Optical band-diagram, Urbach energy tails associated with photoluminescence emission in defected ZnO thin films deposited by sol–gel process dip-coating: effect of precursor concentration

This study employs zinc acetate as the precursor for synthesizing pure zinc oxide thin films through the sol–gel process. Comprehensive characterization and analysis were conducted using X-ray diffraction, field-emission scanning electron microscopy, ultraviolet–visible-infrared spectrophotometry, ellipsometry, and photoluminescence. The X-ray diffraction study confirms the hexagonal wurtzite polycrystalline structure of zinc oxide. Increasing the zinc precursor concentration resulted in a decrease in the observed intensity of diffraction lines, as revealed by XRD studies. Microstructural parameters such as crystallite size and micro-strain were determined using the Scherrer equation and the Williamson–Hall method. The Williamson–Hall method indicated an increase in crystallite size values from 28.3 to 36.9 nm, while micro-strain values decreased from 1.192 × 10⁻³ to 1.12 × 10⁻³ with an increase in zinc acetate concentration from 0.1 M to 0.5 M. Scanning electron microscopy images displayed connected microstructures with wrinkle network architectures influenced by the concentration of zinc acetate, consistent with the texture coefficient results. Optical characteristics were investigated in the 300–2500 nm range through transmission measurements and subsequent photoluminescence measurements. The optical band-gap energy was found to decrease from 3.33 to 3.21 eV and the band tail width (Urbach energy) decreased from 0.344 to 0.105 eV with an increase in zinc concentration.