Controlling morphology of conjugated polymers and the impact on charge transport in flexible organic field-effect transistors

Abstract

The discovery of organic semiconductors has opened new possibilities for the development of large-scale, lightweight and flexible electronic devices that can be fabricated under ambient conditions at low cost. Although significant progress has been achieved, organic semiconductors typically exhibit low charge carrier mobilities, poor environmental stability and short operational lifetime when compared to their inorganic counterparts. Thus, the flexible, stretchable and wearable organic electronic devices still persist at an early stage of development, and many key challenges remain to be addressed. Herein, this thesis focuses on fundamental investigations of the correlations between morphology and charge transport properties into every aspect of a model conjugated polymer, poly(3-hexylthiophene) (P3HT), from intra/inter-molecular interactions to micro/macro-scale thin film structure in order to explore the potential of this conjugated polymer for field-effect transistor applications, continuous roll-to-roll printing device production and flexible device fabrication. First, an improved organic field-effect transistor is demonstrated through a polymeroligomer semiconductor blend approach. The incorporation of an air-stable oligomer into oxygen sensitive P3HT effectively reduced the oxidative doping effect even when the device fabrication was carried out under ambient conditions, resulting in a substantial decrease in threshold voltage and off-current. Additionally, the charges trapped in grain boundaries were minimized since the isolated oligomer crystalline domains were interconnected by long polymer chains. This facile blend approach provides an alternative avenue to combine the advantageous properties of conjugated polymers and oligomers for OFET fabrication, which required no pre- and/or post treatments. In addition to controlling the morphology of the semicrystalline polymer-oligomer thin film structure, the impact of P3HT crystal orientation on charge transport was systematically investigated. Long-range ordering and highly aligned P3HT thin films can be obtained by preprocessing the polymer solution with ultraviolet irradiation/solution aging and then depositing via the blade coating method, which is compatible with roll-to-roll printing processes. The surface morphologies and optical anisotropy of deposited films revealed that the degree of chain alignment was greatly improved with increased levels of polymer assembly that can be precisely controlled by solution-aging time. This highly anisotropic crystalline thin film structure provides a useful platform to explore the intra- and intermolecular charge transport properties of P3HT in OFETs. Finally, we take a further step to develop a fully-flexible OFET device on a plastic substrate by using ultraviolet irradiated regioregular and regiorandom P3HT blend thin films as the device active layer. The transistors not only provide prominent charge carrier mobility but also exhibit impressive mechanical flexibility upon application of high external bending strain. Hence, the insights gained here are expected to facilitate the identification of processing parameters and materials design that will help future breakthroughs in organic electronic device performance.