WO2013105043A1

WO2013105043A1 - Cascaded photon-emitting nanoparticle codoped with terbium and ytterbium and functionalized by an organic ligand

Info

Publication number: WO2013105043A1
Application number: PCT/IB2013/050212
Authority: WO
Inventors: Olivier Raccurt; Nicolas Gauthier; Daniel Imbert; Marinella MAZZANTI EMSLEY
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2012-01-11
Filing date: 2013-01-10
Publication date: 2013-07-18
Also published as: FR2985509B1; FR2985509A1

Abstract

The present invention relates to a cascaded photon-emitting nanoparticle partially consisting or at least one crystal matrix of formula (I): Na_(1-n)XF_(4-n), where n is equal to 0 or 1, and X is a rare earth element selected from yttrium, gadolinium and lanthanum; or of formula (III): MF₂, where M is selected from magnesium, calcium, strontium and barium, said nanoparticle being: codoped, at the crystal matrix(es), with the terbium and ytterbium, with a doping rate for Tb of 1 to 15% relative to X, and a doping rate for Yb of 1 to 30% relative to X; and functionalized at the surface thereof by at least one organic ligand capable of absorbing photons having a wavelength of between 300 and 500 nm and capable of transferring the absorbed photon energy to the terbium. The invention further relates to the use of such nanoparticles for forming a spectral conversion layer in a photovoltaic device.

Description

NANOPARTICLE WITH TRANSMISSION OF PHOTONS IN CASCADE CODOPED IN TERBIUM AND YTTERBIUM, AND FUNCTIONALIZED BY ONE

ORGANIC LIGAND

The present invention relates to the field of photo voltaic devices. It more particularly relates to new nanoparticles and their use to improve the performance of photovoltaic devices.

One of the constant concerns in the research and development of photovoltaic cells is to increase their efficiency, while making them economically viable. For this purpose, we seek more particularly to obtain the best match between the solar spectrum and the sensitivity spectrum of the material used in the photovoltaic device to perform the photovoltaic conversion. To do this, two alternatives have been considered: find the photovoltaic material having a spectral efficiency best suited to the solar spectrum, or adapt the spectrum of incident light arriving on the cell to the spectral response of the photovoltaic material.

With regard to the first alternative, various types of solar cells have been developed, in particular so-called multi-junction cells which are composed of several layers making it possible to convert different parts of the solar spectrum and thus to obtain better yields of solar cells. conversion. Unfortunately, the development of such cells is limited in view of their high manufacturing cost. Moreover, as illustrated in FIG. 1 comparing the solar spectrum (1) to the spectral response of two types of conventional solar cell, one based on silicon (2) and the other based on CIGS (copper, indium, gallium and selenium) (3), the efficiency of these solar cells is not optimum for wavelengths between 300 and 500 nm, in particular below 450 nm, whereas these wavelengths correspond to maximum solar emission.

The second alternative consists in modifying the incident spectrum by absorbing the parts of the solar spectrum which are little converted by the cell, in particular between 300 and 500 nm, and by retransmitting photons of lower energy corresponding to the optimum efficiency of the cell. solar, for example above 500 nm, and more particularly between 800 and 1000 nm for a silicon cell.

In this way, it has been more particularly proposed the use of luminescent materials which, for an absorbed photon, are capable of reemitting one photon more low energy. This mechanism of lower energy photon generation is better known by the English name of "down shifting". For example, the use of europium doped fluoride nanoparticles [1] or the addition of organic phosphor [2] can be used to increase the yield of photosensitive pigment cells (also called DSCC cells or Grâtzel cells). We can also mention the publication of Marchionna et al. [3] which proposes the integration of Eu organo-lanthanide complexes in silicone for encapsulating the silicon photovoltaic cell, to lead to an increase in the photovoltaic conversion efficiency.

Furthermore, the document WO 2011/038335 describes the deposition of a polymer film on the front face and the rear face of a bifacial solar cell in order respectively to perform down- and up-conversion from all kinds of materials. luminescent materials, such as organic fluorophores, inorganic nanoparticles doped with earth, quantum dots and organolanthanides.

We can also mention the document US 2009/0156786, which describes the incorporation of phosphorus in a transparent film to absorb UV radiation to increase the efficiency of solar cells. The absorption of these phosphors takes place over a wavelength range of less than 400 nm for a re-emission between 500 and 780 nm.

The present invention aims to take advantage of another spectral conversion mechanism: that of the emission of photons in cascade, also known under the names of "multi-generation of photons" and, in English, of "quantum cutting" . These designations refer to the ability of certain materials to absorb a high energy photon and to re-emit several photons of lower energy. In other words, this type of material has a quantum efficiency greater than 100%.

Quantum cutting materials have been studied for a long time for lighting. For example, the Ronda article [4] lists a number of quantum cutting materials and their applications for lighting. These materials are generally based on a rare-earth codopage, with a first rare earth chosen as the activator ion, absorbing the incident photons and transmitting its energy to a second rare earth called the emitter ion. Many matrices have been tested for this type of material. For example, the quantum cutting properties between terbium and pyrterbium have been measured in different matrices, such as fluorides [5], phosphates [6], borates [7], oxides [8], surfures, etc. . Unfortunately, the implementation of cascaded photon emission materials in photovoltaic devices poses two major difficulties.

