FR3045946A1 - ALLOY (LI-CU, ZN, SN, SE, S) FOR PHOTOVOLTAIC APPLICATIONS AND METHOD OF MANUFACTURING THE SAME - Google Patents

Abstract

The invention relates to the manufacture of a photovoltaic cell comprising an active photovoltaic component. In particular, the above-mentioned component comprises lithium in an alloy (LixCu1-x) 2ZnSnS4, in which the element S is sulfur and / or selenium. The incorporation of lithium in such an alloy makes it possible to increase the band gap and to reduce the effects of the disorder (Cu, Zn) in this alloy. This results in a material of choice in photovoltaic applications, for example in a top cell, in tandem with a silicon bottom cell in particular.

Description

ALLOY (LICU.Zn.Sn.Se.SI FOR

PHOTO VOLTAIC APPLICATIONS AND METHOD OF MANUFACTURE

The invention is in the field of materials with photovoltaic properties, especially for the design of solar cells.

The needs of the photovoltaic energy production sector are: - a decrease in the production costs of solar panels, - a rise in conversion efficiency, and - a large scale production.

The photovoltaic market is largely dominated by silicon-based technologies with remarkable technical performance, very close to their theoretical limit. The so-called thin-film technology, based on materials with high photovoltaic potential (Cu (In, Ga) Se2 or CdTe), presents very interesting performances in a small-scale laboratory with conversion efficiencies close to those of silicon technologies. However, because of relatively small conversion efficiencies (over large area) and a supply of rare elements (In, Ga and Te), large-scale production is still uncertain.

An increase in conversion efficiencies of solar cells of thin-film technologies or silicon-based solar cells is still sought.

According to one approach of the invention, it is proposed a chalcogenide material effectively absorbing light and consisting of abundant elements (Cu, Li, Zn, Sn and S).

This can increase energy production, reduce the supply constraint of rare elements and ultimately reduce the cost of energy produced.

Thin-film technology based on the abundant Cu2ZnSn (Se, S) 4 material (commonly referred to as "CZTS") has significantly lower conversion efficiencies, with η = 12.6%, than those observed for Cu (In) materials. , Ga) Se2 (or "CIGS") and CdTe for which η = 21%.

The relatively low performances of the CZTS (η record at 12.6%) are related to the complexity of its phase diagram and also to the presence of a very important disorder (Cu, Zn). This disorder is due to the random occupation of a site by Cu or Zn in the crystallographic structure of this material. The disorder is responsible for a deficit of the open circuit voltage of up to 30% of the forbidden bandwidth. The poor performance of this technology makes this solution unattractive at the moment. However, it has the advantage of being composed only of abundant elements whereas the production of CIGS (Cu, In, Ga, Se) and CdTe on a large scale (beyond 20 GW annual) can be hampered by the rarity of some of their constituent elements (In, Te or Se) and the price of these elements.

Another solution is to improve the Si technology by adding a thin film technology forming a solar cell called "tandem". A tandem cell consists of: - a cell called "bottom" (with an active photovoltaic component), and - a cell called "top" (with another active photovoltaic component), located on the front side of the cell. the cell and arranged (directly or indirectly) on the bottom cell.

The tandem cells offer the advantage of having a theoretical limit yield of 46% compared to the theoretical limit yield of conventional silicon cells or in thin layers at 33%.

Considering a so-called "bottom" cell made of Si (with a band gap of 1.1 eV), it has been shown that the cell "top" (located on the side of the front face of the cell and arranged directly or indirectly on the bottom cell) should consist of an absorbent material having a band gap Eg of 1.7 eV. It should also be noted that the integration of a "top" cell based on CZTS requires that it reaches sufficiently high performance not to deteriorate the performance of the tandem cell.

The CZTS material, which has been proposed to compose the top cell, has the advantage of having a crystal lattice bond with silicon. It can therefore be deposited on a substrate (or a "wafer") of silicon without creating crystalline defects at the interface (dislocations in particular). However, its bandgap width is too small to achieve optimum conversion efficiency. The band gap is at most Eg = 1.5eV, with sulfur and no selenium in the composition Cu2ZnSnS4. By adding selenium instead of sulfur, the Eg value obtained is even smaller. Moreover, the disorder (Cu, Zn) degrades its performances.

