METHOD FOR DETERMINING A HEATING TEMPERATURE OF A HETERO-TRAINS PHOTOVOLTAIC CELL IN A PROCESSING PROCESS

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for treating a stack obtained during the manufacture of a heterojunction photovoltaic cell, in order to improve and stabilize the efficiency of the photovoltaic cell. STATE OF THE ART

[0002] There figure 1 represents an example of a silicon heterojunction (SHJ) photovoltaic cell 10. The photovoltaic cell 10 comprises a doped crystalline silicon substrate 11 and two amorphous silicon layers 12-13 arranged on either side of the substrate 11. One of the amorphous silicon layers 12-13 is doped with the same conductivity type as the substrate 11, for example n-type, and the other layer is doped with the opposite conductivity type, i.e. p-type.

[0003] The heterojunction is formed by the n-doped crystalline silicon substrate 11 and the p-doped amorphous silicon layer, this layer forming the emitter of the photovoltaic cell. The emitter can be located on the front or back face of the photovoltaic cell.

[0004] The SHJ photovoltaic cell is particularly sensitive to defects located at the interface between the crystalline silicon substrate 11 and the amorphous silicon layers 12-13. These defects can be dangling bonds or impurities such as metal ions. They introduce energy levels into the silicon band gap and increase the number of electron-hole recombinations at the interfaces, which deteriorates the output parameters of the photovoltaic cell, such as the open-circuit voltage Voc.

[0005] To obtain a high-performance photovoltaic cell, it is therefore necessary to minimize the number of recombinations on the surface of the substrate 11, which is generally accomplished by depositing a passivation layer 14 of intrinsic hydrogenated amorphous silicon on each of the faces of the substrate 11, before the amorphous silicon layer 12, 13. The hydrogen atoms contained in the passivation layers 14 diffuse to the surface of the substrate 11 and neutralize the defects.

[0006] Each of the amorphous silicon layers 12-13 is further covered with a layer of transparent conductive oxide (or TCO, for “Transparent Conductive Oxide” in English) 15.

[0007] SHJ photovoltaic cells are known to improve their energy conversion efficiency by approximately 0.3% absolute under the combined action of illumination and temperature. This phenomenon, known as cell enhancement, results from the improvement of at least one of the hydrogenated amorphous silicon passivation layers 14, as well as the improvement of the interfaces between the amorphous silicon layers 12-13 and the TCO layers 15.

[0008] The observation of the improvement phenomenon of SHJ photovoltaic cells has led to the development of treatment processes including a step of exposing the photovoltaic cells to a light flux. These processes are commonly called light-soaking treatment processes.

[0009] Document WO2013 / 001440 describes an example of a method for treating an SHJ photovoltaic cell comprising an n-doped crystalline silicon substrate. In this treatment method, the photovoltaic cell is subjected to a luminous flux with an irradiance greater than or equal to 500 W / m 2 < for a duration of approximately 10 hours, while being heated to a temperature between 20 °C and 200 °C. Such a treatment duration is incompatible with the rate of current photovoltaic cell production lines.

[0010] Documents CN 107 546 296 A and WO 2020 / 082131 A1 also each describe a method for treating a stack of photovoltaic cell layers, said method comprising a step of exposing one face of the stack to electromagnetic radiation emitted by a radiation source, in order to improve and stabilize the efficiency of the photovoltaic cell.

[0011] It is also known that the amorphous silicon of SHJ photovoltaic cells degrades beyond a limit temperature, which can be between 190 °C and 320 °C (depending on the deposition method of the amorphous silicon in particular). Such a temperature limit can be restrictive when the irradiance of the luminous flux is high. Indeed, the luminous flux causes the SHJ photovoltaic cells to heat up to a temperature that is all the higher as the irradiance of the flux is high. However, it is difficult to estimate this heating temperature quickly and accurately. The design of a treatment process that does not risk degrading the amorphous silicon of SHJ photovoltaic cells is therefore long and complex. SUMMARY OF THE INVENTION

[0012] There is therefore a need to easily and quickly determine a heating temperature of a stack obtained during the manufacture of a heterojunction photovoltaic cell, the heating temperature being reached during a stack treatment process. The stack comprises a crystalline silicon substrate and a hydrogenated amorphous silicon passivation layer disposed on the substrate.

[0013] The treatment method comprises a step of exposing a face of the stack to electromagnetic radiation, the electromagnetic radiation being emitted by a radiation source and applied to the face of the stack along a line, the stack and the radiation source being animated by a relative translational movement such that the line scans at least part of the face of the stack.

[0014] This need is tended to be met by providing a method for determining the heating temperature (T) comprising the following steps: acquiring a plurality of parameters of the treatment process, the plurality of parameters comprising an irradiance of the electromagnetic radiation, a width of the line and a relative displacement speed between the stack and the radiation source; acquiring a thickness (e) of the stack; calculating the heating temperature of the stack using the following relationship: Vρc p ΔT c T L c 1 L = − k ΔT c T L c 2 L 2 + ϕ source E e + ϕ ext in which: V is the relative displacement speed between the stack and the radiation source; ΔT c is a function of the heating temperature (T) and represents a temperature variation characteristic of the heating in a direction perpendicular to the line; L c1 is a function of the width of the line (L) and represents a first characteristic length of the heat convection during the relative displacement between the stack and the radiation source; L c2 is a function of the width of the line and represents a second characteristic length of the heat diffusion in a direction perpendicular to the line; Φ source is a function of the irradiance (E) of the electromagnetic radiation and the thickness (e) of the stack and represents the quantity of heat per unit volume supplied by the radiation source;Φ ext represents the quantity of heat per unit volume exchanged with an external environment, in which the stack is placed during the treatment process; ρ, cp and k are respectively the density, the mass capacity and the thermal conductivity of the stack.

[0015] Using the analytical model described above, the heating temperature of the stack can be determined quickly and easily. The determination method can be easily implemented within a heterojunction photovoltaic cell manufacturing line.

[0016] In a preferred embodiment of the determination method, the plurality of parameters further comprises temperatures relative to the external environment and a convection exchange coefficient, the quantity of heat per unit of volume exchanged with the external environment being a function of said temperatures and said coefficient.

[0017] In one implementation variant, the amount of heat per unit volume exchanged with the external environment is considered to be zero.

[0018] A first aspect of the invention relates to a method for parameterizing the stack treatment method, the parameterizing method comprising determining a plurality of parameters of the treatment method such that the heating temperature of the stack reached during the treatment method is less than or equal to a threshold temperature, the plurality of parameters comprising an irradiance of the electromagnetic radiation, a width of the line and a relative displacement speed between the stack and the radiation source and being determined using the following relationship: Vρc p ΔT c T L c 1 L = − k ΔT c T L c 2 L 2 + ϕ source E e + ϕ ext

[0019] In a first mode of implementation, the parameterization process comprises the following steps: a) acquiring a value for each of the treatment process parameters; b) acquiring a stack thickness value; c) calculating a stack heating temperature using said relationship; d) comparing the calculated heating temperature to a threshold temperature; and as long as the calculated heating temperature is strictly higher than the threshold temperature, the following steps: e) choosing an adjustment parameter from among the treatment process parameters; f) modifying the value of the adjustment parameter; and g) repeating steps c) and d).

