SICLE CELL MORE RESISTANT TO IRRADIATION

Abstract

A crystalline silicon substrate (10) for solar cells, particularly in the space sector, said substrate having a P-type conductivity and being doped with at least one silicon acceptor-type doping species, this doping species having an equilibrium concentration p0 greater than 10¹⁷ atoms per cm³. (See Figure 2 for abbreviations.)

Description

Title of the invention: SI CRYSTALLINE SOLAR CELL MORE RESISTANT TO IRRADIATION TECHNICAL FIELD AND PRIOR ART

[0001] The present application relates to the field of solar cells, also called photovoltaic (PV) cells, and in particular those used in environments subject to strong irradiation by electrons and protons, such as in the space sector.

[0002] The application relates more particularly to an improved substrate for solar cells and a cell equipped with such a substrate and intended in particular for use in space.

[0003] For a space application and a cell with a crystalline silicon substrate, it is generally preferable to provide this substrate with p-type doping rather than n-type conductivity. P-doped crystalline silicon is indeed known to be less sensitive to defects created by irradiation than n-doped crystalline silicon.

[0004] Typically, the resistivity of a crystalline silicon (c-Si) substrate used for the manufacture of solar cells is in a range between 0.3 Q.cm and 30 Q.cm.

[0005] The document "Electron and proton damage coefficients in low-resistivity Silicon", by Srour, J et al. (1975), IEEE Transactions on Nuclear Science, 22(6), 2656-2662, shows that the degradation of the diffusion length of minority carriers of p-type c-Si substrates under electron irradiation is all the greater, for a given fluence, as the resistivity of the substrate is low.

[0006] To improve the resistance of a c-Si-based substrate to irradiation, the article "Investigation of p-type Silicon heterojunction radiation hardness", by Cariou et al. (2023). IEEE Journal of Photovoltaics, proposes reducing the thickness of a silicon substrate.

[0007] The problem arises of finding a new way to make a solar cell less sensitive to irradiation. Description of the invention

[0008] It is therefore an object of the present invention to provide a crystalline silicon (c-Si) substrate for solar cells which exhibits increased resistance to irradiation by electrons and protons.

[0009] The inventors have found, unexpectedly and contrary to the teachings of the prior art, that significant doping of a crystalline silicon substrate, and in particular with an equilibrium hole concentration (p0) greater than 1017 cm3 , made it possible to obtain a significantly improved resistance to irradiation, for a solar cell equipped with such a substrate.

[0010] According to one aspect, the present invention thus relates to a crystalline silicon substrate for solar cell, this substrate having a p-type conductivity and being doped with at least one acceptor-type doping species in silicon, so that the equilibrium hole concentration p0 is greater than 1017 cm3 and typically between 1017 and 1018 cm3.

[0011] Preferably, also to make it less sensitive to the consequences of strong irradiations, the substrate is further provided with a low concentration of oxygen in the crystalline silicon and in particular less than 3*1017 atoms per cm3 and typically between 1014 and 1017 atoms per cm3.

[0012] Preferably, the silicon of the substrate is monocrystalline or predominantly monocrystalline.

[0013] At least one doping acceptor species used to enable p-type doping can be chosen in particular from: B, Ga, Al.

[0014] According to a particular embodiment, the substrate can be further doped:

[0015] - using at least one neutral (isovalent) species such as germanium or tin and / or,

[0016] - using at least one light element such as hydrogen or lithium and / or,

[0017] - using a donor species such as antimony.

[0018] By “light” element is typically meant an element with an atomic number less than or equal to 5.

[0019] The substrate has a thickness measured between a first main face and a second main face which is typically between 20 pm and 180 pm, preferably between 80 pm and 120 pm.

[0020] According to another aspect, the present invention relates to a solar cell having a substrate as defined above.

