U.S. patent application number 12/618741 was filed with the patent office on 2011-09-29 for photovoltaic cell.
Invention is credited to Barry Breen, JONATHAN R. GOLDSTEIN, Ron Paz, Eli Rosh Hodesh, Ilya Yakupov.
Application Number | 20110232736 12/618741 |
Document ID | / |
Family ID | 40002734 |
Filed Date | 2011-09-29 |
United States Patent
Application |
20110232736 |
Kind Code |
A1 |
GOLDSTEIN; JONATHAN R. ; et
al. |
September 29, 2011 |
PHOTOVOLTAIC CELL
Abstract
A photovoltaic cell including: (a) a housing adapted to enclose
the photovoltaic cell, and including an at least partially
transparent cell wall; (b) an electrolyte, disposed within the cell
wall, and containing a corrosive redox species; (c) an at least
partially transparent conductive coating disposed on an interior
surface of the cell wall, within the photovoltaic cell; (d) an
anode disposed on the conductive coating, the anode including a
porous film adapted to make intimate contact with the redox
species, and a dye, absorbed on a surface of the porous film, the
dye and the film adapted to convert photons to electrons; (e) a
cathode, disposed within an interior surface of the housing and
disposed substantially opposite the anode, including a conductive
carbon layer, and a catalytic component, associated with the carbon
layer and adapted to catalyze a redox reaction, the carbon layer
adapted to transfer electrons from the catalytic component to a
current collection component of the cathode, and (f) at least one
metal strip or wire, electrically associated with the anode and
with the conductive coating, the strip or wire having sufficient
thickness to form a protrusion protruding above a plane of the
porous film by at least 50 micrometers, wherein a distance between
a surface of the catalytic component and a surface of the porous
film is less than 20 micrometers.
Inventors: |
GOLDSTEIN; JONATHAN R.;
(Jerusalem, IL) ; Breen; Barry; (Jerusalem,
IL) ; Yakupov; Ilya; (Rehovot, IL) ; Rosh
Hodesh; Eli; (Rishon Lezion, IL) ; Paz; Ron;
(Jerusalem, IL) |
Family ID: |
40002734 |
Appl. No.: |
12/618741 |
Filed: |
November 15, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/IL2008/000671 |
May 15, 2008 |
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12618741 |
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60917941 |
May 15, 2007 |
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Current U.S.
Class: |
136/256 |
Current CPC
Class: |
H01G 9/2031 20130101;
H01G 9/2077 20130101; H01G 9/2059 20130101; H01G 9/2068 20130101;
H01L 51/445 20130101; Y02E 10/542 20130101; H01G 9/2022
20130101 |
Class at
Publication: |
136/256 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 51/44 20060101 H01L051/44 |
Claims
1. A photovoltaic cell for converting a light source into
electricity at a reduced ohmic loss, the cell comprising: (a) a
housing adapted to enclose the photovoltaic cell, said housing
including an at least partially transparent cell wall, said cell
wall having an interior surface and an exterior surface; (b) an
electrolyte, disposed within said cell wall, said electrolyte
containing an iodine based redox species; (c) an at least partially
transparent conductive coating disposed on said interior surface of
said cell wall, within the photovoltaic cell; (d) an anode disposed
on said conductive coating, said anode including: (i) a porous film
adapted to make intimate contact with said redox species, and (ii)
a dye, absorbed on a surface of said porous film, said dye and said
film adapted to convert photons to electrons; (e) a cathode
disposed within an interior surface of said housing, said cathode
disposed substantially opposite said anode, said cathode including:
(i) a conductive carbon layer, and (ii) a catalytic component,
associated with said carbon layer and adapted to catalyze a redox
reaction of said redox species, said conductive carbon layer
adapted to transfer electrons from said catalytic component to a
current collection component of said cathode, said conductive
carbon layer and said catalytic component disposed in electrolytic
communication, via said electrolyte, with said porous film, and (f)
at least one metal strip or wire, electrically associated with said
anode and with said conductive coating, said strip or wire having
sufficient thickness to form a protrusion protruding above a plane
of said porous film by at least 50 micrometers, and wherein a
distance between a surface of said catalytic component and a
surface of said porous film is less than 20 micrometers.
2. The photovoltaic cell of claim 1, wherein said cathode is
mechanically supported by said cell wall in a monolithic
configuration.
3. The photovoltaic cell of claim 1, wherein said distance is an
average distance between said surface of said catalytic component
and said surface of said porous film.
4. The photovoltaic cell of claim 1, wherein said distance is
achieved over at least 80% of the cell surface area.
5. The photovoltaic cell of claim 1, wherein said distance is less
than 15 micrometers.
6. The photovoltaic cell of claim 1, said strip or wire having
sufficient thickness to protrude above a plane of said porous film
by at least 80 micrometers.
7. The photovoltaic cell of claim 1, further comprising: (g) a
porous separator layer interdisposed between at least a portion of
said porous film of said anode and at least a portion of said
catalytic component of said cathode, said porous separator layer
adapted to physically separate and electrically insulate between
said porous film and said cathode.
8. The photovoltaic cell of claim 7, said porous separator layer
including a spacing element that is physically distinct with
respect to said porous film of said anode.
9. The photovoltaic cell of claim 7, wherein said porous separator
layer is attached to said porous film of said anode.
10. The photovoltaic cell of claim 1, said conductive carbon layer
and said catalytic component forming self-supporting strips.
11. The photovoltaic cell of claim 10, said self-supporting strips
being solely physically associated with a porous separator layer
interdisposed between said porous film of said anode and said
catalytic component of said cathode.
12. The photovoltaic cell of claim 1, said at least one metal strip
or wire is a plurality of strips or wires, and said protrusion is a
plurality of protrusions.
13. The photovoltaic cell of claim 12, said conductive carbon layer
and said catalytic component forming self-supporting strips, said
strips disposed between said protrusions.
14. The photovoltaic cell of claim 13, said strips disposed between
said protrusions to make a surface of said catalytic component
substantially flush with an opposing surface.
15. The photovoltaic cell of claim 10, said self-supporting strips
being physically associated with, and juxtaposed against, a porous
separator layer interdisposed between said porous film of said
anode and said catalytic component of said cathode.
16. The photovoltaic cell of claim 7, wherein said at least one
metal strip or wire is a plurality of strips or wires, said
protrusion is a plurality of protrusions, and said porous separator
layer is discontinuous, whereby, between adjacent protrusions of
said protrusions, said catalytic component is juxtaposed against
said porous film of said anode.
17. A photovoltaic cell for converting a light source into
electricity at a reduced ohmic loss, the cell comprising: (a) a
housing adapted to enclose the photovoltaic cell, said housing
including an at least partially transparent cell wall, said cell
wall having an interior surface and an exterior surface; (b) an
electrolyte, disposed within said cell wall, said electrolyte
containing an iodine based redox species; (c) an at least partially
transparent conductive coating disposed on said interior surface of
said cell wall, within the photovoltaic cell; (d) an anode disposed
on said conductive coating, said anode including: (i) a porous film
adapted to make intimate contact with said redox species, and (ii)
a dye, absorbed on a surface of said porous film, said dye and said
film adapted to convert photons to electrons; (e) a cathode
disposed within an interior surface of said housing, said cathode
disposed substantially opposite said anode, said cathode including:
(i) a conductive carbon layer, and (ii) a catalytic component,
associated with said carbon layer and adapted to catalyze a redox
reaction of said redox species, said conductive carbon layer
adapted to transfer electrons from said catalytic component to a
current collection component of said cathode, said conductive
carbon layer and said catalytic component disposed in electrolytic
communication, via said electrolyte, with said porous film, and (f)
at least one electrically-conductive structural component, disposed
within said conductive carbon layer, said structural component
having a specific resistivity below 1200 microohmcm, said component
adapted and disposed to collect current from said cathode, and
wherein a footprint of a single cell of the photovoltaic cell is
defined by a length and a width of said single cell, and wherein
both said length and said width exceed 5 centimeters.
18. The photovoltaic cell of claim 17, wherein said conductive
carbon layer has a thickness of at least 50 micrometers.
19. The photovoltaic cell of claim 17, wherein said structural
component is at least one continuous component at least partially
buried in said conductive carbon layer.
20. A photovoltaic cell for converting a light source into
electricity at a reduced ohmic loss, the cell comprising: (a) a
housing adapted to enclose the photovoltaic cell, said housing
including an at least partially transparent cell wall, said cell
wall having an interior surface and an exterior surface; (b) an
electrolyte, disposed within said cell wall, said electrolyte
containing a redox species; (c) an at least partially transparent
conductive coating disposed on said interior surface of said cell
wall, within the photovoltaic cell; (d) an anode disposed on said
conductive coating, said anode including: (i) a porous film adapted
to make intimate contact with said redox species, and (ii) a dye,
absorbed on a surface of said porous film, said dye and said film
adapted to convert photons to electrons; (e) a cathode disposed
within an interior surface of said housing, said cathode disposed
substantially opposite said anode, said cathode including: (i) a
conductive carbon layer, and (ii) a catalytic component, associated
with said carbon layer and adapted to catalyze a redox reaction of
said redox species, said conductive carbon layer adapted to
transfer electrons from said catalytic component to a current
collection component of said cathode, said conductive carbon layer
and said catalytic component disposed in electrolytic
communication, via said electrolyte, with said porous film, and (f)
at least one metal strip or wire, electrically associated with said
anode and with said conductive coating, said strip or wire having
sufficient thickness to form a protrusion protruding above a plane
of said porous film by at least 50 micrometers, and wherein a
distance between a surface of said catalytic component and a
surface of said porous film is less than 20 micrometers.
Description
[0001] This Continuation-In-part (CIP) application draws priority
from PCT Application No. IL2008/000671, filed May 15, 2008, which
draws priority from U.S. Provisional Patent Application Ser. No.
60/917,941, filed May 15, 2007.
FIELD AND BACKGROUND OF THE INVENTION
[0002] The present invention relates to photovoltaic cells, also
known as solar cells, for producing electricity from sunlight, and
more particularly, to broad footprint photovoltaic cells utilizing
electrolytes containing iodine-based redox species or other
corrosive species.
[0003] The invention has particular relevance for solar cells of
the dye-sensitized type, and is applicable to other types of solar
cells in which a high current density of operation at minimal ohmic
loss is advantageous.
[0004] Dye-sensitized photovoltaic cells for producing electricity
from sunlight have been disclosed by U.S. Pat. No. 5,350,644 to
Graetzel, et al., which is hereby incorporated by reference for all
purposes as if fully set forth herein. U.S. Pat. No. 5,350,644
teaches a photovoltaic cell having a light-transmitting,
electrically-conductive layer deposited on a glass plate or a
transparent polymer sheet to which a series of titanium dioxide
layers have been applied, in which at least the last titanium
dioxide layer is doped with a metal ion that is selected from a
divalent or trivalent metal.
[0005] Following U.S. Pat. No. 5,350,644 is U.S. Pat. No. 6,069,313
to Kay, which is hereby incorporated by reference for all purposes
as if fully set forth herein. U.S. Pat. No. 6,069,313 teaches a
plurality of series-connected cell elements arranged as separate,
parallel, narrow elongated strips on a common transparent
substrate. Each element includes a light facing anode including
nanocrystalline titania, a carbon-based counterelectrode (cathode),
and an intermediate electrically insulating porous layer separating
the anode from the cathode. The pores of the intermediate layer are
at least partially filled with a liquid phase, ion-transferring
electrolyte, following coating of the nanocrystalline titania with
a light sensitive dye.
[0006] A current collecting layer of a tin oxide based transparent,
electrically-conducting material is situated between the
transparent substrate and the anode. The anode and cathode of a
given cell provide a direct-current voltage when the anode is
exposed to light, such that series assemblies of cells may readily
be built up. The cathode of each succeeding element is connected
with the intermediate conducting layer of the preceding anode
element, over a gap separating the respective intermediate layers
of these two elements. The series of cells is then sealed using an
organic polymer, ensuring in particular that each individual strip
cell is sealed from its neighbor cell. In the art of dye cells,
such an arrangement of anode and cathode materials or cells on a
common substrate is termed "monolithic".
[0007] Generally, dye cells of the above-cited prior art
disclosures are much closer conceptually to battery cells than to
conventional photovoltaic cells, since the charge generators are
separated by an electrolyte and are not in direct contact. These
cells have two electrodes separated by an electrolyte, with one
electrode (the photo-electrode or photo-anode) facing the sun or
light source. Each electrode is supported on its own current
collector, usually a sheet of conducting glass, which is glass
coated on one side with a thin (.about.0.5 micron) transparent
layer, usually based on electrically-conductive tin oxide. The
conducting glass sheets act as transparent walls of the dye
cell.
[0008] A transparent polymer may be used in place of glass to
support the tin oxide. The photo-electrode or photoanode includes a
transparent porous layer about 10 microns thick (in contact with
the tin oxide layer) based on titania, having a nanocrystalline
characteristic particle size of 10-50 nm, applied by baking onto
the conductive glass or transparent polymer, and impregnated with a
special dye. The baked-on titania layer is applied in dispersion
form by any of various methods: doctor-blading, rolling, spraying,
painting, electrophoresis, gravure printing, slit coating, screen
printing or printing. The baking step giving highest cell
performance is usually in excess of 450.degree. C., requiring the
use of conducting glass rather than plastic for supporting the
titania layer. Other processing procedures for the titania layer
are feasible, such as reduced temperature baking, or pressing,
usually with some sacrifice in efficiency. It is important to note
that the titania is principally in contact with the tin oxide.
Presence of other conductors (such as many metals, carbon and the
like, even if chemically inert to the electrolyte) on the
photoanode can greatly increase recombination of charge carriers
and provide a serious efficiency loss in the cell. Very few
materials (amongst them tin oxide and titanium metal) combine
chemical inertness to the electrolyte with freedom from
recombination effects so as to be potentially suitable as
construction materials for, or for electrically associating with,
the photoanode.
[0009] For cells that are partially transparent, the other
electrode (the counter electrode) includes a thin layer of catalyst
(usually containing a few micrograms of platinum per sq. cm) on its
respective sheet of tin oxide coated conductive glass or
transparent plastic. If cell transparency is not required, the
counter electrode can be opaque, for example, based on carbon or
graphite advantageously catalyzed with trace platinum or another
catalyst. The electrolyte in the cell is usually an organic solvent
with a dissolved redox species. The electrolyte is typically
acetonitrile or a higher molecular weight reduced volatility
nitrile, with the redox species being dissolved iodine and
potassium iodide--essentially potassium tri-iodide. Other solvents
and phases, for example ionic liquids with no vapor pressure, and
even different redox species, may be used, however.
[0010] U.S. Pat. No. 5,350,644 to Graetzel, et al., discloses
various dye cell chemistries, especially different dyes based on
ruthenium complexes. Photons falling on the photoelectrode excite
the dye (creating activated oxidized dye molecules), causing
electrons to enter the conduction band of the titania and to flow
(via an outer circuit having a load) to the counter-electrode.
There, the electrons reduce tri-iodide to iodide in the
electrolyte, and the iodide is oxidized by the activated dye at the
photoanode back to tri-iodide, leaving behind a deactivated dye
molecule ready for the next photon. U.S. Pat. No. 5,350,644
discloses that such dye cells can attain a solar-to-electric
conversion efficiency of 10%.
[0011] The cells taught by U.S. Pat. No. 5,350,644 to Graetzel, et
al., are based on two generally parallel, closely placed sheets of
conductive glass, a first sheet coated with titania and dye, and a
second sheet coated with catalyst. Electrolyte is provided between
the two sheets. The cells are sealed by means of organic adhesive
at the edges. Current takeoff is achieved via the conductive glass
sheets, which project beyond the adhesive on either side of the
cell. These cells operate at a voltage of about 700 mV and at a
current density of 15 mA/sq. cm under peak solar illumination, with
the counterelectrode being the positive pole. It is asserted
therein that since the materials and preparation methods are low
cost and the titania layer can be prepared in large areas, such
cells could potentially provide a good route to low-cost
photovoltaic cells. It is further argued that there might be
significant cost savings over classical single crystal or
polycrystalline silicon cells and even more recent thin-film
photovoltaic cells, since these are all high cost and rely on
expensive and often environmentally problematic raw materials,
together with complex, costly, semiconductor industry processing
equipment and production techniques. These drawbacks include the
use of vacuum deposition and semiconductor doping methods,
clean-room protocols, use of toxic hydrides such as silane,
phosphine, etc., as raw materials, and the use of toxic
active-layer materials containing cadmium, selenium or
tellurium.
[0012] The ohmic loss via the conductive glass coated with tin
oxide is a major problem of dye cells. The tin oxide coating is
extremely thin, being limited in thickness usually to below one
micron due to the need to maintain a high light transmittance
through to the dye/titania layer of the photoanode. Moreover, tin
oxide is only semiconductive and is bonded to with difficulty.
Consequently, the current takeoff is significantly limited by such
a cell design to very small sized cells having a maximum footprint
of a few square millimeters or strip cells having long narrow
strips of active titania. It should be noted that active strip
cells have certain technical disadvantages. For example, in the
cells of Kay described above, the strips of titania are
disadvantageously narrow (typically 0.6-0.8 cm wide), due to the
ohmic loss restriction. This results in an excessive loss of active
area between cells, due to the practical width of inert materials
needed for inter-cell sealing. Moreover, adequate sealing between
adjacent cells so as to effectively prevent any inter-cell
electrolyte migration remains a serious challenge.
[0013] Efforts have been made to increase the active area and
breadth of cells by laying down parallel conducting strips on a
conducting glass surface, thereby enabling a large-area, broad-cell
construction. A photovoltaic cell having electrically conducting
coatings on spaced, glass support panes is disclosed by U.S. Pat.
No. 6,462,266 to Kurth, which is hereby incorporated by reference
for all purposes as if fully set forth herein. In FIG. 1 is
provided a partial, cross-sectional view of a photovoltaic cell 1
having two generally parallel support panes 2 and 3, set apart at a
distance. The edges of photovoltaic cell 1 are held and sealed by a
sealing system 4 extending along the whole circumference. The inner
surfaces of support panes 2 and 3 are each coated with a conductive
layer 5, and 6, respectively. Layers 5 and 6 are made of a suitable
metal or metal oxide such as tin oxide. On layers 5 and 6, an
arrangement of parallel conductor leads 7 and 8 are provided,
preferentially made from silver or a silver alloy, or from copper
or a copper alloy. These conductor leads are coated each with an
insulating coating 10, which insulates conductor leads 7 and 8
electrically towards the interior of the cell. Coating 10 consists
of a glass free of heavy metals, which was applied as a glass flow
on conductor leads 7 and 8. Onto conductor leads 7 and 8 insulated
by the glass coating 10, a further electrically conductive layer
11, and 12 respectively, made from tin oxide or a similar material,
can be applied in order to obtain a still-higher yield of
photovoltaic cell 1. In the border zones of seals 13 and 14, no
electrically conductive layers 5 and 6 are provided, these layers
having been eliminated from the border zones by means of sand
blasting. In this manner, possible short circuits via the seals 13
and/or 14 are avoided. Onto these two seals, a thin layer 15 of a
low melt soldering tin is applied in such a manner that exterior
weather influences also can not act onto photovoltaic cell 1.
[0014] The photovoltaic cell taught by U.S. Pat. No. 6,462,266 has
reduced ohmic loss with respect to the cell disclosed by U.S. Pat.
No. 5,350,644 to Graetzel, et al., because conductor leads 7 and 8
are good conductors (e.g., silver paste screen-printed on and fired
at 600.degree. C.), and because the overall effective thickness of
conducting materials has been increased. Unfortunately, the high
process temperatures required for the silver and glaze compositions
adversely affect the conductivity and strength properties of the
glass. Glaze materials that can be processed at lower temperatures
are available, but contain toxic heavy metals such as lead, and may
also be attacked or contaminated by the electrolyte. Furthermore,
protection of silver conductors by a glass layer is inadequate over
time, since pinholes cannot be entirely eliminated in the glaze,
especially in mass production processes. In fact, silver is
corroded by the iodine in cell electrolytes, adversely reducing the
iodine inventory in the cell and irreversibly attacking the dye,
and a similar deterioration occurs with silver lines protected by
polymer.
[0015] U.S. Patent Application Publication No. 20050072458 to
Goldstein describes a large-area, broad conductive glass or
conductive plastic for a dye cell. The conductive glass or
conductive plastic carries a set of conductors selected from
materials intrinsically resistant to corrosion and to carrier
recombination in the presence of the cell electrolyte, and onto
this superior glass or plastic (having enhanced current collecting
properties over plain conductive glass or plastic) the titania is
deposited. By way of example, a conductive glass face is first
grooved, giving a set of parallel spaced shallow grooves. Into each
groove is placed a wire of a metal such as titanium, molybdenum,
tungsten, chromium or their alloys (inert to corrosion and to
carrier recombination under the operating conditions of the cell)
and electrical conductivity between the wire and the tin oxide
layer on each side of the groove is achieved using a heat curable
binder paste based on an inert ceramic adhesive (such as alumina)
mixed with an inert, electrically conducting filler (such as
titanium nitride). The paste fills the groove and overlaps on each
side of the groove to make good electrical contact with the tin
oxide layer there following curing. The wires exit from the cell
from the groove extremities at the glass edges and may be welded to
a current-collecting strip. In a separate embodiment of U.S. Patent
Application Publication No. 20050072458 to Goldstein there is
described a set of parallel strips of a metal or metal alloy having
stability under cell operating conditions that is plated onto the
conductive glass or conducting plastic surface. One example given
of a plated metal is chromium. Current take-off from the anode
plate is again made from the side of the cell.
