SOLAR CELL AND SOLAR CELLS MODULE

Abstract

A solar cell including at least a first layer made of a semiconductor material for absorbing photons from light radiation and releasing charge carriers, and at least one conductive layer, overlapping the first layer, adapted to allow the light radiation to enter into the solar cell towards the first layer and to collect the charge carriers released by the first layer, the solar cell where the conductive layer includes at least three overlapped layers, including a transparent intermediate metal layer, made of metal, and two transparent oxide layers, made of a conductive oxide, where the two oxide layers are an inner oxide layer and an outer oxide layer surrounding the transparent intermediate metal layer to provide a low resistance path for the electrical charges and to maximize the amount of light radiation entering the solar cell. The embodiments also include a solar cells module including said solar cell.

Claims

1-9. (canceled)

10. A solar cell, comprising: at least a first layer made of a semiconductor material for absorbing photons from light radiation and releasing charge carriers, and at least one conductive layer, overlapping said first layer, adapted to allow said light radiation to enter into said solar cell towards said first layer and to collect the charge carriers released by said first layer, said conductive layer comprising at least three overlapped layers: a transparent intermediate metal layer, made of metal, and two oxide layers, made of a transparent conductive oxide, wherein said two oxide layers are respectively an inner oxide layer and an outer oxide layer surrounding said transparent intermediate metal layer so as to provide a low resistance path for the electrical charges and to maximize the amount of light radiation entering the solar cell, wherein said transparent intermediate metal layer has a thickness between 5 nm and 10 nm, said inner oxide layer has a thickness substantially of 50 nm; and said outer oxide layer has a thickness between 50 nm and 60 nm.

11. The solar cell according to claim 10, wherein said transparent intermediate metal layer has a thickness between 5 nm and 7 nm, said inner oxide layer has a thickness substantially of 50 nm and said outer oxide layer has a thickness between 50 nm and 60 nm.

12. The solar cell according to claim 10, wherein said transparent intermediate metal layer has a thickness of 5 nm, said inner oxide layer has a thickness substantially of 50 nm and said outer oxide layer has a thickness substantially of 50 nm.

13. The solar cell according to claim 10, wherein at least one of said oxide layers is made of indium trioxide In.sub.2O.sub.3 doped with stannic oxide SnO.sub.2, or is made of aluminum doped zinc oxide.

14. The solar cell according to claim 10, wherein said transparent intermediate metal layer has the shape of a continuous layer.

15. The solar cell according to claim 10, wherein said transparent intermediate metal layer is arranged as a mesh network.

16. The solar cell according to claim 10, wherein the solar cell is a heterojunction bifacial solar cell comprising a front side and a rear side, said at least one first layer comprising a first layer made of crystalline silicon, a second layer and a third layer, said second and third layers being made of hydrogenated amorphous silicon, and overlapping said first layer, surrounding it, said second layer arranged in correspondence of said front side, said third layer arranged in correspondence of said rear side, said solar cell further comprising a fourth layer made of n-type doped hydrogenated amorphous silicon, overlapping said second layer, a fifth layer made of p-type doped hydrogenated amorphous silicon, overlapping said third layer, and two conductive layers of said at least one conductive layer, respectively overlapping said fourth layer and said fifth layer.

17. The solar cell according to claim 10, wherein the solar cell does not comprise a metallization grid or in that it comprises a metallization grid made only by parallel or substantially parallel conductors.

18. A solar cells module, comprising: a first solar cell and a second solar cell according to claim 10, and connectors for electrically connecting a conductive layer of said first solar cell to a conductive layer of said second solar cell, said connectors directly attached to said outer oxide layer of said conductive layer of said first solar cell and to said outer oxide layer of said conductive layer of said second solar cell; or to said parallel or substantially parallel conductors on said conductive layer of said first solar cell and/or on said conductive layer of said second solar cell.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The present invention will now be described, for illustrative but not limitative purposes, according to its preferred embodiment, with particular reference to the figures of the accompanying drawings, wherein:

