Perovskite-based photovoltaic cells and preparation process thereof

Abstract

Perovskite-based photovoltaic cell (or solar cell) wherein the perovskite photoactive layer comprises at least one salified chitosan having general formula (I): wherein: - X- represents a carboxylate anion having general formula (II): R1-COO-, (II), or a sulfonate anion having general formula (III): R2-SO3- (III), or a phosphonate anion having general formula (IV): R3-PO3H- (IV), wherein R1, R2 and R3, equal to or different from each other, represent a C4-C30, preferably C4-C20, linear or branched, saturated or unsaturated, optionally containing heteroatoms alkyl group; an optionally substituted aryl group; an optionally substituted heteroaryl group; an optionally substituted cycloalkyl group; an optionally substituted heterocyclic group; - a is an integer or fractional number between 0.1 and 1, preferably between 0.5 and 1; - b is a fractional number between 0 and 0.9, preferably between 0 and 0.5; 44 - c is a fractional number between 0 and 0.4, preferably between 0 and 0.2; - p is an integer between 3 and 500, preferably between 5 and 400, more preferably between 10 and 250. Preferably, in said general formula (I) the sum of a + b + c is equal to 1. Said perovskite-based photovoltaic cell (or solar cell) can be advantageously used in various applications that require the production of electrical energy through the exploitation of light energy, in particular the energy of solar radiations such as, for example: architecturally integrated photovoltaic systems ("Building Integrated Photo Voltaic" - BIPV); photovoltaic windows; greenhouses; photo-bioreactors; noise barriers; lighting engineering; design; advertising; automotive industry. Said perovskite-based photovoltaic cell (or solar cell) can be used both in "stand alone" mode and in modular systems.

