Effective role of Bi doping in controlling the crystallographic, microstructures morphology and optical properties of Cu₂ZnSnS₄ thin films

Copper-rich Cu₂ZnSnS₄ (CZTS) thin films synthesized by chemical spraying were studied, focusing on the effect of adding bismuth instead of zinc on controlling the microstructural, morphological, and optical properties. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and UV-Visible spectroscopy were measurements. To perform a complete analysis of the X-ray pattern profiles, to determine the lattice parameters, crystallite sizes, phases, and microstrain, "material analysis using diffraction employing the Rietveld method" was used. XRD research confirmed a tetragonal kesterite structure and discovered the production of the Cu₃BiS₃ impurity phase. Measurements of crystallographic parameters indicate that the introduction of controlled bismuth into CZBTS improves crystallite size, degree of crystallinity and crystal deformation up to the percentage of 5%, while weakening other properties. The absorbance measurements revealed that the band gap of Cu₂Zn_1-xBi_xSnS₄ (CZBTS) was decreased systematically from 1.50 to 1.22 eV by increasing the Bi concentration. The optical energy gap in the visible region decreased as the bismuth ion penetrated and replaced the zinc ion within the unit cell, approaching the optimal energy gap for solar cell applications that were made in a less costly manner. In addition, as the doping ratio increases, the average refractive index increases, which is attributed to the larger crystallite size in the doped samples. Chemically sprayed CZBTS samples have demonstrated their suitability for use as an effective absorber layer in solar cells.