EP2255372A2

EP2255372A2 - Photovoltaic cell and substrate for photovoltaic cell

Info

Publication number: EP2255372A2
Application number: EP09713430A
Authority: EP
Inventors: Frank Gromball; Emmanuelle Peter; Stéphane AUVRAY; Eric Mattmann; Ulrich Billert; Nikolas Janke
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: 2008-02-18
Filing date: 2009-02-17
Publication date: 2010-12-01
Also published as: WO2009103929A3; WO2009103929A2; MX2010008809A; KR20100119871A; ZA201005723B; JP2011512665A; BRPI0907513A2; CN102007562A

Abstract

The invention relates to a photovoltaic cell (1) including an absorbent photovoltaic material, in particular a cadmium-based one, wherein said cell includes a front-surface substrate (10), in particular a transparent glass substrate that comprises, on a main surface, a transparent electrode coating (100) that comprises a stack of thin layers including at least one functional metal layer (40), in particular a silver-based one, and at least two antireflection coatings (20, 60), said antireflection coatings each including at least one antireflection layer (24, 26; 66, 68), the functional layer (40) being provided between the two antireflection coatings (20, 60), characterised in that the antireflection coating (60) provided above the functional metal layer (40) opposite the substrate includes at least two antireflection layers (66, 68), wherein the antireflection layer (68) being the farthest from the functional metal layer (40) is more resistive than the antireflection layer (66) being the closest to the functional metal layer (40).

Description

PHOTOVOLTAIC CELL AND PHOTOVOLTAIC CELL SUBSTRATE

The invention relates to a photovoltaic cell front-face substrate, in particular a transparent glass substrate, and to a photovoltaic cell incorporating such a substrate.

In a photovoltaic cell, a photovoltaic photovoltaic material system that generates electrical energy under the effect of incident radiation is positioned between a back-face substrate and a front-face substrate, this front-face substrate being the first substrate which is traversed by the incident radiation before it reaches the photovoltaic material.

In the photovoltaic cell, the front-face substrate conventionally comprises, beneath a main surface facing the photovoltaic material, a transparent electrode coating in electrical contact with the photovoltaic material disposed below when considering that the main direction arrival of incident radiation is from above.

This front face electrode coating is thus, in general, the negative terminal of the photovoltaic cell.

Of course, the photovoltaic cell also comprises, in the direction of the rear-face substrate, an electrode coating which then constitutes the positive terminal of the photovoltaic cell, but in general, the electrode coating of the rear-face substrate is not transparent. For the purposes of the present invention, the term "photovoltaic cell" is intended to mean any set of constituents generating the production of an electric current between its electrodes by conversion of solar radiation, whatever the dimensions of this assembly and whatever the voltage and the intensity of the current produced and in particular that this set of components has, or not, one or more internal electrical connection (s) (in series and / or in parallel). The notion of "cell photovoltaic in the sense of the present invention is here equivalent to that of "photovoltaic module" or "photovoltaic panel".

The material usually used for the transparent electrode coating of the front-face substrate is generally a transparent conductive oxide ("TCO") material, such as for example an indium oxide-based material. tin (ITO), or based on zinc oxide doped with aluminum (ZnO: Al) or doped with boron (ZnO: B), or with tin oxide doped with fluorine (SnO ₂ : F).

These materials are deposited chemically, for example by chemical vapor deposition ("CVD"), optionally enhanced by plasma

("PECVD") or physically, for example by vacuum deposition by cathodic sputtering, possibly assisted by magnetic field

("Magnetron").

However, to obtain the desired electrical conduction, or rather the desired low resistance, the electrode coating of a

TCO must be deposited at a relatively large physical thickness, of the order of 500 to 1000 nm and sometimes even more, which is expensive compared to the price of these materials when they are deposited in layers of this thickness.

When the deposition process requires heat input, this further increases the cost of manufacture.

Another major disadvantage of electrode coatings made of a TCO-based material lies in the fact that for a chosen material, its physical thickness is always a compromise between the electrical conduction finally obtained and the transparency finally obtained because the greater the physical thickness is important the higher the conductivity, the lower the transparency, and the lower the physical thickness, the stronger the transparency but the lower the conductivity.

It is therefore not possible with TCO-based electrode coatings to independently optimize the conductivity of the electrode coating and its transparency. The prior art knows from the international patent application No. WO 01/4204 a photovoltaic cell manufacturing method in which the transparent electrode coating is not made of a TCO-based material but consists of a stack of layers. thin layer deposited on a main face of the front face substrate, this coating comprising at least one functional metal layer, in particular based on silver, and at least two antireflection coatings, said antireflection coatings each comprising at least one antireflection layer, said functional layer being arranged between the two antireflection coatings.This process is remarkable in that it provides that at least one highly refractive oxide or nitride layer is deposited below the metallic functional layer and above the photovoltaic material when the the direction of incident light entering the cell from above is considered. an embodiment in which the two antireflection coatings which surround the metallic functional layer, the antireflection coating disposed under the metal functional layer towards the substrate and the antireflection coating disposed above the metal functional layer opposite the substrate each comprise at least one layer of a highly refractive material, in this case zinc oxide (ZnO) or silicon nitride (Si ₃ N ₄ ).

However, this solution can be further improved. The prior art also knows the US Pat. No. 6,169,246 which relates to a photovoltaic cell with absorbent photovoltaic material based on cadmium, said cell comprising a transparent glass front face substrate comprising on a main surface a transparent electrode coating constituted a transparent conductive oxide TCO.

According to this document, above the TCO electrode coating and below the photovoltaic material is interposed a zinc stannate buffer layer which is therefore neither part of the TCO electrode coating, nor photovoltaic material. - AT -

An important object of the invention is to enable the charge transport between the electrode coating and the photovoltaic material, in particular based on cadmium, to be easily controlled and that the efficiency of the cell can consequently be improved.

Another important goal is also to achieve a thin film-based transparent electrode coating which is simple to make and the cheapest possible to manufacture industrially.

The object of the invention is therefore, in its broadest sense, a photovoltaic cell with absorbent photovoltaic material, in particular based on cadmium, according to claim 1. This cell comprises a front-face substrate, in particular a transparent glass substrate, comprising on a main surface a transparent electrode coating consisting of a stack of thin layers comprising at least one metallic functional layer, in particular based on silver, and at least two antireflection coatings, said antireflection coatings each comprising at least one antireflection layer, said layer the antireflection coating disposed above the metal functional layer opposite the substrate comprises at least two antireflection layers, the antireflection layer furthest from the metallic functional layer being more resistive than the anti-scratch layer flounder closest to the metallic functional layer.

The resistivity p corresponds to the product of the square resistance R of the layer by its real thickness.

In a preferred variant of the invention, the antireflection layer furthest from the metal functional layer has a resistivity equal to at least 5 times, or even at least 10 times, or even at least 50 times, or even at least 100 times at least 200 times, or even at least 500 times, or even at least 1000 times, the resistivity of the antireflection layer closest to the metal functional layer. The antireflection layer furthest from the metallic functional layer, the one that is more resistive, preferably has a resistivity p between 5.10 ³ Ω.cm and 10 Ω.cm, or between 10 ^-2 Ω.cm and 5 Ω.cm, or even between 5.10 ² Ω.cm and 1 Ω.cm. The antireflection layer closest to the metal functional layer, the one that is more conductive, preferably has a resistivity p between 10 ^"5 Ω .cm and 5.10 ³ Ω.cm, excluding this last value, or between 5.10 ⁴ Ω.cm and 2.10 ³ Ω.cm, or even between 10 ^"4 Ω.cm and 10 ^{" 3} Ω.cm. Furthermore, the antireflection layer farthest from the metal functional layer has an optical thickness preferably representing between 2 and 50% of the total optical thickness of the antireflection coating furthest from the substrate and in particular an optical thickness representing between 2 and 25% or even between 5% and 20% of the total optical thickness of the anti-reflective coating furthest from the substrate.

This antireflection layer farthest from the metal functional layer preferably has a real thickness of between 2 and 100 nm, and preferably between 5 and 50 nm, or even between 10 and 30 nm.

An antireflection layer is preferably based on: zinc oxide ZnO optionally doped, for example

ZnO: Al, ZnO: B, ZnO: Ga, optionally doped tin oxide SnO ₂ , for example SnO ₂ : F,

TiO ₂ titanium oxide, optionally doped, for example TiO ₂ : Nb,

gallium oxide Ga ₂ O ₃ possibly doped,

indium oxide In ₂ O ₃ optionally doped,

silicon oxide SiO ₂ possibly doped,

- or based on a mixed oxide of indium and tin ITO, - a mixed oxide of gallium and zinc GZO,

a mixed oxide of indium and zinc IZO, a mixed oxide of zinc and tin Zn ₂ SnO ₄ ,

a mixed oxide of indium, gallium and zinc IGZO, this oxide possibly being non-stoichiometric. The antireflection layer closest to the metal functional layer is preferably generally based on a transparent conductive oxide (TCO) obtained from at least one of the elements of the following list Al, Ga, Sn, Zn, Sb, In, Cd, Ti, Zr, Ta, W and Mo, and in particular an oxide from one of these elements doped with at least one other of these elements, this oxide being optionally under stoichiometric oxygen . Doping refers here to the presence of at least one other metallic element in the layer, in an atomic proportion of metals (or oxygen element) ranging from 0.5 to 10%.

A mixed oxide is here an oxide of metal elements in which each metal element is present in an atomic proportion of metals (excluding oxygen element) of more than 10%.

In a particular variant embodiment, the antireflection layer closest to the metallic functional layer and the antireflection layer furthest from the metallic functional layer are based on the same oxide, in particular based on: zinc oxide ZnO,

- tin oxide SnO ₂ ,

TiO ₂ titanium oxide,

gallium oxide Ga ₂ O ₃ ,

of indium oxide In ₂ O ₃ , of silicon oxide SiO ₂ ,

- or based on a mixed oxide of indium and tin ITO,

a mixed oxide of gallium and zinc GZO,

a mixed oxide of indium and zinc IZO,

a mixed oxide of zinc and tin Zn ₂ SnO ₄ , a mixed oxide of indium, gallium and zinc IGZO, this oxide possibly being non-stoichiometric. The antireflection layer closest to the metallic functional layer, the one which is less resistive, preferably constitutes the first layer of the upper antireflection coating which is disposed above the metal functional layer, opposite the substrate. The antireflection layer furthest from the metallic functional layer, the one that is more resistive, is preferably the last layer of the upper antireflection coating which is disposed above the metal functional layer, opposite the substrate. This antireflection layer furthest from the metallic functional layer thus preferably constitutes the last layer of the electrode coating and is thus directly in contact with the photovoltaic material.

The interface between, on the one hand, the electrode coating according to the invention which incorporates, in particular in its optical definition, the last most resistive layer and, on the other hand, the photovoltaic material, in particular based on cadmium, is preferably as smooth as possible.

The antireflection layer furthest from the metal functional layer thus preferably has a surface roughness of between 5 and 250 Angstroms, in particular between 15 and 100 Angstroms, or between 10 and 50 Angstroms.

Recognizing that the absorption of conventional photovoltaic materials was different from one material to another, the inventors sought to define the essential optical characteristics necessary for the definition of a stack of thin layers of the type discussed above for form a solar cell front face electrode coating.

The antireflection coating disposed above the metallic functional layer opposite the substrate preferably has an optical thickness equal to about half the maximum wavelength λ _m absorption of the photovoltaic material. The antireflection coating disposed below the metal functional layer towards the substrate preferably has a thickness optical equal to about one eighth of the maximum wavelength λ _m absorption of the photovoltaic material.

