We present our research and development of fabricating mu c-Si:H bottom cells on a-Si:H/a-Si:H double-junction solar cells made in a commercial production line to make a-Si:H/a-Si:H/mu c-Si:H triple-junction solar cells. The mu c-Si:H bottom cells were deposited in a single-chamber system. We studied the n/p tunnel junction between the a-Si:H middle and mu c-Si:H bottom cells by optimizing the hydrogen plasma treatment time, the mu c-Si:H n layer, and the mu c-Si:H p layer. We found that a proper combination of the three key elements can form a high-quality n/p tunnel junction with electrical and optical properties suitable for the triple-junction solar cells. We used the optimized n/p tunnel junction in a-Si:H/a-Si:H/mu c-Si:H p-i-n-type triple-junction solar cells and improved the cell performance significantly, mainly from the increased open-circuit voltage and fill factor. We further optimized the mu c-Si:H bottom cells. Combining the optimized n/p tunnel junction and high performance mu c-Si:H bottom cell, we made a-Si:H/a-Si:H/nc-Si:H triple-junction solar cells having an initial efficiency of 8.85%, which is 21% higher than the conversion efficiency of the a-Si:H/a-Si:H tandem solar cells from the current production line. (C) 2012 Elsevier B.V. All rights reserved.