Optimization and characteristics of CIGS solar cells with different evaporated Ga gradient

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The CIGS films were deposited on Mo coated soda-lime glass substrate by multi-source co-evaporation system. The composition of Cu/(In+Ga) within the frame from 0.8 to 0.9 and Ga/(In+Ga) within the frame from 0.25-0.4. The process began with an (In,Ga)2Se3 precursor was formed by evaporating In, Ga and Se on the substrate at 350°C. Subsequently, the precursors were exposed to Cu and Se and fluxed to form Cu-rich CIGS films at 550°C. In the third stage, a small amount of In-Ga-Se was evaporated to form Cu-poor CIGS films with the same substrate temperature (550°C). The analysis was performed about the structure of absorber layer films, surface properties, optical properties, electrical properties, elemental distribution, and conversion efficiency. The obtained CIGS absorber layer possesses unique equiaxed grain with (112) preferred orientation by co-evaporation process, diffraction angle shifts to high-angle, and thus reduced the d-spacing with increasing content of gallium. The content of Ga increases and the band-gap energy increases from 1.22 eV to 1.24 eV. The order vacancy compound (OVC) could be found on the film surface as copper content was within the range between 0.80 and 0.85. For the copper content of exceeding 0.85, the order vacancy compound quantity decreased and the stable CIGS compound was formed. The lowest Cu ratio was found at the absorber surface about 400 nm. After selenium annealing, the CIGS surface layer with copper content could increased from 0.6 to 0.8. The concentration of Ga is clearly increased to the back contact side after selenium annealing. In this research, the optimal condition of Ga/(In+Ga) is 0.36 corresponding to the Eg is 1.24 eV where the solar cell showed the highest efficiency of 13.06%.