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Title:
SYSTEM FOR SENSITIZATION OF METAL OXIDES BY DYES STABILIZED BY DISTINCTIVE ANCHORING GROUPS
Document Type and Number:
WIPO Patent Application WO/2014/026285
Kind Code:
A1
Abstract:
The present invention provides a system for sensitization of metal oxides comprising a ruthenium(II) complex comprising at least one first ligand functionalized to provide high binding stability to the metal oxide, and at least one second ligand functionalized to provide efficient electron injection to the metal oxide. Preferably, the ligands are tridentate and include a phosphonic acid group, which provides the binding stabiltiy, and a carboxylic acid group, which allows for electron injection to the metal oxide.

Inventors:
BERLINGUETTE, Curtis (279 Royal Oak Circle NW, Calgary, Alberta T3G 6A4, CA)
Application Number:
CA2013/050624
Publication Date:
February 20, 2014
Filing Date:
August 14, 2013
Export Citation:
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Assignee:
BERLINGUETTE, Curtis (279 Royal Oak Circle NW, Calgary, Alberta T3G 6A4, CA)
International Classes:
C09C3/08; C07F15/00; H01G9/20
Foreign References:
EP1174891A2
US20090000658A1
Other References:
WOLPHER ET AL.: 'Synthesis and Electron Transfer Studies of Ruthenium?Terpyridine-Based Dyads Attached to Nanostructured TiO2' INORG. CHEM. vol. 46, 2007, pages 638 - 651
KOIVISTO ET AL.: 'Systematic Manipulation of the Light-Harvesting Properties for Tridentate Cyclometalated Ruthenium(II) Complexes' INORG. CHEM. vol. 48, 2009, pages 9644 - 9652
HU ET AL.: 'Intramolecular Hole Transfer at Sensitized TiO2 Interfaces' J. AM. CHEM. SOC. vol. 134, 01 May 2012, pages 8352 - 8355
ROBSON ET AL.: 'Design and Development of Functionalized Cyclometalated Ruthenium Chromophores for Light-Harvesting Applications' INORG. CHEM. vol. 50, 2011, pages 5494 - 5508
BROWN ET AL.: 'Stabilization of Ruthenium Sensitizers to TiO2 Surfaces through Cooperative Anchoring Groups' J. AM. CHEM. SOC. vol. 135, 23 January 2013, pages 1692 - 1695
Attorney, Agent or Firm:
MBM INTELLECTUAL PROPERTY LAW LLP (270 Albert Street, 14th FloorOttawa, Ontario K1P 5G8, CA)
Download PDF:
Claims:
WE CLAIM:

1. A system for sensitization of metal oxides comprising a ruthenium(II) complex comprising at least two ligands bound to a ruthenium(H) center, wherein the at least two ligands comprises at least one first ligand being functionalized to provide high binding stability to the metal oxide, and at least one second ligand functionalized to provide efficient electron injection to the metal oxide.

2. The system according to claim 1 , wherein the first ligand comprises a phosphonic acid or derivative thereof.

3. The system according to claim 1, wherein the first ligand is:

where R2 is PO3H2.

4. The system according to any one of claims 1 to 3, wherein the second ligand comprises a carboxylic acid or derivative thereof.

5. The system according to claim 4, wherein the second ligand is:

where R, is COOH.

6. A system for sensitization of metal oxides which is:

wherein Rj is COOH and R2 is P03H2.

Description:
13 050624

SYSTEM FOR SENSITIZATION OF METAL OXIDES BY DYES STABILIZED BY

DISTINCTIVE ANCHORING GROUPS

FIELD OF THE INVENTION

[0001] The present invention pertains to the field of photovoltaic technology and in particular to dye sensitized solar cells.

BACKGROUND

[0002] Among the suite of emerging photovoltaic (PV) technologies (e.g., organic PV, quantum dots, polymer cells), the dye sensitized solar cell (DSSC) is arguably the best positioned to penetrate the market in the immediate future. The stability of the DSSC nonetheless remains a significant impediment to commercialization due to the hermetic sealing of the electrolyte, and the degradation and desorption of the dye. 1"4 While recent advances in solid-state hole-transport materials (e.g., perovskites, polymers) could help to resolve problems associated with the liquid electrolyte, 5 ' 6 charge-transport limitations have thus far confined performance to efficiencies of less than -8.5%, which is at least 4% less than that achieved with liquid electrolytes. The long-term stability of sensitizers represents another critical problem. 3 ' 7 Indeed, the current set of commercially relevant dyes (e.g., N 3 , N71 , black dye) are susceptible to temporal degradation due to by the loss of the monodentate, labile NCS " ligands. 7 In recognition of these shortcomings, our research program (and others) have sought to replace these NCS " ligands with chelating bidentate and tridentate ligands without compromising device performance. 8 ' 9 This line of inquiry has also unraveled a compelling approach to modulating both the ground and excited state oxidation potentials - an unprecedented accomplishment for high-performance DSSC dyes. 9

