WO2013049019A1

WO2013049019A1 - Antenna ligands for dye-sensitized solar cells

Info

Publication number: WO2013049019A1
Application number: PCT/US2012/057047
Authority: WO
Inventors: Ahmed El-Shafei
Original assignee: North Carolina State University
Priority date: 2011-09-26
Filing date: 2012-09-25
Publication date: 2013-04-04

Abstract

Antenna ligands for use in dye-sensitized solar cells are described. The antenna ligands consist of a bidentate or tridentate heteroaromatic metal-chelating group having at least one independently selected antenna group substituted thereon, each antenna group being an independently selected substituent of the formula -R'₁-Ar' or -R'₁-Ar'-R₂-Ar'₂, wherein: R'₁ is an aliphatic group of the formula -(CH=CH)_n-, where n is from 1 to 3; Ar' is an aromatic group; in some embodiments a heteroaromatic group; R'₂ is a covalent bond or a C1 to C15 linear or branched, saturated or unsaturated alkyl; and Ar'₂ is an aromatic group. Organometallic complexes containing such ligands and dye-sensitized solar cells containing the same are also described.

Description

ANTENNA LIGANDS FOR DYE-SENSITIZED SOLAR CELLS

Ahmed El-Shafei

Field of the Invention

The present invention concerns antenna ligands for dye-sensitized solar cells (DSSCs), metal complexes and solar cells containing the same, and methods of making the same.

Background of the Invention

Dye-sensitized solar cells, or DSSCs, are regenerative photo-electrochemical cells comprising a photoanode, a counter-electrode, and an electrolyte positioned between these electrodes. The principle components of DSSCs are a photosensitizing dye, an electrolyte {e.g., I7I₃ ^"), and a semiconductor, typically in the nanocrystalline form {e.g., Ti0₂). The photoanode is sensitized by the dye or pigment. See, e.g., US Patent Nos. 5,350,644 to Graetzel et al. and 7,932,404 to Zakeeruddin et al.

The solar-to-electric conversion of the cell is dependent on the number of photons harvested across the solar radiation spectrum, and the electron injection from the sensitizer into the semiconductor, which is controlled by the redox potentials of both the sensitizer and electrolyte and the location of the conduction band edge of the semiconductor with respect to the LUMO of the sensitizer, respectively. In one non-limiting example, when the cell is exposed to a light source, electrons are excited from the ground state of the sensitizer to its excited state, then injected into the conduction band of Ti0₂ as shown in Figure 1.

The photosensitizing dye is adsorbed on the photoanode {e.g., nanocrystalline Ti0₂) and is in contact with the electrolyte. However, not all injected electrons into the conduction band of Ti0₂ travel forward to the transporting electrode to the outside circuit to the counter electrode owing to possible charge recombinations at the interfaces between the injected electron/oxidized state of the sensitizer and/or injected electron/electrolyte. Hence, the needs for novel generations of sensitizers that can be molecularly engineered to harvest light across a wide range of the solar radiation spectrum covering the visible and near infra red region up to 1030nm with high optical extinction coefficients coupled with efficient electron injection into the CB and minimum charge recombination(s) are highly desirable. Photosensitizing dyes can be metal-free or metal-complex dyes. Metal complex dyes are known to exhibit significantly better photostability and solar-to-electric conversion compared to metal -free dyes. Metal-complex dyes such as ruthenium-based are known to outperform other transition metals including Cu, Fe, and Os. They typically consist of Ru and an "anchoring" ligand with or without an "antenna" ligand. The anchoring ligand serves to secure the chromophore to the photoanode, and the antenna ligand serves to enhance light harvesting across a wider range of the solar radiation spectrum and to significantly improve the molar absorptivity for efficient light to electrical energy.

Problems with dye-sensitized solar cells include, but are not limited to, inability to harvest photons across a wide range of the solar radiation spectrum (high and low energetic photons), particularly the red region of visible spectrum, inefficient electron injection from the excited state of the dye into the conduction band edge of the semiconductor (Ti0₂), and inferior molar absorptivity. Hence, judicious molecular engineering of sensitizers through fine tuning of antenna molecular structure can address all the aforementioned drawbacks for dye sensitized solar cells, so that novel generation of dyes with the oxidation potential of the ground and excited states thermodynamically favorable can be synthesized for dye sensitized solar cells. Accordingly, there is an immediate need for novel generations of antenna ligands for use in the developments of a novel generation of more efficient and robust dyes for dye- sensitized solar cells.

