ORGANOMETALLIC PEROVSKITE-BASED SOLAR CELL, TANDEM SOLAR CELL AND ASSOCIATED MANUFACTURING PROCESS

Description

[0001] The invention relates to a metal-organic perovskite solar cell, in particular one with a lead- or tin-containing metal-organic photon absorber layer, and a manufacturing process for it.

[0002] Organic solar cells, also called plastic solar cells, are known, for example from EP 2498315 A2, which, unlike inorganic solar cells, can be built on flexible substrates and films.

[0003] Since the demonstration of the first organic solar cell with an efficiency in the percent range, organic materials have been used extensively for various electronic and optoelectronic components. Organic solar cells consist of a series of thin layers, typically ranging from 1 nm to 100 µm in thickness. The bandgap of suitable absorber layers, for example, is at least 1 eV.

[0004] Numerous studies have already been conducted on suitable dopants for the charge carrier transport layers adjacent to the absorber layer, such as the hole transport layer and the electron transport layer. Examples include EP 2443680, DE 102011003192, DE 102012209520, DE 102014210412, and DE 102015121844.

[0005] Organic solar cells have already been the subject of numerous studies, as the prospect of making entire glazing surfaces of high-rise buildings usable for electricity generation by coating them with organic solar cells is very tempting worldwide.

[0006] The well-known plastic solar cells use conjugated polymers as the material for the absorber layer - hydrocarbon polymers in combination with small molecules, for example fullerenes, for charge separation.

[0007] Also known from WO 2014 / 020499 is a structure for a metal-organic perovskite solar cell in which one or more organic-inorganic, here also referred to as "metal-organic", perovskite layers are arranged between two contact layers, for example electrodes, with which the perovskite layers are in electrical, preferably galvanic, contact.

[0008] The use of metal-organic absorber layers instead of purely organic absorber layers, as described above, presents new challenges for the layer sequence of the metal-organic solar cell.

[0009] In WO 2014 / 020499 it is still assumed that the metal-organic absorber layer makes a hole transport layer, as provided in organic solar cells between the absorber layer and the electrode, obsolete.

[0010] However, this has proven to be a disadvantage, so that now the metal-organic solar cell is also realized with an absorber layer of a metal-organic material crystallizing in the perovskite crystal lattice for faster removal of the charge carriers separated by photon radiation with at least one adjacent hole transport layer.

[0011] A metal-organic "pin"-switched solar cell is known from EP 2898553 A1, WO 2018 / 005749 A1, and US 2015 / 295194 A1, the layer sequence of which comprises at least the following layers: a transparent electrode, a hole transport material, then the absorber layer with a metal-organic absorber material ABX 3 crystallizing in a three-dimensional perovskite lattice, then an electron transport layer, and the counter electrode. A hole transport layer usable in a solar cell described here is, for example, "2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,9'-spirobifluorene" or "SpiroOMeTad" for short, with very high concentrations—for example, 30 mol% and higher—of a weak dopant containing lithium ions.

[0012] However, the use of such high doping concentrations of lithium in a metal-organic solar cell has the disadvantage that these layers are highly hygroscopic and have low stability.

[0013] The object of the present invention is therefore to provide alternative p-doping agents, instead of or in addition to the known lithium-containing p-doping agent, whose stability within the hole conductor layer and the entire metal-organic solar cell is higher. This object is achieved by the subject matter of the present invention as disclosed from the description, the figure, and the claims.

[0014] Accordingly, the present invention relates to a metal-organic solar cell comprising at least two contact layers and adjacent to each of these a semiconducting layer in a layer stack with a centrally arranged absorber layer made of a metal-organic material that crystallizes in a three-dimensional perovskite crystal lattice, wherein the absorber layer comprises lead and / or tin as the central atom and a halide as the anion in a metal-organic compound, characterized in that the at least one semiconducting layer between the absorber layer and the anode is a hole-conducting layer comprising a zinc- and / or bismuth-containing dopant.

[0015] Furthermore, the invention relates to a tandem solar cell comprising either two metal-organic solar cells or at least one metal-organic solar cell with a zinc and / or bismuth-containing doping agent in the hole conductor layer.

[0016] Finally, the invention relates to a method for producing a layered body forming a tandem solar cell, in which a layer stack comprising two solar cells is present, wherein a lower and an upper solar cell are produced by producing successive layers, characterized in that at least one of the solar cells is a metal-organic solar cell as is the subject of the invention.

