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Microstructure and Carrier Transport Processes in Semiconducting Polymers

Ban Dong
When03:00 PM05:00 PMApr 13, 2017
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Where G604, Bldg. 10 NCRC
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Thesis Defense

The molecular design and synthesis of semiconducting conjugated polymers was a major achievement in the field of chemistry, enabling the now viable organic electronics industry. Unlike their crystalline inorganic counterparts, the weak van der Waals bonding forces between conjugated polymer chains give rise to significant structural and energetic disorder. This reduces charge carrier mobility and adversely impacts device performances. Efforts to understand carrier transport in conjugated polymer systems have been challenged due to the complexity of the structure and a lack of proper understanding of the manner in which the polymer morphology affects electrical properties. This talk focuses on developing several experimental strategies to manipulate polymer morphologies and studying the impact of polymer microstructure on carrier transport. The talk will be divided into 3 parts. First, I will discuss the role of substrates on the morphology and out-of-plane carrier transport in a low bandgap polymer. We show that the carrier mobilities in organic semiconductors are not intrinsic properties of the materials but rather dictated by local morphology, varying nearly by an order of magnitude depend on the proximity to the substrate. In the second part of the talk, I will show how the polymer structure near polymer/ substrate interface in organic thin film transistor can be selectively manipulated via supercritical carbon dioxide processing. Subsequently, we observe a significant enhancement in the in-plane carrier mobility that is accompanied by rather a subtle change in polymer morphology. Finally, I will discuss crystallization mechanism and carrier transport characteristics of conjugated polymer films fabricated using a novel vacuum deposition technique known as Matrix-Assisted Pulse Laser Evaporation (MAPLE). Using an advanced X-Ray characterization method, we discover that in MAPLE-deposited films, there exists a large population of highly-oriented crystals near the polymer/substrate interface. We find out that the growth of these highly-oriented crystals strongly depends on the chemistry of the underlying substrate and that these highly-oriented crystals control transport in thin-film transistors.

 

DISSERTATION COMMITTEE:

Chair: Prof. Peter Green, MSE

Members:

Prof. Jinsang Kim, MSE Assoc. Prof. Anish Tuteja, MSE

Cognate:

Assoc. Prof. Zhaohui Zhong, EECS