Side‐Chain Deuteration Enables High‐Contrast Neutron Scattering of Solution Aggregates in Organic Photodiodes and Enhances Device Performance

Comprehensive Summary Understanding the solution‐state aggregation of conjugated polymers is essential for controlling their film microstructure and improving optoelectronic device performance. However, resolving aggregate structures in donor:acceptor blends remains challenging due to limited contrast and the multicomponent nature of these systems. Here, we selectively deuterate the side chains of poly(3‐hexylthiophene) (P3HT) to produce P3HT‐D, substantially increasing its neutron scattering length density (SLD). This isotopic modification enables high‐contrast small‐angle neutron scattering (SANS) analysis of polymer aggregation in solutions of polymer:nonfullerene acceptor (NFA) blends. The enhanced contrast overcomes the deficiencies of conventional X‐ray scattering and microscopy, allowing for unambiguous determination of chain conformation and aggregate structure. Photodiodes incorporating P3HT‐D show improved responsivity and achieve a high specific detectivity of 5.67 × 10 13 Jones. These improvements are attributed to refined film morphology, improved miscibility, and reduced nonradiative recombination facilitated by isotopic substitution. Our findings demonstrate that side‐chain deuteration provides a dual benefit: (i) enabling precise structural characterization and (ii) tuning intermolecular interactions to improve device performance. This study establishes side‐chain deuteration as a versatile strategy for structural analysis, microstructure engineering, and performance optimization in conjugated polymer–based organic photodiodes.