WO2013053967A2

WO2013053967A2 - Method for determining the photovoltaic properties of solid materials capable of acting as light absorbers in photovoltaic devices

Info

Publication number: WO2013053967A2
Application number: PCT/ES2012/070701
Authority: WO
Inventors: José Carlos CONESA CEGARRA; Raquel LUCENA GARCÍA; Fernando FRESNO GARCÍA; Perla Wahnon Benarroch; Pablo Palacios Clemente
Original assignee: Consejo Superior De Investigaciones Científicas (Csic); Universidad Politécnica de Madrid
Priority date: 2011-10-11
Filing date: 2012-10-09
Publication date: 2013-04-18
Also published as: ES2407929R2; WO2013053967A3; ES2407929B1; ES2407929A2

Abstract

The present invention relates to a method for determining, by means of a photocatalytic reaction, the photovoltaic properties of solid materials capable of acting as light absorbers in photovoltaic devices, which includes at least the following steps: placing the material in contact with a fluid that contains at least one chemical species capable of reacting by electron transfer with said material; irradiating the material while the latter is in contact with the fluid at least once with a beam of light made up of at least one wavelength within the solar spectrum; and determining by chemical and/or stereoscopic analysis of the fluid the variation in the concentration of the chemical species capable of reacting by electron transfer and/or the presence and concentration of at least one product resulting from the electron transfer reaction between the solid material and the aforementioned chemical species.

Description

METHOD FOR DETERMINING PHOTOVOLTAIC PROPERTIES OF SOLID MATERIALS SUSCEPTIBLE TO ACT AS LIGHT ABSORBERS IN PHOTOVOLTAIC DEVICES TECHNICAL SECTOR

The present invention relates to a method for analyzing the operation of materials that have or may have use in photovoltaic devices. It is therefore within the new materials sector, while its application is mainly located in the energy sector, and more specifically in the renewable energy sector.

STATE OF THE TECHNIQUE

The photovoltaic devices for solar energy use used in the state of the art are generally based on light absorbing materials with semiconductor character, that is, they have an electronic structure with a valence band and a conduction band (which, in absence of defects or impurities, are respectively full and empty of electrons) separated by a range of prohibited energies to electrons (the "bandgap" in the usual English terminology). In these materials, the absorption of a photon of electromagnetic radiation with energy equal to or greater than the width of the bandgap causes the excitation of an electron from the conduction band (in which there is then an empty electronic state, called hollow) to the band from Valencia, crossing the bandgap. Said electron and said gap, properly separated and routed, can produce electric current and voltage with the end result of the conversion of light energy into electrical energy.

Both for the practical use of these systems and for evaluating the efficiency and other characteristics of the light absorbing material, it is usual to place it, which is in a more or less compact state (monocrystalline or no; frequently, in the form of a thin sheet) and with a relatively smooth surface, selective electrical contacts so that in one of them only electrons are transferred between it and the valence band, while in the other contact only electrons are transferred between it and the driving band These two contacts thus tend to balance their electrical potentials with the average electronic potentials of the respective electrons and holes. This requires designing and establishing such selective contacts, which can be difficult especially if the absorbent material is of a new type (so it is not known a priori which contact materials are the most suitable) or / and must be prepared , at least initially, in powder form or with a complete nanostructure surface.

It is worth mentioning that photovoltaic cell variants have recently been proposed in which absorbent materials perform their electronic function in a somewhat different way from the one mentioned above, and in which this difficulty may appear in a particular way. Such is the case in which the principle called intermediate band is used in absorbent materials (see A. Luque, A. Martí; Phys. Rev. Lett. 78, 1997, 5014), also called the impurity or multiband band ( Figure 1) . In these materials, in addition to the aforementioned valence and conduction bands, there is another one that does not superimpose on energy with them but that is energetically located between them and that may be partially occupied by electrons. This intermediate band would then allow, by absorbing two photons with energies less than the width of the bandgap, to take an electron from the valence band to the intermediate band (producing a gap in the valence band) and then from the intermediate band to that of conduction, producing the same final result as that which can be achieved, as described in the first paragraph of this section, absorbing a single photon of energy greater than the width of the basic bandgap. With such a scheme one can obtain, in principle, a total efficiency in the use of solar energy far greater than that achievable with a normal semiconductor. This type of more complex material is relatively new, and it may happen that the materials that are proposed for it are not known to prepare in the compact and smooth way necessary to put on them selective electrical contacts, or / and it is unknown what conductive material is suitable for making such contacts, preventing the evaluation of the photovoltaic property in the usual way.

