Thermally Activated Rubidium Redistribution Limits Carrier Collection in Narrow‐Bandgap CIGS Solar Cells

Rubidium fluoride post‐deposition treatment (RbF‐PDT) is widely used to improve Cu(In,Ga)Se 2 (CIGS) solar cells, but its benefits remain strongly process‐dependent in narrow‐bandgap absorbers for tandem photovoltaics. Here, we identify thermally activated Rb redistribution during ZnO sputtering as a hidden pathway that limits interface stability in RbF‐treated narrow‐bandgap CIGS. Depth‐resolved time‐of‐flight secondary ion mass spectrometry and x‐ray photoelectron spectroscopy reveal that Rb incorporated during PDT can redistribute toward the CdS/ZnO region during window‐layer deposition, accompanied by temperature‐dependent changes in its local chemical environment. This redistribution correlates with selective losses in short‐circuit current density and fill factor, while the open‐circuit voltage remains comparatively stable. External quantum efficiency analyses and SCAPS simulations support an interface‐state‐dominated degradation scenario involving impaired long‐wavelength carrier collection. By controlling thermal accumulation during ZnO sputtering and optimizing RbF‐PDT conditions, we define a processing window that preserves beneficial Rb effects while suppressing detrimental interface modification. Consequently, narrow‐bandgap CIGS solar cells achieve a certified efficiency of 20.09%, enabling mechanically stacked four‐terminal perovskite/CIGS tandem devices with a combined efficiency of 30.21%. This work reveals an overlooked thermal‐redistribution pathway of heavy alkali metals and provides guidance for high‐efficiency, tandem‐relevant CIGS photovoltaics.