Doping and thickness optimization of the blue thermally activated delayed fluorescence organic electroluminescence diode

Thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) can, in principle, reach internal quantum efficiencies (IQEs) of 100%, far beyond the 25% IQE limit of conventional fluorescent OLEDs. However, achieving stable and efficient blue TADF OLEDs continues to be a major challenge. In this work, we report the first comprehensive study on the combined influence of emitter doping concentration and emissive layer (EML) thickness on device performance in the 10,10′-(4,4′-sulfonylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine) (DMAC-DPS) emitter with bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) host system. We show that simultaneous optimization of doping concentration and EML thickness effectively positions the exciton recombination zone away from quenching interfaces, leading to improved charge balance and reduced non-radiative exciton losses. This mechanism is directly supported by time-correlated single-photon counting (TCSPC) measurements, which reveal exciton quenching at both the EML/hole transport layer (HTL) and EML/electron transport layer (ETL) interfaces. With this strategy, our optimized devices achieve an external quantum efficiency (EQE) of 23.3%, a current efficiency exceeding 48 cd/A, and a maximum power efficiency of 41.9 lm/W, representing record-high performance for the DMAC-DPS/DPEPO system. Beyond reporting efficient blue TADF OLEDs, this work provides a clear physical rationale for the observed improvements and establishes a framework for the rational design of high-efficiency and relatively stable blue TADF devices.