U.S. patent application number 11/809274 was filed with the patent office on 2007-10-18 for methods for manufacturing solar cells.
This patent application is currently assigned to Solyndra, Inc.. Invention is credited to Chris M. Gronet.
Application Number | 20070240760 11/809274 |
Document ID | / |
Family ID | 37572162 |
Filed Date | 2007-10-18 |
United States Patent
Application |
20070240760 |
Kind Code |
A1 |
Gronet; Chris M. |
October 18, 2007 |
Methods for manufacturing solar cells
Abstract
A solar cell assembly comprising a plurality of elongated solar
cells is provided. Each solar cell in the plurality of cells
comprises a conductive core configured as a first electrode, a
semiconductor junction circumferentially disposed on the conductive
core, and a TCO layer disposed on the semiconductor junction. The
plurality of solar cells is arranged in a parallel manner in pairs
on a transparent insulating substrate such that (i) the solar cells
in each respective pair are joined together lengthwise by a
corresponding counter-electrode, and (ii) solar cells adjacent
pairs of solar cells do not touch each other. Each solar cell pair
is affixed to the transparent insulating substrate. A first and
second solar cell pair is electrically connected in series by an
electrical contact that electrically connects the first electrode
of each cell in the first pair to the corresponding
counter-electrode of the second pair.
Inventors: |
Gronet; Chris M.; (Portola
Valley, CA) |
Correspondence
Address: |
JONES DAY
222 EAST 41ST ST
NEW YORK
NY
10017
US
|
Assignee: |
Solyndra, Inc.
|
Family ID: |
37572162 |
Appl. No.: |
11/809274 |
Filed: |
May 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11725231 |
Mar 16, 2007 |
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11809274 |
May 31, 2007 |
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11158178 |
Jun 20, 2005 |
7196262 |
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11725231 |
Mar 16, 2007 |
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Current U.S.
Class: |
136/259 ;
257/E31.039 |
Current CPC
Class: |
H01L 31/042 20130101;
Y02E 10/50 20130101; H01L 31/035281 20130101; H01L 31/0504
20130101; H01L 31/022425 20130101; H01L 31/03529 20130101; H01L
31/02008 20130101 |
Class at
Publication: |
136/259 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1-114. (canceled)
115. A method of manufacturing a solar cell comprising: depositing
an absorber layer on a face of a metallic web or a conducting foil;
depositing a window layer on said absorber layer; depositing a
transparent conductive oxide layer on said window layer; and
rolling said metallic web or conducting foil around an elongated
core, thereby forming an elongated solar cell.
116. The method of claim 115, wherein said absorber layer is
copper-indium-gallium-diselenide (Cu(InGa)Se.sub.2) and said window
layer is cadmium sulfide.
117. The method of claim 115, wherein the metallic web is a
polyimide/molybdenum web.
118. The method of claim 115, wherein the conducting foil is steel
foil or aluminum foil.
119. The method of claim 115, wherein the conducting foil is a
transparent conducting material.
120. The method of claim 115, wherein the elongated core is
wire-shaped or tube-shaped.
121. The method of claim 115, wherein the elongated core is made of
a nonconductive material.
122. The method of claim 121, wherein the nonconductive material is
plastic or glass.
123. The method of claim 115, wherein the elongated core is made of
a conductive metal, a metal-filled epoxy, a metal-filled glass, a
metal-filled resin, or a conductive plastic.
124. The method of claim 115, wherein an i-layer is deposited on
the window layer prior to depositing the transparent conductive
oxide layer.
125. The method of claim 115, wherein said rolling step further
comprises gluing said metallic web or conducting foil around said
elongated core.
126-130. (canceled)
131. The method of claim 115, wherein the elongated core is a
plastic rod, a glass rod, a glass tube, a plastic tube, or a metal
tube.
132. The method of claim 115, wherein the elongated core is
hollowed.
133. The method of claim 115, wherein the solar cell is
rod-shaped.
134. The method of claim 115, the method further comprising:
circumferentially depositing an antireflective coating on said
transparent conductive oxide layer.
135. The method of claim 134, wherein said antireflective coating
is made of MgF.sub.2.
136. The method of claim 115, wherein said elongated core is made
of polybenzamidazole, polyimide, polytetrafluoroethylene,
polyetheretherketone, polyamide-imide, glass-based phenolic,
polystryrene, cross-linked polystyrene, polyester, polycarbonate,
polyethylene, acrylonitrile-butadiene-styrene,
polytetrafluoro-ethylen, polymethacrylate, nylon 6,6, cellulose
acetate butyrate, cellulose acetate, rigid vinyl, plasticized
vinyl, or polypropylene.
137. The method of claim 115, wherein the solar cell is between 1
cm and 50,000 cm in length.
138. The method of claim 115, wherein the solar cell is between 1
cm and 50,000 cm in width.
Description
1. FIELD OF THE INVENTION
[0001] This invention relates to solar cell assemblies for
converting solar energy into electrical energy and more
particularly to improved solar cell assemblies.
2. BACKGROUND OF THE INVENTION
[0002] Interest in photovoltaic cells has grown rapidly in the past
few decades. Photovoltaic cells comprise semiconductor junctions
such as p-n junctions. It is well known that light with photon
energy greater than the band gap of an absorbing semiconductor
layer in a semiconductor junction is absorbed by the layer. Such
absorption causes optical excitation and the release of free
electrons and free holes in the semiconductor. Because of the
potential difference that exists at a semiconductor junction (e.g.,
a p-n junction), these released holes and electrons move across the
junction in opposite directions and thereby give rise to flow of an
electric current that is capable of delivering power to an external
circuit. The flow of carriers into the external circuit constitutes
a electrical current density, J amp cm.sup.-2, which, under
short-circuit conditions, is known as the short-circuit current
density, J.sub.sc. At the same time, the separation of the charges
(holes and electrons) sets up a potential difference between the
two ends of the material, .phi., which under open circuit
conditions is known as the open-circuit voltage, .phi..sub.OC. It
is desirable to maximize both J.sub.sc and .phi..sub.OC. For
interaction with the solar spectrum, J.sub.sc and .phi..sub.OC are
optimized when the junction semiconductor absorber has a band gap
of about 1.4 electron volts (eV).
[0003] It is presently common practice to provide an array of solar
cells to generate electrical energy from solar radiation. Many
solar cells are made of silicon. However, cells made of other
materials, e.g., cadmium sulfide and gallium arsenide, have also
been developed and tested. Crystalline silicon has traditionally
been a favored material since it has a band gap of approximately
1.1 eV and thus favorably responds to the electromagnetic energy of
the solar spectrum. However, because of the expense in making
crystalline silicon-based cells, thin film solar cells made of
materials other than silicon have been explored and used.
[0004] Presently solar cells are fabricated as separate physical
entities with light gathering surface areas on the order of 4-6
cm.sup.2 or larger. For this reason, it is standard practice for
power generating applications to mount the cells in a flat array on
a supporting substrate or panel so that their light gathering
surfaces provide an approximation of a single large light gathering
surface. Also, since each cell itself generates only a small amount
of power, the required voltage and/or current is realized by
interconnecting the cells of the array in a series and/or parallel
matrix.
[0005] A conventional prior art solar cell structure is shown in
FIG. 1. Because of the large range in the thickness of the
different layers, they are depicted schematically. Moreover, FIG. 1
is highly schematized so that it will represent the features of
both "thick-film" solar cells and "thin-film" solar cells. In
general, solar cells that use an indirect band gap material to
absorb light are typically configured as "thick-film" solar cells
because a thick film of the absorber layer is required to absorb a
sufficient amount of light. Solar cells that use a direct band gap
material to absorb light are typically configured as "thin-film"
solar cells because only a thin layer of the direct band-gap
material is need to absorb a sufficient amount of light.
[0006] The arrows at the top of FIG. 1 show the direction of the
solar illumination on the cell. Layer (element) 102 is the
substrate. Glass or metal is a common substrate. In thin-film solar
cells, substrate 102 can be-a polymer-based backing, metal, or
glass. In some instances, there is an encapsulation layer (not
shown) coating substrate 102. Layer 104 is the back electrical
contact for the solar cell. It makes ohmic contact with the
absorber layer of semiconductor junction 106.
[0007] Layer 106 is the semiconductor absorber layer. In many but
not all cases it is a p-type semiconductor. Absorber layer 106 is
thick enough to absorb light. Layer 108 is the semiconductor
junction partner--that completes the formation of a p-n junction,
which is a common type of junction found in solar cells. In a solar
cell based on a p-n junction, when absorber 106 is a p-type doped
material, junction partner 108 is an n-type doped material.
Conversely, when layer 106 is an n-type doped material, layer 108
is a p-type doped material. Generally, junction partner 108 is much
thinner than absorber 106. For example, in some instances junction
partner 108 has a thickness of about 0.05 microns. Junction partner
108 is highly transparent to solar radiation. Junction partner 108
is also known as the window layer, since it lets the light pass
down to absorber layer 106.
[0008] In a typical thick-film solar cell, layers 106 and 108 can
be made from the same semiconductor material but have different
carrier types (dopants) and/or carrier concentrations in order to
give the two layers their distinct p-type and n-type properties. In
thin-film solar cells in which copper-indium-gallium-diselenide
(CIGS) is absorber layer 106, the use of CdS to form layer 108 has
resulted in high efficiency cells. Other materials that can be used
for layer 108 includes, but are not limited to, SnO.sub.2, ZnO,
ZrO.sub.2 and doped ZnO.
[0009] Layer 110 is the top transparent electrode, which completes
the functioning cell. Layer 110 is used to draw current away from
the junction since junction partner 108 is generally too resistive
to serve this function. As such, layer 110 should be highly
conductive and transparent to light. Layer 110 can in fact be a
comb-like structure of metal printed onto layer 108 rather than
forming a discrete layer. Layer 110 is typically a transparent
conductive oxide (TCO) such as zinc oxide (ZnO), indium-tin-oxide
(ITO), or tin oxide (SnO.sub.2). However, even when a TCO layer is
present, a bus bar network 114 is typically needed to draw off
current since the TCO has too much resistance to efficiently
perform this function in larger solar cells. Network 114 shortens
the distance charger carriers must move in the TCO layer in order
to reach the metal contact, thereby reducing resistive losses. The
metal bus bars, also termed grid lines, can be made of any
reasonably conductive metal such as, for example, silver, steel or
aluminum. In the design of network 114, there is design a tradeoff
between thicker grid lines that are more electrically conductive
but block more light, and thin grid lines that are less
electrically conductive but block less light. The metal bars are
preferably configured in a comb-like arrangement to permit light
rays through TCO layer 110. Bus bar network layer 114 and TCO layer
110, combined, act as a single metallurgical unit, functionally
interfacing with a first ohmic contact to form a current collection
circuit. In U.S. Pat. No. 6,548,751 to Sverdrup et al., hereby
incorporated by reference in its entirety, a combined silver (Ag)
bus bar network and indium-tin-oxide layer function as a single,
transparent ITO/Ag layer.
[0010] Layer 112 is an antireflection (AR) coating, which can allow
a significant amount of extra light into the cell. Depending on the
intended use of the cell, it might be deposited directly on the top
conductor (as illustrated), or on a separate cover glass, or both.
Ideally, the AR coating reduces the reflection of the cell to very
near zero over the spectral region that photoelectric absorption
occurs, and at the same time increases the reflection in the other
spectral regions to reduce heating. U.S. Pat. No. 6,107,564 to
Aguilera et al., hereby incorporated by reference in its entirety,
describes representative antireflective coatings that are known in
the art.
[0011] Solar cells typically produce only a small voltage. For
example, silicon based solar cells produce a voltage of about 0.6
volts (V). Thus, solar cells are interconnected in series or
parallel in order to get a reasonable voltage. When connected in
series, voltages of individual cells add together while current
remains the same. Thus, solar cells arranged in series reduce the
amount of current flow through such cells, compared to analogous
solar cells arrange in parallel, thereby improving efficiency. As
illustrated in FIG. 1, the arrangement of solar cells in series is
accomplished using interconnects 116. In general, an interconnect
116 places the first electrode of one solar cell in electrical
communication with the counterelectrode of an adjoining solar
cell.
[0012] As noted above and as illustrated in FIG. 1, conventional
solar cells are typically in the form of a plate structure.
Although such cells are highly efficient when they are smaller,
larger planar solar cells have reduced efficiency because it is
harder to make the semiconductor films that form the junction in
such solar cells uniform. Furthermore, the occurrence of pinholes
and similar flaws increase in larger planar solar cells. These
features can cause shunts across the junction.
[0013] A number of problems are associated with solar cell designs
present in the known art. A number of prior art solar cell designs
and some of the disadvantages of each design will now be
discussed.
