FR3021808A1 - IMPROVED METHOD FOR PRODUCING A SOLAR CELL WITH TRANSPARENT OXIDE REGIONS OF MODIFIED CONDUCTIVITY - Google Patents

Abstract

Realization of a solar cell in which a localized heat treatment is carried out by means of light radiation on one or more surface regions (14b) of a transparent conductive oxide layer so as to render these regions (14b) more conducive and subsequently allow selective realization of contacts on these regions.

Description

TECHNICAL FIELD AND PRIOR ART The present invention relates to the field of photovoltaic cells also called solar cells and their manufacture. It applies in particular to the realization, for such cells, localized metal contacts and formed on at least one layer of conductive transparent conductive material (OTC). A photovoltaic cell is generally provided with metal contacts for collecting the current produced by the cell and for connecting this cell to other cells. These metal contacts can be obtained for example by screen printing a conductive paste, or by evaporation or by spraying a metal or by electrolytic deposition of metal.

This last type of process has the particular advantage of enabling, at low cost, to form contact zones with a high aspect ratio (ratio of the width of these zones by their thickness) and in particular to make contact zones of low width which limits the shading caused by the contacts while maintaining a sufficient thickness to ensure good conductivity.

In order to make contacts by electrolytic deposition on localized regions of a conductive surface of the cell, one solution consists in forming a perforated insulating mask on this surface and in increasing the contact areas on regions of this surface which are not exposed. covered by masking. The realization of such masking can be expensive to implement. On the other hand, in the case where the material of the insulating masking is not transparent, an additional step of removing the masking is generally necessary.

Some photovoltaic cells have a layer of Transparent Conductive Oxide (OTC) used as a contact material and on which the metal contact areas are formed. WO2011115206 A1 gives an example of a method of making contacts for such a type of photovoltaic cell. In this process, a transparent insulating layer is formed on the transparent conductive oxide layer, then some areas of the insulating layer are ablated, for example with the aid of a laser, to form openings revealing the layer of transparent conductive oxide. The contacts are then formed in these openings.

The ablation step may degrade the materials beneath the transparent conductive oxide layer which may then cause a decrease in cell performance. Therefore, there is a real need for a photovoltaic cell having at least one transparent oxide layer, which is capable of being manufactured using a process having a reduced number of steps, which is less expensive, and is improved vis-à-vis disadvantages mentioned above. DISCLOSURE OF THE INVENTION The present invention aims in particular to solve these problems. According to one aspect, the present invention provides a method for producing a photovoltaic cell, comprising the steps of: a) forming on a semiconductor medium in which at least one junction is capable of being produced: a first layer to a transparent conductive oxide base having a first resistivity, the first layer being covered with a second transparent oxide-based layer of second resistivity greater than the first resistivity, b) performing a localized heat treatment with the aid of a light radiation applied to one or more regions of the second layer, said treatment being adapted to decrease the resistivity of the regions and to make these regions more conductive.

The junction may be PN type. At the end of step b), the treated regions may have a resistivity between ten and a thousand times lower than that of the remainder of the transparent oxide of the second layer (ie between 10 and 1000 less than the second resistivity) and advantageously between ten and a hundred times lower. The first resistivity may be less than 10-3 Ohm * cm. The second resistivity transparent oxide is preferably resistive or of low conductivity. By low conductivity or resistive is meant in particular a resistivity greater than 1E-2 or 10-2 Ohm * cm.

The regions formed in step b) have a resistivity of less than 10-3 ohm * cm and may be of the order of the first resistivity. According to one possible embodiment, the transparent oxide of the first layer may be chosen based on one of the following materials: ITO, 10, ZnO: Al, ZnO: B, ZnO: Ga, SnO 2: F, IWO , 10: This.

