Extremely efficient sky-blue organic electroluminescence with external quantum efficiency of ≈37% is achieved in a conventional planar device structure, using a highly efficient thermally activated delayed fluorescence emitter based on the spiroacridine-triazine hybrid and simultaneously possessing nearly unitary (100%) photoluminescence quantum yield, excellent thermal stability, and strongly horizontally oriented emitting dipoles (with a horizontal dipole ratio of 83%).

Lead halide-based perovskite thin films have attracted great attention due to the explosive increase in perovskite solar cell efficiencies. The same optoelectronic properties that make perovskites ideal absorber materials in solar cells are also beneficial in other light-harvesting applications and make them prime candidates as triplet sensitizers in upconversion via triplettriplet annihilation in rubrene. In this contribution, we take advantage of long carrier lifetimes and carrier diffusion lengths in perovskite thin films, their high absorption cross sections throughout the visible spectrum, as well as the strong spin-orbit coupling owing to the abundance of heavy atoms to sensitize the upconverter rubrene. Employing bulk perovskite thin films as the absorber layer and spin-mixer in inorganic/organic heterojunction upconversion devices allows us to forego the additional tunneling barrier owing from the passivating ligands required for colloidal sensitizers. Our bilayer device exhibits an upconversion efficiency in excess of 3% under 785 nm illumination.

Photon upconversion via triplet-triplet annihilation (TTA) has achieved high efficiencies in solution and within polymer matrices that support molecular migration systems. It has diverse potential applications including bioimaging, optical sensors, and photovoltaics. To date, however, the reported performance of TTA in rigid solid-state systems is substantially inferior, which may complicate the integration of TTA in other solid-state devices. Here, solid-state loss mechanisms in a green-to-blue upconversion system are investigated, and three specific losses are identified: energy back transfer, sensitizer aggregation, and triplet-charge annihilation (TCA). Strategies are demonstrated to mitigate energy back transfer and sensitizer aggregation, and a completely dry-processed solid-state TTA upconversion system having an upconversion efficiency of ~2.5% (by the convention of maximum efficiency being 100%) at a relatively low excitation intensity of 238 mW/cm 2 is reported. This device is the first demonstration of dry-processed solid-state TTA comparable to solution-processed solid-state systems. The strategies reported here can be generalized to other upconversion systems and offer a route to achieving higher performance solid-state TTA upconversion devices that are compatible with applications sensitive to solvent damage.

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Photon upconversion via triplet–triplet annihilation (TTA) has promise for overcoming the Shockley–Queisser limit for single‐junction solar cells by allowing the utilization of sub‐bandgap photons. Recently, bulk perovskites have been employed as sensitizers in solid‐state upconversion devices to circumvent poor exciton diffusion in previous nanocrystal (NC)‐sensitized devices. However, an in‐depth understanding of the underlying photophysics of perovskite‐sensitized triplet generation is still lacking due to the difficulty of precisely controlling interfacial properties of fully solution‐processed devices. In this study, interfacial properties of upconversion devices are adjusted by a mild surface solvent treatment, specifically altering perovskite surface properties without perturbing the bulk perovskite. Thermal evaporation of the annihilator precludes further solvent contamination. Counterintuitively, devices with more interfacial traps show brighter upconversion. Approximately an order of magnitude difference in upconversion brightness is observed across different interfacial solvent treatments. Sequential charge transfer and interfacial trap‐assisted triplet sensitization are demonstrated by comparing upconversion performance, transient photoluminescence dynamics, and magnetic field dependence of the devices. Incomplete triplet conversion from transferred charges and consequent triplet‐charge annihilation (TCA) are also observed. The observations highlight the importance of interfacial control and provide guidance for further design and optimization of upconversion devices using perovskites or other semiconductors as sensitizers.

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