Efficient electron injection layer based on thermo-cleavable materials for
inverted bottom-emission polymer light emitting diodes
Tengling Ye,
a
Minrong Zhu,
b
Jiangshan Chen,
*
a
Qiang Fu,
a
Fangchao Zhao,
a
Changsheng Shi,
a
Yue Hu,
a
Dongge Ma
*
a
and Chuluo Yang
*
b
Received 22nd November 2011, Accepted 24th January 2012
DOI: 10.1039/c2jm16061g
The electron injection material PF-EP was found to be thermo-cleavable. Its solubility in
chlorobenzene can be adjusted by thermal treatment at different temperatures. By analyzing the
results of TGA, F T-IR, and
13
C solid-state NMR, we interpret that the solubility transition is
caused by partial acidification, crosslinking through a hydrogen-bonded network and coordination
of O, P and Li. Based on this thermo-cleavable approach, efficient fully solution-processed
IBPLEDs were successfully fabricated. The maximum current efficiency of the device with 2.5%
Li
2
CO
3
doped PF-EP as the EIL reaches nearly 1.7 times of that of a conventional device. We
attribute the high performance to the good electron injection and hole blocking abilities of PF-EP
and Li
2
CO
3
.
Introduction
Active matrix organic light-emitting diode (AMOLED)
displays have become attractive recently because of their high
resolution, low power consumption and very thin form. There
are t wo driver technologies in the development of AMOLED
displays, i.e. low-temperature polysilicon thin film transistors
(LTPS T FTs) and a-Si thin film transistors (TF T). Compared
with a-Si TFTs, the LTPS TFTs are more commonly accepted
due to their high mobility, and thus high current, which is
essential for AMOLED displays.
1
However, a-Si TFTs drivers
forAMOLEDdisplaysarealsoattractivebecauseoftheir
cheaper manufacturing cost and lower equipment investment.
To take advantage of a-Si TFTs as a display driver, it is
highly desirable that the bottom contact of the OLED is the
cathode.
2,3
Inverted bottom-emi ssion organi c light- emitting
diodes (IBOLEDs) are one of the general structures that can
be integrated with an a-Si TFT at present. IBOLEDs have
a structure with an ITO bottom cathode and an evaporated
metal top anode to allow light to escape from the ITO glass a t
the bottom. An inherent issue f or IBOLEDs is their high
electron injection barrier due to the use of the high work
function ITO as the cathode. The problem can be overcome
by the doping method, for example a lithium (Li) or cesium
(Cs) doped 4,7-diphenyl-1,10-phenanthroline (Bphen) layer
can be used as a modification layer on ITO.
4,5
However, this
may be difficult for solution-processed inverted bottom-
emission polymer light-emitting diodes (IBPL EDs) due to the
effect of the solution-processed active organic layer on the
doped modification layer on ITO. Although some metal
oxides (for example TiO
2
and ZnO) can be used as the elec-
tron injection layer (EIL) in IBPLEDs, their poor hole
blocking ability results in larger leakage currents and low
device efficiencies.
6–11
Recently, some water/alcohol soluble
conjugated polymers (CPs), which can be spin coated, have
been used as effective EILs in PLEDs with Al and even high
work function Ag or Au as the cathodes.
11–14
However, it has
been shown that Li
2
CO
3
doped CPs (such as Li
2
CO
3
doped
PF-OH/PF-EP) or cationic conjugated polyelectrolytes
(CPEs) are more effective.
12–17
If these kinds of materials can
be applied to IBPLEDs as EILs, it should be of great benefit
to the device performance.
In this article, we reported a neutral conjugated polymer poly
[9,9-bis(6
0
-diethoxylphosphorylhexyl)fluorene] (PF-EP) whose
solubility in chlorobenzene can be adjusted by thermal treatment
at different temperatures. Utilizing this thermo-cleavage depen-
dent characteristic, the PF-EP can be spin-coated on ITO glass as
an EIL. After thermal treatment the PF-EP precursor became
insoluble and then the fully solution-processed IBPLEDs were
successfully fabricated. The current efficiency of the corre-
sponding IBPLED is comparable to the conventional PLED.
What’s more, the performance of the IBPLED can be further
improved by doping Li
2
CO
3
into the PF-EP. The chemical
structures of the polymer materials used in this study are shown
in Fig. 1.
a
State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Graduate School of Chinese Academy of
Sciences, Chinese Academy of Sciences, Changchun 130022, People’s
Republic of China. E-mail: jschen@ciac.jl.cn; mdg1014@ciac.jl.cn; Fax:
+86-431-85262873; Tel: +86-431-85262357
b
Department of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan 430072, People’s
Republic of China. E-mail: clyang@whu.edu.cn
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