U.S. patent application number 14/775580 was filed with the patent office on 2016-01-28 for free-standing metallic article for semiconductors.
The applicant listed for this patent is GTAT CORPOTATION. Invention is credited to Steve Babayan, Robert Brainard, Arvind Chari, Alejandro de la Fuente Vornbrock, Venkatesan Murali, Gopal Prabhu, Arthur Rudin, Venkateswaran Subbaraman, David Tanner, Dong Xu.
Application Number | 20160027947 14/775580 |
Document ID | / |
Family ID | 51521931 |
Filed Date | 2016-01-28 |
United States Patent
Application |
20160027947 |
Kind Code |
A1 |
Babayan; Steve ; et
al. |
January 28, 2016 |
FREE-STANDING METALLIC ARTICLE FOR SEMICONDUCTORS
Abstract
A free-standing metallic article, and method of making, is
disclosed in which the metallic article is electroformed on an
electrically conductive mandrel. The mandrel has an outer surface
with a preformed pattern, wherein at least a portion of the
metallic article is formed in the preformed pattern. The metallic
article is separated from the electrically conductive mandrel,
which forms a free-standing metallic article that may be coupled
with the surface of a semiconductor material for a photovoltaic
cell.
Inventors: |
Babayan; Steve; (Los Altos,
CA) ; Brainard; Robert; (Sunnyvale, CA) ;
Chari; Arvind; (Saratoga, CA) ; de la Fuente
Vornbrock; Alejandro; (San Carlos, CA) ; Murali;
Venkatesan; (San Jose, CA) ; Prabhu; Gopal;
(San Jose, CA) ; Rudin; Arthur; (Morgan Hill,
CA) ; Subbaraman; Venkateswaran; (San Jose, CA)
; Tanner; David; (San Jose, CA) ; Xu; Dong;
(Fremont, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GTAT CORPOTATION |
Merrimack |
NH |
US |
|
|
Family ID: |
51521931 |
Appl. No.: |
14/775580 |
Filed: |
March 10, 2014 |
PCT Filed: |
March 10, 2014 |
PCT NO: |
PCT/US14/22216 |
371 Date: |
September 11, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13798123 |
Mar 13, 2013 |
8916038 |
|
|
14775580 |
|
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|
Current U.S.
Class: |
136/245 |
Current CPC
Class: |
Y02E 10/547 20130101;
H01L 31/0508 20130101; H01L 31/022441 20130101; C25D 1/00 20130101;
H01L 31/022425 20130101; C25D 1/22 20130101; C25D 1/08 20130101;
H01L 31/0682 20130101 |
International
Class: |
H01L 31/05 20060101
H01L031/05; H01L 31/0224 20060101 H01L031/0224; H02S 30/20 20060101
H02S030/20 |
Claims
1.-53. (canceled)
54. A photovoltaic module comprising: a) a backing substrate that
is segmented by a fold line; and b) a plurality of photovoltaic
cells mounted on the backing substrate, each photovoltaic cell
comprising a metallic article, the metallic article comprising: a
plurality of electroformed elements configured to serve as an
electrical conduit for a light-incident surface of the photovoltaic
cell, the plurality of electroformed elements comprising a cell
interconnection element integral with a continuous grid having a
plurality of first elements intersecting a plurality of second
elements; wherein the cell interconnection element directly couples
the grid to a neighboring cell; wherein the electroformed elements
are interconnected and integral, with the continuous grid in
contact with the light-incident surface and the cell
interconnection element configured to extend beyond the
light-incident surface; wherein one of the cell interconnection
elements serves as a foldable interconnection lying across the fold
line, the foldable interconnection electrically coupling a pair of
photovoltaic cells that lies on either side of the fold line.
55. The photovoltaic module of claim 54, wherein the cell
interconnection element comprises a strip spanning a length of the
continuous grid.
56. The photovoltaic module of claim 55, wherein the strip lies
along the fold line.
57. The photovoltaic module of claim 55, wherein the strip
comprises openings along the fold line.
58. The photovoltaic module of claim 55, wherein the cell
interconnection element further comprises a plurality of segments
coupled to the strip lying across the fold line.
59. The photovoltaic module of claim 54, wherein each electroformed
element in the continuous grid has a height and a width, wherein
the ratio of the height to the width is an aspect ratio, and
wherein a majority of the electroformed elements have an aspect
ratio greater than 0.1.
60. The photovoltaic module of claim 59, wherein the aspect ratio
is greater than 1.0.
61. The photovoltaic module of claim 54, further comprising a
plurality of fold lines and a plurality of foldable
interconnections lying across the fold lines, wherein the
photovoltaic cells are electrically connected in series by the cell
interconnection elements and the foldable interconnections.
62. The photovoltaic module of claim 54, further comprising a
plurality of holes positioned along edges of the backing substrate,
the holes configured to accommodate a pull cord.
63. The photovoltaic module of claim 54, wherein the backing
substrate comprises a vertical fold line and a horizontal fold line
configured such that the photovoltaic module has bi-directional
folding capability.
64. The photovoltaic module of claim 54, wherein a first
photovoltaic cell in the plurality of photovoltaic cells comprises
a semiconductor substrate having a scored line such that the first
photovoltaic cell is capable of bending, and wherein the metallic
article of the first photovoltaic cell spans over and remains
intact across the scored line of the semiconductor substrate.
65. The photovoltaic module of claim 64, wherein the semiconductor
substrate comprises a plurality of scored lines, and wherein the
first photovoltaic cell is mounted on a curved or uneven surface of
the photovoltaic module.
66. A photovoltaic module comprising: a) a backing substrate that
is segmented by a fold line into a plurality of segments; b) a
plurality of photovoltaic cells mounted on the backing substrate,
each photovoltaic cell comprising a metallic article, the metallic
article comprising: a plurality of electroformed elements
configured to serve as an electrical conduit for a light-incident
surface of the photovoltaic cell, the plurality of electroformed
elements comprising a cell interconnection element integral with a
continuous grid having a plurality of first elements intersecting a
plurality of second elements; wherein the cell interconnection
element directly couples the grid to a neighboring cell within one
of the segments of the photovoltaic module; wherein the
electroformed elements are interconnected and integral, with the
continuous grid in contact with the light-incident surface and the
cell interconnection element configured to extend beyond the
light-incident surface; and c) a foldable interconnection lying
across the fold line and electrically coupling photovoltaic cells
that are in different segments of the photovoltaic module.
67. A photovoltaic cell comprising: a) a metallic article
comprising: a plurality of electroformed elements, the plurality of
electroformed elements comprising a cell interconnection element
integral with a continuous grid having a plurality of first
elements intersecting a plurality of second elements; wherein the
electroformed elements are interconnected and integral, and are
configured to serve as an electrical conduit for a light-incident
surface of the photovoltaic cell, with the continuous grid in
contact with the light-incident surface and the cell
interconnection element configured to extend beyond the
light-incident surface; and b) a semiconductor substrate to which
the metallic article is attached, the semiconductor substrate
having a scored line such that the photovoltaic cell is capable of
bending, wherein the metallic article spans over and remains intact
across the scored line of the semiconductor substrate.
68. The photovoltaic cell of claim 67, wherein the semiconductor
substrate comprises a plurality of scored lines.
Description
RELATED APPLICATIONS
[0001] This application claims priority to U.S. patent application
Ser. No. 13/798,123, entitled "Free-Standing Metallic Article for
Semiconductors" filed on Mar. 13, 2013, which is owned by the
assignee of the present application and is hereby incorporated by
reference. This application is also related to 1) Babayan et al.,
U.S. patent application Ser. No. 13/798,124, entitled
"Free-Standing Metallic Article for Semiconductors" and filed on
Mar. 13, 2013; 2) Babayan et U.S. Provisional Patent Application
No. 61/778,443, entitled "Free-Standing Metallic Article for
Semiconductors " and filed on Mar. 13, 2013; and 3) Babayan et al.,
U.S. Provisional Patent Application No. 61/778,444, entitled
"Free-Standing Metallic Article for Semiconductors" and filed on
Mar. 13, 2013; all of which are owned by the assignee of the
present application, and are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] A solar cell is a device that converts photons into
electrical energy. The electrical energy produced by the cell is
collected through electrical contacts coupled to the semiconductor
material, and is routed through interconnections with other
photovoltaic cells in a module. The "standard cell" model of a
solar cell has a semiconductor material, used to absorb the
incoming solar energy and convert it to electrical energy, placed
below an anti-reflective coating (ARC) layer, and above a metal
backsheet. Electrical contact is typically made to the
semiconductor surface with fire-through paste, which is metal paste
that is heated such that the paste diffuses through the ARC layer
and contacts the surface of the cell. The paste is generally
patterned into a set of fingers and bus bars which will then be
soldered with ribbon to other cells to create a module. Another
type of solar cell has a semiconductor material sandwiched between
transparent conductive oxide layers (TCO's), which are then coated
with a final layer of conductive paste that is also configured in a
finger/bus bar pattern.
[0003] In both these types of cells, the metal paste, which is
typically silver, works to enable current flow in the horizontal
direction (parallel to the cell surface), allowing connections
between the solar cells to be made towards the creation of a
module. Solar cell metallization is most commonly done by screen
printing a silver paste onto the cell, curing the paste, and then
soldering ribbon across the screen printed bus bars. However,
silver is expensive relative to other components of a solar cell,
and can contribute a high percentage of the overall cost.
