U.S. patent application number 16/112288 was filed with the patent office on 2018-12-20 for high efficiency configuration for solar cell string.
The applicant listed for this patent is SunPower Corporation. Invention is credited to Gilad ALMOGY, Nathan BECKETT, John GANNON, Ratson MORAD.
Application Number | 20180367095 16/112288 |
Document ID | / |
Family ID | 50621231 |
Filed Date | 2018-12-20 |
United States Patent
Application |
20180367095 |
Kind Code |
A1 |
MORAD; Ratson ; et
al. |
December 20, 2018 |
HIGH EFFICIENCY CONFIGURATION FOR SOLAR CELL STRING
Abstract
A high efficiency configuration for a string of solar cells
comprises series-connected solar cells arranged in an overlapping
shingle pattern. Front and back surface metallization patterns may
provide further increases in efficiency.
Inventors: |
MORAD; Ratson; (Palo Alto,
CA) ; BECKETT; Nathan; (Oakland, CA) ; GANNON;
John; (Oakland, CA) ; ALMOGY; Gilad; (Palo
Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SunPower Corporation |
San Jose |
CA |
US |
|
|
Family ID: |
50621231 |
Appl. No.: |
16/112288 |
Filed: |
August 24, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15371677 |
Dec 7, 2016 |
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16112288 |
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13801432 |
Mar 13, 2013 |
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15371677 |
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13672386 |
Nov 8, 2012 |
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13801432 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/0508 20130101;
H01L 31/02327 20130101; H01L 31/035281 20130101; H01L 31/0747
20130101; H01L 31/028 20130101; H02S 40/36 20141201; Y02E 10/52
20130101; H01L 31/044 20141201; H01L 31/0392 20130101; H01L 31/0201
20130101; H01L 31/042 20130101; H01L 31/048 20130101; H01L 31/02245
20130101; H01L 31/052 20130101; H02S 40/22 20141201; H01L 31/0504
20130101 |
International
Class: |
H02S 40/22 20060101
H02S040/22; H01L 31/0224 20060101 H01L031/0224; H01L 31/05 20060101
H01L031/05; H01L 31/0352 20060101 H01L031/0352; H01L 31/044
20060101 H01L031/044; H01L 31/02 20060101 H01L031/02; H01L 31/042
20060101 H01L031/042; H01L 31/0232 20060101 H01L031/0232; H01L
31/0747 20060101 H01L031/0747; H01L 31/028 20060101 H01L031/028;
H01L 31/0392 20060101 H01L031/0392; H01L 31/048 20060101
H01L031/048; H02S 40/36 20060101 H02S040/36; H01L 31/052 20060101
H01L031/052 |
Claims
1. An apparatus comprising: a plurality of rectangular or
substantially rectangular silicon solar cells arranged in line with
long sides of adjacent silicon solar cells overlapping and
conductively bonded to each other with an electrically conductive
bonding material to electrically connect the silicon solar cells in
series to form a string of silicon solar cells, each silicon solar
cell comprising a front surface to be illuminated by light and an
oppositely positioned rear surface; wherein the front surface of
each silicon solar cell comprises: a plurality of fingers oriented
perpendicularly to the edge of a first long side of the silicon
solar cell and spaced along the first long side with a pitch of
about 0.7 millimeters to about 1.2 millimeters; a bypass conductor
that interconnects two or more of the plurality of fingers; and an
end conductor that interconnects two or more of the plurality of
fingers at their ends opposite from the edge of the first long side
of the silicon solar cell; wherein for each pair of adjacent
silicon solar cells in the string of silicon solar cells the
electrically conductive bonding material is bonded to and
electrically interconnects the fingers on the front surface of one
of the pair of solar cells adjacent the edge of the first long side
of the silicon solar cell to perform a current collecting function
of a bus bar, the bypass conductor on the front surface of the
solar cell provides multiple current paths between the fingers and
the electrically conductive bonding material, and the end conductor
provides additional current paths between the fingers and the
electrically conductive bonding material.
2. The apparatus of claim 1, wherein the electrically conductive
bonding material is an electrically conductive adhesive.
3. The apparatus of claim 1, wherein the electrically conductive
bonding material provides more mechanical compliance than provided
by an electrically conductive solder bond.
4. The apparatus of claim 1, wherein the rear surface of each
silicon solar cell that overlaps a front surface of an adjacent
silicon solar cell comprises a bus bar or plurality of contact pads
conductively bonded to the front surface of the adjacent silicon
solar cell by the electrically conductive bonding material.
5. The apparatus of claim 1, comprising a mechanically compliant
electrical interconnect conductively bonded to the front surface of
one of the silicon solar cells.
6. The apparatus of claim 5, wherein the mechanically compliant
electrical interconnect is electrically connected to a bypass
diode.
7. The apparatus of claim 1, comprising a mechanically compliant
electrical interconnect conductively bonded to the rear surface of
one of the silicon solar cells.
8. The apparatus of claim 7, wherein the mechanically compliant
electrical interconnect is electrically connected to a bypass
diode.
9. The apparatus of claim 1, comprising a mechanically compliant
electrical interconnect conductively bonded to the rear surface of
one of the silicon solar cells located at an intermediate position
along the string of silicon solar cells.
10. The apparatus of claim 9, wherein the mechanically compliant
electrical interconnect is electrically connected to a bypass
diode.
11. The apparatus of claim 1, comprising another bypass conductor
arranged in line with the bypass conductor and interconnecting
another two or more fingers to provide multiple current paths from
each of the other two or more interconnected fingers to the
electrically conductive bonding material.
12. The apparatus of claim 1, wherein the end conductor has a width
perpendicular to its long axis that is the same as the width of a
finger.
13. The apparatus of claim 1, wherein the front surface of each
silicon solar cell comprises a bus bar or a plurality of contact
pads positioned adjacent to and running parallel to the edge of the
first long side of the silicon solar cell between the bypass
conductor and the edge, and for each pair of adjacent silicon solar
cells in the string of silicon solar cells the electrically
conductive bonding material is bonded to the bus bar or plurality
of contact pads.
14. The apparatus of claim 13, wherein the bus bar or plurality of
contact pads have a width perpendicular to the edge of the first
long side that is the same as the width of one of the plurality of
fingers.
15. The apparatus of claim 1, wherein the front surfaces of the
silicon solar cells do not comprise bus bars or a plurality of
contact pads running parallel to the edge of the first long side of
the silicon solar cell between the bypass conductor and the
edge.
16. The apparatus of claim 1, wherein the ratio of the length of a
long side of the rectangular or substantially rectangular silicon
solar cells to the length of a short side of the rectangular or
substantially rectangular silicon solar cells is greater than or
equal to three.
17. The apparatus of claim 1, sandwiched between a transparent top
sheet and a glass substrate.
18. The apparatus of claim 1, wherein the front surface of each
silicon solar cell comprises about 125 to about 225 fingers spaced
along the first long side.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 15/371,677 filed Dec. 7, 2016 and titled "High
Efficiency Configuration For Solar Cell String", which is a
continuation of U.S. patent application Ser. No. 13/801,432 filed
Mar. 13, 2013 and titled "High Efficiency Configuration For Solar
Cell String". U.S. patent application Ser. No. 13/801,432 is a
continuation-in-part of U.S. patent application Ser. No. 13/672,386
filed Nov. 8, 2012 and titled "High Efficiency Configuration For
Solar Cell String", and also claims benefit of priority to U.S.
Provisional Application No. 61/734,239 filed Dec. 6, 2012 and
titled "High Efficiency Configuration For Solar Cell String". Each
of the patent applications identified in this paragraph is
incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates generally to solar cells and to their
use in concentrating solar energy collectors.
BACKGROUND
[0003] Alternate sources of energy are needed to satisfy ever
increasing world-wide energy demands. Solar energy resources are
sufficient in many geographical regions to satisfy such demands, in
part, by provision of electric power generated with solar (e.g.,
photovoltaic) cells.
SUMMARY
[0004] High efficiency arrangements of solar cells are disclosed
herein. Solar cells and strings of solar cells as disclosed herein
may be particularly valuable in concentrating photovoltaic systems,
in which mirrors or lenses concentrate sunlight onto a photovoltaic
cell to light intensities greater than one "sun."
[0005] In one aspect, a solar cell comprises a silicon
semiconductor diode structure having rectangular or substantially
rectangular front and back surfaces that have shapes defined by
first and second oppositely positioned long sides of the solar cell
and two oppositely positioned short sides of the solar cell. In
operation, the front surface is to be illuminated by light. The
solar cell comprises an electrically conducting front surface
metallization pattern disposed on the front surface. This
metallization pattern includes a plurality of fingers running
parallel to the short sides of the solar cell for substantially the
length of the short sides. An electrically conducting back surface
metallization pattern is disposed on the back surface.
[0006] In some variations, the front surface metallization pattern
does not include any bus bar interconnecting the fingers to collect
current from the front surface of the solar cell. In such
variations, the back surface metallization pattern may lack any
contact pad conventionally prepared for solder connections to the
solar cell. Alternatively, the back surface metallization pattern
may include, for example, a contact pad positioned adjacent to and
running parallel to a long side of the solar cell for substantially
the length of the long side, or two or more discrete contact pads
positioned adjacent to and arranged parallel to the long side.
[0007] In some variations, the front surface metallization pattern
comprises only a single bus bar, which is positioned adjacent to
and runs parallel to the first long side for substantially the
length of the first long side. The fingers of the front
metallization pattern are attached to and interconnected by the bus
bar. In such variations, the back surface metallization pattern may
lack any contact pad. Alternatively, the back surface metallization
pattern may include, for example, a contact pad positioned adjacent
to and running parallel to the second long side for substantially
the length of the second long side, or two or more discrete contact
pads positioned adjacent to and arranged parallel to the second
long side. These contact pads may have widths measured
perpendicular to the long sides that approximately match the width
of the bus bar, for example. In any of these variations the front
surface metallization pattern may include a bypass conductor that
has a width perpendicular to its long axis narrower than the width
of the bus bar and that interconnects two or more fingers to
provide multiple current paths from each of the two or more
interconnected fingers to the bus bar. The bypass conductor may be
positioned adjacent to and run parallel to the bus bar, for
example.
[0008] In some variations, the front surface metallization pattern
comprises two or more discrete contact pads positioned adjacent to
the first long side. Each of the fingers of the front metallization
pattern is attached and electrically connected to at least one of
the contact pads. In such variations, the back surface
metallization pattern may lack any contact pad. Alternatively, the
back surface metallization pattern may include, for example, a
contact pad positioned adjacent to and running parallel to the
second long side for substantially the length of the second long
side, or two or more discrete contact pads positioned adjacent to
and arranged parallel to the second long side. These contact pads
may have widths measured perpendicular to the long sides that
approximately match the width of the contact pads in the front
surface metallization pattern, for example. In any of these
variations the front surface metallization pattern may include a
bypass conductor that has a width perpendicular to its long axis
narrower than the widths of the front surface metallization contact
pads and that interconnects two or more fingers to provide multiple
current paths from each of the two or more interconnected fingers
to one or more of the contact pads.
[0009] In any of the above variations, the solar cell may comprise
any suitable silicon semiconductor diode structure. For example,
the solar cell may comprise a heterojunction with intrinsic thin
layer (HIT) structure.
[0010] In any of the above variations, the ratio of the length of a
long side of the solar cell to the length of a short side of the
solar cell may be greater than or equal to about three, for
example.
[0011] A concentrating solar energy collector may comprise the
solar cell of any of the above variations and one or more optical
elements arranged to concentrate solar radiation onto the solar
cell.
