Solar cell comprising a polymer layer comprising acrylates

Abstract

The present invention relates to solar cells having a polymer layer arranged adjacent to the protective layer, said polymer layer containing acrylate polymers, manufacturing methods thereof and liquid compositions for use in the manufacturing of said polymer layer.

Description

[0001] Solar Cell Comprising a Polymer Layer Comprising Acrylates

[0002] Technical Field

[0003] The present invention relates to the technical field of solar cells comprising a protective layer arranged on the top of the solar cell structure and a polymer layer arranged adj acent to said layer, and methods for the manufacturing of such solar cells .

[0004] Disclosure of the Invention

[0005] Solar cells are electronic devices that convert light energy into electric energy . The operation of a solar cell requires a photoconversion layer in which the absorption of light energy excites an electron to a higher energy level and an electric circuit that receives the excited electron .

[0006] Typically, a solar cell therefore comprises a photoconversion layer that is arranged between two electrodes . To protect the photoconversion layer and the electrodes , a protecting layer is arranged on top of the solar cell structure . It might be challenging to find a way to add the protecting substrate onto the solar cell structure , since liquid adhesives which are used to bond the protecting layer might destroy one of the layer underneath, in particular the electrode layer or the photoconversion layer . The liquid adhesive material might also destroy the photoconversion layer even i f an electrode separates the photoconversion layer from the adhesive because electrodes are thin layers having small pinholes or microcracks through which components of said adhesive can dif fuse and degrade the photoconversion layer . Components of said adhesive can also di f fuse through the charge transport layers as these also have pinholes or microcracks , especially in the case of nanoparticle-based charge transport layers . The present invention therefore solves the problem to present a solar cell structure and a method to enable the addition of a protecting layer to the solar cell structure .

[0007] Unless otherwise stated, the following definitions shall apply in this speci fication :

[0008] The terms "a" , "an" "the" and similar terms used in the context of the present invention are to be construed to cover both the singular and plural unless otherwise indicated herein or clearly contradicted by the context . The term "containing" shall include all , "comprising" , "essentially consisting of" and "consisting of" . Percentages are given as weight-% , unless otherwise indicated herein or clearly contradicted by the context . " Independently" means that one substituent / ion may be selected from one o f the named substituents / ions or may be a combination of more than one of the above .

[0009] The term "perovskite crystals" is known and particularly includes crystalline compounds of the perovskite structure . Such perovskite structures are known per se and described as cubic, pseudocubic, tetragonal or orthorhombic crystals of general formula M1M2X3 , where Ml are cations of coordination number 12 ( cuboctaeder ) and M2 are cations of coordination number 6 ( octaeder ) and X are anions in cubic, pseudocubic, tetragonal or orthorhombic positions of the lattice . In these structures , selected cations or anions may be replaced by other ions ( stochastic or regularly up to 30 atom-% ) , thereby resulting in doped perovskites or non-stochiometric perovskites , still maintaining its original crystalline structure .

[0010] The term "photoconversion layer" is known and relates to the layer in the solar cell where the absorption of light or solar energy causes the electrons to be excited and to j ump to a higher energy level . An electron-hole pair is formed which is called an exciton .

[0011] A first aspect of the invention refers to a solar cell comprising a photoconversion layer arranged between a first electrode and a second electrode and a polymer layer arranged between the second electrode and a protective layer . The polymer layer is arranged adj acent to the protective layer .

[0012] That the polymer layer is arranged adj acent to the protective layer means in particular that the polymer layer is in direct contact with the protective layer, preferably adhering the protective layer . Further advantageously, the protective layer protects the layer stack of the solar cell against the environment . Further advantageously, the protective layer is transparent . Further advantageously, the protective layer is a glass layer . Further advantageously, the protective layer acts as a barrier layer or barrier film . Further advantageously, the protective layer has a multiple layer structure comprising a polymer substrate , preferably PET and / or an inorganic gas- impermeable layer, and / or a polymeric adhesion layer, wherein the inorganic gas-impermeable layer or the adhesion layer, i f there is any, is in direct contact with the polymer layer .

[0013] The first and the second electrode are in electrical contact with the photoconversion layer . This means that they are directly or indirectly electrically connected with the photoconversion layer . Directly in contact means that the electrode touches directly the photoconversion layer . Indirectly in contact means that there might be a charge transport layer between the electrode and the photoconversion layer, in particular a hole or an electron charge transport layer . Advantageously, there might be a partial contact between the first and the second electrode with the photoconversion layer, e . g . i f the charge transport layer is porous . Anyway, advantageously, there are additional layers, for example charge transport layers, arranged between the photoconversion layer and the first and / or second electrode. The polymer might be in contact, respectively arranged adjacent, at least partially, to the photoconversion layer. Therefore, it might be advantageous if the polymer layer is compatible with the photoconversion layer.

[0014] Surprisingly it has been found that the polymer layer comprising at least 50 wt-%, preferably at least 70 wt-% of acrylate polymers, wherein said acrylate polymers comprise acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups, optionally substituted with one or more cyclic, linear or branched C1-20 alkyl or alkenyl groups, is fully compatible with the other layers present in the solar cell, in particular a perovskite solar cell, according to the present invention (i.e. does not degrade the other layers present in the solar cell during the manufacturing process and does not negatively impact their properties during the solar-cell lifetime) , and ensures excellent adhesion of the protective layer to the other layers present in said solar cell. Furthermore owing to the presence in its composition of the herein described acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups, the polymer layer described herein exhibits a low WVTR (water vapor transmission rate) , thereby ensuring a low water diffusion and thus minimizing water diffusion towards the water-sensitive photoconversion layer (e.g. perovskite layer) , and a low tendency towards yellowing caused by light and temperature influence .

[0015] In one embodiment, the polymer layer further comprises a polyisobutylene (PIB) polymer. In one embodiment the polymer layer consists essentially of the herein described acrylate polymers and optionally the herein described additives. The acrylate polymers are preferably UV-cured . Owing to the high speed UV curing, polymer layers containing UV-cured acrylate polymers are added to the other layers of the solar cell at high speed .

[0016] The polymer layer has preferably a thickness from 10pm to 200pm, more preferably from 20pm to 100pm, most preferably from 30pm to 70pm .

[0017] The polymer layer has preferably a glass transition temperature ( Tg) lower than 100 ° C, more preferably lower than 50 ° C, most preferably lower than 0 ° C . It is believed that the low glass transition temperature results in better resistance to temperature variations ( e . g . temperature variations to which the solar cell is subj ected between summer and winter season) by ensuring softness of the polymer layer also at low temperatures and thereby, preventing the occurrence of temperature dependent cycles between soft and brittle state of the polymer . As well known to the skilled person, glass transition temperature ( Tg) is a well-established parameter in the field of polymers ; it describes the temperature where an amorphous or semi-crystalline polymer changes from a glassy (hard) state to a more pliable , compliant or rubbery state . Polymers with high Tg are considered "hard" , while polymers with low Tg are considered " soft" . On a molecular level , Tg is not a discrete thermodynamic transition, but a temperature range over which the mobility of the polymer chains increase signi ficantly . The convention, however, is to report a single temperature defined as the midpoint of the temperature range , bounded by the tangents to the two flat regions of the heat flow curve of the DSC measurement . Tg may be determined according to DIN EN ISO 11357-2 or ASTM E1356 using DSC . This method is particularly suitable i f the polymer is present in the form of bulk material . Alternatively, Tg may be determined by measuring temperaturedependent micro- or nanohardness with micro- or nanoindentation acording to ISO 14577 - 1 or ASTM E2546- 15 . This method is suited for solar cells as disclosed herein . Suitable analytical equipment is available as MHT (Anton Paar ) , Hysitron TI Premier (Bruker ) or Nano Indenter G200 (Keysight Technologies ) . Data obtained by temperature controlled micro- and nanoindentation can be converted to Tg . Typically, the plastic deformation work or Young' s modulus or hardness is measured as a function of temperature and Tg is the temperature where these parameters change signi ficantly .

