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Difference of conduction band minimum (E C ) between transparent conductive oxide (TCO) and absorber, named ΔE C-TA , in thin-film solar cell is investigated for high cell performance using device simulator. According to the simulation, the optimized ΔE C-TA value is different, depending on the carrier density in buffer layer, N D-B . With ΔE C-TA above 0.6 eV for both N D-B s of 1.0 ' 10 13 and 1.0 ' 10 18 cm %3 , the spike is formed at the TCO/ buffer interface, thus decreasing cell performances, especially short-circuit current density owing to impeding photo-generated carriers to TCO. On the other hand, with ΔE C-TA s below %0.2 and %0.4 eV for N D-B s of 1.0 ' 10 13 and 1.0 ' 10 18 cm %3 , the solar cells demonstrate double diode characteristics, thereby decreasing cell efficiency. Eventually, the optimized ΔE C-TA values for high cell performance are proposed to be in the ranges from %0.2 to 0.6 eV and from %0.4 to 0.6 eV for N D-B s of 1.0 ' 10 13 and 1.0 ' 10 18 cm %3 , respectively.

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16 atom/cm 3 although higher than the threshold value.

The effect of the band-gap profile on the performance of Cu 2 ZnSn(S x ,Se 1%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 (χ) of CZTSSe at various band gaps has not been clarified yet, although two models with different χ 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.