Enhanced brightness of organic light-emitting diodes based on Mg:Ag
cathode using alkali metal chlorides as an electron injection layer
Ye Zou
a
, Zhenbo Deng
a,
n
, Denghui Xu
b
, Zhaoyue L
¨
u
a
, Yuehong Yin
a
,
Hailiang Du
a
, Zheng Chen
a
, Yongsheng Wang
a
a
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
b
Department of Mathematics and Physics, Beijing Technology and Business University, Beijing 100037, PR China
article info
Article history:
Received 4 November 2010
Received in revised form
11 July 2011
Accepted 2 September 2011
Available online 10 September 2011
Keywords:
Organic light-emitting diodes
Electron injection
Alkali metal chlorides
Mg:Ag
abstract
Different thicknesses of cesium chloride (CsCl) and various alkali metal chlorides were inserted into
organic light-emitting diodes (OLEDs) as electron injection layers (EILs). The basic structure of OLED is
indium tin oxide (ITO)/N,N
0
-diphenyl-N,N
0
-bis(1-napthyl-phenyl)-1.1
0
-biphenyl-4.4
0
-diamine (NPB)/
tris-(8-hydroxyquinoline) aluminum (Alq
3
)/Mg:Ag/Ag. The electroluminescent (EL) performance curves
show that both the brightness and efficiency of the OLEDs can be obviously enhanced by using a thin
alkali metal chloride layer as an EIL. The electron injection barrier height between the Alq
3
layer and
Mg:Ag cathode is reduced by inserting a thin alkali metal chloride as an EIL, which results in enhanced
electron injection and electron current. Therefore, a better balance of hole and electron currents at the
emissive interface is achieved and consequently the brightness and efficiency of OLEDs are improved.
& 2011 Published by Elsevier B.V.
1. Introduction
Since the double-layer structure introduced by Tang and
VanSlyke [1], organic light-emitting diodes (OLEDs) have
attracted much attention because of their potential for use in
cheap, lightweight, mechanically flexible, large display and light
source applications [2]. It has been extensively demonstrated that
the interlayer between the electrode and the organic layer plays
an important role in improving the device efficiency and/or
stability of organic light emitting diodes [3–12]. Because of the
poor electron injection of the aluminum (Al) cathode, a thin
insulator electron injection layer (EIL), such as lithium fluoride
(LiF) [5], cesium fluoride (CsF) [9], sodium chloride (NaCl) [6],
potassium chloride (KCl) [7], and cesium chloride (CsCl) [8], was
usually introduced in a standard device with a configuration of
indium tin oxide (ITO)/ N,N
0
-diphenyl-N,N
0
-bis(1-napthyl–phe-
nyl)-1,1
0
-biphenyl-4,4
0
-diamine (NPB)/tris-(8-hydroxyquinoline)
aluminum (Alq
3
)/Al to increase the electron current and improve
the balance of the electron and hole current. And thus the device
luminance and efficiency are improved. One of the other ways to
improve the cathode electron injection is replacing the Al elec-
trode by a lower work function metal electrode or the metal
compound electrode, such as Mg:Ag [1,7] and Li:Al [7]. The
performance of OLEDs with Mg:Ag cathode could be also orderly
enhanced compared to the Al-only electrode device. But com-
pared with the hole injection barrier, there still exists a higher
electron injection barrier height between the lowest unoccupied
molecular orbital (LUMO) of Alq
3
(3.0 eV) and the work function
of Mg:Ag (3.7 eV) cathode. In addition to the enormous mobility
difference between electron in Alq
3
and hole in NPB [13], there is
great possibility to further improve the Mg:Ag cathode-based
OLEDs performance.
Although Mg:Ag electrode is one of the most efficient OLED
cathodes, only a few works have been made to improve the
Mg:Ag electron injection property [10,11]. In this work, alkali
metal chlorides, including CsCl, KCl and NaCl, were selected as
effective EIL in OLEDs based on Mg:Ag cathode. The current
density–voltage–luminance (J–V–L) characteristics of devices
were measured and the results demonstrated that the brightness
and efficiency of OLED could be greatly enhanced by using a
proper thickness of alkaline metal chlorides layer as effective
electron injection interlayer. Because alkali metal chlorides
belong to the same compound group of CsF and LiF, but more
often they are cheaper than CsF and LiF in cost and are easier to
evaporate, we hold our work would provide a consideration for
the future OLED industry.
2. Experiment
Indium tin oxide (ITO) coated glass substrates (with a sheet
resistance of 20
O
/&) after routine cleaning procedure, including
Contents lists available at SciVerse ScienceDirect
journal homepage: www.elsevier.com/locate/jlumin
Journal of Luminescence
0022-2313/$ - see front matter & 2011 Published by Elsevier B.V.
doi:10.1016/j.jlumin.2011.09.009
n
Corresponding author. Tel.: þ 86 10 51684858; fax: þ 86 10 51683933.
E-mail address: zbdeng@bjtu.edu.cn (Z. Deng).
Journal of Luminescence 132 (2012) 414–417