Photovoltaic device and associated fabrication method

A photovoltaic device comprising: a plurality of photovoltaic cells, separated from each other; a support receiving the cells; and a light guide in contact with the cells and comprising a primary guide with a surface that is proximal to the cells, where the proximal surface is oriented towards the cells and the support. The photovoltaic device comprises, between the cells, areas located between the support and the primary guide which comprise a material with an index of refraction less than that of the proximal surface, where the material is in contact with the proximal surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of the International Patent Application No. PCT/FR2015/051460 filed Jun. 2, 2015, which claims the benefit of French Application No. 14 55122 filed Jun. 5, 2014, the entire content of which is incorporated herein by reference.

BACKGROUND

The present invention relates to a photovoltaic device.

This device type is widely used and its purpose is to convert solar energy to electrical energy.

To do that, these devices are provided with photovoltaic cells which will be illuminated by sunlight and convert this light into electrical energy by a photoelectric effect.

In order to improve the yield of this type of device, the use of concentrated light is known; this also has the advantage of reducing the consumption of primary photovoltaic material. To this end, the cells can be coupled to a light guide provided for receiving the photons and for better guiding them to the surface of the photovoltaic cells.

In some of these devices, the cells are arranged on a reflector configured for reflecting photons and allow their recapture by the waveguide. The waveguide is then arranged in contact with this reflector.

However, it was observed that devices of this type had some disadvantages. In fact, the reflectors have a coefficient of reflection that is not ideal and this results in losses on each reflection. Additionally, each reflector generally has local roughnesses, for example because of the roughness of the support on which it is deposited, aging of the reflector or fabrication imperfections. Under some conditions, in particular in terms of dimensions of these roughnesses which are not negligible compared to the wavelength of the photons, these roughnesses produce a local phenomenon of diffusion of the light which induces an uncontrolled variation of the angle of reflection of the photons and therefore an overall reduction of the guiding effect provided by the waveguide.

SUMMARY

The present invention aims to improve the situation.

For this purpose, the invention targets a photovoltaic device comprising:a plurality of photovoltaic cells, separated from each other;a support receiving the cells; anda light guide in contact with said cells and comprising a primary guide with a surface that is proximal to the cells, where the proximal surface is oriented towards the cells and the support.

In particular, the device comprises, between the cells, areas located between the support and the primary guide which comprise a material with an index of refraction less than that of the proximal surface, where the material is in contact with said proximal surface.

In an implementation that is easy to implement, this material is just air and spacing is then provided between the cells.

According to an aspect of the invention, the support is a reflector having a reflecting surface oriented towards the proximal surface of the primary guide.

According to a particular aspect of the invention, the primary guide is a fluorescent concentrator. In this way the guiding effect of the light guide towards the cells can in particular be maximized and the efficiency of the device improved.

According to another aspect of the invention, the support and the proximal surface of the primary guide are separated by a distance included between 1 μm and 20 μm. The nonlinearity effects of the light, which could occur and which would limit the reflection performance, can in particular be limited this way.

In a specific implementation of the invention, the distance between the proximal surface of the primary guide and the support is substantially equal to a multiple of a characteristic wavelength corresponding to a preferred wavelength of emission of the primary guide greater than or equal to two. This makes it possible to limit the aforementioned nonlinearity effects earlier.

According to a particular aspect of the invention, one or more cells are arranged projecting from the support towards the primary guide, where the primary guide is in contact with said cells and kept separated from the support at least by said cells. In that way the cells themselves contribute to forming the areas comprising the material.

In an embodiment, the light guide comprises a plurality of secondary guides separated from each other by said material where each secondary guide is interposed between the proximal surface of the primary guide and a photovoltaic cell. These guides serve in particular to closely select the geometry of the device while also improving the guiding effect of the waveguide by allowing a good optical coupling between the cell and the primary guide and does so even if the surface of the cells is rough.

According to another aspect of the invention, the secondary guides keep the proximal surface of the primary guide away from the support at least in said areas.