[0058] FIG. 1 shows a sectional view of a multilayer structure of a known HJT cell in a back-emitter configuration;

[0059] FIG. 2 shows a sectional view of a multilayer structure of a solar cell according to the present invention;

[0060] FIG. 3A shows the front view of a known solar cell having a metallization grid;

[0061] FIG. 3B shows the front view of a solar cell according to the present invention;

[0062] FIG. 4A shows the front view of the solar cell of FIG. 3A inside a first solar cell module;

[0063] FIG. 4B shows the front view of the solar cell of FIG. 3B inside a second solar cell module;

[0064] FIG. 4C shows the front view of the solar cell of FIG. 3B inside a third solar cell module;

[0065] FIG. 5 shows the percentage of the reflectance of a monolithic layer of aluminum-doped zinc oxide having a thickness equal to 20 nm, as a function of an incident radiation wavelength;

[0066] FIG. 6 shows the percentage of the reflectance and of the absorbance of three different conductive layers of a solar cell, which is not according to the present invention, as a function of an incident radiation wavelength;

[0067] FIG. 7 shows the percentage of the reflectance of a first example of a solar cell, which is not according to the present invention;

[0068] FIG. 8 shows the percentage of the reflectance of a second and of a third example of a solar cell, which is not according to the present invention;

[0069] FIG. 9 shows the percentage of the reflectance of a fourth example of a solar cell, which is not according to the present invention; and

[0070] FIG. 10 shows the percentage of the reflectance of a fifth and a sixth example of a solar cell, which is according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0071] With reference to FIGS. 2, 3B, 4B, and 4C, a solar cell 1 according to the present invention has a conductive layer 2 on a front side 11 and, preferably also on a rear side 10.

[0072] Specifically, the solar cell 1 shown in FIG. 2 is a bifacial n-type HJT solar cell 1, however the conductive layer 2 may be applied also on other kind of solar cells, such as p-type wafer base HJT cells or any other kind of solar cells.

[0073] Solar cell 1 comprises a central layer 3, or first layer 3, made of c-Si, preferably c-Si(n) and two middle layers 4′, 4″ made of a-Si:H, surrounding said central layer 3, respectively a second layer 4′ arranged in correspondence of the front side 11 of the cell 1 and a third layer 4″ arranged in correspondence of the rear side 10 of the cell 1.

[0074] Furthermore, solar cell 1 comprises a fourth layer 5 made of a-Si:H(n+), in contact with said second layer 4′, and a fifth layer 6 made of a-Si:H(p+), in contact with said third layer 4″. Therefore, said fourth layer 5 is arranged in correspondence of the front side 11 of the solar cell 1 and said fifth layer 6 arranged in correspondence of the rear side 10 of the solar cell 1.

[0075] All the aforementioned layers made of silicon have the function of absorbing photons from light radiation and releasing charge carriers. In other embodiments, such layers may have a different structure and made with different type of semiconductors.

[0076] A conductive layer 2 is deposited over both said fourth and fifth layer 5, 6.

[0077] Such conductive layers 2 have the function of allowing the light radiation to enter into said solar cell (1) towards said layer 5, 4′, 3 or 6, 4″, 3 and to collect the charge carriers released by said layers 5, 4′, 3 or 6, 4″, 3 into electrical currents.

[0078] In particular, each conductive layer 2 comprises two oxide layers 21, 22 made of TCO, which can be ITO or AZO, or other suitable oxides, and a transparent intermediate metal layer 23, placed between said two oxide layers 21, 22.

[0079] The transparency of the intermediate metal layer 23 is achieved by minimizing its total thickness, which can be between 5 nm and 20 nm (nanometers), more preferably between 5 nm and 10 nm. Such thickness can be adapted to the required optical and electrical properties of the final device. In an embodiment, the thickness of the intermediate metal layer can be between 5 nm and 7 nm, preferably being equal to 5 nm.

[0080] In fact, the transparent intermediate metal layer 23 is mainly responsible of the resistivity of the structure and thickness below 5 nm would not always guarantee the growth of a continuous interconnected metal film.

