Browsing by Author "Chen, Yen-Chih"

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Characteristics of Cu2ZnSn(SXSe1-x)(4) thin-film solar cells prepared by sputtering deposition using single quaternary Cu2ZnSnS4 target followed by selenization/sulfurization treatment
(2017-04)
Lin, Yu-Pin
;
Hsieh, Tsung-Eong
;
Chen, Yen-Chih
;
Huang, Kun-Ping
;
材料科學與工程學系
This work demonstrates the preparation of Cu2ZnSn(SxSe1-x)(4)(CZTSSe) thin films by sputtering deposition using single-phase Cu2ZnSnS4 (CZTS) target followed by selenization/sulfurization treatment at 570 degrees C for 1 h. Afterward, the CZTSSe thin-film solar cell samples with the Mo/CZTSSe/CdS/i-ZnO/IZO/Al structure were prepared and their performances were evaluated. By varying the ratio of Se and S powders of heat treatment, the Cu-poor/Zn-rich CZTSSe layers with S/(S+Se) ratio in the range of 0.21-1 were achieved and the CZTSSe layers were the mixture of kesterite CZTS and CZTSe phases as revealed by the x-ray diffraction and the Raman spectroscopy analyses. UV-NIR spectroscopy indicated the bandgaps of CZTSSe samples are in the range of 1.06-1.45 eV when the S/(S+Se) ratio varies from 0.21 to 1. Hall measurement observed the best transport property with p-type carrier concentration of 2.17x10(15) cm(-3) and mobility of 8.9 cm(2) V-1 sec(-1) in CZTSSe layer with S/(S+Se) ratio of 0.46. Under the AM1.5 illumination condition, the CZTSSe thin-film solar cell sample with S/(S+Se) ratio of 0.46 exhibited the best performance with open-circuit voltage of 0.506 V, short-circuit current density of 27.41 mA/cm(2), fill factor of 50% and conversion efficiency of 6.9%.
Constraint relaxation and annealed belief propagation for binary networks
(2007)
Chen, Yen-Chih
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Su, Yu T.
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電信工程研究所
;
Institute of Communications Engineering
In this paper, we propose two novel generalized belief propagation (BP) algorithms to improve the convergence behavior of the conventional BP algorithm. By incorporating a dynamic temperature into the free energy formulation, message passing is performed on a dynamic surface of energy cost. The proposed cooling process helps BP converge to a stable fixed point with a lower energy value that leads to better estimations. For decoding turbo-like error correcting codes, we adopt a parametric Gaussian approximation to relax the binary parity check constraints and generalize the conventional binary networks as well. Both the computational complexity and the convergence rate of the proposed annealed BP algorithms are almost the same as those of the conventional BP algorithm. Simulated performance of the proposed algorithms when they are used to decode a low density parity check (LDPC) code and the (23,12) Golay code is presented to validate our proposals.
Formation of gradient Ga distribution in Cu(In,Ga)Se-2 thin-film solar cells prepared by (InGa)(2)Se-3/CuInGaSe2 stacking structure followed by Se-Vapor selenization
(2018-09-15)
Lin, Yu-Pin
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Hsieh, Tsung-Eong
;
Chen, Yen-Chih
;
Huang, Kun-Ping
;
材料科學與工程學系
Cu(In,Ga)Se-2 (CIGS) light absorption layers with gradient Ga distribution were prepared by utilizing the sequential deposition of (InGa)(2)Se-3 (IGS) and CIGS layers. The IGS buffer layers were deposited on Mo/soda lime glass substrate at various working pressures (WP = 1, 10 and 30 mtorr) followed by the deposition of CIGS layers. Afterward, the microstructures and compositions of CIGS layers formed by the selenization of CIGS/IGS stacking layers in Se vapor ambient at 560 degrees C for 1 h were investigated. The CIGS layer with the optimized microstructure and transport properties was then implanted in the thin-film solar cells and the device with the best conversion efficiency of 8.22 +/- 0.0754% was achieved. The satisfactory solar cell performance was ascribed to the insertion of IGS layer which remedies the deficiency of In, Ga and Se elements and forms the gradient Ga distribution in CIGS layer as revealed by the secondary ion mass spectroscopy.
Maximum likelihood DOA estimation based on the cross-entropy method
(2006)
Chen, Yen-Chih
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Su, Yu T.
