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

Abstract— In this paper, we report on the utilization of zirconium (IV) tetras (8‐hydroxyquinoline), Zrq₄, and hafnium (IV) tetras (8‐hydroxyquinoline), Hfq₄, 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 (Mq₄) have a very low dipole moment (<0.1 D), in contrast to Alq₃ which has an estimated dipole moment of 4.7 D. Mobility measurements show that Mq₄ complexes give mobilities of (3.5 ± 0.5) × 10^?6 cm²/V‐sec, which are close to the values reported for Alq₃, i.e., (2.3–4.3) × 10^?6 cm²/V‐sec. OLEDs were prepared with the structure ITO/NPD (400 Å)/Mq_n (500 Å)/LiF/Al (NPD = 4‐4′‐bisN‐(1‐naphthyl)‐N‐phenyl‐amino]bi phenyl, Mq_n = Alq₃, Zrq₄, Hfq₄. The Mq₄‐based OLEDs gave external efficiencies of 1.1%, while the Alq₃‐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 Mq₄ ETLs showed a greatly improved efficiency, when compared to Alq₃‐based PHOLEDs. The Zrq₄‐based PHOLEDs gave a peak external quantum efficiency of 14% at 0.3 mA/cm² (150 cd/m²), while the Hfq₄ based PHOLED gave a peak external quantum efficiency of 15% at 0.6 mA/cm² (300 cd/m²). Comparable PHOLEDs with an Alq₃ ETL give peak external quantum efficiencies of 8.0% at 0.5 mA/cm². 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 Mq₄ emission observed at any bias level.