 Open Access
 Total Downloads : 964
 Authors : Benmoussa Dennai, A Hemmani, HBen Slimane, A.Helmaoui
 Paper ID : IJERTV2IS110575
 Volume & Issue : Volume 02, Issue 11 (November 2013)
 Published (First Online): 09112013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Simulation of multijunction solar cells based on InGaN using AMPS1D
Benmoussa Dennai 1, A Hemmani 1,HBen Slimane 1 and A.Helmaoui 1
1Laboratory in semiconductor device University of Bechar, Algeria
Abstract
During the past few years a great variety of multi junction solar cells has been developed with the aim of a further increase in efficiency beyond the limits of single junction devices. InxGa1xN is one of a few alloys that can meet this key requirement. In this paper, we designed series of InxGa1xN multijunction solar cells. Key properties of InxGa1XN tandem solar cells (for two junctions up to six junctions) are simulated by using AMPS1D software, including IV characteristic, conversion efficiency, band structure etc. Our calculation shows that the efficiency can be improved from 10.09% for a single junction up to 40.05% for six junctions obtained in 1sun AM1.5 illumination and at room temperature, using realistic material parameters..

Introduction
Semiconductors of the type IIIN are of growing interest in the scientific world. This is justified by the fact that IIIN semiconductors are robust, having a high thermal conductivity and a high melting point, and, moreover, a direct forbidden band gap. They currently represent ideal materials for the development of light emitting diodes (LEDs) operating in the greenblue and UV ranges of the electromagnetic spectrum. Among these semiconductors, we find mainly aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN), respectively with a gap of 6.2eV, 3.4eV and 0.7eV [1].
Tandem solar cells and multijunction solar cells, which consist of a stack of p/n solar cells, are sometimes classified as third generation solar cells. They achieve the highest conversion efficiencies, even exceeding 40 % [6].
Recently, InxGa1XN alloys have become very potential for high performance MJ solar cells. Because the band gap of InxGa1XN alloys can be varied continuously from 0.7 to 3.4 eV. This provides an almost perfect fit to the full solar spectrum offering a unique opportunity to design MJ solar cells using a single ternary alloy system. This will be technologically very significant because of easy
fabrication, similarity in thermal expansion coefficient, electron affinity and lattice constant. In addition, InN based alloys are predicted to show high nobilities and lifetime of charge carriers and superior resistance against irradiation damage. These all make InxGa1xN alloys very promising for high performance solar cells.
In order to evaluate the possibilities of these alloys, we tried, in this work, to model and simulate tandem cells made of two, three, four, five and six InxGa1XN junctions with a one dimensional simulation program called a analysis of microelectronic and photonic structures (AMPS1D).
In this work, calculations were all performed under 1sun AM1.5 illumination and a temperature of 300 K using the one diode ideal model, and for convenience, several simplifying assumptions were made, including no reflection losses and no surface recombination velocity.

