19th European Photovoltaic Solar Energy Conference, 7-11 June 2004, Paris, France
PHOSPHORUS DIFFUSION AND GETTERING IN MULTI-CRYSTALLINE SILICON SOLAR CELL PROCESSING
*
Andreas Bentzen a),*, Erik S. Marstein a), Radovan Kopecek b), Arve Holt a)
a)
Section for Renewable Energy, Institute for Energy Technology
P.O. Box 40, NO-2027 Kjeller, Norway
b)
Department of Physics, University of Konstanz
Jakob-Burckhardt-Str. 27, D-78464 Konstanz, Germany
Corresponding author. Phone: +47 63806473, Fax: +47 63812905, E-mail: andreas.bentzen@ife.no
ABSTRACT: Gettering of mc-Si by P diffusion has been studied in the temperature range 840-990°C, in order t o
investigate its effect on the minority carrier recombination lifetime. We find that the recombination lifetime
increases with both increased diffusion temperature and time. However, by correlating the enhancement i n
minority carrier lifetime with the sheet resistance of the emitters, we find that phosphorus penetration alone
cannot not explain the gettering. It is found that for approximately equal emitter sheet resistances, longer
diffusion times at lower temperatures are more efficient for gettering purposes than shorter high temperature
diffusions. For successful gettering of non-recombinative traps, however, deep P penetration is required.
Finally, by applying a temperature cool down to extend the high temperature processing without altering the
emitter sheet resistance, we find that the minority carrier lifetime can be significantly increased, resulting in
increased average solar cell efficiency from 12.53% to 13.43%.
Keywords: Diffusion, Gettering, Multi-Crystalline
1
INTRODUCTION
Solar cell fabrication using multi-crystalline Si
(mc-Si) wafers constitutes a significant portion of the
industrial PV production, due to the relatively low
cost and acceptable performance of this material.
However, the rather rapid directional solidification
process during growth of mc-Si inherently introduces
additional defects in the material besides the grain
boundaries, in particular dislocations induced b y
thermal stress stemming from temperature variations
across the ingot. Moreover, due to the enhanced
solidification rate, reduced segregation of metallic
impurities into the melt can be experienced, resulting
in an elevated impurity concentration within the
crystal. Furthermore, due to enhanced segregation of
impurities towards crystal defects, the presence of
oxygen, carbon and various transition metals in the
material may significantly affect the electrical activity
of grain boundaries and dislocations.
Although the presence of metallic impurities
along dislocations and within the crystal grains in the
mc-Si wafers may impose severe limits on the
performance of processed solar cells, the thermal
properties of the metallic impurities can be exploited.
Through a process known as gettering, electrically
active metallic impurities in the crystal can be
released, diffused away, and captured at gettering sites
in an inactive state, resulting in increased carrier
diffusion lengths in the material [1]. An effective
method for achieving such gettering of metallic
impurities is via heavy P diffusion and extended high
temperature annealing to allow the impurities t o
diffuse away [2]. However, such gettering requires
prolonged high temperature treatments, significantly
contributing to the total thermal budget. Moreover,
the heavy diffusion means that the P layer formed i s
not suitable as an emitter, since shallow junctions are
required for optimal solar cell performance.
In this study, we have investigated P diffusion in
mc-Si wafers from a spray-on diffusant using a beltdriven IR furnace, in order to examine the inherent
gettering efficiency of the P diffusion during emitter
formation in an industrial Si solar cell process line.
2
EXPERIMENTAL DETAILS
P diffused emitters were prepared on 10x10cm2
0.5-2Ωcm p-type mc-Si wafers with a thickness of
about 340µm. The samples were selected as
consecutive wafers from the same block, in order t o
ensure that the initial properties were as identical as
possible. Moreover, reference wafers from the same
wafer series were also retrieved. The as-cut wafers were
chemically polished in a (10:2:5) solution of HNO3,
HF, and CH3COOH to remove surface damages
resulting from wafer sawing and obtain planar
surfaces. Immediately following a dilute HF-dip and
drying to remove any native oxide on the surfaces, the
diffusion source Filmtronics P509 was applied b y
pressurized air spraying followed by a 10 minutes
bake at 120°C to evaporate the solvents from the film.
Then, in-diffusion from the P containing film at 840990°C in an air atmosphere for 4-40 minutes was
achieved using an RTC S-1210 belt furnace. Finally,
the diffusion source residues were removed by etching
in 10% HF.
