WO2013076197A1

WO2013076197A1 - Dye compounds, method of making the same, and their use in dye-sensitized solar cells

Info

Publication number: WO2013076197A1
Application number: PCT/EP2012/073363
Authority: WO
Inventors: Max Josef Braun; Taichi MIYAJI; Mohammad Khaja Nazeeruddin; Il Jung; Michael Graetzel
Original assignee: Solvay Sa
Priority date: 2011-11-22
Filing date: 2012-11-22
Publication date: 2013-05-30

Abstract

Disclosed are novel dye compounds, method of making them, and their use in photoelectric conversion devices, especially in dye-sensitized solar cells. The dye compounds are organometallic compounds of formula ML1L2, wherein L1 and L2 independently indicates a tridentate ligand of specific structures.

Description

Dye compounds, method of making the same, and their use in dye- sensitized solar cells

This application claims priority to European patent application

No. 11190167.4 filed on November 22^nd, 2012, the whole content of this application being incorporated herein by reference for all purposes.

TECHNICAL FIELD

The present invention relates to novel dye compounds, methods of making the same, and their use as dyes in photoelectric conversion devices, especially in dye- sensitized solar cells (DSSC).

BACKGROUND OF THE INVENTION

Conventional solar cells convert light into electricity by exploiting the photovoltaic effect that exists at semiconductor junctions. In other words, the commercial solar cells absorb energy from visible light and convert excited charge carriers thereof to electric energy. At present, the main commercial solar cells are silicon-based solar cells. For the silicon-based solar cell, there are shortcomings in that high energy costs for material processing is required and many problems to be addressed such as environmental burdens and cost and material supply limitations are involved. For an amorphous silicon solar cell, there are also shortcomings in that energy conversion efficiency decreases when used for a long time, due to deterioration in a short period.

Recently, many attempts have been undertaken to develop low-cost organic solar cells. In particular, developments have targeted dye-sensitized solar cells (DSSC) which are based on a semiconductor sensitized electrode, a counterelectrode and an electrolyte. The sensitizer absorbs incoming light to produce excited electrons.

Examples of ruthenium complexes useful as dye molecules in dye sensitized solar cells are dye "N719," i.e. [Ru(NCS)₂(dcbpy)₂]

(dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) and "N749" or "Black Dye," i.e. [Ru(NCS)₃(tctpy)] (tctpy = 4,4',4"-tricarboxy-2,2':6',2"-terpyridine).

Although the DSSCs using the dye N719 exhibit high conversion efficiency, they are insufficient in durability such as weather resistance and heat resistance. Alternatively, still higher efficiencies can be obtained by using Black Dye. However, Black Dye cannot replace N719 because of its shortcomings, e.g. difficult synthesis, isomers formation, relatively low extinction coefficient and troublesome cell manufacturing due to aggregation etc.

Recently, the use of cyclometalated ruthenium complexes, instead of N719 and Black Dye, as efficient sensitizers for T1O₂ semiconductor electrodes for solar cells has been reported (see, e.g. [Sipke H. Wadman et al.,

Chem. Commun., 2007, 1907-1909]). It discloses the use of certain

cyclometalated ruthenium complexes of [Ru(C^AN^AN)(N^AN^AN)] configuration. Also, EP 2,036,955 Al discloses dyestuffs having bi- and tri-dentate ligands with specific structures, for example, the dyestuffs as shown below, DSSC comprising the same, and method for manufacturing the same.

The invention now makes available dyes for improvement of DSSCs, in particular improved absorption range and/or stability. More particularly, the present invention provides dyes exhibiting a broad spectrum of absorbed light (i.e. absorbing as much of the solar spectrum as possible), a high molar extinction coefficient, and/or contributing to the long-term stability of the device. It is an object of the present invention to provide new dyes showing advantageous properties when used in photoelectric conversion devices, in particular in dye sensitized solar cells (DSSC).

DESCRIPTION OF THE INVENTION

The present invention therefore relates to dye compounds of following formula (I) :

ML1L2 (I) wherein M represents a metal, belonging to Group 6, 8, 9, 10 or 11 of the long- format Periodic Table ;

LI and L2 are independently selected from tridentate ligands, at least one of LI and L2 corresponding to following formula (T) :

Aa-Ab-Ac (T) wherein each of Aa, Ab and Ac is an aromatic group, and Aa is connected to M through a metal-carbon bond, Ab is connected to M through a metal-nitrogen bond (M-N), and Ac is connected to M through a metal -X bond,

wherein X is selected from C and N, and

the dye compound of formula (I) has two carbon-metal bonds.

Generally, Ab is connected to Aa or Ac in ortho position relative to a nitrogen atom forming part of the aromatic group Ab and which nitrogen atom forms the metal-nitrogen bond. Preferably, Ab is connected to Aa and Ac in ortho position relative to said nitrogen atom.

In a preferred embodiment, at least one of LI and L2 corresponds to following formula (II) :

wherein

X is selected from C and N,

Rl, Rl', Rl" and Rl"' are independently selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, R3 is selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, NO2, and anchoring groups,

if X is C, R2, R2', R2" and R2'" are independently selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, NO2, and anchoring groups, or

if X is N, R2, R2', R2" and R2'" are independently selected from the group consisting of hydrogen, halogens, NO2, and anchoring groups.

The dye compounds of the present invention are suitable for use in photoelectric conversion devices, in particular for use in semiconducting layer in dye- sensitized solar cell (DSSC). Thus, the present invention also relates to the use of a dye compound of the present invention or its salt in a photoelectric conversion device, especially in DSSC.

