WO2010088723A1

WO2010088723A1 - Dye composition for use in photoelectric material

Info

Publication number: WO2010088723A1
Application number: PCT/AU2010/000105
Authority: WO
Inventors: David Leslie Officer; Gordon George Wallace; George Tsekouras; Attila Janos Mozer; Ying Dong; Pawel Wagner; Klaudia Wagner
Original assignee: University Of Wollongong
Priority date: 2009-02-03
Filing date: 2010-02-03
Publication date: 2010-08-12

Abstract

A dye composition for use in a photoelectric material, the dye composition comprising a plurality of dyes comprising a chromophore and at least one binding group for binding with a semiconductor (preferably a metal oxide semiconductor) wherein the plurality of dyes include a first dye wherein the binding group is linked to the dye chromophore by a linker not in conjugation with the chromophore and a second dye wherein the binding group is attached by a linker in conjugation with the chromophore.

Description

Dye Composition for Use in Photoelectric Material

Field

The invention relates to a dye composition for use in photoelectric material, to photoelectric material such as dye sensitized semiconductor materials and devices, such as dye-sensitized electrodes and dye- sensitized solar cells (DSSCs), incorporating the material and to methods of preparing the compositions, photoelectric material and devices.

Background

Photoelectric devices are devices that function on the basis of the photoelectric effect, namely, the absorption of photon (light) energy by electrons, leading to their release from a surface or otherwise allowing conduction. The efficiency of such devices is measured in terms of photon-to-current conversion.

A dye sensitized solar cell (DSSC) is a photovoltaic system which has a photoelectric material in the form of a metal oxide having an adsorbed dye so as to produce excited electrons from the incident light. In addition to two electrodes, including a light transmissible electrode such as transparent conducting oxide (TCO) and a counter electrode, the DSSC includes an electrolyte separating the dye from the counter electrode and which may be a liquid or solid such as a hole transport material.

Photoelectric materials used in the manufacture of these devices include semiconductors. In these semiconductor-based devices, photon energy is absorbed and excited electrons are injected into the conduction band of the semiconductor. Zinc oxide (ZnO), titanium dioxide (TiO₂), nickel oxide (NiO₂) and tin dioxide (SnO₂) are examples of wide-band-gap (> 3.0 eV) semiconductors. These semiconductors absorb photon energy with wavelengths < 413nm.

The photoelectrical material is generally in the form of semiconductor coated with a thin layer of sensitising dye (chromophore). If the oxidative energy level of the excited state of the dye molecule is favourable (i.e. more negative) with respect to the conduction band energy level of the semiconductor, then there will be electron transfer and injection of an excited electron into the conduction band of the semiconductor.

Titanium dioxide is a preferred substrate for the preparation of dye-sensitised solar cells (DSSCs). It is a chemically inert, non-toxic and biocompatible semiconductor readily available in high purity. It therefore represents an economical and ecologically safe semiconductor for use in the preparation of photoelectric materials.

Thin films of Tiθ2 are prepared by many different physical and chemical techniques such as thermal oxidation, sputtering and chemical vapour deposition. Transparent mesoporous nanocrystalline films of TiO₂ with large surface area may be prepared, for example by depositing nanosized colloidal TiO₂ particles on a support.

Coating mesoporous nanocrystalline films of metal oxide with a thin layer of sensitising dye has provided DSSCs with absorbance in the visible part of the solar light spectrum and improved solar energy conversion efficiency. To date, the most successful DSSCs are ruthenium-polypyhdyl based dyes adsorbed on nanocrystalline films of TiO₂ (M. K. Nazeeruddin, P. Pechy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G. B. Deacon, C. A. Bignozzi, M. Gratzel, J. Am. Chem. Soc. 123 (2000) 1613). Recently, purely organic-based DSSCs have been reported that provide energy conversion efficiencies of >7 % (Z. -S. Wang, Y. Cui, Y. Dan-oh, C. Kasada, A. Shinpo, and K. Hara, J. Phys. Chem. C 111 (2007) 7224 (coumahn dye), S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M. K. Nazeeruddin, P. Pechy, M. Takata, H. Miura, S. Uchida, M. Gratzel, Adv. Mater. (Weinheim, Ger.) 18 (2006) 1202 (indoline dye) and W. M. Campbell, K. W. Jolley, P. Wagner, K. Wagner, P. J. Walsh, K. C. Gordon, L. Schmidt-Mende, M. K. Nazeeruddin, Q. Wang, M. Graetzel, D. L. Officer, J. Phys. Chem. C 111 (2007) 11760 (porphyrin dye)).

Ruthenium-based dyes are perhaps the most widely used in photoelectric materials, however, they are likely to become increasingly more expensive as the demand for ruthenium raw materials increases. Alternatives to ruthenium-polypyridyl complexes for use as sensitising dyes have therefore been sought.

The use of organic dyes as sensitising dyes is attractive since they are inexpensive and readily available. Numerous organic dyes have been used for the photosensitisation of wide-band-gap semiconductors like NiO, ZnO and Tiθ2, the most common being coumahn (A. Nattestad, M. Ferguson, R. Kerr, Y.-B. Cheng and U. Bach, Nanotechnology (2008) 19 295304; Z.-S. Wang, Y. Cui, Y. Dan-oh, C. Kasada, A. Shinpo, and K. Hara, J. Phys. Chem. C 111 (2007) 7224), indoline (D. Kuang, S. Uchida, R. Humphry-Baker, Shaik M. Zakeeruddin, and M. Gratzel, Angew. Chem. Int. Ed. 47 (2008) 1923), thiophene (H. Choi, C. Baik, S. O. Kang, J. Ko, M. -S. Kang, Md. K. Nazeeruddin, and M. Gratzel, Angew. Chem. Int. Ed. 47 (2008) 327), oligoene (T. Kitamura, M. Ikeda, K. Shigaki, T. Inoue, N. A. Anderson, X. Ai, T. Lian, and S. Yanagida, Chem. Mater. 16 (2004) 1806), merocyanine (K. Sayama, S. Tsukagoshi, K. Hara, Y. Ohga, A. Shinpou, Y. Abe, S. Suga, H. Arakawa, J. Phys. Chem. B 106 (2002) 1363), porphyrin (W. M. Campbell, K. W. Jolley, P. Wagner, K. Wagner, P. J. Walsh, K. C. Gordon, L. Schmidt-Mende, M. K. Nazeeruddin, Q. Wang, M. Graetzel, D. L. Officer, J. Phys. Chem. C 111 (2007) 11760) and phthalocyanine (JJ. He, A. Hagfeldt, S.E. Lindquist, H. Grennberg, F. Korodi, L. C. Sun, B. Akermark, Langmuir 17 (2001 ) 2743).

The central role of the dyes is the efficient absorption of light and its conversion to electrical energy. In order for the dyes to provide high efficiency ideally every absorbed photon should be converted to an electron resulting from an excited dye state. In order for the dye to be returned to its initial state, ready for absorption of another photon, it has to accept an electron from the hole transport material. To ensure many turnovers and a long useful life of the device, both electron injection into the electron transport material and hole injection into the hole transport material has to be faster than any other chemistry that the dye is subject to. For example, it is important that the dyes do not recapture electrons injected into the electron transport material or serve as an electronic pathway from the electron transport material to the hole transport material. There is a need for dye compositions and photoelectric materials and devices utilizing dyes which provide high photoelectric conversion.

The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Summary

We provide a dye composition for use in a photoelectric material, the dye composition comprising a plurality of dyes comprising a chromophore and at least one binding group for binding with semiconductor wherein the plurality of dyes include a first dye wherein the binding group is linked to the dye chromophore by a linker not in conjugation with the chromophore and a second dye wherein the binding group is attached by a linker in conjugation with the chromophore.

The semiconductor may be an organic semiconductor, a metal oxide semiconductor or mixture thereof.

In particularly preferred embodiments there is a molar excess of said first dye. For example, in an example of this embodiment there is provided a mixture of the dyes wherein the molar ratio of said first dye to said second dye is in the range of from 1.5:1 to 5:1 preferably from 2:1 to 4:1 and most preferably about 3:1.

We also provide, in other embodiments, a photoelectronic material comprising a semiconductor and a dye composition comprising a plurality of dyes comprising a chromophore and at least one binding group bound with a the semiconductor wherein the plurality of dyes include a first dye wherein the bound group is linked to the dye chromophore by a linker not in conjugation with the chromophore and a second dye wherein the bound group is attached by a linker in conjugation with the chromophore.

In further embodiments there is provided a photo electronic device comprising the photo electronic material which in one set of embodiments are in the form of a dye sensitized solar cell.

In one set of embodiments there is provided a dye-sensitized solar cell comprising a dye sensitized electrode, the dye sensitized electrode comprising a substrate having an electrically conductive surface, an electron transporting layer that is disposed on the electrically conductive surface, and a dye composition as hereinbefore described bound to the electrically conductive surface. The DSSC will also generally comprise a counter electrode; and a hole transporting layer in contact with the dye-sensitized electrode and the counter electrode.

Detailed Description

A number of standard terms are used in the specification and claims and unless the context calls for another meaning, the following terms have the meanings provided.

"Aliphatic" The term aliphatic refers to straight or branched chain non-aromatic groups which in the context of the linking group are bonded at both ends of a chain to nominated portions of the molecule. Preferred examples of aliphatic are Ci to C20 aliphatic, more preferably C2 to Ci₄ aliphatic. Aliphatic includes alkanes, alkenes and alkynes with alkanes and alkenes generally preferred. In the case of long chain alkenes used to provide conjugation the linker will be a polyene.

"Arylene" refers to a aromatic linker which is bonded to two other portions of the molecule. Arylene may optionally be substituted by one or two substituents in addition to the linked portions of the molecule preferably the optional substituents being selected from the group consisting of hydroxyl, Ci to C₄ alkoxy, carboxyl and Ci to C₄ alkoxycarbonyl. "β-substituted porphyrin" means a substituted porphyrin including a substituent at the β- pyrollic carbon(s) of the porphyrin nucleus where the porphyrin exists in the free base, protonated diacid, dianion or metallated forms.

"Bound" means by an ester formation, coordination (syn-syn bridging), chelating, or H- bonding interaction between one or more binding functional groups of the dye and the semiconductor surface.

"Carboxylic acid" means a compound (or substituent) having one or more carboxyl radicals and phosphonic acid and sulfonic acid have corresponding meanings.

"Throughout the description and the claims of this specification the word "comprise" and variations of the word, such as "comprising" and "comprises" is not intended to exclude other additives, components, integers or steps.

"Conjugated" or "conjugation" in the context of the binding group being conjugated with or in conjugation with a chromophore or sparated from the chromophore by a conjugated linker refers to a system of atoms covalently bonded with a continuous chain of atoms possessing aligned p-orbitals providing derealization of electrons across the chain of atoms. Conjugation is commonly achieved by alternating single and multiple bonds. In addition to alternating carbon-carbon single and multiple bonds it will be appreciated that the chain of conjugation may pass through aromatic rings.

Conversely, a reference to "not in conjugation" means there is not a continuous chain of atoms with aligned p-orbitals so that derealization of electrons within the chain is not possible. This may for example be brought about by two or more contiguous carbon- carbon single bonds.

"Hole conducting material" means a material that allows the regeneration of the dye after electron injection in to the conduction band of the semiconductor due to its hole transport properties.

"Non-acceptor" means a substance used to coat the semiconductor surface to raise the conduction band potential at the electrode-electrolyte interface.

The phrase "overall conversion efficiency" means the conversion efficiency measured in a solid state photovoltaic window normalised to provide a corrected "overall conversion efficiency" of 3.0 % for ZnTPP-=-=(CO₂H)₂.

In general we have found that by using a combination of a first dye wherein the binding group is linked to the dye chromophore by a linker not in conjugation with the chromophore and a second dye wherein the binding group is attached by a linker in conjugation with the chromophore provide a synergistic interaction.

The said first and second dyes may be independently selected from compounds of formula I

Ch - L -R¹ (I)

wherein

Ch is the dye chromophore;

R¹ is a binding group;

(A) in said first dye the L is a linker which does not provide conjugation between the chromophore and R¹; and (B) in said second dye the linker group L is a bond or a linker which provides conjugation between the chromophore (Ch) and binding group R¹.

In one embodiment the said first dye of formula I has L as a linker group which does not provide conjugation between the porphyrin chromophore and R¹ and is preferably selected from the group consisting of -X-, -X-CH(R⁶)- and X=C(R⁶) wherein X is optionally substituted non-conjugated hydrocarbyl such as non-conjugated aliphatic, non- conjugated aliphatic-arylene and non-conjugated aliphatic-arylene-aliphatic; and wherein R⁶, when present, is selected from the group consisting of: H, CN and a binding group; and the said second dye of formula I has L¹ as a bond or linker group which provides conjugation between the porphyrin chromophore and R¹; and is preferably selected from -X- and -X=C(R⁶)- wherein X is optionally substituted conjugated hydrocarbyl such as conjugated aliphatic, conjugated arylene, conjugated aliphatic-arylene and conjugated aliphatic-arylene-aliphatic; and wherein R⁶, when present, is selected from the group consisting of: H, CN and a binding group.

The dyes used in the mixture have at least one binding group for binding with the semiconductor. Many dyes having binding groups for binding with semiconductors are individually known in the art and the skilled person will have an array of existing dyes at their disposal for use in formulating the mixture.

The binding group of the first and second dye (R¹ in formula I) is preferably selected from the group consisting of alcohol, amino, nitrile, thiocyanate, acetoacetonate, hydroxyquinolate, alizarin, barbituric acid, carboxylic acid, dicarboxylic acid, phosphoric acid, phosphinic acid, sulphonic acid or hydroxamic acid or combinations thereof and preferably is a carboxyl and more preferably a carboxyl present as a styryl or dehydrostyryl carboxylic acid.

The mixture of dyes called for by the invention may use combinations of dyes previously reported individually. In one embodiment the chromophore (Ch in formula I) is selected from the group consisting of porphyrin dyes, porphyrazine dyes, phthalocyanine dyes, coumahn dyes, indoline dyes, rhodanine dyes, thiophene dyes, xanthene dyes, such as rhodamine B, rose bengal, eosin, and erythrosine, cyanine dyes, such as quinocyanine and kryptocyanine, anthraquinone dyes and polycyclic quinone dyes, azo dyes, basic dyes such as phenosafranine, fog blue, thiosine, and methylene blue, and coordination compounds containing a metal atom, such as ruthenium, rhenium and iridium pyridyl, bipyridyl and terpyridyl complexes preferably from porphyrin dyes, porphyrazine dyes, phthalocyanine dyes and coumarin dyes and most preferably from porphyrins.

A range of dyes complying with the requirements for said first dye or said second dye have previously been reported but to our knowledge have not be reported or used in combination and in particular have not been reported for use in combination in photoelectronic materials or devices.

Examples of publications describing dyes which may be used in mixed dye systems described hereinabove include:

US Application Pub 2007/0073052 describes coumarin, indoline, cyanine and hemicyanine based dyes of formula: D-SpI -Ch-Sp2-Acc-Y wherein the groups D, Ch, Ace and Y are conjugate with each other, the group D is a donor group, the group Ch is a chromophore rendering low HOMO-LUMO gap or a polyaromatic chromophore, the group Ace is an acceptor group, the group Y is a binding group, and each of Sp1 and Sp2 represents a single bond or a spacer group providing conjugation between the groups D and Ch or between the groups Ch and Ace. D-SpI -Ch-Sp2-Acc-Y.

Such compounds may be used together with corresponding compounds in which there is not conjugation between anchoring group and chromophore or other suitable dyes in which the chromophore is not conjugated with the binding group.

Li et al. US 2007/0151600 describe liquid crystalline porphyrins which may be used as the second dye in combination with suitable non-conjugated analogues or other dyes having binding groups not conjugated with the chromophore. US 2008/0015356 describes binuclear metal complex having a substituent carboxyl conjugated with the chromophore substituted with substituents many of which are in conjugation with the chromophore.

US 6359211 describes dyes comprising a chromophore and attachment groups for attachment to a semiconductor. The attachment groups are separated from the chromophore by linkages not in conjugation with the chromophore. The dyes may be cyanine, oxazine, thiazine or acridine dyes.

US2008/0087325 describes ruthenium bipyridyl compounds some of which provide conjugated metal oxide binding groups.

US 6043428 describes phthalocyanine dyes.

Sayama et al. "Efficient sensitization of nanocrystalline TiO₂ films with cyanine and merocyanine dyes" Solar Energy Materials and Solar Cells" 80 (2003) 47-71 describe various dye some having conjugated binding groups and some having non-conjugated binding groups. The paper may be used in preparation of suitable dyes for use in combination as provided hereinbefore.

Campbell et al. "Porphyrins as light harvesters in dye- sensitized TiO₂ solar cells" Coord. Chem. Rev. 248 (2004) 1363-1379 describe a range of carboxylic acid substituted porphyrins some of which have a conjugated carboxyl group and others of which have a carboxyl separated from the chromophore by a non-conjugated linker. This paper may be used to select suitable examples of the first dyes and or the second dyes.

Hasobe et al. "Organization of supramolecular assemblies of fullerene, porphyrin and fluorescein dye derivatives on TIO₂ nanoparticles for light energy conversion Chem. Phys. 319 (2005) 243-252 describe a range of carboxyl substituted dyes. Hasselman et al. "Theoretical Solar-to-Electrical Energy-Conversion Efficiencies of Perylene-Porphyrin Light Harvesting Arrays" J. Phys. Chem. B 2006, 110, 25430-25440 describe a range of porphyrin based dyes including some having conjugated carboxyl binding groups.

Jayasundera at al. "Design of high-efficiency solid-state dye-sensitized solar cells using coupled dye mixtures" Solar Energy Materials and Solar Cells 90 (2006) 864-871 describe ruthenium di- and ter-pyhdyl dyes bonded to a TΪO2 film by carboxyl groups.

Campbell et al. "Highly Efficient porphyrin Sensitizers for Dye-Sensitized Solar Cells" J. Phys. Chem. C 2007, 111 11760-11762 describe zinc porphyrin dyes having a 4,4- dicarboxy-1 ,3-butadien-1-yl substituent in the β-position of the porphyrin.

Campbell et al. International Application WO2006/038823 discloses a range of porphyrin dyes having a binding group or groups linked to the chromophore by a conjugated linker.

The Campbell et al. dyes are preferred for use as said second dyes and are of formula:

and where:

R¹ is selected from the group consisting of: carboxylic acids, phosphonic acids, sulfonic acids, or salts thereof;

R2, R3, R4 and R5 are independently selected from the group consisting of: H, substituted or unsubstituted alkyl, substituted or unsubstituted aryl, and substituted or unsubstituted alkyl aryl;

R⁶ is selected from the group consisting of: H, CN or -COOH; and M is absent (and the porphin exists in the free base, protonated diacid, or dianion form) or is selected from the group consisting of: Cu, Ni or Zn.

Wang et al. "Efficient Light Harvesting by Using Green Zn-Porphyhn-Sensitized Nanocrystalline TiO₂ Films" J. Phys. Chem. B 2005, 109, 15397-15409 describe a range of zinc metalloporphyhn dyes some of which have a carboxyl substituent in conjugation with the chromophore and some of which have a saturated linker between the chromophore and carboxyl so that the carboxyl liker is not conjugated with the chromophore.

Nazeeruddin et al. "Application of Metalllo porphyrins in the Nanocrystalline Dye- Sensitized Solar Cells for Conversion of Sunlight to Electricity" Langmuir 2004, 20, 6514- 6517 describe a range of metalloporphyrins having a carboxyl group in a substituent providing conjugation between the chromophore and carboxyl group.

In one set of embodiments at least one of said first and second dyes comprises a porphyrin. Thus, the first dye may comprise a chromophore which is a porphyrin and the second dye another type of dye chromophore such as selected from the general classes listed above; the first dye may have a non-porphyhn dye chromophore such as selected from these general classes listed above and said second dye may comprise a porphyrin; or both said first and second dyes may comprise a porphyrin chromophore.

In the most preferred embodiments, the first and second dyes are porphyrins comprising a linker to said binding group in the β-position of the porphyrin.

We have found that the combination of dyes including (A) a first dye wherein the binding group is linked to the dye chromophore by a linker not providing conjugation of the binding group with the chromophore and (B) a second dye wherein the binding group is attached by a linker providing conjugation with the chromophore give rise to a synergistic enhancement in photon to current conversion. Each of said first and said second dyes may comprise one or more dyes satisfying the requirements hereinbefore described.

Without wishing to be bound by theory we believe the first dye (comprising a chromophore not in conjugation with the linker) acts as a 'source' of energy and electrons and that the second dye (comprising a chromophore in conjugation with the linker) would act as a 'sink'. The linker of the first dye is not conjugated, for example by including a saturated alkylene linker, which we consider may disrupt direct electron injection into the semiconductor and to facilitate energy and electron transfer to the second dye chromophore, which is provided with a conjugated binding group to provide electron injection into the semiconductor.

Brief Description of Drawings In the drawings:

Figure 1 is a scheme depicting a possible mechanism for the synergistic interaction between said first dye and said second dye.

Figure 2a and Figure 2b show the special arrangement of the first dye and second dye in a ratio of 1 :1 and 3:1 respectively. Figure 3 is a normalised UV-Vis absorption and photoluminescence (PL) spectra for porphyrins A and B in THF solution. PL spectra obtained using 500 nm excitation.

Figure 4 is a graph showing CVs of 2 x 10^"4 M DMF solutions of porphyrins A and B with

0.1 M TBAP supporting electrolyte. Scan rate = 100 mV s^~1.

Figure 5 shows HOMO/LUMO levels compared to vacuum of porphyrins A and B in THF solution.

Figure 6 is UV-Vis absorption spectra of porphyrins A and B bound to TiO₂.

Figure 7 is a graph showing CVs of porphyrins A and B bound to TiO₂ in 0.1 M TBAP in

ACN electrolyte. Scan rate = 100 mV s^"1.

Figure 8 shows the HOMO/LUMO levels compared to vacuum of porphyrins A and B bound to TiO₂. Figure 9 is a graph showing absorbance ratios abs@560nm / abs@520nm measured from UV-Vis spectra in solution and on TiO₂ as a function of % A in sensitisation solution.

