Photovoltaic device

Abstract

The invention relates to a photovoltaic cell (20) comprising a stack (24, 28) of semiconductor layers or semi-metallic layers forming a junction, and at least two contacts (32, 34) for collecting charge carriers generated by the conversion of electromagnetic radiation (40, 50) in the junction, characterized in that at least one semiconductor layer comprises a substance selected from the group consisting of Mg (Z) Pb (x) Si (y), Mg8PbSi3, Mg8Pb (X) Si (y), Mg2Si, Mg64Si3Pb, Mg64Si30Pb (2).

Description

TECHNICAL FIELD

[0001] The present invention relates to photovoltaic devices, such as photovoltaic cells or panels. The present disclosure particularly, but not exclusively, discloses photovoltaic devices and structures configured for converting electromagnetic spectrum visible and infrared region radiation, and materials and structures particularly suitable for this purpose. BACKGROUND

[0002] A variety 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 numerous electronic devices, to power facilities not connected to the grid, and increasingly for general energy production. Photovoltaic systems are among the most promising renewable energy sources and do not emit greenhouse gases into the atmosphere.

[0003] The solar energy received at the Earth's surface is, on average, 341 W / m2 during the day. Only a fraction of this radiation lies in the wavelength range of the visible light radiation (conventionally between 0.4 pm and 0.8 pm), while the majority of the energy arrives in the form of infrared radiation. The maximum lies at a wavelength of 0.9 pm.

[0004] It is known that the Earth is also a source of radiation, emitting mainly in the infrared spectrum due to its lower surface temperature than the Sun. However, the energy emitted is considerable. It is estimated that the Earth's surface emits, on average, 396 W / m2 (corresponding to the excess of solar energy received to the geothermal energy released inside our planet). The emission peak of the Earth's radiation lies at about l=10 pm.

[0005] Photovoltaic systems are based on semiconductor materials. If the electromagnetic radiation has the appropriate wavelength, it is possible to move an electron from the valence band to the conduction band of the material, thus creating an electron-hole pair. Solar cells usually have p-n type junctions, which generate an electric field, such that the electrons and holes are forced to move to one opposite face of the material, where they are collected by conductive electrodes.

[0006] The majority of photovoltaic installations in operation are based on first-generation technology, including panels of polycrystalline or monocrystalline silicon. This is a mature and well-known technology, but has limitations. Silicon photovoltaic panels are relatively expensive, heavy, and only utilize visible light radiation, since silicon is relatively transparent to infrared radiation due to the relatively large energy jump between the conduction and valence bands. It can be seen 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 which allow to manufacture silicon wafers with the required purity characteristics. Among the known processes for the manufacture of single-crystal silicon wafers or other materials, including the Czochralski, micro-pulling-down based on capillary transport of the molten mass, zone-refining technique, vertical Bridgman (VB) and Bridgman-Stockbarger (VBS) and numerous variants thereof, aimed at growing crystals in a temperature gradient. The document CN102312279 A discloses the manufacture of a large number of materials by VGF (Vertical Gradient Freeze), VB and VBS processes.

[0008] Solar cells based on thin-film technology are also known, such as those based on amorphous silicon, cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and perovskite photovoltaic cells.

[0009] Special materials have been proposed for the conversion of infrared electromagnetic radiation, such as indium phosphide (InP), indium antimony tellurium (InSbTe) and many others. It has also been proposed to improve the efficiency of photovoltaic cells for infrared radiation by means of nanostructures, such as "quantum dots". SUMMARY

[0010] The present invention proposes a photovoltaic system as claimed in the independent claim, in particular a photovoltaic cell comprising a stack of semiconducting or semimetallic layers forming a junction, and at least two contacts for collecting the charge carriers generated by the conversion of electromagnetic radiation in the junction, characterized in that at least one semiconducting layer comprises a substance selected from the group consisting of Mg （z） Pb （x） Si （y） , Mg8PbSi3, Mg2Si, Mg2Pb （x） Si （1-x） .

[0011] The present invention is not limited to pure or lightly doped compounds, but can also include solid solution semiconducting layers of compatible chemical species, such as a solid solution of one of the above Mg （z） Pb （x） Si （y） , Mg8PbSi3 or Mg2Pb （x） Si （y） , Mg2Si with other substances.

