Microstructure and Optical Properties of Zinc Sulfide Fabricated by Pulsed Electrochemical Deposition

ZnS is an n-type semiconductor with a wide direct band gap (3.7eV at room temperature), and is potentially used as the window layer in heterojunction photovoltaic solar cell. In this present study, ZnS thin films were electroplated onto ITO from the electrolyte containing zinc sulfate (ZnSO4) and sodium thiosulphate (Na2S2O3) using pulsed electrochemical deposition (PECD), by which the potentials for oxidation and reduction reactions, respectively, were alternatively applied. Experimental results show that when the cathodic potential was set to be -1.2V (vs. SCE), the surface morphology and crystallinity of the deposit were markedly influenced by the anodic potentials that were varied between -0.5 and -0.8V while the duration was set to be 4 s. Moreover, the deposit plated at an anodic potential of -0.7V had the composition close to stoichiometry of ZnS. The surface morphology and optical properties of the deposit underwent insignificant change as the duration time of anodic half-cycle was increased from 4 to 9s. The pH of the solution also played an important role in the properties of ZnS films. That is, zinc hydroxides formed in the solution with pH exceeding 4, while extensive hydrogen evolution occurred in the solution with pH less than 3. Finally, transmittance of the deposit was increased to be around 90% after 1h of annealing at 300℃. The deposits after annealing displayed an amorphous structure as characterized by XRD. Transmission spectrum also revealed the annealed deposit can be amorphous.