U.S. patent application number 14/878628 was filed with the patent office on 2016-08-11 for photovoltaic module fabrication with thin single crystal epitaxial silicon devices.
The applicant listed for this patent is Crystal Solar, Incorporated. Invention is credited to Ashish Asthana, Somnath Nag, Kramadhati V. Ravi, Tirunelveli S. Ravi.
Application Number | 20160233824 14/878628 |
Document ID | / |
Family ID | 50026644 |
Filed Date | 2016-08-11 |
United States Patent
Application |
20160233824 |
Kind Code |
A1 |
Ravi; Kramadhati V. ; et
al. |
August 11, 2016 |
PHOTOVOLTAIC MODULE FABRICATION WITH THIN SINGLE CRYSTAL EPITAXIAL
SILICON DEVICES
Abstract
Photovoltaic modules including a plurality of solar cells bonded
to a module back sheet are described herein, wherein each solar
cell includes a superstrate bonded to a front side of a
photovoltaic device to facilitate handling of very thin
photovoltaic devices during fabrication of the module. Modules may
also include module front sheets and the solar cells may include
bottom sheets. The modules may be made of flexible materials, and
may be foldable. Fabrication processes include tabbing photovoltaic
devices prior to attaching the individual superstrates.
Inventors: |
Ravi; Kramadhati V.;
(Atherton, CA) ; Ravi; Tirunelveli S.; (Saratoga,
CA) ; Asthana; Ashish; (Fremont, CA) ; Nag;
Somnath; (Saratoga, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Crystal Solar, Incorporated |
Santa Clara |
CA |
US |
|
|
Family ID: |
50026644 |
Appl. No.: |
14/878628 |
Filed: |
October 8, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13566906 |
Aug 3, 2012 |
9021164 |
|
|
14878628 |
|
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|
61514641 |
Aug 3, 2011 |
|
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61652063 |
May 25, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/0504 20130101;
H01L 31/02168 20130101; H01L 31/02366 20130101; H01L 31/0488
20130101; Y02E 10/547 20130101; H01L 31/0201 20130101; G06F 13/105
20130101; H01L 31/049 20141201; H01L 31/0284 20130101; H01L 31/1804
20130101; H02S 30/20 20141201; H01L 31/068 20130101; H04B 5/0056
20130101; H01L 31/1876 20130101; H04B 5/0031 20130101; G06F 1/1698
20130101; G06F 1/1632 20130101 |
International
Class: |
H02S 30/20 20060101
H02S030/20; H01L 31/048 20060101 H01L031/048; H01L 31/049 20060101
H01L031/049; H01L 31/18 20060101 H01L031/18; H01L 31/028 20060101
H01L031/028; H01L 31/068 20060101 H01L031/068; H01L 31/0236
20060101 H01L031/0236; H01L 31/0216 20060101 H01L031/0216; H01L
31/02 20060101 H01L031/02; H01L 31/05 20060101 H01L031/05 |
Claims
1. A photovoltaic module, comprising: a plurality of solar cells,
each solar cell comprising: a photovoltaic device with a bus bar on
a front side of said photovoltaic device; a front tab attached to
said front-side bus bar; a superstrate bonded to said front side of
said photovoltaic device, wherein said front tab is between said
photovoltaic device and said superstrate; and a rear tab attached
to a rear side of said photovoltaic device; and a module back
sheet; wherein said plurality of solar cells are arranged in a
planar array and electrically interconnected, and wherein said
module back sheet is bonded to the bottom side of said planar array
of solar cells.
2. The photovoltaic module as in claim 1, wherein said superstrate
is a glass sheet.
3. The photovoltaic module as in claim 1, wherein said superstrate
is a polymer sheet.
4. The photovoltaic module as in claim 1, further comprising a
bonding material between said superstrate and said photovoltaic
device.
5. The photovoltaic module as in claim 1, wherein said module back
sheet is bonded to said rear sides of said photovoltaic devices,
said rear tabs being between said photovoltaic devices and said
module back sheet.
6. The photovoltaic module as in claim 1, wherein spaces between
said solar cells in said array are filled with a sealant.
7. The photovoltaic module as in claim 1, further comprising a
module top sheet, said module top sheet being bonded to the top
surfaces of said superstrates.
8. The photovoltaic module as in claim 7, wherein said module top
sheet and said module back sheet are polymer sheets and said
photovoltaic module is configured to be foldable along a line
through said array, said line passing between solar cells.
9. The photovoltaic module as in claim 1, wherein each said solar
cell further comprises a bottom sheet bonded to said rear side of
said photovoltaic device, wherein said rear tab is between said
photovoltaic device and said bottom sheet.
10. The photovoltaic module as in claim 9, wherein said module back
sheet is bonded to the bottom surfaces of said bottom sheets.
11. The photovoltaic module as in claim 1, wherein said
photovoltaic device is less than 50 microns thick.
12. A method of fabricating a photovoltaic module comprising:
providing a plurality of photovoltaic devices, each of said
photovoltaic devices being attached at a rear side to a substrate,
each of said photovoltaic devices having a bus bar on a front side;
for each of said photovoltaic devices, attaching a front tab to
said bus bar and bonding a superstrate to said front side of said
photovoltaic device, wherein said front tab is between said
photovoltaic device and said superstrate; for each of said
photovoltaic devices with front tab and superstrate, separating
said photovoltaic device from said substrate, to provide a
plurality of solar cells, each solar cell including said
photovoltaic cell, said front tab and said superstrate; assembling
said plurality of solar cells to form an array and electrically
interconnecting said array; and laminating said array to a module
back sheet.
13. The method as in claim 12, wherein said providing a plurality
of photovoltaic devices includes: anodizing a single crystal
silicon substrate to form a porous silicon layer on a top surface;
and growing very thin epitaxial silicon on said porous silicon
layer in an epitaxial reactor.
14. The method as in claim 12, further comprising, for each of said
plurality of solar cells, attaching a rear tab to a rear side of
said solar cell.
15. The method as in claim 12, further comprising bonding a module
top sheet to said array.
16. The method as in claim 12, further comprising filling spaces
between said solar cells in said array with a sealant.
17. A method of fabricating a photovoltaic module comprising:
providing a photovoltaic device attached at a rear side to a
substrate, forming a plurality of bus bars on a front side of said
photovoltaic device, corresponding to a plurality of sub-devices;
attaching a plurality of front tabs to said plurality of bus bars
and bonding a plurality of superstrates to said front side of said
photovoltaic device, each superstrate corresponding to one of said
sub-devices, wherein said plurality of front tabs are between said
photovoltaic device and corresponding ones of said plurality of
superstrates; separating said plurality of sub-devices with tabs
and superstrates from said substrate; separating said plurality of
sub-devices to provide a plurality of solar sub-cells; assembling
said plurality of solar sub-cells to form an array and electrically
interconnecting said array; and laminating said array to a module
back sheet.
18. The method as in claim 17, further comprising, before said
separating from said substrate, scribing said photovoltaic device
to define said sub-devices.
19. The method as in claim 17, further comprising, after said
separating from said substrate, scribing said photovoltaic device
to define said sub-devices.
20. The method as in claim 19, wherein, during said scribing said
plurality of superstrates are held by a fixture.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/566,906 filed Aug. 3, 2012, which claims
the benefit of U.S. Provisional Applications Nos. 61/514,641 filed
Aug. 3, 2011, and 61/652,063 filed May 25, 2012, all applications
incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates generally to photovoltaic
module fabrication, and more particularly to module fabrication for
epitaxially deposited thin single crystal silicon solar cells,
including flexible modules and high voltage modules.
BACKGROUND
[0003] Reducing the manufacturing costs of silicon based
photovoltaics requires a drastic reduction in silicon usage. One
approach to achieving this is to use very thin silicon wafers for
fabricating the solar cells. These thin silicon wafers, less than
50 microns thick, are fabricated using a process that makes very
efficient use of the silicon. This process includes epitaxial
deposition of thin single crystal silicon wafers on single crystal
silicon substrates that have been anodically etched to create a
thin (less than 2 microns) porous silicon release layer which
enables single crystal growth by epitaxy and also enables
exfoliation or peeling of the thin silicon wafer from the silicon
substrate to create very thin high quality single crystal silicon
wafers. However, these very thin single crystal silicon wafers are
mechanically fragile and present challenges for handling,
processing, testing and packaging of the resulting solar cells to
make photovoltaic modules.
[0004] There is a need for new and improved methods and equipment
for handling, processing, testing and packaging of very thin
silicon wafers and solar cells.
[0005] Traditional flexible and light weight photovoltaics have
been based on thin film technologies such as amorphous silicon and
copper indium selenide (CIGS). A significant problem with these
technologies is their relatively low energy conversion
efficiencies--typically 6 to 8% for amorphous silicon and 10 to 11%
for CIGS. In addition, the reliability and long term stability of
these products are questionable, especially due to moisture induced
degradation. Crystalline silicon wafer based photovoltaics have
high efficiency (>15% to .about.18% module efficiencies with
>20% cell efficiencies), high reliability based on over 30 years
of field experience, and use earth abundant, non-toxic raw
materials. However, significant issues exist with conventional
crystalline silicon photovoltaics for lightweight, flexible
applications such as: fabricating the thin silicon required at low
cost; handling the thin silicon during cell and module fabrication;
and wafer thicknesses are typically about 180 microns, making the
wafers inflexible and subject to breaking when flexed, and if the
wafers are mechanically thinned to enable flexibility, the cost of
manufacture increases substantially making such products
non-competitive in the marketplace.
[0006] There are many applications for which small sized (say 1 sq.
ft. and smaller) solar modules are desired in markets for advanced
charging technologies for portable devices, including solar-powered
handsets, cell phones, solar chargers, wireless power units,
fuel-cell battery charging products and public charging kiosks. The
modules need to provide the correct "high voltage" for the portable
devices--voltages of 6 V, 12 V and 24 V are needed. This is also
true for battery charging applications. In order to achieve these
voltages in small modules the individual solar cells have to be
small (roughly between 4 and 8 cm.sup.2) and connected in
series.
[0007] There is a need for new and improved flexible cell
structures and methods and equipment for handling, processing,
testing and packaging of very thin silicon wafers and solar
cells.
SUMMARY OF THE INVENTION
[0008] The present invention provides methods for fabricating
photovoltaic modules comprising a multitude of mechanically fragile
thin solar cells, including photovoltaic devices less than 50
microns thick. Methods for handling, processing, testing and
packaging these mechanically fragile solar cells are described,
which do not involve handling unsupported thin silicon wafers. The
solar cells described herein include epitaxial single crystal
silicon solar cells; furthermore, the teaching and principles of
the present invention may apply to very thin epitaxial solar cells
comprising other semiconductors such as germanium, gallium arsenide
and others. Furthermore, the present invention includes
photovoltaic modules comprising thin glass, modules comprising
multiple layers of laminated thin glass, and also modules
comprising polymer sheets, such as Teflon.RTM., in place of
glass.
[0009] According to aspects of the present invention, a
photovoltaic module, may comprise: a plurality of solar cells; and
a module back sheet; wherein each solar cell comprises: a
photovoltaic device with a bus bar on a front side of the
photovoltaic device, a front tab attached to the front-side bus
bar, a superstrate bonded to the front side of the photovoltaic
device, wherein the front tab is between the photovoltaic device
and the superstrate, and a rear tab attached to a rear side of the
photovoltaic device; and wherein the plurality of solar cells are
arranged in a planar array and electrically interconnected, and
wherein the module back sheet is bonded to the bottom side of the
planar array of solar cells. The superstrate may be a glass or
polymer sheet. Furthermore, a module top sheet may be bonded to the
top surfaces of the superstrates. The photovoltaic module may be
made of sufficiently thin and flexible components to allow the
module to be folded up.
[0010] According to further aspects of the present invention, a
method of fabricating a photovoltaic module including a
multiplicity of very thin silicon solar cells may comprise:
anodizing a single crystal silicon substrate; growing very thin
epitaxial silicon on the anodized surface of the silicon substrate;
processing the exposed surface of the epitaxial silicon to form
front-side structures of the solar cell; tabbing the front-side;
bonding the front-side of the solar cell to a thin glass
superstrate; exfoliating the solar cell from the silicon substrate;
processing the exposed surface of the epitaxial silicon to form
back-side structures of the solar cell; testing the solar cell to
determine a characteristic current-voltage curve in response to
light exposure; sorting the solar cell into a bin based on the
current-voltage characteristic; assembling the solar cell with
other solar cells from the same bin and interconnecting them to
form a solar cell array; laminating the array to a module
back-sheet to form a photovoltaic module; and weather-proofing the
module. Wherein the weatherproofing may including laminating a
protective glass sheet to the top of the module or filling the gaps
between solar cells with a sealant.
[0011] According to further aspects of the present invention, high
voltage flexible panels and methods for making the same are
disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] These and other aspects and features of the present
invention will become apparent to those ordinarily skilled in the
art upon review of the following description of specific
embodiments of the invention in conjunction with the accompanying
figures, wherein:
[0013] FIGS. 1-6 are representations of a process for the
fabrication of a first type of photovoltaic module, according to
some embodiments of the present invention;
[0014] FIGS. 7-9 are representations of a process for the
fabrication of a second type of photovoltaic module, according to
some embodiments of the present invention;
[0015] FIG. 10 is a perspective view of the active layers,
collecting grid, bus bars and tabs for a solar cell, according to
some embodiments of the present invention;
[0016] FIGS. 11-21 are representations of a process for the
fabrication of a flexible photovoltaic module, according to some
embodiments of the present invention;
[0017] FIGS. 22-26 are representations of a process for the
fabrication of series connected cells to form a compact high
voltage module, according to some embodiments of the present
invention;
[0018] FIG. 27 shows a plan view representation of a 2.times.5
array of subcells corresponding to the process step of FIG. 24;
[0019] FIG. 28 shows a schematic representation of a compact high
voltage module corresponding to the device of FIG. 27;
[0020] FIGS. 29-31 show a representation of a further process for
the fabrication of series connected cells to form a compact high
voltage module, according to some embodiments of the present
invention;
[0021] FIGS. 32-38 are representations of a further process for the
fabrication of a first type of photovoltaic module, according to
some embodiments of the present invention; and
[0022] FIGS. 39-41 are representations of an alternative process
for forming ohmic back contacts to a solar device, according to
some embodiments of the present invention.
DETAILED DESCRIPTION
[0023] Embodiments of the present invention will now be described
in detail with reference to the drawings, which are provided as
illustrative examples of the invention so as to enable those
skilled in the art to practice the invention. Notably, the figures
and examples below are not meant to limit the scope of the present
invention to a single embodiment, but other embodiments are
possible by way of interchange of some or all of the described or
illustrated elements. Moreover, where certain elements of the
present invention can be partially or fully implemented using known
components, only those portions of such known components that are
necessary for an understanding of the present invention will be
described, and detailed descriptions of other portions of such
known components will be omitted so as not to obscure the
invention. In the present specification, an embodiment showing a
singular component should not be considered limiting; rather, the
invention is intended to encompass other embodiments including a
plurality of the same component, and vice-versa, unless explicitly
stated otherwise herein. Moreover, applicants do not intend for any
term in the specification or claims to be ascribed an uncommon or
special meaning unless explicitly set forth as such. Further, the
present invention encompasses present and future known equivalents
to the known components referred to herein by way of
illustration.
[0024] The present invention provides methods for fabricating
photovoltaic modules comprising a multitude of mechanically fragile
thin solar cells. Some embodiments of the modules of the present
invention are light weight flexible modules. Methods for handling,
processing, testing and packaging these mechanically fragile solar
cells are described, which do not involve handling unsupported thin
silicon wafers. The solar cells described herein include epitaxial
single crystal silicon solar cells; furthermore, the teaching and
principles of the present invention may apply to very thin
epitaxial solar cells comprising other semiconductors such as
germanium. Furthermore, the present invention includes photovoltaic
modules comprising thin glass, modules comprising multiple layers
of laminated thin glass, and also modules comprising polymer
sheets, such as Teflon.RTM., in place of glass.
[0025] An example of a process flow for forming a first embodiment
of a photovoltaic (PV) module is as follows.
[0026] (1) Single crystal silicon wafers are anodically etched to
create a porous silicon layer approximately 2 microns thick.
[0027] (2) Thin epitaxial silicon layers are grown on the porous
silicon layer utilizing an epitaxial reactor. See U.S. application
Ser. No. 13/483,002 filed May 26, 2012, incorporated by reference
in its entirety herein, for a description of methods and equipment
for epitaxial growth of thin silicon.
[0028] (3) The front side solar cell processing is achieved on the
epitaxial film while it is still attached to the silicon substrate.
For example, device fabrication may be comprised of: (a) texture
etching the surface to minimize reflectivity; (b) forming an n+p
junction using, for example, diffusion using phosphorus oxy
chloride as the dopant source; (c) removing diffusion induced oxide
from the epitaxial film surface by chemical etching; (d) depositing
a layer of silicon nitride utilizing, for example, plasma enhanced
chemical vapor deposition (PECVD); (e) screen printing a silver
metal grid (comprising collecting electrodes and bus bars) on the
surface and firing in a furnace to form ohmic contacts; and (f)
attaching tabs to the bus bars on the front surface of the solar
cell. A perspective view of the general configuration of the active
device layers 10, collecting electrodes 20, bus bars 30 and tabs 40
of a solar cell according to embodiments of the present invention
is provided in FIG. 10. Only two bus bars per cell are shown in
FIG. 10 for simplicity of illustration--there may be more, perhaps
3 bus bars, depending on the cell architecture, or less, just one
bus bar per cell; similarly, only 7 collecting electrodes are shown
for ease of illustration--there are typically many more, of the
order of many tens of collecting electrodes, for example 70. Note
that the tabs 40 in FIG. 10 have been minimized for clear
illustration--the tabs are actually the width of the bus bars and
are much thicker than the bus bars. Furthermore, note that, for the
purpose of illustration, FIGS. 1-10 are not drawn to scale. The
representations of devices in FIGS. 1-9 show cross sections along
the plane defined by X-X in FIG. 10.)
