Lower subcell, tandem solar cell and method for producing a solar cell

Abstract

The invention relates to a lower sub-cell (11) for a tandem solar cell with - a substrate (1) having a front side (12) and a back side (14), - a tunnel oxide layer (2) arranged on the front side (12) of the substrate (1), - a compact poly-Si(n):O layer (7) arranged on a side of the tunnel oxide layer (2) facing away from the substrate (1), and - a porous poly-Si(n):O layer (6) arranged on a side of the compact poly-Si(n):O layer (7) facing away from the tunnel oxide layer (2). Furthermore, the invention relates to a tandem solar cell comprising an upper sub-cell (10) and the lower sub-cell (11), as well as a method for producing a solar cell.

Description

[0001] The invention relates to a lower subcell for a tandem solar cell, a tandem solar cell, and a method for producing a solar cell. In particular, the invention relates to a lower subcell comprising a substrate, a tunnel oxide layer, and a poly-Si layer, a tandem solar cell containing the lower subcell, and a method for producing a solar cell. The substrate has a front side and a back side. During operation, the front side is a side of the substrate facing the light, while the back side is a side facing away from the light, and edges extend between the front side and the back side.

[0002] If the tunnel oxide layer is arranged on the back of the substrate, and the poly-Si layer is arranged on a side of the tunnel oxide layer facing away from the substrate, the solar cell is called a TOPCon (Tunnel Oxide Passivated Contact) solar cell. A TOPCon solar cell exhibits a relatively high efficiency.

[0003] If the tunnel oxide layer is arranged on the front side of the substrate and the poly-Si layer is arranged on a side of the tunnel oxide layer facing away from the substrate, the cell is an “inverted” TOPCon solar cell.

[0004] The use of such an "inverted" TOPCon solar cell is described, for example, by WU, Yiliang [et al.]: 27.6% perovskite / c-Si tandem solar cells using industrially fabricated TOPCon device. In: Advanced energy materials, Vol. 12, 2022, No. 27, Art.-No. 2200821, 9 pp. - ISSN 1614-6832. The "inverted" TOPCon solar cell is used as a bottom cell of a tandem solar cell. Such a tandem solar cell exhibits a relatively high efficiency.

[0005] US 2021 / 0 273 127 A1 describes a tandem solar cell containing, in order of incident light: an outer anti-reflection layer, a transparent conductive oxide layer, a hole transport layer, a perovskite layer, a p + -porous silicon containing electron transport layer and a back contact.

[0006] US 2018 / 0 175 112 A1 describes a tandem solar cell containing an upper subcell with a perovskite layer without open porosity and a lower subcell with a silicon heterojunction.

[0007] KULESZA-MATLAK, Grazyna [et al.]: Interlayer microstructure analysis of the transition zone in the silicon / perovskite tandem solar cell. In: Energies, Vol. 14, 2021, No. 20, Art. No. 6819, 15 pp. - ISSN 1996-1073 describe a tandem solar cell containing an upper perovskite subcell and a lower silicon subcell, with a transition region between these two subcells containing porous silicon in the form of chemically etched wires.

[0008] LUDERER, Christoph [et al.]: PERC-like Si bottom solar cells for industrial perovskite-Si tandem solar cells. In: AIP conference proceedings, Vol. 2487, 2022, No. 1, Art. No. 020012, 8 pp. - ISSN 0094-243X describe a tandem solar cell containing an upper perovskite subcell and a lower silicon subcell, with a tunnel oxide and poly-Si-based passivating front contact being used as a replacement for an emitter of a PERC (Passivated Emitter and Rear Cell) cell as the lower silicon subcell.

[0009] However, there is still a need to improve the reflection and absorption properties of solar cells.

[0010] It is an object of the present invention to provide a lower subcell for a tandem solar cell with improved reflection properties, a tandem solar cell with improved reflection properties, and a method for producing a solar cell with improved reflection properties. At the same time, they should be cost-effective.

