U.S. patent application number 15/722480 was filed with the patent office on 2018-02-01 for metal-foil-assisted fabrication of thin-silicon solar cell.
This patent application is currently assigned to SUNPOWER CORPORATION. The applicant listed for this patent is SUNPOWER CORPORATION. Invention is credited to Gabriel HARLEY, Seung Bum RIM.
Application Number | 20180033894 15/722480 |
Document ID | / |
Family ID | 50973257 |
Filed Date | 2018-02-01 |
United States Patent
Application |
20180033894 |
Kind Code |
A1 |
RIM; Seung Bum ; et
al. |
February 1, 2018 |
METAL-FOIL-ASSISTED FABRICATION OF THIN-SILICON SOLAR CELL
Abstract
One embodiment relates to a method of fabricating a solar cell.
A silicon lamina is cleaved from the silicon substrate. The
backside of the silicon lamina includes the P-type and N-type doped
regions. A metal foil is attached to the backside of the silicon
lamina. The metal foil may be used advantageously as a built-in
carrier for handling the silicon lamina during processing of a
frontside of the silicon lamina. Another embodiment relates to a
solar cell that includes a silicon lamina having P-type and N-type
doped regions on the backside. A metal foil is adhered to the
backside of the lamina, and there are contacts formed between the
metal foil and the doped regions. Other embodiments, aspects and
features are also disclosed.
Inventors: |
RIM; Seung Bum; (Palo Alto,
CA) ; HARLEY; Gabriel; (Mountain View, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SUNPOWER CORPORATION |
San Jose |
CA |
US |
|
|
Assignee: |
SUNPOWER CORPORATION
San Jose
CA
|
Family ID: |
50973257 |
Appl. No.: |
15/722480 |
Filed: |
October 2, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13725580 |
Dec 21, 2012 |
9812592 |
|
|
15722480 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/0682 20130101;
H01L 31/1896 20130101; H01L 31/0216 20130101; H01L 31/022425
20130101; H01L 31/022441 20130101; Y02E 10/547 20130101; H01L
31/02002 20130101 |
International
Class: |
H01L 31/0216 20060101
H01L031/0216; H01L 31/068 20060101 H01L031/068; H01L 31/18 20060101
H01L031/18; H01L 31/02 20060101 H01L031/02; H01L 31/0224 20060101
H01L031/0224 |
Claims
1. A solar cell comprising: a substrate having a front side that is
configured to face the sun during normal operation and a backside
opposite the front side; an encapsulant on the front side of the
substrate; a transparent layer on the encapsulant; a P-type emitter
and an N-type emitter on the backside of the substrate; a first
dielectric on the P-type emitter and the N-type emitter; a metal
foil having a first contact that is electrically connected to the
P-type emitter through the first dielectric and a second contact
that is electrically connected to the N-type emitter through the
first dielectric; and an adhesive layer that the adheres the metal
foil to the first dielectric.
2. The solar cell of claim 1, wherein the adhesive layer comprises
epoxy.
3. The solar cell of claim 1, wherein the transparent layer
comprises glass.
4. The solar cell of claim 1, further comprising: a second
dielectric between the backside of the substrate and the P-type and
N-type emitters.
5. The solar cell of claim 1, wherein the front side of the
substrate is textured.
6. The solar cell of claim 1, wherein the substrate comprises
silicon.
7. The solar cell of claim 1, wherein the metal foil extends beyond
a perimeter of the substrate.
8. The solar cell of claim 1, wherein the metal foil comprises
aluminum.
9. A solar cell comprising: a silicon lamina; P-type and N-type
doped regions on a backside of the silicon lamina; a metal foil
adhered to the backside of the silicon lamina; a first set of
contacts between the metal foil and the P-type doped regions; and a
second set of contacts between the metal foil and the N-type doped
regions.
10. The solar cell of claim 9, wherein the silicon lamina has a
thickness in a range between ten and one-hundred microns.
11. The solar cell of claim 9, further comprising: an adhesive
layer between the metal foil and the P-type and N-type doped
regions on the backside of the silicon lamina, wherein said
contacts go through openings in the adhesive layer.
12. The solar cell of claim 9, further comprising: contact spots
between the metal foil and the P-type and N-type doped regions on
the backside of the silicon lamina.
