Generation of textured surfaces, fabrication of tandem solar cells, and tandem solar cells

Abstract

In a method for generating a textured surface of a semiconductor layer, a surface of the semiconductor layer is etched with a first alkaline etching solution to generate a surface of the semiconductor layer having a pyramid-like texture. The surface having the pyramid-like texture is then etched with a second alkaline etching solution different from the first alkaline etching solution to remove material from the pyramid-like texture, thereby reducing a height difference between a peak and an adjacent valley of the pyramid-like texture. A method for manufacturing a tandem solar cell further comprises generating a first solar cell structure produced in this way having tandem solar cells with textured surfaces, and generating a second solar cell structure in series with solar cells on the side of the first solar cell structure on which the textured surface is arranged.

Description

Technical Field

[0001] The invention relates to a method for generating a textured surface of a semiconductor layer, a method for producing a tandem solar cell having such a textured surface, and a tandem solar cell produced by such a method. Background Art

[0002] In solar cells, the surface of a semiconductor wafer (particularly a silicon wafer) is usually textured to reduce reflectivity (degree of reflection) and thus increase light yield. To achieve such texturing, an alkaline etching solution including an additive is usually used to anisotropically etch the wafer surface, thereby etching pyramids on the surface. Isopropyl alcohol or an additive without isopropyl alcohol is used as an additive to remove silicon, so that the surface is formed with pyramids.

[0003] Multilayer solar cells are known in which several different semiconductor elements / compounds are arranged above each other in order to exploit the different band gaps of the different semiconductor elements / compounds. This results in different degrees of absorption of sunlight of a specific wavelength range. Such multilayer solar cells are also called tandem solar cells. The bottom solar cell structure (the so-called bottom cell) mainly absorbs light in the long wavelength range, while the upper solar cell structure (the so-called top cell) representing the higher cell layer is used to convert the short wavelength part of the light into electrical energy. A solar cell structure can be understood as a structure capable of converting incident light into charge carriers, which can then be at least partially extracted. By selective absorption of the individual layers, the sunlight can be better utilized as a whole and can be better converted into electrical energy than in the case of a single solar cell. This is reflected in the higher efficiency of tandem solar cells compared to simple (or single) solar cells. Tandem solar cells can also have more than two solar cell structures arranged on top of each other.

[0004] The material suitable for the lowermost solar cell structure (i.e., the bottom cell) is c-Si, i.e., crystalline silicon, monocrystalline or polycrystalline. The material suitable for the upper solar cell structure (top cell) or the next higher solar cell layer located above the lowermost solar cell structure (bottom cell) includes perovskite. The perovskite is deposited on the lower solar cell structure. Various methods for depositing the perovskite layer are known, including solution-based deposition methods and vapor-based deposition methods. Examples of commonly used methods for solution-based deposition of perovskite layers are spin coating and slot die coating. An example of a conventional vapor-based deposition method is co-evaporation.

[0005] Vapor-based deposition methods are methods based on physical vapor deposition (PVD), chemical vapor deposition (CVD) or atomic layer deposition (ALD), which usually require high vacuum. Here, the educts of the perovskite are converted into the gas phase, for example by heating, where they react and grow on the surface of the substrate (which can be a c-Si bottom cell) to form the perovskite. The advantages of this vapor-based method are the homogenous deposition of the layer and the precisely adjustable layer thickness of the perovskite. In addition, this coating method does not have any requirements on the surface morphology of the surface on which the deposition occurs, because the perovskite layer can be deposited equally well on smooth surfaces and textured surfaces. On the other hand, the disadvantages of these vapor-based methods are the difficulty in controlling the stoichiometric ratio of the reactants to be evaporated and the associated harsh process control, and on the other hand, the technically complex conditions under high vacuum. In addition, an annealing step (tempering) must be performed to generate the desired crystallinity of the perovskite. In addition to the limited deposition rate, this step also has a production-limiting effect on commercial realization. Generally speaking, vapor-based deposition methods are at a disadvantage in terms of capital expenditure and throughput compared to solution-based deposition methods.

[0006] In a solution-based deposition method, the educt of the perovskite is deposited in its liquid form as a mixture directly on the surface of the bottom cell, wherein the deposited solution is then dried at normal pressure and low temperature (e.g., below 120° C.) and the perovskite is formed. In general, the deposition can be completely or partially solution-based. For example, there are hybrid methods in which one component is deposited in a solution-based manner and the other component is deposited from the gas phase by means of evaporation. The advantage of such a solution-based method is the process conditions that are easy to implement technically, which has a positive impact on costs and further enables high production volumes. In addition, the simple control of the educt composition allows for rapid optimization. A disadvantage of a solution-based deposition method can be the challenge of generating uniform layer thicknesses, which, in addition to handling the educt solution, which may be partially toxic, becomes increasingly difficult as the substrate size increases. In addition, the uniformity of the layer thickness may depend to a large extent on the surface morphology of the surface on which the deposition occurs, because the layer does not grow as a uniform layer as in a vapor-based method, but rather as a thin film deposition that may not necessarily be able to equalize morphological irregularities.

[0007] In order to achieve a strong light absorption leading to a higher efficiency factor and thus to a higher cell efficiency, tandem solar cells are usually also provided with a texture, which is usually formed on the lower solar cell structure (e.g. c-Si bottom cell). Such a texture can in turn be produced mainly wet-chemically by anisotropic etching with the aid of alkali metal hydroxide solutions containing additives, so that a pyramidal texture is produced.

[0008] US 2009 / 0 280 597 A1 discloses a method for smoothing a textured surface, wherein an alkaline etching bath is used to round or smooth the edges of a pyramidal texture, but wherein the material removal is small enough that the reflectivity of the textured surface is not impaired.

[0009] US2018 / 0 158 976A1 and US2021 / 0126147A1 disclose solar cells comprising a combination of a perovskite solar cell and a silicon solar cell. Summary of the invention

[0010] It is an object of the present invention to provide a method for generating a textured surface of a semiconductor layer, the textured surface comprising a texture suitable for depositing further layers. It is a further object of the present invention to provide a method for manufacturing a tandem solar cell comprising such a textured surface, and to provide a corresponding tandem solar cell.

[0011] This object is solved by a method for producing a textured surface according to claim 1 , a method for producing a tandem solar cell according to claim 14 and a tandem solar cell according to claim 16 .

