Probing Photoinduced Halide Segregation in CH₃NH₃Pb(Br_<i>x</i>I_1–<i>x</i>)₃ Perovskites via <i>in Situ</i> Nuclear Quadrupole Resonance Spectroscopy

Mixed-halide perovskites of formula MAPb(Br_<i>x</i>I_1-<i>x</i>)₃, where MA is methylammonium, are of great interest for optoelectronic applications (particularly high-efficiency solar cells) due to their finely tunable bandgap, which enables precise control over light absorption. However, their stability remains a critical challenge, notably due to reversible photoinduced halide segregation. Under continuous illumination, this process leads to the formation of Br- and I-rich domains, which lower device performance by introducing low-bandgap regions that trap charge carriers. Despite extensive efforts, the mechanism of photosegregation remains debated, partly due to the difficulty in characterizing these compositionally disordered systems, especially under illumination. Here, we report a systematic investigation across a series of MAPb(Br_<i>x</i>I_1-<i>x</i>)₃ compositions using ^79/81Br and ¹²⁷I nuclear quadrupole resonance (NQR) spectroscopy. Furthermore, we present the first application of NQR under <i>in situ</i> illumination to directly probe photoinduced halide segregation, focusing on compositions with <i>x</i> = 0.9, 0.8, and 0.6. Our findings highlight the potential of NQR as a powerful, noninvasive technique for studying local environments under illumination. The results reveal composition-dependent light-induced segregation when comparing substantially different stoichiometries. In contrast, the ⁸¹Br NQR spectra of MAPb(Br_0.9I_0.1)₃ and MAPb(Br_0.8I_0.2)₃ show the emergence of comparable local environments, with preferential formation of MAPbBr₃-like domains.