[0018] The acrylate polymers described herein preferably contain at least 30 wt-% , more preferably at least 50 wt-% , much preferably at least 60 wt-% , most preferably at least 70 wt-% of the acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups .

[0019] The acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups are preferably selected from mono- functional acrylates of formula ( I ) : and bi- functional acrylates of formula ( I D , wherein in formula ( I ) :

[0020] R9represents H or CH3,

[0021] R10represents a cyclic C5-25 alkyl or alkenyl , optionally substituted with one or more cyclic, linear or branched C1-20 alkyl or alkenyl , n represents 0 or 1 , and X represents a spacer from the group of alkox- ylates comprising 1-40 carbon atoms and 1-10 oxygen atoms ; and in formula (II)

[0022] R31independently from each other represent H or CH3;

[0023] R33represents a cyclic C5-25 alkylene or a cyclic C5-25 alkenylene, each optionally substituted with one or more cyclic, linear or branched C1-20 alkyl;

[0024] X32are independent from each other and are either not present (R33directly bound to the oxygen by a single bond) or represent a spacer selected from the group of alkylenes and alkoxylates, whereby both substituents X32together comprise 1-8 carbon atoms and if present 1-8 oxygen atoms.

[0025] As used herein, the term cyclic Cx yalkyl / al- kylene includes monocyclic and polycyclic groups, including fused ring systems comprising x-y carbon atoms as ring members. As used herein, the term cyclic Cx yalkenyl / al- kynylene includes monocyclic and polycyclic groups, including fused ring systems comprising x-y carbon atoms as ring members and at least one double bond.

[0026] Mono-functional acrylates of formula (I) include acrylates of formulae (1-1) and (1-2) where R9is H, and methacrylates of formula (1-3) and (1-4) where R9is methyl, collectively also termed acrylates. Further, compounds of formula (I) also include acrylates of formulae (1-1) and (1-3) , wherein n is 0 and X is not present: and alkoxylated acrylates of formulae (1-2) and (1-4) : wherein R10has the meaning described herein. The residue R10is preferably a cyclic C6-20 alkyl or alkenyl, more preferably a cyclic Ce-12 alkyl or alkenyl, optionally comprising 1-6 substituents, each substituent independently selected from Ci-4 alkyl, such as methyl. Specific examples of R10include 3,3,5 trimethylcyclohexyl, 4-tert-butylcyclohexyl, isobornyl, dicy- clopentanyl and dicyclopentenyl.

[0027] Specific examples of acrylates of formula (I-

[0028] 1) and (1-3) include: isobornylacrylate (CAS Nr 5888-33- 5) , isobornylmethacrylate (CAS Nr 7534-94-3) , dicyclopen- tanyl-acrylate (CAS Nr 79637-74-4, FA-513AS (Hitachi Chemical, Japan) ) , dicyclopentanyl-methacrylate (CAS Nr 34759- 34-7, FA-513M (Hitachi Chemical, Japan) ) , dicylopentenyl acrylate (CAS Nr. 33791-58-1, FA-511AS (Hitachi Chemical, Japan) ) , dicylopentenyl acrylate, 3 , 3 , 5-trimethyl cyclohexyl acrylate (CAS Nr 86178-38-3) , 3 , 3 , 5-trimethyl cyclohexyl methacrylate (CAS Nr 7779-31-9) , 4-tert-butylcyclohexyl acrylate (CAS Nr 84100-23-2) , 4-tert-butylcyclohexyl methacrylate (CAS Nr 46729-07-1) .

[0029] Specific examples of acrylates of formula (I-

[0030] 2) and (1-4) include: ethylene glycol dicyclopentenyl ether acrylate (including CAS Nr. : 65983-31-5, such as FA-512AS from Hitachi Chemical, Japan) , ethylene glycol dicyclopentenyl ether methacrylate (including CAS Nr. : 68586-19-6, such as FA-512M, FA-512MT from Hitachi Chemical, Japan) , ethylene glycol dicyclopentanyl ether acrylate, ethylene glycol dicyclopentanyl ether methacrylate, ethylene glycol 3 , 3 , 5-trimethyl cyclohexyl acrylate, ethylene glycol 3 , 3 , 5-trimethyl cyclohexyl methacrylate, ethylene glycol isobornylacrylate, and ethylene glycol isobornylmethacrylate .

[0031] Bi-functional acrylates of formula (II) include acrylates of formula (II) where both R31are H, and methacrylates of formula (II) where R31are methyl, collectively also termed acrylates.

[0032] In one embodiment, R33is directly bound to the oxygen of the (meth) acrylate group by a single bond i.e. neither of X32is present. In another embodiment, both X32are independently of each other Ci-4 alkylene spacers, pref- erably C1-2 alkylene spacers. The X32spacers may be identical or different. In one embodiment the X32spacers are identical .

[0033] The residue R33preferably represents a cyclic Ce-20 alkylene or a cyclic Ce-20 alkenylene, more preferably a cyclic Ce-12 alkylene or a cyclic Ce-12 alkenylene, each optionally substituted with one or more cyclic, linear or branched C1-20 alkyl, preferably linear or branched C1-4 alkyl, such as methyl.

[0034] Specific examples of bi-functional acrylates of formula (IT) include tricyclodecanedimethanol diacrylate (CAS Nr: 42594-17-2) and tricyclodecanedimethanol dimethacrylate .

[0035] In a preferred embodiment, the acrylate polymers contain at least 20 wt-%, preferably at least 40 wt- %, more preferably at least 60 wt-% of the mono-functional acrylates of general formula (I) .

[0036] In a further preferred embodiment, the acrylate polymers consist essentially of acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups. For example, the acrylate polymers may consist of monofunctional acrylate of general formula (I) and bi-functional acrylates of general formula (II) .

[0037] The acrylate polymers described herein may comprise further acrylate repeating units preferably selected from the group of mono-, bi-, tri-, tetra-, penta-, and hexa-functional acrylates. The further acrylate repeating units are structurally different from the acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups i.e. do not contain cyclic C5-25 alkyl and / or alkenyl groups. The content of further acrylate repeating units in the acrylate polymers is preferably lower than the content of the acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups in said acrylate polymers. In embodiments, the acrylate polymers contain at most 30 wt- % of the herein described further acrylate repeating units.

[0038] The further mono-functional acrylate may be a compound of general formula (IV) ( IV) , wherein :

[0039] R9represents H or CH3,

[0040] R10represents a linear or branched C1-25 alkyl, or a Ce-26 aryl group, each optionally substituted with one or more cyclic, linear or branched C1-20 alkyl, phenyl, phenoxy, hydroxyl, or amino groups, n represents 0 or 1, and

[0041] X represents a spacer from the group of alkoxylates comprising 1-8 carbon atoms and 1-4 oxygen atoms, and alkylenes comprising 1 - 8 carbon atoms.

[0042] Mono-functional acrylates of general formula (IV) wherein R10is a linear or branched C3-20 alkyl (e.g. 2-ethyl-hexyl- (meth) acrylate, N-butyl- (meth) acrylate, lauryl (meth) acrylate, stearyl acrylate) ) may act as reactive plasticizers and aid in decreasing the glass transition temperature Tg of the polymer layer. Examples of further mono-functional acrylates of general formula (IV) include, poly ( ethylene glycol) phenyl ether acrylates, such as 2- phenoxyethyl acrylate, O-phenyl phenoxyethyl acrylate, polyethylene glycol o-phenylphenyl ether acrylate (CAS 72009-86-0) , poly ( ethylene glycol) ethyl ether methacrylate, di (ethylene glycol) ethyl ether acrylate, poly (ethylene oxide) nonylphenyl etheracrylate, poly (propylene glycol) 4-nonylphenyl ether acrylate, and ethylene glycol dicyclopentenyl ether acrylate.

