Iwamoto M., Kwon Y.-S., Lee T. (Eds.) Nanoscale Interface for Organic Electronics
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Metal Complexes for Nanoscale OLED Devices 89
Huang et al.
48
prepared a high-efficiency white polymer light-
emitting diode (WPLED) based on carbazole (
59) doped with green
(60) and red emitting (61) iridium complexes as the emitting layer. The
device exhibits pure white light yet with stable sharp blue (430 nm),
green (512 nm), and red (613 nm) emissions and has the maximum current
and external quantum efficiencies of 16.8 cd/A and 8.65%, respectively.
The CIE coordinates are (0.32, 0.33) at 20 V, quiet close to that of pure
white light (0.33, 0.33).
Fig. 9. The chemical structures of color materials for fabricating white OLED.
55
54
53
56
57
58
59
60
61
T. D. Hoanh and B.-J. Lee 90
4. Conclusion
We summarized the background and the basic phenomenon of OLED
together with the device structures in Introduction of this review, which
is necessary to understand the promising next-generation display. In the
devices with multilayer structure, the hole and the electron are tunneled
through the HTL and ETL to meet each others, and the recombination
produces the photons (light emission) with the wavelength belong to
visible light region.
We also gave the general knowledge for the basic ligands,
8-hydroxyquinone, 2-phenylpyridine, benzo-ligands which are well-
known and important to the development of other ligands and their
complexes for application to OLED.
The description of the metal complexes based on Ir(III), Zn(II), and
Sn(IV) was done with the selective complexes of recent reports to give
the new, update information for the OLED.
Acknowledgments
This work was supported by the Inje Research and Scholarship
Foundation in 2008 and Korea Institute of Industrial Technology
Evaluation and Planning (ITEP) through the Biohealth Products
Research Center (BPRC) of Inje University.
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93
CHAPTER 6
STRUCTURE OPTIMIZATION FOR HIGH EFFICIENCY
WHITE ORGANIC LIGHT-EMITTING DIODES
Ji Hoon Seo, Ji Hyun Seo and Young Kwan Kim
*
Department of Information Display, Hongik University,
Sangsu-dong, Mapo-ku, Seoul 121-791, Korea
*
E-mail: kimyk@hongik.ac.kr
White organic light-emitting diodes (WOLEDs) have proven to have
great utility in solid-state light sources (SSLs) as well as backlights in
liquid-crystal displays (LCDs) and full-color OLEDs, due to their light
weight, low operating voltage, thinness, diffusiveness, and harmless-
light source for the restriction of certain hazardous substances
directive/waste electrical and electronic equipment. In this article,
advances in the understanding of WOLEDs are reviewed with dry- and
wet-process and structure optimization for high efficiency WOLEDs.
This article also reviews the highlights of the last few years and
summarizes recent trends on WOLEDs. The technical discussions
explore the question of whether WOLEDs are a viable alternative
component in SSLs and backlights in LCD and full-color OLEDs.
1. Introduction
The first observations of electroluminescence from organic materials
were made in the 1950s.
1
Interest in this phenomenon was fueled by the
work of Pope et al.,
2
who observed electroluminescence from single
crystals of anthracene. A voltage was applied between silver paste
electrodes that were placed on the opposite sides of an anthracene crystal.
A bright blue emission was observed. However, these devices were
impractical for commercial applications because of the high voltages
required for their operation and the need for exceptionally pure crystals.
Thanks to innovations in the fields of vacuum and thin film coating
J. H. Seo, J. H. Seo and Y. K. Kim 94
technologies, Vincett et al.
3
were able to fabricate light-emitting devices
based on evaporated thin films of anthracene in 1982. These were an
order of magnitude thinner than the single crystals used by Pope et al. By
using very thin vapor-deposited films, high fields were generated across
the devices at much lower voltages, thereby substantially improving the
device efficiency.
In recent years, the optimization of organic light-emitting diodes
(OLEDs) as well as the synthesis of electroluminescent materials have
been attracting great attention. In particular, white OLEDs (WOLEDs)
have many advantages that can apply to illuminating light sources,
backlights for liquid-crystal displays, and full-color, flat-panel displays
with a color filter.
4,5
Many attempts to achieve WOLEDs have been
reported, including the use of small organic molecules,
6
specially
synthesized polymers,
7
polymer blends,
8-10
and polymer bilayers,
11
mixing the above with phosphorescent iridium complexes,
12
as well as
luminescent semiconducting polymers blended with organometallic
emitters.
13
WOLEDs can be fabricated by vapor deposition and a solution
process using fluorescent materials and phosphorescent materials.
Therefore, this chapter is organized in the following manner. Following
is very brief introduction into the structure optimization of high
efficiency WOLEDs. Firstly, in Sec. 2, we introduce the different types
of WOLED fabricated by thermal evaporation, which treat only small
molecules because polymers cannot be thermally evaporated in a vacuum
chamber. Polymers generally crosslink or decompose upon heating. In
Sec. 3, we present various types of WOLEDs fabricated by wet-process
for both small molecules and polymers. Finally, we offer a short
conclusion on this topic. (Sec. 4)
2. WOLEDs with Small Molecules Based on Dry-Process
2.1. Two Complementary Colors versus Three Primary Colors
White light emission can be obtained from two complementary colors
(sky blue and yellow-orange) or three primary colors (blue, green,
and red).
