Advances in Materials Physics and Chemistry, 2012, 2, 106-109
doi:10.4236/ampc.2012.24B029 Published Online December 2012 (http://www.SciRP.org/journal/ampc)
Preparation of High Ga Content Cu(In,Ga)Se2 Thin Films by
Sequential Evaporation Process Added In2S3
Toshiyuki Yamaguchi1, Kazuma Tsujita1, Shigetoshi Niiyama2, Toshito Imanishi2
1Department of Electrical and Computer Engineering, Wakayama National College of Technology, Gobo, Wakayama, Japan
2Industrial Technology Center of Wakayama Prefecture, Ogura, Wakayama-shi, Japan
Received 2012
ABSTRACT
High Ga content Cu(In,Ga)Se2 thin films incorporated sulfur were prepared by sequential evaporation from CuGaSe2 and CuInSe2
ternary compounds and subsequently Ga2Se3, In2Se3 and In2S3 binary compounds. The In2S3/(Ga2Se3+ In2Se3) ratio was varied from
0 to 0.13, and the properties of the thin films were investigated. XRD studies demonstrated that the prepared thin films had a chal-
copyrite Cu(In,Ga)Se2 structure. The S/(Se+S) mole ratio in the thin films was within the range from 0 to 0.04. The band gaps of
Cu(In,Ga)Se2 thin films increased from 1.30 eV to 1.59 eV with increasing the In2S3 /(Ga2Se3+ In2Se3) ratio.
Keywords: Cu(In,Ga)Se2 Thin Film; Solar Cell; High Ga Content; Sulfur Incorporation; Sequential Evaporation
1. Introduction
Photovoltaic power system has received considerable attention
for safety and clean energy resources. It is necessary to fabri-
cate low cost and high efficient solar cells in order to spread the
PV system widely. Chalcopyrite Cu(In,Ga)Se2 is a potential
absorber material for high efficiency thin film solar cell be-
cause of its favorable band gap and high absorption coefficient
for solar radiation. The band gap energy of Cu(In,Ga)Se2 thin
films varies from about 1.0eV to 1.7eV according to the in-
crease in CuGaSe2 molar fraction which makes it also promis-
ing for single-junction and multi-junction solar cell applications
[1]. Conversion efficiencies for Cu(In,Ga)Se2 based solar cells
have been significantly improved over recent years and
achieved the value of 20% by three-stage process using a mul-
tisource vacuum evaporation system equipped with elemental
Cu, In, Ga and Se sources [2,3]. The Ga/(In+Ga) ratio of this
absorber was around 0.3, which showed a band gap Eg of about
1.14 eV. It is expected to improve the efficiency by increasing
its band gap until 1.4 eV due to a better matching solar spec-
trum. The conversion efficiencies of Cu(In,Ga)Se2 thin film
solar cells decreased with increasing a Ga/(In+Ga) mole ratio
above 0.3 [4]. For example, the efficiencies of Cu(In,Ga)Se2
thin film solar cells were 12% for Ga/(In+Ga) mole ratio of
0.73 (Eg=1.5 eV) and 10% for that of 0.91 (Eg=1.62 eV), respec-
tively [4]. On the other hand, a performance of Cu(In,Ga)Se2 thin
film solar cell with a Ga/(In+Ga) mole ratio of around 0.3 was
improved by sulfurization of the film surface such as InS treat-
ment by a wet process [5] and annealing in S vapor atmosphere
[6]. We have proposed the process using a vacuum deposition
apparatus with three evaporation boats which was the sequential
evaporation technology from CuGaSe2 and CuInSe2 ternary
compounds [7,8]. Our proposed process has advantages to be
able to easily control a Ga/(In+Ga) mole ratio in Cu(In,Ga)Se2
thin films by changing the amount of CuGaSe2 and CuInSe2
evaporating materials in the first step and to use inexpensive
equipment for preparation of an absorber layer. In this study,
one evaporation source was added in our vacuum deposition
apparatus. In2S3 was added as an evaporation material in the
third step of our sequential evaporation process and the pre-
pared thin films and solar cells were investigated.
2. Experimental
2.1. P reparation of Cu(In,Ga)Se2 Thin Films Added
In2S3
The evaporating materials of CuGaSe2 and CuInSe2 were syn-
thesized by reacting stoichiometric amounts of high-purity
elements (Cu, In, Ga, Se) in sealed and evacuated quartz am-
poules. The detail procedure was described in Reference [9].
The CuInSe2 and CuGaSe2 ingots were removed from the
quartz ampoules. In2Se3, Ga2Se3 and In2S3 compounds available
in the market were used as an evaporating material. Mo layer
used as a back contact was prepared by rf magnetron sputtering
onto soda-lime glass substrate in Ar ambient. Our evaporation
process consists of the four steps, which schematic profile was
shown in Figure 1. Before fabrication of Cu(In,Ga)Se2 thin
films, the Mo/soda-lime glass substrates were heated in vacuum
for 5min at 500oC with infrared lamp. After cooling down to
200oC, in the first step, Cu-In-Ga-Se layer was evaporated from
CuGaSe2 and CuInSe2 compounds onto the Mo/soda-lime glass.
The CuGaSe2/(CuGaSe2+ CuInSe2) mole ratio of the evaporat-
ing materials kept at constant of 0.8. In the second step, In-
Ga-Se layer was deposited from In2Se3 and Ga2Se3 compounds
at a substrate temperature of 490oC. The (In2Se3 + Ga2Se3)/
(CuGaSe2 + CuInSe2) mole ratio kept at constant of 0.2. In the
third step, S was deposited from In2S3 compound at a substrate
temperature of 490oC.The In2S3/(In2Se3+Ga2Se3) mole ratio
was varied from 0 to 0.13 in this experiment. Finally, only Se
was effused at the same substrate temperature.

