High Efficiency Stable a-Si Three Junction 12" 13" Modules
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HIGH EFFICIENCY STABLE a-Si THREE JUNCTION 12" x 13" MODULES MURRAY S. BENNETT, A. CATALANO, J. NEWTON, C. POPLAWSKI, R. ARYA, K. RAJAN, G. WOOD, AND S. WIEDEMAN, Solarex Corporation, Thin Film Division, 826 Newtown-Yardley Road, Newtown, PA 18940 ABSTRACT Three junction a-Si based photovoltaic modules have been made using a design which provides high initial efficiency and good resistance to photodegradation. The cells have a Si/Si/SiGe configuration in which the i-layer inthe middle junction is 4000A thick. The most efficient module measured to date has an aperture area efficiency of 9.82%. This design will limit light-induced efficiency losses to 15% or less, based on small area results, however defect related problems have increased this value to 17 23%. Preliminary data on the effect of shunts on stability is presented. We discuss various concerns related to large area deposition and scale-up. INTRODUCTION Single junction amorphous silicon solar cells with efficiencies around 12% have been made in small areas [1,2] and modules have been made which exceed 10% in aperture area efficiency [3]. However, as a result of light-induced degradation [4], these very efficient cells and modules will not retain their efficiency after a prolonged period of exposure to light. In order to provide resistance to photodegradation it is necessary to use a more complicated multi-junction configuration [5]. Cells made in this way combine high initial efficiency with good resistance to degradation, the price being a much more complicated structure. Small area three junction devices have been made with efficiencies (active area) in excess of 13% [6]. High efficiency three junction cells (9.6%) have been measured after 1000 hours of 100 mW/cm2 light soaking with degradation rates of 10 - 12% [7] where the light soaking was carried out at Voc and 50 0C. We will report herein Initial results of scale-up of these efficient and stable small area cells to 12" x 13" modules. Very efficient modules have been achieved, and in the best cases stability only slightly worse than that seen in small area cells has been measured, however, a number of issues remain to be resolved involving the impact of defects on both efficiency and stability. MODULE DESIGN The silicon was deposited by dc glow discharge in a load-locked deposition system having separate deposition chambers for p-, i-, and n-layer depositions. Feedstock gases were SiH 4 , PH 4 , CH4 , H2 , and for doping PH 3 and B2 H6. Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society
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The structure of the modules was the same as that used for the high efficiency cells. The substrate was conducting tin oxide (CTO) deposited by chemical vapor deposition on glass. The triple structure was Si/Si/SiGe where each of the first two junctions were p-SiC/i-Si/n-Si structures, and the third junction was p-SiC/i-SiGe/n-Si. The back contact was a layer of indium tin oxide overcoated with a layer of silver. The modules were separated into 26 segments by laser patterning using the techni
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