U.S. patent number RE36,156 [Application Number 08/788,660] was granted by the patent office on 1999-03-23 for columnar-grained polycrystalline solar cell and process of manufacture.
This patent grant is currently assigned to Astropower, Inc.. Invention is credited to Allen M. Barnett, Joseph C. Checchi, Sandra R. Collins, David H. Ford, Robert B. Hall, Christopher L. Kendall, Steven M. Lampo, James A. Rand.
United States Patent |
RE36,156 |
Hall , et al. |
March 23, 1999 |
Columnar-grained polycrystalline solar cell and process of
manufacture
Abstract
The invention relates to techniques for manufacturing
columnar-grained polycrystalline sheets which have particular
utility as substrates or wafers for solar cells. The sheet is made
by applying granular silicon to a setter material which supports
the granular material. The setter material and granular silicon are
subjected to a thermal profile all of which promote columnar growth
by melting the silicon from the top downwardly. The thermal profile
sequentially creates a melt region at the top of the granular
silicon and then a growth region where both liquid and a growing
polycrystalline sheet layer coexist. An annealing region is created
where the temperature of the grown polycrystalline silicon sheet
layer is controllably reduced to effect stress relief.
Inventors: |
Hall; Robert B. (Newark,
DE), Barnett; Allen M. (Newark, DE), Collins; Sandra
R. (Chesapeake City, MD), Checchi; Joseph C. (Newark,
DE), Ford; David H. (Wilmington, DE), Kendall;
Christopher L. (Wilmington, DE), Lampo; Steven M.
(Elkton, MD), Rand; James A. (Oxford, PA) |
Assignee: |
Astropower, Inc. (Newark,
DE)
|
Family
ID: |
26964001 |
Appl.
No.: |
08/788,660 |
Filed: |
January 24, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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959009 |
Oct 9, 1992 |
5336335 |
|
|
Reissue of: |
286673 |
Aug 5, 1994 |
05496416 |
Mar 5, 1996 |
|
|
Current U.S.
Class: |
136/258; 257/51;
148/33.2; 148/DIG.122; 428/620; 427/74; 438/96; 438/97;
438/488 |
Current CPC
Class: |
H01L
31/03682 (20130101); H01L 31/036 (20130101); H01L
31/182 (20130101); H01L 31/1864 (20130101); Y02P
70/50 (20151101); Y02E 10/546 (20130101); Y10T
428/12528 (20150115) |
Current International
Class: |
H01L
31/036 (20060101); H01L 31/0368 (20060101); H01L
31/18 (20060101); H01L 031/0392 (); H01L
031/0368 (); H01L 031/18 () |
Field of
Search: |
;136/258PC
;437/4,12,173-174,233,967 ;428/620 ;427/74 ;257/51
;148/33.2,DIG.122 ;438/96,97,488 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
A Eyer et al, Conf. Record, 18.sup.th IEEE Photovoltaic Specialists
Conf. (1985), pp. 1138-1141. .
A. Eyer et al, Conf. Record, 19.sup.th IEEE Photovoltaic
Specialists Conf. (1987), pp. 951-954. .
A. Eyer et al, Conf. Record, 20.sup.th IEEE Photovoltaic
Specialists Conf. (1988), pp. 1565-1568..
|
Primary Examiner: Nguyen; Nam
Attorney, Agent or Firm: Connolly & Hutz
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of Ser. No. 07/959,009,
filed Oct. 9, 1992, now U.S. Pat. No. 5,336,335.
Claims
What is claimed is:
1. A process of making an improved columnar-grained polycrystalline
sheet for functioning as a substrate for a solar cell, comprising
(a) applying granular silicon to a setter material having a release
coating at its upper surface and which supports the granular
silicon, (b) preheating the setter material and granular silicon in
a preheat zone, (c) subjecting the setter material and granular
silicon to a thermal profile which causes melting of the granular
silicon from the top downwardly, wherein 25 to 90% of the granular
silicon depth is melted and the partially melted silicon below the
melted material functions as a net to stabilize the melt and to
minimize molten silicon contact with the underlying setter material
and release coating and to nucleate subsequent crystal growth, (d)
transporting the melt pool on the silicon net into a growth zone
wherein a thermal profile is created to promote columnar growth of
a columnar grain size greater than 20 microns in a direction
approximately perpendicular to the plane of the setter material and
where both liquid and a growing polycrystalline layer coexist and
where the entire layer experiences a liquid state prior to
solidification, (e) transporting the grown sheet into an anneal
zone wherein a linear temperature gradient along the direction of
setter material motion is provided to promote low stress cooling of
the sheet, (f) removing the polycrystalline sheet from the setter
material, facilitated by a release agent, and (g) reusing the
setter material for the making of further columnar-grained
polycrystalline sheet.