T. YAMAGUCHI ET AL. 107
0
100
200
300
400
500
600
Temperature[℃]
Time
CuInSe
2
+
CuGaS e
2
P reheating
In
2
Se
3
+ Ga
2
Se
3
In
2
S
3
Se
Figure 1. Schematic profile of our sequential evaporation process.
2.2. Fabricati on of Sol ar Ce l l s
The solar cells with a configuration of Al/ZnO:Al/i-ZnO/CdS/
Cu(In,Ga)Se2/Mo/SLG substrate were fabricated. CdS buffer
layer with a thickness of 70 nm was deposited by the chemical
bath deposition technique using a CdI2 (2.0x10-3 M)-thiourea
(0.166M)- ammonia (1M) aqueous solution during heating
from room temperature to 65oC. i-ZnO buffer layer with a
thickness of 100 nm was deposited by rf-magnetron sputtering
from non-doped ZnO target in Ar gas at room temperature.
Transparent conductive ZnO:Al film with a thickness of 0.4 μm
was subsequently deposited by rf-magnetron sputtering from a
2wt%Al2O3 doped ZnO target in Ar gas at room temperature.
Al grids for the front electrode were formed by a vacuum
evaporation with W boat using a metal mask. No antireflection
coating was applied. The size of a solar cell is 5 mm x 5 mm.
2.3. Charac t erization
The surface composition of thin films were determined by an
electron probe microanalysis (EPMA). The effective range of
electron for production of the characteristic X-rays in EPMA
analysis is roughly estimated to be around 0.4 μm for Cu
(In,Ga)Se2 thin films [10]. The growth orientation of thin films
was studied by X-ray diffraction (XRD) in the θ-2θ mode using
Cu Kα radiation. The surface and cross-section morphology and
grain size of the thin films were studied by scanning electron
microscopy (SEM). Current-voltage characteristics of solar
cells were measured using standard 1-sun (AM1.5, 100mW/cm2)
illumination. The quantum efficiencies of solar cells were
measured using a spectrophotometer with illumination normal-
ized against calibrated photodiode.
3. Results and Discussion
3.1. Film Composi tion
From EPMA analysis, the thin films prepared at various In2S3/
(In2Se3+Ga2Se3) mole ratio had almost a stoichiometry compo-
sition in I-III-VI2 compound. Figure 2 shows the compositional
ratio of Ga/(In+Ga) and S/(Se+S) in the thin films. In this ex-
periment, the CuGaSe2/(CuGaSe2+CuInSe2) mole ratio in the
evaporating materials was kept at constant of 0.8. The Ga/(In +
Ga) mole ratio in the thin films prepared in the range of In2S3/
(In2Se3+Ga2Se3) mole ratio from 0 to 0.13 was within the range
from 0.855 to 0.747. The Ga/(In+Ga) mole ratio slightly de-
creased with increasing the In2S3/(In2Se3+Ga2Se3) mole ratio
due to the presence of In in the third step. These values are
considered to be a high Ga content which is the purpose of this
study. On the other hand, the S/(Se+S) mole ratio in the tin
films increased from 0 to 0.04 with increasing the In2S3/(In2Se3
+ Ga2Se3) mole ratio. A slightly S incorporation into the thin
films was confirmed from EPMA analysis.
3.2. Crystal Structure
Figure 3 shows XRD patterns for the thin films prepared at
In2S3/(In2Se3+Ga2Se3) mole ratio of 0 to 0.13. XRD spectrum
exhibited several peaks corresponding to diffraction lines of the
chalcopyrite phase in Cu(In,Ga)Se2, in particular split of
220/204 and 312/116 diffraction lines. The 112 diffraction line
was the strongest. The position of X-ray diffraction peaks for
Cu(In,Ga)Se2 thin films prepared at various In2S3/(In2Se3 +
Ga2Se3) mole ratio was almost same although the Ga/(In+Ga)
and S/(Se+S) mole ratio in the thin films was slightly different.
0
0.02
0.04
0.06
0.08
0.1
0
0.2
0.4
0.6
0.8
1
00.050.1 0.15
Ga /(I n+Ga )
In
2
S
3
/(In
2
Se
3
+Ga
2
Se
3
)
Ga /(I n+Ga )S/(Se+S)
S/ (Se +S)
Figure 2. Compositional ratio of the prepared thin films deter-
mined by EPMA.
15 25 35 45 55 65 75 85
Intensity[a.u.]
2θ[deg]
In
2
S
3
/(I n
2
Se
3
+Ga
2
Se
3
)=0.13
0.0
0.04
0.09
112 220 204 204
116
316
400
Figure 3. XRD patterns of the thin film prepared at In2S3/(In2Se3 +
Ga2Se3)=0- 0.13.

