Insights Into Ferroelectric Phase Transition Mechanism for One‐Dimensional Halide Perovskites to Attain Record‐Performance Self‐Powered X‐Ray Detection

ABSTRACT The spontaneous polarization in ferroelectric perovskites offers a promising route toward self‐powered X‐ray detection, yet the microscopic link between ferroelectric phase transition and detector‐relevant carrier dynamics remains largely unexplored. Here, using 1D [3‐(aminomethyl)piperidinium]BiI 5 single crystals (3AMP SCs) as a model system, we uncover how a specific first‐order ferroelectric–paraelectric transition mechanism directly governs polarization‐driven carrier transport and photovoltaic behavior. Structural analyses reveal that room‐temperature ferroelectricity originates from the non‐centrosymmetric ordering of 3AMP 2 + cations and strong organic–inorganic interfacial coupling, while the transition at 364 K is driven by highly anharmonic hydrogen‐bond dynamics and stabilized by the rigidity of the Bi–I framework. This cooperative mechanism generates a robust polarization field along the c‐axis, which reduces exciton binding energy, suppresses trap‐assisted recombination, and enables efficient carrier separation through the bulk and anomalous photovoltaic effects. As a direct consequence of this phase‐transition‐controlled transport behavior, the 3AMP SC‐based detector achieves record‐performance self‐powered X‐ray detection, with a sensitivity of 566.79 µC Gy air −1 cm −2 , an ultra‐low detection limit of 5.24 nGy air s −1 , and excellent long‐term stability. This work establishes a clear structure–phase transition–polarization–transport–performance relationship in Bi‐based halide perovskites, providing a new framework for designing ferroelectric materials for self‐powered optoelectronic devices.