Efficient bluish-green phosphorescent iridium complex for both solution-processed and vacuum-deposited organic light-emitting diodes

Iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2′)picolinate (Firpic) is one typical bluish-green phosphor widely used in phosphorescent organic light-emitting diodes (PhOLEDs). In order to optimize its electroluminescent performance, 3,6-(di-tert-butyl)carbazolyl was introduced into the pyridine ring of the 2,4-difluorophenyl-pyridine ligand via a non-conjugated CH2 linkage. The generated 3,6-di-tert-butyl-9-((6-(2,4-difluorophenyl)pridine-3-yl)methyl)-9H-carbazole (Cz-CH2-dfppy) was used as cyclometalating ligand to prepare iridium complex 1, (Cz-CH2-dfppy)2Ir(pic). In comparison with the case to attach carbazole directly on pyridine, this non-conjugated CH2 linking strategy avoids the unwanted bathochromic shift of the phosphorescence and improves the solubility of the iridium complex. (Cz-CH2-dfppy)2Ir(pic) (1) was used as doped emitter to fabricate OLEDs by both spin-coating and vacuum evaporation methods. Efficient bluish-green electrophosphorescence was obtained with maximum luminance efficiency of 22 cd/A (14 lm/W, 8.7%) and 26 cd/A (12 lm/W, 9.5%) for the solution-processed and vacuum-deposited devices, respectively, which far exceed those of the parent Firpic based device. The improved performance for (Cz-CH2-dfppy)2Ir(pic) was interpreted in terms of improved charge balance brought by the presence of the carbazole groups in the ligands.