METHOD FOR THE PRODUCTION OF PEROWSKIT / SILICON TANDEM SOLAR CELLS

Description

[0001] The invention relates to a method for manufacturing perovskite / silicon tandem solar cells, such as those known from photovoltaic applications. The perovskite / silicon tandem solar cells can also be used without restriction for the formation of solar modules.

[0002] The improvement of solar cells is primarily aimed at improving efficiency, i.e., improving the ratio of the amount of light energy available for absorption to the amount of energy made usable by the solar cell in the form of electric current.

[0003] Besides improving the efficiency of solar cells, another important aspect of their manufacture is to make production as cost-effective and suitable for industrial-scale production as possible.

[0004] Perovskite / silicon tandem solar cells are candidates for high efficiency. Several physical properties of a solar cell contribute to its efficiency. In the review article, Article 1, by S. Akhil et al. (Review on perovskite silicon tandem solar cells: Status and prospects 2T, 3T and 4T for real world conditions, Materials & Design Vol. 211, 2021, 110138_1-36), various state-of-the-art knowledge regarding the efficiency and efficiency improvement of perovskite / silicon tandem solar cells is presented, and aspects of their fabrication are also discussed, such as the deposition of perovskite layers onto previously deposited, structured layers.

[0005] Among other things, the efficiency depends crucially on the proportion of light that is reflected in or by the solar cell and therefore not available for absorption. Reflected light is available for absorption in the solar cell to a significantly reduced extent, and in most cases not at all, thus contributing significantly less, and mostly not at all, to the solar cell's efficiency.

[0006] Several approaches exist for reducing the proportion of reflected light. One approach is to structure the interfaces between layers of different materials in the layer stacks of solar cells, particularly to reduce light reflection and to provide so-called light-trapping structures. This structuring can be achieved by having several successive layers in the stack each adopt the same structure as the layer applied first (either at the front or in a layering sequence, as layers are applied downwards). This is accomplished by applying the same (homogeneous) layer thickness at every point of a predefined structure, i.e., in its troughs and peaks. A structure-conforming layer replicates the structure of the underlying structured layer in the essential characteristics of height and thickness.The depth of the structures and their geometric shape and dimensions are determined by the location of the underlying layer in the same direction as the layer sequence. In this context, the term "structure-conforming deposition of layers" is also used, referring to layers applied in a structure-conforming manner to the layer directly below. The replication of the structures depends on the unavoidable variations in layer thickness inherent in the application of any layering process, caused by experimental conditions that are ultimately not entirely controllable. Therefore, the condition of structure conformity can be understood as a copy of the essential features of the structure while maintaining the functionality resulting from the structuring (e.g., reduction of reflections).

[0007] For perovskite / silicon tandem solar cells, the influence of structured layers on the reflection and light scattering properties of the solar cell is discussed, for example, in paper 2 by K. Jäger et al. (Prospects of light management in perovskite / silicon tandem solar cells, Nanophotonics; Vol. 10(8), 2021, pp. 1991-2000) and also, for example, in paper 3 by F. Sahli et al. (Fully textured monolithic perovskite / silicon tandem solar cells with 25.2% power conversion efficiency, Nature Materials, Vol. 17, 2018, pp. 820-826) for a specific perovskite / silicon tandem solar cell design. The size and shape of the structures are crucial for the reflection and light scattering properties. A special form of structuring using so-called inverted pyramids is described in paper 4 by D. Shi et al. (Perovskite / c-Si tandem solar cell with inverted nanopyramids: realizing high efficiency by controllable light trapping, Scientific Reports, 2015, 5:16504, p.1-10) reveals, which has proven advantageous for so-called "light trapping".

[0008] In paper 5 by Y. Hou et al. (Efficient tandem solar cells with solution-processed perovskite on textured crystalline silicon, Science, Vol. 367, 2020, pp. 1135-1140) the deposition of perovskite layers from solution onto a structured layer to form perovskite / silicon tandem solar cells is examined in more detail.

[0009] In article 6 by B. Chen et al. (Blade-Coated Perovskites on Textured Silicon for 26%-Efficient Monolithic Perovskite / Silicon Tandem Solar Cells, Joule, Vol. 4, 2020, pp. 850-864), the application of a perovskite layer using the so-called "blade-coat" process is presented. This process involves coating structured layers with a layer from slot nozzles using a separate device (squeegee) for smoothing or homogenizing the layer thickness. In this case, the device is for providing a uniform N2 flow.

