U.S. patent application number 10/918806 was filed with the patent office on 2005-01-27 for low temperature interconnection of nanoparticles.
Invention is credited to Chittibabu, Kethinni, Gaudiana, Russell.
Application Number | 20050019414 10/918806 |
Document ID | / |
Family ID | 27609433 |
Filed Date | 2005-01-27 |
United States Patent
Application |
20050019414 |
Kind Code |
A1 |
Chittibabu, Kethinni ; et
al. |
January 27, 2005 |
Low temperature interconnection of nanoparticles
Abstract
A polymeric linking agent enables the manufacture of
photovoltaic cells on flexible substrates, including, for example,
polymeric substrates. Photovoltaic cells may be fabricated by a
relatively simple continuous manufacturing process, for example, a
roll-to-roll process, instead of a batch process.
Inventors: |
Chittibabu, Kethinni;
(Lowell, MA) ; Gaudiana, Russell; (Merrimack,
NH) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Family ID: |
27609433 |
Appl. No.: |
10/918806 |
Filed: |
August 13, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10918806 |
Aug 13, 2004 |
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10350913 |
Jan 24, 2003 |
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10350913 |
Jan 24, 2003 |
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10057394 |
Jan 25, 2002 |
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6706963 |
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Current U.S.
Class: |
424/489 ;
424/617 |
Current CPC
Class: |
H01G 9/2095 20130101;
H01G 9/2086 20130101; Y02P 70/521 20151101; Y02P 70/50 20151101;
Y02E 10/542 20130101; H01G 9/2031 20130101; H01G 9/2059 20130101;
H01L 51/0086 20130101; H01G 9/2068 20130101 |
Class at
Publication: |
424/489 ;
424/617 |
International
Class: |
A61K 009/14; A61K
033/24 |
Claims
What is claimed is:
1-21. (Canceled)
22. A flexible photovoltaic cell comprising a photosensitized
interconnected nanoparticle material and a charge carrier material,
both disposed between first and second flexible, significantly
light transmitting substrates.
23. The photovoltaic cell of claim 22, wherein the photosensitized
interconnected nanoparticle material comprises nanoparticles linked
by a polymeric linking agent.
24. The photovoltaic cell of claim 22, wherein the photosensitized
interconnected nanoparticle material comprises particles with an
average size substantially in the range of 10 nm to 40 nm.
25. The photovoltaic cell of claim 22, wherein the photosensitized
interconnected nanoparticle material comprises interconnected
titanium dioxide nanoparticles.
26. The photovoltaic cell of claim 22, wherein the photosensitized
interconnected nanoparticle material is selected from the group
consisting of zirconium oxides, zinc oxides, tungsten oxides,
niobium oxides, lanthanum oxides, tantalum oxides, tin oxides,
terbium oxides, and combinations thereof.
27. The photovoltaic cell of claim 22, wherein the photosensitized
interconnected nanoparticle material comprises a photosensitizing
agent that comprises a molecule selected from the group consisting
of xanthines, cyanines, merocyanines, phthalocyanines, and
pyrroles.
28. The photovoltaic cell of claim 27, wherein the photosensitizing
agent further comprises a metal ion selected from the group
consisting of divalent and trivalent metals.
29. The photovoltaic cell of claim 27, wherein the photosensitizing
agent comprises at least one of a ruthenium transition metal
complex, an osmium transition metal complex, and an iron transition
metal complex.
30. The photovoltaic cell of claim 22, wherein the charge carrier
material comprises a redox electrolyte system.
31. The photovoltaic cell of claim 22, wherein the charge carrier
material comprises a polymeric electrolyte.
32. The photovoltaic cell of claim 22, wherein the charge carrier
material transmits at least about 60% of incident visible
light.
33. The photovoltaic cell of claim 22, wherein at least one of the
first and second flexible, significantly light transmitting
substrates comprises a polyethylene terephthalate material.
34. The photovoltaic cell of claim 22, further comprising a
catalytic media layer disposed between the flexible, significantly
light transmitting first and second substrates.
35. The photovoltaic cell of claim 34, wherein the catalytic media
layer comprises platinum.
36. The photovoltaic cell of claim 22, further comprising an
electrical conductor material disposed on at least one of the first
and second flexible, significantly light transmitting
substrates.
37. The photovoltaic cell of claim 36, wherein the electrical
conductor material comprises indium tin oxide.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefits of and priority to U.S.
patent application Ser. No. 10/057,394 filed on Jan. 25, 2002, to
U.S. Provisional Patent Application Ser. No. 60/351,691 filed on
Jan. 25, 2002, to U.S. Provisional Patent Application Ser. No.
60/368,832 filed on Mar. 29, 2002, and to U.S. Provisional Patent
Application Ser. No. 60/400,289 filed on Jul. 31, 2002, all of
which are owned by the assignee of the instant application and the
disclosures of which are incorporated herein by reference in their
entireties.
FIELD OF THE INVENTION
[0002] The invention relates generally to the field of photovoltaic
devices, and more specifically to chemical structures and methods
of interconnecting nanoparticles at low temperatures.
BACKGROUND OF THE INVENTION
[0003] Thin film solar cells composed of percolating networks of
liquid electrolyte and dye-coated sintered titanium dioxide were
developed by Dr. Michael Grtzel and coworkers at the Swiss Federal
Institute of Technology. These photovoltaic devices fall within a
general class of cells referred to as dye-sensitized solar cells
("DSSCs"). Conventionally, fabrication of DSSCs requires a high
temperature sintering process (>about 400.degree. C.) to achieve
sufficient interconnectivity between the nanoparticles and enhanced
adhesion between the nanoparticles and a transparent substrate.
Although the photovoltaic cells of Grtzel are fabricated from
relatively inexpensive raw materials, the high temperature
sintering technique used to make these cells limits the cell
substrate to rigid transparent materials, such as glass, and
consequently limits the manufacturing to batch processes and the
applications to those tolerant of the rigid structure. Furthermore,
the high temperature sintering process increases the cost of
manufacturing a photovoltaic cell due to the energy required to
perform the sintering.
SUMMARY OF THE INVENTION
[0004] The invention in, one embodiment, addresses the deficiencies
of the prior art by providing a polymeric linking agent that
enables the fabrication of thin film solar cells at relatively low
temperatures. This enables the manufacture of such cells on
flexible substrates, including, for example, those substrates
constructed from somewhat heat sensitive polymeric materials. In
addition, the invention provides photovoltaic cells and methods of
photovoltaic cell fabrication that facilitate their manufacture by
a relatively simple, continuous manufacturing process. For example,
a roll-to-roll process can be utilized instead of the batch
processes that limited the prior art. More particularly, in one
embodiment, the invention provides a method for interconnecting
metal oxide nanoparticles in DSSCs, with reduced or no heating,
using a polymeric linking agent. By way of example, metal oxide
nanoparticles may be interconnected by contacting the nanoparticles
with a suitable polymeric linking agent dispersed in a solvent,
such as n-butanol, at about room temperature or at elevated
temperatures below about 300.degree. C.
[0005] In one aspect, therefore, the invention provides a method of
interconnecting nanoparticles at low temperature that includes
providing a solution having a polymeric linking agent and a solvent
and contacting a plurality of metal oxide nanoparticles with the
solution at a temperature below about 300.degree. C., where the
solution contains the polymeric linking agent in a concentration
sufficient to interconnect at least a portion of the plurality of
metal oxide nanoparticles. In various embodiments of the method,
the temperature is below about 200.degree. C., below about
100.degree. C., or at about room temperature. In one embodiment,
the polymeric linking agent includes a long chain macromolecule.
The long chain macromolecule may have a backbone structure
substantially similar to the chemical structure of the plurality of
metal oxide nanoparticles, and one or more reactive groups
chemically bonded to the backbone structure. In another embodiment,
the plurality of metal oxide nanoparticles has a chemical structure
of the form M.sub.xO.sub.y, where x and y are integers. By way of
example, M can include Ti, Zr, W, Nb, Ta, Tb, or Sn.
[0006] In one embodiment, the polymeric linking agent is
poly(n-butyl titanate). In another embodiment, the solvent of the
solution is n-butanol. In various embodiments of the method, the
mechanism for interconnecting at least a portion of the plurality
of metal oxide nanoparticles is a mechanical or electrical bridge
formed by the one or more reactive groups binding to the plurality
of metal oxide nanoparticles. The plurality of metal oxide
nanoparticles may be disposed, for example, as a thin film on a
substrate. In various embodiments of the method, the metal oxide
nanoparticles are disposed on the substrate by, for example,
dipping the substrate into the solution including the polymeric
linking agent, spraying the solution including the polymeric
linking agent onto the substrate, or dispersing the solution
including the polymeric linking agent on the substrate. In one
embodiment, the plurality of metal oxide nanoparticles are
dispersed onto the substrate, and then the solution including the
polymeric linking agent is deposited onto the substrate. In another
embodiment, the method includes the step of contacting the metal
oxide nanoparticles with a modifier solution. In yet another
embodiment, the plurality of metal oxide nanoparticles includes
nanoparticles of titanium oxides, zirconium oxides, zinc oxides,
tungsten oxides, niobium oxides, lanthanum oxides, tantalum oxides,
tin oxides, terbium oxides, and one or more combinations
thereof.
[0007] In another aspect, the invention provides a polymeric
linking agent solution including (1) a polymeric linking agent of
the formula --[O-M(OR).sub.i--].sub.m--; (2) a plurality of metal
oxide nanoparticles that have the formula M.sub.xO.sub.y; and (3) a
solvent; where i, m, x, and y are integers greater than zero. In
one embodiment, M is Ti, Zr, Sn, W, Nb, Ta, or Tb. R may be a
hydrogen atom, an alkyl, an alkene, an alkyne, an aromatic, or an
acyl group. In this embodiment, the solution preferably contains
the polymeric linking agent in a concentration sufficient to
interconnect at least a portion of the plurality of metal oxide
nanoparticles at a temperature below about 300.degree. C. In
another embodiment, the polymeric linking agent solution contains
the polymeric linking agent in a concentration sufficient to
interconnect at least a portion of the plurality of nanoparticles
at a temperature below about 100.degree. C. In one embodiment, for
example, the polylinker solution is a 1% (by weight) poly(n-butyl
titanate) in n-butanol.
[0008] In a further aspect, the invention provides a flexible
photovoltaic cell including a photosensitized interconnected
nanoparticle material and a charge carrier material, both of which
are disposed between first and second flexible and significantly
light transmitting substrates. The photosensitized interconnected
nanoparticle material may include nanoparticles linked by a
polymeric linking agent. In one embodiment of the photovoltaic
cell, the photosensitized interconnected nanoparticle material
includes particles with an average size substantially in the range
of about 10 nm to about 40 nm. In another embodiment, the
photosensitized interconnected nanoparticle material is
interconnected titanium dioxide nanoparticles. The photosensitized
interconnected nanoparticle material may be, for example, zirconium
oxides, zinc oxides, tungsten oxides, niobium oxides, lanthanum
oxides, tantalum oxides, tin oxides, terbium oxides, or one or more
combinations thereof. The photosensitized interconnected
nanoparticle material may include a photosensitizing agent such as
a xanthine, cyanine, merocyanine, phthalocyanine, and/or pyrrole.
The photosensitizing agent may include a metal ion, such as
divalent or trivalent metals. The photosensitizing agent may also
include at least one of a rutheniumrtransition metal complex, an
osmium transition metal complex, and an iron transition metal
complex. In one embodiment of the photovoltaic cell, the charge
carrier material includes a redox electrolyte system. In another
embodiment, the charge carrier media is a polymeric electrolyte.
According to one feature, the charge carrier material transmits at
least about 60% of incident visible light.
[0009] In one embodiment of the photovoltaic cell, at least one of
the first and second flexible, significantly light transmitting
substrates includes a transparent substrate (e.g., a polyethylene
terephthalate material). In another embodiment, the photovoltaic
cell includes a catalytic media layer disposed between the first
and second flexible, significantly light transmitting substrates.
The catalytic media layer is, for example, platinum. In another
embodiment, the photovoltaic cell includes an electrical conductor
material disposed on at least one of the first and second flexible,
significantly light transmitting substrates. In another embodiment,
the electrical conductor material is, for example, indium tin
oxide.
[0010] Other aspects and advantages of the invention will become
apparent from the following drawings, detailed description, and
claims, all of which illustrate the principles of the invention, by
way of example only.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing and other objects, features, and advantages of
the invention described above will be more fully understood from
the following description of various illustrative embodiments, when
read together with the accompanying drawings. In the drawings, like
reference characters generally refer to the same parts throughout
the different views. The drawings are not necessarily to scale, and
emphasis instead is generally placed upon illustrating the
principles of the invention.
[0012] FIG. 1 depicts an exemplary chemical structure of an
illustrative embodiment of a polylinker for nanoparticles of an
oxide of metal M, in accordance with the invention;
[0013] FIG. 2 depicts another exemplary chemical structure of an
illustrative embodiment of a polylinker, according to the
invention, for nanoparticles of an oxide of metal M;
[0014] FIG. 3A shows an exemplary chemical structure for an
interconnected nanoparticle film with a polylinker, according to an
illustrative embodiment of the invention;
[0015] FIG. 3B shows the interconnected nanoparticle film of FIG.
