Development of High-Performance Solution-Processable Organic Field-Effect Transistors for Sensor Applications

Abstract

Solution-processable organic field-effect transistors (OFETs) have attracted great attention as the promising alternative to silicon-based devices for future electronics, due to their many advantages including flexibility, light weight, superior processability, and low-cost fabrications. Remarkable progress has been made in the performance of OFETs based on donor-acceptor (D-A) polymers, with charge-carrier mobilities μ > 1 cm2 V-1 s-1, mainly through the rational molecular design of conjugated backbones. On the other hand, side-chain engineering has recently emerged as a powerful strategy to boost the electrical performance of polymer semiconductors. Sophisticated structural modifications of polymer side chains can not only enhance molecular ordering but derive novel functional property in the polymer semiconductors. Here, I present various side-chain engineering strategies employing new side-chains containing the thermally removable group and hybrid siloxane-solubilizing groups in order to achieve novel functionalities together with high solubility in polymer semiconductors. These polymer semiconductors show intriguing charge transport behaviors, such as inversion of dominant polarity in ambipolar FETs, unprecedentedly high ambipolar mobilities, three-dimensional (3-D) charge transport, and highly stable solvent-resistant FETs, resulting from their finely modulated side chains. For the device optimizations, a solution-shearing technique is firstly applied to study polymer FETs to maximize the electrical properties by facilitating the formation of efficient pathways for charge transport in polymer films. The solution-sheared films exhibit significantly improved electrical performance compared with the spin-coated or drop-cast films, most likely due to the highly crystalline and aligned grains in the solution-sheared films. In this thesis, the highest performance ambipolar FETs are demonstrated by a synergistic combination of rational polymer backbone design, side-chain dynamics, and solution processing. Furthermore, the high-performance FETs are further applied to sensor applications because FETs are promising sensing platform, with respect to low cost and simple processing, and continuous monitoring. The direct detection of toxic liquid-phase chemicals is suggested for the solvent-resistant FET platforms, which are highly stable upon exposure to water and toluene. Moreover, the highly sensitive near-infrared (NIR) light detection is successfully demonstrated based on a NIR absorbing high-performance polymer semiconductor. These results provide new insights and key methodologies into the molecular design of polymer semiconductors and processing architecture to expand the scope of FET applications and possibilities for their practical use.