A‐Site Symmetry Engineering in Designing Metal‐Free Perovskites for Ultrahigh Sensitivity X‐Ray Detection

ABSTRACT Metal‐free perovskites (MFPs) have emerged as promising candidates for environmentally benign X‐ray detectors. However, their further developments still face several key challenges, with the most prominent being the trade‐off between structural stability and electrical performances. To address this challenge, an A‐site symmetry engineering strategy, aiming to optimize charge transport properties without compromising their structural stability is proposed. By replacing the C 3v ‐symmetric MDABCO 2+ cation with a higher‐symmetry D 3h DABCO 2+ cation and introducing PF 6 − anions to reinforce the hydrogen bonding, a new MFP single crystal (SC) is designed, DABCO‐NH 4 (PF 6 ) 3 . Both theoretical calculations and experiments confirm that increasing A‐site symmetry can effectively promote carrier transport. As a result, DABCO‐NH 4 (PF 6 ) 3 SC device achieves a record sensitivity of 2701.0 µC Gy a i r −1 cm −2 at 50 V, and an ultralow detection limit of 10.3 nGy a i r s −1 . Moreover, due to the robust structural stability, it exhibits excellent long‐term operational stability under harsh conditions. Even at 200 V, the sensitivity reaches 5874.2 µC Gy a i r −1 cm −2 with a low detection limit of 0.217 µGy a i r s −1 , demonstrating its reliable detection capability. This work proposes an A‐site symmetry engineering to simultaneously achieve high stability and superior electrical performance in MFPs, opening a promising avenue for developing high‐performance, environmentally‐benign X‐ray detector.