FR2969389A1 - CONDUCTIVE SUBSTRATE BASED ON MOLYBDENUM - Google Patents

Abstract

The invention relates to a conductive substrate (2, 4) for a photovoltaic cell comprising a dielectric substrate (2) containing alkali and an electrode coating (4) formed on the dielectric substrate (2). The electrode coating (4) comprising a layer of molybdenum. The molybdenum layer has an average surface porosity less than or equal to 10%, preferably less than or equal to 8%.

Description

1 MOLYBDENE CONDUCTIVE SUBSTRATE

The present invention relates to the field of photovoltaic cells based on Cu (In, Ga) Se2 (CIS or CIGS) deposited by coevaporation bitherme. US-B-5,436,204 discloses a photovoltaic cell of this type. The Cu (In, Ga) Se2 layers of photovoltaic cells are usually produced by isothermal coevaporation. The work done in recent years, however, has shown that the Cu (In, Ga) Se2 layers produced by bitherme coevaporation improve the efficiency of the cell. The invention is more particularly concerned with providing a high-performance conductive substrate on which to form the Cu (In, Ga) Se 2 layer produced by bitherme coevaporation.

US-B-5,626,688, WO-A-02/065554 and WO-A-09/080931 disclose conductive substrates (dielectric substrates on which electrode coatings are deposited) suitable for Cu (In, Ga) Se2 and above generally to chalcopyrite-based layers. An object of the invention is to provide a conductive substrate improving the performance of a photovoltaic cell based on Cu (In, Ga) Se2 deposited by coevaporation bitherme. For this purpose, the subject of the invention is a conductive substrate comprising: a dielectric substrate containing alkalis; an electrode coating formed on the dielectric substrate, the electrode coating comprising a molybdenum layer, wherein the molybdenum layer has an average surface porosity less than or equal to 10%, preferably less than or equal to 8%. In the present invention, the porosity of conductive substrates is described by its apparent average surface porosity ("average" refers to "porosity"), which will be chosen as the reference parameter. The conductive substrate and the semiconductor device according to the invention make it possible to obtain very good performances for these chalcopyrite-type layers formed by bitherme coevaporation. The conductive substrate is adapted to the deposition conditions of such layers, in particular because of its good heat resistance and its good resistance to selenization. The conductive substrate also has a very good electrical conductivity and has the particularity of forming in the presence of selenium a layer of molybdenum diselenide (MoSe2) which promotes the ohmic contact between the molybdenum electrode and the Cu (In, Ga) layer. ) Se 2. The conductive substrate also allows the migration of the alkaline ions from the dielectric substrate to the Cu (In, Ga) Se 2 layer during its formation. The presence of these alkaline ions, especially Na +, improves the performance of the photovoltaic cells. A low surface porosity of the molybdenum layer makes it possible to increase the performance of the cell, as shown by the results below.

According to particular embodiments, the conductive substrate according to the invention also comprises one or more of the following technical characteristics, taken separately or according to all the possible technical combinations: the electrode coating comprises only a single electroconductive layer; the molybdenum layer is in contact with the dielectric substrate; the dielectric substrate contains at least 5% by weight of alkali, the dielectric substrate being, for example, made of soda-lime-silica glass. The invention also relates to a semiconductor device comprising a conductive substrate and a Cu (In, Ga) Se2 (CIS, CGS or CIGS) layer formed on the conductive substrate, wherein said conductive substrate is as described herein. -above. According to particular embodiments, the device according to the invention further comprises one or more of the following technical characteristics, taken separately or according to all the possible technical combinations: the Cu layer (In, Ga Se2 is in contact with the molybdenum layer; the device further comprises an additional electrode coating formed on the Cu (In, Ga) Se2 layer; the Cu (In, Ga) Se2 layer is a CIGS layer. The invention also relates to a photovoltaic cell comprising a semiconductor device as described above and a photovoltaic module comprising a plurality of photovoltaic cells of the kind formed on the same substrate. The invention also relates to a method of manufacturing a semiconductor device, comprising the steps of: - depositing a layer of molybdenum on a dielectric substrate containing alkali to form an electrode coating; forming a layer of Cu (In, Ga) Se2 on the molybdenum layer, in which the molybdenum layer has an average surface porosity of less than or equal to 10%, preferably less than or equal to 8% and in that the Cu (In, Ga) Se2 layer is formed by a bitherme coevaporation method.

According to particular embodiments, the manufacturing method according to the invention also has one or more of the following technical characteristics, taken separately or according to all the possible technical combinations: the molybdenum layer is deposited directly on the dielectric substrate; the Cu (In, Ga) Se 2 layer is formed directly on the molybdenum layer; the electrode coating comprises only one electroconductive layer; The dielectric substrate contains at least 5% by weight of alkali, the dielectric substrate being, for example, made of soda-lime-silica glass. the device further comprises an additional electrode coating formed on the Cu (In, Ga) Se2 layer; the Cu (In, Ga) Se2 layer is a CIGS layer. The invention also relates to a method of manufacturing a photovoltaic cell including a semiconductor device manufactured according to the above method, the method preferably comprising an additional step of laminating the substrate with a counter-substrate protecting the layers of the semiconductor device, through a thermosetting plastic film. The invention will be better understood on reading the description which follows, given solely by way of example, and with reference to the appended drawings, in which: FIGS. 1 and 1a are respectively an electron microscopy image with sectional scanning of a photovoltaic cell according to the invention and a schematic view corresponding thereto; FIG. 2 is a graph illustrating the relationship between the apparent average surface porosity of a moltenbdenum layer deposited by DC magnetron sputtering and the deposition pressure, for a power of 3 kW; FIG. 2bis is a graph illustrating the sodium density of the Cu (In, Ga) Se 2 layer formed on various conductive substrates, the molybdenum layers of which have different average apparent surface porosities, obtained by SIMS; FIGS. 3 to 6 are graphs illustrating, on the basis of five samples, the relationship between the apparent average surface porosity of the molybdenum layer, at a power of 3 kW, and the performance of the Cu-based cell. (In, Ga) Se2 deposited by coevaporation bitherme.

The illustrated cell 1 comprises: a substrate 2 made of silico-soda-lime glass; a first electrode coating 4 formed on the substrate 2, called the rear contact coating; a p-type layer 6 of Cu (In, Ga) Se2 formed on the first electrode coating; a n-type layer 8 called said buffer, for example composed of CdS, formed on the Cu (In, Ga) Se2 layer; a transparent electrode coating 10, with optional interposition between the transparent electrode coating and the buffer layer of a passivating layer 12, for example ZnO. Note however that, alternatively, the cell 1 does not comprise a buffer layer, the Cu (In, Ga) Se2 layer itself being able to form a homojunction p-n.

In a conventional way, the notation (In, Ga) indicates that it is a combination of In1_, Ga, with 0 x 1. We mean, throughout the text, by "a layer A formed (or deposited) on a layer B ", a layer A formed either directly on the layer B and therefore in contact with the layer B, is formed on the layer B with the interposition of one or more layers between the layer A and the layer B. Note the term "electrode coating" is used throughout the text to mean a coating comprising at least one electrically conductive layer, that is to say the electrical conductivity of which is ensured by the mobility of the electrons. The conductive substrate 2, 4 on which the Cu (In, Ga) Se 2 layer is formed is particularly efficient in the case of a Cu (In, Ga) Se 2 based photovoltaic cell formed by coevaporation bitherme and will be described more in detail below.

The first electrode coating 4, or "back contact", comprises a molybdenum layer whose average surface porosity is less than or equal to 10%, preferably less than or equal to 8%. It has been found, as will be seen below with reference to FIGS. 3 to 6, that providing a dense molybdenum layer makes it possible to improve the performance of photovoltaic cells based on Cu (In, Ga) Se2 formed by coevaporation bitherme. The first electrode coating 4 as illustrated comprises only a single electroconductive layer, namely the molybdenum layer. In this case, the molybdenum layer has a thickness for example of between 200 nm and 500 nm, for example between 250 nm and 450 nm. At this thickness, the molybdenum layer is reflective. It should be noted that throughout the text, "a single layer" means 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 dielectric substrate by several targets to ultimately form a single layer of the same material, namely molybdenum. Alternatively, the electrode coating is formed of several electroconductive layers. It is in this case formed of a layer of molybdenum (Mo) and one or more sub-layers between the dielectric substrate 2 and the molybdenum layer. It is for example a layer of a metal other than Mo such as Cu (copper), with possible interposition of a nitride layer of a metal other than Mo such as TiN (titanium nitride) . In this case, the molybdenum layer has for example a thickness between 10 nm and 50 nm.