In a specific implementation of the invention, each secondary guide has a surface arranged in contact with the surface of the corresponding photovoltaic cell and having dimension substantially equal to the dimensions of the surface of said cell, where said surface of a given secondary guide is arranged substantially edge to edge facing the surface of the corresponding photovoltaic cell. In this way, the exposure of the cells to photons coming from the primary guide can be improved.

According to another aspect of the invention, at least one of the secondary guides has the shape of a pad of generally cylindrical shape where the dimensions of the base the cylinder are substantially equal to those of the surface of the corresponding photovoltaic cell. In particular this has the effect of transferring photons from the primary guide to the cells.

In a specific implementation of the invention, the one or each secondary guide has an index of refraction included between the index of refraction of the primary guide and the index of refraction of the surface of the corresponding photovoltaic cell. In that way, the transfer of photons to the cells is further improved because reflections at the various interfaces between the primary guide, the secondary guide and the cells are minimized.

Additionally, the invention relates to a fabrication method for a photovoltaic device comprising:a plurality of photovoltaic cells, separated from each other;a support near which the cells are laid out; anda light guide in contact with said cells and comprising a primary guide with a surface that is proximal to the cells, where the proximal surface is oriented towards the cells and the support.

In particular:the photovoltaic cells are arranged near the support andthe light guide is arranged in contact with the photovoltaic cells by laying out, between the cells, areas located between the primary guide and the support which comprise a material with an index of refraction less than that of the proximal surface, where said material is arranged in contact with said proximal surface.

According to an aspect of the method according to the invention, one or more transparent secondary guides are obtained which are each interposed between the primary guide and one photovoltaic cell. In this way, the geometry of the device can be precisely controlled and nonlinear phenomena can be limited.

In a specific embodiment, each secondary guide is formed by a deposit directly in contact with the corresponding photovoltaic cell. In this way, the fabrication of the device can be made easier and the management of the associated inventory simplified.

According to another aspect of the invention, all or part of the photovoltaic cells are formed by deposition near the support such that the corresponding photovoltaic cells project from the support and the primary guide is deposited in contact with said projecting cells. In that way, the storage constraints due to the parts required for fabrication of the device can be limited. Additionally, the fabrication is simplified because of the reduction of the number of steps needed.

DETAILED DESCRIPTION

FIG. 1shows a photovoltaic device2according to the invention, configured for transforming light into electric energy.

With reference toFIGS. 1 and 2, the device2comprises a substrate4, a support6for the photovoltaic cells8and a light guide10.

The device2operates over a range of wavelengths referred to as useful. This range of useful wavelengths is defined as the spectral range of photons that the cells8are capable of converting into electricity. The upper end of this range therefore depends on the nature of the photovoltaic cells8and more precisely the nature of the material making up the absorbers of these cells. The lower end of this range is commonly set at 350 nm because there are nearly no photons with a wavelength below 350 nm arriving on Earth.

For example, this range is from 350 nm to 1200 nm.

The general shape of the substrate4is a rectangular plate. It is made for example by known methods. The substrate4is in contact with the support6and supports the support6.

In an embodiment, the substrate4is provided with electrical contacts (not shown) configured for connecting all the photovoltaic cells8individually or in a network to an external circuit.

The general shape of the substrate6is a rectangular plate. The lateral and transverse dimensions thereof correspond substantially to that of the substrate. The support6is arranged on the substrate4and substantially parallel to the substrate4. The substrate4and the support6are arranged substantially edge to edge.

The support6receives the cells8. In the embodiment fromFIGS. 1 and 2, the support6is provided with cavities12which open out and whose respective openings are oriented away from the substrate4. Each cavity12receives a photovoltaic cell8. The cavities12have dimensions substantially complementarity to that of the photovoltaic cells8. The cavities12and therefore the cells8are spaced apart from each other on the support. The spacing between the cells should be regular for the optimal performance of the device2. For example, the cavities12and therefore the cells8are arranged in a matrix pattern on the surface of the support6, meaning in regularly spaced rows and columns on this surface. Nevertheless, in some embodiments, the spacing is less regular or even random. This serves in particular to make fabrication of the device2easier.