[0014] As illustrated in FIG. 2, U.S. Pat. No. 6,762,359 B2 to Asia
et al. discloses a solar cell 210 including a p-type layer 12 and
an n-type layer 14. A first electrode 32 is provided on one side of
the solar cell. Electrode 32 is in electrical contact with n-type
layer 14 of solar cell 210. Second electrode 60 is on the opposing
side of the solar cell. Electrode 60 is in electrical contact with
the p-type layer of the solar cell. Light-transmitting layers 200
and 202 form one side of device 210 while layer 62 forms the other
side. Electrodes 32 and 60 are separated by insulators 40 and 50.
In some instances, the solar cell has a tubular shape rather than
the spherical shape illustrated in FIG. 2. While device 210 is
functional, it is unsatisfactory. Electrode 60 has to pierce
absorber 12 in order to make an electrical contact. This results in
a net loss in absorber area, making the solar cell less efficient.
Furthermore, such a junction is difficult to make relative to other
solar cell designs.
[0015] As illustrated in FIG. 3A, U.S. Pat. No. 3,976,508 to
Mlavsky discloses a tubular solar cell comprising a cylindrical
silicon tube 2 of n-type conductivity that has been subjected to
diffusion of boron into its outer surface to form an outer
p-conductivity type region 4 and thus a p-n junction 6. The inner
surface of the cylindrical tube is provided with a first electrode
in the form of an adherent metal conductive film 8 that forms an
ohmic contact with the tube. Film 8 covers the entire inner surface
of the tube and consists of a selected metal or metal alloy having
relatively high conductivity, e.g., gold, nickel, aluminum, copper
or the like, as disclosed in U.S. Pat. Nos. 2,984,775, 3,046,324
and 3005862. The outer surface is provided with a second electrode
in the form of a grid consisting of a plurality of
circumferentially extending conductors 10 that are connected
together by one or more longitudinally-extending conductors 12. The
opposite ends of the outer surface of the hollow tube are provided
with two circumferentially-extending terminal conductors 14 and 16
that intercept the longitudinally-extending conductors 12. The
spacing of the circumferentially-extending conductors 10 and the
longitudinally-extending conductors 12 is such as to leave areas 18
of the outer surface of the tube exposed to solar radiation.
Conductors 12, 14 and 16 are made wider than the
circumferentially-extending conductors 10 since they carry a
greater current than any of the latter. These conductors are made
of an adherent metal film like the inner electrode 8 and form ohmic
contacts with the outer surface of the tube. While the solar cell
disclosed in FIG. 3 is functional, it is also unsatisfactory.
Conductors 12, 14, and 16 are not transparent to light and
therefore the amount of light that the solar cell receives is
proportionally reduced by the amount of surface area occupied by
the cells.
[0016] U.S. Pat. No. 3,990,914 to Weinstein and Lee discloses
another form of tubular solar cell. Like Mlavsky, the Weinstein and
Lee solar cell has a hollow core. However, unlike Mlavsky,
Weinstein and Lee dispose the solar cell on a glass tubular support
member. The Weinstein and Lee solar cell has the drawback of being
bulky and expensive to build.
[0017] Referring to FIGS. 3B and 3C, Japanese Patent Application
Kokai Publication Number S59-125670, Toppan Printing Company,
published Jul. 20, 1984 (hereinafter "S59-125670") discloses a
rod-shaped solar cell. The rod shaped solar cell is depicted in
cross-section in Figure. A conducting metal is used as the core 1
of the cell. A light-activated amorphous silicon semiconductor
layer 3 is provided on core 1. An electrically conductive
transparent conductive layer 4 is built up on top of semiconductor
layer 3. The transparent conductive layer 4 can be made of
materials such as indium oxide, tin oxide or indium tin oxide (ITO)
and the like. As illustrated in FIG. 3B, a layer 5, made of a good
electrical conductor, is provided on the lower portion of the solar
cell. The publication states that this good conductive layer 5 is
not particularly necessary but helps to lower the contact
resistance between the rod and a conductive substrate 7 that serves
as a counter electrode. As such, conductive layer 5 serves as a
current collector that supplements the conductivity of counter
electrode 7 illustrated in FIG. 3C.
[0018] As illustrated in FIG. 3C, rod-shaped solar cells 6 are
multiply arranged in a row parallel with each other, and counter
electrode layer 7 is provided on the surface of the rods that is
not irradiated by light so as to electrically make contact with
each transparent conductive layer 4. The rod-shaped solar cells 6
are arranged in parallel and both ends of the solar cells are
hardened with resin or a similar material in order to fix the rods
in place.
[0019] S59-125670 addresses many of the drawbacks associated with
planar solar cells. However, S59-125670 has a number of significant
drawbacks that limit the efficiency of the disclosed devices.
First, the manner in which current is drawn off the exterior
surface is inefficient because layer 5 does not wrap all the way
around the rod (e.g., see FIG. 3B). Second, substrate 7 is a metal
plate that does not permit the passage of light. Thus, a full side
of each rod is not exposed to light and can thus serve as a leakage
path. Such a leakage path reduces the efficiency of the solar cell.
For example, any such dark junction areas will result in a leakage
that will detract from the photocurrent of the cell. Another
disadvantage with the design disclosed in FIGS. 3B and 3C is that
the rods are arranged in parallel rather than in series. Thus, the
current levels in such devices will be large, relative to a
corresponding serially arranged model, and therefore subject to
resistive losses.
[0020] Referring to FIG. 3D, German Unexamined Patent Application
DE 43 39 547 A1 to Twin Solar-Technik Entwicklungs-GmbH, published
May 24, 1995, (hereinafter "Twin Solar") also discloses a plurality
of rod-shaped solar cells 2 arranged in a parallel manner inside a
transparent sheet 28, which forms the body of the solar cell. Thus,
Twin Solar does not have some of the drawbacks found in S59-125670.
Transparent sheet 28 allows light in from both faces 47A and 47B.
Transparent sheet 28 is installed at a distance from a wall 27 in
such a manner as to provide an air gap 26 through which liquid
coolant can flow. Thus, Twin Solar devices have the drawback that
they are not truly bifacial. In other words, only face 47A of the
Twin Solar device is capable of receiving direct light. As defined
here, "direct light" is light that has not passed through any media
other than air. For example, light that has passed through a
transparent substrate, into a solar cell assembly, and exited the
assembly is no longer direct light once it exits the solar cell
assembly. Light that has merely reflected off of a surface,
however, is direct light provided that it has not passed through a
solar cell assembly. Under this definition of direct light, face
47B is not configured to receive direct light. This is because all
light received by face 47B must first traverse the body of the
solar cell apparatus after entering the solar cell apparatus
through face 47A. Such light must then traverse cooling chamber 26,
reflect off back wall 42, and finally re-enter the solar cell
through face 47B. The solar cell assembly is therefore inefficient
because direct light cannot enter both sides of the assembly.
[0021] Discussion or citation of a reference herein will not be
construed as an admission that such reference is prior art to the
present invention.
3. SUMMARY OF THE INVENTION
[0022] One aspect of the present invention provides a solar cell
assembly comprising a plurality of elongated solar cells. Each
elongated solar cell in the plurality of elongated solar cells
comprises (i) a conductive core configured as a first electrode,
(ii) a semiconductor junction circumferentially disposed on the
conductive core, and (iii) a transparent conductive oxide layer
disposed on the semiconductor junction. Elongated solar cells in
said plurality of elongated solar cells are geometrically arranged
in a parallel or a near parallel manner thereby forming a planar
array having a first face and a second face. The plurality of
elongated solar cells is arranged such that one or more elongated
solar cells in the plurality of elongated solar cells do not
contact adjacent elongated solar cells. The solar cell assembly
further comprises a plurality of electrode strips. Each respective
electrode strip in the plurality of electrode strips is lengthwise
disposed on a first side of a corresponding elongated solar cell in
the plurality of elongated solar cells. The first side of the solar
cell is part of the first face of the planar array. The solar cell
assembly further comprises a transparent electrically insulating
substrate that covers all or a portion of the first face of the
planar array. A first and second elongated solar cell in the
plurality of elongated solar cells are electrically connected in
series by an electrical contact that connects the first electrode
of the first elongated solar cell to the corresponding electrode
strip of the second elongated solar cell. The plurality of
elongated solar cells is configured to receive direct light from
the first face and the second face of the planar array.
[0023] Another aspect of the invention is also directed to a solar
cell assembly. The solar cell assembly comprises a plurality of
elongated solar cells. Each elongated solar cell in the plurality
of elongated solar cells comprises (i) a conductive core configured
as a first electrode, (ii) a semiconductor junction
circumferentially disposed on the conductive core, (iii) and a
transparent conductive oxide layer disposed on the semiconductor
junction. The elongated solar cells in the plurality of elongated
solar cells are geometrically arranged in a parallel or near
parallel manner as a plurality of solar cell pairs so as to form a
planar array having a first face and a second face. The solar cells
in a pair of solar cells do not touch the solar cells in an
adjacent pair of solar cells in the planar array. The solar cell
assembly further comprises a plurality of metal counter-electrodes.
Each respective metal counter-electrode in the plurality of metal
counter-electrodes joins together, lengthwise, elongated solar
cells in a corresponding solar cell pair in the plurality of solar
cell pairs. The solar cell assembly further comprises a transparent
electrically insulating substrate that covers all or a portion of
the first face of the planar array. A first solar cell pair and a
second solar cell pair in the plurality of elongated solar cells
are electrically connected in series by an electrical contact that
electrically connects the first electrode of each elongated solar
cell in the first solar cell pair to the corresponding
counter-electrode of the second solar cell pair.
[0024] Still another aspect of the invention is directed to a solar
cell assembly comprising a plurality of elongated solar cells. Each
elongated solar cell in the plurality of elongated solar cells
comprises (i) a conductive core configured as a first electrode,
(ii) a semiconductor junction circumferentially disposed on the
conductive core, and (iii) a transparent conductive oxide layer
disposed on said semiconductor junction. The plurality of elongated
solar cells is geometrically arranged in a parallel or a near
parallel manner thereby forming a planar array having a first face
and a second face. The plurality of elongated solar cells is
arranged such that one or more elongated solar cells in the
plurality of elongated solar cells do not contact adjacent
elongated solar cells. The solar cell assembly in accordance with
this aspect of the invention further comprises a plurality of metal
counter-electrodes. Each respective elongated solar cell in the
plurality of elongated solar cells is bound to a first
corresponding metal counter-electrode in the plurality of metal
counter-electrodes such that the first metal counter-electrode lies
in a first groove that runs lengthwise on the respective elongated
solar cell. The solar cell assembly further comprises a transparent
electrically insulating substrate that covers all or a portion of
the first face of the planar array. Furthermore, a first and second
elongated solar cell in the plurality of elongated solar cells are
electrically connected in series by an electrical contact that
connects the first electrode of the first elongated solar cell to
the first corresponding counter-electrode of the second elongated
solar cell. In addition, the plurality of elongated solar cells is
configured to receive direct light from the first face and the
second face of the planar array.
[0025] Yet another aspect of the invention provides a solar cell
assembly comprising a plurality of elongated solar cells. Each
elongated solar cell in the plurality of elongated solar cells
comprises (i) a conductive core configured as a first electrode,
(ii) a semiconductor junction circumferentially disposed on the
conductive core, and (iii) a transparent conductive oxide layer
disposed on the semiconductor junction. The plurality of elongated
solar cells is geometrically arranged in a parallel or a near
parallel manner thereby forming a planar array having a first face
and a second face. The plurality of elongated solar cells is
arranged such that one or more elongated solar cells in the
plurality of elongated solar cells do not contact adjacent
elongated solar cells. The solar cell assembly in accordance with
this aspect of the invention further comprises a plurality of metal
counter-electrodes. Each respective elongated solar cell in the
plurality of elongated solar cells is bound to a first
corresponding metal counter-electrode and a second corresponding
metal counter-electrode in the plurality of metal
counter-electrodes such that the first metal counter-electrode lies
in a first groove that runs lengthwise on the respective elongated
solar cell and the second metal counter-electrode lies in a second
groove that runs lengthwise on the respective elongated solar cell.
The first groove and the second groove are on opposite sides of the
respective elongated solar cell. The solar cell assembly in
accordance with this aspect of the invention further comprises a
transparent electrically insulating substrate that covers all or a
portion of the first face of the planar array. In this aspect of
the invention, a first and second elongated solar cell in the
plurality of elongated solar cells is electrically connected in
series.
4. BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 illustrates interconnected solar cells in accordance
with the prior art.
[0027] FIG. 2 illustrates a spherical solar cell including a p-type
inner layer and an n-type outer layer in accordance with the prior
art.
[0028] FIG. 3A illustrates a tubular photovoltaic element
comprising a cylindrical silicon tube of n-type conductivity that
has been subjected to diffusion of boron into its outer surface to
form an outer p-conductivity type region and thus a tubular solar
cell in accordance with the prior art.
