Studies on Charge Transport in Organic Semiconductors: Influence of Microstructure and Processing

Abstract

For decades, studies for improving charge mobility of organic field-effect transistors (OFET) have continued. Recently, OFET performance has been reported to exceed the performance of amorphous silicon and even polycrystalline silicon-based transistors. However, due to weak van der Waals bonds of organic semiconductors, organic semiconductor-based devices usually exhibit poor and non-ideal operation. In order to solve these problems, it is necessary to study the correlation between the electrical characteristics of organic semiconductors and microstructure/processing techniques, developing guidelines for high performance organic semiconductors. In Chapter I, I briefly described the history of developing organic semiconductors and organic electronics. Charge transport mechanism of organic semiconductors, the operating principle of OFET, and recent research trends of organic semiconductors and devices (including the reliability of OFET) were described. Chapter II introduces microstructure analysis methods, such as atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). And various solution-processing techniques to obtain high-quality organic semiconductor thin-films are described. Since the molecular packing properties are greatly influenced by the chemical structure and the processing conditions of organic semiconductors, in-depth study of the relationship between microstructure/processing techniques and mobility is essential to improve the performance of organic electronics. Chapter III describes the effect of side-chain position of (E)-2-(2-(thiophen-2-yl)-vinyl)thiophene (TVT) based polymers on intrinsic properties of charge transport. TVT-based organic semiconductors have been attracting great attention due to their excellent charge mobility and unique microstructure. By comparing two TVT-based polymers with tail-in and tail-out side-chain position, respectively, I have investigated the effect of side-chain regiochemistry on TVT skeleton. In Chapter IV, the in-depth studies on thiophene bipyrrolylidene-2,2′(1H,1′H)-dione (TBPD) based polymers are described. A systematic study was performed to figure out the relationship between thin-film microstructures and charge carrier mobilities of four newly synthesized TBPD-based polymers. With high boiling-point solvent, solution-shearing process and optimal thermal treatment, I could obtain maximum hole mobility of 0.46 cm2 V-1 s-1. In Chapter V, I focused on the comparative analysis of furan-containing materials with thiophene-containing materials. I applied various analyzing techniques to newly synthesized furan spacer-based polymer (PFDPPTT-Si) and a thiophene spacer-based polymer (PTDPPTT-Si). When processed with common chlorinated solvent, The PTDPPTT-Si thin-films exhibited not only higher crystallinity and tight π-spacing but higher hole mobility (3.57 cm2 V-1 s-1) compared with the PFDPPTT-Si thin-films. On the other hand, the higher mobility (1.87 cm2 V-1 s-1) was obtained from non-chlorinated solvent-processed PFDPPTT-Si thin-films. Chapter VI summarizes all the descriptions covered above and provides a brief overview of future research objectives.