Browsing by Author "Tsai, Yu-Lin"

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4 Tunability and Optimization of Coupling Efficiency in Tamm Plasmon Modes
(2015-07-01)
Chang, Che-Yuan
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Chen, Yi-Hsuan
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Tsai, Yu-Lin
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Kuo, Hao-Chung
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Chen, Kuo-Ping
A Tamm plasmon polariton is a plasmonic resonance excited at the boundary between a photonic crystal and a metal. In this paper, a novel approach based on admittance loci is proposed to demonstrate the relation between thin-film structures and the corresponding Tamm plasmon modes. The tunability of the resonance wavelength and optimization of coupling efficiency are demonstrated. In addition, by using different metals to couple Tamm plasmon modes in the visible spectrum, silver has 4.7 times larger Q-factor than gold and 84 times larger than aluminum at 700 nm. The near-field enhancement of silver Tamm plasmon modes could be up to eight times larger than incident EM waves.
A Dual-Gate InGaZnO4-Based Thin-Film Transistor for High-Sensitivity UV Detection
(2019-08-01)
Chen, Po-Hsun
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Tsao, Yu-Ching
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Chien, Yu-Chieh
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Chiang, Hsiao-Cheng
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Chen, Hua-Mao
UV-sensing devices have received significant recent attention for applications in areas such as human health, fire detection, and optical communication. One key factor for product commercialization is determining the optimal materials that allow for integration of excellent UV-sensing properties with compatibility with industrial fabrication processes. However, current UV sensors often fail to achieve this due to either mismatched materials or a device that must be excessively large in order to produce enough photocurrent for UV detection. The UV-light-sensing properties of an amorphous InGaZnO4 (IGZO) thin-film transistor with a dual-gate structure and relatively small device size (width/length = 50 mu m/10 mu m) that achieves high sensitivity through a threshold-voltage-(V-th)-adjustment method is proposed. Comparing the drain currents under UV exposure to those under darkened conditions indicates that the ratio between the photoinduced and dark current reaches 10(6). Furthermore, the UV sensitivity of the dual-gate transistors can be adjusted by varying the bottom gate voltage, with each pixel of the sensor then being read out separately via scan line pulses. This allows the dual-gate a-IGZO transistor to be used for high-performance UV sensing while being effectively integrated in display applications.
A Highly Efficient Hybrid GaAs Solar Cell Based on Colloidal-Quantum-Dot-Sensitization
(2014-07-18)
Han, Hau-Vei
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Lin, Chien-Chung
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Tsai, Yu-Lin
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Chen, Hsin-Chu
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Chen, Kuo-Ju
This paper presents a hybrid design, featuring a traditional GaAs-based solar cell combined with various colloidal quantum dots. This hybrid design effectively boosts photon harvesting at long wavelengths while enhancing the collection of photogenerated carriers in the ultraviolet region. The merits of using highly efficient semiconductor solar cells and colloidal quantum dots were seamlessly combined to increase overall power conversion efficiency. Several photovoltaic parameters, including short-circuit current density, open circuit voltage, and external quantum efficiency, were measured and analyzed to investigate the performance of this hybrid device. Offering antireflective features at long wavelengths and luminescent downshifting for high-energy photons, the quantum dots effectively enhanced overall power conversion efficiency by as high as 24.65% compared with traditional GaAs-based devices. The evolution of weighted reflectance as a function of the dilution factor of QDs was investigated. Further analysis of the quantum efficiency response showed that the luminescent downshifting effect can be as much as 6.6% of the entire enhancement of photogenerated current.
A Novel Structure to Reduce Degradation Under Mechanical Bending in Foldable Low Temperature Polysilicon TFTs Fabricated on Polyimide
(2020-05-01)
Wang, Yu-Xuan
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Chang, Ting-Chang
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Huang, Shin-Ping
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Tai, Mao-Chou
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Zheng, Yu-Zhe
In this study, a novel structural device, named as a wing-shaped TFT, is proposed and is demonstrated to prevent fast deterioration under mechanical bending in foldable devices. After long-term mechanical bending, standard low-temperature polysilicon TFTs exhibit instability behaviors such as sub-channel formation, serious threshold voltage shifts, an on-current (I-ON) increase, and an off-current (I-OFF) decrease. These phenomena are believed to be due to additional oxide traps along the channel-width edges of the gate insulator after bending. In order to suppress this degradation, a wing-shaped structure is proposed. By extending the active layer, degradation regions are shifted away from the main channel. Therefore, the reliability of the devices is enhanced. The transfer characteristics show enhanced electrical behavior after being held for 48 hours in a static bent condition, with an 88% drop in threshold voltage shift as well as a 55% decrease in subthreshold swing degradation, proving better durability of the foldable TFT to bending stress for devices with these wings. Time-dependent transfer characteristics, a COMSOL simulation, and device geometry are discussed to support our findings.
