Deposition potential governed structural, morphological and optical attributes of electrodeposited ZnS thin films

Zinc sulfide (ZnS) is a wide bandgap II – VI semiconductor that has attracted significant attention for applications in photovoltaics, optoelectronics and dielectric coatings due to its high optical transparency, chemical stability, and eco-friendly nature. Among various fabrication techniques, electrodeposition provides an effective route to tailor ZnS thin-film properties by precisely controlling the deposition potential. In this study, ZnS thin films were electrodeposited on indium tin oxide (ITO)-coated glass substrates from aqueous ZnCl₂ and Na₂S₂O₃·5H₂O electrolytes using chronoamperometry at applied potentials ranging from −1 to −4 V vs. Ag/AgCl. The structural, morphological, and optical properties of the deposited films were systematically investigated. X-ray diffraction (XRD) patterns confirmed that all films crystallized in the cubic ZnS phase with a preferred (200) orientation, while the crystallite size decreased at higher negative potentials. Field-emission scanning electron microscopy (FESEM) revealed that the surface morphology and film uniformity were strongly dependent on the applied potential. Optical studies indicated absorption edges in the 320–365 nm range, with the film deposited at −2 V exhibiting lower transmittance due to enhanced uniformity. The dielectric constant increased with wavelength, confirming the semiconducting behavior of the ZnS thin films. Furthermore, the optical band gap increased from 3.87 eV to 3.97 eV with increasing deposition potential, accompanied by an increase in Urbach energy from 1.75 eV to 2.12 eV. These results demonstrate that electrodeposited ZnS thin films possess tunable wide band gaps and low optical absorption, making them promising candidates for buffer-layer applications in photovoltaic devices.