Photovoltaic device

Abstract

The invention relates to a photovoltaic cell (20) comprising: a stack of semiconductor or semimetal layers (24, 28) forming a junction and at least two contacts (32, 34) for collecting charge carriers resulting from the conversion of electromagnetic radiation (40, 50) within the junction, characterised in that at least one of the semiconductor layers contains a substance from among Mg2Pb(x)Si(1-x), Mg(z)Pb(x)Si(y), Mg8PbSi3, Mg2Si, Mg64Si3Pb, Mg64Si30Pb(2).

Description

Photovoltaic device Technical field

[0001] The invention relates to photovoltaic devices such as photovoltaic cells or panels. This disclosure specifically illustrates, but is not limited to, photovoltaic devices and photovoltaic structures configured to convert radiation in the visible and infrared region of the electromagnetic spectrum, as well as materials and structures specially adapted therefor. State of the art

[0002] Several examples of photovoltaic devices capable of converting solar radiation into electrical energy are known in the art. These devices are used in the aerospace industry, in a multitude of electronic devices, to power installations not connected to the electrical distribution network and, increasingly, for the production of general-purpose energy. Photovoltaic systems represent one of the most promising sources of renewable energy and do not release greenhouse gases into the atmosphere.

[0003] Solar energy received at the Earth's surface is measured at 341 W / m 2 on average during the day. Only part of this radiation is in the visible wavelength band (by convention between 0.4 pm and 0.8 pm), while most of the energy reaches us in the form of infrared radiation. The maximum is at a wavelength of 0.9 pm.

[0004] We know that the Earth is also a source of radiation and emits mainly in the infrared spectrum, due to its lower surface temperature than that of the Sun. The energy emitted is, however, considerable. It has been estimated that the Earth's surface emits 396 W / m 2 on average (the excess compared to the solar energy received corresponds to the geothermal energy released inside our planet). The peak emission of terrestrial radiation is located at approximately X=10 pm.

[0005] Photovoltaic systems are based on semiconductor materials. Electromagnetic radiation, if it has the appropriate wavelength, can move electrons from the valence band of the material to the conduction band, thus producing electron-hole pairs. A solar cell typically has pn junctions that generate an electric field, so that electrons and holes are forced to move to opposite sides of the material, where they are collected by conductive electrodes.

[0006] A large proportion of photovoltaic installations in operation are based on the first technology and include polycrystalline or monocrystalline silicon panels. This is a mature and well-known technology, but it does have limitations. Silicon photovoltaic panels are relatively expensive, heavy, and only exploit visible radiation, as silicon is relatively transparent to infrared radiation due to the relatively consistent energy jump between the conduction band and the valence band. It is observed that the intrinsic quantum efficiency of silicon essentially drops to zero when the radiation wavelength exceeds 1.2 pm.

[0007] Silicon solar cells are obtained by industrial crystallization processes that allow the manufacture of silicon wafers with the desired purity properties. Among the processes used to manufacture monocrystalline wafers of silicon or other materials are known the Czockralski process, micro-pulling-down processes based on capillary transport of the molten material, the molten zone refining technique, the vertical Bridgman (VB) Bridgman-Stockbarger (VBS) processes and their numerous variants aim to grow crystals in a temperature gradient. Document CN 102312279 A discloses the manufacture of a large number of materials by VGF (Vertical Gradient Fusion) VB and VBS processes.

[0008] Solar cells based on thin-film technology are also known, for example solar cells based on amorphous silicon, cadmium telluride (CdTe), copper indium gallium selenide (CIGS), perowskite photovoltaic cells.

[0009] Special materials such as indium phosphate (InP), indium antimony telluride (InSbTe) and many others have been proposed for the conversion of infrared electromagnetic radiation. It has also been proposed to improve the efficiency of photovoltaic cells in infrared radiation by nanostructures, for example "quantum dots". Brief summary of the invention

[0010] The invention provides the photovoltaic system which is the subject of the independent claim, and in particular by a photovoltaic cell comprising: a stack of semi-conductor or semi-metallic layers forming a junction and at least two contacts for the collection of charge carriers deriving from the conversion of electromagnetic radiation in the junction, characterized in that at least one of the semi-conductor layers contains Mg(z)Pb( X )Si(y), MgsPbSis, Mg2Si, Mg2Pb, Mg2Pb(x)Si(ix).

[0011] The invention is not limited to pure or lightly doped compounds, but may also comprise semiconductor layers of solid solutions of compatible chemical species, for example comprising one of Mg( Z )Pb( X )If( y ), MgsPbSis or Mg2Pb( X )If( y ), Mg2Si mentioned above in solid solution with other substances.