The present invention improves the situation.

It proposes for this purpose a method of manufacturing a photovoltaic cell comprising an active photovoltaic component. In particular, this component comprises lithium in an alloy (LixCui_x) 2ZnSnS4, in which the element S is sulfur or selenium, or a mixture of these elements.

Thus, the invention proposes to add a proportion of lithium to the CZTS material presented above. It has been observed that the incorporation of lithium in such an alloy advantageously makes it possible to increase the forbidden bandwidth and to reduce the effects of the disorder (Cu, Zn) in this alloy. This results in a material of choice in photovoltaic applications, for example in a top cell, in tandem with a silicon bottom cell in particular.

Preferably, the proportion of lithium (with x = Li / (Cu + Li)) in the alloy (LixCui-x) 2ZnSnS4 is greater than 20% and less than 40% so as to obtain a forbidden band width of the alloy value between 1.7 eV and 2 eV.

In particular, a band gap value of 1.7 eV can be obtained for an amount x of lithium of 0.25 in an alloy in which the element S is sulfur. The partial replacement of sulfur with selenium makes it possible to reduce the bandgap width of the alloy, so that a proportion of lithium in the alloy between 20% and 40% makes it possible to obtain bandgap widths that are typically understood between 1.7 eV and 2 eV, particularly suitable for applications in tandem with another active photovoltaic layer.

As mentioned above, the substitution of sulfur with selenium tends to reduce the width of the forbidden band of the alloy. An alloy containing no lithium and selenium as S element typically has a band gap of 1.1 eV, against a band gap of 1.5 eY for an alloy comprising no lithium and including sulfur as an element S. Thus, the relative proportion of sulfur and selenium in the alloy makes it possible to adjust the value of the forbidden band of the compound in addition to the quantity x of lithium.

In one embodiment, the component may include a layer of said alloy as an active photovoltaic layer (absorber).

Alternatively, the component may include a layer of said alloy in tandem with another layer (optionally separated by a tunneling junction layer). This other layer may comprise for example silicon (typically a wafer (or "wafer") of silicon (bulk material or "bulk"), on which is deposited (directly or indirectly) the layer of said alloy). Such an embodiment is advantageous for a silicon bandgap of 1.1 eV, which is optimal for use in a tandem type photovoltaic cell with a bandwidth layer of 1.7 eV deposited above (in top ").

It turns out that the alloy (LixCui.x) 2ZnSnS4 advantageously has an increasing forbidden bandwidth with the proportion of lithium x. This band gap width of the alloy layer can reach 1.8 eV or 2 eV typically if the element S is sulfur. In one embodiment, the bandgap width of the alloy layer is then between 1.5 and 2 eV. Preferably the bandgap is between 1.7 eV and 2 eV, especially for applications in tandem with another layer.

In an exemplary embodiment, the manufacture of the active photovoltaic component may comprise at least one step of spraying constituents of the alloy (LixCui_x) 2ZnSnS4 on a substrate and an annealing step, the annealing being carried out in the presence of the element S and a layer of a compound comprising lithium. The annealing step, typically carried out under an atmosphere of the element S, then makes it possible not only to organize the various elements deposited by cathodic sputtering on the substrate in order to create the active photovoltaic component, but also to diffuse the lithium from the layer of the compound comprising lithium to the CZTS precursor layer. The layer of the compound comprising lithium can typically be a LiF layer or a layer of a CuxLii.x and / or LixSny mixture.

According to one embodiment, the sputtering step can be a cathodic sputtering of compounds of the columns IB, IIB or IVA of the periodic table chosen from the elemental compounds Cu, zinc Zn and tin Sn, or alloys of these compounds with the element S: CuS, ZnS, SnS, the process may further comprise the deposition of a layer of lithium fluoride LiF.

Sputtering of copper, zinc and tin on a substrate can be performed simultaneously or sequentially. The proportions of copper, zinc and tin are typically chosen so as to provide a desired stoichiometry for the alloy (LixCui_x) 2ZnSnS4, so that the lithium can diffuse into the CZTS precursor to sites unoccupied by the deficit copper.

In one embodiment, the lithium fluoride LiF layer may be deposited on the substrate before the cathodic sputtering step of compounds of columns IB, IIB or IVA of the periodic table.

Alternatively, the lithium fluoride LiF layer is deposited after the cathodic sputtering step of compounds of columns IB, ΠΒ or IVA of the periodic table.

By choosing the position of the LiF layer relative to the CZTS precursor, it is possible to control more finely the distribution of lithium in the alloy (LixCui.x) 2ZnSnS4. Indeed, for a commonly used annealing time typically between 30 minutes and two hours, for annealing temperatures between 450 ° C and 600 ° C, the diffusion dynamics of lithium generally leads to obtaining an alloy (LixCui_x) 2ZnSnS4 having a lithium gradient in the layer thickness of the alloy.

If diffusion occurs from the precursor top layer, the alloy (LixCui-x) 2ZnSnS4 will tend to have a lithium content gradient increasing from the top layer of the precursor to the substrate. Conversely, diffusion from the lower layer of the precursor, close to the substrate, will tend to create an alloy (LixCui_x) 2ZnSnS4 having a decreasing lithium content gradient from the upper layer of the precursor to the substrate.

A gradient of lithium content between the upper and lower layers of the alloy (LixCui.x) 2ZnSnS4 makes it possible to increase the electrical performance of the active photovoltaic component. These increased electrical performance results in particular from the barrier effect to the holes of the alloy (LixCui.x) 2ZnSnS4. The lithium content gradient leads to a gradual evolution in the thickness of the barrier effect at the holes between the upper and lower layers of the alloy (LixCui-x) 2ZnSnS4.

According to one embodiment, the sputtering may be a co-sputtering of compounds of columns IB, ΠΒ or IVA of the periodic table.

Alternatively, the sputtering may be a cathodic sequential sputtering of compounds of columns IB, ΠΒ or IVA of the periodic table.

In particular, the lithium fluoride LiF layer can be deposited between two sputtering sequences of compounds of columns IB, IIB or IVA of the periodic table.

The position of the lithium diffusing LiF source layer in the alloy makes it possible to adjust the properties of the lithium content gradient in the alloy (LixCui-x) 2ZnSnS4 produced.

According to another embodiment, the sputtering step may be a cathodic co-sputtering of stable element targets comprising zinc Zn, copper Cu and tin Sn and in that the layer of the compound comprising lithium is produced during the step of spraying from a target of at least one alloy selected from LixSny and CuxLii.x.

In this variant embodiment, the lithium is present within the precursor of CZTS before annealing, in a compound comprising copper tin or a combination of copper and tin.

According to one embodiment, the layer of the compound comprising lithium can be obtained during the sputtering step from a target comprising a LixSny alloy and a target of a CuxLii_x alloy controlled according to different parameters. The use of two separate lithium sources in a sputter deposition process makes it possible to better control the amount of lithium deposited by applying, for example, different powers to each target comprising the lithium source.

According to one embodiment, the layer of the compound comprising lithium may have a thickness 20 to 10 times smaller than the thickness of the active photovoltaic component in an alloy (LixCui_x) 2ZnSnS4 produced.

The diffusion of lithium from a LiF layer or from a precursor comprising lithium-based compounds leads to a rearrangement of the compounds in the precursor and the disappearance of the LiF layer or the initial lithium-based compound. The quantity x of lithium of the alloy (LixCui_x) 2ZnSnS4 produced can thus be controlled from the ratio between the thickness of the alloy layer produced and the thickness of the initial lithium-containing layer.

Then, the alloy obtained after annealing can be brought back to room temperature by simple natural cooling (or near-natural), typically taking less than five hours for a layer of the order of one micrometer thick, which makes this material compatible with manufacturing on an industrial scale (unlike CZTS cooled slowly to improve its performance).

The present invention also relates to a photovoltaic cell, comprising an active photovoltaic component comprising lithium in an alloy (LixCui_x) 2ZnSnS4, in which the element S is sulfur or selenium, or a mixture of these elements.

In particular, the quantity x of lithium in the alloy is between 0.2 and 0.4, so as to obtain a forbidden bandwidth of the value alloy of between 1.7 eV and 2 eV. Other advantages and characteristics of the invention will appear on examining the detailed description of embodiments given hereinafter by way of examples, and on examining the drawings in which: FIG. 1 represents a structure keratite, quadratic, for the material (LixCui_x) 2ZnSnS4, this material retaining this quadratic structure for x up to about 40%; - Figure 2 represents a wurtz-kesterite structure (quaternary analogue of wurtzite), for the material ( LixCui.x) 2ZnSnS4, where x is greater than about 60%, - Figure 3 shows the increase of the band gap (or "gap") as a function of x in the material (LixCui.x) 2ZnSnS4 (in which S is sulfur here), the increase in the gap having been theoretically estimated by the density functional theory (or "DFT", for "Density Functional Theory"), - Figure 4 compares the NMR line widths pewter 119Sn characterist the disorder (Cu, Zn): in the material of the prior art Cu 2 ZnSnS 4 obtained at high temperature with slow cooling thereafter (approximately -10 ° C./h for at least 24 hours) (CZTS curve (S)), in the material of the prior art Cu 2 ZnSnS 4 obtained at high temperature and following rapid cooling thereafter (curve CZTS (N)), o in the material (LixCui_x) 2ZnSnS4 that the present invention uses (relevant line surrounded by lines dotted), with cooling here at about -100 ° C / h (about five hours), and a composition such that x = 25% (Li-CZTS curve), - Figure 5A shows a first example of application of the material (LixCui.x) 2ZnSnS4 in a photovoltaic cell, as active photovoltaic layer, - Figure 5B shows a second example of application of the material (LixCui_x) 2ZnSnS4 in a photovoltaic cell, as a layer of a cell "top Of a tandem structure with silicon in a cell FIG. 6 schematically illustrates an example of a method for manufacturing a thin layer of (LixCui_x) 2ZnSnS4.

The material proposed for the implementation of the invention is the alloy (LixCui.x) 2ZnSnS4 exhibiting: (i) a direct and authorized optical transition, (ii) a crystal lattice with silicon at 1%, (iii) a significant energy barrier for the holes allowing a reduction of the recombinations on the front face of the photovoltaic cell, contrary to the techniques generally used to create an energy barrier for the electrons, (iv) a band gap adjustable between 1.5 eV and 2 eV and (v) a control of the disorder of the atoms (Cu, Zn) in CZTS.

All these properties make it possible to improve the performance of CZTS-based solar cells and the integration of these cells into tandem cells with higher efficiency.

The solution proposed for the implementation of the invention is to form a Li-CZTS alloy (with a lithium atom occupying a copper atom site) in a lithium concentration range ranging from 0 to 40%, and preferably from 20% to 40%.

New technical effects related to the use of this material in a tandem solar cell structure (silicon based or thin layer) can be listed as shown below.

Conservation of the kesterite phase

The conservation of the so-called "kesterite" quadratic phase (which is usually in competition with the hexagonal wurtzite phase) is controlled by the stoichiometry x of the alloy (LixCui.x) 2ZnSnS4.

The quadratic phase, illustrated in FIG. 1, has the advantage of being crystallographically compatible with crystalline silicon, with in particular a mesh agreement between the alloy (LixCui_x) 2ZnSnS4 and the Si of 1%. Crystallochemical studies have shown that the pure kesterite phase disappeared from 40% of Li in favor of a two-phase structure before the pure wurtz-kesterite phase appears from 60% of Li (hexagonal structure of Figure 2).

Increasing the bandgap The opening of the band gap Eg between 1.5 and 2 eV in the alloy range from 0 to 40% is ensured by the conservation of the kësterite structure. The increase in the bandgap is mainly done by lowering the position of the valence band. Usually, in the CZTS material (without lithium), the maximum of the valence band is dominated by the interactions between the p-orbitals of chalcogen and the orbital d of Cu and in particular the repulsion pd, which "pushes" the maximum of the band from valence to the vacuum level (upwards). The replacement of Cu by Li, which has no orbital d, then results in a lowering of the maximum of the valence band, and hence an increase in bandgap. FIG. 3 shows the increase in the bandgap band associated with lithium incorporation (by taking sulfur, without selenium, as element VI (element S mentioned above)). It will be noted in particular that values of 1.8 eV are reached with just 25 to 30% lithium in CZTS, with a conserved quadratic structure compatible with the mesh with silicon. The material (LixCui.x) 2ZnSnS4 with x = 25% for example, is therefore a material of choice for tuning in tandem with silicon (gap of 1.1 eV, theoretically offering excellent performance in a cell tandem with a gap material of 1.7 eV). It should further be noted that it is possible to incorporate selenium in place of sulfur in the composition (LixCui.x) 2ZnSnS4 to reduce the bandgap to 1.7 eV, when the proportion of lithium is close to 40. % (for example to minimize the disorder (Cu, Zn) as shown below).

In order to use an alloy-based photovoltaic component (LixCui.x) 2ZnSnS4 in applications in tandem with another active layer, a proportion of lithium to obtain a band gap of between 1.7 eV and 2 eV is particularly advantageous. Such a bandgap width may in particular be obtained for an amount x of lithium of between 0.2 and 0.4, and especially for an amount x of 0.25 when the element S is sulfur. In this configuration, a bandwidth of 1.7 eV for the active photovoltaic compound is obtained. Reduction of the disorder (Cu, Zn)

A reduction of the disorder (Cu, Zn) (random occupation of Cu sites by Zn and vice versa) was furthermore observed by Nuclear Magnetic Resonance (NMR) of 119Sn tin. Thus, the presence of Cuzn and Zncu anti-site ionized spot defects creates strong fluctuations in electrostatic potentials as well as an anchoring of the Fermi level in the center of the forbidden band. These two effects may explain the high open-circuit voltage deficit observed in the case of thin-film solar cells based on CZTS compared to those based on CIGS. A control of the disorder thus makes it possible to overcome this problem and to allow the CZTS technology to approach the performances of the CIGS technology in particular in terms of conversion efficiency.

The experimental NMR results are then observed in FIG. 4, and more particularly the 119Sn spectrum for CZTS materials with an ordered keratite (CZTS (S)) and disordered (CZTS (N)) structure, in comparison with the alloy (LixCui. x) 2ZnSnS4 (Li-CZTS). The NMR spectrum for the alloy has several peaks corresponding to neighboring second environments consisting of 0,1,2,3,4 Li atoms. The disorder (Cu, Zn) is materialized by the shape of the NMR peak. In particular, an asymmetric widening is observed on the CZTS (N) curve obtained at high temperature (500-600 ° C) then cooling by quenching effect (less than 1 minute). Such an enlargement is not observed on the other hand in the case of the same material having undergone a slow cooling (-5 to -10 ° C. per hour, ie more than 24 hours of cooling to bring it back to ambient temperature), on the curve CZTS (S).

Finally, it can be observed that in the case of the alloy (Li-CZTS curve), a very low level of disorder is present in this material (dashed lines), and this simply by cooling on the order of -100 ° C. per hour. to room temperature after its manufacture at 500-600 ° C (about five hours). A natural cooling of this material to room temperature is usually faster and is rather in the range of -200 to -300 ° C per hour (ie for about two hours). Nevertheless, such a difference in cooling rate is not really likely to call into question the order obtained in the Li-CZTS material. Thus, the manufacturing time of this material makes it, in any case, compatible with industrial production rates.

Several examples of embodiments of this material are given below.

In a first embodiment, shown diagrammatically in FIG. 6, the alloy (LixCui-x) 2ZnSnS4 can be made by a process comprising cathodic sputtering of constituents of the CZTS absorber in order to produce a precursor layer on a substrate. The substrate may typically be a molybdenum substrate forming a rear electrical contact of the photovoltaic device produced. This precursor layer may in particular comprise copper Cu, zinc Zn and tin Sn, in stoichiometric proportions compatible with the amount x of lithium that it is desired to incorporate into the alloy (LixCui.x) 2ZnSnS4. The desired stoichiometric proportions may in particular be controlled by the amount of copper or copper-based compound deposited. The precursor can be produced either from pure elements Cu, Zn and Sn, or from compounds comprising the element S (CuS, ZnS, SnS) or from a mixture of pure elements and compounds comprising the element S.

Deposition of the precursor layers can be performed either sequentially or all at once in a co-sputtering process of all compounds.

In addition, a layer of lithium fluoride LiF is also deposited (by a conventional technique of growth of these layers). This LiF layer may for example be deposited on the substrate prior to the manufacture of the precursor layer. The LiF layer is then on the substrate and under the precursor layer.

