U.S. patent application number 12/890428 was filed with the patent office on 2012-03-29 for method of fabricating an emitter region of a solar cell.
Invention is credited to Tim Dennis, Helen Liu, Hsin-Chiao Luan, Brenda Pagulayan Malgapu, Jane Manning, Joseph Ramirez, David Smith, Genevieve A. Solomon, Ann Waldhauer.
Application Number | 20120073650 12/890428 |
Document ID | / |
Family ID | 45869397 |
Filed Date | 2012-03-29 |
United States Patent
Application |
20120073650 |
Kind Code |
A1 |
Smith; David ; et
al. |
March 29, 2012 |
METHOD OF FABRICATING AN EMITTER REGION OF A SOLAR CELL
Abstract
Methods of fabricating emitter regions of solar cells are
described. Methods of forming layers on substrates of solar cells,
and the resulting solar cells, are also described.
Inventors: |
Smith; David; (Campbell,
CA) ; Liu; Helen; (El Cerrito, CA) ; Dennis;
Tim; (Canton, TX) ; Manning; Jane; (Woodside,
CA) ; Luan; Hsin-Chiao; (Palo Alto, CA) ;
Waldhauer; Ann; (La Honda, CA) ; Solomon; Genevieve
A.; (Palo Alto, CA) ; Malgapu; Brenda Pagulayan;
(Cavite, PH) ; Ramirez; Joseph; (Batangas City,
PH) |
Family ID: |
45869397 |
Appl. No.: |
12/890428 |
Filed: |
September 24, 2010 |
Current U.S.
Class: |
136/258 ;
257/E31.043; 257/E31.047; 438/96; 438/97 |
Current CPC
Class: |
H01L 31/0376 20130101;
Y02E 10/547 20130101; H01L 31/0368 20130101; H01L 31/02167
20130101; Y02P 70/50 20151101; H01L 31/202 20130101; Y02E 10/546
20130101; H01L 31/1876 20130101; H01L 31/1864 20130101; H01L 31/182
20130101; H01L 31/062 20130101; H01L 31/0745 20130101; H01L 31/1804
20130101; H01L 31/1872 20130101 |
Class at
Publication: |
136/258 ; 438/97;
438/96; 257/E31.043; 257/E31.047 |
International
Class: |
H01L 31/0368 20060101
H01L031/0368; H01L 31/0376 20060101 H01L031/0376; H01L 31/18
20060101 H01L031/18 |
Goverment Interests
[0001] The invention described herein was made with Governmental
support under contract number DE-FC36-07G017043 awarded by the
United States Department of Energy. The Government may have certain
rights in the invention.
Claims
1. A method of fabricating an emitter region of a solar cell, the
method comprising: forming, in a furnace, a tunnel oxide layer on a
surface of a substrate; and, without removing the substrate from
the furnace, forming an amorphous layer on the tunnel oxide layer;
doping the amorphous layer to provide a first region comprising
N-type dopants and a second region comprising P-type dopants; and,
subsequently, heating the amorphous layer to provide a
polycrystalline layer comprising an N-type-doped region and a
P-type-doped region.
2. The method of claim 1, wherein the substrate comprises silicon,
the tunnel oxide layer comprises silicon dioxide, the amorphous
layer comprises silicon, the N-type dopants comprise phosphorous,
and the P-type dopants comprise boron.
3. The method of claim 1, wherein forming the tunnel oxide layer
comprises heating the substrate in the furnace at a temperature of
approximately 900 degrees Celsius.
4. The method of claim 3, wherein heating the substrate in the
furnace at the temperature of approximately 900 degrees Celsius
further comprises heating at a pressure of approximately 500 mTorr
for approximately 3 minutes in an atmosphere of oxygen to provide
the tunnel oxide layer having a thickness of approximately 1.5
nanometers.
5. The method of claim 1, wherein forming the tunnel oxide layer
comprises heating the substrate in the furnace at a temperature
less than 600 degrees Celsius.
6. The method of claim 5, wherein heating the substrate in the
furnace at the temperature of less than 600 degrees Celsius further
comprises heating at a temperature of approximately 565 degrees
Celsius, at a pressure of approximately 300 Torr, for approximately
60 minutes in an atmosphere of oxygen to provide the tunnel oxide
layer having a thickness of approximately 1.5 nanometers.
