Disorder, defects, and their effect on charge transport in organic semiconductors
- Understanding the nature of charge transport and its limitations on device performance has guided the rational design of organic semiconductors for over a decade. This understanding has led to the development of high performance small molecule and polymeric semiconductors for applications in low-cost complementary logic circuits, displays, lighting, and solar cells. Unfortunately, the large catalog of available materials and processing variables make it difficult to outline general design rules. Specifically, in polycrystalline and semicrystalline organic semiconductors, little is understood about the structural order within a grain and between grains, and even less is known about how this structure affects the material's performance. In this thesis, characterization methods are introduced to address these issues. To understand the structural source of electronic defects, a multi-step Fourier transform X-ray line shape analysis with thorough error analysis is adapted to determine crystallite size and intra-grain cumulative disorder. For the first time, the intra-crystalline disorder in transport-relevant directions is quantitatively determined for a variety of high performing organic semiconductors. These van der Waals bonded crystallites exhibit a wide range of disorder (from < 1% to 10% fluctuations from the mean lattice spacing) depending on the packing geometry and chemistry. The degree of lattice disorder is found to be a useful tool for ranking materials quantitatively on a continuous scale, from perfectly ordered to completely disordered. High levels of lattice disorder, evident in many polymeric semiconductors are found to introduce a tail of localized trap states that limit charge transport. At the low-disorder end of the scale, transport-limiting disorder is not significant and grain boundaries are found to limit charge transport. Finally, we suggest that, from the charge transport perspective, the transition from small molecule/oligomer- to polymer-like behavior may be demarcated by the onset of strong, transport-limiting lattice disorder. Thus, understanding the effect of disorder on transport promises to further aid in determining the structural origins of charge trapping, guiding future materials design.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Stanford University, Department of Materials Science and Engineering
|Toney, Michael Folsom
|Toney, Michael Folsom
|Statement of responsibility
|Submitted to the Department of Materials Science and Engineering.
|Thesis (Ph. D.)--Stanford University, 2011.
- © 2011 by Jonathan Rivnay
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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