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

ABSTRACT 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 HfO 2 nanocrystals (NCs) or emissive CdZnS/ZnS (CZS) quantum dots (QDs) to isolate high‐Z and optical sensitization pathways. HfO 2 /F8BT films exhibit 25× radioluminescence (RL) enhancement but decreasing light yield (LY) with increasing NCs loading, indicating that HfO 2 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.