A guide to solar cells using Elkem Solar Silicon (ESS™)

A guide to solar cells using
Elkem Solar Silicon (ESS)
Jan Ove Odden

Outline
• The Elkem Solar process route and ESS compared to polysilicon
from the Siemens technology.
• Qualification of ESS as a new solar grade silicon product
• Summary
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Elkem Solar – position in the value chain
Si-metal
Feedstock Ingot Wafer
Celle Modul
System/
installasjon
Elkem Solar
Silicon (ESS)
Polysilicon from the
Siemens process
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Elkem Solar Silicon ® production process
with comprehensive value chains
Recycling of ingot cuts
Metallurgical
Silicon
Slag
treatment
Leaching
Solidification
Post
treatment
Customerprocess
Specialized products from side streams
Market
• In-house
production only
• Based on Elkem‘s
core competencies
• Three sequential purification steps designed to
reduce the level of impurities for critical elements
• Largely based on Elkem’s core competencies in
high temperature processes, process- and
equipment design
• Ingots cleaned
and cut into
bricks of ~10-15 kg
• Quality control
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Production
of polysilicon
1) No recycling of STC
Production of
Metallurgical grade silicon (MG-Si)
SiO 2 (s) + 2C(s) → Si(l) + 2CO(g)
(Source: A. Schei et al., 1998)
Generation of TCS from
Metallurgical grade Si
Reaction:
Si(s) + 3HCl(g) → SiHCl 3 (g) + H 2 (g)
TCS
Separation of TCS by
distillations
Decomposition
into polysilicon
Removes Boron as BH 3 and Phosphorus as
PCl 3 and POCl 3
Decomposition reaction, simplified:
~1100°C
4SiHCl 3 (g) → Si(s) + 3SiCl 4 (g) + 2H 2 (g)
TCS
(Source:
CDI, www.chemicaldesign.com)
STC
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(Source: www.gtsolar.com.com)
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Energy payback time (EPBT) – Comparison - LCA
1.8
Energy payback time (years)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Siemens
Elkem Solar
Siemens 2006 ESS 2010
inverter
mounting + cabling
frame
laminate
cell
ingot/crystal + wafer
Si feedstock
13.2% multicrystalline Si modules
Schletter on-roof mounting
2500 W inverter from ecoinvent 2.0
performance ratio 0.75
irradiation 1700 kWh/m2/year
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CO2 emissions depend on location and technology
Comparison of direct related emissions of CO 2 equivalents (in MT)
for different processes producing 6000 MT SOG-Si
900000
800000
700000
600000
MT CO2
500000
400000
300000
200000
100000
0
TCS without
recycling
TCS with
dirty
recycling
TCS with
clean
recycling
*) The German electricity mix is used as a base value when otherwise is not stated
**) Note that this comparison accounts for new SOG-Si producers
Elkem Solar
Elkem Solar
(Norwegian
electricity
mix)
* 1700 kWh/m2/yr
TCS nonrecycle
(coal-based
el mix)
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Main difference between ESS and polysilicon
Elkem Solar B & P: Polysilicon B & P:
B < 0.30 ppmw
P < 0.70 ppmw
B = 0 ppmw
P = 0 ppmw
B & P are the elements constituting the pn-junction -
The most important aspect of the solar cell:
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[B] in ingot
increases with
height due to
segregation.
Main difference between ESS and polysilicon
Poly Si doped with B
ESS TM based ingots
[B] and [P] in ingot
increase with height
due to segregation.
Net doping
concentration
(equal to [B])
INCREASES with
ingot height.
Net doping
concentration (i.e.
the difference
between B and P)
DECREASES with
ingot height until
the pn type
changover.
Resistivity is
inversely
proportional with
the net doping,
therefore resistivity
DECREASES along
entire ingot height.
All p-type
p-type
n-type
Resistivity is
inversely
proportional with
the net doping,
therefore resistivity
INCREASES along
ingot height until
the pn changover.
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Main difference between ESS and polysilicon
CarbideCut
Std.TopCut
Std. cut Poly (mm)
% Yield loss
CarbideCut
TopCut
Calculated ingot heigth (mm)
Useable Block
100% yield
Useable Block
BottomCut
BottomCut
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The Elkem Solar process is a robust and flexible process
regarding use of cuts
Polysilicon cuts:
CarbideCut
Std.TopCut
Cuts containing ESS:
CarbideCut
TopCut
Recycling of ingot cuts
Metallurgical
Silicon
Slag
treatment
Leaching
Solidification
Post
treatment
Customerprocess
The robustness and flexibility of the Elkem Solar process regarding
handling of cuts increases the yield in customer processes
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• The Elkem Solar process route and ESS compared to polysilicon
from the Siemens technology.
• Qualification of ESS as a new solar grade silicon product
• Summary
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Qualification of ESS as a new solar grade silicon product –
some challenges
Light Induced Degradation
Boron-Oxygen complexes
Fe-B pairs
Reverse Breakthrough Voltage (RBV)
Shunts
Thermal breakdown
Long-term degradation issues
At [Fe]

