Mitigating Dielectric Confinement Effect for Ultra‐High Gain in Quasi‐2D Ruddlesden‐Popper Perovskite Photodetectors

Quasi‐2D Ruddlesden‐Popper (quasi‐2D RP) perovskites have exhibited remarkable chemical stability, tunable quantum well architectures, and an adaptable dielectric environment. These features offer excellent control over carrier dynamics in optoelectronic devices. However, these materials have intrinsically fragile crystal lattices that constrain the thorough exploration of their inherent optoelectronic properties. Herein, a rapid liquid‐surface crystallization technique to synthesize high‐quality quasi‐2D RP single‐crystal perovskite flakes, BA 2 (MA) n−1 Pb n I 3n+1 ( n = 1, 2, 3), is demonstrated. Terahertz time‐domain spectroscopy reveals that the carrier mobilities of single‐crystal flakes are able to approach the theoretical limit that is predicted by the Mott‐Ioffe‐Regel criterion. Interface‐preserving photodetectors fabricated through robust van der Waals contact have demonstrated a mitigated dielectric confinement effect. This effect enhances carrier mobility, suppresses noise currents, and prolongs exciton lifetimes, leading to a significantly elevated mobility‐lifetime product and enabling a photoconductive gain exceeding 10⁸. These findings highlight the synergistic interaction between quantum confinement and dielectric modulation, paving the way for optimizing intrinsic optoelectronic responses within the organic‐inorganic quantum well framework.