[0020] In a second mode of implementation, the parameterization process comprises the following steps: acquiring a value for each of the parameters of the treatment process; acquiring a value of the thickness of the stack; collecting a change in the value of at least one parameter of the treatment process and / or of the thickness of the stack; selecting an adjustment parameter from among the other parameters of the treatment process; determining a value of the adjustment parameter such that the heating temperature of the stack during the treatment process is less than or equal to the threshold temperature, the value of the adjustment parameter being determined using said relationship.

[0021] Advantageously, the plurality of parameters further comprises a plurality of temperatures relating to the external environment and a convection exchange coefficient and the adjustment parameter is chosen from said temperatures and said coefficient.

[0022] Alternatively, the adjustment parameter is chosen from the irradiance of the electromagnetic radiation, the line width and the relative displacement speed between the stack and the radiation source.

[0023] A second aspect of the invention relates to a method for scanning processing of a stack comprising a crystalline silicon substrate and a hydrogenated amorphous silicon passivation layer arranged on the substrate, the processing method comprising a step of exposing a face of the stack to electromagnetic radiation emitted by a radiation source, the electromagnetic radiation being applied to the face of the stack along a line, the stack and the radiation source being driven by a relative translational movement such that the line scans at least part of the face of the stack, the processing method further comprising a prior step of determining a plurality of parameters to be applied during the exposure step, the plurality of parameters comprising an irradiance of the electromagnetic radiation, a width of the line and a speed of relative movement between the stack and the radiation source,the plurality of parameters being determined so that the heating temperature of the stack reached during the exposure step is less than or equal to a threshold temperature using the following relationship: , Vρc p ΔT c T L c 1 L = − k ΔT c T L c 2 L 2 + ϕ source E e + ϕ ext

[0024] Preferably, the threshold temperature is less than or equal to a degradation temperature of amorphous silicon.

[0025] A third aspect of the invention relates to a data processing device comprising means for implementing a determination method according to the first aspect of the invention or a parameterization method according to the second aspect of the invention.

[0026] A fourth aspect of the invention relates to a computer program product comprising instructions which, when the program is executed by a computer, cause the latter to implement a determination method according to the first aspect of the invention or a parameterization method according to the second aspect of the invention.

[0027] A fifth aspect of the invention relates to a computer-readable data carrier, on which the computer program product according to the fifth aspect of the invention is recorded. BRIEF DESCRIPTION OF THE FIGURES

[0028] Other characteristics and advantages of the invention will emerge clearly from the description given below, for information purposes only and in no way limiting, with reference to the following figures. [ Fig. 1 ] is a schematic perspective view of an example of a heterojunction photovoltaic cell. [ Fig. 2] schematically represents a method of processing a stack by scanning an electromagnetic radiation line according to one aspect of the invention. [ Fig. 3 ] represents a preferred mode of implementation of the scanning treatment method. [ Fig. 4 ] shows calculation results of the maximum temperature reached in the thickness of a stack for each point on a face of the stack, when this face is entirely scanned by electromagnetic radiation. [ Fig. 5 ] represents steps of a method for determining a heating temperature of the stack reached during the treatment method, according to another aspect of the invention. [ Fig. 6 ] represents, as a function of the scanning speed of the electromagnetic radiation line, calculation results obtained by the determination method according to the Figure 5 and comparative results obtained by thermal simulation of the treatment process. [ Fig. 7] represents, depending on the thickness of the stack, other calculation results obtained by the determination method according to the Figure 5 and comparative results obtained by thermal simulation of the treatment process. [ Fig. 8 ] schematically represents a first mode of implementation of a method for parameterizing the scanning processing method, according to another aspect of the invention. [ Fig. 9 ] schematically represents a second mode of implementation of the method for parameterizing the scanning treatment method.

[0029] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED DESCRIPTION

[0030] There figure 2schematically represents a method for treating a stack 10' obtained during the manufacture of a heterojunction photovoltaic cell. This treatment method aims to improve the efficiency of the heterojunction photovoltaic cell. The stack 10' comprises at least one substrate 11 made of crystalline silicon (c-Si) and a first passivation layer 14 arranged on a first face 11a of the substrate 11. The stack 10' may be a precursor of a heterojunction photovoltaic cell, i.e. an intermediate product obtained during the manufacture of a heterojunction photovoltaic cell, or a finalized heterojunction photovoltaic cell (i.e. ready to be interconnected with other heterojunction photovoltaic cells).

[0031] The crystalline silicon (c-Si) of the substrate 11 is preferably n-type doped. Only donor-type doping impurities (for example phosphorus atoms) have been deliberately introduced into the crystalline silicon in order to modify its electrical conductivity. In addition to donor-type doping impurities, the substrate 11 may contain, unintentionally (and therefore in trace amounts), acceptor-type doping impurities (such as boron or gallium atoms). The concentration of acceptor-type doping impurities NA of the substrate 11 is advantageously less than 10 13< cm -3< , preferably less than 3.10 12< cm -3< . The concentration of donor-type doping impurities ND is preferably greater than 50 times the concentration of acceptor-type doping impurities NA (or ND / NA >50).

[0032] The first passivation layer 14 is made of hydrogenated amorphous silicon (a-Si:H). The hydrogenated amorphous silicon of the first passivation layer 14 is preferably intrinsic, i.e. not intentionally doped. Intrinsic hydrogenated amorphous silicon in fact allows better chemical passivation of crystalline silicon surfaces than doped hydrogenated amorphous silicon.

[0033] In reference to the figure 2 , the treatment method comprises a step of exposing one face of the stack 10' to electromagnetic radiation 20 emitted by a radiation source 40. The electromagnetic radiation 20 is preferably directed perpendicular to the surface of the stack 10'. The exposure to the electromagnetic radiation 20 can take place when the stack 10' is in an enclosure (not shown in the figure 2 ).

[0034] Exposure of the stack 10' to the electromagnetic radiation 20 has the effect of reducing the number of interface states between the crystalline substrate 11 and the first passivation layer 14. In other words, the number of neutralized interface defects increases and the quality of the passivation improves. This results in an increase in the open circuit voltage Voc and the form factor (FF) of the photovoltaic cell, and consequently an increase in its conversion efficiency.