[0081] Furthermore, with such thicknesses, the transparent intermediate metal layer 23 can advantageously achieve values of resistivity in the order of 10.sup.−6 Ωcm, when the film is a continuous film. For example, a resistivity below 10.sup.−5 Ωcm has been measured with an intermediate metal layer 23 made of silver having a thickness about 10 nm.

[0082] In addition, such a transparent intermediate metal layer 23 can also be structured, for example being organized in a grid or mesh network or in an ensemble of interconnected nanoparticles. In case of an intermediate metal layer 23 with a mesh network, the presence of apertures in the mesh itself allows a higher transparency of the cell 1, if compared to solar cells 1 having a homogeneous intermediate metal layer 23 with the same thickness. Furthermore, an intermediate metal layer 23 organized in a mesh network allows to achieve very low values of resistivity.

[0083] Finally, when the intermediate metal layer 23 is obtained by depositing metal interconnected nanoparticles or nanostructures (e.g. nanoparticles having dimensions between 2-5 nm), a very good electrical conductivity can be achieved, with an increased optical transparency due both to the lower density of the porous film and to a plasmonic effect of the nanostructures.

[0084] The transparent intermediate metal layer 23 can be made of silver Ag, copper Cu, gold Au, aluminum Al or other suitable metals.

[0085] As already mentioned, the conductive layer 2 can be made by depositing an inner oxide layer 21 over the fourth layer 5 and/or over the fifth layer 6.

[0086] Subsequently, after depositing said transparent intermediate metal layer 23 over said inner oxide layer 21, an outer oxide layer 22 can be deposited over said transparent intermediate metal layer 23.

[0087] The two oxide layers 21, 22 of each conductive layer 2 can be made with different TCOs.

[0088] In particular, the oxides and/or metals used for the conductive layer 2 arranged on the rear side 10 of the solar cell 1 may be different from the ones used on the front side 11 of the same solar cell 1, in order to optimize the electrical and optical properties of the solar cell 1 inside a solar cell module.

[0089] The properties of the conductive layer 2 can also be tuned by modifying the layers thicknesses, in order to find the optimal electrical and optical properties of solar cell 1.

[0090] The obtained conductive layer 2 can therefore be designed for assuming the combination of electrical and optical properties in such a way that neither pure TCO nor pure metal ultra-thin films can achieve.

[0091] In fact, both the pure metal case and the pure TCO case should be excluded, since in both cases the solar cell 1 would not behave in the desired way.

[0092] Specifically, the thickness of TCO deposited as inner and outer oxide layers 21 and 22 may vary between 1 nm and 200 nm, preferably between 1 nm and 80 nm.

[0093] The combination of the thicknesses of the oxide layers 21 and 22 is a key parameter for controlling the reflectivity of the OMO layer. In fact, being the OMO structure a multi-layered structure, the antireflective effect of this material in the spectral range of the solar light is directly determined by these parameters. Although several thicknesses can be used in the range between 1 nm-200 nm and in particular in the range between 1 nm-80 nm, preferred values have been found between 1 nm and 30 nm or 50 nm for the inner oxide layer 21, and between 30 nm-80 nm for the outer oxide layer 22.

[0094] In a preferred embodiment, said inner oxide layer 21 has a thickness substantially of 50 nm and said outer oxide layer 22 has a thickness between 30 nm and 60 nm, more preferably between 50 nm and 60 nm, even more preferably substantially equal to 50 nm.

[0095] Hereinafter, the term “substantially” referred to thicknesses greater than or equal to 30 nm will mean a variability of ±5 nm.

[0096] Such values ensure the appearance of a significantly extended and broad relative minimum with respect to the reflectance typical behavior defining an optimal reflectance condition for an incident radiation having wavelengths comprised between 500 nm and 700 nm, as shown in FIG. 10 and as better discussed below.