Description

[0001]PEROVSKITE-BASED PHOTOVOLTAIC CELLS AND PREPARATION PROCESS THEREOF DESCRIPTION The present invention relates to perovskite-based photovoltaic cells (or solar cells). More specifically, the present invention relates to a perovskite-based photovoltaic cell (or solar cell) wherein the perovskite photoactive layer comprises at least one salified chitosan having the specific general formula (I) reported below. Said perovskite-based photovoltaic cell (or solar cell) can be advantageously used in various applications that require the production of electrical energy through exploitation of light energy, in particular energy of solar radiation such as, for example: architecturally integrated photovoltaic systems (“Building Integrated Photo Voltaic” - BIPV); photovoltaic windows; greenhouses; photo-bioreactors; noise barriers; lighting engineering; design; advertising; automotive industry. Said perovskite-based photovoltaic cell (or solar cell) can be used both in "stand alone" mode and in modular systems. The present invention also relates to a process for the preparation of said perovskite-based photovoltaic cell (or solar cell). A further object of the present invention is also a composition comprising at least one perovskite and at least one salified chitosan having the specific general formula (I) reported below. Photovoltaic cells (or solar cells) are devices capable of converting the energy of a light radiation into electrical energy. Currently, most photovoltaic cells (or solar cells) that can be used for practical applications exploit the chemical-physical properties of inorganic photoactive materials, in particular high purity crystalline silicon. However, said photovoltaic cells (or solar cells), while providing interesting performances, particularly in terms of efficiency and durability, have also shown some drawbacks. For example, rigidity and weight of said silicon-based photovoltaic cells (or solar cells) often make it necessary to install an ad hoc frame for their positioning and this fact severely limit their fields of use. Some of the aforementioned drawbacks can be overcome by using photovoltaic cells (or solar cells) based on organic polymers (“Organic Photovoltaics” – “OPVs”) or based on perovskites (“Perovskite Solar Cells” - PSCs). In particular, perovskite-based photovoltaic cells (or solar cells) ("Perovskite Solar Cells" - PSCs) have rapidly become, in recent years, a promising alternative as they combine high power conversion efficiency (PCE)] which, currently, has reached a certified value of 25.5%, a series of typical characteristics of photovoltaic cells (or solar cells) based on thin film organic polymers (“Organic Photovoltaics” – “OPVs”) such as, for example, the lightness, flexibility and simplicity of the manufacturing process, which starting from appropriate mixtures of the various precursors, can allow the production of photovoltaic cells (or solar cells) through well-known and consolidated printing processes (even continuously) in mild conditions and with sustainable costs. However, even perovskite-based photovoltaic cells (or solar cells) ("Perovskite Solar Cells" - PSCs) can show some drawbacks such as, for example, high sensitivity of perovskites to atmospheric agents (especially humidity), a non-optimal packing of the perovskite crystalline phase which negatively affects charge transport. In order to solve the aforementioned drawbacks, numerous research groups have developed various techniques for the construction of perovskite-based photovoltaic cells (or solar cells) ("Perovskite Solar Cells" - PSCs) which involve, for example, the use of polymeric additives within the perovskite photoactive layer. Over the last few years, many polymers with both thermoplastic and elastomeric and both hydrophobic and hydrophilic characteristics, have been used and the results have been summarized in the review by Kim K. et al., “Solar RRL” (2021), Vol. 5, pg. 2000783, doi.org / 10.1002 / solr.202000783. In said review, the role and contribution of polymeric additives in perovskite- based solar cells is described. In particular, the use of polymers or polymeric materials as additives is described for the purpose of promoting nucleation and crystallization of the perovskite photoactive layers in order to increase the grain size of the perovskite crystals. Thanks to their high molecular weight, these polymers allow for good passivation of defects present on the edges of the perovskite crystals. Furthermore, said polymers, by limiting growth rate of the perovskite crystals, can cause an increase in their grain size, consequently allowing better packing between them. Furthermore, some polymers work as charge carrier materials in interfacial layers in order to effectively separate charge carriers and reduce charge recombination. Furthermore, some hydrophobic polymers can protect perovskite photoactive layers from humidity, while elastomeric polymers can contribute to mechanical resilience of the perovskite photoactive layer via cross-linking and self-healing. Various molecules containing nitrogen groups and many polymers containing neutral or ionic nitrogen groups were also tested. For example, Xue Q. et al., in “RSC Advances” (2015), Vol. 7, pg. 775-783, report a process for the manufacturing of perovskite-based solar cells with the following layout: PEDOT:PSS / MAPbI3-PEOXA / PCBM / Al. The aforementioned procedure, both as regards the manufacturing of the perovskite-based solar cells and as regards the manufacturing of the photoactive layer, is not reported in detail: however, the authors declare that the results obtained are strongly dependent on the type of solvent used to dissolve the perovskite precursors and the amount of poly(2-ethyl-2-oxazoline) (PEOXA) used. By operating in the most favorable conditions (i.e. ^-butyrolactone (GBL) as solvent and 1.5% by weight of PEOXA), perovskite-based solar cells are obtained with a power conversion efficiency (PCE) equal to 6.16% thanks to better control of the crystallization process and of the morphology of the perovskite photoactive layer. Guo Y. et al., in “Advanced Energy Materials” (2016), Vol. 6, 1502317, report a process for the manufacturing of perovskite-based solar cells with the following layout: PEDOT:PSS / MAPbIxCl3-x-PVP / PCBM-PEIE / Ag. The process involves the preparation of the perovskite photoactive layer by depositing via spin coating a DMF solution containing the perovskite precursors: MAI, PbI2and PbCl2(in a molar ratio of 4:1:1) and variable quantities (0% by weight - 6% by weight) of polyvinylpyrrolidone (PVP). By operating in the most favorable conditions (i.e. PVP at 3% by weight), perovskite-based solar cells are obtained with a power conversion efficiency (PCE) equal to 7.91%, also obtaining a sensitive improvement regarding the thermal stability of the perovskite photoactive layer thanks to an enhancement in the size and morphology of the perovskite crystals. Du C-S. et al., in “Sustainable Energy Fuels” (2022), Vol. 6, pg. 3349-3362, report the preparation of a series of celluloses modified with functional groups based on quaternary ammonium halides and use them in a process for the manufacturing of perovskite-based solar cells with the following layout: P3CT-Na / MAPbI3- PQ- X / PCBM / BCP / Ag. The process involves the preparation of the perovskite photoactive layer by depositing via spin coating a solution in DMF / DMSO (4:1 v / v) containing the perovskite precursors: MAI, PbI2 (in a 1:1 molar ratio) and variable quantities (0.03% by weight - 0.12% by weight) of quaternized ethoxylated hydroxyethylcellulose (“Polyquaternium-halide”: PQ-X with X = F, Cl, Br). By operating in the best conditions (i.e. “Polyquaternium- chloride”: PQ-Cl at 0.06% by weight) perovskite-based solar cells are obtained with a power conversion efficiency (PCE) equal to 15.01%, i.e. approximately one point more than the standard device without the additive and also significantly increasing stability over time. Among the polymers containing nitrogen groups, chitosan is also considered, which is a linear polysaccharide composed of acetylglucosamine units (N- acetyl-2-amino-2-deoxy-D-glucopyranosyl linked together via ^-1,4 bonds obtained from deacetylation of chitin generally extracted from cell wall of fungi, or from exoskeleton of insects or crustaceans (for example, crabs, shrimps, etc.). Generally, chitin deacetylation is carried out in the presence of a basic aqueous solution (for example, an aqueous sodium hydroxide solution) as shown in Scheme (1): Scheme 1 Generally, the deacetylation degree is between 60% and 100% while the weight average molecular weight (Mw) is between 3000 and 400000 Dalton. However, it is known that chitosan has intrinsic difficulties linked to its insolubility in solvents commonly used to dissolve perovskite precursors, as reported, for example, by Sashiwa H. et al., in “Chemistry Letters” (2000), Vol. 29, No. 6, pg. 596-597. In order to overcome the aforementioned problem, Chinese patent CN 104744714 reports the use of chitosan salified through the reaction with hydroiodic acid (HI). In particular, a process is reported for the preparation of a perovskite / chitosan nanocomposite film characterized in that it comprises the following steps: (1) preparing chitosan iodide; (2) dissolving methylamine salt, chitosan iodide, lead iodide or lead chloride, in dimethylformamide in a closed environment, introducing protective gas; leaving under stirring at 55°C-65°C for 6-10 hours, so as to obtain a solution of perovskite precursors; (3) depositing