Influence of pre-sulfurization temperature on properties of Cu2ZnSnS4 thin film in two-step sulfurization process

Abstract

In this study, Cu2ZnSnS4 (CZTS) thin films were prepared by magnetron sputtering deposition of the Cu-Zn-Sn metallic precursor (CZT) on flexible molybdenum foils and a two-step sulfurization process. Low-temperature pre-sulfurization was performed before the final high-temperature sulfurization during the two-step sulfurization process to enhance the properties of CZTS. This paper presents a method to improve the properties of CZTS thin films, making them conducive to solar cell applications. The compositions and morphologies of CZTS were confirmed using energy dispersive spectroscopy and scanning electron microscopy, respectively. X-ray diffraction was conducted to investigate the structural properties of the CZTS thin films, and the results revealed that the CZTS phase in all the samples had a preferential plane along (112). Moreover, Raman spectroscopy helped distinguish the ZnS and Cu2SnS3 phases that are close to kesterite CZTS. Compared to the thin film prepared without pre-sulfurization treatment, a suitable increase in pre-sulfurization temperature could reduce the loss of Sn and enhance the crystallinity of CZTS. However, a higher pre-sulfurization temperature could result in the formation of secondary phases. The CZTS thin films exhibited good properties when the CZT precursor was pre-sulfurized at 150 °C. In addition, the CZTS thin films had a single phase and flat and dense surfaces with large grains. The optical parameters were calculated based on the reflectance and transmittance data obtained by UV–Vis spectroscopy. The absorption coefficient was larger than 1 × 104 cm−1, and the optical bandgap was about 1.55 eV; these are suitable properties for CZTS absorption layers in CZTS thin-film solar cells.