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Fabrication and properties of flexible a-Si:H solar cells with textured AlSi alloy electrodes
J O U R N A L OF
NON-C SI ES ELSEVIER
Journal of Non-CrystallineSolids 198-200 (1996) 1109-1112
Fabrication and properties of flexible a-Si:H solar cells with textured A1-Si alloy electrodes Izumi Ikeo
a
Hisao Morooka a Hisato Shinohara a Akemi Takenouchi b Nobuhiro Takagi b, Yasuyuki Arai b,,
a TDK Corporation R & D Center, 2-15-7 Higashi-Ohwada, lchikawa, Chiba 272, Japan b Semiconductor Energy Lab. Co., Ltd., 398 Hase, Atsugi, Kanagawa 243, Japan
Abstract A back-reflective textured electrode was made by sputtering both pure A1 and A1-Si alloy materials. The electrode surface morphology and the light trapping effects related to the a-Si:H based solar cell were studied. The optical properties of the back reflective textured electrodes were examined from diffuse reflectance. The diffuse reflectance could be changed within the range 5% to 70% when the A1-Si alloy material was used. The surface morphology of the textured electrodes was also changed by the use of A1-Si alloy. The characteristics of the solar cell were investigated in respect of the relation between surface morphology of the textured electrode and the enhancement effect of the collection efficiency. Fine surface morphology was obtained when the AI-Si alloy material was used and the short circuit current of the solar cell was increased effectively by the use of the textured electrode.
I. Introduction Hydrogenated amorphous silicon (a-Si:H) based solar cells have been fabricated on various kinds of substrates such as glass, stainless steel and plastic films [1,2]. These solar cells mainly consist of a transparent electrode layer, an a-Si:H based layer and a reflective electrode layer. The properties of the solar cell depend on the quality of these materials and the cell structure. Therefore the conversion efficiency of the solar cell can be increased not only by improving the quality of a-Si:H and its related materials. To improve the conversion efficiency, transparent electrodes with rough surface morphology were applied to an a-Si:H based solar cell [3]. They are
* Correspondingauthor. Tel.: + 81-462 481 131; fax: + 81-462 702 409,
called textured electrodes. The textured electrode can significantly improve the conversion efficiency by enhancing the collection efficiency of the solar cell. We fabricated a-Si:H based solar cells using plastic film as a substrate [4]. The roll-to-roll method was used to fabricate the solar cells. These solar cells are thin and flexible because they have a thin plastic film, so we call them 'flexible solar cells'. The flexible solar ceils have a structure consisting of s u b s t r a t e / A 1 / Z n O / n - i - p / I T O . However the conversion efficiency of the flexible solar cells was less than conventional solar cells made on a glass substrate with textured transparent electrode. To improve the conversion efficiency of the flexible solar cell, we made back-reflective textured electrode, and examined their surface morphology and optical enhancement effect related to the solar cell, in this study.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)0005 6-7
11 l0
1. lkeo et al. / Journal of Non-Crystalline Solids 198 200 (1996) 1109-1112
Back-reflective textured electrodes were made from A1 and Ag [2,5]. The surface morphology of the electrodes depends on the deposition conditions of the sputtering or vacuum vapor deposition method. The surface morphology was changed by using high substrate temperature and by changing the film thickness. But the plastic film has lower heat resistance than other substrate materials, so we examined pure A1 and A1-Si alloy materials to make back-reflective textured electrodes at low substrate temperature region by sputtering method, and therefore obtain an optimum surface structure for a-Si:H based solar cell.
