Blue-shift in optical bandgap of sprayed nanocrystalline Cu2ZnSnS4 thin films induced by 200 MeV Xe swift heavy ions irradiation

Cu2ZnSnS4 (CZTS) thin films deposited on glass substrate using spray pyrolysis technique were irradiated by 200 MeV Xe ions at different fluences. The effect of high-density electronic excitations on the properties of CZTS thin films has been examined using UV–visible spectrophotometry for optical properties, X-ray diffraction and Raman spectroscopy for structural properties. A pronounced blue-shift in the transmittance spectra edges and bandgap widening up to 2.05 eV were observed as the irradiation fluence increased up to 5 × 1013 ions/cm2. Although CZTS crystallinity gradually deteriorated and crystallite size reduced to 16 nm, total amorphization of the films has not been reached even for the highest fluence. Latent track radius of 3.7 nm was determined experimentally from XRD measurements, in reasonable agreement with the inelastic thermal spike model, which predicts the transition of solid CZTS to molten phase leading to the formation of latent tracks. The possible reasons for the observed blue-shift were carefully discussed in the light of previous findings in literature, and finally was ascribed to the contribution of multiple factors such as quantum confinement effect and annihilation of localized defect bands.