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New materials and deposition techniques for highly efficient silicon thin film solar cells
Solar Energy Materials & Solar Cells 74 (2002) 439–447
New materials and deposition techniques for highly efficient silicon thin film solar cells B. Rech*, O. Kluth, T. Repmann, T. Roschek, J. Springer, . J. Muller, F. Finger, H. Stiebig, H. Wagner Institute of Photovoltaics-IPV, Forschungszentrum Julich GmbH, D-52425 Julich, Germany . .
Abstract This paper reviews recent efforts to provide the scientific and technological basis for costeffective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (mc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for mc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates ( respectively. These mc-Si:H solar cells were successfully up-scaled to a substrate of 5 and 9 A/s, area of 30 30 cm2 and applied in a-Si:H/mc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin film solar cell; Silicon; A-Si; mc-Si; Zinc oxide; TCO; PECVD; Sputtering; High rate; Tandem cell; Light trapping; Up-scaling
1. Introduction Today, commercially available large area modules based on a-Si:H typically show efficiencies in the 6–7% range. Significantly higher efficiencies have been reported for prototype modules. However, these high efficiency solar cells often rely on sophisticated cell structures, optimised light-trapping schemes and/or low deposition rates being yet not compatible with the cost or throughput requirements in *Corresponding author. Institute of Photovoltaics-IPV, Forschungszentrum Julich . GmbH, D-52425 Julich, . Germany.. E-mail address: [email protected] (B. Rech). 0927-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 1 1 4 - 9
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production. The latter issues have to be also addressed in the case of tandem cells made of a-Si:H and mc-Si:H. These devices, introduced by the Neuch#atel group [1], combine the advantages of a-Si:H and its technology with the stability and long wavelength spectral sensitivity of crystalline silicon. High efficiencies have been reported by several research groups [1–4] and recent progress in up-scaling has led to the first production announcement by Kaneka [2]. However, due to the indirect band gap of mc-Si:H, i-layer thicknesses of more than 1 mm are required even when applying efficient light-trapping schemes. Therefore, high deposition rates are essential. This contribution reviews two recent approaches followed at our institute to provide the scientific and technological basis for a future generation of cost-effective and highly efficient thin film solar modules based on a-Si:H and mc-Si:H. (i) Textured zinc oxide films prepared by sputtering and post-deposition etching were applied as TCO material for improved light trapping. (ii) We developed microcrystalline silicon solar cells at high deposition rates using plasma enhanced chemical vapour deposition (PECVD) at 13.56 MHz plasma excitation frequency for the application as bottom cell absorber material in a-Si:H/mc-Si:H stacked cells. Both techniques were developed on laboratory scale (substrate size: 10 10 cm2) and are currently being up-scaled to larger substrate sizes in national and European R&D projects.
2. Texture-etched zinc oxide Aluminum-doped zinc oxide (ZnO:Al) films prepared by magnetron sputtering have emerged as one possible alternative to SnO2:F. The sputtering process leads to highly conductive and transparent but smooth ZnO films. A simple chemical etching step in diluted acid yields a textured surface, which can be adjusted to give optimal light scattering over a wide wavelength range [5,6]. To develop optimised lighttrapping schemes for different solar cell structures (pin or nip) and absorber materials (e.g. a-Si:H, mc-Si:H), we studied the relationship between ZnO film growth, structural properties and surface morphology obtained after etching. The ZnO:Al films were prepared by RF- or dc-magnetron sputtering using ceramic or metallic targets, respectively [7]. Reactive dc-sputtering from metallic Zn/Al-targets is of great technological importance, since it is a potentially cost-effective technique due to low target costs and high deposition rates. The deposition pressure and substrate temperature during the sputtering process have the strongest influence on the ZnO:Al material properties. With both deposition techniques (rf, dc) highly conductive and transparent ZnO:Al films were prepared applying low sputter pressures, while a characteristic increase in specific resistance was observed, if the deposition pressure exceeded a certain value. The deposition pressure also controls the structural film properties, which are reflected by distinctly different surface morphologies obtained after the etching step (typically 10–50 s in diluted hydrochloric acid 0.5% HCl in H2O). This is illustrated for the case of three reactively dc-sputtered ZnO:Al films, which were deposited in the low pressure regime (ranging from 0.5 to 10 mTorr for this
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series). All films show low specific resistances (ro5 104 O cm) and high transparency—both not deteriorated by the etching process. Although the optical and electrical film properties are almost identical, the films show distinctly different surface morphologies after etching as can be seen in the AFM micrographs (Fig. 1). For all films deposited in this low pressure regime, craters of characteristic distribution, lateral size and depth arise at the surface due to anisotropic etching. At 0.5 mTorr the craters are comparatively flat even after prolonged etching, which limits the root mean square surface roughness drms to values of 50 nm. Increasing the deposition pressure results in reduced opening angles and greater depth of the craters. The highest drms of 123 nm is obtained for the 10 mTorr sample. The combination of sputtering and etching opened exciting possibilities to design tailored light-trapping schemes and was applied to a variety of different absorber materials and device structures [7–10]. For the application as front contact in pin solar modules with mc-Si absorber layers one has to consider the relationship between ZnO sheet resistance determined by ZnO thickness, carrier concentration and mobility on the one hand and optical losses due to free carrier absorption in the long wavelength region on the other hand. This is well illustrated by the quantum
Fig. 1. Surface morphology of texture etched ZnO:Al films analysed by atomic force microscopy (AFM). The films were reactively dc sputtered at deposition pressures of 0.5, 5 and 10 mTorr, respectively.
