U.S. patent application number 11/638163 was filed with the patent office on 2007-07-05 for light-harvesting discotic liquid crystalline porphyrins and metal complexes.
This patent application is currently assigned to Kent State University. Invention is credited to Quan Li, Xiaoli Zhou.
Application Number | 20070152189 11/638163 |
Document ID | / |
Family ID | 37964125 |
Filed Date | 2007-07-05 |
United States Patent
Application |
20070152189 |
Kind Code |
A1 |
Li; Quan ; et al. |
July 5, 2007 |
Light-harvesting discotic liquid crystalline porphyrins and metal
complexes
Abstract
Novel discotic liquid crystalline porphyrins and discotic liquid
crystalline metal complexes, methods for their preparation, and
device fabrication are disclosed. Materials with partially
perfluorinated alkyl group in the peripheral chains show a strong
tendency towards the formation of homeotropic alignment. These
compounds are capable of being used as high-efficiency photovoltaic
materials, organic semiconducting materials, and organic light
emitting materials.
Inventors: |
Li; Quan; (Stow, OH)
; Zhou; Xiaoli; (Kent, OH) |
Correspondence
Address: |
FAY SHARPE LLP
1100 SUPERIOR AVENUE, SEVENTH FLOOR
CLEVELAND
OH
44114
US
|
Assignee: |
Kent State University
|
Family ID: |
37964125 |
Appl. No.: |
11/638163 |
Filed: |
December 13, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11325478 |
Jan 4, 2006 |
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11638163 |
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60790996 |
Apr 11, 2006 |
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Current U.S.
Class: |
252/299.01 ;
136/263; 252/299.61; 252/299.62; 257/40; 540/145; 540/465 |
Current CPC
Class: |
Y02P 70/50 20151101;
C09K 19/3488 20130101; H01L 51/42 20130101; H01L 51/0076 20130101;
Y02E 10/549 20130101; Y02P 70/521 20151101; C09K 19/40 20130101;
H01L 51/0077 20130101 |
Class at
Publication: |
252/299.01 ;
252/299.61; 252/299.62; 540/145; 540/465; 136/263; 257/40 |
International
Class: |
C09K 19/52 20060101
C09K019/52; C09K 19/32 20060101 C09K019/32; C09K 19/34 20060101
C09K019/34; C07D 487/22 20060101 C07D487/22; C07D 225/00 20060101
C07D225/00; C07B 47/00 20060101 C07B047/00; H01L 29/08 20060101
H01L029/08; H01L 35/24 20060101 H01L035/24; H01L 31/00 20060101
H01L031/00; H01L 51/00 20060101 H01L051/00 |
Claims
1. A liquid crystalline porphyrin or porphyrin metal complex,
having the: structure I or II below: ##STR00004## wherein L is a
linking group selected from COO, OOC, O, S, COS or NHCO; M=Zn, Co,
Cu, Ni, Cr, Mn, Mg, Ce, Ru, Rh, Pt, Au, or a lanthanide metal and
which may be bound to a halogen, O, OH or .dbd.CO; and wherein
R.sub.1, R.sub.2, and R.sub.3 are independently selected from H,
C.sub.6-36 linear or branched alkyl groups, C.sub.60 (fullerene), a
C.sub.60 derivative, or an aromatic moiety.
2. A liquid crystalline porphyrin or porphyrin metal complex
according to claim 1, wherein at least one of R.sub.1, R.sub.2, or
R.sub.3 comprises a perfluoroalkyl group, C.sub.60 (fullerene), a
C.sub.60 derivative, or an aromatic moiety.
3. A liquid crystalline porphyrin or porphyrin metal complex
according to claim 1, wherein at least one of R.sub.1, R.sub.2, or
R.sub.3 comprises one or more --O--, --S--, --CO--, COO--, --OCO--,
--N.dbd.N-- and/or --C.ident.C--.
4. A liquid crystalline porphyrin or porphyrin metal complex
according to claim 1, wherein said porphyrin can be aligned into an
ordered architecture, in which the columns formed by intermolecular
.pi.-.pi. stacks are spontaneously perpendicular or parallel on the
surface.
