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 LT₅₀ of 3483 h at an initial luminance of 100 cd m^-2.