Radiative recombination in CVT-grown CuGaSe2 single crystals and thin films

The theoretically expected high open circuit voltage of CuGaSe2 (CGSe) based thin film solar cells is partly limited by their high deep defect density. To gain more insight on this aspect, we have analyzed the radiative recombination mechanisms of intrinsic CGSe single crystals and polycrystalline films with various compositions using the steady state photoluminescence spectroscopy technique. The single crystals were grown by chemical vapor transport (CVT) in a closed system using iodine as transport agent and polycrystalline CGSe as the raw charge. The CGSe thin films were grown by an open-tube chemical vapor deposition (CVD) technique using Cu2Se and Ga2Se3 as source materials. Iodine and chlorine were used as transport agents for source materials in the growth process. The observed low temperature photoluminescence is described by a well-known defect model based on two shallow acceptors and one shallow donor. However, the model is widened in this study to include additional deep luminescence transitions, observed approximately at hnu = 1.3 eV in nearly stoichiometric CGSe grown with halogen support. The characteristic chemical reactions of the CVD/CVT growth process, analyzed with thermodynamic equilibrium calculations, are shown to be responsible for the presence of these deep defects. (C) 2003 Elsevier Science B.V. All rights reserved.