[0021] Advantageously, this solar cell may comprise at least one passivated contact stack on a first face of the substrate, the passivated contact stack being formed: - a passivation layer, such as a silicon oxide layer or intrinsic amorphous silicon a-Si(i), directly in contact with the first face, - of an electron-selective layer, in particular an n-doped amorphous silicon layer, or an n-doped nanocrystalline silicon layer, or an n-doped polycrystalline silicon layer, directly in contact with the passivation layer.

[0022] According to one possible implementation of the solar cell, this cell may further comprise at least one other passivated contact stack on a second face of the substrate opposite the first face, the other passivated contact stack being formed: - another passivation layer, such as a silicon oxide layer or intrinsic amorphous silicon a-Si(i), directly in contact with the second face, - a selective hole layer, in particular a p-doped amorphous silicon layer, or a p-doped nanocrystalline silicon layer, or a p-doped polycrystalline silicon layer, directly in contact with the passivation layer.

[0023] Advantageously, the passivated contact stack is coated with at least one layer of at least one conductive transparent oxide, with metallic electrodes arranged on the conductive transparent oxide layer.

[0024] According to another aspect, the present invention also relates to a photovoltaic module for a space device such as a satellite, a space station, a space vehicle, a probe, a building, and comprising one or more solar cells as defined above.

[0025] According to another aspect, the present invention also relates to a method for manufacturing a substrate as defined above and comprising one of the following methods: - Directed solidification in a crucible or; - Growth by epitaxy on a parent substrate with a porous layer followed by detachment from the porous substrate or; - Crystallization by molten zone; - Czochralski drawing performed under a magnetic field. Brief description of the drawings

[0026] The present invention will be better understood upon reading the description of the exemplary embodiments given, by way of illustration only and in no way limiting, with reference to the accompanying drawings in which:

[0027] [Fig. 1] serves to illustrate a p-doped crystalline silicon substrate for a photovoltaic cell with a high concentration of dopants and with a low concentration of oxygen in order to make the cell resistant to the effects of strong irradiations such as those found outside the Earth's atmosphere.

[0028] [Fig.2] serves to illustrate an example of a solar cell with passivated contacts and equipped with such a substrate for applications particularly in the space sector.

[0029] [Fig.3] serves to illustrate another example of a solar cell with passivated contacts.

[0030] Identical, similar or equivalent parts of the different figures bear the same numerical references so as to facilitate the transition from one figure to another;

[0031] [Fig.4A] [Fig.4B] serve to illustrate the resistance to irradiation of a solar cell implemented with such a substrate.

[0032] The different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible.

[0033] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0034] Reference is now made to [Fig. 1], which schematically represents a substrate 10 for solar cell, typically in the form of a plate or wafer. The solar cell made from this substrate 10 is in particular a solar cell intended for use in an environment subject to high irradiation, such as a space environment.

[0035] By “high irradiation” environment, we mean in particular an environment whose main effects correspond to exposure to equivalent electron irradiation of near or at least 1 MeV for a fluence greater than 1013 electrons per cm2.

[0036] The substrate 10 is based on crystalline silicon (c-Si), advantageously monocrystalline or predominantly monocrystalline silicon, and is provided with a p-type conductivity.

[0037] The substrate 10 has the particularity of being configured to allow the obtaining of solar cells with increased resistance to irradiation by electrons and protons.

[0038] The inventors discovered that, unexpectedly, strong doping of the substrate 10 and in particular with an equilibrium hole concentration (p0) greater than 1017 cm3, enabled a solar cell with such a substrate 10 to have significantly improved resistance to irradiation.

[0039] The substrate 10 is thus doped here to create a surplus of holes (i.e., a deficit of electrons) in the silicon crystal structure, by means of at least one doping species that creates this surplus of holes. This doping species can be, for example, boron, gallium, or aluminum. The p-type doping is carried out so as to have an equilibrium hole concentration (p0) typically between 10¹⁷ cm³ and 10¹⁸ cm³.

