Technical Field

The present invention relates to a composite coating material, particularly for solar cells.

Background Art

In the energy sector, the use of photovoltaic systems is well known, installed on the roofs of buildings or directly on the ground at suitable areas exposed to sunlight.

Photovoltaic systems consist of a plurality of photovoltaic panels connected to each other and composed of photovoltaic modules, which in turn consist of solar cells, capable of converting solar energy into electrical energy.

As is well known, exposure to sunlight and weathering results in a decrease in the efficiency of solar cells as well as a rapid deterioration of the same. These effects are mainly caused by exposure to water/humidity, oxygen and other substances in the air.

The aforementioned drawbacks are particularly felt in relation to the emerging Perovskite structure solar cells (PSCs).

Perovskite structure solar cells are among the emerging technologies that have marked the greatest development in recent years, due to their potential high efficiency, low production cost and simple way of processing, making them potentially very attractive from an industrial point of view.

Perovskite structure solar cells represent the newest and most promising photovoltaic cell technology by showing a solar energy-to-electricity conversion efficiency rate of up to 25.6% per single junction on a laboratory scale and skimming 15-20% on a module scale. In comparison, silicon-based cells generally used in common photovoltaic panels are provided with an average conversion rate of between 15-20% and can reach a maximum of 26.7% on a laboratory scale (as estimated by the company Oxford PV, Oxford 0X5 1QU, UK and reported by the official NREL chart http s : //w w w . nrel . go v/p v/cell-efficiency .html) . However, the transition to industrial production of photovoltaic panels provided with perovskite structure solar cells has not yet been possible because such solar cells have significant degradation issues, reaching the point of losing as much as 80% of their initial efficiency in the first 1,000 hours of use (under accelerated ageing conditions).

To remedy at least part of the above drawbacks, the use of special coating materials for encapsulating solar cells is known.

The development of innovative encapsulants is critical to provide excellent durability for photovoltaic panels and to enable commercial- scale production of new perovskite solar cell-related technologies.

In fact, encapsulation has been shown to play a key role in preventing degradation and/or improving stability for a variety of PV system types (J. Phys. Energy 2 (2020) 031002).

When properly designed, encapsulation films or coatings can serve as barrier layers by limiting the diffusion of oxygen and humidity, preventing the penetration of UV radiation, reducing sensitivity to strong thermal fluctuations, and also inhibiting the irreversible escape of volatile decomposition products that may have formed from the solar cell components, resulting in protection of the electrode and active layer interface.

Ideally, encapsulating materials should be easy to work, have excellent chemical inertness and high barrier properties for oxygen and humidity. In addition, encapsulants should show high total light transmission (>90% of incident light) and excellent resistance to UV degradation and thermal oxidation. Finally, other important characteristics are good mechanical strength, excellent adhesion to the solar cell to minimize the risk of delamination, thermal expansion coefficients close to those of PSC components to avoid mechanical damage during stability testing and high flexibility to accommodate angular stress changes during bending or related to temperature cycling trend (J. Phys. Energy 2 (2020) 031002).

To date, various encapsulation methods have been developed for solar cells. One of the most popular approaches is based on glass-to-glass encapsulation, in which the solar cell is sandwiched between two sheets of glass by the use of thermo-set sealants (e.g., ethylene vinyl acetate (EVA), Surlyn ionomer, butyl rubber and polyisobutylene (PIB)) or UV-cured sealants (e.g., E132 resin, epoxy resin, epoxy glue). In addition, appropriate sealants (mainly butyl rubber and PIB or UV-cured epoxy adhesives) should be applied to the edges of the solar cell to prevent, or at least delay, humidity and oxygen from entering from the side perimeter, thus prolonging the life of the solar cells. This technique is very cost-effective, relatively simple and extremely efficient because glass has the best water- and oxygen-proof properties as a transparent material, but it is, however, incompatible with flexible applications, the latter being a market that has grown significantly in recent years (Tong G, Jia Z and Chang J 2018 IEEE Int Symp Circuits Syst.1-5).

Recently, some alternative methods have been developed in which glass is replaced by flexible films. Among these, an encapsulation approach using ultrathin flexible glass sheets has been offered. This process usually requires high temperatures (-140 °C) to cross-link the encapsulant, which may be detrimental to the solar cell by inducing thermal degradation of the active material.

Other more popular encapsulation approaches for flexible applications make use of polymer laminates and polymer tapes coated with thin-film barrier. These encapsulation strategies are noticeable for their versatility in terms of choice of the polymeric materials (polymethyl methacrylate (PMMA) (Sol. Energy 139 426-32), polyethylene terephthalate (PET) (J. Phys. D: Appl. Phys. 50 033001), polytetrafluoroethylene (PTFE) (Appl. Mater. Interfaces 7 17330-6), polycarbonate (PC) (Sol. Energy 139 426-32), polydimethylsiloxane (PDMS) (J. Mater. Chem. A 4 10700-9), ethylene vinyl alcohol copolymer (EVOH) (J. Phys. D: Appl. Phys. 50 033001)), as well as flexible hybrid multi-layers also based on polymer.

Other coatings of known type involve combining a polymer matrix with graphene oxide particles. Such coatings can be easily applied to various substrates (Nano Energy 18 118- 25, J. Energy Chem. 27 673-89), such as e.g. by lamination or applied by means of roll-to-roll lamination to obtain flexible solar cells. However, no study to date has reported an easy, scalable and effective process to encapsulate solar cells that maintain their efficiency for a long time. Moreover, the fundamental incorporation with graphene oxide causes unavoidable problems of transparency and uniformity.

Among deposition techniques, currently, the most widely used technology is thin-film encapsulation (TFE). This technology consists in the direct deposition of an individual ultrathin flexible protective layer (e.g., A12O3, SiOx, SiN, TiO2, Zn2SnO4, Parylene-C, ultrathin plasma polymer film or organic- inorganic hybrid polymer layers or a multi-layer stack, consisting of multiple pairs of organic and inorganic layers called a dyad (Chem. Mater. 27 5122-30 and Adv. Energy Mater. 8 1-8)), involving an application of the vacuum coating material such as chemical or physical vapor deposition, plasma-enhanced chemical vapor deposition, atomic layer deposition and other vacuum coating techniques. However, all the aforementioned techniques are expensive because they require high-cost vacuum-based appliances and processes, as well as a detailed understanding of the interaction between the deposition process, the barrier layer material and the device structure. For this reason, its large-scale applicability is still hindered.

To sum up, despite the large size of the solar encapsulation market, current technologies are outdated compared to modern requirements that have emerged with the development of new photovoltaic technologies having numerous drawbacks.

One of these is undoubtedly related to the difficulty in making coating materials provided with ideal mechanical and chemical properties. The common coating materials and related coatings obtained are very rigid (fragile, non-foldable), heavy, with low thermal conductivity (i.e., not ideal for large-sized devices).

In addition, known coating materials provide for complex manufacturing processes involving high energy consumption, high-temperature work (e.g., high lamination temperature for hot-melting adhesives) that increases the manufacturing cost of the final photovoltaic panel and are not suitable for sensitive materials such as perovskite structure solar cells.

In addition, known coating materials provide for expensive and environmentally high-impact manufacturing processes and do not allow for large-scale application.

Description of the Invention

The main aim of the present invention is to devise a composite coating material, particularly for solar cells, which allows for highly shielding coating against humidity, oxygen and other substances potentially harmful to the solar cell.

Another object of the present invention is to devise a composite coating material, particularly for solar cells, which allows for highly transparent, lightweight, homogeneous and flexible coatings that can be adapted to different surfaces such as flexible and foldable ones.

A further object of the present invention is to devise a composite coating material, particularly for solar cells, which is obtainable by means of simple, low-cost processes which have a high potential for scalability.

Still one object of the present invention is to devise a composite coating material, particularly for solar cells, which allows reducing the overall size of the final photovoltaic panel and thus the related cost.

Another object of the present invention is to devise a composite coating material, particularly for solar cells, which can overcome the aforementioned drawbacks of the prior art within the framework of a simple, rational, easy and effective to use as well as affordable solution.

The aforementioned objects are achieved by this composite coating material, particularly for solar cells, having the characteristics of claim 1.

Brief Description of the Drawings

Other characteristics and advantages of the present invention will become more apparent from the description of a preferred, but not exclusive, embodiment of a composite coating material, particularly for solar cells, in combination with the attached tables of drawings in which:

Figures 1 through 6 show experimental data collected for a composite coating material according to the invention in accordance with a first embodiment; Figure 7 shows experimental data collected for a composite coating material according to the invention in accordance with a second embodiment;

Figure 8 shows experimental data collected for a composite coating material according to the invention in accordance with a third embodiment.

Embodiments of the Invention

The composite coating material according to the invention is intended for use in the solar cell coating.

In particular, reference will be made to perovskite structure solar cells in the remainder of this disclosure.

Perovskite structure solar cells represent the newest and most promising photovoltaic cell technology by showing a solar energy-to-electricity conversion efficiency rate of up to 25.6% per single junction on a laboratory scale and skimming 15-20% on a module scale. In comparison, the usual silicon-based solar cells used in common photovoltaic panels are provided with an average conversion rate of between 15-20% and can reach a maximum of 26.7%. However, perovskite structures are subject to rapid degradation in the presence of water/humidity, UV radiation, oxygen, high temperatures, etc., and therefore need to be protected.

It cannot, however, be ruled out that the present composite coating material could be used for solar cells of different types. Likewise, it cannot be ruled out that the present composite coating material could be used in different perovskite-based optoelectronic applications, e.g., in light emitting devices such as the LEDs.

The composite coating material comprises at least one polymer matrix and at least one metal-organic frame (MOF) embedded in the polymer matrix itself.

As known to the industry technician, metal-organic frames, also known as nanosponges, are crystalline materials consisting of metal clusters or ions coordinated with rigid organic ligands so as to form three-dimensional structures with very high porosity. The void space within the material can be up to 90% of the material volume, with very high inner surface areas, even over 6000 m ² /g. Within the empty spaces, the MOF structure is able to contain and retain small molecules, such as precisely, water, oxygen, carbon dioxide, etc.

MOFs can be synthesized starting from a wide variety of organic and inorganic components, resulting in different geometries depending on the specific precursors and the given application.

The metal-organic frame is in the form of nanoparticles dispersed in the polymer matrix.

The polymer matrix serves as a support for the metal-organic frame and allows the composite coating material to be laid down and applied to the item of interest, in this specific case, the solar cell.

In particular, the polymer matrix is provided with high resistance to UV rays and high transparency.

Usefully, the polymer matrix is selected from polymethyl methacrylate (PMMA) and polystyrene (PS).

It cannot be ruled out that the polymer matrix may be of a different type, e.g., polyvinylidene fluoride, polycarbonate, ethylene vinyl alcohol, polyethylene terephthalate, polytetraflluoroethylene, polydimethylsiloxane or similar polymeric materials provided with similar UV-ray resistance and transparency.

Usefully, the metal-organic frame comprises: at least one ion of a metal selected from the list comprising: Zn, Fe, Cr; and at least one organic ligand selected from the list comprising: 2- methylimidazole, trimesic acid and terephthalic acid.

It cannot, however, be ruled out that the metal-organic frame may comprise ions of different metals such as, e.g., Cu, Mg, Al, possibly combined with each other.

It cannot also be ruled out that the metal-organic frame may comprise different organic ligands such as, e.g., amino acids, aromatic amines, azole derivatives, polycarboxylic acids and phosphonate derivatives.

The metal ion and the organic ligand form coordination bonds with each other, leading to the formation of the three-dimensional porous structure.

In accordance with a first embodiment of the composite coating material according to the invention, the ion of a metal is Zn ²⁺ .

Again with reference to this embodiment, the organic ligand is 2- methy limidazole .

Advantageously, the metal-organic frame is ZIF-8.

ZIF-8 is a metal-organic frame in which zinc cations in the presence of imidazole anions assemble into a highly porous three-dimensional structure.

In accordance with the first embodiment, the polymer matrix is PMMA.

The resulting composite coating material combines the properties of PMMA, a transparent, robust material which does not alter the absorption properties of the active perovskite layer and, at the same time, can offer UV protection and improve its hydrophobic behavior, with ZIF-8 lattices that can repel water and increase the water barrier character of the coating.

This solution not only provides for a humidity barrier, but also preserves the chemical structure of the active material of the solar cell and the functionality of the photovoltaic device.

The composite coating material in accordance with the first embodiment allows for high visible light transmission and UV-ray resistance, excellent homogeneity, consequent mechanical stability and ease of manufacture through solution manufacturing processes at low temperatures and reduced costs.

In particular, the composite coating material enables vacuum lamination at temperatures below 100°C, thus avoiding possible degradation of the solar cell. Deposition of the composite coating material can be carried out by means of techniques such as spin coating, drop casting, spray, etc., and can be easily implemented in any solution manufacturing process.

The resulting coating is in the form of a thin film.

Specifically, the coating has a thickness on the order of hundreds of nanometers (using the spin coating technique) to even on the order of hundreds of micrometers (using a spray deposition technique).

Example 1.

Synthesis of the composite coating material:

Two methanolic solutions of precursors of metal ion and organic ligand are prepared in two separate Erlenmeyer flasks.

A first solution A is prepared by mixing 1.467g of Zn/NCh -bFhO in lOOmL of methanol, resulting in a solution at a concentration of 49 mM.

A second solution B is prepared by mixing 3.245 g of 2-methylimidazole in 100 mL of methanol, resulting in a solution at a concentration of 395 mM.

Each solution is mixed at room temperature by means of magnetic stirring until the components are completely dissolved.

Solution A is then quickly poured into solution B and the resulting mixture is kept stirring vigorously at room temperature until cloudy (approx. 5 minutes).

The suspension is quickly divided into four different 50-mL Falcon vials and centrifuged at 9000 rpm for 30 minutes.

The supernatant is discarded and the solid precipitate (ZIF-8), in pellet form, is washed twice with fresh methanol (60 ml and 30 ml) and centrifuged each time at 9000 rpm for 60 min.

After the last centrifugation, the product in pellet form is washed with 30 mL of 1 -butanol, centrifuged at 9000 rpm and dispersed in 1 -butanol to obtain a concentration of 30 mg/mL (approx. 5-10 mL of Lbutanol).

The concentration of the last sample is determined by letting a small volume of the suspension (0.3 ml) drop into a vial and by calculating the residual mass upon evaporation of the solvent.

The mixture of ZIF-8 in 1 -butanol is then mixed 1:1 to a solution of PMMA in toluene at a concentration of 10 mg/mE, resulting in a final mixture of 5 mg/mE PMMA and 15 mg/mL ZIF -8 in 1-butanol/toluene 1:1.

Coating preparation:

The coating is prepared in thin film form by means of spin coating (Polos Spinl05i spin coater) of the composite coating material solution obtained.

Similarly, individual solutions of metal-organic frame (ZIF-8 10 mg/mE in 1- butanol) and polymer matrix (PMMA 10 mg/mE in toluene) are deposited as a reference.

For this purpose, a glass support is mounted on the spin coater, vacuum is applied to ensure stable rotation and 100 pF of the desired solution is deposited on the surface of the glass support.

Finally, the rotation of the spin coater is operated in order to distribute the solution evenly.

The films are deposited with a spin speed of 4000 rpm for 30 seconds and an acceleration of 200 rpm/s.

Then, the coating was analyzed and its physic-chemical parameters were compared with the relevant reference films.

Analysis of results:

Figure 1 shows the diffractograms obtained by means of powder X-ray diffractometry (PXRD) analysis.

The diffractogram shows the intensity values of the diffracted radiation as a function of the diffraction angle 20, for PMMA and PMMA/ZIF-8 thin films (offset). To confirm the presence of the metal-organic frame ZIF-8, the calculated peaks for that material (ZIF-8 calculated) are also shown.

In addition to showing the actual presence of ZIF-8, this comparison also shows the high crystalline quality of the synthesized metal-organic frame and how it is preserved as a result of incorporation into the polymer matrix.

Figure 2 shows the UV-Vis absorption data of the thin films PMMA/ZIF-8 compared with the absorption of a typical active perovskite material.

The comparison shows that there is no absorption by the coating, indicating total transparency of the coating itself.

Images shot by the Scanning Electron Microscopy (SEM) technique are shown in Figure 3.

Analysis of the PMMA-only sample (left) shows at both magnifications investigated (5. OK X and 50. OK X) substantial homogeneity of the PMMA-only coating, unless small holes on the order of nanometers.

Analysis of the PMMA/ZIF-8 sample (right) shows, at a magnification of 5. OK X, but even more so at 50K X, the presence of a homogeneous distribution of ZIF-8 nanoparticles and their possible aggregation between the ZIF-8 nanoparticles and the polymer matrix that depends on the speed of spin coating deposition. The measurements of the contact angle performed on PMMA-only and PMMA/ZIF-8 coating samples are shown in Figure 4.

The measurements show a strong increase in the hydrophobic behavior of the coating containing the metal-organic frame (contact angle of 116.2°) compared to the polymer matrix alone (contact angle of 61.5°). This behavior is further evident from observing the image of a water droplet deposited above the coatings and its trend over time.

The effectiveness of the coating obtained in accordance with the first embodiment was demonstrated through J-V measurements carried out on partly encapsulated high-efficiency Perovskitic solar cells by depositing a thin-film coating of the PMMA/ZIF-8 matrix and comparing the results with those obtained by a reference cell coated with only PMMA and by a third reference cell without any specific coating.

Specifically, the samples were stored in a humidity-controlled climate chamber (50% relative humidity) and the photovoltaic properties were measured periodically to study the protective behavior against humidity of the thin-film coating.

A second similar experiment is carried out by carrying out J-V measurements on a perovskite solar cell partly coated with only ZIF-8 and comparing it again with a solar cell without any specific coating.

The climatic chamber with 50% relative humidity is prepared inside a closed plastic box. For a 50% relative humidity environment, a magnesium nitrate saturated aqueous solution is prepared inside a beaker and placed inside the box. The box is left overnight and the internal humidity is then checked several times over the following days. Once stable humidity measurements are obtained, the box is used specifically without further action.

The photovoltaic parameters of Power Conversion Efficiency (PCE), Fill Factor (FF), short-circuit current density (Jsc) and open-circuit Voltage (Voc) of the samples are studied by carrying out JV measurements on encapsulated PSCs of 0.12 cm ² (active area) under illumination 1 SUN, AM 1.5 G solar spectrum (LED lamp, Wavelab Sinus-70). Voltage is applied with a power meter (Sourcemeter Keithley 2410).

Perovskitic n-i-p solar cells ((Cs0.05(MA0.15FA0.85)0.95Pb(I0.85Br0.15)3) (FTO/TiCh /perovskite/Spiro-OMeTAD/Au) are tested for analysis.

Before measurements are taken, the vertical position of the platform is adjusted by calibration against a reference cell (Centronics LCE-50 provided with a KG- 3 filter) to precisely locate its specifications for the current value under illumination.

Figure 5 shows the photovoltaic parameter analyses carried out on samples of Perovskitic solar cell which is uncoated, coated with only PMMA and coated with PMMA/ZIF-8, in climate chambers at 50% RH humidity.

Each deposition over the PSCs is carried out on the glass side by spin coating (Spin Coater Polos Spinl05i) deposition at 4000 rpm of a PMMA solution (lOmg/mL in toluene) and of a PMMA/ZIF-8 solution (5 mg/mL in toluene + 10 mg/mL in 1 -butanol).

The photovoltaic parameters of each sample are measured before and after deposition. Subsequently, the samples are transferred to the 50% RH controlled humidity chamber.

The photovoltaic parameters of each sample are monitored after 3 days, 7 days, 14 days, 21 days and 28 days.

The acquired data were collected, normalized to the non-encapsulated PSC data and plotted in the graphs in Figure 5.

It is evident from the graphs that the PMMA/ZIF-8 coating offers incredible protection in conditions of high relative humidity (HR=50%).

In particular, the observation of the trend of the PCE curve shows that the PMMA/ZIF-8 coated solar cell undergoes a decay of less than 20% in 28 days. In contrast, the non-encapsulated solar cell (Ref) shows a drastic decrease in PCE values to a complete loss of operation in the investigated time window.

In PMMA-only encapsulated solar cells, although less pronounced, a clear degradation can be observed, particularly in Jsc parameters, thus reducing the overall performance of the photovoltaic device.

Measurements taken on perovskite solar cells coated with only ZIF-8 are shown in Figure 6.

Deposition over the PSC is carried out on the glass side by spin coating deposition (Spin Coater Polos Spinl05i) at 2,500 rpm of a solution of ZIF-8 (10 mg/mL in 1 -butanol).

The photovoltaic parameters of each sample are measured before and after deposition. Next, the samples are transferred to the 50% RH controlled humidity chamber.

The photovoltaic parameters of each sample are monitored after 1 day, 3 days and 7 days.

The acquired data were collected, normalized to the data of the nonencapsulated reference perovskite cell and plotted in the graphs in Figure 6.

As evident from observing the graphs, the coating with only ZIF-8 is not enough to protect perovskite solar cells from high humidity, thus confirming the synergistic effect of the combination of metal-organic frame and polymer matrix in order to achieve effective protection.

In accordance with a second embodiment of the composite coating material according to the invention, the metal-organic frame is ZIF-8 and the polymer matrix is PS.