2. The process of claim 1 including achievement of any or all of
the preheat, melting, growth, and anneal thermal profiles for the
granular silicon and resultant sheet by focused light energy.
3. The process of claim 1 including creating an electrical
resistivity in the sheet layer in the range of 0.1 to 10 ohm-cm by
adding separate constituents to the granular silicon.
4. The process of claim 1 including the use of granular silicon
sized between 100 to 1000 micrometers, which has a purity between
metallurgical grade and electronic grade silicon.
5. The process of claim 1 including stabilizing the outer edges of
the melt zone by thermal shunts, or reduced energy intensity at the
edge of the melt.
6. The process of claim 1 including forming nucleation sites in the
setter material to commence growth by locally placing thermal
shunts in the setter materialk to provide a thermal conduction path
between the top and the bottom of the setter material.
7. The process of claim 1 wherein multi-grained or single crystal
granular silicon is used to nucleate columnar grains having an
average gain size of 0.002 to 1 cm in the subsequent sheet.
8. The process of claim 1 wherein the setter material is selected
from the group consisting of quartz, silica, alumina, graphite, and
SiC.
9. The process of claim 1 wherein the setter material is replaced
by a thin belt material which supports the sheet during formation
and thermal processing, and does not chemically interact with, or
adhere to, the silicon material.
10. The process of claim 1 wherein the resulting silicon sheet has
the characteristics of flatness, a smooth surface, minority carrier
diffusion length greater than 10 microns, low residual stress, and
relatively inactive grain boundaries.
11. The process of claim 1 including utilizing the sheet as a
substrate for a solar cell by forming the additional solar cell
layers on the substrate.
12. A solar cell made by the process of claim 11.
13. A substrate made by the process of claim 1.
14. The process of claim 1 wherein the partially melted silicon net
and the setter material are replaced by a non-melting,
non-reusable, thermal coefficient-matched substrate which is wetted
by and stabilizes the molten silicon over-layer, nucleates
subsequent growth, and serves as a supporting substrate during
subsequent solar cell processing of the grown sheet.
15. The process of claim 14 including utilizing the sheet as a
substrate for a solar cell by forming the additional solar cell
layers on the substrate.
16. A solar cell made by the process of claim 15.
17. A substrate made by the process of claim 14.
18. The process of claim 14 wherein graphite is used as the
substrate.
19. The process of claim 1 including achievement of any or all of
the preheat, melting, growth, and anneal thermal profiles for the
granular silicon and resultant sheet by graphite based heater
technology.
20. A process in claim 1 where an overpressure consisting of at
least 10% nitrogen (by volume) is employed in any or all of the
pre-heat, melting, growth and anneal zones.
21. A process of making an improved columnar-grained
polycrystalline sheet for functioning as a substrate for a solar
cell, comprising (a) applying a net to a setter material, (b)
applying granular silicon to the setter material and the net
whereby to support the granular silicon, (c) preheating the setter
material and the net and the granular silicon in a preheat zone,
(d) subjecting the setter material and the net and the granular
silicon to a thermal profile which causes melting of the granular
silicon from the top downwardly, wherein 25 to 90% of the granular
silicon depth is melted and the partially melted silicon below the
melted material functions as a nucleation site to nucleate
subsequent crystal growth and the net functions to stabilize the
melt and to minimize molten silicon contact with underlying setter
and the net functions as a release coating, (e) transporting the
melt pool on the net into a growth zone wherein a thermal profile
is created to promote columnar growth of a columnar grain size
greater than 20 microns in a direction approximately perpendicular
to the plane of the setter material and where both liquid and a
growing polycrystalline layer coexist and where the entire layer
experiences a liquid state prior to solidification, (f)
transporting the grown sheet into an anneal zone wherein a linear
temperature gradient along the direction of setter material motion
is provided to promote low stress cooling of the sheet, (g)
removing the polycrystalline sheet from the setter material,
facilitated by a release agent, and (h) reusing the setter material
for the making of further columnar-grained polycrystalline
sheets.