[0011] According to the invention, this object is achieved by a lower subcell having the features of claim 1, a tandem solar cell having the features of claim 7, and a method having the features of claim 8. Advantageous developments and modifications are specified in the subclaims.

[0012] A lower subcell with a porous, phosphorus- and oxygen-enriched polysilicon layer (poly-Si(n):O) exhibits advantageous reflection and absorption properties, particularly reduced reflection, compared to a corresponding lower subcell that does not have the porous poly-Si(n):O layer. This eliminates the need for an anti-reflection layer on the front side. In addition, the improved absorption results in an increased short-circuit current Jsc. The porous poly-Si(n):O layer exhibits significantly reduced reflection across the entire wavelength range from 280 to 1000 nm, particularly in the range from 280 to 600 nm. This allows more light to be coupled into the lower subcell, resulting in a higher Jsc of +1 mA / cm 2 reflects.

[0013] When creating the compact poly-Si(n):O layer, it is not only deposited on the front side, but also on the edges and back side (so-called wrap-around) by bending the substrate during the application of the layer using PECVD (plasma-enhanced chemical vapor deposition) or CVD (chemical vapor deposition) processes. Once the compact poly-Si(n):O layer has been applied to the tunnel oxide layer, it must be removed from the back side and edges. For this purpose, the substrate and the applied layers are first immersed in a KOH bath and then in an HF bath. While the compact poly-Si(n):O layer is removed from the back side and edges, the porous poly-Si(n):O layer is created on the front side at the same time. This means that there are no additional costs for creating the porous poly-Si(n):O layer.

[0014] Without wishing to be bound by any theory, it is assumed that enrichment of the compact poly-Si(n) layer with oxygen promotes the formation of a porous layer in the process according to the invention.

[0015] The invention relates to a lower subcell for a tandem solar cell with - a substrate with a front side and a back side, - a tunnel oxide layer arranged on the front side of the substrate, - a compact poly-Si(n):O layer arranged on a side of the tunnel oxide layer facing away from the substrate, and - a porous poly-Si(n):O layer arranged on a side of the compact poly-Si(n):O layer facing away from the tunnel oxide layer.

[0016] In a preferred embodiment, the total porosity of the porous poly-Si(n):O layer is 30 to 90%. This particularly means that the porous poly-Si(n):O layer is formed from 30 to 90 vol.% air and 70 to 10 vol.% poly-Si(n):O. More preferably, the porous poly-Si(n):O layer contains 50 to 90 vol.% air and 50 to 10 vol.% poly-Si(n):O, i.e., the total porosity of the porous poly-Si(n):O layer is 50 to 90%. Even more preferably, the porous poly-Si(n):O layer contains 70 to 90 vol.% air and 30 to 10 vol.% poly-Si(n):O, i.e., the total porosity of the porous poly-Si(n):O layer is 70 to 90%. The compact poly-Si(n):O layer is a poly-Si(n):O layer that contains no or essentially no air or up to 5 vol% air.

[0017] Porosity can be determined indirectly using optical methods for characterizing thin films, such as ellipsometry or reflection measurements. The lower the refractive index n of the porous layer compared to the refractive index of a compact poly-Si(n):O layer, which is preferably 2.9–3.1 at 632 nm, the higher the porosity. Total porosity includes open and / or closed porosity.

[0018] Preferably, the porous poly-Si(n):O layer has a layer thickness in the range of 20 to 40 nm. This layer thickness is advantageous for influencing the reflection within a satisfactory range.

[0019] Preferably, the compact poly-Si(n):O layer has a layer thickness in the range of 40 to 80 nm. Preferably, the tunnel oxide layer has a layer thickness of 1 to 2 nm. These layer thicknesses also provide a lower subcell with a satisfactory efficiency.

[0020] In a preferred embodiment, the porous poly-Si(n):O layer has a refractive index of <2.9 at a wavelength of 632 nm. Preferably, the compact poly-Si(n):O layer has a refractive index of 2.9 to 3.1 at a wavelength of 632 nm. This gradual difference in the refractive indices further achieves improved reflection properties.