According to one possible embodiment, the transparent oxide of the second layer may be chosen based on one of the following materials: ITO, 10, ZnO: Al, ZnO: B, ZnO: Ga, SnO2: F, IWO, 10: This. The heat treatment performed is located in one or more of the transparent oxide layers and does not cause damaging degradation of materials under this or these transparent oxide layers. A laser is particularly suitable for performing such a localized treatment. The thickness of the oxide layers as well as the intensity of the radiation may also be provided to prevent degradation of areas beneath the transparent oxide layers. Advantageously, the localized treatment is carried out by means of an excimer laser. Laser processing conditions with a fluence of between 100 and 500 mJ / cm 2 and pulses of duration between 10 and 200 ns are particularly suitable for oxide layers of the order of several tens of nanometers. After modifying the conductivity of said surface regions, a contacting on said modified conductivity regions by the treatment in step 5 b) can then be performed. Thus, the method may comprise after step b): a step of forming one or more contact zones (s) metal (s) on said regions. Due to the difference in resistivity between the treated regions and the untreated zones of the second layer in step b), a selective realization of contacts can be performed on the surface regions. Selective formation means that the contact zones are formed opposite the regions of modified conductivity and do not completely cover the second layer.

According to one possibility of carrying out the process, the selective formation of the metal contact zones (s) comprises an electrolytic deposition. The formation of the metal contact areas may comprise an ink jet deposition of a metal layer prior to the electrolytic deposition. According to one possible embodiment, to form the first transparent oxide layer 20 and the second transparent second resistivity oxide layer, the first layer is deposited under a first oxygen flow, while the second layer is deposited under a second oxygen flow rate greater than the first flow rate. A particular embodiment provides that the first layer and the second layer are sub-layers of a conductive transparent oxide layer having an oxygen concentration gradient in its thickness, the oxygen concentration being all the more important. that one moves away from the semiconductor support. A method as defined above applies in particular to the implementation of a photovoltaic cell of the heterojunction type. In this case, the semiconductor substrate 30 may be formed of a substrate based on crystalline semiconductor material, for example silicon, bonded to at least one semi-conductor layer based on a semiconductor material. amorphous or having a crystallographic arrangement different from that of the crystalline semiconductor material of the substrate.

This method can also be applied to a photovoltaic cell of homo-junction type. In this case, the semiconductor support may be formed of a substrate based on crystalline semiconductor material such as silicon and having doped regions of different types to form the p / n junction. According to another aspect, the present invention provides a photovoltaic cell comprising: a semiconductor support in which at least one junction is capable of being produced; a first layer based on at least one transparent oxide material of first resistivity the first layer being covered with a second layer based on at least one transparent oxide material having a second resistivity greater than that of the transparent oxide material of the first layer, the second layer comprising one or more regions of resistivity lower than the second resistivity. The cell may further comprise one or more metal contact areas on said regions. Said regions may have a resistivity between ten and a thousand times lower than that of the remainder of the transparent oxide of the second layer (i.e. between 10 and 1000 less than the second resistivity) and advantageously between ten and a hundred times lower.

Said regions have a resistivity of less than 10-3 ohm * cm and which may be of the order of the first resistivity. The first resistivity may be less than 10-3 Ohm * cm. The second resistivity transparent oxide is preferably resistive or of low conductivity. By low conductivity or resistive is meant in particular a resistivity greater than 10-2 Ohm * cm.

