A‐Site Cation‐Induced Hot‐Carrier Lifetime Extension in 2D Perovskites: A New Strategy for X‐Ray Detection Enhancement

2D metal halide perovskites exhibit exceptional potential for X-ray detection due to their high lattice stability and moisture-resistant layered structures. However, energy loss due to rapid hot carrier (HC) cooling during high-energy X-ray irradiation limits the device performance. Herein, we prolong HC lifetimes in 2D perovskites through A-site cation engineering, synthesizing single-crystalline DMePDAPbI₄ (DMePDA) and MPDAPb₂I₆ (MPDA) structures. By tailoring molecular polarity and hydrogen-bonding capability via functional group substitution, MPDA achieves enhanced lattice rigidity and suppressed low-frequency phonon modes. These structural features directly reduce carrier-phonon scattering and phonon-mediated non-radiative energy dissipation, thereby retarding HC cooling. Femtosecond transient absorption confirms MPDA extends HC cooling times to 80 ps (vs. 10 ps in DMePDA) with peak temperatures reaching 535 K. By integrating a hot-electron extraction layer, MPDA achieves 51.4% HC extraction efficiency sixfold-higher than that of DMePDA. Consequently, vertical X-ray detectors exhibit an improved sensitivity of 12834.1 µC Gy_air ^-1 cm^{- 2}. These results establish organic cation design as a transformative strategy for harnessing hot carriers in radiation detection.