Poly(p-phenylene vinylene) light-emitting devices prepared via the precursor route onto indium tin oxide and fluorine-doped tin dioxide substrates

For the preparation of organic light-emitting devices (LEDs) an optically transparent and electrically conducting thin film is needed as anode. Usually, a glass substrate coated with indium tin oxide (ITO) is used. We show that ITO is unsuitable in the case of poly(p-phenylene vinylene) (PPV) prepared by the precursor route. We have found that a reaction in which hydrogen chloride is eliminated during the thermal conversion to PPV and the ITO takes place. Scanning electron microscopy investigations of the ITO-PPV interface demonstrates that indium chloride compounds, e.g., InCl3 crystals with dimensions up to 40 mu m, are produced. Photoluminescence measurements reveal that the fluorescence efficiency is quenched by a factor of 2-23 in the case of ITO compared with PPV converted onto usual glass. In a second step we have investigated LEDs prepared from PPV in the ITO/PPV/Al configuration in order to obtain information about the process responsible for the degradation of these devices. We shall show that the formation of the above-mentioned indium chloride compounds is one possible degradation mechanism and is responsible for the relative short lifetimes of these LEDs. To overcome this problem we propose to use fluorine-doped tin dioxide (FTO) instead of ITO. Finally, we show the results obtained for LEDs in the FTO/PPV/Al configuration and compare them with ITO/PPV/Al devices.