3A attached to a substrate oxide layer, according to an
illustrative embodiment of the invention;
[0016] FIG. 4 depicts the chemical structure of poly(n-butyl
titanate);
[0017] FIG. 5A shows the chemical structure of a titanium dioxide
nanoparticle film interconnected with poly(n-butyl titanate),
according to the invention;
[0018] FIG. 5B shows the interconnected titanium dioxide
nanoparticle film of FIG. 5A attached to a substrate oxide layer,
according to an illustrative embodiment of the invention;
[0019] FIG. 6 is a cross-sectional view of a flexible photovoltaic
cell, according to an illustrative embodiment of the invention;
[0020] FIG. 7 depicts an illustrative embodiment of a continuous
manufacturing process that may be used to form the flexible
photovoltaic cell shown in FIG. 6;
[0021] FIG. 8 depicts a current-voltage curve for an exemplary
solar cell, according to the invention;
[0022] FIG. 9 shows a current-voltage curve for an exemplary solar
cell, according to an illustrative embodiment of the invention;
[0023] FIG. 10 shows current-voltage curves for two additional
exemplary solar cells, according to an illustrative embodiment of
the invention;
[0024] FIG. 11 depicts the chemical structure of gelation induced
by a complexing reaction of Li.sup.+ ions with complexable
poly(4-vinyl pyridine) compounds, in accordance with an
illustrative embodiment of the invention;
[0025] FIG. 12 shows the chemical structure of a lithium ion
complexing with polyethylene oxide segments, according to another
illustrative embodiment of the invention;
[0026] FIGS. 13A-13C depict chemical structures for exemplary
co-sensitizers, according to illustrative embodiments of the
invention;
[0027] FIGS. 14A-14B depict additional exemplary chemical
structures of co-sensitizers, according to illustrative embodiments
of the invention;
[0028] FIG. 15 shows a graph of the absorbance of the 455 nm
cut-off filter (GC455) used to characterize photovoltaic cells
according to the invention;
[0029] FIG. 16 shows a graph of the absorbance of
diphenylaminobenzoic acid; and
[0030] FIG. 17 depicts an illustrative embodiment of the coating of
a semiconductor primer layer coating, according to the
invention.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0031] A. Low Temperature Interconnection of Nanoparticles
[0032] As discussed in the summary above, the invention, in one
embodiment, provides a polymeric linking agent (hereinafter a
"polylinker") that enables the fabrication of thin film solar cells
at relatively low "sintering" temperatures (<about 300.degree.
C.). Although the term "sintering" conventionally refers to high
temperature (>about 400.degree. C.) processes, as used herein,
the term "sintering" is not temperature specific, but instead
refers generally to the process of interconnecting nanoparticles at
any suitable temperature. In one illustrative embodiment, the
invention provides a method for using polylinkers to interconnect
nanoparticles in a thin film solar cells. According to another
illustrative embodiment, the relatively low temperature sintering
process enables the manufacture of such photovoltaic cells using
flexible polymer substrates. By employing flexible substrates, the
invention also enables a continuous roll-to-roll or web
manufacturing process to be employed.
[0033] FIGS. 1 and 2 schematically depict chemical structures of
illustrative polylinkers, according to the invention. The
particular polylinker structures depicted are for use with
nanoparticles of the formula M.sub.xO.sub.y, where M may be, for
example, titanium (Ti), zirconium (Zr), tungsten (W), niobium (Nb),
lanthanum (La), tantalum (Ta), terbium (Tb), or tin (Sn) and x and
y are integers greater than zero. According to the illustrative
embodiment of FIG. 1, the polylinker 100 includes a backbone
structure 102, which is similar in structure to the metal oxide
nanoparticles, and (OR).sub.i reactive groups, where R may be, for
example, acetate, an alkyl, alkene, alkyne, aromatic, or acyl
group; or a hydrogen atom and i is an integer greater than zero.
Suitable alkyl groups include, but are not limited to, ethyl,
propyl, butyl, and pentyl groups. Suitable alkenes include, but are
not limited to, ethene, propene, butene, and pentene. Suitable
alkynes include, but are not limited to, ethyne, propyne, butyne,
and pentyne. Suitable aromatic group include, but are not limited
to, phenyl, benzyl, and phenol. Suitable acyl groups include, but
are not limited to, acetyl and benzoyl. In addition, a halogen
including, for example, chlorine, bromine, and iodine may be
substituted for the (OR).sub.i reactive groups.
[0034] Referring to FIG. 2, the polylinker 110 has a branched
backbone structure that includes two -M-O-M-O-M-O-- backbone
structures, which include (OR).sub.i reactive groups and
(OR).sub.i+1 reactive groups, where R may be, for example, one of
the atoms, molecules, or compounds listed above and i is an integer
greater than zero. The two backbone structures have similar
structures to the metal oxide nanoparticles. Collectively, the
structure depicted in FIG. 2 can be represented by
-M(OR).sub.i--O-(M(OR).sub.i--O).sub.n-M(OR).sub.i+1, where i and n
are integers greater than zero.
[0035] FIG. 3A depicts schematically the chemical structure 300
resulting from interconnecting the M.sub.xO.sub.y nanoparticles 302
with a polylinker 304. In various embodiments, the polylinker 304
has the chemical structure of the polylinkers 100 and 110 depicted
in FIGS. 1 and 2, respectively. According to the illustrative
embodiment, the nanoparticles 302 are interconnected by contacting
the nanoparticles 302 with a polylinker 304 at or below room
temperature or at elevated temperatures that are less than about
300.degree. C. Preferably, the polylinker 304 is dispersed in a
solvent to facilitate contact with the nanoparticles 302. Suitable
solvents include, but are not limited to, various alcohols,
chlorohydrocarbons (e.g., chloroform), ketones, cyclic and linear
chain ether derivatives, and aromatic solvents among others. It is
believed that the reaction between surface hydroxyl groups of the
nanoparticles 302 with alkoxy groups on the polymer chain of the
polylinker 304 leads to bridging (or linking) the many
nanoparticles 302 together through highly stable covalent links,
and as a result, to interconnecting the nanoparticles 302. It also
is believed that since the polylinker 304 is a polymeric material
with a chemical structure similar to that of the nanoparticles 302,
even a few binding (or linking) sites between the nanoparticles 302
and the polylinker 304 leads to a highly interconnected
nanoparticle film with a combination of electrical and mechanical
properties superior to those of a non-sintered or
non-interconnected nanoparticle film. The electrical properties
include, for example, electron and/or hole conducting properties
that facilitate the transfer of electrons or holes from one
nanoparticle to another through, for example, .pi.-conjugation. The
mechanical properties include, for example, improved
flexibility.
[0036] Still referring to FIG. 3A, at low concentrations of the
polylinker 304, a single polylinker 304 polymer can link many
nanoparticles 302 forming a cross-linked nanoparticle network.
However, by increasing the concentration of the polylinker 304
polymer, more polylinker 304 molecules may be attached to the
surface of the nanoparticles 302 forming polymer-coated
nanoparticles 300. Such polymer-coated nanoparticles 300 may be
processed as thin films due to the flexibility of the polymer. It
is believed that the electronic properties of the polymer-coated
nanoparticles are not affected to a significant extent due to the
similar electronic and structural properties between the polylinker
polymer and the nanoparticles.
[0037] FIG. 3B depicts the chemical structure 306 of an
illustrative embodiment of the interconnected nanoparticle film 300
from FIG. 3A formed on a flexible substrate 308 that includes an
oxide layer coating 310, which is an electrical conductor. In
particular, the polylinkers may be used to facilitate the formation
of such nanoparticle films 300 on flexible, significantly light
transmitting substrates 308. As used herein, the term
"significantly light transmitting substrate" refers to a substrate
that transmits at least about 60% of the visible light incident on
the substrate in a wavelength range of operation. Examples of
flexible substrates 308 include polyethylene terephthalates (PETs),
polyimides, polyethylene naphthalates (PENs), polymeric
hydrocarbons, cellulosics, combinations thereof, and the like. PET
and PEN substrates may be coated with one or more electrical
conducting, oxide layer coatings 310 of, for example, indium tin
oxide (ITO), a fluorine-doped tin oxide, tin oxide, zinc oxide, and
the like.
[0038] According to one preferred embodiment, by using the
illustrative polylinkers, the methods of the invention interconnect
nanoparticles 302 at temperatures significantly below 400.degree.
C., and preferably below about 300.degree. C. Operating in such a
temperature range enables the use of the flexible substrates 308,
which would otherwise be destructively deformed by conventional
high temperature sintering methods. In one illustrative embodiment,
the exemplary structure 306 is formed by interconnecting the
nanoparticles 302 using a polylinker 304 on a substrate 308 at
temperatures below about 300.degree. C. In another embodiment, the
nanoparticles 302 are interconnected using a polylinker 304 at
temperatures below about 100.degree. C. In still another
embodiment, the nanoparticles 302 are interconnected using a
polylinker 304 at about room temperature and room pressure, from
about 18 to about 22.degree. C. and about 760 mm Hg,
respectively.
[0039] In embodiments where the nanoparticles are deposited on a
substrate, the reactive groups of the polylinker bind with the
substrate, substrate coating and/or substrate oxide layers. The
reactive groups may bind to the substrate, substrate coating and/or
substrate oxide layers by, for example, covalent, ionic and/or
hydrogen bonding. It is believed that reactions between the
reactive groups of the polylinker with oxide layers on the
substrate result in connecting nanoparticles to the substrate via
the polylinker.
[0040] According to various embodiments of the invention, metal
oxide nanoparticles are interconnected by contacting the
nanoparticles with a suitable polylinker dispersed in a suitable
solvent at or below room temperature or at elevated temperatures
below about 300.degree. C. The nanoparticles may be contacted with
a polylinker solution in many ways. For example, a nanoparticle
film may be formed on a substrate and then dipped into a polylinker
solution. A nanoparticle film may be formed on a substrate and the
polylinker solution sprayed on the film. The polylinker and
nanoparticles may be dispersed together in a solution and the
solution deposited on a substrate. To prepare nanoparticle
dispersions, techniques such as, for example, microfluidizing,
attritting, and ball milling may be used. Further, a polylinker
solution may be deposited on a substrate and a nanoparticle film
deposited on the polylinker.
[0041] In embodiments where the polylinker and nanoparticles are
dispersed together in a solution, the resultant
polylinker-nanoparticle solution may be used to form an
interconnected nanoparticle film on a substrate in a single step.
In various versions of this embodiment, the viscosity of the
polylinker-nanoparticle solution may be selected to facilitate film
deposition using printing techniques such as, for example,
screen-printing and gravure-printing techniques. In embodiments
where a polylinker solution is deposited on a substrate and a
nanoparticle film deposited on the polylinker, the concentration of
the polylinker can be adjusted to achieve a desired adhesive
thickness. In addition, excess solvent may be removed from the
deposited polylinker solution prior to deposition of the
nanoparticle film.
[0042] The invention is not limited to interconnection of
nanoparticles of a material of formula M.sub.xO.sub.y. Suitable
nanoparticle materials include, but are not limited to, sulfides,
selenides, tellurides, and oxides of titanium, zirconium,
lanthanum, niobium, tin, tantalum, terbium, and tungsten, and
combinations thereof. For example, TiO.sub.2, SrTiO.sub.3,
CaTiO.sub.3, ZrO.sub.2, WO.sub.3, La.sub.2O.sub.3, Nb.sub.2O.sub.5,
SnO.sub.2, sodium titanate, and potassium niobate are suitable
nanoparticle materials.
[0043] The polylinker may contain more than one type of reactive
group. For example, the illustrative embodiments of FIGS. 1-3B
depict one type of reactive group OR. However, the polylinker may
include several types of reactive groups, e.g., OR, OR', OR", etc.;
where R, R' and R" are one or more of a hydrogen, alkyl, alkene,
alkyne, aromatic, or acyl group or where one or more of OR, OR',
and OR" are a halide. For example, the polylinker may include
polymer units of formulas such as,
--[O-M(OR).sub.i(OR').sub.j--]--, and
--[O-M(OR).sub.i(OR').sub.j(OR").su- b.k--]--, where i, j and k are
integers greater than zero.
[0044] FIG. 4 depicts the chemical structure of a representative
polylinker, poly(n-butyl titanate) 400 for use with titanium
dioxide (TiO.sub.2) nanoparticles. Suitable solvents for
poly(n-butyl titanate) 400 include, but are not limited to, various
alcohols, chlorohydrocarbons (e.g., chloroform), ketones, cyclic
and linear chain ether derivatives, and aromatic solvents among
others. Preferably, the solvent is n-butanol. The poly(n-butyl
titanate) polylinker 400 contains a branched
--Ti--O--Ti--O--Ti--O-- backbone structure with butoxy (OBu)
reactive groups.
[0045] FIG. 5A depicts the chemical structure of a nanoparticle
film 500, which is constructed from titanium dioxide nanoparticles
502 interconnected by poly(n-butyl titanate) polylinker molecules
504. It is believed that the reaction between surface hydroxyl
groups of the TiO.sub.2 nanoparticles 502 with butoxy groups 506
(or other alkoxy groups) of the polylinker 504 leads to the
bridging (or linking) of many nanoparticles 502 together through
highly stable covalent links, and as a result, interconnecting the
nanoparticles 502. Furthermore, it is believed that since the
polylinker 504 is a polymeric material with a chemical structure
similar to that of TiO.sub.2, even a few binding (or linking) sites
between nanoparticles 502 and polylinker 504 will lead to a highly
interconnected nanoparticle film 500, with electronic and
mechanical properties superior to those of a non-sintered or
non-interconnected nanoparticle film.
[0046] FIG. 5B depicts the chemical structure 508 of the
nanoparticle film 500 from FIG. 5A formed on a substrate 510, which
includes an electrically-conducting oxide layer coating 512, by
applying the polylinker solution to the substrate 510 and then
depositing the nanoparticles 502 on the polylinker 504. In the
illustrative example using titanium dioxide nanoparticles 502, a
polylinker solution including poly(n-butyl titanate) 504 is
dissolved in n-butanol and applied to the substrate 510. The
concentration of the polylinker 504 can be adjusted to achieve a
desired adhesive thickness for the polylinker solution. A titanium
dioxide nanoparticulate film 500 is then deposited on the
polylinker coated substrate 510. Reaction between the surface
hydroxyl groups of the TiO.sub.2 nanoparticles with reactive butoxy
groups 506 (or other alkoxy groups) of poly(n-butyl titanate) 504
results in interconnecting the nanoparticles 502, as well as
connecting nanoparticles 502 with the oxide layers 512 on the
substrate 510.
[0047] FIG. 6 depicts a flexible photovoltaic cell 600, in
accordance with the invention, that includes a photosensitized
interconnected nanoparticle material 603 and a charge carrier
material 606 disposed between a first flexible, significantly light
transmitting substrate 609 and a second flexible, significantly
light transmitting substrate 612. In one embodiment, the flexible
photovoltaic cell further includes a catalytic media layer 615
disposed between the first substrate 609 and second substrate 612.
Preferably, the photovoltaic cell 600 also includes an electrical
conductor 618 deposited on one or both of the substrates 609 and
612. The methods of nanoparticle interconnection provided herein
enable construction of the flexible photovoltaic cell 600 at
temperatures and heating times compatible with such substrates 609
and 612.
[0048] The flexible, significantly light transmitting substrates
609 and 612 of the photovoltaic cell 600 preferably include
polymeric materials. Suitable substrate materials include, but are
not limited to, PET, polyimide, PEN, polymeric hydrocarbons,
cellulosics, or combinations thereof. Further, the substrates 609
and 612 may include materials that facilitate the fabrication of
photovoltaic cells by a continuous manufacturing process such as,
for example, a roll-to-roll or web process. The substrate 609 and
612 may be colored or colorless. Preferably, the substrates 609 and
612 are clear and transparent. The substrates 609 and 612 may have
one or more substantially planar surfaces or may be substantially
non-planar. For example, a non-planar substrate may have a curved
or stepped surface (e.g., to form a Fresnel lens) or be otherwise
patterned.
[0049] According to the illustrative embodiment, an electrical
conductor 618 is deposited on one or both of the substrates 609 and
612. Preferably, the electrical conductor 618 is a significantly
light transmitting material such as, for example, ITO, a
fluorine-doped tin oxide, tin oxide, zinc oxide, or the like. In
one illustrative embodiment, the electrical conductor 618 is
deposited as a layer between about 100 nm and about 500 nm thick.
In another illustrative embodiment, the electrical conductor 618 is
between about 150 nm and about 300 nm thick. According to a further
feature of the illustrative embodiment, a wire or lead line may be
connected to the electrical conductor 618 to electrically connect
the photovoltaic cell 600 to an external load.
[0050] The photosensitized interconnected nanoparticle material 603
may include one or more types of metal oxide nanoparticles, as
described in detail above. In one embodiment, the photosensitized
interconnected nanoparticle material 603 includes nanoparticles
with an average size of between about 2 nm and about 100 nm. In
another embodiment, the photosensitized nanoparticle material 603
includes nanoparticles with an average size of between about 10 nm
and about 40 nm. Preferably, the nanoparticles are titanium dioxide
particles having an average particle size of about 20 nm.
[0051] A wide variety of photosensitizing agents may be applied to
and/or associated with the nanoparticles to produce the
photosensitized interconnected nanoparticle material 603. The
photosensitizing agent facilitates conversion of incident visible
light into electricity to produce the desired photovoltaic effect.
It is believed that the photosensitizing agent absorbs incident
light resulting in the excitation of electrons in the
photosensitizing agent. The energy of the excited electrons is then
transferred from the excitation levels of the photosensitizing
agent into a conduction band of the interconnected nanoparticles
603. This electron transfer results in an effective separation of
charge and the desired photovoltaic effect. Accordingly, the
electrons in the conduction band of the interconnected
nanoparticles are made available to drive an external load
electrically connected to the photovoltaic cell. In one
illustrative embodiment, the photosensitizing agent is sorbed
(e.g., chemisorbed and/or physisorbed) on the interconnected
nanoparticles 603. The photosensitizing agent may be sorbed on the
surfaces of the interconnected nanoparticles 603, throughout the
interconnected nanoparticles 603, or both. The photosensitizing
agent is selected, for example, based on its ability to absorb
photons in a wavelength range of operation, its ability to produce
free electrons (or electron holes) in a conduction band of the
interconnected nanoparticles 603, and its effectiveness in
complexing with or sorbing to the interconnected nanoparticles 603.
Suitable photosensitizing agents may include, for example, dyes
that include functional groups, such as carboxyl and/or hydroxyl
groups, that can chelate to the nanoparticles, e.g., to Ti(IV)
sites on a TiO.sub.2 surface. Examples of suitable dyes include,
but are not limited to, anthocyanins, porphyrins, phthalocyanines,
merocyanines, cyanines, squarates, eosins, and metal-containing
dyes such as, for example,
cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(I-
I) ("N3 dye");
tris(isothiocyanato)-ruthenium(II)-2,2':6',2"-terpyridine-4-
,4',4"- tricarboxylic acid;
cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-
-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium;
cis-bis(isocyanato) (2,2'-bipyridyl-4,4'dicarboxylato) ruthenium
(II); and tris(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium (II)
dichloride, all of which are available from Solaronix.
[0052] The charge carrier material 606 portion of the photovoltaic
cells may form a layer in the photovoltaic cell, be interspersed
with the material that forms the photosensitized interconnected
nanoparticle material 603, or be a combination of both. The charge
carrier material 606 may be any material that facilitates the
transfer of electrical charge from a ground potential or a current
source to the interconnected nanoparticles 603 (and/or a
photosensitizing agent associated therewith). A general class of
suitable charge carrier materials can include, but are not limited
to solvent based liquid electrolytes, polyelectrolytes, polymeric
electrolytes, solid electrolytes, n-type and p-type transporting
materials (e.g., conducting polymers), and gel electrolytes, which
are described in more detail below.
[0053] Other choices for the charge carrier material 606 are
possible. For example, the electrolyte composition may include a
lithium salt that has the formula LiX, where X is an iodide,
bromide, chloride, perchlorate, thiocyanate, trifluoromethyl
sulfonate, or hexafluorophosphate. In one embodiment, the charge
carrier material 606 includes a redox system. Suitable redox
systems may include organic and/or inorganic redox systems.
Examples of such systems include, but are not limited to,
cerium(III) sulfate/cerium(IV), sodium bromide/bromine, lithium
iodide/iodine, Fe.sup.2+/Fe.sup.3+, Co.sup.2+/Co.sup.3+, and
viologens. Furthermore, an electrolyte solution may have the
formula M.sub.iX.sub.j, where i and j are .gtoreq.1. X is an anion,
and M is selected from the group consisting of Li, Cu, Ba, Zn, Ni,
lanthanides, Co, Ca, Al, and Mg. Suitable anions include, but are
not limited to, chloride, perchlorate, thiocyanate, trifluoromethyl
sulfonate, and hexafluorophosphate.
[0054] In some illustrative embodiments the charge carrier material
606 includes a polymeric electrolyte. In one version, the polymeric
electrolyte includes poly(vinyl imidazolium halide) and lithium
iodide. In another version, the polymeric electrolyte includes
poly(vinyl pyridinium salts). In still another embodiment, the
charge carrier material 606 includes a solid electrolyte. In one
version, the solid electrolyte includes lithium iodide and
pyridinium iodide. In another version, the solid electrolyte
includes substituted imidazolium iodide.
[0055] According to some illustrative embodiments, the charge
carrier material 606 includes various types of polymeric
polyelectrolytes. In one version, the polyelectrolyte includes
between about 5% and 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 0.01 M 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 may include, for example, polyethylene oxide
(PEO), polyacrylonitrile (PAN), polymethylmethacrylate
(acrylic)(PMMA), polyethers, and polyphenols. Examples of suitable
plasticizers include, but are not limited to, ethyl carbonate,
propylene carbonate, mixtures of carbonates, organic phosphates,
butyrolactone, and dialkylphthalates.
[0056] Preferably, the flexible photovoltaic cell 600 also includes
a catalytic media layer 615 disposed between the substrates 609 and
612. According to the illustrative embodiment, the catalytic media
layer 615 is in electrical contact with the charge carrier material
606. The catalytic media 615 may include, for example, ruthenium,
osmium, cobalt, rhodium, iridium, nickel, activated carbon,
palladium, platinum, or hole transporting polymers (e.g.,
poly(3,4-ethylene dioxythiophene and polyaniline). Preferably, the
catalytic media 615 further includes titanium, or some other
suitable metal, to facilitate adhesion of the catalytic media to a
substrate and/or substrate coating. Preferably, the titanium is
deposited in regions or a layer about 10 .ANG. thick. In one
embodiment, the catalytic media 615 includes a platinum layer
between about 13 .ANG. and about 35 .ANG. thick. In another
embodiment, the catalytic media 615 includes a platinum layer
between about 15 .ANG. and about 50 .ANG. thick. In another
embodiment, the catalytic media 615 includes a platinum layer
between about 50 .ANG. and about 800 .ANG. thick. Preferably, the
catalytic media 615 includes a platinum layer about 25 .ANG.
thick.
[0057] In another aspect, the invention also provides methods of
forming a layer of interconnected metal oxide nanoparticles on a
substrate using a continuous manufacturing process, such as, for
example, a roll-to-roll or web process. These methods may be used,
for example, to produce DSSCs. The current processes for producing
DSSCs in large numbers, for example using a continuous and cost
effective assembly line process, are extremely difficult at best.
The difficulties associated with a continuous assembly process for
a DSSC may arise from the cell support or substrate, which is
generally rigid and typically includes thermally resistant
materials such as glass and metal. The primary reason for this is
related to the high temperature sintering process for producing
fused nanocrystals (typically about 400-500.degree. C.). Rigid
substrate materials, by their very nature, generally do not lend
themselves to a continuous process for manufacture, but rather to a
more expensive batch process.
[0058] FIG. 7 depicts an illustrative embodiment of a continuous
manufacturing process 700 that may be used to form the photovoltaic
cell shown in FIG. 6. According to the illustrative embodiment, an
interconnected nanoparticle film is formed on an advancing
substrate sheet 705, which may be continuously advanced,
periodically advanced, and/or irregularly advanced during a
manufacturing run using rollers 708. In this illustrative
embodiment, the electrical conductor material 710, which serves as
the basis for one electrode of a photovoltaic cell, is deposited on
the advancing substrate 705. In various embodiments, the electrical
conductor material 710 may be deposited on a target region of the
substrate 705 by thermal evaporation or low temperature sputtering.
In addition, the electrical conductor material 710 may be
deposited, for example, by vacuum deposition.
[0059] According to the illustrative embodiment shown in FIG. 7,
the photosensitized nanoparticle material 715 is then deposited. As
described above, the photosensitized nanoparticle material 715 may
be formed by applying a solution having a polylinker and metal
oxide nanoparticles onto the advancing substrate sheet 705. The
polylinker-nanoparticle solution may be applied by any suitable
technique including, but not limited to, dip tanks, extrusion
coating, spray coating, screen printing, and gravure printing. In
other illustrative embodiments, the polylinker solution and metal
oxide nanoparticles are separately applied to the advancing
substrate sheet 705 to form the photosensitized nanoparticle
material 715. In one illustrative embodiment, the polylinker
solution is applied to the advancing substrate 705 and the metal
oxide nanoparticles (preferably dispersed in a solvent) are
disposed on the polylinker. In another illustrative embodiment, the
metal oxide nanoparticles (preferably dispersed in a solvent) are
applied to the advancing substrate 705 and the polylinker solution
is applied to the nanoparticles to form the photosensitized
nanoparticle material 715. As described above with regard to FIG.
6, a wide variety of photosensitizing agents may be applied to
and/or associated with the nanoparticles to produce the
photosensitized nanoparticle material 715.
[0060] After deposition of the photosensitized nanomatrix material
715, the substrate sheet 705 may proceed to further processing
stations depending on the ultimate product desired. According to
this illustrative embodiment, the charge carrier material 720,
which facilitates the transfer of electrical charge from a ground
potential or a current source to the photosensitized nanoparticle
material 715, is deposited. The charge carrier material 720 may be
applied by, for example, spray coating, roller coating, knife
coating, or blade coating. The charge carrier media 720 may be
prepared 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 the gel/liquid phase transition temperature; and the iodides
and iodine act as redox electrolytes.
[0061] Still referring to FIG. 7, the catalytic media layer 725,
which facilitates the transfer of electrons ejected by the
photoexcited molecules within the photovoltaic cell, is then
deposited. Subsequently, a second electrical conductor layer 730 is
deposited. The second electrical conductor layer 730 serves as the
basis for a second electrode of the photovoltaic cell. A second,
flexible substrate 735 is then unwound and applied to the advancing
sheet 705 to complete the photovoltaic cell using the continuous
manufacturing process 700.
[0062] Further illustrative examples of the invention in the
context of a DSSC including titanium dioxide nanoparticles are
provided below. The following examples are illustrative and not
intended to be limiting. Accordingly, it is to be understood that
the invention may be applied to a wide range of nanoparticles
including, but not limited to, SrTiO.sub.3, CaTiO.sub.3, ZrO.sub.2,
WO.sub.3, La.sub.2O.sub.3, Nb.sub.2O.sub.5, sodium titanate, and
potassium niobate nanoparticles. In addition, it should be realized
that the invention is generally applicable to formation of
interconnected nanoparticles for a wide variety of applications in
addition to DSSC, such as, for example, metal oxide and
semiconductor coatings.
EXAMPLE 1
Dip-Coating Application of Polylinker
[0063] In this illustrative example, a DSSC was formed as follows.
A titanium dioxide nanoparticle film was coated on a SnO.sub.2:F
coated glass slide. The polylinker solution was a 1% (by weight)
solution of the poly(n-butyl titanate) in n-butanol. In this
embodiment, the concentration of the. polylinker in the solvent was
preferably less than 5% by weight. To interconnect the particles,
the nanoparticle film coated slide was dipped in the polylinker
solution for 15 minutes and then heated at 150.degree. C. for 30
minutes. The polylinker treated TiO.sub.2 film was then
photosensitized with a 3.times.10.sup.-4 N3 dye solution for 1
hour. The polylinker treated TiO.sub.2 film coated slide was then
fabricated into a 0.6 cm.sup.2 photovoltaic cell by sandwiching a
triiodide based liquid redox electrolyte between the TiO.sub.2 film
coated slide a platinum coated SnO.sub.2:F glass slide using 2 mil
SURLYN 1702 hot melt adhesive available from DuPont. The platinum
coating was approximately 60 nm thick. The cell exhibited a solar
conversion efficiency of as high as 3.33% at AM 1.5 solar simulator
conditions (i.e., irradiation with light having an intensity of
1000 W/m.sup.2). The completed solar cells exhibited an average
solar conversion efficiency (".eta.") of 3.02%; an average open
circuit voltage ("V.sub.oc") of 0.66 V; an average short circuit
current ("I.sub.sc") of 8.71 mA/cm.sup.2, and an average fill
factor of 0.49 (0.48 to 0.52). FIG. 8 depicts a graph 800 that
shows the current-voltage curve 802 for the dip-coated photovoltaic
cell.
EXAMPLE 2
Polylinker-Nanoparticle Solution Application
[0064] In this illustrative example, a 5.0 mL suspension of
titanium dioxide (P25, which is a titania that includes
approximately 80% anatase and 20% rutile crystalline TiO.sub.2
nanoparticles and which is available from Degussa-Huls) in
n-butanol was added to 0.25 g of poly(n-butyl titanate) in 1 mL of
n-butanol. In this embodiment, the concentration of the polylinker
in the polylinker-nanoparticle solution was preferably less than
about 50% by weight. The viscosity of the suspension changed from
milk-like to toothpaste-like with no apparent particle separation.
The paste was spread on a patterned SnO.sub.2:F coated glass slide
using a Gardner knife with a 60 .mu.m thick tape determining the
thickness of wet film thickness. The coatings were dried at room
temperature forming the films. The air-dried films were
subsequently heat treated at 150.degree. C. for 30 minutes to
remove solvent, and sensitized overnight with a 3.times.10.sup.-4 M
N3 dye solution in ethanol. The sensitized photoelectrodes were cut
into desired sizes and sandwiched between a platinum (60 nm thick)
coated SnO.sub.2:F coated glass slide and a tri-iodide based liquid
electrolyte. The completed solar cells exhibited an average .eta.
of 2.9% (2.57% to 3.38%) for six cells at AM 1.5 conditions. The
average V.sub.oc was 0.68 V (0.66 to 0.71 V); the average I.sub.sc
was 8.55 mA/cm.sup.2 (7.45 to 10.4 mA/cm.sup.2); and the average
fill factor was 0.49 (0.48 to 0.52). FIG. 9 depicts a graph 900
showing the current-voltage curve 902 for the photovoltaic cell
formed from the polylinker-nanoparticle solution.
EXAMPLE 3
DSSC Cells Formed without Polylinker
[0065] In this illustrative example, an aqueous titanium dioxide
suspension (p25) containing about 37.5% solid content was prepared
using a microfluidizer and was spin coated on a fluorinated
SnO.sub.2 conducting electrode (15 .OMEGA./cm.sup.2) that was
itself coated onto a coated glass slide. The titanium dioxide
coated slides were air dried for about 15 minutes and heat treated
at 150.degree. C. for 15 minutes. The slides were removed from the
oven, cooled to about 80.degree. C., and dipped into
3.times.10.sup.-4 M N3 dye solution in ethanol for about 1 hour.
The sensitized titanium dioxide photoelectrodes were removed from
dye solution rinsed with ethanol and dried over a slide warmer at
40.degree. C. The sensitized photoelectrodes were cut into small
pieces (0.7 cm.times.0.5-1 cm active area) and sandwiched between
platinum coated SnO.sub.2:F-transparent conducting glass slides. A
liquid electrolyte containing 1 M LiI, 0.05 M iodine, and 1 M
t-butyl pyridine in 3-methoxybutyronitrile was applied between the
photoelectrode and platinized conducting electrode through
capillary action. Thus constructed photocells exhibited an average
solar conversion efficiency of about 3.83% at AM 1.5 conditions.
The .eta. at AM 1.5 conditions and the photovoltaic characteristics
I.sub.sc, V.sub.oc, voltage at maximum power output ("V.sub.m"),
and current at maximum power output ("I.sub.m") of these cells are
listed in Table 1 under column A. FIG. 10 depicts a graph 1000
showing the current-voltage curve 1002 for the photovoltaic cell
formed without the polylinker.
1 TABLE 1 B C D E A 0.1% poly- 0.4% poly- 1% poly- 2% poly-
Untreated mer soln. mer soln. mer soln. mer soln. .eta. (%) Avg =
3.83 Avg. = 4.30 Avg = 4.55 Avg = 4.15 Avg = 4.15 (3.37-4.15)
(4.15-4.55) (4.4-4.82) (3.48-4.46) (3.7-4.58) I.sub.sc Avg = 10.08
Avg = 10.96 Avg = 10.60 Avg = 11.00 Avg = 11.24 (mA/cm.sup.2)
(8.88-10.86) (10.44-11.5) (9.79-11.12) (10.7-11.28) (10.82-11.51)
V.sub.oc (V) Avg = 0.65 Avg = 0.66 Avg = 0.71 Avg = 0.7 Avg = 0.69
(0.65-0.66) (0.6-0.7) (0.69-0.74) (0.69-0.71) (0.68-0.71) V.sub.m
(V) Avg = 0.454 Avg = 0.46 Avg = 0.50 Avg = 0.45 Avg = 0.44
(0.43-0.49) (0.43-0.477) (0.47-0.53) (0.4-0.47) (0.42-0.46) I.sub.m
Avg = 8.4 Avg = 9.36 Avg = 9.08 Avg = 9.14 Avg = 9.28 (mA/cm.sup.2)
(7.5-8.96) (8.75-9.71) (8.31-9.57) (8.70-9.55) (8.66-9.97)
EXAMPLE 4
DSSC Cells Formed with Various Concentrations of Polylinker
Solution
[0066] In this illustrative example, a P25 suspension containing
about 37.5% solid content was prepared using a microfluidizer and
was spin coated on fluorinated SnO.sub.2 conducting electrode (15
.OMEGA./cm.sup.2) coated glass slide. The titanium dioxide coated
slides were air dried for about 15 minutes and heat treated at
150.degree. C. for 15 minutes. The titanium dioxide coated
conducting glass slide were dipped into a polylinker solution
including poly(n-butyl titanate) in n-butanol for 5 minutes in
order to carry out interconnection (polylinking) of nanoparticles.
The polylinker solutions used were 0.1 wt % poly(n-butyl titanate),
0.4 wt % poly(n-butyl titanate), 1 wt % poly(n-butyl titanate), and
2 wt % poly(n-butyl titanate). After 5 minutes, the slides were
removed from the polylinker solution, air dried for about 15
minutes and heat treated in an oven at 150.degree. C. for 15
minutes to remove solvent. The slides were removed from the oven,
cooled to about 80.degree. C., and dipped into 3.times.10.sup.-4 M
N3 dye solution in ethanol for about 1 hour. The sensitized
titanium dioxide photoelectrodes were removed from dye solution,
rinsed with ethanol, and dried over a slide warmer at 40.degree. C.
The sensitized photoelectrodes were cut into small pieces (0.7
cm.times.0.5-1 cm active area) and sandwiched between platinum
coated SnO.sub.2:F-transparent conducting glass slides. A liquid
electrolyte containing 1 M LiI, 0.05 M iodine, and 1 M t-butyl
pyridine in 3-methoxybutyronitrile was applied between the
photoelectrode and platinized conducting electrode through
capillary action. The .eta. at AM 1.5 conditions and the
photovoltaic characteristics I.sub.sc, V.sub.oc, V.sub.m, and
I.sub.m of the constructed cells are listed in Table 1 for the 0.1
wt % solution under column B, for the 0.4 wt % solution under
column C, for the 1 wt % solution under colunin D, and for the 2 wt
% solution under column E. FIG. 10 depicts the current-voltage
curve 1008 for the photovoltaic cell formed with the
polylinker.
EXAMPLE 5
Modifier Solutions
[0067] In this illustrative example, titanium dioxide coated
transparent conducting oxide coated glass slides were prepared by
spin coating process as described in Example 4. The titanium oxide
coated conducting glass slides were treated with polylinker
solution including a 0.01 M poly(n-butyl titanate) solution in
n-butanol for 5 minutes to interconnect the nanoparticles. The
slides were air dried for about 5 minutes after removing from the
polylinker solution. The slides were later dipped into a modifier
solution for about 1 minute. The modifier solutions used were 1:1
water/ethanol mixture, 1 M solution of t-butyl pyridine in 1:1
water/ethanol mixture, 0.05 M HCl solution in 1:1 water/ethanol
mixture. One of the slides was treated with steam from humidifier
for 15 seconds. The slides were air dried for 15 minutes and
heat-treated at 150.degree. C. for 15 minutes to remove solvent and
then sensitized with a 3.times.10.sup.-4 M N3 dye solution for 1
hour. The sensitized photoelectrodes were sandwiched between
platinized SnO.sub.2:F coated glass slides and studied for
photovoltaic characteristics using a liquid electrolyte containing
1 M LiI, 0.05 M iodine, and 1 M t-butyl pyridine in
3-methoxybutyronitrile. Acid seems to help in increasing the
photoconductivity and efficiency of these photocells. The .eta. at
AM 1.5 conditions and the photovoltaic characteristics of the cells
of this example are listed in Table 2 as follows: slides not dipped
into a modifier solution and not treated with polylinker solution
(column A); slides not dipped into a modifier, but treated with
polylinker solution (column B); slides were first treated with
polylinker solution and then dipped in 1:1 water/ethanol mixture
(column C); slides were first treated with polylinker solution and
then dipped in 1 M solution of t-butyl pyridine in 1:1
water/ethanol mixture (column D); slides were first treated with
polylinker solution and then dipped in 0.05 M HCl solution in 1:1
water/ethanol mixture (column E); and slides were first treated
with polylinker solution and then treated with steam from
humidifier (column F).
2 TABLE 2 D E B C Treated Treated F Treated Treated with 1 M t-
with 0.05 M Steam from A with 0.01 M with 1:1 BuPy/1:1 HCl/1:1
Humidifier Untreated TiBut EtOH/H.sub.2O EtOH/H.sub.2O
EtOH/H.sub.2O for 15 sec. .eta. (%) Avg = 3.92 Avg = 4.41 Avg =
4.11 Avg = 4.34 Avg = 4.67 Avg = 4.41 (3.75-4.15) (4.12-4.74)
(4.06-4.15) (4.27-4.38) (4.61-4.73) (4.38-4.45) V.sub.oc (V) Avg =
0.66 Avg = 0.66 Avg = 0.65 Avg = 0.65 Avg = 0.66 Avg = 0.66
(0.66-0.67) (0.65-0.66) (0.64-0.65) (0.64-0.66) (0.65-0.66)
(0.66-0.67) I.sub.sc Avg = 9.97 Avg = 12.57 Avg = 11.85 Avg = 11.85
Avg = 12.51 Avg = 11.63 (mA/cm.sup.2) (9.48-10.56) (11.7-13.22)
(11.21-12.49) (11.21-12.49) (12.15-12.87) (11.25-12.01) V.sub.m (V)
Avg = 0.468 Avg = 0.434 Avg = 0.44 Avg = 0.45 Avg 0.457 Avg = 0.45
(0.46-0.48) (0.4-0.457) (0.43-0.45) (0.44-0.456) (0.453-0.46)
(0.44-0.46) I.sub.m Avg = 8.36 Avg = 10.08 Avg = 9.27 Avg = 9.52
Avg = 10.23 Avg = 9.67 (mA/cm.sup.2) (7.85-8.89) (9.57-10.37)
(9.01-9.53) (9.22-9.75) (10.17-10.29) (9.38-9.96)
EXAMPLE 6
Post-Interconnection Heating to 150.degree. C.
[0068] In this illustrative example, a titanium-dioxide-coated,
transparent-conducting-oxide-coated glass slide was prepared by a
spin coating process as described in Example 4. The slide was
dipped into 0.01 M poly(n-butyl titanate) in n-butanol for 30
seconds and was air-dried for 15 minutes. The slide was later heat
treated at 150.degree. C. for 10 minutes in an oven. The
heat-treated titanium oxide layer was sensitized with N3 dye
solution for 1 hour, washed with ethanol, and warmed on a slide
warmer at 40.degree. C. for 10 minutes. The sensitized
photoelectrodes were cut into 0.7 cm.times.0.7 cm active area
photocells and were sandwiched between platinized conducting
electrodes. A liquid electrolyte containing 1 M LiI, 0.05 M iodine,
and 1 M t-butyl pyridine in 3-methoxybutyronitrile was applied
between the photoelectrode and platinized conducting electrode
through capillary action. The photocells exhibited an average .eta.
of 3.88% (3.83, 3.9 and 3.92), an average V.sub.oc of 0.73 V (0.73,
0.74 and 0.73 V), and an average I.sub.sc of 9.6 mA/cm.sup.2 (9.88,
9.65 and 9.26), all at AM 1.5 conditions.
EXAMPLE 7
Post-Interconnection Heating to 70.degree. C.
[0069] In this illustrative example, a titanium-dioxide-coated,
transparent-conducting-oxide-coated glass slide was prepared by a
spin coating process as described in Example 4. The slide was
dipped into 0.01 M poly(n-butyl titanate) in n-butanol for 30
seconds and was air-dried for 15 minutes. The slide was later heat
treated at 70.degree. C. for 10 minutes in an oven. The
heat-treated titanium oxide layer was sensitized with N3 dye
solution for 1 hour, washed with ethanol, and warmed on a slide
warmer at 40.degree. C. for 10 minutes. The sensitized
photoelectrodes were cut into 0.7 cm.times.0.7 cm active area
photocells and were sandwiched between platinized conducting
electrodes. A liquid electrolyte containing 1 M LiI, 0.05 M iodine,
and 1 M t-butyl pyridine in 3-methoxybutyronitrile was applied
between the photoelectrode and platinized conducting electrode
through capillary action. The photocells exhibited an average .eta.
of 3.62% (3.55, 3.73 and 3.58), an average V.sub.oc of 0.75 V
(0.74, 0.74 and 0.76 V), and average I.sub.sc of 7.96 mA/cm.sup.2
(7.69, 8.22 and 7.97), all at AM 1.5 conditions.
EXAMPLE 8
Formation on a Flexible, Transparent Substrate
[0070] In this illustrative example, a PET substrate about 200
.mu.m thick and about 5 inches by 8 feet square was coated with ITO
and loaded onto a loop coater. An 18.0 mL suspension of titanium
dioxide (P25 with 25% solid content) in n-butanol and 0.5 g of
poly(n-butyl titanate) in 10 mL of n-butanol were in-line blended
and coated onto the ITO coated PET sheet. After deposition, the
coating was heated at about 50.degree. C. for about 1 minute. The
interconnected nanoparticle layer was then dye-sensitized by
coating with a 3.times.10.sup.-4 M solution of N3 dye in
ethanol.
[0071] B. Gel Electrolytes for DSSCs
[0072] According to further illustrative embodiments, the invention
provides electrolyte compositions that include multi-complexable
molecules (i.e., molecules containing 2 or more ligands capable of
complexing) and redox electrolyte solutions, which are gelled using
metal ions, such as lithium ions. The multi-complexable compounds
are typically organic compounds capable of complexing with a metal
ion at a plurality of sites. The electrolyte composition can be a
reversible redox species that may be liquid by itself or solid
components dissolved in a non-redox active solvent, which serves as
a solvent for the redox species and does not participate in
reduction-oxidation reaction cycle. Examples include common organic
solvents and molten salts that do not contain redox active ions.
Examples of redox species include, for example, iodide/triiodide,
Fe.sup.2+/Fe.sup.3+, Co.sup.2+/Co.sup.3+, and viologens, among
others. The redox components are dissolved in non-aqueous solvents,
which include all molten salts. Iodide based molten salts, e.g.,
methylpropylimidazolium iodide, methylbutylimidazolium iodide,
methylhexylimidazolium iodide, etc., are themselves redox active
and can be used as redox active liquids by themselves or diluted
with non-redox active materials like common organic solvents or
molten salts that do not undergo oxidation-reduction reaction
cycles. Multi-dendate inorganic ligands may also be a source of
gelling compounds.
[0073] FIG. 11 depicts an illustrative embodiment of an electrolyte
gelled using metal ions. Lithium ions are shown complexed with
poly(4-vinyl pyridine). The lithium ions and the organic compounds,
in this instance poly(4-vinyl pyridine) molecules capable of
complexing at a plurality of sites with the lithium ions, can be
used to gel a suitable electrolyte solution. An electrolyte
composition prepared in accordance with the invention may include
small amounts of water, molten iodide salts, an organic polymer,
and other suitable compound gels upon the addition of a metal ion
such as lithium. Gelled electrolytes may be incorporated into
individual flexible photovoltaic cells, traditional solar cells,
photovoltaic fibers, interconnected photovoltaic modules, and other
suitable devices. The dotted lines shown in FIG. 11 represent the
type of bonding that occurs in a photovoltaic gel electrolyte when
the constituent electrolyte solution and organic compounds gel
after the introduction of a suitable metal ion.
[0074] A non-exhaustive list of organic compounds that are capable
of complexing with the metal ion at a plurality of sites, and which
are suitable for use in the invention, include various polymers,
starburst/dendrimeric molecules, and other molecules containing
multiple functional groups, e.g., urethanes, esters,
ethylene/propylene oxide/imines segments, pyridines, pyrimidines,
N-oxides, imidazoles, oxazoles, triazoles, bipyridines, quinolines,
polyamines, polyamides, ureas, .beta.-diketones, and .beta.-hydroxy
ketones.
[0075] More generally, the multi-complexable molecules employed in
various embodiments may be polymeric or small organic molecules
that possess two or more ligand or ligating groups capable of
forming complexes. Ligating groups are functional groups that
contain at least one donor atom rich in electron density, e.g.,
oxygen, nitrogen, sulfur, or phosphorous, among others and form
monodentate or multidentate complexes with an appropriate metal
ion. The ligating groups may be present in non-polymeric or
polymeric material either in a side chain or part of the backbone,
or as part of a dendrimer or starburst molecule. Examples of
monodentate ligands include, for example, ethyleneoxy, alkyl-oxy
groups, pyridine, and alkyl-imine compounds, among others. Examples
of bi- and multidentate ligands include bipyridines, polypyridines,
urethane groups, carboxylate groups, and amides.
[0076] According to various embodiments of the invention,
dye-sensitized photovoltaic cells having a gel electrolyte 1100
including lithium ions are fabricated at or below room temperature
or at elevated temperatures below about 300.degree. C. The
temperature may be below about 100.degree. C., and preferably, the
gelling of the electrolyte solution is performed at room
temperature and at standard pressure. In various illustrative
embodiments, the viscosity of the electrolyte solution may be
adjusted to facilitate gel electrolyte deposition using printing
techniques such as, for example, screen-printing and
gravure-printing techniques. The complexing of lithium ions with
various ligands can be broken at higher temperatures, thereby
permitting the gel electrolyte compositions to be easily processed
during DSSC based photovoltaic module fabrication. Other metal ions
may also be used to form thermally reversible or irreversible gels.
Examples of suitable metal ions include: Li.sup.+, Cu.sup.2+,
Ba.sup.2+, Zn.sup.2+, Ni.sup.2+, Ln.sup.3+(or other lanthanides),
Co.sup.2+, Ca.sup.2+, Al.sup.3+, Mg2+, and any metal ion that
complexes with a ligand.
[0077] FIG. 12 depicts a gel electrolyte 1200 formed by the
complexing of an organic polymer, polyethylene oxide (PEO), by
lithium ions. The PEO polymer segments are shown as being complexed
about the lithium ions and crosslinked with each other. In another
embodiment, the metal ion complexed with various polymer chains can
be incorporated into a reversible redox electrolyte species to
promote gelation. The gel electrolyte composition that results from
the combination is suitable for use in various photovoltaic cell
embodiments such as photovoltaic fibers, photovoltaic cells, and
electrically interconnected photovoltaic modules.
[0078] Referring back to FIG. 6, the charge carrier material 606
can include an electrolyte composition having an organic compound
capable of complexing with a metal ion at a plurality of sites; a
metal ion such as lithium; and an electrolyte solution. These
materials can be combined to produce a gelled electrolyte
composition suitable for use in the charge carrier material 606
layer. In one embodiment, the charge carrier material 606 includes
a redox system. Suitable redox systems may include organic and/or
inorganic redox systems. Examples of such systems include, but are
not limited to, cerium(III) sulfate/cerium(IV), sodium
bromide/bromine, lithium iodide/iodine, Fe.sup.2+/Fe.sup.3+,
Co.sup.2+/Co.sup.3+, and viologens.
[0079] Further illustrative examples of the invention in the
context of a DSSC having a gel electrolyte composition are provided
below. The photoelectrodes used in the following illustrative
examples were prepared according to the following procedure. An
aqueous, titania suspension (P25, which was prepared using a
suspension preparation technique with total solid content in the
range of 30-37%) was spun cast on SnO.sub.2:F coated glass slides
(15 .OMEGA./cm.sup.2). The typical thickness of the titanium oxide
coatings was around 8 .mu.m. The coated slides were air dried at
room temperature and sintered at 450.degree. C. for 30 minutes.
After cooling the slides to about 80.degree. C., the slides were
immersed into 3.times.10.sup.-4 M N3 dye solution in ethanol for 1
hour. The slides were removed and rinsed with ethanol and dried
over slide a warmer at 40.degree. C. for about 10 minutes. The
slides were cut into about 0.7 cm.times.0.7 cm square active area
cells. The prepared gels were applied onto photoelectrodes using a
glass rod and were sandwiched between platinum-coated, SnO.sub.2:F
coated, conducting glass slides. The cell performance was measured
at AM 1.5 solar simulator conditions (i.e., irradiation with light
having an intensity of 1000 W/m.sup.2).
EXAMPLE 9
Effect of Lithium Iodide in Standard Ionic Liquid Based Electrolyte
Composition
[0080] In this illustrative example, the standard, ionic,
liquid-based redox electrolyte composition that was used contained
a mixture containing 99% (by weight) imidazolium iodide based ionic
liquid and 1% water (by weight), combined with 0.25 M iodine and
0.3 M methylbenzimidazole. In various experimental trials,
electrolyte solutions with at least a 0.10 M iodine concentration
exhibit the best solar conversion efficiency. In a standard
composition, butylmethylimidazolium iodide (MeBuImI) was used as
the ionic liquid. Photovoltage decreased with increases in iodine
concentration, while photoconductivity and conversion efficiency
increased at least up to 0.25 M iodine concentration. Adding
lithium iodide to the standard composition enhanced the
photovoltaic characteristics V.sub.oc and I.sub.sc and the .eta..
Therefore, in addition to lithium's use as a gelling agent, it may
serve to improve overall photovoltaic efficiency. Table 3
summarizes the effect of LiI on photovoltaic characteristics.
3 TABLE 3 Standard + Standard + Standard + Standard + Standard 1 wt
% LiI 2 wt % LiI 3 wt % LiI 5 wt % LiI .eta. (%) 2.9% 3.57 3.75
3.70 3.93 V.sub.oc (V) 0.59 0.61 0.6 0.6 0.61 I.sub.sc
(mA/cm.sup.2) 10.08 11.4 11.75 11.79 12.62 V.sub.m (V) 0.39 0.4
0.39 0.4 0.39 Im (mA/ 7.44 9.02 9.64 9.0 10.23 cm.sup.2)
[0081] The fill factor ("FF") is referenced below and can be
calculated from the ratio of the solar conversion efficiency to the
product of the open circuit voltage and the short circuit current,
i.e., FF=.eta./[V.sub.oc*I.sub.sc].
EXAMPLE 10
The Effect of Cations on the Enhancement in Photovoltaic
Characteristics
[0082] In order to ascertain whether the enhancement in
photovoltaic characteristics was due to the presence of lithium or
iodide, controlled experimental trials using various iodides in
conjunction with cations including lithium, potassium, cesium and
tetrapropylammonium iodide were conducted. The iodide concentration
was fixed at 376 .mu.mols/gram of standard electrolyte composition.
The standard composition used was a mixture containing 99% MeBuImI
and 1% water, combined with 0.25 M iodine and 0.3 M
methylbenzimidazole. 376 .mu.mols of various iodide salts per gram
of standard electrolyte composition were dissolved in the
electrolyte. The complete dissolution of LiI was observed. The
other salts took a long time to dissolve and did not dissolve
completely over the course of the experimental trial. DSSC-based
photovoltaic cells were fabricated using prepared electrolytes
containing various cations. Table 4 shows the effect of the various
cations on the photovoltaic characteristics. It is apparent from
the second column of Table 4 that Li.sup.+ ion shows enhanced
photovoltaic characteristics compared to the standard formula,
while the other cations do not appear to contribute to the
enhancement of the photovoltaic characteristics.
4 TABLE 4 Standard + Standard + Standard + Standard + Standard LiI
NPR.sub.4I KI CsI .eta. (%) 3.23 4.39 2.69 3.29 3.23 V.sub.oc (V)
0.58 0.65 0.55 0.58 0.6 I.sub.sc (mA/cm.sup.2) 10.96 12.03 9.8 9.91
10.14 V.sub.m (V) 0.36 0.44 0.36 0.4 0.4 I.sub.m (mA/cm.sup.2) 8.96
9.86 7.49 8.25 8.32
EXAMPLE 11
Effect of Ionic Liquid Type
[0083] In one aspect of the invention, MeBuImI-based electrolyte
compositions have been found to perform slightly better than
MePrImI based electrolytes. In addition, experimental results
demonstrate that a 1/1 blend of MeBuImI and MePrImI exhibit better
performance than MeBuImI, as shown in Table 5.
5TABLE 5 376 .mu.moles of LiI per 1 gram of MeBuImI/ 376 .mu.moles
of LiI per 1 gram of MePrImI based MeBuImI based standard standard
electrolyte electrolyte composition. composition. .eta. (%) 3.64
3.99 V.sub.oc (V) 0.63 0.63 I.sub.sc (mA/cm.sup.2) 11.05 11.23
V.sub.m (V) 0.42 0.42 I.sub.m (mA/cm.sup.2) 8.69 9.57
EXAMPLE 12
Using Li-Induced Gelling in Composition A Instead of a
Dibromocompound
[0084] In this illustrative example, a Composition A was prepared
by dissolving 0.09 M of iodine in a mixed solvent consisting of
99.5% by weight of 1-methyl-3-propyl imidazolium iodide and 0.5% by
weight of water. Then, 0.2 g of poly(4-vinylpyridine) ("P4VP"), a
nitrogen-containing compound, was dissolved in 10 g of the
Composition A Further, 0.2 g of 1,6-dibromohexane, an organic
bromide, was dissolved in the resultant Composition A solution, so
as to obtain an electrolyte composition, which was a precursor to a
gel electrolyte.
[0085] Gelling occurred quickly when 5 wt % of lithium iodide (376
.mu.mols of lithium salt per gram of standard electrolyte
composition) was used as the gelling agent in an electrolyte
composition containing (i) 2 wt % P4VP and (ii) a mixture
containing 99.5% MePrImI and 0.5% water. The gel did not flow when
a vial containing the Li-induced gel was tilted upside down. One
approach using a dibromo compound produced a phase-segregated
electrolyte with cross-linked regions suspended in a liquid, which
flows (even after gelling at 100.degree. C. for 30 minutes). A
comparison of the photovoltaic characteristics of Composition A,
with and without LiI, is presented in the following Tables 6 and 7.
The results demonstrate that functional gels suitable for
DSSC-based photovoltaic cell fabrication can be obtained using
lithium ions, while also improving the photovoltaic
characteristics.
6TABLE 6 MeBuImI based Composition A electrolyte + Composition A
with with 2 2 wt. % dibromohexane wt. % P4VP P4VP + 5 wt. % LiI
.eta. (%) 2.6 3.04 3.92 V.sub.oc (V) 0.59 0.58 0.65 I.sub.sc (mA/
9.73 10.0 11.45 cm.sup.2) V.sub.m (V) 0.38 0.38 0.42 I.sub.m
(mA/cm.sup.2) 6.82 8.04 9.27
[0086]
7 TABLE 7 (a) Composition A where MePrImI:water is 99.5:0.5 and
with 2% (b) Same composition as (a), P4VP and 0.09 M Iodine but
with 5 wt %of LiI Physical Reddish fluid; flows well Non-scattering
Gel; does not Properties flow; can be thinned by applying force
using a glass rod. Efficiency 2.53% 3.63% V.sub.oc 0.55 V 0.62 V
I.sub.sc 9.82 mA/cm.sup.2 12.29 mA/cm.sup.2 V.sub.m 0.343 V 0.378 V
FF 0.47 0.47
EXAMPLE 13
Effect of Anions of Lithium Salts on the Efficiency and
Photovoltage of DSSCs
[0087] Experiments were performed to study the effect of counter
ions on lithium, given lithium's apparent role in enhancing the
overall efficiency of DSSCs. 376 .mu.mols of LiI, LiBr, and LiCl
were used per gram of the electrolyte composition containing
MePrImI, 1% water, 0.25 M iodine and 0.3 M methylbenzimidazole in
order to study the photovoltaic characteristics of the cells. The
photovoltaic characteristics of cells containing these electrolytes
are presented in Table 8.
8TABLE 8 Electrolyte Electrolyte composition composition with
Electrolyte composition with LiI LiBr with LiCl Efficiency 3.26%
3.64% 3.71% V.sub.oc 0.59 V 0.62 V 0.65 V I.sub.sc 10.98
mA/cm.sup.2 11.96 mA/cm.sup.2 11.55 mA/cm.sup.2 V.sub.m 0.385 V 0.4
V 0.40 V FF 0.5 0.49 0.49
EXAMPLE 14
Passivation and Improved Efficiency and Photovoltage of DSSCs
[0088] In the field of photovoltaic cells, the term passivation
refers to the process of reducing electron transfer to species
within the electrolyte of a solar cell. Passivation typically
includes treating a nanoparticle layer by immersion in a solution
of t-butylpyridine in methoxypropionitrile or other suitable
compound. After the nanomatrix layer, such as a titania sponge, of
a photovoltaic cell has been treated with a dye, regions in the
nanomatrix layer where the dye has failed to adsorb may exist. A
passivation process is typically performed on a DSSC to prevent the
reversible electron transfer reaction from terminating as result of
reducing agents existing at the undyed regions. The typical
passivation process does not appear to be necessary when ionic
liquid compositions containing various lithium salts and/or other
alkali metal salts are used in the DSSCs. A photovoltage greater
than 0.65 V was achieved using a chloride salt of lithium without a
passivation process.
[0089] In this illustrative example, a DSSC was passivated by
immersing it in a solution containing 10 wt % of t-butylpyridine in
methoxypropionitrile for 15 minutes. After passivation, the DSSC
was dried on a slide warmer maintained at 40.degree. C. for about
10 minutes. Electrolyte compositions containing MePrImI, 1% water,
0.3 M methylbenzimidazole, and 0.25 M iodine were gelled using 376
.mu.moles of LiI, LiBr, and LiCl per gram of standard electrolyte
composition used during this study. Adding a t-butylpyridine-based
passivation agent to the electrolyte enhanced the DSSC's
photovoltage, but decreased the efficiency of the DSSC by
decreasing the photoconductivity. Table 9 summarizes the effects of
passivation on photovoltaic characteristics of electrolytes
containing various lithium halides.
9TABLE 9 Electrolyte Electrolyte gelled with gelled with
Electrolyte gelled with LiI LiBr LiCl Efficiency 3.5% 3.65% 3.85%
V.sub.oc 0.61 V 0.63 V 0.65 V I.sub.sc 10.96 mA/cm.sup.2 11.94
mA/cm.sup.2 11.75 mA/cm.sup.2 V.sub.m 0.395 V 0.4 V 0.405 V FF 0.52
0.49 0.5
EXAMPLE 15
Lithium's Role in Gelling the Electrolyte Compositions Containing
Polyvinylpyridine and the Effect of Other Alkali Metal Ions on
Gelability
[0090] Lithium cation appears to have a unique effect in gelling
ionic liquid composition containing complexable polymers, e.g.,
P4VP, in as small an amount as 2 wt %. Other alkali metal ions such
as sodium, potassium, and cesium were used to carry out gelling
experiments. Alkali metal salts such as lithium iodide, sodium
chloride, potassium iodide, cesium iodide were added to portions of
electrolyte composition containing propylmethylimidazolium iodide
(MePrImI), 1% water, 0.25 M iodine, and 0.3 M methylbenzimidazole.
Only compositions containing lithium iodide gelled under the
experimental conditions used. The remaining three compositions
containing sodium, potassium, and cesium did not gel at the
experimental conditions used. Divalent metal ions, such as calcium,
magnesium, and zinc, or trivalent metals, such as aluminum or other
transition metal ions, are other potential gelling salts.
EXAMPLE 16
Effect of Iodine and Lithium Concentration on Ionic Liquid
Electrolyte Gels
[0091] In this illustrative example, gels were prepared by adding
lithium salts to an electrolyte composition containing MeBuImI,
iodine, and 2 wt % P4VP. The photovoltaic characteristics of the
gels were tested using high-temperature sintered, N3 dye sensitized
titanium-oxide photoelectrodes and platinized SnO.sub.2:F coated
glass slides. Both LiI and LiCl gelled the ionic liquid-based
compositions that contained small amounts (2% was sufficient) of
complexable polymers like P4VP. In compositions lacking
methylbenzimidazole, the lithium did not effect the photovoltage. 5
wt % corresponds to a composition including about 376 .mu.moles of
lithium salt per gram of ionic liquid and a mixture of 99 wt %
butylmethylimidazolium iodide, 1 wt % water, 0.3 M methyl
benzimidazole, and 0.25 M iodine. Therefore, 1 wt % corresponds to
a 376/5.congruent.75 .mu.moles of lithium salt per gram of ionic
liquid composition. The photovoltaic characteristics are summarized
in Table 10.
10TABLE 10 5% LiI 2.5% LiI 5% LiCl 2.5% LiCl 0.05 M .eta. = 1.6%
.eta. = 1.23% .eta. = 0.64% .eta. = 1.19% Iodine V.sub.oc = 0.6 V
V.sub.oc = 0.59 V V.sub.oc = 0.59 V V.sub.oc = 0.58 V I.sub.sc =
I.sub.sc = 4.21 mA I.sub.sc = 2.95 mA I.sub.sc = 3.87 mA 4.89 mA FF
= 0.54 FF = 0.495 FF = 0.36 FF = 0.53 V.sub.m = V.sub.m = 0.415 V
V.sub.m = 0.4 V V.sub.m = 0.426 V 0.445 V 0.1 M .eta. = 1.22% .eta.
= 1.29% .eta. = 2.83% .eta. = 2.06% Iodine V.sub.oc = 0.48 V
V.sub.oc = 0.56 V V.sub.oc = 0.57 V.sub.oc = 0.58 I.sub.sc =
I.sub.sc = 5.12 mA I.sub.sc = 9.04 mA I.sub.sc = 7.14 mA 6.46 mA FF
= 0.39 FF = 0.45 FF = 0.55 FF = 0.5 V.sub.m =0.349 V V.sub.m =
0.386 V V.sub.m = 0.422 V V.sub.m = 0.42 V 0.25 M .eta. = 2.58%
.eta. = 3.06% .eta. = 3.4% .eta. = 2.6% Iodine V.sub.oc = 0.55 V
V.sub.oc = 0.55 V V.sub.oc = 0.56 V V.sub.oc = 0.56 V I.sub.sc =
I.sub.sc = 10.78 mA I.sub.sc = 11.32 mA I.sub.sc = 10.18 mA 11.49
mA FF = 0.41 FF = 0.52 FF = 0.54 FF = 0.46 V.sub.m = V.sub.m = 0.36
V V.sub.m = 0.369 V V.sub.m = 0.364 V 0.338 V
EXAMPLE 17
Effect of Polymer Concentration on Gelability and Photovoltaic
Characteristics of Redox Electrolyte Gels
[0092] In this illustrative example, polymer concentration was
varied to study its effect on gel viscosity and photovoltaic
characteristics. The electrolyte composition used for this study
was a mixture containing 99% MeBuImI, 1% water, 0.25 M iodine, 0.6
M LiI, and 0.3 M methylbenzimidazole. The concentration of the
polymer, P4VP was varied from 1% to 5%. The electrolyte composition
with 1% P4VP did flow slowly when the vial containing the gel was
tilted down. The gels with 2%, 3%, and 5% did not flow. The gel
with 5% P4VP appeared much more solid when compared to the 2% P4VP
preparation. Table 11 summarizes the photovoltaic characteristics
of the gels containing the various P4VP contents that were
studied.
[0093] The results show that the photovoltaic characteristics do
not vary with the increases in viscosity achieved by increasing the
P4VP content. Therefore, the viscosity of the gel can be adjusted
without causing degradation to the photovoltaic characteristics.
Methylbenzimidazole may be necessary to achieve high .eta..
Increasing the iodine concentration up to 0.25 M also increased the
efficiency. Beyond 0.25 M, the photovoltage decreased drastically,
reducing the overall efficiency. Other metal ions or cations like
cesium, sodium, potassium or tetraalkylammonium ions were not found
to contribute to the efficiency enhancement and did not cause
gelling of the electrolyte solutions. Furthermore, chloride anion
was found to enhance the efficiency along with lithium, by
improving the photovoltage without causing decreased
photoconductivity in compositions containing
methylbenzimidazole.
11TABLE 11 Photovoltaic Characteristics 1% P4VP 2% P4VP 3% P4VP 5%
P4VP .eta. (%) 3.23 3.48 3.09 3.19 I.sub.sc (mA/cm.sup.2) 10.74
10.42 12.03 10.9 V.sub.oc (V) 0.59 0.59 0.6 0.61 V.sub.m (V) 0.39
0.4 0.38 0.40 I.sub.m (mA/cm.sup.2) 8.27 8.69 8.07 8.03 FF 0.51
0.57 0.43 0.48
[0094] C. Co-Sensitizers
[0095] According to one illustrative embodiment, the
photosensitizing agent described above includes a first sensitizing
dye and second electron donor species, the "co-sensitizer." The
first sensitizing dye and the co-sensitizer may be added together
or separately to form the photosensitized interconnected
nanoparticle material 603 shown in FIG. 6. As mentioned above with
respect to FIG. 6, the sensitizing dye facilitates conversion of
incident visible light into electricity to produce the desired
photovoltaic effect. In one illustrative embodiment, the
co-sensitizer donates electrons to an acceptor to form stable
cation radicals, which improves the efficiency of charge transfer
from the sensitizing dye to the semiconductor oxide nanoparticle
material and reduces back electron transfer to the sensitizing dye
or co-sensitizer. The co-sensitizer preferably includes (1)
conjugation of the free electron pair on a nitrogen atom with the
hybridized orbitals of the aromatic rings to which the nitrogen
atom is bonded and, subsequent to electron transfer, the resulting
resonance stabilization of the cation radicals by these hybridized
orbitals; and (2) a coordinating group, such as a carboxy or a
phosphate, the function of which is to anchor the co-sensitizer to
the semiconductor oxide. Examples of suitable co-sensitizers
include, but are not limited to, aromatic amines (e.g., such as
triphenylamine and its derivatives), carbazoles, and other
fused-ring analogues.
[0096] Once again referring back to FIG. 6, the co-sensitizer is
electronically coupled to the conduction band of the
photosensitized interconnected nanoparticle material 603. Suitable
coordinating groups include, but are not limited to, carboxylate
groups, phosphates groups, or chelating groups, such as, for
example, oximes or alpha keto enolates.
[0097] Tables 12-18 below present results showing the increase in
photovoltaic cell efficiency when co-sensitizers are co-adsorbed
along with sensitizing dyes on the surface of high temperature
sintered or low temperature interconnected titania. In Tables
12-18, characterization was conducted using AM 1.5 solar simulator
conditions (i.e., irradiation with light having an intensity of
1000 W/m.sup.2). A liquid electrolyte including 1 M LiI, 1 M
t-butylpyridine, 0.5 M 12 in 3-methoxypropanitrile was employed.
The data shown in the tables indicates an enhancement of one or
more operating cell parameters for both low-temperature-interconne-
cted (Tables 15, 17 and 18) and high-temperature-sintered (Tables
12, 13, 14 and 16) titania nanoparticles. The solar cells
characteristics listed include .eta., V.sub.oc, I.sub.sc, FF,
V.sub.m, and I.sub.m. The ratios of sensitizer to co-sensitizer are
based on the concentrations of photosensitizing agents in the
sensitizing solution.
[0098] In particular, it was discovered that aromatic amines
enhance cell performance of dye sensitized titania solar cells if
the concentration of the co-sensitizer is below about 50 mol % of
the dye concentration. An example of the general molecular
structure of the preferred aromatic amines is shown in FIGS. 13 and
14. Preferably, the concentration of the co-sensitizer is in the
range of about 1 mol % to about 20 mol %, and more preferably in
the range of about 1 mol % to about 5 mol %.
[0099] FIG. 13A depicts a chemical structure 1300 that may serve as
a co-sensitizer. The molecule 1300 adsorbs to the surface of a
nanoparticle layer via its coordinating group or chelating group,
A. A may be a carboxylic acid group or derivative thereof, a
phosphate group, an oxime or an alpha ketoenolate, as described
above. FIG. 13B depicts a specific embodiment 1310 of the structure
1300, namely DPABA (diphenylaminobenzoic acid), where A=COOH. FIG.
13C depicts another specific amine 1320 referred to as DEAPA
(N',N-diphenylaminophenylpropionic acid), with A as the carboxy
derivative COOH.
[0100] FIG. 14A shows a chemical structure 1330 that may serve as
either a co-sensitizer, or a sensitizing dye. The molecule does not
absorb radiation above 500 nm, and adsorbs to a surface of the
nanoparticle layer via its coordinating or chelating groups, A. A
may be a carboxylic acid group or derivative thereof, a phosphate
group, an oxime or an alpha ketoenolate. R.sub.1 and R.sub.2 may
each be a phenyl, alkyl, substituted phenyl, or benzyl group.
Preferably, the alkyl may contain between 1 and 10 carbons. FIG.
14B depicts a specific embodiment 1340 of the structure 1330,
namely DPACA (2,6 bis (4-benzoicacid)-4-(4-N,N-diphenylamino)
phenylpyridine carboxylic acid), where R.sub.1 and R.sub.2 are
phenyl and A is COOH.
[0101] DPACA 1340 may be synthesized as follows. 1.49 g (9.08 mmol)
of 4-acetylbenzoic acid, 1.69 g (6.18 mmol) of
4-N,N-diphenylbenzaldehyde, and 5.8 g (75.2 mmol) of ammonium
acetate were added to 60 ml of acetic acid in a 100 ml round bottom
flask equipped with a condenser and stirring bar. The solution was
heated to reflux with stirring under nitrogen for 5 hours. The
reaction was cooled to room temperature and poured into 150 ml of
water, which was extracted with 150 ml of dichloromethane. The
dichloromethane was separated and evaporated with a rotary
evaporator, resulting in a yellow oil. The oil was then eluted on a
silica gel column with 4% methanol/dichloromethane to give the
product, an orange solid. The solid was washed with methanol and
vacuum dried to give 0.920 g of 2,6 bis
(4-benzoicacid)-4-(4-N,N-diphenylamino)phenylpyri- dine (DPACA).
The melting point was 199.degree.-200.degree. C., the
.lambda..sub.max was 421 nm, and the molar extinction coefficient,
E was 39,200 L mole.sup.-1 cm.sup.-1. The structure was confirmed
by NMR spectroscopy.
[0102] Table 12 shows the results for high-temperature-sintered
titania; photosensitized by overnight soaking in solutions of 1 mM
N3 dye and three concentrations of DPABA. Table 12 also shows that
the average .eta. is greatest for the preferred 20/1
(dye/co-sensitizer) ratio.
12 TABLE 12 I-V CHARACTERIZATION Cell General area I.sub.m I.sub.sc
conditions Conditions cm.sup.2 V.sub.oc V mA/cm.sup.2 V.sub.m V
mA/cm.sup.2 FF .eta. % .sigma. Adsorption 1 mM 0.44 0.62 6.69 0.44
8.38 0.56 2.91 Temp. N3/EtOH, 0.52 0.64 6.81 0.43 8.59 0.54 2.94 RT
.degree. C. Overnight 0.54 0.63 6.95 0.41 8.72 0.52 2.84 Solvent of
Dye CONTROL EtOH Average 0.50 0.63 6.82 0.43 8.56 0.54 2.90 0.05
Dye Concen. 1 mM N3, 0.50 0.64 7.70 0.45 9.31 0.58 3.43 N3, DPABA
0.05 mM 0.53 0.64 7.40 0.45 9.30 0.56 3.31 Sintering DPABA in 0.50
0.64 7.70 0.45 9.38 0.57 3.44 Temp EtOH for 450.degree. C., 30
Overnight; minutes 20/1 Average 0.51 0.64 7.60 0.45 9.33 0.57 3.39
0.07 Thickness of 1 mM N3, 1 mM 0.53 0.63 7.21 0.41 8.58 0.55 2.96
Film DPABA 0.50 0.63 6.75 0.44 8.23 0.57 2.97 TiO.sub.2, .about.10
.mu.m in EtOH for 0.42 0.63 7.11 0.44 8.67 0.57 3.13 Overnight; 1/1
Average 0.48 0.63 7.02 0.43 8.49 0.56 3.02 0.10 Electrolyte 1 mM
N3, 10 mM 0.33 0.58 4.95 0.42 6.02 0.60 2.08 AM 1.5D, l 1 DPABA
0.52 0.60 5.51 0.42 6.67 0.58 2.31 Sun in EtOH for 0.49 0.60 5.53
0.42 6.72 0.58 2.32 Overnight; 1/10 Film Average 0.45 0.59 5.33
0.42 6.47 0.58 2.24 0.14 pretreatment
[0103] Table 13 shows the results of using a cut-off filter (third
and fourth entries) while irradiating a cell to test its I-V
characteristics. Table 13 also shows that the efficiency of the
cell still improves when DPABA is present, indicating that its
effect when no filter is present is not simply due to absorption of
UV light by DPABA followed by charge injection. FIG. 14 shows a
plot 1400 of the absorbance versus wavelength for the cut-off
filter used to characterize the photovoltaic cells, according to an
illustrative embodiment of the invention. FIG. 15 shows a plot 1500
of the absorbance versus wavelength for DPABA, which absorbs below
400 nm. Because the absorbance of the cut-off is large, little
light reaches the absorption bands of DPABA.
13 TABLE 13 I-V CHARACTERIZATION Cell area I.sub.m I.sub.sc
Conditions cm.sup.2 V.sub.oc V mA/cm.sup.2 V.sub.m V mA/cm.sup.2 FF
.eta. % .sigma. 1 mM N3 in 0.49 0.70 8.62 0.46 11.02 0.51 3.97 EtOH
0.49 0.70 8.13 0.45 10.20 0.51 3.66 Overnight 0.49 0.73 7.93 0.51
9.69 0.57 4.04 control Average 0.49 0.71 8.23 0.47 10.30 0.53 3.89
0.20 1 mM N3 0.49 0.71 9.05 0.46 11.53 0.51 4.16 0.05 mM 0.49 0.71
9.24 0.46 11.56 0.52 4.25 DPABA in 0.49 0.71 9.39 0.46 11.50 0.53
4.32 EtOH, 20/1 Overnight Average 0.49 0.71 9.23 0.46 11.53 0.52
4.24 0.08 1 mM N3 in 0.49 0.69 6.35 0.47 7.83 0.55 4.26 455 nm cut
EtOH 0.49 0.69 6.05 0.46 7.44 0.54 3.98 off filter Overnight 0.49
0.72 5.74 0.52 6.94 0.60 4.27 used, 70 mW/cm.sup.2 control Average
0.49 0.70 6.05 0.48 7.40 0.56 4.17 0.17 1 mM N3 0.49 0.70 6.73 0.47
8.21 0.55 4.52 455 nm cut 0.05 mM 0.49 0.70 6.74 0.47 8.19 0.55
4.53 off filter DPABA in 0.49 0.70 6.74 0.49 8.25 0.57 4.72 used,
70 mW/cm.sup.2 EtOH, 20/1 Overnight Average 0.49 0.70 6.74 0.48
8.22 0.56 4.59 0.11
[0104] Table 14:shows that the addition of triphenylamine itself
(i.e., no titania complexing groups such as carboxy) does not
significantly enhance efficiency under the stated conditions.
14 TABLE 14 I-V CHARACTERIZATION Cell area Conditions cm.sup.2
V.sub.oc V I.sub.m mA/cm.sup.2 V.sub.m V I.sub.sc mA/cm.sup.2 FF
.eta. % .sigma. 0.5 mM N3 0.49 0.70 7.96 0.45 9.82 0.52 3.58 in
EtOH, 0.49 0.71 8.09 0.48 9.58 0.57 3.88 Overnight 0.49 0.70 7.47
0.48 8.83 0.58 3.59 Average 0.49 0.70 7.84 0.47 9.41 0.56 3.68 0.17
0.5 mM N3, 0.49 0.69 7.44 0.45 9.21 0.53 3.35 0.025 mM 0.49 0.69
7.61 0.47 9.75 0.53 3.58 TPA in 0.49 0.69 6.98 0.45 8.56 0.53 3.14
EtOH Overnight 20/1 Average 0.49 0.69 7.34 0.46 9.17 0.53 3.36 0.22
0.5 mM N3, 0.49 0.68 4.62 0.44 5.66 0.53 2.03 2.0 mM 0.49 0.66 4.18
0.45 5.38 0.53 1.88 TPA in 0.49 0.66 4.51 0.45 5.82 0.53 2.03 EtOH
Overnight 1/4 Average 0.49 0.67 4.44 0.45 5.62 0.53 1.98 0.09
[0105] Table 15 shows that the effect is present using low
temperature interconnected titania and that the 20/1
(dye/co-sensitizer) ratio is preferred.
15 TABLE 15 I-V CHARACTERIZATION Cell area Conditions cm.sup.2
V.sub.oc V I.sub.m mA/cm.sup.2 V.sub.m V I.sub.sc mA/cm.sup.2 FF
.eta. % .sigma. 0.5 mM 0.49 0.73 8.32 0.50 10.56 0.54 4.16 N3/EtOH,
0.51 0.72 8.13 0.49 10.30 0.54 3.98 overnight, 0.50 0.72 8.56 0.47
10.65 0.52 4.02 control Average 0.50 0.72 8.34 0.49 10.50 0.53 4.06
0.09 0.5 mM N3, 0.49 0.73 8.55 0.51 10.48 0.57 4.36 0.0125 mM 0.53
0.72 8.53 0.50 11.00 0.54 4.27 DPABA in 0.49 0.74 8.08 0.54 10.96
0.54 4.36 EtOH, 40/1, overnight Average 0.50 0.73 8.39 0.52 10.81
0.55 4.33 0.06 0.5 mM N3, 0.49 0.73 9.07 0.49 11.31 0.54 4.44 0.017
mM 0.49 0.75 8.64 0.52 10.97 0.55 4.49 DPABA in 0.52 0.73 8.19 0.52
10.88 0.54 4.26 EtOH, 30/1, overnight Average 0.50 0.74 8.63 0.51
11.05 0.54 4.40 0.12 0.5 mM N3, 0.50 0.75 8.57 0.52 11.56 0.51 4.46
0.025 mM 0.49 0.74 8.88 0.52 11.45 0.54 4.62 DPABA in 0.53 0.74
9.01 0.51 12.08 0.51 4.60 EtOH, 20/1, overnight Average 0.51 0.74
8.82 0.52 11.70 0.52 4.56 0.09 0.5 mM N3, 0.49 0.72 8.85 0.48 10.78
0.55 4.25 0.5 mM 0.51 0.74 8.62 0.47 10.37 0.53 4.05 DPABA in 0.50
0.75 8.38 0.49 10.02 0.55 4.11 EtOH, 1/1, overnight Average 0.50
0.74 8.62 0.48 10.39 0.54 4.14 0.10 0.5 mM N3, 0.49 0.68 7.56 0.44
9.09 0.54 3.33 5 mM 0.51 0.69 7.62 0.46 9.34 0.54 3.51 DPABA in
0.49 0.67 7.25 0.45 8.84 0.55 3.26 EtOH, 1/10, overnight Average
0.50 0.68 7.48 0.45 9.09 0.54 3.36 0.13
[0106] Table 16 shows results for high-temperature-sintered titania
sensitized with a high concentration of N3 dye while maintaining a
20/1 ratio of dye to co-sensitizer. Entries 1 and 2 show the
increase in cell performance due to co-sensitizer. Entry 3 shows
the effect of DPABA alone as a sensitizer, demonstrating that this
material acts as a sensitizer by itself when irradiated with the
full solar spectrum, which includes low-intensity UV radiation.
16 TABLE 16 I-V CHARACTERIZATION General Cell area I.sub.m I.sub.sc
Conditions Conditions cm.sup.2 V.sub.oc V mA/cm.sup.2 V.sub.m V
mA/cm.sup.2 FF .eta. % .sigma. Adsorption 8 mM 0.49 0.68 8.51 0.44
10.07 0.55 3.74 Temp. N3/aprotic 0.49 0.67 8.28 0.44 9.75 0.56 3.64
RT.degree. C. polar solvent, 0.49 0.68 9.16 0.42 10.80 0.52 3.85
Solvent of Dye 1 hour Aprotic polar CONTROL solvent average 0.49
0.68 8.65 0.43 10.21 0.54 3.74 0.10 8 mM N3, 0.4 mM 0.49 0.68 9.52
0.44 11.18 0.55 4.19 DPABA 0.49 0.68 9.96 0.44 11.59 0.56 4.38 in
aprotic polar 0.49 0.65 9.81 0.42 12.13 0.52 4.12 solvent, 20/1 1
hour average 0.49 0.67 9.76 0.43 11.63 0.54 4.23 0.14 5 mM DPABA
0.49 0.55 1.02 0.42 1.22 0.64 0.43 in aprotic polar 0.49 0.55 0.94
0.41 1.13 0.62 0.39 solvent 0.49 0.58 0.89 0.44 1.07 0.63 0.39
Overnight 0.49 0.56 0.95 0.42 1.14 0.63 0.40 0.02
[0107] Table 17 shows results for low-temperature-interconnected
titania. Entry 5 shows the affect of DPACA alone as a sensitizer,
demonstrating that this material acts as a sensitizer by itself
when irradiated with the full solar spectrum, which includes
low-intensity UV radiation.
