WO1994005025A1

WO1994005025A1 - Use of optical brighteners and phthalocyanines as photosensitizers

Info

Publication number: WO1994005025A1
Application number: PCT/EP1993/002180
Authority: WO
Inventors: Roland Entschel; Friedrich Lehr; Peter Matzinger; Horst Schmid
Original assignee: Sandoz Ltd.; Sandoz-Patent-Gmbh; Sandoz-Erfindungen Verwaltungsgesellschaft M.B.H.
Priority date: 1992-08-17
Filing date: 1993-08-16
Publication date: 1994-03-03
Also published as: AU4948393A

Abstract

A photovoltaic cell comprising an electrically conductive element to which one or more titanium dioxide layers have been applied, to which titanium dioxide layers a photosensitizer having at least one interlocking group (preferably a COOH containing group) has been applied (being hereinafter defined as component a); characterized in that the photosensitizer is selected from optical brightener compounds and phthalocyanine compounds.

Description

USE OF OPTICAL BRIGHTENERS AND PHTHALOCYANINES AS

PHOTOSENSITIZERS

The invention relates to the use of optical brighteners and phthalocyanines (including naphthalocyanines) as a photosensitizer in photovoltaic cells. These brighteners can be coated on titanium dioxide films rendering the device effective in the conversion of visible light to electric energy.

For the avoidance of doubt, naphthalocyanines are included in the term phthalocyanine in the specification.

Titanium dioxide films (layers) are known for their semiconductive properties and this property renders them useful for photovoltaic cells. However titanium dioxide has a large band gap and it does not absorb light in the visible region of the spectrum. For solar applications it is preferable that the titanium dioxide film be coated with a photosensitizer which harvests light in the wavelength domain where the sun emits light, i.e. between 300 and 2000 nm so as to increase the efficiency of the cell. Thermodynamic considerations show that conversion of solar energy into electricity is achieved in the most efficient fashion when all the emitted photons with wavelengths below 820 nm are absorbed by the photosensitizer. The optimal photosensitizer for solar conversion should therefore have an absorption onset under 800nm.

A second requirement for efficient solar light energy conversion is that the photosensitizer after having absorbed light and thereby acquired an energy-rich state is able to inject with practically unit quantum yield, an electron in the conduction band of the titanium dioxide film. This requires that the photosensitizer is attached to the surface of the titanium dioxide through suitable interlocking groups. The function of the interlocking group is to provide electronic coupling between the chromophoric group of the photosensitizer and the conduction band of the semiconductor. This type of electronic coupling is required to facilitate electron transfer between the excited state of the photosensitizer band and the conduction band. Suitable interlocking groups (hereinafter defined as interlocking group) are π-conducting substituents such as carboxylate groups, hydroxy groups, mercapto groups, sulphonate groups, cyano groups, phosphate groups or chelating groups with '-conducting character such as oximes, dioximes, hydroxy quinolines, salicylates and alpha keto enolates. The electrons, photoinjected by the photosensitizer generate electrical current in the external circuit when the photovoltaic cell is operated.

According to the invention there is provided an electrically conductive element to which one or more titanium dioxide layers have been applied, to which titanium dioxide layers a photosensitizer having at least one interlocking group (preferably a COOH containing group) has been applied (being hereinafter defined as component a) ;

characterised in that the photosensitizer is selected from optical brightener compounds and phthalocyanine compounds (including naphthalocyanine compounds).

(For the sake of clarity component a includes the electrically conductive element, one or more titanium dioxide layers and photosensitizer ).

Still further according to the invention there is provided a photovoltaic cell comprising:

i) an electrically conductive element to which one or more titanium dioxide layers have been applied, to which titanium dioxide layers a photosensitizer having at least one interlocking group (preferably a COOH containing group) has been applied (hereinafter defined as component a);

ii) a second electrically conductive element, having no Ti0₂ layer present;

at least one of the two electrically conductive layers being transparent (that is to say having a visible light transmittance of at least 60%)

Hi) means for permitting the passage of an electrical current generated by the cell between the two electrically conductive elements;

Preferably the means for permitting the passage of an electrical current is a liquid or solid electrolyte.

Preferably component a) comprises a support, (preferably a glass plate coated with metal oxide, a metal surface or a polymer sheet (preferably an intrinsically conductive polymer)) to which the Ti0₂ layer is applied. Optionally, component a) is transparent. By the term "transparent" is meant that at least 60%, preferably 70%, more preferably at least 80%, especially 80-98% of incident light passes through the support.

Preferably the Ti0₂ layer comprises rutile and anatase, more preferably anatase.

Preferably the titanium dioxide is doped with a metal ion, which may preferably be selected from a divalent or trivalent metal or boron. Preferred dopant is aluminium.

Preferably the titanium dioxide layer is a film. Preferably the film has a roughness factor greater than 20, the roughness factor being defined as the ratio of true to apparent surface area. Roughness factor is defined in USP 5,084,365 and 4,927,721. More preferably the roughness factor is 20-1000. more preferably 50-200.

Preferably the titanium dioxide layers are built up on the surface of the conductive layer of the electrically conductive elements of component a) using one of two methods. One is the sol-gel method described by "Stalder and Augustynski in J. Electrochem. Soc. 1979, 126:2007" and in Example A. Another is the "colloidal method" described in Examples B and C.

Preferably component a) is a metal, polymer or glass plate to which an electrically conductive surface has been applied, to which one or more layers (preferably a film of 0.1- 50 microns) of titanium dioxide has been applied, the Ti0₂ being coated with a photosensitizer selected from optical brightener compounds and phthalocyanine compounds.

The electrically conductive element having no Ti0₂ present (also known as "the counter electrode") may be coated with a thin layer (preferably up to 10 microns thickness) of an electrocatalyst. The role of the electrocatalyst is to facilitate the transfer of electrons from the counterelectrode to the electrolyte. A further possible modification of the counterelectrode (when component a) is transparent) is to make it reflective to the light that infringes thereon, having first passed through the electrolyte and component a).

Preferably both electrically conductive elements have a surface resistance of 5-1000 ohm/cm² more preferably 5-100 ohm/cm², most preferably 5-50 ohm/cm², especially 10 ohmlcm². The transparent conductive layer may alternatively have a surface resistance of less than 10 ohm! cm² in which case it is preferably from 1 to 10 ohm/ cm².

Preferably the electrolyte contains a redox system (charge transfer relay) or is a solid electrolyte system. Preferably such systems include iodine/ iodide solutions, bromine/ bromide solutions, hydroquinone solutions or solutions of transition metal complexes transferring a nonbonding electron. The charge transfer relays present in the electrolyte transport electric charge from one electrode to the other. They act as pure mediators and undergo no chemical alteration during the operation of the cell. It is preferable that the electrolytes in a photovoltaic cell according to the invention are dissolved in an organic medium so that the photosensitizer applied to the titanium dioxide surface are insoluble therein. This has the advantage that the cell has a long-term stability.

Preferred organic solvents for the electrolyte include but are not limited to water containing (preferably however, water-free), alcohols and mixtures thereof, non-volatile solvents such as propylene carbonate, ethylene carbonate and methyl pyrrolidinone, mixtures of non-volatile solvents with viscosity reducing solvents such as acetonitrile, ethylacetate or tetrahydrofuran. Additional solvents are dimethylsulfoxide or dichloroethane. Where miscible, mixtures of any of the above may be used.

The glass or polymer plate which is used for the transparent electrically conductive element of the cell according to the invention is any transparent glass or polymer onto which a light transmitting electrically conductive layer has been deposited, such that the plate preferably has a visible light transmittance of 60-99%, more preferably 85-95%. Preferably the transparent conductive layer used in a photovoltaic cell according to the invention is made of tin dioxide doped with ca. 0.8 atom percent of fluorine and this layer is deposited on a transparent substrate made of low-cost soda lime float glass. This type of conducting glass can be obtained from Asahi Glass Company, Ltd. Tokyo, Japan, under the brand name of TCO glass. The transparent conductive layer can also be made of indium oxide doped with up to 5% tin oxide, deposited on a glass substrate. This is available from Balzers under the brand name of ITO glass.

The photosensitizer can advantageously be used in a photovoltaic cell that requries high transparency, e.g. when integrated in a watch or in a window.

A photovoltaic cell according to the invention has the following advantages when compared to existing cells.

1. It has a higher open circuit voltage than conventional p-n junction solid state solar cells while maintaining a fill factor comparable to that of conventional solar cells. The fill factor is defined as the electrical power output at the optimal cell voltage for light energy conversion divided by the product of open circuit voltage and short circuit current. A high open circuit voltage is very important for practical applications since it allows to operate the cell at lower Ohmic losses than conventional photovoltaic cells which have a smaller open circuit voltage.

2. In contrast to p-n junction solid state solar cells where the semiconductor assumes the function of light absorption and carrier transport simultaneously, a photovoltaic cell according to the invention achieves the separation of these functions. Light is absorbed by the very thin layer of photosensitizer adsorbed onto the surface of the titanium dioxide film while the charge carrier transport is carried out by the titanium dioxide film. As a consequence, a photovoltaic cell according to the present invention operates as a majority carrier device. This has the advantage that imperfections such as grain boundaries or other types of crystalline disorders or impurities and disorder within the Ti0₂ film do not decrease the efficiency of the cell as would be the case if minority carriers were involved in the cell operation. Conventional solar cells operate with minority charge carriers and this explains the need to fabricate these cells from highly pure and ordered materials which are costly. The present invention permits the development of cheap solar cells. All the materials employed in the present cells are inexpensive.

3. A further consequence of the fact that the present cell operates as a majority carrier device is that the cell voltage depends to a smaller degree on the intensity of the impinging light than that of a conventional solar cell. Thus, the present cell maintains its high efficiency under diffuse light or in cloudy weather while the efficiency of a conventional cell decrease sharply under these conditions.

4. By selecting appropriate optical brighteners, the cell can be optimized with respect to solar energy conversion.

5. A further advantage of a preferred photovoltaic cell according to the present invention is that it can be irradiated from the front side, back side or both sides. It can be irradiated by passing the light through the counter- electrode and the electrolyte to the dyestuff absorbed at the Ti0₂ layer or through the Ti0₂ layer to the absorbed dyestuff. If both the photosensitizer coated electrode and the counterelectrode are transparent, then light from all directions can be collected. In this way it is possible in addition to direct sunlight to harvest diffuse reflected light. This will improve the overall efficiency of the solar cell.

6. A further advantage of a photovoltaic cell according to the present invention is that the specific texture and electronic properties of the photosensitizer loaded Ti0₂ layer allow the counterelectrode to be placed directly on top of the working electrode. In other words, there is no need to employ a spacer such as a polymer membrane to keep the two electrodes apart in order to avoid the formation of a short circuit. The dielectric features of the photosensitizer coated Ti0₂ layer are such that even though it may be in direct contact with the counterelectrode there is no break-through current due to short circuiting of the two electrodes. This is an important advantage for practical application of the cell since it simplifies the construction of the device and reduces its cost.