As an example of another advanced photovoltaic system in which it is difficult to make contacts, it is worth mentioning the one that tries to use the so-called hot carriers, or "hot carriers" in English terminology (see P. Würfel, AS Brown, TE Humphrey, MA Green; Prog Photovolt. Res. Appl. 13, 2005, 277). These carriers are the electrons and holes that after being excited with photons of energy greater than the width of the bandgap initially have an excess kinetic energy, which can be used if its transmission abroad is done through compounds (for example, certain molecules or quantum dots), located on its surface that allow to extract and carry only electrical contacts with certain energy value. These systems require a very small absorbent material thickness, typically less than 10 nm (1 nm = one millionth of a millimeter), so to have sufficient light absorption they must be deposited as an ultra-thin coating on a porous or filiform substrate, which makes it difficult Place the contacts.

It is therefore appropriate to have methods to assess the ability of a material to give a good photovoltaic efficiency, in particular in more advanced systems such as those mentioned, without the need to train on them metal contacts with which to measure voltage and current. In some cases photoluminescence measurements can be used, but these usually require very low temperatures, and they are not applicable in systems such as those using semiconductors of indirect bandgap anyway (i.e. those in which the electronic states of the edges of the bands of valence and conduction have a different kinetic moment; the transition of luminescence between these edges is prohibited) or those that use warm carriers, since the easily detectable luminescence occurs when the excess kinetic energy has already been lost. There is, as far as these inventors know, no other method that can be used for it.

As for photocatalytic processes, they have been known for some time (for a review of them, see for example B. Ohtani; Inorg. Photochem. 63, 2011, 395). In them, as Figure 2 shows, the excited and hollow electrons that are produced in a semiconductor when it absorbs photons with energy greater than the width of its bandgap diffuse to the surface of the material, which is in contact with a fluid containing chemical species capable of yielding or capturing electrons. Electronic transfers then occur between the solid and these species, with chemical changes in them that can be detected and measured. These photocatalytic processes are usually used or proposed for the elimination of polluting substances, the obtaining of fuels such as hydrogen (with which solar energy is used and chemically stored) or the synthesis of specific chemical compounds. As far as the authors of this invention know, such processes have not previously been used as a method to analyze the properties of solid materials in their use as light absorbers in a photovoltaic application, although they have been tested in sometimes with materials of known photovoltaic use, in order to check if they also have photocatalytic properties. In particular, they have never been tested on photovoltaic materials for which it is known or assumed to have intermediate band characteristics of the type described herein, nor to verify the possible use of hot carriers in photovoltaic applications.

DESCRIPTION OF THE INVENTION

Brief Description of the Invention

A main aspect of the invention is a method for determining the photovoltaic properties of solid materials capable of acting as light absorbers in photovoltaic devices. Said method is characterized in that it is carried out by a photocatalytic reaction, and comprises at least the following steps:

a) contacting the solid light absorbing material with a fluid containing at least one chemical species capable of reacting by electronic transfer with said solid material;

b) irradiating the solid material at least once, while in contact with the fluid, with a beam of light containing at least one wavelength selected within the range of the solar spectrum between 350 nm and 2000 nm; Y

c) determine by chemical and / or spectroscopic analysis of the fluid the variation in the concentration of the chemical species (s) capable of reacting by electronic transfer with the solid material, and / or the presence and concentration of at least one product resulting from the reaction electronic transfer between the solid material and the chemical species (s) mentioned.

