U.S. patent application number 13/096952 was filed with the patent office on 2011-11-03 for thin film coating pinning arrangement.
This patent application is currently assigned to SKYLINE SOLAR, INC.. Invention is credited to Harold D. Ackler, Marc A. Finot.
Application Number | 20110265869 13/096952 |
Document ID | / |
Family ID | 44814754 |
Filed Date | 2011-11-03 |
United States Patent
Application |
20110265869 |
Kind Code |
A1 |
Finot; Marc A. ; et
al. |
November 3, 2011 |
THIN FILM COATING PINNING ARRANGEMENT
Abstract
In one aspect of the present invention, a photovoltaic cell for
use in a solar collector is described. The photovoltaic cell
includes two electrically conductive layers that are positioned on
a surface of a semiconductor substrate. The first conductive layer
is adhered to and in direct contact with a surface of the
semiconductor substrate. The second conductive layer has a
different composition from and is substantially more electrically
conductive than the first conductive layer. There are multiple
spaced apart pinning regions that are distributed through an
interface between the first and second conductive layers. The
pinning regions, help locally anchor the two layers. Some aspects
of the present invention relate to the use of pinning regions in
other types of optical or electrical components.
Inventors: |
Finot; Marc A.; (Palo Alto,
CA) ; Ackler; Harold D.; (Sunnyvale, CA) |
Assignee: |
SKYLINE SOLAR, INC.
Mountain View
CA
|
Family ID: |
44814754 |
Appl. No.: |
13/096952 |
Filed: |
April 28, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61329482 |
Apr 29, 2010 |
|
|
|
Current U.S.
Class: |
136/256 ;
136/259; 257/E31.124; 438/98 |
Current CPC
Class: |
F24S 2023/86 20180501;
G02B 19/0023 20130101; H01L 31/02168 20130101; Y02E 10/40 20130101;
F24S 23/77 20180501; Y02E 10/52 20130101; G02B 19/0019 20130101;
Y10T 428/249923 20150401; H01L 31/056 20141201; G02B 7/008
20130101; G02B 7/181 20130101; G02B 19/0042 20130101; Y10T
428/24826 20150115 |
Class at
Publication: |
136/256 ;
136/259; 438/98; 257/E31.124 |
International
Class: |
H01L 31/0216 20060101
H01L031/0216; H01L 31/18 20060101 H01L031/18; H01L 31/0232 20060101
H01L031/0232 |
Claims
1. A photovoltaic cell comprising: a semiconductor substrate that
includes a first surface; a first electrically conductive layer
that is adhered to the first surface of the semiconductor
substrate; a second electrically conductive layer that is adhered
to, has a different composition from and is substantially more
electrically conductive than the first conductive layer; and a
multiplicity of spaced apart pinning regions that are distributed
at the interface between the first and second conductive layers,
wherein the pinning regions locally anchor the first conductive
layer to the second conductive layer and wherein the adhesive
strength of the pinning regions to the second conductive layer is
greater than the adhesive strength of the first conductive layer to
the second conductive layer.
2. A photovoltaic cell as recited in claim 1, wherein the centers
of the pinning regions are spaced apart by a distance d, the
distance d being less than approximately an estimated minimum size
of a buckling feature in the interface when the interface lacks
pinning regions.
3. A photovoltaic cell as recited in claim 1 wherein the pinning
regions occupy less than 5% of the interface between the first
layer and the second layer.
4. A photovoltaic cell as recited in claim 1, wherein the pinning
regions are defined by adhesive pinning elements positioned between
the first layer and the second layer and the adhesive pinning
elements are made of one selected from the group consisting of
titanium, chrome, nickel, nickel chrome alloy, glass and
polymer.
5. A photovoltaic cell as recited in claim 1, wherein the pinning
regions are defined by adhesive pinning elements positioned between
the first layer and the second layer and the adhesive pinning
elements are deposited using one selected from the group consisting
of spraying, electroplating, vapor deposition, electron beam
deposition, sputtering, screen printing, electroless plating, and
chemical deposition.
6. A photovoltaic cell as recited in claim 1, wherein the pinning
regions are formed by treating selected areas of the first layer
such that the adhesive properties of the selected areas are
substantially greater than in adjacent untreated areas.
7. A photovoltaic cell as recited in claim 1, wherein the spacing
between the pinning regions is based at least in part on one
selected from the group consisting of: 1) the stiffness of the
second conductive layer, the strength of adhesion between the first
and second conductive layers and an estimation of force that may be
applied to the second conductive layer; and 2) a minimum radius of
a circular buckle a.sub.m, a.sub.m being based at least partly on
the following: a m = 1.106 h f E f .sigma. m , ##EQU00003## wherein
h.sub.f is the thickness of a buckling layer, E.sub.f is the
in-plane Young's modulus of the buckling layer and .sigma..sub.m is
the mismatch stress in the buckling layer, the buckling layer being
selected from the group consisting of the first conductive layer
and the second conductive layer.
8. A photovoltaic cell as recited in claim 1 wherein the pinning
regions are substantially less electrically conductive than the
first and second conductive layers.
9. A photovoltaic cell as recited in claim 1 wherein: the second
conductive layer is formed from silver, the first conductive layer
is formed from aluminum; and the semiconductor substrate is formed
from doped silicon.
10. A photovoltaic cell as recited in claim 1 wherein adjacent
pinning regions of the multiplicity of pinning regions are
substantially uniformly spaced apart.
11. A photovoltaic cell as recited in claim 1 wherein adhesive
pinning elements define the pinning regions and the pinning
elements are formed on and extend out of a surface of the first
conductive layer, there being gaps between the pinning elements
that are filled with portions of the second layer.
12. A photovoltaic cell as recited in claim 1 wherein the pinning
regions are arranged at a periphery of the photovoltaic cell.
13. A photovoltaic cell as recited in claim 1 wherein the thickness
of each of the first and second conductive layers is approximately
between 1 and 35 .mu.m;
14. A method of forming a photovoltaic cell, the method comprising:
applying a first conductive layer on a semiconductor substrate,
wherein the first conductive layer is formed from a first
electrically conductive material; forming at least one pinning
element along the first conductive layer; and applying a second
conductive layer on the at least one pinning element in the
semiconductor substrate, wherein the second conductive layer is
formed from a second electrically conductive material that is
different from and is substantially more conductive than the first
electrically conductive material, wherein the at least one pinning
element locally anchors the first conductive layer to the second
conductive layer.
15. A method as recited in claim 14, further comprising: after
applying the first conductive layer on the semiconductor substrate,
spraying a material onto the first conductive layer to form a
plurality of the pinning elements, wherein the spraying is
performed such that the plurality of pinning elements are
physically isolated from one another and are spaced apart, wherein
applying the second layer over the pinning elements involves
filling in gaps between the pinning elements with the second
conductive layer.
16. A method as recited in claim 14, further comprising:
determining a minimum size for a buckling feature at an interface
between the first and second layers when the interface lacks any
pinning elements; and forming a plurality the pinning elements,
wherein the pinning elements are spaced apart to help minimize a
likelihood of buckling at the interface between the first and
second layers and wherein the spacing of the pinning elements is
based on the determining operation.
17. A component, comprising: a substrate; a thin film coating that
is adhered to and in direct contact with a first surface of the
substrate; and a multiplicity of spaced apart pinning regions that
are distributed through an interface between the substrate and the
thin film coating, the pinning regions locally anchoring the
substrate to the thin film coating, the adhesive strength of the
pinning regions to the substrate being greater than the adhesive
strength of the thin film coating to the substrate, wherein the
interface is suitable for one selected from the group consisting
of: 1) transmitting optical energy; 2) reflecting optical energy;
3) transmitting electrical energy; 4) transmitting electromagnetic
energy; and 5) reflecting electromagnetic energy.