The dyes of the present invention is that the dye compounds are metal complexes bearing two tridentate ligands, the tridentate ligands being composed of three aromatic cycles and being therefore conjugated. Also, the dye compounds of the present invention have two carbon-metal bonds wherein the carbon exists in the terminal side of the tridentate ligand. The carbons in phenyl ring have stronger electron-donating nature than NCS ligand, and thus, contribute to increased electron density on the metal center.

The dyes of the present invention can have a broad absorption spectrum, particularly in the visible and near-IR regions, i.e. absorbing as much of the light as possible. The dyes of the present invention can also exhibit a high molar extinction coefficient. Such dyes can have an improved communication and directionality of the electrons when being transferred from the sensitizer to the semiconductor electrode. Such dyes can also contribute to the long-term stability of such devices, also because of absence of monodentate thiocyanate ligands, for example better resistance to substitution induced by trace amounts of water in the devices and better shielding of the photoelectrode against corrosion through components present in the electrolyte, such as the triiodide/iodide couple.

In preferred embodiments of the present invention, the dye compounds of formula (I) have two carbon-metal bonds wherein the carbons are located in terminal position of tridentate ligand, so that they are located in trans-position of anchoring group. This configuration provides better directionality of electrons in the excited state of the sensitizer. In the present invention, "alkyl groups" is understood to denote in particular a straight chain, branched chain, or cyclic hydrocarbon groups usually having from 1 to 20 carbon atoms. Examples of alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec -butyl, tert-butyl, pentyl, neopentyl, hexyl, cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl.

In the present invention, "alkoxy groups" is understood to denote in particular a straight chain, branched chain, or cyclic hydrocarbon groups usually having from 1 to 20 carbon atoms singularly bonded to oxygen (Alk-O-).

In the present invention, "aryl groups" is understood to denote in particular any functional group or substituent derived from an aromatic ring. In particular, the aryl groups can have 6 to 20 carbon atoms (preferably 6 to 12 due to its easiness of synthesis at a low cost) in which some or all of the hydrogen atoms of the aryl group may or may not be substituted with other groups, especially alkyl groups, alkoxy groups, or hydroxyl groups. The aryl groups are preferably optionally substituted phenyl groups, naphthyl groups, anthryl group and phenanthryl group. A particular aryl group in the present invention is alkoxy substituted phenyl groups, such as the phenyl group in which its ortho- and para- positions are substituted by alkoxy group.

In the present invention, "aryloxy groups" is understood to denote in particular the aryl group as defined above singularly bonded to oxygen (Ar-O-).

In the present invention, "heterocycles" is understood to denote in particular a cyclic compound which has at least one heteroatom as a member of its one or more rings. Frequent heteroatoms within the ring include sulfur, oxygen and nitrogen. The heterocycles can be either saturated or unsaturated, and may be 3-membered, 4-membered, 5-membered, 6-membered

or 7-membered ring. The heterocycles can be further fused with other one or more ring systems. Examples of the heterocycles include pyrrolidines, oxolanes, thiolanes, pyrroles, furans, thiophenes, piperidines, oxanes, thianes, pyridines, pyrans, and thiopyrans, and their derivatives. A particular class of the heterocycles includes substituents comprising thiophene moiety.

In the present invention, "substituents comprising thiophene moiety" is understood to denote in particular the substituents either comprising one thiophene ring or multiple thiophene rings. The substituents comprising one thiophene ring may further comprise other ring(s) connected to the thiophene ring, e.g. 3,4-ethylenedioxythiophene (EDOT), and/or may be substituted by other groups, such as alkyl groups or alkoxy groups. Also, the substituent comprising thiophene ring can be substituted by aryl groups or aryloxy groups optionally substituted by alkyl groups or alkoxy groups. The substituents comprising multiple thiophene rings include oligothiophenes in which the multiple thiophene rings are joined by single bond(s) (e.g. mono-, di-, tri-, and tetra-thiophene) or in which the multiple thiophene rings are fused

(e.g. [n]thienoacenes (wherein, n is an integer from 2 to 7) or [n]thienohelicenes (wherein, n is an integer from 2 to 7)), and oligothiophenes fused with other ring(s) than the thiophene rings, e.g. benzene-thiophene, thiazole-thiophene or cyclopentadiene-thiophene alternating molecules. The oligothiophenes may further be substituted by other groups, such as alkyl groups or alkoxy groups. The substituents may be any combination of the same or different substituents. Non-limiting examples of the substituents comprising thiophene moiety include

wherein, R independently indicates alkyl groups or alkoxy groups.

In the present invention, "amino groups" is understood to denote in particular aliphatic amines or aromatic amines and encompass compounds comprising one or multiple amino groups. The amino groups can be primary amines, secondary amines and tertiary amines, wherein one or more hydrogen atoms may or may not be substituted with other groups, such as alkyl groups or aryl groups. The one or more hydrogen atoms may be substituted with alkyl groups or aryl groups which are further substituted by other groups, such as alkyl groups or alkoxy groups.

In the present invention, "halogenated derivatives" is understood to denote in particular at least one of the hydrogen atoms has been replaced by a halogen atom, preferably selected from fluorine and chlorine, more preferably fluorine. If all the hydrogen atoms have been replaced by halogen atoms, the halogenated derivative is a perhalogenated derivative. For instance, halogenated derivatives of alkyl groups include (per)fluorinated alkyl groups such as (per)fluorinated methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl or tert-butyl ; for instance -CF₃, -C2F5, heptafluoroisopropyl (-CF(CF₃)₂), or hexafluoroisopropyl (-CH(CF₃)₂).

