WO2024104629A1

WO2024104629A1 - Additive for the coating of hydrophobic surfaces with halogenide perovskites

Info

Publication number: WO2024104629A1
Application number: PCT/EP2023/074273
Authority: WO
Inventors: Thomas KIRCHARTZ; Zhifa LIU
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2022-11-15
Filing date: 2023-09-05
Publication date: 2024-05-23
Also published as: DE102022130199A1

Abstract

The invention relates to a method for producing a coating, containing one or more Pb-(II) compounds, on a hydrophobic surface by applying a precursor solution, which contains one or more Pb-(II) compounds in an organic solvent; optionally then applying an organic anti-solvent; and then annealing, characterised in that the precursor solution contains a polymer additive containing polar groups, which is PMMA. The invention also relates to a solar cell which contains a Pb-(II) perovskite layer obtained in accordance with the method according to the invention, and the use of PMMA to produce perovskite layers on hydrophobic surfaces.

Description

Additive for coating hydrophobic surfaces with halide perovskites

Description

In recent decades, solar cells have been increasingly used to generate electricity. Originally, the focus was mainly on solar cells made of silicon (monocrystalline or polycrystalline). However, since a further increase in efficiency, or rather the efficiency ratio, which is currently 26.7%, no longer seemed possible, people increasingly looked for other ways of generating electricity using photovoltaics.

The development of solar modules based on perovskite is considered very promising due to the low cost of production. The cells can be built much thinner than silicon cells. However, the problems so far are the sometimes low durability, the protection against moisture and the amount of lead required in all efficient perovskite cells.

Although it is in principle possible to replace lead with other elements such as tin, as of 2016 such attempts have been largely unsuccessful because tin gradually oxidizes and thus the crystal structure of the perovskite is lost.

After the consistent efficiency improvements in recent years, perovskite modules must be considered a serious potential challenger to other solar technologies. The efficiency was already 20.1% in 2015, over 25% since 2020 and is expected to continue to rise in the coming years (https://www.nrel.gov/pv/cell-efficiency.html), and the technology is also inexpensive. The efficiency figures mentioned refer to pure perovskite solar cells. Although it is still in its early stages of development, the perovskite-based solar cell has shown outstanding potential for sustainability. It has a very short energy payback time and could potentially be the most environmentally friendly photovoltaic technology if further development can increase utilization and durability (J. Gong et al., Perovskite photovoltaics: life-cycle assessment of energy and environmental impacts. Energy and Environmental Science 8, (2015), 1953-1968, doi:10.1039/c5ee00615e). At the beginning of 2022, several manufacturers announced that they were close to series production of silicon perovskite tandem solar cells, with the cell efficiency in the laboratory already at around 31%.

Perovskites are a class of materials that includes an extremely large number of different substances that all have the same crystal structure. They are able to raise the efficiency of solar cells far beyond the limits of the classic silicon cell.

While a simple silicon-based solar cell can only make optimal use of light in a certain wavelength range (red-yellow), a cell with a second semiconductor layer can be optimized so that each layer converts a different part of the light spectrum (green-blue) into electricity in the best possible way. These are then referred to as tandem cells.

Some of today's perovskite cells have such a double structure, which has, for example, perovskite layers and silicon layers as semiconductor layers (also called absorber layers, e.g. in Fig. 1 and 2). These are therefore called silicon-perovskite tandem solar cells. They usually contain a 140 to 160 micrometer thick classic silicon disk, onto which a perovskite layer of a few micrometers is applied (along with other layers described below).

Materials with a perovskite structure - perovskite is originally the name of the natural mineral calcium titanate CaTiCh - have been known for a long time. In the 1980s, oxide perovskites were the basis for the further development of so-called superconductors.

In fact, research into solar cells made of metal halide perovskites, which differ greatly from oxide perovskites in their electronic, optical and mechanical properties, has only been underway since the end of the 2000s. A first report dates back to 2009 (A. Kojima et al., J. Am Chem. Soc. 2009, 131 17, 6050- 6051). One reason for the speed - and further potential - of research into perovskite solar cells is the low non-radiative recombination coefficients in certain lead halide perovskites, as well as the possibility of depositing these materials in good electronic quality from the liquid phase. Normally, semiconductors for good solar cells and LEDs have to be extremely pure in order to have a high level of efficiency. One example of this is single-crystal Si, where there are no grain boundaries. GaAs has been suggested as an alternative, which is produced epitaxially at high temperatures using very complex equipment. With perovskites, on the other hand, you can mix an "ink" from inexpensive chemicals, then apply (or print) it to a substrate at room temperature, and in this very simple way you get a good semiconductor. This means that the lead halide perovskites obtained in this way are semiconductors with numerous crystallographic defects, but these apparently do not have any serious effects and therefore still allow good solar cells to be produced.

