Nuclear Instruments and Methods in Physics Research B 186 (2002) 441±445
www.elsevier.com/locate/nimb
Porous silicon-based passivation and gettering in
polycrystalline silicon solar cells
W. Dimassi, M. Bouaõcha *, M. Sa^
adoun, B. Bessaõs, H. Ezzaouia, R. Bennaceur
Laboratoire des Applications Solaires, Institut National de Recherche Scienti®que et Technique, B.P. 95, 2050 Hammam-Lif, Tunisia
Abstract
In this work, we report on the e€ect of introducing a super®cial porous silicon (PS) layer on the electrical characteristics of polycrystalline silicon solar cells. The PS layer was formed using a vapour etching (VE)-based method. In
addition to its known anti-re¯ecting action, the forming hydrogen-rich PS layer acts as a passivating agent for the
surface of the cell. As a result we found an improvement of the I±V characteristics in dark conditions and AM1 illumination. We show that when the formation of a super®cial PS layer is followed by a heat treatment, gettering of
impurities from the polycrystalline silicon material is possible. After the removal of the PS layer and the formation of
the photovoltaic (PV) structure, we observed an increase of the light-beam-induced-current (LBIC) for treatment
temperatures not exceeding 900 °C. An improvement of the bulk minority carrier di€usion length and the grain
boundary (GB) recombination velocity were observed as the temperature rises, although a global decrease of the LBIC
current was observed for temperatures greater than 900 °C. Ó 2002 Published by Elsevier Science B.V.
Keywords: Polycrystalline silicon; Porous silicon; Passivation; Gettering
1. Introduction
Defects are usually undesirable in mono and
polycrystalline silicon because they a€ect considerably the electrical properties of the semiconductor material by acting as charge carrier
recombination/generation centres. The presence
of electric active grain boundaries [1] in polycrystalline silicon is a serious limitation for the
photovoltaic (PV) eciency in comparison to
monocrystalline silicon. For the latter, surface
*
Corresponding author. Tel.: +216-1-430-160; fax: +216-1430-934.
E-mail address: bouaicha.mongi@inrst.rnrt.tn (M. Bouaõcha).
texturization using a NaOH aqueous solution
followed by a deposition of an anti-re¯ecting
coating (ARC) increases considerably the PV conversion [2]. Unfortunately, texturization of polycrystalline silicon surface does not lead to such
improvements.
In the last few years, the formation of a porous
silicon (PS) layer at the front surface of mono
and polycrystalline cells seems to be a promising
method to improve PV eciencies [2±8]. Due to
the high densities of defects and metallic impurities, additional steps are needed to improve the
quality of polycrystalline silicon solar cells [9]: Al
and/or P-Al co-gettering were necessary to enhance the minority carrier lifetimes and then their
di€usion lengths.
0168-583X/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 8 5 7 - 6
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W. Dimassi et al. / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 441±445
In the ®rst part of this study, we detail how
polycrystalline silicon solar cells may be passivated
using PS. In the second part, our interest is focused
on the possibility of gettering impurities from the
bulk polycrystalline material by forming a PS layer
both at front and back surfaces followed by a
thermal treatment at di€erent temperatures.
2. E€ect of a super®cial PS layer
The used polycrystalline silicon is a p-type
boron-doped material manufactured by Wacker,
having a resistivity of 0.5±2.0 X cm. The phosphorous di€usion (formation of a n‡ ±p junction)
was done at a temperature of about 925 °C for 60
min. We have chosen a rather long di€usion time
to ensure a deep n‡ zone, and then to avoid the
destruction of the junction after PS formation. The
back and the front contacts are in Al and Ag,
respectively. The PS layer is then formed using
a vapour etching (VE)-based method [10] which
consists of exposing the wafers to acid vapours
issued from a mixture of HNO3 /HF. Figs. 1(a) and
(b) depict, respectively, the short-circuit current
(Isc ) and the open-circuit voltage (Voc ) versus exposure time to HNO3 /HF vapours. We found that
Isc increases of about 30% (Fig. 1(a)) when a PS
layer is formed on the front surface of the cell for
an exposure period not exceeding 25 min. However, when the exposure exceeds this limit, the Isc
current begins to decrease considerably. A similar
behaviour was observed for the Voc (Fig. 1(b)); one
can notice an enhancement for exposure periods
less than 25 min. For longer exposure periods, the
front PS layer may further damage the n‡ -doped
zone leading to the destruction of the cell.
The bulk carrier di€usion length L and the
grain boundary (GB) recombination velocity Vr
were carried out using the light-beam-inducedcurrent (LBIC) technique [11±13]. In Fig. 2, we
report the LBIC scans of a polycrystalline cell,
where curves (a) and (b) correspond to the induced
current before and after forming a thin PS layer on
the emitter, respectively. One can notice that the
collected LBIC is 10 times greater when a PS layer
is applied. One may guess that the forming PS
layer acts as a good ARC and an excellent light
Fig. 1. (a) Short-circuit current (Isc ) versus exposure time of the
cell to the HNO3 /HF vapours. The HNO3 /HF volume is 1/5,
the temperature of the acid solution is 25 °C, the cell surface
area is 4 cm2 . (b) Open-circuit voltage (Voc ) versus exposure
time of the cell to the HNO3 /HF vapours. The HNO3 /HF
volume is 1/5, the temperature of the acid solution is 25 °C, the
cell surface area is 4 cm2 .