Summary of the Invention

A first aspect of the invention is also an antenna ligand for use in dye-sensitized solar cells. The antenna ligands are, in general, bidentate or tridentate heteroaromatic metal chelating compounds having at least one (e.g. one, two, three or four) independently selected antenna group substituted thereon, each antenna group being an independently selected substituent of the formula -R' ^Ar' -R'₂-Ar'₂, wherein:

R'i is an aliphatic group of the formula -(CH=CH)_n~, where n is from 1 to 3;

Ar' is an aromatic group;

R'₂ is a covalent bond or a CI to CI 5 linear or branched, saturated or unsaturated alkyl; and

Ar'₂ is an aromatic group.

A second aspect of the invention is also an antenna ligand for use in dye-sensitized solar cells. The antenna ligands are, in general, bidentate or tridentate heteroaromatic metal chelating compounds having at least one (e.g. one, two, three or four) independently selected antenna group substituted thereon, each antenna group being an independently selected substituent of the formula -R'i-Ar', wherein:

R'l is an aliphatic group of the formula -(CH=CH)_n-, where n is from 1 to 3; and

Ar' is a heteroaromatic group.

A further aspect of the invention is metal chelates comprising antenna ligands as described above, and in further detail below.

A still further aspect of the invention is dye-sensitized solar cells comprising or containing antenna ligands, or metal chelates, as described above, and described in further detail below.

The present invention is explained in greater detail in the drawings herein and the specification set forth below. The disclosures of all United States patent references cited herein are to be incorporated by reference herein in their entirety.

Brief Description of the Drawings

Figure 1. Schematic representation of the various components of dye-sensitized nanocrystalline semiconductor solar cell.

Figure 2. IPCE for NCIO (dashed line) and NC-11 (dotted line) compared to known dye N719 (dotted/dashed line) (cw-di(thiocyanato)bis(2,2'-bipyridyl-4,4'- dicarboxylate)ruthenium(II) or di-tetrabutylammonium cw-bis(isothiocyanato)bis(2,2'- bipyridyl-4,4'-dicarboxylato) ruthenium(II)

Detailed Description of the Preferred Embodiments

"Aliphatic group" as used herein includes, but is not limited to, alkyl, alkenyl, alkynyl, and cycloalkyl groups, and combinations thereof such as alkylcycloalkyl; cycloalkylalkyl; etc.

"Alkyl" as used herein alone or as part of another group, refers to a straight or branched chain hydrocarbon containing from 1 to 10 carbon atoms. Representative examples of alkyl include, but are not limited to, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, n-hexyl, 3-methylhexyl, 2,2- dimethylpentyl, 2,3-dimethylpentyl, n-heptyl, n-octyl, n-nonyl, n-decyl, and the like. "Lower alkyl" as used herein, is a subset of alkyl, in some embodiments preferred, and refers to a straight or branched chain hydrocarbon group containing from 1 to 4 carbon atoms. Representative examples of lower alkyl include, but are not limited to, methyl, ethyl, n- propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, and the like. The term "akyl" or "loweralkyl" is intended to include both substituted and unsubstituted alkyl or loweralkyl unless otherwise indicated and these groups may be substituted with groups selected from halo (e.g., haloalkyl), alkyl, haloalkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heterocyclo, heterocycloalkyl, hydroxyl, alkoxy (thereby creating a polyalkoxy such as polyethylene glycol), alkenyloxy, alkynyloxy, haloalkoxy, cycloalkoxy, cycloalkylalkyloxy, aryloxy, arylalkyloxy, heterocyclooxy, heterocyclolalkyloxy, mercapto, alkyl-S(0)_m, haloalkyl-S(0)_m, alkenyl-S(0)_m, alkynyl-S(0)_m, cycloalkyl-S(0)_m, cycloalkylalkyl-S(0)_m, aryl-S(0)_m, arylalkyl-S(0)_m, heterocyclo-S(0)_m, heterocycloalkyl-S(0)_m, amino, carboxy, alkylamino, alkenylamino, alkynylamino, haloalkylamino, cycloalkylamino, cycloalkylalkylamino, arylamino, arylalkylamino, heterocycloamino, heterocycloalkylamino, disubstituted-amino, acylamino, acyloxy, ester, amide, sulfonamide, urea, alkoxyacylamino, aminoacyloxy, nitro or cyano where m= 0, 1, 2 or 3.

"Alkenyl" as used herein alone or as part of another group, refers to a straight or branched chain hydrocarbon containing from 1 to 10 carbon atoms (or in loweralkenyl 1 to 4 carbon atoms) which include 1 to 4 double bonds in the normal chain. Representative examples of alkenyl include, but are not limited to, vinyl, 2-propenyl, 3-butenyl, 2-butenyl, 4- pentenyl, 3-pentenyl, 2-hexenyl, 3-hexenyl, 2,4-heptadiene, and the like. The term "alkenyl" or "loweralkenyl" is intended to include both substituted and unsubstituted alkenyl or loweralkenyl unless otherwise indicated and these groups may be substituted with groups as described in connection with alkyl and loweralkyl above.

