Synthesis of Organic Semiconducting Molecules and Polymers with Various Steric Differences and Study on the Effects of their Aggregation Types on Organic Transistors and Photo-Transistors

Abstract

Organic thin film transistors (OTFTs) based on conjugated polymers are gaining attention due to their potential in solution processability and flexibility on the plastic substrate. To be applied to ubiquitous plastic electronics, a structural insight of organic material such as various steric differences or their intermolecular aggregation features are a key aspect to achieving high charge carrier mobility and photo-detection. In Chapter 1, to investigate the relationship between the aggregation type and charge-transporting properties, we synthesized two copolymers of different steric types: one with mono-TPD and another with bis-TPD as the accepting unit. While the planarity and energy levels are not significantly different from the mono-TPD unit, the aggregation state is quite different and X-aggregation tendency seems to be stronger where the bis-TPD unit was incorporated. The overall characterization of the polymers was performed and it was found that the type of aggregation and orientation of the polymers in the final films, such as H-, J-, X- aggregation, was indeed important for high charge carrier mobility of the polymer. In Chapter 2, the organic photosensitizer is considered as an effective process strategy that can significantly improve the optoelectrical performance of phototransistors. However, it is difficult to achieve broadband photodetection with high field-effect mobility in a device due to the limited absorption and solution-processability of photoactive materials. In this chapter, the photosensitive small molecule was designed with an unsymmetrical molecular structure features and high solubility that enable the formation of bi-layered photosensitizer/semiconductor thin films by a simple solution process. The photodetection in broadband spectral range, determined by the organic photosensitizer, is realized in phototransistors with various semiconductors. Furthermore, p-type charge transport behavior of the organic semiconductor is enhanced through a surface doping effect of the synthesized photosensitizer, caused by the unique molecular configuration including intrinsic charge trapping core. Such structural properties for organic photosensitizers can lead to the advancement of phototransistors with new optoelectronic properties and functionalities for high-performance broadband photosensing system.