According to an implementation variant, the cells are disposed directly on the surface of the support and the support does not have cavities12. The invention is subsequently described without limitation for embodiments in which the support has cavities12.

Additionally, several implementations of the support6are conceivable.

In one implementation, the support6is a reflector. The reflector6has an upper surface (in the direction of the orientation of the Figures). This upper surface is a reflecting surface14oriented away from the substrate4. More precisely, the reflecting surface14is oriented towards the light guide10. The cavities12open out through the reflecting surface14.

The reflector6is for example implemented conventionally. For example, the reflecting surface14includes a silver Ag or aluminum Al layer formed before or after the placement of the cells8and on which is optionally deposited a zinc oxide ZnO layer with or without aluminum doping.

In some embodiments, the reflecting surface14is configured to reflect only a portion of the visible domain. Advantageously, the wavelength range that the reflecting surface14is configured to reflect includes all or part of the emission wavelength range of a primary guide of the light guide10. For example, it is chosen to include all of this emission wavelength range.

This primary guide and the emission wavelength range thereof are described below.

This is advantageous in some types of applications, in particular in the implementation of window panes, and a device having low optical losses in the context of these applications can be obtained.

In some implementations, the substrate4is itself transparent at wavelengths which are not reflected by the support6.

In some specific implementations, the support6is alternately or parallelly reflecting in a wavelength range chosen from the visible domain such that the device has an outside appearance with a hue depending on said selected wavelength range.

In implementations in which the support14is reflecting in a portion of the visible domain including all or part of an emission wavelength range of the primary guide, the chosen range associated with the shade in question is for example chosen to be disjoint from the emission wavelength range of the primary guide.

In some embodiments, the reflecting surface14is configured to reflect the entire visible domain.

In another implementation, the support6has the same geometry as before. However, the support6does not have a reflecting surface, meaning that the upper surface of the support is not reflecting. The support6is made up for example of a material transparent in the visible domain. It is made for example from glass. Advantageously, the substrate4is itself transparent, so as to make the entire device the most transparent possible. This is particularly advantageous for some applications, such as windowpanes for construction, in which this transparency is an important criterion.

The following description is given without limitation for the scenario where the support6is a reflector; the application to a support of another type, for example a transparent support, is immediate.

As previously indicated, the cells8are arranged respectively in one of the cavities12of the reflector6. Each cell8has an upper surface16oriented towards the light guide10and via which the photons coming from the light guide10that the cell transforms into electric energy are received. The cells8are disposed in the cavities12. For example the surfaces16of the cell8are substantially coplanar with each other and/or coplanar with the reflecting surface14of the reflector6. For example, the cells are flush mounted in the reflector. In that way, the upper surfaces16of the cells are leveled near the reflecting surface14. Alternatively, the surfaces16of the cells8are not mutually coplanar. Furthermore, in some embodiments, they are recessed in their respective cavity12, meaning their surface16is at a lower level than that of the associated opening of the cavity12. In other embodiments described below, the cells protrude from their cavity and the reflector.

The upper surface16of the cells8is substantially flat. The upper surface16of the cells8has an index of refraction nc. The index of refraction ncis substantially equal to 1.9, for example. The upper surface16includes for example a conducting transparent oxide layer. This oxide can be zinc oxide ZnO, transparent, doped with aluminum, or indium and tin oxide ITO, or tin oxide SnO2.

In an embodiment, the cells8are microcells.

Advantageously, the cells8have a generally cylindrical shape and their respective upper surface16is circular. The diameter of the cells is then for example included between 10 μm and 500 μm.

Note that “cylinder” is understood to mean a surface defined by a generator passing through a variable point describing a closed planer curve, or directing curve, while keeping a fixed direction. In that way, a cylindrical shape is not necessarily rotationally symmetric.

In some embodiments, such as the one fromFIG. 1, the cells have a general cylindrical shape with rectangular section as shown onFIG. 1. The cells8then half a width and/or length measured in the plane of their upper surface16included between 10 μm and 500 μm.

