- Email: [email protected]
Thin film Si solar cell fabricated at low temperature
Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
www.elsevier.com/locate/jnoncrysol
Section 22. Solar cells
Thin ®lm Si solar cell fabricated at low temperature Kenji Yamamoto *, Masashi Yoshimi, Yuko Tawada, Yoshifumi Okamoto, Akihiko Nakajima Central Research Laboratories, Kaneka Corporation, 2-80 Yoshida-cho, Hyogo-ku, Kobe 652, Japan
Abstract Research and development of our ®lm Si solar cells are reviewed. Our developed ®lm polycrystalline Si (poly-Si) cells are well described by the structure of natural surface texture and enhanced absorption with a back re¯ector (STAR), where the active poly-Si layer is fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 lm demonstrated an intrinsic eciency of 10.7% (aperture 10.1%). By combining poly-Si cell with an a-Si cell, a stabilized eciency of 12% has been reached for a-Si:H/poly-Si/poly-Si cell structure. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The use of silicon ®lms for solar cells is one of the most promising approaches to realize both better performance and lower cost due to material cost and ease of manufacturing. For a-Si solar module, advances made in the laboratory have been transferred to manufacturing operations. We developed stable 8% of a-Si single junction monolithic solar-module sized 910 455 mm2 in our pilot-plant. An estimated 20 MW of annual manufacturing capacity will become available from autumn, 1999. As a next generation of thin ®lm Si solar cell, we have engaged in thin ®lm polycrystalline Si (polySi) and a-Si/poly-Si stacked solar cell. Progress of eciency for KanekaÕs poly-Si thin ®lm solar cell [1±11] are shown in Fig. 1.
*
Corresponding author. Tel.: +81-78 652 4055; fax: +81-78 652 4081. E-mail address: [email protected] (K. Yamamoto).
In the FY of 1997, a signi®cant progress was made in thin ®lm poly-Si solar cell on glass substrate fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 lm, reached an eciency of 10% which was independently con®rmed (Japan Quality Assurance [11]). Now we consider the fabrication of a-Si/poly-Si stacked cell for obtaining a greater eciency and production yield. In this paper, the performances of poly-Si and a-Si/poly-Si solar cells which include I(V), temperature dependence and light induced stability of cells are reported and we discuss key factors for achieving greater cell eciency, especially by increasing the open circuit voltage, Voc , in real polySi cells. 2. Experimental A summary of the cell fabrication process follows. After the formation of the back re¯ector on a glass substrate, a n-type Si ®lm was deposited on it by plasma CVD. Next, intrinsic (i) poly-Si ®lm,
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 9 0 7 - 2
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
Fig. 1. Progress of eciency for KanekaÕs poly-Si thin ®lm solar cell and targets.
acting as an active layer, was deposited on the ntype layer by plasma CVD. The junction was formed by the deposition of p-type Si ®lms. Indium tin oxide (ITO) was deposited on the solar cell as a transparent conductive electrode. An Ag grid electrode was formed on the top. All fabrication processes were carried out with a maximum process temperature of 550°C. It should be noted that the Ôintrinsic (i-)Õ means that i-layer Si is fabricated by plasma CVD without any intentional doping. However, we believe that the i-layer is n-type since it is deposited on the n-type layer and, also, we assume the impurities incorporated into i-poly-Si are oxygen [12]. The depth pro®le of phosphorus concentration of a 4 lm thick poly-Si was investigated by secondary ion mass spectroscopy (SIMS). The detection limit of phosphorus concentration is around 2 1015 =cm3 .
1083
creased absorption with a back re¯ector. One of the properties of this cell is its natural surface texture. An AFM image of the top surface of our poly-Si solar cell with STAR structure shows dendrite-like morphology with a surface roughness of the order of 0.12 lm for an 4 lm thick cell. Another property of our poly-Si is the columnar structure and (1 1 0) preferred orientation as determined by X-ray diraction. Fig. 2(b) shows the schematic view of our second generation of cell with ÔSTAR structureÕ, where a textured back re¯ector is used. This rough back re¯ector is useful for thinner cells. Since the feature size of natural surface texture depends on the thickness of the cell, it is not enough to have good light trapping for thinner cell such as 2 lm. The 2.0 lm thick cell with this ÔSTAR structureÕ demonstrated an intrinsic eciency of 10.7 0.5% (aperture 10.1 0.5%), an open circuit voltage (Voc ) of 0.539 0.005 V and short current density (Jsc ) of 25.8 0.5 mA/cm2 as independently con®rmed (Japan Quality Assurance). This Jsc was obtained from the 2 lm thick cell. However, Voc was ®xed at 539 mV as compared to Voc for single crystal Si cells. To date the largest measured Voc for a poly-Si cell is 549 mV with a larger ®ll factor for a 1.2 lm thick cell.
