U.S. patent application number 09/952939 was filed with the patent office on 2003-03-13 for low cost high solar flux photovoltaic concentrator receiver.
This patent application is currently assigned to The Boeing Company. Invention is credited to Glenn, Gregory S., Sherif, Raed.
Application Number | 20030047208 09/952939 |
Document ID | / |
Family ID | 25493376 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030047208 |
Kind Code |
A1 |
Glenn, Gregory S. ; et
al. |
March 13, 2003 |
LOW COST HIGH SOLAR FLUX PHOTOVOLTAIC CONCENTRATOR RECEIVER
Abstract
A high solar flux photovoltaic concentrator receiver is
disclosed for the generation of high electrical power at high
efficiency for public and private use. The invention uses a
wraparound interconnect to allow direct bonding of concentrator
solar cells to a heat sink with solder or conductive epoxy. This
approach allows series or parallel interconnection between multiple
cells and provides for high thermal conductance to improve cooling
the solar cells. Cooling the solar cells under high concentration
of solar energy increases their electrical efficiency. A highly
conductive di-electric is utilized to insulate the cell backs from
the metal heat sink. The invention minimizes obscuration losses,
improves thermal conduction, reduces coefficient of thermal
expansion stresses, and can be produced at reduced manufacturing
costs.
Inventors: |
Glenn, Gregory S.; (Pacific
Palisades, CA) ; Sherif, Raed; (Valencia,
CA) |
Correspondence
Address: |
DiPINTO & SHIMOKAJI, P.C.
Suite 480
1301 Dove Street
Newport Beach
CA
92660
US
|
Assignee: |
The Boeing Company
Seattle
WA
|
Family ID: |
25493376 |
Appl. No.: |
09/952939 |
Filed: |
September 11, 2001 |
Current U.S.
Class: |
136/246 ;
136/293 |
Current CPC
Class: |
Y02E 10/50 20130101;
H02S 40/42 20141201; H01L 31/052 20130101; H01L 31/044 20141201;
H01L 31/042 20130101; H01L 31/0504 20130101 |
Class at
Publication: |
136/246 ;
136/293 |
International
Class: |
H01L 031/00 |
Claims
We claim:
1. A solar cell concentrator receiver, comprising: a solar cell
having a top side, a front side and a rear side; an electrically
conductive interconnect in proximate contact with the solar cell
top side and wrapped around the solar cell from the top side to
rear side; a di-electric element insulating the front side and rear
side of the solar cell from the electrically conductive
interconnect; a di-electric sheet element sandwiched between a top
metallic sheet element and a rear metallic sheet element; said top
metallic sheet element having an etched electrical circuit pattern;
said rear metallic sheet element having an etched stress relief
thermal conduction pattern; a metallic heat sink element; a
plurality of first electrically conductive adhesive elements
securing the solar cell and electrically conductive interconnect to
the top metallic element on the di-electric sheet element; a
pliable material to fill gaps between adjacent first electrically
conductive adhesive elements; a second electrically conductive
adhesive element securing the metallic heat sink element to the
rear metallic element on the di-electric sheet element; whereby
said solar cell concentrator receiver converts solar energy to
electrical energy, and provides an electrical power transfer and
interconnection path together with a thermally conductive path for
heat dissipation.
2. The solar cell concentrator receiver of claim 1, further
comprising a plurality of solar cells.
3. The solar cell concentrator receiver of claim 1, further
comprising an electrically conductive interconnect which allows
series or parallel interconnection between multiple solar
cells.
4. The solar cell concentrator receiver of claim 1, further
comprising an electrically conductive interconnect which enables
solar cells to be positioned within about 50 microns from each
other.
5. The solar cell concentrator receiver of claim 4, further
comprising an electrically conductive interconnect of evaporated
metal deposited in a pattern from the top side to the rear side of
said solar cell which enables solar cells to be positioned within
about 50 microns from each other.
6. The solar cell concentrator receiver of claim 1, further
comprising a welded metallic electrically conductive interconnect
wrapped around from the top side to the rear side of said solar
cell.
7. The solar cell concentrator receiver of claim 1, further
comprising a thermally conductive di-electric sheet element made
substantially of polyimide material.
8. The solar cell concentrator receiver of claim 1, further
comprising a thermally conductive di-electric sheet element made
substantially of a ceramic material.