In the present invention, "anchoring groups" is understood to denote in particular groups that will allow attachment of the dyes onto a semiconductor, such as Ti(¾. Suitable anchoring groups are for instance selected from, but not limited to, the group consisting of -COOH, -P0₃H₂, -P0₄H₂, -S0₃H, -CONHOH, acetylacetonate, acrylic acid derivatives, malonic acid derivative, rhodanine-3- acetic acid, propionic acid, deprotonated forms of the aforementioned, salts of said deprotonated forms, and chelating groups with π-conducting character, more preferably -COOH or the salts of deprotonated -COOH. Especially preferable salts of -COOH are for instance ammonium salts, or alkali metal salts, more preferably -COOTBA (wherein, TBA indicates tetrabutylammonium). In the sense of the present invention, the dye compound of the present invention can comprise at least one anchoring group. Especially, the dye compound of the present invention may comprise at least two anchoring groups, for example, two or four anchoring groups.

The dye compounds of the present invention have following formula (I) : ML1L2 (I) wherein the dye compound of formula (I) has two carbon-metal bonds.

In the present invention, M is a metal atom, belonging to Group 6, 8, 9, 10 or 11 of the long-format Periodic Table. Preferred elements are iron, ruthenium, osmium, iridium, cobalt, palladium, platinum, and chromium. A dye compound having a ruthenium atom as M is particularly preferred. Generally, ML1L2 is a hexacoordinated complex, in particular an octahedral complex having M as central atom. In the present invention, LI and L2 are independently selected from tridentate ligands, at least one of LI and L2 corresponding to following formula (T) :

Aa-Ab-Ac (T).

In the present invention, each of Aa, Ab and Ac is an aromatic group. Preferred aromatic group is phenyl group or pyridine group. In the present invention, Aa is connected to M through a metal-carbon bond and is preferably phenyl group, whereas Ab is connected to M through a metal-nitrogen bond and is particularly pyridine group. In the present invention, Ac is connected to M through either a metal-carbon bond or a metal-nitrogen bond since X is selected from C and N.

In a particular embodiment, Aa and Ab are connected through a bond between each of their atoms in 2-position of an atom contributing to a formation of the metal-carbon bond and the metal-nitrogen bond in the aromatic group, respectively. In case that both Aa and Ab are 6-membered aromatic ring, such as phenyl ring or pyridine ring, Aa and Ab are connected through a bond formed in each of ortho-position of the rings. Further, Ab and Ac are connected through a bond between an atom in 2-position of an atom contributing to a formation of M-X bond in Ac, and another atom in nearest position than the atom in 2-position of an atom contributing to a formation of a bond to the metal in the aromatic group. For instance, if Ab is 6-membered aromatic ring, such as pyridine ring, the another atom in nearest position is an atom located

in 6-position. In case that both Ab and Ac are 6-membered aromatic ring, Ab and Ac are connected through a bond between 2-position of Ac and 6-position of Ab provided 2-position of Ab is occupied for the connection to Aa.

In a specific embodiment of the present invention, LI and L2 are independently selected from tridentate ligands, at least one of LI and L2

:

(Π) wherein Rl, Rl ', Rl" and Rl"' are independently selected from the group consisting of hydrogen, halogens (in particular fluorine and chlorine), cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives.

Rl, Rl ', Rl" and Rl"' are preferably independently selected from the group consisting of hydrogen, halogens such as chlorine and fluorine, alkoxy groups such as OCH₃, aryloxy groups such as OPh, halogenated alkyl group such as C₆F5, CF₃ and CT^CF^CFs, and the substituents comprising thiophene moiety having following structures :

In particular, Rl is preferably selected from aryl groups substituted by alkoxy group. One particular example for Rl is the phenyl group substituted by alkoxy groups, especially in the ortho- and para-positions of the phenyl ring. On the other hands, Rl can be diarylamino group or arylthienyl group, for example the structures represented below, but the present invention is not limited thereto.

In a particular embodiment, Rl ' and Rl" are the same. In a further particular embodiment, Rl' and Rl" are the same and are selected from hydrogen, halogens (in particular fluorine and chlorine), alkoxy groups such as OCH₃ and optionally halogenated alkyl groups such as CF₃ and CH₃(CF2)₃CF₃. In a still further particular embodiment, Rl is the same or different than Rl' and Rl" and is selected from hydrogen, halogens (in particular fluorine and chlorine), and the substituents comprising thiophene moiety having following structure :

In another particular embodiment, Rl, Rl ', Rl" and Rl"' are different or the same and are selected from hydrogen and halogens (in particular fluorine and chlorine). In a still another particular embodiment, Rl and Rl'" are different or the same and are selected from hydrogen and halogens (in particular fluorine and chlorine).

In the present invention, R3 may be selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, NO₂, and anchoring groups as defined above. Preferably, R3 is selected from the group consisting of hydrogen, chlorine, fluorine, NO₂, -COOH, -PO₃H₂, -PO₄H₂, -SO₃H, -CONHOH, acetylacetonate, acrylic acid derivatives, malonic acid derivative, rhodanine- 3 -acetic acid, propionic acid, and deprotonated forms or salts thereof ; more preferably from -COOH or the salts of deprotonated -COOH, and the substituents comprising thiophene moiety having following structures :

Especially preferable salts of -COOH are for instance ammonium salts, and alkali metal salts, more preferably -COOTBA. Acrylic acid derivatives may for instance be selected from groups of formula -CH=C(R^a)-COOH where R^a is selected from hydrogen, cyano and alkyl groups substituted by at least one halogen atom, preferably from hydrogen, cyano and CF₃. Malonic acid derivatives suitable as anchoring groups may for example be selected from groups of formula -CR^b=C(COOH)₂ where R^b is selected from hydrogen and optionally halogenated alkyl groups, especially from hydrogen and optionally fluorinated alkyl groups. Alkyne groups can be suitably used for extending the anchoring groups. For example, the anchoring group may be of

formula -C≡C-COOH.