the solution of perovskite precursors by spin coating obtaining a perovskite / chitosan nanocomposite film. However, the photovoltaic cells comprising said perovskite / chitosan nanocomposite film have totally insufficient performance because power conversion efficiency (PCE) varies from 0.34% to 3.25%, depending on the quantity of chitosan iodide used. From the above, it is clear the importance of finding other polymers capable of being used as additives in the perovskite photoactive layer which allow to obtain perovskite-based photovoltaic cells (or solar cells)("Perovskite Solar Cell" - PSC) capable of having a good power conversion efficiency (PCE), as well as a process for their manufactuting suitable for use in the "scaling up" phase for the construction of large area photovoltaic cells (or solar cells). The Applicant therefore faced the problem of finding a perovskite-based photovoltaic cell (or solar cell) capable of having a good power conversion efficiency (PCE), as well as a process for its manufacturing suitable for use in the "scaling up" phase for the construction of large area photovoltaic cells (or solar cells). The Applicant has now found a perovskite-based photovoltaic cell (or solar cell) wherein the perovskite photoactive layer comprises at least one salified chitosan having the specific general formula (I) reported below, capable of having a good power conversion efficiency (PCE)(i.e. PCE > 10%), as well as a process for its manufacturing which involves the deposition of the perovskite photoactive layer in an uncontrolled atmosphere, in the presence of air. Said process is, therefore, suitable for use in the "scaling up" phase for the construction of large-area photovoltaic cells (or solar cells). Furthermore, said perovskite-based photovoltaic cell (or solar cell) is capable of maintaining good photoelectric properties, i.e. good values of FF (Fill Factor), Voc (Open Circuit Voltage), Jsc (short-circuit photocurrent density). Said perovskite-based photovoltaic cell (or solar cell) can be advantageously used in various applications that require the production of electrical energy through the exploitation of light energy, in particular the energy of solar radiations such as, for example: architecturally integrated photovoltaic systems (“Building Integrated Photo Voltaic” - BIPV); photovoltaic windows; greenhouses; photo-bioreactors; noise barriers; lighting engineering; design; advertising; automotive industry. Furthermore, said perovskite-based photovoltaic cell (or solar cell) can be used both in "stand alone" mode and in modular systems. The object of the present invention is therefore a photovoltaic cell (or solar cell) based on perovskite wherein the perovskite photoactive layer comprises at least one salified chitosan having general formula (I): wherein: - X- represents a carboxylate anion having general formula (II): R1-COO-, (II), or a sulfonate anion having general formula (III): R2-SO3- (III), or a phosphonate anion having general formula (IV): R3-PO3H- (IV), wherein R1, R2 and R3, equal or different from each other, represent a C4-C30, preferably C4-C20, linear or branched, saturated or unsaturated, possibly containing heteroatoms alkyl group; an optionally substituted aryl group; an optionally substituted heteroaryl group; an optionally substituted cycloalkyl group; an optionally substituted heterocyclic group; - a is an integer or a fractional number between 0.1 and 1, preferably between 0.5 and 1; - b is a fractional number between 0 and 0.9, preferably between 0 and 0.5; - c is a fractional number between 0 and 0.4, preferably between 0 and 0.2; - p is an integer between 3 and 500, preferably between 5 and 400, more preferably between 10 and 250. For the purposes of the present description and the claims that follow, the definitions of the numerical intervals always comprise the extremes unless otherwise specified. For the purposes of this description and the claims that follow, the term “comprising” also includes the terms “essentially consisting of” or “consisting of”. In accordance with a preferred embodiment of the present invention, in said general formula (I) the sum of a + b + c is equal to 1. In accordance with a preferred embodiment of the present invention, said perovskite can be selected, for example, from organometallic trihalides having the general formula ABX3 wherein: - A represents a monovalent organic cation such as, for example, methylammonium (CH3NH3+), formamidinium [CH(NH2)2+], n-butylammonium (C4H9NH3+), tetra- butylammonium (C16H36N+), or mixtures thereof; or A represents a monovalent inorganic cation such as, for example, cesium (Cs+), rubidium (Rb+), potassium (K+), lithium (Li+), sodium (Na+), copper (Cu+), silver (Ag+), or mixtures thereof; or mixtures thereof; - B represents a divalent metal cation such as, for example, lead (Pb2+), tin (Sn2+), or mixtures thereof; - X represents a halide anion such as, for example, iodine (I-), chlorine (Cl-), bromine (Br-), or mixtures thereof. In accordance with a further preferred embodiment of the present invention, said perovskite can be selected, for example from: methylammonium lead iodide (CH3NH3PbI3), methylammonium lead bromide (CH3NH3PbBr3), methylammonium lead chloride (CH3NH3PbCl3), methylammonium lead iodide bromide (CH3NH3PbIxBr3-x), methylammonium lead iodide chloride (CH3NH3PbIxCl3-x), formamidinium lead iodide [CH(NH2)2PbI3], formamidinium lead bromide [CH(NH2)2PbBr3], formamidinium lead chloride [CH(NH2)2PbCl3], formamidinium lead iodide bromide [CH(NH2)2PbIxBr3-x], formamidinium lead iodide chloride [CH(NH2)2PbIxCl3-x], methylammonium formamidinium lead iodide [(CH3NH3)x(CH(NH2)2)1-xPbI3], methylammonium formamidinium lead bromide [(CH3NH3)x(CH(NH2)2)1-xPbBr3], methylammonium formamidinium lead chloride [(CH3NH3)x(CH(NH2)2)1-xPbCl3], methylammonium formamidinium lead iodide chloride [(CH3NH3)x(CH(NH2)2)1-xPbI3-yCly], methylammonium formamidinium lead iodide bromide [(CH3NH3)x(CH(NH2)2)1-xPbI3-yBry], n-butylammonium lead iodide (C4H9NH3PbI3), tetra-butylammonium lead iodide (C16H36NPbI3), n-butylammonium lead bromide (C4H9NH3PbBr3), tetra-butylammonium lead bromide (C16H36NPbBr3), cesium lead iodide (CsPbI3), rubidium lead iodide (RbPbI3), potassium lead iodide (KPbI3), cesium methylammonium lead iodide [Csx(CH3NH3)1-xPbI3), potassium methylammonium lead iodide [Kx(CH3NH3)1-xPbI3), cesium methylammonium lead iodide chloride [Csx(CH3NH3)1-xPbI3-yCly), cesium formamidinium lead iodide [Csx(CH(NH2)2)1-xPbI3], cesium formamidinium lead bromide [Csx(CH(NH2)2)1-xPbBr3], cesium formamidinium lead iodide chloride [Csx(CH(NH2)2)1-xPbI3-yCly], methylammonium tin iodide (CH3NH3SnI3), methylammonium tin bromide (CH3NH3SnBr3), methylammonium tin iodide bromide (CH3NH3SnIxBr3-x), formamidinium tin iodide [CH(NH2)2SnI3], formamidinium tin iodide bromide [CH(NH2)2SnIxBr3-x], n-butylammonium tin iodide (C4H9NH3SnI3), tetra-butylammonium tin iodide (C16H36NSnI3), n-butylammonium tin bromide (C4H9NH3SnBr3), tetra-butylammonium tin bromide (C16H36NSnBr3), methylammonium tin lead iodide (CH3NH3SnxPb1-xI3), formamidinium tin lead iodide [CH(NH2)2SnxPb1-xI3], or mixtures thereof. Methylammonium lead iodide (CH3NH3PbI3), formamidinium lead iodide [CH(NH2)2PbI3], methylammonium formamidinium lead iodide chloride [(CH3NH3)x(CH(NH2)2)1-xPbI3-yCly], cesium methylammonium lead iodide chloride [Csx(CH3NH3)1-xPbI3-yCly), cesium formamidinium lead iodide chloride [Csx(CH(NH2)2)1-xPbI3-yCly], are preferred. Formamidinium lead iodide [CH(NH2)2PbI3],is even more preferred. For the purposes of the present description and the claims that follow, the term "C4-C30 alkyl groups" indicates alkyl groups having from 4 to 30 carbon atoms, linear or branched, saturated or unsaturated. Specific examples of C4-C30 alkyl groups are: n-butyl, iso-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, 2-ethylheptyl, 2-ethylhexyl, 2-butenyl, 2-pentenyl, 2-ethyl-3-hexenyl, 3-octenyl, 1-methyl-4-hexenyl, 2-butyl-3-hexenyl, lauryl, 4-hexyl-lauryl, cetyl, 2-butyl-cetyl, 4-hexyl- cetyl, stearyl, 2,4-dimethyl-stearyl. For the purposes of the present description and the claims that follow, the term "C4-C30alkyl groups optionally containing heteroatoms" indicates alkyl groups having from 4 to 30 carbon atoms, linear or branched, saturated or unsaturated, wherein at least one of the hydrogen atoms is substituted with a heteroatom selected from: halogens such as, for example, fluorine, chlorine, bromine, preferably fluorine; nitrogen; sulfur; oxygen. Specific examples of C4-C30alkyl groups optionally containing heteroatoms are: perfluoropentyl, perfluoroctyl, perfluorodecyl, ethyl-2-methoxy, propyl- 3-ethoxy, butyl-2-thiomethoxy, hexyl-4-amino, hexyl-3- N,N'-dimethylamino, methyl-N,N'-dioctylamino, 2-methyl- hexyl-4-amino. For the purposes of the present description and the claims that follow, the term "aryl groups" indicates aromatic carbocyclic groups containing from 6 to 60 carbon atoms. Said aryl groups can optionally be substituted with one or more groups, equal or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C12 alkyl groups; C1-C12 alkoxy groups; C1-C12 thioalkoxy groups; C3-C24 tri- alkylsilyl groups; polyethyleneoxy groups; cyano groups; amino groups; C1-C12 mono- or di-alkylaminic groups; nitro groups. Specific examples of aryl groups are: phenyl, methylphenyl, trimethylphenyl, methoxyphenyl, hydroxyphenyl, phenyloxyphenyl, fluorophenyl, pentafluorophenyl, chlorophenyl, bromophenyl, nitrophenyl, dimethylaminophenyl, naphthyl, phenylnaphthyl, phenanthrene, anthracene, phenoxy- phenyl, 