18. A component as recited in claim 17 wherein the thin film
coating is reflective for infrared wavelengths and transmissive for
visible wavelengths.
19. A solar receiver for use in a solar energy collector,
comprising: a photovoltaic cell; and a component as recited in
claim 17 wherein the component is a protective layer that covers
the photovoltaic cell.
20. A component as recited in claim 17 wherein the pinning regions
are one selected from the group consisting of 1) substantially less
reflective than adjacent regions; and 2) substantially less
transmissive than adjacent regions.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/329,482, filed Apr. 29, 2010, entitled
"Reflective Coating," which is incorporated herein in its entirety
for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates generally to the use of
pinning regions to help adhere a thin film layer of an optical or
electrical device to an adjacent layer or substrate. One particular
application relates to the use of pinning regions in the reflector
of a concentrating solar energy collector to help adhere a thin
film reflective layer to a supporting surface. In other
applications, similar pinning regions may be used in other types of
optical or electrical components such as photovoltaic cells,
mirrors, reflectors, windows, etc.
BACKGROUND OF THE INVENTION
[0003] Typically, the most expensive component of a photovoltaic
(PV) solar collection system is the photovoltaic cell. To help
conserve photovoltaic material, various concentrating photovoltaic
(CPV) systems use reflectors to concentrate solar radiation on a
smaller cell area. Since the material used to make reflectors is
less expensive than the material used to make the cells, CPV
systems are thought to be more cost-effective than conventional PV
systems. Although existing designs work well, there are continuing
efforts to improve the efficiency and reliability of reflectors and
other components of solar energy collection systems.
SUMMARY OF THE INVENTION
[0004] The present invention relates to the use of pinning regions
to strengthen the adhesive bond between different layers of an
optical or electrical component such that the likelihood of
cracking or delamination is significantly reduced.
[0005] In one aspect of the present invention, a photovoltaic cell
for use in a solar collector is described. The photovoltaic cell
includes two electrically conductive layers that are positioned on
a surface of a semiconductor substrate. The first conductive layer
is adhered to and in direct contact with a surface of the
semiconductor substrate. The second conductive layer has a
different composition from and is substantially more electrically
conductive than the first conductive layer. There are multiple
spaced apart pinning regions that are distributed through an
interface between the first and second conductive layers. The
pinning regions, help locally anchor the two layers. The adhesive
strength of the pinning regions to the second conductive layer is
greater than the adhesive strength of the first conductive layer to
the second conductive layer.
[0006] The pinning regions can be arranged in a wide variety of
ways. Some designs involve pinning regions that are defined by
protrusions or pinning elements that extend out of the first
conductive layer. In other embodiments, the pinning elements are
made of different materials (e.g., titanium, chrome, glass,
polymer, nickel, a metal alloy, etc.) Generally, the pinning
regions cover only a fraction of the interface between the first
and second conductive layers. In some embodiments, the pinning
regions cover less than 5% or 1% of the surface area of the
cell.
[0007] Pinning regions may be utilized in a wide variety of
multilayered components, including reflectors, photovoltaic cells
and windows. In another aspect of the present invention, an optical
or electrical component with pinning regions will be described. The
component includes a substrate and a thin film coating that is in
direct contact with the surface of the substrate. Multiple spaced
apart pinning regions are distributed through an interface between
the substrate and the thin film coating. The pinning regions
locally anchor the substrate to the coating. Generally, the
interface between the substrate and the thin film coating is
arranged to transmit or reflect optical, electrical and/or
electromagnetic energy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The invention and the advantages thereof, may best be
understood by reference to the following description taken in
conjunction with the accompanying drawings in which:
[0009] FIGS. 1A-1C are diagrammatic side views of an example
reflector that is undergoing various types of buckling.
[0010] FIG. 2A is a diagrammatic side view of a reflector according
to a particular embodiment of the present invention.
[0011] FIG. 2B is an enlarged view of a portion of the reflector
illustrated in FIG. 2A.
[0012] FIGS. 2C and 2D are diagrammatic top views of portions of
the reflector illustrated in FIG. 2A according to various
embodiments of the present invention.
[0013] FIGS. 3A-3F are diagrammatic side views of reflectors
according to various embodiments of the present invention.
[0014] FIG. 3G is a diagrammatic top view of a reflector in which
the pinning regions are positioned at the periphery of the
reflector according to a particular embodiment of the present
invention.
[0015] FIG. 3H is a diagrammatic top view of a reflector in which a
continuous pinning structure extends around the periphery of the
reflector according to a particular embodiment of the present
invention.
[0016] FIG. 3I is a diagrammatic side view of the reflector
illustrated in FIG. 3H
[0017] FIGS. 4A-4B are flowcharts illustrating methods for forming
a reflector according to various embodiments of the present
invention.
[0018] FIGS. 5A-5C, 6A-6D, 7A-7C, 8A-8C and 9A-9C are diagrammatic
side views of methods for forming reflectors according to various
embodiments of the present invention.
[0019] FIG. 10A is a diagrammatic perspective view of a
photovoltaic cell with pinning regions according to a particular
embodiment of the present invention.
[0020] FIG. 10B is a diagrammatic side view of the photovoltaic
cell illustrated in FIG. 9A.
[0021] FIG. 10C is an enlarged view of a portion of the
photovoltaic cell illustrated in FIG. 9B.
[0022] FIG. 11 is a diagrammatic side view of an optical component
according to a particular embodiment of the present invention.
[0023] In the drawings, like reference numerals are sometimes used
to designate like structural elements. It should also be
appreciated that the depictions in the figures are diagrammatic and
not to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] In some concentrating photovoltaic systems, a reflector is
used to reflect incident light towards a solar receiver. Ideally,
the reflector has a high quality optical surface. If the reflector
surface is degraded, the incident light may be absorbed or
scattered and never reach the solar receiver.
[0025] In operation, the reflector is exposed to the ambient
environment and various environmental stresses, such as temperature
fluctuation, ultraviolet light exposure, and moisture. These
stresses can cause portions of the reflector to blister or crack.
As a result, the light may be scattered or absorbed.
[0026] An example of this problem is diagrammatically illustrated
in FIGS. 1A-1C. For purposes of comparison, FIG. 1A illustrates an
example of an undamaged reflector 101. The reflector 101 includes a
substrate 106, a reflective coating 104 and a protective layer 102.
The protective layer 102, which is typically made of an optically
transparent material, is formed over the other layers and helps
shield them from environmental stresses. The reflective coating 104
is made of a highly reflective substance, such as aluminum or
silver, and is arranged to redirect incident sunlight. The
substrate 106, which may be made of glass, metal and/or another
suitable material, provides structural support for the overlying
layers. In this example, all of the layers are perfectly aligned
with one another and no degradation has taken place.
[0027] FIGS. 1B and 1C illustrate situations in which there is
buckling in one or more of the thin layers of the reflector 101. In
FIG. 1B, both the reflective coating 104 and the protective layer
102 have separated from the underlying substrate 106 to form a void
within the reflector 101. In FIG. 1C, the protective layer 102 has
lifted off from the underlying reflective coating 104. In both
figures, the deformation in the surface of the reflector forms a
buckling feature 103. As a result, light is reflected in an
unintended and undesirable direction, as indicated by the arrows in
FIG. 1B.
[0028] There are several possible causes for such buckling. For
example, moisture or other contaminants may have penetrated the
reflector and weakened the bonds between some of its layers. There
may be a mismatch in the coefficients of thermal expansion of the
different layers. The reflector can also be damaged while being
manufactured, transported or operated.
