Structural and electronic characteristics of Cu(In,Ga)Se2 thin films sputtered from quaternary targets

Although the advantages of sputter deposition for large area, uniform deposition are well known, it has long been believed that sputtering Cu(In,Ga)Se₂ (CIGS) from a quaternary sputtering target yields films with morphological and electronic properties that make them unsuitable for use in high-efficiency photovoltaic devices. Recent work, however, has demonstrated that this deposition method can produce dense, polycrystalline, highly oriented films with the desired stoichiometry. Devices built with these films exhibit efficiencies >;10%. While effective parameters for target composition and deposition conditions have been achieved, variation from these conditions can result in a wide array of morphologies, even while composition remains near that of stoichiometric CIGS. In this paper, we review the broad range of structural and electronic properties that result from various sets of target compositions and deposition conditions. Films deposited under some conditions are similar in important respects - their composition, a dense structure composed of ~1 μm sized grains, and the presence of a MoSe₂ layer - to those of evaporated CIGS. We discuss how these results point towards the possibility of higher-efficiency sputtered CIGS.