METHOD FOR FORMING A PEROWSKIT LAYER, ESPECIALLY FOR A SOLAR CELL

Abstract

A method for forming a perovskite layer (1) on a support substrate (3) and a method for fabricating a solar cell (29) with such a perovskite layer are described. The method comprises at least the following steps: - Mixing a solution (5) containing a first and a second perovskite precursor (7, 9) at a mixing temperature (T1), - Applying the solution (5) to the support substrate (3), which is optionally pre-tempered to an initial support substrate temperature (T2), - Moving the support substrate (3) together with the applied solution (5) in a two-dimensional motion and simultaneously cooling the support substrate (3) together with the applied solution (5) to a deposition temperature (T3) below the initial support substrate temperature (T2) to produce a deposited layer (11) containing the first and the second perovskite precursor (7, 9) on the support substrate (3).- optional: removal of a non-deposited portion (13) of the solution (5) from the support substrate (3), and - optional: heating the deposited layer (11) above a drying temperature (T4) above the deposition temperature (T3) to initiate a chemical reaction between the first perovskite precursor (7) and the second perovskite precursor (9).

Description

FIELD OF THE INVENTIONThe present invention relates to a method for forming a perovskite layer. The invention further relates to a method for producing a solar cell having a perovskite layer formed in this way.BACKGROUND ARTPerovskite layers, i.e. thin layers of perovskite material, can have inter alia interesting electrical and optical properties, on the basis of which they are suitable for a wide variety of applications.In particular, solar cells have recently been developed in which an absorber layer made of perovskite material is used for absorbing light and for photovoltaic conversion of energy contained in the light into electrical energy. Such so-called perovskite solar cells promising high efficiencies with low material costs and simple production.Various methods are known for forming perovskite layers, in particular for solar cells.For example, chemical substances which can react with one another to form a perovskite material and which are also referred to herein as perovskite precursors, perovskite precursors or perovskite educts can be applied to a substrate in a single process step and then brought to the chemical reaction there, for example by supplying energy in the form of, for example, heat. In such a single-stage process, the perovskite-layer precursors can be vapor-deposited onto a substrate, for example, in a common vapor deposition process, which is also referred to as co-evaporation. However, such vapor deposition processes are often difficult to implement on an industrial scale, require complex apparatus and / or are cost-intensive. Alternatively, wet chemical deposition processes for forming a perovskite layer in a single-stage process have been proposed. However, technical difficulties and / or high costs of an industrial implementation are often also in the case of these processes.Alternatively, perovskite precursors can be applied to a substrate in a two-stage process before they are then again brought to the chemical reaction. In this case, a first perovskite precursor layer is first applied to a substrate and a second perovskite precursor layer is subsequently applied above it.In order to form the two perovskite precursor layers, different technologies can in turn be used.For example, a perovskite precursor layer can be formed by depositing chemical substances suitable for this purpose by means of physical vapor deposition (PVD), such as co-evaporation, or by means of chemical vapor deposition (CVD). However, complex apparatuses are usually necessary for this purpose, and associated high costs and / or low throughput rates often make such processes uninteresting for an industrial reaction.Alternatively, at least one of the perovskite precursor layers can be formed by means of wet chemical methods. In this case, a use of different wet chemical deposition methods is conceivable.For example, a so-called spin coating (spin coating) is frequently used on a laboratory scale in order to apply a viscous solution, in which the substances required for the perovskite precursor are contained, to a rotating substrate, which is then distributed there, due to acting centrifugal forces, to form a perovskite layer precursor layer of homogeneous thickness. However, such an approach is usually only applicable for coating small-area substrates and is not of interest for an industrial conversion for forming perovskite layers, in particular, because of its lack of mass suitability.As an alternative, perovskite layers can also be applied wet-chemically with the aid of so-called extrusion slot dies. Such processes are also referred to as slot die coating. In this case, a solution which should be sufficiently viscous and in which the substances required for the perovskite precursor are in turn contained is extruded through a wide slot die onto a surface of a substrate. However, a relatively high outlay on apparatus and restrictions on the viscous solutions which can be used in this case make it of no interest to carry out such deposition processes industrially in order to form perovskite layers.In addition, in the two wet chemical deposition processes mentioned above, the educts are frequently dissolved in solvents such as dimethylformamide, which are relatively hazardous substances, which are flammable and / or toxic, for example. This can make industrial implementation of the processes mentioned more difficult, for example, on account of high costs or necessary complex apparatuses. In particular, in the case of a design of a plant to be used for this purpose, many technical safety aspects, such as, for example, taking account of regulations (e.g. ATEX), can lead to an increase in costs. The solvents themselves are also frequently expensive, as a result of which production costs of the overall process can be increased.In a further batch, the educts are dissolved in a solvent and applied to a substrate by a spray method, which is also