[0029] (4) Following front side device fabrication, the wafer is
bonded to thin glass (less than 1 mm thick) utilizing ethylene
vinyl acetate (EVA). FIG. 1 shows a schematic cross-section of a
thin epitaxially deposited silicon device 112 grown on a porous
silicon layer 122 on a single crystal silicon substrate 120. The
device 112 has bus bars 114 deposited on the device surface and
tabs 116 have been attached, making electrical contact to all of
the bus bars. See U.S. patent application Ser. No. 13/241,112,
filed Sep. 22, 2011, and U.S. Patent Appl. Pub. No. 2012/0040487,
both incorporated by reference in their entirety herein, for
further details of the fabrication of devices such as shown in FIG.
1. The device 112 has been bonded to a thin glass superstrate 130
using a layer of EVA 132.
[0030] (5) The epitaxial edge wrap around the edge of the silicon
substrate is removed using dicing saws or lasers. FIG. 2 shows a
representation of edge dicing of the device of FIG. 1 using dicing
blades 140 to remove epitaxial material that was grown over the
edge of the substrate 120. The edge removal process has cutting
depth control at least sufficient to avoid cutting the tabs 116.
Mechanical grinding tools or plasma etching tools may also be used
for the edge removal process.
[0031] (6) The device is exfoliated from the silicon substrate.
FIG. 3 shows a cross-sectional representation of the epitaxial
device 112 mounted to glass superstrate 130 after removal of the
substrate 120. Remnants 123 of the porous silicon layer 122 will
remain attached to the epitaxial device 112 after separation from
the substrate 120.
[0032] (7) Rear side processing of the solar cell, comprising
removing residual porous silicon, deposition of dielectric layers
and metal films with laser ablated holes in the dielectric film for
point contacts to the back side of the solar cell. FIG. 4 shows the
completed and mounted solar cell 110 after the porous silicon
remnants 123 have been removed and further metal and dielectric
layers 118 have been added to the back side of the device.
[0033] (8) Individual cells (thin silicon on glass) are tested and
sorted so that modules may be made up of devices with closely
similar characteristics. Individual cell testing involves the
measurement of illuminated current-voltage characteristics of the
device (I-V curves). This is done using standard solar cell
characterization equipment. Solar cell matching is achieved by
matching the current at the peak power point of the I-V
characteristics. FIG. 5 shows the device of FIG. 4 configured for
cell testing. Probes 151 are used to contact interconnect straps
150 which are attached to tabs 116; the interconnect straps may be
attached prior to lamination of the device to superstrate 130; the
interconnect straps have been bent around the glass superstrate 130
as shown prior to testing. Electrical contact is made to the metal
layer in 118 with contact pins 152 from the bottom; to avoid damage
of the device layers 112 and 118, a metal foil 153 may be placed
between the contact pins and device.
[0034] (9) Cells with similar current at peak power point are
assembled into series strings of solar cells. Individual strings
are connected in parallel to make up the cell array.
[0035] (10) The array of cells is laminated to a module back sheet
using EVA or a similar bonding agent.
[0036] (11) To complete the module fabrication the spaces between
solar cells are filled with a low melting temperature glass or
other suitable sealing material. FIG. 6 shows multiple mounted
solar cells 110 which have been attached to a module back sheet 160
with a bonding agent 162. The tabs 116 and 117 and interconnect
straps 150 have been used to series connect the multiple devices.
The module back sheet may be a stiff laminated Tedlar.RTM.
polyvinyl fluoride sheet, fiberglass sheet or even plywood. The
bonding agent 162 may be EVA or PVB {poly(vinyl butyral)}. The
bonding agent is typically available in sheets that are placed on
the solar cell array and then laminated in a lamination
chamber--where a bond is formed by application of pressure at
elevated temperature. A cell to cell seal 164 may be formed using a
low melting point glass, such as a glassy ceramic. Other suitable
materials for the cell to cell seal may include EVA, PVB and
silicones. The cell to cell sealing material may be applied to the
gap between cells using a robotic delivery system, which may
include a delivery nozzle moved along the gaps between cells by a
robotic device. A cell to cell seal is required for
weather-proofing. The total module thickness may be less than 3.5
mm, comprising, for example, in order from top glass to bottom
module back sheet: 700 microns glass, 200 microns EVA, 50 microns
solar cell, 200 microns EVA and 1 mm module back sheet. Note that
the thickness of the top glass (glass superstrates 130) may be
varied from less than 1 mm to up to 3 or 4 mm, depending on the
amount of impact protection the glass needs to provide. (The solar
modules may be used in an environment in which they are exposed to
the weather, including hail stones, and other environmental
factors, such as dust storms, for a period of 30 years or more--the
top glass needs to provide protection against these hazards.) The
module is illuminated as indicated in FIG. 6. Note that for
simplicity of illustration only 2 cells are shown in the
photovoltaic module of FIG. 6--generally modules include 48 to 72
solar cells.
[0037] An example of a process flow for forming a second embodiment
of a PV module is as follows.
[0038] (1) Single crystal silicon wafers are anodically etched to
create a porous silicon layer approximately 2 microns thick.
[0039] (2) Thin epitaxial silicon layers are grown on the porous
silicon layer utilizing an epitaxial reactor.
[0040] (3) The front side solar cell processing is achieved on the
epitaxial film while it is still attached to the silicon substrate.
Device fabrication is comprised of: (a) texture etch the surface to
minimize reflectivity; (b) form an n+p junction using, for example,
diffusion using phosphorus oxy chloride as the dopant source; (c)
remove diffusion induced oxide from the epitaxial film surface by
chemical etching; (d) deposit a layer of silicon nitride utilizing,
for example, PECVD; (e) screen print a silver metal grid
(comprising collecting electrodes and bus bars) on the surface and
fire in a furnace to form ohmic contacts; and (f) attaching tabs to
the bus bars on the front surface of the solar cell.
[0041] (4) Following front side device fabrication the wafer is
bonded to a thin glass (less than 1 mm thick) utilizing ethylene
vinyl acetate (EVA).