[0004] To reduce silver cost, alternate methods for metallizing
solar cells are known in the art. For example, attempts have been
made to replace silver with copper, by plating copper directly onto
the solar cell. However, a drawback of copper plating is
contamination of the cell with copper, which impacts reliability.
Plating throughput and yield can also be issues when directly
plating onto the cell due to the many steps required for plating,
such as depositing seed layers, applying masks, and etching or
laser scribing away plated areas to form the desired patterns.
Other methods for forming electrical conduits on solar cells
include utilizing arrangements of parallel wires or polymeric
sheets encasing electrically conductive wires, and laving them onto
a cell. However, the use of wire grids presents issues such as
undesirable manufacturing costs and high series resistance.
SUMMARY OF THE INVENTION
[0005] A free-standing metallic article, and method of making, is
disclosed in which a metallic article is electroformed on an
electrically conductive mandrel. The mandrel has an outer surface
with a preformed pattern, wherein at least a portion of the
metallic article is formed in the preformed pattern. The metallic
article is separated from the electrically conductive mandrel,
which forms a free-standing metallic article that may be coupled
with the surface of a semiconductor material for a photovoltaic
cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Each of the aspects and embodiments of the invention
described herein can be used alone or in combination with one
another. The aspects and embodiments will now be described with
reference to the attached drawings.
[0007] FIG. 1A is a perspective view of a conventional solar
cell.
[0008] FIG. 1B is a cross-sectional view of a conventional
back-contact solar cell,
[0009] FIG. 2 shows a perspective view of an exemplary
electroforming mandrel in one embodiment.
[0010] FIGS. 3A-3C depict cross-sectional views of exemplary stages
in producing a free-standing electroformed metallic article.
[0011] FIG. 4 provides a cross-sectional view of one embodiment of
an electrically conductive mandrel.
[0012] FIG. 5 provides a cross-sectional view of another embodiment
of an electrically conductive mandrel.
[0013] FIGS. 6A-6B are top views of two embodiments of metallic
articles.
[0014] FIG. 6C is a cross-sectional view of section B-B of FIG.
6B.
[0015] FIGS. 6D-6E are partial cross-sectional views of vet further
embodiments of the cross-section of FIG. 6B.
[0016] FIGS. 6F-6G are top views of yet further embodiments of
metallic articles.
[0017] FIG. 7 is an exemplary flow chart of a process for
manufacturing an electroformed article and forming a semiconductor
device such as a solar cell.
[0018] FIGS. 8A-8B are perspective views of exemplary solar cells
fabricated with a free-standing metallic article.
[0019] FIG. 8C is a cross-sectional view of another embodiment of a
solar cell.
[0020] FIG. 8D is a top view of exemplary metallic articles used in
the solar cell of FIG. 8C.
[0021] FIGS. 9A-9B illustrate an embodiment of tailoring features
of an electroformed element.
[0022] FIGS. 10A-10B illustrate another embodiment of tailoring
features of an electroformed element.
[0023] FIGS. 11A-11C are cross-sectional views of stages of forming
a metallic article with a dielectric transfer layer, in one
embodiment.
[0024] FIG. 12 depicts a cross-sectional view of an embodiment of a
metallic article being removed using a polymer sheet.
[0025] FIG. 13A-13B are cross-sectional views of an exemplary
polymer layer being fabricated into a back-contact solar cell.
[0026] FIG. 14 shows an exemplary flow chart of a method of
manufacturing a polymer layer with an electroformed article and
forming a semiconductor device such as a solar cell.
[0027] FIGS. 15A-15D provide perspective views of exemplary stages
in using a metallic article as a mask for pattering a conductive
layer on a semiconductor material.
[0028] FIG. 16 is an exemplary flow chart of a method of using a
metallic article as a mask for pattering a conductive layer on a
semiconductor material.
[0029] FIG. 17 is a cross-sectional view of an exemplary
cylindrical mandrel.
[0030] FIG. 18 shows a cross-sectional view of an embodiment of a
flat mandrel having patterns on its top and bottom surfaces.
[0031] FIG. 1.9 shows a top view of an embodiment of a flexible
module, with fold lines.
[0032] FIG. 20 shows a top view of another embodiment of a flexible
module.
[0033] FIG. 21 provides a top view of further embodiment of a
flexible module, with bi-directional folding capability.
[0034] FIG. 22 is a top view of a flexible solar cell using a
metallic article as described herein.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0035] FIG. 1A is a simplified schematic of a conventional solar
cell 100 which includes an anti-reflective coating (ARC) layer 110,
an emitter 120, a base 130, front contacts 140, and a rear contact
layer 150. Emitter 120 and base 130 are semiconductor materials
that are doped as p+ or n- regions, and may be referred to together
as an active region of a solar cell. Front contacts 140 are
typically fired through anti-reflective coating layer 110 in order
to make electrical contact with the active region. Incident light
enters the solar cell 100 through ARC layer 110, which causes a
photocurrent to be created at the junction of the emitter 120 and
base 130. It can be seen that shading caused by front contacts 140
will affect the efficiency of the cell 100. The produced electrical
current is collected through an electrical circuit connected to
front contacts 140 and rear contact 150. A bus bar 145 may connect
the front contacts 140, which are shown here as finger elements.
Bus bar 145 collects the current from front contacts 140, and also
may be used to provide interconnection between other solar cells.
The assembly of front contacts 140 and bus bar 145 may also be
referred to as a metallization layer. In other types of solar
cells, a transparent conductive oxide (TCO) layer may be used
instead of a dielectric-type ARC layer, to collect electrical
current. In a TCO type of cell, metallization in the form of, for
example, front contacts 140 and bus bar 145 would be fabricated
onto the TCO layer, without the need for firing through, to collect
current from the TCO solar cell,
[0036] FIG. 1B illustrates a simplified schematic of another type
of solar cell 160, in which the electrical contacts are made on the
back side, opposite of where light enters. Solar cell 160, also
known as an interdigitated back contact cell, includes an ARC layer
110, a base region 130 made of a semiconductor substrate, and doped
regions 120 and 125 having opposite polarities from each
other(e.g., p-type and n-type). Doped regions 120 and 125 are on
the back side of cell 160, opposite of ARC layer 110. A
non-conducting layer 170 provides separation between the doped
regions 120 and 125, and also completes the role of passivation of
the hack surface of cell 160. Electrical contacts 140 and 150 are
interdigitated with each other and make electrical connections to
doped regions 120 and 125, respectively, through holes 175 in the
passivating layer 170. Although the electrical contacts 140 and 150
do not present a shading issue in this back-contact type of solar
cell, they may still present other issues such as manufacturing
yield losses when forming the contacts onto the cell, high material
costs if using silver for the contacts, or degradation of the cell
if using copper for the contacts.
[0037] Metallization of solar cells typically involves screen
printing a silver paste in the desired pattern of the electrical
contacts to be connected to the cell. In FIG. 1A, the front
contacts 140 are configured in a linear pattern of parallel
segments. Because the cost of silver can add greatly to the expense
of the solar cell, it is highly desirable to reduce or even
eliminate the use of silver. Copper is an attractive alternative to
silver because of its high electrical conductivity, but can lead to
contamination of the semiconductor materials and consequently
reduced performance of the solar cell. Known methods of utilizing
copper in solar cells involve depositing copper directly onto the
cell. However, these methods require subjecting the solar cells to
the temperatures and chemicals involved with the many steps during
these plating processes, which can cause damage to the cell. In
other known methods, arrangements of parallel copper wires or woven
grids of wires are produced separately from the cell, and then
joined to the cell. However, with these methods it can be difficult
to align the wires to the cell, or to produce wires small enough to
be functional but yet minimize shading on a solar cell. Wire grids
encapsulated within polymeric films have also been produced, but
these methods can be complex and still present shading and
alignment problems, particularly due to the presence of the
polymeric sheet. Copper paste is another alternative, but these
pastes can be difficult to apply and still present the problem of
diffusion into the solar cell.
[0038] In the present disclosure, electrical conduits for
semiconductors, such as photovoltaic cells, are fabricated as an
electroformed free-standing metallic article. The metallic articles
are produced separately from a solar cell and can include multiple
elements such as fingers and bus bars that can be transferred
stably as a unitary piece and easily aligned to a semiconductor
device. The elements of the metallic article are formed integrally
with each other in the electroforming process. The metallic article
is manufactured in an electroforming mandrel, which generates a
patterned metal layer that is tailored for a solar cell or other
semiconductor device. For example, the metallic article may have
grid lines with height-to-width aspect ratios that minimize shading
for a solar cell. The metallic article can replace conventional bus
bar metallization and ribbon stringing for cell metallization,
cell-to-cell interconnection and module making. The ability to
produce the metallization layer for a photovoltaic cell as an
independent component that can be stably transferred between
processing steps provides various advantages in material costs and
manufacturing.