referred to as spray coating. However, in general, aerosols are generated in this case, which can be laborious to suck off, on the one hand, and / or on the basis of which a proportion of the educts used does not pass into the perovskite layer to be generated, as a result of which an efficiency of the deposition is reduced.SUMMARY OF THE INVENTION AND EMBODIMENTSThus, there may be a need for an alternative approach that can form a perovskite layer and that can avoid or reduce at least some of the aforementioned problems of previous approaches. In particular, there may be a need for a method with the aid of which a perovskite layer can be produced at low costs, with low outlay on apparatus and / or in a manner which is readily implementable industrially. In addition, there may be a need for a method of manufacturing a solar cell using such a method of forming a perovskite layer.The above needs can be met at least partially with the subject matter of any of the independent claims of the present application. Advantageous embodiments are given in the dependent claims and in the following description.According to a first aspect of the present invention, a method of forming a perovskite layer on a carrier substrate is described. The method has at least the following process steps, preferably in the sequence indicated: (i) mixing a solution which contains a first and a second perovskite precursor at a mixing temperature, (ii) applying the solution to the carrier substrate, which is optionally tempered to a carrier substrate initial temperature, (iii) moving the carrier substrate together with the applied solution in a two-dimensional movement and simultaneously cooling the carrier substrate together with the applied solution to a deposition temperature below the mixing temperature in order to produce a deposited layer which contains both the first and the second perovskite precursor and / or a reaction product of the two perovskite precursors, on the carrier substrate, (iv) optionally: removing an undeposited portion of the solution from the carrier substrate, (v) optionally: heating the deposited layer above a drying temperature above the deposition temperature to dry the solution and / or to initiate a chemical reaction between the first perovskite precursor and the second perovskite precursor and / or a conversion of the reaction product formed therefrom.According to a second aspect of the present invention, a method for manufacturing a solar cell is described. The method has at least the following process steps, preferably in the sequence indicated:forming a perovskite layer on a surface of a carrier substrate by the method according to an embodiment of the first aspect of the invention; andforming electrical contacts for discharging electrical charge carriers from the perovskite layer.Embodiments of the invention may be considered, among other things and without limiting the invention, to be based on ideas or findings described below.At the outset, a basic idea of embodiments of the invention described herein is to be briefly explained, wherein this explanation is to be interpreted as merely roughly summarizing and the invention is not to be interpreted as restrictive:As already indicated, the aim is to provide an advantageous and in particular industrially usable process for forming perovskite layers, for example for producing solar cells.For this purpose, a preferably single-stage wet chemical process is proposed herein. In this case, a solution in which a first perovskite precursor and a second perovskite precursor are absorbed or dissolved is mixed in advance. The solution is then heated to a mixing temperature which is generally elevated compared to an ambient temperature. Preferably, the mixing temperature is selected such that a reaction occurs between the first and second perovskite precursors contained in the solution, wherein a reaction product can correspond to the perovskite to be formed with regard to its chemical composition and possibly also with regard to its structural structure. This solution is then applied to the carrier substrate, wherein the carrier substrate can also be tempered to a so-called carrier substrate initial temperature, which is typically increased compared to the ambient temperature. The carrier substrate provided with the applied solution in this way is subsequently successively cooled to a lower deposition temperature and simultaneously moved in a special two-dimensional manner. As a result of the successive cooling, the reaction product formed from the two perovskite precursors can precipitate out of the solution and accumulate on the surface of the carrier substrate, preferably in the form of a layer and further preferably in a crystalline form. By moving the carrier substrate and the solution located thereon simultaneously, it can be achieved that a layer formed thereby is formed uniformly and with a homogeneous layer thickness. As soon as a sufficient layer thickness has been reached in this case, an excess portion of the solution which has not been absorbed in the deposited layer can be removed from the carrier substrate. Finally, the layer deposited in this way can optionally be heated again, for example in order to evaporate residues of the solution remaining therein and / or in order to convert previously unconverted fractions of the first and second perovskite precursors into the reaction product, so that overall the desired perovskite layer is formed.The proposed approach can be implemented here with a technically relatively simple installation and / or in an industrially implementable manner. Moreover, hazardous, harmful and / or expensive solvents can be dispensed with. Furthermore, in the case of the proposed, preferably single-stage wet chemical process, no aerosols generally form.Possible features of embodiments of the invention and advantages to be achieved therewith are described in detail below.Perovskite layers can be used for various technical applications. Accordingly, different perovskite materials can be used in a manner specific to the application.Perovskite solar cells generally consist of different layers. The main layer used here may be an absorber layer of the