First, quantum cutting materials are generally synthesized in the form of micrometer powder. However, the implementation of particles of micrometric size or in aggregate form is likely to cause problems in terms of incident light scattering, which leads to a loss of efficiency of the solar cell.

Also, inorganic materials doped with rare earths have a low absorption coefficient. Thus, even if the quantum yield obtained is important with such materials, the amount of photons absorbed, and therefore converted, remains low.

Consequently, the quantum-cutting materials currently developed do not make it possible to give complete satisfaction for an implementation in photovoltaic devices in order to improve their efficiency.

The present invention aims precisely to provide a new type of material to overcome the aforementioned drawbacks.

Thus, the present invention relates, according to a first of its aspects, to a cascaded photon emission nanoparticle, formed at least in part by at least one crystalline matrix of formula (I):

wherein n is 0 or 1; and X is a rare earth selected from yttrium, gadolinium and lanthanum;

or of formula (III):

MF ₂

wherein M is selected from magnesium, calcium, strontium and barium; said nanoparticle being:

co-doped at the level of said crystalline matrix (s) in terbium (Tb) and ytterbium (Yb), with an atomic doping level for Tb ranging from 1 to 15% relative to X or M, and a Atomic doping rate for Yb ranging from 1 to 30% relative to X or M; and - Functionalized at the surface by at least one organic ligand having a wavelength absorption capacity of photons between 300 and 500 nm and capable of transferring the energy of the photons absorbed to terbium.

The inventors have thus discovered that it is possible to improve the efficiency of a photovoltaic device by implementing inorganic nanoparticles codoped in terbium and ytterbium, exhibiting properties of photon emission in cascade, and functionalized by an organic ligand. .

Functionalization of particles with ligands has already been proposed, for example to sensitize nanoparticles of NaYF ₄ doped with a lanthanide ion such as Nd or Yb by Zhang et al. [9], or for Eu-doped LaF ₃ nanoparticles by Janssen et al. [10]. We can also quote the article by Benisvy et al. [11] proposing the sensitization of Tb by an organic ligand or the publication of Hasegawa et al.

[12], describing the sensitization by organic functions of polymeric materials of Eu oxide clusters.

However, to the inventors' knowledge, the use of organic ligands has never been proposed for the purpose of sensitizing nanoparticles with photon emission in cascade. The implementation of the nanoparticles according to the invention for photo voltaic devices is particularly advantageous.

First of all, the nanometric size of these particles makes it possible to avoid the diffraction losses mentioned above. Such nanoparticles also make it possible to optimize the surface / volume ratio, and therefore the energy that can be absorbed per unit area of the photovoltaic device.

Moreover, as illustrated by the following examples, the nanoparticles of the invention have a high absorption factor in the wavelengths of 300 to 500 nm, corresponds to the maximum solar emission. Also, as demonstrated by Examples 3 and 4 which follow, the implementation of an organic ligand according to the invention makes it possible to obtain an increased luminescence for the nanoparticles of the invention.

Finally, as illustrated in Example 5, the integration of the nanoparticles of the invention with photovoltaic devices does not affect their luminescence properties. Thus, according to another of its aspects, the present invention relates to the use of the nanoparticles of the invention to form, in a photovoltaic device, a layer or a deposit exhibiting luminescence properties and making it possible to modify the spectrum of light incidence. .

It also aims, according to another of its aspects, a device, in particular photovoltaic, comprising a layer or a deposit formed (e) nanoparticles of the invention.

Other characteristics, variants and advantages of the nanoparticles according to the invention, and of their implementation in a photovoltaic device, will emerge more clearly on reading the description, examples and figures which follow, given for illustrative and non-limiting purposes. of the invention.

In the remainder of the text, the expressions "between ... and ...", "ranging from ... to ..." and "varying from ... to ..." are equivalent and mean to mean that terminals are included unless otherwise stated.

Unless otherwise indicated, the expression "comprising / including a" shall be understood as "comprising / including at least one".

NANO

As specified above, the nanoparticles according to the invention are formed at least in part of at least one crystalline matrix of formula (I):

wherein n is 0 or 1; and X is a rare earth selected from yttrium (Y), gadolinium (Gd) and lanthanum (La);

or of formula (III):

MF ₂

wherein M is selected from magnesium, calcium, strontium and barium.

According to a particularly preferred embodiment, the nanoparticles according to the invention are formed at least in part of at least one crystalline matrix of formula (I), in particular in which X is yttrium. In other words, the nanoparticles according to the invention may be formed at least in part of at least one crystalline matrix of formula NaYF ₄ or YF ₃ . In particular, a nanoparticle according to the invention may comprise, or even, according to an alternative embodiment, be formed of a matrix of NaYF ₄ formula.

According to another aspect of the invention, the nanoparticles of the invention are codoped at the crystalline matrix (s) of formula (I) above in terbium (Tb) and in ytterbium (Yb), with a doping level for Tb, expressed with respect to X (for the crystalline matrix of formula (I)) or M (for the crystalline matrix of formula (III)), between 1 and 15% and a doping level for Yb, expressed with respect to X or M, between 1 and 30%, in particular between 1 and 10%.

The terbium and ytterbium doping levels are expressed as an atomic percentage with respect to X or with respect to M, as represented more particularly, in the context of the implementation of a crystalline matrix of formula (I), in the formula (II) which follows.