2. Experiment We examined pure A1 and A1-Si alloy materials to prepare back-reflective textured electrolites. The pure A1 and A1-Si alloy electrodes were made by a DC magnetron sputtering method with pure A1 and A1-Si alloy targets. The pure A1 target had a purity of 99.99% and the A1-Si alloy targets contained Si at a concentration of 0.5 to 2.0 wt%. A polyethylene naphthalate (PEN) film was used as a substrate with 0.075 mm thick and 230 mm wide. We investigated the influence of deposition conditions to obtain an optimum surface morphology. Various deposition conditions were investigated. The parameters were film thickness, substrate temperature during sputtering, concentration of Si and the applied DC power density. A thin ZnO layer was deposited on the pure A1 and AI-Si alloy electrodes to reduce the reaction between A1 and a-Si:H. An a-Si:H based layer was deposited on these electrodes by plasma CVD. An n - i - p junction was formed with n-type txc-Si:H, i-type a-Si:H and p-type Ixc-Si:H layers. Thickness of each layer was 30 nm, 500 nm and 20 nm respectively. Indium thin oxide (ITO) was deposited by sputtering as a front transparent electrode layer. The details of the above processes are described in Ref. [41. The surface morphology of the pure A1 and A1-Si alloy films formed on the PEN film substrate was observed by scanning electron microscope (SEM) and atomic force microscope (AFM) measurement• The optical properties of the textured electrodes were
analyzed from diffuse reflectance at a wavelength of 650 nm. The diffuse reflectance is defined by a formula of an integral reflectance minus a directional reflectance. The effects of surface morphology on the solar cells were examined from optical and electrical measurements. The measurements were carried out for many textured electrodes with different diffuse reflectance.
3. Results
3.1. Surface morphology of sputtered A1 films The surface morphology of sputtered A1 films differed from that of pure A1 and A1-Si alloy materials. We observed the surface morphology from SEM and AFM measurement• The optical property of the textured electrode is important for the solar cell and we examined it by diffuse reflectance. The surface roughness increased with the thickness of film. In particular, the surface roughness of the AI-Si alloy material was strongly dependent on substrate temperature and film thickness. SEM observations revealed that the A1-Si alloy's surface was sharper than pure Al's. Fig. 1 shows the change of diffuse reflectance with mean surface roughness Rz. The mean surface roughness Rz was defined by AFM measurement. The change of diffuse reflectance was confirmed by the surface roughness Rz data. As shown in Fig. 2, the diffuse reflectance saturated at around 20% when
80 o~ o
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•
40 ¢/)
a
•
60
DI
2O
O0
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o
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260 ' 360 ' 460 Roughness (nm)
500
Fig. 1. Surface roughness Rz versus diffuse reflectance of pure A1 and A1 Si alloy films. (E3) Pure AI 70°C, (11) pure A1 150°C, ( O ) A1-Si 70°C, ( 0 ) AI-Si 150°C.
I. Ikeo et al. / Journal of Non- Crystalline Solids 198-200 (1996) 1109-1112
80
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100 °
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= =_
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"'.~ ~ , . . 0
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1 1.5 Thickness (pm)
400
2
Fig, 2. Thickness dependence of diffuse reflectance for pure Al and A1-Si alloy films. (El) Pure AI 70°C, ( B ) pure A1 150°C, ( O ) A1-0.5 wt% Si 70°C, ( 0 ) A1-0.5% Si 150°C, ( A ) AI-1.0% Si 70°C, ( A ) AI-I.0% Si 150°C. Lines are drawn as guides for the eye.
pure Al material was used. The diffuse reflectance, on the other hand, could be widely changed within the range 5% to 70% when A1-Si material was used. These data clarified that the A1-Si alloy makes it possible to change the diffuse reflectance.
3.2. Optical enhancement effects of textured electrodes Single junction a-Si:H solar cells were made on back-reflective textured electrodes having different diffuse reflectance. Fig. 3 shows the dependence of • 0.86 18
mind [] ~
] 0.84
17 16
0.82
~v
0.8°
>o
500
600 700 Wavelength (nm)
800
Fig. 4. Collection efficiency and surface reflectance of a-Si:H based solar cell with textured electrodes having three different diffuse reflectance. (---) Sample A 8%, ( - - - ) sample B 13%, ( - - ) sample C 70%).
the short circuit current (Jsc) and open circuit voltage (Voc) on the diffuse reflectance. The J~c was dependent upon diffuse reflectance. Two different changes of Jsc are seen in divided between region (A) and region (B). Fig. 3 shows that a',~ increases rapidly in the region (A) and slowly in the region (B) corresponding to the diffuse reflectance. The collection efficiency of these samples was measured to investigate these results. The collection efficiency of the samples was measured in the wavelength range 400 to 800 nm. Fig. 4 shows the collection efficiency of three different samples. In this figure, samples A, B and C have different diffuse reflectance of textured electrode, which are 8%, 13% and 70%, respectively. The increase in the collection efficiency of samples B and C were observed in the longer wavelength region over 600 rim, and also in the shorter wavelength region under 550 nm when compared with sample A.
o
3.3. Properties of the solar cells
14 13 0
20 40 60 Diffuse Reflectance(%)
Fig. 3. Short circuit current and open circuit voltage of solar cell dependence on diffuse reflectance. Lines are drawn as guides for the eye.
