Compositional grading of CZTSSe alloy using exponential and uniform grading laws in SCAPS-ID simulation

The SCAPS (Solar cell capacitance simulation) program were employed to analyze the compositional dependence of CZTSSe (Copper Zinc Tin Sulphur Selenium) absorber layers with Cadmium Sulphide (CdS) as buffer layer and Zinc Oxide (ZnO) as window layer for thin film solar cells applications. The primary motivation of this simulation work is to optimize the composition for Se/(S + Se) ratio, which would yield higher efficiency. The exponential and uniform grading law in SCAPS were set for the composition grading y(x) over a layer, as well as to set the composition dependence P(y) of a property. By varying the different compositions with exponential law and by using uniform law for the same composition, best efficiency of 14.97% were achieved for 0.4-0.1 composition of sulphur and selenium with Se/(S + Se) ratio of 0.80. Higher efficiency were obtained with higher Se content. The influences of the band gap of the CZTSSe alloys, which decreases linearly with an increase in its Se content, were further analyzed. The corresponding open circuit voltage, short circuit voltage, fill factor and efficiency of the simulated values on a photovoltaic cell with Mo back contact were studied in detail.