CRYSTALLINE Si SOLAR CELLS FROM COMPENSATED MATERIAL:
BEHAVIOUR OF LIGHT INDUCED DEGRADATION
Radovan Kopecek, Jayaprasad Arumughan and Kristian Peter
International Solar Energy Research Center - ISC Konstanz, Rudolf-Diesel Str.15 , D-78467 Konstanz, Germany
e-mail: radovan.kopecek@isc-konstanz.de
Ethan A. Good
Dow Corning Corporation, 2200 W. Salzburg Road, Midland, Michigan, United States
Joris Libal, Maurizio Acciarri and Simona Binetti
Department of Material Science, Università di Milano-Bicocca, via Cozzi 53, I-20125 Milano, Italy
ABSTRACT: In this paper we focus on the detailed investigation of light induced degradation behaviour of cells
from so called Solar Grade silicon (SoG-Si) material. Different types of materials were studied at various
compensation levels. Large area industrial solar cells without surface texture with high efficiencies on mc-Si (>15%)
as well as Cz-Si (>16%) substrates were fabricated and exposed to long time illumination at 1 sun. We observed that
on one hand the light induced degradation (LID) was comparable to reference substrates and on the other not
dependent on the absolute but on the net boron content only. One possible explanation for this unforeseen finding is
that the phosphorus atoms in the SoG-Si material interact with boron centers reducing the recombination activity of
e.g. B-O complexes.
Keywords: crystalline Si solar cells, SoG-Si, light induced degradation
INTRODUCTION
Presently many silicon purification technologies
based on low-cost approaches are being developed by
different companies leading to so called Solar Grade
silicon (SoG-Si) material [1, 2, 3]. Such Si materials may
have a higher content of metallic impurities as well as of
dopants (e.g. boron, phosphorus) as compared to the
electronic grade Si wafers from highly purified and
chemical vapour deposited poly-Si. However, the so
called Siemens process is currently constrained by capital
expansion costs.
The commonly applied industrial solar cell process
based on P-diffusion and firing through PECVD SiNx by
metal pastes includes various effective gettering and
passivation steps itself which additionally purify the Si
bulk from highly diffusive impurities. These processes
lead to solar cells with high efficiencies without the need
of extremely pure Si base material. A threshold
concentration exists whereby increased dopant
compensation might have a negative impact on cell
performance as well as on the long term stability
compared to p-type devices with a negligible content of
phosphorus.
2
MATERIAL USED
For our studies directionally solidified mc-Si material
as well as mono-crystalline Cz-Si ingots were used.
2.1 mc-Si ingots
Two ingots were fabricated with a similar range in
resistivity as can be seen in Figure 1. For the mc-Si
reference highly pure poly-Si material was used whereas,
for the second ingot, 10% of SoG-Si material was
blended with poly-Si.
2.2 Cz-Si ingot
As for the mc SoG-Si ingot 10% SoG-Si feedsock
from a different supplier was admixed to pure poly-Si
material for Cz-growth. In this case additional
phosphorus was added which resulted in higher substrate
resistivities as depicted in Figure 1.
specific resistivity [Ωcm]
1
Cz SoG-Si
mc SoG-Si
mc reference
10
1
0,1
0
20
40
60
80
100
fraction solidified after caps removal [%]
Figure 1: Specific substrate resistivity as a function of
fraction solidified. The top and bottom parts have been
removed prior to wafering.
The red curve decreases towards the right which is
the part that solidifies the last in the crystallisation
process. For both compensated materials, due to different
segregation coefficients of boron as compared to
phosphorus, the resistivity increases. The lastly solidified
part has the highest compensation level. I case of the CzSi ingot the compensation ratio Rc=(Na+Nd)/( Na-Nd) is
increased from 1.2 at the top of the ingot (left side of
Figure 1) to almost 23.5 at ingot bottom. The calculated
hole carrier concentration (electrically active B-atoms)
increase from 9x1015cm-3 to 1.4x1016cm-3. More details
can be found in [5].
SOLAR CELL PROCESS AND RESULTS
Large area wafers from all three ingots were used for
the processing of industrial solar cells.