[0039] FIG. 2 depicts a perspective view of a portion of an
exemplary electroforming mandrel 200 in one embodiment. The mandrel
200 may be made of electrically conductive material such stainless
steel, copper, anodized aluminum, titanium, or molybdenum, nickel,
nickel-iron alloy (e.g., Invar), copper, or any combinations of
these metals, and may be designed with sufficient area to allow far
high plating currents and enable high throughput. The mandrel 200
has an outer surface 205 with a preformed pattern that comprises
pattern elements 210 and 212 and can be customized far a desired
shape of the electrical conduit element to be produced. In this
embodiment, the pattern elements 210 and 212 are grooves or
trenches with a rectangular cross-section, although in other
embodiments, the pattern elements 210 and 212 may have other
cross-sectional shapes. The pattern elements 210 and 212 are
depicted as intersecting segments to form a grid-type pattern, in
which sets of parallel lines intersect perpendicularly to each
other in this embodiment.
[0040] The pattern elements 210 have a height `H` and width `W`,
where the height-to-width ratio defines an aspect ratio. By using
the pattern elements 210 and 212 in the mandrel 200 to form a
metallic article, the electroformed metallic parts can be tailored
for photovoltaic applications. For example, the aspect ratio may be
between about 0.01 and about 10. In some embodiments, the aspect
ratio can be designed to be greater than 1, such as between about 1
and about 10, or between about 1 and about 5. Having a height
greater than the width allows the metal layer to carry enough
current but reduce the shading on the cell compared to, for
example, standard circular wires which have an aspect ratio of 1,
or compared to conventional screen-printed patterns which are
horizontally flat and have aspect ratios less than 1. Shading
values for screen-printed metal fingers may be, for example, over
6%. With metallic articles having tailored aspect ratios as
described herein, shading values of less than 6% may be achieved,
such as between 4-6%. Thus, the ability to produce electrical
conduits with aspect ratios greater than 1 enable minimal aperture
loss to a photovoltaic cell, which is important to maximizing
efficiency. In embodiments where the electroformed electrical
conduit is used on a back surface of a solar cell, aspect ratios of
other values, such as less than 1, may be used.
[0041] The aspect ratio, as well as the cross-sectional shape and
longitudinal layout of the pattern elements, may be electroformed
to meet desired specifications such as electrical current capacity,
series resistance, shading losses, and cell layout. Any
electroforming process can be used. For example, the metallic
article may be formed by an electroplating process. In particular,
because electroplating is generally an isotropic process, confining
the electroplating with a pattern mandrel to customize the shape of
the parts is a significant improvement for maximizing efficiency.
Furthermore, although tall yet narrow conduit lines typically would
tend to be unstable when placing them on a semiconductor surface,
the customized patterns that may be produced through the use of a
mandrel allows for features such as interconnecting lines to
provide stability for these tall but narrow conduits. In some
embodiments, for example, the preformed patterns may be configured
as a continuous grid with intersecting lines. This configuration
not only provides mechanical stability to the plurality of
electroformed elements that form the grid, but also enables a low
series resistance since the current is spread over more conduits. A
grid-type structure can also increase the robustness of a cell. For
example, if some portion of the grid becomes broken or
non-functional, the electrical current can flow around the broken
area due to the presence of the grid pattern.
[0042] FIGS. 3A-3C are simplified cross-sectional views of
exemplary stages in producing a metal layer piece using a mandrel.
In FIG. 3A, a mandrel 200 with pattern elements 210 is provided.
The mandrel 200 is subjected to an electroforming process, in which
electroformed elements 310 are formed within the pattern elements
210 as shown in FIG. 3B. In the embodiment of FIGS. 3A-3C, the
pattern elements 210 have been designed with a higher aspect ratio
than those in FIG. 2. The electroformed elements 310 may be, for
example, copper only, or in other embodiments, alloys of copper. In
other embodiments, a layer of nickel may be plated onto the mandrel
200 first, followed by copper so that the nickel provides a barrier
against copper contamination of a finished semiconductor device. An
additional nickel layer may optionally be plated over the top of
the electroformed elements 310 to encapsulate the copper, as
depicted by nickel layer 315 in FIG. 3B. In other embodiments,
multiple layers may be plated within the pattern elements 210,
using various metals as desired to achieve the necessary properties
of the metallic article to be produced.
[0043] In FIG. 3B the electroformed elements 310 are shown as being
formed flush with the outer surface 205 of mandrel 200.
Electroformed element 312 illustrates another embodiment in which
the elements may be overplated. For electroformed element 312,
electroplating continues until the metal extends above the surface
205 of mandrel 200. The overplated portion, which typically will
form as a rounded top due to the isotropic nature of
electroforming, may serve as a handle to facilitate the extraction
of the electroformed element 312 from mandrel 200. The rounded top
of electroformed element 312 may also provide optical advantages in
a photovoltaic cell by, for example, being a refractive surface to
aid in light collection. In yet other embodiments not shown, a
metallic article may have portions that are formed on top of the
surface 205, such as a bus bar, in addition to those that are
formed within the preformed patterns 210.
[0044] In FIG. 3C the electroformed elements 310 are removed from
the mandrel 200 as a free-standing metallic article 300. The
electroformed elements 310 may include intersecting elements 320,
such as would be farmed by patterns 212 of FIG. 2. The intersecting
elements 320 may assist in making the metallic article 300 a
unitary, free-standing piece such that it may be easily transferred
to other processing steps while keeping the individual elements 310
and 320 aligned with each other. The additional processing steps
may include coating steps for the free-standing metallic article
300 and assembly steps to incorporate it into a semiconductor
device. By producing the metal layer of a semiconductor as a
free-standing piece, the manufacturing yields of the overall
semiconductor assembly will not be affected by the yields of the
metal layer. In addition, the metal layer can be subjected to
temperatures and processes separate from the other semiconductor
layers. For example, the metal layer may be undergo high
temperature processes or chemical baths that will not affect, the
rest of the semiconductor assembly.
[0045] After the metallic article 300 is removed from mandrel 200
in FIG. 3C, the mandrel 200 may be reused to produce additional
parts. Being able to reuse the mandrel 200 provides a significant
cost reduction compared to current techniques where electroplating
is performed directly on a solar cell. In direct electroplating
methods, masks or mandrels are formed on the cell itself, and thus
must be built and often destroyed on every cell. Having a reusable
mandrel reduces processing steps and saves cost compared to
techniques that require patterning and then plating a semiconductor
device. In other conventional methods, a thin printed seed layer is
applied to a semiconductor surface to begin the plating process.
However, seed layer methods result in low throughputs. In contrast,
reusable mandrel methods as described herein can utilize mandrels
of thick metal which allow for high current capability, resulting
light plating currents and thus high throughputs. Metal mandrel
thicknesses may be, for example, between 0.2 to 5 mm.
[0046] FIGS. 4-5 are cross-sectional views of exemplary mandrels,
demonstrating embodiments of various mandrel and pattern designs.
In FIG. 4, a planar metal mandrel base 420 has a dielectric layer
430 laid over it. The pattern including pattern elements 410 for
forming a metallic article are created in dielectric layer 430. The
dielectric layer 430 may be, for example, a fluoropolymer (e.g.,
Teflon.RTM.), a patterned photoresist (e.g., Dupont Riston.RTM.
thick film resist), or a thick layer of epoxy-based photoresist
(e.g., SU-8). The photoresist is selectively exposed and removed to
reveal the desired pattern. In other embodiments, the dielectric
layer 430 may be patterned by, for example, machining or precision
laser cutting. In this type of mandrel 400 with
dielectric-surrounded pattern elements, electroplating will fill
the trenches of pattern elements 410 from the bottom up, starting
at the metal mandrel base 420. The use of dielectrics or permanent
resists allows for reuse of the mandrel 400, which reduces the
number of process steps, consumable costs, and increases throughput
of the overall manufacturing process compared to consumable
mandrels.
[0047] FIG. 5 shows another mandrel 500 made primarily of metal,
including the cavities for forming a metallic article. When
electroforming with metal mandrel 500, the metal surfaces of a
pattern element 510 allow for rapid plating from all three sides of
the trench pattern. In some embodiments of mandrel 500, a release
layer 520 such as a dielectric or low-adhesion material (e.g., a
fluoropolymer) may optionally be coated onto the mandrel 500, in
various areas as desired. The release layer 520 may reduce adhesion
of the electroformed part to the mandrel 500, or may minimize
adhesion of a substrate, such as an adhesive film, that may be used
to peel the electroformed article from the mandrel. The release
layer 520 may be patterned simultaneously with the metal mandrel,
or may be patterned in a separate step, such as through photoresist
with wet or dry etching. The pattern elements 510, 530 and 540 in
the metal mandrel, may be, for example, grooves and intersecting
trenches, and may be formed by, for instance, machining, laser
cutting, lithography, or electroforming. In other embodiments, the
mandrel 500 may not require a release layer 520 if the surface of
the mandrel that is exposed to the plating solution is selected to
have poor adhesion to the metallic article. For instance, for
electroformed parts that will have a first layer (that is, an outer
layer) of nickel plating, the mandrel 400 may be made of copper.
Copper has low adhesion to nickel and thereby allows the formed,
nickel-coated piece to be easily removed from the copper mandrel.
When applying a release layer 520 to mandrel 500, the relative
depth of the trench pattern element 510 in the metal and the
thickness of the dielectric coating can be selected to minimize
void formation of the metal piece formed within pattern element
510, while still enabling a high plating rate.