As illustrated in the examples which follow, the implementation of the Tb / Yb pair within a crystalline matrix according to the invention advantageously allows efficient energy transfer from the activating ion (Tb) to the emitter ion ( Yb). This charge transfer between the energy levels of the Tb and those of the Yb is shown schematically in FIG. 2. Thus, the Tb which, itself, transfers its energy to the Yb which will emit for an absorbed photon, two photons of lower energy, Tb and Yb are more particularly present in the crystalline matrix of formula (I) above in the substitution position.

In particular, the distance between the Tb and the Yb is preferably of the order of a few nanometers, in particular less than 10 nm.

Several variants of nanoparticles according to the invention are possible. In particular, as developed subsequently, the nanoparticles of the invention may be formed of a single matrix of formula (I) above or, alternatively, two crystalline matrices of formulas (I) distinct.

Thus, according to a first variant embodiment, a nanoparticle of the invention may be formed of a single crystalline matrix of formula (I), codoped homogeneously in Tb and Yb. This variant is represented in FIG. 3a. In other words, a nanoparticle of the invention may be of formula (II) below:

Na (i_ _n ) X (i_ _x _ _y) F (4-n ₎ (Tb) _x (Yb) _y

with:

- n = 0 or 1;

X being a rare earth selected from yttrium, gadolinium and lanthanum, preferably yttrium;

the doping level in Tb of the nanoparticle, x, being between 1 and 15%; and

the Yb doping rate of the nanoparticle, y, being between 1 and 30%, in particular between 1 and 10%.

By way of example, it may be a NaYF ₄ nanoparticle homogeneously coded in Tb and Yb, in particular with a Tb doping level of approximately 15% with respect to Y and a rate of Yb doping ranging from 5 to 20% relative to Y, in particular about 10%.

In other words, this nanoparticle can be of the following chemical formula:

NaYo, 7 ₅ F ₄ Tbo, 15 Yb ₀ , 10 ·

It is up to those skilled in the art to implement the appropriate methods for synthesizing the codoped nanoparticles according to the invention.

By way of example, the nanoparticles of NaYF ₄ codoped Tb and Yb can be prepared according to the protocol described by Gao et al. [13] for the synthesis of nanoparticles of NaYF ₄ codoped Er and Yb, adapting this protocol for codopage in Tb and Yb. These synthetic methods are more particularly developed in the following examples. According to a second variant embodiment of the invention, a nanoparticle of the invention may have a core / shell structure, the terbium being at least present at the shell.

A heart / shell nanoparticle according to the invention may thus be constituted:

- a heart of chemical formula (Ha) following:

Na ₍ i-1) x (i- _x i- _y i ₎ F (4-nl ₎ (Tb) _x i (Yb) _y i; and

- of a shell of chemical formula (Ilb) following:

Na (i_ _n 2) X '(i _x2- y ₂ ) F (4-n) (Tb) _x 2 (Yb) _y 2 with:

- Ni and n2, identical or different, being selected from 0 or 1;

- X and X ', identical or different, being chosen from yttrium, gadolinium and lanthanum;

the terbium doping rate of the nanoparticle, x = xi + x ₂ , being between 1 and

15%; and

the ytterbium doping level of the nanoparticle, y = yi + y ₂ , being between 1 and 30%, in particular between 1 and 10%;

with x2 being non-zero.

Those skilled in the art are able to implement synthesis methods adapted to the synthesis of this type of architecture core / shell as described for example in the publications [14], [15] and [16]. The core and the shell of a nanoparticle according to the invention can be distinguished by the nature of their crystalline matrix of formula (I) above and / or by their respective doping rate Tb and / or Yb.

Thus, according to one particular embodiment, a nanoparticle according to the invention may be formed of a single crystalline matrix of formula (I), the terbium and / or ytterbium doping levels being different at the level of the core and the shell of said nanoparticle.

According to another particular embodiment, a nanoparticle according to the invention may be formed of two crystalline matrices of formulas (I), respectively constituting the core and the shell of the nanoparticle.

In particular, said crystalline matrices of formula (I), forming respectively the core and the shell of the nanoparticle, can be distinguished by the nature of their rare earth X.

By way of example, the core of the nanoparticle may be formed of a crystalline matrix of formula NaYF ₄ , and the shell of a crystalline matrix of formula NaGdF ₄ .

According to another particular embodiment, the doping levels in Tb and / or Yb may be different at the core and the shell of the nanoparticle. In particular, the Yb may be present only at the core of the nanoparticle. In other words, y2 = 0 in the aforementioned formula (IIb).

According to another particular mode, possibly being combined with the previous mode, the Tb may be present only at the level of the shell, in other words x1 = 0 in the above formula (Ha).

It is understood that the various embodiments mentioned above can be combined. In other words, a heart / shell type nanoparticle according to the invention may be formed of a core and a shell having distinct respective chemical formulas, both with regard to the nature of their crystalline matrix (in particular of the nature rare earth) and with regard to their respective doping levels in Tb and / or in Yb.

For example, a nanoparticle of the invention may be composed of a heart formed crystalline matrix doped NaYF ₄ or Yb co-doped with Tb and Yb, and a shell formed of the crystalline matrix doped NaGdF ₄ Tb.

