Encapsulating material for perovskite -based solar cells

Abstract

The present invention relates to an encapsulating material for perovskite -based solar cells comprising a polymer matrix comprising polyisobutylene having a molecular weight comprised between 30000 and 800000 Da and a filler comprising two-dimensional flakes, wherein said two- dimensional flakes are made of a material selected from the group consisting of graphene, graphene oxides, reduced graphene oxide, fluorinated graphene, heteroatom-doped graphene, hexagonal boron nitride, metal chalcogenides, metal oxides, metal chalcogenide halides, metal halides, phosphotrichalcogenides, MXenes, metal carbides, metal nitrides, layered hydroxides, alkaline-earth metal silicides, alkaline-earth metal bromides, alkaline-earth metal germanides and alkaline-earth metal stannides, layered perovskites, phosphorene, silicene, antimonene, germanene, boronene, stanene, bismuthene and mixtures thereof. There is also provided a laminated element comprising at least one layer made with the encapsulating material and a solar cell comprising the encapsulating material.

Description

[0001] "ENCAPSULATING MATERIAL FOR PEROVSKITE -BASED SOLAR CELLS"

[0002] Cross-Reference to Related Applications

[0003] This Patent Application claims priority from Italian Patent Application No. 102023000018468 filed on September 8, 2023, the entire disclosure of which is incorporated herein by reference.

[0004] Technical Field

[0005] The present invention relates to an encapsulating material for perovskite-based solar cells and a laminated element comprising the same.

[0006] Background of the Invention

[0007] Perovskite solar cells (PSC) promise to revolutionise the photovoltaic ("photovoltaic" PV) industry thanks to power conversion efficiencies (PCE) of up to 25.8% in the single junction configuration and to 33.2% in the tandem configuration, as well as to their cost-effectiveness in terms of materials and production processes.

[0008] However, to achieve a Levelized Cost of Energy (LCoE) that can compete with that of the crystalline silicon (c-Si) solar cells that dominate the market, it is necessary to improve the long-term stability of the PSCs, especially when assembled into modules, where further malfunctions such as for example potential induced degradation and reverse polarization effects can occur. In general, the lifespan of a PSC is determined by both intrinsic (such as polymorphism, defects, lattice deformations, and ion migration) and extrinsic (such as humidity, oxygen, heat, UV rays) factors.

[0009] The main causes of degradation include structural transitions and phase segregation of the perovskite films or charge-transporting layers (CTLs) , often accompanied by morphological alterations. Such mechanisms are often triggered by ion migration and degassing of volatile molecular species at perovskite grain boundaries and material interfaces. These effects are commonly exacerbated in the presence of lattice defects and / or deformations in the perovskite, as well as of interfacial stresses deriving from misalignments of the lattices and thermal expansion coefficients (TECs) of cell materials.

[0010] Some of the factors that can lead to perovskite decomposition are humidity and oxygen that can react with the perovskite light absorbers. By forming hydrogen bonds, water generates deprotonated organic cations, which thus weaken the bond between the organic cation and the octahedron of the Pb halide. The proton can thus be transferred to the halide ions (e.g., I~) through the water molecules, producing volatile species (e.g., CH3NH2, HI, and Pbl2) . Meanwhile, the oxygen can diffuse into the perovskite by occupying the vacancies of the halides and, following photoexcitation of the perovskites , the formation of charged superoxides can occur . These processes induce acid-base reactions with the organic cations , which lead to the formation of volatile species .

[0011] In this context , temperature variation, elevated temperature , illumination, and reverse polari zation of the PSC aggravate the ef fects of the intrinsic degradation, as the light-sensitive perovskite absorbers are subj ect to photodissociation i f exposed to elevated temperatures .

[0012] Several strategies have been proposed to improve the intrinsic stability of the PSCs , including engineering composition and si ze of the perovskite ore , defect passivation, grain boundary modi fication, and material interface engineering .

[0013] Among the strategies used to improve stabil ity, encapsulation has been recogni zed as the key to reali zing PSCs and corresponding modules that last at least 20 years when used in the open field . However, encapsulation technologies known for the commercial photovoltaic sector are not applicable to the PSCs , as they require the application of temperatures beyond the thermal stability of the perovskites , as well as of other functional layers of the PSCs .

[0014] The main requirements that an encapsulation system should meet to be applicable to the PSCs are : 1 ) chemical inertness and chemical compatibility with the materials of the underlying cells (e.g., no release of degrading chemicals, such as acetic acid and methacrylic acid in the case of ethylene vinyl acetate -EVA- and Surlyn, respectively) ; 2) low water vapour transmission rate (WVTR) (<10-4g im2day-1) and low oxygen transmission rate (OTR) (<10-3cm3 m-2day-1atm-1) to prevent humidity and oxygen from entering, while limiting degassing of volatile species; 3) resistance to degradation processes (e.g. yellowing and release of degradants for PSC constituent materials) induced by UV radiation; 4) thermal stability up to 85°C to withstand thermal stresses (e.g. day / night thermal cycles) and processability at low temperature (<120°C) to ensure compatibility with the thermal stability of perovskite and common CTLs; 5) electrically insulating properties (i.e. resistivity >1013Q cm and high dielectric constant) to prevent leakage current and thus attenuate potential induced degradation (RID) ; 7) mechanical properties, such as flexibility (i.e. low Young moduli, preferably <20 MPa at 25°C) and adhesiveness (i.e. adhesion strength >0.1 MPa) to withstand thermomechanical stresses caused by daily temperature variations, as simulated by aging tests for thermal cycles / shocks .

[0015] So far, the combination of glass / leakproof polymer / glass encapsulation, wherein the solar cell or the solar module are inserted between two glass plates using an encapsulating polymer adhesive (blanket-cover approach) , has successfully led to the realization of highly stable PSCs .

[0016] The encapsulating adhesives used include ethylene vinyl acetate (EVA) , ionomers (Surlyn, Bynel and Jurasol) , polyisobutylene-based tapes (PIB) , polyolefins (POE) , polyurethanes (PU) and thermoplastic polyurethanes (TPU) . In addition, edge sealants made of butyl rubber-based tapes, polymers polymerizable with UV rays, epoxy resins, silicones and glass frits are commonly used to realize stable devices.

[0017] Early reports have demonstrated that small-sized PSCs encapsulated with the above-reported polymers can pass some accelerated aging tests in compliance with international standards (e.g., International Electrotechnical Commission -IEC- 61215) and the protocols of the International Summit on Organic and Hybrid Photovoltaics Stability (ISOS) , e.g., (resistance to) wet heat (>1000 h at 85°C, relative humidity (RH) =85% for IEC 61215, room temperature for ISOS-D-2; PCE retention >80%) and / or thermal cycles (>200 temperature cycles between -40°C and 85°C for IEC 61215, minimum temperature >-40°C for other laboratory procedures; PCE retention >80%) .

[0018] However, such results have not yet been fully validated in the perovskite solar modules (PSMs) , making such solar cells not yet commercially usable. A recent study (Wang, T. et al . Nat. Commun. 14, 1342 (2023) ) has reported an effective encapsulation strategy based on a self-crosslinked fluorosilicone polymer gel, obtaining a non-destructive encapsulation at room temperature of both PSCs and a 25 cm2PSM (active area = 15.8 cm2) . Using an unspecified epoxy edge sealant, the encapsulated PSMs passed the IEC 61215 wet heat test (1000 h, PCE retention = 98%) , but they were not subjected to other accelerated aging tests. In addition, the corresponding encapsulated PSCs passed the IEC 61215 wet heat test (1000 h, PCE retention = 98%) and the thermal cycles (220 cycles, PCE retention = 95%) , although continued exposure to light at 55 ± 5°C for 1000 h led to a PCE decrease by almost 20%.

[0019] In general, the encapsulation of the PSMs by means of industrializable methodologies (high-throughput) has rarely been described, while the aging tests have not been systematically performed on scales of application interest beyond the direct evaluation of outdoor performance, insufficient for commercialization. Compared to the traditional photovoltaic encapsulants , those for PSMs must also consider thermal management functionalities, due to the low thermal conductivity of the perovskite absorbers. In addition, they must also have Pb containment capabilities because of the risk of release of this element (~40 pg / kWh) into the environment due to the high solubility product constant (Ksp) of the well-known perovskite by-products (e.g., 4.4xl0~9M for Pbl2, which is 11 orders of magnitude higher than that of PbS and PbSe) . In addition, low Young's modulus encapsulants are typically recommended for PSCs and PSMs to avoid delamination problems caused by mismatch of the material TECs. Finally, the encapsulant must not introduce leaks during the assembly of the cells in the form of a module ("cell-to-module losses") caused by the large area occupied by the sealant on the edge, whose width around the edge of the glass should be less than 1 cm for the solar panels sized one square metre. So far, most stability results have been collected with encapsulant areas even larger than the photoactive area, which means that such results still need to be validated on configurations of devices ensuring attractive LCoEs for the market.

[0020] The need to identify new encapsulants that do not have the disadvantages described above is therefore felt in the art. In particular, a technology is being sought that allows the encapsulation of the perovskite cells, that is industrially usable, does not use solvents, does not cause deformations and other mechanical stresses, and, possibly, does not require the use of edge sealants.

[0021] Therefore, aim of the present invention is to provide an encapsulating material for perovskite cells that is industrially usable, that does not make use of solvents, does not cause deformations and does not require the use of edge sealants.

[0022] Summary

[0023] Such object is reached by an encapsulating material according to claim 1. A laminated element according to claim 7 and a solar cell according to claim 8 are also provided.

[0024] According to one aspect of the present invention, there is provided an encapsulating material for perovskite-based solar cells comprising a polymer matrix comprising polyisobutylene having a molecular weight comprised between 30000 and 800000 Da and a filler comprising two-dimensional (2D) flakes, wherein said 2D flakes are made of a material selected in the group consisting of graphene, graphene oxides, reduced graphene oxide, fluorinated graphene, heteroatom-doped graphene, hexagonal boron nitride, metal chalcogenides, metal oxides, metal chalcogenide halides, metal halides, phosphotrichalcogenides, MXenes, metal carbides, metal nitrides, layered hydroxides, alkaline-earth metal silicides, alkaline-earth metal bromides, alkaline- earth metal germanides and alkaline-earth metal stannides, layered perovskites, phosphorene, silicene, antimonene, germanene, boronene, stanene, bismuthene and combination thereof .

[0025] Advantageously the PSM encapsulant of the present invention provides an industrially usable, solvent-free and deformation-free encapsulation strategy based on a viscoelastic ( semi-solid) / highly viscous (liquid) polyolefin. Thanks to the molecular weight selected for the PIB, the latter exhibits a transition from semi-solid (highly viscoelastic) to liquid (highly viscous) as its temperature rises from -40°C to 85°C, like the one used to age the photovoltaic devices through international standardized tests .

[0026] The matrix may, in particular, be homopolymeric polyisobutylene (PIB) or copolymers of polyisobutylene that do not contain additives that lead to the crosslinking of the polymer.

[0027] The use of the PIB is known as an encapsulant for the PSCs, but, the literature does not describe the use of a homopolymer since the PIB is generally used in polymer blends ( isobutylene-isoprene copolymer) and contains several additives (e.g., silanes, adhesives such as glycerol rosin ester, lamellar minerals such as talc and kaolin, metal oxides, carbon black, and even molecular sieve desiccants) , which lead to the cross-linking of the PIB (by altering its molecular weight and by changing the physical state from semi-solid / liquid to solid) , chemically bond to the surfaces, improve anti-aging and waterproofing properties, regulate rheological / mechanical properties, and also control aesthetic characteristics (e.g., colour) . The molecular weight of the PIB encapsulants reported in the literature is therefore greater than 80,000 Da and such encapsulants are presented with a solid physical state.

[0028] Advantageously, the presence of two-dimensional (2D) inorganic fillers, in particular hexagonal boron nitride (h- BN) flakes, preferably produced on an industrial scale through the wet-jet milling (WJM) process of the native powder as described and claimed in patent WO2017089987, improves the adhesion, barrier and thermal management properties of the encapsulant of the invention compared to the known encapsulants.

[0029] The encapsulating material has a Brookfield viscosity at 10 rpm greater than 10000 mPas for a temperature between -45°C and 200°C.

[0030] In one embodiment, the 2D flakes are present in an amount between 0.1 and 30% by weight based on the total weight of the encapsulant, in particular, between 0.5 and 10% by weight based on the total weight of the encapsulant.

[0031] Following encapsulation and without further sealing of the edges, PSM modules encapsulated with the encapsulant of the present invention, (and, for example, based on the perovskite with the chemical composition Cs0.08FA0.80M 0.12Pb ( Io. ssBro.12) 3) , maintain more than 80% of the initial PCE after thermal stresses (e.g., ISOS-D-2 at 85°C, >1000 h) , light soaking (ISOS-L-1, >1000 h) , as well as accelerated aging tests, i.e., thermal shock (200 cycles between -40 / +85°C with abrupt temperature changes) and humidity freeze tests (10 cycles of thermal shock with an intermediate phase of immersion into water at room temperature) as proposed and described below.

[0032] According to a second aspect of the invention, there is provided a laminated element comprising at least one layer made with the encapsulating material of the present invention .

[0033] Finally, there is also provided a perovskite-based solar cell comprising the encapsulating material of the present invention.

[0034] Brief Description of the Drawings

[0035] The present invention will be described in detail with reference to the figures in the accompanying drawings, which show purely illustrative and non-exhaustive examples in which :

[0036] Figure 1 shows a) the potentiodynamic anodic polarization measurements and the corresponding Tafel analysis of bare steel, PIB-coated steel and steel coated with the encapsulating material of the present invention (PIB:h-BN) in a corrosive environment; and b) the corrosion rate and corrosion inhibition efficiency of the encapsulant on samples of PIB-coated steel and steel coated with the encapsulating material of the present invention (PIB:h-BN) . - Figure 2 shows photographs of a drop of water on the surface of the films a) PIB, b) PIB:h-BN and c) the data of the water contact angle which have been measured for the PIB and PIB:h-BN films.

[0037] - Figure 3 shows the maximum temperature over time measured for glass pairs laminated together with PIB or PIB:h-BN encapsulating adhesives, first heated to 85°C (t = 0 s) and then transferred onto an aluminium platform at 25°C.

[0038] Figure 4 shows a diagram of the laser ablation processes (P1-P2-P3) used to determine the layout of the PSM, as well as the layered structure of the module.

[0039] Figure 5 shows the temperature profile of a representative thermal shock test for PSM.

[0040] Figure 6 shows the temperature profile and the environmental exposure conditions of a representative humidity freeze test for PSM.

[0041] Figure 7 shows a) a comparison between the temperature profiles of the IEC 61215 thermal cycle and the thermal shock test of Figure 5; b) an enlargement of the temperature profiles shown in panel a) , with evidence of the first cycle of the thermal shock test of Figure 5; c) Comparison between the temperature and UR profiles of the IEC 61215 test and the humidity freeze test of Figure 6; and d) an enlargement of the profiles of temperature and of the other environmental conditions (immersion into water or exposure to ambient air ) shown in panel a ) , with evidence of the first cycle of the humidity freeze test of Figure 6 , including a step of immersion into water .

[0042] Description of Embodiments

[0043] In the following, the present invention will be illustrated by means of some examples , which are not intended to be considered limiting of the scope of the invention .

[0044] Example .

[0045] For the encapsulation of PSC and PSM two types of encapsulants were prepared, as described below in the Methods section . In particular, the first encapsulant is a transparent high viscosity liquid PIB at room temperature , while the second is an opaque composite of PIB and 2D hexagonal boron nitride (h-BN) flakes , (hereinafter referred to as PIB : h-BN) , the latter produced by wet-j et milling of h-BN crystals . Despite its amorphous and semi-solid / liquid nature ( in the temperature range -40 / + 85 ° C ) , the PIB has a packing of its molecular chains , leading to exceptional barrier properties . Furthermore , the PIB has a resistivity of the order of 1016Q cm, higher than that of the EVA (between 1013and 1015Q cm) , which leads to potential degradation-induced suppression properties . Furthermore, the incorporation of 2D h-BN flakes into the PIB matrix is an ef fective strategy to improve the barrier properties of the pure polymer against the permeation of water ( and thus humidity) and other corrosive species. The barrier properties of 2D h-BN are generally attributed to its morphology with a high specific surface (1488 m2g-1for the monolayer h-BN) and its hydrophobic nature. Furthermore, the dense delocalized electron cloud of the overlapping n orbitals of the h-BN represents a physical barrier against the penetration of molecules or ions, leading to atomic impermeability. In addition, the 2D h-BN flakes exhibit high thermal conductivity (e.g., >700 W nr1Kt1for the atomic monolayer h-BN and >100 W nr1Kt1for h-BN with a few or more atomic layers) , thus improving the thermal management properties of the polymers when used as additives.

[0046] METHODS Materials The highly viscous DIB at room temperature (Brookfield viscosity >100000 mPas at 10 rpm at temperature < 120°C) (LMW-80, average molecular weight 95,000) was supplied by TER Chemicals. The h-BN powder was supplied by Alfa Aesar. All chemicals were used as they were received, unless otherwise indicated.

