Method for producing a solar cell and solar cell for converting incident electromagnetic radiation into electrical power

Abstract

The invention relates to a method for producing a solar cell for converting incident electromagnetic radiation into electrical power, wherein a layer structure is formed, having at least one charge-selective front-side contact structure (1), at least one charge-selective back-side contact structure (2) and an absorber structure (3) comprising at least perovskite, wherein the back-side contact structure (2) is formed such that it comprises at least one back-side graphite layer (9). What is essential is that coated graphite particles are used for forming the back-side graphite layer (9), wherein the coating of the graphite particles comprises at least one layer from the following list – metal-containing layer, in particular a metal oxide layer, – self-assembling monolayer (SAM layer). The invention furthermore relates to a solar cell for converting incident electromagnetic radiation into electrical power.

Description

[0001] Method for producing a solar cell and solar cell for converting incident electromagnetic radiation into electrical power

[0002] Perovskite solar cells enable high efficiency at lower manufacturing costs compared to solar cells made from semiconductor materials such as silicon. Long-term stability of perovskite solar cells presents a challenge. The use of perovskite solar cells with graphite back electrodes is advantageous in achieving low degradation.

[0003] Previously known perovskite solar cells with a graphite back electrode are described in Wagner et al., "High Photovoltage of 1 V on a Steady-State Certified Hole Transport Layer-Free Perovskite Solar Cell by a Molten-Salt Approach," DOI: 10.1039 / D0EE02175J. A major challenge for achieving maximum efficiencies is charge carrier recombination at the graphite back electrode.

[0004] The present invention is therefore based on the object of providing a perovskite solar cell and a method for producing such a perovskite solar cell which enable a high efficiency.

[0005] This object is achieved by a method for producing a solar cell for converting incident electromagnetic radiation into electrical power according to claim 1 and by a solar cell for converting incident electromagnetic radiation into electrical power according to claim 10. Advantageous embodiments can be found in the dependent claims.

[0006] The method according to the invention is preferably designed to form a solar cell according to the invention, in particular a preferred embodiment thereof. The solar cell according to the invention is preferably formed by means of the method according to the invention, in particular a preferred embodiment thereof. The invention is based on the finding that the greatest challenge in perovskite solar cells with graphite back electrodes is reducing charge carrier recombination at the graphite back electrode. Investigations have shown that, in particular, the low charge selectivity of graphite leads to high charge carrier recombination at the perovskite-graphite interface, resulting in, in particular, a reduction in the open-circuit voltage αc.

[0007] The invention is based on the finding that the use of coated graphite particles for forming the graphite back electrode with a metallic layer and / or with a self-organizing monolayer (SAM layer) leads to a reduction in charge carrier recombination and thus to an increase in efficiency.

[0008] In the method according to the invention for producing a solar cell for converting incident electromagnetic radiation into electrical power, a layer structure is formed having at least one charge-selective front-side contact structure, at least one charge-selective back-side contact structure and an absorber structure comprising at least perovskite, wherein the back-side contact structure is formed having at least one back-side graphite layer.

[0009] It is essential that coated graphite particles are used to form the backside graphite layer, wherein the coating of the graphite particles comprises at least one layer from the list of metal-containing layers, in particular a metal oxide layer, self-organizing monolayer (SAM layer).

[0010] The method according to the invention has the advantage that the cost-effective production of a graphite electrode by forming the backside graphite layer using graphite particles that are coated before the backside graphite layer is formed (individually coated graphite particles) can be used. However, due to the at least partial, preferably complete coating of the surface of the porous backside graphite layer as described above, a significantly better charge selectivity can be achieved, resulting in a higher efficiency of the solar cell. Thus, as individually coated graphite particles, the graphite particles have a coating with one or more of the previously and / or subsequently mentioned layers before the graphite particles are used to form the backside graphite layer.

[0011] The solar cell according to the invention for converting incident electromagnetic radiation into electrical power also has these advantages: The solar cell according to the invention is designed to convert incident electromagnetic radiation into electrical power, with at least one charge-selective front-side contact structure, at least one charge-selective back-side contact structure, and at least one absorber support structure, which is at least partially formed from perovskite, wherein the back-side contact structure is formed from at least one back-side graphite layer. It is essential that the back-side graphite layer comprises coated graphite particles, wherein the coating of the graphite particles comprises at least one layer from the list of metal-containing layers, in particular a metal oxide layer, self-assembling monolayer (SAM layer).

