Molecular Engineering of Donor/π-Bridge Enables High-Efficiency and Long-Lifetime Near-Infrared TADF-OLEDs

Abstract

The development of high-efficiency near-infrared (NIR) organic emitters has been severely constrained by strong and unavoidable non-radiative decay governed by the “energy gap law”, which leads to low emission efficiencies and limited options for molecular design. Herein, comparative studies are performed on six- and five-membered π-bridge and a novel benzo[b]thiophene-based donor is designed through elaborate molecular engineering, which outperforms widely-used triphenylamine derivatives in constructing NIR thermally activated delayed fluorescence emitters. The benzo[b]thiophene donor/π-bridge endows the newly synthesized molecule tBTAP with multiple advantages, including longer emission wavelength and enhanced oscillator strength compared with thiophene-free triphenylamine donor, and suppressed high-frequency vibrations compared with thiophene-inserted donor. Consequently, tBTAP doped and nondoped films display pure NIR emission at peak wavelengths of 795 nm and 854 nm with state-of-the-art photoluminescence quantum yields of 17.9 ± 0.3% and 4.6 ± 0.4%. The corresponding NIR OLEDs demonstrate record-high maximum external quantum efficiencies of 3.64% (796 nm) and 1.20% (867 nm). Most importantly, nondoped device exhibits record-long operational lifetime (LT97) of over 2150 h at 10 mA cm−2 for NIR OLEDs. This study signifies an advance in molecular engineering by designing alternative donor/π-bridge structure, breaking through the bottleneck in developing high-performance NIR organic emitters and devices toward practical applications.