Earth-abundant and less toxic elements composed kesterite (Cu2ZnSn(S,Se)4, CZTSSe) semiconductors have great potential as low-cost and environmentally benign thin film solar cell absorbers. However, the performance of kesterite solar cells is currently limited by the large the large open-circuit voltage deficit (Voc,def). Here we show how the control of solution chemistry enables fabrication of low Voc,def and new record efficiency kesterite solar cells. We have found that when SnCl2 was used as Sn precursor, Sn2+ reacts with thiourea and forms strong Sn-S coordination bonds, which leads to the formation of SnS and other secondary phases in the precursor film. This precursor film takes a multi-step phase fusion grain growth and forms a very defective surface, resulting in low Voc and efficiency. On the contrast, when SnCl4 was used as Sn precursor, Sn4+ reacts with DMSO and forms weak and easy breaking Sn-O coordination bonds, resulting in formation of pure kesterite structured precursor film. This precursor film takes direct phase transformation reaction path and forms a uniform absorber with less defective surface, which achieves record low Voc,def for kesterite solar cell. Further, the unique chemistry in Sn4+ solution enables fabrication of solid solutions (AgxCu1-x)ZnSn(S,Se)4 (ACZTS, x=0-1), which significantly suppressed the formation of defects CuZn, [2CuZn+SnZn] and CuSn and achieves new record efficiency (13.0%) kesterite solar cell. Our results demonstrate careful control of solution chemistry is essential to mitigate defect property of kesterite, which can also be applied to other solution processed thin film semiconductors.

Solution chemistry control enables 13% efficient kesterite solar cell

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