Stacked multi-junction solar cell

Abstract

Stacked multi-junction solar cell (S) comprising - at least one first subcell (SC1) with a first band gap and a first lattice constant - a second subcell (SC2) arranged above the first subcell (SC1) with a second band gap and a second lattice constant, - a first tunnel diode (TD1) arranged between the first subcell (SC1) and the second subcell (SC2) with an n + -layer and a p + -layer, where - the second band gap is larger than the first band gap and - the p + -layer of the first tunnel diode (TD1) comprises an As-containing III-V material, wherein - the n + -layer of the first tunnel diode (TD1) InGaP with a Ga content above 20% based on the Ga / In ratio of the n +-layer, and to suppress unwanted incorporation of an n-dopant into the p+ layer of the tunnel diode during deposition, - between the n + -layer and the p + -layer an intermediate layer (ZW) is arranged, - the intermediate layer (ZW) is always thinner than the n + -layer and as the p + -layer is, - the intermediate layer (ZW) has a thickness of less than 6 nm or less than 4 nm, and - the intermediate layer (ZW) comprises As, wherein the As content of the intermediate layer (ZW) is higher than an As content of the n + -layer, and - the intermediate layer (ZW) is formed directly adjacent to the p+ layer and directly adjacent to the n+ layer, - the intermediate layer (ZW) with silicon with a dopant concentration of at least 10 18 N / cm 3 is endowed.

Description

[0001] The invention relates to a stacked multi-junction solar cell.

[0002] A monolithic solar cell stack is known from W. Guter et al, “Investigation and development of III-V-triple-junction concentrator solar cells”, in 22nd European Photovoltaic Solar Energy Conference, 3-7 September 2007, Milan Italy, pp. 122-125.

[0003] EP 2 251 912 A1 discloses a stacked multi-junction solar cell with improved current conduction between the solar cells. For this purpose, a tunnel diode with mutually strained degenerate layers is arranged between two consecutive solar cells.

[0004] A tensile strained degenerate layer is compensated with a compressively strained degenerate layer. The degenerate layers are either carbon-doped degenerate p+ layers or tellurium- or silicon-doped degenerate n+ layers. By compensating the strain, no strain acts on the layers surrounding the tunnel diode; in other words, the total strain is zero.

[0005] US 2010 / 0 319 764 A1 discloses a multi-junction solar cell with a nitride-containing subcell, in which the tunneling current of the multi-junction solar cell is increased through heat treatment. Negative effects of the heat treatment on the tunnel diodes are prevented by the tunnel diodes comprising a layer sequence of several n- and p-doped layers and an ErAs intermediate layer.

[0006] In KANG, S. [et al]: Numerical analysis of p-GaAs / n-GaAs tunnel junction employing InAs intermediate layer for high concentrated photovoltaic applications. In: Journal of Physics: Conference Series, 490, 2014, 012178. - ISSN 1742-6596, research results on a double-junction solar cell employing a GaAs tunnel diode with a thin InAs intermediate layer are presented.

[0007] Various multi-junction solar cells are known from US 2017 / 0 222 066 A1 as well as from EP 1 134 813 A2 or US 2012 / 0 227 797 A1.

[0008] Further III-V multiple cell arrangements with tunnel diodes are known from US 2017 / 0 077 340 A1 and WO 2011 / 042 682 A2.

[0009] Against this background, the object of the invention is to provide a device that further develops the state of the art.

[0010] The object is achieved by a multi-junction solar cell having the features of patent claim 1. Advantageous embodiments of the invention are the subject of subclaims.

[0011] According to the subject matter of the invention, a stacked multi-junction solar cell is provided, comprising at least a first sub-cell, a second sub-cell and at least one first tunnel diode.

[0012] The first subcell has a first band gap and a first lattice constant.

[0013] The second subcell is arranged above the first subcell and has a second band gap and a second lattice constant, wherein the second band gap is larger than the first band gap.

[0014] A first tunnel diode is formed between the first subcell and the second subcell. The first tunnel diode has an n + -layer and a p + -layer.

[0015] The n +-layer of the first tunnel diode comprises InGaP, with the Ga content above 20% based on the Ga / In ratio of the n + -layer.

[0016] The p + -Layer of the first tunnel diode comprises an As-containing III-V material.

[0017] Between the n + -layer and the p + -layer is arranged an intermediate layer, whereby the intermediate layer is thinner than the n + -layer and as the p + -layer.