4-nitro-phenyl, 4-amino-phenyl, 3-chloro- phenyl, trihydroxyphenyl. For the purposes of this description and the claims that follow, the term "heteroaryl groups" indicates aromatic, penta- or hexa-atomic heterocyclic groups, also benzocondensed or heterobicyclic, containing from 4 to 60 carbon atoms and from 1 to 4 heteroatoms selected from nitrogen, oxygen, sulfur, silicon, selenium, phosphorus. Said heteroaryl groups can optionally be substituted with one or more groups, equal or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C12alkyl groups; C1-C12alkoxy groups; C1-C12 thioalkoxy groups; C3-C24 tri- alkylsilyl groups; polyethyleneoxy groups; cyano groups; amino groups; C1-C12mono- or di-alkylaminic groups; nitro groups. Specific examples of heteroaryl groups are: pyridine, methylpyridine, methoxypyridine, phenylpyridine, fluoropyridine, aminopyridine, pyrimidine, pyridazine, pyrazine, triazine, tetrazine, quinoline, quinoxaline, quinazoline, furan, thiophene, hexylthiophene, bromothiophene, dibromothiophene, pyrrole, oxazole, thiazole, isooxazole, isothiazole, oxadiazole, thiadiazole, pyrazole, imidazole, triazole, tetrazole, indole, benzofuran, benzothiophene, benzooxazole, benzothiazole, benzooxadiazole, benzothiadiazole, benzopyrazole, benzimidazole, benzotriazole, triazolopyridine, triazolopyrimidine, coumarin. For the purposes of the present description and the claims that follow, the term "cycloalkyl groups" indicates cycloalkyl groups having from 5 to 60 carbon atoms. Said cycloalkyl groups can optionally be substituted with one or more groups, equal or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C12 alkyl groups; C1-C12 alkoxy groups; C1-C12thioalkoxy groups; C3-C24tri- alkylsilyl groups; polyethyleneoxy groups; cyano groups; amino groups; C1-C12mono- or di-alkylaminic groups; nitro groups. Specific examples of cycloalkyl groups are: cyclopropyl, 2,2-difluorocyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, methylcyclohexyl, methoxycyclohexyl, fluorocyclohexyl, phenylcyclohexyl, decalin, norbornyl, abiethyl. For the purposes of the present description and the claims that follow, the term "heterocyclic groups" indicates rings having from 3 to 12 atoms, saturated or unsaturated, containing at least one heteroatom selected from nitrogen, oxygen, sulphur, silicon, selenium, phosphorus, optionally condensed with other aromatic or non-aromatic rings. Said heterocyclic groups can optionally be substituted with one or more groups, equal or different from each other, selected from: halogen atoms such as, for example, fluorine, chlorine, bromine, preferably fluorine; hydroxyl groups; C1-C12 alkyl groups; C1-C12 alkoxy groups; C1-C12 thioalkoxy groups; C3-C24tri-alkylsilyl groups; polyethyleneoxy groups; cyano groups; amino groups; C1-C12 mono- or di- alkylaminic groups; nitro groups. Specific examples of heterocyclic groups are: pyrrolidine, methoxypyrrolidine, piperidine, fluoropiperidine, methylpiperidine, dihydropyridine, piperazine, morpholine, thiazine, indoline, phenylindoline, 2- ketoazetidine, diketopiperazine, tetrahydrofuran, tetrahydrothiophene. In accordance with a preferred embodiment of the present invention, said salified chitosan having general formula (I) has a weight average molecular weight (Mw) between 3000 Dalton and 500000 Dalton, preferably between 5000 Dalton and 400000 Dalton, more preferably between 10000 Dalton and 300000 Dalton. In accordance with a preferred embodiment of the present invention, said salified chitosan having general formula (I) dissolved in a mixture of water and acetic acid (1% by weight of acetic acid compared to the total weight of the water / acetic acid mixture) in a concentration equal to 1% by weight with respect to the total weight of the water / acetic acid mixture, has a viscosity between 10 cps and 4000 cps, preferably between 15 cps and 3000 cps, more preferably between 18 cps and 2000 cps. In accordance with a preferred embodiment of the present invention, said salified chitosan having general formula (I) has a solubility in dimethyl sulfoxide (DMSO), measured at room temperature (25°C), up to 50 mg / ml, preferably up to 40 mg / ml, more preferably between 15 mg / ml and 35 mg / ml. In accordance with a preferred embodiment of the present invention, said salified chitosan having general formula (I) can be present in the perovskite photoactive layer in an amount between 0.1% by weight and 20% by weight, preferably between 0.2% by weight and 10% by weight, more preferably between 0.25% by weight and 5% by weight, relative to the total weight of the perovskite precursors. The aforementioned salified chitosan having general formula (I) can be obtained according to processes known in the art as described, for example, by Sashiwa H. et al., in “Chemistry Letters” (2000), Vol. 29, No. 6, pg. 596-597, reported above. For the purpose of the present invention, the salified chitosan having general formula (I) can be obtained through a process comprising reacting at least one chitosan in aqueous suspension with at least one acid in aqueous solution selected from: - carboxylic acids having general formula (IIa): R1-COOH (IIa), or - sulfonic acids having general formula (IIIa): R2-SO3H (IIIa), or - phosphonic acids having general formula (IVa): R3-PO3H2 (IVa), wherein R1, R2and R3have the same meanings reported above; at a temperature between 20°C and 60°C, for the time necessary to obtain complete dissolution of the salified chitosan having general formula (I), generally between 30 minutes and 120 minutes. Specific examples of carboxylic acids having general formula (IIa) which can advantageously be used in the aforementioned process are: butyric acid, hexanoic acid, lauric acid, palmitic acid, stearic acid, salicylic acid, cyclopentanecarboxylic acid, cyclohexanecarboxylic acid, cyclopentenecarboxylic acid, or mixtures thereof. Specific examples of sulfonic acids having general formula (IIIa) which can advantageously be used in the aforementioned process are: butanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, cyclohexanesulfonic acid, camphorsulfonic acid, or mixtures thereof. Specific examples of phosphonic acids having general formula (IVa) which can advantageously be used in the aforementioned process are: butanephosphonic acid, benzenephosphonic acid, p-toluenephosphonic acid, cyclohexanephosphonic acid, camphorphosphonic acid, or mixtures thereof. In accordance with a preferred embodiment of the present invention, said perovskite-based photovoltaic cell (or solar cell) comprises: - a glass substrate covered with a layer of transparent conductive oxide (TCO), typically fluorine-doped tin oxide (SnO2:F) (FTO), or indium tin oxide (ITO) which constitutes the anode; - a layer based on a hole transporting material (“Hole Transport Layer” - HTL), preferably a layer of (2- (9H-carbazol-9-yl)ethyl phosphonic acid (2PACz); - optionally, a layer based on a material useful for improving wettability, preferably a layer of aluminum oxide nanoparticles (n-Al2O3); - a photoactive layer comprising at least one perovskite, preferably formamidinium lead iodide (NH2CHNH2PbI3) and at least one salified chitosan having general formula (I), preferably a salified chitosan with camphorsulfonic acid; - a layer based on an electron transporting material (“Electron Transport Layer” - ETL), preferably a layer of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM); - optionally, a layer based on a hole blocking material (“Hole Blocking Layer” - HBL), preferably a layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (Batocuproine - BCP) or ethoxylated polyethylenimine (PEIE); - a metallic contact known as back contact which constitutes the cathode, preferably a layer of metallic gold, silver, or aluminium. In accordance with a preferred embodiment of the present invention, the electrical energy generated by said at least one perovskite-based photovoltaic cell (or solar cell) can be transported using a wiring system which is connected with said perovskite-based photovoltaic cell (or solar cell). As mentioned above, a further object of the present invention is a process for the preparation of said perovskite-based photovoltaic cell (or solar cell). Consequently, a further object of the present invention is a process for the preparation of a perovskite-based photovoltaic cell (or solar cell) comprising the following steps: (a) preparing a glass substrate covered with a layer of transparent conductive oxide (TCO) (anode); (b) depositing a layer based on a hole transporting material (“Hole Transport Layer” - HTL) on the substrate obtained in said step (a); (c) optionally, depositing on the layer based on a hole transporting material (“Hole Transport Layer” - HTL) obtained in said step (b) a layer based on a material suitable for improving wettability; (d) preparing a mixture comprising perovskite precursors and at least one salified chitosan having general formula (I), said salified chitosan having general formula (I) being present in said mixture in an amount between 0.1% by weight and 20% by weight, preferably between 0.2% by weight and 10% by weight, more preferably between 0.25% by weight and 5% by weight, with respect to the total weight of the perovskite precursors; (e) depositing the mixture obtained in said step (d) on the layer based on a hole transporting material (“Hole Transport Layer” - HTL) obtained in said step (b), or on the layer based on a material to improve the wettability obtained in said step (c), obtaining a photoactive layer; (f) depositing a layer based