## 3 Results and Discussion  
### Role of the Buried Layer with the Performance  
Schematic diagrams of the TEG fabrication process are illustrated in Figure 1a. A co-solvent of DMF and acetone was used for the synthesis of the PVDF-TrFE solution, and the mixture of the silicone elastomer and the curing agent was employed for the PDMS friction layer. Each solution was spin coated on a pre-cleaned indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrate. We should note that all the prepared samples were flexible. The detailed fabrication process of friction pairs is described in Section 2. To investigate the role of the buried layer on the performance of a TEG, the TEGs with different buried layers but identical surface friction pairs were prepared as shown in Figure 1b. We should note that the surface friction pairs of all the samples were kept as PVDF-TrFE and PDMS for a fair comparison. The output generation mechanism of the contact mode TEG is described in Figure S1 (Supplementary Materials). Figure 1c schematically shows the different charge trapping mechanisms of the buried PDMS layer and the buried PVDF-TrFE layer. PVDF-TrFE, which is on the relatively negative side of the triboelectric series, has a stronger electronegativity than PDMS, so when the two friction surfaces are contact-electrified, the electrons on the PDMS surface are transferred to the PVDF-TrFE surface [30]. Here, we note that PDMS is known to have deep trap states, while PVDF-TrFE has shallow trap states [31; 32]. Thus, the electrons transferred to the PVDF-TrFE layer could be easily discharged to the ITO electrode layer if the PDMS blocking layer was not formed underneath (Figure 1c (left) and Figure 1b (iii)). On the other hand, when the electrons were transferred to the PVDF-TrFE surface with a buried PDMS layer, they were filling the deep states at the interface between the two layers, reducing the charge recombination and increasing the overall surface potential (Figure 1c (right) and Figure 1b (ii)) [32; 33; 34]. Figure 2a,b shows the measured output voltage and current density, respectively, of the as-fabricated TEGs with identical surface friction pairs but with different buried layers. For the output measurement, a pressure of 100 kPa normal to the contact surface was applied at 1 Hz for all the samples. Overall, the outputs of the TEGs with the buried layers were relatively higher than those of the TEGs with the single-layered friction pairs. Here, the buried layer served as an electron trapping layer. Thus, the TEGs with the buried layer could exhibit relatively high surface potentials during contact electrification (Figure 2c) and thus demonstrate relatively high output performances (Figure 2a,b). We must note that the output of a TEG with a buried PDMS layer (Figure 2a (ii)) was higher than that with a buried PDVF-TrFE layer (Figure 2a (iii)). The buried PDMS layer, which had a higher trap density compared with the PVDF-TrFE, alleviated the discharging of surface charges induced during the contact and separation motion. Therefore, the electrostatic surface potential of the buried PDMS could be higher than that of the buried PVDF-TrFE (Figure 2c (ii,iii)), leading to a similar trend in outputs (Figure 2a (ii,iii), Figure 2b (ii,iii)). Furthermore, as shown in Figure 2d, the output voltage of each TEG was monitored in real-time during the cyclic contact electrification. The results indicated that it took more time to reach the saturation voltage when the buried PDMS layer was adopted, due to the high trap density of the buried PDMS layer. Particularly, the highest output was obtained when the PVDF-TrFE layer was sandwiched between the PDMS layers (Figure 2a (iv)). Likewise, the electrostatic surface potential and voltage saturation time of each sample also represented a similar trend (Figure 2c,d). Figure 1: (**a**) Schematic fabrication process by the solution spin coating method.