Key issues and recent progress of high efficient organic light

Transcription

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 17 (2013) 69–104
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology C:
Photochemistry Reviews
journal homepage: www.elsevier.com/locate/jphotochemrev
Key issues and recent progress of high efficient organic
light-emitting diodes
Jian Wang a , Fujun Zhang a,∗ , Jian Zhang b , Weihua Tang c , Aiwei Tang a , Hongshang Peng a ,
Zheng Xu a , Feng Teng a , Yongsheng Wang a,∗
a
Key Laboratory of Luminescence and Optical Information (Beijing Jiaotong University), Ministry of Education, Beijing 100044, People’s Republic of China
State Key Laboratory of Catalysis, Dalian institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, 457
Zhongshan Road, Dalian 116023, People’s Republic of China
c
Key Laboratory of Soft Chemistry and Functional Materials (Ministry of Education of China), Nanjing University of Science and Technology, Nanjing
210094, People’s Republic of China
b
a r t i c l e
i n f o
Article history:
Received 13 April 2013
Received in revised form 6 July 2013
Accepted 8 August 2013
Available online 1 September 2013
Keywords:
Organic light-emitting diodes
Device physics
Internal quantum efficiency
External quantum efficiency
White emission
Transparent conductive electrodes
a b s t r a c t
Organic light-emitting diodes (OLEDs) are considered as an ideal in next generation of flat panel displays
and solid state lighting source. Still, the stability and efficiency of OLEDs remain great challenges for its
commercialization application. This article provides an overview on working principle of different kinds
of luminescence, effective methods to improve quantum efficiency, recent progress of white emission
OLEDs, novel types of transparency electrode for flexible OLEDs and the stability of OLEDs. A series of
interesting and promising ideas to improve the performance of OLEDs are summarized from physical
engineering based on the recent achievement of high brightness, high efficient and good stability of
OLEDs.
© 2013 Elsevier B.V. All rights reserved.
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exploiting novel roads to increase radiation emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.
Basic theoretical of organic materials radiation emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.
Enhanced fluorescence emission by exploiting triplet states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.
Emission induced by intermolecular interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.
Aggregation induced emission (AIE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Efficient methods to improve efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.
Key factors on internal quantum efficiency (IQE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.
Key factors on external quantum efficiency (EQE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Improve EQE by employing external extraction structures (EES) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1.
EES: microlens array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2.
EES: texturing meshed surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3.
EES: anti-reflection layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4.
EES: sand-blasting substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.
Improve EQE by employing internal extraction structures (IES) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1.
IES: photonic crystal pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2.
IES: the embedded low-index grids (LIG) and ultra LIG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3.
IES: high refractive index substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗ Corresponding authors.
E-mail addresses: fjzhang@bjtu.edu.cn (F. Zhang), yshwang@bjtu.edu.cn (Y. Wang).
1389-5567/$20.00 © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochemrev.2013.08.001
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4.
5.
6.
7.
J. Wang et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 17 (2013) 69–104
3.4.4.
IES: low index layer on microstructured ITO electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.4.5.
IES: refractive index modulation layer (RIML) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.4.6.
IES: improved Bragg diffraction gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
3.4.7.
IES: embedded nanocomposite scattering layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.4.8.
IES: nanostructured indium tin oxide (NSITO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.5.
Surface plasmon extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
84
Key achievements of white emission OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
85
4.1.
Fluorescence/phosphorescence hybrid WOLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
86
4.2.
All phosphorescence based WOLEDs by vacuum evaporation technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.3.
All phosphorescence based WOLEDs by solution process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.4.
All fluorescence based WOLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.5.
Excimer- and exciplex-based WOLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
88
4.6.
AIE based WOLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
4.7.
Down-conversion WOLEDs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
4.8.
High efficient primary blue emission for WOLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
New transparent conductive electrodes (TCEs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
5.1.
New TCEs: modified graphene anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.2.
New TCEs: carbon Nanotube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5.3.
New TCEs: metal nanostructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.4.
New TCEs: dielectric–metal–dielectric and metal–dielectric–metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
5.5.
New TCEs: ZnO or impurity-doped ZnO films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
95
The endeavor to improve the stability of OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
6.1.
Induction factors of degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.1.1.
Extrinsic degradation mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.1.2.
Intrinsic degradation mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.2.
New architecture design of OLEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.2.1.
Encapsulating technology of isolating moisture and oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.2.2.
New process for high efficiency heat dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
97
6.2.3.
Approaches to reduce the heat yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.2.4.
Resisting degradation and eliminating abrupt interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Outlooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
Jian Wang was born in Taian City, Shandong province of
China. He received his master degree from Beijing Jiaotong University and obtained the National Prize for Master
students in 2012. From 2012, he is a Ph.D. candidate in
Beijing Jiaotong University. His research interests focus
on the fabrication of high performance OLEDs and semitransparent OPVs.
Fujun Zhang completed his undergraduate study from
Minzu University of China in 1999. He has started
his research in organic electronics since he joined the
research group of Academician of Chinese Academy of Sciences Prof. Xurong Xu in 2002. In 2006, he joined in the
group of Prof. Norbert Koch as Guest researcher, Humboldt University, Germany. In 2007, he obtained his Ph.D.
degree in Optics from Beijing Jiaotong University and work
in Beijing Jiaotong University. In 2009, he was exceptional promoted as assistant professor due to his excellent
research. His research field focuses mainly on device physical problems on OLEDs, OPVs and organic image sensors.
Jian Zhang obtained his Ph.D. degree in polymer physics
and chemistry from Changchun Institute of Applied chemistry, Chinese Academy of Science in 2004. From 2004 to
2005, he collaborated with Prof. Frederic Fages in Laboratoire des Matériaux Moléculaires et des Biomatériaux
(LMMB), UMR CNRS 6114, Marseille, France. From 2005
to 2009, he collaborated with Dr. Norbert Koch and Prof.
Jürgen P. Rabe in Humboldt University Berlin under finical support of the Alexander von Humboldt Foundation
and DFG. Since 2009, he is a professor in Dalian Institute
of Chemical Physics, Chinese Academy of Sciences. His
research interests focus on organic semiconductors and
organic photoelectronic devices.
Weihua Tang received his early college study from Beijing
University of Chemical Technology, China. He obtained
his Ph.D. degree in chemistry from National University
of Singapore in 2006. He has started his research in
organic electronics since he joined the Institute of Materials Research and Engineering (IMRE) in Singapore since
September 2005. In 2008, he joined the research group of
Professor Andrew B. Holmes at University of Melbourne
and worked on the solution-printable organic solar cells.
In 2009, he returned to China to take a full-time professor
position at Nanjing University of Science and Technology. His research interests focus on the development of
functional materials for optoelectronic devices.
Dr. Aiwei Tang received the Bachelor degree in chemistry from Shandong Normal University in 2003, and the
Master and Ph.D. degrees in optics from Beijing JiaoTong University in 2006 and 2009, respectively. In 2008,
he worked in University of Florida as a visiting scholar.
Since 2009, he joined Institute of Semiconductors, Chinese
Academy of Sciences as a postdoctoral research fellow. He
is currently an associate professor in Beijing JiaoTong University. His current research interests include synthesis of
organic/inorganic nanocomposites and their applications
in OLEDs, OPVs and electrically bistable devices.
Hongshang Peng is the Associate Professor at Key Laboratory of Luminescence and Optical Information, Ministry
of Education at Beijing Jiaotong University. He obtained
a B.E. in Inorganic Material from Shandong Institute of
Light Industry in 1998, then a M.S. in Materials Physics
and Chemistry from Jilin University in 2003. He obtained
a Ph.D. in Optics from Beijing jiaotong University in 2007,
and was subsequently awarded a Humboldt Research Fellowship for postdoctoral research at the University of
Regensburg.
Zheng Xu received his Bachelor degree from Northeast Normal University, School of Physics in 1983. Since
then he has started working on spectroscopy technology
at Changchun Institute of Physics, Chinese Academy of
Sciences. He obtained his Ph.D. degree from Graduate University of the Chinese Academy of Sciences. In 1992–1993,
he visited Konan University and worked on the Spectra
of Organic Crystals. He is professor of Optics Engineering at Beijing Jiaotong University since 2005. His current
research interests focus on optoelectronic materials and
devices, flat panel display and photovoltaic technology.
Feng Teng received his Ph.D. degree from Changchun
Institute of Physics, Chinese Academy of Sciences in 1998.
From 1998 to 2000, he worked on the organic electroluminescent materials at Institute of Chemistry, Chinese
Academy of Sciences. Prof. Teng has received the support
from the National Science Fund for Distinguished Young
Scholars in 2012. His current research interests include
organic electroluminescent materials and device physics,
up-conversion luminescent materials and optical bistable
devices.
Yongsheng Wang received the Bachelor of Science degree
from the Northeast Normal University in 1985, and the
Ph.D. degree from the Changchun Institute of Physics,
Chinese Academy Of Sciences in 1993. From 1993 to
1995, he engaged in postdoctoral researches from Nankai
University. He is a professor in Beijing Jiaotong University as one research group leader. He is also the
director of the Key laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong
University. In 2009, he received the support from the
National Science Fund for Distinguished Young Scholars. His research interesting includes optical information
storage and display materials and understanding the
device physical problems. He is the President of Beijing Institute of Graphic
Communication.
1. Introduction
The rapid developments of organic photoelectronic devices
have attracted much attention due to the low cost, low consumption and environment-friendly. Since 1987 Tang and Vanslyke put
forward thin film structure organic light-emitting diodes (OLEDs)
with high electroluminescence (EL) efficiency and high brightness
under low driving voltage, the OLEDs have showed great potential
application as flat panel displays and solid state lighting source [1].
In 1990, Friend and co-workers at Cambridge reported polymer
light-emitting diodes (PLEDs) based on conjugated polymer poly(pphenylene vinylene) as active layer prepared by solution process,
suggesting that large-area PLEDs could be made in low cost [2].
In 1992, Heeger and co-workers firstly investigated the transient
EL behavior of PLEDs driven by pulse voltage. The pulsed excitation provides important new information: the transient on/off
response will ultimately limit the high frequency modulation of
such light source, and the extension of light intensity dependence
on current characteristics to high injection levels will determine
the potential of PLEDs in application which require multiplexing
or high levels of pumping [3]. In 1994, Parker et al. demonstrated
that the performance of PLEDs was determined by tunneling of
both the hole and the electron through interface barriers caused
by the band offset between the polymer and the electrodes [4,5].
Manipulating these offsets could control the operating voltage and
efficiency of OLEDs. In 1994, Kido et al. reported white emission
OLEDs (WOLEDs) by mixing three fluorescent dyes (blue, green and
orange) into a single emission layer (EML), which was considered
as one of ideal candidates for future energy-saving lighting sources
[6]. In 1997, Hung et al. reported that EL efficiency of OLEDs
was improved by introduction of ultrathin lithium fluoride (LiF)
between an electron-transporting layer (ETL) and an aluminum
(Al) cathode [7]. The improvements were attributed to band
bending of the organic layer in contact with the dielectrics. After
71
that, Friend et al. reported that hole injection barrier was decreased
by utilizing the anionic poly (3,4-ethylenedioxythiophene):poly(4styrenesulphonate) (PEDOT:PSS) as interfacial buffer layer [8].
With the introduction of PEDOT:PSS, the abrupt hole injection
barrier could be partitioned into a series of smaller steps, resulting
in an easier hopping injection of hole. In 1998, Forster and his coworkers reported that internal quantum efficiency (IQE) of OLEDs
could be up to 100% in theory by using phosphorescence materials
as the emission center [9], suggesting that the OLEDs really enter
into the “high efficiency” times. In 2000, the results of So et al. indicated that the OLEDs fabricated by mixing both the hole transport
material and emission material as the emissive layer (EML) had a
much longer lifetime than that of the devices with a heterostructure [10,11]. In 2001, Adachi et al. firstly demonstrated that the
IQE of phosphorescence OLEDs (PhOLEDs) was close to the theory
limit (87 ± 7%) by employing bis(2-phenylpyridine)iridium(III)
acetylacetonate (ppy)2 Ir(acac) as the emission centers [12]. The
space-charge limited transport was observed in simple dualcarrier PLEDs, where the polymer layer was sandwiched between
two injection layers [13,14]. Recently, Lu and co-workers reported
high-efficiency PhOLEDs using a multifunctional anode stacks,
Ta2 O5 /Au/MoO3 , with a lens-based structure to unlock the full
potential of OLEDs on flexible plastic, the maximum EQE reach
63% and remained 60% at brightness more than 10,000 cd/m2 [15].
In this review article, the recent progress and achievement of
OLEDs form device physics were summarized. We highlight the
efficient methods to improve light extraction efficiency, the high
efficient WOLEDs, new electrodes for flexible OLEDs and the stability. The valuable routes for improving the performance of OLEDs
were discussed in detailed.
2. Exploiting novel roads to increase radiation emission
The OLEDs revealed huge superiorities as light source due to its
great advantages, such as high EL efficiency, fast response, low driving voltage, simplicity of fabrication, environmental friendly and
no harmless to eye. In general, OLEDs contains multilayer functional layers: anode, hole injection layer (HIL), hole transporting
layer (HTL), electron blocking layer (EBL), emitting layer (EML),
hole blocking layer (HBL), electron transporting layer (ETL), electron injection layer (EIL) and cathode. When OLEDs are applied a
forward driving voltage, the electron is injected from the cathode
into the lowest unoccupied molecular orbital (LUMO) of the adjacent organic ETL material, while the hole is injected from the anode
into the highest occupied molecular orbital (HOMO) of the organic
HTL material. According to the Parker theoretical model, the Fermi
energy level (EF ) of the metal is usually lower than the LUMO of the
organic ETL, resulting in an injection barrier (˚) for the electrons
injection [4]. Similarly, the work function (W) of the anode materials is higher than the HOMO of organic HTL and there is also an
injection barrier (˚ ) for the hole injection. The numerical value of
˚ and ˚ could be calculated respectively as follows:
˚ = W − EA ,
˚ = IP − W Here, W is the metal cathode work function, EA the electron
affinity of electron transporting materials, IP and W are ionization
potential and work function of anode materials. Due to quantum
tunneling effect, electron and hole could pass through the triangle
potential barriers ˚ and ˚ under applied voltage. Hence, the current density–electric field intensity of OLEDs could be expressed as
the Fowler–Nordheim formula [5]:
−k
,
J ∝ F exp
E
2
k=
8
2m∗ ˚3/2
3qh
72
Here, m* is the effective mass of carrier, q the unit charge, h the
Planck constant and ˚ is the barrier height.
Different from inorganic semiconductor, the interaction of
molecules which called Van der Waals force in organic semiconductor is weak and the excited states are localized within single
molecule or single conjugate segment. Hence, the charge carrier’
transport among the organic molecules is ‘hopping’ due to the weak
intermolecular wave function overlap [16,17]. Electron and hole
are captured by the coulomb attraction and bound together on the
same molecule due to high binding energies and part of them would
like to form exciton [18–20]. Electron on the excited state relaxes
to the ground state by giving light emission.
2.1. Basic theoretical of organic materials radiation emission
Organic exciton is a kind of tight-binding exciton with high binding energy of several hundred meV and short exciton radius less
than 10 nm [21,22]. The exciton radius, namely coulomb radius,
could be calculated as follows [23,24]:
rC =
e2
4εε0 kT
where e is the elementary charge, ␧␧0 the relative (absolute)
permittivity, k the Boltzmann constant and T is the absolute temperature. The ‘hopping’ distance of free electrons and holes is
much shorter than rC for the disordered organic materials with low
mobility ( < 1 cm2 /V s), the recombination of free electrons and
holes into bound pairs was generally considered following Langevin
process. The Langevin recombination coefficient (ˇL ) is proportional to the holes and electrons mobility (p and n ) [23]:
ˇL =
e(p + n )
εε0
In the Langevin process, recombination rate (R) is determined
by encounter probability of the free electrons and holes in space
and proportional to the ˇL multiplied with the charge density
(R ∝ npˇL ), where n and p are electrons and holes concentrations,
respectively. The formed e–h pairs either dissociate back to free
charge carriers or form Coulomb pairs and/or exciton [24]. The
recombination of hole and electron which bound each other is independent of spin and the spin quantum number is half integer (−1/2,
spin up or spin down). Hence, spin wave function of exciton formed
by the Langevin process could be either singlet (S1 ) or triplet (T1 ),
the ratio of S1 and T1 state exciton is about 1:3 according to the
statistical regularity. As a result, there are four possible spin configurations for the exciton: S1(S=0) (↑↓), T1(S=0) (↑↓), T1(S=1) (↑↑) and
T1(S=−1) (↓↓), as shown in Fig. 1(a). The singlet exciton (S1 ) is antiparallel spin while the triplet (T1 ) is parallel. In fluorescence-based
OLEDs, the general transition process of the S1 and T1 exciton could
be summarized in Fig. 1(b).
For organic fluorescence materials, the HOMO is completely
filled and has the singlet character (S = 0) in ground state (S0 ).
Thus, according to the Pauli exclude principle, only the transition
S1 → S0 is spin allowed and give fluorescence emission, while the
transition T1 → S0 is spin forbidden. Meanwhile, the transition
T1 → S0 was attenuated by the way of non-radiative transition
and the released energy would convert into heat instead of light
emission [25]. In addition, there also exist intersystem crossing
(ISC) and reverse intersystem crossing (RISC) process between
T1 and S1 [26]. Based on the above analysis, the singlet state
exciton is about 25% under electrical excitation, namely the IQE of
fluorescence-based OLEDs is no more than 25%. For OLEDs based on
fluorescence material as emission layer, about 75% of exciton has
no contribution to light emission. In 1998, Thompson and Forster
reported an efficient phosphorescent material 2,3,7,8,12,13,17,
Fig. 1. (a) Schematic of the single and triplet states extion, (b) schematic view of
the EL mechanism (: ratio of holes and electrons in carrier injection, transport and
recombination processes; r : S1 and T1 exciton formation ratio; PL : photoluminescence efficiency; h : light extraction efficiency.).
18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) and hewed
out a new field of PhOLEDs [9]. By employing PtOEP as the emission material, the PhOLEDs had peak EQE and IQE of 4% and 23%,
respectively. Phosphorescent materials containing heavy atoms
such as Pt and Ir could efficiently promote the spin orbit coupling
and lead to compound of S1 and T1 states. Therefore, T1 → S0
attenuation change from spin forbidden to allow, that is, PhOLEDs
could utilize both single and triplet state exciton energy and the
IQE could arrive to 100% in theory. The attenuation of T1 → S0 gives
phosphorescent emission.
Phosphorescent material could be excited from S0 to S1 , then
part energy is dissipated in the form of heat and electrons on the
excited state drop to the lowest vibrational state of the S1 by the
way of vibration relaxation (VR). Part electrons on the excited state
could decay from the S1 to S0 state in the form of internal conversion
(IC) process or give fluorescence emission by radiation transition.
The relationship between IC process and fluorescence emission is
competitive. In addition, electrons on the excited sate could transport from the higher S1 level to the lower T1 level by ISC due to
strong spin-orbit coupling. Then the electrons on the excited T1
level could decay to the S0 level by giving phosphorescence emission. Except these processes, electrons on the excited states of one
molecule may also transmit to the excited state or the ground state
of the adjacent molecule via the Forster process [27]. All of the
processes could be described by Jablonski as shown in Fig. 2.
