Cu(InGa)Se2 thin film absorber with high Ga contents and its application to the solar cells

Cu(InGa)Se-2 (CIGS) thin film absorbers with a high Ga content were investigated for solar cell applications. The CIGS and CuGaSe2 (CGS) films were deposited by the three-stage growth process using a molecular beam epitaxy apparatus incorporating an in situ surface temperature monitoring system. Measurements of the growing surface temperature during the second stage showed 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, showed the presence of Cu2-xSe compounds, which are thought to be responsible for the degradation of the open circuit voltage of solar cells. The use of rapid thermal annealing (RTA) was studied to remove the metallic Cu2-xSe compounds from the surface layer with typical annealing conditions of 400 degreesC for 1 s. After the RTA process, the photovoltaic performance was found to be greatly improved, yielding a high conversion efficiency of 11.2% in CIGS solar cells with a high Ga content of 60%. (C) 2003 Elsevier Ltd. All rights reserved.