Of course, the variants previously developed for nanoparticles formed at least in part of at least one crystalline matrix of formula (I) can also be envisaged for nanoparticles formed at least in part of at least one crystalline matrix of formula (III ). For example, a nanoparticle according to the invention may be formed of a single matrix of formula (III) above or, alternatively, two crystalline matrices of formulas (III) distinct. According to another embodiment, a nanoparticle of the invention may be formed of a matrix of formula (I) and of a matrix of formula (III), one of them forming the core and the other forming the shell of the nanoparticle.

Preferably, the nanoparticles of the invention have a core / shell structure, the shell being doped with Tb, and the core being doped with Tb and Yb or with Yb. These variants are illustrated more particularly in FIGS. 3b and 3c.

Such a co-doping distribution of the nanoparticle heart / shell advantageously allows to promote the transfer of ligand / Tb energy and Tb / Yb energy transfer within the nanoparticle. In particular, doping Yb only at the core level of the nanoparticles can make it possible to avoid any phenomenon of de-excitation (also called "quenching" in English) of Yb by the organic ligand functionalizing the surface of the nanoparticle.

According to another particularly preferred embodiment, the crystalline matrices constituting respectively the heart and the shell are of a different nature.

The implementation of two distinct crystalline matrices constituting respectively the core and the shell, and preferably with the Tb-doped shell and the Yb-doped core, can advantageously make it possible to promote the mechanisms of transfer between rare earths by limiting in particular a dissipation by the phonons. The nanoparticles according to the invention may have a size ranging from 1 to

100 nm, in particular from 5 to 80 nm, and more particularly from 20 to 60 nm.

In particular, the shell of the nanoparticles according to the invention may have a thickness of less than or equal to 10 nm, in particular ranging from 1 to 10 nm. ORGANIC LIGAND

According to another aspect of the invention, the nanoparticles according to the invention are functionalized on the surface by at least one organic ligand.

As indicated above, the organic ligands used are capable of absorbing photons of wavelength between 300 and 500 nm, in particular between 300 and 400 nm.

Moreover, the ligands used are able to efficiently transfer the energy of the photons thus absorbed with terbium.

The organic ligands that can be used according to the invention can more particularly have a level of energy 3ππ * and / or 1 ππ *.

According to a particular embodiment, the transfer of energy of the photons absorbed by the organic ligand to the terbium said nanoparticle is a level of energy transfer 3ππ * ligand to level ⁵ D ₄ of Tb, as shown schematically in Figure 2. The organic ligands that can be used to functionalize the surface of the nanoparticles according to the invention can be of varied nature.

These ligands may be more particularly chosen from the following compounds: i. terpyridine derivatives of formula (a)

wherein the R 1 substituents are independently selected from carboxylic and 2H-tetrazol-5-yl (zH) groups; and R represents a group chosen from:

with n-Oct meaning the n-octyl radical.

More particularly, the ligand may be chosen from the terpyridine derivatives of formula (a) above, in which:

Ri = TzH, R = H (L1),

It may also be chosen from the ligands of formula (a) above, in which:

ii. the bipyridine derivatives of formula (b) and pyridine of formula (c) below

wherein R 1 and R ₂ are independently selected from carboxylic and 2H-tetrazol-5-yl (zH) groups.

In particular, the ligand may be chosen from the bipyridine derivatives of formula (b) above, in which:

R ^{1 =} R ² = TzH (L7)

R ^ Tz, R ² = COOH, (L8)

R ^ R ^ COOH (L9)

In particular, the ligand may be chosen from the pyridine derivatives of formula (c) above, in which:

R ^{1 =} R ² = TzH (L10)

R = Tz, R ² = COOH, (LU)

R ¹ = R ² = COOH (L 12) iii. the 8-hydroquinoline derivatives of formula (d)

(D)

wherein R 1 is selected from carboxylic and 2H-tetrazol-5-yl groups; iv. salicylic acid (formula (el) below) and its derivatives of formula (e2) and (e3) below

v. the following derivatives of formula (f), (g) and (h)

(f) (g)

According to a particularly preferred embodiment, said ligand is chosen from the following compounds:

2-carboxy-8-hydroquinoline

-di (2H-tetrazol-5-yl) -2,2 ': 6', 2 "-terpyridine

-di (carboxy) -2,2 ': 6', 2 "-ter ridine

and salicylic acid.

Organic ligands are synthesized according to methods known to those skilled in the art.

In particular, ligands L1 to L10 can be synthesized according to the procedures described in publications [17], [18] and [19]. The 8-hydroquinoline derivatives of formula (d) above can be synthesized according to the protocol described in the publication [20]. The ligands L1 and L2 are more particularly described in the document WO 2009/037277.

Advantageously, the ligands are chosen from ligands having at least one tetrazole unit.

Preparation of the functionalized nanoparticle

It is up to the person skilled in the art to implement the functionalization methods adapted to sensitize the nanoparticles with an organic ligand according to the invention.

In general, the codoped nanoparticles previously prepared as described previously can be dispersed in anhydrous ethanol under agitation or by sonication. The ligand, previously deprotonated in methanol, for example by addition of a base such as potassium hydroxide or triethylamine, can then be added gradually to the solution of particles. The pH is adjusted so as not to lead to a decomplexation of the ligand.

The solution of sensitized nanoparticles can then be centrifuged and rinsed several times with anhydrous ethanol, or dialyzed to remove the amount of unreacted ligands with the particle. The synthetic methods are more particularly developed in the examples which follow.

The organic ligand is more particularly associated with said nanoparticle via electrostatic interactions.

