class="t m5 x15 h13 y67 ff3 fsc fc0 sc0 ls0">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.
Copyright © 2012 SciRes. AMPC
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.
Copyright © 2012 SciRes. AMPC
T. YAMAGUCHI ET AL.
Copyright © 2012 SciRes. AMPC
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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