[0035] The electromagnetic radiation 20 has an irradiance E, also called energy illumination or surface density of luminous power, which is advantageously greater than or equal to 10 3< W / m 2< , preferably between 10 5< W / m 2< and 5.10 6< W / m 2< . The irradiance E represents the power of the electromagnetic radiation received by a unit of surface, this unit of surface being oriented perpendicular to the direction of the electromagnetic radiation 20.

[0036] Such irradiance makes it possible to reduce the duration of exposure to electromagnetic radiation 20, also called treatment time t, for the same gain in efficiency (the kinetics of the gain in efficiency increases with the irradiance of the radiation).

[0037] In order to improve the conversion efficiency of the photovoltaic cell, it is not obligatory to expose more than one face of the stack 10' to the electromagnetic radiation 20. Preferably, all the exposed regions of the face of the stack 10' receive the electromagnetic radiation 20 for the same treatment duration t.

[0038] The radiation source 40 may be a laser system comprising, for example, one or more laser matrices, a set of light-emitting diodes or any other device capable of emitting electromagnetic radiation with an irradiance greater than 10 3< W / m 2< .

[0039] The electromagnetic radiation 20 may be monochromatic, i.e. have only one wavelength, or polychromatic, i.e. have several (monochromatic) components of different wavelengths. Preferably, the electromagnetic radiation 20 has at least one wavelength between 300 nm and 1100 nm, preferably between 400 nm and 1100 nm (in this range, the radiation is absorbed at least in part in the crystalline silicon substrate), and advantageously between 800 nm and 1000 nm.

[0040] The wavelength range [800 nm-1000 nm] represents an excellent compromise between the photon flux arriving on the 10' stack and the probability that these photons are absorbed by the substrate. Indeed, the longer the wavelength, the less energetic the photons and the greater the photon flux received by the stack at a given irradiance. However, the probability of photon absorption decreases sharply when the wavelength becomes greater than 1000 nm. Another advantage of favoring long wavelengths (800-1000 nm) is that the more energetic photons (< 800 nm) transfer part of their energy in the form of heat into the stack, by phonon emission. However, the resulting increase in temperature is not desirable given the risk of degradation of amorphous silicon.

[0041] During the exposure step, also called the irradiation step, the stack 10' and the radiation source 40 are animated by a relative translational movement (in other words, one moves relative to the other). For example, the stack 10' can be placed on a support 30 movable in translation and the source 40 can be fixed. Conversely, the support 30 of the stack 10' can be fixed and the source 40 movable. The stack 10' and the source 40 can also both be movable, but not in the same direction and at the same speed.

[0042] Such a translational movement makes it possible to irradiate a large part of the face of the stack 10' in the case of a radiation source 40 having an irradiation surface area smaller than the surface area of ​​the face of the stack 10' (in particular the case of a laser system). It also makes it possible to irradiate several stacks 10' successively, these stacks being for example arranged on the same support 30. In this case, it may be considered that the stacks 10' pass in front of the radiation source 40.

[0043] The support 30 may in particular be a substrate holder or a conveyor capable of transporting several 10' stacks simultaneously.

[0044] The support 30 may further be coupled to a cooling system (not shown), in order to reduce the temperature of the stack 10' during the exposure step. The support 30 is for example cooled by circulating a heat transfer fluid or using Peltier elements.

[0045] In addition to the substrate 11 and the first passivation layer 14, the stack 10' may comprise a second passivation layer (not shown in the figure 2 ) arranged on a second face 11b of the substrate 11, opposite the first face 11a. The second passivation layer may be made of intrinsic hydrogenated amorphous silicon, silicon nitride, aluminum oxide, titanium oxide or silicon oxide. Like the first passivation layer 14, the second passivation layer contributes to reducing the number of recombinations of the electron-hole pairs on the surface of the substrate 11, thus increasing the injection level into the stack 10' during the irradiation step (and subsequent operation of the photovoltaic cell).

[0046] The stack 10' may also comprise (in the presence or absence of the second passivation layer) a first layer of doped amorphous silicon arranged on the first passivation layer 14, and advantageously, a layer of transparent conductive oxide arranged on the first layer of doped amorphous silicon. The silicon of the first layer of amorphous silicon is doped with a first conductivity type, which may be identical to that of the substrate 11 (for example n-type in the case of a substrate 11 made of n-type doped silicon) or on the contrary opposite to that of the substrate 11 (e.g. p-type). Exposure to the electromagnetic radiation 20 then has the additional effect of improving the quality of the interface between the first passivation layer and the first layer of doped amorphous silicon and / or the interface between the first layer of doped amorphous silicon and the layer of transparent conductive oxide.

[0047] The stack 10' may also comprise a second layer of doped amorphous silicon disposed on the second passivation layer. The silicon of the second layer of amorphous silicon is doped with a second conductivity type opposite to the first conductivity type. Exposure to electromagnetic radiation 20 then has the additional effect of improving the quality of the interface between the second passivation layer and the second layer of doped amorphous silicon.

[0048] In one embodiment of the treatment method, the stack 10' is of the type shown in the figure 1 and includes: a substrate 11 made of crystalline silicon, preferably n-doped; a first passivation layer 14 made of hydrogenated (and preferably intrinsic) amorphous silicon arranged on the first face 11a of the substrate 11; a first layer of amorphous silicon 12, arranged on the first passivation layer 14 and doped with a first type of conductivity; a first layer of transparent conductive oxide 15 arranged on the first doped layer of amorphous silicon 12; a second passivation layer 14 made of hydrogenated (and preferably intrinsic) amorphous silicon arranged on the second face 11b of the substrate 11; a second layer of amorphous silicon 13, arranged on the second passivation layer 14 and doped with a second type of conductivity opposite to the first type of conductivity; and a second layer of transparent conductive oxide 15 arranged on the second doped layer of amorphous silicon 13.

[0049] Such a heterojunction photovoltaic cell stack or precursor is described as asymmetrical due to the two layers of amorphous silicon 12-13 doped with opposite conductivity types and arranged on either side of the substrate 11. The amorphous silicon of the first doped layer 12 and / or of the second doped layer 13 is preferably hydrogenated.

[0050] When a stack of the type of the figure 1 is exposed to electromagnetic radiation 20, a decrease in the series resistance of the photovoltaic cell is further observed.

[0051] Whatever the stack used, the first face 11a of the substrate 11 (covered with the first passivation layer 14) can be turned towards the radiation source 40, as illustrated by the figure 2 Alternatively, the second face 11b of the substrate 11 can be turned towards the radiation source 40.

[0052] Thus, the scanning processing method can be performed at different stages of the manufacture of a heterojunction photovoltaic cell. This photovoltaic cell can be a monofacial or bifacial cell. In a monofacial cell, only the front face captures the solar radiation. In a bifacial cell, the front and rear faces each capture a portion of the solar radiation. The front face captures the incident (i.e., direct) radiation while the rear face captures the diffuse or reflected radiation. The front face of a bifacial cell is the one that allows the maximum electric current to be obtained when it is turned towards the incident radiation. The emitter of the photovoltaic cell, i.e., the layer of amorphous silicon doped with the conductivity type opposite to that of the substrate 11, can be located on the front face or the rear face of the cell.

