Distinguishing metastable changes in bulk CIGS defect densities from interface effects

The deep defect distributions affecting majority carrier trapping in polycrystalline CuIn1-xGaxSe2 (CIGS) have been studied using drive level capacitance profiling. This technique provides, a spatial and energetic profile of sub-band gap defect transitions in the CIGS layer of working photovoltaic devices, while remaining insensitive to surface states. The bulk response was dominated by a defect which varied between 0.1 and 0.3 eV, according to the Meyer-Neldel rule. Devices grown at reduced substrate temperatures had smaller grain sizes and additional defect response. The effect of a light-soaking treatment, using near-band gap optical excitation, was studied. Both the free carrier density and the density of deeper defects were increased by this treatment. Device quality was degraded, predominantly due to a decreased fill factor. (C) 2003 Elsevier Science B.V. All rights reserved.