Analysis of optical band gap and urbach tail of zinc sulphide coated with aqueous and organic dye extracts prepared by chemical bath deposition technique

Abstract Detailed analysis of the optical band gap of zinc sulphide (ZnS) film coated with organic dye extract was conducted in this research. The chemical bath deposition technique was used in the deposition of ZnS coated with aqueous and alkanone dye extracts on glass substrate, annealed at different temperatures (100OC, 150OC and 200OC). Transmittance measurements were taken at room temperature in the wavelength range 300–1000 nm. The plots of the square of the product of absorption coefficient and photon energy were used to estimate the optical band gap energy of the film samples. The band gap exhibited red shift for some of the films and blue shift for other samples. Increasing the concentration of the precursor (Zn2+), increased the band gap of some film samples and decreased for others. The Urbach tail estimated from the plot of natural logarithm of the absorption coefficient as a function of photon energy, depicted a linear relationship for some of the film samples with respect to post-deposition temperature. In terms of the effect of concentration, the Urbach rule did not show a trend. The optical band gaps obtained in this study are in the range suitable for use in optoelectronics and solar photovoltaic applications.