Tunable Dynamic Black Phosphorus/Insulator/Si Heterojunction Direct-Current Generator Based on the Hot Electron Transport

Triboelectrification and Unique Frictional Characteristics of Germanium-Based Nanofilms

In this study, germanium arsenide (GeAs) is investigated as a promising nanomaterial for application in triboelectric nanogenerators and green energy harvesting. The mechanical and electrical properties of mechanically exfoliated GeAs on silica substrates are evaluated through friction force microscopy and Kelvin probe force microscopy, respectively. First, it is observed that the surface potential/work function of GeAs varied with thickness. Second, thickness-dependent friction on GeAs films is found. However, the variation of friction with GeAs thickness followed an inverse trend typically observed for most other 2D material systems: larger friction is measured on thicker GeAs films. The higher friction is attributed to the higher surface potential of thicker GeAs, resulting from the accumulation of electrons on the GeAs surface that also resulted in higher adhesion between GeAs surface and the tip. Finally, history-dependent friction is observed and resulted from a continual increase in the friction force as the surface is scanned and originated from the triboelectrification of the surface. The dynamic triboelectrification behavior of thick GeAs during the scanning process is further verified and visualized by a serial experiment, where the GeAs is tribo-electrified through scanning and gradually de-electrified/discharged upon ceasing the scan.

Harvesting Environment Mechanical Energy by Direct Current Triboelectric Nanogenerators

As hundreds of millions of distributed devices appear in every corner of our lives for information collection and transmission in big data era, the biggest challenge is the energy supply for these devices and the signal transmission of sensors. Triboelectric nanogenerator (TENG) as a new energy technology meets the increasing demand of today's distributed energy supply due to its ability to convert the ambient mechanical energy into electric energy. Meanwhile, TENG can also be used as a sensing system. Direct current triboelectric nanogenerator (DC-TENG) can directly supply power to electronic devices without additional rectification. It has been one of the most important developments of TENG in recent years. Herein, we review recent progress in the novel structure designs, working mechanism and corresponding method to improve the output performance for DC-TENGs from the aspect of mechanical rectifier, tribovoltaic effect, phase control, mechanical delay switch and air-discharge. The basic theory of each mode, key merits and potential development are discussed in detail. At last, we provide a guideline for future challenges of DC-TENGs, and a strategy for improving the output performance for commercial applications.

Tunable Tribovoltaic Effect via Metal-Insulator Transition

Tribovoltaic direct-current (DC) nanogenerator made of dynamic semiconductor heterojunction is emerging as a promising mechanical energy harvesting technology. However, fundamental understanding of the mechano-electronic carrier excitation and transport at dynamic semiconductor interfaces remains to be investigated. Here, we demonstrated for the first time, that tribovoltaic DC effect can be tuned with metal-insulator transition (MIT). In a representative MIT material (vanadium dioxide, VO₂), we found that the short-circuit current (I_SC) can be enhanced by >20 times when the material is transformed from insulating to metallic state upon static or dynamic heating, while the open-circuit voltage (V_OC) turns out to be unaffected. Such phenomenon may be understood by the Hubbard model for Mott insulator: orders' magnitude increase in conductivity is induced when the nearest hopping changes dramatically and overcomes the Coulomb repulsion, while the Coulomb repulsion giving rise to the quasi-particle excitation energy remains relatively stable.

Broadband Insulator-Based Dynamic Diode with Ultrafast Hot Carriers Process

The excitation, rebound, and transport process of hot carriers (HCs) inside dynamic diode (DD) based on insulators has been rarely explored due to the original stereotyped in which it was thought that the insulators are nonconductive. However, the carrier dynamics of DD is totally different from the static diode, which may bring a subverting insight of insulators. Herein, we discovered insulators could be conductive under the framework of DD; the HC process inside the rebounding procedure caused by the disappearance and reestablishment of the built-in electric field at the interface of insulator/semiconductor heterostructure is the main generation mechanism. This type of DD can response fast up to 1 μs to mechanical excitation with an output of ~10 V, showing a wide band frequency response under different input frequencies from 0 to 40 kHz. It can work under extreme environments; various applications like underwater communication network, self-powered sensor/detector in the sea environment, and life health monitoring can be achieved.

