Effect of Centrifugal Force on Power Output of a Spin-Coated Poly(Vinylidene Fluoride-Trifluoroethylene)-Based Piezoelectric Nanogenerator

While poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) film is an excellent piezoelectric material for mechanical energy harvesting, the piezoelectric output varies considerably with the spin coating conditions. Herein, we reported a systematic evaluation of the structural, electrical, mechanical, and microstructural properties of spin-coated P(VDF-TrFE) films obtained at various distances from the center, as well as under different rotational speeds. With increasing distance, the remnant polarization, dielectric constant, and crystallinity of the films increased, which resulted in enhanced piezoelectric power at the largest distance. With increasing rotational speed, the remnant polarization, dielectric constant, and crystallinity of the films initially increased and then decreased, while the Young's modulus continuously increased. This resulted in an enhanced piezoelectric power at a given rotational speed. The piezoelectric power is proportional to the remnant polarization and inversely proportional to the Young's modulus. The highest (2.1 mW) and lowest (0.5 mW) instantaneous powers were obtained at the largest (1.09 mu C/cm(2)center dot GPa(-1)) and smallest (0.60 mu C/cm(2)center dot GPa(-1)) value of remnant polarization over Young's modulus, respectively. We explain these behaviors in terms of the centrifugal force-induced shear stress and grain alignment, as well as the thickness-dependent beta-phase crystallization and confinement. This work implies that the spin coating conditions of distance and rotational speed should be optimized for the enhanced power output of spin-coated P(VDF-TrFE)-based piezoelectric nanogenerators.