In the early days of perovskite solar cells, the lifespan was unclear. One challenge is that the different perovskites can have very different levels of stability depending on their chemical composition.

Metal halide perovskites in particular have been intensively investigated in recent years with regard to their suitability for solar cells.

Metal halide perovskites grow in the perovskite structure, but what is important for their function in solar cells is not primarily the structure, but the lead-halide bond. The Pb-I or Pb-Br bonds have special properties that make them suitable for use in solar cells. However, Pbl2 alone would not be a good solar cell material. It has been shown that the lead and iodine must be brought into a crystal structure - the perovskite structure - that is suitable for solar cells.

Typical metal halide perovskites for photovoltaic applications are made up of a mixture of various cations (e.g. methylammonium MA, formamidinium FA, Cs, Rb, etc.) with oxidation state +1, cations with oxidation state +2 (almost exclusively Pb ²⁺ ) and anions (I; Br, CI'). A "triple cation" perovskite is widely used, which has Cs, FA and MA. Its formula can be represented as Cso.os MAiyFAssjgsPbOssBriy. It is also called CsMAFA (A. Al-Ashouri et al., Energy Environ. Sei 2019, D0l:10.1039/c9ee02268f). The properties of a perovskite are influenced by the anion ratio Br/I. For a Br/I ratio >20%, for example, a larger band gap is achieved. On the other hand, however, at high Br/I ratios, phase instabilities arise due to light-induced halide segregation.

A typical structure of a perovskite solar cell with a p-i-n structure (positively doped layer - intrinsically conductive layer - negatively doped layer) consists of a glass substrate with a transparent conductive oxide layer applied on top, e.g. B. from ITO (indium tin oxide), as an electrode and a conductive hole transport layer applied on top, usually made of an organic polymer such as PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] or polyTPD (poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine], On top of this there is a perovskite layer as a semiconducting absorber layer. Ceo (fullerene), for example, is applied to the perovskite layer as an electron-selective contact. The last layer is an electrode, which often consists of two layers. First comes a thin layer of an organic semiconductor with a strong dipole (e.g. bathocuproine) and then an approximately 100 nm thick metal electrode made of Ag or Cu. An example is a bathocuproine electrode ((BCP)/Cu or BCP/Ag). Such a perovskite solar cell is shown schematically in Fig. 2.

Such solar cells are often combined with silicon solar cells to form so-called tandem solar cells. A perovskite solar cell constructed as above is "stacked" on a silicon solar cell with a p-i-n-i-n structure, for example. The Si solar cell then has no metal contact on the side facing the light. For this, the BCP/Ag contact in the perovskite solar cell must be replaced by several transparent layers. These typically consist of a thin layer of SnC>2 as protection against sputter damage and then a layer of indium tin oxide and a metal grid.

The individual layers of perovskite solar cells are sometimes applied (printed) on top of each other using liquid phase processes. Although some of the layers are well suited for solar cells due to their electronic properties, they are too hydrophobic to allow the subsequent layer to be applied evenly. As a result, the surface is only partially covered by the perovskite layer, which renders the solar cell unusable.

Solutions to this problem have already been proposed, such as ultraviolet ozone treatment (X. Xu, et al., J. Power Sources 360, 157 (2017)) or the use of poly[(9,9-bis(3-(/V, /V-dimethylamino)propyl)-2,7-fluorene)-a/t-2,7-(9,9-dioctyl-fluorene)] (PFN) as an amphiphilic interfacial compatibilizer (J. Lee et al., Adv. Mater.

29, 1606363 (2017)). A specific two-step process has also been proposed, namely a two-step heating-assisted interdiffusion method (J. Höcker et al., Zeitschr. Naturforsch. 2019, 74, 665-672). In the same article, it is also reported that polyTPD was used as a hole transport layer in combination with vacuum-sublimated MAPbh perovskite, but that the hydrophobic character of the polyTPD layer poses a challenge.

P. Tockhorn et al., Nano-optical designs for high-efficiency monolithic perovskite-silicon tandem solar cells, Nature Nanotechnology, publication date 24.10.2022, https://doi.org/10.1038/s41565-022-012228-8) propose to avoid the formation of macroscopic holes in perovskite layers on planar Me-4PACz surfaces by using nanotextured silicon cells with a Me-4PACz coating as substrates. The nanotexturing, which takes the form of sinusoidal elevations in the submicrometer range, forms perovskite layers that do not have holes visible to the naked eye.