[0012] In particular, metal-containing layers for coating the graphite particles are advantageous for good charge carrier selectivity. In an advantageous embodiment of the method, the coating of the graphite particles has at least one metal-containing layer, which can be selected as a layer from the list

[0013] Metal layer, in particular a layer made of at least 90%, preferably at least 95%, more preferably completely metal, metal oxide layer,

[0014] Metal halide layer,

[0015] Metal I thiocyanate layer,

[0016] Metal sulfide layer, in particular one of the layers

[0017] CuS layer, MoS layer, MoS2 layer,

[0018] A metal nitride layer, in particular a TiN layer, or a metal carbide layer, in particular a TiC layer. The percentages for the layer composition here and below refer to weight percent.

[0019] Accordingly, the graphite particles of the solar cell according to the invention are preferably coated as described above.

[0020] It is within the scope of the invention that the graphite particles initially have only a metal layer and that the metal layer is treated during the formation of the backside graphite layer and / or after the formation of the backside graphite layer.

[0021] In an advantageous embodiment, graphite particles coated with a metal layer are used and the metal layer of the coating is treated to convert the metal layer at least partially, preferably completely.

[0022] In an advantageous embodiment, the partial or preferably complete conversion takes place by oxidation into a metal oxide layer.

[0023] It is advantageous that the coating of the coated graphite particles used to form the backside graphite layer has a thickness of at least 0.3 nm, preferably at least 1 nm, more preferably at least 5 nm, in particular at least 20 nm.

[0024] It is within the scope of the invention to carry out the formation of the backside graphite layer basically according to one of the previously known methods, high temperature route or low temperature route:

[0025] The formation of a backside graphite layer itself is known via two routes, which are also referred to as high temperature route and low temperature route (see Bogachuk, Wagner, Hinsch et al., DOI: 10.1039 / D0EE02175J).

[0026] With the high-temperature route, all electrode layers are first applied in porous form. For example, a porous layer of TiO2 is applied to a TCO-coated glass substrate, forming a front-side contact structure. A porous layer of a dielectric material, e.g. zirconium oxide, is then applied. This serves to electrically insulate the front and back contact structures. The back contact structure, based on graphite particles, is then deposited on top. This layer stack is then heated to high temperatures above 400°C to drive out organic residues from the pastes, particularly the graphite pastes. Since perovskite absorbers used for photovoltaic applications are destroyed at such high temperatures, the perovskite absorber is introduced into the porous contact structure in liquid form as a final step, where it is allowed to crystallize. The process is described in Wagner et al.aaO, and shown schematically in Figure 1.

[0027] The low-temperature route uses graphite pastes that can be annealed at low temperatures, typically <=120°C. At these temperatures, the perovskite absorber layer is not damaged. Therefore, the graphite paste can be applied as the final deposition step after the deposition of the perovskite absorber layer. For example, a porous layer of TiO2 is applied to a TCO-coated glass substrate, forming a front-side contact structure. The perovskite layer is then deposited on top, and the backside graphite layer is applied as the final process layer. Such a process flow for the low-temperature route is explained in Zouhair, Hinsch, Wagner et al. (DOI: 10.1002 / aenm.202200837), particularly in Figure 1 and the associated description.

[0028] In an advantageous embodiment, the method is therefore designed as a low-temperature route or as a high-temperature route, particularly preferably as described above.

[0029] In an advantageous embodiment, the method is designed as a low-temperature route, wherein the coated graphite particles are applied after formation of the perovskite-containing absorber structure, in particular by applying a graphite paste containing the coated graphite particles, the layer structure with the graphite particles, in particular the graphite paste, is heated to a temperature of less than 120°C for annealing.

[0030] Preferably, in the low-temperature route for forming the front-side contact, a porous layer is applied to a substrate, particularly preferably a porous TIO2 layer is applied to a glass substrate, preferably a TCO (Transparent Conducting Oxide)-coated glass substrate, and the perovskite layer is formed by partially coating the porous TIO2 layer with perovskite.