Light Induced Degradation due to B-O complex
A model showing the B s O 2i complex after a O-O dimer has diffused through the
Silicon lattice before residing near a substitutional Boron atom:
Si: grey, O i : blue and B s : red balls.
The two most stable configurations.
(J. Schmidt et al.,J. Mater. Res. 21 (2006) 5)
A degradation of 1-10% (relative) due to B-O
related LID within hours of illumination.
(MC Si on the lower and CZ Si on the high end)
(Gosh et al, EUPVSEC, 2006; Peter et al., IEEE, 2010; Petter
Et al., Silicon for the Chemical and Solar Industry X, 2010)
(Bothe et al., 2003)
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Light Induced Degradation due to B-O complex
Poly Si doped with B
ESS TM based ingots
Factor 3
difference
Factor 1.3-1.5
difference
The net doping (i.e. the [B] – [P]) determines the degree of LID from B-O complexes
NOT the total [B].
(Kruehler et al., EUPVSEC, 1988; Peter et al., EUPVSEC, 2008; Kopecek et al., EUPVSEC, 2008; Macdonald et al., EUPVSEC,
2009; Peter et al., IEEE, 2010)
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Light Induced Degradation
Relative LID [%] is shown in the Figure
The absolute degradation in
solar cells would normally be in the range
of 0.10-0.25 %
(Peter et al., 35th IEEE PVSC, Honolulu, 2010)
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Light Induced Degradation due to B-O complex
Oxygen:
LID due to B-O complexes is only significant if the interstitial oxygen [O i ] is above ~5*10 17 /cm 3
(or > ~6 ppm w ) (Ghosh et al., EUPVSEC, 2006).
Q-Cells claims that for good “UMG-Si” qualities the oxygen content in the feedstock
is not a relevant problem. The oxygen content in the ingot is determined by the crystallization
process only (Petter et al., Silicon for the Chemical and Solar Industry X, 2010).
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Light Induced Degradation
1,5
Relative LID (%) measured on
solar cells made of 100% ESS
0
-0.5
33 % 100 % 0 %
Rel. LID (%)
1,3
1,1
0,9
0,7
0,5
0,3
0 100 200 300 400 500 600
Wafer number (bott. to top)
Brick 1
Brick 2
Brick 3
Relative LID [%]
-1
-1.5
-2
-2.5
% ESS TM blended with polysilicon
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Efficiencies obtained with ESS-based solar cells
compared to polysilicon reference
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Long-term degradation
Initial degradation
due to LID
NOT
significantly more
pronounced for ESS TM .
Long term degradation
caused by factors not
related to the Silicon
Solar Cell material
like:
- UV triggered browning
of coatings.
-Delamination (reduced
adhesion between layers)
- Interconnect failure.
(Pictures from Hedström et al.,
Elforsk report 06:71, 2006)
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Long-term degradation
Elkem Solar has on-going research programs with UiA and Teknova on
Long-term degradation including accelerated degradation tests.
Some results from power plant tests of our customers (Q-Cells):
Note the better high-temp
(summer) performance of
compensated material vs
polysilicon.
(Petter et al., Freiburg, 2011)
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Summary
• The Elkem Solar process is energy efficient and robust
with a low carbon footprint.
• Solar cells and modules made from ESS have shown
comparable performance to polysilicon.
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