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Highly Efficient Organic Light-Emitting Diodes by Exciton Harvesting by Yi-Lu Jack Chang A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Materials Science and Engineering University of Toronto © Copyright by Yi-Lu Jack Chang 2014
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Highly Efficient Organic Light-Emitting Diodes by Exciton Harvesting
by
Yi-Lu Jack Chang
A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy
Department of Materials Science and Engineering University of Toronto
© Copyright by Yi-Lu Jack Chang 2014
ii
Highly Efficient Organic Light-Emitting Diodes by Exciton
Harvesting
Yi-Lu Jack Chang
Doctor of Philosophy
Department of Materials Science and Engineering
University of Toronto
2014
Abstract
Tremendous progress has been made on organic light emitting diodes (OLEDs) over the past two
decades. This has enabled the commercialization of active-matrix OLED displays for mobile
phones and even large-area flat panels recently. However, in terms of solid-state lighting, further
reduction in electrical energy consumption at high brightness levels is urgently needed to make
the technology viable to the lighting industry and to compete with its inorganic LED counterpart
as well as compact fluorescent light bulbs. In this respect, considerable effort has been focused
on the use of a variety of complex device architectures including insertion of exciton confining
or carrier blocking layers, doping of the transport layers, as well as implementing gradient or
mixed emissive zone structures in a single OLED device. While effective, these designs are
generally overly cumbersome for large-scale commercial applications.
In this thesis, two effective ways to significantly enhance the efficiency of OLEDs without
compromising device simplicity are presented. These techniques involve firstly effective exciton
harvesting followed by intrazone and interzone energy transfers, respectively. High external
quantum efficiencies (EQEs) of > 20% were achieved at a high brightness of 1,000 cd/m2 for red
and greenish-yellow OLEDs, which are among the best performances reported to date with
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respect to each emissive color. Furthermore, white OLEDs with a superior combination of EQE
(> 20%) and color rendering index (~85) were achieved for the first time at a lighting-suitable
brightness of 5,000 cd/m2, which represents a significant step toward OLEDs in solid-state
lighting. Detailed investigations on the working mechanism of these two techniques as well as
future work related to these strategies will be discussed.
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Acknowledgments
I would like to first thank my parents, Kuang-Hui Chang and Ching-Li Liu, as well as my
brother, Mike Yi-Te Chang, for their continuous support of my study. I would like to thank my
supervisor, Prof. Zheng-Hong Lu, for providing me with the opportunity and guidance to carry
out this research. I would also like to thank Prof. Timothy Bender, Prof. Gregory Scholes, and
Prof. Nazir Kherani for the inspiring discussions and advices throughout this work. I wish to
express my gratitude to Prof. Suning Wang and her students, Dr. Ying-Li Rao, Dr. Zack Hudson,
Dr. Young-Jin Kang and Xiang Wang for the fruitful collaborations. I would further like to
extend my gratitude to my colleagues, Dr. Zhibin Wang, Dr. Michael Helander, Jacky Qiu, Yin
Song, Dr. Brett Kamino, Dr. Daniel Puzzo, Dr. Mark Greiner, Dr. Shaolong Gong, Lilly Chai,
Grayson Ingram, and Robin White for their support and helpful discussions. Additionally, I
would like to thank my girlfriend, Sarah Ya-Shi Zheng, for her patience and support of my study.
Lastly, I would like to thank the Government of Ontario and the University of Toronto for the
funding of my research.
v
Table of Contents
Table of Contents ..................................................................................................................... v
List of Tables ........................................................................................................................... vii
List of Figures ......................................................................................................................... viii
List of Abbreviations and Symbols .......................................................................... xvi
1 Introduction …………………...…………...................….....................………............. 1
1.1 Brief Overview on OLEDs .................................................................................... 1
1.2 Motivation ............................................................................................................. 5
1.3 Outline ................................................................................................................... 6
2 Background .................................................................................................................... 8
2.1 Performance Evaluation ......................................................................................... 8
2.2 Types of Emitters ................................................................................................. 10
2.3 Excitonics ............................................................................................................. 14
3 Experimental Methods ............................................................................................... 19
3.1 Device Fabrication ............................................................................................... 19
3.2 Device Characterization ....................................................................................... 20
3.3 Absolute Quantum Yield ..................................................................................... 22
3.4 Time-Correlated Single Photon Counting ........................................................... 22
4 Interzone Energy Transfer ........................................................................................ 24
4.1 Theory .................................................................................................................. 24
4.2 Efficiency Enhancement on Greenish-Yellow OLEDs ....................................... 26
4.3 Device Working Principle ……...............................................................……… 32
4.3.1 Exciton Harvesting …........…..........................................................…… 32
4.3.2 Efficient energy transfer …..................................................................… 34
4.4 Synthesis of the Greenish-Yellow Emitter .......................................................... 36
5 Intrazone Energy Transfer ............................................................................... 39
5.1 Theory .................................................................................................................. 39
5.2 Efficiency Enhancement on Red OLEDs ............................................................. 41
5.3 Device Working Principle ……..............................................................…….… 45
vi
5.3.1 Exciton Harvesting ……......................................................................… 45
5.3.2 Efficient Energy Transfer ………........................................................… 47
6 Design of High Efficiency and High Color Quality White OLEDs .................. 54
6.1 Brief Overview on White OLEDs ....…....................................................…...… 54
6.2 Cascaded Architecture ......................................................................................... 58
6.3 Performance Enhancement by Intrazone Energy Transfer .................................. 61
7 Conclusions and Future Work ……..………….....................................….............. 68
7.1 Conclusions .......................................................................................................... 68
7.2 Future Work ......................................................................................................... 68
References ………………………............................................................................................ 72
vii
List of Tables
Table 1. Summary of white OLED performances demonstrated in this work. ........................... 63
viii
List of Figures
Figure 1. A low voltage OLED consisting of two organic layers reported by Eastman-Kodak in
1987. ............................................................................................................................................... 2
Figure 2. An illustration of the working principle of a standard OLED. LUMO and HOMO
levels denote lowest unoccupied molecular orbital and highest occupied molecular orbital of the
organics, respectively. Solid yellow arrow represents light directed out of the device. ................ 3
Figure 3. A simple green bottom-emitting OLED, utilizing a well-known phosphorescent green
dopant. ............................................................................................................................................ 4
Figure 4. Device structure and energy level diagram for a three color OLED device featuring an
exciton blocking layer TCTA [4,4', 4"-tris(carbazol-9-yl) triphenylamine], and two doped
transport layers. The blue, green, and red dopants used are FIrpic [iridium bis-(4,6,-
difluorophenyl- pyridinato-N,C2')-picolinate], Ir(ppy)3 [tris(2-phenylpyridine) iridium(III)] and
PQIr [iridium(III) bis(2-phenylquinolyl-N,C') acetylacetonate], respectively. NPB [N,N'-
di(naphthalen-1-yl)-N,N )-diphenyl-benzidine] is used as the electron blocking layer. MeO-TPD
(N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine) doped with NDP-2 is used as the hole transport
layer, and Bphen doped with Cs is employed as the electron transport layer. ............................... 6
Figure 5. a) Illustrations of fluorescence versus phosphorescence processes and (b) their
corresponding efficiency as a function of luminance characteristics. Open and solid arrows
indicate non-radiative and radiative energy transitions, respectively. S1, T1 and So represent
energy states from the lowest singlet, triplet and the ground states, respectively. ISC denotes
intersystem crossing. .................................................................................................................... 11
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Figure 6. Standard Ir-based phosphors for the three primary colors and their corresponding EL
spectra. FIr6 [bis(4,6-difluorophenylpyridinato) tetrakis(1-pyrazolyl)-borate iridium(III)],
Ir(ppy)2(acac), and Ir(MDQ)2(acac) [bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)
iridium(III)] represent the blue, green and red phosphors, respectively. ..................................... 12
Figure 7. An illustration of thermally activated delayed fluorescence. Open and solid arrows
indicate non-radiative and radiative energy transitions, respectively. RISC stands for reverse
intersystem crossing. .................................................................................................................... 13
Figure 8. Kurt J. Lesker LUMINOS® cluster tool used to fabricate all OLED devices in this
thesis. ........................................................................................................................................... 19
Figure 9. The sample holder (left) and the glass substrate (right) with devices fabricated on top.
The cross-section between each cathode horizontal bar and the anode vertical strip defines the
active area of each device. A total of 32 devices on one substrate is thus possible in this design.
...................................................................................................................................................... 20
Figure 10. Device characterization setup with a sample mount, a Minolta LS-110 Luminance
Meter and the HP4140B pA meter. ............................................................................................. 21
Figure 11. A diagram of the external quantum efficiency measurement setup using an integrating
sphere. .......................................................................................................................................... 21
Figure 12. Absolute PL quantum yield setup. ............................................................................. 22
Figure 13. Time-correlated single photon counting setup. ......................................................... 23
Figure 14. Energy transfer rates in a standard host-guest system. EHost and EGuest represent the
lowest energy triplet states of the host and guest molecules, respectively. kF and kR denote host-
x
to-guest forward energy transfer and guest-to-host reverse energy transfer, respectively. ∆E is the
energy difference between EHost and EGuest. Eo denotes the ground state. .................................... 24
Figure 15. An illustration of possible energy transfer processes between two dopants in a
common host. Dotted and dashed arrows represent non-radiative and radiative energy transitions,
respectively. EHost, EA, ED and EO stand for the energy levels of the host, acceptor, donor, and the
ground state, respectively. χA and χD, and ɳD-A are as defined for equation (14) in the text. ....... 26
Figure 16. a) Device configuration and molecular structure of Ir(MDQ)2(Bpz) used as the
greenish-yellow (GY) emitter. All doping concentrations are in weight percentage. (b)
Normalized EL spectra of GY-only and red (R)-only [Ir(MDQ)2(acac)] devices as well as a
normalized photoluminescence (PL) spectrum of the GY emitter in tetrahydrofuran (THF) (~1×
10-5
M). (c) CE-L plot for the GY-only devices. ......................................................................... 28
Figure 17. Device configuration based on interzone exciton transfer with a green (G) emissive
layer incorporated, and the corresponding energy level diagram with respect to the vacuum level
of all molecules considered. ......................................................................................................... 30
Figure 18. a) CE-L plots of the GY + G devices considered, with numbers in the legends
denoting the thickness (in nm) of the device emissive layer. A photo of the optimized device (9
nm GY + 3 nm G) with an active area of 1 mm × 2 mm illuminating at 5,000 cd/m2 is shown in
the inset. (b) EQE-PE-L plots of the optimized GY-only and GY + G devices. (c) Normalized EL
spectra of the optimized GY+ G device (9 nm GY + 3 nm G) at a wide range of luminance
levels. ........................................................................................................................................... 31
Figure 19. Spectral power spectra of the optimized GY-only and GY + G devices at (a) long and
(b) short wavelength ranges. A considerably higher host emission is observed for the optimized
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GY-only device. The numbers shown in the legends stand for the thickness (in nm) of the device
emissive layer displayed in Figure 17. ......................................................................................... 33
Figure 20. Current Density versus Voltage (J-V) plot of the optimized GY-only and GY + G
devices. A lower current density is seen for the GY + G device, suggesting more carrier trapping
is present. The numbers shown in the legends indicate the thickness (in nm) of the device
emissive layer illustrated in Figure 17. ........................................................................................ 34
Figure 21. Calculated energy transfer efficiency from G to greenish-yellow (GY) emitter versus
the thickness of the GY emissive layer. ....................................................................................... 35
Figure 22. a, b) Normalized EL spectra of the GY + G devices considered. The numbers shown
in the legends represent the thickness (in nm) of the device emission layer as depicted in Figure
17. (c) Solid-state PL spectrum of Ir(ppy)2(acac) doped in 50 nm CBP film and solution
absorption spectrum of Ir(MDQ)2(Bpz) dissolved in CH2Cl2 (~1 × 10-5
M). ............................. 36
Figure 23. Geometry optimized structure and predicted highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) distributions of Ir(MDQ)2(Bpz). . 38
Figure 24. Illustration of the possible energy transfer processes in a three dopants system in a
common host. Cases with a higher number of dopants can be derived in a similar fashion. ...... 40
Figure 25. Schematic device structure and corresponding energy-level diagram of the devices as
well as the molecular structure and triplet energies (T1) of the materials used. The EML consists
of co-evaporated Ir(ppy)2(acac) and Ir(MDQ)2(acac) with various doping concentrations by
weight % into CBP. ..................................................................................................................... 42
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Figure 26. a) CE-L plot for selected devices under a fixed red doping and a range of green
doping concentrations, and b) corresponding absolute irradiance spectra at a current density of 10
mA/cm2. ....................................................................................................................................... 43
Figure 27. a) EQE vs Ir(ppy)2(acac) concentration under a range of Ir(MDQ)2(acac)
concentrations at a luminance of 1,000 cd/m2, and b) EQE versus luminance comparison between
the optimized co-doped device and optimized solely red doped device. Inset shows the EL
spectra of the optimized co-doped device under a wide range of current densities. .................... 44
Figure 28. a) Current density versus voltage for selected devices. The inset shows a table of turn
on voltages defined at a luminance of 1 cd/m2. (b) Driving voltage versus Ir(ppy)2(acac) doping
concentration for devices with 2% red doping at a current density of 1 mA/cm2. ...................... 46
Figure 29. Normalized absorption spectra of Ir(ppy)2(acac) and Ir(MDQ)2(acac) in CH2Cl2 (1.0 ×
10-5
M), as well as normalized PL spectra of CBP in solid state and Ir(ppy)2(acac) in CH2Cl2 (1.0
× 10-5
M), where the excitation wavelengths are at 330 nm and 400 nm, respectively. Inset
illustrates the dominant energy transfer processes between the singlet (S) and triplet (T) energy
levels of the host and dopants, where dotted arrows represent Fӧrster transfer, solid arrows
denote ISC, and dashed arrows represent Dexter transfer. So denotes the ground state. ............. 48
Figure 30. EL intensity spectra normalized to the dominant red peak at a current density of 10
mA/cm2
for selected devices under a fixed green doping and a range of red doping
concentrations. Inset shows a ten times magnified spectrum of the region enclosed in the dashed
box, which highlights the green spectral peak evolution with Ir(MDQ)2(acac) concentration
reduction. ..................................................................................................................................... 49
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Figure 31. Solid state transient response of (a) red and green co-doped CBP films and (b) yellow
and green co-doped CBP films at various co-doping concentrations. The solid lines are the
exponential fits to the transient decay responses. The excitation wavelength is at 350 nm. c)
Calculated energy transfer rate and efficiency versus total dopant concentration with the control
sample concentration corresponding to the green donor concentration of the co-doped films.
Triangles (squares) and rhombuses (circles) denote the energy transfer efficiency (energy transfer
rate) of co-doped yellow and red emissive films, respectively. ................................................... 51
Figure 32. Schematic illustration of two dominant energy transfer processes in co-doped films
under high concentrations: (1) direct transfer from donor-to-acceptor, and (2) indirect transfer by
encountering single or multiple donor-to-donor transfers (exciton diffusion) before a donor-to-
acceptor transfer occurs. The green and red circles represent donor and acceptor molecules,
respectively, and the blue arrows denote energy transfer. ........................................................... 53
Figure 33. Current status of energy conversion efficiency of OLEDs (in solid circles) and LEDs
(in open rhombuses) in the visible spectrum. The LED data were taken from Ref. [76]. The
OLED efficiencies are power efficiencies of the device at 1,000 cd/m2 after applying the out-
coupling enhancement technique (a factor of ~2.5) listed in Ref. [77] and normalized to the
theoretical limit for the corresponding wavelength. The dashed grey curve represents the
photopic sensitivity response curve of human eyes. The OLED data are taken from Ref. [44, 57,
71, 77, 78-80]. .............................................................................................................................. 56
Figure 34. Light emission of spectra of typical incandescent bulb, fluorescent tube, white LED,
and white OLED with warm white illuminations. ....................................................................... 57
xiv
Figure 35. Device configurations (a) and energy level diagrams (b) for WOLEDs W1-W4. The
dopants employed are FIrpic for blue (B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow
(Y), and Ir(MDQ)2(acac) for red (R). All doping concentrations are in weight %. (c) A photo of a
large area (80 mm × 80 mm) WOLED (W3) illuminating at 5,000 cd/m2 with a color rendering
index of 85. .................................................................................................................................. 60
Figure 36. Spectral power spectra at 10 mA/cm2 with a progressive addition of each emissive
layer to construct W1. Inset shows EQE of devices at a luminance of 1,000 cd/m2. The dopants
used are FIrpic for blue (B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow (Y), and
Ir(MDQ)2(acac) for red (R). Each device layer thicknesses and doping concentrations are as
shown for W1 in Figure 35. ......................................................................................................... 61
Figure 37. a-d) PE-L-EQE characteristics of the WOLED devices considered in this work. The
insets show the corresponding electroluminance spectra under various luminances normalized to
the green emission peak at 520 nm. ............................................................................................. 62
Figure 38. a) PE-L-EQE plot for W4 with (blue circles) and without (red squares) lens-based
out-coupling enhancement (see Figure 11). b) Normalized EL intensity spectra for W4 under
various luminances with out-coupling enhancement. All spectra are normalized to the green
emission peak at ~520 nm. ........................................................................................................... 65
Figure 39. EQE versus CRI of state-of-the-art white OLED devices at a luminance of 1,000
cd/m2 from literature. Multi-EML represents multiple emissive layers used, Co-Doped represents
several dopants co-deposited simultaneously to construct the emissive layers, Tandem denotes
stacked devices, and FP represents the use of blue fluorophors and other phosphors together in
the device. Device data are taken from Ref. [8, 9, 45, 53, 67-72, 70, 71, 74, 78, 80, 83-85, 86,
xv
87]. ............................................................................................................................................... 66
Figure 40. A proposed P-i-N white OLED device structure based on the optimized four emitter
cascaded design presented in Chapter 6. ...................................................................................... 70
xvi
List of 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
HIL Hole injection layer
EIL Electron injection layer
EML Emissive layer
HOMO Highest occupied molecular orbital
LUMO Lowest unoccupied molecular orbital
UV Ultraviolet
EL Electroluminescence
PL Photoluminescence
EQE External quantum efficiency
PE Power efficiency
CE Current efficiency
CRI Color rendering index
CIE Commission Internationale de l’Eclairage
E Energy level
EF Fermi energy level
xvii
ED Donor's lowest triplet energy level
EA Acceptor's lowest triplet energy level
EHost Host's lowest triplet energy level
Eo Ground state
T Triplet energy level
S Singlet energy level
ISC Intersystem crossing
kF Host to guest forward energy transfer
kR Guest to host reverse energy transfer
Mobility
e Electron charge
kB Boltzmann constant
ɳoc Out-coupling efficiency
γ Charge balance factor
ɳe-p Exciton to photon conversion efficiency
ɳD-A Donor to acceptor energy transfer efficiency
χ Fraction of excitons trapped or received by the dopant
ϕPL PL quantum yield
1
Chapter 1 : Introduction
1.1 Brief Overview on OLEDs
Organic light-emitting diode (OLED) is widely considered as the ultimate technology for
displays due to its unique form factor and its ability to produce vibrant colors. It is actively being
researched for potential use as the most desired broad band light source for next generation solid-
state lighting. Currently, active matrix OLED (AMOLED) displays are becoming popular in
smart phones world-wide and are emerging in large-sized (55") AMOLED televisions. The key
features of OLED include high energy efficiency, high color quality, environmental friendliness
and, most distinctively, its ultra-thin and flexible form factor, which yields a unique opportunity
for a variety of innovative designs such as wearable screens, semi-transparent displays and
lighting panels.