3. Results and discussions 3.1. The performance of poly-Si solar cell with STAR structure Fig. 2(a) shows the schematic view of our ®rst generation of poly-Si cell with ÔSTAR structureÕ, which means that the cell structure and properties are aected by the natural surface texture and in-
Fig. 2. Schematic view of our proposed thin ®lm poly-Si solar cell with STAR (naturally Surface Texture and enhanced Absorption with back Re¯ector) structure. (a ) First generation of poly-Si cell with ¯at back re¯ector. (b) Second generation of poly-Si cell with rough back re¯ector for thinner cell.
1084
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
3.2. Light trapping of our STAR structure at long wavelength A key issue for real poly-Si solar cell design is to achieve greater eciency by increasing optical absorption since it enables the short circuit current density to be suciently large even in a few microns thick poly-Si solar cell. We have reported on a demonstration of increase of optical absorption of poly-Si solar cell [1±11]. We now consider how to get a greater sensitivity at longer wavelengths for the application of stacked cells. The quantum eciency curves of the 1.5 lm thick cells with ¯at and rough back re¯ectors, investigated in previous paper [11], are shown in Fig. 3. Note that the Jsc of the cell with ¯at and rough surface calculated from these curves are 18.5 and 22.1 mA/cm2 , respectively. The interference eect was still observed for the 1.5 lm thick cell. However, by both optimizing the texture size of back re¯ector and increasing the thickness to 2.1 lm, we have been able to get a larger quantum eciency at longer wavelengths, which is also shown in Fig. 3 for the improved texture type. This new texture was prepared by etching after the formation of back re¯ector. There is no interference eect detected. Note that the
Fig. 3. Quantum eciency curves of the 2.1 lm thick cell with a new textured back re¯ector (described by the improved type texture in this ®gure) together with the previous 1.5 lm thick cell on smooth and rough back re¯ector (the solid line and dotted line correspond to the rough and smooth back re¯ector, respectively). Note that the quantum eciency at 800 nm reached more than 60% for the 2.1 lm thick cell.
quantum eciency at 800 nm was >60% for a 2.1 lm thick cell. This quantum eciency at longer wavelength is appreciable for the a-Si/poly-Si stacked cell. 3.3. Temperature dependence of poly-Si cell Temperature coecient data are also important for practical terrestrial cells. The cell temperature was determined by a thermocouple attached directly to the cell. Temperature variation during the I V scan was less than 0.2°C. The temperature dependence of normalized cell performance and the temperature coecients are shown for two kind of cells in Fig. 4. A dierent temperature coecient was observed for poly-Si-1 (2 lm, Voc 0:515 V) and poly-Si-2 (3.5 lm, Voc 0:483). It is not known if this dierence is due to the
Fig. 4. The temperature dependence of three kinds of normalized cell performance together with the temperature coecients for three types of cells. The thickness of poly-Si-1 and poly-Si-2 corresponds to 2.0 and 3.5 lm, respectively. Hybrid cell shown in this ®gure is a a-Si/poly-Si two stack cell. Table in the inset of this ®gure shows the Voc and temperature coecient of each cell. Lines are drawn as guides for the eye.
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
thickness dierence (or Voc dierence). Further systematic experiments will be needed. The Voc dependence on temperature was investigated, which is shown in Fig. 5. The Voc curve has been extrapolated to T 0 K. Obtained Voc of 1.08 0.05 V is less than that of single crystalline Si. This dierence means that the defect connected states of band edge exist in poly-Si prepared at low temperature. This dierence also indicates a greater sensitivity of the quantum eciency curve observed at the wavelength of 1000±1200 nm, where the defect connected absorption near the band edge is observed [9,13]. 3.4. How to increase Voc It is an issue how to increase Voc . As already mentioned, the larger Jsc was obtained from the 2 lm thick cell, while Voc was ®xed at 539 mV. Largest obtained Voc for a poly-Si cell is 549 mV. It is small compared to Voc of a single crystal Si cell.