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1.0
QE , T
0.8
T
0.6 thin ZnO thick ZnO
0.4
QE
0.2 smooth ZnO 0 300
400
500
600
700
λ (nm )
800
900
1000
1100
Fig. 2. Quantum efficiency QE of mc-Si:H cells prepared on smooth or textured-etched ZnO coated glass substrates with different ZnO thicknesses. Additionally, the optical transmission T is included for the thick (1500 nm) and thin (500 nm) ZnO coated glass substrates measured before etching.
efficiency of mc-Si:H solar cells (see Fig. 2) co-deposited on smooth and textured ZnO. The ZnO films had initial thicknesses of 500 and 1500 nm with sheet resistances of 10 and 3 O, respectively, before etching. Fig. 2 also includes the transmission spectra of these ZnO films. The texture etching significantly enhances the long wavelength spectral response in case of the initially 500 nm thick ZnO film. However, using thick ZnO films (1500 nm before texture etching, around 1350 nm after etching) leads to free carrier absorption losses in the ZnO film reducing the red response. At the same time, the FF improves in this case from 68% to 75%, due to the reduced TCO sheet resistance. The best compromise was an initially 750 nm thick ZnO film (not shown in the graph for better readability). The mc-Si:H cells optimised on this latter substrate yielded efficiencies up to 8.1%.
3. High rate deposition of lC-Si:H solar cells The very high frequency (vhf) PECVD technique is widely used as a standard technique on laboratory level to achieve excellent mc-Si:H material quality and solar cell properties at high deposition rates [1,3,4]. However, these high frequencies (typically above 50 MHz) make an up-scaling to production size (B1 m2) more difficult. For this reason we studied mc-Si:H p–i–n solar cells prepared at the standard industrial frequency of 13.56 MHz in the deposition regime of high rfpower and high deposition pressures [11,12]. First high quality intrinsic mc-Si:H films
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deposited in a high pressure regime at high rate have been reported by Guo et al. [13]. In our approach we varied the i-layer deposition conditions over a wide parameter range in p–i–n cells directly. The deposition pressure pdep which turned out to be one of the key parameters for cell performance was varied between 1 and 11 Torr. With increasing pdep it was necessary to reduce the silane concentration (silane to hydrogen gas flow ratio=[SiH4]/[H2]) from 3% to below 1% and increase the deposition power PRF from around 0.25 to 0.7 W/cm2 to maintain mc-Si:H growth ( and deposition rates of at least 4 A/s. This deposition regime is sketched in Fig. 3 showing the transition between a-Si:H and mc-Si:H growth by a solid line. Comprehensive studies by Vetterl et al. [4,14] showed a significant increase of the efficiency when the transition between a-Si:H and mc-Si:H regime is approached by a variation of [SiH4]/[H2]. The best mc-Si:H solar cells are generally prepared in the mcSi:H growth regime close to the a-Si:H transition. To separate the role of the deposition pressure from the shift between the amorphous and microcrystalline growth regime, we prepared solar cells at different pdep and optimised [SiH4]/[H2] for each pdep : Additionally we adjusted the plasma power to maintain similar growth ( rates (571 A/s) over the whole pdep region. Only the mc-Si:H solar cells with the highest FF and VOC at a given pdep are used for the following discussion. FF and VOC served as a measure for the mc-Si:H material quality. The results are plotted in Fig. 4. For both FF and VOC there is an increase with rising deposition pressure from B50% FF and o400 mV at 1.5 Torr to B70% and 520 mV at 10 Torr, respectively. The highest mc-Si efficiencies achieved after an optimisation of i-layer thickness and ( back reflector were 8.1% and 7.5% at deposition rates of 5 and 9 A/s, respectively (see Fig. 5 for the J2V -curves).
