Browsing by Author "Luo, Chih Wei"

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A Comprehensive Study of One-Step Selenization Process for Cu(In1-xGax) Se-2 Thin Film Solar Cells
(2017-03-21)
Chen, Shih-Chen
;
Wang, Sheng-Wen
;
Kuo, Shou-Yi
;
Juang, Jenh-Yih
;
Lee, Po-Tsung
In this work, aiming at developing a rapid and environmental-friendly process for fabricating CuIn1-xGaxSe2 (CIGS) solar cells, we demonstrated the one-step selenization process by using selenium vapor as the atmospheric gas instead of the commonly used H2Se gas. The photoluminescence (PL) characteristics indicate that there exists an optimal location with superior crystalline quality in the CIGS thin films obtained by one-step selenization. The energy dispersive spectroscopy (EDS) reveals that the Ga lateral distribution in the one-step selenized CIGS thin film is intimately correlated to the blue-shifted PL spectra. The surface morphologies examined by scanning electron microscope (SEM) further suggested that voids and binary phase commonly existing in CIGS films could be successfully eliminated by the present one-step selenization process. The agglomeration phenomenon attributable to the formation of MoSe2 layer was also observed. Due to the significant microstructural improvement, the current-voltage (J-V) characteristics and external quantum efficiency (EQE) of the devices made of the present CIGS films have exhibited the remarkable carrier transportation characteristics and photon utilization at the optimal location, resulting in a high conversion efficiency of 11.28%. Correlations between the defect states and device performance of the one-step selenized CIGS thin film were convincingly delineated by femtosecond pump-probe spectroscopy.
Crystalline Engineering Toward Large-Scale High-Efficiency Printable Cu(In,Ga)Se-2 Thin Film Solar Cells on Flexible Substrate by Femtosecond Laser Annealing Process
(2017-04-26)
Chen, Shih-Chen
;
She, Nian-Zu
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Wu, Kaung-Hsiung
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Chen, Yu-Ze
;
Lin, Wei-Sheng
Ink-printing method emerges as a viable way for manufacturing large-scale flexible Cu(In,Ga)Se-2 (CIGS) thin film photovoltaic (TFPV) devices owing to its potential for the rapid process, mass production, and low-cost nonvacuum device fabrication. Here, we brought the femtosecond laser annealing (fs-LA) process into the ink-printing CIGS thin film preparation. The effects of fs-LA treatment on the structural and optoelectronic properties of the ink-printing CIGS thin films were systematically investigated. It was observed that, while the film surface morphology remained essentially unchanged under superheating, the quality of crystallinity was significantly enhanced after the fs-LA treatment. Moreover, a better stoichiometric composition was achieved with an optimized laser scanning rate of the laser beam, presumably due to the much reduced indium segregation phenomena, which is believed to be beneficial in decreasing the defect states of In-se, V-Se, and In-cu. Consequently, the shunt leakage current and recombination centers were enhancement in photovoltaic conversion efficiency. both greatly decreased, resulting in a near 20%
Femtosecond Laser-Induced Formation of Wurtzite Phase ZnSe Nanoparticles in Air
(2012)
Wang, Hsuan I.
;
Tang, Wei Tsung
;
Liao, Li Wei
;
Tseng, Pei Shan
;
Luo, Chih Wei
We demonstrate an effective method to prepare wurtzite phase ZnSe nanoparticles from zincblende ZnSe single crystal using femtosecond pulse laser ablation. The fabricated ZnSe nanoparticles are in spherical shape and uncontaminated while synthesized under ambient environment. By controlling the laser fluences, the average size of ZnSe nanoparticles can be varied from similar to 16 nm to similar to 22 nm in diameter. In Raman spectra, the surface phonon mode becomes dominant in the smaller average particle size with uniform size distribution. The interesting phase transition from the zinc blende structure of ZnSe single crystal to wurtzite structure of ZnSe nanoparticles may have been induced by the ultrahigh ablation pressure at the local area due to the sudden injection of high energy leading to solid-solid transition.