[0029] The different materials used to form the layers of the
reflector can also affect the likelihood of delamination. For
example, silver has excellent reflective properties and thus is
highly suitable for use in the reflective coating 104. However,
silver adheres poorly to many types of glass, metal and polymer.
That is, silver unfortunately bonds poorly with many of the
materials that the protective layer 102 and the substrate 106 would
be made of. Although other metals, such as aluminum, can be used
for the reflective coating and would adhere better to the other
layers of the reflector, they may not reflect light as effectively
as silver. This creates an undesirable trade-off between
reflectivity and resiliency.
[0030] Various embodiments of the present invention address one or
more of the above issues. One of these embodiments is illustrated
in FIGS. 2A-2C. FIG. 2A is a diagrammatic side view of a reflector
201 that includes a thin film protective layer 202, a thin film
reflective coating 204, a substrate 206 and various spaced apart
pinning regions 210. (FIGS. 2B and 2C are, respectively, a
magnified side view and a top view of portions of the reflector 201
illustrated in FIG. 2A.) In the illustrated embodiment, the
reflective coating 204 is formed on the substrate 206 and the
protective layer 202 is formed on the reflective coating 204. The
pinning regions 210 are distributed through an interface 203
between the protective layer 202 and the reflective coating 204.
Each of the pinning regions 210 locally anchors the reflective
coating 204 to the protective layer 202.
[0031] Generally, a material is chosen for the pinning regions 210
that has, relative to the reflective coating 204, superior adhesive
properties, although possibly inferior reflective properties. The
adhesive bond at locations where the pinning regions 210 directly
contact the protective layer 202 is substantially stronger than at
adjacent locations where the reflective coating 204 directly
contacts the protective layer 202. In the illustrated embodiment,
the pinning regions 210 cover only a small fraction of the surface
area of the reflector and thus do not substantially interfere in
the reflection of light.
[0032] FIG. 2A illustrates a particular arrangement in which
multiple pinning elements 208 are formed on a top surface of the
reflective coating 204. (A pinning element is referred to herein as
the structure that defines and fills a pinning region.) In the
illustrated embodiment, the pinning elements 208 form small
protrusions that extend out of the top surface 230 of the
reflective coating 204. There are gaps between the pinning elements
208 that are filled in by portions of the protective layer 202. The
pinning elements 208 may have a wide variety of shapes (e.g.,
elliptical, oval, whisker-like, rectangular, star-like etc.) In
some implementations, the pinning elements 208 are formed by
spraying a material onto the substrate, which may cause the pinning
elements to have dome-like shapes and/or different sizes.
[0033] Since the pinning regions 210 need not be highly reflective,
they can be made of a wide variety of suitably adhesive materials.
By way of example, the pinning regions 210 may contain titanium,
chrome, nickel, nickel-chrome alloy, glass, etc. The pinning
regions 210 may be formed using a variety of techniques including
physical deposition, chemical deposition, electroplating and
spraying. Some designs involve a pinning element 208 that is
integral with and/or made of the same materials as the protective
layer or the substrate.
[0034] Since the material within the pinning regions 210 is
generally less reflective, it is desirable to limit the size of the
pinning regions 210 to increase access of incident light to the
reflective coating 204. In the illustrated embodiment, for example,
the pinning regions 210 are physically isolated from one another
and are separated from one another by large gaps. This can be seen
more clearly in FIG. 2C, which illustrates a top view of a portion
of the reflector 201 of FIG. 2A. (For the purposes of clarity, the
optically transparent protective layer is not shown in this figure
and the relative size of the pinning regions is greatly
exaggerated.) Preferably, the vast majority of the surface area of
the reflector is not covered by pinning regions. Some
implementations involve a reflector 201 with pinning regions 210
that cover less than 10%, 5%, 2% or 1% of the surface area 232 of
the reflector 201. In some embodiments, the pinning regions 210
occupy less than 5% of the interface between the reflective coating
204 and an adjacent layer (i.e. the substrate 206 or the protective
layer 202), while at least 95% of the surface of the reflective
coating 204 is in direct contact with the adjacent layer.
[0035] In various embodiments, the spacing between the pinning
regions 210 is controlled to strike a proper balance between
reduced coverage and adhesive strength. To clarify how the spacing
may be determined, it is useful to again review the buckling
feature in the reflector 101 illustrated in FIG. 1B. Generally, the
likelihood of buckling in a layer is a function of various factors,
including the stiffness of the layer, the strength of the adhesion
between the layer and an underlying structure, the degree of stress
(compressive or tensile) applied to the layer and the distance over
which such stress is applied. In the simplified example illustrated
in FIG. 1B, the compressive forces 105 exist across the surface
between the substrate 106 and the reflective coating 104, which
results in the formation of the buckling feature 103 in the
buckling region "a". For a large film layer of uniform thickness
that is subjected to equibiaxial stress, the criteria for
delamination may be characterized by the following formula, which
is described in L. Freund & S. Suresh, Thin Film Materials,
262-264, 327-333 (2003):
.GAMMA. = ( 1 - v f 2 ) h f .sigma. m 2 2 E f ##EQU00001##
where .GAMMA. is the bonding energy of the interface (J/m.sup.2),
.nu..sub.f is the Poisson ratio of the buckling layer, h.sub.f is
the thickness of the buckling layer (.mu.m), E.sub.f is the
in-plane Young's modulus of the buckling layer (GPa), and
.sigma..sub.m is the mismatch stress in the buckling layer
(MPa).
[0036] Using a related analysis, the minimum size of a buckling
region "a" can be determined. More specifically, if the buckling
region "a" falls below a particular minimum value, a buckling
feature will not be formed, if the other variables (stiffness,
compressive force, etc.) are held constant. For a thin film of
uniform thickness that is subjected to equibiaxial stress, the
minimum radius of a circular buckle (a.sub.m) may be characterized
by the following formula:
a m = 1.106 h f E f .sigma. m ##EQU00002##
For particular types of thin film reflectors without pinning
regions the minimum buckling radius (a.sub.m) may be on the order
of 1 mm. In order for the pinning regions to occupy a relatively
small percentage of the interface surface area, this implies that
the lateral dimensions of the pinning regions be on the order of
100 .mu.m. Larger or smaller pinning regions may be used depending
on the minimum buckling radius.
[0037] The spacing of the pinning regions illustrated in FIGS. 2A
and 2C is based on these concepts. More specifically, based on the
characteristics of a particular thin film layer and an estimation
of the compressive forces that would be applied to the layer,
a.sub.m can be calculated for any thin film layer in a reflector.
The spacing of the pinning regions is then based on a.sub.m. In the
illustrated embodiment, for example, the centers of adjacent
pinning regions 210 are separated from one another by a distance d.
Since no delamination is expected at the pinning regions 210, any
buckling must occur along the distance d. Preferably, distance d is
less than or equal to approximately 2a.sub.m so that the likelihood
of buckling is either eliminated or greatly reduced.
[0038] The spacing between pinning regions 210 may be uniform or
non-uniform. FIG. 2C, which illustrates a top view of the reflector
201 illustrated in FIG. 2A, presents a uniform arrangement of
pinning regions 210. That is, each pinning region 210 is separated
from each of the other adjacent pinning regions 210 by a
substantially equal distance d. This approach helps maximize the
exposure of the reflective coating 204 to incident sunlight while
reducing the likelihood of delamination. Alternatively, the pinning
regions 210 may be arranged in a non-uniform manner. An example of
this is shown in FIG. 2D, in which the distances between adjacent
pinning regions 210 varies. Preferably, even in a non-uniform
arrangement, the distances between the centers of adjacent pinning
regions is less than d. In another embodiment, the average distance
between the centers of adjacent pinning regions is approximately
equal to or less than d.
[0039] Generally, the reflective coating 304 is made of a material
that has excellent reflective properties but which may have poor
adhesive properties. For example, silver works well as a material
for the reflective coating 204, although any other suitable
reflective material may be used, such as aluminum. The protective
layer 202 may be made of a wide variety of organic or inorganic
materials, such as urethane acrylate, polyimide, photopolymers,
silicon dioxide, silicon dioxide/titanium dioxide, aluminum oxide,
sol-gel glass etc. Various implementations involve a substrate that
is made of a metal, glass, a thin film polymer (e.g., Mylar) and/or
any other suitable material. The composition of the substrate 206
and the protective layer 202 may depend on their orientation
relative to the reflective coating 204. For example, in the
illustrated embodiment the reflective coating 204 is formed on the
top surface of the substrate 206. Light passes through a protective
layer 202 and is reflected by the reflective coating 204. That is,
the protective layer 202 in this case must be made of a transparent
material. In another embodiment, however, the reflective coating
204 is on a bottom surface of a substrate 206, which is transparent
and arranged to pass light through (e.g., as shown in FIG. 3A.) It
should be appreciated that the protective layer 202, the reflective
coating 204, the substrate 206 and the pinning regions/elements do
not necessarily have uniform compositions. That is, they each may
include multiple sublayers of different materials. Additionally, in
some embodiments, various intermediate layers may be situated
between the substrate 206 and the reflective coating 204 and the
protective layer 202 and the reflective coating 204.
[0040] Generally, the substrate 206 of the reflector 201 is
substantially thicker than the overlying reflective and protective
layers 204 and 202, although this is not a requirement. By way of
example, the thickness 214 of the reflective coating 204 may be
approximately between 20 and 200 nm, although thicker and thinner
layers are also possible. Some implementations involve a protective
layer 202 with a thickness 212 of approximately between 0.1 .mu.m
and 20 .mu.m and a substrate 206 with a thickness 216 of half a
millimeter or more.
[0041] Depending on the arrangement of the pinning regions 210, the
adhesive strength between adjacent layers may be increased at one
or more interfaces. In the illustrated embodiment of FIG. 2B, for
example, the pinning region 210 is positioned through an interface
203 between the thin film layers (i.e., the reflective coating 204
and the protective layer 202.) Since the pinning region 210 of FIG.
2B is not positioned through an interface 205 between the thin film
reflective coating and the substrate 206, the pinning region 210
does not locally anchor the reflective coating 204 to the substrate
206. In some embodiments, however, the pinning regions 210 are
arranged to extend through both interfaces 203 and 205 and
therefore help secure all of the layers together.
[0042] Referring next to FIGS. 3A-3E, side views of reflectors
according to various embodiments of the present invention will be
described. FIG. 3A illustrates a reflector 301 that is somewhat
similar to the one illustrated in FIG. 2A, except that the layers
of the reflector 301 are rearranged. That is, the pinning regions
310 are formed on a top surface of the protective layer 302, which
is situated at the bottom of the reflector 301. The protective
layer 302 may be strong enough to be mechanically self-supporting.
The reflective coating 304 is positioned over the top surface of
the protective layer 302 and the pinning regions 310. The
reflective coating 304 is attached with the bottom surface of a
substrate 306. When the reflector 301 is in operation, light passes
through and may be slightly refracted by the transparent substrate
306. The light then reaches the reflective coating 304 and is
directed away from the reflector 301 as indicated by the
arrows.
[0043] Referring next to FIG. 3B, a reflector 303 with a treated
reflective coating 304 according to another embodiment of the
present invention will be described. In the illustrated embodiment,
the pinning regions 310 are formed by treating selected portions
320 of the reflective coating 304. This may be done in a wide
variety of ways. For example, selected portions 320 of the
reflective coating may be chemically altered to form an alloy that
adheres substantially more to the overlying protective layer that
untreated portions of the reflective coating 304. Generally, these
treated, selected portions 320 of the reflective coating do not
penetrate entirely through the reflective coating 304, although
this is not a requirement.
[0044] Referring next to FIG. 3C, a reflector 305 according to
another embodiment of the present invention will be described. The
main difference between the reflector 305 illustrated in FIG. 3C
and the reflector 201 illustrated in FIG. 2A is that the pinning
elements 308 penetrate entirely through the reflective coating 304,
instead of being formed on the top surface of the reflective
coating 304. Each pinning element 308 directly contacts and adheres
to the protective layer 302 and the substrate 306. Accordingly, the
reflective layer 304 is firmly sandwiched therebetween.
[0045] FIG. 3D is a side view of a reflector 307 with a protective
layer 302 with elevated regions 322 in accordance with another
embodiment of the present invention will be described. Like the
reflector 305 illustrated in FIG. 3C, the reflector 307 includes a
top protective layer 302, a middle reflective coating 304, a
substrate 306 and multiple pinning regions 310 that extend entirely
through the reflective coating 304. In the illustrated embodiment,
however, the pinning regions 310 are not filled with a material
that is distinct from that of the protective layer 302. Instead,
the pinning elements 308 in the pinning regions 310 are extensions
of and integral with the overlying protective layer 302. The
pinning elements help secure two interfaces within the reflector
307: the interface between the thin film protective layer 302 and
the thin film reflective coating 304 and the interface between the
thin film layers and the substrate 306.
[0046] In the illustrated embodiment, there are elevated regions
322 that extend out of the bottom surface of the protective layer
302 and that come in direct contact with the substrate 306. The
reflective layer 304 covers the substrate 306 and fills in the gaps
between the elevated regions 322 of the protective layer 302. The
adhesive bond at locations where the protective layer 302 directly
contacts the substrate 306 is substantially stronger than at those
locations where the reflective coating 304 directly contacts the
substrate 306.
[0047] Referring now to FIG. 3E, a reflector 309 with a substrate
306 that has elevated regions 324 according to another embodiment
of the present invention will be described. The substrate 306
includes elevated regions 324 that extend out of a top surface of
the substrate 306. The reflective layer 304 is positioned on the
substrate 306 such that it fills gaps between these elevated
regions 324. In various embodiments, a top surface of the
reflective coating 304 is substantially coplanar with top surfaces
of the elevated regions 324 of the substrate 306. A protective
layer 302 is positioned on the reflective layer 304 and is in
direct contact with the elevated regions 324 of the substrate 306.
In comparison to the reflective coating 304, the elevated regions
324 of the substrate 306 adhere substantially better to the
overlying protective layer 302.
[0048] The figures described above generally illustrate reflectors
with planar top and bottom surfaces. The present invention also
contemplates embodiments in which this is not the case. Referring
next to FIG. 3F, a reflector 311 with nonplanar layers according to
another embodiment of the present invention will be described. The
substrate 306 includes elevated regions 324 that extend out of its
top surface. The overlying reflective coating 304 fills gaps
between these elevated regions 324 and also covers the elevated
regions 324. As a result, some portions of the reflective coating
304 are positioned higher than others. The protective layer 302
conforms to the profile of the underlying layers and is similarly
nonplanar. For some applications, the above reflector design may be
useful. Generally, the non-planar portions of the reflector may be
kept so small such that they do not substantially interfere with
the proper reflection of light. One advantage of the above type of
reflector design is that it may remove processing steps and
therefore may be more cost-effective to manufacture.
