Simulation of optimum band-gap grading profile of Cu2ZnSn(S,Se)4 solar cells with different optical and defect properties

The effect of the band-gap profile on the performance of Cu2ZnSn(S-x,Se1-x)(4) (CZTSSe) solar cells was investigated using a solar cell capacitance simulator (SCAPS) device simulation program. The band gap of CZTSSe is tunable from 1.0 to 1.5 eV by changing the S/(S + Se) ratio. Currently, the evolution of the electron affinity (X) of CZTSSe at various band gaps has not been clarified yet, although two models with different X values at various band gaps of CZTSSe have been proposed. We simulated solar cell performance using these two models and the differential rates of efficiency were compared between them. As a result, we were able to design the optimum band-gap profile using both models. Meanwhile, the characteristics of a solar cell with various optical absorption coefficients and defect densities of the CZTSSe absorber were simulated. The superiority of the graded band-gap profile was demonstrated by comparing the cell performances with and without a grading profile structure. (C) 2014 The Japan Society of Applied Physics