## 3 Results and Discussion
### Embedded Ferroelectric Polymer for the Performance Enhancement of TEGs  
Figure 4a,b summarizes the peak-to-peak output voltage and current density measured from each TEG having different buried layers before or after the polarization process. Before polarization, the highest outputs were obtained from the TEG with PVDF-TrFE embedded in the PDMS layer. Even after polarization, the highest outputs were also observed from the same TEG structure. For all the TEGs with the buried layer, the output performance was further enhanced, regardless of the polarization direction, which was partially due to a permittivity increase after polarization. Using the results shown in Figure 4, both the triboelectrically positive and negative layers could be determined. As a negative contact friction layer for the TEG, either the PVDF-TrFE single layer or the PVDF-TrFE layer with buried PDMS layer could be used (Figure 1b (i,ii)). Here, the highest output was observed from the device with the negatively polarized PVDF-TrFE layer with buried PDMS. Negative polarization of the PVDF layer rendered the surface potential more negatively, and the induced surface charges were partially stored at the interface between the PVDF-TrFE layer and the PDMS layer. Next, a more obvious increase was achieved for the positive contact friction layer (Figure 1b (iv)). By positively polarizing the PVDF-TrFE layer embedded in PDMS, the output voltage was increased from 140 V to 425 V, which was approximately a threefold enhancement. Consistent results were also observed from the charge density measurement (Figure 4c). As shown in Figure 4d, by positively polarizing the PVDF-TrFE layer sandwiched between the PDMS layers, the PDMS friction surface could also be rendered more positively; thus, triboelectrically induced positive charges on the PDMS surface were screened less at the surface, contributing to increasing the surface potential and the outputs of the TEG. On the other hand, when the PVDF-TrFE layer was negatively polarized, the PDMS friction surface was terminated with negative charge centers; thus, triboelectrically induced positive charges were partially screened at the surface [19]. Therefore, it can be seen that the surface potential was greatly affected by the polarization direction of the dipoles in the embedded PVDF-TrFE layer. Here, the output of the TEG was rather decreased when the number of buried layers further increased, as shown in Figure S5 (Supplementary Materials). As the number of buried layers increased, the total thickness of the friction layer also increased, causing electrostatic induction losses and weakening the dipolar coupling effect. Thus, there exists an optimal number of layers to maximize the output performance. Figure 4: (**a**) Output voltage, (**b**) current density and (**c**) charge density measured from four different friction pairs, shown in Figure 1b, after non-, negative and positive polarization of the PVDF-TrFE layer. (**d**) Suppression of the triboelectrically induced positive surface charge screening effect by the positively polarized PVDF-TrFE layer (left) and screening of the triboelectrically induced positive surface charges by the negatively polarized PVDF-TrFE layer (right). Lastly, we prepared a friction pair with buried layers to demonstrate the performance improvement of the TEG (Figure 5a). Here, as a negative friction layer, negatively polarized PVDF-TrFE with a buried PDMS layer was prepared. Since the PVDF-TrFE layer innately had a triboelectrically negative nature, this layer was used as a negative friction surface after negative polarization so that the negative center of the dipole was mainly distributed on the surface. During the cyclic contact friction, electrons on the friction surface could be partially stored at the interface with the PDMS layer, increasing the overall charge density of the layer. On the other hand, a positively polarized PVDF-TrFE-embedded PDMS layer was used as a positive friction layer.