Intrinsic strain dependent redshift in optical band gap of Cu2ZnSnS4 nanostructured thin films

Highlights: • Uniform size, single phase CZTS nanoparticles were prepared by hydrothermal method. • Heating affects the structural and optical properties of CZTS nanoparticles. • On heating, crystallite size remains almost similar but intrinsic strain decreases. • Observed red shift in optical band gap was attributed to variation in strain. - Abstract: Single-phase Cu₂ZnSnS₄ (CZTS) thin films have been spray deposited on glass substrate, using uniform size oleic acid coated CZTS nanoparticles and the effect of heating temperature (75 °C to 300 °C) on the structural and optical properties have been investigated. X-ray diffraction results confirmed the formation of the single phase CZTS with a preferred orientation in (112) plane. The crystallite sizes of these films are ~7.0 ± 0.5 nm but the intrinsic strain present in the nanostructured films decreases from 9.1 × 10⁻³ to 1.3 × 10⁻³ with the increase of the heating temperature from room temperature to 300 °C. Optical band gap of the as deposited CZTS films is ~1.72 eV and it decreases to 1.45 eV for the CZTS films heated at 300 °C. The observed red shift in the band gap of CZTS films with the heating temperature has been attributed to a reduction in the intrinsic strain present in the film with the increase in the heating temperature.