U.S. patent application number 14/924625 was filed with the patent office on 2016-06-09 for high-efficiency pv panel with conductive backsheet.
This patent application is currently assigned to SOLARCITY CORPORATION. The applicant listed for this patent is SOLARCITY CORPORATION. Invention is credited to Peter P. Nguyen, Bobby Yang.
Application Number | 20160163903 14/924625 |
Document ID | / |
Family ID | 54979958 |
Filed Date | 2016-06-09 |
United States Patent
Application |
20160163903 |
Kind Code |
A1 |
Yang; Bobby ; et
al. |
June 9, 2016 |
HIGH-EFFICIENCY PV PANEL WITH CONDUCTIVE BACKSHEET
Abstract
One embodiment of the invention can provide a solar panel, which
can include a cover, a backsheet, and a plurality of solar cell
strings. The backsheet can include a first insulation layer, a
second insulation layer, and a conductive interlayer positioned
between the first insulation layer and the second insulation layer.
The solar cell strings can be positioned between the cover and the
first insulation layer of the backsheet. The first insulation layer
can include a plurality of vias, and the conductive interlayer can
be patterned according to locations of the vias, thereby
facilitating electrical interconnections among the solar cell
strings.
Inventors: |
Yang; Bobby; (Los Altos
Hills, CA) ; Nguyen; Peter P.; (San Jose,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SOLARCITY CORPORATION |
San Mateo |
CA |
US |
|
|
Assignee: |
SOLARCITY CORPORATION
San Mateo
CA
|
Family ID: |
54979958 |
Appl. No.: |
14/924625 |
Filed: |
October 27, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62088509 |
Dec 5, 2014 |
|
|
|
62143694 |
Apr 6, 2015 |
|
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Current U.S.
Class: |
136/251 ;
438/59 |
Current CPC
Class: |
Y02E 10/50 20130101;
H01L 31/188 20130101; H02S 50/10 20141201; B23P 19/04 20130101;
H01L 31/049 20141201; H01L 31/0516 20130101; Y02B 10/12 20130101;
H01L 31/0504 20130101; Y02B 10/10 20130101; H01L 31/0443 20141201;
H02S 20/25 20141201; H01L 31/022433 20130101; H01L 31/1876
20130101 |
International
Class: |
H01L 31/05 20060101
H01L031/05; H01L 31/0443 20060101 H01L031/0443; H01L 31/18 20060101
H01L031/18; H01L 31/049 20060101 H01L031/049 |
Claims
1. A solar panel, comprising: a cover; a backsheet comprising a
first insulation layer, a second insulation layer, and a conductive
interlayer positioned between the first insulation layer and the
second insulation layer; and a plurality of solar cell strings
positioned between the cover and the first insulation layer of the
backsheet; wherein the first insulation layer comprises a plurality
of vias, and wherein the conductive interlayer is patterned
according to locations of the vias, thereby facilitating electrical
interconnections among the solar cell strings.
2. The solar panel of claim 1, wherein the first insulation layer
comprises polyethylene terephthalate (PET), fluoropolymer,
polyvinyl fluoride (PVF), polyamide, or any combination thereof;
and wherein the conductive interlayer comprises Al, Cu, graphite,
conductive polymer, or any combination thereof.
3. The solar panel of claim 1, wherein a respective via is filled
with a conductive paste to facilitate: electrical coupling between
a contact pad located on a corresponding solar cell string and the
conductive interlayer; and/or mechanical bonding between a contact
pad located on a corresponding solar cell string and the conductive
interlayer.
4. The solar panel of claim 1, wherein a respective solar cell
string comprises a plurality of cascaded photovoltaic
structures.
5. The solar panel of claim 1, further comprising a plurality of
bypass diodes, wherein a respective bypass diode is coupled to a
photovoltaic structure through the conductive interlayer.
6. The solar panel of claim 1, wherein a conductive path between a
first solar cell string and a second solar cell string comprises: a
first contact pad of the first solar cell string; a first set of
vias within the first insulation layer, wherein the first set of
vias are filled with a conductive paste and are positioned beneath
the first contact pad; a second contact pad of the second solar
cell string; a second set of vias within the first insulation
layer, wherein the second set of vias are filled with the
conductive paste and are positioned beneath the second contact pad;
and a continuous portion of the conductive interlayer that is in
contact with both the first and second sets of vias.
7. The solar panel of claim 1, wherein the second insulation layer
comprises a plurality of vias filled with a conductive paste to
electrically couple the interconnected solar cell strings to a
junction box.
8. A method for manufacturing a solar panel, comprising: obtaining
a backsheet that comprises a first insulation layer, a second
insulation layer, and a conductive interlayer positioned between
the first insulation layer and the second insulation layer, wherein
the first insulation layer comprises a plurality of vias, and
wherein the conductive interlayer is patterned according to
locations of the vias; overlaying a plurality of solar cell strings
on the backsheet, wherein the first insulation layer faces the
solar cell strings, and wherein the solar cell strings are overlaid
in such a way that selected contact pads of the solar cell strings
are positioned above the vias, thereby facilitating electrical
interconnections among the solar cell strings; and laminating the
solar cell strings between the backsheet and a glass cover.
9. The method of claim 8, wherein the first insulation layer
comprises polyethylene terephthalate (PET), fluoropolymer,
polyvinyl fluoride (PVF), polyamide, or any combination thereof;
and wherein the conductive interlayer comprises Al, Cu, graphite,
conductive polymer, or any combination thereof.
10. The method of claim 8, further comprising filling the vias with
a conductive paste, wherein a respective via filled with the
conductive paste is configured to facilitate: electrical coupling
between a contact pad located on a corresponding solar cell string
and the conductive interlayer; and/or mechanical bonding between a
contact pad located on a corresponding solar cell string and the
conductive interlayer.
11. The method of claim 8, wherein a respective solar cell string
comprises a plurality of cascaded photovoltaic structures.
12. The method of claim 8, further comprising coupling a plurality
of bypass diodes to the interconnected solar cell strings, wherein
a respective bypass diode is coupled to a photovoltaic structure
through the conductive interlayer.
13. The method of claim 8, further comprising establishing a
conductive path between a first solar cell string and a second
solar cell string by curing a conductive paste that fills the vias,
wherein the conductive path comprises: a first contact pad of the
first solar cell string; a first set of vias within the first
insulation layer, wherein the first set of vias are filled with the
conductive paste and are positioned beneath the first contact pad;
a second contact pad of the second solar cell string; a second set
of vias within the first insulation layer, wherein the second set
of vias are filled with the conductive paste and are positioned
beneath the second contact pad; and a continuous portion of the
conductive interlayer that is in contact with both the first and
second sets of vias.
14. The method of claim 8, further comprising filling vias included
in the second insulation layer with a conductive paste to
electrically couple the interconnected solar cell strings to a
junction box.