9. The solar cell concentrator receiver of claim 1, further
comprising a top metallic sheet element and a rear metallic sheet
element.
10. The solar cell concentrator receiver of claim 1, further
comprising a top metallic sheet element made of a clad material and
a rear metallic sheet element made of copper.
11. The solar cell concentrator receiver of claim 1, further
comprising first and second electrically conductive adhesive
elements made substantially of a metal loaded adhesive.
12. The solar cell concentrator receiver of claim 1, further
comprising first and second electrically conductive adhesive
elements made substantially of a solder material.
13. The solar cell concentrator receiver of claim 1, further
comprising a heat sink made of plated copper or nickel plated
aluminum.
14. A solar cell, insulator, conductor circuit, comprising: a solar
cell; an electrically conductive interconnect from a solar cell top
side and to a solar cell rear side; a di-electric sheet element
sandwiched between two metallic sheet elements; one of said
metallic sheet elements having an etched electrical circuit
pattern; another of said metallic sheet elements having an etched
thermal conduction pattern; a metallic heat sink element; and
electrically conductive adhesive elements securing said di-electric
sheet element between said solar cell and said heat sink.
15. The solar cell, insulator, conductor circuit of claim 14,
further comprising an electrically conductive interconnect of
evaporated metal.
16. The solar cell, insulator, conductor circuit of claim 14,
further comprising a welded metallic electrically conductive
interconnect.
17. The solar cell, insulator, conductor circuit of claim 14,
further comprising a thermally conductive di-electric sheet
element.
18. Apparatus for the generation of electrical power from a high
solar flux photovoltaic concentrator receiver, comprising: a solar
cell with an electrically conductive interconnect leading from a
solar cell top side to a solar cell rear side; a di-electric sheet
element sandwiched between two metallic sheet elements; one of said
metallic sheet elements having an etched electrical circuit
pattern; another of said metallic sheet elements having an etched
thermal conduction pattern; a metallic heat sink element;
electrically conductive adhesive elements securing said di-electric
sheet element between said solar cell and said heat sink; whereby
said apparatus converts solar energy to electrical energy, and
provides an electrical power transfer and interconnection path
together with a thermally conductive path for heat dissipation.
19. A solar cell concentrator receiver manufactured by a process
comprising the steps of: installing an electrically conductive
wrap-around interconnect on one side of a solar cell; assembling a
solar cell, together with an electrically conductive wrap-around
interconnect, termination tabs, bypass diodes and bypass diode tabs
into an alignment fixture for geometrical orientation; securing the
solar cell and assembly parts in said alignment fixture to prevent
motion; etching patterns on a front metal face and a rear metal
face on each side of a di-electric element sheet; applying adhesive
to both sides of a di-electric element sheet on the front and rear
metal faces; aligning the assembled solar cell concentrator in a
tooling fixture and clamping said assembled solar cell concentrator
to prevent any relative motion between elements; bonding said
di-electric element sheet, together with front and rear metal
faces, to said solar cell on one side and to a heat sink on an
opposite side, using a thermal vacuum bag or oven at elevated
temperatures for curing the adhesive, and electrically and
mechanically connecting the assembled solar cell concentrator to a
substrate heat sink.
20. The process of claim 18, further comprising the step of
applying on one side of a solar cell, over a previously deposited
di-electric film insulator, evaporated metal to form an
electrically conductive wrap-around interconnect.
21. The process of claim 18, further comprising the step of
mechanically wrapping over a previously deposited di-electric film
insulator, an electrically conductive wrap-around interconnect,
which was previously welded to the cell's bus bar (ohmic).
22. The process of claim 18, further comprising the step of etching
an electrical circuit pattern on the front metal face of the
di-electric film insulator, to interconnect the solar cells into
series or parallel circuits (or both), and to provide a path for a
bypass diode and also for circuit end termination.
23. The process of claim 18, further comprising the step of etching
a pattern on the rear metal face of the di-electric film insulator
to provide a thermal conduction path to the heat sink and to absorb
mechanical stresses due to CTE mismatch between materials.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention generally relates to solar cells for
the generation of electrical power and, more specifically, to
improved solar cells used with high solar flux solar cell
concentrators, which are easily manufactured and generate high
power at high efficiency.