ABX 3 type, where "A" is cesium (Cs), methylammonium (MA) or formamidinium (FA), "B" is lead (Pb) and "X" is chlorine, bromine or iodine. For example, MAPbI 3( methyl ammonium lead iodide) may be used for the perovskite layer of a solar cell.The perovskite layer to be formed with the method described herein can in principle be produced with any desired dimensions. For example, an area of the perovskite layer can be between a few square millimeters up to several square meters. Preferably, an area of the perovskite layer may be between 5 cm 2 and 5 m 2, more preferably between 200 cm 2 and 2 m 2. A layer thickness can be between 1 nm and 1 mm, preferably between 100 nm and 1 μm. The perovskite layer is generally not self-supporting here, but rather is supported by a carrier substrate.The carrier substrate can in principle have any dimensions and consist of any material. For example, the carrier substrate can have the same area dimensions as the perovskite layer to be formed thereon. However, the carrier substrate can also be larger-area than the perovskite layer to be formed, so that the latter covers only partial areas of the carrier substrate. The carrier substrate generally has a significantly greater thickness than the perovskite layer. For example, the thickness of the carrier substrate is usually greater than 0.1 mm, often greater than 0.5 mm, greater than 1 mm or even greater than 3 mm, but generally less than 2 cm or less than 1 cm.A material and a geometry of the carrier substrate can be selected such that, on the one hand, it supports the perovskite layer sufficiently mechanically in order to be able to withstand the forces acting on the perovskite layer during operation. On the other hand, the carrier substrate should be able to withstand the conditions occurring during the formation of the perovskite layer, such as temperatures, attacks by chemicals used, etc. Further, a thermal expansion coefficient of the supporting substrate should not be excessively different from that of the perovskite layer formed thereon. In practice, thin glass panes have proven suitable for the carrier substrate for many applications. However, it is also possible to use carrier substrates made of semiconductor material, in particular in the form of silicon wafers or wafers. As described in more detail below, for example, an already completely processed solar cell, for example a solar cell constructed from a silicon wafer, can serve as carrier substrate. In principle, however, carrier substrates made of other materials are also conceivable, for example, made of plastic, ceramic, metal, etc.In the method described herein, a solution is first mixed which contains both a first perovskite precursor and a second perovskite precursor. The two perovskite precursors are typically completely dissolved in a liquid solvent and / or mixed with it. One of the perovskite precursors may also be chemically bound in the solvent. The solution can be saturated with the first and / or the second perovskite precursor to a certain extent, for example to an extent of more than 10%, more than 30%, more than 60%, more than 80%, more than 90%, more than 95% or even with complete saturation or even supersaturation. The solution is here temperature-controlled to a first temperature T 1, which is referred to herein as mixing temperature T 1. This mixing temperature is typically increased compared to an ambient temperature and is selected such that a reaction occurs between the first and second perovskite precursors in order to form the desired perovskite or at least one chemically equivalent precursor thereof as the reaction product. The mixing temperature can therefore also be referred to as the reaction temperature. For example, the mixing temperature may be at least 50° C., at least 60° C., or at least 70° C. The mixing temperature depends, inter alia, on properties of the two perovskite precursors and / or properties of the solvent. In this case, a solubility of the perovskite precursors, i.e. an amount of material which can be dissolved to the maximum extent in the solution up to complete saturation of the solution, typically increases with increasing mixing temperature.After the solution has been mixed and tempered in this way, it is applied to a surface of the carrier substrate. For example, the solution can be applied dropwise, poured on, poured on or otherwise applied to the carrier substrate. In this case, a sufficient amount of solution is preferably applied in order to wet the entire surface of the carrier substrate or at least one partial surface on which the perovskite layer is to be formed with solvent over the entire surface and in a sufficient thickness. Preferably, an amount of solvent which is applied to the carrier substrate or a material amount of the two perovskite precursors contained therein is greater than or even significantly greater (i.e. for example greater than 10%, greater than 20% or even greater than 50% greater) than a material amount of the perovskite layer ultimately to be formed on the carrier substrate. In other words, solvent can be applied to the carrier substrate in an overflow.The carrier substrate is preferably tempered at least initially to an elevated second temperature T 2, which is referred to herein as carrier substrate initial temperature T 2. This carrier substrate initial temperature may be similar to the mixing temperature, but need not be identical to the mixing temperature. Optionally, the carrier substrate initial temperature can be up to 20° C., up to 10° C. or at least up to 5° C. greater or less than the mixing temperature. Preferably, the carrier substrate initial temperature is slightly lower than the mixing temperature, for example by between 1° C. and 20° C., preferably by between 3° C. and 10° C.The carrier substrate covered with the solution in this way is then set into a two-dimensional movement. In such a two-dimensional movement, the carrier substrate is successively transversely displaced in different directions and / or rotated or pivoted about different axes. Such a two-dimensional movement can also be referred to as wobbling or "wobbling". Due to such a two-dimensional movement of the carrier substrate, it can be achieved that the solution applied to its surface is uniformly distributed and ultimately forms a homogeneous layer.Simultaneously with the two-dimensional movement, the carrier substrate together with the solution applied thereto is successively cooled down to a third temperature T 3, which is referred to herein as deposition temperature T 3. The deposition temperature is preferably significantly below the carrier substrate initial temperature. For example, the deposition temperature may be at least 10° C., preferably at least 20° C. or even at least 30° C. below the carrier substrate initial temperature. For example, the deposition temperature may be less than 40° C. The deposition temperature to be achieved depends on properties of the perovskite precursors or reaction products contained in the solution and of the solvent and in particular on a solubility of the perovskite precursors or reaction products in the solvent of the solution. As a result of the successive cooling to the deposition temperature which is lower in comparison with the starting substrate temperature, the temperature-dependent solubility of the two perovskite precursors or reaction products in the solution is reduced in this case, with the result that these precipitate at least partially from the solution and thus deposit as a layer on the surface of the carrier substrate.After a desired layer thickness of the layer deposited in this way has been produced from the first and second perovskite precursor and / or the reaction product, an undeposited residue of the solution is generally removed from the carrier substrate, for example by casting it.The carrier substrate with the layer produced in this way, which is formed from the two perovskite precursors and / or the reaction product formed from the perovskite precursors, can subsequently be heated again in a targeted manner. In particular, heating above a predetermined fourth temperature T4, which is referred to herein as drying temperature T4, is carried out here. This drying temperature can be selected such that remaining solvent evaporates or evaporates. Furthermore, the drying temperature can depend on the properties of the first and the second perovskite precursor or of the reaction product in a material-specific manner and represent a temperature above which a conversion and / or chemical reaction occurs between the two perovskite precursors, so that these ultimately form the desired perovskite layer. The drying temperature can thus also be referred to as the reaction temperature limit. This reaction temperature limit is usually significantly above the deposition temperature, for example by more than 5° C. or even more than 10° C. For example, the reaction temperature limit can be above 50° C., above 60° C. or above 70° C., but usually below 150° C., below 130° C. or below 90° C. During the stated heating, residues of the solvent contained in the solution may evaporate simultaneously.According to one embodiment, the solution is aqueous, wherein the first and the second perovskite precursor and / or the reaction product are soluble in aqueous solution. The aqueous solution may comprise water as solvent. Alternatively or additionally, the aqueous solution can comprise an aqueous solution of an acid. An aqueous solution is usually easy to handle and can be provided cost-effectively. In particular, the aqueous solution should be free of harmful, hazardous and / or difficult to dispose of chemicals. In this case, preferably both the first and the second perovskite precursor and the reaction product should be water-soluble. A solubility of the two perovskite precursors and / or of the reaction product should be sufficiently high in each case in order to be able to dissolve a sufficient amount of each of the two perovskite precursors and / or reaction products in the solution in order ultimately to be able to produce the perovskite layer therefrom in a desired thickness. In particular, the solubilities of perovskite precursors in the solution may be different, but they should be similar to each other within acceptable tolerances. For example, the solubilities of the two perovskite precursors in the solution can differ from one another by less than 50%, preferably less than 20% or less than 10%.For example, according to one embodiment, the first perovskite precursor may contain cesium, methylammonium, or formamidinium. In particular, the first perovskite precursor may contain methyl ammonium iodide. the second perovskite precursor may contain lead, in particular lead acetate and / or a lead halide. These first and second perovskite precursors were recognized on the one hand as being suitable for forming a perovskite layer, in particular an ABX 3- type perovskite layer. On the other hand, these first and second perovskite precursors have sufficiently high solubility in an aqueous solvent. Accordingly, a simple and cost-effective mixture of water and such a perovskite precursor can be used as a solution, to which, for example, no other chemicals or at least only small amounts of other chemicals are added.According to one embodiment, the solution may contain hydrogen halide as solvent. Such hydrogen halide may be provided as hydriodic acid (HI), hydrochloric acid (HCl, also referred to as hydrochloric acid) or hydrobromic acid (HBr). The solvent can thus be a chemical compound which itself comprises one of the perovskite precursors, for example in the form of a halogen such as iodine, chlorine or bromine. Mixtures of hydrogen halides can also be used. Aqueous solutions of such hydrogen halides can usually be handled using industrially well-established processes and plants and can be relatively easily neutralized, for example with the formation of salts.According to one embodiment, the solution is mixed in a mixing tank separately from the carrier substrate and tempered to the mixing temperature. In other words, the first and the second perovskite precursor do not come into contact for the first time only during or after application of the solution to the carrier substrate, but are already mixed with one another and with the solvent in advance in a mixing tank to be provided separately