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Superstrate-type CuInSe2-based thin film solar cells by a low-temperature process using sodium compounds
Solar Energy Matewials and Solar Cells
ELSEVIER
Solar Energy Materials and Solar Cells 50 (1998) 97-103
Superstrate-type CulnSe2-based thin film solar cells by a low-temperature process using sodium compounds Tokio Nakada*, Tomoyuki Kume, Akio Kunioka Department of Electrical Engineering and Electronics, Aoyama Gakuin University, Setagaya-ku, Tokyo 157, Japan
Abstract Superstrate-type solar cells with a Au/CulnSe2(CIS)/InxSe/ZnO : A1/glass structure were investigated. The CIS films were deposited by coevaporation method with intentionally incorporated Na2S at a substrate temperature of 350°C. Even at relatively low substrate temperatures, sodium compounds enhanced the (1 1 2) preferred orientation of the chalcopyrite structure, and also improved the cell performance. The InxSey buffer layers disappeared after CIS deposition by interdiffusion. Preliminary cells yielded an efficiency of 7.5% with Voc = 430 mV, Jsc = 29.4 mA/cm 2 and FF = 0.60. The light soaking and forward bias effects were observed for these cells. Keywords: CulnSe2; Thin film; Sodium compounds
1. Introduction Superstrate-type solar cells with a metal/CIS/buffer/TCO/glass structure have several advantages such as no requirement of surface cover glass for encapsulation and interconnection between unit cells in the manufacturing of modules. However, high efficiency superstrate-type CIS film solar cells have not yet been achieved [1-5i. The main reason for the low efficiency was the p o o r crystallinity of the CIS films when
* Corresponding author. 0927-0248/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 1 28- 1
98
E Nakada el al. Solar Energy Materials and Solar (_'ells 50 (1998) 97 103
they were deposited at substrate temperatures below 3 5 0 C in order to prevent interdiffusion. In previous work [1], we have demonstrated superstrate-type solar cells fabricated using chemically deposited CdS buffer layer and CIS films deposited at higher substrate temperatures of 450 C. Our key prior result was that the open circuit voltage (Voc) was improved to 440 mV, which is the highest value reported in superstrate-type solar cells. However, the efficiency of superstrate CIS cells with a CdS buffer layer is limited to approximately 8% due to interdiffusion and alloy formation at the CIS/CdS interface. Two options can be considered in order to make high quality heterojunctions. The first is to prevent interdiffusion at the CIS/CdS interface using low temperature process. The second is to use compounds such as lnxSey comprising the constituent elements of CIS in a buffer layer, since undesirable phases such as CuCd2lnSe4 or heavily doped layers are not formed even if the interdiffusion occurs [1, 5]. In this paper, the effects of sodium on the grain structure of CIS films at relatively low substrate temperatures is presented. We also demonstrate the fabrication and performance of superstrate-type CIS thin film solar cells with InxS% buffer layers are also described
2. Effects of sodium on the grain structure of CIS films
One of the requirements for the fabrication of superstrate-type CIS solar cells is the low temperature growth of CIS films. For this reason, the influence of the sodium concentration on the crystallographic properties of CIS films grown at relatively low substrate temperatures was investigated. 0.2 I,tm-thick InxS% layers were first deposited on soda-lime glass substrates at 350"C using the coevaporation method. 2 btm thick CIS films were deposited sequentially by coevaporation with NazS on the InxSey layers at substrate temperatures between 300'C and 500°C. The fluxes of Cu, In and Se were measured using a quartz thickness monitor. The Se/metal flux was maintained at more than three times that of the total metal flux. Fig. 1 shows X-ray diffraction patterns of CIS films deposited at 350'C with Na2S on the lnxS%/glass substrates (Cu -K~ line). The amount of NazS which was charged in the evaporation source was varied from 0 to 12 mg. Na2S of 6 mg corresponds to approximately 5 a t % of the CIS film. It is evident that even at a relatively low substrate temperature of 350'C a strong [1 I 2) preferred orientation of the chalcopyrite structure occurred when Na2S was added during C|S deposition. Similar results were obtained for CIS films deposited with NazSe [5]. For this reason, the thin layer of sodium sulfides on the CIS films, which also has a low melting point may cover the film surface, and lead to faster growth for the (1 1 2) plane which is parallel to the film surface. This growth model is supported by the fact that most of the Na atoms were observed at the surface of CIS films. The grain structure was also dependent upon the substrate temperature. The crystallinity was very poor for the films deposited below 300(7, whereas a pronounced (1 1 2) orientation was observed for the films deposited above 3 5 0 C with the addition of Na2S.