The first breakthrough in OLED technology that sparked eventual commercial application was
reported in Applied Physics Letters by researchers from Eastman-Kodak in 1987.1 It was a
simple two-layer heterojunction organic electroluminescent (EL) device, shown in Figure 1,
exhibiting a room temperature operating voltage under 10 V, a high luminance of over 1,000
cd/m2 (a brightness close to that of a modern flat-panel television display), and an efficiency of
about 1%. Since then, most modern OLEDs are developed based on the concept of organic
heterojunctions. Later development in device structure and organic materials have been primarily
driven by the need from the display industry for saturated red, green and blue emission colors, in
addition to higher efficiencies and longer device operating lifetimes. In terms of application as a
broad band light source, the first white OLED was reported in 1995 by incorporating emitters of
the three primary colors into a single OLED device to produce white light.2 Another major
milestone in OLED device technology concerns with the introduction of phosphorescent emitters
2
in OLEDs, which was first reported in 1998.3 These phosphorescent emitters or phosphors
provide a significant boost in device efficiency and have been gradually adopted by the flat-panel
and portable display industry.
Figure 1. A low voltage OLED consisting of two organic layers reported by Eastman-Kodak in
1987.
The first commercial OLED product was made by Pioneer Corporation in 1997. It was a passive
matrix OLED (PMOLED) display for car audio screens. A decade later in 2007, Samsung
Mobile Display introduced the first commercial AMOLED display, which remains to be the
screen of choice for smart phones. For white OLEDs in lighting applications however, only a
handful of prototypes have been demonstrated since 2010. Thus far, OLED for lighting has been
an active target globally. The key challenges evolve around improving the lifetime of blue
phosphors, reducing overall device complexity and enhancing device stability for high brightness
operation under continuous electrical drive. These challenges further translate into a steep cost
barrier for manufacturing commercial lighting products.
3
Figure 2. An illustration of the working principle of a standard OLED. LUMO and HOMO
levels denote lowest unoccupied molecular orbital and highest occupied molecular orbital of the
organics, respectively. Solid yellow arrow represents light directed out of the device.
At the fundamental level, an OLED is an electroluminescent device consisting of layers of
different functional organic materials each having a thickness of a few tens of nanometers
sandwiched between an anode and a cathode. These organic layers are typically deposited by
thermal evaporation in high vacuum (~1 × 10-7
Torr) chambers on a transparent substrate such as
a glass panel. As illustrated in Figure 2, by applying a bias voltage, holes are injected from the
anode through a hole injection layer (HIL) and then transported through a hole transport layer
(HTL) to the host. Concurrently, electrons are injected from the cathode through an electron
injection layer (EIL) and then transported through an electron transport layer (ETL) to reach the
host. In general the bias voltage applied is large enough such that the difference in quasi-Fermi
levels formed between the two electrodes exceed the energy gap of the host (> 3 eV). Once
electrons and holes arrive at the host layer, tightly bounded electron-hole pairs or excitons are
formed due to Coulomb interaction. These excitons are able to promote the host molecules to the
excited states, which then relax back to the ground states by releasing energy either in the form
En
erg
y anode
cathode
HTL
ETL
holes
electrons
luminescent
dopants
hostHIL
EIL
Thickness
HOMO
LUMO
4
of light (radiative recombination of electrons and holes) or in the form of heat (non-radiative
recombination). In order to enhance the radiative recombination, traces of organic dyes (guests)
with an energy level corresponding to the visible wavelengths are incorporated or doped into the
host material. The excitons formed in the host would then transfer their energy to the guests,
thereby exciting guest molecules, which subsequently emit light in a wavelength determined by
the energy gap of the guests. In this case, the host layer that is doped with luminescent guests is
known as the emissive layer (EML). In general, the non-radiative relaxation rate of the guests
considering internal conversions (Kasha's rule) and vibrational relaxations (Franck-Condon
principle) follows the energy gap law:4
𝑘𝑛𝑟 = 1013e−αEg , (1)
where α is a proportionality constant related to the nature of the molecule, and Eg represents
energy gap as determined from the guest molecule's lowest triplet energy level.
Figure 3. A simple green bottom-emitting OLED, utilizing a well-known phosphorescent green
dopant.
5
An example of a simple OLED device structure is shown in Figure 3. In this case, TPBi
[2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)] is used as the electron transport
layer, and CBP [4,4’-bis(carbazol-9-yl)biphenyl] is used as both the host and hole transport
layer. A green phosphorescent guest molecule, Ir(ppy)2(acac) [bis(2-phenylpyridine)
(acetylacetonate) iridium(III)], is doped to ~4 wt% into the host CBP with a thickness of 15 nm
to form the emissive layer. The hole and electron injection layers (~1 nm thick) are MoO3 and
LiF, respectively. The cathode is Al which is typically ~100 nm thick and is highly reflective
optically. The transparent conducting anode is typically an indium tin oxide (ITO) film (~50-120
nm) deposited on a glass substrate. Currently, the best green OLED on glass substrates without
employing additional light extraction techniques exhibits a maximum external quantum
efficiency of 30.2%,5 corresponding to a maximum power efficiency of 127.3 lm/W. The OLED
performance of the other primary colors, i.e. blue and red, are also not far behind that of the
green.6,7
The best white OLED on glass substrates with out-coupling enhancement reaches a
maximum power efficiency of ~90 lm/W,8 which is well-above that of the standard fluorescent
tubes (~70 lm/W).
1.2 Motivation
Although tremendous academic and industrial research over the past two decades has led to a
sizable presence of OLED in displays, devices with high efficiencies under a high luminance
range (1,000 - 5,000 cd/m2) while retaining simple architectures remain crucial for reducing the
high cost barrier and promoting OLED's entrance into the general lighting market. Currently,
majority of the work has been focused on the use of device architectures involving additional
exciton confining or carrier blocking layers,9
doped transport layers,10
as well as gradient or
mixed emissive zone structures5 in a single OLED to improve the efficiency at high brightness.
6
One prominent example demonstrated by Reineke et al. is shown in Figure 4.8 In this device, an
exciton blocking layer in addition to two doped transport layers and two spacers placed between
the three emissive layers are utilized to give a total of nine organic layers. While effective, these
designs are in general too cumbersome for large-scale commercial applications.
The goal of this thesis is therefore to demonstrate new, practical methods of enhancing the
efficiency of OLEDs at high brightness levels without introducing significant complexity into the
device architecture.
Figure 4. Device structure and energy level diagram for a three color OLED device featuring an
exciton blocking layer TCTA [4,4', 4"-tris(carbazolyl) triphenylamine], and two doped transport
layers. The blue, green, and red dopants used are FIrpic [iridium bis-(4,6,-difluorophenyl-
pyridinato-N,C2')-picolinate], Ir(ppy)3 [tris(2-phenylpyridine) iridium(III)] and PQIr [iridium(III)
bis(2-phenylquinolyl-N,C') acetylacetonate], respectively. NPB [N,N'-di(naphthalen-1-yl)-N,N')-
diphenyl-benzidine] is used as the electron blocking layer. MeO-TPD (N,N,N',N'-tetrakis(4-
methoxyphenyl)-benzidine) doped with NDP-2 is used as the hole transport layer, and Bphen
[4,7- diphenyl-1,10-phenanthrolin] doped with Cs is employed as the electron transport layer.
1.3 Outline
-8
-7
-6
-5
-4
-3
-2
En
erg
y (
eV
) Ag
ITO
TPBi
(22 nm)
TCTA
(8 nm)
Bp
hen
: C
s
(50
nm
)
NP
B (
10
nm
)
MeO
-TP
D :
ND
P-2
(6
0 n
m)
Thickness
7
In the following, a background on OLED device performance evaluation, types of emitters used,
and a theoretical framework on device physics, namely excitonics, will be presented in Chapter
2. The experimental methods involved in this work will be discussed in Chapter 3. In Chapters 4
and 5, two effective techniques to enhance the efficiency of OLEDs will be presented, including
details on theory, proof of concept, and device working principle. In Chapter 6, a brief overview
on current status of white OLEDs will be discussed, followed by the demonstration of a record
performance white OLED architecture based on the proposed technique in Chapter 5. Finally, a
summary and future work based on these novel techniques will be presented in Chapter 7.
8
Chapter 2 : Background
2.1 Performance Evaluation
The device external quantum efficiency, defined as the number of photons generated by the
number of charge carriers injected, can be described mathematically as follows:11
𝜂𝑒𝑥𝑡 = 𝛾𝜂𝑜𝑐𝜂𝑒−𝑝 , (2)
= 𝛾𝜂𝑜𝑐𝜒𝜙𝑃𝐿 , (3)
where ɳoc represents the optical out-coupling efficiency, γ is a charge carrier balance factor, and
ɳe-p is the exciton to photon conversion efficiency. ɳe-p can further be represented by the product
of χ, the fraction of emissive excitons received from the host or directly trapped by the emitter
chosen, and ϕPL, the luminescence quantum yield of the emitter. Due to a refractive index
mismatch among the various materials including the ITO anode, glass substrate and organic
layers in the device, significant amount of light is trapped by these materials inside the OLED
through total internal optical reflections. For a typical OLED, the out-coupling efficiency is
limited to under ~0.30. For a fluorescent emitter, χ is ~0.25. For a phosphorescent emitter, the
maximum χ could potentially reach unity. For an optimized device with perfectly matched
electron and hole currents, γ could also reach near unity. For an efficient phosphorescent emitter
(ϕPL ≈ 1) such as Ir(ppy)2(acac), together with a highly compatible host such as CBP, ɳe-p could
reach unity as well. The main factor limiting overall device efficiency is then the optical out-
coupling.
Part of the content in this chapter has been published by Chang and Lu in a chapter titled "Organic Light Emitting
Diodes", Wiley Encyclopedia of Electrical and Electronics Engineering, DOI: 10.1002/047134608X.W8205.
9
In general, three different types of device efficiencies are commonly used in OLED literature:
external quantum efficiency ηEQE, current efficiency ηCE, and power efficiency ηPE.12
While ηEQE
measures the number of photons extracted to air divided by the number of injected charges, both
ηCE and ηPE are photometric quantities that also take into account the photo-sensitivity of human
eyes.
The current efficiency is calculated using a measured luminance L0o in the forward direction
together with a measured current density Jmeas passing through the device:12
𝜂𝐶𝐸 = 𝐿0𝑜
𝐽𝑚𝑒𝑎𝑠 [cd/A], (4)
The power efficiency can then be computed using the operating voltage at the corresponding
current density, V(Jmeas), as follows:12
𝜂𝑃𝐸 = 𝜂𝐶𝐸𝑓𝐷𝜋
𝑉 𝐽𝑚𝑒𝑎𝑠 [lmW
-1] , (5)
with
𝑓𝐷 = 1
𝜋𝐼0 𝐼 𝜃, 𝜙 sin 𝜃 𝑑𝜙 𝑑𝜃
+𝜋
−𝜋
𝜋 2
0 , (6)
where fD represents the angular distribution of the emitted light intensity I(θ,ϕ) in the forward
hemisphere as a function of the azimuthal (θ) and polar (ϕ) angles. I0 denotes the light intensity
measured in the forward direction perpendicular to an OLED emitting surface. In general,
emission spectra may be different at different emission angles. This has to be included in the
above equation.
The external quantum efficiency can be obtained by:12
10
𝜂𝐸𝑄𝐸 = 𝜂𝐶𝐸𝑓𝐷𝜋𝑒
𝐾𝑟𝐸𝑝ℎ
%
100 , (7)
where Eph is the average photon energy of the EL spectrum and e is the electron charge. Kr
represents the luminous efficacy of radiation, which can be calculated by:
𝐾𝑟 = Ф𝑟 𝜆 𝑉 𝜆 𝑑
780 𝑛𝑚380 𝑛𝑚 𝜆
Ф𝑟 𝜆 𝑑𝜆∞
0
[lmW-1
], (8)
where V(λ) is the weighting function that takes into account the photo-sensitivity of human eyes,
and Φr is the radiant flux. Essentially, Kr quantifies lumen per watt for a given spectrum, thereby
also representing the theoretical limit in power efficiency of a particular light source, assuming
no optical and electrical losses. It is important to note that the angular distribution fD has to be
properly measured in order to calculate both ηEQE and ηPE accurately. Otherwise, an integrating
sphere has to be used and will be discussed in Section 3.2. For a long time, however, it has been
a common practice to calculate these efficiencies by assuming a Lambertian emission pattern of
the emitted light, i.e. I(θ,ϕ) = I0 cosθ, which is often not the case. The deviation from a
Lambertian pattern would certainly result in erroneous efficiency values.
2.2 Types of Emitters
Electrically excited luminescent organic molecules emit light either through a fluorescent
process or a phosphorescent process. The fundamental physics of fluorescence and
phosphorescence are shown in Figure 5a. In general, each organic molecule has a set of
characteristic singlet states (S) and triplet states (T), with an electronic state density ratio of 1 : 3,
following the spin statistics in quantum mechanics.3 In a typical fluorescent molecule, the triplet
states are non-emissive, therefore only a quarter of the excitons generated may contribute to light
emission from its lowest singlet state (S1). This singlet energy radiative relaxation occurs on a
11
time scale of ~10-9
s.3 In a phosphorescent molecule, however, the molecule is attached with a
heavy metal such as Ir or Pt, which can induce a spin-orbit coupling effect, resulting in a rapid
exciton energy transfer from the lowest singlet state to the lowest triplet state (intersystem
crossing) as well as a spin-flip that enables a triplet state to relax to the ground state radiatively.
Here, the strength of spin-orbit coupling is directly proportional to the fourth power of the atomic
number of the metal (the heavier the metal, the stronger spin-orbit coupling is).13
In essence,
these processes lead to potentially 100% of the electrically generated excitons contributing to
light emission. The energy relaxation time of the triplet states is in the order of > 10-6
s.3 The use
of a phosphorescent emitter therefore provides a four-fold enhancement in light emission
efficiency (see Figure 5b) over that of a fluorescent emitter.
Figure 5. a) Illustrations of fluorescence versus phosphorescence processes and (b) their
corresponding efficiency as a function of luminance characteristics. Open and solid arrows
indicate non-radiative and radiative energy transitions, respectively. S1, T1 and So represent
energy states from the lowest singlet, triplet and the ground states, respectively. ISC denotes
intersystem crossing.
Inte
rnal
Eff
icie
ncy (
%)
10 100 1,000 10,000
100
25
Luminance (cd/m2)
High Brightness
(a)
(b)
So
S1S1
So
T1 T1
electrical
excitation
electrical
excitation
ISC100%
75%75%
25%25%
fluorescence phosphorescence
12
Examples of well-known green, red and blue phosphorescent emitters14, 15
with the
corresponding EL emission spectra are shown in Figure 6. In terms of addressing primary colors
suitable for commercial applications, green and red phosphorescent emitters are sufficient in
terms of both efficiency and stability for most display applications. It remains a challenge,
however, to make a stable blue phosphorescent OLED due to the fact that the energy required to
excite the blue emitter is close to that of the dissociation energy of the common C-C and C-N
chemical bonds in the organic complex.16
Furthermore, in order to excite the high energy blue
emitters effectively, even higher energy (or wider energy gap) host materials have to be
electrically excited first. This also leads to host molecule instability issues. In general, a repeated
high energy electrical excitation during prolonged OLED operation (constant bias voltage) that
eventually destabilizes the organic molecule is known as electrical aging.