[0012] An embodiment of the present invention provides a method for generating a textured surface of a semiconductor layer, comprising: anisotropically etching the surface of the semiconductor layer with a first alkaline etching solution to generate a surface of the semiconductor layer including a pyramid-shaped texture; and anisotropically etching the surface including the pyramid-shaped texture with a second alkaline etching solution different from the first alkaline etching solution to remove material from the pyramid-shaped texture, thereby reducing the height difference between the peak of the pyramid-shaped texture and the adjacent valley.

[0013] Examples of the present invention are based on the discovery that by using a second alkaline etching solution that is different from the first alkaline etching solution, it is possible to reduce the height of the pyramidal texture generated by using the first alkaline etching solution. This is also referred to herein as an alkaline pyramid cut. This enables the textured surface to be suitable for solution-based deposition of subsequent layers. However, at the same time, a texture that results in sufficiently low reflectivity still exists so that the semiconductor layer can be efficiently used as a solar cell.

[0014] An example of the invention provides a method for manufacturing a tandem solar cell, which, in addition to correspondingly generating a textured surface of a semiconductor layer, also includes generating a first solar cell structure of a tandem solar cell comprising a textured surface, and generating a second solar cell structure of a tandem solar cell on the side of the first solar cell structure on which the textured surface is arranged. Such an example makes it possible to deposit a perovskite layer in a solution-based manner to generate the second solar cell structure, because due to the reduced height of the texture, pyramidal structures that extend out of the perovskite layer after deposition and thus cause short circuits can be avoided.

[0015] According to the present invention, after isotropic texturing in an alkaline environment, post-processing (or post-processing) of the pyramidal texture is performed anisotropically in an alkaline environment (pyramid cutting), thereby making it possible to reduce the height of the pyramidal texture with the help of an alkaline non-toxic etching solution at reduced operating costs.

[0016] In an example, post-treatment, i.e., anisotropic etching with a second alkaline etching solution, is performed so that the height difference between the peak and the adjacent valley of at least some of the pyramidal textures is reduced by at least 5%, at least 10%, or at least 20%. In an example, before the post-treatment, the height of the generated pyramidal structure is in the range of 1 μm to 3 μm. Thus, the post-treatment can perform material removal, resulting in a height reduction of at least 50 nm, at least 100 nm, or at least 200 nm in the case of a pyramid height of 1 μm, a height reduction of at least 100 nm, at least 200 nm, or at least 400 nm in the case of a pyramid height of 2 μm, and a height reduction of at least 150 nm, at least 300 nm, or at least 600 nm in the case of a pyramid height of 3 μm. In an example, in a direction perpendicular to the plane of the semiconductor layer, a material removal of at least 50 nm (and thus a height reduction) of at least some of the pyramidal textures in the pyramidal texture can be achieved by means of pyramid cutting. In an example, the entire pyramid structure may be equalized in its height, in particular by reducing the height of pyramidal textures that protrude further from the plane than other pyramidal textures by means of material removal.

[0017] In an example, by etching with a second etching medium, the angle between the pyramid face of at least some of the pyramidal textures in the pyramidal texture and the plane on the semiconductor layer is reduced. In an example, the pyramid tips of at least some of the pyramidal textures in the pyramidal texture are removed. The corresponding change in the shape of the texture can be easily confirmed by corresponding image recording or measurement, so that the textured surface or tandem solar cell generated by the method of the present invention is different from the tandem solar cell generated by a conventional method. The examples of the present disclosure relate to corresponding manufactured tandem solar cells, wherein the textured surface of the semiconductor layer includes a texture that has been obtained by the post-processing described herein (i.e., by removing the tips of the pyramidal texture), and includes correspondingly removed pyramid tips in the form of an angle of 50° or less between a terrace or pyramid phase and the plane of the semiconductor layer.

[0018] Examples of the present disclosure provide a tandem solar cell manufactured according to any of the methods described herein, comprising: a first solar cell structure comprising a semiconductor layer having a textured surface; and a second solar cell structure located on a side of the first solar cell structure on which the textured surface is arranged, wherein the textured surface of the semiconductor layer comprises a texture comprising a terrace formed by removing the tips of a pyramidal texture, so that the height difference between the peak of the pyramidal texture and the adjacent valley is reduced by at least 5%, preferably at least 10%, compared to a corresponding texture without removing its tips, and / or the textured surface of the semiconductor layer comprises a texture in which at least some of the pyramidal textures have a pyramidal face with an angle of 50° or less with the plane of the semiconductor layer, so that the height difference between the peak of the pyramidal texture and the adjacent valley is reduced by at least 5%, preferably at least 10%, compared to a corresponding texture in which the angle between the pyramidal face and the plane of the semiconductor layer is about 55°. The corresponding pyramidal textures are understood to include the same bottom area.

[0019] In an example, after etching with the second etching solution, the reflectivity (reflection factor) of the textured surface for light of a wavelength of 600 nm is 10 to 20%. Therefore, corresponding light with the same or longer wavelength can effectively enter the first solar cell structure. Therefore, good efficiency can be maintained.

[0020] In an example, the second alkaline etching solution is an alkali metal solution. Thus, the example enables the generation of a corresponding texture of reduced height by using a non-toxic etching solution in an alkaline environment. In other examples, the second alkaline etching solution is a TMAH etching solution. In an example, the semiconductor layer includes crystalline silicon c-Si. In an example, the second solar cell structure includes perovskite. Thus, the examples of the present disclosure enable the generation of a tandem solar cell by using a known material that enables effective absorption of light at a corresponding wavelength.

[0021] In an example, the first alkaline etching solution contains an additive for generating a pyramidal texture, wherein the second alkaline etching solution does not include such an additive, so that anisotropic removal of material from the pyramidal texture is performed by post-processing, thereby changing the morphology of the pyramidal texture to reduce its height.

[0022] In the example, the anisotropic etching of the textured surface is performed under at least one of the following process conditions, preferably all of the following process conditions: alkali metal hydroxide concentration: 0.01 to 40 weight percent; temperature: 15 to 90° C., preferably 20 to 80° C., particularly preferably 20 to 65° C.; etching duration: 3 to 300 s, preferably 30 to 300 s, particularly preferably 30 to 180 s; pH range: 7 to 14, preferably 9 to 14. The etching range and the overall etching removal and thus the height reduction of the pyramidal texture depend on these parameters, so that an optimized interaction between the removal required for forming the structure and an economical process time can be achieved.

