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 composition and work function for different indium—tin oxide (ITO) samples using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Also, the surface morphology of ITO was investigated by atomic force microscopy (AFM). Despite the pronounced differences in surface morphology and the InSn ratio, the variation of the work function between different ITO samples was very small after each cleaning procedure. The work functions of ITO samples cleaned with organic solvents and hydrogen peroxide were 4.4–4.5 and 4.7–4.8 eV, respectively. Ne-ion sputtering preferentially removed oxygen, which resulted 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 and polymer light-emitting devices (LEDs), different ITOs resulted in pronounced differences in the LED performances despite their almost identical work functions after the hydrogen peroxide treatment. This reveals that the work function of the ITO is not the only factor to determine the hole-injection characteristics in polymer-based LEDs.