[SiH 4]/[H 2] (%)
4 a-Si:H
3 2 1 µc-Si:H 0
2
4
6 p dep (Torr)
8
10
Fig. 3. Transition (—) between mc-Si:H and a-Si:H growth for i-layers prepared at different silane to hydrogen gas flow ratios [SiH4]/[H2] and deposition pressures pdep : a-Si:H and mc-Si:H growth are indicated by open circles and closed squares, respectively.
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70
550
65 FF (%)
450
55 50 45
VOC (mV)
500 60
400 1
2
3
4
5 6 7 p dep (Torr)
8
9
10 11
Fig. 4. Fill factor FF and open-circuit voltage VOC of mc-Si:H solar cells optimised at different deposition pressures pdep :
Voltage (V)
0.1
2
J AM1.5 (mA/cm )
-0.1 -5
0.3
0.5
0.7
η = 7.5%
Voc = 0.525 V J sc = 20.8 mA/cm 2 -15
FF = 69.1 % Rate: 9 Å/s
η = 8.1 %
Voc = 0.544 V J sc = 20.5 mA/cm2 FF = 72.6 % Rate: 5 Å/s
i-layer thickness: 1.4 µm -25
cell area: 1 cm 2
Fig. 5. J2V characteristics of mc-Si:H solar cells measured under standard illumination conditions (AM 1.5, 100 mW/cm2, 251C).
In summary, we achieved the best results for solar cells prepared at 13.56 MHz in a deposition regime of high pdep (B10 Torr) and high Prf (B0.5 W/cm2). Similar to an increase of the excitation frequency an increase of the deposition pressure is known to reduce the ion energy and therefore also the damage due to ion bombardment of the growing silicon surface as compared to conventional RF-discharges. ‘‘Soft’’ deposition parameters (i.e. low ion energy) are essential to obtain device quality
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mc-Si:H, while high power densities are required to achieve high growth rates. If these boundary conditions are fulfilled, usually the silane concentration has to be adjusted to grow the mc-Si:H i-layer in a deposition regime close to the transition to a-Si:H growth.
4. Up-scaling issues For both materials, texture-etched ZnO and mc-Si:H, only a small process window provides the required material properties. In the case of ZnO only a certain film structure gives optimal light scattering after texture-etching, and in the case of mcSi:H deposition parameters close to the transition to a-Si:H growth have to be chosen. Additional concerns are the homogeneity requirements on large areas and the need to use cost-effective processes with high deposition rates. 4.1. Textured zno films For the up-scaling of ZnO sputtering and etching, the experience of sputterequipment manufacturers on the one hand and TCO- and solar cell/moduledevelopers on the other hand were combined in a joint R&D project [15]. Partners within this project are: IPV, RWE Solar GmbH, Applied-Films GmbH&Co.KG, Sentech Instruments GmbH and the Fraunhofer Institute for Surface Engineering and Thin Films. ZnO films sputtered in dc-mode from ceramic ZnO targets on large substrate areas already yielded excellent electrical, optical and light-scattering properties leading to first efficient a-Si:H and mc-Si:H pin solar cells on these substrates [15]. Currently, the project focuses on the development of reactive midfrequency sputtering processes [16] promising high quality TCO at low costs, due to the use of cheap metallic Zn:Al targets and high deposition rates. 4.2. mc-Si:H cells and a-Si:H/mc-Si:H tandem cells The up-scaling of mc-Si:H in the high pressure regime was a major challenge since the non-standard deposition parameters with respect to pressure, gas flow, and required rf-power density led to inhomogeneous mc-Si:H material properties and correspondingly inhomogeneous cell performance even in our small area reactor. In co-operation with FAP GmbH (Dresden, Germany) we developed PECVD electrodes providing homogeneous plasma conditions over an area of 30 30 cm2. Using these electrodes in our two-chamber PECVD system (substrate size 30 30 cm2) we succeeded in preparing mc-Si:H solar cells with an efficiency of ( 8.1% (FF ¼ 72:4%; Voc ¼ 555mV ; Jsc ¼ 20:2 mA/cm2) at a deposition rate of 5 A/s. The successful up-scaling of the 13.56 MHz mc-Si:H cell process is further demonstrated by a-Si:H/mc-Si:H tandem cells prepared on a 30 30 cm2 SnO2coated glass substrate (Asahi U-type). The 162 test cells (area: 1 cm2) evenly distributed over the inner 27 27 cm2, yielded an average initial efficiency of
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0 Counts
2
J AM1.5 (mA/cm )
60 -5
-10
40 20 0
10
11
12
η (%)
η = 11.7 % FF=72.1 %, V oc=1.34 V, J sc=12.1 mA/cm
-15 0.0
0.5 1.0 Voltage (V)
2
1.5
Fig. 6. Homogeneity distribution (162 test cells) and best cell J2V characteristics of an a-Si:H/mc-Si:H tandem cell deposition on a 30 30 cm2 substrate.