[0017] In this context, a so-called complex compound is referred to as an organometallic compound. For example, the compound CH₃NH₃PbI₃, which crystallizes in the perovskite crystal lattice, is a prime example of such a compound. Within the crystal lattice, a unit cell can be identified in which the lead atom occupies the center of a cube, while the organic ligands, such as CH₃NH₃, form the eight corners of the cube. An anion, such as a halide anion like iodide, is located at the center of each face of the cube. When many such cells are adjacent to one another in the crystal lattice, the resulting stoichiometry yields the molecular formula CH₃NH₃PbI₃.

[0018] Regarding tandem solar cells, it has proven advantageous for the two solar cells to be matched to each other with respect to their absorption spectra, thus ensuring maximum radiation absorption. It is particularly advantageous if the tandem solar cell is composed of two organometallic solar cells, for example, by differing in the composition of the absorber layer material between the two solar cells.

[0019] Furthermore, the combination of a metal-organic solar cell, as described in the invention, with a c-Si solar cell has also proven advantageous. A c-Si solar cell is a solar cell that incorporates crystalline silicon in the absorber layer. The metal-organic solar cell is preferably located on top, closer to the sun.

[0020] In particular, the c-Si solar cell is used, for example, as a substrate for the construction of a metal-organic solar cell as is the subject of the invention.

[0021] The individual layers of the sheet material, which forms a metal-organic solar cell or a tandem solar cell with a metal-organic solar cell, can be produced using wet chemical processes, for example by spin coating, but not necessarily with solvents. Alternatively, production via vapor deposition, chemical or physical, is possible.

[0022] The general insight of the invention is that, contrary to expectations which would suggest that doping with zinc and / or bismuth compounds in a spiro-OMeTAD hole conductor layer adjacent to a perovskite absorber layer made of lead and / or tin complex compounds would be unstable, stable doping agents for stable hole conductor layers can be produced from zinc and / or bismuth salts with - for example - superacids.

[0023] The doping agent advantageously comprises, in addition to the zinc and / or bismuth cation, an anion of a superacid.

[0024] The hole-conducting layer comprises at least a matrix and a dopant, the latter being zinc- and / or bismuth-based. The addition of conventional additives is also included within the scope of the invention.

[0025] A suitable matrix material for the hole transport layer of a metal-organic perovskite solar cell is, for example, an organic conductor, such as "2,2'7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,9'-spirobifluorene" or "spiro-OMeTAD". Measurements have shown that low concentrations, such as 0.05 to 10 mol%, in particular 0.1 to 7 mol%, and preferably even only 0.1 to 2 mol%, of a dopant containing zinc and / or bismuth in a spiro-OMeTAD layer are sufficient to generate the necessary current densities in the hole transport layer of the solar cell.

[0026] In wet-chemical deposition of the hole-directed layer, i.e., from solution, the doping concentration is specifically adjusted via the mass fraction of, for example, a superacid salt and the mass fraction of the matrix material in the solution prior to deposition. The volume concentration of the p-dopant in the finished hole-directed layer may deviate from this concentration.

[0027] With the material class of zinc and / or bismuth salts according to the invention, for example of superacids, as doping agents, a wet chemical deposition method is preferred over deposition from the gas phase for the production of the individual layers of the layer stack.

[0028] The new materials for p-doping can significantly improve the photon-absorbing properties of p-dopers in metal-organic solar cells. High conductivity is achieved even at low doping concentrations.

[0029] Non-restrictive examples of superacids within the scope of this application are: Inorganic:

[0030] Fluorosulfonic acid (HSO3F) Fluoroantimonic acid (HSbF6) Tetrafluoroboric acid (HBF4) Hexafluorophosphoric acid (HPF6) Trifluoromethylsulfonic acid (HSO3CF3) Organic:

[0031] Pentacyanocyclopentadiene (HC 5 (CN) 5 ) Partially or fully fluorinated derivatives of pentaphenylcyclopentadiene Penta-trifluoromethylpentadiene or analogous derivatives Partially or fully fluorinated derivatives of tetraphenylboric acid or its cyano derivatives Partially or fully fluorinated derivatives of arylsulfonic acids or their cyano derivatives Partially or fully fluorinated derivatives of arylphosphonic acids or their cyano derivatives Anions of carboranes such as [C 2 B 10 H 10 ] 2-< or [C 1 B 11 H 10 ] -<

[0032] Trifluoromethylsulfonic acid (HSO3CF3) is a particularly suitable representative of this.

