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CuInS2 thin-films solar cells fabricated by sulfurization of oxide precursors
Solar Energy Materials and Solar Cells
ELSEVIER
Solar Energy Materials and Solar Cells 49 0997) 343-348
CuInS2 thin-films solar cells fabricated by sulfurization of oxide precursors Takayuki Negami*, Yasuhiro Hashimoto, Mikihiko Nishitani, Takahiro Wada CentralResearch Laboratories, Ma~ushita ElectriclndustrialCo., Ltd., 3-4Hikaridai, Seikacho, Kyoto 619 02, Japan
Abstract C u l n S 2 thin-films were prepared by sulfurization of Cu-In-O precursors in H2S gas. X-ray diffraction patterns showed that I n 2 0 3 phases did not remain in the C u l n S 2 films sulfurized in a HES and H E atmosphere, whereas In203 phase remained in the films sulfurized in a HES and Ar atmosphere. The performance of CuInS 2 solar cells were studied as a function of the H 2 gas pressure during sulfurization. The open-circuit voltage, short-circuit current and fill factor increased with increasing the H 2 gas pressure. The conversion efficiency of the CuInS 2 solar cells is strongly affected by the reduction of the Cu-In-O precursors.
Keywords: CulnS2 thin-films; Sulfurization
I. Introduction CuInS2 thin films are the most promising materials for thin-film photovoltaic devices due to the energy band gap of 1.5 eV which perfectly matches the solar spectrum for energy conversion. Several preparation techniques have been reported [ l ~ ] . Of these techniques, the sulfurization process is much suitable for producing large-area solar cells. However, the binary phases such as CuS and InS are inevitably introduced in the CuInS2 films prepared by the sulfurization of the C u - I n metal precursors. In order to prepare the single-phase CuInS2 thin films, we have already
* Corresponding author. 0927-0248/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved Pll S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 7 6 - 7
344
72 Negami el al. Solar Ene~xv Material,~ and Solar Cells 49 (19972 343 348
proposed the sulfurization process using oxide precursors [5-7]. The sulfurization of Cu-ln O films can suppress the formation of the binary phases. However, 1n203 phase frequently remains in the CulnS2 films sulfurized in a H2S and Ar atmosphere. In this study, the CulnS2 films were prepared by the sulfurization in a H2S and H2 atmosphere in order to promote the reduction of the Cu In-O precursors. The performance of Cu|nS2 solar cells were studied as a function of the H2 gas pressure during the sulfurization.
2. Experimental procedure The Cu l n - O thin films were prepared by sputtering a Cu21n20 5 powder target in an Ar atmosphere containing 5% 02. The thickness of the Cu l n - O films is about 1 gm. The CulnS2 films were formed by sulfurization of the Cu-ln O films at 550'C for I h. Three kinds of films were obtained by the sulfurization in H 2 gas pressure of 55, 68 and 142 Tort and a constant H2S gas pressure of 56 Torr. The thickness of the obtained films is approximately 1.5 ~m. The Cu In O film was also sulfurized in a H2S and Ar gas atmosphere with the same H2S gas pressure. CulnS2 solar cells with a structure of ITO/ZnO/CdS/CulnSi/Mo/glass were fabricated after KCN treatment of the CulnS2 films, since the obtained films have a Cu-rich composition with Cu/In ratios of above 1.2. The CulnS2 films were dipped into 10 wt% KCN aqueous solution to remove Cu S impurities. The Cu/In ratio decreased to about 0.9 from above 1.2 by KCN treatment. CdS buffer layers of 50-100 nm thick were deposited by chemical-bath deposition (CBD). After that, ZnO and ITO layers were deposited by RF-magnetron sputtering with thicknesses of 0.2 and 0.3 gm, respectively.
3. Results and discussion Fig. la and Fig. l b show typical X-ray diffraction patterns of the CulnS2 films sulfurized in the H2S + Ar and H2S--~ He atmospheres, respectively. The peaks of (0 0 4) and (2 0 0), (1 1 6t and {3 1 2t are observed independently in both films due to the difference of the lattice constants of a and c/2 in chalcopyrite CulnS2. In203 phase marked with (°) remains in the CulnS2 film sulfurized in the H2S + Ar gases. On the other hand, In203 phase is not observed in the film sulfurized in the H2S + H2 gases. This is caused by facilitating the reduction of Cu-In O system with H 2. Fig. 2a and Fig. 2b show cross-sectional SEM micrographs of the CulnS 2 films sulfurized in the H2S + Ar and HeN 4-H2 atmospheres, respectively. Two layers consisted of large and small grains are clearly observed in the film sulfurized in the H2S + Ar atmosphere. On the other hand, such a boundary is not observed in the film sulfurized in the H2S + H2 atmosphere. As a result of the XRD pattern, In2Os phase remained in the layer of small grains in Fig. 2a because the sulfurization proceeded from the surface of the precursors.