The open circuit voltage remained constant for diffuse reflectances up to about 40% and rapidly dropped when diffuse reflectance was over 40% as shown in Fig. 3. The fill factor (FF) was constant over the whole range. These results demonstrate that the value of optimum diffuse reflectance for the back-reflective textured electrode is around 25%.
1112
L lkeo et al. / Journal of Non-C©'stalline Solids 198-200 (1996) 1109-1112
The typical solar cell properties (area of 1 cm 2) were; Vo~ = 0.840 V, J,~ = 16.40 m A / c m 2, F F = 0.628 and conversion efficiency of 8.65% under AM1.5 100 m W / c m 2 when the textured electrode was used. W e also made advanced textured electrode for a-Si:H based solar cells to obtain more optical enhancement effect. This electrode had textured-A1S i / S S ( s t a i n l e s s s t e e l ) / A g / Z n O . The reflectance of A g is larger than AI, so that the optical enhancement can increase effectively. The J~ was increased up to 17.10 m A / c m 2 when this textured electrode was used. The conversion efficiency of the solar cell (area of 1 cm 2) was 9.05% under A M I . 5 100 m W / c m 2.
4. Discussion It is well known that the microstructure of sputtered A1 films is influenced by contamination from residual gases in the reaction chamber. W e applied pre-heat processing to the substrate before sputtering to obtain similar surface morphology in the same sputtering conditions with high reproducibility. A difference in surface morphology between pure A1 and A 1 - S i alloy was clearly seen. It can be argued that the A I - S i alloy material is better than pure A1 material because its surface morphology and diffuse reflectance can be widely changed by changing the substrate temperature during sputtering and the film thickness. As a result, it is possible to fabricate textured electrodes on a plastic film substrate. W e can see two different regions of J,c in Fig. 3. It can be argued that the change of the collection efficiency of solar cell corresponds to those of Jsc as shown in Fig. 4. The back-reflective textured electrodes increase the collection efficiency of both longer wavelength and shorter wavelength regions. W e can see that the collection efficiency in the longer wavelength region is saturated even for substrates having diffuse reflectance of over 30%. On the other hand, the collection efficiency in the shorter wavelength region continuously increases as the dif-
fuse reflectance increases. The surface reflectance of the solar cells in the shorter wavelength region decreased as the diffuse reflectance changed from 8% to 70%. It is assumed that the surface reflectance was lowered by the surface roughness of the solar cells that was increased as the surface roughness of textured electrode increased. These results suggest that the change of J~c was due to the optical enhancement effect in the longer wavelength region and reduced surface reflection in the shorter wavelength region.
5. Conclusion W e prepared back-reflective textured electrodes and examined their surface morphology and the optical enhancement effect related to an a-Si:H based solar cell. To obtain the optimum surface morphology, we examined pure A1 and A 1 - S i alloy materials. The diffuse reflectance of the textured electrode can be changed when A1-Si alloy material is used. Single junction a-Si:H based solar cells were prepared on textured electrode having various diffuse reflectances. Short circuit current increased with diffuse reflectance and saturated when the diffuse reflectance was over 30%. The open circuit voltage remained constant as the diffuse reflectance up to 40%. Therefore we can argue that the value of optimum diffuse reflectance for the back-reflective textured electrode is around 25%.
References [1] K. Hoffman, P. Nath, J. Call, G. DiDo, C. Vogeli and S. R. Ovshinski, 20th IEEE PVSC (1988) 293. [2] M. Hirasaka, K. Suzuki, K. Nakatani, M. Asano, M. Yano and H. Okaniwa, Solar Energy Mater. 20 (1990) 99, [3] H. Iida, T. Mishuku, A. Ito and Y. Hayashi, IEEE Trans. Electron Devices ED-34 (1987) 271. [4] H. Shinohara, M. Abe, K. Nishi and Y. Arai, IEEE 1st WCPEC (1994) 682. [5] H. Sannomiya, S. Moriuch, Y. Inoue, K. Nomoto, A. Yokota, M. Nakata and T. Tsuji, PVSEC 5 (1990) 387.