Fig. 5. The Voc dependence on temperature. Open circle and triangle show the poly-Si1 and poly-Si2, respectively. The Voc curve has been extrapolated to T 0 K and a value of 1.08 V is obtained for both cases.
1085
For a ®rst analysis of the changes in Voc , the thickness dependence on Voc is investigated, which is shown in Fig. 6 together with the calculated results by PC1D [14]. Voc increases with decreasing thickness. The Voc reduction with increasing thickness seen in Fig. 3 is assumed to be explained by the reduction of the total number of defects due to the volume reduction of the active layer. Experimental results are evaluated by PC1D analysis. These are well ®tted by the curve calculated by PC1D, where the diusion length (L) is 7 lm, and the carrier concentration of 2 1015 =cm3 . The calculated Voc for two kinds of carrier concentration shows a decrease of Voc with decreasing carrier concentration, for the same diusion length of each solar cell. It is important to know the carrier concentration and the minority carrier life time (diusion length) of the i-layer for further analysis of our low-temperature poly-Si cell. The carrier concentration has not been directly determined yet. On the other hand, phosphorous would be an appropriate candidate for doping of the i-layer since it is deposited on the n-type layer. It would
Fig. 6. The Voc dependence on thickness. Experimental results are plotted with open circles. The three curves (broken line, dotted line, solid line) are the calculated curves for qfi 0%, 70%, and 92%, respectively, based on PC1D for a constant diusion length (L) of 7 lm for two kinds of carrier concentration (Ni) shown in the inset of this ®gure, respectively. Upper lines and lower lines show the calculated curves for 1 1015 =cm3 and 2 1015 =cm3 , respectively. qfi is de®ned as the internal global re¯ectance at the front surface in PC1D [13].
1086
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
be important to measure the phosphorus concentration. The resulting depth pro®le of phosphorous concentration in 4 lm thick poly-Si was investigated by SIMS, which is shown in Fig. 7. A phosphorous concentration of around 5 ´ 1015 /cm3 was observed in the bulk of poly-Si. This concentration is sucient to explain a carrier concentration of 2 1015 =cm3 calculated from PC1D analysis shown in Fig 6. 3.5. Poly-Si fabrication process issues We are now going to develop the poly-Si and aSi/poly-Si stacked monolithic solar-module sized 910 455 mm2 like the a-Si module which has already been developed in our pilot-plant. There are several key factors for our poly-Si cell to be cost eective and acceptable for large-scale terrestrial applications. Among them, process related key issues are: how to get (i) greater growth rate, (ii) large area (>10 cm2 ) and uniform deposition at the same time and (iii) monolithic series interconnection. First concerning the deposition rate, we have reached 1.5 nm/s, while getting device quality poly-Si ®lm. With this growth rate a poly-Si cell in production would be produced approximately ev-
Fig. 7. The depth pro®le of phosphorus (P) concentration for 4 lm thick poly-Si was investigated by SIMS. Note that the detection limit of P by SIMS is around 1±2 1015 =cm3 in our experimental set-up.
ery 5 min. Together with the eort increase the light trapping for a cell 1 lm thick the deposition rate will have to be increased up to 2 nm/s. Second concerning the uniformity of cell eciency, the average eciency of 10% for poly-Si cell has been achieved over a substrate size of 100 100 mm2 , where the cell separation was performed by laser scribing. The uniform deposition by plasma CVD over the size of 300 400 mm2 has already been achieved with thickness deviations within 5%. Finally concerning the monolithic series interconnection, we have prepared 5 5 cm2 , 10 segments monolithically series interconnected a-Si/ poly-Si mini module, which has an initial eciency of 11.3%, Voc of 13.5 V, a short current of 30.4 mA, ®ll factor of 68.8%. 3.6. Application of a-Si/poly-Si hybrid (stacked) solar cell The advantage of a larger Jsc for our poly-Si solar cell with STAR structure was applied to the stacked cell with the combination of a-Si cell. In applying the a-Si cell to the stacked cell we must pay attention to the stabilized eciency, since a-Si photo-degrades, while a poly-Si cell is stable. We have prepared a stacked cell of a-Si:H/poly-Si/ poly-Si (triple), which will be less sensitive to
Fig. 8. The stabilized performance of the a-Si/poly-Si/poly-Si 3-stacked hybrid solar cell after 550 h indoor light soaking.