US 2022/0108847 A1 discloses an electronic device, such as a solar cell, comprising a layer of a perovskite material. In the disclosed solar cell arrangement, an additional layer of an oxysalt is provided between the perovskite layer and the hole transport layer. The hole transport layer used here is PTAA. The additional oxysalt layer is formed by applying a solution of the oxysalt to the perovskite layer and allowing a reaction to take place between the salt and the perovskite. The oxysalt layer influences the electronic properties and increases the stability of the perovskite film.

Polymethyl methacrylate (PMMA), better known as acrylic, is a transparent and rigid thermoplastic. PMMA or acrylic is a widely used transparent plastic material known for its applications in various fields from car windows to smartphone screens to aquariums. It is a durable plastic that is easy to mold and is an alternative to the expensive and less durable glass.

In perovskite solar cells, PMMA has been used as a separate intermediate layer to passivate surface defects of the perovskite layer, but the thickness Due to the low conductivity of PMMA, the PMMA layer must be extremely thin (referred to as “ultrathin” in the publication, but not specified), i.e. probably a few nanometers thick (J. Peng et al., Nanoscale localized contacts for high fill factors in polymer-passivated perovskite solar cells. Science 2021 , 371 (6527), 390 — 395; J. Peng et al., A Universal Double-Side Passivation for High Open-Circuit Voltage in Perovskite Solar Cells: Role of Carbonyl Groups in Poly(methyl methacrylate). Advanced Energy Materials 2018, 8 (30); H. Kim et al., Polymethyl Methacrylate as an Interlayer Between the Halide Perovskite and Copper Phthalocyanine Layers for Stable and Efficient Perovskite Solar Cells. Advanced Functional Materials 2021 , 32 (13)).

In another case, PMMA was incorporated into CsPbXs quantum dots (QDs-CsPbXs) to improve their stability and maintain their excellent optical properties (L. C. Chen, Influence of PMMA on All-Inorganic Halide Perovskite CsPbBr(3) Quantum Dots Combined with Polymer Matrix. Materials (Basel) 2019, 72 (6)).

PMMA has also been added to perovskite solutions to control perovskite crystal size and morphology in perovskite layers.

S. Masi et al., Multiscale morphology design of hybrid halide perovskites through a polymeric template, Nanoscale 2015, 7, 18569 describe the influence of different polymers, namely poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), poly(methyl methacrylate) (PMMA), poly[(9,9-bis(3'-(/V,/V-dimethylamino)propyl)- 2,7-fluoren-a/t-2,7-(9,9-diocytlfluroen)] (PFN), 2,4-dimethyl-poly(triarylamine (PTAA) and polystyrene (PS) on the formation of MAPbh perovskite. The influence on self-aggregation and the influence on film growth were investigated by adding the different polymers to a perovskite precursor solution in DMF, which was then spin coated on (PEDOT:PSS)/glass substrates and then heated to 100 °C for 10 minutes. MEH-PPV and PFN led to a base-acid equilibrium between polymer and MA ⁺ cation due to their basic functional groups. The resulting improved complexation ability led to smooth perovskite films with evenly distributed aggregates of a few 10 nm. PMMA, PS and PTAA show a much poorer affinity in solution to MA ⁺ and Pblß' ions, which leads to severe phase separations during the film formation process and causes stronger aggregation of the perovskite domains. The polymer was in each case in a less polar solvent (THF) was added to the perovskite precursor solution, which greatly affected the film formation step. MAPbh : PMMA blend films showed uneven surface coverage and large roughness due to the unfavorable polymeric environment, which affected the uniform growth of the perovskite phase.

D. Bi et al., Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency > 21%, Nature Energy 2016, 9, 2609) describes the formation of a mixed organic-inorganic Pb perovskite from FAI, Pbl2, MABr and PbBr2 in a mixed solvent of DMF, DMSO and N-methylpyrrolidone (NMP). PMMA is added to the perovskite precursor solution in a chlorobenzene/toluene mixture (volume ratio 9:1) during spin coating. The solution is applied by spin coating to a substrate made of FTO/compact TiO2/mesoporous TiO2, i.e. on a hydrophilic surface. The publication particularly investigates the influence of the PMMA concentration on the perovskite film properties. A PMMA concentration of 0.6 mg/ml has proven to be the best.

Layers of lead halide perovskites have already been applied to various layers, some of which are also hydrophobic (see e.g. A. Al-Ashouri, op. cit., A. Al- Ashouri et al., Science 2020, 1300-1309, J. Höcker et al., op. cit.). However, the publications do not describe the proportion of rejects obtained with the processes used.

PTAA and Me-4PACz are not as hydrophobic as poly-TPD and P3HT, so it may be possible in some labs to spin coat them with perovskites because the environment such as temperature and humidity is very specific. It may therefore be possible for some labs to spin coat perovskites on Me-4PACz. However, this is not yet possible on poly-TPD, at least not for large area production.

