Boosting external quantum efficiency to 38.6% of sky-blue delayed fluorescence molecules by optimizing horizontal dipole orientation

To achieve high electroluminescence efficiency, great efforts are devoted to enhancing photoluminescence quantum yield (Φ(PL)) and exciton utilization of luminescent molecule, while another important factor, light out-coupling efficiency (η(out)), receives less attention in molecule design. Here, we focus on horizontal dipole orientation engineering of the molecule to increase η(out) and external quantum efficiency (η(ext)). A series of tailor-made luminescent molecules consisting of an electron-accepting carbonyl core plus double electron-donating groups of spiro[acridine-9,9′-fluorene] and carbazole derivatives [e.g., 1,3,5-tri(carbazol-9-yl)benzene] are developed and systematically investigated. These molecules hold distinguished merits of strong sky-blue delayed fluorescence with excellent Φ(PL) values, large horizontal dipole ratios, and balanced bipolar carrier transport, which furnish record-high η(ext) values of up to 26.1 and 38.6% in nondoped and doped sky-blue organic light-emitting diodes (OLEDs), respectively. Moreover, the state-of-the-art nondoped hybrid white OLED and all-fluorescence single-emitting layer white OLED are also realized, demonstrating great potentials in OLED industry of these molecules.