[0012] In another aspect, a string of solar cells comprises at
least a first silicon solar cell and a second silicon solar cell.
The first silicon solar cell comprises a front surface to be
illuminated by light, a back surface, and an electrically
conducting front surface metallization pattern disposed on the
front surface. The second silicon solar cell comprises a front
surface to be illuminated by light, a back surface, and an
electrically conductive back surface metallization pattern disposed
on the back surface. The first and second silicon solar cells are
positioned with an edge of the back surface of the second silicon
solar cell overlapping an edge of the front surface of the first
silicon solar cell. A portion of the front surface metallization
pattern of the first silicon solar cell is hidden by the second
silicon solar cell and bonded to a portion of the back surface
metallization pattern of the second silicon solar cell with an
electrically conductive bonding material to electrically connect
the first and second silicon solar cells in series.
[0013] Either or both of the first and second silicon solar cells
may be, for example, any of the variations of the silicon solar
cell summarized above. In such variations, the overlapping edges of
the silicon solar cells may be defined by long sides of the solar
cells, for example, and the edges may be arranged parallel to each
other. If the front surface metallization pattern of the first
silicon solar cell includes a bypass conductor, the bypass
conductor may either be hidden, or not hidden, by the second
silicon solar cell.
[0014] The first and second silicon solar cells may be bonded to
each other at the overlapping portions of the solar cells with an
electrically conductive solder. As an alternative to solder, the
solar cells may instead be bonded to each other with, for example,
an electrically conductive film, an electrically conductive paste,
an electrically conductive epoxy (e.g., an electrically conductive
silver-filled epoxy), an electrically conductive tape, or another
suitable electrically conductive adhesive. These alternatives to
solder may be selected, for example, to provide more mechanical
compliance than would be provided by an electrically conductive
solder bond. The electrically conductive bonding material bonding
the solar cells to each other may also interconnect fingers of the
front surface metallization pattern to perform the current
collecting function of a bus bar. The front surface metallization
pattern on the solar cells may thus lack any such bus bar.
[0015] A concentrating solar energy collector may comprise the
string of solar cells of any of the above variations and one or
more optical elements arranged to concentrate solar radiation onto
the string.
[0016] In another aspect, a solar energy receiver comprises a metal
substrate and a series-connected string of two or more solar cells
disposed on the metal substrate with ends of adjacent solar cells
overlapping in a shingle pattern. Adjacent overlapping pairs of
solar cells may be electrically connected in a region where they
overlap by an electrically conducting bond between the front
surface of one of the solar cells and the back surface of the other
solar cell. The electrically conducting bond may be between a
metallization pattern on the front surface of one solar cell and a
metallization pattern on the back surface of the other solar cell,
for example. The solar cells may be, for example, silicon solar
cells, including any of the variations of the silicon solar cells
summarized above or any of the variations of the back-contact
silicon solar cells described below, or solar cells similarly
configured to any of those variations but utilizing another
material system other than or in addition to silicon. The
electrically conducting bond between the solar cells may be formed,
for example, by any of the methods summarized above. The solar
cells may be disposed in a lamination stack that adheres to the
metal substrate, for example.
[0017] In some variations, the metal substrate is linearly
elongated, each of the solar cells is linearly elongated, and the
string of solar cells is arranged in a row along a long axis of the
metal substrate with long axes of the solar cells oriented
perpendicular to the long axis of the metal substrate. This row of
solar cells may be the only row of solar cells on the
substrate.
[0018] In some variations, the series-connected string of solar
cells is a first string of solar cells, and the solar energy
receiver comprises a second series-connected string of two or more
solar cells arranged with ends of adjacent solar cells overlapping
in a shingle pattern. The second string of solar cells is also
disposed on the metal substrate. A mechanically compliant
electrical interconnect may electrically couple the back surface of
a solar cell at an end of the first string of solar cells to the
front surface of a solar cell at an end of the second string of
solar cells. The interconnection may be between a metallization
pattern on the front surface of one solar cell and a metallization
pattern on the back surface of the other solar cell, for example.
The solar cell at the end of the first string of solar cells may
overlap the solar cell at the end of the second string of solar
cells and hide the mechanically compliant electrical interconnect
from view from the front (illuminated) surface side of the solar
cells. In such variations, the metal substrate may be linearly
elongated, each of the solar cells may be linearly elongated, and
the first and second strings of solar cells may be arranged in line
in a row along a long axis of the metal substrate with long axes of
the solar cells oriented perpendicularly to the long axis of the
metal substrate.
[0019] A concentrating solar energy collector may comprise the
solar energy receiver of any of the above variations and one or
more optical elements arranged to concentrate solar radiation onto
the receiver.
[0020] In another aspect, a string of solar cells comprises a first
group of solar cells arranged with ends of adjacent solar cells
overlapping in a shingle pattern and connected in series by
electrical connections between solar cells made in the overlapping
regions of adjacent solar cells, a second group of solar cells
arranged with ends of adjacent solar cells overlapping in a shingle
pattern and connected in series by electrical connections between
solar cells made in the overlapping regions of adjacent solar
cells, and a mechanically compliant electrical interconnect
electrically coupling the first group of solar cells to the second
group of solar cells in series. The mechanically compliant
electrical interconnect may electrically couple the back surface of
a solar cell at an end of the first group of solar cells to a front
surface of a solar cell at an end of the second group of solar
cells, for example. The interconnection may be between a
metallization pattern on the front surface of one solar cell and a
metallization pattern on the back surface of the other solar cell,
for example. The mechanically compliant electrical interconnect may
be bonded to the solar cells with electrically conducting bonds
made by any of the methods summarized above, for example.
[0021] The solar cells may be, for example, silicon solar cells,
including any of the variations of the silicon solar cells
summarized above or any of the variations of the back-contact
silicon solar cells described below, or solar cells similarly
configured to any of those variations but utilizing another
material system other than or in addition to silicon. The
electrical connections between overlapping solar cells may be made,
for example, with electrically conducting bonds made by any of the
methods summarized above.
[0022] The first and second groups of solar cells may be arranged
in line in a single row. In such variations, a gap between the two
groups of solar cells where they are interconnected by the
mechanically compliant electrical interconnect may have a width
less than or equal to about five millimeters, for example. Also in
such variations, the mechanically compliant electrical interconnect
may comprise a metal ribbon oriented perpendicularly to a long axis
of the row of solar cells and electrically coupled to a back
surface on a solar cell at an end of the first group of solar cells
and to a front surface on a solar cell at an end of the second
group of solar cells.
[0023] The mechanically compliant electrical interconnect in any of
the above variations may comprise a metal ribbon patterned with
slits or openings, for example, to increase its mechanical
compliance.
[0024] In any of the above variations, the solar cell at the end of
the first group of solar cells may overlap the solar cell at the
end of the second group of solar cells and hide the mechanically
compliant electrical interconnect from view from the front surface
side of the string of solar cells.
[0025] A concentrating solar energy collector may comprise the
string of solar cells of any of the above variations and one or
more optical elements arranged to concentrate solar radiation onto
the string.
[0026] In another aspect, a string of solar cells comprises at
least a first solar cell and a second solar cell. The first solar
cell comprises a front surface to be illuminated by light, a back
surface, and (optionally) an electrically conducting front surface
metallization pattern disposed on the front surface. The second
solar cell comprises a front surface to be illuminated by light, a
back surface, and an electrically conductive back surface
metallization pattern disposed on the back surface. The string of
solar cells also comprises at least a first mechanically compliant
electrical interconnect. The first and second solar cells are
positioned with an edge of the back surface of the second solar
cell overlapping an edge of the front surface of the first solar
cell. The mechanically compliant electrical interconnect is bonded
to a portion of the front surface of the first solar cell that is
hidden by the second solar cell and bonded to a portion of the back
surface of the second solar cell to electrically connect the first
and second solar cells in series. In this arrangement the second
solar cell hides the mechanically compliant electrical interconnect
from view from the front surface side of the first solar cell. The
interconnection may be between a metallization pattern on the front
surface of one solar cell and a metallization pattern on the back
surface of the other solar cell, for example.
[0027] Either or both of the first and second solar cells may be,
for example, any of the variations of the silicon solar cells
summarized above or any of the variations of the back-contact
silicon solar cells described below, or solar cells similarly
configured to any of those variations but utilizing another
material system other than or in addition to silicon. In such
variations, the overlapping edges of the solar cells may be defined
by long sides of the solar cells, for example, and the edges may be
arranged parallel to each other. If the first solar cell comprises
a front surface metallization pattern that includes a bypass
conductor, the bypass conductor may either be hidden, or not
hidden, by the second solar cell.
[0028] The mechanically compliant electrical interconnect may be
bonded to the solar cells with electrically conducting bonds made
by any of the methods summarized above, for example. The
electrically conductive bonds may interconnect fingers of a front
surface metallization pattern on the first solar cell, if present,
to perform the current collecting function of a bus bar. A front
surface metallization pattern on the solar cell may thus lack any
such bus bar.
[0029] The mechanically compliant electrical interconnect may
comprise, for example, a flat metal ribbon, a bent metal ribbon, or
a metal ribbon bent to form a loop. The mechanically compliant
electrical interconnect may comprise a metal ribbon patterned to
increase its mechanical compliance.
[0030] The string of solar cells may comprise a second mechanically
compliant electrical interconnect and a third solar cell having a
front surface to be illuminated by light, a back surface, and an
electrically conducting back surface metallization pattern disposed
on the back surface. The second and third solar cells are
positioned with an edge of the back surface of the third solar cell
overlapping an edge of the front surface of the second silicon
solar cell. The mechanically compliant electrical interconnect is
bonded to a portion of the front surface of the second solar cell
that is hidden by the third solar cell and bonded to a portion of
the back surface of the third solar cell to electrically connect
the second and third solar cells in series. The interconnection may
be between a metallization pattern on the front surface of the
second solar cell and a metallization pattern on the back surface
of the third solar cell, for example. The mechanically compliant
electrical interconnect may be bonded to the solar cells with
electrically conducting bonds made by any of the methods summarized
above, for example. The electrically conductive bonds may
interconnect fingers of a front surface metallization pattern of
the second solar cell to perform the current collecting function of
a bus bar. A front surface metallization pattern on the solar cell
may thus lack any such bus bar.
[0031] A concentrating solar energy collector may comprise the
string of solar cells of any of the above variations and one or
more optical elements arranged to concentrate solar radiation onto
the string.
[0032] In another aspect, a solar energy receiver comprises a
substrate, a thermally conductive encapsulant layer adhering to the
substrate, a string of solar cells disposed on the thermally
conductive encapsulant layer, a clear encapsulant layer disposed on
the string of solar cells, and a clear top sheet disposed on the
clear encapsulant layer. The thermally conductive encapsulant layer
comprises pigments. The solar cells may be, for example, any of the
variations of the silicon solar cells summarized above or any of
the variations of the back-contact silicon solar cells described
below, or solar cells similarly configured to any of those
variations but utilizing another material system other than or in
addition to silicon.
[0033] The thermally conductive encapsulant layer may reflect a
substantial portion of solar radiation incident on it. In such
variations, the thermally conductive encapsulant layer may be
white, for example. Further, in such variations the solar cells may
be HIT solar cells, with the reflective encapsulant layer arranged
to reflect toward the HIT cell solar radiation that passed
unabsorbed through the HIT cell to the reflective layer.
Alternatively, the thermally conductive encapsulant layer may
absorb a substantial portion of solar radiation incident on it. In
such variations, the thermally conductive encapsulant layer may be
black, for example. The clear top sheet may have a moisture
transmission rate of less than or equal to about 0.01 grams per
meter-day, for example. The string of solar cells may comprise a
plurality of solar cells arranged with ends of adjacent solar cells
overlapping in a shingle pattern.