[0023] In an example, the method includes alkaline etching with a third etching medium to remove sawing damage of the semiconductor layer before anisotropic etching with the first etching medium. Thus, the example enables the surface of the lower solar cell structure to be prepared for deposition of the upper solar cell structure by using an alkaline medium. In an example, after etching with the second etching medium, the textured surface is wet chemically cleaned to prepare for further process steps.

[0024] In an example, generating a first solar cell test structure comprises generating a pn junction in or on a semiconductor layer, and generating a second solar cell structure comprises generating a pn junction, wherein a conductive layer, an electrical contact and / or an electrical insulator are generated between the first solar cell structure and the second solar cell structure. In an example, the generation of the pn junction of the first solar cell structure is performed by doping, for example, by means of vapor deposition or diffusion. In an example, generating the pn junction of the second solar cell structure comprises depositing a correspondingly doped layer. In this context, a pn junction is understood to be a direct pn junction as well as an indirect transition, for example, with an intrinsic layer in between.

[0025] In an example, generating a second solar cell structure includes depositing a perovskite layer by means of a method that is at least partially solution-based. This can achieve the advantages described initially. The method can be 100% solution-based. Alternatively, a hybrid method can be used in which one component of the perovskite is deposited in a solution-based manner and the other component is deposited from the gas phase by means of evaporation. In an example, generating a second solar cell structure includes generating a perovskite absorber layer between an electron transport layer and a hole transport layer. In operation, the perovskite layer absorbs light, resulting in the generation of free electrons and holes.

[0026] In an example, the method includes adjusting the removal caused by etching of the second etching medium by: adjusting the hydroxide concentration in the second etching medium, wherein at a hydroxide concentration of up to 15 weight percent, the etching rate increases with increasing hydroxide concentration, wherein at a hydroxide concentration of 15 to 25 weight percent, the etching rate is maximum, and wherein at a hydroxide concentration of more than 25 weight percent, the etching rate decreases with increasing hydroxide concentration; adjusting the temperature, wherein the etching rate increases with increasing temperature; or adjusting the etching duration, wherein the total etching removal increases with increasing etching duration. Thus, by adjusting one or more of the parameters, the pyramidal texture can be removed within a process time acceptable for practical applications, in one aspect, the removal amount is sufficient to avoid short circuits and enable sufficiently low reflectivity of 10.0% to 25.0% to be achieved at a wavelength of, for example, 600 nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Examples of the present disclosure are then described with reference to the accompanying drawings, in which:

[0028] Figure 1 A cross-sectional view of a tandem solar cell is shown in a purely schematic manner;

[0029] Figure 2 and Figure 3 shows a schematic diagram of an example of a tandem solar cell for describing a short circuit caused by an upper solar cell structure;

[0030] Figure 4 A flow chart showing an example of the method of the present invention for manufacturing a tandem solar cell;

[0031] Figure 5 A flow chart showing an example of a method for treating the surface of a semiconductor layer when manufacturing a tandem solar cell;

[0032] Figures 6a to 6e A schematic cross-sectional view of an example for describing the method of the present invention is shown;

[0033] Figures 7a to 7dshows a schematic cross-sectional view for describing how material removal of a pyramidal texture occurs in an example of the present invention; and DETAILED DESCRIPTION

[0034] Hereinafter, examples of the present disclosure are described in detail and using the accompanying drawings. It should be noted that the same element or elements with the same function have the same or similar reference numerals, wherein the repeated description of the elements with the same or similar reference numerals is generally omitted. The description of the elements including the same or similar reference numerals is interchangeable. In the following description, many details are described in order to provide a substantial understanding of the examples of the present disclosure. However, it is obvious to those skilled in the art that other examples can be implemented without these specific details. Except that the features of the corresponding combination exclude each other or explicitly exclude such a combination, the features of the different examples described can be combined.

[0035] In the following, examples of the invention will be described in particular in connection with the production of tandem solar cells, wherein a textured surface of a c-Si layer is generated and then a perovskite layer is applied on the textured surface, in particular in a solution-based manner. However, it is noted that the semiconductor layer may comprise other materials instead of c-Si, such as germanium arsenide or gallium arsenide (germanium orgallium arsenide). Furthermore, it is noted that instead of the perovskite layer, different layers made of different amorphous or microcrystalline absorbing materials capable of absorbing the short-wave portion of light may be used. In the examples, further layers are deposited on the textured surface to provide a tandem solar cell, wherein short circuits may be avoided in an effective and cost-effective manner by the upper solar cell structure.

[0036] Figure 1 Examples of tandem solar cell concepts to which examples of the present disclosure may be applicable are shown in a purely schematic manner. Figure 1A cross section of a tandem solar cell having a lower solar cell structure 10 (also commonly referred to as a bottom cell) and an upper solar cell structure 12 (also commonly referred to as a top cell) is shown. In an example, the lower solar cell structure 10 is a crystalline silicon (c-Si) based solar cell. In an example, the upper solar cell structure 12 disposed above the lower solar cell structure 10 is a perovskite solar cell deposited using, for example, a vapor-based or solution-based coating or deposition process as described above. In a further example, additional solar cell structures (not shown) may be arranged below and / or above the solar cell structures 10, 12. The lower solar cell structure 10 has a p-doped hole transport layer 22 and an n-doped electron transport layer 24. The upper solar cell structure 12 has a p-doped hole transport layer 26 and an n-doped electron transport layer 28. A conductive recombination layer 14 is typically arranged between the solar cell structures 10 and 12. A first connecting electrode 18 is formed on the bottom side of the lower solar cell structure 10, and a second connecting electrode 20 is formed on the top side of the upper solar cell structure 12. This tandem solar cell concept is purely exemplary, wherein, for example, the arrangement of the individual layers (n and p) can also be reversed and wherein, for example, the recombination layer can be omitted and the top and bottom cells can instead be contacted on both sides respectively.