[0018] It should also be understood that the term "subcell" is used synonymously with the term "solar cell component." In other words, the solar cell stack consists of several solar cell components connected in series via tunnel diodes.

[0019] It should be noted that the different layers may also comprise additional elements in addition to the specified materials, for example InGaP or As-containing III-V material.

[0020] It should also be noted that the intermediate layer is formed as a single piece and is produced epitaxially using MOVPE. It goes without saying that a tunnel diode is formed between each of the two subcells.

[0021] If layers consist of a given material composition, it is understood that in addition to the stated material composition, dopants such as zinc or carbon or silicon are also included, without the dopants being explicitly mentioned.

[0022] It is understood that the multi-junction solar cell is designed as an upright grown multi-junction solar cell or as an inverted grown multi-junction solar cell.

[0023] Furthermore, it should be noted that the stacked multi-junction solar cell preferably consists predominantly or entirely of subcells made of a III-V material. In one embodiment, the multi-junction solar cell is completely monolithic. In another embodiment, the multi-junction solar cell comprises a wafer bond, in particular to form stacked subcells with different lattice constants.

[0024] One advantage of the intermediate layer according to the invention is that it reduces the undesirable incorporation of an n-type dopant into the p+ layer of the tunnel diode. This suppresses cross-contamination during deposition.

[0025] Surprisingly, it was found that the unwanted incorporation of the n-type dopant can be suppressed more effectively using As-containing layers than with phosphide-containing layers. Experts have previously mistakenly assumed that As-containing layers, especially GaAs, are unsuitable due to absorption losses. These absorption losses reduce the efficiency of the multi-junction cell by reducing the light intensity of the underlying subcells.

[0026] However, the disturbing absorption can be suppressed by making the GaAs layer very thin, ie a few nm thick, so that there are no states or absorbing levels in the quantum wells of the As-containing intermediate layer.

[0027] This makes it possible to achieve a high transparency of the tunnel diode for the sub-cells located below the tunnel diode and at the same time to provide a tunnel diode with a high peak current density and to increase the efficiency of the multi-junction solar cell.

[0028] In another embodiment, the lattice constant of the second subcell is greater than the lattice constant of the first subcell, ie, the second subcell is not lattice-matched to the first subcell. To reduce stresses between the two subcells, a metamorphic buffer is arranged between the first subcell and the second subcell. The first tunnel diode is arranged between the metamorphic buffer and the second subcell. Preferably, the n+ layer of the first tunnel diode comprises In x Ga x-1 P with x > 0.53 and < 0.75.

[0029] It should be noted that the metamorphic buffer preferably comprises a sequence of at least three layers, each layer having a lattice constant and the lattice constants of the layers increasing from layer to layer towards the second subcell.

[0030] Alternatively, the metamorphic buffer between a lower layer and an upper layer exhibits a nearly linear change in lattice constant and / or doping.

[0031] In another embodiment, the first lattice constant and the second lattice constant are equal. Preferably, the n+ layer of the first tunnel diode comprises In x Ga x-1 P with x > 0.45 and < 0.55.

[0032] A reduction in the tunnel current occurring due to the higher band gap of InGaP is counteracted by the intermediate layer according to the invention, preferably a GaAs intermediate layer.

[0033] According to alternative developments, the intermediate layer comprises GaAs or AlGaAs or AlInAs or AlInGaAs.

[0034] According to the invention, the intermediate layer has a thickness of less than 6 nm or less than 4 nm and, according to a further embodiment, is coated with silicon with a dopant concentration of at least 10 18 N / cm 3 endowed.

[0035] According to further training, the n + -Layer of the first tunnel diode with silicon and / or with tellurium and / or with selenium and / or with sulfur with a dopant concentration of ≥ 10 19 N / cm 3 endowed.

[0036] In a further embodiment, the p + -layer of the first tunnel diode AlInAs and according to a further development Al x Ga y In 1-x-y As with x>0.4 or AlGaAs.

[0037] According to the invention In another development, the p +-Layer of the first tunnel diode doped with carbon.

[0038] According to another embodiment, a lattice constant corresponds to the n + -layer of the first tunnel diode of the second lattice constant of the second subcell and a lattice constant of the p + -layer of the first tunnel diode is equal to or smaller than the second lattice constant of the second subcell.