Alternatively, the LiF layer can be deposited after the manufacture of the precursor layer, and is then found on an upper surface thereof.

The relative position of the LiF layer with respect to the precursor layer makes it possible to adjust the manner in which the lithium diffuses through the precursor to occupy the copper-free sites of the alloy (LixCui.x) 2ZnSnS4.

When the LiF layer and the precursor layer have been manufactured, high temperature annealing can be applied to this hetero-structure, in the presence of element VI such as sulfur (preferred because it makes it possible to achieve a bandwidth forbidden higher) or selenium in gaseous form for example. The latter combines with fluorine to form a highly volatile compound SF6 or SeF6 which evacuates to leave a layer of Li-CZTS according to a stoichiometry (LixCui.x) 2ZnSnS4, where x typically depends on the thickness of the layer Li-F initial. The layer obtained Li-CZTS is typically of a thickness of the order of one or a few microns, and can be brought to room temperature by natural cooling. This facility makes this material susceptible of industrial application for the manufacture of photovoltaic cells in particular, with good performance in particular.

It has thus been found that a LiF layer 20 10 times thinner than the layer of the alloy (LixCui_x) 2ZnSnS4 produced makes it possible to include an amount x of lithium typically comprised between 0.2 and 0.4, resulting in a forbidden band width for the alloy (LixCui_x) 2ZnSnS4 between 1.7 eV and 2 eV. These proportions are particularly suitable for applications in a tandem type photovoltaic device.

The position of the LiF layer with respect to the precursor layer makes it possible to control the distribution of lithium in the alloy layer (LixCui.x) 2ZnSnS4. For a standard annealing time typically between 30 minutes and two hours, for annealing temperatures between 450 ° C. and 600 ° C., the diffusion dynamics of the lithium in the precursor can lead to the appearance of a gradient of lithium concentration in the alloy (LixCui.x) 2ZnSnS4.

Thus, for a LiF layer located on the CZTS precursor layer, there will be a tendency to observe a decreasing lithium content gradient from the upper layer of the precursor to the substrate. For a LiF layer located under the CZTS precursor layer, there will be a tendency to observe a lithium content gradient increasing from the upper layer of the precursor to the substrate.

The presence of such a gradient will result in barrier properties to different holes in the final structure obtained, to increase the electrical performance of the photovoltaic device made.

When the constituents of the precursor layer CZTS are deposited sequentially, it is possible to provide a location of the LiF layer between the constituents of the precursor. The diffusion of lithium may then give rise to the appearance of a lithium content gradient around the initial position of this LiF layer.

In another alternative embodiment, it is possible to manufacture the alloy layer (LixCui-x) 2ZnSnS4 by co-spraying stable elements including lithium-based compounds. For example, it is possible to provide simultaneous sputtering of Cu copper, zinc Zn and a LixSny compound. Alternatively, the simultaneous sputtering may comprise a CuxLIIIx compound, Zn zinc and Sn tin.

However, it is preferred to provide simultaneous sputtering from targets comprising two lithium sources: a target comprising a LixSny compound and a target comprising a compound of CuxLii.x type, thus, better control over the quality of the lithium deposit in the precursor is possible by separately controlling the sputtering parameters of each target (adjustment of the power, the amount of material provided, etc.).

Thus, an embodiment in the sense of the invention offers the advantage of good performance of the material chosen, low costs of both constituents and manufacturing.

Possible implementations for this material are:

the direct use of this material as a photovoltaic layer alone, as illustrated in FIG. 5A (CPA "absorber" photovoltaic layer deposited on a SUB substrate, with for example a BUF buffer layer then deposited over it, and then a layer transparent electrode ELE), or - its use in a tandem structure (layer CPA deposited on a wafer (or "wafer") of SiW silicon, optionally separated by a tunnel effect junction layer (JCT), as shown in Figure 5B.

Thus, the invention can be applied to chalcogenide type thin-film solar cells and / or silicon-based tandem solar cells, and in particular for the manufacture of cells and solar panels.

The advantages provided by the invention are numerous, especially with: the use of abundant and non-toxic elements; a position of the valence band allowing the creation of a barrier to the holes and thus preventing the recombination of the holes front face, - the reduction of the disorder (Cu, Zn) which causes an increase of the performances in particular in terms of open circuit voltage.

