METHOD FOR THE MANUFACTURING OF PHOTOVOLTAIC

MATERIAL

The present invention refers to a method for the manufacturing of a photovoltaic material. The exploitation of photovoltaic effect for the manufacturing of industrial devices such as e.g., solar cells for producing electric power, is known for years .

Such effect occurs when an electron in the valence band of a semiconductor moves to the conduction band owing to the absorption of a sufficiently energetic photon directed towards the material.

This phenomenon is normally used for producing electricity by solar cells. Their mechanism is generally based on the use of semiconductors . When a light emission runs into a semiconductor, the transition of certain number of electrons towards the conduction band takes place, corresponding to the formation of an equal number of electron holes in the valence band. Therefore charge carriers are thus available, which can be used to provide an electric current.

Nevertheless, not all photons in the light beam running into the semiconductor are actually absorbed: a big portion thereof passes through the material - as the normally used materials, such as e.g. silica, are transparent to infrared - or it is reflected.

Moreover, even the amount of photons actually

absorbed does not entirely contribute to make electrons free the in the conduction band, allowing to produce electric power, but it is rather converted in large amount into heat . In particular, only that fraction of photons having enough energy to allow the electrons making a band jump - from the valence band to the conduction one - will be available for producing electric power.

Thus, new materials have been developed in order to improve the performance of solar cells, also using various combinations thereof, and allowing a higher exploitation of the light radiation.

In order to evaluate the performances of photovoltaic devices it is normally used the so- called efficiency, namely the ratio between the solar energy provided to the device and electric power obtained therefrom.

Anyhow, efficiency is not normally used for comparing the performance of different photovoltaic devices, as devices manufactured with rather different approaches are known to the state of the art in the photovoltaic field and, as a consequence, they also have remarkably different costs. More precisely, at one side very cheap solar cells based on polymeric semiconductors are known, the better performance thereof being nowadays just below 5%. At the other side multi-junctions cells

(tandem) , obtained from complex and expensive manufacturing methods exists, which are capable of reaching efficiency values higher than 40%. As a matter of fact, from the point of view of a real

exploitation of solar energy, in order to evaluate a photovoltaic device, it is surely more significant using figures of merit also considering differences in the manufacturing costs and therefore defined as the cost of the device in relation to the electrical power that the device is capable to provide, the available surface being equal, the latter quantity being directly connected to the efficiency of the cell. Therefore, two cells, a first one having efficiency, but also costs, ten times higher than the other, could be considered equivalent therebetween .

Accordingly, according to the figures of merit previously defined, for a real exploitation of the solar energy, it will be important for the cell not only to have high efficiency, but also to have low manufacturing costs .

Therefore, the research is directed to find new materials allowing high efficiency together with somehow low manufacturing costs.

One of the most promising way according this point of view has been identified in the exploitation of nanotechnologies for the manufacturing of solar cells.

In particular, recently it has been supposed, and later realized at an experimental stage, a photovoltaic material having an intermediate electronic band between the valence band and the conduction band that essentially allows to diminish

the amount of energy required to realize the band jump, since, in this case, the jump is divided in two smaller steps.

Such material, that will be indicated in the following just as intermediate band material, would allow a remarkable increase in the cell efficiency, allowing theoretical values close to 60% .

Nevertheless, as indicated before, the most proper figures of merit for evaluating the quality of a photovoltaic cell is given by the ratio between the cost and the provided electrical power. So far, the intermediate band solar cells, as the cells provided with this type of material are called, have been manufactured with methods having extremely high costs, without obtaining competitive efficiency values. As a consequence, their use would not prove to be advantageous at the moment .

In fact, the manufacturing of such intermediate band materials, as e.g. described in patent publication US 6,444,897, could be provided by a combination of growing techniques based on films deposition with nanophotolithography techniques. Nevertheless, these techniques are particularly expensive and they could require extreme temperature and/or pressure conditions; in particular, a part of the manufacturing of the material should be performed on vacuum conditions .

It is evident that this characteristic strongly avoid the spreading on an industrial scale of solar cells based on intermediate band materials.

Hence, the technical problem underlying the present invention is to provide a intermediate band photovoltaic material allowing to overcome the drawbacks mentioned above with reference to the known art.

Such problem is solved by the method for the manufacturing of an intermediate band photovoltaic material according to claim 1 and by the photovoltaic device according to claim 14. The present invention provides several relevant advantages. The main advantage lies in that the method according to present invention allows to provide intermediate band materials by means of technologies which are easily industrially applicable and, accordingly, are inexpensive.

Moreover, the solar cells according to the present invention have a high figure of merit, owing to their capacity of combining high efficiency and low manufacturing costs. Other advantages, features and the operation modes of the present invention will be made apparent from the following detailed description of some embodiments thereof, given by way of a non-limiting example. Reference will be made to the figures of the annexed drawings, wherein: figure 1 is a schematic illustration of the nano- structured intermediate band photovoltaic material obtained by the method according to the present invention;

figure IA is a schematic illustration of the

electronic bands along section A-A of the material of figure 1; figure 2 is a schematic illustration showing the operation of the intermediate band material according to the present invention; and figure 3 .is a schematic illustration of a photovoltaic device based on the material of figure 1.

With reference initially to figure 1, it is now shown an example of a possible scheme of the photovoltaic material of the present invention.

The material is formed by a plurality of nanoparticles 1 made of a first semiconductor, confined in a second semiconductor 2, also called barrier semiconductor.

The dimensions of the nanoparticles, as well as the distance therebetween, are particularly relevant, since only at proper scales the electrons are actually affected by the confinement effect within the nanoparticle as the electronic states are quantized.

In particular, for this effect to occur, the indicative sizes of the nanoparticles 1 and their distance therebetween should be preferably comprised between 2 and 100 nanometres. In any case, the size of the nanoparticles should be such that to provide effects of quantum confinement, and, therefore, it could be quite below the abovementioned value of 100 nm. Moreover, the distance between the nanoparticles should be such

that it allows a good carrier transport. Also in this case, it could be quite below the abovementioned value of 100 nm.

The profile of the electronic bands along section A-A of the photovoltaic material of the present invention is shown in figure IA, and it is obtained using couples of materials in which the upper state of the first semiconductor valence band and of the second semiconductor one are substantially aligned, whilst a significant difference occurs between the respective energy of the lower level of their conduction bands .

In other words, the first material and the second material are selected so that their valence bands are aligned (i.e. they have a similar energy) and their conduction bands are different therebetween. In alternative, it is possible to select the materials in a such way that they have aligned conduction bands, and valence bands substantially different.

The use of materials having one band with similar energy, i.e. the aligned one, and the other one with a different energy level, is one of the feature contributing in providing the intermediate band desired in the final material.

Possible couples of such materials are provided by ZnSe/CdSe, ZnTe/CdTe, ZnTe/InP, ZnO/InP, GaAs/InAs, ZnO/CdTe, ZnO/InP, ZnSe/GaSb, ZnSe/GaAs, GaAs/GaSb, AlAs/ZnTe. Nevertheless, this list should be considered as a non-limiting example of the possible couples of materials.

Thanks to the features shown before, the layout of the materials according to the scheme of figure 1 allows to obtain a partial overlapping of the wave functions of the discrete energy levels in the quantum wells formed by the nanoparticles . Therefore, it should be noted that, accordingly, the nanoparticles behave as quantum dots. In particular, such overlapping forms a continuous electronic band between the conduction and valence bands, i.e. the desired intermediate band.

As it will be shown now, such material could be realized by synthesis methods based on exclusively "bench-top" techniques, thus obtained exclusively by chemical processes and, in particular, at pressure and temperatures close to the atmospheric ones .

In detail, the first step for the manufacturing of such material consists in providing a plurality of nanoparticles of the first semiconductor. Such particles could be provided by semiconductor nanoparticles synthesis in colloidal solution. The preparation of the colloidal solution could be made by means of selecting suitable precursors. For example, in the case of InP nanoparticles, InBr3 and InI ₃ with P (SiMe ₃ ) ₃ , or InCl ₃ with P(NMe ₂ ) could be used. The size of the nanoparticles could be controlled by varying the temperature.