[0002] The conversion of solar energy into electrical energy with
photovoltaic cells is a major contributing source for the
production of electrical power for public and private use, as costs
associated with more traditional power plants, such as those
employing conventional energy sources, increase yearly and
environmental concerns restrict the construction of new generating
plants. The solar energy generation of high power at high
efficiencies ultimately results in reduction of most system costs,
such as land acquisition and usage, support structures, operating
manpower and wiring.
[0003] One method used in the past to increase efficiency is to
manufacture solar cells with multiple layers having different
energy band gaps stacked so that each cell or layer can absorb a
different part of the wide energy distribution in the sunlight.
Because of the cell's high voltage and susceptibility to reverse
bias breakdown, there is a requirement to protect each cell with a
bypass diode. Attachment of the diode to each cell, in addition to
attaching interconnects for the purpose of increasing voltage by
series connection, increases complexity and manufacturing
costs.
[0004] Also, in the past, connection of cells has involved multiple
interconnects and diode tabs. The diode tabs have commonly been
separate strips of metal, connecting the diode electrically from
the front of the cell to the rear of the cell. This has required
much handling, attaching, and cleaning, thus increasing
manufacturing costs, as well as solar cell attrition due to
handling.
[0005] Traditionally, once the individual solar cells have been
interconnected in a string, the string is then bonded to a
substrate. Wiring the cell strings together in series for higher
voltage or in parallel for higher current has typically been
accomplished by the use of metal tabs or wire and soldered or
welded joints. However, this method of attachment involves a time
consuming set of manual processes, which require inspection, rework
and cleaning. Along with being time consuming, those steps also
lead to attrition of the fragile and expensive solar cells.
[0006] Past designs of solar cell panels that attempt to address
one or more of the above performance and manufacturing issues have
been numerous. One design includes encapsulating solar cell modules
in a polymer cover film molded to provide an embossed surface
having depressions arranged in a row. Each depression has the same
configuration as a solar cell. Solar cells with positive and
negative contacts on the back surface are preferred and can be
positioned in the depressions with the front surfaces of the cells
that face the light source contacting the bottom of the
depressions. A second polymer film having interconnecting circuitry
metallization is placed over the back surfaces of the cells so that
the cells are electrically connected. A disadvantage of the concept
is the lack of direct bonding between the back surfaces of the
cells and the second polymer film, which leads to a greater
potential for separation from the metallization. Another
disadvantage is that the device may not work in a severe thermal
environment where thermal expansion may result in a loss of
electrical connection due to coefficient of thermal expansion
mismatch.
[0007] Another past design uses a printed circuit substrate whereby
the solar cells are physically and electrically connected to a
substrate via interconnect pads. Positive and negative terminals on
the back side of the cells are connected by soldering to the
interconnect pads. If the terminals are on opposite sides of the
cells, metallic interconnectors are used to connect terminals on
the tops sides, over the cell edges, and to the interconnect pads.
An adhesive may be used to secure the cells to the substrate.
Stress relief loops bound the interconnect pads to electrical
traces encapsulated in the substrate. This results in the solar
cells being effectively mounted to the substrate on coiled springs.
On the back side of the substrate, electrically conductive mounting
pads enable attachment to elements such as blocking and shunting
diodes. If the cell is soldered to the spring shaped conductor then
the solder could bridge across the spring, thus making it lose its
advantage as an absorber of thermal stresses. Another disadvantage
is that the configuration of a coiled loop provides a relatively
weak structure that is susceptible to structural failure when
stressed and, thus, electrical connection failure. Yet another
disadvantage is that this design requires either a wrapthrough
metal configuration to bring both cell contacts to the rear side of
the solar cells or a tab. The tab type described in the patent
bridges off the cell onto an adjacent conductive pad, which
increases the area required for a solar array of a given power
design. The wrapthrough metal configuration has the disadvantage of
being very costly to manufacture because it requires a number of
expensive photomasks and photoresist processes. Other disadvantages
of prior art designs include: obscuration losses resulting from
extensively sized interconnects and ohmic bars; poor thermal
conduction resulting from use of overly thick di-electric adhesives
to bond the solar cell assemblies to the heat sink; and high
stresses resulting from mismatched Coefficients of Thermal
Expansion (CTE) between the solar cells and their substrates.
