Effect of deposited temperatures of the buffer layer on the band offset of CZTS/In_2S_3 heterostructure and its solar cell performance

The effect of the deposition temperature of the buffer layer In₂S₃ on the band alignment of CZTS/In₂S₃ heterostructures and the solar cell performance have been investigated. The In₂S₃ films are prepared by thermal evaporation method at temperatures of 30, 100, 150, and 200℃, respectively. By using x-ray photoelectron spectroscopy (XPS), the valence band offsets (VBO) are determined to be -0.28 ±0.1, -0.28±0.1, -0.34±0.1, and -0.42±0.1 eV for the CZTS/In₂S₃ heterostructures deposited at 30, 100, 150, and 200℃, respectively, and the corresponding conduction band offsets (CBO) are found to be 0.3±0.1, 0.41±0.1, 0.22±0.1, and 0.01±0.1 eV, respectively. The XPS study also reveals that inter-diffusion of In and Cu occurs at the interface of the heterostructures, which is especially serious at 200℃ leading to large amount of interface defects or the formation of CuInS₂ phase at the interface. The CZTS solar cell with the buffer layer In₂S₃ deposited at 150℃ shows the best performance due to the proper CBO value at the heterostructure interface and the improved crystal quality of In₂S₃ film induced by the appropriate deposition temperature. The device prepared at 100℃ presents the poorest performance owing to too high a value of CBO. It is demonstrated that the deposition temperature is a crucial parameter to control the quality of the solar cells.