Numerical simulation and optimization of a-Si:H/a-SiGe:H/µc-Si:H triple junction thin film silicon solar cell designs for high conversion efficiency

Thin film silicon solar cells based on hydrogenated amorphous silicon (a-Si:H), hydrogenated amorphous silicon–germanium (a-SiGe:H) and hydrogenated microcrystalline silicon (μc-Si:H) are promising candidates for low-cost photovoltaic technology due to their low costs and low- temperature processing. However, the conversion efficiency of a thin film silicon solar cell is low compared to that of a silicon solar cell. Conversion efficiency improvements can be obtained through the use of multi-junction solar cells consisting of series of connected subcells. This work focuses on a numerical investigation of triple junction solar cells consisting of a-Si:H top cells, a-SiGe:H middle cells and µc-Si:H bottom cells, with the goal of obtaining a high conversion efficiency. The initial conversion efficiency is 15.79%. After optimizing the top, middle and bottom subcell thicknesses at current matching, the conversion efficiency is enhanced to 16.95%. The band gaps of the top, middle and bottom subcells are then optimized, increasing the conversion efficiency to 18.25%. It is found that the triple junction solar cell is sensitive to defects in the a-Si:H top cell, the conversion efficiency can be improved to 20.3% when defects are reduced. The simulation results reported in this work can be used to optimize and develop thin film silicon solar cells.