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

Abstract

The invention relates to a lower subcell 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 poly-Si layer stack comprising a plurality of poly-Si layers (3, 4, 5), each enriched with phosphorus and oxygen, wherein the poly-Si layer stack is arranged on a side of the tunnel oxide layer (2) facing away from the substrate (1) and 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 layers (3, 4, 5) differ from one another. The invention further relates to 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 the same. The substrate has a front side and a back side. During operation, the front side is a light-incident side or side of the substrate facing toward the light, while the back side is a side facing away from the light.

[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, Y. [et al.]: 27.6% Perovskite / c-Si Tandem Solar Cells Using Industrially Fabricated TOPCon Device. In: Advanced Energy Materials, Vol. 12, Issue 27, July 21, 2022, https: / / doi.org / 10.1002 / aenm.202200821. The "inverted" TOPCon solar cell is used as a bottom cell of a tandem solar cell. Such a tandem solar cell also exhibits a relatively high efficiency.

[0005] CN 1 16 759 473 A describes a back-contact solar cell comprising a wafer and a tunnel oxide layer on the back, a first doped poly-Si layer, a second doped poly-Si layer, a third doped poly-Si layer, a mask layer, a passivation layer, a doped layer, an electrically conductive layer, and an electrode, as well as a passivation layer and an antireflection layer on the front. The three doped poly-Si layers are doped with P, C, and / or O.

[0006] CN 1 15 863 480 A describes a method for producing a TOPCON solar cell with a tunnel oxide layer, an intrinsic polycrystalline Si layer and a poly-Si layer stack with four to eight poly-Si layers, wherein a doping concentration of the poly-Si layers increases with increasing distance from the intrinsic polycrystalline Si layer and phosphorus is a dopant.

[0007] CN 1 14 512 508 A describes a tandem solar cell with an upper perovskite subcell and a lower TOPCON subcell, the lower subcell having a doped poly-Si layer.

[0008] There is still a need to increase the efficiency of tandem solar cells. In particular, there is a need to more optimally adjust the absorption, electrical conductivity, and FSF (front surface field) of solar cells.

[0009] It is an object of the present invention to provide a lower subcell for a tandem solar cell with improved adjusted absorption, electrical conductivity and FSF of the bottom cell or lower subcell, to provide a tandem solar cell with improved efficiency and a method for producing a solar cell which enables adjustment of the absorption, electrical conductivity and FSF.

[0010] 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 5, and a method having the features of claim 6. Advantageous developments and modifications are specified in the subclaims.

[0011] 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 poly-Si layer stack comprising a plurality of poly-Si layers each enriched with phosphorus and oxygen, wherein the poly-Si layer stack is arranged on a side of the tunnel oxide layer facing away from the substrate and 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 layers differ from one another.

[0012] The multiple poly-Si layers represent poly-Si sublayers of the poly-Si layer stack. Because the poly-Si layers of the poly-Si layer stack have different oxygen and phosphorus concentrations, an advantageous phosphorus and oxygen concentration profile can be created to achieve the highest possible transparency for the red and IR (infrared) spectral range, preferably for wavelengths greater than 600 nm, while also generating the highest possible FSF (front surface field). The phosphorus concentration profile is preferably the concentration profile of the electrically active phosphorus.

[0013] The substrate is preferably a silicon substrate. More preferably, the substrate is an n-type or p-type Si wafer. The backside of the substrate is preferably textured. The n-type silicon substrate preferably has a diffused emitter.

[0014] In a preferred embodiment, the poly-Si layer stack comprises three poly-Si layers stacked one above the other. The poly-Si layer stack preferably comprises a first poly-Si layer arranged on a side of the tunnel oxide layer facing away from the substrate, a second poly-Si layer arranged on a side of the first poly-Si layer facing away from the substrate, and a third poly-Si layer arranged on a side of the second poly-Si layer facing away from the substrate. The first Si layer preferably represents a seed layer during the production of the lower subcell, while the second Si layer serves as a barrier layer for phosphorus diffusion from the third Si layer into the substrate during the production of the lower subcell.

[0015] Preferably, the phosphorus concentration profile is designed such that the third poly-Si layer has a concentration of electrically active phosphorus of at least 5 × 10 +19 cm -3 the second poly-Si 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 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 layer therefore has the highest concentration of electrically active phosphorus compared to the concentrations of electrically active phosphorus in the first and third poly-Si layers, while the interface has a decreasing concentration of electrically active phosphorus from the first poly-Si layer to the substrate, and the substrate has the lowest concentration of electrically active phosphorus. 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 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.