[0053] There figure 3is a top view of the 10' stack, taken during the exposure stage.

[0054] The exposed face of the 10' stack includes: a first edge 100a called the leading edge, defined as being the edge to pass first in front of the radiation source 40; a second edge 100b called the trailing edge, opposite the first edge 100a and defined as being the edge to pass last in front of the radiation source 40; and third and fourth edges 100c-100d called transverse (lower and upper edges in the figure) which connect the first and second edges 100a-100b.

[0055] The exposed face of the stack 10' preferably has a square, pseudo-square (truncated angles) or rectangular shape. The leading edge 100a and the trailing edge 100b are thus parallel to each other, as are the transverse edges 100c-100d.

[0056] The radiation source 40 is configured so that the electromagnetic radiation 20 is applied to the face of the stack 10' along a line 50. In other words, the surface of the stack 10' irradiated by the radiation source 40 (called the irradiation surface) at a given time in the treatment process is in the form of a line. The line 50 has a width L which can be between 10 -4< m and 10 -1< m, preferably between 10 -3< m and 10 -2< m.

[0057] The direction D of the relative movement between the stack 10' and the radiation source 40 is such that the line 50 scans at least part of the face of the stack 10' during the treatment process, starting with the leading edge 100a then moving towards the trailing edge 100b. The part exposed to the electromagnetic radiation advantageously occupies more than 60% of the face of the stack 10', and preferably more than 80%.

[0058] The direction of movement D of the radiation source 40 relative to the stack 10', also called the scanning direction D, is for example perpendicular to the leading edge 100a and to the trailing edge 100b, as shown in the figure 3 .

[0059] The line 50 preferably comprises a straight line segment oriented perpendicular to a median M of the exposed face of the stack 10'. The median M is the straight line segment which connects the midpoints of the leading edge 100a and the trailing edge 100b. It is preferably perpendicular to the leading edge 100a and the trailing edge 100b. The radiation source 40 is advantageously configured so that the line 50 irradiates the entire width of the stack 10'.

[0060] Line 50 is thus the intersection of a beam of electromagnetic waves generated by the radiation source 40 with the surface of the stack 10', the rays of the beam being contained in a plane.

[0061] The speed V of the relative movement between the stack 10' and the radiation source 40 is preferably chosen so as to treat a stack 10' by scanning the electromagnetic radiation in a short time t, typically between 0.1 s and 30 s. The movement speed V is advantageously between 0.01 m / s and 1 m / s, and preferably greater than or equal to 0.02 m / s.

[0062] This scanning treatment is simple and quick to implement. By continuously scrolling the 10' stacks (photovoltaic cell precursors or photovoltaic cells) in front of the radiation source 40, the treatment process can be accomplished at a high rate, compatible with the industrial production requirements of photovoltaic cells.

[0063] For example, considering a displacement speed V between 0.01 m / s and 1 m / s and 10' stacks whose main faces measure 156 mm by 156 mm, the processing time t of a 10' stack is between 0.156 s and 15.6 s. The number of 10' stacks processed per hour can be between 200 and 25,000.

[0064] Compared to (simultaneous) irradiation of the entire face of the stack, scanning has the advantage of limiting the heating of the stack beyond a silicon degradation temperature (by suitably choosing the irradiance E and the displacement speed V), and under certain conditions of eliminating it completely. Thus, the degradation of the amorphous silicon of the first passivation layer 14 is avoided or limited to a small so-called overheated portion of the stack. It is then not obligatory to cool the stack 10', for example by means of a cooling system or during an interruption of the exposure. Preferably, the overheated portion occupies less than 40% of the exposed face of the stack 10', and preferably less than 20%.

[0065] The degradation temperature of amorphous silicon, hereinafter called the limit (or critical) temperature T max , can be between 190 °C and 320 °C, depending on the method of deposition of the amorphous silicon. For example, it is substantially equal to 200 °C. By "substantially equal to 200 °C" is meant 200 °C to within 5% (200 °C ± 5%).

[0066] The scanning treatment method, as described in connection with the figures 2 And 3 , can be characterized by the following parameters: E: the irradiance of the electromagnetic radiation 20 (in W / m 2< ); V: the relative displacement speed between the stack 10' and the radiation source 40, also called the scanning speed of the electromagnetic radiation (in m / s); L: the width of the line 50 (in m).

[0067] The surface energy density Q (in J / m 2< ), or surface energy, received at any point on the face of the stack by electromagnetic radiation is defined by the following relationship: Q = E ∗ L / V

[0068] The parameters E, V and L are preferably chosen to obtain a surface energy density Q of between 10 3< J / m 2< and 10 6< J / m 2< , advantageously between 10 4< J / m 2< and 10 5< J / m 2< . The ranges of values ​​given above for the irradiance E of the electromagnetic radiation 20, the scanning speed V and the width L of the line 50 are sufficiently wide to achieve this.

[0069] Thermal simulations have shown that the temperatures at different points of the stack during the exposure step increase with the surface energy density Q. Furthermore, for a given combination of the parameters E, V and L, other thermal simulations have shown that these temperatures depend on the thickness e of the stack. The lower the thickness e of the stack, the higher the temperatures, because the absorbed energy volume density increases. All the simulation results presented below were obtained with a stack thickness e equal to 160 µm.

[0070] There figure 4 is a map of the maximum temperature reached in the thickness of a 10' stack (whose main faces are pseudo-square: 156 mm x 156 mm) during a treatment process according to the figure 3. It was obtained by means of a thermal simulation (by finite volumes) of the scanning treatment process. In this simulation, line 50 scans the entire surface of the stack face. The irradiance E of the electromagnetic radiation is equal to 216 kW / m 2< , the scanning speed V is equal to 2.48 cm / s and the width of line 50 is equal to 6 mm. The isothermal curve T = 200 °C is shown in dotted lines.

[0071] This mapping shows that the maximum temperature of the stack during the treatment is lowest near the leading edge 100a (first exposed to the electromagnetic radiation 20), that it increases progressively towards the trailing edge 100b to reach (and maintain) a so-called plateau temperature of the order of 195°C, then increases further to exceed 200°C in a portion 61 adjacent to the trailing edge 100b of the stack (i.e. the edge last exposed to the electromagnetic radiation 20). Outside this overheated portion 61 (the limit temperature T max considered here is equal to 200°C), the maximum temperature of the stack 10' is therefore less than 200°C.

