Surprising Efficiency Enhancement of Cu2ZnSn(S,Se)4 Solar Cells with Abnormal Zn/Sn Ratios

The elemental proportion of Cu poor and Zn rich in Cu2ZnSn(S,Se)(4)(CZTSSe) is well established for achieving highly efficient CZTSSe solar cells. However, how high Zn/Sn ratio can the complicated CZTSSe thin film tolerate remains a question. Therefore, herein, the well control of Zn/Sn ratio in CZTSSe thin film is obtained by multi-spin-coating and tuning the initial Zn/Sn ratio in the Cu-Zn-Sn-S precursor ink from 1.0 to 1.9. It is found that the Zn/Sn on the surface of CZTSSe absorber can self-regulate to around 1.2 even with Zn/Sn ratio up to 1.9 in the precursor solution. Excess Zn presented as Zn(S,Se) secondary phase not only concentrate near the bottom area, but also widely distribute at the grain boundaries (GBs). In addition, it is found that the Zn(S,Se) secondary phase at GBs can promote current transport as revealed by conductive atomic force microscopy measurement. The surface roughness and grain size of the resulting CZTSSe absorber increased, whereas the MoSe(2)thickness was reduced with increasing Zn/Sn ratio. More importantly, the device performance increased from 4.5% to 10.0% with a significant decrease inV(OC)deficit from 0.73 to 0.58 V when the Zn/Sn ratio increases from 1.0 to 1.5 in the precursor ink.