[0012] Preferably, the invention uses semiconductor or semi-metallic materials with a gap between the valence band and the conduction band much smaller than that of silicon, at a gap smaller than 0.2 eV. This result can be achieved by semiconductor or semi-metallic layers of Mg2Pb( X )If( y ), MgsPbSis, Mg( Z )Pb( X )If( y ), Mg2Si with appropriate doping. Doped MgsPbSis, for example, can have a band gap of about 0.1 eV.

[0013] Another advantage of the semiconductor or semi-metallic materials selected by the invention is that some have a direct band gap, without quasi-momentum shift between the lowest energy state of the conduction band and the highest energy state of the valence band. In these materials, radiation can trigger a transition without requiring interaction with phonons, unlike indirect band gap materials, such as silicon.

[0014] Without a junction, the charge carriers created by the conversion of incident radiation photons would tend to recombine rather than generate an electric current. In the invention, the junction is obtained by different dopings of the same semiconductor, or by the juxtaposition of two semiconductor layers of different composition (heterojunction). Bismuth (or Antimony or Arsenic or Phosphorus or Nitrogen) easily replaces lead in the MgsPbSis lattice by providing an additional electron. It is an excellent "n" type dopant for this application, while sodium (or Lithium or Potassium) can easily replace magnesium and act as an acceptor to generate a "p" type layer.Alternatively, the junction may involve the juxtaposition of materials that differ in their chemical composition (heterojunctions) or the juxtaposition of a first semi-conductor or semi-metallic layer and a second metallic layer (Schottky junctions).

[0015] In variants, the invention has optional but advantageous features such as for example a composition of at at least one of the semiconductor or semi-metallic layers is made up of MgsPbSis a junction comprising the superposition of a layer of MgsPbSis of type "n" doped with atoms of electron donor type, for example Bi, and a layer of MgsPbSis of type "p" doped with atoms of electron acceptor type, for example Na, the use of semiconductors or semi-metals with direct or non-direct gap and less than 0.2 eV, preferably less than or equal to 0.14 eV, the capacity to convert into electrical energy radiation with a wavelength greater than 1 pm, preferably greater than or equal to 8 pm, a transparent or semi-transparent or opaque conductive contact layer, semiconductor or semi-metallic layers are monocrystalline.

[0016] Multiple photovoltaic cells as disclosed above may be combined into a photovoltaic device. In an advantageous embodiment, the photovoltaic device may include a group of stacked photovoltaic cells optimized for nighttime operation and, optionally, one or more photovoltaic cells for photovoltaic conversion of solar energy above the group of photovoltaic cells for nighttime operation.

[0017] The photovoltaic cells of the invention can be manufactured by any suitable process, for example from a semiconductor or semi-metallic wafer containing Mg(z)Pb( X )Si(y), MgsPbSis, Mg2Pb( X )If( y ), Mg2Si by a VGF, VB or VBS process in which the required semi-metallic layers are produced.

[0018] By using selected semiconductor or semi-metallic materials, the invention enables the conversion of infrared radiation into electrical energy with a considerably higher efficiency than known photovoltaic devices. The invention can convert radiation with a wavelength greater than 1 pm, and even greater than 5 pm.

[0019] The ability to convert infrared radiation into energy results in a considerable increase in the energy produced when the devices of the invention are exposed to daylight. Although the peak of solar radiation is at a wavelength of about 0.9 pm, usable by conventional solar cells, the quantum efficiency of silicon is already considerably reduced at this wavelength. A major part of solar energy is irradiated at different and even longer wavelengths and is not usefully converted in ordinary solar installations.

[0020] Another advantage of the invention is the ability to harvest energy irradiated by the earth during the night. The earth emits as much energy as it receives from the sun. This energy is irradiated in the form of infrared radiation, with a peak intensity at 9 pm and cannot be used by conventional photovoltaic installations.

[0021] Advantageously, the invention provides photovoltaic devices that can harvest energy from solar radiation during the day and that radiated by the earth, which is also available at night. In this way, a solar installation based on the invention can generate higher currents and provide greater power, and this energy is available both night and day. Storage batteries can therefore be reduced or completely eliminated.

[0022] In addition, the devices of the invention can provide higher electrical power for the same occupied surface area, compared to conventional photovoltaic solutions. It is then possible to envisage using them to provide autonomous power to devices where space is limited, for example personal electronic devices or vehicles. For example, it would be possible to achieve completely autonomous mobile phones or solar cars capable of operating on photovoltaic energy alone for very long, and potentially indefinite, periods without recharging.