The amounts of ligand are adjusted to allow functionalization to optimize the luminescence of the nanoparticle.

The number of ligands used for a nanoparticle of the invention is of course likely to vary with respect to the size of the nanoparticle, its terbium doping rate and the steric hindrance of the organic ligand used.

However, in general, the ligand / terbium molar ratio for a nanoparticle of the invention can range from 0.01 to 1.

USE

As specified above, the present invention relates, in another of its aspects, to the use of the nanoparticles previously described for forming in a photovoltaic device, in particular in a photovoltaic cell, a layer or a deposit having luminescence properties and making it possible to change the spectrum of the incident light.

The nanoparticles of the invention are of course arranged between the solar radiation and the photovoltaic cell, so as to modify the incident spectrum seen by the cell.

They may be more particularly present on the surface of the device, in particular on the glass, within the glass encapsulating the photovoltaic device, on the surface or within the encapsulation layer of the photovoltaic material, or on the surface of the cell. photovoltaic. Preferably, the nanoparticles of the invention are implemented on the surface or within the encapsulation layer of the photovoltaic material, to minimize the retransmission of the converted photons to the outside of the cell. According to a first variant embodiment, the functionalized particles according to the invention may be deposited on a surface of the solar cell, for example on the encapsulation glass of the photovoltaic cell.

They can be deposited by conventional liquid deposition methods, for example by centrifugal coating (better known under the name of "spin coating", dip coating, coating). spraying, etc.

According to another variant embodiment, the functionalized particles according to the invention can be integrated into a sol-gel layer. The sol-gel deposited may be silica or another material adapted from an optical point of view to the cell, for example Si0 ₂ , Zr0 ₂ , ZnO, Ti0 ₂ .

According to yet another alternative embodiment, the nanoparticles of the invention may be incorporated in an encapsulation polymer for photovoltaic application, for example silicone, EVA (ethylene-vinyl acetate), PMMA (polymethyl methacrylate). The sensitized particles may be premixed with the monomer, and then the polymerization is carried out, optionally in the presence of a catalyst, to form an encapsulation layer.

Advantageously, these methods for implementing the nanoparticles according to the invention within a photovoltaic device do not affect the luminescence properties of said nanoparticles.

The examples and figures presented below are only given by way of non-limiting illustration of the invention. figures

Figure 1: Comparison of the solar spectrum (1) with the spectral responses of two types of conventional silicon-based (2) and CIGS-based (3) solar cells. Figure 2: Schematic representation of the ligand / Tb charge transfer mechanisms and quantum-cutting Tb / Yb for a nanoparticle of the invention Tb / Yb codopee and sensitized with an organic ligand.

FIG. 3: Schematic representation of nanoparticles according to the invention: nanoparticle formed of a single codoped Tb and Yb crystalline organization (FIG. 3a), or of the heart / shell type formed of 2 crystalline organizations, one forming the core being codoped Yb and Tb (Figure 3b) or Yb-doped (Figure 3c), the second forming the shell being Tb-doped.

4: Emission and emission spectra of Tb (FIG. 4a) and Yb (FIG. 4b) in nanoparticles of NaYF ₄ codoped Tb and Yb.

FIG. 5: Emission spectra of Tb (FIG. 5a) and Yb (FIG. 5b) in Tb and Yb codoped nanoparticles with a Tb doping rate of 15% and variable Yb doping levels (0; 1, 3, 7 and 10%).

Figure 6: Emission spectra of Tb in Tb-doped nanoparticles and sensitized with varying amounts of L1 ligand.

Figure 7: Emission spectra of Tb in Tb-doped nanoparticles and sensitized, or not, with salicylic acid.

8: Emission and emission spectra of Tb (FIG. 8a) and Yb (FIG. 8b) in Tb and Yb codoped YF ₃ nanoparticles.

FIG. 9: Emission spectra of Tb (FIG. 9a) and Yb (FIG. 9b) in nanoparticles of NaYF ₄ encoded Tb and Yb and sensitized by the organic ligand L2.

10: Emission spectra of Yb in nanoparticles doped Yb and sensitized by the organic ligand H ₂ hqa, after deposition by evaporation on a quartz slide (FIG. 10a) and after dispersion in silicone (FIG. 10b).

EXAMPLES

EXAMPLE 1

Synthesis of quantum-cutting nanoparticles codoped Tb and Yb

Quantum cutting nanoparticles, formed of a Tb and Yb-coded NaYF ₄ matrix, with a 15% Tb doping rate and a 5% Yb doping level were synthesized according to the following protocol, similar to that described by Li Gao et al. [13]. 0.48 mmol of sodium acetate trihydrate is dissolved in 10 ml of ice-cold acetic acid. The whole is stirred for 5 to 10 minutes at room temperature. 0.3 ml of an aqueous solution of YC1 ₃ (0.5 mol / l) is added to the mixture.

Of a solution of YbCl ₃ (0.5 mol / L) and 60 of a solution of TbCl ₃

(0.5 mol / L) are added.

After stirring for 5 minutes, 0.7 mL of an aqueous NaF solution (1.0 mol / L) is slowly added.

The mixture is stirred for 3 minutes then heated to 106 ° C in 5 to 15 minutes, and finally refluxed for 80 minutes at 120 ° C.

After reaction, the mixture is cooled to room temperature and the precipitate is collected after centrifugation and washing with deionized water and anhydrous ethanol. Luminescence analysis

Luminescence measurements

The emission and excitation spectra are obtained using a fluorimeter (Horiba Jobin Yvon FL3-22 equipped with a double excitation monochromator network) and a photomultiplier (R928P Hamamatsu) for the measurements of emission in the visible. The lamp used is a 450 W Xenon lamp.

Results

The excitation and emission spectra of Tb and those of Yb in the codoped nanoparticles are presented respectively in FIGS. 4a and 4b.

The excitation ( _exc ) and emission wavelengths (λ _m ) used to obtain the emission and excitation spectra are shown in the figures.

For example, the emission spectrum of Yb is obtained by fixing _XC at 377 nm. A maximum of Yb luminescence is observed by exciting at a wavelength _exceeding 377 nm corresponding to the level of absorption. of Tb, which shows that a transfer of energy between Tb and Yb occurs within the codoped nanoparticle. Emission spectra for a variable Yb concentration

The emission spectra are obtained as described above, for codoped nanoparticles Yb and Tb synthesized as described above, having a constant Tb doping rate of 15% and variable Yb doping levels (0; 1; 3; 7 and 10%).

Figure 5 shows the emission spectra of Tb (Figure 5a) and Yb (Figure

5b).

It is observed that the luminescence of Tb decreases, whereas the luminescence of Yb increases, as the Yb doping rate increases, which is an indication of the transfer between the two rare earths.

Measurements of luminescence lifetime of terbium

Terbium luminescence lifetime measurements were also made according to the following protocol, using a pulsed lamp of 25 W Xe / Hg and a photomultiplier R928P (Hamamatsu).

The sample is excited at 377 nm with the pulsed lamp (40 ms tap) and the luminescence decay at 544 nm is recorded for the Tb. The integration of this signal makes it possible to determine the luminescence lifetime.

The results are summarized in Table 1 below.

Table 1

There is a decrease in the lifetime of the Tb when the percentage of Yb doping increases, again evidence of charge transfer between Tb and Yb.

The charge transfer appears optimal (FIG. 5b) for a doping rate in

Yb of 10%. EXAMPLE 2

Sensitization of a Tb doped nanoparticle with an organic ligand

The organic ligand (6,6 "-di (2H-tetrazol-5-yl) -2,2 ': 6', 2" -terpyridine (L1) or salicylic acid) is dissolved in anhydrous methanol in the presence of a at four base equivalents (triethylamine or potassium hydroxide) (ligand concentration around 1.10 ^"3 mol / L) in order to deprotonate the alcohol and amine functions of the ligand.

The Tb doped nanoparticles, synthesized according to a protocol similar to that described in Example 1, are dispersed in anhydrous ethanol at a rate of 5 to 10 mg / mL of ethanol).

Both solutions are then mixed. The pH is maintained during mixing at a value between 8 and 10, in order to avoid decomplexation of the ligand / particle system.

The solution of nanoparticles thus sensitized is then either centrifuged and rinsed several times with anhydrous ethanol, or dialyzed to remove the amount of unreacted ligands with the nanoparticles.

Results

The luminescence of the Tb is measured as described in Example 1 for sensitized nanoparticles obtained by implementing ligand quantities Ll of 0 μΐ, at 80 μί.

6 shows the luminescence nanoparticles doped with Tb by exciting the ligand (AE _XC 320 nm) and by varying the added volume of ligand.

It is observed that the luminescence of Tb increases as the amount of ligand organic ligand implemented increases, which is an indication of the transfer of energy of the ligand on acceptor levels of Tb.

Similarly, the comparison of the luminescence of Tb in a nanoparticle sensitized with salicylic acid with that obtained for a non-sensitized nanoparticle (FIG. 7) shows an increase in the luminescence of Tb for nanoparticles sensitized with salicylic acid, evidence of ligand energy transfer on acceptor levels of Tb. EXAMPLE 3

Nanoparticles of YF ^ codopées Tb and Yb and sensitized by salicylic acid

YF ₃ nanoparticles codoped at Tb and Yb, with a doping level for Tb of 1% and a doping level for Yb of 10% were synthesized according to the following protocol.

0.174 g of YbCl ₃ , 6H ₂ O + 1.213 g of YCI ₃ , 6H ₂ O + 16.9 mg of TbCl ₃ , 6H ₂ O are mixed in 10 ml of anhydrous methanol.

The mixture is stirred for 15 min and then added dropwise to 0.170 g of anhydrous NH ₄ F in 30 ml of anhydrous methanol at 70.degree.

The heating is continued at 70 ° C. until a white haze appears (-3-4 hours) and is then allowed to return to ambient temperature.

A translucent gel is recovered by successive centrifugations (speed> 10000 rpm and time 15-20 min) of the supernatant

The gel is finally dispersed in anhydrous ethanol

The nanoparticles dispersed in anhydrous ethanol are sensitized with salicylic acid, according to the protocol described in Example 2. Results

The luminescence of Tb is measured as described in Example 1 for the same concentration of Tb and Yb codoped nanoparticles sensitized with salicylic acid on the one hand, and non-sensitized Tb and Yb codoped nanoparticles on the other hand.

The excitation and emission spectra of Tb and those of Yb in the codoped and sensitive nanoparticles are presented respectively in FIGS. 8a and 8b.

The luminescence of the Tb and Yb codoped YF ₃ particles and the non-sensitized ones is not visible, unlike that of a solution of the same concentration of codoped nanoparticles sensitized with salicylic acid.

The luminescence is thus about 3.10 ⁵ times greater for particles sensitized according to the invention compared to non-sensitized particles. EXAMPLE 4

Nanoparticles of NaYF ₄ codoped Tb and Yb and sensitized by the ligand

L2

NaYF4 nanoparticles codoped at Tb and Yb are synthesized as described in Example 1.

They are sensitized with the ligand 6.6 "-di (carboxy) -2,2 ': 6', 2" -terpyridine (L2), according to the protocol described in Example 2.

Results

The emission spectra of Tb and Yb in the nanoparticles thus sensitized by the ligand L2, obtained by using variable ligand quantities (10 to 70 μί) are represented in FIG. 9.

An increase in the luminescence of the two rare earths is observed when the amount of ligand L2 implemented increases.

These results show that the luminescence of the codoped nanoparticles Tb and

Yb can be increased by sensitizing them with an organic ligand.

EXAMPLE 5

Resistance of sensitized nanoparticles to an application in a photovoltaic device

Nanoparticles of Yb-doped NaYF ₄ , synthesized according to the protocol described in Example 1, are sensitized with a quinoline derivative, previously deprotonated with a base, according to the protocol described in Example 2, and then washed with ethanol to remove the excess of ligand that would not have reacted.

Test 1

The nanoparticles thus sensitized are dispersed in ethanol and deposited by evaporation of the solvent on a glass slide.

The emission spectrum of the Yb, shown in Figure 10a, shows that the nanoparticles retain their luminescence properties. Test 2

The sensitized nanoparticles are incorporated into an encapsulation polymer (Rhodia RTV silicone).

The emission spectrum of Yb, shown in Figure 10b, shows that the nanoparticles retain their luminescence properties.

Thus, this example demonstrates the resistance of particles sensitized by an organic ligand, and the possibility of implementing them in a photovoltaic device, for example in the encapsulant of the cell or deposited as a layer on the surface, without affecting their properties in terms of luminescence.

References

[1] He et al, "Effects of luminescent downconversion film in dye-sensitized solar cells," APPLIED PHYSICS LETTERS 88, 173119 (2006);

[2] Yum et al., "Incorporating Multiple Energy Relay Dyes in Liquid Dye-

Sensitized Solar Cells, ChemPhysChem 2011, 12, 657-661;

[3] Marchionna et al., "Photovoltaic quantum effi ciency enhancement by light harvesting of organo-lanthanide complexes", Journal of Luminescence 118 (2006) 325-329;

[4] Ronda, "Luminescent Materials with Quantum Efficiency Larger Than 1, Status and Prospects", Journal of Luminescence 100 (2002) 301-305;

[5] (a) Ye et al., "Spectral Modification and Quantum Cutting in RE3 + -Yb3 + (R = Pr, Tm, and Tb) Codoped Transparent Glass-ceramics CaF2 Containing Nanocrystals, IUMRS-ICA 2008, Symposium" AA.Rare -EarthRelatedMaterialProcessingandFunctions ", IOP Conf Series: Materials Science and Engineering 1 (2009) 012008 (b) CHEN et al, CHIN.PHYS.LETT Vol 25, No. 6 (2008) 2078.

[6] P. Vergeer et al., "Quantum cutting by cooperative energy transfer in Yb _x Yi-xP0 ₄ : Tb ³⁺ ", PHYSICAL REVIEW B 71, 014119 (2005);

[7] (a) QY Zhang et al, Concentration-dependent near-infrared quantum cutting in GdB0 ₃ : Tb ³⁺ , Yb ³⁺ nanophosphors, APPLIED PHYSICS LETTERS 90, 061914 (2007); (b) QY Zhang and GF Yang, Cooperative downconversion in GdAl ₃ (B0 ₃ ) ₄ : RE ³⁺ , Yb ³⁺ (RE = Pr, Tb, and Tm), APPLIED PHYSICS LETTERS 91, 051903 (2007); [8] a) Hachanietal, Journal of Luminescence 130 (2010) 1774-1783, (b) Jiang, et al, 18 January 2010 / Vol. 18, No. 2 / OPTICS EXPRESS 639, (c) Huang et al, JOURNAL OF APPLIED PHYSICS 107, 063505 (2010);

[9] Zhang et al, "A Strategy to Protect and Sensitize Near-Infrared Luminescent Nd ³⁺ and Yb ³⁺ : Organic Tropolonate Ligands for the Sensitization of Ln ³⁺ - Doped NaYF ₄ Nanocrystals", J. AM. CHEM. SOC. 2007, 129, 14834-14835;

[10] Janssens et al, Systematic study of sensitized LaF ₃ : Eu ³⁺ nanoparticles, JOURNAL OF APPLIED PHYSICS 109, 023506 (2011);

[11] Benisvy et al, "Efficient near-UV photosensitization of the Tb (III) green luminescence by use of 2-hydroxyisophthalate ligands", Dalton Trans., 2008, 3147-3149

[12] Hasegawa et al, "Strategies for the design of luminescent lanthanide (III) complexes and their photonic applications", Journal of Photochemistry and Photobiology C: Photochemistry Reviews 5 (2004) 183-202;

[13] Gao et al, "Shape-Controlled Synthesis of Octahedral D-NaYF ₄ and Its Rare Earth Doped Submicrometer Particles in Acetic Acid", Nano Res (2009) 2: 565,574;

