Browsing by Author "Chi, Gou-Chung"

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Approaching conversion limit with all-dielectric solar cell reflectors
(2015-02-09)
Fu, Sze Ming
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Lai, Yi-Chun
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Tseng, Chi Wei
;
Yan, Sheng Lun
;
Zhong, Yan Kai
Metallic back reflectors has been used for thin-film and wafer-based solar cells for very long time. Nonetheless, the metallic mirrors might not be the best choices for photovoltaics. In this work, we show that solar cells with all-dielectric reflectors can surpass the best-configured metal-backed devices. Theoretical and experimental results all show that superior large-angle light scattering capability can be achieved by the diffuse medium reflectors, and the solar cell J-V enhancement is higher for solar cells using all-dielectric reflectors. Specifically, the measured diffused scattering efficiency (D.S.E.) of a diffuse medium reflector is >0.8 for the light trapping spectral range (600nm-1000nm), and the measured reflectance of a diffuse medium can be as high as silver if the geometry of embedded titanium oxide(TiO2) nanoparticles is optimized. Moreover, the diffuse medium reflectors have the additional advantage of room-temperature processing, low cost, and very high throughput. We believe that using all-dielectric solar cell reflectors is a way to approach the thermodynamic conversion limit by completely excluding metallic dissipation. (C) 2015 Optical Society of America
Back-Contacted Thin-Film GaAs Solar Cells
(2016-01-01)
Hong, Chung-Yu
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Lin, Yu-Chen
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Ho, Kuan-Ying
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Tsai, Jia-Ling
;
Chien, Zhan-Tien
In this work, we present a thin-film GaAs solar cell employing via contacts for forming a back-contacted solar cell to eliminate light blocking effects and enhance photon recycling at the same time. The fabrication procedure and the characterization of the fabricated devices are discussed in this paper. From our measurement, a conversion efficiency of 6.5%, open circuit voltage of 0.95 V, and a current density of 8.6mA/cm(2) under one-sun illumination were obtained in a 5x5 mm cell.
Characteristics of the epitaxy of InGaN-based light-emitting diodes grown by nanoscale epitaxial lateral overgrowth using a nitrided titanium buffer layer
(2015-05-01)
Shieh, Chen-Yu
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Li, Zhen-Yu
;
Chang, Jenq-Yang
;
Chi, Gou-Chung
;
光電工程學系
In this work, a buffer layer of nitrided titanium (Ti) achieved through the nitridation of a Ti metal layer on a sapphire substrate was used for the epitaxial growth of InGaN-based light-emitting diodes (LEDs) achieved by low pressure metal-organic chemical vapor deposition. The effect of in-situ Ti metal nitridation on the performance of these InGaN-based LEDs was then investigated. It was very clear that the use of the nitrided Ti buffer layer (NTBL) induced the formation of a nanoscale epitaxial lateral overgrowth layer during the epitaxial growth. When evaluated by Raman spectroscopy, this epi-layer exhibited large in-plane compressive stress releasing with a Raman shift value of 567.9 cm(-1). Cathodoluminescence spectroscopy and transmission electron microscopy results indicated that the InGaN-based LEDs with an NTBL have improved crystal quality, with a low threading dislocations density being yielded via the strain relaxation in the InGaN-based LEDs. Based on the results mentioned above, the electroluminescence results indicate that the light performance of InGaN-based LEDs with an NTBL can be enhanced by 45% and 42% at 20 mA and 100 mA, respectively. These results suggest that the strain relaxation and quality improvement in the GaN epilayer could be responsible for the enhancement of emission power. (C) 2015 Published by Elsevier B.V.
Chemical vapor deposition growth of bilayer graphene via altering gas flux geometry
(2019-11-30)
Petrov, Stefan
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Rafailov, Peter M.