A skew freeform reflector design method for prescribed off-axis irradiance
(2019-06-01)
Su, Jian-Jia
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Tien, Chung-Hao
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Tsai, Yu-Lin
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Wu, Chin-Tien
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應用數學系
In this paper we proposed a new methodology for designing freeform reflectors tailored to redistribute the radiant flux emitted by a point light source onto a target plane with the prescribed irradiance pattern. Based on the laws of reflection and energy conservation, a skew freeform model with a set of second-order ordinary differential equations formulating the ray tracing between the source and target domain was presented. A 3D freeform reflector was constructed by using such skew freeform model with quadric surface fitting accordingly. Proposed scheme is applicable for arbitrary geometric-based mapping without the restrictions subject to the target, source, collective solid angle, or target topography, respectively.
A ZnO/InN/GaN Heterojunction Photodetector with Extended Infrared Response
(2015)
Hsu, Lung-Hsing
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Hsu, Shun-Chieh
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Lee, Hsin-Ying
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Tsai, Yu-Lin
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Lin, Da-Wei
An extended infrared photoresponse is observed in a ZnO/InN/GaN heterojunction diode with InN grown by MOCVD. The external quantum efficiency is measured between 1200 and 1800 nm and can be as high as 3.55%.
Abnormal hysteresis formation in hump region after positive gate bias stress in low-temperature poly-silicon thin film transistors
(2020-09-30)
Tu, Hong-Yi
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Chang, Ting-Chang
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Tsao, Yu-Ching
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Tai, Mao-Chou
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Tsai, Yu-Lin
Degradation in low-temperature polycrystalline-silicon thin-film transistors after electrical stress was thoroughly investigated in this work. Main channel degradation, abnormal hump generation and hysteresis appearing in the hump region can be observed after positive bias stress. Furthermore, the difference in subthreshold swing (SS) values between forward/reverse sweep is observed. The electron trapping into the gate insulator (GI) dominates the main degradation and the hump generation. Additionally, the difference in SS values which appears in the hump region is attributed to the interface traps and the hysteresis is caused by electron trapping/detrapping into GI.
Analysis of Negative Bias Temperature Instability Degradation in p-Type Low-Temperature Polycrystalline Silicon Thin-Film Transistors of Different Grain Sizes
(2019-11-01)
Tu, Hong-Yi
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Tsao, Yu-Ching
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Tai, Mao-Chou
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Chang, Ting-Chang
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Tsai, Yu-Lin
This letter investigates degradation after negative bias temperature instability (NBTI) stress applied to LTPS TFTs with different polycrystalline-silicon grain sizes. The initial characteristics of the LTPS TFTs are similar regardless of grain size; however, we observed a different degree of degradation after NBTI depending on grain size. In general, after NBTI, both grain boundary traps and interface traps were generated. We found that the degree of NBTI degradation is dominated by the concentration of grain boundary traps, which themselves are a result of the different grain sizes that occur due to excimer laser annealing energy. At initial, dangling bonds in the grain boundaries and at the interface are passivated by hydrogen atoms, hence the initial characteristics are similar. Since the large grain of poly-Si initially generates more dangling bonds in the grain boundaries, after NBTI, hydrogen depassivation generates more grain boundary traps and causes much more serious degradation in device performance.
Angle-resolved characteristics of silicon photovoltaics with passivated conical-frustum nanostructures
(2011-09-01)
Tseng, Ping-Chen
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Yu, Peichen
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Chen, Hsin-Chu
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Tsai, Yu-Lin
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Han, Hao-Wei
Passivation plays a critical role in silicon photovoltaics, yet how a passivation layer affects the optical characteristics of nano-patterned surfaces has rarely been discussed. In this paper, we demonstrate conical-frustum nanostructures fabricated on silicon solar cells using polystyrene colloidal lithography with various silicon-nitride (SiN(x)) passivation thicknesses. The omnidirectional and broadband antireflective characteristics were determined by utilizing angle-resolved reflectance spectroscopy. The conical-frustum arrays with a height of 550 nm and a SiN(x) thickness of 80 nm effectively suppressed the Fresnel reflection in the wavelength range from 400 to 1000 nm, up to an incidence angle of 60 degrees. As a result, the power conversion efficiency achieved was 13.39%, which showed a 9.13% enhancement compared to that of a conventional KOH-textured silicon cell. The external quantum efficiency measurements confirmed that the photocurrent was mostly contributed by the increased optical absorption in the near-infrared. The angular cell efficiencies were estimated and showed improvements over large angles of incidence. (C) 2011 Elsevier B.V. All rights reserved.
Approach to analytically minimize the LCD moire by image-based particle swarm optimization
(2015-10-01)
Tsai, Yu-Lin
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Tien, Chung-Hao
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光電工程學系
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Department of Photonics
In this paper, we proposed a methodology to optimize the parametric window of a liquid crystal display (LCD) system, whose visual performance was deteriorated by the pixel moire arising in between multiple periodic structures. Conventional analysis and minimization of moire patterns are limited by few parameters. With the proposed image-based particle swarm optimization (PSO), we enable a multivariable optimization at the same time. A series of experiments was conducted to validate the methodology. Due to its versatility, the proposed technique will certainly have a promising impact on the fast optimization in LCD design with more complex configuration. (C) 2015 Optical Society of America
Breakthrough to Non-Vacuum Deposition of Single-Crystal, Ultra-Thin, Homogeneous Nanoparticle Layers: A Better Alternative to Chemical Bath Deposition and Atomic Layer Deposition
(2017-04-01)
Liao, Yu-Kuang
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Liu, Yung-Tsung
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Hsieh, Dan-Hua
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Shen, Tien-Lin
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Hsieh, Ming-Yang
Most thin-film techniques require a multiple vacuum process, and cannot produce high-coverage continuous thin filmswith the thickness of a fewnanometers on rough surfaces. We present a new "paradigm shift" non-vacuum process to deposit high-quality, ultra-thin, single-crystal layers of coalesced sulfide nanoparticles (NPs) with controllable thickness down to a few nanometers, based on thermal decomposition. This provides high-coverage, homogeneous thickness, and large-area deposition over a rough surface, with little material loss or liquid chemical waste, and deposition rates of 10 nm/min. This technique can potentially replace conventional thin-film deposition methods, such as atomic layer deposition (ALD) and chemical bath deposition (CBD) as used by the Cu(In, Ga) Se2 (CIGS) thin-film solar cell industry for decades. We demonstrate 32% improvement of CIGS thin-film solar cell efficiency in comparison to reference devices prepared by conventional CBD deposition method by depositing the ZnS NPs buffer layer using the new process. The new ZnS NPs layer allows reduction of an intrinsic ZnO layer, which can lead to severe shunt leakage in case of a CBD buffer layer. This leads to a 65% relative efficiency increase.
Bridging the "green gap" of LEDs: giant light output enhancement and directional control of LEDs via embedded nano-void photonic crystals
(2016-01-01)
Tsai, Yu-Lin
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Liu, Che-Yu
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Krishnan, Chirenjeevi
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Lin, Da-Wei
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Chu, You-Chen
Green LEDs do not show the same level of performance as their blue and red cousins, greatly hindering the solid-state lighting development, which is the so-called "green gap". In this work, nano-void photonic crystals (NVPCs) were fabricated to embed within the GaN/InGaN green LEDs by using epitaxial lateral overgrowth (ELO) and nano-sphere lithography techniques. The NVPCs act as an efficient scattering back-reflector to outcouple the guided and downward photons, which not only boost the light extraction efficiency of LEDs with an enhancement of 78% but also collimate the view angle of LEDs from 131.5 degrees to 114.0 degrees. This could be because of the highly scattering nature of NVPCs which reduce the interference giving rise to Fabry-Perot resonance. Moreover, due to the threading dislocation suppression and strain relief by the NVPCs, the internal quantum efficiency was increased by 25% and droop behavior was reduced from 37.4% to 25.9%. The enhancement of light output power can be achieved as high as 151% at a driving current of 350 mA. Giant light output enhancement and directional control via NVPCs point the way towards a promising avenue of solid-state lighting.
