DEGRADATION AND FAILURE OF MEH-PPV LIGHT-EMITTING-DIODES

Light-emitting diodes made with oly(2-methoxy-5(2'-ethyl)hexoxy-phenyl enevinylene) (MEH-PPV) using indium-tin-oxide (ITO) as anode and Ca as cathode have been examined as they age during operation in a dry iner t atmosphere. Two primary modes of degradation are identified. First, oxidation of the polymer leads to the formation of aromatic aldehyde, i.e., carbonyl which quenches the fluorescence. The concomitant chain scission results in reduced carrier mobility. ITO is identified as a l ikely source of oxygen. The second process involves the formation of l ocalized electrical shorts which do not necessarily cause immediate co mplete failure because they can be isolated by self-induced melting of the surrounding cathode metal. We have not identified the origin of t he shorts, but once they are initiated, thermal runaway appears to acc elerate their development. The ultimate failure of many MEH-PPV device s occurs when the regions of damaged cathode start to coalesce. (C) 19 96 American Institute of Physics.