Blue Phosphorescent Iridium complexes with a Fluorine-free structure for Efficient Organic Light-Emitting Diodes

Abstract

Organic light-emitting diodes (OLEDs) attracts great attention because of its superiority, such as self-emitting, fast response, high brightness and applicable for flexible displays. Especially, phosphorescent OLEDs using iridium (ΙΙΙ) complexes as emitters have been deeply studied for several decades. Their nature of electrochemical stability, high quantum yield and easy color tuning ability by controlling chelating ligands make iridium complexes as prospective candidates for dopants. Iridium(ΙΙΙ)bis(4,6-(difluorphenyl)pyridinato-N,C2)picolinate (FIrpic), sky-blue emitter, is widely used as a reference material of blue phosphorescent emitters. However, deeper blue em ission is needed than that of FIrpic to extend color rendering range. For deep blue emission, there were many approaches which substituted electron-withdrawing groups, such as trifluoromethyl, trifluoromethylketone and perfluorobutanone, at 4-position of phenyl ring of FIrpic. However, according to previous study, fluorine substitution exhibited degradation process during sublimation and device operation. In our study, we designed and synthesized blue phosphorescent iridium complexes without fluorine moiety. Three new compounds adapt 2-tert-butyl-5-(pyridin-2-yl)pyrimidine (b5ppmH) as a cyclometalated ligand and 2-(3-methyl-1H-1,2,4-triazol-5-yl)pyridine (mptz) and 2-(1H-tetrazol-5-yl)pyridine (ptz) as ancillary ligands to induce blue shift of emission wavelength. All compounds showed blue emission at 455-463nm with moderate quantum efficiencies. Photophysical, thermal and electrochemical properties of the three new complexes were fully characterized. In addition, organic light emitting devices using complexes 1-3 were fabricated successfully and attained high external quantum efficiency of 11.5% with lower color coordinate y value than that of FIrpic.