The formation mechanism of secondary phases in Cu2ZnSnSe4 absorber layer

Secondary phase formation in a Cu2ZnSn(S,Se)4 based p-type layer for photovoltaic applications is one of the major problems which must be overcome to improve solar cell efficiency. To better understand the crystallization mechanism of secondary phases in the material system Cu-Zn-Sn-Se, we investigated the selenization of binary metallic layers as a function of temperature. The sputtered thin film precursors comprise Cu-Zn, Cu-Sn, and Zn-Sn with different sequence. A total six precursors were studied by time-resolved X-ray diffraction while temperature increased from 30 °C to 550 °C. Selenium had been always deposited on top of the metallic precursors in a separate thermal evaporation step. The observed reaction sequences were shown different results depending on the metals which are in first contact with selenium. From these experimental results, the mutual affinities of metals are determined, and a way of reducing the ZnSe crystallization is presented. Preferable precursor composed of ternary metallic layers with Se is also suggested in conclusion with respect to a stacking order of the metals.