EP1913642A1

EP1913642A1 - Antireflection coating, particularly for solar cells, and method for producing this coating

Info

Publication number: EP1913642A1
Application number: EP06792732A
Authority: EP
Inventors: Vladimir Aroutiounian; Khachatur Martirosyan; Patrick Soukiassian
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2005-08-10
Filing date: 2006-08-08
Publication date: 2008-04-23
Also published as: FR2889745B1; CN101238586A; FR2889745A1; WO2007017504A1; CN101238586B

Abstract

The invention relates to an antireflection coating, particularly for solar cells, and to a method for producing this coating. An inventive antireflection coating (4) comprises at least one antireflection inner layer (6) made of porous silicon and comprises an outer layer (8) made of silicon oxynitride, which is essentially non-porous, essentially free from foreign species and which is formed on the inner layer.

Description

ANTI-REFLECTION COATING, ESPECIALLY FOR SOLAR CELLS, AND METHOD OF MAKING THE COATING

DESCRIPTION

TECHNICAL AREA

The present invention relates to an anti-reflective coating (in English, anti-reflective coating) and a method of manufacturing this coating.

It applies in particular to coatings for solar cells.

STATE OF THE PRIOR ART

The reduction of the reflection coefficient

Today, surface reflectance is one of the important ways to improve the performance of solar cells and, to this end, anti-reflective coatings have already been developed.

Since various encapsulants have a refractive index around 1.4-1.5 while the corresponding refractive index of silicon is 3.87, reflection losses can be very easily reduced by an anti-reflective coating. whose refractive index is between 1.5 and 3.87.

A monolayer anti-reflective coating is usually made of titanium dioxide, silicon dioxide or silicon nitride, but other materials are also used in solar cells. Such a coating makes it possible to significantly reduce reflection losses around the specific wavelength for which the coating has been designed.

Double layer anti-reflective coatings, using a combination of the above-mentioned materials or MgF ₂ , ZnS and some other materials, have been studied in the past. In particular, one or two porous silicon layers have been used as a single or double antireflection coating on silicon, in order to improve the solar cell conversion factor by decreasing the amount of solar radiation reflected by the surface of the solar cell. input of such a cell.

The possibility of using different values of the refractive index of porous silicon layers, made at different rates of their manufacture, is very attractive. In this regard, reference is made to documents [1] to [5] which are mentioned at the end of this description, as are the other documents cited later. Such anti-reflective layers of porous silicon, however, have disadvantages. In fact, porous silicon reacts slowly with ambient air and, consequently, its chemical composition and its properties vary continuously over time.

In addition, the oxidation level depends not only on the time spent but also on the ambient conditions. Therefore, the anti-reflective properties of porous silicon are subject to degradation over time. SUMMARY OF THE INVENTION

The present invention aims to overcome these drawbacks and to provide an anti-reflective coating, particularly for solar cells, which is less likely to degrade over time, while not detrimentally affecting cell travel. solar.

Another object of the present invention is to make it possible to adjust and optimize the spectral range in which efficient conversion of light into electrical energy can occur in a solar cell. More particularly, an object of the invention is to reduce the value of the reflection coefficient and to extend the spectral range in the ultraviolet range.

Anti-reflective coatings are also known from documents [9] and [10].

Specifically, the subject of the present invention is an anti-reflection coating, in particular for solar cells, this coating being characterized in that it comprises at least one porous silicon anti-reflection inner layer and an outer layer of silicon oxynitride which is substantially non-porous and substantially free of foreign species and is formed on the inner layer.

In the document [5], it is proposed to use a diamond-like carbon outer layer (DLC), which is substantially non-porous and substantially free of foreign species. The advantages of such a porous silicon / diamond-like double-layered antireflection coating are discussed and lead to requirements on the optical length (refractive index and thickness) of each layer in the main part of the solar spectrum.

In the present invention, it is proposed to use a layer of silicon oxynitride (Si _x O _y N ₂ ) instead of a diamond-like carbon layer (DLC).

In general, silicon oxynitrides can be considered as nitrogen doped silicon dioxide and are promising candidates for replacing pure silicon dioxide in the microelectronic industry, particularly for thin film grids. in the MOS (Metal Oxide Semiconductor) technology.

The oxynitride films are generally prepared by direct oxynitriding of a silicon surface or by nitriding a silicon dioxide layer. This leads to a remarkable decrease in the concentration of surface states as well as an extremely low surface recombination rate (surface passivation) and also leakage currents.

