Processing method for a crystalline silicon wafer used in photovoltaics

Abstract

The invention relates to a process for treating a crystalline silicon wafer leading to a reduction in the density of boron-oxygen defects, which increases sensitivity to illumination. The process comprises a step (i) of heating the crystalline silicon wafer to a temperature in the range of 650°C to 1200°C, where step (i) is followed by an ultra-rapid cooling step (ii) that begins while the crystalline silicon wafer is still at a temperature of at least 600°C, and during which the temperature of the crystalline silicon wafer is lowered at a cooling rate of 150 to 2000°C / s, until a temperature of 400°C or less is reached. The invention also relates to crystalline silicon wafers, methods for manufacturing a photovoltaic cell, and the corresponding photovoltaic cells. Figure for the abstract: Fig. 1.

Description

Title of the invention: Process for treating a crystalline silicon wafer used in photovoltaics. Technical field

[0001] The present invention relates to the technical field of photovoltaics. More specifically, it concerns a process for treating crystalline silicon wafers used in the manufacture of photovoltaic cells. It also relates to processes for preparing photovoltaic cells, as well as the silicon wafers and photovoltaic cells that can be obtained by such processes.

[0002] Technological background of the invention and technical problem

[0003] Photovoltaic cells, the basic components of photovoltaic modules, are electronic devices that convert sunlight into electricity. The photovoltaic principle consists of transforming solar energy, by capturing photons, into electrical energy, resulting in the movement of positive and negative charge carriers within the photovoltaic cell. Photovoltaic cells are made from semiconductors such as silicon and generate a direct current in response to an incident beam of light. Today, technologies using crystalline silicon wafers (monocrystalline sc-Si or multicrystalline mc-Si) are the most widely used.In photovoltaic cells, crystalline silicon can be found in a form called p-type silicon, in which the majority of charge carriers are electron holes, or in a form called n-type silicon, in which the majority of charge carriers are free electrons. P-type silicon is generally doped with boron (B), but other dopants from group 13 of the periodic table, such as aluminum (Al) or gallium (Ga), can also be used instead of or in addition to boron. n-type silicon is generally doped with an element from group 15 of the periodic table, typically phosphorus (P) or arsenic (As).

[0004] It is known that the crystalline silicon used in photovoltaic cells often contains oxygen, in particular so-called interstitial oxygen (O₂), located in interstitial positions within the silicon crystal lattice. The presence of interstitial oxygen can have several origins. A common method for manufacturing crystalline silicon is the Czochralski process, leading to silicon called Czochralski silicon (SiCz). In this process, a silicon crystal is drawn from a crucible of molten silica (SiO₂), and the resulting crystalline silicon necessarily contains oxygen from the crucible. Crystalline silicon can be obtained by other Processes such as the Bridgman method, better known as directional solidification, or the float zone crystallization technique (producing FZ Si), although this technique leads to high-purity crystalline silicon with a lower amount of interstitial oxygen. Interstitial oxygen can also be present as a contaminant, particularly in lower-purity silicon substrates.

[0005] It has been described in the literature (SW Glunz et al., 2001, in particular) that oxygen atoms combine with boron present in crystalline silicon wafers, especially of the SiCz type, to form defects, called "Boron-Oxygen" defects, although the exact stoichiometry is not known. Under the influence of light, these defects induce a decrease in the lifetime of charge carriers and increase the light-induced degradation (LID) of the silicon wafers. The nature and stoichiometry of the defects inducing LID are not fully understood. Several hypotheses are proposed in the literature, such as complexes in which boron is associated with interstitial oxygen dimers.Light-induced dielectric loss (LID) is characterized by a degradation of the lifetime (DL) of the charge carriers in the silicon wafer in question and by a degradation of the photovoltaic properties of the corresponding cell upon exposure to light (more generally, when charge carriers are injected into the device). This problem exists whether boron is present as the primary dopant (p-type silicon), as a secondary dopant (p-type silicon in which boron is an additional dopant to Al or Ga, n-type silicon in which boron is an additional dopant to phosphorus or antimony, this type of silicon being called compensated n-type silicon), or as a contaminant in lower-purity silicon (for example, crystalline silicon obtained from recycled silicon or UMG silicon: solar-grade silicon that contains boron).Traditionally, the SiCz ingots used to produce SiCz wafers are obtained from high-purity silicon. In this case, the SiCz contains few boron atoms as a contaminant. However, it is also possible to carry out the Czochralski process, incorporating a portion of UMG or recycled silicon into the traditionally used high-purity silicon. UMG silicon is obtained by metallurgical purification (the silicon remains in a solid or liquid state) of metallurgical silicon, while recycled silicon comes from waste from the photovoltaic or electronics industries. In such a case, the resulting crystalline silicon contains boron (specifically up to 5 x 10¹⁷ atoms / cm³, also referred to as cm³).

[0006] Heterojunction photovoltaic (HJP) cells predominantly use n-type crystalline silicon wafers which may sometimes include quantities of boron and oxygen forming Boron-Oxygen defects. These defects lead to High light sensitivity (LID) and, after prolonged illumination, a short lifetime of charge carriers on these wafers make them unattractive. SHJ production involves several key steps, each crucial to ensuring cell efficiency and durability. Generally, layers of amorphous silicon (p-type on one side and n-type on the other) are deposited on an n-type crystalline silicon wafer. Often, an intermediate intrinsic amorphous silicon layer containing hydrogen is deposited before the p- and n-type amorphous silicon layers. Currently, doped amorphous silicon layers tend to be replaced by nanocrystalline silicon layers (or even stacks of amorphous silicon and nanocrystalline silicon). Finally, a transparent conductive layer (typically indium tin oxide or zinc oxide) is deposited on each amorphous layer.Then, metallic contacts, such as metal grids, are deposited onto the transparent conductive layer (CEA Challenges, February 2019, No. 234). Annealing (heat treatment) is then carried out to improve the contacts. A light soaking step is also generally performed after the photovoltaic cell has been manufactured. This step, which involves exposing the cell to intense light (often provided by halogen lamps or special LEDs) for an extended period (ranging from a few hours to several days, depending on the cell's specifications and performance requirements), aims to stabilize and improve the cell's performance. During this step, the temperature is controlled to keep it below 250°C.During this stage of exposure to light, the photovoltaic yield can be significantly impacted due to LID caused by the presence of Boron-Oxygen defects.

[0007] Light-induced degradation (LID) is therefore associated with the presence of these boron-oxygen defects. LID is diagnosed by a degradation of the photovoltaic properties of the cell or by a degradation of the lifetime of the charge carriers of the crystalline silicon wafer upon exposure to light (ZY Yeo et al., 2014). A treatment called light-induced regeneration has been proposed to limit, or even neutralize, LID effects. This consisted of strongly illuminating or injecting a current into cells heated to a moderate temperature (120-250°C).

