CuGaxSey Chalcopyrite-related Thin Films Grown by Chemical Close-Spaced Vapor Transport (CCSVT) for Photovoltaic Application - Surface and Bulk Material Properties, Oxidation and Surface Ge-Doping

Device grade ternary Cu-Ga-Se chalcopyrite thin films used for a photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapour transport (CCSVT) technique developed for a thin films growth on areas up to 10 x 10 cm2. A two-step process has been developed is allowing the fine tuning of the film composition and electronic properties appropriate for a subsequent solar cell preparation. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the phase transitions. Films were grown with chalcopyrite (1:1:2) and CuGaSe2-related defect compounds structures (DC) with stoichiometries of CuGa3Se5 and CuGa5Se8. X-ray fluorescence (XRF) spectroscopy and X-ray diffraction (XRD) techniques have been combined for a compositional and structural study of this material system probing both the bulk and near surface properties of the films. This analysis was also extended to the rear-surface investigation of selected two-phase thin films and complemented with surface sensitive X-ray photoelectron spectroscopy (XPS). From these results a growth model is presented for CuGa3Se5 formation in gallium rich CCSVTgrown CuGaxSey-films. In order to derive the defect compound structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe2-related defect compounds (DC) -CuGa3Se5 and CuGa5Se8- grown by elemental synthesis (powder) and CCSVT (thin films), respectively. The influence of an additional homogenization step during the elemental synthesis has been investigated and a dependence of the structural properties, including the lattice parameters and the occupation factors of cationic sites in the lattice was concluded. A good matching was found for the structural properties of CCSVT-grown thin film samples and elementally synthesized powders prepared under offequilibrium conditions and a new structural model has been derived. No hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures could be found.JRC.DDG.F.8-Renewable Energy (Ispra