Si _x OyN ₂ silicon oxynitrides are used in solar cells and integral optics. They are used in particular for the manufacture of silicon solar cells with buried contacts.

It is well known that the four solid phases of silicon (Si, SiO ₂ , Si ₂ N ₂ O and Si ₃ N ₄ ) in the Si-NO system are stable. In turn, silicon oxynitride Si _x OyN ₂ is stable to various chemical influences and is more resistant to hydrofluoric acid and the diffusion of various ions and impurities.

Indeed, it has been found that silicon oxynitride retains its dielectric properties at thicknesses less than 10 nm, and the production of thin gate dielectric and masking layers for metal-insulator-

Semiconductor in Very Large Scale Integration Technology (VLSI). Silicon oxynitride manufacturing technologies are well known, making it possible to control the optical properties of the latter.

The silicon oxynitrides Si _x O _y N _z may have forbidden bandwidth and refractive index values which are intermediate between those of silicon dioxide and silicon nitride; these values depend on the x, y and z parameters.

The refractive index of silicon oxynitrides varies from 1.45 to 2, depending, for example, on the flow rates of N ₂ O and NH ₃ in the manufacturing process of these compounds.

Taking into account that it is also possible to modify the refractive index of porous silicon (from 1.25 to 3) in a wide range of porosity, various combinations of anti-reflective layers, made of both materials different mentioned above (silicon oxynitride and porous silicon), are possible. The number of centers P _b and the density of the interface traps decreases by at least two orders of magnitude. And when the surface of the solar cells is passivated, there is an increase in the diffusion length of the charge carriers, which is very important for thin-film solar cells.

The combination of porous silicon and silicon oxynitride layers for applications to solar cells or other domains had never been disclosed. Surprisingly, the authors of the present invention have discovered that replacing a diamond-like carbon layer on a porous silicon film with a silicon oxynitride layer results in better reflection coefficient values and at a lower cost of the coating.

According to a particular embodiment of the coating object of the invention, this coating comprises a plurality of inner anti-reflective layers which are made of porous silicon and whose refractive indices are different from each other.

Preferably, in a coating according to the present invention, each porous silicon layer has a thickness of at least 42 nm. According to a preferred embodiment, each porous silicon layer has a thickness of between 42 nm and 53 nm.

Preferably, each porous silicon layer has a refractive index of between 2.6 and 2.9. The silicon oxynitride layer preferably has a thickness of between 76 nm and 112 nm.

In addition, it is preferable that the silicon nitride layer has a refractive index of between 1.5 and 1.7.

According to a preferred embodiment of the present invention, the porous silicon layer has a thickness of 52 nm and a refractive index of 2.9, and the silicon oxynitride layer has a thickness of 94 nm and a refractive index of 1.5.

The present invention also relates to a solar cell coating, which comprises the anti-reflection coating object of the invention (but which may also include other elements, including one or more photo-active parts).

The present invention further relates to a solar cell comprising the anti-reflection coating object of the invention.

The present invention also relates to a method of forming an anti-reflective coating on an exposed surface of solid silicon, said method being characterized in that it comprises the following steps:

applying a porosification treatment to the exposed solid silicon surface, over a predetermined thickness, so as to form a porous silicon layer or a plurality of porous silicon layers having different refractive indices, and - depositing, on the free surface of the layer or layers of porous silicon thus obtained, of a solid layer of silicon oxynitride, substantially nonporous and substantially free from foreign species.

The silicon oxynitride layer can be obtained by plasma enhanced chemical vapor deposition, laser ablation or nitrogen ion implantation.

The solid silicon surface may be the surface of a solar cell panel. Each solid layer of porous silicon can be grown on a silicon wafer at different rates of anodization of the silicon wafer.

The invention also relates to the use of an antireflection coating obtained by the method which is the subject of the invention, for a solid silicon surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments, given below purely by way of indication and in no way limiting, with reference to the appended drawings in which:

FIG. 1 schematically illustrates a solar cell panel provided with an anti-reflection coating according to the present invention;

FIG. 2 is a reflection coefficient diagram R (in%) / wavelength λ (in nm) for an example of a coating according to the present invention; (solid lines), compared to a single layer anti-reflective coating of silicon oxynitride (dotted),

FIG. 3 is a reflection coefficient diagram R (in%) / wavelength λ (in nm) for an example of a coating according to the present invention (solid lines), compared with a double-layer anti-reflection coating; SiO ₂ / TiO ₂ (dotted) and

FIG. 4 is a reflection coefficient diagram R (in%) / wavelength λ (in nm) for an example of a coating according to the present invention (solid lines), compared with a double-layer anti-reflection coating; diamond-like carbon / porous silicon (dotted).