[0012] Preferably, the present invention uses a semiconducting or semimetallic material with a band gap between the valence and conduction bands much lower than silicon, less than 0.2 eV. This can be obtained with Mg2Pb （x） Si （y） , Mg8PbSi3, Mg （z） Pb （x） Si （y）semiconductor or semimetallic layer of Mg2Si. For example, doped Mg8PbSi3may exhibit a bandgap of about 0.1 eV.

[0013] Another advantage of the selected semiconducting or semimetallic materials of the application is that some of them exhibit a direct bandgap, 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, the radiation can trigger a transition without the need for interaction with phonons, unlike in indirect bandgap materials such as silicon.

[0014] Without the junction, the charge carriers generated by the conversion of the incident radiation photons would tend to recombine, rather than generating an electric current. In the application, the junction is obtained by different doping of the same semiconductor, or by juxtaposition of two semiconducting layers of different composition (heterojunction). Bismuth (or antimony or arsenic or phosphorus or nitrogen) can easily replace lead in the Mg8PbSi3lattice, providing additional electrons. It is an excellent n-type dopant in this application, while sodium (or lithium or potassium) can easily replace magnesium, acting as an acceptor to create a p-type layer. Alternatively, the junction can comprise a juxtaposition (heterojunction) of materials different in their chemical composition, or a juxtaposition of a first semiconducting or semimetallic layer and a second metallic layer (Schottky junction).

[0015] In some variants, the application has optional but advantageous features, such as the composition of at least one semiconducting or semimetallic layer consisting of Mg8PbSi3; the junction comprising a stack of an n-type Mg8PbSi3layer (doped with electron donor atoms, such as Bi) and a p-type Mg8PbSi3layer (doped with electron acceptor atoms, such as Na), the use of a semiconducting or semimetallic material with a direct or indirect bandgap smaller than 0.2 eV, preferably smaller than or equal to 0.14 eV, the ability to convert radiation with a wavelength greater than 1 pm, preferably greater than or equal to 8 pm, into electrical energy, the contacts comprising transparent, semi-transparent or opaque conductive layers, the semiconducting or semimetallic layers being single crystals.

[0016] A plurality of photovoltaic cells as disclosed above can be combined into a photovoltaic device. In an advantageous variant, the photovoltaic device can comprise a set of stacked photovoltaic cells optimized for night operation, and optionally one or more photovoltaic cells for photovoltaic conversion of solar energy above said set of photovoltaic cells for night operation.

[0017] The photovoltaic cells of the application can be manufactured by any suitable method, such as from a composition comprising Mg （z） Pb （x） Si （y） , Mg8PbSi3, Mg2Pb （x） Si （y）Starting from Mg2Si semiconductor wafers or half-metal wafers, the desired semiconductor layer or half-metal layer is fabricated using VGF, VB, or VBS methods.

[0018] By using selected semiconductor or semi-metallic materials, this invention can convert infrared radiation into electrical energy with a much higher efficiency than known photovoltaic devices. This invention can convert radiation with wavelengths greater than 1 μm and even greater than 5 μm.

[0019] The ability to convert infrared radiation into energy results in a significant increase in energy production when the device of this invention is exposed to sunlight. Although the peak of solar radiation is at a wavelength of approximately 0.9 μm (which can be utilized by conventional solar cells), the quantum efficiency of silicon is significantly reduced at this wavelength. Most solar energy radiates at different and even larger wavelengths and is not efficiently converted in ordinary solar energy installations.

[0020] Another advantage of this invention is its ability to harvest energy radiated by the Earth at night. The Earth does indeed emit as much energy as it receives from the Sun. This energy is radiated in the form of infrared radiation, with a peak intensity at 9 μm, and cannot be utilized by conventional photovoltaic facilities.

[0021] Advantageously, the present invention proposes a photovoltaic device capable of collecting solar radiation energy during the day and Earth's radiation energy at night. Thus, solar energy facilities based on the present invention can generate higher current and provide greater power, and this energy is available both day and night. Therefore, energy storage batteries can be reduced or completely eliminated.

[0022] Furthermore, compared to conventional photovoltaic solutions, the device of this invention can provide higher electrical power while occupying the same area. Therefore, it can be considered for providing autonomous power to devices with limited space (such as personal electronic devices or vehicles). For example, it could enable fully autonomous mobile phones or solar-powered cars that can operate for extended periods, or even indefinitely, solely on photovoltaic energy without the need for charging.