The sheet resistance of the diffused layers was
measured using a four-point probe. Measurements
were conducted at 16 different points across each wafer
to eliminate deviations due to differences in intergrain resistance. Moreover, the P diffusion profiles of
selected samples were measured by secondary ion
mass spectrometry (SIMS) using Cs+ sputtering ions
with a net energy of 13.5keV as well as electrochemical
CV (ECV) profiling, revealing the chemical and
electrical concentration profiles, respectively.
In order to analyze the gettering efficiency of the
different diffusion processes, the emitters were
removed by additional chemical polishing, removing
approximately 10µm from both sides of the wafers.
The minority carrier lifetime in the samples was then
measured by the quasi-steady-state photoconductance
(QssPC) technique [3]. In this technique, the entire
sample is illuminated by a relatively slowly decaying
infrared light pulse while the changes i n
photoconductance is measured by a 2cm diameter
inductively coupled coil, providing an average
minority carrier lifetime over the area measured by the
coil. For reliable measurements of bulk lifetime, the
surfaces need to be passivated to minimize the
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19th European Photovoltaic Solar Energy Conference, 7-11 June 2004, Paris, France
contribution of surface recombination. In the present
study, surface passivation was obtained by a light P
diffusion at 840°C for about 2 minutes on both sides.
This method for surface passivation has allowed
measurements of recombination lifetimes on CZ-Si
above 120µs. The reference wafers received the exact
same treatment as the experiment wafers, except that
there was no initial emitter diffusion in these samples.
To correlate the results with solar cell performance,
additional processing was performed. For selected
samples, saw damage was removed by a 10% NaOH
solution at 78°C, followed by surface texturization in
2%NaOH + 10%IPA at 68°C. Then, emitter diffusion
and residue removal was achieved as described earlier.
Edge isolation was performed in a CF4 plasma,
followed by the deposition of a thin SiNyHy antireflection coating. Front and back metallization was
achieved by screen printing, followed by sintering in a
RTC L-310 belt furnace.
3
RESULTS AND DISCUSSION
3.1 Phosphorus in-diffusion
During in-diffusion of P from a high concentration
surface source at relatively low temperatures, a dual
diffusion process commonly occurs. This gives rise t o
the characteristic kink-and-tail profiles in which a
high concentration, slowly diffusing region i s
surpassed by a faster diffusing tail at lower
concentrations. Moreover, if the P concentration near
the surface exceeds the solid solubility at the
diffusion temperature, the profile of electrically active
dopants will deviate from the chemical profile in the
surface region. In Fig.1, the diffusion profile of both
electrically active and chemical P is shown for an
890°C diffusion for 20 minutes, resulting in an emitter
sheet resistance on textured samples of approximately
45Ω/sq.
Figure 1: Chemical and electrically active P
concentration for an 890°C diffusion for 20 minutes.
The chemical P profile in Fig.1 clearly shows the
dual nature of the diffusion process. In the low
concentration tail, P diffuses in Si primarily via
interactions with Si self-interstitials [4]. For higher P
concentrations, a conversion from an interstitialcy to a
slower vacancy-mediated process occurs, giving rise
to the anomalous P diffusion profiles [5,6]. The kink,
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observed at a depth around 0.15µm and with a
concentration of approximately 2x1019cm-3 represents
a transition between the high concentration and the
low concentration regimes. Moreover, it has been
shown that such a dual diffusion process results in a
significant supersaturation of Si self-interstitals
extending well beyond the P profile itself [7].
Also shown in Fig.1 is the diffusion profile of
electrically active P. It is seen from the figure that this
profile follows that of the chemical concentration
until the latter is sufficiently high, and then abruptly
changes to a flat plateau. For the given profile, this
occurs at a P concentration of about 3x1020cm-3. Above
this concentration, P diffuses in a partially neutral
state, giving rise to the well-known phenomenon of a
dead layer.
3.2 Recombination lifetimes and trapping effects
In the QssPC technique, the minority carrier
lifetime is calculated by measuring the change i n
photoconductance when exposed to a light pulse [3].