7. In the case that the photosensitizer of the present cell is an optical brightener, it may- enable one to produce a colorless photovoltaic cell with improved efficiency.

In a further aspect according to the invention, the photosensitizer can be a three layer system comprising

i) an optical brightener compound (hereinafter defined as component i); ii) a phthalocyanine compound (hereinafter defined as component ii); and Hi) a ruthenium complex compound (hereinafter defined as component Hi)

Preferably in such a three layer system, the amount of component i) is 10-50%, the amount of component ii) is 10-40% and the amount of component Hi) is 10-50%. The percentages are preferably by weight based on 100% being the total combined amount of components i), ii) and Hi).

It goes without saying that other photosensitizers may also be added to the electrically conductive layer of a cell according to the invention. The order of addition of components i) to Hi) is immaterial to this aspect of the invention.

In the sol gel method it is preferable that only the last three, the last two or just the very top layer of the titanium dioxide is doped with a divalent or trivalent metal in an amount of no: more than 15% doping by weight. However, the deposition of the pure dopant in form of a very thin top oxide layer can also be advantageous. In the latter case, a blocking layer is formed which impedes leakage current at the semiconductor- electrolyte junction. All of the Ti0₂ layers are formed by the sol gel process method described in Application Example A. Preferably the number of Ti0₂ layers deposited is 10-11. Preferably tlie total thickness of the Ti0₂film is from 5 to 50 microns (more preferably 8-20 microns).

The photosensitizing layer may be produced by applying to the Ti0₂ layer a photosensitizer, preferably an optical brightener according to the invention defined below.

Preferably the photosensitizer is an optical brightener of the stilbene series, the coumarin series, the pyrazoline series, the benzoxazolyl series (including bisbenzoxazolyl series), the thiophene series, the furane series, the benzimidazoline series, the naphthalic acid imide series, the triazole series, and the oxazole series. The photosensitizer having at least one interlocking group (preferably COOH containing group) present. A preferred -COOH containing group is

where n' is 0 or 1 to 4.

More preferably, tlie photosensitizer is a compound of formula I

SUBSTITUTE SHEET in which R_l0 is a group a), b), c) or d)

in which R₁₂ is hydrogen, C,_-alkyl or phenyl;

R₁₃ is hydrogen, C^alkyl (preferably methyl), C Xalkoxycarbonyl or cyano R₁₄ is hydrogen, C,.₄alkyl (preferably methyl), chloro or cyano; and X is -O- or -S-.

Preferably the -CH-.CH- group is in the 4 position with respect to the -(CH--CH).- COOH group on the R₁₄-bearing phenyl ring, where n' is 0 or 1-4.

The most preferred photosensitizer is of formula II

Alternatively, the photosensitizer can be a compound of formula X or XI

(XI)

in which each Rj independently is -COOH, OH or hydrogen; each R₂ independently is -COOH, OH or hydrogen; with the proviso that on at least one phenyl ring in formula I and formula II

Rj and R₂ cannot both be hydrogen; n' is 0 or 1

R₃ is halogen (preferably Cl), OH,-O-C,.₁₀alkyl, -0-CO-C,.₆alkyl,

-0-Si-(C_5.7 alkyl)₃, O-Si- (C_7.12aryl)₃ or together with R₄ = O

R₄ independently of R₃ has a significance of R₃; each n independently is 0 or 1 and

Me is selected from Cu, Ni, Fe, Co, V, Sn, Si, Ti, Ru, Ge, Cr, Zn, Mg, Al, Pb, Mn, In, Mo,

Y, Zr, Nb, Sb, La, W, Pt, Ta, Tl, Ir, Pd, Os, Ga, Tb, Eu, Rb, Bi, Se, As, Sc, Ag, Cd, Hf, Re,

Au, Hg, Ac, Tc, Te and Rh.

Preferably R, is R,' where R,' is -COOH.

Preferably R₂ is R₂ where R₂ is -COOH or hydrogen.

Preferably R₃ is -O -Si( C_halky I) ₃ or -0-CO-C alkyl.

Preferably R₄ is R₄' where R₄ is -0-Si(C,^alkyl)₃ or -0-CO-C alkyl.

Prefereably Me is Me' where Me' is Cu, Ni, Fe, Co, Ti, Ru, V, Zn, Si or In.

Compounds of formulae X and XI are new.

Compounds of formula I are new and can be made by known methods from known compounds, for example by reacting a compound of formula III

R_W-CH0 (III)

with a compound of formula IV

followed by saponification of the carboxylate group to the free carboxylic acid group by known methods.

The compounds of formula IV can be prepared by reacting tri(C _.₃alkyl)phosphite with a compound of formula V

by known methods.

Preferred ruthenium complex photosensitizers for use in the three layer system photosensitizer are those described in PCT/EP 91/00734, the contents of which are incorporated in their entirety herein by reference.

