- Email: [email protected]
Film-substrate a-Si solar cells with a new monolithic series-connected structure
N
ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 1081 - 1086
Film-substrate a-Si solar cells with a n e w monolithic series-connected structure Yukimi Ichikawa *, Katuya Tabuchi, Akihiro Takano, Shinji Fujikake, Takashi Yoshida, Hiroshi Sakai Fuji Electric" Corporate Research and Del,elopment, Ltd., 2-2-1, Nagasaka, Yokosuka, Kanagawa 240-01, Japan
Abstract An amorphous silicon solar cell deposited on a plastic film-substrate has been studied. The solar cell has a new monolithic series-connected structure named 'SCAF' and was fabricated by a newly developed fabrication process based on the 'stepping-roll' film deposition system. The details of the new process are presented and the advantage of that is discussed. To fabricate the SCAF solar cells, we started developing a prototype line and obtained preliminary results for small- and large-area SCAF solar cells. The results show that the SCAF structure worked well with the new process.
1. Introduction
The performance of hydrogenated amorphous silicon (a-Si:H) based large-area solar cells has improved remarkably last decade, and been reaching a practical application level. The next stage required for R and D on a-Si solar cells is development of mass production technology and application-intensive product. In principle, the a-Si solar cell has an advantage in production cost compared with crystalline silicon (c-Si) based solar cells. To make this advantage real, however, development in the production technology is not avoidable. Another concern is application-intensive products. We have to admit that the performance of a-Si solar cells is inferior to that of c-Si, but they have several superior features; they are large-area thin film devices and are basically suitable
Corresponding author. Tel.: +81-468 57 6730; fax: +81468-57 2791; e-mail: [email protected].
for mass production. In addition, Various materials can be used for the substrate. Using plastic film or stainless steel film for that, we are able to fabricate flexible a-Si solar cells. They are very light and easy to use and are expected to open a door for new applications in the residential and building fields. From Ihe above viewpoints, we concluded that flexible a-Si solar cells formed on plastic film have high potential in both mass production and applications, and started developing plastic film-substrate solar cells in 1992. We considered that there are two major advantages in the plastic film solar cell; one is that roll-to-roll processing is applicable, and the other is that a large-area device with a monolithic series-connected structure is fabricated. The former makes mass production very easy, and the latter enables us to obtain a desirable voltage for applications if we could find a proper way. However, we face a number of difficulties if we simply apply conventional technologies to the fabrication of flexible a-Si solar cells.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 0 4 9 - X
1082
Y. lchikawa et al. / Journal of Non-Crystalline Solids 198-200 (1996) 1081-1086
To overcome these difficulties and to fabricate the practical flexible solar cells efficiently, we proposed new technologies in both fabrication process and series-connected structure. In the following sections, we will present the basic concept and its advantage as well as the present status of research and development of the film-substrate solar cells.
2. What is the best structure for film-substrate a-Si solar cells?
Series-connection holes
Transp~
Laserq for unit cell separation
D,tt,r~-~uc c~ct,uoue
F i g . 1. S c h e m a t i c d i a g r a m o f S C A F structure.