Dandelion-shaped nanostructures for enhancing omnidirectional photovoltaic performance
(2013)
Kuo, Shou-Yi
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Hsieh, Ming-Yang
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Han, Hau-Vei
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Lai, Fang-I
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Tsai, Yu-Lin
Broadband and omnidirectional light harvesting is important in photovoltaic technology because of its wide spectral range of radiation and the sun's movement. This study reports the fabrication and characterization of zinc oxide (ZnO) dandelions on Cu(In,Ga)Se-2 (CIGS) solar cells. The fabrication of dandelions involves the combination of self-assembled polystyrene (PS) nanospheres and the hydrothermal method, which is one of the simplest and cheapest methods of fabricating a three-dimensional, closely packed periodic structure. This study also investigates the optimization on dimension of the PS nanospheres using the rigorous coupled-wave analysis (RCWA) method. This study uses an angle-resolved reflectance spectroscope and a homemade rotatable photo I-V measurement to investigate the omnidirectional and broadband antireflections of the proposed dandelion structure. Under a simulated one-sun condition and a light incident angle of up to 60 degrees, cells with ZnO dandelions arrays enhanced the short-circuit current density by 31.87%. Consequently, ZnO dandelions are suitable for creating an omnidirectionally antireflective coating for photovoltaic devices.
Effect of a-InGaZnO TFT Channel Thickness under Self-Heating Stress
(2019-09-18)
Tai, Mao-Chou
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Chang, Po-Wen
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Chang, Ting-Chang
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Tsao, Yu-Ching
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Tsai, Yu-Lin
In this work, indium-gallium-zinc-oxide thin film transistors (IGZO-TFTs) with different channel thicknesses were compared after self-heating stress (SHS). Although TFTs with a thicker channel have higher current during stress conditions, less degradation is reported. This degradation, manifest as the threshold voltage shift in the transfer characteristics, is well described by the stretched-exponential equation. In addition, the average effective barrier height, Et, is extracted to compare the difference in degradation between both TFTs, and finds that the barrier height in the thick channel device is twice that of the thin channel device. Finally, a COMSOL simulation is performed to demonstrate the electrical difference at the gate insulator layer and confirm such phenomena. (C) 2019 The Electrochemical Society.
Efficiency enhancement of InGaN/GaN multiple quantum well solar cells using CdS quantum dots and distributed Bragg reflectors
(2013)
Tsai, Yu-Lin
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Lin, Chien-Chung
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Han, Hau-Vei
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Chen, Hsin-Chu
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Chen, Kuo-Ju
In recent year, InGaN-based alloy was also considered for photovoltaic devices owing to the distinctive material properties which are benefit photovoltaic performance. However, the Indium tin oxide (ITO) layer on top, which plays a role of transparent conductive oxide (TCO), can absorb UV photons without generating photocurrent. Also, the thin absorber layer in the device, which is consequent result after compromising with limited crystal quality, has caused insufficient light absorption. In this report, we propose an approach for solving these problems. A hybrid design of InGaN/GaN multiple quantum wells (MQWs) solar cells combined with colloidal CdS quantum dots (QDs) and back side distributed Bragg reflectors (DBRs) has been demonstrated. CdS QDs provide down-conversion effect at UV regime to avoid absorption of ITO. Moreover, CdS QDs also exhibit anti-reflective feature. DBRs at the back side have effectively reflected the light back into the absorber layer. CdS QDs enhance the external quantum efficiency (EQE) for light with wavelength shorter than 400 nm, while DBRs provide a broad band enhancement in EQE, especially within the region of 400 nm similar to 430 nm in wavelength. CdS QDs effectively achieved a power conversion efficiency enhancement as high as 7.2% compared to the device without assistance of CdS QDs. With the participation of DBRs, the power conversion efficiency enhancement has been further boosted to 14%. We believe that the hybrid design of InGaN/GaN MQWs solar cells with QDs and DBRs can be a method for high efficiency InGaN/GaN MQWs solar cells.