"Alkynyl" as used herein alone or as part of another group, refers to a straight or branched chain hydrocarbon containing from 1 to 10 carbon atoms (or in loweralkynyl 1 to 4 carbon atoms) which include 1 triple bond in the normal chain. Representative examples of alkynyl include, but are not limited to, 2-propynyl, 3-butynyl, 2- butynyl, 4-pentynyl, 3- pentynyl, and the like. The term "alkynyl" or "loweralkynyl" is intended to include both substituted and unsubstituted alkynyl or loweralknynyl unless otherwise indicated and these groups may be substituted with the same groups as set forth in connection with alkyl and loweralkyl above.

"Cycloalkyl" as used herein alone or as part of another group, refers to a saturated or partially unsaturated cyclic hydrocarbon group containing from 3, 4 or 5 to 6, 7 or 8 carbons (which carbons may be replaced in a heterocyclic group as discussed below). Representative examples of cycloalkyl include, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl. These rings may be optionally substituted with additional substituents as described herein such as halo or loweralkyl. The term "cycloalkyl" is generic and intended to include heterocyclic groups as discussed below unless specified otherwise.

"Heterocyclic group" or "heterocyclo" as used herein alone or as part of another group, refers to an aliphatic (e.g., fully or partially saturated heterocyclo) or aromatic (e.g., heteroaryl) monocyclic- or a bicyclic-ring system. Monocyclic ring systems are exemplified by any 5 or 6 membered ring containing 1, 2, 3, or 4 heteroatoms independently selected from oxygen, nitrogen and sulfur. The 5 membered ring has from 0-2 double bonds and the 6 membered ring has from 0-3 double bonds. Representative examples of monocyclic ring systems include, but are not limited to, azetidine, azepine, aziridine, diazepine, 1 ,3-dioxolane, dioxane, dithiane, furan, imidazole, imidazoline, imidazolidine, isothiazole, isothiazoline, isothiazolidine, isoxazole, isoxazoline, isoxazolidine, morpholine, oxadiazole, oxadiazoline, oxadiazolidine, oxazole, oxazoline, oxazolidine, piperazine, piperidine, pyran, pyrazine, pyrazole, pyrazoline, pyrazolidine, pyridine, pyrimidine, pyridazine, pyrrole, pyrroline, pyrrolidine, tetrahydrofuran, tetrahydrothiophene, tetrazine, tetrazole, thiadiazole, thiadiazoline, thiadiazolidine, thiazole, thiazoline, thiazolidine, thiophene, thiomorpholine, thiomorpholine sulfone, thiopyran, triazine, triazole, trithiane, and the like. Bicyclic ring systems are exemplified by any of the above monocyclic ring systems fused to an aryl group as defined herein, a cycloalkyl group as defined herein, or another monocyclic ring system as defined herein. Representative examples of bicyclic ring systems include but are not limited to, for example, benzimidazole, benzothiazole, benzothiadiazole, benzothiophene, benzoxadiazole, benzoxazole, benzofuran, benzopyran, benzothiopyran, benzodioxine, 1 ,3- benzodioxole, cinnoline, indazole, indole, indoline, indolizine, naphthyridine, isobenzofuran, isobenzothiophene, isoindole, isoindoline, isoquinoline, phthalazine, purine, pyranopyridine, quinoline, quinolizine, quinoxaline, quinazoline, tetrahydroisoquinoline, tetrahydroquinoline, thiopyranopyridine, and the like. These rings include quaternized derivatives thereof and may be optionally substituted with groups selected from halo, alkyl, haloalkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heterocyclo, heterocycloalkyl, hydroxyl, alkoxy, alkenyloxy, alkynyloxy, haloalkoxy, cycloalkoxy, cycloalkylalkyloxy, aryloxy, arylalkyloxy, heterocyclooxy, heterocyclolalkyloxy, mercapto, alkyl-S(0)_m, haloalkyl-S(0)_m, alkenyl- S(0)_m, alkynyl-S(0)_m, cycloalkyl-S(0)_m, cycloalkylalkyl- S(0)_m, aryl-S(0)_m, arylalkyl- S(0)_m, heterocyclo-S(0)_m, heterocycloalkyl-S(0)_m, amino, alkylamino, alkenylamino, alkynylamino, haloalkylaniino, cycloalkylamino, cycloalkylalkylamino, arylamino, arylalkylamino, heterocycloamino, heterocycloalkylamino, disubstituted-amino, acylamino, acyloxy, ester, amide, sulfonamide, urea, alkoxyacylamino, aminoacyloxy, nitro or cyano where m = 0, 1, 2 or 3.

