High Performance Solar Cells Exceeding 17 ... - ISC Konstanz

HIGH PERFORMANCE SOLAR CELLS EXCEEDING 17% EFFICIENCY BASED ON
LOW COST SOLAR GRADE SILICON
P. Preis 1 , P. Diaz-Perez 1 , K. Peter 1 , R. Tronstad 2 , B.R. Henriksen 2 , E. Enebakk 3 , S. Beringov 4 , M. Vlasiuk 4
1 International Solar Energy Research Center Konstanz, Rudolf-Diesel-Str. 15, D-78467 Konstanz, Germany
Phone: +49-(0)7531-36183-6620, Fax: +49-(0)7531-36183-11, Email: pirmin.preis@isc-konstanz.de
2 Elkem Technology, Fiskaaveien 100, N-4675 Kristiansand, Norway
3 Elkem Solar, Fiskaaveien 100, N-4675 Kristiansand, Norway
4 Pillar Ltd, member of Pillar Group, Severo-Syretskaya Str.3, 04136, POB 42, Kiev, Ukraine
ABSTRACT: To further establish solar grade silicon from metallurgical process route in the photovoltaic market it is
crucial to demonstrate the material quality by reaching high cell efficiencies and thus enabling SoG-Si to be
competitive to “traditional” poly-Si. In this work we present Elkem Solar Silicon (ESS) based solar cells with
efficiencies exceeding 17% demonstrating the high feedstock quality and a fine tuned solar cell process, which was
optimized in respect to efficient gettering. Investigations of LID behavior of both ESS and poly-Si solar cells show
even lower losses in cell efficiency on the SoG-Si material, which was confirmed by low interstitial oxygen
concentrations measured using FTIR. Further experiments focused on improvements of the reverse current-voltage
characteristics by applying different texturisation methods, which allow for similar cell efficiencies compared to the
standard acidic iso-texturisation.
Keywords: Solar Grade Silicon, Multicrystalline Silicon, Degradation, Texturisation
1 INTRODUCTION
Within the last few years Elkem Solar as well as ISC
Konstanz were constantly improving their processes with
the aim to achieve solar cell efficiencies above 17% on
multicrystalline silicon solar cells. For this purpose mc
solar grade silicon wafers from metallurgical process
route originating from center and corner bricks of ingots
produced out of three different kinds of feedstock were
investigated and compared with wafers from poly-Si. All
four ingots were grown in industrial directional
solidification furnaces at Pillar Ltd., two of the ESS
ingots were grown using IMCC technology as a previous
step of purification of UMG-silicon [1]. Adaptations of
the solar cell process regarding a more efficient gettering
during the POCl 3 diffusion were carried out to remove
metallic impurities present in SoG-Si, which act as active
recombination centers and thus limit the carrier lifetime.
Due to the higher boron doping of ESS feedstock
compared to poly-Si feedstock, boron oxygen related
light induced degradation plays an important role for
SoG-Si based solar cells. The results presented in this
work support the assumption of other research groups
[2-5], that a possible boron-phosphorous interaction
might prevent BO 2i complex formation and thus LID is
only related to the net-doping and interstitial oxygen
concentration [6].
Because of the higher doping and impurity
concentrations in SoG-Si the reverse current-voltage
characteristics of SoG-Si based modules are more critical
than in 100% poly-Si based modules. The mechanisms of
breakdown in mc-Si and resulting hot spots have been
shown by Kwapil et al. [7] and Bauer et al. [8],
concluding that also the higher impurity concentrations
and the acidic texturisation, leading to deep etch pits and
thus narrow junction radius which induces a strong
electrical field, can cause poor reverse current-voltage
characteristics. To overcome this problem for SoG-Si
both, a more efficient removal of impurities and the
testing of novel approaches for the texturisation have to
be developed.
