. 109, 053107 (2011).
7. Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z.M. Hudson, S. Wang,
Z.W. Liu, and Z. H. Lu, “Unlocking the full potential of organic light-emitting diodes on
flexible plastic”, Nature Photon. 5, 753 (2011).
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