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Marinova, Vera
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Lin, Shiuan-Huei
;
Lai, Yi-Chun
Graphene films with high content of bilayer graphene are obtained by chemical vapor deposition technique on copper (Cu) foil by means of suitably designing the flux geometries of the reactant gases. First single-layer graphene growth is optimized using a preliminary oxygen passivation of the Cu substrate which facilitates the formation of larger graphene grains with low defect density. Then, in order to enhance the transport of reactant species to the catalyst substrate, the gas flux geometry is modified in two ways: (i) fixing a graphite holder next to the Cu substrate perpendicular to the stream of the flux gases or (ii) positioning the Cu foil into a graphite shelter. It is found that both modifications facilitate the growth of a second graphene layer leading to increased formation of bilayer graphene. The obtained graphene layers are characterized by Raman spectroscopy (with special focus on the overtone (2D) of the disorder-induced band), scanning electron microscopy, transmission electron microscopy, atomic force microscope analysis and optical transmission measurements. Narrow symmetric 2D Raman peaks with linewidths ranging down to 22 cm(-1) but blue-shifted with respect to that of single-layer graphene are obtained from graphite holder- and shelter-grown samples indicating high-quality bilayer graphene.
Doping of Monolayer Graphene for Silicon based Schottky Junction Solar Cells
(2013)
Lai, Yi-Chun
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Wu, Bing-Shu
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Yu, Shu-Cheng
;
Yu, Peichen
;
Chi, Gou-Chung
Silicon-based hybrid solar cells have garnered extensive attentions in the photovoltaics industry due to easy processing attributes and high optical absorption and outstanding carrier mobility of silicon. Among all, indium tin oxide (ITO)/silicon solar cells have achieved a power conversion efficiency of 13% due to excellent conductivity, transmittance and applicable surface potential of ITO. However, the cost of ITO has risen significantly recently due to the deficiency of indium. Therefore, graphene has been an inexpensive alternative to ITO. For solar cell applications, graphene plays an important role as transparent electrodes (TE) with tunable work functions for efficient carrier collection. Therefore, graphene-based Schottky junction solar cells (SJSC) on crystalline silicon thin films hold great promises for low-cost photovoltaics owing to potentials for high efficiency and rapid production on flexible substrates. According to previous reports, the key factors to achieve a highly efficient SJSC include excellent transparence and conductance, as well as tunable work functions. Herein, we demonstrate a single layer graphene/n-Si Schottky junction solar cell that exhibits a power conversion efficiency (PCE) of 1.2 % under one-sun AM 1.5G illumination, and an integrated short-circuit photocurrent of 18.3 mA/cm(2) from the external quantum efficiency measurement. The transmittance of the monolayer graphene in this device is over 97 % and the sheet resistance is around 800 to 1200 Omega/square. Furthermore, we investigate a doping method involving bis(trifluoromethanesulfonyl)-amid (TFSA) for the monolayer graphene to improve the separation and collection of photogenerated carriers in the SJSC. The preliminary data show that the sheet resistance is decreased rapidly from 1200 to 300 Omega/square. and the surface potential is also adjusted by the chemical doping. Currently, device fabrication with doped monolayer graphene is still in process and complete characterization data will be presented.
Effect of rapid thermal annealing on Mg(x)Zn(1-x)O films prepared by radio-frequency magnetron sputtering
(2010-07-01)
Hsueh, Kuang-Po
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Tun, Chun-Ju
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Chiu, Hsien-Chin
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Huang, Yu-Ping
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Chi, Gou-Chung
