Microstructure and Carrier Transport Processes in Semiconducting Polymers

Semiconducting polymer;structure-transport relationship;Materials Science and Engineering;Engineering;Materials Science and Engineering

Dong, BanTuteja, Anish ;

University of Michigan

关键词: Semiconducting polymer;    structure-transport relationship;    Materials Science and Engineering;    Engineering;    Materials Science and Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/138715/banxdong_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

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 thesis focuses on developing several experimental strategies to turn polymer morphologies and studying the impact of polymer microstructure on carrier transport. The experimental approaches for morphology manipulation in this thesis include (i) fabricating film with different thickness (ii) using the environmentally benign method supercritical carbon dioxide processing and (iii) using a novel vacuum deposition technique to deposit thin polymer films. By studying thickness dependence of morphology and carrier transport in a low bandgap polymer, we show that the out-of-plane carrier mobilities in conjugated polymer films monotonically increase with thickness in the range of 100 nm to 1 μm due to substrate-induced morphological changes as a function of film thickness. Our findings demonstrate that carrier mobility in conjugated polymers is not intrinsic properties of the materials but rather dictated by local morphology; it could vary nearly by an order of magnitude depend on the proximity to the substrate.For the second route, we show how the polymer structure near polymer/substrate interface in organic thin film transistor can be selectively manipulated using 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. This demonstrates that supercritical carbondioxide processing is an effective way to control polymer structure near the buried interface.Lastly, we introduce a novel vacuum deposition technique Matrix-Assisted Pulse Laser Evaporation (MAPLE) to fabricate conjugated polymer films. Although the structure of MAPLE-deposited samples is highly disordered, transistor devices made from MAPLE-deposited films exhibit superior in-plane transport characteristics. Motivated by this finding, we then employ an advanced X-ray characterization method called complete pole figure construction and characterize the polymer structure at different stage of deposition in order to reveal the structure of MAPLE-deposited films at the buried interface. To our surprise we discover a large population of highly-oriented crystals at the buried interface having structure strongly depending on the substrate chemistry. We also show that this polymer layer dictates transport in thin film transistor, showing the importance of characterizing the structure at the buried interface in order to evaluate transistor performance.