The metal-organic frame is prepared similarly to the above and later mixed with a PS solution.

The resulting mixture is deposited by spin coating and analyzed against reference samples.

The measurements of the contact angle showed an increase in hydrophobicity in the samples coated with PS/ZIF-8, bringing the contact angle to a value of about 90°.

Figure 7 shows the comparison graph between the values of contact angle of a coating with only PS and a coating with PS/ZIF-8, obtained from a solution at a concentration of 1 mg/mL.

In accordance with a third embodiment of the composite coating material according to the invention, the ion of a metal is Fe ³⁺ .

Again with reference to this embodiment solution, the organic ligand is trimesic acid.

Advantageously, the metal-organic frame is MIL- 100.

MIL- 100 is synthesized in nanoparticle form by means of a solution process that involves mixing a reaction mixture between metal ions and organic ligand at 95°C for 12h. The precipitate is centrifuged and washed several times with water and ethanol and finally dried.

Alternatively, the preparation of the metal-organic frame can be carried out by micro wave heating. In that case, the reaction mixture between metal ions and organic ligand is placed in a microwave reactor and stirred at 130°C for 6 min. The precipitate is centrifuged and washed several times with water and ethanol. The formation of the desired product is confirmed in both cases by PXRD analysis in comparison with experimental data reported in literature.

Again, the polymer matrix is PMMA.

Thin-film coatings are prepared from solutions at concentrations of 1 mg/mL and 10 mg/mL deposited by spin coating.

Similarly to what has been described for the previous embodiments, the coatings are then subjected to measurements of the contact angle on samples obtained from the solutions at both concentrations.

Figure 8 shows images of a water droplet on PMMA/MIL-100 samples obtained by deposition of the solutions at the two indicated concentrations subjected to spin coating at a speed of 2,500rpm for 10 seconds, 60 seconds, 180 seconds and 300 seconds.

The sample obtained from the solution at a concentration of lOmg/mL gives better water resistance and hydrophobic character. This result is attributable to a higher homogeneity of the thin film obtained at this concentration.

In accordance with a fourth embodiment of the composite coating material according to the invention, the ion of a metal is Cr ⁶⁺ .

Still with reference to this embodiment solution, the organic ligand is terephthalic acid.

Advantageously, the metal-organic frame is MIL- 101.

MIL- 101 is synthesized in nanoparticle form by means of a microwave process that involves stirring a reaction mixture between metal ions and organic ligand in a microwave reactor at 200°C for 5 min. The nanoparticle precipitate is centrifuged and washed several times with water and ethanol.

The formation of the desired product is confirmed in both cases by PXRD analysis in comparison with experimental data reported in literature.

Again, the polymer matrix is PMMA.

Thin-film coatings are prepared from solutions at concentrations of 1 mg/mL, 10 mg/mL and 30 mg/mL deposited by spin coating.

According to a further aspect, the present invention also relates to a photovoltaic panel.

The photovoltaic panel according to the invention comprises at least one solar cell and at least one coating according to one or more of the embodiments described above.

Usefully, the coating is placed on the outer surface of the solar cell.

Alternatively or in combination thereof, the coating is arranged between at least two layers of the solar cell. In other words, the coating can be applied to the solar cell so as to protect each individual layer of the solar cell.

It has in practice been ascertained that the described invention achieves the intended objects, and in particular the fact is emphasized that the composite coating material according to the invention allows for a highly- shielding coating against humidity, oxygen, carbon dioxide and other substances potentially harmful to the solar cell.

The present composite coating material makes it possible to synergistically combine the chelating properties of the metal-organic frame towards small molecules dissolved in air, with the transparency and mechanical strength properties of the polymer matrix.

Such a combination of materials makes it possible to achieve highly transparent, lightweight, homogeneous and flexible coatings that can be adapted to different surfaces such as flexible and foldable surfaces, thus also adapting to recent technologies related to flexible solar cells.

In addition, the present composite coating material is obtainable by means of simple, low-cost processes with a high potential for scalability. In fact, the metal-organic frame is synthesizable in the form of nanoparticles by means of a solution process involving the simple mixing of two solutions of precursors (metal ions and organic ligand) at room temperature and, subsequently, mixed with the polymer matrix.

Finally, the present composite coating material allows for practical deposition in the form of thin films, which enables the overall dimensions of the final photovoltaic panel to be reduced.