7. The method of claim 1, wherein forming the amorphous layer
comprises depositing the amorphous layer in the furnace at a
temperature less than 575 degrees Celsius.
8. The method of claim 7, wherein depositing the amorphous layer in
the furnace at the temperature less than 575 degrees Celsius
further comprises heating at a temperature of approximately 565
degrees Celsius at a pressure of approximately 350 mTorr in an
atmosphere of silane (SiH.sub.4) to provide the amorphous layer
having a thickness approximately in the range of 200-300
nanometers.
9. The method of claim 1, wherein both the tunnel oxide layer and
the amorphous layer are formed at a temperature of approximately
565 degrees Celsius, and wherein heating the amorphous layer to
provide the polycrystalline layer comprises heating at a
temperature of approximately 980 degrees Celsius.
10. A method of forming layers on a substrate of a solar cell, the
method comprising: loading, into a furnace, a wafer carrier with a
plurality of wafers, the wafer carrier having one or more wafer
receiving slots loaded with two wafers positioned back-to-back;
forming, in the furnace, a tunnel oxide layer on all surfaces of
each of the plurality of wafers; and, without removing the
plurality of wafers from the furnace, forming an amorphous layer on
the tunnel oxide layer, the amorphous layer formed on all portions
of the tunnel oxide layer except on the portions in contact between
wafers positioned back-to-back.
11. The method of claim 10, wherein for the wafers positioned
back-to-back, a ring pattern of the amorphous layer is formed on
the back of each wafer.
12. The method of claim 11, further comprising: subsequent to
forming the amorphous layer, applying a cleaning solution to the
back of each wafer, the cleaning solution comprising an oxidizing
agent; and, subsequently, applying a texturizing solution to the
back of each wafer, the texturizing solution comprising a
hydroxide.
13. The method of claim 12, where in the oxidizing agent is
selected from the group consisting of ozone and hydrogen peroxide
(H.sub.2O.sub.2), and wherein the hydroxide is selected from the
group consisting of potassium hydroxide (KOH) and sodium hydroxide
(NaOH).
14. The method of claim 10, wherein each of the plurality of wafers
comprises silicon, the tunnel oxide layer comprises silicon
dioxide, and the amorphous layer comprises silicon.
15. The method of claim 10, wherein forming the tunnel oxide layer
comprises heating each of the plurality of wafers in the furnace at
a temperature of approximately 900 degrees Celsius.
16. The method of claim 15, wherein heating each of the plurality
of wafers in the furnace at the temperature of approximately 900
degrees Celsius further comprises heating at a pressure of
approximately 500 mTorr for approximately 3 minutes in an
atmosphere of oxygen to provide the tunnel oxide layer having a
thickness of approximately 1.5 nanometers.
17. The method of claim 10, wherein forming the tunnel oxide layer
comprises heating each of the plurality of wafers in the furnace at
a temperature less than 600 degrees Celsius.
18. The method of claim 17, wherein heating each of the plurality
of wafers in the furnace at the temperature of less than 600
degrees Celsius further comprises heating at a temperature of
approximately 565 degrees Celsius, at a pressure of approximately
300 Torr, for approximately 60 minutes in an atmosphere of oxygen
to provide the tunnel oxide layer having a thickness of
approximately 1.5 nanometers.
19. The method of claim 10, wherein forming the amorphous layer
comprises depositing the amorphous layer in the furnace at a
temperature less than 575 degrees Celsius.
20. The method of claim 19, wherein depositing the amorphous layer
in the furnace at the temperature less than 575 degrees Celsius
further comprises heating at a temperature of approximately 565
degrees Celsius at a pressure of approximately 350 mTorr in an
atmosphere of silane (SiH.sub.4) to provide the amorphous layer
having a thickness approximately in the range of 200-300
nanometers.
21. A substrate of a solar cell, comprising: a tunnel oxide layer
comprising silicon dioxide disposed on all surfaces of a silicon
wafer; and a polycrystalline layer disposed on the tunnel oxide
layer, the polycrystalline layer disposed on all portions of the
tunnel oxide layer except on a back side of the silicon wafer which
comprises a ring pattern of the polycrystalline layer.
