# PVDF-TrFE Film on PDMS Substrate for Energy Harvesting Applications
## 3 Results and Discussion  
The schematic diagram of our flexible piezoelectric energy harvester is shown in Figure 2a. The device comprises four layers with the Ti/Ni as the top and bottom electrodes, the P(VDF-TrFE) piezoelectric film, and the PDMS substrate. Previously, a nanogenerator (NG) fabricated by [PERSON] et al. [19] made use of the patterned PDMS-CNT composite to form the bottom electrode, while patterned PDMS was used as micro/nano dual scale by [PERSON] et al. [21] to increase the surface roughness of the NG. [PERSON] et al. [20] also used patterned PDMS-MWCNT composite to obtain increased roughness and lower internal resistance in the device. As mentioned above, these methods included complex fabrication steps, such as chemical etching, photolithography, and plasma treatment, which add more cost and increase the complexity of fabrication. An optical image of our device, which is much simpler in construction, is shown in Figure 2b. All the materials, such as Ti/Ni for electrodes and P(VDF-TrFE) film for active area, were found to adhere well onto the PDMS substrate and withstand repeated cycles of bending and stretching. Once the composition and thickness had been optimized, we performed structural characterization using the XRD technique on the prepared P(VDF-TrFE) film, and the results are shown in Figure 3a. We found that the orientation planes (110) and (200) shown in the XRD spectra are related to the polar \(\beta\)-phase, since the diffraction peak position is shown at a 20 value of 19.7\({}^{\circ}\)[26; 27]. FTIR measurements were also carried out to further confirm the presence of \(\beta\)-phase P(VDF-TrFE). Figure 3b indicates the signature FTIR spectra of the film, with 3 peaks corresponding to the \(\beta\)-phase P(VDF-TrFE): 842 (CH\({}_{2}\) rocking), 1286 (trans band) and 1400 (CH\({}_{2}\) wagging) cm\({}^{-1}\)[18; 28]. Taken together, these tests clearly indicate that the synthesized film is indeed P(VDF-TrFE). To determine the efficacy of the layered energy harvesting structure prepared, it was exposed to external strain using a mechanical shaker, and the output voltage and power were recorded. Figure 4a shows a photographic image of the basic experimental setup for energy harvesting, which consists of an oscilloscope (Tektronix TDS 5054), a Shaker (LDS V203), and the device attached to the shaker. The output voltages were obtained from the device under application of an external force at frequencies of 5, 10, 20 and 30 Hz at 1.75 g acceleration. The force was provided by the shaker. An extended metal arm from the shaker was attached to the bottom center of the device to provide the oscillatory force (see Figure 4a). The active area of the harvesting device is estimated to be \(\sim\)4 cm\({}^{2}\) (2 \(\times\) 2 cm). Figure 3: (**a**) XRD and (**b**) FTIR result of \(\beta\)-phase of P(VDF-TrFE) film. The XRD peak at 19.7 degrees and the transmittance peaks at 842, 1286 and 1400 cm\({}^{-1}\) confirm the material is \(\beta\)-phase P(VDF-TrFE). Figure 2: (**a**) Schematic illustration; (**b**) optical microscopy image; and (**c**) photographic image of the proposed device. A COMSOL-based finite element simulation was performed to theoretically estimate the stress, strain, and voltage output from the device. From the simulation results (see Figure 4c) the peak and average stresses were found to be 8.42 \(\times\) 10\({}^{6}\) and 2.45 \(\times\) 10\({}^{6}\) N/m\({}^{2}\), respectively, for the P(VDF-TrFE) films over the active area. The output voltage calculated from the relation V = g\({}_{33}\)\(\times\)\(\sigma_{3}\)t (g\({}_{33}\): piezoelectric voltage constant, \(\sigma_{3}\): applied mechanical stress, and t: thickness of film) yields the peak and average voltages of 20 and 5.8 V, respectively.