Evidence for I₂ loss from the perovskite-gas interface upon light-induced halide segregation

Sunlight-induced halide segregation in (CH₃NH₃)Pb(Br _x I_1-x )₃ (1 > x > 0.2), which limits obtainable voltages from solar cells with these perovskite absorbers, reverses upon resting in the dark. However, sustained illumination at ca. 1 sun opens a new decomposition pathway, leading to irreversible I₂ loss in an open system. We conclusively show I₂ off-gassing from halide-segregated (CH₃NH₃)Pb(Br_0.75I_0.25)₃ by trapping gaseous I₂ and tracking the electronic conductivity of the perovskite, which increases from electron-doping as iodides are oxidized to iodine. Importantly, we show that this reaction occurs across the perovskite-air solid-gas interface, without confounding effects from solvent or reactive solid interfaces. This characterization was conducted under a nitrogen atmosphere, avoiding vacuum- and oxygen-driven I₂ loss pathways. Consistent observations of I₂ loss upon light-soaking CsPb(Br_0.75I_0.25)₃ films show that this reaction is intrinsic to the inorganic framework. We propose that the disruption of iodide-rich domains in the halide-segregated films through I₂ loss can masquerade as a light-induced healing or apparent remixing of the segregated film, when in fact it leads to irreversible decomposition. Although I₂ off-gassing is less likely in bromide-rich solid solutions, light-induced halide segregation brings the iodides into proximity and forms electronic states that are energetically poised to trap and accumulate holes, providing a driving force for I₂ loss. Thus, even bromide-rich mixed-halide perovskite absorbers will benefit from I₂-impermeable encapsulation for long-term stability.