However, the IQE of PhOLEDs that employing pure phosphorescent materials as EML is very low due to serious concentration
quenching. Hence, phosphorescent materials were usually doped
into proper host material as guest material to form host–guest
doped system. In this system, the concentration quenching is
related to Forster dipole–dipole interactions [28,29]. Forster
dipole–dipole energy transfer rate (k) and concentration quenching constant (kCQ ) could be expressed as the following equations:
k=
1
PL
R 6
0
R
,
PL = F ISC
kr
kr + knr + kCQ
where PL is intrinsic radiative decay lifetime of host material,
R is distance between host and guest and R0 is critical distance
resulting in concentration quenching (Forster radius), PL is photoluminescence (PL) quantum efficiency, F is energy transfer
efficiency from host to dopant, ISC is ISC efficiency, kr is radiative decay rate and knr is non-radiative decay rate [28,29]. The
methods of energy transfer from host to dopant molecule could
be summarized as: Forster energy transfer, Dexter energy transfer
73
Fig. 2. The simplified Jablonski diagram of photophysical processes of phosphorescent materials. The radiative transition is indicated by straight arrows (F = fluorescence,
P = phosphorescence), non-radiative process by wavy and dotted arrows. Reprinted with permission from Ref. [27]© 2007 American Physical Society.
and charge-trapping [30]. Forster energy transfer was realized by
the resonance between the excited state (D* ) and the ground state
(A), which belongs to long distance (40–100 Å) interaction. Dexter
energy transfer is a kind of short distance process (∼10 Å), where
exciton diffuse from D to A sites via intermolecular charge carrier
exchange. Its characteristic could be concluded that the total spin
of the D–A pair is conserved, both singlet-singlet and triplet-triplet
transfers are permitted and Dexter transfer efficiency is rapidly
decreased with the increase of donor–acceptor distance. The energy
transfer efficiency in host–guest system is closely related to the diffusion length of exciton. Both Forster and Dexter energy transfer are
proportional to the spectral overlap degree between PL spectrum
of the host material and absorption spectrum of the guest material. The transfer rate of the Forster and Dexter transfer could be
expressed by the following equations, respectively:
KD→A =
1 1
D R6
KD→A = K exp
3
4
2r L
c4
FD (ω)
A (ω) dω
ω4 n40
c4
ω4 n40
FD (ω)
A (ω) dω
where FD (ω) is the normalized fluorescence spectrum of host
material, A (ω) the absorption cross section of guest material, D
the lifetime of host material, R the distance between host and guest
materials, n0 the dielectric constant, ω the light angular frequency
and K is a constant related to the spectral overlap degree.
phosphorescent sensitizer to excite fluorescent dye is considered as an effective method [34]. The phosphorescent sensitizer
could transfer energy from the triplet state exciton to the singlet
state exciton by Forster energy transfer and the energy transfer
efficiency (ET ) is expressed by:
ET =
kET
kET + kr + knr
Here kET is the Forster energy transfer rate from D to A, kr and knr
are radiative and non-radiative rates on the donor, respectively. The
energy transfer is efficient if kET > kr + knr . According to Forster theory, kET is proportional to the oscillator strength of donor transition,
as is kr , if kr knr , kET is approximately independent of oscillator
strength and the triplet–singlet energy transfer by Forster energy
transfer is possible.
When fluorescent acceptor is directly doped into phosphorescent donor material, the close proximity of the donor and acceptor
could increase the likelihood of Dexter transfer between the donor
and the acceptor triplets, that is, Forster energy transfer becomes to
be inefficient. In order to restrain Dexter energy transfer, both fluorescent acceptor and phosphorescent materials are needed to dope
into suitable host polymer material [34]. Phosphorescent materials
could sensitize the energy transfer from the host (D) and act as the
donor (X), resulting in ideal energy transfer from host material to
the triplet state of the sensitizer and then to the singlet state of the
fluorescent dye (A), the progress could be express as follows and
shown in Fig. 3:
1 D∗
+ 1X → 1D + 1X∗, 1X∗ → 1X∗
2.2. Enhanced fluorescence emission by exploiting triplet states
3X∗
+ 1 A → 1 X + 1 A∗ , 1 A∗ → 1 A + hv
Though PhOLEDs have huge superiority in quantum efficiency,
the cost of phosphorescent materials is rather high compared with
fluorescent materials [31,32]. In addition, PhOLEDs would suffer
from a marked EL efficiency roll-off with the increase of current
density because of various triplet exciton annihilation which
caused by the long lifetime of triplet states exciton [33]. Fluorescent materials are cheaper than phosphorescent materials and are
unaffected by exciton annihilation which decreases EL emission
efficiency at high excitation densities [31]. Hence, exploiting
triplet state exciton of fluorescent materials has attracted more
and more attention to improve the EL emission efficiency. Using
and
3
D∗ + 1 X → 1 D + 3 X ∗ , 3 X ∗ + 1 A1 X + 1 A∗ , 1 A∗ → 1 A + hv
Here, h is photon energy, superscript 3 or 1 represents triplet
and singlet states, asterisk marks the excited states.
Besides using phosphorescent sensitizer to exploit the triplet
state exciton of fluorescent materials, up-conversion T1 into S1
of fluorescent materials by thermally activated delayed fluorescence (TADF) is another potential approach [26,31,35]. The
potential TADF materials have a very small energy gap between
its singlet and triplet excited states, E1–3 , which allows efficient
74
Fig. 3. Energy transfer mechanisms in the sensitized system. Reprinted with permission from Ref. [34]© 2000, Nature Publishing Group.
up-conversion of triplet exciton into singlet state. When the E1–3
of molecules is proportional to exchange integral between HOMO
and LUMO spatial wave functions, the environmental thermal
energy could induce RISC process from T1 to S1 [35]. Therefore,
in principle, heating the OLEDs with TADF materials as emission
center would result in high EL efficiency by increasing the RISC rate
and the efficiency roll-off could be effectively restrained when RISC
rate (kRISC ) is significantly larger than phosphorescence decay rate
(kP ) [26]. It is found that kISC , kR and kP are independent of temperature, while kRISC exhibited strong temperature dependence and be
proportional to exp(−E1–3 /kB T). The ISC rate (kISC ), fluorescence
decay rate (kr ), kRISC and kp could be calculated as follows:
KISC =
kRISC =
˚P
,
F kp p
˚TADF
kISC F2 P kr
∝ (E1−3 /kB T ),
kr =
˚F
F
kp =
˚P
P
Here, F and P are fluorescence and phosphorescence transient
lifetime, ФF , ˚P and ФTADF are fluorescence, phosphorescence and
delayed fluorescence quantum efficiency.
Molecules of TADF materials should have small E1–3 , rigid
structure and heavy atom effects for obtaining the high kRISC .
Adachi and co-workers reported a series of SnF2 –porphyrin
complexes, which exhibited rather strong TADF at room temperature and displayed a clearly apparent fluorescence intensity
enhancement with the increase of temperature [26]. The strategy provided by Prof. Adachi for TADF materials design is based
on the introduction of electron donor and acceptor units in
which the ␲-conjugation is significantly distorted by steric hindrance introduced through bulky substituent [35]. Based on this
strategy, high efficient 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a]
carbazole-11-yl)-1,3,5-triazine] (PIC-TRZ) containing an indolocarbazole donor unit was synthesized, which exhibits a very small
E1–3 of 0.11 eV along with a radiative decay rate of kr ∼ 107 , providing both efficient up-conversion from T1 to S1 levels and intense
fluorescence intensity that leads to high EL efficiency. A significant
contribution of 30% RISC efficiency within molecule was realized
under both PL and EL processes, as shown in Fig. 4.
2.3. Emission induced by intermolecular interaction
It is known that both fluorescence and phosphorescence emission of organic luminescent materials are only single molecular
behaviors due to its large exciton binding energy and small exciton
radius. Emission induced by intermolecular interaction happens
Fig. 4. PL process and EL process. kr , knr , kISC , kRISC , kp , knp , kr(TADF) , Фr , ФTADF , ФISC and
ФRISC represent fluorescence decay rate, non-radiative decay rate from S1 , ISC rate
from S1 to T1 , RISC rate, phosphorescence decay rate, non-radiative decay rate from
T1 to S0 , TADF decay rate, fluorescence efficiency, TADF efficiency, ISC efficiency and
RISC efficiency, respectively. Here kr and kr(TADF) are exactly the same because TADF
takes the same route of radiative decay as fluorescence. Reprinted with permission
from Ref. [35]© 2011, American Institute of Physics.
probably under high exciton density conditions, such as delayed
fluorescence induced by triplet–triplet annihilation (TTA), excimer,
exciplex and electroplex [36–42]. In PhOLEDs, the TTA processes
usually result in the decrease of IQE. However, it could enhance
the IQE in fluorescence-based devices due to production of singlet
exciton. Kondakov et al. demonstrated that exemplary red fluorescence OLEDs gain as much as half of their EL from annihilation of
triplet states generated by recombining charge carriers [43]. The
magnitude of TTA contribution in combination with the remarkably high total efficiency (11% EQE) indicated that the absolute
amount of EL attributable to TTA substantially exceeds the limit
imposed by spin statistics, which was independently confirmed
by investigating magnetic field effects on delayed luminescence.
The value of 1.3 for the rate constants ratio of singlet and triplet
channels of annihilation, which is indeed substantially higher than
the value of 0.33 expected for a purely statistical annihilation
process. In principle, the upper limit on the singlet excited state
yield resulting from the TTA process is 0.5, which makes the
maximum IQE of fluorescence OLEDs to be 25% + 0.5 × 75% = 62.5%.
The estimates of maximum EQE of the fluorescence OLEDs should
be revised to at least 0.2 × 62.5% = 12.5%, likely, even higher to
taking account into optical extraction exceeding 20%. Xiong and
co-worker reported that TTA process produces considerable extra
single states, which accounts for as high as 19% of total single states
at 20 K based on ITO/N,N -Di(naphthalene-1-yl)-N,N -diphenylbenzidine (NPB, 50 nm)/tris(8-hydroxyquinoline) aluminum (Alq3 ,
70 nm)/LiF (0.7 nm)/Al (100 nm) device, 34% at 20 K and 17%
at room temperature based on the ITO/NPB (50 nm)/Alq3 : 4dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran
(DCM, 40 nm)/Alq3 (30 nm)/LiF (0.7 nm)/Al (100 nm) device, resulting in the total singlet generation yields exceeding the classical
0.25 spin statistics limit [44].
Intermolecular radiative recombination maybe occurs when
the intense interaction between adjacent molecules is beneficial
to forming dimer. An excimer (M* M) is a kind of dimer, which is
formed by the interactions of a molecule at the excited state (M* )
with adjacent molecule at the ground state (M), and the dissociative property imparts its basic spectral feature: a broad featureless
band red-shifted from the parent monomer emission [45,46].
Although two identical molecules are involved, the excimer is
not the same as the dimer which is formed in the ground state
without optical or electrical excitation. Only a monomer is formed
when the dopant concentration is low. While both monomer and
excimer emission could be increased along the increase of dopant
concentration. The excimer emission peak is usually located at
the low energy range compared with the monomer emission
peak. White light emission could be generated by a combination
of monomer and excimer emissions by choosing the molecule
material and adjusting the relative intensity ratio between the red
excimer emission and the blue monomer (i.e., exciton) emission.
Usually, the excimer emission has low quantum yield due to
the increase rate of the non-radiative transition and an overall
reduction in the oscillator strength of the dimer.
Forster and co-workers demonstrated that the quantum efficiency of triplet excimer based WOLEDs depends exponentially
on the thickness of the emission layer [47]. To some phosphorescent materials, emission induced by intermolecular interaction,
such as excimer or exciplex emission has been studied and
applied widely [36,37,46,48,49]. Williams and co-workers demonstrated that phosphorescent excimer emission is an efficient
de-excitation process, PhOLEDs with single phosphorescent material as the emission layer could obtain 100% IQE [48]. Li et al. also
reported an intensive excimer emission of phosphorescent material
bis[3,5-bis(2-pyridyl)-1,2,4-triazolato] platinum(II) Pt(ptp)2 [50].
The Pt(ptp)2 exhibits different emission color, structured monomer
emission in the blue-green region (max ∼ 480 nm), unstructured
excimer emission in the yellow region (max ∼ 550 nm) and rather
broad unstructured extended excimer emission in the orange-red
region (max ∼ 600 nm) by varying the doping level in host material
4,4 -bis(N-carbazolyl)-1,1 -biphenyl (CBP). Recently, we reported
the emission color-tunable OLEDs based on single phosphorescent
material Ir(III) bis [2-(4,6-difluorophenyl)-pyridinato-N,C2 ] (picolinate) (FIrpic) as the emission layer [41]. The emission color of
different concentration solutions and their relative films has an
apparent variation from blue to green along with the increase of
FIrpic doping level in polymer host material poly(9-vinylcarbazole)
(PVK) under 380 nm light excitation condition, as shown in Fig. 5.
It is very interesting that emission color of solution and its film
are not completely identical under the same excitation conditions.
The PL spectra of the blended films show marked difference along
with the variation of FIrpic doping level in PVK. When the FIrpic
doping level in PVK is higher than 1:1 (weight ratio), the emission
peak at 476 nm was distinctly weaken due to the self-absorption
effect. Meanwhile, the intensity of excimer emission peak at about
530 nm was markedly enhanced along with the increase of FIrpic
doping levels in PVK [41].
The excimer as having contributions from all allowed intermolecular interactions could be represented as follows:
excimer = c1 (A− A+ ) + c2 (A+ A− ) + c3 (A∗ A) + c4 (AA∗∗ )
where A is the monomer ground state, A* the excited state
monomer and A± are ions of the monomer [46]. This formulation
allows for multiple excimer and aggregate states coexist. The heavy
metal atom such as Pt and Ir in phosphorescent materials could
induce the ISC from S1 to T1 in a very short time (∼1 ns) [51]. Hence,
the exciton are generated in the monomer and then translate into
the excited triplet states (3 T* ) immediately in the neat films under
optical excitation. The molecule in the 3 T* and the other in the
ground state (1 S) coalesce together and form the triplet excimer
(3 E0∗ ). The formation can be expressed by:
3 ∗
K TE
X
T + 1 S −→3 E0∗
75
K TE
X
The −→ is formation rate of 3 E0∗ [46]. Taking into consideration
that the radiative, non-radiative and the excimer formation rates,
the expression could be amended as:
d[3 T ∗ ]
T 3 ∗
= −krT [3 T ∗ ] − knr
[ M ] − krTE [1 S][1 T ∗ ] + G
dt
d[3 E0∗ ]
dt
E 3 ∗
[ E0 ]
= kXTE [1 S][3 T ∗ ] − krE [3 E0∗ ] − knr
where [3 E0∗ ] is optically generated triplet excimer concentration, krT
T are radiative and non-radiative decay rates of the monomer
and knr
E are radiative and non-radiative decay rates
excited state, krE and knr
of the excimer, and G is generation rate of triplet monomers [46].
In the doped system, taking into consideration that nonH , the singlet host state
radiative decay rate of the excited host knr
concentration [1 H], the rate of that exciton is transferred from the
host to the dopant molecule kXHS , and Q is the probability that one
dopant molecule has another dopant molecule as its nearest neighbor. As the host material emission is quenched, the rate equations
for excimer formation in the doped system could be expressed as
the following [46]:
d[1 H ∗ ]
H 1 ∗
[ H ] − kXHS [1 S][1 H ∗ ] + G
= −knr
dt
d[3 T ∗ ]
T 3 ∗
= −krT [3 T ∗ ] − knr
[ T ] − QkXSE [1 S][3 T ∗ ] + kXHS [1 S][1 H ∗ ]
dt
d[3 E0∗ ]
dt
E 3 ∗
= QkXTE [1 S][3 T ∗ ] + krE [3 E0∗ ] − knr
[ E0 ]
For neat films Q = 1 and for doped films Q = 1 − (1 − f)C , where
C is the number of possible lattice positions around one molecule
that another molecule can occupy to form an excimer pair, f is mole
fraction of the dopant [46]. When phosphorescent materials under
electrically excitation, charge transfer between adjacent molecules
also could promote the form of the triplet excimer. The cations (M+ )
and anions (M− ) in the adjacent molecules form the triplet excited
excimer under the direct coulomb interactions and the formation
mechanism could be described as [52]:M+ + M− →3 E∗0
An exciplex is another kind of dimer that may be generally
formed by the interaction of an excited electron donor (acceptor)
molecule D* (A* ) with another unexcited counterpart A (D), which
can be observed both in EL and PL spectrum [53]. An exemplary
exciplex emission could be obtained between electron donor
4,4 ,4 -tris[3-methylpheny(phenyl)amino]triphenylamine
(mMTDATA) and electron acceptor 4,7-diphenyl-1,10-phenanthroline
(Bphen) molecules [54]. The blended films give a broad exciplex
PL band at about 510 nm, which is red-shifted from both the PL
band due to the exciton of m-MTDATA and the PL band due to
the exciton of Bphen. Intermolecular exciplex emission could be
formed by conformational change leading to charge-transfer (CT)
processes, especially in a donor (D) and an acceptor (A) with a
three-carbon methylene chain, as represented by D-(CH2 )3 -A [55].
Most exciplex emission could be observed in solution phases and
special exciplex emission could be observed in crystalline or solid
states [56].
Both excimer and exciplex emission have been used as molecular probes to investigate molecular structure, detection of DNA
sequences and fluorometric analysis of nucleic acids [57,58].
Exciplex- and excimer-based molecules (exci-partners) probe offer
a number of advantages over common detection approaches, which
utilize conventional fluorescence dyes as the reporter groups.
Through adjusting three-dimensional alignment of exci-partners,
these probes can produce specific fluorescence emission at much
longer wavelengths than individual fluorescence partners separated in space by more than ∼4 Å. This means that the emission
76
Fig. 5. (a) Photograph of the mixed soultion and thin films of FIrpic:PVK with different doping level under UV-light excitation, (b) The PL spectra of the films under 380 nm
light excition Reprinted with permission from Ref. [41]© 2013, IOP Publisher.
color of the fully assembled excimer or exciplex detector is visibly different to that of the individual components, and thus direct
visualization approaches may be possible for detection. Another
advantage of excimer- and exciplex-based detection approaches
is substantially reduced background fluorescence at the detection wavelength. Earlier, excimer-based oligonucleotide molecular
probes have been reported for the detection of nucleic acids [59].
Recently, Bichenkova and co-workers developed an alternative
approach based on an exciplex detector: oligonucleotide splitprobes (exciprobes) equipped with the exciplex partners were
shown to be capable of emitting characteristic exciplex fluorescence (at ∼480 nm) on correct self-assembly by their bio-target
[58,60–63]. Exciprobes were also assessed for their ability to detect
certain nucleic acid sequences and discriminate mutations at the
level of PCR products and plasmid DNA molecules. An exceptional
sensitivity of excimer or exciplex detectors to the spatial separation between the exci-partners makes it possible to monitor fine
conformational re-arrangements within molecules. These unique
properties can be used, for example, to develop novel molecular
probes capable of signaling the presence of certain chemical or biological factors (e.g. high cellular levels of H+ , metal ions or certain
enzymes) [58].
As the particular emission species, electroplex emission is different from excimer and exciplex emission, the electroplex emission
means cross-recombination of electrons from ETL molecules and
holes from HTL molecules, and it often occurs under high-electric
field inside OLED, but not under photo-excitation conditions.
Recently, we reported a strong electroplex emission peaking
at 610 nm between the PVK and 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP) interfaces based on the ITO/PVK/BCP
(8 nm)/Alq3 (5 nm)/Al, the EL spectra were measured at different driving voltages, as shown in Fig. 6. The intensity of emission
peaking at 610 nm from electroplex emission was enhanced along
with the increase of driving voltage [64]. When the driving voltage exceeds 10 V, the dominant emission peak has a distinguished
red shift from 410 to 490 nm due to the increased 610 nm emission
intensity. The 490 nm emission should be attributed to the spectral
overlap between PVK emission at 410 nm and electroplex emission
at 610 nm from the PVK and BCP interfaces.
2.4. Aggregation induced emission (AIE)
It is well known that luminescence of organic materials is
often quenched at high concentrations, referred to as “aggregation caused quenching” (ACQ), thus limited its application in real
world which requires organic luminescent materials be solid films.