[0039] In yet another embodiment, the multi-junction solar cell has further sub-cells, wherein each sub-cell has a further band gap, the further sub-cells are arranged between the first tunnel diode and the first sub-cell, and the band gaps of the further sub-cells are each larger than the first band gap of the first sub-cell and each smaller than the second band gap of the second sub-cell.

[0040] In a further development, the multi-junction solar cell has at least one further tunnel diode. According to a further development, the at least one further tunnel diode has a further intermediate layer, wherein an n + -layer of the further tunnel diode InGaP, a p + -layer of the further tunnel diode comprises an As-containing III-V material, the further intermediate layer between n + -layer and p + -layer of the further tunnel diode. The further intermediate layer is always thinner than the n + -layer and as the p + -layer of the further tunnel diode.

[0041] Preferably, the lowest tunnel diode of the further tunnel diodes comprises a different composition or is formed without an intermediate layer.

[0042] According to another embodiment, the multiple cell is designed as a Ge / InGaAs / AlInGaAs / AlInGaP 4-fold cell, wherein the n +The first tunnel diode (TD1) comprises InGaP. It is understood that in the Ge / InGaAs / AlInGaAs / AlInGaP quadruple cell, the sub-solar cell SC2 is formed from AlInGaP, and the tunnel diode TD1 is arranged below the AlInGaP sub-solar cell.

[0043] In another alternative embodiment, the multi-junction solar cell is designed as a Ge / InGaAs / AlInGaAs / InGaP / AlInGaP 5-fold cell, wherein the n + The first tunnel diode (TD1) comprises InGaP. It is understood that in the Ge / InGaAs / AlInGaAs / InGaP / AlInGaP 5-junction cell, the sub-solar cell SC2 is formed from AlInGaP, and the tunnel diode TD1 is arranged below the AlInGaP sub-solar cell.

[0044] In a further development, the first sub-cell consists of more than 50% germanium. Preferably, the first sub-cell is designed as a Ge cell and simultaneously represents the substrate of the multi-junction solar cell.

[0045] In a further development, the multi-junction solar cell is designed as a monolithic multi-junction solar cell.

[0046] The invention is explained in more detail below with reference to the drawings. Similar parts are labeled with identical designations. The illustrated embodiments are highly schematic, meaning that the distances and the lateral and vertical dimensions are not to scale and, unless otherwise stated, do not have any deducible geometric relationships to one another. In the drawings: Fig. 1 a schematic view of a first embodiment of the invention of a stacked monolithic upright metamorphic multi-junction solar cell, Fig. 2 a schematic view of a second embodiment of the multi-junction solar cell according to the invention, Fig. 3 a schematic view of a third embodiment of the multi-junction solar cell according to the invention Fig. 4 a schematic view of a fourth embodiment of the multi-junction solar cell according to the invention.

[0047] The illustration of the Fig. 1 shows a stacked monolithic upright metamorphic multi-junction solar cell S comprising a first subcell SC1 as the lowest subcell followed by a metamorphic buffer MP1, a tunnel diode TD1 and a second subcell SC2 as the top subcell.

[0048] The first subcell SC1 has a first band gap EG1 and a first lattice constant A1 and consists of more than 50% germanium.

[0049] The second subcell has a second band gap EG2 and a second lattice constant A2, wherein the second band gap EG2 is larger than the first band gap EG1 and the second lattice constant A2 differs from the first lattice constant A1.

[0050] The metamorphic buffer MP1 compensates for the differences between the lattice constants A1 and A2 and comprises a sequence of at least three layers with lattice constants increasing from layer to layer in the direction of the second subcell SC2.

[0051] The tunnel diode TD1 is arranged between the metamorphic buffer MP1 and the second subcell SC2 and has an n + -layer, a p + -layer and one between the n + -layer and the p + -layer arranged intermediate layer ZW.

[0052] The n + -layer of the first tunnel diode TD1 comprises InGaP and the p + -Layer of the first tunnel diode TD1 comprises an As-containing III-V material.

[0053] The intermediate layer ZW is always thinner than the n + -layer and as the p + -layer formed.

[0054] In the illustration of the Fig. 2 shows another embodiment. In the following, only the differences to the illustration of the Fig. 1 explained.

[0055] The multi-junction solar cell S additionally comprises a third sub-cell SC3 and a fourth sub-cell SC4, wherein the two further sub-cells SC3 and SC4 are each lattice-matched to the second sub-cell SC2 and are arranged between the metamorphic buffer MP1 and the tunnel diode TD1.

[0056] In the illustration of the Fig. 3 shows another embodiment. In the following, only the differences to the illustration of the Fig. 2 explained.

[0057] In this embodiment, the multi-junction solar cell S comprises, in addition to the third and fourth subcells SC3 and SC4, a further tunnel diode TD3 with an intermediate layer according to the invention. + -layer of the further tunnel diode comprises InGaP, with the p +-Layer of the further tunnel diode comprises an As-containing III-V material.

[0058] While a second tunnel diode TD2 between the metamorphic buffer MP1 and the first subcell SC1 forms an n + -layer and a p + -layer, but no intermediate layer, the third tunnel diode TD3 is constructed between the third subcell SC3 and the fourth subcell SC4 in accordance with the first tunnel diode TD1.

[0059] The third tunnel diode TD3 has a p + -layer and an n + -layer has another intermediate layer ZW2.

[0060] The first subcell SC1 is formed as a Ge solar cell or substrate, the third subcell SC3 comprises InGaAs, the fourth subcell SC4 InAlGaAs, and the second subcell InAlGaP. The p+ layer of the first tunnel diode TD1 comprises AlInGaAs, the n+ layer comprises InGaP, and the intermediate layer ZW comprises Ga(Al)As.

[0061] In the illustration of the Fig. 4 shows another embodiment. In the following, only the differences to the illustration of the Fig. 3 explained.

[0062] The multi-junction solar cell is designed as a 5-junction cell. A second tunnel diode TD2 is arranged on the first subcell SC1, which is designed as a Ge substrate. The second tunnel diode TD2 does not include an intermediate layer.

[0063] The second tunnel diode TD2 is followed by the metamorphic buffer MP1, a third subcell SC3 made of InGaAs, a third tunnel diode TD3, a fourth subcell made of InAlGaAs, a fourth tunnel diode TD4, a fifth subcell made of InGaP, the first tunnel diode TD1 and the second subcell made of InAlGaP.

[0064] The first tunnel diode comprises an AlInGaAs layer as the p+ layer, an InGaP layer as the n+ layer, and an intermediate layer ZW of Ga(Al)As between the n+ layer and the p+ layer.

[0065] The second tunnel diode TD2 has no intermediate layer.

[0066] In the illustrated embodiment, the third and fourth tunnel diodes TD3 and TD4, like the second tunnel diode TD1, have no intermediate layer. In the illustrated embodiment, the n + -Layer of the third and fourth tunnel diode InGaP.

[0067] Alternatively, the fourth tunnel diode TD4 also includes an intermediate layer. Alternatively, in addition to the fourth tunnel diode TD4, the third tunnel diode TD3 also includes an intermediate layer.

[0068] It is understood that the tunnel diode according to the invention can also be readily incorporated into lattice-matched multi-junction solar cells, ie, solar cell stacks without a metamorphic buffer. In this case, the first cell can also be formed from a material other than germanium, e.g., GaAs.

[0069] It should also be noted that the stacked solar cell can also be formed as an interverted multi-junction solar cell with or without a metamorphic buffer. All variants can also be formed monolithically. In another embodiment, the multi-junction solar cell comprises one or more wafer bonds and / or a metamorphic buffer.

Claims

[1] Stacked multi-junction solar cell (S) comprising - at least one first subcell (SC1) with a first band gap and a first lattice constant - a second subcell (SC2) arranged above the first subcell (SC1) with a second band gap and a second lattice constant, - a first tunnel diode (TD1) arranged between the first subcell (SC1) and the second subcell (SC2) with an n + -layer and a p + -layer, where - the second band gap is larger than the first band gap and - the p + -layer of the first tunnel diode (TD1) comprises an As-containing III-V material, wherein - the n + -layer of the first tunnel diode (TD1) InGaP with a Ga content above 20% based on the Ga / In ratio of the n +-layer, and to suppress unwanted incorporation of an n-dopant into the p+ layer of the tunnel diode during deposition, - between the n + -layer and the p + -layer an intermediate layer (ZW) is arranged, - the intermediate layer (ZW) is always thinner than the n + -layer and as the p + -layer is, - the intermediate layer (ZW) has a thickness of less than 6 nm or less than 4 nm, and - the intermediate layer (ZW) comprises As, wherein the As content of the intermediate layer (ZW) is higher than an As content of the n + -layer, and - the intermediate layer (ZW) is formed directly adjacent to the p+ layer and directly adjacent to the n+ layer, - the intermediate layer (ZW) with silicon with a dopant concentration of at least 10 18 N / cm 3 is endowed. [2] Multi-junction solar cell (S) according to claim 1, characterized bythat - the second lattice constant is greater than the first lattice constant and a metamorphic buffer (MP1) is arranged between the first subcell (SC1) and the second subcell (SC2), and wherein the first tunnel diode (TD1) is arranged between the metamorphic buffer (MP1) and the second subcell (SC2). [3] Multi-junction solar cell according to claim 1 or 2, characterized by that the n + -Layer of the first tunnel diode (TD1) In x Ga x-1 P with x > 0.53 and < 0.