[0023] Several semiconductor substances usable in the invention have the formula Mg(z)Pb( X)Si(y). All these materials can be used in the context of the invention, preferably those where the proportion (y) of Silicon is not zero. Among these substances, some have a gap of approximately 0.5 eV, which is not optimal for the direct conversion of terrestrial thermal radiation; however, these materials can be used advantageously, for example in tandem structures comprising several superimposed photovoltaic elements with complementary spectral characteristics, or with conversion layers which transform the incident photons into longer wavelength radiation. The following description presents examples of these arrangements.

[0024] Alternative materials such as Mg2Pb, Hg(ix)Cd( X)Te or Ag2Se are also included within the scope of the invention and may enter into the composition of the semiconductor layer of the invention partially or exclusively in special embodiments. Brief description of the figures

[0025] Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which: - Figure 1 illustrates the structure of a photovoltaic cell according to one aspect of the invention. - Figure 2 shows a photovoltaic device according to one aspect of the invention, with a plurality of cells for converting sunlight. Figure 3 shows a photovoltaic device according to one aspect of the invention optimized for the conversion of infrared radiation. radiated by the Earth's surface, with tandem photovoltaic cells to capture infrared radiation from the ground. - Figure 4 shows the spectrum of solar radiation and the spectrum of infrared radiation emitted by the Earth.

[0026] In figures, notable elements are marked by reference signs used in the text to simplify understanding. The same reference sign is used to designate identical, equivalent or similar elements. When a figure contains several identical, equivalent or similar elements, some of the reference signs may be omitted to avoid cluttering the drawing. Example(s) of embodiment of the invention

[0027] Figure 4 shows the spectrum of solar radiation and the spectrum of infrared radiation emitted by the Earth. Curve 41 corresponds to the energy of solar radiation received per unit area at the Earth's surface, and follows Planck's law for a body at the sun's surface temperature of 6000 K. The radiation peak is in the visible light band 45, which also delimits the region in which silicon photovoltaic devices are effective.

[0028] Curve 51 illustrates the radiation spectrum emitted by a surface at the average ground temperature, i.e. 300 K. This radiation is located entirely in the infrared range, above X=1 pm and has a peak at X=10 pm.

[0029] Figure 1 schematically illustrates a stacked thin-film solar cell structure that could be adopted to carry out the invention. The solar cell comprises a substrate 21, preferably a material transparent to the electromagnetic radiation that it is desired to convert. In one possible embodiment, the substrate 21 is a layer of Mg(z)Pb( X )Si(y), MgsPbSis, Mg2Pb( X )If(i- X ) or Mg2Pb, but other solutions are possible. Layer 24 is a thin layer of "p" type semiconductor, for example a layer of MgsPbSis with added Sodium atoms, or another impurity that behaves as an electron acceptor in the MgsPbSis lattice. Layer 28, above layer 24, is a thin layer of "n" type semiconductor, for example a layer of MgsPbSis with Bismuth doping.

[0030] The sequence of layers could be changed, and the "n" layer could be between the substrate and the "p" layer, thus giving rise to a junction with the opposite polarity. Furthermore, the substrate 21 could be eliminated. The stack of layers 21-24-28 can be obtained by any suitable method, for example by epitaxy, cathode sputtering, evaporation, sublimation, by ablation with a pulsed laser beam, by a chemical bath, or by printing techniques. Fabrication from a semiconductor wafer obtained by the VGF or VBS processes, for example, is also possible and included in the scope of the invention. Monocrystalline cells are preferable, but the invention can also have polycrystalline embodiments.

[0031] The photovoltaic cell 20 also includes conductive structures for collecting the photovoltaic current. In the example of Figure 1, two transparent electrodes 32, 34 can be seen with this function, but other structures and solutions are possible, for example metallized fingers or grids. The solar cell may also include other support and protection layers, for example made of polymer, not shown in the figure.

[0032] Figure 1 is not to scale and is shown only as an example. In a real device, the layers may have different thicknesses, and the proportions between vertical and lateral dimensions may vary considerably from what is shown.

[0033] Figure 2 shows a solar device combining several solar cells according to the invention, optimized for the electrical conversion of sunlight 40. This device comprises a plurality of cells 20 connected together by electrical conductors 35. In the example shown, the solar cells are connected in parallel, but other connection topologies are possible. The device has two terminals 38 for connection to a load or, preferably, to an electronic management circuit configured to balance the cells and make them work close to their optimal charging point.