These calculations assume that the excess minority
and majority carrier concentration are equal, i.e. that
there is negligible non-recombinative trapping in the
samples. However, for mc-Si samples, such trapping i s
a common phenomenon, leading to anomalously high
measured minority carrier lifetimes at low injections
[8]. This anomality is due to the fact that the trapped
minority carriers must be compensated by an equally
large amount of excess majority carriers to obtain
charge neutrality, resulting in an increased majority
carrier density and an enhanced measured
photoconductance than what is expected without such
trapping. A model to physically explain this effect has
been proposed by Hornbeck and Haynes [9], and also
by Fan [10], and has later been applied to mc-Si wafers
by Macdonald and Cuevas [8].
Figure 2: Apparent recombination lifetime of the
reference as measured by QssPC (open circles), and a
fit using the Hornbeck-Haynes model (solid line).
By fitting the Hornbeck-Haynes model to minority
carrier lifetimes measured by the QssPC technique, one
can obtain a value of the average density of nonrecombinative traps as well as the recombination
lifetime, providing the latter is constant with injection
level. This is generally not correct, but in many cases
when the minority carrier lifetimes are relatively low,
the assumption is acceptable.
19th European Photovoltaic Solar Energy Conference, 7-11 June 2004, Paris, France
Fig.2 above shows the apparent minority carrier
lifetime as a function of minority carrier density of the
reference sample measured by QssPC. Also shown i n
the figure is the fitted data using the Hornbeck-Haynes
model as discussed above. The model reveals a
recombination lifetime of 4.0µs and a trap density of
3.9x1014cm-3.
3.3 Gettering efficiency of phosphorus diffusion
To investigate the gettering efficiency of the
different emitter diffusions, the minority carrier
lifetimes in the samples were measured using QssPC
after emitter removal and passivation of the surfaces.
The results of the QssPC measurements were analyzed
by fitting the Hornbeck-Haynes model to the measured
data. In Fig.3 below, the recombination lifetime given
by the fitted data are plotted against diffusion time for
different diffusion temperatures. The broken line at
4.0µs is the recombination lifetime in the reference
sample shown in Fig.2 above.
polished prior to the diffusion. For samples with a
surface texture, the sheet resistance is increased with a
factor of about 1.5. The results in Fig.4 indicate that P
penetration alone cannot completely explain the
enhancement in recombination lifetime. It can be seen
from the figure that for approximately the same emitter
sheet resistance, the gettering is more efficient for
lower diffusion temperatures. Thus, longer processing
times are generally more efficient for lifetime
gettering.
Figure 4: Recombination lifetime as a function of
sheet resistance.
Figure 3: Recombination lifetimes versus diffusion
times for different temperatures.
Fig.3 shows that the minority carrier lifetime
generally increases both with increasing temperature
at the same diffusion time and with increasing time at
the same temperature. An exception is the sample
processed at 940°C for 38 minutes, revealing a
somewhat lower lifetime than what might be expected.
The exact reason for this is not identified. Due to the
increasing P diffusivity at higher temperatures, the P
penetration depth increases with both time and
temperature. Thus, the trend of increasing minority
carrier lifetime with time and temperature as shown i n
Fig.3 above could be explained by the enhanced P
penetration. Nevertheless, long low temperature
diffusions may reveal a similar P penetration as a
shorter high temperature profile. In order to give a
more complete understanding on the relationship
between different diffusions and the gettering
efficiency, we need to consider differences in the
profiles. In Fig.4, the recombination lifetimes are
plotted against the emitter sheet resistance measured
after the diffusions. The figure shows that for a given
temperature, the recombination lifetime increases with
decreasing the sheet resistance. This is in accordance
with expectations, since longer diffusion times will
result in lower sheet resistances.
It should be mentioned that the sheet resistance
was measured on samples that were chemically
In section 3.1 we have seen that P in-diffusion
from a high concentration source can be explained as a
dual process with two different diffusivities; a low
concentration region of high diffusivity and a high
concentration region with a lower diffusivity. The
high concentration region has higher activation
energy for diffusion than the low concentration tail,
thus the latter will be less influenced by changing the
diffusion temperature [6]. In fact, we have previously
found that the evolution of the emitter sheet resistance
is primarily given by the high concentration part of
the diffusion profiles [11]. Hence, two diffusions at
different temperatures and times giving the same
emitter sheet resistance will reveal similar high
concentration regions, whereas the low concentration
tail of the profile diffused at a lower temperature will
penetrate slightly deeper into the sample [11]. Thus,
enhancement of the recombination lifetime due to P
in-diffusion can be only partly explained by the
penetration of P itself. Upon high concentration indiffusion of P, injection of Si self-interstitials into the
bulk of the sample will be significant [7], possibly
assisting in the release of bulk interstitial impurities.