In the present invention, R2, R2', R2" and R2'" in the formula (II) are preferably varied depending on what atom is chosen for X. In case that X is C in the formula (II), R2, R2', R2" and R2'" are generally independently selected from the group consisting of hydrogen, halogens (in particular fluorine and chlorine), cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, NO2, and anchoring groups. R2, R2', R2" and R2'" are preferably independently selected from the group consisting of hydrogen, halogens such as chlorine, fluorine, alkoxy groups such as OCH₃, aryloxy groups such as OPh, halogenated alkyl group such as C₆F5, CF₃ and CH₃(CF2)₃CF₃, and the substituents comprising thiophene moiety having following structures :

as OCH₃ and optionally halogenated alkyl groups such as CF₃ and CH₃(CF2)₃CF₃. In a still further particular embodiment, R2 is the same or different than R2' and R2" and is selected from hydrogen, halogens (in particular fluorine and chlorine), and the substituents comprising thiophene moiety having following structure :

In another particular embodiment, R2, R2', R2" and R2'" are different or the same and are selected from hydrogen and halogens (in particular fluorine and chlorine). In a still another particular embodiment, R2 and R2'" are different or the same and are selected from hydrogen and halogens (in particular fluorine and chlorine).

In case that X is N in the formula (II), R2, R2', R2" and R2'" are independently selected from the group consisting of hydrogen, halogens, NO2, and anchoring groups as defined above. Preferably, R2', R2" and R2'" are hydrogen, and R2 is selected from the group consisting of hydrogen, chlorine, fluorine, N0₂, -COOH, -P0₃H₂, -P0₄H₂, -S0₃H, -CONHOH, acetylacetonate, acrylic acid derivatives, malonic acid derivative, rhodanine- 3 -acetic acid, propionic acid, and deprotonated forms or salts thereof ; more preferably from -COOH or the salts of deprotonated -COOH, especially preferably ammonium salts, and alkali metal salts, still more preferably -COOTBA. Acrylic acid derivatives may for instance be selected from groups of

formula -CH=C(R^a)-COOH where R^a is selected from hydrogen, cyano and alkyl groups substituted by at least one halogen atom, preferably from hydrogen, cyano and CF₃. Malonic acid derivatives suitable as anchoring groups may for example be selected from groups of formula -CR^b=C(COOH)₂ where R^b is selected from hydrogen and optionally halogenated alkyl groups, especially from hydrogen and optionally fluorinated alkyl groups. Alkyne groups can be suitably used for extending the anchoring groups. For example, the anchoring group may be of formula -C≡C-COOH. In this embodiment, Rl can preferably be the phenyl group substituted by alkoxy groups, especially in the ortho- and para- positions of the phenyl ring.

In a first preferred aspect of the present invention, both LI and L2 are represented by formula (II) wherein X is N, resulting in two tridentate ligands of (C^AN^AN) type, each one with a carbon-metal bond between the tridentate ligand and M, the central atom of the formula (I).

In this first preferred aspect, the two carbons in phenyl ring of the tridentate ligands LI and L2 are arranged in cis-configuration. Indeed, in this case, the dye has a higher symmetry, leading to better electron transfer due to an electron-donating carbon anion group in phenyl ring ("PUSH") and an anchoring group in pyridine ring bearing anchoring group ("PULL"), which is trans position to the "PUSH", leading to better photon-to-electron injection onto semiconductor (e.g. T1O₂) conduction band.

One of the features of the first preferred aspect of the present invention is that the dye compounds of formula (I) have two carbon-metal bonds wherein the carbon exists in the terminal side of the each tridentate ligands. This has the advantage with respect to the adsorption on the T1O₂ surface using two to four anchoring groups. In addition, the position of the two carbon coordination results in cis-configuration which is trans to terminal anchoring group providing directionality in the excited state of the sensitizer. In this first preferred aspect, ligands LI and L2 may be the same or different, preferably the same. This is especially advantageous as, in such a case, only one kind of ligand (rather than two or three kinds of ligands) is needed for making the dye compounds, and the resulting molecule is symmetric. This preferred aspect allows for still better electron transfer and better synthetic yields.

As specific examples of the dye compounds of the first preferred aspect of the present invention, the dye compounds can be represented by formula (1-1), (1-2), (1-3), (1-4), (1-5), (1-6), (1-7), (1-8), (1-9), (I- 10), (I-l l), (1-12), (1-13), (1-14),



(1-11)

wherein "Hx" is understood to denote n-hexyl.

In a second preferred aspect of the present invention, one of LI and L2 is represented by formula (II) as defined above, wherein X is C, resulting in two carbon-metal bonds between the tridentate ligand and M, the central atom of the formula (I).

In said second aspect, another ligand of LI and L2, i.e. the ligand other than the one represented by formula (II) wherein X is C, is preferably

la (IV) :

In the formula (IV), R4, R5 and R6 are selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, NO₂, and anchoring groups as defined above. Preferably, R4, R5 and R6 are selected from the group consisting of hydrogen, chlorine, fluorine, NO₂, -COOH, -PO₃H₂, -PO₄H₂, -SO₃H, -CONHOH, acetylacetonate, acrylic acid derivatives, malonic acid derivative, rhodanine-3 -acetic acid, propionic acid, and

deprotonated forms or salts thereof ; more preferably from -COOH or the salts of deprotonated -COOH, and the substituents comprising thiophene moiety having follo in structures

- u

formula -C≡C-COOH.

As specific example of the dye compounds of the second preferred aspect, the d rmula (1-17) :

(1-17).

In a third preferred aspect of the present invention, at least one of, preferably both of, LI and L2 are independently represented by formula (V) as defined below, resulting in two carbon-metal bonds between the tridentate ligand and M, the central atom of the formula (I).

wherein Rl ', Rl", Rl"\ R2, R2', R2", R2'" and R3 have the same meaning as defined above.

