Fabrication of Cu(In,Ga)Se2 thin films by sputtering and selenization using diethylselenide

Full text : Chalcopyrite structure CuIn1-xGaxSe2 (CIGS) alloys are used as light-absorbing medium in high conversion efficiency, low cost, lightweight, and radiation-resistant solar cells. The ideal band gap energy for solar cells is known to be 1.4 eV, which corresponds to the band gap of CuIn0.4Ga0.6Se2. However, growing CuIn0.4Ga0.6Se2 alloys or CIGS solid solutions of high CuGaSe2 (CGS) molar faction is difficult because of unwanted compositional separation caused by the difference in reaction rates of the two end-point compounds. The morphology and crystal structure of the CIGS thin films strongly depend on the heating profile and the time of each step during the selenization process. However, few experimental results have been reported. In this presentation, it will be introduced the fabrication of CIGS thin films by selenization using diethylselenide [(C2H5)2Se: DESe] as a selenization source. Cu-In-Ga precursors were deposited on Mo by RF magnetron sputtering. The precursors were selenized using DESe under atmospheric pressure. The selenization process was performed by two sequential stages. First, the precursors were selenized at 300 degrees Celsium for 20 min (first step). Then, the precursors were heated to 515 degrees Celsium and selenized for 60 min (second step). The precursor consisted of Cu11In9 alloy before selenization. The film selenized at 300 degrees Celsium for 10 min had binary compounds such as Cu-Se, In-Se, Cu-In except CIGS. After the first step selenization at 300 degrees Celsium for 20 min, the Cu-Se alloy was not observed. The film selenized at 515 degrees Celsium for 60 min had only a CIGS chalcopyrite phase without an extra phase. These results indicate that the films selenized at 300 degrees Celsium are due to insufficient of CIGS growth, and phase transformation occurred during the selenization process when using H2Se or elemental Se as a selenization source.