Suppressed Interface Defects by GeSe2 Post‐Deposition Treatment Enables High‐Efficiency Kesterite Solar Cells

Abstract Kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) has emerged as a promising photovoltaic material not only because of its environmentally benign and earth‐abundant constituents, but also its outstanding photoelectronic properties. Unfortunately, the significant open‐circuit voltage ( V oc ) loss and inferior fill factor (FF) resulting from abundant nonradiative carrier recombination at depletion region has become a major obstacle for further improving device performance. Here, an effective strategy to passivate the deep trap and band‐tail states in the heterojunction is proposed, by modifying the CZTSSe absorber layer with GeSe 2 post‐deposition treatment. The results reveal that the Ge 4+ can migrate into the front surface of the absorber, which plays an active role in suppressing the Cu Sn deep defects and [2Cu Zn +Sn Zn ] defect clusters, accordingly dramatically reducing severe interfacial nonradiative carrier recombination of CZTSSe photovoltaic device. Under optimal treatment conditions, the CZTSSe solar cell efficiency increases from 10.36% to 12.22%, mainly benefitting from the increasement of V oc and FF.