Figure 6. Standard Ir-based phosphors for the three primary colors and their corresponding EL
spectra. FIr6 [bis(4,6-difluorophenylpyridinato) tetrakis(1-pyrazolyl)-borate iridium(III)],
Ir(ppy)2(acac), and Ir(MDQ)2(acac) [bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)
iridium(III)] represent the blue, green and red phosphors, respectively.
400 500 600 700 800 900
No
rma
lize
d E
L I
nte
nsity [
a.u
.]
Wavelength [nm]
CH3
IrO
O
CH32
N
Ir(ppy)2(acac)
N
N
Ir
N
N
2 FIr6
BNN
N N
N
F
F
Ir
CH3
2
CH3
O
O
CH3
Ir(MDQ)2(acac)
NN
13
Recently, a new class of fluorescent emitters that exhibit efficiencies close to that of a typical
phosphorescent emitter has been reported.17
The light emitting process of this type of molecules
is based on thermally activated delayed fluorescence (TADF), shown in Figure 7. Here, the
energy level difference between the singlet and triplet states of the emitter is sufficiently close (<
100 meV) such that with a thermal energy activation during device operation (at room
temperature), the slightly lower energy triplet excitons could transfer to the higher singlet energy
level (reverse intersystem crossing) efficiently, resulting in nearly 100% exciton fluorescence
from the singlet states. Such delayed fluorescence process takes place on a time scale of ~10-6
s,
which is close to that of the radiative process in phosphorescence. The use of this new class of
molecules could avoid the inherently high cost of phosphorescent emitters that contain expensive
noble metals such as Ir and Pt. Although this approach is attractive, the efficiency roll-off is
typically worse than phosphorescent based OLEDs due to an even more severe triplet-triplet
annihilation process associated with the accumulation of non-radiative guest triplet states.
Figure 7. An illustration of thermally activated delayed fluorescence. Open and solid arrows
indicate non-radiative and radiative energy transitions, respectively. RISC stands for reverse
intersystem crossing.
14
2.3 Excitonics
In organic semiconductors, the weak van der Waals interactions that hold each molecule together
form a weak dielectric screening of the Coulomb interactions due to randomly oriented
polarizations. This is characterized by a small dielectric constant ɛr of ~3-4, which results in a
large exciton binding energy (0.3-1 eV) and tightly bound electron-hole pairs that are either
localized on a single molecule (0.5-1 nm) called Frenkel excitons or on adjacent molecules
called charge-transfer excitons.18
Such localized electron-hole pairs also result in a strong
electron-hole wavefunction overlap that induces a large exchange energy (0.1-1 eV) which
separates the singlet and the triplet state energies apart. From Pauli's exclusion principle, the
singlet excitons have antisymmetric spin wavefunctions and are therefore allowed to be spatially
bound closer together with a higher energy, whereas the triplet excitons have symmetric spin
wavefunctions such that they are spatially further apart with a lower energy due to strong spin-
spin interactions. These are in stark contrast to inorganic semiconductors which are characterized
by strong ionic and covalent bonds that induce a strong dielectric screening of the Coulomb
interactions owning to well-ordered polarizations, leading to large dielectric constants ɛr in the
range of ~11-16.18
As a result, the exciton binding energies are small (14.7 meV for Si, 4.7 meV
for GaAs, and 2.7 meV for Ge) and the electron-hole pairs are loosely-bound (4-10 nm) or also
known as Wannier excitons. Such loosely-bound electron-hole pairs imply little electron-hole
wavefunction overlap, leading to nearly zero exchange energy, and hence there is no need to
differentiate between a singlet and a triplet exciton (simply called an exciton).18
In addition to the aforementioned radiative recombination of charge carriers in which excitons
are formed as excited states in organic molecules prior to releasing the energy radiatively, two
15
types of non-radiative energy transfer mechanisms, namely Förster-type and Dexter-type, are
also essential to OLED operation.
Here, a molecule involved in an excitonic energy transfer process is referred to either as a donor
D or as an acceptor A depending on whether the molecule donates or accepts energy,
respectively. Additionally, the multiplicities of the excitons are denoted with preceding
superscripts, 1 (1D and
1A) or 3 (
3D and
3A) for singlets and triplets, respectively, and species in
the excited states are marked with asterisks (D* and A*).
The non-radiative energy transfer rate is proportional to the spectral overlap of the donor
emission band ID(ν) and the acceptor absorption band α(ν) and is quantified by the spectral
overlap integral J as follows:
𝐽 = 𝐼𝐷(𝑣)𝛼 𝜈 𝑑𝜈 ,∞
0 (9)
where ID(ν) and α(ν) are normalized intensities.
In the case of Förster energy transfer19
that is driven by Coulomb interactions, the rate constant
representing the most dominant dipole-dipole interaction can be expressed as:20
𝑘𝐹 = 𝑘09 ln 10 𝜅2𝜙𝐷
128𝜋5𝑁𝐴𝑛4 ∙ 𝐽 ∙1
𝑅𝐷𝐴6 = 𝑘0
𝑅𝑜
𝑅𝐷𝐴 6 , (10)
where k0 denotes the rate constant of the excited donor without the presence of an acceptor, κ
represents the orientation factor, NA is the Avogadro's number, n is the refractive index of the
medium, ϕD denotes the luminescence quantum yield of the donor emission, RDA is the
intermolecular distance between a donor and an acceptor, and R0 is known as the Förster radius.
Under the framework of Förster transfer, the following processes are allowed:21
16
1D∗ +
1A →
1D +
1A∗ , [1]
1D∗ +
3A →
1D +
3A∗ . [2]
These processes suggest that a donor molecule with excited singlet states can transfer its energy
to either the singlet or triplet states of an acceptor by Förster transfer. This is thus a critical
energy transfer mechanism between a host and a guest emitter (either fluorescent or
phosphorescent) in an OLED. The proficient singlet-singlet Förster transfer also sets the limit on
the doping concentration of typical fluorophors in a host to be ~1% or less in order to prevent
significant emitter self-quenching or repeated self-absorption and re-emission thereby losing
more energy non-radiatively. Furthermore, if phosphorescent donor molecules are involved, two
additional processes are also possible due to an enhanced triplet recombination induced by a
strong spin-orbit coupling that facilitates a spin-flip of the triplet states:21
3D∗ +
1A →
1D +
1A∗ , [3]
3D∗ +
3A →
1D +
3A∗ . [4]
These processes come into play when two or more phosphorescent dopants are incorporated in
the same host as in the case for multi-color or white emission OLEDs, where significant energy
transfer can take place between the high and low energy phosphors by the Förster-type
mechanism, resulting in considerable quenching of the higher energy phosphor emission by the
lower energy ones.22, 23
Such energy transfer can occur even at low emitter doping concentrations
because Förster energy transfer can be very efficient even at a long range of ~10 nm,23
which is
considerably larger than the typical size of individual organic molecules.24
Process [4] can also
describe concentration dependent phosphorescent emitter self-quenching or repeated self-
absorption and emission, thereby losing more energy non-radiatively.
17
In contrast, Dexter energy transfer25
arises from electron exchange interactions that require
significant orbital overlap between D and A. In addition, Dexter-type energy exchange follows
the Wigner-Witmer spin conservation rules, which require the conservation of total spin
configuration throughout the process. The resulting energy transfer processes are:21
1D∗ +
1A →
1D +
1A∗ , [5]
3D∗ +
1A →
1D +
3A∗
, [6]
3D∗ +
3A∗ →
1D +
1A∗. [7]
Although singlet to singlet energy transition is possible from Dexter exchange interactions as
indicated in process [5], this transition is mostly dominated by highly efficient Förster transfer in
process [1]. Process [6] describes the triplet migration process or "hopping" transport through the
organic host molecules. Essentially, triplets in the host will migrate until a suitable guest
molecule is encountered whereby energy is transferred to the triplet state of the guest by the
same process. Typically, triplets in an organic semiconductor will diffuse a relatively long
distance (~100 nm, on the order of entire device length) without radiative emission to the ground
states since such transition requires a spin-flip, which is not allowed without the help of heavy
metal-induced spin-orbit coupling effect. Process [7] implies that two excited triplet states can
react and form two singlet states, one in the ground state and one in the excited state. This
process is also known as triplet-triplet annihilation,26
which may lead to phosphorescent OLED
efficiency roll-off under high driving voltages or high current densities when the device is filled
with excited triplet states (see Figure 5b).
The Dexter transfer rate constant is expressed as:21
𝑘𝐷 =2𝜋
ћ𝐾2 ∙ 𝐽 ∙ 𝑒−2𝑅𝐷𝐴 /𝐿 , (11)
18
where K is a constant with units of energy, and L is the sum of van der Waals radius. Here, the
exponential dependence on the intermolecular distance RDA reflects the quantum mechanical
nature of closely bound electrons that have sufficient wavefunction overlap to facilitate the
Dexter exchange process. Typical Dexter transfer distance is only up to ~ 2 nm.
In terms of exciton migration throughout organic materials during device operation, both Förster
(mainly singlet-to-singlet) and Dexter (mainly triplet-to-triplet) energy transfers can contribute.27,
28 For exciton diffusion, there is no net charge involved, and the driving force behind exciton
movement is a gradient in exciton concentration,▽n(r,t), which creates a chain of uncorrelated
hopping processes from one molecule to another in a random fashion, i.e. random walk model.
Such particle diffusion phenomenon is described by Fick's 2nd law as follows:29
∂𝑛 𝒓,𝑡
∂t= 𝐺 𝒓, 𝑡 −
𝑛 𝒓,𝑡
𝜏+ 𝐷𝛻2𝑛 𝒓, 𝑡 , (12)
where G(r,t) represents exciton generation, D is the diffusion constant, and τ is the exciton
lifetime.
During device operation under electrical excitation, excitons are typically generated in a close
proximity to an interface between two organic layers such that the width of the exciton
generation zone is considerably smaller than the thickness of total device organic stack.
Therefore, it is a generally practiced method to model exciton generation zone as a delta-
function, i.e. G(x,t) = G·δ(x=x0,t). Under this condition, it is possible to acquire the steady-state
(∂n/∂t = 0) solution of Fick's 2nd law as follows:30
𝑛 𝑥 = 𝑛0 ∙ 𝑒−𝑥/𝐿𝑥 , 𝐿𝑥 = 𝐷𝜏, (13)
where Lx is the diffusion length and n0 is the exciton density at the interface.
19
Chapter 3 : Experimental Methods
3.1 Device Fabrication
Figure 8. Kurt J. Lesker LUMINOS® cluster tool system used to fabricate all OLED devices in
this thesis.
All devices were fabricated by thermal evaporation using a Kurt J. Lesker LUMINOS® cluster
tool (Figure 8) under a base pressure of ~10−7
Torr on a glass substrate (1.1 mm thick) pre-
coated with indium tin oxide, having a thickness and sheet resistance of 120 nm and 15 Ω/sq,
respectively. Prior to loading, the substrate was degreased with standard solvents (acetone and
methanol), blow-dried using a N2 gun, and treated in a UV-ozone chamber for 15 minutes.
Figure 9 shows the sample holder and the ITO patterned glass substrate with devices fabricated
on top. All doping concentrations used in this work are by weight percentage. The active area for
each device is ~2 mm2 as verified with an optical microscope. The deposited layer thickness was
monitored by a quartz crystal microbalance that was calibrated by spectroscopic ellipsometry
(Sopra GES 5E). All dopants were purified by gradient sublimation before use to ensure the
highest purity possible. Precise control of layer thicknesses during the device fabrication as well
20
as the purity of the phosphorescent emitter was found to be critical to obtaining highly efficient
devices with high reproducibility.
Figure 9. The sample holder (left) and the glass substrate (right) with devices fabricated on top.
The cross-section between each cathode horizontal bar and the anode vertical strip defines the
active area of each device. A total of 32 devices on one substrate is thus possible in this design.
3.2 Device Characterization
Luminance-voltage measurements were carried out using a Minolta LS-110 Luminance Meter
and current-voltage characteristics were measured using an HP4140B pA meter as shown in
Figure 10. The radiant flux for calculating EQEs was measured using an integrating sphere
equipped with an Ocean Optics USB 4000 spectrometer with NIST traceable calibration using a
halogen lamp.12
Measurements with out-coupling enhancement used a 10 mm diameter BK7
half-sphere lens mounted on top of the device with index matching gel. The geometry for the
lens-based out-coupling enhancement measurement using an integrating sphere is shown in
Figure 11.
21
Figure 10. Device characterization setup with a sample mount, a Minolta LS-110 Luminance
Meter and a HP4140B pA meter.
Figure 11. A diagram of the external quantum efficiency measurement setup using an integrating
sphere.
Lens
SubstrateOLED
Baffle
Detector
Integrating Sphere
22
3.3 Absolute Quantum Yield
The absolute quantum yield measurements were performed using a custom built setup according
to the procedure reported in Ref. [31], as shown in Figure 12. A 365 nm collimated LED from
Thorlabs (M365L2-C2) served as the excitation source, which was directed onto the sample
consisted of a doped organic film (100 nm thick) deposited on a quartz substrate (1 mm thick)
and mounted inside a calibrated integrating sphere. The light generated was then detected using
an Ocean Optics Maya 2000 Pro spectrometer. The solution PL measurements were conducted
using Perkin Elmer LS55 fluorescence spectrometer and the absorption measurements were
carried out using Perkin Elmer Lambda 25 UV-Vis spectrometer.
Figure 12. Absolute PL quantum yield setup.
3.4 Time-Correlated Single Photon Counting
Time-correlated single photon counting (TCSPC) measurements were conducted using an IBH
Datastation Hub system with an IBH 5000 M PL monochromator and an R3809U-50 cooled
MCP PMT detector. The light source used was a model 3950 ps Ti: sapphire Tsunami laser
(Spectra-Physics), pumped by a Millenium X (Spectra-Physics) diode laser, pulse picked (Model
23
3980 Spectra-Physics), and frequency doubled using a GWU-23PL multi-harmonic generator
(Spectra-Physics). Pulse repetition rates were kept below 100 kHz. A photo of the entire setup is
shown in Figure 13. The samples consisted of doped CBP films (50 nm thick) on quartz that
were encapsulated with a second, identical sized, blank quartz using ultraviolet (UV)-sensitive
epoxy under N2 environment prior to measurement. During measurement, single photons of a
given energy are detected at different times after a laser pulse, which are counted in
accumulation to form a histogram of the transient decay response.
Figure 13. Time-correlated single photon counting setup.
24
Chapter 4 : Interzone Energy Transfer
4.1 Theory
Figure 14. Energy transfer rates between a standard host and guest. EHost and EGuest represent the
lowest energy triplet states of the host and guest molecules, respectively. kF and kR denote host-
to-guest forward energy transfer and guest-to-host reverse energy transfer, respectively. ∆E is the
energy difference between EHost and EGuest. Eo denotes the ground state.
In a standard host-guest system shown in Figure 14, the rate of the forward energy transfer kF,
and reverse energy transfer kR depend critically on the energy difference ∆E between the lowest
triplet energy states of the host and guest molecules. Here are four possible scenarios, assuming a
phosphor is used as the guest:32
(i) ∆E >> 0. In this case, the host and guest triplet energies are non-resonant such that
even though kF >> kR, both rates are considerably smaller than their resonance maxima. This
This chapter is based on a published work by Chang et al., Adv. Funct. Mater. 23, 3204 (2013).
EGuest
Eo
EHost
∆EkF kR
25
means that a strong confinement of triplet energy by the host does not necessarily facilitate
effective energy transfer from the host to the guest.
(ii) ∆E > 0. This is the ideal case where kF > kR, and the system is close to resonance,
where phosphorescence from the triplet states of the guest dominates.
(iii) ∆E < 0. In this case, triplets are expected to mostly reside on the host with kR > kF,
which results in inefficient phosphorescence from the guest's triplet states.
(iv) ∆E << 0. Here, kR >> kF and extremely inefficient phosphorescence is expected from
the guest due to significant quenching of the triplet states of the guest by host's accumulation of
non-emissive triplet states.
From these four cases, it is clear that scenario (ii) is most desired. However, in general a standard
host material such as CBP can most easily reach such resonant condition on only one type of
guest, typically green dopants such as Ir(ppy)2(acac) or Ir(ppy)3 in an OLED. A smaller triplet
energy guest dopant such as yellow or red emitter typically results in scenario (i), which is a less
effective combination. The existence of such resonant excitonic energy transfer is analogous to
the case of electron transfer which follows Marcus theory.32
It is therefore proposed herein that an additional doped layer using the more compatible green
dopant adjacent to the original emissive zone with a lower energy guest in a common host
material may not only assist in exciton harvesting but also provide an intermediate energy
delivery step to boost the lower energy guest emission by interzone energy transfer (i.e. energy
transfer between adjacent EMLs) as shown in Figure 15. This mechanism may be modeled as:
𝜂𝑒𝑥𝑡 = 𝛾𝜂𝑜𝑐 𝑖𝐴𝜒𝐴𝜙𝑃𝐿,𝐴 + 𝑖𝐷𝜒𝐷 𝜂𝐷−𝐴𝜙𝑃𝐿,𝐴 + 1 − 𝜂𝐷−𝐴 𝜙𝑃𝐿,𝐷 , (14)
26
where ηext, ηoc, and ηD-A represent device external quantum efficiency, out-coupling efficiency,
and energy transfer efficiency from donor to acceptor emitter, respectively. , ϕPL, and χ denote
charge balance factor, absolute quantum yield of each emitter, and fraction of emissive excitons
trapped by each emitter in the device, respectively. Here, i is defined as the ideality factor
accounting for the reduction in the fraction of emissive excitons trapped by each emitter with an
emissive layer thickness that deviates from the optimum thickness in a single color device. In the
following sections, this interzone energy transfer technique is used to enhance the efficiency of
an OLED featuring a newly synthesized greenish-yellow dopant.