[0008] As can be seen, there is a need for improved solar cell
concentrator modules that are easily manufactured and generate high
power at high efficiency. One method for increasing efficiency is
to allow direct bonding of concentrator solar cells to a heat sink.
This approach allows series or parallel interconnection between
multiple cells and provides for high thermal conductance to improve
cooling the solar cells. Cooling the solar cells under high
concentration of solar energy increases their electrical efficiency
by increasing their voltage. Another method for increasing
efficiency involves utilization of a high thermally conductive
di-electric to insulate the cell backs from the metal heat
sink.
[0009] These and other objects, features and advantages of the
present invention are specifically set forth in, or will become
apparent from, the following detailed description of the
embodiments of the invention when read in conjunction with the
accompanying drawings.
SUMMARY OF THE INVENTION
[0010] In one aspect of the present invention, a solar cell
concentrator receiver comprises a solar cell, an electrically
conductive interconnect, and a di-electric element sandwiched
between two metallic elements. The top metallic element has an
etched electrical circuit and the rear metallic element has an
etched pattern for thermal conduction. Electrically conductive
adhesives or solder secure the solar cell and electrically
conductive interconnect to the di-electric, and to a heat sink.
[0011] In another aspect of the present invention, a solar cell,
insulator, conductor circuit comprises a solar cell, an
electrically conductive interconnect, and a di-electric element
sandwiched between two metallic sheets. One of the metallic sheets
has an etched electrical circuit pattern and the other metallic
sheet has an etched pattern for thermal conduction. Electrically
conductive adhesives or solder secure the di-electric sheet element
between the solar cell and a heat sink.
[0012] In yet another aspect of the present invention, an apparatus
for the generation of electrical power from a high solar flux
photovoltaic concentrator receiver comprises a solar cell with an
electrically conductive interconnect leading from the top side to
the rear side, and a di-electric sheet sandwiched between two
metallic sheet elements. One metallic sheet element has an etched
electrical circuit pattern and another metallic sheet element has
an etched pattern for thermal conduction. Electrically conductive
adhesives or solder secure the di-electric sheet element between
the solar cell and the heat sink. The apparatus converts solar
energy to electrical energy, and provides an electrical power
transfer and interconnection path together with a thermally
conductive path for heat dissipation.
[0013] In yet a further aspect of the present invention, a process
is disclosed for manufacturing a solar cell concentrator receiver.
The process involves the steps of installing an electrically
conductive wraparound interconnect on one side of a solar cell,
assembling the solar cell into an alignment fixture, etching
patterns on front and back metal faces, applying conductive
adhesive or solder to both sides of the di-electric element,
bonding said di-electric element to the solar cell on one side and
to a heat sink on the opposite side, curing the adhesives or
cleaning off solder flux residue, and electrically and mechanically
connecting the assembled solar cell concentrator to a substrate
heat sink.
[0014] Other aspects, advantages and features of the invention will
become more apparent and better understood, as will equivalent
structures, which are intended to be covered herein, with the
teaching of the principles of the invention in connection with the
disclosure of the embodiments thereof in the specification, claims,
and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic cross sectional view of a solar
cell/insulator/conductor circuit according to an embodiment of the
present invention;
[0016] FIG. 1A is a more detailed schematic cross sectional of a
solar cell/insulator/conductor circuit according to an embodiment
of the present invention;
[0017] FIG. 2 is a plane view of the inventive wrap around
interconnect and solar cells taken along section line A-A of FIG.
1;
[0018] FIG. 3 is a plane view of the di-electric element taken
along section line B-B of FIG. 1 illustrating the relationship of
solar cells with respect to other elements of the solar
cell/insulator/conductor circuit;
[0019] FIG. 4 is a flow diagram of the inventive solar
cell/insulator/conductor circuit manufacturing process.
DETAILED DESCRIPTION OF THE INVENTION
[0020] Various embodiments of the improved low cost high solar flux
photovoltaic concentrator receiver and the method of manufacturing
it are described in detail below. While the present invention may
be particularly useful for the conversion of solar energy into
electrical energy as a major source for the production of
electrical power for public and private use, other applications are
contemplated.
