Investigation of electron transport properties in Li
2
CO
3
-doped Bepp
2
thin
films
Y.P. Wang
a
, X.F. Qiao
a
, D.Z. Yang
a
, J.Y. Huang
a
, J.S. Chen
a
, D.G. Ma
a,
⇑
, L.S. Dong
b,
⇑
a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of
Sciences, Changchun 130022, People’s Republic of China
b
Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Changchun
130022, People’s Republic of China
article info
Article history:
Received 3 February 2015
Received in revised form 17 June 2015
Accepted 10 July 2015
Keywords:
Electron transport
Li
2
CO
3
-doped Bepp
2
films
Admittance spectroscopy
abstract
The admittance spectroscopy investigations showed that doping lithium carbonate (Li
2
CO
3
) into bi
s[2-(2-hydroxyphenyl)-pyridine] beryllium (Bepp
2
) greatly improved the electron mobility compared
with the pure Bepp
2
film. The electron mobility reaches the orders of 10
4
cm
2
V
1
s
1
, almost indepen-
dent of the electric field. The trap states at low frequencies were clearly observed by capacitance–fre-
quency measurement. The current–voltage and current–thickness characteristics indicated the electron
conduction of space-charge-limited current (SCLC) with discrete trap distributions in the intermediate
voltage and the SCLC with exponential trap distribution at the higher voltage in the Li
2
CO
3
-doped
Bepp
2
film. We further estimated the density of trap states to be about 4.54 10
17
cm
3
by the temper-
ature dependent current density characteristics. The investigation of ultraviolet photoemission spec-
troscopy (UPS) and X-ray photoemission spectroscopy (XPS) found that there occurs complicated
chemical reaction between Bepp
2
and Li
2
CO
3
, and the Bepp
2
traps more electrons after Li
2
CO
3
doping.
This is an effective charge transfer between Bepp
2
and Li
2
CO
3
, which greatly reduces the electron injec-
tion barrier and significantly enhances the electron mobility.
Ó 2015 Published by Elsevier B.V.
1. Introduction
A considerable research efforts on organic semiconductors have
recently been attached by the potential application prospect for
low-cost, large area and light weight optoelectronic devices, such
as organic light-emitting diodes (OLEDs) [1–4], organic solar cells
(OSCs) [5,6] and organic thin film transistors (OTFTs) [7–10].In
general, there are two critical factors to improve the organic device
performance: (1) enhancing the carrier injection and transport
abilities; (2) keeping the charge balance. An immense effort has
been made to design and synthesize organic semiconductor mate-
rials with good carrier transport abilities, especially those with
high electron mobility. However, the electron mobility is typically
lower than that of the hole for these materials. Therefore, it is
important to improve the electron injection and transport for
charge balance in organic devices.
Recently, electrical doping in organic semiconductors has been
considerably accepted because it can enhance the charge injection
and transport, thus improving device performance [11–14]. Alkali
metal compounds, such as cesium carbonate (Cs
2
CO
3
) [14–16]
and Li
2
CO
3
[13,17] are the widely studied n-type dopants in
electron transport layers. Zhao et al. [14] investigated the effect
of Cs
2
CO
3
doping on the electron transport properties of
4,7-diphenyl-1,10-phenanthroline (BPhen) in OLEDs, which
showed that the incorporation of Cs
2
CO
3
decreases the activation
energy of the BPhen, resulting in the enhancement of electron
injection and transport by the reduction of injection barrier and
the increase of BPhen conductivity. Clearly, although the utilization
of these doped films can greatly improve the performance of
organic devices, the research on the charge transport properties
is still not clarified. Notably, the admittance spectroscopy (AS)
measurements are a feasible and powerful tool in evaluating not
only the hole transport property but also the electron transport
property in organic semiconductors [18–23]. In this work, we
investigated in detail the electron transport properties and mech-
anism in Li
2
CO
3
-doped bis[2-(2-hydroxyphenyl)-pyridine] beryl-
lium (Bepp
2
) thin films by frequency-dependent admittance
spectroscopy, current–voltage characteristics, ultraviolet photoe-
mission spectroscopy (UPS) and X-ray photoemission spectroscopy
(XPS).
http://dx.doi.org/10.1016/j.orgel.2015.07.023
1566-1199/Ó 2015 Published by Elsevier B.V.
⇑
Corresponding authors.
E-mail addresses: mdg1014@ciac.jl.cn (D.G. Ma), dongls@ciac.jl.cn (L.S. Dong).
Organic Electronics 26 (2015) 86–91
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel