3rd Intenational Workshop on Crystalline Silicon Solar Cells
SINTEF/NTNU, Trondheim NORWAY
3-5 June 2009
DOPANT SPECIFICATION OF COMPENSATED SILICON FOR SOLAR CELLS OF
EQUAL EFFICIENCY AND YIELD AS STANDARD SOLAR CELLS
Erik ENEBAKK, Anne K. SØILAND, John T. HÅKEDAL, Ragnar TRONSTAD
Elkem Solar AS, 4623 Kristiansand, NORWAY
* E-mail: erik.enebakk@elkem.no
ABSTRACT
Pilot production of solar grade silicon has taken place at
Elkem Solar in Kristiansand, Norway, the last year.
Ingot, wafer, and solar cells have been made at institutes
and in industrial scale with good results. Based on this
experience and calculations it is concluded that
compensated SoG-Si with less than 0.45 ppmw B can be
used for solar cells with efficiencies and yield equal to
standard solar cells based on poly silicon.
DOPANT LEVELS IN SOLAR GRADE SILICON
One of the focus areas in the development of ESS from
metallurgical route has been to produce a high quality
silicon feedstock with sufficiently low boron and
phosphorus content for high quality solar cells. When
the boron content in the product is too high, the lifetime
in the wafer and the effciency of the solar cells will
decrease. A high boron content can also lead to
increased light induced degradation related to the B-O
complex formation, and reduced efficiency. Therefore,
to avoid these efficiency losses the boron concentration
is fundamental and the most important criteria for a
SoG-Si feedstock.
Despite the large number of potential new producers of
SoG-Si from metallurgical route, there exist a limited
amount of publications and no well established standard
for the requirements for dopants and impurities in
compensated silicon for solar cells. Here, general
dopant requirements for compensated SoG-Si are
described, and general specification limits for dopants in
compensated SoG-Si is suggested.
SoG-Si from metallurgical route will always contain a
remaining amount of phosphorus in addition to boron,
i.e. the feedstock is compensated. Phosphorus will, as
boron, also contribute to the ingot resistivity but more
importnant is it’s influence on the ingot yield due to the
type-conversion.
Because of different segregation
behaviour of the dopants, a type conversion will be
present in the ingot, which can reduce the overall ingot
yield if there are too much phosphorus in the feedstock.
The boron specification for standard p-type noncompensated multi crystalline silicon solar cells are
typically 0.05-0.15 ppmw B, corresponding to ingot and
wafer resistivity in range 0.5 – 2.0 ohm cm to satisfy
acceptable efficiency targets. One of the challenges
with SoG-Si from metallurgical route are to control the
balance between boron and phosphorus and the
distribution of the dopants in the following ingot growth.
Not only is it challenging by metallurgical refining to
obtain sufficiently low boron and phosphorus levels, but
there is also a need for precice and accurate analytical
methods.
Keywords: Silicon solar cells, feedstock, SoG-Si
INTRODUCTION
Elkem AS has a long history as one of the leading
silicon producers in the world. Together with
Sintef/NTNU in Trondheim, Norway, a high
technological level on process metallurgy and silicon
product knowledge have been developed, and this
knowledge base has been a needed platform for
development of a metallurgical route for solar grade
silicon. Since the first project in the late 70-ties until the
start-up of Elkem Solar in 2001, several different
technologies have been tested. The present process
consists of 3 purification steps; slag treatment, acid
leaching and directional solidification. The product is
cut to size in a post treatment plant and surface
impurities are removed by etching and washing. The
product has been tested and qualified at institutes and in
the industry.
Industrial tests in which more than
150.000 wafers were produced showed efficiencies of
Elkem Solar Silicon® based cells equal to poly-Si based
references. Ingot yield was according to the p/n
transition at the top part of the ingot, due to the typeconversion resulting from the B and P concentration[1].
The philosophy of Elkem Solar is to develop a product
which has equal properties in ingot and solar cells as
poly based ingots and solar cells. A property which is
focused is the resistivity in ingots made from ESS. All
produced multi crystalline ingots should be within the
standard range of 0.5 to 2.0 ohm cm. Therefore, the
needed dopant concentration in feedstock is determined
by requirements relating to solar cell performance, ingot
resistivity and ingot yield.