Negami et al./Solar Energy Materials and Solar Cells 49 (1997) 343 348
345
[~ n H 2 S - + A r
"~
I
I'-
15
30
I in H2S *
45
142
60
2e (degrees) Fig. 1. X-ray diffraction patterns of the CuInSz films sulfurized in: (a) the H2S + Ar and; (b) the H2S + Hz gases.
(a)
(b)
Fig. 2. Cross-sectional SEM micrographs of the CuInS2 films sulfurized in: (a) the H2S + Ar and; (b) the H2S + H2 gases.
346
T. Negami et al. ,'Solar Energ3' Materials and Solar Cells 49 (1997) 343 348
> o 0
>
E 0
E "-a
U. LI.
0.68 0.66 0.64 0.62 0.60 18
l I
t
I
I
[ 1 1 1
I
~
f
f
I
J
I
I
I
I
I
I
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I
I
I
I
1
I
r
i
i
17 16 15 0.55 0.50 0.45 0.40 6
v
.,,~"
5
LU
4 3 , 50
100
150
H 2 Press (Torr) Fig. 3. The dependence of the cell performance on the H2 gas pressure during the sulfurization process.
The cell performance was measured under standard AM1.5 illumination. Fig. 3 shows the dependence of the performance on the H 2 gas pressure during the sulfurization process. Open-circuit voltage (Voc), short-circuit current (Jsc) and fill factor (FF) increase with increasing H2 gas pressure. All parameters of the cell with the CuInS2 film sulfurized at the H 2 gas pressure of 142 Torr, are considerably improved compared with those having the films sulfurized at half the H 2 gas pressure. Although the In203 phase is not observed in any film sulfurized in the HzS + H2 atmosphere, a very small amount of oxygen remained in the CuInS2 films might affect the cell performance. Fig. 4 shows the spectral quantum efficiency of the CuInS2 solar cells fabricated by using the sulfurization under different H 2 pressures. The quantum efficiency in long wavelengths largely increase with H2 gas pressure in the cell with the films sulfurized in low H2 pressure. The quantum efficiency in short wavelengths is much improved in the cell with film sulfurized in high H 2 pressure has a slightly higher quantum efficiency in long wavelengths than that with the film sulfurized in low H 2 pressure. However, the quantum efficiency of the cells decrease with increasing the wavelength in the range between 600 and 800 nm, in spite of the H2 gas pressure during the sulfurization. This is caused by a short carrier diffusion length in the CuInS2 films. The remaining oxygen in the films may reduce the carrier diffusion length.
T. Negami et al./Solar Energy Materials and Solar Cells 49 (1997) 343-348
1.0 e-
¢
H2 Pressure -55 Torr
'
'
400
600
O
347
0.8
0
[E
0.6
E
0.4
UJ
C
0= 0
0.2 o
800
1000
Wavelength (nm)
Fig. 4. Spectral quantum efficiency of the cells fabricated by the sulfurization of the Cu-In-O oxide precursors. 20
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
AM1.5, 100mW/cm 21 Area = 0.1cm 2
E 15
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== I.
-, O
5
FF= 0.657
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/
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Voltage (V) Fig. 5. I V curve of the CulnS 2 solar cell under AM1.5 and 100 mW/cm 2 illumination.
The I - V curve of the highest efficient cell fabricated by the C u - I n - O precursor is shown in Fig. 5. A Cu!nS2 solar cell with an active area efficiency of 7.5% (Vo¢ -- 0.702 V, Jsc = 16.2 mA/cm 2 and F F = 0.657) was obtained.
