Deep‐Blue OLEDs Based on Organoboron–Phenazasiline‐Hybrid Delayed Fluorescence Emitters Concurrently Achieving 30% External Quantum Efficiency and Small Efficiency Roll‐Off

Abstract Deep‐blue organic light‐emitting diodes (OLEDs) featuring thermally activated delayed fluorescence (TADF) are experiencing growing demand, especially for full‐color display applications, as they can harvest both singlet and triplet excitons to achieve high electron‐to‐photon conversion efficiencies. However, deep‐blue TADF materials that can achieve sufficiently high electroluminescence (EL) efficiencies at a practical luminance and high emission color purity remain exceedingly rare. Herein, two deep‐blue TADF emitters are reported, OBO‐I and OBO‐II, combining a polycyclic organoboron and phenazasiline as weak electron acceptor and donor units, respectively. Both emitters exhibit efficient deep‐blue TADF, with high photoluminescence quantum yields of 81% and 98%, respectively, along with rather short emission lifetimes of ≈1 µs in their doped films. TADF‐OLEDs incorporating OBO‐II achieve extremely high maximum external EL quantum efficiencies of up to 33.8% in the blue color gamut. Furthermore, high EL efficiencies far exceeding 20% are retained, even at a high luminance of over 1000 cd m⁻², indicating highly suppressed efficiency roll‐offs. The importance of the proposed fluorophore design, which combines proper weak donor and acceptor units possessing high triplet excited energies for ideal deep‐blue TADF, is highlighted by this study.