[0034] A concentrating solar energy collector may comprise the
solar energy receiver of any of the above variations and one or
more optical elements arranged to concentrate solar radiation onto
the receiver.
[0035] In another aspect, a back-contact silicon solar cell
comprises a front surface to be illuminated by light, a back
surface, one or more n-contacts on the back surface that
electrically contact an n-conductivity type side of a silicon diode
junction, one or more p-contacts on the back surface that
electrically contact a p-conductivity type side of the silicon
diode junction, and one or more electrically conducting vias. The
electrically conducting vias pass through the solar cell from the
back surface to the front surface to provide near an edge of the
front surface one or more electrical connections to either the
p-contacts or the n-contacts.
[0036] The front and back surfaces may have corresponding
rectangular or substantially rectangular shapes defined by two
oppositely positioned long sides and two oppositely positioned
short sides, with upper ends of the vias arranged along a long side
of the front surface. In some such variations, the n-contacts
comprise a plurality of n-fingers arranged side-by-side and running
parallel to the short sides of the back surface, the p-contacts
comprise a plurality of p-fingers arranged side-by-side and running
parallel to the short sides of the back surface, and the n-fingers
and the p-fingers are interdigitated. In other variations, the
n-contacts comprise a plurality of n-fingers arranged side-by-side
and running parallel to each other at an angle to the short sides
of the back surface such that opposite ends of each n-finger are
offset in a direction parallel to the long sides by a distance
equal to a pitch distance between n-fingers, the p-contacts
comprise a plurality of p-fingers arranged side-by-side and running
parallel to each other at an angle to the short sides of the back
surface such that opposite ends of each p-finger are offset in a
direction parallel to the long sides by a distance equal to a pitch
distance between p-fingers, and the n-fingers and the p-fingers are
interdigitated.
[0037] In other variations, upper ends of the vias may be arranged
along a short side of the front surface, and the n-fingers and
p-fingers may be similarly configured to as summarized above except
for running parallel to, or at an angle to, the long sides of the
back surface. In yet other variations the back-contact solar cell
may be substantially square, with vias and fingers arranged
similarly to as summarized above and running parallel to or at an
angle to one pair of sides of the solar cell.
[0038] In any of the above variations, the back contact solar cell
may comprise a bus bar or a plurality of contact pads on the front
surface that electrically interconnect upper ends of the vias.
[0039] A concentrating solar energy collector may comprise the
back-contact solar cell of any of the above variations and one or
more optical elements arranged to concentrate solar radiation onto
the solar cell.
[0040] In another aspect, a string of solar cells comprises a first
back-contact silicon solar cell comprising a front surface to be
illuminated by light, a back surface, one or more n-contacts on the
back surface that electrically contact an n-conductivity type side
of a diode junction, one or more p-contacts on the back surface
that electrically contact a p-conductivity type side of the diode
junction, and a second back-contact silicon solar cell comprising a
front surface to be illuminated by light, a back surface, one or
more n-contacts on the back surface that electrically contact an
n-conductivity type side of a diode junction, and one or more
p-contacts on the back surface that electrically contact a
p-conductivity type side of the diode junction. The first and
second back-contact silicon solar cells are positioned with an edge
of the back surface of the second back-contact silicon solar cell
overlapping an edge of the front surface of the first back-contact
silicon solar cell and electrically connected in series.
[0041] The back-contact silicon solar cells may be, for example,
any of the variations of back-contact silicon solar cells
summarized above.
[0042] In some variations, the first back-contact silicon solar
cell comprises one or more electrically conducting vias that pass
through the solar cell from its back surface to its front surface
to electrically interconnect either the p-contacts or the
n-contacts of the first back-contact silicon solar cell to contacts
of opposite polarity on the back surface of the second back-contact
silicon solar cell. Upper ends of the conducting vias may be
located, for example, in a region of the front surface of the first
back-contact silicon solar cell that is overlapped by the second
back-contact silicon solar cell. The conducting vias may be
electrically connected to the contacts on the back surface of the
second silicon solar cell by one or more electrically conductive
bonds between the front surface of the first back-contact silicon
solar cell and the back surface of the second back-contact silicon
solar cell. The electrically conductive bonds may be made by any of
the methods summarized above, for example. The first back-contact
silicon solar cell may optionally comprise a bus bar or a plurality
of contact pads on its front surface that electrically interconnect
upper ends of the vias to each other, and that are electrically
connected to the contacts on the back surface of the second
back-contact silicon solar cell by the one or more electrically
conductive bonds.
[0043] In other variations, a mechanically compliant electrical
interconnect electrically connects either the p-contacts or the
n-contacts on the back surface of the first back-contact silicon
solar cell to electrical contacts of opposite polarity on the back
surface of the second back-contact silicon solar cell. The
mechanically compliant electrical interconnect may be bonded to the
solar cells with electrically conducting bonds made by any of the
methods summarized above, for example.
[0044] A concentrating solar energy collector may comprising the
string of solar cells of any of the variations described above and
one or more optical elements arranged to concentrate solar
radiation onto the solar cell.
[0045] In another aspect, a solar energy receiver comprises a
substrate, and a series-connected string of two or more solar cells
disposed on the substrate with ends of adjacent solar cells
overlapping in a shingle pattern. The linear coefficient of thermal
expansion of the solar cells differs from that of the substrate by
greater than or equal to about 5.times.10.sup.-6, or by greater
than or equal to about 10.times.10.sup.-6, or by greater than or
equal to about 15.times.10.sup.-6, or by greater than or equal to
about 20.times.10.sup.-6.
[0046] The solar cells may be silicon solar cells, for example. The
solar cells may be, for example, any of the variations of silicon
solar cells summarized above, including variations of HIT and
back-contact silicon solar cells, or solar cells similarly
configured to any of those variations but utilizing another
material system other than or in addition to silicon.
[0047] Adjacent overlapping pairs of solar cells in the string may
be electrically connected in series in a region where they overlap
by an electrically conducting bond between a front surface of one
of the solar cells and a back surface of the other solar cell. Such
electrically conducting bonds may be formed by any of the methods
summarized above, for example. Alternatively, adjacent overlapping
pairs of solar cells may be electrically connected in series in a
region where they overlap by a mechanically compliant electrical
interconnect between a front surface of one of the solar cells and
a back surface of the other solar cell. The mechanically compliant
electrical interconnects may be bonded to the solar cells with
electrically conducting bonds made by any of the methods summarized
above, for example.
[0048] The substrate may be a metal substrate, for example. The
substrate may be an aluminum substrate, for example.
[0049] In some variations, the metal substrate is linearly
elongated, each of the solar cells is linearly elongated, and the
string of solar cells is arranged in a row along a long axis of the
substrate with long axes of the solar cells oriented perpendicular
to the long axis of the substrate. In such variations the string of
solar cells may be a first string of solar cells, and the solar
energy receiver may also comprise a second series-connected string
of two or more solar cells disposed on the substrate with ends of
adjacent solar cells overlapping in a shingle pattern, and a
mechanically compliant electrical interconnect that electrically
connects the first and second strings in series. The linear
coefficient of thermal expansion of solar cells in the second
string may also differ from that of the substrate by greater than
or equal to about 5.times.10.sup.-6, or by greater than or equal to
about 10.times.10.sup.-6, or by greater than or equal to about
15.times.10.sup.-6, or by greater than or equal to about
20.times.10.sup.-6. The second string may be positioned in line
with the first string. Overlapping pairs of solar cells in the
second string may be bonded to each other or otherwise
interconnected as summarized above for the first string, for
example.
[0050] A concentrating solar energy collector may comprise the
solar energy receiver of any of the variations summarized above and
one or more optical elements arranged to concentrate solar
radiation onto the receiver.
[0051] In another aspect, a method of laminating solar cells to a
substrate comprises arranging a plurality of solar cells to form a
series-connected string of solar cells with ends of adjacent solar
cells overlapping in a shingle pattern, disposing the string of
solar cells in a stack of layers on the substrate, and applying a
pressure not greater than about 0.6 atmospheres to force the stack
of layers and the substrate together. The pressure may be, for
example, less than or equal to about 0.4 atmospheres. The pressure
may be, for example, between about 0.2 and about 0.6 atmospheres.
The method may comprise heating the substrate, the stack of layers,
or the substrate and the stack of layers to a temperature of
between about 130.degree. C. and about 160.degree. C. while
applying the pressure. This method may be used with any of the
variations of solar cells, and any of the variations of
series-connected strings of overlapping solar cells, summarized
above.
[0052] In another aspect, a method of preparing a string of solar
cells comprises arranging a plurality of solar cells with ends of
adjacent solar cells overlapping in a shingled manner and with an
uncured electrically conductive epoxy disposed between overlapped
portions of adjacent solar cells in locations selected to
series-connect the solar cells. The method also comprises applying
a pressure to force overlapping ends of the solar cells against
each other while elevating a temperature of the solar cells to cure
the electrically conductive epoxy to form electrically conductive
bonds between the solar cells. In some variations, after the
electrically conductive epoxy is cured, the string of solar cells
is disposed in a stack of layers on a substrate that is then
laminated to the substrate. In other variations, the string of
solar cells is disposed in a stack of layers on a substrate before
the electrically conductive epoxy is cured. The stack is then
laminated to the substrate. The electrically conductive epoxy is
cured (under pressure) during the lamination process. This method
may be used with any of the variations of solar cells summarized
above.
[0053] In any of the strings of overlapping solar cells summarized
above, the amount of overlap between adjacent solar cells may vary
along the string so that the size of the area of the front surface
of each solar cell that is not overlapped by an adjacent solar cell
varies through the string in a manner that matches the electrical
performance of the solar cells. For example, the different sizes of
illuminated (i.e., not overlapped) area for each solar cell may be
selected to compensate for inherent performance differences between
the cells to thereby match the current output by each cell when
under equal illumination.
[0054] Any of the strings of overlapping solar cells summarized
above may be positioned for operation in a solar energy collector
with the string oriented so that for each solar cell that has a
portion of its front surface overlapped by another solar cell, the
overlapped front surface portion is closer to the earth's equator
than is the uncovered front surface portion. With the string in
this orientation, exposed edges of the upper overlapping solar
cells are oriented away from the earth's equator.
[0055] Any of the variations of silicon solar cells summarized
above may be formed from or comprise, for example, mono-crystalline
or poly-crystalline silicon.
[0056] These and other embodiments, features and advantages of the
present invention will become more apparent to those skilled in the
art when taken with reference to the following more detailed
description of the invention in conjunction with the accompanying
drawings that are first briefly described.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] FIG. 1A shows a schematic diagram of an example front
surface metallization pattern for a solar cell.
[0058] FIG. 1B shows a schematic diagram of an example back surface
metallization pattern that may be used, for example, for a solar
cell having the front surface metallization pattern of FIG. 1A.
[0059] FIG. 1C shows a schematic diagram of an example back surface
metallization pattern for a back contact solar cell in which
contacts to both sides of the diode junction are made on the back
surface and in which vias pass through the cell from the back
surface to the front surface to provide electrical connection at an
edge of the front surface to one side of the diode junction.
[0060] FIG. 1D shows an example front surface metallization pattern
for a back contact solar cell in which vias pass through the cell
from the back surface to the front surface to provide electrical
connections from one side of the diode junction to a bus bar along
an edge of the front surface.
[0061] FIG. 1E shows a perspective view of an example back contact
solar cell employing the example front surface and back surface
metallization patterns of FIG. 1C and FIG. 1D, respectively.
[0062] FIG. 1F shows another example back surface metallization
pattern for a back contact solar cell in which contacts to both
sides of the diode junction are made on the back surface and in
which vias pass through the cell from the back surface to the front
surface to provide electrical connection at an edge of the front
surface to one side of the diode junction.
[0063] FIG. 2 shows a fragmentary view schematically illustrating
one end of an example solar energy receiver that comprises a string
of series-connected solar cells arranged in an overlapping manner
on a linearly elongated substrate. Each solar cell has the front
surface metallization pattern illustrated in FIG. 1A.
[0064] FIG. 3A shows a schematic cross-sectional diagram
illustrating the overlap of adjacent solar cells in the string of
solar cells shown in FIG. 2.
[0065] FIG. 3B shows a schematic cross-sectional diagram
illustrating the overlap of adjacent back contact solar cells, with
an electrical interconnection between the back surfaces of
overlapping solar cells made with a flexible electrical
interconnect.
[0066] FIG. 4 shows a schematic diagram of an example string of
solar cells including a first group of overlapped solar cells
electrically connected to a second group of overlapped solar cells
by an electrically conductive mechanically compliant
interconnect.
[0067] FIG. 5A shows a schematic diagram of the example
mechanically compliant interconnect used in the string of solar
cells illustrated in FIG. 4.
[0068] FIG. 5B shows a schematic diagram of another example
mechanically compliant interconnect that may be used, for example,
in place of the interconnect shown in FIG. 5A.
[0069] FIGS. 6A-6C show schematic cross-sectional diagrams
illustrating additional examples of series-connected strings of
overlapping solar cells.
[0070] FIGS. 7A and 7B show front and rear views, respectively, of
another example series-connected string of overlapping solar
cells.
[0071] FIGS. 8A and 8B show front and rear views, respectively, of
another example series-connected string of overlapping solar
cells.
[0072] FIG. 9 shows a rear view of another example series-connected
string of overlapping solar cells.
[0073] FIG. 10 shows a fragmentary schematic diagram of an example
lamination stack, comprising solar cells, disposed on and adhering
to a substrate.
[0074] FIG. 11 shows a schematic diagram of an example bypass diode
flex circuit that may be employed, for example, with shingled solar
cells as described in this specification.
DETAILED DESCRIPTION
[0075] The following detailed description should be read with
reference to the drawings, in which identical reference numbers
refer to like elements throughout the different figures. The
drawings, which are not necessarily to scale, depict selective
embodiments and are not intended to limit the scope of the
invention. The detailed description illustrates by way of example,
not by way of limitation, the principles of the invention. This
description will clearly enable one skilled in the art to make and
use the invention, and describes several embodiments, adaptations,
variations, alternatives and uses of the invention, including what
is presently believed to be the best mode of carrying out the
invention.
[0076] As used in this specification and the appended claims, the
singular forms "a," "an," and "the" include plural referents unless
the context clearly indicates otherwise. Also, the term "parallel"
is intended to mean "parallel or substantially parallel" and to
encompass minor deviations from parallel geometries rather than to
require that any parallel arrangements described herein be exactly
parallel. The term "perpendicular" is intended to mean
"perpendicular or substantially perpendicular" and to encompass
minor deviations from perpendicular geometries rather than to
require that any perpendicular arrangement described herein be
exactly perpendicular.
[0077] This specification discloses high efficiency configurations
for solar cell strings as well as solar cells (e.g., photovoltaic
cells), and electrically conductive interconnects for solar cells,
that may be used in such strings. As further described below, the
high efficiency configuration strings may be advantageously
employed in concentrating solar energy collectors in which solar
radiation is concentrated onto the solar cells with reflectors,
lenses, or other optical components. Such collectors may
concentrate light onto the solar cells to provide illumination
greater than or equal to about seven "suns", for example.
[0078] FIG. 1A shows a schematic diagram of an electrically
conducting front surface metallization pattern on the front surface
of an example solar cell 10. The front surface of solar cell 10 is
rectangular or substantially rectangular. Other shapes may also be
used, as suitable. The front surface metallization pattern includes
a bus bar 15 positioned adjacent to the edge of one of the long
sides of solar cell 10 and running parallel to the long sides for
substantially the length of the long sides, and fingers 20 attached
perpendicularly to the bus bar and running parallel to each other
and to the short sides of solar cell 10 for substantially the
length of the short sides.
[0079] Solar cell 10 comprises a semiconductor diode structure on
which the front surface metallization pattern is disposed. A back
surface metallization pattern is disposed on a back surface of
solar cell 10 as shown, for example, in FIG. 1B and described
further below. The semiconductor structure may be, for example, a
conventional crystalline silicon diode structure comprising an n-p
junction, with the top semiconductor layer on which the front
surface metallization is disposed being, for example, of either
n-type or p-type conductivity. Any other suitable semiconductor
diode structure in any other suitable material system may also be
used.
[0080] Referring now to FIG. 1B, an electrically conducting back
surface metallization pattern on the back surface of solar cell 10
comprises back contact 25, and back contact pad 30 positioned
adjacent to the edge of one of the long sides of solar cell 10 and
running parallel to the long sides for substantially the length of
the long sides. FIG. 1B shows the back side of solar cell 10 as if
it were viewed through the front surface of solar cell 10. As shown
by a comparison of FIG. 1A and FIG. 1B, back contact pad 30 and
front surface bus bar 15 are positioned along opposite long sides
of solar cell 10.
[0081] The front and back surface metallization patterns on solar
cell 10 provide electric contacts to the semiconductor diode
structure by which electric current generated in solar cell 10 when
it is illuminated by light may be provided to an external load. In
addition, the illustrated front and back surface metallization
patterns allow two such solar cells 10 to be positioned in an
overlapping geometry with their long sides parallel to each other
and with the back contact pad 30 of one of the solar cells
overlapping and physically and electrically connected to the front
surface bus bar 15 of the other solar cell. As further described
below, this pattern may be continued, in a manner similar to
shingling a roof, to construct a string of two or more overlapping
solar cells 10 electrically connected in series. Such an
arrangement is referred to below as, for example, series-connected
overlapping solar cells.
[0082] In the illustrated example solar cell 10 has a length of
about 156 millimeters (mm), a width of about 26 mm, and thus an
aspect ratio (length of short side/length of long side) of about
1:6. Six such solar cells may be prepared on a standard 156
mm.times.156 mm dimension silicon wafer, then separated (diced) to
provide solar cells as illustrated. In other variations, eight
solar cells 10 having dimensions of about 19.5 mm.times.156 mm, and
thus an aspect ratio of about 1:8, may be prepared from a standard
silicon wafer. More generally, solar cells 10 may have aspect
ratios of, for example, about 1:3 to about 1:20 and may be prepared
from standard size wafers or from wafers of any other suitable
dimensions. As further explained below, solar cells having long and
narrow aspect ratios, as illustrated, may be advantageously
employed in concentrating photovoltaic solar energy collectors in
which solar radiation is concentrated onto the solar cells.
[0083] Referring again to FIG. 1A, in the illustrated example the
front surface metallization pattern on solar cell 10 also comprises
an optional bypass conductor 40 running parallel to and spaced
apart from bus bar 15. Bypass conductor 40 interconnects fingers 20
to electrically bypass cracks that may form between bus bar 15 and
bypass conductor 40. Such cracks, which may sever fingers 20 at
locations near to bus bar 15, may otherwise isolate regions of
solar cell 10 from bus bar 15. The bypass conductor provides an
alternative electrical path between such severed fingers and the
bus bar. A bypass conductor 40 may have a width, for example, of
less than or equal to about 1 mm, less than or equal to about 0.5
mm, or between about 0.05 mm and about 0.5 mm. The illustrated
example shows a bypass conductor 40 positioned parallel to bus bar
15, extending about the full length of the bus bar, and
interconnecting every finger 20. This arrangement may be preferred
but is not required. If present, the bypass conductor need not run
parallel to the bus bar and need not extend the full length of the
bus bar. Further, a bypass conductor interconnects at least two
fingers, but need not interconnect all fingers. Two or more short
bypass conductors may be used in place of a longer bypass
conductor, for example. Any suitable arrangement of bypass
conductors may be used. The use of such bypass conductors is
described in greater detail in U.S. patent application Ser. No.
13/371,790, titled "Solar Cell With Metallization Compensating For
Or Preventing Cracking," and filed Feb. 13, 2012, which is
incorporated herein by reference in its entirety.
[0084] The example front surface metallization pattern of FIG. 1A
also includes an optional end conductor 42 that interconnects
fingers 20 at their far ends, opposite from bus bar 15. The width
of conductor 42 may be about the same as that of a finger 20, for
example. Conductor 42 interconnects fingers 20 to electrically
bypass cracks that may form between bypass conductor 40 and
conductor 42, and thereby provides a current path to bus bar 15 for
regions of solar cell 10 that might otherwise be electrically
isolated by such cracks.
[0085] Bus bar 15, fingers 20, bypass conductor 40 (if present),
and end conductor 42 (if present) of the front surface
metallization pattern may be formed, for example, from silver paste
conventionally used for such purposes and deposited, for example,
by conventional screen printing methods. Alternatively, these
features may be formed from electroplated copper. Any other
suitable materials and processes may be also used. Bus bar 15 may
have a width perpendicular to its long axis of, for example, less
than or equal to about 3 mm, and in the illustrated example has a
width of about 1.5 mm. Fingers 20 may have widths, for example, of
about 10 microns to about 100 microns. In the illustrated example,
the front surface metallization pattern includes about 125 fingers
spaced evenly along the .about.154 mm length of bus bar 15. Other
variations may employ, for example, less than about 125, about 150,
about 175, about 200, about 225, about 125 to about 225, or more
than about 225 fingers spaced evenly along a bus bar 15 of about
the same (.about.154 mm) length. Generally, the width of the bus
bar and the width, number, and spacing of the fingers may be varied
depending on the intensity of solar radiation to be concentrated on
the solar cell. Typically, higher concentrations of solar radiation
on the solar cell require more and/or wider fingers to accommodate
the resulting higher current generated in the solar cell. In some
variations, the fingers may have widths that are greater near the
bus bar than they are away from the bus bar.
[0086] Referring again to the example back surface metallization
pattern shown in FIG. 1B, back contact 25 may be a conventionally
deposited aluminum contact, for example, and may substantially
cover the back surface of solar cell 10. Alternatively, back
contact 25 may leave islands or other portions of the back surface
of solar cell 10 unmetallized. As yet another alternative, back
contact 25 may comprise fingers similar to those in the front
surface metallization pattern, running parallel to each other and
to the short sides of solar cell 10 for substantially the length of
the short sides. Any other suitable configuration for back contact
25 may also be used. Back contact pad 30 may be formed, for
example, from silver paste conventionally used for such purposes
and deposited, for example, by conventional screen printing
methods. Alternatively, contact 25 and/or back contact pad 30 may
be formed from electroplated copper. Any other suitable materials
and processes may also be used to form back contact 25 and back
contact pad 30. Back contact pad 30 may have a width perpendicular
to its long axis of, for example, less than or equal to about 3 mm,
and in the illustrated example has a width of about 2 mm. Back
contact pad 30 may have a width, for example, matching or
approximately matching the width of front bus bar 15. In such
instances back contact pad 30 may have a width, for example, of
about 1 to about 3 times the width of bus bar 15.
