Engineering Carrier Barriers To Suppress Dark Current in Near-Infrared Organic Photodiodes

A comprehensive understanding and effective suppression of dark current in near-infrared organic photodiodes (NIR-OPDs) are crucial for enhancing their detectability, a topic that remains a persistent challenge in this field. Herein, the origins of dark current in NIR photodetectors from the perspective of carrier dynamics is elucidated. Building on this analysis, an interface engineering-based solution targeting undesirable carrier transport and collection is proposed: a wide-band gap, highly biocompatible anode interfacial layer (D149:CoO_x) with bidirectional carrier barriers. Its modestly deeper highest occupied molecular orbital blocks thermally activated holes, while the shallower lowest unoccupied molecular orbital impedes electron injection from external circuits, collectively suppressing the dark current of the NIR-OPD (active layer: PTB7-Th:TQ_PP2FIC). Compared to conventional PEDOT:PSS, D149:CoO_x achieves effective dark current suppression without compromising responsivity (0.17/0.23 A W^-1 @ PEDOT:PSS/D149:CoO_x-OPD), synergistically enabling a specific detectivity of 10¹² Jones at -1 V. Furthermore, featuring a lower dark current of ∼ 3 × 10^-9 A cm^-2 (20× lower than PEDOT:PSS at ∼ 6 × 10^-8 A cm^-2), the flexible D149:CoO_x NIR-OPD is capable of real-time human heart rate monitoring. This work establishes design principles for low-noise NIR devices while demonstrating significant prospects in wearable NIR optoelectronics.