[0033] Polymer matrix materials for hole transporters, which can be deposited using wet chemical methods to produce the hole conductor layer of the solar cell, include, in addition to the already mentioned "2,2'7,7'-tetrakis-(N, N-di-p-methoxyphenylamine)9,9'-spirobifluorene" or "spiro-OMeTAD", in particular: PEDOT (Poly(3,4 ethylenedioxythiophene)) PVK (poly(9-vinylcarbazole)) PTPD (poly (NN - bis (4-butylphenyl)-N,N-bis (phenyl)-benzidine)) P3HT (poly(3-hexylhtiophene)) PANI (polyaniline) PTAA (Poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine]) and 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene and / or 4,4',4"-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine.

[0034] Mixtures of the aforementioned polymeric hole transport materials are also included within the scope of the invention.

[0035] Organic solvents are preferred for wet chemical processing, such as: Benzene, chlorobenzene, chloroform, toluene, THF, methoxypropyl acetate, anisole, acetonitrile, phenetol or dioxane.

[0036] A further particular advantage of the invention is that the class of superacid salts suitable for p-doping can be deposited with the hole-guide matrix from the same solvent. This represents a significant simplification of the deposition processes for the fabrication of the metal-organic solar cell.

[0037] Furthermore, the use of zinc and / or bismuth salts as dopants makes doping the hole transport layer easier, especially at lower process temperatures, than the previously known lithium-doped hole transport layers. Temperature is a critical factor in the fabrication of metal-organic solar cells because the organic ligands and the crystal structure are highly sensitive to temperature increases. Moreover, these dopants do not require the presence of oxygen during processing to achieve the doping effect. This is advantageous because oxygen has a negative impact on other parts of the metal-organic solar cell's layer system. For example, the fabrication of a hole transport layer with a lithium-containing dopant requires the use of additives such as tert-butylpyridine (TBP).This, together with the strong hygroscopy of lithium compounds, leads to indirect oxidation by atmospheric oxygen.

[0038] In an advantageous embodiment of the tandem solar cell, the metal-organic solar cell is the upper solar cell, onto which the photons first strike. There are two embodiments of this – 2-terminal and 4-terminal structures of a tandem cell, each depending on the number of contact points of the tandem solar cell.

[0039] The absorber layer of the metal-organic solar cell is preferably a layer with an ABX 3 stoichiometry that crystallizes in the three-dimensional perovskite crystal lattice.

[0040] For example, the organometallic ABX3 compound used is CH3NH3PbX3 and / or CH3NH3SnX3, where X can be a halide or pseudohalide, for example, selected from the group comprising fluoride, chloride, cyanide, isocyanide, bromide, and / or iodide, as well as any combination thereof. The perovskite absorber can have a very diverse composition and may, for example, include so-called "mixing cations" such as MA, FA, and / or Cs.

[0041] The halides / pseudohalides exist as anions in the crystal lattice, while the organic ligand (CH₃NH₃)⁺<-, as well as the lead or tin, exist as cations. The absorber layer material can also comprise—partially or entirely—other compounds, such as those listed below (this list is not exhaustive): FA 0.81 Cs 0.15 PbI 2.51 Br 0.45 FA 0.9 Cs 0.1 PbI 3 Cs 0.05 MA 0.1 FA 0.85 Pb(I 0.85 Br 0.15 ) 3 Cs 0.05 MA 0.1 FA 0.85 Pb(I 0.85 Br 0.15 ) 3

[0042] Mixtures of the aforementioned compounds are also suitable for the absorber material within the scope of the invention.

[0043] Surprisingly, it has been found that replacing lithium with zinc and / or bismuth in the dopant, or rather in the hole-conducting layer, not only significantly increases the stability of the hole-conducting layer, but initial tests have also shown that even at considerably lower concentrations, the zinc and / or bismuth dopants lead to higher open-circuit voltages, a high fill factor, and a significantly higher photon conversion efficiency (PCE) of the solar cells. For example, Zn(TFSI)₂ is apparently more active than LiTFSI in the hole-conducting layer, as in spiro-MeOTAD; it conducts charge carriers faster and leads to higher levels of free charge carriers.