[0048] FIG. 5 shows a further embodiment in which the release layer
520 has been extended partially into the depth of pattern element
530. This extension of the coating into pattern element 530 may
enable electroforming rates between that of
dielectrically-surrounded pattern element 410 of FIG. 4 and
metal-surrounded pattern element 510 of FIG. 5. The amount that
release layer 520 extends into the pattern element 530 may be
chosen to achieve a desired electroforming rate. In some
embodiments, release layer 520 may extend into pattern element 530
by, for example, approximately half the amount of the pattern
width. A pattern element 530 with release layer 520 extending into
the trench can allow a more uniform electroplating rate within the
trench, and hence, a more uniform grid can be produced. The amount
that the dielectric or release layer 520 extends into the trench
can be modified to optimize overall plating rate and plating
uniformity.
[0049] FIG. 5 shows yet another embodiment of mandrel 500 in which
the pattern element 540 has tapered walls. The tapered walls are
wider at the outer surface 505 of mandrel 500, to facilitate
removal of a formed metallic element from the patterned mandrel. In
other embodiments not shown, the cross-sectional shape of the
preformed patterns for any of the mandrels described herein may
include shapes such as, but not limited to, curved cross-sections,
beveled edges at the corners of a pattern's cross-section, curved
paths along the length of a pattern, and segments intersecting each
other at various angles to each other.
[0050] FIGS. 6A and 6B illustrate top views of exemplary metal
layers 600a and 600b that may be produced by the electroforming
mandrels described herein. Metal layers 600a and 600b include
electroformed elements embodied here as substantially parallel
fingers 610, which have been formed by substantially parallel
grooves in an electrically conductive mandrel. Metal layer 600b
also includes electroformed elements embodied here as horizontal
fingers 620 that intersect vertical fingers 610, where the fingers
610 and 620 intersect at approximately a perpendicular angle. In
other embodiments, fingers 610 and 620 may intersect at other
angles, while still forming a continuous grid or mesh pattern.
Metal layers 600a and 600b also include a frame element 630 which
may serve as a bus bar to collect current from the fingers 610 and
620. Having a bus bar integrally formed as part of the metallic
article can provide manufacturing improvements. In present
high-volume methods of solar module production, cell connections
are often achieved by manually soldering metal ribbons to the
cells. This commonly results in broken or damaged cells due to
manual handling and stress imparted on the cells by the solder
ribbons. In addition, the manual soldering process results in high
labor-related production costs. Thus, having a bus bar or ribbon
already formed and connected to the metallization layer, as is
possible with the electroformed metallic articles described herein,
enables low-cost, automated manufacturing methods.
[0051] Frame element 630 may also provide mechanical stability such
that metal layers 600a and 600b are unitary, free-standing pieces
when removed from a mandrel. That is, the metal layers 600a and
600b are unitary in that they are a single component, with the
fingers 610 and 620 remaining connected, when apart from a
photovoltaic cell or other semiconductor assembly. Frame element
630 may furthermore assist in maintaining spacing and alignment
between finger elements 610 and 620 for when they are to be
attached to a photovoltaic cell. Frame element 630 is shown in
FIGS. 6A-6B as extending across one edge of metal layers 600a and
600b. However, in other embodiments, a frame element may extend
only partially across one edge, or may border more than one edge,
or may be configured as one or more tabs on an edge, or may reside
within the grid itself. Furthermore, frame element may be
electroformed at the same time as the fingers 610 and 620, or in
other embodiments may be electroformed in a separate step, after
fingers 610 and 620 have been formed.
[0052] FIG. 6C shows a cross-section of metal layer 600b taken at
section B-B of FIG. 6B. Fingers 610 in this embodiment are shown in
as having aspect ratios greater than 1, such as about 1 to about 5,
and such as approximately 2 in this figure. Having a
cross-sectional height greater than the width reduces the shading
impact of metal layer 600b on a photovoltaic cell. In various
embodiments, only a portion of the fingers 610 and 620 may have an
aspect ratio greater than 1, or a majority of the fingers 610 and
620 may have an aspect ratio greater than 1 or all of the fingers
610 and 620 may have an aspect ratio greater than 1. Height IF of
fingers 610 may range from, for example, about 5 microns to about
200 microns, or about 10 microns to about 300 microns. Width `W` of
fingers 610 may range from, for example, about 10 microns to about
5 mm, such as about 10 microns to about 150 microns. The distance
between parallel fingers 610 has a pitch `P`, measured between the
centerline of each finger. In some embodiments the pitch may range,
for example, between about 1 mm and about 25 mm. In FIGS. 6B and
6C, the fingers 610 and 620 have different widths and pitches, but
are approximately equivalent in height. In other embodiments, the
fingers 610 and 620 may have different widths, heights and pitches
as each other, or may have some characteristics that are the same,
or may have all the characteristics the same. The values may be
chosen according to factors such as the size of the photovoltaic
cell, the shading amount for a desired efficiency, or whether the
metallic article is to be coupled to the front or rear of the cell.
In some embodiments, fingers 610 may have a pitch between about 1.5
mm and about 6 mm and fingers 620 may have a pitch between about
1.5 mm and about 25 mm. Fingers 610 and 620 are formed in mandrels
having grooves that are substantially the same shape and spacing as
fingers 610 and 620. Frame element 630 may have the same height as
the fingers 610 and 620, or may be a thinner piece as indicated by
the dashed line in FIG. 6C. In other embodiments, frame element 630
may be formed on above finger elements 610 and 620.
[0053] FIG. 6C also shows that fingers 610 and 620 may be
substantially coplanar with each other, in that the fingers 610 and
fingers 620 have a majority of their cross-sectional areas that
overlap each other. Compared to conventional meshes that are woven
over and under each other, a coplanar grid as depicted in FIG. 6C
can provide a lower profile than overlapping circular wires of the
same cross-sectional area. The intersecting, coplanar lines of
metal layer 600b are also formed integrally with each other during
the electroforming process, which provides further robustness to
the free-standing article of metal layer 600b. That is, the
integral elements are formed as one piece and not joined together
from separate components. FIGS. 6D and 6E show other embodiments of
coplanar, intersecting elements. In FIG. 6D, finger 610 is shorter
in height than 620 but is positioned within the cross-sectional
height of finger 620. Fingers 610 and 620 have bottom surfaces 612
and 622, respectively, that are aligned in this embodiment, such as
to provide an even surface for mounting to a semiconductor surface.
In the embodiment of FIG. 6E, finger 610 has a larger height than
finger 620 and extends beyond the top surface of finger 620. A
majority of the cross-sectional area of finger 610 overlaps the
entire cross-section of finger 620, and therefore fingers 610 and
620 are coplanar as defined in this disclosure.
[0054] FIGS. 6F and 6G show yet other embodiments, in which
electroformed metallic articles enable interconnections between
photovoltaic cells in a module. A typical module has many cells,
such as between 36-60, connected in series. The connections are
made by attaching the front of one cell to the back of the next
cell using solder-coated copper ribbon. Attaching the on in this
way requires a ribbon that is thin, and consequently resistive, so
that the ribbon can bend around the cells without break the cell
edges. The interconnections also typically require three separate
ribbons, each soldered separately. In the embodiment of FIG. 6F, a
metallic article 650 has interconnection elements 660 that have
been integrally electroformed with a first grid region 670.
Interconnection elements 660 have a first end coupled to grid 670,
and are configured to extend beyond the surface of a photovoltaic
cell to allow connection to a neighboring cell. The interconnection
elements 660 replace the need for a separate ribbon to be soldered
between cells, thus reducing manufacturing costs and enabling
possible automation. In the embodiment shown, interconnection
elements 660 are linear segments, although other configurations are
possible. Also, the number of interconnection elements 660 can vary
as desired, such as providing multiple elements 660 to reduce
resistance. Interconnection elements 660 may be bent or angled
after electroforming, such as to enable a front-to-back connection
between cells, or may be fabricated in the mandrel to be angled
relative to the grid 670.
[0055] The opposite end of interconnection elements 660 may be
coupled to a second region 680, where the second region 680 may
also be electroformed in an electrically conductive mandrel as part
of the metallic article 650. In FIG. 6F, the second region 680 is
configured as a tab--a bus bar--that may then be electrically
connected to an electrical conduit 690 of a neighboring cell. The
conduit 690 is configured here as a mesh, but other configurations
are possible. Grid 670 may, for example, serve as an electrical
conduit on a front surface of a first cell while grid 690 may be an
electrical conduit on a rear surface of a second cell. In the
embodiment of FIG. 6G, a metallic article 655 has a mesh instead of
a bus bar type of connection. Metallic article 655 includes first
region 670, interconnection elements 660 and second region 690 that
have all been electroformed as a single component, such that the
inter-cell connections are already provided by metallic article
655. Thus the metallic articles 650 and 655 provide electrical
conduits not only on a surface of one photovoltaic cell, but also
the interconnections between cells.
[0056] Although the mandrels described in FIGS. 2-5 have been
described as flat mandrels, the mandrel may instead be cylindrical
to be conducive to a continuous process. FIG. 17 shows a
cross-sectional view of an exemplary cylindrical mandrel 1700, with
preformed pattern 1710 created on outer surface 1720. In such
embodiments, the cylindrical mandrel 1700 may be dipped and rotated
in an electroforming bath, and the resulting unitary metallic
article may be produced as a continuous strip that can later be
trimmed to into separate, unitary pieces as needed. In other
embodiments, a flat mandrel 1800, exemplified in the
cross-sectional view of FIG. 18, may have a first preformed pattern
1810 in a top surface 1820 and a second preformed pattern 1850 in a
bottom surface 1860. The first and second preformed patterns 1810
and 1850 may be the same or different from each other. For example,
in FIG. 18 the first preformed pattern 1810 has elements with
different width, height and pitch than the second preformed pattern
1850. The two-sided mandrel 1800 may be used to produce the two
patterns at once, or in other embodiments, one side may be masked
while the other side is used to produce an electroformed part. In
one embodiment, the first preformed pattern may be used to produce
a metallic article for the front side of a solar cell, and the
second preformed pattern may be used to form a metallic article for
the back side of the solar cell.
