Metal (IV) tetras (8-hydroxyquinoline) (M = Zr, Hf) used as electroluminescent material and electron-transport layer in OLEDs

In this paper, we report on the utilization of zirconium (IV) tetras (8-hydroxyquinoline), Zrq(4), and hafnium (IV) tetras (8-hydroxyquinoline), Hfq(4), as an electroluminescent material in fluorescent organic light-emitting diodes (OLED) and as electron transport layer (ETL) for high-efficiency electrophosphorescent organic light-emitting diodes (PHOLEDs). Structural studies show that the metal tetraquinolates (Mq(4)) have a very low dipole moment (< 0.1 D), in contrast to Alq(3) which has an estimated dipole moment of 4.7 D. Mobility measurements show that Mq(4) complexes give mobilities of (3.5 +/- 0.5) x 10(-6) cm(2)/V-sec, which are close to the values reported for Alq(3), i.e., (2.3 - 4.3) x 10(-6) cm(2)N-sec. OLEDs were prepared with the structure ITO/NPD (400 angstrom)/Mq(n) (500 angstrom)/LiF/Al (NPD = 4-4'-bis[N-(1-naphthyl)-N-phenyl-amino]bi phenyl, Mq(n) = Alq(3), Zrq(4), Hfq(4). The Mq(4)-based OLEDs gave external efficiencies of 1.1%, while the Alq(3)- based devices of the same structure gave efficiencies of 0.7%. PHOLEDs have been fabricated with the structure ITO/NPD (500 angstrom)/CBP-8% Ir(ppy)(3) (250 angstrom)/BCP (150 angstrom)/Mq(n) (250 angstrom)/LiF/Al (CBP = N, N'-dicarbazolyl-4-4'-biphenyl, Ir(ppy)(3) = fac-tris(2-phenylpyrridine)iridium, BCP = bathocruprione). PHOLEDs with Mq(4) ETLs showed a greatly improved efficiency, when compared to Alq(3)-based PHOLEDs. The Zrq(4)-based PHOLEDs gave a peak external quantum efficiency of 14% at 0.3 mA/cm(2) (150 cd/m(2)), while the Hfq(4) based PHOLED gave a peak external quantum efficiency of 15% at 0.6 mA/cm(2) (300 cd/m(2)). Comparable PHOLEDs with an Alq(3) ETL give peak external quantum efficiencies of 8.0% at 0.5 mA/ cm(2). The devices gave an electroluminescence (EL) spectrum consisting only of fac-tris(2-phenylpyrridine)iridium (Ir(ppy)(3)) dopant emission (CIE coordinates of 0.26, 0.66), with no Mq(4) emission observed at any bias level.