Study on film formation mechanism of conjugated polymers via in situ analysis for flexible and printable organic electronic devices

Abstract

Flexible and printable organic electronics based on conjugated polymers have researched to realize next-generation electronics due to their solution processability and biocompatibility. In order to improve the performance of organic electronic devices, not only developing new strategies in the aspect of the materials and device engineering but understanding the fundamental characteristics of organic semiconductors are strongly required. In this regard, this thesis investigates the optimization of film formation process of conjugated polymers via developing a new method regulating the crystallization rate and in-depth understanding of the film growth mechanism using in situ experiments. Chapter 1 provides a general introduction about flexible and printed electronics including both materials and devices, fundamentals of Organic field-effect transistors (OFETs), and in situ analysis during film formation of organic semiconductors. In Chapter 2, a new strategy, kinetically controlled crystallization (KCC) process, was proposed to optimize film morphology of conjugated polymers using the control of the solidification rate in the metastable state of wet films. KCC process resulted in dramatic morphological changes in large polymer domains, elongated molecular packing structures, and molecular orientation preferable for charge transport in OFETs. These film morphological variations were confirmed with various morphological analysis techniques and the film growth process was theoretically illustrated. Structural changes from nano to macro scale significantly improved electrical properties including electron mobilities with high reliability, small contact resistance for electron injection, low activation energy, and narrow density of states distribution for electron transport. This study can provide important information in the fundamentals of the crystallization of conjugated polymers that can be broadly applied to optimize the film morphology for high-performance organic electronic devices. In Chapter 3, the film formation mechanism of the conjugated polymer during blade coating was investigated through in situ analysis. By combining in situ ultraviolet-visible spectroscopy and in situ grazing-incidence wide-angle X-ray scattering analysis, the evolution of polymer aggregates and crystallites, which plays a critical role in the formation of the charge transporting pathway, was monitored in real-time. I identified five distinct stages during the blade-coating process and the five stages are described in detail with a proposed illustration of film growth. This work can lead to the development of improved methods for controlling the film microstructure as well as a greater understanding of the relationship between the film structure and their charge-transport properties Finally, Chapter 4 represents the overall summary and conclusions of this thesis.