Stability of large-area amorphous silicon alloy tandem solar modules

A dual bandgap, dual junction a-Si:H/a-SiGe:H solar cell has been selected as the structure to be used in Solarex's new 10 MW solar module plant. This structure offers high initial efficiency, good stability, low material usage, and short deposition time. Thousands of one square foot and four square foot tandem modules have been produced on the Solarex pilot line, and an average of 8% stabilized efficiency on 4 square foot modules in the trial production, and 8.6% on 1 square foot modules in R&D were achieved with this structure. Good agreement was found in the stability of modules tested outdoors and indoors, and also with an array of test modules set up at NREL. An improved tandem structure with better stability is being developed by optimizing the i-layer thickness and back junction bandgap. The current loss associated with thinner i-layers was well compensated by improvements in fill factor, open-circuit voltage, and stability. Two environmental degradation modes for modules other than Staebler-Wronski effect were identified and solutions implemented.