In a preferred variant, the maximum wavelength λ _m of absorption of the photovoltaic material is, however, weighted by the solar spectrum. In this variant, the antireflection coating disposed above the metal functional layer opposite the substrate has an optical thickness equal to about half of the maximum wavelength λ _M of the product of the absorption spectrum of the material. photovoltaic by the solar spectrum. In this variant also, the antireflection coating disposed below the metal functional layer in the direction of the substrate has an optical thickness equal to about one-eighth of the maximum wavelength λ _M of the product of the spectrum of absorption of the photovoltaic material by the solar spectrum.

In a preferred version, the antireflection coating disposed above the metallic functional layer has an optical thickness of between 0.45 and 0.55 times the maximum absorption wavelength λ _m of the photovoltaic material, including these values. and preferably said antireflection coating disposed above the metallic functional layer has an optical thickness of between 0.45 and 0.55 times the maximum wavelength λ _M of the product of the spectrum of absorption of the photovoltaic material by the solar spectrum, including these values.

In a version which is also preferred, the antireflection coating disposed beneath the metal functional layer has an optical thickness of between 0.075 and 0.175 times the maximum wavelength λ _m of absorption of the photovoltaic material, including these values and preferably antireflection coating disposed below the metal functional layer has an optical thickness of between 0.075 and 0.175 times the maximum wavelength λ _M of the product of the the absorption of the photovoltaic material by the solar spectrum, including these values.

Thus, according to the invention, an optimum optical path is defined as a function of the maximum wavelength λ _m of absorption of the photovoltaic material or preferably as a function of the maximum wavelength λ _M of the product of the spectrum of the absorption of the photovoltaic material by the solar spectrum, in order to obtain the best efficiency of the photovoltaic cell.

The solar spectrum referred to herein is the AM 1.5 solar spectrum as defined by ASTM. By "coating" in the sense of the present invention, it should be understood that there may be several layers of different materials inside the coating.

By "anti-reflective layer" in the sense of the present invention, it should be understood that from the point of view of its nature, the material is "non-metallic", that is to say is not a metal. In the context of the invention, this term does not intend to introduce any limitation on the resistivity of the material, which may be that of a conductor (in general, p <10 ^"3 Ω.cm), of an insulator ( in general, p> 10 ⁹ Ω.cm) or a semiconductor (generally between these two previous values) In a completely surprising manner and independently of any other characteristic, the optical path of an electrode coating to layer of functional monolayer thin film which has an antireflection coating disposed above the functional metal layer having an optical thickness equal to about four times the optical thickness of the antireflection coating disposed below the metal functional layer, provides the improved efficiency of the solar cell, as well as its improved resistance to the stresses generated during the operation of the cell.

The purpose of the coatings which surround the metallic functional layer is to "antireflect" this functional metallic layer. That's why they are called "anti-reflective coatings". Indeed, if the functional layer alone allows to obtain the desired conductivity for the electrode coating, even at a low physical thickness (of the order of 10 nm), it will strongly oppose the passage of light and electromagnetic radiation. In the absence of such an anti-reflective system, the light transmission would then be much too weak and the light reflection much too strong (in the visible and the near infrared since it is a matter of making a photovoltaic cell).

The expression "optical path" here takes on a specific meaning and is used to denote the summary of the different optical thicknesses of the different antireflection coatings underlying and overlying the (or each) functional metallic layer of the interference filter thus produced. It is recalled that the optical thickness of a coating is equal to the product of the physical thickness of the layer by the index of the material of the layer when there is only one layer in the coating or of the sum of the products of the physical thickness of each layer by the index of the material of each layer when there are several layers.

The optical path according to the invention is, in absolute terms, a function of the physical thickness of the metallic functional layer, but in reality in the physical thickness range of the functional metal layer which makes it possible to obtain the desired conductance, it turns out that it does not vary so to speak. The solution according to the invention is thus suitable when the functional layer (s) is (or are) based on silver present (or have in total) a physical thickness of between 5 and 20 nm, in including these values.

The type of thin film stack according to the invention is known in the field of building or vehicle glazing to produce reinforced thermal insulation glazings of the "low-emissive" type and / or "solar control" type. . The inventors have thus discovered that certain stacks used for low-emissive glazings in particular were suitable for use in to produce electrode coatings for photovoltaic cells, and in particular the stacks known under the name of "hardenable" or "soaking" stacks, that is to say those used when it is desired to undergo a treatment of quenching the carrier substrate of the stack and in particular a quenching heat treatment.

The present invention thus also relates to the use of a stack of thin layers for architectural glazing and in particular a stack of this type according to the invention which is "hardenable" or "to be tempered", in particular a low stack emissifier according to the invention which is in particular a low-emissive "quenchable" stack or a low-emissive "quenching" stack, for producing a photovoltaic cell front-face substrate.

By stacking or "quenchable" substrate in the sense of the present invention, it should be understood that the optical properties and essential thermal properties (expressed by the square resistance which is directly related to the emissivity) are retained during the heat treatment.

Thus, it is possible on the same building facade for example to have in the vicinity of each other glazing incorporating tempered substrates and non-hardened substrates, all coated with the same stack, without it being possible to distinguish them. each other by a simple visual observation of the reflection color and / or light reflection / transmission.

For example, a stack or a substrate coated with a stack that has the following variations before / after thermal treatment will be considered as hardenable because these variations will not be perceptible to the eye:

- a light transmission variation (in the visible) ΔT _L low, less than 3% or even 2%; and or

- a change in light reflection (in the visible) ΔR _L low, less than 3% or even 2%; and or - a color variation (in the Lab system) ΔE = / ((ΔL *) ² + (Δa *) ² + (Δb *) ² ) weak, less than 3, or even 2.

