TADF, how to increase efficiency of OLEDs?
Electron-hole recombination in the organic emitters creates 25% singlet excitons that decay radiatively (producing light) and 75% non-radiative triplet excitons. Complexes of heavy metals have efficiently intermixed singlet and triplet states, via an effective heavy atom spin orbit coupling, yielding close to 100% of light emission by phosphorescence. In the case of fluorescent OLED, the maximal internal efficiency is limited to 25%.
Although the fabrication of highly efficient OLEDs appears possible with the intramolecular spin up-conversion from non-radiative triplet state to radiative singlet state, the realization of an efficient upconversion requires a very small energy gap between singlet and triplet excited states and is difficult to achieve in a single molecule because of chemical and technological problems. There is therefore a strategy to obtain radiative excitons with higher efficiency than the fluorescence limitation of 25%, using the efficient upconversion of an intermolecular excited state (exciplex state) between electron donating and electron accepting molecules.
Thermally Activated Delayed Fluorescence (TADF)
TADF has emerged recently as one of the most suitable methods to harvest triplet states in metal-free organic materials. Numerous TADF molecules have been designed with the purpose of enhancing device efficiency, by converting non-emissive triplet states into emissive singlet states that give origin to delayed fluorescence. Fluorescent OLEDS with internal quantum efficiencies close to 100% are now routinely fabricated in the green spectral regions. However, blue and red emitters still show relative lower efficiencies, and still major challenges persist concerning the full understanding of the TADF mechanism and the stability of these materials, which need to be solved in order to fully implement TADF emitters in the OLED field, and expand their application into other areas.