T Nakada et al.."Solar EnerL~, Materials and Solar Cells 50 (1998) 97-103
99
I"I
o
:-Z. ..... 15
:/.--~/omg
30 45 60 2 0 (degrees)
75
Fig. 1. X-ray diffraction patterns of CulnSe2 films deposited on In~Se/glass substrates at 350 C with and without Na,S.
3. Cell fabrication and performance
A superstrate-type CIS solar cell with a Au/CIS/InxSer/ZnO : A1/glass structure shown in Fig. 2 were fabricated. 2 jam thick transparent conducting ZnO : A1 films were first deposited on soda-lime glass substrates at 540°C by rf-magnetron sputtering using a 2 wt% AlzO3-doped ZnO ceramic target. The films exhibited a sheet resistance of 10~/sq and an optical transmittance of more than 80% for wavelengths between 400 and 900 nm. The 0-0.6 ~tm thick In~S% layers were then grown on the ZnO : AI films at a substrate temperature of 350'~C using the coevaporation method. Subsequently, the 2 lam thick CIS films were deposited sequentially by coevaporation with Na2S on the InxSe~, layers. Finally, the solar cells were completed through the deposition of Au back contacts on the CIS films. The influence of Na on the J - V characteristics for the superstrate-type solar cells with a Au/CIS/In~Ser/ZnO : Al/glass structure is shown in Fig. 3. The J - V characteristics were measured after light-soaking in AM1.5, 100 mW/cm 2 illumination under forward bias. No photovoltaic effect was observed for the cells fabricated without NazS. In contrast, when a suitable amount of NazS was added during CIS deposition, the photovoltaic performance was remarkably improved. The reason for the improvement in cell performance is partly due to the improvement in crystallinity as shown in Fig. 1. Another cause for this is probably due to the increased hole concentration as observed in sodium doped CIGS films [6]. The best cell yielded an efficiency of 7.5% with Voc = 430 mV, Jsc = 29.4 mA/cm 2 and F F = 0.60 after light soaking under forward bias. Fig. 4 shows a dependence of the cell performance on the substrate temperature during CIS deposition. The best cell was obtained using CIS films deposited at 350°C, whereas the cell fabricated above 400°C deteriorated in its performance presumably because of reducing InxSey buffer layer. On the other hand,
100
T. Nakada et al./'Solar Energy Materials and Solar Cells 50 (1998) 97 103
Light
Glass ZnO:A1
Au
I
Y Fig. 2. A structure of a superstrate-type Au/CIS/ln.~Se~/ZnO : Al/glass thin film solar cell.
AMI.5, 100 mW/cm2 Active area: 0.10 em2 - - - - - - - - _ _ _ (b)
~
.~
10_
L~
0
Voc-~).430 V Jsc=29.4 mA]cm2 FF=0.60
Voltage (V) Fig. 3. The influence of Na on the J V characteristics of superstrate-type Au/C1S/InxSe/ZnO : Al/glass solar cells fabricated using Na2S (a) 0 mg, (b) 6 mg and (c) 12 rag.
the poor performance of the cell fabricated at 300°C was due to its poor crystallinity of CIS absorber layer. Fig. 5 shows the spectral response curve of the best cell together with optical transmittance curves of 0.2 tam thick In~Sey and 2 gm thick Z n O : A1 layers. The cut-off at the shorter wavelength for a cell fabricated using CIS film deposited at
T. Nakada et al./Solar Energy Materials and Solar Cells 50 (1998) 97-103
101
Ts~b (0(2)
AM1.5, 100 mW/cm2
"''"--'30- Activea~a:0.10cm' (b) "~o ~
(a) 300 (b) 350
20
..~
g
\\
,
o
o
03
Voltage (V) Fig. 4. J - V characteristics of superstrate-type solar cells fabricated at different substrate temperatures. (dlnxSey= 0.2 ~tm,Na2S = 6 mg).
