Optics and Photonics Journal, 2013, 3, 222-226
doi:10.4236/opj.2013.32B052 Published Online June 2013 (http://www.scirp.org/journal/opj)
Effects of Annealing Conditions on ZnO Buffer
Layer for Inverted Polymer Solar Cells
Chuan Liu, Lihua Zheng, Zhiyang Gao, Yuhui Gan, Jian Zhang*, Chuannan Li*
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering,
Jilin University, Qianjin Street, Changchun, China
Email: *zhangjian@jlu.edu.cn, *licn@jlu.edu.cn
Received 2013
ABSTRACT
A solution-processed zinc oxide (ZnO) thin film as the buffer layer with optimized processes especially the annealing
conditions for inverted polymer solar cells (PSCs) has been demonstrated. Firstly the thickness of ZnO buffer layer was
optimized, and different annealing conditions including temperature and time have also been taken into consideration.
And the best Power Conversion Efficiency (PCE) 3.434% was observed when the ZnO buffer layer was spin–coated at
1500 rpm and annealed at 275℃ for 5 min, and AFM results showed that morphology of this ZnO film has the best
uniformity which was beneficial to form high quality polymer composite active layer.
Keywords: Polymer Solar Cells (PSCs); Zinc Oxide; Buffer Layer; Annealing; Morphology
1. Introduction
Polymer solar cells (PSCs) offer a potentially low-cost,
lightweight, flexible and scalable source of renewable
energy. However, before PSCs can become a marketable
energy technology, further improvements on efficiency
and stability are required. And the Power conversion
efficiencies (PCEs) of PSCs have been improved through
such as designs of the device structure, careful controls
of morphology and the applications of low band gap ma-
terials [1-3]. And there exists significant interests in
polymer bulk heterojunction solar cells by solution proc-
ess due to their low temperature and high efficiency. In
the regular bulk heterojunction PSCs structure, transparent
indium tin oxide (ITO) usually is used as the anode and
low work function metal as the cathode. Thus it brings
the problem of low stability and efficiency. Therefore
highly efficient polymer solar cells using an inverted
structure, in which the positions of anode and cathode are
reversed, have been demonstrated [4]. The low work
function metal e.g. calcium used as the cathode in the
regular structure, is replaced by a relatively nonreactive
electron collection layer and transparent ITO film, and
the stable metal such as Ag or Al can be used as the top
anode of inverted PSCs, this significantly improves the
air stability of the polymer solar cells. Furthermore, ver-
tical phase separation in polymer blends has proven to be
advantageous in the inverted structure [5].
One of the important keys to achieving high perform-
ance inverted PSCs is the selection of the electron collec-
tion layer between the transparent cathode and the active
polymer composite layer. The purpose of the electron
collection layer is to provide hole blocking capability and
a low resistive pathway for efficiently electron extraction.
Inorganic compounds CsCO3 [6-7], TiO2 [8,9] and ZnO
[10] have been demonstrated as the effective electron
collection materials. Particularly, solution-processed ZnO
is an attractive candidate because it easily forms a nanos-
tructure film with more efficient charge extraction and
transporting capability, at the same time the solution-
processed ZnO film reveals a strong hole blocking capa-
bility too [11]. During the fabrication processes of ZnO
buffer layer, the rotating speed and annealing conditions
greatly affect its electron collection and hole blocking
characteristics, the formation of polymer composite active
layer on ZnO film, and the performance of PSCs thereby
[12,13].
Here we demonstrate the fabrication of inverted bulk
heterojunction PSCs utilizing a ZnO interlayer as buffer
layer between the ITO and active layer. The ZnO film is
fabricated by solution processing, the morphology, IV
characteristics with different annealing condition is
thoroughly analyzed, and different annealing conditions
on the ZnO buffer layer are optimized. And we also ex-
plore the best rotation speed of ZnO precursor solution
for ZnO film.
2. Experiments
PSCs in these experiments were fabricated on patterned
*Corres
ondin
author.
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