Abstract Barrier detectors such as nBn and pBp architectures (formed by a n- or p-type contact layer, a barrier layer and a n- or p-type absorber) aim to block one carrier type while allowing the other to pass, but require complex hetero-integration and precise band engineering. Here, we propose an ultra-thin polar barrier strategy using a 0.75 nm water-intercalated WSe2/H2O/PdSe2 heterostructure. The confined water layer forms a clean, well-ordered interface and further generates a precisely oriented polarization field that depletes electrons in WSe2, significantly suppressing dark current to sub-pA levels across all biases, while enabling efficient tunneling of photogenerated holes. The device shows broadband photoresponse from the ultraviolet to mid-wave infrared (MWIR), with a room-temperature average detectivity exceeding 10¹⁰ cm Hz¹/² W⁻¹ in the MWIR. It also features ultrafast response (~3 μs), polarization light sensitivity, and two-year stability. Our work establishes a platform for high-performance infrared photodetection via van der Waals gap engineering.