In 2001, Tang and co-workers reported a group of organometallic molecules called silole. The silole molecules were found to
be virtually non-luminescent when silole material is dissolved in
Fig. 6. EL spectra of ITO/PVK/BCP/Alq3 /Al devices. Reprinted with permission from
Ref. [64] © 2012, IOP Publisher.
good solvents, but became highly emissive when aggregated in
poor solvents or fabricated into thin solid films. They coined the
term of “aggregation-induced emission (AIE)” for this phenomenon
because the silole molecules were induced to emit by aggregate formation [65,66]. The main characteristic of luminescent materials
with AIE attributes, exactly the opposite characteristic of the ACQ
effect, is that the fluorescence intensity is low as isolated species,
while rapidly increase in poor solvents or when made into thin
film by aggregating into nanoparticles. As aggregation is an inherent process when luminescent molecules are in condensed phase,
it would be useful to develop a system in which aggregation plays
a constructive, rather than destructive role, in the light-emitting
processes. Many research groups have successfully synthesized
different types of materials and have explored their applications
in organic sensors, OLED and imaging [67–71]. However, working
mechanisms of the AIE processes are so complicated that they still
remain unclear, although various theories have been advanced to
explain the AIE phenomenon, such as planarity and rotation ability,
restrictions intramolecular rotation (RIR), intermolecular interactions, ACQ-to-AIE transformation [72–75]. Tang and co-workers
demonstrated that the restrictions intramolecular rotation is the
main cause for the AIE effect of their systems through a series of
externally and internally modulated experiments and theoretical
studies [76–78]. According to fundamental physics, any molecular
motion will consume energy. Tang and co-workers have conducted
a number of control experiments (decreasing temperature, increasing viscosity, applying pressure, etc.) to externally activate the
RIR process. They have also used covalent bonds to fasten the
aryl rotors to internally set off the RIR process at the molecular
77
Fig. 7. Molecular structures of TPE-containing polymer, fluorescent photographs of the polymer in THF/water mixtures with different water fractions taken under UV
illumination. Reprinted with permission from Ref. [75] © 2012, Elsevier Publisher.
level. Shuai and co-workers conjectured that the low-frequency
motions are associated with the non-radiative energy dissipations
in the solution state [79]. These motions are readily suppressed
in the aggregate state or at low temperature, leading to recovery of the radiative transitions. In response to both the external
and internal controls, the luminogens become emissive, thus offering experimental evidence to support their mechanistic hypothesis
[77,80,81]. Some fluorescence materials have enhanced emission
in the aggregation state, for the aggregation of the molecules
could induce efficient intermolecular charge transfer and positively
restrict the intramolecular rotation. Hence, the emission intensity could be enhanced by the block of the non-radiative decay
channels [82,83]. Tang and co-workers investigated the synthesis
of conjugated poly(phenylenevinylene)s bearing TPE luminogens.
More importantly, the polymer exhibits an aggregation-enhanced
two-photon excited fluorescence (AETPEF) effect. Fig. 7 shows fluorescent photographs of the polymer in THF/water mixtures with
different water fractions taken under UV illumination. The twophoton excited fluorescence (TPEF) intensity of its nanoaggregate
is ∼9-fold higher than its isolated chain in the THF solution when
excited at 800 nm with a femtosecond laser pulse. The profiles
of the one- and two-photon excited fluorescence spectrum are
similar, manifesting that they are originated from the same radiative species. Since the two-photon technique allows intact tissue
imaging due to its advantages of increased penetration depth, localized low energy excitation, and prolonged observation time, the
polymer is a promising light-emitting material for biological applications. The fast development of AIE research has resulted in the
accumulation of a wealth of information on structural design of AIE
luminogens and mechanistic understanding of AIE processes.
3. Efficient methods to improve efficiency
3.1. Key factors on internal quantum efficiency (IQE)
In general, OLEDs operating under low driving voltage (10 V)
could obtain enough brightness more than 10,000 cd/m2 . However, the obvious efficiency roll-off of OLEDs could be observed
with the increase of current density. The factors, such as charge
carriers balance, lifetime of exciton and intrinsic quantum efficiency of materials, strongly influence the IQE of OLEDs [84–88].
For fluorescence material based OLEDs, the IQE is no more than
25% due to the spin forbidden of transition T1 → S0 . The T1 → S0
was attenuated by the way of non-radiative transition and the
released energy would convert into heat instead of light emission. The IQE of the PhOLEDs could be reached 100% in theory by
utilizing both the S1 and T1 state exciton. While the accelerated
efficiency roll-off of OLEDs at high current density can be ascribed
to be twofold: the imbalance distribution of electrons and holes
in EML and non-radiative exciton quenching processes, such as
TTA and triplet–polaron annihilation (TPA) due to long radiative
lifetime of the triplet exciton, singlet–singlet annihilation (SSA),
singlet-heat annihilation (SHA), singlet–polaron annihilation (SPA),
singlet–triplet annihilation (STA), ISC and field induced quenching [84,89–96]. Efficiency roll-off of OLEDs at high current density
largely restricts the practical application of OLEDs. The actual IQE
could be calculated by the following relation [97]:
int = ıexc ˚p
Here, ı is electron–hole charge balance factor, exc the ratio of
the exciton that could result in radiative attenuation and ˚p is the
intrinsic quantum efficiency of the luminescent materials.
It is known that the carrier mobility of organic semiconducting film is low due to the charge carrier hopping transporting way
[98,99]. Besides this, the mobility strongly depends on the electric
field intensity (E) and temperature, which could be expressed by
Pool–Frenkel formula [100]:
= 0 exp
−
kT
√
exp( E)
where 0 , and are factors that related to the materials, k the
Boltzmann constant and T is the absolute temperature [101–103].
In addition, the charge carrier mobility is related to the chemical structure of materials, such as denoting electron group and
accepting electron group. It is known that most of the organic semiconductors have unipolar transport character, showing greater
tendency for transporting one type of charge carriers. In order
to improve the charge carriers balance in the EML, charge carriers blocking layers, such as hole blocking layer (HBL) or electron
blocking layer (EBL), are introduced into the structure of OLEDs
[104–109]. A good device should have good charge mobility and
comparable balance between the hole and electron. Hence, it is
important to choose suitable HTL and ETL materials to make carriers more balance in EML. The balanced charge carriers mobility
could make less thickness disparity of the HTL and ETL which would
decrease the electric field intensity change induced by charge carriers transporting and blocking layers thickness errors [110–113].
In order to decrease concentration quenching and improve
forming film performance, phosphorescent materials were usually
doped into appropriate host material. Therefore, energy transport
from host material to phosphorescent material and charge carrier
transporting characteristics of host material have great effect on
the exciton forming in phosphorescent materials, which results in
IQE less than 100% in fact [114]. Adachi and co-workers reported a
series of OLEDs have high IQE based on adopting appropriate host
material resulting from high efficiency of energy transport from
host material to phosphorescent material [114–117]. The effective
improvement of exciton forming and radiative emission could be
summarized as three methods: (i) adopting appropriate host materials, (ii) enlarging charge carrier recombination region and (iii)
confining exciton diffusion.
78
3.2. Key factors on external quantum efficiency (EQE)
To obtain high efficiency OLEDs, two major factors should be
taken into account: (i) the IQE would like to be close to 100% and
(ii) the extraction of light emission out from the emission layer must
be as efficient as possible. The EQE (ext ) of OLEDs is codetermined
by the IQE (int ) and the external extraction efficiency (extraction )
[118]: ext = int × extraction . The power efficiency power of OLEDs is
strongly influenced by ext and could be calculated by the following
equation:
power =
POLED
ext
h
=
×
IOLED V
V
e
where V is the applied driving voltage and h e is the average energy
of emitted photon.
Though the IQE of PhOLEDs have been almost 100% by harvesting both the singlet and the triplet excited states emission
[12,119,120], the ultimate energy utilization efficiency characterized by the EQE is still not very ideal. In conventional configuration
of OLEDs that fabricated on ITO coated glass, only a small fraction (∼20–30%) of light emission could be exported [121–123]. The
trapped light distribution is primarily determined by index configuration of multiple thin film layers. According to Snell’s law,
sharp difference of refraction between the two thin film interfaces
leads to small critical angles of total reflection (TR), screening out
most of light with large incident angles. This trapped light, furthermore, is reabsorbed by the functional layers and turned into
heat that may impact the devices performance. extraction could
be estimated by 1/2n2 and is about 20% based on the perfect
conditions, light emission in EML is isotropic and cathode has
a total reflection [124–126]. extraction is strongly influenced by
interfacial loss such as the losses of TR in the interfaces between
glass and air, ITO and glass, organic material layer and ITO, as
shown in Fig. 8 [97,124,127,128]. Through deep research on physical features of OLEDs, the effective methods to improve extraction
focus on employing external extraction structures, internal extraction structures and surface-plasmon cross coupling. The detailed
advantage of each method would be discussed in the following
sections.
3.3. Improve EQE by employing external extraction structures
(EES)
In conventional structure OLEDs, light emission in the EML
would pass through glass substrate. The light loss at glass/air interface is about 20–30% due to the small total reflection angle. The
small total reflection angle is about 41.8◦ due to the refractive index
mismatch between glass (n ∼ 1.5) and air (n = 1). When incidence
angle of light is higher than the total reflection angle, this part of
Fig. 8. Schematic diagram of light extraction losses in conventional OLEDs Reprinted
with permission from Ref. [127] © 2012, the Optical Society.
light would be confined in glass substrate. A simple approach to
decrease the light loss at glass/air interface is directly exporting
light from ITO into the air without glass substrate based on top
emitting structure. However, the process of fabricating ITO thin
films has great negative effects on organic material which mostly
adjoins the ITO layer in the top emitting structure devices, especially by sputtering method [129]. Meanwhile, microresonator in
the top emitting devices with ultrathin metal or metal oxide layers
as top electrode could result in squeezed spectrum of the emission [73,130]. Recently, Lee and co-workers reported that the high
performance top emitting OLEDs with a total maximum luminance
efficiency of 67 cd/A and power efficiency of 67 lm/W was realized
using 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN)
as an organic buffer material. HATCN effectively protected the
underlying organic emission layers from damage caused by sputtering deposition of the indium zinc oxide top electrode, and
simultaneously showed good hole injection from the transparent
top electrode into hole transporting layer. Moreover, the ransparent top electrode showed an average transmittance of around 81%
in the visible range, which is very close to that of ITO/glass [131].
However, realizing highly efficient top emitting OLEDs has proven
to be a significant challenge, primarily because of the difficulty in
finding an efficient bottom-electron-injecting contact and damagefree top transparent anode top emitting OLEDs. Here, the successful
realization of light extraction improvement in the conventional
structure OLEDs are summaried as the following.
3.3.1. EES: microlens array
Forster and co-workers put forward an approach to improve
the light extraction efficiency by using ordered array of microlens
attached to the glass substrate [132]. A series of microlens array
with different diameter, aspect ratio and area coverage, as an
external out-coupler, have improved the light extraction efficiency
by 1.5–3 folds in the conventional structure OLEDs [133–137].
Although the working mechanism of microlens arrays for the light
extraction efficiency enhancement is still ambiguous, it could lead
to randomization of photons and out-coupling of some wide angle
rays from the substrate. In the conventional structure OLEDs, light
emitting from EML layer would suffer from TR at the interface
of glass. When the surface was coated with microlens, such TR
was partly eliminated due to the enlarged critical angle, as shown
in Fig. 9 [138]. Fabrication process of the microlens array can be
very simple, such as soft lithography and micro-contact printing of
hydrophobic self-assembled monolayers [139]. Up to now, a series
of different structural microlens arrays have been fabricated to
improve extraction efficiency of the congenital structural OLEDs
[136,140–142]. Lu and Sturm reported a series of microlens array
coated substrates with the same materials, in which extraction efficiency was increased by a factor of 2.2 for the average and a factor of
3.2 in the normal direction [143]. Furthermore, the use of microlens
array could also adjust the intensity distribution nearly to Lambertian. However, such regular periodic structural microlens arrays fail
to be applicable to a wide variety of materials.
3.3.2. EES: texturing meshed surfaces
Texturing meshed surfaces is another approach to lessen the
trapped light in the substrate. Lee et al. used ray tracing to model
the effect of textured surface on the light extraction [144]. Assuming that the effect of textured surface is to randomize the photon
trajectory, the presence of textured surface by introduction of the
random process could define the orientation of the trajectory of
the photon. Cheng et al. demonstrated that texturing meshed surface on a poly (dimethyl siloxane) (PDMS) film could efficiently
improve light extraction [145]. This meshed surface was fabricated
through a spin-coating process by using a self-organized porous
film as a template. The outcoupling coefficient of OLEDs with such
79
Fig. 9. The improvement diagrammatic sketch of light extraction due to the introduce of microlens array Reprinted with permission from Ref. [138]© 2008, the Optical
Society.
structure was improved to 46%. However, such approach, allowing
the estimation of gross effect of textured surface on light extraction, does not permit to study the effect of the texture parameters
such as shape, dimensions, surface density [146]. The chaotic surface morphology could make the OLEDs having wide bandwidth
and emitting angle, which is important to the OLEDs, especially for
the white OLEDs. For the OLEDs without a meshed surface, the light
is trapped in the film in case the incident angle is larger than the
critical angle. According to the diffraction principles, the grating
structure could effectively improve the light extraction efficiency.
However, such diffraction gratings enhance the extraction of the
guided light only at specific emission wavelengths, which strongly
depend on the particular grating period which makes the application have a huge limitation especially in white OLEDs. As the light
invades on the textured surface, they are scattered by the meshed
structure. The factor of light extraction enhancement is defined by:
g=
Lmeshed − Lplate
Lplate
× 100%
where Lmeshed and Lplate are the total luminescence intensity of
meshed surface and plate glass OLEDs, respectively. Thus, the original trapped light in the film can escape as shown in Fig. 10.
3.3.3. EES: anti-reflection layer
The EES such as microlenses array usually requires special
methods to fabricate the structure and might have an asymmetrically distributed light pattern in the forward direction. The
anti-reflection (AR) technology could effectively avoid the problems and attracted more and more attentions [147–150]. Saxena
et al. used single layer coating of magnesium fluoride (MgF2 ) on the
backside of glass substrate to form simple AR layer and observed
about two-fold enhancement of the luminance [147]. The coating
of MgF2 thin film could generate two reflected waves, one at glassMgF2 boundary and another at MgF2 -air boundary. Light exits out
OLEDs would pass through the two boundaries. In both cases, if the
thickness of AR-coating is ∼/4, the relative phase-shift between
the two-reflected waves at the upper and lower boundary of thin
film is ␲. Hence, light wave reflected at MgF2 -air boundary will
experience a ␲-phase change, whereas light wave at glass–MgF2
interface will be reflected without ␲-phase change, as shown in
Fig. 11. Destructive interference between the two reflected waves
would take place and substrate wave guided modes would cancel
each other, and more light could be coupled out.
3.3.4. EES: sand-blasting substrates
The methods mentioned before are proved to have great
contribution to improving the light extraction efficiency. However,
complex and expensive fabrication in some way do not correspond
with the trend of low cost in OLEDs. Directly using the glass substrate as scattering medium for OLEDs could effectively reduce the
devices complexity and the fabricating cost, which is an attractive
approach to improve the light extraction efficiency. In 2010, Chen
and Kwok reported a very simple and cost-effective method to
rough the glass substrates and hence to scatter the light [151]. By
simply sand-blasting the edges and back-side surface of the glass
substrates, the EQE of OLEDs has a 20% improvement while a constant color over all viewing angles and uniform light pattern with
Lambertian distribution was obtained due to the scattering effect.
The schematic diagram of improvement effect by sand-blasting is
shown in Fig. 12. Ray a with incident angle smaller than the critical angle c at glass/air interface could directly emit to the forward
surface, while ray b with > c , ray c with c , are wave-guided
by the glass due to TR. The wave-guided ray b undergoing many
times reflection/absorption and finally disappear due to absorption, leading to a dark region F. Ray c may escape from the edges
of substrate, resulting in an edge emission with light propagation
direction nearly parallel to the surface and lead to a stripe illumination region E. By sand-blasting the edges of substrates, ray c could
be random scattered, resulting in a bright region E’ by increasing
Fig. 10. (a) Schematic illustration of optical ray trajectories in the thin films without and with a mesh on the surface and (b) SEM of the meshed surface, the upper image
shows the cross section of thin films. Copyright permission by American Institute of Physics [145].
80
Fig. 11. (a) Schematic diagram depicting the propagation of emitted light via various modes and (b) schematic diagram of the phenomenon of AR coating using single-layer
MgF2 for the extraction of substrate-waveguided modes. Reprinted with permission from Ref. [147]© 2008, Elsevier.
the forward efficiency of 5%. Due to the scattering effect, the
original non-emissive regions F exhibit bright emission (regions F’)
resulting in a 10% improvement. Ray d that may be wave-guided
by the substrate is scattered back to the glass substrate (ray d’)
and may find opportunities to escape from the back-side surface
due to scattering effect, resulting in an additional 5% improvement
of forward efficiency. This simple and cost-effective method may
be suitable for mass production of large-area OLEDs for lighting
applications.
3.4. Improve EQE by employing internal extraction structures
(IES)
Theoretical calculation demonstrates that the loss of light generated in EML due to TR between ITO and glass surface could be
more than 50% [97,124]. The lost light is reflected many times inside
the devices, trapped in the OLEDs and translated into heat at last
[143]. The part of light not only causes the waste of energy, but
also results in the instability of the devices due to the sensibility of organic materials to light and heat [152,153]. Therefore, it is
very necessary to reduce the loss of light caused by the TR between
ITO and glass surface. Many effective internal extraction structures
(IES) have been exploited in the recent years.
3.4.1. IES: photonic crystal pattern
Boroditsky et al. described a promising thin-slab light-emitting
diode (LED) design, which used a highly efficient coherent external scattering of trapped light by two-dimensional (2D) photonic
crystals (PCs). The light generation region was an unpatterned
hetero-structure surrounded by the light extraction region, a thin
film patterned as a 2D PC. A six-fold PL enhancement was observed
compared to an unpatterned thin film LED. That corresponded to
70% EQE [154]. Based on the remarkable enhancement, Lee et al.
introduced a PCs pattern into the glass substrate of an OLED [155].
With the use of an optimized PC pattern, the viewing angle range
could be enlarged to 90◦ ± 40◦ , the enhancement of the light extraction efficiency could be more than 80% and 50% in theory and
experiment, respectively. However, to the large area patterns that
required for practical OLEDs, the preparation process of 2D PCs patterning need a very long time. In 2007, Ishihara et al. reported
a simple approach that fabricating tuned 2D PCs by employing
nanoimprint lithography (NIL) technique which is very useful for
the fabrication of the OLEDs with PC [156]. The OLEDs having PCs
showed the improvement of luminescent efficiency by a factor of
1.5 compared to normal devices under the same conditions. In
2012, Yue et al. employed the finite-difference time-domain (FDTD)
method that embedded PCs and surface PCs to improve the light
Fig. 12. (a) Schematic illustration of light propagation inside the devices; photos of devices fabricated on (b) edges sand-blasting substrates, (c) partial surface sand-blasting
substrates and (d) untreated substrates. Reprinted with permission from Ref. [151]© 2010, the Optical Society.
81
Fig. 13. Schematic diagrams of the OLEDs with embedded LIG in the organic layers. Reprinted with permission from Ref. [157]© 2008, Nature Publishing Group.
extraction efficiency [128]. The experimental result showed that
the embedded PCs played a key role in improving the light extraction efficiency. The enhancement of light extraction efficiency could
be in excess of 290% for the optimized OLEDs with double PCs.
3.4.2. IES: the embedded low-index grids (LIG) and ultra LIG
Forster et al. demonstrated a simple and wavelengthindependent method to improve light extraction by embedding a
low-index grids (LIG) into the OLED active organic layers [157]. This
method results in an enhancement of extraction by 2.3 ± 0.2 times
that from a conventional OLED on a glass substrate, and simulations predict this enhancement factor can be improved to 3.4 ± 0.2.
The light with high incidence angle (rays A and B) that would be
trapped by TR at the ITO/glass interface could enter the LIG, and are
refracted into a direction towards the substrate normal. In addition, the LIG would not affect the rays that are originally emitted
into the forward viewing cone (ray C). If the LIG has an index nLIG
lower than or equal to nsub , all light entering this region could be
extracted from the wave guided mode into the glass substrate, as
shown in Fig. 13.
The numerical full-wave electromagnetic field simulations
revealed that the light extraction could be significantly increased
as the refractive index of the LIG material reduced to that of air, theoretically allowing EQE > 50% [158]. However, the refractive index
for visible wavelength light is usually more than 1.35 for the most of
inorganic dielectric materials [159]. In 2001, Tsutsui et al. reported
silica aerogels with an extremely low refractive index equivalent
to air (could be controlled from 1.01 to 1.10) using sol–gel method
[160]. The light extraction efficiency of the OLEDs with the silica
aerogels could be enhanced by a factor about 2. Unfortunately, the
silica aerogels prepared by sol–gel method are highly hydrophilic
and could be degraded by imbibing water, in addition, the silica
aerogel bulks are fragile and quite difficult to handle. These features
determine that the silica aerogels layer is not very suit for OLEDs.