Modelling and simulations
2.1About AMPS1D
AMPS1D is the firstprinciples simulation tool developed by the Penn State/Electric Power Research Institute (EPRI)[15]. AMPS software used in this study is based on the firstprinciples, basic equations of semiconductors and solar cells: Poissons equation, the continuity equation for free holes, and the continuity equation for free electrons.
. Determining transport characteristics then becomes a task of solving the three coupled nonlinear differential equations, each of which has two associated boundary conditions.
In AMPS, these three coupled equations are solved simultaneously to obtain a set of three unknown state variables at each point in the device: the local vacuum level, the electron, and hole quasiFermi levels. From these three state variables, the free carrier concentrations, fields, currents, etc. can then be computed.
Besides the classical continuity equations, other semiclassical or quantum transport equations such as the Boltzmann equation, Quantum Hydrodynamic
(QHD) model, Wigner function method, and non equilibrium Greens function method have also been applied to study the transport processes in photovoltaic devices so far [1618].
four, five, six and seven junctions were simulated. The energy gap and indium fraction for InX Ga1X N alloys computed for a six junction are given in table II.
AMPS1D supplies two different approaches to the process of recombination/generation. One is the density
TABLE I I
ENERGY GAPS FOR A SIXJUNCTION TANDEM CELL
of states (DOS)/capture cross section model and the Junction
indium fraction for InX Ga1X N
Band gap
other is the carrier lifetime model. Zhang et al. [7] NÂ°
alloys
(eV)
In this study, we use the DOS model in the
simulation of InGaN solar cells because the DOS 1
0.11718
2.25
model could provide much more information about
recombination/generation in semiconductors than the 2
0.613
1.79
lifetime. 3
0.766659
1.475
4
0,8535
1.19
the InX Ga1X N SCs 5
0,921
0.927
Band gap[2] 6
1
0.7
AMPS1D supplies two different approaches to the process of recombination/generation. One is the density
TABLE I I
ENERGY GAPS FOR A SIXJUNCTION TANDEM CELL
of states (DOS)/capture cross section model and the Junction
indium fraction for InX Ga1X N
Band gap
other is the carrier lifetime model. Zhang et al. [7] NÂ°
alloys
(eV)
In this study, we use the DOS model in the
simulation of InGaN solar cells because the DOS 1
0.11718
2.25
model could provide much more information about
recombination/generation in semiconductors than the 2
0.613
1.79
lifetime. 3
0.766659
1.475
4
0,8535
1.19
the InX Ga1X N SCs 5
0,921
0.927
Band gap[2] 6
1
0.7
2.2. Parameters for the simulation
Material parameter equations used for the simulation of
Eg x = 0.7x + 3.4 1 x 1.43(1 x) (1)
Electron affinity [8,9]:
= 4.1 + 0.7(3.4 Eg ) (2)
Absorption coefficient
= 2.2 Ã— 105 1.24/ (3) Effective density of states in the conduction band [8]
= [0.9 + 1 2.3] Ã— 1018 (4)
Effective density of states in the valence band [4]
Nv = [5.3x + 1 x 1.8] Ã— 1019 (5)
Relative permittivity [8]:
r = 14.6x + 1 x 10.4 (6)
Carrier mobility [10] :

