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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2016

Month Degree Awarded

February

First Advisor

Professor Todd Emrick

Second Advisor

Professor Thomas P. Russell

Subject Categories

Polymer and Organic Materials | Semiconductor and Optical Materials | Structural Materials

Abstract

This thesis describes the solution behavior and interfacial properties of electronically active polymers. The performance of such polymers in devices is often determined by their chain conformation and morphology in solution and in thin films. For example, the intricate balance between polymer domain size and crystalline packing of electron donor and acceptor components, as well as the properties at the polymer-metal interface, are crucial for achieving optimal performance in devices, such as solar cells.

Chapter 1 presents the current progress in polymer-based solar cells, their fundamental principles, and key factors to improve their efficiency. Literature precedents on the development of materials for the active layer and electrode modifiers are also described in detail.

Chapter 2 centers on the solution-driven assembly of a low band gap polymer (PCDTBT) in a marginal solvent to give semicrystalline nanofibers. In contrast to poly(3-alkylthiophene) nanowires prepared by similar techniques, these truncated nanostructures showed undulated features along the fiber axis. Such morphology suggested the nanofibers were formed from packing of smaller crystalline units, giving valuable insight into the ordering of conjugated polymers in solution-processed thin films.

Chapter 3 highlights zwitterionic polymers bearing pendent azulene groups with unique optoelectronic properties. The orthogonal solubility of these polar copolymers is enabling for multilayer device fabrication, and proving useful for improved charge collection efficiency, affording high performance solar cells.

Chapter 4 describes sulfobetaine (SB) and phosphorylcholine (PC) functionalized zwitterionic poly(acetylene)s (ZIPAs). SB ZIPA proved amenable to nanofiber formation in solution upon addition of a non-solvent, while PC ZIPA remained well-solvated under similar conditions. Both of these polymers significantly reduced the work function of silver, rendering ZIPAs as promising cathode modifiers. Upon incorporating into polymer-based solar cells, the power conversion efficiency significantly increased from 2.5 % to 9.2%.

Lastly, chapter 5 summarizes the thesis and presents a perspective for utilizing interlayer materials to enhance the stability and lifetime of future solar cells. A recent work on employing zwitterionic nanoparticles as interlayer materials is discussed with preliminary results presented in the appendix.

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