[0072] The local overheating near the trailing edge 100b is explained by the formation of a thermal boundary layer at the front of the bundle. This thermal boundary layer is linked to heat diffusion and propagates in the stack from the leading edge 100a to the trailing edge 100b. As the trailing edge 100b approaches, the heat can no longer diffuse towards the front of the bundle, which causes overheating.

[0073] In order to limit the degradation of the amorphous silicon and the resulting reduction in the performance of the photovoltaic cell, the exposure of the face of the stack 10 to the electromagnetic radiation may also be interrupted before the line 50 reaches the trailing edge 100b of the face of the stack (the overheated portion is then reduced or avoided). The face of the stack 10' may cease to be irradiated upon approaching the trailing edge 100b either by stopping the radiation source 40 (and therefore the emission of the electromagnetic radiation 20), or by blocking the continuously emitted electromagnetic radiation 20 (i.e., by preventing it from reaching the stack), for example by means of a shutter.

[0074] There figure 4also shows that the temperature of the stack is equal to the plateau temperature in a major part of the stack, this part being called “central” because it is arranged between the leading edge 100a and the trailing edge 100b and containing the point located in the center of the exposed face, at (x,y)=(0,0). Hereinafter, the “heating temperature T” refers to the plateau temperature reached by the stack in this central part (in particular in the center) during the treatment process.

[0075] Developing a scanning treatment process, for example in a heterojunction photovoltaic cell manufacturing line, requires determining a value for each of the E, V and L parameters of the treatment process. Determining these parameters can be tricky because the operating range that allows both the desired efficiency gain and an acceptable temperature to be achieved is restricted.

[0076] In order to ensure (for example when designing the sweep treatment process) that the heating temperature T (or plateau temperature) will not exceed the limit temperature T max , a method for determining the heating temperature T from the treatment process parameters has been developed.

[0077] There Figure 5 represents steps S11 to S13 of this method for determining the heating temperature T. These steps are preferably coded in the form of a program and implemented by a computer (by executing the program).

[0078] The determination method firstly comprises a step S11 of acquiring the parameters of the treatment method and a step S12 of acquiring a parameter of the stack 10' (or stacks) to be treated, its thickness e. By "acquisition" of a parameter, we mean the measurement (by means of a sensor connected to the computer), the entry (by an operator, typically by means of a keyboard) or the retrieval (typically in a memory) of a value of this parameter. For example, the thickness e of the stack 10' can be measured by a sensor of the non-contact gauge type ("Contactless Wafer Thickness Gauge") or determined via the measurement of the mass of the stack 10' (the density and the other dimensions of the stack 10' being known).

[0079] The parameters of the processing method acquired in step S11 include at least the irradiance E of the electromagnetic radiation 20, the scanning speed V and the width L of the line 50. The values ​​of the parameters E, V, L and e are preferably recorded in a memory of the computer. The acquisition steps can be performed in any order.

[0080] The determination method then comprises a step S13 of calculating the heating temperature T, from the parameter values ​​acquired in steps S11-S12. This calculation step S13 is preferably carried out by the processor of the computer.

[0081] The calculation of the heating temperature T is based on an analytical model constructed from the heat conservation equation. Assuming that the scanning speed V is high enough to establish a quasi-permanent regime, the heat conservation equation at the irradiated zone of the stack is written as follows: Vρc p ∂ T ∂ x = − k ∂ 2 T ∂ x 2 + ϕ source E e + ϕ ext Or : V (in m / s) is the scanning speed of the line 50; Φ source (in W / m 3< ) represents the quantity of heat per unit volume supplied by the radiation source 40, this quantity of heat being a function of the irradiance E of the electromagnetic radiation 20 and the thickness e of the stack 10' (in m); Φ ext (in W / m 3< ) represents the quantity of heat per unit volume exchanged with the external environment in which the stack 10' is placed during the treatment process; ρ (in kg / m 3< ), cp (in J / kg / K) and k (in W / m / K) are respectively the density, the mass capacity and the thermal conductivity of the stack 10' and assumed to be equal to those of silicon.

[0082] An order of magnitude analysis allows us to simplify the heat conservation equation as follows: Vρc p ΔT c T L c 1 L = − k ΔT c T L c 2 L 2 + ϕ source E e + ϕ ext

[0083] In the simplified equation [Math. 3] above, ΔT c (in K) represents a temperature variation characteristic of the heating in a direction perpendicular to the line 50; L c1 (in m) represents a first length characteristic of the heat convection during the relative movement between the stack 10' and the radiation source 40; and L c2 (in m) represents a second length characteristic of the heat diffusion in a direction perpendicular to the line 50.

[0084] The temperature variation ΔT c is a function of the heating temperature T, while the first and second lengths L c1 and L c2 are functions of the width L of the line 50. These three characteristic quantities, as well as the heat quantities Φ source and Φ ext , can be expressed in different ways, depending on the treatment process configurations, the order of magnitude analysis carried out and the physical phenomena considered.

[0085] In a preferred embodiment of the determination method, the quantities ΔT c , L c1 , L c2 , Φ source and Φ ext are expressed as follows: ΔT c = T − T 0 where T 0 (in K) is the initial temperature of the stack 10', in other words before the stack 10' is exposed to the electromagnetic radiation 20. The initial temperature T 0 is then another parameter of the treatment method which can be predetermined or, more advantageously, acquired during step S11 (at the same time as E, V and L), for example by means of a temperature sensor. L c 1 = L L c 2 = L + δ where δ is the characteristic length of the thermal boundary layer in front of the beam, which can be approximated by: δ = k Vρc p ϕ source = E e

[0086] The quantity of heat Φ ext exchanged with the external environment can be considered as zero at first glance, in particular when the exchanges are made by natural convection, for coefficients h < 10 W / m / K, and when the scanning speed V is high (typically greater than 0.2 m / s).

[0087] Conversely, the term Φ ext can be considered non-zero. It can then be assumed to be equal to the sum of the flow exchanged by convection Φ c and the flow exchanged by radiation Φ r: ϕ ext = ϕ c + ϕ r = 2 h T − T c e + 2 εσ T 4 − T r 4 e Or : h is a convective exchange coefficient, for example equal to 10 W / m 2< / K; a is the Boltzmann constant (in W / m 2< / K 4< ); ε is the emissivity of the material considered, here silicon (unitless, 0<ε<1); T r is the temperature of the surfaces with which the stack 10' exchanges by radiation, typically the temperature of the walls of the enclosure in which the stack is exposed to radiation; T c is the temperature of the fluid (typically air) which surrounds the stack 10' and with which it exchanges by convection.

[0088] In the relation [Math. 9], a coefficient 2 was introduced into the expressions of the fluxes Φ c and Φ r because it is assumed that the two faces of the stack exchange in the same way with the environment.