Xi et al. employed glancing-angle vapor deposition to reduce the
refractive index of SiO2 and obtained great success [161,162]. In
2010, Slootsky and Forster manufactured an ultra-low refractive
index (n = 1.15), porous SiO2 grid (Ultra LIG) by using glancingangle vapor deposition and embedded them in the organic layer
of the OLEDs (as shown in Fig. 14) [159]. The light extraction efficiency of OLEDs was increased by 48% over a conventional device
at a luminance of 100 cd/m2 . The EQE and P of the OLEDs were
up to 22.5% and 64 lm/W at the peak efficiency, a nearly threefold
increase over analogous conventional OLEDs.
3.4.3. IES: high refractive index substrates
The occurrence of TR between ITO and glass substrate is due to
the refractive index mismatch of ITO (1.8–2.2) and glass substrate
(∼1.5). According to classical optics theory, the high refractive
index substrate could effectively increase the light extraction efficiency. In 2009, Mladenovski et al. replaced the traditional glass
substrate by high refractive index sapphire substrate (n = 1.8) [163].
As the refractive index of the substrate is similar to that of organic
layers and ITO, TR and light trapping in the organic multilayer are
effectively avoided, resulting in most of generated light could pass
through substrate. After optimizing the device structure, EQE and
luminous efficiency of the OLEDs could be increased to 42% and
183 lm/W at 1000 cd/m2 , respectively. In 2010, Mikami reported
that the light extraction efficiency of OLEDs could be increased
by using weak micro-cavity structure and high refractive index
substrate coupled with micro-lens array simultaneously [164]. The
maximum EQE and power efficiency of the optimized device were
increased to 57% and 200 lm/W.
Fig. 14. The surface (top) and cross-sectional (bottom) SEM images of obliquely
deposited porous SiO2 used for the Ultra LIG Reprinted with permission from Ref.
[159]© 2010, the Optical Society.
82
Fig. 15. (a) Proposed electrode structure consisting of patterned ITO and coated
high-conductivity PEDOT:PSS layer, (b) representative cases that convert light
trapped in organic/ITO layers into out coupled mode Reprinted with permission
from Ref. [165]© 2010, WILEY-VCH.
3.4.4. IES: low index layer on microstructured ITO electrodes
The OLEDs for display applications require high definition, simple production process and low cost. In 2010, Koh et al. introduced
a novel anode structure based on micropatterned ITO coated with
high-conductivity PEDOT:PSS layers [165]. This electrode structure
could improve the light extraction efficiency of OLEDs in a relatively simple way without severe spectral dependence, blurring
(optional), or deviation from the normal angular dependence. In the
OLEDs, a high-conductivity PEDOT:PSS layer was coated throughout the patterned ITO anode area. PEDOT:PSS has a lower refractive
index (n = 1.42 at = 550 nm) than organic material and ITO layers, its incorporation in the micropatterned ITO would provide a
significant index contrast between the organic material and ITO
layers (as shown in Fig. 15). Light emitted with a small angle
within the EML, which would normally be wave-guided throughout the organic material/ITO layers, was guided either solely within
organic layers or solely within ITO layers. Upon hitting the structured region once or multiple times, some portion light will directly
couple out by changing direction and some portion light confined
in the ITO/organic material layer wave-guided mode converted to
the glass/substrate wave-guided mode. Thus, the structuring of
ITO electrodes could have a significant optical effect and lead to
the enhancement of extraction [137,165]. The EQE and the power
efficiency of OLEDs could be increased from 0.91% to 1.59% and
2.90–5.05 lm/W by using of micropatterned ITO.
3.4.5. IES: refractive index modulation layer (RIML)
In 2010, Hong et al. demonstrated a novel way of enhancing
the extraction in OLEDs by using nanofacet-structured refractiveindex-modulating layers (RIMLs) between ITO layer and glass
substrate [166]. The RIMLs materials nanofacet-structured MgO
(n = 1.73) and ZrO2 (n = 1.84) layers that embedded in the OLEDs
could effectively reduce the TR at the glass/ITO interface, as shown
in Fig. 16. The thickness of the RIML was typically /4n, where was the wavelength of the generated light and n is the refractive
index of the RIMLs. The light emission from OLEDs would propagate in all directions due to Lambertian distribution. According
to the classical Snell’s law, ray 1 that with low-angle could be
extracted toward the glass substrate at the ITO/glass interface;
ray 2 that with high-angle would undergo TR at the interface,
resulting in propagation within glass substrate and ITO electrode.
Fig. 16. (a) Schematic diagram of OLEDs with the embedded RML (MgO and
MgO/ZrO2 ) at the glass/ITO interface and (b) schematic explanation of the mechanism for improving device extraction efficiency with a micro-facet-structured RIML
Reprinted with permission from Ref. [166]© 2010, WILEY-VCH.
After embedding the nanofacet-structured RIMLs in the OLEDs,
the ray 2 could enter the RIML region and was refracted into a
direction towards the substrate normal (ray 3) while the rays
that originally emitted into the forward viewing cone were not
affected. In addition, MgO and ZrO2 could be deposited by electron
beam evaporation and the nanofacet structured MgO film could be
formed spontaneously due to the anisotropic material characteristics between the crystal orientations. By inserting RIML between
ITO layer and glass substrate, the luminance value was enhanced
by a factor of 1.19 compared to conventional OLEDs on a glass
substrate and the power efficiency could be improved by 34.7%.
3.4.6. IES: improved Bragg diffraction gratings
The wavelength-scale periodic gratings could increase the
external efficiency of OLEDs effectively due to Bragg diffraction
[156,167]. Only specific wavelength light dependence on the particular grating constant could be enhanced, which results in its
limited application in OLEDs. In addition, Bragg diffraction grating
required incorporation of corrugated structure into OLEDs and was
usually fabricated by using electron-beam, holographic, nanoimprint lithography techniques. However, the fabrication techniques
have high costs, complicated procedures for large periodic pattern
[156,167,168].
In 2010, Koo et al. put forward a simple and effective method,
a quasi-periodic flexibility buckling structure, to extract the light
in a broaden waveguide range [169]. The buckling patterns were
formed spontaneously by thermally evaporating 10 nm Al films
on poly(dimethylsiloxane) (PDMS) substrates preheated to 100 ◦ C
using an external radiation source. The difference between the
thermal expansion coefficients of the PDMS and Al film would
release the compressive stress during cooling to ambient temperature, resulting in the form of buckling structure. Compressive
stress was introduced by further deposition of a 10-nm-thick Al
layer, once or twice more, on a buckled PDMS replica that was
fabricated from a buckled PDMS mold after the first deposition of
Al. The surface area ratio after deposition twice and three times
significantly increases from ∼1.4% to ∼9.0% and 11.3%, respectively, corresponding to buckling structure depths of 40–70 nm and
50–70 nm. The buckled OLEDs exhibited enhancements of ∼83%
(double buckling) and 120% (triple buckling) for current efficiency,
and 120% (double buckling) and 190% (triple buckling) for power
efficiency in the visible light range. The characteristics of the broad
periodicity distribution and the randomly oriented wave vectors
of the buckles provide possible extraction of the waveguide light
propagating along any direction in a wide spectral range without
inducing spectral changes, which is beneficial to full color and white
OLEDs.
In 2012, Koo et al. put forward another light extraction scheme
with an emission profile close to Lambertian emitter by introducing defects into a hexagonal-close-packed (HCP) silica sphere arrays
and randomizing the directionality and broadening the periodicity
[170]. The 1.0 (or 0.5) ␮m diameter silica microspheres and 100 nm
diameter polystyrene nanospheres were dispersed and deposited
on glass substrates with the deposition blade. After annealing treatment on sample at 140 ◦ C, the polystyrene spheres were melted
to form corrugated structures with periodicities corresponding to
the size of the silica spheres, the depth of the corrugated structure
could be tuned by controlling the annealing treatment time. The
defective HCP array that retains the hexagonal fast Fourier transform (FFT) patterns could allow diffractions of waveguide light in all
azimuthal angles. The current and power efficiencies of OLEDs with
0.5 ␮m grating were enhanced by 70% and 90% respectively, without introducing particular spectral change over emission angles.
With the low-cost and large-area processing, the defective HCP silica array pattern can supply a practical solution for light extraction
in the field of OLEDs applications.
3.4.7. IES: embedded nanocomposite scattering layer
Wu and co-workers utilized internal scattering layers and
obtained enhancement in the optical extraction by about two-fold
[171]. The fabrication of nanocomposite scattering films based
on solution-processing was relatively simple and convenient.
There is still plenty of room for further improvement by reducing
the TR at the ITO/photoresist interface. In 2012, Chang et al.
combined TPR with titanium oxide (TiO2 ) nanoparticles (NPs)
to restrain the TR at the ITO/TPR interface [172]. The 25 nm-NPs
could improve the refractive index of the TPR and flatten the
surface of the nanocomposite film as well as the 250 nm-TiO2
83
Fig. 17. The sketch maps of OLEDs with and without light extraction enhancement
layer. Reprinted with permission from Ref. [172]© 2012, Elsevier.
contributed to effective scattering. The nanocomposite substrate
could obtain high refractive index, satisfactory transmittance and
great scattering capability by combining TiO2 in different sizes. The
efficiencies of OLEDs utilizing the embedded nanocomposite film
could be remarkably enhanced to 25.2%, 62.9 cd/A, and 37.1 lm/W
at 103 cd/m2 . In contrast to the pristine ITO substrate, the power
efficiency enhancement at 5000 cd/m2 was as high as 4.3 times
for blue PhOLEDs. The schematic maps of OLEDs with and without
light extraction enhancement layer are shown in Fig. 17.
3.4.8. IES: nanostructured indium tin oxide (NSITO)
The low cost method that could effectively extract the trapped
light at ITO–organic and ITO–glass interfaces is required. In 2010,
Slootsky and Forster used Ultra LIG porous SiO2 (n = 1.15) to
enhance the light extraction efficiency by using glancing angle
deposition techniques and the light extraction efficiency could
be enhanced by about 48% [159]. The glancing angle deposited
Fig. 18. (a) SEM image of the ITO films grown at a glancing angle 85◦ , (b) optical transmission versus wavelength for conducting ITO films without and with NSITO films
deposited at 65◦ , 75◦ , 85◦ , (c) schematic diagram of the OLED showing the extraction scheme without NSITO and (d) schematic diagram of the OLED showing the extraction
scheme with NSITO. Reprinted with permission from Ref. [174]© 2012, the Optical Society.
84
technology had been used in inorganic LEDs to effectively enhance
the efficiencies and have many advantages, such as low cost and
the possibility of roll to roll fabrication [173]. In 2012, Kumar
et al. grew a nanostructured ITO (NSITO) film between the ITO
anode and glass substrate by glancing angle deposition to extract
the light trapped inside the ITO and organic material layer [174].
ITO was deposited on glass substrate by using the RF-sputtering
technique at glancing angles of 65◦ , 75◦ , 85◦ and the corresponding
refractive indices were 1.8–1.9, 1.4–1.5 and 1.2–1.25, respectively.
In the wavelength range of 350–800 nm, the optical transmission
of the film with NSITO was increased in glancing angle from 65◦
to 85◦ . The SEM image of the NSITO and the optical transmission
of ITO films without and with NSITO films are shown in Fig. 18(a)
and (b). At the same time, the addition of nanostructure did not
deteriorate the hole injection property of ITO and the OLEDs
shown a negligible change in CIE coordinates within a viewing
cone of a 75◦ half angle. At last, the light extraction efficiency of the
OLEDs fabricated onto NSITO film was enhanced by 80% without
introducing any detrimental effects to operating voltage, current
density, and angular invariance of emission spectra of OLEDs.
3.5. Surface plasmon extraction
The light loss due to surface plasmon-polaritons (SPPs) is about
20–40% for OLEDs [137,175,176]. SPPs are longitudinal, p-polarized
waves traveling at the interface between metal and dielectric with
evanescent fields decaying exponentially into both adjacent media
[177,178]. On the planar surface the combined electromagnetic
field/surface charge nature means that they are non-radiative in
nature. For semi-infinite layers, the dispersion relation could be
expressed by:
kx = k0
ε ε 1/2
1 2
ε1 + ε1
=
ω
c
ε (ω)ε (ω) 1/2
1
2
ε1 (ω)ε2 (ω)
with ε1,2 (ω) being the complex dielectric functions of the metal
and the adjacent dielectric layer, respectively, and k0 the vacuum
wave vector. As shown in Fig. 19, for metal surface adjacent to air
as dielectric for the range of frequencies and in-plane wave vectors,
the SPP dispersion curve and the light-line in air do not intersect
for finite frequencies. Therefore, energy and momentum conservation cannot be fulfilled simultaneously and as a consequence SPPs
cannot couple to far-field radiation.
Andrew and Barnes reported that the evanescent field of the
SPP could extend through an adjacent thin organic layer and is
sensitive to effective refractive index. The SPPs could exist on both
sides of the metal layer and couple with each other if the metal was
thin enough [179]. The SPPs are guided electromagnetic waves that
propagate along interfaces between metal and dielectric material,
Fig. 19. Schematic SPP dispersion curve for a silver-air interface together with the
light cones for far-field radiation on both sides of the metal layer. Reprinted with
permission from Ref. [127]© 2012, the Optical Society.
resulting in propagation distance up to 10−4 m. The SPPs associated
to a collective oscillation of the free electrons of metal with the incident electromagnetic field could significantly affect the dynamics of
nearby molecules and provide an alternative de-excitation channel
by radiative decay [180].
Since SPP fields are bound to the metal interface, light coupled to the SPP mode remains be trapped. Many research works
have been done in order to reuse this trapped energy. Weber and
Eagen pointed out that the coupling efficiency between the donor
dipole moment and the SPPs mode could be more than 90% under
the suitable conditions [181]. Hobson et al. pointed out that the
device efficiency could be decreased due to SPPs modes, particularly those based on small molecules. However, the efficiency
decrease induced by SPPs modes could be effectively restrained by
using periodic nanostructure [182]. The energy coupled into SPPs
is only lost if it has no radiative outlet. Because the momentum,
or equivalently, the wave vector of a SPP is larger than that of
a freely propagating photon, SPPs require assistance in order to
radiate into free space. Gifford and Hall reported that the cross
coupling between surface plasmon on the opposite sides of the
metal cathode layer could enable the transmission of EL through
the practically opaque metal [183]. In the surface plasmon cross
coupling (SPCC) technique, an excited molecular dipole near a corrugated metal electrode excites directly a SP mode at the interior
metal/dielectric interface, as shown in Fig. 20(a)). The periodicity of metal layer allows the initially excited SP, with wave-vector
ksp1 , to cross couple to a SP, with wave vector ksp2 , at the exterior
metal/dielectric interface. The grating acts again to enable the exterior SP to emit into free space. Cesario et al. reported a method to
Fig. 20. (a) The graphical representation of SPCC and (b) the graphical representation of LSPs/SPPs. Reprinted with permission from Ref. [183]© 2002, American Institute of
Physics. Reprinted with permission from Ref. [180]© 2007, the Optical Society.
Fig. 21. Schematic representation of the energy transfer through a metal film by
SPPs. Reprinted with permission from Ref. [184]© 2012, American Chemical Society.
maximize light transmission through thin metal layers by coupling
of the localized and extended surface plasmon [180]. The localized
surface plasmons (LSPs) are associated with bound electron plasmas in nano-voids or particles with dimensions much smaller than
the incident wavelength and could directly couple with propagating light. The graphical representation of LSPs/SPPs configuration
is shown in Fig. 20(b). Metal film was coupled to a periodic array of
metallic NPs through a thin dielectric spacer. A significant part of
the light emitted by the polymer couples directly to the SPP at the
metal layer interfaces, a maximum out-coupling to the far-field is
expected to be achieved by making the resonances of the array of
plasmon particles overlap with those from the film within the emission band of polymer. In a specific direction, the light transmission
through a metallic film even could be up to 100%.
In 2012, Collini et al. deeply studied the photophysics and
dynamics of SPPs-mediated energy transfer (ET) [184]. The results
indicated that not only the energy could be efficiently transported
from the donor to acceptor for distances up to 150 nm, but also
that the presence of an electric field can cooperatively enhance
the process. The SPP-ET progress could be described as shown in
Fig. 21. It could be summarized as: (i) conversion of the oscillating transition dipole of the initially excited antenna donor into
surface charge-density waves in the first metal–dielectric interface; (ii) cross coupling of the two surface plasmon on the opposite
interfaces of the metal film; (iii) transfer of excitation energy to
the acceptor on the second metal–dielectric interface. SPP modes
extend deeply into both dielectric layers, the range of ET can be up
to hundreds of nanometers. In addition, the presence of an electric
field can cooperatively enhance the process. This SPP-ET is different from the conventional Forster energy transfer whose excitation
energy is transferred in a non-radiative way through the resonant
dipole–dipole interaction between donor and acceptor with a range
of 1–10 nm [185]. The first step of SPPs-ET was proved to be the
main relaxation process for molecules at distance up to 400 nm
from the interface, while it is also recognized as the main cause
of luminescence quenching of dyes in proximity of metallic surfaces [184,186]. When the electronic transition frequency and the
surface plasmon frequency are nearly degenerated, the localized
electronic energy of the dipole can be effectively converted into
surface charge-density waves in the metal surface [187]. If the
thickness of the metal film is thin enough, the SPPs mode created at
the first interface can couple with a plasmon polariton mode at the
other interface. The evanescent wave associated with the plasmon
polariton mode at the second interface can then excite acceptor on
the other side of the metal film with respect to the excited donor.
4. Key achievements of white emission OLEDs
As the worldwide energy crisis is more and more outstanding,
the energy-saving technologies are increasingly valued. China and
85
America, the largest two energy consumption countries, have a
total generated energy about 40% (19.7% and 20.3%) of the world in
2010. Energy consumption report of 2010 revealed that the power
consumption of the two countries in the field of lighting were
about 13% and 19% of the generated energy, respectively. Hence,
developing high efficient lighting technology is a very serious and
imperative task. At the same time, the development of display
technique is very fast, the need of new display terminal is also
urgent. Under this circumstance, WOLEDs which called “green
light source” and “dream display” have gained plenty of attention
throughout the world.
As an important research branch of OLEDs, WOLEDs inherit
almost all the superiority of OLEDs: high efficiency, high contrast
ratio, high responsivity, wide visual angle, low cost and so on
[188–190]. Different from the filament lamp and fluorescent lamp,
this “green light source” is a kind of high brightness area light
source with nature and downy light that is very suit for indoor illumination. WOLEDs are also diffuse light sources with low power
density (e.g., 0.0001 W/mm2 ), which could ensure that glare is not
a critical issue and that thermal management is not required [191].
Because thermal management is needless, WOLEDs light panels
could be made on rigid substrates with total thickness <2.0 mm
and the flexible prototypes with total thickness <100 ␮m have been
demonstrated [191]. Moreover, the new large-area manufacturing
technology such as ink-jet method and roll-to-roll coating would
vastly improve the throughput and drastically reduce the production cost. Recently, Jou et al. reported a kind of physiologically and
friendly candle light-style OLED used for lighting at night, which
may effectively reduce cancer risk [192]. This light source shows
low color temperature (1900 K), high color rendering index (CRI,
93), an efficacy at least two times that of incandescent bulbs and
an 80% resemblance in luminance spectrum to that of a candle.
This kind of superiority is incomparable to the fluorescent lights
and inorganic LED light. As new display terminal, WOLEDs have
no ultraviolet rays which are harmful to human eyes. Moreover,
the WOLEDs display with no backlight is more suitable for display
closed to eyes, which has a great application in game, scientific
research and military field. As a kind of all solid state display, it
could be used in harsh environment such as plateau, high cold, gravity overload and so on. In addition, this display has flexibility and
could be made into narrow side or endless displays which could be
easily joined together. At last, WOLEDs whether used in display or
lighting technique have an incomparable virtue that the light has no
ultraviolet rays, that is, the light has no attraction to the bothersome
winged insect which has a phototaxis of ultraviolet rays.
The ideal WOLEDs have the elementary feature that the emission a spectrum similar to that of the natural sun light and cover
the entire visible range (400–700 nm) [193]. Emission spectrum of
most organic emitting materials could cover only some fraction
of the visible spectrum. By colorimetry definition, white emission
could be obtained by blending of primary colors (red, green, and
blue) or complementary (blue and yellow or orange) [190,194].