[0047] Preparation of the encapsulating materials

[0048] To produce PIB homopolymeric encapsulants , the PIB was first dissolved in toluene with a PIB: toluene weight ratio of 1:1.5 and stirred vigorously (500 rpm) for 12 hours at 800 rpm and 80°C until a homogeneous solution was obtained (hereinafter, PIB resin) . For the production of PIB:h-BN encapsulants , the h-BN flakes with few layers were produced using the patented wet-jet milling (WJM) method by BeDimensional S.p.A (W02017089987 ) . Experimentally, a mixture of N-methyl-2-pyrrolidone (NMP) and h-BN crystals with an NMP:h-BN weight ratio of 98:2 was pressurized in two jet streams, which collided in a nozzle to produce the shear forces responsible for the milling mechanism. The 2D h-BN flake dispersion produced with WJM was dried using a dryer. Finally, the 2D h-BN flakes produced were added to the PIB solution, and subsequently mixed with a planetary centrifugal mixer (Thinky ARE-250 Mixing and Degassing Machine) at 1000 rpm for 5 minutes, to produce a PIB:h-BN composite material (hereinafter PIB:h-BN resin) with a weight percentage, on the total weight of the encapsulant, of h-BN of 5% (excluding the solvent) . The h-BN content has been previously optimized through electrochemical testing conforming to ASTM G5-14, ASTM G59-97, ASTM G61-86, and ASTM G106-89 standards to maximize the barrier properties of the encapsulants incorporating h-BN. Electrochemical characterizations were performed on samples produced using PIB LMW-80. To produce the encapsulants, the PIB resin and the PIB:h-BN resin were deposited by spreading on 1 mm thick glass substrates, to be used for glass / pressure-tight polymer / glass encapsulation of PSC and PSM. The obtained films were dried at room temperature for 1 hour, followed by 15 hours at 60°C to evaporate the residual solvent. The thickness of the homopolymer PIB or PIB:h-BN films after evaporation of the solvent obtained is between 600 and 700 pm, measured with a Trotec BB20 thickness measurement system (for these measurements, the encapsulating films were deposited on metal substrates following the same procedure and the same parameters used for the glass coating) .

[0049] Characterization of the encapsulants

[0050] The water contact angle of the PIB and PIB:h-BN films was measured with an OSSILA L2004A1 contact angle goniometer, imagining a 10 pL water drop deposited on the sample. The measured contact angle is 88.3° ± 0.4° for the PIB films and 97.9° ± 0.4° for the PIB:h-BN films (Fig. 2) .

[0051] To evaluate the barrier properties of the encapsulants, electrochemical measurements were carried out.

[0052] The electrochemical measurements were performed using a BioLogic VMP3 multi-channel potentiostat in a 1 L three- electrode electrochemical cell at room temperature in a 3.5 wt % aqueous NaCl solution, following the procedures described in ASTM G5-14 standard. A radiometric reference electrode Ag / AgCl REF201 Red Rod (Biologic) saturated with KC1 was used as a reference electrode, while a graphite bar was used as a counter electrode. The standard assembly of the working electrode consisted of a cylindrical sample of structural steel substrate (S355) coated with PIB or PIB:h- BN, drilled and tapped with a 3-48 UNC thread and screwed onto the support rod. The PIB or PIB:h-BN films were produced by depositing the corresponding resins by spreading, followed by drying at room temperature for 1 hour and at 60°C for 15 hours to evaporate the residual solvent. The thickness of the PIB or PIB:h-BN films obtained, after evaporation of the solvent, is about 60 pm, measured with a Trotec BB20 magnetic induction measurement system. A Teflon compression gasket ensured a leak-free seal. The opencircuit voltage was monitored for 30 minutes, after which the corrosion performance of the coatings were analysed by means of potentiodynamic anodic polarization measurements and relative Tafel analysis, as described in ASTM G5-14 standard, for the determination of the corrosion current of a metal (icorr) and of the corrosion potential. The corrosion rate of the samples was computed by icorr according to Faraday's law, namely: D > KWeq icorr CR~ D where CR is the corrosion rate (in mm yr-1) , K is a constant with a value of 3.27 x 10~3, Weqis the equivalent weight of iron in the ferrous compounds (27.9 g eq-1) , icorr is the corrosion current density (in pA cur2) and D is the density of steel (7.85 g cur3) . The corrosion rate of the samples was computed from icorr . The corrosion inhibition efficiency (r|p) of the composites was calculated starting from the icorr with the following equation: . 1n0n0 where i°Corr and icorr are the corrosion current densities in the absence and presence of inhibitors, respectively.

[0053] Fig. la shows the potentiodynamic anodic polarization measurements and the corresponding Tafel analysis of the PIB or PIB:h-BN coated steel (bare steel results are shown for comparative purposes) immersed in a corrosive environment (3.5 wt % aqueous NaCl solution) . These experiments were performed according to ASTM G5-14 and ASTM G59-97 standards. The results demonstrate the barrier properties of the PIB and PIB:h-BN films, reducing the corrosion rate from 7.3 x ICG1mm yr-1for structural steel to mean values of 1.5 x 10~1mm yr-1for PIB-coated steels and 1.7 x 10~4mm yr-1for PIB:h-BN coated steels (Figure la) . Noteworthy is the fact that the addition of 2D h-BN flakes in the PIB matrix improved the reproducibility of the anti-corrosion performance of PIB films. In general, the data demonstrated the superior barrier properties of the PIB:h-BN films (average corrosion inhibition efficiency = 99.97%) compared to homopolymer PIB (average corrosion inhibition efficiency = 79.53%) (Figure lb) . This effect is attributed to the higher hydrophobicity deriving from the presence of 2D flakes of h-BN (water contact angle of 88.3° ± 0.4° and 97.9° ± 0.4° for the PIB and PIB:h-BN films, respectively, Fig. 2.

[0054] The exceptional barrier properties of PIB:h-BN can be verified also through calcium corrosion tests ("Ca tests") , which analyse calcium corrosion through strength measurements of a calcium film encapsulated with glass / pressure-tight polymer encapsulation similar to what is proposed for PSC and PSM (blanket-cover approach) . In this configuration, humidity cannot pass through the glass (therefore, its entry takes place laterally from the edge of the device through the encapsulants ) .

[0055] The WVTR calculated through electrical measurements of the Ca film encapsulated with the glass / PIB : h-BN system is less than 5xl0~5g im2d-1.

[0056] The thermal management properties of the encapsulants were evaluated by monitoring the maximum temperature of the glass / PIB / glass and glass / PIB : h-BN / glass systems (area = 5.6 cmx5.6 cm) , using a thermal camera (A655sc, FLIR) located at about 50 cm from the sample surface. Samples were produced with a lamination protocol for PSC and PSM encapsulation. The samples were heated to 90°C in a hot plate with a lid and then transferred to an Al platform at 25°C. The temperature of the system was monitored during cooling. The thermal camera was controlled with the FLIR (Temperature FLIR ResearchIR Max software) , which was also used to process the temperature data. As shown in Fig. 3, the presence of 2D h-BN flakes improves the heat dissipation capacity of the system compared to that based on pure homopolymer PIB, reducing the time to reach 30°C by 11.2%.

[0057] Manufacture of perovskite solar cells and modules

[0058] Mesoscopic PSCs

[0059] The process for preparing PSC is known in the literature (for example see Mariani et al., ACS Appl . Mater. Interfaces 2021, 13, 19, 22368-22380 and Vesce et al., RRL Solar, 2022, vol. 6, number 7) . Regular configurations refer to layered structures of the type: transparent conductive oxide (TCO) / electron-transporting layer (ETL) / perovskite / passivating layer (e.g. phenethylammonium iodide, PEAI ) / hole-transporting layer / back electrode (e.g. gold or carbonaceous electrode, including graphene-based electrodes) . ETL may be mesoporous (e.g., mesoporous TiCh) or compact (e.g., compact TiCh or SnCh) , which is why the resulting PSCs are referred to as mesoporous and planar, respectively. Mesoporous PSCs are sometimes made by coupling a compact ETL layer with a second mesoporous ETL layer (e.g. compact TiO2 + mesoporous TiO2) .

[0060] A representative example of a perovskite absorber is Cso. osFAo. soMAo. i2?b ( Io. ssBro.12 ) 3, easily obtainable with the use of precursors and relative ratios well known in the literature (for example see Mariani et al., ACS Appl. Mater. Interfaces 2021, 13, 19, 22368-22380 and Vesce et al., RRL Solar, 2022, vol. 6, number 7, 2101095) .

[0061] Mesoscopic PSMs

[0062] The manufacture of PSMs occurs similarly to the PSCs, except for the amounts of solution poured onto the substrates and for the three additional laser ablation processes (PIPS— P3, see Fig. 4) to define the layout of serially connected PSCs, as described in the literature (Castriotta et al., Advanced Energy Materials, 2022, vol. 12, issue 12, 2103420) .

[0063] Lamination of perovskite solar cells and modules

[0064] The PIB and PIB:h-BN encapsulants can be applied both on PSC PSM through a lamination process at low temperatures (<120°C) , for example a multi-phase lamination process, at 90°C and at differential pressure, using an automatic two- chamber laminator for solar panels, provided with a cooling system to ensure a high reproducibility of the lamination procedure and reduce the time of exposure of the materials to the temperature that causes their degradation. Experimentally, the entire surface of the devices is covered with the encapsulating glass coated with PIB or PIB:h-BN to ensure the glass / pressure-tight polymer / glass encapsulation of PSC and PSM (blanket-cover approach) . The laminates thus assembled are inserted into the lower chamber of the laminator, exploiting the differential pressure between the upper and lower chamber. After blocking the laminator, the lamination process is carried out. For example, the chambers of the laminator are evacuated under moderate vacuum

[0065] (pressure of ~1 mbar) , while the laminates are heated from room temperature to 50°C, for example in a time interval between 200 and 250 s (e.g. ~7°C min-1) . Then, the upper chamber inflates to apply a final pressure of e.g. 30 mbar on the upper part of the substrates. Then, the temperature of the laminate is increased from 50°C to temperatures between 80 and 120°C in time intervals for example between 400 and 600 s (e.g. 5°C min-1) , and maintaining the final temperature for a time interval for example between 500 and 700s. Subsequently, the temperature of the laminate is lowered to 50°C and the pressure of the chambers returns to 1000 mbar, resulting in the opening of the laminator, as well as the release of the devices.

[0066] Characterization of the devices

[0067] The ISOS-L-1 and ISOS-D-2 tests are described in the literature (Khenkin et al., Nature Energy vol. 5, p. 35-49 (2020) ) . An example of a thermal shock test is performed by oscillating the temperature of the samples between -40°C and +85°C with abrupt temperature variations (from room temperature to +85°C and vice versa, from room temperature to -40°C and vice versa, see Fig. 5) . An example of a humidity freeze test (modified with respect to the protocol reported in the TEC 61215 standard) can be performed after 200 cycles of thermal shock and consists of 10 cycles of thermal shock between -40°C and +85°C, each cycle starting with a step of immersion into water at room temperature (see Fig. 6) .

[0068] Inductively coupled plasma optical emission spectroscopy measurements can be performed e.g. with a ThermoFisher iCAP 7600 DUO spectrometer to measure the Pb loss of PSC and PSM immersed in water, by sampling the aqueous solution over time.

[0069] Preliminary evaluation of the encapsulants in the PSCs

[0070] The PIB:h-BN encapsulants, selected over the PIBs for their superior barrier properties, were preliminarily tested in n-i-p mesoscopic PSC configurations. The PIB:h-BN encapsulants were applied by means of a high-speed lamination protocol (total duration <45 min) compatible with massive production. Experimental evidence indicates that the lamination process marginally affects the overall performance of the cell (absolute PCE drop <1%) . During the ISOS-D-1 test, the PSCs maintain their performance, confirming their stability. Following ISOS-D-2 and ISOS-L-1 tests for >1000 h, the non-encapsulated cells degrade rapidly, typically showing a Tso (defined as the time at which the PCE drops to 80% of the initial one) <100 h and <10 h for the ISOS-D-2 and ISOS-L-1 tests, respectively, and PSCs based on perovskite absorbers Cso. osFAo. soM o. i2?b ( Io. ssBro.12 ) 3. In contrast, the encapsulated cells show Tso >1000 h without the use of edge sealants. The efficacy of the PIB:h-BN encapsulants has been demonstrated also on planar n-i-p PSCs, e.g., PSCs based on compact ETLs of Sn02 deposited at low temperature, reporting Tso of approximately 2000 h during ISOS-D-2 test, while those not encapsulated usually show a lower Tso of 700 h.

[0071] Validation of the encapsulant in PSMs

[0072] The PIB and PIB:h-BN encapsulants were subsequently evaluated in n-i-p mesoscopic PSM as reported in the literature (e.g. Vesce, L. et al., Sol. RRL 5, 2100073 (2021) ) . The encapsulants were applied following the lamination protocols described for the PSCs, taking care to completely cover the porous layers of the module structure. Like in the case of the PSCs, it is not necessary to use edge sealants in combination with the encapsulants of the present invention. Like in the case of the cells, the encapsulation process marginally affects the overall performance of the module (absolute PCE drop <1%) . PSM performances are maintained during the 240-hour ISOS-D-1 tests (absolute PCE drop <1%) . During the ISOS-D-2 and ISOS- L-l tests, the non-encapsulated PSMs degrade rapidly, typically showing Tso <100 h and <10 h, respectively. In contrast, the encapsulated PSMs typically show Tso >1000 h.

[0073] The encapsulated PSMs subjected to thermal shock tests (between -40 / +85°C) and, subsequently, to modified humidity freeze tests confirm the reliability and the barrier functionalities of the PIB:h-BN encapsulants . Figs. 5 and 6 show the temperature profiles and environmental conditions (e.g., immersion into water and exposure to air) for representative heat shock and humidity freeze tests, while the comparison with the heat variation and humidity freeze tests of the IEC 61215 standard is illustrated in Fig. 7. In the accelerated aging stresses, the abrupt temperature changes induce strong thermomechanical stresses caused by thermal expansion and contraction of the materials with different TECs, critically compromising the adhesion between the various layers and the reliability of the electrical interconnections. The PSMs encapsulated with PIB:h-BN withstand at least 200 cycles of thermal shock, preserving >80% of the initial PCE . The PIB:h-BN encapsulant performs better than the PIB encapsulant, indicating that the use of 2D h-BN flakes as thermoconductive additives in the encapsulants is an effective strategy to improve the overall thermal management properties of the PSMs by integrating passive cooling capabilities into the encapsulation system. This is consistent with the thermal properties measured for the encapsulants of the present invention (Fig. 3) . The PSMs encapsulated with PIB:h-BN subjected to the humidity freeze test preserve >85% of the initial PCE after 10 cycles. The efficacy of the PIB:h-BN encapsulant in protecting PSMs from extrinsic factors was also evaluated by measuring the release of Pb from the encapsulated PSMs immersed in water by inductively coupled plasma source optical emission spectroscopy ( ICP-OES ) . After immersion into water, the nonencapsulated PSM rapidly degraded, showing yellowing associated with decomposition of the perovskite to Pbl2 . Due to its high solubility ( 340 mg In1, solubility product constant = 4 . 4 x l 0~9M) , Pbl2 dissolved rapidly in water . The Pb loss detected ( >60 pg cur2after 24 hours ) is consistent with the Pb content in the perovskite , typically comprised between 0 . 1 and 1 g nr2. In contrast to the non-encapsulated devices , perovskite in the encapsulated PSMs retained the starting colour, preserving the perovskite phase . As a result , Pb leakage was drastically inhibited at values lower than 1 pg cur2after 24 hours ( low Pb water contamination is likely associated with the perovskite residues near the edges of the encapsulant and not with the perovskite degradation on the active area of the PSM) .

[0074] Conclusions

[0075] The viscoelasticity / highly viscous liquid state of the PIB inherently limits the thermomechanical stresses caused by both the encapsulation process and the temperature gradients that occur during accelerated aging stres ses . The incorporation of 2D h-BN flakes into the PIB matrix improves the barrier and thermal management properties of the PIB homopolymer . Without the use of edge sealants, the PSCs and PSMs encapsulated with the encapsulants of the invention based on PIB and 2D h-BN flakes withstand several accelerated aging tests, including ISOS-D1 (>2o h) , ISOS-D2 (> 1000 h) , ISOS- LI (>1000 h) , as well as customized thermal shock tests (>200 cycles) and modified humidity freeze tests (>10 cycles) , maintaining more than 80% of their initial PCE .

[0076] The results reported above represent a step forward towards the realization of long-term stable PSMs through semi-solid / liquid encapsulants compatible with low temperature and inexpensive lamination processes that combine high speed processability, compatibility with eCTL perovskite, barrier properties, adhesiveness, thermal and light stability, without the need to take into consideration advanced perovskite formulations with high internal stability or edge-specific sealants.

Claims

CLAIMS1.- An encapsulating material for perovskite-based solar cells comprising a polymer matrix comprising polyisobutylene having a molecular weight comprised between 30000 and 800000 Da and a filler comprising two-dimensional flakes, wherein said two-dimensional flakes are made of a material selected from the group consisting of graphene, graphene oxides, reduced graphene oxide, fluorinated graphene, heteroatom-doped graphene, hexagonal boron nitride metal chalcogenides, metal oxides, metal chalcogenide halides, metal halides, phosphotrichalcogenides, MXenes, metal carbides, metal nitrides, layered hydroxides, alkaline-earth metal silicides, alkaline-earth metal bromides, alkaline-earth metal germanides and alkaline-earth metal stannides, layered perovskites, phosphorene, silicene, antimonene, germanene, boronene, stanene, bismuthene and combination thereof.2.- Encapsulating material according to claim 1 characterised in that said polymeric matrix is made of homopolymeric polyisobutylene.3.- Encapsulating material according to claim 1 characterised in that said two-dimensional flakes are made of hexagonal boron nitride.4.- Encapsulating material according to claim 1 characterised in that said two-dimensional flakes arepresent in an amount between 0.1 and 30% by weight of the total weight of the encapsulating material.5.- Encapsulating material according to claim 4 characterised in that said two-dimensional flakes are present in an amount between 0.5 and 10% by weight on the total weight of the encapsulant.6.- Encapsulating material according to claim 1 characterised in that said encapsulating material has a Brookfield viscosity at 10 rpm greater than 10000 mPas for a temperature between -45°C and 200°C.7.- Laminated element comprising at least one layer made with the encapsulating material according to any one of claims 1 to 6.8.- A perovskite-based solar cell comprising the encapsulating material according to any one of claims 1 to6.

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