Optimization of High Efficiency Amorphous Silicon Alloy based Triple-Junction Modules
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ABSTRACT A systematic approach has been used to scale up high efficiency 0.25cm 2 active-area amorphous Si alloy based triple-junction devices to high-efficiency encapsulated modules of aperture area -920cm 2 . In order to analyze the losses involved in the scale-up, intermediate aperture area, 40cm2 and 450cm 2, modules have also been fabricated. The best stable active-area2 efficiency obtained on the small-area cells is 12.9%. The best initial efficiency of a -920cm aperture area encapsulated module is 12.1%. National Renewable Energy Laboratory (NREL) has independently light soaked three of the -920cm 2 modules. They have measured a stable efficiency of 10.5% which represents a new world record. This paper presents various aspects of the large-area module work. INTRODUCTION We have previously reported [1,2] on the achievement of a 10.2% stable module efficiency as confirmed by NREL using a-Si/a-SiGe/a-SiGe alloy based triple-bandgap triplejunction structure. Since then we have improved the small-area device to obtain a world record stable efficiency of 13.0% [3,4]. We have developed a new optimization scheme which scales up the 0.268cm cell results into large-area -920cm2 aperture-area modules using identical materials and processes. These materials and processes are the same as those used in the manufacture of our commercial product. Intermediate aperture-area, -40cm 2 and -450cm 2 , modules have also
been fabricated for diagnostic purpose. This approach enables the analysis of the devices at both the cell and module levels. Evaluation of the small-area cells using current-voltage (I-V) and quantum efficiency (Q) measurements provide information about the device without complications of electrical and optical losses associated with modules. The analysis of the performance and uniformity of the intermediate-area modules provides a preview of the module efficiency that can be expected from a -920cm 2 module taking into consideration all the losses related to modules. The optimization scheme has culminated in the achievement of an aperture-area efficiency of 12.1% on a -920cm 2 module. NREL has light-soaked three of these modules and measured a stable efficiency of 10.5% which corresponds to a new world record. Since these modules have been fabricated using production technology, it provides the most realistic practical goal for a-Si alloy commercial product. EXPERIMENT A large-area stainless steel substrate is first sputter-coated with a textured Ag/ZnO back reflector layer. The deposition is over an area of -1 sq. ft. This is followed by the deposition over the same area of a triple-junction triple-bandgap a-Si/a-SiGe/a-SiGe alloy cell using conventional glow discharge technique. The top transparent conducting oxide (TCO) is deposited in two different configurations for: (1) small-area devices and (2) modules. 743 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society Downloaded from https://www.cambridge.org/core. Cambridge University Main, on 07 Feb 2020 at 23:25:01, subject to the Cambridge Co
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