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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.