Enhancement of the electroluminescence properties of iridium-complexes by decorating the ligand with hole-transporting carbazole dendrons

Abstract

Iridium complexes are particularly essential and have been intensively utilized as emissive phosphorescence emitters for efficient phosphorescent electroluminescent (EL) devices. In order to improve the EL performance, a series of new iridium complexes decorated with carbazole dendrons in a non-conjugated fashion using an ether linkage were designed and synthesized. The iridium(III) bis(4-methyl-2-phenylpyridinato-N,C2′)picolinate substituted with a hexyl chain (IrG0), N-hexyl carbazole (IrG1), N-hexyl-N′-9,3′:6′,N′′-tercarbazole (IrG2) and N-hexyl-6′,6′′′-di(carbazol-N-yl)-N′′,3′:N′,3′′:6′′,N′′′:3′′′,N′′′′-quinquecarbazole (IrG3) was designed to afford improved hole-transporting properties without affecting the iridium core's photophysical and electronic properties. The synthesized iridium complexes exhibited intense yellowish-green photoluminescence (PL) emissions at 542–561 nm in the film state. The hole-transporting capability of the complexes was found to be improved when carbazole dendrons were incorporated in the ligand and increased as the generation of the substituent carbazole dendrons increased in the order of IrG0 < IrG1 < IrG2 < IrG3. In particular, the use of IrG3, showing the highest hole mobility, in an organic light-emitting diode (OLED) device resulted in a strong and stable green emission peaking at 532 nm (color coordinates CIE x, y of (0.36, 0.56)) with a brightness of 16 170 cd m⁻², the maximum luminous efficiency of 13.59 cd A⁻¹ and a maximum EQE of 4.36%.