POLYMERIC ANODES FOR ORGANIC LIGHT-EMITTING-DIODES

Polymer light-emitting diodes based on PPV, for example MEH-PPV, are k nown to be susceptible to photo-oxidative degradation. The formation o f the carbonyl species in the polymer results in quenching of the lumi nescence. In addition the oxidation process reduces the conjugation of the polymer, leading to lower charge carrier mobilities and consequen tly higher operating voltages. Previous in situ FTIR studies revealed that even in a dry inert atmosphere polymer oxidation occurs, and that ITO can act as the source of oxygen. In order to explore further the nature of. the oxidation mechanism and to provide guidance for its eli mination, we have studied the behavior of MEH-PPV LEDs prepared with c onducting polymer anodes. When a layer of polyaniline is present betwe en the ITO and the MEH-PPV the device characteristics improve dramatic ally: the injection voltage drops, the luminous efficiency increases a nd, most significantly, the rate of decay of the luminance decreases b y up to two orders of magnitude. These data not only confirm that ITO is a source of oxygen, but also imply that the oxidation mechanism is due to direct interfacial reaction. We compare several different forms of polyaniline, with different dopants, as well as a derivative of a polythiophene.