FR3031240A3 - CONDUCTIVE SUBSTRATE FOR PHOTOVOLTAIC CELL - Google Patents

Abstract

The invention relates to a conductive substrate (1) for a photovoltaic cell, comprising a carrier substrate (2) and an electrode coating (6) formed on the carrier substrate (2), the electrode coating (6) comprising: - a main layer ( 8) based on molybdenum formed on the carrier substrate (2); a selenization barrier layer (10) formed on the main layer (8) of 20-40 nm based on molybdenum and based on molybdenum oxynitride; and on the selenization barrier layer (10), an upper layer (12) of 30-35 nm based on molybdenum, the carrier substrate is in an alkaline-containing material, the conductive substrate comprising an alkali barrier layer ( 4) formed on the carrier substrate and under the molybdenum-based main layer, the alkali barrier layer being at most 100 nm based on silicon nitride.

Description

The invention relates to the field of photovoltaic cells, more particularly to the field of molybdenum-based conductive substrates used to fabricate thin-film photovoltaic cells. BACKGROUND OF THE INVENTION Indeed, in known manner, certain thin-film photovoltaic cells, called second-generation, use a conductive substrate based on molybdenum coated with a layer of absorber (ie photoactive material), generally copper chalcopyrite Cu, d indium In, and Se selenium and / or S sulfur. It may be, for example, a CuInSe2 type material. This type of material is known under the abbreviation CIS. It may also be CIGS, that is to say a material that also incorporates gallium, or materials of the Cu2 (Zn, Sn) (S, Se) 4 (ie CZTS) type using zinc and / or tin rather than indium and / or gallium.

For this type of application, the electrodes are most often based on molybdenum (Mo) because this material has a number of advantages. It is a good electrical conductor (relatively low resistivity of the order of 10pacm). It can be subjected to the necessary high heat treatments because it has a high melting point (2610 ° C). It resists, to a certain extent, selenium and sulfur. Deposition of the absorber layer most often requires contact with an atmosphere containing selenium or sulfur which tends to deteriorate most metals. Molybdenum reacts with selenium or sulfur in particular, forming MoSe2, MoS2 or Mo (S, Se) 2, but retains most of its properties, especially electrical properties, and maintains adequate electrical contact with, for example, the CIS layer. , CIGS, or CZTS. Finally, it is a material on which the layers of CIS, CIGS or CZTS type adhere well, molybdenum even tends to promote crystal growth. However, molybdenum has a significant disadvantage when considering industrial production: it is an expensive material. Indeed, the molybdenum layers are usually deposited by sputtering (magnetic field assisted). But molybdenum targets are expensive. This is all the less negligible as to obtain the desired level of electrical conductivity (a resistance per square of less than or equal to 250%, and preferably less than or equal to 1 or even 0.50% after treatment in an atmosphere containing S or Se) requires a relatively thick Mo layer, generally of the order of 400 nm to 15 microns. The patent application WO-A-02/065554 of SAINT-GOBAIN GLASS FRANCE teaches to provide a relatively thin layer of molybdenum (less than 500 nm) and to provide one or more alkaline-impermeable layers between the substrate and the layer based on molybdenum, so as to preserve the qualities of the thin layer based on molybdenum during subsequent heat treatments. This type of conductive substrate is nevertheless relatively expensive. EP 2 777 075 also discloses a conductive substrate of the Mo / MoON / Mo type. The invention more particularly relates to such a type of conductive substrate. An object of the present invention is to provide a new conductive substrate based on molybdenum more efficient. For this purpose, the present invention particularly relates to a conductive substrate for a photovoltaic cell, comprising a carrier substrate and an electrode coating formed on the carrier substrate, the electrode coating comprising: a molybdenum-based main layer formed on the substrate carrier ; a selenization barrier layer formed on the molybdenum-based and molybdenum oxynitride main layer; and on the selenization barrier layer, a molybdenum-based top layer, the carrier substrate is of an alkaline-containing material, the conductive substrate comprising an alkaline barrier layer formed on the carrier substrate and under the main layer. molybdenum base, the alkali barrier layer being based on silicon nitride, in which the molybdenum-based top layer has a thickness of at least 30 nm and at most 35 nm, the selenization barrier layer has a thickness of at least 20 nm and at most 40 nm and the alkali barrier layer 3031240 3 has a thickness of at most 100 nm. The conductive substrate is reliable and relatively fast to manufacture. The choice of the thickness of the alkali barrier layer is particularly important because even if at such thicknesses the barrier layer is likely to pass alkali, this risk is weighted by the advantage of having a more conductive substrate. fast to manufacture, the deposition of a nitrided layer being relatively slow. Previously, it had never been contemplated using such a thin alkali barrier layer with molybdenum-based stacks as thin and as brittle as Mo / MoON / Mo stacks. This choice of thickness being indeed considered by the skilled person as too risky. According to particular embodiments, the molybdenum-based main layer has a thickness of at least 100 nm and at most 140 nm. According to another particular embodiment, the substrate does not comprise other layers, i.e. consists of the above layers. The subject of the invention is also a method for manufacturing a photovoltaic cell or module comprising the use of a conductive substrate according to any one of the preceding claims for effecting the formation of a photoactive layer by selenization and / or sulphurization On said upper molybdenum-based layer, the step of transforming said molybdenum-based top layer being performed before or during the formation of said photoactive layer, preferably during. According to a particular embodiment, the step of forming the photoactive layer (22) comprising a step of selenization and / or sulphurization at a temperature greater than or equal to 300 ° C. The invention also relates to a photovoltaic module from the above method. The invention will be better understood on reading the description which will follow, given solely by way of example, and with reference to the appended drawings, in which: FIG. 1 is a diagrammatic sectional view of a conductive substrate ; Figure 2 is a schematic sectional view of a photovoltaic cell comprising a conductive substrate according to Figure 1; The drawings of FIGS. 1 to 2 are not to scale, for a clear representation, because the differences in thickness between, in particular, the carrier substrate and the deposited layers are important, for example of the order of a factor. 500.

In FIG. 1 there is illustrated a conductive substrate 1 for a photovoltaic cell comprising: - a carrier substrate 2 made of glass; an alkaline barrier layer 4 formed on the substrate 2; and a molybdenum-based electrode coating 6 formed on the alkaline barrier layer 4. Throughout the text, it is meant by "a layer A formed (or deposited on a layer B"), a layer A formed either directly on layer B and therefore in contact with layer B, is formed on layer B with the interposition of one or more layers between layer A and layer B.

It should be noted that throughout the text "electrode coating" means a current feed coating comprising at least one electronically conductive layer, that is to say the conductivity of which is ensured by the mobility of the electrons. In addition, throughout the text, "includes a layer" should of course be understood as "comprises at least one layer". The illustrated electrode coating 6 consists of: - an alkali barrier layer 4; a molybdenum-based main layer 8 formed directly on the alkaline barrier layer 4; A selenization barrier layer 10 formed directly on the molybdenum-based main layer 8 and having a small thickness; and an upper layer 12 based on molybdenum, formed directly on the selenization barrier layer 10. Such a conductive substrate 1 is intended for the manufacture of a photoactive material with addition of sodium (it is known that the sodium improves the performance of CIS or CIGS photoactive materials). The alkali barrier layer 4 prevents the migration of sodium ions from the glass substrate 2, for better control of the addition of sodium in the photoactive material.

Alternatively, the electrode coating 6 comprises one or more interlayers. Thus, in general, the conductive substrate 1 comprises a carrier substrate 2 and an electrode coating 6 comprising: a molybdenum-based main layer 8 formed on the carrier substrate 2; a selenization barrier layer 10 formed on the main layer 8 based on molybdenum and based on molybdenum oxynitride; and an upper molybdenum-based layer 12 formed on the selenization barrier layer 10. The molybdenum is capable of forming, after sulphurization and / or selenization, an ohmic contact layer with a photoactive semiconductor material, particularly with a photoactive semiconductor material based on copper chalcopyrite and selenium and / or sulfur for example a Cu (In, Ga) (S, Se) 2 type of photoactive material, in particular CIS or CIGS, or a material of type Cu2 (Zn, Sn) (S, Se) 4. An ohmic contact layer is understood to mean a layer of a material such that the current / voltage characteristic of the contact is non-rectifying and linear.

Preferably, the upper layer 12 is the last upper layer of the electrode coating 6, i.e. the electrode coating 6 does not have another layer above the layer 12. Also preferably, the coating electrode 6 comprises a single main layer 8 based on molybdenum, a single barrier layer 10 to selenization, and a single layer 12. It should be noted that throughout the text is meant by "a single layer", a layer of the same material. This single layer can nevertheless be obtained by the superposition of several layers of the same material, between which there is an interface that can be characterized, as described in WO-A-2009/080931. Typically, in a magnetron deposition chamber, several layers of the same material will be successively formed on the carrier substrate by several targets to ultimately form a single layer of the same material, namely molybdenum.

