Low temperature growth and properties of Cu–In–Te based thin films for narrow bandgap solar cells

Cu–In–Te based thin films were grown onto soda–lime glass (SLG) substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy, X-ray diffraction and Raman scattering. The crystalline quality of Cu–In–Te based thin films with high Cu/In ratios is superior to that of films with low Cu/In ratios. The films with Cu/In ratios of 0.69 ± 0.04 exhibited a single chalcopyrite phase with random orientation, whereas a defect chalcopyrite phase with a preferred (112) orientation was obtained for thin films with Cu/In ratios of 0.26 ± 0.02. However, the films with high Cu/In ratios of 0.69 ± 0.04 showed nearly constant low resistivity (∼ 10− 2 Ω cm) at temperatures from 80 to 400 K due to high hole concentration (> 1019 cm− 3), resulting in semi-metallic behavior. The hole conduction mechanism of the film (Cu/In atomic ratios = 0.26 ± 0.02) with semi-conductive properties was found to be variable-range-hopping of the Mott type in the wide range of 80–300 K. The optical bandgaps of Cu–In–Te based thin films are determined to be 0.93–1.02 eV at 300 K from transmission and reflection measurements. A solar cell with a ZnO/CdS/CuIn3Te5/Mo/SLG structure showed a total area (0.50 cm2) efficiency of 5.1% under AM1.5 illumination (100 mW/cm2) after light soaking. The conduction band offset at the CdS/CuIn3Te5 interface was estimated to be − 0.14 eV from X-ray photoelectron spectroscopy analysis.