A new electron transporting material for effective hole-blocking and improved charge balance in highly efficient phosphorescent organic light emitting diodes

A highly efficient new benzoimidazole phosphine oxide based electron transporting material, bis(1-phenyl-1H-benzo[d]imidazole)phenylphosphine oxide (BIPO), was designed, synthesized by condensation, nucleophilic substitution and oxidation reactions, and confirmed using various spectroscopic studies. It shows a thermal stability (ΔT5%) of 451 °C with a glass transition temperature of 129 °C from the thermogravimetric analysis and differential scanning calorimetry studies. BIPO used as an efficient electron transport layer (ETL) in a green emitting phosphorescent organic light emitting diode, ITO/4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (30 nm)/4,4′,4′′-tris(carbazole-9-yl)triphenylamine 10 nm)/(4,4′-N,N′-dicarbazole)biphenyl (CBP) host doped with Ir(ppy)3 dopant (5%) (30 nm)/ETL (40 nm)/LiF (1 nm)/Al (100 nm), serves as an effective hole-blocking material and improves the charge balance in the device, resulting in higher device efficiencies of 22.19%, 68.3 cd A−1 and 24.4 lm W−1 with a maximum luminance of 72 080 cd m−2 compared to those (17.03%, 52.0 cd A−1 and 20.9 lm W−1 with a maximum luminance of 33 490 cd m−2) of a device using a widely used ETL, 1,3,5-tris(m-pyrid-3-yl-phenyl)benzene. These results show that the new BIPO ETL could be very useful in efficient organic light emitting diodes.