Solar Energy Materials & Solar Cells 59 (1999) 377}385
Formation of porous silicon for large-area silicon
solar cells: A new method
M. Saadoun , H. Ezzaouia , B. BessamK s *, M.F. Boujmil ,
R. Bennaceur
Institut National de Recherche Scientixque et Technique, Laboratoire de Photovoltan( que et des Mate& riaux
Semiconducteurs, BP 95, 2050 Hammam-Lif, Tunisia
Laboratoire de Physique de la Matie% re Condense& e, Faculte& des Sciences de Tunis, De& partement de Physique,
1006 Le Belve& de% re, Tunis, Tunisia
Received 29 December 1998
Abstract
Luminescent porous silicon (PS) was prepared for the "rst time using a spraying set-up,
which can di!use in a homogeneous manner HF solutions, on textured or untextured (1 0 0)
oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the
front-side surface of silicon solar cells, by supplying very "ne HF drops. The front side of N>/P
monocrystalline silicon solar cells may be treated for long periods without altering the front
grid metallic contact. The monocrystalline silicon solar cells (N>/P, 78.5 cm) which has
undergone the HF-spraying were made with a very simple and low-cost method, allowing
front-side Al contamination. A poor but expected 7.5% conversion e$ciency was obtained
under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying
time and #ow HF-spraying rate, Al contamination favours the formation of a thin and
homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface re#ectivity up to 3% (i.e., increase light
absorption), improves the short circuit current (I ), and the "ll factor (FF) (i.e., decreases the
series resistance), allowing to reach a 12.5% conversion e$ciency. The dramatic improvement
of the latter is discussed throughout the in#uence of HF concentration and spraying time on the
I}< characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer
acts as a good anti-re#ection coating (ARC), it improves the spectral response of the cells,
especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to
surface band gap widening and conversion of a part of UV and blue light into longer
* Corresponding author. Tel.: 216-1-430 044; fax: 216-1-430 934.
E-mail address: brahim.bessais@inrst.rnrt.tn (B. BessamK s)
0927-0248/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 5 7 - 4
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M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
wavelengths (that are more suitable for conversion in a Si cell) throughout quantum con"nement into the PS layer. 1999 Elsevier Science B.V. All rights reserved.
Keywords: Silicon; Porous silicon; Solar cells
1. Introduction
In 1986, Yablonovitch et al. [1] showed that oxidation of silicon followed by an
Fluorhydric acid (HF) etch results in the formation of Si-H bonds, which made
passive the recombination centres. Preparation in this manner gives the lowest carrier
recombination velocity value ever reported for any semiconductor. Since this date,
a brief HF etching has been used to improve the carrier lifetime and hence ameliorate
the current density of silicon solar cells. In solar cells processing, the HF etching could
be applied after sintering the metallic contacts, just before encapsulation of the cells, to
avoid possible degradation. However, in all cases the HF etching time cannot exceed
10 s, otherwise the metallic contacts are damaged. It is evident that in an industrial
solar cells processing, an HF etching step may involve some risks overall regarding
the adhesion and the quality of the ohmic contacts. Recently, it has been shown that
the electrochemical etching of silicon in HF produces a porous silicon (PS) layer rich
in hydrogen [2]. Since this event, several workers searched to take bene"ts from the
Si}H-rich layer forming PS. Thus many attempts have been done to introduce PS in
photovoltaic devices. Primitive solar cells using PS have been demonstrated. However, few higher-e$ciency cells based on PS were reported. In forming PS, highly
textured surfaces are obtained, enhancing light trapping and its potential use as an
anti-re#ection coating [3]. The use of PS as an optimised emitter has been shown to
be possible in a Si solar cell [4]: the PS layer is formed onto the top surface of the
emitter by electrochemical etching of the surface, then the emitter consists of a top
layer of PS and a bulk layer of N> Si. The quantum e$ciency measurements show the
e!ectiveness of the PS layer in Si solar cells. The main problems are due to the series
resistance which limit the "ll factor (FF), limiting the conversion e$ciency. But, in all
these attempts PS was formed using the conventional electrochemical etching process
in an HF solution. This technique is known to be aggressive, so to take bene"ts from
PS as an anti-re#ection coating or as a passivating layer, very short anodisation time
((5 s) must be applied to avoid destroying the junction and damaging the front grid
contacts. Recently, encouraging results have been reported by Schirone et al. [5], who
produced large-area solar cells by converting the Si surface into PS by etching in
controlled solutions; they reported a 100 cm cell with an e$ciency of 10.4% (AM1.5),
with improved photon absorption at near infrared radiation and surface passivation.