[0029] FIG. 3B is a cross-sectional view of an elongated solar cell
in accordance with the prior art.
[0030] FIG. 3C is a cross-sectional view of a solar cell assembly
in which a plurality of elongated solar cells are affixed to an
electrically conductive substrate in accordance with the prior
art.
[0031] FIG. 3D is a cross-sectional view of a solar cell assembly
disposed a distance away from a reflecting wall in accordance with
the prior art.
[0032] FIG. 4A is a cross-sectional view of elongated solar cells
electrically arranged in series and geometrically arranged in a
parallel or near parallel manner on counter-electrodes that contact
a substrate in order to form a bifacial assembly, in accordance
with an embodiment of the present invention.
[0033] FIG. 4B is a cross-sectional view taken about line 4B-4B of
FIG. 4A depicting the serial electrical arrangement of tubular
solar cells in a bifacial assembly in accordance with an embodiment
of the present invention.
[0034] FIG. 4C is a blow-up perspective view of region 4C of FIG.
4B, illustrating various layers in elongated solar cells in
accordance with one embodiment of the present invention.
[0035] FIG. 4D is a cross-sectional view of an elongated solar cell
taken about line 4D-4D of FIG. 4B, in accordance with an embodiment
of the present invention.
[0036] FIG. 4E is a cross-sectional view taken about line 4B-4B of
FIG. 4A that depicts the serial arrangement of tubular solar cells
in a bifacial assembly in accordance with an alternative embodiment
of the present invention.
[0037] FIG. 4F is a cross-sectional view of a elongated solar cell
taken about line 4F-4F of FIG. 4E, in accordance with an embodiment
of the present invention.
[0038] FIGS. 5A-5D depict semiconductor junctions that are used in
various elongated solar cells in various embodiments of the present
invention.
[0039] FIG. 6A is a cross-sectional view of elongated solar cells
electrically arranged in series in a bifacial assembly where
counter-electrodes form interfaces between solar cell pairs, in
accordance with another embodiment of the present invention.
[0040] FIG. 6B is a cross-sectional view taken about line 6B-6B of
FIG. 6A that depicts the serial arrangement of tubular solar cells
in a bifacial assembly in accordance with an embodiment of the
present invention.
[0041] FIG. 6C is a cross-sectional view of an elongated solar cell
taken about line 6C-6C of FIG. 6B, in accordance with an embodiment
of the present invention.
[0042] FIG. 7A is a cross-sectional view of elongated solar cells
electrically arranged in series in a bifacial assembly where
counter-electrodes abut individual solar cells, in accordance with
another embodiment of the present invention.
[0043] FIG. 7B is a cross-sectional view taken about line 7B-7B of
FIG. 7A that depicts the serial arrangement of tubular solar cells
in a bifacial assembly in accordance with an embodiment of the
present invention.
[0044] FIG. 8 is a cross-sectional view of elongated solar cells
electrically arranged in series in a bifacial assembly where
counter-electrodes abut individual solar cells and the outer TCO is
cut, in accordance with another embodiment of the present
invention.
[0045] FIG. 9 is a cross-sectional view of elongated solar cells
electrically arranged in series in a bifacial assembly in which the
inner metal electrode is hollowed, in accordance with an embodiment
of the present invention.
[0046] FIG. 10 is a cross-sectional view of elongated solar cells
electrically arranged in series in a bifacial assembly in which a
groove pierces the counter-electrodes, transparent conducting oxide
layer, and junction layers of the solar cells, in accordance with
an embodiment of the present invention.
[0047] FIG. 11 illustrates how the solar cell assemblies of the
present invention can be used in conjunction with one type of
static concentrator.
[0048] FIG. 12 illustrates how the solar cell assemblies of the
present invention can be used in conjunction with another type of
static concentrator.
[0049] FIG. 13 illustrates a solar cell made by a roll method in
accordance with an embodiment of the present invention.
[0050] Like reference numerals refer to corresponding parts
throughout the several views of the drawings. Dimensions are not
drawn to scale.
5. DETAILED DESCRIPTION
[0051] Disclosed herein are solar cell assemblies for converting
solar energy into electrical energy and more particularly to
improved solar cells and solar cell arrays. The solar cells of the
present invention have a wire shape and are arranged in parallel
but are electrically connected in series.
5.1 Basic Structure
[0052] The present invention provides a solar cell assembly 400 in
which elongated solar cells 402, shown in cross-section in FIG. 4A,
serve to absorb light. A conductive core (elongated conductive
core) 404 serves as the first electrode in the assembly and a
transparent conductive oxide (TCO) 412 on the exterior surface of
each solar cell serves as the counter electrode.
[0053] In general, conductive core 404 is made out of any material
such that it can support the photovoltaic current generated by
solar cell with negligible resistive losses. In some embodiments,
conductive core 404 is composed of any conductive material, such as
aluminum, molybdenum, steel, nickel, silver, gold, or an alloy
thereof. In some embodiments, conductive core 404 is made out of a
metal-, graphite-, carbon black-, or superconductive carbon
black-filled oxide, epoxy, glass, or plastic. In some embodiments,
conductive core 404 is made of a conductive plastic. As defined
herein, a conductive plastic is one that, through compounding
techniques, contains conductive fillers which, in turn, impart
their conductive properties to the plastics system. The conductive
plastics used in the present invention to form conductive core 404
contain fillers that form sufficient conductive current-carrying
paths through the plastic matrix to support the photovoltaic
current generated by solar cell with negligible resistive losses.
The plastic matrix of the conductive plastic is typically
insulative, but the composite produced exhibits the conductive
properties of the filler.
[0054] A semiconductor junction 410 is formed around conductive
core 404. Semiconductor junction 410 is any photovoltaic
homojunction, heterojunction, heteroface junction, buried
homojunction, or p-i-n junction having an absorber layer that is a
direct band-gap absorber (e.g., crystalline silicon) or an indirect
band-gap absorber (e.g., amorphous silicon). Such junctions are
described in Chapter 1 of Bube, Photovoltaic Materials, 1998,
Imperial College Press, London, which is hereby incorporated by
reference in its entirety. Details of exemplary types of
semiconductors junctions 410 in accordance with the present
invention are disclosed in Section 5.2, below. In addition to the
exemplary junctions disclosed in Section 5.2, below, junctions 410
can be multijunctions in which light traverses into the core of
junction 410 through multiple junctions that, preferably, have
successfully smaller bandgaps.
[0055] Optionally, there is a thin intrinsic layer (i-layer) 415
between semiconductor junction 410 and an outer transparent
conductive oxide (TCO) layer 412. The i-layer 415 can be formed
using any undoped transparent oxide including, but not limited to,
zinc oxide or indium-tin-oxide.
[0056] The transparent conductive oxide (TCO) layer 412 is built up
on top of the semiconductor junction layers 410 thereby completing
the circuit. As noted above, in some embodiments, there is a thin
i-layer coating the semiconductor junction 410. In such
embodiments, TCO layer 412 is built on top of the i-layer. In some
embodiments, TCO layer 412 is made of tin oxide SnO.sub.x (with or
without fluorine doping), indium-tin oxide (ITO), doped zinc oxide
(ZnO) or any combination thereof. In some embodiments, TCO layer
412 is either p-doped or n-doped. For example, in embodiments where
the outer semiconductor layer of junction 410 is p-doped, TCO layer
412 can be p-doped. Likewise, in embodiments where the outer
semiconductor layer of junction 410 is n-doped, TCO layer 412 can
be n-doped. In general, TCO layer 412 is preferably made of a
material that has very low resistance, suitable optical
transmission properties (e.g., greater than 90%), and a deposition
temperature that will not damage underlying layers of semiconductor
junction 410 and/or optional i-layer 415. In some embodiments, TCO
412 is an electrically conductive polymer material such as a
conductive polytiophene, a conductive polyaniline, a conductive
polypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or a derivative of
any of the foregoing. In some embodiments, TCO comprises more than
one layer, including a first layer comprising tin oxide SnO.sub.x
(with or without fluorine doping), indium-tin oxide (ITO), zinc
oxide (ZnO) or a combination thereof and a second layer comprising
a conductive polytiophene, a conductive polyaniline, a conductive
polypyrrole, a PSS-doped PEDOT (e.g., Bayrton), or a derivative of
any of the foregoing. Additional suitable materials that can be
used to form TCO layer are disclosed in United States Patent
publication 2004/0187917A1 to Pichler, which is hereby incorporated
by reference in its entirety.
[0057] Rod-shaped (elongated) solar cells 402 are lined up multiply
parallel. The entire assembly is sealed between electrically
resistant transparent substrate 406 and a covering 422 using a
sealant such as ethyl vinyl acetate. Covering 422 is generally made
from the same materials as substrate 406. Suitable materials for
covering 422 and substrate 406 include, but are not limited to
glass or polyvinyl fluoride products such as Tedlar (DuPont,
Wilmington, Del.).
[0058] FIG. 4B provides a cross-sectional view with respect to line
4B-4B of FIG. 4A. As can be seen with FIGS. 4A and 4B, each
elongated cell 402 has a length that is great compared to the
diameter d of its cross-section. An advantage of the architecture
shown in FIG. 4A is that there is no front side contact that shades
solar cells 402. Such a front side contact is found in known
devices (e.g., elements 10 of FIG. 3). Another advantage of the
architecture shown in FIG. 4A is that elongated cells 402 are
electrically connected in series rather than in parallel. In such a
series configuration, the voltage of each elongated cell 402 is
summed. This serves to increase the voltage across the system,
thereby keeping the current down, relative to comparable parallel
architectures, and minimizing resistive losses. A serial electrical
arrangement is maintained by arranging all or a portion of the
elongated solar cells 402 such that they do not touch each other,
as illustrated in FIGS. 4A and 4B. The separation distance between
solar cells 402 is any distance that prevents electrical contact
between solar cells 402. For instance, in some embodiments, the
distance between adjacent solar cells 402 is 0.1 micron or greater,
0.5 microns or greater, or between 1 and 5 microns.
[0059] Another advantage of the architecture shown in FIG. 4A is
that the resistance loss across the system is low. This is because
each electrode component of the circuit is made of highly
conductive material. For example, as noted above, conductive core
404 of each solar cell 402 is made of a conductive metal.
Furthermore, each conductive core 404 has a diameter that is large
enough to carry current without an appreciable current loss due to
resistance. While larger conductive cores 404 ensure low
resistance, TCO layers encompassing such larger conductive cores
404 must carry current further to contacts (counter-electrode
strip) 420. Thus, there is an upper bound on the size of conductive
cores 404. In view of these and other considerations, diameter d is
between 0.5 millimeters (mm) and 20 mm in some embodiments of the
present invention. Thus, conductive cores 404 are sized so that
they are large enough to carry a current without appreciable
resistive loss, yet small enough to allow TCO 412 to efficiently
deliver current to leads 420. With this balanced design, resistive
loss is minimized and an efficient solar cell assembly 400 is
realized.
[0060] The advantageous low resistance nature of the architecture
illustrated in FIG. 4A is also facilitated by the highly conductive
properties of leads 420. In some embodiments, for example, leads
420 are composed of a conductive epoxy (e.g., silver epoxy) or
conductive ink and the like.
[0061] There are a number of different ways in which elongated
cells 402 can be packaged in order to form solar cell assembly 400.
For example, in one embodiment, leads 420 are formed by depositing
a thin metallic layer on substrate 406 and then patterning the
layer into a series of parallel strips, where each strip runs the
length of a solar cell 402. Then, elongated solar cells 402 are
affixed to substrate 406 by leads 420 using a conductive epoxy. In
some embodiments, leads 420 are formed directly on solar cells 402
and solar cells 402 are not affixed to substrate 406. In such
embodiments, there are at least two different ways in which
elongated solar cells 402 can be packaged to form solar cell
assembly 400. In a first approach, elongated solar cells 402,
having leads 420 as illustrated in FIG. 4A, rest on substrate 406
but are not affixed to the substrate. In a second approach,
elongated solar cells 402, having leads 420 as illustrated in FIG.
4A, do not contact substrate 406. This second approach is not
illustrated. In this second approach, a layer of ethyl vinyl
acetate or some other suitable transparent material separates
contacts 420 from substrate 406.
[0062] Still another advantage of the architecture illustrated in
FIG. 4A is that the path length through the absorber layer (e.g.,
layer 502, 510, 520, or 540 of FIG. 5) of semiconductor junction
410 is, on average, longer than the path length through of the same
type of absorber layer having the same width but in a planar
configuration. Thus, the elongated architecture illustrated in FIG.
4A allows for the design of thinner absorption layers relative to
analogous planar solar cell counterparts. In the elongated
architecture, the thinner absorption layer absorbs the light
because of the increased path length through the layer. Because the
absorption layer is thinner relative to comparable planar solar
cells, there is less resistance and, hence, an overall increase in
efficiency in the cell relative to analogous planar solar cells.
