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  • 1. Advances in Physics Theories and Applications ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.22, 2013 30 Numerical simulation of graded band gap GaAs/AlGaAs heterojunction solar cell by AMPS-1D Hassane Ben slimane1 , B. Dennai, A. Helmaoui, Laboratory of Semiconductors Devices Physics, Physics Department, University of Bechar, Algeria 1 E-mil: Abstract The conduction band discontinuity or spike in an abrupt heterojunction p+ GaAs / NAl0.4 Ga0.6As solar cell can hinder the separation of hole-electron by electric field. This paper analyzes the GaAs /AlxGa1-xAs/Al0.4Ga0.6As based solar cell performance by AMPS-1D numerical modeling. The affect of graded band gap region in the interface between the emitter (GaAs) and base (Al0.4Ga0.6As) on the solar cell’s performance is investigated. Among the factors studied are thickness of graded band gap region, thickness of emitter layer of the cells. In this study, a width 0.14µm has been required to eliminate the spike and improved the performance of solar cell. Keywords: heterojunction solar cell; graded band gap; AMPS-1D. 1. Introduction It is known that the dark current play an important role in limiting single heterojunction solar cell open-circuit voltage (Voc) fill factor (FF), and conversion efficiency (η) [1]. So in dark condition the dominant current transport mechanism (at low current density) is the current due to recombination of carriere via deep levels in the space charge region. Because the losses of carrier recombination in the space charge region reduces the efficiency of a similar heterojunction cell, and the conduction band discontinuity or spike in an abrupt heterojunction p+ GaAs / NAl0.4 Ga0.6As solar cell can hinder the separation of hole-electron by electric field of junction, a graded layer inserted between the emitter (p+ GaAs) to base (NAl0.4 Ga0.6As) is used to eliminate the spike and reduces recombination in space charge region [2-5]. The folowing figures show the different between graded and abrubte héterojunction , They show the band diagram of graded and non graded heterojunction, the band diagrams of figures (1-a, 2-a) ignore electrostatic potentials due to arrangement of free carriers which occur near the compositional junction after the semiconductor are placed in contact. Figure 1- P-GaAs/ N-Al0.4 Ga0.6As heterojunction )P(before contact (b) after contact (a) (b)
  • 2. Advances in Physics Theories and Applications ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.22, 2013 31 Figure 2- P-GaAs/ n-AlxGa1-xAs /N-Al0.4 Ga0.6As heterojunction (a) before contact (b) after cotact In this present work, a one dimensional simulation program called analysis of microelectronic and photonic structures (AMPS-1D) [6] is used to simulate the AlxGa1-xAs graded band gap inserted between the emitter and base of heterojunction solar cell shown in figure 3. Figure 3 -A schematic diagram of the structure of p-GaAs/n- AlxGa1-xAs /n-Al0.4Ga0.6As 2. Modelling The heterojunction solar-cell structure used in the simulation consists of several layers including the top contact, bottom contact, window p-AlGaAs layer, p-GaAs absorber layer, graded gap n-AlxGa1-xAs interface layer, and the n-Al0.4Ga0.6As base layer. The computer simulation tool, analysis of microelectronic and photonic structures (AMPS)-1D[6] is employed by specifying the semiconductor parameters in each defined layer of the cell structure as input parameters in the simulation. To proceed with the simulation, the material parameters employed as the inputs are selected based on the reported literature values or constrained to reasonable ranges. The key semiconductor properties of different layers as the input parameters for the simulations are given in Table 1. (a) (b) Window layer p-GaAs Graded band gap layer n-Alx Ga1-x As n-Al0.4Ga0.6As hv
  • 3. Advances in Physics Theories and Applications ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.22, 2013 32 Layers Parameters Window AlGaAs p-GaAs Graded layer(AlGaAs) n/AlGaAs Thickness(μm) 0.04 4 0.14 0.5 Dielectricconstant,ε 5.66 13.10 11.76 11.76 Electron mobility (cm²/Vs) 2956.0000 8500 800 800 Hole mobility (cm²/Vs) 67.7760 400 100 100 Carrier density, P: 8.00e+018 1e18 1.00e+016 1.00e+016 Optical band gap, (eV) 1.75 1.42 1.42-1.89 1.92 Effective density, (cm-3 ) 4.82e+018 4.70e+017 5.50e+018 5.50e+018 Effective density, (cm-3 ) 1.73e+019 7.50e+018 4.50e+019 4.50e+019 Electron affinity ,χ(eV) 4.42 4.07 3.65-4.07 3.63 Table 1: Base parameters for P-GaAs/ n-AlxGa1-xAs /N-Al0.4 Ga0.6As solar cells. Although no attempt was made to match the simulation results with the experimental data, the purpose is to analyze the trend in the performance of GaAs /AlxGa1-xAs/Al0.4Ga0.6As cells versus thicknesses of the gradient layer intended to study. The modelling calculations discussed in the following section uses the software AMPS-1D. It estimates the steady-state band diagram, and caracteristics (current-voltage) Fig.4. AMPS-1D simulation front panel contains the device and layer grid parameters, and general layer parameters 3. Results By calculating the energy band profiles of a series of p+(GaAs)-n(A10.4Ga0.6As) heterojunctions having different grading widths and doping densities, the barrier height lowering factor is related to grading width and doping density. The energy barrier, which is ≈ 0.4 eV for the abrupt case, is almost zero for a grading width of only 0.14µm (fig 5), when the doping density in the n-region is 1016 cm3 . These values are probably quite realistic in practical heterojunctions.
