Proceedings of the 5th WSEAS Int. Conf. on Microelectronics, Nanoelectronics, Optoelectronics, Prague, Czech Republic, March 12-14, 2006 (pp133-137)
Effect of Growth Process on Polycrystalline Silicon
Solar Cells Efficiency.
ZOHRA BENMOHAMED, MOHAMED REMRAM*
Electronic department university of Guelma
Département d’Electronique Université 8 mai 1945 BP 401Guelma 24000
*Electronic department university of Constantine
Département d’Electronique Université Mantouri Constantine25000
ALGERIA
Abstract
In this paper, we present the effect of the dislocation density over the minority-carrier diffusion length for
polycrystalline samples grown by PHOTOWATT POLIX and SUMITOMO SITIX .The analyzes presented
shows that the dislocation is not a dominating factor which limit the electric properties of material. The
presence of impurities and their Interactions with dislocations and other intragrain defects affect the minoritycarrier lifetime. Simulator PC1D is used to calculate the solar cells performances according to the last
technological processes developed.
A comparison between photovoltaic properties in particular polycrystalline silicon solar cells efficiency grown
by POLIX and SITIX is studied taking account of the structural parameters of these materials. The results
shown that an efficiency of over 16% has been obtained for SITIX solar cells, whereas an efficiency not
exceeding 13% has been obtained in the case of POLIX solar cells.
Key words:polycrystalline -silicon - dislocation – impurities- efficiency-interaction
1 Introduction
The potential and value of polycrystalline silicon
solar cells have already been proven for
photovoltaic
applications.Large
grained
polycrystalline silicon solar cells are actually
obtained with wafers made by conventional casting
method supplied from Photowatt Polix or Sumitomo
Sitix. Once fabricated the material, which is
characterised by a columnar structure, is revealed to
be competing with monocrystalline silicon for
terrestrial photovoltaic applications(1]. Due to the
various growth methods involved, structural defects,
such as dislocations, grain boundaries and intragrain
defects, can behave differently on the electric
properties of material. The performance-limiting
impurities, such as oxygen, carbon and transition
metals can be present in different concentrations in
wafers cut out of ingots. The impurities interactions
with dislocations and intragrain defects limit the
minority- carrier diffusion length then they decrease
the solar Cell efficiency. However, the new
technological approaches suggested to realize low
cost and high efficiency are insufficient. The
detailed comprehension of the parameters which
influence the carrier transport remains important in
order to improve the polycrystalline silicon solar
cells performances.
The harmful effect of
impurities and their interactions with structural
defects on polycrystalline silicon solar cells
electrical and photovoltaic properties must be
considered.
In this paper, correlations between minoritycarrier diffusion lengths and dislocation density
have been analysed. A comparison between
photovoltaic properties in particular polycrystalline
silicon solar cells efficiency grown by Photowatt
Polix (France) and Sumitomo Sitix (Japan) is
presented and discussed.The usual electrical and
structural characterization techniques have been
used along with the (SPV) method to evaluate
minority- carrier diffusion length.
Simulator PC1D is used in order to calculate
solar cells performances under standard illumination
(AM1.5G, 100mW/cm 2) .At this stage, we carried
out the calculation on the cell performance of
polycrystalline silicon solar cells using practical
device parameters which have recently been
reported. The technological parameters are selected
according to the recent development in solar cells
fabrication
Proceedings of the 5th WSEAS Int. Conf. on Microelectronics, Nanoelectronics, Optoelectronics, Prague, Czech Republic, March 12-14, 2006 (pp133-137)
2 Experimental procedure
SS1
SS2
SS3
SS4
SS5
SS6
SS7
SS8
SS9
SS10
SS11
SS12
diffusionlength (µm)
82
78
80
72
85
78
62
82
53
60
62
53
Table 1: values of minority-carrier diffusion
length (µm) at ambient temperature
for Sitix wafers.
SP1
SP2
SP3
SP4
SP5
SP6
25°C
30
35
34
33
38
40
400°C
20
30
30
30
35
35
700°
15
22
20
28
32
27
90
ln sitix
ln polix
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
4
10
3 Effect of the dislocation density on
diffusion length
1000°
08
20
18
24
30
24
Table 2: values of minority-carrier diffusion
length (µm) after RTP in Polix
wafers.
longueur de diffusion(µm)
Wafers
We have evaluated the minority-carrier diffusion
length versus dislocations density for the two types
of materials in particular at ambient temperature.
Fig.1 illustrates the variation of diffusion length
versus dislocations density. We notice that the
diffusion length decrease when the dislocation
density increases from 103cm-2 at 105cm-2 for to two
series of wafers, furthermore, we notice that SITIX
samples presents a diffusion length larger than that
of POLIX samples. The accentuated degradation in
the case of POLIX samples can be explained by the
fact that dislocations affect the electronic structure
and induce dangling bonds. In addition, impurities
precipitation or the segregation (transitions metals,
oxygen and carbon) in the vicinity of dislocations
increases their activity recombining by the means of
the deep centres generated by the impurities [4, 5].
