Peripheral Selenium Modification of Multi-Resonance Thermally Activated Delayed Fluorescence Molecules for High-Performance Blue Organic Light-Emitting Diodes

Multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules have attracted much attention in the academia owing to their unique photoelectrical properties. However, MR-TADF emitters usually show slow reverse intersystem crossing (RISC) rate, resulting in high efficiency roll-off of organic light-emitting diodes (OLEDs) and seriously limiting their further development. Here, a peripheral selenium (Se) modification is presented for MR-TADF molecules to promote the RISC process while keeping the narrowband emission for high-performance blue OLEDs. Compared to the parent molecules (NBN and tBuNBN), SeNBN and SetBuNBN exhibited narrower full-width at half maximum (FWHM) value of 23 nm and more obvious delayed fluorescence properties with a high efficiency of delayed fluorescence up to 86%, shorter delayed lifetime of 2.4 mu s as well as a faster RISC rate of 3.34x105 s-1. Therefore, high-performance OLEDs based on these two Se modified MR-TADF emitters are achieved with a high maximum external quantum efficiency (EQE) up to 25.5% and extremely suppressed efficiency roll-offs of 3.9% at 100 cd m-2 and 24.4% at 1000 cd m-2. This work demonstrated that the introduction of peripheral Se atom can achieve high-performance organic semiconductors with both narrowband emission and fast RISC rate constant for high-performance organic optoelectronic devices. The strategy of peripheral Se modification is proposed to disclose the impact of heavy atom on the structure-performance relationship of blue MR-TADF emitters. As the proof of concept, the OLEDs based on Se-modified emitters achieved excellent performances, with a maximum EQE of 25.5% and the EQE still maintains to be 24.5% and 19.6% at the luminance of 100 and 1000 cd m-2, respectively. image