[14] Abel et al., Hard Proof of the NaYF4 / NaGdF4 Nanocrystal Core / Shell Structure, J. AM. CHEM. SOC. 2009, 131, 14644-14645;

[15] Vetrone et al, "The Active-Core / Active-Shell Approach: A Strategy to Enhance the Upconversion Luminescence in Lanthanide-Doped Nanoparticles", Adv. Funct. Mater. 2009, 19, 2924-2929;

[16] Qian et al, "Synthesis of Hexagonal-Phase Core-Shell NaYF4 Nanocrystals with Tunable Upconversion Fluorescence", Langmuir 2008, 24, 12123-12125;

[17] Duati et al, Inorg. Chem., 2003, 42, 8377-8384;

[18] Andreiadis et al, Chem.-Eur. J., 2009, 15, 9458-9476;

[19] Giraud et al, Inorg. Chem., 2008, 47, 3952-3954;

[20] Bozoklu et al, Dalton Trans., 2010, 39, 9112-9122.

Claims

1. Nanoparticle with photon emission in cascade, formed at least in part of at least one crystalline matrix of formula (I):

or of formula (III):

MF ₂

co-doped at the crystalline matrix (s) in terbium and ytterbium, with a doping level for Tb ranging from 1 to 15% relative to X or M, and a doping level for Yb ranging from 1 to 30% with respect to X or M; and

- Functionalized at the surface by at least one organic ligand having a wavelength absorption capacity of photons between 300 and 500 nm and capable of transferring the energy of the photons absorbed to Terbium.

2. Nanoparticle according to claim 1, characterized in that it is formed at least in part of at least one crystalline matrix of formula (I) as defined in claim 1, wherein X is yttrium.

3. Nanoparticle according to claim 1 or 2, characterized in that it is formed of a single crystalline matrix of formula (I) codoped homogeneously in terbium and ytterbium.

4. Nanoparticle according to the preceding claim, characterized in that it is formed of a single crystalline matrix of NaYF ₄ formula codopee homogeneously, Tb and Yb with a Tb doping rate of about 15% relative to at Y and a Yb doping rate ranging from 5 to 20% relative to Y, in particular about 10%.

5. Nanoparticle according to claim 1 or 2, characterized in that it has a core / shell type structure, the terbium being at least present at the shell.

6. Nanoparticle according to claim 5, characterized in that it is formed of a single crystalline matrix of formula (I), the doping levels of terbium and / or ytterbium being different at the heart and the shell of said nanoparticle.

7. Nanoparticle according to claim 5, characterized in that it is formed of two crystalline matrices of formulas (I), respectively constituting the heart and the shell of said nanoparticle.

8. Nanoparticle according to claim 7, characterized in that said crystalline matrices of formula (I) are distinguished by the nature of their rare earth X.

9. Nanoparticle according to claim 7 or 8, characterized in that the doping levels of terbium and / or ytterbium are different at the core and the shell of said nanoparticle.

10. Nanoparticle according to any one of claims 5 to 9, characterized in that the ytterbium is present only at the heart.

11. Nanoparticle according to any one of claims 5 to 10, characterized in that the terbium is present only at the shell.

12. Nanoparticle according to any one of the preceding claims, characterized in that it has a size ranging from 1 to 100 nm, in particular from 5 to 80 nm, and more particularly from 20 to 60 nm.

13. Nanoparticle according to any one of the preceding claims, characterized in that said organic ligand is associated with said nanoparticle via electrostatic interactions.

14. Nanoparticle according to any one of the preceding claims, characterized in that it has a ligand / terbium molar ratio ranging from 0.01 to 1.

15. Nanoparticle according to any one of the preceding claims, characterized in that said organic ligand is chosen from:

Terpyridine derivatives of formula (a):

wherein the R 1 substituents are selected independently of each other from carboxylic and 2H-tetrazol-5-yl groups; and R represents a group chosen from:

the bipyridine derivatives of formula (b) and pyridine of formula (c):

wherein R 1 and R ₂ are independently selected from carboxylic and 2H-tetrazol-5-yl groups; the 8-hydroquinoline derivatives of formula (d):

wherein R 1 is selected from carboxylic and 2H-tetrazol-5-yl groups; salicylic acid (el) and its derivatives of formula (e2) and (e3:

Derivatives of formula (f), (g) and (h)

(f) (g)

and their mixtures.

A nanoparticle according to any one of the preceding claims, wherein said organic ligand is selected from the following compounds

2-carboxy-8-hydroquinoline:

6.6 "-di (2H-tetrazol-5-1) -2,2 ': 6', 2" -terpyridine

6.6 '' -di (carboxy) -2,2 ': 6', 2 "-ter ridine

salicylic acid.

17. Use of nanoparticles as defined in any one of claims 1 to 16 to form, in a photovoltaic device, a layer or a deposit exhibiting luminescence properties and making it possible to modify the spectrum of light incidence.

18. Device, particularly photovoltaic, characterized in that it comprises a layer or a deposit formed (e) of nanoparticles as defined in any one of claims 1 to 16.

19. Photovoltaic device according to the preceding claim, wherein said nanoparticles are present on the surface of the device, within the glass encapsulating the photovoltaic device, on the surface or within the encapsulation layer of the photovoltaic material or on the surface of the photovoltaic device. the photovoltaic cell.