According to a preferred embodiment, the nanoparticles 1 of the first semiconductor are provided with an external shell of the second semiconductor. Nevertheless, it could be also provided the use of nanoparticles made only of the

first semiconductor that will be immerged in an array 2 of the second semiconductor, as will be seen in the following.

The use of nanoparticles formed by cores of the first semiconductor and shells of the second semiconductor anyhow allows to provide the array in a particularly easy and inexpensive way.

The next step of the present method for the manufacturing consists in the self-assembly of nanoparticles layers on a substrate.

The self-assembly is realized by deposition of the colloidal solution on the above mentioned substrate, so as to obtain an ordered array of nanoparticles. This process is anyhow known to the state of the art and, therefore, it will not be described in further detail. In particular, the assembly technique is described e.g. in CB. Murray et al., Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies, Annual Review of Materials Science, vol. 30, p. 545-610, 2000.

Following to this step of self-assembly, the completion of the array 2 is then provided, so that the nanoparticles 1 of the first semiconductor will be dispersed therein, in an ordered array.

Thus, the use of nanoparticles formed by cores of the first semiconductor and shells of the second semiconductor is particularly advantageous in this step, as it allows to provide the array 2 by means of a so-called flash sintering process, by which

shells of the second semiconductor sinters therebetween, thus forming the array 2, while internal cores, made of the nanoparticles 1 of the first semiconductor remain untouched, preserving their characteristics.

Such flash sintering process is particularly advantageous since it can take place substantially at ambient temperature and pressure. In fact, the process consist in providing heat to the substrate intensively and rapidly, so as to obtain the junction of the shells of the second semiconductor by sintering.

Therefore, this process allows to obtain the completion of the array 2 of the second semiconductor, closing at least partially the porosities .

Besides such flash sintering technique, even different processes could be used for the completion of the array 2 and, it could be obtained e.g. by means of electro-chemical techniques, sol- gel, or by means of ALD (Atomic Layer Deposition) . Nevertheless, it should be noted that, in general, these techniques are normally more complex and expensive than the preceding one, as they require particular environments, e.g. - for ALD - vacuum conditions .

Accordingly, the method hitherto described allows to manufacture a photovoltaic material featuring an intermediate band thus offering an additional path for absorbing an higher portion of the light spectrum, in a such way that it is converted in

electrons available for electrical conduction.

The photovoltaic material thus obtained lends therefore itself to several applications, some of them will be briefly described in the following. A first possible use is connected to the manufacturing of photovoltaic devices based on this material. An exemplificative embodiment of such device is shown in figure 3, wherein a layer of intermediate band photovoltaic material according to the present invention is comprised between a N- type layer 31 and a P-type layer 32, forming a p-n junction, such layers being connect e.g. to a conductive film 33, such as an ITO film (Indium Tin Oxide) and to an electrode 34, respectively, so as to provide the contacts of the photovoltaic circuit .

The layers thereby provided will be covered with a further layer made of insulating transparent material 35, e.g. glass. A second exemplificative embodiment consists of the manufacturing of LEDs, i.e. using the photoemission property of the material upon the application of an electrical field.

Also LEDs thereby manufactured have the same advantages stated with reference to photovoltaic materials, i.e. being of easy manufacture and inexpensive, and having flexible emission and absorption optical properties.

A further possible application consists of the manufacturing of a material wherein the position of

the intermediate band is variable along the direction through the thickness of the layer, so that the actual band jump of the nanoparticles is variable as well accordingly. As a consequence, since as previously stated the presence of the intermediate band provides an additional path for absorbing the light radiations, the presence of a variable energy intermediate band will offer more additional paths. In principle, such material has improved absorption properties compared to a single band material and the possible efficiency for a photovoltaic device based on the material according to the present invention is further increased. Such variable intermediate band material could be manufactured by assembling layers of nanoparticles having gradually increasing (or decreasing) size, or with a variable composition.

The present invention has hereto been described with reference to preferred embodiments thereof. It is understood that there could be other embodiments referable to the same inventive kernel, all falling within the protective scope of the claims set forth hereinafter .