on an electron transporting material (“Electron Transport Layer” - ETL), on the photoactive layer obtained in said step (e); (g) optionally, depositing on the layer based on an electron transporting material (“Electron Transport Layer” - ETL) obtained in said step (f), a layer based on a hole blocking material (“Hole Blocking Layer” - HBL); (h) depositing a metallic contact known as back contact which constitutes the cathode, on the layer based on an electron transporting material (“Electron Transport Layer” - ETL)) obtained in said step (f), or on the layer based of a hole blocking material (“Hole Blocking Layer” - HBL) obtained in said step (g); wherein said step (d) is carried out in an uncontrolled atmosphere, in the presence of air. For the purpose of the aforementioned process, said transparent conductive oxide (TCO), said layer based on a hole transporting material (“Hole Transport Layer” - HTL), said layer based on an electrons transporting material (“Electron Transport Layer” - ETL), said layer based on a material to improve wettability, said layer based on a hole blocking material (“Hole Blocking Layer” - HBL) and said metal contact known as back contact, are selected from those listed above. For the purpose of the aforementioned process, said mixture comprising perovskite precursors and at least one salified chitosan having general formula (I), comprises: - at least one halide selected from the halides of the monovalent organic cations or monovalent inorganic cations reported above, preferably iodides, chlorides, bromides, more preferably iodides [for example, formamidinium iodide (NH2CHNH2)I], and at least one halide selected from the halides of the above divalent metal cations, preferably iodides, chlorides, bromides, more preferably iodides [for example, lead iodide (PbI2)] as perovskite precursors; - at least one salified chitosan having general formula (I), preferably a salified chitosan with camphorsulfonic acid. For the purpose of the aforementioned process, said steps (b), (c), (e), (f) and (g), can be carried out according to deposition techniques known in the art such as, for example, spin-coating, spray-coating, ink-jet printing, slot die coating, gravure printing, screen printing. For the purpose of the aforementioned process, said step (h) can be carried out according to techniques known in the art such as, for example, evaporation, cathodic sputtering, electron beam assisted deposition, sputtering, spin coating, gravure printing, flexographic printing, slot die coating. As mentioned above, said perovskite-based photovoltaic cell (or solar cell) can be advantageously used in various applications that require the production of electrical energy through the exploitation of light energy, in particular the energy of solar radiations such as, for example: architecturally integrated photovoltaic systems (“Building Integrated Photo Voltaic” - BIPV); photovoltaic windows; greenhouses; photo-bioreactors; noise barriers; lighting engineering; design; advertising; automotive industry. Furthermore, said perovskite-based photovoltaic cell (or solar cell) can be used both in "stand alone" mode and in modular systems. Consequently, a further object of the present invention is the use of said perovskite-based photovoltaic cell (or solar cell) in: architecturally integrated photovoltaic systems ("Building Integrated Photo Voltaic" - BIPV); photovoltaic windows; greenhouses; photo-bioreactors; noise barriers; lighting engineering; design; advertising; automotive industry. As mentioned above, a further object of the present invention is also a composition comprising at least one perovskite and at least one salified chitosan having general formula (I). Consequently, a further object of the present invention is a composition comprising at least one perovskite and at least one salified chitosan having general formula (I), said salified chitosan having general formula (I) being present in an amount between 0.1% by weight and 20 % by weight, preferably between 0.2% by weight and 10% by weight, more preferably between 0.25% by weight and 5% by weight, with respect to the total weight of the perovskite precursors. Said at least one perovskite can be selected from those listed above. The present invention will now be illustrated in greater detail through an embodiment with reference to Figure 1 below reported. In particular, Figure 1 represents a cross-sectional view of a perovskite-based photovoltaic cell (or solar cell) (1) comprising the following layers: a glass substrate (7) covered by a transparent conductive oxide layer (TCO) (anode) [e.g., indium tin oxide (ITO) or fluorine-doped tin oxide (SnO2:F) (FTO)] (2); a layer based on a hole transporting material (“Hole Transport Layer” - HTL) [e.g., (2-(9H-carbazol-9-yl)ethyl phosphonic acid (2PACz)] (3); optionally, a layer based on a material useful for improving wettability, [e.g., nanoparticles of aluminum oxide (n-Al2O3)] (not represented in Figure 1); a photoactive layer comprising at least one perovskite [e.g., formamidinium lead iodide (NH2CHNH2PbI3)] and at least one salified chitosan having general formula (I) (e.g., chitosan salified with camphorsulfonic acid) (4); a layer based on an electron transporting material (“Electron Transport Layer” - ETL) [e.g., [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)] (5a); a layer based on a hole blocking material (Hole Blocking Layer” - HBL) [e.g., 2,9-dimethyl-4,7- diphenyl-1,10-phenanthroline (Batocuproine - BCP) or ethoxylated polyethylenimine (PEIE)] (5b); a metallic contact known as back contact which constitutes the cathode [e.g., a layer of metallic gold, silver or aluminium] (6). In order to better understand the present invention and to put it into practice, some illustrative and non- limiting examples of the same are reported below. In the following examples, for greater simplicity, the term "solar cell" is used which is intended to have the same meaning as "photovoltaic cell". EXAMPLE 1 Preparation of salified chitosan with salicylic acid (CHI1-sa) 2.0 g of salicylic acid (14.4 mmol) (purity 99% - Merck) and 150 ml of deionized water were introduced into a first 250 ml flask, equipped with a magnetic stirrer: everything was kept, under stirring, at 50°C, for 1 hour, obtaining the complete dissolution of the salicylic acid. 2.5 g of chitosan (CHI1) [Merck - weight average molecular weight (Mw) between 50000 Dalton and 190000 Dalton, viscosity in a mixture of water and acetic acid (Merck) (1% by weight of acetic acid) between 20 cps and 300 cps] and 70 ml of deionized water were introduced in a second 500 ml flask, equipped with a magnetic stirrer: everything was kept, under stirring, at room temperature (25°C), for 1 hour, obtaining a suspension. Subsequently, the solution obtained in the first flask was added to said second flask: everything was kept, under stirring, at room temperature (25°C), for approximately 2 hours. The solution obtained was roughly filtered on a gauze pad in order to remove the impurities present and subsequently inserted into a dialysis tube (“Dialisys Tubing Cellulose Membrane” Mw cut-off 14000 Da - Sigma Aldrich - Code: D9652) and dialyzed against deionized water for 6 days. At the end, the solution obtained was divided into batches of 70 ml and freeze-dried at room temperature (25°C) obtaining 2.15 g of chitosan salified with salicylic acid in the form of white flakes. The chitosan salified with salicylic acid (CHI1-sa) obtained was found to have a solubility in dimethyl sulfoxide (DMSO), measured at room temperature (25°C), equal to 20 mg / ml. EXAMPLE 2 Preparation of salified chitosan with camphorsulfonic acid (CHI2-ca) 3.25 g of camphorsulfonic acid (14.4 mmol) (purity 99% - Merck) and 150 ml of deionized water were introduced into a first 250 ml flask, equipped with a magnetic stirrer: everything was kept, under stirring, at 50°C, for 90 minutes, obtaining the complete dissolution of the camphorsulfonic acid. 2.5 g of chitosan (CHI2) [Merck - weight average molecular weight (Mw) between 50000 Dalton and 150000 Dalton, viscosity in a mixture of water and acetic acid (1% by weight of acetic acid) between 20 cps and 200 cps] and 70 ml of deionized water were introduced into a second 500 ml flask, equipped with a magnetic stirrer: everything was kept, under stirring, at room temperature (25°C), for 1 hour, obtaining a suspension. Subsequently, the solution obtained in the first flask was added to said second flask: everything was kept, under stirring, at room temperature (25°C), for approximately 2 hours. The solution obtained was roughly filtered on a gauze pad in order to remove the impurities present and subsequently inserted into a dialysis tube (“Dialisys Tubing Cellulose Membrane” Mwcut-off 14000 Da - Sigma Aldrich - Code: D9652) and dialyzed against deionized water for 6 days. At the end, the solution obtained was divided into 70 ml batches and freeze-dried at room temperature (25°C) obtaining 3.90 g of chitosan salified with camphorsulfonic acid (CHI2-ca) in the form of white flakes. The chitosan salified with camphorsulfonic acid (CHI2-ca) obtained was found to have a solubility in dimethyl sulfoxide (DMSO), measured at room temperature (25°C), equal to 30 mg / ml. EXAMPLE 3 Preparation of a perovskite-based solar cell For this purpose, a perovskite-based solar cell was prepared on a patterned ITO [Indium Tin Oxide] (Kintec KT18086-1) coated glass substrate (dimensions 15x15x1 mm; surface resistance “sheet resistance” equal to 12 ^ / cm2) previously subjected to a cleaning procedure consisting