A Mechanistic Blueprint for Fast, High‐Yield Green Scintillators Using Conjugated Polymer–Nanocrystal Composites

Nanocrystal-sensitized plastic scintillators offer promise for high-performance radiation detectors, but achieving both high efficiency and ultrafast timing remains challenging. This need grows as the field moves away from blue emitters, whose spectra overlap radio-induced defect bands in polymers, toward green/yellow emitters that avoid these losses. Yet green dyes typically suffer from slow photophysics. Here, we show green-emitting polymer 9,9-dioctylfluorene-alt-benzothiadiazole (F8BT) overcomes this limitation through large Stokes shift and fast emission, making it a compelling scintillator matrix. We use F8BT to clarify sensitization mechanisms in nanocrystal/polymer systems by blending it with non-emissive high-Z HfO2 nanocrystals (NCs) or emissive CdZnS/ZnS (CZS) quantum dots (QDs) to isolate high-Z and optical sensitization pathways. HfO2/F8BT films exhibit 25× radioluminescence (RL) enhancement but decreasing light yield (LY) with increasing NCs loading, indicating that HfO2 acts as passive energy-dissipation centers for secondary electrons. Conversely, CZS/F8BT films achieve 70× RL enhancement with loading-independent LY, demonstrating that emissive QDs recycle secondary electrons and enable dual sensitization. Both systems preserve sub-3 ns decay times due to F8BT's ultrafast emission and QD multiexciton dynamics. α-particle detection further confirms practical applicability. These findings establish a scalable blueprint for fast, efficient, green-emitting scintillators that circumvent defect-band absorption while enabling mechanistically informed dual sensitization.