22. The process of claim 22 wherein the net is made from a graphite
material.
23. The process of claim 22 including achievement of any or all of
the preheat, melting, growth, and anneal thermal profiles for the
granular silicon and resultant sheet by graphite based heater
technology.
24. The process of claim 21 wherein the net is made from silicon
carbide.
25. The process of claim 21 including achievement of any or all of
the preheat, melting, growth, and anneal thermal profiles for the
granular silicon and resultant sheet by graphite based heater
technology.
26. The process of claim 21 including utilizing the sheet as a
substrate for a solar cell by forming the additional solar cell
layers on the substrate.
27. A solar cell made by the process of claim 26.
28. A substrate made by the process of claim 21.
29. A process in claim 21 where an overpressure consisting of at
least 10% nitrogen (by volume) is employed in any or all of the
pre-heat, melting, growth and anneal zones.
30. A process of making an improved columnar-grained
polycrystalline sheet for functioning as a substrate for a solar
cell, comprising (a) providing a layer of graphite material, (b)
applying granular silicon to the graphite material whereby to
support the granular silicon, (c) preheating the graphite material
and the granular silicon in a preheat zone, (d) subjecting the
graphite material and the granular silicon to a thermal profile
which causes melting of the granular silicon from the top
downwardly, wherein 25 to 90% of the granular silicon depth is
melted and the partially melted silicon below the melted material
functions as a nucleation site to nucleate subsequent crystal
growth and the graphite material functions as a release coating,
(e) transporting the melt pool on the graphite material into a
growth zone wherein a thermal profile is created to promote
columnar growth of a columnar grain size greater than 20 microns in
a direction approximately perpendicular to the plane of the
graphite material and where both liquid and a growing
polycrystalline layer coexist and where the entire layer of
experiences a liquid state prior to solidification, (f)
transporting the grown sheet into an anneal zone wherein a linear
temperature gradient along the direction of graphite material
motion is provided to promote low stress cooling of the sheet, (g)
removing of the polycrystalline sheet from the graphite material,
and (h) reusing the graphite material for the making of further
columnar-grained polycrystalline sheets.
31. The process of claim 30 including utilizing the sheet as a
substrate for a solar cell by forming the additional solar cell
layers on the substrate.
32. A solar cell made by the process of claim 31.
33. A substrate made by the process of claim 30.
34. A process in claim 30 where an overpressure consisting of at
least 10% nitrogen (by volume) is employed in any or all of the
pre-heat, melting, growth and anneal zones. .Iadd.
35. A sheet of silicon, said silicon sheet having a pair of
opposing manor surfaces, said silicon of said sheet being of
elongated columnar grain form with the grains having an average
width in the range of 0.002 to 1 cm in size with the columnar axis
thereof extending in the direction of their columnar axis from
major surface to major surface, and said sheet having a thickness
of from 350 to 1000 microns. .Iaddend..Iadd.36. The sheet of claim
35 wherein said grain size is greater than 80 microns.
.Iaddend..Iadd.37. The sheet of claim 36 wherein said sheet has an
electrical resistivity in the range of 0.1 to 10 ohm-cm.
.Iaddend..Iadd. The sheet of claim 37 having the characteristics of
flatness, a smooth surface, minority carrier diffusion length
greater than 10 microns, low residual stress, and low activity
grain boundaries. .Iaddend..Iadd.39. The sheet of claim 38 wherein
said minority carrier diffusion length is greater than 40 microns.
.Iaddend..Iadd.40. The sheet of claim 38 wherein the minimum grain
dimension is at least two times said minority carrier diffusion
length. .Iaddend..Iadd.41. The sheet of claim 40 wherein said sheet
is sized to function as a substrate.
.Iaddend..Iadd. 2. The sheet of claim 35 wherein said sheet has an
electrical resistivity in the range of 0.1 to 10 ohm-cm.
.Iaddend..Iadd.43. The sheet of claim 35 having the characteristics
of flatness, a smooth surface, minority carrier diffusion length
greater than 10 microns, low residual stress, and low activity
grain boundaries. .Iaddend..Iadd.44. The sheet of claim 43 wherein
the minimum grain dimension is at least two times said minority
carrier diffusion length.