[0023] Various semiconductor materials that can be used in this invention have the general formula Mg （z） Pb （x） Si （y） All of these materials are suitable for use within the scope of this invention, with those having a non-zero silicon content (y) being preferred. Some of these materials have a band gap of about 0.5 eV, which is not optimal for the direct conversion of Earth's thermal radiation; however, these materials can be advantageously used, for example, in tandem structures comprising multiple stacked photovoltaic elements with complementary spectral properties, or in conjunction with conversion layers that convert incident photons into longer-wavelength radiation. Examples of these arrangements are described below.

[0024] Alternative materials, such as Mg2Pb and Hg （1-x） Cd （x） Te or Ag2Se are also included within the scope of this invention and may be partially or completely incorporated into the semiconductor layer composition of this invention in particular embodiments.

[0025] Brief description of the attached figures Embodiments of the present invention are shown in the accompanying drawings, wherein: ● Figure 1 The structure of a photovoltaic cell according to one aspect of the present invention is shown.

[0026] ● Figure 2 A photovoltaic device according to one aspect of the present invention is shown, which has a plurality of cells for converting sunlight.

[0027] ● Figure 3 A photovoltaic device optimized for converting infrared radiation from the Earth's surface, according to one aspect of the invention, is shown, having tandem photovoltaic cells for capturing infrared radiation from the ground.

[0028] ● Figure 4 The solar radiation spectrum and the infrared radiation spectrum emitted by the Earth are shown.

[0029] In the accompanying drawings, elements are indicated by reference numerals, which are repeated in the text for ease of understanding. The same reference numerals are used to denote identical, equivalent, or similar elements. When a drawing contains multiple identical, equivalent, or similar elements, some reference numerals may be omitted to avoid overcrowding. Detailed Implementation

[0030] Figure 4 The solar radiation spectrum and the infrared radiation spectrum emitted by the Earth are shown. Curve 41 corresponds to the solar radiation energy received per unit area of ​​the Earth's surface, following the blackbody radiation law of the Sun at a surface temperature of 6000 K. The radiation peak is located in the visible light band 45, which also defines the effective area for silicon photovoltaic devices.

[0031] Curve 51 shows the radiation spectrum emitted by the surface at an average surface temperature (i.e., 300 K). This radiation is entirely in the infrared region, greater than λ = 1 μm, and exhibits a peak at λ = 10 μm.

[0032] Figure 1 The diagram schematically illustrates a structure for a thin-layer tandem solar cell that enables the present invention. The solar cell includes a substrate 21, preferably made of a material transparent to the electromagnetic radiation to be converted. In one possible embodiment, the substrate 21 is Mg. （z） Pb（x） Si （y） Mg8PbSi3, Mg2Pb （x） Si （1-x） Layer 24 could be a Mg2Pb layer, but other solutions are also possible. Layer 24 is a p-type semiconductor thin layer, such as a Mg8PbSi3 layer with added sodium atoms or other impurities that act as electron acceptors in the Mg8PbSi3 lattice. Layer 28, located above layer 24, is an n-type semiconductor thin layer, such as a bismuth-doped Mg8PbSi3 layer.

[0033] The order of the layers can be changed; an n-type layer can be located between the substrate and the p-type layer, thereby forming a junction with opposite polarities. Furthermore, substrate 21 can be omitted. The stack of layers 21-24-28 can be obtained by any suitable method, such as by epitaxy, cathode sputtering, evaporation, sublimation, pulsed laser ablation, chemical bath, or by printing techniques. Fabrication starting from semiconductor wafers obtained by methods such as VGF or VBS is also possible and included within the scope of this invention. Monocrystalline cells are preferred, but the invention can also have polycrystalline embodiments.

[0034] The photovoltaic cell 20 also includes a conductive structure for collecting photovoltaic current. Figure 1 In the example, two transparent electrodes 32 and 34 with this function can be seen, but other structures and solutions are also possible, such as metallized finger electrodes or grids. Solar cells may also include other support and protective layers, such as polymer layers, not shown in the figure.

[0035] Figure 1 This drawing is not to scale and is provided as an example only. In actual devices, the layers may have different thicknesses than each other, and the ratio between the vertical and horizontal dimensions may differ significantly from that shown.

[0036] Figure 2 illustrates a solar energy device according to the present invention, which combines multiple solar cells of the present invention and is optimized for the electrical energy conversion of sunlight 40. The device comprises multiple cells 20 interconnected by wires 35. In the illustrated embodiment, the solar cells are connected in parallel, but other connection topologies also exist. The device has two terminals 38 for connecting to a power source, or more preferably to an electronic management circuit configured to balance the cells and operate them near their point of optimal charge.