Flexible Heteroepitaxy Photoelectrode for Photo-electrochemical Water Splitting
(2018-08-01)
Le Thi Quynh
;
Chien Nguyen Van
;
Tzeng, W. Y.
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Huang, Chun-Wei
;
Lai, Yu-Hong
MICAtronics represents a new research direction by taking the advantages of good mechanical flexibility and optical transparency of muscovite mica, leading to a new platform to use oxide heteroepitaxy for novel transparent soft technology. Here we report a model flexible photoelectrode for photo-electrochemical (PEC) water splitting based on Fe2O3/ZnO/mica heteroepitaxy. The heteroepitaxy was confirmed by a combination of X-ray diffraction and transmission electron microscopy. The PEC performance of this flexible and semitransparent photoelectrode under various bending states was investigated. We found that the photocurrent of the heteroepitaxial system was enhanced by three times relative to pure ZnO and Fe2O3 under visible-light irradiation, and the heteroepitaxial photoelectrodes retain their photocurrent after continuous bending in cycling (>3000 cycles) with a smallest bending radius of 3.5 mm. The energy band alignment and charge dynamics under light excitation were characterized to understand the mechanism of the enhanced PEC performance. This study provides a new platform for design and fabrication of a flexible transparent photoelectrode in the harvest of solar energy based on oxide heteroepitaxy.
Growth and characterization of Cu(In,Ga)Se-2 thin films by nanosecond and femtosecond pulsed laser deposition
(2014-06-02)
Chen, Shih-Chen
;
Hsieh, Dan-Hua
;
Jiang, Hsin
;
Liao, Yu-Kuang
;
Lai, Fang-I
In this work, CuIn1-xGaxSe2 (CIGS) thin films were prepared by nanosecond (ns)- and femtosecond (fs)-pulsed laser deposition (PLD) processes. Different film growth mechanisms were discussed in perspective of the laser-produced plasmas and crystal structures. The fs-PLD has successfully improved the inherent flaws, Cu-2 (-) Se-x, and air voids ubiquitously observed in ns-PLD-derived CIGS thin films. Moreover, the prominent antireflection and excellent crystalline structures were obtained in the fs-PLD-derived CIGS thin films. The absorption spectra suggest the divergence in energy levels of radiative defects brought by the inhomogeneous distribution of elements in the fs-PLD CIGS, which has also been supported by comparing photoluminescence (PL) spectra of ns- and fs-PLD CIGS thin films at 15 K. Finally, the superior carrier transport properties in fs-PLD CIGS were confirmed by fs pump-probe spectroscopy and four-probe measurements. The present results indicate a promising way for preparing high-quality CIGS thin films via fs-PLD.
Growth and characterization of topological insulator Bi2Se3 thin films on SrTiO3 using pulsed laser deposition
(2013-05-01)
Phuoc Huu Le
;
Wu, Kaung Hsiung
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Luo, Chih Wei
;
Leu, Jihperng
;
材料科學與工程學系
Bismuth selenide (Bi2Se3) thin films were grown on SrTiO3(111) (STO) substrates using pulsed laser deposition (PLD). The structural, morphological, electrical, and transport properties were studied at various substrate temperatures (T-S) from 120 to 350 degrees C. Amorphous films grown at T-S < 180 degrees C exhibited semiconducting behavior, and highly c-axis-oriented textured films deposited at T-S >= 180 degrees C exhibited metallic behavior. Bi2Se3 thin-films were epitaxially grown on STO substrates at 300 and 350 degrees C. Thickness-dependent characteristics were also investigated for optimized Bi2Se3 films deposited at T-S = 230 degrees C. The semiconducting or metallic characteristics of Bi2Se3 films prepared using PLD were observed through electrical and transport results. (C) 2013 Elsevier B. V. All rights reserved.