[0043] The further bi-functional acrylate may be a compound of formula (III) (III) , wherein:

[0044] R21independently from each other represent H or CH3; R23represents a linear or branched C1-25 alkyl, or a Ce-26 aryl group, each optionally substituted with one or more linear or branched C1-20 alkyl, phenyl or phenoxy;

[0045] X22independently from each other represent a spacer selected from the group of alkoxylates, whereby both substituents X22together comprise 8-40 carbon atoms and 2- 20 oxygen atoms, tris (2-hydroxy ethyl ) isocyanurate diacrylate or neopentyl glycol hydroxypivalate diacrylate (CAS Nr. 2136366-99-7, such as SR606A) .

[0046] The further bi-functional acrylate is preferably a compound of formula (III-l) wherein R represents H or CH3 and m+n is between 4 and 10.

[0047] Specific examples of further di-functional acrylates include 1 , 4-butanediol diacrylate, 1,4-butane- diol dimethacrylate, 1 , 3-butanediol dimacrylate, 1,3-bu- tanediol dimethacrylate, 3-methyl 1 , 5-pentanediol diacrylate, 3-methyl 1 , 5-pentanediol dimethacrylate, 1,9-nonane- diol diacrylate, 1,9 - neopentylglycol dimethacrylate, 1 , 10-decanediol diacrylate, 1 , 10-decanediol dimethacrylate, 1 , 6-hexanediol diacrylate, 1 , 6-hexanediol dimethacrylate, neopentyl glycol dimethacrylate, polypropyleneglycol diacrylate (including CAS Nr. 52496-08-9, FA-P240A (n=7) , FA-P270A (n = 12) from Hitachi Chemical, Japan) , polyethyleneglycol dimethacrylate (including CAS Nr: 25852-47-5; such as n=4, FA-220M; n=7 FA-240M from Hitachi Chemical, Japan) , ethoxylated Bisphenol A dimethacrylate (including CAS Nr. 24448-20-2, such as FA-320M, FA-321M and FA-3218M from Hitachi Chemical, Japan) , ethoxylated Bisphenol A diacrylate (including CAS 64401-02-1, such as Miramer M240 (Miwon Korea) , Miramer M2100 (Miwon Korea) , Fancryl FA-324A (Hitachi Chemical, Japan) , Fancryl FA-326A (Hitachi Chemical, Japan) , Fancryl FA-328A (Hitachi Chemical, Japan) , Fancryl FA-321A (Hitachi Chemical, Japan) ) , Bisphenol A ethoxylate dimethacrylate (such as Miramer M241 (Miwon Korea) , Miramer M244 (Miwon Korea) , Miramer M2101 (Miwon Korea) , Fancryl FA-324M (Hitachi Chemical, Japan) , Fancryl FA-326M (Hitachi Chemical, Japan) , Fancryl FA-328M (Hitachi Chemical, Japan) Fancryl FA-321M (Hitachi Chemical, Japan) ) , and modified f luorene-9-bisphenol diacrylate (such as Miramer HR6060 (Miwon Korea) .

[0048] Examples of further tri-functional acrylates include ethoxylated trimethylolpropane triacrylate (CAS Nr: 28961-43-5) , isopropoxylated trimethylolpropane triacrylate (CAS Nr: 53879-54-2) , isopropoxylated glycerine triacrylate (CAS Nr: 52408-84-1) and tris (2-hydroxy- ethyl ) isocyanurate-triacrylate (CAS Nr: 40220-08-4; FA- 731A (Hitachi Chemical, Japan) ) .

[0049] Specific examples of further tetrafunctional acrylates include di (trimethylolpropane) tetraacrylate (CAS Nr: 94108-97-1) , and ethoxylated pentaerythritol tetraacrylate (CAS Nr: 51728-26-8) .

[0050] Specific examples of further pentafunctional acrylates include dipentaeritritol pentaacrylate (CAS Nr: 60506-81-2) .

[0051] Specific examples of further hexafunctional acrylates include dipentaerythritol hexaacrylate (CAS 29570-58-9) .

[0052] In a preferred embodiment, the further acrylate repeating units are compounds of formula (III) . In another preferred embodiment, the further acrylate monomer is dipentaerythritol hexaacrylate. In a still preferred embodiment, the further acrylate repeating units / monomers are tris ( 2-hydroxyethyl ) isocyanurate-triacrylate and / or tris (2-hydroxyethyl) isocyanurate-diacrylate .

[0053] In a further advantageous embodiment of the invention, the acrylate polymer described herein may contain sulfur containing residues, preferably residues containing thiol and / or thioether functional groups. Advantageously the sulfur containing residues reduce polymerization shrinkage and increase adhesion to the adjacent layers in the solar cell. The precursor of such sulfur containing residues are preferably polythiols of general formula (V)

[0054] R2(SH)y, (V) , where R2is (hetero ) hydrocarbyl group having a valence of y, and y is 2 - 20, preferably 2 - 10, e.g 4.

[0055] The polythiols of general formula (V) may react with the double bond present in the herein described (further) acrylate repeating units prior or during UV curing to forma a thioether.

[0056] The thiol groups of the polythiols may be primary or secondary, preferably primary. The polythiols of formula (V) may include a mixture of compounds having an average functionality of two or greater.

[0057] R2includes any hydrocarbyl groups, including aliphatic and aromatic polythiols with 2 - 50 carbon atoms.

[0058] R2includes (hetero ) hydrocarbyl groups, ie. hy- drocaboy groups with 2 - 50 carbon atoms further comprising one or more functional groups. Functional groups include pendent hydroxyl, acid, ester, or cyano groups or catenary (in-chain) ether, urea, urethane and ester groups.

[0059] In one embodiment, R2comprises a non-polymeric aliphatic or cycloaliphatic moiety having from 2 to 30 carbon atoms .

[0060] Specific examples of useful polythiols include 2, 3-dimercapto-l-propanol, 2-mercaptoethyl ether, 2- mercaptoethyl sulfide, 1 , 6-hexanedithiol , 1 , 8-octanedi- thiol, 1, 8-dimercapto-3, 6-dithiaoctane, propane-1,2,3- trithiol, and trithiocyanuric acid.

[0061] Another useful class of polythiols includes those obtained by esterification of a polyol with a terminally thiol-substituted carboxylic acid (or derivative thereof, such as esters or acyl halides) including a- or p-mercaptocarboxylic acids such as thioglycolic acid, p- mercaptopropionic acid, 2-mercaptobutyric acid, or esters thereof .

[0062] Useful examples of commercially available compounds thus obtained include ethylene glycol bis (thioglycolate) , pentaerythritol tetrakis ( 3-mercaptopropionate ) , dipentaerythritol hexakis ( 3-mercaptopropionate ) , ethylene glycol bis ( 3-mercaptopropionate ) , trimethylolpropane tris (thioglycolate) , trimethylolpropane tris ( 3-mercaptopropionate) , pentaerythritol tetrakis (thioglycolate) , pentaerythritol tetrakis (3-mercaptopropionate) , pen- taerithrytol tetrakis ( 3-mercaptobutylate ) , and 1,4- bis 3-mercaptobutylyloxy butane, tris [ 2- ( 3-mercapto-propio- nyloxy] ethyl ] isocyanurate, trimethylolpropane tris (mercaptoacetate) , 2 , 4-bis (mercaptomethyl ) -1 , 3, 5,-triazine- 2, 4-dithiol, 2, 3-di ( 2-mercaptoethyl ) thio) -1-pro- panethiol, dimercaptodiethylsuf ide, and ethoxylated trime- thylpropan-tri (3-mercaptopropionate .

[0063] A specific example of a polymeric polythiol is polypropylene ether glycol bis ( 3-mercaptopropionate ) which is prepared by esterification of polypropylene-ether glycol (e.g., Pluracol™ P201, BASF Wyandotte Chemical Corp.) and 3-mercaptopropionic acid by esterification.

