Xin Shi is currently studying at Fujian University of Technology and co-trained at Minjiang University. His research focuses on semiconductor-based triboelectric nanogenerator.
Jun Wang received Ph. D degree from the University of Science and Technology of China (USTC). He is now a professor at Minjiang University. His research interest includes the chemical and physical properties of micro-nano structure, magnetic sensing devices and microfabrication.
Huamin Chen received his B. S. degree from Fudan University in 2014. He received his Ph. D degree from Institute of Semiconductors, Chinese Academy of Sciences in 2019. He is now an associate professor at Minjiang University. His research interest includes smart sensing Materials, stretchable and flexible electronics, energy harvesting devices, and microfabrication.1. Introduction
With the continuous miniaturization and reduced power consumption of electronic devices, coupled with the widespread adoption of 5G networks, the IoT has rapidly advanced[1]. However, the escalating demand for efficient, cost performance, and highly stable power sources for electronic devices presents significant challenges for large-scale IoT deployment[2, 3]. Triboelectric nanogenerators (TENGs) efficiently harvest environmental mechanical energy into electrical energy, advantages such as low production, costs, abundant material options, and compact size, making them highly suitable for small electronic devices[4−6]. TENGs can utilize different types of mechanical energy including human, wind and ocean energy, to power self-sustaining systems. Based on the principles of triboelectrification and electrostatic induction, there are four TENGs modes: contact-separation, horizontal sliding, single-electrode, and freestanding triboelectric-layer mode. These modes rely on the interaction between two dissimilar materials to generate alternating current (AC), which is typically characterized by low current density due to materials’ low surface charge density and high internal impedance. Additionally, the generated AC must be converted to direct current (DC) by a rectifying circuit, limitations that have impeded their broader application[7, 8].
At present, a variety of DC-TENGs with different mechanisms have been studied. For example, the charge space accumulation effect is utilized, and the charge space accumulation is enabled by designing shielding layer and optional blank friction zone, which can improve the charge density of TENG[9]. Using triboelectric effect of ternary medium, constant DC and constant high voltage can be generated without brush and rectifier[10]. For high-performance DC-TENG based on phase coupling, with the increase of phase number and group number, the overall output performance is obviously improved[11]. Based on the air ionization DC-TENG, the air is ionized by a high voltage, and the high voltage is passed to the electrode, so that the electrons always flow in one direction in the external circuit, achieving DC output[12]. Direct charge injection through self-charge excitation technology, i.e., electrostatic breakdown of charge, can significantly improve TENG performance by several times[13].
In contrast, semiconductor-based DC-TENGs (SDC-TENGs) produce DC output directly by rubbing one semiconductor against another, resulting in higher current density and lower impedance[14]. A semiconductor is a material that conducts electricity between a conductor and an insulator. Until now it has been at the heart of development and progress in many fields[15]. As illustrated in Fig. 1, SDC-TENGs differ significantly from traditional TENGs in both structure and current output mechanism. Unlike the traditional TENG, where the horizontal sliding mode alters the contact area between friction layers through external forces to modulate the electrostatic field and charge movement, generating AC signals[16, 17]. SDC-TENGs maintain a constant contact state throughout the friction process. The mechanism involves the formation of atomic bonds during friction, which releases energy that excites carriers at the sliding interface. Similar to the photovoltaic effect, this new effect was originally proposed as the tribovoltaic effect[18, 19].
This review comprehensively summarizes the recent advancements and developments in semiconductor-based SDC-TENGs, which hold significant promise for DC energy harvesting technologies and semiconductor systems. First, the tribovoltaic effect is elucidated, and SDC-TENGs are categorized into six types based on different materials and structures. Subsequent sections detail the operational mechanisms, strengths, and limitations of each category. Additionally, this paper outlines the enhancement mechanisms of SDC-TENGs providing guidance and recommendations for performance improvement. The conclusion highlights potential application scenarios for various types of SDC-TENGs, outlining the prospective benefits and challenges. SDC-TENG technology is poised to drive revolutionary developments in the fields of semiconductors and self-powered systems.