[0031] In an advantageous embodiment, the method is designed as a high-temperature route, wherein a porous absorber carrier structure is formed between the front-side contact structure and the back-side contact structure, in a subsequent method step the layer structure is heated in order to form the back-side graphite layer from the graphite particles, wherein heating is preferably carried out to a temperature greater than 120°C, in particular greater than 300°C, preferably greater than 400°C, the perovskite is added in a method step after heating.

[0032] In the high-temperature route, at least the backside contact structure is preferably porous, and the perovskite is added to the porous absorber support structure through the backside contact structure to form the absorber structure. Preferably, the frontside contact structure is additionally formed as a porous structure.

[0033] In an advantageous embodiment, a treatment for at least partial, preferably complete, conversion of the metal layer takes place before forming the backside graphite layer. In this embodiment, the graphite particles are advantageously treated before they are used to form the backside graphite layer. This results in the advantage that the treatment of the graphite particles does not have a damaging effect on the existing layers of the solar cell. Conversion by oxidation can occur in the ambient atmosphere at room temperature. Preferably, the graphite particles are heated in an oxygen-containing environment.

[0034] In an advantageous embodiment, a treatment for at least partial, preferably complete, conversion of the metal layer is carried out during the formation of the backside graphite layer. This results in the advantage that a process step can be eliminated by treating the metal layer and annealing the backside graphite layer in a single process step.

[0035] In an advantageous embodiment, a treatment for at least partial, preferably complete, conversion of the metal layer is carried out after the formation of the backside graphite layer. This results in the advantage that the formation of the oxide layer at the interfaces between the individual coated graphite particles can be better optimized.

[0036] It is therefore advantageous that, after forming the backside graphite structure, oxidation occurs using graphite particles coated with a metal layer, so that a metal layer / metal oxide layer structure with an external metal oxide layer is formed, or complete oxidation occurs with the metal layer. The oxidation preferably occurs by heating at least the porous backside graphite layer to a temperature of 100°C, preferably greater than 150°C, in particular greater than 200°C. The heating accordingly takes place in an oxygen-containing atmosphere.

[0037] It is also within the scope of the invention that the treatment for at least partial, preferably complete conversion of the metal layer takes place partly before and / or partly during and / or partly after the formation of the backside graphite layer.

[0038] In an advantageous embodiment, the partial or preferably complete conversion into a metal halide layer, preferably a CuI layer, takes place. Advantageously, the conversion takes place before the formation of the backside graphite layer to avoid damaging the existing layers of the solar cell. The conversion preferably takes place by exposing the metal-coated, preferably copper-coated, graphite particles to iodine vapor and / or by contacting the graphite particles with hydroiodic acid (HI), preferably while heating the graphite particles.

[0039] In an advantageous embodiment, the partial or preferably complete conversion into a metal thiocyanate layer, preferably a copper thiocyanate layer (CuSCN), takes place. Advantageously, the conversion takes place before the formation of the backside graphite layer to avoid damaging the existing layers of the solar cell.

[0040] The conversion is preferably carried out by first forming a metal halide layer, in particular a CuCl layer, as described above, and then reacting it using a reaction solution, in particular a potassium thiocyanate solution, to form a CuSCN layer. Alternatively, a metallic copper layer can also be converted to CuSCN in a copper sulfate solution containing potassium thiocyanate. The reaction with the reaction solution preferably takes place at elevated temperature, preferably at least 60°C, in particular at least 80°C.

[0041] Advantageously, residues, in particular KCI residues, are washed off after the reaction, in particular by rinsing with water.

[0042] In an advantageous embodiment, the graphite particles are coated directly with already converted metal-containing materials. This has the advantage of eliminating a subsequent step, such as oxidation.

[0043] Advantageously, the graphite particles are coated with oxidized metal, in particular oxidized metal particles, preferably oxidized metal particles in a solution, so that a metal oxide coating is achieved directly.

[0044] It is therefore advantageous to carry out the coating in the form of a coating of graphite particles with titanium dioxide nanoparticles. Graphite particles are advantageously treated with a solution containing a titanium alkoxide, for example, titanium(IV) isopropoxide. Accordingly, the graphite particles of the solar cell according to the invention are preferably coated as described above.