[0064] Useful soluble, high molecular weight thiols include polyethylene glycol di (2-mercaptoacetate) , LP-3™ resins supplied by Morton Thiokol Inc. (Trenton, N.J.) , and Permapol P3™ resins supplied by Products Research & Chemical Corp. (Glendale, Calif.) and compounds such as the adduct of 2-mercaptoethylamine and caprolactam.

[0065] Preferred polythiols include the following pentaerythrol-derivatives (V-l) - (V-4) :

[0066] (V-l)

[0067]

[0068] (V-2)

[0069] (V-3) (V-4)

[0070] Preferred polythiols include the following isocy- anurate-derivatives (V-5) - (V-7) :

[0071]

[0072] In a preferred embodiment the polythiol comprises 4 valences, i.e y=4, such as Pentaerythritol tera- kis ( 3-mercaptopropionate ) (CAS 7575-23-7, ) .

[0073] In an alternative embodiment, R2is polymeric and comprises a polyoxyalkylene, polyester, polyolefin, polyacrylate, or polysiloxane polymer having pendent or terminal reactive — SH groups. Useful polymers include, for example, thiol-terminated polyethylenes or polypropylenes and thiol-terminated poly ( alkylene oxides. In a further advantageous embodiment of the invention, the polymer layer is transparent , in particular UV-transparent . In addition, also the protective layer might be transparent .

[0074] In a further advantageous embodiment of the invention, the protective layer is transparent and comprises or consists of a barrier film, wherein the barrier film has a WVTR (water vapor transmission rate ) of WVTR < 10A-2 g H2O / m2 / day, preferably WVTR < 10A-3 g H2O / m2 / day, more preferably WVTR < 10A-4 g H2O / m2 / day, more preferably WVTR < 10A-5 g H2O / m2 / day .

[0075] Advantageously, the barrier layer is in direct contact with the polymer layer .

[0076] In a further advantageous embodiment the protective layer comprises a surface that is functionali zed with a silane coupling agent with an acrylate functional group, wherein the surface is facing the polymer layer, for better adhesion between the protective layer and the polymer layer .

[0077] In a further advantageous embodiment of the invention, the protective layer comprises or consists of metal or a metal film .

[0078] In a further advantageous embodiment of the invention, the photoconversion layer comprises perovskite crystals , in particular wherein the perovskite crystals form a polycrystalline layer . Further advantageous , the polycrystalline layer has a thickness of 0 . 3 - 5pm .

[0079] In a further advantageous embodiment of the invention, the solar cell is transmissive or partially transmissive to visible light . This means that all layers of the solar cell are transmissive to visible light or at least portions of the layers are transmissive to visible , wherein said portions are arranged in a way that the visible light can transmit through the solar cell .

[0080] In a further advantageous embodiment of the invention, the protective layer is selected from the group consisting of glass , metal , and a transparent barrier film . The transparent barrier film might be an inorganic gas- impermeable coating on a polymer layer ( such as e . g . PET ) .

[0081] In a further advantageous embodiment of the invention, the first electrode or the second electrode are transparent electrodes . Further advantageously, the first or the second electrode might be non-transparent electrodes . Therefore , either both, the first and the second electrode might be transparent or only one electrode , the first or the second electrode might be transparent and the respective other electrode might be non-transparent .

[0082] In a further advantageous embodiment of the invention, the first and / or the second electrode is selected from the group consisting of indium tin oxide ( ITO) , fluorine tin oxide ( FTO) , carbon and silver nanowires . Advantageously, the first and / or second electrode is selected from a material that allow manufacturing of transparent electrodes .

[0083] In a further advantageous embodiment of the invention, one or more of a first charge transport layer is arranged between the first electrode and the photoconversion layer . The charge transport layer exhibits advantageous energy level alignment with the photoconversion layer . Further advantageously, one or more of a second charge transport layer is arranged between the second electrode and the photoconversion layer .

[0084] Further advantageously, the first charge transport layer is a hole transport layer and the second charge transport layer is an electron transport layer, or the first charge transport layer is an electron transport layer and the second charge transport layer is a hole transport layer .

[0085] Further advantageously, the first and / or second charge transport layer is an electron transport layer, preferably comprising or consisting of a metal oxide , preferably selected from the group consisting of pure or doped zinc oxide , pure or doped tin oxide , zinc tin oxide , zinc magnesium oxide , pure and doped titanium oxide preferably in the form of nanoparticles .

[0086] Further advantageously, the respective other charge transport layer ( first and / or second charge transport layer ) , is a hole transport layer, preferably comprising or consisting of a metal oxide , preferably selected from the group consisting of pure or doped nickel oxide , pure or doped tungsten oxide , and antimony tin oxide (ATO) , preferably in the form of nanoparticles .

[0087] Further examples of advantageous embodiments of the solar cell are :

[0088] An advantageous embodiment of the solar cells comprises the following features :

[0089] • a substrate , in particular a substrate serving as a base substrate to build the layer structure on top of the substrate

[0090] • the first electrode arranged on the substrate , in particular directly adj acent to the substrate ,

[0091] • the second electrode ,

[0092] • the photoconversion layer being a perovskite layer is arranged between the first electrode and the second electrode ,

[0093] • at least one charge transport layer, in particular a hole transport layer, arranged between the first electrode and the photoconversion layer respectively the perovskite layer,

[0094] • at least one charge transport layer, in particular an electron transport layer arranged between the second electrode and the photoconversion layer respectively the perovskite layer, and

[0095] • the polymer layer arranged between the second electrode and the protective layer, wherein the polymer layer is arranged adj acent to the protective layer .

[0096] A further advantageous embodiment of the solar cell comprises the following features :

[0097] • the substrate , in particular a glass substrate , • the first electrode comprising or consisting of fluorine doped tin oxide , wherein the first electrode is arranged on the substrate , in particular directly adj acent to the substrate ,

[0098] • the second electrode comprising or consisting of fluorine doped tin oxide ,

[0099] • the photoconversion layer being a perovskite layer is arranged between the first electrode and the second electrode ,

[0100] • at least one charge transport layer, in particular a hole transport layer, arranged between the first electrode and the photoconversion layer respectively the perovskite layer,

[0101] • at least one charge transport layer, in particular an electron transport layer arranged between the second electrode and the photoconversion layer respectively the perovskite layer, and

[0102] • the polymer layer arranged between the second electrode and the protective layer, wherein the polymer layer is arranged adj acent to the protective layer, and

[0103] • wherein the protective layer is in particular a glass layer .

[0104] A further advantageous embodiment of the solar cells comprises the following features :

[0105] • the substrate , preferably a glass substrate ,

[0106] • the first electrode arranged on the substrate , in particular directly adj acent to the substrate ,

[0107] • the second electrode ,

[0108] • the photoconversion layer , preferably being a perovskite layer, is arranged between the first electrode and the second electrode ,

[0109] • at least one charge transport layer, in particular a hole transport layer, arranged between the first electrode and the photoconversion layer respectively the perovskite layer, • at least one charge transport layer, in particular an electron transport layer, arranged between the second electrode and the photoconversion layer respectively the perovskite layer, and

[0110] • the polymer layer arranged between the second electrode and the protective layer, wherein the polymer layer is arranged adj acent to the protective layer,

[0111] • wherein the protective layer is transparent , advantageously is a barrier film, and further advantageously has a multiple layer structure comprising : o a polymer substrate , very preferably comprising or consisting of PET (polyethyleneterephtalat ) , o an inorganic gas-impermeable layer, with a thickness of < 5pm, preferably comprising or consisting of a metal oxide selected from the group consisting of SiOx, AlOx, SiNx and mixtures thereof , o advantageously a polymeric adhesion layer, o wherein the inorganic gas-impermeable layer or the adhesion layer, i f there is any, is in direct contact with the polymer layer .

[0112] A second aspect of the invention refers to a method for manufacturing of the solar cell comprising the following steps : arranging the photoconversion layer in between the first electrode and the second electrode , and arranging the polymer layer between the second electrode and the protective layer .

[0113] A further advantageous method comprises additionally the steps of arranging the at least one first transport layer between the first electrode and the photoconversion layer, and / or arranging the at least one second transport layer between the second electrode and the photoconversion layer .

[0114] Advantageously, the arrangement of the polymer layer between the second electrode and the protective layer refers to the adhesion of the protective layer on top of the layer structure , by means of the polymer layer

[0115] A third aspect according to the present invention is directed to a use of a liquid formulation for manufacturing the polymer layer of the solar cell claimed and described herein . Said liquid formulation comprises at least 30 wt-% , preferably at least 50 wt-% , more preferably at least 60 wt-% , most preferably at least 70 wt-% of acrylate monomers containing cyclic C5-25 alkyl and / or alkenyl groups . Said acrylate monomers are preferably selected from the herein described mono- functional acrylates of formula ( I ) and herein described bi- functional acrylates of formula ( I I ) . Advantageously, the herein claimed liquid formulation can be directly applied on the other layers of the solar cell and subsequently hardened and is particularly suitable to be used in a large-scale high-speed production of solar cells . Further the liquid formulation does not degrade the layers of the solar cells during the manufacturing process .

[0116] The liquid formulation is preferably UV curable i . e . may be hardened by exposure to ultraviolet (UV) light . This is particularly advantageous because it enables production of the polymer layer in an expedient manner by directly applying ( e . g . by coating or printing) the formulation on the other layers of the solar cell and subsequent UV curing which is a high speed curing process . The obtained polymer layer exhibits excellent water barrier properties . Such UV curable formulation further contains an UV initiator for radical polymeri zation, preferably in an amount lower than 6 wt-% , such as 2 wt-% .

[0117] For environmental reasons and fast manufacturing, the liquid formulation is particularly solvent- free . Thus , in a preferred embodiment , the liquid formulation is and UV-curable solvent- free liquid formulation .

[0118] In one embodiment , the acrylate monomers consist essentially of acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups . For example , the acrylate monomers may consist of monofunctional acrylate of general formula (I) and bi-functional acrylates of general formula (II) .

[0119] The liquid formulation may contain further acrylate monomers preferably selected from the group of the herein described mono-, bi-, tri-, tetra-, penta-, and hexa-functional acrylates. The further acrylate monomers are different from the acrylate monomers containing cyclic C5-25 alkyl and / or alkenyl groups i.e. do not contain cyclic C5-25 alkyl and / or alkenyl groups. The content of further acrylate monomers in the liquid formulation is preferably lower than the content of the acrylate monomers containing cyclic C5-25 alkyl and / or alkenyl groups.

[0120] The further mono-functional acrylate may be a compound of general formula (IV)

[0121] R9represents H or CH3,

[0122] R10represents a linear or branched C1-25 alkyl, or a Ce-26 aryl group, each optionally substituted with one or more cyclic, linear or branched C1-20 alkyl, phenyl, phenoxy, hydroxyl, or amino groups, n represents 0 or 1, and

[0123] X represents a spacer from the group of alkoxylates comprising 1-8 carbon atoms and 1-4 oxygen atoms, and alkylenes comprising 1 - 8 carbon atoms.

[0124] Examples of further mono-functional acrylates of general formula (IV) include, poly ( ethylene glycol) phenyl ether acrylates, such as 2-phenoxyethyl acrylate,

[0125] O-phenyl phenoxyethyl acrylate, polyethylene glycol o- phenylphenyl ether acrylate (CAS 72009-86-0) , polyethylene glycol) ethyl ether methacrylate, di (ethylene glycol) ethyl ether acrylate, poly ( ethylene oxide) nonylphenyl etheracrylate, poly (propylene glycol) 4- nonylphenyl ether acrylate, and ethylene glycol dicyclopentenyl ether acrylate. The further bi-functional acrylate may be a compound of formula (III) III) , wherein :

[0126] R21independently from each other represent H or

[0127] CH3;

[0128] R23represents a linear or branched C1-25 alkyl, or a Ce-26 aryl group, each optionally substituted with one or more linear or branched C1-20 alkyl, phenyl or phenoxy;

[0129] X22independently from each other represent a spacer selected from the group of alkoxylates, whereby both substituents X22together comprise 8-40 carbon atoms and 2-20 oxygen atoms, tris (2-hydroxy ethyl ) isocyanurate diacrylate or neopentyl glycol hydroxypivalate diacrylate (CAS Nr. 2136366-99-7, such as SR606A) .

[0130] The further bi-functional acrylate is preferably a compound of formula (III-l) wherein R represents H or CH3 and m+n is between 4 and 10.

[0131] Specific examples of further di-functional acrylates include 1 , 4-butanediol diacrylate, 1,4-butane- diol dimethacrylate, 1 , 3-butanediol dimacrylate, 1,3-bu- tanediol dimethacrylate, 3-methyl 1 , 5-pentanediol diacrylate, 3-methyl 1 , 5-pentanediol dimethacrylate, 1,9-nonane- diol diacrylate, 1,9 - neopentylglycol dimethacrylate, 1 , 10-decanediol diacrylate, 1 , 10-decanediol dimethacrylate, 1 , 6-hexanediol diacrylate, 1 , 6-hexanediol dimethacrylate, neopentyl glycol dimethacrylate, polypropyleneglycol diacrylate (including CAS Nr. 52496-08-9, FA-P240A (n=7) , FA-P270A (n = 12) from Hitachi Chemical, Japan) , polyethyleneglycol dimethacrylate (including CAS Nr: 25852-47-5; such as n=4, FA-220M; n=7 FA-240M from Hitachi Chemical, Japan) , ethoxylated Bisphenol A dimethacrylate (including CAS Nr. 24448-20-2, such as FA-320M, FA-321M and FA-3218M from Hitachi Chemical, Japan) , ethoxylated Bisphenol A diacrylate (including CAS 64401-02-1, such as Miramer M240 (Miwon Korea) , Miramer M2100 (Miwon Korea) , Fancryl FA-324A (Hitachi Chemical, Japan) , Fancryl FA-326A (Hitachi Chemical, Japan) , Fancryl FA-328A (Hitachi Chemical, Japan) , Fancryl FA-321A (Hitachi Chemical, Japan) ) , Bisphenol A ethoxylate dimethacrylate (such as Miramer M241 (Miwon Korea) , Miramer M244 (Miwon Korea) , Miramer M2101 (Miwon Korea) , Fancryl FA-324M (Hitachi Chemical, Japan) , Fancryl FA-326M (Hitachi Chemical, Japan) , Fancryl FA-328M (Hitachi Chemical, Japan) Fancryl FA-321M (Hitachi Chemical, Japan) ) , and modified f luorene-9-bisphenol diacrylate (such as Miramer HR6060 (Miwon Korea) .

[0132] Examples of further tri-functional acrylates include ethoxylated trimethylolpropane triacrylate (CAS Nr: 28961-43-5) , isopropoxylated trimethylolpropane triacrylate (CAS Nr: 53879-54-2) , isopropoxylated glycerine triacrylate (CAS Nr: 52408-84-1) and tris (2-hydroxy- ethyl ) isocyanurate-triacrylate (CAS Nr: 40220-08-4; FA- 731A (Hitachi Chemical, Japan) ) .

[0133] Specific examples of further tetrafunctional acrylates include di (trimethylolpropane) tetraacrylate (CAS Nr: 94108-97-1) , and ethoxylated pentaerythritol tetraacrylate (CAS Nr: 51728-26-8) .

[0134] Specific examples of further pentafunctional acrylates include dipentaeritritol pentaacrylate (CAS Nr: 60506-81-2) .

[0135] Specific examples of further hexafunctional acrylates include dipentaerythritol hexaacrylate (CAS 29570-58-9) .

[0136] In a preferred embodiment, the further acrylate monomers are compounds of formula (III) . In another preferred embodiment, the further acrylate monomer is dipentaerythritol hexaacrylate. In a still preferred embodiment, the further acrylate monomers are tris (2-hydroxy- ethyl ) isocyanurate-triacrylate and / or tris (2-hydroxy- ethyl) isocyanurate-diacrylate . The liquid formulation described herein may further comprise a polyisobutylene polymer (PIB) . PIB is useful for modifying the viscosity of the liquid formulation and reducing the glass transition temperature of the polymer layer. PIB may be present in an amount of at least 2 wt-%, preferably at least 6 wt-%, most preferably at least 20 wt-%. The amount of polyisobutylene polymer (wt- %) present in the liquid formulation shall not exceed the total amount (wt-%) of the acrylate monomers (i.e. the total amount of the acrylate monomers containing cyclic Cs- 25 alkyl and / or alkenyl groups (wt-%) , or if the liquid composition contains further acrylate monomers, the sum of the total amount of the acrylate monomers containing cyclic C5-25 alkyl and / or alkenyl groups and of the total amount of the further acrylate monomers) .

[0137] In a further advantageous embodiment of the invention, the liquid formulation may contain polythiols of general formula (V) R2(SH)y, (V) , where

[0138] R2is (hetero ) hydrocarbyl group having a valence of y, and y is 2 - 20, preferably 2 - 10, e.g 4.

[0139] The polythiols of general formula (V) may react with the double bond present in the herein described (further) acrylate monomers prior or during UV curing to form a thioether.

[0140] The thiol groups of the polythiols may be primary or secondary, preferably primary. The polythiols of formula (V) may include a mixture of compounds having an average functionality of two or greater.

[0141] R2includes any hydrocarbyl groups, including aliphatic and aromatic polythiols with 2 - 50 carbon atoms.

[0142] R2includes (hetero ) hydrocarbyl groups, ie. hy- drocaboy groups with 2 - 50 carbon atoms further comprising one or more functional groups. Functional groups include pendent hydroxyl, acid, ester, or cyano groups or catenary (in-chain) ether, urea, urethane and ester groups. In one embodiment, R2comprises a non-polymeric aliphatic or cycloaliphatic moiety having from 2 to 30 carbon atoms .

[0143] Specific examples of useful polythiols include 2, 3-dimercapto-l-propanol, 2-mercaptoethyl ether, 2- mercaptoethyl sulfide, 1 , 6-hexanedithiol , 1 , 8-octanedi- thiol, 1, 8-dimercapto-3, 6-dithiaoctane, propane-1,2,3- trithiol, and trithiocyanuric acid.

[0144] Another useful class of polythiols includes those obtained by esterification of a polyol with a terminally thiol-substituted carboxylic acid (or derivative thereof, such as esters or acyl halides) including a- or p-mercaptocarboxylic acids such as thioglycolic acid, p- mercaptopropionic acid, 2-mercaptobutyric acid, or esters thereof .

[0145] Useful examples of commercially available compounds thus obtained include ethylene glycol bis (thioglycolate) , pentaerythritol tetrakis ( 3-mercaptopropionate ) , dipentaerythritol hexakis ( 3-mercaptopropionate ) , ethylene glycol bis ( 3-mercaptopropionate ) , trimethylolpropane tris (thioglycolate) , trimethylolpropane tris ( 3-mercaptopropionate) , pentaerythritol tetrakis (thioglycolate) , pentaerythritol tetrakis (3-mercaptopropionate) , pen- taerithrytol tetrakis ( 3-mercaptobutylate ) , and 1,4- bis 3-mercaptobutylyloxy butane, tris [ 2- ( 3-mercapto-propio- nyloxy] ethyl ] isocyanurate, trimethylolpropane tris (mercaptoacetate) , 2 , 4-bis (mercaptomethyl ) -1 , 3, 5,-triazine- 2, 4-dithiol, 2, 3-di (2-mercaptoethyl) thio) -1-pro- panethiol, dimercaptodiethylsuf ide, and ethoxylated trime- thylpropan-tri (3-mercaptopropionate .

[0146] A specific example of a polymeric polythiol is polypropylene ether glycol bis ( 3-mercaptopropionate ) which is prepared by esterification of polypropylene-ether glycol (e.g., Pluracol™ P201, BASF Wyandotte Chemical Corp.) and 3-mercaptopropionic acid by esterification.

[0147] Useful soluble, high molecular weight thiols include polyethylene glycol di (2-mercaptoacetate) , LP-3™ resins supplied by Morton Thiokol Inc. (Trenton, N.J.) , and Permapol P3™ resins supplied by Products Research & Chemical Corp. (Glendale, Calif.) and compounds such as the adduct of 2-mercaptoethylamine and caprolactam. Preferred polythiols include the following pentaerythrol-derivatives (V-l) - (V-4) :

[0148] (V-2)

[0149] (V-3) (V-4) Preferred polythiols include the following isocyanurate-derivatives (V-5) - (V-7) :

[0150] In a preferred embodiment the polythiol comprises 4 valences, i.e y=4, such as Pentaerythritol tera- kis ( 3-mercaptopropionate ) (CAS 7575-23-7, ) .

[0151] In an alternative embodiment, R2is polymeric and comprises a polyoxyalkylene, polyester, polyolefin, polyacrylate, or polysiloxane polymer having pendent or terminal reactive — SH groups. Useful polymers include, for example, thiol-terminated polyethylenes or polypropylenes, and thiol-terminated poly ( alkylene oxides) .

[0152] The liquid formulation may further contain an additive selected from UV blocking additives, viscosity modifiers, non-reactive plasticizers, and desiccants. Preferably, the amount of such additives is lower than 20 wt-%, preferably lower than 10 wt%.

[0153] The UV blocking additives prevent degradation of the polymer layer upon exposure to sunlight. Suitable UV blocking additives include organic UV absorbing additives and inorganic UV absorbing additives, such as zinc oxide and titanium oxide. The amount of UV blocking additives in the herein described formulation is preferably lower than 5 wt%.

[0154] The liquid formulation has preferably a viscosity of 50-2'000 cP, more preferably of 100-1'000 cP, most preferably of 200 - 700 cP as determined with a a rotational viscometer at 25°C (either cone or plate type viscometer) . To adjust the viscosity, the liquid formulation may further contain viscosity modifiers. Particularly suitable viscosity modifiers include linear polymers, preferably selected from polystyrene, and polymethylmethacrylate, .

[0155] To decrease the glass transition temperature of the polymer layer, the liquid composition may additionally contain non-reactive plasticizers, such as bis ( 2-ethylhexyl ) phthalate. As used herein, the term "non-reactive plasticizers" refer to plasticizers that do not contain functional groups that may react with the herein described acrylate monomers during storing, application or curing.

[0156] The liquid formulation may additionally contain dessicants (or water absorbing additives) for removing the eventual water present in the liquid formulation or penetrating in the polymer layer. Examples of dessicants include, but are not limited to, calcium oxide, zeolites, molecular sieves, silica gel, activated alumina, and montmorillonite clay. Preferably the particle size of desiccants does not exceed 50 pm, preferably 20 pm. Preferably, the amount of dessicants in the liquid formulation does not exceed 20 wt-%, preferably 10 wt-%. Other advantageous embodiments are listed in the dependent claims as well as in the description below .

[0157] Brief Description of the Drawings

[0158] The invention will be better understood and obj ects other than those set forth above will become apparent from the following detailed description thereof . Such description makes reference to the annexed drawings , wherein :

[0159] Fig . 1 shows a schematic drawing of the solar cell according to a speci fic embodiment of the invention; and

[0160] Fig . 2 shows a schematic drawing of the solar cell according to a further speci fic embodiment of the invention .

[0161] Modes for Carrying Out the Invention

[0162] Fig . 1 discloses a solar cell 100 comprising a photoconversion layer 3 arranged between a first electrode 1 and a second electrode 2 . In addition, a polymer layer 4 is arranged between the second electrode 2 and a protective layer 5 . The polymer layer 4 comprises at least 50 wt-% of acrylate polymers and said acrylate polymers comprise acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups , optionally substituted with one or more cyclic, linear or branched C1-20 alkyl or alkenyl groups .