The semiconductor medium may comprise a substrate based on crystalline semiconductor material bonded to at least one semiconductor layer based on an amorphous semiconductor material or having a crystallographic arrangement different from that of the semi-conducting material. crystalline conductor of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS Other characteristics and advantages of the invention will emerge more clearly on reading the following description and with reference to the appended drawings, given solely by way of illustration and in no way limiting. FIGS. 1A-1C illustrate steps of a first example of a method 10 for producing a solar cell according to one embodiment of the invention and in which the conductivity of conductive transparent oxide regions is modified by means of a heat treatment using a light radiation; Figure 1D illustrates selective formation of contact areas for a solar cell on conductive transparent oxide regions whose conductivity has been changed; FIG. 2 illustrates an embodiment of metal contacts for a solar cell, the contacts being formed of several metal layers made by successive deposits on conductive transparent oxide regions whose conductivity has been modified; FIGS. 3A-3B illustrate a variant of the first exemplary embodiment method in which the regions whose conductivity has just been modified are regions of a transparent oxide layer comprising a conductivity gradient; Identical, similar or equivalent parts of the different figures bear the same numerical references so as to facilitate the passage from one figure to the other. The different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS An example of a method of manufacturing a photovoltaic cell or solar cell will now be described in connection with FIGS. 1A-1D. The starting material of this process may be a substrate 1 based on crystalline semiconductor material such as, for example, crystalline Si. The substrate 1 may be doped according to a first type of doping, for example of the N type. In the case where the photovoltaic cell produced is a heterojunction cell, the substrate 1 may comprise on a first face A a semiconductor layer 6 to base of a semiconductor material having a crystallographic arrangement different from that of the crystalline semiconductor material of the substrate 1. This semiconductor layer 6 may be based on amorphous semiconductor material such as, for example, hydrogenated amorphous Si intrinsic (a-: Si: H (i)). The semiconductor layer 6 may itself be covered with another semiconductor layer 7 based on amorphous semiconductor material such as, for example, hydrogenated amorphous Si doped with a second type of doping, for example of P type ( a-Si: H (p)). The substrate 1 may also comprise, on a second face B opposite to the first face A, a semiconductor layer 6 based on a semiconductor material having a crystallographic arrangement different from that of the crystalline semiconductor material of the substrate 1. This semiconductor layer Conductor 8 may be based on amorphous semiconductor material such as, for example, intrinsic hydrogenated amorphous Si (a-Si: H (i)). The semiconductor layer 8 may itself be covered with another semiconductor layer 9 based on amorphous semiconductor material such as, for example, hydrogenated amorphous Si doped according to a first type of doping, for example of the N type. , (a-Si: H (N)). The amorphous semiconductor layers 6, 7, 8 and 9 may be formed for example by a plasma enhanced chemical vapor deposition (PECVD) technique. The semiconductor substrate 1 covered with amorphous semiconductor layers 6, 7, 8 and 9 forms a semiconductor substrate which may also be the starting material of the process.

On the side of the first face A of the substrate 1, a first layer 12 based on transparent conductive oxide 13 is then reported (FIG. 1A). The transparent conductive oxide 13 may have a high transmittance, for example at least 80% in the visible light range (from 400 nm to 800 nm).

The conductive transparent oxide 13 preferably has a low resistivity, particularly a resistivity of less than 1E-3 or 10-3 Ohm * cm. The transparent conductive oxide 13 may for example be based on indium oxide doped with (InO 2: Sn) commonly known as ITO (for "Indium tin oxide"), or based on Indium oxide (10 ), or based on doped Zinc oxide, for example with aluminum (ZnO: Al) or with Boron (ZnO: B), or with Gallium (ZnO: Ga), or with a base of Oxide of Fluorine-doped tin (SnO2: F), or based on tungsten doped indium oxide (IWO), or based on cerium-doped Indium oxide (10: Ce), the first layer 12 has a thickness which may be for example between 20 nm and 80 nm, preferably between 20 nm and 50 nm. The first layer 12 may be formed by physical vapor deposition (PVD for "Physical Vapor Deposition"), and in particular by sputtering. An example of deposition conditions provides a deposition chamber temperature of the order of 200 ° C, power applied to the target of 2kW, a flow rate of O 2 of the order of 4 cm3 per minute. With such conditions, it is possible to obtain for example a first layer 12 based on ITO with a resistivity of about 6E-4 Ohm-cm. The flow rate of O 2 can be adapted according to the oxygen concentration that it is desired to obtain in the transparent oxide. The resistivity that one wishes to confer on the transparent oxide material is all the higher as this concentration is important.

Then, on the first layer 12, a second layer 14 based on transparent oxide 15 is produced (FIG. 1B). The transparent oxide 15 is provided with a higher resistivity than that of the transparent conductive oxide 13. The transparent oxide 15 of the second layer 14 is resistive, or has a low conductivity, in particular a resistivity greater than 10 -2 Ohm * cm and preferably between ten and one or more hundred times that of the oxide 13 of the first layer 12. The transparent oxide 15 may be for example based on ITO, or IO or ZnO: Al or ZnO: B, or ZnO: Ga, or SnO2: F, or IWO, or (10: Ce).

According to one possible embodiment, the transparent oxide 13 and the transparent conductive oxide 15 may be based on similar materials but with different resistivities, the variation in resistivity may be due to different doping levels. The transparent oxide 13 and the transparent conductive oxide 15 may for example be based on ITO but with respective doping of tin 10 different. The second layer 14 has a thickness which may for example be between 20 nm and 80 nm, advantageously between 50 nm and 80 nm, and may also be formed by PVD deposition. Advantageously, it is desired to obtain a thickness of the bilayer 12 and 14 of between 70 and 120 nm.

With deposition conditions, for example such that a deposition chamber temperature of the order of 200 ° C., a power applied to the target of 2 kW, a flow rate of 15 cm.sup.3 per minute, it is possible to form a second layer 14 based on ITO resistivity of the order of 1E-2 Ohm-cm. One method for producing a transparent oxide 15 with a higher resistivity than that of the transparent oxide 13 is to provide a flow rate of oxygen greater than the flow rate of oxygen expected during the production of the first layer 12. It is thus possible to produce oxides transparent 13 and 15 of the same type, for example based on ITO, but having different oxygen concentrations. It is thus possible advantageously to produce transparent oxides 13 and 15 of different composition in the same deposition chamber. The transparent oxides 13 and 15 obtained, although they may be of the same type, for example both based on ITO but with different oxygen concentrations, have different morphologies and in particular different grain sizes. This difference in morphology is observable for example by scanning electron microscope (SEM) or TEM (transmission electron microscope). Then, the conductivity of one or more determined regions 14b of the stack of layers 12 and 14, and in particular of the second superficial layer 14, is modified. This modification is effected here by means of a heat treatment with the aid of a luminous radiation, the treatment being able to modify the crystalline structure of the transparent oxide 15 in order to make it more conductive (FIG. 1C). This treatment is advantageously carried out using a laser which allows localized, precise treatment and without necessarily having to use a mask. The laser exposure can be carried out so that the regions 14b whose conductivity is increased have a thickness el (measured parallel to the vector z of the orthogonal reference [O; x; y; z] given in FIG. 1C) at least equal to to the thickness of the second layer 14.

These regions 14b are thus in contact with the first transparent conductive oxide layer 12 and are flush with an upper face of the second layer, the upper face being the face opposite to a lower face of the second layer in contact with each other. with the first layer 12. Preferably, the laser used as well as the exposure conditions, in particular the laser pulse duration and the fluence of the laser are adapted, depending on the thickness of the layers 12, 14 and 13, 15, and so as to modify the crystallographic structure of regions of the transparent oxide 13 of the second layer 14, superficial, without performing ablation and without damaging layers underlying the stack.

An excimer laser emitting in the ultraviolet range or in a range between 100 nm and 350 nm may be employed. According to a particular embodiment, a XeCI type laser emitting at a wavelength of the order of 308 nm is used. For layers 12 and 14 of thickness, for example between 20 nm and 80 nm, it is also possible to provide a fluence for example between 100 and 500 mJ / cm 2 and a pulse duration of, for example, between 10 and 10 nm. 200 ns. Advantageously, a fluence of, for example, between 100 and 250 mJ / cm 2 and a pulse duration of, for example, 5 to 50 ns is provided. Surface regions 14b of lower resistivity are thus formed than those of the remainder of the second layer 14. The regions 14b may have a low resistivity, in particular a resistivity of less than 1E-3 or 10-3 Ohm * cm. The regions 14b may have a resistivity between ten and a thousand times lower than that of the transparent oxide 15 of the second layer 14. Advantageously, the regions 14b have a resistivity between ten and a hundred times lower than that of the transparent oxide 15 The regions 14b may have a conductivity of the order of that of the first layer 13. The regions 14b of modified resistivity, although being based on the same transparent oxide 15 as those of the rest of the superficial second layer 14, have a different morphology and in particular different grain sizes. Due to the difference in conductivity between the laser-treated regions 14b with respect to the remaining zones 14a of the second layer 14, selective metallization can then be provided on these regions 14b. A so-called "self-aligned" process is thus carried out. Thus, areas 20 of electrical contacts arranged on and in contact with the modified regions 14b of the second transparent oxide layer 14 can then be formed (FIG. 1D). This formation may comprise an electrolytic deposition of copper or other conductive metal such as, for example, Ag or Ni, which is selectively grown on regions 14b. The electrolytic deposition can be achieved by using the first transparent conductive oxide layer 13 as an electrode. The contacts 20 may be for example in the form of a grid in order to allow light to pass through the device.