4. Conclusions
CulnS2 thin films were prepared by the sulfurization of the C u - I n - O precursors in a H2S + H2 atmosphere, l n 2 0 3 phase was eliminated in the CuInS2 films due to the reduction by the addition of the H2 gas. However, a very small amount of oxygen in the films has a large influence on the efficiency of the CuInS2 solar cells. The cell parameters such as Voc, J,c and F F increased with increasing H2 pressure during the sulfurization. The spectral quantum efficiency of the CuInS2 solar cells fabricated using oxide precursor was improved by the sulfurization in high H2 pressure, corresponding to the Jsc. However, the quantum efficiency decreased with increasing the
348
E Negami et al./Solar Eneixv MateriaL~ amt So&#" Cells 49 (1997) 343 346'
wavelength due to a short carrier diffusion length, A sufficient reduction of Cu In O system during the sulfurization process is necessary to achieve higher cell efficiencies. The CulnS2 solar cell fabricated by the sulfurization of the oxide precursor showed a conversion efficiency of 7.5%. Further improvement of the efficiency is expected by optimizing the sulfurization condition and the precursor structure.
Acknowledgements This work was partially supported by the New Energy and Industrial Technology Development Organization as a part of the Newsunshine Program conducted under the MITI. We would like to thank Drs. T. Nitta, H. Ogawa and T. Tohda for their encouragement throughout this work.
References [I] A.N. Tiwari. D.K. Pandya. K . L . ( h o p r a , Sol. EnerG5 Maicr. Sol. Cells 15ll987) 121. [2] Y. Ogawa, A. Jagcr-Waldau. Y. Hashimoio. K. l Io. J pn. J. Appl. Phys. 33 (1994) 1775. [3] T. Walter, D. Brunger, D. Hariskos, ('11. Koble, H.W. Schock. Proc. 13th E.C Photovoh. Solar Energy ('onf.. Nice. 1995, p, 597 [4] D. Braunger. D. Hariskos, T. Walter, H,W. Schock, Sol. Energy Mater. Sol. ('ells 40 (1996) 97. [5] T. Wada, T. Negami. M. Nishiiani. Appl. Phys. kcll. 62 (1993) 1943. [6] T. Wada, T. Negami. M. Nishilani, J. Mater. Res. 8 {19931 545. [7] M. Terauchi. T. Negami, M. Nishitani, M. Ikeda. H. Wada. T. Wada. Sol. Energy Mater. Sol. Cells 35 (19941 121.
ELSEVIER
Solar Energy Materials and Solar Cells 49 0997) 343-348
CuInS2 thin-films solar cells fabricated by sulfurization of oxide precursors Takayuki Negami*, Yasuhiro Hashimoto, Mikihiko Nishitani, Takahiro Wada CentralResearch Laboratories, Ma~ushita ElectriclndustrialCo., Ltd., 3-4Hikaridai, Seikacho, Kyoto 619 02, Japan
Abstract C u l n S 2 thin-films were prepared by sulfurization of Cu-In-O precursors in H2S gas. X-ray diffraction patterns showed that I n 2 0 3 phases did not remain in the C u l n S 2 films sulfurized in a HES and H E atmosphere, whereas In203 phase remained in the films sulfurized in a HES and Ar atmosphere. The performance of CuInS 2 solar cells were studied as a function of the H 2 gas pressure during sulfurization. The open-circuit voltage, short-circuit current and fill factor increased with increasing the H 2 gas pressure. The conversion efficiency of the CuInS 2 solar cells is strongly affected by the reduction of the Cu-In-O precursors.
Keywords: CulnS2 thin-films; Sulfurization
I. Introduction CuInS2 thin films are the most promising materials for thin-film photovoltaic devices due to the energy band gap of 1.5 eV which perfectly matches the solar spectrum for energy conversion. Several preparation techniques have been reported [ l ~ ] . Of these techniques, the sulfurization process is much suitable for producing large-area solar cells. However, the binary phases such as CuS and InS are inevitably introduced in the CuInS2 films prepared by the sulfurization of the C u - I n metal precursors. In order to prepare the single-phase CuInS2 thin films, we have already
* Corresponding author. 0927-0248/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved Pll S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 0 7 6 - 7
344
72 Negami el al. Solar Ene~xv Material,~ and Solar Cells 49 (19972 343 348
proposed the sulfurization process using oxide precursors [5-7]. The sulfurization of Cu-ln O films can suppress the formation of the binary phases. However, 1n203 phase frequently remains in the CulnS2 films sulfurized in a H2S and Ar atmosphere. In this study, the CulnS2 films were prepared by the sulfurization in a H2S and H2 atmosphere in order to promote the reduction of the Cu In-O precursors. The performance of Cu|nS2 solar cells were studied as a function of the H2 gas pressure during the sulfurization.
