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Title: Electrospun nanostructures for solar energy harvesting and motion sensing
Authors: Ma, Shaoyang
Keywords: DRNTU::Engineering::Materials::Nanostructured materials
DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
Issue Date: 3-Jun-2019
Source: Ma, S. (2019). Electrospun nanostructures for solar energy harvesting and motion sensing. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: With the rapid development of human technology, we are continuously facing new problems and challenges. On one hand is the limited natural resources, and on the other hand is the increasing demands for convenient lifestyle. It is not a good idea to sacrifice one aspect to satisfy the other, but we should seek a balance between sustainable development and the comfortable living concept. Thus it becomes a very crucial task to explore the low cost, large scale and environmental friendly fabrication methods for wide applications. Electrospinning is a top-down method in which polymeric or melt components are drawn out from a solution system onto a collector by electrostatic force. In comparison with other methods, including drawing, template synthesis, chemical vapor deposition and so on, electrospinning offers some attractive features. One of the most important advantages is its industrial scalability, which makes it possible to directly transfer the results from lab research to the industry. In general, the unique advantages of electrospun nanostructures including high spatial interconnectivity, high porosity, and large surface-to-volume ratio, make it a promising fabrication method in energy harvesting applications. The objective of this work is to develop nanostructures used for solar energy harvesting and motion sensing through electrospinning techniques. First, titanium dioxide (TiO2) nanoporous hemispheres (NHSs) with a radius of ∼200 nm are fabricated for the construction of mesoporous TiO2 layer of perovskite solar cells (PSCs) by electrospraying. The resulting TiO2 NHSs are highly porous, which are beneficial to the infiltration of perovskites and provide a larger contact area, as building blocks for PSCs. By varying the TiO2 NHSs collecting period (15 s, 30 s, 60 s and 90 s) during the electrospraying process, the performance of PSCs changes with different TiO2 NHSs distribution densities. The optimized PSC employing TiO2 NHSs (60 s) exhibits a PCE as high as 19.3% with a short circuit current density (JSC) of 23.8 mA cm−2, an open circuit voltage (VOC) of 1.14 V and a fill factor (FF) of 0.71. Furthermore, the PSC possesses a reproducible PCE value with little hysteresis in its current density-voltage (J–V) curves. Second, hollow rice grain-shaped TiO2 nanostructures (NSs) with the radius of 200-300 nm are fabricated by electrospinning titanium isopropoxide solution and further calcination. The resulting hollow rice grain-shaped TiO2 NSs are applied as the building blocks to construct the mesoporous TiO2 layer for the large-area PSCs. By varying the spin coating speed (2000 rpm, 4000 rpm, and 8000 rpm, respectively), the performance of PSCs changes with different TiO2 NSs distribution densities. The optimized PSC employing the 4000 rpm spin coating speed exhibits a PCE of 14.2% with the JSC of 21.6 mA cm−2, VOC of 1.07 V and fill FF of 0.61, which is superior to both the plain structure based control group with the PCE of 9.6% and conventional meso-TiO2 based group with the PCE of 12.1%. Furthermore, the PSC possesses a reproducible PCE value with weak hysteresis in its J-V curves. Moreover, photoluminescence (PL) measurements and finite-difference time-domain (FDTD) optical simulations reveal the enhanced fast charge carrier extraction/transport and light absorption in the proposed system, which makes electrospun hollow rice grain-shaped TiO2 NSs a promising electron transportation material for high-efficiency and large-area photovoltaic devices. Last, electrospinning is a widely applied technique to produce an extended length of fiber-shaped piezoelectric devices. A stretching-induced alignment method is demonstrated to achieve highly oriented electrospun poly[(vinylidenefluoride-co-trifluoroethylene] P(VDF-TrFE) fibers on a large scale. These globally aligned electrospun P(VDF-TrFE) fibers exhibit an enhanced piezoelectric property and high mechanical endurance. Using this simple stretching method, a high average output voltage of 80% aligned electrospun P(VDF-TrFE) fibers is 84.96 mV, about 266% of their original randomly distributed counterpart. Furthermore, when woven into an outfit, the aligned electrospun P(VDF-TrFE) fiber bundle can work both individually and combined to monitor body gestures including angles of elbow bending and directions of a swinging arm, which may lead to the further development of motion-tracking technology in wearable smart devices.
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