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
1087
Acknowledgements This work was supported by the NEDO as a part of the New Sunshine Program under MITI. References
Fig. 9. Light-induced changes in eciency for a-Si/poly-Si/ poly-Si 3-stacked hybrid solar cell under AM1.5, 100 mW/cm2 , 48°C, and open-circuit conditions. The line is drawn as a guide for the eye.
degradation by using the thinner a-Si. We have investigated the stability of a-Si:H/poly-Si/poly-Si (triple) cell. These devices were subjected to indoor light soaking under AM1.5, 100 mW/cm2 , 48°C, and open-circuit conditions. The triple cell showed a stabilized eciency of 12% as shown in Fig. 8. Note that a stabilized FF of 76.2% was also obtained. Fig. 9 shows the change in the device eciency during light soaking under the above conditions, where a much smaller photodegradation is observed for our 3-stacked cell.
4. Conclusions The a-Si:H/poly-Si/poly-Si stacked cell showed a stabilized eciency of 12%. The temperature coecient of 0.268% is as small as that of a-Si cell, which is promising result in view of the terrestrial applications of stacked cells.
[1] K. Yamamoto, A. Nakajima, T. Suzuki, M. Yoshimi, H. Nishio, M. Izumina, Jpn. J. Appl. Phys. 33 (1994) L1751. [2] K. Yamamoto, A. Nakajima, T. Suzuki, M. Yoshimi, H. Nishio, M. Izumina, in: Proceedings of the First World Conference on Photovoltaic Energy Conversion 1994, IEEE, Piscataway, 1994, p. 1573. [3] A. Nakajima, T. Suzuki, M. Yoshimi, K. Yamamoto, in: Proceedings of the 13th European Photovoltaic Solar Energy Conference 1995, H.S. Stephens & Associates, UK, 1995, p. 1550. [4] K. Yamamoto, T. Suzuki, M. Yoshimi, A. Nakajima in: Proceedings of the 25th IEEE Photovoltaic Energy Conversion 1996, IEEE, Piscataway, 1996, p. 661. [5] K. Yamamoto, A. Nakajima, T. Suzuki, M. Yoshimi, Jpn. J. Appl. Phys. 36 (1997) L569. [6] K. Yamamoto, A. Nakajima, T. Suzuki, M. Yoshimi, in: Proceedings of the 14th European Photovoltaic Solar Energy Conference, 1997. [7] K. Yamamoto, T. Suzuki, K. Kondo, T. Okamoto, M. Yamaguchi, M. Izumina, Y. Tawada, Solar Energy Mater. Solar Cells 34 (1994) 501. [8] K. Yamamoto, T. Suzuki, M. Yoshimi, A. Nakajima, in: Proceedings of the 14th European Photovoltaic Solar Energy Conference, 1997, p. 1018. [9] K. Yamamoto, T. Suzuki, M. Yoshimi, A. Nakajima, in: Proceedings of 26th IEEE Photovoltaic Energy Conversion 1996, IEEE, Piscataway, 1998, p. 575. [10] K. Yamamoto, T. Suzuki, M. Yoshimi, A. Nakajima, in: Proceedings of 98 MRS Spring Meeting, 507, 1998, p. 131. [11] K. Yamamoto, A. Nakajima, Y. Tawada, M. Yoshimi, Y. Okamoto, S. Igari, in: Proceedings of the Second World Conference, Photovoltaic Energy Conversion, 1998, p. 1284. [12] J. Meier, R. Flueckiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65 (1994) 860. [13] A. Poruba et al., in: Proceedings of the 14th European Photovoltaic Solar Energy Conference, 1997, p. 2105. [14] P.A. Basore, IEEE Trans. Electron. Dev., ED-37 (1990) 337.
