Synthesis, optical and optoelectrical analysis of the Cu₂CoSnS₄ thin films as absorber layer for thin-film solar cells

In this work, the optical and optoelectrical characterizations of the quaternary kesterite Cu₂CoSnS₄ (CCTS₄) thin films were studied. The polycrystalline CCTS₄ films have prepared by the spray pyrolysis method at four different thicknesses (160 nm, 230 nm, 297 nm and 345 nm). The X-ray diffraction charts demonstrated that the as-prepared CCTS₄ films have a polycrystalline nature with tetragonal single phase. The linear optical parameters of the CCTS₄ films were computed by finding the transmission and reflection data. The magnitudes of the absorption coefficient for the CCTS₄ films were high in the order of 10⁵ cm⁻¹ while the energy gap of the CCTS₄ films has been reduced from 1.41 to 1.12 eV by raising the film thickness. The optical conductivity and the Urbach energy of the CCTS₄ films were computed from the absorption coefficient data. In addition, the nonlinear optical parameters represented in the nonlinear refractive index n₂ and the third-order nonlinear optical susceptibility χ⁽³⁾ of the CCTS₄ films were boosted with rising up the film thickness.