By stacking or "quenching" substrate within the meaning of the present invention, it should be understood that the optical and thermal properties of the coated substrate are acceptable after heat treatment when they are not, or at least not all, before .

For example, a stack or a substrate coated with a stack that has after the heat treatment the following characteristics will be considered to be dipping in the context of the present invention, whereas before the heat treatment at least one of these characteristics does not occur. was not fulfilled:

- a light transmission (in the visible) T _L high of at least 65, even 70%, or even at least 75%; and / or a light absorption (in the visible range defined by 1 -T _L -R _L ) that is low, equal to or less than 10%, even equal to or less than 8%, or even equal to or less than 5%; and or

a resistance per square R at least as good as that of the conductive oxides conventionally used, and in particular equal to or less than 20 Ω /, or even equal to or less than 15 Ω /, or even equal to or less than 10 Ω /.

Thus, the electrode coating must be transparent. It must thus present, deposited on the substrate, in the wavelength range between 300 and 1200 nm, a minimum average light transmission of 65%, or even

75% and more preferably 85% or more, especially at least

90%.

If the face substrate has undergone a heat treatment, in particular quenching, after the deposition of the thin layers and before its integration into the photovoltaic cell, it is quite possible that before this heat treatment the substrate coated with the stack acting as a coating electrode is not very transparent. It may for example have, before this heat treatment a light transmission in the visible less than 65%, or even less than 50%.

The important thing is that the electrode coating is transparent before heat treatment and is such that it has after the heat treatment, in the wavelength range between 300 and 1200 nm, an average light transmission (in the visible) to a minimum from 65% to 75% and more preferably 85% or more, especially at least 90%.

Moreover, in the context of the invention, the stack does not have in absolute the best light transmission possible, but has the best possible light transmission in the context of the photovoltaic cell according to the invention.

The antireflection coating disposed beneath the metallic functional layer may also have a function of chemical barrier to diffusion, and in particular to the diffusion of sodium from the substrate, thus protecting the electrode coating, and more particularly the functional metallic layer, in particular during a possible heat treatment, especially quenching. In another particular variant, the substrate comprises, under the electrode coating, a base antireflection layer having a low refractive index close to that of the substrate, said base antireflection layer preferably being based on silicon oxide or based on aluminum oxide, or a mixture of both. In addition, this dielectric layer may constitute a chemical barrier layer to the diffusion, and particularly to the diffusion of sodium from the substrate, thus protecting the electrode coating, and more particularly the functional metal layer, especially during a possible heat treatment, especially quenching or for the implementation of the photovoltaic material. In the context of the invention, a dielectric layer is a layer which does not participate in the displacement of electric charge (electric current) or whose effect of participation in the displacement of electric charge can be considered as zero compared to that of the others. electrode coating layers.

Furthermore, this basic antireflection layer preferably has a physical thickness of between 10 and 300 nm or between 35 and 200 nm and more preferably between 50 and 120 nm.

This functional metal layer may be based on silver, copper or gold, and may optionally be doped with at least one other of these elements.

The term "based on" in a usual manner means a layer containing predominantly the material, that is to say containing at least 50% of this material in molar mass; the term "based on" thus covers doping.

The metal functional layer is preferably deposited in a crystallized form on a thin dielectric layer which is also preferably crystallized (then called "wetting layer" as promoting the proper crystalline orientation of the metal layer deposited thereon). The stack of thin layers producing the electrode coating is preferably a functional monolayer coating, that is to say a single functional layer; however, it can be multi-layer functional and in particular two-layer functional.

The functional layer is thus preferably deposited over one or even directly onto an oxide-based wetting layer, in particular based on zinc oxide, optionally doped, optionally with aluminum.

The physical (or actual) thickness of the wetting layer is preferably between 2 and 30 nm and more preferably between 3 and 20 nm. This wetting layer is dielectric and is a material which preferably has a resistivity p (defined by the product of the resistance per square of the layer by its thickness) such that 0.5 Ω.cm <p <200 Ω. cm or such that 50 Ω.cm <p <200 Ω.cm. The stack is generally obtained by a succession of deposits made by a technique using the vacuum such as sputtering possibly assisted by magnetic field. There may also be one or two very thin coating (s) called "blocking coating", which are not part of the antireflection coatings, arranged directly under, on or on each side of each functional metal layer, in particular a silver-based coating, the coating underlying the functional layer, in the direction of the substrate, as a bonding, nucleation and / or protection coating during the eventual heat treatment thereafter at the deposition, and the coating overlying the functional layer as a protective or "sacrificial" coating to prevent alteration of the functional metal layer by etching and / or oxygen migration of a layer which the overcomes especially during the possible heat treatment, or even by oxygen migration if the layer above it is deposited by cathodic sputtering in the presence of oxygen.

Within the meaning of the present invention when it is specified that a layer or coating deposit (comprising one or more layers) is carried out directly under or directly on another deposit, it is that there can be no interposition of 'no layer between these two deposits. At least one blocking coating is preferably based on Ni or Ti or is based on a Ni-based alloy, in particular is based on a NiCr alloy.

The coating below the metal functional layer in the direction of the substrate preferably comprises a layer based on mixed oxide, in particular based on mixed oxide of zinc and tin or mixed tin oxide and aluminum oxide. Indium (ITO). Furthermore, the coating below the metal functional layer in the direction of the substrate and / or the coating above the metallic functional layer may have a layer with a high refractive index, in particular equal to or greater than 2, such as for example a layer based on silicon nitride, optionally doped, for example aluminum or zirconium.

Moreover, the coating below the metallic functional layer in the direction of the substrate and / or the coating above the metallic functional layer may have a layer with a very high refractive index, in particular equal to or greater than 2, 35, such as a titanium oxide layer.

The substrate may comprise a coating based on photovoltaic material, especially based on cadmium, above the electrode coating opposite the front face substrate. A preferred structure of the front face substrate according to the invention is thus of the type: substrate / (optional antireflection base layer) / electrode coating / photovoltaic material, or else of the type: substrate / (optional antireflection layer) / electrode coating / photovoltaic material / electrode coating. In a particular variant, the electrode coating consists of a stack for architectural glazing, in particular a stack for "hardenable" or "soaking" architectural glazing, and in particular a low-emissive stack, in particular a low-emissive stack. "Quenching" or "soaking", this stack of thin layers having the characteristics of the invention.

The present invention also relates to a substrate for a photovoltaic cell according to the invention, in particular a substrate for architectural glazing coated with a stack of thin layers having the characteristics of the invention, in particular a substrate for architectural glazing "hardenable" or "soaking" with the characteristics of the invention, and in particular a low-emissive substrate, in particular a low-emissive "quenchable" or "quenching" substrate having the characteristics of the invention.

All the layers of the electrode coating are preferably deposited by a vacuum deposition technique, but it is not excluded, however, that the first or the first layers of the stack may be deposited by a another technique, for example by a pyrolytic or CVD type thermal decomposition technique, optionally under vacuum, possibly assisted by plasma.

Advantageously, the electrode coating according to the invention with a thin film stack is moreover much more resistant than a TCO electrode coating. Thus, the lifespan of the photovoltaic cell can be increased. Advantageously also, because of its low physical thickness, compared to that of an electrode made of a TCO-based material, an electrode with functional layer (s) (s) metal (s) according to the invention is much easier to engrave, in particular by laser: less energy and less time not necessary to achieve the longitudinal separations generally operated over the entire thickness of the electrode (so-called "modularization" stage); moreover, this etching step causes less material removal, with identical etching width, than for an electrode made of a TCO-based material and thus reduces the risk of pollution of the cell by the material removed. Advantageously, furthermore, the electrode coating according to the invention can quite well be used as a backside electrode coating, in particular when it is desired that at least a small part of the incident radiation passes completely through the photovoltaic cell.

The details and advantageous characteristics of the invention emerge from the following nonlimiting examples, illustrated with the aid of the attached figures: FIG. 1 illustrates a photovoltaic cell front-face substrate of the prior art coated with a conductive transparent oxide electrode coating and with a base antireflection layer;

FIG. 2 illustrates a photovoltaic cell front face substrate according to the invention coated with an electrode coating consisting of a functional monolayer thin layer stack and a base antireflection layer;

FIG. 3 illustrates the quantum efficiency curve of three photovoltaic materials; FIG. 4 illustrates the real efficiency curve corresponding to the product of the spectrum of the absorption of these three photovoltaic materials by the solar spectrum;

FIG. 5 illustrates the principle of the durability test of photovoltaic cells; and - Figure 6 illustrates a sectional diagram of a photovoltaic cell.

In Figures 1, 2 and 5, 6, the proportions between the thicknesses of the different coatings, layers, materials are not rigorously respected to facilitate their reading.

FIG. 1 illustrates a photovoltaic cell front-facing substrate 10 'of the prior art with absorbent photovoltaic material 200, said substrate 10' comprising on a main surface a transparent electrode coating 100 'consisting of a layer which conducts the current 66 in TCO.

The front-face substrate 10 'is disposed in the photovoltaic cell such that the front-face substrate 10' is the first substrate traversed by the incident radiation R, before reaching the photovoltaic material 200. The substrate 10 'furthermore comprises, under the electrode coating 100', that is to say directly on the substrate 10 ', a base antireflection layer 22 having a low refractive index n ₂₂ close to that of the substrate.

The substrate 10 'may further comprise on the electrode coating 100' and under the photovoltaic material 200 a buffer layer, not shown.

FIG. 2 illustrates a photovoltaic cell front-face substrate 10 according to the invention. The front-face substrate 10 also has a transparent electrode coating 100 on a main surface, but here this electrode coating 100 consists of a stack of thin layers comprising at least one metallic functional layer 40, based on silver, and at least two antireflection coatings 20, 60, said coatings each comprising at least one fine antireflection layer 24, 26; 66, 68, said functional layer 40 being disposed between the two antireflection coatings, one underlying anti-reflective coating 20 located beneath the functional layer, toward the substrate, and the other called antireflection coating 60 located at the above the functional layer, in the opposite direction to the substrate.

The stack of thin layers constituting the transparent electrode coating 100 of FIG. 2 is a structure of a stack of the type of that of a low-emissive, possibly quenchable or quenched, functional monolayer substrate, such as can be commercially available, for applications in the field of architectural glazing for buildings.

Two examples, numbered 1 and 2, were made on the basis of the functional monolayer stacking structure illustrated: for example 1 in FIG. 1, and - for example 2 in Figure 2, except that the stack did not include over-blocking coating.

Moreover, in all the examples below, the layers are deposited on a substrate 10 ', 10 made of clear soda-lime glass with a thickness of 4 mm. The indices given below were measured at the usual wavelength of 550 nm.

The electrode coating 100 'of Example 1 is based on conductive aluminum-doped zinc oxide.

The stack constituting an electrode coating 100 of Example 2 consists of a stack of thin layers comprising in order:

an antireflection layer 24 which is a layer based on dielectric titanium oxide, of index n = 2.4;

an antireflection layer 26 which is an oxide-based wetting layer, in particular based on zinc oxide, optionally doped, dielectric, of index n = 2;

- optionally an underlying blocking coating (not shown), for example based on Ti or based on a NiCr alloy could be disposed directly under the functional layer 40, but is not provided here; this coating is generally necessary if there is no wetting layer 26, but is not necessarily essential;

- The single functional layer 40, silver, is here arranged directly on the wetting coating 26;

an overlying blocking coating 50 based on Ti or based on a NiCr alloy could be placed directly on the functional layer 40 but is not provided in the examples made;

an antireflection layer 66, conductive, based on zinc oxide doped with aluminum, of index n = 2; its resistivity being between 0.35 10 ^-3 Ω.cm and 2.5 10 ^-3 Ω.cm, in particular substantially close to 10 ^-3 Ω.cm, this layer being deposited here from a ceramic target ( composed of about 2% aluminum, 49% zinc and 49% oxygen) under an argon atmosphere directly on the functional layer 40; a dielectric 68, which is anti-reflective and which is based on zinc oxide and tin oxide, Sn _x Zn _y 0 _z , of index n = 2.1, having a resistivity of the order of 1 Ω.cm, this layer being deposited here from a metal target (composed of about 50% tin, 50% zinc) under a compound containing 25% oxygen (O ₂ ) and 75% argon.

It should be noted that the layers based on mixed oxide of zinc and tin throughout their thickness may have on their thickness

Sn: Zn vary or percentages of dopant that vary, depending on the targets used to deposit these layers and in particular when several targets of different compositions are used to deposit a layer.

In the examples, the photovoltaic material 200 is based on cadmium telluride.

The QE quantum efficiency of this material is illustrated in FIG. 3, with that of the microcrystallized silicon (whose crystallite size is of the order of 100 nm), and of the amorphous (that is to say non-crystallized) silicon. , other photovoltaic materials which are also suitable in the context of the invention.

It is recalled here that the quantum efficiency QE is in a known manner the expression of the probability (between 0 and 1) that an incident photon with a wavelength according to the abscissa is transformed into an electron-hole pair . As can be seen in FIG. 3, the maximum absorption wavelength λ _m , that is to say the wavelength at which the quantum efficiency is maximum (that is to say the higher) :

amorphous silicon a-Si, λ _m a-Si, is 520 nm,

microcrystallized silicon μc-Si, λ _m μc-Si, is 720 nm, and cadmium telluride CdTe, λ _m CdTe, is 600 nm.

In a first approach of the optical path of the stack, this maximum absorption wavelength λ _m is sufficient.

The antireflection coating 20 disposed below the metal functional layer 40 in the direction of the substrate then has an optical thickness equal to about one-eighth of the maximum absorption wavelength λ _m of the photovoltaic material and the antireflection coating 60 disposed above the metal functional layer 40 opposite the substrate then has an optical thickness equal to about half of the maximum absorption wavelength λ _m of the photovoltaic material.

Table 1 below summarizes the preferred ranges of the optical thicknesses in nm, for each coating 20, 60, as a function of these three materials.

Table 1

However, it turns out that the optical definition of the stack can be improved by considering the quantum efficiency to obtain an improved real efficiency by convolving this probability by the wavelength distribution of the sunlight at the surface of the earth. . Here we use the standardized solar spectrum AM 1.5.

In this case, the antireflection coating 20 disposed below the metal functional layer 40 in the direction of the substrate has an optical thickness equal to about one-eighth of the maximum wavelength λ _M of the product of the absorption spectrum of the photovoltaic material by the solar spectrum and the antireflection coating 60 disposed above the metal functional layer 40 opposite the substrate has an optical thickness equal to about half the maximum wavelength λ _M of the product of the spectrum of the absorption of photovoltaic material by the solar spectrum.

As can be seen in FIG. 4, the maximum wavelength λ _M of the product of the spectrum of the absorption of the photovoltaic material by the spectrum solar, that is the wavelength at which the efficiency is maximum (that is, the highest):

- amorphous silicon a-Si, a-Si _M λ, is 530 nm,

- microcrystalline silicon μc-Si, μc-Si λ _M, is 670 nm, and - the cadmium telluride CdTe, CdTe _M λ, is 610 nm.

Table 2 below summarizes the preferred ranges of the optical thicknesses in nm, for each coating 20, 60, as a function of these three materials.

Table 2

In all the examples, a base anti-reflective layer 22 based on silicon oxide was deposited directly on the substrate. Its refractive index n ₁₅ is low and close to that of the substrate, its optical thickness is not taken into account in defining the optical path of the stack according to the invention.

The deposition conditions of these layers are known to those skilled in the art since it involves making stacks similar to those used for low-emission or solar control applications. As such, the person skilled in the art can refer to patent applications

EP 718 250, EP 847 965, EP 1 366 001, EP 1 412 300, or EP 722 913.

Table 3 below summarizes the materials and physical thicknesses measured in nanometers of each of the layers of each of Examples 1 and 2 and Table 4 sets out the main features of these examples. The performance characteristic P is calculated by the so-called "TSQE" method where the product of the integration of the spectrum is operated over the entire radiation domain under consideration with the quantum efficiency QE of the cell.

The light reflection characteristic R _L is measured according to the illuminant D65.

All examples, 1 and 2, have been subjected to a resistance test of the electrode coatings to the stresses generated during the operation of the cell (in particular the presence of an electrostatic field), carried out as illustrated in FIG. 5.

For this test, a piece of substrate 10, 10 'for example 5cmx5cm and coated respectively electrode coating 100, 100', but without photovoltaic material 200 is deposited on a metal plate 5 disposed on heat source 6 at about 200 ⁰ C .

It involves applying for 20 minutes an electric field to the substrate

10, 10 'coated electrode coating 100, 100' by making an electrical contact 102 on the surface thereof and connecting this contact 102 and the metal plate 5 to the terminals of a power supply 7 delivering DC current of about 200 V.

At the end of the test, once the sample has cooled, the proportion of remaining coating is measured over the entire surface of the sample.

This proportion of coating remaining post resistance test is noted PRT.