[0049] Referring now to FIG. 3G, a reflector 313 with peripheral
pinning regions 310 according to another embodiment of the present
invention will be described. In the illustrated embodiment, the
pinning regions 310 are arranged only along the periphery of the
reflector 313. In an alternative embodiment, there are pinning
regions 310 across the entire reflector 313, but the density of
pinning regions 310 are greater at the periphery then at the
central region 314 of the reflector 313 (e.g., the aforementioned
distance d between adjacent pinning regions may be smaller at the
periphery than at the central region 314.) For various reasons, it
is particularly important to prevent delamination, deformation, and
cracking at the periphery of the reflector. Generally, the
periphery of the reflector is more prone to bending or twisting.
Cracking or delamination often begins at the edges of the reflector
and then propagates inward towards the center of the reflector.
Additionally, the multiple layers that make up the reflector may be
exposed at the periphery of the reflector, which could allow
moisture and other contaminants to seep between the layers of the
reflector. Concentrating the pinning regions 310 at the periphery
of the reflector 313 can help prevent such problems.
[0050] Referring next to FIGS. 3H and 3I, a reflector 315 with a
continuous pinning structure 312 according to a particular
embodiment of the present invention will be described. FIGS. 3H and
3I are, respectively, top and side views of the reflector 315. The
pinning regions are defined by single continuous pinning structure
312 that extends along the periphery of the reflector 315 and
entirely surrounds a central region 314 of the reflector 315. The
pinning structure 312 may be formed in the same way as any pinning
element described herein. In the illustrated embodiment, for
example, the pinning structure 312 extends past the reflective
coating 304 and directly contacts the reflective coating 304 and
the substrate 306. In another embodiment, the pinning structure 312
may be a wall-like structure that is formed on a top surface of the
reflective coating 304 (e.g., similar to the way in which the
pinning element 208 is formed on the top surface of the reflective
coating 204 in FIG. 2A.) In this embodiment, the protective layer
302 would fill in a large gap between the "walls" of the pinning
structure 312 at the central region 314 of the reflector 315.
[0051] It should be appreciated that the present application is not
limited to the specific examples described above. The present
application also contemplates a wide variety of arrangements
involving combinations of the features shown in different figures.
By way of example, FIGS. 2A-2C and FIGS. 3A-3I illustrate various
types of pinning elements and pinning regions (e.g., pinning
regions that do or do not extend entirely through the reflective
coating, pinning regions that are arranged at the periphery of the
reflector, pinning regions that are extensions of and are integral
with the protective layer and/or the substrate, pinning regions
that are on the backside of a substrate, etc.) Any feature of a
pinning region or reflector illustrated in a particular figure may
be added to or used to modify a pinning region or a reflector
illustrated in another figure.
[0052] Referring now to FIG. 4A and FIGS. 5A-5C, a method for
forming the reflector 201 illustrated in FIG. 2A according to a
particular embodiment of the present invention will be described.
Initially, a thin film reflective coating 204 is applied over a top
surface of a substrate 206 (FIG. 5A and step 402 of FIG. 4A.) The
substrate 206 may be formed from any material that is suitable for
physically supporting the other layers of the reflector 201, such
as metal, glass, polymer, etc. In some embodiments, the substrate
206 is intended to allow the passage of light and is therefore
transparent. Highly reflective metals such as silver and aluminum
work well as materials for the reflective coating 204. The
reflective coating 204 may be applied to a thickness of
approximately between 20 and 200 nm, although larger and smaller
thicknesses are also possible.
[0053] The reflective coating 204 may be applied using a wide
variety of different thin film deposition techniques, depending on
the needs of a particular application. For example, the applying of
the reflective coating 204 may involve physical vapor deposition,
electron beam deposition, sputtering, chemical deposition, screen
printing, electroless plating and/or electroplating. Some
approaches involve the thermal evaporation of metals into a vapor
that is then deposited on the substrate 206 in one or more layers.
Such deposition processes may be conducted in high vacuum with
small quantities of gases such as oxygen or ammonia to react with
the evaporated material to form the thin film reflective coating.
During the deposition process, the underlying layer may be
bombarded by an ion beam to cause microstructural, chemical or
other physical changes in the layer to help form the desired thin
film. In various embodiments, a reactive species or precursors may
be applied to the underlying layer in a liquid form, which reacts
(e.g., in a redox reaction, sol-gel reaction, etc.) to form the
desired thin film. (It should be noted that the above techniques
may be utilized whenever a thin film layer is being formed in
accordance with one of the embodiments described in the present
application, irrespective of whether the underlying layer is the
substrate or another thin film layer.)
[0054] Afterward, one or more pinning regions 210 are formed over a
top surface of the reflective coating (FIG. 5B and step 404.) In
this example, each pinning region 210 is defined by pinning element
208 that is made up of a material with strong adhesive properties.
For example, titanium, chrome, nickel, a metal alloy, nickel chrome
alloy and glass work well for various embodiments. The pinning
elements are physically isolated from one another and are
distributed across the top surface of the reflective coating 204.
There may be a distance d<2a.sub.m separating adjacent pinning
elements 208, where 2a.sub.m is calculated for the thin film
reflective coating 204 based on the factors discussed earlier in
this application.
[0055] The pinning elements 208 may be formed in various ways. For
example, a particular approach involves spraying a suitably
adhesive material onto the reflective coating 204 to form the
spaced apart pinning elements 208. Some approaches involve the
spraying of liquid beads or droplets of the material on the
reflective coating 204 in a non-uniform pattern (e.g., as seen in
FIG. 2C.) The pinning elements 208 may also be formed using any
thin film deposition techniques discussed in the present
application, including chemical deposition, electroplating and
vapor deposition.
[0056] Another approach for forming the pinning elements involves
applying a continuous thin film layer of the pinning element
material over the reflective coating 204. This may be performed
using any of the aforementioned thin film deposition techniques.
This layer may then be etched using any known, suitable method to
form the spaced apart pinning elements 208. By way of example, the
selective removal of portions of the thin film may involve laser
ablation, chemical solvents, mechanical abrasion and/or
photolithography.
[0057] At step 406, a thin film protective layer of FIG. 5C is
applied directly onto the reflective coating 204 and the pinning
elements 208 to form the reflector 201 illustrated in FIG. 2A. The
protective layer 202 may be made from any suitably impermeable,
resilient and/or transparent material (e.g., ceramic, a dielectric
material, polyimide, oxide, glass, urethane acrylate,
fluoropolymer, silicon dioxide, silicon dioxide/titanium dioxide
mixture, aluminum oxide, sol-gel glass etc.) The application of the
protective layer 202 may involve any of the thin film deposition
techniques described above. Some deposition processes involve the
thermal evaporation of ceramics or dielectrics into a vapor that is
deposited over the reflective coating 204. These processes may be
conducted in high vacuum with small quantities of gases such as
oxygen or ammonia to react with the evaporated material to form a
ceramic or dielectric thin film protective layer 202.
[0058] The protective coating 202 covers the spaced apart pinning
elements 208 and fills in gaps between them. Due to the materials
used to form the pinning elements 210, the strength of the adhesive
bond at the interface between the protective layer and the
reflective coating is substantially stronger at the pinning regions
210 than at the gaps between the pinning regions 210. In some
embodiments, the protective coating 202 is applied to form a planar
top surface for the reflector 201. In another embodiment, the
protective coating 202 is applied substantially evenly over the
reflective coating 204 and the pinning elements 208, which may
cause the top surface of the reflector 201 to be non-planar.
[0059] The steps illustrated in FIG. 4A may be reordered and/or
modified in a wide variety of ways to form different types of
reflectors. Referring now to FIG. 4A and FIGS. 6A-6D, a method for
forming the reflector 307 illustrated in FIG. 3D according to
another embodiment of the present invention will be described.
