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To achieve high electroluminescence efficiency, great efforts are devoted to enhancing photoluminescence quantum yield (ΦPL) and exciton utilization of luminescent molecule, while another important factor, light out-coupling efficiency (ηout), receives less attention in molecule design. Here, we focus on horizontal dipole orientation engineering of the molecule to increase ηout and external quantum efficiency (ηext). A series of tailor-made luminescent molecules consisting of an electron-accepting carbonyl core plus double electron-donating groups of spiroacridine-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 ΦPL values, large horizontal dipole ratios, and balanced bipolar carrier transport, which furnish record-high ηext 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.
Fu et al. (Thu,) studied this question.