Figure 15. An illustration of possible energy transfer processes between two dopants in a
common host. Dotted and dashed arrows represent non-radiative and radiative energy transitions,
respectively. EHost, EA, ED and EO stand for the energy levels of the host, acceptor, donor, and the
ground state, respectively. χA and χD, and ɳD-A are as defined for equation (14) in the text.
4.2 Efficiency Enhancement on Greenish-Yellow OLEDs
One way to improve the CRI of a standard three color white (i.e. blue, green and red
combination) phosphorescent OLED is to employ a greenish-yellow emitter to replace the green
emitter such that the gap in emission wavelength between standard green and red emitters is
EHost
ED
EA
χAχD
Eo
ɳD-A
27
eliminated.33, 34
Unfortunately, there are relatively few reports on greenish-yellow emitters for
OLEDs.34-37
As a result, the performance of greenish-yellow emitters is significantly behind
those emitting in the three primary colors, which are driven strongly by the display industry.
Hudson et al.35, 36
demonstrated a triarylboron-functionalized Pt(II) complex greenish-yellow
emitter with a peak emission at 538 nm, that exhibits device external quantum efficiency of
16.5%, current efficiency (CE) of 53.0 cd/A, and power efficiency (PE) of 45.0 lm/W at 1,000
cd/m2. So et al.
37 synthesized a trimethylsilylxylene-based greenish-yellow Ir(III) emitter with a
peak wavelength at 532 nm, and demonstrated OLEDs with maximum EQE and CE of 12.7%
and 45.7 cd/A, respectively, at 10 cd/m2. More recently, Chen et al.
34 reported a yellowish-green
Ir(III) emitter with a peak wavelength at 544 nm, which exhibits a high CE of 63.0 cd/A at a
luminance of 100 cd/m2, corresponding to an EQE of 16.3% and power efficiency of 36.6 lm/W.
Herein, a newly synthesized greenish-yellow emitter (see Section 4.4 for the synthesis) that
exhibits a decent EQE (CE) of ~15.2% (53.6 cd/A) at a luminance of 1,000 cd/m2
has been
demonstrated in an OLED. By introducing a novel design concept featuring interzone exciton
transfer, i.e., molecular energy transfer between adjacent emitting layers, the device performance
was enhanced to a remarkable EQE (CE) of 21.5% (77.4 cd/A) at 1,000 cd/m2. Even at a high
luminance of 5,000 cd/m2
required for solid-state lighting, the EQE (CE) remains as high as
20.2% (72.8 cd/A). Such performance is comparable to that of the state-of-the-art green emitter,
Ir(ppy)3 [tris(2-phenylpyidine) iridium(III)],38
and is the highest reported to date among greenish-
yellow emitting OLEDs.
The newly synthesized molecule, Ir(MDQ)2(Bpz) [bis(2-methyldibenzo[f,h]quinoxaline)
tetrakis(1-pyrazolyl)-borate iridium(III)], exhibits a peak emission at ~539 nm that is ~70 nm
blue-shifted compared to that of Ir(MDQ)2(acac) as shown in Figure 16b. The Commission
28
Internationale de l’Eclairage (CIE) coordinate of Ir(MDQ)2(Bpz) is (0.378, 0.602), which
corresponds to a greenish-yellow (GY) emission. Moreover, a decent absolute quantum yield of
~0.74 is obtained for the GY emitter, which is slightly lower than that of the red emitter (R),
Ir(MDQ)2(acac), at ~0.77. This can be attributed to an increase in the metal-to-ligand charge-
transfer (MLCT) transition energy or a reduction in the mixing of the 3MLCT character into the
lowest excited state, T1, which resulted in a widening of the emission energy, following a
decrease in the HOMO energy.
Figure 16. a) Device configuration and molecular structure of Ir(MDQ)2(Bpz) used as the
greenish-yellow (GY) emitter. All doping concentrations are in weight percentage. (b)
Normalized EL spectra of GY-only and red (R)-only [Ir(MDQ)2(acac)] devices as well as a
normalized photoluminescence (PL) spectrum of the GY emitter in tetrahydrofuran (THF) (~1×
10-5
M). (c) CE-L plot for the GY-only devices.
100
101
102
103
104
0
10
20
30
40
50
60
70
Cu
rre
nt
Eff
icie
ncy [
cd
/A]
Luminance [cd/m2]
6 GY
9 GY
12 GY
15 GY
18 GY
500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
No
rma
lize
d In
ten
sity [a
.u.]
Wavelength [nm]
EL of GY: 8% in CBP
PL of GY in THF
EL of R: 8% in CBP
(a)
(b) (c)
N
TPBi (70 nm)
CBP: 8% GY
(x nm)
ITO/MoO3 (1 nm)
Glass Substrate
LiF/Al (100 nm)
CBP (45 nm)
N N
N
N N
N
N N
N
Ir
CH3
NN
N
N
2
CBP
TPBi
Ir(MDQ)2(Bpz)
B
NN
N N
N
TPBi (70 nm)
CBP: 8% GY
(x nm)
ITO/MoO3 (1 nm)
Glass Substrate
LiF/Al (100 nm)
CBP (45 nm)
N N
N
N N
N
N N
N
Ir
CH3
NN
N
N
2
CBP
TPBi
Ir(MDQ)2(Bpz)
B
NN
N N
29
To exploit the GY emitter in a device, a simple, yet highly effective device architecture as shown
in Figure 16a is constructed. In this design, TPBi is utilized as the electron transport layer and
CBP is employed as both the hole transport layer and the host material. The emissive layer
consists of GY phosphor incorporated with varying thicknesses in host CBP starting from the
CBP/TPBi interface. Standard ITO/MoO3 anode and LiF/Al cathode are applied. In this design,
the majority of the excitons will naturally generate near the HTL and ETL interface (i.e. the
CBP/TPBi interface) on both sides, and are subsequently harvested (i.e. recombination takes
place) on the doped regions of CBP by the GY emitter. After performance optimization as shown
in Figure 16c, a decent CE (EQE) of ~53.6 cd/A (15.2%) was achieved at a luminance of 1,000
cd/m2 with a emissive layer thickness of 15 nm and a GY doping level of 8% in CBP, which is
among the best greenish-yellow OLED performances reported to date.
In order to further improve on the device performance, a new design concept based on interzone
exciton transfer has been implemented as shown in Figure 17. This design involves the
incorporation of an additional thin layer (~3 nm) of doped CBP using a green phosphor (G),
Ir(ppy)2(acac), which is known for not only its high emission efficiency, but also for its excellent
exciton trapping capability in CBP. In this configuration, the majority of the excitons formed
near the CBP/TPBi interface will be harvested first by the GY emitter before the G emitter that is
located farther away with respect to the HTL/ETL interface. More importantly, it is expected that
the higher energy G emitter will naturally transfer its energy to the adjacent lower energy GY
emitter. Since the extent of such interzone exciton transfer is generally accepted to be ~3 nm,
assuming a Dexter-type process takes place, the choice of the G emission zone thickness should
in principle allow for a nearly complete energy transfer to the GY emission zone provided
enough GY emissive sites are available.
30
Figure 17. Device configuration based on interzone exciton transfer with a green (G) emissive
layer incorporated, and the corresponding energy level diagram with respect to the vacuum level
of all molecules considered.
Figure 18 illustrates the performance characterization for devices with GY emission layer
thickness of over 6 nm. It is seen from Figure 18a that with the G emission layer incorporation,
the Current Efficiency versus Luminance (CE-L) plots are characterized by a dramatic increase
in efficiency with luminance. This suggests that with increasing current density, more excitons
formed in the host are able to reach the G emissive layer to be harvested, and subsequently
transferred to the GY emissive sites efficiently. Remarkably, the optimum device with G
emission layer incorporation (9 nm GY + 3 nm G) reaches a record high CE of 77.4 cd/A, which
is ~1.4 times higher than the optimum device without the G emission layer at 53.6 cd/A at 1,000
cd/m2
as shown in Figure 18a. This also corresponds to an unprecedented EQE of 21.5% (see
Figure 18b) for greenish-yellow emitting OLEDs reported to date. Even at a high luminance of
5,000 cd/m2 that is required for solid-state lighting, the EQE (CE) remains as high as 20.2%
(72.8 cd/A). The enhancement in power efficiency (PE) is also impressive as shown in Figure
18b. A high PE of 50.7 lm/W is achieved at 1,000 cd/m2, which is considerably higher than that
of the optimum device without G emission layer at 34.9 lm/W. Furthermore, Figure 18c shows
extremely stable EL spectra under varying luminance (or current density), which indicates that
TPBi (70 nm)
CBP: 8% GY
(x nm)
ITO/MoO3 (1 nm)
Glass Substrate
LiF/Al (100 nm)2.8 eV
CBP TPBi
Ir(ppy)2(acac)
2.7 eV
6.1 eV6.2 eV
3.7 eV
6.0 eV
5.6 eV
CBP (45 nm)
3.2 eV
Ir(MDQ)2(Bpz)
CBP: 8% G (3 nm)
31
excitons are well-confined in the emissive zones in our design owning to the high triplet energy
levels of both CBP and TPBi. The stable spectra also suggest that the energy transfer from the G
to the GY emitter remains proficient under a wide range of current injection levels. It is worth
noting that beyond a GY layer thickness of 9 nm, the device performance reaches saturation
presumably due to increased charge imbalance or reduced optical out-coupling in the device
given the increased total device thickness.
Figure 18. a) CE-L plots of the GY + G devices considered, with numbers in the legends
denoting the thickness (in nm) of the device emissive layer. A photo of the optimized device (9
nm GY + 3 nm G) with an active area of 1 mm × 2 mm illuminating at 5,000 cd/m2 is shown in
the inset. (b) EQE-PE-L plots of the optimized GY-only and GY + G devices. (c) Normalized EL
spectra of the optimized GY+ G device (9 nm GY + 3 nm G) at a wide range of luminance
levels.
450 500 550 600 650 700
0.0
0.5
1.0
No
rma
lize
d E
L I
nte
nsity [
a.u
.]
Wavelength [nm]
100 cd/m2
1,000 cd/m2
5,000 cd/m2
10,000 cd/m2
100
101
102
103
104
0
10
20
30
40
50
60
70
80
90
Cu
rre
nt E
ffic
ien
cy [cd
/A]
Luminance [cd/m2]
6 GY + 3 G
9 GY + 3 G
12 GY + 3 G
15 GY + 3 G
100
101
102
103
104
0
5
10
15
20
25
Po
we
r E
ffic
ien
cy [
lm/W
]
EQ
E [
%]
Luminance [cd/m2]
15 GY
9 GY + 3 G
0
20
40
60
80
100
(a)
(b) (c)
32
4.3 Device Working Principle
4.3.1 Exciton Harvesting
In order to investigate the working principle of the devices in further detail, the spectral power
(total radiant power per wavelength) of the optimized device with and without G emission layer
under a fixed current density is measured as shown in Figure 19. It is observed that the
enhancement in spectral intensity with G emission layer is consistent with the device efficiency
enhancement. More importantly, from Figure 18b it is observed that a considerably reduced host
emission is present from the device with G emission layer incorporation. This suggests that the G
emission layer is able to further utilize the excitons that would have otherwise been wasted by
the GY emission layer. Additionally, from the energy level diagram in Figure 17, a considerably
higher HOMO level for the G emitter at 5.6 eV as compared to those of the host CBP (6.1 eV)
and the GY emitter (6.0 eV) is observed, which suggests an improved hole trapping by the G
emitter in CBP followed by direct exciton formation on the G emitter is highly probable. Such
carrier trapping phenomenon is also evidenced by the lower current density of the device with G
layer incorporation as shown in Figure 20. It be therefore be suggested that the G emitter is not
only capable of directly forming excitons, but also further harnessing excitons in the host that are
unused by or leaked through the GY emission layer, and subsequently perform efficient exciton
transfer to the adjacent GY emissive sites, thereby significantly enhancing the efficiency of the
overall device.
33
Figure 19. Spectral power spectra of the optimized GY-only and GY + G devices at (a) long and
(b) short wavelength ranges. A considerably higher host emission is observed for the optimized
GY-only device. The numbers shown in the legends stand for the thickness (in nm) of the device
emissive layer displayed in Figure 17.
From Equation (14), it is apparent that a high fraction of emissive excitons trapped by the G
emitter, χD, together with high energy transfer efficiency from G to GY emitter, ηD-A, can
significantly enhance the overall device efficiency with a predominant GY emission. Using
optimized device parameters for single-color G and GY devices, it can be deduced that the
fraction of emissive excitons trapped in the device, χD and χA, is ~0.96 and ~0.71, respectively.
(a)
(b)
500 550 600 650 700
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Sp
ectr
al P
ow
er
[W
/nm
]
Wavelength [nm]
15 GY
9 GY + 3 G10 mA/cm
2
350 400 450 500-0.005
0.000
0.005
0.010
0.015
0.020
0.025
Sp
ectr
al P
ow
er
[W
/nm
]
Wavelength [nm]
15 GY
9 GY + 3 G
10 mA/cm2
34
By applying Equation (14), a projected ηext at 1,000 cd/m2
is found to be ~22%, which is in
excellent agreement with that achieved from the optimal GY + G device shown in Figure 18b.
Figure 20. Current Density versus Voltage (J-V) plot of the optimized GY-only and GY + G
devices. A lower current density is seen for the GY + G device, suggesting more carrier trapping
is present. The numbers shown in the legends indicate the thickness (in nm) of the device
emissive layer illustrated in Figure 17.
4.3.2 Efficient Energy Transfer
To investigate the interzone energy transfer, the normalized electroluminescence (EL) spectra of
the devices are measured as shown in Figure 21. It is seen from Figure 21a that with an increase
in thickness of the GY emitting layer, the spectra progressively red shift as more contribution
from the GY emitter is present. Eventually, beyond a GY emitting layer thickness of 6 nm, the
EL spectra becomes constant with a peak emission at 539 nm regardless of the presence of the G
emitting layer as shown in Figure 21b. This suggests the GY emission layer has reached a
thickness with sufficient GY emissive sites available to receive the majority of the excitons
delivered from the G emitter. Using fits from the contribution of the individual EL spectra of the
two emitters, it is possible to quantitatively approximate the energy transfer efficiency from the
3 4 5 6 7 8 90
20
40
60
80
Cu
rre
nt
De
nsity (
mA
/cm
2)
Voltage (V)
15 GY
9 GY + 3 G
35
Figure 21. a, b) Normalized EL spectra of the GY + G devices considered. The numbers shown
in the legends represent the thickness (in nm) of the device emission layer as depicted in Figure
2a. (c) Solid-state PL spectrum of Ir(ppy)2(acac) doped in 50 nm CBP film and solution
absorption spectrum of Ir(MDQ)2(Bpz) dissolved in CH2Cl2 (~1 × 10-5
M).
G to GY emitter as shown in Figure 22. It is apparent that a 6 nm thick layer of GY indicates
over 95% transfer efficiency and 9 nm suggests nearly perfect transfer of over 99%. Such
efficient exciton energy transfer is further supported by a substantial overlap between the
photoluminescence (PL) emission spectrum of the G emitter and the triplet 3MLCT and
3LC
absorption states of the GY emitter as shown in Figure 21c. Interestingly, the fact that it required
6 nm to observe nearly complete energy transfer further suggests that the extent of such energy
transfer could be substantially longer than the generally accepted value of ~3 nm, which could
475 500 525 550 575 600 625 650
0.0
0.5
1.0
No
rma
lize
d E
L I
nte
nsity [
a.u
.]
Wavelength [nm]
3 G
6 GY
6 GY + 3 G
9 GY
9 GY + 3 G
12 GY
12 GY + 3 G
15 GY
15 GY + 3 G
300 400 500 600 700
0.0
0.5
1.0
No
rma
lize
d A
bso
rptio
n [
a.u
.]
No
rma
lize
d P
L I
nte
nsity [
a.u
.]
Wavelength [nm]
PL of Ir(ppy)2(acac)
Abs. of Ir(MDQ)2(Bpz)
3MLCT,
3LC
0.0
0.5
1.0
475 500 525 550 575 600 625 650
0.0
0.5
1.0
No
rma
lize
d E
L In
ten
sity [a
.u.]
Wavelength [nm]
3 G
0.5 GY + 3 G
1 GY + 3 G
2 GY + 3 G
3 GY + 3 G
6 GY + 3 G
Increasing
GY layer
thickness
(a)
(b) (c)
36
have significant implications for the thickness of interlayer (non-doped host layer) required for
the prevention of such interzone exciton transfer. This long energy transfer range further
suggests a Fӧrster-type mechanism is also in place as described by process [4] in Section 2.3,
either governed by angular momentum conservation39
or facilitated by spin-orbit coupling.23
Figure 22. Calculated energy transfer efficiency from G to greenish-yellow (GY) emitter versus
the thickness of the GY emissive layer.