2 EXPERIMENTALS
2.1 Optimization of POCl 3 diffusion towards a more
efficient gettering
In the first half of the work the influence of a more
efficient gettering regarding an optimized POCl 3
diffusion was tested and compared to the standard POCl 3
diffusion. The processed wafers originate from corner
and center bricks of four different ingots, three made out
of a mix with 60% ESS and 40% poly-Si, in which two
of them were grown using IMCC technology as a
previous step of purification [1], one made out of 100%
poly-Si. To investigate the effectiveness of the gettering
the two different POCl 3 diffusions were applied to
neighboring wafers from different ingot heights. The
remaining cell process was identical for all wafers
including acidic texturisation, PECVD SiN x deposition,
full Al BSF screen printing, Ag finger grid screen
printing and firing through. Investigations of light
induced degradation (LID) by both accelerated mode [9]
and long term measurements were carried out as well as
FTIR measurements on wafer level to determine
concentrations of interstitial oxygen and its influence on
the LID. Lifetime measurements of PECVD SiN x
passivated samples from different heights of each of the
four ingots were carried out to screen the feedstock
quality.
2.2 Improvements of the reverse current-voltage
characteristics
The second part of this work focused on the
enhancement of the reverse current-voltage
characteristics by applying different texturisation
methods. Five different textures were used, including the
state of the art iso-texturisation, soft iso-texturisation,
alkaline texturisation, an ISC recipe which is not
disclosed yet, and NaOH flat etching. The experiments
were carried out on both ESS (Resistivity from 1.1 – 2
Ω*cm, blended with 30% poly-Si) and poly-Si
(Resistivity from 1.4 – 2 Ω*cm) based wafers from two
different brick positions. The wafers used in this
investigation are different in both composition and ingot
origin, compared to the wafers which were used in the
experiment described in chapter 2.1. After the

Average solar cell efficiency [%]
Lifetime [s]
texturisation all samples were processed by POCl 3
diffusion, PECVD SiN x deposition, full Al BSF screen
printing, Ag finger grid screen printing and firing
through. Measurements of the solar cell parameters were
carried out and the reverse current was determined at
-14.5V to see the influence of the texturisation on both
results.
3 RESULTS
3.1 Results of diffusion optimization
Minority carrier lifetime measurements were
performed to screen the feedstock quality of the
investigated ingots. Therefore the saw damage of wafers
from the center bricks of each ingot was removed and the
samples were passivated by a PECVD SiN x . The
lifetimes of those samples were determined using QSSPC
measurements and are shown in figure 1.
60
50
40
30
20
10
0
ESS 1
ESS 2
ESS 3
Poly
0 10 20 30 40 50 60 70 80 90 100
Ingot Height [%]
Figure 1: Minority carrier lifetime of wafers from
different ingot positions, measured by QSSPC. Ingots
ESS 1 and 2 were purified by IMCC and thus are not
made from the standard ESS feedstock.
Lifetimes between 10µs and 60µs have been measured on
both ESS and poly-Si wafers. Figure 1 shows that the
minority carrier lifetime of the ESS based wafers is at
least on the same level or even higher compared to the
poly-Si reference wafers. The bottom wafers of both
ingots ESS 3 and poly show reduced lifetimes most
likely due to some crucible induced impurities. For the
highest wafer positions the lifetime of all four ingots is
decreasing. In case of the poly-Si ingot it is likely that the
higher boron doping at the top, because of the
segregation of impurities is a reason for the decrease in
lifetime. For the ESS based samples dislocations,
found at the higher ingot positions, might limit the carrier
lifetime, because they act as active recombination
centers.
and are thus not made from standard ESS feedstock
Ingot Brick
V oc
[mv]
J sc
[mA/cm²]
FF
[%]
η
[%]
ESS 1 Corner 621.9 33.7 79.5 16.7
ESS 1 Center 618.4 33.5 79.7 16.5
ESS 2 Corner 619.1 33.9 79.3 16.6
ESS 2 Center 616.8 33.8 79.1 16.5
ESS 3 Corner 613.6 33.5 79.1 16.3
ESS 3 Center 618.7 33.9 79.3 16.6
poly Corner 615.1 33.7 79.0 16.4
poly Center 617.1 33.9 79.2 16.6
Table II: Average solar cell parameters over at least 18
cells per brick, which were evenly distributed over the
complete ingot height, processed using an optimized
diffusion regarding more efficient gettering. Ingots ESS 1
and ESS 2 have been grown using IMCC technology as a
previous step of purification and are thus not made from
standard ESS feedstock.