15. A photovoltaic structure encapsulation mechanism, comprising: a
transparent cover; and a non-transparent cover comprising a first
insulation layer, a second insulation layer, and a conductive
interlayer positioned between the first insulation layer and the
second insulation layer; wherein the first insulation layer
comprises a plurality of through holes, and wherein the conductive
interlayer is patterned according to locations of the through
holes, thereby facilitating electrical interconnections among solar
cell strings sandwiched between the transparent cover and the
non-transparent cover.
16. The photovoltaic structure encapsulation mechanism of claim 15,
wherein the first insulation layer comprises polyethylene
terephthalate (PET), fluoropolymer, polyvinyl fluoride (PVF),
polyamide, or any combination thereof; and wherein the conductive
interlayer comprises Al, Cu, graphite, conductive polymer, or any
combination thereof.
17. The photovoltaic structure encapsulation mechanism of claim 15,
wherein a respective through hole is filled with a conductive
paste.
18. The photovoltaic structure encapsulation mechanism of claim 17,
wherein the conductive paste comprises: a conductive metallic core
surrounded by a resin; and/or a resin comprising a number of
suspended conductive particles.
19. The photovoltaic structure encapsulation mechanism of claim 17,
wherein the through hole filled with the conductive paste
facilitates: electrical coupling between a contact pad located on a
corresponding solar cell string and the conductive interlayer;
and/or mechanical bonding between a contact pad located on a
corresponding solar cell string and the conductive interlayer.
20. The photovoltaic structure encapsulation mechanism of claim 15,
wherein the second insulation layer comprises a plurality of
through holes filled with a conductive paste to electrically couple
the interconnected solar cell strings to a junction box.
Description
CROSS-REFERENCE TO OTHER APPLICATIONS
[0001] This claims the benefit of U.S. Provisional Patent
Application No. 62/088,509, Attorney Docket Number P103-1PUS,
entitled "SYSTEM, METHOD, AND APPARATUS FOR AUTOMATIC MANUFACTURING
OF SOLAR PANELS," filed Dec. 5, 2014; and U.S. Provisional Patent
Application No. 62/143,694, Attorney Docket Number P103-2PUS,
entitled "SYSTEMS AND METHODS FOR PRECISION AUTOMATION OF
MANUFACTURING SOLAR PANELS," filed Apr. 6, 2015; the disclosures of
which are incorporated herein by reference in their entirety for
all purposes.
[0002] This is also related to U.S. patent application Ser. No.
14/563,867, Attorney Docket Number P67-3NUS, entitled "HIGH
EFFICIENCY SOLAR PANEL," filed Dec. 8, 2014; and U.S. patent
application Ser. No. 14/510,008, Attorney Docket Number P67-2NUS,
entitled "MODULE FABRICATION OF SOLAR CELLS WITH LOW RESISTIVITY
ELECTRODES," filed Oct. 8, 2014; the disclosures of which are
incorporated herein by reference in their entirety for all
purposes.
FIELD OF THE INVENTION
[0003] This is generally related to solar panels. More
specifically, this is related to a solar panel that achieves
inter-cell electrical connections via a conductive backsheet.
DEFINITIONS
[0004] "Solar cell" or "cell" is a photovoltaic structure capable
of converting light into electricity. A cell may have any size and
any shape, and may be created from a variety of materials. For
example, a solar cell may be a photovoltaic structure fabricated on
a silicon wafer or one or more thin films on a substrate material
(e.g., glass, plastic, or any other material capable of supporting
the photovoltaic structure), or a combination thereof.
[0005] A "solar cell strip," "photovoltaic strip," or "strip" is a
portion or segment of a photovoltaic structure, such as a solar
cell. A photovoltaic structure may be divided into a number of
strips. A strip may have any shape and any size. The width and
length of a strip may be the same or different from each other.
Strips may be formed by further dividing a previously divided
strip.
[0006] A "cascade" is a physical arrangement of solar cells or
strips that are electrically coupled via electrodes on or near
their edges. There are many ways to physically connect adjacent
photovoltaic structures. One way is to physically overlap them at
or near the edges (e.g., one edge on the positive side and another
edge on the negative side) of adjacent structures. This overlapping
process is sometimes referred to as "shingling." Two or more
cascading photovoltaic structures or strips can be referred to as a
"cascaded string," or more simply as a string.
[0007] "Finger lines," "finger electrodes," and "fingers" refer to
elongated, electrically conductive (e.g., metallic) electrodes of a
photovoltaic structure for collecting carriers.
[0008] A "busbar," "bus line," or "bus electrode" refers to an
elongated, electrically conductive (e.g., metallic) electrode of a
photovoltaic structure for aggregating current collected by two or
more finger lines. A busbar is usually wider than a finger line,
and can be deposited or otherwise positioned anywhere on or within
the photovoltaic structure. A single photovoltaic structure may
have one or more busbars.
[0009] A "photovoltaic structure" can refer to a solar cell, a
segment, or solar cell strip. A photovoltaic structure is not
limited to a device fabricated by a particular method. For example,
a photovoltaic structure can be a crystalline silicon-based solar
cell, a thin film solar cell, an amorphous silicon-based solar
cell, a poly-crystalline silicon-based solar cell, or a strip
thereof.
BACKGROUND
[0010] Advances in photovoltaic technology, which are used to make
solar panels, have helped solar energy gain mass appeal among those
wishing to reduce their carbon footprint and decrease their monthly
energy costs. However, the panels are typically fabricated
manually, which is a time-consuming and error-prone process that
makes it costly to mass-produce reliable solar panels.
[0011] Solar panels typically include one or more strings of
complete photovoltaic structures. Adjacent photovoltaic structures
in a string may overlap one another in a cascading arrangement. For
example, continuous strings of photovoltaic structures that form a
solar panel are described in U.S. patent application Ser. No.
14/510,008, filed Oct. 8, 2014 and entitled "Module Fabrication of
Solar Cells with Low Resistivity Electrodes," the disclosure of
which is incorporated herein by reference in its entirety.
Producing solar panels with a cascaded cell arrangement can reduce
the resistance due to inter-connections between the cells, and can
increase the number of photovoltaic structures that can fit into a
solar panel.
[0012] Moreover, it has been shown that solar panels based on
strings of strips cascaded in parallel, which are created by
dividing complete photovoltaic structures, provide several
advantages, including but not limited to: reduced shading,
enablement of bifacial operation, and reduced internal resistance.
Detailed descriptions of a solar panel based on cascaded strips can
be found in U.S. patent application Ser. No. 14/563,867, entitled
"HIGH EFFICIENCY SOLAR PANEL," filed Dec. 8, 2014, the disclosures
of which is incorporated herein by reference in its entirety for
all purposes. Conventional inter-string connections, including both
serial and parallel connections, can involve cumbersome wirings,
which often not only complicates the panel manufacturing process
but also leads to extra shading.
[0013] In addition to interconnecting strings of photovoltaic
structures, forming a solar panel also involves connecting each
string or portion of the strings to bypass diodes. The bypass
diodes can be used to prevent currents flowing from good
photovoltaic structures (photovoltaic structures are well-exposed
to sunlight and in normal working condition) to bad photovoltaic
structures (photovoltaic structures that are burning out or
partially shaded) by providing a current path around the bad cells.
