Preparation and characterization of CuIn0.3Al0.7Se2 thin films for tandem solar cells

CuIn0.3Al0.7Se2 thin films were prepared by 4-source co-evaporation technique on soda lime glass substrates held at a substrate temperature of 673 K. The films are found to be near-stoichiometric, polycrystalline, single-phase with chalcopyrite structure. The lattice parameters are found to be a=0.572nm and c=1.128nm. The X-ray photoemission studies showed the presence of surface oxygen. The valence band spectrum of the films was found to match with the calculated band structure of CuInSe2 and CuAlSe2. Optical absorption studies, revealed three optical band gaps viz., 1.85, 1.92 and 2.18 eV. Considering the three-fold optical band structure of chalcopyrite compounds, these are attributed to the fundamental absorption and additional transitions that arise out of crystal field and spin-orbit interactions. The crystal field (ΔCFΔCF) and spin-orbit (ΔSOΔSO) splitting parameters deduced from the quasi-cubic model are found to be −0.09 eV and ΔSO=0.25eV, respectively. The films are found to be p-type conducting with the room temperature resistivity of 140 Ω-cm.