More preferably the ruthenium complex is one in which one ligand comprises a mononuclear, cyano containing pyridyl group. Most preferred ruthenium complexes are selected from formulae 1 to 10:

[M(L^a)(L^b) (μ-(NC)M(CN)(U)(L^d))₂] (1)

[M(L^a)(L^b) (μ-(NC)M(L^c)(L^d)μ-(CN)M(CN)(L^c)(L^d))₂J (2)

[M(L^a)(U) (μ-(NC)M(U)(L^d)μ-(CN))₂M(U)(L^d)] (3)

[(L^a)(L")(X₁)M μ-(NC)M(CN)(L^c)(L^d)] (4)

[M(L^a)(L^b)(X,)₂] (5)

[M(L^a)(U)(U)] (6)

[M(L^a)(L^b) (μ-(NC)M(U)(U))₂) (7)

[M(U)(L^b) (μ-(NC)M(U)(L^d) μ-(CN)M(V)(U))₂] (8) lM(L-)(U) μ-(NC)M(V)(L^b)] (9) lM(L^a)(U'(X,)] (10)

in which each M is independently selected from ruthenium, osmium or iron; μ-(CN) or μ-(NC) indicates that the cyano group bridges two metal atoms;

each L", L^b, L^~ and L^d independently is selected from 2 ,2 '-bipyridyl, unsubstituted or substituted by one or two COOH groups; 2,2' bipyridyl substituted by one or two groups selected from

C ,.,_salkoxy and diphenyl; 2 _2 '-biquinoline unsubstituted or substituted by one or two carboxy groups; phenanthroline, unsubstituted or substituted by one or two carboxy groups and lor one or two hydroxy groups, and/or one or two oxime groups; 4,7- diphenyl-1, 10-phenanthroline disulfonic acid;diazatriphenylene, diaza-hydroxy -carboxy l- triphenylene (for example 1 ,12-diazatriphenylene or 1,12-diaza- (6-hydroxy-l '-carboxy) triphenylene); carboxy pyridine - (for example 2 -carboxypyridine); phenyl pyridine; 2,2'-Bis- (diphenylphosphino) 1.l '-binaphthalene: (pyridyl azo) resorcinol (for example 4-(2-pyridyl(azo) resorcinol); bis (2-pyridyl) C^alkane; /VNN'N'-terrα C^ alkyl ethylene diamine; and di-C ,.₄alky glyoxime; 2,2'-biimidazole; 2,2'-bibenzimidazole: 2 ,-(2 '-pyridyl N-methyl-benzimidazole; 2 ,-(2 '-pyridyDbenzothiozole; 2 ,-(2 '-pyridylmethyl)- benzimidazole;

L^' is selected from terpyridyl. (unsubstituted or substituted by phenyl. which phenyl group is unsubstituted or substituted by COOH)(for example 2,2',6'.2 "-terpyridine) and dicarboxy-pyridine (preferably 2, 6 -dicarboxy -pyridine); 2, 6-bis(benzimidazole-2'-yl)pyridine; 2 ,6-bis(N -methylbenzimidazole- 2 '-yl)pyήdine; 2 ,6-bis(benzothiazol-2 '-yl)pyridine.

each X independently is halide, H₂0, CN^' , NCS^' , amine (primary or preferably secondary alkylamine) and/or pyridine.

Preferably one of L" and L^b has an interlocking group selected as defined above, preferably a -COOH and/or an OH and/or an =N-OH and/or -CO-NH₂ group.

Preferably the terpyridyl when substituted is substitued by C,^alkyl (preferably methyl) and/ or C _lsalkoxy (preferably methoxy) and! or carboxy on one or more of the pyridyl groups - for example 2_r2',6'_r2"-terpyridine.

Preferably any phenanthroline in a L" to L^d is selected from 5-carboxy-6-hydroxy-l ,10- phenanthroline and 5,6-dioxime-l ,10-phenanthroline.

Especially preferred ruthenium complexes are given in Table in or Table 2 below.

Table 1

in which "bpy" = 2 ~' - bipyridyl "Me" = methyl "ph" = phenyl

In Table 2 below, bpy - 22' bipyridyl; biq = 22' biquinoline and phen = 1,10 phenanthroline in example 19, 2-phenylpyridine is used in example 22, straight and branched alkyl groups are used in example 26, N,N -tetramethyl and C,C - tetramethyl ethylene diamine are used in example 27, 22' ' bis(diphenylphosphino)-! , "binaphthylene is used in example 28, 30 and 33 1.10 orthophenanthrolene is used and in example 31 , 4-(2-pyridyl) azo resorcinol is used Table 2

The photosensitizers of the invention will now be illustrated by the Examples below and the cells of the invention by the Application Examples below.

Example 1

a) 78.1 parts of 1 -naphthaldehyde are dissolved in 150 mis of dimethylformamide. Whilst stirring well, 163 parts of 4-diethylphosphonomethyl-cinnamic acid methyl ester, prepared from 4-bromomethyl cinnamic acid methyl ester and triethylphosphite (by known methods), are added. Over 30 minutes and under strong stirring 107 parts of a 30% methanolic sodium methylate solution are slowly let into the above mixture. Through external cooling, the temperature of the reaction mixture is held at <30°C. The resulting suspension is stirred for about 1 hour at room temperature, suction filtered over a glass frit and washed consecutively with 500 parts of water and 500 parts of methanol. 100 parts of light yellow crystals having a melting point of 144- 145°C and having a UV absorption maxiumum of 353nm in ethanol result.

The product is of the compound of formula lb

b) 36.3 parts of the compound of formula lb are slurried in 180 parts of ethanol (80%) and 56 parts of water. 49.1 parts of potassium hydroxide powder are added and the suspension is stirred under reflux for 4 hours. Additional 160 parts of ethane are added and stirring is continued for an additional 2 hours. After cooling to room temperature the pH is brought to 1 by the addition of 80 parts of concentrated HCl and filtered over a paper filter in a suction filler. The product is washed chloride-free using about 3 liters of water. The light yellow compound is dried at 80°C and under 16mm Hg vacumm. 33.1 parts (71% of theory) of light yellow (in solution blue flour escing) crystals are obtained having a melting point of 250-255° C (Sinter point). The compounds are very- soluble in dimethylformamide.