5. A liquid crystalline porphyrin or porphyrin metal complex
according to claim 4, wherein said ordered architecture is stable
within a very wide temperature range and can enhance the charge
carrier mobility.
6. A blend comprising the liquid crystalline porphyrin or porphyrin
metal complex according to claim 1, further comprising an
electron-acceptor material.
7. A blend according to claim 6, wherein said electron-acceptor
material comprises a C.sub.60 compound, a C.sub.60 derivative
compound, a dye, and/or a carbon nanotube.
8. A liquid crystalline porphyrin or porphyrin metal complex
according to claim 1, wherein said porphyrin or porphyrin metal
complex comprises one of the following I, II, or III: ##STR00005##
wherein m is from 0-20 and n is from 0 to 20; ##STR00006## wherein
m is from 0-20 and n is from 0 to 20; ##STR00007## wherein n is
from 5 to 35.
9. A method for producing the porphyrin of claim 1, comprising the
following synthesis steps: ##STR00008##
10. A method according to claim 9, wherein said synthesis is
conducted in an organic solvent.
11. A method according to claim 10, wherein said organic solvent
comprises at least one of propionic acid, propionic anhydride,
pyrrole, methylene chloride, chloroform, N,N-dimethylformamide,
N-methylpyrrolidone, pyridine, triethylamine, ether,
tetrahydrofuran, alcohol, ethyl acetate, acetonitrile, ethyl methyl
ketone, saturated aliphatic hydrocarbons and aromatic
hydrocarbons.
12. A method for producing the porphyrin of claim 1, comprising the
following synthesis steps: ##STR00009##
13. A method according to claim 12, wherein said synthesis is
conducted in an organic solvent.
14. A method according to claim 13, wherein said organic solvent
comprises at least one of methylene chloride, chloroform, ether,
tetrahydrofuran, pyrrole, propionic acid, propionic anhydride,
pyridine, triethylamine, N,N-dimethylformamide,
N-methylpyrrolidone, alcohol, ethyl acetate, acetonitrile, ethyl
methyl ketone, saturated aliphatic hydrocarbons and aromatic
hydrocarbons.
15. A method for producing the porphyrin metal complex of claim 1,
comprising the following synthesis steps: ##STR00010##
16. A photovoltaic cell including the liquid crystalline porphyrin
or porphyrin metal complex of claim 1.
17. A photovoltaic cell according to claim 16, wherein said cell
comprises a first transparent electrode, a second electrode, and a
photoactive layer positioned between said first and second
electrodes comprising the liquid crystalline porphyrin or porphyrin
metal complex.
18. A photovoltaic cell according to claim 17, wherein said first
electrode is an indium tin oxide electrode, wherein said electrode
is coated on a glass or plastic substrate.
19. A photovoltaic cell according to claim 18, wherein said second
electrode comprises aluminum, copper, silver and/or gold.
20. A photovoltaic cell according to claim 17, further comprising a
photosensitizing agent.
21. A photovoltaic cell according to claim 16, further comprising
an electron-acceptor.
22. A photovoltaic cell according to claim 21, wherein said
electron-acceptor comprises C.sub.60 (fullerene), a C.sub.60
derivative, a carbon nanotube or a photosensitive dye.
23. A photo-sensitive electric resistor comprising the liquid
crystalline porphyrin or porphyrin metal complex of claim 1.
24. An organic light emitting device comprising the liquid
crystalline porphyrin or porphyrin metal complex of claim 1.
25. A method for producing a photovoltaic cell, including the steps
of: a) providing a first transparent electrode and a second
electrode; b) positioning the liquid crystalline porphyrin or
porphyrin metal complex of claim 1 between said first and second
electrodes; and c) aligning said porphyrin homeotropically.
26. A method according to claim 25, further comprising: d) sealing
the two electrodes together while maintaining a liquid crystal
uptake opening between the two; e) heating said porphyrin in a
vacuum chamber to melt it; f) placing the cell in said vacuum
chamber to remove air from the cell. g) dipping the cell opening
into the melted porphyrin; and h) reducing the vacuum level in said
vacuum chamber to allow the cell to uptake the melted
porphyrin.