Fig. 2. LBIC pro®les: (a) without PS and (b) after PS formation.
di€usor leading to an improvement of the absorption and hence to an enhancement of the
collected photo-current.
The LBIC improvement observed after PS formation (Fig. 2) is not sucient to con®rm whether
or not a passivation phenomenon occurred. For
this purpose, we analysed the I±V characteristics in
dark conditions (Fig. 3(A)) and AM1 illumination
(Fig. 3(B)). One can observe (Figs. 3(A) and (B))
that the super®cial PS layer acts not only as a good
ARC, but also like a passivating layer.
Several approaches proved that such an improvement (Figs. 3(A) and (B)) is not simply due
to an ARC action, but also to hydrogen-passiva-
443
Normalized LBIC current
W. Dimassi et al. / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 441±445
Fig. 4. Plot of the LBIC pro®les when the laser beam scans the
left region of a GB: (a) before and (b) after PS formation.
Table 1
Di€usion length and GB recombination velocity (a) before and
(b) after PS formation
Vr (104 cm/s)
L (lm)
(A)
(B)
Fig. 3. (A) I±V characteristics under the dark for various exposure times: (a) 0, (b) 2, (c) 4 and (d) 7 min. The HNO3 /HF
volume ratio is 1/5, the temperature of the acid solution is 25
°C. The surface area of the cell is 4 cm2 . (B) I±V characteristics
at AM1 illumination: (a) without PS and (b) with PS. The
HNO3 /HF volume ratio is 1/7, the temperature of the acid
mixture is 25 °C and the time exposure is 8 min. The cell surface
area is 25 cm2 .
Before PS
formation
After PS
formation
10
60
5
60
tion of the GB in a region near the front surface of
the cell [8].
In Fig. 4, we plot the LBIC pro®les when the
laser beam scans the left region of a GB before and
after the formation of a super®cial PS layer. We
detect (Fig. 4) a LBIC improvement in the GB
region when a PS layer was applied.
From the adjustment of the theoretical LBIC
pro®les [11±13] to experimental ones (Fig. 4), we
pointed out a decrease of the GB recombination
velocity Vr (see Table 1) when a super®cial PS layer
was formed on the front surface of the cell, while,
no changes of the minority carrier di€usion length
L was found. Consequently, the enhancement of Vr
could be attributed to the passivation of the GB
near the PS/Si interface.
3. E€ect of PS and thermal treatment combination:
gettering of impurities
The polycrystalline material used is the same as
previously described (Section 2). In this part, care
must be taken to choose approximately identical
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W. Dimassi et al. / Nucl. Instr. and Meth. in Phys. Res. B 186 (2002) 441±445
samples from ®ve consecutive polycrystalline silicon wafers sharpened successively in the same ingot. All solar cells based on the ®ve polycrystalline
Si samples were elaborated in the same conditions.
One of them was used as a reference. After the
formation of PS both on the front and the back
surfaces of the wafers, all samples (excepting one
of them) are subjected to a thermal treatment at
temperatures ranging from 600 to 1000 °C. The PS
layer was formed using the VE method [10] (the
volume ratio is 1/5). The heat treatment was done
in an infrared furnace. After that, the PS layers
were removed using a 1 N NaOH solution.
Fig. 5 shows the LBIC pro®les around the same
selected GB (in all used cells). An increase of the
photo-current was observed for temperatures below 900 °C. Beyond 900 °C, the LBIC decreases
considerably. Adjustments of theoretical LBIC
pro®les [11±13] to experimental ones, show (cf.
Table 2) an enhancement of L and Vr when the
heat treatment temperature rises. These improvements could be attributed to the migration of the
impurities from the bulk to the PS structures.
Unfortunately, when the migration of impurities
was not achieved (i.e., a part of them is not be able
to access to the PS layer), the latter will run as
recombination sites for the photo-induced carriers
and a global decay of the measured LBIC values
was observed (Fig. 5).
4. Conclusions
In this work, we have investigated the e€ect of
forming a front super®cial PS layer on the electrical characteristics of polycrystalline silicon solar
cells. We have pointed out that in addition to its
good ARC action, the hydrogen-rich PS layer acts
as a passivating agent for surface and subsurface
regions of the cell. Both I±V characteristics (under
the dark and AM1 illumination) and GB recombination velocity were improved.
From the adjustments of the theoretical LBIC
pro®les to experimental ones, we found an enhancement of the di€usion length and the GB
recombination velocity when combining PS formation and heat treatment. A global decrease of
the induced photo-current was observed for temperatures greater than 900 °C.
Acknowledgements
Fig. 5. LBIC measurements around the same GB after di€erent
heat treatment. The HNO3 /HF volume ratio is 1/5, the temperature of the acid solution is 25 °C.
Table 2
Numerical values of the di€usion length and GB recombination
velocities obtained from the adjustment of the theoretical LBIC
pro®les to experimental ones
Samples
Temperature (°C)
L (lm)
Vr (104 cm=s)
1
2
3
4
5
No heat treatment
600
750
900
1000
80
90
105
102
105
100
40
11
10
8
This work was supported by the Sectretariat
d'Etat a la Recherche Scienti®que et a la Technologie under contract PRC-98 ± INRST and
TWAS ± RG no. 98-274 RG/PHYS/Af/AC. The
authors would like to thank Dr. A. Karoui for his
help in the use of the LBIC technique.
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