VDF-content-guided selection of piezoelectric P(VDF-TrFE) films in sensing and energy harvesting applications

Abstract Worldwide energy crisis and environmental pollution issues are driving people to seek for clean energy harvesting technologies as alternative and/or complementary energy sources for traditional fossil fuel, in which piezoelectric nanogenerators are extensively studied for mechanical energy harvesting due to their high energy density, simple architectures and excellent scaling-down capacity. Poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)) are mostly studied piezoelectric polymers. Recent work indicated that the largest longitudinal coefficient occurred at the morphotropic phase boundary of P(VDF-TrFE) with 50 mol.% VDF content (CVDF), while P(VDF-TrFE) with higher CVDF was mostly studied. Thus it is indispensable to further understand the influence of CVDF on piezoelectric performance in order to optimize the design of P(VDF-TrFE) based sensors and energy harvesters in various practical applications. Here we made a systematic investigation of the dependence of longitudinal and transverse piezoelectric properties on CVDF in commercially available 50/50, 55/45 and 70/30 copolymers. Experimental results indicated that 50/50 copolymer presented the largest longitudinal piezoelectric property and thus provided the best electric response for tactile sensing while 70/30 one showed the most outstanding transverse piezoelectricity and applied for monitoring the bending of fingers. In energy harvesting experiments at maximum strain of 1.2% and frequency of 0.6 Hz, 70/30 copolymer showed the largest power density of 4.96 nW/mm2, while 55/45 copolymer had the highest energy conversion efficiency of 1.03%. Device performance was expected to be further improved by controlling film quality and microstructures.