Initially, masking features 326 are formed on a top surface of the
substrate 306 of FIG. 6A. The masking features 326 may be formed
from any suitable material that can later be readily removed from
the substrate 306, such as a dissolvable photoresist. In some
implementations, the masking features 326 are formed by first
depositing a layer of the material over the reflective coating and
then etching the material to define the masking features. In
another implementation, the masking features 326 are spot deposited
over the surface of the reflective coating. For example, they may
be sprayed over the reflective coating to form spaced apart masking
features 326 in the form of liquid beads or droplets. Some
approaches involve forming masking features 326 that are made of
small particles and/or have whisker- or filament-like shapes. In
another embodiment, the masking feature takes the form of a tape
that is arranged to be pulled off after the reflective coating is
applied. This approach works well when forming a reflector in which
the pinning regions are concentrated at the periphery of the
reflector (e.g., the reflector 315 illustrated in FIG. 3H.) In such
a case, the tape can be applied at the edges and along the
periphery of the substrate, so that any reflective coating that
forms on the tape can be later removed by pulling off the tape.
[0060] Afterward, a reflective coating 304 is applied over the
masking features (FIG. 6B and step 402 of FIG. 4A.) The reflective
coating 304, which may be applied using any of the aforementioned
thin film deposition techniques, covers the top surface of the
substrate 306 and possibly the masking features 326. In an
embodiment in which the masking feature 326 is dissolvable, the
reflective coating 304 preferably does not entirely encapsulate
each masking feature so that a suitable solvent has access to it,
although this is not a requirement.
[0061] In FIG. 6C, the masking features 326 are removed. The
techniques used to remove the masking features may vary, depending
on the composition of the masking features 326 and the way in which
they were applied. By way of example, a chemical solvent may be
applied to dissolve the masking features 326. Some approaches
involve burning away masking features that are made of organic or
carbonaceous materials. In another approach, the masking features
326 (e.g., a tape) are mechanically pulled or peeled off from the
underlying surface. Once the masking features 326 are removed,
multiple recesses 328 are left in the reflective coating where the
masking features were. As a result, underlying portions of the
substrate 306 are exposed.
[0062] In various embodiments, recesses 328 are formed in the
reflective coating 306 without the use of masking features 326. By
way of example, after the initial deposition of the thin film
reflective coating 304 (e.g., as seen in FIG. 6A), suitable
portions of the reflective coating 304 can be removed using any
technique familiar to those of ordinary skill in the art. Some
embodiments involve etching the reflective coating 304 using laser
ablation or mechanical abrasion. These techniques also form
recesses 328 that expose underlying portions of the substrate
306.
[0063] After the removal of portions of the reflective coating 304,
a thin film protective layer 302 is formed over the reflective
coating 304 (FIG. 6D and step 406 of FIG. 4A.) The application of
the protective layer 302 may involve any of the aforementioned thin
film deposition techniques. The protective layer 302 covers the
reflective coating 304, fills the recesses 328 in the coating and
therethrough comes in direct contact with the top surface of the
substrate 306. The portions of the protective layer 302 that fill
the recesses define the pinning regions 310 for the reflector. That
is, protrusions that extend from the bottom surface of the
protective layer 302 act as pinning elements 308 for the reflector
309.
[0064] A particular implementation of the method illustrated in
FIGS. 6A-6D involves filling the recesses 328 with a distinct
pinning element material before applying the protective layer. In
this implementation, the pinning element material differs from the
materials used to form both the overlying protective layer 302 and
the underlying reflective coating 304. Examples of suitable pinning
element materials include titanium, chrome, nickel, an alloy,
nickel-chrome alloy and glass. In this approach, before the
protective layer 302 is formed over the reflective coating 304, a
pinning element material is deposited into the recesses 328 in the
reflective coating 304. To fill the recesses 328, the pinning
element material may be initially applied as layer that covers both
the reflective 304 coating and fills the recesses 328. The layer
may then be etched such that the top surface of the reflective
coating 304 is exposed and/or the top surfaces of the pinning
elements 308 are coplanar with the top surface of the reflective
coating 304. In another embodiment, each of the recesses 328 are
spot-filled in a manner that leaves the top surface reflective
coating 304 uncovered. Afterward, the protective layer 302 is
formed over the pinning elements 308 and the reflective coating
304. In this implementation, the protective layer 302 is optionally
flat and has a substantially uniform thickness, unlike the
protective layer 302 of the reflector 309 illustrated in FIG. 6D.
The above approach may be used to form the reflector 305
illustrated in FIG. 3C.
[0065] Referring next to FIGS. 7A-7C, a method for forming the
reflector 307 illustrated in FIG. 3E according to another
embodiment of the present invention will be described. Initially, a
substrate 306 of FIG. 7A is provided that includes elevated regions
330 in its top surface. There are gaps between the elevated regions
330, which are distributed along the length of the underlying
substrate 306. In various embodiments, the centers of adjacent
elevated regions 330 are separated by a distance d<2a.sub.m,
whose calculation was discussed elsewhere herein.
[0066] A thin layer reflective coating 304 is then applied over the
substrate 306 (step 402 of FIG. 4A and FIG. 7B.) There are various
ways to arrange the reflective coating 304 over the substrate. In a
particular embodiment, the reflective coating 304 is selectively
applied only within the gaps between the elevated regions 330 of
the substrate, such that the top surfaces 332 of the elevated
regions 330 are uncovered. In another embodiment, the reflective
coating is deposited over the entire substrate 306. Then, portions
of the reflective coating 304 are removed to expose the top
surfaces 332 of the elevated regions 330. The selective etching of
portions of the reflective coating may be performed using any
technique known in the art (e.g., laser ablation, mechanical
abrasion, photolithography, etc.) Generally, the top surfaces 332
of the elevated regions 330 and a top surface of the reflective
coating 304 are substantially coplanar, although this is not a
requirement.
[0067] Afterward, a protective layer 302 is deposited over the
reflective coating 304 and the substrate 306 (step 404 of FIG. 4A
and FIG. 7C.) The elevated regions 330 of the substrate 306 come in
direct contact with the overlying protective layer 302 and help
anchor the protective layer 302 to the rest of the reflector. That
is, the elevated regions 330 of the substrate 306 define the
pinning regions 310 for the reflector.
[0068] Referring now to FIG. 4A and FIGS. 8A-8C, a method for
forming the reflector 305 illustrated in FIG. 3B according to
another embodiment of the present invention will be described.
Initially, a reflective coating 304 is formed over a substrate 306
(step 402 of FIG. 4A and FIG. 8A.) Selected portions 334 of the
reflective coating are then treated to form pinning regions 310
(step 404 of FIG. 4A and FIG. 8B.) That is, the selected portions
334 of the reflective coating 304 are chemically altered such that
their composition or physical properties differ from that of the
rest of the reflective coating 304. As a result of the treatment,
the selected portions 334 become less reflective and/or
substantially more adhesive. A variety of techniques may be used to
treat the reflective coating 304 in this manner. By way of example,
selected portions of the reflective coating 304 may be chemically
altered to form an alloy. In one embodiment, the selected portions
334 of the reflective coating 304 may be heat treated. Various
applications involve altering the selected portions 334 of the
reflective coating 304 by exposing them to UV radiation and/or
immersing them in a solution. In the illustrated embodiment, the
treated pinning regions 310 extend into but not entirely through
the reflective coating 304, although in other embodiments, the
pinning regions 310 extend entirely through the reflective coating
such that they come in direct contact with the underlying substrate
306.
