Ph.D Thesis


Ph.D StudentEyal Aharon
SubjectGuest-Host Nanocomposits based on Semiconducting Polymers
for Optoelectronic Applications
DepartmentDepartment of Materials Science and Engineering
Supervisor Full Professors Frey Gitti
Full Thesis textFull thesis text - English Version


Abstract

The research describes the design, synthesis and characterization of novel conjugated polymer/inorganic nanocomposites and their integration into optoelectronic devices. In the nanocomposites, conjugated polymers were confined into a layered inorganic material host. Following the synthesis and characterization of the new nanocomposite materials, suitable nanocomposites were integrated into light-emitting diodes.

The inorganic hosts in this project are members of the layered transition metal dichalcogenides, such as SnS2 and MoS2, characterized by strong covalent bonds within the layers and weak van der Waals forces between the layers. The interlayer space can be separated considerably to incorporate the guest polymer while preserving the integrity of the layer structure. Several semiconducting polymers were used as the guest species includes the blue- and green- emitting polyfluorenes and red-emitting polyphenylenevinylenes.

The structure of the host/guest nanocomposites was confirmed by XRD and optical absorption measurements. It was found that a single conjugated polymer monolayer, with an overall extended planar morphology conformation, is isolated between the inorganic sheets, so that polymer aggregation or π-π inter-chain interactions are significantly reduced.

The photo-physical processes in the incorporated polymer chains were investigated using UV/Vis optical absorption spectroscopy, photoluminescence excitation, steady state and time-resolved photoluminescence. The effect of solvent on the conformation of isolated chains was studied by incorporating MEH-PPV from different solvents. It was found that conjugated polymer chains incorporated from o-xylene adopt more twisted conformation compared to more extended chain morphologies when incorporated from chloroform. The differences between the PL spectra of the nanocomposites prepared using the different solvents were in good agreement with those obtained for the PL spectra of the same solutions, indicating that the conformation of the polymer chains in the solutions is retained upon incorporation into the inorganic host.

The role of energy transfer in the nanocomposite was studied by incorporating conjugated polymers with different band-gaps into the inorganic matrix and also incorporating the same conjugated polymer but with different concentrations. The reduction of π-π inter-chain interactions significantly inhibited energy transfer rate between high band-gap chromophores to the lower band-gap chromophores, due to limiting the energy transfer to 2D only. The reduced energy transfer was utilized to design and fabricate light emitting diodes with superior color-tunability properties.

The above projects support the proposed hypothesis that this system provided a platform to study two-dimensional intra-chain photo-physical processes in conjugated polymer chains.