Self-Organized Back Surface Field to Improve the Performance of Cu₂ZnSn(S,Se)₄ Solar Cells by Applying P-Type MoSe₂:Nb to the Back Electrode Interface

Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells have been encountering a bottleneck period since the champion power conversion efficiency (PCE) of 12.7% was achieved by Kim et al. in 2014. One of the critical factors that impede its further development is the relatively low open-circuit voltage (VOC) caused by serious interface carrier recombination. In this regard, back surface field (BSF) employment is a feasible strategy to address the VOC issue of CZTSSe solar cells to some extent. Here, we demonstrated a self-organized BSF introduced by prompting interfacial MoSe2 layer transition from inherent n-type to desirable p-type with Nb doping (p-MoSe2:Nb). The BSF application can significantly reduce the carrier recombination at the back electrode interface (BEI) and lower down the back contact barrier height. The PCE of the corresponding cell was improved from 4.72 to 7.15% because of the enhancement of VOC and fill factor, primarily stemming from the doubling aspects of increased shunt resistance (RSh), decreased series resistance (RS), and alleviative recombination velocity of the BEI induced by the BSF. Our results suggest that introducing a BSF fulfilled with p-MoSe2:Nb is a facile and promising route to improve the performance of CZTSSe thin-film solar cells.