Modified spontaneous emission in oligo(p-phenylene vinylene) planar microcavities

This paper reports on the photoluminescence properties of phenylene vinylene oligomers films (OPV) the emission of which was modified by microcavity effect. Films with 4 and 6 vinyl groups (respectively denoted OPV-4 and OPV-6) were investigated. A strong blue luminescence at 470 nm is observed for OPV-4, OPV-6 shows an emission spectrum quite similar to that of poly phenylene vinylene (PPV). Photoluminescence excitation measurements (PLE) give similar behaviors for the two samples and do not show any change in the shape of the spectra. Ageing under UV irradiation (380 nm) of the unencapsulated OPV-4 layers was evidenced by an exponential decrease of the photoluminescence intensity of about 40% after 3 h a continuous illumination. OPV-6 samples, aged on shelf, for weeks does only show a small decrease (15%) of the PL level over more than 7 h continuous UV irradiation. Microcavity related effects were observed by inserting the films between two mirrors. These are generally a TiO2/SiO2 distributed Bragg reflectors (DBR) as bottom mirror and a Ag film as top mirror. However best results were obtained with two dielectric DBR. As expected, a strong enhancement of the emission at the cavity resonance and pronounced angular effects (emission peaking in the direction normal to the surface of the device) were observed. The importance of the position of the emitting material within the microcavity is evidenced with a thin tris(8-hydroxy) quinoline aluminum (Alq) embedded in SiO2 with various cavity configurations. (C) 1998 Elsevier Science B.V.