Inverted metamorphic multijunction solar cell with barrier layer and method for its fabrication

Abstract

Method for forming a multijunction solar cell with an upper subcell, a middle subcell and a lower subcell, wherein the method comprises: Provision of a first substrate (101) for epitaxial growth of the semiconductor material; Forms of a first solar subcell (A) on the first substrate (101) with a first band gap; Shapes of a second solar subcell (B) above the first solar subcell (A) with a second band gap that is smaller than the first band gap; Forms of a barrier layer (116a) composed of InGa(Al)P over the second subcell (B); Forms of a grading interlayer (116) composed of InGaAlAs above and directly adjacent to the barrier layer (116a), wherein the grading interlayer (116) has a third band gap that is larger than the second band gap; and forms of a third solar subcell (C) above the grading interlayer (116) with a fourth band gap smaller than the second band gap such that the third subcell (C) is lattice mismatched with respect to the second subcell (B), wherein The barrier layer is configured to prevent the propagation of threading dislocations into a solar subcell.

Description

Background of the invention 1. Field of the invention

[0001] The present invention relates to the field of solar cell semiconductor devices, and more particularly to multijunction solar cells (solar cells with multiple (pn) junctions) including metamorphic layers. Such devices include inverted metamorphic solar cells. 2. Description of related technology

[0002] Photovoltaic cells, also known as solar cells, are one of the most important new energy sources to become available in recent years. Considerable effort has been put into solar cell development. As a result, solar cells are currently used in a number of commercial and consumer applications. Although considerable progress has been made in this field, the need for solar cells for more complex applications has not been met in line with demand. Applications such as satellites and data transmission have dramatically increased the demand for solar cells with improved performance and energy conversion characteristics.

[0003] In satellites and other space applications, the size, mass, and cost of the satellite power system depend on the performance and energy conversion efficiency of the solar cells used. In other words, the size of the payload and the availability of on-board services are proportional to the amount of power intended or delivered. Therefore, as payloads become more complex, solar cells, which serve as the power conversion devices for the on-board power systems, become increasingly important.

[0004] Solar cells are often manufactured in vertical multijunction structures, arranged in horizontal arrays with individual solar cells connected in series. The shape and structure of an array, as well as the number of cells in the array, are determined in part by the required output voltage and current.

[0005] Inverted metamorphic solar cell structures, as described in the following reference: In the publication Wanless MW, [et al.]: LATTICE-MISSMATCHED APPROACHES FOR HIGH-PERFORMANCE. III-V PHOTOVOLTAIC ENERGY CONVERTERS. In: Conference Proceedings of the 31st IEEE Photovoltaic Specialists Conference, 2005, 530 - 535, provide an important starting point for the development of future commercial high-efficiency solar cells. The structures described in this prior art have a number of practical difficulties relating to the appropriate selection of materials and manufacturing steps, and in particular the difficulties arise with regard to the mismatched layers between the "bottom" subcell (the subcell with the lowest band gap) and the adjacent subcell. Before the present invention, the materials and manufacturing steps were.The state of the art was not adequate to create commercially viable and energy-efficient solar cells using an inverted metamorphic cell structure. In particular, the threading dislocations resulting from the metamorphic layers posed a processing challenge.

[0006] EP 1 863 099 A2 discloses a method for forming a multijunction solar cell having an upper subcell, a middle subcell, and a lower subcell. The described method comprises the following steps: providing a first substrate for the epitaxial growth of semiconductor material, forming a first solar subcell on the substrate with a first band gap; forming a second solar subcell over the first subcell with a second band gap that is smaller than the first band gap, and forming a grading interlayer over the second subcell with a third band gap that is larger than the second band gap, wherein a third solar subcell is formed with a fourth band gap that is smaller than the second band gap, such that the third subcell is lattice mismatched with respect to the second subcell.

[0007] US 2006 / 0 144 435 A1 discloses a monolithic tandem multi-bandgap solar photovoltaic converter comprising at least one and preferably at least two subcells grown in a lattice-matched manner on a substrate having a bandgap in the medium to high energy band of the sunlight spectrum, and at least one subcell grown in a lattice-mismatched manner on the substrate and having a bandgap in the low energy region of the sunlight spectrum, for example of approximately 1 eV.