13. The solar cell of claim 9, further comprising: a textured and
passivated surface at a frontside of the silicon lamina; a glass
layer over the frontside of the silicon lamina; and encapsulant
material between the frontside of the silicon lamina and the glass
layer.
14. The solar cell of claim 9, wherein the metal foil comprises
aluminum.
15. The solar cell of claim 14, wherein the metal foil comprises
Al-x% Si, where x% is in a range from zero percent to three
percent.
16. The solar cell of claim 9, wherein the metal foil extends
beyond a perimeter of the silicon lamina.
17. A solar cell comprising: a primary substrate having a front
side that faces the sun during normal operation and a backside
opposite the front side; a P-type emitter and an N-type emitter on
the backside of the primary substrate; a first dielectric layer on
the P-type and N-type emitters; a metal foil on the dielectric
layer, the metal foil having a first contact finger that is
electrically connected to the P-type emitter through the dielectric
layer and a second contact finger that is electrically connected to
the N-type emitter through the dielectric layer; and a secondary
substrate on the metal foil, wherein the metal foil includes a
finger separation area between the first dielectric layer and the
secondary substrate.
18. The solar cell of claim 17, wherein the metal foil includes an
extended area that extends beyond a perimeter of the primary
substrate.
19. The solar cell of claim 17, further comprising a second
dielectric layer between the primary substrate and the P-type and
N-type emitters.
20. The solar cell of claim 17, wherein the primary substrate is a
silicon lamina.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 13/725,580, filed on Dec. 21, 2012, which is incorporated
herein by reference in its entirety.
TECHNICAL FIELD
[0002] Embodiments of the subject matter described herein relate
generally to solar cells. More particularly, embodiments of the
subject matter relate to solar cell fabrication processes and
structures.
BACKGROUND
[0003] Solar cells are well known devices for converting solar
radiation to electrical energy. A solar cell has a front side that
faces the sun during normal operation to collect solar radiation
and a backside opposite the front side. Solar radiation impinging
on the solar cell creates electrical charges that may be harnessed
to power an external electrical circuit, such as a load.
[0004] Solar cell fabrication processes typically include numerous
steps involving masking, etching, deposition, diffusion, and other
steps. Embodiments of the present invention provide advantageous
solar cell processes.
BRIEF SUMMARY
[0005] One embodiment relates to a method of fabricating a solar
cell. A silicon lamina is cleaved from the silicon substrate. The
backside of the silicon lamina includes the P-type and N-type doped
regions. A metal foil is attached to the backside of the silicon
lamina. The metal foil may be used advantageously as a built-in
carrier for handling the silicon lamina during processing of a
frontside of the silicon lamina.
[0006] Another embodiment relates to a solar cell that includes a
silicon lamina having P-type and N-type doped regions on the
backside. A metal foil is adhered to the backside of the lamina,
and there are contacts formed between the metal foil and the doped
regions.
[0007] Another embodiment relates to a method of fabricating a
solar cell that involves adhering a metal foil to a backside of a
silicon substrate. A silicon lamina may then be separated from the
backside of the silicon substrate. The metal foil is used as a
built-in carrier for handling the silicon lamina during processing
of a frontside of the silicon lamina.
[0008] These embodiments and other embodiments, aspects, and
features of the present invention will be readily apparent to
persons of ordinary skill in the art upon reading the entirety of
this disclosure, which includes the accompanying drawings and
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] A more complete understanding of the subject matter may be
derived by referring to the detailed description and claims when
considered in conjunction with the following figures, wherein like
reference numbers refer to similar elements throughout the figures.
The figures are not drawn to scale.
[0010] FIGS. 1-6 are cross-sectional views schematically
illustrating fabrication of a solar cell in accordance with an
embodiment of the present invention.
[0011] FIG. 7 is a flow diagram of a method of fabricating a solar
cell in accordance with an embodiment of the present invention.
[0012] FIG. 8 is a flow diagram of a method of fabricating a solar
cell in accordance with an alternate embodiment of the present
invention.
[0013] FIG. 9 is a cross-sectional view of a fabricated solar cell
as fabricated in accordance with the method of FIG. 8.
[0014] FIG. 10 is a planar view of a metal foil over the backside
of a silicon lamina in accordance with an embodiment of the present
invention.
[0015] FIG. 11 is a flow diagram of a method of fabricating a
thin-silicon solar cell in accordance with another embodiment of
the present invention.