[0037] In such a tandem solar cell, each solar cell structure generates electron-hole pairs independently of each other, wherein the extraction of holes occurs from the connection electrode 18 of the bottom cell 10, and the extraction of electrons occurs from the connection electrode 20 of the top cell 12. The rest of the generated charge carriers (electrons from the bottom cell 10 and holes from the top cell 12) can be recombined within the tandem solar cell via the recombination layer 14. The recombination layer 14 is usually very thin and transparent, so that it does not absorb too much light and has conductivity, so that it can be recombined. The recombination layer can, for example, have one or more layers of indium tin oxide (ITO). Alternatively, mutually insulated conductive layers can be provided between the lower solar cell structure 10 and the upper solar cell structure 12 instead of the recombination layer. These mutually insulated conductive layers can be provided with external terminals to extract the electrons generated by the lower solar cell structure 10, and to extract the holes generated by the upper solar cell structure 12. At this point, it should be noted that the respective arrangements of the electron transport layer and the hole transport layer are exemplary and can also be interchanged.

[0038] As described above, in order to achieve high light absorption, the lower solar cell structure is usually provided with a texture. Such a texture can in turn be carried out wet chemically by anisotropic etching with the aid of an alkali metal hydroxide solution containing additives, thereby generating a pyramidal texture. In the example, the height of the individual pyramids of the texture (the dimension perpendicular to the main plane of the semiconductor layer) can be controlled via the process conditions and the height can be less than 3 μm.

[0039] Depending on the process and the desired structure of the tandem solar cell, the perovskite layer can be deposited with different thicknesses between 20 nm and 1000 nm and above. For example, solution-based perovskites can typically have a thickness of about 500 nm. Therefore, the combination of a textured bottom cell and a perovskite top cell generated by means of a solution-based method is difficult to achieve because the pyramid tips of the lower solar cell extend through the perovskite layer and may generate an electrical short (shunt).

[0040] Figure 2 A schematic cross-sectional view of a tandem solar cell with a lower c-Si bottom cell and a perovskite top cell 12 is shown. A pyramidal texture 30 is formed on the top side of the bottom cell 10. After the recombination layer 14 is generated on the bottom cell 10, the top cell 12 can be deposited onto the c-Si bottom cell by means of a solution-based method. In turn, the bottom cell 10 includes a hole transport layer 22 and an electron transport layer 24. The top cell 12 includes a hole transport layer 26, an electron transport layer 28, and an electron-hole generation layer 29 (e.g., perovskite). The electron transport layer 24 can be composed of C60, while the hole transport layer can be composed of Spiro-OMeTAD. The order of these layers can be interchanged.

[0041] like Figure 2 As shown in FIG. 1 , one of the pyramidal textures 30 has such a height that it extends through the upper solar cell structure 12 and forms a short circuit (shunt) 32 with the terminal electrode or metal contact 20 arranged on the surface of the upper solar cell structure 12. After the pyramidal texture is generated by means of anisotropic etching, the height of the pyramidal texture can vary, such as Figure 3 as shown in . Figure 3 Also shown in a purely schematic manner Figure 2 , wherein several pyramidal textures 30 of the bottom cell 10 extend through the top cell 12 and form corresponding short circuits 32.

[0042] Depending on the thickness of the perovskite layer, for example < 1 μm, the above reference can be solved by not using any texturing or by using texturing with very small pyramids. Figure 2 and Figure 3described problem. A further possibility could be to deposit a textured light-transmitting layer on top of the perovskite, wherein this layer remains untextured between the bottom cell and the perovskite. However, these possibilities are associated with a number of disadvantages, which ultimately lead to efficiency losses in the tandem solar cell. A further possibility would be to post-treat the textured surface with the aid of HF / ozone, which would result in rounding of the pyramid tips. However, the pyramid tips are so small that the tips would still extend through the perovskite layer. In this regard, a method has been described in which the pyramid tips and pyramid valleys are rounded, wherein the angle of the pyramid sides relative to the substrate surface is reduced to a size slightly smaller than 55°, thereby reducing the pyramid height while keeping the bottom area the same. This is performed solely by an isotropic etching process, which can be carried out with the aid of HF / HNO 3 , by means of reactive ion etching, by means of vapor phase etching or by means of sputtering etching in a wet-chemical manner. This known method is used in methods in which perovskites are deposited in a vapor-based manner, in which the Gibb-Thompson effect is to be avoided and a more uniform layer is formed.

[0043] Examples of the invention relate to methods for generating textured surfaces of semiconductor layers and methods for manufacturing tandem solar cells suitable for preventing the short circuit problem discussed above. To this end, after generating the pyramidal texture, it is post-processed, which is also called pyramid cutting.

[0044] As in Figure 4As shown at 100 in the figure, anisotropic etching is first performed with a first alkaline etching solution to generate a textured surface, wherein a pyramidal texture is anisotropically generated on a semiconductor layer (e.g., a c-Si bottom cell). The pyramidal structure may have a height of <3 μm. An alkali metal solution containing an additive may be used as the first alkaline etching solution. Any known alkaline etching solution suitable for generating a corresponding pyramidal texture on a semiconductor surface may be used as the first alkaline etching solution. In the next step 102, post-treatment is also performed anisotropically with the aid of a second alkaline etching solution to remove material from the pyramidal texture. An alkali metal solution may be used as a second alkaline etching solution, which can be said to be a polishing method. The second alkaline etching solution does not include any additives that may cause the generation of a pyramidal structure. For example, the components of the second alkaline etching solution may substantially correspond to the components of the first alkaline etching solution, but without corresponding additives. In the example, due to post-treatment with the second alkaline etching solution, the morphology of the pyramid has one or more of the following changes: the tip of the pyramid is removed, the tip of the pyramid is smoothed, the angle of the pyramid is changed, and there is a platform. Therefore, the core idea of ​​the present invention is to perform anisotropic post-treatment in an alkaline environment after anisotropic texturing in an alkaline environment. The morphological changes generated by the anisotropic post-treatment are also referred to herein as pyramid cutting. Due to the pyramid cutting, similar light absorption properties as conventional textured surfaces can be achieved and perovskites can be deposited with the help of solution-based techniques, because the removed / smoothed tips no longer extend through the perovskite and therefore no longer have any short circuits.

[0045] In an example of the invention, a first solar cell structure (bottom cell) is produced in the semiconductor layer after a corresponding processing of the semiconductor layer, Figure 3 104 in. For this purpose, a pn junction can be generated in the semiconductor layer by means of doping, for example by means of vapor deposition or diffusion. To generate the first solar cell structure, an emitter or any other cell layer of the first solar cell structure can be deposited depending on the cell type. In the example, a conductive recombination layer is generated on the textured surface of the first solar cell structure. Then, a second solar cell structure is generated on the first solar cell structure, Figure 4 106. Generating the second solar cell structure includes generating a perovskite layer between the hole transport layer and the electron transport layer. In an example, oppositely doped layers are deposited.

