Abstract: Two-dimensional (2D) van der Waals heterostructures offer a promising platform for advanced optoelectronics. However, conventional type-II configurations with Fermi level offsets unavoidably introduce band bending and trap-assisted recombination, restricting photoelectric conversion efficiency. Here, a size-effect-controlled strategy is employed to intrinsically align Fermi levels across the MoS₂/ReS₂ interface while preserving type-II band alignment. The resulting heterojunction exhibits nearly zero Fermi level offset, thereby eliminating interfacial barriers and prolonging carrier lifetime. Under low-power ultraviolet excitation, the device delivers a responsivity of 4.53 × 10⁴ A/W, an external quantum efficiency of 1.5 × 10⁷ %, and a specific detectivity of 1.6 × 10¹⁴ Jones. Spectroscopic and ultrafast carrier dynamics analyses demonstrate that the precise Fermi level plays a more decisive role in governing charge separation than conventional built-in fields. Comparative investigations further confirm that increasing the Fermi level mismatch induces pronounced band bending, trap-assisted recombination, and substantial photoresponse degradation. In contrast, optimized alignment maximizes charge transfer efficiency, as evidenced by 77.3 % photoluminescence quenching. Additionally, the optimized heterostructure supports broadband detection and enables high-resolution imaging, demonstrating strong application potential. This work establishes interfacial electronic equilibrium as a pivotal design principle and introduces a general, doping-free framework for Fermi level regulation in 2D heterostructures, offering mechanistic insights and scalable guidance for next-generation optoelectronic and imaging devices.

Key words: MoS₂/ReS₂ heterojunction, Fermi level offset, Size effect, Photodetector, Optical imaging