White light mainly has three evaluation parameters: CIE (Commission Internationale d’Eclairage) coordinates, color temperature (CT)
and color rendering index (CRI). The white point or equal energy
point on the CIE chart is defined as (0.33, 0.33) for WOLEDs [195].
In order for light to be perceived as white by the human eye, the
emission spectrum should closely match the spectrum emitted by
an incandescent blackbody with a CT between 2500 and 6500 K
such as sunlight has a CT of 5800 K [189]. According to the conclusion of Jou et al., the light used at night with low CT is more
beneficial to the health of human [192]. The CIE coordinates and
CTs have internal physics relation, as shown in Fig. 22. Hence, ideal
CIE of WOLEDs is not required to be (0.33, 0.33) [196,197]. The
CRI, represented by a number between 0 and 100, more than 80 is
required for lighting applications for natural light such as sunlight
86
Fig. 22. (a) CTs of white light area in 1931standard and (b) isotemperature line of white light in 1931standard.
and firelight is more recipient by human eyes as a result of long
term evolution. The CRI of WOLEDs could be easily in excess of 80
for the whole visible spectrum could be covered [195,198]. Some
important achievements WOLEDs have been obtained in the recent
years [188,189,195,199,200].
4.1. Fluorescence/phosphorescence hybrid WOLEDs
Fluorescence/phosphorescence hybrid WOLEDs use fluorescent
emitting dopant to harness all electrically generated high energy
singlet exciton for blue emission, and phosphorescent dopants to
harvest the remainder of lower energy triplet exciton for green and
red emission. This hybrid configuration could minimize exchange
energy losses of the energy transfer in host–guest doped system,
thereby maximizing device power efficiency and IQE. In addition,
this approach has a stable white emission balance dependence
on the current density, high efficiency at high brightness due to
the reduced geminate exciton recombination, and an enhanced
lifetime due to the combined use of a stable fluorescent blue,
and long lived phosphorescent green and red, dopants in a single emissive region [201–203]. In 2006, Sun et al. introduced
WOLEDs that exploited blue fluorescent dopant 4,4 -bis(9-ethyl3-carbazovinylene)-1,1 -biphenyl (BCzVBi), in combination with
green and red phosphorescent dopants fac-tris(2-phenylpyridine)
iridium (Ir(ppy)3 ) and iridium(III) bis(2-phenyl quinolyl-N,C2 ) acetylacetonate (PQIr) [201]. All the emitting materials are doped into
host material 4,4 -bis(N-carbazolyl)-1-1 -biphenyl (CBP) to form
extended EML. Under the electro-excitation, exciton formed on
BCP molecule. The singlet state exciton could be transferred to
BCzVBi directly following a resonant Forster process as the opposed
to direct trap formation. Host triplets state exciton which cannot efficiently transfer to the BCzVBi by the Forster mechanism
or by Dexter transfer could migrate into the center of the EML
where they transfer onto Ir(ppy)3 and PQIr due to long diffusion
lengths (∼100 nm). Finally, placing an undoped host spacer with a
thickness larger than the Forster radius (∼3 nm) between BCzVBi
and the phosphors prevented direct energy transfer from the blue
dopant to the green (6 nm) and red (4 nm) phosphorescent materials. Based on this architecture, both the singlet and triplet exciton
could be harvested along completely independent channels with
optimized energy transfer. WOLEDs were fabricated by doping the
middle region of the EML with both the green and red phosphorescent dopants. The architecture and energy transport process of the
WOLED are shown in Fig. 23. The peak EQE and power efficiencies
were 18.7 ± 0.5% and 37.6 ± 0.6 lm/W. The CIE coordinates could
shift from (0.40, 0.41) at 1 mA/cm2 to (0.38, 0.40) at 100 mA/cm2
with a CRI = 85 at all studied current densities.
In 2009, He et al. reported a high-efficiency and long-lifetime
stacked hybrid WOLEDs by using PIN technology [204]. The
white emission was obtained by mixing blue fluorescent emission together with green- and red- phosphorescent emission. The
current efficiency of the WOLEDs used PIN technology could be
doubled or tripled depending on the number of emission units,
lifetime of WOLEDs could be vastly improved. When applied an
external bias, the p-n junction actually worked under a reverse
bias which could lower the LUMO level of ETL with respect to the
HOMO level of HTL. When the applied voltage was high enough,
the LUMO of the n-doped ETL will come close to or be even lower
Fig. 23. (a) Architecture of the fluorescent/phosphorescent WOLED and (b) energy transfer mechanisms in the WOLED. Reprinted with permission from Ref. [201]© 2006,
Nature Publishing Group.
87
than the HOMO of the p-doped HTL. Because all HTLs and ETLs
in the stacked WOLEDs were highly electrically doped, the voltage drops over these layers was negligible which offered greater
freedom to tune emission color easily by selecting emitting materials and varying the transport-layer thicknesses. With the help of
PIN technology, the operating voltage was only 8.5 V at 1000 cd/m2
and 11 V at 10,000 cd/m2 (J = 13 mA/cm2 ). Under the improved outcouple technology, the CIE was (0.43, 0.44) with a CRI of 90 and
almost without color shift from 1000 to 20,000 cd/m2 . The power
efficiency of 38 lm/W at 1000 cd/m2 and 30 lm/W at 10,000 cd/m2
were achieved. The extrapolated lifetime at an initial luminance of
1000 cd/m2 was above 100,000 h. However, the complex fabrication process of WOLEDs limits its actual application.
4.2. All phosphorescence based WOLEDs by vacuum evaporation
technology
In conventional phosphorescent based WOLEDs, the exciton
formation zone was located in the EML adjacent to one or both
adjacent carrier transport layers, the WOLEDs would might suffer from efficiency roll-off [205]. In 2007, Sun and Forster pointed
out that the IQE would be kept at high level if the triplet exciton formed in multiple expanded regions, the pileup of exciton
would be restrained which was benefit to the decrease TTA and
efficiency roll-off [206]. A three-emission-layers (3-EMLs) WOLED
structure was designed, which allowed the red (PQIr), green
(Ir(ppy)3 ) and blue (bis-4 ,6 -difluorophenylpyridinato) tetrakis(1pyrazolyl)borate, FIr6) phosphorescent dopants, each doped in a
separate host (4,4 ,4 -tris-(N-carbazolyl)-triphenylamine (TCTA),
N,N -dicarbazolyl-3,5-benzene (mCP) and UGH2, to achieve optimum efficiency and color rendition. The detailed structure
of the EMLs was 4 wt% PQIr:TCTA (5 nm)/2 wt% Ir(ppy)3 :mCP
(8 nm)/20 wt% FIr6:UGH2 (20 nm). The HOMO level of 5.9 eV for
mCP aligns well with that of FIr6 (6.1 eV) and TCTA (5.7 eV) with
Ir(ppy)3 (5.4 eV), whereas the LUMO of 2.8 eV for UGH2 also aligns
well with that of Ir(ppy)3 (2.6 eV). The unique energy level configuration would promote resonant injection of holes/electrons into
the HOMO/LUMO of the dopant in the adjacent EML and widen the
exciton-formation zone due to the exciton effectively formed in
all EMLs. The 3-EMLs WOLEDs had a CRI of 81, the peak total EQE
and power efficiencies were 28 ± 1% and 54 ± 3 lm/W. When an ndoped ETL was used, the total PE peaks at 64 ± 3 lm/W and rolls off
to 34 ± 2 lm/W at 1000 cd/m2 .
In 2009, Reineke et al. reported another kind of high efficient all phosphorescent multilayer WOLED by combining a double
EMLs structure [194]. The energy level diagram of the double
EML is shown in Fig. 24. Both charge carrier could be effectively injected without facing any energy barrier into the EMLs
TCTA:Ir(MDQ)2 (acac) and TPBi:Ir(ppy)3 , respectively. Hole could
be transported directly within the HOMO level of Ir(MDQ)2 (acac)
due to its high concentration (10wt%). Here, TPBi is 2,2 ,2 (1,3,5benzenetriyl) tris-(1-phenyl-1H-benzimidazole), Ir(MDQ)2 (acac)
is iridium(III)bis (2-methyldibenzo[f,h]quinoxaline) (acetylacetonate). Both charge carrier was accumulated at the double EML
interface forming exciton. The different sublayers were separated
by 2 nm intrinsic interlayers (TCTA and TPBi) to decouple the sublayers from unwanted energy transfer [196]. Exciton created in the
blue region on host or dopant had various decay channels. The blue
host–guest system was surrounded by red and green sublayers of
the EML to harvest unused exciton (kb-r and kb-g ). The transfer rate
kb-r and kb-g were strongly reduced by the TCTA and TPBi interlayers
because the Forster energy transfer was repressed, restricting diffusive exciton migration and triplet exciton could move freely within
the TPBi:FIrpic layer, resulting in a back-energy transfer rate kBT .
The EML was nearly barrier-free until both kinds of charge carrier
reach the region of exciton formation, which kept the operating
Fig. 24. Energy level diagram of the phosphorescent multilayer WOLEDs. Reprinted
with permission from Ref. [194]© 2009, Nature Publishing Group.
voltage low. The outermost layers in contact with the electrodes
were chemically p- and n-doped, which reduced Ohmic losses to
a negligible level. With the excellent architectonics and extraction
structure, the IQE was about 100% and the power efficiency were
up to 90 lm/W (EQE, 34%) at 1000 cd/m2 which would be up to
124 lm/W (EQE, 46%) if the light extraction was further improved.
In 2012, Chang et al. presented an all-phosphorescent material,
four-color (blue, green, yellow and red emissions) WOLEDs with
high EQE and high CRI simultaneously by employing molecular
energy transfer or, specifically, triplet exciton conversion (TEC) in
a cascaded emissive zone [207]. The emission efficiency of yellow
phenylbenzothiozolatoN,C2 )(acetylacetonate)
iridium(III)bis(2
Ir(BT)2 (acac) and red Ir(MDQ)2 (acac) phosphorescent materials
were significantly enhanced by a high efficient TEC from green
phosphorescent material Ir(ppy)2 (acac), respectively. This intrazone molecular energy transfer from high energy donor to low
energy acceptor molecules was found to be more than 90% through
Forster energy transfer process. In this kind of OLEDs, TPBi was
used as ETL, CBP [2,2 ,2 -(1,3,5-benzinetriyl)-tris(1-phenyl-1-Hbenzimi-dazole)] was used as HTL and triplet host. Both TPBi and
CBP are wide band gap materials with high triplet energy levels,
the majority of exciton generated near the CBP/TPBi interface on
the CBP side and were well-confined onto the TPBi. Some exciton
could be transferred to FIrpic molecule, the rest could be harvested
by the green, yellow and red emission materials which placed
sequentially next to blue in a cascaded fashion. Based on the TEC
process, the EQE of OLEDs obtained a 24.5% EQE at 1000 cd/m2
with a color CRI of 81, and an EQE at 5000 cd/m2 of 20.4% with a
CRI of 85. Aided with a lens-based extraction enhancement, the
power efficiency could be increased to 76.0 lm/W.
4.3. All phosphorescence based WOLEDs by solution process
Besides vacuum deposition technology, solution process is
another very promising technology to prepare WOLEDs due to
the following advantages: straightforward fabrication procedure,
large area coverage, low power consumption, screen printing and
ink-jet deposition and so on. However, for RGB hybrid WOLEDs
with solution-processed EMLs are generally less efficient in terms
of power and current efficiency compared with the ones based
on vacuum deposition technology [208,209]. In 2011, Zou et al.
demonstrated highly efficient WOLEDs fabricated by using solution
process technology with a peak forward-viewing power efficiency
(PE) close to 40 lm/W, an EQE of 28.8%, a CRI of 76 and a peak
current efficiency of 60 cd/A [210]. The WOLEDs were fabricated
88
with two newly synthesized yellow-emitting iridium complexes
and saturated red iridium (III) based dendrimer Ir-G2 as the single EML. The high efficiency was obtained by combining a carefully
designed spectrum of the EML with a modified hole-injection layer
that ensures reduced charge leakage and remarkably improved
charge-carrier balance. The total PE at the relevant brightness level
(1000 cd/m2 ) has the potential to exceed 40 lm/W if light extraction technology was applied, suggesting that the performance of
WOLEDs fabricated by using solution process technology could
be comparable with that of fluorescent lamps (with typical PE of
40–70 lm/W).
Using complementary colors, such as blue (B) and orange (O),
provides an efficient approach for obtaining WOLEDs. The device
fabrication process could be generally simplified by reducing
the number of used phosphorescent emitting materials. Cao
and Jen et al. demonstrated that the optimized EL efficacy of
WOLEDs by solution process could be obtained by using a blueyellow complementary color system [211,212]. In 2012, Zhang
et al. reported solution-processed all phosphorescence based
WOLEDs of very high efficiency by using a recently developed
dendrimer host H2 with a high triplet level (2.89 eV) and a
novel efficient orange-emitting iridium-based dopant containing
5-trifluoromethy l-2-(9,9-diethylfluoren-2-yl)pyridine ligand
Ir(Flpy-CF3 )3 which has good miscibility with H2 [213]. The single
EML included FIrpic and Ir(Flpy-CF3 )3 simultaneously doped H2.
H2 has high-lying HOMO level and better film-forming properties
over other polymeric host materials such as PVK. WOLEDs have
a configuration of ITO/PEDOT:PSS (50 nm)/EML (40 nm)/2,7(SPPO13)
bis(diphenylphosphoryl)-9,9 -spirobi[fluorene]
(50 nm)/LiF (1 nm)/Al (100 nm). The thermally deposited ETL of
SPPO13 could eliminate the cathode interface issue. The solutionprocessed WOLEDs demonstrated a forward-viewing LE, PE and
EQE of 70.6 cd/A, 47.6 lm/W and 26% at a luminance of 100 cd/m2 ,
respectively. It is worth noting that this is the best record that the
light-emitting efficiency of the solution-processed WOLEDs really
approaches to the fluorescent lamp level (40–70 lm/W).
4.4. All fluorescence based WOLEDs
Most WOLEDs were obtained based on all-phosphorescence
or fluorescence/phosphorescence hybrid systems. However, these
kinds of WOLEDs usually suffer from color-stability problems associated with significant roll-off at high brightness or high current
density due to strong triplet–triplet or triplet–charge annihilation.
In addition, the widely used phosphorescent emission materials are
iridium (Ir) and platinum (Pt) complexes. These precious metals
are quite rare and already widely used in much other important industrial application. Therefore, the price of phosphorescent
materials is continuously increasing. Besides this, the recycling use
of Ir and Pt is very difficult. In a word, the long-term development of WOLEDs based on phosphorescent materials has a barrier
that is hard to be overcome. Therefore, the development of allfluorescence based WOLEDs is still important for the potential
application [32].
In 2011, Yang et al. reported that two-component highly efficient WOLEDs were fabricated by employing two commercial
fluorescent materials, deep-blue emitting material bis(2-(2hydroxyphenyl)-pyridine)beryllium (Bepp2 ) as host and orange
organic dye DCM as dopant [214]. In the EML, the energy transfer from Bepp2 to DCM is efficient but incomplete. The WOLEDs
were fabricated with a high CRI of 79–81, a Correlated Color Temperature (CCT) of about 5500 and stable CIE coordinates close to
the ideal equal-energy white (0.33, 0.33) at brightness from 10 to
10,000 cd/m by adjusting the doping ratio of DCM in Bepp2 . Beside
this, the devices not only with a peak efficiency of 14.0 ± 0.35 cd/A
for LE, 9.2 ± 0.25 lm/W for PE and 5.6 ± 0.15% for EQE but also high
LE and EQE values such as 12 cd/A and 5.0%, respectively, upon a
variation in practical brightness from 20 to 2000 cd/m2 . The parameters are comparable to those of an incandescent light bulb.
Almost at the same time, Duan et al. also reported a kinds of
WOLEDs with an extremely long lifetime by wisely controlling
the recombination zone [215]. The key feature of the WOLEDs is
the utilization of double blue-EMLs to stabilize the blue emission.
A mixed-host blue-EML consisting of 78% ␣,␤-ADN:20% NPB:2%
ENPN was utilized to broaden the recombination zone and dilute
the concentration of any degradation related quenching species
(where ␣,␤-ADN is 9-(1-naphthyl)-10-(2-naphthyl)anthracene,
ENPN is 6,6 -(1,2-ethenediyl)bis(N-2-naphthalenyl-N-phenyl-2naphthalenamine). A second blue-EML of 98% ␣,␤-ADN:2% ENPN
was deposited onto the first blue-EML to prevent hole penetration
into the ETL and to attain better confinement of carrier recombination. In order to obtain high efficiency WOLEDs, mixed-host
yellow EML of 10 nm 98% NPB:2% 3,11-Diphenylamino-7,14diphenylacenaphtho[1,2-k]fluoranthene (DDAF) was fabricated.
Then EMLs of the WOLEDs were 78% ␣,␤-ADN:20% NPB:2% DDAF
10 nm/78% ␣,␤-ADN:20% NPB:2% ENPN 10 nm/98% ␣,␤-ADN:2%
ENPN 15 nm. At last, the WOLEDs had a record high lifetime of
over 150,000 h at an initial brightness of 1000 cd/m, 40% higher efficiency and 40 times longer than the conventional bilayer WOLEDs
together with a stable color over the whole lifespan.
4.5. Excimer- and exciplex-based WOLEDs
Excimer- and exciplex-based emission offers another route for
WOLEDs. The emission from an excimer/exciplex is red-shifted
and shows the broader emission range than that from a single
molecule emission. Both excimer and exciplex lack bound ground
state, which can prevent the cascade of energy from both the host
and higher-energy dopants to them. The unique features could
make the energy transfer more efficient and minimize the fabrication complexity of WOLEDs. In addition, the excimer- and
exciplex-based WOLEDs may have a stable emission color due to
the emission originating from the same dopant [216]. For excimerbased devices, doping concentration is a key parameter to control
relative monomer and excimer emission intensity, and emission
color is codetermined by the monomer and excimer emission intensity [41,216].
Adamovich and his co-workers pointed out that platinum(II)[2(4 ,6 -difluorophenyl)pyridinato-N,C2](2,4-pentanedionato) (FPt)
could give blue monomer emission and orange excimer emission, WOLEDs based on this kind of Pt complex have high CRI
[216,217]. In order to improve the efficiency of WOLEDs based
on excimer emission from Pt phosphorescence materials, Jabbour’s
group developed a new host material 2,6-bis(N-carbazolyl)pyridine
(26mCPy) [48]. Such new host material could improve charge transfer from the host material to dopant and balance hole and electron.
The EQE, CRI and CIE of the single EML WOLEDs were 18% (29 lm/W,
42.5 cd/A, at 1 cd/m2 ), 69 and (0.46, 0.47). These experimental
results indicated that the IQE of the WOLEDs was nearly 100%.
In 2009, Zhu et al. presented WOLEDs with high CRI using
only exciplex emissions based on the synthetized new material tris(dibenzoyl methane)-aluminum (Al(DBM)3 ) [218]. In the
devices, very broad white EL band at the 400–760 nm regions
was obtained by well overlapped four exciplex emission bands,
monomer emission could not be observed. The four emission
bands (blue, green, orange and red) were generated from exciplex emission TPD/Bphen, m-MTDATA/Bphen, TPD/Al(DBM)3 and
m-MTDATA/Al(DBM)3 , respectively. Here m-MTDATA and TPD are
4,4 ,4 -tris[methylpheny(phenyl)amino]triphenylamine, and N,N bis(3-methylphenyl)-N,N -diphenylbenzidine. The energy level
diagram, chemical structure of Al(DBM)3 , four exciplex emission
EL spectra, monomer EL spectra of the materials and EL spectrum
89
Fig. 25. (a) Energy-level diagram of the WOLED and the chemical structure of Al(DBM)3 , (b) EL spectra of four exciplexes emission, (c) PL spectra of m-MTDATA, TPD, Bphen,
and Al(DBM)3 films excited at 360 nm and (d) EL spectrum of the WOLED (solid) and four decomposed bands corresponding to the four exciplex emissions (dashed). Reprinted
with permission from Ref. [218]© 2012, the Optical Society.
of the WOLEDs are shown in Fig. 25. The CRI, CIE coordinates and
CT were 94.1, (0.33, 0.35) and 5477 K at bias voltage of 10 V.