Ideally, there would be one bypass diode connected to each
photovoltaic structure, but electrical connections can be too
complicated and expensive. In most cases, one bypass diode can be
used to protect a group of serially connected strips, which can be
a string or a portion of a string. However, connecting strings or
cascaded strips to bypass diodes can be challenging because the
strings do not have exposed busbars, except at the very end of the
string. In other words, it can be difficult to access a
photovoltaic structure that is in the middle of a string.
SUMMARY
[0014] One embodiment of the invention can provide a solar panel,
which can include a cover, a backsheet, and a plurality of solar
cell strings. The backsheet can include a first insulation layer, a
second insulation layer, and a conductive interlayer positioned
between the first insulation layer and the second insulation layer.
The solar cell strings can be positioned between the cover and the
first insulation layer of the backsheet. The first insulation layer
can include a plurality of vias, and the conductive interlayer can
be patterned according to locations of the vias, thereby
facilitating electrical interconnections among the solar cell
strings.
[0015] In a variation on the embodiment, the first insulation layer
can include polyethylene terephthalate (PET), fluoropolymer,
polyvinyl fluoride (PVF), polyamide, or any combination thereof.
The conductive interlayer can include Al, Cu, graphite, conductive
polymer, or any combination thereof.
[0016] In a variation on the embodiment, a respective via can be
filled with conductive paste to facilitate electrical coupling
between contact pad located on a corresponding solar cell string
and the conductive interlayer and/or mechanical bonding between a
contact pad located on a corresponding solar cell string and the
conductive interlayer.
[0017] In a variation on the embodiment, a respective solar cell
string can include a plurality of cascaded photovoltaic
structures.
[0018] In a variation on the embodiment, the solar panel can
further include a plurality of bypass diodes, wherein a respective
bypass diode can be coupled to a photovoltaic structure through the
conductive interlayer.
[0019] In a variation on the embodiment, a conductive path between
a first solar cell string and a second solar cell string can
include: a first contact pad of the first solar cell string, a
first set of vias within the first insulation layer, wherein the
first set of vias can be filled with conductive paste and
positioned beneath the first contact pad, a second contact pad of
the second solar cell string, a second set of vias within the first
insulation layer, wherein the second set of vias can be filled with
conductive paste and positioned beneath the second contact pad, and
a continuous portion of the conductive interlayer that can be in
contact with both the first and second sets of vias.
[0020] In a variation on the embodiment, the second insulation
layer can include a plurality of vias to electrically couple the
interconnected solar cell strings to a junction box.
BRIEF DESCRIPTION OF THE FIGURES
[0021] FIG. 1A shows an exemplary conductive grid pattern on the
front surface of a photovoltaic structure.
[0022] FIG. 1B shows an exemplary conductive grid pattern on the
back surface of a photovoltaic structure.
[0023] FIG. 2A shows a string of strips stacked in a cascaded
pattern.
[0024] FIG. 2B shows the side-view of the string of the cascaded
strips.
[0025] FIG. 3 shows the structure of an exemplary backsheet with a
conductive interlayer.
[0026] FIG. 4A shows exemplary electrical coupling between a string
and the conductive interlayer in the backsheet, according to an
embodiment of the invention.
[0027] FIG. 4B shows a cross-sectional view of a string sandwiched
between both covers of a solar panel, according to an embodiment of
the invention.
[0028] FIG. 5A shows a cross-sectional view of two strings
connected in series, according to an embodiment of the
invention.
[0029] FIG. 5B shows the top view of two strings connected in
series, according to an embodiment of the invention.
[0030] FIG. 6A shows a cross-sectional view of two strings
connected in parallel, according to an embodiment of the
invention.
[0031] FIG. 6B shows the top view of four strings connected in
parallel, according to an embodiment of the invention.
[0032] FIG. 7A shows the back side of a string comprising cascaded
strip, according to an embodiment of the invention.
[0033] FIG. 7B shows the back side of a string comprising cascaded
strip, according to an embodiment of the invention.
[0034] FIG. 8A shows a cross-sectional view of a string
mechanically bonded to the backsheet, according to an embodiment of
the invention.
[0035] FIG. 8B shows the top view of a string mechanically bonded
to the backsheet, according to an embodiment of the invention.
[0036] FIG. 9A shows the inter-string connections and the bypass
protection strategy of a solar panel, according to an embodiment of
the invention.
[0037] FIG. 9B shows the inter-string connections and the bypass
protection strategy of a solar panel, according to an embodiment of
the invention.
[0038] FIG. 10 shows the flowchart of manufacturing a solar panel,
according to an embodiment of the invention.
[0039] In the figures, like reference numerals refer to the same
figure elements.
DETAILED DESCRIPTION
[0040] The following description is presented to enable any person
skilled in the art to make and use the embodiments, and is provided
in the context of a particular application and its requirements.
Various modifications to the disclosed embodiments will be readily
apparent to those skilled in the art, and the general principles
defined herein may be applied to other embodiments and applications
without departing from the spirit and scope of the present
disclosure. Thus, the invention is not limited to the embodiments
shown, but is to be accorded the widest scope consistent with the
principles and features disclosed herein.
Overview
[0041] Embodiments of the invention provide a solar module that
includes a conductive backsheet used for inter-cell electrical
coupling. More specifically, the conductive backsheet can include a
conductive (Cu or Al) middle layer sandwiched between multiple
insulating layers. At the cell-facing side of the backsheet, vias
can be formed by removing the insulating layers to expose the
underneath conductive interlayer at selected locations. Conductive
adhesive can fill the vias, thus enabling formations of conductive
paths between the photovoltaic structure surface and the backsheet.
To achieve inter-string electrical connections and connections to
bypass diodes, the conductive middle layer can be patterned
according to the solar panel layout design (including how the
strings are placed and interconnected and what type of bypass
protection strategy is used).
Solar Panel Based on Cascaded Strips
[0042] As described in U.S. patent application Ser. No. 14/563,867,
a solar panel can have multiple (such as 3) strings, each string
including cascaded strips, connected in parallel. Such a
multiple-parallel-string panel configuration can provide the same
output voltage with a reduced internal resistance. In general, a
cell can be divided into a number of (e.g., n) strips, and a panel
can contain a number of strings (the number of strings can be the
same as or different from number of strips in the cell). If a
string has the same number of strips as the number of regular
photovoltaic structures in a conventional single-string panel, the
string can output approximately the same voltage as a conventional
panel. Multiple strings can then be connected in parallel to form a
panel. If the number of strings in a panel is the same as the
number of strips in the cell, the solar panel can output roughly
the same current as a conventional panel. On the other hand, the
panel's total internal resistance can be a fraction (e.g., 1/n) of
the resistance of a string. Therefore, in general, the greater n
is, the lower the total internal resistance of the panel is, and
the more power one can extract from the panel. However, a tradeoff
is that as n increases, the number of connections required to
inter-connect the strings also increases, which increases the
amount of contact resistance. Also, the greater n is, the more
strips a single cell needs to be divided into, which increases the
associated production cost and decreases overall reliability due to
the larger number of strips used in a single panel.
