Preparation of Cu2ZnSnS4 (CZTS) sputtering target and its application to the fabrication of CZTS thin-film solar cells

The CuS, ZnS and SnS2 raw powders were adopted as the starting materials for preparing the 2-inch Cu2ZnSnS4 (CZTS) sputtering target. The powders were mixed at various molar ratios of Cu:Zn:Sn:S = 1.6:1:1:4, 1.8:1:1:4 and 2:1:1:4, pressed into 6-mm pellet forms, and then sintered at different temperatures for 8 h. Accordingly, the influence of powder constitution and heat-treatment conditions on the microstructure, composition and morphology of CZTS pellets were investigated. Microstructure and composition analyses indicated that the sample prepared with the Cu molar ratio of 1.6 and sintered at 600 °C for 8 h forms the kesterite CZTS single-phase structure with the Cu-poor/Zn-rich composition. The 2-inch CZTS sputtering target was prepared by utilizing the method established by the study of 6-mm pellets and adopted for depositing the CZTS thin films. The CZTS layer deposited at the working pressure of 1 mtorr followed by the sulfurization at 570 °C for 1 h possessed the stoichiometry of Cu24.15Zn14.73Sn10.75S50.37 which is the closest to the ideal stoichiometric ratio of 2:1:1:4. Moreover, it exhibited the bandgap of 1.5 eV, p-type carrier concentration of 2.8 × 1017 cm−3 and mobility of 8.61 cm2 V−1 s−1. Such a CZTS layer was implanted in the thin-film solar cells with the device structure of Mo/CZTS/CdS/i-ZnO/IZO/Al. Under the AM1.5 illumination condition, the CZTS thin-film solar cell sample with the open-circuit voltage of 0.52 V, the short-circuit current density of 19.17 mA/cm2, the fill factor of 52.7% and the conversion efficiency of 5.2% was achieved.