Cellulose acetate and PVDC used as protective layers for organic diodes P. Le Rendu, T.P. Nguyen * , L. Carrois Laboratoire de Physique Cristalline, IMN, BP 32229, Nantes 44322, France Abstract 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 0 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. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Polymer; Diode; Encapsulation; Degradation 1. Introduction Organic light emitting diodes (OLEDs) are one of the most promising applications of conducting polymers because of their advantages such as low weight, high bright- ness and flexibility. To be used as industrial products, the devices should have not only good physical performance but also a sufficiently long operating lifetime. A 10 000 h life- time is usually considered as necessary for commercial products. In spite of great progresses made in this field, since the last decade, the reliability of OLEDs is still to be improved. The mechanisms of the degradation are still not well understood, but in many cases the growth of dark spots on the device surface has been mentioned [1]. It has been demonstrated by several investigations that these black spots have high concentration in oxygen [2]. This oxidation can result from several processes such as air contact, film deposition, polymer synthesis or decomposition of transpar- ent anode. In order to limit the formation of the black spots and to increase the lifetime of the diodes, one may try to prevent the emitting layer from contamination after the fabrication of the device by insulating the film against ambient air. Poly( p-phenylene vinylene) (PPV) and its derivatives are good candidates for OLEDs active layer. Among these poly(2-methoxy-5-(2 0 ethyl-hexyloxy) p-phenylene vinylene) (MEH-PPV) is currently used in OLEDs. In this paper, we have investigated the behaviour of PPV and MEH-PPV films covered with two different polymer films used as a protec- tive layer: cellulose acetate (CA) and polyvinylidene chlor- ide (PVDC). The optical properties (UV-Vis, Raman, IR) of the encapsulated films were studied as a function of time for 21 or 28 days and compared to those of bare polymer films in order to test the efficiency of the protective layers and to understand the possible degradation mechanisms in the emitting films. 2. Experimental MEH-PPV was prepared by an improved synthesis technique [3] and dissolved in xylene solution. PPV was prepared using the Wessling–Zimmermann method, the polymer films were obtained by conversion of the precursor at 300 8C in vacuum for 3 h. CA was purchased from Good Fellow and PVDC from Aldrich. Synthetic Metals 138 (2003) 285–288 * Corresponding author. Tel.: þ33-2-40-37-39-76; fax: þ33-2-37-39-91. E-mail address: nguyen@cnrs-imn.fr (T.P. Nguyen). 0379-6779/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01294-8