In-Situ Probing Plasmonic Energy Transfer in Cu(In, Ga)Se-2 Solar Cells by Ultrabroadband Femtosecond Pump-Probe Spectroscopy
(2015-12-18)
Chen, Shih-Chen
;
Wu, Kaung-Hsiung
;
Li, Jia-Xing
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Yabushita, Atsushi
;
Tang, Shih-Han
In this work, we demonstrated a viable experimental scheme for in-situ probing the effects of Au nanoparticles (NPs) incorporation on plasmonic energy transfer in Cu(In, Ga)Se-2 (CIGS) solar cells by elaborately analyzing the lifetimes and zero moment for hot carrier relaxation with ultrabroadband femtosecond pump-probe spectroscopy. The signals of enhanced photobleach (PB) and waned photoinduced absorption (PIA) attributable to surface plasmon resonance (SPR) of Au NPs were in-situ probed in transient differential absorption spectra. The results suggested that substantial carriers can be excited from ground state to lower excitation energy levels, which can reach thermalization much faster with the existence of SPR. Thus, direct electron transfer (DET) could be implemented to enhance the photocurrent of CIGS solar cells. Furthermore, based on the extracted hot carrier lifetimes, it was confirmed that the improved electrical transport might have been resulted primarily from the reduction in the surface recombination of photoinduced carriers through enhanced local electromagnetic field (LEMF). Finally, theoretical calculation for resonant energy transfer (RET)-induced enhancement in the probability of exciting electron-hole pairs was conducted and the results agreed well with the enhanced PB peak of transient differential absorption in plasmonic CIGS film. These results indicate that plasmonic energy transfer is a viable approach to boost high-efficiency CIGS solar cells.
Nanomechanical, structural, and transport properties of Bi3Se2Te thin films
(2016-09-15)
Phuoc Huu Le
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Chiu, Shao-Pin
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Jian, Sheng-Rui
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Luo, Chih Wei
;
Lin, Jiunn-Yuan
New ternary phase of Bi3Se2Te thin films were successfully grown through pulsed laser deposition with a single Bi2Se2Te crystal as the target. Bi-rich and Se-and Te-deficient compositions deposited at a substrate temperature (T-s) of 250 degrees C and helium gas pressures (P-He) ranging from 2.0 x 10(-5) to 6.5 x 10(-1) Torr aided the formation of the dominant Bi-rich phase: Bi3Se2Te. The films were grown epitaxially on Al2O3 (001) substrates with Bi3Se2Te [001]//Al2O3 [001] and Bi3Se2Te [110]//Al2O3 [210]. An increase in P-He remarkably enhanced the hardness and Young\'s modulus of the films, primarily because of the decrease in nanograin size, following the HallePetch relationship. Moreover, the Bi3Se2Te films present linear magnetoresistance under a high perpendicular magnetic field (B >= 4 T) and twodimensional weak antilocalization effect under a low B (+/- 1 T) which may be attributed to topological insulator surface state (TSS). Clearly, further theoretical calculations and experiments are needed to confirm the suggested interpretation. (C) 2016 Elsevier B.V. All rights reserved.
Optimal Te-doping in GaSe for non-linear applications
(2012-02-27)
Ku, Shin An
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Chu, Wei-Chen
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Luo, Chih Wei
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Andreev, Yu. M.
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Lanskii, Grigory
Centimeter-sized Te-doped GaSe ingots were grown from the charge compositions of GaSe with nominals 0.05, 0.1, 0.5, 1, and 3 mass% Te, which were identified as e -GaSe: Te (0.01, 0.07, 0.38, 0.67, and 2.07 mass%) single crystals. The evolution of the absorption peaks of the phonon modes E'((2)) (similar to 0.584 THz) and E"((2)) (1.77 THz) on Te-doping in GaSe: Te crystals was studied by THz time-domain spectroscopy. This study proposes that the evolution of both E'((2)) and E"((2)) absorption peaks correlates well with the optical quality of Te-doped GaSe crystals, which was confirmed by experimental results on the efficiency of THz generation by optical rectification. Maximal intensity of the absorption peak of the rigid layer mode E'((2)) is proposed as a criterion for identification of optimal Te-doping in GaSe crystals. (C) 2012 Optical Society of America
Superconductivity in textured Bi clusters/Bi2Te3 films
(2014-09-01)
Le, Phuoc Huu
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Tzeng, Wen-Yen
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Chen, Hsueh-Ju
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Luo, Chih Wei
;
Lin, Jiunn-Yuan
We report superconductivity at an onset critical temperature below 3.1 K in topological insulator similar to 200-nm-thick Bi2Te3 thin films grown by pulsed laser deposition. Using energy-dispersive X-ray spectroscopy elemental mapping and Auger electron spectroscopy elemental depth profiling, we clearly identified bismuth (Bi) precipitation and Bi cluster signatures. Superconductivity in the Bi2Te3 films was attributed to the proximity effect of Bi clusters precipitated on the surface of the Bi2Te3 films. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
Thermoelectric properties of bismuth-selenide films with controlled morphology and texture grown using pulsed laser deposition
(2013-11-15)
Phuoc Huu Le
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Liao, Chien-Neng
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Luo, Chih Wei
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Lin, Jiunn-Yuan
;
Leu, Jihperng
Polycrystalline, thermoelectric thin films of bismuth selenide (Bi2Se3) were grown on SiO2/Si (1 1 1) substrates, using pulsed laser deposition (PLD). Bi2Se3 films with highly c-axis-oriented and controlled textures were fabricated by maintaining the helium gas pressure (P) between 0.7 and 173 Pa and the substrate temperature (T-s) between 200 and 350 degrees C. The carrier concentration (n) of films decreased with increasing P, which was attributed to the increase of Se concentration from Se deficiency (P <= 6.7 Pa) to stoichiometry to slight Se enrichment (P >= 40 Pa). The Seebeck coefficient (S) was enhanced considerably because of the reduction in n, following the S similar to n(-2/3) relation approximately. The average grain size increased from approximately 100 to 500 nm when T-s was raised from 200 to 350 degrees C, resulting in enhanced carrier mobility (mu) and electrical conductivity (sigma) and a reduced full width at half maximum of (0 0 6) peaks. The shape of grains transformed from rice-like at T-s of 200-250 degrees C to layered-hexagonal platelets (L-HPs) or super-layered flakes (S-LFs) at T-s of 300-350 degrees C. Films that were grown at 300 degrees C and 40 Pa and contained highly c-axis oriented L-HPs possessed the highest power factor (PF =S-2 sigma), which reached 5.54 mu W cm(-1) K-2, where S=75.8 mu V/K and sigma = 963.8 S cm(-1). (C) 2013 Elsevier B.V. All rights reserved.
Thermoelectric properties of nanocrystalline Bi3Se2Te thin films grown using pulsed laser deposition
(2016-07-15)
Tuyen, Le Thi Cam
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Phuoc Huu Le
;
Luo, Chih Wei
;
Leu, Jihperng
;
材料科學與工程學系
This study reports the material and thermoelectric properties of n-type nanocrystalline Bi-Se-Te thin films grown using pulse laser deposition (PLD) with a Bi2Se2Te single crystal as the target. In order to optimize the transport and thermoelectric properties, Bi-Se-Te thin films are fabricated by maintaining the substrate temperature (T-S) between 200 and 350 degrees C and a helium gas pressure (P-He) from 0.027 to 86.7 Pa. Unlike the target, all deposited films unexpectedly exhibit Bi2Se2Te phase with highly c-axis orientation, because of the high re-evaporation rates of Se and Te at the elevated T-S >= 200 degrees C. The estimated grain (crystalline) size ranges between 18.4 and 39.4 nm, and this monotonically increases as TS increases. The Bi3Se2Te films demonstrate excellent electrical conductivities owning to the high carrier concentration in the order of 10(20) cm(-3). A window of deposition conditions for high thermoelectric power factors (PF) is T-S of 250-350 degrees C and P-He of 40 Pa. The optimal PF is 8.3 mu W/cmK(2) for the Bi3Se2Te film prepared at 250 degrees C and 40 Pa. (C) 2016 Elsevier B.V. All rights reserved.
