A new device captures the energy of individual raindrops and converts it into direct current electricity, offering a compact solution for powering small devices.
(Nanowerk Spotlight) Raindrops, condensation, and other small volumes of moving water contain mechanical energy due to their mass and velocity. When a droplet falls or slides across a surface, it exerts a force and deforms slightly on impact, releasing kinetic energy. Though the energy carried by a single droplet is small—typically in the microjoule range—it is concentrated and repeatable, especially in environments with continuous exposure to moisture, rainfall, or spraying systems.
The challenge lies in capturing this energy efficiently and converting it into a form that can power electronics. Unlike steady flows of water used in large-scale hydropower, droplet motion is brief, discontinuous, and difficult to harness using conventional means.
In principle, any mechanical action—such as a droplet impacting or spreading on a surface—can be used to drive charge separation. This is the basis of triboelectric generation, where contact between materials leads to localized electron transfer. Devices known as triboelectric nanogenerators (TENGs) have been built to capture this process, using insulating surfaces that accumulate charge when rubbed or struck by water.
However, these systems typically produce alternating current with high internal resistance and low charge density, which limits their utility for powering sensors or microelectronic devices without bulky rectifiers or energy storage circuits.
To address this, researchers have turned to semiconductor-based approaches that can guide carrier motion using internal electric fields. By integrating metal–semiconductor junctions into water-responsive devices, it becomes possible to generate direct current directly from droplet motion. These tribovoltaic nanogenerators (TVNGs) use the triboelectric potential from droplet impacts to dynamically modulate energy bands in the semiconductor, allowing charge carriers to flow across junctions in a controlled way.
The result is a more stable and efficient electrical output. However, realizing this in practice has proved difficult, due primarily to the challenge of engineering reliable metal–semiconductor interfaces with predictable barrier properties.
A team of researchers in China addressed this problem with a new design that uses a Schottky metal–semiconductor–metal (MSM) architecture, engineered for high charge transfer and low resistance. By controlling the interfacial oxide layer and tuning the electronic properties of the junction, the researchers demonstrate a device that efficiently converts droplet impact into direct current with record-setting output. This approach shows promise for powering small-scale systems where conventional energy sources are impractical.
In a study published in Advanced Materials (“Schottky MSM‐Structured Tribovoltaic Nanogenerator Enabling Over 25,000 nC Charge Transfer via Single Droplet Impact”), researchers from Shanghai Jiao Tong University and the Chinese Academy of Sciences report a tribovoltaic nanogenerator capable of producing over 25,000 nanocoulombs of charge from a single droplet. This is achieved by introducing a finely tuned oxide interface between platinum and p-type silicon in an MSM structure, enabling stable Schottky behavior and directional charge transport under mechanical excitation.
Concept of a static Schottky MSM-based tribovoltaic nanogenerator. a) Schematics of a static M-S junction driven by light and water and a dynamic M-S junction driven by sliding friction. b) I-V curve of a static SchottkyMSM structure, the topM-S junction is Schottky contact, while the bottom junction is featured as an ohmic-like contact. c) Band diagram of the p-Si-based static SchottkyMSM structure, the built-in electric field is formed in depletion region and causes the band bending. d) Electron-cloud-potential model of the electron transfer process from the sliding state to the separate state. e) The band diagram of the p-Si-based dynamic Schottky M-S junction shows how the built-in electric field competes with the interfacial electric field generated by contact electrification, illustrating carrier transport when the friction-induced interfacial electric field becomes dominant. f) Carrier transport behavior in static MSM structure, which is based on the photovoltaic effect. g) The negative current signal of MSM structure driven by ultraviolet light. h) Carrier transport behavior in dynamic M-S junction, which is based on the tribovoltaic effect and dominated by ECE-induced. i) The positive current signal generated by sliding friction. j) Carrier transport behavior in static MSM structure, which is controlled by triboelectric potential. k) The positive current signal of MSM structure driven by impinging water droplet. (Image: reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
The device uses an asymmetric configuration where the top junction behaves as a Schottky contact and the bottom junction as an ohmic contact. When a droplet impacts the silicon surface and moves toward the platinum electrode, charge is transferred due to contact electrification. The resulting triboelectric potential modulates the Schottky barrier, reducing its width and height and allowing holes in the silicon to move toward the metal electrode. This movement constitutes a direct current output, unlike the alternating signals observed in traditional triboelectric systems.
The authors systematically explored how varying the thickness of the interfacial SiO₂ layer affects performance. At 500 nanometers, the device achieves the best balance of rectification, barrier modulation, and charge transfer. The dominant transport mechanism depends on barrier width: thin barriers favor thermionic emission and tunneling, while thicker ones rely on diffusion. The optimized device produces a short-circuit current of 358 microamperes and a peak charge density of 349.3 coulombs per cubic meter. The energy density of 79.4 joules per cubic meter is also substantially higher than in previous TVNGs or TENG-based systems.
The material of the top electrode influences performance due to differences in triboelectric interaction with water. Platinum, with high electronegativity, generates the strongest bias and best output. Substituting silver or copper lowers both the induced voltage and current. These variations stem from the triboelectric series, which governs the amount of charge transferred between materials upon contact.
Chemical properties of the droplet also affect output. Using an alkaline solution (pH 14) instead of neutral water increases charge transfer from 410 to 25,500 nanocoulombs. The high ionic strength enhances the electric double layer at the droplet–solid interface, boosting bias potential. However, long-term exposure to strong base degrades the silicon surface, reducing device performance irreversibly. Neutral and mildly acidic conditions cause only temporary changes. This suggests a tradeoff between maximizing performance and ensuring durability, which future designs may resolve through more robust materials such as silicon-based diamond-like carbon.
To demonstrate scalability, the researchers fabricated a 3-inch wafer array containing 60 MSM units connected in parallel. Unlike isolated cells that produce pulsed output, the array yields a continuous current as droplets slide across multiple junctions. Under excitation with NaOH droplets, the module produces up to 2.5 milliamperes of current and charges a 220-microfarad capacitor to 0.6 volts in under two seconds. This is sufficient to power many types of remote or embedded microelectronic systems.
This study demonstrates that performance bottlenecks in droplet-based energy harvesting can be overcome by precise interfacial engineering and careful material selection. By tailoring the state density at the Schottky interface and using the triboelectric potential to dynamically bias the junction, the researchers achieve stable and efficient direct current generation from a mechanical input as fleeting as a single falling droplet. The findings point to a viable path forward for integrating energy harvesting capabilities into devices exposed to environmental moisture, especially where traditional power sources are unsuitable.
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