27. A bulk heterojunction cell having homeotropically aligned
architecture comprising a blend of the liquid crystalline porphyrin
or porphyrin metal complex of claim 1 and an electron-acceptor
comprising C.sub.60, a C.sub.60 derivative, a dye and/or a carbon
nanotube,
28. A double- or multi-layered photovoltaic cell comprising: i) a
donor layer comprising a homeotropically aligned liquid crystalline
porphyrin or porphyrin metal complex of claim 1 or a
homeotropically aligned blend comprising said liquid crystalline
porphyrin or porphyrin metal complex; and ii) an acceptor layer
comprising a C.sub.60 compound, a C.sub.60 derivative compound, a
dye, and/or a carbon nanotube.
Description
[0001] The present application claims priority to U.S. Provisional
Patent Application Ser. No. 60/790,996, filed Apr. 11, 2006, and is
a continuation in part and claims priority from prior U.S. patent
application Ser. No. 11/325,478, filed Jan. 4, 2006.
BACKGROUND
[0002] The present exemplary embodiments relate to discotic liquid
crystalline porphyrins and discotic liquid crystalline porphyrin
metal complexes. In certain embodiments, their homeotropically or
homogeniously aligned architecture, in which the columns formed by
intermolecular stack are spontaneously perpendicular or parallel on
the surface respectively, is a crucial point for their
applications. They find use as high-efficiency photovoltaic
materials, organic semiconducting materials, organic light emitting
materials, materials for organic transistors and in solar cell
device implementation. However, it is to be appreciated that the
present exemplary embodiments are also amenable to other like
applications.
[0003] In the long term, solar energy is the only source of
renewable energy that has the capacity to fill humanity's
technological needs. A grand challenge is to convert solar energy
into green electric energy in an inexpensive and efficient way.
Crystalline silicon photovoltaic cells, though efficient, are too
expensive to compete with primary fossil energy. Organic
photovoltaic (OPV) technology would hold the promise for cost
reduction since the OPV materials are potentially cheap, easy to
process, and capable of being deposited on flexible substrates such
as plastics and bending, where their inorganic competitors e.g.
crystalline silicon would crack.
[0004] New OPV materials able to efficiently absorb sunlight and
new approaches based on nanostructured architectures holds the
potential to revolutionize the technology used to produce solar
electricity; however the availability of such new materials with
tailored properties has undoubtedly posed a bottleneck to the OPV
technology. A breakthrough of new material development is urgently
needed to boost the feasibility and prevalence of OPV technology.
Currently widely used OPV materials, e.g. polycrystalline Cu
phthalocyanine, suffer from the scattering of electron/exciton
between small crystal grain boundaries in which random arrangement
of molecules results in poor charge mobility. The existing grain
boundaries and defects act as deep traps that dramatically reduce
the charge mobility. In addition, polycrystalline materials are
intrinsically inhomogeneous. The attainment of large defect-free
single crystals or single crystalline film of large area of either
inorganic (e.g. silicon) or organic molecules is difficult and
costly.
[0005] A challenge for OPV, with the possibility of very
significant cost reduction, is to make them in desired macroscopic
order to improve charge transportation etc. Discotic liquid
crystals (LCs) capable of being homeotropically aligned (i.e. the
columns formed by intermolecular strong stack are perpendicular to
the electrode surface) would be a desirable candidate to meet the
challenge since they can form ordered nanostructures at macroscopic
scale for photovoltaic application. Unfortunately, the preferable
homeotropic-alignment of discotic LCs, especially those having
large conjugated systems, is difficult to achieve due to their high
viscosity and poor affinity to substrates, as compared to the well
established technologies for their calamitic counterparts in the
display industry.
[0006] In order to make discotic LC with more efficient absorption
of sunlight, one should consider porphyrin as the building block of
the potentially most viable discotic material since it is the basic
structure of the best photoreceptor in nature, chlorophyll.
Porphyrin and its derivatives have many desirable features such as
highly conjugated plane, high stability, intense absorption of
sunlight, and the small gap between the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy level.