[0008] Several other strategies have also been proposed in the literature to try to limit Boron-Oxygen defects leading to LID, by applying a heat treatment.

[0009] In SW Glunz et al., J. Appl. Phys. 2001, strategies aimed at reducing or eliminating LID in boron-doped p-type Si Cz were investigated. Various approaches were proposed to decrease the boron-oxygen defect density: The reduction of oxygen concentration, the substitution or reduction of boron, and the use of high-temperature treatments have been proposed. Specifically, the following have been suggested: either annealing combined or not with oxidation in a conventional tube furnace at 750°C or 1050°C, or annealing at 850°C in an infrared lamp furnace with a 120-second pass through time and a silicon nitride or silicon oxide barrier layer. When these heat treatments were applied, a simultaneous reduction in silicon quality could occur. Therefore, this publication concludes that heat treatment processes must be optimized either to limit contamination (by using a barrier layer) or to limit oxygen precipitation (by limiting the high-temperature treatment time).When indications of Boron-Oxygen defect concentrations are reported, in the form of an N* value, a quantity proportional to the defect density, the concentrations after treatment are in the order of 25 to 65% of the pre-treatment concentrations.

[0010] Furthermore, LJ Caballero et al., 2005 showed a reduction in Boron-Oxygen defect density to approximately 80%, 50% and 80% after a heat treatment step at 875°C for 40 minutes corresponding, respectively, to a phosphorus diffusion process (heating under POC13), dry oxidation and annealing in a nitrogen environment.

[0011] FE Rougieux et al., 2011, examined the boron-oxygen defect density in compensated n-type silicon (boron-phosphorus co-doping) using carrier lifetime measurements. The effect of annealing at an intermediate temperature (500°C to 700°C) on the boron-oxygen defect density was also explored. In this publication, the boron-oxygen defect density is presented as being effectively improved by annealing at these temperatures, probably due to the dissociation of oxygen dimers. Thus, the concentration of oxygen dimers available to interact with boron atoms is decreased, which reduces the boron-oxygen defect density (these explanations are based on the assumption that the boron-oxygen defect is of the form B-Oi2).The authors determined an activation energy for this process of 0.14 eV, which is relatively low, and suggested that the temperature range of 500°C to 700°C was favorable for its implementation.

[0012] More recently, N. Nampalli et al., 2017 defined a four-state model for Boron-Oxygen defects: state A, which is the annealed state, inactive in terms of possible recombination (i.e., no effect on the lifetime of charge carriers); state B, which is a degraded state that is active in terms of possible recombination; and state C, which is a regenerated or passivated state that is also inactive in terms of recombination, but unlike state A, state C is relatively stable and shows no further degradation upon charge carrier injection. In addition to these three already known states, the authors introduce a state D, termed the dissociated state, also inactive in terms of recombination, to model the transformation kinetics of boron-oxygen defects. This state D represents the state of the defect after thermal deactivation and an initial state leading to state A after thermal annealing. The authors thus proposed two strategies to limit the LID due to the presence of boron-oxygen defects: either favoring the state named D, or favoring the final inactive state C, obtained by the activation of state A leading to state B followed by its deactivation, this sequence being called degradation-regeneration or regeneration.The transition to state D is achieved by thermally deactivating state A, which requires rapid cooling following high-temperature annealing. It is noted that an annealing temperature of 450°C may be sufficient. To promote regeneration (state C), annealing in a flow furnace to allow hydrogen diffusion throughout the volume and subsequent annealing under illumination (more generally, under carrier injection) are usually necessary. Regeneration could involve hydrogen passivation of the fully formed boron-oxygen defect. In contrast, thermal deactivation likely involves the dissociation of the defect back to its precursor state, with no apparent role for hydrogen. During deactivation for the transition to state D, annealing is performed at a maximum temperature of 750°C.No details are given on the concept of rapid cooling, but reference is made to the following three publications mentioned below (SW. Glunz et al., 2001a, K. Bothe et al., 2002 and DC Walter et al., 2014).

[0013] SW Glunz et al., Sol. Energy Mater. Sol. Cells 2001, proposed solutions consisting of avoiding boron or oxygen in the starting silicon (use of MCz-Si or FZ-Si with a very low quantity, or even an absence, of interstitial oxygen, substitution of boron with gallium, in particular). In cases where boron and interstitial oxygen are present, SW Glunz et al. considered carrying out a high-temperature heat treatment to permanently reduce the concentration of boron-oxygen defects, which resulted in an increase in photovoltaic efficiency or an increase in the lifetime of charge carriers. The heat treatment consisted of high-temperature oxidation (ranging from 850 to 1050°C). No details of the cooling conditions are mentioned in this publication.

[0014] K. Bothe et al., 2002, studied the effects of different heat treatments on the Boron-Oxygen defect density associated with LID, starting from N*. These authors studied a crystalline silicon initially of p-type containing boron and oxygen (O-doped FZ). It was found that a treatment prolonged up to 32 hours at 450°C reduced the N* defect density. At a higher temperature (850°C in a quartz furnace), K. Bothe et al. showed that oxidation treatment and phosphorus diffusion treatment (POC13) had the same effect on N* reduction, provided the heat balance was similar. Bothe et al. also reported that using a faster cooling ramp resulted in a greater N* reduction, corresponding to approximately a 70% reduction compared to the initial silicon (N* divided by 3.5). Two cooling conditions, described as slow and fast, were used, without further clarification of these terms. It should be noted that, generally speaking, conventional cooling ramps result in cooling rates of 3 to 70 °C / s (see, for example, H. Wagner et al., 2016).

[0015] Finally, DC Walter et al., 2014, identified the cooling rate after pass-through furnace annealing as a parameter influencing the degradation of boron-oxygen defects and their regeneration. The cooling rate was modulated by changing the conveyor speed of the pass-through furnace. The authors also demonstrated that these effects were independent of the presence of hydrogen, since studies were conducted with and without hydrogenated dielectric layers.

[0016] Thus, it appears that the prior art does not offer a method for satisfactorily reducing the density of boron-oxygen defects leading to LID in a crystalline silicon wafer containing boron and interstitial oxygen. One of the objectives of the invention is precisely to offer a new solution leading to crystalline silicon wafers which, although containing boron and oxygen, exhibit a low density of boron-oxygen defects leading to LID. In particular, the invention proposes a new process for treating crystalline silicon wafers, making it possible to drastically reduce the density of boron-oxygen defects leading to LID, compared to the density present before said treatment. Thus, the invention also aims to provide silicon wafers and photovoltaic cells with, respectively, improved charge carrier lifetime and photovoltaic efficiency.

