International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
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Laboratory of Semiconductors and Solar Energy, Physics Department, Faculty of Science and Technology
University Cheikh Anta Diop – Dakar - SENEGAL
Abstract — The search for a material which makes it possible to obtain free-Cadmium solar cells pushed us to study in this article the influence of the donor doping density of the buffer layer on the macroscopic electric parameters of the Cu(In, Ga)Se
2
thin film solar cell. We use two types of buffer layer: Zn(O,S) and CdS. the study of the density current-voltage characteristic (J-V) according to the doping density shows us the interest of the use of Zn(O,S). Then by studying the open circuit voltage Voc, the short-circuit current density Jsc, the form factor FF and the cell efficiency η, we clearly perceive the doping density which gives optimal electric parameters. Indeed we reach a cell efficiency of 19.53% for the solar cell with Zn(O,S) buffer layer with a donor doping density
Nd=10
18 cm
-3
.With the use of CdS as buffer layer we reached a cell efficiency 18.79% for a donor doping density Nd=10
17 cm
-3
.
Keywords
—
buffer layer, donor density, CdS,
Zn(O,S), Cu(In,Ga)Se
2
, macroscopic electric parameters.
I.
I NTRODUCTION
The technology of the solar cell undergoes many changes at the rhythm of research. These experimental changes pass by the use of new materials. Basing on the solar cell buffer layer, we notice that during years the Cadmium Sulphide CdS occupied the first place. It makes it possible to obtain layers of good quality [1]-
[3]. Indeed CdS has the capacity to increase the optical properties of the Cu(In, Ga)Se
2
buffer layer and the ability to protect it from the environmental bad weather. However CdS remains a very toxic product.
The search for other types of materials undertook to the use of Zn(O,S) like buffer layer [4]-[5]. The principal interest of the use of Zn(O,S) instead of CdS, except its non toxicity is its gap higher which plays also an optical role of window layer. Its physical parameters are known with a Conduction Band Offset which can be optimized while exploiting the ratio
Oxygen/Sulfide [6].
In our work we are not interested by the method of elaboration of these buffer layers. Several publications already elucidated these processes [7]-[9].
The interest of our study is to show the influence of the type and the doping density of the buffer layer on the J-V characteristic, the open circuit voltage Voc, the short circuit current density Jsc, the form form FF and the cell efficiency η. We use for that Zn(O,S) and CdS like buffer layer .
II.
E XPERIMENTAL PROCESS
For this study we use the AMPS (analysis of microelectronic and photonic structures). the AMPS is a software developed at the Illinois University by
Fonash et al..[10]-[11]. Its speciality is thin film solar cells. The configurations of the studied solar cells are traditional and the Table 1 shows these configurations.
Tab. 1 : Solar cells Configurations for the study of the donor doping density influence on the electric parameters
Window layer
Nature
ZnO
Thickness
0.2µm
Buffer layer CdS or Zn(O, S) 0.05µm
Absorber layer Cu(In, Ga)Se2 3µm
For each layer we introduce the optical properties, the electric properties and the defects. All these data are result from published experimental results. To study the influence of the donor doping density on the electric parameters, we use doping densities varying from the medium doping levels to the doping strong levels: 10
15 cm
-3 to 10
20 cm
-3
.
The results obtained are treated with Matlab. The study is carried out under the solar spectrum AM1.5 with an incidental light power of 1000W.m
-2
. We take into account the totality of the radiation wavelengths which reach the layer window layer. At the end of this process we will be able to judge the effectiveness of the cells according to the nature of the window layer and its doping density. The studied physical parameters are the current density-voltage characteristic J-V, the open circuit voltage Vco, the short circuit current density Jsc, the form factor FF and the cell efficiency η.
III.
R ESULTS AND D ISCUSSION
A. Study of the current density-voltage characteristic
J-V according to the donor doping density of the buffer layer.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
Fig. 1 : Current density-Voltage characteristics of the solar cells with the CdS and Zn(O, S) buffer layers doped N with Nd=10
15 cm
-3
.
1) For a donor doping density Nd=10
15 cm
-3
of the buffer layer: The Figure 1 gives the J-V characteristics of the two cells with CdS or Zn(O,S) buffer layers with a donor density Nd=10
15 cm
-3
. We note two conventional characteristics which give a short-circuit current density of 37.74mA.cm
-2 for CdS and 37.73mA.cm
-2 for Zn(O,S).These two values are very close for the density doping of 10
15 cm
-3
. The open circuit voltage Vco is equal to 0.643V for CdS and 0.637V for Zn(O,S). This is a difference of 6mV.
The maximum power point is weaker for CdS than for
Zn(O,S). characteristics of the cells with the Zn(O,S) and CdS buffer layers with a still medium donor density of
10
16 cm
-3
. The two profiles obtained are similar to the precedents but we note a bringing together of the open circuit voltages Voc, a distance of the short-circuit current densities Jsc and a proximity of the maximum power. This is visible if we take to account the numerical values. We find Jsc=37.73mA.cm
-2 for the solar cell with a CdS buffer layer and 37.77mA.cm
-2 with the Zn(O,S) buffer layer. The Voc of the cell with Zn(O,S) varies very slightly compared to that of the cell with CdS buffer layer.
