EP2675938A1

EP2675938A1 - Method for producing a photocatalytic material

Info

Publication number: EP2675938A1
Application number: EP12709913.3A
Authority: EP
Inventors: Rosiana Aguiar; Bernard Nghiem; Anne Durandeau
Original assignee: Saint Gobain Glass France SAS; Compagnie de Saint Gobain SA
Current assignee: Saint Gobain Glass France SAS; Compagnie de Saint Gobain SA
Priority date: 2011-02-16
Filing date: 2012-02-15
Publication date: 2013-12-25
Also published as: FR2971519A1; CN103370441A; WO2012110746A1; CN103370441B

Abstract

The invention relates to a method for producing a material including a substrate, at least one of the surfaces of which is coated with a titanium-oxide photocatalytic layer, said method including depositing said photocatalytic layer by a chemical vapour deposition method in which a gaseous mixture, including at least one titanium alkoxide and at least one compound including at least one carboxyl or ester group, is placed in contact with said substrate.

Description

METHOD FOR OBTAINING PHOTOCATALYTIC MATERIAL

The invention relates to the field of materials comprising a substrate provided with a photocatalytic coating, in particular intended to be incorporated in photovoltaic cells.

Photocatalytic coatings, in particular those based on titanium dioxide, are known to impart self-cleaning and anti-fouling properties to the substrates which are provided with them. Two properties are at the origin of these advantageous characteristics. Titanium oxide is first of all photocatalytic, that is to say that it is capable under suitable radiation, generally ultraviolet radiation, of catalyzing the degradation reactions of organic compounds. This photocatalytic activity is initiated within the layer by the creation of an electron-hole pair. In addition, the titanium dioxide has an extremely pronounced hydrophilicity when it is irradiated by this same type of radiation. This strong hydrophilicity, sometimes called "super-hydrophilic", allows the evacuation of mineral soils under water runoff, for example rainwater. Such materials, in particular used to form glazings, are described, for example, in application EP-A-0 850 204.

Titanium dioxide has a high refractive index, which results in important light reflection factors for substrates with photocatalytic coatings. This is a disadvantage in the field of glazing for the building, and especially in the field of photovoltaic cells, for which it is necessary to maximize the transmission to the photovoltaic material, and thus minimize any absorption and reflection of solar radiation. However, there is a need to provide photovoltaic cells with a photocatalytic coating, because the deposition of dirt is able to reduce the efficiency of photovoltaic cells of ΘΠνΐΓΟΠ 6 "6 per year.This figure is obviously dependent on the geographical location cells.

To reduce the light reflection factor, it is possible to reduce the thickness of the photocatalytic coatings, but this is done to the detriment of their photocatalytic activity.

The aim of the invention is to propose a process for obtaining photocatalytic materials based on titanium oxide having low light reflection factors that can be used in photovoltaic cells.

For this purpose, the object of the invention is a process for obtaining a material comprising a substrate coated on at least a part of at least one of its faces with a titanium oxide-based photocatalytic layer, said method comprising depositing said photocatalytic layer by a chemical vapor deposition process wherein a gaseous mixture comprising at least one titanium alkoxide and at least one compound comprising at least one carboxyl or ester group is contacted with said substrate.

The substrate is typically a glass sheet, especially extra-clear glass, as described in more detail in the following text.

Through the use of this method, the photocatalytic layer generally has an index of refraction of at most 1.9 for a wavelength of 550 nm. The refractive index can be measured by variable angle spectroscopic ellipsometry (VASE).

The subject of the invention is also a process for obtaining a photovoltaic cell comprising a front-face substrate which is a substrate coated on at least a part of at least one of its faces with a photocatalytic layer based on titanium oxide, said process comprising a step of depositing said photocatalytic layer by a chemical vapor deposition process in which a gaseous mixture comprising at least one titanium alkoxide and at least one compound comprising at least one carboxyl or ester group is contacted with said substrate.

"Front face substrate" means the substrate which is the first crossed by solar radiation. The photocatalytic coating is then generally positioned outward, so that the self-cleaning effect can be usefully demonstrated.

The subject of the invention is also a material that can be obtained by the process according to the invention. The various preferred features described below are therefore characteristics applicable both to the processes according to the invention and to the material according to the invention.

Chemical vapor deposition, generally referred to by its acronym CVD, is a pyrolysis process in which a gaseous mixture comprising a carrier gas and diluted precursors is contacted with a hot substrate, the precursors decomposing and / or chemically reacting under the effect of the heat of the substrate. The carrier gas is generally nitrogen.

