Metal precursor with bi-layer indium for Cu(In,Ga)Se2 thin film preparation

• Bi-layer In film possess smoother surface and better coverage on CuGa layer.
• Surface of Cu(In,Ga)Se2 film becomes smoother and with less voids.
• A more uniform lateral surface composition distribution has been obtained.
• We improve performance of the solar cell by using a bi-layer In film technique.

A novel process was developed to prepare a sequentially stacked CuGa/In precursor using DC magnetron sputtering to make absorber for Cu(In,Ga)Se2 (CIGS) solar cell. Compared with precursor made of normally used single-layer In, CuGa/In film with bi-layer In was found to be with smoother surface and better coverage of the CuGa film by In islands. CIGS absorber prepared by selenizing the precursor with bi-layer In film showed a surface with less roughness, which indicated strong influence of the surface texture of the precursor on the morphology of the absorber. Furthermore, the thickness inhomogeneity of the In islands on the CuGa layer in the precursor was found to induce different degree of mixing for Ga and In, resulting in a lateral composition fluctuation among CIGS grains in addition to the normally observed vertical Ga grading in each grain. By using a precursor with bi-layer In film, an improvement of cell efficiency is always obtained resulting from the increased open circuit voltage (Voc) and larger fill factor (FF).