U.S. patent application number 10/828959 was filed with the patent office on 2004-10-07 for photovoltaic cell.
This patent application is currently assigned to University of Massachusetts Lowell, a Massachusetts corporation. Invention is credited to Balasubramanian, Srinivasan, Chittibabu, Kethinni G., He, Jin-An, Kumar, Jayant, Li, Lian, Samuelson, Lynne Ann, Tripathy, Sukant, Tripathy, Susan.
Application Number | 20040194821 10/828959 |
Document ID | / |
Family ID | 26861772 |
Filed Date | 2004-10-07 |
United States Patent
Application |
20040194821 |
Kind Code |
A1 |
Chittibabu, Kethinni G. ; et
al. |
October 7, 2004 |
Photovoltaic cell
Abstract
A method of making a photovoltaic cell includes contacting a
cross-linking agent with semiconductor particles, and incorporating
the semiconductor particles into the photovoltaic cell.
Inventors: |
Chittibabu, Kethinni G.;
(Nashua, NH) ; He, Jin-An; (Lowell, MA) ;
Samuelson, Lynne Ann; (Marlboro, MA) ; Li, Lian;
(N. Chelmsford, MA) ; Tripathy, Sukant; (Acton,
MA) ; Tripathy, Susan; (Acton, MA) ; Kumar,
Jayant; (Westford, MA) ; Balasubramanian,
Srinivasan; (Woburn, MA) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Assignee: |
University of Massachusetts Lowell,
a Massachusetts corporation
|
Family ID: |
26861772 |
Appl. No.: |
10/828959 |
Filed: |
April 21, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10828959 |
Apr 21, 2004 |
|
|
|
10165877 |
Jun 10, 2002 |
|
|
|
60298858 |
Jun 15, 2001 |
|
|
|
Current U.S.
Class: |
136/263 ;
136/252; 438/57; 438/82 |
Current CPC
Class: |
H01G 9/2059 20130101;
H01G 9/2031 20130101; Y02P 70/50 20151101; Y02E 10/542
20130101 |
Class at
Publication: |
136/263 ;
136/252; 438/082; 438/057 |
International
Class: |
H01L 031/00 |
Goverment Interests
[0002] Funding for the work described herein was provided by the
federal government, which has certain rights in the invention.
Claims
1. A method of making a photovoltaic cell, the method comprising:
contacting a cross-linking agent with semiconductor particles; and
incorporating the semiconductor particles into the photovoltaic
cell.
2. The method of claim 1, wherein the cross-linking agent comprises
an organometallic molecule.
3. The method of claim 1, wherein the cross-linking agent and the
semiconductor particles each comprise an identical chemical
element.
4. The method of claim 3, wherein the chemical element is a
metal.
5. The method of claim 3, wherein the chemical element is selected
from a group consisting of titanium, zirconium, and zinc.
6. The method of claim 1, wherein the cross-linking agent and the
semiconductor particles comprise an identical chemical bond.
7. The method of claim 6, wherein the chemical bond is a metal to
non-metal bond.
8. The method of claim 6, wherein the chemical bond is a
metal-oxygen bond.
9. The method of claim 1, wherein the cross-linking agent is a
material selected from a group consisting of metal alkoxides, metal
acetates, and metal halides.
10. The method of claim 1, wherein the cross-linking agent
comprises a gel precursor.
11. The method of claim 1, further comprising applying a dye on the
semiconductor particles.
12. The method of claim 1, wherein the semiconductor particles are
disposed on a first substrate.
13. The method of claim 12, further comprising electrically
connecting a second substrate to the first substrate.
14. The method of claim 13, wherein the semiconductor particles are
disposed between the first and second substrates.
15. The method of claim 13, wherein the second substrate is
flexible.
16. The method of claim 13, wherein the second substrate comprises
a polymeric material.
17. The method of claim 16, wherein the polymeric material is
selected from a group consisting of polyethyleneterephthalate and
polyethylenenaphthalate.
18. The method of claim 16, wherein the second substrate comprises
a polyimide.
19. The method of claim 12, further comprising heating the first
substrate to less than about 400.degree. C.
20. The method of claim 12, wherein the first substrate is
flexible.
21. The method of claim 12, wherein the first substrate comprises a
polymeric material.
22. The method of claim 21, wherein the polymeric material is
selected from a group consisting of polyethyleneterephthalate and
polyethylenenaphthalate.
23. The method of claim 21, wherein the substrate comprises a
polyimide.
24. The method of claim 1, further comprising incorporating a
polymeric electrolyte into the photovoltaic cell.
25. A method of making a photovoltaic cell, the method comprising:
(a) contacting titanium oxide particles with a first flexible
polymeric substrate to form a titanium oxide film on the first
substrate; (b) contacting the titanium oxide film with titanium
alkoxide to cross-link the particles; (c) contacting the titanium
oxide film with a dye; (d) contacting the titanium oxide film with
a polyelectrolyte; and (e) applying a second flexible polymeric
substrate on the polyelectrolyte to form the cell.