4.4 Synthesis of the Greenish-Yellow Emitter
It has been well-established that upon photoexcitation of an Ir-based metal-organic complex, two
main electronic transitions will arise: 1) metal-to-ligand charge-transfer (MLCT) transition,
where an electron is promoted from a metal d orbital to a vacant π* orbital on one of the ligands,
and 2) ligand-centered (LC) transition, where an electron is promoted between π orbitals on one
of the coordinated ligands.40, 41
More importantly, due to the strong spin-orbit coupling exerted
by the Ir metal core, triplet MLCT and LC transitions become dominant, yielding a total of four
electronic states, i.e., singlet 1MLCT and
1LC as well as triplet
3MLCT and
3LC transition states.
The lowest excited state or the highly emissive state is generally consisted of an admixture (or a
0 2 4 6 8 100
20
40
60
80
100
Tra
nsfe
r E
ffic
ien
cy [%
]
Thickness [nm]
37
linear combination) of the lowest triplet states, 3MLCT and
3LC, or also known as a hybrid triplet
state, T1.42
Scheme 1. Synthesis and structure of the greenish-yellow dopant used in this work.
In order to tune the emission wavelength of the complex, substantial work has been carried out to
alter the LC transition state energy by changing the ligand structure such as the incorporation of
an electron-donating or electron-withdrawing substituent.34, 43-46
This change in ligand structure
can effectively change the frontier orbital energies, thereby shifting the lowest unoccupied
molecular orbital (LUMO) level that is localized on the cyclometalating ligands, and hence
tuning the triplet state energy, T1. Alternatively, one can also tune T1 by changing the highest
occupied molecular orbital (HOMO) level through the incorporation of various ancillary ligands
that can affect the MLCT transition state energy.47-49
Herein, in order to achieve a greenish-
yellow emission, we have adopted the latter approach and successfully employed a well-known
ancillary ligand, Bpz [tetrakis(1-pyrazolyl)-borate],50
to modify a highly efficient, standard red
phosphorescent emitter, Ir(MDQ)2(acac) [bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)
iridium(III)],51
as schematically depicted in Scheme 1. The ancillary ligand Bpz is known
38
previously for its remarkable blue-shifting capability by reducing the HOMO level of the Ir-
complex that also led to a very high efficiency deep-blue emitter, Fir6.48, 52, 53
The proton and
carbon NMR, mass spectrometry, and elemental analysis results for this complex are shown
below. A geometry optimized structure for Ir(MDQ)2(Bpz) with predicted HOMO and LUMO
distributions from time dependent density functional theory (TD-DFT) calculations is also shown
in Figure 23.
Figure 23. Geometry optimized structure and predicted HOMO and LUMO distributions of
Ir(MDQ)2(Bpz) by time-dependent density functional theory calculations.
Ir(MDQ)2(Bpz) : Yield: 70%. 1H NMR (400 MHz, CDCl3): δ 9.24 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz,
2H), 8.55 (d, J = 7.2 Hz, 2H), 8.02 (d, J = 8.2 Hz, 2H), 7.77 (m, 6H), 7.39 (s, 2H), 7.20 (d, J =
2.5 Hz, 2H), 7.12 (t, J = 7.7 Hz, 2H), 7.00 (d, J = 1.9 Hz, 2H), 6.18 (m, 6H), 6.02 (t, J = 1.9 Hz,
2H), 2.77 (s, 6H). 13
C NMR (100 MHz, CDCl3): δ 153.7, 149.7, 146.9, 144.8, 142.4(8), 142.4(6),
142.1, 141.9, 139.0, 138.8, 133.7, 133.0, 131.4, 130.0, 129.2, 129.1, 127.45, 125.3, 123.3, 115.7,
107.0, 105.4. HRMS (Dart) calc’d for C46H35BN12Ir [M + H]+ : 959.2824, found 959.2810. Anal.
calc’d for C46H34BN12Ir: C 57.68, H 3.58, N 17.55, found: C 57.49, H 3.51, N 17.37.
HOMO LUMO
39
Chapter 5 : Intrazone Energy Transfer
5.1 Theory
Although the interzone energy transfer technique has been quite effective as demonstrated in the
last chapter, it is only applicable to a single color OLED due to the limited extent of the energy
transfer distance. Herein, a more versatile approach known as intrazone energy transfer is
proposed based on a similar concept, where the exciton harvesting donor molecule is co-
deposited with the acceptor molecule into a common host to form the emissive layer. This
technique can be described in a similar fashion:
𝜂𝑒𝑥𝑡 = 𝛾𝜂𝑜𝑐 𝜒𝐴𝜙𝑃𝐿,𝐴 + 𝜒𝐷 𝜂𝐷−𝐴𝜙𝑃𝐿,𝐴 + 1 − 𝜂𝐷−𝐴 𝜙𝑃𝐿,𝐷 , (15)
where ϕPL is the luminescence quantum yield of the two emitters, χD (χA) represents the fraction
of emissive excitons that are trapped by the higher energy donor (lower energy acceptor) emitter
in the device, and ɳD-A represents the energy transfer efficiency from donor D to acceptor A. This
equation accounts for the energy transfer between two emitters, which is presumably of a
Fӧrster-type as typical emitter doping concentrations are low. High doping concentrations would
be required to provide sufficient orbital overlap for Dexter-type transfer between the emitters.
Furthermore, the amount of energy that are not transferred from the high energy donor to the low
energy acceptor will also contribute to the emission from the donor as indicated by the last term.
An additional advantage of this technique is that it can be applied to the case of multiple acceptor
molecules in the same host as illustrated in Figure 24, or even in a white OLED device as shown
later in Chapter 6. A generalized equation in this case may be represented by:
This chapter is based on a published work by Chang et al., Org. Electron. 13, 925 (2012).
40
𝜂𝑒𝑥𝑡 = 𝛾𝜂𝑜𝑐 Ӽ1𝜙𝑃𝐿,1 + Ӽ
𝑖 𝜂𝑖 ,𝑖−𝑗
𝑖−1𝑗=1 𝜙𝑃𝐿,𝑖−𝑗 + 1 − 𝜂𝑖 ,𝑖−𝑗
𝑖−1𝑗 =1 𝜙𝑃𝐿,𝑖
𝑛𝑖=2 , (16)
where n denotes the total number of dopants, and i and j are indexing terms that count each
dopant starting from the lowest energy one. Here, the first term in the bracket accounts for the
contribution from the lowest triplet energy dopant. The first summation accounts for all the
contribution from the second lowest triplet energy dopant up to the highest triplet energy dopant.
The second summation accounts for the total energy transferred to each of the lower triplet
energy dopants from each dopant starting from the second lowest triplet energy one. The final
term in the first summation is simply attributed to non-transferred energy or directly-emitted
emission from each dopant, once again starting from the second lowest triplet energy one. In the
following sections, the intrazone energy transfer technique will be investigated for the efficiency
enhancement of red OLEDs.
Figure 24. Illustration of the possible energy transfer processes in a three dopants system in a
common host. Cases with a higher number of dopants can be derived in a similar fashion.
5.2 Efficiency Enhancement on Red OLEDs
Eo
EHost
E2
E1
Ӽ1Ӽ2
E3
ɳ 2-1
Ӽ3
ɳ 3-2
ɳ 3-1
41
Among the three primary colors, red OLEDs are currently lagging behind green and blue
OLEDs9, 54-57
in terms of device performance presumably due to the energy gap law, which states
that an increase in non-radiative rate constant is associated with the reduction of the energy gap,
leading to a lower emission quantum yield.58
Furthermore, the significantly lower energy gap of
the red dopants as compared with the typical host material results in charge trapping and requires
higher doping concentration in order to form a secondary transport channel and facilitate charge
transport through dopant molecules, which in turn leads to concentration self-quenching.40, 59-64
There are thus very few reports of red OLEDs exhibiting a high external quantum efficiency of >
20%.
Here, it is demonstrated that the incorporation of green emitter Ir(ppy)2(acac) and red emitter
Ir(MDQ)2(acac) simultaneously into CBP host in a simplified wide-bandgap platform led to red
phosphorescent OLEDs with a high EQE of > 20% over a broad luminance range (10 - 5,000
cd/m2). In particular, a remarkable maximum EQE of 24.8% was achieved, which remained as
high as 20.8% at a high luminance of 5,000 cd/m2. To our knowledge, such high performance
device is considered the best red OLED reported to date using commercially available
phosphors. Such achievement can be attributed to the effective exciton harvesting function of the
green molecules in the wide-bandgap architecture, followed by efficient intrazone exciton energy
transfer to the red emitter to further activate a higher amount of red triplet emissive sites.
A schematic diagram of such simplified device structure and the corresponding energy-level
diagram are depicted in Figure 25, where TPBi serves as the electron transport layer and CBP
functions as both the hole transport layer and the host. The emissive layer consists of co-
evaporated green emitter (G) Ir(ppy)2(acac) and red emitter (R) Ir(MDQ)2(acac) with various
doping concentrations into the CBP host. All doping concentrations reported in this work are by
42
weight percentage. One of the key features of this design is the use of wide-bandgap and high
triplet energy electron transport layer, hole transport layer and host materials that can effectively
confine generated excitons in the emissive layer. There are also noticeably very small energy
barriers between the electron and hole transport layers, i.e., there is only ~0.1 eV difference in
the highest occupied molecular orbital level and lowest unoccupied molecular orbital level
between the two materials, which effectively prevents significant charge accumulation at the
electron/hole transport layer interface that could potentially induce various quenching processes.
Figure 25. Schematic device structure and corresponding energy-level diagram of the devices as
well as the molecular structure and triplet energies (T1) of the materials used. The EML consists
of co-evaporated Ir(ppy)2(acac) and Ir(MDQ)2(acac) with various doping concentrations by
weight % into CBP.
Figure 26 shows the current efficiency versus luminance (CE-L) plot of the OLED devices at a
fixed red doping concentration (2%) and varied green doping concentrations (from 0 to 12%). It
is clear that the co-doped devices show progressively better performance with reduced efficiency
roll-off at lower green doping concentrations as a result of minimized self-quenching. For the
optimized device with 2% doping each, a high current efficiency (power efficiency) of 37.0 cd/A
O
TPBi (75 nm)
EML (15 nm)
ITO/MoO3 (1 nm)
Glass Substrate
LiF/Al (100 nm)2.8 eV
CBP
TPBi
Ir(ppy)2(acac)
Ir(MDQ)2(acac)
2.75 eV2.7 eV
6.1 eV6.2 eV
3.0 eV
5.35 eV
5.6 eVCBP (65 nm)
N N
N
N N
N
N N
CH3
Ir
CH3
ON
N
CH3
2
CH3
Ir
O
O
CH3
2
N
CBP (T1 = 2.55 eV)
TPBi (T1 = 2.6 eV)
Ir(ppy)2(acac) (T1 = 2.3 eV)
Ir(MDQ)2(acac) (T1 = 2.0 eV)
43
(30.2 lm/W), 35.3 cd/A (21.7 lm/W), and 31.0 cd/A (15.7 lm/W) at 100 cd/m2, 1,000 cd/m
2, and
5,000 cd/m2, respectively, have been achieved. These correspond to an impressive EQE of
24.8%, 23.7%, and 20.8% at 100 cd/m2, 1,000 cd/m
2, and 5,000 cd/m
2, respectively.
Figure 26. a) CE-L plot for selected devices under a fixed red doping and a range of green
doping concentrations, and b) corresponding absolute irradiance spectra at a current density of 10
mA/cm2.
100
101
102
103
104
0
5
10
15
20
25
30
35
40
Luminance (cd/m2)
Cu
rre
nt E
ffic
ien
cy (
cd
/A)
R: 2%, G: 0%
R: 2%, G: 1%
R: 2%, G: 2%
R: 2%, G: 4%
R: 2%, G: 8%
R: 2%, G: 12%
500 600 700 800
0.0
0.2
0.4
0.6
0.8
Irra
dia
nce
(W
/nm
)
Wavelength (nm)
R: 2%. G: 0%
R: 2%. G: 1%
R: 2%. G: 2%
R: 2%. G: 4%
R: 2%. G: 8%
R: 2%. G: 12%
10 mA/cm2
(a)
(b)
44
Figure 27. a) EQE vs Ir(ppy)2(acac) concentration under a range of Ir(MDQ)2(acac)
concentrations at a luminance of 1,000 cd/m2, and b) EQE versus luminance comparison
between the optimized co-doped device and optimized solely red doped device. Inset shows the
EL spectra of the optimized co-doped device under a wide range of current densities.
The results shown in Figure 26 clearly demonstrate that the co-doped green phosphor can
significantly enhance red emission. However, it may be arguable that the solely red doped device
has not yet been optimized, i.e., a different doping concentration may also enhance the red
emission efficiency. Therefore, a comprehensive study on the performance of the devices under a
wide range of red and green doping concentrations is conducted. The EQEs of which at a
luminance of 1,000 cd/m2 are summarized in Figure 27a. Without any green doping, the red
0 2 4 6 8 10 120
5
10
15
20
25
EQ
E (
%)
Ir(ppy)2(acac) Concentration (%)
Ir(MDQ)2(acac): 1%
Ir(MDQ)2(acac): 2%
Ir(MDQ)2(acac): 4%
Ir(MDQ)2(acac): 8%
Ir(MDQ)2(acac): 12%
Ir(MDQ)2(acac): 16% 1,000 cd/m
2
100
101
102
103
104
0
5
10
15
20
25
EQ
E (
%)
Luminance (cd/m2)
R: 2%, G: 2%
R: 4%, G: 0%
(a)
(b)500 600 700 800
0.0
0.5
1.0
EL Inte
nsity (
a.u
.)
Wavelength (nm)
1 mA/cm2
10 mA/cm2
100 mA/cm2
45
doped device exhibits the highest EQE of only 17.3% at 4% doping concentration. The highest
EQE device was realized at 2% red and 2% green doping, which is equivalent to ~1.35 times that
of the optimized solely red doped device (see Figure 27b). The inset of Figure 27b shows the
electroluminescence spectra of the optimized co-doped device, which remain stable under a wide
range of current densities with constant Commission Internationale de l’Eclairage coordinates of
(0.61, 0.39). In the next section, the working mechanism behind this enhancement will be
discussed.
5.3 Device Working Principle
5.3.1 Exciton Harvesting
It is generally believed that the significantly lower energy gap of the red dopants as compared to
the typical host material results in charge trapping and requires higher doping concentrations in
order to facilitate charge transport through dopant molecules, which in turn leads to
concentration self-quenching and higher driving voltages. However, in our devices, the current
density of the 2% green and red co-doped device is significantly lower than that of the 2% and
4% solely red doped devices as shown in Figure 28a, which suggests that even more charge
traps exist in the co-doped device, yet the efficiency of the co-doped device is considerably
higher (see Figure 27b). To further illustrate that the green dopant does function as traps, the
driving voltage of the devices at 1 mA/cm2 is plotted as a function of the green doping
concentration in Figure 28b. At low doping concentrations, the hopping among the green dopants
is not preferable and the dopants act mainly as carrier traps, i.e. carrier mobility decreases
significantly from zero to 1% doping concentration. However, when the doping concentration
increases, the hopping among the dopants becomes favorable and the mobility increases as a
function of the doing concentration, which leads to a decrease in driving voltage.
46
Figure 28. a) Current density versus voltage for selected devices. The inset shows a table of turn
on voltages defined at a luminance of 1 cd/m2. (b) Driving voltage versus Ir(ppy)2(acac) doping
concentration for devices with 2% red doping at a current density of 1 mA/cm2.
The above results suggest that even though the red dopants are known to trap charges, the
excitons are not directly formed on the red dopant but rather in the host at the CBP/TPBi
interface and subsequently the energy is transferred to the dopant. This is evidenced by the fact
that the turn on voltage of 3.1 V is significantly higher than the photon energy of the red
emission at ~2.05 eV (peak of EL spectrum). In the co-doped device, however, the turn on
voltage is roughly 0.2 V lower (see the inset of Figure 28a), which indicates that the green
phosphors help form excitons as well. In fact, it has been shown that excitons can be directly
2 3 4 510
-5
10-4
10-3
10-2
10-1
100
101
Cu
rre
nt D
en
sity (
mA
/cm
2)
Voltage (V)
R: 2%, G: 2%
R: 2%, G: 0%
R: 4%, G: 0%
Turn On Voltage at 1 cd/m2
R: 2%, G: 2% 2.9 V
R: 2%, G: 0% 3.1 V
R: 4%, G: 0% 3.1 V
(a)
0 2 4 6 8 10 124.2
4.4
4.6
4.8
5.0
5.2
Dri
vin
g V
olta
ge
(V
)
Ir(ppy)2(acac) Concentration (%)
(b)
1 mA/cm2
47
formed on Ir(ppy)2(acac),65
which is consistent with the results reported herein. It is also worth
noting that the device structure employed in this study is without any significant barrier or
blocking layers, which has been believed to be necessary to confine the carriers and excitons for
high EL efficiency. The high performance of the co-doped device suggests that the green
phosphors function not only as carrier traps, but also as exciton formation sites, thereby acting as
effective exciton harvesters.