DETAILED DESCRIPTION
[0016] In the present disclosure, numerous specific details are
provided, such as examples of apparatus, structures, materials, and
methods, to provide a thorough understanding of embodiments of the
invention. Persons of ordinary skill in the art will recognize,
however, that the invention can be practiced without one or more of
the specific details. In other instances, well-known details are
not shown or described to avoid obscuring aspects of the
invention.
[0017] The present disclosure provides techniques for forming
thin-silicon solar cells using a metal foil. Advantageously, the
metal foil may be used as a built-in carrier for handling the
otherwise fragile silicon lamina during processing of a frontside
of the lamina Subsequently, the metal foil may be re-used to form
metal fingers and contacts to the P-type and N-type emitters on the
backside of the lamina.
[0018] FIGS. 1-6 are cross-sectional views schematically
illustrating fabrication of a thin-silicon solar cell in accordance
with an embodiment of the present invention. Shown in FIG. 1 is a
silicon substrate 102 having formed on it a P-type doped (P+)
region 104 and an N-type doped (N+) region 106 formed on the
backside of the substrate 102. The P+ and N+ doped regions may be
referred to as P-type and N-type emitters in the context of the
solar cell being fabricated. In the backside contact solar cell,
which is shown in FIG. 1, the emitters and corresponding contacts
are on the backside of the solar cell. The doped regions may be
formed, for example, by diffusing dopants from dopant sources.
[0019] A thin dielectric layer 108 may be formed over the P+ and N+
regions on the backside for electrical insulation, passivation,
and/or other purposes. The dielectric layer 108 may comprise, for
example, silicon oxide and/or silicon nitride. Alternatively, the
emitter surface may be passivated by means other than forming the
dielectric layer 108, such as by chemical passivation, for
example.
[0020] The solar cell structure of FIG. 1 may be placed in an ion
implantation tool, which is also referred to as an "ion implanter."
The implanter may be used to implant ions at a predetermined
implant depth 202, as depicted in FIG. 2. The ions may be hydrogen
ions (i.e. protons). In alternate embodiments, other ions may be
implanted or co-implanted with the hydrogen. For example, helium
ions may be implanted instead of hydrogen ions, or may be
co-implanted with hydrogen ions. The dose of the implantation
induces defects at the implant depth so that the planar lamina of
silicon above the implant depth may be separated or exfoliated from
the remainder of the silicon substrate below the implant depth. The
energy of the implantation controls the implant depth and so
controls the thickness of the thin silicon substrate after the
exfoliation. For example, the energy of the implantation may be
calibrated to cleave a thin lamina with a thickness within a range
from 10 microns to 100 microns. The exfoliation may be accomplished
by heating the substrate at an elevated temperature.
[0021] As depicted in FIG. 3, a metal foil 306 may be adhered to
the dielectric layer 108 on the backside of the silicon lamina 302.
The metal foil 306 may be an aluminum foil. To facilitate the foil
being used as a carrier for the lamina, an extended area (handling
area) of the metal foil 306 may extend beyond a perimeter of the
silicon lamina 302. In an exemplary implementation, the composition
of the metal foil 306 may be Al-1% Si (99% aluminum and 1%
silicon), or more generally Al-x% Si, where x% is from 0% to 3%.
Other compositions for aluminum foil may be used. It is also
possible to use metal foils other than aluminum, such as silver
foil, for example.
[0022] In one embodiment, an adhesive layer 304 may be used to
adhere the metal foil 306 to the backside of the silicon lamina
302. The adhesive layer 304 may be a thin layer of epoxy, silicone,
ethelyne vinyl acetate (EVA) or other encapsulant material which is
applied to the backside of the substrate. In one implementation,
the adhesive layer may be a coating pre-applied to the metal foil
prior to the adhesion.
[0023] In an alternate embodiment, the metal foil 306 may be
adhered to the backside of the substrate using an array of contact
spots between the metal foil 306 and the backside of the substrate.
The contact spots may be formed by spot melting of the metal foil
using a pulsed laser, for example. In this embodiment, the adhesion
layer 304 is not needed. Air gaps beneath the foil between the
contact spots may be removed by flattening the foil.
[0024] As depicted in FIG. 4, using the foil as a built-in or
integrated carrier to support the lamina, the surface 402 at the
front-side of the lamina 302 may then be textured and passivated.