Hot Carrier Transport and Carrier Multiplication Induced High Performance Vertical Graphene/Silicon Dynamic Diode Generator

Dynamic semiconductor diode generators (DDGs) offer a potential portable and miniaturized energy source, with the advantages of high current density, low internal impedance, and independence of the rectification circuit. However, the output voltage of DDGs is generally as low as 0.1-1 V, owing to energy loss during carrier transport and inefficient carrier collection, which requires further optimization and a deeper understanding of semiconductor physical properties. Therefore, this study proposes a vertical graphene/silicon DDG to regulate the performance by realizing hot carrier transport and collection. With instant contact and separation of the graphene and silicon, hot carriers are generated by the rebounding process of built-in electric fields in dynamic graphene/silicon diodes, which can be collected within the ultralong hot electron lifetime of graphene. In particular, monolayer graphene/silicon DDG outputs a high voltage of 6.1 V as result of ultrafast carrier transport between the monolayer graphene and silicon. Furthermore, a high current of 235.6 nA is generated due to the carrier multiplication in graphene. A voltage of 17.5 V is achieved under series connection, indicating the potential to supply electronic systems through integration design. The graphene/silicon DDG has applications as an in situ energy source for harvesting mechanical energy from the environment.

Friction-Dominated Carrier Excitation and Transport Mechanism for GaN-Based Direct-Current Triboelectric Nanogenerators

The semiconductor triboelectric nanogenerator (TENG) based on the tribovoltaic effect has the characteristics of direct current and high current density, but the energy transfer and conversion mechanism is not completely clear. Here, a series of gallium nitride (GaN)-based semiconductor direct-current TENGs (SDC-TENGs) are investigated for clarifying the carrier excitation and transport mechanism. During the friction process, the external output current always flows from GaN to silicon or aluminum, regardless of the direction of the built-in electric field, because of the semiconductor types. These results reveal that the carrier transport direction is dominated by the interfacial electric field formed by triboelectrification, which is also verified under different bias voltages. Moreover, the characteristics dependent on the frictional force have been systematically investigated under different normal forces and frictional modes. The open-circuit voltage and short-circuit current of SDC-TENG are both increased with a larger frictional force, which shows that the more severe friction results in both a larger interface electric field and more excited carriers. The maximum voltage can reach 25 V for lighting up a series of LEDs, which is enhanced by four times compared to the cutting-edge reported SDC-TENGs. This work has clarified the friction-dominated carrier excitation and transport mechanism for the tribovoltaic effect, which demonstrates the great potential of semiconductor materials for frictional energy recovery and utilization.

Mechanism of In-Plane and Out-of-Plane Tribovoltaic Direct-Current Transport with a Metal/Oxide/Metal Dynamic Heterojunction

Interfacial layer engineering has been demonstrated as an effective strategy for boosting power output in semiconductor-based dynamic direct-current (DC) generators, although the underlying mechanism of power enhancement remains obscure. Here, such ambiguity has been elucidated by comparing fundamental tribovoltaic DC output characteristics of prototypical metal-oxide-metal heterojunctions prepared by atomic-layer deposition (ALD) with a vertical (out-of-plane carrier transport through the interfacial layer) and a horizontal (in-plane carrier transport along the interfacial layer) configuration such that the influences from nonequilibrium electronic excitation and interfacial capacitive amplification can be individually tuned and investigated. It is found in the case of Al/TiO₂/Ti vertical configurations that the open-circuit voltage (V_OC) increases linearly from -0.03 to -0.52 V as the thickness of titanium oxide (t_TiO2) increases from 0 to 200 nm with a linear amplification coefficient of -2.31 mV nm^-1, which is validated by a parallel-capacitor theoretical model with tribovoltaic electronic excitation. In contrast, the V_OC output with the horizontal configuration is ∼55 mV, where the potential difference is merely associated with the accumulation of surface charges and the subsequent charge rearrangement in the depletion region. Meanwhile, it is measured that the short-circuit current density (J_SC) shows an initial increasing trend when t_TiO2 increases, reaches its peak value at 0.21 A m^-2 at t_TiO2 = 20 nm, and then decreases as t_TiO2 increases further. From current-voltage (I-V) characterization, it is proposed that such DC output variation with an optimal interfacial layer thickness stems from the competition of amplified voltage and increased resistance with increasing interfacial layer thickness, with the main charge transport mechanism switching from quantum tunneling to thermionic emission/trap-assisted transport. In contrast, tribovoltaic excitation is proven to be significantly weaker when a wide band-gap insulator (Al₂O₃) is involved. The elucidation of the fundamental mechanism of power enhancement by the interfacial layer in this work is of great significance in providing instructional direction for the development and optimization of high-performance DC nanogenerators.

Wind driven semiconductor electricity generator with high direct current output based on a dynamic Schottky junction

With the fast development of the internet of things (IoTs), distributed sensors are frequently used and small and portable power sources are highly demanded. However, current portable power sources such as lithium batteries have low capacity and need to be replaced or recharged frequently. A portable power source which can continuously generate electrical power in situ will be an ideal solution. Herein, we demonstrate a wind driven semiconductor electricity generator based on a dynamic Schottky junction, which can output a continuous direct current with an average value of 4.4 mA (with a maximum value of 8.4 mA) over 740 seconds. Compared with a previous metal/semiconductor generator, the output current is one thousand times higher. Furthermore, this wind driven generator has been used as a turn counter, due to its stable output, and also to drive a graphene ultraviolet photodetector, which shows a responsivity of 35.8 A W^-1 under 365 nm ultraviolet light. Our research provides a feasible method to achieve wind power generation and power supply for distributed sensors in the future.