[0069] At step 406 of FIG. 4A and FIG. 8C, a thin film protective
layer 302 is applied over the reflective coating 304. The
protective layer 302 may be deposited using any of the
aforementioned techniques for forming thin film layers. Compared to
the adjacent untreated regions, the treated regions of the
reflective coating 304 (i.e., the pinning regions 310) adhere
substantially more strongly to the overlying protective layer
302.
[0070] Referring next to FIG. 4B and FIGS. 9A-9C, a method for
forming a reflector according to another embodiment of the present
invention will be described. This method may be somewhat similar in
several respects to the method illustrated in FIG. 4A and FIGS.
5A-5C (e.g., the composition of the materials used, the manner in
which some materials are deposited, etc.), although the steps are
in a different order.
[0071] Initially, multiple spaced apart pinning regions are formed
over a surface of the substrate 306. The pinning regions and their
corresponding pinning elements may be formed and spaced in any
manner described herein (e.g., in a manner similar to the way in
which pinning regions 308 were formed and distributed over the
reflective coating 304 in FIGS. 5A-5C.)
[0072] Afterward, the reflective coating is applied over the
pinning regions 308 and the substrate 306 (step 414 of FIG. 4B and
FIG. 9B.) The reflective coating may be applied using any thin film
deposition technique described herein. In the illustrated
embodiment, top surfaces of the pinning elements 308 are
substantially coplanar with top surfaces of the reflective coating,
although this is not a requirement. In another embodiment, the
reflective coating completely covers the pinning elements. The
pinning regions are distributed along an interface between the
reflective coating and the substrate, thereby helping to locally
anchor the reflective coating to the substrate. At step 416 of FIG.
4B and FIG. 9C, a protective layer 302 is applied over the
reflective coating 304. The reflective coating 304 may be deposited
using any thin film deposition technique described herein.
[0073] Although the methods described in connection with FIGS.
5A-5C, 6A-6D, 7A-7C, 8A-8C and 9A-9C involve placing pinning
regions and a reflective coating on the top surface of a substrate,
it should be appreciated that the steps of the aforementioned
methods may be reordered and modified to also form a reflector
whose reflective coating is attached with the bottom surface of a
transparent substrate. (An example of such a reflector is the
reflector 301 of FIG. 3A.)
[0074] The use of pinning regions is not limited to reflectors, but
may also be utilized to form a wide variety of multilayered,
components. Generally, these components have in common a surface
covered by a least one thin film coating and designed to reflect or
transmit some form of energy through the coating. The energy may be
in the form of an electrical current, electromagnetic radiation,
heat, a mechanical wave, such as sound or other type of energy. By
way of example, FIGS. 10A-10B are diagrammatic perspective and side
views of a photovoltaic cell 1001 according to a particular
embodiment of the present invention. FIG. 10C is an enlarged view
of a portion of the photovoltaic cell 1001 illustrated in FIG. 10B.
The photovoltaic cell 1001 includes a substrate 1006 and two
additional layers. Generally, the substrate 1006 is formed from a
semiconductor material. The two layers (the first layer 1004 and
the second layer 1002) are formed from different electrically
conductive materials and are stacked over the back surface of the
substrate 1006. Multiple spaced apart pinning regions 1010 are
distributed at an interface between the two layers. These pinning
regions locally anchor the layers.
[0075] The photovoltaic cell 1001 is arranged to receive light at
the front surface 1030 of the semiconductor substrate 1006. The
light generates free electrons in the semiconductor material, which
then flow through the first and second layers 1004 and 1002 as
indicated by the arrows in FIG. 10C. Ideally, the first layer 1004
is formed from an electrically conductive material that adheres
well to the semiconductor material. Such materials, however, are
not known to be the best conductors 1004. For example, aluminum
bonds well with the semiconductor substrate but conducts
electricity about half as well as silver, which bonds poorly with
the semiconductor substrate and aluminum.
[0076] It is desirable to use a highly conductive material for the
second layer 1002, so that the electrical current can be directed
quickly to the edges of the photovoltaic cell 1001, where it can be
used for power generation. To help prevent delamination between the
first and second layers 1004 and 1002 and maximize current flow,
pinning regions 1010 are positioned along the interface between the
first and second layers 1004/1002 to help bond the layers together.
In various embodiments, the pinning regions 1010 are defined by
pinning elements 1008 that are electrically non-conductive or are
poor conductors of electricity. In such embodiments, the footprint
of the pinning regions 1010 is preferably minimized so that the
free electrons have easier access to the overlying, highly
conductive second layer 1002. In the illustrated embodiment, for
example, the centers of adjacent pinning regions 1010 are separated
by a distance d, where d<2a.sub.m, where 2 a.sub.m is based on
the factors (e.g., estimated compressive forces, stiffness of the
thin film layer, etc.) that were discussed earlier. Various
embodiments involve a photovoltaic cell 1001 where the pinning
regions occupy less than 5% of the interface between the first and
second layers 1004/1002 and at least 95% of a surface of the first
layer 1004 is in direct contact with a surface of the second layer
1002.
[0077] Various materials may be used for the layers and the pinning
regions 1010 of the photovoltaic cell, depending on the needs of a
particular application. In various embodiments, the second layer
1002 is formed from silver and the first layer 1004 is formed from
aluminum. The pinning regions 1010 may be formed from electrically
non-conductive or conductive materials (e.g., glass, titanium,
chrome, nickel, nickel chrome alloy, etc.) The dimensions of the
layers can vary widely between different implementations. By way of
example, the substrate 1006 may be a semiconductor wafer or a thin
film semiconductor. In some embodiments, the thickness 1032 of the
substrate 1006 is approximately between 100 and 300 .mu.m. The
thicknesses 1036/1034 of the first and second layers 1004/1002 may
be between approximately 5 and 35 .mu.m, although lower and higher
thicknesses are also possible for particular applications. Some
implementations involve a first layer that is positioned to
entirely separate the second layer from the substrate, although in
other implementations, portions of the second layer may extend
through the first layer to come in (direct) contact with the
substrate.
[0078] The photovoltaic cell 1001 may be formed using any method
and/or with any feature that was previously discussed in connection
with various reflector designs. By way of example, the photovoltaic
cell 1001 may be formed using any suitable technique discussed in
FIG. 4A, FIGS. 5A-5C, 6A-6D, 7A-7C and 8A-8C. The pinning regions
1010 may be arranged relative to the second layer 1002, first layer
1004 and the substrate 1006 in the same manner that the pinning
regions 210/310 were arranged relative to the protective layer
202/302, the reflective coating 204/304 and the substrate 206/306
in FIGS. 2A-2D and 3B-3I. In a particular embodiment, pinning
elements 1008 are formed on a top surface of a first layer 1004
(e.g., similar to the way in which pinning elements 308 were formed
over the reflective coating 306 in FIG. 5B). In still another
embodiment, portions of the first layer 1004 are treated to form
pinning regions (e.g., similar to the way in which pinning regions
310 were formed by treating selected portions of the reflective
coating 304 in FIG. 8B) In yet another embodiment, pinning elements
1008 are formed that extend entirely through the first layer 904 to
come in direct contact with both the second layer 1004 and the
semiconductor substrate (e.g., similar to the way in which pinning
regions 308 may be formed to extend through the reflective coating
304, which was discussed earlier in connection with FIGS. 6A-6D.)
For the purpose of applying the methods for forming reflectors that
were discussed earlier to the forming of the photovoltaic cell,
insofar as the methods refer to a substrate, a reflective coating
and a protective layer of a reflector, the same methods may be
understood as referring to the semiconductor substrate, the first
layer and the second layer of the photovoltaic cell,
respectively.