"Aryl" as used herein alone or as part of another group, refers to a monocyclic carbocyclic ring system or a bicyclic carbocyclic fused ring system having one or more aromatic rings. Representative examples of aryl include, azulenyl, indanyl, indenyl, naphthyl, phenyl, tetrahydronaphthyl, and the like. The term "aryl" is intended to include both substituted and unsubstituted aryl unless otherwise indicated and these groups may be substituted with the same groups as set forth in connection with alkyl and loweralkyl above.

"Heteroaryl" as used herein is as described in connection with heterocyclo above.

"Alkoxy" as used herein alone or as part of another group, refers to an alkyl or loweralkyl group, as defined herein (and thus including substituted versions such as polyalkoxy), appended to the parent molecular moiety through an oxy group, -0-. Representative examples of alkoxy include, but are not limited to, methoxy, ethoxy, propoxy, 2-propoxy, butoxy, tert-butoxy, pentyloxy, hexyloxy and the like.

"Halo" as used herein refers to any suitable halogen, including -F, -CI, -Br, and -I.

"Amino" as used herein means the radical -NH₂.

"Alkylamino" as used herein alone or as part of another group means the radical - NHR, where R is an alkyl group.

"Arylalkylamino" as used herein alone or as part of another group means the radical - NHR, where R is an arylalkyl group.

"Disubstituted-amino" as used herein alone or as part of another group means the radical -NR_aR_b, where R_a and R_b are independently selected from the groups alkyl, haloalkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heterocyclo, heterocycloalkyl.

Antenna groups and antenna ligands.

As noted above, a first aspect of the invention is an antenna ligand for use in a dye- sensitized solar cell, the antenna ligand consisting of a bidentate or tridentate heteroaromatic metal-chelating group having at least one (e.g., one, two, three or four) independently selected antenna groups substituted thereon, at least one of said antenna groups (e.g., one, two, three or four) being an independently selected substituent of the formula -RVAr', wherein: R'i is an aliphatic group of the formula -(CH=CH)_n-, where n is from 1 to 3; and Ar' is a heteroaromatic group.

A second aspect of the invention is an antenna ligand for use in a dye-sensitized solar cell, the antenna ligand consisting of a bidentate or tridentate heteroaromatic metal-chelating group having at least one (e.g., one, two, three or four) independently selected antenna groups substituted thereon, at least one of said antenna groups (e.g., one, two, three or four) being an independently selected substituent of the formula -RVAr'-RYAr^, wherein:

R'i is an aliphatic group of the formula -(CH=CH)_n-, where n is from 1 to 3;

Ar' is an aromatic group;

Ar'₂ is an aromatic group.

In some embodiments, the antenna group has or comprises a "core" bidentate or tridentate heteroaromatic metal-chelating group structure selected from the group consisting of Formulas A-J:

wherein:

from one to four of R_l5 R₂, R₃, R4, R₅, Rg, R₇, R₈, R₉, R₁₀, Rn, R₁₂, and R¹³, when present, is the antenna group; and

the remainder of R_1; R₂, R₃, R4, R₅, R<;, R₇, R₈, R9, Rio, Rn, R₁₂, and R¹³, when present, are selected from the group consisting of H, halo, loweralkyl, alkoxy, amino, aminoalkyl, Ν,Ν-dialkylamino, and N,N-diarylamino.

In some embodiments of the foregoing, each Ar' is independently selected from the group consisting of carbazolyl, 5H-benzo[b]carbazole, indolyl, biindolyl, -2-(lH-indol-2-yl)- lH-benzo[fjindole, dibenzo[b,d]thiophene, benzo[b]naphtho[2,3-d]thiophene, dibenzo[b,d]furan, and benzo[b]naphtho[2,3-d]furan.

In some embodiments of the foregoing, each Ar'₂ is independently selected from the group consisting of phenyl, naphthyl, anthracyl, and pyrenyl, and perylenyl.

In some embodiments of the foregoing, the antenna group has the structure:

Thus, in some embodiments of the foregoing, the antenna ligand has the structure:

Anchoring ligands and metal complexes.