11.070.5% (see Fig. 6 for the statistical distribution). Latest improvements lead to initial test cell efficiencies of 11.9% (Voc ¼ 1:36 V, Jsc ¼ 12:1 mA/cm2, FF ¼ 71:7%).
5. Conclusions and outlook Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes for silicon thin film solar cells. Necessary prerequisite was the knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. In a national TCO project the laboratory results are currently being up-scaled to large areas applying cost-effective sputtering processes. High rate deposition processes using 13.56 MHz PECVD were developed for mc-Si:H solar cells yielding efficiencies of ( 8.1% and 7.5% at deposition rates of 5 and 9 A/s, respectively. These mc-Si:H solar cells were successfully up-scaled to a substrate size of 30 30 cm2 and applied as bottom cells in tandem devices showing initial test cell efficiencies up to 11.9%. Our current technological goal is the realisation of efficient prototype modules on a substrate size of 30 30 cm2 comprising the described materials and deposition techniques. For this purpose, a complete technology for 30 30 cm2 including laser scribing, inline-sputtering and texture-etching is currently under construction at the IPV.
Acknowledgements The authors thank their colleagues at the IPV for providing excellent technical and scientific support. We gratefully acknowledge financial support from the BMBF, the
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BMWi (contracts: 0329885 and 0329854A), RWE Solar GmbH and the European Commission (contract: ENK6-CT-2000-00321).
References [1] J. Meier, E. Vallat-Sauvain, S. Dubail, U. Kroll, J. Dubail, S. Golay, L. Feitknecht, P. Torres, S. Fay, D. Fischer, A. Shah, Sol. Energy Mater. Sol. Cells 66 (2001) 73. [2] K. Yamamoto, M. Yoshimi, T. Suzuki, T. Nakata, T. Sawada, A. Nakajima, K. Hayashi, Proceedings of the 28th IEEE PVSC, Anchorage, USA, 2000, p. 1428. [3] K. Saito, M. Sano, K. Matzuda, T. Kondo, M. Higasikawa, T. Kariya, Technical Digest of the PVSEC-11, Sapporo, Japan, 1999, p. 229. [4] O. Vetterl, F. Finger, R. Carius, P. Hapke, L. Houben, O. Kluth, A. Lambertz, A. Muck, . B. Rech, H. Wagner, Sol. Energy Mater. Sol. Cells 62 (2000) 97. . S. Wieder, B. Rech, O. Kluth, C. Beneking, H. Wagner, Proceedings of the 14th EC-PVSEC, [5] A. Loffl, Barcelona, 1997, p. 2089. . [6] O. Kluth, A. Loffl, S. Wieder, C. Beneking, W. Appenzeller, L. Houben, B. Rech, H. Wagner, S. Hoffmann, R. Waser, J.A. Anna Selvan, H. Keppner, Proceedings of the 26th IEEE PVSC, Anaheim, USA, 1997, p. 715. . . [7] O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schope, C. Beneking, H. Wagner, A. Loffl, H.W. Schock, Thin Solid Films 351 (1999) 247. . [8] B. Rech, S. Wieder, C. Beneking, A. Loffl, O. Kluth, W. Reetz, H. Wagner, Proceedings of the 26th IEEE PVSC, Anaheim, USA, 1997, p. 619. [9] O. Kluth, O. Vetterl, R. Carius, F. Finger, S. Wieder, B. Rech, H. Wagner, Mater. Res. Soc. Symp. Proc. 557 (1999) 731. . [10] J. Muller, . O. Kluth, S. Wieder, H. Siekmann, G. Schope, W. Reetz, O. Vetterl, D. Lundszien, A. Lambertz, F. Finger, B. Rech, H. Wagner, Sol. Energy Mater. Sol. Cells 66 (2001) 275. [11] B. Rech, T. Roschek, J. Muller, . S. Wieder, H. Wagner, Sol. Energy Mater. Sol. Cells 66 (2001) 267. [12] T. Roschek, T. Repmann, J. Muller, . B. Rech, H. Wagner, Proceedings of the 28th IEEE PVSC, Anchorage, USA, 2000, p. 150. [13] L. Guo, M. Kondo, M. Fukawa, K. Saitoh, A. Matsuda, Jpn. J. Appl. Phys. 37 (1998) L1116. [14] O. Vetterl, R. Carius, L. Houben, C. Scholten, M. Luysberg, A. Lambertz, F. Finger, H. Wagner, Mater. Res. Soc. Symp. Proc. 609 (2000) A152. . . [15] J. Muller, . G. Schope, O. Kluth, B. Rech, M. Ruske, J. Trube, B. Szyszka, Th. Hoing, X. Jiang, G. Br.auer, Proceedings of the 28th IEEE PVSC, Anchorage, USA, 2000, p. 758. [16] B. Szyszka, Thin Solid Films 351 (1999) 164.
