Ph.D Thesis


Ph.D StudentOlga Epshtein
SubjectModulation Spectroscopy of Transient Photogenerated Species
in Conjugated Polymers
DepartmentDepartment of Physics
Supervisors Professor Emeritus Ehrenfreund Eitan
Professor Eichen Yoav


Abstract

In this work we present a study of the optical and electro-optical properties of electroactive conjugated polymers. Conjugated polymers form a specific class of organic materials offering semiconducting properties and high optical nonlinearity. The semiconductivity and optical properties plus the attributes of organic materials  (processability, flexibility, strength, compatibility with other polymers, relative ease of structure modification via synthetic organic methodologies) make conjugated polymers very prospective electronic materials.

The present work reports on the study of three groups of new conjugated materials:

(a) Bipyridine-containing PPV derivatives are studied from the point of view of their optical and electro-optical properties. We have investigated the effect of changes in electronic structure (protonation-deprotonation processes) and interchain interaction (aggregation phenomenon) on the optical and electro-optical properties of the polymers. The structural changes are caused by the introducing various acids or bases into the polymers. Owing to their acid/base sensitivity the new polymers can potentially be used as sensors. Particular emphasis has been placed on the study of the photoexcitation kinetics in the polymers. We explain the frequency dependence of the photo-induced signal in the terms of “dispersive” (or diffusive) relaxation. We interpret the recombination mechanism in the investigated polymers as a monomolecular recombination process with an inhomogeneous distribution of lifetimes. The distribution is extracted using the analogous behavior of the non-Debye frequency dependence of the dielectric constant in glasses. That approach is novel, as applied to photoexcitation processes in conjugated polymers. We also propose the physical model based on the presence of trapping centers and explaining the temperature and photoexcitation intensity dependencies of the photo-induced signal. This model is consistent with the “dispersive” relaxation model.

(b) Polypyrrole having pendant bipyridinium electron acceptor groups is of interest to us because we can observe electron transfer from polymer chains to electron acceptor groups. Electron transfer results in long lived charge separation, which in turn allows fabrication of photoconductors and photovoltaic devices. The evidences of the electron transfer are obtained by both photo-induced absorption and photoconductivity measurements. The sandwich device Gold|polymer|ITO based on the investigated polymer offers photovoltaic properties.

(c) Complexes of a conjugated oligomer with rare-earth ions Eu3+. In these materials there is an energy transfer from a polymer (oligomer) ligand to rare-earth ions Eu3+. We have shown that energy is absorbed by the oligomer itself and emitted by Eu3+ ions. The optical and electro-optical properties of such a complex, are reported.