The minimum requirements we made in the development of the film-substrate solar cell were as follows: (1) A monolithic series-connected structure can be formed by a completely dry process. (2) A roll-to-roll system can be applied to all the fabrication processes. What we tried first was to replace the substrate simply from glass to transparent plastic film such as PET and PEN in the conventional monolithic structure, and we succeeded in obtaining an aperture-area efficiency of about 6% in a 94 cm 2 cell. However, we did not employ this structure because we were not able to succeed in applying a laser-scribing process to the metal electrode patterning; we used a wet etching process for that. This technique means the above requirements are not satisfied. To overcome this following, we had to find a new technology. An idea for the monolithic structure was incubated by using a property of plastic film; it is easy to make holes or cut the plastic film. The conceptual structure of the new monolithic structure is shown in Fig. 1. The structure was named as ' S C A F (series-connection through apertures formed on film)'. In this structure, a substrate type a-Si:H based solar cell is fabricated on one surface of the plastic film; the cell is composed of metal electrode, a-Si:H based layers, and transparent electrode stacked in the order on the substrate. The plastic films used were heat-resisting plastics such as poly-imide with a thickness of tens of micrometers. The remarkable feature of the S C A F structure is a number of apertures made on the substrate. These apertures are divided into two groups and have different functions. One function is through-hole contact between the transparent electrode and the back-
side electrode formed on the other surface of the substrate; they are named current-collection holes and distributed in the active-area region, the transparent electrode region. The other is for the connection between the backside electrode and the metal electrode; they are named series-connection holes and are on both sides out of the transparent electrode region. As shown in Fig. 1, the S C A F solar cell consists of a number of divided unit cells. The current generated from the unit cell has to flow in the transparent electrode having relatively large sheet resistance. To avoid power loss there, the current-collection holes collect the current generated near them and transport it to the backside electrode having low sheet resistance. The current collected into the backside electrode flows to both sides with little joule loss, and then moves into the metal electrode of the next unit cell through the series-connection holes. This is the concept of the S C A F structure. The most important advantages of this structure are as follows: (1) Both surfaces of the plastic film are covered with metal before fabricating a-Si layers: thus we can avoid the influence of degassing and diffusion of impurities from the film. (2) The laser-scribing process is needed only once after all the film deposition processes are completed; thus a-Si layers and electrodes can be formed in successive processes. This sequence is very important to reduce not only the number of fabrication steps and but also the short-circuit defects. In addition, for patterning processes, selective scribing and
E lchikawa et al./Journal of Non-Crystalline Solids 198-200 (1996) 1081-1086
precise position control of the laser beam are not required. (3) To make the through-hole contact, no additional process is needed; it is automatically made by depositing most of layers in a proper order.
3. What is the best process for SCAF structure a-Si solar cells?
3.1. New roll-to-roll system A roll-to-roll system is a very attractive method for the process for flexible a-Si solar cells. The conventional roll-to-roll system, however, has several problems for such applications [ 1]. It is excellent for mass production, but isolation among reactors is not perfect in an in-line multi-reactor system. As a result, interdiffusion of material gases is not avoidable, and also the gas pressures and the deposition rates are not chosen independently among the reactors. These difficulties become barriers to improvement of solar cell efficiency. What we proposed to solve these difficulties is a combination of the conventional roll-to-roll system and a batch system named 'stepping-roll (SR)' system. Fig. 2 shows the schematic diagram of the SR system. The basic configuration of the apparatus consists of a common chamber and several film deposition reactors in it. In this example, all the reactors are for plasma CVD. Each reactor has a movable grounded electrode (GE). When the GEs are at down position and press the chambers, the reactors are completely isolated from the common chamber by holding the substrate film between Orings hooked up on the edges of the GE and the reactor. On the other hand, when the GEs are at Film
Grounded Electrode Common Chamber
Unwii!:fllRo I~er~~ ~iowlrll ~:ectWri~! ingRoller Fig.2.Basicconceptofsteppingrollfilmdepositionapparatus. Dep. Chamber
1083
upper position, the film-substrate is released and can be wound up as the conventional roll-to-roll system. Using this system, we can deposit various film layers simultaneously according to the following sequence: (1) Pressing down the GEs to isolate the reactors from the common chamber. (2) Film deposition. (3) Moving up the GEs. (4) Winding up film by one flame (reactor length of film). As one can understand easily, the SR system has the following advantages compared with the conventional roll-to-roll system: (1) Interdiffusion of raw material gases among reactors is completely prevented. (2) The configuration of the reactors is compact and simple because no intricate mechanisms for transport of the substrate are required. (3) Precise control of the composition in the depth direction can be made. (4) Deposition conditions in each reactor can be chosen independently from those of other reactors. Thus, not only are all the technologies developed for high efficiency glass substrate cells applicable, but also the SR system can have plasma CVD reactors and sputtering reactors in a single line. This feature of the SR system is very convenient for the SCAF structure solar cell as described in the following sections.
