Preventing Triple Halide Intermediate Formation for Segregation-Resistant Wide Band Gap Perovskites

Abstract Mixed I-Br perovskites are promising top absorbers for tandem solar cells due to their tunable band gap and solution processability, yet photoinduced halide segregation—a critical but poorly understood degradation mechanism—hinders their reliability. Here, we demonstrate how the enhancement of the Br− binding energy in the perovskite inhibits I2Br− formation—the key intermediate species driving halide segregation. The incorporation of densely charged cations like Sr2+ in the perovskite strongly localizes halide anions, inhibiting their electron donation to photogenerated I2 and preventing triple halide ion formation, which eventually suppresses halide segregation. Additionally, this also reduces undercoordinated Pb at grain surfaces, suppressing nonradiative recombination. These synergistic effects yield high-performance solar cells with champion efficiencies of 20.18% at 1.77 eV and 19.02% at 1.84 eV, alongside enhanced stability. Our work establishes a design principle to mitigate halide segregation in mixed-halide perovskites, overcoming a major challenge in perovskite optoelectronics.