2. Experimental procedure The Cu l n - O thin films were prepared by sputtering a Cu21n20 5 powder target in an Ar atmosphere containing 5% 02. The thickness of the Cu l n - O films is about 1 gm. The CulnS2 films were formed by sulfurization of the Cu-ln O films at 550'C for I h. Three kinds of films were obtained by the sulfurization in H 2 gas pressure of 55, 68 and 142 Tort and a constant H2S gas pressure of 56 Torr. The thickness of the obtained films is approximately 1.5 ~m. The Cu In O film was also sulfurized in a H2S and Ar gas atmosphere with the same H2S gas pressure. CulnS2 solar cells with a structure of ITO/ZnO/CdS/CulnSi/Mo/glass were fabricated after KCN treatment of the CulnS2 films, since the obtained films have a Cu-rich composition with Cu/In ratios of above 1.2. The CulnS2 films were dipped into 10 wt% KCN aqueous solution to remove Cu S impurities. The Cu/In ratio decreased to about 0.9 from above 1.2 by KCN treatment. CdS buffer layers of 50-100 nm thick were deposited by chemical-bath deposition (CBD). After that, ZnO and ITO layers were deposited by RF-magnetron sputtering with thicknesses of 0.2 and 0.3 gm, respectively.
3. Results and discussion Fig. la and Fig. l b show typical X-ray diffraction patterns of the CulnS2 films sulfurized in the H2S + Ar and H2S--~ He atmospheres, respectively. The peaks of (0 0 4) and (2 0 0), (1 1 6t and {3 1 2t are observed independently in both films due to the difference of the lattice constants of a and c/2 in chalcopyrite CulnS2. In203 phase marked with (°) remains in the CulnS2 film sulfurized in the H2S + Ar gases. On the other hand, In203 phase is not observed in the film sulfurized in the H2S + H2 gases. This is caused by facilitating the reduction of Cu-In O system with H 2. Fig. 2a and Fig. 2b show cross-sectional SEM micrographs of the CulnS 2 films sulfurized in the H2S + Ar and HeN 4-H2 atmospheres, respectively. Two layers consisted of large and small grains are clearly observed in the film sulfurized in the H2S + Ar atmosphere. On the other hand, such a boundary is not observed in the film sulfurized in the H2S + H2 atmosphere. As a result of the XRD pattern, In2Os phase remained in the layer of small grains in Fig. 2a because the sulfurization proceeded from the surface of the precursors.
Negami et al./Solar Energy Materials and Solar Cells 49 (1997) 343 348
345
[~ n H 2 S - + A r
"~
I
I'-
15
30
I in H2S *
45
142
60
2e (degrees) Fig. 1. X-ray diffraction patterns of the CuInSz films sulfurized in: (a) the H2S + Ar and; (b) the H2S + Hz gases.
(a)
(b)
Fig. 2. Cross-sectional SEM micrographs of the CuInS2 films sulfurized in: (a) the H2S + Ar and; (b) the H2S + H2 gases.
346
T. Negami et al. ,'Solar Energ3' Materials and Solar Cells 49 (1997) 343 348
> o 0
>
E 0
E "-a
U. LI.
0.68 0.66 0.64 0.62 0.60 18
l I
t
I
I
[ 1 1 1
I
~
f
f
I
J
I
I
I
I
I
I
I
I
I
I
I
1
I
r
i
i
17 16 15 0.55 0.50 0.45 0.40 6
v
.,,~"
5
LU
4 3 , 50
100
150
H 2 Press (Torr) Fig. 3. The dependence of the cell performance on the H2 gas pressure during the sulfurization process.
