Patent Application
20170022072
The present invention relates to the field of inorganic semiconductor compounds intended in particular for providing a photocurrent, especially via a photovoltaic effect.
Nowadays, photovoltaic technologies using inorganic compounds are mainly based on silicon technologies (more than 80% of the market) and on “thin layer” technologies (mainly CdTe and CIGS (Copper Indium Gallium Selenium), representing 20% of the market). The growth of the photovoltaic market appears to be exponential (40 GW cumulative in 2010, 67 GW cumulative in 2011).
Unfortunately, these technologies suffer from drawbacks that limit their capacity to satisfy this growing market. These drawbacks include poor flexibility as regards silicon from a mechanical and installation viewpoint, and the toxicity and scarcity of the elements for the “thin layer” technologies. In particular, cadmium, tellurium and selenium are toxic. Moreover, indium and tellurium are rare, which has an impact especially on their cost.
For these reasons, it is sought to dispense with the use of indium, cadmium, tellurium and selenium or to reduce their proportion.
One route that has been recommended for replacing indium in CIGS is to replace it with the couple (Zn2+, Sn4+). In this context, the compound Cu2ZnSnSe4 (known as CZTS) has especially been proposed. This material is nowadays considered as being the most serious successor to CIGS in terms of efficacy, but has the drawback of the toxicity of selenium.
As regards selenium and tellurium, few substitution solutions have been proposed, and they generally prove to be disadvantageous. Compounds such as SnS, FeS2 and Cu2S have indeed been tested, but, although they have advantageous intrinsic properties (gap, conductivity, etc.), they do not prove to be sufficiently chemically stable (e.g.: Cu2S is very readily transformed into Cu2O on contact with air and moisture).
To the inventors' knowledge, no satisfactory solution for obtaining good photovoltaic efficacy without problems associated with the toxicity and/or scarcity of the elements used in a photovoltaic system has been published to date.
One aim of the present invention is, precisely, to provide alternative inorganic compounds to those used in the current photovoltaic technologies, which make it possible to avoid the abovementioned problems.
To this end, the present invention proposes using a novel family of inorganic materials, for which the inventors have now demonstrated, surprisingly, that they prove to have good efficacy, and that they have the advantage of not needing to use, or of using in a very low content, rare or toxic metals such as the abovementioned In, Te or Cd, and also offer the possibility of using anions, such as Se or Te in a reduced content, or even of not using anions of this type.
One of the subjects of the present invention is a novel material comprising at least one compound of formula (I):
Bi1-xMxCu1-y-εM′yOS1-zM″z (I)
in which:
When they are present, the elements M, M′ and M″ are generally substitution elements occupying, respectively, the place of the element Bi, of the element Cu and of the element S.
The term “material comprising at least one compound of formula (I)” means a solid, generally in divided form (powder, dispersion) or in the form of a coating or of a continuous or discontinuous layer on a support, and which comprises, or even consists of, a compound corresponding to formula (I).
The term “rare earth metal” means the elements from the group consisting of yttrium and scandium and the elements of the Periodic Table with an atomic number of between 57 and 71 inclusive.
According to the invention, the element M may preferably be chosen from the elements Sb, Pb, Ba and rare earth metals. The element M may, for example, be lutetium.
According to the invention, the element M′ may preferably be chosen from the elements Ag, Zn and Mn. The element M′ may, for example, be the element Ag.
According to the invention, the element M″ may especially be the element I.
In a first variant of the invention, the compound of formula (I) according to the invention corresponds to the following formula: Bi1-xMxCu1-εOS (Iε), in which x≠0, ε is a zero or non-zero number and M is an element or a mixture of elements chosen from group (A) consisting of Pb, Sn, Hg, Ca, Sr, Ba, Sb, In, Tl, Mg, rare earth metals.
According to one embodiment of this variant of the invention, M is an element or a mixture of elements chosen from rare earth metals.
The compound may then correspond, for example, to the formula Bi1-xLuxCuOS in which x≠0 and ε=0.