Alternatively, the cells have shapes and respective upper surfaces16of arbitrary shape. The cells are for example inscribed in a cylinder with circular section and diameter included between 10 μm to 500 μm.

The cells8are, for example, thin layer cells, which can have advantages in terms of ease of fabrication. For example, they are of the type referred to as CIGS (for Cu, In, Ga and Se) and their composition is Cu(In, Ga)Se2, meaning they are made from copper, indium, gallium and selenium. They can also be CdTe or CZTS type, which are other thin layer cells.

Nonetheless, the invention is not limited to a specific type of cell. The cells can be chosen arbitrarily among the existing cells. For example, the cells could be crystalline, polycrystalline or amorphous silicon, or cells of type III-V semiconductor, like for example GaAs, le GaInP ou le GalnAs.

The light guide10is configured to receive photons and guide them to the cells8. The light guide10is common to the cells8. Additionally, it is in contact with all cells8. The light guide is configured to guide photons therewithin to the upper surface16of the cells8.

The light guide10comprises a primary guide18and a plurality of secondary guides20.

The primary guide18is a fluorescent concentrator. It is configured to absorb and re-emit photons in response to photons at another wavelength. This is described in more detail later.

The general shape of the primary guide18is a flat rectangular plate. This configuration makes assembly of the device easy and reduces the bulk of the device.

This primary guide18has for example a thickness of order a millimeter or even a centimeter.

The primary guide18is disposed substantially parallel to the reflecting surface14of the reflector6.

The primary guide18has a surface22proximal to the cells8which is oriented towards the reflector6. With reference to the orientation inFIGS. 1 and 2, the proximal surface22corresponds to the lower surface of the primary guide18. As shown on theFIGS. 1 and 2, the reflecting surface14of the reflector6is oriented towards the proximal surface22.

The proximal surface22and the reflecting surface14of the reflector6are substantially parallel.

The primary guide18has longitudinal and transverse dimension substantially equal to that of the reflector6. More precisely, the dimensions of the primary guide, the reflector and the substrate are dependent on the application of the device2. For example, the proximal surface area22of the guide (and therefore the area of the reflector and the substrate) is of order tens of square centimeters for some applications or of order a square meter for other applications. The ratio of the proximal surface area22to the sum of the upper surface areas16of the cells8, also known under the name of geometric gain of the device2, is for example included between 2 and 100 and is for example 20.

Advantageously, the cells8are facing the central part of the primary guide18. This allows adjustment of the dimensions of the primary guide18without having to modify the arrangement of the cells8near the support6.

Advantageously, the cells8are thus arranged near the support6opposite the primary guide18in a way that the upper surface16thereof is not facing a lateral end of the primary guide18, meaning edges delimiting the lateral surfaces23(FIG. 2) of the primary guide18.

The primary guide18comprises at least one dye and also a material forming the majority of the primary guide and in which the or each coloring is immersed and homogeneously distributed. The dye is phosphorescent or florescent, meaning a material which absorbs light in a first wavelength range, called absorption range of the device2. In response, it re-emits within it and principally isotropically photons in a second wavelength range or emission wavelength range. This range is centered on a characteristic wavelength λ of the device2.

The absorption wavelength range designates the spectral range of the photons that the dye is capable of absorbing. Ideally, the lower limit thereof corresponds to the lower limit of the useful wavelength range, and the upper limit thereof is slightly below that of the useful wavelength range.

The emission wavelength range designates the spectral range of the photons emitted by the dye. This range is offset towards longer wavelengths compared to the absorption range. It must ideally have an upper limit coinciding with that of the useful wavelength range. Additionally, this range is generally narrow, such that the range can be associated with and is centered around a specific wavelength—the characteristic wavelength λ. As will be seen subsequently, this characteristic wavelength λ is used to define the height of the secondary guides included in the device2and also the spacing between the primary guide and the reflector. The characteristic wavelength λ of the device2is a function of the primary guide18and the dye(s) that it contains. It is chosen for being included in a spectral range where the photovoltaic cells8perform well.