In this work, we demonstrate, for the "rst time, that PS may be formed on the top
surface of large area N>/P monocrystalline silicon solar cells by spraying in an
homogeneous manner concentrated HF solutions for long periods. From I}< characteristics, we optimise the HF concentration and the HF-spray-etching time. The
e!ects of both parameters on the Photovoltaic (PV) features of the cells (current
density, "ll factor (FF) etc.) are discussed. The bene"ts of forming thin PS layer on
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
379
both surface re#ectivity and internal quantum e$ciency are also shown. This work
does not consist, from a technological point of view, to ameliorate the e$ciency of
monocrystalline silicon solar cells. Our aim is simply to demonstrate qualitatively the
dramatic improvement that can provide the application of a thin PS layer, formed
within an HF-spray-etching process, on the characteristics of silicon solar cells.
2. Experimental
The N>/P monocrystalline silicon solar cells (78.5 cm, p-type solar-grade cells)
were prepared by the usual phosphorus di!usion technology. The phosphorus source
is a POCl /Propanol-2 solution. The latter is spread by the spinning technique onto
p-type NaOH-textured Si wafers. Both front grid and back side metallic contacts were
realised by screen printing a silver paste and an aluminium/silver paste, respectively.
The front grid contact must be realised before forming PS to limit the high series
resistance. The metallic contacts undergo a co-"ring in an Infrared furnace, in air,
without any further precautions (such as ventilation and controlled atmosphere...).
Owing to I}< characteristics, we have optimised the ratio POCl /Propanol-2 to and
temperature and time exposure within di!usion to 9253C and 20 min, respectively. The
edges of the cells were mechanically etched. This low-cost technique allows us to achieve
expected poor quality cells having a conversion e$ciency of about &7.5%. This poor
e$ciency is needed, to show the dramatic bene"ts that can provide a thin PS top layer
on the characteristics of the Si cells. A speci"c HF-spraying set-up was built to supply
"ne HF drops in order to treat the top surface of the cells for long periods. The speci"c
spraying nozzle is mounted so that it can execute an automated X}> scanning.
3. Results and dicussion
3.1. Formation of porous silicon
In the preparation of PS by stain etching, it was reported that pure HF cannot
produce alone the required holes to start forming PS. It was always recommended to
add HNO to the etching solution. However, a new method was reported recently [6]
to prepare thin PS layers (&1000 As ) with HF/HNO : a thin Al "lm was deposited by
evaporation prior to etching, the reaction between Al and HNO produces the very
fast start of the chemical etching of Si. Following these recent results, we have
intentionally contaminated the front of the N> side of the cells by performing a simple
co-"ring of the metallic contacts, so that Al vapour contamination occurs, prior to
form PS by HF-spray-etching. The advantage of our technique is to use HF solution
only, instead of HF/HNO , because of the aggressive etch of the latter, which may
seriously damage the front metallic contacts of the cells.
The HF-spray-etching that we have performed consists to treat for long periods the
front-side surface of the cells to make it passive by Si-H bonds, following the
statements of Yablonovitch et al. [1]. As previously said, this HF treatment should be
done after screen printing and sintering the metallic contacts, to take bene"ts from Al
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M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
vapour contamination. The co-"ring is a key step in the formation of the PS layer.
Indeed, we have noticed that when the front side of the cells undergo an HF-sprayetching after screen printing and sintering the contacts, a good and uniform surface
darkening occurs, leading to the formation of a homogeneous PS layer. On the
contrary, if the HF-spray-etching is performed before sintering the contacts, one may
obtain at the most dispersed darkish stains, due to very weak Al contamination of the
front side of the cells, and hence the formation of an inhomogeneous and poor PS
layer. During the HF-spray-etching step, the cells are uniformly heated with a temperature-regulated hot plate. The temperature of the hot plate is determinant to obtain
an homogeneous frontal surface darkening. When the darkish surface is excited with
an UV light, it emits bright red}orange luminescence which can be seen with the
naked eye. On the other hand, the darkish aspect of the surface of the cells is not
removed when etching is performed in concentrated HF solution, while removed
when the wafers are etched in NaOH (1 N) solution. These two latter "ndings con"rm
the formation of PS. Fig. 1 shows the PL spectrum emitted by a thin PS layer
prepared by HF-spray-etching. One can notice that the forming PS layer emit in the
red-orange spectral region with a typical peak at 1.72 eV.