[0040] According to a particular embodiment, the doping of crystalline silicon can be achieved by the joint addition of distinct acceptor dopant elements. Thus, for example, a p-type doping can be achieved, using both boron and gallium.

[0041] In order to improve resistance to irradiation, in addition to high levels of p-type doping with acceptor dopants, other types of dopants can also be introduced into the c-Si substrate. Thus, besides acceptor dopants, "neutral" (isovalent) impurities can be introduced into the silicon. By neutral impurities, we mean elements, in particular atoms, that do not add free charge carriers (electrons or holes) to the substrate material.

[0042] Alternatively or in combination, light dopant elements such as lithium or hydrogen may be introduced.

[0043] Alternatively or in combination, elements that can exhibit an electrically active state may be introduced. Elements of a donor species such as, for example, antimony. When introducing dopant elements with donor characteristics, care is nevertheless taken to ensure that the overall high level of p-type doping is maintained.Thus, if we consider the addition of a donor species D such as, for example, antimony (Sb) to substrate 10 with a major acceptor element A, for example Boron, we ensure that the following condition is met: p0=[A ]-[D+], with here for example p0=[B ]-[ Sb+] with p0 G [1017 cm 3 ; 1018 cm3], [A ] and [D+] corresponding to the respective concentrations of ionized acceptor and donor atoms in the general case (we consider here for the sake of simplicity that doping species release only one charge), and [B ] and [Sb+ ] to the respective concentrations of ionized B and Sb atoms in a particular embodiment example. .

[0044] With a doping level such as the above mentioned and so as to have a hole concentration (p0) at equilibrium greater than 1017 cm3, a substrate of relatively high thickness eio can be expected, for example between 60 pm and 120 pm, in particular between 80 pm and 120 pm and preferably between 90 pm and 120 pm, while maintaining a good conversion efficiency potential, i.e. a potential of at least 12.2% (AM1.5G spectrum; temperature of 300 K) for irradiation by electrons of energy 1 MeV and a fluence of 1.5 x 0.15 cm2.

[0045] The thickness ei0 of the substrate 10 is a dimension given on the [Fig.1] measured parallel to an axis z between a first main face also called "front face" 12 and a second main face also called "back face" 14 opposite to the first main face 12 of the substrate 10.

[0046] To avoid the formation of AO complexes (A for acceptor, i denoting an interstitial position of the atom in the crystal lattice), in particular B-O and / or Ga-O, which can degrade cell performance, particularly in terms of reducing charge carrier lifetime, it is further provided that, jointly at high concentration of dopants, low concentration of oxygen, and in particular less than 3*1017 atoms * cm 3.

[0047] The oxygen concentration in the crystalline silicon of the substrate 10 is typically between 1014 and 3xl017 atoms per cm3.

[0048] Thus, protection against the effects of irradiation, particularly irradiation such as that encountered in a space environment, requires both high p-doping and a low oxygen concentration. The p-doping can be achieved conventionally, for example by an in-situ process during the production of an ingot and before cutting the ingot to obtain the substrate.

[0049] With regard to obtaining a low oxygen concentration, a different process than a conventional Czochralski drawing process is preferably used here.

[0050] A first method that can be used to obtain a low oxygen concentration is directed crucible solidification, also known as the unidirectional solidification method, and in particular with seed-assisted crystallization known as "Mono-like" or "Cast-Mono". Such a method involves crucibleing purified silicon. The silicon is heated above its melting point, for example, to a temperature between 1410°C and 1550°C. This melting is carried out in a controlled atmosphere. Doping is typically performed during the silicon melting phase. The crucible containing the molten silicon is then cooled in a controlled manner. An intermediate coating, for example of silicon nitride, is provided between the crucible and the silicon.This coating limits the interactions between the crucible, which can be made of silica and be a source of oxygen contamination, and the silicon. Once the silicon has solidified, an ingot is removed from the crucible. The resulting ingot is then sliced ​​into wafers, i.e., substrates such as the one described previously. This method yields a silicon substrate with a predominantly single-crystal structure but with some polycrystalline regions. This means that the majority of the silicon exhibits uniform crystal alignments, similar to those of single-crystal silicon.