[0008] US 2004 / 0 166 681 A1 discloses a method for forming a multijunction solar cell with lattice-mismatched layers and lattice-matched layers, in which an upper subcell with a first band gap grows on a growth semiconductor substrate. A middle subcell with a second band gap grows on the upper subcell, and a lower subcell with a third band gap grows on the middle subcell. The lower subcell is lattice-mismatched with respect to the growth semiconductor substrate. The first band gap of the upper subcell is greater than the second band gap of the middle subcell. The second band gap of the middle subcell is greater than the third band gap of the lower subcell. A carrier substrate is formed over the lower subcell, and the growth semiconductor substrate is removed.

[0009] The task is to create an improved solar cell. Summary of the invention

[0010] The object is achieved by the method for forming a multijunction solar cell having the features of patent claim 1 and the solar cell having the features of the independent patent claim 14. Further embodiments emerge from the subclaims.

[0011] A method is described for forming a multijunction solar cell including an upper subcell, a middle subcell and a lower subcell, namely by firstly providing a substrate for the epitaxial growth of semiconductor material; forming a first solar subcell on the substrate having a first band gap; forming a second solar subcell over the first solar subcell having a second band gap which is smaller than the first band gap; forming a barrier layer over the second subcell for blocking threading dislocations; forming a grading layer.A grading interlayer over the barrier layer, wherein the grading interlayer has a third band gap that is greater than the second band gap; and forming a third solar cell over the grading interlayer having a fourth band gap that is smaller than the second band gap, and wherein the third subcell is lattice misaligned with respect to the second subcell.

[0012] According to a further aspect, the invention also provides a multijunction solar cell comprising: a substrate; a first solar subcell on the substrate having a first band gap; a second solar subcell disposed over the first subcell and having a second band gap that is smaller than the first band gap; a barrier layer disposed over the second subcell; a grading interlayer disposed over the barrier layer and having a third band gap that is larger than the second band gap; and a third solar subcell disposed over the grading interlayer that is lattice misaligned with respect to the middle subcell and has a fourth band gap that is smaller than the third band gap.The barrier layer is constructed of a suitable material and with a lattice constant to prevent or inhibit threading dislocations associated with the grading interlayer due to propagation. Short description of the drawings

[0013] The present invention can be better and more fully appreciated by reference to the following detailed description taken in conjunction with the drawings. In the drawing: Fig. 1 is an enlarged cross-sectional view of a solar cell constructed in accordance with the present invention; Fig. 2 a cross-sectional view of the solar cell in Fig. 1 after the next processing step; Fig. 3 a cross-section of the solar cell of the Fig. 2 after the next processing step; Fig. 4 a cross-section of the solar cell of the Fig. 3 after the next processing step; Fig. 5A a cross-section of the solar cell of the Fig. 4 after the next processing or process step in which the original substrate is removed; Fig. 5B shows another cross-section of the solar cell of the Fig. 5A with the surrogate substrate on the bottom of the figure; Fig. 6A is a plan view of a wafer in which the solar cells are manufactured; Fig. 6B is a bottom view of a wafer in which the solar cells are manufactured; Fig. 7 a plan view of the wafer of the Fig. 6B after the next process step; Fig. 8 a cross-section of the solar cell of the Fig. 5A after the next processing step; Fig. 9 a cross-section of the solar cell of the Fig. 8 after the next processing step; Fig. 10 a cross-section of the solar cell of the Fig. 9 after the next processing step; Fig. 11 a cross-section of the solar cell of the Fig. 10 after the next processing step; Fig. 12 a cross-section of the solar cell of the Fig. 11 after the next processing step; Fig. 13 a cross-section of the solar cell of the Fig. 12 after the next processing step; Fig. 14 a cross-section of the solar cell of the Fig. 13 after the next processing step; Fig. 15 a cross-section of the solar cell of the Fig. 14 after the next processing step; Fig. 16 is an external quantum efficiency (EQE) graph of the inverted metamorphic solar cell without barrier layers according to the present invention; Fig. 17 an EQE graph of the middle solar subcell with and without barrier layers; and Fig. 18 an EQE graph of an inverted metamorphic solar cell with barrier layers according to the present invention.

[0014] Description of the prior art and a preferred embodiment.