[0010] In paper 7 by M. Saliba et al. (Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency, Energy & Environmental Science, Vol. 9, 2016, pp. 1989-1997) the application of perovskite layers by spin coating is disclosed.

[0011] In addition to the "blade-coat" process and solution deposition, the so-called "spin coating" method should also be mentioned for the application of perovskite layers. Within the scope of the present invention, "spin coating" refers to a single-stage process step in which the material required to form a perovskite layer is not applied separately according to the different components of the perovskite composition, but rather as a single material in one step. Although high-quality layers that contribute to highly efficient solar cells can be produced using spin coating, as described, for example, in Paper 2, structurally conformal perovskite layers, as described, for example, in Paper 3, cannot be produced using this type of coating according to the prior art.

[0012] In article 8 by BW Schneider et al. (Pyramidal surface textures for light trapping and antireflection in perovskite-on-silicon tandem solar cells, Optics Express, Vol. 22, 2014, pp. A1422-A1430), the improvement of efficiency through enhanced light management is discussed, and it is theoretically demonstrated that the structurally conformal deposition of perovskite layers onto structured silicon layers is promising. The silicon structure under consideration consists of inverted pyramids.

[0013] The properties of a silicon layer structured with inverted pyramids and coated with gold for infrared absorption are further demonstrated in paper 9 by J. Hu et al. (Efficient infrared sunlight absorbers based on gold-covered, inverted silicon pyramid arrays, Materials Advances, Vol. 3, 2022, pp. 2364-2372), which also describes a method for fabricating the inverted pyramids using nanoimprint lithography. US Patent 4,528,260 A describes a photolithographic method for fabricating inverted pyramids in silicon.

[0014] Finally, EP 3 893 285 A1 should be mentioned, which discloses a method for manufacturing silicon / perovskite tandem solar cells. The perovskite layers are formed, in particular, from laminated, i.e., layered, perovskites of different compositions. The tandem solar cell as a whole is, in particular, monolithic and can be built on a silicon layer textured with irregular pyramids, onto which the remaining functional layers of the tandem solar cell are deposited layer by layer. The figures suggest a structurally conformal deposition of the layers. Another method for manufacturing silicon / perovskite tandem solar cells is known from WO 2021 / 030491 A1.

[0015] The object of the invention is to provide a method for manufacturing perovskite / silicon tandem solar cells that features a structure optimized for reduced reflection and simultaneously allows the deposition of perovskite layers, resulting in high efficiency of the manufactured solar cells. This object is achieved by the features of claim 1.

[0016] Surprisingly, a method for manufacturing perovskite / silicon tandem solar cells was discovered at HZB. This method incorporates both a structuring optimized for reduced reflection and the ability to apply a structurally conformal perovskite layer via spin coating. The structuring is characterized by a defined area of ​​inverted pyramids, created using nano-embossing lithography or photolithography and dry or wet chemical etching. This structure enables the spin coating of a structurally conformal perovskite layer.

[0017] For the purposes of the invention, perovskite is defined as all compounds that can be described by the molecular formula ABX3 and are crystallized in the crystal structure of the mineral perovskite, comprising, for example, methylammonium (MA), cesium (Cs), and / or formamidinium (FA) as component A, while component B is frequently lead (Pb). X is, for example, iodine (I), bromine (Br), and / or chlorine (Cl), or a mixture of these elements. Preferably, Cs 0.05 (FA 0.79 MA 0.21 ) 0.95 Pb(I 0.79 Br 0.21 ) 3 or similar compositions are used.

[0018] The inventive method for manufacturing perovskite / silicon tandem solar cells involves first providing a silicon wafer which is oriented along (100) and is monocrystalline. Such silicon wafers for the production of solar cells are familiar to those skilled in the art. The silicon wafer forms the absorber of the rear subcell (silicon subcell) in the tandem solar cell.

[0019] Further layers, especially for completing the solar cell, such as for forming a pn junction with the silicon absorber by means of a correspondingly doped additional silicon layer, are to be provided in later steps of the process as required.

[0020] In a further step, the first side of the silicon wafer is structured, thus defining it as the front side. This is done using a lithography process, such as nano-embossing lithography or photolithography, followed by a dry and wet chemical etching step. The second side of the silicon wafer, opposite this first side, is then referred to as the back side.

