The  primary  objective  of  this  modeling  investigation  is  to  optimize  a  multi-junction tandem  device under  the AM1.5G  spectrum. Based on previous studies, GaAs and Ge cells, because of their energy band gaps, can be combined together to achieve high-efficiency double-junction devices. In this study, the top cell is made of GaAs (1.42 eV) while the bottom cell is made of Ge (0.66 eV). In order to avoid the losses and design constraints observed in two-terminal and four-terminal devices. In order to determine the optimal structure of the device,  the top and bottom junctions were investigated and  optimized with regard to the thicknesses . The optimum configuration of the device shows an efficiency of 32.07% under the AM1.5G spectrum and one sun, which is higher than the efficiency of an optimized single-junction Si cell under the same illumination conditions. Keywords: AMPS-1D, multi-junction, Ge, GaAs, optimization

Abderrachid, Helmaoui

Benmoussa, Dennai

Hassane, Ben Slimane

English

International Institute for Science, Technology and Education (IISTE): E-Journals

Journal of Energy Technologies and Policy                                                                   www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.3, No.3, 2013  9 Optimization of a Tandem Solar Cell GaAs / Ge using AMPS-1D  Dennai Benmoussa* Ben Slimane Hassane,  Helmaoui Abderrachid Physics laboratory in semiconductor devices, University of  Bechar, Algeria  * E-mail of the corresponding author: deennai_benmoussa@yahoo.com  Abstract The  primary  objective  of  this  modeling  investigation  is  to  optimize  a   mult i-junction tandem  device under  the AM1.5G  spectrum. Based on previous studies, GaAs and Ge cells, because of their energy band gaps, can be combined together to achieve high-efficiency double-junction devices. In this study, the top cell is made of GaAs (1.42 eV) while the bottom cell is made of Ge (0.66 eV). In order to avoid the losses and design constraints observed in two-terminal and four-terminal devices.  In order to determine the optimal structure of the device,  the top and bottom junctions were investigated and  optimized with regard to the thicknesses . The optimum configuration of the device shows an efficiency of 32.07% under the AM1.5G spectrum and one sun, which is higher than the efficiency of an optimized  single-junction Si cell under the same illumination conditions.   Keywords: AMPS-1D, multi-junction, Ge, GaAs, optimizat ion.  1. Introduction Single-junction photovoltaic devices have limitations in the ability to utilize efficiently the photons of the broad solar spectrum ranging from 300 nm to 2500 nm. For  instance, in the case of  Si  solar  cells, they cannot absorb photons with wavelengths longer than 1100 nm, which  represents more than 20% of  the  standard  terrestrial normal rad iation at AM1.5 [1]. Short wavelengths in the ultraviolet region also are not converted efficiently by Si solar cells because of thermalizat ion effects. Because solar cells only  convert photons with specific wavelengths efficiently, stacking solar cells made of different materials (i.e . d ifferent  energy band gaps) together proved to be a good approach to increase the efficiency of photovoltaic devices, and many devices made of a stack of single-junction cells have been  proposed by many research groups [2]. Indeed, efficiencies as high as 41.3% and 32.6% have been reported for  triple-junction and double-junction devices under 343 and 1026 suns, respectively, using the AM1.5 spectrum [3] The vertical mult i-junctions (VMJs) cell consists of a number of non-monolithic edge-illuminated junctions connected together in parallel or in series. The series VMJs cell is fabricated by metalizing, stacking and alloying a number of silicon wafers together. Then, cutting and sizing processes take place. The series VMJs cell has the property of high voltage at a low current. The light is incident on the VMJs cell parallel to the junctions rather than perpendicular to them and the carriers generated by the long wavelength part of the light spectrum have the same probability to be collected by the junction as those generated by short wavelength part. Thus, these VMJs cells have uniformly wide spectral responses. The output current of the series connected VMJs cell is limited by the lowest current of a  certain junction. The parameters of top illuminated monolithic series connected multi-junctions such as the energy gap, the thickness, the doping concentration and others have to be carefully  designed to ensure current matching between junctions. How-ever, since all the wafers are similar for edge-illuminated series VMJs, the use of uniform light ensures similar currents in each unit cell. The important benefit of the series VMJs solar cell lies in its capability of operation at very high intensities [4]. The application of such high light intensities requires good cooling techniques for these solar cells and good design to the concentrators. Based on the above, we expect a  tandem device with materials with energy band gaps close to 0.66 eV and 1.42 eV to be highly efficient. In this paper, we propose a two-junction solar cell device having GaAs (1.42 eV) as a top cell and Ge (0.66 eV) as a bottom cell. The two cells are connected back to back. In the next section, we present the optimization of the structure of the top cells that lead to the best tandem GaAs / Ge device.   Journal of Energy Technologies and Policy                                                                   www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.3, No.3, 2013  10 2.Optimal Device Structure The major objectives of numerical modeling and simulation in solar cell research are testing the validity of proposed physical structures, geometry on cell performance and fitting of modelling output to experimental results. Any numerical program capable of solving the basic semiconductor equations could be used for modeling thin film solar cells. The fundamental equations for such numerical programs are (i) Poisson’s equation for the distributions of electric field (φ) inside the device and (ii) the equation of continuity for conservation of electrons and holes currents. [5] The AMPS-1D program has been developed for pragmat ically simulate the electrical characteristics of multi-junction solar cells. It has been proven to be a very powerful tool in understanding device operation and physics for single crystal, poly -crystal and amorphous structures. To date, more than 200 g roups worldwide have been using AMPS-1D for solar cell design [6].  