Cu(In1−xGax)Se2 co-evaporated thin films from simple tungsten baskets-Influence of the gallium source

Cu(In1−xGax)Se2 (CIGSe) thin films with 0 ≤ x ≤ 1 are grown by co-evaporation. Cu, In and Ga elements are evaporated from simple tungsten baskets, while Se is evaporated from broad Ta basket. Different combinations of the metal sources have been tested using three and two tungsten baskets. It is shown that, when deposited on a substrate heated at 500 °C, the Ga is present throughout the thickness of the films whatever the technique used. X-ray photoelectron spectroscopy (XPS measurements have shown that Ga depth profile is more reliable in that case. X-ray diffraction shows that the films crystallize in the expected chalcopyrite structure. The lattice parameters decrease with increasing Ga atomic percentage. It is revealed that the optical band gap increases with the Ga content and yielded a bowing parameter around 0.28. The best results have been obtained with the four sources technique. Thin film solar cells, Mo/CIGSe/In2S3−xO3x/i-ZnO/ZnO:Al/Ni-Al grid, have been fabricated and probed. The efficiency of the cells depends strongly in the film composition but also in the metal source number.