Excitonic luminescence of polycrystalline CuInS 2 solar cell material under the influence of strain

Using molecular beams, polycrystalline CuInS 2 (CIS) films were deposited on Mo-covered Si substrates. In order to investigate the influence of growth-induced strain on the optical and structural properties, detailed photoluminescence, photoreflectance and x-ray diffraction (XRD) measurements were performed. The transition energy of the free A -exciton ( FX A ) transition decreases with (i) decreasing thickness of the CIS layer at a constant thickness of the Mo buffer layer and (ii) increasing thickness of the Mo buffer layer at a constant CIS layer thickness. This appreciable redshift of FX A is accompanied by an increase of the energetic splittings between FX A , FX B , and FX C . When we compare theoretically predicted valence band splittings as a function of the crystal field-obtained from the calculated relative valence band energies-to our experimental values, a completely coherent picture is obtained. We also derived the structure of the conduction band as a function of crystal field, based on the theoretically expected valence band structures combined with the measured transition energies of FX A , FX B , and FX C . The XRD data show the increasing strain to occur with decreasing lattice spacings in growth direction.