[0089] The term ϕ ext may also include a term related to the cooling system, the mathematical formulation of which depends on the type of cooling used. For example, when the stack is placed on a temperature-controlled support, the additional term may be a diffusion term on the scale of the thickness of the stack, of the form: ϕ dif = k T − T support e where T support is the temperature of the support.

[0090] This term becomes significant as soon as the characteristic diffusion time on the scale of the thickness of the stack, τ dif = e 2< (ρc p / k), is of an order of magnitude less than or equal to the characteristic time of the treatment process τ p = L / V. This term can also be weighted by a thermal contact resistance between the stack and its support.

[0091] Thus, when we wish to take into consideration the quantity of heat Φ ext exchanged with the external environment, this is advantageously determined from the temperatures T r, T c relating to this environment and the convective exchange coefficient h. A value for each of these parameters is advantageously acquired during step S11. The temperatures T r and T c can be measured by means of temperature sensors, while the coefficient h can be entered by the operator or calculated by the computer on the basis of the temperature T c and known relationships.

[0092] In order to determine the heating temperature T, the equation [Math. 3] can be solved analytically or numerically using an iterative method.

[0093] Because it is based on an analytical model, the heating temperature determination method T allows for a quick and simple estimation of the heating temperature that the stack can reach during its scanning treatment. It can be easily implemented in equipment within a heterojunction photovoltaic cell production line.

[0094] There figure 6represents calculation results of the heating temperature T obtained using the analytical model described above and, for comparison, results of thermal simulations (by finite volumes). The calculation results are given with and without taking into account the quantity of heat Φ ext exchanged with the external environment (relation [Math. 9]). The parameter values ​​are as follows: a surface energy density Q equal to 60 kJ / m 2< , an irradiance E variable between 10 kW / m 2< and 10000 kW / m 2< , a scanning speed V variable between 0.1 cm / s and 1 m / s, a line width L equal to 6 mm, a stack thickness e equal to 160 µm, temperatures T 0 , T r, T c equal to 25 °C and a convection coefficient h equal to 10 W / m 2< / K. The heating temperature T is represented as a function of the scanning speed V.

[0095] It can be seen that the temperature values ​​calculated with the analytical model are close to the simulated temperature values. The relative difference between the heating temperature T calculated using the model (with Φ ext = 0) and the simulated heating temperature T is less than 17% when the scanning speed V is greater than 1 cm / s. It can also be seen that the amount of heat Φ ext exchanged with the external environment has practically no influence from a scanning speed greater than about 20 cm / s. However, for other configurations (lower temperatures T r, T c, higher convective exchange coefficient h, addition of a term related to the temperature-regulated support, etc.), this term Φ ext can have a significant influence on the heating temperature T calculated even at high scanning speeds.

[0096] There figure 7represents other calculation results and thermal simulations of the heating temperature T, this time as a function of the thickness e of the stack. The parameter values ​​are as follows: a surface energy density Q equal to 52.3 kJ / m 2< , an irradiance E equal to 216 kW / m 2< , a scanning speed V equal to 2.48 cm / s, a line width 50 equal to 6 mm, a stack thickness e varying between 60 µm and 160 µm, temperatures T 0 , T r, T c equal to 25 °C and a convection coefficient h equal to 10 W / m 2< / K.

[0097] Again, we see that the temperature values ​​calculated with the analytical model are close to the simulated temperature values.

[0098] The method for determining the heating temperature T is therefore precise.

[0099] The analytical model described above and represented by equation [Math. 3] can also be used to determine one or more parameters of the treatment process, knowing the thickness e of the 10' stack to be treated and the degradation temperature of the amorphous silicon T max (or another threshold temperature, as described later).

[0100] This other use of the analytical model corresponds to a method of parameterizing the scanning treatment process (in other words, a method for determining parameters of the treatment process).

[0101] In general, the method of parameterizing the scanning treatment process comprises the determination of a set of parameters (including the irradiance E, the speed V and the width L, as well as, where appropriate, the initial temperature T 0 , the temperatures T r, T c relating to the environment and the convection coefficient h) such that the heating temperature T of the stack during the treatment process is less than or equal to the limit temperature T max . This set of parameters is obtained by solving the equation [Math. 3]. One or more of these parameters may be set according to considerations other than temperature, in particular the treatment time of a stack, the improvement of the expected yield, constraints on the radiation source or the cooling system, etc.

[0102] The parameterization process is preferably implemented by a computer. It can be considered as a preliminary step of the scanning processing process. The parameter set determined during this preliminary step is then applied during the next step of exposure to electromagnetic radiation.

[0103] In a first mode of implementation illustrated by the figure 8 , the setting method comprises steps S21 to S23 identical to steps S11 to S13 of the determination method illustrated by the Figure 5 As a reminder, these steps consist respectively of acquiring a value for each of the parameters E, L and V (at least) of the treatment process, acquiring a value of the thickness e of the stack and calculating the heating temperature T from the acquired parameter values, using the equation [Math. 3].

[0104] The acquisition step S21 of the parameterization method advantageously comprises the following operations: the determination of a surface energy density Q to be supplied to the stack 10' by means of the electromagnetic radiation 20, in order to obtain the desired improvement in efficiency; the determination of a displacement speed V, as a function of the desired treatment time and the dimensions of the stack 10'; and the dimensioning of the radiation source 40, in other words the determination of an irradiance E of the electromagnetic radiation 20 and a width L of the line 50 which are compatible with the surface energy density Q and the displacement speed V determined previously (i.e. which verify the relationship [Math. 1]).

[0105] When the values ​​of width L and irradiance E determined during the last operation are not compatible with any radiation source, the scanning speed V is reduced and then new values ​​of irradiance E and width L are determined (in other words, the last two operations are repeated).

[0106] As described previously, the initial temperature T 0 , the temperatures T r, T c relating to the environment and the convection coefficient h can also be acquired during step S21.

[0107] The setting process according to the figure 8 then comprises a step S24 of comparing the heating temperature T (calculated in step S23) with the limit temperature T max , which may have been predefined or acquired during a prior step of the parameterization method.

[0108] The following steps S25 and S26 of the parameterization method are only performed when the heating temperature T is strictly greater than the limit temperature T max . In step 25, at least one adjustment parameter is chosen from the parameters of the treatment method. Then, the value of the adjustment parameter is modified in the direction of a decrease in the heating temperature T during step S26. At the end of step S26, the parameterization method returns to step S23 to perform a new calculation of the heating temperature T, taking into account the modified value of the adjustment parameter.

[0109] Steps S23 to S26 of the parameterization process are thus repeated in a loop as long as the heating temperature T calculated in step S23 is strictly greater than the limit temperature T max.

[0110] When, on the contrary, the heating temperature T is less than or equal to the limit temperature T max , the scanning treatment process can be considered as parameterized, because the determined set of parameters satisfies the temperature condition T ≤ T max .