[0016] The inventors have found that by enriching the third Si layer with oxygen to create a poly-Si(n):O layer, absorption in the wavelength range > 600 nm can be reduced. However, this also reduces the electrical conductivity of the third poly-Si layer, since oxygen is a sink for the "active," i.e., electrically conductive, phosphorus. To ensure sufficient electrical conductivity of the third poly-Si layer, the third poly-Si layer preferably has a concentration of the electrically active phosphorus of at least 5 × 10 19 cm -3In addition to the absorption and conductivity of the third poly-Si layer, the doping profile of the electrically active phosphorus at the substrate surface is also important. For an efficient front surface field (FSF), which reduces electron recombination at the interface of the poly-Si layer stack, the strong concentration gradient of the electrically active phosphorus is formed in the form of the kink at the interface of the first poly-Si layer / tunnel oxide layer / substrate or the deep extension at the substrate surface down to a depth of approximately 200 to 500 nm of the substrate.

[0017] In a preferred embodiment, the first and third poly-Si layers are formed as an oxygen-enriched layer. Preferably, the second poly-Si layer is formed as an oxygen-poor layer. When comparing the oxygen concentration of the poly-Si layer stack among the three poly-Si layers, the first and third poly-Si layers preferably have a high oxygen concentration, while the second poly-Si layer has a significantly lower oxygen concentration relative thereto.

[0018] The invention further relates to a tandem solar cell which has an upper sub-cell and a lower sub-cell according to one or more of the embodiments described above. The upper sub-cell is preferably a perovskite sub-cell. For maximum efficiency of the tandem solar cell, it is necessary for the currents in the upper sub-cell and the lower sub-cell to be as high as possible and as equal as possible, since otherwise one sub-cell would limit the overall current, so that so-called “current matching” is necessary. In the case of a lower sub-cell which has a single poly-Si layer instead of the poly-Si layer stack, a portion of the light which is transmitted by the upper sub-cell during operation, in particular wavelengths > approx.700 nm, i.e. in particular the IR range of the solar spectrum, are already absorbed in the poly-Si layer without generating free charge carriers, which significantly reduces the current in the lower sub-cell. The inventors have found that the absorption of the poly-Si(n) can be reduced by enriching the third Si layer with oxygen to create a poly-Si(n):O layer, and that the electrical conductivity of the third poly-Si(n):O layer can be increased by an increased concentration of the electrically active phosphorus. As a result, the lower sub-cell according to the invention can be matched to the upper sub-cell or can be matched during its production. As described above, the efficient front surface field (FSF), which is created by the concentration gradient of the electrically active phosphorus in the form of the kink at the interface of the first poly-Si layer / tunnel oxide layer / substrate orthe deep extension on the surface of the substrate to a depth of approximately 200 to 500 nm of the substrate is advantageous.

[0019] The tunnel oxide layer preferably has a layer thickness of 1 to 2 nm. The first poly-Si layer preferably has a layer thickness of 1 to 2 nm. The second poly-Si layer preferably has a layer thickness in the range of 2 to 10 nm. The third poly-Si layer preferably has a layer thickness in the range of 50 to 150 nm.

[0020] 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, which is formed e.g. from SiOx or AlOx, c) applying an amorphous silicon (a-Si) layer stack comprising several a-Si layers or amorphous Si layers to the tunnel oxide layer, using oxygen and phosphorus as enrichment substances, so that the amorphous silicon is enriched by oxygen and phosphorus, d) annealing the substrate with applied tunnel oxide layer and applied amorphous Si layer stack at a temperature > 800 °C, wherein steps c) and d) are carried out such that the poly-Si layer stack produced by means of step d) with a plurality of poly-Si layers has, following step d), an oxygen concentration profile and a phosphorus concentration profile such that the oxygen concentration and the phosphorus concentration of the plurality of poly-Si layers differ from one another.

[0021] Steps b) and c) are preferably carried out in a tube PECVD system at a temperature of 400 to 450°C. Preferably, steps b) and c) are carried out without interrupting a vacuum.

