Properties of Cu2ZnSnS4 Films with Respect to Sulfurization Time Investigated using Cu-Zn-Sn Films Deposited by Co-sputtering

Abstract

Cu2ZnSnS4 (CZTS) thin films have been used as an absorber layer for solar cells because they have a large absorption coefficient and a direct band gap of suitable energy. We investigated the properties of CZTS thin films with respect to the sulfurization time by using Cu-Zn-Sn (CZT) metal precursor films. The CZT metal precursor films were deposited onto soda-lime glass by co-sputtering with single targets of copper (Cu), zinc (Zn), and tin (Sn). The sulfurization of the CZT films was performed in evacuated and sealed quartz ampoules using sulfur powder. The sealed samples were annealed at 550°C for times ranging from 1 to 60 min.The crystallization of the CZTS films as a function of annealing time was investigated using X-ray diffraction (XRD). The XRD peaks of CZTS appeared in the XRD patterns of the samples that were annealed for at least 3 min. Furthermore, the intensity of the CZTS peaks increased with annealing time. The increase in peak intensity and decrease in the full width at half maximum (FWHM) indicate that crystallization increases with annealing time. The morphology and the atomic compositional ratios of the CZTS films were also analyzed by using field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. The band gap energy of the CZTS films was determined using a UV-Vis-NIR spectrophotometer. The estimated optical energy band gap of the CZTS films that were annealed for more than 10 min ranged from 1.4 to 1.5 eV, which is close to the previously reported value of 1.45 eV.