Thus, in general, the electrode coating 4 comprises a molybdenum layer whose average surface porosity is less than or equal to 10%, preferably less than or equal to 8%. The electrode coating 4 is in contact with the dielectric substrate 2. The alkaline ions can thus migrate easily from the dielectric substrate 2 and through the electrode coating 4, to the upper layers, that is to say up to the layer 6 of Cu (In, Ga) Se2. In the case where the electrode coating 4 comprises only an electroconductive layer, the molybdenum layer is then in contact with the dielectric substrate 2.

Also advantageously, the layer 6 of Cu (In, Ga) Se2 is formed directly on the molybdenum layer and therefore in contact therewith. The dielectric substrate 2 is for example a sheet with a glass function, for example transparent. This is for example a glass sheet, for example made in a silico-soda-lime glass.

The sheet may have any type of size, especially at least one dimension greater than 1 meter. Silica-soda-lime glass is understood to mean a glass whose composition comprises silica (SiO 2) as forming oxide and oxides of sodium (sodium Na 2 O) and calcium (lime CaO). This composition preferably comprises the following constituents in a content varying within the weight limits defined below: SiO 2 60-75% Al 2 O 3 0-10% B2O 3 0-5%, preferably 0

7 CaO 5-15% MgO 0-10% Na2O 5-20% K2O 0-10% BaO 0-5%, preferably 0. The glass has for example the following mass composition: SiO2 72% Al2O3 1 0/0 B203 0% CaO 9% MgO 3% Na2O 14% K2O 0.5% BaO 0% 15 Impurities: 0.5% In general, the substrate is a dielectric sheet containing alkalis, in particular a substrate having a mass composition with at least 5% alkaline. The substrate has, for example, a content of less than or equal to 30% by weight of alkali, for example less than or equal to 20% by weight of alkali. The substrate is intended to be at the rear of the photoactive layer with respect to the light source, that is to say to receive the incident light after the photoactive layer. It is thus a conductive substrate called "back". The p-type layer 6 of Cu (In, Ga) Se2 will now be described. The Cu (In, Ga) Se 2 layer 6 is formed by bitherme coevaporation. This process is for example described in US Pat. No. 5,436,204. A bithermal coevaporation method comprises the steps of: depositing precursors of Cu and (In, Ga) on the conductive substrate 2, 4; - heating the conductive substrate 2, 4 in the presence of Se or H2Se vapor at an intermediate temperature between 350 ° C and 500 ° C, for example 450 ° C, and maintain it at this intermediate temperature; Feeding the vapor chamber of (In, Ga) in the presence of Se or H2Se while heating the conductive substrate 2, 4 to a higher recrystallization temperature between 500 ° C. and 650 ° C., for example 620 ° C. ; maintaining the conductive substrate 2, 4 at the higher recrystallization temperature in the presence of (In, Ga) vapor and Se or H2Se; - Stop the supply of (In, Ga) and cool the conductive substrate 2, 4 in the presence of Se or H2Se vapor to a low temperature between 250 ° C and 350 ° C, for example 300 ° C. By way of comparison, an isothermal coevaporation process, for which the molybdenum-based conductive substrates 2, 4 are intended in the prior art, comprises successive steps of: bringing the conductive substrate 2, 4 into a chamber of selenization containing H2Se or Se vapor; - Heating the conductive substrate 2, 4 at a constant temperature, for example of the order of 575 ° C, for example by means of halogen lamps; feed the chamber with steam of In and / or Ga, that is to say (In, Ga), for example for 20 minutes; closing the supply of ln and / or Ga and supplying the Cu vapor chamber with a power variation necessary for heating the conductive substrate 2, 4 indicating a phase transition between a Cu-poor phase and a rich phase in Cu; closing the Cu feed, supplying the ln and / or Ga vapor chamber again and stopping the heating of the conductive substrate 2, 4; - Stopping the supply of steam ln and / or Ga, H2Se or Se when the temperature of the conductive substrate 2, 4 becomes lower than a predetermined temperature, for example of the order of 350 ° C. In an isothermal coevaporation process, the conductive substrate 2, 4 is therefore maintained at a single constant temperature, unlike the bitherme coevaporation method in which the conductive substrate 2, 4 is maintained at two different constant temperatures (intermediate temperature and recrystallization temperature ). The invention thus also relates to a method of manufacturing a semiconductor device, comprising the steps of: - depositing a layer of molybdenum on a dielectric substrate 2 containing alkali to form an electrode coating 4; - To form a layer 6 of Cu (In, Ga) Se2 (CIS, CGS or CIGS) on the molybdenum layer.

