Improved performance of Cu(InGa)Se/sub 2/ thin film solar cells with high Ga composition using rapid thermal annealing process

Cu(InGa)Se/sub 2/ (CIGS) and CuGaSe/sub 2/ (CGS) films were deposited by the three-stage growth process using a molecular beam deposition (MBD) apparatus incorporating an in-situ surface temperature monitoring system. Measurements of the growing surface temperature during the second stage showed that the chemical reaction between Cu and Ga to be slower than that between Cu and In. Raman spectroscopy of as-grown samples with a high Ga content revealed that metallic Cu/sub 2-x/Se compounds existed on the surface and the compounds are thought to be responsible for the degradation of the open circuit voltage of solar cells. The use of rapid thermal annealing (RTA) was widely studied to remove the metallic Cu/sub 2-x/Se compounds from the surface layer. It was found from the experiments that hydrogen plays an important role in reducing the Cu/sub 2-x/Se compounds in the RTA process. By using the RTA process, the photovoltaic performance was greatly improved, yielding a high conversion efficiency of 11.7% for a Cu(In/sub 0.4/Ga/sub 0.6/)Se/sub 2/ solar cell.