For use in a dye-sensitized solar cell, antenna ligands as described herein are typically complexed with a metal and at least one anchoring ligand. In general, such complexes comprise or consist of:

( ) a metal selected from the group consisting of Ru, Os, Ir, Re, Rh, and Fe,

(b) from one to two bidentate or tridentate organic anchoring ligands {e.g., as described below);

(c) from one to two bidentate or tridentate organic antenna ligands {e.g., as described above), and

(d) from zero to two monodentate ligands selected from the group consisting of H₂0, halo, cyano, -NCO-, -NCS, and -NCSe.

Anchoring ligands of the present invention are, in general, a bidentate or tridentate heteroaromatic metal-chelating group having at least one {e.g., one, two, three or four) independently selected anchoring groups substituted thereon. Anchoring ligands are known and include, but are not limited to, those described in Zakeeruddin et al, PCT Application Publication No. WO 2010/055471. Suitable anchoring groups for anchoring ligands include, but are not limited to, -COOH, -P0₃H₂, -P0₄H₂, -S0₃H₂, -S0₄H₂, -CONHOH, and deprotonated forms thereof.

Anchoring ligands for use in complexes of the present invention include, but are not limited to, compounds of Formulas A- J as given above, wherein: from one to four of R_ls R₂, R₃, R4, R₅, R₆, R₇, R₈, R9, Rio, Rn, R₁₂, and R¹³, when present, are independently selected anchoring groups; and the remainder of R_ls R₂, R₃, R^, R₅, R^, R₇, R₈, R9, Rio, R11, R12, and R¹³, when present, are selected from the group consisting of H, halo, loweralkyl, alkoxy, amino, aminoalkyl, Ν,Ν-dialkylamino, and N,N-diarylamino.

Examples of metal complexes of the present invention include, but are not limited to:

and

Dye-sensitized solar cells.

Dye-sensitized solar cells of the present invention can be prepared from the antenna groups and metal complexes described above in accordance with known techniques or variations thereof that will be apparent to those skilled in the art. See, e.g., US Patent Nos. 5,350,644 to Graetzel et al. and 7,932,404 to Zakeeruddin et al; see also US Patent Nos. 6,479,745; 7,019,209; 7,118,936; 7,126,054; 7,145,071 ; 7,157,788; 7,332,782; and 7,989,691.

Thus the present invention provides a regenerative photoelectrochemical cell comprising a photoanode, said photoanode comprising at least one semi-conductive metal oxide layer on a conductive substrate, sensitized by a photosensitizing dye, a counter electrode and an electrolyte arranged between said semi-conductive metal oxide layer and said counter electrode, wherein the photosensitizing dye is an organometallic complex as described above.

In some embodiments, an amphiphilic compacting compound whose molecular structure comprises at least one anchoring group, a hydrophobic portion and a terminal group is co-adsorbed with said photosensitizing dye on said semi-conductive metal oxide layer in a mixed monolayer. In some embodiments, the compacting compound is selected from the group consisting of alkyl carboxylic acids, alkyl dicarboxylic acids, alkyl carboxylates, alkyl phosphonic acids, alkyl phosphonates, alkyl diphosphonic acids, alkyl diphosphonates, alkyl sulphonic acids, alkyl sulphonates, alkyl hydroxamic acids and alkyl hydroxamates, wherein said alkyl is linear or branched from CI to C20.

In some embodiments, the molar ratio of said photosensitizing dye to said co- adsorbed compacting compound is of between 10 and 1/2, and in that said self-assembled monolayer is a dense packed monolayer having an order-disorder transition temperature above 80 degree C.

In some embodiments, the length of said hydrophobic chain portion of the compacting compound allows said terminal group to protrude above the sensitizing dye in said monolayer.

In some embodiments, the electrolyte comprises a redox system, that said redox system comprises an electrochemically active salt and a first compound forming a redox couple with either the anion or the cation of said electrochemically active salt, wherein said salt is a room temperature molten salt, said molten salt being liquid at least between standard room temperature and 80 degree C above said room temperature.

In some embodiments, the electrolyte further comprises a polar organic solvent having a boiling point of 100 degree C or greater than 100 degree C at normal atmospheric pressure.

In some embodiments, the solvent is a nitrile selected from 3-methoxypropionitrile 3- ethoxypropionitrile,3-butoxupropionitrile, and butyronitrile.

In some embodiments the electrolyte further comprises, as an additive, a compound formed by a neutral molecule comprising one or more nitrogen atom(s) with a lone electron pair.

The present invention is explained in greater detail in the following non-limiting Examples.