The present invention is based on the consideration made by the inventors from their experience technique, that, based on the photovoltaic and photocatalytic phenomena on physical principles and similar initial processes, the second of these phenomena can be used to examine the properties that a solid material that may want to apply as a light absorber may have in the first phenomenon in a photovoltaic device, this use being advantageous on many occasions by not requiring the photocatalytic method that the solid to be examined is in a compact form or that an electrical contact is established on it, with which such evaluation can be abbreviated and facilitated.

The invention thus consists of a method of examining the properties of photovoltaic interest of a material (which may be constituted by a single phase or by more than one) by performing photocatalytic tests on it. The method requires having a photocatalytic reaction device, with a chemical reactor in which the material to be studied is placed in contact with a fluid that contains at least one chemical species whose chemical transformation can be triggered by irradiation of the assembly, with the consequent formation in the solid of electrons and holes, followed by diffusion of these to the surface of the material and the realization of electronic transfers between it and said species or species. By provoking the electronic transfer reaction between the solid light absorbing material and the fluid with which it is in contact, by irradiating a beam of light, it is possible to study how the concentration in said fluid of the reactant molecules evolves over time or of the products of said reaction. Ultimately, the fluid is analyzed chemically to see to what extent the concentration of any substance in the fluid decreases or increases during irradiation (and not in the absence of the solid), so that the process can only be assigned, according to knowledge existing technicians, to direct or indirect transfers of electrons and holes between the solid and chemical species present in the fluid.

Since the method is aimed at studying the possible use of the solid material in question as a light absorber in a photovoltaic device, it is understood that the beam of light with which it is irradiated must contain a discrete wavelength or a range of lengths of wave that are within the range of the solar spectrum that has an interest in the photovoltaic application, which is between 350 and 2000 nm, including both limits.

The measurement of the speed at which such a change in concentration takes place can be used as a measure of the ability of the solid to generate, by absorbing light, electrons and holes that are transferable outside it, and therefore will also give information about its greater or less ability to act effectively in a photovoltaic application as a light absorber and generator of electrons and usable holes; all this, without the need to establish an electrical contact with the solid.

In this sense, the present method allows analyzing the effectiveness of solid materials capable of absorbing light, not by the usual measurement of the relationship between electric current and voltage produced by irradiating the material with light, which requires making electrical contacts on surfaces more or less smooth of said material, but by detecting and quantifying photocatalytic chemical reactions, that is, produced on the surface of the material when it is irradiated in contact with a fluid containing molecules capable of reacting by electronic transfer.

The method is advantageous because it allows the study to be carried out without the need to build electrical contacts on the material or to have it in a compact or smooth surface, being possible to apply it, in particular and preferably, to powdery or highly porous material. The invention is preferable and especially useful for analyzing the behavior of materials or systems that perform the photovoltaic function according to advanced photonic utilization schemes and that can be difficult to prepare in appropriate ways for the formation of such contacts, such as intermediate band materials. or the systems that take advantage of the excess energy that the newly excited holes and electrons have in a material (the so-called hot carriers or "hot carriers").

A second aspect of the present invention is the use of a photocatalytic activity measuring device to determine the photovoltaic properties of a solid material capable of acting as a light absorber in photovoltaic devices according to the method described above, in any of the variants and alternatives that are presented here. Said device for measuring photocatalytic activity is of the type known in the field of photocatalytic technique, and comprises at least the following elements in an essential way in order to carry out the method of protection:

a) a chemical reactor where the solid material and the fluid with which it is in contact are housed, and where the photocatalytic reaction takes place;

b) irradiation means, comprising at least one light source for irradiating the solid material while in contact with the fluid;

c) means for chemically and / or spectroscopically determining the variation and concentration of the species or species capable of reacting with the solid material, and / or the presence and concentration of products resulting from the reaction of the material solid with the chemical species or species during irradiation.