The absorption wavelength range and the emission wavelength range generally have a shared frequency range. However, this shared range is preferably as narrow as possible. This serves to limit the phenomenon of reabsorption by the waveguide18of photons emitted by the primary guide18itself since these reabsorptions result in losses.

The reflecting surface14of the reflector6is chosen for optimally reflecting photons having a wavelength included in the emission wavelength range of the primary guide.

Preferably the phosphorescence yield of the dye, meaning the ratio of the number of photons re-emitted by the dye to the number of photons absorbed, is over 90% and advantageously 95%.

In a first variant, the primary waveguide is composed of one or more polymers doped by one or more dyes. For example, the primary guide is made from poly methyl methacrylate or PMMA. In some of embodiments, the dye is implemented from organic molecules such as for example Lumogen®, sold by BASF, and is for example Lumogen® RED 305. Alternatively, the dye is made from rhodamine, perylene, 4-butylamino-N-allyl-1,8-naphthalimide, poly(9,9-di-(2-ethylhexyl)-9H-fluorene-2,7-vinylene, poly((9,9-di-(2-ethylhexyl)-9H-fluorene-2,7-vinylene)-co-(1-methoxy-4-(2-ethylhexyloxy)-2,5 or chelates of lanthanide ions.

Alternatively, the dye is made from semiconductor nano crystals (known in English as “quantum dots”), such as for example nanoparticles of PbS or PbSe or core/shell type structures of CdSe/ZnS, CdSe/CdS, CdSe/CdS/CdZnS/ZnS or CdTe/CdSe.

Alternatively, the dye is made from organic/inorganic hybrid compounds.

In some embodiments, the dye is made from several elements described above, which serves to expand the absorption range of the concentrator.

According to another variant, the primary guide is an oxide doped with luminescent elements.

In other embodiments, the dye is implemented from nanoparticles of oxides doped with rare earth metals, like yttrium orthovanadate doped with europium or oxides doped with neodymium (Nd3+) or ytterbium (Yb3+) or doped with other rare earth metals, for example with lanthanides.

The primary guide18, and therefore the proximal surface22thereof, have an index of refraction ng1. The index of refraction ng1is substantially equal to 1.5, for example.

According to the invention, the proximal surface22of the primary guide18is away from the reflector6. The device2comprises one or more areas23located between the cells and comprising a material24with an index of refraction less than that of the proximal surface22of the primary guide18. The material24fills the delimited space between the reflector6and the primary guide18and extends between the cells8. The one or more areas23are located between two portions belonging respectively to the primary guide18and to the reflector6and which are opposite one another.

Preferably, the material24has the smallest possible index of refraction. The preferred material24is therefore air (with an index of refraction equal to 1).

The effect of the presence of material24is to induce a Fresnel reflection at the interface between the primary guide18and the material24, meaning near the proximal surface22. This reflection is specular and the efficiency is substantially equal to 100% for photons with an angle of incidence greater than or equal to a critical angle. One then talks about total internal reflection, TIR. The value of this angle then depends only on the indices of refraction of the materials forming the interface, meaning the primary guide18and the material24. For the phenomenon of total internal reflection to occur, the light has to pass from a high index medium to a lower index of refraction medium, which explains the addition of the material24below the primary guide. In the implementations where the material24is air and the material of the primary waveguide has an index of refraction of 1.5 then the critical angle is substantially 42°, which corresponds to 75% of the incident photons subsequently reflected by TIR and 25% of the photons not reflected assuming isotropic emission, which is the case here.

Preferably, the material24has an index of refraction equal or substantially equal to 1. This has the effect of minimizing the value of the critical angle and therefore maximizing the proportion of photons reflected by total internal reflection.

As a variant, the material24is a porous material, for example made of SiO2or TiO2nanostructures, so as to minimize the effective index of refraction. Alternatively, the material24is made from a polymer with an index of refraction less than 1.4 and equal to 1.3, for example. In another variant, the material24is made from magnesium fluoride MgF2or even silicon oxide SiO2.