Thermoelectric properties of nanostructured bismuth-telluride thin films grown using pulsed laser deposition
(2014-12-05)
Phuoc Huu Le
;
Liao, Chien-Neng
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Luo, Chih Wei
;
Leu, Jihperng
;
交大名義發表
Nanostructured n-type bismuth telluride (Bi2Te3) thin films were grown on SiO2/Si (100) substrates at argon ambient pressure (P-Ar) of 80 Pa by pulsed laser deposition (PLD). The effects of film morphologies, structures, and compositions on the thermoelectric properties were investigated. At a substrate temperature (T-s) of 220-340 degrees C, stoichiometric films with highly (001)-oriented and layered structures showed the best properties, with a carrier mobility mu of 83.9-122.3 cm(2)/Vs, an absolute Seebeck coefficient vertical bar alpha vertical bar of 172.8-189.7 mu V/K, and a remarkably high power factor (PF) of 18.2-24.3 mu W cm(-1) K-2. By contrast, the Te-rich films deposited at T-s <= 120 degrees C with (015)-preferred orientations and columnar-small grain structures or the Te-deficient film deposited at 380 degrees C with Bi4Te5 polyhedron structure possessed poor properties, with mu < 10.0 cm(2)/Vs, vertical bar alpha vertical bar < 54 mu V/K, and PFs <= 0.44 mu W cm(-1) K-2. The morphology of highly (001) oriented-layered structures and the stoichiometry predominantly contribute to the substantial enhancement of mu and alpha vertical bar, respectively, resulting in remarkable enhancement in PF. (C) 2014 Elsevier B.V. All rights reserved.
Thickness-dependent magnetotransport properties and terahertz response of topological insulator Bi2Te3 thin films
(2017-01-25)
Phuoc Huu Le
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Liu, Po-Tsun
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Luo, Chih Wei
;
Lin, Jiunn-Yuan
;
Wu, Kaung Hsiung
We report the thickness (d) dependence of structural, morphological, magneto-transport properties as well as terahertz (THz) response in the 0.25-2.25 THz range of topological insulator Bi2Te3 thin films grown by pulsed laser deposition (PLD). The Bi2Te3 films exhibit a highly c-axis orientation (textured films) and two Raman-active modes of E-g(2) at 102.5 cm(-1) and A(1g)(2) at 134.8 cm(-1). The films obtain relatively low carrier concentration between 2.0 x 10(19) and 4.4 x 10(19) cm(-3). The films present two-dimensional weak (2D WAL) antilocalization magnetoresistance in a low magnetic field (B) and the 2D WAL shows thickness dependence, suggesting the presence of topological surface states (TSS). The THz transmittance increases owing to increased resistivity (associated with a decreased carrier concentration) when film thickness decreases from 152 to 16 QL. The complex conductance is well described by the Drude-Lorentz model with three components of the Drude surface, Drude bulk, and phonon. The Drude spectral weight (SW) of the surface is dominant and independent of the thickness. Besides, the Drude SW of the bulk and phonon SW linearly increase with increasing thickness. (C) 2016 Elsevier B.V. All rights reserved.
THz Generation and Detection on Dirac Fermions in Topological Insulators
(2013-11-01)
Luo, Chih Wei
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Chen, Hsueh-Ju
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Tu, Chien Ming
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Lee, Chia Ching
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Ku, Shin An
Tuning Electronic Transport in a Self-Assembled Nanocomposite
(2014-06-01)
Chang, Wei Sea
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Liu, Heng-Jui
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Vu Thanh Tra
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Chen, Jhih-Wei
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Wei, Tzu-Chiao
Self-assembled nanocomposites with a high interface-to-volume ratio offer an opportunity to overcome limitations in current technology, where intriguing transport behaviors can be tailored by the choice of proper interactions of constituents. Here we integrated metallic perovskite oxide SrRuO3 - wurzite semiconductor ZnO nanocomposites to investigate the room-temperature metal - insulator transition and its effect on photoresponse. We demonstrate that the band structure at the interface can be tuned by controlling the interface-to-volume ratio of the nanocomposites. Photoinduced carrier injection driven by visible light was detected across the nanocomposites. This work shows the charge interaction of the vertically integrated multiheterostructures by incorporating a controllable interface-to-volume ratio, which is essential for optimization of the design and functionality of electronic devices.
