Tunable Near‐Infrared IIa Narrowband Organic Photodetectors Enabled by Microcavity–Charge Transfer State Coupling

ABSTRACT Organic photodetectors (OPDs) with narrowband response in the near‐infrared (NIR) region are essential for cost‐effective spectroscopic sensing. Here, we report a microcavity‐enhanced OPD based on the PTB7‐Th:BTPSeV‐4F system, where charge transfer (CT) transitions are resonantly amplified by a Fabry–Pérot cavity. By optimizing the optical thickness, the device achieves a first‐order resonance at 1208 nm with a narrow full width at half maximum (FWHM) of 17 nm and an external quantum efficiency (EQE) of 10.7%, compared to ≈1% for the non‐cavity reference. The detector exhibits a low dark current density of 38 nA cm − 2 at 0 V bias, a detectivity of 1.39 × 10 10 Jones, and a noise‐equivalent power (NEP) of 18.75 pW. By tuning the cavity length, the resonance wavelength can be continuously adjusted from 1050 to 1350 nm, covering the biologically relevant NIR‐IIa window and extending beyond the response limit of conventional Si detectors while avoiding costly InGaAs devices. Benefiting from CT state–cavity synergistic enhancement, the device operates in a self‐powered mode and enables clear spectral discrimination of water, ethanol, and propylene glycol. This work establishes an effective strategy for narrowband, low‐noise, deep‐NIR organic photodetection, highlighting the potential of organic materials for compact spectroscopic sensing.