Study on side chain engineering for controlling charge transport and chirality of organic semiconductors

Abstract

Over the past few decades, researchers have sought to enhance the charge mobility of organic field-effect transistors (OFETs). Despite recent reports of OFET performance exceeding that of amorphous silicon and polycrystalline silicon-based transistors, devices based on organic semiconductors often exhibit poor and non-ideal operation due to the weak van der Waals bonds of these materials. To address these issues, it is necessary to investigate the relationship between the electrical characteristics of organic semiconductors and their microstructure and processing techniques, and to develop guidelines for producing high-performance organic semiconductors. Conjugated polymers are a type of plastic that can absorb/emit light and conduct electrical currents, and have been utilized in various organic electronics, including organic photovoltaics (OPVs), field-effect transistors (FETs), light-emitting diodes, and electrochromic devices, among others. The advent of flexible and printed electronics has led to increased interest in conjugated polymers, and as a result, the number of synthesized conjugated polymers has exponentially increased in recent years. These materials have demonstrated improved performance, and commercial applications appear to be on the horizon. These advancements in performance have been achieved through a better understanding of materials design, processing, and device fabrication. When designing conjugated polymers, the selection of side chains is as important as the selection of the conjugated backbones. Solution-processable conjugated polymers typically consist of two parts: π-conjugated backbones and peripheral flexible solubilizing side chains. The optoelectronic properties of the resulting polymers are determined by the π-conjugated backbones, which have been the focus of most research efforts. However, the side chains have not been fully exploited, despite numerous side chain substituents being tested over the years. In this doctoral thesis, I will emphasize the significance of this strategy. In Chapter 1, a brief overview of the research background and the purpose of this paper is provided, including fundamental information on side chain engineering, organic electronics, charge transport, and supramolecular chirality. In Chapter 2, DPP-based small molecules and polymers were synthesized and applied in OFET devices. In DPP small molecules, siloxane side groups showed the best device performances and phosphonate-end side groups exerted a negative influence on charge-transport properties. In DPP conjugated polymers, various comonomers were introduced and two consecutive thiophene comonomers exhibited the best charge transport due to a combination of edge-on packing, fibrillar intercalating networks, and large crystalline π-stacking. In Chapter 3, a new series of ultralow-bandgap DAP-based donor-acceptor type (D-A) copolymers with chalcogenophene counterparts were synthesized. The copolymers showed unipolar p-channel operation and selenophene (Se) counterpart showed the best hole mobility up to 4.79 × 10−1 cm2 V−1 s−1 as a result of 3D charge-conduction channel with high crystallinity. It also had the highest near infra-red (NIR) photoresponsive properties, showing that DAP-based copolymers are highly promising for use in NIR sensors. In Chapter 4, a set of PDI-based monomers were synthesized and self-assembled into one-dimensional nanowires. I changed the spacer length from chiral point, the type of the chiral substituents, and symmetry (whether only one side has the chiral pendant or both sides have the chiral pendants) to explore the structure-chirality relationship. Through the circular dichroism spectrum analysis of the prepared wires, I could find out that the spacer length was significant in determining the stacking manner of the molecules, which was stacked in a helical wire with amplified chirality. In addition, molecular chirality was doubled when the conjugation length got longer, but in supramolecular chirality, where molecular stacking is an important determinant, it showed no proportion to the conjugation length. Lastly, alkyl chain substituted on one side of the PDI core helped the better stacking of the molecules via additional space, leading to high electron mobility when fabricated as OFET devices.