π‐Bridge Engineering of Nonfullerene Acceptors for Organic Photodiodes: Extension of the Near Infrared Response Beyond 1200 nm

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Abstract Novel nonfullerene acceptors (NFAs) are designed that enable the realization of high‐specific‐detectivity organic photodiodes (OPDs) operating effectively at wavelengths beyond 1200 nm in the near‐infrared (NIR) region. Previous studies on NIR OPDs have rarely addressed the influence of π‐bridge engineering in NFAs on optoelectronic performance. To address this gap, the effects of π‐bridge substitution in NFAs are systematically investigated, focusing on how enhanced intramolecular charge transfer (ICT) influences energy level modulation and molecular aggregation behavior. By synthesizing three NFAs with different π‐bridge structures, the optical bandgap is successfully tuned from 1.12 to 1.05 eV, thereby achieving extended absorption into the longer‐wavelength region. Furthermore, by suppressing excessive molecular aggregation, exciton dissociation is enhanced and nonradiative recombination is reduced. These optimizations culminated in the realization of a highly efficient NIR OPD with a specific detectivity of 3.43 × 10 11 Jones and a −3 dB cutoff frequency of 58.6 kHz at 1050 nm under a low operating bias of −2 V. Finally, the high‐performance NIR OPD is successfully applied in both photoplethysmography (PPG) and optical wireless communication sensor prototypes, highlighting its strong potential for integration into practical optoelectronic systems for real‐time wireless biosignal monitoring and transmission.

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