3.2. Process flow for SCAF solar cells The SCAF cell is fabricated by a process based on roll-to-roll systems as shown in Fig. 3. First, apertures are made on the plastic film by a punching unit and then the substrate is cleaned by a cleaning unit in a pretreatment apparatus. After that, the roll is transferred to a roll-to-roll sputtering apparatus to deposit metal electrodes. Then the pretreated filmsubstrate roll is set in the SR apparatus composed of plasma CVD reactors and sputtering reactors to form the a-Si layers, the transparent electrode and backside electrode. Finally, the layers formed on both surfaces of the film are laser-scribed to make unit cells by a roll-to-roll laser-scriber. The fabrication process for the SCAF solar cell presented here is simplified further compared with that for the glass substrate solar cells. Moreover
1084
Y.lchikawa et al. /
Journal of Non-Crystalline Solids 198-200 (1996) 1081-1086
Punchingunit Cleaningunit
Target/
I
I o o o o ol
Cleaning & making apertures (Pretreatment apparatus)
Metal electrode formation (Roll-to-roll sputtering apparatus)
ol I
Separation to unit cells (Roll-to-rolllaser-scriber)
Sputteringreactors 1
Deposition of a-Si layers, transparent electrode and back-side electrode (Stepping-roll apparatus)
Fig. 3. Fabricationprocessflowfor SCAFsolarcells. since all the process units are roll-to-roll systems, an automated line will be easily constructed.
4. Fabrication of SCAF solar cells
4.1. Prototype SR apparatus To make the S C A F solar cell with the roll-to-roll process described in the previous sections, we started developing a prototype line. The critical part of the technology is the SR apparatus. W e first constructed a simplified prototype SR apparatus to study whether the fundamental functions work as expected. The prototype SR apparatus has two deposition chambers as shown in Fig. 4; one is for plasma-CVD and the other is for sputtering. Both chambers have an effective deposition area of 40 cm × 40 cm, in which a-Si:H or an electrode is deposited on plastic film-
Film substrate Unwinder /
!
Winder
W"
Common Chamber
Fig. 4. Schematicdiagramof prototypesteppingroll deposition apparatus.
substrates with a width of 50 cm. The a-Si films and ITO for the transparent electrode deposited by this apparatus demonstrated good uniformities and qualities in thickness and optoelectrical properties over a wide area as described in Refs. [2,3]. Large-area devices were studied with the prototype SR apparatus. As already mentioned, this apparatus has only one reactor for a-Si film deposition. Thus, with this apparatus, we do not expect high efficiencies, but can study the process technology for the S C A F solar cell and fundamental structure of the SR apparatus for designing a full specification apparatus. Generation of short-circuit points due to pin holes of the a-Si layer was one of the typical defects for glass substrate cells. In the beginning, we were concerned about pinhole shorts in the S C A F cells generated through roll-to-roll conveyance. Hence we fabricated 64 1 cm 2 single-junction solar cells on an area of 40 cm × 40 cm area using the prototype SR apparatus. Fig. 5 shows the histogram of open-circuit voltages, Voc, of 512 cells; we fabricated 8 samples and the summation of them is shown. As seen from the figure, 99% of the cells exceed 0.85 V. This means that the number of short-circuit points is negligibly small. From the above results, we concluded that largearea a-Si solar cells can be formed on plastic filmsubstrate by the SR apparatus, though the efficiencies of the small area cells are lower than those
E lchikawa et al./Journal ofNon-Co, stalline Solids 198-200 (1996) 1081-1086
99% 500
' ' ' i ! i ~
'
'
L
' !i
i~
1 0
.
3 . . . . i !
' }
. . . .
8~,
0
,,~ 300
i
. . . .
i
iE
J
. . . .
.......... Horizontal .....
Diagonal
4 ............
| I |
I
ILl
E 0
. . . .
6 "
O
"~
.~ 200
~
1085
='i
~ ,
0.55 0.70
~
J,i
~ ~..IIR
0.75 0.80 voc
0.85 0.90
=
0.95
=
1.0
0 20 0
(v)
, .... -" 1 ' 0 0 0 .... 100 P o s i t i o n (ram)
200
Fig. 5. Histogram of open-circuit voltage, Voc, of 512 cells formed on 40 cm × 40 cm substrate.
Fig. 6. Distribution of efficiency for 1 cm 2 cells formed on 40 cm × 40 cm film-substrate.
deposited by the multi-chamber system. The full specification SR apparatus under construction will have a multi-reactor system, Thus we will improve the efficiency in the near future.
side and back-side electrode were scribed by the roll-to-roll laser-scriber. The fabricated SCAF cells had an aperture-area of 980 cm 2 and the conversion efficiency in aperture-area was 7.2%. This value is not so low if we take into account that the n - i - p junction was made by single reactor.