In order to effect the metallization and to form the contact zones 20, several variants can be provided. The zones 20 of contacts may be formed of a stack of several metal layers 20a, 20b (Figure 2). According to one variant, a deposit is made by localized dispensing, for example of inkjet type, so as to form a thin metal layer 20a of adhesion reinforcement for example based on Cu, or Ni or Ag or Au. Then, another electrolytic layer 20b is formed by electrolytic deposition, for example based on Ni or Cu or Ta or W. The process which has been described previously with reference to FIGS. 1A-1C is not not necessarily limited to the formation of a transparent oxide bilayer and may further include a number of transparent oxide layers greater than two. An embodiment variant illustrated in FIGS. 3A-3B, provides, in place of the stack of layers 12, 14 described previously, a transparent oxide layer 34 comprising an oxygen concentration gradient. In this layer 34, the oxygen concentration in the layer 34 is greater as one moves away from the substrate 1. The layer 34 thus comprises a corresponding gradient of resistivity, the resistivity of the layer 34 As in the example illustrated in FIG. 3, the oxygen concentration and the resistivity of the layer 34 increase according to the direction and the direction of the vector k of a orthogonal reference [0; 1>; i here A way of making the layer 34 is to make a deposit for example of PVD type in which the flow of oxygen in the deposition chamber is gradually increased during the deposition.

Then, the conductivity of one or more determined regions 34b of the layer 34 is modified in a localized manner by means of a laser treatment as described above. The crystallographic arrangement of regions 34b of a surface sublayer 342 of transparent oxide is thus modified so as to make it more conductive. This underlayer 342 rests on another transparent oxide underlayer 341 which has not undergone laser processing (FIG. 3B).

In the examples of the method which have just been given, the conductivity of conductive transparent oxide regions is modified on the side of the first face A of the substrate and then contacts are formed on the side of this first face A. One or the other of the methods which have just been described can also be applied to forming contacts on the side of the second face B of the substrate. Thus, a method according to the invention can be applied to the implementation of metal contacts both in the front face and in the rear face of a substrate.

Claims (23)

