6 research outputs found

    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

    3D-Printed Self-Folding Electronics

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    Self-transforming structures are gaining prominence due to their general ability to adopt programmed shapes each tailored for specific functions. Composites that self-fold have so far relied on using the stimuli–responsive mechanisms focusing on reversible shape change. Integrating additional functions within these composites can rapidly enhance their practical applicability; however, this remains a challenging problem. Here, we demonstrate a method for spontaneous folding of three-dimensional (3D)-printed composites with embedded electronics at room temperature. The composite is printed using a multimaterial 3D-printing process with no external processing steps. Upon peeling from the print platform, the composite self-shapes itself using the residual forces resulting from polymer swelling during the layer-by-layer fabrication process. As a specific example, electrochromic elements are printed within the composite and can be electrically controlled through its folded legs. Our shape-transformation scheme provides a route to transform planar electronics into nonplanar geometries containing the overhangs. Integrating electronics within complex 3D shapes can enable new applications in sensing and robotics

    Speed Limit for Triplet-Exciton Transfer in Solid-State PbS Nanocrystal-Sensitized Photon Upconversion

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    Hybrid interfaces combining inorganic and organic materials underpin the operation of many optoelectronic and photocatalytic systems and allow for innovative approaches to photon up- and down-conversion. However, the mechanism of exchange-mediated energy transfer of spin-triplet excitons across these interfaces remains obscure, particularly when both the macroscopic donor and acceptor are composed of many separately interacting nanoscopic moieties. Here, we study the transfer of excitons from colloidal lead sulfide (PbS) nanocrystals to the spin-triplet state of rubrene molecules. By reducing the length of the carboxylic acid ligands on the nanocrystal surface from 18 to 4 carbon atoms, thinning the effective ligand shell from 13 to 6 Å, we are able to increase the characteristic transfer rate by an order of magnitude. However, we observe that the energy transfer rate asymptotes for shorter separation distances (≤10 Å) which we attribute to the reduced Dexter coupling brought on by the increased effective dielectric constant of these solid-state devices when the aliphatic ligands are short. This implies that the shortest ligands, which hinder long-term colloidal stability, offer little advantage for energy transfer. Indeed, we find that hexanoic acid ligands are already sufficient for near-unity transfer efficiency. Using nanocrystals with these optimal-length ligands in an improved solid-state device structure, we obtain an upconversion efficiency of (7 ± 1)% with excitation at λ = 808 nm

    The Spatial Resolution Limit for an Individual Domain Wall in Magnetic Nanowires

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    Magnetic nanowires are the foundation of several promising nonvolatile computing devices, most notably magnetic racetrack memory and domain wall logic. Here, we determine the analog information capacity in these technologies, analyzing a magnetic nanowire containing a single domain wall. Although wires can be deliberately patterned with notches to define discrete positions for domain walls, the line edge roughness of the wire can also trap domain walls at dimensions below the resolution of the fabrication process, determining the fundamental resolution limit for the placement of a domain wall. Using a fractal model for the edge roughness, we show theoretically and experimentally that the analog information capacity for wires is limited by the self-affine statistics of the wire edge roughness, a relevant result for domain wall devices scaled to regimes where edge roughness dominates the energy landscape in which the walls move

    Thermally Activated Delayed Fluorescence and Aggregation Induced Emission with Through-Space Charge Transfer

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    Emissive molecules comprising a donor and an acceptor bridged by 9,9-dimethylxanthene, were studied (XPT, XCT, and XtBuCT). The structures position the donor and acceptor with cofacial alignment at distances of 3.3–3.5 Å wherein efficient spatial charge transfer can occur. The quantum yields were enhanced by excluding molecular oxygen and thermally activated delayed fluorescence with lifetimes on the order of microseconds was observed. Although the molecules displayed low quantum yields in solution, higher quantum yields were observed in the solid state. Crystal structures revealed π–π intramolecular interactions between a donor and an acceptor, however, the dominant intermolecular interactions were CH···π, which likely restrict the molecular dynamics to create aggregation-induced enhanced emission. Organic light emitting devices using XPT and XtBuCT as dopants displayed electroluminescence external quantum efficiencies as high as 10%