OLEDs without Iridium. 100% efficient triplet harvesting by Thermally Activated Delayed Fluorescence.

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The energy agenda demands more efficient display and lighting technologies to meet the UK government's targets. Professors Monkman, Dias and Bryce propose a new paradigm for highly efficient organic LEDs (OLEDs) using only organic fluorescent emitters, rather than Ir based organometallic phosphorescent emitters that are currently very fashionable. Phosphorescent emitters have major limitations. Blue-emitting phosphorescent complexes especially suffer from poor stability and short lifetimes, and they have not yet produced the deep-blue emission required for both display and solid-state lighting applications. Further, they mostly contain Ir as the core heavy metal, this is the fourth most scarce element on the planet, and so basing high volume mass produced lighting technology on such a scarce resource is highly risky. Our new devices will have as their emitters organic internal charge transfer (ICT) molecules. The excited states of ICT molecules have strong charge transfer character which can have vanishingly small electron exchange energies resulting in nearly equivalent singlet and triplet energies. This means that it is efficient for triplet CT states to (thermally) reverse intersystem crossing back to the singlet manifold, thereby giving a method of 'harvesting' up to 100% of triplet states formed by charge recombination in an OLED device, i.e. they can be as efficient as the best phosphorescent emitter. Thus ICT emitters can combine the most desirable properties of phosphorescent emitters, namely 100% triplet harvesting, with the added benefit of the long term stability of a fluorescent emitter. This is what both display and lighting manufactures demand but as yet do not have in the blue spectral region. It is particularly important to replace phosphorescent blue emitters, as they chemically degrade during the vacuum deposition process used in device fabrication; they have short working lifetimes and do not emit deep-blue light which is essential for both high quality displays and lighting. Our Initial studies have so far demonstrated that TADF molecules can harvest triplets with up to 100% efficiency, and also in the solid state, isolation of TADF molecules in a host affords ideal conditions for very efficient TADF emission. Thus these materials are perfectly suited for OLED applications. Furthermore, our collaborators in Japan have demonstrated simple monochrome TADF OLEDs having up to 85% internal quantum efficiency. TADF emitters are looking well set to provide a unique challenge to Ir phosphors.During this project we shall undertake detail photophysical investigations of ICT molecules that show very efficient TADF in the solid state. We will develop models to understand fully this triplet harvesting method and structure-property relationships to optimise the design and synthesis of new families of efficient TADF emitters covering the whole visible spectrum. OLEDs will be fabricated from these new materials and fully characterized. Finally, device architectures that combine two or more TADF emitters will be evaluated to determine the best routes to producing white emission from TADF emitters in simple device structures.