Size‐Dependent Energy Transfer From CsPbBr 3 Perovskite Nanocrystal to Organic Dye

Lead halide perovskite nanocrystals (PNCs) are promising energy donors for organic dyes, yet the factors determining whether Förster resonance energy transfer (FRET) or Dexter‐type electron‐exchange transfer (DET) dominates remain unclear. Here, we investigate energy transfer (ET) mechanisms in CsPbBr 3 PNC–Cy3 linkages as a function of PNC size using ensemble spectroscopy and single‐PNC measurements. In small PNC–Cy3 linkages, the experimentally determined ET efficiencies from PL quenching and lifetime shortening agree well with theoretical FRET efficiencies, and the donor decay time matches the acceptor rise time, identifying FRET as the dominant pathway. In contrast, large PNC–Cy3 linkages exhibit ET efficiencies exceeding theoretical FRET values and a faster donor decay than the acceptor rise, with both dynamics much faster than expected for FRET, supporting the involvement of a stepwise DET pathway. PLQY analysis further indicates that a nonemissive quenching pathway competes with ET to Cy3 and supports a model in which stepwise DET is initiated by hole transfer from the PNC to Cy3, followed by electron transfer. Single‐PNC measurements corroborate the ensemble results. These findings reveal that PNC size controls the competition between FRET and DET by modulating the donor–acceptor distance, providing a design principle for selectively controlling ET pathways in PNC–dye assemblies.