CuInSe2 (CIS)-based semiconductors which are strong candidates for high-efficiency low-cost thin-film solar cells, have been investigated for device applications. First, high-quality CIS and CuGaSe2 (CGS) epitaxial films were grown by molecular beam epitaxy. CIS films showing predominantly free exciton emissions in photoluminescence spectra were grown for the first time by optimizing the growth conditions. Next, native defects, control of which is critical for solar cell performance, have been systematically investigated. A common acceptor-type defect has been identified as Cu-vacancies (V-Cu) from positron lifetime measurements and the Cu/In composition ratio of the CIS films. Unlike films grown under Cu-excess conditions, films grown under an In-excess tend to be heavily compensated, and the origin of the donor-type defects present in these films was found to be Cu-Se divacancies (VCu-Se). Reduction of Se-vacancies (V-Se) was found to be critical for improving solar cell performance. Air-annealing, the presence of a Cu-Se second phase and the interdiffusion of Na from the sodalime glass substrate were all found to be effective in suppressing V-Se formation. Post-growth annealing has been extensively used to understand the oxidation process of CIS and CGS. Annealing of CGS films at temperatures higher than 450degreesC resulted in a significant improvement in CGS film quality as well as the formation of a Ga-O second phase. The Cu/Ga ratios of oxygen-annealed films after the selective etching of Ga-O by H2SO4 were found to become nearly unity, suggesting that the formation of the Ga-O phase enhanced Cu outdiffusion leaving Cu-deficient CGS regions. (C) 2002 Elsevier Science B.V. All rights reserved.