4.2. Preliminar
results
On the basis of the above results, we tried to fabricate a SCAF cell with a double-junction structure. In this fabrication, all the thin film layers such as metals, a-Si, and ITO were deposited by using the prototype SR-apparatus. Fig. 6 shows the spatial distribution of efficiency for 1 c m 2 tandem cells on the effective deposition area. The efficiencies were not high, but the uniformity was relatively good. This low efficiency seems to come from phosphorus contamination because the prototype SR apparatus has only a reactor for plasma CVD. After all the film deposition processes were completed, both the cell
5. H o w high efficiency can we achieve with S C A F structure a-Si solar cells?
The first question we had at the beginning stage of development was whether the efficiency of the film-substrate cell can reach the same level as that of the glass substrate cells or not. To answer the question, we studied the device structure and fabrication conditions for the substrate type double-junction solar cell with a conventional multi-chamber deposition system named IVE apparatus [4]. Fig. 7 summa-
~)ptimum design of thicknes~ ~Iigh quality a-SiO:I~
"~
Top Cell
~-SiO:H p/i interface l a y e t ~ ide gap a-Si:H (>l.8eV) deposited] m highly H 2 diluted Sill4 I
Bottom Cell ~Iigh quality a-SiO:HI- ~ ~a-SiO:H p/i interface layet~J ~ . 7
~arrow g a ~ [at higher substrate temperature ~Iigh reflectivity and
/ [" 7
l
/
~ptimum textured morphology[
/
Fig. 7. Structure of film-substrate double-junction solar cell and applied techniques for improving efficiency.
1086
Y. lchikawa et al. / Journal of Non-Crystalline Solids 198-200 (1996) 1081-1086
rizes the techniques applied to the solar cell. Adopting this structure, we have obtained a conversion efficiency of 11.1% for a 1 cm 2 solar cell. This value is less than that of our superstrate type glass substrate cells; the best data of our glass substrate double junction cells is about 12%. W e believe, however, there is room for further improvement in the film-substrate cell since the optical confinement by textured electrode has not been optimized yet. With regard to the efficiency, therefore, we conclude that the film-substrate solar cell has the same potential as the glass one. The second question is whether the SR system really works or not when it is expanded to a multireactor system and for larger cell size. To confirm that, we have been developing a new SR apparatus composed o f 6 plasma CVD reactors and 2 sputtering reactors with an effective deposition area o f 40 cm × 80 c m for each reactor as shown in Fig. 8. W e have started preliminary fabrication by this apparatus, and already confirmed that the S C A F structure Unwinder
Winder I '°
°'1
\ / Plasma CVD reactors
Sputtering reactors
Fig. 8. Schematic diagram of full specification stepping roll fihn deposition apparatus.
double-junction cell of 40 cm × 80 cm is formed with an efficiency of about 7%. After completing the apparatus, we expect that the circumstance where 11.1% was obtained in small size cell will be realized. According to our simulation based on the I - V characteristics of the small size cell, we estimate that an aperture-area efficiency of 10% or higher is achieved in 40 cm × 80 cm S C A F solar cells.
Acknowledgements
This work was supported by the New Energy and Industrial Technology Development Organization under the New Sunshine Program o f the Ministry of International Trade and Industry.
References
[1] K. Nakatani, K. Suzuki and H. Okaniwa, Tech. Digest Photovoltaic Sci. and Eng. Conf., Tokyo (1987) p. 391. [2] Y. Ichikawa, K. Tabuchi, S. Kato, A. Takano, T. Sasaki, M. Tanda, S. Saito, H. Sato, S. Fujikake, T. Yoshida and H. Sakai, Proc. I st World Conf. Photovoltaic Energy Conversion, Hawaii (1994) p. 441. [3] S. Fujikake, K. Tabuchi, T. Yoshida, Y. Ichikawa and H. Sakai, Proc. Mater. Res. Soc. 377 (1995) 609. [4] Y. Ichikawa, S. Fujikake, T. Takayama, S. Saito, H. Ota, T. Yoshida, T. Ihara and H. Sakai, Proc. 23rd IEEE Photovoltaic Specialists Conf., LouisVille (1993) p. 27.
ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 1081 - 1086
Film-substrate a-Si solar cells with a n e w monolithic series-connected structure Yukimi Ichikawa *, Katuya Tabuchi, Akihiro Takano, Shinji Fujikake, Takashi Yoshida, Hiroshi Sakai Fuji Electric" Corporate Research and Del,elopment, Ltd., 2-2-1, Nagasaka, Yokosuka, Kanagawa 240-01, Japan
Abstract An amorphous silicon solar cell deposited on a plastic film-substrate has been studied. The solar cell has a new monolithic series-connected structure named 'SCAF' and was fabricated by a newly developed fabrication process based on the 'stepping-roll' film deposition system. The details of the new process are presented and the advantage of that is discussed. To fabricate the SCAF solar cells, we started developing a prototype line and obtained preliminary results for small- and large-area SCAF solar cells. The results show that the SCAF structure worked well with the new process.
1. Introduction
The performance of hydrogenated amorphous silicon (a-Si:H) based large-area solar cells has improved remarkably last decade, and been reaching a practical application level. The next stage required for R and D on a-Si solar cells is development of mass production technology and application-intensive product. In principle, the a-Si solar cell has an advantage in production cost compared with crystalline silicon (c-Si) based solar cells. To make this advantage real, however, development in the production technology is not avoidable. Another concern is application-intensive products. We have to admit that the performance of a-Si solar cells is inferior to that of c-Si, but they have several superior features; they are large-area thin film devices and are basically suitable
Corresponding author. Tel.: +81-468 57 6730; fax: +81468-57 2791; e-mail: [email protected].
for mass production. In addition, Various materials can be used for the substrate. Using plastic film or stainless steel film for that, we are able to fabricate flexible a-Si solar cells. They are very light and easy to use and are expected to open a door for new applications in the residential and building fields. From Ihe above viewpoints, we concluded that flexible a-Si solar cells formed on plastic film have high potential in both mass production and applications, and started developing plastic film-substrate solar cells in 1992. We considered that there are two major advantages in the plastic film solar cell; one is that roll-to-roll processing is applicable, and the other is that a large-area device with a monolithic series-connected structure is fabricated. The former makes mass production very easy, and the latter enables us to obtain a desirable voltage for applications if we could find a proper way. However, we face a number of difficulties if we simply apply conventional technologies to the fabrication of flexible a-Si solar cells.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 0 4 9 - X
1082
Y. lchikawa et al. / Journal of Non-Crystalline Solids 198-200 (1996) 1081-1086
To overcome these difficulties and to fabricate the practical flexible solar cells efficiently, we proposed new technologies in both fabrication process and series-connected structure. In the following sections, we will present the basic concept and its advantage as well as the present status of research and development of the film-substrate solar cells.
2. What is the best structure for film-substrate a-Si solar cells?
Series-connection holes
Transp~
Laserq for unit cell separation
D,tt,r~-~uc c~ct,uoue
F i g . 1. S c h e m a t i c d i a g r a m o f S C A F structure.