[0017] Description of the invention

[0018] In this context, the present invention relates to a method for treating a crystalline silicon wafer comprising boron and interstitial oxygen forming defects that increase the illumination sensitivity of the crystalline silicon wafer, called Boron-Oxygen defects, said treatment method leading to a reduction in the density of said Boron-Oxygen defects, which comprises a step i) of heating the crystalline silicon wafer to a temperature in the range of 650°C to 1200°C, characterized in that step i) is followed by an ultra-rapid cooling step ii) which starts while the silicon wafer crystalline is still at a temperature of at least 600°C, and, during which, the temperature of the crystalline silicon wafer is lowered by a cooling rate which belongs to the range of 150 to 2000 °C / s, until reaching a temperature of 400°C or less, preferably until reaching a temperature of 300°C or less.

[0019] Within the scope of the invention, the notion of "process of a crystalline silicon wafer comprising boron and interstitial oxygen forming defects which increase the sensitivity to illumination of the crystalline silicon wafer, called Boron-Oxygen defects" may be replaced in any place in the present description of the invention by "process of a crystalline silicon wafer comprising boron and interstitial oxygen to decrease the sensitivity to illumination of said crystalline silicon wafer".Within the framework of the invention, the inventors propose a specific heat treatment cycle targeting the elimination of the LID effect due to Boron-Oxygen defects (corresponding possibly to complexes formed between boron atoms and interstitial oxygen dimers, according to previous studies) by dissociating the complex precursors at high temperature, during step i) of heating and blocking them in this state, thanks to step ii) of ultra-rapid cooling.

[0020] The solution proposed within the framework of the invention was by no means obvious, because the inventors first observed that the silicon wafers were subjected to excessively high thermomechanical stresses and broke when quenched in water directly from a thermal heater, which leads to a cooling rate much higher than that proposed within the framework of the invention.Surprisingly, according to the invention, ultra-rapid cooling from a temperature that can be as high as 1000°C, notably obtained by immersing the thin wafer (particularly with a thickness <180 pm) of large-dimension crystalline silicon (with a surface area of ​​156x156 mm2 in the examples that follow) in liquid nitrogen does not generate breakage (this has been visually checked, in the examples that follow), nor does it generate slip planes that could be due to excessively high thermo-mechanical stresses (this has been checked by the DDV obtained).

[0021] According to particular embodiments, the treatment process according to the invention includes, upstream of step i), a preliminary preparation of the surface of the crystalline silicon wafer to reduce the risks of contamination of the wafer by impurities, during step i) and / or ii).

[0022] According to certain embodiments, the treatment process according to the invention comprises, upstream of step i), a preliminary preparation of the surface of the crystalline silicon wafer, said preparation comprising or consisting of a A surface texturing step for the crystalline silicon wafer and / or a step for removing a work-hardened area present on the surface of the crystalline silicon wafer and / or a step for forming a barrier layer or a getter layer on the surface of the crystalline silicon wafer. In particular, the pre-preparation of the crystalline silicon wafer surface may include the formation of a phosphosilicate (PSG) getter layer under a POC13 atmosphere, followed by annealing leading to phosphorus diffusion into the crystalline silicon wafer from its surface; the phosphosilicate layer may then be removed or not.

[0023] According to other embodiments, the treatment process according to the invention comprises, upstream of step i), a preliminary preparation of the surface of the crystalline silicon wafer, said preparation comprising or consisting of a step of texturizing the surface of the crystalline silicon wafer and / or a step of removing a work-hardened area present on the surface of the crystalline silicon wafer, and, during step i), the formation on the surface of the crystalline silicon wafer of a layer acting as a barrier layer or a layer developing a getter effect is carried out. In particular, step i) can be carried out under a POC13 atmosphere, leading to the formation of a phosphosilicate (PSG) layer developing a getter effect and resulting in phosphorus diffusion into the silicon wafer from its surface.

[0024] Regardless of the embodiment, in the treatment process according to the invention, steps i) and ii) can be carried out under the following temperature conditions: - during step i), the heating of the crystalline silicon wafer is carried out at a temperature belonging to the range of 700°C to 1050°C, - during step ii), the ultra-rapid cooling starts while the crystalline silicon wafer is still at a temperature of at least 600 °C and the crystalline silicon wafer is lowered according to a cooling rate in the range of 150 to 2000 °C, until reaching a temperature of 400 °C or less, preferably until reaching a temperature of 300 °C or less.

[0025] Typically, the process for treating a crystalline silicon wafer according to the invention comprises the following steps i) and ii): - In step i), heating to a temperature of approximately 1000°C is carried out, and - in step ii), ultra-rapid cooling begins while the crystalline silicon wafer is still at a temperature of at least 800°C, and a higher cooling rate in the range of 200 to 600°C / s is applied until a temperature lower than or equal to that is reached. at 400°C and preferably until reaching a temperature of 300°C or less.

[0026] Regardless of the embodiments, in the treatment process according to the invention, during step i) the crystalline silicon wafer is advantageously heated to a temperature in the range of 650°C to 1200°C, preferably in the range of 700°C to 1050°C, and typically to about 1000°C, for a period of 0.5 s to 120 min, preferably for a period of 1 s to 30 min.

[0027] In particular, in the treatment process according to the invention, during step ii), ultra-rapid cooling is achieved by immersing the crystalline silicon wafer in liquid nitrogen.

[0028] In the processing methods according to the invention, the crystalline silicon may be n-type silicon or p-type silicon, in particular boron-compensated n-type silicon. In particular, the crystalline silicon is Cz silicon, in particular n-type Cz silicon, in particular boron-compensated n-type Cz silicon, or n-type Cz silicon obtained from recycled or solar-grade silicon.

[0029] In the processing methods according to the invention, the crystalline silicon of the crystalline silicon wafer contains, in particular, from 5.1013 to 5.1017 atoms / cm3 of boron, preferably from 1014 to 1017 atoms / cm3, and even more preferably from 1015 to 5.1016 atoms / cm3 of boron and the crystalline silicon of the crystalline silicon wafer contains, in particular, from 8.1016 to 2.2.1018 atoms / cm3 of interstitial oxygen, preferably from 2.1017 to 2.2.1018 atoms / cm3 of interstitial oxygen, and even more preferably from 2.1017 to 7.1017 atoms / cm3 of interstitial oxygen. Note that 2 x 10¹⁷ to 7 x 10¹⁷ atoms / cm³ can also be written as 2 x 10¹⁷ to 7 x 10¹⁷ cm³. All the ranges given above for boron and interstitial oxygen can be combined. In one particular embodiment, the crystalline silicon of the crystalline silicon wafer contains from 10¹⁵ to 5 x 10¹⁶ atoms / cm³ of boron and from 2 x 10¹⁷ to 7 x 10¹⁷ cm³.1017 atoms / cm3 of interstitial oxygen.

