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

Kesterite Cu2ZnSn(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 GeSe2 post-deposition treatment. The results reveal that the Ge4+ 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.