Solution‐Processed Blue Narrowband OLED Devices with External Quantum Efficiency Beyond 35 % through Horizontal Dipole Orientation Induced by Electrostatic Interaction

The multiple resonance thermally activated delayed fluorescence (MR-TADF) device has drawn great attention due to their outstanding efficiency and color purity. However, the efficiency of solution-processed MR-TADF devices is still far behind their vacuum-deposited counterparts, due to the uncontrollable horizontal emitting dipole orientation for emitters during solution process, resulting in low light out-coupling efficiency. Here, we proposed a new strategy namely electrostatic interaction between a dendritic host with high positive electrostatic potential (ESP) and dendritic emitter with multiple negative ESP sites, which could induce high horizontal dipole ratio (Θ_||) up to 83.0 % in solution-processed films. For this couple, the largest plane of dendritic host tends to anchor on the substrate, and thus the strong positive electrostatic site mainly lies at the exposed tetraphenylsilicon, which could electrostatically attract the multiple negative electrostatic sites of the dendritic emitter, realizing horizontal dipole orientation. Moreover, the highly twisted structure of dendritic host and dendron encapsulation of emitter could effectively suppress aggregation, leading a high photoluminescence quantum yield of 98.6 %. As a result, the solution-processed blue MR-TADF devices exhibit a record-break external quantum efficiency of 35.3 %, as well as narrow bandwidth of 17 nm and pure blue color with CIE coordinates of (0.137, 0.176).