[0030] Advantageously, the treatment processes according to the invention lead to a reduction of at least 85%, and preferably to a reduction of 90% or more, in the density of "Boron-Oxygen" defects.

[0031] Another aspect of the invention relates to the crystalline silicon Pt wafers that can be obtained by the processing method according to the invention. The higher DDV obtained, while the amounts of boron and interstitial oxygen are significant in such crystalline silicon Pt wafers, clearly highlights the difference compared to prior art silicon wafers having similar amounts of boron and interstitial oxygen.

[0032] The invention also relates to a method for manufacturing a photovoltaic cell, and in particular a heterojunction photovoltaic cell, comprising the use of a Pt crystalline silicon wafer according to the invention or comprising a step consisting of implementing the processing method according to the invention leading to a Pt crystalline silicon wafer.

[0033] Such a method for manufacturing a photovoltaic cell includes, in particular, a step of associating the crystalline silicon wafer Pt with at least one semiconductor layer.

[0034] Advantageously, a method for manufacturing a photovoltaic cell according to the invention does not include any heating step at a temperature exceeding 300°C applied to the crystalline silicon wafer Pt and, preferably, it does not include any heating step at a temperature exceeding 250°C applied to the crystalline silicon wafer Pt. A heating step applied to the crystalline silicon wafer Pt is understood to mean a heating step once the crystalline silicon wafer Pt is obtained, it being understood that the absence of such a heating step also applies to the crystalline silicon wafer Pt possibly modified or associated with one or more layers, depending on the intended use.

[0035] The invention also relates to photovoltaic cells manufactured from a crystalline silicon wafer Pt according to the invention or capable of being obtained by the manufacturing process of a photovoltaic cell according to the invention. Detailed description of the invention

[0036] Other features, details and advantages of the invention will become apparent from the description made with reference to the accompanying figures given by way of example.

[0037] Detailed description of the figures

[0038] Figure 1 schematically illustrates the various stages that a processing method and a manufacturing method for a photovoltaic cell proposed according to the invention may comprise. The crystalline silicon wafer Po can, after surface treatment (A in the Figure), yield the crystalline silicon wafer Pi, which itself yields the crystalline silicon wafer Pt, by implementing step i) heating, followed by step ii) ultra-rapid cooling (B in the Figure). Finally, the crystalline silicon wafer Pt is subjected to the stages of a manufacturing process for a photovoltaic cell, for example, deposition of semiconductor and dielectric layers, annealing at a temperature < 300°C, exposure to light, known as "light soaking"... (C in the Figure) to produce the photovoltaic cell.

[0039] Figure 2 illustrates the reduction in the N* defect concentration using a treatment process according to the invention on a Cz silicon wafer. of type n. The reference is a similar plate which has not undergone the treatment according to the invention.

[0040] Figure 3 illustrates the increase in the effective lifetime of charge carriers (reff in ps shown as a function of the injection level in cm-3) obtained in Example 2 using a treatment process according to the invention (curve □) on a p-type crystalline silicon wafer (boron doped), with formation of a getter layer by phosphorus diffusion, compared with slow cooling (curve □). The injection level is the concentration of excess carriers (p / r at equilibrium) in the silicon.

[0041] Fig. 4 is an illustration of the increase in the effective lifetime of the charge carriers (r eff in ps presented as a function of the injection level in cm-3) obtained in Example 2 using a treatment process according to the invention (curves with symbol A) on a Cz p-type crystalline silicon wafer (boron doping), without formation of a getter layer by phosphorus diffusion, compared with slow cooling (curve o).

[0042] Figure 5 shows the effects of annealing (step i)) and the cooling method (step ii)), compared with another type of slow cooling, on the N* values ​​obtained in Example 2. Negative N* values ​​indicate that the effective lifetime increased under illumination. The N* values ​​were calculated from reff measurements extracted at an injection level of 3 x 1014 cm-3.

[0043] Definitions

[0044] Unless otherwise specified, the terms used in this description have the meaning generally recognized by a person skilled in the art.

[0045] The terms "of the order" or "approximately" mean that a variation of at most 5%, 4%, 3%, 2%, and preferably 1%, around the given value is tolerated. Identity with the given value is, however, preferred.

[0046] The crystalline silicon of the wafers subjected to the treatment process according to the invention can be monocrystalline or multicrystalline silicon. Advantageously, the silicon is monocrystalline silicon, in particular obtained by Czochralski drawing. The crystalline silicon can be obtained by various solidification methods: Czochralski silicon (or Si Cz), crystalline silicon obtained by the Bridgman method (or directional solidification), crystalline silicon obtained by the float zone crystallization technique (FZ-Si), and magnetic Czochralski silicon (MCz).

[0047] The boron present in crystalline silicon may originate from a desired doping, in the case of boron-doped p-type crystalline silicon or boron-compensated n-type crystalline silicon. Boron may also be present as a contaminant, This is, in particular, the case for crystalline silicon obtained from solar grade silicon (UMG), recycled silicon or certain polycrystalline silicon obtained by fluidized bed.

[0048] In the context of the invention, the amount of boron or phosphorus in crystalline silicon can be determined by secondary ion mass spectrometry (SIMS). The amount of interstitial oxygen in crystalline silicon can be determined by Fourier transform infrared (FTIR) spectroscopy, according to ASTM 1188, as amended on July 1, 2024.

[0049] A characteristic quantity of the amount of boron-oxygen defects that increase the light sensitivity (LID) of the crystalline silicon wafer can be calculated from the lifetime (LID) of the charge carriers, as described by Walter et al., 2014. As in prior art publications, said characteristic quantity of the concentration or amount is generally denoted N*. The effective lifetime of the charge carriers (reff) is measured before (rini) and after (rfin) a sufficiently long illumination time (experimentally verified) to form all the boron-oxygen complexes. The effective lifetime of the charge carriers (reff) was determined by photoconductivity, using Sinton WCT-120 equipment, as a function of the injection level. Typically, in the context of the invention, 0.5 suns of illumination for 60 minutes is applied.From these measurements, carried out for an identical selected injection level, the characteristic data of the relative concentrations of Boron-Oxygen complexes formed (N*) are extracted.

