Note: Descriptions are shown in the official language in which they were submitted.
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CRYSTALLINE SI SOLAR CELLS MADE FROM UPGRADED
METALLURGICAL SILICON
BACKGROUND OF THE INVENTION
The shortage of available polysilicon for the production of ingots, wafers and
solar
cells has resulted in the last two years in the development of upgraded
metallurgical
silicon (UMG-Si). The lower production cost and energy usage of UMG-Si, lower
capital requirements and shorter time to completion are major advantages
compared
to polysilicon. However, UMG-Si contains impurities, in particular boron and
phosphorous, that lower efficiency and yield if not compensated. This
invention
describes how to compensate UMG-Si (silicon containing both boron and
phosphorus ) with dopants to increase the yield in the production of ingots.
SUMMARY OF THE INVENTION
The invention describes how to adjust boron and phosphorus concentration in
feedstock made of upgraded metallurgical silicon to get the largest amount of
p-type
silicon in the 0.5 0-cm (ohm-cm) to 3 SZ-cm resistivity range. This can be
achieved by
diluting UMG-Si with poly-silicon and/or by adding p-type dopant to the UMG-
Si.
Upgraded metallurgical silicon (UMG-Si) is silicon obtained from the direct
purification of metallurgical silicon to a high purity level (generally >99.99
%Si).
UMG-Si contains boron and phosphorus, two chemical elements that are generally
used in the doping of silicon to make solar cells.
Boron and phosphorus level in UMG-Si:
0.1 ppmw < Boron (ppmw) < 3 ppmw
0.1 ppmw < Phosphorus (ppmw) < 10 ppmw
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Conversion of ppmw to ppma
Definition:
ppma: part(s) per million atomic
ppmw: part(s) per million by weight
28.0855
[Pl ppma -[P]ppmw x 30.97376 = [P]ppmw x 0.91
[B]ppma = [B]ppmw x 28.0855 10.811 - [B]ppmw x 2.60
Dopant concentration in polysilicon ingot (Scheil's equation)
C, =k=Ca -(1- fy.~k 1
where:
CS: Concentration of the solute in the solid;
Co: Initial concentration of the solute in the liquid;
k: Distribution coefficient;
f: Solid fraction.
Phosphorus Boron
kP = 0.35 kB = 0.80
[P]ppma,s = 0.35 = [P]ppma,o . (I - f ~0.35-1 [B]ppma,s = 0.80. [B]ppma,a . (1-
f )0.80-1
s r
Proportion of p-type silicon in the polysilicon ingot
Condition: [P]ppma,s = [B]ppma,s (p-n junction)
[I']ppmw - 30.974 0.80 = (1- fs) 0.80-1
[B]ppmw 10.811 0.35 = (1- 0. fs)o.35-1
Quantity of p-type [P]/[8] ratio
% of the in ot mw/ mw
0 6.55
80 3.17
90 2.32
95 1.70
99 0.82
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Dopant concentrations in usable compensated of p-type silicon
Resistivity criteria : 0.5 0=cm to 30=cm
NCC (Net current carrier)
NCC = [B]ppma,.r - [P]pp,na,s
To obtain 0.5 Q=cm min., NCC <_ 3.3 x 1016a1cm3
To obtain 30=cm max., NCC>_4.6x1015a1cm3
16 a lcm3 28.0855g
NCC _ 3.3 x 10 3 13 = 1000000 = 0.66 ppma
cm 2.33g 6.02 x 10 a
a lcm3 28.0855g
NCC _ 4.6 x 10 3 23 = 1000000 = 0.09 ppma
cm 2.33g 6.02 x 10 a
15 10.09ppma ? NCC >_ 0.66ppma
From these calculations, we are able to see that the more interesting range of
boron
and phosphorus to make solar cells is:
0.1 ppmw < Boron (ppmw) < 1 ppmw
0.1 ppmw < Phosphorus (ppmw) < 2.5 ppmw
The complete results are shown in Graph 2. So, starting with a known level in
boron
and phosphorus, we can adjust the average chemistry of the melt of silicon by
adding
boron or phosphorus and/or diluting with poly-silicon (silicon at 99.9999999%
Si
purity) to get the most quantity of p-type material having a resistivity of
0.5 to 30=cm
in the ingot.
The upgraded metallurgical silicon may be diluted at different ratios with
poly-silicon
(silicon produced by Siemens process) to be in the best area of the graph.