New materials and deposition techniques for highly efficient silicon thin film solar cells B. Rech*, O. Kluth, T. Repmann, T. Roschek, J. Springer, . J. Muller, F. Finger, H. Stiebig, H. Wagner Institute of Photovoltaics-IPV, Forschungszentrum Julich GmbH, D-52425 Julich, Germany . .
Abstract This paper reviews recent efforts to provide the scientific and technological basis for costeffective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (mc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for mc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates ( respectively. These mc-Si:H solar cells were successfully up-scaled to a substrate of 5 and 9 A/s, area of 30 30 cm2 and applied in a-Si:H/mc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin film solar cell; Silicon; A-Si; mc-Si; Zinc oxide; TCO; PECVD; Sputtering; High rate; Tandem cell; Light trapping; Up-scaling
1. Introduction Today, commercially available large area modules based on a-Si:H typically show efficiencies in the 6–7% range. Significantly higher efficiencies have been reported for prototype modules. However, these high efficiency solar cells often rely on sophisticated cell structures, optimised light-trapping schemes and/or low deposition rates being yet not compatible with the cost or throughput requirements in *Corresponding author. Institute of Photovoltaics-IPV, Forschungszentrum Julich . GmbH, D-52425 Julich, . Germany.. E-mail address: [email protected] (B. Rech). 0927-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 2 ) 0 0 1 1 4 - 9
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production. The latter issues have to be also addressed in the case of tandem cells made of a-Si:H and mc-Si:H. These devices, introduced by the Neuch#atel group [1], combine the advantages of a-Si:H and its technology with the stability and long wavelength spectral sensitivity of crystalline silicon. High efficiencies have been reported by several research groups [1–4] and recent progress in up-scaling has led to the first production announcement by Kaneka [2]. However, due to the indirect band gap of mc-Si:H, i-layer thicknesses of more than 1 mm are required even when applying efficient light-trapping schemes. Therefore, high deposition rates are essential. This contribution reviews two recent approaches followed at our institute to provide the scientific and technological basis for a future generation of cost-effective and highly efficient thin film solar modules based on a-Si:H and mc-Si:H. (i) Textured zinc oxide films prepared by sputtering and post-deposition etching were applied as TCO material for improved light trapping. (ii) We developed microcrystalline silicon solar cells at high deposition rates using plasma enhanced chemical vapour deposition (PECVD) at 13.56 MHz plasma excitation frequency for the application as bottom cell absorber material in a-Si:H/mc-Si:H stacked cells. Both techniques were developed on laboratory scale (substrate size: 10 10 cm2) and are currently being up-scaled to larger substrate sizes in national and European R&D projects.
2. Texture-etched zinc oxide Aluminum-doped zinc oxide (ZnO:Al) films prepared by magnetron sputtering have emerged as one possible alternative to SnO2:F. The sputtering process leads to highly conductive and transparent but smooth ZnO films. A simple chemical etching step in diluted acid yields a textured surface, which can be adjusted to give optimal light scattering over a wide wavelength range [5,6]. To develop optimised lighttrapping schemes for different solar cell structures (pin or nip) and absorber materials (e.g. a-Si:H, mc-Si:H), we studied the relationship between ZnO film growth, structural properties and surface morphology obtained after etching. The ZnO:Al films were prepared by RF- or dc-magnetron sputtering using ceramic or metallic targets, respectively [7]. Reactive dc-sputtering from metallic Zn/Al-targets is of great technological importance, since it is a potentially cost-effective technique due to low target costs and high deposition rates. The deposition pressure and substrate temperature during the sputtering process have the strongest influence on the ZnO:Al material properties. With both deposition techniques (rf, dc) highly conductive and transparent ZnO:Al films were prepared applying low sputter pressures, while a characteristic increase in specific resistance was observed, if the deposition pressure exceeded a certain value. The deposition pressure also controls the structural film properties, which are reflected by distinctly different surface morphologies obtained after the etching step (typically 10–50 s in diluted hydrochloric acid 0.5% HCl in H2O). This is illustrated for the case of three reactively dc-sputtered ZnO:Al films, which were deposited in the low pressure regime (ranging from 0.5 to 10 mTorr for this
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series). All films show low specific resistances (ro5 104 O cm) and high transparency—both not deteriorated by the etching process. Although the optical and electrical film properties are almost identical, the films show distinctly different surface morphologies after etching as can be seen in the AFM micrographs (Fig. 1). For all films deposited in this low pressure regime, craters of characteristic distribution, lateral size and depth arise at the surface due to anisotropic etching. At 0.5 mTorr the craters are comparatively flat even after prolonged etching, which limits the root mean square surface roughness drms to values of 50 nm. Increasing the deposition pressure results in reduced opening angles and greater depth of the craters. The highest drms of 123 nm is obtained for the 10 mTorr sample. The combination of sputtering and etching opened exciting possibilities to design tailored light-trapping schemes and was applied to a variety of different absorber materials and device structures [7–10]. For the application as front contact in pin solar modules with mc-Si absorber layers one has to consider the relationship between ZnO sheet resistance determined by ZnO thickness, carrier concentration and mobility on the one hand and optical losses due to free carrier absorption in the long wavelength region on the other hand. This is well illustrated by the quantum
Fig. 1. Surface morphology of texture etched ZnO:Al films analysed by atomic force microscopy (AFM). The films were reactively dc sputtered at deposition pressures of 0.5, 5 and 10 mTorr, respectively.
