Reflection electro-optical measurements on electroluminescent polymer films: A good tool for investigating charge injection and space charge effects

We present experimental results on the electro-optic response of electroluminescent polymer films, pertaining to the family of the poly-phenylene-vinylenes. The samples used in the experiments were obtained by sandwiching polymer films between chromium-gold and indium tin oxide electrodes. The technique that was adopted is based on the Teng and Man setup, that is usually adopted for the determination of the electro-optic Pockels coefficient in poled polymers. As the structure of the polymer used is not provided with polar groups and, moreover, is completely disordered and therefore centrosymmetric, as expected, only quadratic electro-optical effects were observed (Kerr), which are connected to the presence of third-order optical nonlinearities (chi((2))equivalent to 0, chi((3))not equal 0). The existence of charge injection and trapping in the samples, giving rise to built-in static electric fields in the polymer layers, is put into evidence. The experimental data show the presence of a third-order susceptibility for Kerr electro-optic effect which is comparable to values reported in literature, namely chi((3))(- omega;omega,0,0) = 7.9 X 10(-21) m(2)/V-2 at the wavelength lambda = 0.6328 mu m. The possibility that the measured signals are due to a mechanical compression of the films is taken into;account and discarded on the basis of the experimental results. The technique can be usefully applied in order to perform charge injection and trapping measurements in electroluminescent devices. Moreover, the results show that in the case of measurements performed on poled polymers for X(2) applications, the measured values for the electro-optic coefficient can be offset by a third-order contribution, even in those cases when usually one assumes it may be neglected. In the article, the main models reported in literature, describing the effects contributing to the electro-optic response, are critically reviewed. The measurement on the centrosymmetric polymer films were used in order to check those models and permit us to evaluate. (C) 1998 American Institute of Physics.