IT202100002186A1 - SOLAR ENERGY CONVERSION DEVICE - Google Patents

Description

SOLAR ENERGY CONVERSION DEVICE

TECHNICAL FIELD

This disclosure refers generally to devices that take advantage of sunlight and more? particularly a solar energy conversion device having a top cover consisting of a layer of AZO (aluminum zinc oxide) deposited over an intermediate layer of material, such as alumina, which forms a substantially crystalline flat top surface for depositing the 'AZO.

BACKGROUND

There are many solar applications aimed at the energy conversion of sunlight into heat. Among the most used are solar panels for the production of hot water for domestic and/or industrial use (medium-low operating temperatures), concentrated solar power for the production of electricity using a vector fluid and turbines (high operating temperatures) and hybrid systems photovoltaic/thermal for domestic production of electricity and hot water (low operating temperatures).

In all these applications ? necessary to reduce the thermal dispersions as much as possible, why? they constitute losses in terms of system efficiency.

Typically, these devices have a top glass plate that is ? crossed by the sun's rays that come from above and are captured by the solar panel.

In many devices, the space between the collector and the glass plate above it is under vacuum in order to minimize conduction and convection losses. In these cases, however, radiative thermal losses remain which, especially for solar panels operating at high temperatures, can become considerable, significantly reducing the final efficiency of the device.

For this reason, so-called heat mirrors are used in many cases, i.e. layers of material placed on top of the glass plate that overlooks the solar panel and facing the panel itself, or directly grown/deposited on top of it. Heat mirrors exploit the different spectral range between sunlight and the heat emitted in the form of loss from the device. In particular, an efficient heat mirror must have a high optical transmittance for the spectral region occupied by sunlight (between about 250 and 2500 nm) and at the same time a high reflectance for the spectral region in which the solar device emits heat radiatively (typically in the medium and far infrared, the spectrum varies according to the working temperature).

Are there some materials that inherently possess properties? satisfactory optics for their use as heat mirrors. These materials are the so-called transparent conductive oxides (TCO), which are typically used in the form of thin films deposited on the encapsulation glass of the solar device. Among them the most performing and the pi? reported in the literature? doped indium tin oxide (ITO). In some cases are also made pi systems? layers to increase the properties? of the material.

However ? known as the ITO is an expensive material, destined to become less and less sustainable. The scarcity? of its main content (indium) and its massive use in other technologies, such as displays and certain types of solar cells, make it an impractical material in large-scale technologies.

A less expensive material? aluminum-doped zinc oxide, also known by its acronym AZO. It allows to obtain acceptable values of reflectance of infrared radiation when deposited with thicknesses of some hundreds of nanometers. Would it be desirable to have the possibility? to create solar devices covered by a layer of AZO with a thickness of around 100 nm and with improved performance in terms of reflectance.

SUMMARY

Excellent results have been obtained in solar energy conversion devices as defined in the attached claim 1. In particular, the aforementioned limits on the use of AZO as a reflective material with thicknesses around 100 nm are exceeded when the upper layer is deposited of AZO over an intermediate layer of a first material having a first thickness and defining a substantially crystalline top surface, wherein the intermediate layer has a first thickness to be substantially transparent to visible light.

The middle layer can be deposited for example by means of an Atomic Layer Deposition (ALD) technique on a glass substrate for the encapsulation of the solar energy conversion device or, particularly for photovoltaic panels, it can be deposited directly over an active layer of the device itself.

Preferably, the intermediate layer ? of alumina, for example with a thickness of 5nm to 50nm, or with a thickness of 10nm to 30nm, or preferably with a thickness of 20nm.

Further embodiments are defined in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 ? a sectional view of a multilayered ?heat mirror? applicable to a solar device according to this disclosure.

Figure 2 ? a sectional view of a multilayer structure according to the present disclosure with a top AZO layer over an intermediate layer deposited directly on an active layer of a solar energy conversion device. Figure 3 ? a comparative graph of the reflectance spectrum of a multilayer structure composed of an AZO layer deposited via ALD on a glass substrate, or on an intermediate layer of 10 nm and 20 nm alumina, with a deposition time of 15 minutes.

Figure 4 ? a comparative graph of the reflectance spectrum of a multilayer structure composed of an AZO layer deposited via ALD on a glass substrate, or on an intermediate layer of 10 nm and 20 nm alumina, with a deposition time of 30 minutes.

Figure 5 ? a comparative graph of the reflectance spectrum of a multilayer structure composed of an AZO layer deposited via ALD on a glass substrate, or on an intermediate layer of 10 nm and 20 nm alumina, with a deposition time of 45 minutes.

Figure 6 ? a comparative graph of the reflectance spectrum of a multilayer structure composed of an AZO layer deposited by magnetron sputtering on a glass substrate or on a monocrystalline silicon substrate with deposition times of 100, 150, 200, 250 seconds.

