Ph.D Thesis

Ph.D StudentShany Neyshtadt
SubjectExciton Dissociation at Organic-Inorganic Interface
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Berger Shlomo
Full Thesis textFull thesis text - English Version


The following research describes the synthesis and the optoelectronic properties of mesostructured three dimensional composite materials based on titanium oxide and conjugated polymer, motivated by the quest for nanostructured hybrid donor/acceptor materials for photovoltaic applications. Control of the final structure is achieved by utilizing amphiphilic surfactants, which under certain conditions can self-organize into long range ordered structures with a lattice parameter of several nanometers, termed mesophases, and a 3D symmetry. The formed mesostructure can serve as a template for the conjugated polymer, an electron donor, and titanium oxide, electron acceptor.

This mesostructure allows exciton dissociation and charge conduction towards electrodes. The synthesis developed in this study begins from dilute solution, containing the surfactant, conjugated polymer and Titania precursor in a common solvent, tetrahydrofuran. Solvent evaporation, upon which the film is formed, promotes the polymerization of titanium oxide and the self assembly of the surfactant, resulting in an organic mesophases surrounded by a Titania framework.

The study was conducted on two levels. On the first level, a mechanism of mesophase formation in the composite system was investigated in detail. For this purpose, four non-ionic surfactants were selected, characterized by different chemical formulas and block lengths, to demonstrate the generality of the synthesis and to investigate the effect of the surfactant of the film properties. Various factors that affect the formation of the mesostructure were examined, such as the type and concentration of the surfactant, environmental conditions, etc., in relation to surfactant/water phase diagrams. Considerable attention has been paid to the location of the conjugated polymer within the film, investigated with advanced electron microscopy and optical methods. On the second level, opto-electronic properties of the mesostructured films, chiefly the exciton dissociation into positive and negative charge carriers at the interface between the conjugated polymers and titanium oxide, were examined using optical and electrical measurements techniques. The role of the surfactant, present at the Titania / conjugated polymer interface, was investigated in detail, with special emphasis on its effect on the conjugated polymer position relative to Titania and on the electron transfer between the two.

In conclusion, this study demonstrates a novel, simple way to create functional materials based on conjugated polymer and titanium oxide, with a nanometric separation of continuous phases, for application in photovoltaic devices. The surfactant, used as a template for the mesostructure, plays an important role not only in determination of the final structure, but also in the electron transfer from the polymer to the Titania. Surmounting the inhibition of electron transfer through the surfactant, and low polymer quantities can pave the way of new and efficient solar cells.