The researchers spin-coated a 80μm thick paper substrate with a co-polymer solution to yield a thin piezoelectric film of poly(vinylidene fluoride trifluoroethylene) or P(VDF-TrFE), only 1μm thin.
The deposition process was layered between two platinum electrodes deposited at a 200nm thickness (see figure 1). Key to obtaining the right piezoelectric film density and efficiency was thermal annealing, carried out at 150ºC with a specific cooling profile.
Fig. 1: The flexible P(VDF-TrFE) thin film-based paper power generators in (a) bending and (b) releasing modes. Power is generated both during the bending (c) and the releasing processes.
Flexing back and forth the very thin structure, the researchers were able to generate an open-circuit voltage of 1.5V at a frequency of about 1Hz, with a short-circuit current of 0.38μA for a device only a few centimetres square. Under this test scenario, the output power density was evaluated to be 2.85mW/cm3.
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 0.4V 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 0.6V at a 2Hz 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 array devices without any process temperature limitation up to 250°C. What’s more, a passivation layer of biocompatible parylene-c polymer can be deposited through CVD to protect the final device from moisture contamination, making it even suitable for implantable medical devices.
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