2) For a donor doping density Nd=10
16 cm
-3
of the buffer layers : The Figure 2 gives the J-V
Fig. 2: Current density-Voltage characteristics of the solar cells with the CdS and Zn(O, S) buffer layers doped N with Nd=10
16 cm
-3
.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
Fig. 3 : Current density-Voltage characteristics of the solar cells with the CdS and Zn(O, S) buffer layers doped N with Nd=10
17 cm
-3
.
3) For a donor doping density Nd=10
17 cm
-3
of the buffer layers: The figure 3 shows the J-V characteristics of the cells with a donor doping density of the buffer layers of 10
17 cm
-3
. We are on the limit of strong doping densities. The two profiles obtained correspond to that of the cell with a CdS buffer layer and that of the cell with a Zn(O,S) buffer layer. These characteristics present differences compared to the other characteristics obtained with weaker doping densities. The open circuit voltage Voc is the same one for the two cells and it is of 0.637V. The difference between the two short circuit current densities Jsc circuit increases. The Jsc is equal to
37.46mA.cm
-2 for the CdS buffer layer and
38.05mA.cm
-2 for Zn(O,S) buffer layer. The proximity of the two characteristics means proximity of their maximum power points.
4) For a donor doping density Nd=10
18 cm
-3
of the buffer layers: The figure 4 shows the J-V characteristics of the two cells with CdS and Zn(O,S) buffer layers doped Nd=10
18 cm
-3
.
Fig. 4: Current density-Voltage characteristics of the solar cells with the CdS and Zn(O, S) buffer layers doped
N with Nd=10
18 cm
-3
.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
The obtained characteristics show variations compared to the preceding ones. The open circuit voltages of the two cells are not equal any more. That of the cell with a CdS window layer is of 0.636V whereas that of the cell with a Zn(O,S) window layer is of 0.638V. As for the short circuit current densities, the difference between the two Jsc always increases. It is of 36.76mA.cm
-2
for the cell with the plug layer in
CdS and of 38.25mA.cm
-2
for the cell with Zn(O,S) buffer layer.
5) For a donor doping density Nd=10
19 cm
-3
of the buffer layers: The J-V characteristics of the solar cells with a donor doping density Nd=10
19 cm
-3 of the CdS and Zn(O,S) buffer layers are given by the figure 5.
We are in a situation of strong doping of the buffer layers. We note a difference with the characteristics obtained with the lower dopings levels. The difference between the short circuit current densities decreases.
We find 36.85mA.cm
-2 for the cell with a CdS buffer layer and 38.22mA.cm
-2 for the cell with a Zn(O,S) buffer layer.
Fig. 5: Current density-Voltage characteristics of the solar cells with the CdS and Zn(O, S) buffer layers doped
N with Nd=10
19 cm
-3
.
Fig. 6 : Current density-Voltage characteristics of the solar cells with the CdS and Zn(O, S) buffer layers doped
N with Nd=10
20 cm
-3
.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
It is of 0.636V for the cell with a CdS buffer layer and 0.638V for the cell Zn(O,S) buffer layer. The intersection of the two characteristics causes a bringing together of their maximum power points.
B. Study of the macroscopic electric parameters according to the donor doping densities of the buffer layers.
V oc
R sh
J ph
J s qV co e nk
B
T
1 (1)
6) For a donor doping density Nd=10
20 cm
-3
of the buffer layers : The figure 6 shows the J-V characteristics of the cells with the CdS or Zn(O,S) buffer layers doped Nd=10
20 cm
-3
. We note a proximity of the open circuit voltages Voc and a very light variation of the short circuit current densities Jsc.
The Voc is of 0.636V for the cell with a CdS buffer layer and of 0.638V for the cell with a Zn(O, S) buffer layer. We note also a light increase in Jsc which is of
36.80mA.cm
-2
for the cell with a CdS buffer layer and
38.22mA.cm
-2 for Zn(O,S) buffer layer. The high doping of the buffer layers give two profiles which show maximum power point very close. For better determining the influence of the donor doping density of the buffer layers on the electric parameters of the solar cell, we study the behavior of the macroscopic electric parameters.
1) Influence of donor doping density of the buffer layers on the open circuit voltage Voc: The open circuit situation in a solar cell is established when the minority carriers photogenerated are stored at the junction located at the interface transmitter/base. The cell behaves as a voltage source when any output current is generated. It is a quantifiable quantity [12]:
The Figure 7 gives the variation of the open circuit voltage Voc according to the doping density of the
CdS and Zn(O,S) buffer layers.
