How should we reduce efficiency roll-off in TADF OLEDs? A figure of merit approach

The efficiency of all LEDs decreases as their light output increases, and this effect is known as efficiency roll-off. It is a significant effect in phosphorescent OLEDs, and can be severe in TADF OLEDs, leading to efficiencies at useful brightnesses far below reported maximum efficiencies. The literature suggests that the main approaches to improving the situation are to reduce the energy difference between singlet and triplet, and to increase the rate of reverse intersystem crossing. We show this is incomplete and by considering the dynamic equilibrium between singlet and triplet, propose a figure of merit to guide the development of improved materials. The possibility of extending the approach to hyperfluorescence will also be considered.