Chlorinated Nonfullerene Acceptor Stabilizes Compact Solution Networks for Organic Photovoltaics and Photodetectors ^†

Comprehensive Summary

Solution aggregation has emerged as a key structural variable in organic optoelectronic blends, yet stronger aggregation does not necessarily lead to more favorable film morphology. This issue is particularly relevant when the same donor–acceptor system is used for both organic photovoltaics and photodetectors, where charge generation, charge transport, and dark‐current suppression require a careful balance between molecular order and phase separation. Here, we show that the chlorinated nonfullerene acceptor BO‐4Cl stabilizes a compact solution network in PM6‐based photoactive blends. Variable‐temperature small‐angle X‐ray scattering (SAXS) reveals that the blend solution maintains a nearly unchanged mass‐fractal polymer network over a broad temperature range, with a fractal dimension close to 3, a cylindrical aggregate length of approximately 20 nm, and a radius of approximately 0.9 nm. This temperature‐insensitive solution structure is translated into smooth blend films with moderate phase separation, enhanced molecular ordering, and face‐on orientation. As a result, the blend supports stable photovoltaic operation and also functions effectively in organic photodetectors. These results identify BO‐4Cl as a chlorinated acceptor that defines a compact‐network regime, in which stable solution organization is coupled with morphological compatibility. This work clarifies the structural role of chlorination and extends solution‐aggregation control from organic photovoltaics to photodetectors.