The electroconductive molybdenum layer has an average surface porosity of less than or equal to 10%, preferably less than or equal to 8%, and the CIGS layer is formed by the bithermal coevaporation method described above. The CdS buffer layer mentioned above is obtained, for example, by immersing the Cu (In, Ga) Se 2 -conductive substrate 2, 4 in a solution at 60 ° C. formed of an ammonia solution containing cadmium salts. and thiocarbamide (or thiourea) ((NH2) 2CS) The cadmium salts first react with ammonia to form a cadmium hydroxide CdOH2, and then thiocarbamide-substituted sulfur is substituted for CdOH2 oxygen. leading to the formation of CdS. The optional passivation layer is for example formed by the magnetron sputtering deposition of a ZnO layer, for example of the order of 100 nm, on the CdS layer. The transparent electrode coating 10 is for example a layer of ZnO: Al (eg atomic percentage of 3% Al) deposited on the passivation layer. For example, it is deposited by reactive DC magnetron sputtering under a 2 pBar atmosphere in the presence of an argon-oxygen mixture (20% oxygen, 80% argon). For a good electrical connection and a good conductance, a metal grid is then optionally deposited on the transparent electrode coating 4, for example through a mask, for example by electron beam. This is for example a grid of Al (aluminum) for example about 2pm thick on which is deposited a grid of Ni (nickel) for example about 50nm thick to protect the Al layer. The cell 1 is then protected. It comprises for example for this purpose a counter-substrate covering the front electrode coating 10 and laminated to the substrate 2 via a lamination interlayer of thermosetting plastic material. This is for example a lamination interlayer EVA, PU or PVB.

EXAMPLES and RESULTS: DC magnetron sputtering power and deposition pressure influence the apparent surface porosity of the deposited molybdenum layer. Indeed, the power (W / cm2) applied to the target influences the following parameters: the spraying rate is directly proportional to the power; the mobility of the atoms on the substrate increases because of the increase of the flow of the atoms and the flow of gas coming to be reflected on the substrate. The pressure, meanwhile, determines the average free path of Mo atoms from the target to the substrate. The mean free path corresponds to the length between two successive collisions of Mo atoms with the gas atoms. It is proportional to the inverse of the pressure p (usually between 1 and 20 pBar). Thus, the average number N of Mo atom collisions with the gas atoms between the target and the substrate is proportional to d.p where d is the substrate - target distance. The kinetic energy of the atoms arriving on the substrate decreases exponentially with the product d.p. The product d.p is therefore a relevant criterion for comparing the deposition conditions and predicting the apparent surface porosity of the molybdenum layer. At constant power and constant substrate-target distance, the apparent surface porosity of the deposited layer therefore depends on the deposition pressure. At constant pressure and constant substrate-target distance, an increase in the deposition power is equivalent to a rise in substrate temperature during deposition. The experiments carried out were intended to characterize the apparent surface porosity of the molybdenum layer for a given power and distance, by varying the deposition pressure, and then to determine the impact of this apparent surface porosity. the molybdenum layer on the performance of photovoltaic cells based on a layer of CIGS deposited by co-evaporation bitherme.