1
Extended abstract
When SoG-Si from metallurgical route is used in ingot
production, there will exist different opinions and ways
of using the feedstock depending on the specifications at
ingot- and solar cell producers. The feedstock may be
mixed with poly silicon, various off-cuts, scrap or other
dopants to adjust resistivity and yield, or it can be
processed directly to solar cell ingots without blending.
A reliable control scheme of dopant levels and their
limits in the feedstock are needed in any case to support
a stable ingot and solar cell production. Therefore, this
paper is focusing on dopant control in compensated
SoG-Si and how this can influence on ingot yield and
resistivity.
As a simplification the mobility can be assumed constant
across ingot heigth, or the dependence of NA and ND can
be modelled by simpler or experimentally determined
relations. The resistivity calculations in this paper are
based on carrier mobilities calculated by Equation (1.4).
Table 1: Parameters for mobility calculation
B (p-type)
P (n-type)
44.9
68.5
µmin
470.5
1414
µmax
n0
2.23x1017
9.20x1016
0.719
0.711
α
In addition to the base resistivity in solar cell ingots, the
ingot yield is important to focus on. Ingot yield can be
defined in two ways:
(1) the position of type
changeover, or (2) the cut-off position where the base
resistivity exceed a given value depending on the criteria
from the different solar cell producers. The latter
definition relates directly to the amount of usable
material for further solar cell processing from the ingot,
and a typical cut-off value would be 2 ohm cm. The
ingot yield in terms type-changeover is calculated by
Equation (1.5).
Resistivity across the ingot height is calculated by
simple formulas and assumptions.
The general
expression for resistvity in a silicon semiconductor is
ρ=
1
(1.1)
p ⋅ µ ⋅e
where p is the concentration (cm-3) of majority carriers,
µ is the corresponding carrier mobility (cm2/Vs), and e
is the charge (C) of an electron. In compensated p-type
Si for solar cells, the carrier concentration can be
assumed equal to the net dopant concentration as given
in Equation (1.2),
p = NA − ND
1
 N A0 ⋅ k A
Type changeover = 1 −  0
 N D ⋅ kD
 k D −k A


(1.5)
(1.2)
Examples of a resistivity calculation are shown in
Figure 1. Here, the feedstock melt contains 0.45
ppmw B and 1.00 ppmw P, and the resulting resistivity
is above 0.5 ohm cm and the ingot yield is close to 90%
of the total ingot. In the lower example the same
feedstock has been mixed with 50% poly and some
added boron. It results in a solar cell ingot with a higher
resistivity and approximately the same yield.
where NA and ND is the concentrations (atoms/cm3) of
acceptors and donors, respectively.
The Scheil
equation is used to find NA and ND across the ingot
height
N i = N i0 ⋅ k i ⋅ (1 − g ) ki −1
(1.3)
where N0 is initial melt concentration (atoms/cm3), i is a
dopant element, e.g. B or P, and ki is the segregation
coefficient for element i in silicon, and g represents the
ingot height given by the fraction solidified.
Segregation coefficients used in calculations in this
paper are kB =0.80 and kP =0.35. The mobility values
that goes into Equation (1.1) for both for p-type and ntype silicon is calculated by
3.0
0.45 ppmw B
Resistivity (ohm cm)
2.5
1.00 ppmw P
2.0
1.5
1.0
0.5
µ max − µ min
 N +N
1 +  A
D
 
n
0
 



α




0.0
(1.4)
3.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.241 ppmw B
2.5
Resistivity (ohm cm)
µ = µ min +
where the respective parameters are given in Table 1.
Equation (1.4) accounts to some degree for reduced
mobility in compensated silicon, which has been
reported by others[2]. There are also reported that
mobility in mc-Si ingots are lower due to grain
boundaries than values obtained by Equation (1.4)[3].
However, the dependence between dopants and mobility
in compensated and multicrysalline silicon is not clear.