[0044] Measurements at EPFL, Lausanne, have shown that the TFSI derivatives of zinc and bismuth used here for the first time in connection with spiro-MeOTAD produce significant electrical improvements in the hole conductor layer that cannot be explained solely by improved conductivity.

[0045] Figure 1 Figure 1 shows the structure of a metal-organic solar cell 1, in a nip layout, comprising at least the following layers: A transparent conductive electrode 7, such as an electrode made of doped indium tin oxide or another transparent conductive layer. This can be applied to a substrate such as glass or be self-supporting.

[0046] On this layer is an n-conducting layer 2, for example made of titanium dioxide. On this is the absorber layer, for example layer 3 made of CH3NH3PbI3 and / or CH3NH3SnI3 in a three-dimensional perovskite structure. The absorber layer 3 can be planar or in the form of a framework structure. Adjacent to this layer is the hole transport layer 4, which in this case is made of a matrix material, for example spiro-MeOTAD with a dopant containing zinc and / or bismuth, in particular with Zn(TFSI)2 and / or Bi(TFSI)3, as known from DE 10 2015 121844.

[0047] In the case of the dopant Zn(TFSI) 2 and / or Bi(TFSI) 3, an advantageous embodiment provides a thin barrier layer, not shown here, between the hole conductor layer 4 and the absorber layer 3. This can be advantageous if the dopant tends to diffuse into the absorber layer.

[0048] Instead of or together with Zn(TFSI) 2, for example Bi-(3,5-TFMBZ)3, Bismuth(III) tris (3,5-bistrifluoromethyl)benzoate, Bi(4-pFbz) 3 , Bismuth (III) tris(4-pentafluoro)benzoate, K(TFSI), K(I)bis(trifluoromethanesulfonyl)imide and / or Zn(II)bis (trifluoromethanesulfonyl)imide and / or Sodium(I)bis(trifluoromethanesulfonyl)imide are also present as dopants.

[0049] Furthermore, trifluoromethanesulfonates such as Zn(TFMS)₂ can also be advantageously used as dopants. Alternatively or additionally, so-called ionic liquids can also be used as effective dopants.

[0050] Finally, the counter electrode, made of materials such as aluminum, silver and / or gold, is located on the hole conductor layer 4.

[0051] The entire structure is preferably protected from moisture and / or air by an encapsulation 6.

[0052] Figure 2 shows the increase in the open-circuit voltage of a metal-organic solar cell when switching from a lithium-doped hole conductor layer to a zinc-doped hole conductor layer.

[0053] Figure 3This shows four different characteristic photovoltaic parameters (JSC (short-circuit current), VOC (open-circuit voltage), FF (fill factor), and PCE (photocurrent efficiency)) of perovskite solar cells. Here, a comparison is made between a perovskite solar cell with spiro-MeOTAD / LiTFSI (black) and spiro-MeOTAD / Zn(TFSI)2 (red) as the hole conductor layer.

[0054] These measurements compare metal-organic solar cells with lithium-doped and zinc-doped hole layers under otherwise identical construction and measurement conditions. These measurements clearly demonstrate that the solar cells built with a zinc-doped hole layer are at least equivalent to those with conventional lithium doping. This is all the more remarkable given the significant decrease in doping concentration from lithium to zinc and / or bismuth, which offers a considerable economic advantage.

[0055] Figure 4The figure shows the measurement of individual hole conductor layers without a solar cell setup. The current density of different doping concentrations at different voltages can be seen in the figure, with the result that above 0.2 mol of dopant per mol of matrix compound, no significant increase in current density is possible by increasing the doping concentration.

[0056] In Figure 4 In addition to the current-voltage curves shown on the left, the corresponding photovoltaic parameters such as JSC, VOC, FF and PCE are shown on the right as a function of the concentration of the dopant Zn (TFSI) 2 in the matrix material Spiro-MeOTAD.

[0057] What is particularly noticeable is the significant improvement in the "fill factor". The fill factor is the quotient of the maximum power output of a solar cell at its maximum power point and the product of its open-circuit voltage and short-circuit current.

[0058] Overall, the results of the measurements show that the metal-organic solar cells built with a hole conductor layer containing the doping agent according to the invention, based on zinc and / or bismuth, and with an absorber layer made of a material crystallizing in the three-dimensional perovskite structure, exhibit very good efficiency in the conversion of light into electricity.