The resulting product is of formula la

Example 2

Example 1 is repeated using 140 parts of 4-diethylphosphonomethyl-cinnamic acid nitrile in place of the 163 parts of 4-diethylphosphono-methyl-cinnamic acid methyl ester.

The product of formula la above results also in good yield.

Examples 3-6

The following compound can be prepared by a method analogous to that of Example 1 or 2 from known compounds.

Example 3

Example 4

Example 5

Example 6

The optical brighteners of Examples 1 to 6 are found to be useful as photosensitizers and can be used as such in photovoltaic cells according to the invention.

Example 7

Preparation of copper tetracarboxyphthalocyanine

10 parts of trimellitic acid anhydride, 30 parts of urea, 4 parts of Cu(II) chloride and 1 part of ammonium molybdate are suspended in 150 parts of nitrobenzene. While stirring the mixture is heated 3-4 hours to 160-170°C. After allowing to cool, 100 parts ofmethanol are added and the mixture is filtered. The residue is then washed with 200 mis of ethanol and dried under vacuum in a drying cupboard.

The product (12 parts) is stirred together with 150 parts of potassium hydroxide in 300 parts of water. The mixture is heated to 100° C for about 15 hours. After the addition of 100 parts of water, the mixture is filtered. The filtrate is then brought to pH 2 by the addition of about 250 parts of 6N HCl and the dyestuff precipitates out in fine form.

The dyestuff is then filtered off and washed with 200 parts of 0.1 HCl and finally washed with 200 parts water.

Drying under vacuum results in 6 parts of product (copper tetracarboxy phthalocyanine) that is soluble in dimethyl sulphoxide.

Examples 8-21

Compounds of formula X

in which R,-R₄ and Me are as defined in the Table below where n= l . can be made from appropriate reactants by a method analogous to that of Example 7.

Example 22

lg (4.22 mmols) of 6-methoxycarbonyl-2 ,3-dicyanonaphthalene is suspended in 0.4g (2.11 mmols) of SnCl₂ + 35ml of chloronaphthalene. This is stirred for 8 hours at 230-240°C. After cooling, the mixture is poured into 100 ml of diethylether and filtered. The dark-green to black residue is boiled for 3 hours in 50ml of 30% NaOH solution. The pH is brought to 1 by adding a suitable amount of 30%HCl. 400mg of a black residue results of tin-tetracarboxynaphthalocyanine.

Example 23-27

Compounds of the formula

in which R_y-R and Me are as defined in the Table below. This can be made from appropriate reactants by a method analogous to that of Example 22.

Application Example A

A photovoltaic device, as shown in Figure 1, based on the sensitization of a aluminum- doped titanium dioxide film supported on conducting glass is fabricated as follows:

A stock solution of organic titanium dioxide precursor is prepared by dissolving 21 mmol of freshly distilled TiCl₄ in WmL of absolute ethanol. TiCl₄ in ethanol solution gives titanium alkoxide spontaneously which on hydrolysis gives TiO_: The stock solution is then diluted with further absolute ethanol to give two solutions (solution A and solution B having) titanium contents of 25 mg/ml (solution A) and 50 mg/ml (solution B). A third solution (C) is prepared from solution B by addition of AlCl₃ to yield an aluminium content of 1.25 mg/ml. A conducting glass sheet provided by Asahi Inc. Japan, surface area 10 cm² and having a visible light transmittance of at least 85% and a surface resistance smaller than 10 ohms per square cm is used as support for a deposited Ti0₂ layer. Prior to use, the glass is cleaned with alcohol. A droplet of solution A is spread over the surface of the conducting glass to produce a thin coating. Subsequently the layer is hydrolyzed at 28°C for 30 minutes in a special chamber where the humidity is kept at 48% of the equilibrium saturation pressure of water. Thereafter, the electrode is heated in air in a tubular oven kept at 450°C, preheating it in the entrance of the oven for 5 minutes followed by 15 minutes of heating in the interior. Three more layers are produced in the same way. Subsequently, 5 thicker layers are deposited by using solution B. The same procedure as for the first layers is applied. Finally, solution C is used to deposit the last two layers containing the aluminum dopant. The heating of the last layer in the tubular oven is extended from 15 to 30 minutes. The total thickness of the titanium dioxide film is between 10 and 20 microns.

Prior to deposition of the photosensitizer the film is subjected to a sintering treatment in highly purified 99.997% argon. A horizontal tubular oven composed of quartz tubes with suitable joints is employed. After insertion of the glass sheet with the Ti0₂ film. the tube is twice evacuated and purged with argon. The glass sheet is then heated under argon flux at a flow rate of 25Lih and a temperature grandient of 500° Clh up to 550°C at which temperature it maintained for 35 minutes. This treatment produces anatase films with a surface roughness factor of 80-200.

After cooling under a continuous argon flow, the glass sheet is immediately transferred to an alcoholic solution of a photosensitizer. The photosensitizer of Example 1 in a concentration of ^' 5xl0^~ *M in absolute ethanol is employed. Prolonged exposure of the film to the open air prior to photosensitizer adsorption is avoided in order to prevent hydroxylation of the Ti0₂ surface as the presence of hydroxyl groups at the electrode surface interferes with dye uptake. The adsorption of photosensitizer from the ethanolic solution is allowed to continue for 30 minutes after which time the glass sheet is withdrawn and washed briefly with absolute ethanol.

The photocurrent action spectrum obtained with such a film using a conventional three electrode electrochemical cell containing an ethanolic solution of 0.5M Lil and 3xlO^'3M iodine is shown in the attached figure together with the AM 1 spectral distribution of solar light emission. The incident monochromatic photon to current conversion efficiency (IPCE) is plotted qs a function of the excitation wavelength. This was derived from the equation:

[(1.24x10^s) x photocurrent density (μA/cm²)]

(1) IPCE(%)=

[wavelength (nm) x photon flux (W/m²)]

From the overlap of the photocurrent action spectrum with solar emission the overall efficiency for the conversion of solar light to electricity η is calculated from the formula

(2) η = 12 x OCV x FF(Ψc)

where OCV is the open circuit voltage and FF is the fill factor of the photovoltaic cell.

For experimental verification of equation 2, a photovoltaic cell, shown in the drawing attached, is constructed, using a photosensitizer (4)-loaded Ti0₂ (5) film supported on a conducting glass (the working electrode) comprising a transparent conductive tin dioxide layer (6) and a glass substrate (7) as a photoanode. The cell has a sandwich-like configuration, the working electrode (4-7) being separated from the counter electrode (1 ,2) by a thin layer of electrolyte (3) having a thickness of ca. 20 microns. The electrolyte used is an ethanolic solution of 0.5 M Lil and 3xl0^'3M iodine. The electrolyte (3) is contained in a small cylindrical reservoir mot shown) attached to the side of the cell from where capillary forces attract it to the inter-electrode space. The counter-electrode comprises the conductive tin dioxide layer (2) deposited on a glass substrate (1 ) made also of Asahi conducting glass and is placed directly on top of the working electrode. A monomolecular transparent layer of platinum is deposited on to the conducting glass of the counter electrode (1 ,2) by- electroplating from an aqueous hexachloroplatinate solution. The role of the platinum is to enhance the electrochemical reduction of iodine at the counter electrode. The transparent nature of the counterelectrode is an advantage for photovoltaic applications since it allows the harvesting of light from both the forward and the backward direction. Experiments are carried out with a high pressure Xenon lamp equipped with appropriate filters to simulate AMI solar radiation.

The action spectrum of the compound of formula la shows a maximum around 400 nm. Incident photon to current conversion efficiencies are high i.e. between 50 and 60%. Figure 2 shows the limit of the current yield is 80-85% due to light absorption by the conducting glass of the cell of Application Example A. The photocurrent voltage current and power characteristics of the cell of Application Example A are greatly improved (see Figure 3).

Application Example B

A transparent Ti0₂ film from colloidal titanium dioxide particles which are deposited on a conducting glass support and sintered to yield a coherent highly porous semiconducting film that is transparent and can be used instead of the Ti0₂ layer film in Application Example A.

Colloidal titanium oxide particle of approximately lOnm are prepared by hydrolysis of titanium isopropoxide as follows :

125 ml of titanium isopropoxide is added to a solution of 0.1 M nitric acid in 750ml of water whilst stirring. A precipitate of amorphous titanium dioxide is formed under these conditions. This is heated to 80°C for approximately 8 hours, stirring vigorously, resulting in peptisation of the precipitate and formation of a clear solution of colloidal anatase. The anatase structure of the titanium dioxide particles is established by Raman spectroscopy. The sol is concentrated by evaporation of the solvent in vacuum at room temperature until a viscous liquid is obtained containing the colloidal particles. At this stage the nonionic surfactant TRITON X-100 (40% weight ofTi0₂) is added to reduce cracking of the film when applied to a substrate.

The titanium dioxide films are formed by spin coating the concentrated sol on to a conducting glass substrate. Usually it is sufficient to apply 6 to 10 layers in order to obtain semiconductor membranes of sufficient surface area to give excellent visible light harvesting efficiencies after deposition of a monolayer of the sensitizer.

Low resolution electron microscopy confirms the presence of the three layer structure, the lowest being the glass support followed by the 0.5 micron thick fluorine-doped Sn0₂ and the 2.7 micron thick titanium dioxide layer. High resolution electron microscopy (Fig. 3) reveals the Ti0₂ film to be composed of a three dimensional network of interconnected particles having an average size of approximately 16nm. Apparently, significant particle growth occurs during sintering.

The transparent Ti0₂ films are tested in conjunction with a regenerative cell containing the photosensitizer of Example 1 for the generation of electricity from visible light. The results can be represented where the photocurrent under simulated sunlight (intensity ca 30Wlm²) is plotted as a function of cell voltage.

Application Example C

A sheet of conducting glass (ASAHI) area resistance ca 10 Ohm/ square) having a size of 2x9.6 cm² is coated with a colloidal titanium dioxide film according to the procedure of Application Example B. A total of 7 layers of Ti0₂ colloid are deposited successively by spin coating and the film is subjected each time to calcination at 500°C for 30 minutes. 30% iw/w) of TRITON X 405 surfactant is added in order to avoid cracking of the film. The final thickness of the titanium dioxide film is 5 micron as determined from the optical interference pattern. It is important to note that the conducing glass sheet after deposition of the Ti0₂ remains clear and transparent to visible and near infrared light.

Immediately before coating with the photosensitizer of Example 1 , the film is fired for 1 hour at 500°C. The coating of Ti0₂ with photosensitizer is performed by immersing the glass sheet for 16 hours in an ethanolic solution containing the optical brightener of Example 1.

After photosensitizer deposition, the glass sheet is cut into two parts each having a size of ca 9 cm₂. These sheets serve as working electrodes (photo-anodes) in the module whose assembly is described further below.

Transparent counter electrodes are made of the same type of ASAHI conducting glass as the working electrodes. The counterelectrode is not coated with Ti0₂. Instead, the equivalent of 10 monolayer of Pt is electrochemically deposited on to conducting glass. The transparent nature of the counterelectrode is not affected by the deposition of the Pt its transmission in the visible and near infrared remaining greater that 60%. The Pt acts as an electrocatalyst, enhancing the rate of reduction of the electron transfer mediator, i.e. triiodide, at the counterelectrode. Two ca. 1mm deep and 1.5mm wide and 20mm long indentations are engraved into the surface of the counterelectrode close to the edges of the glass sheets. These serve as a reservoir for the electrolyte.

The counter electrode is placed directly on top of the working electrode to yield a sandwich-type configuration. After filling the reservoirs with electrolyte, the cell is sealed with epoxy resin. The wetting of the space between the two electrodes by the electrolyte occurs spontaneously by capillary action. The electrolyte is a solution of 0.5M tetrapropyl ammonium iodide and 0.02M iodine in ethanol.

Two cells are fabricated in this way, each having a surface area of ca 9cm². Subsequently they are connected in series by electrically contacting the photoanode of one cell to the cathode of the second cell. In this way a module is constructed, having a total surface area of 18 cm².

Application Example D

A photovoltaic device, as shown in Figure, 1 based on the sensitization of a transparent Ti0₂ film, made from colloidal titanium dioxide particles which are deposited on a conducting glass support and sintered to yield a coherent highly porous semiconducting film.

Colloidal titanium oxide particles of approximately 8 nm are prepared by hydrolysis of titanium isopropoxide as follows:

125ml titanium isopropoxide is added to a solution of 0.1 M nitric acid in 750ml water while stirring. A precipitate of amorphous titanium dioxide is formed under these conditions. This is heated to 80° C for approximately 8 hours, stirring vigorously, resulting in peptisation of the precipitate and formation of a clear solution of colloidal anatase. the propanol formed by the hydrolysis is allowed to evaporate during the heating. The colloidal solution is then autoclaved at 140 to 250°C, preferably 200°C, in a pressure vessel of titanium metal or teflon for 2 to 20 hours, preferably 16 hours. The resultant sol, containing some precipitate is stirred or shaken to resuspend the precipitate. The resulting sol, minus any precipitate that will not resuspend, is concentrated by evaporation of the solvent in vacuum at room temperature until a viscous liquid is obtained containing the colloidal particles. A typical concentration at this point is 200g/L. At this stage a polyethylene oxide polymer, for example Union Carbide Carbowax 20M or Triton X-405 can be added to increase the thickness of the layer that be deposited without cracks. The polymer is added in amount of 30 to 50, preferably 40. weight percent Ti0₂.

The electrodes for sensitization are formed from the colloidal solution as follows:

A suitable substrate, for example a 3 x 6 cm piece of conductive tin oxide coated glass, for example from Asahi Corp. (but also titanium metal or any flat conductive surface), is placed with the conductive surface up and with suitable spacers, for example 50 to 100 micron. preferably 80 micron thick plastic tape, placed along each edge. A suitable amount of the sol. for example 150 microliters of sol with 200 g/L Ti0₂ and 40% Carbowax 20M for the above substrate, is pipetted along one end of the substrate. The sol is spread across the substrate by drawing with a flat edged piece of glass whose ends ride along the spacers. Thus the spacers, the viscosity of the sol, and the concentration of the sol control the amount of Ti0₂ deposited. The spread film is allowed to dry in room air till visibly dry and preferable and additional 20 minutes. After drying the electrode is fired at 400 to 500°C. preferably 450, for a minimum of 20 minutes. In the case of sols autoclaved below 170° C the spacers less than 40 micron must be used and the process must be repeated twice to achieve an 8 to 10 micron thick Ti0₂ film.

Electrodes of up to 10 cm by 10 cm have been fabricated by this method. The sol can also be applied to substrates by spin coating and dip coating.

The electrode can then be cut to the size desired by normal glass cutting techniques. Immediately before applying the photosensitizer, the electrode is fired again at 450 to 550, preferably 500° C for 2 to 12, preferably 6 hours. For some solvent and dye combinations the surface of the electrode is improved (with respect to electron injection) by firing the electrode 5 to 10, preferably 7 times at 500 oC for 2 to 6 hours with either 10 hours in air or soaking up to 1 hour in water, 0.5M nitric acid or 0.5M HCl, between each firing. The acid solutions are saturated with dissolved Ti0₂ before use. After the last firing, immediately after cooling, the electrode is placed in the sensitizer solution. Preferably an ethanolic solution containing the optical brightener of Example 1. Between 4 and 24 hours are required for the electrode to be fully coated.

After removal from the photosensitizer solution, the electrode is made into a photovoltaic cell as follows:

Transparent counterelectrodes are made of the same type of ASAHI conducting glass as the workinς electrodes. The counterelectrode is not coated with Ti0₂. Instead the equivalent of 10 monolayer s of Pt is electrochemically deposited onto conducting glass. The transparent nature of the counterelectrode is not affected by the deposition of the Pt, its transmissions is visible and near infrared remains greater that 60%. The Pt acts as an electrocatalyst enhancing the rate of reduction of the electron transfer mediator, i.e. triiodide, at the counterelectrode. Alternatively, a thin titanium sheet, which may be porous coated as above with Pt. may be used as a counter electrode. In the case of a porous sheet, another sheet of impervious material is required behind the counter electrode, such as plastic, glass or metal.

