Electroluminescence: enhanced injection using ITO electrodes coated with a self assembled monolayer

In a typical organic bilayer electroluminescent device the hole injecting electrode is almost invariably ITO glass, but a number of electron injecting metal electrodes are possible. Unfortunately the low work function materials used readily oxidise and restrict the lifetime of the device. It is known that appropriate monolayers can change the work function of a solid and also that phosphonic acids can form self assembled monolayers on ITO glass. Using an ITO glass electrode coated with a self assembled monolayer of an electron accepting phosphonic acid (2-chloroethanephosphonic acid) and aluminium as the electron injecting electrode, it was found that the threshold voltage was significantly reduced to the same value as achieved with the less stable Mg:Ag electrode. The use of such modified ITO electrodes would obviate the use of highly reactive metal electrodes and help to overcome one of the factors which limit device lifetime. (C) 1998 Elsevier Science B.V.