[0043] Another consideration in determining n is the contact
resistance between the electrode and the photovoltaic structure on
which the electrode is formed. The greater this contact resistance
is, the greater n might need to be to reduce effectively the
panel's overall internal resistance. Hence, for a particular type
of electrode, different values of n might be needed to attain
sufficient benefit in reduced total panel internal resistance to
offset the increased production cost and reduced reliability. For
example, conventional silver-paste or aluminum based electrode may
require n to be greater than 4, because process of screen printing
and firing silver paste onto a cell does not produce ideal
resistance between the electrode and underlying photovoltaic
structure. In some embodiments of the present invention, the
electrodes, including both the busbars and finger lines, can be
fabricated using a combination of physical vapor deposition (PVD)
and electroplating of copper as an electrode material. The
resulting copper electrode can exhibit lower resistance than an
aluminum or screen-printed-silver-paste electrode. Consequently, a
smaller n can be used to attain the benefit of reduced panel
internal resistance. In some embodiments, n is selected to be
three, which is less than the n value generally needed for cells
with silver-paste electrodes or other types of electrodes.
Correspondingly, two grooves can be scribed on a single cell to
allow the cell to be divided to three strips.
[0044] In addition to lower contact resistance, electro-plated
copper electrodes can also offer better tolerance to micro cracks,
which may occur during a cleaving process. Such micro cracks might
adversely impact silver-paste-electrode cells. Plated-copper
electrode, on the other hand, can preserve the conductivity across
the cell surface even if there are micro cracks in the photovoltaic
structure. The copper electrode's higher tolerance for micro cracks
allows one to use thinner silicon wafers to manufacture cells. As a
result, the grooves to be scribed on a cell can be shallower than
the grooves scribed on a thicker wafer, which in turn helps
increase the throughput of the scribing process. More details on
using copper plating to form a low-resistance electrode on a
photovoltaic structure are provided in U.S. patent application Ser.
No. 13/220,532, entitled "SOLAR CELL WITH ELECTROPLATED GRID,"
filed Aug. 29, 2011, the disclosure of which is incorporated herein
by reference in its entirety.
[0045] FIG. 1A shows an exemplary grid pattern on the front surface
of a photovoltaic structure, according to one embodiment of the
present invention. In the example shown in FIG. 1A, grid 102
includes three sub-grids, such as sub-grid 104. This three sub-grid
configuration allows the photovoltaic structure to be divided into
three strips. To enable cascading, each sub-grid needs to have an
edge busbar, which can be located either at or near the edge. In
the example shown in FIG. 1A, each sub-grid includes an edge busbar
("edge" here refers to the edge of a respective strip) running
along the longer edge of the corresponding strip and a plurality of
parallel finger lines running in a direction parallel to the
shorter edge of the strip. For example, sub-grid 104 can include
edge busbar 106, and a plurality of finger lines, such as finger
lines 108 and 110. To facilitate the subsequent laser-assisted
scribe-and-cleave process, a predefined blank space (i.e., space
not covered by electrodes) is inserted between the adjacent
sub-grids. For example, blank space 112 is defined to separate
sub-grid 104 from its adjacent sub-grid. In some embodiments, the
width of the blank space, such as blank space 112, can be between
0.1 mm and 5 mm, preferably between 0.5 mm and 2 mm. There is a
tradeoff between a wider space that leads to more tolerant scribing
operation and a narrower space that leads to more effective current
collection. In a further embodiment, the width of such a blank
space can be approximately 1 mm.
[0046] FIG. 1B shows an exemplary grid pattern on the back surface
of a photovoltaic structure, according to one embodiment of the
invention. In the example shown in FIG. 1B, back grid 120 can
include three sub-grids, such as sub-grid 122. To enable cascaded
and bifacial operation, the back sub-grid may correspond to the
front sub-grid. More specifically, the back edge busbar needs to be
located at the opposite edge of the frontside edge busbar. In the
examples shown in FIGS. 1A and 1B, the front and back sub-grids
have similar patterns except that the front and back edge busbars
are located adjacent to opposite edges of the strip. In addition,
locations of the blank spaces in back conductive grid 120
correspond to locations of the blank spaces in front conductive
grid 102, such that the grid lines do not interfere with the
subsequent scribe-and-cleave process. In practice, the finger line
patterns on the front and back side of the photovoltaic structure
may be the same or different.
[0047] In the examples shown in FIGS. 1A and 1B, the finger line
patterns can include continuous, non-broken loops. For example, as
shown in FIG. 1A, finger lines 108 and 110 both include connected
loops with rounded corners. This type of "looped" finger line
pattern can reduce the likelihood of the finger lines from peeling
away from the photovoltaic structure after a long period of usage.
Optionally, the sections where parallel lines are joined can be
wider than the rest of the finger lines to provide more durability
and prevent peeling. Patterns other than the one shown in FIGS. 1A
and 1B, such as un-looped straight lines or loops with different
shapes, are also possible.
[0048] To form a cascaded string, cells or strips (e.g., as a
result of a scribing-and-cleaving process applied to a regular
square-shaped cell) can be cascaded with their edges overlapped.
FIG. 2A shows a string of cascaded strips, according to an
embodiment of the invention. In FIG. 2A, strips 202, 204, and 206
are stacked in such a way that strip 206 partially overlaps
adjacent strip 204, which also partially overlaps (on an opposite
edge) strip 202. Such a string of strips forms a pattern that is
similar to roof shingles. Each strip includes top and bottom edge
busbars located at opposite edges of the top and bottom surfaces,
respectively. Strips 202 and 204 are coupled to each other via an
edge busbar 208 located at the top surface of strip 202 and an edge
busbar 210 located at the bottom surface of strip 204. To establish
electrical coupling, strips 202 and 204 are placed in such a way
that bottom edge busbar 210 is placed on top of and in direct
contact with top edge busbar 208.
[0049] FIG. 2B shows a side view of the string of cascaded strips,
according to one embodiment of the invention. In the example shown
in FIGS. 2A and 2B, the strips can be part of a 6-inch
square-shaped photovoltaic structure, with each strip having a
dimension of approximately 2 inches by 6 inches. To reduce shading,
the overlapping between adjacent strips should be kept as small as
possible. In some embodiments, the single busbars (both at the top
and the bottom surfaces) are placed at the very edge of the strip
(as shown in FIGS. 2A and 2B). The same cascaded pattern can extend
along an entire row of strips to form a serially connected
string.
[0050] From FIGS. 2A and 2B one can see that, other than at both
ends of a string, all busbars are sandwiched between the overlapped
strips. This no-busbar configuration reduces shading. However,
hiding the busbars makes it difficult to electrically access the
photovoltaic structures, especially the strips that are in the
middle of a string. In addition, although a string can be connected
to a different string via busbars at either ends of the string,
connecting the strings may sometimes require flipping over a string
of cascaded strips, which is not an easy task considering that a
string may include tens of cascaded strips and the strips are made
of fragile Si wafers.