26. A method of making a photovoltaic cell, the method comprising:
(a) continuously forming a first electrode comprising: a flexible
polymeric first substrate; a titanium oxide film disposed on the
first substrate; a dye comprising ruthenium disposed on the
titanium oxide film; and a polyelectrolyte disposed on the titanium
oxide film; (b) continuously forming a second electrode comprising:
a flexible polymeric second substrate; and a catalyst layer
comprising platinum disposed on the second substrate; and (c)
continuously connecting the first and second electrodes to form the
cell.
43. The method of claim 43, wherein step (a) comprises contacting
the semiconductor particles with a cross-linking agent.
44. The method of claim 43, wherein step (a) comprises heating the
first electrode to less than about 400.degree. C.
45. The method of claim 45, wherein heating is performed after
contacting the particles with a cross-linking agent.
46. The method of claim 43, wherein step (a) comprises applying a
polymeric polyelectrolyte to the first electrode.
47. The method of claim 47, wherein the polyelectrolyte comprises
about 5% to about 100% by weight of a polymer, about 5% to about
95% by weight of a plasticizer and about 0.5M to about 10M of a
redox electrolyte.
48. The method of claim 43, wherein the second substrate is
flexible.
49. The method of claim 43, wherein step (b) comprises forming a
catalyst on the second substrate.
50. The method of claim 43, further comprising contacting the
semiconductor particles with a dye.
51. A method of fabricating a photovoltaic cell, the method
comprising: forming a first electrode comprising (a) applying
semiconductor particles onto a flexible first substrate; and (b)
applying a polymeric electrolyte onto the first substrate, wherein
forming the first electrode is performed in a continuous
process.
52. The method of claim 52, further comprising contacting a
cross-linking agent with the semiconductor particles.
53. The method of claim 53, further comprising heating the first
electrode to less than about 400.degree. C. after contacting the
cross-linking agent with the semiconductor particles.
54. The method of claim 52, further comprising contacting the
particles with a dye.
55. The method of claim 52, further comprising forming a second
electrode having a catalyst disposed thereon.
56. The method of claim 56, wherein the second electrode is formed
in a continuous process.
57. The method of claim 57, further comprising continuously joining
the first and second electrodes to form the photovoltaic cell.
58. The method of claim 57, further comprising continuously joining
the first and second electrodes to form the photovoltaic cell.
Description
CLAIM OF PRIORITY
[0001] This application claims priority under 35 USC .sctn.119(e)
to U.S. Patent Application Serial No. 60/298,858, filed on Jun. 15,
2001, the entire contents of which are hereby incorporated by
reference.
FIELD OF THE INVENTION
[0003] The invention relates to photovoltaic cells.
BACKGROUND
[0004] Photovoltaic cells, sometimes called solar cells, can
convert light, such as sunlight, into electrical energy. One type
of photovoltaic cell is sometimes called a dye-sensitized solar
cell (DSC). Referring to FIG. 1, one embodiment of a DSC 10
includes a first glass substrate 12 and a second glass substrate
14. Each substrate 12 and 14 has deposited thereon a transparent
conducting coating, such as a layer of fluorine-doped tin oxide, 16
and 18, respectively. DSC 10 further includes, sandwiched between
substrates 12 and 14, a semiconductor layer 20 (e.g., TiO.sub.2
particles), a sensitizing dye layer 22, an electrolyte 24 (e.g.,
I.sup.-/I.sub.3.sup.-), and a catalyst layer 26 (e.g., Pt).
Semiconductor layer 20 is deposited on coating 16 of first
substrate 12. Dye layer 22 is sorbed on semiconductor layer 20,
e.g., as a monolayer. Together, substrate 12, coating 16,
semiconductor layer 20, and dye layer 22 form a working electrode.
Catalyst layer 26 is deposited on coating 18 of second substrate
14, and together these components 14, 18, and 26 form a counter
electrode. Electrolyte 24 acts as a redox mediator to control the
flow of electrons from the counter electrode to the working
electrode.
[0005] During use, cell 10 undergoes cycles of excitation,
oxidation, and reduction that. produce a flow of electrons, i.e.,
electrical energy. Incident light excites dye molecules in dye
layer 22. The photoexcited dye molecules then inject electrons into
the conduction band of semiconductor layer 20, which leaves the dye
molecules oxidized. The injected electrons flow through
semiconductor layer 20 to an external load 28 to provide electrical
energy. After flowing through load 28, the electrons reduce
electrolyte 24 at catalyst layer 26. The reduced electrolyte can
then reduce oxidized dye molecules back to their neutral state.
This cycle of excitation, oxidation, and reduction is repeated to
provide continuous electrical energy to the load.
[0006] In some cell fabrication processes, substrate 12, coating 16
and semiconductor layer 20 are sintered at relatively high
temperatures, e.g., about 450-500.degree. C., to provide good
contact between the semiconductor particles and between the
semiconductor layer and the coating. As a result, certain
components of a photovoltaic cell can be limited to materials that
are stable at relatively high temperatures, such as rigid glass.
Limitations on useable materials can, in turn, limit the selection
of manufacturing processes, e.g., to batch processes.