Thus, the di!erence of results between HF-spray-etching the cells before and after
sintering the metallic contacts seems to be due to the co-"ring step where we have
intentionally contaminated the N>-type front layer by Al vapours (no ventilation
done during the co-"ring). Paradoxically this simple and low-cost co-"ring step
contributes (intentionally) to the poor quality of the junction (i.e., conversion e$ciency), but plays an important role in the formation of PS and in the darkish aspect of the
front surface of the cells. Indeed, in optimising the HF #ow rate and the temperature
of the hot plate (which may have a weak oxidising e!ect) HF may attack at a very low
rate the Al-based aggregates formed at the front N>-type surface during the co-"ring,
leading to the formation a thin PS layer rich in passivating species (i.e., Si}H species)
V
and acting as a good anti-re#ecting coating (Fig. 2). Fig. 2 depicts the re#ectivity of the
Fig. 1. Room temperature PL spectra of a thin PS layer prepared by HF-spray-etching the N> emitter of
N>/P monocrystalline silicon solar cell.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
381
Fig. 2. (a) Re#ectivity of a textured monocrystalline silicon solar cell; (b) Re#ectivity of a textured
monocrystalline silicon solar cell in the presence of a thin porous silicon layer onto the front-side surface.
front-side surface of a textured monocrystalline silicon solar cell without (Fig. 2a) and
with (Fig. 2b) a thin porous silicon layer. After PS formation, the re#ectivity of the cell
decreases from &12% (textured cell without PS layer) to &3% (textured cell with
a PS layer). It should be noted that the darkish colour of the surface occurs after 5 mn
of HF-spray-etching. Beyond 5 mn of HF treatment the front surface re#ectivity is
independent of HF-spraying time whatever the HF concentration may be.
Now, it is important to optimise the HF concentration and the HF-spray-etching
time to obtain the maximum bene"ts from the thin PS layer, without destroying the
N>/P junction and damaging the metallic contacts.
3.2. Ewect of HF concentration on the I}< characteristics
Fig. 3 shows the evolution of the I}< characteristics taken under AM1 illumination
for sprayed HF concentration varying between 0% and 40%, for a "xed spraying time
of 5 mn.
In Table 1, we report the variation of the PV parameters (< , I , J , FF and
conversion e$ciency g) with HF concentration. From Table 1, one may notice that
the current density (J ) increases with HF concentration. Since the re#ectivity (Fig. 2)
is independent of HF concentration and spraying time (as previously said), the
increase of the current density with HF concentration is mainly due to the presence of
further Si}H bonds. So, this phenomenon seems to be due to further surface passivation as HF concentration increases. However, we note an improvement of FF and the
conversion e$ciency (g) with HF concentration up to 20% of HF concentration.
Beyond this value, we notice that FF and g decrease.
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M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
Fig. 3. E!ect of the sprayed HF concentration on the I}< characteristics. Spraying time is "xed to 5 min.
The I}< characteristics are measured under AM1 illumination.
Table 1
In#uence of HF concentration on the photovoltaic parameters of the cells. Spraying time was "xed to 5 mn
HF concentr.
Without HF
10%
20%
30%
40%
0.580
1.665
21.26
60.11
7.83
0.573
1.883
23.96
64.32
9.48
0.584
2.012
25.64
66.82
10.03
0.561
2.106
26.84
56.31
8.07
0.538
2.214
28.21
42.25
7.68
Parameters
< (V)
I (A)
J (mA/cm)
FF(%)
g(%)
The deterioration of these characteristics beyond 20% of HF concentration may be
due to a notable increase of the series resistance (cf. Fig. 3) and to the damaging of the
contact resistance. The open circuit voltage (< ) breaks down at 40% HF; probably
at this concentration the N>/P junction begins to degrade. As shown in Table 1, the
better performances are achieved for an HF concentration of 20%. Now, we search to
optimise the HF-spray-etching time that gives the best I}< characteristic at 20% HF
concentration.
3.3. Ewect of HF-spray-etching time on the I}< characteristics
Fig. 4 depicts the in#uence of HF-spray-etching time on the I}< characteristics
taken under AM1 illumination for an optimised HF concentration of 20%.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
383
Fig. 4. E!ect of Spraying time on the I}< characteristics. HF concentration was optimised to 20%. The
I}< characteristics are measured under AM1 illumination.
Table 2
In#uence of HF-spray-etching time on the photovoltaic parameters of the cells. HF concentration was
optimised to 20%
t(mn)
0
5
10
15
20
25
0.580
1.692
21.56
55.38
7.47
0.583
2.037
25.95
70.07
10.61
0.580
2.129
27.13
72.23
11.68
0.585
2.234
28.46
74.17
12.47
0.585
2.163
27.56
69.54
11.28
0.548
2.182
27.80
47.37
7.39
Parameters
< (V)
I (A)
J (mA/cm)
FF(%)
g(%)
The PV parameters values versus HF-spray-etching time are shown in Table 2. We
notice an improvement of the cell parameters up to 15 mn of HF-spray-etching.
