Production and characterization of Cu2SnS3 films for solar cell applications: The effect of the sulfurization temperature on CuS secondary phase

Cu2SnS3 is a good alternative to solve the problems related to CdTe and CIGS absorber layers having toxic, expensive and rare-earth elements. In this study, CTS films were obtained by a two-stage process that includes sulfurization of Sn-Cu metallic precursors stacked by thermal evaporation. Sulfurization process was carried out in the temperature range of 400-550 degrees C, and the role of the sulfurization temperature on secondary phases of CTS films was reported. XRD and Raman analyzes revealed that CTS film sulfurized at 550 degrees C has a highly crystalline tetragonal-CTS phase, and undesirable CuS secondary phase can be significantly minimized due to increased sulfurization temperature. Elemental analyses showed that desired Cu-poor stoichiometry was reached at 550 degrees C and 550 degrees C. Optical analyzes indicated that optical band gap values approached the optimum value for photovoltaic applications, and 1.39 eV was reached especially for CTS-550 film. Thickness and optical constants of the films were determined using spectroscopic ellipsometry. CuS secondary phase in Cu-rich CTS-400 and CTS-450 films brought metallic behavior to the materials, and electrical resistivity of the Cu-poor CTS-550 film approached the appropriate value for photovoltaic applications. Besides, surface analyzes proved that sulfurization temperature has a strong effect on the surface properties and the film surface became more compact at 550 degrees C. As a result, this study showed that higher sulfurization temperatures (especially 550 degrees C) contribute to the solution of the CuS secondary phase problem, which limits the performance of CTS-based solar cells.