A carboxyl group is a -CO ₂ H group, especially present in carboxylic acids. An ester group is a -CO ₂ R group, where R is a carbon group.

The inventors have been able to demonstrate that the addition of a compound comprising at least one carboxyl or ester group, in particular a carboxylic acid, made it possible to obtain layers having reduced light reflection factors, in certain cases of the order those of the uncoated substrate, or even lower thereto, without appreciable loss of photocatalytic activity. It is thus possible to obtain, by a CVD process, titanium oxide layers having both a satisfactory photocatalytic activity and a low reflection.

Preferably, the substrate is a glass or glass-ceramic sheet. The sheet may be flat or curved, and have any type of dimensions, especially greater than 1 meter. The glass is preferably of the silico-soda-lime type, but other types of glasses, such as borosilicate glasses or aluminosilicates can also be used. The glass may be clear or extra-clear, or tinted, for example blue, green, amber, bronze or gray. The thickness of the glass sheet is typically between 0.5 and 19 mm, especially between 2 and 12 mm, or even between 4 and 8 mm. In the field of photovoltaic cells, the glass is preferably extra-clear; it preferably comprises a total weight content of iron oxide of at most 150 ppm, or even 100 ppm and even 90 ppm, or even a redox of at most 0.2, especially 0.1 and even a zero redox. "Redox" means the ratio of the ferrous iron oxide content by weight (expressed as form FeO) and the total weight content of iron oxide (expressed as Fe ₂ O ₃ ).

Chemical vapor deposition is achieved by contacting the gas mixture with the generally hot substrate. According to a first embodiment, the carrier gas, the or each titanium alkoxide and the or each compound comprising at least one carboxyl or ester group are mixed to form the gaseous mixture, which is passed through a nozzle in the deposition chamber. , close to the substrate, generally at a distance from the substrate ranging from 1 to 10 mm, in particular from 3 to 6 mm. According to a second embodiment, the carrier gas and the titanium alkoxide are mixed to form a first gaseous mixture, the carrier gas and the compound comprising at least one carboxyl or ester group are mixed to form a second gaseous mixture, then the first and second gaseous mixtures are each separately passed through a different nozzle in the deposition chamber, thus obtaining the final mixture in the deposition chamber. The second embodiment has the advantage of avoiding any premature reaction between the titanium alkoxide and the compound comprising at least one carboxyl or ester group, which could lead to plugging of the nozzle.

The mixture of the carrier gas with the precursor (the compound comprising at least one carboxyl or ester group or the titanium alkoxide) is generally carried out by passing the carrier gas through the liquid precursor at a suitable temperature for driving the precursor in gaseous form.

Generally, the or each nozzle is fixed and located above the moving substrate. The chemical vapor deposition is preferably carried out at atmospheric pressure on a heated substrate at a temperature ranging from 400 to 700 ° C., preferably from 500 to 600 ° C., or even from 500 to 560 ° C. . Temperatures ranging from 500 to 560 ° C make it possible to maximize the crystallization in anatase form of the titanium oxide and therefore the photocatalytic activity.

The chemical vapor deposition is advantageously implemented on a flat glass production line, in particular on a glass floating line, when the glass substrate is within the floating device (that is to say in the enclosure where the glass ribbon is poured onto the molten tin), or when the glass substrate is between said float device and the lehr, or when the glass substrate is within the lehr. Preferably, the deposition is implemented when the glass substrate is between the float device and the lehr, this zone corresponding to the preferred deposition temperature ranges. The lehr is the enclosure in which the glass is annealed in order to evacuate any mechanical stresses within it. The deposit can also be implemented on a flat glass manufacturing line by rolling between metal or ceramic rolls, a method used to form textured glass sheets in particular. Alternatively, the chemical vapor deposition can be implemented in recovery, that is to say in a dedicated and decoupled installation of the flat glass manufacturing line.

The titanium alkoxide is preferably chosen from the compounds of formula Ti (OR ₁ ) (OR ₂ ) (OR ₃ ) (OR ₄ ), each radical R 1 being a linear or branched alkyl radical, identical or different, in C 1 C12. Preferably, R 1 radicals are identical and are linear or branched C ₂ -C ₅ alkyls. A particularly preferred titanium alkoxide is titanium tetraisopropoxide. Other interesting alkoxides are tetraethoxytitanium and tetrabutoxytitanium.

