Excitonic luminescence of polycrystalline CuInS2 solar cell material under the influence of strain

Abstract

Using molecular beams, polycrystalline CuInS2 (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 (FXA) 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 FXA is accompanied by an increase of the energetic splittings between FXA, FXB, and FXC. 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 FXA, FXB, and FXC. The XRD data show the increasing strain to occur with decreasing lattice spacings in growth direction.