Sn⁴⁺ precursor enables 12.4% efficient kesterite solar cell from DMSO solution with open circuit voltage deficit below 0.30 V

The limiting factor preventing further performance improvement of the kesterite (sulfide Cu₂ZnSnS₄ (CZTS), selenide Cu₂ZnSnSe₄ (CZTSe), and their alloying Cu₂ZnSn(S,Se)₄ (CZTSSe)) thin film solar cells is the large open-circuit voltage deficit (V_oc,def) issue, which is 0.345 V for the current world record device with an efficiency of 12.6%. In this study, SnCl₄ and SnCl₂·2H₂O were respectively used as tin precursor to investigate the V_oc,def issue of dimethyl sulfoxide (DMSO) solution processed CZTSSe solar cells. Different complexations of tin compounds with thiourea (Tu) and DMSO resulted in different reaction pathways from the solution to the absorber material and thus dramatic differences in photovoltaic performance. The coordination of Sn²⁺ with Tu led to the formation of SnS, ZnS and Cu₂S in the precursor film, which converted to selenides first and then fused to CZTSSe, resulting in poor film quality and device performance. The highest efficiency obtained from this film was 8.84% with a V_oc,def of 0.391 V. The coordination of Sn⁴⁺ with DMSO facilitated direct formation of CZTS phase in the precursor film which directly converted to CZTSSe during selenization, resulting in compositional uniform absorber and high device performance. A device with an active area efficiency of 12.2% and a V_oc,def of 0.344 V was achieved from the Sn⁴⁺ solution processed absorber. Furthermore, CZTSSe/CdS heterojunction heat treatment (JHT) significantly improved the performance of the Sn⁴⁺ device but had slightly negative effect on the Sn²⁺ device. A champion CZTSSe solar cell with a total area efficiency of 12.4% (active area efficiency of 13.6%) and a low V_oc,def of 0.297 V was achieved from the Sn⁴⁺ solution. Our results demonstrate the preformed uniform CZTSSe phase enabled by Sn⁴⁺ precursor is the key for the highly efficient CZTSSe absorber. The lowest V_oc,def and high efficiency achieved here shines new light on the future of CZTSSe solar cell.