Phase evolution pathways of kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 thin films during the annealing of sputtered Cu-Sn-Zn metallic precursors

The formation of secondary phases in kesterite-based solar cells plays a critical role, affecting the performances of devices primarily due to the open circuit voltage deficient characteristics. Systematic studies of the phase evolution pathways and the formation of secondary phases in kesterite thin films are desirable. In this paper, the phase evolution pathways and the formation of secondary phases in kesterite Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) thin films during thermal treatment of sputtered Cu-Sn-Zn metallic precursors have been studied using high-resolution X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). A possible phase evolution process for the kesterite thin films is proposed based on the experimental observations. The Cu-Sn-Zn metallic precursor thin films were prepared using the sputtering technique followed by thermal treatments at different temperatures ranging from 250 °C to 580 °C in a chalcogen atmosphere. XRD and Raman characterizations of the thin films annealed under chalcogen atmospheres indicated that the precursor thin films were completely transformed to the CZTS and CZTSe kesterite phases above 500 °C. However, the TEM analysis of the thin films annealed above 450 °C revealed Cu-based secondary phases. The formation of phase pure CZTS and CZTSe kesterites was observed at annealing temperatures above 500 °C. Based on the experimental observations, the phase evolution pathways for the formation of kesterite and secondary phases from metallic precursors are proposed in this paper.