303 12 40 6 Note that the term "molybdenum-based" refers to a material composed of a substantial amount of molybdenum, that is to say either a material consisting solely of molybdenum, or an alloy comprising mostly molybdenum, that is a compound comprising mainly molybdenum but with a content of oxygen and / or nitrogen, for example a content leading to a resistivity greater than or equal to 20 μOhm.cm. The layer 12 is intended to be completely transformed by selenization and / or sulphurization in Mo (S, Se) 2, which material is not on the other hand considered a "molybdenum-based" material but a disulfide-based material. molybdenum, molybdenum diselenide or a mixture of molybdenum disulfide and diselenide. Conventionally, the notation (S, Se) indicates that it is a combination of SxSei_x with 0 <x <1. It is important to note that the substrate shown in FIG. 1 and described above is an intermediate product in the manufacture of a photovoltaic module. This intermediate product is then transformed because of the method of manufacturing the photoactive material. The conductive substrate 1 described above is understood to be the intermediate product before processing, which can be stored and conveyed to other production sites for the manufacture of the module. The upper layer 12 has a thickness of at least 30 nm and at most 35 nm. Mo (S, Se) 2 molybdenum disulfide and / or diselenide compounds are materials whose effectiveness as an ohmic contact layer is proven. The selenization barrier layer 10 protects the molybdenum-based main layer 8 from possible selenization and / or sulfidation. Note that a layer protecting selenization also protects from sulfurization.

Selenization barrier layer is understood to mean a layer of a material of any type suitable for preventing or reducing the selenization of the layers covered by the barrier layer at the selenization stage during the deposition, on the selenization barrier layer, of layers semiconductor materials formed by selenization and / or sulfurization. The selenization barrier layer 3031240 7 within the meaning of the invention shows a proven effectiveness even at a thickness of 3 nm. A possible test as to whether a material is suitable or not for a selenization barrier role is to compare a sample with and without a 5 nm layer of this material between the molybdenum top layer 12 and the main layer 8. and selenizing the samples, for example by heating at 520 ° C in a 100% selenium atmosphere. If the selenization of the main layer 8 is reduced or prevented and the upper layer 12 is fully selenized, the material is effective. The material of the selenization barrier layer 10 is a molybdenum oxynitride, any oxygen stoichiometry and suitable nitrogen. It may be may be substoichiometric, stoichiometric or superstoichiometric, respectively in nitrogen and oxygen.

In general, it is a material of any type suitable for protecting the molybdenum-based main layer 8 from selenization or possible sulphurisation. With x = 0 / (0 + N), there is for example 0.05 <x <0.95, for example 0, 1 <x <0.9.

The selenization barrier layer 10 is of small thickness, chosen from at least 20 nm and at most 40 nm. The selenization barrier layer 10 has a lower conductivity than the molybdenum-based main layer 8. For example, it has a resistivity of between 200 .mu.m.cm and 500 .mu.m.cm.

Due to the small thickness of the selenization barrier layer 10, a high resistivity does not affect the performance of the cell, the electric current passing transversely. The selenization barrier layer 10 is further preferably capable of limiting the back diffusion of the sodium ions to the carrier substrate 2, i.e. the diffusion of sodium ions from the top of the top layer 12, to through the upper layer 12 and to the carrier substrate 2. This property is advantageous in several respects. It makes reliable the manufacturing process of adding alkali to form the photoactive material, for example by deposition of sodium diselenide on the top layer 12 of the electrode coating 6 or by addition of sodium during the deposition of the photoactive material, for example using targets containing sodium or other alkalis, as described in US-B-5,626,688.

The molybdenum-based main layer 8 has a thickness sufficient so that the electrode coating 6 has, after a selenization test as described above, a square resistance of less than or equal to 20%, preferably less than or equal to at 1 Q / ^. The presence of the molybdenum-based upper layer 12 and the molybdenum oxynitride-based selenization barrier layer achieves such performance. Assuming an electrode coating 6 not comprising other electroconductive layers than the molybdenum-based main layer 8, the selenization barrier layer 10 and the upper layer 12 with the molybdenum-based main layer 8a preferably an upper thickness of at least 100 nm and at most 140 nm. Decreasing the thickness of the molybdenum-based main layer 8 has the advantage of allowing this relatively thin layer to be deposited by sputtering with deposition parameters leading to a strongly stressed layer, without the delamination problems that can be encountered. meet with thick layers. The molybdenum-based main layer 8 consists for example of molybdenum, that is to say that it comprises only molybdenum. The carrier substrate 2 and the alkali barrier layer 4 will now be described.

