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 detailed photophysical measurements of intramolecular charge transfer (ICT) states we have made both in solution and solid state,1 focusing on temperature dependent time resolved emission, delayed emission, phosphorescence and photoinduced absorption to map the energy levels involved in rISC,2 and through dynamic quantum chemical modelling, the real electron exchange energies and other energy barriers of the systems are determined and the vibronic coupled spin orbit coupling model3 underpinning the reversed intersystem crossing mechanism was elucidated.
The ramifications of the second order vibronic coupling spin orbit mechanism will be discussed, and from this, I will show how the molecular structure of an emitter, and more importantly the how multiple donor configurations separately control maximum device EQE and roll off behaviour not just rISC rate. To finish, I will show new results for exciplex TADF systems which demonstrate how we can control the energy of emission and PLQY,4 which directly relates back to the simple Coulombic nature of a CT excited state.