Preferably, the distancedbetween the proximal surface22of the primary guide18and the support6is greater than or equal to the characteristic wavelength λ of the device2. The effect of this is to minimize the nonlinear effects of the behavior of the photons induced because this distance can not be negligible compared to the wavelength of the photons after their emission by the primary guide18. Preferably the distancedbetween the proximal surface22and the reflector6is greater than or equal to a multiple of the characteristic wavelength λ which is strictly greater than 1. This makes it possible to minimize the aforementioned nonlinearity effects earlier. For example, the characteristic wavelength of the device2can be about 1 μm and the distance between the proximal surface22and the reflector6is for example taken greater than or equal to two, three or four times this wavelength and is for example 5 μm.

Additionally, preferably, the distance between the proximal surface22and the reflector6is less than or equal to a few times the characteristic wavelength λ of the device2, for example 20 times this wavelength. This serves in particular to limit the phenomena of photon loss by the sides of the secondary guides20and also to minimize the probability of the occurrence of reflections near lateral surfaces of the secondary guides20as will be seen subsequently. Thus, for example, the distance between the proximal surface22and the reflector6is taken less than or equal to 20 μm and is, for example, included between 5 μm and 10 μm.

Preferably, the zones23form a single area23which is continuous and in contact with the proximal surface22over substantially the entire surface of the proximal face22which is not facing a secondary guide20. The effect of this is to improve the efficiency of the reflections over a surface of maximum area.

The secondary guides20are transparent. They are respectively associated with one of the cells8. Preferably, the secondary guides20are identical to each other. This makes it easy to fabricate them and therefore to fabricate the device2in general.

Each secondary guide20has the shape of a pad. Each secondary guide20is interposed between a proximal surface22of the primary guide18and the upper surface16of a cell8. The secondary guides20keep the proximal surface22of the primary guide18away from the reflector6. The secondary guides20are separated from each other laterally by the material24.

Preferably, each secondary guide20has a surface, or base, in contact with the associated cell8which has a shape substantially identical to that of the upper surface16of the cell8. This has the effect of maximizing the percentage of photons which pass from the primary guide18to the secondary guides20and from the secondary guides20to the cells8. For example, each secondary guide has a generally cylindrical or prismatic shape of arbitrary section and whose base has a shape substantially identical to that of the upper surface16of the associated cell8. For example, as shown inFIG. 2for cells8with rectangular upper surface16, each secondary guide20has a right prismatic shape with rectangular section having dimensions substantially identical to that of the upper surface16of the cells. Alternatively, for cells8with cylindrical shape and circular section, the secondary guides20also have a generally cylindrical shape with a circular section.

Alternatively, the secondary guides20can have concave or convex sides, with a trapezoidal or other shape.

Each secondary guide20is arranged in contact with the upper surface16of the associated cell8with the base of the secondary guide20arranged in contact with the upper surface16and edge to edge, as shown inFIGS. 1 and 2.

The secondary guides20are for example made from a photosensitive resin. For example, the photosensitive resin is the resin sold under the name AZ® nLOF™ 2070 by MicroChemicals or the resin 40XT or the resin SU8.

The secondary guides20have an index of refraction ng2. The index of refraction ng2is greater than the index of refraction ng1of the primary guide18. Additionally, the index of refraction ng2of the secondary guides20is less than the index of refraction ncof the upper surfaces16of the cells8. The effect of this is to enhance the transfer of photons towards the cells8, since the secondary guides provide an anti-reflection for the cells because of their intermediate index between the index of the primary guide and that of the cells.

Preferably, the index of refraction ng2of the secondary guides20is substantially equal to the geometric average of the index of refraction ng1of the primary guide18and the index of refraction ncof the upper surfaces16of the cells8. This has the effect of simultaneously enhancing the transfer of photons from the primary guide18to the secondary guides20and the transfer of photons from the secondary guides20to the cells8. In other words, this is the preferred relation:
ng2≅√{square root over (ng1*nc)}

For example, the index ng1is substantially 1.5 and the index ncis substantially 1.9. Preferably, the index ng2is then substantially 1.69.