Tuning of the Electro-Optical Properties of Tetraphenylcyclopentadienone via Substitution of Oxygen with Sterically-Hindered Electron Withdrawing Groups
(2019-09-04)
Coluccini, Carmine
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Anusha, Puliparambil Thilakan
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Chen, Hsin-Yi Tiffany
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Liao, Sheng-Lun
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Ko, Ying Kuan
In this report, the substitution of the oxygen group (=O) of Tetraphenylcyclopentadienone with =CR2 group (R= methyl ester or nitrile) was found to have tuned the electro-optical properties of the molecule. Although both groups are electrons withdrawing in nature, their absorption from UV-vis spectra analysis was observed to have been blue-shifted by methyl ester substitution and red-shifted by nitrile substitution. Interestingly, these substitutions helped to enhance the overall intensity of emission, especially in the context of methyl ester substitution whereby the emission was significantly boosted at higher concentrations due to hypothesized restrictions of intramolecular motions. These observations were explained through detailed descriptions of the electron withdrawing capability and steric properties of the substituents on the basis of density functional theory calculations.
Ultrafast relaxation dynamics of photoexcitations in poly(3-hexylthiophene) for the determination of the defect concentration
(2010-09-30)
Lee, Yu Hsien
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Yabushita, Atsushi
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Hsu, Chain Shu
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Yang, Sheng Hsiung
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Iwakura, Izumi
Ultrafast pump-probe spectroscopy of poly(3-hexylthiophene) (P3HT) films was performed using a 9-fs laser and a broadband lock-in detection system. The fast geometrical relaxation (GR) time was attributed to the transition from a free exciton (FE) to form a bound polaron pair (BPP), and the time constant was estimated to be tau(GR) = 90 +/- 2 fs. The relaxation time constant of BPP was determined as tau(BPP) = 710 +/- 40 fs. The measurement of pump-power dependence enabled us to distinguish the defect trapping process of BPPs from other parallel decay processes and to determine the defect concentration of a P3HT thin film. (C) 2010 Elsevier B.V. All rights reserved.
Ultrafast spectroscopy of poly-thiophene derivative by using sub-10 fs visible pulse
(2012)
Yabushita, Atsushi
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Lee, Yu Hsien
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Hsu, Chain Shu
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Yang, Sheng Hsiung
;
Luo, Chih Wei
Ultrafast spectroscopy was performed for a poly(3-hexylthiophene) (P3HT) film changing pump power. The result clarified the femtosecond time constant for generation and relaxation of the bound polaron pair in the film. (C) 2010 Optical Society of America
Understanding Multiferroic Hexagonal Manganites by Static and Ultrafast Optical Spectroscopy
(2013)
Wang, Yu Ting
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Luo, Chih Wei
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Kobayashi, Takayoshi
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電子物理學系
;
Department of Electrophysics
Multiferroic hexagonal manganites ReMnO3 studied by optics are reviewed. Their electronic structures were revealed by static linear and nonlinear spectra. Two transitions located at similar to 1.7 eV and similar to 2.3 eV have been observed and attributed to the interband transitions from the lower-lying Mn(3+)d(xy)/d(x2-y2) and d(xz)/d(yz) states to the Mn(3+)d(3z2-r2) state, respectively. These so-called d-d transitions exhibit a blueshift as decreasing temperatures and an extra blueshift near T-N. This dramatic change indicates that the magnetic ordering seriously influences the electronic structure. On the other hand, the ultrafast optical pump-probe spectroscopy has provided the important information on spin-charge coupling and spin-lattice coupling. Because of the strongly correlation between electronic structure and magnetic ordering, the amplitude of the initial rising component in Delta R/R shows striking changes at the vicinity of T-N. Moreover, the coherent optical and acoustic phonons were observed on optical pump-probe spectroscopy. Both the amplitude and dephasing time of coherent phonons also exhibit significant changes at T-N, which provide the evidence for spin-lattice interaction in these intriguing materials.