Beyond this time, we observe a degradation of the PV performances. We should note
that the open-circuit voltage undergoes a little variation during the HF treatment.
From Figs. 3 and 4 (i.e., Tables 1 and 2), we conclude that the dependence of the I}<
characteristics on HF concentration and HF-spray-etching time are similar. Two
steps characterise the evolution of the I}< characteristics. The "rst step corresponds
to an improvement of the PV performances: when both HF concentration and
HF-spray-etching time increase the PV performances attain a parametric limit corresponding to a maximum e$ciency of about 12.5% (for the maximum e$ciency HF
concentration and HF-spray-etching time were optimised to 20% and 15 min, respectively). The second step corresponds to a degradation of the PV performances when
384
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
the treatment parameters go beyond the optimal limit. At "rst sight, the improvement
of the PV parameters may be due to the existence of two simultaneous phenomena:
surface passivation by Si}H bonds and formation of an anti-re#ecting coating.
It is well known [1] that the hydrogen provided by HF improves the short-circuit
current and the FF. This phenomenon is ampli"ed by the formation of a PS layer
which, owing to its large internal surface contains an important quantity of hydrogen
in surface as well as in volume. The formation of the PS layer decreases the N>-layer
thickness and hence reduce the undesired dead layer. However, beyond a certain HF
concentration and HF-spray-etching time limits (i.e., 20% and 15 min, respectively)
HF vapours attack the metallic contacts and weakens their adherence to the cell.
3.4. Spectral response
Fig. 5a and b shows the in#uence of incorporating PS on the spectral response (SR)
(i.e., the internal quantum e$ciency) of a Si solar cell.
The spectral response SR is expressed as
J (j)
,
SR(j)"
qN(j)(1!R(j))
where J (j) is the photocurrent density, N(j) is the monochromatic photon #ux and
R(j) the re#ectivity (di!use and specular) at a given wavelength j.
As shown in Fig. 2, when PS is formed, the surface re#ectivity fall down from
&12% to &3%. The decrease of R(j) increases light absorption and hence J (j).
Fig. 5. A comparison between the spectral responses of (a) SiO }passivated monocrystalline Si solar cell
and (b) in presence of a thin PS layer formed by HF-spray-etching the front surface of the cell.
M. Saadoun et al. / Solar Energy Materials & Solar Cells 59 (1999) 377}385
385
Fig. 5 clearly demonstrates that the internal quantum e$ciency (SR) of the HFspray-etched Si solar cell (formation of PS) is higher than that of SiO -passivated one.
In fact, despite increasing light absorption from the highly textured surface that
provides the thin PS layer, the photoluminescence property of the latter allows to
convert (with a certain e$ciency) UV and blue light into longer absorbable
wavelengths (in the red region). This may generate a small additional photo-current,
in the UV and blue part of the solar spectrum, that may improve the internal quantum
e$ciency in this spectral region, as shown in Fig. 5b. However, the main contribution
to the improvement of the internal quantum e$ciency is signi"cantly due to the
surface passivation of the cells by the hydrogen of the PS layer.
4. Conclusion
We have shown for the "rst time that spraying for long periods the front side of
N>/P monocrystalline silicon solar cells with "ne HF drops leads to the formation of
a darkish thin porous silicon layer. It was shown that during the co-"ring step of the
metallic contacts, Al contamination of the N> layer occurs and plays an important
role in the start of the homogeneous chemical etching of Si, leading to PS formation. It
was found that at optimised HF-spray-etching conditions, the presence of the thin PS
layer signi"cantly improves the characteristics of the cells, allowing to improve the
conversion e$ciency from 7.5% to 12.5% and the FF from &60% to &74%,
suggesting that the thin Si}H rich PS layer acts not only as a good anti-re#ection
coating, but also as a passivating layer. A signi"cant improvement of the internal
quantum e$ciency is also shown, overall in the UV } blue spectral zone. This
HF-spray-etching method seems to be promising in polycrystalline silicon solar cells
technology.
Acknowledgement
This work was supported by the SecreH tariat d'Etat à la Recherche Scienti"que et
à la Technologie (P96EN01). The authors would like to thank M.Oueslati, Pr at the
faculty of Sciences of Tunis for his help in PL measurements.
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