Breaking Crystallinity for Optimal Dark Current: Nonfullerene Acceptor Dilution as a Strategy for High‐Performance Organic Photodetectors

Abstract Organic photodetectors (OPDs) based on polymer donor:non‐fullerene‐acceptor (NFA) bulk heterojunction (BHJ) blends demonstrate great potential for light‐sensing applications, thanks to optoelectronic tunability and strong absorption. However, state‐of‐the‐art organic photoconversion devices lag behind silicon counterparts due to the laborious dark current, J_d, and photodetection speed optimization required in OPDs. Herein, the donor/acceptor blend ratio is controlled to optimize PM6:Y6‐based OPDs, with diluted Y6 blends (1:0.1 D/A weight ratio) strongly suppressing J_d to 6.5 × 10⁻⁹ A cm⁻² (−4 V). Diluted OPDs exhibit faster photoresponse and poor short‐circuit photocurrent generation is dramatically improved under reverse bias. In‐depth photophysical and energetic analysis reveal the origin of high performance. In diluted blends, dissociation of emissive strongly bound excitonic charge‐transfer states within Y6 require electric field assistance, while the polymer matrix provides a continuous charge‐transport pathway. The generality of this strategy is tested with various NFAs, with NFA dilution being most effective for highly crystalline acceptors. It is determined that the reduction in dark current is predominantly driven by disruption of NFA crystallinity and intermolecular interactions. Polymer‐rich BHJ devices, therefore, provide an effective optimization method for reducing dark current in OPDs and are highly promising for industrial scalability due to facile processability, robust mechanical properties, and superior thermal stability.