Ingot Brick
V oc
[mv]
J sc
[mA/cm²]
FF
[%]
η
[%]
ESS 1 Corner 623.8 33.8 79.5 16.7
ESS 1 Center 620.2 33.6 79.6 16.6
ESS 2 Corner 620.5 33.9 79.2 16.7
ESS 2 Center 618.4 33.9 79.0 16.6
ESS 3 Corner 614.5 33.5 78.9 16.3
ESS 3 Center 619.8 34.0 79.2 16.7
poly Corner 616.2 33.8 78.9 16.4
poly Center 618.5 34.0 79.2 16.6
The ingots ESS 1 and ESS 2, which were purified using
IMCC technology, show the highest achieved solar cell
efficiencies, due to high FF, because of the low resistivity
and thus a low series resistance, high V oc caused by the
Fermi level shift as a result of the higher boron
concentration and respective higher doping levels [6],
and J sc similar to the reference wafers. Ingot ESS 3
also shows a good performance at the centre brick,
whereas the corner brick shows the lowest average
efficiency of all investigated bricks.
17,0
16,8
16,6
16,4
16,2
Optimized
Standard
IV measurements show average efficiencies, over at
least 18 solar cells from each brick, between 16.3% and
16.7% for the ESS based solar cells and between
16.4% and 16.6% for the poly-Si based solar cell
reference (see table I and II).
Table I: Average solar cell parameters over at least 18
cells per brick, which were evenly distributed over the
complete ingot height, processed using the standard
diffusion. Ingots ESS 1 and ESS 2 have been grown
using IMCC technology as a previous step of purification
16,0
ESS 1 Center
ESS 1 Corner
ESS 2 Center
ESS 2 Corner
ESS 3 Center
ESS 3 Corner
Poly Center
Poly Corner
Figure 2: Comparison of average solar cell efficiency of
solar cells diffused by either an optimized or a standard
POCl 3 diffusion. The optimized diffusion was improved
towards a more efficient gettering.
As it can be seen in table I and II and figure 2, the
optimized diffusion leads to an absolute gain of 0.1% in

Interstitial oxygen [ppma]
Reverse current @ -12V [A]
Relative loss in cell efficiency [%]
Relative loss in cell efficiency [%]
cell efficiency for most of the groups, due to enhanced
V oc and J sc values, which we claim to be an effect of a
more efficient gettering. The optimization of the
diffusion leads to solar cell efficiencies exceeding 17%,
with best solar cells even reaching 17.1% efficiency. The
results of the best cells of each ingot and brick are shown
in table III.
Table III: Solar cell parameter of the best solar cells of
each group. All of the best solar cells were processed
using the optimized diffusion.
Ingot Brick
V oc
[mv]
J sc
[mA/cm²]
FF
[%]
η
[%]
ESS 1 Corner 625.2 33.9 79.5 16.8
ESS 1 Center 628.5 34.1 79.7 17.1
ESS 2 Corner 625.0 34.2 79.3 16.9
ESS 2 Center 628.1 34.3 79.4 17.1
ESS 3 Corner 615.8 33.8 79.2 16.5
ESS 3 Center 622.1 34.1 79.2 16.8
poly Corner 618.7 34.1 79.3 16.7
poly Center 619.4 34.1 79.4 16.8
Another indication for a more efficient gettering resulting
in lower impurity concentrations on cell level is the
improved reverse current-voltage characteristics. The
reverse breakdown is often caused by metallic impurities
[7]. As shown in figure 3 the solar cells with the
optimized diffusion show an improved reverse current
2
Optimized
Standard
for the SoG-Si based solar cells for most ingot positions,
even though the boron concentrations are higher
compared to the poly-Si. The ingots ESS 1 and ESS 2
show LID mostly below 1%, whereas ingot ESS 3 and
the poly reference show LID up to 1.7% at the bottom
wafers.
