Electrical properties of homogeneous Cu(In,Ga)S2 films with varied gallium content

Abstract Indirect synthesis via a copper-free precursor film leads not only to superior transport properties but also to an inherent grading of the Ga/(In + Ga) ratio when Cu-rich Cu(In,Ga)S 2 films have been deposited for thin film solar cells by two-stage sequential multi-source evaporation (sequence (In,Ga)–S/Cu–S). An inherent grading of the Ga/(In + Ga) ratio could be avoided by including a certain amount of Cu in the precursor. We have for the first time investigated the electrical properties and microstructure of the homogeneous Cu(In,Ga)S 2 films with changing Ga content. The carrier densities in Cu(In,Ga)S 2 films with Ga/(In + Ga) > 30% are found to be one order of magnitude larger than those in films with lower Ga content. The resistivity, its activation energy and mobility in Cu(In,Ga)S 2 films decreased with Ga/(In + Ga) > 30%. Electrical data are supplemented by micro-structural characterization using a transmission electron microscope. A reduction in grain size with increasing Ga content has been observed.