Exceeding 100 % EQE with fast-response in a self-powered WS₂/InSe photodetector for broadband detection

Two-dimensional (2D) van der Waals heterostructures (vdWHs) overcome the limitations of traditional lattice matching and are widely applied in high-performance optoelectronic devices. However, the presence of interface traps and the limited built-in electric field strength pose significant challenges in enhancing the sensitivity and response speed of self-powered photodetectors based on 2D vdWHs. In this study, we designed a WS₂/InSe semi-vertical heterojunction photodetector. The synergistic integration of a type-II band alignment and a graphene bottom electrode facilitates the formation of an extensive unidirectional depletion region. This architecture leverages the potent built-in electric field to drive the efficient separation and rapid extraction of photogenerated carriers, thereby ensuring superior response dynamics. Simultaneously, the asymmetric electrode configuration directs photogenerated holes towards the Au/InSe interface, where impact ionisation occurs under the strong local electric field. This process leads to carrier multiplication and thus yields substantial internal gain. The device spatially decouples the carrier separation process from the gain mechanism while maintaining their functional synergy. This design successfully surmounts the conventional trade-off between sensitivity and response speed. Consequently, this device demonstrates exceptional self-powered performance, characterised by an external quantum efficiency (EQE) that exceeds the classical theoretical limit (up to 141.1 %) alongside microsecond-scale response times (19.6/20.1 μs). Furthermore, the device demonstrates robust broadband detection capabilities, extending from the ultraviolet to the near-infrared spectrum (300–1100 nm). These findings offer a sophisticated and viable design strategy for the realisation of high-performance, self-powered broadband photodetectors based on 2D van der Waals heterostructures.