FR2981591A1 - PROCESS FOR THE MANUFACTURE BY MECANOSYNTHESIS OF A CZTSE POWDER, ITS USE FOR FORMING A THIN LAYER - Google Patents

Abstract

A method of manufacturing by mechanosynthesis of a powder of the compound Cu ZnSnSe, the process comprising a step in which a grinding is carried out of a mixture containing: the chemical elements Cu, Sn and Se in elemental form; a precursor ZnSe; the mixture containing Cu, Zn, Sn and Se according to the stoichiometric proportions in which they are in the Cu ZnSnSe 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

MECANOSYNTHESIS MANUFACTURING PROCESS OF A CZTSE 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. In particular, it relates to a method of manufacturing by mechanosynthesis of a powder of the compound Cu2ZnSnSe4, 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), CuIn0.7Ga0.3Se (CIGS) or Cu2ZnSnSe4 (CZTSe). Among these compounds, Cu 2 ZnSnSe 4 is particularly interesting because it exhibits neither the toxicity of CdTe nor the high cost of CuIn0.7Ga0.3Se. The compound Cu 2 ZnSnSe 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 Cu2ZnSnSe4 to be available in the form of a powder, possibly in solid form following a sintering or melting operation. In order to have such a powder, the authors of the article "R. Adhi Wibowo et al., Synthesis of Cu2ZnSnSe4 compound powders by solid state reaction using elemental powders, Journal of Physics and Chemistry of Solids 71 (2010) 1702- 1706) "have proposed to obtain a powder of Cu 2 ZnSnSe 4 by mechanically synthesizing a mixture of elemental powders of Cu, Zn, Sn and Se. However, the Cu 2 ZnSnSe 4 powder obtained has secondary phases, in particular phases composed of CuSe, Zn, Sn, Se, CuSe 2, SnSe, Cu 2 Si, Cu 2 SnS 3, Cu 5 Zn 8, ZnSe or Cul, 8Se. However, the presence of secondary phases should be avoided at best in order to optimize the performance of photovoltaic cells based on thin layers made of Cu2ZnSnSe4. In order to eliminate as much as possible these impurities, the authors anneal up to 700 ° C. However, they do not make it possible to obtain a stoichiometric Cu 2 ZnSnSe 4 powder and tend to increase the size of the grains (and / or crystallites) of the powder, which can be detrimental to the quality of the thin layer produced from this powder. 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 ZnSnSe 4 powder whose degree of purity is improved without this requiring an annealing step. The present invention thus relates to a method of manufacturing by mechanosynthesis of a powder of the compound Cu2ZnSnSe4, the process comprising a step in which a grinding is carried out of a mixture containing: the chemical elements Cu, Sn and Se in elemental form; a precursor ZnSe; the mixture containing Cu, Zn, Sn and Se according to the stoichiometric proportions in which they are in the compound Cu2ZnSnSe4. The manufacturing process of the invention is characterized in particular by the presence of ZnSe in the mixture to be milled. The use of this precursor makes it possible to obtain a Cu2ZnSnSe4 powder with improved degree of purity while avoiding an additional annealing step. 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 ZnSnSe 4 powder by means of the manufacturing method of the invention, a mixture containing the Cu, Sn and Se chemical elements and the ZnSe precursor is ground in order to produce a mechanical alloying. The chemical elements Cu, Sn and Se are in elemental form, that is to say in the pure state or practically pure (that is to say typically more than 99% pure). These chemical elements or the ZnSe precursor can occur, independently of each other, in the form of powder or nugget, it being understood that the nugget is converted to powder during grinding. For example, when they are in powder form, the chemical elements Cu, Sn, Se and / or the precursor ZnSe 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. Mobile bodies are generally composed of a dense material of high hardness (tungsten carbide, steel, etc.). 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 24 hours and 72 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 ZnSnSe 4 in order to limit the oxidation and / or 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 effect a so-called "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 compound Cu 2 ZnSnSe 4 obtained at the end of grinding is nanostructured, since it consists of nanocrystallites (possibly aggregated and / or agglomerated), ie of crystallites with a mean size of, for example, between 5 nm and 15 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 Cu 2 ZnSnSe 4 and the amplitude of the diffraction peak of greater amplitude characteristic of the majority impurity. Regarding its implementation, the powder of the compound Cu2ZnSnSe4 synthesized can be used as such, in the form of ink or also in the form of compact material. The compact material may for example be obtained by means of the manufacturing method of the invention further comprising, after the grinding step, a step in which the powder of the compound Cu 2 ZnSnSe 4 is then hot-compacted, or cold compacted and 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 particular embodiments of the method of the invention, given by way of illustration and not limitation, with reference to Figures 1 to 2 attached.