www.elsevier.com/locate/jnoncrysol
Section 22. Solar cells
Thin ®lm Si solar cell fabricated at low temperature Kenji Yamamoto *, Masashi Yoshimi, Yuko Tawada, Yoshifumi Okamoto, Akihiko Nakajima Central Research Laboratories, Kaneka Corporation, 2-80 Yoshida-cho, Hyogo-ku, Kobe 652, Japan
Abstract Research and development of our ®lm Si solar cells are reviewed. Our developed ®lm polycrystalline Si (poly-Si) cells are well described by the structure of natural surface texture and enhanced absorption with a back re¯ector (STAR), where the active poly-Si layer is fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 lm demonstrated an intrinsic eciency of 10.7% (aperture 10.1%). By combining poly-Si cell with an a-Si cell, a stabilized eciency of 12% has been reached for a-Si:H/poly-Si/poly-Si cell structure. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction The use of silicon ®lms for solar cells is one of the most promising approaches to realize both better performance and lower cost due to material cost and ease of manufacturing. For a-Si solar module, advances made in the laboratory have been transferred to manufacturing operations. We developed stable 8% of a-Si single junction monolithic solar-module sized 910 455 mm2 in our pilot-plant. An estimated 20 MW of annual manufacturing capacity will become available from autumn, 1999. As a next generation of thin ®lm Si solar cell, we have engaged in thin ®lm polycrystalline Si (polySi) and a-Si/poly-Si stacked solar cell. Progress of eciency for KanekaÕs poly-Si thin ®lm solar cell [1±11] are shown in Fig. 1.
*
Corresponding author. Tel.: +81-78 652 4055; fax: +81-78 652 4081. E-mail address: [email protected] (K. Yamamoto).
In the FY of 1997, a signi®cant progress was made in thin ®lm poly-Si solar cell on glass substrate fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 lm, reached an eciency of 10% which was independently con®rmed (Japan Quality Assurance [11]). Now we consider the fabrication of a-Si/poly-Si stacked cell for obtaining a greater eciency and production yield. In this paper, the performances of poly-Si and a-Si/poly-Si solar cells which include I(V), temperature dependence and light induced stability of cells are reported and we discuss key factors for achieving greater cell eciency, especially by increasing the open circuit voltage, Voc , in real polySi cells. 2. Experimental A summary of the cell fabrication process follows. After the formation of the back re¯ector on a glass substrate, a n-type Si ®lm was deposited on it by plasma CVD. Next, intrinsic (i) poly-Si ®lm,
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 9 0 7 - 2
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
Fig. 1. Progress of eciency for KanekaÕs poly-Si thin ®lm solar cell and targets.
acting as an active layer, was deposited on the ntype layer by plasma CVD. The junction was formed by the deposition of p-type Si ®lms. Indium tin oxide (ITO) was deposited on the solar cell as a transparent conductive electrode. An Ag grid electrode was formed on the top. All fabrication processes were carried out with a maximum process temperature of 550°C. It should be noted that the Ôintrinsic (i-)Õ means that i-layer Si is fabricated by plasma CVD without any intentional doping. However, we believe that the i-layer is n-type since it is deposited on the n-type layer and, also, we assume the impurities incorporated into i-poly-Si are oxygen [12]. The depth pro®le of phosphorus concentration of a 4 lm thick poly-Si was investigated by secondary ion mass spectroscopy (SIMS). The detection limit of phosphorus concentration is around 2 1015 =cm3 .
1083
creased absorption with a back re¯ector. One of the properties of this cell is its natural surface texture. An AFM image of the top surface of our poly-Si solar cell with STAR structure shows dendrite-like morphology with a surface roughness of the order of 0.12 lm for an 4 lm thick cell. Another property of our poly-Si is the columnar structure and (1 1 0) preferred orientation as determined by X-ray diraction. Fig. 2(b) shows the schematic view of our second generation of cell with ÔSTAR structureÕ, where a textured back re¯ector is used. This rough back re¯ector is useful for thinner cells. Since the feature size of natural surface texture depends on the thickness of the cell, it is not enough to have good light trapping for thinner cell such as 2 lm. The 2.0 lm thick cell with this ÔSTAR structureÕ demonstrated an intrinsic eciency of 10.7 0.5% (aperture 10.1 0.5%), an open circuit voltage (Voc ) of 0.539 0.005 V and short current density (Jsc ) of 25.8 0.5 mA/cm2 as independently con®rmed (Japan Quality Assurance). This Jsc was obtained from the 2 lm thick cell. However, Voc was ®xed at 539 mV as compared to Voc for single crystal Si cells. To date the largest measured Voc for a poly-Si cell is 549 mV with a larger ®ll factor for a 1.2 lm thick cell.