1.0 [ /
In.Set/glass
1100 [
0.4
40
- - . d 20 0 300 5;0
7;0
900 1100 1300 15'00 1700 Wavelength (rim)
0
Fig. 5. Spectral response curves of Au/CIS/InxSey/ZnO: A1/glasssuperstrate-type solar cells fabricated at 350°C. Optical transmittance curves of a 0.2 gm thick InxSer layer and a 2 gm thick ZnO : AI filmare added for comparison. 350°C shifted toward wavelengths shorter than that expected from the transmittance curve of the InxSer buffer layer alone. This implies that the thickness of the InxSer layer was reduced by interdiffusion taking place during the CIS deposition process even at the relatively low substrate temperature of 350°C. The decrease in quantum efficiency at long wavelengths is attributed to an optical absorption of Z n O : AI film. The cell efficiency can thus be improved by optimizing the Z n O : A1 film. Fig. 6 shows the dependence of J - V characteristics on In,Set layer thickness before CIS deposition, indicating that the o p t i m u m In~Sey thickness is 0.2 gm. As the InxSey thickness increased to over 0.4 gin, the Jsc decreased by the optical absorption of the In~Sey layers. This was confirmed by the spectral response curves shown in Fig. 5. The Voc was independent on InxSer thickness of 0.2~).6 gm, whereas it deteriorated if In~Sey layer was not used. The reason for the increased Voc of the cell fabricated with
102
1~ Nakada (,1 al. Solar EneiXy Matertal.~ and Solar ( "ell,~ 50 (19981 97 103
AMI.5, 100 mW/cm: Active area: 0.10 cm 2
dimscy(~ m)
V
(b) 0.2 |(c) 0.4
~
:
~o 0
Ol
0 . 2 ~ 0.3
Voltage(V) Fig. 6. The d e p e n d e n c e of .I V characleristics for superstrate-type solar cells on the ln,Se,, thickness (Ts,,h = 350 C, Na=S = 6 nag).
;> >8
0.6
0.41 ~
O
0
0
0
O
•
A
Ak
A
I
I
I
O
O
O
0,2 3o(
10 i ~
0.6
J O
O
rL
0.2 (
A•
~" 6.0 2.0 I
i
',I
I
1 2 14 Time (hour)
II
l
18
Fig. 7. L i g h t - s o a k i n g effect of a superstrate-type Au.'CIS:In,Se~,'ZnO : Allglass solar cell.
the InxSe~, thin layer is not understood fully at the present. The improved I<~c may be related to the surface passivation of sulfur atoms. Further experiments are needed to understand this behavior. The Au/CIS/In:,SeffZnO : Al/glass cells described above exhibited a light soaking effect under AM 1.5 illumination and forward bias. Fig. 7 shows a result of this effect
T. Nakada et aL /Solar Energy Materials and Solar Cells 50 (1998) 97-103
103
for the 7.5% efficiency cell. A remarkable increase in Voc and Jsc was observed after light soaking. This p h e n o m e n o n suggests that recombination centers are present near the surface of the CIS layers.
4. Conclusions The cell efficiency of superstrate-type solar cells with a CdS buffer layer is limited by c a d m i u m diffusion into the CIS layers. This has motivated us to investigate the possibility of superstrate-type CIS thin film solar cells with Cd-free buffer layers such as InxSe r The merit of the InxSey c o m p o u n d s is that undesirable phases are eliminated even if interdiffusion occurs since the elements comprising In~S% are constituent elements of CIS. The superstrate-type solar cells with A u / C I S / I n x S e f f Z n O : A1/glass structure were fabricated using CIS films deposited by the c o e v a p o r a t i o n m e t h o d with intentionally incorporated NaES at 350°C. Even at relatively tow substrate temperatures, the addition of the sodium c o m p o u n d enhanced the (1 1 2) preferred orientation of the chalcopyrite structure and also i m p r o v e d the cell performance. After CIS deposition the In,Set layers disappeared by interdiffusion. A preliminary cell yielded an efficiency of 7.5% with Voc = 430 mV, Jsc = 29.4 m A / c m 2 and F F = 0.60 after light soaking under forward bias. Further i m p r o v e m e n t in the cell efficiency can be achieved t h r o u g h optimization of the deposition conditions of the CIS and InxSe:, layers.
