HLCT‐Type Acceptor Molecule‐Based Exciplex System for Highly Efficient Solution‐Processable OLEDs with Suppressed Efficiency Roll‐Offs

Exciplex systems are promising candidates for thermally activated delayed fluorescence (TADF) molecules because of the small energy difference between the lowest singlet and triplet excited states (ΔE_ST). However, realizing high-efficiency and low-external-quantum-efficiency (EQE) roll-off in solution-processed organic light-emitting diodes (OLEDs) using an exciplex system remains a formidable challenge. In this study, two (HLCT)-type isomers with a spiro skeleton, 2-^tBuspoCz-TRZ and 10-^tBuspoCz-TRZ, are designed and synthesized as acceptors of exciplexes, where tert-butylspirofluorene indole is regarded as a donor and the triazine unit as an acceptor. Green exciplex emissions are observed for the 2-^tBuspoCz-TRZ:TAPC and 10-^tBuspoCz-TRZ:TAPC exciplexes, indicating distinct TADF characteristics with a very small ΔE_ST of 35 ± 5 meV. By using the TADF exciplex system based on the HLCT acceptor as an emitter, solution-processable OLEDs achieve a maximum external quantum efficiency (EQE_max) of 20.8%. Furthermore, a high EQE_max > 25% with a very low-efficiency roll-off (≈3.5% at 1000 cd m^-2) is obtained for solution-processable phosphorescent devices using HLCT-based exciplexes as the host matrix of phosphors. This study paves the way for a novel strategy for designing acceptor exciplex molecules for effective TADF molecules and host matrices in solution-processable OLEDs.