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電信工程研究所
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Institute of Communications Engineering
In this paper, we propose two simulation based maximum likelihood (ML) methods to estimate the direction of arrival (DOA) by a novel combination of the Cross-Entropy (CE) method and the polynomial parameterization scheme. The CE method is an efficient stochastic approximation method for solving both discrete and continuous optimization problems. We bridge the ML approach and the stochastic search algorithm by properly randomizing the desired parameters. Numerical results show that the proposed CE-based algorithms yield highly accurate DOA estimation with fast convergence rate while requiring only linear processing complexity. Compared with the conventional iterative quadratic maximization likelihood (IQML) method, the proposed algorithms can alleviate the error propagation effect in low signal to noise ratio (SNR) region and asymptotically approach the Cramer-Rao bound in high SNR region.
MIMO Channel Estimation in Correlated Fading Environments
(2010-03-01)
Chen, Yen-Chih
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Su, Yu T.
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電子工程學系及電子研究所
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Department of Electronics Engineering and Institute of Electronics
This paper presents two analytic correlated multiple-input multiple-output (MIMO) block fading channel models and their time-variant extensions that encompass the popular Kronecker model and the more general Weichselberger model as special cases. Both static and time-variant models offer compact representations of spatial- and/or time-correlated channels. When the transmit antenna array is such that the associated MIMO channel has a small angle spread (AS), which occurs quite often in a cellular downlink, our models admit reduced-rank channel representations. They also provide compact channel state information (CSI) descriptions which are needed in feedback systems and in many post channel estimation applications. The latter has the important implication of reduced feedback channel bandwidth requirement and lower post-processing complexity. Based on one of the proposed channel models we present novel iterative algorithms for estimating static and time-variant MIMO channels. The proposed models make it natural to decompose each iteration of our algorithms into two successive stages that are responsible for estimating the correlation coefficients and the signal direction, respectively. Using popular industry-approved standard channel models, we verify through simulations that our algorithms yield good MSE performance which, in many practical cases, is better than that achievable by a conventional least-square estimator. The mean-squared error (MSE) performance of our estimators are analyzed and the resulting predictions are consistent with those estimated by simulations.
Model-Based Channel Estimation and Codeword Selection for Correlated MIMO Channels
(2012)
Cheng, Chien-Chun
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Chen, Yen-Chih
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Su, Yu T.
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Sari, Hikmet
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電機工程學系
For a closed loop MIMO system to provide outstanding performance, the receiver needs to feedback accurate channel state information, which in turn requires a large codebook size. In this paper, we present a new codeword selection scheme based on a model-based channel estimator. Our scheme performs simultaneous channel estimation and codeword selection by exploiting the structures of the associated MIMO spatial channel and the codebook used. For a DFT-based codebook with arbitrary size, only a quantization operation is needed to select a proper codeword in a correlated fading environment. Using industry-approved standard channel models in simulating the system performance, we show that our low-complexity codeword selection scheme does outperform the scheme based on the conventional least-square estimator.
Modelling and Estimation of Correlated MIMO-OFDM Fading Channels
(2011)
Cheng, Chien-Chun
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Chen, Yen-Chih
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Su, Yu T.
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傳播研究所
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Institute of Communication Studies
This paper presents a general reduced-rank channel model and a corresponding low-complexity estimation scheme for wideband spatial-correlated multiple-input multiple-output (MIMO) systems. We focus on orthogonal frequency division multiplex (OFDM) based systems. The proposed reduced-rank model is useful for many post-channel-estimation applications such as channel state information (CSI) feedback, precoder design and user/channel selection. Our work is an extension of an earlier investigation on narrowband MIMO channels. Like the narrowband case, the proposed wide-band channel estimator also offer the advantage of rendering both channel coefficients and mean angle of departure (AoD) simultaneously. By exploiting the time, frequency, and spatial correlations of the channel and with continuous-type pilot symbols, we found that, even with as high as a compression ratio of 1%, our channel estimator is capable of maintaining an acceptable mean squared errors (MSE) in highly correlated environments. Both mathematical analysis and computer simulation, based on some industry-approved standard channel models, indicate that our algorithm outperform the conventional least-square estimator within most range of interest.
