Optical properties and band gap energy of CuInSe2 thin films prepared by two-stage selenization process

Chalcopyrite polycrystalline CuInSe 2 (CIS) films with various elemental composition, synthesized by the two-stage selenization process on soda-lime glass substrates, were studied using photoluminescence (PL), photoluminescence excitation (PLE) and optical absorption (OA) techniques in the wide temperature range from 4.2 to 300 K in order to correlate the elemental composition with the optical properties of the material. For near-stoichiometric films (Cu/In≈1) intense free- and bound-exciton PL emission lines were observed at 4.2 K. At this temperature the A- and B-free exciton lines at E A =1.0409 and E B =1.0444 eV, respectively were found to be very narrow, with the full-width at half-maximum of about 2.5 meV, which is close to that of CIS single crystals. Well-resolved spectral lines of the A- and B-excitons were also observed in the OA and PLE spectra taken at 4.2 K. These excitonic lines remained intense at temperatures up to 78 K in both the OA and PL spectra. Deviations from stoichiometry significantly changed the PL spectra. The origin of the features observed in the PL spectra is discussed.