In 2013, Xiong et al. developed high CRI WOLEDs based on
the monomer and excimer emission of platinum [1,3-difluoro4,6-di(2-pyridinyl) benzene]chloride (Pt-4) by incorporating green
fluorescence emission to form double EMLs structure [219]. The
emission from N,N -di(n-butyl)-1,3,8,10-tetramethylquinacridone
(TMDBQA) could make up the lack of green emission. The
structure of the WOLEDs was ITO/NPB/TCTA/mCP:12.5 wt% Pt4/mCP (0.5 nm)/Alq3:TMDBQA (1 wt%)/Bphen/LiF/Al. The 0.5 nm
mCP interlayer was inserted as an exciton blocking layer to separate
the phosphorescent EML and the fluorescent EML. Furthermore, the
mCP interlayer also suppressed the Dexter transfer of the phosphorescent exciton to the non-radiative triplet state of TMDBQA
and played an important role in the chromatic stability of the
devices. The devices exhibited a maximum current efficiency of
11.9 cd/A. The breakthrough was that the CRI up to 94 and 3 V and
92 at 9 V with an only ± (0.01, 0.01) CIE coordinates shift and a
low efficiency roll-off which was mainly due to the triplet exciton
confinement.
4.6. AIE based WOLEDs
Performances of traditional doped-WOLEDs are sensitive to the
parameters of each layer and fabrication conditions, such as film
morphology, doping concentration, dopant distribution and layer
thickness [220]. Most emission materials in the solid state would
suffer from the aggregation-caused quenching (ACQ), which is
a thorny problem in the development of efficient WOLEDs. The
AIE or AIE enhancement (AIEE) is opposite to ACQ and provides
a promising approach to the fabrication of efficient OLEDs. Tang
et al. had developed two kinds of new emission materials that
have AIEE feature by attaching tetraphenylethene (TPE) units into
pyrene’s periphery (mark “64”, TTPEPy) and introducing electronwithdrawing groups to TPE (mark “66”) [220,221]. The EL emission
images of 64 and 66 are shown in Fig. 26. The TPE units in 64 not
only suppressed the excimer formation but also enhanced the solid
state emission via the restriction of intramolecular rotation, the
introduction of electronwithdrawing groups in 66 could furnish
“redder” emissions [221]. The non-doped WOLED based on 64
and 66 had high CRI of 90 and moderate color stability with CIE
Fig. 26. Photographs of cyan, orange and WOLEDs using pure 64 and 66 and a mixture of them (64/66) as emitting layers, respectively. Reprinted with permission from Ref.
[221]© 2010 Bentham Science Publishers.
90
coordinates changed from (0.41, 0.41) to (0.38, 0.40) over a wide
range of driving voltage with a reduced efficiency roll-off due to
the AIE nature of the emission materials. The maximum current
and power efficiency were 7.4 cd/A and 4 lm/W, respectively.
4.7. Down-conversion WOLEDs
WOLEDs made by coupling blue-emitting OLEDs utilizing a
single emissive species with one or more down-conversion layers
could provide high illumination quality light with no efficiency
loss from the down-conversion process [222,223]. Using the
down-conversion approach, part of the blue light emitted from
OLEDs is converted to yellow and red light, resulting in white
light emission. The down-conversion WOLEDs have some unique
advantages such as simple device structure, stable color coordinates under operational lifetime and ease of color tunability
by varying the thickness of down conversion layers [224]. In
addition, the color coordinates might not shift within operating
lifetime. Duggal et al. developed a simple model to describe
phosphorescent material down conversion [222]. In the model,
each phosphorescent material layer absorbed a fraction of the
input photons and emitted them at a different wavelength. The
output of the device (in photons) upon exiting the nth phosphorescent material layer was
given by:Sn () = Sn−1 () exp[−˛n ()ın ] +
Wn Cn ()Pn () Wn = Qn Sn−1 (){1 − exp[−˛n ()ın ]} dCn () =
1−Qn
exp[−˛n ()ın ]
Pn (){1−exp[−˛n ()ın ]}
Here, S0 () was output spectrum of OLED, ˛n () was absorption coefficient of phosphorescent material in the nth layer, ın was
effective optical path length, Pn () was normalized so that its integral over all wavelengths was unity and was multiplied by a weight
factor Wn . Qn was the quantum yield and Cn () was self-absorption
correction which was assumed that the effective path lengths were
equal to that for the luminescence process.
In 2011, So and co-workers reported a kind of efficient downconversion WOLEDs with blue microcavity OLEDs [225]. The blue
emission materials were FIrpic and the down conversion materials were yellow and red phosphorescent materials. In order
to achieve high efficiency of OLEDs, the emission spectrum of
the FIrpic-based blue OLEDs needs to match the phosphorescent material excitation spectrum. However, the FIrpic emission
spectrum does not well match the excitation spectra of the
yellow and red phosphorescent materials. So and co-workers
introduced a microcavity structure in the FIrpic-based OLEDs
to tune the FIrpic emission spectrum to shorter wavelengths.
Besides this, the microcavity structure also redistributed the optical modes in the device, resulting in a significantly enhanced
light extraction efficiency of OLEDs. The OLEDs with microcavity structure is described as the following: glass substrate
(1 mm)/SiO2 (79 nm)/TiO2 (48 nm)/SiO2 (79 nm)/TiO2 (48 nm)/ITO
(50 nm)/PEDOT: PSS) (50 nm)/TAPC (20 nm)/mCP (25 nm): 20 wt%
FIrpic/3TPYMB (40 nm)/Cs2 CO3 (0.8 nm)/Al (100 nm). To further
increase light extraction efficiency, macrolens were attached to the
top of the down-conversion phosphorescent material film with an
index-matching gel. The power efficiency and CRI of WOLEDs was
enhanced to 87 lm/W and 83 at brightness of 100 cd/m2 .
4.8. High efficient primary blue emission for WOLEDs
As an indispensable primary color of white emission, the corresponding blue OLEDs have crucial influence on the performance
of WOLEDs. Blue phosphorescent materials are the best choice
because their quantum efficiency could be up to 100% in theory.
However, for the experimental results, the quantum efficiency of
corresponding blue OLEDs are usually poorer than those of green
and red OLEDs, and thus limiting the performance of WOLEDs [226].
The reasons for this could be summarized as follows: (i) design
and synthesis of stable and high quantum efficiency phosphorescent blue emission materials is a great challenge [227]; (ii) blue
light emission has high photon energy (2.6–2.8 eV), it is difficult to
find mutually matched host and guest materials with large triplet
state energy (>2.8 eV) to avoid non-radiative quenching [228]; (iii)
charge carrier balance not only influences the peak efficiency in
the device but also the efficiency roll-off. Therefore, effort should
be made to maintain the charge balance in the devices because the
hole mobility of HTL is usually higher than the electron mobility
of ETL [229]. Chi et al. summarized the mainly problems as the
followings [227]: (i) the commonly used blue emission materials
cannot give deep blue emission; (ii) the factual quantum efficiency
of blue emission material is inferior to that of green and red emission materials; (iii) the lifetime or long-term stability of materials
need to be improved. The deep blue color is defined as CIEy < 0.2,
and deep-blue emission material can effectively reduce the power
consumption of a full-color OLEDs and can be employed in WOLEDs
for improving CRI [230,231]. The reduction of emission efficiency
and lifetime could be explained by one major deactivation mechanism, the emissive state is in proximity to the metal-centered
d␲ d␴* state. Therefore, the phosphorescence emission is prone to
be quenched by the repulsive dd state via contact with the potential
energy surface (PES) with respect to that of the ground state [227].
So far, the efficient PhOLEDs with blue light emission are
mainly based on Ir-based complexes. A series of excellent blue
emission phosphorescent materials have been synthesized by
either adopting high-triplet-energy ligands or using electronwithdrawing ancillary ligands, such as iridium(III)bis(4,6-difluorophenylpyridinato)-3(trifluoromethyl)-5-(pyridine-2-yl)-1,2,4triazolate (FIrtaz), iridium(III)bis(4,6-difluorophenylpyridinato)5-pyridin-2-yl)-1H-terazolate (FIrN4), FIrpic and FIr6 [227,232].
Among them, the sky-blue emission phosphorescent material
FIrpic is considered as the most efficient one [226,233,234]. In
2011, Tao et al. synthesized a new blue phosphorescent emission
material, iridium(III) bis[(3,4,5-rifluorophenyl)-pyridinato-N,C2 ]
picolinate (F3 Irpic), with a similar molecular structure of FIrpic
except for the three fluorine atoms substituted at the 3,4,5-position
of the phenylpyridine ligand [235]. The PL quantum yield of F3 Irpic
was increased compared with FIrpic due to more fluorine atoms
substituted in the ligand. However, the PL emission peak of F3 Irpic
has a red-shift of 8 nm. As a whole, the design and fabrication
of deep blue emission phosphorescent materials is still ongoing
challenge.
Chi et al. summarized the road map to synthesize deep blue
emission material with high quantum yield (QY) and photostability [227]: (i) increase metal-to-ligand charge-transfer (MLCT)
contribution in S1 to increase the spin-orbit coupling matrix as
well as to reduce E (the band gap between the lowest energy
p
state of S1 and T1 ) and increase kISC and kr ; (ii) increase the
metal–ligand coordination strength to destabilize the dd energy;
(iii) a certain degree of ligand-to-ligand charge-transfer (LLCT) in
the proximity of the dd state to mitigate possible decomposition;
(iv) localize intra-ligand charge-transfer (ILCT) in T1 to reduce the
non-radiative transition as well as to facilitate color tenability. A
series of deep blue emission materials were synthesized according to this concept. Cyclometalated Pt complexes are also high
effective blue phosphorescent materials [232]. Jabbour et al. had
reported the photophysics, electrochemistry, and EL properties of
a novel platinum [1,3-difluoro- 4,6-di(2-pyridinyl)benzene] chloride (Pt-4). The photophysical studies of Pt-4 suggest that the high
QY and narrow emission spectra of Pt-4 could be mainly attributed
to strong mixing of 1 MLCT character to the lowest excited state.
By utilizing 26mCPy and OXD-7 with a weight ratio of 1:1 as
co-host, the Pt-4 based OLEDs showed a peak EQE of 16% and
CIE coordinates of (0.15, 0.26). Single-dopant WOLEDs based on
this complex showed a peak EQE of 9.3% and CIE coordinates of
(0.33, 0.36).
The high photon energy of blue phosphorescent materials is an
important reason to the lag of high efficient blue emission [236].
Utilizing excellent host material with high triplet energy and good
charge carrier transport properties is an effective solution channel
[236–238]. In 2011, Qiu and co-workers designed and synthesized
a series of novel solution processable bipolar host materials
by incorporating carbazole and diphenylphosphine oxide units
into the star-shaped structure [239]. Utilization of bipolar host
materials could balance the charge carriers and broaden the recombination zone in the EML, which could reduce the efficiency roll-off.
Qiu et al. synthesized novel star-shaped host materials, 9,9 -(5(diphenylphosphoryl)-1,3-phenylene)bis(9H-carbazole) (CzPO1)
and
9-(3,5-bis(diphenylphosphoryl)phenyl)-9H-carbazole
(CzPO2) which has high triplet energies, excellent morphological
stability and bipolar nature. The solution processed PhOLEDs with
the new host materials showed current efficiency of 23.6 cd/A and
EQE of 12.2%, current efficiency of 33.8 cd/A and EQE of 12.0% for
WOLEDs. Another effective solution channel is incorporating of
a high performance ETL between EML and cathode in the blue
PhOLEDs. To be effective in enhancing the performance, this
electron transport material should have: (i) high electron affinity,
(ii) high electron mobility, (iii) high ionization potential, (iv)
high triplet energy to confine exciton within the EML. In 2011,
Jenekhe and co-workers synthesized novel wide band gap n-type
solution processed organic semiconductors based on dendritic
oligoquinolines [236]. The band gaps of the materials were ∼3.4 eV
and the HOMO energy levels were ∼−6.1 eV which would provide
excellent hole-blocking properties. The high electron affinity
and high electron mobility (3.3 × 10−3 cm2 /V s) of the solutiondeposited thin films facilitated good electron injection/transport
properties. Blue PhOLEDs based on FIrpic doped PVK host EML and
solution processed oligoquinoline ETL gave high current efficiency
of 30.5 cd/A at brightness of 4130 cd/m2 with EQE of 16.0%.
For the most cases, wide band gap host materials with high
triplet energy and high electron mobility are generally a dilemma
for organic molecules, because wide band gap means the conjugation of molecules is weak, and then the charge mobility might
be low [119]. Padmaperuma and co-workers also pointed out that
the high triplet energy of host material came at the expense of
other important material parameters that play a crucial role in
obtaining either high EQE or other desirable device characteristics such as low drive voltage and long lifetime [121]. At this case,
wide band gap with a low enough HOMO energy to block holes
is more crucial than high electron mobility for the host materials because hole mobility is usually much higher than the electron
mobility. In 2009, Kido and co-workers synthesized a wide band
gap silane with a weak triplet energy, diphenyl-bis[4-(pyridin-3yl) phenyl]silane (DPPS), as a combined triplet energy and hole
blocking layer in the FIrpic-based OLED [119]. Although DPPS was
a weak triplet energy material, nearly 100% IQE was obtained due
to the charge balance and high charge recombination efficiency. In
2011, Padmaperuma and co-workers reported a new host material 4-(diphenylphosphoryl)-N,N-di-p-tolylaniline (DHM-A2) with
a triplet energy (2.6 eV) lower than FIrpic and demonstrated that
the triplet energy of the ETL had a larger effect on the magnitude of the EQE than the triplet energy of the host material [121].
High performance blue phosphorescent OLEDs (3.75 V, 16.5% EQE,
and 440 cd/m2 at 1 mA/cm2 ) were obtained using DHM-A2, which
had the triplet energy level lower than that of FIrpic. Since there
was a pathway for the triplet exciton to get back to the FIrpic and
decay radiatively, significant quenching does not occur if the DHMA2:FIrpic EML was surrounded by high triplet energy transport
layers. Thus, host materials with the triplet energy level equal to
or slightly lower than the phosphorescent dopant can be used to
91
achieve high EQEs if coupled with ETLs and HTLs of sufficiently high
triplet energy and appropriate blocking properties. These results
emphasize the fact that the design of host materials do not need to
adhere strictly to the high triplet energy and wide band gap rule.
5. New transparent conductive electrodes (TCEs)
Transparent electrodes with a remarkable combination of high
electrical conductivity and optical transparency are very important
to the area light source OLEDs, because they have crucial influence on the injection of charge carriers without affecting the light
extraction efficiency [240]. The doped metal oxide film ITO has
single-handedly dominated the field for almost decades due to
its high transparency in the visible range of the electromagnetic
spectrum (>90%), good electrical conductivity (>103 S/cm) and relatively high work function (∼4.8 eV) [241–243]. In recent years, the
tendency of OLEDs is large area and low cost, as well as the growing demand for high EQE. Hence, the tendency requires TCEs to be
lightweight, flexible, cheap, and compatible with large scale manufacturing methods, besides conductivity and transparence [241].
In terms of such tendency, ITO has exposed some flaws in the
application as the anode of OLEDs: (i) the ITO is brittle and easily generates cracks under bending stress [244], (ii) the migration
of indium and oxygen from ITO into organic semiconductors during the OLED operation would cause device degradation [245,246],
(iii) the transmittance in the blue region is low relatively (∼80%)
[242], (iv) the raw materials of ITO (particularly of In) is increasingly expensive and the cycling use efficiency is low [247], (v) the
electrical properties greatly depend on the film preparation [248],
(vi) the detergent/solvent-cleaned ITO would has a relatively low
work function which leads to a significant barrier for the hole injection while the work function of the ITO would decay with time after
plasma-treated [249,250], (vii) ITO cannot be used for some specific
devices which need electrodes transparent in UV and mid IR regions
[251], (viii) the EQEs of the conventional OLEDs fabricated on glass
coated with ITO and without light extraction enhanced methods
are only 20–30%. Thus, there is an urgent need for a replacement
of ITO. Through unremitting efforts, researchers have developed
some TCEs with excellent performances up to now. There are some
typical representatives such as carbon nanotube (CNT), silver/Au
nanowire and graphene flakes as shown in Fig. 27.
5.1. New TCEs: modified graphene anode
Graphene, two-dimensional graphite, is the most potential candidate as TCEs due to high conductivity, high charge mobility, a
tunable band gap, quantum electronic transport, high elasticity,
high transmittance (∼97.7% per monolayer) in the entire visible
range and high mechanical compliance [252,253]. In addition, a
few atoms’ thick graphene is mechanically compliant enough to
be employed as the TCE for flexible OLEDs [254]. Hence, it is an
ideal substitute for ITO electrode. However, the practical application of graphene as the anode of OLEDs showed poorer current
efficiencies than ITO-based devices [255,256]. The shortages of
graphene for the practical application in OLEDs are summarized
as: the low work function (∼4.4 eV) and high sheet resistance
(>300 /sq) [120,255]. The low work function of graphene could
lead to high hole injection barrier at the interface. The low conductivity of pristine graphene films could result in high turn on
voltage of graphene-based OLEDs. To achieve practical graphene
anodes, these disadvantages of graphene need to be overcome.
In 2010, Wu et al. predicted that the sheet resistance of graphene
varies with the number of layers as Rs ∼ 62.4/N /sq for highly
doped grapheme, and Nair et al. predicted that the transmittance
will vary as T ∼ 100–2.3N (%), where N is the number of layers
92
Fig. 27. (a) CNT film, (b) silver nanowire network film, (c) Au nanowire grating and (d) graphene flakes. Reprinted with permission from Ref. [241]© 2010, American Chemical
Society.
[256,257]. In 2012, Han et al. used conducting polymer compositions to modify the surface of graphene films [258]. The work
function of graphene films could be adjusted to be about 5.95 eV
by modification, and the sheet resistance could be reduced to be
about 30 /sq, respectively. In addition, the work function gradient
from the graphene to the overlying organic layer would be formed
by doping with p-dopants HNO3 or AuCl3 . The higher work function would enable hole to be easily injected into the organic layer
despite the high hole-injection barrier at the interface between
the graphene anode and the organic layer. The current and power
efficiency of the flexible fluorescent and phosphorescent OLEDs
with four-layered graphenes by modifying the surface, which had
a gradient work function could be up to 30.2 and 98.1 cd/A, 37.2
and 102.7 lm/W. Fig. 28 shows the schematic illustration of holeinjection process from graphene anode via self-organized HIL with
work-function gradient to the NPB layer and the optical image of
light emission from flexible fluorescent OLED with four-layered
graphene anode.
These remarkable device efficiencies vastly increase the feasibility of using graphene anodes to make extremely high-performance
flexible OLEDs. However, a prerequisite for the wide applications of
graphene is the availability of processable graphene sheets in large
quantity. Unfortunately, the prerequisite is hard to realize for the
particular characteristics that graphene sheets have a high specific
surface area and easily tend to form irreversible agglomerates or
even restack to form graphite [259]. In 2007, Li et al. reported that
chemically converted graphene sheets obtained from graphite can
readily form stable aqueous colloids through electrostatic stabilization [259]. This discovery could make it possible to obtain graphene
by using low-cost solution process techniques.
5.2. New TCEs: carbon Nanotube
Carbon nanotube (CNT) made from the most abundant element
carbon is another promising alternative electrode material. Besides
the resource superiority, CNT also has some unique advantages
Fig. 28. (a) Schematic illustration of a hole-injection process from a graphene anode via a self-organized HIL with work-function gradient to the NPB layer and (b) optical
image of light emission from a flexible fluorescent OLED with a four-layered graphene anode. Reprinted with permission from Ref. [258]© 2012, Nature Publishing Group.
for the TCE of OLEDs: (i) CNT films could be bent to acute angles
without fracturing [260]; (ii) individual CNTs have mobility in
excess of 105 cm2 /V s at room temperature, current carrying
capacity of 109 A/cm2 and the ON/OFF current ratios higher than
105 [261–264]; (iii) CNTs could selectively facilitate or block the
transport of charge carriers and effectively improve the OLEDs’
performance by introducing additional energy levels or forming
carrier traps in the host polymers at the optimized dopant concentrations [260]; (iv) CNT films have a mixture properties that with
1/3 metallic and 2/3 semiconducting, the sheet resistances of CNT
electrodes could be adjusted by the large junction resistances with
an optical transmittance of 80–90% [265–267]; (v) the transmission spectrum of CNT films is relatively flat over the visible range
which is very suit to application in WOLEDs [268]; (vi) the work
function of CNT films could be adjusted in the 4.7–5.2 eV range
[269]. Due to these amazing electrical, optical and mechanical
properties, CNTs are attracting more and more attention.