This study investigates the effects of thermal annealing on the MgxZn1-xO films. MgxZn1-xO films were deposited by a radio-frequency magnetron sputtering system using a 6 in. ZnO/MgO (80/20wt%) target. The Hall results, x-ray diffraction (XRD), transmittance, and x-ray photoelectron spectroscopy (XPS) were measured. The XRD results indicate that the appearance of only (111) peaks in the as-grown MgxZn1-xO film is a sign of the cubic single phase, whereas the appearance of ZnO (002) peaks in MgxZn1-xO films annealed at 700 and 800 C confirms the formation of a wurtzite single-phase crystal. The existence of a weak (002) -wurtzite peak besides the (111)-cubic peak indicates the coexistence of two phases. The absorption spectra of MgxZn1-xO annealed at 700 and 800 C show two stages at wavelengths of 357 and 261 nm. The XPS spectra of MgxZn1-xO films were also demonstrated. The results of this study show that the ZnO films were separated from MgxZn1-xO films after higher thermal annealing. c 2010 American Vacuum Society. [DOI: 10.1116/1.3442476]
Effect of rapid thermal annealing on MgxZn1-xO films prepared by radio-frequency magnetron sputtering
(2010-07-01)
Hsueh, Kuang-Po
;
Tun, Chun-Ju
;
Chiu, Hsien-Chin
;
Huang, Yu-Ping
;
Chi, Gou-Chung
This study investigates the effects of thermal annealing on the MgxZn1-xO films. MgxZn1-xO films were deposited by a radio-frequency magnetron sputtering system using a 6 in. ZnO/MgO (80/20wt%) target. The Hall results, x-ray diffraction (XRD), transmittance, and x-ray photoelectron spectroscopy (XPS) were measured. The XRD results indicate that the appearance of only (111) peaks in the as-grown MgxZn1-xO film is a sign of the cubic single phase, whereas the appearance of ZnO (002) peaks in MgxZn1-xO films annealed at 700 and 800 C confirms the formation of a wurtzite single-phase crystal. The existence of a weak (002) -wurtzite peak besides the (111)-cubic peak indicates the coexistence of two phases. The absorption spectra of MgxZn1-xO annealed at 700 and 800 C show two stages at wavelengths of 357 and 261 nm. The XPS spectra of MgxZn1-xO films were also demonstrated. The results of this study show that the ZnO films were separated from MgxZn1-xO films after higher thermal annealing. c 2010 American Vacuum Society. [DOI: 10.1116/1.3442476]
Effects of semiconductor processing chemicals on conductivity of graphene
(2012-07-01)
Chen, Chung Wei
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Ren, F.
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Chi, Gou-Chung
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Hung, S. C.
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Huang, Y. P.
"Graphene layers on SiO2/Si substrates were exposed to chemicals or gases commonly used in semiconductor fabrication processes, including solvents (isopropanol, acetone), acids, bases (ammonium hydroxide), UV ozone, H2O, and O-2 plasmas. The recovery of the initial graphene properties after these exposures was monitored by measuring both the layer resistance and Raman 2D peak position as a function of time in air or vacuum. Solvents and UV ozone were found to have the least affect, while oxygen plasma exposure caused an increase of resistance of more than 3 orders of magnitude. Recovery is accelerated under vacuum but changes can persist for more than 5 h. Careful design of fabrication schemes involving graphene is necessary to minimize these interactions with common processing chemicals. (C) 2012 American Vacuum Society. [http://dx.doi.org/10.1116/1.4732517]"
Efficiency and droop improvement in green InGaN/GaN light-emitting diodes on GaN nanorods template with SiO2 nanomasks
(2012-12-03)
Lin, Da-Wei
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Lee, Chia-Yu
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Liu, Che-Yu
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Han, Hau-Vei
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Lan, Yu-Pin
This study presents the green InGaN/GaN multiple quantum wells light-emitting diodes (LEDs) grown on a GaN nanorods template with SiO2 nanomasks by metal-organic chemical vapor deposition. By nanoscale epitaxial lateral overgrowth, microscale air voids were formed between nanorods and the threading dislocations were efficiently suppressed. The electroluminescence measurement reveals that the LEDs on nanorods template with SiO2 nanomasks suffer less quantum-confined Stark effect and exhibit higher light output power and lower efficiency droop at a high injection current as compared with conventional LEDs. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4768950]
Efficiency droop behavior improvement through barrier thickness modification for GaN-on-silicon light-emitting diodes
(2015-03-05)
Tzou, An-Jye
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Lin, Bing-Cheng