This model of impurity gettering was abandoned b y
Kang and Schroder, partly due to their finding that the
gettering efficiency is decreased at temperatures
exceeding 1050°C [1]. We believe that further
investigations are needed to identify the role of Si
self-interstitials in P gettering.
We now consider the evolution of the density of
non-recombinative traps upon P diffusion. In Fig.5
below, trap densities found by fitting the HornbeckHaynes model to the QssPC data are plotted against
sheet resistance for different diffusion temperatures
and times. The broken line at 3.9x1014cm-3 is the trap
density in the reference sample shown in Fig.2.
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19th European Photovoltaic Solar Energy Conference, 7-11 June 2004, Paris, France
Figure 5: Density of non-recombinative traps as a
function of sheet resistance.
It is seen from Fig.5 that the evolution of the trap
density shows a different behavior than the
recombination lifetime shown in Fig. 4. It appears
from the figure that a very low sheet resistance i s
needed to obtain significant gettering of nonrecombinative traps. This indicates that the Si selfinterstitials injected from the high concentration
region of the phosphorus diffusion could play an
important role in the gettering of such traps.
Moreover, since all the temperatures show a slight dip
in the trap density at the longest diffusion time
(lowest sheet resistance for a given temperature), i t
could be possible that the formation and migration of
dislocations are important for efficient gettering of
these traps.
3.4 Effect of processing time on gettering efficiency
In order to further explore the role of processing
time alone on the minority carrier recombination
enhancement, we now consider different diffusions all
giving the same emitter sheet resistance on polished
wafers of about 30Ω/sq. When processed on samples
with a surface texture, emitter sheet resistances
suitable for fabrication of screen printed solar cells at
approximately 45Ω/sq are found. In addition, we
considered diffusion at 890°C for 20 minutes, after
which the temperature was abruptly lowered to 800°C
in order to slow down the P diffusion and then
gradually ramped down to 700°C during 58 minutes.
The samples diffused at 890°C, both with and without
the ramp down, were processed through to finished
screen printed solar cells as described in section 2, and
their performance were measured in a solar simulator
under AM1.5 conditions. The results of these
investigations are summarized in Table 1 below.
Table I: Minority carrier lifetime (τ), trap density, and
average processed cell efficiency (η) of samples with
emitter sheet resistance of 45Ω/sq on textured wafers.
Trap density
τ
Cell η
Diffusion
(cm-3)
(µs)
(%)
— (Reference)
4.0
3.9x1014
—
940°C, 4min
4.4
4.1x1014
—
890°C, 20min
7.8
4.5x1014
12.53
Ibid + ramp
14.8
3.0x1014
13.43
down
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It is shown in Table I above that the recombination
lifetime increases nearly by a factor of two, from 7.8 t o
14.8µs, by adding the ramped cool down after the
diffusion. Furthermore, as already discussed before, we
see that a 20 minutes diffusion at 890°C results in a
notably higher recombination lifetime than a 4
minutes diffusion at 940°C. These results further
indicate that the total processing time at elevated
temperatures is more important than the P penetration
in respect to gettering efficiency. Contrarily, although
a slight decrease in trap density is observed after the
ramped cool down, it seems that by comparing with
the results found in the previous section, a deep P
penetration is necessary to obtain significant
gettering of traps.
Finally, we show the importance of achieving
successful gettering by fabricating screen printed
solar cells on textured wafers. The average solar cell
efficiency on a batch of 25 samples was observed t o
increase from 12.53 to 13.43% by the addition of the
ramped cool down.
4
CONCLUSIONS
We have found that the recombination lifetime
increases with both increasing temperature and time of
the phosphorus emitter diffusion. Moreover, b y
comparing the recombination lifetimes to the emitter
sheet resistances, we find that a longer duration of the
diffusion process is more favorable than high
temperatures for lifetime gettering purposes. For the
gettering of non-recombinative traps, however, a deep
P penetration is needed to observe significant decrease
in the trap density.
ACKNOWLEDGEMENTS
This project was financed by the Norwegian
Research Council. The authors wish to thank Dr.
Margareta Linnarson at the Royal Institute of
Technology (KTH, Sweden) for assistance with the
SIMS measurements.
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