5.3.2 Efficient Energy Transfer
Now that the increase in carrier trapping and exciton formation by the green dopant has been
established, it is worth looking into more detail at the energy transfer process between the two
emitters in the emissive layer. Figure 29 shows the room temperature absorption spectra of
Ir(ppy)2(acac) and Ir(MDQ)2(acac), as well as the photoluminescence spectra of CBP and
Ir(ppy)2(acac). The considerable overlap of the CBP emission spectrum with the absorption
spectra of both Ir(ppy)2(acac) and Ir(MDQ)2(acac) suggests effective Förster and/or Dexter
energy transfer from the host to both guest molecules. Due to the presence of the heavy metal Ir
atom in the emitters, intersystem crossing from the singlet charge transfer state to the triplet
metal ligand charge transfer state (3MLCT) occurs rapidly for both emitters. More importantly,
the substantial phosphorescent emission spectrum overlap of Ir(ppy)2(acac) with the 3MLCT
absorption of Ir(MDQ)2(acac) (at ~525 nm) implies that efficient Förster and/or Dexter energy
transfer from the green emitter to the red emitter are highly favorable depending on the
respective doping concentrations. The higher the co-doping concentration, the shorter the
distance is between the two dopant molecules, and both Dexter and Förster processes could
occur. At low co-doping concentrations however, only the Förster process will prevail due to its
considerably longer transfer range. It worth noting that energy transfer from the singlet states of
48
Ir(ppy)2(acac) to the singlet states of Ir(MDQ)2(acac) is also possible, however, cannot be
dominant due to the efficient ISC process of each emitter.
Figure 29. Normalized absorption spectra of Ir(ppy)2(acac) and Ir(MDQ)2(acac) in CH2Cl2 (1.0
× 10-5
M), as well as normalized PL spectra of CBP in solid state and Ir(ppy)2(acac) in CH2Cl2
(1.0 × 10-5
M), where the excitation wavelengths are at 330 nm and 400 nm, respectively. Inset
illustrates the dominant energy transfer processes between the singlet (S) and triplet (T) energy
levels of the host and dopants, where dotted arrows represent Fӧrster transfer, solid arrows
denote ISC, and dashed arrows represent Dexter transfer. So denotes the ground state.
To further investigate the energy transfer process, the normalized EL spectra at a fixed green
doping level of 2% under a range of red doping levels are measured as shown in Figure 30. It is
observed that an increase in green emission intensity associates with the reduction of red doping
(inset), which can be attributed to the saturation of the red triplet emission sites by the excitons
from the green emitter. This is expected since the typical Förster and/or Dexter energy transfer
processes occur on a much faster time scale than the excited state lifetime of the phosphors. It is
worth noting that the observed blue shift arises from red doping concentration reduction,
resulting in a reduced aggregation, similar to the trend observed for the devices without any
green doping. This cascaded energy transfer evidently is quite long range, given the levels of
300 400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
PL
In
ten
sity (
a.u
.)
Ab
so
rptio
n (
a.u
.)
Wavelength (nm)
PL of CBP
PL of Ir(ppy)2(acac)
Abs. of Ir(ppy)2(acac)
Abs. of Ir(MDQ)2(acac)
3MLCT
0.0
0.2
0.4
0.6
0.8
1.0
TR
So
TCBP
TG
SCBP
SGSR
49
phosphor doping in our devices. This suggests a Förster-type mechanism19
is involved, either
promoted by spin-orbit coupling23
or allowed by angular momentum conservation.39
It can
therefore be deduced that the efficiency enhancement is attributed to improved host exciton
utilization by the green phosphor, followed by efficient triplet energy transfer from the green to
lower energy red emitters as expressed by equation (15). Using Equation (15) and device
parameters from optimized single emitter devices, it can be derived that the fraction of emissive
excitons trapped by each emitter, Ӽ, are ~0.96 and ~0.77 for green and red devices, respectively.
Figure 30. EL intensity spectra normalized to the dominant red peak at a current density of 10
mA/cm2
for selected devices under a fixed green doping and a range of red doping
concentrations. Inset shows a ten times magnified spectrum of the region enclosed in the dashed
box, which highlights the green spectral peak evolution with Ir(MDQ)2(acac) concentration
reduction.
In order to quantify the energy transfer process, time correlated single photon counting (TCSPC)
measurements were conducted. Here, a yellow acceptor dopant, Ir(BT)2(acac) [iridium (III) bis(2
phenylbenzothiozolato N,C2′) (acetylacetonate)], is also studied as a comparison. Using TCSPC,
the transient decay time of the green donor emission at 520 nm under various co-doping
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0 R: 16%, G: 2%
R: 12%, G: 2%
R: 8%, G: 2%
R: 4%, G: 2%
R: 2%, G: 2%
R: 1%, G: 2%
EL Inte
nsity (
a.u
.)
Wavelength (nm)
500 550
0.00
0.05
0.10
EL Inte
nsity (
a.u
.)
Wavelength (nm)
Ir(ppy)2(acac)
Reducing
Ir(MDQ)2(acac)
concentration
50
concentrations for both red and yellow doped CBP films are conducted as shown in Figure 31.
Control samples of green donor-doped only films at various concentrations (2%, 4%, 6%, and
8%) revealed similar decay time constants of 1.15~1.20 µs, which include both the non-radiative
and radiative relaxation processes of the green donor triplet states. In co-doped films, it is
anticipated that any energy transfer from the green donor to either red or yellow acceptor
molecules will induce an additional green donor triplet relaxation path, leading to a shorter decay
time. The transient donor emission intensity can then be defined by:
𝐼 𝑡 = 𝑒−𝐾𝑐𝑡 𝐶1 + 𝐶2𝑒−𝑘𝑒𝑡 𝑡 , (17)
where Kc represents the decay rate constant of the donor emission (from the control samples), ket
denotes the energy transfer rate from donor to acceptor, and C1 and C2 are related to the donor
and acceptor concentrations, respectively. Using equation (17), it is possible to describe the
transient response of the donor emission in co-doped films as illustrated in Figure 31a and 31b,
and obtain the energy transfer rate as shown in Figure 31c. The energy transfer efficiency can
then be expressed as:
𝜂𝐷−𝐴 = 𝑘𝑒𝑡
𝑘𝑒𝑡 + 𝑘𝑟+ 𝑘𝑛𝑟=
𝑘𝑒𝑡
𝑘𝑒𝑡 + 𝐾𝑐 , (18)
where the Kc term consists of the sum of radiative (kr) and non-radiative (knr) rate constants of
the donor triplet states of the control films. From Figure 31a and 31b, it is clearly observed for
both red and yellow emissive films a faster transient decay with increasing co-doping
concentration, which corresponds to a reduction in donor-to-acceptor molecule distance that
promotes the energy transfer process. It is worth noting that in co-doped films, the transient
decay response of the lower energy red and yellow emissions does not alter significantly
compared to those from single doped red and yellow films, suggesting no other non-radiative
51
energy transfer path took place. This is expected since any increase in the excited state
population of the lower energy emitters should not affect their triplet radiative decay lifetimes.
Figure 31. Solid state transient response of (a) red and green co-doped CBP films and (b) yellow
and green co-doped CBP films at various co-doping concentrations. The solid lines are the
exponential fits to the transient decay responses. The excitation wavelength is at 350 nm. c)
Calculated energy transfer rate and efficiency versus total dopant concentration with the control
sample concentration corresponding to the green donor concentration of the co-doped films.
Triangles (squares) and rhombuses (circles) denote the energy transfer efficiency (energy
transfer rate) of co-doped yellow and red emissive films, respectively.
As shown in Figure 31c, the ηD-A is calculated to be as high as ~90.2 and ~92.1% for red and
yellow emissive films, respectively, at low co-doping concentrations (2% each). The ηD-A further
reaches ~99.6% at high co-doping concentrations (8% each), which represents nearly perfect
0.0 0.1 0.2 0.3 0.4
10-2
10-1
100
Y: 8%, G: 8%
Y: 6%, G: 6%
Y: 4%, G: 4%
Y: 2%, G: 2%
Y: 0%, G: 4%
No
rma
lize
d In
ten
sity (
a.u
.)
Time (s)
0 5 10 15 2010
6
107
108
109
1010
En
erg
y T
ran
sfe
r E
ffic
ien
cy (
%)
Total Dopant Concentration (%)
En
erg
y T
ran
sfe
r R
ate
(1
/s)
80
85
90
95
100
c
0.0 0.1 0.2 0.3 0.4
10-2
10-1
100
No
rma
lize
d In
ten
sity (
a.u
.)
Time (s)
R: 8%, G: 8%
R: 6%, G: 6%
R: 4%, G: 4%
R: 2%, G: 2%
R: 0%, G: 4%
a
b
(c)
30 35 40 45 50 5510
6
107
108
109
1010
Energ
y T
ransfe
r E
ffic
iency (
%)
Distance ( )
Energ
y T
ransfe
r R
ate
(1/s
)
80
85
90
95
100
(c)
0.0 0.1 0.2 0.3 0.4
10-2
10-1
100
No
rma
lized
In
tensity (
a.u
.)
Time (s)
R: 8%, G: 8%
R: 6%, G: 6%
R: 4%, G: 4%
R: 2%, G: 2%
R: 0%, G: 4%
(a)
0.0 0.1 0.2 0.3 0.4
10-2
10-1
100
Y: 8%, G: 8%
Y: 6%, G: 6%
Y: 4%, G: 4%
Y: 2%, G: 2%
Y: 0%, G: 4%
Norm
aliz
ed
In
ten
sity (
a.u
.)
Time (s)
(b)
30 35 40 45 50 5510
6
107
108
109
1010
Energ
y T
ransfe
r E
ffic
iency (
%)
Distance ( )
Energ
y T
ransfe
r R
ate
(1/s
)
80
85
90
95
100
(c)
0.0 0.1 0.2 0.3 0.4
10-2
10-1
100
No
rma
lized
In
tensity (
a.u
.)
Time (s)
R: 8%, G: 8%
R: 6%, G: 6%
R: 4%, G: 4%
R: 2%, G: 2%
R: 0%, G: 4%
(a)
0.0 0.1 0.2 0.3 0.4
10-2
10-1
100
Y: 8%, G: 8%
Y: 6%, G: 6%
Y: 4%, G: 4%
Y: 2%, G: 2%
Y: 0%, G: 4%
Norm
aliz
ed
In
ten
sity (
a.u
.)
Time (s)
(b)
52
energy transfer. This high energy transfer efficiency together with an increased exciton
utilization rate can well-explain the observed spectral EL intensity enhancement of the lower
energy red emission.
It is worth noting that for high co-doping concentrations, an extra exponential term is included
as shown in equation (19) to account for donor-to-donor exciton diffusion before eventually
transferring to an acceptor, which is a relatively slow process. This is because green donor
emission transient response under high co-doping concentrations (6% and 8% each) for both red
and yellow co-doped films reveal a more complicated decay response that can be more
accurately described by introducing a third exponential term to equation (17) as:
𝐼 𝑡 = 𝑒−𝐾𝑐𝑡 𝐶1 + 𝐶2𝑒−𝑘𝑒𝑡 𝑡 + 𝐶3𝑒
−𝜅𝑒𝑡 𝑡 , (19)
where 𝜅et is the relatively lower energy transfer rate ascribed to the donor-to-donor energy
transfer or exciton diffusion processes66
taking place prior to the eventual donor-to-accepter
energy transfer as illustrated in Figure 32 (process 2). C3 is related to both donor and acceptor
concentrations. In this case, the energy transfer rate is taken as the average of ket and 𝜅et.
Although exciton diffusion exists in samples of all co-doping concentrations, the third
exponential term was not critical at low co-doping concentrations (2% and 4% each for both
yellow and red emissive films), which suggests that direct energy transfer (process 1 in Figure
32) is pre-dominant. This can be understood by the fact that the spectral overlap for the donor-to-
donor energy transfer is considerably smaller than the case for donor-to-acceptor energy transfer
(see Figure 29). As a result, only at higher co-doping concentrations or shorter dopant-to-dopant
distances will such donor-to-donor energy transfer (process 2 in Figure 32) become substantial.
Additionally, since the transient response of the control samples for the doping concentration
53
considered (CBP: G: 2%-8%) revealed very similar decay times (~1.15-1.20 s), we can assume
that the contribution from concentration-induced donor-to-donor self-quenching process is
negligibly small.
Figure 32. Schematic illustration of two dominant energy transfer processes in co-doped films
under high concentrations: (1) direct transfer from donor-to-acceptor, and (2) indirect transfer by
encountering single or multiple donor-to-donor transfers (exciton diffusion) before a donor-to-
acceptor transfer occurs. The green and red circles represent donor and acceptor molecules,
respectively, and the blue arrows denote energy transfer.
54
Chapter 6 : Design of High Efficiency and High Color Quality
White OLEDs
6.1 Brief Overview on White OLEDs
White organic light-emitting diodes (WOLEDs) are considered the most promising technology
for next generation solid-state lighting due to their many attributes such as high energy
efficiency, eye-friendly diffusive warm light, ultrathin form factor, etc. To produce high
efficiency WOLEDs, the use of phosphors has become indispensable owning to their ability to
generate light from both singlet and triplet excitons, thereby achieving nearly 100% internal
quantum efficiencies.3
In addition to high efficiency, a high color rendering capability for objects
viewed under such white illumination source is another equally important parameter for solid-
state lighting. In particular, a color rendering index of over 80 is required to qualify WOLEDs as
suitable illumination sources. To increase CRI, a number of groups have developed hybrid
WOLEDs by employing a blue fluorophore along with green and red phosphors.67-69
Schwartz et
al.68
have employed a blue fluorophore N,N′-di-1-naphthalenyl-N,N'-diphenyl-[1,1'':4'',1'''-
quaterphenyl]-4,4'''-diamine (4P-NPD) together with a green and an orange phosphor to fabricate
WOLEDs having a power efficiency at 1000 cd/m2 (ηp,1000) of 37.5 lm/W, an external quantum
efficiency (EQE1000 ) of 16.1%, and a CRI of 86. More recently, Chen et al.69
have employed
4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) as the blue fluorophore along with a
yellowish-green and a red phosphor to obtain a ηp,1000 of 11.3 lm/W, an EQE1000 of 10.7%, and a
CRI of 91.2. Here, high CRI values are achieved at a cost of lower device efficiency.
This chapter is based on published works by Chang et al., IEEE J. Display Technol. 9, 459 (2013) (Section 6.1) and
Chang et al., Adv. Funct. Mater. 23, 3204 (2013) (Sections 6.2 and 6.3).
55
To increase the device efficiency, several groups have taken the approach of using only two
phosphorescent emitters to achieve very high efficiencies.9, 45, 70
Su et al.9 have reported a two-
color WOLED employing a blue and an orange phosphor together with a carrier and exciton
confining design to achieve high ηp,1000 and EQE1000 of 44.0 lm/W and 25.0%, respectively.
Wang et al.45
have incorporated a fluoro-modified Ir(BT)2(acac) yellow phosphor together with a
blue phosphor to obtain a maximum power efficiency (ηp,max) of 34.0 lm/W and external
quantum efficiency (EQE max ) of 26.2%. While high in energy efficiency, these devices have
extremely low CRI (< 70) which is insufficient for illumination sources. Therefore, the use of
three or more phosphorescent emitters has become a prerequisite for high color-rendering and
highly efficient lighting applications.71, 72
To solve this problem, the current prevailing wisdom has been designing WOLEDs by co-doping
multiple phosphorescent emitters with different colors into one emissive layer, i.e., as a single
unit, while preserving all emission colors with the advantage of having a reduced total number of
organic layers.53, 73-75
However, such approach makes it more difficult to tune the emission
spectrum as most of the energy will naturally transfer to the lower energy emitters. This typically
results in the use of high concentration high energy dopants (e.g., blue phosphors) and low
concentration low energy dopants (e.g., red phosphors) with respect to the host, which further
limits the degree of control over the emission efficiency for each color.
56
Figure 33. Current status of energy conversion efficiency of OLEDs (in solid circles) and LEDs
(in open rhombuses) in the visible spectrum. The LED data were taken from Ref. [76]. The
OLED efficiencies are power efficiencies of the device at 1,000 cd/m2 after applying the out-
coupling enhancement technique (a factor of ~2.5) listed in Ref. [77] and normalized to the
theoretical limit for the corresponding wavelength. The dashed grey curve represents the
photopic sensitivity response curve of human eyes. The OLED data were taken from Ref. [44,
57, 71, 77, 78-80].
During the past decade, traditional LED technology has also made incredible progress in
improving the energy conversion efficiency, in particular using GaN-based blue LEDs. However,
traditional compound semiconductor LEDs have been problematic in producing LEDs in green-
yellow color, or known as “Green-Yellow” gap. This is unfortunate as human eyes have the
highest sensitivity (photopic sensitivity) for these wavelengths, i.e. least amount of energy is
required to produce a discernible signal for a given task. OLEDs, however, face no such
constraints. Figure 33 shows the efficiency of the state-of-the-art OLEDs as compared to state-
of-the-art inorganic LEDs.76
It is seen that OLEDs are superior in the range of the visible
spectrum that is most sensitive to the human eye, particularly in the green portion, as compared
400 450 500 550 600 650 7000
10
20
30
40
50
60
70
Effic
ien
cy (
%)
Wavelength (nm)
InGaNAlInGaP
OLED
57
to inorganic LEDs, although there is still much room for improvement to be made on yellow,
orange and red OLEDs. This is mainly due to the strong display industry in developing OLEDs
emitting in the primary colors. In fact, from the energy gap law,4 it is anticipated that the
efficiency of yellow and orange OLEDs can surpass that of the red OLEDs. The blue OLEDs
currently are closely catching up in terms of efficiency compared to inorganic LEDs, however
they are well-known to have rather short device lifetime due to inherent instability of the blue
phosphors, leading to a rapid efficiency drop-off at higher luminance levels needed for lighting.
Figure 34. Light emission of spectra of typical incandescent bulb, fluorescent tube, white LED,
and white OLED with warm white illuminations.