22. A substrate of a solar cell, comprising: a tunnel oxide layer
comprising silicon dioxide disposed on all surfaces of a silicon
wafer; and an amorphous layer disposed on the tunnel oxide layer,
the amorphous layer disposed on all portions of the tunnel oxide
layer except on a back side of the silicon wafer which comprises a
ring pattern of the amorphous layer.
Description
TECHNICAL FIELD
[0002] Embodiments of the present invention are in the field of
renewable energy and, in particular, methods of fabricating emitter
regions of solar cells.
BACKGROUND
[0003] Photovoltaic cells, commonly known as solar cells, are well
known devices for direct conversion of solar radiation into
electrical energy. Generally, solar cells are fabricated on a
semiconductor wafer or substrate using semiconductor processing
techniques to form a p-n junction near a surface of the substrate.
Solar radiation impinging on the surface of the substrate creates
electron and hole pairs in the bulk of the substrate, which migrate
to p-doped and n-doped regions in the substrate, thereby generating
a voltage differential between the doped regions. The doped regions
are connected to metal contacts on the solar cell to direct an
electrical current from the cell to an external circuit coupled
thereto.
[0004] Efficiency is an important characteristic of a solar cell as
it is directly related to the solar cell's capability to generate
power. Accordingly, techniques for increasing the efficiency of
solar cells are generally desirable. Embodiments of the present
invention allow for increased solar cell efficiency by providing
novel processes for fabricating solar cell structures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 illustrates a flowchart representing operations in a
method of fabricating an emitter region of a solar cell, in
accordance with an embodiment of the present invention.
[0006] FIG. 2A illustrates a cross-sectional view of a stage in the
fabrication of a solar cell including an emitter region, in
accordance with an embodiment of the present invention.
[0007] FIG. 2B illustrates a cross-sectional view of a stage in the
fabrication of a solar cell including an emitter region,
corresponding to operation 102 of the flowchart of FIG. 1, in
accordance with an embodiment of the present invention.
[0008] FIG. 2C illustrates a cross-sectional view of a stage in the
fabrication of a solar cell including an emitter region,
corresponding to operation 104 of the flowchart of FIG. 1, in
accordance with an embodiment of the present invention.
[0009] FIG. 2D illustrates a cross-sectional view of a stage in the
fabrication of a solar cell including an emitter region,
corresponding to operation 106 of the flowchart of FIG. 1, in
accordance with an embodiment of the present invention.
[0010] FIG. 2E illustrates a cross-sectional view of a stage in the
fabrication of a solar cell including an emitter region,
corresponding to operation 108 of the flowchart of FIG. 1, in
accordance with an embodiment of the present invention.
[0011] FIG. 3 illustrates a flowchart representing operations in a
method of forming layers on a substrate of a solar cell, in
accordance with an embodiment of the present invention.
[0012] FIG. 4A illustrates a cross-sectional view of a stage in the
fabrication of solar cells, corresponding to operation 302 of the
flowchart of FIG. 3, in accordance with an embodiment of the
present invention.
[0013] FIG. 4B illustrates a magnified view of a portion of FIG.
4A, in accordance with an embodiment of the present invention.
[0014] FIG. 4C illustrates a cross-sectional view of a stage in the
fabrication of solar cells, corresponding to operations 304 and 306
of the flowchart of FIG. 3, in accordance with an embodiment of the
present invention.
[0015] FIG. 5 illustrates both a cross-sectional view and a
top-down view of a substrate of a solar cell, the substrate having
layers formed thereon, in accordance with an embodiment of the
present invention.
DETAILED DESCRIPTION
[0016] Methods of fabricating emitter regions of solar cells are
described herein. In the following description, numerous specific
details are set forth, such as specific process flow operations, in
order to provide a thorough understanding of embodiments of the
present invention. It will be apparent to one skilled in the art
that embodiments of the present invention may be practiced without
these specific details. In other instances, well-known fabrication
techniques, such as lithographic and etch techniques, are not
described in detail in order to not unnecessarily obscure
embodiments of the present invention. Furthermore, it is to be
understood that the various embodiments shown in the figures are
illustrative representations and are not necessarily drawn to
scale.