[0021] In a preferred embodiment, the compact poly-Si(n):O layer is formed as a poly-Si(n) layer stack comprising a plurality of poly-Si(n) layers, each enriched with phosphorus and oxygen. The poly-Si layers are sublayers of the compact poly-Si(n):O layer. Preferably, the poly-Si(n) layer stack has an oxygen concentration profile and a phosphorus concentration profile, such that the oxygen concentration and the phosphorus concentration of the plurality of poly-Si(n) layers differ from one another.The poly-Si(n) layer stack preferably comprises a first poly-Si(n) layer arranged on a side of the tunnel oxide layer facing away from the substrate and constituting a seed layer, a second poly-Si(n) layer arranged on a side of the first poly-Si(n) layer facing away from the substrate and constituting an intermediate layer, and a third poly-Si(n) layer arranged on a side of the second poly-Si(n) layer facing away from the substrate and constituting a main layer. In a preferred embodiment, the first and third poly-Si(n) layers are formed as an oxygen-enriched layer. The second poly-Si(n) layer is preferably formed as an oxygen-poor layer.In a comparison of the oxygen concentration of the poly-Si(n) layer stack between the three poly-Si(n) layers, the first and third poly-Si(n) layers preferably have a high oxygen concentration, while the second poly-Si(n) layer has a significantly lower oxygen concentration relative thereto.

[0022] Preferably, the phosphorus concentration profile is designed such that the third poly-Si(n) layer has a concentration of an electrically active phosphorus of at least 5 × 10 +19 cm -3 the second poly-Si(n) layer has a higher concentration of electrically active phosphorus, preferably at least 1 × 10 +20 cm -3, and an interface at the first poly-Si layer, the tunnel oxide layer and the substrate has a concentration gradient of the electrically active phosphorus, which has a kink and a subsequent tail down to a depth in the range of 200 to 500 nm of the substrate. The kink represents a rapid drop in the concentration of the electrically active phosphorus at the tunnel oxide barrier. The concentration of the electrically active phosphorus is preferably formed such that the second poly-Si(n) layer has a concentration of the electrically active phosphorus that is formed as a peak in the electrically active phosphorus doping profile.The second poly-Si(n) layer therefore has the highest concentration of electrically active phosphorus compared to the concentrations of electrically active phosphorus of the first and third poly-Si(n) layers, while the interface has a decreasing concentration of electrically active phosphorus from the first poly-Si(n) layer to the substrate and the substrate has the lowest concentration of electrically active phosphorus.

[0023] For the purposes of the invention, the electrically active phosphorus is phosphorus that contributes to the conductivity of the respective layer in which it is contained. In addition to the electrically active phosphorus, the poly-Si(n) layers can contain electrically inactive phosphorus that does not contribute to the conductivity of the respective layer. ToF SiMS (time-of-flight secondary ion mass spectroscopy) measurements combined with ECV (electrochemical capacitance-voltage) measurements, which only detect the electrically active phosphorus, allow the concentrations of the electrically active phosphorus and the electrically inactive phosphorus to be determined.

[0024] In a preferred embodiment, pores of the porous Si(n):O layer are at least partially or completely filled with TCO (transparent electrically conductive oxides). The porous Si(n):O layer can contain 30 to 90 vol. % TCO. TCO is preferably selected from the group consisting of ITO (indium tin oxide), AZO (aluminum-doped zinc oxide) or ZnO (zinc oxide). If the pores are at least partially filled with TCO, a TCO layer is preferably arranged on a side of the porous Si(n):O layer facing away from the tunnel oxide layer, which acts as a TCO intermediate layer to an upper sub-cell in a tandem solar cell. This preferably has a layer thickness of 5 to 30 nm and / or a refractive index of approximately 1.8 at a wavelength of 632 nm. With the application of the TCO layer, the open pores orHoles of the porous Si(n):O layer are at least partially filled with TCO, whereby TCO penetrates into or fills the open pores of the porous Si(n):O layer during the deposition of the TCO layer onto the porous Si(n):O layer.

[0025] The invention further relates to a tandem solar cell, comprising an upper subcell and the lower subcell according to one or more of the previously described embodiments. The upper subcell is preferably formed as an upper perovskite subcell. An intermediate layer, e.g. a TCO layer such as an ITO, AZO or ZnO layer, is preferably arranged between the upper subcell and the lower subcell. This intermediate layer preferably has a layer thickness of 5 to 30 nm. It preferably has a refractive index of approximately 1.8 at a wavelength of 632 nm.

[0026] The invention further relates to a method for producing a solar cell, comprising: a) providing a substrate having a front side and a back side, b) Applying a tunnel oxide layer on the front side of the substrate, c) Applying an amorphous, phosphorus and oxygen-enriched silicon layer (a-Si(n):O) layer onto the tunnel oxide layer, d) annealing the substrate at a temperature > 800 °C so that the deposited a-Si(n):O layer is converted into a compact poly-Si(n):O layer, e) Immerse the substrate in a KOH bath, and then f) Immersing the substrate in an HF bath.

[0027] The substrate is of course subjected to steps d), e) and f) together with the layers applied in steps b) and c).

[0028] During steps e) and f), the edge and backside wraparound of the compact poly-Si(n):O layer is also removed. The compact poly-Si(n):O layer must be removed at the edge and backside after its deposition, as otherwise shunts to the emitter will occur. Steps e) and f) are preferably carried out in a batch process. The batch process is lower in cost than an in-line process. In an inline process, several substrates are usually moved through the KOH bath and the HF bath using transport rollers. Particularly if only the backside is immersed in the baths, the front side of the treated substrate is protected by a resulting "water cap" and is therefore not treated.

[0029] In a preferred embodiment, the KOH (potassium hydroxide) bath has a concentration of 3-4%. This concentration is sufficient to achieve the desired effects. Preferably, the KOH bath contains a polishing additive. In a preferred embodiment, step e) is carried out for a period of 100 to 150 seconds. This period is sufficient to achieve the desired effects.

[0030] The HF bath (hydrofluoric acid) preferably has a concentration of 1 to 25%, preferably 15 to 22%. This concentration is sufficient to achieve the desired effects. Step f) is preferably carried out for a period of up to 10 minutes, preferably 40 to 60 seconds. This period is sufficient to achieve the desired effects. The duration depends on the HF concentration. The lower the HF concentration, the longer the duration.

[0031] In a preferred embodiment, step c) comprises applying an amorphous Si layer stack comprising a plurality of amorphous Si sublayers to the tunnel oxide layer, wherein oxygen and phosphorus are used as enrichment substances.

[0032] In a preferred embodiment, step b) is carried out by plasma oxidation. The tunnel oxide layer is therefore preferably SiO x -layer. Alternatively, a tunnel oxide layer made of aluminum oxide (AlOx) can also be formed.

[0033] Steps b) and c) can be performed using PECVD (plasma-enhanced chemical vapor deposition) or LPCVD (low-pressure chemical vapor deposition). Preferably, steps b) and c) are performed using PECVD in a single step, preferably without vacuum interruption, in a tubular PECVD system.

[0034] A method for manufacturing further comprises the following steps: First, a substrate such as a Si wafer, e.g., an n-type Si wafer, is provided, which is subjected to texturing. Then, an emitter diffusion such as a boron diffusion with BBr 3 or BCl 3 carried out. Subsequently, a borosilicate glass etching on the back, chemical edge isolation, and polishing of the front are carried out. A substrate produced in this way is preferably provided in step a). Following step f), a passivation of the back, e.g., with AlO x (aluminum oxide) and / or SiN x (silicon nitride).

[0035] Subsequently, an LCO (laser contact opening) process can optionally be performed on the rear side to form holes in the produced passivation layer. Preferably, after removing the edge wrap or the LCO process, a full or partial metallization of the rear side is performed. Subsequently, an intermediate layer, e.g., a TCO layer, preferably an ITO layer, is preferably applied to the front side. Then, the upper subcell, which is preferably formed as a perovskite subcell, is preferably applied to the intermediate layer in one or more steps.