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

FIG. 1 schematically shows a solar cell flat panel 2. It is provided with an anti-reflection coating 4 according to the present invention. The coating 4 comprises an inner layer

6 which is formed on the solar cell flat panel 2 and an outer layer 8 which is formed on the inner layer 6.

The inner layer 6 is a porous silicon anti-reflection layer and the outer layer 8 is a silicon oxynitride layer which is substantially non-porous and substantially free of foreign species. The porous silicon layer 6 can be obtained by various methods known in the state of the art.

The porous silicon is preferably formed on the solar cell panel by an electrochemical anodizing process. In the present example, electrochemical etching, or anodizing method, is performed on the panel surface, usually a silicon wafer, as described below, after defatting and cleaning this wafer with pure water.

An electrolyte composed of 4M-dimethylformamide in hydrofluoric acid (HF) in a molar ratio of 1: 1 with water can be used to obtain macro-porous silicon (pore size between 200 nm and 2 μm).

Alternatively, an electrolyte composed of an equal volume ratio of HF, at a concentration of 48%, and ethanol (C ₂ H ₅ OH), at a concentration of 96%, can be used to obtain microporous silicon ( pore size between 10 nm and 100 nm).

Several samples were prepared with different etching times and different current densities. In particular, it is possible to use current densities of between lmA / cm ² and 15mA / cm ² for durations of between 5 seconds and 10 minutes, and it is possible to implement an anodizing process with constant illumination by a halogen lamp having a power of IkW, placed at a distance of 20cm from the surface which is anodized. The porous silicon layer 6 may have a thickness (denoted d _PS ) of several tens of nanometers, preferably between approximately 42 nm and 53 nm. The anodizing conditions are further chosen so that the refractive index n _PS of this layer 6 is between approximately 2.6 and 2.9.

The preparation of the silicon oxynitride layer 8 is explained below. This silicon oxynitride layer 8 is directly formed on the layer 6. It can be obtained in various ways. Non-exhaustively, we can use:

a plasma-assisted chemical vapor deposition using a mixture of silane SiH ₄ gas and N ₂ O gas at a remarkably low temperature,

implantation of nitrogen ions, using doses and corresponding energies of the ions, at corresponding temperatures,

a chemical vapor deposition of Si _x O _y N ₂ at low pressure,

an evaporation of silicon in an atmosphere of N ₂ -O ₂ or a standard growth of a layer of silicon dioxide on porous silicon, followed by a rapid thermal nitriding (processes called RTO and RTN) in an atmosphere of N ₂ O, NO or NH ₃ .

For example, Si _x OyN ₂ layers can be grown by a plasma enhanced chemical vapor deposition (PECVD) method and its remote (in-line) version (PVD plasma vapor deposition), on porous silicon wafers, using a high purity gas mixture, composed of silane (2% in argon), nitrous oxide or nitric and ammonia gas.

The gas mixture can be excited in a parallel plate reactor, and the PVD system can have a 300W magnetron power supply and a 13.56MHz radio frequency (RF). Layeres having various compositions can be grown by changing the N ₂ O: N ₂ O + NH ₃ flow ratio, temperature or pressure. The substrate temperature is, when it is usually maintained between 100 ⁰ C and 300 ⁰ C. Can be formed a film of a-SiO _x N _y reacting dichlorosilane (SiH ₂ Cl ₂ ) with nitrous oxide (N ₂ O) and ammonia gas (NH ₃ ). The relative gaseous flow ratio r = QN2O / QNH3, the deposition temperature and the pressure have a great influence on the composition of the film. For example, the relative gas flow ratio may be between 0 and 8 for a deposition temperature of 86O ⁰ C; when the parameter r is kept constant (r = 3.5), the deposition temperature can be increased from 82O ⁰ C to 88O ⁰ C.

Silicon oxynitride layers can also be obtained by sputtering onto a silicon target, for example a silicon wafer, with a radio frequency plasma, only the two reactive gases N ₂ and O ₂ . Concentrations gaseous can be approximately 99% N ₂ and 1% O ₂ .

The composition of the silicon oxynitride can be varied by changing the gaseous concentration ratio. This gas concentration ratio can be easily modified by changing partial pressures or gas concentrations.