[0007] As will be described in detail, the present disclosure is an
important extension of our previous application above. The
materials described herein provide a broad range of discotic LCs
capable of forming homeotropically/homogeneous aligned
architecture. Chemical modifications can provide a way to control
and improve their alignment which is a crucial point for their
applications. The materials are capable of being used as
photovoltaic materials, organic semiconductors and organic light
emitting materials. Although the drawings, discussions and
descriptions are mainly directed toward the preparation of the said
materials, photovoltaic devices and methods, it is to be understood
that the principles of the present invention are applicable to any
type of devices that uses homeotropically or homogenously aligned
architecture of any a discotic liquid crystal or the blend composed
of a discotic liquid crystal and one or more other components as a
layer.
BRIEF DESCRIPTION
[0008] In accordance with one aspect of the present exemplary
embodiments, there is provided a discotic liquid crystalline
porphyrin or complex, having the structure set forth in the
claims.
[0009] In a second aspect, there is provided, a method for
producing the porphyrin.
[0010] In a third aspect, there is provided a photovoltaic cell
including the porphyrin.
[0011] In a fourth aspect, there is provided either bulk
heterojunction cells with homeotropically aligned architecture of
blends composed of a discotic liquid crystal and one or more other
components which may be a material such as C.sub.60, its
derivative, dye and carbon nanotube, or double- or multi-layered
cells in which a donor layer includes a homeotropically aligned
discotic liquid crystal or its homeotropically aligned blend
together with an acceptor layer which may composed of a material
such as C.sub.60, its derivative, dye and carbon nanotube.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is the molecular structure of one present embodiment
compound.
[0013] FIG. 2 is the molecular structure of another present
embodiment compound.
[0014] FIG. 3 is general schematic of a presently conceived
photovoltaic cell.
[0015] FIGS. 4-9 are molecular structures of specific exemplary
compounds.
[0016] FIG. 10 is a UV-vis absorption spectrum of one of the
exemplary compounds.
[0017] FIG. 11 is a fluorescent emission spectrum of the same
exemplary compound.
[0018] FIG. 12 is a depiction showing crossed polarized optical
textures with homeotropic alignment (dark area) of one of the
exemplary compounds at room temperature (A and B with different
cooling rate).
DETAILED DESCRIPTION
[0019] There is disclosed herein design and synthesis of novel
liquid crystalline porphyrins that can be aligned to form an
ordered architecture. These aligned architectures can facilitate
charge transport in the direction along the columns, can be
processed to form a large area single crystalline thin film, can
respond to external light irradiation by changing their
resistivity, and can convert light to electric energy etc.
[0020] Light-harvesting discotic liquid crystalline porphyrins and
discotic crystalline porphyrin metal complexes, method for
preparation, and device fabrication are disclosed. The discotic
liquid crystalline porphyrins and metal complexes can be aligned
into an ordered architecture, in which the columns formed by
intermolecular stack are spontaneously perpendicular on the
surface, i.e. homeotropic alignment. The aligned architecture,
which is stable within a wide temperature range, can greatly
enhance the charge carrier mobility, and thus can dramatically
improve the light induced electric generation. Also, the discotic
liquid crystalline porphyrins and metal complexes can be aligned
into another ordered architecture, in which the columns formed by
intermolecular stack are spontaneously parallel on the surface,
i.e. homogenous alignment.
[0021] These compounds are capable of being used as high-efficiency
photovoltaic materials, organic semiconducting materials, and
organic light emitting materials. Materials with partially
perfluorinated alkyl group in the peripheral chains show a
particularly strong tendency towards the formation of homeotropic
alignment. Applications of such materials may be made in
electricity generation, photovoltaic devices, light emitting
devices, photosensors, transistors, memory arrays and the like.
[0022] These materials may be mixed with other electron-acceptor
materials such as, e.g., C.sub.60 or its derivatives, dyes, and/or
nanotubes. The blend, which can retain a homeotropically aligned
architecture, can be used for achieving a high conversion
efficiency of its heterojunction blend cell.
[0023] These materials and their blends may present advantages,
including large area single crystal domain and high charge carrier
mobility in the directions along columns.
[0024] The structures of the materials are porphyrin based
molecules with 4 side chain groups of 3,4,5-alkylphenyl connected
to a tetraphenylporphyrin core by linkage groups. The straight or
branched alkyl chains R may include perfluoroalkyl, C.sub.60
(fullerene), an aromatic moiety, one or more O, S, CO, COO, OOC,
--N.dbd.N-- and/or --C.ident.C-- linkages. In one embodiment, the
general structure is shown in FIG. 1.
[0025] In the disclosed structure of the exemplary light-harvesting
discotic liquid crystalline porphyrins of FIG. 1, L is a linkage
which may be COO, OOC, O, S, COS or NHCO;
R.sub.1.dbd.R.sub.2.dbd.R.sub.3.dbd.C.sub.6-36 linear or branched
alkoxy which may include perfluoroalkyl, C.sub.60 (fullerene), an
aromatic moiety, or one or more O, S, CO, COO, N.dbd.N and/or
C.dbd.C. Alternately, R.sub.1.dbd.H,
R.sub.2.dbd.R.sub.3.dbd.C.sub.6-36 linear or branched alkoxy which
may include perfluoroalkyl group, C.sub.60 (fullerene), aromatic
moiety, or one or more O, S, CO, COO, N.dbd.N and/or C.dbd.C. In
another embodiment, R.sub.2.dbd.H,
R.sub.1.dbd.R.sub.3.dbd.C.sub.6-36 linear or branched alkoxy which
may include perfluoroalkyl group, C.sub.60 (fullerene), an aromatic
moiety, or one or more O, S, CO, COO, N.dbd.N and/or C.dbd.C. In
still another embodiment, R.sub.1.dbd.R.sub.3.dbd.H,
R.sub.2.dbd.C.sub.6-36 linear or branched alkoxy which may include
perfluoroalkyl, C.sub.60 (fullerene), aromatic moiety, one or more
O, S, CO, COO, N.dbd.N and/or C.dbd.C. In a preferred embodiment,
the materials contain at least partially perfluorinated alkyl group
in the peripheral chains, which show a strong tendency towards the
formation of homeotropic alignment.
[0026] In a second main embodiment, the inventive materials are
discotic crystalline porphyrin metal complexes as shown in FIG. 2
wherein M=Zn, Co, Cu, Ni, Cr, Mn, Mg, Ce, Ru, Rh, Pt, Au, or
lanthanide metals and may be bound to a halogen atom, O, --OH or
.dbd.CO. As detailed above, these compounds are capable of being
used as high-efficiency photovoltaic materials, organic
semiconducting materials, and organic light emitting materials. In
a preferred embodiment, the materials contain at least partially
perfluorinated alkyl group in the peripheral chains, which show a
strong tendency towards the formation of homeotropic alignment.
[0027] The general procedure for the preparation of one class of
compounds according to one of the embodiments of this invention is
provided below. In a first method, the target compounds are
synthesized by 5,10,15,20-tetra(p-X-phenyl)porphyrin reacting with
3,4,5-tris-alkyl benzoic acid or benzoyl chloride in organic media.
The intermediate tetra(p-X-phenyl)porphyrin is prepared by
cyclocondensation of 4-X-benzaldehyde and pyrrole, wherein X and Y
are reactive groups that react to form target molecule I as defined
above (FIG. 1).
##STR00001##
[0028] In a second method, the target compounds are synthesized by
a cyclocondensation process in organic solvent or without solvent
as shown more generally below:
##STR00002##
[0029] Suitable organic solvents include, e.g., propionic acid,
propionic anhydride, pyrrole, methylene chloride, chloroform,
N,N-dimethylformamide, N-methylpyrrolidone, pyridine,
triethylamine, ether, tetrahydrofuran, alcohol, ethyl acetate,
acetonitrile, ethyl methyl ketone, saturated aliphatic hydrocarbons
and aromatic hydrocarbons.
[0030] A general procedure for the preparation of metal complexes
as shown in FIG. 2 is provided below. Metal complexes are
synthesized by metal-free porphyrin reacting with a metal salt.
##STR00003##
[0031] The metal atom linked to porphyrin may enhance mesophase,
optical and electric properties, and affect the charge
transportation process.
[0032] The present porphyrin molecules can be aligned into an
ordered architecture, in which the columns formed by intermolecular
.pi.-.pi. stacks are spontaneously perpendicular or parallel on the
surface, i.e. homeotropic alignment or homogenous alignment. The
ordered aligned archtecture, which is stable, can excellently
enhance the charge carrier mobility, and thus can dramatically
improve the light induced electric generation.
[0033] The intermolecular interaction between discotic mesogens
might mainly come from 1) .pi. conjugated, core-core attraction,
and 2) hydrophobic interaction between the flexible chains.
[0034] As described above, the materials with partially
perfluorinated alkyl groups in the peripheral chains show a strong
tendency towards the formation of homeoptropic alignment. This is
thought to be due to the fact that the fluorine atom is unique in
that its size is only little larger than hydrogen, while it has the
highest electronegativity among all atoms. This unique combination
of steric and polarity effects enables some significant tuning of
physical properties without much disruption to the liquid crystal
phase stability.
[0035] Generally the efficiency of the charge-generation process is
extremely low when a single material forms the organic layer. This
is because in conjugated materials the binding energy of the lowest
singlet exciton (i.e., the strength of the Coulombic attraction
between the electron and hole) is significant; this makes excitons
(electron and hole) rather stable species. As a result, current
organic solar cells rely on either blends made from an
electron-donor component and an electron-acceptor component, or
double- or multi-layered heterojunction structures. Porphyrin,
which is the basic structure of chlorophyll, is a superior electron
donor (p-type material). A suitable electron acceptor (n-type
material) for use therewith may be dye, carbon nanotube, and/or
C.sub.60 (fullerene) or a derivative thereof. For example, C.sub.60
or its derivative is an excellent electron acceptor, so liquid
crystalline porphyrin-C.sub.60 blend with aligned architecture
would make a perfect marriage. The porphyrin absorbs the light and
transfers an electron from its excited state to n-type material
e.g. C.sub.60, C.sub.60 derivative or carbon nanotube.
[0036] With regard to photovoltaic cells, in general, crystalline
molecular organic materials exhibit better transport properties
than their polymeric counterparts. However, large single crystals
are difficult and costly to process, while polycrystalline
materials suffer from the grain boundaries and defects. This
disadvantage can be overcome by utilizing the discotic liquid
crystals capable of being spontaneously homeotropically aligned,
because their aligned structure resembles the aromatic stacking in
single crystal domains.
[0037] Discotic liquid crystals have recently been used as
hole-transporting layer to construct an organic photovoltaic cell;
however, their alignment is a crucial point for the resulting high
conductivity and possible applications. We have achieved two
important alignments on our synthesized liquid crystalline
porphyrins disclosed here, namely homogeneous and homeotropic
alignments. Homeotropically aligned architecture can provide a most
efficient path for electrons and holes along the columnar axis
which is most favorable for high conductivity and its applications
in photovoltaic cells and organic light emitting diodes etc.
Homogenously aligned architecture has potential applications such
as organic thin-film transistors.
[0038] In one embodiment, the present discostic liquid crystals are
used to form a photovoltaic cell. As seen in FIG. 3, the structure
of a photovoltaic cell 10 includes at least one photoactive layer
12 sandwiched between first 14 and second 16 electrodes, the first
of which is transparent or substantially transparent. In an
embodiment, the photoactive heterojunction blend layer, or double-,
or multi-layers is sandwiched between the two electrodes with
different work functions. For improved cell performance, the cell
is either a bulk heterojunction cell with homeotropically aligned
architecture of the blend composed of a discotic liquid crystal and
one or more other components which may be any material such as
C.sub.60, its derivative, a dye or a carbon nanotube, or a
double-layered cell in which a donor layer is homeotropically
aligned architecture of a discotic liquid crystal or
homeotropically aligned architecture of its blend thereof together
with an acceptor layer which may composed of any material such as
C.sub.60, its derivative, a dye or carbon nanotube. Also, for
improved solar cell performance the number of layers and junctions
can be multiple.
[0039] In one embodiment, the electrodes are positioned on first
and second substrates 18, 20.
[0040] Thus, in one embodiment, substrates which are transparent
and have insulating properties, such as a glass plate, quartz
plate, plastic plate or other organic polymers, can be used as the
first transparent substrate 18. The transparent electrode
positioned on a surface of the transparent substrate can be
composed of common electrodes such as those of indium tin oxide
(ITO), tin oxide doped with Sb, F or P, indium oxide doped with Sn,
Zn and/or F, antimony oxide, zinc oxide and noble metals, which may
be coated with a transparent conductive polymer layer for hole
collection.
[0041] The non-transparent substrate 20 may be a combined
substrate/electrode and can be formed of metals such as titanium,
aluminum, copper, silver, gold and nickel, and which may also be
coated with an interface layer; or conducting metal oxide, such as
zinc oxide, titanium oxide, etc; or conducting polymer.
Alternately, a separate electrode can be positioned on a
non-conducting substrate.
[0042] For example, the electrode material can be any of platinum,
rhodium, metallic ruthenium and- ruthenium oxide. Further,
conductive materials, such as tin oxide, tin oxide doped with Sb, F
or P, indium oxide doped with Sn and/or F and antimony oxide,
having their surfaces overlaid with the above electrode materials
by plating or vapor deposition can also be used as the electrode
layer. Still further, common electrodes, such as carbon electrode,
can be used for constituting the electrode layer.
[0043] As discussed above, the photoactive layer of the
photovoltaic cell generally includes either one heterojunction p-n
blend layer or two distinct layers forming a p-n junction.
[0044] In one embodiment, one photovoltaic cell comprises the
homeotropically aligned heterojunction blend made from a discotic
liquid crystal and one or more other components which may be any
material such as C.sub.60, its derivative, a dye and/or a carbon
nanotube (e.g. 22a). Another photovoltaic cell comprises double
layers in which homeotropically aligned discotic liquid crystal or
its blend is one necessary layer, and C.sub.60, its derivatives,
dyes or carbon nanotubes as other layer (e.g. 22b). The number of
layers and junctions can be more than one. In a specific
embodiment, the cell is a dye sensitized device wherein the
photoactive layer includes one or more dyes and a discostic liquid
crystal material 22 associated with the dye.
[0045] The photosensitizing agent can be sorbed (e.g., chemisorbed
and/or physisorbed) on the nanoparticles. The photosensitizing
agent may be sorbed on the surfaces of the nanoparticles, within
the nanoparticles, or both. The photosensitizing agent is selected,
for example, based on its ability to absorb photons in a wavelength
range of operation (e.g., within the visible spectrum), its ability
to produce free electrons (or holes) in a conduction band of the
nanoparticles, and its effectiveness in complexing with or sorbing
to the nanoparticles. Suitable photosensitizing agents may include,
for example, dyes that include functional groups, such as carboxyl
and/or hydroxyl groups.
[0046] Examples of dyes include black dyes (e.g.,
tris(isothiocyanato)-ruthenium
(II)-2,2':6',2''-terpyridine-4,4',4''-tricarboxylic acid,
tris-tetrabutylammonium salt), orange dyes (e.g.,
tris(2,2'-bipyridyl-4,4'-dicarboxylato)ruthenium (II) dichloride,
purple dyes (e.g.,
cis-bis(isothiocyanato)bis-(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium
(II)), red dyes (e.g., an eosin), green dyes (e.g., a merocyanine)
and blue dyes (e.g., a cyanine). Examples of additional dyes
include anthocyanines, perylenes, porphyrins, phthalocyanines,
squarates, and certain metal-containing dyes.
[0047] The discotic liquid crystal or its mixture with any
electron-acceptor material such as C.sub.60 derivative, carbon
nanotube or dye is sandwiched in between these two substrates and
aligned homeotropically. With further detail, the two electrodes
are glued or otherwise attached and sealed to form a cell.
Depending on the method of filling the cell, a small slit may be
maintained for liquid crystal uptake. A typical gap thickness
between the two electrodes is about 0.01-10 .mu.m. The liquid
crystal is then deposited inside the cell using known methods.
After dryness of the film, the other electrode is laminated on top
of the film to form a cell.