[0046] At this point it is noted that any known method may be used to produce the first solar cell structure (bottom cell), as well as to produce the second solar cell structure (top cell).

[0047] When manufacturing solar cells, the silicon surface of the wafer is altered in order to efficiently convert light into electrical energy. In this case, the single-crystal silicon wafer can be processed using different etching methods. Figure 5 An example of a process flow for generating a textured surface of a semiconductor layer for producing a tandem solar cell is shown in FIG. In the example, the semiconductor layer is a monocrystalline or multicrystalline silicon wafer. The silicon wafer is usually sawn from a so-called ingot. In order to remove the sawing damage caused and to generate a surface that is as flat and uniform as possible, the wafer can be planarized in a preparatory etching step, wherein the sawing damage of the wafer is removed. Figure 5 The etching is typically performed in an alkaline environment using KOH at 70°C to 90°C and with a processing time of less than 10 minutes. This can effectively remove existing indentations caused by sawing the original silicon. In this case, the overall thinning of the wafer due to the etching removal is typically between 2 and 6 μm per wafer side.

[0048] The resulting uniform homogeneity of the surface with respect to smoothness and lack of defects can be advantageous for a subsequent etching step 112 for generating a pyramidal texture, since a consistent starting condition is provided for the subsequent etching medium. Step 112 includes alkaline etching with an alkaline etching solution (a first alkaline etching solution) to form pyramids. The first alkaline etching solution can be KOH (potassium hydroxide) and can contain isopropanol or an additive without isopropanol as an additive. This removes silicon to the extent that pyramids are formed in the surface. In an example, the first alkaline etching solution is a mixture containing long-chain alcohols, wherein the precise composition is known to the manufacturer. An example of such an etching solution is distributed under the German trademark "Celltex".

[0049] If the wafer already has a smooth surface after a separate previous wafer manufacturing process, the step 110 of removing saw damage may also be omitted. In an example, the first etching solution may contain additional additives that enable step 112 to be performed without step 110 being required.

[0050] After the pyramid texture is generated in step 112, Figure 5 In step 114 of the method, an alkaline etching for processing the pyramids is performed, which is performed by means of anisotropic etching with a second alkaline etching solution and is referred to herein as pyramid cutting. With the help of this additional alkaline etching step, more silicon is selectively removed from the pyramids so that the pyramids include a structure and morphology different from the original pyramids. In an example, in addition to water, the second alkaline etching solution can also include only reactive substances. The reactive substance can be an alkali metal hydroxide, such as KOH, NaOH (sodium hydroxide) or TMHA (tetramethylammonium hydroxide).

[0051] Subsequently, a wet chemical cleaning step can be performed, which has no or very little etching effect, so that only an atomic layer is removed, but the pyramid side structure is not changed, Figure 5 Alternatively, a post-cleaning with a stronger etching effect can be performed, for example using HF or ozone, resulting in a stronger removal but without changing the texture generated by the pyramid cutting with respect to its structure.

[0052] Before and after each of these etching steps, rinsing steps may be performed with water, such as pre-cleaning before step 110 and pre-cleaning / hydrophilization after step 110. Such rinsing steps are necessary if an acidic wet chemical post-cleaning is performed, which is why a considerable amount of water is required for rinsing. This, in turn, has a negative impact on operating costs and pollutes the environment, since the adhering alkaline residues lead to partial neutralization, thereby reducing the cleaning effect and / or increasing the acid concentration of the subsequent acid cleaning step.

[0053] After the post-cleaning in step 116, further process steps for producing the bottom cell can be performed depending on the cell concept. Subsequently, the perovskite can be deposited to provide the top solar cell structure. The further process steps required to produce the bottom cell (first solar cell structure) and the top cell (second solar cell structure) in each case are known to the person skilled in the art. Cleaning can be performed using known processes such as SC1, pSC1 and SC2. For example, pre-cleaning can be performed using the SC1 method or the pSC1 method. For example, post-cleaning can be performed using the following methods: SC1+SC2+HF-Dip, pSC1+HF / HCl, DiO 3 +HF / HCl, HF / HCl+DiO 3 +HF-Dip, HF / HCl / O 3 +HF / HCl.

[0054] Figures 6a to 6e Schematic cross-sectional views of structures resulting from implementation of examples of methods disclosed herein are shown. Figure 6a A semiconductor layer 120 in the form of c-Si bulk silicon of the bottom cell is shown, wherein a portion 120a in which the texture is generated is shown separated from a portion 120b purely for illustrative purposes. Figure 6b It shows that Figure 4 Step 100 and Figure 5 After step 112 in FIG. 1 , a semiconductor layer 120 having a pyramidal texture 122 is formed. Figure 6a Compared with the thickness of the portion 120a, Figure 6b The height of the pyramidal texture 122 is shown in FIG. Figure 6b Starting with the structure shown in Figure 4 Step 102 or Figure 5 In step 114, anisotropic etching is performed with a second alkaline etching solution. This results in Figure 6c truncated pyramidal structure 124 shown in . Even though only one side of the semiconductor layer is shown as textured in the figures, in the examples, anisotropic etching with a second alkaline etching solution generates corresponding textures on two opposite main surfaces of the semiconductor layer or on all sides of the semiconductor layer. Subsequently, various process steps can be performed to generate the first solar cell structure, which process steps can be different depending on the bottom cell concept. For example, the surface(s) in which the texture is generated can be doped in order to create a pn junction. In addition, it can be deposited Figure 6d After these process steps, a second solar cell structure is produced on the textured side or on one of the two textured sides. For this purpose, in the example, a solution-based deposition of, for example, a perovskite as the top cell is performed. Figure 6e The corresponding perovskite layer 128 is shown in FIG. At this point, it should be noted that Figure 6e The perovskite layer 128 in FIG. 1 is only schematically shown without any pn junctions which are generated during the generation of the second solar cell structure.

[0055] Depending on the respective solar cell concept, different further layers may be provided between the first solar cell structure and the second solar cell structure and on the outer surfaces of these structures. For example, a light-transmitting electrode layer, such as comprising indium tin oxide, may be provided on the surface of the first solar cell structure and the second solar cell structure. For example, the recombinant layer provided between the solar cell structures may also comprise indium tin oxide. In this respect, it should be noted that the present disclosure relates in particular to post-processing of the generated pyramidal texture, i.e. pyramid cutting, wherein for further details of the tandem solar cell and the tandem solar cell concept, reference may be made to the existing solar cells and solar cell concepts.

[0056] In the post-processing step, Figure 4 Step 102 and Figure 5In step 114, the semiconductor (such as silicon) is anisotropically etched. The removal in the post-processing step can be adjusted by changes in the process parameters. For example, by reducing the temperature, a reduction in reactivity can be observed. In this case, the combination and control of different process parameters can lead to a uniform etching pattern and uniform processing of the wafer structure. In the example, the process conditions for pyramid cutting can be defined as follows: alkali metal hydroxide concentration: 0.01 to 40 weight percent; temperature: 15 to 90°C, preferably 20 to 80°C, particularly preferably 20 to 65°C; etching time: 3 to 300s, preferably 30 to 300s, particularly preferably 30 to 180s; pH range is 7 to 14, preferably 9 to 14. It has been shown that an optimized interaction between the removal necessary for forming the structure and the economical process time can be achieved by adjusting the process conditions accordingly.

[0057] For example, the etching rate can be adjusted by adjusting the concentration of the etching medium. If an alkali metal hydroxide is used as an etching medium, at a concentration of 0.01 to about 15 weight percent, the etching rate increases with increasing concentration, that is, the pyramid cut increases. At a concentration of 15 to 25 weight percent, the etching weight is maximum, and at a concentration above 22 weight percent to 40 weight percent, the etching weight decreases with increasing concentration, that is, the pyramid cut decreases. The higher the temperature, the higher the etching rate, that is, the stronger the pyramid cut. The longer the etching pattern, the higher the etching rate, that is, the stronger the pyramid cut. By adjusting the corresponding process conditions, a material removal rate sufficient to prevent the peaks of the pyramidal texture from causing short circuits can be achieved.

[0058] The level of pyramid cutting can be determined and adjusted by using surface reflection measurement. The level of pyramid cutting can be adjusted so that the reflection range of the surface texture using pyramid cutting is in the range of 10.0 to 25.0% at a wavelength of 600nm. Another way to determine material removal is to measure the pyramid angle. The pyramid angle is the angle of the side of the pyramid with the horizontal substrate surface. The pyramid angle of the pyramid texture before pyramid cutting is about 55°. Due to pyramid cutting, the pyramid angle can be reduced and is in the range of 10 to <55°, preferably in the range of 10 to 50°. A single pyramid or pyramid valley can be completely polished so that the angle can be as low as 0°. In order to determine the pyramid angle in each case, the angle between the side surface of the pyramid in question and a line parallel to the main plane of the semiconductor layer of the wafer (which passes through the base of the pyramid) can be determined in a cross section. If the side surface of the pyramid is not straight, the side surface can be approximated in a cross section by a line connecting the base of the side surface of the pyramid to the highest point of the pyramid.

[0059] Figures 7b to 7d Possible shapes of textures obtained by pyramid cutting are shown. Figure 7a The initial shape after generating a pyramidal texture by anisotropic etching with a first alkaline etching solution is shown. The pyramid angle α between the side of the pyramid and the plane of the semiconductor layer is about 55°. In the example, the pyramid cut generates a more obtuse angle α of <55° or <50°. 1 Pyramids, such as Figure 7b As shown in Figure 7c As shown in , in an example, the pyramid cut generates a texture with removed pyramid tips, that is, a texture 124 with a terrace 130 by removing the tips of the pyramid-shaped texture 122. The terrace 130 can be arranged substantially parallel to the substrate plane. The pyramid angle can be ≤55°. In an example, in addition to the removed pyramid tips, the pyramid cut can also generate a terrace 132 in the valley between the pyramid tips, and the pyramid angle is ≤55°, Figure 7d It should be noted that the mesas 132 are not generated by filling the valleys, but rather by increasing the overall anisotropy removal.

[0060] Due to the different etching rates of the crystal planes, the first etching solution contains Figure 7a The second alkaline etching solution does not contain such an additive, so during etching, the etching rate of the crystal planes affected by the additive in the previous step is directly and only exposed to the second alkaline etching solution (and the main reaction products, such as silicates). Therefore, material removal of the existing pyramidal texture can be performed, which changes the shape, structure and morphology of the pyramidal texture. Figure 7b to Figure 7d The shapes shown in can also exist in parallel and / or as mixed shapes. In any case, pyramid cutting reduces the height of the pyramidal texture to allow the material of the second solar cell structure (e.g., perovskite) to be subsequently deposited by a solution-based method without the risk of short circuiting. In an example, to achieve this, the reduction in the height difference between the peak and the adjacent valley can be at least 5%, at least 10%, or at least 20%. By comparing the texture before and after the pyramid cutting or by comparing the texture after the pyramid cutting with respect to the complete pyramid with a pyramid angle of 55°, the corresponding reduction in height can be determined. In other words, the corresponding reduction corresponds to the corresponding difference relative to the complete pyramid with a pyramid angle (e.g., 55°) produced by etching with the first etching solution. Obviously, the size and shape of the pyramidal texture and the texture after pyramid cutting can be confirmed without further trouble, for example, by means of an image of a cut-through tandem solar cell, by measuring the surface of the textured surface, or by weight analysis.

[0061] Anisotropic etching with a second alkaline etching solution causes material removal, which results in a change in the pyramid structure. In the example, the formed platform results in modified optical properties, in particular a measurable and noticeable increase in the reflection level, i.e. reflectivity. The reflectivity can be determined using various optical measuring devices, such as a spectrophotometer. For comparability of the analysis, the difference in reflectivity at a fixed wavelength, such as 600 nm, is usually used. In the example, anisotropic etching with a second alkaline etching solution may result in an increase in the reflectivity of light with a wavelength of 600 nm (absolutely) of at least 0.5% to 10%, preferably 1% to 8%, particularly preferably 2% to 6%. The reflectivity can be determined with the aid of a surface reflectance measurement associated with all parts of the reflected light consisting of diffuse reflection and direct reflection. For example, the reflectivity can be determined by using a product called The spherical spectrometer of the Ci62 measures reflectivity in the SPIN (specular included) measurement mode.