[0059] In Figure 5Finally, the stability of the hole-conducting layers made with zinc on the one hand and with lithium on the other is compared. It turns out that the conventional lithium-doped hole-conducting layers are far less stable than the corresponding hole-conducting layers with zinc and / or bismuth. This is partly due to the fact that the small lithium ion naturally diffuses more easily and quickly under increased temperature and / or in an electric field, thus reducing the homogeneity of the hole-conducting layers. In particular, the PCE (Power Conversion Efficiency) measurement clearly shows how the efficiency of the lithium-doped hole-conducting layer decreases with increasing operating hours.

[0060] The present invention discloses for the first time a metal-organic solar cell comprising an absorber layer with a compound crystallizing in a perovskite crystal lattice, which has a lithium-poor hole conductor layer.

Claims

1. Metal-organic solar cell having at least two contact layers (5,7), wherein one contact layer is an anode (5), and, adjoining these, in each case a semiconducting layer (2,4) in a layer stack having a centrally arranged absorber layer (3) composed of a metal-organic material which crystallizes in the three-dimensional perovskite crystal lattice, where the absorber layer (3) comprises lead as central atom and a halide as anion in a metal-organic compound, characterized in that the at least one semiconducting layer (4) between the absorber layer (3) and the anode (5) is a hole-conducting layer which comprises a zinc-containing dopant.

2. Metal-organic solar cell having at least two contact layers (5,7), wherein one contact layer is an anode (5), and, adjoining these, in each case a semiconducting layer (2,4) in a layer stack having a centrally arranged absorber layer (3) composed of a metal-organic material which crystallizes in the three-dimensional perovskite crystal lattice, where the absorber layer (3) comprises tin as central atom and a halide as anion in a metal-organic compound, characterized in that the at least one semiconducting layer (4) between the absorber layer (3) and the anode (5) is a hole-conducting layer which comprises a bismuth-containing dopant.

3. Solar cell according to either Claim 1 or 2, wherein the zinc or bismuth compound in the dopant is the salt of a superacid.

4. Solar cell according to any of claims 1 to 3, wherein the solar cell comprises a diffusion barrier layer between the absorber layer and a semiconducting layer.

5. Solar cell according to Claim 4, wherein the diffusion barrier layer has a layer thickness of less than 150 nm.

6. Solar cell according to any of Claims 1 to 5 which in the matrix material of the hole conductor layer comprises one or more compounds selected from the group consisting of the following compounds: spiro-OMeTAD - 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene, PEDOT - poly(3,4-ethylenedioxythiophene), PVK - poly(9-vinylcarbazole), PTPD - poly(N,N-bis(4-butylphenyl)-N,N-bis(phenyl)benzidine), P3HT - poly(3-hexylthiophene), PANI - polyaniline, PTAA - poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], 9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene, 4,4',4"-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine and / or ionic liquids, and also mixtures of the abovementioned compounds.

7. Solar cell according to any of Claims 1 to 6, wherein the absorber layer comprises a metal complex having tin and / or lead as central atom which contains at least one anion in the form of a halide or pseudohalide, selected from the group of the following elements: fluoride, chloride, bromide, iodide, cyanide, isocyanide.

8. Solar cell according to any of Claims 1 to 7, wherein the absorber layer comprises a metal complex having tin and / or lead as central atom to which a (CH3NH3)+ ligand is coordinated.

9. Tandem solar cell comprising at least two superposed solar cells in a layer stack, wherein one solar cell is a metal-organic solar cell according to any of Claims 1 to 8.

10. Tandem solar cell according to Claim 9, wherein the metal-organic solar cell according to any of Claims 1 to 9 is the upper solar cell on which the photons impinge first.

11. Tandem solar cell according to either Claim 9 or 10 which comprises a solar cell having crystalline silicon in the absorber layer.

12. Tandem cell according to any of Claims 9 to 11 which comprises two metal-organic solar cells according to any of Claims 1 to 8, wherein the two solar cells differ in terms of the composition of the material which forms the absorber layer.

13. Process for producing a layer body forming a tandem solar cell, in which a layer stack comprising two solar cells can be produced by layer deposition in a wet-chemical process, where a lower solar cell and an upper solar cell are produced by the production of sequential layers, characterized in that one of the solar cells is a metal-organic solar cell according to any of Claims 1 to 8.