Green-emitting iridium(iii) complexes containing pyridine sulfonic acid as ancillary ligands for efficient OLEDs with extremely low efficiency roll-off

Abstract

A novel ancillary ligand of pyridine sulfonic acid (PySO₃) was developed for two green-emitting iridium(III) compounds, Ir1 (λ_max = 496 nm) and Ir2 (λ_max = 504 nm), with trifluoromethyl-substituted 2-phenylpyridine derivatives as the main ligands. Due to the strong electron-withdrawing ability of PySO₃, both complexes have relatively low LUMO energy levels and good electron mobility, which benefit the charge balance in the organic light-emitting diodes (OLEDs) during the electroluminescence process. Therefore, all devices with double light-emitting layers exhibit good performances. In particular, the device using Ir2 as an emitter obtains a maximum luminance above 92 000 cd m⁻², a maximum external quantum efficiency (EQE_max) of 25.5% with an extremely low efficiency roll-off, and the EQE still remains at 22.9% at the high luminance of 20 000 cd m⁻². These results demonstrate that pyridine sulfonic acid is a potential and charming ligand for Ir(III) complexes and high-performance OLEDs.