[0022] Step d) is a tempering step, which is preferably carried out in a tube furnace at a temperature > 800 °C, more preferably at a temperature > 900 °C, e.g. 920 / 930 °C for a time of approx. 15 - 60 min., so that a porous poly-Si or poly-silicon is formed from the silicon, preferably amorphous silicon, preferably with a concentration profile of the electrically active phosphorus as follows: the third poly-Si layer has an electrically active phosphorus concentration or concentration of the electrically active phosphorus of at least 5 × 10 19 cm -3the second poly-Si layer has a higher electrically active phosphorus concentration and an interface between the first poly-Si layer, the tunnel oxide layer and the substrate has an electrically active phosphorus concentration gradient or concentration gradient of the electrically active phosphorus, which has a kink preferably at an electrically active phosphorus concentration of 10 18 cm -3 in the first 100 nm after the interface between the tunnel oxide layer and a surface of the substrate and a tail following the kink to a depth in the range of 200 to 500 nm of the substrate. The electrically active phosphorus concentration in the third poly-Si layer > 5 × 10 19 cm -3 has a positive effect on the FSF and the electrical conductivity, while the electrically active phosphorus concentration in the second poly-Si layer of > 1×10 20 cm -3is also beneficial for optimal FSF.

[0023] In a preferred embodiment, step c) comprises c1) Applying a first amorphous Si layer to the tunnel oxide layer, c2) applying a second amorphous Si layer to the first amorphous Si layer, and c3) Applying a third amorphous Si layer onto the second amorphous Si layer.

[0024] Preferably, step c1) comprises applying the first amorphous Si layer in the form of an amorphous a-Si(n):O seed layer with enrichment with oxygen and enrichment or doping with phosphorus. The produced a-Si(n):O seed layer preferably has a layer thickness of 1 to 2 nm. During step c1), SiH 4 and H 2 as the main process gases, with oxygen and phosphorus being added as enrichment or doping agents. Oxygen can be added, for example, by using O 2, N 2 O or CO 2 or NO 2 in the PECVD process, phosphorus e.g. in the form of PH 3 , if necessary diluted in H 2 The addition of oxygen to the seed layer further increases robustness against layer thickness variations of the plasma oxide, e.g., depending on the substrate's position in a support device such as a graphite boat during PECVD. The first amorphous Si layer is preferably relatively weakly enriched with oxygen, which can be achieved, e.g., via a low N 2 O flow during step c1). Also, the phosphorus enrichment or doping is preferably relatively weak, which is achieved via a low pH 3 -flux is achievable. The terms "strong," "weak," "low," and "high" are to be understood in relation to the first, second, and third Si layers.

[0025] Preferably, step c2) comprises applying the second amorphous Si layer in the form of an oxygen-free a-Si(n) layer while enriching or doping with phosphorus. The second amorphous Si layer is therefore preferably not doped or enriched with oxygen. The enrichment or doping with phosphorus is preferably carried out with a relatively lower pH. 3 -Flux. The oxygen-free a-Si(n) layer is preferably formed with a layer thickness of 2 to 10 nm as a barrier layer for the phosphorus from the third amorphous Si layer in the substrate. Without the second amorphous Si layer, there is a risk of a "shallow" kink or a weak FSF, since too much electrically active phosphorus diffuses from the third amorphous Si layer into the substrate during step d).

[0026] Preferably, step c3) comprises applying the third amorphous Si layer in the form of an a-Si(n):O layer with enrichment or doping with oxygen and phosphorus. A relatively high oxygen enrichment or doping, e.g., via relatively high N 2 O flow or other gas used for oxygen enrichment or doping, which is, for example, 3 to 4 times as high as the flow of the gas used for oxygen enrichment or doping in the production of the first amorphous Si layer. This lowers the refractive index and the absorption coefficient > 600 nm and increases the porosity of the third poly-Si layer, which is obtained following step d). In step c3), a high pH is preferably used. 3 -Flux is used for phosphorus enrichment or doping, which is 4 to 5 times higher than the PH 3-Flux to create the first amorphous Si layer. The enrichment or doping with oxygen binds the phosphorus and reduces the "active," i.e., electrically conductive, phosphorus in the third Si layer. The third Si layer is preferably formed as an a-Si(n):O layer with a layer thickness of 50 to 150 nm. It exhibits reduced absorption and reflection in the IR spectral range.

[0027] The enrichment or doping of the amorphous Si layers during steps c1) to c3) differs from that of the poly-Si layers obtained by step d). Annealing, or annealing, results in a different phosphorus and oxygen concentration profile through diffusion than that originally deposited in steps c1) to c3). Furthermore, in step d), poly-Si layers are formed from the amorphous silicon (i.e., a-Si:H) layers.

