Effects of ZnO fabricating process on the performance of inverted
organic solar cells
Jia Hu, Zhongwei Wu, Huaixin Wei, Tao Song, Baoquan Sun
⇑
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University,
199 Ren’ai Road, Suzhou 215123, PR China
article info
Article history:
Received 14 January 2012
Received in revised form 27 February 2012
Accepted 17 March 2012
Available online 7 April 2012
Keywords:
Zinc oxide
Inverted solar cell
Morphology
Charge extraction
Work function
abstract
The effects of zinc oxide (ZnO) fabricating process on the performance of the inverted bulk
heterojunction (BHJ) solar cells were explored in this study. The ZnO layers were prepared
by either sputtering or solution-processed method. These ZnO films on the indium tin
oxide (ITO) substrates were used as the cathode of the inverted solar cells. It was found that
the topography of the ZnO films played a leading role on the device performance. The
devices based on solution-processed ZnO films displayed better electric output compared
with that of sputtered ones. The measurement of capacitance against bias voltage indicated
that ZnO film with certain degree of roughness exhibited high charge extraction efficiency,
which resulted in improved device performance. The measurement of ultraviolet photo-
electron spectro scopy revealed that a shift of work function was observed due to the fab-
ricating process of ZnO films.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
In recent years, great efforts have been made to im-
prove the performance of the organic bulk-heterojunction
(BHJ) solar cells [1–3]. The BHJ structure provided a facial
approach to fabricate devices with high performance. Con-
ventional BHJ solar cells were fabricated with a transpar-
ent conductive anode (e.g. indium tin oxide, ITO) and a
low-work-function metal cathode (e.g. Al) to collect holes
and electrons, respectively. Normally, a hole conducting
layer of poly(3,4-ethylenedioxythiophene)/poly(styrene
sulfonate) (PEDOT:PSS) was deposited onto the ITO sub-
strate to enhance the collection efficiency of holes as well
as smoothing the roughness of ITO. However, a drawback
of instability was accompanied simultaneously due to the
gradual etching of ITO if directly contacted with PED-
OT:PSS [4]. In addition, the low-work-function cathode
was susceptible to degradation by oxygen and moisture,
which deteriorated the device performance [5,6]. In order
to overcome these problems, inverted device architecture,
in which the positions of the anode and cathode were re-
versed, was employed [7,8]. In this structure, a layer of
PEDOT:PSS or high-work-function transition metal oxide
(e.g. V
2
O
5
, MoO
x
) was deposited onto the active layer act-
ing as hole-collecting layer before the evaporation of the
relatively nonreactive electron collection layer (e.g. Ag,
Au) [6,9]. In addition, the high-work-function transition
metal oxide could also act as electron-blocking layer. This
approach alleviated the device instability problem and en-
hanced the device performance effectively [2,7].
Although dramatic improvement of stability has been
obtained by using inverted structure, many challenges still
remained. Typically, the inverted BHJ solar cells exhibited
inferior performance compared with the conventional
ones. Inserting an electron collection layer of metal oxide
(e.g. ZnO, TiO
2
,Al
2
O
3
, PbO) between active layer and cath-
ode could alleviate this problem [10–14]. In addition, ce-
sium carbonate (Cs
2
CO
3
) and polyethylene oxide (PEO)
have also been used to modify the ITO substrate to enhance
the electronic performance of devices [15,7,16].
Among the documented electron collection materials,
ZnO was one of the most widely explored due to its high
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.03.021
⇑
Corresponding author. Tel.: +86 512 6588 0951.
E-mail address: bqsun@suda.edu.cn (B. Sun).
Organic Electronics 13 (2012) 1171–1177
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel