TECHNICAL FIELD

The present disclosure relates in general to devices that exploit sunlight and more particularly to 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 of AZO deposit.

BACKGROUND

There are many solar applications aimed at the energy conversion of sunlight into heat. Among these applications there are solar panels for the production of hot water for domestic and / or industrial use (medium-low operating temperatures), concentrating solar panels for the production of electrical energy through carrier fluid and turbines (high operating temperatures) and hybrid photovoltaic / thermal systems for the domestic production of electricity and hot water (low operating temperatures).

In all these applications it is necessary to reduce heat losses as much as possible because they reduce system efficiency.

Typically, these devices have an upper glass plate which is crossed by the sun’s rays coming from above and which are captured by the solar panel.

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

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

There are some materials that intrinsically possess satisfactory optical properties 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 best performing and the most cited in literature is tin doped indium oxide (ITO). In some cases, multi-layer systems are also created to increase properties of the material.

However, it is known that ITO is an expensive material, destined to become less and less sustainable. The scarcity of its main container (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 is aluminum doped zinc oxide, also known by the English acronym AZO. It allows to obtain acceptable reflectance values of infrared radiation when deposited with thicknesses of a few hundred nanometers.

It would be desirable to have the possibility of making solar devices covered by an AZO layer 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 depositing the upper layer of AZO over an intermediate layer of a first material having a first thickness and defining a substantially crystalline upper surface, wherein the intermediate layer has a first thickness such as to be substantially transparent to visible light.

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

Preferably, the intermediate layer is of alumina, for example with a thickness from 5nm to 50nm, or with a thickness from 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 is a sectional view of a heat mirror multilayer structure applicable to a solar device according to this disclosure.

Figure 2 is a sectional view of a multilayer structure according to the present disclosure with an upper AZO layer on top of an intermediate layer deposited directly on an active layer of a solar energy conversion device.

Figure 3 is a comparative graph of the reflectance spectrum of a multilayer structure composed of an AZO layer deposited by ALD on a glass substrate, or on an intermediate 10 nm and 20 nm alumina layer, with a deposition time of 15 minutes.

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

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

Figure 6 is 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 EXEMPLARY EMBODIMENTS

A multilayer reflective structure applicable to an active layer of a solar device 1 is schematically illustrated in Figure 1. It is 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, AZO for short. The intermediate layer 2 is deposited so as to define a flat crystalline surface S on which the layer of zinc oxide doped with aluminum 3 is deposited.

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

The crystalline material used to make the intermediate layer 2 is chosen 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 conformably 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, the AZO layer 3 is deposited on this crystalline flat surface S, which can be deposited with any conforming deposition technique, for example with an Atomic Layer Deposition (ALD) technique or magnetron sputtering technique. Surprisingly, the inventors found that depositing an AZO layer on a crystalline surface improves the optical properties of the AZO 3 layer. Without being bound to a theory, the unexpected improvement of the properties of the AZO 3 layer could be due to the fact that in this way the AZO layer has a more crystalline structure. According to current techniques, the AZO layer is deposited directly on a glass substrate which, not having a crystalline deposition surface, causes the AZO layer to have a partially amorphous structure, which worsens its properties.

If, on the other hand, the AZO layer is deposited on a flat crystalline surface, the AZO layer also has a crystalline structure and this would explain the best 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 on the support, which may be a glass substrate 4 for encapsulating a solar device or an active layer 1 of the solar- device itself, an intermediate layer 2 is deposited first on the support. The material and thickness of the intermediate layer 2 are chosen so that the intermediate layer 2 is transparent to visible light. The minimum thickness of this intermediate layer will be 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), in general the part of the intermediate layer 2 in direct contact with the support will be polycrystalline, thus it must have a minimum thickness sufficient to form a flat surface S with better crystallinity on which the AZO layer is to be deposited.

The minimum thickness of the intermediate layer 2 will depend on the material used and will correspond to the thickness of the thinnest 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 the visible light, thus its maximum thickness will also depend on the used material and will be determined in such a way as not to substantially attenuate the incident visible light.

Tests carried out by the inventors on different materials have shown that alumina is a suitable material for making the intermediate layer 2 because already a layer of alumina of only 10 nm deposited on a glass substrate 4 has a flat crystalline surface S. Furthermore, such a thin alumina layer is practically transparent to visible light, so it does not reduce the characteristics of the solar device.

Preferably, an intermediate layer 2 of alumina with a thickness of 20 nm will be formed, in order to further improve its crystallinity and consequently to be sure that the AZO layer 3 which is deposited on the flat surface S have optimal reflectance properties.

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 lOnm 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 layers of AZO deposited with the same thicknesses directly in contact on a glass substrate. It is immediately noted that the reflectance R% in the infrared increases considerably for the same thickness of the AZO layer, when it is not deposited directly on the glass substrate but is deposited on the alumina substrate. From the graphs of figure 2 it clearly emerges that a layer of AZO deposited directly on a glass substrate with 15 nm of deposition has an unacceptable R% reflectance because it is lower than 50%, while the same AZO 3 layer has a practically double R% reflectance when deposited on an intermediate layer 2 of 10nm alumina and a reflectance R% higher than 80% when deposited on an intermediate layer 2 of 20nm alumina. Better R% reflectance values are obtained with a thicker AZO 3 layer deposited on the alumina intermediate layer 2, even if the differences are less marked than in the case in which the AZO layer is directly deposited on the glass substrate (figures 4 and 5).

In order to corroborate the theory according to which the characteristics of the flat surface of amorphous or crystalline material on which the AZO layer is deposited influence its R% reflectance properties, tests have been carried out on multilayer structures in which an AZO layer has been 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 related graphs represented in figure 6 it clearly emerges that the AZO layers deposited on silicon have better reflectance properties of the radiation with a wavelength from 2500 nm upwards compared to those deposited directly on the substrate.

Comparing the graphs of figures 3 to 6 it may be inferred that with the same thickness of the AZO layer the worst performances are obtained when it is deposited directly on a glass substrate than when it is deposited on a flat crystalline surface. Consequently, it is believed that other materials, other than alumina, may be used to constitute the intermediate layer 2 on which to deposit the AZO layer 3, provided that they are suitable for forming a fiat 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 that a flat surface S with these characteristics is formed, the material will be chosen so that an intermediate layer 2 of such thickness is not opaque to visible light.

According to one aspect, said upper layer of zinc oxide doped with aluminum 3 has a thickness comprised between lOOnm 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 AZO layer 3 deposited on the flat surface S has the desired crystallinity characteristics.

The multilayer structure so realized may be used to encapsulate a solar device (photovoltaic panel or heat collector), as shown for example in figure 1. It is particularly suitable for devices intended to work at relatively high temperatures, such as heat collectors with concentrated solar power.

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

One or more anti-reflective layers may be deposited on the AZO 3 layer, such as for example one or more layers of magnesium fluoride and / or one or more layers of alumina. The present invention has so far been described with reference to preferred embodiments. It is understood that there may be other embodiments which refer to the same inventive concept defined by the scope of the following claims.