Electrical Interconnection Based on a Conductive Backsheet
[0051] The manufacture of a solar panel typically involves
encapsulating photovoltaic structures between two layers of
protective material, which are the front and back covers. The
light-facing side of the panel often includes a glass cover, and
the side facing away from light often includes a non-transparent
cover, known as the backsheet. Typical backsheets for solar panels
are made of polyvinyl fluoride (PVF) or polyethylene terephthalate
(PET) films, which are electrical insulating. Alternatively, a
solar panel backsheet may include a conductive interlayer
sandwiched between layers of insulating materials. The conductive
interlayer can include a conductive interlayer, which can include
metallic materials (e.g., Al, Cu, or their alloy) or non-metallic
conductive materials (e.g., graphite or conductive polymer).
[0052] FIG. 3 shows the structure of an exemplary backsheet with a
conductive interlayer. In FIG. 3, backsheet 300 includes a
plurality of layers, including primer layer 302 facing the
photovoltaic structures, electrical-grade PET layer 306, adhesive
layer 304 positioned between primer layer 302 and electrical-grade
PET layer 306, conductive interlayer 310, adhesive layer 308
positioned between conductive interlayer 310 and electrical-grade
PET layer 306, PET layer 314 that is hydrolysis resistant and UV
stable, and adhesive layer 312 positioned between conductive
interlayer 310 and hydrolysis-resistant PET layer 314. The outer
PET layers 302 and 314 provide excellent electrical insulation,
which is essential to protect the photovoltaic structures from
being exposed to external voltage. In addition to PET, other
insulating materials, such as PVF, Polyamide, and Tedlar.RTM.
(registered trademark of E. I. du Pont de Nemours and Company of
Wilmington, Del.), may also be used as outer layers that
encapsulate the conductive interlayer.
[0053] The usage of a backsheet having a conductive interlayer is
originally motivated by the need of a moisture barrier inside a
solar panel. However, the existence of a conductive interlayer
inside the backsheet can also provide the possibility of
establishing electrical paths through the backsheet. More
specifically, when two solar strings need to be coupled
electrically, one may electrically couple an exposed busbar of one
string to a point on the conductive interlayer and couple an
exposed busbar of the other string to another point on the
conductive interlayer. If a continuous layer of conductive material
exists between these two points, these two strings can be
electrically coupled. This way, there is no need for additional
tabbing wires between the two strings, which not only saves
material cost but also simplifies the panel fabrication
process.
[0054] Electrically coupling between a string and a conductive
interlayer of the backsheet requires a way to bypass the insulating
PET layer positioned between the string and the conductive
interlayer. In some embodiments, vias (or through holes) can be
created in the insulating PET layer, and conductive paste can be
used to fill these vias to establish a conductive path between the
busbar of an edge strip of the string and the conductive
interlayer.
[0055] FIG. 4A shows exemplary electrical coupling between a string
of photovoltaic structures and the conductive interlayer in the
backsheet, according to an embodiment of the invention. In FIG. 4A,
backsheet 410 can include top insulation layer 412, conductive
interlayer 414, and bottom insulation layer 416. A cascaded string
402 can include edge busbar 404 facing backsheet 410. Other busbars
within string 402 (with the exception of a busbar at the other
edge, not shown in FIG. 4A) are sandwiched between the cascaded
strip edges, and are not electrically accessible. For simplicity,
the individual photovoltaic structures or strips in string 402 are
not drawn in detail. One can refer to FIGS. 2A and 2B for details
about how the strips are cascaded.
[0056] Establishing electrical coupling between string 402 and
conductive interlayer 414 can involve creating a conductive path
between edge busbar 404 and conductive interlayer 414. In the
example shown in FIG. 4A, such a conductive path can be formed by
creating via 418 (or through hole 418) within top insulation layer
412 at a location that is directly beneath busbar 404, and by
filling via 418 with a conductive material. The conductive material
is then in direct contact with both busbar 404 and conductive
interlayer 414, thus forming a conductive path between them.
[0057] Via 418 can be formed by selective etching top insulation
layer 412. The conductive material used to fill via 418 can include
a conductive paste (or adhesive), which can not only provide
conductivity but can also enhance the bonding between string 402
and backsheet 410. The conductive adhesive or paste can have
various forms. In one embodiment, the conductive adhesive can
include a conductive metallic core surrounded by resin. When the
conductive paste fills via 418, the metallic core can establish
electrical connections, while the resin that surrounds the metallic
core can function as an adhesive. In another embodiment, the
conductive paste may be in the form of resin that includes a number
of suspended conductive particles, such as Ag or Cu particles.
These conductive particles may be coated with a protection layer
that evaporates when the paste is thermally cured, thereby
resulting in electrical conductivity among the conductive particles
suspended inside the resin. The volume fraction of the conductive
particles can be approximately between 50 and 90%
[0058] Also shown in FIG. 4A is sealant layer 420, which when
thermally cured can seal the strings between the covers. Sealant
layer 420 can be formed using various materials, such as
ethylene-vinyl acetate (EVA), acrylic, polycarbonate, polyolefin,
and thermal plastic. In some embodiments, sealant layer 420 may be
part of backsheet 410.
[0059] FIG. 4B shows a cross-sectional view of a string sandwiched
between both covers of a solar panel, according to an embodiment of
the invention. In FIG. 4B, cascaded string 450 can be sandwiched
between glass cover 460 and backsheet 440, and can be also embedded
in sealant layer 470. Backsheet 440 can include top insulation
layer 442, conductive interlayer 444, and bottom insulation layer
446. Cascaded string 450 can include edge busbars 452 and 454 that
are on opposite (top and bottom) sides of the string, and
conductive tab 456. In some embodiments, conductive tab 456 can be
"L" shaped, as shown in FIG. 4B, which enables electrical access to
busbar 454 from the opposite side of cascaded string 450. In
further embodiments, conductive tab 456 may include a conductive
core wrapped with a layer of insulating material. One end of the
conductive core electrically couples to busbar 454, and the other
end of the conductive core can be used to electrically access
busbar 454. Conductive tab 456 may have other shapes or forms, as
long as it can enable electrical access to busbar 454 from a side
of cascaded string 450 that is opposite to the side where busbar
454 is located. For example, conductive tab 456 can shaped as a
rectangular prism or two-step stairs. In some embodiments,
conductive tab 456 can include a conductive core partially wrapped
with an EPE film layer, which is a multi-layer film consisting of
Vinyl Acetrate resin (EVA) bonded to both sides of a Polyester
film. More pacifically, other than the string-facing surface, the
surface that faces away from the string and sidewalls of conductive
tab 456 can be completely covered by the EPE film. The EPE film can
be chosen to be black to ensure consistence in appearance of the
tabbed solar string.