[0045] In an advantageous embodiment, the coating of graphite particles used to form the backside graphite layer comprises a self-assembling monolayer (SAM layer). Such a layer has the advantage that particularly high charge selectivity can be achieved. At the same time, the very thin monolayer minimizes losses due to interfacial resistance. Examples of SAMs can be found in DOI: 10.1126 / science.abd4016, particularly in Fig. 1, in particular 2PACz, MeO-2PACz, or Me-4PACz. The coating preferably comprises a SAM layer according to one of these examples.

[0046] In an advantageous embodiment, the graphite particles have a multilayer coating consisting of several different layers. It is therefore advantageous for the coated graphite particles used to form the backside graphite layer to be at least partially, preferably completely, coated with a multilayer coating system. Multilayer coatings have the advantage of allowing the benefits of the individual layers to be combined. Furthermore, the wetting properties of SAM on metal oxide layers are improved.

[0047] Accordingly, the graphite particles of the solar cell according to the invention are preferably coated as described above.

[0048] A multilayer coating is preferably formed by treating graphite particles coated with a metal layer, preferably during the formation of the backside graphite layer and / or after the formation of the backside graphite layer, so that the side of the metal layer facing away from the graphite particle is converted, in particular into a metal oxide layer, a metal halide layer, or a metal thiocyanate layer, so that a coating system with at least two, preferably exactly two, layers is formed. This is particularly advantageous when the process is configured as a high-temperature route as described above.It is therefore advantageous that the layer system has at least two layers, preferably exactly two layers, with a metal-containing layer facing the graphite particle, in particular a metal oxide layer, and a SAM layer arranged indirectly or preferably directly on the side of the metal-containing layer facing away from the graphite particle.

[0049] Accordingly, the graphite particles of the solar cell according to the invention are preferably coated as described above.

[0050] The backside graphite layer is preferably formed by applying a graphite paste containing the coated graphite particles, particularly by means of a printing process. This results in a simple and cost-effective process.

[0051] The backside graphite layer is preferably formed as a porous layer, wherein preferably adjacent graphite particles are partially connected and partially cavities are arranged between adjacent graphite particles, so that a porous layer structure results. It is within the scope of the invention that the cavities of the porous layer structure are filled with gas, in particular ambient air. It is also within the scope of the invention that the cavities are filled with a filling material, which preferably does not contain graphite and does not contain metal. It is within the scope of the invention that the cavities are filled with a filling material from the list

[0052] Carbon Black

[0053] Organic binders

[0054] - Inorganic binders, in particular titanium(IV) isopropoxide or soda silicate,

[0055] Glass frits (ie low-melting glass powder) are filled.

[0056] The backside graphite layer preferably has a porosity in the range of 0.1 to 0.7. The porosity is specified as the ratio of the void volume (of the areas between the graphite particles) to the total volume. Advantageously, the backside graphite layer is arranged directly on the absorber structure. The method according to the invention has the advantage that the backside graphite layer can be formed due to the coating of the graphite particles with charge selectivity, and thus it is not necessary for an additional charge-selective layer to be arranged between the backside graphite layer and the absorber structure. In particular, it is therefore advantageous that the solar cell is formed without the interposition of a charge-selective layer between the absorber structure and the backside graphite layer.

[0057] The omission of a separate charge-selective layer leads to cost savings.

[0058] A charge-selective layer or structure is characterized by different conductivities for different charge carriers, particularly holes and electrons, as described in U. Würfel et al., "Charge Carrier Separation in Solar Cells," 10.1109 / JPHOTOV.2014.2363550. A layer or structure with n-type charge selectivity exhibits a higher conductivity for electrons, while a layer or structure with the opposite p-type charge selectivity exhibits a higher conductivity for holes.

[0059] Advantageously, the charge-selective backside contact structure is formed with a charge selectivity opposite to that of the charge-selective frontside contact structure. In particular, it is advantageous for the charge-selective frontside contact structure to have at least one charge-selective frontside layer, and for the charge-selective backside contact structure to have at least one charge-selective backside contact layer. In particular, the coating of the graphite particles of the backside graphite layer is formed as a single- or multi-layer charge-selective backside layer, which charge-selective backside layer is formed with a charge selectivity opposite to that of the charge-selective frontside layer.

[0060] As described above, it is within the scope of the invention to use graphite particles with a coating which has a metal oxide layer, wherein the metal oxide layer is preferably formed as one of the following layers or a layer structure of the following layers: As a hole-selective metal oxide layer, comprising one of the metals: nickel, copper, molybdenum, chromium, vanadium, tungsten, indium, tantalum, cobalt, niobium, titanium or alloys of the metals mentioned.

