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We review the progresses and issues towards manufacturing hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) based thin film multi-junction solar cells using a roll-to-roll process on flexible substrates. United Solar has been heavily involved in the research and development of high efficiency a-Si:H and nc-Si:H multi-junction solar cells since 2001. We have resolved several critical issues limiting nc-Si:H solar cell performance, such as nanocrystalline evolution, impurities, and porosity/ambient degradation. We have developed new cell designs, including the proper optimization of the seeding layer for nc-Si:H growth and proper n/i and i/p buffers for improvement of cell efficiency. We have optimized Ag/ZnO back reflectors for nc-Si:H cell performance. Combining all of the efforts in the improvement of material quality and optimization of device structure, we have advanced thin film silicon solar cell efficiency. We reported 16.3% initial active-area efficiency using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cell. Furthermore, we attained 12.5% stable total area (0.27 cm2) efficiency using a-Si:H/nc-Si:H/nc-Si:H triple-junction solar cells and 11.3% stable aperture area (800 cm2) efficiency using the same cell structure, where the efficiencies were measured by NREL and were the records for thin film silicon photovoltaic technology.
Date of Conference: 4-6 July 2012