The method consists in placing the solid to be studied inside the photocatalytic reactor, in contact with the chosen fluid, irradiating it with light and chemically analyzing the fluid by means of chemical or spectroscopic analysis means.

Detailed description of the invention

In general, the solid material to which the present method is applied is any of the semiconductor type, or improved semiconductor to enhance its light absorption or its ability to transfer electrons and holes in its surface, which is to be applied as an absorbent element of Light in a photovoltaic device. Although, as said, in a preferred embodiment the method described above is used to determine the photovoltaic properties of intermediate band materials; and, in another preferred embodiment of the invention, the method described above is used to determine the operation of a material (combination of a light absorber and one or more surface transmitting compounds of carriers with determined energy) that is to be used in a photovoltaic device based on the use of hot carriers.

In a preferred embodiment, the solid material is in powder form; In general, the powder may have a grain size of less than 1 rom, although this aspect is not decisive for the present invention. It should be taken into account that the solid material may be in powder form when it is, inter alia, an intermediate band material, but not when it is a material capable of being used in a photovoltaic device based on the use of hot carriers. In another preferred embodiment, the light absorbing solid material is a porous material. As for the fluid, this may be a liquid, such as an aqueous solution or based on an organic solvent such as ethanol or acetone, or a gas such as air or an inert gas containing another gas or a vapor capable of being oxidized or decomposed by the photocatalytic reaction.

In one of the preferred embodiments the solid material to be studied is deposited, for example as a porous layer or compacted powder, on the surface of a solid substrate that serves as a base and does not react with the solid material to be studied or with the fluid, and is irradiated with the light beam while in contact with the liquid or gas containing the species or chemical species that can react by electronic transfer with the material.

In another particular embodiment of the invention, the fluid is a liquid and the solid material is irradiated with the light beam while it is in suspension, agitated or not, within a liquid solution (liquid fluid) containing the chemical species or species liable to react by electronic transfer with the material; In this particular case, the solid material is preferably prepared in powder form so that it can be kept in suspension. It is in this liquid or solution where it is determined, by chemical and / or spectroscopic analysis, how the concentration of the reactant species or the products of said reaction evolves over time between the solution and the solid material when irradiated with light.

The light source within the solar spectrum may be natural, but more frequent and preferably is an artificial source, which provides photons whose wavelength or wavelengths are in the range of interest of the photovoltaic application (typically between 350 nm and 2000 nm, both limits included). As an artificial light source Any known device in the field of photocatalysis can be used. Irradiation is carried out for sufficient time (generally not exceeding a few hours if appropriate devices from the field of photocatalysis, such as that described in the memory) are used to check if the photocatalytic reaction takes place.

In a preferred embodiment, the light beam is composed of a continuous and variable range of wavelengths (polychromatic light). Thus, it can vary from the range corresponding to sunlight itself (which in terms of this invention, is comprised between 350 nm and 2000 nm, including both limits) to another which, provided by artificial devices, also covers several hundreds of nanometers such as that of sunlight; such as that of an Xenon arc lamp, which includes the aforementioned range between 350 nm and 2000 nm, including both limits, although it also includes other longer wavelengths that are not of interest for this invention. Preferably, the continuous range of wavelengths of the irradiation beam will comprise at least the range between 400 nm and 1500 nm, including both limits.

In another preferred embodiment, the beam of light with which the material is irradiated is composed of a narrower wavelength range, not more than 100 nm wide, more preferably no more than 50 nm wide (monochromatic light ), within the range of interest of the final photovoltaic application (between 350 nm and 2000 nm, including both limits). In another preferred embodiment, the light beam with which the material is irradiated is composed of a single discrete wavelength. These two preferred embodiments can be achieved through the use of wavelength selection means, such as filters, monochromators, light emitting diodes or other devices that allow the wavelength of the radiation used to be selected at will. This irradiation with a beam of light of a single wavelength, or monochromatic, is advantageous because it allows determining how the use of photons in the solid material varies with said wavelength, thereby clarifying the nature and effectiveness of the electronic processes induced by said photons in such material.

