Purely organic pyridium-based materials with thermally activated delayed fluorescence for orange-red light-emitting electrochemical cells

Light-emitting electrochemical cells (LECs) based on ionic transition metal complexes (iTMCs) have become a trend in the field of electroluminescence research due to their simple device structure. However, high cost has become a major obstacle to the development of these precious metal-containing materials. Addressing this challenge, two pyridium-based thermally activated delayed fluorescence (TADF) emitters were designed and synthesized to obtain high-efficiency orange-red-emitting light-emitting electrochemical cells (LECs). Pyridine moiety is used as the precursor of electron acceptor, as well as the embedded ionic part, connecting with different donors, carbazole (CZ) and tert-butyl carbazole (tBuCZ), named Pym-CZ and Pym-tBuCZ. The lifetime measurements showed delayed fluorescence at 2887 and 1746 ns for Pym-CZ and Pym-tBuCZ, respectively, confirming their TADF characteristics. The orange-red emission of Pym-CZ is considered to be due to the strong intermolecular interaction in high concentration, while green emission occurs when the molecules are in monomer form. Notably, compounds Pym-CZ and Pym-tBuCZ are the first pure ionic organic TADF materials for orange-red LECs, and the maximal EQE of LEC based on Pym-CZ reaches approximately 1.2%, demonstrating the potential of a new method for manufacturing low-cost orange-red LECs.