The effect of thermal annealing of Mo film on the CuInSe
2
layer texture
and device performance
Jun Tong
a,b,d
, Hai-Lin Luo
b
, Zhu-An Xu
a
, Hao Zeng
d
, Xu-Dong Xiao
b,c
, Chun-Lei Yang
b,
n
a
Department of Physics, Zhejiang University, Hangzhou 310027, China
b
Center for Photovoltaic Solar Cells, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 510275, China
c
Department of Physics, The Chinese University of Hong Kong, Shatin, New Territory, Hong Kong, China
d
Department of Physics, University at Buffalo, the State of New York, New York 14222, USA
article info
Available online 16 July 2013
Keywords:
Molybdenum annealing treatment
CuInSe
2
solar cell
Texture
abstract
The influence of annealing treatment of molybdenum (Mo) film on the growth of CuInSe
2
(CIS) thin film
has been studied. We show that Mo film is with much better electrical conductivity and adhesion after
being annealed in vacuum. It is found that annealing of Mo film not only modifies the orientation of
In
2
Se
3
(IS) precursor, but also the texture of CIS crystal. Thermally treated Mo film favors the growth of
(220/204) oriented CIS film. CIS solar cell grown on annealed Mo film is found to be with higher device
efficiency, which is attributed to the increased fill factor (FF).
& 2013 Elsevier B.V. All rights reserved.
1. Introduction
CuInSe
2
(CIS) based polycrystalline thin- film has been regarded
to be promising material for photovoltaic applications [1]. A record
efficiency of 20.3% for small-area thin film solar cell has been
achieved for its alloy Cu(In,Ga)Se
2
(CIGS) [2] which is prepared
using the so-called “three-stage” co-evaporation technique. Typical
structure of this kind of solar cell takes Mo film as the back contact
layer, since Mo shows low contact-resistance to the absorber layer
and is with strong chemical and physical stability. Most often, Mo
film is deposited on soda-lime glass (SLG) using direct current (DC)
sputtering which is proved to be superior to other methods such
as radio frequency (RF) sputtering [3].
Mo film not only acts as the back electrode, but also behaves as
the substrate for the following absorber growth. Then the nuclea-
tion and growth of CIS/CIGS are certainly influenced by the
orientation and surface structure/morphology of the Mo film.
To find optimized Mo film for solar cell, many research works have
been done to study its adhesion, sheet resistivity, stress and texture.
Sputtering power and gas pressure have been studied in detail to
show their influence on the mechanical, microstructure and elec-
tronic properties of Mo films and the corresponding effect on the
performance of solar cell devices [3–6]. However, few works have
been done to check the annealing effect on Mo film [7].
In this work, we have studied the influence of annealing Mo on
the solar cell device performance. To reduce uncertainties in the
experiments, we will only focus on CIS without adding Ga to
eliminate alloy fluctuation. Certainly, the conclusions drawn will
also be instructive to the growth of CIGS. The X-ray diffraction
patterns show that Mo annealing will firstly modify the orienta-
tion of In
2
Se
3
(IS) precursor, then change the orientation of CIS
thin film. We demonstrate that higher efficiency device can be
fabricated on annealed Mo film which is mainly due to the
increased fill factor (FF).
2. Experiments
2.1. Materials growth
The solar cell device has a structure of 2 mm SLG=1 μmbi-layer
structure Mo=2:2 μm CIS/50 nm CdS/50 nm i-ZnO/500 nm AZO=3 μm
Al grids, which is similar to that of NREL [4].
SLG (10 cm 10 cm 2 mm) substrate is firstly washed using
soap. Then it is cleaned in ultrasound bath using DI water and
dried with high pressure N
2
gas. It is de-gassed by heating in a
high vacuum chamber. After that the SLG could be transported
into the Mo film sputtering chamber without being exposed to air.
Mo film is deposited by an in-line DC magnetron sputtering
system with a 10 cm 24.5 cm rectangular Mo target (purity,
99.99%). The SLG substrate sitting 7 cm above the target moves
back and forth to get a homogeneous Mo fi lm with desired
thickness. The base pressure of the chamber was close to
1 10
−5
Pa and the argon (Ar) gas (99.999%) was injected into
the chamber at a flow rate of 10 sccm during deposition. A series
of Mo films have been deposited with the working pressure
varying from 0.3 to 0.05 Pa, and the power varying from 350 to
500 W. After deposition, Mo film is transported into a high
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Solar Energy Materials & Solar Cells
0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.solmat.2013.06.039
n
Corresponding author. Tel.: + 011 86 13632398049.
E-mail addresses: cl.yang@siat.ac.cn, tj198754@sina.com (C.-L. Yang).
Solar Energy Materials & Solar Cells 119 (2013) 190–195