Therefore, preferably, wavelength selection elements are used, so that the solid material in contact with the fluid is irradiated more than once with a beam of light that is composed of a single discrete wavelength , or of a reduced range of wavelengths that does not exceed the width of 100 nm, more preferably 50 nm, said discrete wavelength being different at each irradiation or the average value of the reduced range of wavelengths that make up the beam . Then the speed at which the concentration change of the chemical species susceptible to react and / or, alternatively, of the compounds that are formed in the reaction, and therefore the photovoltaic capacity, can be measured, depending on each length of wave to which it radiates; the relationship between both magnitudes constitutes the spectral response of the photocatalytic phenomenon. This allows checking the photovoltaic effectiveness and potential of the material for different wavelengths of light.

In the case where the material is of intermediate band, obtaining the spectral response by irradiation with different monochromatic light beams will also allow to verify the effective action of such intermediate band, if it is found that photocatalytic action occurs with photons whose energy is less than the width of the base semiconductor bandgap. On the other hand, the comparison between the reaction rate found for materials with and without intermediate band but with the same semiconductor of base, when using photons that have energy greater than the width of the bandgap, it will allow to see if the intermediate band causes recombination between the electrons and the holes, with the consequent loss of photovoltaic capacity, thus obtaining additional information.

If the method is applied to materials that constitute a system designed to take advantage of hot carriers, certain molecules (not necessarily those that are finally transformed) or quantum dots that could only capture by the location of their discrete electronic levels could only capture electrons or holes in the material if they have excess kinetic energy, and if no other species capable of capturing carriers can interact with the material, observing photocatalytic activity, even with polychromatic radiation, will demonstrate that hot carrier capture is effectively produced. If the radiation is monochromatic and only photocatalytic activity is found with photons that have energy significantly greater than the width of the bandgap, this will give additional demonstration of the specific use of hot carriers, and will also report the additional kinetic energy needed in them.

As regards the use of the photocatalytic device used to carry out the method of the present invention, the chemical reactor that is part of said catalytic reaction device can be, for example, a pyrex glass reactor.

The irradiation media used are the usual ones in the field of photocatalysis. Thus, the light source, when not natural, can be any system known in this field, for example a 450 W Xe lamp. Other elements may optionally be included in addition to the light source, such as a collimating lens, a filter. of water, and a mirror. As mentioned in describing the method, it is preferable that, in order to obtain more complete results, the device, specifically the irradiation means, includes wavelength selecting means of said incident light on the solid to be studied, such as monochromators, filters or light emitting diodes, or any other element known in the field and commonly used.

The means of chemical and / or spectroscopic analysis of the material and the fluid can be any of those customary in the art, which serve to determine chemically and / or spectroscopically the variation of the concentration of the reactant species or of the products of the reaction in the fluid

BRIEF DESCRIPTION OF THE FIGURES

Figure 1. Explanatory scheme of the operation of an intermediate band material in a photovoltaic device using said band (state of the art). The valence bands BV (occupied by electrons), the conduction BC (empty of them) and the intermediate band BI (partially occupied) are represented; the electrical contacts located on the material and communicating selectively with the BV and BC respectively; and the three electronic transitions that can be induced by the absorption of photons of different energy.

Figure 2. Explanatory scheme of the principle of photocatalysis. A semiconductor particle is shown in which they are produced, by absorption of a photon, electrons and holes; after diffusion to the surface, electrons that do not recombine with holes are transferred to an acceptor chemical species A (which can be an oxygen molecule) and the holes receive electrons from a donor species D (which can be, in aqueous solutions , an OH group ^" ). Figure 3. Scheme of the equipment used in the example of the invention.