[0017] Disclosed herein are methods of fabricating emitter regions
of solar cells. In one embodiment, a method of fabricating an
emitter region of a solar cell includes forming, in a furnace, a
tunnel oxide layer on a surface of a substrate. Without removing
the substrate from the furnace, an amorphous layer is formed on the
tunnel oxide layer. The amorphous layer is doped to provide a first
region having N-type dopants and a second region having P-type
dopants. Subsequently, the amorphous layer is heated to provide a
polycrystalline layer having an N-type-doped region and a
P-type-doped region. In one embodiment, a method of forming layers
on a substrate of a solar cell includes loading, into a furnace, a
wafer carrier with a plurality of wafers, the wafer carrier having
one or more wafer receiving slots loaded with two wafers positioned
back-to-back. In the furnace, a tunnel oxide layer is formed on all
surfaces of each of the plurality of wafers. Without removing the
substrate from the furnace, an amorphous layer is formed on the
tunnel oxide layer, the amorphous layer formed on all portions of
the tunnel oxide layer except on the portions in contact between
wafers positioned back-to-back.
[0018] Also disclosed herein are solar cells. In such embodiments,
a solar cell includes a substrate or wafer. In one embodiment, a
tunnel oxide layer including silicon dioxide is disposed on all
surfaces of a silicon wafer. A polycrystalline layer is disposed on
the tunnel oxide layer, the polycrystalline layer disposed on all
portions of the tunnel oxide layer except on a back side of the
silicon wafer which has a ring pattern of the polycrystalline
layer. In one embodiment, a tunnel oxide layer including silicon
dioxide is disposed on all surfaces of a silicon wafer. An
amorphous layer is disposed on the tunnel oxide layer, the
amorphous layer disposed on all portions of the tunnel oxide layer
except on a back side of the silicon wafer which has a ring pattern
of the amorphous layer.
[0019] In accordance with an embodiment of the present invention,
in order to fabricate a passivated emitter of a solar cell, a thin
tunnel oxide and heavily doped poly-silicon, both n-type and
p-type, are used. Although such films may conventionally be formed,
individually, in furnaces, the combination has not been applied to
fabrication of a solar cell, and the manufacturing cost to do may
be prohibitive for the solar cell market. Instead, in an
embodiment, the oxidation and subsequent silicon deposition are
combined into a single process operation. In an embodiment, this
approach can also be used to double the throughput by loading two
wafers per slot in a furnace boat. In an embodiment, the silicon is
first deposited as an undoped and amorphous layer. In that
embodiment, the silicon is doped and crystallized in a later
processing operation to provide a poly-silicon layer. In an
alternative embodiment, the silicon layer is formed as a
poly-silicon layer in the single process operation.
[0020] Embodiments of the present invention may address
conventional fabrication issues such as, but not limited to, (1)
control of oxide thickness, and oxide quality, (2) contamination
between oxidation and poly deposition, (3) excessive preventative
maintenance requirements, (4) throughput, or (5) control of n-poly
and p-poly sheet resistance. In accordance with an embodiment of
the present invention, several features for a method of solar cell
manufacturing are combined, namely the combining of the oxidation
and poly (as amorphous silicon first) deposition in a single
process. In one embodiment, silicon carbide (SiC) parts are used in
the furnace to extend maintenance intervals. In one embodiment, two
wafers are loaded per slot to increase throughput. The above
embodiments may all contribute to the feasibility of manufacturing
solar cells.
[0021] In an embodiment, depositing the silicon as an amorphous
layer and then doping and crystallizing the layer in a later
operation makes the process more controllable and improves the
passivation. In an embodiment, throughput is improved by loading
two wafers per slot in a furnace handling boat. In an embodiment,
an SiC boat is used for dimensional stability. In an embodiment,
control of sheet resistance is achieved by, instead of in-situ
doped poly-silicon, depositing undoped amorphous silicon. The n and
p regions are then formed selectively and crystallized at a later,
higher, temperature operation. In an embodiment, by following one
or more of the approaches described herein, grain size may be
maximized, sheet resistance may be minimized, and counter-doping
may be avoided.
[0022] It is to be understood that a furnace for film fabrication
is not limited to a conventional furnace. In an embodiment, the
furnace is a chamber for wafer processing such as, but not limited
to, a vertical furnace chamber, a horizontal furnace chamber, or a
plasma chamber. It is also to be understood that reference to an
amorphous film or layer herein is not limited to an amorphous
silicon film or layer. In an embodiment, the amorphous film or
layer is a film or layer such as, but not limited to, an amorphous
silicon-germanium film or layer or an amorphous carbon-doped
silicon film or layer.
