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 kesterite (CZTSSe) thin film solar cell performance further improvement is the large open-circuit voltage deficit (Voc,def) issue, which is 0.345V for the current world record device with an efficiency of 12.6%. In this work, SnCl4 and SnCl2_2H2O are respectively used as tin precursor to investigate the Voc,def issue of dimethyl sulfoxide (DMSO) solution processed CZTSSe solar cells. Different complexations of tin compounds with thiourea and DMSO lead to different reaction pathways from solution to absorber material and thus dramatic difference in photovoltaic performance. The coordination of Sn2+ with Tu leads to the formation of SnS and 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 is 8.84% with a Voc,def of 0.391V. The coordination of Sn4+ with DMSO facilitates direct formation ofkesterite CZTS phase in the precursor film which directed converted to CZTSSe during selenization, resulting in compositional uniform absorber and high device performance. A device with active area efficiency 12.2% and a Voc,def of 0.344 V was achieved from Sn4+ solution processed absorber. Furthermore, CZTSSe/CdS heterojunction heat treatment (JHT) significantly improved Sn4+ device performance but had slightly negative effect on Sn2+ device. A champion CZTSSe solar cell with a total area efficiency of 12.4% (active are efficiency 13.6%) and low Voc,def of 0.297 V was achieved from Sn4+ solution. Our results demonstrate the preformed uniform kesterite phase enabled by Sn4+ precursor is the key in achieving highly efficient kesterite absorber material. The lowest Voc-def and high efficiency achieved here shines new light on the future of kesterite solar cell.