Additional advantages of having a thinner absorption layer that
still absorbs sufficient amounts of light is that such absorption
layers require less material and are thus cheaper. Furthermore,
thinner absorption layers are faster to make, thereby further
lowering production costs.
[0063] Another advantage of elongated solar cells 402 illustrated
in FIG. 4A is that they have a relatively small surface area,
relative to comparable planar solar cells, and they possess radial
symmetry. Each of these properties allow for the controlled
deposition of doped semiconductor layers necessary to form
semiconductor junction 410. The smaller surface area, relative to
conventional flat panel solar cells, means that it is easier to
present a uniform vapor across the surface during deposition of the
layers that form semiconductor junction 410. The radial symmetry
can be exploited during the manufacture of the cells in order to
ensure uniform composition (e.g., uniform material composition,
uniform dopant concentration, etc.) and/or uniform thickness of
individual layers of semiconductor junction 410. For example, the
conductive core 404 upon which layers are deposited to make solar
cells 402 can be rotated along its longitudinal axis during such
deposition in order to ensure uniform material composition and/or
uniform thickness.
[0064] The cross-sectional shape of solar cells 402 is generally
circular in FIG. 4B. In other embodiments, solar cell 402 bodies
with a quadrilateral cross-section or an elliptical shaped
cross-section and the like are used. In fact, there is no limit on
the cross-sectional shape of solar cells 402 in the present
invention, so long as the solar cells 402 maintain a general
overall rod-like or wire-like shape in which their length is much
larger than their diameter and they possess some form of
cross-sectional radial symmetry.
[0065] As illustrated in FIG. 4B, assembly 400 comprises many
elongated solar cells 402 geometrically arranged in parallel
fashion and electrically connected in series. For example, a first
and second elongated solar cell (rod-shaped solar cell) 402 are
electrically connected in series by an electrical contact 433 that
connects the conductive core 404 (first electrode) of the first
elongated solar cell 402 to the corresponding counter-electrode
strip 420 electrode strip of the second elongated solar cell. Thus,
as illustrated in FIG. 4A, elongated solar cells 402 are the basic
unit that respectively forms the semiconductor layer 410, the TCO
412, and the metal conductive core 404 of the elongated solar cell
402. The elongated solar cells 402 are multiply arranged in a row
parallel or nearly parallel with respect to each other and rest
upon independent leads (counter electrodes) 420 that are
electrically isolated from each other. Advantageously, in the
configuration illustrated in FIG. 4A, elongated solar cells 402 can
receive direct light either through substrate 406, covering 422, or
both substrate 406 and covering 422.
[0066] In some embodiments, not all elongated solar cells 402 in
assembly 400 are electrically arranged in series. For example, in
some embodiments, there are pairs of elongated solar cells 402 that
are electrically arranged in parallel. A first and second elongated
solar cell can be electrically connected in parallel, and are
thereby paired, by using a first electrical contact (e.g., an
electrically conducting wire, etc., not shown) that joins the
conductive core 404 of a first elongated solar cell to the second
elongated solar cell. To complete the parallel circuit, the TCO 412
of the first elongated solar cell 402 is electrically connected to
the TCO 412 of the second elongated solar cell 402 either by
contacting the TCOs of the two elongated solar cells either
directly or through a second electrical contact (not shown). The
pairs of elongated solar cells are then electrically arranged in
series. In some embodiments, three, four, five, six, seven, eight,
nine, ten, eleven or more elongated solar cells 402 are
electrically arranged in parallel. These parallel groups of
elongated solar cells 402 are then electrically arranged in
series.
[0067] In some embodiments, rather than packaging solar cells 402
between a substrate 406 and cover 422 using a sealant such as ethyl
vinyl acetate, solar cells 402 arranged in the same planar parallel
configuration illustrated in FIGS. 4A and 4B are encased in a rigid
transparent film. Suitable materials for such a rigid transparent
film include, but are not limited to, polyvinyl fluoride products
such as Tedlar (DuPont, Wilmington, Del.).
[0068] FIG. 4C is an enlargement of region 4C of FIG. 4B in which a
portion of conductive core 404 and transparent conductive oxide
(TCO) 412 have been cut away to illustrate the positional
relationship between counter-electrode strip 420, elongated cell
402, and electrically resistant transparent substrate 406.
Furthermore FIG. 4C illustrates how electrical contact 433 joins
the conductive core 404 of one elongated solar cell 402 to the
counter-electrode 420 of another solar cell 402.
[0069] One advantage of the configuration illustrated in FIG. 4 is
that electrical contacts 433 that serially connect solar cells 402
together only need to be placed on one end of assembly 400, as
illustrated in FIG. 4B. Thus, referring to FIG. 4D, which is a
cross-sectional view of a elongated solar 402 cell taken about line
4D-4D of FIG. 4B, it is possible to completely seal far-end 455 of
solar cell 402 in the manner illustrated. In some embodiments, the
layers in this seal are identical to the layers circumferentially
disposed lengthwise on conductive core 404, namely, in order of
deposition on conductive core 404, semiconductor junction 410,
optional thin intrinsic layer (i-layer) 415, and transparent
conductive oxide (TCO) layer 412. In such embodiments, end 455 can
receive sun light and therefore contribute to the electrical
generating properties of the solar cell 402.
[0070] FIG. 4D also illustrates how the various layers deposited on
conductive core 404 are tapered at end 466 where electrical
contacts 433 are found. For instance, a terminal portion of
conductive core 404 is exposed, as illustrated in FIG. 4D. In other
words, semiconductor junction 410, optional i-layer 415, and TCO
412 are stripped away from a terminal portion of conductive core
404. Furthermore, a terminal portion of semiconductor junction 410
is exposed as illustrated in FIG. 4D. That is, optional i-layer 415
and TCO 412 are stripped away from a terminal portion of
semiconductor junction 410. Such a configuration is advantageous
because it prevents a short from developing between TCO 412 and
conductive core 404. In FIG. 4D, elongated solar cell 402 is
positioned on counter-electrode strip 420 which, in turn, is
positioned onto electrically resistant transparent substrate 406.
However, there is no requirement that counter-electrode strip 420
make contact with electrically resistant transparent substrate 406.
In fact, in some embodiments, elongated solar cells 402 and their
corresponding electrode strips 420 are sealed between electrically
resistant transparent substrate 406 and covering 422 in such a
manner that they do not contact substrate 406 and covering 422. In
such embodiments, elongated solar cells 402 and corresponding
electrode strips 420 are fixedly held in place by a sealant such as
ethyl vinyl acetate.
[0071] FIG. 4D further provides a perspective view of electrical
contacts 433 that serially connect elongated solar cells 402. For
instance, a first electrical contact 433-1 electrically interfaces
with counter-electrode 420 whereas a second electrical contact
433-2 electrically interfaces with conductive core 404 (the first
electrode of elongated solar cell 402). First electrical contact
433-1 serially connects the counter-electrode of elongated solar
cell 402 to the conductive core 404 of another elongated solar cell
402 in assembly 400. Second electrical contact 433-2 serially
connects the conductive core 404 of elongated solar cell 402 to the
counter-electrode 420 of another elongated solar cell 402 in
assembly 400.
[0072] FIG. 4E provides a cross-sectional view with respect to line
4B-4B of FIG. 4A in accordance with another embodiment of the
present invention. FIG. 4E is similar to FIG. 4B. However, in FIG.
4E, elongated solar cells 402 facing end 455 are not sealed as they
are in FIG. 4B and FIG. 4D. Thus, the ends of elongated solar cells
402 facing end 455 cannot contribute to the photovoltaic potential
of solar cell 402. However, the embodiment illustrated in FIG. 4E
has the advantage of being easier to make than the embodiment
illustrated in FIGS. 4B and 4D. Furthermore, in many instances, the
loss of contribution to the photovoltaic potential from end 455 is
negligible because the surface area of such ends is so small. FIG.
4F is a cross-sectional view of a elongated solar 402 cell taken
about line 4F-4F of FIG. 4E which further illustrates the
configuration of end 455 of elongated solar cell 402 in accordance
with the embodiment of the invention illustrated in FIG. 4E.
[0073] FIG. 6 illustrates a solar cell assembly 600 in accordance
with the present invention. Specifically, FIG. 6A is a
cross-sectional view of rod-shaped (elongated) solar cells 402
electrically arranged in series in a bifacial assembly 600 where
counter-electrodes 420 form interfaces between solar cell pairs
402. As illustrated in FIG. 6A, solar cell assembly 600 comprises a
plurality of elongated solar cells 402. There is no limit to the
number of solar cells 402 in this plurality (e.g., 1000 or more,
10,000 or more, between 5,000 and one million solar cells 402,
etc.). As in the embodiment of the invention illustrated in FIG. 4
and described above, each elongated solar cell 402 comprises a
conductive core 404 with a semiconductor junction 410
circumferentially disposed on the conductive core. A transparent
conductive oxide layer 412 circumferentially disposed on the
semiconductor junction 412 completes the circuit.
[0074] As illustrated in FIGS. 6A and 6B, the plurality of
elongated solar cells 402 are geometrically arranged in a parallel
or near parallel manner as a plurality of solar cell pairs so as to
form a planar array having a first face (on side 633 of assembly
600 as illustrated in FIG. 6A) and a second face (on side 655 of
assembly 600 as illustrated in FIG. 6A). Solar cells 402 in a pair
of solar cells do not touch the solar cells 402 in an adjacent pair
of solar cells. However, in the embodiment illustrated in FIG. 6,
solar cells 402 within a given pair of solar cells are in
electrical contact with each other through their common
counter-electrode 420. Accordingly, assembly 600 comprises a
plurality of metal counter-electrodes 420. Each respective metal
counter-electrode in the plurality of metal counter-electrodes
joins together, lengthwise, elongated solar cells 402 in a
corresponding solar cell pair in the plurality of solar cell pairs.
As such, elongated solar cells 402 in a solar cell pair are
electrically arranged in parallel, not series.
[0075] In some embodiments there is a first groove 677-1 and a
second groove 677-2 that each runs lengthwise on opposing sides of
solar cell 402. In FIG. 6A, some but not all grooves 677 are
labeled. In some embodiments, the counter-electrode 420 of each
pair of solar cells 402 is fitted between opposing grooves 677 in
the solar cell pair in the manner illustrated in FIG. 6A. The
present invention encompasses grooves 677 that have a broad range
of depths and shape characteristics and is by no means limited to
the shape of the grooves 677 illustrated in FIG. 6A. In general,
any type of groove 677 that runs along the long axis of a first
solar cell 402 in a solar cell pair and that can accommodate all or
part of counter-electrode 420 in a pairwise fashion together with
an opposing groove on the second solar cell 402 in the solar cell
pair is within the scope of the present invention.
[0076] As illustrated in FIG. 6A, a transparent electrically
insulating substrate 406 covers all or a portion of face 655 of the
planar array of solar cells. In some embodiments, solar cells 402
touch substrate 406. In some embodiments, solar cells 402 do not
touch substrate 406. In embodiments in which solar cells 402 do not
touch substrate 406, a sealant such as ethyl vinyl acetate is used
to seal substrate 406 onto solar cells 402.
[0077] FIG. 6B provides a cross-sectional view with respect to line
6B-6B of FIG. 6A. As can be seen in FIGS. 6A and 6B, each elongated
solar cell 402 has a length that is great compared to the diameter
of its cross-section. Typically each solar cell 402 has a rod-like
shape (e.g., has a wire shape). Each solar cell pair is
electrically connected to other solar cell pairs in series by
arranging the solar cell pairs such that they do not touch each
other, as illustrated in FIGS. 4A and 4B. The separation distance
between solar cells pairs is any distance that prevents electrical
contact between the cells. For instance, in some embodiments, the
distance between adjacent solar cell pairs is 0.1 micron or
greater, 0.5 microns or greater, or between 1 and 5 microns. Serial
electrical contact between solar cell pairs is made by electrical
contacts 677 that electrically connect the conductive cores 404 of
each elongated solar cell in a one solar cell pair to the
corresponding counter-electrode 120 of a different solar cell pair
as illustrated in FIG. 6B. FIG. 6B further illustrates a cutaway of
conductive core 404 and semiconductor junction 410 in one solar
cell 402 to further illustrate the architecture of the solar
cells.
[0078] Referring back to FIG. 6A, in some embodiments, solar cell
assembly 600 further comprises a transparent insulating covering
422 disposed on face 633 of the planar array of solar cells 402,
thereby encasing the plurality of elongated solar cells 402 between
the transparent insulating covering 422 and the transparent
electrically insulating substrate 406. In such embodiments,
transparent insulating covering 422 and the transparent insulating
substrate 406 are bonded together by a sealant such as ethyl vinyl
acetate. Although not illustrated in FIGS. 6A and 6B, in preferred
embodiments, there is an intrinsic layer circumferentially disposed
between the semiconductor junction 410 and TCO 412. In some
embodiments, this intrinsic layer is formed by an undoped
transparent oxide such as zinc oxide, indium-tin-oxide, or a
combination thereof.
