Multi-junction solar cells are known to have a considerably increased efficiency potential over their typical single junction counterparts. In order to produce low cost and lightweight multi-junction devices, the availability of suitable narrow (< 1.1 eV) bandgap bottom cells is paramount. A possible absorber for such a bottom cell is the Cu(In, Ga)Se-2 (CIGS) compound semiconductor, one of the most efficient thin film materials to date.

In this contribution we report on the RbF post deposition treatment of narrow bandgap CIGS absorbers grown with a single bandgap grading approach. We discuss the necessary deposition conditions and the observed improvements on solar cells performance. A certified record efficiency of 18.0% for an absorber with 1.00 eV optoelectronic bandgap is presented and its suitability for perovskite/CIGS tandem devices is shown.