and are tempered to the desired mixing temperature. The mixing temperature is preferably selected to be so high that chemical reactions to reaction products occur between the perovskite precursors already within the mixing tank. A suitable stirrer or the like can be provided for this purpose in the mixing tank in order to be able to mix the components of the solution with one another as homogeneously as possible. Furthermore, a heater or the like can be provided in the mixing tank in order to temperature control the entire solution to the mixing temperature.In addition, the mixing tank can be supplied with gas, in particular inert gas. In this case, an increased gas pressure can be adjustable, in order to accelerate, for example, a metering of precursors. The solution should be distributed as quickly and uniformly as possible on the substrate. For this purpose, the 2-dimensional movement can already be effected during the transfer of the solution from the mixing tank to the substrate. In addition, for example, an amount to be dispensed or a liquid jet can be dispensed or divided into a region of the substrate in a targeted manner by a device in order to initially reach a plurality of regions of the substrate. In addition, a larger volume can optionally be applied without liquid striking the substrate at points with high kinetic energy. Possible damage to sensitive underlying layers is thereby avoided and more homogeneous wetting for the initial deposition is achieved. In addition, a device for providing a required quantity can be provided, for example as a metering pump, a gravity metering, a control of a switching time of a valve, etc. The volume can be measured and / or regulated, for example, with an impeller counter or ultrasonic flow meter.According to one embodiment, during the production of the deposited layer, process parameters including a process duration and a temporal temperature profile during cooling to the deposition temperature are selected such that the deposited layer is produced as a crystalline layer. In other words, process parameters which influence a manner in which the deposited layer is produced from the two perovskite precursors and / or the reaction product can be adjusted in a targeted manner such that the deposited layer is produced as a crystalline layer, i.e. crystallization occurs during the deposition. Such process parameters include, among other things, the temperature of the solution and in particular a time profile with which this temperature is reduced in the course of the cooling process. A process duration during which the previously applied solution is successively cooled from the carrier substrate initial temperature to the deposition temperature and is then optionally kept stable at this final temperature or at intermediate temperatures typically also influences the manner in which the perovskite precursors are deposited as a layer and whether crystal formation occurs in the process. The crystalline layer to be produced can be monocrystalline, multicrystallin, polycrystalline or nanocrystalline, for example. Such a crystalline layer can subsequently react particularly well upon heating above the reaction temperature limit to form a perovskite layer or be converted into a perovskite layer.According to one embodiment, during the mixing of the solution, saturation of the solution with the first and the second perovskite precursors is adjusted in such a way that, during the subsequent cooling of the solution to the deposition temperature, supersaturation of the solution is established, in particular supersaturation with the reaction product formed from the two perovskite precursors. In other words, already when the solution is prepared, so much of the first and / or the second perovskite precursor is dissolved in the solvent that a high saturation of the solution is achieved even at the high mixing temperature prevailing in that case. The degree of saturation is preferably set to be so high that, during the subsequent cooling of the solution, in addition to the lower deposition temperature, supersaturation of the solution occurs at least with respect to one of the two perovskite precursors and / or preferably with respect to the reaction product formed from the two perovskite precursors due to the temperature-dependent solubility which then decreases. When such supersaturation is reached, a portion of the amount of the respective perovskite precursor or of the reaction product contained in the solution then precipitates out of the solution and is preferably deposited on the surface of the carrier substrate or on previously produced parts of the layer deposited there. As a result, the deposited layer can be grown particularly quickly.In order to produce the deposited layer with a high homogeneity and in particular with a constant layer thickness, the carrier substrate is moved in a targeted manner simultaneously with the cooling. The movement takes place two-dimensionally in this case, i.e. a position and / or orientation of the carrier substrate is changed in at least two dimensions. In particular, the movement should take place not only in a direction orthogonal to a plane of extension of the carrier substrate, but also transversely to this direction. The movement can be translatory and / or rotatory. The movement can take place continuously and / or periodically. In particular, the movement can take place in repeated cycles. For example, the motion may be in the form of a mixture of a repeating back and forth motion and a repeating up and down motion. Due to such a movement, forces act on the solution applied to the carrier substrate in different directions, in particular in different directions parallel to the surface of the carrier substrate, so that the solution is distributed along this surface as homogeneously as possible. The forces can act as inertial forces and / or gravitational forces.According to one embodiment, the two-dimensional movement is a movement pivoting about at least two different axes. In other words, during cooling to the deposition temperature, the carrier substrate is repeatedly pivoted about two different axes, in order to change its orientation in this