| Summary: | Abstract Commercial photoplethysmography (PPG) sensors and other biomedical devices that require near‐infrared (NIR) light detection are limited by rigid photodetector architectures that cannot interface comfortably with skin. Flexible organic photodetectors could potentially solve this problem; however, organic photodetectors suffer from low NIR sensitivity, largely due to the early absorption cutoffs originating from the use of fullerene acceptors. This work details a new flexible organic photodiode (OPD) based on narrow bandgap non‐fullerene‐acceptor (NFA) bulk heterojunction (BHJ) incorporating a PTB7‐Th and COTIC‐4F blend. The photodetector developed for this study reaches an external an external quantum efficiency (EQE) to 60% in the 600–1000 nm range and responsivity (R) as high as ≈0.42 A W–1. Through the integration of a PEDOT:PSS and PDINO interfacial layers, undesired carrier injection is suppressed, and a detectivity (D*) in the order of ≈1013 Jones is achieved. In addition, oxygen saturation and heart rate are successfully measured by integrating the developed photodiode in a PPG sensor.
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