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INVESTIGATION OF InGaN/Si DOUBLE JUNCTION TANDEM SOLAR CELLS
F. Bouzid
1*
and L. Hamlaoui
2
1
Laboratory of Metallic and Semiconducting Materials, University of Biskra, Algeria
2
Faculty of sciences, El Hadj Lakhdar University, Batna, Algeria
Received: 01 September 2012 / Accepted: 28 November 2012 / Published online: 31 December 2012
Abstract
In this work, the solar power conversion efficiency of InGaN/Si double junction tandem solar
cells was investigated under

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INVESTIGATION OF InGaN/Si DOUBLE JUNCTION TANDEM SOLAR CELLS
F. Bouzid
1*
and L. Hamlaoui
21
Laboratory of Metallic and Semiconducting Materials, University of Biskra, Algeria
2
Faculty of sciences, El Hadj Lakhdar University, Batna, Algeria
Received: 01 September 2012 / Accepted: 28 November 2012 / Published online: 31 December 2012
Abstract
In this work, the solar power conversion efficiency of InGaN/Si double junction tandem solarcells was investigated under 1-sun AM1.5 illumination, using realistic material parameters.With this intention, the current-voltage curves are calculated for different front recombinationvelocities and the influence of the bottom cell thickness on efficiency has been studied. Theresults show that a front recombination velocity value of 1e
3
cm/s is most advantageous andthe use of relatively thick bottom cell is necessary to obtain conversion efficiency greater than27%, at 300°k cell temperature. This efficiency will decrease as the operating temperatureincrease.© 2012 University of El Oued. All rights reserved.
Keywords:
Photovoltaic, Efficiency, Carrier lifetimes, Recombination velocity, Temperature.
Introduction
Photovoltaic (PV) tandem cells have been widely demonstrated in recent years as aneffective pathway to realize higher conversion efficiency, showing promising prospects inboth terrestrial and space applications. Theoretical modelling of two-junction tandem solarcells shows that for optimal device performance, the bandgap of the top cell should be in therange of 1.6 to 1.8ev [1].
Author Correspondence, e-mail: faycal.bouzid@ymail.com
Tel.: +213(0)33741087; fax: +213(0)33741087.
Available online at www.jfas.univ-eloued.dz
Journal of Fundamental and Applied Sciences
J. Fund. App. Sci.
F. Bouzid et al. Journal of Fundamental and Applied Sciences, 2012, 4(2), 59-71 60
60A tandem cell using Indium Gallium Nitride (In
x
Ga
1-x
N) for the top cell and Silicon (Si) forthe bottom cell is advantageous in two respects: The direct bandgap of the In
x
Ga
1-x
N alloysystem, extends continuously from Indium Nitride (InN) bandgap which is 0.7eV in themedium infrared, to that of the Gallium Nitride (GaN) which is 3.42eV [2] in the nearultraviolet, makes the In
x
Ga
1-x
N alloy a promising candidate for radiation and temperatureresistant single or multi-junction solar cells [3]. In the other hand, Si is relatively cheap andplentiful and its processing techniques are well established, in addition to the fact that the Sibandgap of 1.1ev is ideally suited for the bottom junction of high efficiency two-junctionsolar cells [4].In this paper, we have modelled the photovoltaic conversion efficiency of series-connected,two-junction, two-terminal In
x
Ga
1-x
N on Si solar cells in terms of their physical parameters,employing a simulation program developed for this reason, where the In
x
Ga
1-x
N has an alloyfraction close to In
0.53
Ga
0.47
N.
Model description
Figure 1 show a simplified structure of the In
0.53
Ga
0.47
N/Si tandem, where x
j
is the junctiondepth, w is the depletion region width and d is the cell thickness.
Figure 1:
Simplified configuration of the In
0.53
Ga
0.47
N/Si tandem.In this work, calculations were all performed under 1-sun AM1.5 illumination and atemperature of 300°k using the one diode ideal model, and for convenience, severalsimplifying assumptions were made, including no series resistance losses, no reflection lossesand contact shadowing. Currents calculation follows the general methodology described inref. [5].