In a second variant of the invention, the compound of formula (I) according to the invention corresponds to the following formula: BiCu1-y-εM′yOS (Ib), in which y≠0, ε is a zero or non-zero number and M′ is an element or a mixture of elements chosen from group (B) consisting of Ag, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Mg, Al, Cd.
According to one embodiment of this variant of the invention, M′ is an element or a mixture of elements chosen from the elements Ag and Zn.
The compound may then correspond, for example, to the formula BiCu1-yAgyOS or to the formula BiCu1-yZnyOS in which y≠0 and ε=0.
In a third variant of the invention, the compound of formula (I) according to the invention corresponds to the following formula: BiCuOSzM″1-z (Ic), in which z≠0, ε is a zero or non-zero number and M″ is a halogen.
According to one embodiment of this variant of the invention, M″ is the element I, and the compound then corresponds to the formula BiCuOSzI1-z in which z≠0 and ε=0.
A subject of the invention is also various routes of access to the material according to the invention.
Thus, in a first variant, a subject of the invention is a first process for preparing the material according to the invention, comprising a step of solid milling of a mixture comprising at least inorganic compounds of bismuth and copper, and
According to this variant, a mixture in solid form comprising at least inorganic compounds of bismuth and copper is milled. Preferably, the inorganic compounds of bismuth and copper present in the mixture are at least the compounds Bi2O3, Bi2S3 and Cu2S.
This milling may be performed according to any means known per se. This mixture may especially be placed in an agate mortar. The milling may be performed, for example, with a planetary mill.
To facilitate the milling, it is possible to add to the mixture in solid form milling beads that consist, for example, of stainless-steel beads, special chromium steel beads, agate beads, tungsten carbide beads or zirconium oxide beads.
The milling time may be adjusted according to the desired product. It may especially be between 20 minutes and 96 hours, especially between 1 hour and 72 hours.
The inorganic compounds of bismuth and copper in the mixture may be in the form of particles with a particle size of less than 50 μm, in particular less than 10 μm, for example less than 1 μm.
The dimensions of the particles to which reference is made here may typically be measured by scanning electron microscopy (SEM).
In a second variant, a subject of the invention is a second process for preparing the material according to the invention by performing a precipitation reaction comprising the following steps:
This process consists in performing a precipitation reaction using soluble metallic precursors so as to obtain a homogeneous mixture of the substitution elements in the material comprising the compound of formula (I).
The various solutions of precursors are prepared separately, and then mixed together, whereby a homogeneous mixture and submicron particle sizes are obtained.
In certain cases, the precipitation may be performed by raising the temperature especially to obtain better crystallization.
By way of illustration, such a precipitation may be performed in the following manner:
In a third variant, a subject of the invention is a third process for preparing the material according to the invention, comprising the following steps:
The aqueous medium used in step (b′) may especially be a solvent, for example a mixture of ethylene glycol or an ionic liquid at reflux.
On conclusion of step (c′), a deagglomeration step may be performed, for example using an ultrasonication probe.
Preferably, the inorganic compounds of bismuth and copper supplied in the mixture of step (a′) are at least Bi2O3 and Cu2O. According to another possible embodiment, bismuth and copper soluble salts may be used. In particular, in the case of absence of oxide of the inorganic compounds in step (a′), step (b′) is advantageously performed in the presence of a source of oxygen, such as water, nitrates or carbonates.
The source of sulfur used in step (a′) may be chosen from sulfur, hydrogen sulfide H2S and salts thereof, an organosulfur compound (thiol, thioether, thioamide, etc.), preferably an anhydrous or hydrated sodium sulfide.
Preferentially, irrespective of their exact nature, the oxides in dispersed form are used in step (a′) in the form of particles, typically in the form of powders, having a particle size of less than 10 μm, in particular less than 5 μm and preferentially less than 1 μm. This particle size may be obtained, for example, by previous milling of the oxides (separately or, more advantageously in the case of mixtures of oxides, this milling may be performed on the mixture of oxides), for example using a device such as a micronizer or wet ball mill.
