FR2981592A1 - PROCESS FOR THE MANUFACTURE BY MECANOSYNTHESIS OF A CZTS POWDER, ITS USE FOR FORMING A THIN LAYER - Google Patents

Abstract

A method of manufacturing by mechanosynthesis of a powder of the compound Cu ZnSnS, the process comprising a step in which a grinding is carried out of a mixture containing: the chemical elements Sn and S in elemental form; the ZnS and Cu Sn precursors; the mixture containing Cu, Zn, Sn and S according to the stoichiometric proportions in which they are in the Cu ZnSnS compound. The invention also relates to the use of the method for producing a thin layer, particularly for the composition of a photovoltaic cell.

Description

A PROCESS FOR THE MANUFACTURING BY MECANOSYNTHESIS OF A CZTS POWDER, ITS USE TO FORM A THIN LAYER TECHNICAL FIELD The present invention is in the field of photovoltaic cells based on a thin layer. It relates in particular to a method of manufacturing by mechanosynthesis of a powder of the compound Cu2ZnSnS4, and its use to make a thin layer. STATE OF THE ART The depletion of fossil fuels encourages the development of renewable energies. Among these energies, the production of electricity by the conversion of sunlight is a promising way. This conversion requires the use of photovoltaic cells which are composed of a semiconductor material, generally silicon. Nevertheless, in recent years, a new generation of thin-film-based photovoltaic cells does not include silicon. These cells have the particular advantages of having a low manufacturing cost, while offering acceptable performance, long-term performance and a light and flexible structure for their implementation in a large number of configurations. The thin layers composing these new photovoltaic cells are for example based on CdTe (cadmium telluride), CuIn0,2Ga0,2Se (CIGS) or Cu2ZnSnS4 (CZTS). Among these compounds, Cu 2 ZnSnS 4 is particularly interesting because it exhibits neither the toxicity of CdTe nor the high cost of CuIn0.7Ga0.3Se. The compound Cu 2 ZnSnS 4 can be deposited in the form of thin layers by vacuum or liquid methods. The implementation of these thin film deposition processes most often requires the Cu2ZnSnS4 to be available in the form of a powder, possibly in solid form following a sintering or melting operation. However, the presence in such a powder of secondary phases constituting impurities must be best avoided in order to optimize the performance of photovoltaic cells based on thin layers made of Cu 2 ZnSnS 4. Furthermore, it should also avoid obtaining a Cu2ZnSnS4 powder whose average grain size (and / or crystallites) is too large in order not to degrade the quality of the thin layer made from this powder, in particular when the deposition of the thin layer is carried out by a liquid route. SUMMARY OF THE INVENTION One of the aims of the invention is therefore to avoid or mitigate the disadvantages described above, by proposing a process which makes it possible in particular to manufacture a Cu 2 ZnSnS 4 powder having a good degree of purity. The present invention thus relates to a method of manufacturing by mechanosynthesis of a powder of the compound Cu2ZnSnS4, the process comprising a step in which a grinding is carried out of a mixture containing: the chemical elements Sn and S in elemental form; The ZnS and Cu3Sn precursors; the mixture containing Cu, Zn, Sn and S in the stoichiometric proportions in which they are in the compound Cu2ZnSnS4.