Of course, the present invention is not limited to the embodiments described above as examples; it extends to other variants.

For example, a different embodiment of the alloy (LixCui.x) 2ZnSnS4 may further provide electrochemical treatment of CZTS in a lithium salt bath with Li / Cu exchange. In yet another variant, provision can be made for dipping a thin layer of CZTS in a butyllithium bath with a Li / Cu exchange.

Claims (20)

A method of manufacturing a photovoltaic cell comprising an active photovoltaic component, characterized in that said component comprises lithium in an alloy (LixCui.x) 2ZnSnS4, wherein the element S is sulfur or selenium, or mixture of these elements. 2. Method according to claim 1, characterized in that the proportion of lithium in the alloy (LixCui.x) 2ZnSnS4 is greater than 20% and less than 40%, so as to obtain a bandgap width of the alloy value between 1.7 eV and 2 eV. 3. Method according to one of the preceding claims, characterized in that the component comprises a layer of said alloy as an active photovoltaic layer. 4. Method according to one of claims 1 and 2, characterized in that the component comprises a layer of said alloy in tandem with another layer. 5. Method according to claim 4, characterized in that said other layer comprises silicon. 6. Method according to claim 5, characterized in that said other layer is a silicon wafer, on which is deposited the layer of said alloy. 7. Method according to one of claims 4 to 6, characterized in that said other layer and the layer of said alloy are separated from a tunneling junction layer. 8. Method according to one of the preceding claims, characterized in that it comprises at least one step of spraying constituents of the alloy (LixCui_x) 2ZnSnS4 on a substrate and an annealing step, the annealing being carried out in the presence of element S and a layer of a compound comprising lithium. 9 Process according to claim 8, characterized in that the sputtering step is a cathodic sputtering of the compounds of the columns IB, IIB or IVA of the periodic table chosen from the elemental compounds Cu, zinc Zn and tin Sn, or alloys of these compounds with the element S: CuS, ZnS, SnS, the process further comprising depositing a layer of lithium fluoride LiF. 10. The method of claim 9, characterized in that the LiF lithium fluoride layer is deposited on the substrate before the sputtering step of compounds of columns IB, IIB or IVA of the periodic table. 11. The method of claim 9, characterized in that the LiF lithium fluoride layer is deposited after the sputtering step of compounds of columns IB, IIB or IVA of the periodic table. 12. Method according to any one of claims 8 to 11, characterized in that the sputtering is a co-sputtering of compounds of columns IB, IIB or IVA of the periodic table. 13. Method according to any one of claims 8 to 11, characterized in that the sputtering is a cathodic sequential spray of compounds of columns IB, IIB or IVA of the periodic table. 14. The method of claim 13, characterized in that the LiF lithium fluoride layer is deposited between two sputtering sequences of compounds of columns IB, IIB or IVA of the periodic table. 15. The method of claim 9, characterized in that the sputtering step is a co-sputtering of stable element targets comprising zinc Zn, copper Cu and tin Sn and that the layer of compound comprising lithium is produced during the sputtering step from a target of at least one alloy selected from LixSny and CuxLii_x. 16. The method of claim 15, characterized in that the layer of the compound comprising lithium is obtained during the sputtering step from a target comprising a LixSny alloy and a target of a controlled CuxLii.x alloy. according to different parameters. 17. Method according to any one of claims 8 to 16, characterized in that the layer of the compound comprising lithium has a thickness 20 to 10 times smaller than the thickness of the active photovoltaic component of an alloy (LixCui.x) 2ZnSnS4 achieved. 18. Method according to any one of claims 8 to 17, characterized in that the alloy obtained after annealing is brought to room temperature by cooling for less than five hours for a layer of thickness of the order of one micrometer . Photovoltaic cell, characterized in that it comprises an active photovoltaic component comprising lithium in an alloy (LixCui_x) 2ZnSnS4, in which the element S is sulfur or selenium, or a mixture of these elements. Photovoltaic cell according to claim 10, characterized in that the quantity x of lithium in the alloy is between 0.2 and 0.4, so as to obtain a forbidden bandwidth of the alloy of value between 1.7 eV and 2 eV.