Efeitos de ordenamento na polarização da luz emitida por filmes de polímeros semicondutores

Abstract

This work reports on the study about the optical properties of oriented and non-oriented poly(p-phenylene vinylene) (PPV) films. We investigated the influence of the molecular organization on the state of polarization of the emitted light of PPV films. To accomplish this task we performed ellipsometry experiments in the scope of the Stokes theory for electromagnetic field. The oriented films of PPV had been produced via Langmuir-Blodgett technique (LB), while the non-oriented one had been produced via casting technique, both in association with Instituto de Física de São Carlos - Universidade de São Paulo. Trough the ellipsometry experiments we obtain the Stokes parameters of emitted light for PPV films. That allows us to obtain the polarization degree, the angle orientation, the ellipticity and the asymmetry factor. We observe that the LB films (oriented) emit light with high degree of polarization ( % 70 ~ ), in the other hand the casting films present low degree of polarization ( % 10 ~ ) in the emitted light. For both films, it was, also, verified a circularly polarized light emission; the LB films present a dissymmetry factor 10 times higher than casting films. We also observe that the dissymmetry factor strongly depend on the lattices vibration modes. In order to explain the PPV circular polarization light emission, we performed the computational simulations to calculate polymeric chain structure and to obtain the energies for the molecular conformation. The calculations were implemented with HyperChem Release 7 for Windows software. The geometry structure for polymers we employed the molecular mechanics method using AMBER (Assisted Model Building with Energy Refinement) force field. To obtain the conformational energies we use AM1 (Austin Model 1) calculation in the single point option. The results showed an improper torsion angle for both models simulated; this can be explain a possible PPV symmetry break, in which possibly to explain the PPV circular light emission. The energy difference found between the structures and PPV with planar geometry is around 1.0eV that explains the polarization state lost with the phonon replicas. We also compared the improper torsion angles; the casting films exhibit a value twice times less than the LB films. This result furnish a possible explanation for the higher circularly polarized light presented by LB films.