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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

Spring 2014

First Advisor

Kenneth Carter

Subject Categories

Polymer Chemistry

Abstract

VISUALIZAING AND CONTROLLING CHARGE TRANSPORT IN CONJUGATED POLYMER NETWORKS AND FILMS

MAY 2014

ANDREW R. DAVIS,

B.S., UNIVERSITY OF VIRGINIA

M.S., UNIVERSITY OF MASSACHUSETTS AMHERST

Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST

Directed by: Professor Kenneth R. Carter

The desire for more commercially feasible flexible electronic plastics has led to the development of increasingly complex conjugated polymer architectures and device geometries. Through these efforts, tremendous advances have been made in the design and performance of electronic devices fabricated with solution-processable semiconducting polymers. However, none of these materials have yet reached commercial maturity, so the opportunity for their further exploration from both a fundamental science and an application-driven point of view motivates this dissertation.

Chapter 1 presents a background introduction to many of the concepts, ideas, and existing research necessary to set the context of this dissertation’s work. The first component of this work (Chapters 2-6) investigates thiol-ene cross-linked conjugated polymer networks. By installing vinyl functionalities in poly(fluorene) molecules at chain ends (Chapters 2 and 3) and along the polymer backbone (Chapters 4 and 5), the polymers can be rapidly and efficiently cross-linked by photo-reaction with thiol cross-linkers into highly tunable semiconducting polymer networks. It is shown that the thiol-ene cross-linking reaction allows for a new molecular handle on modifying interchain electronic communication via network density, which is visualized using characteristic and unambiguous photoemission from low-energy fluorenone species. Light emitting diodes fabricated using these networks as an emissive layer show enhanced color stability compared to as-spun counterparts, and the robustness of the networks allows for solution processing of multiple stratified emissive layers for controlling color emission. Furthermore, the highly efficient thiol-ene cross-linking reaction is shown to work as an effective means for grafting poly(fluorene)s onto functionalized surfaces.

This work’s second component in Chapter 7 details the direct visualization of charge carrier density in polymeric thin film transistors using Modulation-Amplified Reflectance Spectroscopy (MARS) measurements. Owing to the unique changes in optical behavior following the formation of a charged state within the conjugated polymer film, optical spectroscopy coupled to a CCD camera offers a powerful visualization tool for observing and mapping charges across large areas as they interact with electrodes, defects, and film morphologies in active devices. The MARS technique illuminates the spatial distribution of carriers in electronic polymer films, allowing direct spatial visualization of charge density and film defects.

Finally, a brief concluding comment on this work and an outlook for the field in general is presented in Chapter 8.

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