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

초록

The novel dinuclear Pt(II) complexes, <bold>Di-Pt-CH3</bold> and <bold>Di-Pt-CD</bold>(3) with non-fluorinated n-hetero cyclic (<bold>NHC)</bold> ligands are developed. They exhibit phosphorescent emission in the range of 440-460 nm in film state with approximate to 60% photoluminescence quantum yield (<bold>PLQY)</bold> and a shorter lifetime due to a moderate Pt-Pt distance of 3.21 & Aring;. By suitably combining with blue multiresornance thermally activated delay fluorescence ( <bold>MR-TADF)</bold> emitters, <bold>t-DABNA</bold> and nu<bold>-DABNA</bold>, efficient energy transfer is achieved from the triplet intraligand state (<bold>(IL)-I-3)</bold> and triplet metal ligand change transfer (<bold>(MLCT)-M-3)</bold> mixed states of Pt(II) complexes to the singlet state of the emitters. Importantly, the delayed triplet lifetime of the <bold>TADF</bold> emitter is shortened through the fast relaxation of triplet metal-metal to ligand charge transfer ((MMLCT)-M-3) states, possessing 0.07 eV lower energy compared to the triplet states of the <bold>TADF</bold> emitters. <bold>Di-Pt-CH3</bold> and <bold>Di-Pt-CD</bold>(3) are employed in phosphorescent and phosphorescent sensitized <bold>TADF</bold> (<bold>PS-TADF)</bold> blue OLEDs, resulting in high external quantum efficiency (<bold>EQE)</bold> of 18.8% and 25.4%, respectively. An extremely low roll-off characteristic of 9.8% is observed in the <bold>PS-TADF OLED</bold>. 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 <bold>Ir(cb)(3)-</bold>based devices. The mechanism for the increased lifetime is extensively studied through the magneto-electroluminescence (MEL) and transient electroluminescence (TrEL) measurements.

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