[0003] Another key degradation pathway within conventional DSSCs is the desorption of the dye from the Ti0 2 . The electrolyte in state-of-the-art devices is typically dissolved in an organic medium, and requires the rigorous exclusion of water to avoid hydrolysis of the anchoring groups of the dye that are bound to Ti0 2 in order to operate efficiently over prolonged periods of time (e.g., ca. 10 yrs). The exclusion of adventitious water due to environmental factors over the required cell lifetime cannot be guaranteed, however, therefore providing the imperative to design dyes that are inherently tolerant of water.

[0004] A perusal of the literature finds that all high performance DSSCs contain sensitizers with carboxylate anchoring group(s) that bind in a bidentate fashion to Ti0 2 . Carboxylate anchoring groups are, however, inherently sensitive to water as they do not have sufficient binding strength to Ti0 2 to avoid desorption in aqueous solutions. The hydrolysis of the carboxylate-Ti0 2 linker can be suppressed by spatially "blocking" access of water to the titan ia/carboxyl ate interface — a common strategy for making high performance dyes. Alternatively, the carboxylate linker can be replaced with an alternative chemical moiety, but the rates of electron injection are often compromised. For example, phosphonates exhibit a binding constant that is a 10-fold greater than that of the carboxylate linker due to the tri-dentate chelating mode of the phosphonate. However, the sp 3 hybridization of the phosphorus center compromises the injection of electrons from the excited state of the dye into titania.

[0005] This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide a system for sensitization of metal oxides. In accordance with an aspect of the present invention, there is provided a system for sensitization of metal oxides comprising a ruthenium(il) complex comprising at least two ligands bound to a ruthenium(Ii) center, wherein the at least two ligands comprises at least one first ligand being functionalized to provide high binding stability to the metal oxide, and at least one second ligand functionalized to provide efficient electron injection to the metal oxide. BRIEF DESCRIPTION OF THE FIGURES

[0007] Figure 1 illustrates a schematic showing how the phosphonates and carboxylate anchoring groups can be used in conjunction with each to achieve injection.

[0008] Figures 2 and 3 depict the synthetic scheme for the synthesis of the complexes of the present invention.

[0009] Figure 4 illustrates X-ray structures of complexes 1 and 2 and a molecular model of complex 3.

[0010] Figure 5a depicts the UV-vis spectrum of complexes 1-3 absorbed to mesoporous Ti0 2 substrates (10 μιη thick), and the inset depicts the optical spectra of complexes 1-3 recorded in MeOH.

[0011] Figure 5b depicts the FTIR data of complexes 1-3 absorbed to Ti0 2 . The vibrational spectrum of the fully deprotonated form of complex 3 pressed in a KBr pellet is provided for comparison.

[0012] Figure 6 depicts the temporal stability of complex 3 absorbed to a mesoporous Ti0 2 substrate in water. Data for N 3 and the black dye are also shown to highlight the enhanced stability engendered by the phosphonate groups. Inset: Zoomed in region of temporal stability.

[0013] Figure 7 depicts incident-photon-to-current efficiency (IPCE) data for complex 3 absorbed to a mesoporous Ti0 2 substrate. Cell conditions: AM 1 .5 with a 0.88 cm 2 active area.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

[0014] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. [0015] The present invention provides a sensitizer system formed of a ruthenium(II) complex comprising at least two ligands attached to the ruthenium(II) center, one of which is functionalized to provide high binding stability to the metal oxide, while the other is functionalized to provide efficient electron injection. Each of these functions is provided by respective linking groups, each of which binds to the surface of the metal oxide through a suitable anchor moiety.

[0016] As used herein, the term "anchor" is used to describe the functional group that binds to the surface of the metal oxide. The anchor may be attached directly to the ligand or indirectly through a bridging moiety. Accordingly, as used herein, the term "linker" or "linking group" is used to describe the portion of the sensitizer system that comprises an anchor portion as defined above and an optional bridging moiety between the anchor and the ligand bound to the ruthenium(II) center.

[0017] Suitable anchoring groups for binding to metal oxides include phosphonic acids and derivatives thereof (such as phosphonate esters and salts), carboxylic acids and derivatives thereof (such as esters, acid chlorides, carboxylate salts or amides), silanes, ethers, acetylacetonate and salicylates.

[0018] Examples of suitable linkers include, but are not limited to -C0 2 H, -P(0)(OH) 2 , -OH, -CH(C0 2 H) 2 , -C(H)=C(C0 2 H) 2 , -C(H)=C(CN)(C0 2 H), -SiCl 3 , -COCH 2 COH, and -SiOEt 3 .

[0019] In one embodiment, the sensitizer system utilizes phosphonate anchors to bind to the surface while maintaining efficient electron injection into the metal oxide substrate using a carboxylate linker, as shown in Figure 1. This molecular design keeps the carboxylate linker on the electron deficient ligand that will be the site responsible for injecting electrons into the metal oxide, while positioning the phosphonate on the adjacent chelating ligand. The resultant dye is then poised to retain a strong dye-surface interaction while also enabling facile charge transfer into the metal oxide.

[0020] In one embodiment, the electron injection linker comprises a carboxylic acid or a derivative thereof, including but not limited to -C0 2 H, esters, acid chlorides, carboxylate salts, amides, -CH(C0 2 H) 2 , -C(H)=C(C0 2 H) 2 , and -C(H)=C(CN)(C0 2 H). In one embodiment, the binding stability linker comprises a phosphonic acid or its derivatives.

[0021] In one embodiment, the electron injection linker and the high binding stability linker are located on separate ligands in the dye complex. In one embodiment, the electron injection linker and the high binding stability linker are located on the same ligand in the dye complex.

[0022] In one embodiment of the present invention, there is provided a ruthenium(II) complex having the structure:

wherein Ri is an anchoring moiety that provides the efficient electron injection, for example, carboxylic acid and its derivatives, such as esters, acid chlorides, carboxylate salts or amides; and R 2 is an anchoring moiety that provides high binding stability, for example, phosphonic acid and its derivatives, such as phosphonate esters and salts. In a preferred embodiment, Ri is COOH and R is Ρθ3¾.

[0023] Three model compounds were prepared and tested, including a compound containing only the carboxylate group (complex 1 ), a compound containing only the phosphonate group (complex 2), and a derivative with both (complex 3). As demonstrated herein, the complexes bearing both acid groups produce a relatively higher performance in the DSSC, and also exhibit much longer term stability in the DSSC relative to N 3 and the black dye.

[0024] In one embodiment of the present invention, there is therefore provided a sensitizer system that combines the efficient electron injection afforded by carboxylate linkers, with the binding stability of phosphonate groups.

[0025] It is of course understood that the sensitizer system of the present invention is also suitable for use with metal oxides other than Ti0 2 , for example, ZnO, Fe 2 0 3 , W0 3 , Sn0 2 and Zr0 2 . Such alternative metal oxides are considered to be within the scope of the present invention.

[0026] It is also contemplated that other ligand/anchoring group configurations are also within the scope of the present invention. Alternative tridentate ligands suitable for use in the present invention are as follows:

[0027] Alternative bidentate ligands suitable for use in the present invention are as follows:

[0028] For the tridentate and bidentate ligands shown above:

Yi,Y 2 = (C,N) or (N,N) or (N,C);

Χι,Υι,Ζ = (C,N,N) or (C,C,N) or (Ν,Ν,Ν); or

X,,X 2 ,X 3 =(N,N,N) or (N,C,N) or ( ,C, ) or (N,N,C) or (C,N,C) or (C,N,N);

where a,c =1-3; b = 0-2 ■ anc j

[0029] For each of the R x and R y groups for the bidentate and tridentate ligands set out above, the anchoring group A may be defined as follows:

-C0 2 H

A = -P(0)(OH) 2

-OH

-CH(C0 2 H) 2

-C(H)=C(C0 2 H) 2

-C(H)=C(CN)(C0 2 H)

-S1CI3

-COCH2COH

-SiOEt 3

O

H HN - OH

[0030] The invention will now be described with reference to specific examples. It will be understood that the following examples are intended to describe embodiments of the invention and are not intended to limit the invention in any way.

EXAMPLES

EXAMPLE 1 :

[0031] Synthesis of the dyes was carried out using the following reaction scheme (Scheme 1) with moderate yields of the final products. Compounds 1 -3 were synthesized employing selective transmetalation reactions between silver-activated forms of the carbene ligands [namely, silver(I) complexes of 2',6'-¾«'( l -(2,4,6-trimethyl-phenyl)-3-methyl-l ,2,3- triazol-4-yl-5-idene)pyridine (C A N A C-R 2 )](R 2 ^PO(OEt) 2 ) derivatives and cw-[Ru(terpy- Ri)(DMSO)Cl 2 ] (Ri = -C0 2 Me positioned at the 4'-position of terpy) derivatives (Scheme 1). The final step involved saponification of the ester groups to their acid derivatives. The synthetic schemes are shown in Figures 2 and 3. EXAMPLE 2:

[0032] Figure 4 depicts structures of complexes 1 and 2 determined by single crystal X- ray diffraction techniques, and a molecular model of complex 3 showing that the phosphonate and/or carboxylate anchoring groups are appropriately positioned for collective binding. Hydrogens, counter ions and ester linkages omitted for clarity.

EXAMPLE 3:

[0033] The UV-vis data was collected for the complexes 1-3 in MeOH solution and once adsorbed onto the titania surface (Figure 5). Figure 5a depicts the UV-vis spectrum of complexes 1-3 absorbed to mesoporous Ti0 2 substrates (10 μπι thick), and the inset depicts the optical spectra of complexes 1-3 recorded in MeOH. The UV-vis absorption spectra of solutions of complexes 1-3 (Figure 5a) each reveal MLCT bands centered at ca. 460 nm with moderate extinction coefficients (e.g., ~1 .O x 10 4 M "1 cm "1 ). The narrow MLCT band in the visible region for each complex is a manifestation of the C 2v symmetry and the nearly degenerate π* orbitals of both ligands. The electrochemical behaviour was investigated by square-wave cyclic voltammetry where the working electrode was the dye covered titania fluorine doped tin oxide (FTO) glass. The title complexes 1-3 were observed to show a ruthenium (II/III) oxidation potential at -1.2 V.

[0034] The binding modes of complexes 1-3 were investigated by diffuse reflectance Fourier transform infrared spectroscopy (FTIR) by looking for the disappearance or shifting of carboxylate C=0 and C-0 and/or phosphonates P=0 and P-0 stretching frequencies.— Initial conclusions for complex 3 seem to indicate that all three anchoring groups are binding simultaneously to the Ti0 2 surface (Figure 5b). The binding mode for complex 1 appears to be monodentate (bound through former -OH) in nature as the C=0 stretching frequencies around 1650-1700 cm "1 are still present. The methyl groups at the 4 position of the mesityl rings appear to be hindering the ability of the carboxylate linker to form a bidentate covalent bond with the titania surface. Figure 5b depicts the FTIR data of complexes 1-3 absorbed to Ti0 2 . The vibrational spectrum of the fully deprotonated form of complex 3 pressed in a KBr pellet is provided for comparison. EXAMPLE 4:

Table 1. Photophysical and electrochemical data for complexes 1-3.

1 2 3

465 458 461

*? (x l 0 4 M-' cm " ') * 1.0 1 .0 1 .0

1.12 1.1 8 1.14

J sc (mA/cm 2 0.090 0.12 1 .7

V 0C (V) f 0.332 0.163 0.427

FF f 45.7 34.6 43.7 η (%Υ 0.014 0.006 0.316

"Measured in MeOH at 298 K. ^Maximum absorption of lowest energy MLCT band. c Emission maxima measured in deareated EtOH:MeOH (v/v 80:20). ^Measured in MeCN using a 0.1 M NBu 4 BF 4 supporting electrolyte and [Fc] + /[Fc]° as a standard with the working electrode as the FTO dye dipped titania. ^Reported vs NHE. ^Characterized under AM 1.5 light conditions.

EXAMPLE 5:

[0035] Electron injection efficiency was demonstrated by construction of dye-sensitized solar cells that were characterized under AM 1 .5 conditions. Complex 3 exhibited the highest η at 0.316%. The incident photon to current efficiency (IPCE) was also investigated which demonstrated the ability of the complexes to produce photo injected electrons. As expected complex 2 exhibited poor energy conversion efficiency with no carboxylate linker to provide the critical electron injection pathway from the terpy fragment. The η of complex 1 was intermediate due to the steric hindrance the methyl groups P T/CA2013/050624

exhibited with the titania surface. This correlated with the poor dye uptake observed by the absorption of the film and the FTIR data which showed the binding to be monodentate in nature.

REFERENCES

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(2) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Prog. Photovoltaics 2010, 18, 144.

(3) Nguyen, P. T.; Andersen, A. R.; Skou, E. M.; Lund, T. Sol. Energy Mate?'. Sol. Cells 2010, 94, 1582.

(4) Asghar, M. I.; Miettunen, K.; Halme, J.; Vahermaa, P.; Toivola, M.; Aitola, K.; Lund, P. Energy Environ. Sci. 2010, 3, 41 8.

(5) Wang, M.; Moon, S.-J.; Xu, M.; Chittibabu, K.; Wang, P.; Cevey-Ha, N.-L.; Humphry-Baker, R.; Zakeeruddin, S. M.; Gratzel, M. Small 2010, 6, 319.

(6) Yanagida, S.; Yu, Y.; Manseki, K. Acc. Chem. Res. 2009, 42, 1 827.

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(9) Bomben, P. G.; Robson, K. C. D.; Berlinguette, C. P. Coord. Chem. Rev. 2012, DOI: 0.1016/j. bbr.2011.03.031.

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It is obvious that the foregoing embodiments of the invention are examples and can be varied in many ways. Such present or future variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.