This application claims priority to European patent application

No. 12155142.8 filed on 13 February, 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 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.

Recently, developments have targeted low-cost organic solar cells, in particular dye-sensitized solar cells (DSSC) which are based on a dye-sensitized semiconductor electrode, and a counterelectrode sandwiched with 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 N719 dye 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 also has several shortcomings, e.g. difficult synthesis, isomers formation, relatively low extinction coefficient and troublesome cell manufacturing due to aggregation etc.

Instead of N719 or N749 as dyes for DSSC, US 2008/01 14174 Al discloses ruthenium complex of formula of RuLiL ₂ X having bi- and tri-dentate ligands of specific structures, as shown below.

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 formula (I)

ML1L2 (I) wherein M represents a metal belonging to Group 6, 8, 9, 10 or 1 1 of the long- format Periodic Table ;

LI and L2 are independently selected from tridentate ligands, at least one of LI and L2 corresponding to any one of the formula (II- 1), (Π-2), (Π-3), (II-4), (II-5)

(II-l ) Rl is selected from the group consisting of hydrogen, halogens, cyano, alkyl groups, halogenated alkyl groups, alkoxy groups, halogenated alkoxy groups, aryl groups, halogenated aryl groups, aryloxy groups, halogenated aryloxy groups, heterocycles, halogenated heterocycles, heterocyclic alkyl, heterocyclic alkoxy, hetercyclic aryloxy, amino groups, halogenated amino groups, N0 ₂ , and anchoring groups,

R2 is selected from the group consisting of halogens, cyano, alkyl groups, halogenated alkyl groups, alkoxy groups, halogenated alkoxy groups, aryl groups, halogenated aryl groups, aryloxy groups, halogenated aryloxy groups,

heterocycles, halogenated heterocycles, heterocyclic alkyl groups, halogenated heterocyclic alkyl groups, heterocyclic alkoxy groups, halogenated heterocyclic alkoxy groups, heterocyclic aryl groups, halogenated heterocyclic aryl groups, heterocyclic aryloxy groups, halogenated heterocyclic aryloxy groups, amino groups, halogenated amino groups, N0 ₂ , and anchoring groups, and

R3, R3', R4, R4', R5, R5\ R6, R7 and R8 are independently selected from the group consisting of hydrogen, halogens, cyano, alkyl groups, halogenated alkyl groups, alkoxy groups, halogenated alkoxy groups, aryl groups, halogenated aryl groups, aryloxy groups, halogenated aryloxy groups, heterocycles, halogenated heterocycles, heterocyclic alkyl groups, halogenated heterocyclic alkyl groups, heterocyclic alkoxy groups, halogenated heterocyclic alkoxy groups,

heterocyclic aryl groups, halogenated heterocyclic aryl groups, heterocyclic aryloxy groups, halogenated heterocyclic aryloxy groups, amino groups, and halogenated amino groups.

The invention 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.

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 are metal complexes bearing two tridentate ligands composed of three aromatic cycles and therefore conjugated. Also, the dye compounds of the present invention have at least one 5-membered aromatic heterocycle of pyrazole, imidazole or triazole in the terminal side of the tridentate ligand. In the present invention, triazole is understood to include both 1,2,3-triazole and 1 ,2,4-triazole. The 5-membered aromatic heterocycle in the terminal side has stronger electron-donating nature than monodentate ligand, e.g. NCS ligand, and thus, contributes 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 anchored to the semiconductor electrode. Such dyes can also contribute to the long-term stability of such devices, for example better resistance to substitution induced by trace amounts of water in the devices because of absence of monodentate ligands, such as NCS, and better shielding of the photoelectrode against corrosion through components present in the electrolyte, such as the triiodide/iodide couple.

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, aryl groups, or hydroxyl groups. The aryl groups are preferably optionally substituted phenyl groups, naphthyl groups, anthryl group and phenanthryl 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. The

heterocycles can further be substituted by other groups, such as alkyl groups, alkoxy groups, aryl groups or aryloxy groups as defined above. In the present invention, those substituted heterocycles may alternatively be named as

"heterocyclic alkyl groups" when alkyl groups is substituted, "heterocyclic alkoxy groups" when alkoxy groups is substituted, "heterocyclic aryl groups" when aryl groups is substituted, or "heterocyclic aryloxy groups" when aryloxy groups is substituted.