[0079] In some embodiments, the semiconductor junction 410 of solar
cells 402 in assembly 600 comprise an inner coaxial layer and an
outer coaxial layer, where the outer coaxial layer comprises a
first conductivity type and the inner coaxial layer comprises a
second, opposite, conductivity type. In some embodiments, the inner
coaxial layer comprises copper-indium-gallium-diselenide (CIGS) and
the outer coaxial layer comprises CdS, SnO.sub.2, ZnO, ZrO.sub.2,
or doped ZnO. In some embodiments, conductive core 404 and/or
electrical contacts 677 and/or counter-electrodes 420 are made of
aluminum, molybdenum, steel, nickel, silver, gold, or an alloy
thereof. In some embodiments, transparent conductive oxide layer
412 is made of tin oxide SnO.sub.x, with or without fluorine
doping, indium-tin oxide (ITO), zinc oxide (ZnO) or a combination
thereof. In some embodiments, transparent insulating substrate 406
and transparent insulating covering 422 comprise glass or Tedlar.
Although not shown in FIG. 6, in some embodiments, conductive core
404 is hollowed as depicted in FIG. 9.
[0080] FIG. 6C illustrates a cross-sectional view of an elongated
solar 402 cell taken about line 6C-6C of FIG. 46. FIG. 6C
illustrates how the various layers deposited on conductive core 404
are tapered at either end 687 or 688 (end 687 as illustrated in
FIG. 6C). For instance, a terminal portion of conductive core 404
is exposed, as illustrated in FIG. 6C. In other words,
semiconductor junction 410, an optional i-layer (not shown), and
TCO 412 are stripped away from a terminal portion of conductive
core 404 at an end of the solar cell. Furthermore, a terminal
portion of semiconductor junction 410 is exposed as illustrated in
FIG. 6C. That is, optional i-layer (not shown) and TCO 412 are
stripped away from the terminal portion of semiconductor junction
410 at an end of the solar cell (end 687 in FIG. 6C). Such a
configuration is advantageous because it prevents an electrical
short from developing between TCO 412 and conductive core 404. In
FIG. 6C, elongated solar cell 402 is positioned on electrically
resistant transparent substrate 406. However, there is no
requirement that elongated solar cell 402 make direct contact with
electrically resistant transparent substrate 406. In fact, in some
embodiments, elongated solar cells 402 are sealed between
electrically resistant transparent substrate 406 and covering 422
in such a manner that they do not contact substrate 406 and
covering 422. In such embodiments, elongated solar cells 402 are
fixedly held in place by a sealant such as ethyl vinyl acetate.
[0081] In some embodiments, not all elongated solar cell pairs in
assembly 600 are electrically arranged in series. For example, in
some embodiments, two or more pairs of elongated solar cells are
themselves paired such that all the elongated solar cells in the
paired pairs are electrically arranged in parallel. This can be
accomplished by joining the conductive core 404 of each of the
solar cells by a common electrical contact (e.g., an electrically
conducting wire, etc., not shown). To complete the parallel
circuit, the TCO 412 of each of the elongated solar cell 402 are
electrically joined together either by direct contact or by the use
of a second electrical contact (not shown). The paired pairs of
elongated solar cells are then electrically arranged in series. In
some embodiments, three, four, five, six, seven, eight, nine, ten,
eleven or more pairs of elongated solar cells are electrically
arranged in parallel. These parallel groups of elongated solar
cells 402 are then electrically arranged in series.
[0082] FIG. 7 illustrates solar cell assembly 700 in accordance
with another embodiment of the present invention. Solar cell
assembly 700 comprises a plurality of elongated solar cells 402.
Each elongated solar cell 402 in the plurality of elongated solar
cells has a conductive core 404 configured as a first electrode, a
semiconductor junction 410 circumferentially disposed on the
conductive core 402 and a transparent conductive oxide layer 412
disposed on the semiconductor junction 410. The plurality of
elongated solar cells 402 are geometrically arranged in a parallel
or a near parallel manner thereby forming a planar array having a
first face (facing side 733 of assembly 700) and a second face
(facing side 766 of assembly 700). The plurality of elongated solar
cells is arranged such that one or more elongated solar cells in
the plurality of elongated solar cells do not contact adjacent
elongated solar cells. In preferred embodiments, the plurality of
elongated solar cells is arranged such that each of the elongated
solar cells in the plurality of elongated solar cells does not
directly contact (through outer the TCO layer 412) adjacent
elongated solar cells 402.
[0083] In some embodiments there is a first groove 777-1 and a
second groove 777-2 that each runs lengthwise on opposing sides of
solar cell 402. In FIG. 7A, some but not all grooves 777 are
labeled. In some embodiments, there is a counter-electrode 420 in
one or both grooves of the solar cells. In the embodiment
illustrated in FIG. 6A, there is a counter-electrode fitted
lengthwise in both the first and second grooves of each solar cell
in the plurality of solar cells. Such a configuration is
advantageous because it reduces the pathlength of current drawn off
of TCO 412. In other words, the maximum length that current must
travel in TCO 412 before it reaches a counter-electrode 420 is a
quarter of the circumference of the TCO. By contrast, in
configurations where there is only a single counter-electrode 420
associated with a given solar cell 402, the maximum length that
current must travel in TCO 412 before it reaches a
counter-electrode 420 is a full half of the circumference of the
TCO. The present invention encompasses grooves 777 that have a
broad range of depths and shape characteristics and is by no means
limited to the shape of the grooves 777 illustrated in FIG. 7A. In
general, any groove shape 777 that runs along the long axis of a
solar cell 402 and that can accommodate all or part of
counter-electrode 420 is within the scope of the present invention.
For example, in some embodiments not illustrated by FIG. 7A, each
groove 777 is patterned so that there is a tight fit between the
contours of the groove 777 and the counter-electrode 420.
[0084] As illustrated in FIG. 7A, there are a plurality of metal
counter-electrodes 420, and each respective elongated solar cell
402 in the plurality of elongated solar cells is bound to at least
a first corresponding metal counter-electrode 420 in the plurality
of metal counter-electrodes such that the first metal
counter-electrode lies in a groove 777 that runs lengthwise along
the respective elongated solar cell. Furthermore, in the solar cell
assembly illustrated in FIG. 7A, each respective elongated solar
cell 402 is bound to a second corresponding metal counter-electrode
420 such that the second metal counter-electrode lies in a second
groove 777 that runs lengthwise along the respective elongated
solar cell 402. As further illustrated in FIG. 7A, the first groove
777 and the second groove 777 are on opposite or substantially
opposite sides of the respective elongated solar cell 402 and run
along the long axis of the cell.
[0085] Further illustrated in FIG. 7A, is a transparent
electrically insulating substrate 406 that covers all or a portion
of face 766 of the planar array. The plurality of elongated solar
cells 402 are configured to receive direct light from both face 733
and face 766 of the planar array. Solar cell assembly 700 further
comprises a transparent insulating covering 422 disposed on face
733 of the planar array, thereby encasing the plurality of
elongated solar cells 402 between the transparent insulating
covering 422 and the transparent electrically insulating substrate
406.
[0086] FIG. 7B provides a cross-sectional view with respect to line
7B-7B of FIG. 7A. Solar cell 402 are electrically connected to
other in series by arranging the solar cells such that they do not
touch each other, as illustrated in FIGS. 7A and 7B and by the use
of electrical contacts as described below in conjunction with FIG.
7B. The separation distance between solar cells 402 is any distance
that prevents electrical contact between the TCO layers 412 of
individual cells 402. For instance, in some embodiments, the
distance between adjacent solar cells is 0.1 micron or greater, 0.5
microns or greater, or between 1 and 5 microns.
[0087] Referring to FIG. 7B, serial electrical contact between
solar cells 402 is made by electrical contacts 788 that
electrically connect the metal conductive core 404 of one elongated
solar cell 402 to the corresponding counter-electrodes 120 of a
different solar cell 402 as illustrated in FIG. 7B. FIG. 7B further
illustrates a cutaway of metal conductive core 404 and
semiconductor junction 410 in one solar cell 402 to further
illustrate the architecture of the solar cells 402.
[0088] The solar cell assembly illustrated in FIG. 7 has several
advantages. First, because of the positioning of counter-electrodes
420 and the transparency of both substrate 406 and covering 422,
there is almost zero percent shading in the assembly. For instance,
the assembly can receive direct sunlight from both face 733 and
face 766. Second, in embodiments where a sealant such as EVA is
used to laminate substrate 406 and covering 422 onto the plurality
of solar cells, the structure is completely self-supporting. Still
another advantage of the assembly is that is easy to manufacture.
Unlike solar cells such as that depicted in FIG. 3A, no complicated
grid or transparent conductive oxide on glass is needed. For
example, to assemble a solar cell 402 and its corresponding
counter-electrodes 420 together to complete the circuit illustrated
in FIG. 7A, counter-electrode 420, when it is in the form of a
wire, can be covered with conductive epoxy and dropped in the
groove 777 of solar cell 402 and allowed to cure. As illustrated in
FIG. 7B, conductive core 404, junction 410, and TCO 412 are flush
with each other at end 789 of elongated solar cells 402. In
contrast, at end 799 conductive core protrudes a bit with respect
to junction 410 and TCO 412 as illustrated. Junction 410 also
protrudes a bit at end 799 with respect to TCO 412. The protrusion
of conductive core 404 at end 799 means that the sides of a
terminal portion of the conductive core 404 are exposed (e.g., not
covered by junction 410 and TCO 412). The purpose of this
configuration is to reduce the chances of shorting
counter-electrode 420 (or the epoxy used to mount the
counter-electrode in groove 777) with TCO 412. In some embodiments,
all or a portion of the exposed surface area of counter-electrodes
420 are shielded with an electrically insulating material in order
to reduce the chances of electrical shortening. For example, in
some embodiments, the exposed surface area of counter-electrodes
420 in the boxed regions of FIG. 7B is shielded with an
electrically insulating material.
[0089] Still another advantage of the assembly illustrated in FIG.
7 is that the counter-electrode 420 can have much higher
conductivity without shadowing. In other words, counter-electrode
420 can have a substantial cross-sectional size (e.g., 1 mm in
diameter when solar cell 402 has a 6 mm diameter). Thus,
counter-electrode 420 can carry a significant amount of current so
that the wires can be as long as possible, thus enabling the
fabrication of larger panels.
[0090] The series connections between solar cells 402 can be
between pairs of solar cells 402 in the manner depicted in FIG. 7B.
However, the invention is not so limited. In some embodiments, two
or more solar cells 402 are grouped together (e.g., electrically
connected in a parallel fashion) to form a group of solar cells and
then such groups of solar cells are serially connected to each
other. Therefore, the serial connections between solar cells can be
between groups of solar cells where such groups have any number of
solar cells 402 (e.g., 2, 3, 4, 5, 6, etc.). However, FIG. 7B
illustrates a preferred embodiment in which each contact 788
serially connects only a pair of solar cells 402.
[0091] In some embodiments, there is a series insulator that runs
lengthwise between each solar cell 402. In one example, this series
insulator is a 0.001'' thick sheet of transparent insulating
plastic. In other examples this series insulator is a sheet of
transparent insulating plastic having a thickness between 0.001''
and 0.005''. Alternatively, a round insulating clear plastic
separator that runs lengthwise between solar cells 402 can be used
to electrically isolate the solar cells 402. Advantageously, any
light that does enter the small gap between solar cells 402 will be
trapped and collected in the "double-divet" area formed by facing
grooves 777 of adjacent solar cells 402.
[0092] Yet another embodiment of solar cell assembly 700 is that
there is no extra absorption loss from a TCO or a metal grid on one
side of the assembly. Further, assembly 700 has the same
performance or absorber area exposed on both sides 733 and 766.
This makes assembly 700 symmetrical.
[0093] Still another advantage of assembly 700 is that all
electrical contacts 788 end at the same level (e.g., in the plane
of line 7B-7B of FIG. 7A). As such, they are easier to connect and
weld with very little substrate area wasted at the end. This
simplifies construction of the solar cells 402 while at the same
time serves to increase the overall efficiency of solar cell
assembly 700. This increase in efficiency arises because the welds
can be smaller. Smaller welds take up less of the electrically
resistant transparent substrate 406 surface area that is otherwise
occupied by solar cells 402.
[0094] Although not illustrated in FIG. 7, in some embodiments in
accordance with FIG. 7, there is an intrinsic layer
circumferentially disposed between the semiconductor junction 410
and the transparent conductive oxide 412 in an elongated solar cell
402 in the plurality of elongated solar cells 402. This intrinsic
layer can be made of an undoped transparent oxide such as zinc
oxide, indium-tin-oxide, or a combination thereof. In some
embodiments, the semiconductor junction 410 of solar cells 402 in
assembly 700 comprise an inner coaxial layer and an outer coaxial
layer where the outer coaxial layer comprises a first conductivity
type and the inner coaxial layer comprises a second, opposite,
conductivity type. In an exemplary embodiment the inner coaxial
layer comprises copper-indium-gallium-diselenide (CIGS) whereas the
outer coaxial layer comprises CdS, SnO.sub.2, ZnO, ZrO.sub.2, or
doped ZnO. In some embodiments not illustrated by FIG. 7, the
conductive cores 404 in solar cells 402 are hollowed.
[0095] FIG. 8 illustrates a solar cell assembly 800 of the present
invention that is identical to solar cell assembly 700 of the
present invention with the exception that TCO 412 is interrupted by
breaks 810 that run along the long axis of solar cells 402 and cut
completely through TCO 412. In the embodiment illustrated in FIG.
8, there are two breaks 810 that run the length of solar cell 402.
The effect of such breaks 810 is that they electrically isolate the
two counter-electrodes 420 associated with each solar cell 402 in
solar cell assembly 800. There are many ways in which breaks 800
can be made. For example, a laser or an HCl etch can be used.
[0096] In some embodiments, not all elongated solar cells 402 in
assembly 800 are electrically arranged in series. For example, in
some embodiments, there are pairs of elongated solar cells 402 that
are electrically arranged in parallel. A first and second elongated
solar cell can be electrically connected in parallel, and are
thereby paired, by using a first electrical contact (e.g., an
electrically conducting wire, etc., not shown) that joins the
conductive core 404 of a first elongated solar cell to the second
elongated solar cell. To complete the parallel circuit, the TCO 412
of the first elongated solar cell 402 is electrically connected to
the TCO 412 of the second elongated solar cell 402 either by
contacting the TCOs of the two elongated solar cells either
directly or through a second electrical contact (not shown). The
pairs of elongated solar cells are then electrically arranged in
series. In some embodiments, three, four, five, six, seven, eight,
nine, ten, eleven or more elongated solar cells 402 are
electrically arranged in parallel. These parallel groups of
elongated solar cells 402 are then electrically arranged in
series.
[0097] FIG. 9 illustrates a solar cell assembly 900 of the present
invention in which conductive cores 402 are hollowed. In fact,
conductive cores 402 can be hollowed in any of the embodiments of
the present invention. One advantage of such a hollowed core 402
design is that it reduces the overall weight of the solar cell
assembly. Core 402 is hollowed when there is a channel that extends
lengthwise through all or a portion of core 402. In some
embodiments, conductive core 402 is metal tubing.
[0098] In some embodiments, not all elongated solar cells 402 in
assembly 900 are electrically arranged in series. For example, in
some embodiments, there are pairs of elongated solar cells 402 that
are electrically arranged in parallel. A first and second elongated
solar cell can be electrically connected in parallel, and are
thereby paired, by using a first electrical contact (e.g., an
electrically conducting wire, etc., not shown) that joins the
conductive core 404 of a first elongated solar cell to the second
elongated solar cell. To complete the parallel circuit, the TCO 412
of the first elongated solar cell 402 is electrically connected to
the TCO 412 of the second elongated solar cell 402 either by
contacting the TCOs of the two elongated solar cells either
directly or through a second electrical contact (not shown). The
pairs of elongated solar cells are then electrically arranged in
series. In some embodiments, three, four, five, six, seven, eight,
nine, ten, eleven or more elongated solar cells 402 are
electrically arranged in parallel. These parallel groups of
elongated solar cells 402 are then electrically arranged in
series.
[0099] FIG. 10 illustrates a solar cell assembly 1000 of the
present invention in which counterelectrodes 420, TCOs 412, and
junctions 410 are pierced, in the manner illustrated, in order to
form two discrete junctions in parallel.
5.2 Exemplary Semiconductor Junctions
[0100] Referring to FIG. 5A, in one embodiment, semiconductor
junction 410 is a heterojunction between an absorber layer 502,
disposed on conductive core 404, and a junction partner layer 504,
disposed on absorber layer 502. Layers 502 and 504 are composed of
different semiconductors with different band gaps and electron
affinities such that junction partner layer 504 has a larger band
gap than absorber layer 502. In some embodiments, absorber layer
502 is p-doped and junction partner layer 504 is n-doped. In such
embodiments, TCO layer 412 is n.sup.+-doped. In alternative
embodiments, absorber layer 502 is n-doped and junction partner
layer 504 is p-doped. In such embodiments, TCO layer 412 is
p.sup.+-doped. In some embodiments, the semiconductors listed in
Pandey, Handbook of Semiconductor Electrodeposition, Marcel Dekker
Inc., 1996, Appendix 5, hereby incorporated by reference in its
entirety, are used to form semiconductor junction 410.
5.2.1 Thin-Film Semiconductor Junctions Based on Copper Indium
Diselenide and Other Type I-III-VI Materials
[0101] Continuing to refer to FIG. 5A, in some embodiments,
absorber layer 502 is a group I-III-VI.sub.2 compound such as
copper indium di-selenide (CuInSe.sub.2; also known as CIS). In
some embodiments, absorber layer 502 is a group I-III-VI.sub.2
ternary compound selected from the group consisting of
CdGeAs.sub.2, ZnSnAs.sub.2, CuInTe.sub.2, AgInTe.sub.2,
CuInSe.sub.2, CuGaTe.sub.2, ZnGeAs.sub.2, CdSnP.sub.2,
AgInSe.sub.2, AgGaTe.sub.2, CuInS.sub.2, CdSiAs.sub.2, ZnSnP.sub.2,
CdGeP.sub.2, ZnSnAs.sub.2, CuGaSe.sub.2, AgGaSe.sub.2, AgInS.sub.2,
ZnGeP.sub.2, ZnSiAs.sub.2, ZnSiP.sub.2, CdSiP.sub.2, or CuGaS.sub.2
of either the p-type or the n-type when such compound is known to
exist.
[0102] In some embodiments, junction partner layer 504 is CdS, ZnS,
ZnSe, or CdZnS. In one embodiment, absorber layer 502 is p-type CIS
and junction partner layer 504 is n-type CdS, ZnS, ZnSe, or CdZnS.
Such semiconductor junctions 410 are described in Chapter 6 of
Bube, Photovoltaic Materials, 1998, Imperial College Press, London,
which is hereby incorporated by reference in its entirety.
[0103] In some embodiments, absorber layer 502 is
copper-indium-gallium-diselenide (CIGS). In some embodiments,
absorber layer 502 is copper-indium-gallium-diselenide (CIGS) and
junction partner layer 504 is CdS, ZnS, ZnSe, or CdZnS. In some
embodiments, absorber layer 502 is p-type CIGS and junction partner
layer 504 is n-type CdS, ZnS, ZnSe, or CdZnS.
5.2.2 Semiconductor Junctions Based on Amorphous Silicon or
Polycrystalline Silicon
[0104] In some embodiments, referring to FIG. 5B, semiconductor
junction 410 comprises amorphous silicon. In some embodiments this
is an n/n type heterojunction. For example, in some embodiments,
layer 514 comprises SnO.sub.2(Sb), layer 512 comprises undoped
amorphous silicon, and layer 510 comprises n+ doped amorphous
silicon.
[0105] In some embodiments, semiconductor junction 410 is a p-i-n
type junction. For example, in some embodiments, layer 514 is
p.sup.+ doped amorphous silicon, layer 512 is undoped amorphous
silicon, and layer 510 is n.sup.+ amorphous silicon. Such
semiconductor junctions 410 are described in Chapter 3 of Bube,
Photovoltaic Materials, 1998, Imperial College Press, London, which
is hereby incorporated by reference in its entirety.
[0106] In some embodiments of the present invention, semiconductor
junction 410 is based upon thin-film polycrystalline. Referring to
FIG. 5B, in one example in accordance with such embodiments, layer
510 is a p-doped polycrystalline silicon, layer 512 is depleted
polycrystalline silicon and layer 514 is n-doped polycrystalline
silicon. Such semiconductor junctions are described in Green,
Silicon Solar Cells: Advanced Principles & Practice, Centre for
Photovoltaic Devices and Systems, University of New South Wales,
Sydney, 1995; and Bube, Photovoltaic Materials, 1998, Imperial
College Press, London, pp. 57-66, which is hereby incorporated by
reference in its entirety.
[0107] In some embodiments of the present invention, semiconductor
junctions 410 based upon p-type microcrystalline Si:H and
microcrystalline Si:C:H in an amorphous Si:H solar cell are used.
Such semiconductor junctions are described in Bube, Photovoltaic
Materials, 1998, Imperial College Press, London, pp. 66-67, and the
references cited therein, which is hereby incorporated by reference
in its entirety.
5.2.3 Semiconductor Junctions Based on Gallium Arsenide and Other
Type III-V Materials
[0108] In some embodiments, semiconductor junctions 410 are based
upon gallium arsenide (GaAs) or other III-V materials such as InP,
AlSb, and CdTe. GaAs is a direct-band gap material having a band
gap of 1.43 eV and can absorb 97% of AM1 radiation in a thickness
of about two microns. Suitable type III-V junctions that can serve
as semiconductor junctions 410 of the present invention are
described in Chapter 4 of Bube, Photovoltaic Materials, 1998,
Imperial College Press, London, which is hereby incorporated by
reference in its entirety.
[0109] Furthermore, in some embodiments semiconductor junction 410
is a hybrid multijunction solars cells such as a GaAs/Si
mechanically stacked multijunction as described by Gee and Virshup,
1988, 20.sup.th IEEE Photovoltaic Specialist Conference, IEEE
Publishing, New York, p. 754, which is hereby incorporated by
reference in its entirety, a GaAs/CuInSe.sub.2 MSMJ four-terminal
device, consisting of a GaAs thin film top cell and a
ZnCdS/CuInSe.sub.2 thin bottom cell described by Stanbery et al.,
19.sup.th IEEE Photovoltaic Specialist Conference, IEEE Publishing,
New York, p. 280, and Kim et al., 20.sup.th IEEE Photovoltaic
Specialist Conference, IEEE Publishing, New York, p. 1487, each of
which is hereby incorporated by reference in its entirety. Other
hybrid multijunction solar cells are described in Bube,
Photovoltaic Materials, 1998, Imperial College Press, London, pp.
131-132, which is hereby incorporated by reference in its
entirety.
5.2.4 Semiconductor Junctions Based on Cadmium Telluride and Other
Type II-VI Materials
[0110] In some embodiments, semiconductor junctions 410 are based
upon II-VI compounds that can be prepared in either the n-type or
the p-type form. Accordingly, in some embodiments, referring to
FIG. 5C, semiconductor junction 410 is a p-n heterojunction in
which layers 520 and 540 are any combination set forth in the
following table or alloys thereof. TABLE-US-00001 Layer 520 Layer
540 n-CdSe p-CdTe n-ZnCdS p-CdTe n-ZnSSe p-CdTe p-ZnTe n-CdSe n-CdS
p-CdTe n-CdS p-ZnTe p-ZnTe n-CdTe n-ZnSe p-CdTe n-ZnSe p-ZnTe n-ZnS
p-CdTe n-ZnS p-ZnTe
[0111] Methods for manufacturing semiconductor junctions 410 are
based upon II-VI compounds are described in Chapter 4 of Bube,
Photovoltaic Materials, 1998, Imperial College Press, London, which
is hereby incorporated by reference in its entirety.
5.2.5 Semiconductor Junctions Based on Crystalline Silicon
[0112] While semiconductor junctions 410 that are made from thin
semiconductor films are preferred, the invention is not so limited.
In some embodiments semiconductor junctions 410 is based upon
crystalline silicon. For example, referring to FIG. 5D, in some
embodiments, semiconductor junction 410 comprises a layer of p-type
crystalline silicon 540 and a layer of n-type crystalline silicon
550. Methods for manufacturing crystalline silicon semiconductor
junctions 410 are described in Chapter 2 of Bube, Photovoltaic
Materials, 1998, Imperial College Press, London, which is hereby
incorporated by reference in its entirety.
