Bandgap grading via sputtering and post-selenization using SeS2 powder enabling Sb2(S,Se)3 solar cells with 7.1% efficiency

This paper describes the fabrication of bandgap grading Sb2(S,Se)3 solar cells via sputtering and post-selenization/sulfurization using SeS2 powder as a sulfur source. The material characteristics and photovoltaic performances of the solar cells were assessed as functions of S/(S + Se) ratio. In experiments, an appropriate S/(S + Se) ratio resulted in grains with the preferred vertical orientation and fewer grain boundaries (GB) to hinder carrier transport, which together reduced charge transfer resistance. A suitable S/(S + Se) ratio also resulted in an S depth profile with a V-shaped distribution, which allowed bandgap grading in the depletion region. An excessively high S/(S + Se) ratio gradually altered the energy band at GBs from downward bending to upward bending, which increased the likelihood of electron-hole recombination at GBs. A high S/(S + Se) ratio also promoted excessive grain growth in various directions, which increased surface roughness and prevented the uniform coverage of the CdS layer, thereby reducing device performance. The optimal S/(S + Se) ratio of 0.27 resulted in a power conversion efficiency (PCE) of 7.1%, which is a new efficiency record for antimony chalcogenide-based thin film solar cells prepared via sputtering and post-selenization.