[0163] Advantageous embodiments of the polymer layer 4 are speci fied in detail in the speci fication of the first , second and third aspect above .

[0164] In a further advantageous embodiment of the invention, the polymer layer 4 might be transparent , in particular transparent for visible light . Therefore solar energy can pass through the polymer layer 4 to interact with the photoconversion layer . In addition, also the protective layer 5 might be transparent , with the same ef fect .

[0165] Further advantageously, the first electrode 1 and / or the second electrode 2 might not cover the complete surface of the photoconversion layer 3 . As shown in Fig . 1 , e . g . the first electrode only covers part of the photoconversion layer 3 . Therefore , i f the polymer 4 is applied onto the first electrode 1 , it might get in contact not only with the first electrode 1 but also with the photoconversion layer 3 . Therefore , it is important , that the polymer layer 3 is compatible with the photoconversion layer 3 , which can be for example a perovskite layer .

[0166] In a further advantageous embodiment of the invention, the photoconversion layer 3 might comprise or consist of organic small molecules , organic polymer, or perovskite crystals .

[0167] In a further advantageous embodiment of the invention, the photoconversion layer 3 might comprise perovskite crystals .

[0168] In particular, the photoconversion layer 3 might have a thickness t of 0 . 3-5pm .

[0169] In a further advantageous embodiment of the invention, the solar cell 100 is partially transmissive to visible light as a whole . This means that most of the layers comprised in the solar cell 100 are transmissive to light . In particular, a partial transmissive solar cell 100 would comprise a first 1 and second 2 transmissive electrode and a transmissive polymer 3 , a transmissive polymer layer 4 , and a transmissive protective layer 5 .

[0170] In a further advantageous embodiment of the solar cell 4 , the protective layer 5 is selected from the group consisting of glass , metal and a transparent barrier film .

[0171] Further advantageously, the first 1 and the second 2 electrode might be transparent , therefore might be made of ITO or FTO or silver nanowires or further materials with similar properties .

[0172] In a further advantageous embodiment of the invention, a first charge transport layer is arranged between the first electrode 1 and the photoconversion layer 3 (not shown in the figure ) . Further advantageously, a second charge transport layer is arranged between the second electrode 2 and the photoconversion layer 3 (not shown in the figure ) . Advantageously, the first 11 and / or the second 21 charge transport layer comprise metal oxide nanoparticles .

[0173] Fig . 2 shows a further advantageous embodiment of the solar cell 100 . The solar cell 100 comprises a substrate 6 , advantageously a glass substrate , wherein the second electrode 2 and an electrode section 1 ' are evaporated onto the substrate 6 . In particular, the evaporated second electrode 2 and electrode section 1 ' might be ITO or FTO electrodes . The photoconversion layer 3 is then applied onto the second 2 electrode and the electrode section 1 ' , at least partly covering the second electrode and / or the electrode section 1 ' . Advantageously, a charge transport layer is arranged between the second 2 electrode and the photoconversion layer 3 (not shown in the figure ) . On a side of the photoconversion layer 3 opposite to the side facing the second electrode 2 , the first electrode 1 is covering the photoconversion layer 3 at least partly and is in electrical contact with the photoconversion layer 3 . The first electrode 1 is preferably a metallic layer and is applied in a way that it is in electrical contact with the electrode section 1 ' . Advantageously, a charge transport layer is arranged between the first electrode 1 and the photoconversion layer (not shown in the figure ) . The polymer 4 is then applied on top of the photoconversion layer 3 and first electrode 1 , to protect the first electrode 1 and the photoconversion layer 3 and further to adhere the protecting layer 5 . The solar cell 100 might now be connected to an electrical circuit via the second electrode 2 and the electrode section 1' .

[0174] Examples

[0175] In all the following experiments perovskite photoconversion layers were deposited on a glass substrate, then overcoated with UV curable liquid formulations, then covered with a second glass substrate and then subjected to UV light for UV curing of the liquid formulations.

[0176] Then it was checked if the perovskite layer changed its color / appearance directly after UV curing and after heat treatment for Ih at 100°C to see if the liquid formulations are chemically compatible with the perovskite layer and if the cured solid polymers degrade the perovskite layers at high temperature treatment.

[0177] All chemicals were analytical grade and purchased from Sigma Aldrich.

[0178] Formamidinium lead iodide (FAPbI3) perovskite photoconversion layers were deposited on glass as following :

[0179] A lead iodide (Pbl2) precursor solution was made consisting of 1.2 M Pbl2 in DMF:DMSO with a volume ratio of 4:1. The Pbl2 / DMF : DMSO precursor solution was preheated at 75 °C prior to its deposition in order to fully dissolve the Pbl2. The Pbl2 / DMF : DMSO precursor and glass substrates were preheated on a hot plate at 75°C. The Pbl2 precursor was deposited on the glass substrate by spin-coating (spin speed, 3000 rpm; spin time, 30s) , and then annealed at 75 °C for 20 min. After the annealing treatment, 80 pL of a 50 mg / mL (0.315 M) formamidinium iodide / isopropanol (FAI / IPA) solution was applied onto the Pbl2 film. Then, after 20s, the substrates were spun for 30 s to remove the excess FAI / IPA solution from the surface (spin speed, 3000 rpm) . Finally, the substrates with perovskite films were immediately transferred to a hot plate and annealed at 100 ° C for 15 min . After annealing, the perovskite films were cooled naturally to room temperature before further use .

[0180] The final perovskite photoconversion layers were dark brown indicating the semiconducting FAPbI 3 phase .

[0181] Polymer layers were produced as follows :

[0182] The corresponding liquid UV curable formulation for each of the following examples was deposited on the perovskite layer by doctor blading at a wet film thickness of about 100 micron . Then the wet film was covered with a second glass substrate and afterwards the liquid formulations were UV cured in a UV Box (Hdnle ; mercury lamp ; 150W) for 2 min .

[0183] Evaluation of the produced solar cells :

[0184] Then it was checked visually i f the perovskite layer inside the sandwich structure of glass / perovskite layer / UV cured f ormulation / glass changed its color . A change in color means that there is an unwanted reaction and thus degradation of the perovskite layer with the liquid formulation before or after UV curing .

[0185] I f the perovskite layer did not show any change in color, then the whole sandwich structure was put on a hotplate for Ih at 100 ° C ( in air ) and afterwards it was checked again i f the color of the perovskite layer changed . The conditions to which the sandwich structure was subj ected ( 100 ° C at ambient air for Ih) simulate an accelerated temperature-based solar cell aging ( solar cells have to withstand high temperatures in operation when exposed to intense sunlight and high air temperatures e . g . in summer ) .

[0186] A good UV curable liquid formulation results in no color change after formulation deposition, UV curing and after heat treatment . Comparative example 1 :

[0187] Composition of the UV curable liquid formulation: 69 wt% Hydroxyethyl acrylate (CAS: 818-61-1)

[0188] - 29 wt% Miramer M240 (Bisphenol A (EC) 4 Diacrylate; CAS: 64401-02-1)

[0189] - 2 wt% diphenyl ( 2 , 4 , 6-trimethylbenzoyl ) phosphine oxide (TPO; CAS: 75980-60-8)

[0190] Conclusion about the chemical compatibility between the liquid formulation and the perovskite layer: Not compatible

[0191] Comparative example 2 :

[0192] Composition of the UV curable liquid formulation:

[0193] 69 wt% 2- (diethylamino) ethyl methacrylate (CAS: 105-16-8)

[0194] - 29 wt% Miramer M240 (Bisphenol A (EC) 4 Diacrylate;

[0195] CAS: 64401-02-1)

[0196] - 2 wt% diphenyl ( 2 , 4 , 6-trimethylbenzoyl ) phosphine oxide (TPO; CAS: 75980-60-8) Conclusion about the chemical compatibility between the liquid formulation and the perovskite layer: Not compatible

[0197] Inventive example 1 :

[0198] Composition of the UV curable liquid formulation: 69 wt% FA-513AS ( dicyclopentanyl acrylate; CAS Nr: 79637-74-4; )