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1.0
QE , T
0.8
T
0.6 thin ZnO thick ZnO
0.4
QE
0.2 smooth ZnO 0 300
400
500
600
700
λ (nm )
800
900
1000
1100
Fig. 2. Quantum efficiency QE of mc-Si:H cells prepared on smooth or textured-etched ZnO coated glass substrates with different ZnO thicknesses. Additionally, the optical transmission T is included for the thick (1500 nm) and thin (500 nm) ZnO coated glass substrates measured before etching.
efficiency of mc-Si:H solar cells (see Fig. 2) co-deposited on smooth and textured ZnO. The ZnO films had initial thicknesses of 500 and 1500 nm with sheet resistances of 10 and 3 O, respectively, before etching. Fig. 2 also includes the transmission spectra of these ZnO films. The texture etching significantly enhances the long wavelength spectral response in case of the initially 500 nm thick ZnO film. However, using thick ZnO films (1500 nm before texture etching, around 1350 nm after etching) leads to free carrier absorption losses in the ZnO film reducing the red response. At the same time, the FF improves in this case from 68% to 75%, due to the reduced TCO sheet resistance. The best compromise was an initially 750 nm thick ZnO film (not shown in the graph for better readability). The mc-Si:H cells optimised on this latter substrate yielded efficiencies up to 8.1%.
3. High rate deposition of lC-Si:H solar cells The very high frequency (vhf) PECVD technique is widely used as a standard technique on laboratory level to achieve excellent mc-Si:H material quality and solar cell properties at high deposition rates [1,3,4]. However, these high frequencies (typically above 50 MHz) make an up-scaling to production size (B1 m2) more difficult. For this reason we studied mc-Si:H p–i–n solar cells prepared at the standard industrial frequency of 13.56 MHz in the deposition regime of high rfpower and high deposition pressures [11,12]. First high quality intrinsic mc-Si:H films
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deposited in a high pressure regime at high rate have been reported by Guo et al. [13]. In our approach we varied the i-layer deposition conditions over a wide parameter range in p–i–n cells directly. The deposition pressure pdep which turned out to be one of the key parameters for cell performance was varied between 1 and 11 Torr. With increasing pdep it was necessary to reduce the silane concentration (silane to hydrogen gas flow ratio=[SiH4]/[H2]) from 3% to below 1% and increase the deposition power PRF from around 0.25 to 0.7 W/cm2 to maintain mc-Si:H growth ( and deposition rates of at least 4 A/s. This deposition regime is sketched in Fig. 3 showing the transition between a-Si:H and mc-Si:H growth by a solid line. Comprehensive studies by Vetterl et al. [4,14] showed a significant increase of the efficiency when the transition between a-Si:H and mc-Si:H regime is approached by a variation of [SiH4]/[H2]. The best mc-Si:H solar cells are generally prepared in the mcSi:H growth regime close to the a-Si:H transition. To separate the role of the deposition pressure from the shift between the amorphous and microcrystalline growth regime, we prepared solar cells at different pdep and optimised [SiH4]/[H2] for each pdep : Additionally we adjusted the plasma power to maintain similar growth ( rates (571 A/s) over the whole pdep region. Only the mc-Si:H solar cells with the highest FF and VOC at a given pdep are used for the following discussion. FF and VOC served as a measure for the mc-Si:H material quality. The results are plotted in Fig. 4. For both FF and VOC there is an increase with rising deposition pressure from B50% FF and o400 mV at 1.5 Torr to B70% and 520 mV at 10 Torr, respectively. The highest mc-Si efficiencies achieved after an optimisation of i-layer thickness and ( back reflector were 8.1% and 7.5% at deposition rates of 5 and 9 A/s, respectively (see Fig. 5 for the J2V -curves).
[SiH 4]/[H 2] (%)
4 a-Si:H
3 2 1 µc-Si:H 0
2
4
6 p dep (Torr)
8
10
Fig. 3. Transition (—) between mc-Si:H and a-Si:H growth for i-layers prepared at different silane to hydrogen gas flow ratios [SiH4]/[H2] and deposition pressures pdep : a-Si:H and mc-Si:H growth are indicated by open circles and closed squares, respectively.
