Sensitive infrared photodetection enabled by in-sensor background-offset cancellation in a mixed-dimensional van der Waals heterostructure.

Room-temperature infrared photodetection has long been constrained by broadband thermal radiation, which injects background photons and drives detectors toward a background-limited regime. Beyond stochastic photon fluctuations, this radiation produces a deterministic mean photocurrent offset that can overwhelm weak signals under ambient conditions. This limitation originates from unipolar spectral integration, where background photons across the response band contribute with the same photocurrent polarity. Here we demonstrate an in-sensor background-offset cancellation mechanism enabled by spectral polarity inversion. By engineering wavelength-dependent photocurrent reversal, photons from different spectral bands generate opposite-signed responses within a single device, suppressing the mean background-induced current under fixed blackbody illumination without external subtraction or balanced readout. We realize this concept using an optically reconfigurable mixed-dimensional Ge/black phosphorus van der Waals heterostructure photodetector. The zero-bias device achieves a room-temperature detectivity of 7.89 × 1010 Jones, within a factor of 4.11 of the background-limited benchmark for unity-quantum-efficiency integration-type detectors.