Dual-controlled piezoelectric composite film with enhanced crystallinity and defect-free via solvent vapor treatment

A simple, cost-effective solvent vapor annealing (SVA) with a 2-butanone atmosphere-based PVDF/BaTiO₃ NPs composite film (treated for 5 h) exhibits a crystallinity and β-phase fractions of PVDF in the film of 25.6 % and 79.8 %, respectively, which are higher compared to the traditional thermal annealing (TA)-treated composite film. Moreover, the SVA-treated composite films show better control of roughness, pores, defects/cracks than the TA-treated film. The superior properties of SVA-treated composite films were due to the re-orientation of -CH₂/-CF₂ molecular chains by stretching/rotation leads to the recrystallization of PVDF in the composite film. The SVA-treated composite flexible piezoelectric harvester (f-PEH) exhibits 2 folds higher electrical response than the TA-treated f-PEH device upon mechanical bending force. Higher piezoelectric performance was achieved when the pores are minimized and higher crystallinity of film, which are somewhat correlated with the theoretical simulations. Also tested the f-PEH response with periodic water droplets and finger bending forces suggests that f-PEH has potential to work as harvester and sensor. The proposed SVA technique may help to generate novel polymer/composite films with enhanced functional properties for wearable and skin-adaptable energy harvesters and sensor electronics.