[0111] At the end of the parameterization process, one or more parameters can be optimized empirically or using thermal simulations.

[0112] The setting process according to the figure 6 is advantageously accomplished when a user, for example a manufacturer of heterojunction photovoltaic cells, wishes to implement the scanning processing method for the first time (in other words, at the design of the processing method).

[0113] In a second mode of implementation illustrated by the figure 9 , the setting process includes: a step S31 of acquiring a value for each of the parameters E, V and L (at least) of the treatment method; a step S32 of acquiring a value of thickness e of the stack 10'; a step S33 of collecting a modification of the value of at least one parameter of the treatment method and / or of the thickness e of the stack 10'; a step S34 of selecting an adjustment parameter from among the other parameters of the treatment method; and a step S35 of calculating a (new) value of the adjustment parameter such that the heating temperature T of the stack is less than or equal to the limit temperature T max , the value of the adjustment parameter being calculated from the values ​​(modified and unmodified) of the other parameters using the equation [Math. 3].

[0114] The setting process according to the figure 9allows an existing treatment process to be recalibrated after one or more parameters have been modified (intentionally or not). Such a modification can occur in many situations. Examples include the treatment of a 10' stack (or batch of stacks) of different thickness, a change of radiation source 40 following a breakdown, a drift in the treatment process parameters, an increase in the ambient temperature and therefore in the initial temperature T 0 of the 10' stack, etc.

[0115] In step S33, the modified value of the parameter(s) can be entered by the operator or measured by a sensor (temperature sensor, optical thickness measurement or mass measurement of the stack, etc.)

[0116] Step S34 of selecting the adjustment parameter can be performed before step S33 of receiving the modified parameter.

[0117] The step S35 of calculating the value of the adjustment parameter can be triggered by receiving the modified parameter (step S33) and the calculated value can be automatically applied in the subsequent exposure step, so that the recalibration of the scanning processing method is performed autonomously.

[0118] Finally, the setting process according to the figure 9 may further comprise a step of measuring the heating temperature T during the exposure step in order to control and / or improve the adjustment of the treatment process.

[0119] In the first and second modes of implementation, the parameterization method comprises the definition of an adjustment parameter (step S25 and S34) in order to find a heating temperature T lower than the limit temperature T max.

[0120] In some cases, for example when a stack cooling system can be added or optimized, the adjustment parameter can be chosen from the initial temperature T 0 of the stack, the temperatures T r, T c relative to the environment and the convection coefficient h. For example, we will seek to reduce the initial temperature T 0 of the stack, increase the exchanges by convection (increase of the coefficient h), reduce the temperature T c of the fluid surrounding the stack (for example, with a forced convection system by ventilation), increase the exchanges by radiation by reducing the temperature T r of the enclosure. This choice avoids reducing the surface energy density Q on which the improvement in efficiency depends (up to a certain value).

[0121] In other cases, the adjustment parameter can be chosen from the irradiance E, the scanning speed V and the width L, which will have the consequence of reducing the surface energy density Q (relation [Math. 1]). When the surface energy density Q has been chosen to be higher than the minimum surface energy density Q min to obtain the desired efficiency gain (for example +0.3% absolute), such a choice is not detrimental (up to a certain adjustment limit).

[0122] Generally speaking (but especially when the targeted surface energy density Q is equal to the minimum surface energy density Q min ), it is preferable to act on the initial temperature T 0 or the parameters relating to heat exchanges with the environment (T r , T c and h), typically by means of a cooling system.

[0123] Several parameters of the treatment process can be defined as adjustment parameters in step S25 of the process according to the figure 8 or in step S34 of the method according to the figure 9 . Furthermore, the adjustment of certain parameters may be limited by the equipment that makes up the scanning processing system (support 30, radiation source 40, cooling system, etc.). It may then be useful to define a main adjustment parameter and secondary adjustment parameters that are only modified when the main adjustment parameter has reached its limit.

[0124] In the parameterization process, and more particularly in the comparison step S24 of the process according to the figure 8 or at calculation step S35 of the method according to the figure 9, it is possible to consider a threshold temperature (i.e. not to be exceeded during the exposure step) other than the limit temperature T max (degradation temperature of amorphous silicon). This threshold temperature is lower than the limit temperature T max (and higher than the initial temperature T 0 of the stack). The parameterization method then advantageously comprises an additional step of acquiring this threshold temperature (before said comparison step S24 or calculation step S35).

[0125] Two examples of implementing the parameterization method will now be described. The first example concerns the parameterization method according to the figure 8 (design process) and the second concerns the parameterization process according to the figure 9 (recalibration process). Example 1 :

[0126] Experiments have shown that a yield enhancement effect can be obtained for a surface energy density Q min = 60 kJ / m 2< .

[0127] We want to process stacks of pseudo-square format “M2” (156.75 mm x 156.75 mm, diameter 210 mm) in a time less than or equal to 5 s. For this, the scanning speed is fixed at V = 3.2 cm / s.

[0128] In order to carry out the treatment, a radiation source is chosen to generate a line of width L = 6 mm with an irradiance E = 320 kW / m 2< , which verifies the equation [Math. 1].

[0129] The stacks have a thickness e = 160 µm (manually entered supplier data, possibly verified at the line entry by a mass measurement).

[0130] Since the treatment is carried out continuously on stacks moving under the radiation source and without cooling, the temperatures T 0 , T c , T r are estimated to be equal to 50 °C (heating of the environment linked to the operation of the source), with h = 10 Wm -2< .K -1< (order of magnitude for a natural convection regime).

[0131] The analytical model then allows us to estimate that the stacks will reach a heating temperature T equal to 231 °C. Considering a threshold temperature of 200 °C (equal to T max ), it is decided to cool the processing environment. Assuming that the temperatures T 0 , T c , T r are equal, the equation [Math. 3] indicates that temperatures T 0 , T c , T r less than or equal to 18 °C are necessary so that the heating temperature T does not exceed 200 °C. Example 2:

[0132] The following treatment process is considered as a reference: Q = 52.3 kJ / m 2< , E = 216 kW / m 2< , V = 2.48 cm / s, L = 6 mm, e =160 µm; T 0 = 25 °C and Φ ext = 0.

[0133] The analytical model (without the term φ ext ) predicts a temperature of 184 °C. The threshold temperature is set at 200 °C.

[0134] It is assumed that a new series of stacks is to be processed, with a thickness of 120 µm. The analytical model then predicts a temperature of 238 °C. It is decided to adapt the scanning speed V in order to maintain an acceptable temperature. The numerical resolution of the equation [Math. 3] indicates that the scanning speed V allowing a temperature of 200 °C is 3.05 cm / s. In order to take into account the model error (thermal simulations predict an initial temperature of 197 °C, instead of 184 °C), a significantly higher speed can be chosen as a starting point for an empirical adjustment. Increasing the scanning speed V implies a reduction in the surface energy density Q.