[0037] Figure 3 illustrates an optimized device for converting infrared light 50 emitted from the Earth's surface. It is advantageous to arrange multiple batteries in series so that the infrared light can pass through them sequentially. Multiple battery packs 27 can be seen stacked in this manner in the figure. Each battery pack 27 can contain a large number of batteries 20—the figure shows three batteries per pack, but this is just an example. Within each battery pack, the batteries can be connected in series to increase the generated voltage.

[0038] Alternatively, a combined device could be considered, integrating the solar panel shown in Figure 2 with the device shown in Figure 3 for nighttime optimization. During the day, the upper solar cells efficiently capture sunlight; at night, all photovoltaic cells, particularly photovoltaic array 27, convert infrared radiation from the ground into energy.

[0039] The performance of the photovoltaic device of the present invention can be further improved by adding a conversion layer. This conversion layer comprises one or more materials capable of converting incident electromagnetic radiation with a wavelength of approximately 1 μm into infrared radiation with a longer wavelength, for example, λ ≥ 8 μm. As previously mentioned, the junction of the present invention is particularly suitable for converting radiation in this region of the electromagnetic spectrum. Currently, there are various photofluorescent materials that can absorb visible or ultraviolet light and emit infrared photons through fluorescence. The conversion layer can be a thin layer prepared using appropriate deposition techniques or a layer containing a suspension of infrared phosphor particles. This includes nonlinear materials, such as UV2IR1 phosphor produced by Maxmax LLC, USA.

[0040] Reference numbers used in the attached figures 20 photovoltaic cells 21 Base 24 p-type semiconductor layer 27 Photovoltaic cell modules 28 n-type semiconductor layer 32 Lower electrode 34 Upper electrode 35 conductor 38 electrical terminals 40 Solar radiation 41 Solar Radiation Spectrum 45 Visible light band 50 Earth Radiation 51 Earth's Radiation Spectrum

Claims

1. A photovoltaic cell, comprising: A stack of semiconductor or half-metal layers forming a junction, and at least two junctions for collecting charge carriers generated by electromagnetic radiation conversion in the junction, characterized in that at least one semiconductor layer comprises a material selected from: Mg( Z Pb(x)Si(y), Mg8PbSi3, Mg8Pb( X Si(y), Mg2Si, Mg 64 Si3Pb,Mg 64 Si 30 Pb (2) .

2. The photovoltaic cell according to the preceding claim, wherein at least one semiconductor layer or half-metal layer is composed of Mg8PbSi3, or is composed of a solid solution of Mg8PbSi3 and other substances.

3. The photovoltaic cell according to any one of the preceding claims, wherein the junction comprises a stack of n-type and p-type layers, wherein the n-type layer is doped with electron donor atoms such as Bi, and the p-type layer is doped with electron acceptor atoms such as Na.

4. The photovoltaic cell according to any one of the preceding claims, wherein the semiconductor layer or half-metal layer is composed of a semiconductor or half-metal with a direct bandgap or indirect bandgap and a bandgap of less than 0.2 eV, preferably less than or equal to 0.14 eV.

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

6. The photovoltaic cell according to any one of the preceding claims, wherein at least one contact comprises a transparent, translucent or opaque conductive layer.

7. The photovoltaic cell according to any one of the preceding claims, wherein the semiconductor layer or half-metal layer is a single crystal.

8. A photovoltaic device comprising a plurality of photovoltaic cells according to any one of the preceding claims.

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

10. The photovoltaic device according to the preceding claim further includes one or more photovoltaic cells for solar photovoltaic conversion above the photovoltaic cell array for nighttime operation.

11. The photovoltaic device according to any one of the preceding claims has a conversion layer comprising one or more materials capable of converting incident electromagnetic radiation into infrared radiation.

12. A method for manufacturing a photovoltaic cell according to any one of claims 1 to 7, comprising manufacturing a cell containing Mg2Pb(x)Si(1-x), Mg8PbSi3, or Mg by a VGF, VB, or VBS method. (z) Pb( X Si(y), Mg2Si, Mg 64 Si3Pb,Mg 64 Si 30 Pb (2) The semiconductor wafer or half-metal wafer, and the semiconductor layer or half-metal layer formed in the wafer to form a junction.

Images