The principle of operation of the device2is now going to be described with reference toFIGS. 1 and 2.

During operation of the device2, the primary guide18is illuminated by photons coming from the environment thereof.

Referring toFIG. 2which shows a sample optical path T, the photons around the device2enter into the primary guide18. As previously indicated, they are absorbed by the primary guide18. Some photons for example are absorbed near the point A within the thickness of the primary guide18. In response, the primary guide18isotropically emits photons from the point A in the primary guide18, meaning in all directions. These photons are emitted at a wavelength belonging to the primary guide emission range18.

Once emitted within the primary guide18, these photons move in it towards an interface of the primary guide18.

The photons coming to a zone of the proximal surface22facing a secondary guide20pass into the secondary guide and then, all or part thereof, arrive at the upper surface16of the associated cell8, as described below.

As shown by the optical path T, the photons arriving near an interface of the primary guide18in an area which is not facing a secondary guide20are reflected. More specifically, as is well known, on each reflection, only a portion of the photons are reflected, the other portion of these photons escape from the primary guide18. For reflections occurring near the proximal face22, the photons which are not reflected propagate towards the reflector6where they are reflected towards the primary guide18. There they enter it and propagate in it again.

According to the invention, as previously indicated, the proportion of the photons effectively reflected near the proximal surface22is increased because of the presence of areas23comprising the material24. In fact, in a scenario in which the reflector is positioned directly under the primary guide18, all the rays would be reflected with the coefficient of reflection of the reflector, which is not perfect. With the presence of material24, a large portion of the rays are perfectly reflected by total internal reflection. Those which escape encounter the reflector6and are therefore reflected at the rate of the reflecting surface thereof and reenter into the primary guide again.

The movement of the photons in the primary guide18, whether or not following one or more reflections on the reflecting surface14of the reflector, continues until arriving within the primary guide18near the proximal surface22in a zone located facing the secondary guide20. Because the values of the indices of refraction ng1, ng2and nc, the photons enter into the secondary guide20in question in which they move towards the corresponding cell8. Depending on the angle at which a photon enters a secondary guide20, it can be subject to one or more reflections near the lateral side of the secondary guide20. As before, only a portion of the photons undergoing these reflections are effectively reflected since another portion passes into the material24. Depending on the path thereof and the position thereof in the device2, in particular the proximity thereof to an edge of the device2, once they have left the secondary guide20, these photons enter into another secondary guide20(with or without reflection by the reflector6), enter the primary guide18again (after reflection on the reflector6) or escape from the device2(with or without reflection on the reflector6). The photons reaching the upper surface16of a cell8are then converted into electrical energy by the cell8.

In the embodiments in which the support6is only reflecting for a portion of the spectrum, the photons whose wavelength is located in the range of wavelengths reflected by the support6behave as described above. The photons with a wavelength which is not reflected by the support are then not reflected by the support and escape from the device during operations thereof.

In the embodiments in which the support6is transparent, the photons are not reflected by the support6during operation of the device.

The fabrication of the device2is now going to be described with reference toFIGS. 1 and 2.

In a first step, the substrate4, support6and cells8are fabricated by any known method and they are arranged as previously described. In other words, the support6is arranged on the substrate4and the cells8are placed near the support6.

In some embodiments, the upper surface16of the cells8include a layer of zinc oxide ZnO doped with aluminum Al. In the corresponding embodiments, this layer is deposited on the cells8once they are arranged near the support, either selectively only on the cells8or both on the cells8and on the reflecting surface14of the reflector6.

Additionally, the secondary guides20are fabricated, for example, by optical photolithography. The positioning of the secondary guides is then done during the photolithography. Finally, the proximal surface22of the primary guide18is arranged in contact with the free end of the secondary guides20. Optionally, a final annealing is also done in order to rigidly join the secondary guides20to the primary guide18, which improves the mechanical strength of the device2.