2. Tribovoltaic effect and SDC-TENG classification
Triboelectrification process involves mechanical contact or sliding of two materials, against each other, during which various local physical and chemical processes occur. It represents a unified process of tribology and interfacial charge transfer, and is one of the most fundamental phenomena in electricity generation. When a glass rod is rubbed with silk, electrons transfer from the glass to the silk, leaving the silk negatively charged and the rod positively charged owing to electron loss[20]. Although triboelectrification has been known for a long time, in-depth research into it remained limited until 2012, When Wang Zhonglin’s team pioneered the triboelectric nanogenerator (TENG), bringing widespread attention to friction-based power generation. Since then, SDC-TENGs have emerged as a research focus[21]. Intensive development began in 2018 with the introduction of a M−I−S point contact SDC-TENG. This was followed by the development of a graphene/metal−Si heterostructure, a M−S system and a more complex structure that utilized polarized water drive a dynamic PN junction. Subsequent innovations have included various semiconductor heterojunctions (such as GaN−Si), which have significantly enhanced output performance[22]. Depending on their material and structural characteristics, and the presence of an insulating layer, SDC-TENGs can be classified into six categories. Through the introduction of these six kinds of devices, researchers can have a clear understanding of SDC-TENG.
3. Various types of SDC-TENG
3.1 Semiconductor−semiconductor (S−S) DC-TENG
A novel type of generator, composed of n- and p-type semiconductors, initiates direct current generation as soon as one semiconductor slides over the other. A triboelectric cell with n-Si/p-Si heterojunction was studied by Zhang et al. A voltage of 0.35 V can be achieved[23]. A carbon aerogel/silicon dynamic heterojunction with voltage up to 2 V, designed by Liu et al.[24], and Lin et al. documented the highest DC voltage achieved to date, 6.1 V, using a black phosphorus/AlN/Si heterojunction[25]. As displayed in Fig. 2(a), the compressed metal/conductive polymer and the sliding metal/MoS2 Schottky contact to produce a direct current[26]. As illustrated in Fig. 2(b), a n-doped silicon was slid over a p-doped silicon back and forth on a translation stage, controlled by a linear motor. Fig. 2(b) shows a band diagram where two electrodes are disconnected[23]. There is a positive space charge for the depletion region of n-type semiconductor and an opposite space charge on p-type side. This creates an n-type to p-type electric field inside. Due to the action of electric field, direct current is formed. Since the charge is electrically neutral as a whole, it can be thought of as a dipole with two opposite charges. The positive charge is located in the n-type semiconductor depletion region, and the negative charge is located in semiconductor depletion region. The direction is from the negative charge to the positive charge, and is perpendicular to the contact surface. In the case of two electrodes with different Fermi levels, sliding a doped semiconductor onto another inversely doped electrode generates DC[27−29]. Ma et al. proposed a dynamic halide perovskite heterojunction to generate direct current. A heterogeneous structure composed of perovskite and helicoid material. As shown in Fig. 2(c), the resulting performance is measured by contacting the perovskite electrode and sliding the spiral electrode. A DC voltage of ~0.4 V and a current of ~1.2 mA are obtained, and this DC power is thought to be related to the band alignment between the perovskite and the helix, between which charge carriers can be transferred in a directional way[30]. South Korean researchers proposed CsFAMA and PEDOT:PSS, and developed a DC-TENG owing to p−n junction. The DC-TENG generates a high DC of about 2.1 µA∙cm−2 and a voltage of 0.33 V. Fig. 2(d) shows the state of PEDOT:PSS before contact with CsFAMA. With slow contact, the surface charge of these material transfers and reaches thermodynamic equilibrium, thus forming a p−n junction. In addition, when the two materials slide, tribo-excitons is generated at the interface. These electron−hole pairs are separated and excited, resulting in DC output[31]. As shown in Fig. 2(e), a p-Si slides onto the n-GaN to form heterojunction. The device exhibits an ultrahigh voltage of 130 V, which is dozens of times compared with that of previous reports[32].
