Anisotropic photoluminescence properties of oriented poly(p-phenylene-vinylene) films: Effects of dispersion of optical constants

We report on the effects of dispersive optical constants on the anisotropic photoluminescence spectra of highly oriented poly-phenylene-vinylene films. Polarized transmittance, reflectance, and photoluminescence PL spectra have been measured over a broad spectral range, at different temperatures. Due to the high degree of chain orientation in the polymer films, the emission properties are highly anisotropic. The PL spectral shape and external quantum efficiency are significantly affected by self-absorption inside the polymer film and by refractive effects at the polymer-air interface. In order to elucidate these aspects we have determined the dispersion of the parallel and perpendicular components of the complex dielectric constant, by which the PL spectra have been corrected according to Fresnel equations. After correction, the PL intensity is found to be higher when the excitation is polarized perpendicular to the stretching direction, a fact that we attribute to charge-induced PL quenching. We have also considered the influence of the refractive index on the PL spontaneous emission rate and on the light extraction from the polymer film. The spectral dependence of the emission anisotropy functions of the corrected PL spectra indicates the presence of two distinct emitting species, which may originate from intramolecular and intermolecular states.