Ethoxylated polyethylenimine as an efficient electron injection
layer for conventional and inverted polymer light emitting
diodes
Xiaohui Yang
a,b,
⇑
, Ruixue Wang
a,1
, Changjun Fan
a,1
, Guoqing Li
a,1
, Zuhong Xiong
a,1
,
Ghassan E. Jabbour
b,
⇑
a
School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China
b
Material Science and Engineering, Arizona State University, 7700 S. River Parkway, Tempe, AZ 85284, USA
article info
Article history:
Received 17 May 2014
Received in revised form 6 July 2014
Accepted 6 July 2014
Available online 16 July 2014
Keywords:
Conventional polymer light emitting
devices
Inverted polymer light emitting devices
Work function
Electron injection layer
abstract
We report inverted light emitting devices using ethoxylated polyethylenimine (PEIE) as a
single electron injection layer for indium tin oxide cathode, which possess comparable
efficiency to those using ZnO/PEIE double electron injection layers. Implementation of a
PEIE layer between light emitting polymer layer and aluminum has been shown to
significantly enhance device efficiency as well. Improvement of device efficiency can be
attributed to increased electron injection due to the reduced work function of PEIE
modified cathode as well as the hole blocking effect of PEIE layer. Furthermore, PEIE
serves as an efficient electron injector for a range of light emitting polymers with wide
distribution of energy levels.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
Polymer light emitting diodes have been actively stud-
ied for applications of flat-panel displays and solid-state
lighting due to their unique advantages of flexibility,
self-emitting, low-cost and large area processing [1]. The
typical configuration of conventional devices is indium
tin oxide (ITO)/hole injection layer/light emitting polymer
(LEP)/metal, where commonly adopted hole injection layer
is poly(3,4-ethylenedioxythiophene): poly(styrenesulfo-
nate) (PEDOT: PSS). Efficient injection of electrons and
holes from the electrodes into LEPs is required to achieve
high efficiency. To facilitate electron injection, low work-
function (WF) metals such as calcium and barium are gen-
erally used [2], which on the other hand cause devices very
sensitive to ambient moisture and oxygen. Various
materials such as alkaline/alkaline earth metal fluorides
[3], poly(ethylene glycol) [4] or polyelectrolytes [5] have
been incorporated into devices for efficient electron
injection from high WF metals such as aluminum. Among
them, poly(ethylene glycol) and polyelectrolytes can be
deposited onto LEP layer from alcohol or aqueous solution,
representing a facile approach toward fully solution-
processed light emitting devices. Alternatively, inverted
devices with the structure of ITO/metal oxide/LEP/metal
oxide/metal, which utilize air-stable low WF metal oxides
such as ZnO and TiO
2
as electron injection layers and high
WF metal oxides such as MoO
3
as hole injection layers,
exhibit improved environmental stability [6]. In such
devices, ITO serves as the cathode as the work function
difference is important for the polarity of the devices [7].
The conduction band minimum of TiO
2
and ZnO lies at
http://dx.doi.org/10.1016/j.orgel.2014.07.009
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
⇑
Corresponding authors. Address: School of Physical Science and
Technology, Southwest University, Chongqing 400715, PR China. Tel.:
+86 2368254913 (X. Yang).
E-mail addresses: xhyang@swu.edu.cn (X. Yang), Jabbour@asu.edu
(G.E. Jabbour).
1
Tel.: +86 2368254913.
Organic Electronics 15 (2014) 2387–2394
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel