WO2013057451A2

WO2013057451A2 - Process for manufacturing, by mechanosynthesis, a powder of cztse, the use thereof for forming a thin layer

Info

Publication number: WO2013057451A2
Application number: PCT/FR2012/052398
Authority: WO
Inventors: Fabrice LEGENDRE; Anne CARO-VERRON
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2011-10-20
Filing date: 2012-10-19
Publication date: 2013-04-25
Also published as: FR2981591B1; WO2013057451A3; FR2981591A1

Abstract

Process for manufacturing, by mechanosynthesis, a powder of the compound Cu₂ZnSnSe₄, the process comprising a step in which the milling of a mixture is carried out, said mixture containing: - the chemical elements Cu, Sn and Se in elemental form; a ZnSe precursor; the mixture containing Cu, Zn, Sn and Se in the stoichiometric proportions in which they are found in the compound Cu₂ZnSnSe₄. The invention also relates to the use of the process for producing a thin layer, which is incorporated in particular into the composition of a photovoltaic cell.

Description

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

DESCRIPTION

TECHNICAL AREA

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 Cu ₂ ZnSnSe ₄ , and its use to make a thin layer.

TECHNICAL BACKGROUND

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 that are composed of a semi conductor 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), Culn ₀ , ₇ Gao, ₃ Se (CIGS) or Cu ₂ nSnSe ₄ (CZTSe). Among these compounds, Cu ₂ ZnSnSe ₄ is particularly interesting because it has neither the toxicity of CdTe nor the high cost of Culno, ₇ Gao, ₃ Se.

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 Cu ₂ ZnSnSe _{4 to be} in the form of a powder, possibly formed into a 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 Cu ₂ ZnSnSe ₄ 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 performing the mechanosynthesis of a mixture of elemental powders of Cu, Zn, Sn and Se.

However, the powder obtained has secondary phases, especially phases consisting of CuSe, Zn, Sn, Se, CuSe ₂ , SnSe, Cu ₂ Se, Cu ₂ SnSe ₃ , Cu ₅ Zn ₈ , ZnSe or Cui, sSe. A powder corresponding to the Cu ₂ ZnSnSe ₄ stoichiometry is therefore not really obtained.

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 Cu ₂ ZnSnSe ₄ .

In order to eliminate as much as possible these impurities, the authors anneal up to 700 ° C.

These anneals still 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 may be unfavorable to the quality of the thin layer produced. from this powder, in particularly when the deposition of the thin layer is performed 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 Cu ₂ ZnSnSe ₄ , the process comprising a step in which a grinding 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 ₂ ZnSnSe ₄ compound.

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 Cu ₂ ZnSnSe ₄ 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 may 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 ("screen- printing "), spin coating (" spin coating ") or tape casting.

DETAILED DESCRIPTION OF THE INVENTION In order to produce the Cu ₂ ZnSnSe ₄ powder using 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 mechanosynthesis.

The chemical elements Cu, Sn and Se 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 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 lease 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 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 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 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 ZnSnSe ₄ compound obtained at the end of grinding is nanostructured, since it consists of nanocrystallites (possibly aggregated and / or agglomerated), that is to say of crystallites of average size, for example between 5 nm and 15 nm.

The crystallites of the compound Cu ₂ ZnSnSe ₄ can be of different crystalline forms (polymorphism) or identical. The crystalline structure is for example of the stannite type.

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.

Thus, the degree of purity may be 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 impurity maj oritaire. The production process of the invention makes it possible to obtain large quantities of powder of the compound Cu ₂ ZnSnSe ₄ , for example several tens to several hundreds of grams, using in particular a ball mill.

Regarding its implementation, the powder of the compound Cu ₂ nSnSe ₄ 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 using the manufacturing method of the invention further comprising, after the grinding step, a step in which the powder of the Cu ₂ ZnSnSe ₄ compound 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

Figures 1 and 2 show the X-ray diffraction pattern of a powder of Cu ₂ ZnSnSe ₄ obtained respectively using a vertical vibration mill and a ball mill. The characteristic crystallographic planes of Cu ₂ ZnSnSe ₄ are shown in the diagrams.

EXAMPLE OF PARTICULAR EMBODIMENTS 1. Synthesis of a Cu ₂ ZnSnSe ₄ Powder Using a Small Capacity Ball Mill. A Cu ₂ ZnSnSe ₄ 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 provided with a valve allowing to make the vacuum if necessary or to control the atmosphere inside the enclosure.

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 100C6 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, Mr. 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 = (M _b .V _max .f) / M _p

, in which :

I: grinding intensity

M _b : mass of the ball

M _p : mass of the mixture to be ground (in powder form, nugget, ...) V _max : 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 2Π 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 M _b = 500 g and M _p = 5 g:

amplitude of 0.9 mm: 1 = 500 m / s ²

- amplitude of 1.3 mm: I = 1000 m / s ²

An intensity of I = 1000 m / s ² 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 (marketed by GoodFellow under the number CU 06045/10, purity of 99.8%);

0.95 g of tin powder (marketed by GoodFellow under the number LS 279681, purity of 99.9%);

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, 99.99% purity).

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 Cu ₂ ZnSnSe ₄ compound to be synthesized.

The mixture and the ball are then introduced into the crucible of the mill. After sealing the lid of 1 'enclosure, the valve is closed to preserve an atmosphere of argon.

The mill is extracted from the glove box and fixed on the vibratory plate 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 72 hours of grinding, the Cu ₂ ZnSnSe ₄ powder obtained is extracted from the mill for later analysis.

2. Synthesis of Cu ₂ ZnSnSe ₄ powder using a large 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 Cu ₂ ZnSnSe ₄ compound to be synthesized:

81.08 g of copper powder (marketed by Strem Chemicals under the number B7572032, 99.9% purity);

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, 99.99% purity).

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 Cu ₂ ZnSnSe ₄ powders obtained.

The two Cu ₂ ZnSnSe ₄ powders obtained in the preceding examples are analyzed with an X-ray diffractometer (Model "X 'Pert Pro" sold by the company PANalytical) whose analysis parameters are as follows:

X-ray emission tube (anode) made of cobalt emitting radiation with a wavelength of 1.78897 A;

- divergence slot = 1;

- mask = 10;

- anti-diffusion slot = 2;

- receiving slot = 6.6;

- no measurement = 0.02 ° for 1000 seconds;

- scanning angles = 20 ° <2Θ <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 Cu ₂ ZnSnSe ₄ powders of crystalline structure of stannite type.

From the width of the peaks at mid-height, the average crystallite size of 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 about 10 nm for the powders of Examples 1 and 2.

The powders obtained also have a very high degree of purity as evidenced by the absence of parasitic peaks on the X-ray diffraction patterns, and the Raman spectra not reproduced here.

According to the method known to those skilled in the art, the optical properties of the compound Cu ₂ ZnSnSe ₄ 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 a ² is then plotted (absorption coefficient determined from the absorbance) as a function of 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 Cu ₂ ZnSnSe ₄ thus calculated is Egap = 1 ev (theoretical value of Egap = 1 ev). It follows from the foregoing description that the manufacturing method of the invention makes it possible to manufacture a nanostructured Cu ₂ ZnSnSe ₄ powder whose degree of purity is improved, without this requiring an annealing step.

Claims

1) Method of manufacturing by mechanosynthesis of a powder of the compound Cu ₂ ZnSnSe ₄ , the process comprising a step in which a grinding of a mixture containing:

the chemical elements Cu, Sn and Se in elemental form;

a precursor ZnSe;

said mixture containing Cu, Zn, Sn and Se according to the stoichiometric proportions in which they are in the Cu ₂ ZnSnSe ₄ compound.

2) The 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) The manufacturing method 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. 4) A method of manufacture according to any one of the preceding claims, wherein the mixture does not contain any additive, in order to achieve a dry grinding. 5) A manufacturing method 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. 6) 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 Cu ₂ nSnSe ₄ .

7) A manufacturing method according to any one of the preceding claims, wherein the grinding time is at least 24 hours.

8) The manufacturing method according to claim 7, wherein the grinding time is between 24 hours and 144 hours.

9) The manufacturing method according to claim 8, wherein the grinding time is between 24 hours and 72 hours. 10) A manufacturing method according to any one of the preceding claims, wherein the powder of the compound Cu ₂ ZnSnSe ₄ is composed of crystallites with a mean size of between 5 nm and 15 nm. 11) A method of manufacture 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 Cu ₂ ZnSnSe ₄ is hot-compacted, or cold-compacted then sintered.

12) The manufacturing method according to claim 11, wherein the compact material obtained is used to achieve a target for sputtering. 13) Use of the manufacturing method as defined in any one of claims 1 to 12, to achieve a thin layer. 14) Use according to claim 13, wherein the thin layer is made using a vacuum deposition process.

15. Use according to claim 14, wherein the vacuum deposition process is sputtering or evaporation.

16) Use according to claim 13, wherein the thin layer is produced using a liquid deposition process.

17) Use according to claim 16, wherein the liquid deposition process is screen printing, spin coating or strip casting.

18) Use according to any one of claims 13 to 17, wherein the thin layer is in the composition of a photovoltaic cell.