The carrier substrate 2 is, for example, a glass-silica-type glass sheet obtained by floatation, a glass of relatively low cost and which has all the qualities known to this type of material, for example its transparency, its impermeability to water and its hardness. The alkali ion content of the substrate 2 is in this case a disadvantage that the alkali barrier layer minimizes. The alkali barrier layer 4 is based on silicon nitride (SiN). Importantly, the alkali barrier layer, based on SiN, has a thickness of at most 100 nm. The carrier substrate 2 is intended to serve as a rear contact in the photovoltaic module 30 and does not need to be transparent. The sheet constituting the carrier substrate 2 may be flat or curved, and have any type of size, especially at least one dimension greater than 1 meter.

The invention also relates to a method of manufacturing the conductive substrate 1 described above. The process comprises the steps of: depositing the molybdenum base layer 8 on the carrier substrate 2, with optional preliminary deposition of the alkali barrier layer 4; Depositing the selenization barrier layer on the molybdenum-based main layer 8, for example directly on it; depositing the top layer 12 based on molybdenum on the selenization barrier layer 10; and - converting said molybdenum-based layer to a molybdenum sulfide and / or selenide. This transformation step may be a separate step prior to the formation of the CIS, CGS or CZTS semiconductor layer or a step performed during the selenization and / or sulphidation of the CIS, CGS or CZTS semiconductor layer, such as this selenization and / or sulphurization either during the deposition of said semiconductor layer or after the deposition of metal components called precursors of the semiconductor layer. The deposition of the various layers is for example carried out by magnetron assisted sputtering, but it is alternatively another method of any suitable type. The invention also relates to a semiconductor device 20 (FIG. 2) using the conductive substrate 1 described above to form one or more photoactive layers 22, 24. The first photoactive layer 22 is typically a doped layer of type p, for example based on Cu copper chalcopyrite, indium In, and Se selenium and / or S sulfur. It may be, for example, as previously explained, CIS, CIGS or CZTS. The second photoactive layer 24 is n-type doped and called buffer. It is for example composed of CdS (cadmium sulphide) and formed directly on the first photoactive layer 22. In a variant, the buffer layer 24 is for example based on InxSy, Zn (O, S) or ZnMgO or in another material of any suitable type. In a variant also, the cell does not comprise a buffer layer, the first photoactive layer 22 itself being able to form a homojunction p-n. In general, the first photoactive layer 22 is a p-type or homojunction p-n layer obtained by the addition of alkaline elements. The deposition of the photoactive layer comprises selenization and / or sulfurization steps, as explained in more detail below. The deposition can be carried out by evaporation of the elements Cu, In, Ga and Se (or Cu, Sn, Zn, S).

During these selenization and / or sulphidation steps, the molybdenum-based top layer 12 is converted into a Mo (S, Se) 2 -based layer 12 '. This transformation concerns for example the entirety of the upper layer 12. The semiconductor device 20 therefore comprises: the carrier substrate 2 and the electrode coating 6 'formed on the carrier substrate 2 and whose upper layer 12' has been transformed. The electrode coating 6 'comprises: the main layer 8 based on molybdenum; the selenization barrier layer 10 formed on the molybdenum-based main layer 8; and - the M (S, Se) 2 -based ohmic contact top layer 12 formed on the selenization barrier layer 10. On the ohmic contact layer 12 'and in contact therewith, the device semiconductor material comprises the photoactive semiconductor layer (s) 14, 16.

The invention also relates to a photovoltaic cell 30 comprising a semiconductor device 20 as described above. The cell comprises for example, as illustrated in FIG. 2: the semiconductor device 20 formed by the layers 8, 10, 12 ', 22 and 24; A transparent electrode coating 32, for example ZnO: Al, formed on the first photoactive layer 22, and on the buffer layer 24 in the presence of the latter, with optional interposition between the transparent electrode coating 32 and the semi device conductor 20 of a passivating layer 34, for example intrinsic ZnO or intrinsic ZnMgO.