The surface texturing serves to increase the capacity of the
silicon surface to absorb light, and the surface passivation serves
to reduce charge recombination at the surface. The surface
texturing may be accomplished using a wet surface etching process,
for example. The surface passivation may be accomplished by
chemical passivation or by other means.
[0025] Thereafter, a glass encapsulation process may be performed
on the frontside of the silicon lamina 302. FIG. 5 shows the
resultant glass layer 502 which is attached to the frontside using
encapsulant material 503.
[0026] As shown in FIG. 6, further steps may then be performed on
the backside of the silicon lamina 302. These steps include forming
metal contacts 604 and 606 in contact holes to electrically couple
to corresponding P+ regions 104 and N+ regions 106, respectively. A
first set of metal contacts 604 may be from the metal foil 306 to
the P+ region 104, and a second set of metal contacts 606 may be
formed from the metal foil 306 to the N+ region 106. In one
embodiment, the metal contacts 604 and 606 may be formed using a
laser-based contact formation process. In such a process, a laser
scanner may controllably scan a pulsed laser beam across the
backside of the solar cell being fabricated. The pulsed laser beam
may form the contact openings through the adhesive layer 304 and
the dielectric layer 108, and the contact openings may be filled by
melted metal from the foil 306.
[0027] In addition, a finger separation 608 pattern may be formed
on the foil area to electrically separate the first set of metal
contacts 604 from the second set of metal contacts 606. The finger
separation 608 may be configured so that the fingers of the foil
that lead to the contacts are interdigitated.
[0028] FIG. 7 is a flow diagram of an exemplary method 700 of
fabricating a thin-silicon solar cell in accordance with an
embodiment of the present invention. In the exemplary method 700 of
FIG. 7, emitter regions may be first formed on a silicon wafer per
block 702. The silicon wafer may be of a thickness of several
hundred microns or more and may be referred to as a thick handle
wafer. The emitter regions include both P-doped and N-doped regions
and may be formed on the backside of the wafer as shown in FIG.
1.
[0029] Per block 704, a thin silicon lamina may be cleaved from the
silicon wafer. For example, the silicon lamina may be of a
thickness between 10 microns to 100 microns. In one implementation,
the cleaving may be performed using ion implantation and
exfoliation as described above in relation to FIG. 2.
Alternatively, the cleaving may be performed by spalling or etching
a sacrificial layer from the frontside of the wafer.
[0030] In block 706, metal foil may be adhered to the silicon
lamina, as described above in relation to FIG. 3. In particular,
the metal foil may be adhered to the backside surface of the
silicon lamina. The metal foil may be of a thickness between 50
microns and 1 millimeter so as to provide mechanical support for
the thin silicon lamina. To facilitate the foil being used as a
carrier for the lamina, an extended area (handling area) of the
metal foil may extend beyond a perimeter of the silicon lamina. In
one implementation, the adhesion may be accomplished by using a
laser to fire contacts between the metal foil and the silicon
lamina. In another implementation, the adhesion may be accomplished
using a thin adhesive layer coated on the metal foil.
[0031] Per block 708, the metal foil may be used as an integrated
carrier for handling the silicon lamina so that the frontside
surface of the silicon lamina may be processed. The frontside
surface processing may include texturing and passivation, as
described above in relation to FIG. 4. The surface texturing and
passivation may be accomplished, for example, by dipping the lamina
into chemical solutions to etch and passivate the frontside
surface. Subsequently, the metal-foil-supported silicon lamina may
have its frontside processed with a glass lamination procedure, as
described above in relation to FIG. 5. Subsequent to the frontside
processing, the extended area (handling area) of the metal foil may
be trimmed.
[0032] Per block 710, contacts may be formed from the metal foil to
the emitter regions. As described above in relation to FIG. 6, the
contacts formed may include a first set of contacts 604 to P-doped
emitter regions 104 and a second set of contacts 606 to N-doped
emitter regions 106. In addition, a finger separation 608 pattern
may be formed on the foil to electrically separate the first set
and the second set of contacts.
[0033] In an alternate embodiment, instead of adhering a continuous
metal foil layer to the backside and subsequently creating the
finger separation pattern while the foil is attached to the
backside, the finger separation pattern may be pre-formed in the
metal foil before the metal foil is applied to the backside of the
silicon lamina FIG. 8 is a flow diagram of an alternate method 800
of fabricating a thin-silicon solar cell which uses such a
pre-patterned metal foil in accordance with an embodiment of the
present invention.