[0021] Referring to the accompanying drawings (in which like
reference numerals indicate like parts throughout the several
views), and in particular to FIG. 1 and FIG. 1A, there is shown a
schematic cross sectional view of the solar
cell/insulator/conductor circuit 10. For purposes of illustration,
only three solar cells 11 are shown. The actual number of solar
cells may vary depending on the particular application of the solar
cell/insulator/conductor circuit 10
[0022] The solar cells 11 are generally positioned in a common
plane operatively adjacent to one another and are constructed
utilizing designs well known to those skilled in the art. The solar
cells may be positioned out of plane, for example inside a
cone-shaped concentrator receiver. Each of the solar cells 11 is
described by a top side 13, a rear side 14, and a front side 15,
and each is typically on the order of about 7 mils thick.
[0023] A thin di-electric film insulator 16, which may be of
polyimide, such as Kapton.TM. about 12.5 to 25 microns thick,
comprised of rear side di-electric film element 16A and front side
di-electric film element 16B is placed in proximate relationship
with the corresponding rear side 14 and front side 15 of solar cell
11. The inventive wraparound interconnect 17 may be generally a
rectangular portion comprised of a top side 17A in proximate
relationship with the top side 13 of solar cell 11, a front side
17B in proximate relationship with di-electric film front side
element 16B, and a rear side 17C in proximate relationship with
di-electric film rear side element 16A. It may be etched in a mesh
pattern for both promoting adhesion of the metal filled adhesive
20A and for stress relief.
[0024] The wrap around interconnect 17 wraps the top solar cell 11
polarity to the rear of the cell and allows either series or
parallel interconnection between multiple solar cells 11. In one
embodiment, the wrap around interconnect 17 may be comprised of
evaporated metal deposited over the thin di-electric film 16
insulator in a precise pattern from the top of the solar cell 11 to
the rear side 14 of the solar cell, thus eliminating the need for a
separate interconnect. This feature of the invention allows the
solar cells 11 to be positioned within about 50 microns from each
other. Additionally, the ohmic bar, which acts as a bus bar to
collect current from the cell gridlines, on the front cell surface
is minimized, thus resulting in lower obscuration losses.
[0025] Still referring to FIG. 1 and FIG. 1A, there is shown
sandwiched between multiple adhesive layers 20A and 20B under solar
cell 11 a di-electric element 19, which provides the dual function
of thermal conduction of heat to the heat sink as well as the
electrical power interconnection path between solar cells. In one
embodiment, the di-electric element 19 may be made from a polyimide
material, such as Kapton.TM., on the order of 12.5 to 25.0 microns
thick. For yet another optional embodiment, a thicker, on the order
of 250 to 750 mils, but higher thermal conductance material, such
as aluminum oxide or boron nitride ceramic, may be used for the
di-electric element 19. The electrical power interconnection
function, which is further described below, is comprised of a
circuit pattern etched into a front 19A and a rear 19B metal face
which may be directly bonded to both sides of the di-electric
element 19, using processes well known to those skilled in the
art.
[0026] A significant improvement over prior art is to metalize both
sides of the di-electric element 19. This feature facilitates heat
conduction and simplifies the interconnection path between adjacent
solar cells 11. The front 19A and back 19B metal faces may be made
of different metals. For example, copper can be used on the heat
sink side and a composite material, such as silver clad or plated
kovar can be used on the cell side. The metals chosen to face each
side of the di-electric can be of different materials to lessen CTE
mismatch stresses. Both front metal face 19A and rear metal face
19B may be etched to provide a stress relief between the heat sink,
which traditionally has a high CTE and the solar cells, which have
a much lower CTE.
[0027] The inventive di-electric element 19 increases thermal
conductance to the heat sink 18 thereby improving cooling of the
solar cells with a corresponding increase in electrical efficiency.
It is a significant improvement over prior art, which typically
uses a 50 to 150 micron thick thermally conductive di-electric
adhesive to bond the solar cell assemblies to the heat sink.
Additionally, the use of a di-electric sheet material, such as
polyimide Kapton.TM., results in a reduction in CTE stresses over
bonding the cells directly to a high CTE metal heat sink such as
aluminum, an inexpensive heat sink material, which has a CTE of
23.2.times.10.sup.-6. A more expensive, but even more improved
option, may be to use a ceramic di-electric. Direct bonded metal to
ceramic, such as alumina, has a CTE of approximately
7.times.10.sup.-6, which is closely matched to the CTE of germanium
in high efficiency solar cells at 5.7.times.10.sup.-6.
