Efficient Orange–Red Delayed Fluorescence Organic Light-Emitting Diodes with External Quantum Efficiency over 26%

Highly efficient orange–red/red thermally activated delayed fluorescence (TADF) materials have been rarely reported due to the restriction of their molecular design in the energy gap law. An effective strategy is to use large rigid planar units and strong intramolecular charge transfer for synthesizing efficient long-wavelength TADF materials. Three novel orange–red to red TADF materials are obtained using a large rigid planar dibenzo[a,c]phenazine (BP) acceptor core and different numbers of strong electron-donating phenoxazine (PXZ) moieties, namely 1PXZ-BP, 2PXZ-BP, and 3PXZ-BP. The increase in the number of PXZ units red-shifts the emission from 602 to 682 nm in solution. Moreover, these emitters are endowed with high electroluminescence performance due to small singlet–triplet energy difference (ΔE_ST), fast radiative rate, and high photoluminescence quantum yields. 1PXZ-BP-based orange–red organic light-emitting diodes (OLEDs) achieve an excellent external quantum efficiency of 26.3%, which is one of the highest efficiencies reported for orange–red TADF OLEDs.