A reservoir is provided for the electrolyte by engraving two ca 1mm deep and 1.5mm wide and 20mm long clefts into the surface of the counterelectrode close to the edges of the glass sheet. The reservoir can also be added external to the glass sheets or be behind the counter electrode in the case of porous counter electrode.

The counter eletr ode is place directly on top of the working electrode to yield a sandwich type configuration. The reservoirs are filled with electrolyte solution, selected from the list above buy preferably 85% by weight ethylene carbonate, 15% propylene carbonate 0.5M potassium iodide and 40mM iodine. An amount of Lil or Tetraalkylamonium Iodide can be present (preferably 20mM) depending on the voltage desire. The cell is sealed around the edge with a sealant compatible with the solvent chosen and bonded closed with an adhesive. The sealant and the adhesive may be the same material for example silicon adhesive in the case of the alcohol solvents, or for example, polyethylene and epoxy resin (or mechanical closure) in the case of ethylene carbonate. The wetting of the space between the two electrodes by the electrolyte injected into the reservoirs occurs spontaneously by capillary action.

The same amount of a photosensitizer of any one of Examples 2 to 37 can be used in place of the optical brightener of Example 1 in Application Examples A to D in the photovoltaic cell.

Application Example E Application Example A is repeated, except that the Ti0₂ surface is coated with photosensitizer by immersing the Ti0₂ film for 3 to 5 hours in a 3xl0^'"mol solution of each of the following compounds sequentially:

1) 33% of an optical brightener of the formula la (component i)

ii) 33% of the copper tetracarboxyphthalocyanine of Example 7 (component ii)

Hi) 34% of a ruthenium complex of the formula a)

[Ru(L₂((CN₂)Ru(L')₂)₂

in which L is 4 ,4'-dicarboxybipyridyl and L, is 2,2'bipyridyl (component Hi)

the percentages being by weight based on the combined amount of components i), ii) and Hi) being 100%.

Alternatively a 3x10" 'mol solution contain components i), ii) and Hi) above can be made up and the Ti0₂ film is immersed therein for 3 to 5 hours to apply the photosensitizer.

Claims

Claims;

1. A photovoltaic cell comprising:

an electrically conductive element to which one or more titanium dioxide layers have been applied, to which titanium dioxide layers a photosensitizer having at least one interlocking group has been applied (hereinafter defined as component a) ;

characterised in that the photosensitizer is selected from optical brightener compounds and phthalocyanine compounds.

2. A photovoltaic cell comprising:

i) an electrically conductive element to which one or more titanium dioxide layers have been applied, to which titanium dioxide layers a photosensitizer having at least one interlocking group has been applied (hereinafter defined as component a);

ii) a second electrically conductive element, having no Ti0₂ layer present;

at least one of the two electrically conductive layers being transparent

characterized in that the photosensitizer is selected from optical brighteners and phthalocyanine compounds.

3. A cell according to claim 1 or claim 2 in which the Ti0₂ layer is anatase.

4. A cell according to any one of the preceding claims in which the titanium dioxide layer is doped with a metal ion.

5. A cell according to claim 1 in which the or both electrically conductive elements have a surface resistance of 5-1000ohm/cm².

6. A cell according to claim 1 in which the photosensitizer comprises a three layer system comprising:

i) an optical brightener compound (hereinafter defined as component i) ii) a phthalocyanine compound (hereinafter defined as component ii) and Hi) a ruthenium complex compound (hereinafter defined as component Hi)

7. A cell according to claim 6 in which the amount of component i) is 10-50%, the amount of component ii) is 10-40% and the amount of component Hi) is 10-50%.

8. A cell according to claim 1 in which the photosensitizer is an optical brightener of the stilbene series, the coumarin series, the pyrazoline series, the benzoxazolyl (including bisbenzoxazolyl) series, the thiophene series, the furane series, the benzimidazoline series, the naphthalic acid imide series, the triazole series, and the oxazole series.

9. A compound of formula I

in which n is 0 or 1 to 4.

SUBSTITUTE SHEET R₁₀ is a group a), b). c) or d)

in which R₁₂ is hydrogen, C,.₄alkyl or phenyl;

^■ R₁₃ is hydrogen, C_halky I, C alkoxycarbonyl or cyano R_]4 is hydrogen, C,^alkyl, chloro or cyano; and X is -O- or -S-.

10. A compound according to claim 9 of formula II

11. A compound of formula X or XI

in which n is 0 or 1 each R_ independently is -COOH. OH or hydrogen; each R_: independently is -COOH, OH or hydrogen; with the proviso that on at least one phenyl ring in formula I and formula II

R, and R₂ cannot both be hydrogen;

R₃ is halogen. OH,-0-C,__walkyl, -0-CO-C,__eμlkyl.OC₅.₇aryl, -0-CO-C^ryl, -0-Si-(C_._t alkyl)₃, O-Si- (C_5.7aryl)₃ or together with R₄ = O

Me is selected from Cu. Ni, Fe, Co, V, Sn, Si, Ti, Ru, Ge, Cr, Zn. Mg, Al, Pb, Mn, In,

Mo, Y, Zr, Nb, Sb, La. W, Pt, Ta, Tl, Ir, Pd, Os, Ga, Tb, Eu, Rb, Bi, Se, As, Sc, Ag, Cd,

Hf, Re. Au, Hg, Ac, Tc, Te and Rh.

12. A cell according to any one of claims 1 to 8 in which the photosensitizer comprises a compound of formula I defined in claim 9.

13. A cell according to any one of claims 1 to 8 and 12, in which the photosensitizer is comprises a compound of formula X or XI defined in claim 10.