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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Physics

Year Degree Awarded

Spring 2014

First Advisor

Michael D. Barnes

Subject Categories

Condensed Matter Physics | Optics | Physical Chemistry

Abstract

Plastic electronics have an essential role in the future technologies owing to their compelling characteristics such as light weight, biocompatibity, low cost fabrication, and tunable optoelectronic properties. However, the performance of polymer-based devices strongly depends on the efficiency of exciton formation and dynamics that are themselves strongly sensitive to polymer molecular packing and structural order. Therefore, the current challenge in achieving high efficiency is establishing a correlation between molecular packing and exciton coupling.

P3HT nanofibers represent an attractive platform for studying optical and electronic properties of exciton coupling because their nominal (highly crystalline) internal chain packing structure is known. A combination of wavelength-, time-, and polarization- resolved photoluminescence imaging on isolated nanofibers made of P3HT with varying molecular weight (from 10 to 65 kDa) has revealed a transition in dominant exciton coupling from primarily interchain (H-aggregation) for low molecular weight nanofibers, to predominantly intrachain (J- aggregation) coupling for high molecular weight nanofibers. Based on nanofiber width measurements from TEM imaging, the driving force for this transition appears to be folding of individual polymer chains within the lamellae, resulting in enhanced chain planarity and reduced torsional disorder.

In addition, it is shown that mechanically and chemically robust functionalized poly- (3-hexylthiophene) (P3HT) nanofibers can be made via chemical cross-linking. Dramatically different photophysical properties are observed depending on the choice of functionalizing moiety and cross-linking strategy. It is shown by a variety of photophysical metrics that by controlling the size of the cross-linking agent relative to alkyl side chain length, exciton coupling in P3HT nanofibers can be controlled, either by including a relatively large linker, which reduces inter- chain coupling, or alternatively using a linker of com- parable length to the lamellar spacing which minimally perturbs the aggregate structure.

KPFM measurements have revealed an interesting dependence of surface potential contrast (SPC) with P3HT nanostructure morphology. These results indicate that changes in the structural order are positively correlated with HOMO level energies in structures with effective one-dimensional excitonic coupling (high molecular weight regime), and are negatively correlated in effectively two-dimensional structures (low molecular weight regime). Moreover, it is recognized that the cross-linked aggregates show optical and electronic properties that are analogous to solvated chains because of their perturbed interchain coupling. We believe the results presented in this thesis provide new insights into polymer morphology “design rules” for optimizing OPV device performance.

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