From Event: SPIE Organic Photonics + Electronics, 2016
Detailed photophysical measurements of intramolecular charge transfer (ICT) states have been made both in solution and solid state. Temperature dependent time resolved emission, delayed emission and photoinduced absorption are used to map the energy levels involved in molecule decay, and through detailed kinetic modelling of the thermally activated processes observed, true electron exchange energies and other energy barriers of the systems determined with the real states involved in the reversed intersystem crossing mechanism elucidated.
For specific donor acceptor molecules, the CT singlet and local triplet states (of donor or acceptor) are found to be the lowest lying excited states of the molecule with very small energy barrier between them kT. In these cases the decay kinetics of the molecules become significantly different to normal molecules, and the effect of rapid recycling between CT singlet and local triplet states is observed which gives rise to the true triplet harvesting mechanism in TADF. Using a series of different TADF emitters we will show how the energy level ordering effects or does not effect TADF and how ultimate OLED performance is dictated by energy level ordering, from 5% to 22% external quantum efficiency. From this understanding, we are able to define three criterion for TADF in different molecules and these will be discussed.
Andrew P. Monkman, Marc Etherington, David Graves, Przemyslaw Data, Paloma Lays Dos Santos, Roberto Nobuyasu, and Fernando M. Baiao Dias, "Explaining rISC and 100% efficient TADF
(Conference Presentation)," Proc. SPIE 9941, Organic Light Emitting Materials and Devices XX, 994108 (Presented at SPIE Organic Photonics + Electronics: August 28, 2016; Published: 4 November 2016); https://doi.org/10.1117/12.2236460.5167077772001.
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