[0061] As an electron-selective metal oxide layer comprising one of the metals: titanium, tin, zinc, chromium, molybdenum, indium, tungsten, vanadium, iron, cerium, strontium, barium, aluminum, silicon, zirconium or alloys of the aforementioned metals.

[0062] It is particularly advantageous to form the metal oxide layer of the graphite particle coating as a nickel oxide layer. This offers the advantage of achieving particularly good charge selectivity at low cost.

[0063] The basic structure of the solar cell according to the invention can be designed in a manner known per se, in particular as described in Wagner et al., op. cit., and shown schematically in Figure 1.

[0064] It is also within the scope of the invention to form the basic structure as described in Zouhair, Hinsch, Wagner et al., DOI: 10.1002 / aenm.202200837, and in particular Figure 1 and the associated description. This formation takes place at lower temperatures compared to the previously described approach (Wagner et al.) and is therefore referred to as a "low-temperature" approach in contrast to the previously described "high-temperature" approach.

[0065] Advantageously, the layer structure of the solar cell is arranged on a carrier substrate, in particular a glass carrier substrate or a plastic carrier substrate, in particular a plastic film. The layer structure is preferably arranged with the front-side contact structure on the carrier substrate. It is therefore advantageous for the carrier substrate to be substantially transparent, preferably substantially transparent at least in the wavelength range from 300 nm to 1200 nm. The carrier substrate is preferably designed to be conductive in order to conduct charge carriers to or to supply them to the front-side contact structure. In particular, the carrier substrate is preferably designed as conductive glass and / or the carrier substrate has a carrier layer, in particular a glass substrate, and at least one conductive layer, preferably a conductive oxide (TCO), in particular a material from the list of fluorine-doped tin oxide (FTO) and indium tin oxide (ITO).This enables the electrical connection of the front contact structure and at the same time avoids shadowing losses in the incident electromagnetic radiation.

[0066] Advantageously, the front-side contact structure comprises a charge-selective layer. This front charge-selective layer is preferably arranged between a carrier substrate as described above and the absorber structure.

[0067] It is within the scope of the invention to form the front charge-selective layer of the front contact structure as one or a layer system of the following layers:

[0068] Electron-selective layers: Metal oxides as mentioned above, particularly preferred: titanium oxide, tin oxide, zinc oxide. Also of interest: fullerenes and fullerene derivatives such as PCBM.

[0069] Hole-selective layers: Metal oxides as mentioned above, particularly preferred: nickel oxide, copper oxide, vanadium oxide. Other preferred hole-selective layers: CuI, CuSCN, organic materials such as phthalocyanine, PEDOT:PSS, spiro-OMeTaD, P3HT, PTAA, self-assembled monolayers (SAM) such as 2PACz, MeO-4PACz.

[0070] As previously described, it is within the scope of the invention for the absorber structure to have a porous absorber support structure that is at least partially coated with perovskite. It is therefore within the scope of the invention for the absorber structure to be designed as a porous absorber structure coated with perovskite. It is also within the scope of the invention for the absorber structure to be designed as a non-porous perovskite layer.

[0071] When forming the absorber structure as a porous absorber structure, it is advantageous to first form a porous absorber support structure, which is then at least partially coated with perovskite in a subsequent process step. The porous absorber structure preferably has a thickness in the range of 0.5 pm to 5 pm.

[0072] The absorber support structure is preferably formed from one or more of the following materials: oxides of zirconium, silicon, or aluminum, particularly preferably from porous ZrO2. The introduction of the absorber using perovskite is preferably carried out by using a liquid perovskite precursor solution to apply perovskite to the absorber structure. Advantageously, the solvent is subsequently expelled by heating, particularly preferably under a protective atmosphere. It is also within the scope of the invention for the expulsion to be carried out in a negative pressure atmosphere (lower pressure than the ambient atmosphere).

[0073] In the method according to the invention and the solar cell according to the invention, perovskite is used as an absorber. It is within the scope of the invention that perovskite is used in the form of a compound from the list of metal halide perovskites (ABX3), preferably with A: monovalent cation, B: metal / divalent cation, X: halide, in particular with a monovalent cation A of methylammonium, formamidinium, or cesium, in particular with B: lead or tin, in particular with X: chlorine, bromine, or iodine.