SUMMARY OF THE INVENTION
[0007] The invention relates to photovoltaic cells. In particular,
the invention relates to photovoltaic cells having one or more
flexible substrates that can be manufactured at relatively low
temperatures in a continuous process, such as a roll-by-roll or
sheet-by-sheet process. Flexible photovoltaic cells can be used,
for example, in canopies for defense, commercial, residential, and
agricultural applications.
[0008] In one aspect, the invention features a method of making a
photovoltaic cell. The method includes contacting a cross-linking
agent with semiconductor particles, and incorporating the
semiconductor particles into the photovoltaic cell.
[0009] Embodiments may include one or more of the following
features. The cross-linking agent includes an organometallic
molecule, e.g., a metal alkoxide, a metal acetate, or a metal
halide. The cross-linking agent and the semiconductor particles
include an identical chemical element, e.g., a metal such as
titanium, zirconium, or zinc. The cross-linking agent and the
semiconductor particles include an identical chemical bond, e.g., a
metal to non-metal bond such as a metal-oxygen bond. The
cross-linking agent includes a sol-gel precursor.
[0010] The semiconductor particles can be disposed on a first
substrate. The method can further include electrically connecting a
second substrate to the first substrate. The semiconductor
particles can be disposed between the first and second substrates.
The first and/or the second substrate can be flexible, e.g.,
including a polymeric material such as poly(ethyleneterephthalate)
or poly(ethylenenaphthalate). The substrate(s) can include a
polyimide.
[0011] The method can further include applying a dye on the
semiconductor particles. The method can further include heating the
first substrate to less than about 400.degree. C. The method can
further include incorporating a polymeric electrolyte into the
photovoltaic cell.
[0012] In another aspect, the invention features a photovoltaic
cell including a first substrate having cross-linked semiconductor
particles disposed thereon and a second substrate electrically
connected to the first substrate.
[0013] Embodiments may include one or more of the following
features. One or both of the substrates are flexible. The
substrate(s) includes a polymeric material, e.g., a polyimide. The
semiconductor particles are between the first and second
substrates. The cell further includes a polymeric polyelectrolyte
between the first and second substrates. The polyelectrolyte can
include, for example, about 5% to about 100%, e.g., 5-60%, 5-40%,
or 5-20%, by weight of a polymer, e.g., an ion-conducting polymer,
about 5% to about 95%, e.g., about 35-95%, 60-95%, or 80-95%, by
weight of a plasticizer and about 0.05 M to about 10 M of a redox
electrolyte, e.g., about 0.05 M to about 10 M, e.g., 0.05-2 M,
0.05-1 M, or 0.05-0.5 M, of organic or inorganic iodides, and about
0.01M to about 1 M, e.g., 0.05-5 M, 0.05-2 M, or 0.05-1 M, of
iodine. The cell further includes a dye disposed on the
semiconductor particles.
[0014] The semiconductor particles can be crosslinked by a material
including an identical chemical element, e.g., a metal such as
titanium, zirconium, or zinc, as in the semiconductor particles.
The semiconductor particles can be crosslinked by a material
including an identical chemical bond, e.g., a metal to non-metal
bond such as a metal-oxygen bond, as in the semiconductor
particles.
[0015] In another aspect, the invention features a method of
fabricating a photovoltaic cell including (a) forming a first
electrode comprising semiconductor particles disposed on a flexible
substrate, (b) forming a second electrode comprising a second
substrate, and (c) continuously joining the first and second
electrodes to form the photovoltaic cell.
[0016] Embodiments may include one or more of the following
features. Step (a) includes contacting the semiconductor particles
with a cross-linking agent. Step (a) includes heating the first
electrode to less than about 400.degree. C., wherein, for example,
heating is performed after contacting the particles with a
cross-linking agent. Step (a) includes applying a polymeric
polyelectrolyte to the first electrode. The polyelectrolyte can
include, for example, about 5% to about 100%, e.g., 5-60%, 5-40%,
or 5-20%, by weight of a polymer, e.g., an ion-conducting polymer,
about 5% to about 95%, e.g., about 35-95%, 60-95%, or 80-95%, by
weight of a plasticizer and about 0.05 M to about 10 M of a redox
electrolyte, e.g., about 0.05 M to about 10 M, e.g., 0.05-2 M,
0.05-1 M, or 0.05-0.5 M, of organic or inorganic iodides, and about
0.01M to about 1 M, e.g., 0.05-5 M, 0.05-2 M, or 0.05-1 M, of
iodine. The second substrate is flexible. Step (b) includes forming
a catalyst on the second substrate. The method can further include
contacting the semiconductor particles with a dye.
[0017] In another aspect, the invention features a method of
fabricating a photovoltaic cell including forming a first electrode
which includes applying semiconductor particles onto a flexible
first substrate and applying a polymeric electrolyte onto the first
substrate, wherein forming the first electrode is performed in a
continuous process.
[0018] Embodiments may include one or more of the following
features. The method further includes contacting a cross-linking
agent with the semiconductor particles. The method further includes
heating the first electrode to less than about 400.degree. C. after
contacting the cross-linking agent with the semiconductor
particles. The method further includes contacting a dye with the
particles. The method further includes forming a second electrode
having a catalyst disposed thereon. The second electrode is formed
in a continuous process. The method further includes continuously
joining the first and second electrodes to form the photovoltaic
cell.
[0019] In another aspect, the invention features a photovoltaic
cell including a first electrode, a second electrode, and a
polymeric electrolyte between the first and second electrodes. The
polyelectrolyte can include, for example, about 5% to about 100%,
e.g., 5-60%, 5-40%, or 5-20%, by weight of a polymer, e.g., an
ion-conducting polymer, about 5% to about 95%, e.g., about 35-95%,
60-95%, or 80-95%, by weight of a plasticizer and about 0.05 M to
about 10 M of a redox electrolyte, e.g., about 0.05 M to about 10
M, e.g., 0.05-2 M, 0.05-1 M, or 0.05 -0.5 M, of organic or
inorganic iodides, and about 0.01M to about 1 M, e.g., 0.05-5 M,
0.05-2 M, or 0.05-1 M, of iodine.
[0020] Embodiments may include one or more of the following
advantages. Cross-linking the semiconductor particles can provide
particles with good integrity and stability. As a result, in some
circumstances, the particles need not be sintered at high
temperatures, thereby increasing the selection of materials that
can be used for the substrates. For example, the substrates can
include materials with relatively low temperature stability such
polymeric materials, which can make the cells relatively light in
weight. Flexible substrates are amendable to continuous or
semi-continuous manufacturing processes with relatively high
throughput rates. The polymeric polyelectrolyte can also be
conveniently applied during a continuous or semi-continuous
fabrication process. As a result, manufacturing can be performed
with relatively inexpensive processing techniques and materials,
and unit cost can be relatively cost-effective and
cost-competitive.
[0021] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. Although
methods and materials similar or equivalent to those described
herein can be used in the practice or testing of the present
invention, suitable methods and materials are described below. All
publications, patent applications, patents, and other references
mentioned herein are incorporated by reference in their entirety.
In case of conflict, the present specification, including
definitions, will control. In addition, the materials, methods, and
examples are illustrative only and not intended to be limiting.
[0022] Other features and advantages of the invention will be
apparent from the following detailed description, and from the
claims.
DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a cross-sectional view of an embodiment of a
photovoltaic cell.
[0024] FIG. 2 is an exploded view of an embodiment of a
photovoltaic cell.
[0025] FIG. 3 is a block diagram of an embodiment of a process for
making a photovoltaic cell.
[0026] FIG. 4 is a schematic diagram of an embodiment of
cross-linked metal oxide particles.
[0027] FIG. 5 is a schematic diagram of an embodiment of a
continuous process for manufacturing a photovoltaic cell.
DETAILED DESCRIPTION
[0028] Referring to FIG. 2, one embodiment of a flexible
photovoltaic cell 30 includes a first polymeric substrate 32 and a
second polymeric substrate 34. Both substrates 32 and 34 are
flexible and have a transparent conductive coating (not shown)
deposited thereon. Cell 30 further includes, between substrates 32
and 34, a dye-sensitized semiconductor layer 36, a polymeric
polyelectrolyte layer 38, and a catalyst layer 40. Substrate 32 and
semiconductor layer 36 together form a working electrode or a
photoelectrode; and substrate 34 and catalyst layer together form a
counter electrode.
[0029] FIG. 3 shows one embodiment of a process 50 for fabricating
cell 30. Generally, semiconductor particles are deposited on first
substrate 32, and the particles and the substrate are treated with
a chemical cross-linking agent. Substrate 32 and the particles are
then sintered, for example, at a relatively low temperature, such
as about room temperature to less than about 200.degree. C.,
depending on the materials used for the substrate. After sintering,
the semiconductor particles are treated with a dye solution and
then with a polymeric electrolyte. Separately, catalyst layer 40 is
formed on second substrate 34. Substrates 32 and 34 are then
applied together to form cell 30.
[0030] Substrate 32 is formed of a transparent and flexible
material, such as a polymeric material. Preferably, substrate 32
has a thermal coefficient of expansion that is relatively low
and/or comparable to the thermal coefficient of expansion of
semiconductor layer 36 to minimize the occurrence of defects, such
as cracks, during fabrication. Substrate 32 can be formed, for
example, of polyethylene terephthalate (PET), polyethylene
naphthalate (PEN), or a polyimide. An example of a polyimide is a
KAPTON.RTM. polyimide film (available from E. I. du Pont de Nemours
and Co.), which has a thermal stability up to about 400.degree. C.
and can be incorporated into flexible solar cells that are
substantially non-transparent in the visible range. Substrate 32
can have a thickness of about 50 to about 1,000 microns, such as,
for example, about 100 to about 500 microns, or about 150 to about
250 microns.
[0031] Substrate 32 includes a transparent and conductive coating
formed thereon. In some embodiments, the transparent and conductive
coating can be patterned on substrate 32 to define the voltage and
current of cell 10. The coating, for example, indium tin oxide
(ITO), can be deposited on an active area of substrate 32 by
thermal evaporation or low temperature sputtering, for example,
ambient sputtering. The active area of substrate 32 corresponds
approximately to a predetermined area covered by semiconductor
layer 36. A suitable conductor, e.g., a wire, can be connected to
the conductive coating to be connected later to a load (not shown).
The transparent and conductive coating can be about 100 nm to about
500 nm thick, e.g., about 100 to about 300 nm, or about 100 to
about 200 nm.
[0032] Colloidal semiconductor particles are then deposited onto
the transparent and conductive coating of substrate 32 to form a
transparent, nanocrystalline semiconductor layer. The semiconductor
particles, in part, provide substrate 32 with high surface area,
thereby maximizing the amount of dye that can sorb to the
semiconductor layer and the absorption of light. Deposition
generally includes forming a colloidal semiconductor solution,
masking substrate 32 to expose the active area, and applying the
solution to the active area of the substrate.
[0033] The colloidal solution is generally prepared (step 52) by
dispersing semiconductor nanoparticles (here, TiO.sub.2) in a
suitable solvent, such as water, alcohols, ketones, and other
organic solvents. For example, the solution can be prepared by
incrementally adding about 20 mL of dilute nitric acid or acetic
acid (pH about 3-4 in de-ionized water) to about 12 grams of
TiO.sub.2 in a mortar and pestle while grinding. For large-scale
preparation of TiO.sub.2 dispersions, techniques such as internal
mixing or ball milling can be used. The titanium dioxide can have
an average particle size of, e.g., about 21 nm and is available
from Huls Degussa AG, D-6000, Frankfurt, Germany. Other particle
sizes can be used. The acid is added in 1 mL increments after the
preceding mixing and grinding has produced a uniform paste free of
lumps. Alternatively, about 0.2 mL of acetylacetone can be added to
about 1 mL of water and added to 12 grams of TiO.sub.2 powder,
followed by adding about 19 mL of water in 1 mL increments, while
grinding. Alternatively, nanoparticles colloids, e.g., 2-100 nm,
can be prepared from appropriate precursors using standard sol-gel
techniques.
[0034] Other semiconductor particles can also be used including,
for example, other oxides such as zirconium oxide, zinc oxide, and
tungsten oxide, mixed oxides, sulfides, selenides, and
tellurides.
[0035] The semiconductor particles can be deposited by a number of
techniques, including, for example, spray coating, blade or knife
coating, casting, spin casting, screen printing, and stencil
printing.
[0036] Alternatively, substrate 32 is masked, for example, with
adhesive tape or with a masking frame, to define a mold into which
the colloidal solution can flow and to mask a portion of the
conductive coating to which electrical contact can be made.
[0037] A portion of the colloidal solution is then applied (step
51) to the unmasked portion of the conductive coating of substrate
32. The solution is then drawn with a glass stirring rod or a
drawdown bar to distribute the solution to a generally uniform
thickness.
[0038] Substrate 32, with the colloidal solution deposited thereon,
is then heated between about room temperature and about 70.degree.
C. for less than about 10 minutes to yield a thin film of the
semiconductor particles adhered to the substrate. The thin film is
partially dried.
[0039] After heating substrate 32 (step 57), the thin film of the
semiconductor particles on the substrate is treated with a
cross-linking agent. Without wishing to be bound to theory, it is
believed that the cross-linking agent provides electronic and/or
mechanical connections between the semiconductor particles and/or
between the particles and the conductive coating on substrate 32.
It is believed that these connections can provide semiconductor
layer 36 with integrity and stability that, for example, mimic or
approximate integrity and stability achievable by relatively high
temperature sintering. The cross-linking agent, however, provides
cell 30 with good performance without high temperature sintering,
thereby permitting more materials, e.g., a flexible polymeric
substrate with relatively low temperature stability, to be used in
the construction of the cell.
[0040] Generally, semiconductor layer 36 can be cross-linked by
contacting the layer with an agent that can bond, chemically and/or
mechanically, with the semiconductor particles. The agent can be an
appropriate semiconductor precursor or a sol-gel processable
precursor. Preferably, the agent can exhibit similar electronic
conductivity as the semiconductor particles. For example, for
TiO.sub.2 particles, the agent preferably includes Ti--O bonds,
such as those present in titanium alkoxides. Referring to FIG. 4,
it is believed that titanium tetraalkoxide can react with each
other, with TiO.sub.2 particles, and with the conductive coating on
substrate 32, to form titanium oxide bridges that connect the
particles with each other and with the conductive coating (not
shown). As a result, the cross-linking agent enhances the stability
and integrity of the semiconductor layer. The cross-linking agent
can include, for example, an organometallic species such as a metal
alkoxide, a metal acetate, or a metal halide. In other embodiments,
the cross-linking agent can include a different metal than the
metal in the semiconductor.
[0041] In an exemplary cross-linking step, a cross-linking agent
solution is prepared (step 53) by mixing a sol-gel precursor agent,
e.g., a titanium tetra-alkoxide such as titanium tetra-butoxide,
with a solvent, such as ethanol, propanol, butanol, or higher
primary, secondary, or tertiary alcohols, in a weight ratio of
0-100%, e.g., about 5 to about 25%, or about 20%. Generally, the
solvent can be any material that is stable with respect to the
precursor agent, e.g., does not react with the agent to form metal
oxides (e.g. TiO.sub.2). The solvent preferably is substantially
free of water, which can cause precipitation of TiO.sub.2.
[0042] The cross-linking agent can be applied (step 55) by
contacting substrate 32 and the semiconductor particles with the
cross-linking agent solution, e.g., by soaking the particles in the
solution for about 10 minutes at room temperature. Other methods of
applying the cross-linking agent include, for example, spraying the
agent, e.g., as an aerosol, in neat form or in a suitable solvent,
or passing semiconductor layer 36 through a solution of the
cross-linking agent, or passing the semiconductor particles through
an atmosphere having the cross-linking agent, e.g., an oven having
100% TiCl.sub.4 gas.
[0043] Next, substrate 32, with a cross-linked semiconductor layer
formed thereon, is heated (step 59). Heating dries substrate 32 by
evaporating solvent, if any, from the cross-linking solution.
Depending on the heating temperature, heating can also sinter the
semiconductor particles to connect particles together and/or to
connect the particles with the conductive coating of substrate 32.
Generally, the heating temperature is dependent on the
cross-linking agent used and the material of substrate 32. The
heating temperature is selected to be less than the decomposition
temperature of the cross-linking agent, or the temperature at which
substrate 32 becomes unstable, e.g., melts or decomposes, whichever
is less. For example, for a TiO.sub.2 semiconductor layer
cross-linked with titanium alkoxide, the heating temperature is up
to 150.degree. C. for a polymeric substrate such as PET, up to
400.degree. C. for a KAPTON.RTM. substrate, and up to 500.degree.
C. for a glass substrate, although lower temperatures can be used.
Heating time ranges from about 10 minutes to about 60 minutes.
Referring again to FIG. 3, after heating, substrate 32 is a
sintered electrode 54 having the substrate with deposited thereon a
dried, cross-linked semiconductor layer.
[0044] Sintered electrode 54 is then treated with sensitizing dye.
As discussed above, the dye is the primary absorber that harvests
light and injects electrons to the semiconductor layer. Generally,
the dye is selected based on its ability to provide optimum
absorption in the light range, e.g., sunlight, that cell 30 is
exposed to, its ability to transfer electrons to the semiconductor
particles, and its effectiveness in complexing or sorbing to the
semiconductor particles. For example, the dye can include
functional groups, such as multiple carboxyl or hydroxyl groups,
that can chelate to the semiconductor particles, e.g., to the
Ti(IV) sites on a TiO.sub.2 surface. Exemplary dyes include
anthocyanins, porphyrins, phthalocyanins, eosins, and
metal-containing dyes such as
cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate) ruthenium
(II).
[0045] A dye solution can be prepared (step 56) by dissolving a dye
in an appropriate solvent. For example, cis-di(thiocyanato)bis(2,
2'-bipyridyl-4, 4'-dicarboxylate) ruthenium (II) can be dissolved
in ethanol, e.g., in the order of about tens to hundreds of
micromolar concentration.
[0046] The dye can be applied (step 61) to sintered electrode 54 by
soaking cross-linked semiconductor layer 36 in the dye solution for
about 5 minutes up to a few hours. In some embodiments, the rate at
which the dye adsorbs on semiconductor layer 36 can be increased by
adsorbing a single molecular layer of a polycation on the
semiconductor layer. The polycation can be any polymer with
multiple positive charges on either the polymer backbone or on side
chains, such as polyallylamine hydrochloride (PAH) or
poly(diallylmethylammonium chloride) (PDAC). Other methods of
applying the dye solution include, for example, spray coating.
After application of the dye, excess dye is removed by washing the
electrode with a solvent, such as ethanol.
[0047] The electrode is then heated (step 63) to remove residual
solvent, e.g., ethanol. For example, the electrode can be heated at
about 50.degree. C. to about 80.degree. C., for about 10 minutes.
The resulting electrode is sensitized electrode 58 having, e.g., a
deep brownish to red (dyed) semiconductor film.
[0048] After drying, polymeric polyelectrolyte layer 38 is
deposited onto sensitized layer 36 of sensitized electrode 58.
Referring again to FIG. 2, the electrolyte allows electrons to be
transferred from a counter electrode (substrate 34 and catalyst
layer 40) to sensitized semiconductor layer 36, thereby allowing
the excitation-oxidation-reducti- on cycle of cell 30 to continue
with exposure to light. The polyelectrolyte is preferably a
polymeric, relatively viscous, e.g., jelly-like, material. As a
result, the polyelectrolyte can be applied to sensitized layer 36
using techniques that may not be practical or feasible if, for
example, the electrolyte were a liquid. These techniques include,
for example, those that can be used in a continuous or
semi-continuous process such as spray coating, roller coating, or
knife or blade coating.
[0049] The polyelectrolyte can be prepared (step 60) by forming a
solution having an ion-conducting polymer, a plasticizer, and a
mixture of iodides and iodine. The polymer provides mechanical
and/or dimensional stability; the plasticizer helps to provide
relatively high ionic conductivity; and the iodides and iodine act
as the redox electrolytes. The polyelectrolyte can include about
5-100%, e.g., 5-60%, 5-40%, or 5-20%, by weight of an
ion-conducting polymer, about 0-95% e.g., 35-95%, 60-95%, or
80-95%, by weight of a plasticizer, about 0.05 M to about 10 M.
e.g., 0.05-2 M, 0.05-1 M, or 0.05-0.5 M, of organic or inorganic
iodides, and about 0.01M to about 1 M e.g., 0.05-5 M, 0.05-2 M, or
0.05-1 M, of iodine. The ion-conducting polymer can be, for
example, polyethylene oxide (PEO), polyacrylonitrile (PAN),
polymethylmethacrylate (acrylic) (PMMA), polyethers, and
polyphenols. Examples of plasticizers include ethyl carbonate,
propylene carbonate, mixtures of carbonates, organic phosphates,
and dialkylphthalates. Redox electrolytes may include other
reversible organic and/or inorganic redox systems, such as
Fe.sup.2+/Fe.sup.3+, Co.sup.2+/Co.sup.3+ or viologens.
[0050] A useful polymeric electrolyte includes about 10% of
polyethylene oxide, about 90% of 1:1 by weight mixture of ethyl
carbonate: propylene carbonate, about 0.05 M iodine, and about 0.5
M lithium tetramethylammonium iodide. The polymeric electrolyte has
a relatively long shelf life (e.g., up to a few years), experience
minimal phase segregation during processing or while cell 30 is in
use, and can be used without processing, such as melting, prior to
deposition on semiconductor layer 36.
[0051] The polyelectrolyte is deposited (step 65) on semiconductor
layer 36 by blade coating techniques to yield an
electrolyte-deposited, sensitized electrode 62. In some
embodiments, electrode 62 can then be heated to remove residual
solvent, e.g., acrylonitrile. For example, electrode 62 can be heat
at about 60.degree. C. to about 80.degree. C., for about 5 minutes.
Electrode 62, which includes substrate 32, dye-sensitized
semiconductor layer 36, and polyelectrolyte layer 38, is then
applied to a metallized substrate 64, which includes substrate 34
and catalyst layer 40.
[0052] Substrate 34 can be generally similar to substrate 32. That
is, substrate 34 can be formed of a transparent and flexible
material. For example, substrate 34 can be formed of polyethylene
terephthalate (PET) or polyethylene naphthalate (PEN). Substrate 34
can have a thickness of about 50 to about 1,000 microns, such as,
for example, about 100 to about 500 microns.
[0053] Similar to substrate 32, substrate 34 can also include a
transparent and conductive coating deposited thereon. The coating,
for example, indium tin oxide (ITO), can be deposited on the active
area of substrate 34 by low temperature sputtering, for example,
room temperature sputtering. A suitable conductor can be connected
to the conductive coating. The transparent and conductive coating
can be about 100 to about 500 nm thick, e.g., about 100 to about
300 nm, or about 100 to about 200 nm.
[0054] Metallized substrate or electrode 64 is formed by forming
catalyst layer 40 (step 69) on the transparent and conductive
coating on substrate 34. Catalyst layer 40 can include, for
example, carbon, or preferably, platinum. For substrates 34 that
are stable up to relatively low temperatures, catalyst layer 40 can
be formed, for example, by thermal evaporation, room temperature
sputtering or electrodeposition. For example, a thin platinum
catalyst layer can be electrodeposited on the conductive coating of
substrate 0.34 by using a solution with about 0.05 g/L PtCl.sub.4
and 0.025 M HCl, a current density of about 1 mA/cm.sup.2 for about
2-5 minutes. Catalyst layer 40 can be about 2 to about 10 nm thick,
e.g., about 2 to about 3 nm thick.
[0055] Metallized substrate 64 is then applied (step 67) to
electrolyte-deposited sensitized electrode 62 by contacting
catalyst layer 40 with polyelectrolyte layer 38 to form a
sensitized solar cell 66.
[0056] Solar cell 66 is then sealed at its perimeter with a polymer
sealant, e.g., an epoxy or Surlyn.RTM. hot melt (available from
DuPont), to minimize permeation of water and/or air into the cell,
which can adversely affect cell performance. Alternatively, or in
addition, cell 66 can be thermally sealed about its perimeter.
[0057] Sealed cell 66 is then laminated with a barrier to moisture
and/or. oxygen such as one ore more polymeric overlayers, e.g., a
Surlyn.RTM. polymer (available from DuPont). Sealed and laminated
cell 66 is then packaged, e.g., mounted on a support structure, to
provide a finished solar module 68. The barrier may include UV
stabilizers and/or UV absorbing luminescent chromophores (which
emit at higher wavelengths) and antioxidants to protect and improve
the efficiency of the cell.
[0058] The methods describe above include features that allow cell
30 to be manufactured in a continuous process. One feature, for
example, is the cross-linked semiconductor layer that can exhibit
good performance without the need for high temperature sintering.
This expands the selection of substrates, such as flexible
polymeric substrates with relatively low temperature stability,
that can be used without sacrificing performance. Another feature,
for example, is the viscous polymeric polyelectrolyte that can be
applied to a dye-sensitized electrode during a continuous process,
vis--vis, e.g., a liquid electrolyte that can run and may be
difficult to apply.
[0059] Referring to FIG. 5, a schematic diagram of a continuous
roll-to-roll or sheet-by-sheet process 100 for manufacturing cell
30 is shown. Generally, metallized electrode 64 and
electrolyte-deposited sensitized electrode 62, on two separate
assembly lines, are formed continuously from reels having webs of
substrate material. The electrodes are then brought together to
form cells 66. Individual cells 66 or sets of cells 66 can then be
cut from the joined web, sealed, and packaged.
[0060] Metallized electrode 64 and electrolyte-deposited sensitized
electrode 62 can be formed according to the methods described above
and in FIG. 3. Metallized electrode 64 can be continuously formed
by providing a reel or spool of a flexible web of substrate 34,
e.g., ITO-coated PET film, guiding the web along rollers 102, and
depositing catalyst layer 40 by sputtering. Metallized electrode 64
can be fed to be applied to electrode 62. Similarly, electrode 62
can be continuously formed by providing a reel of a flexible web of
substrate 32, and guiding the web along rollers 102 to subject
predetermined areas, i.e., the active areas, of the web to the
processes described above (FIG. 5). Electrodes 62 and 64 can then
be applied together to form solar cell 66.
[0061] The following examples are illustrative and not intended to
be limiting.
EXAMPLE 1
[0062] A dye-sensitized solar cell having flexible substrates was
made according to the following procedures.
[0063] A PET substrate (200 microns thick, 10 mm.times.10 mm) was
coated with a 200 .mu.m thick ITO coating by thermal evaporation. A
colloidal TiO.sub.2 solution was prepared by dispersing P25
particles in water (pH 3-4). The TiO.sub.2 was deposited on the PET
substrate by spin coating. After deposition, the electrode was
heated at about 50.degree. C. for about 10 minutes.
[0064] The TiO.sub.2-coated PET substrate was then soaked in a 20%
titanium tetrabutoxide solution in ethanol (wt/wt) for 15 minutes.
After soaking, the substrate was removed from the solution, dried
at about 50.degree. C. for about 30 minutes, and dried at about
120.degree. C. for about 30 minutes.
[0065] The sintered substrate was dye-sensitized by soaking the
substrate in a solution of cis-di(thiocyanato)bis(2,
2'-bipyridyl-4, 4'-dicarboxylate) ruthenium (11) in ethanol (1
mg/mL) overnight.
[0066] A redox polyelectrolyte (a liquid electrolyte including 1 M
LiI, 0.05 M iodine, 1 M tbutyl pyridine in 3-methoxy propionitrile)
was coated on the dye-sensitized, sintered substrate, and
sandwiched with a second PET substrate coated with an ITO
conductive layer (200 nm) and a platinum catalyst layer (2.5 nm
thick).
[0067] The cell exhibited a solar conversion efficiency of about
3%.
EXAMPLE 2
[0068] A dye-sensitized solar cell having rigid substrates was made
according to the following procedures.
[0069] An ITO-coated glass slide (surface resistance 8
.OMEGA./cm.sup.2) was coated with TiO.sub.2 nanoporous film by spin
coating from a TiO.sub.2 dispersion (7 micron thick). The coated
slide was dried at room temperature for 30 minutes and sintered at
450.degree. C. in an oven for 1 hour.
[0070] The sintered slide was dye-sensitized by soaking the slide
in a solution of cis-di(thiocyanato)bis(2, 2'-bipyridyl-4,
4'-dicarboxylate) ruthenium (II) in ethanol (1 mg/mL)
overnight.
[0071] A redox polyelectrolyte (10% polyethylene oxide, 90% 1:1
ethyl carbonate:propylene carbonate, 0.05 M iodine and 0.5 M LiI in
acetonitrile) was coated on the substrate, and sandwiched with a
second glass slide coated with an ITO conductive layer (200 .mu.m)
and a platinum catalyst layer (2.5 nm thick).
[0072] The cell exhibited a solar conversion efficiency of about
7.3%. The polyelectrolyte was stable up to at least 180 days with
no observable evidence of phase segregation.
Other Embodiments
[0073] It is to be understood that while the invention has been
described in conjunction with the detailed description thereof, the
foregoing description is intended to illustrate and not limit the
scope of the invention, which is defined by the scope of the
appended claims.
[0074] For example, while the above features and methods have been
described as applied to photovoltaic cells, the features and
methods can be applied to other applications, such as
electrochromic windows and ceramic coatings, e.g., TiO.sub.2
coatings.
[0075] Other embodiments of photovoltaic cells can include
substrates made of rigid material, such as glass, and, for example,
the polyelectrolyte described above.
[0076] In other embodiments, substrates 32 and/or 34 can be thin
foils, e.g., relatively transparent metal foils such as titanium or
molybdenum foils about 5-50 microns thick. The transparent and
conductive coating can include other materials besides ITO, such as
fluorine-doped tin oxide.
[0077] Cell 30 can be a hybrid system having, for example, one
rigid substrate and one flexible substrate. For example, the
photoelectrode can include a flexible substrate formed in a
continuous process as described above, and applied to a rigid
substrate, e.g., SnO.sub.2:F coated glass, in a separate,
non-continuous process.
[0078] Other aspects, advantages, and modifications are within the
scope of the following claims.
* * * * *