Figure 10 is a graph showing I-V curves of DSSCs based on Zn salt A, free base B, and an optimised mixture of both A and B. AM1.5, 100 mW cm^"2 illumination. Figure 11 shows representative IPCE profiles of DSSCs based on Zn salt A, free base B, and an optimised mixture of both A and B.

Figure 12 shows representative Nyquist plots obtained from EIS characterisation of

DSSCs based on Zn salt A, free base B and an optimised mixture of A and B. AM1.5,

100 mW cm^"2 illumination. Figure 13 shows a proposed mechanism involving light-induced energy and electron transfer between porphyrins in DSSCs based on mixtures of Zn salt A and free base B.

Figure 14 is a graph of efficiency as a function of % A on TiO₂ for DSSCs based on mixtures of Zn salt A and free base B. Efficiencies of DSSCs based on Zn salt A (i.e. 100

% A) and free base B (i.e. 0 % A) included. The A and B referred to in describing the figures are the dyes A and B referred to in the examples.

A possible mechanism for the synergistic interaction between said first dye and said second dye will be discussed with reference to Figures 1 , 2a and 2b using the example of a first dye comprising a schematically represented zinc porphyrin chromophore not conjugated with the binding group and a porphyrin free base chromophore conjugated with the binding group. Referring to Figure 1 , excitation of a zinc porphyrin (A) could lead to energy transfer to the free base porphyrin (B). This state could also be achieved by excitation of the free base porphyrin (C). Electron injection (D) from the excited free base porphyrin followed by electron transfer from a zinc porphyrin to free base porphyrin (E) would then lead to an oxidized zinc porphyrin, which would undergo normal reduction by an electrolyte (F) such as the iodide/iodine couple as shown in Figure 1. Note that we have found that the free base porphyrins provide relatively poor contribution to the photoelectrical current generating process but are important in the transfer of electrons via conjugation to the metal oxide. In order to optimize the synergy observed from the combination we have found that the dye having a chromophore not in conjugation with the binding group is preferably present in excess (based on molar ratio) of the dye having a chromophore in conjugation with the binding group, preferably the molar ratio is from 1.5:1 to 5:1 more preferably from 2:1 to 4:1 and most preferably about 3: 1.

The reason for the improved performance in the presence of an excess of dye with a non-conjugated binding group may be explained with reference to the spatial arrangement depicted in Figures 2a and 2b (again using the example of a first dye comprising a schematically represented zinc porphyrin chromophore not conjugated with the binding group and a porphyrin chromophore conjugated with the binding group). In a 1 :1 molar ratio of dyes of conjugated and non-conjugated binding group each free base conjugated porphyrin is surrounded by four zinc porphyrin and four free base porphyrins and energy transfer could occur from either of the porphyrins. However, only the transfer from the non-conjugated zinc porphyrin leads to photocurrent. In contrast the 3:1 mixture represented in Figure 6b would allow the conjugated free base porphyrin to have only non-conjugated zinc porphyrin neighbours allowing photocurrent generation from every oxidized free base porphyrin.

We therefore consider, without wishing to be bound by theory, that exciton coupling, as occurs in photosynthesis, may account for the synergistic interaction between the dyes.

In an embodiment of the invention the first and second dyes are independently chosen from non-metallated porphyrins or porphyrins metallated with a metal selected from the group selected from zinc, magnesium, nickel, copper, cobalt, iron, tin, ruthenium, cadmium, palladium, platinum and more preferably said first dye is a metallated porphyrin containing zinc and said second dye is a non-metallated porphyrin.

The more preferred dyes for use in the mixed dye compositions are compounds comprising (A) at least a first dye comprising at least one compound of formula I

wherein:

R¹ is a binding group a or salts thereof;

R₂, R3, R₄ and R₅ are independently selected from the group consisting of: H, substituted or unsubstituted alkyl, substituted or unsubstituted aryl, and substituted or unsubstituted alkyl aryl;

L is a linker group which does not provide conjugation between the porphyrin chromophore and R¹; and is preferably selected from -X-, XCH(R⁶)- and -X=C(R⁶)- wherein X is optionally substituted non-conjugated hydrocarbyl such as non-conjugated aliphatic, non-conjugated aliphatic-phenylene and non- conjugated aliphatic-phenylene-aliphatic ;

R⁶ when present is selected from the group consisting of: H, CN and a binding group; and

M is absent (and the porphyrin exists in the free base, protonated diacid, or dianion form) or is selected from the group consisting of: Cu, Ni or Zn and preferably M is zinc; and (B) a second dye comprising at least one compound of formula III

R¹ is a binding group or salts thereof;

L¹ is a bond or linker group which provides conjugation between the porphyrin chromophore and R¹; and is preferably selected from -X- and X=C(R⁶)- wherein X is optionally substituted conjugated hydrocarbyl such as conjugated aliphatic, conjugated aliphatic-phenylene and conjugated aliphatic-phenylene-aliphatic ;

R⁶ is selected from the group consisting of: H, CN or a binding group; and

M is absent (and the porphyrin exists in the free base, protonated diacid, or dianion form) or is selected from the group consisting of: Cu, Ni or Zn and preferably M is absent.

In a further set of embodiments said first dye is of formula (IV)

wherein:

R¹ is a binding group a or salts thereof;

X is optionally substituted non-conjugated hydrocarbyl such as non-conjugated aliphatic, non-conjugated aliphatic phenylene and non-conjugated aliphatic phenylene aliphatic; and

R⁶ is selected from the group consisting of: H, CN and a binding group;

In one set of embodiments said second dye includes at least one compound of formula V

R¹ is a binding group a or salts thereof;

X is optionally substituted conjugated hydrocarbyl such as conjugated aliphatic, conjugated aliphatic-phenylene and conjugated aliphatic-phenylene-aliphatic; and

R⁶ is selected from the group consisting of: H, CN and a binding group;

In one set of embodiments said first dye is of formula Vl

wherein:

A and B do not provide conjugation of the prophyrin chromophore to the group R¹ and are independently selected from aliphatic and the bond between B and adjacent carbon C may be a single or double bond and when the bond is a double bond x is 0;

R⁶ is selected from the group consisting of H, CN and a bonding group;

R⁷ when present is hydrogen; and

x is 0 or 1. In one set of embodiments said second dye includes at least one compound selected from formula VII:

wherein: A¹ and B¹ provide conjugation of the prophyrin with the group R¹ and wherein

R¹ to R⁶ are as hereinbefore defined.

The best results will generally be obtained using a dye combination which exhibits long- lived (>1 ns) π^* singlet excited states and only weak single/triplet mixing. The combination will preferably have an appropriate LUMO level that resides above the conduction band of the TiO₂ and a HOMO level that lies below the redox couple in the electrolyte solution. This provides charge separation at the semiconductor-dye- electrolyte surface.

Specific examples of said first and second dyes include:

(VIIIB)

The compositions of the present invention are useful as photosensitizers for applications in optoelectronic devices, optical sensors, devices for hydrogen preparation by water splitting, and as absorptive contrast agents. In one set of embodiments, the device comprises a dye-sensitized electrode. In a further embodiment, the compositions of the present invention are comprised within the dye component of a dye-sensitized electrode present in a dye-sensitized solar cell.

Thus, in one embodiment, the present invention provides a dye-sensitized electrode comprising a substrate having a transparent conductive layer, a semiconductor disposed on the transparent conductive layer and a composition comprising the dye mixture as hereinbefore described bound to the surface of the semiconductor.

A photosensitive electrode may be associated with a substrate such as a glass or transparent plastic substrate. At least one surface of the substrate is coated with a substantially transparent, electrically conductive material. Suitable materials that can be for coating are substantially transparent conductive oxides, such as indium tin oxide (ITO), tin oxide, indium oxide, zinc oxide, antimony oxide, and mixtures thereof. A substantially transparent layer, a thin film, or a mesh structure of metal such as silver, gold, platinum, titanium, aluminum, copper, steel, or nickel may be also suitable.

The semiconductor facilitates transfer of charge across the cell by transferring the electron ejected from the dye to the electrode. It is thus desirable for the electron transporting layer to have a lowest unoccupied molecular orbital (LUMO) energy level or conduction band edge that closely matches the LUMO of the metal complex to facilitate the transport of electrons between the metal complex and said electron transporting layer.

Examples of suitable semiconductor materials for an electron transporting layer include, but are not limited to, metal oxide semiconductors; tris-8-hydroxyquinolato aluminum (AIQ. sub.3); cyano-polyphenylene vinylene (CN-PPV); and oligomers or polymers comprising electron deficient heterocyclic moieties, such as 2,5-diaryloxadiazoles, diaryl trazoles, thazines, pyridines, quinolines, benzoxazoles, benzthiazoles, or the like. Other exemplary electron transporters are particularly functionalized fullerenes (e.g., 6,6- phenyl-C61 -butyl acid-methylester), difluorovinyl-(hetero)arylenes, 3-(1 ,1-difluoro- alkyl)thiophene group, pentacene, n-decapentafluoroheptyl-methylnaphthalene-1 , 4,5,8- tetracarboxylic diimide, poly(3-hexylthiophene), poly(3-alkylthiophene), dihexyl- anthradithiophene, phthalocyanine, C60 fullerene, or the like, or a combination comprising at least one of the foregoing electron transporters.

The semiconductor may be an organic semiconductor, metal oxide semiconductor or combination thereof.

Examples of preferred organic semiconductors include fullerenes, oxadiazoles, carbon nanotubes, graphene and organic polymeric semiconductors such as polymers containing CN groups and organic polymers containing CF3 groups.

The semiconductor may be a mixture of a metal oxide semiconductor and another semiconductor. For example in some embodiments the semiconductor may be a porous metal oxide semiconductor with pores at least partly filled by another semiconductor such as an organic semiconductor.

Suitable metal oxide semiconductors are oxides of the transition metals and oxides of the elements of Group III, IV, V, and Vl of the Periodic Table. Oxides of titanium, zirconium, hafnium, strontium, zinc, indium, yttrium, lanthanum, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, iron, nickel, silver or mixed oxides of these metals may be employed. Other suitable oxides include those having a perovskite structure such as SrTiO₃ or CaTiO₃. The semiconductor layer is coated by adsorption of the dye composition the surface thereof. The dyes interact with the surface of the semiconductor layer via the binding groups present in each of said first and second dyes of the dye composition. In one particularly preferred set of embodiments titanium dioxide (Tiθ2) is used as the semiconductor in an electron-transporting layer.