[0023] A solar cell may be fabricated to include an emitter region.
For example, FIG. 1 illustrates a flowchart 100 representing
operations in a method of fabricating an emitter region of a solar
cell, in accordance with an embodiment of the present invention.
FIGS. 2A-2E illustrate cross-sectional views of various stages in
the fabrication of a solar cell including an emitter region,
corresponding to operations of flowchart 100, in accordance with an
embodiment of the present invention.
[0024] Referring to FIG. 2A, a substrate 202 for solar cell
manufacturing is provided. In accordance with an embodiment of the
present invention, substrate 202 is composed of a bulk silicon
substrate. In one embodiment, the bulk silicon substrate is doped
with N-type dopants. In an embodiment, substrate 202 has a textured
surface, although not depicted in FIG. 2A.
[0025] Referring to operation 102 of flowchart 100, and
corresponding FIG. 2B, a method of fabricating an emitter region of
a solar cell includes forming, in a furnace, a tunnel oxide layer
204 on a surface of substrate 202. In accordance with an embodiment
of the present invention, forming tunnel oxide layer 204 includes
heating substrate 202 in the furnace at a temperature of
approximately 900 degrees Celsius. In a specific embodiment,
heating substrate 202 in the furnace at the temperature of
approximately 900 degrees Celsius further includes heating at a
pressure of approximately 500 mTorr for approximately 3 minutes in
an atmosphere of oxygen to provide tunnel oxide layer 204 having a
thickness of approximately 1.5 nanometers. In accordance with
another embodiment of the present invention, forming tunnel oxide
layer 204 includes heating substrate 202 in the furnace at a
temperature less than 600 degrees Celsius. In a specific
embodiment, heating substrate 202 in the furnace at the temperature
of less than 600 degrees Celsius further includes heating at a
temperature of approximately 565 degrees Celsius, at a pressure of
approximately 300 Torr, for approximately 60 minutes in an
atmosphere of oxygen to provide tunnel oxide layer 204 having a
thickness of approximately 1.5 nanometers. In an alternative
embodiment, the atmosphere include N.sub.2O.
[0026] Referring to operation 104 of flowchart 100, and
corresponding FIG. 2C, the method of fabricating an emitter region
of a solar cell further includes, without removing substrate 202
from the furnace, forming an amorphous layer 206 on tunnel oxide
layer 204. In accordance with an embodiment of the present
invention, forming amorphous layer 206 includes depositing
amorphous layer 206 in the furnace at a temperature less than 575
degrees Celsius. In a specific embodiment, depositing amorphous
layer 206 in the furnace at the temperature less than 575 degrees
Celsius further includes heating at a temperature of approximately
565 degrees Celsius, at a pressure of approximately 350 mTorr, and
in an atmosphere of silane (SiH.sub.4) to provide amorphous layer
206 having a thickness approximately in the range of 200-300
nanometers.
[0027] Referring to operation 106 of flowchart 100, and
corresponding FIG. 2D, the method of fabricating an emitter region
of a solar cell further includes doping amorphous layer 206 with
dopants 208 to provide a doped amorphous layer 210 having a first
region (left side of p-n junction 212) including N-type dopants and
a second region (right side of p-n junction 212) including P-type
dopants. In one embodiment, the dopants are introduced from a
solid-state source. In another embodiment, the dopants are
introduced as implanted atoms or ions.
[0028] Referring to operation 108 of flowchart 100, and
corresponding FIG. 2E, the method of fabricating an emitter region
of a solar cell further includes, subsequently, heating doped
amorphous layer 210 to provide a polycrystalline layer 214 having
an N-type-doped region 218 and a P-type-doped region 216. In
accordance with an embodiment of the present invention, substrate
202 is composed of silicon, tunnel oxide layer 204 is composed of
silicon dioxide, amorphous layer 206 is composed of silicon, the
N-type dopants are phosphorous dopants, and the P-type dopants are
boron dopants. In an embodiment, both tunnel oxide layer 204 and
amorphous layer 206 are formed at a temperature of approximately
565 degrees Celsius, and heating doped amorphous layer 210 to
provide polycrystalline layer 214 includes heating at a temperature
of approximately 980 degrees Celsius.