EXAMPLE 1

Synthesis of the antenna ligand

4,4'-bis((E)-2-(9-ethyl-9H-carbazol-3-yl)vinyl)-2,2'-bipyridine for NC-10

The antenna ligand for NC-10 was synthesized in a pressure tube containing 4,4'- dimethyl-2,2'-bipyridine (2g, 0.0108 mol) 9-ethyl-9H-carbazole-3-carbaldehyde (4.86g, 0.0217mol) and 0.065mol of trimethylchlorosilane, and a magnetic stirrer bar in 70ml DMF. The reaction temperature was raised to 100 °C and allowed to run for 48 hours with continuous stirring. During the course of the reaction, the color of the reaction mixture changed to yellow and turned orange on cooling and release of pressure from the tube. The solvent was removed using rotary evaporator, and the orange product was recovered by addition of water and filtration under vacuum to furnish the ancillary ligand in 81% yield. Scheme 1 shows a schematic of the synthesis of the antenna ligand for NC-10.

Scheme 1 Synthesis of antenna ligand for NC-10.

EXAMPLE 2

Reaction of the antenna ligand with 2,2'-bipyridine-4,4'-dicarboxylic acid

and dichloro-(p-cymene)-Ruthenium (II) dimer to furnish NC-10

The Synthesis of NC-10 was carried out in a one-pot three-step reaction, outlined by schematic diagram in Scheme 2. The reactions were carried out in a 250 ml reaction flask equipped with a condenser and magnetic stirrer bar under Argon. The flask was charged with anhydrous DMF, dichloro-( -cymene)-Ruthenium (II) dimer (0.2g, 3.265xl0^"4mol) and 4,4'- bis((E)-2-(9-ethyl-9H-carbazol-3-yl) vinyl-2,2'-bipyridine (0.388g, 6.53xl0^_4mol). The reaction mixture was stirred at 90 °C for 4h. After 4 hours 2,2'-bipyridyl-4,4'-dicarboxalic acid was added (0.159g, 6.53xl0^"4mol) and the temperature was raised to 130°C and allowed to run for 5 hours. After the 5 hours, excess of NH₄NCS was added to the reaction mixture, and the reaction mixture was allowed to run for extra 4h at 130°C. The last two steps of the reaction were monitored for completion by taking aliquots from the reaction mixture every 60 minutes and measuring its absorption spectrum until there was no increase in the absorbance of the MLCT peak with respect to the π-π* peak. The reaction mixture was cooled down to room temperature and DMF was removed using a rotary evaporator. Water was added to the flask and the insoluble solid was vacuum filtered and washed with de-ionized water and ether. The product was dried overnight to give a yield of 80%.

Scheme 2 Synthesis for NC-10. EXAMPLE 3

Synthesis of the antenna ligand

4,4'-bis(^-2-(9-benzyl-9iT-carbazol-3-yl)vinyl)-2,2^,-bipyridine for NC-11

The antenna ligand for NC-11 was synthesized in a pressure tube containing 4,4'- dimethyl-2,2'-bipyridine (2g, 0.0108 mol) and 9-benzyl- H-carbazole-3-carbaldehyde (6.19g, 0.0217mol) and 0.065mol of trimethylchlorosilane, and magnetic stirrer bar in 70ml DMF. The reaction temperature was raised to 100°C and allowed to run for 48 hours with continuous stirring. During the course of the reaction, the color of the reaction mixture changed to yellow and turned orange on cooling and release of pressure from the tube. The solvent was removed using rotary evaporator, and the orange product was recovered by addition of water and filtration under vacuum to furnish the ancillary ligand in 84% yield. Scheme 3 shows a schematic of the synthesis of the antenna ligand for NC-11.

Scheme 3 Synthesis for the antenna ligand for NC-11. EXAMPLE 4

Reaction of the antenna ligand with 2,2'-bipyridine-4,4'-dicarboxylic acid

and dichloro-(p-cymene)-Ruthenium (II) dimer to furnish NC-11 The Synthesis of NC-10 was carried out in a one-pot three-step reaction, outlined by schematic diagram in Scheme 4:

Scheme4. Synthesis for NC-11.

The reactions were carried out in a 250 ml reaction flask equipped with a condenser and magnetic stirrer bar under Argon. The flask was charged with anhydrous DMF, dichloro-(/?- cymene)-Ruthenium (II) dimer (0.2g, 3.265xl0^"4mol) and 4,4'-bis(fE 2-(9-benzyl-9H- carbazol-3-yl)vinyl)-2,2'-bipyridine (0.469g, 6.53x10^"4mol). The reaction mixture was stirred at 90 °C for 4h. After 4 hours, 2,2'-bipyridyl-4,4'-dicarboxalic acid was added (0.159g, 6.53xl0^"4mol) and the temperature was raised to 130°C and allowed to run for 5 hours. After the 5 hours, excess of NH₄NCS was added to the reaction mixture, and the reaction mixture was allowed to run for extra 4h at 130°C. The last two step of the reaction were monitored for completion by taking aliquots from the reaction mixture every 60 minutes and measuring its absorption spectrum until there was no increase in the absorbance of the MLCT peak with respect to the π-π* peak. The reaction mixture was cooled down to room temperature and DMF was removed using a rotary evaporator. Water was added to the flask and the insoluble solid was vacuum filtered and washed with de-ionized water and ether. The product was dried overnight to give a yield of 85%.