[0015] Details of the present invention will now be described, including exemplary aspects and embodiments of the invention. Referring to the drawings and the following description, it should be noted that the same reference numerals are used to designate similar or functionally similar elements, with key features of the exemplary embodiments being illustrated in a highly simplified schematic manner. Furthermore, the drawings are not intended to disclose every feature of the actual embodiment, nor the relative dimensions of the illustrated elements, and are not to scale.

[0016] Fig. Figure 1 shows the multijunction solar cell according to the present invention after the formation of three subcells A, B, and C on a substrate. In particular, a substrate 101 is shown, which is made of either gallium arsenide (GaAs), germanium (Ge), or another suitable material. In the case of a Ge substrate, a nucleation layer 102 is deposited on the substrate. On the substrate, or over the nucleation layer 102, are a buffer layer 103 and an etch stop layer 104. A contact layer 105 is then deposited on layer 104, and a window layer 106 is deposited on the contact layer. Subcell A, consisting of an n+ emitter layer 107 and a p-type base layer 108, is then deposited on the window layer 106.

[0017] It should be noted that the multijunction solar cell structure can be formed by any combination of Group III-V elements according to the periodic table, taking into account the lattice constant and band gap requirements. Group III includes: boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (T). Group IV includes: carbon (C), silicon (Si), germanium (Ge), and tin (Sn). Group IV includes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi).

[0018] In the preferred embodiment, the emitter layer 107 is constructed of InGa(Al)P, and the base layer is constructed of InGa(Al)P.

[0019] The Al expression in parentheses means that Al is an optional ingredient and that in this case it can be used in a range of 0% to 30%.

[0020] On top of the base layer 108, a back surface field (BSF) layer 109 (a built-in electric field that drives the minority carriers to the collecting pn junction) is deposited, used to reduce recombination loss.

[0021] The BSF layer 109 drives minority carriers from the zone or region near the base / BSF junction or interface surface to minimize the effect of recombination loss. In other words, the BSF layer 109 reduces recombination loss at the backside of solar subcell A, thereby reducing recombination in the base.

[0022] On top of the BSF layer 109, a sequence of heavily doped p-type and n-type layers 110 is deposited, forming a tunnel diode, which is a circuit element for connecting subcell A to subcell B.

[0023] A window layer 111 is deposited on top of the tunnel diode layers 110. The window layer 111, used in subcell B, also functions to reduce recombination loss. The window layer 111 also improves the passivation of the cell surface of the underlying junctions (pn junctions). It will be apparent to those skilled in the art that additional layer(s) may be added or omitted from the cell structure without departing from the scope of the invention.

[0024] On top of the window layer 111, the layers of cell B are deposited: the emitter layer 112 and the p-type base layer 113. These layers are preferably constructed of InGaP and Ga(In)As, respectively, although any other suitable materials consistent with the lattice constant and bandgap requirements may be used.

[0025] On top of cell B, a BSF layer 114 is deposited, which performs the same function as the BSF layer 109. A p ++ / n ++ Tunnel diode 115 is deposited over the BSF layer 114 similar to layers 110, again forming a switching element for connecting cell B to cell C.

[0026] A barrier layer 116a, preferably consisting of InGa(Al)P, is deposited over the tunnel diode 115 to a thickness of approximately 1.0 micron. Such a barrier layer serves to prevent threading dislocations from propagating, either opposite to the direction of growth into the middle and top subcells B and C or in the direction of growth into the bottom subcell A. The barrier layer can be any combination of III-V compound semiconductor layers with a band gap energy greater than or equal to the grading interlayer 116 and a thickness sufficient to reduce the propagation of threading dislocations. Typical materials are As, P, N, or Sb-based III-V semiconductor materials.

[0027] A grading intermediate or metamorphic layer 116 is deposited over the barrier layer 116a. Layer 116 is preferably a compositionally graded series of InGaAlAs layers with monotonically varying lattice constants to achieve a transition in lattice constant from subcell B to subcell C. The band gap of layer 116 is 1.5 eV, corresponding to a value slightly larger than the band gap of the middle subcell B.

[0028] A grading intermediate layer may be provided, constructed of any of the As, P, N, Sb-based III-V compound semiconductors with the restrictions that the in-plane lattice parameter (planar lattice parameter) is greater than or equal to that of the second solar cell B and less than or equal to that for the third solar cell C and having a band gap energy greater than that of the second solar cell B.