(1) Device for measuring activity or photocatalytic reaction (the means of chemical and / or spectroscopic analysis are not represented);

(2) reactor (pyrex glass);

(3) suspension of the solid material (in the example, V: In ₂ S ₃ ) in a solution of the reactant (which in the example is HCOOH);

(4) magnetic stirrer;

(5) air bubbling device in the suspension;

(6) sampling syringe;

(7) irradiation means;

(8) light source: 450 W Xe lamp;

(9) collimating lens;

(10) water filter;

(11) wavelength selector filter;

(12) mirror.

The equipment is completed with a commercial spectrometer (not shown in the drawing) capable of measuring in the ultraviolet range the optical absorbance of the solution in aliquots of it extracted with the syringe.

Figure . Results of photocatalytic degradation tests of formic acid on In ₂ S ₃ with or without partial replacement of In by V (lower and upper graphs, respectively). Each curve shows how the relative concentration of formic acid (C) decreases, expressed as a fraction of the initial (Co), from the moment when the suspension begins to radiate with light selected by a nominal wavelength filter X _F . Figure 5. Graph of the velocity constant k (in continuous curves with symbols of points or squares included in them) of the photocatalytic degradation of formic acid on In ₂ S ₃ with or without partial replacement of In by V, obtained from the data represented in Figure 4, represented as a function of the nominal wavelength of the filter. The absorption spectrum measured for each of the two solids is also represented in the form of a dashed dashed line.

EXAMPLES OF THE EMBODIMENT OF THE INVENTION

More specific experimental details on the method and the device used to determine the photovoltaic properties of a light-absorbing solid material by photocatalytic reaction are provided below, and thus establish its ability to be used in photovoltaic devices, in accordance with the present invention.

Example 1. Method based on an otocatalytic reaction according to the present invention, to determine the ability of an indium sulfide partially substituted with vanadium to act according to the intermediate band principle in photon absorption processes, and therefore in photovoltaic applications .

In previous works (see P. Palacios, I. Aguilera, K. Sánchez, JC Conesa, P. Wahnón; Phys. Rev. Lett. 101, 2008, 046403, and patent application ES 200702008) was shown using mechano-theoretical theoretical calculations that by replacing vanadium ions part of the indium ions with octahedral coordination existing in an indium sulphide with spinel structure, a material with intermediate band characteristics suitable for making a high-efficiency photovoltaic cell should result. In subsequent works (see R. Lucena, I. Aguilera, P. Palacios, P. Wahnón, JC Conesa; Chem. Maters. 20, 2008, 5125, and the aforementioned patent application) a compound was synthesized in the laboratory this type, and it was shown that its optical absorption characteristics agreed with the intermediate band structure predicted by the previous calculations. Such material has only been synthesized so far as a powder. This example shows how the method of the present invention makes it possible to verify for said compound, without the need to deposit a metallic contact thereon, the participation of the intermediate band in the additional generation of electrons and holes that can be exploited by transferring the material outside, which therefore contributes to improve the photovoltaic performance of the material.

For this purpose, a laboratory photocatalytic reaction equipment has been developed, the configuration of which is shown in Figure 3. The equipment uses a 450 W power Xenon lamp (Spectratech brand) as a light source (8), which emits light in the range between the near ultraviolet and the middle infrared. After being collimated in the form of a horizontal parallel beam with a quartz lens (9), the light passes through a water bucket (10) with quartz windows, which acts as an infrared thermal radiation filter; then through a filter (11) wavelength selector (Andover brand) that only lets light with a wavelength in a range of about 70 nm in width around a nominal value A _F of that wavelength pass through , the value of A _F being chosen between 400 and 900 nm; and finally the light is turned down with a mirror (12). Under this is the reactor (2), a container of about 150 mL capacity made of pyrex glass provided with a tight lid and thermostated with water, so that light enters it through its upper face.