[0074] The use of mixed forms of the materials mentioned is also within the scope of the invention.

[0075] The use of perovskite in the form of 2D perovskites or double perovskites is also within the scope of the invention.

[0076] In an advantageous embodiment, the absorber structure is formed as a non-porous layer containing perovskite, in particular as a perovskite layer. The thickness of the absorber structure is preferably in the range of 200 nm to 800 nm, in particular when the absorber structure is formed as a non-porous absorber structure. When the absorber structure is formed as a porous absorber structure, the thickness is preferably in the range of 500 to 4,000 nm.

[0077] Advantageously, graphite particles coated with a graphite layer on the back are used with a configuration according to variants a) to e) of the following table, wherein preferred value ranges are indicated in square brackets (starting from layer 1 arranged directly on the graphite particle):

[0078] The graphite particles typically have a flat structure with a small thickness in a first spatial direction and a comparatively greater extent in the two spatial directions perpendicular to this. The thickness is preferably in the range 0.3 nm to 1000 nm, preferably 1 nm to 1000 nm, in particular 10 nm to 500 nm. The extent in the spatial directions perpendicular to the thickness is preferably in the range 0.05 pm to 50 pm. These size specifications refer to the primary graphite particles. It is within the scope of the invention for several primary particles to form aggregates and for the primary particles or the aggregates or a mixture of primary particles and aggregates to be used to form the back-side graphite layer. In an advantageous embodiment, the back-side graphite layer is formed in multiple layers.In particular, it is advantageous for the backside graphite layer to be formed on the front side (the side facing the front side contact structure) with the previously described graphite layer, wherein the coated graphite particles are used, as the first graphite layer of the backside graphite layer. In addition, at least one second graphite layer of the backside graphite layer is formed on the backside of the backside graphite layer facing away from the front side, wherein uncoated graphite particles are used.

[0079] Further advantageous features and embodiments are explained below with reference to Figure 1 and an embodiment.

[0080] Figure 1 shows an embodiment of a solar cell according to the invention, which was formed using an embodiment of a method according to the invention. The figure shows a schematic representation, not to scale. Like reference numerals in the figures denote like or equivalent elements.

[0081] The exemplary embodiment of a solar cell according to the invention for converting incident electromagnetic radiation into electrical power, shown schematically in Figure 1, has a charge-selective front-side contact structure 1, a charge-selective back-side contact structure 2 and a porous absorber structure 3 arranged between front-side contact structure 1 and back-side contact structure 2.

[0082] The solar cell in Figure 1 is shown in the order in which the individual layers were manufactured, so that the front side facing the incident electromagnetic radiation during use is shown at the bottom.

[0083] To form the front-side contact structure 1, an electrically conductive oxide is applied as a TCO layer 5 to a glass substrate 4. A compact charge-selective layer 6 and a porous charge-selective layer 7 are applied to the TCO layer 5, each made of TiO2. The TiO2 in these layers exhibits the properties of an electron-selective layer due to its high electron conductivity and low hole conductivity. The compact TiO2 (layer 6) serves in particular to completely passivate the TCO layer in order to minimize losses due to charge carrier recombination, which occur at TCO / perovskite interlayers. The mesoporous TiO2 (layer 7) serves to create a large TiO2 / perovskite surface and thus achieve efficient electron transfer from the perovskite to the TiO2.

[0084] A porous layer made of ZrO2 is applied to the porous charge-selective layer 7 to form the absorber structure in a later process step. This layer thus represents the porous absorber layer 8.

[0085] A paste containing graphite particles is applied to the porous absorber layer 8 by means of a printing process, wherein the graphite particles have a nickel coating.

