Mechanism of contact electrification and improved performance of triboelectric nanogenerators via surface modification with functional materials

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Wang, Aurelia Chi
Lin, Zhiqun
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Efforts to establish a universal explanation of contact electrification (CE) have spanned many centuries, and yet our understanding of this commonplace phenomenon remains to be explored. With the landscape of triboelectrification research changing so dramatically in the past few years due to the invention of triboelectric nanogenerators (TENGs) and their wide gamut of applications and innovations, it is imperative that we make efforts to understand CE moving forward. Through our research, we have unraveled the underlying mechanisms of CE using high temperature resistant TENGs. We have correlated the exponential decay trends of surface charges on the contact separation Ti-SiO2 and Ti-Al2O3 TENGs at high temperatures to the model of thermionic emission of electrons. With this knowledge in hand, we have developed a surface states model for CE in metal-semiconductor pairs with clear energy levels and an electron cloud potential-well model for CE in metal-dielectric or dielectric-dielectric pairs, which could be extended to universal solid-solid CE. Investigating sliding mode Ti-SiO2 TENGs at high temperature with a similar material scheme resulted in an extension of those models. We also determined that the sliding mode TENGs were even able to generate charges at high temperature without any pre-charging of the SiO2 substrates, which was essential for maintaining any surface charges in the contact-separation mode. We determined that the model for the sliding mode TENG should be area-dependent, and when the displaced area is greater than the area in contact, thermionic emission has the potential to overtake the charge generation capabilities of the TENG. Following our studies on thermionic emission and charge degradation on surfaces in high temperature, we developed a high temperature-resistant rotating TENG that was capable of generating and maintaining an appreciable level of charge even at 673 K through pre-annealing at similarly high temperatures to achieve extremely tight contact. Functional materials that are robust and capable of being tuned for optimal triboelectric charge generation are of great interest to the development of hybrid and sensing TENG applications. We investigated the effects of precursor modulation on the surface morphology of CsFAMAPb(IxBr1-x)3 thin films fabricated through a facile, one-step spin coating process. We found that lowering the CsI molar ratio in the precursor solution to 0.02 produced the highest improvement in TENG output, resulting in a 26.5% increase in VOC, 24.4% increase in ISC, and 17.5% increase in QSC over the lowest performing sample with a CsI molar ratio of 0.14. Also, a unique TENG triboelectric material design with CsFAMAPb(IxBr1-x)3 paired with interfacial layers of SnO2 and P3HT exhibited improved electrical output performance, particularly in VOC, under solar simulator illumination compared to without any carrier charge transport layers. Finally, effective electrical output improvements were made on a PAzoMA microsphere-based TENG by using 450 nm blue LPL to irradiate said microspheres and produce elongations of up to an aspect ratio of 1.82 in the direction of light polarization. It was found that irradiation at 300 mW cm-2 for 3 hours produced the greatest TENG output improvement over microspheres without light treatment, with a 16% increase in VOC, 20.8% increase in ISC, and 10.7% increase in QSC. The understanding of functional materials and the factors that impact their performance in TENGs for sensors and energy harvesting is of great importance to their eventual commercial usage.
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