EXAMPLE 5

Extinction coefficients

Table 1 shows a comparison between the extinction coefficient of NC-10, NC-11 versus N-719. It is clear that NC-10 harvests more photons, both high and low energetic photons, and hence greater light than NC-10 or N-719 at all wavelengths of maximum absorptions.

EXAMPLES 6-7

Device Performance

NC-10 and NC-11 were tested against N-719 under the same conditions for incident- photon-to-current efficiency (IPCE) conversion, short-circuit photocurrent density (Jsc)_> open-cell photovoltage (Voc), fill factor (FF), and total solar-to-electric conversion (η) under standard illuminating conditions of AMI.5 (lOOmW/cm ).

IPCE Performance. Figure 2 shows that the IPCE conversion of NC-11 exceeds that of N-719 and NC-10. It should be noted that the IPCE of NC-11 is before device optimization while the IPCE of NC-10 and N-719 were collected after device optimization. It is expected that the IPCE of NC-11 would increase even more after device optimization.

Total Solar-to-Electric Conversion. Table 2 shows the device performance data collected for NC-10 and N-719 after optimization versus that of NC-11 before optimization. It is expected that the η% of NC-11 should reach ~11.0 after device optimization.

refers to device after optimization; ^v refers to device before optimization; n.a. not available.

The foregoing is illustrative of the present invention, and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims

THAT WHICH IS CLAIMED IS:

1. An antenna ligand for use in a dye-sensitized solar cell, said antenna ligand consisting of a bidentate or tridentate heteroaromatic metal-chelating group having at least one independently selected antenna group substituted thereon, each antenna group being an independently selected substituent of the formula -RVAr'-RVAr^, wherein:

R'j is an aliphatic group of the formula -(CH=CH)_n-, where n is from 1 to 3;

Ar' is an aromatic group;

Ar'₂ is an aromatic group.

2. An antenna ligand of claim 1, said antenna ligand having a structure selected from the group consisting of:

wherein:

from one to four of R_1? R₂, R₃, R4, R₅, R6, R₇, R_g, R₉, R₁₀, Rn, R₁₂, and R¹³, when present, is said antenna group; and

the remainder of R_1} R₂, R₃, R4, R₅, R^, R₇, R₈, R9, R₁₀, Rn, R12, and R¹³, when present, are selected from the group consisting of H, halo, loweralkyl, alkoxy, amino, aminoalkyl, alkylamino, arylalkylamino, and disubstituted-amino.

3. The antenna ligand of claim 1 to 2, wherein Ar' is selected from the group consisting of carbazolyl, 5H-benzo[b]carbazole, indolyl, biindolyl, -2-(lH-indol-2-yl)-lH- benzo[f]indole, dibenzo[b,d]thiophene, benzo[b]naphtho[2,3-d]thiophene, and dibenzo[b,d]furan, benzo[b]naphtho[2,3-d]furan

4. The antenna ligand of claim 1 to 3, wherein Ar'₂ is selected from the group consisting of phenyl, naphthyl, anthracyl, and pyrenyl, and perylenyl.

5. The antenna ligand of claim 1 to 4, wherein said antenna group has the structure:

6. The antenna ligand of claim 1 having the structure:

7. An antenna ligand for use in a dye-sensitized solar cell, said antenna ligand consisting of a bidentate or tridentate heteroaromatic metal-chelating group having at least one independently selected antenna group substituted thereon, each antenna group being an independently selected substituent of the formula -R'j-Ar', wherein:

R'i is an aliphatic group of the formula -(CH=CH)_n-, where n is from 1 to 3; and Ar' is a heteroaromatic group.

8. An antenna ligand of claim 7, said antenna ligand having a structure selected from the group consisting of:

wherein:

from one to four of R_ls R₂, R₃, R4, R₅, Re, R₇, R₈, R₉, R₁₀, Rn, R12, and R¹³, when present, is said antenna group; and

the remainder of Ri, R₂, R₃, R4, R₅, R^, R₇, Rg, R₉, Rio, Rn, R12, and R¹³, when present, are selected from the group consisting of H, halo, loweralkyl, alkoxy, amino, aminoalkyl, alkylamino, arylalkylamino, and disubstituted-amino.

