High-Efficiency Sky Blue to Ultradeep Blue Thermally Activated Delayed Fluorescent Diodes Based on Ortho-Carbazole-Appended Triarylboron Emitters: Above 32% External Quantum Efficiency in Blue Devices

A novel approach to highly efficient, blue thermally activated delayed fluorescent (TADF) emitters is proposed. A series of ortho-carbazole-appended triarylboron compounds (1–5) are prepared with various substituents, including tert-Bu, Me, and OMe, introduced to the carbazole donor and/or to the dimesitylphenylboron (PhBMes2) acceptor. Depending on the substituents on the donor and/or acceptor moieties, the emission color is finely tuned over the entire blue region from sky blue to ultradeep blue. Retention of a twisted donor (D)–acceptor (A) structure enabled by the ortho D–A connectivity and bulky triarylboron leads to small energy splitting (ΔEST) between singlet and triplet excited states, resulting in very efficient TADF. High-efficiency blue TADF organic light-emitting diodes (OLEDs) are realized by using the proposed ortho D–A compounds as emitters. Among them, the blue OLEDs incorporating a BuCzMeoB (2) emitter exhibit a very high external quantum efficiency (EQE) of 32.8% at Commission Internationale de l'Eclairage (CIE) color coordinates of (0.135, 0.266). The high device efficiency is attributed to both the high photoluminescence quantum yield of 2 in a given host film (93%) and the high horizontal transition dipole ratio (Θ) of 0.76. Ultradeep blue OLEDs with a record-high efficiency are also fabricated with a CzOMeoB (5) emitter, exhibiting an EQE of 14.9% at CIE coordinates of (0.151, 0.058).