Ga-grading profiles formed by incorporation of gallium into Cu(In1−xGax)Se2 absorber thin films

Highlights: ► CIGS thin films with Ga-grading profile were prepared by evaporation of GaxSe for incorporation of gallium. ► The process was found to have little effect on the film structure. ► No new impurity phases were detected. ► The proposed process was feasible for forming a 'notch' Ga-graded structure in CIGS thin films. - Abstract: Cu(In1−xGax)Se2 (CIGS) polycrystalline thin films were prepared by the two-stage method for two different precursors corresponding to the sequences CuGa/CuIn/CuGa and CuGa/In/Cu/CuGa, respectively. In order to obtain the required Ga-grading profile, co-evaporation of GaxSe for incorporation of gallium into CIGS was used. Experimental results show that CIGS films were prepared by the proposed process with a 'notch' Ga-graded distribution, and the surface morphologies of the films after gallium incorporation were significantly changed compared with that of the as-selenized films, and the process was found to have little effect on the film structure. No new impurity phases have been found. The main XRD peaks shows a slight shift to higher diffraction angles with increasing Ga content, which could be attributed to Ga atoms substituting for In atoms in the chalcopyrite structure. The result of the present work implied that the proposed process was feasible for forming a 'notch' Ga-graded structure in CIGS thin films.