Efficiency loss processes in hyperfluorescent OLEDs: a kinetic Monte Carlo study

In hyperfluorescent OLEDs, fluorescence emitter molecules are sensitized by molecules utilizing thermally activated delayed fluorescence (TADF). In principle, obtaining an internal quantum efficiency (IQE) approaching 100% combined with a small IQE roll-off should be feasible. However, the actual device performance depends on the balance between the transfer of singlet and triplet excitons from the TADF emitters to the fluorescent molecules and on the role of excitonic loss processes. Here, we study these factors governing the IQE using kinetic Monte Carlo simulations, for prototypical OLEDs based on the green TADF emitter (2s,4r,6s)-2,4,5,6-tetrakis(3,6-dimethyl-9H-carbazol-9-yl)isophthalonitrile (4CzIPN-Me) and the yellow fluorescent emitter 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene. Making use of the experimental photophysical interaction rates, the simulated voltage versus current density characteristics and IQE roll-off agree well with experiment. The simulations show that the IQE can be enhanced by carefully avoiding the formation of charge-transfer excitons.