of manual cleaning, rubbing with a lint-free cloth soaked in a detergent diluted with deionized water. The substrate was then rinsed with deionized water. Subsequently, the substrate was thoroughly cleaned using the following methods in sequence: ultrasonic baths in (i) deionized water plus detergent (followed by manual drying with a lint-free cloth); (ii) distilled water [followed by manual drying with a lint-free cloth]; (iii) acetone (Merck) and (iv) iso-propanol (Merck) in sequence. In particular, the substrate was placed in a beaker containing the solvent, placed in an ultrasonic bath, maintained at 40°C, for a 10 minutes treatment. After treatments (iii) and (iv), the substrate was dried with a compressed nitrogen flow. Subsequently, the glass / ITO was further cleaned by treatment in an ozone device (UV Ozone Cleaning System EXPO3 - Astel), immediately before proceeding to the next step. The substrate thus treated was ready for the deposition of the layer based on a hole transporting material (“Hole Transport Layer” - HTL). For this purpose, a solution of (2-(9H-carbazol-9-yl)ethyl phosphonic acid (2PACz) (Merck) in anhydrous ethanol (max 0.003% by weight of water - VWR) at a concentration equal to 0.5 mg / ml, was deposited, through spin coating operating at a rotation speed equal to 3000 rpm (acceleration equal to 100 rpm / s), for 30 seconds: everything was subjected to heat treatment (annealing), at 100°C, for 10 minutes. The layer based on a hole transporting material (“Hole Transport Layer” - HTL) is a “self-assembly monolayer” having a thickness < 1 nm. A material was deposited on the substrate thus obtained to improve wettability. For this purpose, a solution of aluminum oxide nanoparticles (n-Al2O3) (particle size < 50 nm - Merck) in iso-propanol (purity 99.5% - Merck) at a concentration equal to 0.2 mg / ml, was deposited, through spin coating operating at a rotation speed equal to 3000 rpm (acceleration equal to 1000 rpm / s), for 30 seconds, then everything was subjected to heat treatment (annealing), at 100 °C, for 5 minutes. Above the layer based on a material to improve wettability, the layer of formamidinium lead iodide (NH2CHNH2PbI3) and chitosan salified with camphorsulfonic acid (CHI2-ca) obtained as reported in Example 2, was deposited operating as follows. For this purpose, lead iodide (PbI2) (ultra dry purity 99.999% - Alfa Aesar) (461.0 mg - 1.0 mmol), formanmidinium iodide (FAI) (NH2CHNH2I) (GreatCell Solar) (172.0 mg - 1.0 mmol), methylammonium chloride (MACl) (CH3NH3Cl) - (Merck) (13.5 mg - 0.2 mmol) and chitosan salified with camphorsulfonic acid (CHI2-ca) (2.0 mg), were dissolved in anhydrous dimethyl sulfoxide (purity 99.9% - Merck) (1 ml), operating under stirring, at a temperature of 80°C, for 3 hours, obtaining a solution containing 37% by weight of perovskite precursors and 0.17% by weight of chitosan salified with camphorsulfonic acid (CHI2-ca) obtained as reported in Example 2, i.e. 0.31% by weight of chitosan salified with camphorsulfonic acid (CHI2-ca) with respect to the total weight of the other solid components [i.e. lead iodide (PbI2) + formamidinium iodide (FAI) (NH2CHNH2I)]. The solution thus obtained was deposited on said layer based on a material useful for improving wettability, through spin coating operating at a rotation speed equal to 4000 rpm (acceleration equal to 1000 rpm / s), for 20 seconds, with the addition of 300 ^l of antisolvent (i.e. ethyl acetate - Merck) after 11 seconds from the start of the operation, and everything was subjected to heat treatment (annealing), at 150°C, for 10 minutes, operating in uncontrolled atmosphere, in the presence of air. The thickness of the layer of perovskite and chitosan salified with camphorsulfonic acid (CHI2ca) was found to be equal to 300 nm. The substrate thus obtained was ready for the deposition of the layer based on an electron transporting material (“Electron Transport Layer” -ETL). For this purpose, a filtered solution of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) (Nano-C Products) (25 mg) in anhydrous chlorobenzene (purity 99.8% - Merck) (1 ml), was deposited, via spin coating operating at a rotation speed equal to 1000 rpm (acceleration equal to 500 rpm / s), for 60 seconds: the substrate obtained was left to rest, at room temperature (25 °C), for 5 minutes. The thickness of the layer based on an electron transporting material (“Electron Transport Layer” - HTL) was found to be equal to 50 nm. The substrate thus obtained was ready for the deposition of the layer based on a hole blocking material (“Hole Blocking Layer” - HBL). For this purpose, a solution of 2,9-dimethyl-4,7-diphenyl-1,10-phenatroline (Batocuproine - BCP) (purity 96% - Merck) (9 mg) in anhydrous iso-propyl alcohol (purity 99.5% - Merck) (18 ml) obtained by operating under stirring at 60°C, for 3 hours, was deposited through spin coating operating at a rotation speed equal to 6000 rpm (acceleration equal to 1000 rpm / s), for 20 seconds, the substrate obtained was left to rest, at room temperature (25°C), for 5 minutes. The thickness of the layer based on a hole blocking material (“Hole Blocking Layer” - HBL) was found to be equal to 5 nm. Subsequently, the silver (Ag) back contact (cathode) was deposited on top of said layer based on a hole blocking material (“Hole Blocking Layer” - HBL), via evaporation. For this purpose, a Kurt J. Lesker evaporator was used, operating at a pressure equal to 2x10-6mmHg and at a speed equal to 0.1 Angstrom / sec, appropriately masking the area of the solar cell in order to obtain an active area equal to 4 mm2. The thickness of the silver (Ag) back contact (cathode) was found to be equal to 80 nm. The thicknesses were measured by scanning electron microscopy using a Sigma-Zeiss scanning electron microscope (SEM), equipped with a field emission electron gun, operating with an accelerating voltage equal to 5 kV, and exploiting the signal coming from secondary electrons. The electrical characterization of the perovskite- based solar cell thus obtained was carried out at room temperature (25°C). Current density-voltage (J-V) curves were acquired with a Keithley®2400 digital multimeter connected to a personal computer for data collection. The photocurrent was measured by exposing the solar cell to the light of a Newport 91160A solar simulator (Newport Corp), placed at a distance of 10 mm from said solar cell, equipped with a 300 W Xenon light source, using a lighting spot equal to 100 mm x 100 mm: in Table 1, the characteristic parameters are reported as average values. The light intensity was calibrated with a standard silicon solar cell (“VLSI Standard” - SRC-100-RTD-KG5). In particular, Table 1 shows, in order: the number of the reference Example; the composition of the perovskite photoactive layer and chitosan salified with camphorsulfonic acid (CHI2-ca); FF (Fill Factor); Voc (Open Circuit Voltage); Jsc (short-circuit photocurrent density); PCE (Power Conversion Efficiency). EXAMPLE 4 Preparation of a perovskite-based solar cell The perovskite-based solar cell was obtained using the same procedure reported in Example 3, with the only difference deriving from the use of a different quantity of chitosan salified with camphorsulfonic acid (CHI2-ca) obtained as reported in Example 2. For this purpose, lead iodide (PbI2) (ultra dry purity 99.999% - Alfa Aesar) (461.0 mg – 1.0 mmol), formamidinium iodide (FAI) (NH2CHNH2I) (GreatCell Solar) (172.0 mg – 1.0 mmol), methylammonium chloride (MACl) (CH3NH3Cl) (Merk) (13.5 mg – 0.2 mmol), chitosan salified with caphorsulfonic acid (CHI2-ca) obtained as reported in Example 2 (4 mg), were dissolved in anhydrous dimethyl sulfoxide (purity 99.9% - Merck) (1 ml), operating under stirring, at a temperature of 80°C, for 3 hours, obtaining a solution containing 37% by weight of perovskite precursors and 0.34% by weight of chitosan salified with camphorsulfonic acid (CHI2-ca) obtained as reported in Example 2, i.e. 0.62% by weight of chitosan salified with camphorsulfonic acid (CHI2-ca) with respect to the total weight of the other solid components (i.e. lead iodide (PbI2) + formamidinium iodide (FAI) (NH2CHNH2I) + methylammonium chloride (MACl) (CH3NH3Cl) . The electrical characterization of the perovskite- based solar cell obtained was carried out as described above: in Table 1, the characteristic parameters are reported as average values. Table 1 Examp Photoactive layer FF(1)Voc Jsc(3)PCE(4le (%) (2) (mA / cm ) (V)2) (%) 3 NH2CHNH2PbI3 (37)(5)+ 77.4 1.0 22.3 18.1 CHI2-ca (0.3)(6)5 4 NH2CHNH2PbI3 (37)(5)+ 77.8 1.0 22.6 18.7 CHI2-ca (0.62)(6)6(1): “Fill Factor”;(2): “Open Circuit Voltage”;(3): “short-circuit photocurrent density”;(4): “Power Conversion Efficiency”;(5): formamidinium lead iodide (NH2CHNH2PbI3) [(in brackets % by weight of perovskite precursors (i.e. lead iodide (PbI2) + formamidinium iodide (FAI) (NH2CHNH2I) + methylammonium chloride (MACl) (CH3NH3Cl)];(6): chitosan salified with camphorsulfonic acid (CHI2- ca) (in brackets % weight of chitosan salified with camphorsulfonic acid (CHI2-ca) with respect to the total weight of the other solid components [i.e. lead iodide (PbI2) + formamidinium iodide (FAI) (NH2CHNH2I) + methylammonium chloride (MACl) (CH3NH3Cl)]. From the data reported in Table 1 it can be seen that the perovskite-based solar cell object of the present invention has both a good power conversion efficiency (PCE)(i.e. PCE > 10%), and good electrical properties, i.e. good values of FF (Fill Factor), Voc (Open Circuit Voltage); Jsc (short-circuit photocurrent density).

