Effect of deposition time on the structural, morphological, and optical properties of CBD-grown flexible CuS thin films

This study demonstrates a simple, scalable, and cost-effective chemical bath deposition (CBD) strategy for the low-temperature (80 ∘ C) growth of crystalline films consistent with predominantly covellite-type CuS phase thin films on flexible polyester substrates, offering lightweight, mechanical flexibility, and low-cost processing for portable electronics. Systematic variation of the deposition time (2h-8h) revealed its critical role in governing the structural defect density, morphological evolution, and optical properties of the thin films. Prolonging the deposition time significantly enhanced crystallinity, increased the crystallite size from 32.04 nm to 43.99 nm, and reduced the lattice microstrain from 1.99 × 10 −3 to 1.45 × 10 −3 . Field-emission scanning electron microscopy (FESEM) images showed a distinct change in surface morphology from discontinuous grain clusters to a dense, three-dimensional, cauliflower-like porous network with improved surface coverage. This morphological evolution improved light-trapping ability, resulting in higher measured optical absorbance across the UV-Vis range and reducing the direct optical bandgap from 1.82 to 1.61 eV. Overall, the film deposited for 8 h presented the most favorable structural and optical characteristics, highlighting deposition time as a critical parameter for tailoring flexible CuS thin films for flexible optoelectronic applications.