The manufacturing process of the invention is characterized in particular by the presence of ZnS and Cu3Sn in the mixture to be ground. The use of these precursors makes it possible to obtain a powder of Cu 2 ZnSnS 4 having a good degree of purity, without this requiring the implementation of an additional annealing step which would have the disadvantage of increasing the average grain size ( and / or crystallites) of Cu2ZnSnS4 powder. The invention also relates to the use of the manufacturing method for producing a thin layer, which enters for example into the composition of a photovoltaic cell. The thin layer can be produced by means of a vacuum deposition process such as, for example, sputtering or evaporation; or a liquid deposition process such as, for example, screen printing, spin coating or tape casting. DETAILED DESCRIPTION OF THE INVENTION In order to produce the Cu 2 ZnSnS 4 powder using the manufacturing method of the invention, a mixture containing the Sn and S chemical elements and the ZnS and Cu 3 Sn precursors is milled in order to produce a mechanical alloying. The chemical elements Sn and S are in elemental form, that is to say in the pure state or practically pure (ie typically more than 99% pure). These chemical elements, or the ZnS or Cu3Sn precursors may be independently of one another in the form of a powder or nugget, it being understood that the nugget is converted to powder during grinding. For example, when they are in the form of a powder, the chemical elements Sn, S, the precursors ZnS and / or Cu3Sn have grains with an average size of between 100 nm and 3 mm, for example measured using a laser granulometer in the dry or liquid process. Regarding mechanosynthesis, also called high energy milling (or "high energy ball milling"), it is an alloy synthesis process well known to those skilled in the art. It is described for example in the document "Nanomaterials - Structure and Development, Paul Costa, Engineering Techniques, reference NM 3.010, chapter 3.3".

Mechanosynthesis essentially consists in using the mechanical energy resulting from grinding to induce chemical reactions between the components of the mixture to be ground. These reactions usually occur at low temperatures. Mechanosynthesis can therefore be carried out without the addition of thermal energy. In practice, the mechanosynthesis consists of violently stirring the mixture to effect grinding in the presence of one or more movable bodies such as for example a ball or a ball.

The moving bodies are generally composed of a dense material and of high hardness (tungsten carbide, steel, ...). The stirring of the moving bodies can be carried out using different means which vary according to the type of mill used. The grinding of the mixture can thus be carried out using a grinder conventionally used to carry out a mechanosynthesis, for example chosen from a vertical vibration mill, a ball mill, a planetary mill or an attritor. Under the action of grinding, the constituents of the mixture to be ground undergo shocks occurring between the moving bodies moving at high speed, or between these movable bodies and the walls of the tank.

The frequency and power of the shocks affect the intensity and power of the grinding, and can be increased by increasing the size or number of moving bodies, stirring intensity, or decreasing the amount of powder to be milled.

It should be noted, however, that one skilled in the art can operate in a wide range of intensity and grinding power in order to carry out the mechanosynthesis step. The duration of grinding also influences the mechanosynthesis. Although it depends on the grinding conditions, the grinding time is generally at least 24 hours, for example between 24 hours and 144 hours, preferably between 72 hours and 120 hours. The grinding may be carried out under vacuum or in a chemically inert atmosphere, in particular with respect to the constituents of the mixture and the compound Cu 2 ZnSnS 4 in order to limit the oxidation thereof and / or the contamination by any other substance. The chemically inert atmosphere includes, for example, a gas such as nitrogen or argon. If necessary, an additive may be added to the mixture to be milled in order to carry out "wet" milling. However, preferably, the mixture does not contain any additive, so as to carry out grinding said "dry". During grinding, the grains obtained are alternately deformed, fractured and welded to each other. The resulting crystal structure modification allows the diffusion of the chemical elements, which leads to the formation of a powder of a new alloy. The powder of the Cu 2 ZnSnS 4 compound obtained at the end of the grinding is nanostructured, since it consists of nanocrystallites (possibly aggregated and / or agglomerated), ie crystallites with a mean size of, for example, between 1 nm and 10 nm. This powder has a good degree of purity, which results, for example, in an X-ray diffraction pattern and / or in a Raman spectrum which has no peak (s) of secondary phase (s) which is easily detectable.

