This study presents the successful preparation of highly oriented Cs₃Bi₂I₉ films with different thicknesses using sol–gel spin-coating method. X-ray diffraction analysis confirmed a single hexagonal crystal phase with a high orientation along the direction. The crystallite size in the range of 27 and 41 nm increases with increasing film thickness. The bandgap energy ranged from 1.91 to 1.97 eV and was found to decrease with increasing film thickness, which was associated with the film relaxation with increasing film thickness. The refractive index, absorption index, complex dielectric coefficients, complex optical conductivity coefficients, and the ratio of free charge concentration also were found to increase with film thickness. The first-order linear optical susceptibility ranged from 0.22 to 0.38, the non-linear optical susceptibility χ⁽³⁾ and the nonlinear refractive index parameters n⁽²⁾ were respectively within the range of 0.4–3.77 ×10^-12 esu 0.96–7.97 × 10^-11 esu, the highest value of all the parameters was found in the visible region at 500 nm. These findings position Cs₃Bi₂I₉ as a highly promising material for optoelectronics.

Inorganic Lead-free Perovskite, Non-linear Optical Parameters, Highly Oriented Films, Dispersion Oscillator Analysis