Organic Light Emitting Diodes, OLEDs, are now a common feature in mobile phones and ultrathin televisions. Light generation by electroluminescence in the best OLEDs can have 100% internal charge to photon conversion efficiency. This requires very efficient triplet to singlet excited state harvesting, and has been the strict preserve of electrophosphorescence heavy metal complex emitters until now. However, recently it has been discovered that all organic, donor-acceptor (DA) charge transfer molecules can also yield such efficient triplet harvesting and OLEDS with 100% internal efficiency can be fabricated. Here the process of triplet harvesting is by thermally activated delayed fluorescence, ‘TADF’, i.e. E-type delayed fluorescence, and in this talk I shall elucidate how this triplet harvesting mechanism works, including the mechanism that allows very efficient reverse intersystem crossing in a non heavy metal containing molecule, second order vibronic coupling spin orbit coupling. 1, 2
Detailed photophysical measurements of intramolecular charge transfer (ICT) states in the solid state will be used to guide our interpretation. Temperature dependent time resolved emission, delayed emission and photoinduced absorption are used to map the energy levels involved in molecule decay, and through detailed quantum chemical modelling, electron exchange energies and other energy barriers of the systems are determined with the various excited states involved in the reversed intersystem crossing mechanism elucidated. From these measurements rates of rISC can be obtained. This will be explained.
One concern over TADF has been the potential trade off between rISC rate and PLAY because of the orthogonality of the mechanisms controlling these two key photophysical processes. From a new design of TADF molecule, we will demonstrate that it is indeed possible to achieve both high PLQY (100%) and a rISC rate > 107 s-1, seemingly impossible from the original description of rISC and TADF. This gives a new design criterion for TADF emitters.
Our vibronic coupling second order spin orbit mechanism has been used to explain the observed photophysical phenomena and from further quantum chemical helps to explain how this paradox can be overcome. With very fast risk and high PLQY comes low efficiency roll-off at high brightness.
1. Etherington, M. K., Gibson, J., Higginbotham, H. F., Penfold, T. J. & Monkman, A. P. Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence. Nat Commun 7, 13680 (2016).
2. Gibson, J., Monkman, A. P. & Penfold, T. J. The Importance of Vibronic Coupling for Efficient Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence Molecules. ChemPhysChem 1–7 (2016). doi:10.1002/cphc.201600662
3. Dias, F. B. et al. The Role of Local Triplet Excited States in Thermally-Activated Delayed Fluorescence: Photophysics and Devices. Adv. Sci. 3, 1600080 (2016).
4. M.K. Etherington, F. Franchello, J. Gibson, T. Northey, J. Santos, J.S. Ward, H.F. Higginbotham, P. Data, A. Kurowska, P.L. Dos Santos, D.R. Graves, A.S. Batsanov, F.B. Dias, M.R. Bryce, T.J. Penfold, A.P. Monkman, Regio- and conformational isomerization critical to design of efficient thermally-activated delayed fluorescence emitters, Nature Communications 8, 14987 (2017).