14,15
The electroluminescence spectra of organic materials
generally are broader than that of inorganic materials. Therefore, two
Structure Optimization for High Efficiency WOLEDs 95
complementary colors can produce white light emission. In general,
WOLEDs with two complementary colors can be used as solid-state
lighting (SSL). However, its weak point is a low color rendering index
(CRI) because it lacks green emitting material. To serve as a backlight
in thin film transistor liquid-crystal displays or full-color OLEDs,
WOLEDs must be produced with organic emitting materials of three
primary colors. WOLEDs with three primary colors had lower efficiency,
more complex process, and more difficulty in controlling the location of
the exciton recombination zone than WOLEDs with two complementary
colors. So, the development of one or two emitting materials with three
primary colors is necessary due to simple process. However, despite their
potential capabilities, no one emitting material with three primary colors
have yet been considered as proper WOLEDs, due to their extremely low
efficiency and difficulty in finding a proper host material.
Seo et al. recently reported highly efficient WOLEDs using two
emitting materials for three primary colors (red, green, and blue).
16
The 1,4-bis[2-(7-N-diphenyamino-2-(9,9-diethyl-9H-fluoren-2-yl)))vinyl]
benzene used in the study had blue and green emissions at a peak of 478
and 510 nm, respectively. Iridium(III) bis(5-acetyl-2-phenylpyridinato-
N,C2') acetylacetonate also had red emissions at the peak of 568 nm.
Figure 1 shows the electroluminescence (EL) spectrum of WOLEDs at
the driving voltage of 8, 10, and 12 V.
Fig. 1. EL spectra of WOLEDs at driving voltages from 8 to 12 V.
J. H. Seo, J. H. Seo and Y. K. Kim 96
2.2. All Fluorescent- versus All Phosphorescent-
versus Hybrid-WOLEDs
After the first fluorescent WOLEDs were reported with multilayer
structure in 1994 by Kido et al., WOLEDs have drawn increasing
attention as a potential source of SSLs and backlights in liquid-crystal
displays and full-color OLEDs.
6,17-22
There are three ways of realizing
WOLEDs as SSLs, depending on the materials used. One type uses
all fluorescent WOLEDs. The other two types are phosphorescent
WOLEDs (PHWOLEDs), which consist solely of phosphorescent
emitters, and hybrid WOLEDs (HWOLEDs), which use both fluorescent
blue and phosphorescent red-green emitters. In 2002, Forrest et al.
23
developed entirely phosphorescent WOLEDs in a multilayered structure.
Researchers have also conducted intense investigations of PHWOLEDs,
because PHWOLEDs allow them to harvest both singlet and triplet
excitons.
24-28
Unfortunately, despite the possibility of achieving 100%
internal quantum efficiency (IQE) with PHWOLEDs, researchers have
not yet found a phosphorescent blue emitter with a sufficiently high
energy gap, long lifetime, and host materials. However, a few researchers
have recently reported data concerning highly efficient phosphorescent
deep-blue OLEDs and also investigated the synthesis of novel blue
Ir- and Pt-complexes. Sun et al. have demonstrated highly efficient
HWOLEDs using fluorescent blue and phosphorescent red-green
emitters, where a spacer of 4,4′-N,N′-dicarbazole-biphenyl was used to
prevent singlet energy transfer from a fluorescent blue emitter to
phosphorescent red-green emitters and the exchange energy losses were
minimized to increase efficiency, brightness, and lifetime.
29
Figure 2(a) shows forward-viewing external quantum efficiency
(EQE) (filled squares) and power efficiency (open circles) versus current
density of the HWOLED shown in the inset. White device showed an
EQE of 11.0% ± 0.3% at 1.0 mA/cm
2
and a slight decrease to an EQE of
10.8% ± 0.3% at 500 cd/m
2
. The white device also showed a power
efficiency of 22.1 ± 0.3 lm/W. Figure 2(b) shows proposed energy
transfer mechanisms in the HWOLED. The HWOLED was harvested
completely in independent channels as the singlet and tripelt excitons and
the energy transfer from host to dopant could be separately optimized.
Structure Optimization for High Efficiency WOLEDs 97
Fig. 2. (a) Forward-viewing EQE (filled squares) and power efficiency (open circles)
versus current density of the HWOLED shown in the inset. (b) Proposed energy transfer
mechanisms in the fluorescent/phosphorescent WOLED.
Therefore, the efficiency of WOLEDs was demonstrated with fluorescent
dopants with IQE of 25% and phosphorescent dopant with IQE of 100%,
resulting in a total WOLED IQE of 100%.
2.3. Excimer- versus Exciplex-WOLEDs
WOLEDs are obtained from a blue emitter and its excimer- and
exciplex-based emissions with multiple-wavelength emissions.
30,31
The
J. H. Seo, J. H. Seo and Y. K. Kim 98
emissions of excimer and exciplex are red-shifted from blue emitter. An
excimer is formed from the formation of complexes formed by the
combination between an excited-state molecule and a ground-state
molecule into one molecule. In contrast, an exciplex is a donor-acceptor
complex between an excited-state donor and ground-state acceptrors
in two molecules. Williams et al. reported excimer-based PHWOLEDs
with nearly 100% IQE using platinum(II) [2-(4′,6′-difluorophenyl)
pyridinato-N,C2′)](2,4-pentanedionato) as phosphorescent blue dopant
and 2,6-bis(N-carbazolyl)pyridine as phosphorescent host, respectively,
as shown in Fig. 3(a).
32
The energy diagram of the excimer-based
PHWOLED is shown in Fig. 3(a). The optimized PHWOLED
demonstrated an EQE of 15.9% and a power efficiency of 29 lm/W
at 500 cd/m
2
. Figure 3(b) shows the EL spectrum of excimer-based
PHWOLED at 500 cd/m
2
. The monomer was observed from 470 nm to
500 nm and the excimer peaks showed around 600 nm. The device
showed an emission of CIE
x,y
coordinates of (0.46, 0.47) at 500 cd/m
2
and a color rendering index of 69.
Fig. 3. (a) Energy diagram of excimer-based WOLEDs. (b) EL spectrum of excimer-
based WOLED at 500 cd/m
2
.