Sweet spot of energy-level alignment in hyperfluorescent organic light-emitting diodes

We investigate loss mechanisms in hyperfluorescent organic light-emitting diodes (HF-OLEDs) with the emissive layer consisting of a host, a thermally activated delayed fluorescence (TADF) sensitizer, and a terminal emitter. We focus on understanding how the relative energy levels between the TADF sensitizer and terminal emitter impact device efficiency and roll-off through the formation and subsequent dissociation of an intermolecular state. Using a combined experimental and kinetic Monte Carlo (KMC) simulation-based approach, we analyzed HF-OLEDs incorporating either multiresonant (MR) or fluorescent (non-MR) terminal emitters. We find that selecting terminal emitters with ionization potential and electron affinity values that position the intermolecular state at least 150 meV above the singlet energy of the terminal emitter effectively suppresses the losses due to exciton dissociation. Furthermore, we show that using the MR emitter, which exhibits reverse intersystem crossing (RISC), significantly reduces residual triplet-related losses compared with the non-MR emitter. This further mitigates exciton dissociation losses, thereby improving the external quantum efficiency (EQE). These findings provide clear guidelines for selecting terminal emitters and optimizing energy level alignment to address intermolecular state-related loss pathways in HF-OLEDs.