[0097] These values have also been selected in order to minimize the thickness of the layers interposed between the layers made of semiconductor 3, 4′, 4″, 5, 6 and the outer surface of the solar cell 1. At the same time, such thicknesses ensure the maximum protection of solar cell 1 in correspondence of its outer region, which is usually in touch with a final glass protecting, when the solar cell 1 is inside a solar cell module.

[0098] Furthermore, the OMO layer 2 selected thicknesses advantageously allow to increase the transparency of the solar cell 1 in the spectral range of the visible light and in the near infra-red.

[0099] Specifically, the solar cell 1 has been designed in order to increase the transmission of the red part of the solar spectrum at its rear side 10 of the bifacial solar cell 1, which receives the albedo of the light after reflection on the ground (poor in blue and rich in red wavelength).

[0100] Furthermore, the outer oxide layer 22 advantageously facilitates the connection a solar cell 1 within a solar cell module, being able to connect by means of interconnection ribbons 100, as shown in FIGS. 4B and 4C.

[0101] In fact, since the outermost layer is the outer oxide layer 22 made of TCO, the adhesion between the solar cell 1 and the ribbons 100 is advantageously improved. On the other hand, a direct connection between the transparent intermediate metal layer 23 and the ribbons 100, even if possible, would result in a worse adhesion, due to the possible discontinuities on said transparent intermediate metal layer and to its reduced thickness.

[0102] In addition, the configuration of said conductive layer 2 allows to avoid the use of a metal grid on the surface of the solar cell 1. Therefore, such configuration allows to advantageously maximize the amount of light which enters in the solar cell 1, while achieving the required conductibility in order to collect and transport charges up to the ribbons 100, without any further external metallization grid. However, few bus bars may be used to improve the collection of the charge carriers, as explained below.

[0103] The particular configuration of said conductive layer 2 results, in fact, in a distributed collection of charges on the whole exterior surface of the cell 1, leading to a better metallization of the solar cell 1 itself.

[0104] The combination of these features, allow to advantageously reduce the consumption of silver paste, reducing the costs related to the solar cell 1 manufacturing.

[0105] Moreover, also expensive tools, such as screen printers, are not required for producing said solar cell 1, furtherly reducing the costs for manufacturing it.

[0106] Furthermore, as mentioned above, the proposed conductive layer 2 allows to attach the ribbons 100 directly on the outer oxide layer 22, without any further intermediate contact.

[0107] Specifically, it is advantageously possible to glue said interconnection ribbons 100 directly on said outer oxide layer 22, without any electric loss. In fact, the mechanical strength of the contact between the ribbons 100 and the outer oxide layer 22 is higher than the mechanical strength between ribbons 100 and bus bars made of silver paste.

[0108] With reference to FIGS. 4B and 4C, two different metallization approaches can be adopted for connecting said solar cell 1 in a solar cell module.

[0109] In particular, a standard approach, as shown in FIG. 4B, uses a classic stringer with four bus bars and four ribbons 100 on top of said bus bars. On the other hand, a more distributed scheme using a multi-wire approach is shown in FIG. 4C, which may be applied on a solar cell 1 according to the present invention.

[0110] In both cases there is no need of alignment between ribbons and bus bars, since there is no metal grid on the solar cell 1.

[0111] Finally, the conductive layer 2 and the relative interconnection schemes are also advantageously independent from the orientation of the cell, allowing any kind of metallization scheme in a cell module.

[0112] In the following paragraphs, some measurements and experiments are described, which allowed to identify an optimal conductive layer 2 for a solar cell 1.

[0113] In FIG. 5 the percentage of the reflectance of a monolithic AZO layer is shown as a function of an incident radiation wavelength, wherein the monolithic AZO layer has a thickness equal to 20 nm. It is evident that a local maximum in reflectance is achieved for a wavelength of approximately 375 nm, while for greater wavelengths there is a plateau or an increasing monotonic behavior.