.Iaddend..Iadd.45. The sheet of claim 35 wherein said sheet is
sized to function as a substrate. .Iaddend..Iadd.46. The sheet of
claim 35 wherein said grains from at one of said surfaces having
features of a silicon net.
.Iaddend..Iadd.47. A substrate made from a sheet of silicon, said
silicon sheet having a pair of opposing major surfaces, said
silicon sheet being of elongated columnar grain form with the
grains having an average width in the range of 0.002 to 1 cm in
size with the columnar axis thereof extending in the direction of
their columnar axis from one of said surfaces to the other of said
surfaces, and said silicon sheet having a thickness of from 350 to
1000 microns. .Iaddend..Iadd.48. The substrate of claim 47 wherein
said grain size is greater than 80 microns.
.Iaddend..Iadd.49. In a solar cell having a substrate, the
improvement being in that said substrate comprises a sheet of
silicon, said silicon sheet having a pair of opposing major
surfaces, said silicon sheet being of elongated columnar grain form
with the grains having an average width in the range of 0.002 to 1
cm in size with the columnar axis thereof extending in the
direction of their columnar axis from one of said surfaces to the
other of said surfaces, and said silicon sheet having a thickness
of from 350 to 1000 microns. .Iaddend..Iadd.50. The solar cell of
claim 49 wherein said substrate has a thickness in the range of
0.01 to 0.10 cm, and said solar cell being capable of achieving
voltages in excess of 560 mV and fill factors in excess of 0.72.
.Iaddend..Iadd.51. The solar cell of claim 50 wherein said solar
cell includes a plurality of layers made entirely of silicon
supported by said substrate. .Iaddend.
Description
BACKGROUND OF THE INVENTION
Photovoltaic solar cells are semiconductor devices which convert
sunlight into electricity. Solar cells based on crystalline silicon
offer the advantage of high performance and stability. The
principal barrier to expanded utilization of silicon solar cells
for electric power generation is the present high cost of the solar
cells.
In conventional solar cells based on single crystal or large grain
polycrystalline silicon ingot processes, the major cost factor is
determined by the requirement of sawing ingots into wafers. Sawing
is an expensive processing step, and furthermore results in the
loss of approximately half the costly ingot material as silicon
dust. The problem to be solved requires the development of a
low-cost process, that efficiently employs low-cost materials while
maintaining solar cell performance.
The technical requirements for a solution to the problem are based
on the achievement of a process that is controllable, has high
areal throughput, and generates material with adequate crystalline
morphology. The prior art includes several processes which either
effectively achieve controlled growth, or high areal throughput of
silicon sheet or ribbons. All these approaches eliminate the costly
process of sawing large areas to create wafers from ingots. For
example, publications by Hopkins (WEB), Ettouney, et al. (EFG),
Gurtler (RTR) and Eyer, et al. (SSP) describe processes that
achieve controlled polycrystalline growth of grains greater than 1
mm in size at low linear speeds (and consequently low areal
generation rates). Common to these sheet growth processes is the
fact that the sheet pulling direction and the direction of sheet
growth are collinear. All of these processes employ a large
temperature gradient (>500 degrees Centigrade per centimeter)
along the sheet growth direction. This gradient is necessary to
achieve the practical linear speed indicated (typically less than 2
cm/min), but also introduces large thermal-induced stresses. In
many cases these stresses limit the practical sheet width that can
be achieved by causing sheet deformations which make solar cell
fabrication untenable. Thermal stresses can also create crystalline
defects which limit solar cell performance. Each of these processes
attempts to achieve grain sizes that are as large as possible in
order to avoid the deleterious effects of grain boundaries on solar
cell performance.
Another set of processes has been developed that can achieve high
areal throughput rates. For example, publications by Bates, et al.
(LASS), Helmreich, et al. (RAFT), Falckenberg, et al. (S-Web),
Hide, et al. (CRP) and Lange, et al. (RGS) describe processes that
achieve polycrystalline sheet growth with grain sizes in the 10
microns to 3 mm range at high linear rates (10 to 1800 cm/min).
Typically, these processes have difficulty maintaining geometric
control (width and thickness) (e.g. (LASS, RAFT, RGS), and/or
experience difficulty with contamination of the silicon by the
contacting materials (e.g. RAFT, S-Web, CRP). Common to these sheet
growth processes is the fact that the sheet pulling direction and
the direction of crystalline growth in the sheet are nearly
perpendicular. It is this critical feature of these processes that
allows the simultaneous achievement of high linear pulling speeds
and reduced crystal growth speeds. Reduced crystal growth speeds
are necessary for the achievement of materials with high
crystalline quality.
The prior art regarding the fabrication of solar cells from
polycrystalline materials requires that the grain size be greater
than 1.0 mm. This requirement on grain size was necessitated by the
need to minimize the deleterious effects of grain boundaries
evident in prior art materials. Historically, small-grained
polycrystalline silicon (grain size less than 1.0 mm) has not been
a candidate for photovoltaic material due to grain boundary
effects. Grain boundary recombination led to degradation of
voltage, current and fill factors in the solar cell. Previous
models, for example Ghosh (1980) and Fossum (1980), based on
recombination at active grain boundaries correctly predicted
performance of historical materials. By inference these models
teach that the achievement of inactive grain boundaries permits the
utilization of small grained materials.
SUMMARY OF THE INVENTION
It is the object of this invention to provide a low cost process
for forming low stream columnar-grained sheets that are employed in
high performance solar cells.
A further object of this invention is to provide techniques for
manufacturing columnar-grained polycrystalline silicon sheets for
use as a substrate in solar cells, which overcomes the
disadvantages of the prior art.
A yet further object of this invention is to provide a process for
manufacturing a low-cost solar cell that employs small-grained
polycrystalline silicon with low-activity grain boundaries.
A still further object of this invention is to provide a substrate
and a solar cell made from such process.
In accordance with this invention the sheet is formed by using a
columnar growth technique that controls the details of heat flow,
and thus growth speed of the polycrystalline material. The process
begins with granular silicon that is applied to a setter material;
the setter and silicon are then subjected to a designed thermal
sequence which results in the formation of a columnar-grained
polycrystalline silicon sheet at high areal throughput rates. The
equipment employed to accomplish the process includes a line source
of energy and a polycrystalline sheet growth and annealing
technology.
The invention may also be practiced with a process which includes a
more distributed source of energy application than a line source,
such as by graphite-based infrared heating.
BRIEF DESCRIPTION OF THE DRAWING
The single FIGURE illustrates a perspective view showing the
sequence for fabricating low stress, columnar-grained silicon
sheets usable as solar cells substrates in accordance with this
invention.
DETAILED DESCRIPTION
The present invention is directed to techniques used for making
improved columnar grain polycrystalline sheets which are
particularly adaptable for use as substrates or wafers in solar
cells. The ability to use the sheet as a solar cell substrate makes
possible the provision of a solar cell consisting entirely of
silicon material where the sheet would function as a substrate made
of silicon and the remaining layers of the solar cell would also be
made of silicon.
The desired properties of the columnar-grained silicon sheet or
substrate fabricated for inclusion in a low-cost solar cell in
accordance with the teachings of this invention are: flatness, a
smooth surface, minority carrier diffusion length greater than 40
microns, minimum grain dimension at least two times the minority
carrier diffusion length, low residual stress and relatively
inactive grain boundaries. Since the minimum grain dimension of the
columnar grain silicon sheet is at least two times the minority
carrier diffusion length which in turn is greater than 40 microns,
the columnar grains would have a grain size greater than 80
microns. Grain sizes down to 10 microns can be employed with
minority carrier diffusion length greater than 10 microns, and will
lead to solar cells having lower currents, and lower power. The
desired properties of a process for fabricating columnar-grain
silicon material appropriate for inclusion in a low-cost solar cell
in accordance with the teachings of this invention are: low thermal
stress procedure, controlled nucleation, high areal throughput, and
simple process control.
The criteria for the columnar-grain silicon material product of
flatness and smoothness are required to make solar cell fabrication
tenable. The requirements on diffusion length and grain size are to
minimize recombination losses in the bulk and at grain surfaces
(i.e. grain boundaries), respectively. The requirement of
relatively inactive grain boundaries is to effect the minimization
of grain boundary recombination. The requirement of low residual
stress is to minimize mechanical breakage and to maintain high
minority carrier diffusion lengths.
The criteria for the columnar-grained silicon process of a low
thermal stress procedure is to effect minimization of bulk
crystalline defects. The requirement of controlled nucleation is to
affect the achievement of the required grain morphology and size.