Claims

CLAIMS 1. Perovskite-based photovoltaic cell (or solar cell) wherein the perovskite photoactive layer comprises at least one salified chitosan having general formula (I):wherein: - X- represents a carboxylate anion having general formula (II): R1-COO-, (II), or a sulfonate anion having general formula (III): R2-SO3- (III), or a phosphonate anion having general formula (IV): R3-PO3H- (IV), wherein R1, R2 and R3, equal to or different from each other, represent a C4-C30, preferably C4-C20, linear or branched, saturated or unsaturated, optionally containing heteroatoms alkyl group; an optionally substituted aryl group; an optionally substituted heteroaryl group; an optionally substituted cycloalkyl group; an optionally substituted heterocyclic group; - a is an integer or fractional number between 0.1 and 1, preferably between 0.5 and 1; - b is a fractional number between 0 and 0.9, preferably between 0 and 0.5; - c is a fractional number between 0 and 0.4, preferably between 0 and 0.2;- p is an integer between 3 and 500, preferably between 5 and 400, more preferably between 10 and 250.

2. Perovskite-based photovoltaic cell (or solar cell) according to claim 1, wherein in said general formula (I) the sum of a + b + c is equal to 1.

3. Perovskite-based photovoltaic cell (or solar cell) according to claim 1 or 2, wherein said perovskite is selected from organometallic trihalides having the general formula ABX3wherein: - A represents a monovalent organic cation such as methylammonium (CH3NH3+), formamidinium [CH(NH2)2+], n-butylammonium (C4H9NH3+), tetra-butylammonium (C16H36N+), or mixtures thereof; or A represents a monovalent inorganic cation such as cesium (Cs+), rubidium (Rb+), potassium (K+), lithium (Li+), sodium (Na+), copper (Cu+), silver (Ag+), or mixtures thereof; or mixtures thereof; - B represents a divalent metal cation such as lead (Pb2+), tin (Sn2+), or mixtures thereof; - X represents a halide anion such as iodine (I-), chlorine (Cl-), bromine (Br-), or mixtures thereof.

