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High-efficiency orange and white phosphorescent organic light-
emitting diodes based on homojunction structure
Bingye Chen, Liangmei Zuo, Asu Li, Ren Sheng, Yu Duan, Yi Zhao, Ping Chen
*
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, People's Republic of
China
article info
Article history:
Received 22 December 2016
Received in revised form
14 February 2017
Accepted 14 February 2017
Available online 17 February 2017
Keywords:
Homojunction organic light-emitting diodes
Phosphorescence
Step-graded p- and n-type doping structure
White light
Superior color-stability
abstract
We demonstrate high-efficiency orange and white phosphorescent organic light-emitting diodes based
on homojunction structure. Excellent performance is realized by using step-graded p- and n-type doping
structure in orange homojunction device. The resulting orange homojunction device exhibits a
maximum current efficiency of 30.0 cd/A and low effi ciency roll-off. The improvements are mainly
attributed to the utilization of step-graded doped profile, which facilitates balanced charge carrier in-
jection and transport. Moreover, one optimized white homojunction device based on two comple-
mentary colors shows a maxi mum efficiency of 15.4 cd/A, and superior color-stability in a wide range of
luminance.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
Organic light-emitting diodes (OLEDs) are finding commercial
applications in flat panel display and solid-state lighting due to
their high efficiency, high brightness, and low manufacturing cost
[1e4]. In order to achieve efficient OLEDs, phosphorescent organ-
ometallic materials are usually employed as dopants to harvest
both singlet and triplet excitons and reach a maximum internal
quantum efficiency of 100% in principle [5,6]. In general, a typical
phosphorescent device always adopts intricate architecture with
various organic functional layers such as hole injection layer (HIL),
hole transport layer (HTL), emitting layer (EML), hole blocking
layer, electron transport layer (ETL) and electron injection layer
(EIL) [7,8]. So many functional layers inevitably increase the fabri-
cation cost for future mass production and hamper OLEDs
commercialization application in field of display or lighting.
Therefore, it's necessary to further simplify the devices structure for
the potential commercial use [9,10]. To overcome the complexity
and reduce the production cost, homojunction OLEDs which
contain a single organic material have been executed to date
[11e19]. However, the absence of suitable ambipolar host materials
leads to poor performance due to the unbalanced charge carrier
injection. To improve the performance of the homojunction OLEDs,
almost all studies focus on the synthesis of new ambipolar host
materials. Tsuji et al. reported efficient p-i-n homojunction OLEDs
based on a new ambipolar material bis(carbazolyl)benzodifuran
[18]. The efficiency of the red phosphorescent homojunction device
was comparable to that of the multiple-layer heterojunction device.
In addition, Wang et al. demonstrated high-performance green,
orange, and red top-emitting homojunction OLEDs based on an
excellent ambipolar host 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-
1,3,4-oxadiazole [19]. However, the performance of the current
homojunction OLEDs is still below their counterparts with
complicated heterojunction architecture. Presently, a lot of effective
heterojunction OLEDs fabricated by electrical doped in organic
layers has been reported [20,21]. Such p-i-n structure could
evidently enhance the injection and transport of both electrons and
holes. Unfortunately, there are few reports about white homo-
junction OLEDs using p/n doping structure in literature.
In this work, we propose efficient orange and white homo-
junction OLEDs based on a common host 4,4
0
-Bis(carbazol-9-yl)
biphenyl (CBP) used as the only active organic material. The key
feature of the designed homojunction OLEDs is using step-graded
p-/n-type doping structure. The resulting orange OLED exhibits a
maximum current efficiency of 30.0 cd/A with low efficiency roll-
off. The improvements are mainly attributed to the step-graded
* Corresponding author.
E-mail address: pingchen@jlu.edu.cn (P. Chen).
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
http://dx.doi.org/10.1016/j.orgel.2017.02.019
1566-1199/© 2017 Elsevier B.V. All rights reserved.
Organic Electronics 44 (2017) 183e188