Examples of suitable materials for hole transporting layer includes, but are not limited to, oligo- and poly-thiophenes, hydrazone compounds, styryl compounds, diamine compounds, aromatic tertiary amine compounds, butadiene compounds, indole compounds, carbazole derivatives, triazole derivatives, imidazole derivatives, oxadiazole derivatives having an amino group, or the like, or a combination comprising at least one of the foregoing materials. Yet other examples of suitable hole transporters are alpha., .omega. -substituted sexithiophenes, n-dihexyl-quinquethiophene, poly(3- hexylthiophene), poly(3-alkylthiophene), poly(ethylenedioxythiophene) (PEDOT), dihexyl- hexathiophene, triphenylmethane, bis(4-diethylamine-2-methylphenyl) phenylmethane, stylbene, hydrozone; aromatic amines comprising tritolylamine; arylamine; enamine phenanthrene diamine; N,N'-bis-(3,4-dimethylphenyl)-4-biphenyl amine; N,N'-bis-(4- methylphenyl)-N,N'-bis(4-ethylphenyl)-1 ,1'-3,3'-dimeth- ylbiphenyl)-4,4'-diamine; 4-4'- bis(diethylamino)-2,2'-dimethyltriphenylmethane; N,N'-diphenyl-N,N'-bis(3-methylphenyl)- [1 ,1'-biphenyl]-4,4'-diamine; N,N'-diphenyl-N,N'-bis(4-methylphenyl)-1 ,1'-biphenyl-4,4'- diamine; N,N'-diphenyl-N,N'-bis(alkylphenyl)-1 ,1'-biphenyl-4,4'-diamine; and N, IST- diphenyl-N,N'-bis(chlorophenyl)-1 ,1'-biphenyl-4,4'-diamine; 1 ,1-bis(4-di-p- tolylaminophenyl)cyclohexane; 1 , 1 -bis(4-di-p-tolylaminophenyl)-4-phenylcyclohexane; 4,4'-bis(diphenylamino)quadhphenyl; bis(4-dimethylamino-2-methylphenyl)- phenylmethane; N,N,N-Tri(p-tolyl)amine; 4-(di-p-tolylamino)-4'-[4(di-p-tolylamino)- styryl]stilbene; N,N,N',N'-tetra-p-tolyl-4-4'-diaminobiphenyl; N,N,N',N'-tetraphenyl-4,4'- diaminobiphenyl; N,N,N',N'-tetra-1-naphthyl-4,4'-diaminobiphenyl; N,N,N',N'-tetra-2- naphthyl-4,4'-diaminobiphenyl; N-phenylcarbazole; 4,4'-bis[N-(1 -naphthyl)-N- phenylamino]biphenyl; 4,4'-bis[N-(1 -naphthyl)-N-(2-naphthyl)amino]biphenyl; 4,4"-bis[N- (1-naphthyl)-N-phenylaamino]p-terphenyl; 4,4'-bis[N-(2-naphthyl)-N- phenylamino]biphenyl; 4,4'-bis[N-(3-acenaphthenyl)-N-phenylamino]biphenyl; 1 ,5-bis[N- (1-naphthyl)-N-phenylamino]naphthalene; 4,4'-bis[N-(9-anthryl)-N-phenylamino]biphenyl; 4,4"-bis[N-(1 -anthryl)-N-phenylamino]-p-terphenyl; 4,4'-bis[N-(2-phenanthryl)-N- phenylamino]biphenyl; 4,4'-bis[N-(8-fluoranthenyl)-N-phenylamino]biphenyl; 4,4'-bis[N-(2- pyrenyl)-N-phenylamino]biphenyl; 4,4'-bis[N-(2-naphthacenyl)-N-phenylamino]biphenyl; 4,4'-bis[N-(2-perylenyl)-N-phenylamino]biphenyl; 4,4'-bis[N-(1-coronenyl)-N- phenylamino]biphenyl; 2,6-bis(di-p-tolylamino)naphthalene; 2,6-bis[di-(1 - naphthyl)amino]naphthalene; 2,6-bis[N-(1 -naphthyl)-N-(2-naphthyl)amino]naphthalene; N,N,N',N'-tetra(2-naphthyl)-4,4"-diamino-p-terphenyl; 4,4'-bis{N-phenyl-N-[4-(1-naphthyl)- phenyl]amino}biphenyl; 4,4'-bis[N-phenyl-N-(2-pyrenyl)amino]biphenyl; 2,6-bis[N,N-di(2- naphthyl)amine]fluorine; 1 ,5-bis[N-(1-naphthyl)-N-phenylamino]naphthalene; or the like, or a combination comprising at least one of the foregoing hole transporters.

The hole-transporting layer may be liquid or solid. In the case of a liquid hole transporting layer an ionic liquid or an electrolyte may be used. Suitable examples of ionic liquids that may used as the hole transporter are methylpropylimidazolium thaflate, methylpropylimidazolium bistriflimide, methylpropylimidazolium nanoaflate, methylpropylimidazolium ethersulfonate, methylpropylimidazolium iodide methylpropylimidazolium triiodide, methylpropylimidazolium halides, metal complex cations with phosphonium anion, or the like, or a combination comprising at least one of the foregoing hole transporters.

In one embodiment a redox electrolyte is used as a hole-transporting layer. The redox electrolyte can be, for example, a I. sup. -/I. sub.3. sup.- system, a Br.sup.-/Br.sub.3.sup.- system, or a quinone/hydroquinone system. The electrolyte can be liquid or solid. The solid electrolyte can be obtained by dispersing the electrolyte in a polymeric material. In the case of a liquid electrolyte, an electrochemical inert solvent such as acetonithle, propylene carbonate or ethylene carbonate may be used. In one set of embodiments there is provided a dye-sensitized solar cell comprising a dye sensitized electrode, the dye sensitized electrode comprising a substrate having an electrically conductive surface, an electron transporting layer that is disposed on the electrically conductive surface, and a dye composition as hereinbefore described bound to the electrically conductive surface. The DSSC will also generally comprise a counter electrode; and a hole-transporting layer in contact with the dye-sensitized electrode and the counter electrode.

The following abbreviations are used:

AFM atomic force microscopy

AM1.0 air mass 1.0 (shortest path length for solar radiation through the atmosphere, 1000 Wm^"2)

AM1.5 air mass 1.5 (1.5 times the shortest path length for solar radiation through the atmosphere, 1000 Wm^"2)

AR analytical reagent app apparent aq. aqueous

Ar aryl group avg average

Au gold

BAP 5,15-bis-ary/-porphyrin

BAcP bis-acefø/-porphyrin

BCP bis-ca/t>oxy-porphyrin

BCMP bis-cafiboxy-bis-/τ7e#7oxy-porphyrin

BEP bis-ester-porphyrin

BDP bis-c//su///c/e-porphyrin

BFP bis-fo/777y/-porphyrin

BP 3,5-di-te/t-butylphenyl group

B2TP bis-2-tf7/eny/-porphyrin

B3TP bis-3-tf7/eny/-porphyrin BTTP bιs-te/t/7/eny/-porphyπn

Calcd calculated

CHCA α-cyano-4-hydroxycinnamic acid cone. concentrated

COSY correlated spectroscopy d doublet

DBU 1 ,8-diazabicyclo[5.4.0]undec-7-ene

DCE 1 ,2-dichloroethane

DCM dichloromethane or CH₂CI₂

DMF Λ/,Λ/-dimethylformamide

DMSO dimethylsulfoxide dia circular diameter

DPM dipyrrylmethane

El electron ionisation eq equivalent

ES electrospray

Et ethyl

Et₂O diethyl ether

EDW electron donating group

Et₃N triethylamine

EWG electron withdrawing group

FAB fast atom bombardment

FET field-effect-transistor

FF fill factor (ratio of the maximum output of the photovoltaic device, to the product of l_sc and V_oc)

GaAs gallium arsenide

GP general-purpose reagent h hours hept heptet hex hextet

HMTA hexamethylenetetramine HR high resolution

HRMS high resolution mass spectrometry

HOMO highest occupied molecular orbital

IPCE incident photon-to-current conversion efficiency

Uc short circuit current

ITO indium-tin-oxide (conductive glass coating)

LUMO lowest unoccupied molecular orbital

LR low resolution (MS) or long range (NMR)

LRMS low resolution mass spectrometry min minutes

M mol L^"1

M a metal ion m multiplet, milli

MALDI matrix assisted laser desorption ionisation spectroscopy

Me methyl

MeOH methanol mp melting point

MS mass spectrometry

NMR nuclear magnetic resonance

[O] oxidation

OCt octet

P(A-D) TiO₂ coated ITO glass (batches A-D)

Pc phthalocyanine

PEC photoelectrochemical cell pent pentet

Ph phenyl ppm parts per million ps phosphonium salt q quartet

[R] reduction

Rf retention factor RT room temperature

RO reverse osmosis

ROSEY rotating frame overhauser enhancement spectroscopy sat. saturated

SC semiconductor

SEM scanning electron microscopy sh shoulder

SP "sticky" porphyrin

SS steady state

STM scanning tunnelling microscopy t triplet

TAcP tetra-acefø/-porphyrin

TAP 5,10,15,20-tetra-ary/-porphyrin

TBM tetrabutyltetramethyl

TBMP 2,8,12,18-tetra-n-butyl-3,7, 13, 17-tetramethylporphyrin

TBP 5,10,15,20-tetrakis(3\5'-di-te/t-butylphenyl)porphyrin

TBP 5,10,15,20-tetra-n-butylporphyrin

TCA trichloroacetic acid

TCP tetra-4'-ca/t>oxy-porphyrin

T3CP tetra-3'-ca/t>oxy-porphyrin

T3,5CP tetra-3',5'-c//ca/t>oxy-porphyrin

TEP tetra-4'-ester-porphyrin

T3EP tetra-3- ester-porphyrin

T3,5EP tetra-3', 5 -c//ester-porphyrin

TFA thfluoroacetic acid

TFPP tetra-(4'-formylphenyl)porphyrin

THF tetrahydrofuran

Tiθ2 titanium dioxide

TLC thin layer chromatography

TMS tetramethylsilane

TOF time-of-flight TOPP 5,10,15,20-tetra(4-octylphenyl)porphyrin

TPP 5,10,15,20-tetraphenylporphyιϊn

TR all-frans-retinoic acid

TTTP tetra-3"-te/t/7/eny/-porphyrin TXP tetra-xy/y/-porphyrin or

5,10,15,20-tetrakis(3',5'-dimethylphenyl)porphyrin UV-vis ultraviolet-visible spectroscopy

XyI xylyl (3,5-dimethylphenyl group)

V₀C open circuit voltage

The invention will now be described with reference to the following examples. It is to be understood that the examples are provided by way of illustration of the invention and that they are in no way limiting to the scope of the invention.

EXAMPLES Example 1 : Porphyrin dye synthesis

Porphyrins A and B were synthesized according to the following Scheme.

Hydrolysis

Me

H

TXP-CH₂PS TXP-CHCHPhCO₂Me TXP-CHCHPhCO₂H (B)

Hydrogenation

1 Metallation

2 Hydrolysis

Me

H TXP-CH₂CH₂PhCO₂Me ZnTXP-CH₂CH₂PhCO₂H (A)

General Methods

¹H NMR characterizations were performed with a Varian lnova-500 instrument, working at 499.913, or with a Direct Drive Varian-500 NMR system, working at 499.753 and with a Varian-300 Mercury system, working at 299.957 MHz. The chemical shifts are relative either to tetramethylsilane (TMS) or to the residual proton signal in deuterated solvents (CDCI₃ δ 7.26 ppm; CD₂CI₂ δ 5.30 ppm; DMSO-d₆ δ 2.49 ppm) when TMS is not present. ¹³C NMR chemical shifts are relative to CD₂CI₂ (δ 53.52 ppm) or CDCI₃ (δ 77.36 ppm). Chemical shifts are reported as position (δ), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublets, m = multiplet) relative integral, coupling constant (J in Hz), and assignment. Preparation of TXP-CH₂PS

The tetraxylylporphyrin phosphonium salt TXP-CH₂PS was prepared according to the procedure of Bonfantini et al., J. Porphyrins Phthalocyanines 2002, 6, 708-719.

Preparation of TXP-CHCH-PhCO₂Me

A solution of TXP-CH₂PS (540 μmol) and methyl 4-carboxybenzoate (2.16 mmol) in CHCI₃ (60 ml_) was heated to reflux under N₂. DBU (242 μl_, 3.0 eq) was added and the resulting mixture was refluxed for 15 to 30 minutes (the progress of reaction was monitored by TLC) then cooled to room temperature. The crude isomeric mixture was precipitated with MeOH. The crude product was dissolved in CH₂CI₂ (30 ml_) and I₂ (3 eq.) added. After stirring at room temperature for 3 h in darkness, excess saturated aqueous solution of Na₂S₂O₃ was added and stirring continued for 15 min. The organic layer was separated, dried over MgSO₄ and the solvent was removed in vacuo at 5O⁰C. The residue was purified on silica (CH₂CI₂:hexane) and recrystallized from CH₂CI₂/methanol to give TXP-CHCH-PhCO₂Me. Yield: 75%; ¹H NMR (400 MHz, CDCI₃, TMS): d -2.60 (br s, 2H, NH), 3.95 (s, 3H, CO₂CH₃), 7.09 and 7.29 (ABq, 2H, J = 16.0, 15.5 Hz, H₂.,_r), 7.28 (d, 2H, J = 8.5 Hz, H_styryι), 7.71 -7.85 (m, 12H, H_m,_p-Ph), 8.00 (d, 2H, J = 8.4 Hz, H_styryι), 8.17-8.26 (m, 8H, H_0-Ph), 8.72 and 8.78 (ABq, 2H, J = 4.8, 4.9 Hz, H_b. pyrrohc), 8.78 (d, 1 H, J = 4.8 Hz,

8.81 -8.84 (m, 3H,

9.01 (s, 1 H, H₃- _{b. py_rro_hcUV-vis (CH₂CI₂): λ_max [nm] (ε x 10^"3) 301 (21.9), 427 (204), 524 (18.2), 564 (10.2), 600 (6.31 ), 655 (2.14). FAB-LRMS: m/z (%, assignment) cluster at 773-778, 775 (100, MH⁺). HRMS: Requires for MH⁺ (C₅₄H₃₉N₄O₂): 775.3073, found: 775.2994.

Preparation of TXP-CHCH-PhCO₂H (B)

KOH (174 mg, 3.10 mmol) in MeOH (36 mL) and H₂O (3.6 mL) was added to a solution of TXP-CHCHPhCO₂Me (155 μmol) in THF (36 mL). The mixture was refluxed overnight under N₂. On cooling to room temperature, H₂O (49 mL), CHCI₃ (65 mL) and 2.0 M H₃PO₄ (aq) (1.6 mL) were added with stirring. The resulting red coloured organic layer was washed with H₂O (80 ml x 3), and then separated, dried over MgSO₄, the solvent was removed in vacuo and the residue was recrystallized from CHCI₃/hexane to give B. Yield: 96%; ¹H NMR (400 MHz, CDCI₃ + Et₃N, TMS): d -2.623 (br s, 2H, NH), 2.514 (s, 6H, H_Me-xyi), 2.597 (s, 12H, H_Me-x_yι), 2.626 (s, 6H, H_Me-x_yι), 7.086 (d, 1 H, ³J = 16.2 Hz, H₂-), 7.28-7.47 (m, 7H, 1 H_r + 2H_styryι + 4H_P-Xyι), 7.78-7.86 (m, 8H, H_0-Xyι), 8.058 (d, 2H, ³J = 8.2 Hz, Hstyryi), 8.77-8.86 (m, 6H, Hi,_-Py_rroi_ιc), 9.018 (s, 1 H, H₃- (b-pyπ-ohc))- Et₃N was required to solubilize porphyrin. UV-vis (CH₂CI₂): λ_max [nm] (ε x 10^"3) 430 (243), 526 (22.1 ), 566 (13.1 ), 601 , (8.46), 657 (3.81 ). FAB-LRMS: m/z (%, assignment) cluster at 871 -876, 873 (100, M⁺). HRMS: Calcd for M⁺ (C₆i H₅₂N₄O₂): 872.4062, found: 872.4090.

Preparation of TXP-CH₂CH₂-PhCO₂Me A mixture of TXP-CHCH-PhCO₂Me (383 mg, 0.432 mmol) and 10% palladium on carbon (335 mg) in formic acid (33.5 ml) was heated under H₂ atmosphere at 5O⁰C for 24 hours and TLC analysis indicated all starting material was consumed. The reaction suspension was filtered through Celite and the filtrate was diluted with H₂O (190 ml) and neutralized with aqueous NaOH (35.4 g in 190 ml of H₂O), followed by the addition of sat. aq. NaHCO₃ adjusting the pH of the solution to * 7. The porphyrin was extracted into CH₂CI₂ (200 ml) and the organic layer was washed with sat. aq. NaHCO₃ (200 ml), H₂O (200 ml) and then again with sat. aq. NaHCO₃ (200 ml). The organic layer was separated, dried (MgSO₄) and the solvent removed in vacuo. The crude material was purified by column chromatography (silica gel, eluted with 2:1 to 1 :1 of hexane:CH₂CI₂) and TXP-CH₂CH₂- PhCO₂Me was obtained as a purple powder. Yield: 51 %; ¹H NMR (500 MHz, CDCI₃): δ - 2.769 (br s, 2H, NH), 2.527 (s, 6H, H_Meχ_yι), 2.586 (s, 12H, H_Meχ_yι), 2.593 (s, 6H, H_Meχ_yι), 3.126 (t, 2H, J = 8.0 Hz, PhCH₂CH₂), 3.288 (t, 2H, J = 8.0 Hz, PhCH₂CH₂), 3.904 (s, 3H, CO₂Me), 7.062 (d, 2H, J = 7.5 Hz, H₃,₅), 7.390 (br s, 4H, H_p-Xyι), 7.757 (br s, 4H, H_0-Xyι), 7.824-7.829 (m, 4H, H_0-Xy,), 7.894 (d, 2H, J = 8.0 Hz, H_2,6), 8.603 (s, 1 H, H₃-_{(/3- pyrr0}|_IC)), 8.678 (d, 1 H, J = 4.5 Hz,

8.749-8.784 (ABq, 2H, J = 5.0, 5.0 Hz,

8.817 (d, 1 H, J = 4.5 Hz, H /3-_pyrroiιc), 8.880 (br s, 2H,

Assignments aided by gCOSY spectra. UV-vis (CH₂CI₂) λ_max [nm] (log ε) 371 (4.47), 402 (sh), 420 (5.81 ), 516 (4.22), 551 (3.95), 590 (3.90), 645 (3.74). HRMS: Found [M+H]⁺, 889.4502 (ESI). C₆₂H₅₇N₄O₂ requires [M+H]⁺, 889.4482. Preparation of ZnTXP-CH₂CH₂-PhCO₂Me

A solution of Zn(OAc)₂-2H₂O (44.6 mg, 0.203 mmol, 1.2 eq) in MeOH (3 ml) was added to a solution of TXP-CH₂CH₂PhCO₂Me (150 mg, 0.169 mmol) in CHCI₃ (15 ml) with stirring at RT. After 1.5 h, the reaction was completed and the solvent was removed in vacuo. The crude material was purified by column chromatography (silica gel, eluted with 2:1 to 1 :1 of Hexane:CH₂CI₂) and ZnTXP-CH₂CH₂-PhCO₂Me was obtained as a purple powder (153 mg, yield 95%). ¹H NMR (500 MHz, CDCI₃): 2.521 (s, 6H, H_Meχ_yι), 2.590 (s, 18H, H_Meχyi), 3.171 -3.201 (m, 2H, PhCH₂CH₂), 3.260-3.290 (m, 2H, PhCH₂CH₂), 3.901 (s, 3H, CO₂CH₃), 7.099 (d, 2H, J = 8.3 Hz, H₃,₅), 7.375-7.395 (m, 4H, Hp_-Xyι), 7.752 (br s, 2H, H_0-Xy,), 7.770 (br S, 2H, H_0-Xy,), 7.833 (br S, 4H, H_0-Xy,), 7.895 (d, 2H, J = 8.0 Hz, H_2,6), 8.708 (s, 1 H, H₃^_{- pyrrohc)}), 8.801 (d, 1 H, J = 4.9 Hz, H^_-pyrrohc), 8.898- 8.924 (ABq, 2H, J = 4.9, 4.4 Hz, Hβ-pyπohc), 8.946 (s, 2H, Hβ-pyπohc), 8.968 (d, 1 H, J = 4.4 Hz, Hβ-pyπonc). UV-vis (CH₂CI₂) λ_max [nm] (log ε) 350 (3.89), 400 (sh), 421 (5.86), 549 (4.41 ), 586 (3.38). HRMS: Found [M+H]⁺, 951.3664 (ESI). C₆₂H₅₅N₄O₂Zn requires [M+H]⁺, 951.3616.

Preparation of ZnTXP-CH₂CH₂-PhCO₂H (A)

A solution of NaOH (126 mg, 20 equiv, 3.14 mmol) in H₂O (2.6 ml) and MeOH (6.5 ml) was added to a refluxing solution of porphyrin ZnTXP-CH₂CH₂-PhCO₂Me (150 mg, 0.157 mmol) in THF (21 ml) and MeOH (6.5 ml) under argon. After 1 h, TLC analysis indicated that all of the starting material had been consumed. On cooling to RT, CH₂CI₂ (150 ml), H₂O (150 ml) and aq. 2 M H₃PO₄ (3.77 ml) was added and stirred vigorously in the flask, observing the porphyrin transferring from the aqueous layer to the organic layer. The organic layer was washed with H₂O (150 ml) and separated carefully. The solvent was removed in vacuo and the residue was recrystallized from CHCI₃/Hexane to give A as a purple powder (136 mg, yield 92%). ¹H NMR (500 MHz, CDCI₃): 2.520 (s, 6H, H_Meχ_yι), 2.586 (s, 18H, H_Meχ_yι), 3.186-3.216 (m, 2H, PhCH₂CH₂), 3.268-3.298 (m, 2H, PhCH₂CH₂), 7.125 (d, 2H, J = 8.0 Hz, H_3,5), 7.376-7.389 (m, 4H, H_p-Xyι), 7.755 (s, 2H, H_0- xy,), 7.772 (s, 2H, Ho-xy,), 7.832 (s, 4H, H_0-Xy,), 7.953 (d, 2H, J = 8.0 Hz, H₂,₆), 8.708 (s, 1 H, H₃-G*. pyrrohc)), 8.799 (d, 1 H, J = 4.5 Hz, Hβ-pyrrohc), 8.894-8.920 (ABq, 2H, J = 4.5, 4.5 Hz, Hβ-pyπo_hc), 8.941 (s, 2H, H^_0I10), 8.963 (d, 1 H, J = 4.5 Hz, H^_0I10). UV-vis (CH₂CI₂) λ_max [nm] (log ε) 331 (4.51 ), 402 (sh), 421 (5.75), 548 (4.30), 587 (3.09). HRMS: Found [M+H]⁺, 937.3460 (ESI). C_6IH₅₃N₄O₂Zn requires [M+H]⁺, 937.3462.

Preparation of Photoelectic Material and UV-Vis, photoluminescence and cyclic voltammetry

UV-Vis solution spectra were recorded using 5 x 10^~6 M solutions of A and B in THF. Spectra of TiO₂-bound porphyrins were measured for dyes adsorbed on thin (-1 -2 μm) sintered TiO₂ films on FTO glass prepared by screen-printing "SOLARONIX" Ti-Nanoxide T paste to single layer thickness through a fine mesh. Sensitisation was carried out in 5 x 10^~4 M porphyrin solutions for 3 hrs. Wavelength at absorption onset (λ_onSet) values were obtained based on the lowest energy Q band absorption. The optical band gap of porphyrins was calculated using UV-Vis data according to:

band gap (eV) = 1243 / λ_onSet (nm)

where 1243 = h.c / e and h = Planck constant = 6.63 x 10^~34 J s c = speed of light = 3.00 x 10⁸ m s^"1 e = elementary charge = 1.6O x 10^"19 C

Photoluminescence (PL) spectra were obtained using 5 x 10^~6 M solutions of A and B and 500 nm excitation wavelength, which corresponded to energy greater than the highest energy Q band absorption of both porphyrins.

Solution CVs were recorded using solutions containing 5 X i O^-4 M porphyrin and 0.1 M tetrabutylammonium perchlorate (TBAP) supporting electrolyte in dimethylformamide (DMF). The 3-electrode cell consisted of a glassy carbon disk working electrode, Pt mesh auxiliary electrode and Ag/Ag⁺ reference electrode. The half-potential (E₁/₂) of ferrocene (Fc) was determined from the CV of a solution containing 1 mM Fc and 0.1 M TBAP supporting electrolyte in DMF. Example 2

CVs of TiO₂-bound porphyrins were carried out in a 3-electrode cell using as the working electrode dye-sensitised, thin TiO₂ films on FTO glass as described above, and the same auxiliary and reference electrodes as described above. The electrolyte consisted of 0.1 M TBAP in acetonitrile (ACN). EiZ₂(Fc) was determined from the CV of a solution containing 1 mM Fc and 0.1 M TBAP supporting electrolyte in ACN.

Oxidation onset potential (E_onset^ox) values for porphyrins in solution or bound to TiO₂ were obtained from the onset potential of the anodic peak in CVs. Potentials vs. Ag/Ag⁺ reference electrode in all cases were converted to potentials vs. Fc/Fc⁺ electrochemical standard according to:

E (vs. Fc/Fc⁺) = E (vs. Ag/Ag⁺) - Ei_/2(Fc/Fc⁺ (vs. Ag/Ag⁺))

The highest-occupied-molecular-orbital (HOMO) of porphyrins was calculated using CV data according to Kulkarni et al. Chemistry of Materials, 16 (2004) 4556-4573. HOMO = Eonset^ox (vs. Fc/Fc⁺) + 4.5588 eV

Calculated values for optical band gap and HOMO were finally used to back calculate the lowest-unoccupied-molecular-orbital (LUMO) according to:

LUMO (eV) = HOMO (eV) - band gap (eV)

Example 3: Dye-sensitised solar cell fabrication and testing Solaronix Ti-Nanoxide TiO₂ paste was screen-printed on Asahi FTO glass (8 Ω) to size 8 x 8 mm to 3 layers, giving films of ~ 18 μm thickness. Layers were allowed to 'flow' at room temperature for ~ 5 min and then dried at ~ 120 °C for ~ 10 min prior to the printing of subsequent layers. TiO₂ films were sintered using a maximum temperature of 500 °C. For single porphyrin devices, sintered, transparent films at ~ 120 °C were placed in 0.2 mM THF solutions of A or B and left to sensitise for 3 hrs. For mixed porphyrin devices, sensitisation was carried out for 3 hrs in 0.2 mM total concentration THF solutions containing 50, 65, 75, 85 or 95 % of A with the remainder made up by B. Sealed DSSCs were fabricated using a 60 μm hotmelt spacer. The liquid electrolyte contained 0.6 M 2,3- dimethyl-1-propylimidazolium iodide (DMPII), 0.03 M I₂, 0.1 M LiI and 0.5 M t-butyl pyridine (TBP) in 85:15 ACN:valeronitrile (VN). Counter electrodes were based on 8 nm sputtered Pt on Delta Technologies ITO glass (10 Ω) and had a small hole drilled to allow for introduction of liquid electrolyte via vacuum backfill. Devices were sealed by covering the hole in the counter electrode with a piece of 60 μm hotmelt and a glass slide. A mask of slightly larger size than the active area was also used. Current-voltage (I-V) characteristics of DSSCs were measured on a Newport Solar Simulator under AM1.5 and 100 mW cm^"2 illumination intensity.

Example 4: Electrochemical impedance spectroscopy of dye-sensitised solar cells

EIS of DSSCs was carried out using a Solartron SI 1287 electrochemical interface and Solartron SI 1260 impedance/gain-phase analyser. Spectra were analysed using ZView v. 2.90 software (Scribner Associates, Inc.). The V₀₀ of devices under AM1.5, 100 mW cm^"2 illumination was first monitored until stable within ~ ± 2 mV (typically reached within ~ 2 min of illumination). This stable value for V₀₀ was then applied as the DC bias on top of which was applied an AC perturbation of 10 mV over the frequency range 100 kHz - 0.1 Hz.

Example 5: UV-Vis, photoluminescence and cyclic voltammetry

Part (a) Porphyrins in Solution

Figure 3 shows the overlay of normalised UV-Vis absorption and PL spectra for porphyrins A and B in THF solution. Both A and B showed intense Soret band absorptions with ε values of 5.4 x 10⁵ M^"1 cm^"1 and 2.4 x 10⁵ M^"1 cm^"1, respectively. Zn salt A showed 2 distinct Q band absorptions at 560 nm (ε = 2.2 x 10⁴ M^"1 cm^"1) and 600 nm, while free base B showed 4 Q band absorptions at 525 nm (ε = 2.1 x 10⁴ M^"1 cm^"1), 565 nm, 605 nm and 665 nm. Both A and B showed 2 distinct PL peaks at 610 nm and 660 nm, and at 670 nm and 735 nm, respectively. An important observation to make was that the emission of the Zn salt A at 610 nm could conceivably have enough energy to 'pump' the lower energy Q band absorptions of the free base B at 650 nm and 665 nm. This would constitute light-induced energy transfer and raises the possibility of energy transfer from the Zn salt A to the free base B for these porphyrins bound to TiO₂.

The overlayed CVs of porphyrins A and B in DMF solution are illustrated in Figure 4. The Zn salt A showed 2 reversible redox couples at + 0.494 V / + 0.426 V and at + 0.682 V / + 0.590 V compared to the single, rather irreversible redox couple at + 0.698 V / + 0.640 V of the free base B. The slight anodic response commencing at ~ + 0.4 V in the CV of B was due to the background response of the electrolyte.

Values for λ_onSet and E_onset^ox for porphyrins A and B measured from Figures 3 and 4, respectively, were used to calculate HOMO/LUMO levels of these porphyrins in THF solution, which are summarised in Figure 5. The HOMO/LUMO levels of the Zn salt A were ~ 0.3 eV higher compared to the free base B, raising the possibility of light-induced electron transfer from A to B on the surface of TiO₂.

Characterisation of porphyrins A and B in THF solution by UV-Vis, PL and CV, and subsequent calculation of HOMO/LUMO levels, suggested that both energy and electron transfer may be possible between the Zn salt A and free base B bound in close proximity on TiO₂. The following considers whether the conclusions reached for the Zn salt A and free base B in solution hold when these porphyrins are bound to TiO₂.

Part (b) Porphyrins bound to TiO₂

Figure 6 shows the overlay of UV-Vis spectra of TiO₂-bound porphyrins A and B. Practically no peak shift or broadening was observed for the Q band absorptions of Zn salt A or free base B compared to THF solution spectra (Figure 3). This shows that the electronic properties of porphyrins A and B were largely unchanged when bound to TiO₂, supporting the possibility of energy transfer from Zn salt A to free base B on TiO₂ within devices.

The overlayed CVs of TiO₂ thin films sensitised with porphyrins A and B are illustrated in Figure 5. The CV of TiO₂-bound Zn salt A demonstrated a single reversible redox couple at + 0.618 V / + 0.394 V, quite different compared to the 2 reversible redox couples observed in THF solution (Figure 4). This was likely due to mass transport limitations associated with the use of a high surface area, A-sensitised TiO₂ working electrode. Tiθ₂-bound free base B showed a single irreversible redox couple at + 0.796 V / + 0.614 V, similar to the CV of B in THF solution (Figure 4).

The HOMO/LUMO levels of TiO₂-bound porphyrins A and B calculated from λ_onSet and Eonset^ox values taken from Figures 6 and 7, respectively, are shown in Figure 8. Compared to porphyrins A and B in THF solution, the HOMO/LUMO levels of TiO₂- bound porphyrins were shifted to between 0.03-0.1 eV higher energy likely due to binding-induced stress on the porphyrin structures. However, the HOMO/LUMO levels of Zn salt A and free base B relative to each other barely changed when bound to TiO₂ compared to in THF solution. This suggests that the predictions in section 3.1.1 regarding light-induced energy and electron transfer from Zn salt A to free base B hold when these porphyrins are bound to TiO₂.

The following considers first the measurement of the relative amounts of Zn salt A and free base B bound to TiO₂ for the various porphyrin mixtures investigated. DSSCs based on photoanodes sensitised with various mixtures of the Zn salt A as 'source' dye and the free base B as 'sink' dye are considered in detail thereafter.

Example 6: Measurement of relative amounts of porphyrins on TiO₂ surface The overlayed UV-Vis spectra of porphyrins A and B in THF solution in Figure 3 suggested that the ratio of peak absorption intensities at ~ 560 nm and ~ 520 nm would give a good estimate of the ratio of Zn salt A to free base B on TiO₂. The ratio abs@560nm / abs@520nm was calculated from the UV-Vis spectra of sensitisation solutions containing 50, 65, 75, 85 or 95 % A and of thin TiO₂ films sensitised in these solutions. Figure 9 shows the ratio abs@560nm / abs@520nm in solution and on TiO₂ as a function of % A in each sensitisation solution. An exponential function was fitted to the solution data and rearranged to allow calculations of % Zn salt A on TiO₂ for the various sensitisation solutions used. The result of these calculations is summarised in Table 1. From the results in Table 1 it may be concluded that free base B bound to TiO₂ with a slight preference compared to Zn salt A.

Table 1 : Calculated % A bound to TiO₂ depending on % A in sensitisation solution.

Example 7: Dye-sensitised solar cells based on single porphyrin and mixed porphyrins

Figure 8 shows overlayed representative I-V curves obtained from the testing of DSSCs based on Zn salt A alone, free base B alone, and an optimised mixture of A and B (optimisation discussed below). A synergistic effect was observed, whereby the DSSC based on a photoanode sensitised with porphyrins A and B showed much better I-V properties compared to DSSCs based on A or B alone.

The dominant improvement for mixed porphyrin DSSCs was in l_sc, which is better illustrated in the overlayed representative IPCE profiles of DSSCs based on A, B or an optimised mixture of A and B (Figure 9). Remarkably, the IPCE of the device based on an optimised mixture of A and B was greater than the sum of the IPCE of devices based on A or B at all wavelengths where these porphyrins absorb light (i.e. between 400-700 nm). This, together with the prominent IPCE contributions at 525 nm and 655 nm due to the free base B, proved that the contributions to photocurrent generation of both Zn salt A and free base B porphyrins were enhanced synergistically in mixed porphyrin DSSCs.

Figure 10 shows overlayed Nyquist plots obtained from the EIS characterisation of representative DSSCs based on A, B or an optimised mixture of A and B, under illumination. The relatively large impedance of the DSSC based on B was due to the free base form of this porphyrin, which, according to our knowledge, greatly increases the charge-transfer-resistance (R_CT) associated with the reduction of photooxidised porphyrin, porphyrin^"1", by I^" in the liquid electrolyte. The intermediate impedance of the DSSC based on Zn salt A was attributed to the saturated linker of this porphyrin, which, also according to our knowledge, raises considerably the R_CT associated with electron injection from photoexcited porphyrin, porphyrin^*, into Tiθ2. The overall low R_CT associated with the photoanode / liquid electrolyte interface, Rcτ(TiO₂), of the DSSC based on an optimised mixture of A and B appeared to be due to the combination of the favourable Zn salt form of A and the favourable conjugated linker of B. This is further evidence of the interaction between porphyrins A and B within mixed porphyrin DSSCs.

The results of I-V, IPCE and EIS characterisations given in Figures 10, 11 and 12, respectively, all point towards the existence of a synergistic effect, whereby porphyrins A and B appear to be interacting on the surface of TiO₂ within mixed porphyrin DSSCs. It may be considered that the apparent synergy of porphyrins A and B in mixed porphyrin DSSCs is due to the light-induced inter-porphyrin energy and electron transfers. A possible mechanism accounting for the observed synergy within mixed porphyrin DSSCs is proposed here and illustrated in Figure 13. The mechanism involves light absorption by A (a), interporphyrin energy transfer from A^* to B (b), electron injection from B^* into TiO₂ (c), interporphyrin electron transfer from A to B⁺ (d) and finally reduction of A⁺ by I^" (e). In this mechanism the porphyrins are of mutual benefit to each other, whereby the conjugated linker of B provides an outlet for the energy absorbed by A, and where B⁺ is reduced by A at a rate presumably more rapid than reduction by I^".

Table 2: I-V testing data averaged over 4 devices for DSSCs based on Zn salt A, free base B, and various mixtures of A and B.

As alluded to above, the main influence of having a porphyrin mixture was to dramatically raise l_sc values. In addition, significant increases in V₀₀ and FF were also observed for mixed porphyrin DSSCs compared to single porphyrin DSSCs. The generally higher V₀₀ and FF values of mixed porphyrin DSSCs suggested a lower incidence of recombination at the porphyrin / TiO₂ interface, which in turn supports the charge separated state proposed in Figure 13(e).

Figure 14 is a graphical representation of DSSC efficiencies in Table 2 plotted against the known (in the case of single porphyrin DSSCs) or calculated (in the case of mixed porphyrin DSSCs) % A on the TiO₂ surface. The optimum mixed porphyrin DSSC efficiency was observed at 72 % A on the TiO₂ surface, which is very close to 75 % A.

Significantly, this proportion of A on the TiO₂ surface corresponds to an A:B ratio of 3:1 , an apparently 'magic' ratio. From our estimations the 'magic' ratio of 3:1 between Zn salt A and free base B would correspond to a situation on the TiO₂ surface whereby every porphyrin 'source' A would be next to a porphyrin 'sink' B, and visa versa. Statistically, a lower A:B ratio would increase the chance of a recombination between TiO₂ ^" and B⁺ due to a lack of A molecules and a subsequent retardation of the process depicted in Figure

13(d). Similarly, a higher A:B ratio would increase the chance of internal recombination of A^* due to a lack of nearby B molecules and a subsequent retardation of the process depicted in Figure 13(b). A synergistic effect was observed for DSSCs based on the mixture of a Zn salt porphyrin A and a free base porphyrin B. Such devices displayed overall efficiencies greater than the sum of efficiencies observed for devices based on Zn salt A or free base B alone. Evidence for this was provided by I-V, IPCE and EIS characterisations.

Light-induced energy and electron transfer from Zn salt A to free base B on the Tiθ2 surface was predicted from UV-Vis, PL and CV characterisations and subsequent HOMO/LUMO calculations. This prediction and the observed synergistic effect in mixed porphyrin devices led to the proposal of a mechanism describing inter-porphyrin light- induced energy and electron transfer. Experiments aimed at proving the various aspects of this proposed mechanism are currently underway in our laboratories. In addition, considerations are underway in order to establish a strategy for exploiting the synergistic effects within mixed-porphyrin DSSCs revealed in this study.

Example 8: Dye-sensitised solar cells based on porphyrin and phthalocyanine dye mixture

Phthalocyanine dye synthesis

Phthalocyanine dye P2 was synthesized according to the following Scheme.

General Methods

General methods are given in Example 1. Preparation of P1

P1 was prepared according to the method of Gouloumis, Andreas; Liu, Shen-Gao; Sastre, Angela; Vazquez, Purificacion; Echegoyen, Luis; Torres, Tomas. Synthesis and electrochemical properties of phthalocyanine-fullerene hybrids. Chemistry—A European Journal (2000), 6(19), 3600-3607.

Preparation of P2

P1 (100 mg, 0.13 mmol) and cyanoacrylic acid (55 mg, 0.65 mmol) were dissolved in acetic acid - THF mixture (12 mL, 1 :1 ) then ammonium actetate (50 mg, 0.65 mmol) was added. The resulting mixture was stirred at 70°C for 24 h then the reaction was treated with cold water (50 mL). The blue solid was filtered off, washed by water and dried to give P2 (100 mg, 99%), mp > 300⁰C.

One of the porphyrin of Examples 2 to 6 dyes was replaced by a phthalocyanine dye in the mixture prepared from 0.2 mM P2 and 0.2 mM A in THF to provide the 1 : 3 ratio. The dye-sensitised solar cells prepared and their photovoltaic characteristics measured as for Example 7. The PV data is given in Table 3.

Table 3: I-V testing data for DSSCs based on Zn porphyrin A, phthalocyanine P2, and 1 :3 mixtures of P2 and A.

Notes:

1. The Device ID is the identification number for the individual device.

2. The Calcd l_s/_c is the current calculated based on the surface coverage of each dye in the mixture, e.g. 25% of P2 and 75% of A. If more than one device is reported, the % of the average current of the devices is used.

3. The Calcd Eff. is the efficiency calculated based on the surface coverage of each dye in the mixture, e.g. 25% of P2 and 75% of A. If more than one device is reported, the % of the average efficiency of the devices is used.

As the previous results with the two porphyrins A and B, the 1 :3 mixtures of the phthalocyanine P2 and porphyrin A show a synergistic effect and an overall improvement in both the short circuit current (l_s/c) and overall cell efficiency (Eff.). This is apparent by comparison of the Calcd l_s/_c data and the Calcd Eff. data with the experimental l_s/_c and Eff. data. The calculated data is based on the expected 1 :3 contribution to each parameter from the individual dyes, whose PV data for cells with full dye surface coverage (100%) was collected at the same time and is also shown in Table 3.

Example 9: Dye-sensitised solar cells based on porphyrin and fluorine dye mixtures

Fluorene dye syntheses

Fluorenes R4 and R5 were synthesized according to the following Scheme.

R4 (X = COOH, Y = CN)

General Methods

General methods are given in Example 1.

Preparation of R1

R1 was purchased from Sigma Aldrich and used as received.

Preparation of R2

R1 (5.40 g, 0.03 mols) and 2,2-dimethyl-1 ,3-propandiol (5.79 g, 0.06 mols) were dissolved in benzene (100 ml_) then a catalytic amount of p-toluenesulfonic acid monohydrate (50 mg) was added. The resulting mixture was heated under reflux for 5 hours using a Dean-Stark trap. Afterwards the solvent was removed under vacuo and the oil remaining was purified on silica (DCM) to give R2 (7.70 g, 98%).

Preparation of R3

R2 (7.7 g, 0.03 mols) and 4-dimethylaminobenzaldehyde (4.92 g, 0.03 mol) were dissolved in mixture of ethanol and THF (130 ml_, 3:1 ) then sodium ethoxide was added (5.00 g). The resulting mixture was heated under reflux for 5 h. Following this, the reaction mixture was cooled, the red solid filtered off and washed with a small amount of cold methanol. The solid was redissolved in dichloromethane (20 ml_), trifluoroacetic acid (20 ml_) and water (10 ml_) added and the resulting mixture stirred vigorously for 30 min. The mixture was washed with water and sodium bicarbonate solution then dried over magnesium sulphate and evaporated to dryness. The residue was purified on silica (DCM) to give R3 as a 1 :1 E and Z isomer mixture as determined by NMR spectroscopy.

Preparation of R4 and R5

R3 (0.33 g, 1 mmol) and the appropriate CH acid (6 mmol) were dissolved in acetic acid (7 ml_). Ammonium acetate (0.46 g, 6 mmol) was added and the resulting mixture was stirred at 70°C for 1.5 h, the resulting red solid was removed by filtration, washed with a small amount of acetic acid, then dried to give R4 (0.33 g, 94%) or R5 (0.44 g, 94%).

One of the porphyrin dyes of Examples 2 to 6 was replaced by a fluorene dye in the mixture. The sensitised solar cells were prepared and their photovoltaic characteristics measured as for Example 7. The PV data is given in Tables 4 and 5.

Table 4: I-V testing data for DSSCs based on Zn porphyrin salt A, fluorene R4, and 1 :3 mixtures of A and R4.

Notes:

1. The Device ID is the identification number for the individual device.

2. The Calcd l_s/_c is the current calculated based on the surface coverage of each dye in the mixture, e.g. 25% of A and 75% of R4. If more than one device is reported, the % of the average current of the devices is used.

3. The Calcd Eff. is the efficiency calculated based on the surface coverage of each dye in the mixture, e.g. 25% of A and 75% of R4. If more than one device is reported, the % of the average efficiency of the devices is used.

As the previous results with the two porphyrins A and B, the 1 :3 mixtures of the porphyrin A and fluorene R4 show a synergistic effect and an overall improvement in both the short circuit current (l_s/c) and overall cell efficiency (Eff.). This is apparent by comparison of the Calcd l_s/_c data and the Calcd Eff. data with the experimental l_s/_c and Eff. data. The calculated data is based on the expected 1 :3 contribution to each parameter from the individual dyes, whose PV data for cells with full dye surface coverage (100%) was collected at the same time and is also shown in Table 4.

