Emerging wearable and implantable biomedical energy harvesters require efficient power conversion from flexible, biocompatible, lightweight and tailorable piezoelectric materials. Many piezoelectric materials, e.g. ZnO, barium titanate (BTO, BaTiO3), Lead zirconate titanate (PZT, Pb[ZrxTi1-x]O3), NaNbO3, (1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT), etc., have been explored as energy harvesters. Unfortunately, most of them are bulky, rigid, toxic, difficult to fabricate and expensive. Polyvinylidene difluoride (PVDF) and its copolymers PVDF-TrFE eliminate these disadvantages and provide a much better choice because it is biocompatible, cost-effective and intrinsically super flexible. However, PVDF-TrFE owns lower piezoelectric charge coefficients d31, and d33. Thus, it would be very prominent to enhance larger power output provided with same volumes. Till now, various methods have been explored and achieved to enhance piezoelectric charge coefficients of PVDF-TrFE polymer, e.g. β-phase enhancement by annealing and mixing with nanoparticles, sponge-like structures by etching ZnO nanoparticles, etc., however, their electrical performance are still not satisfactory. In addition, although some porous structures have been studied, there’re still missing literature reported on pore-size control and optimization in piezoelectric power output and also no complete understanding in corresponding size-controlled mesoporous structures for electrical output power enhancement.
Herein, we proposed a facile method to increase open-circuit voltage and short-circuit current output of PVDF-TrFE energy harvesters through optimizing mesoporous structures in PVDF-TrFE body. The mesoporous PVDF-TrFE films were easily fabricated using close “Breath Figure” method when the solvents were gradually dissolved by condensation water molecules and evaporated away with relative humidity (RH) under careful control and finally mesoporous matrix were left behind. It is found that the mesoporous diameters range from 1 µm to 4 µm at the condition of ambient RH controlled from 11.3% to 97.3%. Compared to solid PVDF-TrFE thin film devices, the open-circuit voltage increases about 22.3 times Vpp values, and output power after rectifying circuit enlarges about 15.5 times. It is worth noting that Voc attained highest value of 20.1V at RH of 51.4% while Isc arrived lowest of 857.4 µA. The output power measured at 10Hz achieves 0.53mW/cm2 at RH of 51.4%. COMSOL simulation confirmed the theory on piezoelectric charge coefficient enhancement induced by mechanical stress enhancement. Time-domain and frequency-domain simulations are also achieved with simulation to verify the experimental results. All these unique properties of RH controlled mesoporous PVDF-TrFE thin films open the possibility of substituting battery for various wearable personal electronic devices and final biological energy sources for self-powered various implantable biomedical electronic devices.