[0048] In one embodiment, the discotic liquid crystal as the
hole-transporting layer and a photosensitizing agent as electron
transporting layer may be prepared in a solvent and spin-coated
onto an indium tin oxide electrode. Suitable solvents may be, e.g.,
water, alcohols, oligoethers, carbonates such as propione
carbonate, phosphoric esters, dimethylformamide, dimethyl
sulfoxide, N-methylpyrrolidone, N-vinylpyrrolidone, sulfur
compounds such as sulfolane 66, ethylene carbonate, methylene
chloride, chloroform, chlorobenzene, toluene, acetonitrile and
.gamma.-butyrolactone.
[0049] Alternately, in another embodiment, the discotic liquid
crystal or its blend may be heated to melting inside a vacuum
chamber. The cell is then placed in the vacuum chamber to remove
any air inside the cell. To fill the cell, the opening slit of the
cell is dipped into the melted material. The vacuum level is then
slowly reduced to allow the cell to uptake the material. Of course,
other methods of filling the cell are also possible.
[0050] In specific embodiments, the photovoltaic device is composed
of an ITO coated transparent electrode and an aluminum, copper,
silver or gold coated reflective electrode. In specific embodiment
of the invention, the transparent substrate can be glass or
plastic. In specific embodiment of the invention, the alignment of
the liquid crystalline porphyrin or its blend is homeotropic.
[0051] With patterned and individually addressable electrode on
certain substrate, the claimed liquid crystal material could be
prepared in the form of a film on top of these patterned electrode
substrate in the same way as mentioned. The photosensitive
resistance plus the photo-voltage produced at different site of the
substrate can map the intensity of the object in front of the film.
In this way, the liquid crystal material can be used as a
photo-image receiver.
[0052] In one embodiment, a small area solar cell can act as a
simple photosensor in conjunction with a Schmidt trigger circuit,
which can set a tunable threshold voltage for detection and act as
a photosensor.
EXAMPLES
[0053] In accordance with the present embodiments, a series of
novel light-harvesting discotic liquid crystalline porphyrins (I)
and discotic liquid crystalline porphyrin metal complexes (II) were
synthesized and characterized.
[0054] The structures of the following compounds are shown in FIGS.
4-9. C.sub.228H.sub.274F.sub.84N.sub.4O.sub.20 (FIG. 4),
C.sub.228H.sub.272F.sub.84N.sub.4O.sub.20Zn (FIG. 5),
C.sub.228H.sub.272F.sub.84N.sub.4O.sub.20Cu (FIG. 6),
C.sub.228H.sub.356N.sub.4O.sub.20Zn (FIG. 7),
C.sub.228H.sub.356N.sub.4O.sub.20Cu (FIG. 8), and
C.sub.228H.sub.356N.sub.4O.sub.20Ni (FIG. 9). Their structures were
identified and confirmed by .sup.1H NMR, .sup.13C NMR, elemental
analysis and MS. The UV-vis absorption spectrum of the compound of
FIG. 5 in CH.sub.2Cl.sub.2 was measured and is shown in FIG. 10.
The very strong absorption at about 420 nm enables it to be a very
efficient absorber for blue photons. Another absorption peak at 549
nm enables this material to function as a good absorber for a large
spectrum of sunlight. The fluorescent emission of this same
compound was determined and is shown in FIG. 11 under a 420 nm
excitation wavelength. Peaks can be seen at 595 and 643 nm. One of
the exemplary compounds can be homeotropically aligned into an
ordered architecture cooling from its isotropic phase, as shown in
FIG. 12. The dark areas in FIG. 12 represent homeotropic alignment.
The bright domains represent defects which appear where the
porphyrin plane is not parallel to the substrate. Cooling of the
material from isotropic liquid at a specific rate can provide a way
to achieve a defect-free single crystal thin film.
[0055] It is to be understood that the principles of the present
embodiments are applicable to any type of device that uses
homeotropically or homogenously aligned architecture of any a
discotic liquid crystal or a blend composed of a discotic liquid
crystal and one or more other components as a layer.
[0056] The exemplary embodiment has been described with reference
to the preferred embodiments. Obviously, modifications and
alterations will occur to others upon reading and understanding the
preceding detailed description. It is intended that the exemplary
embodiment be construed as including all such modifications and
alterations insofar as they come within the scope of the appended
claims or the equivalents thereof.
* * * * *