High-efficiency near-infrared organic light-emitting devices based on an iridium complex with negligible efficiency roll-off

Abstract

For NIR-emitting organic light-emitting devices (OLEDs), platinum complexes have the record maximum external quantum efficiency (EQE), although such devices generally suffer from severe efficiency roll-off with increasing current densities. Here, we report on iridium complexes as a competent alternative for NIR dyes with high EQEs and negligible efficiency roll-off. A simple, charge-neutral iridium complex, iridium(III) bis(2-methyl-3-phenylbenzo[g]quinoxaline-N,C′) acetylacetonate (Ir(mpbqx-g)₂acac, 1), has been synthesized and characterized by a strong NIR emission with λ_max,peak at 777 nm and λ_max,shoulder at 850 nm in CH₂Cl₂ solutions. The single-crystal and electronic structure as well as photophysical and electrochemical properties were systematically studied in comparison with its cationic counterpart [Ir(mpbqx-g)₂(Bphen)]⁺PF₆⁻ (2, Bphen = 4,7-diphenyl-1,10-phenanthroline). Complex 1 has seven times the quantum efficiency of complex 2 because of its much stronger spin–orbit coupling. NIR-emitting OLEDs based on complex 1 have been fabricated with a bipolar gallium complex as the host. The devices achieved a maximum EQE of up to 2.2% (J = 13 mA cm⁻²) and a maximum radiant emittance (R_max) of 1.8 mW cm⁻². In particular, the EQEs remained around 2% over a wide range of current densities from 3 to 100 mA cm⁻².