Browsing by Author "Tsai, Hung-Wei"

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Design of novel TiO2-SiO2 core-shell helical nanostructured anti-reflective coatings on Cu(In,Ga)Se-2 solar cells with enhanced power conversion efficiency
(2019-05-14)
Chen, Chia-Wei
;
Tsai, Hung-Wei
;
Wang, Yi-Chung
;
Su, Teng-Yu
;
Yang, Chen-Hua
Introducing an anti-reflective coating (ARC) is the last step of fabricating Cu(In,Ga)Se-2 solar cells with enhanced photo-generated current by reducing the reflectance of visible light. The use of nanostructures in ARC has been intensively investigated because this can prolong the optical path length by a scattering effect or by the formation of a refractive index gradient. Herein, we proposed the use of TiO2 helical nanostructures using glancing angle deposition (GLAD), and then synthesized SiO2 surrounding the helical nanostructures to further decrease the reflectance by producing a refractive index gradient. The optimized values of TiO2 and SiO2 thicknesses are 150 nm and 3 nm determined by the finite-difference time-domain (FDTD) simulation on Cu(In,Ga)Se-2 solar cells. The suppression of reflectance led to enhanced light absorption across a broad wavelength range, and boosted the J(SC) from 20.66 to 22.51 mA cm(-2), resulting in improved power conversion efficiency (PCE) from 6.32 to 7.00% after applying the TiO2-SiO2 core-shell nanostructures as the ARC. This 10.75% PCE enhancement suggests that the novel TiO2-SiO2 core-shell nanostructures have great potential as ARC for Cu(In,Ga)Se-2 solar cells, and it would be beneficial for development in the photovoltaic field in both research and applications.
Fabrication of large-scale single-crystal bismuth telluride (Bi2Te3) nanosheet arrays by a single-step electrolysis process
(2014)
Tsai, Hung-Wei
;
Wang, Tsang-Hsiu
;
Chan, Tsung-Cheng
;
Chen, Pei-Ju
;
Chung, Chih-Chun
Nanolizing of thermoelectric materials is one approach to reduce the thermal conductivity and hence enhance the figure of merit. Bismuth telluride (Bi2Te3)-based materials have excellent figure of merit at room temperature. For device applications, precise control and rapid fabrication for the nanostructure of thermoelectric materials are essential issues. In the present study, we demonstrate a one-step electrolysis process to directly form Bi2Te3 nanosheet arrays (NSAs) on the surface of bulk Bi2Te3 with controllable spacing distance and depth by tuning the applied bias and duration. The single sheet of NSAs reveals that the average thickness and electrical resistivity of single crystalline Bi2Te3 in composition are 399.8 nm and 137.34 mu Omega m, respectively. The formation mechanism of NSAs has been proposed. A 1.12% efficiency of quantum dot-sensitized solar cells with Bi2Te3 NSAs for counter electrode has been demonstrated, indicating that Bi2Te3 NSAs from top-down processing with a high ratio of surface area to volume are a promising candidate for possible applications such as thermoelectrics, dye-sensitized solar cells (DSSCs), and lithium-ion batteries.
Resistive switching of Au/ZnO/Au resistive memory: an in situ observation of conductive bridge formation
(2012-10-08)
Peng, Chung-Nan
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Wang, Chun-Wen
;
Chan, Tsung-Cheng
;
Chang, Wen-Yuan
;
Wang, Yi-Chung
A special chip for direct and real-time observation of resistive changes, including set and reset processes based on Au/ZnO/Au system inside a transmission electron microscope (TEM), was designed. A clear conducting bridge associated with the migration of Au nanoparticles (NPs) inside a defective ZnO film from anode to cathode could be clearly observed by taking a series of TEM images, enabling a dynamic observation of switching behaviors. A discontinuous region (broken region) nearby the cathode after reset process was observed, which limits the flow of current, thus a high resistance state, while it will be reconnected to switch the device from high to low resistance states through the migration of Au NPs after set process. Interestingly, the formed morphology of the conducting bridge, which is different from the typical formation of a conducting bridge, was observed. The difference can be attributed to the different diffusivities of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting path. The current TEM technique is quite unique and informative, which can be used to elucidate the dynamic processes in other devices in the future.