[0199] - 29 wt% Miramer M240 (Bisphenol A (EC) 4 Diacrylate; CAS: 64401-02-1)

[0200] - 2 wt% diphenyl ( 2 , 4 , 6-trimethylbenzoyl ) phosphine oxide (TPO; CAS: 75980-60-8)

[0201] Conclusion about the chemical compatibility between the liquid formulation and the perovskite layer: Compatible

[0202] Inventive example 2 :

[0203] Composition of the UV curable liquid formulation: 85 wt% IBOA (Isobornylacrylate; CAS: 5888-33-5) 13 wt% SR833S (tricyclodecanedimethanol diacrylate;

[0204] CAS: 42594-17-2)

[0205] - 2 wt% diphenyl ( 2 , 4 , 6-trimethylbenzoyl ) phosphine oxide (TPO; CAS: 75980-60-8) Conclusion about the chemical compatibility between the liquid formulation and the perovskite layer: Compatible

[0206] Inventive example 3:

[0207] Composition of the UV curable liquid formulation:

[0208] 69 wt% FA-DCPA ( Dicyclopentenyl acrylate; CAS: 33791- 58-1)

[0209] - 29 wt% Miramer M600 (Dipentaerythritol Hexaacrylate;

[0210] CAS: 29570-58-9)

[0211] - 2 wt% Irgacure 819 (Photoinitiator; Bis ( 2 , 4 , 6-trimethylbenzoyl ) -phenylphosphineoxide )

[0212] Conclusion about the chemical compatibility between the liquid formulation and the perovskite layer: Compatible

[0213] Inventive example 4 :

[0214] Composition of the UV curable liquid formulation: 69 wt% SR421A (3,3,5 Trimethylcyclohexyl Methacrylate; CAS: 7779-31-9)

[0215] - 29 wt% Miramer M240 (Bisphenol A (EC) 4 Diacrylate; CAS: 64401-02-1)

[0216] - 2 wt% diphenyl ( 2 , 4 , 6-trimethylbenzoyl ) phosphine oxide (TPO; CAS: 75980-60-8)

[0217] Conclusion about the chemical compatibility between the liquid formulation and the perovskite layer : Compatible

Claims

Claims1. A solar cell (100) comprising: a photoconversion layer (3) arranged between a first electrode (1) and a second electrode (2) , and a polymer layer (4) arranged between the second electrode (2) and a protective layer (5) , wherein : the polymer layer (4) is arranged adjacent to the protective layer (5) , and the polymer layer (4) comprises at least 50 wt-% of acrylate polymers and said acrylate polymers comprise acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups, optionally substituted with one or more cyclic, linear or branched C1-20 alkyl or alkenyl groups.

2. The solar cell (100) according to claim 1, wherein said acrylate polymers contain at least 30 wt-%, preferably at least 50 wt-%, more preferably at least 60 wt-%, most preferably at least 70 wt-%, of the acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups.

3. The solar cell (100) according to claim 1 or 2, wherein the acrylate repeating units containing cyclic C5-25 alkyl and / or alkenyl groups are selected from mono-functional acrylates of formula (I) :( I ) , wherein :R9represents H or CH3,R10represents a cyclic C5-25 alkyl or alkenyl, optionally substituted with one or more cyclic, linear or branched C1-20 alkyl or alkenyl, n represents 0 or 1, andX represents a spacer from the group of alkoxylates comprising 1-40 carbon atoms and 1-10 oxygen atoms ; and bi-functional acrylates of formulaR31independently from each other represent H or CH3;R33represents a cyclic C5-25 alkylene or a cyclic C5-25 alkenylene, each optionally substituted with one or more cyclic, linear or branched C1-20 alkyl;X32are independent from each other and are either not present (R33 directly bound to the oxygen by a single bond) or represent a spacer selected from the group of alkylenes and alkoxylates, whereby both substituents X32together comprise 1-8 carbon atoms and if present 1-8 oxygen atoms.

4. The solar cell (100) according to claim 3, wherein said acrylate polymers contain at least 20 wt-%, preferably at least 40 wt-%, more preferably at least 60 wt-%, of the mono-functional acrylates of general formula (I) .

5. The solar cell (100) according to one of the preceding claims, wherein the acrylate polymers comprise further acrylate repeating units selected from the group of mono-, bi-, tri-, tetra-, penta-, and hexafunctional acrylates.

6. The solar cell (100) according to claim 5, wherein the further acrylate repeating units are selected from bi-functional acrylates of formula (ITT)III) , wherein :R21independently from each other represent H or CH3;R23represents a linear or branched C1-25 alkyl, or a Ce-26 aryl group, each optionally substituted with one or more linear or branched C1-20 alkyl, phenyl or phenoxy;X22independently from each other represent a spacer selected from the group of alkoxylates, whereby both substituents X22together comprise 8-40 carbon atoms and 2-20 oxygen atoms.

7. The solar cell (100) according to one of the preceding claims, wherein the polymer layer (4) further comprises a polyisobutylene (RIB) polymer.

8. The solar cell (100) according to one of the preceding claims, wherein the polymer layer (4) is transparent and / or the protective layer (5) is transparent.

9. The solar cell (100) according to one of the preceding claims, wherein the photoconversion layer (3) comprises or consists of organic small molecules, organic polymer, or perovskite crystals.

10. The solar cell (100) according to claim 9, wherein the photoconversion layer (3) comprises perovskite crystals, in particular, wherein the perovskite crystals form a polycrystalline layer, in particular a polycrystalline layer having a thickness t of 0.3-5pm.

11. The solar cell (100) according to one of the preceding claims, wherein said solar cell (100) is partially transmissive to visible light.

12. The solar (100) cell according to one of the preceding claims, wherein the protective layer (5) is selected from the group consisting of glass, metal and a transparent barrier film, in particular, wherein the transparent barrier film is an inorganic gas-impermeable layer.

13. The solar cell (100) according to one of the preceding claims, wherein the first electrode (1) or the second electrode (2) is a transparent electrode.

14. The solar cell (100) according to one of the preceding claims, wherein the first (1) and / or second electrode (2) is selected from the group consisting of indium tin oxide (ITO) , fluorine tin oxide (FTO) , carbon and silver nanowires.

15. The solar cell (100) according to one of the preceding claims, wherein a first charge transport layer is arranged between the first electrode (1) and the photoconversion layer (3) , and / or wherein a second charge transport layer is arranged between the second electrode (2) and the photoconversion layer (3) , in particular, wherein the first (11) and / or the second (21) charge transport layer comprises metal oxide nanoparticles.

16. A method for manufacturing the solar cell (100) according to claims 1 to 15, comprising:• arranging the photoconversion layer (3) in between the first electrode (1) and the second electrode( 2 ) , and• arranging the polymer layer (4) between the second electrode (2) and the protective layer (5) .

17. Method according to claim 16, further comprising :• arranging the first charge transport layer (11) between the first electrode (1) and the photoconversion layer (3) , and / or• arranging the second charge transport layer (21) between the second electrode (2) and the photoconversion layer ( 3 ) .

18. Use of a liquid formulation, preferably a UV curable liquid formulation, for manufacturing the polymer layer (4) of the solar cell (100) according to any one of claims 1 to 16 wherein the liquid formulation comprises at least 30 wt-%, preferably at least 50 wt%, more preferably at least 60 wt-%, most preferably at least 70 wt-% of acrylate monomers containing cyclic C5-25 alkyl and / or alkenyl groups, wherein preferably said acrylate monomers are selected from mono-functional acrylates of formula (I) and bi-functional acrylates of formula (II) .

19. The use according to claim 18 wherein• the liquid formulation additionally comprises polyisobutylene; and / or• the liquid formulation is solvent-free; and / or• the liquid formulation further comprises a UV initiator for radical polymerization; and / or• the liquid formulation further comprises an additive selected from UV blocking additives, viscosity modifiers, plasticizers, and desiccants.

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