Embedded biomimetic nanostructures for enhanced optical absorption in thin-film solar cells
(2011-07-04)
Tsai, Min-An
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Han, Hao-Wei
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Tsai, Yu-Lin
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Tseng, Ping-Chen
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Yu, Peichen
Light-management is critical to thin film solar cells due to their usually limited optical absorption in the active layer. Conventional approaches involve employing separate techniques for anti-reflection and light trapping. Here, we demonstrate an embedded biomimetic nanostructure (EBN) that achieves both effects for hydrogenated amorphous silicon (a-Si: H) solar cells. The fabrication of EBNs is accomplished by patterning an index-matching silicon-nitride layer deposited on a glass substrate using polystyrene nanospheres lithography, followed by reactive ion etching. The profile of EBN is then reproduced layer by layer during the deposition of a-Si: H cells. We show that a solar cell with an optimized EBN exhibits a broadband enhanced external quantum efficiency due to both antireflection and light-trapping, with respect to an industrial standard cell using an Asahi U glass substrate which is mostly optimized for light trapping. Overall, the cell with an optimized EBN achieves a large short-circuit current density of 17.74 mA/cm(2), corresponding to a 37.63% enhancement over a flat control cell. The power conversion efficiency is also increased from 5.36% to 8.32%. Moreover, the light management enabled by the EBN remains efficient over a wide range of incident angles up to 60 degrees, which is particularly desirable for real environments with diffused sun light. The novel patterning method is not restricted to a-Si:H solar cells, but is also widely applicable to other thin film materials. (C) 2011 Optical Society of America
Enhance Current Density and Power Conversion Efficiency in Solar Cells by Using Luminescent Downshifting Phosphors
(2014-01-01)
Han, Hau-Vei
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Lin, Chien-Chung
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Lin, Wein-Yi
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Tsai, Yu-Lin
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Shen, Tien-Lin
The silicate phosphor on the surface of GaAs solar cell are demonstrated to have promising potential for efficient solar spectrum utilization. The phosphor is a common utilized material in white light LED package, so the cost of phosphor is relatively low and the stability is good. The solar cells showed an enhancement of 22.8% in short-circuit current density and approximately 25.3% in power conversion efficiency when coated with silicate phosphor particles. The current analyses and experiments conclude that the phosphor particles not only play as luminescent downshifting centers in the ultraviolet region but also serve as an anti-reflection coating for enhancing the light absorption in the overall spectral response of external quantum efficiency.
Enhanced efficiency for c-Si solar cell with nanopillar array via quantum dots layers
(2011-09-12)
Chen, Hsin-Chu
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Lin, Chien-Chung
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Han, Hao-Wei
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Tsai, Yu-Lin
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Chang, Chia-Hua
The enhanced efficiency of the crystalline silicon (c-Si) solar cell with nanopillar arrays (NPAs) was demonstrated by deployment of CdS quantum dots (QDs). The NPAs was fabricated by the colloidal lithography and reactive-ion etching techniques. Under a simulated one-sun condition, the device with CdS QDs shows a 33% improvement of power conversion efficiency, compared with the one without QDs. For further investigation, the excitation spectrum of photoluminescence (PL), absorbance spectrum, current-voltage (I-V) characteristics, reflectance and external quantum efficiency of the device was measured and analyzed. It is noteworthy that the enhancement of efficiency could be attributed to the photon down-conversion, the antireflection, and the improved electrical property. (C) 2011 Optical Society of America
Enhanced light harvesting of nitride-based nanopillars covered with ZnO using indium-tin oxide nanowhiskers
(2014-04-01)
Hsu, Lung-Hsing
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Lin, Chien-Chung
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Lee, Hsin-Ying
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Huang, Jhih-Kai
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Han, Hau-Vei
Nitride-based nanopillars were successfully fabricated by nanoimprint lithography. A nanowhisker of indium-tin oxide (ITO) deposited by on oblique evaporation method was investigated in nitride-based nanopillars and thin ZnO layers grown by atomic layer deposition (ALD). From the results of field-emission scanning electron microscopy (SEM) measurement, it was found that ITO whiskers grew on nitride-based nanopillars covered with ZnO. Moreover, from the results of UV-visible spectrophotometry and bidirectional reflectance distribution function (BRDF) measurements, it was found that this hybrid structure of ITO nanowhiskers above a ZnO medium enhanced the broadband and angle-independent antireflection in the range between 380 and 600nm. We used the hybrid design of the ITO/ZnO structure to achieve the lowest reflectance value between 3.8 and 10.9% in a quantum well absorption range. (C) 2014 The Japan Society of Applied Physics
Enhanced power conversion efficiency in InGaN-based solar cells via graded composition multiple quantum wells
(2015-11-30)
Tsai, Yu-Lin
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Wang, Sheng-Wen
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Huang, Jhih-Kai
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Hsu, Lung-Hsing
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Chiu, Ching-Hsueh
This work demonstrates the enhanced power conversion efficiency (PCE) in InGaN/GaN multiple quantum well (MQWs) solar cells with gradually decreasing indium composition in quantum wells (GQWs) toward p-GaN as absorber. The GQW can improve the fill factor from 42% to 62% and enhance the short current density from 0.8 mA/cm(2) to 0.92 mA/cm(2), as compares to the typical MQW solar cells. As a result, the PCE is boosted from 0.63% to 1.11% under AM1.5G illumination. Based on simulation and experimental results, the enhanced PCE can be attributed to the improved carrier collection in GQW caused by the reduction of potential barriers and piezoelectric polarization induced fields near the p-GaN layer. The presented concept paves a way toward highly efficient InGaN-based solar cells and other GaN-related MQW devices. (C) 2015 Optical Society of America