4. Perovskite-based photovoltaic cell (or solar cell) according to any one of the preceding claims, wherein said perovskite is selected from: methylammonium lead iodide (CH3NH3PbI3), methylammonium lead bromide (CH3NH3PbBr3), methylammonium lead chloride (CH3NH3PbCl3), methylammonium lead iodide bromide (CH3NH3PbIxBr3-x), methylammonium lead iodide chloride (CH3NH3PbIxCl3-x), formamidinium lead iodide [CH(NH2)2 PbI3], formamidinium lead bromide [CH(NH2)2PbBr3], formamidinium lead chloride [CH(NH2)2PbCl3],formamidinium lead iodide bromide [CH(NH2)2PbIxBr3-x], formamidinium lead iodide chloride [CH(NH2)2PbIxCl3-x], methylammonium formamidinium lead iodide [(CH3NH3)x(CH(NH2)2)1-xPbI3], methylammonium formamidinium lead bromide [(CH3NH3)x(CH(NH2)2)1-xPbBr3], methylammonium formamidinium lead chloride [(CH3NH3)x(CH(NH2)2)1-xPbCl3], methylammonium formamidinium lead iodide chloride [(CH3NH3)x(CH(NH2)2)1-xPbI3-yCly], methylammonium formamidinium lead iodide bromide [(CH3NH3)x (CH(NH2)2)1-xPbI3-y Bry], n- butylammonium lead iodide (C4H9NH3PbI3), tetra- butylammonium lead iodide (C16H36NPbI3), n-butylammonium lead bromide (C4H9NH3PbBr3), tetra-butylammonium lead bromide (C16H36NPbBr3), cesium lead iodide (CsPbI3), rubidium lead iodide (RbPbI3), potassium lead iodide (KPbI3), cesium methylammonium lead iodide [Csx(CH3NH3)1-xPbI3), potassium methylammonium lead iodide [Kx(CH3NH3)1-xPbI3), cesium methylammonium lead iodide chloride [Csx(CH3NH3)1-xPbI3-yCly), cesium formamidinium lead iodide [Csx(CH(NH2)2)1-xPbI3], cesium formamidinium lead bromide [Csx(CH(NH2)2)1-xPbBr3], cesium formamidinium lead iodide chloride [Csx(CH(NH2)2)1-xPbI3-yCly], methylammonium tin iodide (CH3NH3SnI3), methylammonium tin bromide (CH3NH3SnBr3), methyl ammonium tin iodide bromide (CH3NH3SnIxBr3-x), formamidinium tin iodide [CH(NH2)2SnI3], formamidinium tin iodide bromide [CH(NH2)2SnIxBr3-x], n-butylammonium tin iodide (C4H9NH3SnI3), tetra-butylammonium tin iodide (C16H36NSnI3), n-butylammonium tin bromide (C4H9NH3SnBr3), tetra-butylammonium tin bromide (C16H36NSnBr3), methylammonium tin lead iodide (CH3NH3SnxPb1-xI3), formamidinium tin lead iodide [CH(NH2)2SnxPb1-xI3], ormixtures thereof; is preferably selected from methylammonium lead iodide (CH3NH3PbI3), formamidinium lead iodide [CH(NH2)2PbI3], methylammonium formamidinium lead iodide chloride [(CH3NH3)x(CH(NH2)2)1-xPbI3-yCly], cesium methylammonium lead iodide chloride [Csx(CH3NH3)1-xPbI3-yCly), cesium formamidinium lead iodide chloride [Csx(CH(NH2)2)1-xPbI3-yCly]; more preferably is formamidinium lead iodide [CH(NH2)2PbI3].

5. Perovskite-based photovoltaic cell (or solar cell) according to any one of the preceding claims wherein said salified chitosan having general formula (I): - has a weight average molecular weight (Mw) between 3000 Dalton and 500000 Dalton, preferably between 5000 Dalton and 400000 Dalton, more preferably between 10000 Dalton and 300000 Dalton; and / or - dissolved in a mixture of water and acetic acid (1% by weight of acetic acid with respect to the total weight of the water / acetic acid mixture) in a concentration equal to 1% by weight with respect to the total weight of the water / acetic acid mixture, has a viscosity between 10 cps and 4000 cps, preferably between 15 cps and 3000 cps, more preferably between 18 cps and 2000 cps; and / or - has a solubility in dimethyl sulfoxide (DMSO), measured at room temperature (25°C), up to 50 mg / ml, preferably up to 40 mg / ml, more preferably between 15 mg / ml and 35 mg / ml.

6. Perovskite-based photovoltaic cell (or solar cell) according to any one of the preceding claims, wherein said salified chitosan having general formula (I) is present in the perovskite photoactive layer in an amount between 0.1% by weight and 20% by weight, preferablybetween 0.2% by weight and 10% by weight, more preferably between 0.25% by weight and 5% by weight, with respect to the total weight of the perovskite precursors.

7. Perovskite-based photovoltaic cell (or solar cell) according to any one of the preceding claims, comprising: - a glass substrate covered with a layer of Transparent Conductive Oxide (TCO), typically fluorine-doped tin oxide (SnO2:F), Fluorine-doped Tin Oxide (FTO),or indium tin oxide (ITO) which forms the anode; - a layer based on a hole transporting material ((“Hole Transport Layer” - HTL), preferably a layer of (2-(9H-carbazol-9-yl)ethyl phosphonic acid (2PACz) ; - optionally, a layer based on a material useful for improving wettability, preferably a layer of aluminum oxide nanoparticles (n-Al2O3); - a photoactive layer comprising at least one perovskite, preferably formamidinium lead iodide (NH2CHNH2PbI3) and at least one salified chitosan having general formula (I), preferably a chitosan salified with camphorsulphonic acid; - a layer based on an electron transporting material (“Electron Transport Layer” - ETL), preferably a layer of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM); - optionally, a layer based on a hole blocking material (“Hole Blocking Layer” - HBL), preferably a layer of 2,9-dimethyl-4,7-diphenyl-1,10- phenanthroline (batocuproine-BCP) orpolyethyleneimine ethoxylate (PEIE); - a metallic contact known as back contact which constitutes the cathode, preferably a layer of metallic gold, silver or aluminium.

8. Perovskite-based photovoltaic cell (or solar cell) according to any one of the preceding claims, wherein the electrical energy generated by said at least one perovskite-based photovoltaic cell (or solar cell) is transported using a wiring system which is connected with said perovskite-based photovoltaic cell (or solar cell).

9. Process for the preparation of a perovskite-based photovoltaic cell (or solar cell) comprising the following steps: (a) preparing a glass substrate covered with a layer of transparent and conductive oxide (TCO) (anode); (b) depositing a layer based on a hole transporting material (“Hole Transport Layer” - HTL) on the substrate obtained in said step (a); (c) optionally, depositing on the layer based on a hole transporting material “Hole Transport Layer” - HTL) obtained in said step (b) a layer based on a material suitable for improving wettability; (d) preparing a mixture comprising perovskite precursors and at least one salified chitosan having general formula (I), said salified chitosan having general formula (I) being present in said mixture in an amount between 0.1% by weight and 20% by weight, preferably between 0.2% by weight and 10% by weight, more preferably between 0.25% by weight and 5% by weight, with respect to the total weight of the perovskite precursors;(e) depositing the mixture obtained in said step (d) on the layer based on a hole transporting material (“Hole Transporting Material” - HTL) obtained in said step (b), or on the layer based on a material to improve the wettability obtained in said step (c), obtaining a photoactive layer; (f) depositing a layer based on an electron transporting material (“Electron Transport Layer” - ETL), on the photoactive layer obtained in said step (e); (g) optionally, depositing on the layer based on an electron transporting material (“Electron Transport Layer” - ETL) obtained in said step (f), a layer based on a hole blocking material (“Hole Blocking Layer” - HBL); (h) depositing a metallic contact known as back contact which constitutes the cathode, on the layer based on an electron transporting material (“Electron Transport Layer” - ETL) obtained in said step (f), or on the layer based of a hole blocking material (“Hole Blocking Layer” - HBL) obtained in said step (g); wherein said step (d) is carried out in an uncontrolled atmosphere, in the presence of air.

10. Use of a perovskite-based photovoltaic cell (or solar cell) according to any one of the preceding claims in: architecturally integrated photovoltaic systems (Building Integrated Photo Voltaic-BIPV); photovoltaic windows; greenhouses; photo-bioreactors; noise barriers; lighting engineering; design; advertising; automobile industry.

11. Composition comprising at least one perovskite andat least one salified chitosan having general formula (I), said salified chitosan having general formula (I) being present in an amount between 0.1% by weight and 20% by weight, preferably between 0 2% by weight and 10% by weight, more preferably between 0.25% by weight and 5% by weight, with respect to the total weight of the perovskite precursors.

12. Composition according to claim 11, wherein said perovskite is selected from those referred to in claim 3 or 4 and said salified chitosan having general formula (I) is selected from those referred to in claim 5.

Images