Results and discussions

Simulations for a sixjunction tandem cell
InxGa1xN tandem cells comprising two, three, four, five and six junctions were simulated.
Hereaftr, the results computed for a InxGa1xN tandem structure comprising six junctions are given.
The following table (Table III) shows the energy gaps of the identified materials, the thicknesses of the junctions and that of the nside.
TABLE III
=
+ ,+ ,
(7)
ENERGY GAPS AND THICKNESSES FOR A SIXJUNCTION TANDEM
,
1+ ,
CELL
Junction
Band gap
nside thickness
Junction thickness
NÂ°
(eV)
(m)
(m)
1
2.25
0.1
0.3
2
1.79
0.1
0.3
3
1.475
0.1
0.3
4
1.19
0.1
0.5
5
0.927
0.1
0.5
6
0.7
0.1
0.5
Junction
Band gap
nside thickness
Junction thickness
NÂ°
(eV)
(m)
(m)
1
2.25
0.1
0.3
2
1.79
0.1
0.3
3
1.475
0.1
0.3
4
1.19
0.1
0.5
5
0.927
0.1
0.5
6
0.7
0.1
0.5
The above formulae with asterisk are obtained from the linear fitting of the corresponding parameters of InN and GaN. The carrier mobility of InGaN is assumed to be similar to GaN, where i= n, p denotes electrons and holes, respectively, and N the doping concentration, while the model parametersmin ,i , max ,i
, Ng,i and i depend on the type of semiconductor [10].
TABLE I
MODEL PARAMETERS USED IN THE CALCULATIONS OF THE CARRIER MOTILITIES
Type of
,
max ,i
2 1 1
Ng,i
3 i
carriers
()
(cm V S )
cm
We can see from Table III that the energy gaps of the junctions decrease from the top to the bottom of the tandem cell.
Electrons 100
55
1
Holes 170
3
2
Electrons 100
55
1
Holes 170
3
2
. InxGa1xN tandem cells comprising two, three,
Table IV gives the computations of the photocurrent
60
60
densities, the opencircuit voltages and the output peak power for a sixjunction InxGa1xN tandem cell.
50
50
Simulations show that the sixjunctions InxGa1xN tandem cell could reach an efficiency of more than 40% with a shortcircuit current density of 52.27mA/cm2 and
current density(mA/cm2)
current density(mA/cm2)
an opencircuit voltage of 0.51V. 40
TABLE IV
SIMULATION RESULTS FOR A SIXJUNCTIONS TANDEM CELL
30
Junction N Isc mA/cm2 Voc
Fill
Efficiency (%)
(V)
factor(%)
1
5.79
1.87
0.92
10.09
2
19.66
1.31
0.90
23.35
3
27.29
1.00
0.88
24.30
4
40.74
0.86
0.86
30.64
5
46.32
0.63
0.83
36.57
6
52.27
0.51
0.80
40.05
(V)
factor(%)
1
5.79
1.87
0.92
10.09
2
19.66
1.31
0.90
23.35
3
27.29
1.00
0.88
24.30
4
40.74
0.86
0.86
30.64
5
46.32
0.63
0.83
36.57
6
52.27
0.51
0.80
40.05
20
10
0
1 2 3 4 5 6
The junction number
Fig.2: Variation of the shortcircuit current density versus the number of junctions in the cell
We can notice from Table VI and Fig.1 that the opencircuit voltages produced by the junctions of the tandem cell decrease almost linearly from the top to the bottom.
2,0
1,8
0,92
0,90
Fill factor(%)
Fill factor(%)
0,88
0,86
0,84
0,82
0,80
Open circuit voltage (V)
Open circuit voltage (V)
1,6
1,4
1,2
1,0
0,8
0,6
0,4
1 2 3 4 5 6
Number of the junction
1 2 3 4 5 6
Number of junctions
Fig.3. Variation of Fill factor versus the number of junctions in the cell
Fig. 1: Variation of the Opencircuit voltage versus the junction number for a sixjunction tandem structure.
40
40
InxGa1xN tandem cells have an additional advantage as they can be produced with a simpler technology than the ones used to produce tandem junctions made of different materials. In fact, the InxGa1xN alloys have
35 similar properties, which make their deposition in
successive films easier.
Efficiency (%)
Efficiency (%)
30
25
20 5.Acknowledgements
15
15
We would like to acknowledge the use of AMPS1D program that was developed by Dr. Fonashs group at
10 Pennsylvania State University (PSU).
1 2 3 4 5 6
Number of junctions
Fig. 4: Variation of the efficiency with the number of junctions included in the tandem cell.
We notice from Table 2and Fig. 1that the achievable opencircuit voltages decreases as the tandem cell. Contains more layers However, the increase of the shortcircuit current density as a function of the number of junctions is almost linear; hence, it compensates the decrease of the opencircuit voltages, which is a function of the inverse of the same variable; this explains the increase of the output maximum power and the cell efficiency with the number of junctions.
The highest efficiency (40.05%) was reached for the sixjunction cell (Fig. 4)with a shortcircuit current density of 52.27mA/cm2 and an open circuit voltage of about 0.51V.
InxGa1xN tandem cells comprised of two and three junctions have also interesting potentials with efficiency of 23.35% for the twojunction tandem cell and of 24.3% for the threejunction one.
It is noticeable that the increase of the efficiency is more important when we move from a twojunction to a threejunction InxGa1xN tandem cell.


Conclusion
The theoretical design and performance of InxGa1xN based MJ solar cells for high efficiency have been studied by developing a simulatio model. The simulation with AMPS1d result shows that the InxGa1 xN alloys have interesting performances for tandem cells applications. The efficiency is evaluated from 10.09% for single junction to 40.05% for six junctions.
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