Large-Scale Deposition of Transparent Conducting Oxides by Hollow Cathode Sputtering
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Large-Scale Deposition of Transparent Conducting Oxides by Hollow Cathode Sputtering Alan E. Delahoy1, Kai Jansen1, Chris Robinson1, Anthony Varvar1, Paul Fabiano1, Rajesh Kappera2, Sheyu Guo3, Hongmei Li3 and Shaohua Yang3 1 New Millennium Solar Equipment Corp., 8 Marlen Dr., Robbinsville, NJ 08691, U.S.A. 2 Rutgers University, Dept. of Electrical Engineering Piscataway, NJ 08854, U.S.A. 3 Yiri Solartech (Suzhou) Co., Ltd., Wujiang Hi-Tech Park, 2358 Chang An Road, Wujiang City, Jiangsu Province, P.R. China 215200 ABSTRACT This paper reviews the status of hollow cathode sputtering as an evolving technology for production of thin-film transparent conducting oxides for PV applications. A large market segment for PV TCOs is represented by thin-film a-Si:H and tandem a-Si:H/nc-Si:H modules. For superstrate devices, textured SnO2:F produced on-line by APCVD is currently the market leader, although alternative off-line methods and materials are now emerging. In particular, zinc oxide can be produced by LPCVD, APCVD, magnetron sputtering, and hollow cathode sputtering (HCS). HCS is a stable process featuring low-cost metal targets and a soft deposition process. We discuss the deposition principles and the film results obtained using linear hollow cathodes 0.5 m and 1.0 m in length. We report the direct deposition of highly textured doped ZnO having an electron mobility in excess of 50 cm2/Vs. The production cost of textured ZnO is estimated for several competing techniques. INTRODUCTION Transparent conducting oxides are used as a window electrode in most types of thin-film PV modules and in the heterojunction cell utilizing a-Si:H on c-Si wafers [1]. In the case of superstrate-type thin Si:H modules (a-Si and a-Si/nc-Si tandems) the TCO needs to be of substantial thickness in order to realize the required values of sheet resistance and haze. In the case of the heterojunction cell, high-mobility ITO or related materials are of interest. In both cases, free-carrier absorption needs to be minimized. The cost of high performance commercial tin oxide (SnO2:F or FTO) coated glass suitable for a-Si/nc-Si tandem modules is approximately $18/m2, corresponding to a direct cost element of $0.18/Wp for a 10% efficient PV product. A TCO product with lower haze suitable for a-Si modules represents a similar cost element in $/Wp. Compared to a target module selling price of $1.00/Wp, the TCO cost alone represents a substantial burden. The annual production volume for a-Si and a-Si/nc-Si modules combined is about 500 MW, and if 75% is on TCO-coated glass and has an average total-area efficiency of 7.5%, then the world-wide consumption of textured TCO-coated glass for this market segment alone is greater than 5 x 106 m2/y. The production rate of CdTe modules is > 1 GW/y, thereby demanding 1 x 107 m2/y of appropriate TCO. On-site production of high-quality TCOs suited to these technologies could represent an attractive lower cost option, provided appropriate manufacturing processes can be devised.
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This paper will focus on the potenti
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