75. [4] Multi-junction solar cell according to claim 1, characterized by that the first lattice constant and the second lattice constant are equal. [5] Multi-junction solar cell according to claim 4, characterized by that the n + -Layer of the first tunnel diode (TD1) In x Ga x-1 P with x > 0.45 and < 0.

55. [6] Multi-junction solar cell according to one of the preceding claims, characterized bythat the intermediate layer (ZW) comprises GaAs or AlGaAs or AlInAs or InGaAs or AlInGaAs. [7] Multi-junction solar cell according to one of the preceding claims, characterized by that the intermediate layer (ZW) has an energy band gap of ≥ 1.08 eV or ≥ 1.35 eV. [8] Multi-junction solar cell according to one of the preceding claims, characterized by that the n + -Layer of the first tunnel diode (TD1) with silicon and / or with tellurium and / or with selenium and / or with sulfur with a dopant concentration of ≥10 19 N / cm 3 is endowed. [9] Multi-junction solar cell according to one of the preceding claims, characterized by that the p + -Layer of the first tunnel diode (TD1) comprises AlInAs. [10] Multi-junction solar cell according to one of the preceding claims, characterized by that the p + -Layer of the first tunnel diode (TD1) Al x Ga y In 1-x-y As with x>0.

4. [11] Multi-junction solar cell according to one of the preceding claims, characterized by that the p + -layer of the first tunnel diode (TD1) is doped with carbon. [12] Multi-junction solar cell according to one of the preceding claims, characterized by that a lattice constant of the n + -layer of the first tunnel diode (TD1) corresponds to the second lattice constant of the second subcell (SC2) and a lattice constant of the p + -layer of the first tunnel diode (TD1) is equal to or smaller than the second lattice constant of the second subcell (SC2). [13] Multi-junction solar cell according to one of the preceding claims, characterized by that the first sub-cell (SC1) consists of more than 50% germanium or is designed as a Ge cell and forms the substrate of the multi-junction solar cell. [14] Multi-junction solar cell according to one of the preceding claims, characterized bythat the multi-junction solar cell has further sub-cells (SC3, SC4), each with a further band gap, wherein the further sub-cells (SC3, SC4) are arranged between the first tunnel diode (TD1) and the first sub-cell (SC1) and the band gaps of the further sub-cells (SC3, SC4) are each larger than the first band gap of the first sub-cell (SC1) and each smaller than the second band gap of the second sub-cell (SC2). [15] Multi-junction solar cell according to claim 14 characterized by that the multi-junction solar cell has at least one additional tunnel diode (TD2, TD3). [16] Multi-junction solar cell according to claim 15 characterized by that the at least one further tunnel diode (TD3) has a further intermediate layer (ZW2), wherein an n + -layer of the further tunnel diode (TD3) comprises InGaP, a p + -layer of the further tunnel diode (TD3) comprises an As-containing III-V material, the further intermediate layer (ZW2) between n + -layer and p+ -layer of the further tunnel diode (TD3) and the further intermediate layer (ZW2) is thinner than the n + -layer and as the p + -layer of the further tunnel diode (TD3). [17] Multi-junction solar cell according to one of claims 14 to 16, characterized by that the multi-junction solar cell is designed as a Ge / InGaAs / AlInGaAs / AlInGaP 4-fold cell. [18] Multi-junction solar cell according to one of claims 14 to 16, characterized by that the multi-junction solar cell is designed as a Ge / InGaAs / AlInGaAs / InGaP / AlInGaP 5-fold cell.

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