1,8
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
0 10 20 30 40 50 60 70 80 90 100
Figure 4: Relative losses in solar cell efficiencies of solar
cells from the center bricks, depending on the ingot
height.
1,8
1,6
1,4
1,2
1,0
Ingot Height [%]
ESS 1
ESS 2
ESS 3
Poly
ESS 1
ESS 2
ESS 3
Poly
0,8
0,6
1
0
ESS 1 Center
ESS 1 Corner
ESS 2 Center
ESS 2 Corner
ESS 3 Center
ESS 3 Corner
Poly Center
Poly Corner
Figure 3: Comparison of reverse current, as an indicator
of more efficient gettering, of solar cells diffused by
efficient POCl 3 gettering compared to the standard POCl 3
diffusion.
0,4
0,2
0,0
0 10 20 30 40 50 60 70 80 90 100
Ingot Height [%]
Figure 5: Relative losses in solar cell efficiencies of solar
cells from the corner bricks, depending on the ingot
height.
All of the ingots show a decreasing tendency of LID
towards higher ingot positions, what is in good agreement
with FTIR measurements of interstitial oxygen, which
can be seen in figure 6.
LID behavior of solar cells, originating from different
ingot heights ranging from bottom to top from both
center and corner bricks, was investigated. Therefore the
solar cells were exposed to 1 sun illumination at elevated
temperature using a Semilab PV2000 tool, to reach a
completely degraded state due to the formation of BO 2i
complexes. The exact measurement procedure is
explained in [9]. The solar cell parameters were measured
before and after this procedure and the relative loss in
cell efficiency was calculated out of both measurement
results. The results of the relative loss in cell efficiency
for solar cells from the center bricks are shown in figure
4, the results for the solar cells from the corner bricks in
figure 5. The measurement results show lower LID losses
7,00E+017
6,00E+017
5,00E+017
4,00E+017
3,00E+017
2,00E+017
1,00E+017
0 10 20 30 40 50 60 70 80 90 100
Ingot Height [%]
ESS 1
ESS 2
ESS 3
Poly

Reverse current @-14.5V [A]
Figure 6: Concentrations of interstitial oxygen of wafers
from different ingot heights, measured using FTIR.
In consideration that bulk resistivity is lower (see table
IV) and respectively the doping level is higher for ESS
based solar cells, these results lead to the conclusion that
LID is rather relating to the net doping and the interstitial
oxygen concentration than to the absolute boron
concentration. This supports the assumption that
phosphorous in compensated silicon somehow prevents
the formation of BO 2i , what was stated by different
research groups before [2-5].
Table IV: Bulk resistivity of investigated wafers
Ingot
Resisitvity [Ω*cm]
Center brick Corner brick
ESS 1 1.0 – 1.2 1.0 – 1.4
ESS 2 1.4 – 2.3 1.4 – 2.0
ESS 3 1.5 – 1.7 1.5 – 1.7
poly 1.5 – 1.8 1.4 – 1.9
3.2 Results of reverse current-voltage characteristics
optimization
Table V shows the average solar cell parameters over
at least 12 solar cells of each group from positions
covering the complete ingot height, which were
processed using different texturisation methods. The soft
iso texturisation was processed with the texturisation
bath, but less silicon was removed compared to the
standard iso-texturisation. The ISC recipe is not disclosed
yet.
Table V: Average solar cell parameters of solar cells,
processed by different texturisations.