In 2011, Inigo et al. introduced the acid oxidised multiwall
CNTs (COOH-MWCNT) as hole injection buffer layer in OLEDs
[270]. With COOH-MWCNT as a buffer layer, the OLEDs have high
brightness under low operating voltage and hole injection was
enhanced by several orders of magnitude. In addition, the increase
of current injection and brightness does not alter the emission
spectrum at different operating voltage in these devices. In 2012,
Park et al. reported a tiny amount (less than 0.1 wt%) of chemically doped CNTs into the ZnO charge transport layers to enhance
the electro-conductivity of ZnO [271]. The ZnO transport layer
with 0.08 wt% N-CNT showed a five-fold enhancement of electron
mobility, while maintaining the intrinsic band gap energy levels, optical transparency and solution processability of pure ZnO.
By doping the N-CNT to adjust the work function, the inverted
OLEDs employing ZnO/N-CNT nanocomposite as electron transport layers had a more balanced electron–hole injection, at last
the maximum luminance and efficiency could be enhanced by
more than two-fold (from 21,000 cd/m2 at 14.6 V to 46,100 cd/m2
at 14.0 V and from 6.9 cd/A at 13.4 V to14.3 cd/A at 13.6 V). In
2012, Peng et al. prepared multiwalled CNT (MWCNT)/graphene
hybrids with two three-dimensional microstructures which had an
interconnected network and a double-layer structure [272]. The
IN-MWCNT/graphene hybrid with porous structures and strong
␲–␲ interaction is an excellent conductive network because the
conductivity and performance could be controlled by different
microstructures.
However, CNTs still have a series of problems for practical
application. CNTs usually mixed with various materials including
catalyst particles, catalyst support, amorphous or non-tubular carbon during the preparation process, and is usually the mixture of
nanotubes of various lengths, diameters, tube number, and chiralities [264]. To pick the anticipant needle from these mixtures in
large scale and at low cost is still a huge challenge. As a kind of
TCE, the performances of CNT films are largely characterized by
the sheet resistance and the visible light transmission. For a given
film, the sheet resistance and transmission are mainly controlled
by the DC conductivity (
dc ) and optical conductivity (
op ), respectively. Obtaining the high dc and op at the same time required that
CNT films have high conductivity. The conductivity of CNT films is
mainly decided by band gap, purity, length, diameter, stable doping,
lattice perfection, bundle size, wall number, metal/semiconductor
ratio, doping level chirality and so on [264].
Due to the above mentioned impact factors on the CNTs conductivity characteristics, the production of the anticipant CNTs in
large scale is very difficult to be realized. Up to now, there are
three major approaches to dispersing CNTs: (a) dispersing CNTs
in neat organic solvents or superacids [273–275]; (b) dispersing
CNTs in aqueous media with the use of dispersing agents such
as surfactants, dispersants, or other solubilization agents [276];
93
(c) adding functional groups which will help draw the CNTs in
solution [277].
5.3. New TCEs: metal nanostructure
Metals are the most conductive materials on earth due to their
typically high free electron density. However, the metals could not
be used as TCE directly due to their highly reflective in the visible range. Usually, the metals used as TCE are in the forms of
metallic nanostructures such as metal films, metal grid and metallic nanowire [278–281]. When the metal films are sufficiently thin
(∼10 nm or less), they become transparent to visible light [282].
Various metals can be used to make these transparent films including metal alloys, thin noble metals, alkaline earth metals protected
from oxidation by noble metal layers, multi-component metals, and
single-component metals such as chromium and nickel. However,
the conductivity of thin metallic films will sharply decrease when
the films thickness become smaller than the mean free path length
due to scattering of free charge carriers, free charge carriers scattering could be further enhanced by the substrate surface roughness
[282]. Hence, the performances of metal films are usually not very
ideal compared to ITO films [283]. The surface roughness is not a
critical issue for patterned metal grids. The area between grid lines
is 100% transparent and the total transmittance is defined by the
percentage of the total area that is covered by the metal grid. The
transmittance and conductivity could be easily tuned by varying
the line width and the thickness of the metal, the work function
could be easily changed by choosing different metal materials. The
similar structure could be made on flexible plastic substrate, the
manufacture could be in the forms of nanoimprint lithography and
roll-to-roll printing in large scale [280].
Metal nanowire (NW) networks could maintain the advantages
of patterned metal films and combine that with the low cost manufacturing available with solution deposited roll-to-roll techniques,
especially Ag NW. A commonly used figure of merit for transparent conductors is the ratio of DC to optical conductivity, dc /
op
[284]. In order for Ag NW networks to be applied, some challenges must be simultaneously resolved: wire to wire junction
resistance; surface roughness; gaps between Ag NW causing parasitic lateral current flow; work function; mechanical robustness
including adhesion and flexibility and process compatibility [285].
In 2009, De et al. used aqueous dispersions to prepare Ag NW
films, at last the dc /
op , optical transmittance and sheet resistance were up to 500, 85% and 13 /sq, respectively [284]. In 2011,
Gaynor and his co-workers proved that Ag NW mesh roughness was
the reason these films are incompatible with efficient devices and
transformed the Ag NW into truly effective transparent electrodes
by embedding Ag nanowires into conducting polymer PEDOT:PSS
[281]. By varying the polymer thickness, the morphology could be
controlled precisely, and the films could have sheet resistances and
transmittance comparable to ITO on glass and better than ITO on
plastic. In 2012, Chung and his co-workers embedded the AgNW
network in the ITO nanoparticle matrix by employing solutiondeposition of ITO nanoparticles onto pre-existing AgNW networks
at low temperatures [285]. At last, the Ag NW films had low wire
to wire junction resistance, smooth surface morphology, excellent
mechanical adhesion and flexibility while maintaining low sheet
resistance and high transmittance.
5.4. New TCEs: dielectric–metal–dielectric and
metal–dielectric–metal
The hybrid dielectric/metal/dielectric (DMD) multilayer is a
very forceful competitor for the TCE in the future which was first
reported in 1974 by Fan et al. [286]. A metal (such as Au and Ag)
layer sandwiched between dielectric materials with a high n value
94
Fig. 29. (a) Schematic of the OLED structure on low-cost flexible plastic with metal electrodes, (b) photograph of a large-area flexible OLED (50 mm × 50 mm) working at
high luminance (>5000 cd/m2 ) and (c) the EQE of the optimized OLEDs as a function of luminance. Reprinted with permission from Ref. [15]© 2011, Nature Publishing Group.
can show improved transmittance due to the multiple reflections
and interference. The DMD multilayer system could suppress the
reflection from the metal layer in the visible region and provide
a selective transparent effect. These DMD electrodes could have
optical transparency and sheet conductance properties that are
comparable to or better than those of ITO films. In addition, their
injection property could be tuned by varying their inner dielectric
layers which interface with the organic semiconductors to realize
near-Lambertian emission and ultrahigh EQE [15,287]. Hence, the
DMD multilayer has obtained great development.
In 2008, Yook et al. developed WO3 /Ag/WO3 as a transparent
cathode by using a thermally evaporable, the transparency was over
80% and the sheet resistance was only 12 /sq [288]. In 2009, Park
et al. pointed out that the DMD could keep steady due to there was
no severe interfacial reaction between metal layers and dielectric
layers at room temperature [289]. In 2011, Tian et al. made the
MoO3 /Ag/MoO3 stacks by using simple thermal evaporation at relatively low temperatures without causing deposition damage to
the organic layers, the optical transmittance and the sheet resistance of the stacks were about 65–80% in the 400–700 nm and
9 /sq [290]. At the same year, Wang and his co-workers made
the Ta2 O5 /Au/MoO3 stacks, as shown in Fig. 29, where the Ta2 O5
and MoO3 could eliminate the strong interfacial dipole formed at
the metal/organic interface and the optical microcavity formed
between the semitransparent metal anode and the highly reflective Al cathode besides maintaining the high transmittance and
low sheet resistance [15]. It was wondrous that a record high EQE
of be about 40% at a very high brightness of 10,000 cd/m2 could be
achieved for OLEDs fabricated on flexible plastic and the EQE/power
efficiency at 10,000 cd/m2 could up to 60%/126 lm/W by using a
lens-based structure, respectively. In 2012, Kim et al. embedded
an MgO nano-facet into WO3 /Ag/WO3 multilayer, as shown in
Fig. 30, the transmittance was 93% (92.5% than that of ITO (86.4%)
in the blue region <500 nm) and conductivity was 1.3 × 105 S/cm
[242]. Due to the nano-facet structured MgO (n = 1.73) layer and
a ZrO2 (n = 1.84) layer as a graded index layer, the luminance
of OLEDs is enhanced by 24% compared to that of devices with
ITO.
Although the DMD multilayer has very excellent performance,
the mechanism is not very clear. Park et al. considered that the
enhanced transparency could be attributed to surface plasmon
resonance (SPR) effects at the two metal/metal oxide interfaces
[289]. In 2011, Hong et al. explained the enhanced transparency
of WO3 /Ag/WO3 by means of “zero-reflection” model and admittance diagram technique [291]. Theoretical simulation have shown
that if the refractive index of bare metal is zero, the loss of light
transmitted metal thin film is mainly due to the absorption of the
metal and is proportional to nkd/ [291,292]. In the scale factor,
n is the refractive index, is the wavelength of incident light, k
is the extinction coefficient and d is the thickness of the metal
film. However, such metal is inexistence, the metal has the lowest n and absorption in the visible spectrum is Ag (nAg = 0.05–2.90
in the visible region). The optical transmittance of this DMD multilayer could be expounded by the admittance diagram technique
as shown in Fig. 31 [292]. For a “zero reflection” condition, the
admittance of a DMD structure starts from the substrate (nsub ,
0) and ends in the air (1, 0). In the case of WO3 film deposited
on a glass substrate, the starting point is (nsub , 0) as nWO3 > nsub
[137,291]. As the thickness of WO3 is increased to a quarter-wave,
a semicircle is traced in a clockwise direction and intersects the
real axis again at the (n2WO /nsub ) point. Increasing the thickness of
3
the WO3 layer resulted in the rotation of admittance on this circle. Different from the WO3 films, Ag has an imaginary part of the
refractive index which could result in the admittance diagram distorted with a loop bowing out along the direction of the real axis, as
shown in Fig. 31(b). Because the transmittance of film increased by
decreasing the distance from admittance to the air (1, 0), increasing the metal layer thickness would decrease the transmittance of
the metal film. When a dielectric film with high refractive index is
coated on the metal film to form a DMD structure, the distance from
the admittance to the air (1, 0) is reduced and the optical transmittance is enhanced due to multiple reflections and interferences as
95
Fig. 30. Schematic illustrations of OLEDs with (a) WAW and (b) MgO/ZrO2 /WAW structure and the schematic representations of the mechanism used for improving device
extraction efficiency. Inset: The cross section TEM image of MgO nano-facet structure. Reprinted with permission from Ref. [242]© 2012 the Optical Society.
shown in Fig. 31(c). In addition, the using of the dielectric materials
with high refractive indices could make the Ag thickness increasing to avoid forming a discontinuous film problem and having poor
electrical conductivity while the high transmittance could be maintained [293]. According to Fig. 31, due to the diameter of the circle in
the admittance diagrams will be larger with higher n, the DMD multilayer with high refractive index dielectric layer could fulfill the
optimum “zero-reflection” condition with relatively thick Ag film
that has improved transparency characteristics and low sheet resistance [242,290,291]. After optimization according to the model, the
WO3 (300 Å)/Ag (120 Å)/WO3 (300 Å) has the best results, the transmittance was be about 93.5% and the sheet resistance was about
7.22 /sq.
Opposite to the DMD structure, the metal–dielectric–metal
(MDM) structure electrode had also been studied. Lee et al. pointed
out that as the conductivity of materials decreased from metals to
insulators, their ability to transmit light tended to increase significantly and it was possible to dope the insulators (wide-band-gap
oxide materials) with a suitable electrical conducting dopant while
still remaining transparent. [294]. The high optical transmittance
and conductivity were attributed to surface and compositional
modifications of the structure. Lee et al. directly used the ultrathin metals films such as Ca, Ba and Sr whose work function could
be matched well with the LOMO of the organic layer to cover the
organic layer. On top of the metal films, high work function metal
(such as Ag) ultrathin films covered to form combination electrode (for example, Ca–Ag, Ba–Ag) [294–296]. After optimization,
the optical transparency of Ca (10 nm)/Ag (10 nm) electrode could
over 70% with the reflectivity was about 14% in the visible region
and the electrical sheet resistance was about 12 /sq, the optical
transparency of Ba (10 nm)/Ag (8 nm) could over 60% in the visible
region and the sheet resistance was about 15 /sq in the structure,
the optical transparency of Sr (8–10 nm)/Ag (10 nm) were 55–76%
in the visible spectral region and the sheet resistance was about
12 /sq. The metal with low work function were very sensitive to
the oxygen or moisture, the formation of transparent hydroxide or
oxide layer between the duplex metal ultrathin layers could produce a high level of optical transparency in the composite structures
[295]. However, these metals with low work function are always
susceptible to atmospheric oxidation, including the formation of
dark spots and degradation of the operational life span.
5.5. New TCEs: ZnO or impurity-doped ZnO films
Recently, the transparent conducting zinc oxide (ZnO) or
impurity-doped ZnO films have been actively investigated as alternative materials to ITO [297–299]. ZnO, a non-toxic, inexpensive,
abundant material, is an n-type semiconductor with an optical band
gap of approximately 3.3 eV at room temperature [299–301]. It is
also chemically stable under hydrogen plasma processes and could
be produced by various methods including pulsed laser deposition, chemical vapor deposition, spray pyrolysis, and magnetron
sputtering [301]. Impurity doped ZnO could have high electrical conductivity and excellent optical transparency in the near
infrared and visible regions. As reported, Al-, Ga- and Zr-doped
ZnO thin films deposited on glass substrates could have a minimum resistivities of 3.9 × 10−4 , 4.0 × 10−4 and 5.8 × 10−4 /cm,
respectively [300]. Among the impurity doped ZnO, Al-doped ZnO
Fig. 31. Admittance diagrams of WO3 , Ag, and WO3 /Ag/WO3 multilayers. n and k are the refractive index and extinction coefficient of materials. Reprinted with permission
from Ref. [291]© 2011, American Chemical Society.
96
(AZO) is an inexpensive, non-toxic and abundant material with
electrical and optical properties comparable to those of ITO [302].
In recently, the study on AZO is more and more deeply. Meyer et al.
reported the performance of OLED with AZO as transparency electrode, the power and current efficiencies could exceed 27 lm/W
and 44 cd/A at brightness of 100 cd/m2 [302]. The transmittance of
the devices was above 73% in the visible spectrum and the leakage current densities were only 3 × 10−5 mA/cm2 at a reverse bias
of 6 V after the structure optimization. Murdoch et al. demonstrated that ozone treatment would lower the resistivity of AZO
films due to a reduction in the number of oxygen vacancies in
the AZO lattice which would lead to reduced impurity scattering
and higher electron mobility [301]. In 2010, Ruske et al. pointed
out that high temperature treatments could strongly increase the
conductivity of AZO films and the key to prevent degradation of
the films during heat treatment is to use a capping layer such as
amorphous silicon [297]. The treated films could repeatedly exhibit
remarkably high mobility well above 60 cm2 /Vs and resistivity less
than150 ␮/cm, values that are difficult to be achieved for films
with carrier concentrations in the 1020 cm−3 range fabricated by
any deposition method. After annealing treatment at 650 ◦ C temperature, the charge carrier mobility was increased from values
of around 40 cm2 /V s up to 67 cm2 /V s, resulting in a resistivity of
1.4 × 10−4 /cm, which was most likely obtained by reduced grain
boundary scattering. In 2011, Chen et al. proved that an ultrathin
Ni capping layer with a thickness at percolation threshold could
significantly stabilize an underlying AZO layer in harsh environment [298]. The stability and the performance of AZO films was
increased by using Ni capping layer inhibiting the penetration of
oxygen and moisture into the AZO’s grain boundaries. In 2012,
Tseng et al. fully investigated the hole transportation at the interface between the AZO anode and the organic layer and pointed out
that the OLEDs with AZO as anode showed good performance under
high applied driving voltage [299]. In 2012, Park et al. reported
that electron mobility of ZnO films was increased about five-fold
by mixing the ZnO film with 0.08 wt% N-CNT [271]. The inverted
configuration OLEDs employing ZnO/N-CNT nanocomposite electron transport layers could facilitate well-balanced electron–hole
injection and had more than two-fold enhancement of maximum
luminance (from 21,000 cd/m2 at 14.6 V to 46,100 cd/m2 at 14.0 V)
and efficiency (from 6.9 cd/A at 13.4 V to 14.3 cd/A at 13.6 V). This
method offered an unprecedented opportunity to enhance the
device performance of inverted OLEDs with minimized alteration
of the device architecture and fabrication process.
6. The endeavor to improve the stability of OLEDs
OLEDs, especially WOLEDs, have drawn increasing attention
because of their potential applications in full color displays, backlights for liquid crystal displays and solid-state lightings. WOLEDs
with fluorescent-tube efficiency have been successfully fabricated
by long-term endeavor. Up to now, the stability of OLEDs is not very
ideal due to various influencing factors including the diffusion of
moister, oxygen and metal ion. Therefore, it is very necessary to
research the degradation mechanism of OLEDs and how to maintain high stability. In 2010, So and Kondakov have ever discussed
the degradation mechanisms of OLEDs [303]. In this part, degradation mechanisms and methods to maintain the stability of OLEDs
are summarized.
6.1. Induction factors of degradation
The degradation mechanisms of OLEDs have been intensive
investigated during the last decades. Because organic materials
and cathode materials with low work function are unstable
under ambient conditions, hence, the factors that could induce
degradation of OLEDs are varied and complicated. As a whole,
the degradation factors of OLEDs mainly could be classified into
extrinsic and intrinsic factors.
6.1.1. Extrinsic degradation mechanisms
Usually, the degradation of unencapsulated OLEDs is mainly
due to the combined action of moisture, oxygen and even light.
Moisture/oxygen combination might is the number one slayer to
OLEDs. The prototypical exemplar of degradation is the notorious dark spots [304,305]. Recently, Aziz et al. researched that the
formation of dark spots carefully [306,307]. The foreign materials or asperities on the substrate, cathode pin-holes formed due
to non-homogenous metal deposition could lead to the formation of channels or conduits for moisture and oxygen propagation
in the OLEDs, which leads to the metal cathode interface oxidation to form hydroxide, or organic layer damage. Moreover, this
penetrating moisture and oxygen can also result in gas evolution
at the cathode-organic interface to form bubble or domelike structures due to chemical reactions as well as induce the organic material crystallization. The bubble and organic material crystallization
could in return induce the local delamination of cathode and result
in the appearance of dark spots at the cathode surface. The form of
hydroxide and dark spots could be further enhanced by the electrolysis in the OLEDs under the applied bias. The study results also
revealed that stronger adhesion could slow down the growth of
dark spots. In addition, the dark spot growth is mainly related to
the degradation of cathode, not the underlying organic layers [303].
The degradation of OLEDs induced by light especially ultraviolet rays is more and more obvious because the effect induced
by moisture/oxygen could be well controlled due to the development of encapsulation technology [308–310]. In 2009, Achete
and co-workers investigated the stability of thermally deposited
Alq3 films by exposing them under UV radiation and analyzed
through Fourier transform infrared spectroscopy (FTIR) and optical
absorption in UV–Vis region [308]. Different degradation products were proposed, the feasibility of them was analyzed by Gibbs
free energy calculations and by comparing theoretical and experimental IR spectra. Theoretical IR spectra of these products were
all in well agreement with experimental results, while Gibbs free
energy calculations indicated the degradation of Alq3 formed carboxylate groups which bound to Al. In 2010, Aziz et al. studied the
photostability of OLEDs under external illumination in the optical
wavelengths range [310]. Irradiating OLEDs by external illumination could cause changes at the organic/metal cathode interface, the
changes could deteriorate electron injection and result in a gradual
increase in driving voltage and decrease in EL efficiency of OLEDs.