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 with or without an anchoring group. 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, alkoxy groups, aryl groups or aryloxy 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 usually an integer from 2 to 7) or [n]thienohelicenes (wherein, n is usually 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 optionally halogenated alkyl groups, alkoxy groups, aryl groups or aryloxy groups. The substituents may be any combination of the same or different substituents. Non-limiting examples of the substituents comprising thiophene moiety include at least one structure selected from the following structures :

wherein, R independently indicates optionally halogenated alkyl groups, alkoxy groups, aryl groups or aryloxy groups. The substituents comprising thiophene moiety may have an anchoring group as defined herein later on. For example, the substituents comprising thiophene moiety can be for instance thiophene in which 5-position is substituted by the anchoring group, such as cyanoacrylate, having following structure :

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.

In the present invention, "halogenated" is understood to denote in particular at least one of the hydrogen atoms of the following chemical group has been replaced by a halogen atom, preferably selected from fluorine and chlorine, more preferably fluorine. If all of the hydrogen atoms have been replaced by halogen atoms, the halogenated chemical group is perhalogenated. For instance, "halogenated alkyl groups" include (per)fluorinated alkyl groups such as (per)fluorinated methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl or tert-butyl ; and for instance -CF ₃ , -C ₂ F ₅ , heptafluoroisopropyl (-CF(CF ₃ ) ₂ ), hexafluoroisopropyl (-CH(CF ₃ ) ₂ ) or -CF ₂ (CF ₂ ) ₄ CF ₃ . Non-limiting example of "halogenated aryl groups" include -C ₆ F ₅ .

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 Ti0 ₂ . Suitable anchoring groups are for instance selected from, but not limited to, the group consisting of -COOH, -P0 ₃ H ₂ , -P0 ₄ H ₂ , -S0 ₃ H, -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) In the present invention, M is a metal atom, belonging to Group 6, 8, 9, 10 or 1 1 of the long-format Periodic Table as defined by the International Union of Pure and Applied Chemistry (IUPAC). 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 one embodiment of the present invention, both LI and L2 are independently selected from the formula (II- 1), (II-2), (II-3), (II-4), (II-5) or (II-6). According to this embodiment, two 5-membered aromatic heterocycles independently selected from pyrazole, imidazole or triazole which have stronger electron-donating nature than NCS ligand are located in each of the terminal side of the tridentate ligand, and thus, contribute to more increased electron density on the metal center. Also, in this embodiment, the 5-membered aromatic heterocycles selected from pyrazole, imidazole or triazole in the terminal side 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 pyrazole, imidazole or triazole anion group ("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. Ti0 ₂ ) conduction band and to directionality in the excited state of the sensitizer.

In another embodiment, ligands LI and L2 are the same. This is especially advantageous as, in such a case, only one kind of ligand (rather than two kinds of ligands) is needed for making the dye compounds, and the resulting molecule is symmetric. This embodiment allows for still better electron transfer and better synthetic yields.

In the present invention, Rl may be selected from the group consisting of hydrogen, halogens, cyano, alkyl groups, halogenated alkyl groups, alkoxy groups, halogenated alkoxy groups, aryl groups, halogenated aryl groups, aryloxy groups, halogenated aryloxy groups, heterocycles, halogenated heterocycles, heterocyclic alkyl groups, halogenated heterocyclic alkyl groups, heterocyclic alkoxy groups, halogenated heterocyclic alkoxy groups,

heterocyclic aryl groups, halogenated heterocyclic aryl groups, heterocyclic aryloxy groups, halogenated heterocyclic aryloxy groups, amino groups, halogenated amino groups, N0 ₂ , and anchoring groups as defined above.

Preferably, Rl is selected from the group consisting of hydrogen, chlorine, fluorine, N0 ₂ , -COOH, -P0 ₃ H ₂ , -P0 ₄ H ₂ , -S0 ₃ H, -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 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 CF ₃ . 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 may be selected from the group consisting of halogens, cyano, alkyl groups, halogenated alkyl groups, alkoxy groups, halogenated alkoxy groups, aryl groups, halogenated aryl groups, aryloxy groups, halogenated aryloxy groups, heterocycles, halogenated heterocycles, heterocyclic alkyl groups, halogenated heterocyclic alkyl groups, heterocyclic alkoxy groups, halogenated heterocyclic alkoxy groups, heterocyclic aryl groups, halogenated heterocyclic aryl groups, heterocyclic aryloxy groups, halogenated heterocyclic aryloxy groups, amino groups, halogenated amino groups, N0 ₂ , and anchoring groups as defined above. Preferably, R2 is selected from the group consisting of chlorine, fluorine, N0 ₂ , -COOH, -P0 ₃ H ₂ , -P0 ₄ H ₂ , -S0 ₃ H, -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 CF ₃ . 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 particular, R2 is selected from an anchoring group, preferably -COOH or its salts, or heterocycles, particularly substituents comprising thiophene moiety, especially substituents comprising thiophene moiety with an anchoring group. In the present invention, R3, R3\ R4, R4\ R5, R5', R6, R7 and R8 are independently selected from the group consisting of hydrogen, halogens, cyano, alkyl groups, halogenated alkyl groups, alkoxy groups, halogenated alkoxy groups, aryl groups, halogenated aryl groups, aryloxy groups, halogenated aryloxy groups, heterocycles, halogenated heterocycles, heterocyclic alkyl groups, halogenated heterocyclic alkyl groups, heterocyclic alkoxy groups, halogenated heterocyclic alkoxy groups, heterocyclic aryl groups, halogenated heterocyclic aryl groups, heterocyclic aryloxy groups, halogenated heterocyclic aryloxy groups, and amino groups, halogenated amino groups, preferably independently selected from the group consisting of hydrogen, halogens such as chlorine and fluorine, alkoxy groups such as OCH ₃ , aryloxy groups such as OPh, halogenated alkyl group such as CF ₃ and CH ₂ (CF2) ₃ CF ₃ , and the substituents comprising thiophene moiety having following structures :

In a further embodiment of the present invention, at least one of LI and L2 corresponds to the formula (II- 1).

(II- 1)

In this further embodiment, R3 is preferably selected from hydrogen, the optionally halogenated alkyl groups or aryl groups, preferably -CF ₃ ,

-CF ₂ (CF ₂ ) ₄ CF ₃ , -C ₆ F ₅ , or -C ₆ F ₄ H, or aryloxy groups, such as -OC ₆ H ₃ (OPh) ₂ . R3 ' can be a hydrogen. Also in this embodiment, R2 is selected from an anchoring group, preferably -COOH or its salts, or heterocycles, particularly substituents comprising thiophene moiety, especially substituents comprising thiophene moiety with an anchoring group.

In a certain embodiment of the present invention, at least one of LI and L2 corresponds to the formula (II-4).

In this further embodiment, R2 is selected from an anchoring group, preferably -COOH or its salts, or heterocycles, particularly substituents comprising thiophene moiety, especially substituents comprising thiophene moiety with an anchoring group. Also, in this embodiment, R6 is preferably selected from hydrogen, the optionally halogenated alkyl groups or aryl groups, preferably -CF ₃ , -CF ₂ (CF ₂ ) ₄ CF ₃ , -C ₆ F ₅ , or -C6F4H, or aryloxy groups, such as -OC ₆ H ₄ (OPh) ₂ .

In another embodiment, at least one of LI and L2 corresponds to the formula (II-5).

(II-5)

In the another embodiment, R7 is preferably selected from hydrogen, the optionally halogenated alkyl groups or aryl groups, preferably -CF ₃ ,

-CF ₂ (CF ₂ ) ₄ CF ₃ , -C ₆ F ₅ , or -C ₆ F ₄ H, or aryloxy groups, such as -OC ₆ H ₄ (OPh) ₂ . Also in this embodiment, R2 is selected from an anchoring group, preferably -COOH or its salts, or heterocycles, particularly substituents comprising thiophene moiety, especially substituents comprising thiophene moiety with an anchoring group.

As specific examples of the dye compounds of the present invention, the dye compounds can be represented by formula (I-l), (1-2), (1-3), (1-4), (1-5), (1-6), (1-7), (1-8), (1-9), (1-10), (I-l 1), (1-12), (1-13) or (1-14) : - 15 -

COOH

HOOC COOH

wherein n-Hx denotes n-hexyl and R' denotes alkyl or aryl groups.

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.

In the present invention, the dye compounds according to the present invention can be manufactured by a process comprising a step of reacting the corresponding tridentate ligand and metal ion source, such as RuCl ₂ (p-cymene) ₂ and tetrakis(dimethylsulfoxide)dichloro-Ru(II). Therefore, the present invention also relates to process for the manufacture of the dye compounds or their salt according to the present invention which comprises reacting the corresponding tridentate ligand and metal ion source, in particular RuCl ₂ (p-cymene) ₂ and tetrakis(dimethylsulfoxide)dichloro-Ru(II).

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 dye compound of the present invention or its salt in photoelectric conversion devices, especially in DSSC.

The DSSC offers the prospect of a low cost 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 Ti0 ₂ (titanium dioxide) sensitized with dye molecules to form the core of a DSSC. Ti0 ₂ is a preferred semiconductor material since its surface is highly resistant to the continued electron transfer. However, Ti0 ₂ 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 organic moiety 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 excites electrons from highest occupied molecular orbitals (HOMO) to lowest unoccupied molecular orbitals (LUMO), which is rapidly injected to the semiconductor particles (usually Ti0 ₂ ). 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 Ti0 ₂ ) 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 between the two electrodes. The material for the cathode is not limited as long as the cathode 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 anode 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 anode, a glass comprising the materials described above for the cathode 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 Ti0 ₂ and Sn0 ₂ are included. TiOF ₂ (titanyl oxyfluoride, titanium oxyfluoride or titanium fluoride oxide) can also be envisaged as a suitable semiconductor. TiOF ₂ 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 such as galium oxide etc.

The dye compound of the present invention is caused to be adsorbed on the semiconductor. The dye is adsorbed by causing the anode 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 compound and the semiconductor material. Other methods than the adsorption for achieving association of 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 Ti0 ₂ and the dye compound of the present invention or its salt or TiOF ₂ 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 I ₃ ^" + 2 e ^" 3 Γ + I ₂ . 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 with the electrolytic solution between the anode and cathode face to face. 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 its salt 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.

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 : Normalized UV-absorption spectra for the formula (1-1) prepared according to Example 1 , measured on a Hewlett-Packard 8453 UV -visible spectrometer in ethanol solution

Figure 2 : Current-voltage data for the DSSCs fabricated according to Example 2 Figure 3 : Synthetic route for the formulae (1-1) to (1-3)

Figure 4 : Synthetic route for the formulae (Ti l) to (1-13)

Figure 5 : Synthetic route to manufacture a tridentate ligand for the formula (1-4) Examples

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

4,4'-dimethyl-2,2'-bipyridine (2.03 g, 1 1.02 mmol) and Iron(II) sulfate heptahydrate (0.15 g, 0.55 mmol) were dissolved in MeCN (50 ml).

Trifluoroacetic acid (0.93 ml, 12.12 mmol), t-BuOOH (2.1 ml, 22.04 mmol), and acetaldehyde (7.4 ml, 132 mmol) were added, and the solution was heated under reflux for overnight. After cooling to room temperature, the volatiles were removed under vacuum. The residue was taken up in an aqueous 1M NaOH and extracted with ethylacetate. The product was purified by column

chromatography on Silica gel as white powder (Yield = 1.01 g (40.5 %)).

Ή NMR (400 MHz, CDC1 ₃ ) : 8.52 (d, J= 4.8 Hz, 1H), 8.40 (s, 1H), 8.30 (s, 1H), 7.85 (s, 1H), 7.14 (d, J= 1.2 Hz, 1H), 2.81 (s, 3H), 2.46 (s, 3H), 2.45 (s, 3H). MS (GC-EI) : m/z : calcd for C, ₄ Hi ₄ N ₂ 0 : 226.1 1 ; found : 226. 4,4'-dimethyI-6-(3-(trifluoromethyI)-lH-pyrazol-5-yI)-2,2'-b ipyridine

To a stirred mixture of NaOEt (0.78 ml, 9.94 mmol) and THF (50 ml) at 0°C was added a 20 mL solution of 4,4'-dimethyl-6-acetyl-2,2'-bipyridine (1.5 g, 6.63 mmol) in THF, followed by addition of ethyl trifluoroacetate (0.95 ml, 7.95 mmol). The mixture was refluxed for 12 h and the reaction was quenched with 2 M HC1 until pH8 ~ 9. Then the mixture was extracted with ethyl acetate. The extracts were then washed with deionized water, dried over anhydrous MgS0 ₄ , and concentrated in vacuum to give the diketone. Without further purification, the hydrazine monohydrate (2.9 ml, 59.7 mmol) was added into a solution of diketone reagent in EtOH (60 ml). After heating under reflux for 12 h, the solvent was removed under vacuum. The residue was dissolved in CH2CI2, and the solution was washed with water, dried over anhydrous MgS0 ₄ , and concentrated. Finally, the product was purified by silica-gel column chromatography using a 1 : 1 mixture of hexane and ethyl acetate, giving product as colorless solid (Yield = 1.26 g (59.7 %)).

H NMR (400 MHz, CDC1 ₃ ) : 8.55 (d, J= 4.8 Hz, 1H), 8.12 (s, 1H), 7.42 (s, 1H), 7.18 (d, J= 1.2 Hz, 1H), 6.95 (s, 1H), 2.46 (s, 3H), 2.45 (s, 3H). m/z : calcd for Ci ₆ Hi ₃ F ₃ N ₄ : 318.1 1 ; found : 318.

6-(3-(trifluoromethyl)-lH-pyrazol-5-yI)-2,2'-bipyridine-4,4 ^, -dicarbox Iic acid

A solid sample of 4,4'-dimethyl-6-(3-(trifluoromethyl)-lH-pyrazol-5-yl)- 2,2'-bipyridine (0.732 g, 2.30 mmol) was slowly added to a vigorously stirred solution of sulfuric acid (20 ml). K ₂ Cr ₂ 0 ₇ (2.86 g, 11.5 mmol) was added in small portions, keeping the temperature below 80°C. The mixture was stirred until the temperature gradually decreased back to room temperature. The deep green mixture was poured into ice-water mixture, and left overnight at 5°C. The precipitate was filtered and washed with water. This solid was transferred into 1 : 1 water and 98 % nitric acid (20 ml) and refluxed for 5 h. The solution was allowed to cool to room temperature and was poured into ice-water and kept at 5°C overnight. The precipitate was filtered, washed with water and Et ₂ 0, giving a colorless solid (Yield = 0.71 g (81.6 %)).

'H NMR (400 MHz, DMSO-d ₆ ) : 9.18 (s, 1H), 8.93 (d, J= 4.8 Hz, 1H), 8.85 (s, 1H), 8.43 (s, 1H), 7.94 (d, J= 1.2 Hz, 1H), 7.64 (s, 1H). m/z : calcd for C, ₆ H ₉ F ₃ N ₄ 0 ₄ : 378.06 ; found : 378. Bis[6-(3-(trifluoromethyl)-pyrazol-5-yl)-2,2'-bipyridine-4,4 '-dicarboxyIic acid]ruthenium(II)

6-(3-(trifluoromethyl)-lH-pyrazol-5-yl)-2,2'-bipyridine-4,4' -dicarboxylic acid (4 equivalents) and [RuCl ₂ (p-cymene)] ₂ (2 equivalents) were dissolved in DMF (75 mL), and the reaction mixture was heated to 160°C under Ar until UV- Absorption spectrum did not change. After the reaction finished, solvent was removed by evaporator. Reaction mixture was dissolved in hot acetone and kept it in the refrigerator. The resulting precipitation was collected on a sintered glass crucible by suction filtration to get a black powder (Yield = 82 mg (18 %)).

Ή NMR (400 MHz, DMSO-d ₆ ) : 9.08 (s, 1H), 9.03 (s, 1H), 8.71 (s, 1H), 7.49 (m, 2H), 7.46 (s, 1H). m/z : calcd for C ₃₂ H, ₆ F ₆ N80 ₈ Ru : 856.00 ;

found : 856.

Example 2 : Fabrication of Dye-Sensitized Solar Cells

The FTO glass substrates were immersed in 40 mM TiCl ₄ aq. at 70°C for 30 min and washed with water and ethanol. The 1 1 mm thick mesoporous nano-Ti0 ₂ films composed of 20 nm anatase Ti0 ₂ particles were coated on the FTO glass plates by repetitive screen printing. After drying the nanocrystalline Ti0 ₂ layer at 125°C, a 4 mm thick second layer of 400 nm sized light scattering anatase particles (CCIC, HPW-400) was deposited by screen printing onto the transparent layer. The Ti0 ₂ electrodes were gradually heated under an air flow at 325°C for 5 min, at 375°C for 5 min, at 450°C for 15 min and 500°C for 15 min. The Ti0 ₂ electrodes were treated again by TiCl ₄ and sintered at 500°C for 30 min. The Ti0 ₂ electrodes were immersed into the

Bis[6-(3-(trifluoromethyl)-pyrazol-5-yl)-2,2'-bipyridine- 4,4'-dicarboxylic acid]ruthenium(II) solutions (0.3 mM in DMF with 3 mM of chenodeoxycholic acid) and kept at room temperature for 20 h. The chenodeoxycholic acid is added to reduce aggregation of dye molecules leading to higher efficiency. A platinized FTO glass was used as counter electrode. Two different kinds of electrolytes were incorporated for DSSCs. An electrolyte solution, Z960 was composed of 1.0 M 1 ,3-dimethyl imidazolium iodide, 0.03 M iodine,

0.5M tert-butylpyridine, 0.1M guanidinium thiocyanate, 0.05M lithium iodide in 85/15(v/v) acetonitrile/valeronitrile. The active area of DSSCs was 0.16 cm ² Measurements of current and voltage charecteristics showed power conversion efficiency (PCE) of 6.43 % (in average).