Alternatively, the degree of purity is greater than 98%, preferably 99%, even more preferentially 99.5%. It can for example be measured by calculating the ratio, multiplied by 100, between the amplitude of the diffraction peak of greater amplitude characteristic of the compound Cu2ZnSnS4 and the amplitude of the diffraction peak of greater amplitude characteristic of the majority impurity. As regards its implementation, the powder of the synthesized compound Cu 2 ZnSnS 4 can be used as it is, in the form of ink or also in the form of a compact material. The compact material may for example be obtained using the manufacturing method of the invention further comprising, after the grinding step, a step in which the powder of the compound Cu 2 ZnSnS 4 is then hot-compacted, or cold-compacted. then sintered. The compact material can be used to make a target for sputtering. Other objects, features and advantages of the invention will now be specified in the following description of a particular embodiment of the method of the invention, given by way of illustration and not limitation, with reference to the appended FIG. . BRIEF DESCRIPTION OF THE FIGURES FIG. 1 represents the X-ray diffraction pattern of a Cu2ZnSnS4 compound powder obtained using a vertical vibration mill. The characteristic crystallographic planes of the Cu2ZnSnS4 compound are shown in the diagram. EXAMPLE OF PARTICULAR EMBODIMENTS 1. Synthesis of a Cu2ZnSnS4 powder using a ball mill. A Cu2ZnSnS4 powder is manufactured by carrying out the mechanosynthesis in a small capacity vibrating mill (model "Pulveris PO" marketed by the company Fritsch).

The "Pulveriser PO" crusher comprises an enclosure which can be sealed with a gasket which is covered with a cover sealed by screw-nuts. This cover is equipped with a valve allowing to make the vacuum if necessary or to control the atmosphere inside pregnant.

A crucible is placed in the enclosure. It is intended to receive the mixture to grind, as well as a ball of 500 grams and diameter 50 mm. The crucible and the ball are made of 10006 steel. Thermocouples make it possible to measure the outside temperature (as a reference) and the walls of the crucible which is placed in the enclosure. A vibrating tray supports the speaker to make it vibrate vertically and the ball contained in the crucible. As stated in the document "Y. Chen, M. Bibole, R. Lehazif, G. Martin, Phys. Rev. B, vol. 48, pages 14 to 21 (1993) ", the intensity of the grinding can be modulated according to the amplitude of the vertical vibrations according to the following formula: I = (MM - Vma x - f) / Mp, in which: I: intensity of grinding Mb: mass of the ball Mp: mass of the mixture to grind (in the form of powder, nugget, ...) Vma x: maximum speed of the vibrating plate, which corresponds to the product of the amplitude of vibration and pulsation of the vibrating tray (fixed at 2H x 50 Hz). f: shock frequency that can be measured with a differential transducer on the top of the enclosure.

Depending on the amplitude chosen, the intensity is for example the case where Mb = 500 g and Mp = 4 g: amplitude of 0.9 mm: I = 600 m / s2 amplitude of 1.3 mm: I = 1200 m / s2 An intensity of I = 1200 m / s2 is retained for the following grinding. In a glove box under an argon atmosphere, the grinder is placed and a mixture of 4 g is prepared from the following constituents: 0.877 g of sulfur powder (sold by Prolabo under the number 28260, purity of 99.5 %); 0.362 g of tin powder (marketed by GoodFellow under the number LS 279681, purity of 99.9%); 1.874 g of powder of a precursor Cu3Sn obtained by mechanosynthesis, by grinding for 72 hours under argon a mixture of 3.084 g of copper powder and 1.916 g of tin powder with the mill "Spray PO". The resulting Cu3Sn powder is composed of crystallites of average size between 5 nm 10 nm; 0.887 g of a ZnS precursor in the form of nuggets (marketed by GoodFellow under the number ZN536100, purity of 99.99%). The amounts of each component in the mixture thus obtained make it possible to ensure that Cu, Zn, Sn and S are present in the stoichiometric proportions of the Cu 2 ZnSnS 4 compound to be synthesized.

The mixture and the ball are then introduced into the crucible of the mill. After sealing the enclosure lid, the valve is closed to preserve an argon atmosphere. The mill is removed from the glove box and fixed on the vibratory tray whose amplitude of vibration is fixed at 1.3 mm. Shocks produced during grinding only result in a very small increase in temperature, which can be considered constant.

After 120 hours of grinding, the Cu 2 ZnSnS 4 powder obtained is extracted from the mill for later analysis. 2. Characterization of the Cu2ZnSnS4 powder obtained.