For this purpose, it is possible to use a gaseous flow mixing chamber (in English, flow mixing chamber). This method is widely used in microelectronics to obtain thin films by a chemical vapor deposition process or a plasma enhanced chemical vapor deposition process. According to a second method, thin films of silicon oxynitride can be deposited by laser ablation of a sintered target of Si ₃ N ₄ , in a gaseous O ₂ environment, or a silicon target, in an atmosphere of O ₂ and N ₂ gas. The high oxidation rate of silicon nitride can be used to control the composition of the film by varying the partial pressures of oxygen and nitrogen.

By doing so, the refractive index of the deposited material can be adjusted to any value, from 1.47 (SiO ₂ ) to 2.3 (Si ₃ N ₄ ).

The refractive index adjustment for double layers is well known (see for example document [6]). For example, when directly depositing a layer of silicon oxynitride on a polysilicon layer, using SiH ₄ and NH ₃ as reactive gases, obtaining refractive indices in the range of 1.95 to 2.50 is guaranteed. A refractive index range of 1.72 to 3.1 is obtained for a four-layer stack; a range of 1.78 to 2.93 is detected for a two-layer stack by varying the SiH ₄ / NH ₃ ratio. In our case, we are sure to have a range of 1.47 to 2.3.

It should be noted that ethylene vinyl acetate, a material encapsulating for many silicon solar cells, has a refractive index of 1.4, which is close to that of the refractive index of SiO ₂ .

A third method for preparing the silicon oxynitride layers consists of implanting nitrogen ions, with doses and corresponding energies, at temperatures not exceeding 500 ^° C.

Amorphous Si _x OyN ₂ films can be deposited at 300 ^° C. by decomposition of a mixture of SiH ₄ , O ₂ and NH ₃ by a radio-frequency discharge plasma (in English, RF glow discharge), in a hot wall type reactor, using the inductive coupling of radio frequency power. Then, the platelets can be annealed in high purity ammonia gas at elevated temperature for up to 10 hours at a gas flow rate of 0.5 liters per minute; then the wafers are oxidized for nearly 2 hours at substantially the same temperature, to form a layer of silicon oxynitride. In any case, the technique used to deposit the silicon oxynitride layer results in a layer that is substantially non-porous (in particular, porosity less than 30%), and substantially free of foreign species such as hydrogen or nitrogen (these species being at least undetectable by usual methods).

For example, a gaseous mixture of high purity silane (2% in argon), very pure nitrous oxide and ammonia gas can be used.

The presence of foreign species depends essentially on the deposition process.

The absence of porosities in the silicon oxynitride layer 8 allows the porous silicon layer 6 to be effectively protected against degradation (in particular, after chemical oxidation degradation during, for example, a week, a short-term double-layer test mentioned above was carried out under different environmental conditions, eg a dry air environment or with a relative humidity of around 55%), and the lack of a significant amount of foreign species ensures that satisfactory and stable physical and chemical properties can be obtained, which themselves influence the optical properties of the layer.

The following is considered hereinafter the complete coating.

It should be noted that the spectral range, in which the light conversion of a solar cell, provided with the coating represented by FIG. 1 is effective and depends on the values of the thickness and the refractive index of each of the layers 6 and 8.

At present, no mathematical relationship has been established between the values of the optical parameters, but those skilled in the art can perform experimental work to obtain the desired light conversion curves.

Some preferred but non-limiting characteristics of an antireflection coating according to the invention, with a double layer of silicon oxynitride / porous silicon, for which the reflection coefficient is less than 5.5% in the range are given below. from 380 nm to 900 nm: the porous silicon layer has a thickness of between approximately 42 nm and 53 nm,

the porous silicon layer has a refractive index of between approximately 2.6 and 2.9,

the silicon oxynitride layer has a thickness of between approximately 76 nm and 112 nm, and preferably between 76 nm and 88 nm when the refractive index is close to 1.7, the silicon oxynitride layer has an index of refraction between about 1.5 and 1.7.

In a specific embodiment, the porous silicon layer has a thickness of about 52 nm and a refractive index of 2.9, while the silicon oxynitride layer has a thickness of about 94 nm and a refractive index of about 1.5.