Acknowledgements This work was supported in part by the P h o t o v o l t a i c P o w e r Generation Technology Research Association (PVTEC).
References [1] T. Nakada, N. Okano, Y. Tanaka, H. Fukuda, A. Kunioka, Sperstrate-type CulnSe 2 solar cells with chemically deposited CdS window layers, Proc. 1st World Conf. Photovoltaic Energy Conversion, 1994, pp. 95-98. [2] T. Negami, M. Nishitani, M. Ikeda, T. Wada, Preparation of CuInSe2 films on large grain CdS films for superstrate-type solar cells, Solar Energy Materials and Solar Cells 35 (1994) 215-222. [3] T. Negami, M. Nishitani, M. Ikeda, T. Wada, T. Hirao, Preparation of CuInSe 2 films for photovoltaic application, Proc. 1lth European Photovoltaic Solar Energy Conf., 1992, pp. 783-786. [4] T. Yoshida, R.B. Birkmire, Fabrication of CuInSe 2 solar cells in a superstrate configuration, Proc. 1lth European Photovoltaic Solar Energy Conf., 1992, pp. 811-814. [5] T. Nakada, T. Kume, A. Kunioka, Superstrate-type CuInSe2 thin film solar cells with selenide buffer layers, Proc. 25th IEEE Photovoltaic Specialists Conf., 1996, pp. 893-896. [6] T. Nakada, H. Ohbo, M. Fukuda, A. Kunioka, Improved compositional flexibility of Cu(In, Ga)S%based thin film solar cells by sodium control technique, presented in PVSEC-9.
ELSEVIER
Solar Energy Materials and Solar Cells 50 (1998) 97-103
Superstrate-type CulnSe2-based thin film solar cells by a low-temperature process using sodium compounds Tokio Nakada*, Tomoyuki Kume, Akio Kunioka Department of Electrical Engineering and Electronics, Aoyama Gakuin University, Setagaya-ku, Tokyo 157, Japan
Abstract Superstrate-type solar cells with a Au/CulnSe2(CIS)/InxSe/ZnO : A1/glass structure were investigated. The CIS films were deposited by coevaporation method with intentionally incorporated Na2S at a substrate temperature of 350°C. Even at relatively low substrate temperatures, sodium compounds enhanced the (1 1 2) preferred orientation of the chalcopyrite structure, and also improved the cell performance. The InxSey buffer layers disappeared after CIS deposition by interdiffusion. Preliminary cells yielded an efficiency of 7.5% with Voc = 430 mV, Jsc = 29.4 mA/cm 2 and FF = 0.60. The light soaking and forward bias effects were observed for these cells. Keywords: CulnSe2; Thin film; Sodium compounds
1. Introduction Superstrate-type solar cells with a metal/CIS/buffer/TCO/glass structure have several advantages such as no requirement of surface cover glass for encapsulation and interconnection between unit cells in the manufacturing of modules. However, high efficiency superstrate-type CIS film solar cells have not yet been achieved [1-5i. The main reason for the low efficiency was the p o o r crystallinity of the CIS films when
* Corresponding author. 0927-0248/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 7 - 0 2 4 8 ( 9 7 ) 0 0 1 28- 1
98
E Nakada el al. Solar Energy Materials and Solar (_'ells 50 (1998) 97 103
they were deposited at substrate temperatures below 3 5 0 C in order to prevent interdiffusion. In previous work [1], we have demonstrated superstrate-type solar cells fabricated using chemically deposited CdS buffer layer and CIS films deposited at higher substrate temperatures of 450 C. Our key prior result was that the open circuit voltage (Voc) was improved to 440 mV, which is the highest value reported in superstrate-type solar cells. However, the efficiency of superstrate CIS cells with a CdS buffer layer is limited to approximately 8% due to interdiffusion and alloy formation at the CIS/CdS interface. Two options can be considered in order to make high quality heterojunctions. The first is to prevent interdiffusion at the CIS/CdS interface using low temperature process. The second is to use compounds such as lnxSey comprising the constituent elements of CIS in a buffer layer, since undesirable phases such as CuCd2lnSe4 or heavily doped layers are not formed even if the interdiffusion occurs [1, 5]. In this paper, the effects of sodium on the grain structure of CIS films at relatively low substrate temperatures is presented. We also demonstrate the fabrication and performance of superstrate-type CIS thin film solar cells with InxS% buffer layers are also described
2. Effects of sodium on the grain structure of CIS films
