Enhancement of power conversion efficiency of Cu2ZnSn(S,Se)4 Solar Cells by pretreating Mo electrode with H2O2

It is proved that secondary phases, such as Cu2Se, and ZnSe etc., and MoSe2 layer can form at Cu2ZnSn(S,Se)4 (CZTSSe)/Mo electrode interface, due to that Mo electrode reacts with CZTSSe and Se vapor during selenization process. The secondary phases and MoSe2 with inappropriate thickness can decrease power conversion efficiency (PCE) of CZTSSe solar cell. In this work, a simple method was proposed to suppress the reaction between CZTSSe and Mo, and to optimize the thickness of MoSe2. This was achieved by growing a layer of Mo oxide in situ on the Mo electrode surface through soaking the Mo electrode in H2O2. The Mo oxide layer consisted of MoO3, MoOx and MoO2. It could suppress the reaction between Mo and CZTSSe, thus preventing the formation of secondary phases in CZTSSe. By further tuning the soaking time, the thickness and density of the Mo oxide layer were optimized, resulting in an optimized thickness of the MoSe2 layer and a significant improvement in the contact quality at both the front and back interfaces. This improvement is beneficial for suppressing carrier recombination and reducing contact resistance. Furthermore, the Cu/(Zn+Sn) ratio in CZTSSe was optimized, leading to an expansion of the depletion region width. As a result, the reverse saturation current density and series resistance decrease, while the photogenerated current density increases. This ultimately led to a rise in PCE from 7.66% for CZTSSe solar cells with Mo electrodes that were not soaked to 8.95% for those with Mo electrodes soaked for 20 seconds.