  • 4. Advances in Physics Theories and Applications ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.22, 2013 33 Fig.5. The conduction band profiles of a p+ GaAs / AlxGa1-xAs /N Al0.4Ga0.6As heterojunction solar cell Figure 6 shows the J-V characteristics of graded and non graded heterojunction solar cell while Table 2 shows the efficiencies for two cells. The short-circuit current density (Jsc) is reduced due to the spike barrier for photogenerated electrons (≈ 0.40 eV) and recombination at the interface GaAs/Al0.4Ga0.6As in comparison to GaAs/AlxGa1-xAs/Al0.4Ga0.6As cell Fig.6. Current density vs. voltage of grabased solar cell . (%)ηFFVCO(V)JSC(mA/cm2 )Photovoltaic characteristics 27.310.7871.1529.94Heterojunction solar cell withow graded interface 31.110.891.1630.01Heterojunction solar cell with graded interface Table 2: Photovoltaic parameters of graded and non graded heterojunction solar cells. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 0 5 10 15 20 25 30 Currentdensity(mA/cm 2 ) Voltage (V) Graded heterojunction solar cell Abrupte heterojunction solar cell 3 4 5 -3,4 -3,2 -3,0 -2,8 -2,6 -2,4 -2,2 -2,0 -1,8 Electronenergy(eV) Distance (祄 ) Conduction band with interface graded layer Conduction band without interface graded layer
  • 5. Advances in Physics Theories and Applications ISSN 2224-719X (Paper) ISSN 2225-0638 (Online) Vol.22, 2013 34 4. Conclusion We report on the results of an extensive analysis employing one-dimensional simulation in order to optimize the performances of GaAs /AlxGa1-xAs/Al0.4Ga0.6As graded band gap solar cell. The simulation results of graded heterojunction cell show that when we use sufficient width of graded layer the photovoltaic performance improves remarkably. The best efficiency is obtained for graded thickness equals to 0.14 µm,the energy barrier in the conduction band is reduced from 0.40 eV to zero as the grading layer width increases from zero (abrupt case) to 0.14µm. A graded band gap layer at the interface has a little influence on the heterojunction cells performances. Therefore, the obtained simulation results may offer the possibility of elaborating heterojuncion solar cells with relatively high conversion efficiencies. Acknowledgments We acknowledge the use of AMPS-1D program developed by Dr. Fonash's group of Pennsylvania State University. 5. References [1] Kitt C. Reinhardt, Clay S. Mayberry, Brendan P. Lewis, « Multijunction solar cells iso-junction dark- current study » IEEE photovoltaic specialist conference, 2000 [2] O. V. Konstantinov, O, A, Merzin, B. V. Egorov, V. M. Lantratov, S. I. Troshkov, « Theory of a solar cell with band gap gradient in the space charge region of p-n heterojunction », Sov. Phy: Semicond. 20(7), 1986 [3] J. J. Liou « An analytical model for the current transport of graded heterojunction bipolar transistors », Solid-State Elecronics. Vol. 38. No. 4, pp.944-948. 1995 [4] D. T. Cheung, S. Y. Chiang and O. L. Pearson « A simplified model for graded-gap heterojunctions, Solid-Stat, Electronics, I975. Vol. 18, pp. 263-266. [5] A. Moraless-Acevedo, « Varibale band-gap semicoductors as the basis of new solar cells » Solar Energy, 83, 2009, pp. 1466–1471 [6] S.J. Fonash, A manual for One-Dimensional Device Simulation Program for the Analysis of Microelectronic and Photonic Structures (AMPS-1D), The Center for Nanotechnology Education and Utilization, The Pennsylvania State University, University Park, PA 16802).
  • 6. 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: CALL FOR JOURNAL 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. Prospective authors of IISTE journals can find the submission instruction on the following page: The IISTE editorial team promises to the review and publish all the qualified submissions in a fast manner. All the journals articles are available online to the readers all over the world without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself. Printed version of the journals is also available upon request of readers and authors. MORE RESOURCES Book publication information: Recent conferences: IISTE Knowledge Sharing Partners EBSCO, Index Copernicus, Ulrich's Periodicals Directory, JournalTOCS, PKP Open Archives Harvester, Bielefeld Academic Search Engine, Elektronische Zeitschriftenbibliothek EZB, Open J-Gate, OCLC WorldCat, Universe Digtial Library , NewJour, Google Scholar