The best curves (reflection coefficient R (in%) as a function of wavelength λ (in nm)) relative to a porous silicon / silicon oxynitride antireflection coating corresponding to the four values mentioned above, are represented in FIG. solid lines in Figures 2, 3 and 4 by comparison

with a single layer of silicon oxynitride having a thickness of 63 nm and a refractive index of 2 (dotted line in FIG. 2),

with a coating commonly used, with two SiO ₂ -TiO ₂ layers having thicknesses of 55.5 nm and 53.2 nm respectively and refractive indices of 1.41 and 2.24 (dashed line in FIG. 3), and

with a porous diamond / silicon carbon two-layer coating having thicknesses of 86.9 nm and 47.9 nm respectively and refractive indices of 1.6 and 2.8 (dotted line in FIG. 4).

As can be seen in FIG. 4, compared to a porous diamond / silicon carbon double-layer anti-reflective coating, the porous silicon / silicon oxynitride double-layer anti-reflective coating is characterized by a low coefficient reflection in the wavelength range from about 470nm to 650nm, where solar radiation is maximum. Therefore, the use of the SiO _x N _y double layer anti-reflective coating / porous silicon allows to increase the conversion efficiency of solar cells into silicon.

Advantageously, the coating anti ^¬ reflection according to the invention may comprise a single layer of porous silicon or at least two porous silicon layers having different refractive indices.

An anti-reflective coating according to the present invention may be formed on a solid silicon surface as follows:

- a porosification treatment is applied to the exposed surface of solid silicon to a predetermined thickness, so as to form a porous silicon layer or at least two porous silicon layers having different refractive indices, and

depositing a solid layer of silicon oxynitride which is substantially nonporous and substantially free of foreign species on the porous silicon layer.

A dual layer or multilayer anti-reflective coating according to the present invention may be applied to a monocrystalline, polycrystalline or microcrystalline silicon solar cell. The present invention is not limited to the foregoing embodiments or the accompanying drawings: many variations and modifications can be made thereto.

In particular, the anti-reflective coating of the present invention can be advantageously used in all cases where it is desirable to limit the reflection of radiation, such as visible, infrared or ultraviolet radiation by a surface on which it falls.

In addition, it is possible to use more than one inner layer of porous silicon: in FIG. 1, the dotted line illustrates the possibility of replacing the layer 6 with two (or more) porous silicon layers whose optical indices are different.

In another example of the present invention, a silicon solar cell is provided with a dual anti-reflective coating system according to the present invention. The porous silicon layer (s) is (are) formed on a polished monocrystalline silicon wafer, using an anodizing current density of approximately 6mA / cm ² .

The anodization is carried out in a suitable Teflon (registered trademark) cell comprising two electrodes. As a counter electrode, a Pt wire is used.

The rate of formation of the porous silicon layer varies from 5 nm / s to 5.5 nm / s and the thickness of this layer strongly depends on the HNO ₃ / HF ratio.

The silicon oxynitride film is prepared by the plasma enhanced chemical vapor deposition (CVD) technique or by the photo-CVD version of this technique, using silane and a gas comprising nitrogen (N ₂ O, NO or NH ₃ ), at a decomposition temperature ranging from 15O ⁰ C to 35O ⁰ C. The growth time and refractive index of silicon oxynitride films depend on N ₂ O and NH ₃ flow rates.

As mentioned above, a gaseous mixture of high purity silane (2% in argon), very pure nitrous oxide and ammonia gas can be used.

The refractive index characterization curve for the SiH ₄ / NH _{3 process} obtained in document [7] is used.

The deposition time of the silicon oxynitride is chosen so that the porous layer properties remain invariable.

Reference is made to FIG. 9 of document [8] which shows the modification of the forbidden bandwidth (Eg) as a function of the variation of the percentage of ammonia gas in the gaseous mixture NH ₃ / SiH 4 + NH 3 + H ₂ . This Eg parameter increases linearly from 2.96eV to 4.17eV when this percentage increases from 48.2% to 66.9%. The gas flow, the elemental composition and the thickness of? -SiO _x N _y : H are studied in detail in document [7] (Table 1). Similar corresponding investigations are carried out for the present invention. The reflection coefficient curve, which determines the proportion of radiation reflected by a solar cell provided with such a coating as a function of wavelength, is shown in FIG. 2. Note that in the case of a single layer antireflection, the solar conversion of the cell is high in a significant part of the visible range, that the efficiency decreases (that is to say that the reflection increases) towards the ultraviolet and infrared domains.

It should be noted that the reflection coefficient curves of FIGS. 2, 3 and 4 were obtained by simulation, in accordance with the so-called optical matrix technique, as described for example in document [5].