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70
550
65 FF (%)
450
55 50 45
VOC (mV)
500 60
400 1
2
3
4
5 6 7 p dep (Torr)
8
9
10 11
Fig. 4. Fill factor FF and open-circuit voltage VOC of mc-Si:H solar cells optimised at different deposition pressures pdep :
Voltage (V)
0.1
2
J AM1.5 (mA/cm )
-0.1 -5
0.3
0.5
0.7
η = 7.5%
Voc = 0.525 V J sc = 20.8 mA/cm 2 -15
FF = 69.1 % Rate: 9 Å/s
η = 8.1 %
Voc = 0.544 V J sc = 20.5 mA/cm2 FF = 72.6 % Rate: 5 Å/s
i-layer thickness: 1.4 µm -25
cell area: 1 cm 2
Fig. 5. J2V characteristics of mc-Si:H solar cells measured under standard illumination conditions (AM 1.5, 100 mW/cm2, 251C).
In summary, we achieved the best results for solar cells prepared at 13.56 MHz in a deposition regime of high pdep (B10 Torr) and high Prf (B0.5 W/cm2). Similar to an increase of the excitation frequency an increase of the deposition pressure is known to reduce the ion energy and therefore also the damage due to ion bombardment of the growing silicon surface as compared to conventional RF-discharges. ‘‘Soft’’ deposition parameters (i.e. low ion energy) are essential to obtain device quality
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mc-Si:H, while high power densities are required to achieve high growth rates. If these boundary conditions are fulfilled, usually the silane concentration has to be adjusted to grow the mc-Si:H i-layer in a deposition regime close to the transition to a-Si:H growth.
4. Up-scaling issues For both materials, texture-etched ZnO and mc-Si:H, only a small process window provides the required material properties. In the case of ZnO only a certain film structure gives optimal light scattering after texture-etching, and in the case of mcSi:H deposition parameters close to the transition to a-Si:H growth have to be chosen. Additional concerns are the homogeneity requirements on large areas and the need to use cost-effective processes with high deposition rates. 4.1. Textured zno films For the up-scaling of ZnO sputtering and etching, the experience of sputterequipment manufacturers on the one hand and TCO- and solar cell/moduledevelopers on the other hand were combined in a joint R&D project [15]. Partners within this project are: IPV, RWE Solar GmbH, Applied-Films GmbH&Co.KG, Sentech Instruments GmbH and the Fraunhofer Institute for Surface Engineering and Thin Films. ZnO films sputtered in dc-mode from ceramic ZnO targets on large substrate areas already yielded excellent electrical, optical and light-scattering properties leading to first efficient a-Si:H and mc-Si:H pin solar cells on these substrates [15]. Currently, the project focuses on the development of reactive midfrequency sputtering processes [16] promising high quality TCO at low costs, due to the use of cheap metallic Zn:Al targets and high deposition rates. 4.2. mc-Si:H cells and a-Si:H/mc-Si:H tandem cells The up-scaling of mc-Si:H in the high pressure regime was a major challenge since the non-standard deposition parameters with respect to pressure, gas flow, and required rf-power density led to inhomogeneous mc-Si:H material properties and correspondingly inhomogeneous cell performance even in our small area reactor. In co-operation with FAP GmbH (Dresden, Germany) we developed PECVD electrodes providing homogeneous plasma conditions over an area of 30 30 cm2. Using these electrodes in our two-chamber PECVD system (substrate size 30 30 cm2) we succeeded in preparing mc-Si:H solar cells with an efficiency of ( 8.1% (FF ¼ 72:4%; Voc ¼ 555mV ; Jsc ¼ 20:2 mA/cm2) at a deposition rate of 5 A/s. The successful up-scaling of the 13.56 MHz mc-Si:H cell process is further demonstrated by a-Si:H/mc-Si:H tandem cells prepared on a 30 30 cm2 SnO2coated glass substrate (Asahi U-type). The 162 test cells (area: 1 cm2) evenly distributed over the inner 27 27 cm2, yielded an average initial efficiency of
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0 Counts
2
J AM1.5 (mA/cm )
60 -5
-10
40 20 0
10
11
12
η (%)
η = 11.7 % FF=72.1 %, V oc=1.34 V, J sc=12.1 mA/cm
-15 0.0
0.5 1.0 Voltage (V)
2
1.5
Fig. 6. Homogeneity distribution (162 test cells) and best cell J2V characteristics of an a-Si:H/mc-Si:H tandem cell deposition on a 30 30 cm2 substrate.