As specific examples of the dye compounds of the third preferred aspect, the dye compounds can be represented by formula (1-18) and (1-19) :

The dye compounds of the present invention can be electrically neutral or neutralized with other counter-ion resulting in a salt form. Examples of these salts include ammonium salts and alkali metal salts of the dye compounds of the present invention. Therefore, the present invention also relates to a salt of the dye compound according to the present invention.

The compounds of the present invention described herein can be a dye which may be suitable for use in photoelectric conversion devices, especially in dye- sensitized solar cells (DSSC). The present invention therefore also relates to the use of a compound of the present invention or its salt in photoelectric conversion devices, especially in DSSC.

The DSSC offers the prospect of a cheap and versatile technology for large scale production of solar cells. The dye- sensitized solar cell (DSSC) is formed by a combination of organic and inorganic components that could be produced at a low cost. The dye-sensitized solar cells have advantages over silicon-based solar cells in terms of simplified processing steps, low fabrication cost, and transparency. The dye- sensitized solar cells can be fabricated from flexible substrates to function as cells of mobility and portability. The dye- sensitized solar cells have also the advantage to be lightweight.

One of the objectives that dye- sensitized solar cells are facing over the silicon-based solar cells is to increase relatively lower energy (photoelectric) conversion efficiency. In order to improve the energy conversion efficiency, extension of absorption spectra wavelength up to infrared regions would be of interest.

One of the basic elements of a DSSC is generally a T1O₂ (titanium dioxide) sensitized with dye molecules to form the core of a DSSC. T1O₂ is a preferred material for the particles since its surface is highly resistant to the continued electron transfer. However, T1O₂ only absorbs a small fraction of the solar photons (those in the UV). The dye compounds attached to the semiconductor surface are used to harvest a great portion of the solar light.

The dye compounds usually consist of one metal atom and an organic structure that provides the required properties (e.g., wide absorption range, fast electron injection, and stability). The dye is sensible to the visible light. The light creates and excites in the dye highly energetic electron, which is rapidly injected to the semiconductor particles (usually T1O₂). The particulate semiconductor functions as the transporter of light induced electrons towards the external contact, a transparent conductor that lies at the basis of the

semiconductor (usually T1O₂) film.

Construction of a DSSC would be well known by a person skilled in the art. Generally, the DSSC comprises an anode, a cathode, and an electrolyte. The anode and cathode are arranged in a sandwich-like configuration, and the electrolyte is inserted to separate the two electrodes.

The material for the anode is not limited as long as the anode is formed from a material having conductivity. For a non-limiting example, a substrate comprising electrically conductive transparent glass which contains a small amount of platinum or conductive carbon adhering to the surface can be suitably used. As the electrically conductive transparent glass, a glass made of tin oxide or indium-tin oxide (ITO) can be used. The cathode has a substrate made of electrically conductive transparent glass and a semiconducting layer comprising a semiconductor and the dye compound according to the present invention adsorbed thereto. As an example of the electrically conductive transparent glass for the cathode, a glass comprising the materials described above for the anode can be used, but not limited to them.

As non- limiting examples of materials for the semiconductor, metal oxides such as titanium oxides, niobium oxides, zinc oxides, tin oxides, tungsten oxides, and indium oxides, preferably Τί(¾ and SnC>2 are included. T1OF2 (titanyl oxyfluoride, titanium oxyfluoride or titanium fluoride oxide) can also be envisaged as a suitable semiconductor. T1OF2 might be especially suitable when combined with fluorinated dyes of the present invention. The materials may be used as the sole semiconductor in the DSSC semiconductor layer or may be combined in mixture with any other suitable semiconductor layer material.

The dye compound of the present invention is caused to be adsorbed on the semiconductor. The dye is adsorbed by causing the cathode comprising substrate of electrically conductive transparent glass and a semiconducting layer formed on the surface to come in contact with a dye solution containing the dye compound of the present invention and a solvent. The association of the dye compound and the semiconductor can be maintained through chemical bond or electrostatic interaction achieved between the anchoring group of the dye complex and the semiconductor material. Other methods than the adsorption for achieving association the dye and the semiconductor would be known to a person skilled in the art. The present invention therefore also relates to a

semiconducting element comprising a semiconductor and the dye compound of the present invention or its salt, more particularly to a semiconducting layer comprising Ti(¾ and the dye compound of the present invention or its salt or T1OF2 and the dye compound of the present invention or its salt.

As the electrolyte, a liquid electrolyte, a solid electrolyte, or a solution containing the electrolyte can be used. The electrolyte is preferably a redox electrolyte, containing a substance forming a redox system, for example a solution containing iodine and imidazolium salt of iodide, which forms a redox system of I3^" + 2 e^" <→^■ 3 Γ + I2. As the suitable solvent for the solution, an electrochemically inert substance, such as acetonitrile or propionitrile can be used. The DSSC can be formed, for example, by filling a container with the electrolytic solution and disposing the anode and cathode face to face in the electrolytic solution. The anode and the cathode can be thus disposed with a desired distance between them by securing them by sandwiching a spacer between them. Nonetheless, other methods of manufacturing the DSSC would be well understood by a person skilled in the art.

The present invention further relates to a photoelectric conversion device, preferably to a dye-sensitized solar cell, which comprises the dye compound of the present invention or to the semiconducting element, in particular to the semiconducting layer comprising a semiconductor and the dye compound of the present invention or its salt. The dye compound of the present invention is used as a dye, in particular as a sensitizing dye, in such device or cell.

While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of systems and methods are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

SHORT DESCRIPTION OF THE FIGURES

Figure 1 : Synthetic scheme of preparation of (C^AN^AN)-type ligands, wherein Rl, Rl' and Rl" are independently selected from the group consisting of hydrogen, alkoxy groups, halogens, and optionally halogenated alkyl groups.