Table 5: I-V testing data for DSSCs based on Zn porphyrin salt B, fluorene R5, and various mixtures of B and R5.

Notes:

1. The Device ID is the identification number for the individual device.

2. The Calcd l_s/_c is the current calculated based on the surface coverage of each dye in the mixture, e.g. 25% of B and 75% of R5. If more than one device is reported, the % of the average current of the devices is used.

3. The Calcd Eff. is the efficiency calculated based on the surface coverage of each dye in the mixture, e.g. 25% of B and 75% of R5. If more than one device is reported, the % of the average efficiency of the devices is used.

As the previous results with the two porphyrins A and B, the 1 :3 mixtures of the porphyrin B and fluorene R5 show a synergistic effect and an overall improvement in both the short circuit current (l_s/_c) and overall cell efficiency (Eff.). This is apparent by comparison of the Calcd l_s/_c data and the Calcd Eff. data with the experimental l_s/_c and Eff. data. The calculated data is based on the expected 1 :3 contribution to each parameter from the individual dyes, whose PV data for cells with full dye surface coverage (100%) was collected at the same time and is also shown in Table 5. Example 10: Dye-sensitised solar cells based on fluorene dye mixtures

Both of the porphyrin dyes were replaced by fluorene dyes in the mixture and and dye- sensitised solar cells prepared and their photovoltaic characteristics measured as for Example 7. The PV data is given in Table 6.

Table 6: I-V testing data for DSSCs based on fluorene R4, fluorene R5 and 1 :3 mixtures of R4 and R5.

Notes:

1. The Device ID is the identification number for the individual device.

2. The Calcd l_s/_c is the current calculated based on the surface coverage of each dye in the mixture, e.g. 25% of R4 and 75% of R5. If more than one device is reported, the % of the average current of the devices is used.

3. The Calcd Eff. is the efficiency calculated based on the surface coverage of each dye in the mixture, e.g. 25% of R4 and 75% of R5. If more than one device is reported, the % of the average efficiency of the devices is used. As the previous results with the two porphyrins A and B, the 1 :3 mixtures of the porphyrin A and fluorene R4 show a synergistic effect and an overall improvement in both the short circuit current (l_s/c) and overall cell efficiency (Eff.) although not as great as previously seen. In contrast to the porphyrin experiments, these cells have not been optimised to achieve a 1 :3 binding ratio on the surface. The calculated data is based on the expected 1 :3 contribution to each parameter from the individual dyes, whose PV data for cells with full dye surface coverage (100%) was collected at the same time and is also shown in Table 6.

Comparative Example 11 : Dye-sensitised solar cells based on a mixture of two conjugated dyes.

Both of the porphyrin dyes were replaced by the conjugated phthalocyanine dye P2 and the conjugated fluorene dye R4 in the mixture and and dye-sensitised solar cells prepared and their photovoltaic characteristics measured as for Example 7. The PV data is given in Table 7.

Table 7: I-V testing data for DSSCs based on phthalocyanine P2, fluorene R4 and 1 :3 mixtures of P2 and R4.

Notes:

1. The Device ID is the identification number for the individual device. 2. The Calcd l_s/_c is the current calculated based on the surface coverage of each dye in the mixture, e.g. 25% of P2 and 75% of R4. If more than one device is reported, the % of the average current of the devices is used.

3. The Calcd Eff. is the efficiency calculated based on the surface coverage of each dye in the mixture, e.g. 25% of P2 and 75% of R4. If more than one device is reported, the % of the average efficiency of the devices is used.

In contrast to the previous results with the two porphyrins A and B, the 1 :3 mixtures of the two conjugated dyes, phthalocyanine P2 and fluorene R4 DO NOT show an overall improvement in the short circuit current (l_s/_c) and overall cell efficiency (Eff.). Rather they show a decrease in these parameters based on the expected 1 :3 contribution to each parameter from the individual dyes, whose PV data for cells with full dye surface coverage (100%) was collected at the same time and is also shown in Table 7. This supports the notion that synergistic effects are only observed with mixtures of conjugated and unconjugated dyes.

Claims

1. A dye composition for use in a photoelectric material, the dye composition comprising a plurality of dyes comprising a chromophore and at least one binding group for binding with a semiconductor (preferably a metal oxide semiconductor) wherein the plurality of dyes include a first dye wherein the binding group is linked to the dye chromophore by a linker not in conjugation with the chromophore and a second dye wherein the binding group is attached by a linker in conjugation with the chromophore.

2. A dye composition according to claim 1 wherein the binding group of the first and second dye is selected from the group consisting of alcohol, amino, nitrile, thiocyanate, acetoacetonate, hydroxyquinolate, alizarin, barbituric acid, carboxylic acid, dicarboxylic acid, phosphoric acid, phosphinic acid, sulphonic acid or hydroxamic acid or combinations thereof and preferably is a carboxyl and more preferably a styryl or dehydrostyryl carboxylic acid.

3. A dye composition according to any one of the previous claims wherein the chromophore is selected from the group consisting of porphyrin dyes, porphyrazine dyes, phthalocyanine dyes, coumarin dyes, indoline dyes, rhodanine dyes, thiophene dyes, xanthene dyes, such as rhodamine B, rose bengal, eosin, and erythrosine, cyanine dyes, such as quinocyanine and kryptocyanine, anthraquinone dyes and polycyclic quinone dyes, azo dyes, basic dyes such as phenosafranine, fog blue, thiosine, and methylene blue, and coordination compounds containing a metal atom, such as ruthenium, rhenium and iridium pyridyl, bipyridyl and terpyhdyl complexes preferably from porphyrin dyes, porphyrazine dyes, phthalocyanine dyes and coumarin dyes and most preferably from porphyrins.

4. A dye composition according to any one of the previous claims wherein there is a molar excess of said first dye.

5. A dye composition according to any one of the previous claims wherein the molar ratio of said first dye to said second dye is in the range of from 1.5:1 to 5:1 preferably from 2:1 to 4:1 and most preferably about 3:1.

6. A dye composition according to any one of the previous claims wherein at least one of said first and second dyes comprises a porphyrin chromophore.

7. A dye composition according to any one of the previous claims wherein the first and second dyes are non-metalated porphyrins or porphyrins metallated with a metal selected from the group selected from zinc, magnesium, nickel, copper, cobalt, iron, tin, ruthenium, cadmium, palladium, platinum and more preferably said first dye is a metallated porphyrin containing zinc and said second dye is a non-metallated porphyrin.

8. A dye composition according to any one of the previous claims wherein the first and second dyes are porphyrins comprising a linker to said binding group in the β- position of the porphyrin.

9. A dye composition according to any one of the previous claims wherein the linker group of the first and second dyes is selected from the group consisting of C₂ to C30 hydrocarbyl wherein the hydrocarbyl may be straight or branched chain.

10. A dye composition according to any one of the previous claims wherein the linker group of the first and second dyes is selected from C₂ to C₁₂ aliphatic, (C₂ to C₁₂ aliphatic)aryl wherein the aryl is optionally substituted in the aryl ring by from one to three substituents selected from selected from the group consisting of one or more said binding group and Ci to C₆ alkyl and the group C₂ to Ci₂ aliphatic(aryl)C₂ to Ci₂ aliphatic wherein the aryl is optionally substituted in the aryl ring by from one to three substituents selected from the group consisting of one or more said binding group and wherein said aliphatic may be straight or branched chain.

11. A photoelectric material comprising:

a semiconductor, preferably comprising a metal oxide; and a dye composition comprising a plurality of dyes comprising a chromophore and at least one binding group bound with the semiconductor wherein the plurality of dyes include a first dye wherein the bound group is linked to the dye chromophore by a linker not in conjugation with the chromophore and a second dye wherein the bound group is attached by a linker in conjugation with the chromophore.

12. A photoelectric material according to claim 11 wherein the bound group of the first and second dye is selected from the group consisting of alcohol, amino, nitrile, thiocyanate, acetoacetonate, hydroxyquinolate, alizarin, barbituric acid, carboxylic acid, dicarboxylic acid, phosphoric acid, phosphinic acid, sulphonic acid or hydroxamic acid and conjugate bases and mixtures thereof and preferably is a carboxyl and more preferably a styryl or dehydrostyryl carboxylic acid.

13. A photoelectric material according to claim 11 or claim 12 wherein the chromophore is selected from the group consisting of porphyrin dyes, porphyrazine dyes, phthalocyanine dyes, coumarin dyes, indoline dyes, rhodanine dyes, thiophene dyes, xanthene dyes, such as rhodamine B, rose bengal, eosin, and erythrosine, cyanine dyes, such as quinocyanine and kryptocyanine, anthraquinone dyes and polycyclic quinone dyes, azo dyes, basic dyes such as phenosafranine, fog blue, thiosine, and methylene blue, and coordination compounds containing a metal atom, such as ruthenium, rhenium and iridium pyridyl, bipyridyl and terpyridyl complexes preferably from porphyrin dyes, porphyrazine dyes, phthalocyanine dyes and coumarin dyes and most preferably from porphyrins.

14. A photoelectric material according to any one of claims 11 to 13 wherein there is a molar excess of said first dye.

15. A photoelectric material according to any one of claims 11 to 14 wherein the molar ratio of said first dye to said second dye is in the range of from 1.5:1 to 5:1 preferably from 2:1 to 4:1 and most preferably about 3:1.

16. A photoelectric material according to any one of claims 11 to 15 wherein the first and second dyes are non-metalated porphyrins or porphyrins metallated with a metal selected from the group selected from zinc, magnesium nickel, copper, cobalt, iron, tin, ruthenium, cadmium, palladium, platinum and more preferably said first dye is a metallated porphyrins containing zinc and said second dye is a non-metallated porphyrin.

17. A photoelectric material according to any one of claims 11 to 16 wherein the first and second dyes are porphyrins comprising a linker to said bound group in the β- position of the porphyrins.

18. A photoelectric material according to any one of claims 11 to 17 wherein the linker group of the first and second dyes is selected from the group consisting of C2 to

C30 hydrocarbyl wherein the hydrocarbyl may be straight or branched chain.

19. A photoelectric material according to any one of claims 11 to 18 wherein the linker group of the first and second dyes is selected from C₂ to C12 aliphatic, (C₂ to C12 aliphatic)aryl wherein the aryl is optionally substituted in the aryl ring by from one to three substituents selected from selected from the group consisting of one or more said binding group and Ci to C₆ alkyl and the group C₂ to Ci₂ aliphatic(aryl)C₂ to Ci₂ aliphatic wherein the aryl is optionally substituted in the aryl ring by from one to three substituents selected from the group consisting of one or more said binding group and wherein said aliphatic may be straight or branched chain.

20. A photoelectric material according to any one of claims 11 to 19 wherein the semiconductor comprises at least one metal oxide selected from the group consisting of, titanium oxide, zinc oxide, niobium oxide, tungsten oxide, indium oxide, tin oxide, nickel oxide and zirconium oxide and preferably titanium oxide or zinc oxide and most preferably titanium oxide.

21. A photoelectric material according to any one of claims 11 to 20 in the form of a dye sensitized solar cell.