Microscale Schottky superlubric generator with high direct-current density and ultralong life

Miniaturized or microscale generators that can effectively convert weak and random mechanical energy into electricity have significant potential to provide solutions for the power supply problem of distributed devices. However, owing to the common occurrence of friction and wear, all such generators developed so far have failed to simultaneously achieve sufficiently high current density and sufficiently long lifetime, which are crucial for real-world applications. To address this issue, we invent a microscale Schottky superlubric generator (S-SLG), such that the sliding contact between microsized graphite flakes and n-type silicon is in a structural superlubric state (an ultra-low friction and wearless state). The S-SLG not only generates high current (~210 Am-2) and power (~7 Wm-2) densities, but also achieves a long lifetime of at least 5,000 cycles, while maintaining stable high electrical current density (~119 Am-2). No current decay and wear are observed during the experiment, indicating that the actual persistence of the S-SLG is enduring or virtually unlimited. By excluding the mechanism of friction-induced excitation in the S-SLG, we further demonstrate an electronic drift process during relative sliding using a quasi-static semiconductor finite element simulation. Our work may guide and accelerate the future use of S-SLGs in real-world applications.

Polarized Water Driven Dynamic PN Junction-Based Direct-Current Generator

There is a rising prospective in harvesting energy from the environment, as in situ energy is required for the distributed sensors in the interconnected information society, among which the water flow energy is the most potential candidate as a clean and abundant mechanical source. However, for microscale and unordered movement of water, achieving a sustainable direct-current generating device with high output to drive the load element is still challenging, which requires for further exploration. Herein, we propose a dynamic PN water junction generator with moving water sandwiched between two semiconductors, which outputs a sustainable direct-current voltage of 0.3 V and a current of 0.64 μA. The mechanism can be attributed to the dynamic polarization process of water as moving dielectric medium in the dynamic PN water junction, under the Fermi level difference of two semiconductors. We further demonstrate an encapsulated portable power-generating device with simple structure and continuous direct-current voltage output of 0.11 V, which exhibits its promising potential application in the field of wearable devices and the IoTs.

Scanning Probing of the Tribovoltaic Effect at the Sliding Interface of Two Semiconductors

Contact electrification (CE or triboelectrification) is a common phenomenon, which can occur for almost all types of materials. In previous studies, the CE between insulators and metals has been widely discussed, while CE involving semiconductors is only recently. Here, a tribo-current is generated by sliding an N-type diamond coated tip on a P-type or N-type Si wafers. The density of surface states of the Si wafer is changed by introducing different densities of doping. It is found that the tribo-current between two sliding semiconductors increases with increasing density of surface states of the semiconductor and the sliding load. The results suggest that the tribo-current is induced by the tribovoltaic effect, in which the electron-hole pairs at the sliding interface are excited by the energy release during friction, which may be due to the transition of electrons between the surface states during contact, or bond formation across the sliding interface. The electron-hole pairs at the sliding interface are subsequently separated by the built-in electric field at the PN or NN heterojunctions, which results in a tribo-current, in analogy to that which occurs in the photovoltaic effect.

Interfacial Built-In Electric Field-Driven Direct Current Generator Based on Dynamic Silicon Homojunction

Searching for light and miniaturized functional device structures for sustainable energy gathering from the environment is the focus of energy society with the development of the internet of things. The proposal of a dynamic heterojunction-based direct current generator builds up new platforms for developing in situ energy. However, the requirement of different semiconductors in dynamic heterojunction is too complex to wide applications, generating energy loss for crystal structure mismatch. Herein, dynamic homojunction generators are explored, with the same semiconductor and majority carrier type. Systematic experiments reveal that the majority of carrier directional separation originates from the breaking symmetry between carrier distribution, leading to the rebounding effect of carriers by the interfacial electric field. Strikingly, NN Si homojunction with different Fermi levels can also output the electricity with higher current density than PP/PN homojunction, attributing to higher carrier mobility. The current density is as high as 214.0 A/m², and internal impedance is as low as 3.6 kΩ, matching well with the impedance of electron components. Furthermore, the N-i-N structure is explored, whose output voltage can be further improved to 1.3 V in the case of the N-Si/Al₂O₃/N-Si structure, attributing to the enhanced interfacial barrier. This approach provides a simple and feasible way of converting low-frequency disordered mechanical motion into electricity.