[0042] (5) The device is exfoliated from the silicon substrate
after removing the epitaxial edge wrap around using dicing saws or
lasers. FIG. 7 shows a schematic cross-section of a thin epitaxial
silicon device mounted to a glass superstrate 230 using a bonding
agent 232, such as EVA. The epitaxial device may have been formed
as shown in FIG. 1, edge trimmed as shown in FIG. 2 and then had
remnant porous silicon removed. The device of FIG. 7 comprises a
BSF layer 211, a p-type epitaxial layer 212, a n+emitter 213, a
silicon dioxide passivation layer 214, a silicon nitride ARC layer
215 and metal contacts 216. See U.S. patent application Ser. No.
13/241,112, filed Sep. 22, 2011, and U.S. Patent Appl. Pub. No.
2012/0040487, both incorporated by reference in their entirety
herein, for further details of the fabrication of devices such as
shown in FIG. 7. The device has tabs 217 attached for making
electrical contact.
[0043] (6) The rear side of the solar cell is processed, the
processing comprising deposition of dielectric layers and metal
films and laser ablating holes in the dielectric film for making
point contacts to the back side of the solar cell. FIG. 8 shows
completed mounted solar cell 210, which has dielectric layers 218
and back contact 219 with point contacts. Details of the
fabrication of this device are provided in U.S. patent application
Ser. No. 13/241,112, filed Sep. 22, 2011, incorporated by reference
in its entirety herein.
[0044] (7) Individual cells (thin silicon on glass) are tested and
sorted so that modules may be made up of devices with closely
similar characteristics. Individual cell testing involves the
measurement of illuminated current-voltage characteristics of the
device (I-V curves). This is done using standard solar cell
characterization equipment. Solar cell matching is achieved by
matching the current at the peak power point of the I-V
characteristics. The testing of solar cells is carried out as
described above with reference to FIG. 5.
[0045] (8) Cells with similar current at peak power point are
assembled into series strings of solar cells. Individual strings
are connected in parallel to make up the cell array.
[0046] (9) The array of cells is laminated between a glass top
sheet and a back sheet using EVA to complete module fabrication.
FIG. 9 shows a PV module 200 comprising multiple devices 210 which
have been strung together (series connection of devices using tabs
217 and 220, and interconnect straps, 221) and lamination between a
module back sheet 250 and a glass top sheet 254 using a bonding
material 252, such as EVA. The module back sheet 250 may be glass
or another suitable material such as Teflon.RTM. sheets. The module
is illuminated, through the glass top sheet 254, as indicated in
FIG. 9. The total module thickness may be less than 3.0 mm,
comprising, for example, in order from top glass down: 1 mm glass,
200 microns EVA, 700 microns glass, 200 microns EVA, 50 microns
solar cell, 200 microns EVA and 1 mm module back sheet. Note that
for simplicity of illustration only 3 cells are shown in the
photovoltaic module of FIG. 9--generally modules include 48 to 72
solar cells.
[0047] Although two specific examples of solar cell and module
configurations are provided herein, the present invention is
generally applicable to the handling, processing, testing and
packaging of very thin solar cells to make photovoltaic modules.
For example, the present invention is applicable to thin silicon
devices with ceramic, glass and glass-bonded ceramic handling
layers--see U.S. Patent Appl. Publ. No. 2011/0186117, incorporated
by reference in its entirety herein, for details of the fabrication
of silicon devices with ceramic, glass and glass-bonded ceramic
handling layers--where the handling layer is formed on the thin
silicon device prior to separation from the growth substrate. Note
for handling layers that are not transparent, the handling layer
will be laminated to the module back sheet.
[0048] Although the processes and structures described above
include thin epitaxial layers which wrap around the edge of the
silicon substrate and thus require a dicing step, the teaching and
principles of the present invention may also be applied to
substrates with epitaxial layers which do not wrap around the edge
of the silicon substrate.
[0049] Note that a top sheet of glass or other suitable transparent
material may be laminated to the top surface of the array of cells
in FIG. 6, in order to provide protection and weather proofing,
similar to glass top sheet 254 in FIG. 9, in which case the cell to
cell seal will not be required. Furthermore, if a top sheet is
used, the thickness of the glass superstrates 130 may be reduced,
since they are no longer the only material providing impact
protection for the active layers of the solar cells. (Multiple
sheets--two or more--of thin glass laminated together may be
expected to provide better impact resistance than a single sheet of
thick glass, even where the total glass thickness is the same, thus
providing an opportunity to reduce the overall thickness of the PV
module while maintaining good impact resistance. The module with
multiple sheets of laminated glass may also be more flexible than
the module with only a single sheet of glass.) Similarly, the top
sheet of glass 254 in FIG. 9 may be dispensed with if sufficiently
thick glass superstrates 230 are used, so as to provide impact
protection for the active layers of the solar cell, and the gaps
between cells are filled as described with reference to FIG. 6
above.
[0050] Note that in principle wherever glass superstrates and glass
sheets are used in the embodiments of the present invention they
may be replaced with other materials, such as Teflon.RTM. sheets
available from DuPont and Gorilla.RTM. Glass available from
Corning; although, use of thin sheets of these materials for
flexible modules may require modification of the fabrication
process to provide extra support at certain steps, as described in
detail below.
[0051] According to a third embodiment of the present invention
flexible photovoltaic modules are fabricated using thin
silicon--typically 50 microns and below--as will be described in
detail below. In the present invention the thin silicon is
epitaxially deposited. Note that due to using very thin silicon,
flexible photovoltaics have reduced efficiency compared with
thicker silicon cells due to transmission, rather than absorption,
of long wavelength (red) light. However, with good light trapping
(back reflectors and front texturing) and surface passivation,
efficiencies greater than 19% are theoretically possible even with
25 micron thick silicon wafers.
[0052] An example of a process flow for forming the third
embodiment of a PV module--the flexible PV module--is as follows.
FIGS. 11-14 and 16-19 show cross-sectional representations, not
drawn to scale, of the fabrication process.
[0053] (1) Single crystal silicon wafers are anodically etched to
create a porous silicon layer approximately 2 microns thick.
[0054] (2) Thin epitaxial silicon layers are grown on the porous
silicon layer utilizing an epitaxial reactor.
[0055] (3) The front side solar cell processing is achieved on the
epitaxial film while it is still attached to the silicon substrate.