Figure 2 : Synthetic route for formula (1-1)

Figure 3 : Normalized UV-absorption spectra for the formula (1-1) prepared according to Example 1, measured on a Hewlett-Packard 8453 UV- visible spectrometer in 1.0 X 10^"4 M ethanol solution

Figure 4 : Normalized UV-absorption spectra for the formula (1-3) prepared according to Example 2, measured on a Hewlett-Packard 8453 UV- visible spectrometer in 1.0 X 10^"4 M ethanol solution

Figure 5 : Synthetic route for formula (Ti l) Examples

Example 1 : Synthesis of Dye 1 (formula (1-1))

Dye 1 is summarized in Figure 2

(Dye 1)

4-(methoxycarbonyl)-2-(4-(methoxycarbonyl)pyridin-2-yl)pyridine-l-oxide

3-chloroperoxybenzoic acid (mCPBA, 1.33 g, 7.71 mmol) was added slowly to a solution of the corresponding dimethyl 2,2'-bipyridine-4,4'- dicarboxylate (2 g, 7.35 mmol) in CHC1₃ (100 mL) at 0 °C. The resulting solution was stirred for 1 hr at this temperature and then, the reaction mixture was allowed to warm to r.t. and stirred overnight. After end of the reaction, solvent was evaporated under reduced pressure. The product was purified by column chromatography on silica gel as colorless powder (R_f = 0.5 in MeOH : DCM = 1 : 19, Yield = 1.65 g (77.9 %)).

^JH NMR (400 MHz, CDC1₃) : 9.34 (s, 1H), 8.91 (d, / = 4.8 Hz, 1H), 8.79 (d, J = 2.8 Hz, 1H), 8.35 (d, J = 6.8 Hz, 1H), 7.95 (dd, J = 4.8, 1.6 Hz, 1H), 7.89 (dd, / = 6.8, 2.4 Hz, 1H), 3.98 (s, 3H), 3.97 (s, 3H). MS (GC-EI) : m/z : calcd for Ci₂H₇BrO : 245.97 ; found : 246.

Dimethyl 6-chloro-2,2'-bipyridine-4,4'-dic

Phosphoryl chloride (4 ml, 42.36 mmol) was added slowly

to 4-(methoxycarbonyl)-2-(4-(methoxycarbonyl)pyridin-2-yl)pyridine- 1 -oxide (1 g, 3.47 mmol) at 0 °C. The mixture was heated to 110°Cwith stirring overnight. After cooling, water (50 ml) was added to this mixture then organic compounds were extracted with dichloromethane (50 ml, 3 times) and then the organic phase was washed by brine (50 ml). The organic extract was dried over MgS0₄, and solvents were removed on a rotary evaporator. The product was purified by column chromatography on silica gel as colorless powder

(R_f = 0.55 in MeOH : DCM = 1 : 39, Yield = 0.63 g (56.2 %)).

^JH NMR (400 MHz, CDC1₃) : 8.92 (dd, / = 1.6, 0.8 Hz, 1H), 8.89

(d, 7 = 1.2 Hz, 1H), 8.85 (dd, J = 4.8, 0.8 Hz, 1H), 7.93 (d, 7 = 1.6 Hz, 1H), 7.92 (dd, / = 4.8, 1.6 Hz, 1H), 4.01 (s, 3H), 4.00 (s, 3H). MS (GC-EI) : mJz : calcd for : 245.97 ; found : 246.

Dimethyl 6-(2,4-difluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylate

To a mixture of 2,4-difluorophenylboronic acid (0.57 g, 3.61 mmol), dimethyl 6-chloro-2,2'-bipyridine-4,4'-dicarboxylate (1 g, 3.26 mmol), potassium carbonate (0.54 g, 3.91 mmol), and Pd(PPli₃)₄ (0.19 g, 0.16 mmol) were added degassed THF (50 ml) and water (30 ml). The solution was refluxed overnight. After cooling, solvent was evaporated in vacuo. A methanol (50 ml) and sulfuric acid (0.5 ml) were added to the crude mixture. The solution was refluxed overnight again. Water (50 ml) was added to this mixture and neutralized with NaHCC>3 after finished the reaction, then organic compounds were extracted with dichloromethane (50 ml, 3 times) and then the organic phase was washed by brine (50 ml). The product was purified by column chromatography on silica gel as white solid. (R_f = 0.2 in DCM : Hx = 1 : 1, Yield = 0.41 g (32.7 %)).

JH NMR (400 MHz, CDC1₃) : 9.03 (dd, / = 1.6, 0.8 Hz, 1H), 8.95

(d, / = 1.2 Hz, 1H), 8.89 (dd, / = 4.8, 0.8 Hz, 1H), 8.39 (t, / = 1.6 Hz, 1H), 8.25 (td, / = 8.8, 6.4 Hz, 1H), 7.92 (dd, / = 4.8, 1.6 Hz, 1H), 7.09 (m, 1H), 6.98 (m, 1H), 4.02 (s, 3H), 4.01 (s, 3H). MS (GC-EI) : mJz : calcd for : 384.09 ; found : 384. 6-(2,4-Difluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylic acid

Potassium hydoxide (0.15 g, 2.67 mmol) was added to dimethyl 6-(2,4- difluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylate (0.3 g, 0.78 mmol) in methanol (50 ml) solution. The mixture was refluxed and stirred overnight. The mixture solution was neutralized by HCl solution. The precipitated product was filtered and washed by methanol and water (Yield = 0.26 g (93.5 %)).

^JH NMR (400 MHz, DMSO-d₆) : 8.92 (d, J = 4.4 Hz, 1H), 8.84 (s, 1H), 8.79 (s, 1H), 8.22 (s, 1H), 8.17 (q, J = 8.0 Hz, 1H), 7.92 (d, J = 4.4 Hz, 1H), 7.46 (t, J = 10.4 Hz, 1H), 7.34 (t, J = 8.0 Hz, 1H), 3.38 (bs, 2H). MS (GC-EI) : mJz : calcd for : 356.06 ; found : 357.

Bis[6-(2,4-difluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylic acid] ruthenium(I

6-(2,4-difluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylic acid (0.25 g,

0.65 mmol) and [RuCi₂(p-cymene)]2 (0.1 g, 0.164 mmol) were dissolved in DMF (75 mL), and the reaction mixture was heated to 160°Cunder Ar for 48hr. After this time, the solvent was removed by a rotary evaporator. To the reaction flask, diethylether was added resulting precipitate, which was collected on a sintered glass filter by suction filtration. The precipitate on the filter was dissolved in methanol and then the solution was concentrated in vacuo. The solution was purified on a Sephadex LH-20 column chromatography and the isolated solid color was as black powder (Yield = 47 mg (35.3 %). Ή NMR (400 MHz, DMSO-d₆) : 9.02 (s, 2H), 8.73 (s, 1H), 7.57

(d, 7 = 5.6 Hz, 1H), 7.47 (d, 7 = 5.6 Hz, 1H), 6.43 (t, 7 = 11.2 Hz, 1H), 5.24 (d, 7 = 8.0 Hz, 1H), 4.09 (bs, 2H). MS (GC-EI) : m/z : calcd for 812.01 ;

found : 812.

Example 2 ; Synthesis of Dye 2 (formula (1-3))

Dimethyl 6-(2,3,4-trifluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylate

To a mixture of 2,3,4-trifluorophenylboronic acid (0.631 g, 3.59 mmol), Dimethyl 6-chloro-2,2'-bipyridine-4,4'-dicarboxylate (1 g, 3.26 mmol), sodium carbonate (0.38 g, 3.59 mmol), and Pd(PPh₃)₄ (0.188 g, 0.08 mmol) were added degassed and distilled THF (150 ml). Then the solution was refluxed overnight. After cooling, the solvent was evaporated under vacuum. The resulting solid product was purified by column chromatography on silica gel as white solid (R_f = 0.2 in DCM : Hx = 1 : 1). This product was a mixture with starting material, which was recrystallized using DCM. The precipitate was filtered by filtration (Yield = 0.54 g (41.2 %)).

^JH NMR (400 MHz, CDC1₃) : 9.06 (m, 1H), 8.98 (d, 7 = 1.2, 1H), 8.88 (dd, 7 = 4.8, 0.8 Hz, 1H), 8.37 (t, 7 = 1.6, 1H), 7.97 (m, 1H),

7.92 (dd, 7 = 4.8, 1.6 Hz, 1H), 7.17 (m, 1H), 4.03 (s, 3H), 4.02 (s, 3H).

MS (GC-EI) : m/z : calcd for 402.08 ; found : 402. 6-(2,3,4-trifluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylic acid

Potassium hydoxide (0.23 g, 4.13 mmol) was added to dimethyl 6-(2,3,4- trifluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylate (0.5 g, 1.03 mmol) in water (20 ml) and methanol (50 ml) solution. The mixture was refluxed and stirred overnight. Then the solution was neutralized by HC1 solution. The precipitated product was filtered and washed using methanol and water

(Yield = 0.425 g (90.2 %)).

^JH NMR (400 MHz, DMSO-d₆) : 8.92 (d, J = 5.2 Hz, IH),

8.87 (t, J = 1.2 Hz, IH), 8.26 (t, , / = 1.6 Hz IH), 7.97 (m, IH), 7.93 (dd, J = 4.8, 1.6 Hz, IH), 7.57 (m, IH). MS (GC-EI) : m/z : calcd for 374.05 ; found : 374. Bis[6-(2,3,4-trifluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylic acid] rutheni

6-(2,3,4-trifluorophenyl)-2,2'-bipyridine-4,4'-dicarboxylic acid

(0.3 g, 0.802 mmol) and [RuCl₂(p-cymene)]₂ (0.124 g, 0.203 mmol) were dissolved in DMF (75 mL), and the reaction mixture was heated to 160°Cunder Ar for 48hr. After this time the solvent was removed by evaporator and was added diethylether to the reaction mixture. The resulting precipitate was collected on a sintered glass filter by suction filtration. The precipitate was dissolved in methanol and then the solution was concentrated in vacuo. The solution was purified on a Sephadex LH-20 column chromatography as black powder (Yield = 68 mg (39.5 %)). Ή NMR (400 MHz, DMSO-d₆) : 9.04 (m, 2H), 8.72 (s, 1H),

7.56-7.47 (m, 2H), 5.33 (m, 1H), 7.28 (d, / = 8.0 Hz, 1H). MS (GC-EI) : m/z : calcd for 847.99 ; found : 848.

Example 3 : Synthesis of the ligand for Dye 3 (formula (1-6))

To a mixture of 2,4-dichlorophenylboronic acid (0.631 g, 3.59 mmol), Dimethyl 6-chloro-2,2'-bipyridine-4,4'-dicarboxylate (1 g, 3.26 mmol), sodium carbonate (0.38 g, 3.59 mmol), and Pd(PPh₃)₄ (0.188 g, 0.08 mmol) were added degassed and distilled THF (150 ml). The solution was refluxed overnight.

After cooling, the solvent was evaporated in vacuo. The product was purified by column chromatography on silica gel as white solid

(R_f = 0.2 in DCM : Hx = 1 : 1). This product was mixture with starting material.

White solid was recrystallized in DCM, then take a precipitate by filteration

(Yield = 0.54 g (41.2 %)).