This
action does not change the phosphorus to boron ratio. This ratio can be
modified by
adding small amounts of phosphorus or boron.
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Producers of solar cells do not like to add boron because a phenomenon called
"boron degradation": the initial rapid light-induced degradation of cell
performance.
The quantity of usable p-type silicon can be increased by adding other p-type
dopant
(other than boron):
p-type dopant Distribution coefficient Atomic weight
Al 2x10" 26.98
Zn 1 x10" 65.37
Ga 8x10" 69.72
I n 4x 0-4 114.82
These p-type dopants are able to increase the proportion of usable p-type
silicon
after the multi-crystalline solidification of the ingot. Gallium (Ga) and
Aluminum (AI)
are very interesting because of their high value of distribution coefficient:
they have a
very good compensation effect at the end of the crystallization to compensate
for the
rapid increase in the phosphorus concentration.
The amount of aluminum (Al) or gallium (Ga) to add to the silicon melt is
preferably:
0 ppmw < Gallium (ppmw) < 250 ppmw
0 ppmw < Aluminum (ppmw) < 100 ppmw
NCC = [B]PPma,e. + [Ga]PPma,.c - [r ]ppma,s
[Ga]ppma = [Ga]PPmw x 28=0855 69.723 = [Ga] Pm'" x 0.403
kUa = 0.008
[Ga]pPma,s = 0.008 - [Ga]PPma,o . (1 - fs)o.oos-1
Gallium is also known to have a better stability than boron. So, in the case
of a
silicon feedstock having a high phosphorus to boron ratio, it would be
beneficial to
add p-type dopant, like gallium, to increase the proportion of usable ingot in
the
production of solar cells instead of boron.
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DESCRIPTION OF THE BEST MODE OF REALISATION
Example 1:
5 Upgraded metallurgical silicon with initial dopant concentration of 1.5 ppmw
of boron
and 4.5 ppmw of phosphorus is melted in a crystallization furnace. The amount
of p-
type silicon having a resistivity in between 0.5 0=cm and 30=cm is
approximately
7.7% of the ingot.
Example 2:
Upgraded metallurgical silicon with initial dopant concentration of 1.5 ppmw
of boron
and 4.5 ppmw of phosphorus is melted with poly-silicon in a crystallization
furnace.
The ratio of UMG-Si to poly-Si is 1:2. The amount of p-type silicon having a
resistivity in between 0.5 0=cm and 3 0=cm is approximately 79.5% of the
ingot, an
increase of 72% of ingot usage (vs example 1).
Example 3:
Upgraded metallurgical silicon with initial dopant concentration of 0.5 ppmw
of boron
and 1.5 ppmw of phosphorus is melted in a crystallization furnace. The amount
of p-
2 0 type silicon having a resistivity in between 0.5 f2=cm and 3Q=cm is
approximately
79.5% of the ingot.
Example 4:
Upgraded metallurgical silicon with initial dopant concentration of 0.5 ppmw
of boron
and 1.5 ppmw of phosphorus is melted in a crystallization furnace. The
equivalent of
approximately 25 ppmw of gallium is added to the melt and crystallization is
carried
out. The amount of p-type silicon having a resistivity in between 0.5 f2=cm
and
3 0=cm is approximately 97.5% of the ingot, an increase of 18% of ingot usage
(vs example 3).
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Example 5:
Upgraded metallurgical silicon with initial dopant concentration of 0.5 ppmw
of boron
and 2.5 ppmw of phosphorus is melted in a crystallization furnace. The amount
of p-
type silicon having a resistivity in between 0.5 0=cm and 3 0-cm is
approximately
33.7% of the ingot.
Example 6:
Upgraded metallurgical silicon with initial dopant concentration of 0.5 ppmw
of boron
and 2.5 ppmw of phosphorus is melted in a crystallization furnace. The
equivalent of
approximately 65 ppmw of gallium is added to the melt and crystallization is
carried
out. The amount of p-type silicon having a resistivity in between 0.5 0=cm and
3 0=cm is approximately 96.1% of the ingot, an increase of 62% of ingot usage
(vs example 5).
It should be understood that the values quoted above are approximate. By
"approximate", it is meant that the value can vary within a certain range, for
example
the value can vary from 0% to 5%, 0% to 10%, or 0% to 25%.
Of course, these examples are given by way of illustrating the invention and
are in no
way to be deemed as limitative.
