Vacancy Passivation of Zinc Oxide via an Imide-Bridge Dipole for Efficient Bulk-Heterojunction Inverted Organic Photodetectors

In this study, an inverted organic photodetector was fabricated by introducing an imide-bridge passivation layer that synergizes with zinc oxide (ZnO), which enhances the charge-injection barrier and suppresses the dark current. Organic photodetectors (OPDs) are attracting increasing interest for next-generation biomedical and wearable sensing platforms because of their solution processability, spectral tunability, and compatibility with flexible substrates. However, their performance is limited by intrinsic defects in ZnO electron transport layers, where oxygen vacancies and trap states induce trap-assisted tunneling, which increases dark current and degrades the signal-to-noise ratio. To address this issue, we propose an imide-bridge-based hole-injection barrier layer with vacancy passivation, which forms strong dipole interactions and chemical bonds that effectively suppress oxygen vacancies. This interfacial engineering substantially reduces trap-assisted leakage and lowers the dark current density by approximately 3.5 times while maintaining comparable photocurrent levels. As a result, the optimized OPD achieves a detectivity of 7.38 × 10 12 Jones at 0 V bias, accompanied by improved noise suppression. Under 1-sun illumination, the device also recorded clear and stable photoplethysmography (PPG) signals from a fingertip, which demonstrates its feasibility for reliable cardiovascular monitoring under realistic ambient light conditions.