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Lee, Chia-Yu
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Lin, Da-Wei
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Liao, Yu-Kuang
Crack-free GaN-based light-emitting diodes (LEDs) were grown on 150-mm-diameter Si substrates by using low-pressure metal-organic chemical vapor deposition. The relationship between the LED devices and the thickness of quantum barriers (QBs) was investigated. The crystal quality and surface cracking of GaN-on-Si were greatly improved by an AlxGa1-xN buffer layer composed of graded Al. The threading dislocation density of the GaN-on-Si LEDs was reduced to < 7 x 10(8) cm(-2), yielding LEDs with high internal quantum efficiency. Simulation results indicated that reducing the QB thickness improved the carrier injection rate and distribution, thereby improving the droop behavior of the LEDs. LEDs featuring 6-nm-thick QBs exhibited the lowest droop behavior. However, the experimental results showed an unanticipated phenomenon, namely that the peak external quantum efficiency (EQE) and light output power (LOP) gradually decreased with a decreasing QB thickness. In other words, the GaN-on-Si LEDs with 8-nm-thick QBs exhibited low droop behavior and yielded a good peak EQE and LOP, achieving a 22.9% efficiency droop and 54.6% EQE. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
Efficiency Enhancement of Organic/GaAs Hybrid Photovoltaic Cells Using Transparent Graphene as Front Electrode
(2016-03)
Huang, Chi-Hsien
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Yu, Shu-Chen
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Lai, Yi-Chun
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Chi, Gou-Chung
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Yu, Peichen
Large-area graphene of high quality and uniformity was successfully grown by chemical vapor deposition (CVD) using surface oxidation treatment of copper foil prior to the graphene growth step. The graphene was transferred to the polyethylene terephthalate (PET) substrate (G/PET) to act as a transparent front electrode of hybrid heterojunction photovoltaic (PV) cells; these cells were based on a structural motif of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as the p-type semiconductor, GaAs (1 0 0) as the inorganic n-type semiconductor, and a silver comb electrode atop of PEDOT: PSS. By using G/PET as a transparent front electrode, the power conversion efficiency (PCE), under simulated AM1.5G illumination conditions, was greatly enhanced by up to 26% (from 6.85% to 8.60%). All PV characteristics, including open-circuit voltage (V-oc), short-circuit current (J(sc)), and fill factor (FF), contributed to this PCE enhancement. The reflectance, external quantum efficiency, and dark current were investigated to explain this observed PCE enhancement. Although two layers of graphene can efficiently reduce the sheet resistance, the reduction of transmittance in multilayer cells resulted in lower short-circuit current density, leading to lower PCE, in comparison with those with only one layer of graphene.
Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate
(2014-09-16)
Lee, Chia-Yu
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Tzou, An-Jye
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Lin, Bing-Cheng
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Lan, Yu-Pin
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Chiu, Ching-Hsueh
The flip chip ultraviolet light-emitting diodes (FC UV-LEDs) with a wavelength of 365 nm are developed with the ex situ reactive plasma deposited (RPD) AlN nucleation layer on patterned sapphire substrate (PSS) by an atmospheric pressure metal-organic chemical vapor deposition (AP MOCVD). The ex situ RPD AlN nucleation layer can significantly reduce dislocation density and thus improve the crystal quality of the GaN epitaxial layers. Utilizing high-resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the crystalline quality of the epitaxial layer with the (RPD) AlN nucleation layer is better than that with the low-temperature GaN (LT-GaN) nucleation layer. The threading dislocation density (TDD) is estimated by transmission electron microscopy (TEM), which shows the reduction from 6.8 x 10(7) cm(-2) to 2.6 x 10(7) cm(-2). Furthermore, the light output power (LOP) of the LEDs with the RPD AlN nucleation layer has been improved up to 30 % at a forward current of 350 mA compared to that of the LEDs grown on PSS with conventional LT-GaN nucleation layer.