[0028] A first adhesive layer, or solder layer 20A is utilized to
bond the dielectric element 19 to the solar cell 11 at the rear
side 14 and to the wrap around interconnect 17. A second adhesive
layer, or solder layer 20B is utilized to bond the rear side of the
di-electric element 19 to the heat sink 18. The heat sink 18 may be
made of plated copper or nickel plated aluminum. A nickel plating,
or other suitable metal, on the aluminum may be required if solder
is used to attach the metalized di-electric to the heat sink.
Otherwise, the coating may be an anodize on the aluminum or a tin
plate on copper for corrosion resistance if the metal loaded epoxy
is utilized. The adhesive layers 20A and 20B may be made of a
conductive adhesive, such as a metal loaded epoxy, such as Tra-Con
2902, or alternatively of a solder paste, such as Sn62 with
no-clean flux. By using solder paste or a conductive epoxy, the
invention utilizes a manufacturing friendly process for passing
high current between solar cells. Previous designs have used
interconnects between the solar cells and thermally conductive but
electrically isolating adhesives for bonding the solar cell
assemblies to heat sinks. The thermal conductance of prior use
adhesives is usually on the order of 20 times less than that of
solder, which is one of the embodiments used in this invention. In
fact, previously used thermally conductive silicone or acrylic
adhesives have a thermal conductance of approximately only 2W/m-K,
even when loaded with conductive oxides.
[0029] Still referring to FIG. 1 and FIG. 1A, there is shown a low
viscosity silicone underfill 21 which may be applied between
adjacent first adhesive layers 20A to seal the cavity between said
layers. This underfill 21 may be used to reduce thermal hot spots
in the cavity between adhesive layers 20A and to help adhere the
solar cells to the di-electric substrate.
[0030] Referring now to FIG. 2, there is shown a plane view of the
inventive wrap around interconnect and solar cells taken along
section line A-A of FIG. 1. A plurality of solar cells 11 is viewed
looking in the direction of the rear side 14 of each. The view also
shows the rear side of the wrap around interconnects 17C overlying
the di-electric sheet rear side 16A. Only a small portion of the
di-electric film insulator 16 is visible in FIG. 2.
[0031] FIG. 3 is also a plane view taken along section line B-B of
FIG. 1 illustrating a di-electric element 19 of sufficient size to
underlie a plurality of solar cells 11. The solar cells 11 will
ultimately be bonded to the di-electric element 19 at the locations
shown by the dashed lines 35. The front metal faces 19A (shown in
FIG. 1) have etched metal traces 30 which interconnect the solar
cells into a series or parallel (or both) circuit pattern. The
pattern may be such that the rear side 14 of the solar cell 11,
which is divided into both negative and positive polarity portions,
is interconnected to adjacent solar cells 11 through the etched
metal traces 30 circuitry on the di-electric element 19. The etched
metal traces 30 pattern may also include an etched metal trace
connection path 30A for a bypass diode 31 (including a bypass diode
tab 31A) for reverse bias protection and also for circuit end
termination. A bypass diode 31 (or a switching device in the case
of smart solar panels) may be attached, such as by soldering or by
metal loaded adhesive, to the rear side 14 of each of the solar
cells 11 to minimize the effects of a reverse bias voltage. The use
of bypass diodes is known in the art and shown, for example, in
U.S. Pat. No. 5,616,185.
[0032] The rear metal faces 19B (shown in FIG. 1) also have etched
metal traces, which may be serpentine or diamond shaped. The bottom
metal trace pattern provides a thermal conduction path to the heat
sink 18, and also absorbs mechanical stresses due to CTE mismatches
between the di-electric element 19 material and the heat sink
18.
[0033] The solar cells 11 are ultimately bonded to the di-electric
element 19 at the locations shown by the dashed lines 35. Gaps
between adjacent fist adhesive layers 20A may be filled with a low
viscosity silicone underfill 21 running the full width of each
solar cell 11. Also shown are the bypass diodes 31, bypass diode
end tabs 31A, tubule end tabs 32, and wires 34.
[0034] As can be seen, the inventive solar cell/insulator/conductor
circuit reduces manufacturing costs from the prior art. As no
separate interconnects or wire bonds are used, the invention saves
the cost of the series circuiting process. The solar cell bonding
process also connects the solar cells together into a circuit
through the flex circuit or ceramic metalization pattern. The
bonding process uses a fast cure metal loaded epoxy or a solder
process, which can be completed in minutes using a thermal vacuum
bag or vacuum oven.
