Identifying the origin of the Voc deficit of kesterite solar cells from the two grain growth mechanisms induced by Sn2+ and Sn4+ precursors in DMSO solution

Kesterite Cu2ZnSn(S,Se)(4) solar cells fabricated from DMSO molecular solutions exhibit very different open circuit voltage (V-oc) when the tin precursor has a different oxidation state (Sn(2+)vs. Sn4+). Here, the grain growth mechanism of the two absorbers was used as a platform to investigate the large voltage deficit issue that limits kesterite solar cell efficiency. The secondary sulfide composed Sn2+ precursor film took a multi-step phase fusion reaction path with secondary SnSe2 existing on the film surface during the whole grain growth, which forms in a very defective surface whereas a uniform kesterite structured Sn4+ precursor film took a direct transformation reaction path along with a top down and bottom up bi-direction grain growth that forms a uniform and less defective surface. Characterizations show that both absorber films exhibit similar bulk electronic properties with comparable band and potential fluctuations, Cu-Zn disorder level and tail states, and the much lower V-oc of the Sn2+ device than the Sn4+ device primarily comes from the serious recombination near the junction as revealed by the large ideality factor and reverse saturation current. Our results demonstrate that the large V-oc deficit of the kesterite solar cell mainly comes from surface deep defects that originated from the multi-phase fusion grain growth mechanism. The high efficiency (>12%) and low V-oc deficit (<300 mV) of Sn4+ processed CZTSSe solar cells highlight that direct phase transformation grain growth is a new strategy to fabricate high quality kesterite absorbers, which can also be applied to other multi-element thin film semiconducting materials.