[0079] To better clarify how the reflector methods may be applied
in the above manner, an example method for forming a photovoltaic
cell 1001 based upon the method illustrated in FIGS. 5A-5C will be
described. Initially, the first layer 1004 is formed over the
semiconductor substrate 1006 (i.e., similar to the way in which the
reflective coating 304 is formed over the substrate 306 of FIG.
5A.) Afterward, pinning elements 1008 are formed on a surface 1038
of the first layer 904 (i.e., similar to the way in which the
pinning elements 308 are formed over the reflective coating 304 of
FIG. 5B.) The pinning elements 1008 extend out of the surface 1038
of the first layer 904 and may be separated by a distance d, where
d<2a.sub.m, whose calculation for a given thin layer was
previously discussed. The second layer 1002 is then deposited over
the pinning regions 1010, which are defined by the pinning elements
1008, and the underlying first layer 1004 (i.e., similar to the way
in which the protective layer is formed over the reflective coating
of FIG. 5C.) The first and second layers 1004/1002 may be deposited
using any of the techniques for thin film deposition that were
discussed earlier.
[0080] The idea of using small pinning regions to anchor two or
more layers together may be applied to almost any type of optical
component (e.g., a window, photovoltaic cell, reflector etc.) in
which delamination is an issue and the transmission or reflection
of light is desirable. Referring next to FIG. 11, an optical
component 1101 according to a particular embodiment of the present
invention will be described. FIG. 11 is a side view of the optical
component 1101 that includes a substrate 1106, a thin film layer
1104 and multiple spaced apart pinning regions 1110. The pinning
regions 1110 are arranged along an interface 1122 between the thin
film layer 1104 and the substrate 1106 to locally anchor the thin
film layer 1104 to the substrate 1106.
[0081] The composition of the layers may vary widely, depending on
the needs of a particular application. In various embodiments, the
substrate 1106 and the thin film layer 1104 may be made of metal,
polymer, glass, thin film polymer, ceramic, a transparent material
and/or an electrically conductive material. Generally, the
interface 1122 between the substrate 1106 and the thin film layer
1104 is arranged to transmit or reflect electrical, optical and/or
electromagnetic energy. In a particular embodiment, the optical
component 1101 is a window and the substrate 1106 is transparent.
In another embodiment, the thin film layer 1104 is reflective for
infrared wavelengths and transmissive for visible wavelengths. Such
a design is useful for windows and housing applications, where it
is desirable to allow in exterior light and limit thermal losses.
It is also useful for the protective cover on a solar receiver in a
solar collection system (e.g., as described in U.S. Pat. No.
7,280,906, entitled "Photovoltaic Receiver," filed May 20, 2008,
which was filed by the assignee of the present application and is
incorporated herein by reference in its entirety for all purposes.)
In various embodiments involving such a protective cover, the
infrared wavelengths are often not converted into electrical energy
in the receiver and they cause additional waste heat that must be
removed by the receiver. Reflecting these wavelengths in the
protective cover thus reduces the receiver operating temperature,
which may improve receiver performance and lifetime.
[0082] The thin film layer 1004 may be formed on the substrate 1006
using any of the thin film deposition techniques discussed earlier.
The pinning regions 1010 may have any feature or any arrangement
previously discussed in connection with various types of reflector
designs (e.g., the pinning regions may be formed on a top surface
of the thin film layer, they may be arranged along the periphery of
the optical component, they may be extensions of the substrate or
another layer or be made of different materials, etc.) There may be
additional layers (e.g., a protective layer over the thin film
layer) in the optical component 1001.
[0083] Although only a few embodiments of the invention have been
described in detail, it should be appreciated that the invention
may be implemented in many other forms without departing from the
spirit or scope of the invention. By way of example, the present
invention contemplates that the features of one figure may be used
to modify or rearrange the features of another figure. For example,
FIG. 3A is a side view of a reflector 301 in which the reflective
coating 304 is attached directly to the backside of the substrate
306. The pinning regions in FIG. 3A do not extend entirely through
the reflective coating 304 and are defined by pinning elements 308
that are formed over a top surface of the protective layer 302.
Other figures, however, illustrate many types of pinning elements,
such as pinning elements that extend entirely through the
reflective coating and pinning elements that are integral with and
extend out of a surface of the protective layer and/or the
substrate. The present invention therefore also contemplates
various embodiments in which the pinning regions 310 illustrated in
FIG. 3A have similar such features. Additionally, various parts of
the present application refer to thin film deposition techniques
and etching techniques. Whenever thin film deposition is referred
to in connection with a particular embodiment, it should be
understood that any thin film deposition technique discussed in the
present application may be applied to said embodiment. Whenever
etching or the removal of portions of a layer is referred to in
connection with a particular embodiment, it should be understood
that any etching technique discussed in the present application may
be applied to said embodiment. Many of the figures are diagrammatic
side views of a reflector or an optical component where the pinning
regions are separated by an equal distance d. However, it should be
noted that the arrangement of the pinning regions for any given
embodiment may be modified based on any arrangement of pinning
regions that was discussed elsewhere in the present application. By
way of example, FIGS. 3G and 3H relate to pinning regions 310 that
are situated at the periphery of the reflector. Other parts of the
application discuss how the pinning regions are in a uniform or
non-uniform arrangement. Some parts of the application described
how the pinning regions may be separated by a distance d that is
less than or equal to a calculated 2a.sub.m. In some approaches,
where the arrangement of the pinning regions is non-uniform, the
average distance between adjacent pinning regions may be
approximately d or less than d. It should be understood that the
arrangement and characteristics of pinning regions in any figure or
any description in the present application may have any of the
above features. Although the reflectors in the drawings are
depicted as having a somewhat flat shape, the present invention
also contemplates reflector layers and reflectors with a curved,
substantially concave shape. In the foregoing application, there
are many references to "pinning regions" and "pinning elements."
Pinning elements may be understood as the structures that fill the
pinning regions. In various embodiments, the pinning region is the
exact space that the corresponding pinning region occupies.
Accordingly, if a pinning region is described as being positioned
at a certain location, the corresponding pinning element in the
pining region is also positioned at that location. If the pinning
region is described as containing or being filled with a particular
material, then the corresponding pining element includes that
material. Various drawings and descriptions in this application
refer to discrete layers or elements (e.g., a first layer, a second
layer, a substrate, a reflective coating, a protective layer, etc.)
It should be noted that each of these layers or elements is not
necessarily limited to a single layer having a uniform composition.
In some implementations, a layer may contain multiple sublayers
that are made of different materials. It should also be appreciated
that the present application uses the terms, "substrate" and
"layer" to refer to a wide variety of possible structures. For
example, the term "substrate" may relate to a support substrate
that physically supports other thin film layers, a transparent
substrate that allows the passage of light, or any other type of
layer. The term "layer" may refer to a substrate, a protective
layer, a thin film reflective coating or any other type of layer.
Also, the embodiments contemplated in the present application are
not necessarily limited to what is shown in the figures. The
figures can be modified in various ways, depending on the needs of
a particular application. For example, FIGS. 10A-10B illustrate
layers stacked on the back face of a semiconductor substrate; in
another embodiment, however, the layers are (also) patterned and/or
positioned on the front face of the semiconductor substrate. In
various figures, the reflective coating is adhered directly to a
substrate and/or a protective layer; in some embodiments, however,
there are one or more intermediate layers between the coating and
the substrate and/or the coating and the protective layer, and the
pinning regions may extend into, through and/or be positioned on an
intermediate layer. Therefore, the present embodiments should be
considered as illustrative and not restrictive and the invention is
not limited to the exact details given herein, but may be modified
within the scope and equivalents of the appended claims.
* * * * *