[0021] For this purpose, a silicon dioxide layer with a thickness in the range of 200 nm to 500 nm is first applied to the silicon wafer on the first side. This is advantageously done by plasma-enhanced chemical vapor deposition (PECVD). Other methods for applying the silicon dioxide layer, such as physical vapor deposition (PVD) or oxidation at high temperatures, are known to those skilled in the art and are equally applicable in the process according to the invention. A layer of photoresist is then applied to the silicon dioxide layer, for example by spin coating or other methods. The photoresist advantageously consists of an organic material and is applied with a thickness in the range of 100 nm to 500 nm.

[0022] In a further step according to the invention, a regular structure of cylindrical holes is introduced into the photoresist by nano-embossing lithography or photolithography to form a mask. The holes have a diameter in the range of 1 µm to 5 µm and are spaced 1.5 µm to 6 µm apart. The corresponding cylinders in the die, which are used to create the holes (with a diameter in the range of 1 µm to 5 µm and a spacing of 1.5 µm to 6 µm), have a height in the range of 100 nm to 500 nm. The structuring is carried out using the die, which is pressed into the photoresist, after which the photoresist is cured by irradiation with UV light. The production of a stamp and further information on the nanoimprint lithography process can be found, for example, in article 10 by LJ Guo (Nanoimprint Lithography: Methods and Material Requirements, Advanced Materials, Vol.: 19(4), 2007, p.495-513).

[0023] The structure of the photoresist mask is then transferred into the silicon dioxide layer by directed (anisotropic), reactive ion etching. The etching time must be adjusted experimentally so that the etching only occurs until the silicon wafer is reached. For example, reactive ion etching with a plasma consisting of argon and CHF 3 or SF 6 is advantageously used. This creates a mask in the silicon dioxide layer that corresponds to the mask introduced into the photoresist. This silicon dioxide mask is sufficiently stable against etching with potassium hydroxide or other etchants. The photoresist is typically removed with an oxygen plasma or by wet chemical etching.

[0024] In the subsequent step, wet chemical etching is performed through the mask, advantageously using, for example, a potassium hydroxide or sodium hydroxide solution, or tetramethylammonium hydroxide. Etching continues until inverted pyramids are formed, i.e., until the lateral faces of the pyramids meet at a point. The etching time determined by this condition must be determined experimentally in the specific application by verifying the etching results, for example, using scanning electron microscopy, and adjusting the etching time accordingly. In this wet chemical etching step, the entire silicon wafer is exposed to the etching process. This results in etching also occurring on the back surface of the silicon wafer, the second side, which textures the surface of this second side randomly through the formation of pyramids.

[0025] The inverted pyramids thus created on the surface of the front face of the silicon wafer have the following dimensions: The base of the pyramids has edge dimensions ranging from 1.5 µm to 6 µm, and the depth ranges from 1 µm to 4 µm, with a spacing of 1.5 µm to 6 µm. An inverted pyramid, as defined by the invention, is the reversed spatial shape (the negative) of a pyramid as a depression in a layer (here, the silicon layer), wherein the base of the inverted pyramid is determined by the opening in the layer. It should be noted that the holes, which are transferred into the photoresist by the stamp and, through directional etching, also into the silicon dioxide layer, are smaller in diameter than the edge lengths of the pyramids formed by the dry or wet chemical etching.

[0026] Functional layers are first deposited on the second side of the silicon wafer. These layers are required for the final completion of the tandem solar cell, and in particular, the silicon subcell. They include at least one layer for forming a corresponding pn junction with the silicon wafer and a contact layer. Then, functional layers are deposited on the first side of the silicon wafer, which is structured with inverted pyramids. These functional layers include at least a passivation layer, a recombination layer, and a hole-conducting layer. Further layers are possible. The methods for depositing these layers are known to those skilled in the art and can be found in the relevant literature.

[0027] Subsequently, a perovskite layer is applied by spin-spraying a perovskite solution, with the applied perovskite layer having a thickness in the range of 300 nm to 700 nm. The perovskite layer is then finished by salting out the applied perovskite solution and subsequent baking.

[0028] To complete the perovskite / silicon tandem solar cell, at least the following functional layers are applied to the perovskite layer, conforming to the structure: an electronic conductor layer, a buffer layer, a transparent conductive oxide, a metal contact, and an antireflective layer. Further layers are possible. The methods for applying the layers are known to those skilled in the art and can be found in the relevant literature.

