Efficient narrow green organic light-emitting diodes with low efficiency roll-offs based on iridium(iii) complexes containing indolo[3,2,1-jk]carbazole and pyrimidine units

Abstract

In this work, two iridium(III) complexes, (2-pymICz)₂Ir(tmd) and (4-pymICz)₂Ir(tmd), using 2-(pyrimidine-2-yl)indolo[3,2,1-jk]carbazole (2-pymICz) and 2-(6-(methyl)pyrimidine-4-yl)indolo[3,2,1-jk]carbazole (4-pymICz) as the main ligands, which incorporate the rigid indolo[3,2,1-jk]carbazole (ICz) unit and 2,2,6,6-tetramethyl-3,5-heptanedione (tmd) as an ancillary ligand were synthesized. The Ir(III) complexes exhibit green photoluminescence (PL) with emission peaks at 515 and 523 nm, and relatively narrow full width at half maximum (FWHM) bands of 53 and 57 nm, and PL quantum yields (PLQYs) of 70% and 73%, respectively, in dichloromethane solutions. When these complexes were doped into the bipolar host 2,6DCzPPy (2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine), the PLQYs of the resulting films significantly increased to 98.5% and 92.5%, accompanied by narrower FWHMs of 38 and 43 nm. Organic light-emitting diodes (OLEDs) based on these two emitters display good performance characteristics. Notably, the device based on (2-pymICz)₂Ir(tmd) exhibits better performances with a maximum external quantum efficiency (EQE_max) of 31.3%. Even at a high brightness of 10 000 cd m⁻², the EQE of this device still can reach 30.4%, indicating an extremely low efficiency roll-off of below 3%. Both devices show narrow electroluminescence FWHMs of 40 and 44 nm, respectively. Overall, the study highlights the practicality of incorporating rigid ICz groups and nitrogen atoms into the main ligands of Ir(III) complexes as a viable strategy for achieving efficient OLEDs with narrow emission spectra, high efficiencies, and low efficiency roll-offs.