Breaking the Gain–Speed Trade‐Off in Phototransistors With Sub‐10 nm Single‐Crystalline Organic–Inorganic van der Waals Heterojunctions at the Exciton‐Diffusion Limit

ABSTRACT Phototransistors offer intrinsic gate‐controlled gain amplification, providing much higher responsivity than photodiodes, but this advantage is generally limited by the gain‐speed trade‐off: high gain typically relies on long‐lived trapped carriers, which inevitably slows the photoresponse. Although low‐dimensional semiconductors and hybrid heterojunctions have improved responsivity and detectivity, they still predominantly operate through slow, trap‐mediated gain, leaving the trade‐off unresolved. Here, sub‐10 nm organic–inorganic single‐crystalline van der Waals heterojunctions are introduced to decouple gain from slow carrier trapping. Specifically, a bilayer molecular crystal (C 6 ‐DPA) is integrated onto a monolayer MoS 2 channel. Reducing the heterojunction thickness to the exciton diffusion limit, localized Frenkel excitons in C 6 ‐DPA efficiently undergo interfacial charge separation and transfer across the type‐II interface, while the separated charges rapidly modulate the MoS 2 channel conductance through the field effect. Gain therefore arises from efficient channel gating rather than prolonged trapped‐carrier circulation. The resulting phototransistors achieve a responsivity of 1.07 × 10 5 A W −1 , rise/decay times of 771.2/809.8 µs, and a specific detectivity of 2.19 × 10 15 Jones under their respective optimized operating conditions. These results demonstrate that the gain–speed trade‐off in phototransistors arises primarily from trap‐mediated gain mechanisms and can be mitigated through exciton‐engineered ultrathin single‐crystalline organic–inorganic van der Waals heterojunctions.