[0029] The invention relates to the production of perovskite / silicon tandem solar cells according to the method described above, with the following features. A silicon wafer as absorber of the silicon subcell is structured with regularly arranged inverted pyramids with dimensions of 1.5 µm to 6 µm for the base of the pyramids and 1 µm to 4 µm for the depth of the pyramids, spaced 1.5 µm to 6 µm apart, and wherein all layers following the silicon absorber as the lower layer in the tandem solar cell are structurally conformal to the structure in the silicon layer, in particular at least one perovskite layer as absorber in the perovskite subcell.

[0030] Such a perovskite / silicon tandem solar cell has the advantage that it can be manufactured using the inventive method and thus has a structure suitable for reducing reflections, while at the same time the perovskite layer in the tandem solar cell can be applied by spin coating, resulting in improved efficiency. In the perovskite / silicon tandem solar cell manufactured according to the inventive method, the perovskite layer is structurally textured on both sides.

[0031] The invention will be explained in more detail in the following exemplary embodiment and with reference to 2 figures. This shows

[0032] Fig. 1: Schematic representation of a perovskite / silicon tandem solar cell produced according to the inventive method. Fig. 2: Schematic representation of steps (a)-c)) of the nano-imprint lithography process used according to the invention (prior art) and d) SEM image of such a structured silicon dioxide layer for producing the inverted pyramids in a silicon wafer according to the invention.

[0033] In the Fig. 1 The layers of a perovskite / silicon tandem solar cell produced according to the inventive method in the exemplary embodiment are shown. The layers are to be referred to as follows, from bottom to top: Functional layers: 1: Backside reflector (silver) 2: Doped SnO (tin(II) oxide) 3: (p) a-Si:H 4: (i) a-Si:H Silicon subcell absorber: 5: Silicon wafer (shown in dashed lines) Functional layers: 6: (i) a-Si:H 7: (n) nc-Si:H 8: Doped SnO (tin(II) oxide) 9: Hole conductor layer made of Me-4PACz Perovskite subcell absorber: 10: Perovskite (Cs 0.05 (FA 0.79 MA 0.21 ) 0.95 Pb(I 0.79 Br 0.21 ) 3 ) Functional layers: 11: C 60 12: SnO 2 (tin(IV) oxide) 13: IZO (Indium zinc oxide) 14: Silver ring (see also below) 15: LiF (lithium fluoride)

[0034] The layer thicknesses are not shown to scale and are for orientation purposes only. The layer numbering is for reference only and corresponds to the layer sequence in the completed tandem solar cell, but does not refer to the layer deposition sequence. The silicon wafer 5, acting as the absorber, is structured with inverted pyramids on a first side (the top side in the figure) according to the inventive method. The silicon wafer 5 is considered the bottom layer for layers 6-15, particularly with regard to the layer deposition sequence. The silicon wafer 5 is also considered the bottom layer for the back-side layers 1-4, but not for the layer sequence in the final tandem solar cell. The silicon wafer 5 is structured on both sides by the wet chemical etching step of the inventive method.On the first side, shown at the top of the figure, it is structured using the inventive method, and on the second, lower side, it is provided with a random pyramid structure by the wet chemical etching step. Both the lower layers (1-4) and the upper layers (6-15) are applied to the respective surfaces of the silicon wafer 5 in a structurally conformal manner.

[0035] In the exemplary embodiment of the inventive method for producing perovskite / silicon tandem solar cells, a monocrystalline silicon wafer with (100) orientation (the lattice planes {100} of silicon coincide in orientation with the surface of the wafer) is first provided. The silicon wafer 5 is coated on a first side with a silicon dioxide layer S (in Fig. 1 (not shown, as it was removed during the process) is coated with a layer thickness of 260 nm using PECVD. A commercially available organic photoresist mr-NIL210 from micro resist technology GmbH F is then applied to this layer with a thickness of 500 nm (in Fig. 1 (not shown, as it is removed during the process). A hole structure is introduced into the photoresist F using nano-embossing lithography ( Fig. 2a This is done using a polydimethylsiloxane (PDMS) stamp P, which is formed by transferring a structure from a master. The stamp P is pressed into the UV-curable photoresist F, after which the photoresist F is cured with UV light. After removing the PDMS stamp S, the structure reproduced in the photoresist F is etched in a directed (anisotropic) manner into the silicon oxide layer S previously applied to the silicon wafer 5 by means of reactive ion etching with a plasma of argon and CHF 3, and thus transferred ( Fig. 2b )). The photoresist is removed with an oxygen plasma. The resulting silicon oxide mask S' is sufficiently stable against etching with potassium hydroxide and exhibits a periodic hole structure introduced by the stamp P, with holes having a diameter of 2.5 µm and a spacing of 4 µm ( Fig. 2d The holes in the structured silicon dioxide layer S' extend down to the silicon wafer and thus have a depth corresponding to the thickness of the silicon dioxide layer. Inverted pyramids are produced in the silicon wafer 5 at the locations defined by the holes in the silicon dioxide layer S' using potassium hydroxide etching ( Fig. 2c)), whose periodicity is determined by the mask of the silicon dioxide layer S'. Pyramid formation occurs due to the highly selective etching preference of the (100) planes of monocrystalline silicon 1 during wet chemical etching, carried out in this embodiment with potassium hydroxide. The same angles of 54.74° are always formed on the pyramid flanks, whereby the depth of the pyramid structures is determined by the distance to the next pyramid. In this embodiment, the pyramids have the following dimensions: 4 µm for the edges of the base of the pyramids and a depth of 2.8 µm for the pyramids, with a spacing of 4 µm (the pyramids are in contact).

