Semiconducting Polymer Design and Interface Engineering for Efficient Charge Transport

Abstract

A major challenge to achieve macroscopic conjugated polymer (CP) alignment leading to efficient charge transport stems from the intrinsic disordered and entangled nature of CP chains. Liquid crystalline (LC) CP design principles for directed alignment and their application to build a better understanding of charge transport in plastic electronics are discussed in this dissertation. First, molecular design parameters affecting the CP alignment are thoroughly investigated. The identified parameters, correlating with alignment characteristics via LC properties, are (a) the planarity of polymer chains; (b) intramolecular interaction moieties for induced chain planarity; (c) the effective bulkiness of side chains; and (d) surface energy of CPs. Second, cleavable side chains were introduced to the LC CPs as another design factor to achieve solvent-resistant highly aligned polymer films. The alignment behavior of the resulting new LC CP was examined in detail by adopting the floating film transfer method (FTM). In this method, an optimum amount of a high boiling point solvent was found to be vital to provide enough time for CPs to align. A high mobility anisotropy of ~14 was obtained through well-aligned CPs under the optimized condition. The subsequent side chain removal led to the formation of solvent-resistant highly aligned CP films. Overall, the outcomes provide insights into the realization of anisotropic properties of CPs in the solid thin films and offer an opportunity to enable a wide range of applications in organic electronics. FTM was employed as an interface engineering tool to investigate the charge transport of CPs in organic field effect transistors (OFETs). Although the transistor performance has been known to be critically affected by the polymer film-dielectric interface, it has been very difficult to isolate the contribution of CP alignment near the interface from that of the bulk film to the device performance. FTM has the capability of modulating CP alignment directions discretely in multilayered films, providing an opportunity to solve the daunting task. The resulting CP films prepared by FTM consisted of a bottom layer close to the polymer-dielectric interface and a top layer in contact with the source-drain electrodes. When the bottom layer had a parallel CP orientation and the CPs in the top layer were oriented perpendicular to the source-drain direction, the average hole mobility was larger by a factor of 3.3 than that of the opposite case. Moreover, OFET devices with combinations of the various bottom and top layer CP orientation directions revealed that the CP orientation direction of the bottom layer governed the overall device performance with a much smaller contribution from that of the top layer. These findings support that the CP alignment near the polymer-dielectric interface is a decisive factor for the charge transport in OFETs. Possible device performance enhancement through interface engineering is also demonstrated by investigating how the work function of electrodes can be modulated. Changes in the work function were demonstrated by means of electrophoretic deposition of ionic polyelectrolytes, electrospraying of a neutral polymer, and even after inserting an insulating spacer layer between a work function modifying layer and an electrode. The results consistently show that the work function can be controlled via a combination of the surface interaction and the charge-based through-space interaction, which can lead to the precise work function modification of electrodes for effective charge injection and extraction in organic electronics.