## 1 Introduction  
The triboelectric generator (TEG), which converts different mechanical stimuli into electrical energy through frictional electrification and electrostatic induction, has attracted great attention as a kinetic energy harvesting device, thanks to its relatively simple device structure and higher outputs compared with other energy harvesting devices [1, 2, 3, 4, 5, 6, 7, 8]. Since triboelectric power generation is highly dependent on the triboelectric surface properties of the friction layers, proper treatment of the friction surface is the key to enhancing the performance of a TEG. To this end, several methods, such as surface patterning [9, 10, 11, 12], chemical functionalization [13, 14, 15], and dielectric property modulation [16, 17, 18], have been introduced, which have demonstrated notable output performance enhancement of TEGs. Most of these approaches, however, focus only on the surface properties of the friction layers, but the role of the layer beneath the friction surface has been largely ignored. Recently, an interesting approach to embedding the charge storage inside or beneath the friction surface was tried, and significant output performance enhancement was achieved [19, 20]. In addition, the improvement of the surface potential through the dipolar coupling effect by adopting ferroelectric materials was also recently reported [21, 22, 23]. These recent approaches exploit charge trapping or dipolar coupling of the sublayer to increase the surface charge density or potential of the triboelectric friction surface. However, the effective use of these methods is still unclear, and only a few results have been reported [24, 25]. Furthermore, no attempts have been made to apply both methods simultaneously. Therefore,further studies are necessary to develop the triboelectric friction layers that synergistically improve the performance of TEGs. In particular, only limited performance enhancement methods are available for the TEGs made of only flexible polymeric layers [26; 27; 28; 29].  
In this work, we report on a multi-layered (sandwich) polymer structure consisting of polydimethylsiloxane (PDMS) and polarized poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) layers to boost the surface charge density of the friction surfaces of a TEG. Here, the PDMS layer functions as a charge blocking layer and charge reservoir, providing charge trapping states at the interface between the PVDF-TrFE and the PDMS. In addition, the ferroelectric PVDF-TrFE layer plays dual roles as a negative triboelectric friction layer and surface charge modulation layer by polarizing the internal dipoles in a preferred direction. Our findings show that the outputs of TEGs can be greatly improved by forming a multilayered (sandwich) structure in the friction layer and properly polarizing the PVDF-TrFE layer. To determine the optimal multilayer structure composed of PVDF-TrFE and PDMS, we examined four different friction pairs of TEGs. Using these approaches, the TEG with the polarized sandwich structure generated the maximum peak-to-peak output voltage and current density of \(\sim\)420 V and \(\sim\)50 \(\upmu\)A/cm\({}^{2}\), respectively, which were approximately values about five times higher than the TEG with a non-polarized single layer.