## 3 Results and Discussion
### Embedded Ferroelectric Polymer for the Performance Enhancement of TEGs
Here, since the PVDF-TrFE layer was triboelectrically negative compared with the PDMS layer, it was buried inside the PDMS layer to avoid the friction between the same surfaces and indirectly charge the PDMS surface positively through the polarized dipoles. We should note that both the PVDF-TrFE layers were polarized in a preferable direction. Figure 5b shows the output voltage and current density of the TEGs measured using the same friction pair with or without buried layers. The outputs measured after polarization of the PVDF-TrFE are shown next to each plot (red color). For the TEGs with the buried layers, the output voltage and current density of the TEG were clearly enhanced from 110 V to 360 V and from 15 \(\upmu\)A/cm\({}^{2}\) to 47 \(\upmu\)A/cm\({}^{2}\), respectively, after polarization of the PVDF-TrFE layer in a preferred direction. On the other hand, in the case of the TEG with single-layer friction pairs, an output voltage and current density of 100 V and 10 \(\upmu\)A/cm\({}^{2}\) were measured, even after polarization. Therefore, the polarization effect could be boosted by forming a friction pair with buried layers. Next, the output voltage and current density of the TEG were also evaluated by varying the load resistances, as shown in Figure 5c. The output voltage increased with the connected load resistance while the current density decreased. A maximum output power density of \(\sim\)3.3 mW/cm\({}^{2}\) was obtained by connecting a \(\sim\)10 M\(\Omega\) resistor (Figure 5d). Figure 5: **(a)** Schematic representation of the TEG consisting of a friction pair with buried layers. **(b)** Output voltage and current density, measured using a single-layer friction pair and a friction pair with buried layers. The red colored lines show the outputs after polarization. **(c)** Dependence of the output voltage and current density on the load resistances. **(d)** Dependence of the power density on the load resistances. **(e)** Charging test using a capacitor (1 \(\upmu\)F) for each contact friction pair. The inset is a schematic diagram of a full-wave bridge circuit connected to a capacitor or Light emitting diode (LED). **(f)** Instantaneous lighting of 20 (top) and 100 (bottom) blue LEDs during the contact–separation motions of two different contact friction pairs. To explicitly demonstrate the performance of the TEGs, a capacitor (1 uF) was charged using each TEG. It can be clearly noticed that a capacitor could be quickly charged by adopting friction pairs with a buried PDMS layer and a polarized PVDF-TrFE layer (Figure 5e). In addition, while 20 LEDs can be turned on when using the TEG with single-layered friction pairs, the TEG with multilayered (sandwich) friction pairs could instantaneously turn on at least 100 LEDs (Figure 5f). Here, we constructed the load by the series and parallel connection of 100 commercial blue LEDs, as shown in Figure 5e (inset). Since the turn-on voltage of an LED is \(\sim\)2.5 V, the total forward turn-on voltage of 100 LEDs connected as shown was \(\sim\)100 V, which was lower than the open-circuit voltage of the TEG. By connecting this load to the output of the TEG, the output current through the circuit was measured as \(\sim\)20 uA (Figure S6 in Supplementary Materials). Thus, the total resistance of 100 LEDs was \(\sim\)5 M\(\Omega\), which was slightly smaller than the impedance at the maximum power density.