way. The two axes extend transversely to one another, preferably perpendicularly to one another. The two axes can extend within the plane of extent of the carrier substrate or parallel to this plane of extent. The pivoting movement brought about in this case is also referred to as a wobbling movement or "wobbling" and enables a particularly effective distribution of the solution along the surface of the carrier substrate.According to one embodiment, the carrier substrate is tempered during the application of the solution, during the production of the deposited layer and / or during the heating of the deposited layer by means of a tempering device which makes planar contact with the carrier substrate. The temperature control device can be supplied with power in a controlled manner, in particular with electrical power, in order to be able to be heated to an elevated temperature and / or in order to be cooled to a reduced temperature. For this purpose, a power control can be sufficiently precise to control the temperature of the carrier substrate to the starting carrier substrate temperature T 2 in a targeted manner with the aid of the temperature control device, then to bring about a cooling of the carrier substrate to the deposition temperature T 3 by targeted reduction of the heating power, increasing a cooling line or switching off the temperature control device and / or subsequently to heat the carrier substrate beyond the drying temperature T 4. The temperature control device and / or its controller can be configured to set and / or maintain the mentioned temperatures precisely within a tolerance of, for example, less than ± 10° C., preferably less than ± 5° C. or less than ± 3° C. A technical configuration of the temperature control device and / or of the controller used for its power supply can be relatively simple and thus enable a cost-effective and / or reliable temperature control of the carrier substrate. For example, the temperature control device can be designed as a simple heating plate. Alternatively, the temperature control device can be designed as a thermal station in which a liquid medium that has been temperature controlled to a desired temperature is circulated under a plate in order to effect heating or cooling of the plate.According to one embodiment, the carrier substrate is covered by a cover during the application of the solution and during the movement of the carrier substrate in such a way that the cover abuts the carrier substrate in an annularly sealing manner along its edges and a receiving volume is formed between the carrier substrate and the cover, in which receiving volume the solution applied to the carrier substrate is received. In other words, the cover can be formed in such a way that it can lie with its edges hermetically sealingly against the carrier substrate along an annularly closed line. Due to the fact that the cover rests hermetically tightly against the carrier substrate at its edges, the solution accommodated in the surrounded receiving volume can be prevented from moving away from the surface of the carrier substrate to be coated and dripping off from the carrier substrate, for example, laterally during the movement of the carrier substrate.With the aid of the method according to the first aspect of the invention, a perovskite layer can be formed, which can then be used in the production of a solar cell according to the second aspect of the invention.The solar cell can be embodied as a pure perovskite solar cell. For this purpose, electrical contacts in the form of electrodes must additionally be attached to the perovskite layer formed, with the aid of which electrical charge carriers which are generated by absorption of light in the perovskite layer can be discharged and fed to an external circuit. In addition to the electrical contacts, further layers or components can be formed on the perovskite layer, for example in the form of an electron conductor layer, a hole conductor layer or a transparent, electrically conductive oxide layer (TCO). The method described herein is suitable in particular for manufacturing perovskite solar cells based on inorganic perovskite layers (e.g. CsPbBr3). However, perovskite solar cells with perovskite layers, which contain organic compounds, in particular organometallic compounds, can also be manufactured therewith (e.g. MAPbl3).According to a further embodiment, the carrier substrate can comprise a first solar cell, wherein the perovskite layer together with the electrical contacts is formed as a second solar cell. In other words, the carrier substrate itself can be a solar cell or can have a solar cell, so that a further solar cell can then be formed on this existing solar cell with the aid of the perovskite layer formed thereon by the method described herein. The first solar cell can be referred to as bottom solar cell and the second solar cell as top solar cell, wherein both solar cells together form a tandem solar cell.In particular, the first solar cell can be a silicon solar cell, preferably a solar cell formed on the basis of a silicon wafer. Such silicon solar cells can be produced using technologies that have been established for a long time, have high efficiencies and are very reliable. A perovskite solar cell can be formed as a top solar cell on such a silicon solar cell. The two solar cells supplement one another well with regard to their respective absorption spectra.Alternatively, the first solar cell can be a thin-film solar cell, for example based on a III-V semiconductor. As another example, the first solar cell may be a CdTe solar cell or a CIGS solar cell.It is pointed out that possible advantages and configurations of embodiments of the invention are described herein in part with reference to a method according to the invention for forming a perovskite layer and in part with reference to a method for manufacturing a solar cell having such a perovskite layer. One skilled in the art will recognize that the described features may be suitably transferred, adapted, interchanged or modified to arrive at further embodiments of the invention.BRIEF DESCRIPTION OF THE DRAWINGSEmbodiments of the invention are described below with reference to the attached drawings, wherein neither the drawings nor the description should be interpreted as restricting the invention. Figures 1(a)-(e) show successive stages of a process according to one embodiment of the invention.The figures are schematic only and not to scale. Identical reference numerals designate identical or identically acting features in the different figures.DESCRIPTION OF PREFERRED EMBODIMENTSFIG. 