17 TABLE 17 I-V CHARACTERIZATION Cell area Conditions cm.sup.2
V.sub.oc V I.sub.m mA/cm.sup.2 V.sub.m V I.sub.sc mA/cm.sup.2 FF
.eta. % .sigma. 0.5 mM 0.51 0.73 8.40 0.50 10.84 0.53 4.20 N3/EtOH,
0.53 0.72 8.13 0.49 10.30 0.54 3.98 overnight, 0.50 0.72 8.77 0.47
10.87 0.53 4.12 control average 0.51 0.72 8.43 0.49 10.67 0.53 4.10
0.11 0.5 mM N3, 0.49 0.73 8.10 0.51 10.39 0.54 4.13 0.01 mM 0.50
0.74 7.95 0.50 10.01 0.54 3.98 DPACA in 0.49 0.72 8.10 0.50 9.85
0.57 4.05 EtOH, 50/1, overnight average 0.49 0.73 8.05 0.50 10.08
0.55 4.05 0.08 0.5 mM N3, 0.49 0.74 8.38 0.50 10.48 0.54 4.19 0.02
mM 0.52 0.73 8.18 0.48 9.74 0.55 3.93 DPACA in 0.49 0.76 8.08 0.54
9.45 0.61 4.36 EtOH, 25/1, overnight average 0.50 0.74 8.21 0.51
9.89 0.57 4.16 0.22 0.5 mM N3, 0.49 0.73 9.07 0.46 11.31 0.51 4.17
0.5 mM 0.49 0.75 7.41 0.53 9.24 0.57 3.93 DPACA in 0.52 0.76 7.93
0.52 9.12 0.59 4.12 EtOH, 1/1, overnight average 0.50 0.75 8.14
0.50 9.89 0.56 4.07 0.13 0.5 mM N3, 0.56 0.73 6.36 0.49 7.59 0.56
3.12 5.0 mM 0.52 0.73 6.63 0.49 7.84 0.57 3.25 DPACA in 0.50 0.72
6.53 0.49 7.59 0.59 3.20 EtOH, 1/10, overnight average 0.53 0.73
6.51 0.49 7.67 0.57 3.19 0.07 5.0 mM 0.43 0.65 3.12 0.49 3.77 0.62
1.53 DPACA in 0.45 0.65 2.93 0.49 3.51 0.63 1.44 EtOH, 0.49 0.66
2.83 0.49 3.40 0.62 1.39 overnight average 0.46 0.65 2.96 0.49 3.56
0.62 1.45 0.07
[0108] Table 18 shows results for low-temperature-interconnected
titania. Entry 6 shows the affect of DEAPA alone as a sensitizer,
demonstrating that this material acts as a sensitizer by itself
when irradiated with the full solar spectrum, which includes
low-intensity UV radiation.
18TABLE 18 I-V CHARACTERIZATION General Cell area I.sub.m I.sub.sc
conditions Conditions cm.sup.2 V.sub.oc V mA/cm.sup.2 V.sub.m V
mA/cm.sup.2 FF .eta. % .sigma. Adsorption 0.5 mM 0.51 0.72 8.67
0.49 10.60 0.56 4.25 Temp. N3/EtOH, 0.49 0.75 8.15 0.47 10.50 0.49
3.83 RT .degree. C. overnight, 0.49 0.74 8.74 0.44 10.63 0.49 3.85
Solvent of control Dye average 0.50 0.74 8.52 0.47 10.58 0.51 3.97
0.24 EtOH Dye Concen. 0.5 mM N3, 0.49 0.70 8.68 0.44 11.00 0.50
3.82 N3, DEAPA 0.01 mM 0.52 0.71 8.57 0.45 11.11 0.49 3.86
Sintering DEAPA in 0.50 0.72 8.40 0.45 10.61 0.49 3.78 Temp EtOH,
50/1, 120.degree. C., 10 overnight minutes average 0.50 0.71 8.55
0.45 10.91 0.49 3.82 0.04 Thickness of 0.5 mM N3, 0.51 0.74 8.90
0.44 10.92 0.48 3.92 Film 0.02 mM 0.53 0.73 8.76 0.44 10.51 0.50
3.85 TiO.sub.2, .about.7 .mu.m DEAPA in 0.49 0.73 8.40 0.45 10.21
0.51 3.78 Liquid EtOH, 25/1, overnight average 0.51 0.73 8.69 0.44
10.55 0.50 3.85 0.07 Electrolyte 0.5 mM N3, 0.49 0.71 8.94 0.43
10.78 0.50 3.84 AM 1.5 D, 1 0.5 mM 0.51 0.71 8.83 0.44 10.37 0.53
3.89 Sun DEAPA in 0.50 0.70 8.18 0.42 9.71 0.51 3.44 EtOH, 1/1,
overnight Film average 0.50 0.71 8.65 0.43 10.29 0.51 3.72 0.25
pretreatment 0.5 mM N3, 0.52 0.60 0.88 0.45 1.08 0.61 0.40 5.0 mM
0.49 0.59 0.71 0.44 0.85 0.62 0.31 DEAPA in 0.49 0.59 0.75 0.44
0.91 0.61 0.33 EtOH, 1/10, overnight average 0.50 0.59 0.78 0.44
0.95 0.62 0.35 0.04 5.0 mM 0.49 0.54 0.41 0.42 0.49 0.65 0.17 DEAPA
in 0.49 0.54 0.35 0.39 0.46 0.55 0.14 CHCl3, 0.51 0.52 0.45 0.40
0.52 0.67 0.18 overnight average 0.50 0.53 0.40 0.40 0.49 0.62 0.16
0.02
[0109] D. Semiconductor Oxide Formulations
[0110] In a further illustrative embodiment, the invention provides
semiconductor oxide formulations for use with DSSCs formed using a
low temperature semiconductor oxide nanoparticle interconnection,
as described above. The semiconductor oxide formulations may be
coated at room temperature and, upon drying at temperatures between
about 50.degree. C. and about 150.degree. C., yield mechanically
stable semiconductor nanoparticle films with good adhesion to the
transparent conducting oxide (TCO) coated plastic substrates. In
one embodiment, the nanoparticle semiconductor of the
photosensitized interconnected nanoparticle material 603 is formed
from a dispersion of commercially available TiO.sub.2 nanoparticles
in water, a polymer binder, with or without acetic acid. The
polymer binders used include, but are not limited to,
polyvinylpyrrolidone (PVP), polyethylene oxide (PEO), hydroxyethyl
cellulose (HOEC), hydroxypropyl cellulose, polyvinyl alcohol (PVA)
and other water-soluble polymers. The ratio of semiconductor oxide
particles, e.g., TiO.sub.2, to polymer can be between about 100:0.1
to 100:20 by weight, and preferably is between about 100:1 to
100:10 by weight. The presence of acetic acid in the formulation
helps to improve the adhesion of the coating to the TCO coated
substrate. However, acetic acid is not essential to this aspect of
the invention and semiconductor oxide dispersions without acetic
acid perform satisfactorily. In another embodiment, the TiO.sub.2
nanoparticles are dispersed in an organic solvent, such as, e.g.,
isopropyl alcohol, with polymeric binders such as, e.g., PVP,
butvar, ethylcellulose, etc.
[0111] In another illustrative embodiment, the mechanical integrity
of the semiconductor oxide coatings and the photovoltaic
performance of the dye sensitized cells based on these coatings can
be further improved by using a crosslinking agent to interconnect
the semiconductor nanoparticles. The polylinkers described above
may be used for this purpose. These crosslinking agents can be
applied, e.g., in the titania coating formulation directly or in a
step subsequent to drying the titania coating as a solution in an
organic solvent such as ethanol, isopropanol or butanol. For
example, subsequent heating of the films to temperatures in the
range of about 70.degree. C. to about 140.degree. C. leads to the
formation of TiO.sub.2 bridges between the TiO.sub.2 nanoparticles.
Preferably, the concentration of the polylinker in this example
ranges from about 0.01 to about 20 weight % based on titania.
[0112] E. Semiconductor Primer Layer Coatings
[0113] In another illustrative embodiment, the invention provides
semiconductor oxide materials and methods of coating semiconductor
oxide nanoparticle layers on a base material to form DSSCs. FIG. 16
depicts an illustrative embodiment 1600 of the coating process,
according to the invention. In this illustrative embodiment, a base
material 1610 is coated with a first primer layer 1620 of a
semiconductor oxide, and then a suspension of nanoparticles 1630 of
the semiconductor oxide is coated over the primer layer 1620. The
primer layer 1620 may include a vacuum-coated semiconductor oxide
film (e.g., a TiO.sub.2 film). Alternatively, the primer layer 1620
may include a thin coating with fine particles of a semiconductor
oxide (e.g. TiO.sub.2, SnO.sub.2). The primer layer 1620 may also
include a thin layer of a polylinker or precursor solution, one
example of which is the Ti (IV) butoxide polymer 400 shown in FIG.
4 above. According to one illustrative embodiment of the invention,
the base material 1610 is the first flexible, significantly light
transmitting substrate 609 referred to in FIG. 6. Additionally, the
base material 1610 is a transparent, conducting, plastic substrate.
According to this illustrative embodiment, the suspension of
nanoparticles 1630 is the photosensitized interconnected
nanoparticle material 603 of FIG. 6. Numerous semiconducting metal
oxides, including SnO.sub.2, TiO.sub.2, Ta.sub.2O.sub.5,
Nb.sub.2O.sub.5, and ZnO, among others, in the form of thin films,
fine particles, or precursor solutions may be used as primer layer
coatings using vacuum coating, spin coating, blade coating or other
coating methods.
[0114] The primer layer 1620 improves the adhesion of
nano-structured semiconductor oxide films, like layer 1630, to the
base material 1610. Enhancements in the performance of DSSCs with
such primer layers have been observed and will be described below.
The enhancement arises from an increase in the adhesion between the
semiconductor oxide nanoparticles (or photoelectrodes) and the
transparent conducting oxide coated plastic substrates, as well as
from higher shunt resistance.
[0115] Examples of various illustrative embodiments of this aspect
of the invention, in the context of a DSSC including a titanium
dioxide nanoparticle layer, are as follows.
EXAMPLE 18
Vacuum Coated TiO.sub.2 as Prime Layers for Nanoparticle TiO.sub.2
Photoelectrodes
[0116] In this illustrative example, thin TiO.sub.2 films with
thicknesses ranging from 2.5 nm to 100 nm were sputter-coated under
vacuum on an ITO layer coated on a polyester (here, PET) substrate.
A water based TiO.sub.2 (P25, with an average particle size of 21
nm) slurry was spin-coated on both the ITO/PET with sputter-coated
thin TiO.sub.2 and on the plain ITO/PET (i.e., the portion without
sputter-coated thin TiO.sub.2). The coated films were soaked in
poly [Ti(OBu).sub.4] solution in butanol and then heat treated at
120.degree. C. for 2 minutes. The low-temperature reactively
interconnected films were placed into an aprotic, polar
solvent-based N3 dye solution (8 mM) for 2 minutes. Photovoltaic
cells were made with platinum (Pt) counter-electrodes, an
I.sup.-/I.sub.3.sup.- liquid electrolyte, 2 mil SURLYN, and copper-
conducting tapes. I-V characterization measurements were performed
with a solar simulator.
[0117] Adhesion of nanostructured TiO.sub.2 films from the P25
slurry coated on the ITO/PET with sputter-coated, thin TiO.sub.2
was superior to films on the plain ITO/PET. Better photovoltaic
performance was also observed from the PV cells prepared on the
ITO/PET with sputter-coated, thin TiO.sub.2 as compared to those on
the plain ITO/PET. Improvement on the fill-factor was achieved as
well. A FF as high as 0.67 was measured for the photovoltaic cells
made on the ITO/PETs with sputter-coated, thin TiO.sub.2. For the
photovoltaic cells made on the plain ITO/PET, the FF observed was
not greater than 0.60. Higher photovoltaic conversion efficiencies
(about 17% higher than the photoelectrodes made from the plain
ITO/PET) were measured for the photoelectrodes prepared on the
ITO/PET with thin sputter-coated TiO.sub.2. Improvement in shunt
resistance was also observed for the photovoltaic cells made on the
ITO/PET with thin sputter-coated TiO.sub.2.
EXAMPLE 19
Fine Particles of TiO.sub.2 as Primer Layer for TiO.sub.2
Suspensions
[0118] In this illustrative example, fine particles of TiO.sub.2,
small enough such that they would stick in the valleys between
spikes of ITO on the PET substrate, were prepared by hydrolyzing
titanium (IV) isopropoxide. The fine particles were then spin
coated at 800 rpm onto the ITO layer. A 37% TiO.sub.2 (P25)
suspension of approximately 21 nm average particle size was then
spin coated at 800 rpm onto the fine particle layer. The coated
TiO.sub.2 was low temperature interconnected by dipping in 0.01
molar Ti (IV) butoxide polymer in butanol for 15 minutes followed
drying on a slide warmer at 50.degree. C. before heating at
120.degree. C. for 2 minutes. The interconnected coating was dyed
with N3 dye by dipping into an 8 mM aprotic polar solvent solution
for 2 minutes, then rinsed with ethanol and dried on a slide warmer
at 50.degree. C. for 2 minutes. Control coatings were prepared in
the same way, except without the fine particle prime coat. The
cells' performance characteristics were measured using a solar
simulator. Results for test and control are listed below in Table
19. Fine particles of tin oxide as primer coating for TiO.sub.2
suspensions yielded similar improvements.
19 TABLE 19 V.sub.oc I.sub.sc .eta. FF Control 0.64 4.86 1.67% 0.54
Invention 0.66 6.27 2.36% 0.57
EXAMPLE 20
Titanium (IV) Butoxide Polymer in Butanol (Precursor Solution) as
Primer Layer for TiO.sub.2
[0119] In another test, titanium (IV) butoxide polymer in butanol
at 0.01 molar was spin coated on an ITO/PET plastic base at 800
rpm. A 43% TiO.sub.2 (P25) suspension of approximately 21 mn
average particle size was spin coated at 800 rpm. The coated
TiO.sub.2 was interconnected at low temperature by dipping in 0.01
M titanium (IV) butoxide polymer in butanol for 15 minutes and then
drying on a slide warmer at 50.degree. C. before heating at
120.degree. C. for 2 minutes. The sintered coating was dyed with N3
dye by dipping into an 8 mM aprotic, polar solvent solution for 2
minutes, then rinsed with ethanol and dried on a slide warmer at
50.degree. C. for 2 minutes. Control coatings were prepared in the
same way only without the primer layer coating. The I-V properties
of the cells were measured with a solar simulator. Results for test
and control are listed below in Table 20.
20 TABLE 20 V.sub.oc I.sub.sc .eta. FF Control 0.66 7.17 2.62% 0.56
Invention 0.70 8.11 3.38% 0.59
[0120] While the invention has been particularly shown and
described with reference to specific illustrative embodiments, it
should be understood that various changes in form and detail may be
made without departing from the spirit and scope of the invention
as defined by the appended claims. By way of example, any of the
disclosed features may be combined with any of the other disclosed
features to form a photovoltaic cell or module.
* * * * *