[0087] Solar cells 10 may be HIT (heterojunction with intrinsic
thin layer) silicon solar cells. In such cases, the HIT cells may
employ, for example, the front surface metallization patterns
described above with respect to FIG. 1A or any variations of those
front surface metallization pattern described herein. The HIT cells
may employ, for example, the back surface metallization patterns
described above with respect to FIG. 1B or any variations of those
back surface metallization patterns described herein. The HIT cell
back surface metallization pattern may comprise fingers (e.g.,
silver fingers) similar to those in the front surface metallization
pattern of FIG. 1A. In such cases the fingers of the back surface
metallization pattern may be disposed on a layer of transparent
conducting oxide (TCO), which in turn is disposed on the back
surface of the semiconductor diode structure. Alternatively, the
back surface metallization pattern for HIT cells may comprise a
thin copper layer disposed on a TCO layer, which is in turn
disposed on a back surface of the semiconductor diode structure.
The copper layer may be deposited by electroplating, for example.
The TCO in this or the previous variation may be or comprise indium
tin oxide, for example. Any other suitable back surface
metallization pattern may also be used.
[0088] For HIT cells employed in solar cell strings as described
herein, a thin copper layer back surface metallization pattern may
handle high current density with low resistance and therefore
results in low I.sup.2R loss at the back contact. Light passing
unabsorbed through the HIT cell is typically is typically absorbed
by the copper layer, however, leading to optical loss. HIT cells in
which the back surface metallization pattern comprises fingers
deposited on a TCO layer may be positioned with their back surfaces
on or above a reflecting surface, such as a white surface. Light
which passes unabsorbed through the HIT cell may thereby be
reflected back into the HIT cell, past the fingers and through the
TCO, to be absorbed in the HIT cell and generate additional
current. The I.sup.2R loss in the fingers may be greater than that
for the thin copper layer back surface metallization variation,
however. The choice of back surface metallization pattern generally
depends on which such pattern performs best when the HIT cells are
illuminated at a desired level of concentration (e.g., at greater
than or equal to about seven "suns").
[0089] Referring now to FIG. 2, an example solar energy receiver 45
comprises a string of series-connected solar cells 10 arranged in
an overlapping manner on a linearly elongated substrate 50. Each
solar cell 10 in solar energy receiver 45 has the front and back
surface metallization patterns illustrated in FIGS. 1A and 1B,
respectively. FIG. 3A shows a cross-sectional view illustrating the
overlap of adjacent solar cells in solar energy receiver 45. As
shown in FIG. 3A, for each pair of overlapping solar cells the
bottom contact pad 30 of one solar cell overlaps the front surface
bus bar 15 of the other solar cell. Exposed front surface bus bar
15 at one end of the string and exposed bottom contact pad 30 at
the other end of the string may be used to electrically connect the
string to other electrical components as desired. In the example
illustrated in FIG. 2, bypass conductors 40 are hidden by
overlapping portions of adjacent cells. Alternatively, solar cells
comprising bypass conductors 40 may be overlapped similarly to as
shown in FIG. 2 and FIG. 3A without covering the bypass
conductors.
[0090] Front surface bus bar 15 and bottom contact pad 30 of an
overlapping pair of solar cells 10 may be bonded to each other
using any suitable electrically conductive bonding material.
Suitable conductive bonding materials may include, for example,
conventional electrically conductive reflowed solder, and
electrically conductive adhesives. Suitable electrically conductive
adhesives may include, for example, interconnect pastes, conductive
films, and anisotropic conductive films available from Hitachi
Chemical and other suppliers, as well as electrically conductive
tapes available from Adhesives Research Inc., of Glen Rock Pa., and
other suppliers. Suitable electrically conductive adhesives may
also include silver-filled conductive epoxies or other conductive
epoxies. In some variations such electrically conductive adhesives
may be selected, for example, to remain flexible over a temperature
range between about -40.degree. C. and about 115.degree. C., have
an electrical resistivity less than or equal to about 0.04
ohm-centimeters, exhibit elongation at break greater than or equal
to about 20%, have a dispensable viscosity, or have any combination
of the preceding characteristics.
[0091] The illustration of FIG. 3A labels front bus bars 15 with a
minus sign (-), and bottom contact pads 30 with a plus sign (+), to
indicate electrical contact to n-type and p-type conductivity
layers in the solar cell, respectively. This labeling is not
intended to be limiting. As noted above, solar cells 10 may have
any suitable diode structure.
[0092] Referring again to FIG. 2, substrate 50 of solar energy
receiver 45 may be, for example, an aluminum or other metal
substrate, a glass substrate, or a substrate formed from any other
suitable material. Solar cells 10 may be attached to substrate 50
in any suitable manner. For example, solar cells 10 may be
laminated to an aluminum or other metal substrate 50 with
intervening adhesive, encapsulant, and/or electrically insulating
layers disposed between solar cells 10 and the surface of the metal
substrate. Substrate 50 may optionally comprise channels through
which a liquid may be flowed to extract heat from solar energy
receiver 45 and thereby cool solar cells 10, in which case
substrate 50 may preferably be an extruded metal substrate. Solar
energy receiver 45 may employ, for example, lamination structures,
substrate configurations, and other receiver components or features
as disclosed in U.S. patent application Ser. No. 12/622,416, titled
"Receiver for Concentrating Solar Photovoltaic-Thermal System", and
filed Nov. 19, 2009, which is incorporated herein by reference in
its entirety. Although in the illustrated example substrate 50 is
linearly elongated, any other suitable shape for substrate 50 may
also be used.
[0093] Receiver 45 may include only a single row of solar cells
running along its length, as shown in FIG. 2. Alternatively,
receiver 45 may include two or more parallel rows of solar cells
running along its length.
[0094] Strings of overlapping series-connected solar cells as
disclosed herein, and linearly elongated receivers including such
strings, may be used, for example, in solar energy collectors that
concentrate solar radiation to a linear focus along the length of
the receiver, parallel to the string of solar cells. Concentrating
solar energy collectors that may advantageously employ strings of
series-connected overlapping solar cells as disclosed herein may
include, for example, the solar energy collectors disclosed in U.S.
patent application Ser. No. 12/781,706 titled "Concentrating Solar
Energy Collector" and filed May 17, 2010, and the solar energy
collectors disclosed in U.S. patent application Ser. No. 13/740,770
titled "Concentrating Solar Energy Collector" and filed Jan. 14,
2013. Each of these patent applications is incorporated herein by
reference in its entirety. Such concentrating solar energy
collectors may, for example, employ long narrow flat mirrors
arranged to approximate a parabolic trough that concentrates solar
radiation to a linear focus on the receiver.
[0095] Referring again to FIGS. 1A and 1B, although the illustrated
examples show front bus bar 15 and back contact pad 30 each
extending substantially the length of the long sides of solar cell
10 with uniform widths, this may be advantageous but is not
required. For example, front bus bar 15 may be replaced by two or
more discrete contact pads which may be arranged, for example, in
line with each other along a side of solar cell 10. Such discrete
contact pads may optionally be interconnected by thinner conductors
running between them. There may be a separate (e.g., small) contact
pad for each finger in the front surface metallization pattern, or
each contact pad may be connected to two or more fingers. Back
contact pad 30 may similarly be replaced by two or more discrete
contact pads. Front bus bar 15 may be continuous as shown in FIG.
1A, and back contact pad 30 formed from discrete contact pads as
just described. Alternatively, front bus bar 15 may be formed from
discrete contact pads, and back contact pad 30 formed as shown in
FIG. 1B. As yet another alternative, both of front bus bar 15 and
back contact pad 30 may be replaced by two or more discrete contact
pads. In these variations, the current-collecting functions that
would otherwise be performed by front bus bar 15, back contact pad
30, or by front bus bar 15 and back contact pad 30 may instead be
performed, or partially performed, by the conductive material used
to bond two solar cells 10 to each other in the overlapping
configuration described above.
[0096] Although FIG. 1B and FIG. 3A show back contact pad 30
located adjacent a long edge of the back surface of solar cell 10,
contact pad 30 may have any suitable location on the back surface
of the solar cell. For example, FIGS. 6A-6C, 7B, and 8B, further
described below, show example solar cells 10 that each have a
contact pad 30 located near the center of the back surface of the
solar cell and running parallel to the solar cell's long axis.
[0097] Further, solar cell 10 may lack front bus bar 15 and include
only fingers 20 in the front surface metallization pattern, or lack
back contact pad 30 and include only contact 25 in the back surface
metallization pattern, or lack front bus bar 15 and lack back
contact pad 30. In these variations as well, the current-collecting
functions that would otherwise be performed by front bus bar 15,
back contact pad 30, or front bus bar 15 and back contact pad 30
may instead be performed by the conductive material used to bond
two solar cells 10 to each other in the overlapping configuration
described above.
[0098] Solar cells lacking bus bar 15, or having bus bar 15
replaced by discrete contact pads, may either include bypass
conductor 40, or not include bypass conductor 40. If bus bar 15 is
absent, bypass conductor 40 may be arranged to bypasses cracks that
form between the bypass conductor and the portion of the front
surface metallization pattern that is conductively bonded to the
overlapping solar cell.
[0099] To this point solar cells 10 have been described as having
front and back surface metallization patterns that provide
electrical contact to opposite sides of a diode junction.
Alternatively, solar cells 10 may be back-contact solar cells in
which one set of contacts on the back surface of the solar cell
electrically contacts one side of the diode junction, and another
set of contacts on the back surface of the solar cell electrically
contacts the other side of the diode junction. When such solar
cells are deployed conventionally, typically no electrical contact
is made to the front surface of the solar cells. This back-contact
geometry advantageously increases the amount of light incident on
active portions of the solar cell by eliminating front surface
metallization that would block light. Such back-contact solar cells
are available, for example, from SunPower Inc.
[0100] When used in shingled strings of solar cells as described
herein, such a back-contact solar cell may further include
conducting vias that pass through the solar cell from its back
surface to its front surface to provide, at an edge of the front
surface, one or more electrical connections to one side of the
diode junction. When the solar cell is arranged in a shingled
manner with an adjacent similarly configured solar cell, the front
surface electrical connections at the edge of one cell overlap with
and may be electrically connected to back surface contacts on the
other cell to electrically connect the two overlapped back-contact
solar cells in series.
[0101] FIGS. 1C-1E schematically depict an example all-back-contact
solar cell 10 configured for use in a series-connected string of
overlapping (i.e., shingled) solar cells. The example back surface
metallization pattern shown in FIG. 1C and FIG. 1E includes an
optional p-line 22 running parallel to and adjacent to a long side
of the solar cell, a plurality of p-fingers 24 connected to the
p-line and running parallel to the short sides of the solar cell,
an optional n-line 26 running parallel to and adjacent to the other
long side of the solar cell, and a plurality of n-fingers 28
connected to the n-line, running parallel to the short sides of the
solar cell, and interdigitated with p-fingers 24. The regions of
the semiconductor structure beneath and contacted by the n-fingers
and the p-fingers are correspondingly doped n-type or p-type to
form a diode junction.
[0102] As seen in FIG. 1C-1E, the example back-contact solar cell
10 also includes conducting vias 32 that pass through solar cell 10
to provide electrical contact from n-line 26 and n-fingers 28 on
the back surface of solar cell 10 to an optional bus bar 34 that
runs parallel to and adjacent to a long side of the solar cell on
the front surface of the solar cell. FIG. 1D depicts the front
surface of the solar cell 10 as if that front surface were viewed
through the back surface of the solar cells. As shown by a
comparison of FIGS. 1C-1E, in the illustrated example bus bar 32
and n-line 26 are positioned along the same long side of the solar
cell, with p-line 22 positioned along the opposite long side. Solar
cells configured in this manner may be positioned with the p-line
22 on the back surface of one solar cell overlapping and
electrically connected to the bus bar on the front surface of an
adjacent solar cell to connect the solar cells in series. In this
arrangement bus bar 34 is covered by an active portion of the
overlapping solar cell. Thus there is no exposed front surface
metallization blocking light from active regions of the solar
cell.
[0103] Alternatively, the polarities n and p in the above
description may be swapped so that vias 32 provide electrical
contact from p-contacts on the back surface of solar cell 10 to bus
bar 34 on the front surface. Solar cells configured in this manner
may be positioned with the n-line on the back surface of one solar
cell overlapping and electrically connected to the bus bar on the
front surface of an adjacent solar cell to connect the two solar
cells in series.
[0104] Although the illustrated examples show one via for each
finger on the back surface that is to be electrically connected to
the front surface, there may be more or fewer vias than fingers so
long as the fingers to be connected to the front surface are
interconnected on the back surface in such a manner that each is
electrically connected to one or more vias. Though bus bar 34 is
shown as extending substantially the length of the long sides of
solar cell 10 with uniform width, this may be advantageous but is
not required. For example, bus bar 34 may be replaced by two or
more discrete contact pads which may be arranged, for example, in
line with each other along a side of solar cell 10. Such discrete
contact pads may optionally be interconnected by thinner conductors
running between them. There may be a separate (e.g., small) contact
pad on the front surface for each via, or each contact pad may be
connected to two or more vias. Bus bar 34 may also be absent.
P-line 22 and/or n-line 26 may similarly be replaced by two or more
discrete contact pads, or may be absent. Some variations lack a bus
bar 34 at the front surface end of the vias, or lack an
interconnecting conductor such as a p-line or an n-line at the back
surface end of the vias, or lack a bus bar 34 at the upper surface
end of the vias and also lack an interconnecting conductor at the
back surface end of the vias. In variations in which bus bar 34,
p-line 22, and/or n-line 26 are formed from discrete contact pads
or are absent, the current-collecting functions that would
otherwise be performed by these features may instead be performed,
or partially performed, by conductive material used to bond two
solar cells together in the overlapping configuration described
above.
[0105] To shorten the current path between overlapped back contact
solar cells through the vias described above, it may be desirable
to configure and/or arrange the solar cells so that each via is
aligned at one end with the end of a (n or p) finger on the back
surface of one solar cell and aligned at its other end with the end
of a (p or n) finger of opposite polarity on the back surface of an
adjacent overlapped solar cell. With fingers configured as shown in
FIG. 1C, the vias may be aligned in this manner by positioning the
overlapped solar cells so that one is translated with respect to
the other along their overlapping long sides by a distance equal to
the pitch between fingers. Alternatively, the fingers may be
configured as shown in FIG. 1F, for example, so that they extend at
an angle across the solar cell back surface such that opposite ends
of each finger are offset along the long sides of the solar cell by
a distance equal to the pitch between fingers. Solar cells
configured in this manner may be overlapped with their short sides
flush to provide the desired via alignment with fingers on the
overlapped solar cells. Although FIG. 1F shows the back surface
metallization pattern including p-line 22 and n-line 26, either or
both may be absent.
[0106] Vias 32 may thus interconnect two overlapped back-contact
solar cells finger to finger, finger to line (e.g., bus bar,
p-line, or n-line), or line to line, for example.
[0107] The formation of vias 32 may be integrated into the
conventional manufacturing processes for all-back-contact solar
cells. Holes for the vias may be formed, for example, by
conventional laser drilling and may be filled, for example, with
any suitable conventional conducting material deposited by any
suitable conventional method. The conducting material may be an
electroplated metal or a printed conductive metal paste, for
example.
[0108] Back-contact solar cells may also be employed in
series-connected overlapped strings of solar cells without the use
of the vias described above. Referring to the cross-sectional view
of FIG. 3B, for example, two such overlapped back-contact solar
cells may be electrically connected in series by a mechanically
compliant electrical interconnect 90 which interconnects a back
contact on one of the solar cells and a back contact of opposite
polarity on the other solar cell.
[0109] The strings of overlapping series-connected solar cells
disclosed herein, and linearly elongated receivers including such
strings, may operate with higher efficiency than conventional
arrangements, particularly under concentrated illumination. In some
variations, the strings of overlapping solar cells disclosed herein
may provide, for example, >15% more output power than analogous
conventionally arranged strings of solar cells.
[0110] Dicing a wafer to provide solar cells having smaller areas
reduces the current "I" generated in the solar cells and can
thereby reduce "I.sup.2R" power losses that result from resistance
"R" internal to the solar cells and resistance in connections
between the solar cells in a string. However, conventional strings
of series-connected solar cells require gaps between adjacent solar
cells. For a string of a given physical length, the number of such
gaps increases as the solar cells are made shorter. Each gap
reduces the power generated by the string, thereby at least
partially defeating the advantage that might otherwise result from
using solar cells of smaller areas. Further, the power loss
resulting from the gaps increases when such a conventional string
is employed in a concentrating solar energy collector.
[0111] In contrast to conventional strings of solar cells, the
strings of series-connected overlapping solar cells disclosed
herein do not have gaps between solar cells. The solar cells in
such strings may therefore be diced into smaller areas to reduce
I.sup.2R losses without accumulating power losses due to gaps. For
example, it may be advantageous to use solar cells having a longest
side that has a length that spans a standard wafer, as in solar
cells 10 depicted in the various figures herein, because such solar
cells may be oriented with their longest sides perpendicular to the
long axis of the string to provide a wider focal region in a linear
focus concentrating solar energy collector. (Making the focal
region wider relaxes tolerances on optical elements in the
concentrating solar energy collector, and may facilitate
advantageous use of flat mirrors). For conventional strings of
solar cells, the optimal length of the short side of the solar
cells would then be determined in part by a trade-off between
I.sup.2R power losses and losses due to gaps between cells. For the
strings of overlapping solar cells disclosed herein, the length of
the short sides of the solar cells (and thus the areas of the solar
cells) may be selected to reduce I.sup.2R losses to a desired level
without concern for losses due to gaps.
[0112] Conventional solar cells typically employ two or more
parallel front surface bus bars which shade the underlying portions
of the solar cells and thus reduce the power generated by each
solar cell. This problem is exacerbated by the copper ribbons,
typically wider than the bus bars, which are used in conventional
strings to electrically connect the front surface bus bars of a
solar cell to the back surface contact of an adjacent solar cell in
the string. The copper ribbons in such conventional strings
typically run across the front surface of the solar cells, parallel
to the string and overlying the bus bars. The power losses that
result from shading by the bus bars and by the copper ribbons
increase when such conventional solar cells are employed in a
concentrating solar energy collector. In contrast, the solar cells
disclosed herein may employ only a single bus bar on their front
surfaces, as illustrated, or no bus bar, and do not require copper
ribbons running across the illuminated front surface of the solar
cells. Further, in strings of overlapping solar cells as disclosed
herein, the front surface bus bar on each solar cell, if present,
may be hidden by active surface area of an overlapping solar cell,
except at one end of the string. The solar cells and strings of
solar cells disclosed herein may thus significantly reduce losses
due to shading of underlying portions of the solar cells by the
front surface metallization, compared to conventional
configurations.
[0113] One component of I.sup.2R power losses is due to the current
paths through the fingers in the front surface metallization. In
conventionally arranged strings of solar cells, the bus bars on the
front surfaces of solar cells are oriented parallel to the length
of the string, and the fingers are oriented perpendicularly to the
length of the string. Current within a solar cell in such a
conventional string flows primarily perpendicularly to the length
of the string along the fingers to reach the bus bars. The finger
lengths required in such geometries may be sufficiently long to
result in significant I.sup.2R power losses in the fingers. In
contrast, the fingers in the front surface metallization of solar
cells disclosed herein are oriented parallel to the short sides of
the solar cells and parallel to the length of the string, and
current in a solar cell flows primarily parallel to the length of
the string along the fingers. The finger lengths required in this
arrangement may be shorter than required for conventional cells,
thus reducing power losses.
[0114] Another component of I.sup.2R power losses is due to the
length of the current path between adjacent solar cells through the
conventional copper ribbon interconnects. The current paths between
adjacent solar cells in the overlapping configurations disclosed
herein may be shorter than in conventional arrangements, thus
reducing I.sup.2R losses.
[0115] The solar cell metallization patterns and/or overlapping
cell geometries disclosed herein may be advantageously used with
crystalline silicon solar cells disposed on a metal substrate, as
in receiver 45 of FIG. 2, for example. One of ordinary skill in the
art may find this surprising, however. If formed using conventional
reflowed solder, for example, the bond between the front surface
bus bar and the back surface contact pad of overlapping solar cells
in a string as disclosed herein may be significantly more rigid
than the electrical connections between adjacent solar cells
provided by copper ribbon tabbing in conventionally tabbed strings
of solar cells. Consequently, in comparison to copper ribbon
tabbing, the solder connections between adjacent solar cells in
such a string may provide significantly less strain relief to
accommodate mismatch between the coefficient of thermal expansion
(CTE) of the silicon solar cells and that of the metal substrate.
That mismatch may be quite large. For example, crystalline silicon
has a CTE of .about.3.times.10.sup.-6, and aluminum has a CTE of
.about.23.times.10.sup.-6. One of ordinary skill in the art may
therefore expect such strings of overlapping silicon solar cells
disposed on a metal substrate to fail through cracking of the
silicon solar cells. This expectation would be even stronger for
such strings of overlapping solar cells employed in a concentrating
solar energy collector in which they may cycle over larger
temperature ranges, and therefore experience greater strain from
thermal expansion mismatch with the substrate, than typically
experienced in a non-concentrating solar energy collector.
[0116] Contrary to such expectations, however, the inventors have
determined that strings of series-connected overlapping silicon
solar cells may be bonded to each other with conventional reflowed
solder, attached to an aluminum or other metal substrate, and
reliably operated under concentrated solar radiation. Such strings
may have a length, for example, of greater than or equal to about
120 mm, greater than or equal to about 200 mm, greater than or
equal to about 300 mm, greater than or equal to about 400 mm, or
greater than or equal to about 500 mm, or between about 120 mm and
about 500 mm.
[0117] Further, the inventors have also determined that solder
substitutes such as those described above, including electrically
conducting tapes, conductive films, interconnect pastes, conductive
epoxies (e.g., silver-filled conductive epoxies), and other similar
conducting adhesives, for example, may be used to bond solar cells
to each other to form even longer strings of series-connected
overlapping solar cells on a metal substrate. In such variations
the conductive bonding material that bonds overlapping cells
together is selected to be mechanically compliant, by which it is
meant that the bonding material is easily elastically
deformed--springy. (Mechanical compliance is the inverse of
stiffness). In particular, the conductive bonds between solar cells
in such strings are selected to be more mechanically compliant than
solar cells 10, and more mechanically compliant than conventional
reflowed solder connections that might otherwise be used between
overlapping solar cells. Such mechanically compliant conductive
bonds between overlapping solar cells deform without cracking,
detaching from the adjacent solar cells, or otherwise failing under
strain resulting from thermal expansion mismatch between solar
cells 10 and substrate 50. The mechanically compliant bonds may
therefore provide strain relief to a string of interconnected
overlapping solar cells, thereby accommodating CTE mismatch between
solar cells 10 and substrate 50 and preventing the string from
failing. The difference between the CTE of the (e.g., silicon)
solar cell and the substrate may be, for example, greater than or
equal to about 5.times.10.sup.-6, greater than or equal to about
10.times.10.sup.-6, greater than or equal to about
15.times.10.sup.-6, or greater than or equal to about
20.times.10.sup.-6. Such strings of series-connected overlapping
silicon solar cells disposed on a substrate with mismatched CTEs
may have a length, for example, greater than or equal to about 1
meter, greater than or equal to about 2 meters, or greater than or
equal to about 3 meters.