T. YAMAGUCHI ET AL.
108
3.3. Grain Si ze
SEM micrographs of the cross section of Cu(In,Ga)Se2 thin
films prepared at In2S3/(In2Se3+Ga2Se3) mole ratio of 0 to 0.13
are shown in Figure 4. This Cu(In,Ga)Se2 thin films had a high
Ga content such as Ga/(In+Ga) mole ratio of the range from
0.855 to 0.747. The grain size in Cu(In,Ga)Se2 thin film was
estimated to be smaller than 1.0 μm. It is well known in general
that efficiencies of polycrystalline solar cells increase with
increasing grain sizes in the absorber materials. Therefore, the
large grain growth in Cu(In,Ga)Se2 thin films is required for the
fabrication of high-performance photovoltaic devices. In com-
parison with Figures 4(a) and (b), the grain size in Figure 4(b)
was seemed to be larger than that in Figure 4(a), suggesting the
promotion of the grain growth by slightly In2S3 supplying.
3.4. Band gap Engineering
The solar cells with a configuration of ZnO: Al/i-ZnO/CdS/Cu
(In,Ga)Se2/Mo/soda-lime glass substrate were fabricated by
using Cu(In,Ga)Se2 thin films prepared at In2S3/(In2Se3 +
Ga2Se3) = 0-0.13. The best solar cell demonstrated Voc=500mV,
Isc= 19.07mA/cm2, FF = 0.39 and η = 4.1% without AR-coating,
which used Cu(In,Ga)Se2 thin film prepared at In2S3/(In2Se3 +
Ga2Se3)=0.04. The efficiencies for Cu(In,Ga)Se2 thin film solar
cells were not so good. However, the remarkable change was
observed in the quantum efficiency of Cu(In,Ga)Se2 thin film
solar cells, which was shown in Figure 5. The quantum effi-
ciency from 400 nm to 600nm for Cu(In,Ga)Se2 thin film solar
cells prepared at In2S3/(In2Se3+Ga2Se3) = 0.04 and 0.09 in-
creased rather than that at In2S3/(In2Se3+Ga2Se3) = 0. Moreover,
the absorption band edge in the long wavelength region shifted
to the short wavelength range with increasing In2S3/(In2Se3+
Ga2Se3) mole ratio. For a direct transition, the dependence of
the absorption coefficient α on the photon energy h

is given
by

12
hAhEg    (1)
(a) In2S3/(In2Se3+Ga2Se3)=0.0 (b) 0.04
(c) 0.09 (d) 0.13
Figure 4. SEM micrographs of the cross-section of Cu(In,Ga)Se2
thin films prepared at In2S3/(In2Se3+Ga2Se3) =0-0.13.
Assuming a very short minority carrier diffusion length Ln,
the quantum efficiency QE can be approximated by