3.2 Metal−semiconductor (M−S) DC-TENG
N-type doped Si/stainless steel heterostructure is constructed to form M−S DC-TENG. The structure of this device is shown in Fig. 3(a). The energy band diagram of M−S is displayed. The generator consistently outputs DC during one motion period[32].
The utility model relates to a miniature DC windmill generator using wind energy for DC power generation using dynamic semiconductor/metal Schottky contact. Fig. 3(b) illustrates the schematic diagram of our designed windmill SDC-TENG’s power generation unit. At equilibrium status, PEDOT:PSS contacts with the Al friction layer to form a Schottky heterojunction. With a rotation of wind cup, the heterojunction interface rotates and slides. Based on the frictional voltaic effect, friction generated by relative rotation can excite charge carriers at the interface[33]. An independent mode TENG consisting of freestanding metal part and Si friction part is shown in Fig. 3(c). When the metal simultaneously slides onto P-Si and N-Si, DC output from P-Si to N-Si is found[34]. Fig. 3(d) shows the plane structure of rolled TENG. The reciprocating movement of metal rollers on the surface of perovskite film generates DC. Yuan et al. successfully realized DC output with 3.69 V voltage and 5.73 μA current[35].
By sliding a titanium plate on the depleted-type gallium nitride heterojunction structure, we've come up with the M−S DC-TENG. It can generate a high voltage of 45.5 V. Fig. 3(e) shows the simulated three-dimensional structure of our MSDC-TENG, which uses Ti and D-GaN HEMT as the triboelectric layers. This D-GaN HEMT DC-TENG can light up all 14 green LEDs, which glow brightly in the dark (inset), and also drive a digital watch to function properly[36].
3.3 Metal−insulator−semiconductor (M−I−S) DC -TENG
In a triboelectrically charged M−I−S point contact system, they used a p-type silicon, silicon oxide, and a metal tip to explore this phenomenon. At the nanoscale, the AFM probe slides on a p-Si substrate, as shown in Fig. 4(a)[37]. The tunnel current during sliding is measured using the conductive mode AFM, which becomes measurable when the force exceeds 300 nN. Silicon is usually covered with a thin layer of natural oxides[38]. A stainless-steel base test lead was driven onto a silicon substrate to further demonstrate the phenomenon. The bottom was coated with aluminum ohmic contacts. The probe is driven in linear reciprocating and circular rotation modes and measures the short circuit current (Isc) in each mode[39, 40]. Liu et al. proposed a DC in a M−I−S sliding system using microtips, which the results are better than those of traditional polymer-based TENG. In the AFM experiments, p-type Si substrates coated with aluminum base electrodes are used. KPFM scanning was exhibited in Fig. 4(b)[41]. To explain the frictional tunneling current. The C-AFM tunnel current signals in peak force knock mode and contact mode indicate that DC is formed only under continuous sliding contact, and C-AFM current distribution is assessed by changing the external bias (Vbias) at a force of 500 nN. When current signal is decreased to approximately 0, the Voc is estimated to be between 300−400 mV[41, 37].
3.4 Semiconductor−insulator−semiconductor (S−I−S) DC-TENG
Electronic devices based on static heterojunction are widely used[42]. Lu et al. proposed a black phosphorus/aluminum nitride/silicon heterojunction with high performance. As can be seen from the structural diagram in Fig. 5(a), when the black phosphorus moves along the contact surface of the silicon substrate, the peak current of 6.2 μA can be observed, and the intensity reaches 124 A/m2[25].
