The past, present, and future of piezoelectric fluoropolymers: Towards efficient and robust wearable nanogenerators

Polyvinylidene difluoride (PVDF) derivatives in metal/PVDF/metal (MPM) sandwich structures have been studied extensively since 1969. Cousin copolymers of the same family have been discovered with fascinating piezoelectric, pyroelectric, electrocaloric, and ferroelectric properties. Solution processing, flexibility, lightweight, and thermal stability make this class of materials complementary to inorganics. Thus, PVDF based polymers potentially compete with inorganic materials for a broad range of technologies such as energy generators, loudspeakers, coolers, and memories. However, the stable non-electroactive alpha-phase and hydrophobic nature of PVDF are the main barriers for developoing high performing and robust MPM devices in electronic applications. In this review, we present an up-to-date overview on different methods to induce the electroactive beta-phase and improve the adhesion strength with metals to ensure robust and durable MPM devices. We go through advantages and disadvantages of several methods and pinpoint future opportunities in this research area. A special attention is paid to wearable piezoelectric nanogenerators for energy harvesting from human body motion, where flexible PVDF derivatives are compared with rigid piezoelectric ceramics. While the piezoelectric coefficient of PVDF (d(33) similar to 24-34 pm/V) is one order lower than ceramic materials, novel copolymers of PVDF display d(33) > 1000 pm/V upon bias. This shows promise to bring piezoelectrics to flexible and large-area applications such as smart textiles. We also discussed challenges to improve wearability, such as light weight, breathability, and flexibility.