[0060] In the example shown in FIG. 4B, electrical accesses to both
sides, thus both polarities, of cascaded string 450 can come from
the bottom side (or the side facing backsheet 440) of cascaded
string 450. This is important in enabling inter-string connections
via backsheet 440. In other words, the introduction of conductive
tabs can enable single-sided electrical connections at the string
level. This is different from other rear contact photovoltaic
structures, such as interdigitated back contact (IBC) solar cells,
emitter warp through (EWT) solar cells, and metallization wrap
through (MWT) solar cells. More specifically, IBC and EWT solar
cells require formations of oppositely doped regions at the back
side of the photovoltaic structures, and EWT and MWT solar cells
require drilling holes through the photovoltaic structures. All
these can lead to a much more complex manufacturing process. On the
other hand, in embodiments of the invention, the current-collecting
finger lines and busbars are on both sides of the photovoltaic
structures, and access to the busbars can be achieved from the same
side of the photovoltaic structures without the need to form
oppositely doped regions on one side or drill holes through the
photovoltaic structures.
[0061] In FIG. 4B, top insulation layer 442 includes vias 462 and
464, with via 462 beneath busbar 452 and via 464 beneath conductive
tab 456. Both vias 462 and 464 are filled with conductive paste,
meaning that both busbars 452 and 454 are electrically coupled to
conductive interlayer 444. For any photovoltaic structure to
operate normally, the two polarities (i.e., the two busbars on the
opposite sides) cannot be shorted. To prevent shorting between
busbars 452 and 454 through conductive interlayer 444, gap 448 can
be created within conductive interlayer 444 to electrically
insulate the conductive portion coupled to busbar 452 from the
conductive portion coupled to busbar 454.
[0062] On the other hand, when two strings need to be electrically
coupled, either in series or in parallel, a continuous portion of
the conductive interlayer can serve as a bridge to create a
conductive path between busbars of the two strings. FIG. 5A shows a
cross-sectional view of two strings connected in series, according
to an embodiment of the invention. In FIG. 5A, string 510 and
string 520 are sandwiched between glass cover 570 and backsheet
550. More specifically, strings 510 and 520 are placed adjacent to
each other, with bottom busbar 512 of string 510 adjacent to
conductive tab 526 that is coupled to top busbar 524 of string 520.
Backsheet 550 can include top insulation layer 552, conductive
interlayer 554, and bottom insulation layer 556. Considering all
strings may be oriented the same way, meaning that busbars of the
same polarity of all strings are facing the same side (top or
bottom) of the panel, connecting strings 510 and 520 in series can
require establishing a conductive path between bottom busbar 512 of
string 510 and top busbar 524 of string 520. To do so, vias 562 and
564 underneath bottom busbar 512 and conductive tab 526,
respectively, can be created within top insulation layer 552, and
can be filled with conductive paste. As a result, a conductive
path, as indicated by double arrow 566, can be established via
conductive interlayer 554. Gaps 572 and 574 within conductive
interlayer 554 can ensure that this conductive path is isolated
from other circuitry within conductive interlayer 554.
[0063] FIG. 5B shows the top view of two strings connected in
series, according to an embodiment of the invention. For purposes
of illustration, the different layers are overlaid on each other in
a transparent manner, although they are not transparent in real
life. The vertical sequence of the layers can be seen in FIG. 5A.
For example, string 510 is drawn transparently to reveal bottom
busbar 512 and via 562. The dotted background is conductive
interlayer 554. Also shown in FIG. 5B are vias 562 and 564 filled
with conductive paste. These vias are within the top insulation
layer, which is not explicitly shown in FIG. 5B. The conductive
paste filled in vias 562 and 564 can electrically couple busbar 512
and conductive tab 526 to conductive portion 568 carved out of
conductive interlayer 554. From FIG. 5B, one can see that portion
568 is insulated from the rest of conductive interlayer 554 by a
gap surrounding portion 568. Consequently, although conductive
portion 568 can provide a conductive path between busbar 512 and
conductive tab 526, such a conductive path does not lead to
anywhere else, thus preventing possible shorting of the
strings.
[0064] FIG. 6A shows a cross-sectional view of two strings
connected in parallel, according to an embodiment of the invention.
In FIG. 6A, string 610 and string 620 can be sandwiched between
glass cover 670 and backsheet 650. Unlike what is shown in FIG. 5A,
in FIG. 6A, strings 610 and 620 can be placed in such a way that
bottom busbar 612 of string 610 is adjacent to bottom busbar 622 of
string 620. Backsheet 650 can include top insulation layer 652,
conductive interlayer 654, and bottom insulation layer 656. Because
bottom busbars 612 and 622 are of the same polarity, the parallel
connection of strings 610 and 620 can require establishing a
conductive path between bottom busbar 612 of string 610 and bottom
busbar 622 of string 620. Similarly, a conductive path needs to be
established between top busbars 614 and 624.
[0065] Similar to what is shown in FIG. 5A, vias 662 and 664
underneath bottom busbars 612 and 622, respectively, can be created
within top insulation layer 652, and can be filled with conductive
paste. Through the conductive paste and conductive interlayer 654,
a conductive path, as indicated by double arrow 666, can be
established between bottom busbars 612 and 622. Gaps 672 and 674
within conductive interlayer 654 ensure that this conductive path
is isolated from other circuitry within conductive interlayer 654.
Similarly, a conductive path can be established between top busbars
614 and 624 through conductive tabs 616 and 626, vias 682 and 684
that are filled with conductive paste, and conductive interlayer
654.
[0066] In certain cases, the strings connected in parallel may be
connected to a junction box located on the opposite side (the side
facing away from the strings) of backsheet 650 to enable
connections to other circuitries outside of the panel. In some
embodiments, the electrical coupling between inter-connected
strings and the junction box can also be achieved via the
conductive interlayer in the backsheet. In the example shown in
FIG. 6A, vias 676 and 678 are created within bottom insulation
layer 656, and are filled with conductive paste, thus enabling
electrical access from the outside of the panel to the strings.
More specifically, via 676 enables electrical coupling to bottom
busbars of the strings and via 678 enables electrical coupling to
top busbars of the strings. As one can see from FIG. 6A, the
pattern of conductive interlayer 654 can be designed in a way to
allow vias 676 and 678 to be in close vicinity of each other,
making it easier to establish connections to the junction box.
[0067] FIG. 6B shows the top view of four strings connected in
parallel, according to an embodiment of the invention. For purposes
of illustration, the different layers are overlaid on each other in
a transparent manner, although they are not transparent. The
vertical sequence of the layers can be seen in FIG. 6A.
[0068] In FIG. 6B, conductive interlayer 654 include portion 655
that is carved out and insulated from the rest of conductive
interlayer 654. Carved out portion 655 can form an equal potential
plane that electrically couples the bottom busbars of the four
strings through conductive paste filled in strategically positioned
vias, e.g., vias 662 and 664, which couple the bottom busbars of
strings 610 and 620 to conductive portion 655. On the other hand,
the remaining portion of conductive interlayer 654 can form a
second equal potential plane that electrically couples the top
busbars of the four strings. For example, conductive paste filled
in vias 682 and 684 can couple the top busbars of strings 610 and
620 to the remaining portion of conductive interlayer 654. As a
result, the four strings shown in FIG. 6B are connected in parallel
with one polarity (the bottom busbars) coupled to carved out
portion 655 and the other polarity (the top busbars) coupled to the
remaining portion of conductive interlayer 654. Conductive paste
filled in vias 676 and 678 within the bottom insulation layer can
enable electrical access to the two polarities of the strings from
the back side of the backsheet.
