U.S. patent application number 12/207463 was filed with the patent office on 2009-01-08 for elongated photovoltaic cells in casings with a filling layer.
This patent application is currently assigned to Solyndra, Inc.. Invention is credited to Markus E. Beck, Benyamin Buller, Chris M. Gronet, Ratson Morad.
Application Number | 20090007963 12/207463 |
Document ID | / |
Family ID | 38516505 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090007963 |
Kind Code |
A1 |
Buller; Benyamin ; et
al. |
January 8, 2009 |
ELONGATED PHOTOVOLTAIC CELLS IN CASINGS WITH A FILLING LAYER
Abstract
A solar cell unit comprising a cylindrical shaped solar cell and
a transparent tubular casing is provided. The tubular shaped solar
cell comprises a back-electrode, a semiconductor junction
circumferentially disposed on the back-electrode and a transparent
conductive layer disposed on the semiconductor junction. The
transparent tubular casing is circumferentially sealed onto the
transparent conductive layer of the cylindrical shaped solar cell.
A solar cell unit comprising a cylindrical shaped solar cell, a
filler layer, and a transparent tubular casing is provided. The
cylindrical shaped solar cell comprises a cylindrical substrate, a
back-electrode circumferentially disposed on the cylindrical
substrate, a semiconductor junction circumferentially disposed on
the back-electrode, and a transparent conductive layer disposed on
the semiconductor junction. The filler layer is circumferentially
disposed on the transparent conductive layer and the transparent
tubular casing is circumferentially disposed onto the filler
layer.
Inventors: |
Buller; Benyamin;
(Cupertino, CA) ; Gronet; Chris M.; (Portola
Valley, CA) ; Morad; Ratson; (Palo Alto, CA) ;
Beck; Markus E.; (San Jose, CA) |
Correspondence
Address: |
JONES DAY
222 EAST 41ST ST
NEW YORK
NY
10017
US
|
Assignee: |
Solyndra, Inc.
Fremont
CA
|
Family ID: |
38516505 |
Appl. No.: |
12/207463 |
Filed: |
September 9, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11378847 |
Mar 18, 2006 |
|
|
|
12207463 |
|
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Current U.S.
Class: |
136/256 ;
257/E31.038 |
Current CPC
Class: |
Y02P 70/50 20151101;
Y02E 10/50 20130101; Y02E 10/52 20130101; Y02P 70/521 20151101;
H01L 31/035281 20130101; Y02E 10/541 20130101 |
Class at
Publication: |
136/256 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1-65. (canceled)
66. A solar cell unit comprising: (A) a solar cell comprising: a
rigid substrate, wherein the substrate is either (i) tubular or
(ii) a solid rod; a back-electrode circumferentially disposed on
said substrate; a semiconductor junction circumferentially disposed
on said back-electrode; and a transparent conductive layer disposed
on said semiconductor junction; (B) a filler layer
circumferentially disposed on said transparent conductive layer;
and C) a transparent tubular casing circumferentially disposed onto
said filler layer.
67. The solar cell unit of claim 66, wherein said rigid substrate
has a solid core.
68. The solar cell unit of claim 66, wherein the rigid substrate
comprises plastic, metal or glass.
69. The solar cell unit of claim 66, wherein the rigid substrate
comprises a urethane polymer, an acrylic polymer, a fluoropolymer,
polybenzamidazole, polymide, polytetrafluoroethylene,
polyetheretherketone, polyamide-imide, glass-based phenolic,
polystyrene, cross-linked polystyrene, polyester, polycarbonate,
polyethylene, polyethylene, acrylonitrile-butadiene-styrene,
polytetrafluoro-ethylene, polymethacrylate, nylon 6,6, cellulose
acetate butyrate, cellulose acetate, rigid vinyl, plasticized
vinyl, or polypropylene.
70. The solar cell unit of claim 66, wherein the rigid substrate
comprises aluminosilicate glass, borosilicate glass, dichroic
glass, germanium/semiconductor glass, glass ceramic, silicate/fused
silica glass, soda lime glass, quartz glass, chalcogenide/sulphide
glass, fluoride glass, a glass-based phenolic, flint glass, or
cereated glass.
71. The solar cell unit of claim 66, wherein said semiconductor
junction comprises an absorber layer and a junction partner layer,
wherein said junction partner layer is circumferentially disposed
on said absorber layer; and said absorber layer is
circumferentially disposed on said back-electrode.
72. The solar cell unit of claim 71, wherein said absorber layer is
copper-indium-gallium-diselenide and said junction partner layer is
In.sub.2Se.sub.3, In.sub.2S.sub.3, ZnS, ZnSe, CdInS, CdZnS,
ZnIn.sub.2Se.sub.4, Zn.sub.1-xMgO, CdS, SnO.sub.2, ZnO, ZrO.sub.2,
or doped ZnO.
73. The solar cell unit of claim 71, wherein the absorber layer
comprises CIGS having a <110> crystallographic orientation a
<112> crystallographic orientation, or no crystallographic
orientation.
74. The solar cell unit of claim 66, wherein said solar cell unit
further comprises: (D) an antireflective coating circumferentially
disposed on said transparent tubular casing.
75. The solar cell unit of claim 74, wherein the antireflective
coating comprises MgF.sub.2, silicon nitrate, titanium nitrate,
silicon monoxide, or silicon oxide nitrite.
76. The solar cell unit of claim 66, wherein r i .gtoreq. r o .eta.
outer ring ##EQU00003## wherein r.sub.i is a radius of the solar
cell; r.sub.o is the radius of the transparent tubular casing; and
.eta..sub.outer ring is the refractive index of the transparent
tubular casing.
77. The solar cell unit of claim 66, wherein the transparent
tubular casing is formed directly on said filler layer by casting
or UV curing.
78. The solar cell unit of claim 66, wherein the transparent
tubular casing comprises a plurality of transparent tubular casing
layers including a first transparent tubular casing layer and a
second transparent tubular casing layer, and wherein the first
transparent tubular casing layer is circumferentially disposed on
said filler layer and the second transparent tubular casing layer
is circumferentially disposed on said first transparent tubular
casing layer.
79. A solar cell unit comprising: (A) a solar cell comprising: a
rigid substrate, wherein said substrate is either (i) tubular or
(ii) a solid rod; a back-electrode circumferentially disposed on
the rigid substrate; a semiconductor junction circumferentially
disposed on the back-electrode; and a transparent conductive layer
disposed on the semiconductor junction; (B) a water resistant layer
circumferentially disposed on the transparent conductive layer; (C)
a filler layer circumferentially disposed on the water resistant
layer; and (D) a transparent tubular casing circumferentially
disposed on the filler layer.
80. The solar cell unit of claim 79, wherein said rigid substrate
is a tube.
81. The solar cell unit of claim 79, wherein r i .gtoreq. r o .eta.
outer ring ##EQU00004## wherein r.sub.i is a radius of the solar
cell; r.sub.o is the radius of the transparent tubular casing; and
.eta..sub.outer ring is the refractive index of the transparent
tubular casing.
82. The solar cell unit of claim 79, wherein the transparent
tubular casing comprises a plurality of transparent tubular casing
layers including a first transparent tubular casing layer and a
second transparent tubular casing layer, and wherein the first
transparent tubular casing layer is circumferentially disposed on
said filler layer and the second transparent tubular casing layer
is circumferentially disposed on said first transparent tubular
casing layer.
83. The solar cell unit of claim 79, wherein the transparent
tubular casing is formed by casting or UV curing directly onto the
filler layer.
84. A solar cell unit comprising: (A) a solar cell comprising: a
rigid substrate, wherein said substrate is tubular or a solid rod
shaped; a back-electrode circumferentially disposed on said rigid
substrate; a semiconductor junction circumferentially disposed on
said back-electrode; and a transparent conductive layer disposed on
said semiconductor junction; (B) a filler layer circumferentially
disposed on said transparent conductive layer; (C) a water
resistant layer circumferentially disposed on said filler layer;
and (D) a transparent tubular casing circumferentially disposed
onto said water resistant layer.
85. The solar cell unit of claim 84, wherein r i .gtoreq. r o .eta.
outer ring ##EQU00005## wherein r.sub.i is a radius of the solar
cell; r.sub.o is the radius of the transparent tubular casing; and
.eta..sub.outer ring is the refractive index of the transparent
tubular casing.
86. The solar cell unit of claim 84, wherein said rigid substrate
is a tube.
87. The solar cell unit of claim 84, wherein the transparent
tubular casing is formed by casting or UV curing directly onto the
water resistant layer.
88-94. (canceled)
95. The solar cell unit of claim 66, wherein the transparent
tubular casing comprises aluminosilicate glass, borosilicate glass,
dichroic glass, germanium/semiconductor glass, glass ceramic,
silicate/fused silica glass, soda lime glass, quartz glass,
chalcogenide/sulphide glass, fluoride glass, flint glass, or
cereated glass.
96. The solar cell unit of claim 66, wherein the transparent
tubular casing comprises a urethane polymer, an acrylic polymer, a
fluoropolymer, a silicone, a silicone gel, an epoxy, a polyamide,
or a polyolefin.
97. The solar cell unit of claim 66, wherein the transparent
tubular casing comprises polymethylmethacrylate (PMMA),
poly-dimethyl siloxane (PDMS), ethylene vinyl acetate (EVA),
perfluoroalkoxy fluorocarbon (PFA), nylon, cross-linked
polyethylene (PEX), polypropylene (PP), polyethylene terephtalate
glycol (PETG), polytetrafluoroethylene (PFTE), polyvinyl chloride
(PVC), or polyvinylidene fluoride (PVDF).
98. The solar cell unit of claim 79, wherein the transparent
tubular casing comprises aluminosilicate glass, borosilicate glass,
dichroic glass, germanium/semiconductor glass, glass ceramic,
silicate/fused silica glass, soda lime glass, quartz glass,
chalcogenide/sulphide glass, fluoride glass, flint glass, or
cereated glass.
99. The solar cell unit of claim 79, wherein the transparent
tubular casing comprises a urethane polymer, an acrylic polymer, a
fluoropolymer, a silicone, a silicone gel, an epoxy, a polyamide,
or a polyolefin.
100. The solar cell unit of claim 79, wherein the transparent
tubular casing comprises polymethylmethacrylate (PMMA),
poly-dimethyl siloxane (PDMS), ethylene vinyl acetate (EVA),
perfluoroalkoxy fluorocarbon (PFA), nylon, cross-linked
polyethylene (PEX), polypropylene (PP), polyethylene terephtalate
glycol (PETG), polytetrafluoroethylene (PFTE), polyvinyl chloride
(PVC), or polyvinylidene fluoride (PVDF).
101. The solar cell unit of claim 84, wherein the transparent
tubular casing comprises aluminosilicate glass, borosilicate glass,
dichroic glass, germanium/semiconductor glass, glass ceramic,
silicate/fused silica glass, soda lime glass, quartz glass,
chalcogenide/sulphide glass, fluoride glass, flint glass, or
cereated glass.
102. The solar cell unit of claim 84, wherein the transparent
tubular casing comprises a urethane polymer, an acrylic polymer, a
fluoropolymer, a silicone, a silicone gel, an epoxy, a polyamide,
or a polyolefin.
103. The solar cell unit of claim 84, wherein the transparent
tubular casing comprises polymethylmethacrylate (PMMA),
poly-dimethyl siloxane (PDMS), ethylene vinyl acetate (EVA),
perfluoroalkoxy fluorocarbon (PFA), nylon, cross-linked
polyethylene (PEX), polypropylene (PP), polyethylene terephtalate
glycol (PETG), polytetrafluoroethylene (PFTE), polyvinyl chloride
(PVC), or polyvinylidene fluoride (PVDF).
104. The solar cell unit of claim 66, wherein the filler layer
comprises ethylene vinyl acetate (EVA), silicone, silicone gel,
epoxy, polydimethyl siloxane (PDMS), RTV silicone rubber, polyvinyl
butyral (PVB), thermoplastic polyurethane (TPU), a polycarbonate,
an acrylic, a fluoropolymer, or a urethane.
105. The solar cell unit of claim 79, wherein the filler layer
comprises ethylene vinyl acetate (EVA), silicone, silicone gel,
epoxy, polydimethyl siloxane (PDMS), RTV silicone rubber, polyvinyl
butyral (PVB), thermoplastic polyurethane (TPU), a polycarbonate,
an acrylic, a fluoropolymer, or a urethane.
106. The solar cell unit of claim 84, wherein the filler layer
comprises ethylene vinyl acetate (EVA), silicone, silicone gel,
epoxy, polydimethyl siloxane (PDMS), RTV silicone rubber, polyvinyl
butyral (PVB), thermoplastic polyurethane (TPU), a polycarbonate,
an acrylic, a fluoropolymer, or a urethane.
107. The solar cell unit of claim 66, wherein the solar cell has a
length and a diameter, wherein the length is much larger than the
diameter and wherein a cross-section of the solar cell has
cross-sectional radial symmetry or approximate cross-sectional
radial symmetry.
108. The solar cell unit of claim 79, wherein the solar cell has a
length and a diameter, wherein the length is much larger than the
diameter and wherein a cross-section of the solar cell has
cross-sectional radial symmetry or approximate cross-sectional
radial symmetry.
109. The solar cell unit of claim 84, wherein the solar cell has a
length and a diameter, wherein the length is much larger than the
diameter and wherein a cross-section of the solar cell has
cross-sectional radial symmetry or approximate cross-sectional
radial symmetry.
110. The solar cell unit of claim 66, wherein the semiconductor
junction comprises a first composition that comprises a first
semiconductor element and said second layer comprises a second
composition that comprises a second semiconductor element, wherein
said first semiconductor element is different than said second
semiconductor element and wherein the first composition in the
first layer forms a junction with said second composition in said
second composition in said second layer.
111. The solar cell unit of claim 79, wherein the semiconductor
junction comprises a first composition that comprises a first
semiconductor element and said second layer comprises a second
composition that comprises a second semiconductor element, wherein
said first semiconductor element is different than said second
semiconductor element and wherein the first composition in the
first layer forms a junction with said second composition in said
second composition in said second layer.
112. The solar cell unit of claim 79, wherein said semiconductor
junction comprises an absorber layer and a junction partner layer,
wherein said junction partner layer is circumferentially disposed
on said absorber layer; and said absorber layer is
circumferentially disposed on said back-electrode.
113. The solar cell unit of claim 112, wherein said absorber layer
is copper-indium-gallium-diselenide and said junction partner layer
is In.sub.2Se.sub.3, In.sub.2S.sub.3, ZnS, ZnSe, CdInS, CdZnS,
ZnIn.sub.2Se.sub.4, Zn.sub.1-xMgO, CdS, SnO.sub.2, ZnO, ZrO.sub.2,
or doped ZnO.
114. The solar cell unit of claim 84, wherein the semiconductor
junction comprises a first composition that comprises a first
semiconductor element and said second layer comprises a second
composition that comprises a second semiconductor element, wherein
said first semiconductor element is different than said second
semiconductor element and wherein the first composition in the
first layer forms a junction with said second composition in said
second composition in said second layer.
115. The solar cell unit of claim 84, wherein said semiconductor
junction comprises an absorber layer and a junction partner layer,
wherein said junction partner layer is circumferentially disposed
on said absorber layer; and said absorber layer is
circumferentially disposed on said back-electrode.
116. The solar cell unit of claim 115, wherein said absorber layer
is copper-indium-gallium-diselenide and said junction partner layer
is In.sub.2Se.sub.3, In.sub.2S.sub.3, ZnS, ZnSe, CdInS, CdZnS,
ZnIn.sub.2Se.sub.4, Zn.sub.1-xMgO, CdS, SnO.sub.2, ZnO, ZrO.sub.2,
or doped ZnO.
117. The solar cell unit of claim 66, wherein the transparent
tubular casing is made out of plastic or glass.
118. The solar cell unit of claim 79, wherein the transparent
tubular casing is made out of plastic or glass.
119. The solar cell unit of claim 84, wherein the transparent
tubular casing is made out of plastic or glass.
Description
CROSS REFERENCE TO RELATED PATENT APPLICATION
[0001] This application claims priority to U.S. patent application
Ser. No. 11/378,847, filed on Mar. 18, 2006.
1. FIELD OF THE INVENTION
[0002] 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
[0003] Solar cells are typically 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.
[0004] 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 schematic so that it represents 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 needed
to absorb a sufficient amount of light.
[0005] The arrows at the top of FIG. 1 show the source of direct
solar illumination on the cell. Layer 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.
[0006] Layer 106 is the semiconductor absorber layer. Back
electrical contact 104 makes ohmic contact with absorber layer 106.
In many but not all cases, absorber layer 106 is a p-type
semiconductor. Absorber layer 106 is thick enough to absorb light.
Layer 108 is the semiconductor junction partner-that, together with
semiconductor absorber layer 106, completes the formation of a p-n
junction. A p-n junction is a common type of junction found in
solar cells. In p-n junction based solar cells, when semiconductor
absorber layer 106 is a p-type doped material, junction partner 108
is an n-type doped material. Conversely, when semiconductor
absorber layer 106 is an n-type doped material, junction partner
108 is a p-type doped material. Generally, junction partner 108 is
much thinner than absorber layer 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.
[0007] In a typical thick-film solar cell, absorber layer 106 and
window layer 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 the absorber layer 106,
the use of CdS to form junction partner 108 has resulted in high
efficiency cells. Other materials that can be used for junction
partner 108 include, but are not limited to, In.sub.2Se.sub.3,
In.sub.2S.sub.3, ZnS, ZnSe, CdInS, CdZnS, ZnIn.sub.2Se.sub.4,
Zn.sub.1-xMg.sub.xO, CdS, SnO.sub.2, ZnO, ZrO.sub.2 and doped
ZnO.
[0008] Layer 110 is the counter electrode, which completes the
functioning cell. Counter electrode 110 is used to draw current
away from the junction since junction partner 108 is generally too
resistive to serve this function. As such, counter electrode 110
should be highly conductive and transparent to light. Counter
electrode 110 can in fact be a comb-like structure of metal printed
onto layer 108 rather than forming a discrete layer. Counter
electrode 110 is typically a transparent conductive oxide (TCO)
such as doped zinc oxide (e.g., aluminum doped zinc oxide, gallium
doped zinc oxide, boron doped zinc oxide), indium-tin-oxide (ITO),
tin oxide (SnO.sub.2), or indium-zinc oxide. However, even when a
TCO layer is present, a bus bar network 114 is typically needed in
conventional solar cells 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 charge 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 trade off 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 herein in its entirety, a
combined silver bus bar network and indium-tin-oxide layer function
as a single, transparent ITO/Ag layer.
[0009] Layer 112 is an antireflective coating that 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 in FIG. 1. Alternatively or additionally,
antireflective coating 112 made be deposited on a separate cover
glass that overlays top electrode 110. Ideally, the antireflective
coating reduces the reflection of the cell to very near zero over
the spectral region in which 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 herein in its entirety,
describes representative antireflective coatings that are known in
the art.
[0010] 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 achieve greater voltages. 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 counter-electrode of an adjoining solar
cell.
[0011] 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.
[0012] 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.
[0013] As illustrated in FIG. 2A, 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.
[0014] As illustrated in FIG. 2B, 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 3,005,862. 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. 2 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
reduced.
[0015] 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.
[0016] Referring to FIGS. 2C and 2D, 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 FIG. 2C. A conducting metal is used as 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. 2C, 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. 2D.
[0017] As illustrated in FIG. 2D, 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.
[0018] 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. 2C). 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. 2C and 2D 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.
[0019] Referring to FIG. 2E, 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.
[0020] Although tubular designs of solar cells have addressed many
of the drawbacks associated with planar solar cells, some problems
remain unresolved. The capacity of solar cells to withstand
physical shock is one unresolved problem. Conventional solar cell
panels often crack after a certain number of years, often even
before the gained energy benefit can balance their production
costs. Solar cell assemblies are often built from small individual
solar cell units. This approach provides efficiency and
flexibility. Smaller solar cells are easier to manufacture at a
large scale, and they can also be assembled into different sizes
and shapes to suit the ultimate application. Inevitably, the
smaller solar cell unit design also comes with the price of
fragility. The smaller solar cell units easily break under pressure
during transportation or routine handling processes. What are
needed in the art are methods and systems that provide support and
strength to solar cell units while maintaining the advantages of
the small design.
[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 invention provides a solar cell unit
comprising a cylindrical shaped solar cell and a transparent
tubular casing. The cylindrical shaped solar cell comprises a
back-electrode, a semiconductor junction layer circumferentially
disposed on the back-electrode, and a transparent conductive layer
disposed on the semiconductor junction. The transparent tubular
casing is circumferentially sealed onto the cylindrically shaped
solar cell so that there is no air between the transparent tubular
casing and the cylindrically shaped solar cell in the solar cell
unit. In some embodiments, the transparent tubular casing is made
of plastic or glass. In some embodiments, the cylindrically shaped
solar cell further comprises a cylindrical substrate and the
back-electrode is circumferentially disposed on the cylindrical
substrate. The cylindrical substrate can be made of a wide variety
of materials including plastic, metal, or glass. Typically, the
cylindrical substrate is hollowed (e.g., a tube). Therefore, fluids
such as air, nitrogen, or helium can be passed through the
cylindrical substrate in many embodiments of the present invention.
In some embodiments, however, the cylindrical substrate is
solid.
[0023] In some embodiments, the semiconductor junction comprises a
homojunction, a heterojunction, a hetero face junction, a buried
homojunction, a p-i-n junction, or a tandem junction. In some
embodiments, the conductor junction comprises an absorber layer and
a junction partner layer and the junction partner layer is
circumferentially disposed on the absorber layer. In some
embodiments, the absorber layer is copper-indium-gallium-diselenide
and the junction partner layer is In.sub.2Se.sub.3,
In.sub.2S.sub.3, ZnS, ZnSe, CdInS, CdZnS, ZnIn.sub.2Se.sub.4,
Zn.sub.1-xMg.sub.xO, CdS, SnO.sub.2, ZnO, ZrO.sub.2, or doped
ZnO.
[0024] In some embodiments, the cylindrical shaped solar cell
further comprises an intrinsic layer circumferentially disposed on
the semiconductor junction. In such embodiments, the transparent
conductive layer is disposed on the intrinsic layer rather than the
semiconductor junction.
[0025] In some embodiments, the solar cell unit further comprises a
filler layer circumferentially disposed on the transparent
conductive layer. In such embodiments, the transparent tubular
casing is circumferentially disposed on the filler layer thereby
circumferentially sealing the tubular shaped solar cell.
[0026] In some embodiments, a water resistant layer is
circumferentially disposed on the transparent conductive layer. In
such embodiments, the transparent tubular casing is
circumferentially disposed on the water resistant layer thereby
circumferentially sealing the cylindrical shaped solar cell.
[0027] In some embodiments, a water resistant layer is
circumferentially disposed on the transparent conductive layer and
a filler layer circumferentially is disposed on the water resistant
layer. In such embodiments the transparent tubular casing is
circumferentially disposed on the filler layer thereby
circumferentially sealing the cylindrical shaped solar cell.
[0028] In some embodiments, the solar cell unit further comprises a
filler layer circumferentially disposed on the transparent
conductive layer and a water resistant layer circumferentially
disposed on the water resistant layer. In such embodiments, the
transparent tubular casing is circumferentially disposed on the
water resistant layer thereby circumferentially sealing the
cylindrical shaped solar cell. In some embodiments, the solar cell
unit further comprises an antireflective coating circumferentially
disposed on the transparent tubular casing.
[0029] In some embodiments the cylindrically shaped solar cell
further comprises at least one electrode strip, where each
electrode strip in the at least one electrode strip is overlayed on
the transparent conductive layer of the solar cell along the long
cylindrical axis of the solar cell. In some embodiments, the at
least one electrode strip comprises a plurality of electrode strips
that are positioned at spaced intervals on the transparent
conductive layer such that the plurality of electrode strips run
parallel or approximately parallel to each other along the
cylindrical axis of the solar cell. The plurality of electrode
strips can be spaced out at, for example, sixty degree intervals on
a surface of the transparent conductive layer of the solar cell. In
fact, the electrode strips in the plurality of electrode strips can
be spaced out at any type of even interval or uneven interval on
the surface of the transparent conductive layer of the solar cell.
In some embodiments, a length of the cylindrical shaped solar cell
is between 0.3 meters and 2 meters. In some embodiments, an outer
surface of the transparent tubular casing is textured.
[0030] Another aspect of the present invention provides a solar
cell assembly comprising a plurality of solar cell units. Each
solar cell unit in the plurality of solar cell units has the
structure of any of the solar cell units described above. The solar
cell units in the plurality of solar cell units are arranged in
coplanar rows to form the solar cell assembly. In some embodiments,
the solar assembly further comprises an albedo surface positioned
to reflect sunlight into the plurality of solar cell units. In some
embodiments, the albedo surface has an albedo that exceeds 95%. In
some embodiments, the albedo surface is a Lambertian, diffuse, or
involute reflector surface. In some embodiments, a first solar cell
unit and a second solar cell unit in the plurality of solar cell
units is electrically arranged in series or in parallel.
[0031] Still another aspect of the present invention comprises a
solar cell assembly comprising a plurality of solar cell units and
a plurality of internal reflectors. Each of the solar cell units in
the plurality of solar cell units has the structure of any of the
solar cell units described above. In this embodiment, the plurality
of internal reflectors are arranged in coplanar rows in which
internal reflectors in the plurality of solar cell units abut solar
cell units in the plurality of solar cell units thereby forming the
solar cell assembly. In some embodiments, an internal reflector in
the plurality of internal reflectors has a hollow core. In some
embodiments, an internal reflector in the plurality of internal
reflectors comprises a plastic casing with a layer of reflective
material deposited on the plastic casing. In some embodiments, an
internal reflector in the plurality of internal reflectors is a
single piece made out of a reflective material. In some
embodiments, a cross-sectional shape of an internal reflector in
the plurality of internal reflectors is astroid. In some
embodiments, a cross-sectional shape of an internal reflector in
the plurality of internal reflectors is four-sided and a side of
the four-sided cross-sectional shape is linear, parabolic, concave,
circular or elliptical. In some embodiments, a cross-sectional
shape of an internal reflector in the plurality of internal
reflectors is four-sided and a side of the four-sided
cross-sectional shape defines a diffuse surface on the internal
reflector.
[0032] Still another aspect of the invention provides a solar cell
unit comprising a cylindrical shaped solar cell, a filler layer,
and a transparent tubular casing. In some embodiments the
cylindrical shaped solar cell comprises a cylindrical substrate, a
back-electrode circumferentially disposed on the cylindrical
substrate, a semiconductor junction circumferentially disposed on
the back-electrode, and a transparent conductive layer disposed on
the semiconductor junction. The cylindrical substrate can be a
hollowed cylinder (e.g., tube) or a solid cylinder. The filler
layer is circumferentially disposed on the transparent conductive
layer and the transparent tubular casing is circumferentially
disposed onto the filler layer. In some embodiments in accordance
with this aspect of the invention, the semiconductor junction
comprises an absorber layer and a junction partner layer and the
junction partner layer is circumferentially disposed on the
absorber layer while the absorber layer is circumferentially
disposed on the back-electrode. In some embodiments in accordance
with this aspect of the invention, the solar cell unit further
comprises an antireflective coating circumferentially disposed on
the transparent tubular casing.
[0033] Yet another aspect of the invention comprises a solar cell
unit comprising a cylindrical shaped solar cell, a water resistant
layer, a filler layer, and a transparent tubular casing. The
cylindrical shaped solar cell comprises a cylindrical substrate, a
back-electrode circumferentially disposed on the cylindrical
substrate, a semiconductor junction circumferentially disposed on
the back-electrode, and a transparent conductive layer disposed on
the semiconductor junction. The cylindrical substrate can be a
solid cylinder or a hollowed cylinder (e.g., a tube). The water
resistant layer is circumferentially disposed on the transparent
conductive layer. The filler layer circumferentially disposed on
the water resistant layer. The transparent tubular casing is
circumferentially disposed onto the filler layer.
[0034] Still another aspect of the invention provides a solar cell
unit comprising a cylindrical shaped solar cell, a filler layer, a
water resistant layer, and a transparent tubular casing. The
cylindrical shaped solar cell comprises a cylindrical substrate, a
back-electrode circumferentially disposed on the cylindrical
substrate, a semiconductor junction circumferentially disposed on
the back-electrode, and a transparent conductive layer disposed on
the semiconductor junction. The cylindrical substrate can be solid
or hollowed (e.g., a tube). The filler layer is circumferentially
disposed on the transparent conductive layer. The water resistant
layer is circumferentially disposed on the filler layer. The
transparent tubular casing is circumferentially disposed onto the
water resistant layer.
4. BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 illustrates interconnected solar cells in accordance
with the prior art.
[0036] FIG. 2A illustrates a spherical solar cell including ap-type
inner layer and an n-type outer layer in accordance with the prior
art.
[0037] FIG. 2B 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.
[0038] FIG. 2C is a cross-sectional view of an elongated solar cell
in accordance with the prior art.
[0039] FIG. 2D 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.
[0040] FIG. 2E is a cross-sectional view of a solar cell assembly
disposed a distance away from a reflecting wall in accordance with
the prior art.
[0041] FIG. 3A illustrates a photovoltaic element with tubular
casing, in accordance with an embodiment of the present
invention.
[0042] FIG. 3B illustrates a cross-sectional view of an elongated
solar cell in a transparent tubular casing, in accordance with an
embodiment of the present invention.
[0043] FIG. 3C illustrates the multi-layer components of an
elongated solar cell in accordance with an embodiment of the
present invention.
[0044] FIG. 3D illustrates a transparent tubular casing, in
accordance with an embodiment of the present invention.
[0045] FIG. 4A is a cross-sectional view of elongated solar cells
in tubular casing that are electrically arranged in series and
geometrically arranged in a parallel or near parallel manner, in
accordance with an embodiment of the present invention.
[0046] FIG. 4B is a cross-sectional view taken about line 4B-4B of
FIG. 4A depicting the serial electrical arrangement of solar cells
in an assembly, in accordance with an embodiment of the present
invention.
[0047] 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.
[0048] 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.
[0049] FIGS. 5A-5D illustrate semiconductor junctions that are used
in various elongated solar cells in various embodiments of the
present invention. art.
[0050] FIG. 6B illustrates an injection blow molding method, in
accordance with the prior art.
[0051] FIG. 6C illustrates a stretch blow molding method, in
accordance with the prior art.
[0052] FIG. 7A is a cross-sectional view of elongated solar cells
electrically arranged in series in an assembly where
counter-electrodes abut individual solar cells, in accordance with
another embodiment of the present invention.
[0053] FIG. 7B is a cross-sectional view taken about line 7B-7B of
FIG. 7A that depicts the serial arrangement of the cylindrical
solar cells in an assembly, in accordance with an embodiment of the
present invention.
[0054] FIG. 7C is a perspective view an array of alternating
tubular casings, in accordance with an embodiment of the present
invention.
[0055] FIG. 8 is a cross-sectional view of elongated solar cells
electrically arranged in series in an assembly where
counter-electrodes abut individual solar cells and the outer TCO is
cut, in accordance with another embodiment of the present
invention.
[0056] FIG. 9 is a cross-sectional view of elongated solar cells
electrically arranged in series in an assembly in which the inner
metal electrode is hollowed, in accordance with an embodiment of
the present invention.
[0057] FIG. 10 is a cross-sectional view of elongated solar cells
electrically arranged in series in an 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.
[0058] FIG. 11 illustrates a static concentrator for use in some
embodiments of the present invention.
[0059] FIG. 12 illustrates a static concentrator used in some
embodiments of the present invention.
[0060] FIG. 13 illustrates a cross-sectional view of a solar cell
in accordance with an embodiment of the present invention.
[0061] FIG. 14 illustrate molded tubular casing in accordance with
some embodiments of the present invention.
[0062] FIG. 15 illustrates a perspective view of an elongated solar
cell architecture with protruding electrode attachments, in
accordance with an embodiment of the present invention.
[0063] FIG. 16 illustrates a perspective view of a solar cell
architecture in accordance with an embodiment of the present
invention.
[0064] FIG. 17A illustrates light reflection on a specular surface,
in accordance with the prior art.
[0065] FIG. 17B illustrates light reflection on a diffuse surface,
in accordance with the prior art.
[0066] FIG. 17C illustrates light reflection on a Lambertian
surface, in accordance with the prior art.
[0067] FIG. 18A illustrates a circle and an involute of the circle,
in accordance with the prior art
[0068] FIG. 18B illustrates a cross-sectional view of a solar cell
architecture in accordance with an embodiment of the present
invention.
[0069] FIG. 19 illustrates a cross-sectional view of an array of
alternating tubular casings and internal reflectors, in accordance
with an embodiment of the present invention.
[0070] FIG. 20A illustrates a suction loading assembly method in
accordance with the present invention.
[0071] FIG. 20B illustrates a pressure loading assembly method in
accordance with the present invention.
[0072] FIG. 20C illustrates a pour-and-slide loading assembly
method in accordance with the present invention.
[0073] FIG. 21 illustrates a partial cross-sectional view of an
elongated solar cell in a transparent tubular casing, in accordance
with an embodiment of the present invention.
[0074] FIG. 22 illustrates Q-type silicone, silsequioxane, D-type
silicon, and M-type silicon, in accordance with the prior art.
[0075] Like reference numerals refer to corresponding parts
throughout the several views of the drawings. Dimensions are not
drawn to scale.
5. DETAILED DESCRIPTION
[0076] 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 an elongated cylindrical shape.
5.1 Basic Structure
[0077] The present invention provides individually
circumferentially covered cylindrical solar cell units 300 that are
illustrated in perspective view in FIG. 3A and cross-sectional view
in FIG. 3B. In a solar cell unit 300, an elongated cylindrical
solar cell 402 (FIG. 3C) is circumferentially covered by a
transparent tubular casing 310 (FIG. 3D). In some embodiments,
solar cell unit 300 comprises a solar cell 402 coated with a
transparent tubular casing 310. In some embodiments, only one end
of elongated solar cell 402 is exposed by transparent tubular
casing 310 in order to form an electrical connection with adjacent
solar cells 402 or other circuitry. In some embodiments, both ends
of elongated solar cell 402 are exposed by transparent tubular
casing 310 in order to form an electrical connection with adjacent
solar cells 402 or other circuitry. As used herein, the term
cylindrical means objects having a cylindrical or approximately
cylindrical shape. In fact, cylindrical objects can have irregular
shapes so long as the object, taken as a whole, is roughly
cylindrical. Such cylindrical shapes can be solid (e.g., a rod) or
hollowed (e.g., a tube). As used herein, the term tubular means
objects having a tubular or approximately tubular shape. In fact,
tubular objects can have irregular shapes so long as the object,
taken as a whole, is roughly tubular.
[0078] Although most discussion in the present application
pertaining to solar cell units 300 are in the context of either the
encapsulated embodiments or circumferentially covered embodiments,
it is to be appreciated that such discussion serves as no
limitation to the scope of the present invention. Any transparent
tubular casing that provides support and protection to elongated
solar cells and permits electrical connections between the
elongated solar cells are within the scope of the systems and
methods of the present invention.
[0079] Descriptions of exemplary solar cells 402 are provided in
this section as well as Sections 5.2 through 5.8. For instance,
examples of semiconductor junctions that can be used in solar cells
402 are discussed in Section 5.2. Exemplary systems and methods for
manufacturing transparent tubular casing 310 are described in
Section 5.1.2. Systems and methods for coating solar cells 402 with
transparent tubular casing 310 in order to form solar cell units
300 are found in Section 5.1.3. Solar cell units 300 can be
assembled into solar cell assemblies of various sizes and shapes to
generate electricity and potentially heat water or other
fluids.
[0080] FIG. 3B illustrates the cross-sectional view of an exemplary
embodiment of solar cell unit 300. Other exemplary embodiments of
solar cells (e.g., 402 in FIG. 4A) are also suitable for coating by
transparent tubular casing 310.
[0081] Substrate 403. Substrate 403 serves as a substrate for solar
cell 402. In some embodiments, substrate 403 is made of a plastic,
metal, metal alloy, or glass. Substrate 403 is cylindrical shaped.
In some embodiments, substrate 403 has a hollow core, as
illustrated in FIG. 3B. In some embodiments, substrate 403 has a
solid core. In some embodiments, the shape of substrate 403 is only
approximately that of a cylindrical object, meaning that a
cross-section taken at a right angle to the long axis of substrate
403 defines an ellipse rather than a circle. As the term is used
herein, such approximately shaped objects are still considered
cylindrically shaped in the present invention. In some embodiments,
substrate 403 is made of a urethane polymer, an acrylic polymer, a
fluoropolymer, polybenzamidazole, polyimide,
polytetrafluoroethylene, polyetheretherketone, polyamide-imide,
glass-based phenolic, polystyrene, cross-linked polystyrene,
polyester, polycarbonate, polyethylene, polyethylene,
acrylonitrile-butadiene-styrene, polytetrafluoro-ethylene,
polymethacrylate, nylon 6,6, cellulose acetate butyrate, cellulose
acetate, rigid vinyl, plasticized vinyl, or polypropylene. In some
embodiments, substrate 403 is made of aluminosilicate glass,
borosilicate glass (e.g., PYREX, DURAN, SIMAX, etc.), dichroic
glass, germanium/semiconductor glass, glass ceramic, silicate/fused
silica glass, soda lime glass, quartz glass, chalcogenide/sulphide
glass, fluoride glass, PYREX glass, a glass-based phenolic,
cereated glass, or flint glass. In some embodiments, substrate 403
is a solid cylindrical shape. Such solid cylindrical substrates 403
can be made out of a plastic, glass, metal, or metal alloy.
[0082] Back-electrode 404. A back-electrode 404 is
circumferentially disposed on substrate 403. Back-electrode 404
serves as the first electrode in the assembly. In general,
back-electrode 404 is made out of any material such that it can
support the photovoltaic current generated by solar cell unit 300
with negligible resistive losses. In some embodiments,
back-electrode 404 is composed of any conductive material, such as
aluminum, molybdenum, tungsten, vanadium, rhodium, niobium,
chromium, tantalum, titanium, steel, nickel, platinum, silver,
gold, an alloy thereof, or any combination thereof. In some
embodiments, back-electrode 404 is composed of any conductive
material, such as indium tin oxide, titanium nitride, tin oxide,
fluorine doped tin oxide, doped zinc oxide, aluminum doped zinc
oxide, gallium doped zinc oxide, boron doped zinc oxide,
indium-zinc oxide, a metal-carbon black-filled oxide, a
graphite-carbon black-filled oxide, a carbon black-filled oxide, a
superconductive carbon black-filled oxide, an epoxy, a conductive
glass, or 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 plastic. In some embodiments, the conductive
plastics used in the present invention to form back-electrode 404
contain fillers that form sufficient conductive current-carrying
paths through the plastic matrix to support the photovoltaic
current generated by solar cell unit 300 with negligible resistive
losses. The plastic matrix of the conductive plastic is typically
insulating, but the composite produced exhibits the conductive
properties of the filler.
[0083] Semiconductor junction 410. A semiconductor junction 410 is
formed around back-electrode 404. Semiconductor junction 410 is any
photovoltaic homojunction, heterojunction, hetero face junction,
buried homojunction, a p-i-n junction or a tandem 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,
as well as Lugue and Hegedus, 2003, Handbook of Photovoltaic
Science and Engineering, John Wiley & Sons, Ltd., West Sussex,
England, each of which is hereby incorporated by reference herein
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 band gaps. In some embodiments, semiconductor junction 410
include a copper-indium-gallium-diselenide (CIGS) absorber
layer.
[0084] Optional intrinsic layer 415. Optionally, there is a thin
intrinsic layer (i-layer) 415 circumferentially coating
semiconductor junction 410. The i-layer 415 can be formed using any
undoped transparent oxide including, but not limited to, zinc
oxide, metal oxide, or any transparent material that is highly
insulating. In some embodiments, i-layer 415 is highly pure zinc
oxide.
[0085] Transparent conductive layer 412. Transparent conductive
layer 412 is circumferentially disposed on the semiconductor
junction layers 410 thereby completing the circuit. As noted above,
in some embodiments, a thin i-layer 415 is circumferentially
disposed on semiconductor junction 410. In such embodiments,
transparent conductive layer 412 is circumferentially disposed on
i-layer 415. In some embodiments, transparent conductive layer 412
is made of tin oxide SnO.sub.x (with or without fluorine doping),
indium-tin oxide (ITO), doped zinc oxide (e.g., aluminum doped zinc
oxide, gallium doped zinc oxide, boron dope zinc oxide),
indium-zinc oxide or any combination thereof. In some embodiments,
transparent conductive layer 412 is either p-doped or n-doped. In
some embodiments, transparent conductive layer is made of carbon
nanotubes. Carbon nanotubes are commercially available, for example
from Eikos (Franklin, Mass.) and are described in U.S. Pat. No.
6,988,925, which is hereby incorporated by reference herein in its
entirety. For example, in embodiments where the outer semiconductor
layer of junction 410 is p-doped, transparent conductive layer 412
can be p-doped. Likewise, in embodiments where the outer
semiconductor layer of junction 410 is n-doped, transparent
conductive layer 412 can be n-doped. In general, transparent
conductive 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, transparent conductive
layer 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, transparent conductive
layer 412 comprises more than one layer, including a first layer
comprising tin oxide SnO.sub.x (with or without fluorine doping),
indium-tin oxide (ITO), indium-zinc oxide, doped zinc oxide (e.g.,
aluminum doped zinc oxide, gallium doped zinc oxide, boron doped
zinc oxide) 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 transparent conductive layer are disclosed in U.S.
patent publication 2004/0187917A1 to Pichler, which is hereby
incorporated by reference herein in its entirety.
[0086] Optional electrode strips 420. In some embodiments in
accordance with the present invention, counter-electrode strips or
leads 420 are disposed on transparent conductive layer 412 in order
to facilitate electrical current flow. In some embodiments,
electrode strips 420 are thin strips of electrically conducting
material that run lengthwise along the long axis (cylindrical axis)
of the cylindrically shaped solar cell, as depicted in FIG. 4A. In
some embodiments, optional electrode strips are positioned at
spaced intervals on the surface of transparent conductive layer
412. For instance, FIG. 3B, electrode strips 420 run parallel to
each other and are spaced out at ninety degree intervals along the
cylindrical axis of the solar cell. In some embodiments, electrode
strips 420 are spaced out at five degree, ten degree, fifteen
degree, twenty degree, thirty degree, forty degree, fifty degree,
sixty degree, ninety degree or 180 degree intervals on the surface
of transparent conductive layer 412. In some embodiments, there is
a single electrode strip 420 on the surface of transparent
conductive layer 412. In some embodiments, there is no electrode
strip 420 on the surface of transparent conductive layer 412. In
some embodiments, there is two, three, four, five, six, seven,
eight, nine, ten, eleven, twelve, fifteen or more, or thirty or
more electrode strips on transparent conductive layer 412, all
running parallel, or near parallel, to each down the long
(cylindrical) axis of the solar cell. In some embodiments electrode
strips 420 are evenly spaced about the circumference of transparent
conductive layer 412, for example, as depicted in FIG. 3B. In
alternative embodiments, electrode strips 420 are not evenly spaced
about the circumference of transparent conductive layer 412. In
some embodiments, electrode strips 420 are only on one face of the
solar cell. Elements 403, 404, 410, 415 (optional), and 412 of FIG.
3B collectively comprise solar cell 402 of FIG. 3A. In some
embodiments, electrode strips 420 are made of conductive epoxy,
conductive ink, copper or an alloy thereof, aluminum or an alloy
thereof, nickel or an alloy thereof, silver or an alloy thereof,
gold or an alloy thereof, a conductive glue, or a conductive
plastic.
[0087] In some embodiments, there are electrode strips that run
along the long (cylindrical) axis of the solar cell and these
electrode strips are interconnected to each other by grid lines.
These grid lines can be thicker than, thinner than, or the same
width as the electrode strips. These grid lines can be made of the
same or different electrically material as the electrode
strips.
[0088] In some embodiments, electrode strips 420 are deposited on
transparent conductive layer 412 using ink jet printing. Examples
of conductive ink that can be used for such strips include, but are
not limited to silver loaded or nickel loaded conductive ink. In
some embodiments epoxies as well as anisotropic conductive
adhesives can be used to construct electrode strips 420. In typical
embodiments, such inks or epoxies are thermally cured in order to
form electrode strips 420.
[0089] Optional filler layer 330. The addition of counter-electrode
strips or leads 420 often renders the shape of the circular solar
cells irregular. Care is taken to exclude air from the solar cell
unit to avoid oxidation. Accordingly, in some embodiments of the
present invention, as depicted in FIG. 3B, a filler layer 330 of
sealant such as ethylene vinyl acetate (EVA), silicone, silicone
gel, epoxy, polydimethyl siloxane (PDMS), RTV silicone rubber,
polyvinyl butyral (PVB), thermoplastic polyurethane (TPU), a
polycarbonate, an acrylic, a fluoropolymer, and/or a urethane is
coated over transparent conductive layer 412 to seal out air and,
optionally, to provide complementary fitting to a transparent
tubular casing 310. In some embodiments, filler layer 330 is a
Q-type silicone, a silsequioxane, a D-type silicon, or an M-type
silicon. However, in some embodiments, optional filler layer 330 is
not needed even when one or more electrode strips 420 are present.
Additional suitable materials for optional filler layer 330 are
disclosed in Section 5.1.4, below.
[0090] Transparent tubular casing 310. Transparent tubular casing
310 is circumferentially disposed on transparent conductive layer
412 and/or optional filler layer 330. In some embodiments tubular
casing 310 is made of plastic or glass. In some embodiments,
elongated solar cells 402, after being properly modified for future
packaging as described below, are sealed in the transparent tubular
casing 310. As shown in FIG. 4A, transparent tubular casing 310
fits over the outermost layer of elongated solar cell 402. In some
embodiments, elongated solar cell 402 is inside transparent tubular
casing 310 such that adjacent elongated solar cells 402 do not form
electric contact with each other except at the ends of the solar
cells. Methods, such as heat shrinking, injection molding, or
vacuum loading, can be used to construct transparent tubular casing
310 such that they exclude oxygen and water from the system as well
as to provide complementary fitting to the underlying solar cell
402. In some embodiments, transparent tubular casing 310, for
example as depicted in FIG. 14, can be used to cover elongated
solar cells 402.
[0091] In some embodiments, transparent tubular casing 310 is made
of a urethane polymer, an acrylic polymer, polymethylmethacrylate
(PMMA), a fluoropolymer, silicone, poly-dimethyl siloxane (PDMS),
silicone gel, epoxy, ethylene vinyl acetate (EVA), perfluoroalkoxy
fluorocarbon (PFA), nylon/polyamide, cross-linked polyethylene
(PEX), polyolefin, polypropylene (PP), polyethylene terephtalate
glycol (PETG), polytetrafluoroethylene (PTFE), thermoplastic
copolymer (for example, ETFE.RTM., which is a derived from the
polymerization of ethylene and tetrafluoroethylene: TEFLON.RTM.
monomers), polyurethane/urethane, polyvinyl chloride (PVC),
polyvinylidene fluoride (PVDF), Tygon.RTM., vinyl, Viton.RTM., or
any combination or variation thereof. Additional suitable materials
for optional filler layer 330 are disclosed in Section 5.1.4,
below.
[0092] In some embodiments, transparent tubular casing 310
comprises a plurality of transparent tubular casing layers. In some
embodiments, each transparent tubular casing is composed of a
different material. For example, in some embodiments, transparent
tubular casing 310 comprises a first transparent tubular casing
layer and a second transparent tubular casing layer. Depending on
the exact configuration of the solar cell, the first transparent
tubular casing layer is disposed on the transparent conductive
layer 412, optional filler layer 330 or the water resistant layer.
The second transparent tubular casing layer is disposed on the
first transparent tubular casing layer.
[0093] In some embodiments, each transparent tubular casing layer
has different properties. In one example, the outer transparent
tubular casing layer has excellent UV shielding properties whereas
the inner transparent tubular casing layer has good water proofing
characteristics. Moreover, the use of multiple transparent tubular
casing layers can be used to reduce costs and/or improve the
overall properties of transparent tubular casing 310. For example,
one transparent tubular casing layer may be made of an expensive
material that has a desired physical property. By using one or more
additional transparent tubular casing layers, the thickness of the
expensive transparent tubular casing layer may be reduced, thereby
achieving a savings in material costs. In another example, one
transparent tubular casing layer may have excellent optical
properties (e.g., index of refraction, etc.) but be very heavy. By
using one or more additional transparent tubular casing layers, the
thickness of the heavy transparent tubular casing layer may be
reduced, thereby reducing the overall weight of transparent tubular
casing 310.
[0094] Optional water resistant layer. In some embodiments, one or
more layers of water resistant layer are coated over solar cell 402
to prevent the damaging effects of water molecules. In some
embodiments, this water resistant layer is circumferentially coated
onto transparent conductive layer 412 prior to depositing optional
filler layer 330 and encasing the solar cell 402 in transparent
tubular casing 310. In some embodiments, such water resistant
layers are circumferentially coated onto optional filler layer 330
prior to encasing the solar cell 402 in transparent tubular casing
310. In some embodiments, such water resistant layers are
circumferentially coated onto transparent tubular casing 310
itself. In embodiments where a water resistant layer is provided to
seal molecular water from solar cell 402, it is important that the
optical properties of the water resistant layer not interfere with
the absorption of incident solar radiation by solar cell 402. In
some embodiments, this water resistant layer is made of clear
silicone, SiN, SiO.sub.xN.sub.y, SiO.sub.x, or Al.sub.2O.sub.3,
where x and y are integers. In some embodiments, water resistant
layer is made of a Q-type silicone, a silsequioxane, a D-type
silicon, or an M-type silicon.
[0095] Optional antireflective coating. In some embodiments, an
optional antireflective coating is also circumferentially disposed
on transparent tubular casing 310 to maximize solar cell
efficiency. In some embodiments, there is a both a water resistant
layer and an antireflective coating deposited on transparent
tubular casing 310. In some embodiments, a single layer serves the
dual purpose of a water resistant layer and an anti-reflective
coating. In some embodiments, antireflective coating, made of
MgF.sub.2, silicon nitrate, titanium nitrate, silicon monoxide
(SiO), or silicon oxide nitrite. In some embodiments, there is more
than one layer of antireflective coating. In some embodiments,
there is more than one layer of antireflective coating and each
layer is made of the same material. In some embodiments, there is
more than one layer of antireflective coating and each layer is
made of a different material.
[0096] In some embodiments, some of the layers of multi-layered
solar cells 402 are constructed using cylindrical magnetron
sputtering techniques. In some embodiments, some of the layers of
multi-layered solar cells 402 are constructed using conventional
sputtering methods or reactive sputtering methods on long tubes or
strips. Sputtering coating methods for long tubes and strips are
disclosed in for example, Hoshi et al., 1983, "Thin Film Coating
Techniques on Wires and Inner Walls of Small Tubes via Cylindrical
Magnetron Sputtering," Electrical Engineering in Japan 103:73-80;
Lincoln and Blickensderfer, 1980, "Adapting Conventional Sputtering
Equipment for Coating Long Tubes and Strips," J. Vac. Sci. Technol.
17:1252-1253; Harding, 1977, "Improvements in a dc Reactive
Sputtering System for Coating Tubes," J. Vac. Sci. Technol.
14:1313-1315; Pearce, 1970, "A Thick Film Vacuum Deposition System
for Microwave Tube Component Coating," Conference Records of 1970
Conference on Electron Device Techniques 208-211; and Harding et
al., 1979, "Production of Properties of Selective Surfaces Coated
onto Glass Tubes by a Magnetron Sputtering System," Proceedings of
the International Solar Energy Society 1912-1916, each of which is
hereby incorporated by reference herein in its entirety.
[0097] Optional fluorescent material. In some embodiments, a
fluorescent material (e.g., luminescent material, phosphorescent
material) is coated on a surface of a layer of solar cell 300. In
some embodiments, the fluorescent material is coated on the luminal
surface and/or the exterior surface of transparent tubular casing
310. In some embodiments, the fluorescent material is coated on the
outside surface of transparent conductive oxide 412. In some
embodiments, solar cell 300 includes an optional filler layer 300
and the fluorescent material is coated on the optional filler
layer. In some embodiments, solar cell 300 includes a water
resistant layer and the fluorescent material is coated on the water
resistant layer. In some embodiments, more than one surface of a
solar cell 300 is coated with optional fluorescent material. In
some embodiments, the fluorescent material absorbs blue and/or
ultraviolet light, which some semiconductor junctions 410 of the
present invention do not use to convert to electricity, and the
fluorescent material emits light in visible and/or infrared light
which is useful for electrical generation in some solar cells 300
of the present invention.
[0098] Fluorescent, luminescent, or phosphorescent materials can
absorb light in the blue or UV range and emit visible light.
Phosphorescent materials, or phosphors, usually comprise a suitable
host material and an activator material. The host materials are
typically oxides, sulfides, selenides, halides or silicates of
zinc, cadmium, manganese, aluminum, silicon, or various rare earth
metals. The activators are added to prolong the emission time.
[0099] In some embodiments, phosphorescent materials are
incorporated in the systems and methods of the present invention to
enhance light absorption by solar cell 300. In some embodiments,
the phosphorescent material is directly added to the material used
to make optional transparent tubular casing 310. In some
embodiments, the phosphorescent materials are mixed with a binder
for use as transparent paints to coat various outer or inner layers
of solar cell 300, as described above.
[0100] Exemplary phosphors include, but are not limited to,
copper-activated zinc sulfide (ZnS:Cu) and silver-activated zinc
sulfide (ZnS:Ag). Other exemplary phosphorescent materials include,
but are not limited to, zinc sulfide and cadmium sulfide (ZnS:CdS),
strontium aluminate activated by europium (SrAlO.sub.3:Eu),
strontium titanium activated by praseodymium and aluminum
(SrTiO3:Pr, Al), calcium sulfide with strontium sulfide with
bismuth ((Ca,Sr)S:Bi), copper and magnesium activated zinc sulfide
(ZnS:Cu,Mg), or any combination thereof.
[0101] Methods for creating phosphor materials are known in the
art. For example, methods of making ZnS:Cu or other related
phosphorescent materials are described in U.S. Pat. Nos. 2,807,587
to Butler et al.; 3,031,415 to Morrison et al.; 3,031,416 to
Morrison et al.; 3,152,995 to Strock; 3,154,712 to Payne; 3,222,214
to Lagos et al.; 3,657,142 to Poss; 4,859,361 to Reilly et al., and
5,269,966 to Karam et al., each of which is hereby incorporated by
reference herein in its entirety. Methods for making ZnS:Ag or
related phosphorescent materials are described in U.S. Pat. Nos.
6,200,497 to Park et al., 6,025,675 to Ihara et al.; 4,804,882 to
Takahara et al., and 4,512,912 to Matsuda et al., each of which is
hereby incorporated herein by reference in its entirety. Generally,
the persistence of the phosphor increases as the wavelength
decreases. In some embodiments, quantum dots of CdSe or similar
phosphorescent material can be used to get the same effects. See
Dabbousi et al., 1995, "Electroluminescence from CdSe
quantum-dot/polymer composites," Applied Physics Letters 66 (11):
1316-1318; Dabbousi et al., 1997 "(CdSe)ZnS Core-Shell Quantum
Dots: Synthesis and Characterization of a Size Series of Highly
Luminescent Nanocrystallites," J. Phys. Chem. B, 101: 9463-9475;
Ebenstein et al., 2002, "Fluorescence quantum yield of CdSe:ZnS
nanocrystals investigated by correlated atomic-force and
single-particle fluorescence microscopy," Applied Physics Letters
80: 4033-4035; and Peng et al., 2000, "Shape control of CdSe
nanocrystals," Nature 404: 59-61; each of which is hereby
incorporated by reference herein in its entirety.
[0102] In some embodiments, optical brighteners are used in the
optional fluorescent layers of the present invention. Optical
brighteners (also known as optical brightening agents, fluorescent
brightening agents or fluorescent whitening agents) are dyes that
absorb light in the ultraviolet and violet region of the
electromagnetic spectrum, and re-emit light in the blue region.
Such compounds include stilbenes (e.g., trans-1,2-diphenylethylene
or (E)-1,2-diphenylethene). Another exemplary optical brightener
that can be used in the optional fluorescent layers of the present
invention is umbelliferone (7-hydroxycoumarin), which also absorbs
energy in the UV portion of the spectrum. This energy is then
re-emitted in the blue portion of the visible spectrum. More
information on optical brighteners is in Dean, 1963, Naturally
Occurring Oxygen Ring Compounds, Butterworths, London; Joule and
Mills, 2000, Heterocyclic Chemistry, 4.sup.th edition, Blackwell
Science, Oxford, United Kingdom; and Barton, 1999, Comprehensive
Natural Products Chemistry 2: 677, Nakanishi and Meth-Cohn eds.,
Elsevier, Oxford, United Kingdom, 1999.
[0103] Circumferentially disposed. In the present invention, layers
of material are successively circumferentially disposed on a
cylindrical substrate 403 in order to form a solar cell. As used
herein, the term circumferentially disposed is not intended to
imply that each such layer of material is necessarily deposited on
an underlying layer. In fact, the present invention teaches methods
by which such layers are molded or otherwise formed on an
underlying layer. Nevertheless, the term circumferentially disposed
means that an overlying layer is disposed on an underlying layer
such that there is no annular space between the overlying layer and
the underlying layer. Furthermore, as used herein, the term
circumferentially disposed means that an overlying layer is
disposed on at least fifty percent of the perimeter of the
underlying layer. Furthermore, as used herein, the term
circumferentially disposed means that an overlying layer is
disposed along at least half of the length of the underlying
layer.
[0104] Circumferentially sealed. In the present invention, the term
circumferentially sealed is not intended to imply that an overlying
layer or structure is necessarily deposited on an underlying layer
or structure. In fact, the present invention teaches methods by
which such layers or structures (e.g., transparent tubular casing
310) are molded or otherwise formed on an underlying layer or
structure. Nevertheless, the term circumferentially sealed means
that an overlying layer or structure is disposed on an underlying
layer or structure such that there is no annular space between the
overlying layer or structure and the underlying layer or structure.
Furthermore, as used herein, the term circumferentially sealed
means that an overlying layer is disposed on the full perimeter of
the underlying layer. In typical embodiments, a layer or structure
circumferentially seals an underlying layer or structure when it is
circumferentially disposed around the full perimeter of the
underlying layer or structure and along the full length of the
underlying layer or structure. However, the present invention
contemplates embodiments in which a circumferentially sealing layer
or structure does not extend along the full length of an underlying
layer or structure.
5.1.1 Solar Cell Unit Assemblies
[0105] FIG. 4A illustrates a cross-sectional view of the
arrangement of three solar cell units 300 arranged in a coplanar
fashion in order to form a solar cell assembly 400. FIG. 4B
provides a cross-sectional view with respect to line 4B-4B of FIG.
4A. In FIG. 4, back-electrode 404 is depicted as a solid
cylindrical substrate. However, in some embodiments in accordance
with FIG. 4, rather than being a solid cylindrical substrate,
back-electrode is a thin layer of electrically conducting material
circumferentially disposed on substrate 403 as depicted in FIG. 3B.
All other layers in FIG. 4 are as illustrated in FIG. 3B. Like in
FIG. 3B, optional filler layer 330 in the embodiments depicted in
FIG. 4 is optional.
[0106] 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. 2B). 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 minimizes
resistive losses. A serial electrical arrangement is maintained by
arranging all or a portion of the elongated solar cells 402 as
illustrated in FIGS. 4A and 4B. 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 material such as metal. In the alternative, where
conductive core 404 is not a solid, but rather comprises a
back-electrode layer circumferentially deposited on substrate 403,
the back-electrode layer 404 is highly conductive. Regardless of
whether back-electrode 404 is in a solid configuration as depicted
in FIG. 4 or a thin layer as depicted in FIG. 3B, such
back-electrodes 404 carry current without an appreciable current
loss due to resistance. While larger conductive cores 404 (FIG. 4)
and/or thicker back-electrode 404 (FIG. 3B) ensure low resistance,
transparent conductive layers encompassing such larger conductive
cores 404 must carry current further to contacts (counter-electrode
strips or leads) 420. Thus, there is an upper bound on the size of
conductive cores 404 and/or substrate 403. 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 core 404 (FIG. 4) and/or substrate 403 (FIG. 3B) are
sized so that they are large enough to carry a current without
appreciable resistive losses, yet small enough to allow transparent
conductive 412 to efficiently deliver current to counter-electrode
strip 420.
[0107] The advantageous low resistance nature of the architecture
illustrated in FIG. 4A is also facilitated by the highly conductive
properties of counter-electrode strip 420. In some embodiments, for
example, counter-electrode strips 420 are composed of a conductive
epoxy (e.g., silver epoxy) or conductive ink and the like. For
example, in some embodiments, counter-electrode strips 420 are
formed by depositing a thin metallic layer on a suitable substrate
and then patterning the layer into a series of parallel strips.
Each counter-electrode strip 420 is affixed to a solar cell 402
with a conductive epoxy along the length of a solar cell 402, as
shown in FIG. 4D. In some embodiments, counter-electrode strips 420
are formed directly on solar cells 402. In other embodiments,
counter-electrode strips 420 are formed on the outer transparent
conductive layer 412, as illustrated in FIG. 3B. In some
embodiments, connections between counter-electrode strip 420 to
electrodes 433 are established in series as depicted in FIG.
4B.
[0108] 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.
[0109] 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.
[0110] 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 shape in which their length is much larger than
their diameter and they possess some form of cross-sectional radial
symmetry or approximate cross-sectional radial symmetry.
[0111] In some embodiment, as illustrated in FIG. 4A, 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 back-electrode 404 (first electrode) of the first
elongated solar cell 402 to the corresponding counter-electrode
strip 420 of the second elongated solar cell 402. Thus, as
illustrated in FIG. 4A, elongated solar cells 402 are the basic
unit that respectively forms the semiconductor layer 410, the
transparent conductive layer 412, and the metal conductive core 404
of the elongated solar cell 402. In some embodiments, 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 through
transparent tubular casing 310.
[0112] 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, transparent
conductive 412 of the first elongated solar cell 402 is
electrically connected to transparent conductive layer 412 of the
second elongated solar cell 402 either by contacting the
transparent conductive layers 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.
[0113] FIG. 4C is an enlargement of region 4C of FIG. 4B in which a
portion of back-electrode 404 and transparent conductive layer 412
have been cut away to illustrate the positional relationship
between counter-electrode strip 420, electrode 433, back-electrode
404, semiconductor layer 410, and transparent conductive layer 412.
Furthermore, FIG. 4C illustrates how electrical contact 433 joins
back-electrode 404 of one elongated solar cell 402 to
counter-electrode 420 of another solar cell 402.
[0114] 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. However, encapsulation shields each solar
cell 402 from unwanted electrical contacts from adjacent solar
cells 402, making encapsulation relatively simple. Thus, referring
to FIG. 4D, which is a cross-sectional view of an 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 with transparent
tubular casing 310 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 and/or
substrate 403, semiconductor junction 410, optional thin intrinsic
layer (i-layer) 415, and transparent conductive layer 412. In such
embodiments, end 455 can receive sunlight and therefore contribute
to the electrical generating properties of the solar cell 402. In
some embodiments, transparent tubular casing 310 is open at both
ends of solar cell 402 such that electrical contacts can be
extended from either end of the solar cell.
[0115] FIG. 4D also illustrates how, in some embodiments, 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 back-electrode 404 is exposed, as illustrated
in FIG. 4D. In other words, semiconductor junction 410, optional
i-layer 415, and transparent conductive layer 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
transparent conductive layer 412 are stripped away from a terminal
portion of semiconductor junction 410. The remaining portions of
the conductive core 404, semiconductor junction 410, optional
i-layer 415, and transparent conductive layer 412 are coated by
transparent tubular casing 310. Such a configuration is
advantageous because it prevents a short from developing between
transparent conductive layer 412 and conductive core 404. In FIG.
4D, elongated solar cell 402 is positioned on counter-electrode
strip 420 which, in turn, is positioned against electrically
resistant transparent tubular casing 310. However, there is no
requirement that counter-electrode strip 420 make contact with
electrically resistant transparent tubular casing 310. In fact, in
some embodiments, elongated solar cells 402 and their corresponding
counter-electrode strips 420 are sealed within transparent
conductive layer 412 such that there is no unfavorable electrical
contact. In such embodiments, elongated solar cells 402 and
corresponding electrode strips 420 are fixedly held in place by a
sealant such as ethylene vinyl acetate or silicone. In some
embodiments in accordance with the present invention,
counter-electrode strips 420 are replaced with metal wires that are
attached to the sides of solar cell 402. In some embodiments in
accordance with the present invention, solar cells 402 implement a
segmented design to eliminate the requirement of additional wire-
or strip-like counter-electrodes. Details on segmented solar cell
design are found in copending U.S. patent application Ser. No.
11/378,847, attorney docket number 11653-007-999, entitled,
"Monolithic Integration of Cylindrical Solar Cells," filed Mar. 18,
2006, which is hereby incorporated by reference herein in its
entirety.
[0116] 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 back-electrode 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 back-electrode 404 of another elongated solar cell.
Second electrical contact 433-2 serially connects the
back-electrode 404 of elongated solar cell 402 to the
counter-electrode 420 of another elongated solar cell 402, as shown
in FIG. 4B. Such an electrical configuration is possible regardless
of whether back-electrode 404 is itself a solid cylindrical
substrate or is a layer of electrically conducting material on a
substrate 403 as depicted in FIG. 3B. Each solar cell 402 is coated
by a transparent tubular casing 310.
[0117] In addition, FIG. 4D provides an encapsulated solar cell 402
where an optional filler layer 330 and a transparent tubular casing
310 cover the solar cell, leaving only one end 466 to establish
electrical contracts. It is to be appreciated that, in some
embodiments, the optional filler layer 330 and transparent tubular
casing 310 are configured such that both ends (e.g., 455 and 466 in
FIG. 4D) of the elongated solar cell 402 are available to establish
electrical contacts.
[0118] FIG. 7A illustrates a 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 encapsulated in transparent tubular casing 310. Each elongated
solar cell 402 in the plurality of elongated solar cells has a
back-electrode 404 configured as a first electrode. In the
embodiments depicted in FIG. 7A, back electrode 404 is a solid
cylindrical electrically conducting substrate. However, in
alternative embodiments in accordance with FIG. 7, back-electrode
404 is a thin film of electrically conducting material deposited on
a hollowed tubular shaped substrate as in the case of FIG. 3B. The
principles taught in FIG. 7 apply to each such form of
back-electrode 404. In FIG. 7, a semiconductor junction 410 is
circumferentially disposed on the conductive core 402 and a
transparent conductive layer 412 is circumferentially disposed on
semiconductor junction 410. In some embodiments, 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 some 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 transparent conductive layer 412)
adjacent elongated solar cells 402. In some 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 directly contact the outer transparent tubular casing 310 of
adjacent elongated solar cells 402.
[0119] 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 path length of current drawn
off of transparent conductive layer 412. In other words, the
maximum length that current must travel in transparent conductive
layer 412 before it reaches a counter-electrode 420 is a quarter of
the circumference of the transparent conductive layer. 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 transparent conductive layer 412 before it
reaches a counter-electrode 420 is a full half of the circumference
of the transparent conductive layer 412. 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.
[0120] 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.
[0121] In some embodiments, transparent tubular casing 310, such as
the transparent tubular casing 310 depicted in FIG. 14, is used to
encase elongated solar cells 402. Because it is important to
exclude air from the solar cell unit 402, an optional filler layer
330 is circumferentially disposed between solar cell 402 and
transparent tubular casing 310 in the manner illustrated in FIG. 7A
in some embodiments of the present invention. In some embodiments,
filler layer 330 prevents the seepage of oxygen and water into
solar cells 402. In some embodiments, filler layer 330 comprises
EVA or silicone. In some embodiments, the individually encased
solar cells 402 are assembled into a planar array as depicted in
FIG. 7A. The plurality of elongated solar cells 402 are configured
to receive direct light from both face 733 and face 766 of the
planar array.
[0122] FIG. 7B provides a cross-sectional view with respect to line
7B-7B of FIG. 7A. Solar cells 402 are electrically connected to
each 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. Although the individual solar cells are shown separate
from each other to reveal the encasing by transparent tubular
casing 310, no actual separation distance between solar cells 402
is required since transparent tubular casing 310 shields the
individual solar cells 402 of solar cell unit 300 from any
unfavorable electrical contacts. However, tight space or no space
packing is not a required for individually shielded solar cell unit
300. In fact, the presence of the transparent tubular casing 310
provides more versatility in the solar cell assembly. For instance,
in some embodiments, the distance between adjacent solar cell units
300 is 0 microns or greater, 0.1 microns or greater, 0.5 microns or
greater, or between 1 and 5 microns, or optimally correlated with
the size and dimensions of the solar cell units 300.
[0123] Referring to FIG. 7B, serial electrical contact between
solar cells 402 is made by electrical contacts 788 that
electrically connect the back-electrode 404 of one elongated solar
cell 402 to the corresponding counter-electrodes 120 of a different
solar cell 402. 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 solar cells
402.
[0124] The solar cell assembly illustrated in FIG. 7 has several
advantages. First, the planar arrangement of the solar cells 402
leaves 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 individually encapsulated
solar cells 402 are aligned parallel to each other with no or
little space separation, the structure is completely
self-supporting. Still another advantage of the assembly is ease of
manufacture. Unlike solar cells such as that depicted in FIG. 2B,
no complicated grid or transparent conductive oxide on glass is
required. 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.
[0125] As illustrated in FIG. 7B, conductive core 404, junction
410, and transparent conductive layer 412 are flush with each other
at end 789 of elongated solar cells 402. In contrast, at end 799,
the conductive core protrudes a bit with respect to junction 410
and transparent conductive layer 412 as illustrated. Junction 410
also protrudes a bit at end 799 with respect to transparent
conductive layer 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
transparent conductive layer 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 transparent conductive layer
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.
[0126] 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.
[0127] 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.
[0128] Yet another advantage of the assembly illustrated in FIG. 7B
is that transparent tubular casing 310 is circumferentially
disposed on solar cells 402. In some embodiments, an optional
filler layer 330 lies between the outer surface of solar cell 402
and the inner surface of transparent tubular casing 310. Although
FIG. 7B only depicts electrical circuitry at one end of adjacent
solar cell units 300, it is possible for electrical circuitry to be
established at both ends of solar cell units 300 or between the two
ends of solar cell units 300.
[0129] The solar cell design in accordance with the present
invention is advantageous in that each individual solar cell 402 is
encapsulated by transparent tubular casing 310. transparent tubular
casing 310 is at least partial transparent and made of
non-conductive material such as plastics or glass. Accordingly,
solar cell assemblies made according to the present design do not
require insulator lengthwise between each solar cell 402. Yet
another embodiment of solar cell assembly 700 is that there is no
extra absorption loss from a transparent conductive layer 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.
[0130] 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.
[0131] Although not illustrated in FIG. 7, in some embodiments in
accordance with FIG. 7, there is an intrinsic layer 415
circumferentially disposed between the semiconductor junction 410
and the transparent conductive layer 412 in an elongated solar cell
402 in the plurality of elongated solar cells 402. Intrinsic layer
415 can be made of an undoped transparent oxide such as zinc oxide,
metal oxide, or any transparent metal that is highly insulating. 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 In.sub.2Se.sub.3,
In.sub.2S.sub.3, ZnS, ZnSe, CdInS, CdZnS, ZnIn.sub.2Se.sub.4,
Zn.sub.1-xMg.sub.xO, CdS, SnO.sub.2, ZnO, ZrO.sub.2, or doped ZnO.
In some embodiments not illustrated by FIG. 7, conductive cores 404
in solar cells 402 are hollowed.
[0132] 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 transparent conductive
layer 412 is interrupted by breaks 810 that run along the long axis
of solar cells 402 and cut completely through transparent
conductive layer 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.
[0133] 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, transparent
conductive layer 412 of the first elongated solar cell 402 is
electrically connected to transparent conductive layer 412 of the
second elongated solar cell 402 either by contacting the
transparent conductive layers of the two elongated solar cells
either directly or through a second electrical contact (not 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.
[0134] In some embodiments, transparent tubular casing 310, such as
depicted in FIG. 14, is used to encase elongated solar cells 402.
Because it is important to exclude air from the solar cell unit
402, a filler layer 330 may be used to prevent oxidation of the
solar cell 402. As illustrated in FIG. 8, filler layer 330 (for
example EVA) prevents seepage of oxygen and water into solar cells
402. Filler layer is disposed between solar cell 402 and the inner
layer of transparent tubular casing 310. In some embodiments, the
individually encapsulated solar cells 402 are assembled into a
planar array as depicted in FIG. 8.
[0135] FIG. 9 illustrates a solar cell assembly 900 of the present
invention in which back-electrodes 404 are hollowed. In fact,
back-electrode 404 can be hollowed in any of the embodiments of the
present invention. One advantage a hollowed back-electrode 404
design is that it reduces the overall weight of the solar cell
assembly. Back-electrode 404 is hollowed when there is a channel
that extends lengthwise through all or a portion of back-electrode
404. In some embodiments, back-electrode 404 is metal tubing. In
some embodiments, back-electrode 404 is a thin layer of
electrically conducting material, e.g. molybdenum, that is
deposited on a substrate 403 as illustrated in FIG. 3B.
[0136] 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. 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.
[0137] In some embodiments, a transparent tubular casing 310, for
example as depicted in FIG. 14, can be used to circumferentially
cover elongated solar cells 402. Because it is important to exclude
air from the solar cell unit 402, additional sealant may be used to
prevent oxidation of the solar cell 402. Alternatively, as
illustrated in FIG. 9, an optional filler layer 330 (for example,
EVA or silicon, etc.) may be used to prevent seepage of oxygen and
water into solar cells 402. In some embodiments, the individually
encased solar cells 402 are assembled into a planar array as
depicted in FIG. 9. FIG. 10 illustrates a solar cell assembly 1000
of the present invention in which counter-electrodes 420,
transparent conductive layers 412, and junctions 410 are pierced,
in the manner illustrated, in order to form two discrete junctions
in parallel. In some embodiments, transparent tubular casing 310,
for example as depicted in FIG. 14, may be used to encase elongated
solar cells 402 with or without optional filler layer 330.
[0138] FIG. 15 illustrates an elongated solar cell 402 in
accordance with the present invention. A transparent tubular casing
310 encases the elongated solar cell 402, leaving only ends of
electrodes 420 exposed to establish suitable electrical
connections. The ends of the elongated solar cell 402 are stripped
and conductive layer 404 is exposed. As in previous embodiments,
back-electrode 404 serves as the first electrode in the assembly
and transparent conductive layer 412 on the exterior surface of
each elongated solar cell 402 serves as the counter-electrode. In
some embodiments in accordance with the present invention as
illustrated in FIG. 15, however, protruding counter-electrodes 420
and electrodes 440, which are attached to the elongated solar cell
402, provide convenient electrical connection.
[0139] In typical embodiments as shown in FIG. 15, there is a first
groove 677-1 and a second groove 677-2 that each runs lengthwise on
opposing sides of elongated solar cell 402. In some embodiments,
counter-electrodes 420 are fitted into grooves 677 in the manner
illustrated in FIG. 15. Typically, such counter-electrodes 420 are
glued into grooves 677 using a conductive ink or conductive glue.
For example, CuPro-Cote (available from Lessemf.com, Albany, N.Y.),
which is a sprayable metallic coating system using a non-oxidizing
copper as a conductor, can be used. In preferred embodiments,
counter-electrodes 420 are fitted in to grooves 677 and then a bead
of conductive ink or conductive glue is applied. As in previous
embodiments, 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.
15. In general, any type of groove 677 that runs along the long
axis of a first solar cell 402 and that can accommodate all or part
of counter-electrode 420 is within the scope of the present
invention. Counter-electrodes 420 conduct current from the combined
layer 410/(415)/412. At the regions at both ends of elongated solar
cell 402, counter-electrodes 420 are sheathed as shown in FIG. 15
so that they are electrically isolated from conductive layer 404.
The ends of protruding counter-electrodes 420, however, are
unsheathed so they can form electric contact with additional
devices.
[0140] In the embodiments as depicted in FIG. 15, a second set of
electrodes 440 are attached to the exposed back-electrode 404. The
second set of electrodes 440 conduct current from back-electrode
404. As illustrated in FIG. 15, an embodiment in accordance with
the present invention has two electrodes 440 attached at two
opposing ends of each elongated solar cell 402. Typically,
electrodes 440 are glued onto back-electrode 404 using a conductive
ink or conductive glue. For example, CuPro-Cote can be used. In
some embodiments, electrodes 440 are glued to layer 404 and then a
bead of conductive ink or conductive glue is applied. Care is taken
so that electrodes 440 are not in electrical contact with layer
410/(415)/412. Additionally, electrodes 440 in the present
invention have a broad range of lengths and widths and shape
characteristics and are by no means limited to the shape of 440
illustrated in FIG. 15. In the embodiments as shown in FIG. 15, the
two electrodes 440 on opposite ends of the elongated solar cell 402
are not on the same side of the solar cell cylinder. The first
electrode 440 is on the bottom side of the elongated solar cell 402
while the second electrode 440 is on the top side of the elongated
solar cell 402. Such an arrangement facilitates the connection of
the solar cells in a serial manner. In some embodiments in
accordance with the present invention, the two electrodes 440 can
be on the same side of elongated solar cell 402.
[0141] In some embodiments, each electrode 440 is made of a thin
strip of conductive material that is attached to conductive layer
404/1304 (FIG. 15). In some embodiments, each electrode 440 is made
of a conductive ribbon of metal (e.g., copper, aluminum, gold,
silver, molybdenum, or an alloy thereof) or a conductive ink. As
will be explained in conjunction with subsequent drawings,
counter-electrode 420 and electrodes 440 are used to electrically
connect elongated solar cells 402, preferably in series.
5.1.2 Transparent Tubular Casing
[0142] A transparent tubular casing 310, as depicted in FIGS. 3A
through 3C, seals a solar cell unit 402 to provide support and
protection to the solar cell. The size and dimensions of
transparent tubular casing 310 are determined by the size and
dimensions of individual solar cells 402 in a solar cell assembly
unit 402. Transparent tubular casing 310 may be made of glass,
plastic or any other suitable material. Examples of materials that
can be used to make transparent tubular casing 310 include, but are
not limited to, glass (e.g., soda lime glass), acrylics such as
polymethylmethacrylate, polycarbonate, fluoropolymer (e.g., Tefzel
or Teflon), polyethylene terephthalate (PET), Tedlar, or some other
suitable transparent material.
5.1.2.1 Transparent Tubular Casing Construction
[0143] In some embodiments, transparent tubular casing 310 is
constructed using blow molding. Blow molding involves clamping the
ends of a softened tube of polymers, which can be either extruded
or reheated, inflating the polymer against the mold walls with a
blow pin, and cooling the product by conduction or evaporation of
volatile fluids in the container. Three general types of blow
molding are extrusion blow molding, injection blow molding, and
stretch blow molding. Extrusion blow molding is used to make items
of weight greater than twelve ounces. Injection blow molding
achieves accurate wall thickness. Stretch blow molding is typically
used for difficult to blow crystalline and crystallizable polymers
such as polypropylene and polyethylene terephthalate. U.S. Pat. No.
2,371,68 describes a process for blow molding (e.g., 602 in FIG.
6A). Other forms of blow molding include low density polyethylene
(LDPE) blow molding, high density polyethylene (HDPE) blow molding
and polypropylene (PP) blow molding
[0144] Extrusion blow molding. As depicted in FIG. 6A, the
extrusion blow molding method comprises a Parison (e.g., 602 in
FIG. 6A) and mold halves that close onto the Parison (e.g., 604 in
FIG. 6A). In extrusion blow molding (EBM), material is melted and
extruded into a hollow tube (e.g., a Parison as depicted in FIG.
6A). The Parison is then captured by closing it into a cooled metal
mold. Air is then blown into the Parison, inflating it into the
shape of the hollow bottle, container or part. After the material
has cooled sufficiently, the mold is opened and the part is
ejected.
[0145] EBM processes consist of either continuous or intermittent
extrusion of the Parison 602. The types of EBM equipment may be
categorized accordingly. Typical continuous extrusion equipment
usually comprises rotary wheel blow molding systems and a shuttle
machinery that transports the finished products from the Parison.
Exemplary intermittent extrusion machinery comprises a
reciprocating screw machinery and an accumulator head machinery.
Basic polymers, such as PP, HDPE, PVC and PET are increasingly
being coextruded with high barrier resins, such as EVOH or Nylon,
to provide permeation resistance to water, oxygen, CO.sub.2 or
other substances.
[0146] Compared to injection molding, blow molding is a low
pressure process, with typical blow air pressures of 25 to 150 psi.
This low pressure process allows the production of economical
low-force clamping stations, while parts can still be produced with
surface finishes ranging from high gloss to textured. The resulting
low stresses in the molded parts also help make the containers
resistant to strain and environmental stress cracking.
[0147] Injection blow molding. In injection blow molding (IBM), as
depicted in FIG. 6B, material is injection molded onto a core pin
(e.g., 612 in FIG. 6B); then the core pin is rotated to a blow
molding station (e.g., 614 in FIG. 6B) to be inflated and cooled.
The process is divided in to three steps: injection, blowing and
ejection. A typical IBM machine is based on an extruder barrel and
screw assembly which melts the polymer. The molten polymer is fed
into a manifold where it is injected through nozzles into a hollow,
heated preform mold (e.g., 614 in FIG. 6B). The preform mold forms
the external shape and is clamped around a mandrel (the core rod,
e.g., 612 in FIG. 6B) which forms the internal shape of the
preform. The preform consists of a fully formed bottle/jar neck
with a thick tube of polymer attached, which will form the
body.
[0148] The preform mold opens and the core rod is rotated and
clamped into the hollow, chilled blow mold. The core rod 612 opens
and allows compressed air into the preform 614, which inflates it
to the finished article shape. After a cooling period the blow mold
opens and the core rod is rotated to the ejection position. The
finished article is stripped off the core rod and leak-tested prior
to packing. The preform and blow mold can have many cavities,
typically three to sixteen depending on the article size and the
required output. There are three sets of core rods, which allow
concurrent preform injection, blow molding and ejection.
[0149] Stretch blow molding In the stretch blow molding (SBM)
process, as depicted in FIG. 6C, the material is first molded into
a "preform," e.g., 628 in FIG. 6C, using the injection molded
process. A typical SBM system comprises a stretch blow pin (e.g.,
622 in FIG. 6C), an air entrance (e.g., 624 in FIG. 6C), mold vents
(e.g., 626 in FIG. 6C), a preform (e.g., 628 in FIG. 6C), and
cooling channels (e.g., 632 in FIG. 6C). These preforms are
produced with the necks of the bottles, including threads (the
"finish") on one end. These preforms are packaged, and fed later,
after cooling, into an EBM blow molding machine. In the SBM
process, the preforms are heated, typically using infrared heaters,
above their glass transition temperature, then blown using high
pressure air into bottles using metal blow molds. Usually the
preform is stretched with a core rod as part of the process (e.g.,
as in position 630 in FIG. 6C). The stretching of some polymers,
such as PET (polyethylene terepthalate), results in strain
hardening of the resin and thus allows the bottles to resist
deforming under the pressures formed by carbonated beverages, which
typically approach 60 psi.
[0150] FIG. 6C shows what happens inside the blow mold. The preform
is first stretched mechanically with a stretch rod. As the rod
travels down low-pressure air of 5 to 25 bar (70 to 350 psi) is
introduced blowing a `bubble`. Once the stretch rod is fully
extended, high-pressure air of up to 40 bar (580 psi) blows the
expanded bubble into the shape of the blow mold.
[0151] Plastic tube manufacturing. In some embodiments, transparent
tubular casing 310 is made of plastic rather than glass. Production
of transparent tubular casing 310 in such embodiments differs from
glass transparent tubular casing 310 production even though the
basic molding mechanisms remain the same. A typical plastic
transparent tubular casing 310 manufacturing process comprises the
following steps: extrusion, heading, decorating, and capping, with
the latter two steps being optional.
[0152] In some embodiments, transparent tubular casing 310 is made
using extrusion molding. A mixture of resin is placed into an
extruder hopper. The extruder is temperature controlled as the
resin is fed through to ensure proper melt of the resin. The
material is extruded through a set of sizing dies that are
encapsulated within a right angle cross section attached to the
extruder. The forming die controls the shape of transparent tubular
casing 310. The formed plastic sleeve cools under blown air or in a
water bath and hardens on a moving belt. After cooling step, the
formed plastic sleeve is ready for cutting to a given length by a
rotating knife.
[0153] The forming die controls the shape of the transparent
tubular casing 310. In some embodiments in accordance with the
present invention, as depicted in FIG. 14, the forming dies are
custom-made such that the shape of transparent tubular casing 310
complements the shape of the solar cell unit 402. The forming die
also controls the wall thickness of the transparent tubular casing
310. In some embodiments in accordance with the present invention,
transparent tubular casing 310 has a wall thickness of 2 mm or
thicker, 1 mm or thicker, 0.5 mm or thicker, 0.3 mm or thicker, or
of any thickness between 0 and 0.3 mm.
[0154] During the production of one open-ended transparent tubular
casing 310, the balance of the manufacturing process can be
accomplished in one of three ways. The most common method in the
United States is the "downs" process of compression, molding the
head onto the tube. In this process, the sleeve is placed on a
conveyor that takes it to the heading operation where the shoulder
of the head is bound to the body of the tube while, at the same
time, the thread is formed. The sleeve is then placed on a mandrel
and transferred down to the slug pick-up station. The hot melt
strip or slug is fused onto the end of the sleeve and then
transferred onto the mold station. At this point, in one operation,
the angle of the shoulder, the thread and the orifice are molded at
the end of the sleeve. The head is then cooled, removed from the
mold, and transferred into a pin conveyor. Two other heading
methods are used in the United States and are found extensively
worldwide: injection molding of the head to the sleeve, and an
additional compression molding method whereby a molten donut of
resin material is dropped into the mold station instead of the hot
melt strip or slug. Transparent tubular casing 310 with one
open-end are suitable to encase solar cell embodiments as depicted
in FIGS. 3, 4, 7, 8, 9, 10 or 11. Plastic tubing with both ends
open may be used to encase solar cell embodiments as depicted in
FIGS. 3 and 15.
[0155] The headed transparent tubular casing 310 is then conveyed
to the accumulator. The accumulator is designed to balance the
heading and decorating operation. From here, the transparent
tubular casing 310 may go to the decorating operation. Inks for the
press are premixed and placed in the fountains. At this point, the
ink is transferred onto a plate by a series of rollers. The plate
then comes in contact with a rubber blanket, picking up the ink and
transferring it onto the circumference of the transparent tubular
casing 310. The wet ink on the tube is cured by ultra-violet light
or heat. In the embodiments in accordance with the present
invention, transparency is required in the tube products so the
color process is unnecessary. However, a similar method may be used
to apply a protective coating to transparent tubular casing
310.
[0156] After decorating, a conveyor transfers the tube to the
capping station where the cap is applied and torqued to the
customer's specifications. The capping step is unnecessary for the
scope of this invention.
[0157] Additional glass fabrication methods. Glass is a preferred
material choice for transparent tubular casing 310 relative to
plastics because glass provides a complete seal against molecular
water from solar cell 402 and therefore provides protection and
helps to maintain the performance and prolong the lifetime of solar
cell 402. Similar to plastics, glass may be made into transparent
tubular casing 310 using the standard blow molding technologies. In
addition, techniques such as casting, extrusion, drawing, pressing,
heat shrinking or other fabrication processes may also be applied
to manufacture suitable glass transparent tubular casing 310 to
circumferentially cover and/or encapsulate solar cells 402. Molding
technologies, in particular micromolding technologies for
microfabrication, are discussed in greater detail in Madou,
Fundamentals of Microfabrication, Chapter 6, pp. 325-379, second
edition, CRC Press, New York, 2002; Polymer Engineering Principles:
Properties, Processes, and Tests for Design, Hanser Publishers, New
York, 1993; and Lee, Understanding Blow Molding, first edition,
Hanser Gardner Publications, Munich, Cincinnati, 2000, each of
which is hereby incorporated herein by reference in its
entirety.
5.1.2.2 Exemplary Materials for Transparent Tubular Casing
[0158] Transparent tubular casing made of glass. In some
embodiments, transparent tubular casing 310 is made of glass. In
its pure form, glass is a transparent, relatively strong,
hard-wearing, essentially inert, and biologically inactive material
that can be formed with very smooth and impervious surfaces. The
present invention contemplates a wide variety of glasses for
transparent tubular casing 310, some of which are described in this
section and others of which are know to those of skill in the
relevant arts. Common glass contains about 70% amorphous silicon
dioxide (SiO.sub.2), which is the same chemical compound found in
quartz, and its polycrystalline form, sand. Common glass is used in
some embodiments of the present invention to make transparent
tubular casing 310. However, common glass is brittle and will break
into sharp shards. Thus, in some embodiments, the properties of
common glass are modified, or even changed entirely, with the
addition of other compounds or heat treatment.
[0159] Pure silica (SiO.sub.2) has a melting point of about
2000.degree. C., and can be made into glass for special
applications (for example, fused quartz). Two other substances are
always added to common glass to simplify processing. One is soda
(sodium carbonate Na.sub.2CO.sub.3), or potash, the equivalent
potassium compound, which lowers the melting point to about
1000.degree. C. However, the soda makes the glass water-soluble,
which is undesirable, so lime (calcium oxide, CaO) is the third
component, added to restore insolubility. The resulting glass
contains about 70% silica and is called a soda-lime glass.
Soda-lime glass is used in some embodiments of the present
invention to make transparent tubular casing 310.
[0160] Besides soda-lime, most common glass has other ingredients
added to change its properties. Lead glass, such as lead crystal or
flint glass, is more `brilliant` because the increased refractive
index causes noticeably more "sparkles", while boron may be added
to change the thermal and electrical properties, as in PYREX.
Adding barium also increases the refractive index. Thorium oxide
gives glass a high refractive index and low dispersion, and was
formerly used in producing high-quality lenses, but due to its
radioactivity has been replaced by lanthanum oxide in modem
glasses. Large amounts of iron are used in glass that absorbs
infrared energy, such as heat absorbing filters for movie
projectors, while cerium(IV) oxide can be used for glass that
absorbs UV wavelengths (biologically damaging ionizing radiation).
Glass having on or more of any of these additives is used in some
embodiments of the present invention to make transparent tubular
casing 310.
[0161] Common examples of glass material include but are not
limited to aluminosilicate, borosilicate (e.g., PYREX, DURAN,
SIMAX), dichroic, germanium/semiconductor, glass ceramic,
silicate/fused silica, soda lime, quartz, chalcogenide/sulphide,
cereated glass, and fluoride glass and transparent tubular casing
310 can be made of any of these materials.
[0162] In some embodiments, transparent tubular casing 310 is made
of soda lime glass. Soda lime glass is softer than borosilicate and
quartz, making scribe cutting easier and faster. Soda Lime glass is
very low cost and easy to mass produce. However, Soda lime glass
has poor thermal shock resistance. Thus, soda lime glass is best
used for transparent tubular casing 310 in thermal environments
where heating is very uniform and gradual. As a result, when solar
cells 402 are encased by transparent tubular casing 310 made from
soda lime glass, such cells are best used in environments where
temperature does not drastically fluctuate.
[0163] In some embodiments, transparent tubular casing 310 are made
of glass material such as borosilicate glass. Trade names for
borosilicate glass include but are not limited to PYREX.RTM.
(Corning), DURAN.RTM. (Schott Glass), and SIMAX.RTM. (Kavalier).
Like most glasses, the dominant component of borosilicate glass is
SiO.sub.2 with boron and various other elements added. Borosilicate
glass is easier to hot work than materials such as quartz, making
fabrication less costly. Material cost for borosilicate glass is
also considerably less than fused quartz. Compared to most glass,
except fused quartz, borosilicate glass has low coefficient of
expansion, three times less than soda lime glass. This makes
borosilicate glass useful in thermal environments, without the risk
of breakage due to thermal shock. Like soda lime glass, a float
process can be used to make relatively low cost optical quality
sheet borosilicate glass in a variety of thickness from less than 1
mm to over 30 mm thick. Relative to quartz, borosilicate glass is
easily moldable. In addition, borosilicate glass has minimum
devitrification when molding and flame working. This means high
quality surfaces can be maintained when molding and slumping.
Borosilicate glass is thermally stable up to 500.degree. C. for
continuous use. Borosilicate glass is also more resistant to
non-fluorinated chemicals than household soda lime glass and
mechanically stronger and harder than soda lime glass. Borosilicate
is usually two to three times more expensive than soda lime
glass.
[0164] Soda lime and borosilicate glass are only given as examples
to illustrate the various aspects of consideration when using glass
material to fabricate transparent tubular casing 310. The preceding
discussion imposes no limitation to the scope of the invention.
Indeed, transparent tubular casing 310 can be made with glass such
as, for example, aluminosilicate, borosilicate (e.g., PYRAX.RTM.,
DURAN.RTM., SIMAX.RTM.), dichroic, germanium/semiconductor, glass
ceramic, silicate/fused silica, soda lime, quartz,
chalcogenide/sulphide, cereated glass and/or fluoride glass.
[0165] Transparent tubular casing made of plastic. In some
embodiments, transparent tubular casing 310 is made of clear
plastic. Plastics are a cheaper alternative to glass. However,
plastic material is in general less stable under heat, has less
favorable optical properties and does not prevent molecular water
from penetrating through transparent tubular casing 310. The last
factor, if not rectified, damages solar cells 402 and severely
reduces their lifetime. Accordingly, in some embodiments, the water
resistant layer described in Section 5.1.1. is used to prevent
water seepage into the solar cells 402 when transparent tubular
casing 310 is made of plastic.
[0166] A wide variety of materials can be used in the production of
transparent tubular casing 310, including, but not limited to,
ethylene vinyl acetate (EVA), perfluoroalkoxy fluorocarbon (PFA),
nylon/polyamide, cross-linked polyethylene (PEX), polyolefin,
polypropylene (PP), polyethylene terephtalate glycol (PETG),
polytetrafluoroethylene (PTFE), thermoplastic copolymer (for
example, ETFE.RTM., which is a derived from the polymerization of
ethylene and tetrafluoroethylene: TEFLON.RTM. monomers),
polyurethane/urethane, polyvinyl chloride (PVC), polyvinylidene
fluoride (PVDF), Tygon.RTM., Vinyl, and Viton.RTM..
5.1.2.3 Available Commercial Sources of Transparent Tubing
Products
[0167] There are ample commercial sources for obtaining or custom
manufacturing transparent tubular casing 310. Technologies for
manufacturing plastic or glass tubing have been standardized and
customized plastic or glass tubing are commercially available from
numerous companies. A search on GlobalSpec database for "clear
round plastic or glass tubing," a web center of engineering
resources (www.globalspec.com; GlobalSpec Inc. Troy, N.Y.), results
in over 950 catalog products. Over 180 companies make specialty
pipe, tubing, hose and fittings. For example, Clippard Instrument
Laboratory, Inc. (Cincinnati, Ohio) provides Nylon, Urethane or
Plastic Polyurethane tubing that is as thin as 0.4 mm. Coast Wire
& Plastic Tech., Inc. (Carson, Calif.) manufactures a
comprehensive line of polyvinylidene fluoride clear round plastic
tubing product under the trademark SUMIMARK.TM.. Their product has
a wall thickness as thin as 0.3 mm. Parker Hannifin/Fluid
Connectors/Parflex Division (Ravenna, Ohio) provides vinyl, plastic
polyurethane, polyether base, or polyurethane based clear plastic
tubing of 0.8 mm or 1 mm thickness. Similar polyurethane products
may also be found in Pneumadyne, Inc (Plymouth, Minn.).
Saint-Gobain High-Performance Materials (U.S.A) further provides a
line of 30 Tygon.RTM. tubing products of 0.8 mm in thickness.
Vindum Engineering, Inc. (San Ramon, Calif.) also provides clear
PFA Teflon tube of 0.8 mm in thickness. NewAge Industries, Inc.
(Southampton, Pa.) provides 63 clear round plastic tubing products
that have a wall thickness of 1 mm or thinner. In particular,
VisiPak Extrusion (Arnold, Mo.), a division of Sinclair & Rush,
Inc., provides clear round plastic tubing product as thin as 0.5
mm. Cleartec Packaging (St. Louis, Mo., a division of MOCAP Inc.)
manufactures clear round plastic tubing as thin as 0.3 mm.
[0168] In addition, numerous companies can manufacture clear round
plastic or glass tubing with customized specification such as even
thinner wall. Some examples are Elasto Proxy Inc. (Boisbriand,
Canada), Flex Enterprises, Inc. (Victor, N.Y.), Grob, Inc.
(Grafton, Wis.), Mercer Gasket & Shim (Bellmawr, N.J.), New
England Small Tube Corporation (Litchfield, N.H.), Precision
Extrusion, Inc. (Glens Falls, N.Y.), and PSI Urethanes, Inc.
(Austin, Tex.).
5.1.3 Integrating Solar Cells into Transparent Tubular Casings
[0169] In the present invention, gaps or spaces between transparent
tubular casing 310 and solar cell 402 are eliminated in order to
avoid adverse effects such as oxidation. Thus, in the present
invention, there is no annular space between the inside wall of
transparent tubular casing 310 and the outer wall of solar cell
402. In some embodiments (e.g., FIG. 3B), a filler layer 330 is
provided to seal a solar cell unit 402 from adverse exposure to
water or oxygen. In some embodiments, where transparent tubular
casing 310 is made of glass and therefore exposure to water is no
longer a concern, a filler layer 330 may be eliminated such that
solar cells 402 directly contact transparent tubular casing
310.
[0170] In some embodiments, custom-designed transparent tubular
casing 310, made of either glass or plastics or other suitable
transparent material, may be used to encase the corresponding
embodiments of solar cell 402 to achieve tight fitting and better
protection. FIG. 14 depicts exemplary embodiments of transparent
tubular casing 310 that provides proper encapsulation to the solar
cell embodiments depicted in FIGS. 4, 7, 8, 9, 10, 11 and 13.
[0171] Rod or cylindrical shaped solar cells 402, individually
encased by transparent tubular casing 310 (for example those shown
in FIGS. 3, 4, 7, 8, 9, 10, 13, and 15), can be assembled into
solar cell assemblies of any shape and size. In some embodiments,
the assembly can be bifacial arrays 400 (FIG. 4), 700 (FIG. 7), 800
(FIG. 8), 900 (FIG. 9), or 1000 (FIG. 10). There is no limit to the
number of solar cells 402 in this plurality (e.g., 10 or more, 100
or more, 1000 or more, 10,000 or more, between 5,000 and one
million solar cells 402, etc.).
[0172] Alternatively, instead of being encapsulated individually
and then being assembled together for example into planar arrays,
solar cells 402 may also be encapsulated as arrays. For example, as
depicted in FIG. 7C, multiple transparent tubular casings 310 may
be manufactured as fused arrays. This method is advantageous in
that little or no additional connection between the individual
solar cells 402 is required. There is no limit to the number of
transparent tubular casings 310 in the assembly as depicted in FIG.
7C (e.g., 10 or more, 100 or more, 1000 or more, 10,000 or more,
between 5,000 and one million transparent tubular casings 310,
etc.). A solar cell assembly is further completed by loading
elongated solar cells 402 (for example 402 in FIG. 4A) into all or
a portion of the transparent tubular casing 310 in the array of
tubular casings.
5.1.3.1 Integrating Solar Cells having a Filler Layer into
Transparent Tubular Casings
[0173] In some embodiments in accordance with the present
invention, a solar cell 402 having a filler layer coated thereon is
assembled into a transparent tubular casing 310. In some
embodiments in accordance with the present invention, filler layer
330 comprises one or more of the properties of: electrical
insulation, oxidation eliminating effect, water proofing, and/or
physical protection of transparent conductive layer 412 of solar
cell 402 during assembly of solar cell units.
[0174] In some embodiments in accordance with the present
invention, an elongated solar cell 402, optional filler layer 330,
and a transparent tubular casing 310 are assembled using a suction
loading method illustrated in FIG. 20A. Transparent tubular casing
310, made of transparent glass, plastics or other suitable
material, is sealed at one end 2002. Materials that are used to
form filler layer 330, for example, silicone gel, is poured into
the sealed transparent tubular casing 310. An example of a silicone
gel is Wacker SILGELL.TM. 612 (Wacker-Chemie GmbH, Munich,
Germany). Wacker SILGEL.RTM. 612 is a pourable, addition-curing,
RTV-2 silicone rubber that vulcanizes at room temperature to a soft
silicone gel. Still another example of silicone gel is SYLGARD.RTM.
silicone elastomer (Dow Corning). Another example of a silicone gel
is Wacker ELASTOSIL.RTM. 601 (Wacker-Chemie GmbH, Munich, Germany).
Wacker ELASTOSIL.RTM. 601 is a pourable, addition-curing, RTV-2
silicone rubber. Referring to FIG. 22, silicones can be considered
a molecular hybrid between glass and organic linear polymers. As
shown in FIG. 22, if there are no R groups, only oxygen, the
structure is inorganic silica glass (called a Q-type Si). If one
oxygen is substituted with an R group (e.g. methyl, ethyl, phenyl,
etc.) a resin or silsequioxane (T-type Si) material is formed.
These silsequioxanes are more flexible than the Q-type materials.
Finally, if two oxygen atoms are replaced by organic groups a very
flexible linear polymer (D-type Si) is obtained. The last structure
shown (M-type Si) has three oxygen atoms replaced by R groups,
resulting in an end cap structure. Because the backbone chain
flexibility is increasing as R groups are added, the modulus of the
materials and their coefficients of thermal expansion (CTE) also
change. In some embodiments of the present invention the silicone
used to form filler layer is a Q-type silicone, a silsequioxane, a
D-type silicon, or an M-type silicon. The elongated solar cell 402
is then loaded into transparent tubular casing 310. Optional
suction force may be applied at the open end 2004 of transparent
tubular casing 310 to draw the filler material upwards to
completely fill the space between solar cell 402 and transparent
tubular casing 310.
[0175] In some embodiments in accordance with the present
invention, an elongated solar cell 402, filler layer 330, and a
transparent tubular casing 310 may be assembled using the pressure
loading method illustrated in FIG. 20B. Transparent tubular casing
310, made of transparent glass, plastics or other suitable
material, is dipped in container 2008 containing optional filler
layer material (e.g., silicone gel) used to form optional filler
layer 330. Elongated solar cell 402 is then loaded into transparent
tubular casing 310. Pressure force is applied at filler material
surface 2006 to push the filler material upwards to completely fill
the space between solar cell 402 and transparent tubular casing
310.
[0176] In yet other embodiments in accordance with the present
invention, an elongated solar cell 402, filler layer 330 and a
transparent tubular casing 310 is assembled using the
pour-and-slide loading method depicted in FIG. 20C. A transparent
tubular casing 310, made of transparent glass, plastics or other
suitable material, is sealed at one end 2002. A container 2010,
containing filler material (e.g., silicone gel), is used to pour
the filler layer material into the sealed transparent tubular
casing 310 while solar cell 402 is simultaneously slid into
transparent tubular casing 310. The filler material that is being
poured into transparent tubular casing 310 fills up the space
between solar cell 402 and transparent tubular casing 310.
Advantageously, the filler material that is being poured down the
side of transparent tubular casing 310 provides lubrication to
facilitate the slide-loading process.
5.1.3.2 Integrating Solar Cells without an Optional Filler Layer
into Transparent Tubular Casings
[0177] In some embodiments in accordance with the present
invention, a tubular casing 310 is assembled onto solar cell 402
without a filler layer 330. In these embodiments, tubular casing
310 may directly contact solar cell 402. Tight packing and tubular
casing 310 against solar cell 402 may be achieved by using one of
the following methods. It will be appreciated that the methods for
assembling a solar cell unit 300 described in this section can be
used with solar cells 402 that are encased with a filler layer 330.
However, in such embodiments, layer 330 must be deposited onto
transparent conductive layer 412 of solar cells 402 prior to
integrating the solar cell 402 with a transparent tubular
casing.
[0178] Heat Shrink Loading. In some embodiments, transparent
tubular casing 310 is heat shrinked onto solar cell 402. The heat
shrink method may be used to form both plastic and glass
transparent tubular casings 310. For example, heat-shrinkable
plastic tubing made of polyolefin, fluoropolymer (PVC, FEP, PTFE,
KYNAR.RTM. PVDF), chlorinated polyolefin (Neoprene) and highly
flexible elastomer (Viton.RTM.) heat-shrinkable tubing may be used
to form transparent tubular casing 310. Among such materials,
fluoropolymers offer increased lubricity for easy sliding, and low
moisture absorption for enhanced dimensional stability. Three such
materials are commercially accessible: PTFE
(polytetrafluoroethylene), FEP (fluorinated ethylene propylene) and
PVDF (polyvinylidene fluoride, tradename Kynar.RTM.). Transparent
heat-shrinkable plastic tubing is available. In some embodiments,
the heat shrink tubing is available in an expandable range of 2:1
to 3:1. In some embodiments, the heat shrink ratio of the tubing
material is smaller than 2:1, for example, fluorinated
ethylene-propylene (FEP) at 1.3:1. In other embodiments, a heat
shrink tubing suitable for the manufacture of transparent tubular
casing 310 may have a heat shrink ratio greater than 3:1.
[0179] Injection molding to construct transparent tubular casing.
In some embodiments, transparent tubular casing 310 may be
circumferentially disposed onto solar cell 402 by using the method
of injection molding. A more detailed description of the method is
already included above. In these embodiments, solar cells 402 may
be used as the preformed mold and transparent tubular casing 310
(e.g., made of plastic material) is directly formed on the outer
surface of solar cells 402. Plastic material does not completely
seal molecular water from solar cells 402. Because water interferes
with the function of a solar cell 402, it is therefore important to
make the solar cell 402 resistant to water. In the embodiments
where plastic transparent tubular casings 310 are used to cover
solar cells 402, this is accomplished by covering either the solar
cell 402 or transparent tubular casing 310 with one or more layers
of transparent water-resistant coating 340 (FIG. 21). In some
embodiments, both solar cell 402 and transparent tubular casing 310
are coated with one or more layers of transparent water-resistant
coating 340 to extend the functional life time of the solar cell
unit 300. In other embodiments, an optional antireflective coating
350 is also disposed on transparent tubular casing 310 to maximize
solar cell efficiency.
[0180] Liquid Coating Followed by Polymerization. In some
embodiments, solar cell 402 is dipped in a liquid-like suspension
or resin and subsequently exposed to catalyst or curing agent to
form transparent tubular casing 310 through a polymerization
process. In such embodiments, materials used to form transparent
tubular casing 310 comprise silicone, poly-dimethyl siloxane
(PDMS), silicone gel, epoxy, acrylics, and any combination or
variation thereof
5.1.4 Optical and Chemical Properties of the Materials for
Transparent Tubular Casing and the Optional Filler Layer
[0181] In order to maximize input of solar radiation, any layer
outside a solar cell 402 (for example, optional filler layer 330 or
transparent tubular casing 310) should not adversely affect the
properties of incident radiation on the solar cell. There are
multiple factors to consider in optimizing the efficiency of solar
cells 402. A few significant factors will be discussed in detail in
relation to solar cell production.
[0182] Transparency. In order to establish maximized input into
solar cell absorption layer (e.g., semiconductor junction 410),
absorption of the incident radiation by any layer outside a solar
cell 402 should be avoided or minimized. This transparency
requirement varies as a function of the absorption properties of
the underlying semiconductor junction 410 of solar cells 402. In
general, transparent tubular casing 310 and optional filler layer
330 should be as transparent as possible to the wavelengths
absorbed by the semiconductor junction 410. For example, when the
semiconductor junction 410 is based on CIGS, materials used to make
transparent tubular casing 310 and optional filler layer 330 should
be transparent to light in the 500 nm to 1200 nm wavelength
range.
[0183] Ultraviolet Stability. Any material used to construct a
layer outside solar cell 402 should be chemically stable and, in
particular, stable upon exposure to UV radiation. More
specifically, such material should not become less transparent upon
UV exposure. Ordinary glass partially blocks UVA (wavelengths 400
and 300 nm) and it totally blocks UVC and UVB (wavelengths lower
than 300 nm). The UV blocking effect of glass is usually due to
additives, e.g. sodium carbonate, in glass. In some embodiments,
additives in transparent tubular casings 310 made of glass can
render the casing 310 entirely UV protective. In such embodiments,
because the transparent tubular casing 310 provides complete
protection from UV wavelengths, the UV stability requirements of
the underlying optional filler layer 330 are reduced. For example,
EVA, PVB, TPU (urethane), silicones, polycarbonates, and acrylics
can be adapted to form a filler layer 330 when transparent tubular
casing 310 is made of UV protective glass. Alternatively, in some
embodiments, where transparent tubular casing 310 is made of
plastic material, UV stability requirement should be strictly
followed.
[0184] Plastic materials that are sensitive to UV radiation should
not be used as transparent tubular casing 310 because yellowing of
the material and/or optional filler layer 330 blocks radiation
input into the solar cells 402 and reduces their efficiency. In
addition, cracking of transparent tubular casing 310 due to UV
exposure permanently damages solar cells 402. For example,
fluoropolymers like ETFE, and THV (Dyneon) are UV stable and highly
transparent, while PET is transparent, but not sufficiently UV
stable. In some embodiments, transparent tubular casing 310 is made
of fluoropolymer based on monomers of tetrafluoroethylene,
hexafluoropropylene and vinylidene fluoride. In addition, polyvinyl
chloride ("PVC" or "vinyl"), one of the most common synthetic
materials, is also sensitive to UV exposure. Methods have been
developed to render PVC UV-stabilized, but even UV stabilized PVC
is typically not sufficiently durable (for example, yellowing and
cracking of PVC product will occur over relative short term usage).
Urethanes are better suited, but depend on the exact chemical
nature of the polymer backbone. Urethane material is stable when
the polymer backbone is formed by less reactive chemical groups
(e.g., aliphatic or aromatic). On the other hand when the polymer
backbone is formed by more reactive groups (e.g., double bonds),
yellowing of the material occurs as a result of UV-catalyzed
breakdown of the double bonds. Similarly, EVA will yellow and so
will PVB upon continued exposure to UV light. Other options are
polycarbonate (can be stabilized against UV for up to 10 years OD
exposure) or acrylics (inherently UV stable).
[0185] Reflective Properties. Referring to FIG. 21, an incident
beam L.sub.1 hits the surface of transparent tubular casing 310.
Part of the incident beam L.sub.1 is reflected as L.sub.2 while the
remainder of incident beam L.sub.1 (e.g., as refracted beam L.sub.3
in FIG. 21) travels through transparent tubular casing 310. In some
embodiments in accordance with the present invention, the refracted
beam L.sub.3 directly hits transparent conductive layer 412 of
solar cell 402 (e.g., when optional filler layer 330 is absent).
Alternatively, when filler layer 330 is present, as depicted in
FIG. 21, L.sub.3 hits the outer surface of the filler layer 330,
and the processes of reflection and refraction is repeated as it
was when L.sub.1 hit the surface of transparent tubular casing 310,
with some of L.sub.3 reflected into filler layer 330 and some of
L.sub.3 refracted by filler layer 330.
[0186] In order to maximize input of solar radiation, reflection at
the outer surface of transparent tubular casing 310 should be
minimized. Antireflective coating, either as a separate layer 350
or in combination with the water resistant coating 340, may be
applied on the outside of transparent tubular casing 310. In some
embodiments, this antireflective coating is made of MgF.sub.2. In
some embodiments, this antireflective coating is made of silicon
nitrate or titanium nitrate. In other embodiments, this
antireflective coating is made of one or more layers of silicon
monoxide (SiO). For example, shiny silicon can act as a mirror and
reflects more than thirty percent of the light that shines on it. A
single layer of SiO reduces surface reflection to about ten
percent, and a second layer of SiO can lower the reflection to less
than four percent. Other organic antireflective materials, in
particular, one which prevents back reflection from the surface of
or lower layers in the semiconductor device and eliminates the
standing waves and reflective notching due to various optical
properties of lower layers on the wafer and the photosensitive
film, are disclosed in U.S. Pat. No. 6,803,172, which is hereby
incorporated by reference herein in its entirety. Additional
antireflective coating materials and methods are disclosed in U.S.
Pat. Nos. 6,689,535; 6,673,713; 6,635,583; 6,784,094; and
6,713,234, each of which is hereby incorporated herein by reference
in its entirety.
[0187] Alternatively, the outer surface of transparent tubular
casing 310 may be textured to reduce reflected radiation. Chemical
etching creates a pattern of cones and pyramids, which capture
light rays that might otherwise be deflected away from the cell.
Reflected light is redirected down into the cell, where it has
another chance to be absorbed. Material and methods for creating an
anti-reflective layer by etching or by a combination of etching and
coating techniques are disclosed in U.S. Pat. Nos. 6,039,888;
6,004,722; and 6,221,776; each of which is hereby incorporated
herein by reference in its entirety.
[0188] Refractive Properties. As depicted in FIG. 21, part of
incident beam L.sub.1 is refracted as refracted beam L.sub.3. How
much and to which direction incident beam L.sub.1 is bent from its
path is determined by the refractive indices of the media in which
beams L.sub.1 and L.sub.3 travel. Snell's law specifies:
.eta..sub.1 sin(.theta..sub.1)=.eta..sub.2 sin(.theta..sub.2),
where .eta..sub.1 and .eta..sub.2 are the refractive indices of the
two bordering media 1 and 2 while .theta..sub.1 and .theta..sub.2
represent the angle of incidence and the angle of refraction,
respectively.
[0189] In FIG. 21, the first refraction process occurs when
incident beam L.sub.1 travels from air through transparent tubular
casing 310 as L.sub.3. Ambient air has a refractive index around 1
(vacuum space has a refractive index of 1, which is the smallest
among all known materials), which is much smaller than the
refractive index of glass material (ranging from 1.4 to 1.9 with
the commonly used material having refractive indices around 1.5) or
plastic material (around 1.45). Because .eta..sub.air is always
much smaller than .eta..sub.310 whether tubular casing is formed by
glass or plastic material, the refractive angle .theta..sub.310 is
always much smaller than the incident angle .theta..sub.air, i.e.,
the incident beam is always bent towards solar cell 402 as it
travels through transparent tubular casing 310.
[0190] In the presence of a filler layer 330, beam L.sub.3 becomes
the new incident beam when it travels through the filler layer 330.
Ideally, according to Snell's law and the preceding analysis, the
refractive index of the filler layer 330 (e.g., .eta..sub.310 in
FIG. 21) should be larger than the refractive index of transparent
tubular casing 310 so that the refracted beam of incident beam
L.sub.3 will also be bent towards solar cell 402. In this ideal
situation, every incident beam on transparent tubular casing 310
will be bent towards solar cell 402 after two reflection processes.
In practice, however, optional filler layer 330 is made of a
fluid-like material (albeit sometimes very viscous fluid-like
material) such that loading of solar cells 402 into transparent
tubular casing 310 may be achieved as described above. In practice,
efficient solar radiation absorption is achieved by choosing filler
material that has refractive index close to those of transparent
tubular casing 310. In some embodiments, materials that form
transparent tubular casing 310 comprise transparent materials
(either glass or plastic or other suitable materials) with
refractive indices around 1.5. For example, fused silica glass has
a refractive index of 1.46. Borosilicate glass materials have
refractive indices between 1.45 and 1.55 (e.g., PYRAX.RTM. glass
has a refractive index of 1.47). Flint glass materials with various
amounts of lead additive have refractive indices between 1.5 and
1.9. Common plastic materials have refractive indices between 1.46
and 1.55.
[0191] Exemplary materials with the appropriate optical properties
for forming filler layer 330 further comprise silicone,
polydimethyl siloxane (PDMS), silicone gel, epoxy, and acrylic
material. Because silicone-based adhesives and sealants have a high
degree of flexibility, they lack the strength of other epoxy or
acrylic resins. Transparent tubular casing 310, optional filler
layer 330, optional antireflective layer 350, water-resistant layer
340, or any combination thereof form a package to maximize and
maintain solar cell 402 efficiency, provide physical support, and
prolong the life time of solar cell units 402.
[0192] In some embodiments, glass, plastic, epoxy or acrylic resin
may be used to form transparent tubular casing 310. In some
embodiments, optional antireflective 350 and/or water resistant
coating 340 are circumferentially disposed on transparent tubular
casing 310. In some such embodiments, filler layer 330 is formed by
softer and more flexible optically suitable material such as
silicone gel. For example, in some embodiments, filler layer 330 is
formed by a silicone gel such as a silicone-based adhesives or
sealants. In some embodiments, filler layer 330 is formed by GE RTV
615 Silicone. RTV 615 is an optically clear, two-part flowable
silicone product that requires SS4120 as primer for polymerization.
(RTV615-1P), both available from General Electric (Fairfield,
Conn.). Silicone-based adhesives or sealants are based on tough
silicone elastomeric technology. The characteristics of
silicone-based materials, such as adhesives and sealants, are
controlled by three factors: resin mixing ratio, potting life and
curing conditions.
[0193] Advantageously, silicone adhesives have a high degree of
flexibility and very high temperature resistance (up to 600.degree.
F.). Silicone-based adhesives and sealants have a high degree of
flexibility. Silicone-based adhesives and sealants are available in
a number of technologies (or cure systems). These technologies
include pressure sensitive, radiation cured, moisture cured,
thermo-set and room temperature vulcanizing (RTV). In some
embodiments, the silicone-based sealants use two-component addition
or condensation curing systems or single component (RTV) forms. RTV
forms cure easily through reaction with moisture in the air and
give off acid fumes or other by-product vapors during curing.
[0194] Pressure sensitive silicone adhesives adhere to most
surfaces with very slight pressure and retain their tackiness. This
type of material forms viscoelastic bonds that are aggressively and
permanently tacky, and adheres without the need of more than finger
or hand pressure. In some embodiments, radiation is used to cure
silicone-based adhesives. In some embodiments, ultraviolet light,
visible light or electron bean irradiation is used to initiate
curing of sealants, which allows a permanent bond without heating
or excessive heat generation. While UV-based curing requires one
substrate to be UV transparent, the electron beam can penetrate
through material that is opaque to UV light. Certain silicone
adhesives and cyanoacrylates based on a moisture or water curing
mechanism may need additional reagents properly attached to the
solar cell 402 without affecting the proper functioning of solar
cells 402. Thermo-set silicone adhesives and silicone sealants are
cross-linked polymeric resins cured using heat or heat and
pressure. Cured thermo-set resins do not melt and flow when heated,
but they may soften. Vulcanization is a thermosetting reaction
involving the use of heat and/or pressure in conjunction with a
vulcanizing agent, resulting in greatly increased strength,
stability and elasticity in rubber-like materials. RTV silicone
rubbers are room temperature vulcanizing materials. The vulcanizing
agent is a cross-linking compound or catalyst. In some embodiments
in accordance with the present invention, sulfur is added as the
traditional vulcanizing agent.
[0195] In some embodiments, for example, when optional filler layer
330 is absent, epoxy or acrylic material may be applied directly
over solar cell 402 to form transparent tubular casing 310
directly. In such embodiments, care is taken to ensure that the
non-glass transparent tubular casing 310 is also equipped with
water resistant and/or antireflective properties to ensure
efficient operation over a reasonable period of usage time.
[0196] Electrical Insulation. An important characteristics of
transparent tubular casing 310 and optional filler layer 330 is
that these layers should provide complete electrical insulation. No
conductive material should be used to form either transparent
tubular casing 310 or optional filler layer 330.
[0197] Dimension requirement. The combined width of each of the
layers outside solar cell 402 (e.g., the combination of transparent
tubular casing 310 and/or optional filler layer 330) in some
embodiments is:
r i .gtoreq. r o .eta. outer ring ##EQU00001##
where, referring to FIG. 3B,
[0198] r.sub.i is the radius of solar cell 402, assuming that
semiconductor junction 410 is a thin-film junction;
[0199] r.sub.o is the radius of the outermost layer of transparent
tubular casing 310 and/or optional filler layer 330; and
[0200] .eta..sub.outer ring is the refractive index of the
outermost layer of transparent tubular casing 310 and/or optional
filler layer 330.
As noted above, the refractive index of many of the materials used
to make transparent tubular casing 310 and/or optional filler layer
330 is about 1.5. Thus, in typical embodiments, values of r.sub.o
are permissible that are less than 1.5*r.sub.i. This constraint
places a boundary on allowable thickness for the combination of
transparent tubular casing 310 and/or optional filler layer
330.
5.1.3.5 Additional Methods for Forming Transparent Tubular
Casing
[0201] In some embodiments, transparent tubular casing 310 is
formed on an underlying layer (e.g., is formed on transparent
conductive layer 412, filler layer 330 or a water resistant layer)
by spin coating, dip coating, plastic spraying, casting, Doctor's
blade or tape casting, glow discharge polymerization, or UV curing.
These technologies are discussed in greater detail in Madou,
Fundamentals of Microfabrication, Chapter 3, pp. 159-161, second
edition, CRC Press, New York, 2002, which is hereby incorporated by
reference in its entirety. Casting is particularly suitable in
instances where transparent tubular casing 310 is formed from
acrylics or polycarbonates. UV curing is particularly suitable in
instances where transparent tubular casing 310 is formed from an
acrylic.
5.2 Exemplary Semiconductor Junctions
[0202] Referring to FIG. 5A, in one embodiment, semiconductor
junction 410 is a heterojunction between an absorber layer 502,
disposed on back-electrode 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, transparent conductive 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, transparent
conductive layer 412 is p.sup.+-doped. In some embodiments, the
semiconductors listed in Pandey, Handbook of Semiconductor
Electrodeposition, Marcel Dekker Inc., 1996, Appendix 5, which is
hereby incorporated by reference herein 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.sub.2 Materials
[0203] 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.
[0204] 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.
[0205] In some embodiments, absorber layer 502 is
copper-indium-gallium-diselenide (CIGS). Such a layer is also known
as Cu(InGa)Se.sub.2. 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. Such semiconductor junctions 410 are
described in Chapter 13 of Handbook of Photovoltaic Science and
Engineering, 2003, Luque and Hegedus (eds.), Wiley & Sons, West
Sussex, England, Chapter 12, which is hereby incorporated by
reference in its entirety. In some embodiments, CIGS is deposited
using techniques disclosed in Beck and Britt, Final Technical
Report, January 2006, NREL/SR-520-39119; and Delahoy and Chen,
August 2005, "Advanced CIGS Photovoltaic Technology," subcontract
report; Kapur et al., January 2005 subcontract report,
NREL/SR-520-37284, "Lab to Large Scale Transition for Non-Vacuum
Thin Film CIGS Solar Cells"; Simpson et al., October 2005
subcontract report, "Trajectory-Oriented and Fault-Tolerant-Based
Intelligent Process Control for Flexible CIGS PV Module
Manufacturing," NREL/SR-520-38681; and Ramanathan et al., 31.sup.st
IEEE Photovoltaics Specialists Conference and Exhibition, Lake
Buena Vista, Fla., Jan. 3-7, 2005, each of which is hereby
incorporated by reference herein in its entirety.
[0206] In some embodiments CIGS absorber layer 502 is grown on a
molybdenum back-electrode 404 by evaporation from elemental sources
in accordance with a three stage process described in Ramanthan et
al., 2003, "Properties of 19.2% Efficiency ZnO/CdS/CuInGaSe.sub.2
Thin-film Solar Cells," Progress in Photovoltaics: Research and
Applications 11, 225, which is hereby incorporated by reference
herein in its entirety. In some embodiments layer 504 is a
ZnS(O,OH) buffer layer as described, for example, in Ramanathan et
al., Conference Paper, "CIGS Thin-Film Solar Research at NREL: FY04
Results and Accomplishments," NREL/CP-520-37020, January 2005,
which is hereby incorporated by reference herein in its
entirety.
[0207] In some embodiments, layer 502 is between 0.5 .mu.m and 2.0
.mu.m thick. In some embodiments, the composition ratio of
Cu/(In+Ga) in layer 502 is between 0.7 and 0.95. In some
embodiments, the composition ratio of Ga/(In+Ga) in layer 502 is
between 0.2 and 0.4. In some embodiments the CIGS absorber has a
<110> crystallographic orientation. In some embodiments the
CIGS absorber has a <112> crystallographic orientation. In
some embodiments the CIGS absorber is randomly oriented.
5.2.2 Semiconductor Junctions Based on Amorphous Silicon or
Polycrystalline Silicon
[0208] 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.
[0209] 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.
[0210] 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.
[0211] 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.
[0212] In some embodiments, of the present invention, semiconductor
junction 410 is a tandem junction. Tandem junctions are described
in, for example,
[0213] Kim et al., 1989, "Lightweight (AlGaAs)GaAs/CuInSe2 tandem
junction solar cells for space applications," Aerospace and
Electronic Systems Magazine, IEEE Volume 4, Issue 11, November 1989
Page(s):23-32; Deng, 2005, "Optimization of a-SiGe based triple,
tandem and single-junction solar cells Photovoltaic Specialists
Conference, 2005 Conference Record of the Thirty-first IEEE 3-7
Jan. 2005 Page(s):1365-1370; Arya et al., 2000, Amorphous silicon
based tandem junction thin-film technology: a manufacturing
perspective," Photovoltaic Specialists Conference, 2000. Conference
Record of the Twenty-Eighth IEEE 15-22 Sep. 2000 Page(s):1433-1436;
Hart, 1988, "High altitude current-voltage measurement of GaAs/Ge
solar cells," Photovoltaic Specialists Conference, 1988, Conference
Record of the Twentieth IEEE 26-30 Sep. 1988 Page(s):764-765 vol.
1; Kim, 1988, "High efficiency GaAs/CuInSe2 tandem junction solar
cells," Photovoltaic Specialists Conference, 1988, Conference
Record of the Twentieth IEEE 26-30 Sep. 1988 Page(s):457-461 vol.
1; Mitchell, 1988, "Single and tandem junction CuInSe2 cell and
module technology," Photovoltaic Specialists Conference, 1988.,
Conference Record of the Twentieth IEEE 26-30 Sep. 1988
Page(s):1384-1389 vol. 2; and Kim, 1989, "High specific power
(AlGaAs)GaAs/CuInSe2 tandem junction solar cells for space
applications," Energy Conversion Engineering Conference, 1989,
IECEC-89, Proceedings of the 24.sup.th Intersociety 6-11 Aug. 1989
Page(s):779-784 vol. 2, each of which is hereby incorporated by
reference herein in its entirety.
5.2.3 Semiconductor Junctions Based on Gallium Arsenide and Other
Type III-V Materials
[0214] In some embodiments, semiconductor junctions 410 are based
upon gallium arsenide (GaAs) or other ITT-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.
[0215] Furthermore, in some embodiments semiconductor junction 410
is a hybrid multijunction solar cell such as a GaAs/Si mechanically
stacked multijunction as described by Gee and Virshup, 1988,
20.sup.th IEEE Photovoltaic Specialist Conference, IEEE Publishing,
N.Y., p. 754, which is hereby incorporated by reference herein 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 herein 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 herein in its
entirety.
5.2.4 Semiconductor Junctions Based on Cadmium Telluride and Other
Type II-VI Materials
[0216] 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
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
[0217] While semiconductor junctions 410 that are made from thin
film semiconductor films are preferred, the invention is not so
limited. In some embodiments semiconductor junctions 410 [[is]] are
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 herein in its entirety.
5.3 Albedo Embodiments
[0218] The solar cell design of the present invention is
advantageous because it can collect light through the entire
circumferential surface. Accordingly, in some embodiments of the
present invention, these solar cell assemblies (e.g., solar cell
assembly 400, 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
herein 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. In general, any Lambertian or diffuse reflector surface
can be used to provide a high albedo surface.
[0219] 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.
[0220] 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.
[0221] 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 herein 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.
In some embodiments, internal or external reflectors are
implemented in the solar cell assembly to take advantage of the
albedo effect and enhance light input into the solar cell assembly.
An exemplary embodiment of the internal reflectors (e.g.,
reflectors 1404) is depicted in FIG. 16. More description of albedo
surfaces that can be used in conjunction with the present invention
is disclosed in U.S. patent application Ser. No. 11/315,523, which
is hereby incorporated by reference in its entirety.
5.4 Dual Layer Core Embodiments
[0222] 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 inner core can be referred to as a substrate 403 while the
outer core can be referred to as back-electrode 404 in such
embodiment. In such embodiments, the outer conductive core is
circumferentially disposed on substrate 403. In such embodiments,
substrate 403 is typically nonconductive whereas the outer core is
conductive. Substrate 403 has an elongated shape consistent with
other embodiments of the present invention. For instance, in one
embodiment, substrate 403 is made of glass fibers in the form of a
wire. In some embodiments, substrate 403 is an electrically
conductive nonmetallic material. However, the present invention is
not limited to embodiments in which substrate 403 is electrically
conductive because the outer core can function as the electrode. In
some embodiments, substrate 403 is tubing (e.g., plastic or glass
tubing).
[0223] In some embodiments, substrate 403 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, substrate 403 is made
of polyamide-imide (e.g., Torlon.RTM. PAI, Solvay Advanced
Polymers, Alpharetta, Ga.).
[0224] In some embodiments, substrate 403 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, substrate 403 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.
[0225] In some embodiments, substrate 403 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 herein in its
entirety. In still other embodiments, substrate 403 is made of
cross-linked polystyrene. One example of cross-linked polystyrene
is Rexolite.RTM. (C-Lec Plastics, Inc). Rexolite is a thermoset, in
particular a rigid and translucent plastic produced by cross
linking polystyrene with divinylbenzene.
[0226] In some embodiments, substrate 403 is a polyester wire
(e.g., a MYLAR.RTM. wire). MYLAR.RTM. is available from DuPont
Teijin Films (Wilmington, Del.). In still other embodiments,
substrate 403 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).
[0227] In still other embodiments, substrate 403 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.
[0228] In some embodiments, substrate 403 is made of polyethylene.
In some embodiments, substrate 403 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 herein in its entirety. In some
embodiments, substrate 403 is made of
acrylonitrile-butadiene-styrene, polytetrifluoro-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 6-175,
which is hereby incorporated by reference herein in its
entirety.
[0229] Additional exemplary materials that can be used to form
substrate 403 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 herein in its entirety.
[0230] 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. In some
embodiments, inner core is made out of a conductive material and
outer core is made out of molybdenum. In some embodiments, inner
core is made out of a nonconductive material, such as a glass rod,
and outer core is made out of molybdenum.
5.5 Exemplary Dimensions
[0231] 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 Additional Solar Cell Embodiments
[0232] Using FIG. 3B for reference to element numbers, 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, back-electrode 404 can be made
of molybdenum. In some embodiments, back-electrode 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) 415 is then deposited
on the semiconductor junction 410. The i-layer 415 can be formed
using a material including but not limited to, zinc oxide, metal
oxide or any transparent material that is highly insulating. Next,
transparent conductive layer 412 is disposed on either the i-layer
(when present) or the semiconductor junction 410 (when the i-layer
is not present). Transparent conductive layer 412 can be made of a
material such as aluminum doped zinc oxide (ZnO:Al), gallium doped
zinc oxide, boron doped zinc oxide, indium-zinc oxide, or
indium-tin oxide.
[0233] 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 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 herein in its entirety.
[0234] 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, 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, optional i-layer 415 and transparent conductive layer 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 transparent conductive layer 412 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 transparent conductive layer 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.
[0235] 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 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.
[0236] In some embodiments, a transparent conducting oxide
conductive film is deposited on a tubular shaped or rigid solid rod
shaped core rather than wrapping a metal web or foil around the
elongated core. In such embodiments, the tubular shaped or rigid
solid rod shaped 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 or back contact of the semiconductor junction. In some
embodiments, divots in the tubular shaped or rigid solid rod shaped
elongated core are filled with a conductive metal in order to
provide such a conductor. The conductor can be inserted in the
divots prior to depositing the transparent conductive layer or
conductive back contact film onto the tubular shaped or rigid solid
rod shaped elongated core. In some embodiments such a conductor is
formed from a metal source that runs lengthwise along the side of
the elongated solar cell 402. This metal can be deposited by
evaporation, sputtering, screen printing, inkjet printing, metal
pressing, conductive ink or glue used to attach a metal wire, or
other means of metal deposition.
[0237] More specific embodiments will now be disclosed. In some
embodiments, the elongated core is a glass tubing having a divot
that runs lengthwise on the outer surface of the glass tubing, and
the manufacturing method comprises depositing a conductor in the
divot prior to the rolling step. In some embodiments, the glass
tubing has a second divot that runs lengthwise on the surface of
the glass tubing. In such embodiments, the first divot and the
second divot 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 divot
prior to the rolling or, in embodiments in which rolling is not
used, prior to the deposition of an inner transparent conductive
layer or conductive film, junction, and outer transparent
conductive layer onto the elongated core.
[0238] In some embodiments, the elongated core is a glass rod
having a first divot that runs lengthwise on the surface of the
glass rod and the method comprises depositing a conductor in the
first divot prior to the rolling. In some embodiments, the glass
rod has a second divot that runs lengthwise on the surface of the
glass rod and the first divot and the second divot 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 divot prior to the rolling or,
in embodiments in which rolling is not used, prior to the
deposition of an inner transparent conductive layer or conductive
film, junction, and outer transparent conductive layer 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, titanium, steel, nickel, silver,
gold, or an alloy thereof.
[0239] FIG. 13 details a cross-section of a solar cell 402 in
accordance with the present invention. 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 divot 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 divot 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,
titanium, 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 divots prior to wrapping the metallic web
or conducting foil 1304 around the elongated core 1306. In
embodiments where 1304 is a transparent conductive oxide or
conductive film, the conductive wiring 1302 is inserted into the
divots prior to depositing the transparent conductive oxide or
conductive film 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 layer. Such a
layer is advantageous because it allows for thinner absorption
layers in the semiconductor junction. In embodiments where layer
1304 is a transparent conductive layer, the transparent conductive
layer, semiconductor junction 410 and outer transparent conductive
layer 412 are deposited using deposition techniques.
[0240] 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 are not in electrically
conductive contact with adjacent elongated solar cells. In some
embodiments, the elongated solar cells can be in physical contact
with each other if there is an insulative layer between 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 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 the second face of the planar array.
5.7 Static Concentrators
[0241] Encapsulated solar cell unit 300 may be assembled into
bifacial arrays as, for example, any of assemblies 400 (FIG. 4),
700 (FIG. 7), 800 (FIG. 8), 900 (FIG. 9), or 1000 (FIG. 10). 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 embodiment 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, for example, any
of assemblies 400 (FIG. 4), 700 (FIG. 7), 800 (FIG. 8), 900 (FIG.
9), or 1000 (FIG. 10) of other assemblies of solar cell units 300
of the present invention. 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 herein in its entirety.
[0242] 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 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 herein in its entirety.
[0243] In some embodiments, a static concentrator as illustrated in
FIG. 12 is used. The bifacial solar cells illustrated in FIG. 12
can be any bifacial solar cell assembly of the present invention
including. but not limited to assembly 400 (FIG. 4), 700 (FIG. 7),
800 (FIG. 8), 900 (FIG. 9), or 1000 (FIG. 10). The static
concentrator illustrated in FIG. 12 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 are
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 herein in its entirety. Additional static
concentrators that can be used with the present invention are
discussed in Handbook of Photovoltaic Science and Engineering,
2003, Luque and Hegedus (eds.), Wiley & Sons, West Sussex,
England, Chapter 12, which is hereby incorporated by reference
herein in its entirety.
5.8 Internal Reflector Embodiments
[0244] After elongated solar cells 402 are encapsulated as
depicted, for example, in FIG. 15, they may be arranged to form
solar cell assemblies. FIG. 16 illustrates a solar cell assembly
1600 in accordance with the present invention. In this exemplary
embodiment, an internal reflector 1404 is used to enhance solar
input into the solar cell system. As shown in FIG. 16, elongated
solar cells 402 and an internal reflector 1404 are assembled into
an alternating array as shown. Elongated solar cells 402 in solar
cell assembly 1600 have counter-electrodes 420 and electrodes 440.
As illustrated in FIG. 16, solar cell assembly 1600 comprises a
plurality of elongated solar cells 402. There is no limit to the
number of solar cells 402 in this plurality (e.g., 10 or more, 100
or more, 1000 or more, 10,000 or more, between 5,000 and one
million solar cells 402, etc.). Accordingly, solar cell assembly
1600 also comprises a plurality of internal reflectors 1404. There
is no limit to the number of internal reflectors 1404 in this
plurality (e.g., 10 or more, 100 or more, 1000 or more, 10,000 or
more, between 5,000 and one million reflector 1404, etc.).
[0245] Within solar cell assembly 1600, internal reflectors 1404
run lengthwise along corresponding elongated solar cells 402. In
some embodiments, internal reflectors 1404 have a hollow core. As
in the case of elongated conductive core 404, a hollow
nonconductive core (e.g. substrate 403 of FIG. 3B) is advantageous
in many instances because it reduces the amount of material needed
to make such devices, thereby lowering costs. In some embodiments,
internal reflector 1404 is a plastic casing with a layer of highly
reflective material (e.g., polished aluminum, aluminum alloy,
silver, nickel, steel, etc.) deposited on the plastic casing. In
some embodiments, internal reflector 1404 is a single piece made
out of polished aluminum, aluminum alloy, silver, nickel, steel,
etc. In some embodiments, internal reflector 1404 is a metal or
plastic casing onto which is layered a metal foil tape. Exemplary
metal foil tapes include, but are not limited to, 3M aluminum foil
tape 425, 3M aluminum foil tape 427, 3M aluminum foil tape 431, and
3M aluminum foil tape 439 (3M, St. Paul, Minn.). Internal reflector
1404 can adopt a broad range of designs, only one of which is
illustrated in FIG. 16. Central to the design of reflectors 1404
found in a preferred embodiment of the present invention is the
desire to reflect direct light that enters into both sides of solar
cell assembly 1600 (i.e., side 1620 and side 1640).
[0246] In general, reflectors 1404 of the present invention are
designed to optimize reflection of light into adjacent elongated
solar cells 402. Direct light that enters one side of solar cell
assembly 1600 (e.g., side 1940, above the plane of the solar cell
assembly drawn in FIG. 16) is directly from the sun whereas light
that enters the other side of the solar cell (e.g., side 1620,
below the plane of the solar cell assembly drawn in FIG. 16) will
have been reflected off of a surface. In some embodiments, this
surface is Lambertian, a diffuse or an involute reflector. Thus,
because each side of the solar cell assembly faces a different
light environment, the shape of internal reflector 1404 on side
1620 may be different than on side 1640.
[0247] Although internal reflector 1404 is illustrated in FIG. 16
as having a symmetrical four-sided cross-sectional shape, the
cross-sectional shape of the internal reflectors 1404 of the
present invention are not limited to such a configuration. In some
embodiments, a cross-sectional shape of an internal reflector 1404
is astroid. In some embodiments, a cross-sectional shape of an
internal reflector 1404 is four-sided and at least one side of the
four-sided cross-sectional shape is linear. In some embodiments, a
cross-sectional shape of an internal reflector 1404 is four-sided
and at least one side of the four-sided cross-sectional shape is
parabolic. In some embodiments, a cross-sectional shape of an
internal reflector 1404 is four-sided and at least one side of the
four-sided cross-sectional shape is concave. In some embodiments, a
cross-sectional shape of an internal reflector 1404 is four-sided;
and at least one side of the four-sided cross-sectional shape is
circular or elliptical. In some embodiments, a cross-sectional
shape of an internal reflector in the plurality of internal
reflectors is four-sided and at least one side of the four-sided
cross-sectional shape defines a diffuse surface on the internal
reflector. In some embodiments, a cross-sectional shape of an
internal reflector 1404 is four-sided and at least one side of the
four-sided cross-sectional shape is the involute of a
cross-sectional shape of an elongated solar cell 402. In some
embodiments, a cross-sectional shape of an internal reflector 1404
is two-sided, three-sided, four-sided, five-sided, or six-sided. In
some embodiments, a cross-sectional shape of an internal reflector
in the plurality of internal reflectors 1404 is four-sided and at
least one side of the four-sided cross-sectional shape is
faceted.
[0248] Additional features are added to reflectors 1404 to enhance
the reflection onto adjacent elongated solar cells 402 in some
embodiments. Modified reflectors 1404 are equipped with a strong
reflective property such that incident light is effectively
reflected off the side surfaces 1610 of the reflectors 1404. In
some embodiments, the reflected light off surfaces 1610 does not
have directional preference. In other embodiments, the reflector
surfaces 1610 are designed such that the reflected light is
directed towards the elongated solar cell 402 for optimal
absorbance.
[0249] In some embodiments, the connection between an internal
reflector 1404 and an adjacent elongated solar cell is provided by
an additional adaptor piece. Such an adapter piece has surface
features that are complementary to both the shapes of internal
reflectors 1404 as well as elongated solar cells 402 in order to
provide a tight fit between such components. In some embodiments,
such adaptor pieces are fixed on internal reflectors 1404. In other
embodiments, the adaptor pieces are fixed on elongated solar cells
402. In additional embodiments, the connection between elongated
solar cells 402 and reflectors 1404 may be strengthened by
electrically conducting glue or tapes.
[0250] Diffuse Reflection. In some embodiments in accordance with
the present invention, the side surface 1610 of reflector 1404 is a
diffuse reflecting surface (e.g., 1610 in FIG. 16). The concept of
diffuse reflection can be better appreciated with a first
understanding of specular reflection. Specular reflection is
defined as the reflection off smooth surfaces such as mirrors or a
calm body of water (e.g., 1702 in FIG. 17A). On a specular surface,
light is reflected mainly in the direction of the reflected ray and
is attenuated by an amount dependent upon the physical properties
of the surface. Since the light reflected from the surface is
mainly in the direction of the reflected ray, the position of the
observer (e.g., the position of the elongated solar cells 402)
determines the perceived illumination of the surface. Specular
reflection models the light reflecting properties of shiny or
mirror-like surfaces. In contrast to specular reflection,
reflection off rough surfaces such as clothing, paper, and the
asphalt roadway leads to a different type of reflection known as
diffuse reflection (FIG. 17B). Light incident on a diffuse
reflection surface is reflected equally in all directions and is
attenuated by an amount dependent upon the physical properties of
the surface. Since light is reflected equally in all directions the
perceived illumination of the surface is not dependent on the
position of the observer or receiver of the reflected light (e.g.
the position of the elongated solar cell 402). Diffuse reflection
models the light reflecting properties of matt surfaces.
[0251] Diffuse reflection surfaces reflect off light with no
directional dependence for the viewer. Whether the surface is
microscopically rough or smooth has a tremendous impact upon the
subsequent reflection of a beam of light. Input light from a single
directional source is reflected off in all directions on a diffuse
reflecting surface (e.g., 1704 in FIG. 17B). Diffuse reflection
originates from a combination of internal scattering of light,
e.g., the light is absorbed and then re-emitted, and external
scattering from the rough surface of the object.
[0252] Lambertian reflection. In some embodiments in accordance
with the present invention, surface 1610 of reflector 1404 is a
Lambertian reflecting surface (e.g., 1706 in FIG. 17C). A
Lambertian source is defined as an optical source that obeys
Lambert's cosine law, i.e., that has an intensity directly
proportional to the cosine of the angle from which it is viewed
(FIG. 17C). Accordingly, a Lambertian surface is defined as a
surface that provides uniform diffusion of incident radiation such
that its radiance (or luminance) is the same in all directions from
which it can be measured (e.g., radiance is independent of viewing
angle) with the caveat that the total area of the radiating surface
is larger than the area being measured.
[0253] On a perfectly diffusing surface, the intensity of the light
emanating in a given direction from any small surface component is
proportional to the cosine of the angle of the normal to the
surface. The brightness (luminance, radiance) of a Lambertian
surface is constant regardless of the angle from which it is
viewed.
[0254] The incident light {right arrow over (l)} strikes a
Lambertian surface (FIG. 17C) and reflects in different directions.
When the intensity of {right arrow over (l)} is defined as
I.sub.in, the intensity (e.g., I.sub.out) of a reflected light
{right arrow over (v)} can be defined as following in accordance to
Lambert's cosine law:
I out ( v .fwdarw. ) = I i n ( l .fwdarw. ) .PHI. ( v .fwdarw. , l
.fwdarw. ) cos .theta. i n cos .theta. out ##EQU00002##
where .phi.({right arrow over (v)}, {right arrow over (l)})=k.sub.d
cos .theta..sub.out and k.sub.d is related to the surface property.
The incident angle is defined as .theta..sub.in, and the reflected
angle is defined as .theta..sub.out. Using the vector dot product
formula, the intensity of the reflected light can also be written
as:
I.sub.out({right arrow over (v)})=k.sub.dI.sub.in({right arrow over
(l)}){right arrow over (l)}{right arrow over (n)},
where {right arrow over (n)} denotes a vector that is normal to the
Lambertian surface.
[0255] Such a Lambertian surface does not lose any incident light
radiation, but re-emits it in all the available solid angles with a
2.pi. radians, on the illuminated side of the surface. Moreover, a
Lambertian surface emits light so that the surface appears equally
bright from any direction. That is, equal projected areas radiate
equal amounts of luminous flux. Though this is an ideal, many real
surfaces approach it. For example, a Lambertian surface can be
created with a layer of diffuse white paint. The reflectance of
such a typical Lambertian surface may be 93%. In some embodiments,
the reflectance of a Lambertian surface may be higher than 93%. In
some embodiments, the reflectance of a Lambertian surface may be
lower than 93%. Lambertian surfaces have been widely used in LED
design to provide optimized illumination, for example in U.S. Pat.
No. 6,257,737 to Marshall, et al; U.S. Pat. No. 6,661,521 to Stern;
and U.S. Pat. No. 6,603,243 to Parkyn et al., which are hereby
incorporated by reference in their entireties.
[0256] Advantageously, Lambertian surfaces 1610 on reflector 1404
effectively reflect light in all directions. The reflected light is
then directed towards the elongated solar cell 402 to enhance solar
cell performance.
[0257] Reflection on involute surfaces. In some embodiments in
accordance with the present invention, surface 1610 of the
reflector 1404 is an involute surface of the elongated solar cell
tube 402. In some embodiments, the elongated solar cell tube 402 is
circular or near circular. Reflector surface 1610 is preferably the
involute of a circle (e.g. 1804 in FIG. 18A). The involute of
circle 1802 is defined as the path traced out by a point on a
straight line that rolls around a circle. For example, the involute
of a circle can be drawn in the following steps. First, attach a
string to a point on a curve. Second, extend the string so that it
is tangent to the curve at the point of attachment. Third, wind the
string up, keeping it always taut. The locus of points traced out
by the end of the string (e.g. 1804 in FIG. 18) is called the
involute of the original circle 1802. The original circle 1802 is
called the evolute of its involute curve 1804.
[0258] Although in general a curve has a unique evolute, it has
infinitely many involutes corresponding to different choices of
initial point. An involute can also be thought of as any curve
orthogonal to all the tangents to a given curve. For a circle of
radius r, at any time t, its equation can be written as:
x=r cos t
y=r sin t
Correspondingly, the parametric equation of the involute of the
circle is:
x.sub.i=r(cos t+t sin t)
y.sub.i=r(sin t-t cos t)
Evolute and involute are reciprocal functions. The evolute of an
involute of a circle is a circle.
[0259] Involute surfaces have been implemented in numerous patent
designs to optimize light reflections. For example, a flash lamp
reflector (U.S. Pat. No. 4,641,315 to Draggoo, hereby incorporated
by reference herein in its entirety) and concave light reflector
devices (U.S. Pat. No. 4,641,315 to Rose, hereby incorporated by
reference herein in its entirety), which are hereby incorporated by
reference in their entireties, both utilize involute surfaces to
enhance light reflection efficiency.
[0260] In FIG. 18B, an internal reflector 1404 is connected to two
elongated solar cells 402. Details of both reflector 1404 and solar
cell 402 are omitted to highlight the intrinsic relationship
between the shapes of the elongated solar cell 402 and the shape of
the side surface 1610 of the internal reflector 1404. Side surfaces
1610 are constructed such that they are the involute of the
circular elongated solar cell 402.
[0261] Advantageously, the involute-evolute design imposes optimal
interactions between the side surfaces 1610 of reflectors 1404 and
the adjacent elongated solar cell 402. When the side surface 1610
of the reflector 1404 is an involute surface corresponding to the
elongated solar cell 402 that is adjacent or attached to the
reflector 1404, light reflects effectively off the involute surface
in a direction that is optimized towards the elongated solar cell
402.
[0262] In some embodiments not illustrated in FIG. 16, elongated
solar cells 402 are swaged at their ends such that the diameter at
the ends is less than the diameter towards the center of such
cells. Electrodes 440 are placed on these swaged ends.
[0263] Solar Cell Assembly. As illustrated in FIG. 16, solar cells
in the plurality of elongated solar cells 402 are geometrically
arranged in a parallel or near parallel manner. In some
embodiments, elongated conductive core 404 is any of the dual layer
cores described in Section 5.4. In some embodiments, rather than
forming a conductive core 404, back-electrode 404 is a thin layer
of metal deposited on a substrate 403 as illustrated, for example,
in FIG. 3B. In some embodiments, the terminal ends of elongated
solar cells 402 can be stripped down to the outer core. For
example, consider the case in which elongated solar cell 402 is
constructed out of an inner core made of a cylindrical substrate
403 and an outer core (back-electrode 404) made of molybdenum. In
such a case, the end of elongated solar cell 402 can be stripped
down to the molybdenum back-electrode 404 and electrode 440 can be
electrically connected with back-electrode 404.
[0264] In some embodiments, each internal reflector 1404 connects
to two encapsulated elongated solar cells 402 (e.g., depicted as
300 in FIGS. 15 and 16), for example, in the manner illustrated in
FIG. 16. Because of this, elongated solar cells 402 are effectively
joined into a single composite device. In FIG. 16, electrodes 440
extend the connection from back-electrode 404. In some embodiments,
internal reflector units 1404 are connected to encapsulated solar
cells 300 via indentations on transparent tubular casing 310. In
some embodiments, the indentations on transparent tubular casing
310 are created to complement the shape of the internal reflector
unit 1404. Indentations on two transparent tubular casing 310 are
used to lock in one internal reflector unit 1404 that is positioned
between the two encapsulated solar cells 300. In some embodiments,
adhesive materials, e.g., epoxy glue, are used to fortify the
connections between the internal reflector unit 1404 and the
adjacent encapsulated solar cell units 300 such that solar
radiation is properly reflected towards the encapsulated solar cell
units 300 for absorption.
[0265] In some embodiments in accordance with the present
invention, internal reflector unit 1404 and transparent tubular
casing 310 may be created in the same molding process. For example,
an array of alternating transparent tubular casing 310 and astroid
reflectors 1404, e.g., shown as 1900 in FIG. 19, can be made as a
single composite entity. Additional modifications may be done to
enhance the albedo effect from the internal reflector unit 1404 or
to promote better fitting between transparent tubular casing 310
and solar cell 402. The tubular casing 310 may contain internal
modifications that complement the shapes of some embodiments of the
solar cell 402. There is no limit to the number of internal
reflectors 1404 or tubular casing 310 in the assembly as depicted
in FIG. 19 (e.g., 10 or more, 100 or more, 1000 or more, 10,000 or
more, between 5,000 and one million internal reflectors 1404 and
tubular casing 310, etc.).
6. REFERENCES CITED
[0266] 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.
[0267] 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. 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.
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