There are currently no published methods that allow the coating of hydrophobic surfaces with a Pb perovskite layer with high reproducibility and good surface coverage. The object of the invention was therefore to provide a process for producing Pb(II)-containing layers on hydrophobic substrates, in which a complete, gap-free coating is obtained.

The object is achieved by the method according to claim 1. It is characterized in that the Pb(II)-containing solution contains a polymer additive which has polar groups, namely polymethyl methacrylate (PMMA).

Furthermore, the invention provides a method for producing a perovskite layer on a hydrophobic layer and a solar cell having such a perovskite layer. The use of PMMA as an additive for producing a Pb(II)-containing layer, in particular a Pb(II)-containing perovskite layer on a hydrophobic surface is also disclosed.

Fig. 1 is a schematic representation of a perovskite-based tandem solar cell.

Fig. 2 is a schematic representation of a perovskite solar cell.

Fig. 3 shows the film quality of a perovskite layer according to the invention on a Me-4PACz surface as a function of the PMMA concentration of the perovskite precursor solution.

Description of the preferred embodiments

According to the invention, PMMA is used as the polymer additive.

Tests have shown that monomeric methyl methacrylate and polyethylene oxide are unsuitable as polymer additives according to the invention.

The PMMA typically has an average weight molecular weight of about 10,000 to about 200,000 g/mol, preferably about 15,000 to about 100,000 g/mol.

Any commercially available PMMA, e.g. from Sigma Aldrich, is suitable as PMMA. The average weight-average molecular weight (determined by GC) is approx. 10,000 to approx. 120,000 g/mol, preferably approx. 15,000 to approx. 100,000 g/mol. In tests, both PMMA with an average weight-average Mw of approx. 15,000 g/mol and PMMA with an average weight-average Mw of approx. 120,000 g/mol showed very good results, ie perovskite layers were obtained that completely covered the hydrophobic surface. It is assumed that PMMA with an average weight-average Mw that is too high, ie higher than approx. 120,000 g/mol, does not dissolve completely in the solvent of the precursor solution, so that weight-average molecular weights that are too high appear unsuitable.

The polymer additive PMMA is added to the Pb(II)-containing solution, which is then applied to the hydrophobic surface. The polymer additive dissolves completely in the Pb(II)-containing or perovskite precursor solution, resulting in a clear, viscous solution.

The solvent is any solvent commonly used for Pb(II) compounds or perovskite precursors, in particular dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), 2-methoxyethanol or γ-butyrolactone and mixtures thereof. DMF or a mixture of DMF/DMSO in a volume ratio of 3:1 to 5:1, e.g. 4:1, is particularly preferably used.

Chlorobenzene and toluene are not suitable solvents for perovskite precursors. On the contrary, they are used as antisolvents, as described below.

The concentration of PMMA added to the Pb(II)-containing solution is generally 0.01 mg/ml to 1.5 mg/ml, preferably 0.03 to 1.2 mg/ml, particularly preferably 0.5 to 1.0 mg/ml, based on the Pb(II)-containing solution. The higher the PMMA concentration in the perovskite precursor solution, the better the quality of the resulting perovskite film, measured in terms of the uninterrupted area. This can be clearly seen from Figure 3.

The hydrophobic surface, which according to the invention acts as a hole conductor or transport layer in the case of solar cells, can be any hydrophobic surface. According to the invention, a surface is particularly hydrophobic if the constituent polymer or molecule has numerous alkyl groups and/or longer-chain alkyl groups and/or aryl groups, as is the case with Me6-PACz or PTAA, for example, while Me-2PACz, 2PACz and V1036, for example, are not classified as hydrophobic. and Me-4-PACz is only slightly hydrophobic. According to the invention, surfaces are considered to be hydrophobic in particular if their constituent polymer or molecule has one or more alkyl groups with a number of >3 carbon atoms and/or aryl groups.

For example, the hydrophobic surface is a surface made of poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (polyTPD). polyTPD is transparent and colorless, so it has no influence on the absorption behavior of the solar cell and is particularly suitable for perovskite solar cells.

In addition to polyTPD, surfaces made of polytriarylamine (PTAA), poly(3-hexylthiophene-2,5-diyl) (P3HT), [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid (Me-4PACz) and 6-(3,6-dimethyl-9/7-carbazol-9-yl)hexyl)phosphonic acid (Me-6PACz) are also suitable according to the invention.

