# PVDF-TrFE Film on PDMS Substrate for Energy Harvesting Applications
## 3 Results and Discussion
Moreover, the peak and average surface charge densities of \(\sim\)355 and 105 uC/m\({}^{2}\) for 20 and 5.8 V, respectively, were obtained using the equation V = ot/\(\varepsilon\) (V: voltage, \(\sigma\): surface charge density, t: thickness, and \(\varepsilon\): dielectric constant). The calculated values of the voltages are well matched with the experimental data (see Figure 5), as well as simulation data for potential (see Figure 4d). The simulation figures show positive charge (and voltage) developing on the P(VDF-TrFE) positioned on top of the PDMS substrate, with negative charge and voltage at the interface of P(VDF-TrFE) and PDMS. The charges on the top surface of the P(VDF-TrFE) meet air, while the bottom surface has PDMS, so although the charge densities are the same, the voltage magnitudes generated are different. Figure 5a-d shows the frequency-dependent output voltage and current of the device under an external force at the various frequencies. The oscilloscope and low-noise current preamplifier were used to measure the electrical signal from the device. It is evident that the average voltage and current (solid parts of peaks in each figure) gradually increase with the increase of frequency, while the maximum upper peak voltage remains almost same at around 20 V. The increase in average voltage and current is attributed to the increase in the average surface charge as the bending frequency is increased. This occurs due to the slower discharge rate of the device as compared to the input pulse rate, resulting in a continuous rise in the output voltage and current as frequency is increased [29; 30]. Figure 4: (**a**) Photographic image of the basic experimental setup and (**b**) image of the device under test; (**c**) Finite element (COMSOL) simulation of strain and stress distribution and (**d**) piezoelectric output voltages and surface charge densities over the active area (2 \(\times\) 2 cm\({}^{2}\)). At 30 Hz, the peak value of average voltage and current is 5.8 V and 3.2 \(\upmu\)A (see Table 2), while the maximum upper peak value of voltage and current is 20 V and 6.5 \(\upmu\)A. These voltages are well matched with the calculated output potential, as mentioned above. Additionally, the value of average voltage of 5.8 V matches the final output voltage available across the full wave bridge rectifier circuit of 4.4 V (steady-state voltage value on the capacitor), considering a total drop across two diodes of \(\sim\)1.4 V. This result is further discussed below. Figure 5: (a–d) the frequency-dependent (5, 10, 20 and 30 Hz, respectively) output (i) voltage and (ii) current of the device under application of an external force. The aforementioned energy harvesting performance is quite impressive, compared to the previously reported performance of flexible energy harvesesters on PDMS substrate. [PERSON] et al. [22] developed PDMS/PET-based PVDF (5 \(\upmu\)m thick) piezoelectric NG with an output voltage of 150 mV. [PERSON] et al. [20] fabricated patterned PDMS-MWCNT composite-based P(VDF-TrFE) (85 \(\upmu\)m thick) hybrid NG, and piezoelectric output voltage was 2.5 V. Micropatterned (pyramid-shaped) P(VDF-TrFE) piezoelectric-pyroelectric NG (7 \(\upmu\)m thick) on the CNT/PDMS composite substrate demonstrated by [PERSON] et al. [19] led to an output voltage of only 1.4 V. By comparison, our device provides a significantly better performance, while requiring a much simpler fabrication process. Table 1 compares the output performance of the current device with various PVDF and P(VDF-TrFE) based NG reported earlier [19; 20; 21; 22; 23; 24]. The output voltage transient across the output capacitors with different capacitance values of 4.7 \(\upmu\)F, 10 \(\upmu\)F, and 47 \(\upmu\)F, under an external force with 30 Hz frequency, is shown in Figure 6a.