One-dimensional AMPS-1D simulator has been used to investigate the effect of different top cell layers. the structure of conventional solar cell are shown in the Fig.1. The GaAs layers thickness was varied from 1 μm to 5 μm and the change of performance parameters are observed.     Figure 1.Structure of the solar cell The base parameters used for different structures adopted from some standard references are shown in Table I Table1 Parameters used In simulation. Layers   Parameters      P-Ge n- Ge Thickness(μm) 0.1-0.2 0.5-3 1 3 Dielectric constant, ε 13.10 13.10 16.2 16.2 Electron mobility   (cm²/Vs) 8500 8500 3900 3900 Hole mobility (cm²/Vs) 400 400 1900 1900 Carrier ensity,  P:1E17 n:8E16 p:5E17 n:5E17 Optical band gap, (eV) 1.42 1.42 0.66 0.66 Effective density,  4.7E+17 4.7E+17 1.1E19 1.1E19 Effective density,  7.5E+18 7.5E+18 5.0E018 5.0E018 Electron affinity ,χ(eV) 4.07 4.07 4.38 4.38  3. Results and Discussions The simulation work has been performed aiming to compare the different types of cell structure made by changing thickness of The emitter layers GaAs bottom cell and base layers GaAs bottom cell to  find out best structure for higher efficiency and more stable GaAs / Ge solar cells. The effect of bottom cell  on performance such as effect on general performance parameters, quantum efficiency (QE), shunt and series resistance, light and dark I-V characteristics. Journal of Energy Technologies and Policy                                                                   www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.3, No.3, 2013  11 0,10 0,12 0,14 0,16 0,18 0,202526272829303132Device total efficiency(%) layer thicknesses(µm)a Figure 2.Perfo rmance depending on the thickness of the emitting layer of the top cell. Figure 3. Performance depending on the thickness of the base layer of the top cell. Figure 2 shows the device total efficiency (i.e. the sum of the efficiency of top and bottom  cells) versus the thickness of the emitter layer o f the top cell. The results on the figure show that the optimum thickness is 0.1 microns. Figure 3shows  the device  total efficiency versus  the thickness  of  the  base  layer  of  the  top  cell.  The results indicate that the optimum thickness is about 1.7microns as for the bottom cell. Device Operation From the above results are obtained using the AMPS-1D software we can determine the solar cell which has the best performance while giving the thickness of each layer o Table 2. 0,5 1,0 1,5 2,0 2,5 3,02526272829303132Device total efficiency(%) layer thicknesses(µm)bJournal of Energy Technologies and Policy                                                                   www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.3, No.3, 2013  12 Table 2.Thickness of each layer         µm  µm  µm The current-voltage and power-voltage characteristics generated by the GaAs /Ge optimized device  under  the  AM1.5G  spectrum  and  one  sun  are  displayed  in  figure  3  for  mult ijunction solar cell. The corresponding PV parameters (open-circuit vo ltage Voc, short-circuit current Isc, fill-factor FF and efficiency (η) are all summarized in Table 3.  Table 3. PV parameters of the optimized GaAs / Ge device. Grandeurs photovoltaïc      1.10 32.74 0.89 32.07  Figure 4. Current-voltage characteristics GaAs /Ge device   4. Conclusions In this investigation, we have shown that a relatively thin double-junction GaAs /Ge device can achieve a remarkably  high power output. An extended spectral coverage due to a carefu l choice of the materials and optimization of  the thickness and doping  levels of each layer led to an enhanced overall power output from the GaAs /Ge tendem device. Under the standard solar spectrum and one sun,  the efficiency of the device is 36.4%, which is much higher than the efficiency of an optimized single-junction  Ge based device under the same illumination conditions.  Acknowledgment We would like to acknowledge the use of AMPS-1D program that was developed by Dr. Fonash’s group at Pennsylvania State University (PSU).    0,0 0,2 0,4 0,6 0,8 1,0 1,205101520253035  courant(mA)VOLTAGE(V)Journal of Energy Technologies and Policy                                                                   www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.3, No.3, 2013  13 References [1] J. A. Duffie, W. A. Beckman, Solar Engineering of Thermal Processes, 3rd Ed ition,  John Wiley & Sons, 2006, pp. 3.  [2] J.  M.  Olson,  D.  J.  Friedman  and  S.  Kutrz,  in  Handbook  of  Photovoltaic  Science  and engineering, edited by A. Luque and S. Hegedus, John Wiley & Sons, 2002, pp. 359.  [3] M. A. Green, K. Emery, Y. Hishikawa and W. Warta, Prog. Photovolt: Res. Appl. 18 (2010) 346.  [4] Araujo, G.L., Marti, A., 1994. Absolute limit ing efficiencies for photovoltaic energy conversion. Sol. Energy Mater. Sol. Cells33, 213–240. (2002)  [5] M. Burgelman, John Verschraegen, Stefaan Degrave and Peter Nollet, Prog.  in  Photovolt:  Res. and Appl. 12,143 (2004). / [6] Hong Zhu, Ali Kaan Kalkan, Jingya Hou and Stephen J. Fonash, AIP Conf. Proc., March 5,   1999. Osaka, Japan.       This academic article was published by The International Institute for Science, Technology and Education (IISTE).  The IISTE is a pioneer in the Open Access Publishing service based in the U.S. and Europe.  The aim of the institute is Accelerating Global Knowledge Sharing.  More information about the publisher can be found in the IISTE’s homepage:  http://www.iiste.org  CALL FOR PAPERS The IISTE is currently hosting more than 30 peer-reviewed academic journals and collaborating with academic institutions around the world.  There’s no deadline for submission.  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Optimization of a Tandem Solar Cell GaAs / Ge using AMPS-1D

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Optimization of a Tandem Solar Cell GaAs / Ge using AMPS-1D

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