[0036] Further properties and advantages of the solar cell according to the invention are explained in more detail in the context of the preferred embodiments described below. It shows schematically and not to scale: Fig. 1 a cross-sectional view of a tandem solar cell according to the invention with a lower subcell according to the invention; and Fig. 2 a flowchart of a method according to the invention.

[0037] Fig. Figure 1 shows a cross-sectional view of a tandem solar cell according to the invention with a lower subcell according to the invention. The tandem solar cell has an upper subcell 10 and the lower subcell 11 according to the invention.

[0038] The lower subcell 11 comprises a substrate 1 with a front side 12 and a back side 14. The front side 12 is a light incidence side, which during operation faces light emitted by the sun 13, as indicated by arrows, while the back side 14 represents a side facing away from the light. Edges (not shown) extend between the front side 12 and the back side 14. It further comprises a tunnel oxide layer 2 arranged on the front side 12 of the substrate 1, and a compact poly-Si(n):O layer 7 consisting of several poly-Si(n) sublayers 3, 4, 5. The compact poly-Si(n):O layer 7 is arranged on a side of the tunnel oxide layer 2 facing away from the substrate 1.

[0039] The compact poly-Si(n):O layer 7 has the following layer structure: a first poly-Si(n) layer 3, which is arranged on a side of the tunnel oxide layer 2 facing away from the substrate 1, a second poly-Si(n) layer 4, which is arranged on a side of the first poly-Si(n) layer 3 facing away from the substrate 1, and a third poly-Si(n) layer 5, which is arranged on a side of the second poly-Si(n) layer 4 facing away from the substrate 1. The compact poly-Si(n) layer 7 has an oxygen doping or concentration profile and a phosphorus doping or concentration profile 7, so that the oxygen content and the phosphorus content of the plurality of poly-Si(n) layers 3, 4, 5 differ from one another.

[0040] The lower subcell 11 further comprises a porous poly-Si(n):O layer 6, which is arranged on a side of the compact poly-Si(n):O layer 7 facing away from the tunnel oxide layer 2.

[0041] On the back side 14 of the substrate 1, a passivation layer 15 is also arranged, e.g. made of AlO x (aluminum oxide) and SiN x (silicon nitride).

[0042] An intermediate layer 9, e.g., a TCO layer such as an ITO layer, is arranged between the upper subcell 10 and the lower subcell 11. A material of the intermediate layer 9, such as ITO, can penetrate the porous structure of the porous poly-Si(n):O layer 6. This means that open pores or holes in the porous structure, which are filled with air before the application of the intermediate layer 9, can also be at least partially filled with the material of the intermediate layer 9, such as ITO, in the finished tandem cell.

[0043] The upper subcell 10 can be designed as a perovskite solar cell. The upper subcell 10 and the lower subcell 11 also have local electrical contacts 8.

[0044] Fig.2 shows a flow diagram of a method according to the invention. The method for producing a solar cell comprises the following steps. A first step comprises providing 20 a substrate having a front side and a back side. Providing 20 is followed by applying 21 a tunnel oxide layer to the front side of the substrate. Applying 21 is followed by applying 22 an amorphous Si:O layer. The amorphous Si:O layer can be applied in the application 22 as a poly-Si(n) layer stack consisting of three amorphous Si layers with different oxygen and phosphorus concentrations. Applying 22 is followed by annealing 23 the substrate with applied tunnel oxide layer and applied amorphous Si:O layer at a temperature > 800 °C, such that the applied amorphous Si:O layer is converted into a compact poly-Si(n):O layer.