The criteria for high areal throughput and simple process control
arc to achieve low-cost and manufacturability.
The single Figure is a perspective view illustrating the sequence
for fabricating low stress, columnar-grained silicon sheets. The
process as depicted moves from left to right. In general, a setter
material 100, which serves as a mechanical support, is coated with
a granular silicon layer 200, and is passed through a prescribed
thermal profile. The prescribed thermal profile first creates a
melt region 300 at the top of the granular silicon 200, and then
creates a growth region 400 where both liquid and a growing layer
of polycrystalline layer coexist. Finally, there is an annealing
region 500 where the temperature of the polycrystalline silicon
sheet layer 600 is reduced in a prescribed manner to effect stress
relief.
The setter material 100 is selected based on the following
requirements. It must: maintain its shape during the sheet
formation thermal processing; not chemically interact with, or
adhere to, the silicon material; and possess the proper thermal
characteristics to effect the required sheet growth and annealing.
The form of the setter material may either be as a rigid board or
as a flexible thin belt.
Several materials including, but not limited to, quartz, refractory
boards (e.g. silica and/or alumina), graphite, and silicon carbide
have been employed and maintained the proper geometric shape during
thermal processing.
To assure that the setter 100 does not adhere to the final
polycrystalline silicon sheet 600, a release agent coating 110 is
applied to the setter. Either, or a combination of, silicon
nitride, silicon oxynitride, silica, or alumina have been employed
as this agent. A low-cost method for applying this coating is to
form a liquid slurry that is painted or sprayed on the bare setter,
and then subsequently dried in an oxidizing atmosphere before use.
The release agent facilitates separation of the sheet and permits
reuse of the setter material.
In the process design the thermal characteristics of the setter 100
play a key role in managing the melt and growth processes. In the
melt region 300 it is preferred that the thermal conductivity of
the setter be low to assure the efficient deployment of the energy
being used to melt the granular silicon 200. The thermal properties
of the setter may be tailored to possess a strip of higher thermal
conductivity under the outer edges 210 of the strip of granular
silicon. The effect of this strip is to define the outer edges of
the growing sheet. The thermal conductivity of the setter may also
be tailored to assist in defining nucleation sites to commence
growth. This can be accomplished by locally placing thermal shunts
120 in the setter. These shunts 120 provide a thermal conduction
path between the top and bottom of the setter, effecting a local
path for removing the heat of solidification, and result in sites
where nucleated growth occurs.
In a preferred embodiment the setter material is low density 1.5 cm
thick silica board. The setter preparation is completed by coating
the top surface with a release agent 110. This is accomplished
using an aqueous colloidal solution of silicon nitride that is
painted on the top surface and baked in an oxidizing atmosphere to
form a non-wetting, non-adhering oxynitride layer, before the
initial application of granular silicon.
The granular silicon 200 is selected based on the following
requirements. It must: be properly sized; be of adequate purity;
and contain a chemical ingredient to provide a p-type resistivity
of the grown silicon sheet 600 in the range of 0.1.[.,.]. to 10
ohm-cm.
The range of proper sizes for the granular silicon 200 employed in
the process is between 100 and 1000 micrometers. The upper limit is
determined by the design thickness for the silicon sheet material.
As a rule the minimum dimension of the largest silicon particles
should be equal to or less than the desired thickness of the
silicon sheet material. The lower size limit of the particle
distribution is dependent on the dynamics of the melting process,
and the need to limit the amount of silicon oxide. The silicon
oxide is a source of sheet contamination, and naturally occurs at
all silicon surfaces.
The purity level necessary in the sheet silicon is determined by
the requirements for the efficient operation of a solar cell device
fabricated on the sheet. Whereas the employment of low-processed
metallurgical grade silicon is not adequate, utilization of highly
processed semiconductor grade silicon is not necessary. In
practice, the preferred process can be executed with off-grade
semiconductor grade silicon. It is also an advantage of the
preferred process that an additional degree of impurity reduction
is accomplished during sheet growth by segregation of impurities to
the sheet surface, where they may easily be removed by a subsequent
chemical etch. This mechanism for purification by segregation is
operative in the preferred process as the actual crystalline growth
rate is less than 0.1 cm/min in the crystal growth direction,
comparable to that employed in the single crystal float zone
process. This mechanism is not operative in sheet growth
technologies that have the crystalline growth rate equal to the
sheet pulling speed (approx., 2 cm/min). At these higher growth
velocities, there is not sufficient time for effective segregation
to occur between liquid and solid as the process is diffusion
limited.