[0051] Another method for producing a silicon substrate with a low oxygen concentration, this time single-crystal, can employ a zone melt crystallization technique. Such a method does not use a silica crucible and is based on the movement of a zone melt along a polycrystalline silicon rod, here a rod heavily doped with a dopant concentration allowing the hole concentration to reach an equilibrium between 10 cm⁻¹ and 10 cm⁻¹. It proceeds by the localized melting of the polycrystalline silicon rod.

[0052] Another method for fabricating a silicon substrate with a low oxygen concentration involves implementing a magnetic field Czochralski drawing (MCz). This method uses a magnetic field to influence convection in the molten silicon. By applying a magnetic field, it is possible to reduce convective motion in the molten silicon, thereby reducing oxygen incorporation into the crystal. When the process is carried out using a silica crucible, the magnetic field helps to limit erosion of the oxygen-source crucible containing the molten silicon and / or confine oxygen-containing areas near the crucible.

[0053] Another method for manufacturing a silicon substrate with a low oxygen concentration involves forming this substrate by epitaxial growth on a "parent" support, typically made of silicon. A layer is grown on this "parent" support, and the epitaxial layer is then subsequently detached. Detachment is facilitated, for example, by incorporating a porous zone in the parent substrate. This so-called "kerf-less" technique eliminates the need for ingot formation and the subsequent cutting steps. It does not involve the interaction of liquid silicon with an oxygen source, as is the case with a silica crucible.

[0054] Starting from a substrate 10 as described above, certain steps are planned for manufacturing a solar cell, including the creation of at least one junction on the front side. When a junction is formed by thermal diffusion or dopant implantation (the implantation typically being followed by annealing) in the silicon of the substrate 10, as p0 increases, the junction depth may tend to decrease. This is particularly the case when manufacturing homojunction cells. In order to avoid the occurrence of short-circuit effects ("shunts") of the junction by one or more metallic electrodes (also called grids) in contact with the surface of the c-Si substrate, so-called "passivated" contacts can be formed, at least on the front side and preferably on the front side 12 and the rear side 14 of the substrate 10.

[0055] Such contacts typically include a passivation layer directly in contact with the substrate 10, and a charge carrier selective layer, here of electrons for the front face, made on the passivation layer.

[0056] One or more layers of transparent conductive oxide (TCO), for example based on indium tin oxide (ITO) or zinc oxide (ZnO), are then formed to serve as a transparent contact, thus allowing light to enter while collecting electrical charges. Metallic grids, for example of silver, are then formed. One or more protective and / or anti-reflective layers may also be provided.

[0057] An example of a heterojunction solar cell structure 101 having a substrate 10 as described above, with an equilibrium hole concentration (p0) greater than 1017 cm3 and an oxygen concentration less than 3*1017 atoms*cm3, is given in [Fig.2].

[0058] In this particular embodiment, in order to decrease the reflectivity of the surface of cell 101 and to increase the optical path of light in cell 101, a texturing of the front face 12 of the substrate 10 is provided.

[0059] Such texturing is optional and also possible on the rear face 14. Texturing is typically carried out before junction formation, for example by mechanical action and / or plasma etching and / or by chemical etching, for example using a KOH solution. It is also possible to perform texturing on both faces 12, 14 and then chemically polish the rear face to maintain a flat rear face 14. Cleaning of the main faces 12, 14, for example using HF / HCl, can then be carried out.

[0060] In the embodiment illustrated in [Fig.2], the cell 101 comprises a first passivation layer 21 on and in contact with the front face 12 of the substrate 10 and a second passivation layer 23 on and in contact with the rear face 14 of the substrate 10. The passivation layers 21, 23 typically have a small thickness, for example between 1 nm and 10 nm, and are for example made of intrinsic amorphous silicon a-Si(i).