3. Results and discussions 3.1. The performance of poly-Si solar cell with STAR structure Fig. 2(a) shows the schematic view of our ®rst generation of poly-Si cell with ÔSTAR structureÕ, which means that the cell structure and properties are aected by the natural surface texture and in-
Fig. 2. Schematic view of our proposed thin ®lm poly-Si solar cell with STAR (naturally Surface Texture and enhanced Absorption with back Re¯ector) structure. (a ) First generation of poly-Si cell with ¯at back re¯ector. (b) Second generation of poly-Si cell with rough back re¯ector for thinner cell.
1084
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
3.2. Light trapping of our STAR structure at long wavelength A key issue for real poly-Si solar cell design is to achieve greater eciency by increasing optical absorption since it enables the short circuit current density to be suciently large even in a few microns thick poly-Si solar cell. We have reported on a demonstration of increase of optical absorption of poly-Si solar cell [1±11]. We now consider how to get a greater sensitivity at longer wavelengths for the application of stacked cells. The quantum eciency curves of the 1.5 lm thick cells with ¯at and rough back re¯ectors, investigated in previous paper [11], are shown in Fig. 3. Note that the Jsc of the cell with ¯at and rough surface calculated from these curves are 18.5 and 22.1 mA/cm2 , respectively. The interference eect was still observed for the 1.5 lm thick cell. However, by both optimizing the texture size of back re¯ector and increasing the thickness to 2.1 lm, we have been able to get a larger quantum eciency at longer wavelengths, which is also shown in Fig. 3 for the improved texture type. This new texture was prepared by etching after the formation of back re¯ector. There is no interference eect detected. Note that the
Fig. 3. Quantum eciency curves of the 2.1 lm thick cell with a new textured back re¯ector (described by the improved type texture in this ®gure) together with the previous 1.5 lm thick cell on smooth and rough back re¯ector (the solid line and dotted line correspond to the rough and smooth back re¯ector, respectively). Note that the quantum eciency at 800 nm reached more than 60% for the 2.1 lm thick cell.
quantum eciency at 800 nm was >60% for a 2.1 lm thick cell. This quantum eciency at longer wavelength is appreciable for the a-Si/poly-Si stacked cell. 3.3. Temperature dependence of poly-Si cell Temperature coecient data are also important for practical terrestrial cells. The cell temperature was determined by a thermocouple attached directly to the cell. Temperature variation during the I V scan was less than 0.2°C. The temperature dependence of normalized cell performance and the temperature coecients are shown for two kind of cells in Fig. 4. A dierent temperature coecient was observed for poly-Si-1 (2 lm, Voc 0:515 V) and poly-Si-2 (3.5 lm, Voc 0:483). It is not known if this dierence is due to the
Fig. 4. The temperature dependence of three kinds of normalized cell performance together with the temperature coecients for three types of cells. The thickness of poly-Si-1 and poly-Si-2 corresponds to 2.0 and 3.5 lm, respectively. Hybrid cell shown in this ®gure is a a-Si/poly-Si two stack cell. Table in the inset of this ®gure shows the Voc and temperature coecient of each cell. Lines are drawn as guides for the eye.
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
thickness dierence (or Voc dierence). Further systematic experiments will be needed. The Voc dependence on temperature was investigated, which is shown in Fig. 5. The Voc curve has been extrapolated to T 0 K. Obtained Voc of 1.08 0.05 V is less than that of single crystalline Si. This dierence means that the defect connected states of band edge exist in poly-Si prepared at low temperature. This dierence also indicates a greater sensitivity of the quantum eciency curve observed at the wavelength of 1000±1200 nm, where the defect connected absorption near the band edge is observed [9,13]. 3.4. How to increase Voc It is an issue how to increase Voc . As already mentioned, the larger Jsc was obtained from the 2 lm thick cell, while Voc was ®xed at 539 mV. Largest obtained Voc for a poly-Si cell is 549 mV. It is small compared to Voc of a single crystal Si cell.