The S−I−S structure also contributes to tribo-tunneling DC output. Fig. 5(b) presents the voltage output of the Si/SiO2/MoS2 junction by varying Si doping types/concentrations. The surface potential of the Si samples appears to influence the surface band bending of MoS2, with upward band bending induced in a p-type Si/MoS2 structure and downward bending in an n-type Si/MoS2 structure[43, 44]. In the former, the ultra-thin SiO2 layer is quantum mechanically tunneled by excited electrons, while the electric field sweeps through the space charge region and outputs as a DC. Conversely, hole tunneling and conduction in the latter generate a negative DC output. Additionally, heavily doped n-/p-type Si samples show markedly lower outputs[26, 41].
3.5 Liquid−semiconductor (L−S) DC-TENG
Recent studies on electron transfer at liquid-solid interfaces have aroused the interest of researchers[23, 45]. Researchers have proved DC can be generated by sliding metal on semiconductor surface based on tribovoltaic effect[32, 46]. The mechanism is similar to that of the L−S interface, where there exists a built-in electric field at the liquid−solid interface, and DC current can be generated when an electronic transition occurs[47]. As illustrated in Fig. 6(a), in the voltage waveform diagram of a cycle, when the water drop begins to slide, a voltage of up to 200 mV can be obtained, and when the drop stops on the surface, the voltage will disappear. As the silicon begins to move backwards, a new voltage is generated again. When the droplet slides on the surface, it can generate an electric current of about 40 nA[48].
Laser-induced graphene (LIG) can be used to build electrodes for flexible droplet generators (DEG). Due to its flexible characteristics, the graphene network after multiple process treatments still maintains good integrity after various mechanical deformation, as illustrated in Fig. 6(b). In the experiment of flexible liquid generator, the liquid drops from the air and collides with the surface of the material for a moment, making the upper surface of the material and the lower surface respectively have opposite charges. Due to the existence of electrostatic induction, the charge accumulates on the surface, resulting in interface contact points. As the droplet expands and begins to slide, the entire setup forms a loop. The trigger charge is rapidly transferred, producing an instantaneous current of approximately 268.9 µA and a peak voltage of 198 V[49].
3.6 Metal/semiconductor–liquid–semiconductor (M/S–L–S) DC-TENG
Recently, water has been utilized to obtain electricity by placing water molecules over various nanostructured materials such as carbon nanotubes[50, 51], graphene[52], polymers, and other nanostructures[53]. The dynamic PN junction using water as a medium for power generation is a great advance in this field. Its operation is characterized by water droplets sliding between p-type and n-type silicon with different Fermi energy levels, producing a continuous DC output. The advantages are also obvious, regardless of the direction of the water drop sliding between the semiconductors, and the output is stable. Fig. 7(a) shows an illustration of the dynamic semiconductor–water–semiconductor structure generator. The water droplet is positioned between two types silicon layers, separated by two 1 mm thick polyvinylchloride layers that form a narrow channel for the water droplet to traverse. Asymmetric molecular structures of the liquid used can enhance the functionality[54, 55].
The invention relates to a SDC-TVNG, which consists of n-type WS2 and p-type silicon. By adding a liquid layer to enhance its electrical conductivity, the maximum DC output at operation is increased by 142 times to 4.2 μA. The schematic illustration of this device is depicted in Fig. 7(b). The effects of dimethylformamide (DMF) on TVNG output were compared with various NaCl solution and tap water as polar liquid mixture. The results show that both deionized water and polar liquid can enhance the output of TVNG. Ionized water has the strongest enhancement effect, and different concentrations of NaCl solutions will also affect the output. When the concentration is 100 g/L, the current can be higher than that of other solutions, reaching 4.2 μA. When 100 g/L NaCl ionic liquid is used as the interlayer, the power density of 5.09 μW/m2 can be generated under different external resistors. In contrast, non-polar liquids inhibit output[56].
