Band gap optimization in Cu(In1 − xGax)(Se1 − ySy)2 by controlled Ga and S incorporation during reaction of Cu-(In,Ga) intermetallics in H2Se and H2S

A large number of companies around the world are developing a variety of manufacturing approaches aimed at low-cost, high throughput, large area CIS-based photovoltaic modules that maintain laboratory-scale cell efficiencies. The most critical technological issue, which directly impacts on the cost-of-ownership of large-scale production, is the specific technology employed for the deposition of the chalcopyrite absorber film. In standard reactive annealing processes, the complex reaction kinetics during the chalcogenization of the precursor film, results in phase segregated multinary alloys. This in turn results in compositionally graded absorber films, which could adversely affect the performance of devices, if grading is not carefully controlled through proper process control. Against this background, a clear understanding of the reaction paths for the formation of the chalcopyrite multinary alloys is essential. In this paper, the details of a fast solid-state reaction process producing single-phase homogeneous Cu(In1 − xGax)(Se1 − ySy)2 alloys, are discussed. The most significant material properties of the resulting single-phase chalcopyrite alloys, as well as the corresponding device characteristics, are also reviewed. This technology has been successfully demonstrated in a pilot facility at the University of Johannesburg and is currently been applied on a commercial level by Johanna Solar Technology GmbH.