[0029] In order to further or complete fabrication of a solar cell,
the method above may further include forming a metal contact above
polycrystalline layer 214. In an embodiment, a completed solar cell
is a back-contact solar cell. In that embodiment, N-type-doped
region 218 and P-type-doped region 216 are active regions.
Conductive contacts may be coupled to the active regions and
separated from one another by isolation regions, which may be
composed of a dielectric material. In an embodiment, the solar cell
is a back-contact solar cell and further includes an
anti-reflective coating layer disposed on a light-receiving
surface, such as on a random textured surface of the solar
cell.
[0030] In another aspect of the present invention, unique
approaches to forming layers on a substrate of a solar cell are
provided. For example, FIG. 3 illustrates a flowchart 300
representing operations in a method of forming layers on a
substrate of a solar cell, in accordance with an embodiment of the
present invention. FIGS. 4A-4C illustrate cross-sectional views of
various stages in the fabrication of solar cells, corresponding to
operations of flowchart 300, in accordance with an embodiment of
the present invention.
[0031] Referring to operation 302 of flowchart 300, and
corresponding FIGS. 4A and 4B, a method of forming layers on a
substrate of a solar cell includes loading, into a furnace, a wafer
carrier 402 with a plurality of wafers 404, wafer carrier 402
having one or more wafer receiving slots loaded with two wafers
positioned back-to-back, such as wafers 406 and 408. In accordance
with an embodiment of the present invention, 50 wafers are loaded
into 25 slots of a carrier 402.
[0032] Referring to operation 304 of flowchart 300, and
corresponding FIG. 4C, the method of forming layers on a substrate
of a solar cell further includes forming, in the furnace, a tunnel
oxide layer 410 on all surfaces of each of the plurality of wafers
404, e.g., on all surfaces of wafers 406 and 408, as depicted in
FIG. 4C. In accordance with an embodiment of the present invention,
forming tunnel oxide layer 410 includes heating each of the
plurality of wafers 404 in the furnace at a temperature of
approximately 900 degrees Celsius. In a specific embodiment,
heating each of the plurality of wafers 404 in the furnace at the
temperature of approximately 900 degrees Celsius further includes
heating at a pressure of approximately 500 mTorr for approximately
3 minutes in an atmosphere of oxygen to provide tunnel oxide layer
410 having a thickness of approximately 1.5 nanometers. In
accordance with another embodiment of the present invention,
forming tunnel oxide layer 410 includes heating each of the
plurality of wafers 404 in the furnace at a temperature less than
600 degrees Celsius. In a specific embodiment, heating each of the
plurality of wafers 404 in the furnace at the temperature of less
than 600 degrees Celsius further includes heating at a temperature
of approximately 565 degrees Celsius, at a pressure of
approximately 300 Torr, for approximately 60 minutes in an
atmosphere of oxygen to provide tunnel oxide layer 410 having a
thickness of approximately 1.5 nanometers. In an alternative
embodiment, the atmosphere include N.sub.2O.
[0033] Referring to operation 304 of flowchart 300, and
corresponding FIG. 4C, the method of forming layers on a substrate
of a solar cell further includes, without removing the plurality of
wafers 404 from the furnace, forming an amorphous layer 412 on
tunnel oxide layer 410, amorphous layer 412 formed on all portions
of tunnel oxide layer 410 except on the portions in contact between
wafers positioned back-to-back, e.g., as depicted with reference to
wafers 406 and 408 in FIG. 4C. In accordance with an embodiment of
the present invention, for the wafers positioned back-to-back, a
ring pattern of the amorphous layer is formed on the back of each
wafer, as described in more detail below with respect to FIG. 5. In
an embodiment, each of the plurality of wafers 404 is composed of
silicon, tunnel oxide layer 410 is composed of silicon dioxide, and
amorphous layer 412 is composed of silicon. In an embodiment,
forming amorphous layer 412 includes depositing amorphous layer 412
in the furnace at a temperature less than 575 degrees Celsius. In a
specific embodiment, depositing amorphous layer 412 in the furnace
at the temperature less than 575 degrees Celsius further includes
heating at a temperature of approximately 565 degrees Celsius at a
pressure of approximately 350 mTorr in an atmosphere of silane
(SiH.sub.4) to provide amorphous layer 412 having a thickness
approximately in the range of 200-300 nanometers. In an embodiment,
the temperature is kept below 575 degrees Celsius to avoid
crystallization of the formed layer, but not substantially below
575 degrees Celsius for the sake of maintaining a deposition rate
suitable for high volume manufacturing.
