Fabrication of Cu2ZnSnSe4 solar cells through multi-step selenization of layered metallic precursor film

In this study, we proposed a 4-step selenization process for the RF-sputtered Cu-Zn/Sn metallic stack to prepare Cu2ZnSnSe4 (CZTSe) absorber. We applied a pre-heating treatment for the metal stack under vacuum prior to the selenization, which plays an important role to form a well inter mixed alloy with relatively smooth thin film morphology. The nucleation temperatures were controlled precisely from 150 °C to 500 °C during 4-step selenization to avoid the formation of secondary phases and to improve the crystal quality of CZTSe with a greater homogeneity in the composition. The formation of various phases during each step in 4-step selenization process were studied by X-ray Diffraction, Raman analysis and we proposed a possible reaction mechanism of the CZTSe formation with binary and ternary compounds as intermediates. We also performed optical analysis, including Uv-Visible absorption and low temperature photoluminescence, and scanning transmission electron microscope analysis for the CZTSe samples. Finally, an efficiency of 5.8% CZTSe solar cell is fabricated with an open circuit voltage of 370 mV, short circuit current of 31.99 mA/cm2, and a fill factor of 48.3%.