It is necessary to provide for the addition of a separate
constituent in, or with, the granular silicon to effect in
electrical resistivity in the range of 0.1 to 10 ohm-cm in the
sheet material. Typically, for p-type conductivity in the sheet
material the preferred elements are boron, aluminum, or indium. As
an example of the preferred embodiment, the addition of powdered
boron silicide followed by mechanical mixing of the granular
silicon provides for the accomplishment of the required p-type
resistivity in the subsequently grown silicon sheet.
The properly doped p-type granular silicon 200 is uniformly layered
on the coated setter 100. For example, this process can be
effectively accomplished by using a doctor blade. The spacing
between the edge of the doctor blade and the setter surface needs
to be at least two times the minimum dimension of the largest
particle in the granular silicon size distribution. Furthermore,
the thickness of the final silicon sheet 600 can be as little as
the minimum dimension of the largest particle in the granular size
distribution.
The silicon-coated setter is transported into an environmental
chamber with an argon or nitrogen overpressure. In a preferred
embodiment a mixture of argon and hydrogen gas is employed to
effectively limit the amount of silicon oxide that is formed during
the growth process. The percent of hydrogen employed is determined
by the water vapor content in the chamber. The ratio of hydrogen to
water vapor controls the magnitude of silicon oxide formation. The
chamber may include a pre-heat zone employed to raise the
temperature to 1100 to 1400.degree. C., which in combination with
the hydrogen present has the effect of reducing the native oxide of
silicon that exists on the granular silicon. An overpressure
consisting of at least 10% nitrogen (by volume) is employed in any
or all of the pre-heat, melting, growth and anneal zones.
After the granular silicon 200 has been pre-heated it is then
brought into a thermal zone 300 where the top portion of the
granular silicon layer 200 is melted. In the preferred embodiment,
this thermal zone and the melting of the top portion of the layer
is accomplished using a focussed beam of light. The length of the
focussed beam along the direction of setter motion is about 1
centimeter. The depth of the granular silicon that is melted
depends on the intensity of the input energy from thermal zone 300,
the thickness of the granular silicon layer, the linear speed of
the granular silicon coated setter through thermal zone 300, and
the details of heat transfer between the granular silicon 200 and
the seer 100. The outer edges of the melt zone are stabilized by
the thermal shunts 120 engineered into the setter 100 or by
reducing energy intensity at the edges. These thermal shunts 120
inhibit the depth of melting at the outer edges 210 and thus
promote edge stabilization. Between 25 and 90% (and preferably
between 50% and 90%) of the granular silicon depth is melted. The
invention may also be practiced where the entire layer may have
experienced a liquid state prior to solidification into a sheet.
The material at the bottom of the granular layer is partially
melted by liquid silicon penetrating from the molten silicon layer
above. This partially melted layer of silicon forms a net 220.
Other materials incorporated in a substrate designed to be
thermally-matched to silicon can be employed as a non-reusable net
220 material. Other materials including fabrics that are woven or
non-woven, such as graphite, can be employed as the net 220. Other
granular materials that are partially melted or unmelted, such as
silicon carbide, can be employed as the net 220 material. The net
220 is responsible for four key process features. First because it
is wetted by the molten silicon above, this layer stabilizes the
melt and growth zones by defeating the surface tension of the
molten silicon over-layer. This allows the production of wide
sheets, with smooth surfaces. Second, this layer serves as a plane
to nucleate subsequent growth. Third, this layer minimizes molten
silicon contact with the supporting setter and release coating,
thereby minimizing any potential contamination by impurities.
Fourth, this layer serves as highly defected back plane,
intrinsically gettering impurities from the active silicon layer
above, allowing the employment of lower purity, lower-cost grades
of silicon raw material.
Where the net 220 is made from a material such as graphite, the
graphite could be unrolled and applied over the setter material
before the granular silicon is applied. Thus, the net is between
the granular silicon and the setter material. The later melted
silicon would function as a nucleation site. The net would function
to stabilize the melt, minimize molten silicon contact with the
underlying setter and act as a release coating. Any or all of the
preheat, melting, growth, and anneal thermal profiles for the
granular powder and resultant sheet could be achieved by graphite
based heater technology.