4. Performance enhancement mechanism of SDC-TENG
The output performance of SDC-TENGs will directly affect its practical application. The properties are affected by the structure and working mode of the material. Table 1 summarizes the output performance of six different types of SDC-TENG[57]. Various SDC-TENGs have been developed using various materials and device structures, each with unique working mechanisms. The primary factors affecting these mechanisms include material choice and structural design[22]. Although many experimental demonstrations have showcased energy harvesting using TENGs and highlighted the generation of heat and charge carriers through frictional contact between two materials, a fundamental understanding of the charge transfer process during friction remains limited[58−60].
| Materials | Contact area (cm2) |
Sliding velocity (cm∙s−1) |
Voltage (V) |
Current (μA) |
Power density (W∙m−2) |
Ref. |
| Carbon aerogel /Si/SiO2 | 0.785 | 5 | 2 | 15 | / | [24] |
| Al/CsPbBr3 | / | 70 | 3.69 | 5.73 | / | [35] |
| PEDOT:PSS/Al | 2.5 | 700 | 0.6 | 3.6 | / | [33] |
| GaN-based heterostructure/ titanium |
2.4 | 100 | 45.5 | 26 | 2.32 | [36] |
| Si/stainless steel | / | 40 | 0.045 | 3.5 | 146.4 | [34] |
| Black phosphorus /AlN/Si | 0.5 × 10–5 | 8 | 6.1 | / | 201 | [25] |
| Si/Al2O3/Si | 0.1 × 10–4 | 10 | 1.3 | 21.4 | 33.6 | [61] |
| Al/Si/silicon oxide | / | / | 0.4 | 5 | / | [37] |
| PEDOT:PSS/CsFAMA | / | / | 0.8 | / | 0.18 × 10–4 | [30] |
| MoS2/mental/semiconductor/ insulator |
0.017 | / | 0.3 | 0.6 | / | [26] |
| Si/Si | 1 | 5 | 0.35 | 0.089 | [23] | |
| GaN/Si | 0.64 | 12 | 130 | 21.5 | 2.8 | [31] |
| Perovskite/CTL | 1 | / | 1.3 | 1200 | / | [29] |
| Al/TiO2/Ti | / | 2.5 | 0.52 | / | / | [62] |
| Droplet/FEP/F-LIG | / | / | 198 | 268.9 | 47.5 | [49] |
| Polytetrafluoroethylene/ferrofluid | / | / | 0.98 | / | / | [63] |
| WS2/Si | / | / | 4.2 | 4.2 | 5.09 × 10–6 | [56] |
| Cu−Si/Metal | / | 10 | 0.3 | 4000 | 1.6 × 10–3 | [64] |
| Si/water/Si | / | 15 | 0.3 | 0.64 | / | [54] |
| Water/Si | / | 5 | 0.4 | 0.3 | / | [65] |
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A significant example involves the Schottky contact between the tip and MoS2，which plays a crucial role in the transport of triboexcited charges. Fig. 8(a)[66] illustrates three adjacent MoS2 crystal grains exhibiting varying triboexcited current responses. It is certain that C-AFM can induce strongly enhanced electric field, resulting in ultra-high current values, due to its unique electrode geometry, namely its nanotip structure[38, 40]. At the tip−sample interface, the strength can be seen to reach 7 × 105 V/m, and through the contact interface, penetrating the contact interface and persisting on the order of 105 V/m. To achieve the expected results of the experiment. In the surface region generated by an inhomogeneous electric field, the excited charge drifts to the AFM tip, which is critical for considering the effective electrical contact area when calculating current density[67]. This simulated result aligns well with the prediction from numerical methods, which estimate a distance of approximately 3 radius[38]. In the process of scanning, with the increase of force, the DC response of the entire surface will be enhanced. The current of C-AFM is related to the contact force. As a whole, the local friction current density of the nanogenerator system increases on account of enhanced electric field at the nanoscale tip[68].