The documents cited in this description are:

[1] Bilyalov R., Stalmans L., Poortmans J., Comparable Analysis of Chemically and Electrochemically Formed Porous Antireflection Coating for Solar Cells, J. Electrochem. Soc. 150 (2003) G216-G222

[2] Adamian Z. N., Hakhoyan A.P., Arutyunian V.M., Barseghian R.S., Touryan K., Investigations of solar cells with antireflection layer, Solar Energy Materials and Solar Cells 64 (2000) 347-351

[3] Arutyunian V.M., Maroutyan K.R., Zatikyan A.L., Touryan K. J., Calculations of the reflectance of porous silicon and other antireflection coating to silicon solar cells, Thin Solid Films 403-404 (2002) 517-521

[4] Arutyunian VM, Marutyan K. R, Levy-Clement C, Zatikyan AL, Touryan KJ, Proc. SPIE on Solar and Switching Materials, 4458, pp. 61-68 (2001) [5] Arutyunian V., Martirosyan Kh. and Soukiassian P., Low reflectance of diamond-like carbon / porous silicon double layer antireflection coating for silicon solar cells, J. Phys. D: Appl. Phys. 37 (2004) L25-L28

[6] S. Winderbaum, F. Yun, 0. Reinhold, Application of plasma enhanced chemical vapor deposition silicon nitride and double layer antireflection coating and passivation layer for polysilicon solar cells, J. Vac. Sci. Technol. A 15 (1997) 1020

[7] H. Kato, M. Fujimaki, T. Homa, Y. Ohki, Photo-induced refractive index change in hydrogenated amorphous silicon oxynitride, J. Appl. Phys. 91 (2002) 6350

[8] H. Nagel, A. Aberle, R. Hezel, Optimized antireflection coating for planar silicon solar cells using remote PECVD silicon nitride and porous silicon dioxide, Progress in Photovoltaics 7 (1999) 245

[9] S. Strehlke et al., Design of porous silicon antireflection coatings for silicon solar cells, Materials Science and Engineering B 69-70 (2000) 81-86

[10] A Mahjoub et al., New designs for graded refractive antireflective coatings index, Thin solid films, 478 (2005) 299-304.

Claims

1. Anti-reflection coating (4), characterized in that it comprises at least an inner layer anti reflection ^¬ (6) of porous silicon and an outer layer (8) of silicon oxynitride that is substantially non-porous and substantially free of foreign species and which is formed on the inner layer.

The coating of claim 1 comprising a plurality of inner anti-reflective layers which are made of porous silicon and whose refractive indices are different from each other.

3. A coating according to any one of claims 1 and 2, wherein each porous silicon layer has a thickness of at least 42nm.

4. The coating of claim 1, wherein each porous silicon layer has a thickness of between 42 nm and 53 nm.

The coating of any one of claims 1 to 4, wherein each porous silicon layer has a refractive index of between 2.6 and 2.9.

6. A coating according to any one of claims 1 to 5, wherein the silicon oxynitride layer has a thickness of between 76nm and 112nm.

The coating of any one of claims 1 to 6, wherein the silicon oxynitride layer has a refractive index of between 1.5 and 1.7.

8. The coating of claim 1, wherein the porous silicon layer has a thickness of 52 nm and a refractive index of 2.9, and the silicon oxynitride layer has a thickness of 94 nm and a refractive index of 1 5.

9. A solar cell coating comprising the anti-reflective coating of any one of claims 1 to 8.

A solar cell comprising the anti-reflective coating of any one of claims 1 to 8.

11. A method of forming an anti-reflective coating on an exposed surface of solid silicon, said method being characterized in that it comprises the following steps:

applying a porosification treatment to the exposed solid silicon surface, over a predetermined thickness, so as to form a porous silicon layer or a plurality of porous silicon layers having different refractive indices, and

- Deposition, on the free surface of the layer or porous silicon layers thus obtained, a solid layer of silicon oxynitride, substantially nonporous and substantially free of foreign species.

The method of claim 11, wherein the silicon oxynitride layer is obtained by plasma enhanced chemical vapor deposition, laser ablation, or nitrogen ion implantation.

The method of any of claims 11 and 12, wherein the solid silicon surface is the surface of a solar cell panel.

14. A method according to any one of claims 11 to 13, wherein each solid layer of porous silicon is grown on a silicon wafer, at different rates of anodization of the silicon wafer.

15. Use of an anti-reflective coating obtained by the method according to any one of claims 11 to 14, for solid silicon surface.