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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2017

Month Degree Awarded

September

First Advisor

Ryan C. Hayward

Subject Categories

Nanoscience and Nanotechnology | Polymer and Organic Materials | Semiconductor and Optical Materials

Abstract

Solution-based crystallization of conjugated polymers offers a scalable and attractive route to develop hierarchical structures for electronic devices. The introduction of well-defined nucleation sites into metastable solutions provides a way to regulate the crystallization behavior, and therefore the morphology of the material. A crystallization method for generating metastable solutions of poly(3-hexylthiophene) (P3HT) was established. These metastable solutions allow P3HT to selectively crystallize into nanofibers (NFs) on graphene-coated surfaces. It was found that the crystallization kinetics is faster with increasing P3HT molecular weight and concentration. Through in situ atomic force microscopy, it was confirmed that NFs grow vertically in a face-on chain orientation (i.e., the π orbitals parallel to the substrate normal) from highly oriented pyrolytic graphite and graphene. Moreover, the P3HT crystal structure observed on the surface of graphene was identified to be the same one formed by solution crystallization. However, as confirmed by X-ray scattering and scanning electron microscopy the crystals transitioned from face-on to edge-on orientation (i.e., the π orbitals perpendicular to the substrate normal) as the film grew thicker. As determined by X-ray scattering. the initial face-on conformation was partially preserved by embedding the P3HT structures in an indene C60 bisadduct matrix when compared to pristine P3HT films. The resulting organic field effect transistors had hole mobilities (μ = 20 x 10-3 cm2 V-1 s-1) two orders of magnitude higher than the devices fabricated from spin casted P3HT (μ = 0.9 x 10-3 cm2 V-1 s-1). The solution-processable fabrication of electrodes and semiconductors is potentially scalable and amenable to roll-to-roll manufacturing.

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