The Cu 2 ZnSnS 4 powder obtained is analyzed with an X-ray diffractometer ("X'Pert Pro" model marketed by the company PANalytical) whose analysis parameters are as follows: X-ray emission tube (anode) in cobalt emitting a radiation of a wavelength of 1.78897 Å; - divergence slot 1; - mask = 10; - anti-diffusion slot = 2; -10- - receiving slot = 6.6; - no measurement = 0.02 ° for 1000 seconds; - scanning angles = 20 ° <20 <160 °; - filters placed upstream of the divergence slot and downstream of the receiving slot. The diffraction pattern X obtained is reproduced in FIG. 1. The comparison of the peaks identified in this diagram with those of reference JCPDS 00-026-0575b confirms the obtaining of the Cu 2 ZnSnS 4 powder. From the width of the peaks at mid-height, the average crystallite size of the Cu 2 ZnSnS 4 powders is calculated using the Scherrer relation. For this calculation, the form factor "K" is set at 0.89 considering that the crystallites are spherical. The average crystallite size of the powder of the Cu 2 ZnSnS 4 powder is between 1 nm and 10 nm. The Cu2ZnSnS4 powder obtained also has a very high degree of purity as evidenced by the absence of parasitic peaks on the X-ray diffraction pattern, and the Raman spectrum not reproduced here. According to the method known to those skilled in the art, the optical properties of the compound Cu2ZnSnS4 were determined from the calculation of the gap energy. For this, the absorbance was measured by dispersion in ethanol then exposure to an incident radiation scan of between 200 nm and 1200 nm. The curve a2 (absorption coefficient determined from the absorbance) is then plotted against the energy of the incident radiation. The linear part of this curve is then extrapolated to determine the value of the energy for which the absorbance is zero. This value corresponds to that of the gap energy.

The gap energy of the compound Cu2ZnSnS4 thus calculated is Egap = 1.5 ev (theoretical value of Egap = 1.5 ev). It follows from the foregoing description that the manufacturing method of the invention makes it possible to manufacture a nanostructured Cu2ZnSnS4 powder whose degree of purity is improved, without this requiring an annealing step.

Claims (14)

1) Method of manufacturing by mechanosynthesis of a powder of the compound Cu2ZnSnS4, the method comprising a step in which is carried out a grinding of a mixture containing: - the chemical elements Sn and S in elemental form - the precursors ZnS and Cu3Sn; said mixture containing Cu, Zn, Sn and S in stoichiometric proportions in which they are in the compound Cu2ZnSnS4. 2) The manufacturing method according to claim 1, wherein the chemical elements Sn, S, the precursors ZnS or Cu3Sn are, independently of each other, in the form of powder or nugget. 3) The manufacturing method according to claim 2, wherein, when in the form of a powder, the chemical elements Sn, S, the precursors ZnS and / or Cu3Sn have grains of average size between 100 nm and 3 mm. 4) A method of manufacture according to any one of the preceding claims, wherein the grinding of the mixture is carried out using a grinder selected from a vertical vibration mill, a ball mill, a planetary mill or an attritor. 5) A method of manufacture according to any one of the preceding claims, wherein the grinding is carried out under vacuum or in an atmosphere chemically inert vis-à-vis the constituents of the mixture and the compound Cu2ZnSnS4.-13- 6) The manufacturing method according to any one of the preceding claims, wherein the powder of the compound Cu2ZnSnS4 is composed of crystallites with a mean size of between 1 nm and 10 nm. 7) A manufacturing method according to any one of the preceding claims, wherein after the grinding step, the method further comprises a step wherein the powder of the compound Cu2ZnSnS4 is hot compacted, or cold compacted and sintered. 8) The manufacturing method according to claim 7, wherein the compact material obtained is used to achieve a target for sputtering. 9) Use of the manufacturing method as defined in any one of claims 1 to 8, for producing a thin layer. 10) Use according to claim 9, wherein the thin layer is made using a vacuum deposition process. 11) Use according to claim 10, wherein the vacuum deposition process is sputtering or evaporation. 12) Use according to claim 9, wherein the thin layer is produced using a liquid deposition process. 13) Use according to claim 12, wherein the liquid deposition process is screen printing, spin coating or strip casting. 14) Use according to any one of claims 9 to 13, wherein the thin layer is in the composition of a photovoltaic cell.