Device fabrication is comprised of: (a) texture etch the surface to
minimize reflectivity; (b) form an n+p junction using, for example,
diffusion using phosphorus oxy chloride as the dopant source; (c)
remove diffusion induced oxide from the epitaxial film surface by
chemical etching; (d) deposit a layer of silicon nitride utilizing,
for example, PECVD; (e) screen print a silver metal grid
(comprising collecting electrodes and bus bars) on the surface and
fire in a furnace to form ohmic contacts; and (f) attaching tabs to
the bus bars on the front surface of the solar cell. FIG. 11 shows
a cross-sectional representation of the thin epitaxial silicon
solar cell 1101 formed on the silicon substrate 1102, with bus bars
1103. Note that tabs are attached to the tops of the bus bars as
shown in FIG. 10, but for clarity of illustration are not shown in
FIGS. 11-19 and 20A; furthermore, collecting electrodes are
connected to the bus bars as shown in FIGS. 10 and 15, but for
clarity of illustration are not shown in FIGS. 11-14 and 16-21.
[0056] (4) Following front side device fabrication the front side
of the wafer is bonded to a thin Teflon.RTM. sheet 1104 (typically
50 microns thick) by a lamination process using a bonding agent
such as EVA, as shown in FIG. 12. Note that for simplicity of
illustration the bonding material is not shown in FIGS. 11-20B;
however, it is shown in FIG. 21.
[0057] (5) Following lamination the wafer is attached to a frame.
The frame is designed to hold the laminate under tension sufficient
to keep the laminate flat after exfoliation of the silicon
substrate. See FIG. 13 for an example of a square frame 1105 which
is attached to the Teflon.RTM. sheet by a bonding material between
the laminate and the frame. Suitable bonding materials include
adhesives and waxes, which may conveniently soften at elevated
temperature to release the frame when desired. Alternatively, the
frame may comprise two pieces which fit within each other and
clamp/cinch the Teflon.RTM. sheet in place under tension.
Furthermore, the Teflon.RTM. sheet may be captured at opposite
edges on parallel drums and then tensioned.
[0058] (6) The laminate is exfoliated from the silicon substrate,
the exfoliation occurring at the porous silicon layer between the
substrate 1102 and solar cell device 1101. During and after
exfoliation the laminate is kept flat by the frame 1105, as shown
in FIG. 14. FIG. 15 shows a perspective view of the laminate held
in place by the frame 1105 after exfoliation; note that collecting
electrodes 1106, bus bars 1103 and device 1101 are visible through
the transparent Teflon.RTM. sheet 1104.
[0059] (7) The rear side of the solar cell is processed, as shown
in FIGS. 16-18. The processing may comprise deposition of
dielectric and metal layers 1107 and attachment of rear tabs 1108.
The processing also includes laser ablating holes in the dielectric
layer(s) for making point contacts to the rear side of the solar
cell.
[0060] (8) A second Teflon.RTM. sheet 1109 is laminated to the rear
side of the cell using a bonding material such as EVA, as shown in
FIG. 19. (The EVA between the cell and the Teflon.RTM. sheet is not
shown for ease of illustration).
[0061] (9) The frame 1105 is removed by mechanical releasing,
softening of the bonding material or by cutting the Teflon.RTM.
sheet 1104 within the frame, depending on how the frame was
attached to the Teflon.RTM. sheet. The Teflon.RTM. sheets are then
cut to size (matching the size of the solar device). The resulting
device is shown in cross-section in FIG. 20A, and in side view in
FIG. 20B--the latter shows the rear tabs 1108 and front tabs 1110.
(The cross-section of FIG. 20A is defined by a plane perpendicular
to the page and including the dashed line in FIG. 20B.)
[0062] (10) The devices are tested and binned according to their
I-V characteristics so that modules may be made up of devices with
closely similar characteristics, as described above.
[0063] (11) An array of cells is selected as described above and
then combined together to form a module. The cells are laminated
between a Teflon.RTM. top sheet 1111 and a Teflon.RTM. back sheet
1112 using EVA 1113. The cells are shown connected in series--tab
to tab. See cross-sectional view of FIG. 21. A module such as shown
in FIG. 21 may be approximately 1 mm thick. Note that the EVA used
to bond the sheets 1109 and 1104 to the solar devices is also shown
in this view.
[0064] In an alternative embodiment of the above process, at step
(4) the silicon device may be bonded to a sufficiently rigid sheet
of material to avoid the need for the use of a frame. For example,
the Si can be bonded to glass or a stiff sheet, such as a PV5300
ionomer encapsulant sheet available from DuPont. The glass is
removed later, after attaching the cells to a back sheet, whereas
the PV5300 sheets are part of the finished product, being bonded to
a Teflon.RTM. front sheet with EVA)--the PV5300 provides the
required stiffness at the individual cell level along with the
flexibility required of the end product.
[0065] The requirements of polymer materials for module cover
sheets and back sheets in flexible silicon photovoltaics are: thin
and flexible; excellent light transmission characteristics; exhibit
good thermal and thermo-mechanical properties; excellent UV
resistance; good oxygen and moisture barrier properties and
excellent dimensional stability. Note that silicon cells have
substantially less stringent packaging requirements when compared
with other thin film based flexible PV materials (e.g. CIGS,
organic photovoltaics). Materials are available that meet the above
requirements, such as DuPont's Teflon.RTM. fluoropolymer
sheets.
[0066] The table below provides examples of flexible module layer
thicknesses for modules fabricated according to some embodiments of
the present invention. The thicknesses of the different layers are
provided for two examples. The first thickness column provides data
for a module that may be readily fabricated and the second column
provides data for a module that in theory could be fabricated using
the teaching and methods of the present invention. As can be seen
from the table, the total thickness of these modules can be
approximately 1 mm, with prospects for <0.5 min--compare this
with the typical thickness of a conventional rigid, glass based
module of approximately 4 mm. (Note that, the example used in the
table is for a module of the basic configuration shown in FIG. 9.
For a module with the basic configuration shown in FIG. 21, the
thickness can be estimated from the table by adding the thicknesses
of another fluoropolymer layer and another glue layer.)