[0057] FIG. 7 depicts an exemplary flow chart 700 for fabricating a
free-standing electroformed metallic article for use with a
semiconductor assembly such as a photovoltaic cell. In this
disclosure, reference to semiconductor materials in formation of a
semiconductor device or photovoltaic cell may include amorphous
silicon, crystalline silicon or any other semiconductor material
suitable for use in a photovoltaic cell. In a step 710, an
electroforming process is performed using an electrically
conductive mandrel. The mandrel has one or more preformed patterns
in which to form a metallic article. In some embodiments, the
metallic article is configured to serve as an electrical conduit
within a photovoltaic cell. In certain embodiments, the metallic
article may include features to enable connections between
photovoltaic cells of a solar module. The preformed pattern may
have an aspect ratio of greater than 1, and may include multiple
parallel patterns intersecting each other. At least a portion of
the finished electroformed metallic article is created within the
preformed patterns. Other portions of the metallic article, such as
a bus bar, may be formed within preformed patterns or on a top
surface of the mandrel.
[0058] The electroforming step 710 may include contacting the outer
surface of the mandrel with a solution comprising a salt of a first
metal, where the first metal may be, for example copper or nickel.
The first metal may form the entire metallic article, or may form a
metallic precursor for layers of other metals. For example, a
solution of a salt comprising a second metal may be plated over the
first metal. In some embodiments, the first metal may be nickel and
the second metal may be copper, where the nickel provides a barrier
for copper diffusion. A third metal may optionally be plated over
the second metal, such as the third metal being nickel over a
second metal of copper, which has been plated over a first metal of
nickel. In this three-layer structure, the copper conduit is
encapsulated by nickel to provide a barrier against copper
contamination into a semiconductor device. Electroforming process
parameters in step 710 may be, for example, currents ranging from 1
to 3000 amps per square foot (ASF) and plating times ranging from,
for example, 1 minute to 200 minutes. Other electrically conductive
metals may be applied to promote adhesion, promote wettability,
serve as a diffusion barrier, or to improve electrical contact,
such as tin, tin alloys, indium, indium alloys, bismuth alloys,
nickel tungstate, or cobalt nickel tungstate.
[0059] After the metallic article is formed, the metallic article
is separated step 720 from the electrically conductive mandrel to
become a free-standing, unitary piece. The separation may involve
lifting or peeling the article from the mandrel, with or without
the use of a temporary polymeric sheet, or with or without the use
of vacuum handling. In other embodiments, removal may include
thermal or mechanical shock or ultrasonic energy to assist in
releasing the fabricated part from the mandrel. The free-standing
metallic article is then ready to be formed into a photovoltaic
cell or other semiconductor device, by attaching and electrically
coupling the article as shall be described below. Transferring of
the metallic article to the various manufacturing steps may be done
without need for a supporting element, such as a plastic or
polymeric substrate, which can reduce cost.
[0060] The free-standing metallic article may be mounted directly
to a solar cell or may undergo additional processing steps prior to
being attached. Note that for the purposes of this disclosure, the
term "metallic article" may also be interchangeably referred to as
a grid or mesh, even though some embodiments may not include
intersecting cross-members. If the metallic article has been formed
without a barrier layer, the separated, free-standing metallic
article may optionally undergo additional plating operations in
step 730. For example, nickel plating may be performed by, for
example, electroless or electroplating. In some embodiments, the
metallic article may also be plated with nickel-cobalt-tungsten or
cobalt-tungsten-phosphorous to create a diffusion barrier for
copper material at high temperatures, while the standard nickel
plating prevents copper migration in the cell below 300.degree.
C.
[0061] After any additional plating has been completed, in step 740
an attachment mechanism may be applied to the free-standing
metallic article to prepare it for being mounted to a cell surface.
For a standard solar cell model, a reactive metal layer such as a
fire-through silver paste may be applied to the surface of the
metallic article that is to be coupled to the solar cell. The
reactive paste provides the electrical connection between the
metallic article and the semiconductor layer, and may be thinly
applied. The paste may be applied to the electroformed metallic
article by, for example, screen printing. The amount of silver that
is applied to the grid is much less than that which is required
when forming the metallization layer solely from fire-through
paste. Because the fire-through paste is applied onto the grid
rather than the solar cell, the electrical coupling between the
grid and solar cell is self-aligned. That is, there is no need to
align the fingers of the electrical conduit to conductive lines of
paste that have been applied onto the solar cell, thus simplifying
the manufacturing process. Furthermore, in conventional methods,
extra paste is often applied to ensure alignment with electrical
contacts. In contrast, the present methods enable the application
of silver paste only where necessary. Additional methods of
applying the attachment mechanism include electroplating;
electroless plating; wave soldering; physical vapor deposition
techniques such as evaporation or sputtering; dispensing via
ink-jet or pneumatic dispensing techniques; or thin film transfer
techniques such as stamping the grid onto a thin film of molten
solder or metal.
[0062] While some types of solar cells use dielectric ARC's, other
types use conductive ARC's, such as TCO's. For TCO types of solar
cells, such as those coated with indium-tin-oxide (ITO), the
attachment mechanism in step 740 may be solder, such as a low
temperature solder. The solder is applied to the surface of the
grid that will be in contact with the cell. By applying solder to
the grid, a minimal amount of solder is used, thus reducing
material cost. In addition, the solder is self-aligned with the
grid pattern. The type of solder on the metallic article may be
chosen for characteristics such as good ohmic contact and
electrical conductivity, strong adhesion, rapid thermal
dissipation, low coefficient of thermal expansion (CTE) mismatch
with the targeted surface, robust mechanical stress relief, high
mechanical strength, solid electrical migration barrier, adequate
wettability, and chemically sound material inter-diffusion barriers
between the metallic electroformed grid and the surface of the
solar cell. In one embodiment, a no-clean solder may be applied. In
another embodiment, an electroless or electroplated low melting
point metal or alloy--such as, but not limited to, indium,
indium-tin, indium-bismuth, lead-tin-silver-copper,
lead-tin-silver, and lead-indium--may be applied to the grid. In a
further embodiment, a solder paste may be printed onto the grid.
The solder paste may require a drying process before the grid and
the solar cell can be coupled together. In yet another embodiment,
the tips--that is, the bottom surface--of the grid may be dipped or
immersed into a liquid solder, which will selectively attach to the
mesh surface.
[0063] Although the attachment mechanisms above have been described
as being applied to the electroformed article, in other
embodiments, step 740 may include applying the fire-through paste
or solder material to the solar cell. The electroformed article
would then be brought into contact with the conductive patterns
made by the paste or solder. The metallic article may be prepared
for contacting with the cell by optionally applying an indium metal
or indium alloy to the article. The indium can be electroplated
onto the surface of the grid by dipping the grid into the
electrolyte while providing current. In another embodiment, the
grid may be coated by an electroless plating method by dipping it
into a solution of indium. The grid can be dipped first into a
molten flux, which removes oxide on the tips of the grid, and then
into an indium tin solder such that only the tips of the grid are
wetted with the indium tin solder. In another embodiment, the grid
can be dipped into indium tin paste followed by an anneal step,
again with only the tips of the grid being coated. Coating of only
the tip, and not the entire grid, with indium preserves precious
indium while still achieving a contactable surface. Once
indium-tipped, the fingers or elements of the electroformed article
may then be aligned with the fire-through paste or solder on the
cell by, for example, optical alignment marks on edges of the solar
cell.
[0064] In further embodiments, the metallic articles may be
utilized in back-contact types of solar cells, such as those
illustrated in FIG. 1B, using similar methods. An attachment
mechanism, which would typically be solder, is applied to either
the metallic article or the solar cell in step 740, and the
metallic article is then contacted with the cell. The attachment
mechanism is heated to electrically couple the metallic article
with the cell. In one embodiment of back contact solar cells, the
electroformed elements of a first metallic article would be coupled
to the p-type regions on the rear surface of the cell, while the
electroformed elements of a second metallic article would be
coupled to the n-type regions. For example, the metallic articles
could be configured with linear fingers, as in FIG. 6A, and the
fingers of the first metallic article would be interdigitated with
the fingers of the second metallic article.
[0065] After an attachment mechanism has been applied to the
metallic article, the metallic article is coupled to the cell or
semiconductor device surface in step 750. The metallic article is
brought into contact with the surface of the solar cell. If the
grid article has been tipped with fire-through silver paste, the
assembly is heated to the fire-through temperature of the paste,
such as to temperatures of at least 400.degree. C., or at least
800.degree. C. The grid may be held mechanically stable during
firing by the use of rollers or clamps. Once the fire-through paste
is set, neighboring solar cells in a module may be interconnected.