Ingot Texture V oc
[mV]
70%
ESS
100%
poly
J sc
[mA/cm²]
FF
[%]
Eta
[%]
Iso 618.7 33.1 79.2 16.2
Soft iso 615.7 33.2 79.4 16.2
ISC 616.8 33.3 78.8 16.2
Alkaline 617.8 33.4 79.1 16.3
NaOH 620.5 32.6 79.4 16.1
Iso 614.9 34.2 78.9 16.6
Soft iso 612.6 34.3 78.8 16.5
ISC 614.2 34.2 78.7 16.5
Alkaline 613.6 34.2 78.8 16.5
NaOH 616.0 33.5 78.8 16.3
As table V shows the solar cell efficiencies of almost all
groups are similar. Only the NaOH flat etched solar cells
show lower cell efficiencies, due to higher reflectivity
resulting in lower J sc values. Depending on the
texturisation the open-circuit of the solar cells is varying
as a consequence of the different surface areas and thus
different surface recombination velocities. The NaOH flat
etched cells show the highest V oc due to the lowest
surface area, whereas the wafers treated by the soft isotexturisation
show the lowest V oc , most likely because the
saw damage was not removed sufficiently. In general the
ESS based solar cells show lower performance than the
poly-Si based solar cells, due to significantly lower short
circuit currents.
The influence of the different texturisation methods
on the reverse current-voltage characteristics is shown in
figure 7. For the poly-Si solar cells there is only a slight
influence of the texturisation on the reverse current. Both
versions of the iso-texturisation lead to reverse current
values of 1.9 A and 1.5 A respectively, whereas the other
three texturisations lead to reverse currents below 1.2 A.
Even the higher values of the iso-texturisations are in a
non critical region regarding possible hot spots in solar
modules. For the ESS based solar cells the reverse
current generally is higher, due to higher doping levels
and assumable higher concentrations of impurities. The
SoG-Si solar cells treated by the two versions of the isotexturisation
show high reverse currents above 10 A,
measured at a reverse voltage of -14.5 V. The mixed
texturisation, the alkaline texturisation and the NaOH flat
etching leads to reduced reverse current values between
1.9 A and 3.6 A.
12
10
8
6
4
2
0
Figure 7: Average reverse currents, measured at -14.5 V,
depending on the used wafer material and the applied
texturisation.
The lowest reverse current was obtained on the NaOH
flat etched solar cells, due to the reduced influence of the
solar cells surface, but at the same time the solar cell
efficiency of those cell is the lowest. The alkaline
texturisation shows the best combination of solar cell
efficiency with 16.3% and a low reverse current of 3.2 A
at -14.5V reverse voltage and thus seems to be a
sufficient solution to prevent possible hot spots in ESS
based solar modules, while allowing for high efficiencies.
4 CONCLUSIONS
iso soft iso ISC alka NaOH
Texturisation
70% ESS
100% Poly
The high quality of ESS feedstock was confirmed
achieving solar cell efficiencies exceeding 17% with best
cells showing 17.1% cell efficiency. The obtained cell
results demonstrate that the ingots purified by IMCC
show high solar cell performance and low LID, indicating
that this technology can be an effective alternative for
final purification of SoG-Si. An industrially applicable,
regarding more efficient gettering optimized, POCl 3
diffusion was investigated leading to a gain of 0.1% in
absolute cell efficiency, due to higher V oc and J sc . The
reverse current could also be lowered by the optimized
diffusion, indicating a more efficient removal of
impurities. Generally V oc and FF is higher for ESS based
solar cell, because of the higher boron doping and thus a
Fermi level shift and low resistivity, whereas Jsc appears

to be comparable to poly-Si based solar cells. The
obtained results indicate the potential of ESS for high
efficiency approaches.
Applying an alkaline texturisation to mc wafers lead
to an improvement of the reverse current-voltage
characteristics of ESS based solar cells, while solar cell
efficiency is similar or even higher compared to the state
of the art acidic iso-texturisation, what can help to
prevent hot spots in ESS based solar modules.
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technology and parameters of solar cells
produced with application thereof, 27 th EUPVSEC,
Hamburg, Germany, (2012).
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compensated p-type Czochralski silicon, 24 th
EUPVSEC, Hamburg, Germany, (2009).
[3] R. Kopecek et al., Crystalline Si solar cells from
compensated material; behavior of light induced
degradation, 23 rd EUPVSEC, Valencia, Spain,
(2008).
[4] 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 104,
(2008).
[5] K. Peter et al., Future potential for SoG-Si feedstock
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solar grade silicon solar cells, 25 th EUPVSEC,
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