Phosphor‐Sensitized Deep Blue Narrowband OLED Devices With EQE Over 40% by Using a Horizontal‐Oriented Host

Phosphor‐sensitized blue multiple resonance thermally activated delayed fluorescence (MR‐TADF) devices represent a promising approach for simultaneously achieving high efficiency and color purity through rapid exciton conversion and narrowband emission. However, realizing such devices with external quantum efficiency (EQE) exceeding 40% remains a challenge due to limited light out‐coupling efficiency caused by low horizontal dipole ratios (Θ || ) of the emitters. Herein, we reported a new horizontal‐oriented host 4CzTPS developed by symmetrically linking two bicarbazole units to a tetraphenylsilane core to achieve a linear geometry, thereby inducing horizontal alignment on the substrate. Consequently, the designed host with strong positive electrostatic potential at the tetraphenylsilane core attracts the emitter D1‐DBN with negative electrostatic potential on the B/N core through electrostatic interaction, yielding a high Θ || of 94.5% and enabling the phosphor‐sensitized blue device to achieve an EQE of 40.5%. Furthermore, by using an exciplex‐type co‐host based on 4CzTPS to decrease electron injection barrier and balance charge transport, the blue device achieved a record‐breaking EQE of 42.0%, narrowband emission with a small full width at half‐maximum of 16.2 nm, deep blue CIE coordinates of (0.116, 0.112), and the operational stability with a LT 50 of 3483 h at an initial luminance of 100 cd m −2 .