3.1 Solar cell process
The process flow chart shown in Figure 2 was used
for both – mc- and Cz-Si solar cells.
saw damage removal and wafer cleaning
17
solar cell efficiency [%]
3
16
15
14
13
Cz SoG-Si
mc SoG-Si
mc reference
12
11
10
POCl3 emitter-diffusion
0
20
40
60
80
100
fraction solidified after removal of caps [%]
PECVD SiNx
Figure 3: Efficiencies of untextured mc- and Cz-Si solar
cells as a function of solidified fraction.
screen printing of front and rear, drying
co-firing
4
edge isolation
Figure 2: Solar cell process flow chart (top) and large
area mc-Si (left) and Cz-Si (right) solar cells processed
with these steps (bottom).
The process starts with alkaline saw damage etching and
wafer cleaning. Texturing was not included as for our
investigations it was not a priority to reach highest
efficiencies possible. The P-emitter was formed on both
sides in an open tube furnace and subsequently one side
was covered by a PECVD SiNx. The front and rear
contacts were screen-printed and co-fired in an metal belt
furnace. At the end the pn-junction was isolated by a
dicing saw. In the bottom part of Figure 2 two pictures
demonstrate the finished mc-Si and Cz-Si solar cells.
3.2 Results
Wafers selected through the entire ingot were
processed and characterised. Figure 3 summarises the
results of solar cell efficiencies as a function of ingot
position. Even without textured front surface high
efficiencies such as >15% for mc-Si substrates and >16%
for Cz-Si substrates were reached. In the case of the mcSi solar cells the influence of the bottom part of the ingot
is visible through an efficiency drop. However the drop is
more pronounced for the mc-Si reference suggesting that
the drop is due to the in-diffusion of impurities from the
crucible.
LIGHT INDUCED DEGRADATION STUDIES
In the last paragraph we have shown that high
efficiencies can be achieved for solar cells on SoG-Si
material on wafers from all parts of the ingot. However
there is some concern that such cells could behave
differently compared to “classical ones” (from pure polySi) when illuminated for long time. Therefore we have
performed light induced degradation studies for the three
types of solar cells and from different ingot positions.
4.1 Experimental set up
An array of six 500W lamps was illuminating the
cells from a distance of about 40 cm (Figure 4) exposing
them to a light intensity about 1 sun and heating them to
around 50°C (typical temperature in a module).
Figure 4: Setup for the measurement of light induced
degradation. Six 500W lamps are mounted in a distance
of about 40 cm from the cells illuminating them with
about 1sun and heating to 50°C.
4.2 Measurements on solar cells
The solar cells were measured before and after such
long term illumination and the difference of the cell
parameters was calculated. Two measuring set-ups were
used for characterisation: a sun simulator measuring all
relevant cell parameters and a sunsVoc set-up from
Sinton measuring the Voc drop of the cells.
10
illumination: one week at 1sun
measurement: sun-simulator
∆Voc [mV]
2
∆Jsc [mA/cm ]
∆η [%]
ingot. As the interstitial oxygen concentration is
comparable for both ingots and the boron concentration
higher for the SoG-Si ingot the formation of B-O
complexes must be somehow suppressed.
4.4 Results for mono Cz-Si cells
Additional studies on mono c-Si solar cells from SoG
Cz-Si wafers have confirmed our previous findings on
mc-Si cells. Five Cz-Si cells from different ingot
positions were selected and LID was measured as a
function of time. The resulted decay in Voc is depicted in
Figure 6.
0
position from top
100 (first solidified)
800
900
1000
1100 (last solidified)
T=50°C
-5
∆ Voc [mV]
4.3 Results for mc-Si cells
In our first experiments we measured the drop in
efficiency of cells from the mc SoG-Si ingot in order to
gain information about all relevant cell parameters.
Figure 5 (on top) depicts the average drop in Voc, Jsc and
in the cell efficiency for 3 ingot positions. A clear trend,
namely a decrease in degradation, of all cell parameters is
observed toward the top of the ingot. Such a trend implies
that the degradation is dominated by the formation of B-O
complexes as the interstitial oxygen concentration
decreases towards the ingot top. After temperature
treatment of the cells at 200°C for 10 minutes the
degradation was regenerated completely which underlines
our assumption.
It is much more accurate to measure the Voc as it is not
dependent on light intensity as Jsc thus in our following
measurements the sunsVoc set-up was used for
comparative studies.
high boron content
high compensation
(∆Voc=8 mV)
-10
-15
degradation
1
-20
-25
0,1
low boron content
low compensation
(∆Voc=20mV)
0
10
20
time [h]
0,01
bottom
middle
illumination: 2 days at >1sun
6
∆Voc [mV]
5
Figure 6: Light induced degradation curves as a function
of time for solar cells from different positions of the CzSi ingot. The plotted lines are guides to the eyes only.
top
sunsVoc
reference
mc SoG-Si
sun simulator
mc SoG-Si
4
3
2
1
0
bottom
middle
top
Figure 5: Degradation in Voc, Jsc and efficiency for
SoG-Si cell (top) and in Voc (bottom) as a function of
fraction solidified for reference and SoG-Si solar cells.