Preparation of Cu(In,Ga)Se-2 nanoparticles via solvothermal method in conjunction with ball milling process and its applications to thin-film solar cells
(2019-06-30)
Chen, Yen-Chih
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Lin, Yu-Pin
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Hsieh, Tsung-Eong
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Huang, Mei-Wen
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材料科學與工程學系
Single-phase chalcopyrite CIGS particles with the sizes of 40-60 nm and the stoichiometry of Cu-26(.)01(In17.00Ga8.01)Se-48(.98) were synthesized by the solvothermal method. The CIGS precursor layers were prepared via the doctor-blade coating process utilizing the slurry containing CIGS nanoparticles and the CIGS layers were then formed by a selenization at 540 degrees C for various times in the Se vapor ambient. Microstructure and composition analyses indicated the CIGS layers are of the singe-phase chalcopyrite phase and the Cu-poor composition feature with stoichiometry close to the ideal ratio of 1:1:2. Moreover, the increase of selenization time benefited the coarsening of grain structure in CIGS layer as revealed by the scanning electron microscopy. Accordingly, the CIGS thin-film solar cell samples with the Mo/CIGS/ CdS/i-ZnO/AZO/Ag structure were prepared and their performance were evaluated. Under the AM1.5 illumination condition, the best PV performance with open-circuit voltage of 0.49 V, short-circuit current density of 29.46 mA/cm(2), fill factor of 54% and conversion efficiency of 7.75% was accomplished in the thin-film solar cell sample containing CIGS light absorption layer prepared by the 540 degrees C-selenization for 15 min. (C) 2019 Elsevier B.V. All rights reserved.
Preparation of Cu2ZnSnS4 (CZTS) sputtering target and its application to the fabrication of CZTS thin-film solar cells
(2016-01-05)
Lin, Yu-Pin
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Chi, Yi-Fang
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Hsieh, Tsung-Eong
;
Chen, Yen-Chih
;
Huang, Kun-Ping
The CuS, ZnS and SnS2 raw powders were adopted as the starting materials for preparing the 2-inch Cu2ZnSnS4 (CZTS) sputtering target. The powders were mixed at various molar ratios of Cu:Zn:Sn:S = 1.6:1:1:4, 1.8:1:1:4 and 2:1:1:4, pressed into 6-mm pellet forms, and then sintered at different temperatures for 8 h. Accordingly, the influence of powder constitution and heat-treatment conditions on the microstructure, composition and morphology of CZTS pellets were investigated. Microstructure and composition analyses indicated that the sample prepared with the Cu molar ratio of 1.6 and sintered at 600 degrees C for 8 h forms the kesterite CZTS single-phase structure with the Cu-poor/Zn-rich composition. The 2-inch CZTS sputtering target was prepared by utilizing the method established by the study of 6-mm pellets and adopted for depositing the CZTS thin films. The CZTS layer deposited at the working pressure of 1 mtorr followed by the sulfurization at 570 degrees C for 1 h possessed the stoichiometry of Cu24.15Zn14.73Sn10.75S50.37 which is the closest to the ideal stoichiometric ratio of 2:1:1:4. Moreover, it exhibited the bandgap of 1.5 eV, p-type carrier concentration of 2.8 x 1017 cm(-3) and mobility of 8.61 cm(2) V-1 s(-1). Such a CZTS layer was implanted in the thin-film solar cells with the device structure of Mo/CZTS/CdS/i-ZnO/IZO/Al. Under the AM1.5 illumination condition, the CZTS thin-film solar cell sample with the open-circuit voltage of 0.52 V, the short-circuit current density of 19.17 mA/cm(2), the fill factor of 52.7% and the conversion efficiency of 5.2% was achieved. (C) 2015 Published by Elsevier B.V.
Turbo Equalization of Nonlinear TDMA Satellite Signals
(2009-03-01)
Chen, Yen-Chih
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Su, Yu Ted
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電信工程研究所
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Institute of Communications Engineering
In this paper, we investigate a coded solution to compensate for the nonlinear distortion of TDMA satellite waveforms. Based on a Volterra-type channel model and the turbo principle, we present a turbo-like that includes a simple rate-I encoder at the transmit side in addition to a conventional channel encoder; the receive side iteratively equalizes the nonlinear channel effect and decodes the received symbols. Some other design alternatives are also explored and computer simulated performance is presented. Numerical results show that significant improvement over conventional approaches can be achieved by the proposed turbo system.