Transformation of ZnS microspheres to ZnO, their computational (DFT) validation and dye-sensitized solar cells application

ZnO spheres were synthesized by solid state oxidation of ZnS microspheres at different oxidation temperatures of 600 and 700 °C in an ambient atmosphere. The high temperature transformation of ZnS microspheres into hexagonal structured and highly crystalline ZnO microspheres comprising interconnected nanoparticles facilitating efficiently electron transport as well as charge collection through an intended path in the solar cell. The comparative physico-chemical and photovoltaic studies were done for synthesized ZnO microspheres. The structural study by x-ray diffraction of ZnO confirmed the hexagonal ZnO; the UV–Visible spectroscopy study showed λmax varying from 410 to 413 nm and 397 to 407 nm for ZnO microspheres synthesized by heating at 600 and at 700 °C, respectively. Additionally, crystalline and electronic structures were validated by density functional theory studies. The computational studies also revealed growth of hexagonal ZnO, where the bandgap varied with the oxidation temperature. The photovoltaic properties of ZnO microspheres synthesized at 600 °C exhibited better performance than the ones synthesized at 700 °C due to high surface roughness leading to enhanced dye loading and favorable charge collection. Dye-sensitized solar cells fabricated from ZnO microspheres synthesized at 600 and 700 °C exhibited the maximum power conversion efficiency of 3.38% and 3.06% correspondingly.