[0028] 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.

[0029] Preferably, following step d), an upper subcell is arranged on the poly-Si layer stack. The solar cell is thus formed as a tandem solar cell. The upper subcell can be mechanically stacked on the lower subcell or built monolithically on top of it.

[0030] Preferably, the tunnel oxide layer is applied to the substrate in step b) with a layer thickness of 1 to 2 nm. Preferably, the first amorphous Si layer is applied to the tunnel oxide layer with a layer thickness of 1 to 2 nm. This layer thickness is sufficient for it to function as a seed layer. Preferably, the second amorphous Si layer is applied to the first amorphous Si layer with a layer thickness in the range of 2 to 10 nm. This layer thickness is sufficient for it to function as a barrier layer for the phosphor. Preferably, the third amorphous Si layer is applied to the second amorphous Si layer with a layer thickness in the range of 50 to 150 nm.

[0031] 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 lower subcell according to the invention; Fig. 2 a cross-sectional view of a tandem solar cell according to the invention; Fig. 3 a phosphorus concentration profile of the electrically active phosphorus in Fig. 1 shown lower subcell depending on its layer structure; Fig. 4 an oxygen concentration profile of the Fig. 1 shown lower subcell.

[0032] Fig. 1 shows a cross-sectional view of a lower subcell according to the invention for a tandem solar cell. The subcell has 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 shown by arrows, while the back side 14 represents a side facing away from the light. Furthermore, it has a tunnel oxide layer 2 arranged on the front side 12 of the substrate 1, and a poly-Si layer stack comprising a plurality of poly-Si layers 3, 4, 5 arranged on a side of the tunnel oxide layer 2 facing away from the substrate 1.

[0033] The poly-Si layer stack has the following layer structure: a first poly-Si layer 3, which is arranged on a side of the tunnel oxide layer 2 facing away from the substrate 1, a second poly-Si layer 4, which is arranged on a side of the first poly-Si layer 3 facing away from the substrate 1, and a third poly-Si layer 5, which is arranged on a side of the second poly-Si layer 4 facing away from the substrate 1. The poly-Si layer stack has an oxygen doping profile and a phosphorus doping profile 7, such that the oxygen content and the phosphorus content of the plurality of poly-Si layers 3, 4, 5 differ from one another. The phosphorus doping profile 7 extends through the poly-Si layer stack, the tunnel oxide layer 2, into a surface of the substrate.

[0034] On the back side 14 of the substrate 1, a passivation layer 6 is also arranged, e.g. made of AlO x (aluminum oxide) and SiN x (silicon nitride).

[0035] Other layers required for the function of the lower subcell, such as an upper subcell and metallization, are omitted.

[0036] Fig. Figure 2 shows a cross-sectional view of a tandem solar cell according to the invention. Fig. 2 tandem solar cell has an upper subcell 10 and a lower subcell 11, between which an intermediate layer 9, e.g., an ITO (indium tin oxide) layer, is arranged. The upper subcell 10 can be designed as a perovskite solar cell with local contacts 8. The lower subcell 11 corresponds to the Fig. 1 shown lower subcell with the difference that it still has local contacts 8.

[0037] Fig. 3 shows a concentration profile of the electrically active phosphorus in Fig. 1 shown lower subcell depending on its layer structure. The third poly-Si layer 5 has an electrically active phosphorus concentration above 5 × 10 19 cm -3 The second poly-Si layer 4 has a higher electrically active phosphorus concentration of 2 × 10 20 cm -3 An interface between the first poly-Si layer 3, the tunnel oxide layer 2, and the substrate 1 exhibits an electrically active phosphorus concentration gradient, which includes a kink 15 and an adjoining extension 16 extending to a depth in the range of 200 to 500 nm of the substrate 1. In the second poly-Si layer 4, the electrically active phosphorus concentration forms a peak 17.