[0061] The first passivation layer 21 is coated with an electron-selective layer 32 to form a pn junction with the substrate. The electron-selective layer 32, with a thickness of, for example, between 2 nm and 40 nm, can in particular be an n-doped amorphous silicon layer (a-Si(n)), or an n-doped nanocrystalline silicon layer. "Nanocrystalline" here refers to a form of silicon where the individual crystals (or grains) have a nanometric size, typically on the order of several nanometers to several tens of nanometers.

[0062] The other passivation layer 23 is, for its part, coated with a hole-selective layer 34, in particular a layer of p-doped amorphous silicon (a-Si (p)) with a thickness of, for example, between 2 nm and 40 nm.

[0063] Each of the layers 32, 34 is coated with a transparent conductive oxide (TCO) based layer, for example indium tin oxide (ITO) based, with a thickness of for example between 5 nm and 150 nm.

[0064] Metallic grids 52, 54 for example in silver are provided respectively on the layer 41 of transparent conductive oxide (TCO) and the other layer 43 of transparent conductive oxide (TCO).

[0065] Another example of cell 103 is given in [Fig.3] and differs from the previous one in particular by the composition of the passivated contacts.

[0066] The cell 103 comprises a first passivation layer 25 disposed on and in contact with the front face 12 of the substrate 10 and a second passivation layer 27 disposed on and in contact with the rear face 14 of the substrate 10, in this example based on silicon dioxide SiOx. The passivation layers 25, 27 have a thickness of, for example, between 1 nm and 5 nm.

[0067] The passivation layer 25 is coated here with an electron-selective layer 36, which, together with the p-doped substrate 10, allows a junction to be formed. The electron-selective layer 36 is here made of n-doped polysilicon (or polycrystalline silicon) (poly-Si(n)), with a thickness, for example, between 5 nm and 90 nm.

[0068] The other passivation layer 27 is here coated with a hole-selective layer 38, this time in p-doped polysilicon (poly-Si(p)) with a thickness of, for example, between 5 nm and 90 nm.

[0069] To improve the transparency of the passivated contacts, it is possible to introduce oxygen and / or carbon atoms into the passivation layer(s) 21, 23, 25, 27 and / or into the charge-selective layer(s) 32, 34, 36, 38.

[0070] The performance of a cell such as cell 103 of [Fig.3], having a c-Si-based substrate 10 coated with SiOx on its two faces 12, 14, and an n-doped poly-Si layer on the front face and a p-doped poly-Si layer on the back face, was evaluated after electron irradiation.

[0071] Such irradiation was modeled using the PC1D numerical simulation software. In particular, the diffusion effects of the phosphorus dopant for layer 36 on the front face and the boron dopant for layer 38 on the back face were taken into account in the model. The optical properties of the device were modeled using the OPAL software and experimental results that take into account the experimental variation of the short-circuit current density with the substrate thickness 10.

[0072] Finally, the post-irradiation charge carrier recombination mechanisms were modeled by Shockley-Read-Hall type recombinations using defect parameters such as the energy position in the band gap, the ratio of hole and electron capture cross sections, such as the parameters presented for example in the document "Effect of illumination intensity on cell parameters of a Silicon solar cell", Khan et al, SolMat, 2019.

[0073] The relevance of the developed cell model was validated by comparing the results of PC1D simulations with the PV parameters of irradiated cells. Figures 4A and 4B show the results of simulations run on the basis of the aforementioned model by varying the substrate thickness 10 and the parameter p0 over a wide range. ranges of values ​​and this respectively for a first fluence of irradiation by electrons (energy 1 MeV) of l,5*1014 cm2 ([Fig.4A]) and for a second fluence of irradiation by electrons (energy 1 MeV) of l,5*1015 cm2 ([Fig.4B]).

