Enhancement in efficiency of CZTS solar cell by using CZTSe BSF layer

Copper Zinc Tin Sulphide (CZTS) thin film solar cell achieved a great attention of researchers due to its optimum direct energy gap, higher absorption coefficient and good electronic properties. In this work, the CZTS/CdS based solar cell structure with and without back surface field (BSF) layer has been investigated numerically by using SCAPS-1D. Firstly, a basic experimentally reported structure (FTO/ZnO/CdS/CZTS/Mo) without BSF layer is considered and replicated. Then, by inserting p-CZTSe material as BSF layer in between the absorber and back contact, a new device structure (FTO/ZnO/CdS/CZTS/CZTSe/Mo) has been proposed. The performance of both structures; with and without BSF layer, have been analyzed with respect to different layer parameters like: thickness, carrier concentration, and defect densities (Nt). A negative effect on cell performance has been observed with working temperature. From these analyses the optimum conversion efficiency of CZTS based PV cell without BSF layer has been found to be around 8.55% using CZTS absorber. The cell with CZTSe BSF layer (FTO/ZnO/CdS/CZTS/CZTSe/Mo) gives an overall conversion efficiency of 22.03%. The efficiency of both cells improved by replacing back contact metal with higher work function. This study gives the highest efficiency of about 29.86% for FTO/ZnO/CdS/CZTS/CZTSe/Pt structure. These simulation results could be an important guideline to a researcher for fabricating cost effective, non-toxic, and highly efficient CZTS solar cells.