Design of CZTS/CZTSe tandem solar cells with enhanced performance

Abstract

The replacement of toxic and expensive materials with non-toxic, earth-abundant alternatives is an urgent necessity in the field of solar cells. Due to their environmentally friendly and optical properties, CZTS and CZTSe kesterite materials have been proposed as promising alternative compounds in thin film solar cell applications. In this study, a numerical investigation of a CZTS/CZTSe tandem solar cell structure is conducted using SILVACO software. In this structure, CZTS and CZTSe serve as the main absorbers in the top and bottom sub-cells, respectively. However, the bottom sub-cells require thick absorbers, which leads to a decrease in overall performance due to the limited diffusion length of minority carriers in the applied materials. A separate study of individually top and bottom cells was conducted, and their results closely matched those reported by world champions. Effects of the top absorber thickness were examined for the tandem configuration, which resulted in a conversion efficiency (η) of 15.71%. In order to enhance the performance of the tandem solar cell, a creation of a diffusion electric field in the absorbers of the sub-cells through gradient doping is proposed. The results demonstrated a significant improvement in performance, with gradient doping densities of 2 × 1014-1019 cm−3 and 1.5 × 1015-1019 cm−3 in the CZTS and CZTSe layers, respectively, yielding a remarkable conversion efficiency of 23.38%.