[0016] In another embodiment, the parallel conductors are inert
strips or wires of titanium, molybdenum, tungsten, chromium or
their alloys bonded directly to the conducting surface of the glass
by means of an inert, electrically conducting ceramic adhesive.
[0017] U.S. Patent Application Publication No. 20050072458 extends
the use of wires in a grooved conductive glass or of strips plated
on conductive glass also for use in the counterelectrode (cathode)
of the cell. The glass plate, provided with wires bonded in grooves
or on top of the glass, or with strips electroplated on the
conductive surface, is used as a base for a broad cathode in the
dye cell, and the conductivity-augmented plate is covered with a
catalytic layer electroactive to iodine. Such a cathode,
unfortunately, although fitted with adequate conducting means for
current takeoff from a large area broad cell, necessarily includes
a second layer of conducting glass in the cell, with associated
cost, weight and thickness penalties. Broad dye cells of at least
10-15 cm per side are made possible, however.
[0018] To date, there has been no real commercialization of
photovoltaic dye cells, despite the great techno-economic potential
thereof. The principal problems remaining include scale-up of cells
to widths much above one centimeter--and areas much above 50 sq.
cm--due to excessive ohmic losses from the poorly conducting tin
oxide layers on the glass or plastic, long term stability of silver
based conductors in the cell, and difficulties of sealing the cells
against long term dryout and performance degradation. Many
approaches rely on costly in-house coating of the transparent
substrate with conductive tin oxide rather than using
commercially-available conductive glass in bulk such as fluorine
doped tin oxide (FTO) glass.
[0019] A further problem in prior art dye cells and modules has
been excessive surface area wasted in seals, protective layers and
conducting paths on the sun-facing side of the cell or module. The
active current-producing area in such cases is often less than 70%
of the geometric area (footprint) of the cell or module, providing
a poor effective efficiency from the available area. A yet further
problem in prior art cells, especially if the counterelectrode is a
separate element from the anode, is the relatively large spacing
between anode and cathode--approaching or exceeding 100
microns--which can result in excessive ohmic loss from the
electrolyte. This problem is particularly acute in the dye cells
disclosed by U.S. Patent Application Publication No. 20050072458 to
Goldstein, and is even more particularly acute in the disclosed dye
cells having a large footprint or cell width.
[0020] Moreover, it should be evident from the above that when the
counterelectrode is a separate element from the anode in the cell,
the counterelectrode usually requires the use of a second glass
support in the cell, or even worse, a second conductive glass in
the cell. These greatly add to the cost, weight and thickness of
the cell.
[0021] There is therefore a recognized need for, and it would be
highly advantageous to have, an electrochemical cell, powered by
sunlight, that is simple, large-area, broad, efficient, low-cost,
lightweight and robust, and successfully addresses the shortcomings
of the prior art.
SUMMARY OF THE INVENTION
[0022] According to the teachings of the present invention there is
provided a photovoltaic cell including: (a) a housing adapted to
enclose the photovoltaic cell, and including an at least partially
transparent cell wall; (b) an electrolyte, disposed within the cell
wall, and containing a corrosive redox species; (c) an at least
partially transparent conductive coating disposed on an interior
surface of the cell wall, within the photovoltaic cell; (d) an
anode disposed on the conductive coating, the anode including a
porous film adapted to make intimate contact with the redox
species, and a dye, absorbed on a surface of the porous film, the
dye and the film adapted to convert photons to electrons; (e) a
cathode, disposed within an interior surface of the housing and
disposed substantially opposite the anode, including a conductive
carbon layer, and a catalytic component, associated with the carbon
layer and adapted to catalyze a redox reaction, the carbon layer
adapted to transfer electrons from the catalytic component to a
current collection component of the cathode, and (f) at least one
metal strip or wire, electrically associated with the anode and
with the conductive coating, the strip or wire having sufficient
thickness to form a protrusion protruding above a plane of the
porous film by at least 50 micrometers, wherein a distance between
a surface of the catalytic component and a surface of the porous
film is less than 20 micrometers.
[0023] According to further features in the described preferred
embodiments, the cathode is mechanically supported by the cell wall
in a monolithic configuration.
[0024] According to still further features in the described
preferred embodiments, the distance is an average distance between
the surface of the catalytic component and the surface of the
porous film.
[0025] According to still further features in the described
preferred embodiments, the distance is achieved over at least 80%
of the cell surface area.
[0026] According to still further features in the described
preferred embodiments, the distance is achieved over at least 85%
of the cell surface area.
[0027] According to still further features in the described
preferred embodiments, the distance is achieved over at least 90%
of the cell surface area.
[0028] According to still further features in the described
preferred embodiments, the distance is achieved over at least 95%
of the cell surface area.
[0029] According to still further features in the described
preferred embodiments, the distance is less than 15
micrometers.
[0030] According to still further features in the described
preferred embodiments, the distance is less than 10
micrometers.
[0031] According to still further features in the described
preferred embodiments, the distance is less than 7 micrometers.
[0032] According to still further features in the described
preferred embodiments, the strip or wire has sufficient thickness
to protrude above a plane of the porous film by at least 80
micrometers, by at least 100 micrometers, or by at least 120
micrometers.
[0033] According to still further features in the described
preferred embodiments, the photovoltaic cell further includes: (g)
a porous separator layer interdisposed between the porous film of
the anode and the catalytic component of the cathode, the porous
separator layer adapted to physically separate and electrically
insulate between the porous film and the cathode.
[0034] According to still further features in the described
preferred embodiments, the porous separator layer includes a
spacing element that is physically distinct with respect to the
porous film of the anode.
[0035] According to still further features in the described
preferred embodiments, the porous separator layer includes a
spacing element that is bonded or attached to the porous film of
the anode.
[0036] According to still further features in the described
preferred embodiments, the porous separator layer includes a
spacing element that is sintered to the porous film of the
anode.
[0037] According to still further features in the described
preferred embodiments, the porous separator layer is discontinuous,
whereby, between adjacent protrusions, the catalytic component is
juxtaposed against the porous film of the anode. The porous
separator layer may typically be bonded, sintered, or otherwise
attached to the porous film of the anode.
[0038] According to still further features in the described
preferred embodiments, the spacing element, in an uncompressed
state, has a height of less than 50 micrometers.
[0039] According to still further features in the described
preferred embodiments, the spacing element is selected from the
group of structural elements consisting of glass fiber and
microporous polymer.
[0040] According to still further features in the described
preferred embodiments, the at least one metal strip or wire is
bonded to the transparent conductive coating by a conductive
binding material.
[0041] According to still further features in the described
preferred embodiments, the conductive binding material includes a
ceramic adhesive.
[0042] According to still further features in the described
preferred embodiments, the conductive binding material includes
titanium nitride.
[0043] According to still further features in the described
preferred embodiments, the conductive carbon layer is sintered on
to the current collection component of the cathode.
[0044] According to still further features in the described
preferred embodiments, the conductive carbon layer and the
catalytic component form self-supporting strips.
[0045] According to still further features in the described
preferred embodiments, the self-supporting strips are solely
physically (i.e., not chemically) associated with a porous
separator layer interdisposed between the porous film of the anode
and the catalytic component of the cathode.
[0046] According to still further features in the described
preferred embodiments, the at least one metal strip or wire is a
plurality of wires, and the protrusion is a plurality of
protrusions.
[0047] According to still further features in the described
preferred embodiments, the conductive carbon layer and the
catalytic component form self-supporting strips, the strips
disposed between the protrusions.
[0048] According to still further features in the described
preferred embodiments, the strips are disposed between the
protrusions to make a surface of the catalytic component
substantially flush with an opposing surface.
[0049] According to still further features in the described
preferred embodiments, the opposing surface is a porous separator
layer interdisposed between the porous film of the anode and the
catalytic component of the cathode.
[0050] According to still further features in the described
preferred embodiments, the self-supporting strips are physically
associated with, and juxtaposed against, a porous separator layer
interdisposed between the porous film of the anode and the
catalytic component of the cathode.
[0051] According to yet another aspect of the present invention
there is provided a photovoltaic cell for converting a light source
into electricity at a reduced ohmic loss, the cell including: (a) a
housing adapted to enclose the photovoltaic cell, the housing
including an at least partially transparent cell wall, the cell
wall having an interior surface and an exterior surface; (b) an
electrolyte, disposed within the cell wall, the electrolyte
containing an iodine based redox species; (c) an at least partially
transparent conductive coating disposed on the interior surface of
the cell wall, within the photovoltaic cell; (d) an anode disposed
on the conductive coating, the anode including: (i) a porous film
adapted to make intimate contact with the redox species, and (ii) a
dye, absorbed on a surface of the porous film, the dye and the film
adapted to convert photons to electrons; (e) a cathode disposed
within an interior surface of the housing, the cathode disposed
substantially opposite the anode, the cathode including: (i) a
conductive carbon layer, and (ii) a catalytic component, associated
with the carbon layer and adapted to catalyze a redox reaction of
the redox species, the conductive carbon layer adapted to transfer
electrons from the catalytic component to a current collection
component of the cathode, the conductive carbon layer and the
catalytic component disposed in electrolytic communication, via the
electrolyte, with the porous film, and (f) at least one
electrically-conductive structural component, disposed within the
conductive carbon layer, the structural component having a specific
resistivity below 1200 microohmcm, the component adapted and
disposed to collect current from the cathode, and wherein a
footprint of a single cell of the photovoltaic cell is defined by a
length and a width of the single cell, and wherein both the length
and the width exceed 5 centimeters.
[0052] According to further features in the described preferred
embodiments, the structural component is selected to have a
specific resistivity below 500 microohmcm or below 200
microohmcm.
[0053] According to still further features in the described
preferred embodiments, the distance between a surface of the
catalytic component and a surface of the porous film is less than
50 micrometers, less than 30 micrometers, less than 20 micrometers,
less than 15 micrometers, or less than 10 micrometers.
[0054] According to still further features in the described
preferred embodiments, the metal strip or wire is disposed in
grooves situated in the interior surface of the cell wall.
[0055] According to further features in the described preferred
embodiments, the conductive carbon layer has a thickness of at
least 50 micrometers or at least 100 micrometers.
[0056] According to further features in the described preferred
embodiments, the conductive carbon layer and said catalytic
component form self-supporting strips.
[0057] According to further features in the described preferred
embodiments, a distance between a surface of said catalytic
component and a surface of said porous film is less than 50
micrometers or less than 20 micrometers.
[0058] According to further features in the described preferred
embodiments, the structural component is at least one continuous
component at least partially buried in said conductive carbon
layer.
[0059] According to further features in the described preferred
embodiments, the structural component is at least one continuous
component selected from the group of components consisting of a
wire, a mesh, and a perforated foil.
[0060] According to further features in the described preferred
embodiments, the structural component is at least one continuous
component selected from the group of components consisting of a
metal wire, a metal mesh, and a perforated metal foil.
[0061] According to further features in the described preferred
embodiments, the structural component extends through said
housing.
[0062] According to another aspect of the present invention there
is provided a photovoltaic cell for converting a light source into
electricity, the cell including: (a) a housing adapted to enclose
the photovoltaic cell, the housing including an at least partially
transparent cell wall, the cell wall having an interior surface;
(b) an electrolyte, disposed within the cell wall, the electrolyte
containing an iodine-based redox species; (c) an at least partially
transparent conductive coating disposed on the interior surface of
the cell wall, within the photovoltaic cell; (d) an anode disposed
on the conductive coating, the anode including: (i) a porous film
adapted to make intimate contact with the redox species, the porous
film including chemically bonded titanium dioxide nanoparticles,
and (ii) a dye, absorbed on a surface of the porous film, the dye
and the film adapted to convert photons to electrons; (e) a cathode
disposed within an interior surface of the housing, the cathode
disposed substantially opposite the anode, the cathode including:
(i) a conductive carbon layer, and (ii) a catalytic component,
associated with the carbon layer and adapted to catalyze a redox
reaction of the redox species, the conductive carbon layer adapted
to transfer electrons from the catalytic component to a current
collection component of the cathode; the conductive carbon layer
and the catalytic component disposed in electrolytic communication,
via the electrolyte, with the porous film; the conductive carbon
layer including (A) carbon black particles; (B) graphite powder;
(C) expanded graphite particles, and (D) titanium dioxide
particles, the titanium dioxide particles of the conductive carbon
layer being chemically bonded to the expanded graphite
particles.
[0063] According to yet another aspect of the present invention
there is provided a method of producing a photovoltaic cell for
converting a light source into electricity, the method including
the steps of: (a) providing: (i) a housing adapted to enclose the
photovoltaic cell, the housing including an at least partially
transparent cell wall, the cell wall having an interior surface;
(ii) an at least partially transparent conductive coating disposed
on the interior surface of the cell wall, within the photovoltaic
cell; (iii) an anode disposed on the conductive coating, the anode
including a porous film adapted to make intimate contact with a
redox species, the porous film including titanium dioxide
nanoparticles, and (b) providing a cathode generally opposite the
anode, the cathode including a sintered conductive carbon layer,
the conductive carbon layer including: (i) carbon black particles;
(ii) graphite powder; (iii) expanded graphite particles, and (iv)
titanium dioxide particles, the titanium dioxide particles of the
conductive carbon layer being chemically bonded to the expanded
graphite particles.
[0064] According to yet another aspect of the present invention
there is provided a method of producing a photovoltaic cell for
converting a light source into electricity, the method including
the steps of: (a) providing: (i) a housing adapted to enclose the
photovoltaic cell, the housing including an at least partially
transparent cell wall, the cell wall having an interior surface;
(ii) an at least partially transparent conductive coating disposed
on the interior surface of the cell wall, within the photovoltaic
cell; (iii) an anode disposed on the conductive coating, the anode
including a porous film adapted to make intimate contact with a
redox species, the porous film including titanium dioxide
nanoparticles; (b) disposing a conductive carbon paste opposite the
anode, the paste including: (A) carbon black particles; (B)
graphite powder; (C) expanded graphite particles, (D) at least one
ceramic oxide precursor, the precursor selected from the group of
precursors consisting of a titania precursor and a zirconia
precursor, and (E) a liquid vehicle, the carbon black particles,
the graphite powder, the expanded graphite particles, and the
titanium dioxide particles being intimately mixed, and (c)
sintering the conductive carbon paste at a temperature above 300 C,
to convert the at least one ceramic oxide precursor to ceramic
oxide particles, and producing a conductive carbon cathodic layer
from the conductive carbon paste.
[0065] According to yet another aspect of the present invention
there is provided a method of producing a photovoltaic cell for
converting a light source into electricity, the method including
the steps of: (a) providing: (i) a housing adapted to enclose the
photovoltaic cell, the housing including an at least partially
transparent cell wall, the cell wall having an interior surface;
(ii) an at least partially transparent conductive coating disposed
on the interior surface of the cell wall, within the photovoltaic
cell; (iii) an anode disposed on the conductive coating, the anode
including a porous film adapted to make intimate contact with a
redox species, the porous film including titanium dioxide
nanoparticles; (b) providing a conductive carbon paste, the paste
including: (A) carbon black particles; (B) graphite powder; (C)
expanded graphite particles; (D) at least one ceramic oxide
precursor, the precursor selected from the group of precursors
consisting of a titania precursor and a zirconia precursor, and (E)
a liquid vehicle, the carbon black particles, the graphite powder,
the expanded graphite particles, and the titanium dioxide particles
being intimately mixed; (c) disposing the conductive carbon paste
on a matrix, and (d) sintering the conductive carbon paste at a
temperature above 300 C, to convert the at least one ceramic oxide
precursor to ceramic oxide particles, and to produce a cathodic
layer in which the ceramic oxide particles are sintered to the
expanded graphite particles, and in which the ceramic oxide
particles are sintered to the matrix.
[0066] According to further features in the described preferred
embodiments, the conductive carbon layer is a sintered conductive
carbon layer.
[0067] According to still further features in the described
preferred embodiments, the porous film is sintered to the
conductive coating.
[0068] According to still further features in the described
preferred embodiments, the carbon black particles, graphite powder,
expanded graphite particles, and titanium dioxide particles are
distributed in a substantially uniform manner within the sintered
conductive carbon layer.
[0069] According to still further features in the described
preferred embodiments, the conductive carbon layer is in a state of
rigid attachment to the porous film.
[0070] According to still further features in the described
preferred embodiments, the cell further includes: a porous
separator layer interdisposed between the porous film of the anode
and the catalytic component of the cathode, the porous separator
layer adapted to physically separate and electrically insulate
between the porous film and the cathode.
[0071] According to still further features in the described
preferred embodiments, the porous separator layer includes a
spacing element that is physically distinct with respect to the
porous film of the anode.
[0072] According to still further features in the described
preferred embodiments, the porous separator layer includes a
spacing element that is chemically bonded to the porous film of the
anode.
[0073] According to still further features in the described
preferred embodiments, the porous separator layer includes a
spacing element that is sintered to the porous film of the
anode.
[0074] According to still further features in the described
preferred embodiments, the spacing element is selected from the
group of structural elements consisting of glass fiber and
microporous polymer.
[0075] According to still further features in the described
preferred embodiments, the chemically bonded titanium dioxide
nanoparticles include sintered titanium dioxide nanoparticles.
[0076] According to still further features in the described
preferred embodiments, the conductive carbon layer is in a state of
rigid attachment with respect to the porous separator layer.
[0077] According to still further features in the described
preferred embodiments, the conductive carbon layer is chemically
bonded to the porous separator layer.
[0078] According to still further features in the described
preferred embodiments, the conductive carbon layer is sintered to
the porous separator layer.
[0079] According to still further features in the described
preferred embodiments, the cell further includes at least one metal
strip or wire, electrically associated with the anode and with the
conductive coating, the strip or wire having sufficient thickness
to form a protrusion protruding above a plane of the porous film by
at least 50 micrometers.
[0080] According to still further features in the described
preferred embodiments, the at least one metal strip or wire is
bonded to the transparent conductive coating by a conductive
binding material.
[0081] According to still further features in the described
preferred embodiments, the conductive carbon layer further includes
zirconium oxide particles, preferably distributed in a
substantially uniform manner with the carbon black particles, the
graphite powder, the expanded graphite particles, and the titanium
dioxide particles.
[0082] According to still further features in the described
preferred embodiments, the carbon black and the expanded graphite
have a weight ratio in a range of 1.5:1 to 1:1.5.
[0083] According to still further features in the described
preferred embodiments, the total carbon content of the conductive
carbon layer includes the graphite powder, the carbon black
particles, and the expanded graphite particles, and the total
carbon content and the titanium dioxide particles have a weight
ratio in a range of 1.2:1 to 3.5:1.
[0084] According to still further features in the described
preferred embodiments, the graphite powder has a weight ratio with
respect to the sum of the carbon black particles and the expanded
graphite particles, the weight ratio falling in a range of 1.5:1 to
3.5:1.
[0085] According to still further features in the described
preferred embodiments, the graphite powder has a weight ratio with
respect to a sum of the carbon black particles, the expanded
graphite particles, and the zirconium oxide particles, the weight
ratio falling in a range of 1.2:1 to 3:1.
[0086] According to still further features in the described
preferred embodiments, the total carbon content of the conductive
carbon layer includes the graphite powder, the carbon black
particles, and the expanded graphite particles, and the weight
ratio of the total carbon content and the zirconium oxide particles
to the titanium dioxide particles falls in a range of 1.5:1 to
4:1.
[0087] According to still further features in the described
preferred embodiments, the width (i.e., the narrow dimension of the
face) of the conductive carbon layer is at least 5 cm.
[0088] According to still further features in the described
preferred embodiments, the width of the conductive carbon layer is
at least 8 cm.
[0089] According to still further features in the described
preferred embodiments, the width of the conductive carbon layer is
at least 10 cm.
[0090] According to still further features in the described
preferred embodiments, the width of the conductive carbon layer is
at least 15 cm.
[0091] According to still further features in the described
preferred embodiments, the expanded graphite particles have a long
dimension of at least 15 micrometers, on average.
[0092] According to still further features in the described
preferred embodiments, the expanded graphite particles have a long
dimension of at least 25 micrometers, on average.
[0093] According to still further features in the described
preferred embodiments, the expanded graphite particles have a long
dimension of at least 40 micrometers, on average.
[0094] According to still further features in the described
preferred embodiments, the expanded graphite particles have a long
dimension of at least 60 micrometers, on average.
[0095] According to still further features in the described
preferred embodiments, the titania particles have an average
primary particle size below 50 nm.
[0096] According to still further features in the described
preferred embodiments, the titania particles have an average
primary particle size below 30 nm.
[0097] According to still further features in the described
preferred embodiments, the method further includes, prior to
providing the cathode having the sintered conductive carbon layer,
the step of disposing a conductive carbon paste opposite the anode,
the paste including: (i) carbon black particles; (ii) graphite
powder; (iii) expanded graphite particles, (iv) at least one
ceramic oxide precursor, the precursor selected from the group of
precursors consisting of a titania precursor and a zirconia
precursor, and (v) a liquid vehicle, wherein the sintered
conductive carbon layer is produced by sintering the conductive
carbon paste at a temperature above 300 C, to convert the at least
one ceramic oxide precursor to sintered ceramic oxide
particles.
[0098] According to still further features in the described
preferred embodiments, the disposing of the conductive carbon paste
opposite the anode is performed by screenprinting the paste.
[0099] According to still further features in the described
preferred embodiments, the method further includes the step of
disposing a porous, electrically insulative separator on the anode,
to electrically insulate between the anode and the cathode.
[0100] According to still further features in the described
preferred embodiments, the method further includes the step of
disposing a porous, electrically insulative separator on the anode,
to electrically insulate between the anode and the cathode, wherein
the disposing of the conductive carbon paste is performed directly
on a surface of the porous, electrically insulative separator.
[0101] According to still further features in the described
preferred embodiments, the ceramic oxide precursor includes a
titanium or zirconium chelate.
[0102] According to still further features in the described
preferred embodiments, the ceramic oxide precursor is a titania
precursor.
[0103] According to still further features in the described
preferred embodiments, the titania precursor includes an organic
titanate.
[0104] According to still further features in the described
preferred embodiments, the titania precursor includes a compound
selected from the group of titania precursors consisting of organic
titanium chelates, aqueous titanium chelates, organic titanates and
ortho-titanate esters.
[0105] According to still further features in the described
preferred embodiments, the titania precursor includes a titanium
acetylacetonate.
[0106] According to still further features in the described
preferred embodiments, the liquid vehicle includes an alcohol.
[0107] According to still further features in the described
preferred embodiments, the sintering of the conductive carbon paste
to convert the ceramic oxide precursor to sintered ceramic oxide
articles is performed in an air-containing environment, the ceramic
oxide precursor includes a fumeless precursor that reacts in the
air-containing environment in a substantially non-fuming
fashion.
[0108] According to still further features in the described
preferred embodiments, the cathode includes a support matrix
structurally supporting the conductive carbon layer and sintered
thereto.
[0109] According to still further features in the described
preferred embodiments, the conductive carbon layer includes
zirconia particles, intimately dispersed within the conductive
carbon layer.
[0110] According to still further features in the described
preferred embodiments, the conductive carbon layer includes
zirconia particles, intimately dispersed within the conductive
carbon paste.
[0111] According to still further features in the described
preferred embodiments, the method further includes the step of
disposing an electrolyte within the cell wall, the electrolyte
containing an iodine-based redox species.
[0112] According to still further features in the described
preferred embodiments, the screenprinting of the paste is performed
over a cell width of at least 5 cm, and preferably, at least 10 or
at least 15 cm.
[0113] According to still further features in the described
preferred embodiments, the sintered conductive carbon layer is
produced by sintering the conductive carbon paste at a temperature
above 350 C.
[0114] According to still further features in the described
preferred embodiments, the sintered conductive carbon layer is
produced by sintering the conductive carbon paste at a temperature
above 400 C.
[0115] According to still further features in the described
preferred embodiments, the matrix is selected from a group of
matrices consisting of glass fiber mat, carbon fiber mat, zirconia
felt, titania felt and carbon foam.
[0116] According to yet another aspect of the present invention
there is provided a method for producing photovoltaic cells of the
present invention, substantially as disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0117] The invention is herein described, by way of example only,
with reference to the accompanying drawings. With specific
reference now to the drawings in detail, it is stressed that the
particulars shown are by way of example and for purposes of
illustrative discussion of the preferred embodiments of the present
invention only, and are presented in the cause of providing what is
believed to be the most useful and readily understood description
of the principles and conceptual aspects of the invention. In this
regard, no attempt is made to show structural details of the
invention in more detail than is necessary for a fundamental
understanding of the invention, the description taken with the
drawings making apparent to those skilled in the art how the
several forms of the invention may be embodied in practice.
Throughout the drawings, like-referenced characters are often used
to designate like elements.
[0118] In the drawings:
[0119] FIG. 1 is a partial cross-sectional view of a photovoltaic
cell of the prior art;
[0120] FIG. 2 is a schematic cross-sectional view of a photovoltaic
cell having an open-face structure, according to one aspect of the
present invention;
[0121] FIG. 3 is a schematic top view of the anode substrate of
FIG. 2;
[0122] FIG. 4 is a schematic cross-sectional view of a photovoltaic
cell having an open-face structure, in which the substrate of the
anode includes a conductive-glass sheet having grooves containing
conductive metal wires configured to boost current take-off;
[0123] FIG. 5 is a schematic cross-sectional view of an inventive
photovoltaic cell having solely a single sheet of conducting glass
per cell, supporting the anode, and a conducting metal foil,
associated with the cathode, as a current takeoff component;
[0124] FIG. 6 is a schematic cross-sectional view of a photovoltaic
cell having an inventive bi-plate construction;
[0125] FIG. 7 is a schematic cross-sectional view of another
embodiment of a photovoltaic cell having an inventive bi-plate
construction;
[0126] FIG. 8 is a schematic cross-sectional view of another
embodiment of an inventive photovoltaic cell having a highly
conductive, corrosion-resistant foil as the current collector of
the cathode;
[0127] FIG. 9 is a schematic cross-sectional view of a photovoltaic
cell having a highly conductive, metal, corrosion-resistant plate
as the current collector of the cathode, according to another
embodiment of the present invention;
[0128] FIG. 10A is a schematic cross-sectional representation of a
porous carbon paste layer following sintering, according to the
present invention;
[0129] FIG. 10B is a schematic cross-sectional representation of a
porous carbon paste layer following sintering, according to the
prior art;
[0130] FIG. 11 is a schematic cross-sectional view of an
"open-faced" photovoltaic dye cell, according to one preferred
embodiment of the present invention;
[0131] FIG. 12 provides a schematic cross-sectional view of a dye
cell according to another preferred embodiment of the
invention;
[0132] FIG. 13A provides a schematic cross-sectional view of an
inventive photovoltaic dye cell having a bi-plate structure;
[0133] FIG. 13B is a schematic top view of the cell of FIG. 13A,
showing the disposition of the anchoring points of the cell;
[0134] FIG. 14 is a schematic cross-sectional view of a
photovoltaic dye cell having a bi-plate structure, according to
another preferred embodiment of the present invention;
[0135] FIG. 15 provides a schematic cross-sectional view of a
photovoltaic dye cell having conductor-filled grooves as cathodic
current take-off means;
[0136] FIG. 16 is a schematic cross-sectional view of a monolithic
dye cell assembly of the prior art;
[0137] FIG. 17 is a schematic, expanded side view of a junction
between two adjacent cells of a monolithic multiple dye cell
assembly according to the present invention, in which junction are
disposed a pair of electrodes;
[0138] FIG. 18 provides a schematic, partially cut-open
representation of one preferred embodiment of the inventive
multiple-cell module, in which the intercell junctions are adapted
to receive an imposed voltage to prevent or reduce intercell ion
migration;
[0139] FIG. 19 is an exemplary, schematic, cross-sectional side
view of a large-area monolithic single cell that might be
fabricated based on the prior art;
[0140] FIG. 20 is a schematic top view of the cell of FIG. 19,
showing printed areas in long strip form, wherein adjacent printed
areas are separated by conductor structures;
[0141] FIG. 21 provides an exemplary, schematic cross-sectional
side view of a portion of the inventive structure, in which
separate strips of a titania layer and insulating spacer layer are
disposed on a glass substrate, so as to leave a gap between the
strips, and
[0142] FIG. 22 is an exemplary schematic cross-sectional side view
of one embodiment of a photovoltaic dye cell of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0143] The principles and operation of the photovoltaic cells
according to the present invention may be better understood with
reference to the drawings and the accompanying description.
[0144] Before explaining at least one embodiment of the invention
in detail, it is to be understood that the invention is not limited
in its application to the details of construction and the
arrangement of the components set forth in the following
description or illustrated in the drawings. The invention is
capable of other embodiments or of being practiced or carried out
in various ways. Also, it is to be understood that the phraseology
and terminology employed herein is for the purpose of description
and should not be regarded as limiting.
[0145] The present invention provides an improved structure for
large-area, broad, single dye cells. A schematic cross-sectional
view of a first aspect of the invention, referred to as a
monolithic photovoltaic cell 200, is provided in FIG. 2. A
substantially transparent anode support such as a conventional
photoanode support glass 30, having a typical thickness of about 2
mm, has a thin, substantially transparent conducting surface layer
or coating 32. Conducting surface layer 32 may be advantageously
based on tin oxide, and also may be provided with electrical
conductivity enhancing features. Conducting surface layer 32 is
used as the substrate and basic building block for photovoltaic
cell 200, as disclosed in our above-described U.S. Patent
Application Publication No. 20050072458, which is hereby
incorporated by reference for all purposes as if fully disclosed
herein.
[0146] Photoanode support glass 30 is coated with a porous,
nanocrystalline, high band-gap layer such as a porous
nanocrystalline titania layer 38, such that titania layer 38
physically and electrically communicates with conducting surface
layer 32 over most of the area (footprint) of cell 200. The surface
of titania layer 38 facing the interior of cell 200 is stained by a
dye to produce an ultra-thin (typically a monolayer) dye layer 40
along the interior surface of titania layer 38. Titania layer 38,
along with ultra-thin dye layer 40, essentially make up a
photoanode 42 of cell 200.
[0147] Titania layer 38 may be applied using various techniques
known in the art, including screenprinting of a
titania-powder-based paste or a sol-gel-based titania precursor
paste followed by drying and sintering in air. Other techniques of
depositing a titania coating are applicable prior to the sintering
step, for example, electrophoretic deposition and/or pressing. The
thickness of the nanocrystalline titania layer, after sintering, is
typically about 15 microns.
[0148] In order to appreciably boost or enhance current collection,
the cell may employ a plurality of preferably parallel, conducting
plated-on metal strips 35. Metal strips 35 may advantageously
transfer current from photoanode 42 towards a side wall of cell
200.
[0149] Conducting plated-on metal strips 35 may be made of a metal
or metal alloy selected to resist corrosion by the electrolyte and
also to resist carrier recombination, even under the harsh working
conditions of the cell, preferably for a period of at least 7-10
years, which is often considered to be a reasonable lifetime for
cells to be economically viable.
[0150] The width of conducting plated-on strips 35 is limited such
that strips 35 may shade only about 15% or less of the available
cell area. Preferably, strips 35 shade less than about 10%, or more
preferably, shade less than about 5% of the available cell
area.
[0151] Above titania layer 38 and dye layer 40 is disposed an
electrically-insulating spacer layer 44, which is advantageously
coarse, porous and thin, and serves to insulate photoanode 42.
During the manufacturing of cell 200, screenprinting or
electrophoresis may be used to apply electrically-insulating spacer
layer 44 directly on top of titania layer 38 (with dye layer 40
being introduced much later in the process). Typically, spacer
layer 44 contains coarsely divided oxides that are substantially
inert to the electrolyte and to dye cell processes. Typical raw
materials for the spacer are alumina, magnesia, silica and/or
zirconia powders. For non-nano-sized compositions based on
relatively coarse powders selected from a particle size range of
about 0.5 microns to 5 micron particle size, and incorporating an
inert ceramic binder and/or a heat fugitive organic/inorganic
binder/pore former, spacer layer 44 can have a thickness ranging
from as little as 2 micrometers to 15 micrometers or more,
following sintering. This spacer layer bonds well to titania layer
38 thereunder, and provides adequate electrical insulation, as well
as good porosity and electrolyte permeation at minimal ohmic
drop.
[0152] Above spacer layer 44, on the cathodic side of cell 200, is
disposed a porous, catalytic carbon layer 48, which is adapted to
catalyze the iodine redox reaction in the cell. The thickness of
catalytic carbon layer 48 may be less than about 5 micrometers, and
typically, may be about 2-3 micrometers or less.
[0153] In terms of cell construction, catalytic carbon layer 48 may
be screenprinted onto spacer layer 44. It may be advantageous to
use a sinterable carbon paste including an inert ceramic binder
and/or a heat fugitive organic/inorganic binder/pore former,
together with a catalytic material adapted to catalyze the iodine
redox reaction in the cell. This catalytic material may include
high surface area carbon (at least 50 m.sup.2/g), platinum, and/or
a catalytic platinum substitute. An example of an inert ceramic
binder is a titania powder or titania precursor; an exemplary heat
fugitive organic binder/pore former is cellulose; an exemplary heat
fugitive inorganic binder/pore former is ammonium carbonate. The
paste may include a heat fugitive vehicle such as terpineol, to
enhance printability.
[0154] Above catalytic carbon layer 48 is disposed a thick
(typically at least 50-100 microns), conductive carbon layer 52.
Conductive carbon layer 52 may be made from a sinterable paste
including primarily highly conductive graphite powder together with
an inert ceramic binder and/or a heat fugitive organic/inorganic
binder/pore former. For thick carbon layers (above 100 microns in
thickness) the paste can contain an inert bulking filler such as
glass microspheres, to ensure an adherent, cohesive layer following
sintering. In some cases, for instance, where catalytic carbon
layer 48 provides adequate cell performance, even in the absence of
platinum catalyst, carbon layer 48 and carbon paste 52 may be
printed as a single layer including both catalytic and conductive
carbons. Current collection over large broad areas from the carbon
cathode is achieved by at least one highly conductive metal
component 54, which may include a plurality of inert, preferably
parallel or near-parallel wires, a metal mesh, or a perforated
metal foil. Conductive metal component 54 may be at least partly
buried in conductive carbon layer 52.
[0155] We have found very few metals that show adequate stability
for the job of cathode current collector, amongst them titanium,
titanium clad on copper, tungsten, molybdenum, chromium and their
alloys. Various sheet forms of carbon or graphite may also be
used.
[0156] Surprisingly, specific stainless steel compositions appear
to provide acceptable corrosion resistance during long-term
operation of the highly corrosive iodine-based dye cells, despite
containing over 50% iron. One of these compositions is a ferritic
stainless steel composition based on chromium, iron and molybdenum.
This alloy may contain, by weight, about 29-30% chromium and 3-5%
molybdenum, the balance consisting primarily of iron. Higher values
of molybdenum (e.g., 5-10%) and/or chromium (e.g., 30-35%) may
enhance the corrosion resistance of the alloy.
[0157] Another such ferritic stainless steel alloy contains
chromium, niobium and iron. This niobium-strengthened alloy may
contain, by weight, about 17-19% chromium and 0.5-1% niobium, the
balance consisting primarily of iron. Higher values of niobium
(e.g., 1-3%) and/or chromium (e.g., 19-25%) may enhance the
corrosion resistance of this alloy.
[0158] It must be emphasized that the above-disclosed alloys
represent specific metallurgical structures that appear to provide
satisfactory long-term corrosion resistance in the environment of
the cathode of iodine-based photovoltaic cells, and specifically as
current collectors. These alloys are preferably free, or
substantially free, of nickel.
[0159] It should be emphasized that the various layers in the cell
may be applied and dried sequentially, and then bonded together in
a single sintering operation. This option offers considerable
inbuilt process advantages. It should be appreciated, however, that
if single step sintering is in fact desired, all component
materials in the various layers must be selected with the
capability of withstanding the usual sintering temperature for
nanocrystalline titania (typically 450.degree.-500.degree. C.) with
no loss of critical properties such as inertness, bonding or
electrical conductivity. If one-step sintering is not required,
however, other, lower-temperature curable binders may be used in
the spacer and carbon layers, for example Teflon.TM.,
polyvinylidene difluoride (PVDF) or other inert organic or
inorganic binders, and other application techniques (spray,
calendaring) may be utilized.
[0160] The cell is activated by dye staining of the titania layer
followed by electrolyte addition. Both steps can be carried out by
direct printing of the respective dye or electrolyte fluids onto
the outer porous carbon layer. The high porosity of the carbon,
spacer, and titania layers ensures fast and thorough transport of
the fluids through the layers. The cell is then covered over and
sealed at the edges using for example hot sealing via a polymer
film or via a spray-on polymer powder formulation 65. Polyethylene
and polyethylene vinyl acetate (EVA) based thermoplastic polymers
such as Surlyn.TM. and Bynel.TM., as well as UV-curable epoxies and
silicones have proven to be effective sealants, especially if
backed up by an outer epoxy layer 70, and long term sealing is
achieved using an outer foil or foil laminate layer 75, based, for
example, on aluminum. In an alternative embodiment, the sealants
are confined to the cell edges and bond to a sheet of window glass
(not shown) or the like to seal the cell.
[0161] Photoanode support glass 30 and the external element of the
cathode arrangement, such as foil layer 75, together form a housing
substantially enclosing cell 200.
[0162] As may be seen from FIG. 3, which is a top view of the
plated-on current collectors on the anode substrate (not shown),
current take-off from the cell anode is via a section of a plated
metal strip 36 common to and disposed perpendicularly to metal
strips 35, and preferably situated at the cell edge outside the
cell seal area. Plated strip 36 can take a welded-on or soldered-on
lead, and if situated outside the cell seal area, may even be of a
different plated composition or a fired silver paste.
[0163] We have discovered that metal strips 35, 36 may achieve
long-term stability, even in the presence of the corrosive
electrolyte containing the iodine-based redox species, using a
W--Ni--Co alloy. Metal strips 35, 36 may be prepared using a
commercial electroplating bath containing tungsten, nickel, and
cobalt such as Enloy.RTM. Ni-150, provided by Enthone Inc.
[0164] In the case of the cathode, current takeoff is via the
wires, mesh or perforated foil emerging via the seal that may be
welded or soldered onto a current collecting tab external to the
cell housing. The large cell is then ready for encapsulation,
preferably including a UV filtering layer on the surface facing the
sun, and this is followed by cell modulizing into a structural
support and the building up of photovoltaic panels.
[0165] FIG. 4 provides a cross-sectional view of a photovoltaic
cell 400 according to another embodiment of the present invention.
Cell 400 is similar to the cell of FIG. 2, but has at least one
significant distinguishing arrangement: an anode substrate 80 has a
plurality of grooves 85, each of which is much deeper than thin,
conducting surface layer 82. Consequently, the electrical
continuity of conducting surface layer 82 is broken in the areas of
grooves 85. As taught in our above-referenced published patent
application, each groove 85 is fitted with a current take-off
arrangement, such as conductive wires 95 bonded to anode substrate
80 and specifically to conducting surface layer 82 by a chemically
inert (with respect to the electrolyte, iodine-based redox species,
etc.), electrically conducting binder 90. Electrically conducting
binder 90 is disposed within grooves 85 and over grooves 85 so as
to electrically bridge or connect between the severed portions of
conducting surface layer 82. Conducting surface layer 82 may
advantageously include, or at least largely consist of, tin oxide.
The rest of the construction of cell 400 may be substantially as
shown in FIG. 2.
[0166] In FIG. 5 is provided a cross-sectional view of a
photovoltaic cell 500 that represents a departure from the
monolithic construction taught hereinabove, but nonetheless enables
the use of only a single conducting glass sheet per cell, and a
relatively close placement of anode and cathode. However, in
contrast to the cathodic arrangements disclosed hereinabove (by way
of example, in FIG. 4), cell 500 has a distinct, self-supporting
(relative to printed layers), laid on current takeoff component for
withdrawing current from conductive carbon layer 52. As in some of
the previous embodiments, e.g., the cell of FIG. 2, photoanode
support glass 30 has a thin, substantially transparent conducting
surface layer or coating 32 (e.g., a tin oxide layer in various
commercially available FTO glasses), which is in turn partially
covered by plated conductors such as plated parallel conductors 35.
On the exposed surface is disposed nanocrystalline titania layer
38, essentially as described hereinabove. Spacer layer 44 and
carbon layers 48, 52 are printed on. After sintering, dye and
electrolyte may be introduced via carbon paste layer 52, producing
dye layer 40. Cell 500 is then closed off with a conductive current
collecting component such as conductive metal sheet 100, which
presses onto and makes good electrical contact with conductive
carbon layer 52. Metal sheet 100, which acts as a current collector
for cathode 62, may be selected from the following metals:
titanium, tungsten, molybdenum or chromium and alloys in which
these metals are the principal elements, or higher alloys
containing, principally or essentially, chromium, niobium and iron,
or chromium, iron and molybdenum, as described hereinabove. Metal
sheet 100 may be hot sealed to the edges of photoanode support
glass 30 using sealants, for example, a polymeric, internal sealing
layer 110 as an internal sealing layer and an external epoxy layer
120 as an external sealing layer adapted to seal cell 500 from an
external environment. Current take-off out of cell 500 via metal
sheet 100 may be effected by means of a projecting strip or a
welded-on element such as a metal strip 89, which electrically
communicates with metal sheet 100.
[0167] In related embodiments (not shown), the sheet may be
prepared with a conductive carbon layer to better mate conductively
with carbon layer 52 on the cell assembly. Alternatively, carbon
layers 52, 54 may even be entirely obviated if the sheet is
catalyzed directly with platinum or platinum substitutes. As an
alternative embodiment to a sheet as a current takeoff component, a
metal mesh may be used (passing sealably out through the edge seal
of the cell) and the cell is sealed off by a plain glass, or by
another suitable sheet (e.g., plastic) covering the mesh, as shown
in FIG. 8 hereinbelow.
[0168] Although in-assembly dye staining and electrolyte filling is
preferred, it is possible to vacuum-fill cells of this type with
electrolyte, even after assembly, via holes pre-drilled in the
glass or sheet faces, and followed by sealing off the holes with
polymer.
[0169] Ideally, the height of the strips above the conductive glass
surface may be less than the thickness of the sintered titania, in
order to avoid steric interference with subsequent layers or
possible short circuits. However, during reduction to practice, we
have discovered that to achieve adequate current takeoff using
metal strips that are substantially inert to the highly corrosive
electrolyte in iodine-based dye cells, and for use in broad, large
area cells of 10-15 cm on a side, the requisite height of the metal
strips may be considerably larger than the height of the titania
layer. Moreover, the metal wires are bonded to the conductive layer
of tin oxide using a conductive adhesive, such that the wire and
adhesive structure may protrude above the conductive layer of the
glass surface by at least about 200 micrometers, and typically, by
at least about 250 or even about 300 or more micrometers.
[0170] Consequently, the wire and adhesive structure may protrude
above layer of sintered titania by at least about 180 micrometers,
and typically, by at least about 230 or even about 280 micrometers
or more. While it is possible to increase the thickness of the
layer of sintered titania above 15-20 micrometers, a significant
increase in the thickness may result in delamination of the layer,
wastes expensive dye, and may introduce various mass transfer
limitations with respect to the redox species. From a practical
standpoint, therefore, the problem of the wire and adhesive
structure protruding well above the titania layer cannot be
remedied by increasing the thickness of the sintered titania.
[0171] Thus, in reducing to practice the teachings of U.S. Patent
Application Publication No. 20050072458, protrusions of the wire
and adhesive structure towards the interior of the cell may dictate
a relatively large spacing between anode and cathode--approaching
or exceeding 100 microns--which can result in excessive ohmic loss
from the electrolyte.
[0172] FIG. 6 is a schematic cross-sectional view of a photovoltaic
cell 600 having an inventive construction enabling use of only one
conducting glass sheet per cell and enabling a fairly close
placement of the anode and the counterelectrode. This assembly may
be termed a biplate construction because the anode and cathode
components in the cell build are physically distinct and not bonded
together. As in FIG. 2, the photoanode includes a porous
nanocrystalline titania layer or film 138 and a dye monolayer
(shown above in FIG. 4), absorbed on a surface of the porous
titania layer. In the photoanode of cell 600, the dye and film are
adapted to convert photons to electrons.
[0173] A photoanode support glass 30 with transparent conductor
layer 32 has spaced, preferably parallel wires or strips such as
wire 110 bonded in place on the surface of the conductive glass by
a conducting adhesive such as an inert conducting ceramic adhesive
115. Porous titania layer 138 is bonded to transparent conductor
layer 32 in most of the area between adjacent wires 110. Both the
titania and the conducting adhesive bonding the wires may be
optionally covered by an electrically insulating layer, for
example, consisting of, consisting essentially of, or including
zirconia, for additional protection against short-circuiting.
[0174] A porous separator, such as a porous, laid-on separator 125,
such as a glass fiber veil or a microporous polymer membrane, may
electrically insulate the photoanode from a counterelectrode 192,
which includes a plurality of strips, each of which may include
conducting carbon layer 185 and a catalytic layer such as catalytic
carbon layer 190 (which may alternatively form a single layer).
Porous separator 125 may be as thin and porous as possible
(preferably having a thickness of less than 50 microns in an
uncompressed state), and of a material that is chemically stable in
the cell (for example: polyethylene, PET, PPS, PEN, fluoropolymer
or glass fiber). Cathodic current collector such as collector sheet
310 may be made of, consist essentially of, or include, a metal
sheet selected from the cathode metals and alloys provided
hereinabove. Collector sheet 310 is adapted to firmly press against
porous separator 125. The metal sheet of collector sheet 310 can be
directly catalyzed with platinum or a platinum substitute, and may
be structured (by corrugation or machining, for example) with
periodically spaced projecting areas (not shown) to ensure close
proximity (via the porous membrane) of catalyst to titania between
the wires of the photoanode.
[0175] Referring again to FIG. 6, a plurality of strips, each
including conducting carbon layer 185 and catalytic carbon layer
190, may be sintered or otherwise bonded to collector sheet 310. In
the assembled cell 600, counterelectrode 192 is disposed
substantially opposite the titania printings, and may be of
pre-designed thickness, such that there is a relatively close
proximity (across the porous separator) of the catalyzed carbon
layer and the titania layer between the wires after cell closure
and sealing. Following dyeing of the titania, the two halves of the
biplate cell are juxtaposed with the porous separator between them,
metal collector sheet 310 is hot sealed to the glass anode
base-plate edges using, by way of example, an inner polymer sealing
layer or seal 165 and an outer epoxy sealing layer or seal 170.
After checking that cell 600 is free from short circuiting, cell
600 is filled with electrolyte, one possibility being adding
electrolyte to the cell via fill holes and later sealing off the
fill holes with polymer. Current take-off from collector sheet 310
out of the cell may be enabled by a projecting strip or a welded-on
element.
[0176] Together, photoanode support glass 30 and collector sheet
310 form a housing substantially enclosing cell 600.
[0177] FIG. 7 provides a cross-sectional view of a photovoltaic
cell 700 according to another embodiment of the present invention.
The construction of this biplate cell enables the use of only a
single conducting glass sheet per cell. This construction further
enables a very close juxtaposition of anode and cathode. A
photoanode support glass 30 coated with substantially transparent
conductor layer 32 has spaced, preferably parallel wires 110 bonded
in place on the surface of the conductive glass by inert conducting
ceramic adhesive 115. Between wires and at the cell edges is a
sintered nanoporous titania layer 138, typically 10-15 microns
thick and covered with an electrically insulating layer 220, e.g.,
including zirconia, typically having a relatively uniform thickness
in a range of 2-15 micrometers. Since the wires may typically have
a diameter of at least 100 microns, and the combined height of the
titania and zirconia layers is typically below about 30 microns,
the wires project substantially above the titania layer and must be
covered also by an electrically insulating layer 225, for example
including zirconia, to avoid short circuiting to a counterelectrode
230 of cell 700.
[0178] Counterelectrode 230 is a distinct component laid onto the
anode element and disposed in close proximity thereto.
Counterelectrode 230 includes a porous support matrix 240
supporting impregnated carbon 238. Porous matrix 240 may include a
mat, woven and/or non-woven, foam, or possibly other matrices known
in the art. A preferred material for the mat is glass fiber since
it is low cost, flexible and conformable to the cell geometry,
inert in the cell environment, and can withstand elevated curing or
sintering temperatures. Other fibers such as carbon fibers may also
be used. Impregnated carbon 238 is preferably bonded to porous
support matrix 240 by an inert binder, which may be selected from
inorganic materials such as alumina, or polymeric materials such as
polytetrafluoroethylene (PTFE, or Teflon.TM.). Counterelectrode 230
may include a conductive layer 235 and a catalytic layer 245
(catalytic layer 245 being disposed towards titania layer 138), or
a combination thereof. Current takeoff from counterelectrode 230
may be effected via an electrically conducting sheet 250, which
presses against the carbon structure and may be structured with
periodic projecting or protruding areas (not shown), to ensure
close proximity of counterelectrode 230 to titania layer 138
between adjacent wires 110. Conducting sheet 250 may be selected
from the cathode metals and alloys provided hereinabove.
[0179] Following dyeing of the titania, counterelectrode 230 and
current collecting sheet 250 may be pressed together and conducting
sheet 250 is then hot sealed to the glass anode base-plate edges
using a polymeric sealing layer 165 (e.g., containing Surlyn.TM.)
and an outer epoxy layer 170 adapted to seal the cell from an
external environment. After checking that the cell does not
short-circuit, the cell is filled with electrolyte, for example, by
addition of electrolyte to the cell via fill holes in sheet 250
followed by sealing off the fill holes with polymer. Current
take-off from photovoltaic cell 700 via sheet 250 may be effected
by a projecting conductive strip or a welded-on conductive
structural component.
[0180] While various sandwich structures, in which a carbon layer
is printed on top of a zirconia spacer layer on top of a titania
layer, may allow close spacing of carbon and titania surfaces, it
must be emphasized that such structures may be extremely prone to
penetration of the carbon printing layer to the conducting glass
surface, via any holes or defects in the printed layers of titania
or spacer. The penetration of the carbon printing layer to the
conducting surface of the FTO glass effects short-circuiting of the
cell, thereby reducing or limiting cell performance. This problem
is particularly acute in view of the at least 7-10 year lifetime
often required for cells to attain commercial viability. The
inventive biplate construction, as described herein, avoids this
significant and potentially critical problem, because the carbon
counterelectrode is not sintered (i.e., chemically bonded at high
temperature) to the printed layers of titania or spacer. Rather,
the carbon counterelectrode is a distinct and separate entity that
is lies on top of, and physically contacts, the separator or spacer
layer, but is not bonded thereto, as in printed carbon layer
technologies.
[0181] FIG. 8 is a schematic cross-sectional view of another
embodiment of an inventive photovoltaic cell 800 having a cathodic
current collector including, or consisting essentially of, a
conductive, chemically inert mesh or foil 195, preferably disposed
above, and physically contacting, counterelectrode 320. In this
embodiment, counterelectrode 320 may be sintered onto electrically
insulating layer 220, typically zirconia, which has been described
hereinabove with respect to FIG. 7. For configurations in which
counterelectrode 320 includes more than one layer, e.g., a
conductive layer and a catalytic layer, the catalytic layer is
disposed towards electrically insulating layer 220.
[0182] The anodic arrangement of photovoltaic cell 800 may be
substantially identical to the arrangement provided in FIG. 7 and
has been described hereinabove. The sealing arrangement of cell 800
may be substantially identical to the arrangement provided in FIGS.
6 and 7 and has been described hereinabove.
[0183] Briefly, photoanode support glass 30 coated with
substantially transparent conductor layer 32 may have spaced,
preferably parallel wires 110 bonded in place on the surface of the
conductive glass by inert conducting ceramic adhesive 115. Between
wires and at the cell edges is a sintered nanoporous titania layer
138, typically 10-15 micrometers thick, which is covered with
electrically insulating layer 220, e.g., including zirconia. Since
the wires may typically have a diameter of at least 100
micrometers, and the combined height of the titania and zirconia
layers is typically below about 30 micrometers, the wires project
substantially above the titania layer and may be covered also by an
electrically insulating layer 225, for example including zirconia,
to avoid short circuiting to counterelectrode 320.
[0184] The superior current collection performance of conductive
mesh or foil 195 obviates the need for sealing the cathodic
arrangement with expensive conductive glass such as FTO glass.
Instead, cell 800 may be sealed on top by a sheet such as ordinary,
inexpensive glass sheet 190, disposed above mesh or foil 195, and
may also be adapted to provide the requisite structural support for
mesh or foil 195 as well as the requisite compressive force against
mesh or foil 195.
[0185] Conductive mesh or foil 195 may pass sealably out through
seals 165, 170 of photovoltaic cell 800. Conductive mesh or foil
195 may include, or consist essentially of, a metal mesh or foil,
preferably selected from the cathode metals and alloys disclosed
hereinabove.
[0186] Conductive mesh or foil 195 may include, or consist
essentially of, a graphite mesh or foil, or a carbon mesh or foil.
Composite cathodic current collectors including graphite and/or
carbon and/or metal meshes or foils may also be practical. It may
be particularly advantageous to use a graphite or carbon foil to
cover the central area of photovoltaic cell 800, and to boost
current takeoff near the edges of cell 800 by electrically
connecting the edges of the carbon or graphite foil to a metal mesh
and/or foil disposed near a perimeter of cell 800 and passing
sealably out through seals 165, 170 of cell 800.
[0187] FIG. 9 is a schematic cross-sectional view of another
embodiment of an inventive photovoltaic cell 900, which may have a
metal, conductive, substantially chemically inert sheet 330 as the
current collector of the cathode. The anodic arrangement of
photovoltaic cell 900 and the sealing arrangement of cell 900 may
be substantially identical similar to those provided in FIG. 7 and
is described hereinabove.
[0188] Beneath sheet 330 is disposed a counterelectrode 305, which
is juxtaposed, but preferably not bonded to, sheet 330.
Counterelectrode 305 may be substantially similar to
counterelectrode 230, which is provided in FIG. 7 and is described
hereinabove. However, counterelectrode 305 is provided as a
plurality of substantially self-supporting strips, which are laid
down on top of porous, electrically insulating layer 220, which is
typically a zirconia printing. Counterelectrode 305 may be of a
pre-designed thickness to achieve a relatively close proximity
(across electrically insulating layer 220) of the catalyzed carbon
layer and the titania layer between the wires after closure and
sealing of cell 900. Moreover, a counterelectrode mat that is
continuous over the entire area of the cell may have trouble
conforming to the protrusions formed, inter alia, by wires 110 and
ceramic adhesive 115, increasing the distance between the catalyst
and the titania in the area immediately around the protrusions.
Thus, by laying down counterelectrode 305 in strip form, between
these protrusions, the distance between the catalyst and the
titania in the area immediately around the protrusions may remain
low and substantially constant over the entire area of cell 900,
thereby minimizing ohmic losses and improving power
performance.
[0189] With reference again to FIG. 8, the prior art does not
appear to provide carbon pastes that can be successfully applied by
screenprinting to large area cells of the monolithic or single cell
design with cell areas significantly above 10 sq. cm.
[0190] Screenprinting, rather than doctor blading, is the
application method of choice for industrial production of broad,
large-area dye cells. However, when water based formulations are
attempted for screenprinting of the carbon for dye cells there
emerge several obstacles. Firstly, the prior art water-based carbon
pastes tend to be quick drying and can clog the screens used in
production scale screenprinting. Certain additives may ease this
situation somewhat.
[0191] A more critical problem, however, is that after sintering,
these pastes have insufficient electrical conductivity for
practically useful printing thicknesses. Even more problematic is
the fact that after screenprinting and sintering--and even with
application of multiple layers--the layers have insufficient
mechanical strength (bonding within the layer itself and to the
substrate beneath) and can delaminate. Often, cracks and holes are
evident in the porous structure and carbon particles may dislodge
or delaminate from the layer, in some cases shorting out the cell
between the conducting glass of the anode and the
counterelectrode.
[0192] The insufficient mechanical strength is principally a result
of the incorporation of particulate titania intrinsically as the
binder, since this material does not sinter strongly enough to the
carbon materials in the paste nor to the substrate beneath.
[0193] In the present invention, a titania precursor is used
(instead of titania powder) which, during the usual sintering in
air, provides titania centers in situ throughout the layer and
forms stronger bonds with the carbon materials used and, in some
preferred embodiments, with the substrate beneath (usually a spacer
layer of porous zirconia). We have further discovered that many
precursors that oxidize acceptably to titania on sintering in air,
for example the titanium alkoxides, appear to be unsuitable for
paste formulation, because they fume strongly in air and are
difficult to work with. A non-limiting example of a benign,
suitable precursor is the family of organic titanates supplied by
DuPont under the trade name Tyzor.TM.. These organic titanates are
available in the form of organic chelates, aqueous chelates, and
ortho-titanate esters. These precursors contain about 15 wt % of
titania equivalent, and oxidation to titania is usually complete on
heating in air at above 300 C for 30 minutes. We have found
particularly useful the Tyzor.TM. grade AA-75, which is based on
titanium acetylacetonates in an alcoholic solvent. However, other
grades of Tyzor.TM. are also applicable. The parallel zirconium
chelates may also be used (which sinter in situ to zirconia), but
these are more expensive than their titanium analogs.
[0194] We have further discovered that the conductivity and
strength of the conductive carbon layer may be appreciably improved
by the addition of a material known as expanded graphite to the
paste. Prior art carbon pastes for the dye cell cathode (which were
not conducting or robust enough) normally consist of high surface
area conductive grade carbon black (at least 150 sq. meter per
gram) with typical particle size of up to 50 nanometers, together
with graphite powder having an average particle diameter of several
micrometers as the main additive for boosting conductivity.
[0195] Expanded graphite, which is graphite that has been
chemically exfoliated to give large, flat, anisometric,
two-dimensional platelets of particle size selectable from about
20-100 micrometers and having a surface area of about 10-30 sq.
meters per gram, is a much more effective conductivity booster and
also provides improved anchoring and mechanical stability for the
layer. The carbon, graphite and titania precursor components are
conveniently dispersed in a printing vehicle such as terpineol or
ethyl hexanol. The paste may be made more viscous and printable,
giving a print with improved green strength, by the incorporation
into the terpineol of a heat fugitive thickening agent such as
ethyl cellulose. Additional conducting filler powders that are
chemically stable in the cell working conditions, for example doped
tin oxides, titanium nitride and metal powders or metal coated
carbons, may be added to supplement the carbons. The various
carbons (such as the carbon black) can also be optionally
catalyzed, for example, with platinum, or alternatively a special
thin layer (few micrometers) of catalyzed carbon may be printed
adjacent to the titania or zirconia layers, situated underneath a
conducting carbon layer that is substantially free of precious
metal catalyst. For such thin layers, the catalyzed carbon paste
will advantageously be made up containing a fine zirconia powder
(typically having an average particle size of about 0.5
micrometers), which enhances the adhesion of the catalytic layer.
Similarly, non-conducting bulking materials such as glass spheres
(e.g., having a diameter of 40-80 micrometers) may be included in
the paste in order to facilitate printing of thick adhesive carbon
layers of thickness 100 micrometers and above.
[0196] A schematic cut away representation of an inventive,
sintered, porous, conductive carbon layer 100 is shown in FIG. 10A,
for typical layer thicknesses of 50-100 micrometers. Porous carbon
layer 100 includes at least three different types of carbon-based
materials: graphite powder particles such as graphite particle 1,
carbon black particles such as carbon black particle 2, and large
leaf-like platelets of expanded graphite such as expanded graphite
particle 3. The various carbons (such as the carbon black) may
optionally be catalyzed. Porous carbon layer 100 further includes
titania particles, such as titania particle 4 which has been
freshly generated in-situ. Without wishing to be bound by theory,
we believe that such freshly generated titania strongly bonds the
different materials together and to the substrate below, in those
embodiments in which carbon layer 100 is sintered thereto (such as
electrically insulating layer 220, shown in FIG. 8). FIG. 10A shows
that there is good electrical continuity of the carbon particles
through the sintered paste, conferring a high conductivity. We
further believe that the expanded graphite particles provide a
conduit for removing the cathodic current at minimal ohmic
loss.
[0197] Some titania particles may bond directly to the graphite
particles, others may bond directly to the carbon black particles.
We have discovered that in the photovoltaic cell of the present
invention, titania particles such as titania particles 4a may bond
directly to the expanded graphite platelets such as expanded
graphite particle 3, or to carbon black particles 2 or to graphite
powder particles 1. In sintered porous carbon layer 100, such bonds
are chemical bonds, which provide for a more robust, mechanically
solid carbon layer.
[0198] In a preferred embodiment, the conductive paste includes
zirconia particles and/or a zirconia precursor. FIG. 10A further
provides a schematic representation of zirconia particles 5
disposed within sintered porous carbon layer 100. The zirconia
particles may be sintered to particles 1,2,3,4, providing for a
more robust, mechanically solid carbon layer.
[0199] Conductive carbon layer 100 may advantageously be disposed
on a substrate (e.g., in the form of strips) such as flexible mesh,
fiber such as glass fiber, and foam. Such materials may adapt to
fit the contours of the anode within cell, enabling the catalytic
layer proximate to carbon layer 100 to be juxtaposed against the
porous film of the anode (e.g., a titania layer). Carbon layer 100
may be urged against the porous film by the weight of the cover of
the cell, which may advantageously be made of a glass such as
inexpensive window glass.
[0200] In addition, strips of such substrates, impregnated with
conductive carbon layer 100 and a catalytic layer or component, may
advantageously be disposed in between protrusions in the anodic
side, whereby an exterior surface of the catalytic layer is
substantially flush with the opposing surface (typically titania,
or titania covered with an insulating layer such as zirconia) on
the anodic side.
[0201] In sharp contrast to the microstructure of inventive
sintered carbon layer 100, an exemplary, schematic cross-sectional
cut away illustration of a prior art sintered porous carbon layer
105 is provided in FIG. 10B. As may be seen, spheroidal graphite
particles 1 and carbon black particles 2 do not provide an adequate
electrically conductive network, while titania particles 14, which
were not generated in-situ, are somewhat isolated and do not confer
much mechanical strength to the layer 105, nor can they provide
effective bonding to the substrate below, in those embodiments in
which carbon layer 105 is sintered thereto (such as electrically
insulating layer 220, shown in FIG. 8). Moreover, the pastes used
to produce the prior art sintered porous carbon layers may be
generally difficult to screenprint, may be mechanically weak, and
may not have adequate conductivity for dye cells, and more
particularly, for broad, large-area dye cells.
[0202] The graphite powder used to produce sintered carbon layer
100 may typically include spheroidal particles having an average
particle size of at least 2 micrometers, and more typically, at
least 5 micrometers, up to a maximum average particle size of 10 to
15 micrometers. The surface area of the graphite particles is
typically 15-40 m.sup.2/g, and more typically, 20-30 m.sup.2/g.
[0203] The expanded graphite particles used to produce sintered
carbon layer 100 may be typically anisotropic flakes having a long
dimension of at least 15 micrometers, more typically at least 40
micrometers, and yet more typically at least 60 micrometers, on
average. The surface area of the expanded graphite particles is
typically 5-50 m.sup.2/g, and more typically, 25-35 m.sup.2/g.
[0204] The carbon black particles used to produce sintered carbon
layer 100 may be typically disposed in chainlike clusters of
carbon, in which the individual particles may have an average
primary particle size of 5-100 nm, and more typically, 10-75 nm.
The surface area of the carbon black particles is typically 50-2000
m.sup.2/g, more typically, 100-600 m.sup.2/g, and most typically,
150-300 m.sup.2/g.
[0205] The titania particles in sintered carbon layer 100 may be
particles having an average primary particle size of at least 10
nm, and more typically, at least 20 nm to about 30 nm. The surface
area of the titania particles is typically 20-130 m.sup.2/g, and
more typically, 50-90 m.sup.2/g.
[0206] The zirconia particles in sintered carbon layer 100 may be
spheroidal particles, the zirconia particles having an average
particle size of at least 0.3 micrometers, and more typically, at
least 0.5 micrometers, up to an upper bound of typically one
micrometer. The surface area of the zirconia particles is typically
5-40 m.sup.2/g, and more typically, 10-20 m.sup.2/g. When formed
in-situ, the zirconia particles are typically much smaller.
[0207] In reducing sintered carbon layer 100 of the present
invention to practice, we have discovered that various weight
ratios may be of particular importance. The weight ratio of
spheroidal graphite to the sum of carbon black and expanded
graphite may be preferably in the range of 1.5:1 to 3.5:1.
[0208] The weight ratio of carbon black particles to expanded
graphite particles may be preferably in the range of 1.5:1 to
1:1.5.
[0209] The weight ratio of the total carbon content of sintered
carbon layer 100 (including the graphite powder, carbon black
particles, and expanded graphite particles) to titanium dioxide
particles, may be preferably in the range of 1.2:1 to 3.5:1.
[0210] In those preferred embodiments in which zirconia particles
are present in sintered carbon layer 100, the weight ratio of
carbon black particles to expanded graphite particles to zirconia
particles may be preferably in the range of 1.5:1:0.5 to
1:1.5:1.
[0211] The weight ratio of the total carbon content and zirconia
content of sintered carbon layer 100 to the titania content may be
preferably in the range of 1.5:1 to 4:1. The weight ratio of the
typically spheroidal graphite particles to a sum of the carbon
black particles, expanded graphite particles, and zirconia
particles may be preferably in the range of 1.2:1 to 3:1.
EXAMPLES
[0212] Reference is now made to the following examples, which
together with the above descriptions, illustrate the invention in a
non-limiting fashion.
[0213] Preparation of a dye cell carbon cathode according to the
prior art is described in the following example:
Example 1
Preparation of a Dye Cell Carbon Cathode According to the Prior
Art
[0214] Conductive carbon black (Cabot type Vulcan XC72R, 5 gm),
spherical graphite powder (Timcal, type KS4, 20 gm), and titania
nanocrystalline powder (Degussa type P 90, particle size 14 nm, 4
gm) were dry blended in an agate ball mill for 30 minutes. A
quantity of water (80 gm) was then added to the ball mill
containing the carbon, graphite and the titania powder and milling
was continued overnight.
[0215] The resulting carbon paste (as a cathode paste) was
evaluated for fabrication of small area dye cells (typically below
1-5 sq. cm). The paste could be doctor-bladed and sintered onto
conductive glass, or onto conductive glass with a lower sintered
layer of porous titania covered by an upper sintered layer of
porous zirconia. The carbon layer was adherent for these small
cells, with a conductivity of 10 ohms per square for a thickness of
50 micrometers. On dye and electrolyte application and sealing of
cells, a good efficiency of 8% under one sun illumination (100 mW
per sq. cm) was achieved. However, when screenprinting was
attempted with the paste for larger area cells typically above 10
sq. cm. with a view to industrial scale up, the paste tended to
clog the printing screens, and the print following sintering had
inadequate mechanical strength and conductivity. Particularly
problematic was poor adhesion to the substrate, delamination and
formation of cracks and holes over the surface.
[0216] Preparation of a dye cell carbon cathode according to the
present invention is described in the following example:
Example 2
Preparation of a Screenprintable Carbon Dye Cell Cathode
[0217] Spherical graphite powder (Timcal, type KS4, 20 gm), carbon
furnace black (Cabot, type Vulcan XC72R, 5 gm) and expanded
graphite (Timcal, type BNB90, 2.5 gm) were dry blended in an agate
ball mill for 30 minutes. A titania precursor (Dupont, Tyzor.TM.
type AA-75, 30 gm) was stirred with terpineol (Aldrich, 50 gm) that
contained 10 wt % dissolved ethyl cellulose (Dow, ethocell type
standard industrial 45). This liquid mixture was then added to the
ball mill containing the carbon, spherical graphite powder, and
expanded graphite, and milling was continued overnight. The
resulting carbon paste was screenprinted in a single printing using
a plastic mesh onto a sintered porous zirconia spacer upper layer,
itself part of a conducting glass carrying a sintered-on layer of
nanocrystalline titania (15 micrometers thick) covered by the
porous sintered zirconia layer and appropriate for dye cell
fabrication for cell areas in excess of 10 sq. cm. The wet
thickness of the carbon print was 100 micrometers. The carbon print
was dried at 150 C for 10 minutes. The print was then sintered in
air at 450 C for 30 minutes, giving a sintered thickness of 50
micrometers. In order to achieve the desired conductivity, a second
printing of carbon paste, drying and sintering was carried out, and
the final sintered carbon thickness was 50 micrometers. Adherence
of the carbon layer to the underlying zirconia layer was excellent,
there was good integrity of the layer with no flaking off, and
there were no cracks or holes. The electrical conductivity of the
layer was 10 ohms per square at this thickness. Since the
conducting glass used to support the titania layer had a similar
conductivity of 10 ohms per square there was good matching of
photoanode and counter electrode conductivities, one of the
requisites for high dye cell efficiencies. The carbon layer was
quite porous and allowed fast transfer of dye and electrolyte
printings to the titania layer below. The dye cell, after dye
staining of the titania, electrolyte addition and sealing, gave 7%
efficiency under one sun illumination of 100 mW per sq. cm.
Example 3
Preparation of a Screenprintable Carbon Dye Cell Cathode
[0218] Spherical graphite powder (Timcal, type KS4, 20 gm), carbon
furnace black (Cabot, type Vulcan XC72R, 5 gm), expanded graphite
(Timcal, type BNB90, 5 gm) and zirconium dioxide (IAM, type
40R-080, 3 gm) were dry blended in an agate ball mill containing
agate balls for 30 minutes. A titania precursor (Dupont, Tyzor.TM.
type AA-75, 60 gm) was stirred with terpineol (Aldrich, 25 gm) that
contained 10 wt % dissolved ethyl cellulose (Dow, ethocell type
standard industrial 45). This liquid mixture was then added to the
ball mill containing the carbon, spherical graphite powder, and
expanded graphite, and milling was continued overnight. The
resulting carbon paste was screenprinted in a single printing using
a plastic mesh onto a sintered porous zirconia spacer upper layer,
itself part of a conducting glass carrying a sintered-on layer of
nanocrystalline titania (15 micrometers thick) covered by the
porous sintered zirconia layer and appropriate for dye cell
fabrication for cell areas in excess of 10 sq. cm. The wet
thickness of the carbon print was 100 micrometers. The carbon print
was dried at 150 C for 10 minutes. The print was then sintered in
air at 450 C for 30 minutes, giving a sintered thickness of 50
micrometers. In order to achieve the desired conductivity, a second
printing of carbon paste, drying and sintering was carried out, and
the final sintered carbon thickness was 50 micrometers. Adherence
of the carbon layer to the underlying zirconia layer was excellent,
there was good integrity of the layer with no flaking off, and
there were no cracks or holes. The electrical conductivity of the
layer was 10 ohms per square at this thickness. Since the
conducting glass used to support the titania layer had a similar
conductivity of 10 ohms per square there was good matching of
photoanode and counter electrode conductivities, one of the
requisites for high dye cell efficiencies. The carbon layer was
quite porous and allowed fast transfer of dye and electrolyte
printings to the titania layer below. The dye cell, after dye
staining of the titania, electrolyte addition and sealing, gave 7%
efficiency under one sun illumination of 100 mW per sq. cm.
[0219] Preparation of a dye cell carbon cathode in a single
printing according to the present invention is described in the
following example:
Example 4
Preparation of a Screenprintable Carbon Dye Cell Cathode in a
Single Printing
[0220] Spherical graphite powder (Timcal, type KS4, 21 gm), carbon
furnace black (Cabot, type Vulcan XC72R, 4 gm) and expanded
graphite (Timcal, type BNB90, 3 gm) were dry blended in an agate
ball mill for 30 minutes. A titania precursor (Dupont, Tyzor.TM.
type AA-75, 30 gm) was stirred with terpineol (Aldrich, 50 gm)
containing 10 wt % dissolved ethyl cellulose (Dow, ethocell, type
standard industrial 45). This liquid mixture was then added to the
ball mill containing the carbon, spherical graphite powder, and
expanded graphite, and milling was continued overnight. The
resulting carbon paste was screenprinted using a plastic mesh onto
a sintered porous zirconia spacer upper layer, itself part of a
conducting glass carrying a sintered-on layer of nanocrystalline
titania (15 micrometers thick) covered by the porous sintered
zirconia layer and appropriate for dye cell fabrication for cell
areas in excess of 10 sq. cm. The wet thickness of the carbon print
was 100 micrometers. The carbon print was dried at 150 C for 10
minutes. The print was then sintered in air at 450 C for 30
minutes, giving a sintered thickness of 50 micrometers. Adherence
of the carbon layer to the underlying zirconia layer was excellent,
there was good integrity of the layer with no flaking off, and
there were no cracks or holes. The electrical conductivity of the
layer was 10 ohms per sq. at this thickness, similar to that of the
conductive glass supporting the titania layer and adequate for good
dye cell performance. The carbon layer was quite porous and allowed
fast transfer of dye and electrolyte printings to the titania layer
below. The dye cell, after dye staining of the titania, electrolyte
addition and sealing, gave 7% efficiency under one sun illumination
of 100 mW per sq. cm.
[0221] Preparation of a dye cell catalyzed carbon cathode in a
single printing according to the present invention is described in
the following example:
Example 5
Preparation of a Screenprintable Catalyzed Carbon Dye Cell Cathode
in a Single Printing
[0222] Spherical graphite powder (Timcal, type KS4, 17 gm), carbon
furnace black (Cabot, type Vulcan XC72R, 4 gm), carbon furnace
black catalyzed with platinum (Cabot, type Vulcan XC72R containing
1% platinum, 4 gm), and expanded graphite (Timcal, type BNB90, 3
gm) were dry blended in an agate ball mill for 30 minutes. A
titania precursor (Dupont, Tyzor.TM. type AA-75, 80 gm) was stirred
with terpineol (Aldrich, 50 gm) that contained 10 wt % dissolved
ethyl cellulose (Dow, ethocell, type standard industrial 45). This
liquid mixture was then added to the ball mill containing the
carbon materials, spherical graphite powder, and expanded graphite,
and milling was continued overnight. The resulting carbon paste was
screenprinted using a plastic mesh onto a sintered porous zirconia
spacer upper layer, itself part of a conducting glass carrying a
sintered-on layer of nanocrystalline titania (15 micrometers thick)
covered by the porous sintered zirconia layer and appropriate for
dye cell fabrication for cell areas in excess of 10 sq. cm. The wet
thickness of the carbon print was 100 micrometers. The carbon print
was dried at 150 C for 10 minutes. The print was then sintered in
air at 450 C for 30 minutes giving a sintered thickness of 50
micrometers. Adherence of the carbon layer to the underlying
zirconia layer was excellent, there was good integrity of the layer
with no flaking off, and there were no cracks or holes. The
electrical conductivity of the layer was 10 ohms per square at this
thickness, similar to that of the conductive glass supporting the
titania layer and adequate for good dye cell performance. The
carbon layer was quite porous and allowed fast transfer of dye and
electrolyte printings to the titania layer below. The dye cell,
after dye staining of the titania, electrolyte addition and
sealing, gave 8% efficiency under one sun illumination of 100 mW
per sq. cm.
[0223] Another aspect of the present invention enables the use of a
lower cost paste based on inexpensive titania powder commercially
available in bulk. The paste is readily prepared and is
screenprintable over large areas, and may also give acceptable
performance in dye cells. The paste includes a mild titania
precursor that acts as a sinterable binder, and this mild precursor
(or another mild titania precursor) is used also in surface
treatments of the conducting glass substrate and the sintered
titania paste as an alternative to the inherently problematic
titanium tetrachloride.
[0224] This aspect of the present invention may specifically relate
to a screenprintable titania photoanode composition and paste that
may be especially suitable for producing photovoltaic dye cells; to
a method of producing a sintered, photoanodic titania layer from
such a paste; and/or to a titania precursor treatment method for
treating a surface of, or associated with, a photoanode.
[0225] It is known to synthesize titania via a sol-gel route. This
route may not provide a screenprintable titania paste that is low
cost or readily available. Attempts to prepare a photoanode from
commercially available bulk titania powders such as P90 of Degussa,
have not proved particularly successful when the material is
sintered alone, and non-adherent layers or low-performance cells
may result. The Degussa product, which is cheap and readily
available, being prepared in vast quantities by simple pyrolysis of
a low cost chloride precursor of titania, is well chosen in
principle, since it mainly consists of the anatase phase that is
much preferred for high-performance dye cells. Additionally, this
commercially available bulk titania powder has a high surface area
of about 90 square meters per gram, and a mean primary particle
size of 14-15 nm, both of which are in the optimal range for
performance purposes. There are to be expected some performance
improvements over the other applicable commercially available
titania powder product of Degussa, known as P25, which contains
only 75% of the anatase phase, by weight, the balance being the
less active rutile phase. Compared with the P90 product, the P25
material is characterized by a coarser primary particle size (21
nm), and by a lower surface area (50 square meters per gram).
[0226] It should be understood, however, that the choice of this
Degussa material is in no way limiting, and other titania powders
from different manufacturers having similar properties may also be
used. We have found a significant improvement in performance when
the P90 material is prepared as a paste with a dedicated titania
precursor as a binder, preferably together with a non-ionic
surfactant and a heat fugitive pore-former, followed by
screenprinting, drying and sintering.
[0227] We have further discovered that many precursors that oxidize
acceptably to titania on sintering in air, for example, various
titanium alkoxides, are unsuitable for paste formulation, because
they fume strongly in air and are difficult to work with. A
non-limiting example of a benign, suitable precursor is the family
of organic titanates supplied by DuPont under the trade name
Tyzor.RTM.. These organic titanates are available in the form of
organic titanium chelates, aqueous titanium chelates, and ortho
titanate esters. These may contain about 15-25 wt % of titania
equivalent, and oxidation to titania is usually complete on heating
in air at above 300 C for 30 minutes. We have found particularly
useful for photoanode pastes, the Tyzor.RTM. grade AA-105, which is
based on a titanium chelate. However, we have found other types and
grades of Tyzor.RTM. to be suitable for use in, or in conjunction
with, the present invention.
[0228] Without wishing to be limited by theory, the improvement in
performance is thought to be the result of better
particle-to-particle contact, interconnection and continuity of the
titania particles (as in the P90 powder) in a sufficiently porous,
adherent, crack-free layer achieved on sintering with the organic,
non-fuming titanate, as compared with sintering of the titania
alone. We believe that the organic, non-fuming titanates may
decompose in-situ in the paste to provide new titania centers that
can effectively bond, bridge and coat the bulk titania particles,
thereby improving the dye coverage and the photovoltaic efficacy of
the dye cell. The use of a titania precursor like those of the
Tyzor.RTM. series is intrinsically advantageous over binders that
are not titania precursors: such titania precursors, upon
sintering, form titania in additional to the bulk titania
particles, and the organic moieties leave the solid phase without
leaving contaminants or organic fingerprints of any kind. The use
of other binding materials may contaminate the cell or interfere
with the semi-conducting properties of the layer of bulk titania
particles. Residual material from such other binding materials may
even be attacked or corroded by the cell electrolyte and
contaminate the cell.
[0229] The titania powder, titania precursor, non-ionic surfactant,
and the heat fugitive pore former components may be conveniently
dispersed in a printing vehicle such as terpineol, ethyl hexanol,
dibutylphthalate, or various other organic solvents, to provide a
screenprintable paste. The presence of one or more ionic
surfactants has been found to be advantageous. We have found
particularly useful acetyl acetone, which, as well as acting as a
non-ionic surfactant, is also a chelating agent, and can modify the
surface of the titania particles, thereby enhancing their
sinterability, whilst for the heat fugitive thickening agent and
pore former we have found ethyl cellulose to be suitable.
[0230] Regarding the treatment of conducting glass and the sintered
titania layer with a titania precursor, we have found organic
titanates such as the commercially available Tyzor.RTM. materials
much more convenient and less hazardous to work with than titanium
tetrachloride. In the preparation of photoanodes, we have found
particularly useful in this respect, titanium chelates such as
Tyzor.RTM. grade AA-75. However, other types of organic titanates
have been found to be suitable. As an alternative to dip coating of
the conductive glass substrate or sintered titania layer in
Tyzor.RTM. compositions (if required, diluted with the respective
thinner recommended by the manufacturer), the organic titanates can
also be applied in a screenprinting composition based on a printing
vehicle such as terpineol, optionally thickened with a heat
fugitive thickening agent and pore former such as ethyl
cellulose.
[0231] We have further discovered that improved cell performance
can be achieved by treating the bare conducting glass photoanode
substrate, with a non-fuming organic titanate that is a titania
precursor, such as those described herein. The conducting glass
photoanode substrate may first be immersed in, or otherwise
intimately contacted with, a liquid containing the non-fuming
organic titanate. The substrate may then be rinsed prior to
applying the titania paste, e.g., by screenprinting.
[0232] This treatment method is thought to provide blocking off or
passivation of the exposed conductive glass surface and to reduce
charge carrier recombination, and also enables improved bonding to
the subsequently applied photoactive titania layer. Without wishing
to be bound by theory, we believe that the non-fuming organic
titanate becomes chemically attached to the substrate.
[0233] We have further discovered that improved cell performance
can be achieved by treating the sintered titania photoanode layer
with a non-fuming organic titanate that is a titania precursor,
such as those described herein. The sintered titania photoanode
layer on the conducting glass photoanode substrate may be immersed
in, or otherwise intimately contacted with, a liquid containing the
non-fuming organic titanate. The wetted titania photoanode layer
may then be resintered to produce a sintered titania photoanode
layer having improved mechanical strength and improved electrical
conductivity between adjacent titania particles in the photoanode
layer.
[0234] As used herein in the specification and in the claims
section that follows, the term "precursor", with respect to
"titania", refers to a titanium-containing compound, which, upon
heating above at least 300 C, reacts to produce titania. The term
"organic", with respect to a titania precursor, is specifically
meant to exclude titanium tetrachloride.
[0235] As used herein in the specification and in the claims
section that follows, the term "non-fuming titania precursor", with
respect to a titania precursor, refers to a titania precursor that
does not fume in air at 25.degree. C.
[0236] As used herein in the specification and in the claims
section that follows, the terms "titania" and "titanium dioxide"
are used interchangeably.
[0237] As used herein in the specification and in the claims
section that follows, the term "titania powder", refers to a powder
containing at least 98% titania. Typically, titania powders used in
conjunction with the present invention contain at least 99%
titania, or even at least 99.5% titania.
[0238] At least 60% of the titania in the titania powders used in
conjunction with the present invention may be in the anatase phase.
Typically, these titania powders contain at least 70% titania of
the anatase phase. For high-performance dye cells utilizing the
pastes and methods of the present invention, at least 80% of the
titania in the titania powders used in conjunction with the present
invention may be in the anatase phase, and preferably, at least 90%
or at least 95% may be in the anatase phase.
[0239] An exemplary preparation of a screenprintable titania
photoanode paste incorporating a titania powder and an organic
titania precursor according to the present invention, is described
hereinbelow. Also described are treatments of the photoanode with
an organic titania precursor and the resulting effect on the
performance of the dye cell.
Example 6
Preparation of an Inventive Titania Photoanode Paste and
Construction of a Dye Cell Therewith
[0240] A titania precursor (Dupont, Tyzor.RTM. type AA-105, 20 gm)
from the family of organic titanates (a titanium acetylacetonate)
was stirred into a solution of screenprinting vehicle terpineol
(Aldrich, 80 cc) containing dissolved thickener ethyl cellulose
(Dow, Ethocel.TM., 2.5 wt %) to give Component I. The non-ionic
surfactant acetyl acetone (Aldrich, 4 cc) was mixed with the
solvent isopropyl alcohol (Aldrich, 5 cc) to give Component II.
Component II was then mixed into Component I. To this mixture,
commercial titania powder (Degussa, type VP TiO.sub.2 P90, 50 gm)
was stirred in, and the slurry was then ball-milled overnight in an
agate ball mill. The resulting titania paste was screenprinted onto
an FTO glass (in order to provide a dye cell photoanode) by means
of three printings, each printing followed by drying at 130 C for
30 minutes, and finally the printed layer was sintered at 450 C in
air for 30 minutes. The titania layer had a sintered thickness of
15 microns and was homogeneous, adherent and crack-free. SEM work
confirmed that the titania crystallites in the layer were sized
around 14 nm, thus proceeding from the P90 titania material, which
has a mean primary particle size of 14-15 nm, as described
hereinabove. Furthermore, elemental analysis of the titania,
carried out by dissolution of a portion of a sample of the layer in
acid and using ICP spectroscopy, confirmed an iron content of 7.0
ppm, matching closely the iron content of the P90 titania batch
used for the paste preparation. This analysis methodology may be
used as a fingerprint check of the source of the titania in a dye
cell.
[0241] The sintered titania layer was coated with ruthenium dye
sensitizer (Solaronix SA, type Ruthenium 535-bis TBA) by absorption
from an alcoholic solution, and a dye test cell of active area 1.9
sq. cm was assembled with a platinum-catalyzed FTO glass as the
counterelectrode and using a DuPont Surlyn.TM. polymer gasket to
seal the cell edges. Nitrile-based electrolyte containing dissolved
iodine (Solaronix SA, type AN-50) was introduced into the cell by
vacuum filling via holes previously drilled in the counterelectrode
glass, and these holes were then sealed off using Surlyn.TM.
polymer. Current takeoff from the cell was achieved by means of
photoanode and counterelectrode FTO surfaces projecting from the
cell that were coated with conductive silver paint.
Example 7
Performance of the Dye Cell Construction of Example 6
[0242] Three cells were built according to the method described in
Example 1 and performance was tested in a solar simulator providing
one sun illumination (100 mW per sq. cm). The averaged performance
results for the three cells were: [0243] open circuit voltage: 704
mV; [0244] short circuit current: 15.3 mA per sq. cm; [0245] fill
factor: 59.4%; [0246] solar-to-electricity conversion efficiency:
6.27%.
[0247] Further testing and comparative control experiments
confirmed the promising performance of the low-cost,
screen-printable titania-powder based paste of the present
invention. When a paste was attempted without including a
non-fuming organic titanate such as a titanium acetonate (e.g., a
commercially available titanium acetylacetonate such as Tyzor.RTM.
type AA-105, Tyzor.RTM. type AA-75), a titanium acetoacetate such
as titanium ethyl acetoacetate (Tyzor.RTM. PITA), or acetyl
acetone, there was insufficient adhesion following sintering. Cells
that were assembled exhibited conversion efficiencies below 3%.
When expensive specialty pastes of sol-gel titania (via the
hydrothermal route of the prior art) were used, there was only a
moderate gain in efficiency to 7%.
[0248] A further test was made to check the effect of the two
treatments using titanium chloride according to the literature
referred to previously, as compared with treatments using a member
of the Tyzor.RTM. family of materials:
Example 8
Additional Treatments with Titanium Tetrachloride Solution
[0249] In a comparative test run, conductive glass photoanode
substrates were first immersed for 30 minutes in a 40 mM aqueous
titanium tetrachloride solution at 50 C, rinsed and dried.
Following screenprinting of the inventive titania paste and
sintering in the usual way, the photoanodes were again immersed for
30 minutes in the 40 mM aqueous titanium chloride solution at 50 C
and then sintered (i.e., resintered) at 450 C for 30 minutes. Cell
efficiencies under one sun illumination increased to 6.8% (as
compared to 6.27% without the titanium tetrachloride treatment, as
reported in Example 2).
[0250] Preparation of the immersion solution was hazardous and
unpleasant as the source material for the titanium tetrachloride
solution (anhydrous titanium tetrachloride) is very corrosive and
gave off acidic fumes in air.
Example 9
Additional Treatments with an Organic Titanate Solution
[0251] Instead of using a titanium tetrachloride immersion
solution, a relatively benign, non-fuming titanium acetylacetonate
(DuPont Tyzor.RTM. AA-75) was used, diluted 50 times with isopropyl
alcohol. After the two treatments of the photoanode were carried
out, as in Example 3, a similar cell efficiency increase to 6.75%
was obtained.
[0252] Thus, the results demonstrate that the solar-to-electricity
conversion efficiencies obtained using non-fuming organic titanate
treatments are quite similar to the efficiencies obtained using
titanium tetrachloride treatments.
[0253] Another aspect of the present invention relates to a
counter-electrode for broad, large-area, single dye cells of
typically 15 cm per side, and an inventive photovoltaic cell
structure incorporating such a counter-electrode. We have found
that when broad, single cell assemblies of this type are attempted
with the usual carbon-based compositions of the prior art
counter-electrode (as exemplified by the above-referenced patent to
Kay, which uses titania powder as binder to bond the carbon layer),
there emerges, in addition to the expected current takeoff
limitation with cell widths much above 1 cm, an additional problem
resulting from the rather fragile nature of the carbon layer. It is
very difficult to embed, even partially, a current-collecting
structure such as a metal mesh, a metal foil or metal wires in a
broad area carbon layer, without causing the entire structure to
delaminate. This is principally due to the non-elastic nature of
inorganic binders such as titania used to bond the carbon layer.
More flexible organic type binders are generally unsuitable for
this application because of the high sintering temperature of 450 C
needed for both anode and cathode layers in the cell. The high
sintering temperature may destroy organic binders and possibly
contaminate the cell with organic decomposition residues.
Furthermore, many metals are corroded by the cell electrolyte under
the dye cell working conditions and thus appear unsuitable for use
in the counter-electrode as current collectors. In the dye-cell
technology taught by Kay, these issues do not arise because of the
narrow width of the strip cells. In such narrow cells: [0254] the
sintered-on carbon layer may display adequate mechanical stability,
and [0255] the carbon layer alone may convey current, without the
need for a supplemental current collector.
[0256] The cell structure of the present invention may include a
graphite foil, in combination with a corrosion-resistant metal or
metal alloy current takeoff element at least partly embedded in the
graphite foil, which structure is conductively bonded to, or in
direct electrical conductive contact with, an underlying cathode
carbon layer. This structure is acceptably robust and enables a
large area, broad cell construction, while simultaneously
eliminating the need for a second layer of conducting glass in the
dye cell or a second, physically separate counter-electrode. The
corrosion-resistant current takeoff element embedded in the
graphite foil may be selected from various geometries including a
foil, mesh, strips or wires of a number of metals or alloys such as
titanium, titanium-clad copper, tungsten, a higher alloy of iron
and chromium, or a higher alloy of iron, chromium and
molybdenum.
[0257] In another preferred embodiment, the corrosion-resistant
metal or metal alloy current takeoff element may be used as a
standalone counter-electrode in the cell, without the need for
combination with a carbon or graphite foil layer, and in such a
case, the counter-electrode is catalyzed directly with trace
platinum.
[0258] A schematic cross-sectional view of one embodiment of an
inventive photovoltaic dye cell 100, which may be of an
"open-faced" sandwich design, is shown in FIG. 11. Onto a support
glass such as a conventional anode support glass 102 (of typical
thickness 1 mm-3 mm), a thin, transparent conducting surface layer
104 is disposed, the layer based, for example, on tin oxide.
Support glass 102 and conducting surface layer 104 may be provided
with electrical conductivity enhancing features (e.g., as described
in above-referenced U.S. Patent Application Publication No.
20050072458, which is incorporated by reference for all purposes as
if fully disclosed herein). Support glass 102 is used as the
substrate and basic building block for the dye cell. The substrate
is grooved and fitted with wire current take-off means as taught in
the above-referenced patent publication. Thus, wires pass sealably
out of the cell through the side of the cell, and are braided
together and/or connected, for example, by welding, to a current
collecting strip (not shown), so as to form the cell anode
terminal. In each groove 108 there is applied an inert,
electrically conducting binder composition 112 that bonds a wire
116 into the groove and makes good bridging electrical contact with
the tin oxide layer on each side of the groove. As an example, a
titanium, tungsten, or higher alloy wire may be inserted into the
groove. The conducting binder, which preferably includes a
conductor such as titanium nitride and a binder such as alumina, is
also added into the groove. Onto the tin oxide surface is applied a
nanocrystalline titania layer, typically by screen-printing from a
paste, followed by drying and sintering.
[0259] Porous, sintered nanocrystalline titania film or layer 120
is designed to have a typical thickness of about 15 micrometers.
Titania film or layer 120 may include several sublayers of titania,
each of which may be individually screen-printed and sintered. We
have found that covering the upper surface of the conducting binder
112 with an insulating layer 124 prior to the application of the
nanocrystalline titania layer, may help to eliminate possible short
circuits developing between the anode and the cathode. Insulating
layer 124 may be especially important for cells in which the
conducting binder surface is appreciably above the level of
sintered titania layer 120, as depicted in FIG. 11.
[0260] Insulating layer 124 may be selected from a glaze or a
binder composition containing fairly coarse titania, zirconia,
alumina or silica particles preferably having a characteristic
particle size on the order of several micrometers, or
alternatively, from a high temperature polymer such as polyimide or
silicone. Titania layer 120 may then be covered with a
screen-printed, porous, insulating spacer layer from a paste
containing relatively coarse titania, zirconia silica or alumina
particles. The screen-printed spacer layer is designed to yield a
sintered insulating spacer layer 128 having a thickness of about 5
micrometers. Sintered insulating spacer layer 128 also acts as a
light scattering layer, directing light back to sintered
nanocrystalline titania layer 120.
[0261] Cell construction continues with application of the
counter-electrode layers, initially by screen-printing. Spacer
layer 128 may be narrow, typically on the order of 2-10
micrometers, to ensure a very small anode/cathode separation in the
cell, a low ohmic resistance, and high cell fill factor, all of
which contribute to increased cell performance. Onto insulating
spacer layer 128 may be applied a thin porous layer of carbon,
optionally catalyzed with a trace amount of platinum or a platinum
catalyst substitute via a screen-printable paste containing an
inert binder. This thin layer is dried and sintered to produce a
sintered, porous, catalytic carbon layer 132, which has a
characteristic thickness of only a few micrometers.
[0262] The sintering of the screen-printable, porous, catalytic
carbon paste layer is followed by the application of a relatively
thick layer of porous, conducting carbon via a screen-printable
carbon paste containing an inert binder. Another sintering step is
performed, producing a sintered, porous, conducting carbon layer
136. The screen-printing of the carbon paste is typically designed
such that the thickness of sintered, conducting carbon layer 136 is
typically about 50-100 micrometers.
[0263] Conducting carbon layer 136 may be sufficiently active for
the iodine redox reaction in the cell such that the need for a
separate catalytic layer such as catalytic carbon layer 132 may be
obviated.
[0264] In a preferred embodiment, the various layers may be
sintered, if possible in a single sintering step at 450.degree. C.,
and following partial cooling, a sensitizer dye is introduced into
titania layer 120 via porous carbon layer 136. A sheet of graphite
foil 140 having a prepared, sintered-on, porous, conducting carbon
(and chemically inert) binder layer 144 disposed thereunder may be
laid onto carbon layer 136 so as to be in good conductive contact
therewith.
[0265] Porous carbon layer 136, graphite foil 140 (including binder
layer 144), and optionally, catalytic layer 132 may form a cathode
or counter-electrode 160 of cell 100.
[0266] At least partially embedded into the graphite foil may be a
corrosion-resistant metal or metal alloy current collector 152 in
the form of a wire, mesh, strip, perforated strip or foil that
protrudes through the peripheral cell seal (described hereinbelow)
to act as a counter-electrode terminal of the cell. We have found
that under normal cell operating conditions, titanium,
titanium-clad copper, and tungsten, and some higher alloys
principally consisting of chromium and iron, or chromium, iron and
molybdenum, may serve as chemically stable counter-electrode
current collecting materials.
[0267] Graphite foil 140 may advantageously be equipped with one or
more perforations 146 to facilitate electrolyte distribution into
the cell during a subsequent filling operation.
[0268] A sheet 154 may be laid on top of the assembly to close the
cell. Sheet 154 may be made of inexpensive window glass or various
metals or alloys. Underneath sheet 154, and on top of graphite foil
140, may advantageously be disposed an elastic sheet 148,
preferably including or consisting of foam, fiber mat or elastomer
mat or a swelling precursor material or swelling polymeric
material. Elastic sheet 148, which may include a polymeric, carbon
or metallic material, is compressible and spring-like, to help
maintain a fairly uniform pressure between graphite foil 140 and
carbon layer 136 while ensuring sufficient electrical contact over
the large requisite area, without delaminating layer 136.
[0269] The edges of cell 100 may be sealed by a peripheral seal
156, e.g., using a liquid-phase sealant. Electrolyte may then be
introduced into cell 100, typically by vacuum means via a hole (not
shown) in sheet 154, and the hole may be sealed using a sealing
composition. The cell is then ready for testing/modulizing.
[0270] A schematic cross-sectional view of a photovoltaic dye cell
200 according to another preferred embodiment of the invention, is
shown in FIG. 12. On a support glass such as a conventional anode
support glass 202 (of typical thickness 1 mm-3 mm), a thin,
transparent conducting surface layer 204 is disposed, the layer
based, for example, on tin oxide. Support glass 202, which may be
provided with electrical conductivity enhancing features, serves as
the substrate and basic building block of dye cell 200. Disposed on
conducting surface layer 204 is a set of spaced, preferably
substantially parallel strips 216 of a metal or alloy inert to the
cell electrolyte and to charge carrier recombination. Strips 216,
which may be deposited by means of electroplating, provide current
take-off means for cell 200. Strips 216 may pass sealably through
the side of cell 200, and may be electrically connected together,
e.g., by a current collecting strip (not shown) outside of the
seal, thereby forming the anode terminal of the cell. Substantially
as described hereinabove with respect to the filled grooves
provided in FIG. 11, the plated strips may be also advantageously
coated with an insulating layer 228, for example, including a glaze
or a binder composition of titania, zirconia, alumina and/or
silica, to prevent anode/cathode short-circuiting. Onto conducting
surface layer 204 is applied a nanocrystalline titania layer,
typically by screen-printing from a paste, drying and
sintering.
[0271] Porous, sintered nanocrystalline titania film or layer 220
is designed to have a typical thickness of about 15 micrometers.
Titania film or layer 220 may include several sublayers of titania,
each of which may be individually screen-printed and sintered.
[0272] Titania layer 220 is then covered with a screen-printed,
porous, insulating spacer layer, which undergoes sintering to
produce a sintered insulating spacer layer 228. The screen-printed
paste may contain relatively coarse titania, zirconia, silica,
and/or alumina particles. After drying and sintering, sintered
insulating spacer layer 228 may have a thickness of about 2-10
micrometers. This thickness is designed to ensure a very small
anode/cathode separation in the cell. This thickness is further
designed to achieve a low internal resistance and a high cell fill
factor, thereby raising cell performance. Spacer layer 228 also
acts as a light back-scattering layer, directing light back to
sintered titania layer 220.
[0273] On insulating spacer layer 228 is disposed a cathode or
counter-electrode 260 of cell 200. Cathode 260 includes a sintered,
porous, catalytic conducting carbon layer 232, which may contain a
binder that is chemically inert to the constituents of cell 200,
and which may be catalyzed with a trace amount of platinum or of a
platinum catalyst substitute. Carbon layer 232 may be produced by
screen-printing or otherwise applying a thin layer of porous,
catalytic carbon paste, after which the layer is dried and
sintered. Sintered, porous, conducting carbon layer 232 may have a
thickness of several micrometers, typically less than about 10
micrometers. Disposed above catalytic carbon layer 232 is a
relatively thick, sintered, conducting carbon layer 236, which may
also form a part of cathode 260. Carbon layer 236 may be produced
by screen-printing, after which the layer is dried and sintered to
achieve a typical thickness of 50-100 micrometers.
[0274] Some high surface area carbons and graphites are so active
for the iodine redox reaction in the cell that the need for a
separate catalytic or platinized layer 228 may be obviated.
[0275] In a preferred embodiment, the various layers are sintered,
typically or preferably in a single sintering step at 450 C, and
following partial cooling, a sensitizer dye is introduced into
titania layer 220 via porous carbon layer 236. A sheet of graphite
foil 240 is then conductively bonded to carbon layer 236 via a
conducting carbon adhesive layer 245, which may be selected to be
curable at a temperature below 120 C, thereby avoiding any damage
to heat-sensitive dyes in titania layer 220. Suitable components
for adhesive layer 245 may include carbon powder and either an
inert (with respect to the electrolyte) inorganic binder based on
alumina, or an inert (with respect to the electrolyte) organic
binder based on silicone or polyimide, for example.
[0276] Both graphite foil 240 and conducting carbon adhesive layer
245 may be considered to form a part of cathode 260.
[0277] Into graphite foil 240 is embedded, at least partially, a
corrosion-resistant metal or metal alloy current collector 252 in
the form of a wire, mesh, strip, perforated strip or foil that
passes out through the peripheral cell seal (described below) to
act as the counter-electrode terminal of cell 200. We have found
that titanium, titanium-clad copper, and tungsten, or some higher
alloys principally consisting of chromium and iron, or chromium,
iron and molybdenum, may serve as chemically stable
counter-electrode current collecting materials. Graphite foil 240
may advantageously have perforations 246 in order to facilitate
electrolyte distribution into the cell. The electrolyte-filling
step may be carried out at this stage.
[0278] The cell is structurally completed by laying down a sheet of
plastic laminated foil 258 and applying a peripheral sealant 252
between elements 202 and 258 of the cell, or alternatively a
polymer sealing layer may be sprayed on to initially seal the cell,
and additional sealing provided using an outer metal foil (not
shown). However, if a lightweight construction is not particularly
crucial, window glass or metal/alloy sheet can be used to close the
cell, and in the latter case, current collector 252 may be
dispensed with.
[0279] As developed hereinabove, large area, broad dye cells having
physically separated anode and counter-electrodes may suffer from
variously characteristic performance limitations. Reducing or
eliminating this physical separation is one motivation behind the
open-faced sandwich design provided in FIGS. 11 and 12, in which
all the active layers are bonded onto a single (conductive) glass
sheet. In broad cells of industrial applicability, and particularly
in cells using non-volatile electrolytes, inter-electrode spacing
tends to be at least 50-100 micrometers, at which point ohmic
losses from the cell electrolyte become excessive. Similarly,
temperature changes between day and night may cause
expansion/contraction variations in interelectrode spacing, which
may lead to current irregularities and to stressing of seals.
[0280] In a preferred embodiment, however, we disclose a
photovoltaic dye cell having a bi-plate design that may overcome
these limitations.
[0281] While various sandwich structures, in which a carbon layer
is printed on top of a zirconia spacer layer on top of a titania
layer, may allow close spacing of carbon and titania surfaces, it
must be emphasized that such structures may be extremely prone to
penetration of the carbon printing layer to the conducting glass
surface, via any holes or defects in the printed layers of titania
or spacer. The penetration of the carbon printing layer to the
conducting surface of the FTO glass effects short-circuiting of the
cell, thereby reducing or limiting cell performance. This problem
is particularly acute in view of the at least 7-10 year lifetime
often required for cells to attain commercial viability. The
inventive biplate construction, as described herein, avoids this
significant and potentially critical problem, because the carbon
counterelectrode is not sintered (i.e., chemically bonded at high
temperature) to the printed layer(s) of titania or spacer. Rather,
the carbon counterelectrode is a distinct and separate entity that
lies on top of, and may physically contact, the spacer layer, or
the upper surface of the titania layer, but is not chemically
bonded thereto, as in printed carbon layer technologies.
[0282] A schematic cross-sectional view of one embodiment of such
an inventive photovoltaic dye cell 300 is provided in FIG. 13A. An
anode glass 302 of typical thickness 1 mm-3 mm has a thin,
transparent conducting surface layer 304 based on a conductive
material such as tin oxide. Anode glass 302 and conducting surface
layer 304 may be provided with electrical conductivity enhancing
features. The conductive glass is grooved and fitted with wire
current take-off means substantially as described hereinabove.
Thus, wires pass sealably out of the cell at the side of the cell
and are braided together and/or connected, for example, by welding,
to a current collecting strip (not shown), to form an anode
terminal of cell 300. In each groove 308 there is applied an
electrically-conducting binder layer 312, which is preferably
chemically inert to the cell electrolyte and serves to bond a wire
316 into groove 308 while making good bridging electrical contact
with the tin oxide layer (conducting surface layer 304) on each
side of groove 308. As an example, a titanium, tungsten, or higher
alloy wire may be inserted into groove 308 and a conducting binder,
which preferably includes a conductor such as titanium nitride and
a binder such as alumina, is also added into groove 308.
[0283] In constructing cell 300, an upper surface of conducting
binder layer 312 may be covered with an insulating layer 324 prior
to the application of the nanocrystalline titania layer, to prevent
short-circuits developing between the anode and the cathode. This
is especially important when the surface of conducting binder layer
312 is appreciably above the level of a sintered nanocrystalline
titania layer 320, as depicted in FIG. 13A.
[0284] Insulating layer 324 may be selected from a glaze or a
binder composition containing fairly coarse titania, zirconia,
alumina or silica particles having a characteristic particle size
of several micrometers, or alternatively, from a high temperature
polymer such as polyimide or silicone. Onto the tin oxide surface
(conducting surface layer 304) is applied a nanocrystalline titania
layer, typically by screen-printing a paste, followed by drying and
sintering to produce sintered nanocrystalline titania layer 320.
This sintered layer is designed to have a typical thickness of
about 15 micrometers after the sintering step. Titania layer 320
can be optionally coated with a coarse particle layer (not shown)
based on titania, zirconia, alumina or silica, for purposes of
electrical insulation and/or light back-scattering. During
construction of cell 300, titania layer 320 may then be covered
with sensitizer dye, or this step can be carried out later on,
before electrolyte filling. The cell is closed off with a
corrosion-resistant metal sheet 368 that has been catalyzed
conventionally with a catalyst such as trace platinum crystallites
332, and this sheet is sealed at the edges to anode glass 300 by
electrolyte-resistant sealant 336. Suitable construction materials
for metal sheet 368 may include titanium, titanium-clad copper,
tungsten, or higher alloys including principally chromium and iron,
or chromium, iron and molybdenum.
[0285] Importantly, metal sheet 368 is bonded to the upraised,
electrically insulated material of insulating layer 324 by means of
a layer of electrolyte-resistant adhesive 340. Suitable adhesives
are available, based on silicones or polyimides, for example.
Adhesive layer 340 may be placed at periodic spacing intervals
along the materials covering grooves 308 to maintain a strong
anchoring and uniform spacing between sheet 368 and anode glass
302. A lateral continuous layer of adhesive is operational, but may
be disadvantageous, however, as it hydraulically isolates
electrolyte between adjacent grooves and makes the filling of cell
300 with electrolyte rather laborious.
[0286] FIG. 13B shows the placing of the anchoring points in a
schematic top view of cell 300, in which each "+" mark 303
represents a spaced location along the grooves in the anode plate
where adhesive is placed.
[0287] Sheet 368 in FIG. 13A may be grooved in complementary
fashion to anode glass 302, in order to accept the upraised profile
of insulating layer 324 as required and thereby enable the
desirable close approach (a few tens of micrometers only) between
the surface of catalyst 332 and the surface of titania layer 320.
The cell may be filled with electrolyte via holes (not shown) in
metal sheet 368, which holes are then sealed off.
[0288] Various alternative embodiments in the building of the
two-plate design according to the desired spacing criteria are
possible. In FIG. 13A, the anode plate may be fitted with
conducting metal strips instead of buried wires. Similarly, the
metal sheet based counter-electrode may simply be stamped out to
give a groove-like topography. In one preferred embodiment, the
counter-electrode may be a suitably catalyzed flat metal sheet from
which slits have been punched or cut out, such that the slits fit
over the upraised elements situated on the grooves, while
simultaneously enabling the catalyzed surface between the slits to
attain close proximity to the titania surface below. In such a
case, a glass or polymer sheet placed above the slitted plate would
be used to close off the cell, and this glass or polymer sheet
would be anchored in place at multiple points on the upraised
elements.
[0289] FIG. 14 provides a schematic cross-sectional view of a
bi-plate photovoltaic dye cell 400 according to another embodiment
of the present invention. The construction of this biplate cell
enables the use of only a single conducting glass sheet per cell.
This construction further enables a very close juxtaposition of
anode and cathode. A photoanode support glass or substrate 402
coated with a substantially transparent conductor layer 404 may
have spaced, preferably parallel wires 410 bonded in place on a
surface of the conductive glass (e.g., on top of conductor layer
404) by means of a substantially inert conducting adhesive layer
such as a conducting ceramic adhesive layer 415. Between chemically
inert current collecting wires 410 and at the edges of cell 400 is
a sintered, nanocrystalline, porous titania layer 406, typically
10-15 micrometers thick. Since wires 410 may typically have a
diameter of at least 100 micrometers, and the height of titania
layer 406 is typically below about 15 micrometers, wires 410 may
project above titania layer 406 by over 80 micrometers. To avoid
short-circuiting to a counter-electrode 430 of cell 400, ceramic
adhesive layer 415 may advantageously be covered with an
electrically insulating layer 418, e.g., a layer including
zirconia. Electrically insulating layer 418 may typically have a
thickness in a range of 20-50 micrometers.
[0290] Cell 400 may have two physically discrete electrodes. Thus,
a cathode or counter-electrode 430 may be a distinct component that
is laid onto the anode element and may be disposed in close
proximity thereto. Counter-electrode 430 may advantageously include
a porous, conductive cathodic layer 425, e.g., a porous support
matrix supporting impregnated carbon. The porous matrix may include
or essentially consist of a mat, woven and/or non-woven, foam, or
possibly other matrices known in the art. A preferred material for
the mat is glass fiber since it is low cost, flexible and
conformable to the cell geometry, chemically inert in the cell
environment, and may withstand elevated curing or sintering
temperatures. Other fibers such as carbon fibers may also be used.
The impregnated carbon may be bonded to the porous support matrix
by an inert binder, which may be selected from inorganic materials
such as alumina, or polymeric materials such as
polytetrafluoroethylene (PTFE, or Teflon.RTM.).
[0291] Counter-electrode 430 may include porous cathodic layer 425
and a catalytic layer 432 (catalytic layer 432 being disposed
towards titania layer 406), or a combination thereof. Porous
cathodic layer 425 may be disposed within cell 400 as a plurality
of strips, each strip having a width enabling the strip to fit
between neighboring protrusions of layer 418, or, adjacent to the
cell perimeter, between a protrusion of layer 418 and an inner wall
of an (inner) edge seal 465.
[0292] Current takeoff from cathode or counter-electrode 430 may be
effected via a graphite foil 435 laid on (and typically physically
discrete with respect to) the strips of porous cathodic layer 425.
At its periphery, graphite foil 435 may have an at least partially
embedded inert metal mesh or foil 440 that may protrude outside
cell 400, to facilitate cathodic current collection.
[0293] Dye cell 400 may be produced according to the following
inventive method: onto a surface of photoanode support glass 402
having transparent conductor layer 404 (e.g., an electrically
conducting, transparent tin oxide layer), are screen-printed strips
of titania paste. Upon sintering, spaced-apart strips of sintered,
porous titania layer 406 are produced. In each gap between adjacent
titania strips is situated a current collecting wire 410. A
chemically inert, electrically conducting ceramic paste may be
printed around wire 410. On top of the ceramic material may be
printed an electrically insulating paste. A sintering operation
causes ceramic adhesive layer 415 to be chemically bonded to
transparent conductor layer 404. Sintering s electrically
insulating layer 418 to be chemically bonded to ceramic adhesive
layer 415.
[0294] At this point the titania strips may be stained with
dye.
[0295] The counter-electrode of cell 400 includes strips of porous,
conductive layer 425, which are laid on to the strips of titania
layer 406. These strips may advantageously include glass fiber mats
impregnated with electrically conducting carbon paste, which have
undergone sintering. Strips of conductive layer 425 may be
advantageously catalyzed by means of catalytic layer 432, which may
include trace platinum catalyst (or other catalyst materials such
as carbon-based catalyst materials) on a surface thereof. These
strips may be directly laid on sintered titania layer 406 of the
photoanode, with catalytic layer 432 being disposed towards titania
layer 406. Despite this direct contact, we have found that no
further insulation of titania from the carbon in the
carbon-impregnated strips is necessary.
[0296] Current takeoff is advantageously accomplished via a
graphite foil 435 laid on the strips of conductive layer 425.
Graphite foil 435 has an embedded inert metal mesh or foil 440 at a
periphery thereof, which can pass sealably out of the cell. Cell
400 is sealed by means of a cover typically a glass cover such as
window glass cover 445 in conjunction with an inner edge seal 465
and an outer edge seal 470, preferably produced by means of two
sequentially applied polymers. Glass cover 445 applies adequate
pressure to the graphite foil underneath to ensure good electrical
contact and close interspacing of the elements in the cell.
Electrolyte addition to the cell may be by means of fill holes in
the window glass (not shown) which holes are later sealed off by
polymer. Cathode current withdrawal from the cell is via a takeoff
terminal 442 outside the walls of cell 400, formed by a sealable
protrusion of metal mesh or foil 440 through the cell wall, e.g.,
through edge seals 465 and 470. Individual wires, emerging sealably
from the photoanode via edge seals 465 and 470, can be welded to a
current collecting strip (not shown) to form the photoanode current
collector.
[0297] In a solar module, individual cells may be electrically
connected and suitably mounted in a support structure.
[0298] It is evident from the above that cathode 430 forms a
discrete, physically separate layer with respect to porous titania
film 406 of the anode. Cathode 430 is also adapted to display a
measure of compressibility and elasticity. In sharp contrast to the
substantially unyielding cathodes of the prior art, which, to
prevent damage, must be held at a distance from the surface of the
fragile porous titania film of the anode, cathode 430 (and in
particular, strips of conductive layer 425) may be held against the
surface of porous titania film 406, and may absorb moderate
pressures normal to porous titania film 406, so as to protect
titania film 406. Moreover, the bottommost surface of cathode 430
may adapt or conform to a contour of titania film 406, such that
the significant ohmic inefficiency characterizing various prior art
dye cells is appreciably reduced.
[0299] In order to strips of attain the requisite degree of
conformability, strips of conductive layer 425 may have a Shore D
hardness below 90. In some cases, strips of conductive layer 425
may have a Shore D hardness below 80, or below 70.
[0300] In a schematic cross-sectional view of a photovoltaic dye
cell 500 according to another preferred embodiment of the
invention, shown in FIG. 15, the anodic section may be structured
substantially as in FIG. 13A. A counter-electrode plate or cover
includes a window glass 578, which is bonded at the edges to an
anode glass 302 by a peripheral seal 556, e.g., using a liquid
sealant. A nanocrystalline titania layer 320 of the anode may be
coated with a coarse particle layer 322 based on titania, zirconia,
alumina or silica, for purposes of electrical insulation and light
back-scattering. Nanocrystalline titania layer 320 may then be
covered with a sensitizer dye, or this step can be carried out
subsequently, prior to electrolyte filling. On the cathodic side of
cell 500, each of at least one groove such as groove 588 contains a
wire 596 bonded in place by a conducting adhesive layer 592, which
layer extends also as a continuous layer across the glass face in
order to provide electrical conductivity between adjacent grooves.
The anode and counter plates may be bonded together at various
points (e.g., periodically spaced intervals) along anode and
counter grooves (as in FIG. 13A) by a layer of adhesive 566.
Between adjacent grooves, a layer 570 of carbon or graphite,
optionally coated with a catalyst such as trace platinum, is
provided, and the thickness of this layer is selected so as to
allow close proximity of the carbon or catalyst surface to
nanocrystalline titania layer 320. Cell 500 may be filled with
electrolyte through fill holes (not shown) in the
counter-electrode, which is then sealed, at which point cell 500
may be ready for testing and modulizing. The presence of closely
placed anchoring points (e.g., about one cm apart on a large area
cell sized 15 cm.times.15 cm), and the use of a strong adhesive
such as silicone, maintain spacing and prevent short-circuiting
within cell 500.
[0301] Another aspect of the present invention provides a means for
prevention or reduction of ion migration through intercell seals of
monolithic dye cell assemblies. A cross-sectional view of a prior
art monolithic dye cell assembly of the above-referenced patent to
Kay is shown in FIG. 16. A glass substrate 1 is provided having a
substantially transparent, electrically conductive layer 2, usually
based on tin oxide. The conductive layer is cut in a set of
parallel lines at equidistant spacing across the face of the
conducting substrate, usually by laser grooving means, so as to
provide a series of narrow, parallel, conductive strips 2a, 2b, 2c,
several millimeters wide, along the length of substrate 1, but
electrically insulated from each other on the substrate by
electrically insulated regions 3.
[0302] A layer of porous nanocrystalline titania 4 is
screen-printed on each of the strips to act as photoanode, followed
by a porous insulating layer 5 based on zirconia, silica, alumina
or titania. A counter-electrode 6 of the cell, which is printed
over the groove so as to make electrical contact with the adjacent
conducting strip, consists of a porous layer of carbon containing a
suitable binder such as titania (and optionally containing a
precious metal catalyst), which is screen-printed on top of an
insulator layer 5 in each individual cell 150, 160, 170. This
arrangement gives a series connection of adjacent cells on a single
substrate (monolithic assembly) without the need for additional
cell interconnection means.
[0303] Printing, drying and sintering schedules for layers 4, 5, 6
can be preferably designed to involve a single sintering step for
the layers. Typical thicknesses for the sintered layers are 15
microns for the titania, 5 microns for the insulator, and 50
microns for the counter-electrode (carbon). Dye is introduced into
the titania layer of all the cells by printing application of a
solution of dye in an organic solvent onto the porous carbon layer
of each cell, following which this solution flows down to the
titania layer, where dye is absorbed. Following evaporation off of
the dye solvent, electrolyte is introduced into the cells by
printing application onto the porous carbon layer in each cell. A
monolithic series stack, assembly, or multiple-cell module 100 is
made up of a plurality of single cells such as individual cells
150, 160, 170, connected in series. Each of individual cells 150,
160, 170 includes a titania layer 4, insulating layer 5 disposed
thereon, and a counter-electrode 6 disposed generally above
insulating layer 5. A sealant layer such as polymer sealant layer
or coating 7 is disposed so as to electrically seal adjacent cells
(e.g., individual cells 150 and 160) one from another, and to cover
the large area face of series stack 100. Additional sealing
protection is achieved by bonding on top of the polymer a sheet 8
of glass, plastic or laminate. Voltage takeoff from the series
stack 100 is via bus bars 9, 10 situated at the extremities of
stack 100 (bus bar 9 for the positive pole and bus bar 10 for the
negative pole). Bus bars 9, 10 can be based on silver or solder, by
way of example, since these structural elements are external to the
cell electrolyte and are not exposed to the corrosive
conditions.
[0304] The above-described monolithic assembly suffers from
inherent lifetime limitations. A maximum width for the titania
strips of about one centimeter is typical, since this gives an
acceptable ohmic loss from the cell under practical operating
conditions and as dictated by the limited electrical conductivity
of the tin oxide glass and the carbon counter-electrode. In order
to achieve an acceptable minimal shading (of 10%, by way of
example) from such strip assemblies, the polymer sealant layer
between adjacent cells cannot be more than one millimeter thick.
Known polymer sealants may be adequate for sealing between the
cells and bonding to the tin oxide glass, however, such sealants
cannot, at such a narrow spacing, sufficiently resist iodine
migration between adjacent cells for more than several months. This
is unacceptable in view of the lifetime requirements for solar
cells.
[0305] The problem is compounded by the fact that the strong
potential field (about 700 mV/cell) that exists between adjacent
cells can drive the iodine migration. The iodine, in the form of
the iodide ion I.sup.- or the tri-iodide ion I.sub.3.sup.-, is a
critical component of the cell electrolyte since it comprises the
redox couple on which the operation of the cell depends. The iodine
inventory of a cell (the total iodine present) and the iodine
concentration in the electrolyte are well-defined parameters
operating between well-defined limits for optimum cell performance.
Any loss of iodine from the inventory of a particular cell (by
migration to the adjacent cell) will cause a fall-off in
performance of that cell, and this poorly performing cell will
limit the performance of the entire stack.
[0306] In particular, we have found that the iodine migration tends
to occur along the surface of the tin oxide layer at the very base
of the sealant. Iodine migration may also occur through the bulk
sealant itself. Improved sealing techniques and materials
combinations, or successive application of layers of different
adhesives, may increase material and fabrication costs of the cell,
without providing the requisite long-term protection against iodine
migration.
[0307] The present invention may utilize an imposed voltage field
to prevent or reduce iodine migration between adjacent cells. FIG.
17 shows an expanded side view (not to scale) of the junction
between two adjacent cells shown in FIG. 16. Within polymer sealant
layer 7 between adjacent cells are embedded two electrodes, i.e.,
conductive elements 20 and 25, typically in the form of wires or
strips. Each of these conductive elements may be coated with a
porous, insulating coating 30 to prevent short-circuiting (between
wires and/or between wires and transparent, electrically conductive
layer 2).
[0308] Wire 20 (preferably within coating 30) is positioned close
to, or juxtaposed against, transparent, electrically conductive
layer 2 on glass substrate 1. Wire 25 is disposed at a larger
distance than wire 20, with respect to conductive layer 2. In FIG.
17, wire 25 is situated substantially above wire 20, and within the
intercell junction defined as the region between adjacent
photovoltaic units (e.g., between counter-electrode 6 associated
with conductive strip 2a, and insulating layer 5 and
counter-electrode 6 associated with conductive strip 2b). A voltage
is applied across the electrodes such as wires 20 and 25 by means
of a direct current source. The direct current source may be the
photovoltaic assembly itself, as shown hereinbelow in FIG. 18. The
direct current source may include an external direct current source
(not shown).
[0309] In the case of an iodine-based, or other negatively charged,
redox species, the voltage is applied such that wire 20 has a
negative polarity. Similar wire couples under a similarly applied
voltage field are embedded in the other intercell junctions, as
shown in FIG. 18 hereinbelow.
[0310] The electrodes, such as conductive elements 20 and 25 may be
made of an iodine-resistant material such as tungsten, titanium,
stainless steel or titanium-clad copper. The specific electrical
resistivity of the conductive elements is preferably below, or well
below, 10.sup.-4 ohmcm. The insulating coating is porous, and may
include, by way of example, zirconia, silica, alumina or titania,
applied by a coating procedure based on a suitable process, for
example by dip-coating or electrophoresis followed by drying and
baking. The insulating coating may also be any porous material
resistant and preferably substantially inert to chemical attack of
the electrolyte. Such insulating coatings may include polymeric
and/or fibrous materials. A narrow gauge of wire and coating (for
example 50-150 microns for the wire and 10-30 microns for the
coating) may enable secure embedding in the polymer sealing between
cells without necessarily increasing the seal width and
shading.
[0311] Each wire or electrode pair is supplied with a direct
current voltage or potential, as described above. The voltages may
be supplied by the illuminated monolithic dye cell module itself,
but it is also possible to provide the voltage using a battery or
by other means known to those of ordinary skill in the art. In a
typical monolithic dye cell module of size 30 cm.times.30 cm, and
assuming each cell element is 1 cm wide with an operating voltage
of 600 mV/cell, there is about 18V available to be applied across
the wires of each intercell junction. Leads or connections are
preferably provided so as to connect the module bus bars (shown in
FIG. 16) to each of the embedded electrode pairs 20, 25.
[0312] FIG. 18 shows, in schematic form, a preferred embodiment of
a multiple-cell module such as a 3-cell module or stack 40 having
cells I, II, and III, adapted so as to prevent or appreciably
reduce intercell ion migration. The details of the cell structure
of may be essentially as shown in FIGS. 16 and 17, and as described
in the description associated therewith. Bus bars 50 and 55 provide
the main output of power from the module under illumination, via
leads such as regular heavy duty leads 60 and 65. However, from
(negative pole) bus bar 50 extend two additional, typically low
power leads 52 and 54, which are electrically connected,
respectively, to embedded electrode or wire 90 in a junction 72
(whose volume is at least partially filled by sealant coating 7, as
described with respect to FIG. 17) between cell I and cell II, and
to embedded electrode or wire 95 in a junction 77 (whose volume,
like that of junction 72, is at least partially filled by sealant
coating 7 (referenced hereinabove) between cell II and cell III,
these wires being positioned closest to the transparent, conductive
(e.g., tin oxide coating) layer in the cell. Similarly, from
(positive pole) bus bar 55 extend two additional, typically low
power leads, which are electrically connected, respectively, to
embedded electrode or wire 80 in junction 77 between cells II and
III, and to embedded electrode or wire 85 in junction 72 between
cells I and II. Wires 80, 85 are preferably positioned relatively
far (with respect to embedded wires 90 and 95) from the tin oxide
layer in the cells, so as to ensure the requisite polarity for
inhibiting the ion migration. The connection scheme provides a high
voltage field and high negative potential at the intercell
junctions close to the tin oxide surface and prevents or retards
iodine migration across the interfaces.
[0313] The ion-migration reduction system of the present invention
includes a current source such as a monolithic photovoltaic dye
assembly, or an external current source (such as a battery or DC
current source), and at least one electrode pair such as electrodes
80, 90, disposed within a sealant layer such as sealant coating 7,
preferably within an intercell junction such as junction 72, the
electrode pair being electrically connected to the current
source.
[0314] With reference now to both FIGS. 17 and 18, the magnitude of
the negative potential is preferably adapted so as to inhibit or
significantly reduce migration of iodide and tri-iodide ions (which
are both negatively charged) across a sealant layer filled junction
such as junction 72 and into the adjacent cell. More generally, the
magnitude of the potential is preferably adapted so as to inhibit
or significantly reduce migration of the redox ions (which may be
positively charged or negatively charged, depending on the system)
across sealant layer 7 (filling a junction such as junction 72) and
into the adjacent cell.
[0315] Preferably, the magnitude of the applied potential is at
least 60% of the open-circuit potential of a single cell such as
cell I, or at least 80% of the operating potential of such a cell.
More preferably, the magnitude of the applied potential is at least
80% of the open-circuit potential of a single cell such as cell I,
or more typically, at least about 100% of the operating potential
of such a cell.
[0316] The requisite magnitude of the applied potential may be
related to the operating potential of the entire stack. Preferably,
the magnitude of the applied potential is at least 30% of the
operating potential of the entire stack or module such as module
40, yet more preferably, at least 50% of the operating potential of
the entire module, and most preferably, at least 70% of the
operating potential of the entire module.
[0317] Given that bus bars 50 and 55 are often advantageously
disposed at the extremities of module 40, practically the magnitude
of the applied potential may be substantially equal to the
operating potential of the entire module.
[0318] In terms of absolute potential, the applied voltage is
preferably at least about 1.5 Volts, and more preferably, at least
about 3 Volts. Higher voltages (e.g., above 5 Volts, above 10
Volts, above 20 Volts) tend to improve the reduction of ion
migration.
[0319] It will be apparent to one skilled in the art that redox
species other than iodine-based species may be used in conjunction
with the present invention. For example, in the Journal of the
American Chemical Society, Volume 124, page 11215 (2002), Elliot
described the use of cobalt II/III complexes as an alternative to
the iodide/tri-iodide redox couple in dye cells, since they are
less corrosive and less volatile than the iodine-based systems. In
the cobalt system, both redox species have a positive charge.
[0320] If such a cobalt based redox system were to be used in
monolithic cell assemblies according to the present invention, the
imposed potential would be configured such that the positive lead
having the higher potential would be embedded at the base of the
intercell seal close to the tin oxide layer, thereby limiting or
preventing migration of the positively charged redox species
between adjacent cells.
[0321] Another aspect of the present invention relates to an
improved monolithic structure for large-area, broad, single dye
cells. In a monolithic dye cell design, generally there is a single
sheet of conducting glass required per cell, with accompanying cost
savings. Onto that single sheet of conducting glass are printed
sequentially a porous titania photoanode layer, a porous insulating
spacer layer and then a porous carbon cathode (counter-electrode)
layer. After dye staining of the titania and electrolyte addition,
the cell may be sealed using an outer sheet of glass, polymer,
metal foil or laminate. Since the spacer layer between the titania
and cathode layers can be very thin, of the order of several
micrometers only, this ensures a low electrolyte resistance and
hence, a lower ohmic resistance of the cell. The result is a much
lower cell resistance relative to cells in which the cathode
element is a separate structure, wherein such a close spacing
between photoanode and cathode may be extremely difficult to
achieve. The fact that the cell active layers are built up on the
same support also avoids interelectrode spacing variations
resulting from thermal cycling of the cell, which can be a
performance limiting problem in cells having a separate cathode.
Large area monolithic single cells according to the present
invention have, additionally, a higher fraction of cell footprint
that is optically active, relative to the monolithic multi-cell
design of the above-referenced patent to Kay. Thus, the inactive
opaque seal areas and conductor areas are proportionally reduced
for large area single cells, and the active titania area can
approach more completely the carbon cathode area in the design of
the present invention. Both of these factors positively impact the
cell efficiency.
[0322] FIG. 19 provides a schematic, exemplary view of a large area
monolithic single cell 100 that might be fabricated based on the
prior art (elements not drawn to scale). Onto a glass sheet 1
having a conductive surface layer 4 based on tin oxide, a set of
evenly spaced parallel conductor structures 8 is laid down prior to
the printing of the titania layers. Conductor structures 8 jut
above conductive surface layer 4. In the embodiment shown, each
conductor structure 8 includes a substantially chemically inert
metal wire 12 bonded in place on conductive surface layer 4 by a
substantially chemically inert, electrically conductive binder 16,
and covered with an electrically insulating layer 20 that prevents
electrical shorting-out of the conductor structures to subsequently
applied layers.
[0323] By way of example, adjacent conductor structures 15 cm long
and spaced 1 cm apart preferably have an ohmic drop of less than
0.5 ohms to achieve adequate dye cell current collection on a tin
oxide glass having a surface resistance of 10 ohm/sq.
[0324] In a monolithic cell build, the critical cell components,
which preferably include a titania photoanode layer 24, an
insulating spacer layer 28 and a carbon counter-electrode layer 32,
are typically built up, layer by layer, on glass substrate 1, by
successive printing operations, including drying and sintering of
these layers, to form substantially continuous printed areas 36.
FIG. 20 is a schematic top view of cell 100 showing continuous
printed areas 36 in long strip form, wherein adjacent printed areas
36 are separated by conductor structures 8.
[0325] When such printings are attempted by screen-printing from
the appropriate pastes, there is inevitably a lack of uniformity in
the layer thicknesses following sintering, due to the presence of
the upraised features of conductor structures 8 on the glass
surface. Conductor structures 8 may protrude well above the surface
of conductive surface layer 4. The titania and spacer layers are
less prominent, for example only 15 and 10 micrometers thick,
respectively, following sintering. Even a conductor structure
having a height of only several tens of micrometers can spoil the
printing uniformity of the critical titania layer, however. Due to
projecting conductor structures 8, the mesh or screen via which the
paste is applied cannot be made to lie flat on the glass surface. A
flat disposition on the glass surface is the optimum orientation
for correct dispensing of the paste by, for example, squeegee
pressure.
[0326] Consequently, after drying and sintering have been
performed, a significant portion of the photoactive area of the
cell is not fully parallel to the support glass. As shown in FIG.
19, in the central area of each strip, for example, area A between
adjacent conductors 8, the layers are optimally thin, uniform and
parallel to the substrate surface, but at areas B and C
proximately-disposed to adjacent conductor structures 8, the layers
(e.g., titania photoanode layer 24) are much thicker and not fully
parallel to the support glass. In a typical printing, titania layer
24 of area A is 15.+-.2 micrometers thick following sintering.
However, we have found that titania layer 24 of areas B and C may
have a maximum thickness of 30-200 micrometers or more. The main
outcome of this lack of homogeneity in the thickness of the
(typically screenprinted) layers is reduced cell performance. The
areas close to conductor structures 8 are effectively inactive,
since they may have a considerably longer ionic path characterized
by higher electrolyte resistance, a longer recombination-prone ion
diffusion route and reduced light transmittance, due to the
excessive thickness of titania layer 24. In typical printings,
where the printed strip width between the conductor structures is
about 8 mm, the inactive width can be 1 mm (or more) on each side
of the conductor structures, such that the cell performance loss
compared with the case of uniform printing of strips across the
width can approach 20%.
[0327] A similar result may be obtained using other methods of
application of conductor structures in large area cells, for
example, bonding of wires into grooves on the substrate or
electroplating of conducting metal or metal alloy strips onto the
substrate, since here as well, the conductor structures may be
situated well above the surface of the substrate.
[0328] In the present invention, a strategy for uniform printing of
the active layers may be adopted, by which strategy the active
layer printings are made prior to the application of the conductor
structures. FIG. 21 provides an exemplary, schematic
cross-sectional side view of a portion of an inventive structure,
in which separate strips 45 of a titania layer 40 and insulating
spacer layer 44 are disposed (e.g., by screenprinting) on a glass
substrate 48 having a conducting tin oxide layer 52, leaving at
least one gap 56 between strips 45 that may subsequently be at
least partially filled by conductor structures. In this case, there
is no problem in obtaining active layers having a substantially
uniform or homogeneous thickness, even in the areas adjacent to
such conductor structures.
[0329] Gaps 56 are preferably substantially parallel to one
another.
[0330] An exemplary schematic cross-sectional side view of one
embodiment of a monolithic cell 200 of the present invention is
provided in FIG. 22. Highly electrically conductive structural
elements or cores such as wires 60 are positioned in the gaps (such
as gaps 56 shown in FIG. 21) between the layers and/or printings
(e.g., titania layer 40 and optional insulating spacer layer 44),
or adjacent to a termination or end 55 of the printings.
Preferably, sufficient tension is applied to wires 60 to ensure
close and substantially parallel placement thereof to the tin oxide
surface. This placement procedure may be performed using a jig or
other means known in the art.
[0331] Wires 60 are then permanently bonded in place by an
electrically conducting adhesive layer (e.g., containing a ceramic
adhesive), which may be added by a dosing dispenser, to produce
uncured conductor structures. These uncured conductor structures
may undergo treatment (e.g., a heat treatment) to produce a cured
or at least partially sintered conductor structure such as
conductor structures 98. Conductor structures 98 may include highly
electrically conductive structural elements such as wires 60, at
least partially surrounded by, and preferably completely surrounded
by, an electrically conducting binder layer such as electrically
conductive ceramic or binder layer 64 formed by the treating of the
electrically conducting adhesive layer. Binder layer 64 may contain
a ceramic material and one or more electrically conductive
materials such as tungsten, titanium nitride, zirconium nitride,
and titanium boride.
[0332] Conductor structures 98 may also have an electrically
insulating layer such as an insulating ceramic layer 68, which at
least partially and preferably completely envelops or surrounds
wire 60 and electrically conducting binder layer 64.
[0333] In the photovoltaic cell and method of the present
invention, depending on which conductor application method is
employed, the conductor structures may be between 0.1 mm and 2 mm
wide, may be spaced about 5-20 mm apart on the conducting glass,
and may be at least 50 micrometers to 200 micrometers high (or
more) above the surface of the conducting glass. Preferably, the
conductor structures may have a width of less than 1 mm, and more
preferably, less than 0.7 mm.
[0334] A cathode layer such as porous carbon-based cathode 72,
optionally catalyzed, may be screenprinted or laid directly on top
of insulating spacer layer 44. Alternatively, porous carbon-based
cathode 72 may be screenprinted or laid directly upon titania layer
40.
[0335] Current takeoff from the cathode may be achieved by various
means, for example by bonding to carbon-based cathode 72, a sheet
of graphite foil 76 carrying an embedded metal mesh or strip tab
80, and the layers beneath may be kept well compressed following
sealing by additional inclusion of an optional sponge element (not
shown).
[0336] Titania layer 40 may be coated by a dye using a dye solution
printed onto porous carbon-based cathode 72, which enables the dye
to percolate through to titania layer 40, where it chemisorbs
strongly. Following evaporation off of the dye solvent, the cell
electrolyte is added to the cell by printing onto porous cathode
72. In the exemplary embodiment provided in FIG. 22, cell 200 is
substantially closed off and sealed at the edges using a sealant
layer such as polymer sealant layer 84 backed by a housing such as
a metal foil 88 (for a lightweight design), in which case, a metal
tab 80 may be brought through foil 88 via an insulating grommet 92
that may be attached to foil 88. More standard closures, such as a
glass sheet sealed at the edges with polymer or adhesive, may also
be feasible. Current takeoff from the wire-based structures of the
photoanode or the embedded tab of the cathode, which pass out of
the inside of the cell via the sealed edges of the cell, may be
effected by welded metal strips that can make connection to the
adjacent cell in a modular assembly of cells (not shown).
[0337] Thus, in the cell according to this aspect of the present
invention, the active layers may have a substantially uniform or
homogeneous thickness, even including the areas adjacent to the
conductor structures. Along the entire width of the strips disposed
between the conductor structures, and adjacent (within 1 mm) to
conductor structures 98 in particular, the thickness of a strip of
strips 45 is within 50%, preferably within 30%, and more preferably
within about 20%, of the nominal thickness of the strip. Similarly,
with regard to each of the individual components of strips 45, such
as titania layer 40 and insulating spacer layer 44, the thickness
of a particular component is within 50%, preferably within 30%, and
more preferably, within about 20%, of the nominal thickness of the
strip along the entire width of the strips disposed between the
conductor structures, and in particular, in the areas adjacent
(within 1 mm) to conductor structures 98.
[0338] By way of example, in a dye cell of the present invention,
and given a titania layer 40 screenprinted onto a substrate and
having a nominal thickness of 15 micrometers, strip 40 would have a
thickness of no more than 22.5 micrometers along the entire width
of the strip, including the areas adjacent to the conductor
structures. Preferably, strip 40 would have a thickness of no more
than 19.5 micrometers along the entire width of the strip, and more
preferably, no more than about 18 micrometers.
[0339] Typical printing accuracy of a layer (such as a titania
layer) onto flat glass may be about +/-2 micrometers.
[0340] As used herein in the specification and in the claims
section that follows, the term "nominal thickness", with respect to
a strip such as strip 45, a component of the strip, or a porous
layer such as a titania layer, refers to an average thickness,
within a substantially flat area A, of the strip, component, or
layer, respectively, that is situated at least 2.5 mm from any of
the conductor structures.
[0341] In absolute terms, along the entire width of the strips
disposed between the conductor structures, and adjacent (within 1
mm) to conductor structures 98 in particular, the thickness of a
strip of strips 45 is within 15 micrometers, preferably within 10
micrometers, and more preferably within about 5 micrometers, of the
nominal thickness of the strip. Similarly, with regard to each of
the individual components of strips 45, such as titania layer 40
and insulating spacer layer 44, the thickness of a particular
component is within 15 micrometers, preferably within 10
micrometers, and more preferably within about 5 micrometers, of the
nominal thickness of the strip along the entire width of the strips
disposed between the conductor structures, and in particular, in
the areas adjacent (within 1 mm) to conductor structures 98.
[0342] By way of example, in a dye cell of the present invention,
and given a titania layer 40 screenprinted onto a substrate and
having a nominal thickness of 10 micrometers, strip 40 would have a
thickness of no more than 25 micrometers along the entire width of
the strip, including the areas adjacent to the conductor
structures. Preferably, strip 40 would have a thickness of no more
than 20 micrometers along the entire width of the strip, and more
preferably, no more than about 15 micrometers.
[0343] The invention is not limited to the exemplary sequence of
operations provided hereinabove, and various modifications will be
apparent to those of ordinary skill in the art. For example, the
active layers may be printed on the substrate in one large area
printing without any separations and the gaps cleared in a
subsequent ablation step. Similarly, it is possible to print the
carbon layers on the spacer layers prior to the laying down of
conductor structures. The sequence may also be adjusted in order to
enable proper coordination of drying and sintering steps in cell
preparation, or in order to better accommodate the placing of
conductors in grooves on the substrate surface or plated onto it.
Removable masking layers may also be laid down in order to prevent
contamination of prior placed active layers or electrical shorting
of subsequent layers.
[0344] As used herein, the term "monolithic" and the like, with
regard to a dye cell, refers to a dye cell structure in which both
the photoanode and the cathode layers of the cell are supported by
a common conducting glass support. The term "monolithic" and the
like, is specifically meant to exclude dye cell structures in which
the photoanode is supported by a first glass support and the
cathode is supported by a second glass support, such that the
photoanode and the cathode are substantially disposed therebetween.
Typically, monolithic dye cell structures are produced in a
screenprinting process, and have a porous insulating spacer layer
disposed between the photoanode and cathode layers.
[0345] Below we provide a list of various materials that may be
used in photovoltaic dye cells, along with their specific
electrical resistivities (in units of ohmcm), as available in the
literature.
TABLE-US-00001 Specific Electrical Resistivities (ohm cm) Silver
1.5 .times. 10.sup.-6 Copper 1.5 .times. 10.sup.-6 Nickel 6.2
.times. 10.sup.-6 Platinum 9.6 .times. 10.sup.-6 Aluminum 2.4
.times. 10.sup.-6 Titanium 39 .times. 10.sup.-6 Bismuth 107 .times.
10.sup.-6 Titanium Clad Copper ~3 .times. 10.sup.-6 Molybdenum 4.9
.times. 10.sup.-6 Chromium 11.8 .times. 10.sup.-6 Tantalum 12.2
.times. 10.sup.-6 Tungsten 4.8 .times. 10.sup.-6 Carbon 3000
.times. 10.sup.-6 Graphite 1000 .times. 10.sup.-6 Titanium nitride
25 .times. 10.sup.-6 Tin oxide 500 .times. 10.sup.-6 Titanium
dioxide ~10.sup.12 Alumina binder ~10.sup.14 Sensitizer dye
~10.sup.9
[0346] Conductivity is inversely related to the resistivity. It is
evident from these values that metals such as silver, copper,
aluminum, tungsten are intrinsically highly conducting, while other
metals such as titanium, and some conducting fillers such as
titanium nitride are somewhat less conducting. Carbon, graphite,
and tin oxide are much poorer conductors. Materials such as
titanium dioxide, alumina binder and sensitizer dyes, are properly
classed as insulators, having resistivities that are at least 13
orders of magnitude higher than materials that are considered to be
genuine conductors.
[0347] Not only is the specific resistivity of a material important
in determining the resistance of a layer of the material, but also
the layer thickness, its length and width, and the continuity of
the layer components is crucial. Thus, in dye cells, the conductive
tin oxide layer on the glass is an exceedingly poor conductor, not
just because its specific resistivity is much higher than the
specific resistivity of metals, but also because the layer has to
be extremely thin (typically 0.5 micrometers) in order for the
layer to remain transparent and for light to be able to enter the
cell with adequate transmittance. Consequently, the conductive tin
oxide layer on the glass is a poor vehicle for conveying current
out of the cell along the broad plane of the tin oxide layer.
[0348] Conductor structures 98, such as metal wires bonded in place
on a tin oxide glass by an electrically conductive ceramic
adhesive, may be beneficial as current takeoff elements on the
basis of their intrinsic conductivity. However, other criteria for
the structures include low contact resistance to the tin oxide
surface and minimal shading of light to the cell. By way of
example, we can consider a dye cell having a square geometry of 15
cm per side, which may generate, at 7% conversion efficiency, a
peak current of about 3 amperes. Parallel conductor structures
disposed across the face of the device, each of length 15 cm, width
1 mm and spaced 1 cm apart, yield an acceptably low shading of 10%.
For adequate current takeoff on tin oxide glass having a surface
resistance of 10 ohm/sq., however, the resistance between adjacent
conductor structures should preferably not exceed about 0.5
ohms.
[0349] Generally speaking, the highly electrically conductive
structural elements such as wires 60, disposed within conductor
structures 98, have specific electrical resistivities of less than
1200.times.10.sup.-6 ohm cm, preferably below 500.times.10.sup.-6
ohm cm, more preferably, below 200.times.10.sup.-6 ohm cm, yet more
preferably, less than 100.times.10.sup.-6 ohm cm, and most
preferably, below 50.times.10.sup.-6 ohm cm.
[0350] With regard to the cured layer produced from conducting
adhesive paste or layer 64, the specific electrical resistivity is
less than 1.0 ohm cm, preferably, less than 0.1 ohm cm, more
preferably, less than 0.05 ohm cm, and most preferably, less than
0.01. Some materials suitable for use in, or with, conducting
adhesive layer 64 may have specific electrical resistivities that
are several orders of magnitude lower.
[0351] With regard to the electrically insulating layer (such as
ceramic layer 68) that generally envelops the electrically
conductive structural element and the conducting ceramic layer, and
with regard to insulating spacer layer 44 as well, the specific
electrical resistivity is generally at least 10.sup.6 ohm cm,
preferably, at least 10.sup.8 ohm cm, and more typically, at least
10.sup.10-10.sup.14 ohm cm.
[0352] As used herein in the specification and in the claims
section that follows, the term "precursor", with respect to titania
or zirconia, refers to a titanium-containing or
zirconium-containing compound that, upon heating above at least 300
C, reacts to produce titania or zirconia.
[0353] As used herein in the specification and in the claims
section that follows, the term "specific surface area", and the
like, refers to the specific surface area measured by a standard
BET analysis using a nitrogen medium.
[0354] As used herein in the specification and in the claims
section that follows, the terms "titania" and "titanium dioxide",
are used interchangeably, and the terms "zirconia" and "zirconium
dioxide", are used interchangeably.
[0355] As used herein in the specification and in the claims
section that follows, the term "elongated conductive structural
component" refers to a conductive strip or to a conductive
wire.
[0356] As used herein in the specification and in the claims
section that follows, the term "elongated metallic structural
component" refers to a metal strip or to a metal wire.
[0357] As used herein in the specification and in the claims
section that follows, the term "immediately adjacent", with respect
to a surface of the cathode and a surface of the anode (e.g., a
carbon or catalyzed carbon surface of the cathode and a porous
titania film of the anode) refers to surfaces that are not
separated by an interceding layer.
[0358] As used herein in the specification and in the claims
section that follows, the term "solely physically associated", with
respect to layers of the cathode and/or layers of the anode, refers
to layers that are in contact, but are not sintered together and
are otherwise chemically disattached.
[0359] As used herein in the specification and in the claims
section that follows, the term "self-supporting", with respect to
strips or layers of the cathode and/or layers of the anode, refers
to strips or layers that are held in place within the cell in a
disattached structure with respect to the opposing electrode.
[0360] As used herein in the specification and in the claims
section that follows, the term "discrete", with respect to adjacent
layers in the cell, refers to layers that may be in contact with
one another, yet are physically distinct.
[0361] As used herein in the specification and in the claims
section that follows, the terms "positive potential" and "negative
potential", with regard to an electrode pair, are used in a
relative sense, and not necessarily in an absolute sense.
[0362] Throughout this disclosure, various aspects of this
invention can be presented in a range format. It should be
understood that the description in range format is merely for
convenience and brevity and should not be construed as an
inflexible limitation on the scope of the invention. Accordingly,
the description of a range should be considered to have
specifically disclosed all the possible subranges as well as
individual numerical values within that range. For example,
description of a range such as from 1 to 10 should be considered to
have specifically disclosed subranges such as from 1 to 2, from 1
to 5, from 1 to 8, from 3 to 4, from 3 to 8, from 3 to 10, etc., as
well as individual numbers within that range, for example, 1, 2, 3,
4, 5, 6, 7, 8, 9, and 10. This applies regardless of the breadth of
the range.
[0363] Similarly, the terms "at least", "exceeds", and the like,
followed by a number (including a percent or fraction), should be
considered to have specifically disclosed all the possible
subranges above that number, as well as individual numerical values
above that number. For example, the term "at least 75" should be
considered to have specifically disclosed subranges such as 80 and
above, 90 and above, etc, as well as individual numbers such as 85
and 95.
[0364] Similarly, the terms "less than", "below", and the like,
followed by a number (including a percent, fraction, or ratio such
as a weight ratio), should be considered to have specifically
disclosed all the possible subranges below that number, as well as
individual numerical values below that number. For example, the
term "below 75%" should be considered to have specifically
disclosed subranges such as 70% and below, 60% and below, etc, as
well as individual numbers such as 65% and 50%.
[0365] Whenever a numerical range is indicated herein, the range is
meant to include any cited numeral (fractional or integral) within
the indicated range. The phrase "ranging/ranges between" a first
number and a second number and "within a range of" a first number
to a second number, and the like, are used herein interchangeably
and are meant to include the first and second indicated numbers and
all the fractional and integral numerals therebetween.
[0366] It will be appreciated by one of ordinary skill in the art
that certain features of the invention, which are, for clarity,
described in the context of separate embodiments, may also be
provided in combination in a single embodiment. Conversely, various
features of the invention, which are, for brevity, described in the
context of a single embodiment, may also be provided separately or
in any suitable sub-combination.
[0367] Although the invention has been described in conjunction
with specific embodiments thereof, it is evident that many
alternatives, modifications and variations will be apparent those
skilled in the art. Accordingly, it is intended to embrace all such
alternatives, modifications and variations. All publications and
patents mentioned in this specification, including each and every
priority document, are herein incorporated in their entirety by
reference into the specification, to the same extent as if each
individual publication or patent was specifically and individually
indicated to be incorporated herein by reference. In addition,
citation or identification of any reference in this application
shall not be construed as an admission that such reference is
available as prior art to the present invention.
* * * * *