DESCRIPTION OF EXAMPLE EMBODIMENTS

A multilayer reflective structure applicable to an active layer of a solar device 1 ? schematically illustrated in figure 1. It ? substantially composed of an intermediate layer 2, deposited in a conformal manner on a support 4, and of a layer of zinc oxide doped with aluminum 3, more? briefly AZO. The middle layer 2 ? deposited so as to define a flat crystalline surface S on which the aluminum doped zinc oxide layer 3 is deposited.

In the embodiment shown in figure 1, the intermediate layer 2 has a first thickness and ? obtained by depositing material on the support 4 in a conformal manner to define the flat crystalline surface S on which the AZO layer 3 is deposited. The multilayer structure thus? obtained is arranged above the active layer 1 and at a distance from it, with the AZO oxide layer 3 oriented towards the active layer 1 and the support 4 facing towards solar radiation directed at the active layer 1.

The crystalline material used to make the intermediate layer 2 ? selected in such a way that the intermediate layer 2 having the first thickness is substantially transparent to visible light.

According to the embodiment shown in figure 2, the intermediate layer 2 is conformally deposited directly on top of an active layer 1 of the solar energy conversion device. Also in this embodiment, the intermediate layer 2 will define? a flat crystalline surface S.

In both embodiments exemplified in figures 1 and 2, above this flat crystalline surface S ? deposited the layer of AZO 3, which pu? be deposited with any compliant deposition technique, for example with an Atomic Layer Deposition (ALD) or magnetron sputtering technique. Surprisingly, the inventors have found that depositing an AZO layer on a crystalline surface improves the properties of the crystal. AZO layer optics 3.

Without committing ourselves to any theory, the unexpected improvement of the properties? of the layer of AZO 3 could be due to the fact that in this way the layer of AZO has a structure more? crystalline. According to current techniques, the AZO layer is deposited directly on a glass substrate which, not having a crystalline deposition surface, makes s? that the AZO layer has a partially amorphous structure, which worsens its properties.

If instead the AZO layer is deposited on a flat crystalline surface, the AZO layer also has a crystalline structure and this would explain the better characteristics obtained with the device of this disclosure. In practice, according to this theory, the structure of the AZO layer depends on the crystalline or amorphous nature of the flat surface of the support on which it is deposited.

According to this disclosure, instead of depositing the AZO layer directly onto the substrate, which can be an encapsulation glass substrate 4 of a solar device or an active layer 1 of the solar device itself, an intermediate layer 2 is first deposited on the support. The material and thickness of the intermediate layer 2 are selected so that the intermediate layer 2 is transparent to visible light. The minimum thickness of this intermediate layer sar? determined in such a way as to be substantially transparent to visible light and to form a flat crystalline surface S. Being deposited on a support (for example an encapsulation glass substrate 4 or an active layer 1 of a photovoltaic panel), the part of the layer intermediate 2 in direct contact with the support in general sar? polycrystalline, so it must have a minimum thickness sufficient to form a flat surface S with better crystallinity? on which to deposit the AZO layer.

The minimum thickness of the intermediate layer 2 will depend? from the material used and correspond? to the thickness of the pi? thin layer of this material which has a flat crystalline surface S even if deposited on a glass substrate 4. Furthermore, the intermediate layer 2 must be practically transparent to visible light, so that its maximum thickness will depend on the also from the material used and sar? determined so as not to substantially attenuate the incident visible light.

Tests carried out by the inventors on various materials have shown that alumina is a suitable material to make the intermediate layer 2 why? already a layer of alumina of only 10 nm deposited on a glass substrate 4 has a flat crystalline surface S. Furthermore, an alumina layer like this? subtle ? practically transparent to visible light, so it does not reduce the characteristics of the solar device.

Preferably, will it be realized? an intermediate layer 2 of alumina with a thickness of 20 nm, so as to further improve its crystallinity? and consequently be certain that the layer of AZO 3 which is deposited on the flat surface S has the best properties? of reflectance.

Figures 3 to 5 are test graphs for measuring the reflectance of multilayer structures according to the present disclosure composed of a glass substrate 4, a 10nm or 20nm thick alumina layer deposited by means of ALD on the glass substrate 4, and by a layer of AZO 3 deposited again by ALD on the flat surface S of the alumina layer with deposition times of 15, 30 and 45 minutes. The properties? of these multilayer structures have been compared with a classical structure formed by AZO layers deposited with the same thicknesses directly in contact on a glass substrate. It is immediately noted that the infrared reflectance R% increases considerably for the same? thickness of the AZO layer, when it is not? deposited directly on the glass substrate but ? deposited on the alumina substrate. From the graphs of figure 2 it clearly emerges that an AZO layer deposited directly on a glass substrate with a 15 nm deposition has an unacceptable R% reflectance because? less than 50%, while the same layer of AZO 3 has a practically double reflectance R% when deposited on an intermediate layer 2 of 10nm alumina and a reflectance R% greater than 80% when deposited on an intermediate layer 2 of 20nm alumina . Better R% reflectance values are obtained with a layer of AZO 3 pi? often deposited on the intermediate layer 2 of alumina, even if the differences are less marked than in the case in which the layer of AZO ? directly deposited on top of the glass substrate (figures 4 and 5).

