Optimizing Na and Rb Incorporation Along With Bandgap Grading for High Efficiency Ultra‐Thin CIGSe Solar Cells

ABSTRACT

This work investigates the role of Na and Rb incorporation in ultra‐thin Cu(In,Ga)Se ₂ (CIGSe) solar cells, focusing on performance optimization. By systematically varying the Rb content and combining predeposited Na with postdeposited Rb, we identify the optimal configuration, achieving efficiencies of 11.72% on Mo and 10.48% on ITO. Comprehensive structural and electrical characterization reveals that Rb not only enhances the absorber quality through superior grain boundary passivation, following a Na–Rb exchange mechanism, but also modifies the CIGSe/CdS interface by influencing the absorber surface and Cd incorporation. While combined Na and Rb postdeposition treatments suppress Cd incorporation, predeposited Na can promote it. In both cases, the added Na and Rb support the creation of an ordered vacancy compound (OVC) layer, which improves the band alignment. Temperature‐dependent IV (IVT) measurements distinguish between interface and bulk recombination, revealing a Schottky barrier originating from possible GaO _x formation at the ITO/CIGSe interface, as well as a current‐blocking behavior at the CIGSe/CdS interface at low temperatures related to Rb. Tuning the Ga distribution further enhances device performance: An In‐Ga‐In sequence in the first stage of absorber growth suppresses Rb/CdS‐related barriers and further promotes alkali metal incorporation, supporting OVC layer growth and thereby possibly improving band alignment. A mechanistic model showcasing this interplay between Na, Rb, OVC, and Cd is made for visualizing the structural formation during absorber growth. These results underline the relevance of the first systematic Rb analysis in ultra‐thin CIGSe solar cells and highlight the synergistic potential of combining alkali metal doping with Ga grading to improve ultra‐thin CIGSe solar cells.