In addition, independently of the preceding test, Example 2 was subjected to a heat treatment (TT) consisting of annealing at a temperature of about 620 ^° C. for 6 minutes, followed by abrupt cooling with ambient air (20 ^° C.). C), simulating a quenching operation. The data measured after this heat treatment are in the last column of Table 4. The heat treatment applied is thus more demanding than the usual heat treatment undergone by the electrode coating as part of the process of deposition of the photovoltaic coating based on cadmium.

Table 3

Table 4

In Example 2, the optical thickness of the coating 60 over the functional metal layer is 291 nm (= 135 × 2 + 10 × 2.1), and the optical thickness of the coating 20 below the functional metal layer is 78.8 nm (= 27 x 2.4 + 7 x 2).

This example shows that it is possible to obtain an electrode coating consisting of a thin layer stack and coated with Cadmium telluride which has a resistance R (-2.6 ohms /) and better performance P ( + 0.2%) a TCO electrode coating coated with the same material (Example 1). The optical thicknesses of coatings 20 and 60 of Example 2 fall within the recommended ranges for a photovoltaic material 200 in CdTe according to Table 1 and Table 2.

The use of a photovoltaic material based on cadmium, and especially combining CdTe and CdS, requires the electrode coating to withstand a heat treatment because the implementation of this photovoltaic material requires a step at a temperature of between 300 ^° C. C and 700 ⁰ C, generally conducted in a controlled atmosphere, non-oxidizing.

It turns out that this step is, in a surprising way, quite similar to a quenching step as known to those skilled in the art of glass substrates for vehicles or buildings, even if generally the quenching atmosphere does not occur. is not controlled.

It is thus particularly interesting, when the photovoltaic material is based on cadmium, to choose a thin film stack known for vehicle or building applications and resistant to heat treatment quenching, called "quenching" or "soaking".

It can thus be seen that for example 2, the variations of the data during the heat treatment applied are small. The chosen stack can therefore be considered as "heatable".

In addition, it is interesting to note that the stacks of thin layers forming electrode coating in the context of the invention exhibit a light reflection without the lower photovoltaic material both before and after heat treatment, than that of the electrode TCO coating without the photovoltaic material.

FIG. 6 illustrates a photovoltaic cell 1 in section provided with a front-face substrate 10 according to the invention, through which incident radiation R and a rear-face substrate 20 penetrate. Photovoltaic material 200, for example made of silicon amorphous or in crystalline or microcrystalline silicon or in Cadmium tellurium or in Copper Diselenide lndium (CuInSe ₂ - CIS) or Copper-Indium-Gallium-Selenium, is located between these two substrates. It consists of a layer of n-doped semiconductor material 220 and a p-doped semiconductor material layer 240, which will produce the electric current. The electrode coatings 100, 300 interposed respectively between firstly the front-face substrate 10 and the layer of n-doped semiconductor material 220 and secondly between the p-doped semiconductor material layer 240 and the substrate of FIG. rear face 20 complete the electrical structure.

The electrode coating 300 may be based on silver or aluminum, or may also consist of a thin film stack comprising at least one metallic functional layer and according to the present invention.

The present invention is described in the foregoing by way of example. It is understood that the skilled person is able to realize different variants of the invention without departing from the scope of the patent as defined by the claims.

Claims

Photovoltaic cell (1) with absorbent photovoltaic material, in particular based on cadmium, said cell comprising a substrate (10) having a front face, in particular a transparent glass substrate, comprising on a main surface a transparent electrode coating (100) consisting of a stack of thin layers comprising at least one metallic functional layer (40), in particular based on silver, and at least two antireflection coatings (20, 60), said antireflection coatings each comprising at least one antireflection layer (24, 26); 66, 68), said functional layer (40) being disposed between the two antireflection coatings (20, 60), characterized in that the antireflection coating (60) disposed above the metallic functional layer (40) at the opposite of the substrate has at least two antireflection layers (66, 68), the antireflection layer (68) furthest from the metallic functional layer (40) being more resistive than the anti-reflective layer (66); reflection (66) closest to the metallic functional layer (40).

2. Photovoltaic cell (1) according to claim 1, characterized in that the antireflection layer (68) farthest from the metal functional layer (40) has a resistivity equal to at least 5 times, or at least 10 times, or at least 100 times, the resistivity of the antireflection layer (66) closest to the metal functional layer (40).

Photovoltaic cell (1) according to claim 1 or claim 2, characterized in that the antireflection layer (68) farthest from the metal functional layer (40) has a resistivity p between 5.10 ^"3 Ω.cm and 10 Ω.cm.

4. Photovoltaic cell (1) according to any one of claims 1 to 3, characterized in that the antireflection layer (66) closest to the metal functional layer (40) has a resistivity p between 10 ^"5 Ω. cm and 5.10 ³ Ω.cm excluding this last value.

Photovoltaic cell (1) according to any one of claims 1 to 4, characterized in that the antireflection layer (66) closest to the metallic functional layer (40) and the antireflection layer (68) furthest from the functional metal layer (40) are based on the same oxide, in particular based on zinc oxide ZnO, tin oxide SnO ₂ , titanium oxide TiO ₂ , gallium oxide Ga ₂ O ₃ , of indium oxide In ₂ O ₃ , of silicon oxide SiO ₂ , or based on a mixed oxide of indium and tin ITO, a mixed oxide of gallium and zinc GZO, d a mixed oxide of indium and zinc IZO, a mixed oxide of zinc and tin Zn ₂ SnO ₄ , a mixed oxide of indium, gallium and zinc IGZO, this oxide being optionally non-stoichiometric .

6. photovoltaic cell (1) according to any one of claims 1 to 5, characterized in that the antireflection layer (68) farthest from the metal functional layer (40) has an optical thickness of between 2 and 50% of the optical thickness of the antireflection coating (60) furthest from the substrate and in particular an optical thickness representing between 2 and 25% of the optical thickness of the antireflection coating (60) farthest from the substrate.

Photovoltaic cell (1) according to any one of claims 1 to 6, characterized in that the antireflection layer (68) furthest from the metal functional layer (40) has a real thickness of between 2 and 100 nm, and preferably between 5 and 50 nm.

Photovoltaic cell (1) according to one of claims 1 to 7, characterized in that the antireflection coating (60) disposed above the metal functional layer (40) opposite the substrate has an optical thickness. equal to about half the maximum absorption wavelength λ _m of the photovoltaic material, and preferably the antireflection coating (60) disposed above the metallic functional layer (40) opposite the present substrate thickness optical equal to about half of the maximum wavelength λ _M of the product of the spectrum of the absorption of the photovoltaic material by the solar spectrum.

9. Photovoltaic cell (1) according to any one of claims 1 to 8, characterized in that said antireflection coating (60) disposed above the metal functional layer (40) has an optical thickness of between 0.45 and 0.55 times the maximum absorption wavelength λ _m of the photovoltaic material, including these values and preferably said antireflection coating (60) disposed above the metallic functional layer (40) has an optical thickness of between 0.45 and 0.55 times the maximum wavelength λ _M of the product of the spectrum of the absorption of the photovoltaic material by the solar spectrum, including these values.

10. Photovoltaic cell (1) according to any one of claims 1 to 9, characterized in that the antireflection coating (20) disposed below the metal functional layer (40) towards the substrate has an optical thickness of about one eighth of the maximum absorption wavelength λ _m of the photovoltaic material, and preferably the antireflection coating (20) disposed below the metal functional layer (40) towards the substrate has an optical thickness of about one eighth of the maximum wavelength λ _M of the product of the spectrum of the absorption of the photovoltaic material by the solar spectrum.

Photovoltaic cell (1) according to any one of claims 1 to 10, characterized in that said antireflection coating (20) disposed below the metal functional layer (40) has an optical thickness of between 0.075 and 0.175 times the maximum wavelength λ _m absorption of the photovoltaic material, including these values and preferably said antireflection coating (20) disposed below the metal functional layer (40) has an optical thickness included between 0.075 and 0.175 times the maximum wavelength λ _M of the product of the spectrum of absorption of the photovoltaic material by the solar spectrum, including these values.

Photovoltaic cell (1) according to any one of claims 1 to 11, characterized in that the electrode coating (100) comprises directly on said substrate (10) a base antireflection layer (22) having a refractive index n _The base anti-reflective layer (22) preferably is based on silicon oxide or aluminum oxide base or is based on a mixture of the two.

13. Photovoltaic cell (1) according to claim 12, characterized in that said base antireflection layer (22) has a physical thickness of between 50 and 300 nm.

Photovoltaic cell (1) according to any one of claims 1 to 13, characterized in that the functional layer (40) is deposited on top of an oxide-based wetting layer (26), in particular zinc oxide base, optionally doped.

Photovoltaic cell (1) according to any one of claims 1 to 14, characterized in that the functional layer (40) is arranged directly on at least one underlying blocking coating (30) and / or directly under the at least one overlying blocking coating (50).

Photovoltaic cell (1) according to claim 1 to 15, characterized in that at least one blocking coating (30, 50) is based on Ni or Ti or is based on a Ni-based alloy. , in particular is based on a NiCr alloy.

Photovoltaic cell (1) according to any one of claims 1 to 16, characterized in that the coating (20) under the metal functional layer in the direction of the substrate comprises a layer based on mixed oxide, in particular based on mixed oxide of zinc and tin (ZTO) or mixed oxide of indium and tin (ITO).

Photovoltaic cell (1) according to any one of claims 1 to 17, characterized in that the coating (20) under the metallic functional layer towards the substrate and / or the coating (60) above the layer metallic functional comprises (s) a very high refractive index layer, especially greater than or equal to 2.35, such as a titanium oxide-based layer.

19. Photovoltaic cell (1) according to any one of claims 1 to 18, characterized in that it comprises a coating based on photovoltaic material (200), in particular based on cadmium, above the electrode coating (100). ), opposite the substrate (10).

Photovoltaic cell (1) according to any one of claims 1 to 21, characterized in that said electrode coating (100) consists of a stack for architectural glazing, in particular a stack for architectural glazing "hardenable" or " to soak >>, and in particular a low-emissive stack, including a low-emissive stack "hardenable" or "to soak".

21. Substrate (10) coated with a stack of thin layers for a photovoltaic cell (1) according to any one of claims 1 to 22, in particular architectural glazing substrate, in particular architectural glazing substrate "hardenable" or "to soak >>, and in particular a low-emissive substrate, in particular a low-emissive "hardenable" or "quenching" substrate.

22. Use of a substrate coated with a stack of thin layers for producing a substrate (10) of front face of photovoltaic cell (1), in particular a photovoltaic cell (1) according to any one of claims 1 to 20 , said substrate comprising a transparent electrode coating (100) consisting of a stack of thin layers comprising at least one metallic functional layer (40), in particular silver-based, and at least two antireflection coatings (20, 60), said antireflection coatings each having at least one antireflection layer (24, 26; 66, 68), said functional layer (40) being disposed between the two antireflection coatings (20, 60), the antireflection coating (60) disposed above the metallic functional layer (40) opposite the substrate having at least two antireflection layers ( 66, 68), the antireflection layer (68) furthest from the metallic functional layer (40) being more resistive than the antireflection layer (66) closest to the metal functional layer (40).

23. Use according to claim 22 wherein the substrate (10) comprising the electrode coating (100) is a substrate for architectural glazing, including a substrate for architectural glazing "hardenable" or "to soak", and in particular a substrate low-emissive including "soaking" or "soaking".

The use of claim 22 or 23 wherein the antireflection coating (60) disposed above the metallic functional layer (40) opposite the substrate having an optical thickness of about half the wavelength. maximum absorption of the photovoltaic material.