The polymer or molecule that forms the hydrophobic surface is selected according to the perovskite layer to be applied. The choice of material depends on the ionization potential of the molecule or polymer and the perovskite to be applied subsequently or on the position of its valence band. These data are already known from the literature for perovskites and hole conductors (e.g. A. Al-Ashouri et al., Conformal monolayer contacts with lossless interfaces for perovskite single junction and monolithic tandem solar cells, Energy and Environ. Sci. 2019, 12, 3356; S. Tao et al., Absolute energy level positions in tin-and lead-based halide perovskites, Nature Comm. 2019, 10, 2560) or can be determined by ultraviolet

Photoelectron spectroscopy. However, since this method has a significant measurement uncertainty, the hole conductors are often selected by fabricating perovskite solar cells and measuring the current-voltage characteristic.

According to the invention, the surface to be coated with the precursor solution or the hole conductor in particular does not consist of PEDOT:PSS (poly-3,4-ethylenedioxythiophene: sodium polystyrene sulfonate). This polymer is hygroscopic and therefore hydrophilic.

The hydrophobic surface is prepared by any conventional method, e.g. by applying a solution containing the constituent polymer, e.g. in toluene, chlorobenzene, dichlorobenzene, isopropanol, ethanol or another skilled known conventional solvents, on a substrate, e.g. glass/ITO or plastic/ITO, by e.g. spin coating, and subsequent drying.

According to the invention, all Pb(II) salts come into consideration as Pb(II)-containing compounds, in particular lead(II) halides and lead acetate, particularly preferably Pbl2, PbBr2, and Pb(OAc) ₂ .

According to the invention, the perovskite precursor compounds are selected from halides of metals, in particular Pb and Cs, and halides of organic cations, in particular methylammonium (MA) and formamidinium (FA). The halides are selected from bromide, iodide and chloride and are preferably iodide and/or bromide, particularly preferably iodide or both iodide and bromide. According to the invention, the perovskite precursors are particularly preferably Pbl2, PbBr2, MAI, FAI and Csl.

The molar ratio of iodide : bromide, when both are present, in the perovskite precursor solution is arbitrary. It determines the size of the band gap and is therefore selected appropriately to achieve the desired band gap. It is e.g. 5:1 to 1:5, 4:1 to 1:4, 3.5:1 to 1:3.5, e.g. 3.5:1.

For example, the molar ratio of the cations Pb:MA:FA:Cs in the perovskite precursor solution is 1.2:0.26:0.88:0.06.

The Pb(II) compound is preferably selected from Pbl2, PbBr2 and Pb(CH3COO)2. The perovskite obtained according to the invention by the claimed process is preferably FAPbh, MAPbh, Cs _x FAi. _x Pbl3 or Cs _x FA _y MAi. _xy Pb(l _z Bri. _z )3, where x + y < 1, 0 < x < 1, 0 < y < 1 and 0 < z < 1, and x denotes the proportion of Cs in all cations with the oxidation state + 1, y denotes the proportion of FA in all cations with the oxidation state +1 and z denotes the proportion of iodine in all anions.

Particularly preferred according to the invention is the perovskite Cso,o5MAo,22FAo.73Pb(lo,78Bro,2)3 or Cso,o5(MAi7FA83)o,95Pb(l83Bri7)3. These have proven to be particularly suitable for use in perovskite solar cells in tests carried out.

The precursor solution is obtained by dissolving the Pb(II) compounds or perovskite precursors in a suitable solvent, in particular a polar aprotic solvents, for example in dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), 2-methoxyethanol or y-butyrolactone. DMF or a mixture of DMF/DMSO, e.g. in a volume ratio of 3:1 to 5:1, in particular 4:1, is preferably used.

The Pb-(II) and any other compounds are dissolved in the solvent by stirring, e.g. at room temperature for 2 hours, and the solution is usually filtered before further use, e.g. with a 0.45 pm PTFE filter.

The precursor solution is then applied to the hydrophobic surface by a conventional process, e.g. by spin coating, slot die coating, blade coating, inkjet printing or roller coating. For example, spin coating is applied in a two-step process at different speeds, e.g. 1400 rpm for 15 seconds with a ramp rate of 350 rpm/second and then 6000 rpm for 30 seconds with a ramp rate of 767 rpm/second.

Some time before the end of the second step, an antisolvent is applied, if desired, preferably by dripping. The antisolvent is e.g. anisole. However, other common antisolvents are also suitable, such as chlorobenzene, dichlorobenzene, toluene or trimethylbenzene. The antisolvent initiates the crystallization of the perovskite (see A. D. Taylor et al., Nature Communications https://doi.org/10.1038/s41467-021-22049-8). The amount and duration of the application of the antisolvent are known to the person skilled in the art and can be suitably selected, for example, based on the aforementioned publication by Taylor.

Immediately after application with a doctor blade, the precursor solution according to the invention containing the polymer additive forms a liquid film on the hydrophobic surface that is visible to the naked eye and that does not run off when tilted but adheres to the surface. A drop of the additive-containing precursor solution according to the invention applied with a pipette cannot be completely sucked off the surface again with the pipette because the solution adheres too strongly.