[0118] Further still, the inventors have developed mechanically
compliant electrical interconnects that may be used to interconnect
two or more strings of series-connected overlapping solar cells to
form longer strings of series-connected solar cells. The resulting
longer strings may be disposed on a metal substrate or other
substrate and reliably operated under concentrated solar radiation.
Referring now to FIG. 4, an example string 55 of series connected
solar cells comprises a first group 60 of series-connected
overlapping solar cells 10 that is electrically and physically
connected to a second group 65 of series-connected overlapping
solar cells 10 by a mechanically compliant electrically conductive
interconnect 70. Additional such interconnects 70 are located at
the ends of string 55 to allow additional groups of
series-connected overlapping solar cells to be added to either end
of string 55 to extend the length of the string. Alternatively,
interconnects 70 located at the ends of a string may be used to
connect the string to other electrical components or to an external
load. Overlapping solar cells within groups 60 and 65 may be bonded
to each other with electrically conductive reflowed solder or with
electrically conductive adhesives, as described above, or in any
other suitable manner.
[0119] The spacing between the adjacent ends of two groups of
series-connected overlapping solar cells 10 interconnected with a
mechanically compliant interconnect 70 may be, for example, less
than or equal to about 0.2 mm, less than or equal to about 0.5 mm,
less than or equal to about 1 mm, less than or equal to about 2 mm,
less than or equal to about 3 mm, less than or equal to about 4 mm,
or less than or equal to about 5 mm.
[0120] The variation of mechanically compliant electrical
interconnect shown in FIG. 4 is also shown, in more detail, in FIG.
5A. Another variation of mechanically compliant electrical
interconnect 70 having similar features is shown in FIG. 5B.
Referring now to FIG. 5A and FIG. 5B as well as to FIG. 4, the
example mechanically compliant electrical interconnects 70 are
ribbon-like and have a long and narrow aspect ratio with a length
approximately equal to or greater than the length of the long sides
of solar cells 10. Each interconnect 70 comprises two sets of tabs
75, with each set of tabs positioned on an opposite side of the
long axis of the interconnect. As shown in FIG. 4, an interconnect
70 may be positioned between two strings of series-connected
overlapping solar cells with its tabs 75 on one side making
electrical contact to the bus bar 15 on the front surface of an end
solar cell of one string of overlapping solar cells, and with its
tabs 75 on the other side making electrical contact to contact pad
30 on the back surface of an end cell of the other string of
overlapping solar cells. Tabs 75 may be attached to bus bar 15 or
to contact pad 30 with conventional electrically conductive solder,
electrically conductive adhesives as described above, or by any
other suitable method.
[0121] In the example of FIG. 4, interconnects 70 at the end of
string 55 also each include a bypass diode tap 80 at one end, in
addition to tabs 75. Bypass diode taps 80 provide connection points
for bypass diodes. In the illustrated example, bypass diode 85 is
configured to bypass both groups of series-connected overlapping
solar cells in the event that a solar cell in string 55 fails.
Alternatively, interconnects 70 having bypass diode taps 80 may be
used at any desired interval in a string to bypass one, two, or
more groups of series-connected overlapping solar cells. The
maximum number of solar cells that may be arranged to be bypassed
by a bypass diode is determined by the performance characteristics
of the bypass diode. The bypass diodes may be configured to bypass,
for example, approximately 25 solar cells 10, which may be
distributed in any desired number of series-connected groups of
series-connected overlapping solar cells. For example, each bypass
diode may b configured to bypass about 25 solar cells, all of which
are part of a single group of series-connected overlapping solar
cells. Although in the illustrated example the bypass diode is
connected to the string with interconnects 70, alternative
configurations may also be used. For example, bypass diodes may be
connected to the string by a conductor (other than an interconnect
70) that is electrically connected to the bottom metallization
pattern of one solar cell, and by another conductor (other than an
interconnect 70) that is electrically connected to a bus bar on the
front surface of another solar cell. Such connections may be made
to solar cells that are not at the end of a group of
series-connected overlapping solar cells, but instead somewhere in
between.
[0122] Referring now to FIG. 11, bypass diode 85 may be mounted to
a flex circuit 87 comprising two physically separated electrical
contacts 92 sandwiched between two insulating sheets. The
insulating sheets are patterned to expose adjacent regions 93 of
the two contacts to which the diode is attached to electrically
interconnect the contacts, and to expose regions 97 of the contacts
allowing the flex circuit to be electrically connected to bypass a
portion of the solar cell string. Each of contacts 92 is shaped or
patterned to increase its mechanical compliance. In particular,
contacts 92 include narrow necks and oval-shaped regions which make
the contacts very compliant. Contacts 92 may be formed, for
example, from solder-coated metal (e.g., copper) ribbon. The
insulating sheets may be formed, for example, from a polyimide.
Flex circuit 87 may comprise in addition a bottom adhesive layer by
which it may be attached to a substrate supporting a string of
solar cells.
[0123] Referring again to FIG. 4, FIG. 5A, and FIG. 5B,
interconnects 70 are mechanically compliant. In particular, they
are more mechanically compliant than solar cells 10 and more
mechanically compliant than solder connections between bus bar 15
and back contact pad 30 of overlapping solar cells 10.
Interconnects 70 may also be more mechanically compliant than bonds
between overlapping solar cells formed from electrically conductive
adhesives as described above. Interconnects 70 deform without
cracking, detaching from the adjacent solar cells, or otherwise
failing under strain resulting from thermal expansion mismatch
between solar cells 10 and substrate 50. Interconnects 70 may
therefore provide strain relief to a string of interconnected
groups of overlapping solar cells, thereby accommodating the
thermal expansion mismatch between solar cells 10 and substrate 50
and preventing the string from failing.
[0124] In the illustrated examples each interconnect 70 is a
solder-coated metal (e.g., copper) ribbon that has been shaped or
patterned to enhance its mechanical compliance. In particular, the
illustrated interconnect 70 of FIG. 5A includes a central portion
having the form of a series of two or more flattened ovals
interlinked at their ends. Each flattened oval includes a pair of
tabs 75 on opposite flattened sides of the oval, to make contact
with solar cells as described above. The flattened ovals make each
interconnect 70 very compliant ("springy") in directions parallel
and perpendicular to the long axis of the interconnect. In the
illustrated example, the strips of metal forming the walls of the
ovals have a width W1 of approximately 1.5 mm, but any suitable
width may be used. The illustrated interconnect 70 of FIG. 5B
includes a series of slots running down the center of the metal
ribbon parallel to its long axis. The slots make the interconnect
of this variation very compliant, as well. Interconnects 70 may be
formed from highly conductive materials such as copper, for
example, and/or from materials such as Invar (a nickel-iron alloy)
and Kovar (a nickel-cobalt-iron alloy) that have a low coefficient
of thermal expansion. Each metal ribbon may be sandwiched between
thin insulating sheets of material to form a flex circuit, with the
insulating sheets patterned to expose portions of the metal ribbon
(e.g., tabs 75) intended to make electrical contact with the solar
cells. The insulating sheets may be formed from a polyimide, for
example.
[0125] Any other suitable materials and configurations may also be
used for the interconnects 70 that interconnect two
series-connected strings of overlapping solar cells. For example,
interconnects 70 may be similar or identical to any of the
mechanically compliant interconnects 90 described below with
respect to FIG. 6A-6C, 7A, 7B, 8A, 8B, or 9. Also, two or more
interconnects 70 may be arranged in parallel similarly to as shown
in FIGS. 7A and 7B described below to interconnect two groups of
series-connected overlapping solar cells.
[0126] Although the use of interconnects 70 is described above with
respect to solar cells 10 that include front surface bus bars 15
and back contact pads 30, such interconnects 70 may be used in
combination with any of the variations of solar cell 10 described
herein. In variations lacking bus bars 15, back contact pads 30, or
both, interconnects 70 may be bonded to solar cells 10 using
electrically conductive adhesives as described above, for
example.
[0127] Mechanically compliant electrical interconnects similar or
identical to interconnects 70 may also be used between every solar
cell in a string of series-connected solar cells, or between every
solar cell in a three solar cell or longer contiguous portion of
series-connected string of solar cells. As shown in FIGS. 6A-6C,
7A, 7B, 8A, 8B, and 9, for example, each pair of overlapping solar
cells 10 in a series-connected string of overlapping solar cells
may be physically and electrically connected by mechanically
compliant interconnects 90, each of which interconnects the front
surface metallization of a solar cell with the back surface
metallization of an adjacent solar cell. Such strings differ from
conventionally tabbed strings at least because the adjoining solar
cells in the illustrated strings overlap, and because the locations
at which interconnects 90 are bonded to the front surfaces of solar
cells 10 may be hidden from illumination by an overlapping solar
cell. Mechanically compliant interconnects 90 may be attached to
solar cells 10 with, for example, conventional electrically
conductive solder, electrically conductive adhesives, adhesive
films, or adhesive tapes as described above, or by any other
suitable method.
[0128] Interconnects 90 are mechanically compliant. In particular,
they are more mechanically compliant than solar cells 10 and more
mechanically compliant than solder connections between bus bar 15
and back contact pad 30 of overlapping solar cells 10.
Interconnects 90 may also be more mechanically compliant than bonds
between overlapping solar cells formed from electrically conductive
adhesives as described above. Interconnects 90 deform without
cracking, detaching from the adjacent solar cells, or otherwise
failing under strain resulting from thermal expansion mismatch
between solar cells 10 and a substrate to which they are attached.
Interconnects 90 may therefore provide strain relief to a string of
interconnected groups of overlapping solar cells, thereby
accommodating thermal expansion mismatch between solar cells 10 and
a substrate and preventing the string from failing.
[0129] Interconnects 90 may be formed, for example, from highly
conductive materials such as copper, for example, and/or from
materials such as Invar and Kovar that have a low coefficient of
thermal expansion. Interconnects 90 may be or comprise
solder-coated copper ribbons, for example. Alternatively,
interconnects 90 may be or comprise copper ribbons sandwiched
between polyimide layers (for example, Kapton films) or other
insulating layers, with the sandwiching layers patterned to expose
the copper ribbon at locations to be bonded to solar cells. Any
other suitable materials and configurations may be used for
interconnects 90, in addition to those disclosed herein.
[0130] FIGS. 6A-6C show example cross-sectional views illustrating
the interconnection of a string of overlapping solar cells 10 with
mechanically compliant electrical interconnects 90. As illustrated
in these examples, interconnects 90 may have a flat cross-sectional
profile (FIG. 6A), a bent cross-sectional profile (FIG. 6B), or a
looped cross-sectional profile (FIG. 6C). Any other suitable
cross-sectional profile may also be used. Bent or looped
cross-sectional profiles may increase mechanical compliance,
compared to a flat cross-sectional profile.
[0131] In the examples illustrated in FIGS. 6A-6C and in later
figures, back contact pad 30 is located away from the edge of solar
cell 10, near the middle of the back surface. This is not required.
Contact pad 30 may be positioned at any suitable location on the
back surface of the solar cell. For example, contact pad 30 may be
positioned adjacent to the overlapping edge of solar cell 10, as
shown in FIG. 1B, or adjacent to the edge opposite from the
overlapping edge.
[0132] FIGS. 7A and 7B show front and rear views, respectively, of
an example string of series-connected overlapping solar cells. As
shown in these figures, two or more interconnects 90 may be
arranged in parallel with each other to interconnect adjacent
overlapping solar cells. In the illustrated example, interconnects
90 have the form of ribbons with their long axes oriented
perpendicular to the overlapping edges of adjacent solar cells. As
another example (not shown), parallel interconnects 90 may have the
form of two or more ribbons arranged in line with each other with
their long axes oriented parallel to the overlapping edges of
adjacent solar cells.
