A review on barrier layers used in flexible stainless-steel based CIGS photovoltaic devices

Two primary engineering challenges are en route to fabricating high-performance flexible stainless-steel based Cu(In,Ga)(S,Se)2 solar cells; Growing absorbers without contamination from the substrate, and providing alkali dopants to the absorber. The former is chiefly addressed by introducing a barrier layer, and the latter by post-deposition treatment or including dopant-containing layers in the stack. Here we organize these solutions and different approaches in an accessible scheme. Additionally, reports on interaction between contamination and alkali elements are discussed, as is the impact of barrier layer properties on the interconnect technology. Lastly, we make recommendations to consolidate the multitude of sometimes inharmonious solutions.