Cu(In1-xGax)S2 Thin Films Prepared by Sulfurization of Precursors Consisting of Metallic and Gallium Sulfide Layers

Cu(In1-x Gax)S2 alloy thin films were prepared by annealing Cu/In/GaS precursors under a H2S atmosphere. The adhesion of the film to a Mo-coated soda-lime glass substrate was improved by introducing a GaS deposit as the first precursor layer. Optical transmission measurements revealed that the band gap of the thinner alloy film shifts from 1.42 to 2.41 eV with increasing composition ratio x. This sulfurization process, however, does not give rise to a single-phase thick Cu(In1-x Gax)S2 alloy film when x is larger than 0.2. Therefore, the composition of Ga in the film should be restricted to a smaller amount. A solar cell of over 10% efficiency has been obtained using the thin film prepared from the precursor with composition ratios of Cu/(In+Ga) = 1.2 and Ga/(In+Ga) = 0.05.