For solder-tipped grids, the grid is similarly coupled to the solar
cell and heated to temperatures required for the particular solder
typically ranging between 100.degree. C. and 300.degree. C. A
thermal and/or pressure process in atmosphere or vacuum may be used
to reflow the solder and form the contacts between the metallic
article and the solar
[0066] In some embodiments, the independent grid or metallic
article, after being plated with the desired barrier layers, can be
attached to a solar cell prior to anti-reflective coating layer
deposition. In a standard cell, the grid can be contacted to the
emitter surface (e.g., doped silicon) and heated to create a nickel
silicide chemical bond. The ARC, such as a nitride, can then be
deposited after grid attachment, in optional step 760. A bus bar of
the grid can then be connected to another cell in the module. This
embodiment of attaching the grid before the ARC layer eliminates
the need for any silver fire-through usage, in addition, this
embodiment may be applied to silicon heterojunction solar cells.
For instance, the free-standing metallic article, such as a grid,
can be coupled to the surface of the heterojunction cell amorphous
silicon layer. It can then be heated to create a nickel silicide
bond, and the ITO layer can be deposited on the grid
afterwards.
[0067] After the completed photovoltaic cell has been formed in
step 750, the multiple cells that form a solar module may be
interconnected in step 770. In some embodiments, the bus bars or
tabs that have been electroformed as part of the metallic article
may be utilized for these interconnections.
[0068] It can be seen that the free-standing electroformed metallic
article described herein is applicable to various cell types and
may be inserted at different points within the manufacturing
sequence of a solar cell. Furthermore, the electroformed electrical
conduits may be utilized on either the front surface or rear
surface of a solar cell, or both. When electroformed articles are
used on both front and back surfaces, they may be applied
simultaneously to avoid any thermal expansion mismatch which may
cause mechanical bending of the cells.
[0069] FIGS. 8A-8B illustrate a schematic of an exemplary
photovoltaic cell 800 produced with a free-standing metallic
article 810. Metallic article 810 in this embodiment includes
electroformed elements 812 and a frame element 814 that spans an
edge near the perimeter of the electroformed elements 812.
Electroformed elements 812 are shown as parallel lines that
intersect perpendicularly in this embodiment to form a continuous
grid pattern, but in other embodiments they may be configured with
lines intersecting at other angles, or as one set of parallel
lines, or as other patterns. The tips of electroformed elements 812
have an attachment material 820, such as solder or fire-through
silver paste, applied to them. The attachment material 820
electrically couples the metallic article 810 to a photovoltaic
component 830, where the photovoltaic component 830 may include
light incident layer 832 (e.g., ARC and/or TCO), active region 834
(emitter and base), and rear contact layer 836. FIG. 8B Shows
another embodiment of a photovoltaic cell 800 in which layer 832 is
an ARC, in which the attachment material 820 is a silver paste that
has been fired through the ARC. In FIGS. 8A-8B, an encapsulant (not
shown) may be applied over metallic article 810 to seal the
completed photovoltaic cell 800, with interconnection with other
cells being made with the frame element 814. In other embodiments,
a second metallic article 810 may be similarly coupled to rear
contact layer 836, which is a non-incident light surface, to
provide an electrical contact of opposite polarity for the
photovoltaic cell 800.
[0070] FIGS. 8C-8D show simplified schematics of an exemplary
back-contact solar cell 801 produced with free-standing metallic
articles. In the cross-sectional view of FIG. 8C, solar cell 801
includes transparent layer 831 (e.g., an ARC), semiconductor
substrate 833, doped regions 835 and 837, and passivating layer
840. Two free-standing metallic articles 850 and 860 have
electroformed elements that are positioned in an alternating
fashion. The electroformed elements of metallic articles 850 and
860 provide electrical contact with doped regions 835 and 837,
respectively, through the holes 845 in passivating layer 840. FIG.
8D shows a top view of metallic articles 850 and 860 used in solar
cell 801. Metallic article 850 has fingers 852 that arc
interdigitated with fingers 862 of metallic article 860. Frame
elements 854 of metallic article 850 and frame clement 864 of
metallic article 860 serve as an electrical connection point for
each metallic article, and also provide mechanical stability.
[0071] FIGS. 9A-9B illustrate yet another embodiment in which the
shading impact of solder applied between the metallic article and
solar cell can be reduced. FIG. 9A shows a vertical cross-sectional
view of a standard solder joint 910 that may result from soldering
a metal element 920, having a rectilinear cross-section, to a solar
cell 930. Because solder naturally forms a wetting angle between
the surfaces that it is joining, the solder has a footprint with a
width `F1`. The width of this footprint will block light from
entering the solar cell 930, and thus causes shading. In FIG. 9B,
the cross-sectional shape of electroformed element 925 has been
altered compared to electroformed element 920, in that
electroformed element 925 has chamfered corners 927 on its lower
surface. The chamfering changes the wetting angle of the solder
joint 910, such that the footprint width `F2` is less than `F1`.
Thus, the tailored shape of electroformed element 925 reduces
shading. The ability to customize the cross-sectional Shape of
electroformed element 925 is made possible by the use of an
electroforming mandrel, as described in the various embodiments
above. Features such as chamfering, filleting, dimples, nubs, and
the like may be formed in the mandrel to impart these features to
the electroformed part that is to be produced.
[0072] FIGS. 10A-10B show top views of another embodiment of
reducing the shading impact from solder applied to a metallic
article. FIG. 10A shows a conventional solder joint 1010 applied to
two obliquely intersecting linear segments 1020. The total
footprint of the solder joint 1010 has a width `F3`. FIG. 10B shows
electroformed elements 1025 where concave cut-out features 1027
have been incorporated into the corners where the electroformed
elements 1025 intersect, through features of the mandrel in which
the elements 1025 have been formed. The concave features 1027
changes the wetting angle of the solder 1010, such that the
footprint width `F4` is reduced compared to `F3`. Shapes other than
the concave features shown here are possible. Thus, the ability to
tailor the shape of the electroformed elements, by incorporating
features into a forming mandrel, can reduce the shading impact of
the solder that is used to couple the electroformed elements to a
photovoltaic surface.
[0073] In another embodiment shown in FIGS. 11A-11C, a portion of
the mandrel in which the metallic article is formed may become part
of a final semiconductor device. FIG. 11E shows a cross-sectional
view of a mandrel 1100 similar to previously described mandrel 400
of FIG. 4, having a metal base 1120 and a dielectric layer 1130
with patterns for farming electroformed elements 1110.
Electroformed elements 1110 have been formed in dielectric layer
1130 during the electroforming process. In addition, the plating
thickness may also exceed the height of the mandrel patterns to
form overplated heads 1112. In other embodiments, no overplating is
performed, as in electroformed elements 1111. When removing the
metallic article comprising electroformed elements 1130 from the
mandrel 1100, dielectric layer 1130 may be peeled off, along with
the electroformed elements 1130, from mandrel metal base 1120 as
indicated by arrow 1140. The heads 1112 may help secure the
electroformed elements 1110 to the dielectric layer 1130.
[0074] In FIG. 11B the separated metallic article 1150, which is a
combination of electroformed elements 1110 and is surrounded by the
dielectric layer 1130, may then be coupled to a semiconductor
surface to form, for example, a photovoltaic cell. One embodiment
of a solar cell 1160 is depicted in the simplified schematic of
FIG. 11C. Solar cell 1160 includes a semiconductor assembly 1170.
Metallic article 1150 is coupled to semiconductor assembly 1170,
and is overlaid by an encapsulant 1180 and a window layer 1190 such
as an anti-reflective coating. Encapsulant 1180 may he, for
example, ethylene vinyl acetate (EVA), thermoplastic polyolefin
(TPO) or polyvinyl butyral (PVB). The dielectric layer 1130 of
FIGS. 11A-11C can be chosen to be suitable for the appropriate
semiconductor application. For a photovoltaic cell, the target
characteristics of the transferable dielectric will depend on the
reliability specifications of the intended solar module. Because
the dielectric will be incorporated into the module, it must have a
durability to withstand the lifetime of a solar module. The
dielectric must also be transparent to allow light to be
transmitted to the solar cell, and should also be resistant to
copper diffusion into the cell. One type of suitable dielectric is,
for example solder resistant dielectrics that are know n the
electronic packaging industry.
[0075] In other embodiments, the metallic article described herein
may be combined with a polymer sheet to form a polymer layer. FIG.
12 shows one embodiment of such a method in which a metallic
article having electroformed elements 1210 has been formed with a
mandrel 1220. Electroformed elements 1210 may be configured, for
example, as a set of parallel lines, or sets of intersecting lines
forming a grid. For this embodiment, electroformed elements 1210
have been overplated to form a rounded head 1212 at their top
surface, as has been described above in relation to electroformed
element 312 of FIG. 3B. A polymer sheet 1230 is placed over the
surface of the mandrel and is used to remove the electroformed
elements 1210 from the mandrel 1220. FIG. 12 shows a state in which
the polymer sheet 1230 and electroformed elements 1210 have been
lifted from the mandrel 1220. The polymer sheet 1230 is contacted
to the mandrel such that the heads 1212 of electroformed elements
1210 are at least partially embedded into the polymer sheet 1230.
The heads 1212 enable the polymer sheet 1230 to grip the
electroformed elements 1210 because of the larger surface area, and
the heads 1212 also may serve as anchor points. Note that although
the heads 1212 are embodied with curved surfaces, other shapes are
possible. In addition, for some metallic articles and mandrels,
overplating may not be needed. The polymer sheet 1230 with the
embedded heads 1212 of electroformed elements 1210 is lifted from
the mandrel 1220, which pulls the heads 1212 upward, which in turn
lifts the electroformed elements 1210 of the metallic article off
the mandrel 1220. The bottom of the electroformed elements 1212
remains exposed from the polymer sheet 1230, hanging from these
anchor points, which allows them to be subsequently coated or
plated as needed.