One of the requirements for the fabrication of superstrate-type CIS solar cells is the low temperature growth of CIS films. For this reason, the influence of the sodium concentration on the crystallographic properties of CIS films grown at relatively low substrate temperatures was investigated. 0.2 I,tm-thick InxS% layers were first deposited on soda-lime glass substrates at 350"C using the coevaporation method. 2 btm thick CIS films were deposited sequentially by coevaporation with NazS on the InxSey layers at substrate temperatures between 300'C and 500°C. The fluxes of Cu, In and Se were measured using a quartz thickness monitor. The Se/metal flux was maintained at more than three times that of the total metal flux. Fig. 1 shows X-ray diffraction patterns of CIS films deposited at 350'C with Na2S on the lnxS%/glass substrates (Cu -K~ line). The amount of NazS which was charged in the evaporation source was varied from 0 to 12 mg. Na2S of 6 mg corresponds to approximately 5 a t % of the CIS film. It is evident that even at a relatively low substrate temperature of 350'C a strong [1 I 2) preferred orientation of the chalcopyrite structure occurred when Na2S was added during C|S deposition. Similar results were obtained for CIS films deposited with NazSe [5]. For this reason, the thin layer of sodium sulfides on the CIS films, which also has a low melting point may cover the film surface, and lead to faster growth for the (1 1 2) plane which is parallel to the film surface. This growth model is supported by the fact that most of the Na atoms were observed at the surface of CIS films. The grain structure was also dependent upon the substrate temperature. The crystallinity was very poor for the films deposited below 300(7, whereas a pronounced (1 1 2) orientation was observed for the films deposited above 3 5 0 C with the addition of Na2S.
T Nakada et al.."Solar EnerL~, Materials and Solar Cells 50 (1998) 97-103
99
I"I
o
:-Z. ..... 15
:/.--~/omg
30 45 60 2 0 (degrees)
75
Fig. 1. X-ray diffraction patterns of CulnSe2 films deposited on In~Se/glass substrates at 350 C with and without Na,S.
3. Cell fabrication and performance
A superstrate-type CIS solar cell with a Au/CIS/InxSer/ZnO : A1/glass structure shown in Fig. 2 were fabricated. 2 jam thick transparent conducting ZnO : A1 films were first deposited on soda-lime glass substrates at 540°C by rf-magnetron sputtering using a 2 wt% AlzO3-doped ZnO ceramic target. The films exhibited a sheet resistance of 10~/sq and an optical transmittance of more than 80% for wavelengths between 400 and 900 nm. The 0-0.6 ~tm thick In~S% layers were then grown on the ZnO : AI films at a substrate temperature of 350'~C using the coevaporation method. Subsequently, the 2 lam thick CIS films were deposited sequentially by coevaporation with Na2S on the InxSe~, layers. Finally, the solar cells were completed through the deposition of Au back contacts on the CIS films. The influence of Na on the J - V characteristics for the superstrate-type solar cells with a Au/CIS/In~Ser/ZnO : Al/glass structure is shown in Fig. 3. The J - V characteristics were measured after light-soaking in AM1.5, 100 mW/cm 2 illumination under forward bias. No photovoltaic effect was observed for the cells fabricated without NazS. In contrast, when a suitable amount of NazS was added during CIS deposition, the photovoltaic performance was remarkably improved. The reason for the improvement in cell performance is partly due to the improvement in crystallinity as shown in Fig. 1. Another cause for this is probably due to the increased hole concentration as observed in sodium doped CIGS films [6]. The best cell yielded an efficiency of 7.5% with Voc = 430 mV, Jsc = 29.4 mA/cm 2 and F F = 0.60 after light soaking under forward bias. Fig. 4 shows a dependence of the cell performance on the substrate temperature during CIS deposition. The best cell was obtained using CIS films deposited at 350°C, whereas the cell fabricated above 400°C deteriorated in its performance presumably because of reducing InxSey buffer layer. On the other hand,
100
T. Nakada et al./'Solar Energy Materials and Solar Cells 50 (1998) 97 103
Light
Glass ZnO:A1
Au
I
Y Fig. 2. A structure of a superstrate-type Au/CIS/ln.~Se~/ZnO : Al/glass thin film solar cell.