[0074] Unexpectedly compared to what the state of the art suggested, these results highlight areas corresponding to high p0 (in particular greater than 1017 cm 3), i.e. resistivities <0.2 Q.cm, for which the cells exhibit increased resistance to irradiation by demonstrating higher yields.

[0075] It is also observed that with such p0 levels, it is possible to use relatively thick samples, particularly between 90 pm and 120 pm, while maintaining good conversion efficiencies. Furthermore, with such a thickness of photovoltaic cells, the risks of deformation and / or breakage are reduced.

[0076] A particular application of a solar cell as described above relates to photovoltaic modules intended for use in space, for example in a satellite, a space station, a space vehicle, a probe, an orbiting photovoltaic power plant, a lunar or Martian base building.

Claims

Demands

1. A crystalline silicon substrate (10) for a solar cell, said substrate having a p-type conductivity and being doped with at least one acceptor-type doping species in silicon, such that the equilibrium hole concentration p0 is greater than 10 cm⁻¹ and typically between 10 and 10 cm⁻¹, the substrate having an oxygen concentration less than 3*10¹⁷ atoms per cm⁻¹ and typically between 10 and 10 atoms per cm⁻¹.

2. Substrate (10) according to claim 1, the silicon being monocrystalline or predominantly monocrystalline.

3. Substrate (10) according to claim 1 or 2, the acceptor-type doping species being selected from: B, Ga, Al.

4. Substrate (10) according to any one of claims 1 to 3, substrate (10) being further doped: - with at least one neutral species such as germanium or tin and / or - with a light element such as hydrogen or lithium and / or - with a donor-type species such as antimony.

5. Substrate (10) according to any one of claims 1 to 4, wherein the substrate has a first main face (12) and a second main face (14) opposite the first main face, the substrate having a thickness (ei0) measured from the first main face to the second main face, between 20 pm and 180 pm, preferably between 80 pm and 120 pm.

6. Solar cell (101, 103) comprising a substrate (10) according to any one of the preceding claims.

7. Solar cell (101, 103) according to claim 6, comprising a substrate according to any one of the preceding claims and comprising at least one passivated contact stack on a first face of the substrate, the passivated contact stack being formed of: - a passivation layer (21, 25), such as a silicon oxide layer or intrinsic amorphous silicon a-Si(i) layer, directly in contact with the first face (12), - an electron-selective layer (32, 36), in particular an n-doped amorphous silicon layer, or a layer of n-doped nanocrystalline silicon, or a layer of n-doped polycrystalline silicon, directly in contact with the (21, 25) passivation layer.

8. Solar cell (101, 103) according to claim 7, comprising at least one other passivated contact stack on a second face (14) of the substrate (10) opposite the first face (10), the other passivated contact stack being formed of: - another passivation layer (23, 27), such as a silicon oxide layer or intrinsic amorphous silicon a-Si(i), directly in contact with the second face, - a hole-selective layer (34, 38), in particular a p-doped amorphous silicon layer, or a p-doped nanocrystalline silicon layer, or a p-doped polycrystalline silicon layer, directly in contact with the passivation layer (23, 27).

9. Solar cell (101, 103) according to any one of claims 7 or 8, wherein the passivated contact stack is coated: - with at least one layer of at least one conductive transparent oxide (41, 43), metallic electrodes (52, 54) being arranged on the conductive transparent oxide layer.

10. Photovoltaic module for space device such as a satellite, a space station, a space vehicle, a probe, a building comprising one or more solar cells according to any one of claims 7 to 9.

11. A method for manufacturing a substrate according to any one of claims 1 to 5 or a cell according to any one of claims 6 to 9, the production of the substrate comprising one of the following methods: - Directed solidification in a crucible or; - Growth by epitaxy on a parent support having a porous layer and then detachment from the porous support or; - Crystallization by melt zone; - Czochralski pulling carried out under a magnetic field.

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