NON-C SI ES ELSEVIER
Journal of Non-CrystallineSolids 198-200 (1996) 1109-1112
Fabrication and properties of flexible a-Si:H solar cells with textured A1-Si alloy electrodes Izumi Ikeo
a
Hisao Morooka a Hisato Shinohara a Akemi Takenouchi b Nobuhiro Takagi b, Yasuyuki Arai b,,
a TDK Corporation R & D Center, 2-15-7 Higashi-Ohwada, lchikawa, Chiba 272, Japan b Semiconductor Energy Lab. Co., Ltd., 398 Hase, Atsugi, Kanagawa 243, Japan
Abstract A back-reflective textured electrode was made by sputtering both pure A1 and A1-Si alloy materials. The electrode surface morphology and the light trapping effects related to the a-Si:H based solar cell were studied. The optical properties of the back reflective textured electrodes were examined from diffuse reflectance. The diffuse reflectance could be changed within the range 5% to 70% when the A1-Si alloy material was used. The surface morphology of the textured electrodes was also changed by the use of A1-Si alloy. The characteristics of the solar cell were investigated in respect of the relation between surface morphology of the textured electrode and the enhancement effect of the collection efficiency. Fine surface morphology was obtained when the AI-Si alloy material was used and the short circuit current of the solar cell was increased effectively by the use of the textured electrode.
I. Introduction Hydrogenated amorphous silicon (a-Si:H) based solar cells have been fabricated on various kinds of substrates such as glass, stainless steel and plastic films [1,2]. These solar cells mainly consist of a transparent electrode layer, an a-Si:H based layer and a reflective electrode layer. The properties of the solar cell depend on the quality of these materials and the cell structure. Therefore the conversion efficiency of the solar cell can be increased not only by improving the quality of a-Si:H and its related materials. To improve the conversion efficiency, transparent electrodes with rough surface morphology were applied to an a-Si:H based solar cell [3]. They are
* Correspondingauthor. Tel.: + 81-462 481 131; fax: + 81-462 702 409,
called textured electrodes. The textured electrode can significantly improve the conversion efficiency by enhancing the collection efficiency of the solar cell. We fabricated a-Si:H based solar cells using plastic film as a substrate [4]. The roll-to-roll method was used to fabricate the solar cells. These solar cells are thin and flexible because they have a thin plastic film, so we call them 'flexible solar cells'. The flexible solar ceils have a structure consisting of s u b s t r a t e / A 1 / Z n O / n - i - p / I T O . However the conversion efficiency of the flexible solar cells was less than conventional solar cells made on a glass substrate with textured transparent electrode. To improve the conversion efficiency of the flexible solar cell, we made back-reflective textured electrode, and examined their surface morphology and optical enhancement effect related to the solar cell, in this study.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)0005 6-7
11 l0
1. lkeo et al. / Journal of Non-Crystalline Solids 198 200 (1996) 1109-1112
Back-reflective textured electrodes were made from A1 and Ag [2,5]. The surface morphology of the electrodes depends on the deposition conditions of the sputtering or vacuum vapor deposition method. The surface morphology was changed by using high substrate temperature and by changing the film thickness. But the plastic film has lower heat resistance than other substrate materials, so we examined pure A1 and A1-Si alloy materials to make back-reflective textured electrodes at low substrate temperature region by sputtering method, and therefore obtain an optimum surface structure for a-Si:H based solar cell.
2. Experiment We examined pure A1 and A1-Si alloy materials to prepare back-reflective textured electrolites. The pure A1 and A1-Si alloy electrodes were made by a DC magnetron sputtering method with pure A1 and A1-Si alloy targets. The pure A1 target had a purity of 99.99% and the A1-Si alloy targets contained Si at a concentration of 0.5 to 2.0 wt%. A polyethylene naphthalate (PEN) film was used as a substrate with 0.075 mm thick and 230 mm wide. We investigated the influence of deposition conditions to obtain an optimum surface morphology. Various deposition conditions were investigated. The parameters were film thickness, substrate temperature during sputtering, concentration of Si and the applied DC power density. A thin ZnO layer was deposited on the pure A1 and AI-Si alloy electrodes to reduce the reaction between A1 and a-Si:H. An a-Si:H based layer was deposited on these electrodes by plasma CVD. An n - i - p junction was formed with n-type txc-Si:H, i-type a-Si:H and p-type Ixc-Si:H layers. Thickness of each layer was 30 nm, 500 nm and 20 nm respectively. Indium thin oxide (ITO) was deposited by sputtering as a front transparent electrode layer. The details of the above processes are described in Ref. [41. The surface morphology of the pure A1 and A1-Si alloy films formed on the PEN film substrate was observed by scanning electron microscope (SEM) and atomic force microscope (AFM) measurement• The optical properties of the textured electrodes were
analyzed from diffuse reflectance at a wavelength of 650 nm. The diffuse reflectance is defined by a formula of an integral reflectance minus a directional reflectance. The effects of surface morphology on the solar cells were examined from optical and electrical measurements. The measurements were carried out for many textured electrodes with different diffuse reflectance.