Where the net 220 is made from the materials incorporated in a
non-reusable substrate designed to be thermal coefficient-matched
to silicon, the substrate could be positioned on the setter
material before the silicon is applied. Thus, the non-reusable
substrate is between the granular silicon and the setter material.
The non-reusable substrate would act as a nucleation site and
stabilize the melt. Any or all of the preheat, melting, growth and
anneal thermal profiles for the granular powder and resultant sheet
could be achieved by graphite-based heater technology.
After leaving the melt creation zone 300 of the thermal profile,
the melt pool on the partially melted silicon net 220 moves into
the growth zone 400 of the thermal profile. In this zone the growth
is initiated on the silicon net 220. Because growth is nucleated
from the partially melted silicon net 220, the grain size of the
granular silicon 200 is an important parameter in determining the
size of the columnar grains in the grown sheet 600. In the
preferred embodiment, multi-grained or single crystal granular
silicon 200 is used to achieve relatively large columnar grains
(average grain size 0.002 to 1.0 cm) in the grown sheet 600. In one
embodiment, growth may also be preferentially initiated at sites
210 in the granular silicon layer where the beat transfer is
controlled by thermal shunting areas designed in the setter. The
direction of the growth front is approximately perpendicular to the
plane of the setter. The length of the growth zone along the
direction of setter motion is from 2 to 20 centimeters, and is
slightly less than the entire length of the melt pool. The length
of the growth zone is determined by controlling the rate of loss of
heat (and therefore growth rate) attending the solidification
process. As a consequence of the growth process, the grains that
are grown are columnar in nature. Typically, individual grains in
the resulting sheet 600 extend from the top surface to the bottom,
and are at least as wide as they are high. Sheet thicknesses in the
range of 400 to 500 microns can be achieved at sleet pulling speeds
in excess of 30 cm/min.
After leaving the growth zone 400 of the thermal profile, the sheet
600 moves into the annealing zone 500 of the thermal profile. In
this zone the grown sheet, still at approximately 1400.degree. C.,
is subjected to a linear temperature gradient along the direction
of setter motion. The linear temperature profile eliminates
buckling and cracking of the as-grown sheet, and minimizes the
generation of dislocations. The thickness of the grown sheet is in
the range of 350 to 1000 microns in the preferred process. Because
the thickness of the final grown sheet 600 is determined by the
precise application of granular silicon 200 to the setter 100,
exceptional sheet thickness control and process stability are
achieved in comparison to sheet technologies pulled from a melt,
where thickness is controlled by the melt meniscus. After cool
down, the sheet is removed from the setter, and appropriately sized
by sawing or scribing, for fabrication into solar cells. The setter
is reused for making further columnar-grained polycrystalline
sheets.
The properties of the sheet material fabricated with the above
process are quite amenable to the fabrication of efficient solar
cells. This process generates material that has unique properties
of size and character. Although the grains are columnar, and have
average sizes in the range of 0.002 to 1.000 cm in extent, solar
cells fabricated on material in the range of 0.01 to 0.10 cm may
achieve voltages in excess of 560 mV, and fill factors in excess of
0.72. The achievement of these values on such small .[.ground.].
.Iadd.grained .Iaddend.material indicate that this material is not
being limited by recombination at grain boundaries as had been
previously predicted by Ghosh. Previously, columnar grains were
dismissed as being ineffective since columnar grains were always
small, and small grains were thought not to work. The process
herein described achieves columnar grams that yield material with
relatively benign grain boundaries with the result that efficient,
low-cost solar cells can be manufactured.
The process herein described can be carried out in a continuous
manner, resulting in continuous sheets that can be appropriately
sized using an in line scribe or a saw. Impurity content in the
melt and grown sheet quickly reaches steady-state; it does not
increase during continuous processing. Since all embodiments
include application of granular silicon to the setter, and since
material enters the melt creation zone in this form, melt
replenishment is not a problem, unlike sheet technologies pulled
from a melt pool. After being properly sized, the sheets function
as a substrate by having the remaining layers formed thereon to
produce solar cells. Where the remaining layers are of silicon, a
completely silicon solar cell results.
* * * * *