The cell performance was measured under standard AM1.5 illumination. Fig. 3 shows the dependence of the performance on the H 2 gas pressure during the sulfurization process. Open-circuit voltage (Voc), short-circuit current (Jsc) and fill factor (FF) increase with increasing H2 gas pressure. All parameters of the cell with the CuInS2 film sulfurized at the H 2 gas pressure of 142 Torr, are considerably improved compared with those having the films sulfurized at half the H 2 gas pressure. Although the In203 phase is not observed in any film sulfurized in the HzS + H2 atmosphere, a very small amount of oxygen remained in the CuInS2 films might affect the cell performance. Fig. 4 shows the spectral quantum efficiency of the CuInS2 solar cells fabricated by using the sulfurization under different H 2 pressures. The quantum efficiency in long wavelengths largely increase with H2 gas pressure in the cell with the films sulfurized in low H2 pressure. The quantum efficiency in short wavelengths is much improved in the cell with film sulfurized in high H 2 pressure has a slightly higher quantum efficiency in long wavelengths than that with the film sulfurized in low H 2 pressure. However, the quantum efficiency of the cells decrease with increasing the wavelength in the range between 600 and 800 nm, in spite of the H2 gas pressure during the sulfurization. This is caused by a short carrier diffusion length in the CuInS2 films. The remaining oxygen in the films may reduce the carrier diffusion length.
T. Negami et al./Solar Energy Materials and Solar Cells 49 (1997) 343-348
1.0 e-
¢
H2 Pressure -55 Torr
'
'
400
600
O
347
0.8
0
[E
0.6
E
0.4
UJ
C
0= 0
0.2 o
800
1000
Wavelength (nm)
Fig. 4. Spectral quantum efficiency of the cells fabricated by the sulfurization of the Cu-In-O oxide precursors. 20
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
AM1.5, 100mW/cm 21 Area = 0.1cm 2
E 15
o <[
E
v10
== I.
-, O
5
FF= 0.657
\
/
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Voltage (V) Fig. 5. I V curve of the CulnS 2 solar cell under AM1.5 and 100 mW/cm 2 illumination.
The I - V curve of the highest efficient cell fabricated by the C u - I n - O precursor is shown in Fig. 5. A Cu!nS2 solar cell with an active area efficiency of 7.5% (Vo¢ -- 0.702 V, Jsc = 16.2 mA/cm 2 and F F = 0.657) was obtained.
4. Conclusions
CulnS2 thin films were prepared by the sulfurization of the C u - I n - O precursors in a H2S + H2 atmosphere, l n 2 0 3 phase was eliminated in the CuInS2 films due to the reduction by the addition of the H2 gas. However, a very small amount of oxygen in the films has a large influence on the efficiency of the CuInS2 solar cells. The cell parameters such as Voc, J,c and F F increased with increasing H2 pressure during the sulfurization. The spectral quantum efficiency of the CuInS2 solar cells fabricated using oxide precursor was improved by the sulfurization in high H2 pressure, corresponding to the Jsc. However, the quantum efficiency decreased with increasing the
348
E Negami et al./Solar Eneixv MateriaL~ amt So&#" Cells 49 (1997) 343 346'
wavelength due to a short carrier diffusion length, A sufficient reduction of Cu In O system during the sulfurization process is necessary to achieve higher cell efficiencies. The CulnS2 solar cell fabricated by the sulfurization of the oxide precursor showed a conversion efficiency of 7.5%. Further improvement of the efficiency is expected by optimizing the sulfurization condition and the precursor structure.
Acknowledgements This work was partially supported by the New Energy and Industrial Technology Development Organization as a part of the Newsunshine Program conducted under the MITI. We would like to thank Drs. T. Nitta, H. Ogawa and T. Tohda for their encouragement throughout this work.
References [I] A.N. Tiwari. D.K. Pandya. K . L . ( h o p r a , Sol. EnerG5 Maicr. Sol. Cells 15ll987) 121. [2] Y. Ogawa, A. Jagcr-Waldau. Y. Hashimoio. K. l Io. J pn. J. Appl. Phys. 33 (1994) 1775. [3] T. Walter, D. Brunger, D. Hariskos, ('11. Koble, H.W. Schock. Proc. 13th E.C Photovoh. Solar Energy ('onf.. Nice. 1995, p, 597 [4] D. Braunger. D. Hariskos, T. Walter, H,W. Schock, Sol. Energy Mater. Sol. ('ells 40 (1996) 97. [5] T. Wada, T. Negami. M. Nishiiani. Appl. Phys. kcll. 62 (1993) 1943. [6] T. Wada, T. Negami. M. Nishilani, J. Mater. Res. 8 {19931 545. [7] M. Terauchi. T. Negami, M. Nishitani, M. Ikeda. H. Wada. T. Wada. Sol. Energy Mater. Sol. Cells 35 (19941 121.