Optical properties of thermally evaporated In2S3 thin films measured using photoacoustic spectroscopy

In2S3 is a III-VI group semiconductor with n-type conductivity and a wide band gap energy, which can be suitable as a buffer layer alternative to CdS in thin film solar cell fabrication. In the present study, In2S3 thin films were grown on glass substrates by the thermal evaporation method at a constant substrate temperature of 200 °C. The deposited layers were post annealed in vacuum in the temperature range of 200-300 °C for 1 h. The grazing incident X-ray diffraction and scanning electron microscopy studies revealed polycrystalline nature of the layers with a good surface morphology. The optical properties of these annealed layers were investigated by using photoacoustic spectroscopy and independently by using conventional optical transmission spectroscopy. The photoacoustic spectra of In2S3 films showed a sharp fall in the photoacoustic signal at photon energies that increased from 1.95 to 2.74 eV with increasing annealing temperature, which corresponds to the band gap energy of corresponding films. The band gap energy and the refractive index values calculated by using photoacoustic spectra are in good agreement with that determined from transmission spectra.