As a variant, all or part of the secondary guides20are formed directly in contact with the cells8by electrochemical means. More specifically, after laying out the cells8on the reflector6, secondary guides20are made by electrodeposition of zinc oxide ZnO that is selectively deposited on the surface of the cells8.

In the corresponding embodiments, the secondary guides20formed directly on the cells8are made of zinc oxide ZnO.

The device2according to the invention was implemented with a geometric gain of order20. It was thus observed that the concentration factor of the device was three times better than a device from the state-of-the-art. For example, the concentration factor of the device from the state-of-the-art in which the primary guide is adhered to the reflector was measured at 1.8 as compared to the concentration factor of the device2according to the invention which was measured at 5.3.

This is explained by the direct efficiency gain arising from the improvement of the efficiency of the reflections near the proximal surface22of the primary guide18and also from an indirect efficiency gain near the upper surface of the primary guide18resulting from the specular nature of the reflections near the proximal surface22for photons arriving with an angle of incidence sufficient for being reflected by total internal reflection.

With reference toFIG. 3, in a variant of the invention, the cells8project from the reflector6towards the primary guide18. The proximal surface22is arranged directly in contact with the cells8. In other words, the device2does not have a secondary guide20. The cells8keep the primary guide18away from the reflector6.

In this embodiment, the cells8are formed for example directly in the cavities12of the reflector6, for example by deposition. Alternatively, they are formed directly on the reflector6which does not have cavities12. During fabrication of the device2, the reflecting surface14of the reflector6is formed for example after formation of the cells8by deposition. This serves to minimize the impact of cell8formation on the quality of the reflecting surface14of the reflector6.

Because it does not have a secondary guide20, the device2according to this variant has a lower cost and is easier to manufacture.

Referring toFIG. 4, in another embodiment, the cells8project from the reflector6and the device2includes secondary guides20such as previously described. The dimensions of the secondary guides20can be limited and therefore fabrication of the device2also simplified because of the projection of the cells8in this embodiment.

In another variant (not shown), the embodiments described above are combined. For example, the upper surface of some cells8are leveled near the reflecting surface of the reflector6such as shown inFIG. 2whereas other cells project out of the reflector. The device2then comprises secondary guides20of a first size interposed between the cells8whose upper surface is leveled and the proximal surface22of the primary guide, and secondary guides20of a second size interposed between the projecting cells and the proximal surface. The peaks of all the secondary guides are then substantially the same height with the primary guide in contact with each of them.

In another example of this variant, the primary guide18is arranged in direct contact with the cells projecting out of the reflector. Further, the cells8whose upper surface is flattened are then each coupled to a secondary guide20such as previously described and interposed between the cell8in question and the proximal surface22.

Other embodiments of the device2according to the invention are also conceivable.

For example, in some embodiments, the upper surface of the primary guide18is covered with a bandpass filter (dashed inFIG. 2) configured for allowing maximum passage of surrounding photons into the primary guide18but for blocking the exit of photons by the upper surface of the primary guide18in particular photons reemitted by the primary guide18and having a wavelength in the emission range of the primary guide. Thus the filter has good properties for reflection of photons having wavelengths located around the characteristic wavelength and a high transmission for other wavelengths.

Additionally, among the optical mechanisms with which to concentrate the light on the photovoltaic devices, imaging devices are noted, which serve to obtain an image of the object through the optical system thereof and therefore in this case an image of the sun on the cell, and non-imaging devices, which for their part concentrate the light without forming an image.

The imaging devices have the specific feature of only concentrating sunlight if it arrives directly on the device in question, meaning if it is oriented along the axis formed by the sun and the optical device, and being unable to use diffuse light, which arrives along arbitrary directions, for example because of diffusion phenomena generated by clouds.

The non-imaging devices are insensitive to the direction of the incident solar light and therefore have the advantage of not having to precisely follow the course of the sun with special systems.

Also, preferably, the device2is a non-imaging device. It is then attached to the support thereof in a non-mobile manner. This serves to free the device from an orienting mechanism configured for orienting it according to the course of the sun, which is necessary for imaging devices. The cost of the device2is therefore substantially reduced compared to an imaging device.