Optimisation of physical properties in spray-pyrolyzed CdS: in thin films on polyimide substrates

In this study, high-quality pristine and Indium-doped Cadmium Sulfide (CdS: In) thin films were successfully synthesised on flexible polyimide substrates using the cost-effective chemical spray pyrolysis technique. This research systematically evaluates the optimisation of physical properties by investigating the impact of Indium doping concentrations (0%, 1%, and 3% wt.) on the structural, morphological, optical, and electrical characteristics to address film-substrate compatibility challenges in flexible electronics. XRD analysis confirmed a hexagonal wurtzite phase in all films with a dominant (002) orientation. Doping significantly enhanced crystallinity, expanding crystallite size from 45.54 nm to 63.88 nm, while dislocation density and microstrain declined. AFM and SEM micrographs verified that Indium incorporation modulates surface topography, showing a transition from nanogranular roughness to a compact, large-grained coalesced surface. Optical characterisation demonstrated high visible transparency, with maximum transmittance reaching approximately 81% for the 3% In-doped sample, alongside a systematic blue shift in the bandgap from 2.40 eV to 2.51 eV due to the Burstein-Moss effect. Hall Effect measurements confirmed n-type conductivity, revealing a substantial surge in carrier concentration and Hall mobility (34.73 cm²·V−¹·s−¹). These optimised films are excellent window layer candidates for next-generation flexible photovoltaic devices.