Preparation of Narrow Band-Gap Cu₂Sn(S,Se)₃ and Fabrication of Film by Non-Vacuum Process

We successfully prepared a Cu₂Sn(S_1-xSe_x)₃ (CTSSe) solid solution with 0≤x≤1.0. CTSSe solid solution powders were synthesized by mixing the elemental powders and post-annealing at 600 °C. The crystal structure of Cu₂SnS₃ (CTS) was characterized by Rietveld refinement of the powder X-ray diffraction data and determined to be a monoclinic crystal system. The band gaps of CTSSe solid solution were determined by the diffuse reflectance spectra of the powder samples and the transmittance spectrum of the film fabricated by a non-vacuum thin-film fabrication process called printing and high-pressure sintering (PHS). The band gap (E_g) of CTS is 0.87 eV, which is in good agreement with the recently reported value of monoclinic CTS film. The band gap of the Cu₂Sn(S_1-xSe_x)₃ solid solution linearly decreases from 0.87 eV (x = 0.0) to 0.67 eV (x = 0.6) with increasing Se content. The CTSSe solid solution has potential as a narrow band-gap absorber material for thin-film full spectrum solar cells.