[0076] The polymer sheet 1230 may be made of, for example, EVA, TPO
or PVB. Polymer sheet 1230 may optionally be structured as a
substrate layer 1232 covered by an adhesive layer 1234. The
adhesive layer 1234 faces the mandrel, to engage the electroformed
elements 1210. The substrate layer 1232 may be, for example,
polyethyelene, polyester or polyester films (e.g., Mylar.RTM.) and
the adhesive layer 1234 may be, for example, EVA or TPO. If the
polymer sheet 1230 includes an adhesive, mandrel 1220 may include
an optional release layer 1225 to allow the polymer sheet 1230 to
be easily peeled from the mandrel 1220. Release layer 1225 may be,
for example, a fluoropolymer, or other low-adhesion materials. The
adhesive layer 1234 is made with a thickness to enable the heads
1212 to be at least partially embedded in it.
[0077] In some embodiments, the polymer sheet 1230 is used
primarily to remove the electroformed elements 1212 from the
mandrel, such as to serve as a transfer material. The polymer sheet
1230 can then be separated from the electroformed elements 1212,
resulting in a free-standing metallic article as has been described
in previous embodiments. Using a polymer sheet to remove the
metallic article from the electrically conductive mandrel can make
the processing conducive to automation, which enables high
throughputs. The polymer sheet can also provide support for the
electroformed metallic article while the article undergoes
additional manufacturing steps. For example, because the bottom
surfaces of the electroformed elements 1210 remain exposed after
being extracted from mandrel 1220, the polymer sheet 1230 may be
used to hold the metallic article while the bottom surfaces are,
for example, plated with barrier layers or applied with solder or
fire-through paste. The polymer sheet 1230 may also provide
additional mechanical support to preserve the dimensions of the
grid during handling.
[0078] in other embodiments, the polymer sheet may become a
component in a final semiconductor device in which the metallic
article is to be placed. FIGS. 13A-B show an exemplary embodiment,
in which a polymer layer 1315 is placed on a semiconductor
component 1370 to form a photovoltaic cell 1300. In this
embodiment, the polymer layer 1315 serves as an electrical conduit
for the rear surface of the photovoltaic cell 1300. However, the
process described for FIGS. 13A-B may also be utilized for the
polymer layer 1315 serving as a front contact, or both front and
rear. Polymer layer 1315 includes polymer sheet 1330 and
electroformed elements 1310, which are similar to polymer sheet
1230 and electroformed elements 1210 of FIG. 12. The semiconductor
component 1370 may be, for example, a solar cell with layers such
as an active region, rear contact, and TCO layers. In some
embodiments, the polymer layer 1315 may have a reactive metal layer
(not shown) applied to the exposed surface of electroformed
elements 1310, or the reactive metal layer may be applied to the
surface of semiconductor component 1370 that is receiving the
electroformed elements 1310. The polymer layer 1315 is mechanically
and electrically coupled to the cell 1370 using heat and pressure.
The applied heat and pressure pushes the grid into the polymer
material 1330, as shown in FIG. 13B. The electroformed elements
1310 create mechanical anchor points in the polymer 1330 and
provide solid stabilization of the electroformed elements 1310
within polymer layer 1315. The polymer material 1330 is chosen to
have the necessary characteristics of a solar encapsulant material,
such as transparency, durability, wettability and corrosion
resistance, among other constraints which may be necessary
depending on the cell type. The material for polymer sheet 1330 may
be, for example, EVA, TPO, PVB and ionomer.
[0079] FIG. 14 is an exemplary flow chart 1400 for using a
polymeric substrate in combination with an electroformed metallic
article, such as a grid or mesh. In step 1410, a metallic article
is fabricated by an electroforming process using an electrically
conductive mandrel with preformed patterns. The metallic article is
contacted with a polymer sheet in step 1420, where a portion of the
metallic article is embedded within the polymer sheet. In step 1430
the polymer sheet and electroformed elements are lifted or peeled
from the mandrel to separate the polymer layer from the mandrel,
where the polymer layer is a composite of the polymer sheet and the
electroformed grid partially contained in it. In optional step
1440, additional plating or other processes can be performed on the
exposed portions of the electroformed elements. For example, step
1440 may include plating nickel or another barrier material on the
exposed portions of the grid, if nickel was not layered during the
electroforming process. Step 1440 may also include cleaning steps,
such as to remove oxides to prepare the grid for soldering.
[0080] If the polymer sheet is used primarily as a transfer
material, the polymer sheet may be detached from the metallic
article in step 1450. The metallic article can then be processed
into a photovoltaic cell or other semiconductor device in step
1460, which may include performing steps 740 to 770 of FIG. 7. In
other embodiments in which the polymer sheet is to be incorporated
into the finished device, in step 1470 an attachment mechanism may
be applied to either the grid or the semiconductor device, as has
been described in step 740 of FIG. 7. The polymer layer is then
coupled to the semiconductor device, such as by bonding using heat
and pressure, in step 1480. This bonding process results in the
polymer material encapsulating the electroformed grid, and also
electrically couples any solder or fire-through paste between the
grid and the solar cell. The bonding process may include subjecting
the cell and polymer layer to a lamination process with vacuum,
elevated temperature and pressure. Under the lamination conditions,
solder reflows and forms an electrical contact between the cell and
polymer-supported metal grid, while the polymer bonds to the cell
surface and makes a robust mechanical contact. The photovoltaic
cell may then be completed in step 1490 by performing any finishing
steps, such as applying an anti-reflective layer and forming
interconnections with other cells in a solar module. The process of
flow chart 1400 is applicable for both front and backside
connections, as well as to various types of solar cells including
standard, non-standard TCO-coated, and back-contact (e.g.,
interdigitated back contact) cells.
[0081] In yet another embodiment, the metallic article disclosed
herein may be used as a mask for a conductive layer on a
semiconductor surface, wherein the metallic article is consequently
self-aligned with the pattern produced on the conductive layer.
FIG. 15A shows a perspective view of portion of a semiconductor
device 1510, which includes layers for a solar cell. The
semiconductor device 1510 has a conductive metal layer 1520 placed
on its top surface. Conductive metal layer 1520, which may also be
referred to in the industry as a contact layer, may substantially
cover the full surface of semiconductor device 1510. The surface
that is covered by conductive metal layer 1520 may be a light
incident top surface of a solar cell. Conductive metal layer 1520
may be, for example a thin fain of metal deposited onto a standard
solar cell processed just prior to ARC layer deposition or through
completion of a fired-through metal layer. Conductive layer 1520
may alternatively be a TCO layer. In one embodiment, conductive
layer 1520 may be a thin layer of titanium with nickel deposited
over it. The conductive metal layer 1520 is chosen to make good
ohmic contact to the semiconductor device 1510, and provide
excellent adhesion to the semiconductor device 1510 and to the
metal grid that shall be subsequently attached. Conductive metal
layer 1520 may be, for example, titanium, tungsten, chromium,
molybdenum, or combinations thereof, and may be provided on the
semiconductor device using any method known in the art, including
deposition methods such as physical vapor deposition or
electroplating. The thickness of conductive metal layer 1520, in
some embodiments, can be only as thick as necessary to provide a
uniform film that can maintain the required electrical and
mechanical properties.
[0082] A metallic article, embodied as grid 1530 in FIG. 15B, can
be mechanically and electrically coupled to the assembly comprising
the semiconductor device 1510 and conductive metal layer 1520. This
coupling (not shown) can be adhesion through the use of a solder
paste, electrically conductive adhesive, or conventional solder
such that the metal grid 1530 has good electrical and mechanical
contact to the conductive metal layer 1520. The solder, solder
paste, or adhesive may be applied to the grid 1530, such as to the
bottom surface of grid 1530. This grid 1530 is designed such that
it is highly conductive, yet provides a relatively low amount of
shading over the cell. Grid 1530, for example, may have lines with
a tall height to provide sufficient conductivity but a narrow width
to minimize shading.
[0083] The metallic article attached to the conductive metal layer
1520 can be used as a mask to pattern the conductive metal layer
1520, so that the bulk of the solar cell area can be cleared for
light absorption. For example, as shown, a masked region 1540 is
formed directly beneath the grid 1530, while an exposed portion
1545 comprises the remaining portions of conductive metal layer
1520, where the grid 1530 is absent. The exposed portion 1545 can
be removed such that conductive metal layer 1520 becomes patterned
into the shape of the grid 1530. The conductive metal layer 1520
can be patterned by, for example, removing exposed portion 1545
with a wet chemical etch process, a dry etch process such as
reactive ion etching, or by a physical etch process such as, but
not limited to, ion milling. The etching process may remove all or
a portion of the exposed region 1545.
[0084] FIG. 15C shows the assembly after etching, such that only
the masked region 1540 remains on the surface of semiconductor
device 1510 in this embodiment. The masked region 1540 has a
substantially similar pattern as grid 1530, and is coincident with
grid 1530. Thus, the metal grid 1530 provides a chemically
resistant mask in the case of wet or reactive ion etching, and a
mechanical mask in the case of physical etching, allowing for the
coupling and alignment of a separate metallic article to the
semiconductor assembly. In FIG. 15D, a further embodiment is shown
in which the semiconductor device 1510 is a standard cell, and in
which the grid 1530 has been coupled to silicon instead of a TCO.