FIG. 2 illustrates the apparent surface porosity results obtained for various thin layers of molybdenum (Mo) deposited on respective 3 mm thick silico-soda-lime glass substrates (SGG PLANILUX) in a magnetron sputtering deposition machine. DC at a power of 3 kW at five different deposition pressures (0.75 mTorr, 1.5 mTorr, 4.5 mTorr, 7.5 mTorr and 11.25 mTorr). The mass composition of SGG PLANILUX glass is as follows: SiO2 72% AI2O3 10% B203 0% CaO 9% MgO 3% Na2O 14% K2O 0.5% BaO 0% Impurities: 0.5% Each layer of molybdenum a a thickness of 335 nm. The apparent surface porosity of the films was evaluated by the treatment of surface SEM images. The images were previously binarized to produce two classes of pixels, white pixels and black pixels. Black pixels are considered as pores. In this document, the reference parameter for surface apparent porosity is the density of black pixels in binarized SEM images. It is a JEOL 7600 SEM, the apparent surface porosity is the ratio of the number of black pixels to the total number of pixels. The images used have a magnification of 60,000, they have a format of 1280 x 1024 pixels, and represent a length of 1997 nm, or 1.56 nm / pixel. In the example, the images were processed by Quantinet Q550 Leica software. The evolution of the apparent average surface porosity is shown as ordinates in percent, as a function of the deposition pressure in mTorr. The target-substrate distance was 5cm. The respective values of apparent surface porosity are Pressure (mTorr) Apparent porosity of + -15 area (%) 0.75 4.92 1.023 1.5 6.02 1.613 4.5 10.7 1.918 7.5 15 2.69 11 16.9 1.285 Table 1

At constant power, the apparent surface porosity of the Mo layer increases with increasing pressure.

Above all, these experiments made it possible to characterize the apparent surface porosity of the layer for known deposition pressures. CIGS layers were then deposited by bitherme coevaporation on different conductive substrates 2, 4 of FIG. 2 with apparent surface porosities of 6%, 15% and 17%, respectively.

By way of example, the process for obtaining the CIGSe layer is described below. The CIGSe layer was synthesized according to a three-step process: Step 1: The elements ln, Ga and Se were co-evaporated on a substrate whose temperature was measured at 400 ° C. (heating face). This step lasts until the thin layer of (In, Ga) 2Se3 thus obtained has a thickness of 1 μm. Step 2: The substrate is heated to 620 ° C. The elements Cu and Se are provided on the substrate until the composition of the thin film in growth corresponds to [Cu] / [In + Ga]> 1. Step 3: The elements ln, Ga and Se are provided on the substrate until the composition of the thin film in growth corresponds to [Cu] / [In + Ga] <1. The CIGSe layer has a standard elemental composition Cu (22%), Ga (8%), In (20%) and Se (50%). The grain size is on the order of 1 μm. A thin layer of CdS (thickness 50 nm) is then deposited on the glass / Mo / CIGSe structure by chemical bath, in order to make the pn junction.

A bilayer ZnO (50 nm) / ZnO: Al (200 nm) is then deposited on the CdS layer at room temperature. These structures were analyzed by SIMS (Secondary Ion Mass Spectroscopy), as shown in FIG. 2bis, which shows the sodium content of the ZnO layer and of the CIGSe layer for conducting substrates whose molybdenum layers have apparent porosities. respective areas of 6%, 15% and 17%. These experiments show that the sodium content in the CIGS layer increases with the apparent surface porosity of the molybdenum layer. Experiments were then carried out to characterize the performances of CIGSe cells obtained by bitherme coevaporation as a function of the apparent surface porosity of the molybdenum layer. The CIGSe layers were obtained under the same conditions as described above for FIG. 2. Five layers of Mo were deposited at 3kW on respective soda-lime glass substrates under the same conditions as above. , ie in the same machine and at respective pressures of 0.75 mTorr, 1.5 mTorr, 4.5 mTorr, 7.5 mTorr and 11.25 mTorr.