BRIEF DESCRIPTION OF THE FIGURES FIGS. 1 and 2 show the X-ray diffraction pattern of a powder of the Cu 2 ZnSnSe 4 compound obtained respectively by means of a vertical vibration mill and a ball mill. The characteristic crystallographic planes of the Cu2ZnSnSe4 compound are shown in the diagrams. EXAMPLE OF PARTICULAR EMBODIMENTS 1. Synthesis of a Cu2ZnSnSe4 powder using a small-capacity ball mill. A Cu2ZnSnSe4 powder is manufactured by carrying out the mechanosynthesis in a small capacity vibratory mill ("PO spray" model marketed by the Fritsch company). 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 -8- 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 : milling intensity Mb: mass of the ball Mp: mass of the mixture to be milled (in the form of powder, nugget, ...) Vma x: maximum speed of the vibratory plate, which corresponds to the product of the amplitude of vibration and the pulsation of the vibratory plate (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 = 5 g: - amplitude of 0.9 mm: I = 500 m / s2 amplitude of 1.3 mm: I = 1000 m / s2 An intensity of I = 1000 m / s2 is retained for the following grinding. In a glove box under an argon atmosphere, the grinder is placed and a mixture of 5 g is prepared from the following constituents: 1.01 g of copper powder (sold by the company GoodFellow under the name of CU number 06045/10, purity 99.8%); 0.95 g of tin powder (marketed by GoodFellow under the number LS 279681, purity of 99.9%); 10 - 1.90 g of selenium powder (sold by the company GoodFellow under the number SE 006010/4, purity of 99.95%); 1.14 g of a ZnSe precursor in the form of nuggets (marketed by Strem Chemicals under number 93-3041, purity of 99.99%). The amounts of each constituent in the mixture thus obtained make it possible to ensure that Cu, Zn, Sn and Se are present in the stoichiometric proportions of the compound Cu 2 ZnSnSe 4 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 secured to the vibrating tray whose vibration amplitude is set at 1.3 mm. Shocks produced during grinding only result in a very small increase in temperature, which can be considered constant. After 72 hours of grinding, the resulting Cu 2 ZnSnSe 4 powder was extracted from the mill for later analysis. 2. Synthesis of a Cu 2 ZnSnSe 4 powder using a high capacity ball mill. In a flask placed in a glove box under an argon atmosphere, a mixture of 400 g is prepared from the following constituents, present in the stoichiometric proportions of the compound Cu 2 ZnSnSe 4 to be synthesized: - 81.08 g of copper powder (marketed by Strem Chemicals under number B7572032, purity of 99.9%); 75.72 g of tin powder (marketed by GoodFellow under the number LS 360425, purity of 99.9%); 151.12 g of selenium powder (marketed by Strem Chemicals under the number B7518115, purity of 99.99%). 92.08 g of a ZnSe precursor in the form of nuggets (marketed by Strem Chemicals under number 93-3041, purity of 99.99%). The mixture is introduced under air into the steel tank of a large-capacity ball mill (model BB6 marketed by LESSINES). The tank contains 83 steel balls with a diameter of 25.40 mm which represent a total weight of 5.5 kg. After cleaning the various seals with a cloth soaked in ethanol, the atmosphere of the tank is replaced by an argon atmosphere. The mechanosynthesis of the mixture is then carried out by alternating a grinding phase (tank speed = 60 rpm) with a declogging phase (tank speed = 49 rpm). The declogging phase allows the balls to be dragged along the walls of the crucible to detach the powder.

After 72 hours of grinding, the opening of the tank and the recovery of the Cu 2 ZnSnSe 4 powder obtained are done in air. 3. Characterization of Cu2ZnSnSe4 powders obtained. The two powders of Cu 2 ZnSnSe 4 obtained in the preceding examples are 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) cobalt emitting radiation of a wavelength of 1.78897 Å; - divergence slot = 1; - mask = 10; - anti-diffusion slot = 2; - 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 X-ray diffraction patterns obtained in Example 1 and 2 are respectively reproduced in FIGS. 1 and 2.

The comparison of the peaks identified in these diagrams with those of reference JCPDS 01-070-8930 confirms the production of Cu2ZnSnSe4 powders. From the width of the peaks at mid-height, the average crystallite size of the Cu 2 ZnSnSe 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 sizes calculated are approximately 10 nm for the powders of Example 1 and 2. The powders obtained moreover have a very high degree of purity as evidenced by the absence of parasite peaks on the diagrams of X-ray diffraction, and Raman spectra not reproduced here.

According to the method known to those skilled in the art, the optical properties of the compound Cu2ZnSnSe4 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 Cu2ZnSnSe4 thus calculated is Egap = 1 ev (theoretical value of Egap = 1 ev).

It follows from the above description that the manufacturing method of the invention makes it possible to manufacture a nanostructured Cu2ZnSnSe4 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 Cu2ZnSnSe4, the method comprising a step in which is carried out a grinding of a mixture containing: - chemical elements Cu, Sn and Se in elemental form; a precursor ZnSe; said mixture containing Cu, Zn, Sn and Se in the stoichiometric proportions in which they are in the compound Cu2ZnSnSe4. 2) A manufacturing method according to claim 1, wherein the chemical elements Cu, Sn, Se or the precursor ZnSe are, independently of each other, in the form of powder or nugget. 3) A method of manufacture according to claim 2, wherein, when in the form of a powder, the chemical elements Cu, Sn, Se and / or the precursor ZnSe have grains of average size between 100 nm and 3 mm. 25 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. 30 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 Cu2ZnSnSe4. 6) A method of manufacture according to any one of the preceding claims, wherein the powder of the compound Cu2ZnSnSe4 is composed of crystallites with a mean size of between 5 nm and 15 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 Cu2ZnSnSe4 is hot compacted or cold compacted and then 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.