After etching, a nitride layer 1550 has been deposited onto the
areas that were previously occupied by the exposed portions of
conductive metal layer 1520, to form an ARC layer for the
photovoltaic cell. While not shown, metal grid 1530 may also be
coated.
[0085] FIG. 16 illustrates an exemplary flow chart 1600 for using a
metallic article as a mask. In step 1610, a conductive metal layer
is provided on a surface of a semiconductor material. In step 1620,
a metallic article is electrically and mechanically coupled to the
conductive metal layer. The metallic article may be electroformed
in an electrically conductive mandrel having preformed patterns, as
has been described above and shown, for example, in FIGS. 2-7. The
portions of the surface of the semiconductor material that are
covered by the metallic article are masked regions, and the
uncovered portions are exposed regions. In step 1630, the exposed
regions are partially or fully removed by, for example, one of
various etching processes as has been described in relation to FIG.
15B-15C. The resulting assembly, with the conductive metal layer
that is patterned and self aligned with the metallic article, can
now be processed further for fabrication into a finished
semiconductor device assembly such as a solar cell. By using the
grid as a mask, the total number of process steps is greatly
reduced compared to conventional masking techniques in which a
separate masking and patterning process must be undertaken in order
to pattern the contact layer. Furthermore, the need for alignment
between the metal grid and the conductive lines is eliminated since
the mask is self-aligned with the patterned conductive lines that
are produced. The metallic grid also provides an added level of
robustness compared to conventional fired-through silver
contacts.
[0086] Thus, it can be seen that the use of an electroformed
metallic article as described herein enables the preparation of a
wide variety of different photovoltaic cells and solar cell
modules. The electroformed metallic article may be inserted at
different points within the manufacturing sequence. In addition,
the metallic articles can be specifically designed in order to
efficiently produce cells and modules with additional combinations
of benefits and properties that are not readily possible currently.
For example, since the metallic article can be a unitary piece
spanning and crossing essentially the entire surface of the cell,
improved durability results. In particular, should the solar cell
develop a crack, such as during handling or module production, the
metallic article enables the fractured cell to be held intact due
to the grid-like nature of the metallic article, with minimal
functional loss to the cell. In addition, the spanning of the
metallic article across the cell surface reduces the impact of
solder joint failures. Furthermore, since an electroformed metallic
article can be produced with consistent and predictable thicknesses
throughout, current is carried evenly across a cell. This even
distribution of current dramatically reduces the development of hot
spots on the cell surface, which is presently a primary cause of
degradation and damage of solar cells.
[0087] In some embodiments, by including specific design features
into the metallic article, flexible modules can be prepared as
exemplified in FIGS. 19-21. Such modules can be folded in a compact
form and made easy to carry, such as in a backpack, to be unfolded
and used later, such as in a more remote location. In other
embodiments, the flexible modules may be folded for storage, such
as in a rooftop or awning installation.
[0088] For example, FIG. 19 shows a module 1900 that is foldable
along parallel lines. The module 1900 includes thirty-two separate
cells 1910 in this embodiment, each comprising a metallic article
1920 attached to a semiconductor substrate. The cells 1910 are
positioned on a backing substrate 1930, which may be made of known
backing materials for photovoltaic modules, and may be rigid or
flexible. Backing substrate 1930 is segmented, such as by folding
or scoring, to form fold lines 1941, 1942 and 1943. The cells 1910
are electrically connected in series, in a serpentine order from
the first cell 1910a to the fourth cell 1910b, to the fifth cell
1910c, to the eighth cell 1910d, and so on to the last cell 1910e.
Electrical connections between cells 1910 can be achieved using
features of the metallic articles as described above, such as by
using interconnection elements 660 and 680 of FIGS. 6F-6G.
[0089] For interconnections between cells that lie across fold
lines 1941, 1942, and 1943 in FIG. 19, foldable interconnections
1950 are provided. For example, the connection from cell 1910d to
the next set of cells crosses fold line 1941. Thus, the metallic
article for 1910d is designed with a foldable interconnection 1950,
while the interconnection between cells 1910b and 1910c does not
cross a fold line, and therefore does not have a foldable
interconnection between them. The foldable interconnections 1950
can be a solid piece of material, such as a sheet or strip of
copper, with a thickness sufficient to allow it to be readily
folded without cracking or breaking. Thus, foldable interconnection
1950 serves as a living hinge. In some embodiments, foldable
interconnection 1950 may include openings 1960 that provide
additional flexibility. The foldable interconnection 1950 may be,
for example, an elongated version of the interconnections between
non-folding cells. In some embodiments, the foldable
interconnections 1950 can be integral components that are
electroformed as part of the metallic articles. In other
embodiments, the foldable interconnections 1950 can be elements
that are formed separately from the metallic articles, such as by
electroforming or stamping, and subsequently joined to the metallic
articles of the required cells. By arranging cells 1910 and
foldable interconnections 1950 on substrate 1930 as shown, with
interconnections 1950 straddling fold lines 1941, 1942 and 1943,
the resulting module 1900 can be folded. In the embodiment of FIG.
19, fold lines 1941 and 1943 are foldable as a mountain fold, while
fold line 1942 is foldable as a valley fold, as indicated by the
curved arrows. Consequently, the module 1900 is folded such that
panels A, B, C, and D stack on top of each other.
[0090] FIG. 20 shows another embodiment of a flexible module 2000
similar to FIG. 19, but with a greater number of cells. Module 2000
has fold lines 2041, 2042 and 2043 between panels A, B, C and D,
with foldable interconnections 2050 across the fold lines 2041,
2042 and 2043. Module 2000 may be folded accordion-style similarly
to module 1900, such as with fold lines 2041, 2042 and 2043
alternating between mountain folds and valley folds. Also shown in
FIG. 20 are holes 2070 which enable a pull cord such as a cable or
guide wire to contract the module into a folded configuration.
Holes 2070 in this embodiment are positioned at the edges of the
module 2000, and near the fold lines 2041, 2042 and 2043 to apply
tension at the folding joints. Holes 2070 may include
reinforcements such as eyelets or grommets, to increase durability.
A cable mounting system as described with folding module 2000 may
be used, for example, for opening and storage of an awning type of
photovoltaic module.
[0091] Although the foldable interconnections in FIGS. 19 and 20
are shown as approximately rectangular, other shapes are possible.
Additionally, although the foldable interconnections in FIGS. 19
and 20 are shown as centered along the edge of a cell and
encompassing approximately most of the edge length, in other
embodiments the foldable interconnections may extend along only a
portion of an edge of a cell, or may be off centered along the
edge, such as at a corner. The specific configuration of the
foldable interconnect may be designed to accommodate the fold
geometry of a particular module.
[0092] FIG. 21 shows a further embodiment of a flexible module 2100
that has bi-directional folding capability. In addition to vertical
fold lines 2141, 2142 and 2143, module 2100 has a horizontal fold
line 2145 that extends through approximately the mid-line of the
module 2100 in this embodiment. Accordingly, foldable
interconnections 2151 are utilized between adjacent cells that lie
across the fold line 2145. The module 2100 may consequently be
folded to a compact size in two directions, similar to a road map.
For example, the module 2100 may be folded in half along fold line
2145, and then accordion folded along fold lines 2141, 2142 and
2143, as indicated by the curved arrows.
[0093] FIG. 22 illustrates an alternative method of forming
flexible modules that takes advantage of the mechanical support
provided by the metallic article attached to the cell. For example,
the semiconductor substrate 2210 of solar cell 2200 can be scored
or otherwise cut into separate pieces along dashed line 2240 while
metallic article 2220 is attached. As long as the grid of the
metallic article 2220 remains intact, the separate pieces of the
semiconductor substrate 2210 will remain attached to the cell 2200,
and as a result, the cell 2200 is capable of bending or flexing
along the cut line 2240. Additional scoring and cut line formation
would provide additional degrees of flexibility. For example, the
semiconductor substrate can be scored into 2 to 36 sections in some
embodiments. In this way, an individual cell with an attached
metallic article as described herein can be made to be flexible,
allowing it to fit along a curved or uneven surface as part of a
module, particularly when combined with foldable interconnections
such as is shown in FIGS. 19-21. Other additional benefits and
properties will become apparent to one of ordinary skill in the art
given the detailed description provided herein.
[0094] Although the embodiments herein have primarily been
described with respect to photovoltaic applications, the methods
and devices may also be applied to other semiconductor applications
such as redistribution layers (RDL's) or flex circuits.
Furthermore, the flow chart steps may be performed in alternate
sequences, and may include additional steps not shown.
[0095] While the specification has been described in detail with
respect to specific embodiments of the invention, it will be
appreciated that those skilled in the art, upon attaining an
understanding of the foregoing, may readily conceive of alterations
to, variations of, and equivalents to these embodiments. These and
other modifications and variations to the present invention may be
practiced by those of ordinary skill in the art, without departing
from the scope of the present invention, which is more particularly
set forth in the appended claims. Furthermore, those of ordinary
skill in the art will appreciate that the foregoing description is
by way of example only, and is not intended to limit the
invention.
* * * * *