[0035] FIG. 4 is a flow diagram illustrating the specific steps
involved in the process of manufacturing inventive solar
cell/insulator/conductor circuits. The following manufacturing
process description refers primarily to that figure as well as FIG.
1 and FIG. 1A, which illustrate geometrical relationships of the
various elements.
[0036] The initial step, wrap around interconnect attachment 40,
involves attaching both electrical contacts (wrap around
interconnect 17) on one side of the solar cell 11 by use of one of
two optional methods. The first, evaporated metal contact, involves
application of evaporated metal (utilizing techniques well known in
the art) over previously deposited di-electric film insulator
elements 16 from the solar cell top side 13 to the solar cell rear
side 14. The second optional method, mechanical wraparound,
involves welding a metal tab 17 to the ohmic from the top side 13
of the solar cell 11 and mechanically forming it around to the rear
side 14 of the solar cell 11 over the di-electric film insulator
element 16.
[0037] The second step, solar cell alignment 41, involves assembly
of the solar cell 11, together with wrap around interconnect 17
installed, into an alignment fixture designed to geometrically
orient the assembly in all three planes such that subsequent
operations, described below, may be accomplished. Other solar cells
11, tubule end tabs 32, bypass diodes 31 and metal bypass diode
tabs 31A are added as required to the alignment fixture. Once in
the alignment fixture, a vacuum hold down 42 may be used to prevent
any motion. Other types of well known hold down systems may be used
for that purpose.
[0038] The di-electric material is obtained with metal material
attached to both sides. The metal is bonded in the case of the
polyimide material with a high temperature epoxy or acrylic. The
metal is typically direct deposited in the case of the ceramic
material using a screen print and sinter process or a chemical
vapor deposition process. Patterns are next etched 43 into the
front metal face 19A and rear metal face 19B located on each side
of the di-electric element 19. The etching process can utilize a
strong chemical etch, such as an acid bath, or a mechanical etch,
such as with microblasting small abrasive particles through a mask.
A circuit pattern, which will interconnect the solar cells 11 into
a series or parallel circuit (or both) may be etched into metal,
which will be direct bonded to the top side 19A of the di-electric
element 19. The pattern may include a connection path for a bypass
diode 31 for reverse bias protection and also for circuit end
termination. The pattern on the rear metal face 19B of the
di-electric provides a thermal conduction path to the heat sink and
also absorbs mechanical stresses due to CTE mismatch between the
two materials. The backside pattern may be serpentine, diamond
shaped, oval shaped, or other patterns known to absorb thermal
stresses in a lateral direction.
[0039] A metal loaded adhesive or solder is applied 45 to both
sides of the di-electric element 19, which includes both front and
rear metal faces 19A and 19B. The application of adhesive or solder
may be accomplished by use of dispensers or squeegeed through a
patterned screen. Both processes are well known to those skilled in
the art. The silicone underfill 21 may then be applied between
adjacent first adhesive layers 20A to seal the cavity between said
layers. It may be deposited by use of a manual or automatic
dispenser at the cell edge opening between adhesive or solder
layers 20A.
[0040] The complete solar cell/insulator/conductor circuit assembly
is next aligned in a tooling jig 46 and clamped with vacuum to
prevent any relative motion between elements. When all the
materials are properly aligned, the assembly is aligned with the
heat sink 18 using alignment pins and then may be placed in a
thermal vacuum bag or oven 47. A vacuum may be pulled to press the
materials together and then heat of sufficient temperature applied
to cure the epoxy or melt the solder.
[0041] As can be seen, the invention is ideally suited for use by
those interested in high power and high efficiency concentrator
modules made with a cost effective process. It uses high thermal
conductivity materials, which allow the high solar flux
photovoltaic concentrator receiver to run at lower temperatures,
thus improving operating efficiency and reducing system costs. The
concentrator receiver, when placed under high solar concentration,
will transfer heat from the solar cells to the heat sink where it
can be easily removed through convection, conduction, and
radiation.
[0042] Although the present invention has been described in
considerable detail with reference to certain versions thereof,
other versions are possible. Therefore, the spirit and scope of the
appended claims should not be limited to the description of the
versions contained therein.
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