POLYMER-BASED LIGHT-EMITTING DEVICES - INVESTIGATIONS ON THE ROLE OF THE INDIUM-TIN OXIDE (ITO) ELECTRODE

We have studied the cleaning procedure dependence of the chemical comp osition and work function for different indium-tin oxide (ITO) samples using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoele ctron spectroscopy (XPS). Also, the surface morphology of ITO was inve stigated by atomic force microscopy (AFM). Despite the pronounced diff erences in surface morphology and the In/Sn ratio, the variation of th e work function between different ITO samples was very small after eac h cleaning procedure. The work functions of ITO samples cleaned with o rganic solvents and hydrogen peroxide were 4.4-4.5 and 4.7-4.8 eV, res pectively. Ne-ion sputtering preferentially removed oxygen, which resu lted in a partial reduction of the surface and a lowering of the work function to 4.0-4.1 eV. Used as hole-injecting electrodes in organic a nd polymer light-emitting devices (LEDs), different ITOs resulted in p ronounced differences in the LED performances despite their almost ide ntical work functions after the hydrogen peroxide treatment. This reve als that the work function of the ITO is not the only factor to determ ine the hole-injection characteristics in polymer-based LEDs. (C) 1998 Elsevier Science S.A. All rights reserved.