TABLE-US-00001 Sheet Thickness/microns Thickness/microns
Fluoropolymer Front Sheet 125 75 EVA Glue Layer 200 100
Fluoropolymer Superstrate 125 50 EVA Glue Layer 200 100 Silicon
Solar Cell 50 25 EVA Glue Layer 200 100 Fluoropolymer Back Sheet
125 75 Total Thickness/microns 1025 525
[0067] The process flows and structures described herein may all be
used to form high voltage (flexible) PV modules; however, some
specific examples of process flows and structures for high voltage
(flexible) PV modules are provided herein. An example of a process
flow for forming a fourth embodiment of a PV module--a high voltage
flexible PV module--is as follows. FIGS. 22-26 show cross-sectional
representations, not drawn to scale, of the fabrication
process.
[0068] (1) Single crystal silicon wafers are anodically etched to
create a porous silicon layer approximately 2 microns thick.
[0069] (2) Thin epitaxial silicon layers are grown on the porous
silicon layer utilizing an epitaxial reactor.
[0070] (3) The front side solar cell processing, forming an
epitaxial device 2201 is achieved on the epitaxial film while it is
still attached to the silicon substrate 2202, as shown in FIG. 22.
Device fabrication is comprised of: (a) texture etch the surface to
minimize reflectivity; (b) form an n+p junction using, for example,
diffusion using phosphorus oxy chloride as the dopant source; (c)
remove diffusion induced oxide from the epitaxial film surface by
chemical etching; (d) deposit a layer of silicon nitride utilizing,
for example, PECVD; (e) screen print metal on the surface using a
pattern that will generate multiple sub-cells, each with its own
busbar(s) and collecting electrodes and fire in a furnace to form
ohmic contacts; and (f) attach tabs to the bus bars 2203 on the
front surface of the solar cell using standard solder or low
temperature conductive adhesive paste. Note that for simplicity of
illustration the front tabs are not shown in FIGS. 22-26. FIG. 27
shows a top view of an example of a substrate after completion of
the front side device fabrication. FIG. 27 is an example of a 125
mm cell divided into a 5.times.2 array of sub-cells each being
1''.times.2.45'' with one bus bar per sub-cell and a multiplicity
of collecting electrodes 2210. Tabs 2209 are attached to the bus
bars (hidden under the tabs). Note the gutters 2211 around each
sub-cell which are free of screen-printed metal, for ease of
separating the sub-cells in a later process step.
[0071] (4) Following front side device fabrication the epitaxial
device 2201 is bonded simultaneously to a plurality of glass sheets
(100 micron thick, for example) or Teflon.RTM. sheets 2204 by a
lamination process utilizing a bonding material such as EVA. The
glass/Teflon.RTM. sheets are cut to the sub-cell size and are
placed on the surface in alignment with the screen printed
electrode pattern and with even gaps (gutters) between the sheets.
See cross section of FIG. 23 and plan view of FIG. 27.
[0072] (5) Following lamination, the epitaxial film is cut/scribed
into individual devices around the borders of the laminated sheets,
using a laser, for example, as shown in FIG. 24. The laser cuts
through the thickness of the epitaxial silicon--roughly 50
microns--down to the porous silicon, forming scribe lines 2205.
[0073] (6) The devices are exfoliated from the silicon substrate
2202, either one at a time or all at once, as shown in FIG. 25, and
then separated into individual devices 2206.
[0074] (7) The rear sides of the solar cells are processed, as
shown in FIG. 26. The processing comprises deposition of dielectric
layers and metal films 2207, laser ablating holes in the dielectric
film for making point contacts to the back side of the solar cell
and rear side tab 2208 attachment.
[0075] (8) The sub-cells are tested and binned according to their
I-V characteristics so that modules may be made up of devices with
closely similar characteristics, as described above.
[0076] (9) The sub-cells are connected in series and laminated
simultaneously to flexible front and back sheets. The flexible
sheets may be sheets of fluoropolymers such as Teflon.RTM. or
Tefzel.RTM. ETFE, available from DuPont, for example. FIG. 28 shows
a schematic representation of a module with a 2.times.5
configuration of series connected solar sub-cells 2801. The high
voltage module 2800 shown in FIG. 28 is equipped with a voltage
regulator 2802 and a USB connector 2803. The USB connector is shown
as an example outlet which may be convenient for connection to
small electronic devices such as cellular phones, smart phones,
etc. These high voltage modules when fabricated from one 125 mm
silicon wafer are capable of charging a cell phone -0.6
V.times.10=6V.sub.oc, I.sub.sc=0.37 A, 2.5 W. Note the dashed lines
2804 in FIG. 28 which indicate where the module may readily be
folded (through 180 degrees); the module may be folded in between
sub-cells 2801 where the flexible front and back sheets may act
like hinges.
[0077] An alternative method of fabricating a high voltage flexible
PV module is illustrated in FIGS. 29-31, which is illustrated for a
125 mm wafer to be disaggregated into ten 1''.times.2.45''
sub-cells. The method starts with front side processing as
described above for an array of sub-cells followed by laminating an
individual glass/polymer sheet 2905 to each sub-cell using a
bonding material 2906, such as EVA. FIG. 29 shows an array of
epitaxial silicon devices 2901 formed on a porous silicon layer
2902 on a silicon substrate 2903. Front side bus bars 2904 are
shown for each device in the array--the bus bars run perpendicular
to the plane of the page. Each bus bar has collecting electrodes
(not shown) which run perpendicular to the bus bar over the front
surface of the device--see FIGS. 10 and 27 for examples of the bus
bar and collecting electrode configuration. Note that tabs are
attached to the tops of the bus bars as shown in FIG. 10, but for
clarity of illustration are not shown in FIGS. 29-31. A fixture
2907 is attached to the array of individual glass sheets 2906 for
holding the array of sub-cells during subsequent processing
steps--the fixture may be configured to hold each sub-cell
individually, as shown in FIG. 30. The fixture may be based on
vacuum, electrostatics, mechanical adhesives or other and may cover
partially or entirely the individual front glass pieces. The
silicon substrate 2903 is delaminated and then back-side low
temperature processing is completed, all while the fixture holds
the sub-cells, thus keeping the thin epitaxial silicon devices 2901
from deforming. The back-side processing comprises deposition of
dielectric layers and metal films 2908, and laser ablating holes in
the dielectric film for making point contacts to the back side of
the solar cell. Next, a laser 2909 is used from the backside to
singulate the sub-cells by cutting through the entire 50 micron or
so thickness of the epitaxial silicon. FIG. 31 shows the laser cuts
2910. Processing then proceeds as described above. Note that a
single busbar is patterned along the length of each sub-cell, with
current collecting fingers running perpendicular to the busbar;
here the resistance of 0.5'' long fingers (collecting electrodes)
will not limit current collected. Further note that the porous Si
can be either left in place after substrate removal as a Lambertian
light diffractor or may be removed if other optical enhancement
techniques are sufficient.