[0069] In the examples shown in FIGS. 5B and 6B, each busbar is
coupled to the conductive interlayer through one via filled with
conductive paste. In practice, multiple vias filled with conductive
paste can be used to establish the conductive path to the
conductive interlayer. Using multiple vias not only can reduce the
series resistance, but also can strengthen the bonding between the
string and the backsheet.
[0070] In addition to enabling inter-string connections, the
conductive backsheet may also be used to provide electrical access
to middle strips of a string. Accessing the middle strips can be
important, especially if one wants to provide bypass protections at
a higher granularity than an individual string. For example, to
provide bypass protections to half of the strips within a string,
one may need to connect a bypass diode in parallel to the half
string; that is, electrically couple to a strip in the middle of
the string. In some embodiments, some of the strips within a string
can include specially designed "contact pads" (sometimes also
called "landing pads") to enable electrical access to a strip, even
when such a strip is positioned in the middle of a cascaded string,
as shown in FIGS. 7A and 7B.
[0071] FIG. 7A shows the back side of a string comprising cascaded
strips, according to an embodiment of the invention. In the example
shown in FIG. 7A, strip 702 can be positioned in the middle of
string 700, and can include a number of exposed contact pads, such
as contact pad 704. These contact pads can be part of the
electrodes on strip 702, thus enabling electrical connections to
strip 702. Similarly, FIG. 7B shows the back side of a string
comprising cascaded strips, according to an embodiment of the
invention. In the example shown in FIG. 7B, strip 712 can be
positioned in the middle of string 710, and can include additional
non-edge busbar 716. Non-edge busbar 716 is exposed and can have
multiple widened areas, such as widened area 716. These widened
areas can act as contact pads to enable electrical connections to
strip 712. More specifically, the landing or contact pads, such as
the ones shown in FIGS. 7A and 7B, allow a bypass diode to be used
to protect a portion of the cascaded string. Detailed descriptions
of the contact/landing pads can be found in co-pending U.S. patent
application No. TBA, Attorney Docket Number P142-1NUS, entitled
"PHOTOVOLTAIC ELECTRODE DESIGN WITH CONTACT PADS FOR CASCADED
APPLICATION," filed ______, 2015, the disclosure of which is
incorporated herein by reference in its entirety for all
purposes.
[0072] In addition to enabling the sub-string level bypass
protection, these contact pads can also facilitate mechanical
bonding between the string and the backsheet. FIG. 8A shows a
cross-sectional view of a string mechanically bonded to the
backsheet, according to an embodiment of the invention. In FIG. 8A,
string 810 can be positioned between front cover 820 and backsheet
830. Backsheet 830 can include top insulation layer 832, conductive
interlayer 834, and bottom insulation layer 836. Backsheet 830 may
optionally include sealant layer 838. For simplicity, FIG. 8A only
shows one additional busbar 812 located in the middle of string
810, and does not show the edge busbars.
[0073] To facilitate mechanical bonding between string 810 and
backsheet 830, via 842 can be created in top insulation layer 832
underneath additional busbar 812. By filling via 842 with adhesives
(which can include conductive paste or other insulating adhesives),
one can mechanically bond string 810 to backsheet 830. More
specifically, the adhesives bond string 810 to conductive
interlayer 834. Since the adhesives most likely include conductive
paste (to keep the paste application process consistent), to
prevent undesired electrical coupling, portion 844 that is in
contact with the conductive paste can be insulated from the rest of
conductive interlayer 834 via gaps 846 and 848. As a result,
adhesives within via 842 merely serve the purpose of establishing
mechanical bonding, and do not provide any electrical coupling.
[0074] FIG. 8B shows the top view of a string mechanically bonded
to the backsheet, according to an embodiment of the invention. For
purposes of illustration, the different layers are overlaid on each
other in a transparent manner, although they are not transparent.
For example, string 810 is shown as transparent to reveal
additional busbar 812. The vertical sequence of the layers can be
seen in FIG. 8A. As shown in FIG. 8B, a number of vias, such as via
842, are created under additional busbar 812 of string 810.
Adhesives, such as conductive paste, filled in these vias couple
string 810 to portion 844 within conductive interlayer 834. Because
portion 844 is segregated from other portions of conductive
interlayer 834, no electrical coupling to additional busbar 812 can
be established through portion 844. The examples shown in FIGS. 8A
and 8B can also be applied to scenarios where the contact pads are
widened areas of the edge busbars.
[0075] In the example shown in FIG. 6B, four strings are
interconnected to each other through the conductive interlayer. In
practice, a solar panel may include more inter-connected strings,
and multiple bypass diodes can be used to bypass protect strings or
portions of a string. FIG. 9A shows the inter-string connections
and the bypass protection strategy of a solar panel, according to
an embodiment of the invention.
[0076] In FIG. 9A, solar panel 900 includes a number of cascaded
strings, such as strings 902 and 904. Each cascaded string can
include a series of strips stacked in a cascaded manner. In most
cases, the strips are orientated in such a way that they are
connected in series (similar to the example shown in FIG. 5A).
Multiple cascaded strings can also be connected in series to form a
larger string, which can output a larger voltage than a single
cascaded string. In the example shown in FIG. 9A, cascaded strings
902 and 904 and two other strings are connected in series to form
larger string 912. The serial connection can be formed through
portions of the conductive interlayer in the backsheet. For
example, the negative polarity (as indicated by the "-" sign) of
cascaded string 902 can be coupled to the positive polarity (as
indicated by the "+" sign) of cascaded string 904 via portion 906,
which can be insulated from other portions of the conductive
interlayer. The serial connections between other cascaded strings
can be formed similarly. Solar panel 900 can include multiple
similarly connected larger strings, such as larger strings 912 and
914. To enhance the current output of solar panel 900, the multiple
larger strings can be connected in parallel. Such parallel
connections can also be achievable via the conductive interlayer in
the backsheet. In the example shown in FIG. 9A, the positive
polarities of all larger strings, including larger strings 912 and
914, can be coupled to portion 910, and the negative polarities of
all larger strings can be coupled to portion 920. Portions 910 and
920 can be separated from each other to prevent shorting. On the
other hand, the panel output can be obtained from the back side of
the panel via contacts 922 and 924, which are coupled electrically
to portions 910 and 920, respectively. More specifically, contacts
922 and 924 can be formed by creating vias at the bottom insulation
layer of the backsheet and filling these vias with conductive
paste. In addition, bypass diode 950 can also be coupled to
contacts 922 and 924, which provide bypass protection to the entire
panel. Shaded or partially shaded strings can be bypassed to
prevent heating of the shaded strips. In the example shown in FIG.
