This document summarizes a numerical simulation of a graded band gap GaAs/AlGaAs heterojunction solar cell performed with AMPS-1D modeling software. The simulation analyzed how a graded band gap region at the interface between the GaAs emitter and AlGaAs base layers affects solar cell performance. Results showed that a 0.14μm graded band gap region was needed to eliminate the conduction band spike and improve cell performance over an abrupt heterojunction. Photovoltaic parameters, including efficiency, of 31.1% were obtained for the cell with a graded interface, compared to 27.3% for an abrupt heterojunction cell.

1. Advances in Physics Theories and Applications www.iiste.org
ISSN 2224-719X (Paper) ISSN 2225-0638 (Online)
Vol.22, 2013
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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: hassane_ben@Yahoo.fr
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)

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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

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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.

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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

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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).

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