Cellulose acetate and PVDC used as protective layers for organic diodes

In order to increase the lifetime of organic light emitting diodes (OLEDs), it is necessary to prevent the emitter from degradation induced by oxygen contamination from the contact with ambient air. Encapsulation of OLEDs by sealed glass is efficient but not adapted for flexible devices. In this study, we have investigated the behaviour of two polymeric materials used as a protecting layer: cellulose acetate (CA) and polyvinylidene chloride (PVDC). These materials were deposited onto active polymer layers such as poly(p-phenylene vinylene) (PPV) and poly(2-methoxy-5-(2'ethyl-hexyloxy) p-phenylene vinylene) (MEH-PPV). The optical properties (UV-Vis, Raman, IR) of the luminescent polymers were studied as a function of time, up to 21 or 28 days, and compared to those of bare films. The results clearly show the efficiency of the protecting layers. For PPV thin films, both protecting materials delay the effect of air contact, the better efficiency was obtained using CA films. In contrast. MEH-PPV is less sensitive to oxidation than PPV and no clear conclusion could be drawn on the efficiency of the encapsulating layers. (C) 2003 Elsevier Science B.V. All rights reserved.