5.3 Albedo Embodiments
[0113] The solar cell assemblies of the present invention are
advantageous because they can collect light through either of their
two faces. Accordingly, in some embodiments of the present
invention, theses bifacial solar cell assemblies (e.g., solar cell
assembly 400, 600, 700, 800, 900, etc.) are arranged in a
reflective environment in which surfaces around the solar cell
assembly have some amount of albedo. Albedo is a measure of
reflectivity of a surface or body. It is the ratio of
electromagnetic radiation (EM radiation) reflected to the amount
incident upon it. This fraction is usually expressed as a
percentage from 0% to 100%. In some embodiments, surfaces in the
vicinity of the solar cell assemblies of the present invention are
prepared so that they have a high albedo by painting such surfaces
a reflective white color. In some embodiments, other materials that
have a high albedo can be used. For example, the albedo of some
materials around such solar cells approach or exceed ninety
percent. See, for example, Boer, 1977, Solar Energy 19, 525, which
is hereby incorporated by reference in its entirety. However,
surfaces having any amount of albedo (e.g., five percent or more,
ten percent or more, twenty percent or more) are within the scope
of the present invention. In one embodiment, the solar cells
assemblies of the present invention are arranged in rows above a
gravel surface, where the gravel has been painted white in order to
improve the reflective properties of the gravel.
[0114] In some embodiments, the bifacial solar cell assemblies of
the present invention are placed in a manner such that one surface
(e.g., face 633 of solar cell assembly 600) is illuminated in a way
similar to a conventional flat-panel solar cell panel. For example,
it is installed facing South (in the northern hemisphere) with an
angle of inclination that is latitude dependent (e.g., in general
is not very different from the latitude). The opposing surface of
the bifacial solar cell assembly (e.g., face 655 of solar cell
assembly 600) of the present invention receives a substantial
amount of diffuse light reflected from the ground and neighboring
walls in the vicinity of the solar cell assembly.
[0115] By way of example, in some embodiments of the present
invention, the bifacial solar cell assemblies (panels) of the
present invention have a first and second face and are placed in
rows facing South in the Northern hemisphere (or facing North in
the Southern hemisphere). Each of the panels is placed some
distance above the ground (e.g., 100 cm above the ground). The
East-West separation between the panels is somewhat dependent upon
the overall dimensions of the panels. By way of illustration only,
panels having overall dimensions of about 106 cm.times.44 cm are
placed in the rows such that the East-West separation between the
panels is between 10 cm and 50 cm. In one specific example the
East-West separation between the panels is 25 cm.
[0116] In some embodiments, the central point of the panels in the
rows of panels is between 0.5 meters and 2.5 meters from the
ground. In one specific example, the central point of the panels is
1.55 meters from the ground. The North-South separation between the
rows of panels is dependent on the dimensions of the panels. By way
of illustration, in one specific example, in which the panels have
overall dimensions of about 106 cm.times.44 cm, the North-South
separation is 2.8 meters. In some embodiments, the North-South
separation is between 0.5 meters and 5 meters. In some embodiments,
the North-South separation is between 1 meter and 3 meters.
[0117] In some embodiments of the present invention, the panels in
the rows are each tilted with respect to the ground in order to
maximize the total amount of light received by the panels. There is
some tradeoff between increasing the amount of light received by
one face versus the amount of light received on the opposing face
as a function of tilt angle. However, at certain tilt angles, the
total amount of light received by the panels, where total amount of
light is defined as the sum of direct light received on the first
and second face of the bifacial panel, is maximized. In some
embodiments, the panels in the rows of panels are each tilted
between five degrees and forty-five degrees from the horizontal. In
some embodiments, the panels of the present invention are tilted
between fifteen degrees and forty degrees from the horizontal. In
some embodiments, the panels of the present invention are tilted
between twenty-five degrees and thirty-five degrees from the
horizontal. In one specific embodiment, the panels of the present
invention are tilted thirty degrees from the horizontal.
[0118] In some embodiments, models for computing the amount of
sunlight received by solar panels as put forth in Lorenzo et al.,
1985, Solar Cells 13, pp. 277-292, which is hereby incorporated by
reference in its entirety, are used to compute the optimum
horizontal tilt and East-West separation of the solar panels in the
rows of solar panels that are placed in a reflective
environment.
5.4 Dual Layer Core Embodiments
[0119] Embodiments of the present invention in which conductive
core 404 of the solar cells 402 of the present invention is made of
a uniform conductive material have been disclosed. The invention is
not limited to these embodiments. In some embodiments, conductive
core 404 in fact has an inner core and an outer conductive core.
The outer conductive core is circumferentially disposed on the
inner core. In such embodiments, the inner core is typically
nonconductive whereas the outer core is conductive. The inner core
has an elongated shape consistent with other embodiments of the
present invention. For instance, in one embodiment, the inner core
is made of glass fibers in the form of a wire. In some embodiments,
the inner core is an electrically conductive nonmetallic material.
However, the present invention is not limited to embodiments in
which the inner core is electrically conductive because the outer
core can function as the electrode. In some embodiments, the inner
core is tubing (e.g., plastic tubing).
[0120] In some embodiments, the inner core is made of a material
such as polybenzamidazole (e.g., Celazole.RTM., available from
Boedeker Plastics, Inc., Shiner, Tex.). In some embodiments, the
inner core is made of polymide (e.g., DuPont.TM. Vespel.RTM., or
DuPont.TM. Kapton.RTM., Wilmington, Del.). In some embodiments, the
inner core is made of polytetrafluoroethylene (PTFE) or
polyetheretherketone (PEEK), each of which is available from
Boedeker Plastics, Inc. In some embodiments, the inner core is made
of polyamide-imide (e.g., Torlon.RTM. PAI, Solvay Advanced
Polymers, Alpharetta, Ga.).
[0121] In some embodiments, the inner core is made of a glass-based
phenolic. Phenolic laminates are made by applying heat and pressure
to layers of paper, canvas, linen or glass cloth impregnated with
synthetic thermosetting resins. When heat and pressure are applied
to the layers, a chemical reaction (polymerization) transforms the
separate layers into a single laminated material with a "set" shape
that cannot be softened again. Therefore, these materials are
called "thermosets." A variety of resin types and cloth materials
can be used to manufacture thermoset laminates with a range of
mechanical, thermal, and electrical properties. In some
embodiments, the inner core is a phenoloic laminate having a NEMA
grade of G-3, G-5, G-7, G-9, G-10 or G-11. Exemplary phenolic
laminates are available from Boedeker Plastics, Inc.
[0122] In some embodiments, the inner core is made of polystyrene.
Examples of polystyrene include general purpose polystyrene and
high impact polystyrene as detailed in Marks' Standard Handbook for
Mechanical Engineers, ninth edition, 1987, McGraw-Hill, Inc., p.
6-174, which is hereby incorporated by reference in its entirety.
In still other embodiments, inner core is made of cross-linked
polystyrene. One example of cross-linked polystyrene is
Rexolite.RTM. (available from San Diego Plastics Inc., National
City, Calif.). Rexolite is a thermoset, in particular a rigid and
translucent plastic produced by cross linking polystyrene with
divinylbenzene.
[0123] In some embodiments, the inner core is a polyester wire
(e.g., a Mylar.RTM. wire). Mylar.RTM. is available from DuPont
Teijin Films (Wilmington, Del.). In still other embodiments, the
inner core is made of Durastone.RTM., which is made by using
polyester, vinylester, epoxid and modified epoxy resins combined
with glass fibers (Roechling Engineering Plastic Pte Ltd.
(Singapore).
[0124] In still other embodiments, the inner core is made of
polycarbonate. Such polycarbonates can have varying amounts of
glass fibers (e.g., 10%, 20%, 30%, or 40%) in order to adjust
tensile strength, stiffness, compressive strength, as well as the
thermal expansion coefficient of the material. Exemplary
polycarbonates are Zelux.RTM. M and Zelux.RTM. W, which are
available from Boedeker Plastics, Inc.
[0125] In some embodiments, the inner core is made of polyethylene.
In some embodiments, inner core is made of low density polyethylene
(LDPE), high density polyethylene (HDPE), or ultra high molecular
weight polyethylene (UHMW PE). Chemical properties of HDPE are
described in Marks' Standard Handbook for Mechanical Engineers,
ninth edition, 1987, McGraw-Hill, Inc., p. 6-173, which is hereby
incorporated by reference in its entirety. In some embodiments, the
inner core is made of acrylonitrile-butadiene-styrene,
polytetrfluoro-ethylene (Teflon), polymethacrylate (lucite or
plexiglass), nylon 6,6, cellulose acetate butyrate, cellulose
acetate, rigid vinyl, plasticized vinyl, or polypropylene. Chemical
properties of these materials are described in Marks' Standard
Handbook for Mechanical Engineers, ninth edition, 1987,
McGraw-Hill, Inc., pp. 6-172 through 1-175, which is hereby
incorporated by reference in its entirety.
[0126] Additional exemplary materials that can be used to form the
inner core are found in Modern Plastics Encyclopedia, McGraw-Hill;
Reinhold Plastics Applications Series, Reinhold Roff, Fibres,
Plastics and Rubbers, Butterworth; Lee and Neville, Epoxy Resins,
McGraw-Hill; Bilmetyer, Textbook of Polymer Science, Interscience;
Schmidt and Marlies, Principles of high polymer theory and
practice, McGraw-Hill; Beadle (ed.), Plastics, Morgan-Grampiand,
Ltd., 2 vols. 1970; Tobolsky and Mark (eds.), Polymer Science and
Materials, Wiley, 1971; Glanville, The Plastics's Engineer's Data
Book, Industrial Press, 1971; Mohr (editor and senior author),
Oleesky, Shook, and Meyers, SPI Handbook of Technology and
Engineering of Reinforced Plastics Composites, Van Nostrand
Reinhold, 1973, each of which is hereby incorporated by reference
in its entirety.
[0127] In general, outer core is made out of any material that can
support the photovoltaic current generated by solar cell with
negligible resistive losses. In some embodiments, outer core is
made of any conductive metal, such as aluminum, molybdenum, steel,
nickel, silver, gold, or an alloy thereof. In some embodiments,
outer core is made out of a metal-, graphite-, carbon black-, or
superconductive carbon black-filled oxide, epoxy, glass, or
plastic. In some embodiments, outer core is made of a conductive
plastic. In some embodiments, this conductive plastic is inherently
conductive without any requirement for a filler.
[0128] In embodiments where an inner core and an outer core is
present, semiconductor junction 410 and TCO 412 are stripped from
the inner core at a terminal end of the solar cell where an
electrical contact serially joins the solar cell to another solar
cell. For example, in some embodiments, the semiconductor junction
410 and TCO are stripped in the manner illustrated in FIGS. 4D, 4F,
6B, 6C, and 7B.
5.5 Exemplary Dimensions
[0129] The present invention encompasses solar cell assemblies
having any dimensions that fall within a broad range of dimensions.
For example, referring to FIG. 4B, the present invention
encompasses solar cell assemblies having a length l between 1 cm
and 50,000 cm and a width w between 1 cm and 50,000 cm. In some
embodiments, the solar cell assemblies have a length l between 10
cm and 1,000 cm and a width w between 10 cm and 1,000 cm. In some
embodiments, the solar cell assemblies have a length l between 40
cm and 500 cm and a width w between 40 cm and 500 cm.
5.6 Solar Cells Manufactured Using a Roll Method or Having an Inner
TCO
[0130] In some embodiments, copper-indium-gallium-diselenide
(Cu(InGa)Se.sub.2), referred to herein as CIGS, is used to make the
absorber layer of junction 110. In such embodiments, conductive
core 404 can be made of molybdenum. In some embodiments, core 404
comprises an inner core of polyimide and an outer core that is a
thin film of molybdenum sputtered onto the polyimide core prior to
CIGS deposition. On top of the molybdenum, the CIGS film, which
absorbs the light, is evaporated. Cadmium sulfide (CdS) is then
deposited on the CIGS in order to complete semiconductor junction
410. Optionally, a thin intrinsic layer (i-layer) is then deposited
on the semiconductor junction 410. The i-layer can be formed using
any undoped transparent oxide including, but not limited to, zinc
oxide or indium-tin-oxide. Next, TCO 412 is disposed on either the
i-layer (when present) or the semiconductor junction 410 (when the
i-layer is not present). TCO can be made of a material such as
aluminum doped zinc oxide (ZnO:Al).
[0131] ITN Energy Systems, Inc., Global Solar Energy, Inc., and the
Institute of Energy Conversion (IEC), have collaboratively
developed technology for manufacturing CIGS photovoltaics on
polyimide substrates using a roll-to-roll co-evaporation process
for deposition of the CIGS layer. In this process, a roll of
molybdenum-coated polyimide film (referred to as the web) is
unrolled and moved continuously into and through one or more
deposition zones. In the deposition zones, the web is heated to
temperatures of up to .about.450.degree. C. and copper, indium, and
gallium are evaporated onto it in the presence of selenium vapor.