Figure 5 (at bottom) shows the summarised results of
degradation measurements on the mc-Si cells from bottom,
middle and top ingot position. The Voc from the top graph
is included as well (blue dashed bars). It is visible that the
measurements with sunsVoc apparatus and sun-simulator
are in a very good agreement. In addition, what is quite
surprising, the degradation in Voc compared to the
reference mc-Si solar cells is identical for bottom positions
and even much lower for cells from the upper part of the
It is striking that the degradation in Voc is decreasing
toward ingot bottom (in contrast to the mc-Si ingot in CzSi growth the bottom is the latest solidified part). A
remarkable difference of 12 mV was observed after a LID
after 20 hours illumination. Note that the oxygen content
was identical (18ppma) for top and bottom ingot position
as shown in reference [5] which is not typical for Cz-Si
ingots.
The segregation coefficient of boron is lower than 1
thus the overall boron concentration in the bottom part is
the highest. But, as the segregation coefficient for
phosphorus is lower than that of boron, the compensation
rate is also higher in the bottom part leading to a lower
net B-content and thus higher resistivities. Not only that
P is compensating the free charge carriers leading to
lower conductance of the substrates. By its presence it
also somehow reduces the LID. Our hypothesis is that it
forms a complex with B-atoms shielding them from
interstitial oxygen and therefore preventing from the
formation of B-O complexes. An indication for this was
found in our photoluminescence spectra, where an
additional peak was observed with an intensity dependent
on the degree of compensation. Experiments are
underway to prove our findings. In addition we have
studied the regeneration process on our Cz-Si solar cells
finding slower regeneration behaviour as compared to A.
Herguth [6]. This difference can be explained by different
O-, P- and B-concentrations (and possibly other species)
in our solar cells.
5
CONCLUSIONS
In the present study we have focused on the light
induced degradation behaviour of industrially processed
cells on mc- as well as on Cz-Si wafers from SoG-Si
originating from different metallurgical purification
techniques. Efficiencies above 15% have been reached
for mc-Si cells and above 16% for Cz-Si cells. Solar cells
from selected ingot positions have been illuminated at 1
sun and the degradation monitored in comparison with
reference cells on substrates from pure poly-Si material.
As one of the most prominent degradation mechanisms is
based on the formation of recombinative B-O complexes,
the compensated material with a higher B content was
expected to show increased degradation behaviour.
However this was not confirmed as the degradation
magnitude was found to be dependent on the resistivity,
which means on the net B-content only. We assume that
the formation of B-O recombination centers during
illumination can be reduced by the appearance of P-B
pairs which was already suggested by Kruehler in the late
80s [4]. The P-atoms seem to form a bond with the Batoms shielding them from interstitial oxygen. An
indication for this was found in our photoluminescence
spectra, where an additional peak was observed with
intensity dependent on the degree of compensation. We
believe that this peak corresponds to P-B pairs which
reduce the formation of B-O complexes under
illumination and thus the light induced degradation based
on this mechanism.
ACKNOWLEDGEMENTS
This work was supported by the European
Commission within the FoXy project under the contract
number 019811(SES6).
REFERENCES
[1] N. Yuge et al., Purification of metallurgical silicon
up to solar grade, Sol. Energy Mater. Sol. Cells, 34
(1994) 243
[2] K. Peter et al., Investigation of Multicrystalline
Silicon Solar Cells from Solar Grade Silicon
Feedstock, Proc. Of the 20th EU PVSCEC, Barcelona
(2005), 615
[3] K. Peter et. al., Future Potential for SoG-Si
Feedstock from Metallurgical Process Route, these
proceedings
[4] W. Kruehler et al., Effect of PhosphorousCompensation on the Electronic Properties of SolarGrade Silicon, Proc. 8th EU PVSEC (1988), 1181
[5] J. Libal et al., Effect of Compensation and of metallic
impurities on the electrical properties of Cz-grown
solar grade silicon, Journal of Applied Physics, in
press
[6] A. Herguth et al., Further Investigations on the
Avoidance of Boron-Oxygen Related Degradation by
Means of Regeneration, Proc. Of the 22nd EU
PVSCEC, Milan (2007)