1 shows successive stages of a method for forming a perovskite layer 1, with the aid of which a solar cell 29 is ultimately formed.In a first step (see FIG. 1 ( a) ), a solution 5 is produced by dissolving methylammonium iodide (CH 3 NH 3 I) as the first perovskite precursor 7 and lead acetate trihydrate (Pb(CH 3 COO) 2 ·3H 2 O) as the second perovskite precursor 9 in an aqueous solution of hydriodic acid (HI) in a mixing tank 15 and tempering it to a mixing temperature T 1 of 70° C. The three substances mentioned react with one another, the HI also serving to dissolve the other two substances. Instead of lead acetate, lead iodide may also be used. For this purpose, the mixing tank 15 has a stirrer 31 and a heater 33.In a second step (see FIG. 1( b) ), the solution 5 is applied to the carrier substrate 3 through an outlet 35 to the mixing tank 15. In this case, the carrier substrate 3 has optionally already been tempered beforehand to a carrier substrate initial temperature T 2 of 65° C. For this purpose, a temperature control device 17, for example in the form of a heating plate, makes contact with the carrier substrate 3 in a planar manner along its rear side surface. On its front surface, a receiving volume 39 is created by means of a cover 37, into which the solution 5 is introduced. The cover 37 is hermetically sealed with its edges 41 against a front surface of the carrier substrate 3.In a third step (see FIG. 1( c) ), the carrier substrate 3 is then moved together with the applied solution 5 in a two-dimensional movement. This is symbolized in the figure by movement arrows 45. At the same time, the carrier substrate 3 and the applied solution 5 are successively cooled to a deposition temperature T 3 of below 40° C. Since a solubility of the two perovskite precursors 7, 9 or of the reaction products formed therefrom in the solution 5 decreases during this temperature reduction, their degree of saturation increases until at the latest when supersaturation is reached, a part of the perovskite precursors 7, 9 or reaction products contained in the solution 5 precipitates and precipitates on the front-side surface of the carrier substrate 3. As a result, a deposited layer 11 is formed on the carrier substrate 3. Due to the two-dimensional, preferably tumbling movement of the carrier substrate 3, this deposited layer 11 is produced with a high homogeneity and layer thickness. Process parameters are selected in such a way that the deposited layer 11 is produced as a crystalline layer 12.After the deposited layer 11 has reached a sufficient layer thickness, an excess, non-deposited portion 13 of the solution 5 is discharged from a discharge connection 43 and thus removed from the carrier substrate 3.Subsequently, in a fourth step (see FIG. 1( d) ), the deposited layer 11 together with the carrier substrate 3 is heated by means of the temperature control device 17 above a drying temperature T 4 of 50° C., for example. As a result, remaining solvent is evaporated. In addition, a chemical reaction occurs between any remaining residues of the two perovskite precursors 7, 9 and finally, possibly starting from a previously formed intermediate product (MA4PbI6·2H2O), the perovskite layer 1 is formed from methylammonium lead iodide (MAPbI 3).In order to finally produce a solar cell 29 with the perovskite layer 1 formed in this way, electrical contacts 19 in the form of a rear electrical contact 21 and a front electrical contact 23, which electrically contact the perovskite layer 1 on opposite sides, are produced in a fifth step (see FIG. 1( e)). In the example shown, for this purpose, the back contact 21 can be applied to a side of the carrier substrate 3 opposite the perovskite layer 1 and electrically connected to a conductive layer (not shown) possibly previously produced. Alternatively and for practical applications most preferably, a back contact 21 can be formed which directly engages such a conductive layer which is located between the carrier substrate 3 and the perovskite layer 1, or the rear side of the perovskite layer 1. The back contact 21 can be provided here, for example, in the form of a metal layer. The front contact 23 may be provided in the form of a plurality of thin finger-like metal contacts. Alternatively or additionally, the front contact 23 can be formed with the aid of an electrically conductive, transparent layer. Overall, a solar cell 29 in the form of a perovskite solar cell can be formed in this way. Optionally, the side facing the sun may also be the side covered with the glass substrate. In this case, the metal layer can act as a back contact and can possibly be formed over the whole area.Alternatively, instead of being formed with a glass substrate, the carrier substrate 3 can be formed by an already pre-processed first solar cell 25. For example, this first solar cell 25 may be an efficient wafer-based silicon solar cell. The perovskite layer 1 can then be produced on a front side surface of this first solar cell 25 and in this case form a second solar cell 27. The two solar cells 25, 27 are optionally electrically connected to one another via a conductive layer (not shown) formed therebetween and can thereby overall form a solar cell 29 in the form of a tandem solar cell.It is pointed out that both the method steps and the structures produced thereby have been illustrated and described only in a greatly simplified manner. In a real implementation, further method steps can be used and / or further structures can be produced, for example, in the form of additional layers, electrodes or the like.It is also pointed out that the earlier patent DE 10 2006 007 446 B3 describes, inter alia, a device which is designed to be suitable for carrying out embodiments of the method described