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

Abstract 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.