Enhanced Piezoelectric Performance and Stability via Solvent Vapor of Organic/Inorganic Piezoelectric Composites

Flexible piezoelectric energy harvesters (f-PEHs) attached to curved surfaces, which can harvest the electrical energy from not only tiny mechanical deformations but also biomechanical energies in anywhere and anytime, are promising candidates as the permanent power sources for self-powered biomedical monitoring system, internet of things sensors, and wearable devices. Among the many kinds of the f-PEHs, piezoelectric nanocomposites-based f-PEHs fabricated by simple spin-coating of the perovskite structured inorganic particles-dispersed organic polymers has the advantages of not only the high performance with excellent mechanical stability but also a cost-effective and scalable fabrication processes. Recently, ferroelectric polymers such as poly(vinyli-dene fluoride) (PVDF) and poly(vinylidene fluoride-trifluoroethylene) showing naturally flexible and piezoelectric features were also adopted to as a matrices for creating organic-inorganic piezo-composites. Although the organic-inorganic hybrid type f-PEH showed superior mechanical stability and higher output performance compared to previously reported energy devices, challenges remain in enhancing the crystallinity and controlling morphology of the polymer matrix for achieving highly efficient energy generators.<br/>Solvent vapor annealing (SVA) process technology has been used to realize the high output performance and physical/chemical stability of films in organic semiconductors, solar cells, and other fields. To improve the crystallinity and alleviate the internal pores and voids, the as-prepared composite films were exposed to vapor of organic solvents (such as chloroform, tetrahydrofuran, N, N- Dimethylformamide, and chlorobenzene) in a vacuum environment. The solvent vapors can penetrate both the top surface of composites and inside of the polymer matrix, promoting recrystallization of the polymer chain; this behavior led to the improvement of the crystallinity and morphology of active composites.<br/>In this study, we have reported improved output performance of f-PEHs by enhancing the crystallinity and morphology of the piezoelectric layer of organic/inorganic composite-based f-PEHs in a simple and cost-effective manner through adopting the SVA process. We observed changes in the crystallinity and morphology of the piezoelectric layer in composite-based f-PEHs, which are composed of a PVDF matrix with dispersed BaTiO₃ NPs, according to the SVA process, in the piezoelectric devices fabricated based on the aforementioned process. Our study proposes the superiority of the SVA process by comparing it with the TA process that has been previously studied. Furthermore, we predicted the performance of the piezoelectric devices derived from films with enhanced crystallinity and stability using multiphysics simulation.