Efficient hybrid white light-emitting diodes by organic-inorganic materials at different CCT from 3000K to 9000K
(2015-04-06)
Chen, Kuo-Ju
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Lai, Yi-Chun
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Lin, Bin-Cheng
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Lin, Chien-Chung
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Chiu, Sheng-Huan
The hybrid white light-emitting didoes (LED) with polyfluoren (PFO) polymer and quantum dot (QD) was investigated using dispensing method at the different correlated color temperature (CCT) for cool and warm color temperature. This result indicates that the hybrid white LED device has the higher luminous efficiency than the convention one, which could be attributed to the increased utilization rate of the UV light. Furthermore, the CIE 1931 coordinate of high quality white hybrid LED with different CCT range from 3000K to 9000K is demonstrated. Consequently, the angular-dependent CCT and the thermal issue of the hybrid white LED device were also analyzed in this study. (C) 2015 Optical Society of America
Enhanced Light Output Power and Growth Mechanism of GaN-Based Light-Emitting Diodes Grown on Cone-Shaped SiO2 Patterned Template
(2013-04-01)
Lin, Da-Wei
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Huang, Jhih-Kai
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Lee, Chia-Yu
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Chang, Ruey-Wen
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Lan, Yu-Pin
In this study, we successfully transferred the patterns of a cone-shaped patterned sapphire substrate (CPSS) into SiO2 layer to fabricate a cone-shaped SiO2 patterned template by using nanoimprint lithography (NIL). The GaN-based light-emitting diodes (LEDs) were grown on this template by metal-organic chemical vapor deposition (MOCVD). The transmission electron microscopy (TEM) images suggest that the stacking faults formed near the cone-shaped SiO2 patterns during the epitaxial lateral overgrowth (ELOG) can effectively suppress the threading dislocations, which results in an enhancement of internal quantum efficiency. The Monte Carlo ray-tracing simulation reveals that the light extraction efficiency of the LED grown on cone-shaped SiO2 patterned template can be enhanced as compared with the LED grown on CPSS. As a result, the light output power of the LED grown on cone-shaped SiO2 patterned template outperformed the LED grown on CPSS.
Fabrication of Flexible White Light-Emitting Diodes from Photoluminescent Polymer Materials with Excellent Color Quality
(2017-10-11)
Lin, Huang-Yu
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Sher, Chin-Wei
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Lin, Chih-Hao
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Tu, Hsien-Hao
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Chen, Xin Yin
This study developed flexible light-emitting diodes (LEDs) with warm white and neutral white light. A simple ultraviolet flip-chip sticking process was adopted for the pumping source and combined with polymer and quantum dot (QD) films technology to yield white light. The polymer-blended flexible LEDs exhibited higher luminous efficiency than the QD-blended flexible LEDs. Moreover, the polymer-blended LEDs, achieved excellent color-rendering index (CRI) values (Ra = 96 and R9 = 96), with high reliability, demonstrating high suitability for special applications like accent, down, or retrofit lights in the future. In places such as a museum, kitchen, or surgery room, its high R9 and high CRI characteristics can provide high-quality services.
Growth of InN on Si (111) by atmospheric-pressure metal-organic chemical vapor deposition using InN/AlN double-buffer layers
(2008-07-01)
Li, Zhen-Yu
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Lan, Shan-Ming
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Uen, Wu-Yih
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Chen, Ying-Ru
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Chen, Meng-Chu
Indium nitride (InN) epilayers have been successfully grown on Si (111) substrates with low-temperature (450 degrees C) grown InN and high-temperature (1050 degrees C) grown AlN (InN/AlN) double-buffer layers by atmospheric-pressure metal-organic chemical vapor deposition (AP-MOCVD). X-ray diffraction characterizations indicated that highly (0001)-oriented hexagonal InN was grown on Si (111) substrate. Photoluminescence (PL) analyses performed at room temperature showed a strong emission at 0.72 eV with a full width at half maximum of 121 meV. Excitation intensity dependent measurements demonstrated the PL mechanism to be the band-to-band transition. Time-resolved PL could be fitted by a single exponential exhibiting an ordered film and a recombination lifetime of around 0.85 ns. In particular, transmission electron microscopy characterizations indicated that the use of AlN first buffer is very important to achieve a structurally uniform (0001)-oriented InN epilayer on Si (111) by AP-MOCVD. (C) 2008 American Vacuum Society.
High Color-Conversion Efficiency CdSe Quantum Dots Hybrid Light Emitted Diode
(2016)
Liu, Che-Yu
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Kao, Tsung Sheng
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Chen, Tzu-Pei
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Chi, Gou-Chung
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Kuo, Hao-Chung
In this work, we have successfully demonstrated that the color-conversion efficiency in colloidal quantum dots can be greatly enhanced by light-emitting diodes (LEDs) with novel structures of nanorods embedded in microholes. Via an integrated technology of the nano-imprint and photolithography, nanorods structures can be fabricated at specific locations, generating functional nanostructured LEDs for high-efficiency light-emitting performance. With such a structured LED, the color-conversion efficiency of the existing quantum dots can be enhanced up to 12.5 %.