In contrast, if a precursor solution without additive is applied to the hydrophobic surface in the same way with a doctor blade, no film is formed, but the solution forms a drop on the surface and runs off at A drop of the precursor solution without additive applied with the pipette can be completely absorbed again, since the solution without additive does not adhere well to the hydrophobic surface.

After the precursor solution has been applied, the resulting film is dried and then heated. Heating is carried out, for example, to 80 to 120 °C, preferably 90 to 110 °C, particularly preferably about 100 °C for 15 to 45 minutes, preferably 25 to 35 minutes, particularly preferably about 30 minutes.

The layer thickness of the resulting Pb(II) layer or perovskite layer is, for example, approximately 500 to 600 nm. The layer thickness can be increased by increasing the concentration of the perovskite precursor solution, e.g. up to 1.0 pm. The layer thickness can be determined, for example, by measuring a section of a sample obtained using an electron microscope. The layer thickness can be determined in a known manner, e.g. by examining a section using a scanning electron microscope. The layer thickness can also be determined using a profilometer, in which the height of a section through the layer is scanned with a needle that is slowly moved over the sample.

No special environmental conditions need to be observed for the production of the perovskite layer. The coatings are usually produced on a laboratory scale in a glove box at 18 to 25 °C and in a nitrogen atmosphere with a maximum relative humidity of 1%. However, this is mainly done in this way to ensure that no Pb gets into the environment and that the employees carrying out the work are protected from Pb. The expert knows the suitable conditions. For example, work is carried out at a room temperature of 18 to 25 °C and a relative humidity of 30 to 80%.

The resulting Pb(II)-containing layer or perovskite layer completely covers the hydrophobic surface. This means that no holes or gaps in the perovskite coating can be seen with the naked eye. Holes and even very small holes (so-called pinholes) are particularly clearly visible when the coated substrate is held against a light source, for example. The perovskite layer is black in color and its completeness is therefore easily visible with the naked eye. Cracks can be present in the edge area, as can be seen in Fig. 3, for example; the entire area covered with the perovskite layer is large enough, however, that the The solar cell sizes specified below can be achieved and these covered areas have no holes or pinholes.

The layer quality can also be confirmed by an optical microscope or a scanning electron microscope. This clearly shows the complete coverage of the surface of the hydrophobic layer.

The presence of a perovskite crystal structure can be confirmed by X-ray diffraction in a manner known to those skilled in the art. Sharp peaks appear in the X-ray diffraction spectrum as evidence of the presence of polycrystalline perovskite films. The formation of the perovskite structure can already be recognized in known perovskites by the black color of the coating.

The area of the uncoated substrate is usually up to 2 cm x 2 cm on a laboratory scale. On such a substrate, for example, either one solar cell with an area of 1 cm x 1 cm or four solar cells with an area of 4 mm x 4 mm each can be applied.

For industrial application scale, substrates of up to 150 cm x 150 cm, e.g. 100 cm x 100 cm, 40 cm x 40 cm or 30 cm x 30 cm are coated.

Also claimed is a perovskite solar cell which contains a perovskite layer produced according to the method according to the invention. Otherwise, the solar cell contains the usual layers present in perovskite solar cells or tandem solar cells, which are produced in the usual way.

The perovskite solar cell comprises in particular a transparent substrate, a transparent anode applied thereon, an organic polymer hole transport layer provided thereon, the perovskite layer obtained according to the invention applied thereon, and thereon in this order an electron transport layer, a buffer layer and a cathode.

The transparent substrate consists, for example, of glass or transparent plastic, e.g. PET (polyethylene terephthalate), PI (polyimide) or PEN (polyethylene naphthalate). Depending on the material and intended use of the solar cell, the substrate can be planar or curved. It can be rigid or flexible.

The transparent electrode consists, for example, of a transparent conductive metal oxide, preferably antimony-doped tin oxide (ATO), indium tin oxide (ITO), fluorine-doped tin oxide (FTO) or aluminum-doped zinc oxide (AZO), preferably indium tin oxide (ITO).

The hole transport layer (hole conductor) is preferably a hydrophobic layer as defined above, for example a polyTPD layer or PTAA layer.

The electron transport layer consists, for example, of Ceo or PCBM ([6,6]-phenyl-Cei-butyric acid methyl ester), ICBA (T, 1 ", 4',4"-tetrahydro-di[1 ,4]methano-naphthaleno[1 ,2:2',3',56,60:2",3"][5,6]fullerene-C6o, also known as indene-Ceo-bisadduct, molecular formula CysH ) or ZnO.