For the photoinduced degradation of polymer based OLEDs, singlet oxygen was effective reactive intermediate and could result in
extensive chain scission of the polymer [311,312].
6.1.2. Intrinsic degradation mechanisms
The intrinsic degradation factors are mainly from the interior of
OLEDs such as Joule heat and impurities. Stocking et al. pointed
out that the lifetime of OLEDs under 60–70 ◦ C was one or two
orders of magnitude lower than that under room temperature
[313]. However, the temperature of the OLEDs with epoxy-filled
glass or conventional glass encapsulation might easily higher than
60 ◦ C [314,315]. Therefore, exploring the mechanism of degradation induced by Joule heat is necessary [315]. In 2000, Lee and his
co-workers carried out real-time temperature and radiant power
measurements by using an infrared image and analysis system to
investigate the thermally activated degradation process [316]. The
defects such as non-planarity at interface between organic layer
and electrode, surface roughness of the ITO electrode, particulate
contamination and inhomogeneity of organic layer could induce
the leakage currents, even electrical short circuits. For ITO based
OLEDs, the detailed process of the thermal breakdown could be
described as follows: (i) regional high electric field resulted by
non-planarity of ITO induced the decomposition of ITO, resulting
in the production of oxygen and indium metal; (ii) the indium
atoms or ions diffused through the organic layer towards to cathode would form microscopic conduction paths through the organic
layer, the microscopic conduction paths could resulted in a large
leakage current and rapidly accumulation of Joule heat; (iii) the
rising temperature due to Joule heat could enhance the decomposition of ITO, accelerate the electromigration of indium towards
the cathode, and might lead to the crystallization of organic materials; (iv) the electrical short circuit, Joule heat and crystallization
of the organic materials jointly resulted in the thermal breakdown
of OLEDs.
The impurities that maybe post-exist in the emission zone,
introduced either during device fabrication or produced by
electrochemical reactions have unique effect to the stability of
OLEDs. The impurities might induce exciton quenching, charge
trapping or catalysis of degradative reactions [317,318]. Antoniadis
et al. reported that co-evaporation of 0.5 wt% quinacridone (98%
nominal purity) with Alq3 as the emission layer, the lifetime of
OLEDs could be reduced by a factor of 104 , together with a much
faster voltage increase [318]. Ionic impurities in the OLEDs might
build up an internal field during devices working. When a forward
bias is applied, cations would towards the cathode and anions
towards the anode. The internal electric field could reduce effective
electric field for carrier injection. Thus, the current density and
brightness would decrease under a constant bias voltage [319].
So and Kondakov reported that bulk degradation in the EML of
many PLEDs is also an obvious culprit for device degradation, during the process of PLEDs working, bulk traps would form [303].
The traps could lead to the form of non-radiative recombination centers and decrease the effective carrier mobility. Besides
these, the large HOMO difference of HTL and EML in some fluorescence based OLEDs could make the interface is susceptible to
degradation.
6.2. New architecture design of OLEDs
6.2.1. Encapsulating technology of isolating moisture and oxygen
Because moisture and oxygen have huge damage to OLEDs, for
OLEDs have a satisfactory lifetime, gas diffusion barriers and a water
vapor transmission rate (WVTR) on the order of 10−6 g/m2 day is
mandatory [314,320]. In 2009, Kowalsky et al. reported highly efficient Al2 O3 /ZrO2 nanolaminate gas permeation barrier for OLEDs
without introducing any damage to the organic functional layers [320]. The Al2 O3 and ZrO2 sublayers were grown by atomic
layer deposition (ALD) at 80 ◦ C and could retain more than 95% of
brightness of the OLEDs. In addition, it provided permeation rates
4.7 × 10−5 g/m2 day for water and 1.6 × 10−2 cm3 /m2 day for oxygen at 70 ◦ C with 70% humidity. The lifetime of the OLEDs could be
substantially in excess of 10,000 h. When the starting brightness
was 1000 cd/m2 , the lifetime could be improved to 22,000 h.
Organic–inorganic hybrid materials could have higher barrier
properties for moisture/oxygen than commercially available polymers. The amount of packing materials and the number of dyads
could be decreased. Hence, organic–inorganic hybrid materials
are considered as the potential encapsulating materials. Bae and
co-workers synthesized UV curable cycloaliphatic epoxy functionalized oligosiloxane resin for application in encapsulation of OLEDs
[321]. A single hybrimer coating on a PET film has high optical
transparent and low permeability of 0.68 g/m2 day per mil. Moreover, the fabricated hybrimers had a highly dense structure without
any defects. In 2012, Bae et al. again reported a transparent moisture barrier coating fabricated with silica nanoparticle-embedded
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organic/inorganic hybrid (S–H) nanocomposite [322]. A sol–gel
synthesized organic/inorganic hybrid material as the matrix and
Nanopox® E600 as the filler reinforcement could make tortuous
diffusive paths for moisture penetrants. The S–H nanocomposite
barrier coating exhibits WVTR of 0.24 g/m2 day and optical transparency of 90% in the visible range.
In 2011, Kamalasanan et al. reported completely organic
encapsulating layer by stacking alternately two organic materials, N,N -diphenyl-N,N -bis-3-methylphenyl[1,1 -bipheny]-4,4 diamine (TPD) and newly synthesized material 2.2.6. 5, 5 -(4,4 (2,6-di-tert-butylanthracene-9,10-diyl)bis(4,1-phenylene)) bis(2(4-hexylphenyl)-1,3,4-oxadiazole (XP) with different morphologies deposited by simple vacuum thermal evaporation technique
in four periods [323]. The ultrathin organic films deposited on the
OLEDs showed excellent barrier properties, high transparency in
visible wavelength, high safety for underlying organic layers in
OLEDs without any cover glass and desiccant. The technology may
easily be used in flexible OLEDs as well as top emitting OLEDs.
In 2013, Kim and co-workers reported a single-layer barrier of
co-sputtered Al2 O3 /ZrO2 and a bilayer barrier consisting of ALDSiO2 and co-sputtered Al2 O3 /ZrO2 as moisture barrier for OLEDs
[324]. WVTR of the single-layer showed a strong correlation with
the normalized film density and had a dramatic decrease for the
bilayer barrier film. Because ALD-SiO2 film had superior coverage
on particles and pinholes, the ALD-first barrier showed lower WVTR
than the Al2 O3 /ZrO2 -first layer. The WVTR of the later was only
0.06 g/m2 day.
6.2.2. New process for high efficiency heat dissipation
Joule heat may cause formation of bubbles, melt the metal electrode and form dark spots because the temperature of the OLEDs
could be more than 86 ◦ C [316]. Joule heat not only shortens device
lifetime but also causes spectral shift of the OLEDs [153]. Furthermore, Joule heat could cause exciton dissociation, which could
result in the EL efficiency decrease [90,325]. Therefore, it is great
benefit to the stability of the OLEDs if the Joule heat could be efficiently dissipated.
Heat transfers through the encapsulation layers to the heat sink,
therefore, the encapsulation must have high gas diffusion barrier
and superior heat transfer character simultaneously. In 2011, Park
et al. made a comparative analysis of the thermal performance of
the conventional glass encapsulation, epoxy-filled glass encapsulation and thin-film encapsulation (TFE) in the presence of a slim
and flexible heat sink [314]. The TFE shown the best thermal performance, followed by the epoxy-filled glass encapsulation and the
conventional glass encapsulation, which are the most commonly
used in the glass-capped OLEDs. The multi-heterojunction configuration and/or the low thermal conductivity of a polymer material
in the TFE film had no impact on the thermal performance due to its
extremely short heat transfer pathway. The WVTR could as low as
2.7 × 10−6 g/m2 day. Furthermore, a significant temperature gradient appeared inside the TFE layers when the thermal conductivity
of the polymer was lower than 1 × 10−3 W/m K.
In 2012, Leo and co-workers reported a simple and efficient heat
dissipation approach by immersing OLEDs into hydrofluoroether
(HFE) fluid which has high thermal conductivity, low viscosity and
can efficiently dissipate the heat by means of natural convection
with laminar flow [153]. Fabrication of HFE layer would not induce
any damage to organic functional layer. Therefore, it can be used
as excellent encapsulation media to replace of N2 , which is widely
used in the encapsulation technology of OLEDs. By employing HFE,
the lifetimes of OLEDs at high currents can be improved by about
a factor of 8. Furthermore, HFE fluid significantly improved the
light extraction by a factor of 70% due to the high refractive index
(n = 1.3).
98
6.2.3. Approaches to reduce the heat yield
An important approach to improve the stability is reducing the
heat yield especially the Joule heat. According to Joule’s law, the
Joule heat yield has a direct correlation to the current density.
The PIN structure OLEDs contains p- and n-doped highly conductive layers is a kind of selection. The PIN (p-doped hole-transport
layer/intrinsically conductive emission layer/n-doped electrontransport layer) structure could strongly increase charge-carrier
injection at the contacts and minimize the voltage drop over these
transport layers. Wellmann et al. have ever demonstrated that the
lifetime of the PIN OLEDs could more than 220,000 h (equivalent
to 23 years) at a brightness of 150 cd/m2 [326]. The superiority
is also in the tandem WOLEDs, which is accomplished by vertically stacking several individual EL units, each with HTL/EML/ETL
structure, with the entire device driven by a single power source
[327]. In the tandem WOLEDs with N EL units (N > 1), only about 1/N
times of the current density that used in the conventional WOLED
is needed to obtain the same brightness, the low current density
could result in reduced Joule heat yield and an operational lifetime
N times that of the conventional WOLEDs. In 2011, Brown and coworkers reported exceptional power efficiency, lifetime and color
stability for all-phosphorescent stacked WOLEDs with two emissive units connected in series by a charge generation layer (CGL)
[328]. The 15 cm × 15 cm WOLEDs had power efficiency of 48 lm/W,
CRI of 86 and exceptional lifetime about 13,000 h at 3000 cd/m2 . The
operating temperature of the WOLEDs was only 6.4 ◦ C higher than
ambient temperature. However, the structure of tandem OLEDs
may add cost and complication to the manufacturing process.
Other approaches to restrain heat yield are reducing nonradiative transitions of exciton and improving the light extraction
efficiency to stop the trapped light from transforming into heat.
In 2012, Forster et al. reported a pair of high efficiency single
stack (with no internal junction) all-phosphorescent 15 cm × 15 cm
WOLEDs light panels [191]. The chromaticity is kept constant
due to the stable recombination zone. The power efficiency of
WOLEDs was 49 lm/W with CRI of 83 and lifetime about 4000 h
at 3000 cd/m2 . When the WOLEDs was further used as a building block to demonstrate an all-phosphorescent OLEDs luminaire
for under-cabinet lighting applications, the WOLEDs had 52 lm/W
total system efficiency, CCT of 2940 K and CRI of 86 at brightness
approximately 3000 cd/m2 . The temperature change of OLEDs was
kept within 10 ◦ C during working at the brightness 3000 cd/m2 due
to the high IQE.
6.2.4. Resisting degradation and eliminating abrupt interface
Increasing the oxidation resistance of OLEDs is an unavoidable tendency to the long run development of OLEDs due to the
acknowledged instability of organic function materials. In 2011,
Krishnamoorti et al. incorporated varying doping ratios of dispersed SWNTs into PPV derivatives, MEH-PPV (poly[4-methoxy-1(2 -ethylhexyloxy)-p-phenylene vinylene]), MEH-PPCNV (poly[4methoxy-1-(2 -ethylhexyloxy)-p-phenylene cyanovinylene] and
DH-PPCNV (poly(1,4-dihexyloxy)-p-phenylene cyanovinylene), to
study the performance variation of the PPV derivatives based OLEDs
[329]. When the PPV doped with the SWNTs in proper ratio, the lifetime of the OLEDs were improved due to antioxidant effects of the
SWNTS. The oxidative degradation of the polymers occurs via radical chain reactions of both alkyl and alkylperoxyl radicals. SWNTs
could effectively trap the radicals and inhibit these degradation
routes of PPVs.
Charge buildup at the heterojunction due to abrupt interface
between HTL and ETL may seriously limit the reliability of OLEDs
[330]. OLEDs used mixed host materials by blending of a hole transport material and an electron transport material as the host of
the EML could effectively reduce the electric field across HTL/EML
interface. With rational design of device structure, the OLEDs with
mixed host material could have ideal efficiency and stability simultaneously [215]. In 2011, Qiu et al. reported WOLEDs with an
extremely long lifetime by wisely utilize of double blue-emitting
layers based on mixed host material. A mixed-host blue-emitting
layer consisting of 78% ␣, ␤-ADN:20% NPB:2% ENPN was utilized
to broaden the recombination zone and dilute the concentration of
any degradation related quenching species. A second blue-emitting
layer of 98% ␣, ␤-ADN:2% ENPN was then deposited onto the MHBEL
to achieve better charge confinement. Combined with a mixed-host
yellow-emitting layer, lifetime of the WOLED was over 150,000 h
at an initial brightness of 1000 cd/m2 together with a stable color
over the whole lifespan.
Robust Ohmic contact on the cathode/organic interface also
could effectively improve the lifetime of OLEDs. Lu et al. reported a
kinds of high efficient OLEDs using nanostructured carbon fullerene
(nanOLED) as an ETL and an Ohmic cathode (nanoCathode) [331].
Fullerene (C60 ) or nanobuckyball (NBB) have high electron mobility and could form Ohmic contact to LiF/Al cathode (nanoCathode).
The elimination of interface potential barrier of electrons injection
could reduce Joule yield at the cathode interface and lead to a more
stable device. However, C60 is very sensitive to moisture and oxygen. LiF has been reported as an effective oxygen diffusion barrier
in OLEDs, the C60 :LiF nanocomposite is considered as an potential
ETL candidate for the improvement of OLEDs performance [332].
More importantly, the high concentration of LiF may also significantly change the optical properties and make the C60 :LiF suitable
as a highly conductive ETL in OLEDs. In 2013, Liu et al. reported
that the five-stacked C60 /LiF films had a better protection of the
active layer from oxygen and moisture after exposure to ambient
air [333].
7. Outlooks
In last decades, OLEDs have already been incorporated into
some commercial products, like MP3 players, mobile phones, digital cameras, PDAs and so on, as shown in Fig. 32. In theory, OLED
displays could exhibit more than 16 million colors with the pixels
independently fast turn on and off. Moreover, the active displays
can refresh at more than three times the rate of the standard
video, resulting in more fluid full-motion video. The statistical data
based on INNOGRAPHY technology demonstrated that there are
more than 95,466 awarded patents about OLEDs in more than 70
countries and regions up to the end of 2011. These patents mainly
distribute in raw material (15,696), devices (33,831), equipment
(27,685), drive circuit (13,512), packaging technique (9183) and
application (4617). Compared to the development of inorganic light
emitting diodes (LEDs) and other light source technology, the development speed of OLEDs is rather rapid. At present, the applications
of OLEDs have included display and white lighting technology.
For last decades, many lighting devices have been fabricated,
such as filament lamp and fluorescent lamp, however, the power
efficiency of these devices were not very ideal (usually lower than
100 lm/W). The power efficiency was improved by the development
of LEDs. However, the incidental heavy metal such as cadmium pollution of LED might restrict the long-term development. Besides
this, the extra heat sink would easily make the actual efficiency
much lower than the theoretical value. WOLEDs as a kind of new
lighting source, a power efficiency of >70 lm/W and the lifetime
at least 10,000 h at 1000 cd/m2 with a CRI greater than 80 are
preferred. Fortunately, WOLEDs could thoroughly overcome these
defects. The potential preparation technology such as inkjet printers and roll-roll nano-lithography would make the WOLEDs easily
govern the lighting area. The more interesting thing is that WOLEDs
have no attraction to some winged insects which are keen on ultraviolet rays. In 2010, Osram and LG Chemical showed the WOLEDs at
99
Fig. 32. Some classical OLED products in the phylogeny of OLED. Reprinted with permission from Acuity Brands, Inc.
Light Fair International in Las Vegas, respectively, which are innovative designs that will drive end-user demand as shown in Fig. 33(a)
and (b). Recently, audio unveiled a concept car that has hundreds of
triangular OLED panels on the car’s body (shown in Fig. 33c). Osram
and Philips, etc. showed bran-new transparent OLEDs, making the
OLEDs have some unique and interesting application (as shown in
Fig. 33d).
Parallel to the lighting application, the OLEDs display applications also have attracted worldwide attention. In 2008, SONY
Corporation showed the world’s first commercial OLED TV, SONY
XEL-1, with a 3-mm thick panel and ultrahigh contrast ratio of
1,000,000:1. In 2009, Universal Display Corporation showed its
4-in., flexible, wrist-worn OLED display designed for military use
(and nerds) at CES. In 2010, Samsung revealed its transparent OLED
screens during CES. An 14 inch screen was attached to a laptop,
showing a 100,000:1 contrast ration, 40% transparency, and a
960 × 540 resolution. In 2011, LG announced the debut of a 31
prototype while Samsung introduced a 42 prototype. At the same
year, Fujitsu revealed its conceptual transparent flat computer,
Fujitsu Iris, which could be charged by wireless charging technology, save the scanned entities file. This new computer could realize
live navigation and even be shared by multi-user, as shown in
Fig. 34. Recently, LG announced that its 55 in. OLED HDTV has been
commercial product, which taps the wireless Internet to provide an
unparalleled entertainment experience as shown in Fig. 35. In addition, Smart Share Plus also enables the seamless sharing of video,
photos and music from your computer, phone and other compatible
devices. On the other hand, GE is exploiting a pilot line where they
are doing roll-to-roll production at atmosphere, like a newspaper
printing line, to make panels. If they are successful in doing that,
they are likely to reinvent the manufacturing of flexible electronics
in the US. Panasonic Idemitsu OLED Lighting Co. Ltd. (PIOL) a joint
venture of Panasonic Electric Works Co., Ltd. and Idemitsu Kosan
Co., Ltd. will begin shipping high-color rendering OLED lighting
panels to domestic and international markets. The product is a
light source with a panel section as thin as approximately 2 mm,
featuring high luminance (3000 cd/m2 ) as well as 30 lm/W luminous efficiency, 10,000 h durability (with 70% lumen maintenance
factor), 3000 K color temperature, and CRI of no less than Ra 90.
Though, tremendous strides have been made in the science and
technology of OLEDs, some problems are still need to be overcome.
The challenges on large-area OLEDs include avoiding increased
Fig. 33. (a and b) Two OLED luminaires unveiled by Osram and LG Chemical, (c) triangular OLED panels on the car’s body and (d) transparent OLEDs.
100
Fig. 34. Fujitsu Iris of Fujitsu.
operating voltages, resistive heating, weensy degradation in the
active area, while retaining stable high-quality emission during in
the whole life period. For the function materials, more endeavor
need to expend in maintaining charge balance at high currents to
avoid quenching effects and QY roll-off. The luminous mechanism
is still not very clear for the harvesting of the triplet states by phosphorescent emission materials in the active layer. The compromise
of high QY, low operating voltage and long-term stability has great
room for further research. Some excellent OLEDs products made by
vacuum evaporation had to face some problems such as the serious
waste of raw materials and the high cost, while the solution-based
OLEDs often suffer from the obsession of materials purification
and solubility. The research on OLEDs from innovative design in
raw materials, devices structure, light extraction approaches have
been intensively investigated, even the 3D WOLEDs would turn
up. We would like looking forward to exciting developments in
the coming years. The large scale application of OLEDs would have
a bright future.
Acknowledgements
The authors express our thanks to the Fundamental Research
Funds for the Central Universities (2013JBZ004); National Natural
Science Foundation of China (613770029); Beijing Natural Science
Foundation (2122050); National Key Basic Research Program of
China (2010CB327704); Outstanding Youth National Natural Science Fund (61125505). J. Zhang acknowledges financial support
by 100 Talents Program of the Chinese Academy of Science and
the Innovation Program of CAS under Grant No KGCX2-YW-395.
F.J. Zhang thanks the support from the State Key Laboratory of
Catalysis and the Key Laboratory of Photochemical Conversion and
Optoelectronic Materials, TIPC, CAS.
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