Lead‐Free Bandgap‐Engineered Perovskite with Thermally Activated SHG Switching and Enhanced X‐Ray Detection

Abstract The bandgap in hybrid perovskites serves as a core parameter governing not only photon absorption but also coupled optoelectronic‐thermal phenomena. However, the strategic bandgap manipulation to achieve synergistic optimization of optoelectronic functionalities remains a great challenge. Here, a lead‐free hybrid perovskite ( t ‐MCHA) 2 CdCl 4 ( t ‐MCHA = trans ‐4‐methylcyclohexylammonium) is reported that exhibits a thermally driven bandgap transition (3.08 → 1.22 eV and direct → indirect) at 396K through synergistic organic cation ordering and inorganic framework reconstruction. This transition triggers three groundbreaking phenomena: (1) an “OFF→ON” SHG switch (0.2 → 1.1) with one of the highest T c for such anomalous behavior in hybrid perovskites, (2) exceptional X‐ray detection performance ( µτ = 1.04×10 −4 cm 2 V −1 , sensitivity = 294 µC Gy −1 cm −2 , LOD = 83 nGy air s −1 ) at 10 V bias with thermally enhanced response, (3) reversible dielectric switching ( ɛ ' = 3.3→4.6 at 1 MHz) with excellent reversibility and durability. This work pioneers a thermal‐driven bandgap engineering strategy that enables reversible switching between direct/indirect and wide/narrow bandgap states within a single perovskite material, offering unprecedented versatility for on‐demand optoelectronic applications. Furthermore, the discovered anomalous thermal enhancement effect establishes a new design paradigm for developing radiation‐hardened detectors capable of stable operation under extreme conditions.