3. Results
3.1. Surface morphology of sputtered A1 films The surface morphology of sputtered A1 films differed from that of pure A1 and A1-Si alloy materials. We observed the surface morphology from SEM and AFM measurement• The optical property of the textured electrode is important for the solar cell and we examined it by diffuse reflectance. The surface roughness increased with the thickness of film. In particular, the surface roughness of the AI-Si alloy material was strongly dependent on substrate temperature and film thickness. SEM observations revealed that the A1-Si alloy's surface was sharper than pure Al's. Fig. 1 shows the change of diffuse reflectance with mean surface roughness Rz. The mean surface roughness Rz was defined by AFM measurement. The change of diffuse reflectance was confirmed by the surface roughness Rz data. As shown in Fig. 2, the diffuse reflectance saturated at around 20% when
80 o~ o
°l
•
40 ¢/)
a
•
60
DI
2O
O0
•
o
D
%0 '
260 ' 360 ' 460 Roughness (nm)
500
Fig. 1. Surface roughness Rz versus diffuse reflectance of pure A1 and A1 Si alloy films. (E3) Pure AI 70°C, (11) pure A1 150°C, ( O ) A1-Si 70°C, ( 0 ) AI-Si 150°C.
I. Ikeo et al. / Journal of Non- Crystalline Solids 198-200 (1996) 1109-1112
80
1.0
60
~" 0.8 0
v
l 111
100 °
i.~
¢-
/ I f % 8O
o
¢-
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= =_
0.6
20
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~/
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~ 0.2
a
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•
' ".,.%
I
20
oc ¢0
~ ¢r
"'.~ ~ , . . 0
I
0
0.5
I
0.0
I
1 1.5 Thickness (pm)
400
2
Fig, 2. Thickness dependence of diffuse reflectance for pure Al and A1-Si alloy films. (El) Pure AI 70°C, ( B ) pure A1 150°C, ( O ) A1-0.5 wt% Si 70°C, ( 0 ) A1-0.5% Si 150°C, ( A ) AI-1.0% Si 70°C, ( A ) AI-I.0% Si 150°C. Lines are drawn as guides for the eye.
pure Al material was used. The diffuse reflectance, on the other hand, could be widely changed within the range 5% to 70% when A1-Si material was used. These data clarified that the A1-Si alloy makes it possible to change the diffuse reflectance.
3.2. Optical enhancement effects of textured electrodes Single junction a-Si:H solar cells were made on back-reflective textured electrodes having different diffuse reflectance. Fig. 3 shows the dependence of • 0.86 18
mind [] ~
] 0.84
17 16
0.82
~v
0.8°
>o
500
600 700 Wavelength (nm)
800
Fig. 4. Collection efficiency and surface reflectance of a-Si:H based solar cell with textured electrodes having three different diffuse reflectance. (---) Sample A 8%, ( - - - ) sample B 13%, ( - - ) sample C 70%).
the short circuit current (Jsc) and open circuit voltage (Voc) on the diffuse reflectance. The J~c was dependent upon diffuse reflectance. Two different changes of Jsc are seen in divided between region (A) and region (B). Fig. 3 shows that a',~ increases rapidly in the region (A) and slowly in the region (B) corresponding to the diffuse reflectance. The collection efficiency of these samples was measured to investigate these results. The collection efficiency of the samples was measured in the wavelength range 400 to 800 nm. Fig. 4 shows the collection efficiency of three different samples. In this figure, samples A, B and C have different diffuse reflectance of textured electrode, which are 8%, 13% and 70%, respectively. The increase in the collection efficiency of samples B and C were observed in the longer wavelength region over 600 rim, and also in the shorter wavelength region under 550 nm when compared with sample A.
o
3.3. Properties of the solar cells
14 13 0
20 40 60 Diffuse Reflectance(%)
Fig. 3. Short circuit current and open circuit voltage of solar cell dependence on diffuse reflectance. Lines are drawn as guides for the eye.