In step (b′), the dissolution is performed under “hydrothermal conditions”. For the purposes of the present description, the term “hydrothermal conditions” means that the step is performed at a temperature above 180° C. under the saturating vapor pressure of water.
When milling is performed, the temperature of step (b′) may be less than 240° C., or even less than 210° C., for example between 180° C. and 200° C.
Alternatively, step (b′) may be performed without previous milling, in which case it is, however, preferable to perform the step at a temperature above 240° C., preferably above 250° C.
Preferably, in step (b′), the mixture is placed in water at a temperature below the hydrothermal conditions (typically at room temperature and at atmospheric pressure), and the temperature is then raised slowly, advantageously at a rate of less than 10° C./minute, for example between 0.5 and 5° C./minute, typically at 2.5° C./minute, typically operating in a closed medium (using a device such as a hydrothermal bomb, especially a Parr bomb) until the operating temperature is reached.
In step (b′), the dissolution is specifically performed with stirring. This stirring may especially be performed by magnetic stirring, for example by placing the hydrothermal bomb on a magnetic stirrer, the assembly being placed in a heating chamber (such as an oven).
Step (b′) is performed for a time sufficient to obtain dissolution. Typically, the temperature is maintained at at least 190° C. for at least 12 hours, for example for 48 hours, or even 7 days.
On conclusion of the dissolution performed in step (b′), the solution obtained is typically brought, in step (c), to room temperature or more generally to a temperature of between 10 and 30° C. by cooling, for example by reducing the temperature at a rate of at least 1° C./minute, preferably by more rapid cooling, with a decrease typically of at least 3° C./minute, for example from 3 to 5° C./minute. This type of cooling typically leads to particles with a length of between 50 nm and 5 μm, typically between 100 nm and 1 μm, and a thickness of 50 nm. Moreover, without wishing to be bound to a particular theory, the abovementioned high cooling rates generally lead to very low contents of impurities (especially Cu2S, Bi2O3 and Cu2BiS3).
Advantageously, the material according to the invention is obtained via the first solid milling process presented above.
A subject of the present invention is also the use of a material comprising at least one compound of formula (I):
Bi1-xMxCu1-y-εM′yOS1-zM″z (I)
in which:
That which is indicated in the above presentation, especially as regards the elements M, M′ and M″, applies to the present use according to the invention.
The compound that is present in the semiconductor material is a substituted inorganic material, especially of the p type.
The chemical substitutions of bismuth, copper and/or sulfur may have several roles.
In the case of isoelectronic substitutions such as substitution of the element Bi with rare earth metals or with the element Sb or alternatively substitution of the element Cu with the element Ag, these substitutions may, especially by modifying the lattice parameters and/or by modifying the extension of the orbitals and their energetic position, thus lead to modifications of the gap (valency band-conduction band).
As regards aliovalent substitutions, they modify the oxidation state of the element Cu.
The introduction of substituents into the structure of the semiconductor may, depending on the case, lead to a reduction or an increase in the number of charge carriers. The substituted materials may especially have higher conductivity, which induces improved conduction capacity, relative to its unsubstituted form, or, on the contrary, lower conductivity.
In the context of the present invention, the inventors have now demonstrated that the materials corresponding to the abovementioned formula (I), in particular when they are of p type, are capable of providing a photocurrent when they are irradiated at a wavelength longer than their gap (namely the generation of an electron-hole pair in the material under the effect of an incident photon of sufficient energy, the charged species formed (the electron and the “hole”, namely the absence of an electron) being free to move to generate a current).
In particular, the inventors have now demonstrated that the materials of the invention appear to be capable of producing a photovoltaic effect.
In general, a photovoltaic effect is obtained via the combined use of two semiconductor compounds of different type, namely:
In the context of the invention, the photovoltaic effect is typically obtained by placing a semiconductor-based material of the abovementioned formula (I), which is also specifically of p type, in contact with an n-type semiconductor between two electrodes, in direct contact or optionally connected to at least one of the electrodes via an additional coating, for example a charge collector coating; and by irradiating the photovoltaic device thus made with suitable electromagnetic radiation, typically with light from the solar spectrum. To do this, it is preferable for one of the electrodes to allow passage of the electromagnetic radiation used.