[0036] The silicon dioxide etching mask is removed after wet chemical etching with potassium hydroxide by buffered oxide etching (BOE). After cleaning by standard RCA cleaning, as also used in microelectronics, the double-sided textured silicon wafer is further processed into a perovskite / silicon tandem solar cell.

[0037] To complete the tandem solar cell and especially the silicon subcell, the layers (i) a-Si:H 4, (p) a-Si:H 3, doped tin(II) oxide SnO 2 and a rear reflector made of silver 1 are applied to the second side of the silicon wafer using standard procedures.

[0038] In the exemplary embodiment, the first, front-facing side of the silicon wafer 5, structured with inverted pyramids, is first passivated by hydrogenated intrinsic amorphous silicon (by means of PECVD) ((i) a-Si:H, 6) and then the following functional layers are deposited on it in conformal structure: (n) nc-SiOx:H (hydrogenated n-doped nanocrystalline silicon oxide, applied by means of PECVD) 7, a recombination layer of transparent conductive oxide (here doped tin oxide by sputtering) 8 and subsequently a layer of Me-4PACz ([4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid) 9 as a selective hole contact by means of spin coating.

[0039] A perovskite layer is then spin-deposited at 3500 rpm for 40 s. 5 s before the end of the program, 500 µL of ethyl acetate is poured onto the rotating substrate as an anti-solvent for salting out. A triple cation perovskite Cs 0.05 (FA 0.79 MA 0.21 ) 0.95 Pb(I 0.79 Br 0.21 ) 3 is used as the perovskite layer. The spin-deposited layer is then dried at 100°C for 30 min. The resulting structurally conformal perovskite layer completely (uninterruptedly) covers the underlying texture.

[0040] The perovskite / silicon tandem solar cell is completed on the front side by the structure-conforming application of the functional layers: electron conductor layer made of C 60 (

[60] fullerene) 11, a buffer layer made of tin(IV) oxide SnO 2 12, transparent conductive oxide layer made of indium zinc oxide 13, a contact ring consisting of silver14 and an antireflective lithium fluoride layer LiF 15 in standard procedures.

Claims

1. Method for manufacturing perovskite / silicon tandem solar cells, comprising at least the steps of: - providing a (100)-oriented, monocrystalline silicon wafer (5), - applying a silicon dioxide layer (S) to a first side of the silicon wafer (5), - applying a layer of photoresist (F) to the silicon dioxide layer (S); - introducing a regular structure into the photoresist (F) by nanoimprint lithography with a stamp (P) to form a mask, - Directed reactive ion etching of the silicon dioxide layer (S) through the mask made of the photoresist layer (F) to transfer the structure of the mask into the silicon dioxide layer (S); - Subsequent wet chemical etching of the silicon wafer; - Applying rear functional layers to a second side of the silicon wafer (5) and applying at least the following functional layers: passivation layer, recombination layer, and hole conductor layer (9) to the first side of the silicon wafer; - Subsequently, structure-compliant application of a perovskite layer (10) by spinning a perovskite solution onto the hole conductor layer (9) - Desalting the perovskite solution and subsequent baking, wherein the applied perovskite layer (10) has a layer thickness in the range of 300 nm to 700 nm; - Applying front-side functional layers to complete the perovskite / silicon tandem solar cell, wherein the regular structure is formed from cylindrical holes, wherein the holes have a diameter in the range from 1 µm to 5 µm and the holes are spaced apart from each other in the range from 1.5 µm to 6 µm.