Thermally Activated Delayed Fluorescence; from photophysics to next generation OLED devices.

Andy Monkman

Dept. of Physics, Durham University, South Road Durham, UK

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 overview, I shall elucidate on the triplet harvesting mechanism and how it can be controlled by molecular architecture.

Starting from the photophysical measurements of intramolecular charge transfer (ICT) states,¹ focusing on temperature dependent time resolved emission, delayed emission and phosphorescence I will show how we have understood the reverse intersystem crossing mechanism (rISC),² and through dynamic quantum chemical modelling, how vibronic coupling drives spin orbit coupling.³

Finally, I will discuss TADF OLEDs, their problems and how they are being overcome to produce blue devices reaching beyond 30% external quantum efficiency and how these will be used in a new panel architecture in the next generation of high resolution OLED displays.

References

1. 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).

2. 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).

3. 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