Blue Thermally Activated Delayed Fluorescent OLEDs With EQE up to 25% via ³MMLCT-Induced and Short-Lived Phosphorescent Dinuclear Pt(II) Sensitizers

The novel dinuclear Pt(II) complexes, Di-Pt-CH₃ and Di-Pt-CD₃ with non-fluorinated n-hetero cyclic (NHC) ligands are developed. They exhibit phosphorescent emission in the range of 440–460 nm in film state with ≈60% photoluminescence quantum yield (PLQY) and a shorter lifetime due to a moderate Pt-Pt distance of 3.21 Å. By suitably combining with blue multiresornance thermally activated delay fluorescence (MR-TADF) emitters, t-DABNA and ν-DABNA, efficient energy transfer is achieved from the triplet intraligand state (³IL) and triplet metal ligand change transfer (³MLCT) mixed states of Pt(II) complexes to the singlet state of the emitters. Importantly, the delayed triplet lifetime of the TADF emitter is shortened through the fast relaxation of triplet metal-metal to ligand charge transfer (³MMLCT) states, possessing 0.07 eV lower energy compared to the triplet states of the TADF emitters. Di-Pt-CH₃ and Di-Pt-CD₃ are employed in phosphorescent and phosphorescent sensitized TADF (PS-TADF) blue OLEDs, resulting in high external quantum efficiency (EQE) of 18.8% and 25.4%, respectively. An extremely low roll-off characteristic of 9.8% is observed in the PS-TADF OLED. Additionally, deuterium substitution of the methyl group improved phosphorescent device lifetime by 2.6 times. Notably, Di-Pt-CD₃ resulted in significant lifetime enhancements: 4.7 times in phosphorescent devices and 6.6 times in PS-TADF devices, compared with Ir(cb)₃-based devices. The mechanism for the increased lifetime is extensively studied through the magneto-electroluminescence (MEL) and transient electroluminescence (TrEL) measurements.