The minimum requirements we made in the development of the film-substrate solar cell were as follows: (1) A monolithic series-connected structure can be formed by a completely dry process. (2) A roll-to-roll system can be applied to all the fabrication processes. What we tried first was to replace the substrate simply from glass to transparent plastic film such as PET and PEN in the conventional monolithic structure, and we succeeded in obtaining an aperture-area efficiency of about 6% in a 94 cm 2 cell. However, we did not employ this structure because we were not able to succeed in applying a laser-scribing process to the metal electrode patterning; we used a wet etching process for that. This technique means the above requirements are not satisfied. To overcome this following, we had to find a new technology. An idea for the monolithic structure was incubated by using a property of plastic film; it is easy to make holes or cut the plastic film. The conceptual structure of the new monolithic structure is shown in Fig. 1. The structure was named as ' S C A F (series-connection through apertures formed on film)'. In this structure, a substrate type a-Si:H based solar cell is fabricated on one surface of the plastic film; the cell is composed of metal electrode, a-Si:H based layers, and transparent electrode stacked in the order on the substrate. The plastic films used were heat-resisting plastics such as poly-imide with a thickness of tens of micrometers. The remarkable feature of the S C A F structure is a number of apertures made on the substrate. These apertures are divided into two groups and have different functions. One function is through-hole contact between the transparent electrode and the back-
side electrode formed on the other surface of the substrate; they are named current-collection holes and distributed in the active-area region, the transparent electrode region. The other is for the connection between the backside electrode and the metal electrode; they are named series-connection holes and are on both sides out of the transparent electrode region. As shown in Fig. 1, the S C A F solar cell consists of a number of divided unit cells. The current generated from the unit cell has to flow in the transparent electrode having relatively large sheet resistance. To avoid power loss there, the current-collection holes collect the current generated near them and transport it to the backside electrode having low sheet resistance. The current collected into the backside electrode flows to both sides with little joule loss, and then moves into the metal electrode of the next unit cell through the series-connection holes. This is the concept of the S C A F structure. The most important advantages of this structure are as follows: (1) Both surfaces of the plastic film are covered with metal before fabricating a-Si layers: thus we can avoid the influence of degassing and diffusion of impurities from the film. (2) The laser-scribing process is needed only once after all the film deposition processes are completed; thus a-Si layers and electrodes can be formed in successive processes. This sequence is very important to reduce not only the number of fabrication steps and but also the short-circuit defects. In addition, for patterning processes, selective scribing and
E lchikawa et al./Journal of Non-Crystalline Solids 198-200 (1996) 1081-1086
precise position control of the laser beam are not required. (3) To make the through-hole contact, no additional process is needed; it is automatically made by depositing most of layers in a proper order.
3. What is the best process for SCAF structure a-Si solar cells?
3.1. New roll-to-roll system A roll-to-roll system is a very attractive method for the process for flexible a-Si solar cells. The conventional roll-to-roll system, however, has several problems for such applications [ 1]. It is excellent for mass production, but isolation among reactors is not perfect in an in-line multi-reactor system. As a result, interdiffusion of material gases is not avoidable, and also the gas pressures and the deposition rates are not chosen independently among the reactors. These difficulties become barriers to improvement of solar cell efficiency. What we proposed to solve these difficulties is a combination of the conventional roll-to-roll system and a batch system named 'stepping-roll (SR)' system. Fig. 2 shows the schematic diagram of the SR system. The basic configuration of the apparatus consists of a common chamber and several film deposition reactors in it. In this example, all the reactors are for plasma CVD. Each reactor has a movable grounded electrode (GE). When the GEs are at down position and press the chambers, the reactors are completely isolated from the common chamber by holding the substrate film between Orings hooked up on the edges of the GE and the reactor. On the other hand, when the GEs are at Film
Grounded Electrode Common Chamber
Unwii!:fllRo I~er~~ ~iowlrll ~:ectWri~! ingRoller Fig.2.Basicconceptofsteppingrollfilmdepositionapparatus. Dep. Chamber
1083
upper position, the film-substrate is released and can be wound up as the conventional roll-to-roll system. Using this system, we can deposit various film layers simultaneously according to the following sequence: (1) Pressing down the GEs to isolate the reactors from the common chamber. (2) Film deposition. (3) Moving up the GEs. (4) Winding up film by one flame (reactor length of film). As one can understand easily, the SR system has the following advantages compared with the conventional roll-to-roll system: (1) Interdiffusion of raw material gases among reactors is completely prevented. (2) The configuration of the reactors is compact and simple because no intricate mechanisms for transport of the substrate are required. (3) Precise control of the composition in the depth direction can be made. (4) Deposition conditions in each reactor can be chosen independently from those of other reactors. Thus, not only are all the technologies developed for high efficiency glass substrate cells applicable, but also the SR system can have plasma CVD reactors and sputtering reactors in a single line. This feature of the SR system is very convenient for the SCAF structure solar cell as described in the following sections.