[0133] FIGS. 8A and 8B show front and rear views, respectively, of
another example string of series-connected overlapping solar cells.
FIG. 9 shows a rear view of yet another example string of
series-connected overlapping solar cells. As shown in FIGS. 8A, 8B,
and 9, interconnects 90 may have the form of ribbons oriented
parallel to and extending along the length of the overlapping edges
of adjacent solar cells.
[0134] Example interconnects 90 illustrated in FIGS. 8A and 8B are
similar or identical to interconnects 70 illustrated in FIG. 4 and
FIG. 5. In the variation illustrated in FIGS. 8A and 8B, each
interconnect 90 includes two sets of tabs 75, with each set of tabs
positioned on an opposite side of the long axis of the
interconnect. Such an interconnect 90 may be positioned between two
overlapping solar cells with its tabs 75 on one side making
electrical contact to the bus bar 15 on the front surface of one of
the solar cells, and with its tabs 75 on the other side making
electrical contact to contact pad 30 on the back surface of the
other solar cell. Also as illustrated in FIGS. 8A and 8B,
interconnects 90 may optionally include bypass diode taps 80 that
provide connection points for bypass diodes configured to bypass
one or more solar cells in the event that one of the solar cells
fails.
[0135] Example interconnects 90 illustrated in FIG. 9 have the form
of rectangular ribbons patterned with slits or openings 95 that
increase their mechanical compliance. The illustrated interconnects
90 also include contact pads 100 to be bonded to solar cells. Such
interconnects 90 may, for example, be or comprise copper ribbons
sandwiched between polyimide layers (for example, Kapton films) or
other insulating layers, with the sandwiching layers patterned to
expose the copper ribbon at the locations of contact pads 100.
[0136] Although the use of interconnects 90 is described above with
respect to solar cells 10 that include front surface bus bars 15
and back contact pads 30, such interconnects 90 may be used in
combination with any of the variations of solar cell 10 described
herein. In variations lacking bus bars 15, back contact pads 30, or
both, interconnects 90 may be bonded to solar cells 10 using
electrically conductive adhesives as described above, for
example.
[0137] Referring now to FIG. 10, a string of solar cells 10 may be
disposed on a substrate 50 in a lamination stack 105 that adheres
to the substrate. The lamination stack may comprise, for example, a
thermally conductive encapsulant layer 110 disposed between the
solar cells and the substrate, a clear encapsulant layer 115
disposed on the thermally conductive encapsulant layer, and a clear
top sheet 120 disposed on the clear encapsulant layer 115. Solar
cells 10 are typically disposed within the clear encapsulant layer
115 at its boundary with the thermally conductive encapsulant layer
110.
[0138] Thermally conductive encapsulant layer 110 comprises one or
more materials that are selected to facilitate heat transfer from
solar cells 10 to substrate 50 and/or to adhere to substrate 50, to
solar cells 10, and to clear encapsulant layer 115. Material in
encapsulant layer 110 may be selected to adhere to aluminum or
aluminum-based alloys, for example. Thermally conductive
encapsulant layer 110 may have a thickness for example, of about
0.1 millimeters to about 2.0 millimeters.
[0139] In the illustrated example, thermally conductive encapsulant
layer 110 comprises a first thermally conductive adhesive layer
125, a dielectric layer 130, and a second thermally conductive
adhesive layer 135. Dielectric layer 130 typically melts at a
higher temperature than the surrounding adhesive layers, and
consequently provides a barrier to physical and electrical contact
between solar cells 10 and substrate 50 that survives a lamination
process, further described below, by which lamination stack 105 is
bonded to substrate 50. Adhesive layer 125 may comprise, for
example, one or more thermally conductive polyolefins and may have
a thickness, for example, of about 0.1 millimeters to about 2.0
millimeters. Dielectric layer 130 may comprise, for example, one or
more fluoropolymers. The fluoropolymers may be selected, for
example, from the group including, but not limited to, polyvinyl
fluoride (PVF), polyvinylidene fluoride (PVDF), ethylene
tetrafluoroethylene, and mixtures thereof. Dielectric layer 130 may
have a thickness, for example, of about 0.1 millimeters to about
2.0 millimeters. Adhesive layer 135 may comprise, for example, one
or more thermally conductive polyolefins and may have a thickness,
for example, of about 0.1 millimeters to about 2.0 millimeters.
[0140] Any other suitable materials and configuration may be used
for thermally conductive encapsulant layer 110 and its component
layers 125, 130, and 135 described above. For example, in some
variations dielectric layer 130 is absent. In such variations,
encapsulant layer 115 may be, for example, a single layer of
thermally conductive polyolefin.
[0141] Thermally conductive encapsulant layer 110 may be
substantially reflective to solar radiation incident on it. For
example, materials in encapsulant layer 110 may include pigments
that make encapsulant layer 110 appear white. Such a reflective
encapsulant layer 110 may reduce the heat absorbed by lamination
stack 105, which may advantageously improve the efficiency with
which solar cells 10 operate. In addition, if solar cells 10 are
HIT solar cells with back surface metallization comprising fingers,
as described above, then such a reflective encapsulant layer may
reflect light that has passed unabsorbed through the HIT solar cell
back into the solar cell where it may be absorbed to generate
additional current, increasing the efficiency with which the solar
cells operate. Alternatively, thermally conductive encapsulant
layer 110 may be substantially absorbing for solar radiation
incident on it. For example, materials in encapsulant layer 110 may
include pigments that make encapsulant layer 110 appear black. Such
an absorbing encapsulant layer 110 may increase the heat absorbed
by lamination stack 105 and subsequently transferred to substrate
50, which may be advantageous if the collected heat is commercially
valuable.
[0142] Referring again to FIG. 10, clear encapsulant layer 115 may
comprise, for example a clear polyolefin, a clear polyimide, or a
mixture thereof, and may have a thickness, for example, of about
0.1 millimeters to about 2.0 millimeters. Any other suitable
materials and thicknesses may be used for clear encapsulant layer
115.
[0143] Clear top sheet 120 may comprise, for example, one or more
clear fluoropolymers. The fluoropolymers may be selected, for
example, from the group including, but not limited to, polyvinyl
fluoride (PVF), ethylene tetrafluoroethylene, and mixtures thereof.
Clear top sheet 120 may be selected to have a moisture transmission
rate less than or equal to about 0.01 grams/meter-day, for example.
Clear top sheet 120 may have a thickness, for example, of about 0.1
millimeters to about 1.0 millimeters. Any other suitable materials
and thicknesses may be used for clear top sheet 120.
[0144] Solar cells 10 in lamination stack 105 may be or comprise
any of the solar cells disclosed herein, and may be arranged in any
of the configurations of series-connected overlapping solar cell
strings disclosed herein. Any other suitable solar cells and string
configurations may also be disposed in lamination stack 105,
however. For example, although solar cells 10 in FIG. 10 are shown
as overlapping in a shingle pattern, solar cells disposed in stack
105 may instead be configured in a non-overlapping manner and
conventionally tabbed.
[0145] The component layers of lamination stack 105 may be
positioned on a substrate 50 and then bonded to substrate 50 in a
conventional laminator, for example, at an elevated temperature and
with the application of pressure directed to force lamination stack
105 and substrate 50 together. During this lamination process, the
temperature of substrate 50 and/or lamination stack 105 may be
raised, for example, to between about 130.degree. C. and about
160.degree. C. If the solar cells in lamination stack 105 are
configured in a non-overlapping manner, the pressure applied during
the lamination process may be about 1.0 atmosphere, for example.
The inventors have determined, however, that if the solar cells in
lamination stack 105 are configured in an overlapping manner, as
described herein for example, the maximum pressure applied during
the lamination process may preferably be less than or equal to
about 0.6 atmospheres, less than or equal to about 0.5 atmospheres,
less than or equal to about 0.4 atmospheres, less than or equal to
about 0.3 atmospheres, or between about 0.2 atmospheres and about
0.6 atmospheres.
[0146] In variations in which overlapped solar cells are bonded to
each other with a conductive epoxy such as a silver-filled
conductive epoxy, for example, it may be preferable to cure the
epoxy while applying pressure to force the solar cells against each
other. Curing the conductive bond under pressure in this manner may
reduce the thickness of the conductive bond, thereby reducing the
current path between solar cells and consequently reducing I.sup.2R
losses in the string of solar cells. In one approach, the
conducting bonds are cured under pressure to provide a
series-connected string of overlapping solar cells before the
string is laminated to a substrate. In this approach, the
conducting bonds may be cured at a temperature of, for example,
about 150.degree. C. to about 180.degree. C., and under a pressure
of, for example, about 0.1 atmospheres to about 1.0 atmospheres, or
about 0.1 to about 0.5 atmospheres, or about 0.1 to about 0.2
atmospheres. In another approach, the conducting bonds are cured
under pressure during a lamination process similar to that
described above. In this approach, the conducting bonds may be
cured at a temperature of, for example, about 140 C to about 170 C,
and under a pressure of, for example, about 0.1 atmospheres to
about 1.0 atmosphere, or about 0.3 atmospheres to about 1
atmosphere, or about 0.5 atmospheres to about 1.0 atmosphere.
Generally, the higher the temperature at which the conducting epoxy
is cured, the more conductive the bond.
[0147] In some variations, the substrate and/or one or more
lamination layers disposed beneath a series-connected string of
overlapping solar cells is configured to have a surface that
conforms in shape to the underside of the shingled string of solar
cells. For example, a metal substrate may be patterned to have a
surface with a saw-tooth cross section conforming to the shape of
the underside of the shingled string of solar cells. In addition or
alternatively, one or more dielectric sheets disposed between the
substrate and the solar cells may be arranged or patterned to
provide such a conforming surface. For example, such dielectric
sheets may be overlapped in a shingle pattern providing an upper
surface that conforms to the underside of the shingled solar cells.
Supporting the shingled string of solar cells with a conforming
support surface may improve thermal contact between the solar cells
and the substrate.
[0148] Solar energy collectors comprising series-connected strings
of overlapping solar cells as described herein may preferably be
oriented with the exposed edges of the solar cells (e.g., edges 12
in FIG. 3A) away from the equator. With the shingled solar cells
oriented in this manner, solar radiation incident on the cells will
illuminate only the upper surfaces of the cells, not the exposed
edges. This may increase the efficiency with which the collector
converts incident solar radiation into electric power, because
solar radiation incident on the exposed edges of the solar cells
might not be efficiently converted to electricity.
[0149] The performance characteristics of solar cells may vary
between solar cells even when the cells have essentially identical
designs. Hence, two solar cells of identical design that are
illuminated identically may produce currents of two different
magnitudes. In a string of series-connected solar cells, however,
all cells must handle an identical current. Mismatches between the
performances of cells in the string decrease the overall efficiency
of the string. This problem may be readily addressed with
series-connected strings of overlapping solar cells as described
herein. In any of the variations described above, the area of each
solar cell not overlapped by adjacent solar cells may be selected
to match or substantially match the electrical performance (e.g.,
the current) of all of the other solar cells in the string. That
is, the overlap between adjacent cells may be adjusted to vary the
illuminated area of each solar cell so that the electrical
performance of each solar cell substantially matches that of the
other solar cells. This may improve the overall efficiency of the
string.
[0150] This disclosure is illustrative and not limiting. Further
modifications will be apparent to one skilled in the art in light
of this disclosure and are intended to fall within the scope of the
appended claims.
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