Fig. 5. The Voc dependence on temperature. Open circle and triangle show the poly-Si1 and poly-Si2, respectively. The Voc curve has been extrapolated to T 0 K and a value of 1.08 V is obtained for both cases.
1085
For a ®rst analysis of the changes in Voc , the thickness dependence on Voc is investigated, which is shown in Fig. 6 together with the calculated results by PC1D [14]. Voc increases with decreasing thickness. The Voc reduction with increasing thickness seen in Fig. 3 is assumed to be explained by the reduction of the total number of defects due to the volume reduction of the active layer. Experimental results are evaluated by PC1D analysis. These are well ®tted by the curve calculated by PC1D, where the diusion length (L) is 7 lm, and the carrier concentration of 2 1015 =cm3 . The calculated Voc for two kinds of carrier concentration shows a decrease of Voc with decreasing carrier concentration, for the same diusion length of each solar cell. It is important to know the carrier concentration and the minority carrier life time (diusion length) of the i-layer for further analysis of our low-temperature poly-Si cell. The carrier concentration has not been directly determined yet. On the other hand, phosphorous would be an appropriate candidate for doping of the i-layer since it is deposited on the n-type layer. It would
Fig. 6. The Voc dependence on thickness. Experimental results are plotted with open circles. The three curves (broken line, dotted line, solid line) are the calculated curves for qfi 0%, 70%, and 92%, respectively, based on PC1D for a constant diusion length (L) of 7 lm for two kinds of carrier concentration (Ni) shown in the inset of this ®gure, respectively. Upper lines and lower lines show the calculated curves for 1 1015 =cm3 and 2 1015 =cm3 , respectively. qfi is de®ned as the internal global re¯ectance at the front surface in PC1D [13].
1086
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
be important to measure the phosphorus concentration. The resulting depth pro®le of phosphorous concentration in 4 lm thick poly-Si was investigated by SIMS, which is shown in Fig. 7. A phosphorous concentration of around 5 ´ 1015 /cm3 was observed in the bulk of poly-Si. This concentration is sucient to explain a carrier concentration of 2 1015 =cm3 calculated from PC1D analysis shown in Fig 6. 3.5. Poly-Si fabrication process issues We are now going to develop the poly-Si and aSi/poly-Si stacked monolithic solar-module sized 910 455 mm2 like the a-Si module which has already been developed in our pilot-plant. There are several key factors for our poly-Si cell to be cost eective and acceptable for large-scale terrestrial applications. Among them, process related key issues are: how to get (i) greater growth rate, (ii) large area (>10 cm2 ) and uniform deposition at the same time and (iii) monolithic series interconnection. First concerning the deposition rate, we have reached 1.5 nm/s, while getting device quality poly-Si ®lm. With this growth rate a poly-Si cell in production would be produced approximately ev-
Fig. 7. The depth pro®le of phosphorus (P) concentration for 4 lm thick poly-Si was investigated by SIMS. Note that the detection limit of P by SIMS is around 1±2 1015 =cm3 in our experimental set-up.
ery 5 min. Together with the eort increase the light trapping for a cell 1 lm thick the deposition rate will have to be increased up to 2 nm/s. Second concerning the uniformity of cell eciency, the average eciency of 10% for poly-Si cell has been achieved over a substrate size of 100 100 mm2 , where the cell separation was performed by laser scribing. The uniform deposition by plasma CVD over the size of 300 400 mm2 has already been achieved with thickness deviations within 5%. Finally concerning the monolithic series interconnection, we have prepared 5 5 cm2 , 10 segments monolithically series interconnected a-Si/ poly-Si mini module, which has an initial eciency of 11.3%, Voc of 13.5 V, a short current of 30.4 mA, ®ll factor of 68.8%. 3.6. Application of a-Si/poly-Si hybrid (stacked) solar cell The advantage of a larger Jsc for our poly-Si solar cell with STAR structure was applied to the stacked cell with the combination of a-Si cell. In applying the a-Si cell to the stacked cell we must pay attention to the stabilized eciency, since a-Si photo-degrades, while a poly-Si cell is stable. We have prepared a stacked cell of a-Si:H/poly-Si/ poly-Si (triple), which will be less sensitive to
Fig. 8. The stabilized performance of the a-Si/poly-Si/poly-Si 3-stacked hybrid solar cell after 550 h indoor light soaking.
