How Halide Segregation Governs the Ion Density Evolution and Ionic Performance Losses: From Degradation to Recovery

ABSTRACT Halide segregation (HS) is considered to be one of the most significant hurdles for the commercialization of tandem solar cells. However, despite significant research on this matter, the exact impact of HS on the performance degradation and the ion density evolution is yet to be established. In this work, we investigate the intriguing correlation between HS, ion‐induced efficiency losses, and ion density evolution in wide‐bandgap (WBG) triple cation perovskite cells. Our results highlight that all three phenomena evolve on similar timescales and follow the same trend across all studied bandgaps. This implies that the poor energy‐lifetime product observed for devices prone to halide segregation is a result of enhanced ionic losses rather than, for instance, charge carrier funneling. Furthermore, reminiscent of the recovery of HS observed previously, we demonstrate that ionic losses also recover after light exposure and dark storage, which occurs along with a receding ion density. However, we also observe irreversible ionic losses, especially after prolonged illumination, which are critical for device operation. These findings present an important new understanding of the interplay between halide segregation and ionic processes and provide a rational explanation for the performance and stability of mixed halide WBG perovskites.