herein. The method described herein can thus be carried out using such a device. Features as described for the device can be applied analogously to the method described herein. The contents of the prior patent are incorporated herein by reference in their entirety.In addition, it is pointed out that the proposed method can also be carried out as part of a two-stage process. In this case, the proposed method can first be used to generate a first sublayer, onto which a further sublayer can then be applied in a second subprocess. In this case, the two sublayers may only be subsequently heated to an elevated temperature at which a chemical reaction and / or a reforming of the precursors and / or reaction products contained in the two sublayers occurs, such that the perovskite layer ultimately forms. The second sublayer can be produced here using the same or a similar method as the first sublayer. Alternatively, the second partial layer can also be produced by another method, such as, for example, spinning on or rolling on.Finally, it should be noted that terms such as "having", "comprising", etc. do not exclude other elements or steps and terms such as "a" or "an" do not exclude a plurality. It should also be noted that features or steps that have been described with reference to one of the above exemplary embodiments can also be used in combination with other features or steps of other exemplary embodiments described above. Reference signs in the claims should not be regarded as limiting.LIST OF REFERENCE CHARACTERS1 Perovskite layer 3 Carrier substrate 5 Solution 7 First perovskite precursor 9 Second perovskite precursor 11 Deposited layer 12 Crystalline layer 13 Non-deposited portion of the solution 15 Mixing tank 17 Heating plate 19 Electrical contacts 21 Electrical rear contact 23 Electrical front contact 25 First solar cell 27 Second solar cell 29 Solar cell 31 Stirrer 33 Temperature control device 35 Outlet 37 Cover 39 Accommodation volume 41 Edges 43 Outflow connection 45 Movement arrows T 1 Mixing temperature T 2 Carrier substrate initial temperature T 3 Deposition temperature T 4 Drying temperatureReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2006 007 446 B3

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Claims

Method for forming a perovskite layer (1) on a carrier substrate (3), comprising: - mixing a solution (5) containing a first and a second perovskite precursor (7, 9) at a mixing temperature (T1), - applying the solution (5) to the carrier substrate (3), - moving the carrier substrate (3) together with the applied solution (5) in a two-dimensional movement and simultaneously cooling the carrier substrate (3) together with the applied solution (5) to a deposition temperature (T3) below the mixing temperature (T1) to produce a deposited layer (11) containing both the first and the second perovskite precursor (7, 9) and / or a reaction product of the two perovskite precursors (7, 9) on the carrier substrate (3).The method according to claim 1, wherein the solution (5) is aqueous and wherein the first and the second perovskite precursor (7, 9) and / or the reaction product are soluble in aqueous solution.The method according to any one of the preceding claims, wherein the first perovskite precursor (7) contains cesium, methyl ammonium or formamidinium.The method according to any one of the preceding claims, wherein the first perovskite precursor (7) contains methylammonium iodide.Method according to one of the preceding claims, wherein the second perovskite precursor (9) contains lead.Method according to one of the preceding claims, wherein the second perovskite precursor (9) contains lead acetate and / or a lead halide.The method according to any one of the preceding claims, wherein the solution (5) contains hydrogen halide.Method according to one of the preceding claims, wherein the solution (5) is mixed in a mixing tank (15) separately from the carrier substrate (3) and tempered to the mixing temperature (T1).Method according to one of the preceding claims, wherein, during the production of the deposited layer (11), process parameters including a process duration and a temporal temperature profile during the cooling to the deposition temperature (T3) are selected such that the deposited layer (11) is produced as a crystalline layer (12).Method according to one of the preceding claims, wherein, during the mixing of the solution (5), saturation of the solution (5) with the first and the second perovskite precursor (7, 9) is adjusted such that, during the subsequent cooling of the solution (5) to the deposition temperature (T3), supersaturation of the solution (5) occurs.Method according to one of the preceding claims, wherein the two-dimensional movement is a movement pivoting about at least two different axes.Method according to one of the preceding claims, wherein the carrier substrate (3) is tempered during the application of the solution (5), during the production of the deposited layer (11) and / or during heating of the deposited layer (11) by means of a tempering device (17) which makes planar contact with the carrier substrate (3).Method according to one of the preceding claims, wherein the carrier substrate (3) is covered by a cover (37) during the application of the solution (5) and during the movement of the carrier substrate (3) in such a way that the cover (37) sealingly abuts the carrier substrate (3) in an annular manner along its edges (41), and a receiving volume (39) is formed between the carrier substrate (3) and the cover (37), in which receiving volume the solution (5) applied to the carrier substrate (3) is received.Method for producing a solar cell (29), comprising forming a perovskite layer (1) on a surface of a carrier substrate (3) by means of the method according to one of the preceding claims, and forming electrical contacts (19) for discharging electrical charge carriers from the perovskite layer (1).Method according to claim 14, wherein the carrier substrate (3) comprises a first solar cell (21) and wherein the perovskite layer (1) together with the electrical contacts (19) is formed as a second solar cell (23).

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