80 mg of In ₂ S ₃ indium sulphide powder, or of the same material partially substituted by vanadium in a cationic ratio V: In¾l: 9 (materials both prepared in the laboratory by the method described above) are introduced into the reactor work of Lucena et al.), together with 80 mL of a solution (3) aqueous formic acid HCOOH in 1.5 mM concentration, with pH adjusted to 2.5 by means of a sodium phosphate buffer. The powder is kept in suspension in the aqueous solution by a magnetic stirrer (4). The solution can be saturated with air or inert gas by bubbling it (5). The system includes a syringe (6) with which a sample of the suspension (typically 1-1.5 mL each time) that is filtered to separate the powder can be taken, then measuring the concentration of HCOOH by spectrometry in the liquid UV at a wavelength of 205 nm, in which only HCOOH absorbs light. The entire assembly is surrounded by an opaque envelope that ensures that the reactor does not receive ambient light from the laboratory during the experiment.

In a typical experiment of the method, after keeping the suspension in the dark, stirred and saturated with air, for the time necessary to ensure that the concentration of HCOOH remains stable (in this case half an hour), the irradiation of the suspension is started with the lamp light passed through the filters. At the initial moment, and in later ones, a sample of the suspension is taken with the syringe and the concentration of HCOOH is analyzed in it. Irradiation is maintained for a period of several hours, sufficient in this case for the majority of the HCOOH present to be transformed when there is a photocatalytic reaction. The results of this experiment, carried out for both In ₂ S ₃ and for that sulfide in which partially indium has been replaced by vanadium, are summarized in Figure 4. As can be seen, the concentration of HCOOH decreases over time at different rates according to be the value of A _F of the filter used, and it is different for both solids. It can be assumed, as in the generality of the photocatalytic processes of degradation of organic substances in the presence of air, that the net chemical process that occurs is that of total oxidation:

HCOOH + ½ 0 ₂ → C0 ₂ + H ₂ 0 the process being triggered by the transfer of electrons photogenerated molecules adsorbed oxygen to give radicals Q> 2 ^~ very reactive, and transfer to solid electrons from molecules HCOOH, or from ions ^OH- from the solution generating new reactive radicals. Further reactions of these radicals end up giving as final products the aforementioned CO ₂ and ¾0 molecules. The exact mechanism of the process, however, is of little relevance to the validity and use of the method.

From each of these curves, the initial velocity constant of the reaction can be obtained by measuring its initial slope, assuming that it follows a kinetic law of pseudo-first order in the concentration of the oxidized molecule (HCOOH) as usual in Photocatalytic reactions Said constant k, which measures the photocatalytic activity observed in the experiment, is represented as a function of the A _F value of the filter used; Such representation is shown in Figure 5, which also includes the light absorption spectra of the two solids studied, determined by diffuse reflectance techniques in the work of Lucena et al. cited above.

It can be seen how for the In ₂ S ₃ sample there is photocatalytic activity, and therefore formation of usable pairs of electron and hole in its surface, for the entire range of wavelengths that can be absorbed by the material (whose width of bandgap, according to the work of K. Kambas, A. Anagnostopoulos, S. Ves, B. Ploss, J. Spyridelis; Phys. Stat. Sol. (b) 127, 1985, 201, is about 2.0 eV, corresponding to a 620 nm wavelength). That would serve to show that this material is capable of acting as a light absorber in photovoltaic cells that use light of that wavelength range, although in the In the case of In ₂ S ₃ the state of the art already allows to know that capacity by other means.

In the same figure it can be seen that in the case of In ₂ S ₃ partially substituted with vanadium the range of wavelengths in which photocatalytic reaction occurs extends to 750 nm, in line with the greater range of light absorption shown the spectrum of this material, and that is due to the intermediate band characteristic introduced into it by vanadium replacement. The method thus allows to verify that with this material it is possible to obtain electrons and holes usable in interfacial electronic transfer processes, and therefore photovoltaic efficiency, even when the incident photons do not have enough energy to produce the complete electronic jump from the valence band to The driving band. It is thus possible to infer the efficiency of the intermediate band in this material to effectively absorb photons in a greater range of wavelengths, and thus obtain an improved efficiency if used as an absorber in a photovoltaic device.

