Environmental aging of poly(p-phenylenevinylene) based light-emitting diodes

We report the effect of environmental aging of poly(p-phenylenevinylene) (PPV) on the efficiency of light-emitting diodes (LEDs) with calcium and aluminium cathodes. We prepared both single- and double-layer structures with an electron-transporting/hole-blocking layer of 2-(4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole (PBD) blended with poly(methyl metacrylate), inserted between the PPV and the calcium cathode. PPV was prepared via the precursor route and aged prior to the cathode deposition by exposing it to nitrogen (in a glove box) or air, and either in the dark, or in visible daylight. We have found that the reduction of electroluminescence (EL) efficiency after illumination in air is large for the single-layer devices, but much smaller, and comparable to the reduction of the photoluminescence (PL) efficiency, for the double-layer diodes. Surprisingly, aging in air, in the dark, does not alter significantly the efficiency of the Ca devices, whereas it increases that of Al/PPV/ITO by almost an order of magnitude. We interpret the influence of the different aging conditions in terms of a combination of photooxidation and doping of PPV by oxygen and water vapor.