Evolution of detrimental secondary phases in unstable Cu₂ZnSnS₄ films during annealing

The formation and phase evolution of Cu-S based compounds in kesterite Cu₂ZnSnS₄ (CZTS) absorbing thin films is a critical factor affecting the performance of these materials in thin film solar cells (TFSCs). However, to the best of our knowledge, few studies have investigated the segregation of Cu-S based compounds in kesterite thin films during the sulfurization process. In this study, stacked Cu/SnS₂/ZnS precursor thin films were annealed to systematically study the segregation and phase evolution of Cu-S based compounds in kesterite thin films subjected to functional sulfurization times at 550°C. The stacked precursor thin films appeared to be fully transformed to the pure kesterite phase when the sulfurization times are over 30 min. when analyzed using X-ray diffraction and Raman spectroscopy. However, transmission electron microscopy (TEM) characterization revealed that Cu-S based compounds segregated in the kesterite CZTS thin films annealed for 120 min. at 550°C. Based on the experimental results obtained for functional sulfurization times, a mechanism for Cu-S based compounds segregation and the phase evolution process is proposed. [Figure not available: see fulltext.]