9. The antenna ligand of claim7 to 8, wherein Ar' is selected from the group consisting of carbazolyl, 5H-benzo[b]carbazole, indolyl, biindolyl, -2-(lH-indol-2-yl)-lH- benzo[f]indole, dibenzo[b,d]thiophene, benzo[b]naphtho[2,3-d]thiophene, and dibenzo[b,d]furan, benzo[b]naphtho[2,3-d]furan

10. The antenna ligand of claim 7 to 9, wherein said antenna group has the structure:

11. The antenna ligand of claim 7, said antenna ligand having the structure:

12. In an organometallic complex useful as a dye in a dye-sensitized solar cell, the metal complex consisting of: (a) a metal selected from the group consisting of Ru, Os, Ir, Re, Rh, and Fe,

(b) from one to two bidentate or tridentate organic anchoring ligands;

(c) from one to two bidentate or tridentate organic antenna ligands, and

(d) from zero to two monodentate ligands selected from the group consisting of H₂0, halo, cyano, -NCO-, -NCS, and -NCSe;

the improvement comprising: employing an antenna ligand of claim 1 to 11 as at least one of said antenna ligands.

13. The organometallic complex of claim 12, wherein said anchoring ligands are bidentate or tridentate heteroaromatic metal-chelating group having at least one independently selected anchoring groups substituted thereon, said anchoring groups selected from the group consisting of -COOH, -P0₃H₂, -P0₄H₂, -S0₃H₂, -S0₄H₂, -CONHOH, and deprotonated forms thereof.

14. The organometallic complex of claim 12, wherein said anchoring ligands are selected from the group consisting of compounds of Fomulas A- J:

wherein:

from one to four of R_1; R₂, R₃, R4, R₅, R6, R₇, R₈, R₉, Ri₀, Rn, Ri₂, and R¹³, when present, are independently selected anchoring groups; and

the remainder of Rj, R₂, R₃, R), R₅, Rg, R₇, R₈, R₉, R₁₀, Rn, R₁₂, and R¹³, when present, are selected from the group consisting of H, halo, loweralkyl, alkoxy, amino, aminoalkyl, alkylamino, arylalkylamino, and disubstituted-amino.

šnd

16. A regenerative photoelectrochemical cell comprising a photoanode, said photoanode comprising at least one semi-conductive metal oxide layer on a conductive substrate, sensitized by a photosensitizing dye, a counter electrode and an electrolyte arranged between said semi-conductive metal oxide layer and said counter electrode, wherein said photosensitizing dye is an organometallic complex as claimed in claim 12 to 15.

17. A cell as claimed in claim 16, wherein an amphiphilic compacting compound whose molecular structure comprises at least one anchoring group, a hydrophobic portion and a terminal group is co-adsorbed with said photosensitizing dye on said semi-conductive metal oxide layer in a mixed monolayer.

18. A cell as claimed in claim 16, wherein said compacting compound is selected from the group consisting of alkyl carboxylic acids, alkyl dicarboxylic acids, alkyl carboxylates, alkyl phosphonic acids, alkyl phosphonates, alkyl diphosphonic acids, alkyl diphosphonates, alkyl sulphonic acids, alkyl sulphonates, alkyl hydroxamic acids and alkyl hydroxamates, wherein said alkyl is linear or branched from CI to C20.

19. A cell as claimed in claim 16, wherein the molar ratio of said photosensitizing dye to said co-adsorbed compacting compound is of between 10 and 1/2, and in that said self- assembled monolayer is a dense packed monolayer having an order-disorder transition temperature above 80 degree C.

20. A cell as claimed in claim 16, wherein the length of said hydrophobic chain portion of the compacting compound allows said terminal group to protrude above the sensitizing dye in said monolayer.

21. A cell as claimed in claim 16, wherein said electrolyte comprises a redox system, that said redox system comprises an electrochemically active salt and a first compound forming a redox couple with either the anion or the cation of said electrochemically active salt, wherein said salt is a room temperature molten salt, said molten salt being liquid at least between standard room temperature and 80 degree C above said room temperature.

22. A cell as claimed in claim 16, wherein said electrolyte further comprises a polar organic solvent having a boiling point of at least 100 degree C at normal atmospheric pressure.

23. A cell as claimed in claim 16, wherein said solvent is a nitrile selected from 3- methoxypropionitrile 3-ethoxypropionitrile,3-butoxupropionitrile, and butyronitrile.

24. A cell as claimed in claim 16, wherein said electrolyte further comprises, as an additive, a compound formed by a neutral molecule comprising one or more nitrogen atom(s) with a lone electron pair.