The open circuit voltage remained constant for diffuse reflectances up to about 40% and rapidly dropped when diffuse reflectance was over 40% as shown in Fig. 3. The fill factor (FF) was constant over the whole range. These results demonstrate that the value of optimum diffuse reflectance for the back-reflective textured electrode is around 25%.
1112
L lkeo et al. / Journal of Non-C©'stalline Solids 198-200 (1996) 1109-1112
The typical solar cell properties (area of 1 cm 2) were; Vo~ = 0.840 V, J,~ = 16.40 m A / c m 2, F F = 0.628 and conversion efficiency of 8.65% under AM1.5 100 m W / c m 2 when the textured electrode was used. W e also made advanced textured electrode for a-Si:H based solar cells to obtain more optical enhancement effect. This electrode had textured-A1S i / S S ( s t a i n l e s s s t e e l ) / A g / Z n O . The reflectance of A g is larger than AI, so that the optical enhancement can increase effectively. The J~ was increased up to 17.10 m A / c m 2 when this textured electrode was used. The conversion efficiency of the solar cell (area of 1 cm 2) was 9.05% under A M I . 5 100 m W / c m 2.
4. Discussion It is well known that the microstructure of sputtered A1 films is influenced by contamination from residual gases in the reaction chamber. W e applied pre-heat processing to the substrate before sputtering to obtain similar surface morphology in the same sputtering conditions with high reproducibility. A difference in surface morphology between pure A1 and A 1 - S i alloy was clearly seen. It can be argued that the A I - S i alloy material is better than pure A1 material because its surface morphology and diffuse reflectance can be widely changed by changing the substrate temperature during sputtering and the film thickness. As a result, it is possible to fabricate textured electrodes on a plastic film substrate. W e can see two different regions of J,c in Fig. 3. It can be argued that the change of the collection efficiency of solar cell corresponds to those of Jsc as shown in Fig. 4. The back-reflective textured electrodes increase the collection efficiency of both longer wavelength and shorter wavelength regions. W e can see that the collection efficiency in the longer wavelength region is saturated even for substrates having diffuse reflectance of over 30%. On the other hand, the collection efficiency in the shorter wavelength region continuously increases as the dif-
fuse reflectance increases. The surface reflectance of the solar cells in the shorter wavelength region decreased as the diffuse reflectance changed from 8% to 70%. It is assumed that the surface reflectance was lowered by the surface roughness of the solar cells that was increased as the surface roughness of textured electrode increased. These results suggest that the change of J~c was due to the optical enhancement effect in the longer wavelength region and reduced surface reflection in the shorter wavelength region.
5. Conclusion W e prepared back-reflective textured electrodes and examined their surface morphology and the optical enhancement effect related to an a-Si:H based solar cell. To obtain the optimum surface morphology, we examined pure A1 and A 1 - S i alloy materials. The diffuse reflectance of the textured electrode can be changed when A1-Si alloy material is used. Single junction a-Si:H based solar cells were prepared on textured electrode having various diffuse reflectances. Short circuit current increased with diffuse reflectance and saturated when the diffuse reflectance was over 30%. The open circuit voltage remained constant as the diffuse reflectance up to 40%. Therefore we can argue that the value of optimum diffuse reflectance for the back-reflective textured electrode is around 25%.
References [1] K. Hoffman, P. Nath, J. Call, G. DiDo, C. Vogeli and S. R. Ovshinski, 20th IEEE PVSC (1988) 293. [2] M. Hirasaka, K. Suzuki, K. Nakatani, M. Asano, M. Yano and H. Okaniwa, Solar Energy Mater. 20 (1990) 99, [3] H. Iida, T. Mishuku, A. Ito and Y. Hayashi, IEEE Trans. Electron Devices ED-34 (1987) 271. [4] H. Shinohara, M. Abe, K. Nishi and Y. Arai, IEEE 1st WCPEC (1994) 682. [5] H. Sannomiya, S. Moriuch, Y. Inoue, K. Nomoto, A. Yokota, M. Nakata and T. Tsuji, PVSEC 5 (1990) 387.