[0034] In accordance with an embodiment of the present invention,
the method of forming layers on a substrate of a solar cell further
includes, subsequent to forming amorphous layer 412, applying a
cleaning solution to the back of each wafer, the cleaning solution
including an oxidizing agent. A texturizing solution is then
applied to the back of each wafer, the texturizing solution
including a hydroxide. In one embodiment, the oxidizing agent is a
species such as, but not limited to, ozone or hydrogen peroxide
(H.sub.2O.sub.2), and the hydroxide is a species such as, but not
limited to, potassium hydroxide (KOH) or sodium hydroxide
(NaOH).
[0035] The texturizing solution may provide a randomly textured
(rantex) surface on a light-receiving portion of a fabricated solar
cell. In accordance with an embodiment of the present invention, by
introducing a cleaning solution having an oxidizing agent prior to
introducing the texturizing solution, the texturing of the solar
cell is uniform despite the initial presence of a ring portion of a
layer fabricated on the solar cell substrate, as described below in
association with FIG. 5.
[0036] A ring feature, as mentioned with respect to FIG. 4C, may be
retained on a substrate of a solar cell, or may be subsequently
removed. Nonetheless, a solar cell structure may ultimately retain,
or at least temporarily include, such a ring feature. For example,
FIG. 5 illustrates both a cross-sectional view and a top-down view
of a substrate of a solar cell, the substrate having layers formed
thereon, in accordance with an embodiment of the present
invention.
[0037] Referring to FIG. 5, in accordance with an embodiment of the
present invention, a substrate of a solar cell includes a tunnel
oxide layer 504 disposed on all surfaces of a wafer 502. A
polycrystalline layer 506 is disposed on tunnel oxide layer 504,
polycrystalline layer 506 disposed on all portions of tunnel oxide
layer 504 except on a back side of wafer 502 which includes a ring
pattern of polycrystalline layer 506. In accordance with an
embodiment of the present invention, the ring pattern results from
the back-to-back handling of pairs of wafers, as described in
association with FIG. 4C. In an embodiment, tunnel oxide layer 504
is composed of silicon dioxide, wafer 502 is composed of silicon,
and polycrystalline layer 506 is composed of silicon.
[0038] Referring again to FIG. 5, in accordance with another
embodiment of the present invention, a substrate of a solar cell
includes a tunnel oxide layer 504 disposed on all surfaces of a
wafer 502. An amorphous layer 506 is disposed on tunnel oxide layer
504, amorphous layer 506 disposed on all portions of tunnel oxide
layer 504 except on a back side of wafer 502 which includes a ring
pattern of amorphous layer 506. In accordance with an embodiment of
the present invention, the ring pattern results from the
back-to-back handling of pairs of wafers, as described in
association with FIG. 4C. In an embodiment, tunnel oxide layer 504
is composed of silicon dioxide, wafer 502 is composed of silicon,
and amorphous layer 506 is composed of silicon.
[0039] Thus, methods of fabricating emitter regions for solar cells
have been disclosed. In accordance with an embodiment of the
present invention, a method of fabricating an emitter region of a
solar cell includes forming, in a furnace, a tunnel oxide layer on
a surface of a substrate. The method also includes, without
removing the substrate from the furnace, forming an amorphous layer
on the tunnel oxide layer. The method also includes doping the
amorphous layer to provide a first region having N-type dopants and
a second region having P-type dopants. Subsequently, the amorphous
layer is heated to provide a polycrystalline layer having an
N-type-doped region and a P-type-doped region. In one embodiment,
the substrate is composed of silicon, the tunnel oxide layer is
composed of silicon dioxide, the amorphous layer is composed of
silicon, the N-type dopants are phosphorous, and the P-type dopants
are boron. In one embodiment, both the tunnel oxide layer and the
amorphous layer are formed at a temperature of approximately 565
degrees Celsius, and heating the amorphous layer to provide the
polycrystalline layer includes heating at a temperature of
approximately 980 degrees Celsius.
* * * * *