High performance 380-nm ultraviolet light-emitting-diodes with 3% efficiency droop by using free-standing GaN substrate manufacturing from GaAs substrate
(2014-07-07)
Shieh, Chen-Yu
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Tsai, Ming-Ta
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Li, Zhen-Yu
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Kuo, Hao-Chung
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Chang, Jeng-Yang
We investigated the influence of free-standing GaN (FS-GaN) substrates on the performance of ultraviolet light-emitting-diodes (UV-LEDs) grown on top by atmospheric pressure metal-organic chemical vapor deposition. High-resolution double-crystal x-ray diffraction (HRDCXD) analysis demonstrated high-order satellite peaks and clear fringes between them for UV-LEDs grown on the FS-GaN substrate, from which the interface roughness was estimated. In addition, the full width at half maximum of the HRDCXD rocking curve in the (0002) and the (1012) reflections were reduced to below 90 arc sec. The Raman results indicated that the GaN-based epilayer of strain free was grown. Additionally, the effect of the FS-GaN substrate on the crystal quality of the UV-LEDs was examined in detail by transmission electron microscopy (TEM). The TEM characterizations revealed no defects and V-pits were found in the scanned area. Based on the results mentioned above, the light output power of UV-LEDs on the FS-GaN substrate can be enhanced drastically by 80% and 90% at 20 and 100 mA, respectively. Furthermore, an ultralow efficiency degradation of about 3% can be obtained for the UV-LEDs on the FS-GaN substrate at a high injection current. The use of an FS-GaN substrate is suggested to be effective for improving the emission efficiency and droop of UV-LEDs grown thereon.
High-Efficiency and Crack-Free InGaN-Based LEDs on a 6-inch Si (111) Substrate With a Composite Buffer Layer Structure and Quaternary Superlattices Electron-Blocking Layers
(2014-05-01)
Li, Zhen-Yu
;
Lee, Chia-Yu
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Lin, Da-Wei
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Lin, Bing-Cheng
;
Shen, Kun-Ching
In this paper, a composite buffer layer structure (CBLS) with multiple AlGaN layers and grading of Al composition/u-GaN1/(AlN/GaN) superlattices/u-GaN2 and InAlGaN/AlGaN quaternary superlattices electron-blocking layers (QSLs-EBLs) are introduced into the epitaxial growth of InGaN-based light-emitting diodes (LEDs) on 6-inch Si (111) substrates to suppress cracking and improve the crystalline quality and emission efficiency. The effect of CBLS and QSLs-EBL on the crystalline quality and emission efficiency of InGaN-based LEDs on Si substrates was studied in detail. Optical microscopic images revealed the absence of cracks and Ga melt-back etching. The atomic force microscopy images exhibited that the root-meansquare value of the surface morphology was only 0.82 nm. The full widths at half maxima of the (0002) and (1012) reflections in the double crystal X-ray rocking curve were similar to 330 and 450 arcs, respectively. The total threading dislocation density, revealed by transmission electron microscopy, was <6 x 10(8) cm(-2). From the material characterizations, described above, blue and white LEDs emitters were fabricated using the epiwafers of InGaNbased LEDs on 6-inch Si substrates. The blue LEDs emitter that comprised blue LEDs chip and clear lenses had an emission power of 490 mW at 350 mA, a wall-plug efficiency of 45% at 350 mA, and an efficiency droop of 80%. The white LEDs emitter that comprised blue LEDs chip and yellow phosphor had an emission efficiency of similar to 110 lm/W at 350 mA and an efficiency droop of 78%. These results imply that the use of a CBLS and QSLs-EBL was found to be very simple and effective in fabricating high-efficiency InGaN-based LEDs on Si for solid-state lighting applications.
Highly Efficient InGaN-based LED with Embedded Cubic Airvoids
(2014)
Lin, Da-Wei
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Huang, Jhih-Kai
;
Liu, Che-Yu
;
Chang, Ruey-Wen
;
Wang, Sheng-Wen
Highly efficient InGaN-based LEDs with embedded sidewall passivation cubic airvoids made by nanoimprint lithography were demonstrated. The LEDs with embedded airvoids exhibit a 45% enhancement of light output at 20 mA compared with conventional LEDs.