4.1 Intrinsic material parameters
The output performance of DC-TENGs is continuously improved with the optimization of materials or structures[22]. The metal−insulator transition (MIT) can affect the tribovoltaic effect. In MIT's representative material vanadium dioxide (VO2), when the material changes from a non-conductive state to a conductive state, its short circuit current can be significantly increased by more than 20 times. The open circuit voltage remains constant during this process. Under the action of external factors such as electric field, temperature, and pressure, a transition from a metal state to an insulating state can occur[69]. The transition of monocline insulators to rutile metal is due to band gap narrowing and lattice symmetry breakdown, which is a characteristic manifestation of specific changes in electron and crystal structure[70]. The Mott−Hubbard mechanism describes the phenomenon that electrons in the outer layers of a material cannot move freely between each other due to strong coulomb repulsive forces. In a Mott transition, the electron needs to gain enough energy to overcome this repulsive force to change its state, and this energy can be provided by changing temperature or applying pressure[61]. VO2, a material with associated electronic properties whose metal−insulator transition (MIT) can be triggered by changing temperature, applying pressure, an electric field, or by photothermal irradiation. This transition is very rapid and can occur in a very small temperature range of 0.1 K[71−73].
In addition, metal-related power output can be evaluated using different contact metal materials. It was determined that the short-circuit current corresponds well with the work functions of different contact metals. Additionally, short-circuit current amplification was observed by heating all the tested metal materials. This universality precisely validates the generation of tunable friction volts DC with metal/VO2 sliding contacts. The open-circuit voltage with different metals shows a weak dependence on work function, potentially due to surface pinning effects associated with surface states[74−76]. The 20-fold enhancement in DC output of the Al/VO2 sliding system because the sharp MIT effect triggered by change in temperature. However, the voltage value remains constant during this process[77].
4.2 Environment test conditions
As shown in Fig. 8(b), the tribovoltaic effect under fast MIT can be studied using the single-channel sliding method[77, 78]. When t = 0 s, the probe passes through the VO2 surface, and the temperature rapidly rises to >357 K. With the probe moving, when t = 1 s, the temperature has a rapid decline process and is lower than 325 K. Then at t = 2, 3, and 5 s, the surface temperature gradually drops to 306, 304, and 298 K, respectively, and when t reaches 10 s, the surface temperature drops to room temperature (297 K). At the same time, the short-circuit current is measured at the temperature of 298 and 405 K respectively. The ISC values of 0.38 and 8.01 μA, respectively, are equivalent to approximately 21-fold enhancement of the current output[79, 80].
5. Applications
SDC-TENGs can be applied in various field, such as harvesting small scale environmental and human activity energy[81]. For instance, as displayed in Fig. 9(a), the two layers of a separated TENG device are connected through a spring. Under environmental vibrations, the device generates movement causing the upper and lower materials to engage triboelectrically, resulting in periodic electrical output[82]. Fig. 9(b) demonstrates the collection of wave energy in seawater. Through a series of network designs, significant energy output can be achieved[83]. Fig. 9(c) showcases a single-electrode design used to harvest raindrops energy. Integrated transparently over a solar cell, it forms a composite energy collection system—utilizing sunlight to generate electricity through the photovoltaic principle on sunny days and raindrop kinetic energy during rainy conditions[84]. Fig. 9(d) illustrates the TENG’s capacity to efficiently collect and store mechanical energy from various sources in a real-world environment, capable of powering electronic devices such as calculators, electronic watches, and hygrothermostats[85]. As shown in Fig. 9(e), a self-powered environmental monitoring network has been established using TENG devices. This network monitors temperature, humidity, and air pressure in real-time across a monitoring area exceeding 2 square kilometers[86]. Fig. 9(f) reveals the TENG can effectively harvest energy from human activities such as walking and running. Beyond integration directly into the soles of shoes, the device can also be designed as an insole to collect energy from walking[87, 88]. Fig. 9(g) details the application of TENG in wind power generation. The device features a blade structure, where the interface is integrated into the blade material and electrode material. As the blades touch, they trigger triboelectrification with the electrode material, leading to an electrical charge flow. At the speed of 27 m/s, the device’s output power is approximately 2.37 W/m2[89]. Ye et al. with the help of electrospinning technology, developed a method of making coarse yarn. It consists of a rough nano-scale dielectric surface and a high mechanical strength and conductive core yarn owing to leather core structure, and demonstrated in wearable energy supply applications[90, 91], as show in Fig. 9(h). Ye et al. developed a two-dimensional bionic scale knit friction-generating fabric using PTFE coated conductive silver yarn and nylon coated conductive silver yarn as contact electrokinetic materials. A high power three-dimensional orthogonal woven friction nanogenerating fabric composed of warp stainless steel/polyester blend conductive yarn, weft PDMS coated conductive yarn and thickness direction insulation bonded yarn has been created. Additionally, Sheng et al. designed several air-based percussion models through the effect of DC friction power generation fabric, exhibiting high electrical output as shown in Fig. 9(i)[92−94].
