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) × 10 −6 cm 2 /V‐sec, which are close to the values reported for Alq 3 , i.e. , (2.3–4.3) × 10 −6 cm 2 /V‐sec. OLEDs were prepared with the structure ITO/NPD (400 Å)/Mq n (500 Å)/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 Å)/CBP‐8% Ir(ppy) 3 (250 Å)/BCP (150 Å)/Mq n (250 Å)/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.