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 (Voc) when the tin precursor has a different oxidation state (Sn2+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 Voc 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 Voc 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 Voc 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.