11.070.5% (see Fig. 6 for the statistical distribution). Latest improvements lead to initial test cell efficiencies of 11.9% (Voc ¼ 1:36 V, Jsc ¼ 12:1 mA/cm2, FF ¼ 71:7%).
5. Conclusions and outlook Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes for silicon thin film solar cells. Necessary prerequisite was the knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. In a national TCO project the laboratory results are currently being up-scaled to large areas applying cost-effective sputtering processes. High rate deposition processes using 13.56 MHz PECVD were developed for mc-Si:H solar cells yielding efficiencies of ( 8.1% and 7.5% at deposition rates of 5 and 9 A/s, respectively. These mc-Si:H solar cells were successfully up-scaled to a substrate size of 30 30 cm2 and applied as bottom cells in tandem devices showing initial test cell efficiencies up to 11.9%. Our current technological goal is the realisation of efficient prototype modules on a substrate size of 30 30 cm2 comprising the described materials and deposition techniques. For this purpose, a complete technology for 30 30 cm2 including laser scribing, inline-sputtering and texture-etching is currently under construction at the IPV.
Acknowledgements The authors thank their colleagues at the IPV for providing excellent technical and scientific support. We gratefully acknowledge financial support from the BMBF, the
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BMWi (contracts: 0329885 and 0329854A), RWE Solar GmbH and the European Commission (contract: ENK6-CT-2000-00321).
References [1] J. Meier, E. Vallat-Sauvain, S. Dubail, U. Kroll, J. Dubail, S. Golay, L. Feitknecht, P. Torres, S. Fay, D. Fischer, A. Shah, Sol. Energy Mater. Sol. Cells 66 (2001) 73. [2] K. Yamamoto, M. Yoshimi, T. Suzuki, T. Nakata, T. Sawada, A. Nakajima, K. Hayashi, Proceedings of the 28th IEEE PVSC, Anchorage, USA, 2000, p. 1428. [3] K. Saito, M. Sano, K. Matzuda, T. Kondo, M. Higasikawa, T. Kariya, Technical Digest of the PVSEC-11, Sapporo, Japan, 1999, p. 229. [4] O. Vetterl, F. Finger, R. Carius, P. Hapke, L. Houben, O. Kluth, A. Lambertz, A. Muck, . B. Rech, H. Wagner, Sol. Energy Mater. Sol. Cells 62 (2000) 97. . S. Wieder, B. Rech, O. Kluth, C. Beneking, H. Wagner, Proceedings of the 14th EC-PVSEC, [5] A. Loffl, Barcelona, 1997, p. 2089. . [6] O. Kluth, A. Loffl, S. Wieder, C. Beneking, W. Appenzeller, L. Houben, B. Rech, H. Wagner, S. Hoffmann, R. Waser, J.A. Anna Selvan, H. Keppner, Proceedings of the 26th IEEE PVSC, Anaheim, USA, 1997, p. 715. . . [7] O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schope, C. Beneking, H. Wagner, A. Loffl, H.W. Schock, Thin Solid Films 351 (1999) 247. . [8] B. Rech, S. Wieder, C. Beneking, A. Loffl, O. Kluth, W. Reetz, H. Wagner, Proceedings of the 26th IEEE PVSC, Anaheim, USA, 1997, p. 619. [9] O. Kluth, O. Vetterl, R. Carius, F. Finger, S. Wieder, B. Rech, H. Wagner, Mater. Res. Soc. Symp. Proc. 557 (1999) 731. . [10] J. Muller, . O. Kluth, S. Wieder, H. Siekmann, G. Schope, W. Reetz, O. Vetterl, D. Lundszien, A. Lambertz, F. Finger, B. Rech, H. Wagner, Sol. Energy Mater. Sol. Cells 66 (2001) 275. [11] B. Rech, T. Roschek, J. Muller, . S. Wieder, H. Wagner, Sol. Energy Mater. Sol. Cells 66 (2001) 267. [12] T. Roschek, T. Repmann, J. Muller, . B. Rech, H. Wagner, Proceedings of the 28th IEEE PVSC, Anchorage, USA, 2000, p. 150. [13] L. Guo, M. Kondo, M. Fukawa, K. Saitoh, A. Matsuda, Jpn. J. Appl. Phys. 37 (1998) L1116. [14] O. Vetterl, R. Carius, L. Houben, C. Scholten, M. Luysberg, A. Lambertz, F. Finger, H. Wagner, Mater. Res. Soc. Symp. Proc. 609 (2000) A152. . . [15] J. Muller, . G. Schope, O. Kluth, B. Rech, M. Ruske, J. Trube, B. Szyszka, Th. Hoing, X. Jiang, G. Br.auer, Proceedings of the 28th IEEE PVSC, Anchorage, USA, 2000, p. 758. [16] B. Szyszka, Thin Solid Films 351 (1999) 164.