[0034] Figure 3 shows a device optimized for converting infrared light 50 emitted by the Earth's surface. It is advantageous to arrange a plurality of cells one after the other in tandem, so that the infrared light passes through them in succession. Several groups 27 of cells can be seen stacked in this way. Each group 27 can have a large number of cells 20 — the figure shows three cells per group, but this is only an example. Within each group, the cells can be electrically connected in series, to increase the electrical voltage generated.

[0035] Another possible solution is to combine the solar system in Figure 2 with the night-optimized system in Figure 3, one on top of the other. During the day, the solar cells on the upper level efficiently capture sunlight, while at night all the photovoltaic cells, and especially those in groups 27, convert infrared radiation from the ground into energy.

[0036] The performance of the photovoltaic devices of the invention can be further increased by adding a conversion layer comprising one or more materials capable of converting incident electromagnetic radiation with a wavelength of the order of 1 pm into infrared radiation with a longer wavelength, for example that λ > 8 pm. As seen previously, the junction of the invention is particularly suitable for converting radiation in this region of the electromagnetic spectrum. There are several photofluorescent materials capable of absorbing visible or ultraviolet radiation and emitting infrared radiation quanta by fluorescence. The conversion layer can be a thin film made using suitable deposition techniques, or a layer comprising a suspension of IR phosphor particles. This includes non-linear materials, for example the UV2IR1 phosphor produced by Maxmax LLC, USA. Reference numbers used in the figures

[0037] 20 photovoltaic cells 21 substrate 24 p-type semiconductor layer 27 group of photovoltaic cells 28 n-type semiconductor layer 32 lower electrode 34 upper electrode 35 driver 38 electrical terminal 40 solar radiation 41 spectrum of solar radiation 45 band of visible light 50 Earth radiation 51 spectrum of the earth's radiation

Claims

Claims 1. Photovoltaic cell comprising: a stack of semi-conductor or semi-metallic layers forming a junction and at least two contacts for collecting charge carriers deriving from the conversion of electromagnetic radiation in the junction, characterized in that at least one of the semi-conductor layers contains a substance among: Mg( Z )Pb(x)Si(y), MgsPbSis, Mg8Pb( X )Si(y), Mg2Si, Mg64Si3Pb, Mg64Si3oPb(2).

2. Photovoltaic cell according to the preceding claim, in which at least one of the semiconducting or semi-metallic layers is constituted by MgsPbSis or by a solid solution of MgsPbSis with another substance.

3. Photovoltaic cell according to one of the preceding claims, in which the junction comprises the superposition of an “n” type layer doped with electron donor type atoms, for example Bi, and a “p” type layer doped with electron acceptor type atoms, for example Na.

4. Photovoltaic cell according to any one of the preceding claims, in which the semiconductor or semi-metallic layers are constituted by semiconductors or semi-metals with a direct or non-direct gap and less than 0.2 eV, preferably less than or equal to 0.14 eV.

5. Photovoltaic cell according to any one of the preceding claims, capable of converting into electrical energy radiation with a wavelength greater than 1 pm, preferably greater than or equal to 8 pm.

6. Photovoltaic cell according to any one of the preceding claims, in which at least one of the contacts comprises a transparent or semi-transparent or opaque conductive layer.

7. Photovoltaic cell according to any one of the preceding claims, in which the semi-conductor or semi-metallic layers are monocrystalline.

8. Photovoltaic device comprising several photovoltaic cells according to one of the preceding claims.

9. Photovoltaic device according to the preceding claim, comprising a group of photovoltaic cells for nighttime operation, comprising a superposition of photovoltaic cells according to one of claims 1 to 7 for converting infrared radiation from the Earth's surface into electrical energy.

10. Photovoltaic device according to the preceding claim, further comprising one or more photovoltaic cells for photovoltaic conversion of solar energy above the group of photovoltaic cells for nighttime operation.

11. Photovoltaic device according to one of the preceding claims, with a conversion layer comprising one or more materials capable of converting incident electromagnetic radiation into infrared radiation.

12. Method for manufacturing a photovoltaic cell according to one of claims 1 to 7, comprising the manufacture of a semiconductor or semi-metallic wafer containing Mg2Pb( X )If(i- X ), MgsPbSis, Mg(z)Pb( X )Si(y), Mg2Si, Mg64Si3Pb, Mg64Si3oPb(2) by a VGF, VB or VBS process and the manufacture of layers or semi-metallics forming a junction in the wafer.

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