3.2. Process flow for SCAF solar cells The SCAF cell is fabricated by a process based on roll-to-roll systems as shown in Fig. 3. First, apertures are made on the plastic film by a punching unit and then the substrate is cleaned by a cleaning unit in a pretreatment apparatus. After that, the roll is transferred to a roll-to-roll sputtering apparatus to deposit metal electrodes. Then the pretreated filmsubstrate roll is set in the SR apparatus composed of plasma CVD reactors and sputtering reactors to form the a-Si layers, the transparent electrode and backside electrode. Finally, the layers formed on both surfaces of the film are laser-scribed to make unit cells by a roll-to-roll laser-scriber. The fabrication process for the SCAF solar cell presented here is simplified further compared with that for the glass substrate solar cells. Moreover
1084
Y.lchikawa et al. /
Journal of Non-Crystalline Solids 198-200 (1996) 1081-1086
Punchingunit Cleaningunit
Target/
I
I o o o o ol
Cleaning & making apertures (Pretreatment apparatus)
Metal electrode formation (Roll-to-roll sputtering apparatus)
ol I
Separation to unit cells (Roll-to-rolllaser-scriber)
Sputteringreactors 1
Deposition of a-Si layers, transparent electrode and back-side electrode (Stepping-roll apparatus)
Fig. 3. Fabricationprocessflowfor SCAFsolarcells. since all the process units are roll-to-roll systems, an automated line will be easily constructed.
4. Fabrication of SCAF solar cells
4.1. Prototype SR apparatus To make the S C A F solar cell with the roll-to-roll process described in the previous sections, we started developing a prototype line. The critical part of the technology is the SR apparatus. W e first constructed a simplified prototype SR apparatus to study whether the fundamental functions work as expected. The prototype SR apparatus has two deposition chambers as shown in Fig. 4; one is for plasma-CVD and the other is for sputtering. Both chambers have an effective deposition area of 40 cm × 40 cm, in which a-Si:H or an electrode is deposited on plastic film-
Film substrate Unwinder /
!
Winder
W"
Common Chamber
Fig. 4. Schematicdiagramof prototypesteppingroll deposition apparatus.
substrates with a width of 50 cm. The a-Si films and ITO for the transparent electrode deposited by this apparatus demonstrated good uniformities and qualities in thickness and optoelectrical properties over a wide area as described in Refs. [2,3]. Large-area devices were studied with the prototype SR apparatus. As already mentioned, this apparatus has only one reactor for a-Si film deposition. Thus, with this apparatus, we do not expect high efficiencies, but can study the process technology for the S C A F solar cell and fundamental structure of the SR apparatus for designing a full specification apparatus. Generation of short-circuit points due to pin holes of the a-Si layer was one of the typical defects for glass substrate cells. In the beginning, we were concerned about pinhole shorts in the S C A F cells generated through roll-to-roll conveyance. Hence we fabricated 64 1 cm 2 single-junction solar cells on an area of 40 cm × 40 cm area using the prototype SR apparatus. Fig. 5 shows the histogram of open-circuit voltages, Voc, of 512 cells; we fabricated 8 samples and the summation of them is shown. As seen from the figure, 99% of the cells exceed 0.85 V. This means that the number of short-circuit points is negligibly small. From the above results, we concluded that largearea a-Si solar cells can be formed on plastic filmsubstrate by the SR apparatus, though the efficiencies of the small area cells are lower than those
E lchikawa et al./Journal ofNon-Co, stalline Solids 198-200 (1996) 1081-1086
99% 500
' ' ' i ! i ~
'
'
L
' !i
i~
1 0
.
3 . . . . i !
' }
. . . .
8~,
0
,,~ 300
i
. . . .
i
iE
J
. . . .
.......... Horizontal .....
Diagonal
4 ............
| I |
I
ILl
E 0
. . . .
6 "
O
"~
.~ 200
~
1085
='i
~ ,
0.55 0.70
~
J,i
~ ~..IIR
0.75 0.80 voc
0.85 0.90
=
0.95
=
1.0
0 20 0
(v)
, .... -" 1 ' 0 0 0 .... 100 P o s i t i o n (ram)
200
Fig. 5. Histogram of open-circuit voltage, Voc, of 512 cells formed on 40 cm × 40 cm substrate.
Fig. 6. Distribution of efficiency for 1 cm 2 cells formed on 40 cm × 40 cm film-substrate.
deposited by the multi-chamber system. The full specification SR apparatus under construction will have a multi-reactor system, Thus we will improve the efficiency in the near future.
side and back-side electrode were scribed by the roll-to-roll laser-scriber. The fabricated SCAF cells had an aperture-area of 980 cm 2 and the conversion efficiency in aperture-area was 7.2%. This value is not so low if we take into account that the n - i - p junction was made by single reactor.
