AbstractOrganic light-emitting diodes utilizing thermally activated delayed fluorescence is a potential solution for achieving stable blue devices. Sky-blue devices (CIE_y < 0.4) with high stability and high external quantum efficiency (>15%) at 1000 cd m⁻² based on either delayed fluorescence or phosphorescence are still limited and very hard to achieve simultaneously. Here, we report the design and synthesis of a new thermally activated delayed fluorescence emitter, 3Ph₂CzCzBN. A sky-blue device based on 3Ph₂CzCzBN exhibits a high external quantum efficiency of 16.6% at 1000 cd m⁻². The device shows a sky-blue electroluminescence of 482 nm and achieves Commission Internationale de l’ Eclairage coordinates of (0.17, 0.36). The sky-blue device exhibits a superb LT₉₀ of 38 h. This is the first demonstration of high-efficiency and stable sky-blue devices (CIE_y < 0.4) based on delayed fluorescence, which represents an important advance in the field of blue organic light-emitting diode technology.