0.500 ppmw P
2.0
1.5
1.0
0.5
0.0
0.0
0.1
0.2
0.3
Fraction solidified
Figure 1: Examples on resistivity profile and typeconversion calculated from feedstock values.
2
Extended abstract
Resistivity calculations can be used as a tool to visualize
and model relations between chemical composition of
the SoG-Si-feedstock and the expected resistivity and
yield in ingot. Figure 2 shows how ingot resistivity and
ingot yield can be found directly from the initial B and P
concentration in the melt feedstock. For a SoG-Si
feedstock containing 0.45 ppmw B and 1.00 ppmw P,
the calculated resistivity and ingot yield can be found
directly by interpolation between pairs of iso-resistivity
and iso-yield lines. In this example, the corresponding
ingot resistivity close to the ingot bottom is slightly
below 0.6 ohm cm, and the type-changeover position is
at >90% of ingot height. The diagram can also be used
the other way around in order to determine the actual
initial melt concentration from observed ingot resistivity
and changeover position.
1.6
Red: Iso-resistivity lines (@5% above ingot bottom)
Blue: Iso-yield lines (yield definition: pn changeover)
75
%
%
80
85
%
90
4.0
3.5
ppmw P in initial melt
3.0
oh
m
0.
5
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
ppmw P in initial melt
cm
SoG-Si
Can be used directly
>90
e
-typ
%p
UMG-Si
High boron
Low resistivity
Blending or refining needed
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
CONCLUSION
Based on calculations and supported by experience, it
can be concluded that typical maximum limits for
dopants in SoG-Si are <0.45 ppmw B and <1 ppmw P.
Even though the requirements for B and P are clear,
attention will still be needed towards precise analytical
methods and control mechanisms for product quality and
stability to ensure an efficienct ingot and solar cell
production.
%
cm
cm
oh
m
1.
0
0.25
m
oh
Figure 3: The hatched area illustrates the required B
and P range for SoG-Si feedstock which will result in an
ingot with resistivity above 0.5 ohm cm and ingot yield
above 90%.
0.4
0.2
1.5
5
ppmw B in initial melt
0.6
0.15
2.0
0.1
0.8
0.1
.
>0
0.0
1.0
0.0
High phosphorus and boron
Low resistivity and ingot yield
Blending or refining needed
0.5
95 %
0.2
UMG-Si
High phosphorus
Low ingot yield
Blending or refining needed
2.5
1.0
1.4
1.2
UMG-Si
0.7
ppmw B in initial melt
Figure 2: Example of a dopant – resistivity digram.
Resistivity lines (red lines) indicates ingot resistivity at
5% of ingot heigth, i.e. at bottom of ingot. The ingot
yield (blue lines) is here defined as type-changeover
position.
REFERENCES
[1] V. Hoffmann, 23rd European Photovoltaic Solar
Energy Conference, Valencia, 2008
[2] K. Peter, 23rd European Photovoltaic Solar Energy
Conference, Valencia, 2008
To establish a general dopant specification for SoG-Si
there are two targets that must be fulfilled. First, the
base resistvity in the finished solar cell ingot should be
in the range 0.5-2 ohm cm. This represents the limits
where industrial and high efficiency cells often are
made. Some producers may accept higher values, but
few would accept lower. Secondly, the ingot yield – or
amount of usable ingot – should be above 90%. The
definition of yield can vary among producers and the
impact on ingot yield for different technologies can also
vary. The area in Figure 3 defines where these resistvity
and yield targets are fulfilled at the same time, thereby
defining the absolute dopant requirements for
compensated SoG-Si feedstock.
[3] A. Cuevas, Conference on Optoelectronics and
Microelectronic Materials and Devices (COMMAD
’08), Sydney, July 2008
Utilizing boron and phosphorus compensated SoG-Si
material require adequate control of both level and ratio
of the two dopants, as can be concluded from Figure 2
and 3. When the requirements for resistivity and yield
are set, the B and P concentration must be within certain
limits. Higher concentrations of the two dopants lead to
narrower uncertainty limits for the feedstock
concentration. Precise and accurate analytical methods
are needed to meet theese requirements for
administration of dopants in order to fulfill the
expectations for resistivity and yield.
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