9A, only one bypass diode is connected to the larger strings via
the backsheet because the backsheet only provides access to ends of
the larger strings. In practice, portions 910 and 920 can each be
divided into smaller pieces to allow additional bypass diodes to be
connected to a subset of cascaded strips in a larger string. For
example, a bypass diode may be coupled to portion 906 and the back
side of a strip in the middle of string 904 to protect a portion of
string 904. One can see from FIG. 9A that connections to bypass
diodes via the conductive interlayer can be much simpler compared
to conventional approaches, thus making it possible to incorporate
a relatively large number of bypass diodes in the panel. One
extreme example is to couple each regularly sized photovoltaic
structure (which can include multiple strips, such as three strips)
with a bypass diode. In most cases, a number of strips connected in
series (which can be a portion of a cascaded string) are coupled to
a bypass diode. In FIG. 9A, vias that mechanically bond the strings
and the backsheet are not shown.
[0077] In the example shown in FIG. 9A, gaps are formed in the
conductive interlayer of the backsheet to electrically insulate
different portions of the conductive interlayer, and vias are
formed in the insulation layer to electrically couple the different
portions of the conductive interlayer to corresponding electrodes
of strings. These strategically located gaps and vias achieve the
desired circuitry connection. More specifically, locations of the
gaps and vias in the conductive interlayer are predetermined based
on the layout of the panel. During the manufacturing process of the
panel, the strips are first cascaded to form strings, then
conductive paste is applied on the backsheet at locations of the
vias, and at least the cascaded strings are carefully aligned to
the backsheet to ensure that appropriate electrodes (busbars or
contact pads) are in contact with the corresponding vias.
[0078] One drawback in the solution shown in FIG. 9A is that the
large continuous conductive areas, such as portions 910 and 920, in
the backsheet means that a relatively large amount of conductive
material (e.g., Cu) is needed to manufacture the backsheet. To
further reduce cost, instead of coupling the electrodes or contact
pads to large sheets of conductive material to achieve electrical
interconnections, some embodiments rely on smaller, continuous
pieces of conductive material to achieve desired circuit
connections. FIG. 9B shows the inter-string connections and the
bypass protection strategy of a solar panel, according to an
embodiment of the invention. In FIG. 9B, the panel layout is
similar to what is shown in FIG. 9A, with solar panel 900 including
a number of larger strings, such as larger strings 912 and 914.
Each larger string includes a number of cascaded strings connected
in series. For example, larger string 912 can include strings 902
and 904 connected in series, and two other strings, also connected
in series. The inter-string serial connections are made through
small and isolated pieces of conductive material within the
conductive interlayer of the backsheet. For example, portion 906
connects strings 902 and 904 in series. On the other hand, instead
of using larger sheets of conductive material, the parallel
connections among the larger strings are achieved using smaller
pieces 930 and 940. In some embodiments, conductive pieces 930 and
940 can be designed to be as small as possible, as long as they can
achieve the desired electrical conductivity. Similarly, contacts
932 and 942, which are located at the back side of the panel, can
provide panel output and enable coupling to bypass diode 950.
Compared to FIG. 9A, the example shown in FIG. 9B relies on a
backsheet that uses significantly less conductive material. In
other words, the backsheet shown in FIG. 9B can be cheaper to make
than the backsheet shown in FIG. 9A. Cheaper backsheets can lead to
reduced overall cost for manufacturing solar panels. The positive
and negative polarities shown in FIGS. 9A and 9B are for exemplary
purposes only. In reality, the locations of the positive and
negative polarities can be arbitrary, and can depend upon the layer
structure and orientation of the cascaded cells.
[0079] FIG. 10 shows the flowchart of manufacturing a solar panel,
according to an embodiment of the invention. The operation starts
with obtaining a backsheet that can include the patterned
conductive interlayer and a number of vias in the top and bottom
insulation layers (operation 1002). The pattern of the conductive
interlayer and locations of vias within the top (cell facing)
insulation layer of the backsheet can be determined based on the
panel layout, such as the interconnections of the cascaded strings
and pre-designed bypass protection strategies. For example, if a
portion of a string is designed to have bypass protection, one or
more vias will be created at appropriate locations within the top
insulation layer to facilitate coupling between the portion of the
string and the bypass diode. Similarly, vias can be created at
appropriate locations to facilitate serial or parallel connections
among the cascaded strings. In addition, vias can also be created
at locations where electrical connections are not needed to
facilitate mechanical bonding. Portions of the conductive
interlayer underneath such vias can be isolated from other
portions. Locations of vias within the bottom (facing outside of
the panel) insulation layer of the backsheet are determined based
on the pattern of the conductive interlayer and the location of the
junction box.
[0080] Subsequently, conductive paste can be applied to fill the
vias within the top insulation layer of the backsheet (operation
1004), and the cascaded strings can be placed on the backsheet
(operation 1006). The placement of the strings can be carefully
controlled to ensure that the contact pads on the strings are
placed above corresponding vias in the backsheet. Once the strings
are placed, the system can apply heat to cure the paste to activate
electrical conductivity (operation 1008). In some embodiments, a
number of heaters can come in contact with photovoltaic structures
at locations of the vias to cure the paste filled in the vias. The
cured paste can provide mechanical bonding and electrical coupling
between the strings and backsheet. Subsequently, the front glass
cover can be applied to seal the strings within the glass cover and
the backsheet (operation 1010). In some embodiments, the
application of the glass cover is performed from beneath, and the
backsheet and the strings are flipped over to allow the glass cover
to be lifted from below to make soft contact with the strings.
[0081] Additional heat and pressure can be applied to laminate the
strings between the glass cover and the backsheet (operation 1012),
and the laminated module can be placed in a frame (operation 1014).
A junction box can then be attached to provide panel output and
bypass protection circuitry (operation 1016). In some embodiments,
attaching the junction box can involve filling vias within the
bottom insulation layer of the backsheet with conductive paste,
connecting lead wires to such vias, and curing the paste.
[0082] In general, compared to conventional approaches for
interconnecting strings and for coupling bypass diodes, embodiments
of the invention provide a solution that achieves inter-string
electrical coupling through a conductive interlayer within the
backsheet of the solar panel, thus eliminating cumbersome wiring
via metal wires (or tabs) at the panel surface. Placing all
electrical connections within the backsheet can reduce shading, and
the elimination of inter-string metal wires (or tabs) can prevent
occurrences of thermal and mechanical stresses introduced by the
wires. Moreover, having all contacts at one side of the strings can
significantly simplify the manufacturing process by eliminating the
need to flip over individual photovoltaic structures, as needed in
the manufacturing of conventional solar panels. When strings are
flipped, they can be flipped as a whole, and the mechanical bonding
provided by a number of paste-filled vias situated between the
contact pads and the conductive interlayer can ensure that the
strings are securely bonded to the backsheet during the flipping
process.
[0083] The foregoing descriptions of various embodiments have been
presented only for purposes of illustration and description. They
are not intended to be exhaustive or to limit the invention to the
forms disclosed. Accordingly, many modifications and variations
will be apparent to practitioners skilled in the art. Additionally,
the above disclosure is not intended to limit the invention.
* * * * *