High-efficiency a-Si:H/μc-Si:H solar cells by optimizing A-Si:H and μc-Si:H sub-cells

The performance of a-Si:H/μc-Si:H tandem solar cell was improved by optimizing the a-Si:H top cell and μc-Si:H bottom cell, respectively. For the a-Si:H top cell, we focused on opto-electrical and structural properties of phosphorous-doped hydrogenated silicon (Si:H) films and their effect on the open circuit voltage (V_oc). The experimental results indicated that when nanosized silicon crystalline grains existed in amorphous silicon matrix, the V_oc of a-Si:H solar cells was much improved. An initial efficiency of 9.4% for a-Si:H solar cell was obtained. For the μc-Si:H bottom cell, we investigated the structural evolution along the growth direction of the intrinsic μc-Si:H layers. We introduced a high-quality initial seed layer at p/i interface to reduce the incubation layer thickness by lowering the silane concentration and very-high-frequency (VHF) power simultaneously. This initial seed layer acted as a seed layer for bulk μc-Si:H i-layer and the process reduced the ion bombardment on the p/i interface. We demonstrated a VHF power profiling technique by decreasing the VHF power step by step during the μc-Si:H deposition to control the structural evolution along the growth direction in the bulk i-layer. The advantage of this VHF power profiling technique was the reduced ion bombardments on growth surface because of the reduced VHF power. A high conversion efficiency of 9.36% was obtained for μc-Si:H p-i-n solar cell. Using a double n-layer (a-Si:H&μc-Si:H) in n/p tunnel recombination junction, we achieved the best conversion efficiency of 11.63% for a-Si:H/μc-Si:H tandem solar cells.