4.2. Preliminar
results
On the basis of the above results, we tried to fabricate a SCAF cell with a double-junction structure. In this fabrication, all the thin film layers such as metals, a-Si, and ITO were deposited by using the prototype SR-apparatus. Fig. 6 shows the spatial distribution of efficiency for 1 c m 2 tandem cells on the effective deposition area. The efficiencies were not high, but the uniformity was relatively good. This low efficiency seems to come from phosphorus contamination because the prototype SR apparatus has only a reactor for plasma CVD. After all the film deposition processes were completed, both the cell
5. H o w high efficiency can we achieve with S C A F structure a-Si solar cells?
The first question we had at the beginning stage of development was whether the efficiency of the film-substrate cell can reach the same level as that of the glass substrate cells or not. To answer the question, we studied the device structure and fabrication conditions for the substrate type double-junction solar cell with a conventional multi-chamber deposition system named IVE apparatus [4]. Fig. 7 summa-
~)ptimum design of thicknes~ ~Iigh quality a-SiO:I~
"~
Top Cell
~-SiO:H p/i interface l a y e t ~ ide gap a-Si:H (>l.8eV) deposited] m highly H 2 diluted Sill4 I
Bottom Cell ~Iigh quality a-SiO:HI- ~ ~a-SiO:H p/i interface layet~J ~ . 7
~arrow g a ~ [at higher substrate temperature ~Iigh reflectivity and
/ [" 7
l
/
~ptimum textured morphology[
/
Fig. 7. Structure of film-substrate double-junction solar cell and applied techniques for improving efficiency.
1086
Y. lchikawa et al. / Journal of Non-Crystalline Solids 198-200 (1996) 1081-1086
rizes the techniques applied to the solar cell. Adopting this structure, we have obtained a conversion efficiency of 11.1% for a 1 cm 2 solar cell. This value is less than that of our superstrate type glass substrate cells; the best data of our glass substrate double junction cells is about 12%. W e believe, however, there is room for further improvement in the film-substrate cell since the optical confinement by textured electrode has not been optimized yet. With regard to the efficiency, therefore, we conclude that the film-substrate solar cell has the same potential as the glass one. The second question is whether the SR system really works or not when it is expanded to a multireactor system and for larger cell size. To confirm that, we have been developing a new SR apparatus composed o f 6 plasma CVD reactors and 2 sputtering reactors with an effective deposition area o f 40 cm × 80 c m for each reactor as shown in Fig. 8. W e have started preliminary fabrication by this apparatus, and already confirmed that the S C A F structure Unwinder
Winder I '°
°'1
\ / Plasma CVD reactors
Sputtering reactors
Fig. 8. Schematic diagram of full specification stepping roll fihn deposition apparatus.
double-junction cell of 40 cm × 80 cm is formed with an efficiency of about 7%. After completing the apparatus, we expect that the circumstance where 11.1% was obtained in small size cell will be realized. According to our simulation based on the I - V characteristics of the small size cell, we estimate that an aperture-area efficiency of 10% or higher is achieved in 40 cm × 80 cm S C A F solar cells.
Acknowledgements
This work was supported by the New Energy and Industrial Technology Development Organization under the New Sunshine Program o f the Ministry of International Trade and Industry.
References
[1] K. Nakatani, K. Suzuki and H. Okaniwa, Tech. Digest Photovoltaic Sci. and Eng. Conf., Tokyo (1987) p. 391. [2] Y. Ichikawa, K. Tabuchi, S. Kato, A. Takano, T. Sasaki, M. Tanda, S. Saito, H. Sato, S. Fujikake, T. Yoshida and H. Sakai, Proc. I st World Conf. Photovoltaic Energy Conversion, Hawaii (1994) p. 441. [3] S. Fujikake, K. Tabuchi, T. Yoshida, Y. Ichikawa and H. Sakai, Proc. Mater. Res. Soc. 377 (1995) 609. [4] Y. Ichikawa, S. Fujikake, T. Takayama, S. Saito, H. Ota, T. Yoshida, T. Ihara and H. Sakai, Proc. 23rd IEEE Photovoltaic Specialists Conf., LouisVille (1993) p. 27.