AMI.5, 100 mW/cm2 Active area: 0.10 em2 - - - - - - - - _ _ _ (b)
~
.~
10_
L~
0
Voc-~).430 V Jsc=29.4 mA]cm2 FF=0.60
Voltage (V) Fig. 3. The influence of Na on the J V characteristics of superstrate-type Au/C1S/InxSe/ZnO : Al/glass solar cells fabricated using Na2S (a) 0 mg, (b) 6 mg and (c) 12 rag.
the poor performance of the cell fabricated at 300°C was due to its poor crystallinity of CIS absorber layer. Fig. 5 shows the spectral response curve of the best cell together with optical transmittance curves of 0.2 tam thick In~Sey and 2 gm thick Z n O : A1 layers. The cut-off at the shorter wavelength for a cell fabricated using CIS film deposited at
T. Nakada et al./Solar Energy Materials and Solar Cells 50 (1998) 97-103
101
Ts~b (0(2)
AM1.5, 100 mW/cm2
"''"--'30- Activea~a:0.10cm' (b) "~o ~
(a) 300 (b) 350
20
..~
g
\\
,
o
o
03
Voltage (V) Fig. 4. J - V characteristics of superstrate-type solar cells fabricated at different substrate temperatures. (dlnxSey= 0.2 ~tm,Na2S = 6 mg).
1.0 [ /
In.Set/glass
1100 [
0.4
40
- - . d 20 0 300 5;0
7;0
900 1100 1300 15'00 1700 Wavelength (rim)
0
Fig. 5. Spectral response curves of Au/CIS/InxSey/ZnO: A1/glasssuperstrate-type solar cells fabricated at 350°C. Optical transmittance curves of a 0.2 gm thick InxSer layer and a 2 gm thick ZnO : AI filmare added for comparison. 350°C shifted toward wavelengths shorter than that expected from the transmittance curve of the InxSer buffer layer alone. This implies that the thickness of the InxSer layer was reduced by interdiffusion taking place during the CIS deposition process even at the relatively low substrate temperature of 350°C. The decrease in quantum efficiency at long wavelengths is attributed to an optical absorption of Z n O : AI film. The cell efficiency can thus be improved by optimizing the Z n O : A1 film. Fig. 6 shows the dependence of J - V characteristics on In,Set layer thickness before CIS deposition, indicating that the o p t i m u m In~Sey thickness is 0.2 gm. As the InxSey thickness increased to over 0.4 gin, the Jsc decreased by the optical absorption of the In~Sey layers. This was confirmed by the spectral response curves shown in Fig. 5. The Voc was independent on InxSer thickness of 0.2~).6 gm, whereas it deteriorated if In~Sey layer was not used. The reason for the increased Voc of the cell fabricated with
102
1~ Nakada (,1 al. Solar EneiXy Matertal.~ and Solar ( "ell,~ 50 (19981 97 103
AMI.5, 100 mW/cm: Active area: 0.10 cm 2
dimscy(~ m)
V
(b) 0.2 |(c) 0.4
~
:
~o 0
Ol
0 . 2 ~ 0.3
Voltage(V) Fig. 6. The d e p e n d e n c e of .I V characleristics for superstrate-type solar cells on the ln,Se,, thickness (Ts,,h = 350 C, Na=S = 6 nag).