According to another particular aspect, a subject of the present invention is photovoltaic devices comprising, between a hole-conducting material and an electron-conducting material, a layer based on a p-type compound of formula (I) and a layer based on an n-type semiconductor, in which:
For the purposes of the present description, the term “hole-conducting material” means a material which is capable of circulating current between the p-type semiconductor and the electrical circuit.
The n-type semiconductor used in the photovoltaic devices according to the invention may be chosen from any semiconductor which has more pronounced electron-acceptor nature than the compound of formula (I) or a compound which promotes the removal of electrons. Preferably, the n-type semiconductor may be an oxide, for example ZnO or TiO2, or a sulfide, for example ZnS.
The hole-conducting material used in the photovoltaic devices according to the invention may be, for example, a suitable metal, for instance gold, tungsten or molybdenum; or a metal deposited on a support, or in contact with an electrolyte, such as Pt/FTO (platinum deposited on fluorine-doped tin dioxide); or a conductive oxide such as ITO (tin-doped indium oxide), for example deposited on glass; or a p-type conductive polymer.
According to a particular embodiment, the hole-conducting material may comprise a hole-conducting material of the abovementioned type and a redox mediator, for example an electrolyte containing the I2/I− couple, in which case the hole-conducting material is typically Pt/FTO.
The electron-conducting material may be, for example, FTO or AZO (aluminum-doped zinc oxide), or an n-type semiconductor.
In a photovoltaic device according to the invention, the holes generated at the p-n junction are extracted via the hole-conducting material and the electrons are extracted via the electron-conducting material of the abovementioned type.
In a photovoltaic device according to the invention, it is preferable for the hole-conducting material and/or the electron-conducting material to be a material that is at least partially transparent, which allows passage of the electromagnetic radiation used. In this case, the at least partially transparent material is advantageously placed between the source of the incident electromagnetic radiation and the p-type semiconductor.
To this end, the hole-conducting material may be, for example, a material chosen from a metal or a conductive glass.
Alternatively or in combination, the electron-conducting material may be at least partially transparent, and it is then chosen, for example, from FTO (fluorine-doped tin dioxide), or AZO (aluminum-doped zinc oxide), or an n-type semiconductor.
According to another advantageous embodiment, the layer based on an n-type semiconductor which is in contact with the layer based on a p-type compound of formula (I) may also be at least partially transparent.
The term “partially transparent material” means here a material which allows the passage of at least part of the incident electromagnetic radiation, useful for providing the photocurrent, and which may be:
The compound of formula (I) used according to the present invention is advantageously used in the form of isotropic or anisotropic objects having at least one dimension of less than 50 μm, preferably less than 20 μm, typically less than 10 μm, preferentially less than 5 μm, generally less than 1 μm, more advantageously less than 500 nm, for example less than 200 nm, or even 100 nm.
Typically, the dimension less than 50 μm may be:
According to a first variant, the objects based on a compound of formula (I) are particles, typically having dimensions of less than 10 μm.
These particles are preferably obtained according to one of the preparation processes of the invention.
The term “particles” means herein isotropic or anisotropic objects, which may be individual particles, or aggregates.
The dimensions of the particles to which reference is made here may typically be measured by scanning electron microscopy (SEM).
Advantageously, the compound of formula (I) is in the form of anisotropic particles of platelet type, or of agglomerates of a few dozen to a few hundred particles of this type, these platelet-type particles typically having dimensions that remain less than 5 μm (preferentially less than 1 μm, more advantageously less than 500 nm), with a thickness that typically remains less than 500 nm, for example less than 100 nm.
The particles of the type described according to the first variant may typically be used in the form deposited on an n-type conductive or semiconductor support.
An ITO or metal plate covered with p-type particles of formula (I) according to the invention may thus, for example, act as a photoactive electrode for a device of photoelectrochemical type that may be used especially as a photodetector.
Typically, a device of photoelectrochemical type using a photoactive electrode of the abovementioned type comprises an electrolyte that is generally a salt solution, for example a KCl solution, typically having a concentration of about 1 M, in which are immersed:
According to a possible embodiment, the electrochemical device may comprise:
When an electrochemical device of this type is placed under a light source, under the effect of irradiation, electron-hole pairs form and are dissociated.
When the electrolyte is an aqueous solution, which is usually the case, the water in the electrolyte is reduced close to the photoactive electrode by the electrons generated, producing hydrogen and OH− ions. The OH− ions thus produced will migrate toward the counter-electrode via the electrolyte; and the holes of the compound of formula (I) will be extracted via the ITO-type conductor and will enter in the external electrical circuit. Finally, oxidation of the OH− ions is performed using holes close to the counter-electrode, producing oxygen. The placing in motion of these charges (holes and electrons), induced by the absorption of light by the compound of formula (I), generates a photocurrent.
The device may especially be used as a photodetector, the photocurrent being generated only when the device is illuminated.
A photoactive electrode as described above may especially be prepared using a suspension, comprising particles of a compound of formula (I) of the abovementioned type dispersed in a solvent, and depositing this suspension on a support, for example a glass plate covered with ITO or a metal plate, via the wet route or any coating method, for example by drop-casting, spin-coating, dip-coating, ink-jet printing or screen printing. For further details regarding this subject, reference may be made to the article: R. M. Pasquarelli, D. S. Ginley, R. O'Hayre, in Chem. Soc. Rev., vol 40, pages 5406-5441, 2011. Preferably, the particles based on a compound of formula (I) which are present in the suspension have a mean diameter as measured by laser granulometry (for example using a Malvern type laser granulometer) which is less than 5 μm.
According to a preferential embodiment, the particles of compound of formula (I) may be previously dispersed in a solvent, for example terpineol or ethanol.
The suspension containing the particles of compound of formula (I) may be deposited on a support, for example a plate covered with conductive oxide.
According to a second variant of the invention, which proves to be well suited to producing photovoltaic devices, the compound of formula (I) is in the form of a continuous layer based on the compound of formula (I), whose thickness is less than 50 μm, preferably less than 20 μm, more advantageously less than 10 μm, for example less than 5 μm and typically greater than 500 nm.
The term “continuous layer” means herein a homogeneous deposit produced on a support and covering said support, not obtained by simple deposition of a dispersion of particles onto the support.
The continuous layer based on a p-type compound of formula (I) according to this particular variant of the invention is typically placed close to a layer of an n-type semiconductor, between a hole-conducting material and an electron-conducting material, to form a photovoltaic device intended to provide a photovoltaic effect.
An n-type semiconductor in the use according to the invention may be a conductive oxide, for example ZnO or TiO2, or a sulfide, for example ZnS.
Moreover, the term layer “based on the compound of formula (I)” means a layer comprising the compound of formula (I), preferably in a proportion of at least 50% by mass, or even in a proportion of at least 75% by mass.
According to one embodiment, the continuous layer according to the second variant is essentially constituted by the compound of formula (I), and it typically comprises at least 95% by mass, or even at least 98% by mass and more preferentially at least 99% by mass of the compound of formula (I).
The continuous layer based on a compound of formula (I) used according to this embodiment may take several forms.
The continuous layer may especially comprise a polymer matrix and, dispersed in this matrix, particles based on a compound of formula (I), typically with dimensions of less than 10 μm, or even less than 5 μm, especially of the type used in the first embodiment of the invention.
Typically, the polymer matrix comprises a p-type conductive polymer, which may be chosen especially from polythiophene derivatives, more particularly from poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) derivatives.
The particles based on the compound of formula (I) present in the polymer matrix preferably have dimensions of less than 5 μm, which may especially be determined by SEM.
The invention will now be illustrated in greater detail with reference to the illustrative examples given below and to the attached figures, in which:
The placing in contact of a compound of formula (I) according to the invention with an n-type semiconductor ZnO forms a p-n junction. When the device is placed under a light source, the electrons generated move into the ZnO and the holes generated remain in the compound of formula (I) according to the invention. The ZnO is in contact with FTO (electron conductor) to extract the electrons therefrom and the compound of formula (I) according to the invention is in contact with gold (hole conductor) to extract the holes therefrom.
The examples that follow illustrate the invention without, however, limiting the scope.
A BiCu0.5Ag0.5OS powder was prepared by reactive milling at room temperature, according to the following protocol:
1.028 g of Bi2S3, 1.864 g of Bi2O3, 0.477 g of Cu2S and 0.744 g of Ag2S are placed in an agate mortar in the presence of agate milling beads.
The mortar is then covered and placed in a Fritsch No. 6 planetary mill with a spin speed of about 500 rpm. Milling is continued for 120 minutes until a pure phase is obtained.
The compound C1 obtained characterized by x-ray diffraction has the following tetragonal lattice parameters: a=3.866 Å, c=8.5805 Å, V=128.27 Å3.
A BiCuOS0.5I0.5 powder was prepared by reactive milling at room temperature, according to the following protocol:
1.028 g of Bi2S3, 1.864 g of Bi2O3, 0.906 g of Cu2S and 0.114 g of CuI are placed in an agate mortar in the presence of agate milling beads.
The mortar is then covered and placed in a Fritsch No. 6 planetary mill with a spin speed of about 500 rpm. Milling is continued for 120 minutes until a pure phase is obtained.
The compound C2 obtained characterized by x-ray diffraction has the following tetragonal lattice parameters: a=3.88 Å, c=9.595 Å, V=129.47 Å3.
A BiCu0.7Zn0.3OS powder was prepared by reactive milling at room temperature, according to the following protocol:
0.720 g of Bi2S3, 1.584 g of Bi2O3, 0.668 g of Cu2S and 0.349 g of ZnS are placed in an agate mortar in the presence of agate milling beads.
The mortar is then covered and placed in a Fritsch No. 6 planetary mill with a spin speed of about 500 rpm. Milling is continued for 120 minutes until a pure phase is obtained.
The compound C3 obtained characterized by x-ray diffraction has the following tetragonal lattice parameters: a=3.870 Å, c=8.571 Å, V=128.36 Å3.
4 mL of concentrated HNO3 (commercial 52.5%) are added to 2.425 g of BiNO3.5H2O in a container, and the mixture is then diluted with 10 mL of water. In another beaker, 3 g of sodium hydroxide are mixed with 3 g of dibasic sodium tartrate (C4H4Na2O6.2H2O).
The two solutions obtained are mixed rapidly. A white precipitate forms and disappears immediately. The solution obtained is of transparent color. It is then diluted to a volume of 50 mL with water.
0.992 g of copper sulfate pentahydrate (CuSO4.5H2O) and 0.285 g of zinc sulfate heptahydrate are dissolved in 30 mL of distilled water. 1.5 mL of concentrated ammonia (28%) are added and a dark blue solution is obtained. 15 g of sodium thiosulfate pentahydrate are then added.
The mixture is heated moderately (50° C.) for four hours. A colorless solution is obtained. It is preferable to use closed containers to avoid oxidation of the copper(I).
12.25 g of Na2S.9 H2O are dissolved in 100 mL of distilled water.
The solutions prepared previously containing Bi and (Cu(+Zn) are mixed rapidly. A white precipitate forms and disappears immediately. The mixture is heated to a temperature of 90° C. The Na2S solution is heated to 90° C.
When the two solutions are at the desired temperature, the solution of the cations (Bi,Cu,Zn) is added to the Na2S solution. A black precipitate forms immediately. The solution is stirred at 90° C. for four hours. It is then filtered, washed with distilled water and dried at 80° C. in an oven.
The product obtained has a single phase when it is observed by x-ray diffraction.
The device described in
For each of the compounds C1 to C5, the measurements of the photocurrent obtained are as follows:
| Number | Date | Country | Kind |
|---|---|---|---|
| 14/00832 | Apr 2014 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/097023 | 4/3/2015 | WO | 00 |
