Trap passivation and injection barrier tuning via self-assembled dipole interfaces for starlight-sensitive organic photodetectors

Organic photodetectors (OPDs) are promising candidates for low-cost, high-sensitivity imaging and sensing applications, yet their performance remains limited by interfacial recombination and high dark currents. Here, we introduce a molecular interfacial engineering strategy based on a phosphonic acid-functionalized self-assembled monolayer (2PACz) to simultaneously modulate energy-level alignment and suppress trap-assisted leakage at the bulk heterojunction/MoO x interface. The 2PACz layer reduces the MoO x work function from 4.96 eV to 4.26 eV, increasing the electron injection barrier from 0.21 eV to 1.49 eV and effectively minimizing reverse leakage current. Concurrently, interfacial trap passivation broadens the depletion region and enhances exciton dissociation and carrier extraction, leading to prolonged carrier lifetimes. As a result, the optimized OPDs exhibit a two-order-of-magnitude reduction in dark current density (from 10 −8 to 10 −10 A cm −2 ), a high specific detectivity exceeding 1.5 × 10 13 Jones in the near-infrared range, and fast response times (τᵣ/τ f = 5.8 μs/5.9 μs). These low-noise devices enable high-fidelity photoplethysmography (PPG) biosensing with an improved signal-to-noise ratio (10 to 18 dB), multi-channel spectral laser decoding, and ultra-weak starlight (~10 −10 W cm −2 ) imaging using an 8 × 8-pixel OPD array. This work provides a generalizable and scalable interfacial design framework for advancing solution-processed OPDs toward next-generation ultrasensitive optoelectronics.