The compound comprising at least one carboxyl or ester group is preferably a carboxylic acid or an ester. The carboxylic acid is preferably chosen from the compounds of formula X ₁ X ₂ X ₃ C-CO ₂ H, where X ₁ , X ₂ , X ₃ , which are identical or different, are chosen from the hydrogen atom, the carbon atoms and halogen, especially fluorine, chlorine or bromine, or straight or branched carbon chains, saturated or unsaturated, optionally hydroxylated, in particular alkyl radicals C ₁ -C 6, particularly C ₁ -C 3. The carboxylic acid can be weak or strong, preferably low. The constancy of acidity Ka at 25 ° C. of the carboxylic acid is preferably less than or equal to 5.10 ^-5 , especially 2.10 ^-5 .

The carboxylic acid is preferably chosen from ethanoic acid (Ka = 1.73 × 10 ^-5 ), propanoic acid (Ka = 1.38 × 10 ^-5 ) and butanoic acid (Ka = 1.48 × 10 ^-5 ). and pentanoic acid (Ka = 1.44 × 10 ^-5 ).

The carboxylic acid may also be chosen from hydroxyacetic acid (Ka = 1.5 × 10 ^-4 ), trifluoroacetic acid (Ka = 1.0) and monofluoroacetic acid (Ka = 2.10 × ³ ). difluoroacetic acid.

Despite the presence of fluorine in these latter acids, no trace of fluorine is detected within the layer by X-ray photoelectron spectrometry (XPS). The fluorine is probably evacuated with the carrier gas, without being incorporated in the layer. The titanium alkoxide and the compound comprising at least one carboxyl or ester group are preferably the only precursors used for CVD deposition. Good results have been obtained when the alkoxide is titanium tetraisopropoxide and the compound comprising at least one carboxyl or ester group is ethanoic acid or trifluoroacetic acid.

The ratio R between the molar flow rate of compound comprising at least one carboxyl or ester group and the molar flow rate of titanium alkoxide is preferably at least 0.1%, in particular 0.2% or even 0.3%, or even 0.5%. The ratio R is preferably at most 80%, especially 70% or 60%. In some embodiments, this ratio R is at most 5%, especially 3%, or even 2%. Low R ratios do not sufficiently reduce the refractive index of the photocatalytic layer and therefore the light reflection factor. R ratios too high are accompanied by a decrease in photocatalytic activity and degradation of crystallization of titanium oxide.

Preferably, the substrate is coated on all of one of its faces of the photocatalytic layer based on titanium oxide.

The photocatalytic layer based on titanium oxide is preferably composed of titanium oxide, in particular crystallized in anatase form, which is the most active form. A mixture of anatase and rutile phases is also conceivable. The titanium oxide may be pure or doped, for example by transition metals (especially W, Mo, V, Nb), lanthanide ions or noble metals (such as, for example, platinum or palladium), or by nitrogen, carbon or fluorine atoms. These different forms of doping make it possible to increase the activity photocatalytic material, either to shift the gap of titanium oxide to wavelengths near the visible range or included in this area. Preferably, the photocatalytic layer based on titanium oxide does not contain nitrogen atoms, as this contributes to reducing the optical transmission of the layer.

The titanium oxide layer is normally the last layer of the stack deposited on the substrate, ie the layer of the stack farthest from the substrate. It is important that the photocatalytic layer is in contact with the atmosphere and its pollutants. It is however possible to deposit on the photocatalytic layer a very thin layer, generally discontinuous or porous. For example, it may be a layer based on noble metals intended to increase the photocatalytic activity of the material. It may also be thin hydrophilic layers, for example silica, as taught in applications WO 2005/040058 or WO 2007/045805.

Different layers can be deposited, cumulatively or alternatively, under the titanium oxide layer:

One or more layers acting as a barrier to the migration of alkaline ions from the substrate. Such layers can be deposited by CVD before the photocatalytic layer. They are preferably based on or consisting of an oxide, a nitride, an oxynitride or an oxycarbide of at least one of the following elements: Si, Al, Sn, Zn, Zr. Of these materials, silica or silicon oxycarbide are preferred because of their ease of deposition by the CVD technique. One or more low emissivity layers, such as fluorine or antimony doped tin oxide layers. Such layers make it possible to limit the condensation (fog and / or frost) on the surface of the multiple glazings, in particular when they are inclined (for example when they are integrated with roofs or verandas). The presence of a low-emissivity layer in face 1 makes it possible to limit heat exchanges with the outside during the night, and thus to maintain a surface temperature of the glass greater than the dew point. The appearance of mist or frost is therefore strongly reduced or completely eliminated. The photocatalytic layer can be deposited directly on the doped tin oxide layer. The latter usually imposes the less active rutile form, but the crystallization in the gas phase obtained by the process according to the invention overcomes this disadvantage. An additional advantage of the process according to the invention in this case is therefore to allow the deposition of layers in which the titanium oxide is crystallized in the anatase form (the most active) and deposited directly on a tin oxide layer dope.

The thickness of the photocatalytic layer is preferably between 2 and 1000 nanometers, in particular between 5 and 150 nm, or even between 8 and 50 nm. A high thickness makes it possible to increase the photocatalytic activity of the layer but increases the luminous reflection.

The material (obtained) according to the invention preferably has a light transmittance (within the meaning of ISO 9050: 2003) of at least 80%, even 85% and even 90%.

The material (obtained) according to the invention preferably has a light reflection factor (within the meaning of ISO 9050: 2003) of at most 15%, preferably 10%, in particular 8%. The light reflection factor of the material may therefore be less than or equal to that of the uncoated substrate.

The invention also relates to a glazing unit or a photovoltaic cell comprising at least one material according to the invention.

The glazing may be single or multiple (in particular double or triple), in the sense that it may comprise several glass sheets leaving a space filled with gas. The glazing can also be laminated and / or tempered and / or hardened and / or curved.

The other face of the material according to the invention, or possibly a face of another substrate of the multiple glazing, may be coated with another functional layer or a stack of functional layers. It may especially be another photocatalytic layer. It may also be layers or stacks with thermal function, in particular antisolar or low-emissive, for example stacks comprising a silver layer protected by dielectric layers. It may still be a mirror layer, in particular based on silver. It can finally be a lacquer or an enamel intended to opacify the glazing to make a facade facing panel called lighter. The lighter is arranged on the facade alongside the non - opaque glazings and allows to obtain facades entirely glazed and homogeneous from the aesthetic point of view.

In the photovoltaic cell according to the invention, the material according to the invention is preferably the front face substrate of the cell, that is to say the one which is the first crossed by solar radiation. The photocatalytic coating is then positioned outwards, so that the self-cleaning effect can be useful.

For applications as photovoltaic cells, and in order to maximize the energy efficiency of the cell, several improvements can be made, cumulatively or alternatively:

The glass sheet may advantageously be coated, on the face opposite to the face provided with the coating according to the invention, with at least one transparent and electroconductive thin layer, for example based on Sn0 ₂ : F, Sn0 ₂ : Sb, ZnO: Al, ZnO: Ga. These layers may be deposited on the substrate by various deposition methods, such as chemical vapor deposition (CVD) or sputtering deposition, in particular assisted by magnetic field (magnetron process). In the CVD process, halide or organometallic precursors are vaporized and transported by a carrier gas to the surface of the hot glass, where they decompose under the effect of heat to form the thin layer. The advantage of the CVD process is that it is possible to implement it in the glass sheet forming process, especially when it is a floating process. It is thus possible to deposit the layer when the glass sheet is on the tin bath, at the exit of the tin bath, or in the lehr, that is to say when the glass sheet is annealed to eliminate mechanical stress.

The glass sheet coated with a transparent and electroconductive layer may in turn be coated with an amorphous or polycrystalline silicon semiconductor, with chalcopyrites (in particular of the CIS-CuInSe2 or CIGS-CuInGaSe2 type) or with CdTe for to form a photovoltaic cell. In this case, another advantage of the CVD process resides in obtaining a rougher roughness, which generates a phenomenon of trapping of light, which increases the amount of photons absorbed by the semiconductor. - The surface of the glass sheet may be textured, for example have patterns (especially pyramid), as described in WO 03/046617, WO 2006/134300, WO 2006/134301 or WO 2007/015017. These textures are generally obtained using a glass forming by rolling.

The photovoltaic cell is typically formed by joining the front-face substrate (obtained according to the invention) and a back-face substrate, for example by means of a lamination interlayer made of thermosetting plastics material, in particular PVB, PU or EVA. Between the front and rear face substrates are arranged electrodes, generally in the form of thin layers, surrounding a material with photovoltaic properties. The backside substrate is typically a glass sheet.

The material with photovoltaic properties may be solid or in thin film form, depending on the technology, on the front-face substrate or on the back-face substrate. When the material with photovoltaic properties is amorphous or polycrystalline silicon or CdTe, it is generally deposited on the front face substrate, framed by electroconductive thin layers and transparent, typically based on tin oxide (doped with fluorine or with antimony), zinc oxide (doped with aluminum or gallium), or with tin oxide and indium (ITO). When the material with photovoltaic properties is based on Cu (In, Ga) Se2 (CIGS), it is generally deposited on a layer molybdenum conductor, itself deposited on the back-face substrate. By (In, Ga) we mean that the material can comprise In and / or Ga, according to any possible combinations of contents: Ini_ _x Ga _x , x being able to take any value of 0 to 1. In particular, x can be zero (material of type CIS).

The invention will be better understood in the light of the nonlimiting examples which follow, illustrated by FIGS. 1 to 3.

Figures 1 to 3 are scanning electron micrographs.

On 3 mm thick extra-clear silico-soda-lime glass substrates marketed by the Applicant under the name "SGG Diamant®" is deposited a layer of silica 80 nm thick, a layer acting as a barrier to alkaline migration.

On each substrate is deposited a layer of titanium oxide by the CVD technique, using as titanium oxide precursor titanium tetraisopropoxide (TiPT) and as a compound comprising at least one carboxyl group or ester trifluoroacetic acid ( TFA).

According to the examples, the ratio R between the flow rate of TFA and the flow rate of TiPT varies from 0 (Comparative Example C1 without the use of a carboxylic acid) to 3%.

The gas mixture is carried out by passing nitrogen (carrier gas) in TiPT heated to 70-80 ° C, and in TFA at 5 ° C.

The temperature of the substrate is 570 ° C during deposition. The deposit is carried out at atmospheric pressure.

Table 1 below summarizes the results obtained, indicating for each example: the ratio R between the flow rate of TFA and the TiPT flow rate, expressed in%, the photocatalytic activity, denoted Kb, measured in the following manner: an aqueous solution of methylene blue is placed in contact in a sealed cell with the substrate coated (the latter forming the bottom of the cell). After exposure to ultraviolet radiation for 30 minutes, the concentration of methylene blue is evaluated by a light transmission measurement. The value of photocatalytic activity, expressed in gl ^-1 · min ^-1, corresponding to the decrease in the concentration of methylene blue per unit of time of exposure, the light reflection in the sense of ISO 9050: 2003, and rated RL expressed in%, the refractive index at 550 nm, denoted n, measured by variable angle spectroscopic ellipsometry, the mass quantity of T1O ₂ in the layer, denoted "q T1O ₂ ", evaluated by means of a microprobe, expressed in μg / cm ² , the mean size of the anatase grains, evaluated by atomic force microscopy (AFM), the roughness Ra, also evaluated by atomic force microscopy (AFM) on a surface of 1 * 1μη ² .

Cl 1 2 3 4 5 6

R 0 0.3% 0.7% 1.3% 1.7% 2% 2.7%

Kb 40 40 40 40 39 40 34

RL (%) 11 9.3 8.3 7.8 7.8 7.8 7.5 Index 2.4 2.1 2.1 9 1.4

refraction q Ti0 ₂ 4.3 2, 6 2.0 2.7 1.2 g / cm ² ) size 17 20 24 25

grains (nm)

Ra (nm) 0.8 0, 9 1, 6 2.2

Table 1

Figures 1, 2 and 3 are scanning electron micrographs illustrating the morphology of the layers of the respective examples Cl, 3 and 5.

In a second series of examples, a layer of silica with a thickness of 80 nm is deposited on extra-clear silico-soda-lime glass substrates 3 mm thick sold by the applicant under the name SGG Diamant Solar®. thickness, layer acting as a barrier to the migration of alkali.

On each substrate is deposited a layer of titanium oxide by the CVD technique, using as titanium oxide precursor titanium tetraisopropoxide (TiPT) and as a compound comprising at least one carboxyl group ethanoic acid (also called acetic acid) . According to the examples, the ratio R between the molar flow rates of acetic acid and TiPT varies from 0 (Comparative Example C2) to 80%.

The gas mixture is made by passing nitrogen (carrier gas) in TiPT heated to 85 ° C, and in ethanoic acid at 25 ° C.

The temperature of the substrate is 520 ° C during the deposition. The deposit is carried out at atmospheric pressure.

Table 2 below summarizes the results obtained in terms of light reflection.

Table 2

The substrates obtained are particularly suitable for serving as substrates for the front face of a photovoltaic cell. Their transmission TSQE (corresponding to the convolution of the transmission spectrum, the solar emission spectrum, and the quantum efficiency of the photovoltaic material) for an amorphous silicon photovoltaic material thus passes from 83.9% for Example C2 to 88.2% for Example 9.

In a third series of examples, a layer of silica with a thickness of 80 nm is deposited on 3 mm thick extra-clear silico-soda-lime glass substrates marketed by the applicant under the name "SGG Diamant Solar®". thickness, layer acting as a barrier to the migration of alkali. On each substrate is deposited a layer of titanium oxide by the CVD technique, using as titanium oxide precursor titanium tetraisopropoxide (TiPT) and as a compound comprising at least one carboxyl group trifluoroacetic acid (TFA) .

According to the examples, the ratio R between the molar flow rates of trifluoroacetic acid and TiPT varies from 0 (comparative example C3) to 90%.

The gas mixture is made by passing nitrogen (carrier gas) in TiPT heated at 85 ° C, and in TFA at 15 ° C.

Table 3 below summarizes the results obtained in terms of light reflection.

Table 3

The substrates obtained are particularly suitable for serving as substrates for the front face of a photovoltaic cell. Their transmission TSQE (corresponding to the convolution of the transmission spectrum, the solar emission spectrum, and the quantum efficiency of the photovoltaic material) for an amorphous silicon photovoltaic material thus passes from 83.9% for the example C3 to 88.6% for Example 12. These various results show that the addition of a compound comprising at least one carboxyl or ester group makes it possible to very significantly reduce the luminous reflection of the material, until it reaches a reflection lower than that of the bare glass substrate, without a significant decrease in photocatalytic activity.

Claims

A method for obtaining a material comprising a substrate coated on at least a portion of at least one of its faces with a titanium oxide-based photocatalytic layer, said method comprising depositing said photocatalytic layer by a chemical vapor deposition process in which a gaseous mixture comprising at least one titanium alkoxide and at least one compound comprising at least one carboxyl or ester group is brought into contact with said substrate.

2. Method according to the preceding claim, such that the substrate is a glass sheet or glass-ceramic.

3. Method according to the preceding claim, such that the chemical vapor deposition is carried out at atmospheric pressure on a heated substrate at a temperature ranging from 400 to 700 ° C, preferably from 500 to 600 ° C. .

4. Method according to the preceding claim, such that the chemical vapor deposition is implemented on a glass floating line, when the glass substrate is within the float device, or when the glass substrate is between said float device and the lehr, or where the glass substrate is within the lehr.

5. Method according to one of the preceding claims, such that the titanium alkoxide is chosen from compounds of formula Ti (ORi) (OR ₂ ) (OR ₃ ) (OR ₄ ), each radical R ± is an alkyl radical, linear or branched, identical or different, C ₁ -C ₁ 2.

6. Process according to the preceding claim, such that the titanium alkoxide is titanium isopropoxide.

7. Method according to one of the preceding claims, such that the compound comprising at least one carboxyl group is chosen from compounds of formula X ₁ X ₂ X ₃ C-CO2H, where X ₁ , X ₂ , X ₃ , which are identical or different, are chosen. among the hydrogen atom, the halogen atoms, in particular fluorine, chlorine or bromine, or the linear or branched, saturated or unsaturated, optionally hydroxylated carbon chains, in particular C ₁ -C 6 alkyl radicals, in particular in C ₁ -C ₃ .

8. Method according to the preceding claim, such that the compound comprising at least one carboxyl group is trifluoroacetic acid.

9. The method of claim 7, wherein the compound comprising at least one carboxyl group is selected from ethanoic acid, propanoic acid, butanoic acid and pentanoic acid.

10. Method according to one of the preceding claims, such that the photocatalytic layer based on titanium oxide is made of titanium oxide, in particular crystallized in anatase form.

11. Method according to one of the preceding claims, such that the photocatalytic layer based on titanium oxide does not contain nitrogen atoms.

12. Method according to one of the preceding claims, such that the photocatalytic layer has a refractive index of at most 1.9 for a wavelength of 550 nm.

13. Method according to one of the preceding claims, such that the material has a light reflection factor, within the meaning of ISO 9050: 2003, of at most 10%, especially 8%.

14. A method for obtaining a photovoltaic cell comprising a front-face substrate which is a substrate coated on at least a part of at least one of its faces with a titanium oxide-based photocatalytic layer, said method comprising a step of depositing said photocatalytic layer by a chemical vapor deposition process in which a gaseous mixture comprising at least one titanium alkoxide and at least one compound comprising at least one carboxyl or ester group is brought into contact with said substrate .

15. Material obtainable by the method of one of claims 1 to 13.

16. Glazing or photovoltaic cell comprising at least one material according to the preceding claim.