The same results also show that the photocatalytic activity that occurs when photons with a wavelength less than 600 nm are used, that is to say with energy greater than the width of the bandgap, is very similar in both materials. This indicates that the introduction of vanadium does not lead to a significant additional recombination of electrons with the holes, so there is no decrease in photoactivity by that route. This gives interesting information about an important aspect of the behavior that this new material may have in a photovoltaic application.

Therefore, using the method of this invention, it is possible to demonstrate, without the need to obtain the material in a compact form and with a smooth surface or to deposit on he metallic metallic contact some, that vanadium substituted indium sulfide is a suitable radiation absorber for the construction of an intermediate band photovoltaic device with improved efficiency.

Claims

RE IVINDICATIONS

1. A method to determine the otovoltaic properties of solid materials capable of acting as light absorbers in photovoltaic devices, characterized in that it is carried out through a photocatalytic reaction and includes at least the following steps: a) putting the solid light-absorbing material in contact with light with a fluid that contains at least one chemical species capable of reacting by electronic transfer with said solid material;

b) irradiating the solid material at least once, while it is in contact with the fluid, with a beam of light containing at least one wavelength selected within the range of the solar spectrum between 350 and 2000 nm; and

c) determine by chemical and/or spectroscopic analysis of the fluid the variation in the concentration of the chemical species capable of reacting by electronic transfer with the solid material, and/or the presence and concentration of at least one product resulting from the reaction. of electronic transfer between the solid material and the aforementioned chemical species.

2. The method according to claim 1, wherein the solid material is an intermediate band material.

3. The method according to claim 1, wherein the solid material is in turn a combination of a light-absorbing material and one or more carrier-transmitting surface compounds with a given energy, where said combination can be used in a photovoltaic device. that takes advantage of hot carriers.

4. The method according to any one of claims 1 or 2, wherein the material is in powder form.

5. The method according to any one of claims 1 to 4, wherein the solid material is a porous material.

6. The method according to any one of claims 1 to 5, wherein the fluid is a gas that contains the chemical species(s) to react with the solid material.

7. The method according to any one of claims 1 to 5, wherein the fluid is a liquid that contains the chemical species(s) to react with the solid material.

8. The method according to any one of claims 1 to 7, wherein the solid material is deposited on the surface of a solid substrate, and is irradiated with the light beam while in contact with the fluid containing the species or chemical species capable of reacting by electronic transfer.

9. The method according to claim 7, wherein the solid material is irradiated with the light beam while it is in suspension within a liquid solution containing the chemical species or species capable of reacting by electronic transfer with the material.

10. The method according to any one of 1 to 9, where the light beam is composed of a continuous range of wavelengths between 350 nm and 2000 nm, including both limits.

11. The method according to claim 10, wherein the continuous range of wavelengths is between 400 nm and 1500 nm, including both limits.

12. The method according to any one of claims 1 to 9, wherein the light beam is composed of a range of wavelengths with a width equal to or less than 50 nm.

13. The method according to any one of claims 1 to 9, wherein the light beam is composed of a single discrete wavelength.

1 . The method according to any one of claims 12 or 13, wherein the solid material and the fluid are irradiated more than once, the discrete wavelength or the average value of the range of wavelengths that make up the light beam being different in each irradiation.

15. Use of an otocatalytic activity measurement device for measuring otovoltaic properties of a solid material capable of acting as a light absorber in otovoltaic devices according to the method described in any one of claims 1 to 14.

16. Use according to the previous claim, wherein the photocatalytic reaction device comprises:

b) irradiation means, comprising at least one light source for irradiating the solid material while it is in contact with the fluid;

c) means to determine chemically and/or spectroscopically the variation and concentration of the species or species capable of reacting with the solid material, and/or the presence and concentration of products resulting from the reaction of the solid material with the chemical species or species during irradiation.

17. Use according to the preceding claim, wherein the irradiation means comprise means for selecting the wavelength of the light beam.