[0062] In an example, the second alkaline etching solution is a KOH solution having a KOH concentration of less than 5%. It has been found that a corresponding removal of material in pyramid cutting can be achieved by using such an etching solution, wherein the resulting reduced etching rate can be compensated by increasing the temperature and / or etching time.

[0063] In an example, during etching with the second etching solution, the method may comprise applying ultrasound to the second etching solution. For this purpose, a corresponding ultrasonic transducer may be provided. By using ultrasound, the etching rate during pyramid cutting may be increased, hydrogen bubbles may be destroyed and a more uniform etching in relation to the surface area may be achieved. For example, the use of ultrasound to support an alkaline etching process is known from US 6224 713 B1.

[0064] Thus, the present invention makes it possible to manufacture tandem solar cells, wherein the bottom cell has a texture and the subsequent deposition process of, for example, a perovskite is performed using a solution-based method, without the tips of the pyramidal texture protruding from the deposited perovskite layer. Thus, the example enables a solution-based deposition method of, for example, a perovskite on a c-Si solar cell with a textured surface, and an associated solution to the problem of short circuits caused by the tips of the pyramids. This is expected to increase the energy yield of tandem solar cells. The present invention achieves this by the described pyramid cutting, which is a process step after and coupled to the texturing process, wherein the surface morphology produced in the texturing process is changed in such a way that the generated pyramids are blunted, the tips of the pyramids are cut off, and / or a platform is formed both in the valley and on the pyramids. By morphologically modifying the original texture using pyramid cutting, the next solar cell layer, for example a perovskite, can be deposited using a solution-based deposition technique. This is possible because the modified pyramids, in particular the tips of the pyramids, no longer protrude through the deposited solar cell layer, for example a perovskite, and therefore do not cause any electrical short circuits.

[0065] Known methods for processing textures have been described in J. Du et al., "Selective rounding for pyramid peaks and valleys improves the performance of SHJ solar cell", Energy Sci Eng. 2021, 00, 1-7 and L. Mohr et al., "Numerical Simulation of an Ozone-Based Wet-Chemical Etching", Industrial & Engineering Chemistry Research, 2020, 59 (40), 17680-17688. Compared to known methods in which HF and ozone water are used to perform pyramid rounding processing (removal), the examples of the present invention make this possible with a non-toxic etchant, which protects the environment and requires less cleaning solution or cleaning steps, which reduces operating costs. According to the invention, an anisotropic etching mechanism is used, whereas the mechanism of the known solutions is different, in which ozone or nitric acid has an oxidizing effect on silicon by forming silicon dioxide, which is then removed by etching the silicon dioxide with hydrofluoric acid, which is an anisotropic etching mechanism and is described as chemical polishing. In the known processes, the tip is rounded with a given radius of curvature compared to the total height, without significant removal. In contrast, the method according to the invention generates different structures, in particular terraces, as described in reference Figure 7c and 7d According to the invention, a second alkaline etching solution is used for pyramid cutting, which enables etching rates of >50 nm / min to be achieved. In contrast, etching rates in known acid etching are in the range of 5, 9 or 11 nm / min at ozone concentrations of 15, 30 or 40 mg / l. 3 In the known process of hydrofluoric acid and HF, porous silicon is formed, which must be removed in an additional KOH step. This is unnecessary in the method according to the invention. The impact of these two toxic components on the environment is considered to be a particular disadvantage of the known process. In addition, compared with the present disclosure in which pyramid cutting is performed by means of an alkaline etching solution, two active substances are required here in addition to water. Removal with hydrofluoric acid and ozone is less obvious and only leads to a significant microscopic rounding of the tip without changing the angle and without a significant reduction in height. The typical removal rate is 47.7nm in 10 minutes (80ppm ozone, 0.08% HF, 20°C). For industrial applications, this process time is not economical because high production volumes cannot be achieved in solar cell production. In addition, additional production and equipment are required to generate ozone, and the cost of generating such an amount of ozone is huge, while anti-ozone materials are also costly. Chemical polishing can also be achieved by using a mixture of nitric acid and hydrofluoric acid. Here, too, two active substances are inevitably required, both of which are classified as toxic, wherein the highly exothermic reaction generates nitrogen oxides as reaction products and leads to environmentally harmful nitrate-containing wastewaters, together with increased treatment effort and costs. Thus, compared to such known methods, the method according to the invention enables the manufacture of pyramidal textures with modified morphology in a time-saving, cost-effective and more environmentally friendly manner, thereby preventing short circuits and tandem solar cells.

[0066] The invention can be applied in many different application areas, for example all c-Si based solar cell concepts such as HJT (heterojunction), PERC (passivated emitter rear contact), TOPCON (tunnel oxide passivated contact), etc., on which any solar cell layers suitable for a tandem solar cell concept can be deposited.

[0067] Examples of the present disclosure relate in particular to the generation of textured surfaces of semiconductor layers, wherein the pyramid cutting step according to the present invention is different from a conventional post-cleaning process, such as a post-cleaning process performed after each chemical treatment step in a conventional process. For example, the wafer surface is cleaned with deionized water after each chemical treatment step. Other contaminants such as metal impurities can be effectively removed with a mixture of water, hydrochloric acid and / or hydrofluoric acid. Since ozone has a high oxidation potential, ozone can also be added to the mixture. Organic contaminants can preferably be removed with a mixture of hydrogen peroxide and / or an alkaline medium (such as ammonia, hypochlorite solution or ozone). Ozone has a low resistance in an alkaline environment, wherein an acid is preferably added to the ozone, particularly preferably hydrochloric acid (HCl) and / or hydrofluoric acid (HF). Adding an acid to ozonated water can also achieve cleaning of metal impurities. However, such a cleaning process does not remove enough etching products to change the pyramidal structure of the silicon wafer.

[0068] Although some aspects have been described as features in the context of an apparatus, it is clear that such a description may also be viewed as a description of corresponding features of a method. Although some aspects have been described as features in the context of a method, it is clear that such a description may also be viewed as a description of corresponding features of functionality of an apparatus. Some or all of the method steps may be performed by (or using) a hardware device, such as a microprocessor, a programmable computer, or an electronic circuit. In some embodiments, some or several of the method steps may be performed by such a device. According to certain implementation requirements, embodiments of the present invention may be implemented in hardware or software, or may be implemented at least in part in hardware or software.