;> >8
0.6
0.41 ~
O
0
0
0
O
•
A
Ak
A
I
I
I
O
O
O
0,2 3o(
10 i ~
0.6
J O
O
rL
0.2 (
A•
~" 6.0 2.0 I
i
',I
I
1 2 14 Time (hour)
II
l
18
Fig. 7. L i g h t - s o a k i n g effect of a superstrate-type Au.'CIS:In,Se~,'ZnO : Allglass solar cell.
the InxSe~, thin layer is not understood fully at the present. The improved I<~c may be related to the surface passivation of sulfur atoms. Further experiments are needed to understand this behavior. The Au/CIS/In:,SeffZnO : Al/glass cells described above exhibited a light soaking effect under AM 1.5 illumination and forward bias. Fig. 7 shows a result of this effect
T. Nakada et aL /Solar Energy Materials and Solar Cells 50 (1998) 97-103
103
for the 7.5% efficiency cell. A remarkable increase in Voc and Jsc was observed after light soaking. This p h e n o m e n o n suggests that recombination centers are present near the surface of the CIS layers.
4. Conclusions The cell efficiency of superstrate-type solar cells with a CdS buffer layer is limited by c a d m i u m diffusion into the CIS layers. This has motivated us to investigate the possibility of superstrate-type CIS thin film solar cells with Cd-free buffer layers such as InxSe r The merit of the InxSey c o m p o u n d s is that undesirable phases are eliminated even if interdiffusion occurs since the elements comprising In~S% are constituent elements of CIS. The superstrate-type solar cells with A u / C I S / I n x S e f f Z n O : A1/glass structure were fabricated using CIS films deposited by the c o e v a p o r a t i o n m e t h o d with intentionally incorporated NaES at 350°C. Even at relatively tow substrate temperatures, the addition of the sodium c o m p o u n d enhanced the (1 1 2) preferred orientation of the chalcopyrite structure and also i m p r o v e d the cell performance. After CIS deposition the In,Set layers disappeared by interdiffusion. A preliminary cell yielded an efficiency of 7.5% with Voc = 430 mV, Jsc = 29.4 m A / c m 2 and F F = 0.60 after light soaking under forward bias. Further i m p r o v e m e n t in the cell efficiency can be achieved t h r o u g h optimization of the deposition conditions of the CIS and InxSe:, layers.
Acknowledgements This work was supported in part by the P h o t o v o l t a i c P o w e r Generation Technology Research Association (PVTEC).
References [1] T. Nakada, N. Okano, Y. Tanaka, H. Fukuda, A. Kunioka, Sperstrate-type CulnSe 2 solar cells with chemically deposited CdS window layers, Proc. 1st World Conf. Photovoltaic Energy Conversion, 1994, pp. 95-98. [2] T. Negami, M. Nishitani, M. Ikeda, T. Wada, Preparation of CuInSe2 films on large grain CdS films for superstrate-type solar cells, Solar Energy Materials and Solar Cells 35 (1994) 215-222. [3] T. Negami, M. Nishitani, M. Ikeda, T. Wada, T. Hirao, Preparation of CuInSe 2 films for photovoltaic application, Proc. 1lth European Photovoltaic Solar Energy Conf., 1992, pp. 783-786. [4] T. Yoshida, R.B. Birkmire, Fabrication of CuInSe 2 solar cells in a superstrate configuration, Proc. 1lth European Photovoltaic Solar Energy Conf., 1992, pp. 811-814. [5] T. Nakada, T. Kume, A. Kunioka, Superstrate-type CuInSe2 thin film solar cells with selenide buffer layers, Proc. 25th IEEE Photovoltaic Specialists Conf., 1996, pp. 893-896. [6] T. Nakada, H. Ohbo, M. Fukuda, A. Kunioka, Improved compositional flexibility of Cu(In, Ga)S%based thin film solar cells by sodium control technique, presented in PVSEC-9.