Correlation of current mismatch and fill factor in amorphous and nanocrystalline silicon based high efficiency multi-junction solar cells

We present a systematic analysis of the correlation of current mismatch in a-Si:H/nc-Si:H double-junction solar cell fill factor (FF) measured under colored lights. First, we propose a diagnostic method to evaluate the FFs of component cells in multi-junction solar cells using current-voltage measurements under colored lights. The analysis reveals that the over-estimated FF when a colored light is illuminated results from a significant current mismatch. A mathematical calculation demonstrates that the diagnostic method can be used to estimate the FFs of single-junction component cells in a multi-junction structure. Second, we present our recent progress in the optimization of a-Si:H and nc-Si:H based solar cells. We emphasize the importance of improving the compactness of nc-Si:H materials and controlling the nanocrystalline evolution. Finally, we report our new record initial active-area efficiency of 15.4% achieved using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure.