Top-emitting organic light-emitting diodes with Ba/Ag/indium tin oxide cathode and built-in potential analyses in these devices

Abstract

Top-emitting organic light-emitting diodes (TEOLEDs) with a thin semitransparent conducting cathode (STCC) of Ba/Ag/indium tin oxide (ITO) were fabricated and their electric/optical characteristics were investigated. At the wavelength of 520 nm, optical properties of STCC of the Ba(3 nm)/Ag(15 nm)/ITO (100 nm) structure showed the transmittance of 63% and the reflectance of 37%. The light out-coupling properties of the TEOLED, which is composed of glass/Ag(150 nm)/ITO (130 nm)/4,4',4 ''-tris[2-naphthylphenyl-1-phenylamino]triphenylamine (2-TNATA, 30 nm)/4,4'-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (18 nm)/tris(8-quinolinolato)aluminum (III) (62 nm)/Ba (x nm,x=3,2, and 1 nm)/Ag(15 nm)/ITO (100 nm), was increased as the deposition thickness of Ba is increased. This driving performance of the devices could be interpreted on the base of carrier injection barrier by measuring built-in voltage as well as both the optical properties and electric properties of the cathode. The optical properties (e.g., transmittance and reflectance) and electric properties of all STCCs (Ba/Ag/ITO) used in this study were nearly equal. However, built-in voltage studied using modulated photocurrent technique was increased as the thickness of barium composing of STCC was increased. This rising of a built-in voltage means a lowering of barrier height for electron injection in the devices. (C) 2008 American Vacuum Society.