[0050] N*=l / r fin - 1 / r ini

[0051] Detailed description of the treatment process of the invention and of certain embodiments

[0052] As can be seen from [Fig. 1], the treatment process according to the invention, which reduces the light sensitivity (LID) of a crystalline silicon wafer by decreasing the Boron-Oxygen defect density that causes the LID, comprises a first step (i) of high-temperature heat treatment, followed by a second step (ii) of ultra-rapid cooling. The resulting Pt crystalline silicon wafer can then undergo the other steps necessary for the manufacture of a heterojunction photovoltaic cell, which include, in particular, the deposition of semiconductor layers, transparent conductive oxide, annealing at a temperature below 250°C, and exposure to light, known as "light soaking".Prior to step i), the crystalline silicon wafer will generally undergo a surface treatment to prevent chemical contamination during the first heat treatment step i), as well as during the cooling of step ii). The various possible treatments will be detailed later. The silicon wafer. Crystalline silicon, before surface treatment (which may include one or more steps), is designated Po, while that obtained after surface treatment is designated P;. The P; crystalline silicon wafer typically has a thickness of 5 µm to 2 mm, particularly from 40 µm to 200 µm. Its surface area is 0.02 to 0.05 m², preferably from 0.03 to 0.045 m².

[0053] Step i) of heating can be carried out in any suitable heating device capable of reaching the target temperature, which falls within the range of 650°C to 1200°C. In particular, an infrared (IR) furnace, a resistive furnace, a vertical quartz tube furnace, or a pass-through furnace can be used. The heating time of step i), corresponding to the duration for which the crystalline silicon wafer Pi is held in the heating device, will depend on the nature of the device and the time required for the silicon wafer to reach the target heating temperature at which the heating device is maintained. To obtain the desired result, holding the silicon wafer at a temperature Te of 650°C to 1200°C, preferably 700°C to 1050°C, for 0.5 to 2 seconds, and typically for about 1 second, may be sufficient.The heating time in step i) will generally be longer than this duration, to account for the time it takes for the wafer to reach the target temperature. Thus, the duration for which the wafer is held in the heating device will depend on the device and will vary from 0.5 s (seconds) to 120 min (minutes), preferably from 1 s to 30 min, generally.

[0054] Step i) of heating can be carried out in any type of atmosphere: in particular, under air, under a phosphorus diffusion atmosphere (for example of type POC13) or under an inert atmosphere of the nitrogen or argon type. The heating is generally carried out under atmospheric pressure (i.e. at a pressure of 1013.25 hPa).

[0055] Next, the still hot crystalline silicon wafer is subjected to cooling with a cooling rate which belongs to the range of 150 to 2000°C / s, advantageously to the range of 200 to 1000°C / s, and even more preferably to the range of 200°C / s to 600°C / s.

[0056] This speed is very rapid and completely unusual, which is why it is described as ultra-fast in the context of the invention. Ultra-fast cooling can begin directly at temperature Te or at a lower temperature due to initial cooling occurring when the silicon wafer is removed from the heating system. The important point is that ultra-fast cooling starts when the silicon wafer is at a sufficiently high temperature, at which the Boron-Oxygen defects have been deactivated / dissociated. The purpose of the ultra-fast cooling step is to prevent the formation of precursors that cause Boron-Oxygen defects, due to a sudden drop in temperature. the temperature. Thus, the ultra-rapid cooling phase can take place over a temperature range starting at temperature Te, or at a lower temperature, in particular starting at a temperature belonging to the range from 600 to 850°C, depending on the heating temperature Te applied during step i).

[0057] The ultra-rapid cooling phase within the desired speed range is advantageously achieved by placing the silicon wafer in a suitable cooling environment or in contact with a suitable cooling element, said cooling environment or cooling element having a temperature of -40°C or less, in particular a temperature in the range of -100°C to -250°C and typically on the order of -200°C. Step ii) can be carried out by immersing the crystalline silicon wafer, still at a high temperature, in a cooling environment having a temperature of -40°C or less, this immersion being also called quenching. This cooling environment is, in particular, liquid nitrogen (which has a temperature of -196°C) or gaseous nitrogen. Cooling can be achieved directly by quenching the silicon wafer in liquid nitrogen, as illustrated in the examples, with an intermediate passage through gaseous nitrogen at the interface.It is also possible to place the silicon wafer in a cooling environment consisting of a gaseous environment cooled to a temperature close to that of liquid nitrogen, using liquid nitrogen. Such a gaseous environment could be air, nitrogen, argon, hydrogen, or helium, for example. It is also possible to place the silicon wafer in contact with a cooling element, such as a preemption element or a support cooled to a temperature of -40°C or lower, for example, with liquid nitrogen.

[0058] The positioning of the cooling environment or the cooling element used in step ii) relative to the hot zone of the heating device used in step i) will be adapted to minimize the temperature drop before step ii) allowing the dissociation of the precursors causing the Boron-Oxygen defects obtained in step i) to stop. In particular, it may be provided that the cooling environment, in particular the liquid nitrogen bath, or a chamber intended to hold it, is positioned directly under the heating device and that said chamber is filled with the cooling environment by actuation of a valve, and then the crystalline silicon wafer is dropped into said environment by the action of a trapdoor.

[0059] It could also be envisaged to remove the silicon wafer very quickly from the heating device and subject it to a diffusion or projection of gas cooled with liquid nitrogen. Another ultra-rapid cooling method would be to deposit the silicon wafer, at the exit of the heating device, onto a cooled support whose The surface temperature would be below -40°C, notably through the use of liquid nitrogen. Alternatively, it is possible to use a silicon wafer gripping device within the heating device, which itself is either pre-cooled to a temperature below -40°C or continuously cooled to a temperature below -40°C, for example with liquid nitrogen.

[0060] The cooling rate in the range of 150 to 2000°C / s can be applied over a temperature range of 800°C to 400°C, or over a wider temperature range, which depends in particular on the heating temperature Te, which may be higher or lower than 800°C. In particular, the cooling rate of 150 to 2000°C / s can be applied up to a temperature of 400°C, or even 300°C, 250°C, or 200°C. The cooling rate during step ii) can, in particular, be from 200 to 600°C / s, especially from 300 to 400°C, and typically in the order of 300°C / s. The important thing is that the cooling rate, which belongs to the range of 150 to 2000°C / s, advantageously to the range of 200 to 1000°C / s, and even more preferably to the range of 200°C / s to 600°C / s, is maintained over a temperature range in which Boron-Oxygen defects have the capacity to reform.This temperature range corresponds, in particular, to the temperature range of 600 to 400°C, but the ultra-fast cooling rate can be maintained over a wider temperature range, especially for practical reasons. Furthermore, even if the silicon wafer is kept in the cooling environment or in contact with the cooling element, the temperature drop is slower. Specifically, when cooling is achieved by immersion in liquid nitrogen, the silicon wafer can be kept immersed until it reaches the temperature of the liquid nitrogen or removed earlier; this is not critical as long as the cooling rate of 150 to 2000°C / s is applied until a temperature of 400°C is reached, or even 300°C, 250°C, or 200°C.Once the silicon wafer has reached a temperature of 400°C, or even 300°C, 250°C or 200°C, the cooling rate can be slower, without influencing the result obtained within the framework of the Boron-Oxygen defect reduction invention.

[0061] The temperature of the crystalline silicon wafer, and therefore the cooling rate, can be measured by any suitable means. Conventionally, a thermocouple placed on the crystalline silicon wafer is used.

[0062] The crystalline silicon wafer Pi subjected to heating step i) most often results from a prior treatment step which thus allows heating of the wafer while minimizing, or even avoiding, contamination mechanisms by impurities, particularly metallic ones, during the heating of step i). Indeed, such contamination by impurities, particularly metallic ones, would have to This can lead to a reduction in the lifespan of the charge carriers in the wafer. If a sufficiently clean oven, heated under an inert atmosphere, is used, this treatment step could be omitted.

[0063] Such treatments are standard practice for those skilled in the art. Thus, to produce the crystalline silicon wafer Pi subjected to successive steps i) and ii), a crystalline silicon wafer, referred to as the original Po, can undergo a surface texturing step, a work-hardening step, a deposition step of a layer acting as a barrier layer, or the formation of a Getter layer. It is possible to first perform a work-hardening step, then a deposition / formation step of a layer acting as a barrier layer and forming a Getter layer, or to first perform a work-hardening step, then a texturing step, and then a deposition step of a layer acting as a barrier layer and enabling the formation of a Getter layer.

[0064] In particular, the crystalline silicon wafer Po is most often obtained directly from the cutting of a crystalline silicon ingot. A work-hardened area on a crystalline silicon wafer Po refers to a region of the wafer where the silicon has undergone work hardening, that is, deformation due to mechanical treatment. This phenomenon generally occurs during the sawing, polishing, or other mechanical processes involved in manufacturing silicon wafers from the ingot. The removal of a work-hardened area can be achieved, in particular, by chemical treatment, for example, with solutions containing KOH.

[0065] The texturing step, when implemented, is carried out according to techniques well known to those skilled in the art. This may involve treatment with an alkaline solution (typically potassium hydroxide or sodium hydroxide) or an acidic solution (typically hydrofluoric acid or nitric acid), or texturing by laser, plasma, wet etching, sandblasting, or shot peening. The treatment will be adapted according to the crystalline silicon wafer (Po) used. In particular, texturing by treatment with an alkaline solution is more suitable for a monocrystalline silicon wafer (Po), whereas texturing by treatment with an acidic solution is more suitable for a polycrystalline silicon wafer (Po).Preferably, the chemical treatment steps to remove the work-hardened layer are followed by one or more steps to clean the surfaces of the silicon wafer, for example via HF baths.

[0066] The choice of this preparation step will be adapted by a person skilled in the art, depending on the heating implemented in step i) and its potential for contamination, in particular depending on the atmosphere used during this heating, the equipment or the support with which the crystalline silicon wafer is in Contact occurs during heating. In particular, the deposition of a surface barrier layer is recommended when heating is carried out in an oxidizing atmosphere, especially in air. Such a barrier layer is known to be, for example, a layer of silicon nitride (which may have varying nitrogen content, commonly referred to as SiNx), silicon dioxide (SiOx), aluminum oxide, HfO2, or SiOxNy. Such a barrier layer typically has a thickness of 2 nm to 500 nm, particularly 10 to 300 nm. It can be formed by any suitable technique, including thin-film deposition. In the case of a SiOx layer, it can also be formed by oxidation.

[0067] The formation of a Getter layer (more precisely, an external Getter layer) will be implemented, in particular, in the presence of metallic contaminants directly in the silicon wafer or during step i). A Getter layer captures or immobilizes metallic impurities on the surface of the silicon substrate. Such a layer can be created, in a manner known to those skilled in the art, in particular by chemical or physical treatment. It can be a phosphorus-doped surface layer obtained by phosphorus diffusion from a phosphosilicate glass (PSG) layer formed on the surface of the silicon wafer, using POC13, during heat treatment at a temperature most often in the range of 850 to 900°C. After phosphorus diffusion, the PSG layer can be retained or removed (for example, using a hydrofluoric acid solution) to obtain the crystalline silicon wafer Pi subjected to steps i) and ii).Advantageously, PSG glass can be retained as it can act as a barrier layer. An additional barrier layer, particularly silicon nitride, can also be used.

[0068] It is also possible that a getter effect layer, in particular of the PSG type, is formed during step i). In this case, the heat treatment of step i) is carried out under a POC13 atmosphere and, thus, the formation of the PSG and then the diffusion of the phosphorus zone take place during step i).

[0069] In a conventional manner, before and / or after the surface treatment step, the silicon wafer is cleaned to remove surface contaminants such as particles, native oxides, and organic residues. This may involve chemical baths (e.g., sulfuric acid and hydrogen peroxide, also HF / HCl baths) and rinsing with deionized water. After cleaning, the silicon wafer is dried, often using a nitrogen dryer. Note that if a getter coating has been used, it can be removed, for example, by chemical pickling with a solution containing KOH.

[0070] Method for manufacturing a photovoltaic cell and corresponding photovoltaic cell

[0071] The treatment process according to the invention can be integrated into a manufacturing process for a photovoltaic cell, and in particular for a SHJ (Silicon-to-Joseph-Ion). It occurs before the bonding of the crystalline silicon wafer (Pi) with the other layer(s) of semiconductor material (typically amorphous silicon layers), transparent conductive oxide, or even dielectric material, required. In particular, by intervening before the light soaking step carried out at the end of the manufacturing process of a photovoltaic cell, the treatment process according to the invention prevents a degradation of photovoltaic efficiency during this light exposure.

[0072] The treatment process according to the invention is perfectly suited for obtaining Pi crystalline silicon wafers used in any photovoltaic cell manufacturing process, in which no further treatment takes place at a temperature above 300°C, or even above 250°C, after the Pi crystalline silicon wafer has been integrated into the manufacturing process. This is particularly the case for SHJ manufacturing processes. Indeed, heating to a temperature above 250°C to 300°C, depending on the heating time, could have the effect of reforming the Boron-Oxygen defects that increase the sensitivity of the crystalline silicon wafer to illumination.

[0073] In such processes, the Pi crystalline silicon wafer is associated on each of its faces with layers based on hydrogenated amorphous silicon or hydrogenated nanocrystalline silicon or based on oxide(s) of transition metals.

[0074] Further details on the manufacturing processes for photovoltaic cells that can be used within the scope of the invention can be found in S. Zhaoking et al., 2022.