K. Yamamoto et al. / Journal of Non-Crystalline Solids 266±269 (2000) 1082±1087
1087
Acknowledgements This work was supported by the NEDO as a part of the New Sunshine Program under MITI. References
Fig. 9. Light-induced changes in eciency for a-Si/poly-Si/ poly-Si 3-stacked hybrid solar cell under AM1.5, 100 mW/cm2 , 48°C, and open-circuit conditions. The line is drawn as a guide for the eye.
degradation by using the thinner a-Si. We have investigated the stability of a-Si:H/poly-Si/poly-Si (triple) cell. These devices were subjected to indoor light soaking under AM1.5, 100 mW/cm2 , 48°C, and open-circuit conditions. The triple cell showed a stabilized eciency of 12% as shown in Fig. 8. Note that a stabilized FF of 76.2% was also obtained. Fig. 9 shows the change in the device eciency during light soaking under the above conditions, where a much smaller photodegradation is observed for our 3-stacked cell.
4. Conclusions The a-Si:H/poly-Si/poly-Si stacked cell showed a stabilized eciency of 12%. The temperature coecient of 0.268% is as small as that of a-Si cell, which is promising result in view of the terrestrial applications of stacked cells.
[1] K. Yamamoto, A. Nakajima, T. Suzuki, M. Yoshimi, H. Nishio, M. Izumina, Jpn. J. Appl. Phys. 33 (1994) L1751. [2] K. Yamamoto, A. Nakajima, T. Suzuki, M. Yoshimi, H. Nishio, M. Izumina, in: Proceedings of the First World Conference on Photovoltaic Energy Conversion 1994, IEEE, Piscataway, 1994, p. 1573. [3] A. Nakajima, T. Suzuki, M. Yoshimi, K. Yamamoto, in: Proceedings of the 13th European Photovoltaic Solar Energy Conference 1995, H.S. Stephens & Associates, UK, 1995, p. 1550. [4] K. Yamamoto, T. Suzuki, M. Yoshimi, A. Nakajima in: Proceedings of the 25th IEEE Photovoltaic Energy Conversion 1996, IEEE, Piscataway, 1996, p. 661. [5] K. Yamamoto, A. Nakajima, T. Suzuki, M. Yoshimi, Jpn. J. Appl. Phys. 36 (1997) L569. [6] K. Yamamoto, A. Nakajima, T. Suzuki, M. Yoshimi, in: Proceedings of the 14th European Photovoltaic Solar Energy Conference, 1997. [7] K. Yamamoto, T. Suzuki, K. Kondo, T. Okamoto, M. Yamaguchi, M. Izumina, Y. Tawada, Solar Energy Mater. Solar Cells 34 (1994) 501. [8] K. Yamamoto, T. Suzuki, M. Yoshimi, A. Nakajima, in: Proceedings of the 14th European Photovoltaic Solar Energy Conference, 1997, p. 1018. [9] K. Yamamoto, T. Suzuki, M. Yoshimi, A. Nakajima, in: Proceedings of 26th IEEE Photovoltaic Energy Conversion 1996, IEEE, Piscataway, 1998, p. 575. [10] K. Yamamoto, T. Suzuki, M. Yoshimi, A. Nakajima, in: Proceedings of 98 MRS Spring Meeting, 507, 1998, p. 131. [11] K. Yamamoto, A. Nakajima, Y. Tawada, M. Yoshimi, Y. Okamoto, S. Igari, in: Proceedings of the Second World Conference, Photovoltaic Energy Conversion, 1998, p. 1284. [12] J. Meier, R. Flueckiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65 (1994) 860. [13] A. Poruba et al., in: Proceedings of the 14th European Photovoltaic Solar Energy Conference, 1997, p. 2105. [14] P.A. Basore, IEEE Trans. Electron. Dev., ED-37 (1990) 337.