Design, synthesis and characterization of a new blue phosphorescent Ir complex

Abstract

Being incompatible with host materials in a physically blended emitting layer, phosphorescent dyes are prone to form aggregates induced by Joule heat in devices under work. In this work, a new and efficient blue phosphorescent dye Cz-C8-FIrpic was designed and synthesised by incorporating 9-phenyl-9H-carbazole into a commonly used blue emissive iridium complex bis(4,6-(difluorophenyl)pyridine-N,C²′)picolinate (FIrpic) via an alkyl chain linkage. This phosphorescent dye exhibits similar photophysical properties to the units of FIrpic and 9-phenyl-9H-carbazole in solutions. In solid films of Cz-C8-FIrpic, the energy transfer from 9-phenyl-9H-carbazole to FIrpic units is effective. The Cz-C8-FIrpic doped emissive layer was investigated by AFM, STEM-EDS, transient photoluminescence decay curves and molecular dynamics simulations. The results show that in the Cz-C8-FIrpic doped film the phase aggregation of FIrpic units is less severe than that in the typically used FIrpic film. In addition, the optimized Cz-C8-FIrpic based device achieved a maximum luminance of 25 142 cd m⁻², a maximum EQE of 8.5% and a maximum current efficiency of 22.5 cd A⁻¹ which is about 15% higher than that of the control device based on FIrpic. We conclude that grafting a typically used dye to functional groups with alkyl chains is useful to restrict phase separation in physically blended emitting layers, and thus can achieve high electroluminescence performances.