The Role of Electron–Hole Separation in Thermally Activated Delayed Fluorescence in Donor–Acceptor Blends

Abstract

Thermally activated delayed fluorescence (TADF) is becoming an increasingly important OLED technology that extracts light from nonemissive triplet states via reverse intersystem crossing (RISC) to the bright singlet state. Here we present the rather surprising finding that in TADF materials that contain a mixture of donor and acceptor molecules the electron–hole separation fluctuates as a function of time. By performing time-resolved photoluminescence experiments, both with and without a magnetic field, we observe that at short times the TADF dynamics are insensitive to magnetic field, but a large magnetic field effect (MFE) occurs at longer times. We explain these observations by constructing a quantum mechanical rate model in which the electron and hole cycle between a near-neighbor exciplex state that shows no MFE and a separated polaron-pair state that is not emissive but does show magnetic field dependent dynamics. Interestingly, the model suggests that only a portion of TADF in these blends comes fr...