Technology transfer challenges in the manufacturing of a-Si tandem solar cells
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Technology transfer challenges in the manufacturing of a-Si tandem solar cells D. E. Carlson, G. Ganguly, G. Lin, M. Gleaton, M. Bennett and R. R. Arya BP Solar, Toano, VA 23168 ABSTRACT BP Solar started commercial production of amorphous silicon tandem solar cells in Toano, Virginia in 1997. The scale-up process has involved overcoming technology challenges in several areas. It was necessary to develop high speed, multiple-beam laser scribing systems that could pattern photovoltaic modules over an area of 0.8 square meters with close dimensional tolerances. In addition, the deposition rate of amorphous silicon was doubled while production was being ramped up, and the deposition system was modified to meet capacity requirements. The utilization of the germane feedstock gas was increased by about 25% in order to reduce material costs. In addition, the dimensional tolerances of the deposition system geometry were determined and controlled in order to assure uniform deposition of the amorphous silicon alloys. Effects of contaminants (such as pump oils and residual dopants) on device performance were quantified. Sources of debris that could cause shunts and shorts in the devices were identified and minimized. Current research efforts are focused on further increases in the amorphous silicon deposition rate, improvements in device performance and the development of in-situ diagnostic tools to monitor and control the manufacturing process. INTRODUCTION The amorphous silicon (a-Si) program at BP Solar has its origin in the early work at RCA Laboratories performed by Carlson and Wronksi [1]. The RCA program started with the invention of the a-Si solar cell in 1974 [2] and continued through 1983 when the technology was sold to Solarex (which was wholly owned by Amoco Corp. at the time). Solarex started commercial production of small-area (few cm2), single-junction a-Si p-i-n solar cells for lowlight level applications in 1984 and then focused on larger monolithic modules (up to 0.1 m2 in size) for terrestrial applications in 1986. A highly automated production line capable of producing 1 MWP of 0.1 m2 single-junction a-Si modules per year was started in 1990 [3]. The line was initially configured so that glass plates were automatically loaded at one end, and about 4 hours later, an encapsulated, monolithic, series-connected module came out the other end without any manual handling. Solarex started research on a-Si multijunction structures in the late 1980’s, but it was not until 1995 that the company decided to commercialize the multijunction technology using an a-Si/a-SiGe tandem structure. This decision followed the successful demonstration of an average 8% stable conversion efficiency for 0.4 m2 tandem modules produced on a pilot line [4]. Solarex started production of tandem modules (total module area = 0.805 m2) at the TF1 facility in Toano, Virginia in 1997. At this time, Solarex was part of a new joint venture (Amoco/Enron Solar) between Amoco and Enron Corporations. In 1998 BP acquired Amoco, and subsequently Solarex
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