Effect of Cu–Sn–Se Liquid Phase on Grain Growth and Efficiency of CZTSSe Solar Cells

Herein, a liquid‐assisted grain growth (LGG) mechanism for a vacuum‐processed Cu 2 ZnSn(S 1− x Se x ) 4 (CZTSSe) absorber that is enabled by the presence of a liquid phase containing predominantly Cu, Sn, and Se (L‐CTSe) is suggested to explain the large grain size of up to ≈6 µm obtained at low temperatures, such as 480 °C. In this system, LGG plays a key role in achieving a large grain CZTSSe absorber, but the residual L‐CTSe, a key factor in LGG, deteriorates the device performance. L‐CTSe residue can possibly remain when using metal precursors for reasons such as local composition nonuniformity due to the agglomeration of Cu–Sn alloys, uncontrolled Zn volatilization. It is expected that CZTSSe cells with L‐CTSe residues have lower efficiency and fill factor than CZTSSe cells without L‐CTSe due to the role of L‐CTSe as a shunt path and compositional misfit. These tendencies are observed in the statistical results. Thus, when synthesizing CZTSSe using a metal precursor in a two‐step process, it is considered important to design the precursors to remove unwanted L‐CTSe residues and optimization of composition.