Boosting External Quantum Efficiency to 12.0% of an Ultraviolet OLED by Engineering the Horizontal Dipole Orientation of a Hot Exciton Emitter

Abstract Currently, much research effort has been devoted to improving the exciton utilization efficiency and narrowing the emission spectra of ultraviolet (UV) fluorophores for organic light‐emitting diode (OLED) applications, while almost no attention has been paid to optimizing their light out‐coupling efficiency. Here, we developed a linear donor‐acceptor‐donor (D–A–D) triad, namely CDFDB, which possesses high‐lying reverse intersystem crossing (hRISC) property. Thanks to its integrated narrowband UV photoluminescence (PL) ( λ PL : 397 nm; FWHM: 48 nm), moderate PL quantum yield ( ϕ PL : 72 %, Tol), good triplet hot exciton (HE) conversion capability, and large horizontal dipole ratio (Θ // : 92 %), the OLEDs based on CDFDB not only can emit UV electroluminescence with relatively good color purity ( λ EL : 398 nm; CIE x,y : 0.161, 0.040), but also show a record maximum external quantum efficiency (EQE max ) of 12.0 %. This study highlights the important role of horizontal dipole orientation engineering in the molecular design of HE UV‐OLED fluorophores.