A Thermodynamic and Surface Chemistry Perspective on Halide Segregation in Metal Halide Perovskites

Halide segregation in metal halide perovskites (MHPs) limits their ability to tune the band gap, creating a major barrier to achieving stable and efficient MHP-based optoelectronic devices. To understand halide segregation, we performed first-principles simulations on slabs of MAPb(Br_xI_1−x)₃, FAPb(Br_xI_1−x)₃, and FA_0.8Cs_0.2Pb(Br_xI_1−x)₃ with varying bromine and iodine distributions. We find that Br-rich surface configurations are energetically preferred compared to I-rich ones or those with homogeneously distributed Br and I anions, particularly in MAPbI₃. Incorporating Cs and FA cations at the A site substantially suppresses this behavior in agreement with previous experiments. We further evaluated formation energies of Br antisite defects across different layers and again found a strong thermodynamic preference for Br accumulation at the surfaces. Our hole localization analysis reveals that I-rich regions tend to trap holes, which further promotes segregation. Our work elucidates the atomistic mechanism of halide segregation, thereby providing a clear pathway toward designing segregation-resistant MHPs.