As mentioned above, one important but often overlooked parameter is the color rendering
capability of the light source as defined by the color rendering index.81
The CRI is a measure of
how natural the colors of objects can be reproduced under a given illumination condition.
Typically, as shown in Figure 34, the CRI of the spectrum produced by currently prevalent
fluorescent tubes has a value lower than 80, which is considered as poor light sources. The CRI
of typical white LEDs, which use a blue InGaN LED to excite a yellow phosphor to produce a
400 500 600 700 800
Inte
nsity (
a.u
.)
Wavelength (nm)
OLED
3725K, CRI: 82
InGaN LED
3362K, CRI: 75
Fluorescent Tube
3294K, CRI: 76
Incandescent Bulb
3300K, CRI: 100
58
spectrum shown in Figure 34, is also below the minimum of 80 required to qualify it as an
adequate light source. However, a continuous broadband spectrum with a high CRI of over 80
can be readily obtained using OLEDs with three or more emitters each producing an intrinsically
wide bands as shown in Figure 34.71
This suggests a significant advantage of using OLEDs as
high quality light sources.
In the following, by employing intrazone exciton transfer to boost the efficiency of lower energy
emitters (e.g. yellow and red) together with the implementation of a cascaded architecture, a
simultaneous improvement in efficiency and CRI of the WOLED device has been demonstrated
with record performance.
6.2 Cascaded Architecture
Figure 35a shows a schematic illustration of four WOLED device structures (W1-W4) used in
this work, and Figure 35b shows the corresponding energy level diagram. In each device, TPBi
serves as the electron transport layer, and CBP functions as a hole transport layer, and as a triplet
host. Standard ITO/MoO3 anode and LiF/Al cathode are applied. In this configuration, the
majority of excitons will be generated near the CBP/TPBi interface (on both sides) before being
harvested by the emitters (i.e. recombination occurs) on the CBP side. Further, as TPBi has a
higher triplet energy than that of CBP, the generated excitons can be well-confined on the CBP
side where the dopants are incorporated. Since the blue emitter, FIrpic, has the closest energy
levels to both CBP and TPBi, direct exciton formation on the blue dopant is unlikely and it is
critical to place the blue emitter closest to the CBP/TPBi interface to harvest excitons first. Other
lower energy green, yellow and red emitters are placed sequentially next to blue to harvest
excitons in a cascaded fashion as shown by the energy level diagram in Figure 35b. This
cascaded design using a single host allows for only a single site for exciton generation and
59
recombination without introducing other barrier layers (i.e. a second or third host material) that
could induce undesirable charge accumulation in the device, leading to notorious triplet-polaron
and polaron-polaron quenching processes. It is also important that there is no interlayer between
two adjacent emitting layers so that the surplus excitons from the higher energy emitter can be
readily transferred into an adjacent emitter having a lower energy through the emitters. This
interzone free flow of excitons through not only the host material but also through the dopants is
in stark contrast to the widely accepted design involving the use of interlayers, and is key to
maximize our device overall quantum efficiency.
To demonstrate this point, a series of devices have been fabricated with one emitter (blue), two
emitters (blue and green), three emitters (blue, green, and yellow), and four emitters (blue, green,
yellow, and red) as shown in Figure 36. It is found that with each additional emitter
incorporated, the EQE progressively improves from 8.5% to 19.2% as the emissive zone
increases from one to four, respectively. In particular, it is observed that for blue doped only
device, the emission efficiency is fairly low (< 10%), indicating that a considerable portions of
the excitons are not being transferred from CBP to FIrpic. However, with the inclusion of a green
doped region adjacent to the blue doped region, the device shows a nearly twofold increase in
efficiency without sacrificing the emission from FIrpic, which demonstrates that the energy
transfer from CBP to FIrpic, and then to the adjacent Ir(ppy)2(acac) is less significant compared
to direct CBP energy transfer to the Ir(ppy)2(acac) after exciton diffusion in host CBP from blue
to green doped region. With the inclusion of the yellow and red emissive zones adjacent to the
green, the emission of the green emitter is clearly reduced, which suggests facile energy transfer
to the adjacent lower energy emitters. This shows that excitons generated near the CBP/TPBi
interface are effectively harvested by the cascaded emissive zones.
60
Figure 35. Device configurations (a) and energy level diagrams (b) for WOLEDs W1-W4. The
dopants employed are FIrpic for blue (B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow
(Y), and Ir(MDQ)2(acac) for red (R). All doping concentrations are in weight %. (c) A photo of a
large area (80 mm × 80 mm) WOLED (W3) illuminating at 5,000 cd/m2 with a color rendering
index of 85.
CBP : y% R : z% G (17 nm)
CBP : w% Y : x% G (3.5 nm)
CBP : 8% G (3 nm)
CBP : 20% B (10 nm)
W1: w = 8%, x = 0%, y = 8%, z = 0%
W2: w = 8%, x = 8%, y = 8%, z = 0%
W3: w = 8%, x = 8%, y = 8%, z = 8%
W4: w = 4%, x = 4%, y = 4%, z = 4%
TPBi (55 nm)
LiF/Al (100 nm)
CBP (35 nm)
ITO/MoO3 (1 nm)
Glass Substrate
(a)
2.8 eV
CBP TPBi
2.7 eV
6.1 eV 6.2 eV
3.2 eV
5.8 eV
5.5 eV
3.0 eV3.3 eV
5.35 eV
5.6 eV
R Y G B
(b) (c)
61
Figure 36. Spectral power spectra at 10 mA/cm2 with a progressive addition of each emissive
layer to construct W1. Inset shows EQE of devices at a luminance of 1,000 cd/m2. The dopants
used are FIrpic for blue (B), Ir(ppy)2(acac) for green (G), Ir(BT)2(acac) for yellow (Y), and
Ir(MDQ)2(acac) for red (R). Each device layer thicknesses and doping concentrations are as
shown for W1 in Figure 35.
6.3 Performance Enhancement by Intrazone Energy Transfer
A summary of device performance is listed in Table 1, and the power efficiency-luminance-
external quantum efficiency (PE-L-EQE) characteristics as well as the corresponding
electroluminance (EL) spectrum (insets) of each device are shown in Figure 37. The inter-zone
exciton harvesting concept led to device W1 with decent EQE100 (ηp,100) and EQE1000 (ηp,1000) of
16.8% (32.1 lm/W) and 19.2% (28.1 lm/W), respectively. The high efficiency at high luminance
is mainly due to the elimination of accumulated carriers across the entire device, i.e. the unique
design of using CBP as both the host and hole transport layer, which has been demonstrated in a
previous work in Ref. [36]. Also noted is the spectral shift with a reduction in blue emission and
improvement in yellow and red emissions at a higher luminance as shown in the inset of Figure
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
Sp
ectr
al P
ow
er
(w
/nm
)
Wavelength (nm)
BGYR
BGY
BG
B
10 mA/cm2
Device EQE1000
BGYR 19.2%
BGY 17.1%
BG 16.2%
B 8.6%
62
37a. This can be attributed to a shift of the exciton generation towards the yellow and red doped
regions at higher driving voltages. Since CBP can also transport electrons quite effectively, at a
higher driving voltage, relatively more electrons can be injected deeper into the CBP side to form
excitons in the host which are subsequently transferred to the yellow and red dopants, resulting
in the emission intensity enhancement.
Figure 37. a-d) PE-L-EQE characteristics of the WOLED devices considered in this work. The
insets show the corresponding electroluminance spectra under various luminances normalized to
the green emission peak at 520 nm.
100
101
102
103
104
0
10
20
30
40
50
EQ
E (
%)
Po
we
r E
ffic
ien
cy (
lm/W
)
Luminance (cd/m2)
0
5
10
15
20
100
101
102
103
104
0
10
20
30
40
50
EQ
E (
%)
Po
we
r E
ffic
ien
cy (
lm/W
)
Luminance (cd/m2)
0
5
10
15
20
100
101
102
103
104
0
10
20
30
40
50
EQ
E (
%)
Po
we
r E
ffic
ien
cy (
lm/W
)
Luminance (cd/m2)
0
5
10
15
20
25
100
101
102
103
104
0
10
20
30
40
50
EQ
E (
%)
Po
we
r E
ffic
ien
cy (
lm/W
)
Luminance (cd/m2)
0
5
10
15
20
25
(d)(c)
(a)
400 500 600 700 800
No
rma
lize
d
EL
In
ten
sity (
a.u
.)
Wavelength (nm)
5,000 cd/m2
1,000 cd/m2
400 500 600 700 800
Norm
aliz
ed
EL Inte
nsity (
a.u
.)
Wavelength (nm)
5,000 cd/m2
1,000 cd/m2
400 500 600 700 800
No
rma
lize
d
EL
In
ten
sity (
a.u
.)
Wavelength (nm)
5,000 cd/m2
1,000 cd/m2
400 500 600 700 800
No
rma
lized
EL
Inte
nsity (
a.u
.)
Wavelength (nm)
5,000 cd/m2
1,000 cd/m2
(b)
W1 W2
W3 W4
4627 K 4419 K
3332 K 3363 K
63
Table 1. Summary of white OLED performances demonstrated in this work.
Device ηp,100/ηc,100/EQE100a
[lm W-1
/cd A-1
/%]
ηp,1000/ηc,1000/EQE1000b
[lm W-1
/cd A-1
/%]
ηp,5000/ηc,5000/EQE5000c
[lm W-1
/cd A-1
/%]
CRId CIE(x,y)
e
W1 32.1/39.2/16.8 28.1/44.8/19.2 20.5/41.5/17.8 71,72 (0.37,0.48)
W2 37.3/45.6/19.1 32.2/50.2/21.0 23.1/46.0/19.2 70,69 (0.38,0.48)
W3 40.5/53.7/23.0 31.0/53.9/23.3 20.8/47.0/20.4 84,85 (0.44,0.45)
W4 42.6/55.1/23.5 33.8/57.7/24.5 23.2/51.2/21.9 81,82 (0.44,0.46)
a Power efficiency (PE), current efficiency (CE) and external quantum efficiency (EQE) at 100
cd/m2.
b PE, CE, and EQE at 1,000 cd/m
2.
c PE, CE and EQE at 5,000 cd/m
2.
d Color rendering
index at 1,000 cd/m2
and 5,000 cd/m
2.
e Commission Internationale de L’Eclairage coordinates at
5,000 cd/m2.
In order to further improve upon the efficiency of the device, a higher energy (green) phosphor
was incorporated into the yellow emissive layer (W2) to enable intrazone energy transfer, i.e.
molecular energy transfer within a common emissive layer. From our previous study on single
color red OLED devices, we learned that incorporation of the green phosphor will improve the
emission efficiency of a red OLED, while preserving the overall emission spectrum, i.e. the EL
spectrum remains predominantly in red.82
Similarly, it is apparent here that with the green
phosphor incorporation in device W2, the yellow emission is significantly enhanced, becoming
the dominant emission peak as shown in the inset of Figure 37b. This spectral intensity
enhancement corresponds to a considerable improvement in EQE100 and EQE1000 to 19.1% (37.3
lm/W) and 21.0% (32.2 lm/W), respectively. However, devices W1 and W2 exhibit CRI values
64
of only 71 and 70 (see Table 1), respectively, which do not qualify them as adequate illumination
sources.
To improve the CRI, the green phosphor was incorporated into the red emissive layer in addition
to the yellow emissive layer (W3). From the EL spectrum in the inset of Figure 37c, it is
observed that the red emission at ~610 nm became the most dominant peak, leading to a high
CRI of 84 at 1,000 cd/m2. The green phosphor incorporation in the red emissive region also
enhanced EQE100 and EQE1000 to 23.0% (40.5 lm/W) and 23.3% (31.0 lm/W), respectively. At a
high luminance of 5,000 cd/m2 that is critical for solid-state lighting, the EQE remains as high as
20.4% with a high CRI of 85, Commission Internationale de L’Eclairage (CIE) coordinates of
(0.44, 0.45) and a correlated color temperature (CCT) of 3332 K, corresponding to a desirable
warm white illumination. To the best of our knowledge, this is the first report of a WOLED
achieving EQE5000 of over 20% with a CRI of 85 in the scientific literature. A photo of a large
area (80 mm × 80 mm) device, W3, illuminating on arrays of closely colored objects is shown in
Figure 35c, where excellent color rendering capability is displayed by the fact that the color of
each object can be clearly identified.
To further relieve the triplet-triplet annihilation and triplet-polaron quenching processes at high
luminance, the co-doping concentrations in both yellow and red emissive regions are lowered as
demonstrated in W4. It is observed in Figure 37d that the spectrum is characterized by a slightly
increased yellow emission compared to W3. Notably, the EQE100, EQE1000 and EQE5000 have
improved to 23.5% (42.6 lm/W), 24.5% (33.8 lm/W), and 21.9% (23.2 lm/W), respectively.
Even at an ultra-high luminance of 10,000 cd/m2, the EQE remains as high as 20.1% with a CRI
of 82. The EQEs achieved represent the highest reported to date among WOLEDs of single or
multiple emitters exhibiting the corresponding decent CRI values in the scientific literature.
65
To reduce the loss in optical out-coupling, a simple lens-based out-coupling enhancement
technique has been used to obtain ηp,100 (EQE100), ηp,1000 (EQE1000) and ηp,5000 (EQE5000) of 76.0
lm/W (41.5%), 61.7 lm/W (44.3%) and 42.9 lm/W (40.6%), respectively, for W4 as shown in
Figure 38. The corresponding CRI values are 81, 83 and 85, respectively. The resulting
efficiency enhancement factor was ~1.8. These power efficiencies are in the range of standard
fluorescent tubes (40-70 lm/W), but the color rendering index is far superior for lighting
applications.
Figure 38. a) PE-L-EQE plot for W4 with (blue circles) and without (red squares) lens-based
out-coupling enhancement (see Figure 11). b) Normalized EL intensity spectra for W4 under
various luminances with out-coupling enhancement. All spectra are normalized to the green
emission peak at ~520 nm.
100
101
102
103
104
101
102
103
EQ
E [
%]
Po
we
r E
ffic
ien
cy [
lm/W
]
Luminance [cd/m2]
100
101
102
400 500 600 700 800
Norm
aliz
ed E
L I
nte
nsity [
a.u
.]
Wavelength [nm]
5,000 cd/m2
1,000 cd/m2
100 cd/m2
Luminance CRI
5,000 cd/m2 85
1,000 cd/m2 83
100 cd/m2 81
(a)
(b)
66
Figure 39. EQE versus CRI of state-of-the-art white OLED devices at a luminance of 1,000
cd/m2 from literature. Multi-EML represents multiple emissive layers used, Co-Doped represents
several dopants co-deposited simultaneously to construct the emissive layers, Tandem denotes
stacked devices, and FP represents the use of blue fluorophors and other phosphors together in
the device. Device data are taken from Ref. [8, 9, 45, 53, 67-72, 70, 71, 74, 78, 80, 83-85, 86,
87].
Comparing with state-of-the-art literature work, Figure 39 shows an EQE-CRI plot of the
various white OLED devices, where four quadrants denoted by A-D are formed by drawing one
dotted line across CRI of 80 as an indicator for an acceptable color quality of light and another
across EQE of 20% to indicate good energy efficiency. It can be observed that fluorescence plus
phosphorescence (FP) devices are characterized by high CRIs owning to the saturated blue
fluorophores used, yet they possess relatively lower efficiencies (quadrant B), whereas co-doped
emissive layer devices are mainly in quadrant D because of the additional complication in
minimizing energy transfer between multiple dopants in a common emissive layer. Tandem
devices typically show high efficiency from charge carrier recycling but with a poor color quality
due to difficulty in managing micro-cavity effects, in addition to having a higher turn on voltage
60 70 80 90 1000
5
10
15
20
25
30
35
EQ
E (
%)
CRI
FPTandemCo-DopedMulti-EML
83
6867
78
85
7080
53
74
8
71
8684
9
45
A
B
C
D
69
87
This Work
67
that leads to very low power efficiencies. It is clear that multiple emissive layer devices are much
more versatile in terms of achieving either high efficiency or high color quality. In fact, our
design of multiple cascaded emissive layer device aided with intrazone triplet energy transfer
technique (W3) (Ref. 83) makes it the first white OLED in quadrant A with a unique
performance combination of efficiency and color quality.
68
Chapter 7 : Conclusions and Future Work
7.1 Conclusions
In summary, two methods to enhance the efficiency of OLEDs without compromising device
simplicity are presented. These techniques involve firstly effective exciton harvesting followed
by intrazone and interzone energy transfers, respectively. The main principle involves the use of
a highly compatible host-dopant system utilizing shallow charge carrier traps to initiate direct
exciton formation on the dopant while providing an intermediate energy transfer step to
minimize losses associated with a larger energy difference between the host and other lower
energy dopants, thereby improving overall device exciton utilization. Detailed investigations on
the working principles of these two techniques have been conducted, wherein donor to acceptor
dopant energy transfer efficiencies of over 90% was found predominantly from a Förster-type
long range transfer mechanism. Two original equations have been proposed to govern the two
processes. In addition, a generalized equation have been developed for a multi-dopant in a
common host scenario. High external quantum efficiencies of > 20% were achieved at a lighting-
suitable brightness of 5,000 cd/m2 for red phosphorescent OLEDs, which are among the best
performances reported to date using commercially available phosphors. Furthermore, record
external quantum efficiencies of > 20% at 1,000 cd/m2 for greenish-yellow phosphorescent
OLEDs were also achieved, which features a newly synthesized, carefully designed Ir-based
emitter. Additionally, four emitter-based white OLEDs with a record combination of external
quantum efficiency (> 20%) and color rendering index (~85) were achieved for the first time at a
lighting-suitable brightness of 5,000 cd/m2
by employing the intrazone exciton harvest and
transfer approach, which represents a monumental step toward OLEDs in solid-state lighting.
