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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2015

Month Degree Awarded

May

First Advisor

Prof. Alfred J. Crosby

Second Advisor

Prof. Alejandro L. Briseno

Third Advisor

Prof. Ashwin Ramasubramaniam

Subject Categories

Polymer and Organic Materials | Semiconductor and Optical Materials

Abstract

The study of the physical properties of organic crystalline semiconductors has allowed the advent of a new generation of high-performance organic electronic devices. Exceptional charge-transport properties and recent developments in large-area patterning techniques make organic single crystals (OSCs) excellent candidates for their utilization in the next-generation of electronic technologies, including flexible and conformable organic thin-film devices. In spite of the profound knowledge of the structural and electrical properties of OSCs, knowledge of the mechanical properties and the effects of mechanical strain is almost non-existent. This dissertation aims to bring new understanding of the intrinsic mechanical properties and the effect of mechanical strains in charge transport phenomena in organic semiconductors.

The wrinkling instability is chosen as the metrology tool for the effective in-plane elastic constants of OSCs. We demonstrate that the wrinkling instability can be used to obtain the elastic constants of single crystals of rubrene, tetracene, PDIF-CN$_2$ (N,N'-1H,1H-perfluorobutyldicyanoperylene-carboxydi-imide) and perylene. We demonstrate that wrinkling is a practical method to map the in-plane mechanical anisotropy in OSCs. In addition, we utilize wrinkling to characterize how the elastic modulus of pBTTT (poly(2,5-bis(3-alkylthiophen- 2-yl)thieno[3,2-b]thiophene)) changes with increasing molecular weight, from the monomer to the pentamer and the high molecular weight polymer.

To elucidate the effects of mechanical strain on charge transport, we first demonstrate and quantify the existence of a piezoresistive effect in rubrene crystals by the application of bending strains along its b [010] axis. A piezoresistive coefficient of approximately 11.26 is determined and confirmed through density functional theory (DFT) calculations. Second, we take advantage of wrinkling as a unique way to strain the conducting channel of field-effect transistors in a non-destructive, reversible, and predictable manner. We observe field-effect mobility modulation upon wrinkling and establish that it is controlled by the strain experienced by the insulator-semiconductor interface upon deformation. Finally, we propose a model based on plate bending to quantify the net strain at the insulator-semiconductor interface and predict the change in mobility. These contributions are the first to quantitatively correlate the crystal structure and the mechanical properties of OSCs, as well as the first to study electro-mechanical behavior in OSCs.

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