As shown in FIGS. 3 to 6, which respectively represent the following values on the ordinate, as a function of the deposition pressure of the Mo layer (on the abscissa): - Vos, the open circuit voltage in mV as a function of the deposit pressure due to the Mo layer; FF, the form factor (in%); - Jsc, the short-circuit current (in mA / cm 2); and - Eff, the energy conversion efficiency (in%). The values reported are an average of twelve cells. The energy conversion efficiency Eff has a maximum for an apparent surface porosity of the molybdenum layer of 6% and then decreases continuously for apparent upper surface porosities. The open circuit voltage Voc does not appear to be influenced by the apparent surface porosity of the molybdenum layer. On the other hand, the shape factor FF and the short-circuit current Jsc follow the same evolution as the efficiency Eff and have a maximum for an apparent surface porosity of 6% and then decrease continuously for apparent surface porosities. These results are explained by taking into account the content of the sodium layers which is directly related to the apparent surface porosity of the molybdenum layer. In the CIGS layers, sodium is present both inside the grains and also at the grain boundaries. The solubility of sodium in CIGS is low, so when the amount of sodium exceeds the solubility limit, excess sodium segregates at the grain boundaries. As a result, the sodium signal that can be observed in Figure 2a is mainly due to sodium at the grain boundaries. This is confirmed by the fact that the open circuit voltage remains unchanged with the apparent surface porosity and thus the amount of sodium. In fact, the sodium content inside the grains influences the acceptor density in the CIGS and therefore the Voc. On the other hand, the modeling of the intensity-voltage curve of the photovoltaic cells produced shows that the drop in performance for high apparent surface porosities is mainly due to a decrease in the parallel resistance of the equivalent circuit. The grain boundaries containing too much sodium, due to an apparent surface porosity of the molybdenum layer which is too high, favor short-circuits and therefore reduced performance. The conductive substrate and the semiconductor device according to the invention therefore make it possible to obtain very good performances for these chalcopyrite-type layers formed by bitherme coevaporation. In addition, despite a high density, the molybdenum layer has been found to have sufficient mechanical stability, including the "Scotch test" (ISO2409).

Claims (13)

REVENDICATIONS1. Conductive substrate (2, 4) comprising: - a dielectric substrate (2) containing alkalis; an electrode coating (4) formed on the dielectric substrate (2), the electrode coating (4) comprising a molybdenum layer, wherein the molybdenum layer has an average surface porosity of less than or equal to 10%, preferably less than or equal to 8%. The conductive substrate (2, 4) according to claim 1, wherein the electrode coating (4) comprises only a single electroconductive layer. 3. Conductive substrate (2, 4) according to claim 1 or 2, wherein the molybdenum layer is in contact with the dielectric substrate (2). 4. conductive substrate (2, 4) according to any one of the preceding claims, wherein the dielectric substrate (2) contains at least 5% by weight of alkali, the dielectric substrate (2) being for example silicosodium glass calcium. A semiconductor device (2, 4, 6, 8, 10, 12) comprising a conductive substrate (2, 4) and a layer (6) of Cu (In, Ga) Se2 (CIS, CGS or CIGS) formed on the conductive substrate (2, 4), wherein said conductive substrate (2, 4) is according to any one of the preceding claims. 6. Device (2, 4, 6, 8, 10, 12) according to claim 5, wherein the layer (6) of Cu (In, Ga) Se2 is in contact with the molybdenum layer. The device (2, 4, 6, 8, 10, 12) according to claim 5 or 6, further comprising an additional electrode coating (10) formed on the Cu (In, Ga) Se2 layer. 8. Device (2, 4, 6, 8, 10, 12) according to any one of claims 5 to 7, wherein the Cu layer (In, Ga) Se2 is a layer of CIGS. Photovoltaic cell (1), comprising a semiconductor device (2, 4, 6, 8, 10, 12), wherein the semiconductor device is according to any one of claims 5 to 8. Photovoltaic module comprising a plurality of photovoltaic cells (1) formed on a same substrate (2) and connected together in series, wherein the photovoltaic cells (1) are according to claim 9. 11. A method of manufacturing a semiconductor device (2, 4, 6, 8, 10, 12), comprising the steps of: - depositing a layer of molybdenum on an alkaline-containing dielectric substrate (2) to form an electrode coating (4); forming a layer (6) of Cu (In, Ga) Se2 on the molybdenum layer, in which the molybdenum layer has an average surface porosity of less than or equal to 10%, preferably less than or equal to 8%, and in that the Cu (In, Ga) Se2 layer is formed by a bitherme coevaporation method. 12. The method of claim 11, wherein the molybdenum layer is deposited directly on the dielectric substrate (2). The method of claim 11 or 12, wherein the Cu (In, Ga) Se 2 layer is formed directly on the molybdenum layer.