The buffer layer consists, for example, of 2,9-dimethyl-4,7-diphenyl-1,10-phenathroline (BCP), LiF, polyethyleneimine (PEIE), tris-(8-hydroxyquinoline)aluminum (Alq ₃ ), SnO _x , or Ti(Nb)O _x .

The cathode consists of a metal such as silver, copper or gold or alloys of these metals. Alternatively, cathodes made of known non-metallic materials are also suitable for electrodes, such as graphite, ITO, FTO and the like.

The use of polymethyl methacrylate as an additive for producing a perovskite layer on a hydrophobic surface is also claimed.

EXAMPLES

A typical process for producing perovskite solar cells according to the invention looks like this:

example 1 The pre-structured ITO substrates (2.0 x 2.0 cm ² ) were cleaned ultrasonically for 10 minutes with soap (Hellmanex III), deionized water, acetone and isopropanol (IPA) one after the other. The cleaned ITO substrates were treated with oxygen plasma for 12 minutes and placed in a glove box filled with N2. 80 μl of 75 °C poly-TPD [poly (N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine, 1.5 mg/ml in toluene] solution were spin coated onto the ITO substrates using a step program at 4000 rpm for 30 s (with a ramp rate of 800 rpm s ^-1 ), then the samples were thermally annealed at 100 °C for 10 min and then cooled to room temperature.

The perovskite precursor solution, which was prepared by mixing Pbl2 (0.80 mmol), PbBr2 (0.4 mmol), MAI (0.26 mmol), FAI (0.88 mmol), and Csl (0.06 mmol) in 1 mL of DMF, was added with 0.5 mg/mL of PMMA based on the perovskite precursor solution and stirred for 2 h at room temperature and filtered with a 0.45 pm PTFE filter before use. To prepare the perovskite layer, 120 μl of the perovskite precursor solution was spin coated onto the hydrophobic poly-TPD layer in two consecutive steps at 1400 rpm for 15 s (with a ramp rate of 350 rpm s ^-1 ) and 6000 rpm for 30 s with a ramp rate of 767 rpm s ^-1 . 10 s before the end of the program, 300 μl of anisole was dropped onto the film as an antisolvent. The samples were immediately annealed on a hot plate at 100 °C for 30 min.

Subsequently, 20 nm of Ceo was thermally evaporated onto the perovskite film at a rate of 0.15 Ä s ^-1 . Finally, BCP (8 nm) and Ag (80 nm) were thermally evaporated sequentially in a separate vacuum chamber (<5X10 ^-6 Pa).

2 to 4

The procedure was carried out exactly as in Example 1 except that the following hydrophobic polymers, solvents and concentrations were used for coating the hydrophobic layer: Example 2: P3HT [poly(3-hexylthiophene)] 1.5 mg/ml in chlorobenzene Example 3: Me-4PACz 1 mmol/ml (0.331 mg/ml) in ethanol Example 4: PTAA, 1.5 mg/ml in toluene. Example 5: Me-4PACz 1 mml/ml (0.331 g/ml) in ethanol, different concentrations of PMMA, namely 0.05 mg/ml, 0.1 mg/ml, 0.15 mg/ml and 0.2 mg/ml, each in DMF. The results are shown in Fig. 3.

Embodiments

The present application discloses the following embodiments:

1. Process for producing a coating containing one or more Pb(II) compounds on a hydrophobic surface by applying a precursor solution containing one or more Pb(II) compounds in an organic solvent; optionally subsequently applying an organic anti-solvent; and subsequently heating, characterized in that the precursor solution contains a polymer additive containing polar groups.

2. The method according to [1], wherein the precursor solution contains perovskite precursors, at least one of which is a Pb(II) compound, and the coating is a perovskite layer.

3. Process according to [1] or [2], wherein the polymer additive is selected from polyalkyl methacrylate, polyalkyl acrylate, polyacrylate or a copolymer of polyalkyl methacrylate, polyalkyl acrylate and/or polyacrylate.

4. Process according to one or more of the preceding embodiments, wherein the polymer additive is selected from polymethyl methacrylate (PMMA) and polyethyl methacrylate and is preferably PMMA.

5. Process according to one or more of the preceding embodiments, wherein the polymer additive is present in the perovskite precursor solution in a concentration of 0.01 mg/ml to 2.0 mg/ml, preferably 0.05 mg/ml to 1.0 mg/ml, based on the perovskite precursor solution.

6. Process according to one or more of the preceding embodiments, wherein the solvent of the precursor solution is a polar aprotic solvent, preferably dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), 2-methoxyethanol or γ-butyrolactone or a mixture thereof, particularly preferably DMF. 7. Process according to one or more of [2] to [6], wherein the perovskite precursors are selected from halides of metals, in particular Pb and Cs, and halides of organic cations, in particular methylammonium (MAI) and formamidinium (FAI), wherein the halides are preferably selected from bromide, iodide and chloride, particularly preferably are iodide and/or bromide, and most preferably are iodide and bromide.

