Highly Efficient Deep‐Blue OLEDs using a TADF Emitter with a Narrow Emission Spectrum and High Horizontal Emitting Dipole Ratio

New blue (DBA‐SAB) and deep‐blue (TDBA‐SAF) thermally activated delayed fluorescence (TADF) emitters are synthesized for blue‐emitting organic‐light emitting diodes (OLEDs) by incorporating spiro‐biacridine and spiro‐acridine fluorene donor units with an oxygen‐bridged boron acceptor unit, respectively. The molecules show blue and deep‐blue emission because of the deep highest occupied molecular energy levels of the donor units. Besides, both emitters exhibit narrow emission spectra with the full‐width at half maximum (FWHM) of less than 65 nm due to the rigid donor and acceptor units. In addition, the long molecular structure along the transition dipole moment direction results in a high horizontal emitting dipole ratio over 80%. By combining the effects, the OLED utilizing DBA‐SAB as the emitter exhibits a maximum external quantum efficiency (EQE) of 25.7% and 1931 Commission Internationale de l'éclairage (CIE) coordinates of (0.144, 0.212). Even a higher efficiency deep blue TADF OLED with a maximum EQE of 28.2% and CIE coordinates of (0.142, 0.090) is realized using TDBA‐SAF as the emitter. New blue‐emitting thermally activated delayed fluorescence (TADF) emitters are designed and synthesized for blue‐emitting organic light‐emitting diodes (OLEDs) by incorporating rigid donor units with an oxygen‐bridged boron acceptor unit. As a result, a deep‐blue OLED with a maximum external quantum efficiency of 29.3% and CIE coordinates of (0.142, 0.090) is realized.