[0075] Implementation examples

[0076] Example 1 - n-type silicon

[0077] Preliminary results

[0078] Initial experiments showed that phosphorus diffusion or high-speed furnace annealing treatments applied to boron-contaminated n-type crystalline silicon wafers for SHJ cells were insufficient in terms of reducing the boron-phosphorus defect density, as measured by N*. The evolution of charge carrier lifetime over time under LID degradation conditions showed: - For reference 1 (boron-free n-type crystalline silicon Cz), no degradation of the DDV is observed. - For a reference 2 (Cz type n crystalline silicon with presence of boron) without pre-treatment, a strong degradation of the DDV was observed. The boron concentration was approximately 1.5 x 10¹⁶ cm³. The interstitial oxygen concentration was in the range of 6-8 x 10¹⁷ cm³. - For reference 2, subjected to phosphorus diffusion with preservation of the PSG layer, followed by heat treatment at 700°C and 1000°C in a continuous IR furnace, with two feed speeds of 7.2 m / min and 4 m / min, the reduction of N* was significant in both cases with application of the heat treatment. However, the elimination of N* defects was not satisfactory, and contrary to the results of the prior art publications discussed earlier, the reduction of N* was of the same order of magnitude at low and high temperatures (700 / 1000°C) or at medium and high feed speeds. The reduction obtained in N* defects was indeed limited to around 80 ± 5% of the initial defect density, even at the highest feed speed, which a priori corresponds to the highest cooling rate according to DC Walter et al. 2014.

[0079] Implementation of the treatment process according to the invention

[0080] A boron-compensated n-type silicon wafer (boron content: 6 x 10¹⁵ cm³ to 8 x 10⁻³ cm³ and interstitial oxygen content: 5 x 10⁻³ cm³ to 7 x 10⁻³ cm³) was subjected to phosphorus diffusion in a resistive tube furnace, followed by annealing at 1000 °C for 5 s, and then quenching in liquid nitrogen directly from the furnace. The PSG glass formed for phosphorus diffusion had not been removed prior to the annealing and quenching steps. For comparison, cooling by quenching in liquid nitrogen corresponds to a cooling rate of -300°C / s, for 0.65s to go from the temperature of 600°C obtained at the outlet of the furnace to 400°C, whereas a natural cooling of the same type as that described in example 2, from the temperature of 600°C obtained at the outlet of the furnace to 400°C occurs with a cooling rate of -30°C / s and lasts about 6.7s.

[0081] It appears from [Fig.2] that the process according to the invention led to a reduction in the normalised concentration of Boron-Oxygen defect (N*) of 91 ±5%.

[0082] Furthermore, no appearance of various defects was observed due to the presence in the volume of the substrate of electrically active metals (i.e., affecting the lifetime of the carriers), not trapped on the getter layer, which could have been caused by the annealing and ultra-fast cooling steps.

[0083] Example 2 - p-type silicon

[0084] In this example, the wafers used were boron-doped, p-type, Cz single-crystal wafers, in which the quantity of boron atoms was 153 17183 9.1 x 10⁻³ cm³ and the interstitial oxygen quantity of 6 x 10⁻² x 10⁻³ cm³. The resistivity of these platelets was 1.6 ohm·cm, the surface area 125 x 125 mm². The surface area of ​​these The plates were cleaned by chemical polishing, leading to the removal of the work-hardened area.

[0085] Each wafer was cut into 4 smaller wafers (called samples) with a surface area of ​​50x50 mm². Since the crystallographic, electrical, and compositional properties of silicon Cz vary circularly across the surface of a wafer, with this variation centered on the wafer's center, the 4 samples from the same large wafer can be considered identical. All the samples studied were identified by laser engraving.

[0086] Half of the samples underwent a step of forming a PSG (phosphosilicate) layer on the silicon surface in a tube furnace, in the presence of POC13, which, after annealing at a temperature of 1000°C, generated a phosphorus-diffused layer. The PSG glass was then removed by chemical treatment (with an HF solution). Subsequently, all samples underwent PECVD deposition of silicon nitride layers (barrier layer) to electrically passivate the surfaces.

[0087] Next, the four samples were subjected to the following steps: - a sample was used as a reference (no annealing at 750°C) - a sample was annealed at 750°C under air for 10 minutes (heat treatment of step i)), the oven door was then opened and the sample was removed from the oven and placed on a plate allowing the sample to cool "at room temperature" (this condition was described as "slow" cooling compared to the other cooling conditions studied). - a sample was annealed at 750°C under air for 10 minutes (heat treatment of step i)), the oven door was then opened and the sample was plunged into an ice water bath. - a sample was annealed at 750°C under air for 10 minutes (heat treatment of step i)), the oven door was then opened and the sample was immersed in a bath of liquid nitrogen, for the implementation of the treatment process according to the invention of example 1.

[0088] First, it should be noted that immersion in the ice water bath did not produce usable wafers. This step shattered the wafers, most likely due to the very abrupt temperature change, far greater than that resulting from quenching in liquid nitrogen. In contrast, immersion in liquid nitrogen did not cause the wafers to break, most likely due to the presence of nitrogen gas at the interface with the liquid nitrogen, leading to a cooling rate according to the invention.

[0089] Figures 3 and 4 highlight the effect of cooling, carried out after annealing at 750°C for 10 minutes, on the effective lifetime (r eff) of the carriers of charge, respectively for p-type silicon wafers that underwent the phosphorus diffusion step and for wafers that did not. It is clear that immersion in liquid nitrogen consistently and significantly increases the charge carrier resistance. Thus, the passivation properties of the silicon nitride layers do not appear to be altered by this step, and annealing in air did not affect the volumetric properties of the wafers either.

[0090] Figure 5 shows the effects of annealing and cooling on the N* values. It appears that annealing followed by quenching in liquid nitrogen eliminated (or at least significantly reduced) the effects of LID. For some wafers, an increase in the effective resistance was even observed under illumination. Annealing followed by "slow" cooling also gives positive results. It should be noted, however, that this type of cooling is still rapid compared to that obtained, for example, at the outlet of a traditional tube furnace, since in the case of the slow cooling used here for comparison, the wafers were quickly removed from the furnace using tongs and placed on a silica support outside the furnace, which is not typical. Quenching in liquid nitrogen according to the invention gives, by far, the best results. References

[0091] L.J. Caballero et al., Influence of Pgettering thermal step on light-induced dégradation in Cz Si, Solar Energy Materials & Solar Cells 88 (2005) 247-256.

[0092] K. Bothe et al., Effective réduction of the metastable defect concentration in boron-doped Czochralski Silicon for solar cells, in: Proceedings of Conf. Rec. Twenty-Ninth IEEE Photovolt. Spec. Conférence, IEEE, New Orléans, (2003) 194-197, https: / / dx.doi.org / 10.1109 / PVSC.2002.1190489.

[0093] S. W. Glunz et al., Minority carrier lifetime dégradation in boron-doped Czochralski Silicon, J. Appl. Phys., Vol. 90, No. 5, 1 September (2001).

