2 research outputs found

    Triplet vs Singlet Energy Transfer in Organic Semiconductors: The Tortoise and the Hare

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    Current bilayer organic photovoltaics cannot be made thick enough to absorb all incident solar radiation because of the short diffusion lengths (≈10 nm) of singlet excitons. Thus, the diffusion length sets an upper bound on the efficiency of these devices. By contrast, triplet excitons can have very long diffusion lengths (as large as 10 ÎŒm) in organic solids, leading some to speculate that triplet excitonic solar cells could be more efficient than their singlet counterparts. In this paper, we examine the nature of singlet and triplet exciton diffusion. We demonstrate that although there are fundamental physical upper bounds on the distance singlet excitons can travel by hopping, there are no corresponding limits on triplet diffusion lengths. This conclusion strongly supports the idea that triplet diffusion should be more controllable than singlet diffusion in organic photovoltaics. To validate our predictions, we model triplet diffusion by purely ab inito means in various crystals, achieving good agreement with experimental values. We further show that in at least one example (tetracene), triplet diffusion is fairly robust to disorder in thin films, as a result of the formation of semicrystalline domains and the high internal reorganization energy for triplet hopping. These results support the potential usefulness of triplet excitons in achieving maximum organic photovoltaic device efficiency

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

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    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 from direct RISC from triplet to singlet exciplex. A substantial contribution comes from an indirect path, where the electron and hole separate, undergo RISC from hyperfine interactions, and then recombine to form a bright singlet exciplex. These observations have a significant impact on the design rules for TADF materials, as they imply a separate set of electronic parameters that can influence efficiency
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