A π–D and π–A Exciplex‐Forming Host for High‐Efficiency and Long‐Lifetime Single‐Emissive‐Layer Fluorescent White Organic Light‐Emitting Diodes

Abstract Exciplex‐forming hosts with thermally activated delayed fluorescence (TADF) provide a viable opportunity to unlock the full potential of the yet‐to‐be improved power efficiencies (PEs) and stabilities of all‐fluorescent white organic light‐emitting diodes (WOLEDs), but this, however, is hindered by the lack of stable blue exciplexes. Here, an advanced exciplex system is proposed by incorporating bipolar charge‐transport π‐spacers into both the electron‐donor (D) and the electron‐accepter (A) to increase their distance for hypsochromic‐shifted emission while maintaining the superior transporting ability. By using spirofluorene as the π‐spacer, 3,3′‐bicarbazole as the D‐unit, and 2,4,6‐triphenyl‐1,3,5‐triazine as the A‐unit, a π–D and π–A exciplex with sky‐blue emission and fast reverse intersystem crossing process is thereof constructed. Combining this exciplex‐forming host, a blue TADF‐sensitizer, and a yellow conventional fluorescent dopant in a single‐emissive‐layer, the fabricated warm‐white‐emissive device simultaneously exhibits a low driving voltage of 3.08 V, an external quantum efficiency of 21.4%, and a remarkable T80 (time to 80% of the initial luminance) of >8200 h at 1000 cd m⁻², accompanied by a new benchmark PE of 69.6 lm W⁻¹ among all‐fluorescent WOLEDs.