[0094] S. W. Glunz et al., Dégradation of carrier lifetime in Cz Silicon solar cells, Sol. Energy Mater. Sol. Cells 65 (2001) 219-229.

[0095] N. Nampalli et al., Multiple pathways for permanent deactivation of boron-oxygen defects in p-type Silicon, Solar Energy Materials and Solar Cells 173 (2017) 12-17.

[0096] F.E. Rougieux et al., Influence of net doping, excess carrier density and annealing on the boron oxygen related defect density in compensated n-type Silicon, J. Appl. Phys., Vol. 110, 063708 (2011).

[0097] H. Wagner et al., Optimizing phosphorus diffusion for photovoltaic applications: Peak doping, inactive phosphorus, gettering, and contact formation, J. Appl. Phys. 119, 185704 (2016), https: / / doi.Org / 10.1063 / l.4949326.

[0098] D.C. Walter et al., Effect of rapid thermal annealing on recombination centers in boron-doped Czochralski-grown Silicon, Appl. Phys. Lett. 104 (2014) 42111, https: / / dx.doi.org / 10.1063 / 1.4863674.

[0099] Z.Y. Yeo et al., Status review and future perspectives on mitigating light-induced dégradation on silicon-based solar cells, Solar Energy Materials & Solar Cells 131 (2014)51-57.

[0100] S. Zhaoking et al., Toward Efficiency Limits of Crystalline Silicon Solar Cells: Recent Progress in High-Efficiency Silicon Heterojunction Solar Cells, Advanced Energy Materials, vol. 12(23), 2200015, (2022), https: / / onlinelibrary.wiley.com / doi / pdfdirect / 10.1002 / aenm.202200015.

[0101] Les défis du CEA, février 2019, N°234.

Claims

Demands

1. A process for treating a crystalline silicon wafer comprising boron and interstitial oxygen forming defects that increase the sensitivity of the crystalline silicon wafer to illumination, called Boron-Oxygen defects, said processing leading to a reduction in the density of said Boron-Oxygen defects, which comprises a step (i) of heating the crystalline silicon wafer to a temperature in the range of 650°C to 1200°C, characterized in that step (i) is followed by a step (ii) of ultra-rapid cooling which starts while the crystalline silicon wafer is still at a temperature of at least 600°C, and, during which, the temperature of the crystalline silicon wafer is lowered at a cooling rate in the range of 150 to 2000 °C / s, until reaching a temperature of 400°C or less,preferably until a temperature of 300°C or less is reached.

2. Processing method according to claim 1, characterized in that it comprises upstream of step i), a pre-preparation of the surface of the crystalline silicon wafer allowing to reduce the risks of contamination of the wafer by impurities, during step i) and / or ii).

3. Processing method according to claim 1 or 2, characterized in that it comprises upstream of step i), a pre-preparation of the surface of the crystalline silicon wafer, said preparation comprising or consisting of a step of texturing the surface of the crystalline silicon wafer and / or a step of removing a work-hardened area present on the surface of the crystalline silicon wafer and / or a step of forming on the surface of the crystalline silicon wafer a layer acting as a barrier layer or a layer developing a getter effect.

4. A processing method according to claim 1 or 2, characterized in that it comprises, upstream of step i), a preliminary preparation of the surface of the crystalline silicon wafer, said preparation comprising or consisting of a step of texturing the surface of the crystalline silicon wafer and / or a step of removing a work-hardened area present on the surface of the crystalline silicon wafer and, during step i), the surface forming of the silicon wafer crystalline structure of a layer acting as a barrier layer or a layer developing a getter effect is created.

5. A treatment method according to any one of the preceding claims characterized in that steps i) and ii) are carried out under the following temperature conditions: - during step i), the heating of the crystalline silicon wafer is carried out at a temperature in the range of 700°C to 1050°C, - during step ii), ultra-rapid cooling starts while the crystalline silicon wafer is still at a temperature of at least 600°C and the crystalline silicon wafer is lowered at a cooling rate in the range of 150 to 2000°C / s, until a temperature of 400°C or less is reached, preferably until a temperature of 300°C or less is reached.

6. A treatment method according to any one of claims 1 to 4, characterized in that during step i) the crystalline silicon wafer is heated to a temperature in the range of 650°C to 1200°C, preferably in the range of 700°C to 1050°C, and typically about 1000°C, for a period of 0.5 s to 120 min, preferably for a period of 1 s to 30 min.

7. A treatment method according to any one of the preceding claims, characterized in that during step ii), ultra-rapid cooling is achieved by immersion of the crystalline silicon wafer in liquid nitrogen.

8. A treatment method according to any one of the preceding claims, characterized in that the crystalline silicon is n-type silicon or p-type silicon, in particular boron-compensated n-type silicon.

9. Processing method according to any one of the preceding claims, characterized in that the crystalline silicon is Cz silicon, in particular n-type Cz silicon, especially boron-compensated n-type Cz silicon or n-type Cz silicon obtained from recycled or solar-grade silicon.

10. A treatment method according to any one of the preceding claims, characterized in that the crystalline silicon of the wafer crystalline silicon contains from 5.1013 to 5.1017 atoms / cm3 of boron, preferably from 1014 to 1017 atoms / cm3 of boron, and even more preferably from 1015 to 5.1016 atoms / cm3 of boron and from 8.1016 to 2.2.1018 atoms / cm3 of interstitial oxygen, preferably from 2.1017 to 2.2.1018 atoms / cm3 of interstitial oxygen, and even more preferably from 2.1017 to 7.1017 atoms / cm3 of interstitial oxygen.

11. A treatment method according to any one of the preceding claims, characterized in that it leads to a reduction of at least 85%, and preferably to a reduction of 90% or more, in the density of "Boron-Oxygen" defects.

12. Pt crystalline silicon wafer obtained by the process according to any one of claims 1 to 11.

13. A method for manufacturing a photovoltaic cell, and in particular a heterojunction photovoltaic cell, comprising the use of a Pt crystalline silicon wafer as defined in claim 12 or comprising a step consisting of carrying out the method according to any one of claims 1 to 11 leading to a Pt crystalline silicon wafer.

14. A manufacturing method according to claim 13, characterized in that it comprises a step of associating the crystalline silicon Pt wafer with at least one semiconducting layer.

15. A manufacturing method according to claim 13 or 14, characterized in that it does not include any heating step to a heating temperature above 300°C carried out on the Pt crystalline silicon wafer and, preferably, it does not include any heating step carried out on the Pt crystalline silicon wafer to a heating temperature above 250°C.

16. Photovoltaic cell made from a Pt crystalline silicon wafer as defined in claim 12 or obtained by the process defined in any one of claims 13 to 15.

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