Cooling Bromine‐Contained Photovoltaic Perovskites: The Impact of Phase Segregation and Lattice Expansion

Perovskite solar cells are promising next‐generation photovoltaics, yet instability under extreme temperature cycling prevents their applications in multiple scenarios to date. This study investigates the impact of bromine (Br) incorporation on the lattice stability of perovskites during thermal cycling (ranging from 173 to 358 K). After 50 temperature cycles, we find that 5% bromine loading suppresses lead/halide vacancy formation due to a more energetically rigid lattice. However, overloading (10–25%) bromine induces an ionic‐radius mismatch that disrupts halide distribution, causing lattice disorder and destabilizing ionic equilibrium. Such disruption outweighs the benefits of vacancy suppression, ultimately triggering phase segregation. Additionally, our study experimentally reveals for the first time that low‐temperature cycling (173 and 298 K) induces lattice expansion in perovskite, which suppresses nonradiative recombination primarily by enhancing defect relaxation at iodine vacancies (Iᵢ), therefore enhancing the open‐circuit voltage. This study emphasizes the critical role of dynamic behavior of lattice structure in optimizing the stability of photovoltaic perovskites and offers valuable insights for designing stable Br‐contained perovskite solar cells under extreme temperature applications, such as photovoltaic devices in near‐space or polar region.