Alternative back contact designs for Cu(In,Ga)Se2 solar cells on polyimide foils

A multilayer Mo stack is conventionally used as the back contact for flexible Cu(In,Ga)Se2 (CIGS) solar cells on polyimide foils, where record efficiencies of 18.7% were reported on lab-scale. The aim of this work was to develop alternative back contact designs on polyimide foils with the objective to reduce material costs, and to increase production throughput and yield. Two different back contact concepts are discussed, which are either based on Cu or Ti. The Cu-based back contact is a cost-effective design providing low sheet resistance of 0.2 Ω/square at ~ 200 nm stack thickness. To prevent Cu diffusion into the CIGS absorber, a TiN diffusion barrier was integrated into the back contact design. In addition, Cr was evaluated as a candidate of an alternative Cu-diffusion barrier material. Best cell efficiencies of 16.3% were achieved without antireflection coating on a Cu-based back contact design with a TiN diffusion barrier. For the Ti-based design, which is well adapted to the physical properties of the polyimide foil, efficiencies up to 18.1% were measured with antireflective coating.

► Investigation of Cu- and Ti-based back contact designs on polyimide foils
► The ~ 200 nm thin Cu-based contacts have a sheet resistance of only 0.2 Ω/square.
► TiN shows a good diffusion barrier performance against Cu.
► Cu-based contacts: best cell efficiency of 16.3% (without antireflective coating)
► Ti-based contacts: best cell efficiency of 18.1% (with antireflective coating)