[0034] As shown in FIG. 8, after the thin silicon lamina is cleaved
from the wafer per block 704, a pre-patterned metal foil may be
sandwiched 806 between the backside of the silicon lamina and a
secondary substrate. The patterning of the metal foil achieves the
finger separation between the P-type and N-type contacts. The
secondary substrate may be transparent such that laser light may be
transmitted through it. The secondary substrate may be, for
example, a stiff polymer layer, such as a polyethylene
terephthalate (PET) layer or a fluoropolymer layer. Thereafter, per
block 807, the contacts may be formed between the metal foil and
the emitter regions. The formation of the contacts may be
accomplished, for example, using a pulsed laser which is
transmitted through the secondary substrate to create the contact
openings and flow melted metal from the foil into those openings.
Per block 708, the front surface may then be processed, as
described above in relation to FIG. 7. Subsequent to the frontside
processing, the extended area (handling area) of the metal foil may
be trimmed.
[0035] FIG. 9 is a cross-sectional view of a fabricated
thin-silicon solar cell as fabricated in accordance with the method
800 of FIG. 8. As depicted in FIG. 9, the metal foil 306 with the
pre-patterned finger separation 908 is sandwiched between the
secondary substrate 902 and the backside of the silicon lamina 302.
In addition, a P-type contact 904 and an N-type contact 906 are
shown. As described above, these contacts may be formed by
transmission of a pulsed laser through the transparent secondary
substrate 902.
[0036] FIG. 10 is a planar view of a metal foil over the backside
of a silicon lamina in accordance with an embodiment of the present
invention. The view of FIG. 10 shows a portion 1004 of the foil
over the backside of the lamina and an extended area 1006 of the
foil which extends beyond a perimeter 1002 of the lamina Note that
the extended area 1006 may extend over one or more sides of the
perimeter and need not necessarily extend over all sides of the
perimeter.
[0037] FIG. 11 is a flow diagram of a method 1100 of fabricating a
thin-silicon solar cell in accordance with another embodiment of
the present invention. In the exemplary method 1100 of FIG. 11, a
sacrificial layer may be formed on a silicon substrate per block
1102.
[0038] The sacrificial layer may be composed of porous silicon,
such as formed in a HF bath with bias. Alternatively, the
sacrificial layer may be silicon with, for example, germanium
doping and/or a carbon doping, either of which can be formed by
epitaxial deposition or a chemical vapor deposition (CVD) process.
The sacrificial layer may be thin, on the order of approximately
700 micrometers, although it may be slightly or significantly
larger or smaller, as desired for a particular embodiment to
perform the functions described herein. For example, in certain
embodiments, the sacrificial layer may be as thin as 10
micrometers. Smaller thicknesses may also be used in certain
instances.
[0039] An epitaxial layer of silicon may then be grown over the
sacrificial layer per block 1104. The emitter regions may be formed
in the epitaxial layer per block 1106, and a dielectric layer may
be formed over the emitter regions per block 1108.
[0040] A metal foil may then be adhered over the emitter regions
per block 1110. Subsequently, epitaxial lift-off per block 1112 may
be performed by selective wet etching or otherwise removing the
sacrificial layer. After lift-off, the epitaxial layer becomes the
silicon lamina of the solar cell. A cross-sectional view of the
structure at this point in the process corresponds to the view
shown in
[0041] FIG. 3. As disclosed herein, the metal foil provides
structural support and an integrated carrier functionality to the
silicon lamina.
[0042] Subsequently, the front surface may be processed per block
708. The contacts between the metal foil and the emitter regions
may then be formed per block 710. In other words, after the
epitaxial lift-off per block 1110, the processing may proceed as
described above in relation to FIGS. 4-6.
[0043] Techniques for forming thin-silicon solar cells using a
metal foil have been disclosed. Advantageously, the metal foil may
be used as a built-in carrier for handling the otherwise fragile
silicon lamina during processing of a frontside of the lamina
Subsequently, the metal foil may be re-used to form the P-type and
N-type emitter contacts and metal fingers on the backside of the
lamina.
[0044] While specific embodiments of the present invention have
been provided, it is to be understood that these embodiments are
for illustration purposes and not limiting. Many additional
embodiments will be apparent to persons of ordinary skill in the
art reading this disclosure.
* * * * *