The transparent electrode coating 32 alternatively comprises a layer of zinc oxide doped with gallium, or boron, or a layer of ITO. In general, it is a transparent conductive material (TCO) of any suitable type. For good electrical connection and good conductance, a metal grid (not shown in Figure 2) is then optionally deposited on the transparent electrode coating 32, for example through a mask, for example by electron beam. This is for example a grid 10 of Al (aluminum) for example of about 2pm thick on which is deposited a grid of Ni (nickel) for example of about 50nm thick to protect the layer of Al. The cell 30 is then protected from external aggression. For example, it comprises a counter-substrate (not shown) covering the front electrode coating 32 and laminated to the carrier substrate 2 via a lamination interlayer (not shown) made of thermoplastic material. This is for example a sheet of EVA, PU or PVB. The subject of the invention is also a photovoltaic module comprising a plurality of photovoltaic cells formed on the same substrate 2, 20 connected in series with each other and obtained by shading the layers of the semiconductor device 20. The subject of the invention is also a method of manufacture of the semiconductor device 20 and the photovoltaic cell 30 above, which method comprises a step of forming a photoactive layer by selenization and / or sulfurization. There are many known methods for producing a Cu (In, Ga) (S, Se) 2 photoactive layer. The photoactive layer 22 is for example a layer of CIGS formed as follows. In a first step, the precursors of the layer are deposited on the electrode coating 6. A metal stack consisting of alternating layers of CuGa and In type is for example deposited by magnetron sputtering at room temperature, on the electrode coating. 6. A selenium layer is then deposited at room temperature directly on the metal stack, for example by thermal evaporation. In a variant, the metal stack has, for example, a Cu / In / Ga / Cu / In / Ga type multilayer structure. In a second step, the substrate undergoes a high temperature heating treatment called RTP ("Rapid Thermal Process"). RTP "in English), for example at about 520 ° C in an atmosphere composed for example of sulfur gas, for example based on S or H2S, thus forming a layer of CulnxGai_x (S, Se) 2. An advantage of this method is that it does not require an external source of selenium vapor. The loss of some of the selenium during heating is compensated by a deposit of excess selenium on the metal stack. The selenium required for selenization is provided by the deposited selenium layer. Alternatively, the selenization is obtained without the deposition of a selenium layer but by an atmosphere containing selenium gas, for example based on Se or H2Se, prior to exposure to a sulfur-rich atmosphere. The sulphurization step may optionally abstain from a buffer layer, for example CdS.

As explained above, it may be advantageous to deposit an alkali-based layer, for example sodium, for a precise determination of sodium in the photoactive layer. Prior to the deposition of the CuGa and In metal stack, the alkalis are introduced, for example, by depositing a layer of sodium selenide or a compound containing sodium way to introduce for example of the order of 2.1015 sodium atoms per cm2. The metal stack is deposited directly on this layer of sodium selenide. It should be noted that there are numerous possible variants for forming the layers of CI (G) S or CZTS, which include, for example, coevaporation of the elements mentioned above, chemical vapor deposition, electrochemical deposition of metals, selenides or chalcopyrites, reactive sputtering of metals or selenides in the presence of H2Se or H2S. In general, the method of manufacturing the photoactive layer 22 is of any suitable type. All methods of manufacturing CIS or CZTS type layers use a high temperature heating step in the presence of selenium and / or sulfur in the vapor state or in the liquid state. 5

Claims (5)

REVENDICATIONS1. Conductive substrate (1) for a photovoltaic cell, comprising a carrier substrate (2) and an electrode coating (6) formed on the carrier substrate (2), the electrode coating (6) comprising: - a main layer (8) based molybdenum formed on the carrier substrate (2); a selenization barrier layer (10) formed on the molybdenum-based main layer (8) and based on molybdenum oxynitride; and on the selenization barrier layer (10), a molybdenum-based top layer (12), the carrier substrate is in an alkaline-containing material, the conductive substrate comprising an alkaline barrier layer (4) formed on the carrier substrate and under the molybdenum-based main layer, the alkali-barrier layer being based on silicon nitride, wherein the molybdenum-based top layer has a thickness of at least 30 nm and a maximum of 35 nm, the selenization barrier layer has a thickness of at least 20 nm and a maximum of 40 nm and the alkali barrier layer 20 has a thickness of at most 100 nm. The conductive substrate (1) according to claim 1, wherein the molybdenum-based main layer (8) has a thickness of at least 100 nm and at most 140 nm. 3. conductive substrate (1) according to claim 1 or 2, comprising no other coatings. 4. A method of manufacturing a photovoltaic cell or module comprising the use of a conductive substrate according to any one of the preceding claims for forming a photoactive layer (22) by selenization and / or sulphidation, on said molybdenum-based top layer (12), the step of transforming said molybdenum-based top layer being performed before or during the formation of said photoactive layer (22), preferably during. 5, A manufacturing method according to claim 4, wherein the step of forming the photoactive layer (22) comprises a step of selenization and / or sulfurization at a temperature greater than or equal to 300 ° C.