[0078] FIGS. 32-38 illustrate a variation in the fabrication
process for epitaxially deposited silicon solar devices, according
to further embodiments of the present invention. FIGS. 32-38 show
cross-sectional representations, not drawn to scale, of the
fabrication process. A silicon substrate 3301 is provided as shown
in FIG. 32. The edges of the silicon substrate are masked prior to
anodization of the silicon surface. FIG. 33 shows a mask 3202
covering the edges of the silicon substrate 3201. The mask is part
of a fixture used for holding the silicon substrate during the
anodization process. The mask is held in place and forms a fluid
seal to the silicon substrate, such that areas under the mask are
not exposed to the electrolyte (generally a hydrofluoric acid
solution) used in anodization. Suitable mask materials are
hydrofluoric acid resistant polymers, such as Teflon.RTM., and the
typical width of the masked regions at the edge of the wafer is
between 1 and 4 mm. The surface of the silicon substrate is then
anodically etched to create a porous silicon layer approximately 2
microns thick in the ummasked area. FIG. 34 shows a porous silicon
layer 3203 formed by anodization on the surface of the silicon
substrate; the mask protects the substrate edges during anodization
so that porous silicon is not formed in masked areas of the
substrate surface. The mask is then removed and the substrate is
loaded into an epitaxial reactor. A thin film of epitaxial silicon
3204 is grown on the surface of the silicon substrate over the
porous silicon layer and crystalline silicon edges of the substrate
(where the substrate surface was masked) as shown in FIG. 35. See
U.S. application Ser. No. 13/483,002 filed May 26, 2012 for a
description of methods and equipment for epitaxial growth of thin
silicon. Front side solar cell processing is carried out, as
described above; FIG. 36 shows the silicon substrate with a solar
device 3205. The solar device is laminated to a thin glass
superstrate (less than 1 mm thick) using EVA; the resulting
structure with thin glass superstrate 3206 and EVA layer 3207 is
shown in FIG. 37. Note that the thin glass superstrate 3206 is
sized to match the porous silicon area 3203, and thus has a smaller
area than the surface of the silicon substrate. The solar device
attached to the thin glass superstrate is exfoliated from the
silicon substrate, where the porous silicon acts as a separation
layer; FIG. 38 shows the solar device separated from the silicon
substrate. Note that, before the exfoliation, a light scribe of the
silicon device is performed, using a diamond scribe tool or laser,
for example, around the edge of the superstrate where the
superstrate is laminated to the solar device. Note that generally
in this process flow edge trimming of the solar device is not
required, since the portions of the epitaxial silicon device layers
attached directly to the silicon substrate, and not to the porous
silicon area, remain attached to the silicon substrate during
exfoliation. Furthermore, although not shown in FIG. 38, there will
be some remnants of the porous silicon layer on both the silicon
substrate and the exfoliated solar device.
[0079] FIGS. 39-41 illustrate a variation in the fabrication
process for epitaxially deposited silicon solar devices, according
to further embodiments of the present invention. FIGS. 39-41 show
cross-sectional representations, not drawn to scale, of the
fabrication process. A solar device 3205 with front side processing
complete, laminated to a thin glass superstrate 3206 with an EVA
layer 3207 is provided as shown in FIG. 39. The solar device
comprises epitaxially deposited silicon layers, where the silicon
layer on the rear side of the device is an epitaxially deposited
highly-doped silicon layer, which functions as a BSF layer. The
highly doped epitaxial silicon layer may be deposited with a
resistivity of 0.1 ohm-cm or less. A dielectric stack 3208--for
example, 20 nm of an oxide, such as silica, and 70 nm of a nitride,
such as silicon nitride--is deposited on the rear side of the solar
device, followed by a layer of metal 3209, for example, an aluminum
alloy, as shown in FIG. 40. Contact regions 3210 are formed through
the metal layer and dielectric stack, for making ohmic contact to
the BSF layer in the solar device, as shown in FIG. 41; the
formation process utilizes a laser, which is used to create roughly
100 micron diameter contact regions. Good ohmic contact is achieved
without the need for thermal treatment after aluminum deposition;
furthermore, since the BSF is highly doped, there is no need to
diffuse aluminum metal into the silicon of the solar device.
Furthermore, the density of roughly 100 micron diameter laser
drilled holes, and thus rear side contacts, may be as low as 1 per
mm.sup.2, or even lower, due to the low electrical resistivity of
the highly doped epitaxial silicon BSF layer.
[0080] The methods for fabricating solar modules described herein
may be adapted for fabrication of either conventional modules or
bifacial modules. Bifacial modules have transparent encapsulant
materials (such as DuPont.TM. ETFE) on both sides enabling light to
enter the module from the front sun facing side and reflected light
to enter the rear.
[0081] The modules described herein describe attaching polymer
sheets to solar devices/sub-cells using a bonding agent such as
EVA. However, according to further embodiments of the present
invention polymer sheets might be bonded directly to solar
devices/sub-cells without the use of a bonding agent. It is
expected that a combination of elevated temperature and pressure
may be used for such a direct bonding process.
[0082] Although the solar cells described herein are thin epitaxial
single crystal silicon solar cells, the teaching and principles of
the present invention may apply to thin epitaxial single crystal
solar cells comprising other semiconductors such as germanium,
gallium arsenide and others. Furthermore, the teaching and
principles of the present invention may apply to standard CZ wafers
which can also be cut into pieces of the type described herein and
encapsulated in a flexible module as taught herein.
[0083] Although the present invention has been particularly
described with reference to certain embodiments thereof, it should
be readily apparent to those of ordinary skill in the art that
changes and modifications in the form and details may be made
without departing from the spirit and scope of the invention.
* * * * *