These have a direct consequence on the decreasing
of minority-carrier diffusion length and their
lifetime.
Diffusion length(µm)
In order to study the effect of structural and
electrical parameters on the polycristalline solar
cells photovoltaic conversion, we have used two
series of polycrystalline silicon wafers cut out of
ingots grown respectively by Photowatt Polix and
Sumitomo Sitix. The wafers are P type; the carrier
concentration was 1x 1016 Cm-3. To evaluate
minority-carrier diffusion length, we have used the
surface photovoltage"SPV"[2,3];that consists in
subjecting the polycrystalline silicon wafers to an
optical excitation, under this excitation, minoritycarrier diffuse through surface establishing a
potential (SPV). The potential thus established is
proportional to the excess carriers density; that one
being in direct relationship to the minority- carrier
diffusion length.
The Polix samples were treated by rapid
thermal processing (RTP) during 20s in an FV4
furnace of JIPILEC France, with a cooling speed of
-50°C/s at a temperature range from 400°C to
1000°C.
The minority carrier diffusion length for SITIX
samples was evaluated at room temperature.The
Table 1 and Table 2 respectively gather the results
of this characterization.
Dislocation
density
(cm-2)-2)
densité
de dislocation(cm
5
10
Fig. 1: Minority-carrier diffusion length
versus dislocations density.
Proceedings of the 5th WSEAS Int. Conf. on Microelectronics, Nanoelectronics, Optoelectronics, Prague, Czech Republic, March 12-14, 2006 (pp133-137)
Mathematical models describing the diffusion
length according to the structural parameters are in
accordance with the results which we obtained.
Indeed, M. Yamaguchi et al. [6] have described
minority carrier diffusion length by the relation (1)
given below:
BaseP
N+ diffused
emitter
1/L2 = 1/L02 + 1/LD2 +1/Li2
(1)
Were:
L0 is the bulk minority-carrier diffusion length. In
addition to the effect of dislocations expressed by
LD, Li describes the various possible interactions
(grain boundaries - dislocations, dislocationimpurities) which can occur in material and
deteriorate the minority carrier diffusion length.
In conclusion, our opinion is that the parameter
dislocation remains limited to describe the transport
of the carriers in polycrystalline silicon. However
the quantification of such a phenomena remains
difficult to realize, given the evolution of the
unstable state of various interactions.
The results of electrical and structural
characterization that we have obtained are used for
the calculation of the polycrystalline solar cells
photovoltaic
performances
.However
the
determination of the optimal parameters remains an
important stage before passing to the realization.
Thus we used the PC1D version 5 [7] to simulate
the solar cells photovoltaic properties.
P+ BSF
Fig.2: Schematic of poly-Si cell unit used
for simulation.
4 Results and Discussions
Fig.3 and Fig.4 shows the polycrystalline silicon
solar cells efficiency evolution versus cell thickness
as a function of dislocation density, elaborate
respectively by Sumitomo and Photowatt. As
expected, the increase of the dislocation density
from 103cm-2 to 5.105cm-2 reduces significantly the
energetic efficiency. This degradation is more
important for Photowatt solar cells compared to
Sumitomo solar cells. Moreover for the two series
solar cells there is an optimum cell thickness for
energetic efficiency and it increases as the
dislocation density decreases. For this result, a
polycrystalline Sitix solar cells conversion
efficiency of 16% is feasible and 13% for Polix
solar cells even at a cell thickness of 20µm if the
dislocation density is less then 105cm-2.
18
16
Efficiency(%)
Fig.2 shows the schematic polycrystalline silicon
unit cell used here for simulation. The structure
assumes an N+PP+ junction, the cell area is set to be
1 cm2 which consist of a thin n emitter region
(0.15µm) doped at 1020cm-3a p base region with
thickness of 20µm and a 5µm thick P+layer serving
as a back surface field (BSF).the doping of p and p+
regions is fixed at 1016cm-3 and 5.1018cm-3,
respectively. The reflectance of 10% and the rear
and the front surface recombination velocities of
500cm/s and 100cm/s are respectively used. The
technological parameters are selected according to
the recent development in solar cells fabrication
[8,9].
The numerical simulation of this structure is
carried out using the simulator PC1D, under global
irradiance conditions AM1.5 (100mW/cm) at room
temperature. The material parameters such as the
dielectric constant, refractive index, absorption
coefficient, etc., used in this calculation are the
default settings in the model.
