Polarized photoluminescence of highly stretch-oriented poly(p-phenylene-vinylene)

We report on the anisotropic photoluminescence (PL) properties of stretch-oriented free standing films of poly(p-phenylene-vinylene) (PPV) at different temperatures. The PL quantum efficiency is strongly dependent on the pump polarization; it is higher when the pump is polarized perpendicularly to the polymer chain orientation. Independently of the pump polarization, we find that the PL emission spectra are mainly polarized along the polymer chain axis. The PL spectra show high-energy features, close to the onset of the HOMO-LUMO transition, that are significantly affected by self-absorption of the emitted light in the optically thick samples as well as by refractive effects at the polymer-air interface. In order to clarify the origin of these features, we have made a detailed characterization of the anisotropic optical constants of the PPV film. The optical constants have been derived from polarized reflectance and transmittance measurements and were used for the renormalisation of the PL spectra using the Fresnel equations. Frank-Condon analysis for the absorption oscillator strength and for the corrected emission spectra suggests that two different emitting states contribute to the optical properties. The connection of these states with film morphology and intermolecular interactions is described