To validate the theory according to which the characteristics of the flat surface of amorphous or crystalline material on which the AZO layer is deposited influence the properties? of reflectance R% of the same, tests have been carried out on multilayer structures in which a layer of AZO ? was deposited on a monocrystalline silicon or glass surface, by means of a magnetron sputtering technique and with deposition times of 100, 150, 200 and 250 seconds. From the relative graphs represented in figure 6 it clearly emerges that the layers of AZO deposited on silicon have better properties? of reflectance of the radiation with a wavelength from 2500 nm upwards compared to those deposited directly on the substrate.

By comparing the graphs of figures from 3 to 6, can one? conclude that on equal terms? thickness of the layer of AZO the worst performances are obtained when it? deposited directly on a glass substrate that does not when ? deposited on a flat crystalline surface. Consequently, it is believed that other materials, different from alumina, can be used to form the intermediate layer 2 on which the AZO layer 3 is deposited, provided that the AZO layer 3 is deposited. they are suitable to form a flat crystalline surface S when deposited on a glass substrate 4. Since it will be? necessary to deposit an intermediate layer 2 having at least a thickness such as to form a flat surface S with these characteristics, the material will be? chosen so that an intermediate layer 2 of this thickness is not opaque to visible light.

According to one aspect, said upper layer of aluminum-doped zinc oxide 3 has a thickness of between 100nm and 400nm, preferably 200nm. Tests carried out by the Applicant have shown that with such thicknesses, even if they are currently considered too small to obtain acceptable results, the layer of AZO 3 deposited on the flat surface S has the desired crystallinity characteristics.

The multilayer structure so? made can? be used to encapsulate a solar device (photovoltaic panel or heat collector), as for example shown in figure 1. It is particularly suitable for devices designed to work at relatively high temperatures, such as concentrating heat collectors.

For photovoltaic panels, which work at relatively low temperatures, is it possible? optionally deposit the intermediate layer 2, preferably of alumina, directly on the active layer 1 of the device avoiding the glass substrate 4, as for example shown in figure 2.

On the layer of AZO 3 can be deposited one or more? anti-reflective layers, such as one or more? layers of magnesium fluoride and/or one or more? layers of alumina.

The present invention ? heretofore described with reference to preferred embodiments. ? understood that there could be other embodiments referring to the same inventive concept defined by the scope of the following claims.

Claims (7)

1. Solar energy conversion device, including: at least one active layer (1) configured to capture radiant solar energy; a multilayer structure transparent to visible light, arranged above said active layer (1) so as to reflect towards the active layer (1) infrared radiation emitted by the active layer (1) itself and so as to let the visible light pass through the multilayer structure directed towards said underlying active layer (1), said multilayer structure comprising: - an intermediate layer (2) of a first material having a first thickness, said intermediate layer (2) being deposited either directly on said active layer (1) or on a substrate (4), said intermediate layer (2) defining a first flat crystalline surface (S) on a side opposite to a side in contact with the active layer (1) or with the substrate (4), - a layer of aluminum doped zinc oxide (3) having a second thickness and deposited on said first flat crystalline surface (S) of said first material; wherein said intermediate layer (2) has said first thickness such that the intermediate layer (2) ? substantially transparent to visible light; wherein said aluminum-doped zinc oxide upper layer (3) has a thickness of between 100nm and 400nm. 2. Device according to claim 1, wherein said substrate is a glass substrate (4) and in which said intermediate layer (2) of the first material ? deposited in a conforming manner on said glass substrate (4) and ? sandwiched between the glass substrate (4) and the aluminum doped zinc oxide layer (3). 3. Device according to claim 2, wherein said multilayer structure ? arranged above said active layer (1) and at a distance from it, with said aluminum-doped zinc oxide layer (3) oriented towards the active layer (1). 4. Device according to claim 1, wherein said intermediate layer (2) of the first material ? conformally deposited directly in contact over said active layer (1) of the solar energy conversion device and ? sandwiched between said aluminum-doped zinc oxide layer (3) and said active layer (1). 5. Device according to one of claims 1 to 4, wherein said intermediate layer (2) of the first material ? made of alumina and between 5 nanometers and 50 nanometers thick, preferably 20 nanometers. 6. Device according to one of the preceding claims, wherein said aluminum-doped zinc oxide layer (3)? covered by one or more layers of magnesium fluoride and/or alumina. 7. Device according to one of the preceding claims, wherein said intermediate layer (2) is deposited with an Atomic Layer Deposition (ALD) technique or with a magnetron sputtering technique.