[0086] The back-side graphite layer 9 is created by producing a paste from nickel-coated graphite particles. This paste is applied to the existing layer stack using a screen printer. The entire layer stack is then heated to a temperature of 500°C in air. This serves, on the one hand, to burn out the organic components of the printing pastes. It also oxidizes the nickel on the surface to nickel oxide. The layer thickness of the nickel oxide layer of 10 nm is adjusted by the duration of the temperature step. In this exemplary embodiment, the graphite particles thus have a coating consisting of a two-layer system, with the nickel layer being arranged on the side of the coating facing the graphite particle and the nickel oxide layer being arranged on the side of the coating facing away from the graphite particle.In addition, the backside graphite layer 9 is formed in two layers by forming a further, second graphite layer made of uncoated graphite particles on the aforementioned first graphite layer. This second graphite layer is thus arranged on the side of the first graphite layer facing away from the front-side contact structure 1 (the back side). Subsequently, in the illustration according to Figure 1, the perovskite precursor is introduced from above, i.e., starting from the porous backside graphite layer 9, so that it wets, in particular, the porous absorber layer 8. By heating at a temperature of 100°C for a period of 30 minutes, the solvent of the precursor is removed, so that the perovskite absorber structure 3 is formed.

[0087] The layer stack has the following layer thicknesses:

[0088] Glass (4): 2.2 mm. TCO (5): 1 pm, c-TiO2: 30 nm (layer 6), m-TiO2 (7): 500 nm, m-ZrO2 (8): 2 pm. Metal oxide / graphite (first graphite layer of backside graphite layer 9): 1 pm, graphite (second graphite layer of backside graphite layer 9): 40 pm.

[0089] The manufacturing method of the previously described embodiment thus essentially corresponds to the manufacturing method described in Wagner et al., op. cit., therein for Figure 2, although nickel-coated graphite particles were used to produce the porous backside graphite layer 9, so that the surface of the backside contact structure 2 is coated with a nickel oxide layer and thus has a charge-selective functionality.

[0090] A second embodiment of a method according to the invention for forming a second embodiment of a solar cell according to the invention is explained below with reference to Figure 1:

[0091] Alternative process for the low temperature manufacturing approach:

[0092] An alternative embodiment of a solar cell according to the invention is as follows, see Zouhair et al. DOI: 10.1002 / aenm.202200837, in particular Fig. 1.:

[0093] In this second embodiment, the layer stack for the front-side contact structure 1 is manufactured as described above (layers 4 to 7). The mesoporous TiO2 (layer 7) is annealed at 500°C for 30 minutes. The perovskite layer is then applied, in this case by spin-coating a liquid precursor solution under a nitrogen inert gas atmosphere. The precursor solvent is then expelled at 150°C for 13 minutes. This forms a porous perovskite layer in the porous TiO2 (layer 7) and a compact perovskite layer above it (layer 8). In this case, a solution containing salts of large organic cations such as octylammonium iodide is then spin-coated onto the perovskite layer. This forms a 70 nm thick passivation layer of 2D perovskite.

[0094] Subsequently, the backside contact structure 2 is applied, in this case by doctor blade coating with a graphite paste at a doctor blade distance from the substrate of 220 pm. The layer is then baked at 120 °C for 10 minutes. A graphite paste is used for this purpose, which can be baked at temperatures below 150 °C.

[0095] Nickel-coated graphite particles are used for the paste. Exposure to heat in air oxidizes the nickel on its surface to nickel oxide. The thickness of the nickel oxide layer of 10 nm is adjusted by the duration of the temperature step. The coated nickel oxide particles are then immersed in a bath containing SAM (2PACz in ethanol, 0.3 mg / ml). This forms a SAM monolayer on the nickel oxide layer. Excess SAM is then washed off by placing the coated graphite powder in a second bath of pure ethanol. The coated graphite powder is then dried. A graphite paste consisting of the coated graphite and other additives is produced from this coated graphite powder.

[0096] It is within the scope of the invention to use a graphite paste for the backside contact structure according to a recipe as described in https: / / pubs.rsc.org / en / content / articlelanding / 2019 / cc / c8cc09905g or https: / / www.nature.com / articles / s41560-023-01205-y. The graphite paste advantageously consists of coated graphite particles and carbon black, as well as organic solvents such as, preferably, terpineol.

[0097] List of reference symbols 1 Front contact structure

[0098] 2 Rear contact structure

[0099] 3 Absorber structure

[0100] 4 Glass substrate

[0101] 5 TCO layer 6 compact charge-selective layer

[0102] 7 porous charge-selective layer

[0103] 8 porous absorber layer

[0104] 9 Backside graphite layer

Claims

Claims 1. A method for producing a solar cell for converting incident electromagnetic radiation into electrical power, wherein a layer structure is formed, with at least one charge-selective front-side contact structure (1), at least one charge-selective back-side contact structure (2) and an absorber structure (3) comprising at least perovskite, wherein the back-side contact structure (2) is formed having at least one back-side graphite layer (9), characterized in that coated graphite particles are used to form the back-side graphite layer (9), wherein the coating of the graphite particles has at least one layer from the list of metal-containing layers, in particular a metal oxide layer, self-organizing monolayer (SAM layer).