[0135] Many variations and modifications of the scanning treatment method, the method for determining the heating temperature T and the parameterization method will be apparent to those skilled in the art.

[0136] For example, the beam of electromagnetic waves emitted by the radiation source 40 may not be oriented perpendicular to the surface of the stack 10'. By tilting it relative to the normal of the face of the stack, by an angle of less than 90° (in absolute value), it is possible to vary the surface irradiated by the beam as well as the power density received per unit area. The tilt may be constant during the treatment process or be modulated by means of a rotation of the radiation source, for example so as to reduce the surface energy density received by the stack at the end of scanning (i.e. when approaching the trailing edge 100b) and to further limit overheating.

[0137] Line 50 can be oriented at an angle other than 90° relative to the median M (cf. Fig.3). It can also include several line segments. These line segments can be connected, thus forming a broken line, or on the contrary be disjoint. The rays of the beam are then contained in several planes (intersecting or not). The equations of the model can then be adapted to take into account the orientation and / or the number of line segments.

[0138] The treatment method is applicable to a stack 10' comprising a substrate 11 made of p-type doped crystalline silicon (rather than n-type doped crystalline silicon).

[0139] It can also be applied to a heterojunction photovoltaic cell within a photovoltaic module, especially if the scanning speed is sufficiently high so that heat diffusion in the encapsulation material and the support plates (typically glass) can be neglected.

[0140] Finally, the analytical model can be refined (or enriched) in order to improve the accuracy of the results, for example: by establishing other expressions for the terms ΔT c , L c1 , L c2 , Φ source and Φ ext of the model, in particular by adding to the quantity of heat Φ ext a term related to the cooling system, by considering the same length scale (L+δ) for the convective and diffusive terms of the model, by neglecting the thermal boundary layer (δ = 0) (in particular for high scanning speeds)...; empirically based on numerical or experimental results; and / or by using machine learning methods with experimental data or obtained by simulation.

Claims

1. Method for setting parameters of a method for treating a stack (10') obtained upon manufacturing a heterojunction photovoltaic cell, the stack (10') comprising a crystalline silicon substrate (11) and a hydrogenated amorphous silicon passivation layer (14) disposed on the substrate, the treatment method aiming to improve efficiency of the heterojunction photovoltaic cell and comprising a step of exposing a face of the stack (10') to electromagnetic radiation (20), the electromagnetic radiation (20) being emitted by a radiation source (40) and applied to the face of the stack (10') along a line (50), the stack (10') and the radiation source (40) being relatively translationally moving such that the line (50) sweeps at least part of the face of the stack (10'), the method for setting parameters comprising determining a plurality of parameters of the treatment method so that the heat-up temperature (T) of the stack (10') reached during the treatment method is less than or equal to a threshold temperature (Tmax), the plurality of parameters comprising an irradiance (E) of the electromagnetic radiation (20), a width (L) of the line (50) and a speed (V) of relative displacement between the stack (10') and the radiation source (40) and being determined using the following relationship: Vρc P ΔT C T L C 1 L = − k ΔT C T L C 2 L 2 + ∅ source E e + ∅ ext wherein: - V is the speed of relative displacement between the stack (10') and the radiation source (40); - ΔTc is a function of heat-up temperature (T) and represents a temperature variation characteristic of the heat-up (10') in a direction perpendicular to the line (50); - Lc1 is a function of width (L) of the line (50) and represents a first length characteristic of heat convection during the relative displacement between the stack (10') and the radiation source (40); - Lc2 is a function of width (L) of the line (50) and represents a second length characteristic of heat diffusion in a direction perpendicular to the line; - Øsource is a function of irradiance (E) of the electromagnetic radiation (20) and thickness (e) of the stack (10') and represents the quantity of heat per unit volume supplied by the radiation source (40); - Øext represents the quantity of heat per unit volume exchanged with an external environment in which the stack (10') is placed during the step of exposing to the electromagnetic radiation (20); - ρ, Cp and k are respectively the density, specific heat capacity and thermal conductivity of the stack (10').

2. Method for setting parameters according to claim 1, comprising the following steps of: a) acquiring a value for each of the parameters of the treatment method; b) acquiring a thickness value (e) for the stack (10'); c) calculating a heat-up temperature (T) of the stack using said relationship; d) comparing the heat-up temperature (T) calculated with a threshold temperature (Tmax); and as long as the heat-up temperature (T) calculated is strictly greater than the threshold temperature (Tmax), the following steps of: e) selecting an adjustment parameter from the parameters of the treatment method; f) changing value of the adjustment parameter; and g) repeating steps c) and d).

3. Method for setting parameters according to claim 1, comprising the following steps of: - acquiring a value for each of the parameters of the treatment method; - acquiring a thickness value (e) for the stack (10'); - collecting a change in the value of at least one parameter of the treatment method and / or the thickness (e) of the stack (10'); - selecting an adjustment parameter from the other parameters of the processing method; - determining a value of the adjustment parameter such that the heat-up temperature (T) is less than or equal to the threshold temperature (Tmax), the value of the adjustment parameter being determined using said relationship.

4. Method for setting parameters according to one of claims 2 and 3, wherein the plurality of parameters further comprises a plurality of temperatures (Tr, Tc) relative to the external environment and a convective heat exchange coefficient (h) and wherein the adjustment parameter is selected from said temperatures and said coefficient.

5. Method for setting parameters according to one of claims 2 and 3, wherein the adjustment parameter is selected from irradiance (E) of the electromagnetic radiation (20), width (L) of the line (50) and speed (V) of relative displacement between the stack (10') and the radiation source (40).

6. Method for treating a stack (10') obtained upon manufacturing a heterojunction photovoltaic cell, the stack (10') comprising a crystalline silicon substrate (11) and a hydrogenated amorphous silicon passivation layer (14) disposed on the substrate (11), the treatment method aiming to improve efficiency of the heterojunction photovoltaic cell and comprising a step of exposing a face of the stack (10') to electromagnetic radiation (20) emitted by a radiation source (40), the electromagnetic radiation (20) being applied to the face of the stack (10') along a line (50), the stack (10') and the radiation source (40) being relatively translationally moving such that the line (50) sweeps at least part of the face of the stack (10'), the treatment method further comprising a prior step of determining a plurality of parameters to be applied during the exposure step, the plurality of parameters being determined according to claim 1.

7. Method according to any of claims 1 to 6, wherein the threshold temperature (Tmax) is less than or equal to an amorphous silicon degradation temperature.

8. Data processing device comprising means for implementing a method according to any of claims 1 to 5.

9. Computer program product comprising instructions which, when the program is executed by a computer, cause the same to implement a method according to any of claims 1 to 5.

10. Computer-readable data medium, having the computer program product according to claim 9 recorded thereon.