THE PHYSICS GRADUATE PROGRAM invites everyone to the seminar:
Effect of singlet triplet recycling in the charge transfer state manifold and molecular geometry on thermally activated delayed fluorescence
Prof. Andy P. Monkman
Dept. of Physics, Durham University, South Road Durham, UK
Figure 1: Temperature dependent delayed fluorescence measurements of a near zero exchange energy ICT TADF
Detailed photophysical measurements of intramolecular charge transfer states have been made both in solution and solid state1. Temperature dependent emission and delayed emission 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.
For specific donor acceptor molecular configurations, the CT singlet and triplet states are found to be the lowest lying excited states of the molecule with very small electron exchange energies = 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 triplet states is seen to greatly extend the lifetime of the ‘excited state’ and yield non-exponential decay. Quantum yields increase markedly, even though the intersystem crossing rate is fast, ? 109 s-1. The decay kinetics is found to be very sensitive to both temperature and sample inhomogeneity2, see figure 1. Temperature dependent delayed emission measurements reveal very different time domain behaviour and the effects of ICT emitter inhomogeneity is revealed. Clear evidence will be given to show that TADF reaches 100% efficiency at harvesting triplet states1,3, and device having > 15% EQE discussed.
We will then go on to show the results for an ICT molecule with highly controlled structure i.e. the donor and acceptor fragments are held rigidly orthogonal. In this molecule the CT states can clearly be seen to be the lowest energy states of the molecule with a very small exchange energy (singlet triplet gap). Here we find that there are substantial differences between optical and electroluminescent photophysics resulting in device being far more efficient than is suggested by the molecules PLQY. For the first time we can show that the PLQY of the emitter is not the figure of merit to use in a device, because the excited states are formed in a different way within the device which avoids a major excited state quenching mechanism. This will be discussed in a new molecule that has a PLQY of 30% but gives devices having >19% EQE.
TADF thus not only enables 100% of triplet states to be harvested, the charge transfer states which give rise to emission and TADF also enable more efficient singlet emission than ‘PLQY’ would lead us to believe.
1 V. Jankus, C. J. Chiang, F. Dias, and A. P. Monkman, Adv Mater 25, 1455 (2013).
2 F. B. Dias, K. N. Bourdakos, V. Jankus, K. C. Moss, K. T. Kamtekar, V. Bhalla, J. Santos, M. R. Bryce, and A. P. Monkman, Adv Mater 25, 3707 (2013).
3 D. Graves, V. Jankus, F. B. Dias, and A. Monkman, Adv Funct Mater 24, 2343 (2014).
4 Vygintas Jankus, Przemyslaw Data, David Graves, Callum McGuinness, Jose Santos, Martin R. Bryce, F. B. Dias, and A. P. Monkman, Advanced Functional Materials 24, 6178. (2014).
Date: 24/April/2015 – (friday) – Place: Sala 212 – Auditório do Departamento de Física- Time: 10h15min