Achieving 34.3% External Quantum Efficiency for Red Thermally Activated Delayed Fluorescence Organic Light-Emitting Diode by Molecular Isomer Engineering

The development of red organic emissive materials with high-efficiency and low-cost is of great significance but formidable challenge for organic light-emitting diodes (OLEDs). Herein, through isomer engineering, a pair of red thermally activated delayed fluorescence (TADF) isomers with Y-shape and cruciform structures, namely TPA-APQDCN-Y and TPA-APQDCN-C, are developed by integrating two triphenylamine (TPA) donor moieties into different positions of rigid planar acenaphtho[1,2-b]quinoxaline-9,10-dicarbonitrile (APQDCN) acceptor core. Compared to common Y-shape structure, the unique cruciform structure can result in the formation of intramolecular H-bonding, the limited molecular packing, the balance of the contradiction between the small spatial overlap of frontier molecular orbitals, and high oscillator strength, contributing to a higher photoluminescence quantum yield of 95%, a smaller singlet-triplet energy split of 0.24 eV and a larger horizontal emitting dipole ratio of 86%. An external quantum efficiency of 34.3% with an emission peak at 610 nm is achieved for TPA-APQDCN-C based red electroluminescent device, which is the highest value for red TADF-OLEDs with emission maximum beyond 600 nm ever reported.

Automated summary

A pair of red thermally activated delayed fluorescence (TADF) isomers were developed through isomer engineering, achieving high photoluminescence quantum efficiency, small singlet-triplet energy split, and large horizontal emitting dipole ratio. The red electroluminescent device based on TPA-APQDCN-C achieved the highest external quantum efficiency of 34.3% with an emission peak at 610 nm among red TADF-OLEDs with emission maximum beyond 600 nm ever reported.