Analytical model
The total output current drawn from single cells under illumination is given by [5] as:
Dark LightTotal
III
−=
(1)
d
wxjP- In
0.53
Ga
0.47
NN- In
0.53
Ga
0.47
NN - SiP - Si
F. Bouzid et al. Journal of Fundamental and Applied Sciences, 2012, 4(2), 59-71 61
61
( ) ( ) ( )
[ ]
d
λ λ
I
λ
I
λ
II
λ
max
λ
mindrnpLight
∫
++=
(2)Where:I
p
(
λ
) is the photocurrent due to holes collected at the depletion edge xj;I
n
(
λ
) is the photocurrent due to electrons collected at the depletion edge xj+w;I
dr
(
λ
) is the contribution of the depletion region to the photocurrent;
λ
min
is the wavelength corresponding for the bottom cell in case of double junction system tothe top cell bandgap, and equals zero for the top cell;
λ
max
is the wavelength corresponding to the cell bandgap.The dark current can be expressed as:
−=
×
1kTqvexpIDark I
0
(3)Where:I
0
is the saturation current which was calculated following the method described in the ref.[5], v is the applied voltage, k is the Boltzmann constant and T is the temperature.The open circuit voltage is given by [5] as:
+=
1IIsclnqkTV
0oc
(4)Where I
sc
is the short circuit current. The open circuit voltage of the tandem is taken to be thesum of the open circuit voltages of the tandem junctions:
∑
=
=
n1iioc,oc
vV
(5)Where n is the number of junctions incorporated in the tandem.The cell output power is given as:
VIp
Total
×=
(6)The cell conversion efficiency is usually taken to be:
incmm
pVI
η
×=
(7)Where I
m
and V
m
are coordinates of the maximum power point, P
inc
is the total incident solarpower.The fill factor is defined by:
ocscmm
vIVIFF
××=
(8)
F. Bouzid et al. Journal of Fundamental and Applied Sciences, 2012, 4(2), 59-71 62
62
In
x
Ga
1-x
N parameter equations used in our program
The equation relating the bandgap energy to the mole fraction x is given by [6,7] as:Eg(x)= x × Eg(InN) + (1-x) × Eg(GaN) – x × (1-x) × C (9)Where: Eg(InN) = 0.7eV, Eg(GaN) = 3.42eV and C is a bowing parameter which is taken tobe equal to 1.43.The electron and hole mobilities are calculated as a function of doping using [8]:
( )
( )
γ
iig,imin,imax,
imin,i
NN1
μμμ
N
μ
+−+=
(10)Where i represent either electrons (e) or holes (h), N is the doping concentration and thespecific parameters µ
min
, µ
max
,
γ
and N
g
are given in
Table 1
.
Table 1:
Parameters used in the simulation of the InN and GaN carrier mobilities.
µ
min,e
[cm²/vs]µ
max,e
[cm²/vs]µ
min,h
[cm²/vs]µ
max,h
[cm²/vs]
γ
e
γ
h
N
g,e
[cm
-3
]
N
g,h
[cm
-3
]
GaNInN
5530100011003317034011222e2e
17
3e3e
17
In
x
Ga
1-x
N electron mobilities are taken as a linear interpolation between the InN and GaNvalues; however, hole mobilities of the In
x
Ga
1-x
N alloys are assumed to be similar to the GaNhole mobility.The absorption coefficient of the In
x
Ga
1-x
N alloys is taken to be [9]:
( ) ( ) ( )
125
cmEgEbEgEa10E
α
−
−×+−××=
(11)Where E is the incoming photon energy given in ev, a and b are dimensionless fittingparameters. The fitting parameters used in our program are shown in
Table 2
.
Table 2:
Fitting parameters used to calculate the absorption coefficient of the In
x
Ga
1-x
Nalloys.
Indium composition a b0.570.690.83
0.609460.581080.667960.621820.669020.68886

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