Fabrication and characterization of P(VDF-TrFE) thin films and one dimensionally aligned core-shell structures for flexible piezoelectric devices

Abstract

Great attention has been paid to ferroelectric polymers because they are inexpensive, light, flexible, and easily processed materials. Among these polymers, poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE), has a stable ferroelectric $\beta$ -phase at room temperature that exhibits strong ferroelectric properties. For this reason, it has been applied to transducers, actuators, sensors and energy-harvesting devices. Currently, organic flexible electronics are being developed for practical applications such as computer displays and radio-frequency identification tags. However, there have been fewer investigations of P(VDF-TrFE) materials on both flexible and transparent substrates. In this study, we investigated both P(VDF-TrFE) thin films and P(VDF-TrFE) core-shell structures for flexible piezoelectric devices. In chapters 1 and 2, we focus on the effects of electrode materials which are mainly used on flexible substrates on both the ferroelectric properties and the formation of self-polarization alignment in P(VDF-TrFE) ultrathin films. In chapter 3, we introduce one-dimensionally aligned P(VDF-TrFE) core-shell structures. These P(VDF-TrFE) core-shell structures open up new possibilities for their use as flexible nanogenerators. Chapter 1. We demonstrate spin-cast ultrathin poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), films on flexible and transparent substrates. Importantly, the PEDOT:PSS electrode leads to a superior piezoresponse value $(17.14 \pm 2.37 pm V^{-1})$ and a low coercive voltage (1.76 $\pm$ 0.79 V) in 20 nm thick P(VDF-TrFE) films. The use of PEDOT:PSS creates a the coherent interface between PEDOT:PSS and P(VDF-TrFE), strong adhesion at the interface, and no interfacial interaction when compared to the use of indium-tin oxide (ITO). Chapter 2. We report self-polarization alignment without external poling in spin-coated poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), thin films on transparent and flexible substrates. Piezoresponse force microscopy (PFM) facilitates the quantitative analysis of preferentially aligned polarization in ferroelectric thin films. We found that as-received P(VDF-TrFE) thin films on transparent poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) electrodes showed stronger self-polarization alignment than those on indium-tin oxide (ITO) electrodes. The relative ratios of the aligned polarization per unit volume on PEDOT:PSS and ITO electrodes were 18.6% and 4%, respectively. Chapter 3. We report a new method that can be used to fabricate one-dimensionally aligned P(VDF-TrFE) core-shell structures using polyurethane acrylate (PUA) pillars as a template. The PUA pillar structures were easily fabricated by a stamping process, which is less complex and less expensive than the conventional etching process, which sacrifices the template. We characterized the current generation properties of the P(VDF-TrFE) core-shell structures by means of nanoindentation, which could measure both the mechanical and electrical properties. A nano-scale piezoelectric current was generated by applying force to the P(VDF-TrFE) core-shell structure without any poling process. These P(VDF-TrFE) core-shell structures can be widely used not only in (air or gas) pressure sensors but also in piezoelectric nanogenerators