8. The process according to one or more of [2] to [7], wherein the perovskite precursors are Pbl ₂ , PbBr ₂ , MAI, FAI and Csl.

9. Process according to one or more of [2] to [8], wherein the Pb(II) compound is selected from _Pbl2 , _PbBr2 and Pb(CH3COO) ₂ or the perovskite is selected from FAPbh, MAPbh, _CsxFAi.xPbl3 and _{CsxFAyMAi.xyPb} ( _lzBri.z )3, where x + y _< ₁ , ₀ < x < 1, 0 < y < 1 and 0 < z < 1, and x is the proportion of Cs in all cations with _the oxidation state +1, y is the proportion of FA in all cations with the oxidation state +1 and z is the proportion of iodine in all anions.

10. Process according to one or more of the preceding embodiments, wherein the hydrophobic layer is selected from electrically conductive hydrophobic polymers, preferably from P3HT, polyTPD (poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine], PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]), Me-4PACz ([4-(3,6-dimethyl-9/7-carbazol-9-yl)butyl]phosphonic acid and Me-6PACz ([6-3,6-dimethyl-9H-carbazol-9-yl)hexyl]phosphonic acid.

11. Process according to one or more of the preceding embodiments, wherein the perovskite obtained has the empirical formula Cso.o5MAo.22FAo.73Pb( _lo.78Bro.2 ₎ 30or Cso,o5(MAi7FA83)o,95Pb(ls3Bri7)3.

12. Perovskite solar cell, characterized in that it contains a perovskite layer produced by the process according to one or more of [2] to [11].

13. Use of polymethyl methacrylate as an additive for producing a Pb(II) perovskite layer on a hydrophobic surface.

Claims

Patent claims

1. A process for producing a coating containing one or more Pb(II) compounds on a hydrophobic surface by applying a precursor solution containing one or more Pb(II) compounds in an organic solvent; optionally subsequently applying an organic anti-solvent; and then heating, characterized in that the precursor solution contains a polymer additive containing polar groups, wherein the polymer additive is polymethyl methacrylate (PMMA).

2. The method of claim 1, wherein the precursor solution contains perovskite precursors, at least one of which is a Pb(II) compound, and the coating is a perovskite layer.

3. Process according to one or more of the preceding claims, wherein the polymer additive is present in the perovskite precursor solution in a concentration of 0.01 mg/ml to 2.0 mg/ml, preferably 0.05 mg/ml to 1.0 mg/ml, based on the perovskite precursor solution.

4. Process according to one or more of the preceding claims, wherein the solvent of the precursor solution is a polar aprotic solvent, preferably dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), 2-methoxyethanol or γ-butyrolactone or a mixture thereof, particularly preferably DMF.

5. Process according to one or more of claims 2 to 4, wherein the perovskite precursors are selected from halides of metals, in particular Pb and Cs, and halides of organic cations, in particular methylammonium (MAI) and formamidinium (FAI), wherein the halides are preferably selected from bromide, iodide and chloride, particularly preferably are iodide and/or bromide, and most preferably are iodide and bromide.

6. Process according to one or more of claims 2 to 5, wherein the perovskite precursors are Pbl2, PbBr2, MAI, FAI and Csl. Process according to one or more of claims 2 to 6, wherein the Pb(II) compound is selected from Pbl2, PbBr2 and Pb(CHsCOO)2 or the perovskite is selected from FAPbh, MAPbh, Cs _x FAi. _x Pbl3 and Cs _x FA _y MAi. _xy Pb(l _z Bri. _z )3, where x + y < 1, 0 ^ x 5 1, 0 y 1 and 0 z 1, and x is the proportion of Cs in all cations with the oxidation state + 1, y is the proportion of FA in all cations with the oxidation state +1 and z is the proportion of iodine in all anions. Method according to one or more of the preceding claims, wherein the hydrophobic layer is selected from electrically conductive hydrophobic polymers, preferably from P3HT, polyTPD (poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine], PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]), Me-4PACz ([4-(3,6-dimethyl-9/7-carbazol-9-yl)butyl]phosphonic acid and Me-6PACz ([6-3,6-dimethyl-9H-carbazol-9-yl)hexyl]phosphonic acid. Method according to one or more of the preceding claims, wherein the perovskite obtained has the empirical formula Cso.o5MAo.22FAo.73Pb(lo.78Bro.2)30or Oso,o5(MAi7FA83)o,95Pb(l83Bri7)3 Perovskite solar cell, characterized in that it contains a perovskite layer produced by the process according to one or more of claims 2 to 9.