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 degrees 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 degrees 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.