After passing out of the deposition zone(s), the web cools and is
wound onto a take-up spool. See, for example, 2003, Jensen et al.,
"Back Contact Cracking During Fabrication of CIGS Solar Cells on
Polyimide Substrates," NCPV and Solar Program Review Meeting 2003,
NREL/CD-520-33586, pages 877-881, which is hereby incorporated by
reference in its entirety. Likewise, Birkmire et al., 2005,
Progress in Photovoltaics: Research and Applications 13, 141-148,
hereby incorporated by reference, disclose a polyimide/Mo web
structure, specifically, PI/Mo/Cu(InGa)Se.sub.2/CdS/ZnO/ITO/Ni--Al.
Deposition of similar structures on stainless foil has also been
explored. See, for example, Simpson et al., 2004, "Manufacturing
Process Advancements for Flexible CIGS PV on Stainless Foil," DOE
Solar Energy Technologies Program Review Meeting, PV Manufacturing
Research and Development, P032, which is hereby incorporated by
reference in its entirety.
[0132] In some embodiments of the present invention, an absorber
material is deposited onto a polyimide/molybdenum web, such as
those developed by Global Solar Energy (Tucson, Ariz.), or a metal
foil (e.g., the foil disclosed in Simpson et al.). In some
embodiments, the absorber material is any of the absorbers
disclosed herein. In a particular embodiment, the absorber is
Cu(InGa)Se.sub.2. In some embodiments, the elongated core is made
of a nonconductive material such as undoped plastic. In some
embodiments, the elongated core is made of a conductive material
such as a conductive metal, a metal-filled epoxy, glass, or resin,
or a conductive plastic (e.g., a plastic containing a conducting
filler). Next, the semiconductor junction 410 is completed by
depositing a window layer onto the absorber layer. In the case
where the absorber layer is Cu(InGa)Se.sub.2, CdS can be used.
Finally, an optional i-layer 415 and TCO 412 are added to complete
the solar cell. Next, the foil is wrapped around and/or glued to a
wire-shaped or tube-shaped elongated core. The advantage of such a
fabrication method is that material that cannot withstand the
deposition temperature of the absorber layer, window layer, i-layer
or TCO layer can be used as an inner core for the solar cell. This
manufacturing process can be used to manufacture any of the solar
cells 402 disclosed in the present invention, where the conductive
core 402 comprises an inner core and an outer conductive core. The
inner core is any conductive or nonconductive material disclosed
herein whereas the outer conductive core is the web or foil onto
which the absorber layer, window layer, and TCO were deposited
prior to rolling the foil onto the inner core. In some embodiments,
the web or foil is glued onto the inner core using appropriate
glue.
[0133] An aspect of the present invention provides a method of
manufacturing a solar cell comprising depositing an absorber layer
on a first face of a metallic web or a conducting foil. Next a
window layer is deposited on to the absorber layer. Next a
transparent conductive oxide layer is deposited on to the window
layer. The metallic web or conducting foil is then rolled around an
elongated core, thereby forming an elongated solar cell 402. In
some embodiments, the absorber layer is
copper-indium-gallium-diselenide (Cu(InGa)Se.sub.2) and the window
layer is cadmium sulfide. In some embodiments, the metallic web is
a polyimide/molybdenum web. In some embodiments, the conducting
foil is steel foil or aluminum foil. In some embodiments, the
elongated core is made of a conductive metal, a metal-filled epoxy,
a metal-filled glass, a metal-filled resin, or a conductive
plastic.
[0134] In some embodiments, a transparent conducting oxide is
deposited on a wire-shaped or tube-shaped elongated core rather
than wrapping a metal web or foil around the elongated core. In
such embodiments, the wire-shaped or tube-shaped elongated core can
be, for example, a plastic rod, a glass rod, a glass tube, or a
plastic tube. Such embodiments require some form of conductor in
electrical communication with the interior face of the
semiconductor junction. In some embodiments, divits in the
wire-shaped or tube-shaped elongated core are filled with a
conductive metal in order to provide such a conductor. The
conductor can be inserted in the divits prior to depositing the
transparent conductive oxide onto the wire-shaped or tube-shaped
elongated core.
[0135] More specific embodiments will now be disclosed. In some
embodiments the elongated core is a glass tubing having a divet
that runs lengthwise on the outer surface of the glass tubing, and
the manufacturing method comprises depositing a conductor in the
divit prior to the rolling step. In some embodiments the glass
tubing has a second divit that runs lengthwise on the surface of
the glass tubing. In such embodiments, the first divit and the
second divit are on approximate or exact opposite circumferential
sides of the glass tubing. In such embodiments, accordingly, the
method further comprises depositing a conductor in the second divit
prior to the rolling or, in embodiments in which rolling is not
used, prior to the deposition of an inner TCO, junction, and outer
TCO onto the elongated core.
[0136] In some embodiments the elongated core is a glass rod having
a first divet that runs lengthwise on the surface of the glass rod
and the method comprises depositing a conductor in the first divit
prior to the rolling. In some embodiments the glass rod has a
second divit that runs lengthwise on the surface of the glass rod
and the first divit and the second divit are on approximate or
exact opposite circumferential sides of the glass rod. In such
embodiments, accordingly, the method further comprises depositing a
conductor in the second divit prior to the rolling or, in
embodiments in which rolling is not used, prior to the deposition
of an inner TCO, junction, and outer TCO onto the elongated core.
Suitable materials for the conductor are any of the materials
described as a conductor herein including, but not limited to,
aluminum, molybdenum, steel, nickel, silver, gold, or an alloy
thereof.
[0137] FIG. 13 details a cross-section of a solar cell 402 in
accordance with the present invention. The solar cell 402 can be
manufactured using either the rolling method or deposition
techniques. Components that have reference numerals corresponding
to other embodiments of the present invention (e.g., 410, 412, and
420) are made of the same materials disclosed in such embodiments.
In FIG. 13, there is an elongated tubing 1306 having a first and
second divit running lengthwise along the tubing (perpendicular to
the plane of the page) that are on circumferentially opposing sides
of tubing 1306 as illustrated. In typical embodiments, tubing 1306
is not conductive. For example, tubing 1306 is made of plastic or
glass in some embodiments. Conductive wiring 1302 is placed in the
first and second divit as illustrated in FIG. 13. In some
embodiments the conductive wiring is made of any of the conductive
materials of the present invention. In some embodiments, conductive
wiring 1302 is made out of aluminum, molybdenum, steel, nickel,
silver, gold, or an alloy thereof. In embodiments where 1304 is a
conducting foil or metallic web, the conductive wiring 1302 is
inserted into the divits prior to wrapping the metallic web or
conducting foil 1304 around the elongated core 1306. In embodiments
where 1304 is a transparent conductive oxide, the conductive wiring
1302 is inserted into the divits prior to depositing the
transparent conductive oxide 1304 onto elongated core 1306. As
noted, in some embodiments the metallic web or conducting foil 1304
is wrapped around tubing 1306. In some embodiments, metallic web or
conducting foil 1304 is glued to tubing 1306. In some embodiments
layer 1304 is not a metallic web or conducting foil. For instance,
in some embodiments, layer 1304 is a transparent conductive oxide
(TCO). Such a layer is advantageous because it allow for thinner
absorption layers in the semiconductor junction. In embodiments
where layer 1304 is a TCO, the TCO, semiconductor junction 410 and
outer TCO 412 are deposited using deposition techniques.
[0138] One aspect of the invention provides a solar cell assembly
comprising a plurality of elongated solar cells 402 each having the
structure disclosed in FIG. 13. That is, each elongated solar cell
402 in the plurality of elongated solar cells comprises an
elongated tubing 1306, a metallic web or a conducting foil (or,
alternatively, a layer of TCO) 1304 circumferentially disposed on
the elongated tubing 1306, a semiconductor junction 410
circumferentially disposed on the metallic web or the conducting
foil (or, alternatively, a layer of TCO) 1304 and a transparent
conductive oxide layer 412 disposed on the semiconductor junction
410. The elongated solar cells 402 in the plurality of elongated
solar cells are geometrically arranged in a parallel or a near
parallel manner thereby forming a planar array having a first face
and a second face. The plurality of elongated solar cells is
arranged such that one or more elongated solar cells in the
plurality of elongated solar cells do not contact adjacent
elongated solar cells. The solar cell assembly further comprises a
plurality of metal counter-electrodes. Each respective elongated
solar cell 402 in the plurality of elongated solar cells is bound
to a first corresponding metal counter-electrode 420 in the
plurality of metal counter-electrodes such that the first metal
counter-electrode lies in a first groove that runs lengthwise on
the respective elongated solar cell 402. The apparatus further
comprises a transparent electrically insulating substrate that
covers all or a portion of said the face of the planar array. A
first and second elongated solar cell in the plurality of elongated
solar cells are electrically connected in series by an electrical
contact that connects the first electrode of the first elongated
solar cell to the first corresponding counter-electrode of the
second elongated solar cell. In some embodiments, the elongated
tubing 1306 is glass tubing or plastic tubing having a one or more
grooves filled with a conductor 1302. In some embodiments, each
respective elongated solar cell 402 in the plurality of elongated
solar cells is bound to a second corresponding metal
counter-electrode 420 in the plurality of metal counter-electrodes
such that the second metal counter-electrode lies in a second
groove that runs lengthwise on the respective elongated solar cell
402 and such that the first groove and the second groove are on
opposite or substantially opposite circumferential sides of the
respective elongated solar cell 402. In some embodiments, the
plurality of elongated solar cells 402 is configured to receive
direct light from the first face and said second face of the planar
array.
5.7 Static Concentrators
[0139] In some embodiments, static concentrators are used to
improve the performance of the solar cell assemblies of the present
invention. The use of a static concentrator in one exemplary
embodiments is illustrated in FIG. 11, where static concentrator
1102, with aperture AB, is used to increase the efficiency of
bifacial solar cell assembly CD, where solar cell assembly CD is
any of 400 (FIG. 4), 600 (FIG. 6), 700 (FIG. 7), 800 (FIG. 8), 900
(FIG. 9), or 1000 (FIG. 10). Static concentrator 1102 can be formed
from any static concentrator materials known in the art such as,
for example, a simple, properly bent or molded aluminum sheet, or
reflector film on polyurethane. Concentrator 1102 is an example of
a low concentration ratio, nonimaging, compound parabolic
concentrator (CPC)-type collector. Any (CPC)-type collector can be
used with the solar cell assemblies of the present invention. For
more information on (CPC)-type collectors, see Pereira and Gordon,
1989, Journal of Solar Energy Engineering, 111, pp. 111-116, which
is hereby incorporated by reference in its entirety.
[0140] Additional static concentrators that can be used with the
present invention are disclosed in Uematsu et al., 1999,
Proceedings of the 11.sup.th International Photovoltaic Science and
Engineering Conference, Sapporo, Japan, pp. 957-958; Uematsu et
al., 1998, Proceedings of the Second World Conference on
Photovoltaic Solar Energy Conversion, Vienna, Austria, pp.
1570-1573; Warabisako et al., 1998, Proceedings of the Second World
Conference on Photovoltaic Solar Energy Conversion, Vienna,
Austria, pp. 1226-1231; Eames et al., 1998, Proceedings of the
Second World Conference on Photovoltaic Solar Energy Conversion,
Vienna Austria, pp. 2206-2209; Bowden et al., 1993, Proceedings of
the 23.sup.rd IEEE Photovoltaic Specialists Conference, pp.
1068-1072; and Parada et al., 1991, Proceedings of the 10.sup.th EC
Photovoltaic Solar Energy Conference, pp. 975-978, each of which is
hereby incorporated by reference in its entirety.
[0141] In some embodiments, a static concentrator as illustrated in
FIG. 12 is used. The bifacial solar cells illustrated in FIG. 12
can be any of the bifacial solar cell assemblies of the present
invention, including but not limited to assembly 400 (FIG. 4), 600
(FIG. 6), 700 (FIG. 7), 800 (FIG. 8), 900 (FIG. 9), or 1000 (FIG.
10). The static concentrator uses two sheets of cover glass on the
front and rear of the module with submillimeter V-grooves that are
designed to capture and reflect incident light as illustrated in
the Figure. More details of such concentrators is found in Uematsu
et al., 2001, Solar Energy Materials & Solar Cell 67, 425-434
and Uematsu et al., 2001, Solar Energy Materials & Solar Cell
67, 441-448, each of which is hereby incorporated by reference in
its entirety.
7. REFERENCES CITED
[0142] All references cited herein are incorporated herein by
reference in their entirety and for all purposes to the same extent
as if each individual publication or patent or patent application
was specifically and individually indicated to be incorporated by
reference in its entirety for all purposes.
[0143] Many modifications and variations of this invention can be
made without departing from its spirit and scope, as will be
apparent to those skilled in the art. For example, in some
embodiments the TCO 412 is circumferentially coated with an
antireflective coating. In some embodiments, this antireflective
coating is made of MgF.sub.2. The specific embodiments described
herein are offered by way of example only, and the invention is to
be limited only by the terms of the appended claims, along with the
full scope of equivalents to which such claims are entitled.
* * * * *