1expQE W    (2)
where W is the width of the space charge region. From eu-
qations (1) and (2), the following equation is deduced
 
12
ln 1QEhWA hEg   (3)
so that a plot of [h

xIn(1-QE)]2 against h

can be used to
extrapolate the band gap Eg [11]. From this manner, the band
gaps estimated from the QE spectra were changed from 1.30 eV
to 1.59 eV, which shown in Figure 6 including the open circuit
voltage Voc of Cu(In,Ga)Se2 thin film solar cells. The value of
band gap is expected to be 1.56eV for Cu(In,Ga)Se2 thin film
with Ga/III=0.855 from the data reported by Paulson et al [12].
However, the band gap of 1.30eV obtained from Cu(In,Ga)Se2
thin film prepared at In2S3/(In2Se3+Ga2Se3)=0 in this experi-
ment was extremely a small value. It has been reported that
efficient Cu(In,Ga)Se2 thin film solar cells with Ga/(In+Ga)
mole ratio of 0.3 fabricated by three stage process had a double
0
20
40
60
80
100
300 500 700 9001100
Quantum Efficiency[%]
Wa v e le ngth[nm]
In S=0
InS=0.04
InS=0.09
InS=0.13
Figure 5. Quantum efficiency of Cu(In,Ga)Se2 thin film solar cells
prepared at In2S3/(In2Se3+Ga2Se3)=0-0.13.
0
200
400
600
800
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
00.05 0.10.15
Band ga p [eV]
In
2
S
3
/(In
2
Se
3
+Ga
2
Se
3
)
Bandgap Voc
Voc[mV]
Figure 6. Dependence of band gap and open circuit voltage on
In2S3/(In2Se3+Ga2Se3) mole ratio.

T. YAMAGUCHI ET AL.
109
efore, it is presumed that
plications, Cu(In,Ga)Se2 thin films
part by a Grant-in-Aid for Scien-
graded band gap structure [13]. Ther
the value of 1.3 eV demonstrates the bottom of the double
graded band gap structure. This result is suggestive that Cu
(In,Ga)Se2 thin film solar cells with a high Ga/(In+Ga) mole
ratio have a deep valley structure. The deep valley prevents the
carrier collection and causes the deterioration of solar cell per-
formance. The similar tendency for Cu(In,Ga)Se2 thin film
solar cells with Ga/(In+Ga) mole ratio of 0.3 fabricated on Mo
coated Ti foils by three stage process has been reported [14].
On the other hand, Cu(In,Ga)Se2 thin film prepared at In2S3/
(In2Se3+Ga2Se3)=0.04 demonstrated a band gap of 1.4 eV,
which was suitable for a better matching solar spectrum. Thus
the cell performance was improved. Therefore, In2S3 slightly
supplying is one of the promising methods to improve the per-
formance of Cu(In,Ga)Se2 thin film solar cells.
4. Conclusion
For photovoltaic device ap
were prepared by sequential evaporation process. The effect of
In2S3 supplying in the third step was examined. XRD study
showed that Cu(In,Ga)Se2 thin films had a chalcopyrite struc-
ture. EPMA analysis demonstrated that Cu(In,Ga)Se2 thin films
have Ga/(In+Ga) mole ratio of 0.855-0.747 and S/(Se+S) mole
ratio of 0-0.04. From SEM micrograph, Cu(In,Ga)Se2 thin films
were formed with small grains. From the quantum efficiency
analysis, Cu(In,Ga)Se2 thin film solar cells with a high Ga/
(In+Ga) mole ratio prepared by sequential evaporation process
had a deep valley structure, which was the most remarkable
point in this study. This result indicates that it is expected to
obtain the improvement in Cu(In,Ga)Se2 thin film solar cells
with a high Ga/(In+Ga) mole ratio by controlling an adequate
double graded band gap structure. The performance of Cu
(In,Ga)Se2 thin film solar cell was improved by using slightly
In2S3 compound in the third step.
5. Acknowledgements
This study was supported in
tific Research from the Ministry of Education, Culture, Sports,
Science and Technology in Japan.
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