Triboelectric nanogenerators (TENGs) have emerged as promising energy-harvesting technologies in recent years. In 2012, Wang et al. reported a groundbreaking study on the first TENG, which efficiently converts mechanical energy into electrical energy. Since then, extensive efforts have been dedicated to studying various aspects of TENGs, including their mechanisms, device structures, materials, and applications. TENGs were classified into four working modes: contact-separation mode, lateral-sliding mode, single-electrode mode, and freestanding triboelectric-layer mode. These TENGs primarily rely on insulating dielectric materials as triboelectric layers, generating electricity through the synergy of triboelectrification and electrostatic induction. The diverse working modes of TENGs offer flexible options for harvesting triboelectric energy in various scenarios.

In addition to insulating dielectric materials, non-insulating materials can also be adopted as triboelectric layers, offering unique properties that differ from conventional TENGs. These include the transistor-inspired bulk effect at the water-polymer interfaces and the tribovoltaic effect at the semiconductor-based interfaces. The transistor-inspired bulk effect, initially reported in a droplet-based electricity generator, generates a remarkably high instantaneous power density, surpassing those based on interfacial effects by several orders of magnitude. On the other hand, the tribovoltaic effect involves electricity generation at dynamic semiconductor interfaces. In this effect, non-equilibrium electron-hole pairs generated at the dynamic interfaces are efficiently separated by the dynamic semiconductor-based junctions, resulting in direct-current electricity generation. These innovations significantly expand the capabilities of TENGs across diverse dynamic interfaces.

Firstly, this thesis provides a comprehensive review of the recent progress in TENGs operating at various dynamic interfaces, including solid-solid, liquid-solid, liquid-liquid, and bubble-solid interfaces. Special emphasis is given to two significant advancements: the transistor-inspired bulk effect at water-polymer interfaces and the tribovoltaic effect at semiconductor-based interfaces. This review lays a theoretical foundation for the development of high-performance devices and the exploration of underlying mechanisms.

Secondly, to enhance electrical outputs from dynamic water-polymer interfaces, we developed a hybrid energy harvester that allows for the collection of triboelectric and kinetic energy simultaneously based on the synergy of the transistor-inspired bulk effect and the piezoelectric effect. Impinged by a water droplet, the hybrid energy generator produces a boosted transferred charge value (101 nC) and high output power density (82.66 W m−2). We showed that such performances enable the continuous operation of self-powered wireless sensor systems. We proposed a new analytical model to elucidate the effect of key parameters on the transistor-inspired bulk effect, including droplet volume, dripping height, surface charge density, and droplet ion concentration. The hybrid energy harvester is also low-cost and facile for fabrication, paving a new way towards high-efficiency power generation from raindrops as well as other water sources.

Thirdly, to promote the large-scale application, we developed high-performance panel designs that effectively amplify the droplet energy harvesting capability in a spatially scalable manner. The proposed droplet energy harvesting panel possesses the superiorities of (1) high performance, originating from the bulk effect of the new coplanar-electrode transistor-inspired structure; (2) ease of integration, requiring only one patterning process to form the coplanar electrodes and one full-wave rectifier in external rectification; and (3) full transparency, endowing it with great potential for integration with solar panels and smart windows. We also developed a wireless forest monitoring system fully powered by the proposed droplet energy harvesting panel, showing its great potential in self-powered environmental monitoring. The droplet energy harvesting panel represents a new pathway for high-efficiency droplet energy harvesting to power wireless sensor systems and the emerging Internet of Things.

Fourthly, to reveal the underlying mechanism of dynamic metal-semiconductor interfaces, we defined a “triboelectric junction” model for analyzing the tribovoltaic effect at these interfaces. The model proposes that a space charge region induced by the triboelectric effect dominates the electron-hole separation process. Through theoretical analysis and experiments, we concluded that the triboelectric junction affects the electric output in two aspects: 1) the junction direction determines the output polarity; 2) the junction strength determines the output amplitude. The junction direction and junction strength are both related to the electron-affinity difference between the contact metal and semiconductor. The “triboelectric junction” model provides a new perspective on the mechanism of the tribovoltaic effect, which might further trigger new fundamental discoveries and applications.

In summary, this thesis provides a comprehensive investigation into high-performance triboelectric energy harvesting technologies at dynamic interfaces containing non-insulating materials from both device and mechanism aspects. In terms of the water-polymer interfaces, a hybrid energy harvester was developed to enhance the electrical output through the synergy of the transistor-inspired bulk effect and the piezoelectric effect. Additionally, a large-scale droplet energy harvesting panel based on the transistor-inspired bulk effect was proposed for energy harvesting from multiple droplet impacts. Furthermore, we demonstrated the potential of the droplet energy harvester as a promising power supply for wireless sensor systems and the emerging Internet of Things. Regarding the metal-semiconductor interfaces, we conducted an in-depth investigation of the underlying mechanism. We proposed a novel "triboelectric junction" model to analyze dynamic metal-semiconductor interfaces, with both formulaic analysis and experiment verification. Overall, this thesis is of great significance in advancing triboelectric energy harvesting technologies, while also powering the era of the Internet of Things and promoting the realization of the "blue energy" dream.