^JH NMR (400 MHz, CDC1₃) : 9.06 (m, 1H), 8.98 (d, / = 1.2, 1H),

8.88 (dd, J = 4.8, 0.8 Hz, 1H), 8.37 (t, 7 = 1.6, 1H), 7.97 (m, 1H),

7.92 (dd, / = 4.8, 1.6 Hz, 1H), 7.17 (m, 1H), 4.03 (s, 3H), 4.02 (s, 3H).

6-(2,4-dichlorophenyl)-2,2'-bipyridine-4,4'-dicarboxylic acid

Potassium hydoxide (0.23 g, 4.13 mmol) was added to dimethyl 6-(2,4- bis(trifluoromethyl) phenyl)-2,2'-bipyridine-4,4'-dicarboxylate

(0.4 g, 0.96 mmol) in water (20 ml) and methanol (50 ml) solution. The mixture was refluxed and stirred overnight, followed the reaction solution was neutralized by HCl solution. The precipitated product was filtered and washed using methanol and water (Yield = 0.31 g (83.0 %)).

^JH NMR (400 MHz, DMSO-d₆) : 8.93 (d, / = 4.8 Hz, 1H), 8.87 (s, 1H), 8.82 (s, 1H), 8.14 (s, 1H), 7.93 (dd, J = 5.2, 1.2 Hz, 1H), 7.82 (m, 2H), 7.64 (dd, J = 8.4, 2.0 Hz, 1H).

Claims

C L A I M S

1. A dye compound of formula (I) :

ML1L2 (I) wherein M represents a metal belonging to Group 6, 8, 9, 10 or 11 of the long- format Periodic Table ;

LI and L2 are independently selected from tridentate ligands, at least one of LI and L2 corresponding to formula (T) :

Aa-Ab-Ac (T) wherein each of Aa, Ab and Ac is an aromatic group, and Aa is connected to M through a metal-carbon bond, Ab is connected to M through a metal-nitrogen bond, and Ac is connected to M through a metal-X bond, wherein X is selected from C and N, and the dye compound of formula (I) has two carbon-metal bonds.

2. The dye compound according to claim 1, wherein at least one of LI and L2 corresponds to formula (II) or formula (V) :

(Π)

wherein

X is selected from C and N,

Rl, Rl ', Rl" and Rl"' are independently selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives,

R3 is selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, NO₂, and anchoring groups, and if X is C, R2, R2' , R2" and R2'" are independently selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, NO₂, and anchoring groups, or if X is N, R2, R2', R2" and R2'" are independently selected from the group consisting of hydrogen, halogens, NO₂, and anchoring groups.

3. The dye compound according to claim 1 or 2, wherein the M in the formula (I) is selected from the group consisting of iron, ruthenium, osmium, iridium, cobalt, palladium, platinum, and chromium, preferably ruthenium.

4. The dye compound according to any one of claims 1 to 3, wherein the anchoring group is selected from the group consisting of -COOH, -PO₃H₂, -PO₄H₂, -SO₃H, -CONHOH, acetylacetonate, acrylic acid derivatives, malonic acid derivative, rhodanine- 3 -acetic acid, propionic acid, deprotonated forms of the aforementioned, salts of said deprotonated forms, and chelating groups with π-conducting character, preferably -COOH and salts of -COOH, more preferably -COOH and ammonium or alkali metal salts of -COOH.

5. The dye compound according to any one of claims 1 to 4, wherein Rl ' and Rl" are the same, and preferably are hydrogen, fluorine, chlorine, or CF₃.

6. The dye compound according to any one of claims 1 to 5, wherein R2' and R2" are the same, and preferably are hydrogen, fluorine, chlorine, or CF₃.

7. The dye compound according to any one of claims 1 to 6, wherein both LI and L2 are represented by formula (II) wherein X is N.

8. The dye compound according to any one of claims 1 to 7, wherein LI and L2 are the same.

9. The dye compound according to any one of claims 1 to 8, wherein X is N, and R2 is the anchoring group, preferably -COOH or its salts.

10. The dye compound according to any one of claims 1 to 9, wherein the dye compound is selected from the formula (I-l), (1-2), (1-3), (1-4), (1-5), (1-6), (1-7), (1-8), (1-9), (1-10), (1-11), (1-12), (1-13), (1-14), (1-15), (1-16), (1-18) or (1-19).

(I-l)

25

25

11. The dye compound according to any one of claims 1 to 6, wherein one of LI and L2 is represented by formula (II) wherein X is C.

12. The dye compound according to claim 11, wherein the ligand other than the one represented by formula (II) is represented by formula (IV) :

(IV) wherein R4, R5 and R6 are independently selected from the group consisting of hydrogen, halogens, cyano, and alkyl groups, alkoxy groups, aryl groups, aryloxy groups, heterocycles, amino groups and their halogenated derivatives, NO₂, and anchoring groups.

13. The dye compound according to claim 12, wherein the anchoring group is selected from the group consisting of -COOH, -PO₃H₂, -PO₄H₂, -SO₃H, -CONHOH, acetylacetonate, acrylic acid derivatives, malonic acid derivative, rhodanine-3 -acetic acid, propionic acid, deprotonated forms of the

aforementioned, salts of said deprotonated forms, and chelating groups with π-conducting character, preferably -COOH and salts of -COOH, more preferably -COOH and ammonium or alkali metal salts of -COOH.

14. The dye compound according to any one of claims 11 to 13, wherein the dye compound is selected from the formula (1-17)

15. A semiconducting element, in particular a semiconducting layer, comprising a semiconductor and the dye compound according to any one of claims 1 to 14.

16. A dye-sensitized solar cell comprising the dye compound according to any one of claims 1 to 14 or the semiconducting element according to claim 15.