Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
  • G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
  • G02B1/11—Anti-reflection coatings
  • G02B1/113—Anti-reflection coatings using inorganic layer materials only
  • G02B1/115—Multilayers
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
  • G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
  • G02B1/11—Anti-reflection coatings
  • G02B1/113—Anti-reflection coatings using inorganic layer materials only
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
  • G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
  • G02B1/14—Protective coatings, e.g. hard coatings
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
  • H01L31/02—Details
  • H01L31/0216—Coatings
  • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
  • H01L31/02167—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells
  • H01L31/02168—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells the coatings being antireflective or having enhancing optical properties for the solar cells
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B2207/00—Coding scheme for general features or characteristics of optical elements and systems of subclass G02B, but not including elements and systems which would be classified in G02B6/00 and subgroups
  • G02B2207/107—Porous materials, e.g. for reducing the refractive index
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/50—Photovoltaic [PV] energy

Definitions

• the present invention relates to an anti-reflective coating (in English, anti-reflective coating) and a method of manufacturing this coating.
• 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.
• 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 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].
• 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.
• 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.
• MOS Metal Oxide Semiconductor
• 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.
• Si-NO system 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.
• 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.
• 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-
• VLSI Very Large Scale Integration Technology
• 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.
• porous silicon and silicon oxynitride layers for applications to solar cells or other domains had never been disclosed.
• 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.
• 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.
• each porous silicon layer has a thickness of at least 42 nm.
• each porous silicon layer has a thickness of between 42 nm and 53 nm.
• 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.
• the silicon nitride layer has a refractive index of between 1.5 and 1.7.
• 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:
• 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.
• 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 ⁇ (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 ⁇ (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 ⁇ (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).
• 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
• 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.
• 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 ⁇ m).
• 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).
• 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.
• silicon oxynitride layer 8 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:
• 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.
• PECVD plasma enhanced chemical vapor deposition
• PVD plasma vapor deposition remote (in-line) version
• 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.
• the relative gaseous flow ratio r QN2O / QNH3, the deposition temperature and the pressure have a great influence on the composition of the film.
• 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.
• 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.
• 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.
• 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.
• 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.
• a radio-frequency discharge plasma in English, RF glow discharge
• 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).
• a gaseous mixture of high purity silane 2% in argon
• very pure nitrous oxide and ammonia gas can be used.
• 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.
• 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.
• 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.
• 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.
• 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).
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
• a Pt wire is used as a counter electrode.
• 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.
• the deposition time of the silicon oxynitride is chosen so that the porous layer properties remain invariable.
• FIG. 9 of document [8] 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.

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