[0045] Annealing 23 is followed by immersion 24 of the substrate with applied tunnel oxide layer and the compact poly-Si(n):O layer in a KOH bath, preferably with a polishing additive, and immersion 25 in an HF bath. This forms a porous poly-Si(n):O layer, which is arranged on a side of the compact poly-Si(n):O layer facing away from the tunnel oxide layer and is formed therefrom. Furthermore, step 25 can be followed by a step 26 in which an intermediate layer is applied to a side of the porous poly-Si(n):O layer facing away from the tunnel oxide layer, wherein a material of the intermediate layer, e.g., ITO, penetrates into open pores or holes in the porous layer and / or fills these holes. List of reference symbols: 1 substrate 2 Tunnel oxide layer 3,4,5 each poly-Si(n) layer 6 porous poly-Si(n):O layer 7 compact poly-Si(n):O layer 8 Contact 9 Intermediate layer 10 upper subcell 11 lower subcell 12 Front 13 Sun 14 Back 15 Passivation layer 20 Provision 21 Application 22 Application 23 Tempering 24 Immersion 25 Immersion

Claims

[1] Lower subcell (11) for a tandem solar cell with - a substrate (1) having a front side (12) and a back side (14), - a tunnel oxide layer (2) arranged on the front side (12) of the substrate (1), - a compact poly-Si(n):O layer (7) arranged on a side of the tunnel oxide layer (2) facing away from the substrate (1), and - a porous poly-Si(n):O layer (6) arranged on a side of the compact poly-Si(n):O layer (7) facing away from the tunnel oxide layer (2). [2] The lower subcell (11) according to claim 1, wherein a total porosity of the porous Si(n):O layer (6) is 30 to 90%. [3] Lower subcell (11) according to claim 1 or 2, wherein the porous Si(n):O layer (6) has a layer thickness in the range of 20 to 40 nm, the compact poly-Si(n):O layer (7) has a layer thickness in the range of 40 to 80 nm and / or the tunnel oxide layer (2) has a layer thickness of 1 to 2 nm. [4] Lower subcell (11) according to one of the preceding claims, wherein the porous poly-Si(n):O layer (6) has a refractive index < 2.9 at a wavelength of 632 nm and / or the compact poly-Si(n):O layer (7) has a refractive index of 2.9 to 3.1 at a wavelength of 632 nm. [5] Lower subcell (11) according to one of the preceding claims, wherein the compact poly-Si(n):O layer (7) is formed as a poly-Si(n) layer stack from a plurality of poly-Si(n) layers (3, 4, 5) which are each enriched with phosphorus and oxygen, wherein preferably the poly-Si(n) layer stack has an oxygen concentration profile and a phosphorus concentration profile such that the oxygen concentration and the phosphorus concentration of the plurality of poly-Si(n) layers (3, 4, 5) differ from one another. [6] Lower subcell (11) according to one of the preceding claims, wherein pores of the porous Si(n):O layer (6) are at least partially filled with TCO, wherein TCO is preferably selected from ITO, AZO or ZnO. [7] Tandem solar cell, comprising an upper subcell (10), which is preferably designed as an upper perovskite subcell, and the lower subcell (11) according to one of the preceding claims. [8] A method for producing a solar cell, comprising a) providing (20) a substrate (1) having a front side (12) and a back side (14), b) applying (21) a tunnel oxide layer (2) on the front side (12) of the substrate (1), c) applying (22) an amorphous Si(n):O layer onto the tunnel oxide layer (2), d) annealing (23) the substrate (1) at a temperature > 800 °C, so that the applied amorphous Si(n):O layer is converted into a compact poly-Si(n):O layer (7), e) immersing (24) the substrate (1) in a KOH bath, and then f) Immersing (25) the substrate (1) in an HF bath. [9] Method according to claim 8, wherein the KOH bath has a concentration of 3-4% and / or the HF bath has a concentration of 1 to 25%, preferably 15 to 22%. [10] Method according to claim 8 or 9, wherein step e) is carried out for a period of 100 to 150 seconds and / or step f) is carried out for a period of up to 10 minutes, preferably 40 to 60 seconds. [11] Method according to one of claims 8 to 10, wherein step c) comprises applying an amorphous Si layer stack of a plurality of amorphous Si layers (3, 4, 5) to the tunnel oxide layer (2) as the amorphous Si(n):O layer, wherein oxygen and phosphorus are used as enrichment substances.

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