2. Method according to claim 1, characterized in that the coating of the graphite particles has at least one metal-containing layer, which is selected as a layer from the list metal layer, metal oxide layer, Metal halide layer, Metal I-thiocyanate layer.

3. Method according to one of the preceding claims, characterized in that the materials used to form the backside graphite layer are layered graphite particles are at least partially, preferably completely coated with a multi-layer coating system.

4. The method according to claim 3, characterized in that the layer system has at least two layers, preferably exactly two layers, with a metal-containing layer facing the graphite particle, in particular a metal oxide layer, and a SAM layer arranged indirectly or preferably directly on the side of the metal-containing layer facing away from the graphite particle.

5. The method according to claim 4, characterized in that the layer system has at least three, preferably exactly three layers, wherein the metal-containing layer is free of metal oxide, preferably is formed as a metal layer and between the metallic layer and the SAM layer there is a metal-containing layer, preferably from the list metal oxide layer, Metal halide layer, Metal I-thiocyanate layer.

6. Method according to one of the preceding claims, characterized in that the surfaces of the coated graphite particles used to form the backside graphite layer are coated to at least 50%, preferably at least 75%, more preferably at least 90%, in particular that the surfaces of the coated graphite particles used to form the backside graphite layer are completely coated.

7. Method according to one of the preceding claims, characterized in that the coating of the coated graphite particles used to form the backside graphite layer has a thickness of at least 0.3 nm, preferably at least 1 nm, more preferably at least 5 nm, in particular of at least 20 nm.

8. Method according to one of the preceding claims, characterized in that the backside graphite layer is formed by applying a graphite paste which contains the coated graphite particles, in particular by means of a printing process.

9. Method according to one of the preceding claims, characterized in that the back-side graphite layer is formed in multiple layers, in particular that when forming the back-side graphite layer on the side facing the front-side contact structure, the graphite layer, in the formation of which the coated graphite particles are used, is formed as the first graphite layer of the back-side graphite layer and at least one second graphite layer of the back-side graphite layer is formed on the back side of the back-side graphite layer facing away from the front, wherein uncoated graphite particles are used to form the second graphite layer.

10. Solar cell for converting incident electromagnetic radiation into electrical power, with at least one charge-selective front-side contact structure (1), at least one charge-selective back-side contact structure (2) and at least one absorber support structure (3), which is formed at least partially comprising perovskite, wherein the back-side contact structure (2) is formed comprising at least one back-side graphite layer (9), characterized in that the back-side graphite layer (9) has coated graphite particles, wherein the coating of the graphite particles has at least one layer from the list of metal-containing layers, in particular a metal oxide layer, self-organizing monolayer (SAM layer).

11. Solar cell according to claim 10, characterized in that the coating of the graphite particles has at least one metal-containing layer which can be selected as a layer from the list metal layer, metal oxide layer, Metal halide layer, Metal I-thiocyanate layer.

12. Solar cell according to one of claims 10 to 11, characterized in that the coated graphite particles of the backside graphite layer are at least partially, preferably completely, coated with a multilayer coating system.

13. Solar cell according to one of claims 12, characterized in that the layer system has at least two layers, preferably exactly two layers, with a metal-containing layer facing the graphite particle, in particular a metal oxide layer, and a SAM layer arranged indirectly or preferably directly on the side of the metal-containing layer facing away from the graphite particle.

14. Solar cell according to one of claims 13, characterized in that the layer system has at least three, preferably exactly three layers, wherein the metal-containing layer is free of metal oxide, preferably is formed as a metal layer and between the metallic layer and the SAM layer there is a metal-containing layer, preferably from the list metal oxide layer, Metal halide layer, Metal I-thiocyanate layer.

15. Solar cell according to one of claims 10 to 14, characterized in that the backside graphite layer (9) is porous and the surface of the porous backside graphite layer (9) is coated at least 25%, preferably at least 50%, more preferably at least 75%, in particular completely.

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