SDC-TENG faces similar challenges to AC-TENG, with the primary issue being the loss caused by excessive friction. In SDC-TENGs, improving surface charge often involves treating the surface of polytetrafluoroethylene films with nanostructures. However, after repeated friction, these nanostructures may degrade, resulting in diminished power generation efficiency. AC-TENGs have traditionally employed solid-liquid and solid-gas contact modes to reduce friction loss[95]. Nevertheless, SDC-TENG requires a fixed nanoscale void between the charge collector and the triboelectric layer to capitalize on the special power generation properties of gases. This configuration means that liquids and gases cannot fully validate the reduction of friction loss, and there have been no studies explicitly removing solid−solid contact outside this structural form. Following the traditional AC-TENG approach, SDC-TENG can also be developed in the direction of composite friction nanogenerators. This would combine AC-TENG and SDC-TENG technologies to enhance output power, which represents a promising composite form. Additionally, the integration of different energy forms, such as the hybrid structure developed by Liu et al.[96], which combines solar cells with AC-TENG, facilitates both solar and raindrop power generation. This hybrid approach can significantly improve the output efficiency compared to standalone SDC-TENG systems.
6. Conclusions and outlook
The arise of TENGs offers a new technological approach for environmental energy harvesting and has garnered extensive attention across various research fields including energy science, self-powered sensors, wearable electronics, robotics, environmental science, implantable devices, and artificial intelligence. Given the diverse application scenarios, the output requirements for TENGs vary significantly. To directly power electronic devices, various DC output TENGs have been developed, featuring mechanisms such as mechanical rectification bridges, phase superposition, and air-air-punch direct current nanogenerators. Since Liu et al.[97] first proposed SDC-TENG, considerable progress has been made to enhance its power generation efficiency from different perspectives, including surface structure, material selection for devices, working temperature, and working pressure. On the application front, SDC-TENG provides a stable DC power supply for electronic devices, demonstrating its unique advantages. Moreover, combining it with traditional AC-TENG can enhance the overall energy output[98].
To address the absence of a charge accumulation process, which limits methods for enhancing output efficiency, it is crucial that both electrodes of the SDC-TENG are interconnected. Otherwise, an external circuit cannot be formed. In the single-electrode mode and independent layer mode of AC-TENG, only one conducting electrode is grounded through an external resistor, allowing the other friction layer to move freely. This configuration can harvest more forms of energy and significantly reduce the volume of the TENG. Although the SDC-TENG has made significant achievements, it proposes issues and challenges in mechanism, manufacture and application. The friction of semiconductor interfaces involves multiple physical mechanisms, so the theory is not yet clear. Furthermore, advanced semiconductor structure is required for wearable and integrated SDC-TENG, which demands higher manufacturing methods. Currently, most SDC-TENG is utilized for the traditional applications of TENG, which not effectively exert advantages of high current density. And the reliability and power density also limit its applications. As research on TENGs deepens, improvements in energy conversion efficiency and power output density are expected to continue, and the issues encountered in applications will increasingly be resolved. TENG will undoubtedly play a vital role in the future as wearable emergency power supplies and become an integral part of the future energy system.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 52201043, 12174172), the Natural Science Foundation of Fujian (No. 2023J011396), the Fuzhou City Science and Technology Cooperation Project (No. 2022-R-003), Fuzhou Industry Technology Innovation Center for Flexible Functional Materials.
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