[0114] In FIG. 6 the behavior of three different conductive layers 2 is shown, in terms of reflectance and absorbance as a function of an incident radiation wavelength, wherein each of the three conductive layers 2 comprises a transparent intermediate metal layer 23 made of Ag and the two oxide layers 21, 22 made of AZO. Specifically, the layers 21, 23, 22 of a first AZO/Ag/AZO conductive layer 2 have thicknesses respectively equal to 10 nm/5 nm/10 nm; the layers 21, 23, 22 of a second AZO/Ag/AZO conductive layer 2 have thicknesses respectively equal to 10 nm/10 m/10 nm; and the layers 21, 23, 22 of a third AZO/Ag/AZO conductive layer 2 have thicknesses respectively equal to 10 nm/15 nm/10 nm.

[0115] It is evident that for each conductive layer 2, there is a local maximum in reflectance approximately at 375 nm (and consequently a local minimum in absorbance), and an increasing monotonic behavior for greater wavelengths, similarly to the behavior of the monolithic AZO layer shown in FIG. 6.

[0116] Such behavior in a solar cell is generally associated with poor performances. In fact, a higher reflectance means that less radiation passes through the conductive layer 2 and reaches the silicon layer 3, which in turn lowers the release of charge carriers by the photoelectric effect.

[0117] It is therefore desirable to have low reflectance for wavelengths comprised between 500 nm and 700 nm. Furthermore, it is desirable that the percentage of reflectance associated to wavelengths greater than 800 nm is between 30% and 40%, and that it increases with a relatively small slope.

[0118] FIGS. 7-10 show the percentage of reflectance as a function of the wavelength of an incident radiation for several solar cells 1 comprising a first layer 3 made of silicon and a conductive layer 2 overlapping said first layer 3, wherein each conductive layer 2 comprises a transparent intermediate metal layer 23 made of Ag and the two oxide layers 21, 22 made of AZO, having different thicknesses.

[0119] Specifically,

[0120] in a first solar cell 1, the reflectance thereof is shown in FIG. 7, the thicknesses of the layers 21, 23, 22 of the AZO/Ag/AZO conductive layer 2 are respectively 30 nm, 20 nm, and 30 nm;

[0121] in a second solar cell 1, the reflectance thereof is shown in FIG. 8, the thicknesses of the layers 21, 23, 22 of the AZO/Ag/AZO conductive layer 2 are respectively 50 nm, 15 nm, and 70 nm;

[0122] in a third solar cell 1, the reflectance thereof is also shown in FIG. 8, the thicknesses of the layers 21, 23, 22 of the AZO/Ag/AZO conductive layer 2 are respectively 50 nm, 20 nm, and 70 nm;

[0123] in a fourth solar cell 1, the reflectance thereof is shown in FIG. 9, the thicknesses of the layers 21, 23, 22 of the AZO/Ag/AZO conductive layer 2 are respectively 30 nm, 20 nm, and 50 nm;

[0124] in a fifth solar cell 1, the reflectance thereof is shown in FIG. 10, the thicknesses of the layers 21, 23, 22 of the AZO/Ag/AZO conductive layer 2 are respectively 50 nm, 5 nm, and 50 nm; and

[0125] in a sixth solar cell 1, the reflectance thereof is also shown in FIG. 10, the thicknesses of the layers 21, 23, 22 of the AZO/Ag/AZO conductive layer 2 are respectively 50 nm, 10 nm, and 50 nm.

[0126] It is evident that the reflectance of the solar cells 1 shown in FIGS. 7-9, which is dependent on the respective thicknesses of the AZO/Ag/AZO conductive layers 2, has a non-desirable and particularly efficient behavior.

[0127] Surprisingly, the reflectance of the solar cells 1 shown in FIG. 10 has a local minimum for incident radiations having a wavelength between 500 nm and 700 nm, i.e. a desirable behavior in terms of reflectance, and therefore a greater efficiency, was achieved in these solar cells 1. In this way, it is possible to significantly increase the incident radiation at the silicon first layer 3.

[0128] The present invention has been described for illustrative but not limitative purposes, according to its preferred embodiment, but it is to be understood that variations and/or modifications can be made by those skilled in the art without departing from the scope of the claims, as defined by the annexed claims.