The researchers spin-coated a 80-μm thick paper substrate with a co-polymer solution to yield a thin piezoelectric film of poly(vinylidene fluoride trifluoroethylene) or P(VDF-TrFE), only 1 μm thick. The deposition process was layered between two platinum electrodes deposited at a 200-nm thickness (see figure 1).
Flexing back and forth the very thin structure, the researchers were able to generate an open-circuit voltage of 1.5 V at a frequency of about 1 Hz, with a short-circuit current of 0.38 μA for a device only a few centimetres square. Under this test scenario, the output power density was evaluated to be 2.85 mW/cm 3.
The piezoelectric potential is generated in the P(VDF-TrFE) thin film from tensile stress-induced deformation, making the electrons flow and accumulate at the bottom electrode to balance the electric field induced by dipoles. Repeated bending and releasing of the device generates output voltages and current pulses as shown in figure 2.
Attached to the back of a human hand underneath a latex glove, the energy harvester was able to generate a maximum output open-circuit voltage of 0.4 V at low bending frequencies of 0.25 Hz (say closing and opening your hand once every four seconds) and that despite the lower deformation of the piezoelectric structure as it conformed to the skin. The output open-circuit voltage reached 0.6 V at a 2-Hz bending frequency.
As well as being flexible, the structure is lightweight and easier to fabricate compared with the typical ZnO-based paper device structures described in other research papers. The versatility of paper as a substrate means this new form of low-frequency energy harvesting device could be laminated into numerous wearable applications, including disposable sensors for ephemeral monitoring applications.
These films, explain the researchers, are easy to fabricate into