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The Kestrite semiconductor material Cu2ZnSnSe4 (CZTSe) is believed to be a suitable candidate for replacing the CuIn1-xGaxSe2 (CIGS) absorber for the abundance and the non-toxicity of its components. However, the record efficiency of solar cells based on this material reaches 11% which is lower than the conversion efficiency of the CIGS based solar cell for which the efficiency has reached 25%. The aim of this study is to model and optimize the electrical performances of a superstrate type solar cell based on the kestrite material Cu2ZnSn(SxSe1-x)4 (CZTSSe). The goal is to investigate the effect of mixing the sulfide (S) component with selenide (Se) on the conversion efficiency η, band gap Eg open circuit voltage Voc, short circuit current density Jsc, fill factor FF and maximum power density P of the device, through the evaluation of their behavior as a function of the ratio S/(S+Se), which represents the concentration of sulfur in the absorber material CZTSSe. It is also shown in this work, through the calculation of the mismatch strain ε at the interface between the absorber and the buffer layers, that the zinc sulfide (ZnS) is a more appropriate buffer than cadmium sulfide (CdS) for the CZTSSe absorber. The effect of strain at the interface buffer/absorber on the bandgap energy of CZTSSe and then on the cell performances is evaluated. This evaluation is based on the strain theory in order to obtain more realistic results close to experimental results. It is noted that adding 72% of Sulfur in the absorber material, meaning that x=0.72, increases the efficiency to 13.1% therefore an improvement of 21.3% is obtained compared to the efficiency of the CZTSe solar cell with a strain equal to 0 meaning no deformation, Jsc= 15.35mA/cm², Voc= 0.800 V, FF = 74.1% and Pmax=9.45mW/cm².

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