Multifunctional Materials for High-Performance Double-Layer Organic Light-Emitting Diodes: Comparison of Isomers with and without Thermally Activated Delayed Fluorescence

Chiral Spiro-Axis Induced Blue Thermally Activated Delayed Fluorescence Material for Efficient Circularly Polarized OLEDs with Low Efficiency Roll-Off

A spiro-axis skeleton not only introduces circularly polarized luminescence (CPL) into thermally activated delayed fluorescence (TADF) molecules but also enhances the intramolecular through space charge transfer (TSCT) process. Spiral distributed phenoxazine and 2-(trifluoromethyl)-9H-thioxanthen-9-one-10,10-dioxide act as donor and acceptor units, respectively. The resulting TADF enantiomers, (rac)-OSFSO, display emission maxima at 470 nm, small singlet-triplet energy gap (ΔE_ST ) of 0.022 eV and high photoluminescence quantum yield (PLQY) of 81.2 % in co-doped film. The circularly polarized OLEDs (CP-OLEDs) based on (R)-OSFSO and (S)-OSFSO display obvious circularly polarized electroluminescence (CPEL) signals with dissymmetry factor up to 3.0×10^-3 and maximum external quantum efficiency (EQE_max ) of 20.0 %. Moreover, the devices show remarkably low efficiency roll-off with an EQE of 19.3 % at 1000 cd m^-2 (roll-off ca. 3.5 %), which are among the top results of CP-OLEDs.

Chiral Octahydro-Binaphthol Compound-Based Thermally Activated Delayed Fluorescence Materials for Circularly Polarized Electroluminescence with Superior EQE of 32.6% and Extremely Low Efficiency Roll-Off

Circularly polarized organic light-emitting diodes (CP-OLEDs) are particularly favorable for the direct generation of CP light, and they demonstrate a promising application in 3D display. However, up to now, such CP devices have suffered from low brightness, insufficient efficiency, and serious efficiency roll-off. In this study, a pair of octahydro-binaphthol (OBN)-based chiral emitting enantiomers, (R/S)-OBN-Cz, are developed by ingeniously merging a chiral source and a luminophore skeleton. These chirality-acceptor-donor (C-A-D)-type and rod-like compounds concurrently generate thermally activated delayed fluorescence with a small ΔE_ST of 0.037 eV, as well as a high photoluminescence quantum yield of 92% and intense circularly polarized photoluminescence with dissymmetry factors (|g_PL |) of ≈2.0 × 10^-3 in thin films. The CP-OLEDs based on (R/S)-OBN-Cz enantiomers not only display obvious circularly polarized electroluminescence signals with a |g_EL | of ≈2.0 × 10^-3 , but also exhibit superior efficiencies with maximum external quantum efficiency (EQE_max ) up to 32.6% and extremely low efficiency roll-off with an EQE of 30.6% at 5000 cd m^-2 , which are the best performances among the reported CP devices to date.

Unveiling the Role of Langevin and Trap-Assisted Recombination in Long Lifespan OLEDs Employing Thermally Activated Delayed Fluorophores

Recent research studies on noble-metal-free thermally activated delayed fluorescent (TADF) materials have boosted the efficiencies of organic light-emitting diodes (OLEDs) to unity. However, the short lifespan still hinders their further practical application. Carrier recombination pathways have been reported to have a significant influence on the efficiencies of TADF devices, though their effects on device lifetimes remain rarely studied. Here, we have designed and synthesized five pyrimidine or pyrazine/carbazole isomers as hosts for TADF OLEDs to explore the inherent role of Langevin recombination (LR) and trap-assisted recombination (TAR) in device lifetimes. It is revealed that for LR dominant devices, lifetimes would increase by reducing the host triplet energy levels, whereas for TAR dominant devices, lifetimes are insensitive to the host triplet excitons as recombination mainly takes place on dopants. Still, LR dominant devices are favored as they offer more room for optimization. We further apply this concept in designing a stable LR dominant blue TADF device, achieving a long LT50 (lifespan up to 50% of the initial luminance) of 269 h and high external quantum efficiency of 17.9% at 1000 cd m^-2 simultaneously.