10
For medium doping densities of 10
15 cm
-3 or
16 cm
-3
the open circuit voltage of the cell with a
CdS buffer layer are higher than that of the cell with a
Zn(O,S) buffer layer. When we pass from the medium at the strong doping densities, the Voc approaches to be equal for a doping density of 10
17 cm
-3
. Then it follows an inversion of the two characteristics when we pass at the strong doping densities 10
18 cm
-3 and to
10
20 cm
-3
. Indeed for the medium donor dopings the minority carriers are stored at the junction in the cell with CdS buffer layer. But when we pass at the strong donor dopings the storage of the carriers to the junction becomes more significant for the cell with a
Zn(O,S) buffer layer .
2) Influence of donor doping density of the buffer layers on the short circuit current density Jsc: The short circuit situation in a solar cell is established when the majority of the minority carriers crossed the junction to be collected at the front face. The cell behaves as a generator of electrical current when the output tension is null.
Fig. 7 : Variation of the open circuit voltage Voc according to the donor doping density Nd of the buffer layers.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
Fig. 8: Variation of short circuit current density Jsc according to the donor doping density Nd of the buffer layers.
We can determine it while applying:
J sc
J ph
J s e qJ cc
R s nk
B
T
R
R s sh
(2)
10
For the medium doping densities 10
15 cm
-3 and
16 cm
-3 the form factor FF is more significant for the cells with a Zn(O,S) buffer layer that with a CdS buffer layer. When we pass at the strong rates of doping, the performances of the solar cell with the
CdS buffer layer increase considerably whereas those of the Zn(O,S) buffer layer vary slightly while remaining significant.
The figure 8 gives the variation of the short circuit current density Jsc according to the doping density of the buffer layers.
For the donor doping densities of 10
3
15 cm
-3 and 10
16 cm
the short circuit current densities are very close for the two solar cells with CdS and Zn(O,S) buffer layers.
More the buffer layers are doped more the Jsc is significant for the cell with a Zn(O,S) buffer layer, whereas it decreases for the cell with a CdS buffer layer.
Indeed the carriers have more the capacity to cross the junction to be collected at the front face in the cell with a Zn(O,S) buffer layer.
3) Influence of donor doping density of the buffer layers on the form factor FF: The form factor FF is a physical parameter which helps us to judge the performance of the solar cell. It corresponds to the largest rectangle than we can register in the J-V characteristic. It is expressed in % and is quantifiable by:
FF
V m
J m
(3)
V co
J sc
The figure 9 shows the variation of the form factor according to the donor doping of the buffer layer.
4) Influence of donor doping density of the buffer layers on the solar cell efficiency η: The solar cell efficiency η is the principal characteristic which makes it possible to judge the quality of the cell without ambiguity. It is expressed in % and can be given while applying:
FF V oc
I sc
(4)
P in
It is a physical parameter which takes into account the other preceding macroscopic electric parameters and the power of the incident light power.
The figure 10 shows the variation of the cell efficiency according to doping density of the buffer layers.
For any doping density, the efficiency with the
Zn(O,S) buffer layer is more significant. However we can specify that for the medium doping densities the cell efficiency with a CdS buffer layer is about 17%. It increases with the doping density and reaches a maximum of 18.79% for Nd=10
17 cm
-3
. For the cell with Zn(O,S) buffer layer the cell efficiency remains higher than 19% and is even improved with the increase of the donor doping. It reaches a maximum of
19,53% for a doping density Nd=10
18 cm
-3
.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
Fig. 9: Variation of the form factor FF according to the donor doping density Nd of the buffer layers.
Fig. 10: Variation of the form factor FF according to the donor doping density Nd of the buffer layers.
IV.
CONCLUSIONS
At the end of this study we can affirm that the solar cell with a Zn(O,S) window layer gives better electric parameters that those of the solar cell with a CdS buffer layer. The donor doping density of the buffer layers has a remarkable effect on the performances of the cells. This doping density affects more the electric parameters of the cells with a CdS buffer layer. the maximum difference of the open circuit voltage Voc when we pass from doping density of 10
15 cm
-3 to
10
20 cm
-3 is 7mV with CdS whereas with Zn(O, S) it is
1mV. The maximum difference of the short circuit current density Jsc is 0.98mA.cm
-2 for CdS whereas with Zn(O,S) it is 0.52mA.cm
-2
. The study of the form factor FF enable us to obtain with the CdS buffer layer a maximum of 80.12% for Nd=10
20 cm
-3
. With the
Zn(O,S) buffer layer the maximum of the form factor is 80.12% also for the same doping density. About the cell efficiency with a Zn(O,S) buffer layer, the maximum form factor is 19.53% for Nd=10
18 cm
-3
.
According to the cell with CdS buffer layer the maximum efficiency is 18.79% for Nd=10
17 cm
-3
.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 5 - October 2015
Former studies which were undertaken in our laboratory[13]-[14], gave results which are confirmed by the results published in this paper.
Then Zn(O,S) remains a promising alternative for the free-Cadmium solar cell elaboration.
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