REVENDICATIONS1. A method for producing a photovoltaic cell, comprising the steps of: a) forming on a semiconductor medium (1-6-7-8-9) in which at least one junction is capable of being produced: a first layer (12, 341) based on a conductive transparent oxide (13) having a first resistivity, the first layer being covered with a second layer (14, 342), based on a transparent oxide (15) and having a second resistivity, greater than the first resistivity of the first layer, b) performing a localized heat treatment using light radiation applied to one or more regions (14b, 34b) of the second layer (14, 342), said treatment being adapted to decrease the resistivity of the regions and to make these regions (14b, 34b) more conductive. 2. The method of claim 1, wherein at the end of step b) the regions (14b, 34b) have a resistivity between 10 and 1000 times lower than that of the transparent oxide (15) of the second layer. 14 formed in step a), advantageously between 10 and 100 times less than other areas of the transparent oxide (15) of the second layer (14) formed in step a). 3. Method according to one of claims 1 or 2, wherein the transparent conductive oxide (13) of the first layer (12) formed in step a) has a resistivity less than 10-3 Ohm * cm. 4. Method according to one of claims 1 to 3, wherein the transparent oxide (15) of the second layer (14) formed in step a) has a resistivity greater than 10 -2 Ohm * cm. 3021808 15 5. Method according to one of claims 1 to 4, wherein the regions (14b) formed in step b) have a resistivity of less than 10-30hm * cm. 5 6. Method according to one of claims 1 to 5, wherein the transparent oxide (13) of the first layer is selected based on one of the following materials: ITO, 10, ZnO: Al, ZnO: B, ZnO: Ga, SnO2: F, IWO, 10: Ce, the transparent oxide (15) of the second layer being selected based on one of the following materials: ITO, 10, ZnO: Al, ZnO: B, ZnO : Ga, SnO 2: F, IWO, 10: Ce. 10 7. Method according to one of claims 1 to 6, comprising after step b): a step of forming one or more metal contact zones (s) (20) on the regions of the second layer ( 14b, 34b). 15 The method of claim 7, wherein forming the metal contact areas is a selective formation comprising electrolytic deposition. The method of claim 8, wherein the formation of the metal contact areas comprises an ink jet deposition of a metal layer (20a) prior to electrolytic deposition of at least one other metal layer (20b). The method of one of claims 1 to 9, wherein the first layer is formed by deposition under a first oxygen flow rate, the second layer being formed by deposition under a second oxygen flow rate larger than the first flow rate. The method according to one of claims 1 to 10, wherein the first layer (341) and the second layer (342) form sub-layers of a conductive transparent oxide layer (34) having a gradient of oxygen concentration in its thickness. 12. Method according to one of claims 1 to 11, wherein the spot treatment is performed by laser, in particular an excimer laser. 13. The method of claim 12, wherein the laser treatment is performed with a fluence of between 100 and 500 mJ / cm 2 and pulses of duration between 10 and 200 ns. 10 14. Method according to one of claims 1 to 13, wherein the photovoltaic cell is a heterojunction cell, the support comprising a substrate (1) based on crystalline semiconductor material attached to at least one semiconductor layer ( 6, 8) based on a semiconductor material, amorphous or having a different crystallographic arrangement from that of the crystalline semiconductor material of the substrate (1). Photovoltaic cell comprising: a semiconductor support (1-6-7-8-9) in which at least one junction is capable of being made; a first layer (12, 341) based on a at least one transparent oxide material (13) of first resistivity, the first layer being covered with a second layer (14, 342) based on at least one transparent oxide material (15) having a second resistivity greater than that of the transparent oxide material of the first layer, the second layer having one or more regions (14a, 14b) of resistivity lower than the second resistivity. Photovoltaic cell according to claim 15, wherein the regions (14b, 34b) have a resistivity between ten and a thousand times lower than that of the transparent oxide (15) of second resistivity, advantageously between ten and a hundred times lower. to that of the transparent oxide (15) of second resistivity. Photovoltaic cell according to one of claims 15 or 16, wherein the transparent conductive oxide (13) of first resistivity has a resistivity of less than 10-3 ohm * cm. Photovoltaic cell according to one of claims 15 to 17, wherein the transparent oxide (15) of second resistivity has a resistivity greater than 10 2 ohm * cm. 19. Photovoltaic cell according to one of claims 15 to 18, wherein the regions (14b) have a resistivity of less than 10-30hm * cm. 15 Photovoltaic cell according to one of Claims 15 to 19, in which the transparent oxide (13) of the first layer is chosen based on one of the following materials: ITO, 10, ZnO: Al, ZnO: B , ZnO: Ga, SnO2: F, IWO, 10: Ce, the transparent oxide (15) of the second layer being selected based on one of the following materials: ITO, 10, ZnO: Al, ZnO: B, ZnO: Ga, SnO2: F, IW0, 10: Ce. 20 21. Photovoltaic cell according to one of claims 15 to 20, further comprising one or more areas of contact (s) metal (s) (20) on the regions. 25 Photovoltaic cell according to one of claims 15 to 21, wherein the first layer (341) and the second layer (342) are sub-layers of a conductive transparent oxide layer (34) having a gradient of oxygen concentration. 3021808 18 23. Photovoltaic heterojunction cell according to one of claims 15 to 22, the support comprising a substrate (1) based on crystalline semiconductor material attached to at least one semiconductor layer (7, 8) based on a material amorphous semiconductor or having a crystallographic arrangement 5 different from that of the crystalline semiconductor material of the substrate (1).