[0038] Fig. 4 shows an oxygen concentration profile of the Fig.1. From the oxygen concentration profile along the lower subcell, it can be seen that the third poly-Si layer 5 is formed as an oxygen-enriched layer, the second poly-Si layer 4 is formed as an oxygen-poor layer, the first poly-Si layer 3 and the tunnel oxide layer 2 are formed as oxygen-enriched layers, and the oxygen concentration in the substrate 1 is low. List of reference symbols: 1 substrate 2 Tunnel oxide layer 3,4,5 each Si sublayer 6 Passivation layer 7 Phosphorus profile 8 Contact 9 Intermediate layer 10 upper subcell 11 lower subcell 12 Front 13 Sun 14 Back 15 bend 16 foothills 17 Peak

Claims

[1] Lower subcell 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 poly-Si layer stack comprising a plurality of poly-Si layers (3, 4, 5), each enriched with phosphorus and oxygen, wherein the poly-Si layer stack is arranged on a side of the tunnel oxide layer (2) facing away from the substrate (1) and 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 layers (3, 4, 5) differ from one another. [2] Subcell according to claim 1, wherein the poly-Si layer stack comprises a first poly-Si layer (3) arranged on a side of the tunnel oxide layer (2) facing away from the substrate (1), a second poly-Si layer (4) arranged on a side of the first poly-Si layer (3) facing away from the substrate (1) and a third poly-Si layer (5) arranged on a side of the second poly-Si layer (4) facing away from the substrate (1). [3] Partial cell according to claim 1 or 2, wherein the phosphorus concentration profile is designed such that the third poly-Si layer (5) has an electrically active phosphorus concentration of at least 5 × 10 19 cm -3the second poly-Si layer (4) has a higher electrically active phosphorus concentration and an interface at the first poly-Si layer (3), the tunnel oxide layer (2) and the substrate (1) has an electrically active phosphorus concentration gradient which has a kink (15) and an adjoining extension (16) to a depth in the range of 200 to 500 nm of the substrate (1). [4] Subcell according to one of the preceding claims, wherein the first poly-Si layer (3) is formed as an oxygen-enriched layer, the second poly-Si layer (4) is formed as an oxygen-poor layer and the third poly-Si layer (5) is formed as an oxygen-enriched layer. [5] Tandem solar cell, comprising an upper subcell (10) and a lower subcell according to one of the preceding claims, wherein the upper subcell (10) is preferably an upper perovskite subcell. [6] A method for producing a solar cell, comprising a) providing a substrate (1) having a front side (12) and a back side (14), b) applying a tunnel oxide layer (2) on the front side (12) of the substrate (1), c) applying an amorphous Si layer stack comprising several amorphous Si layers (3, 4, 5) to the tunnel oxide layer (2), using oxygen and phosphorus as enrichment substances, d) annealing the substrate (1) with applied tunnel oxide layer (2) and applied amorphous Si layer stack at a temperature > 800 °C, wherein steps c) and d) are carried out such that a poly-Si layer stack produced from the amorphous Si layer stack by means of step d) and comprising a plurality of poly-Si layers (3, 4, 5) has, following step d), an oxygen concentration profile and a phosphorus concentration profile such that the oxygen concentration and the phosphorus concentration of the plurality of poly-Si layers (3, 4, 5) differ from one another. [7] The method of claim 6, wherein step c) comprises c1) applying a first amorphous Si layer (3) to the tunnel oxide layer (2), c2) applying a second amorphous Si layer (4) to the first amorphous Si layer (3), and c3) Applying a third amorphous Si layer (5) onto the second amorphous Si layer (4). [8] Method according to claim 7, wherein step c1) comprises applying the first amorphous Si layer (3) in the form of an a-Si(n):O seed layer with enrichment or doping with oxygen and phosphorus, step c2) comprises applying the second amorphous Si layer (4) in the form of an oxygen-free a-Si(n) layer with enrichment or doping with phosphorus, and / or step c3) comprises applying the third amorphous Si layer (5) in the form of an a-Si(n):O layer with enrichment or doping with oxygen and phosphorus. [9] A method according to any one of claims 6 to 8, wherein step b) is carried out by means of plasma oxidation. [10] Method according to one of claims 6 to 9, wherein following step d) an upper subcell (10) is arranged on the poly-Si layer stack. [11] Subcell according to one of claims 1 to 4 or method according to one of claims 6 to 9, wherein the tunnel oxide layer (2) has a layer thickness of 1 to 2 nm, the first poly-Si layer (3) or the first amorphous Si layer (3) has a layer thickness of 1 to 2 nm, the second poly-Si layer (4) or the second amorphous Si layer (4) has a layer thickness in the range of 2 to 10 nm and / or the third poly-poly-Si layer (5) or the third amorphous Si layer (5) has a layer thickness in the range of 50 to 150 nm.

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