[0069] In the foregoing detailed description, various features are grouped together in the examples for the purpose of simplifying the present disclosure. This method of disclosure should not be interpreted as reflecting the intention that the examples claimed for protection include more features than those explicitly stated in each claim. On the contrary, as reflected in the following claims, the subject matter may be less than all the features of a single disclosed example. Therefore, the following claims are incorporated into the detailed description, where each claim can be independently used as a separate example. Although each claim can be independently used as a separate example, it should be noted that although a dependent claim can be cited in a claim as a specific combination with one or more other claims, other examples can also include a combination of a dependent claim with the subject matter of each other dependent claim or a combination of each feature with other dependent or independent claims. Unless it is stated that a specific combination is not intended, such a combination should be included herein. In addition, even if a claim is not directly subordinate to an independent claim, it is intended to include a combination of the features of the claim with any other independent claim.

[0070] The examples described above represent only the basic principles of the present invention. It should be understood that other persons skilled in the art will appreciate modifications and variations of the arrangements and details described herein. This is why the present invention is intended to be limited only by the scope of the following claims, rather than by the specific details presented herein by means of the description and discussion of the embodiments.

Claims

1. A method for generating a textured surface of a semiconductor layer, comprising: Anisotropically etching the surface of the semiconductor layer with a first alkaline etching solution to generate a surface of the semiconductor layer including a pyramid-shaped texture; as well as anisotropically etching the surface including the pyramidal texture with a second alkaline etching solution different from the first alkaline etching solution to remove material from the pyramidal texture to reduce a height difference between a peak and an adjacent valley of the pyramidal texture, The height difference between the peak and the adjacent valley of at least some of the pyramidal textures is reduced by at least 5%, preferably at least 10%, by anisotropic etching with the second alkaline etching solution.

2. The method according to claim 1, wherein the reflectivity of the surface of the semiconductor layer including the texture for light having a wavelength of 600 nm is increased by 0.5% to 10% in absolute value, preferably by 1% to 8% in absolute value, and particularly preferably by 2% to 6% in absolute value by performing anisotropic etching with the second alkaline etching solution.

3. The method according to claim 1 or 2, wherein by etching with a second etching medium, the angle between the pyramid faces of at least some of the pyramid-shaped textures and the plane of the semiconductor layer is reduced, and / or the pyramid tips of at least some of the pyramid-shaped textures are removed.

4. The method according to any one of claims 1 to 3, wherein after etching with the second etching solution, the reflectivity of the textured surface to light having a wavelength of 600 nm is 10% to 25%. 5 . The method according to claim 1 , wherein the second alkaline etching solution is an alkali metal solution, an alkaline earth metal solution or a TMAH etching solution.

6. The method of any one of claims 1 to 5, wherein the semiconductor layer comprises crystalline silicon.

7. The method according to any one of claims 1 to 6, wherein the anisotropic etching of the surface comprising the pyramidal texture is performed under at least one of the following process conditions: Alkali metal hydroxide concentration: 0.01 to 40 weight percent, Temperature: 15 to 90° C., preferably 20 to 80° C., particularly preferably 20 to 65° C., Etching duration: 3 to 300 s, preferably 30 to 300 s, particularly preferably 30 to 180 s; pH range: 7 to 14, preferably 9 to 14.

8. The method according to any one of claims 1 to 7, comprising, before the anisotropic etching with the first etching medium, performing alkaline etching with a third etching medium for removing saw damage from the semiconductor layer.

9. The method of any one of claims 1 to 8, comprising wet chemical cleaning the textured surface after etching with the second etching medium.

10. The method of any one of claims 1 to 9, further comprising adjusting removal due to etching with the second etching medium by at least one of: adjusting the potassium hydroxide concentration in the second etching medium, wherein at a concentration of up to 15 weight percent of hydroxide, the etching rate increases with increasing hydroxide concentration, at a hydroxide concentration of 15 to 20 weight percent, the etching rate reaches its maximum value, and at a hydroxide concentration of more than 25 weight percent, the etching rate decreases with increasing hydroxide concentration, adjusting the temperature, wherein the etching rate increases with increasing temperature, or The etching duration is adjusted, wherein the total etching removal increases as the etching duration increases.

11. The method according to any one of claims 1 to 10, comprising applying ultrasound to the second etching solution during etching with the second etching solution.

12. A method for manufacturing a tandem solar cell, comprising: Performing a method according to any one of claims 1 to 11; generating a first solar cell structure of said tandem solar cells comprising said textured surface; as well as A second solar cell structure of the tandem solar cells is generated on the side of the first solar cell structure on which the textured surface is arranged.

13. The method according to any one of claims 1 to 12, wherein generating the first solar cell structure comprises generating a pn junction in or on the semiconductor layer, wherein generating the second solar cell structure comprises generating a pn junction, wherein a conductive layer, an electrical contact and / or an electrical insulator are generated between the first solar cell structure and the second solar cell structure.

14. The method according to any one of claims 12 or 13, wherein the second solar cell structure comprises a perovskite layer.

15. The method of claim 14, comprising depositing the perovskite layer by means of an at least partially solution-based method.

16. A series solar cell comprising: a first solar cell structure comprising a semiconductor layer having a textured surface; and a second solar cell structure, the second solar cell structure being located on a side of the first solar cell structure on which the textured surface is arranged, wherein the textured surface of the semiconductor layer comprises a texture comprising terraces formed by removing tips of the pyramidal texture, such that the height difference between the peaks of the pyramidal texture and adjacent valleys is reduced by at least 5%, preferably at least 10%, compared to a corresponding texture in which the tips are not removed, and / or the textured surface of the semiconductor layer comprises a texture wherein at least some of the pyramidal textures have a pyramid face with a plane of the semiconductor layer at an angle of 50° or less, such that the height difference between the peaks of the pyramidal texture and adjacent valleys is reduced by at least 5%, preferably at least 10%, compared to a corresponding texture wherein the angle between the pyramid face and the plane of the semiconductor layer is approximately 55°. 17 . The tandem solar cell of claim 16 , wherein the textured surface of the semiconductor layer has a reflectivity of 10% to 25% for light having a wavelength of 600 nm.

Images