7.2 Future Work
69
As demonstrated in this thesis, both interzone and intrazone energy transfer techniques are highly
effective and sufficiently simple for practical applications to improve the external quantum
efficiency of monochromatic as well as broad band OLED devices. Nevertheless, there are
various areas of improvement worth investigating further, which are outlined below.
1.) Both of these techniques take advantage of the highly compatible green emitter and host
combination that can efficiently harvest excitons and transfer the energy to longer wavelength
yellow and red emitters. However, This leaves out the shorter wavelength blue emitters, which
remain to be the weakest link in terms of efficiency at high luminance in OLEDs. Future work
may therefore involve the development of superior deep blue emitter and host combinations that
can perform the same functions of efficient exciton harvest and energy transfer to improve the
efficiency of existing sky-blue and green emitters.
2.) It has been demonstrated that the intrazone energy transfer technique can be readily
implemented in a white OLED device to enhance the external quantum efficiency dramatically.
However, the power efficiency is enhanced only at a modest level due to the increased device
total thickness with multiple emissive layer insertion. In this regard, it is possible to improve the
power efficiency by improving the conduction through the transport layers. This can be done by
incorporating superior transport materials having considerably higher carrier mobilities into the
device design W4 shown in Figure 35 to replace the original CBP and TPBi transport layers.
Alternatively, exploiting doping strategies to improve the carrier mobilites of the existing
transport layers with minimal disruptions to the already optimized structure as shown in Figure
40 may be a more attractive approach. Here, the hole transport property of CBP may be
improved with the doping of MoO3, and the electron transport property of TPBi may be
enhanced with the doping of LiF in order to form a P-i-N junction.10
70
Figure 40. A proposed P-i-N white OLED device structure based on the optimized four emitter
cascaded design presented in Chapter 6.
3.) Although the devices demonstrated in this thesis are all based on Ir-complex emitters, the
concepts can in principle be applied to highly efficient emitters based on Pt35
or considerably
cheaper Cu complexes88
either as high energy exciton harvesting donor molecules or as the low
energy exciton acceptor molecules. It is worth noting that Pt-based complexes typically exhibit a
larger Stokes shift35
than those of Ir-based complexes, which suggests that a Pt-based acceptor
would require a donor molecule having a significantly greater energy difference than that
required for an Ir-based acceptor in order to acquire sufficient spectral overlap to facilitate
energy transfer processes.
4.) Lastly, as shown from processes [3] and [4] in Section 2.3, due to a strong spin-orbit coupling
that mixes the singlet and triplet characters, thereby facilitating a partially allowed spin-flip of
LiF (1 nm) / Al (100 nm)
CBP : MoO3
ITO (120 nm) / MoO3 (1 nm)
Glass Substrate
CBP : Blue
CBG : Green CBP : Yellow
CBP : Red
TPBi : LiF
TPBi
CBP
N
P
i
71
the triplet states upon relaxation, the triplet exciton energy of one molecule is able to be
transferred to not only a lower triplet energy state89
but also a lower singlet energy state90, 91
of
another molecule by a long-range Förster mechanism. This suggests that the techniques
developed in this thesis could be extended to enhance the efficiency of fluorescent-based
acceptor dopants. For example, the chosen CBP host combined with the green exciton harvesting
molecule Ir(ppy)2(acac), could in principle be used to enhance the performance of longer
wavelength yellow and red fluorescent dopants including those based on the relatively new
concept of thermally assisted delayed fluorescence.17
72
References
1. C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett. 51, 913 (1987).
2. J. Kido, M. Kimura and K. Nagai, Science 267, 1332 (1995).
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. Okada, K. Okinaka, H. Iwawaki, M. Furugori, M. Hashimoto, T. Mukaide, J.
Kamatani, S. Igawa, A. Tsuboyama, T. Takiguchi and K. Ueno, Dalton Trans. 9, 1583,
(2005).
5. S. Y. Kim, W.-I. Jeong, C. Mayr, Y.-S. Park , K.-H. Kim, J.-H. Lee, C.-K. Moon, W.
Brütting, and J.-J. Kim, Adv. Funt. Mater. DOI: 10.1002/adfm.201300104 (2013).
6. C. W. Lee and J. Y. Lee, Adv. Mater. DOI: 10.1002/adma.201301091 (2013).
7. C. H. Fan, P. P. Sun, T. H. Su and C. H. Cheng, Adv. Mater. 23, 2981 (2011).
8. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem and K. Leo,
Nature 459, 234-U116 (2009).
9. S. J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater. 20, 4189 (2008).
10. K. Walzer, B. Maennig, M. Pfeiffer and K. Leo, Chem. Rev. 107, 1233 (2007).
11. Y.-L. Chang and Z.-H. Lu, J. Disp. Technol. 9, 459 (2013).
12. S. R. Forrest, D. D. C. Bradley and M. E. Thompson, Adv. Mater. 15, 1043 (2003).
13. J. Singh, Phys. Rev. B 76, 085205 (2007).
14. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H. E. Lee, C. Adachi, P. E.
Burrows, S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc. 123, 4304 (2001).
15. L. X. Xiao, Z. J. Chen, B. Qu, J. X. Luo, S. Kong, Q. H. Gong and J. J. Kido, Adv. Mater.
23, 926 (2011).
73
16. S. Schmidbauer, A. Hohenleutner and B. Konig, Adv. Mater. 25, 2114 (2013).
17. H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi, Nature 492, 7428 (2012).
18. A. Kohler and H. Bassler, Mater. Sci. Eng. R 66, 71 (2009).
19. T. Forster, Ann. Physik 2, 55 (1948).
20. S. E. Braslavsky, E. Fron, H. B. Rodriguez, E. S. Roman, G. D. Scholes, G. Schweitzer,
B. Valeur and J. Wirz, Photoch. Photobio. Sci. 7, 1444 (2008).
21. S. Reineke, M. Thomschke, B. Lüssem, and K. Leo, Rev. Mod. Phys. 85, 1245 (2013).
22. Y.-L. Chang, B. A. Kamino, Z. B. Wang, M. G. Helander, Y. Rao, L. Chai, S. Wang, T.
P. Bender and Z.-H. Lu, Adv. Funct. Mater. 23, 3204 (2013).
23. G. D. Scholes, Annu. Rev. Phys. Chem. 54, 57 (2003).
24. M. Pope and C. E. Swenberg, eds., Electronic processes in organic crystals (Oxford
University Press, New York, 1999).
25. D. L. Dexter, J. Chem. Phys. 21, 836 (1953).
26. M. A. Baldo, C. Adachi and S. R. Forrest, Phys. Rev. B 62, 10967 (2000).
27. A. Kohler and H. Bassler, J. Mater. Chem. 21, 4003 (2011).
28. R. R. Lunt, N. C. Giebink, A. A. Belak, J. B. Benziger and S. R. Forrest, J. Appl. Phys.
105, 053711 (2009).
29. A. Fick, J. Membrane Sci. 100, 33 (1995).
30. J. Wunsche, S. Reineke, B. Lussem and K. Leo, Phys. Rev. B 81, 245201 (2010).
31. Y. Kawamura, H. Sasabe and C. Adachi, Jpn. J. Appl. Phys. 44, 1160 (2005).
32. M. A. Baldo and S. R. Forrest, Phys. Rev. B 62, 10958 (2000).
74
33. Y. S. Park, J. W. Kang, D. M. Kang, J. W. Park, Y. H. Kim, S. K. Kwon and J. J. Kim,
Adv. Mater. 20, 1957 (2008).
34. S. M. Chen, Z. J. Zhao, Z. M. Wang, P. Lu, Z. Gao, Y. G. Ma, B. Tang and H. S. Kwok,
J. Phys. D Appl. Phys. 44, 145101 (2011).
35. Z. M. Hudson, C. Sun, M. G. Helander, H. Amarne, Z. H. Lu and S. N. Wang, Adv.
Funct. Mater. 20, 3426 (2010).
36. Z. B. Wang, M. G. Helander, Z. M. Hudson, J. Qiu, S. Wang and Z. H. Lu, Appl. Phys.
Lett. 98, 213301 (2011).
37. K. H. So, R. Kim, H. Park, I. Kang, K. Thangaraju, Y. S. Park, J. J. Kim, S. K. Kwon and
Y. H. Kim, Dyes Pigments 92, 603 (2012).
38. S. Watanabe, N. Ide and J. Kido, Jpn. J. Appl. Phys. 46, 1186 (2007).
39. D. Guo, T. E. Knight and J. K. McCusker, Science 334, 1684 (2011).
40. A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T.
Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino and K. Ueno, J. Am. Chem.
Soc. 125, 12971 (2003).
41. M. S. Lowry and S. Bernhard, Chem.-Eur. J. 12, 7970 (2006).
42. Y. You and W. Nam, Chem. Soc. Rev. 41, 7061 (2012).
43. W. C. Chang, A. T. Hu, J. P. Duan, D. K. Rayabarapu and C. H. Cheng, J. Org. Chem.
689, 4882 (2004).
44. D. H. Kim, N. S. Cho, H. Y. Oh, J. H. Yang, W. S. Jeon, J. S. Park, M. C. Suh and F. H.
Kwon, Adv. Mater. 23, 2721 (2011).
45. R. J. Wang, D. Liu, H. C. Ren, T. Zhang, H. M. Yin, G. Y. Liu and J. Y. Li, Adv. Mater.
23, 2823 (2011).
75
46. S. J. Yeh, M. F. Wu, C. T. Chen, Y. H. Song, Y. Chi, M. H. Ho, S. F. Hsu and C. H.
Chen, Adv. Mater. 17, 285 (2005).
47. M. K. Nazeeruddin, R. Humphry-Baker, D. Berner, S. Rivier, L. Zuppiroli and M.
Graetzel, J. Am. Chem. Soc. 125, 8790 (2003).
48. J. Li, P. I. Djurovich, B. D. Alleyne, M. Yousufuddin, N. N. Ho, J. C. Thomas, J. C.
Peters, R. Bau and M. E. Thompson, Inorg. Chem. 44, 1713 (2005).
49. L. Q. Chen, C. L. Yang, J. G. Qin, J. Gao, H. You and D. G. Ma, J. Organomet. Chem.
691, 3519 (2006).
50. J. Li, P. I. Djurovich, B. D. Alleyne, I. Tsyba, N. N. Ho, R. Bau and M. E. Thompson,
Polyhedron 23, 419 (2004).
51. J. P. Duan, P. P. Sun and C. H. Cheng, Adv. Mater. 15, 224 (2003).
52. R. J. Holmes, B. W. D'Andrade, S. R. Forrest, X. Ren, J. Li and M. E. Thompson, Appl.
Phys. Lett. 83, 3818 (2003).
53. B. W. D'Andrade, R. J. Holmes and S. R. Forrest, Adv. Mater. 16, 624 (2004).
54. J. J. Lin, W. S. Liao, H. J. Huang, F. I. Wu and C. H. Cheng, Adv. Funct. Mater. 18, 485
(2008).
55. C. H. Cheng and H. H. Chou, Adv. Mater. 22, 2468 (2010).
56. Y. T. Tao, Q. Wang, C. L. Yang, C. Zhong, K. Zhang, J. G. Qin and D. G. Ma, Adv.
Funct. Mater. 20, 304 (2010).
57. S. J. Su, C. Cai and J. Kido, Chem. Mater. 23, 274 (2011).
58. M. C. Suh, D. H. Kim, N. S. Cho, H. Y. Oh, J. H. Yang, W. S. Jeon, J. S. Park and F. H.
Kwon, Adv. Mater. 23, 2721 (2011).
59. C. L. Li, Y. J. Su, Y. T. Tao, P. T. Chou, C. H. Chien, C. C. Cheng and R. S. Liu, Adv.
Funct. Mater. 15, 387 (2005).
76
60. C. H. Cheng, D. K. Rayabarapu, B. M. J. S. Paulose and J. P. Duan, Adv. Mater. 17, 349
(2005).
61. Y. Chi, Y. L. Tung, S. W. Lee, L. S. Chen, C. F. Shu, F. I. Wu, A. J. Carty, P. T. Chou, S.
M. Peng and G. M. Lee, Adv. Mater. 17, 1059 (2005).
62. H. Kanno, K. Ishikawa, Y. Nishio, A. Endo, C. Adachi and K. Shibata, Appl. Phys. Lett.
90, 123509 (2007).
63. J. H. Lee, H. H. Tsai, M. K. Leung, C. C. Yang and C. C. Chao, Appl. Phys. Lett. 90,
(2007).
64. C. L. Ho, W. Y. Wong, Z. Q. Gao, C. H. Chen, K. W. Cheah, B. Yao, Z. Y. Xie, Q.
Wang, D. G. Ma, L. A. Wang, X. M. Yu, H. S. Kwok and Z. Y. Lin, Adv. Funct. Mater.
18, 319 (2008).
65. C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl. Phys. 90, 5048
(2001).
66. D. L. Huber, Phys. Rev. B 24, 893 (1979).
67. Y. R. Sun, N. C. Giebink, H. Kanno, B. W. Ma, M. E. Thompson and S. R. Forrest,
Nature 440, 908 (2006).
68. G. Schwartz, M. Pfeiffer, S. Reineke, K. Walzer and K. Leo, Adv. Mater. 19, 3672
(2007).
69. S. M. Chen, G. P. Tan, W. Y. Wong and H. S. Kwok, Adv. Funct. Mater. 21, 3785
(2011).
70. B. H. Zhang, G. P. Tan, C. S. Lam, B. Yao, C. L. Ho, L. H. Liu, Z. Y. Xie, W. Y. Wong,
J. Q. Ding and L. X. Wang, Adv. Mater. 24, 1873 (2012).
71. H. Sasabe, J. Takamatsu, T. Motoyama, S. Watanabe, G. Wagenblast, N. Langer, O.
Molt, E. Fuchs, C. Lennartz and J. Kido, Adv. Mater. 22, 5003 (2010).
72. Y. B. Zhao, J. S. Chen and D. G. Ma, Appl. Phys. Lett. 99, 163303 (2011).
77
73. Q. Wang, J. Q. Ding, D. G. Ma, Y. X. Cheng, L. X. Wang and F. S. Wang, Adv. Mater.
21, 2944 (2009).
74. S. H. Eom, Y. Zheng, E. Wrzesniewski, J. Lee, N. Chopra, F. So and J. G. Xue, Appl.
Phys. Lett. 94, 153303 (2009).
75. C. H. Hsiao, Y. H. Lan, P. Y. Lee, T. L. Chiu and J. H. Lee, Org. Electron. 12, 547
(2011).
76. J. Y. Tsao et al., “Ultra-efficient solid-state lighting: Likely characteristics, economic
benefits, technological approaches,” Chapter 1B in T-Y Seong, H. Amano, J. Han, and H.
Morkoc, Eds., “III-nitride based lighting emitting diodes and applications” (Springer),
2013
77. 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, Nature Photon. 5, 753 (2011).
78. S. O. Jeon and J. Y. Lee, Org. Electron. 12, 1893 (2011).
79. Y. L. Chang, B. A. Kamino, Z. B. Wang, M. G. Helander, Y. L. Rao, L. Chai, S. N.
Wang, T. P. Bender and Z.-H. Lu, Adv. Funct. Mater. 23, 3204 (2013).
80. S. L. Gong, Y. H. Chen, J. J. Luo, C. L. Yang, C. Zhong, J. G. Qin and D. G. Ma, Adv.
Funct. Mater. 21, 1168 (2011).
81. “International lighting vocabulary,” 1987, CIE 17.4, item 845-02-59.
82. Y.-L. Chang, Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo and Z.-H. Lu, Org.
Electron. 13 (5), 925-931 (2012).
83. Y.-L. Chang, Y. Song, Z. B. Wang, M. G. Helander, J. Qiu, L. Chai, Z. W. Liu, G. D.
Scholes and Z.-H. Lu, Adv. Funct. Mater. 23, 705 (2013).
84. H. Kanno, R. J. Holmes, Y. Sun, S. Kena-Cohen and S. R. Forrest, Adv. Mater. 18, 339
(2006).
78
85. J. H. Jou, S. M. Shen, C. R. Lin, Y. S. Wang, Y. C. Chou, S. Z. Chen and Y. C. Jou, Org.
Electron. 12, 865 (2011).
86. X. F. Qi, M. Slootsky and S. Forrest, Appl. Phys. Lett. 93, 193306 (2008).
87. N. Li, S. Oida, G. S. Tulevski, S.-J. Han, J. B. Hannon, D. K. Sadana1 and T.-C. Chen,
Nature Comm. DOI: 10.1038/ncomms3294, (2013).
88. Q. S. Zhang, T. Komino, S. P. Huang, S. Matsunami, K. Goushi and C. Adachi, Adv.
Funct. Mater. 22, 2327 (2012).
89. W. Staroske, M. Pfeiffer, K. Leo and M. Hoffmann, Phys. Rev. Lett. 98, 197402 (2007).
90. Y. Divayana and X. W. Sun, Phys. Rev. Lett. 99, 143003 (2007).
91. R. G. Bennett, R. P. Schwenker, and R. E. Kellogg, J. Chem. Phys. 41, 3040 (1964).
