Top-emitting organic light-emitting diodes based on semitransparent conducting cathode of Ba/Al/ITO

Top-emitting organic light-emitting diodes (TEOLEDs) with a thin semitransparent conducting cathode (STCC) of Ba/Al/ITO were fabricated and their electrical/optical characteristics were investigated. At the wavelength of 519 nm, optical properties of STCC of the Ba (5 nm)/Al (10 nm)/ITO (100 nm) structure showed the transmittance of 75% and the reflectance of 13%. The light out-coupling properties of the TEOLED, which is consisted of glass/Ag (100 nm)/ITO (125 nm)/4,4′,4″-tris[2-naphthylphenyl-1-phenylamino]triphenylamine (2-TNATA, 30 nm)/4,4′-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (NPB, 18 nm)/tris(8-quinolinolato)aluminum (III) (Alq3, 62 nm)/Ba (x nm, x = 15, 10, and 5 nm)/Al (10 nm)/ITO (100 nm), was increased as the deposition thickness of Ba is reduced. This driving performance of the devices could be interpreted on the base of carrier injection barrier by Fowler–Nordheim tunneling theory as well as the optical properties of the cathode. A constant barrier height of 0.4 eV was obtained for all the devices regardless of the cathode structure composed of Ba (x nm)/Al (10 nm)/ITO (100 nm), which indicates the electron-only devices where the injection barrier is at the ITO/2-TNATA interface.