THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Monday, July 20, 2020

1:30 PM
via

BlueJeans Video Conferencing

https://bluejeans.com/998041277

will be held the

DISSERTATION DEFENSE

for

Aurelia Wang

"Mechanism of Contact Electrification and Improved Performance of Triboelectric Nanogenerators via Surface Modification with Functional Materials”

Committee Members:

Prof. Zhiqun Lin, Advisor, MSE

Prof. Yulin Deng, ChBE

Prof. Meilin Liu, MSE

Prof. Preet Singh, MSE

Prof. Vladimir Tsukruk, MSE

Abstract:

Efforts to establish a general mechanism of contact electrification (CE) have spanned many centuries, and yet our understanding of this commonplace phenomenon remains to be largely explored. The past few years have witnessed significant advances in triboelectrification research due to the invention of triboelectric nanogenerators (TENGs) and their wide gamut of applications and innovations. Thus, it is imperative to understand CE. In this thesis, we unraveled the underlying mechanisms of CE using high temperature resistant TENGs. We 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 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. Investigation into sliding mode of 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 are even able to generate charges at high temperature without any pre-charging of the SiO2 substrates, which is essential for maintaining any surface charges in the contact-separation mode. The model for the sliding-mode TENG was found to 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 at high temperature, we developed a high temperature-resistant rotating TENG that is 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. With the progress achieved in this research, the published studies in this thesis have paved the way for further investigation of CE theory. 

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 perovskite CsyFAzMA1-y-zPb(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 open-circuit voltage VOC, 24.4% increase in short-circuit current ISC, and 17.5% increase in transferred charge QSC over the lowest performing sample with a CsI molar ratio of 0.14. This was due to increased RMS roughness and reduced grain size of the perovskite thin film layer. Also, a unique TENG triboelectric material design with CsyFAzMA1-y-zPb(IxBr1-x)3 paired with interfacial layers of semiconducting SnO2 as an electron transporting layer and P3HT as a hole transporting layer 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 polyazo microsphere-based TENG by using 450 nm blue linearly polarized light to irradiate said microspheres and yield elongations of up to an aspect ratio of 1.82 in the direction of light polarization. Interestingly, the 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, due to the elongation of the microspheres to increase surface contact area and maintenance of robustness of the microstructure layer. Clearly, the understanding of functional materials and the factors that impact the performance of TENGs for sensors and energy harvesting is of great importance to their eventual commercial applications.