Contacts
rendement de conversion(%)
3 Calculation
P region
sitix
14
12
10
8
-2
3cm
ND=10
4cm
ND=10
5cm
ND=10
5
-2
ND=5.10 cm
-2
6
-2
4
1
10
Cell
thickness(µm)
épaisseur
de la cellule(µm)
100
Fig.3 : Sitix solar cells conversion efficiency vs
dislocation density .
Proceedings of the 5th WSEAS Int. Conf. on Microelectronics, Nanoelectronics, Optoelectronics, Prague, Czech Republic, March 12-14, 2006 (pp133-137)
We also notice that conversion efficiency
profile shows different paces for the two series of
samples .Indeed one remarkable point is that for a
given thickness; the SITIX solar cells efficiency is
more importantly reduced than that for POLIX solar
cells when the dislocation density increases. That’s
confirms the non linearity of the photovoltaic
conversion versus the minority –carrier diffusion
length.
polix
14
Efficiency(%)
rendement de conversion (%)
12
10
8
-2
3cm
ND=10
4cm
ND=10
5cm
ND=10
5cm
ND=5.10
6
-2
-2
-2
4
2
0
1
10
100
épaisseur de la cellule (µm)
Cell thickness(µm)
Efficiency(%
Fig.4 : Polix solar cells conversion efficiency vs
dislocation density.
18
16
14
12
10
8
polix
sitix
6
4 103 104 105 5.105
2 Dislocation density(cm-2)
0
103
104
105
5.105
-2
Dislocation density (cm )
Fig.5: comparison of conversion efficiency for the
two solar cells series.
A comparison between solar cells
conversion efficiency for the two series of samples
is illustrated by the Fig.5. An efficiency of varying
from 11% to 16% is reached for polycrystalline
silicon solar cells grown by
Sumitomo sitix
whereas that obtained for solar cells based of
Photowatt polix samples vary from 10% to 13%.
We notice that the efficiency obtained for samples
POLIX decrease little when the density of
dislocation increases of two decades, contrary with
that obtained for SITIX samples where the
efficiency variation is larger.
4 Conclusion
In this article we presented and discuss the influence
of the dislocation density over the minority-carrier
diffusion length in polycrystalline silicon.
A
comparison of polycrystalline silicon solar cells
photovoltaic performances grown by Photowatt
POLIX and Sumitomo SITIX is analyzed. The
results obtained show that the behaviour of
dislocations in the two series of samples differs
according to the technique growth .this can be
certainly to explain by the fact why the cooling
process
is certainly responsible for the
inhomogeneous formation of microscopic structures
in particular the growth of oxygen precipitates. This
confirms that the dislocation density is not the only
factor which influences the solar cell conversion
efficiency, the possible interaction of the impurities
with the intragrain defects is also the seat of the
limitation of polycrystalline silicon solar cells
performances.
References:
[1] G. Beaucarne, S. Bourdais, A. Slaoui and
J.Poortmans, Thin film polysilicon solar
cells on foreign substrates using direct
thermal CVD, Material and Solar Cell
Design, Thin Solid Films, 2002, pp.403-404.
[2] I. Baser, Semiconductor material and
devices characterization, edition wiley
intersciences, 1998.
[3] S.C. Choo, L.S.Tan, Theory of the
photovoltage at semiconductor surfaces and
its applications to diffusion length
measurements, Solid State Elect.., Vol.35,
N° 3 ,1992,pp.269-283.
[4] Z. Benmohamed,Etude de l’influence des
impuretés sur les performances des cellules
solaires au silicium multicristallin,Thèse de
Magistère, Université de Constantine
, Algérie ,1999.
[5] Z.Benmohamed,M.Remram,A.Laugier,
Effect of oxygen and carbon on efficiency
of multicristalline silicon solar cells, World
Proceedings of the 5th WSEAS Int. Conf. on Microelectronics, Nanoelectronics, Optoelectronics, Prague, Czech Republic, March 12-14, 2006 (pp133-137)
Renewable Energie Network Brighton, 1-7
July, 2000, UK.
[6] S.Pizini, P.Cagnoni, Grain Boundary
Segregation of Oxygen and Carbon, Applied
Physics Letters, Vol 51,pp.676-677
[7] M. Yamaguchi and C. Amano, Efficiency
calculations of thin-film GaAs solar cells on
Si substrates, Journal of applied
physics,Vol.58, 1985,pp.3601-3606.
[8] P.A.Basore, Proceedings of the 25th IEEE
Photovoltaic Specialists
Conference,1996,(unpublished),pp.377.
[9] A. Zerga, E. Christoffel, A. Slaoui, Twodimensional modelling of polycrystalline
silicon thin film solar cells, 3rd world
conference on photovoltaic energy
conversion, may 11-18, 2003,pp.1053-1056.
Osaka. Japan.