[0029] In one embodiment, as suggested in the paper by Wanless et al., the step grading is provided with nine compositionally graded InGaP steps, each step layer having a thickness of 0.25 μm. In the preferred embodiment, layer 116 is composed of InGaAlAs with a monotonically varying lattice constant over at least nine steps.

[0030] According to another embodiment of the invention, a second barrier layer 116 may optionally be deposited over the InGaAlAs metamorphic layer 116. The second barrier layer 116b will have a different composition than the composition of the barrier layer 116a, and again, the base region may be GaInAs, GaAsSb, or GaInAsN.

[0031] A window layer 117 is deposited over the barrier layer 116b, which window layer reduces the recombination loss in subcell "C." Those skilled in the art will appreciate that additional layers may be added or omitted from the cell structure without departing from the scope of the invention.

[0032] On top of the window layer 117, the layers of cell C are deposited: The n + -Emitter layer 118 and the P-type base layer 119. These layers are preferably made of InGaP and Ga(In)As, respectively, although other suitable materials consistent with the lattice constant and bandgap requirements may be used.

[0033] A BSF layer 120 is deposited on top of cell C, the BSF layer performing the same function as BSF layers 109 and 114.

[0034] Finally, a p +-Contact layer 121 deposited on the BSF layer 120.

[0035] One skilled in the art will recognize that one or more additional layers could be added to or omitted from the cell structure without departing from the scope of the invention.

[0036] Fig. 2 is a cross-section of the solar cell of the Fig. 1 after the next processing step, wherein a metal layer 122 is deposited over the p + -Semiconductor contact layer 121 is deposited. The metal is preferably Ti / Au / Ag / Au.

[0037] Fig. 3 is a cross-section of the solar cell of the Fig. 2 after the next processing step, in which an adhesive layer 123 is deposited over the metal layer 122. The adhesive is preferably Genlak 330.

[0038] Fig. 4 is a cross-section of the solar cell of the Fig. 3 after the next processing step, in which a surrogate substrate, preferably sapphire, is applied. The surrogate substrate has a thickness of approximately 40 mils and is perforated with holes approximately 1 mm in diameter and spaced 4 mm apart to assist in the subsequent removal of the adhesive and substrate.

[0039] Fig. 5A is a cross-section of the solar cell of the Fig. 4 after the next processing step, in which the original substrate is removed by a sequence of lapping and / or etching steps in which the substrate 101, the buffer layer 103, and the etch stop layer 104 are removed. The etchant depends on the substrate growth.

[0040] Fig. 5B is a cross-section of the solar cell of the Fig. 5A from the solar cell of the Fig. 5A of the orientation with the surrogate substrate 124 at the bottom of the figure.

[0041] Fig. Figure 6A is a top view of a wafer in which the solar cells are implanted.

[0042] In each cell there are grid lines 501 (which are particularly visible in cross-section in Fig. 10) and a connecting bus line 502 and a contact field 503.

[0043] Fig. Figure 6B is a bottom plan view of the wafer with four solar cells shown in Fig. 6A.

[0044] Fig. 7 is a plan view of the wafer of the Fig. 6A after the next processing step in which a mesa 510 is etched around the perimeter of each cell using phosphide and arsenide etchants.

[0045] Fig. 8 is a simplified cross-section of the solar cell of the Fig. 5B, where just a few of the upper layers and lower layers above the surrogate substrate 124 are shown.

[0046] Fig. 9 is a cross-section of the solar cell of the Fig. 8 after the next processing step in which the etch stop layer 104 is removed by an HCl / H2O solution.

[0047] Fig. 10 is a cross-section of the solar cell of the Fig. 9 after the next sequence of processing steps, in which a photoresist mask (not shown) is disposed over the contact layer 105 to form the grid lines 501. The grid lines 501 are deposited via evaporation and lithographically patterned and deposited over the contact layer 105. The mask is lifted off the metal grid lines 501.

[0048] Fig. 11 is a cross section of the solar cell of the Fig. 10 after the next processing step in which the grid lines are used as a mask to etch down the surface of the window layer 106 using a citric acid / peroxide etching mixture.

[0049] Fig. 12 is a cross-section of the solar cell of the Fig. 11 after the next processing step in which an anti-reflective (ARC) dielectric cap layer 130 is applied over the entire surface of the “bottom” side of the wafer with the grid lines 501.

[0050] Fig. 13 is a cross-sectional view of the solar cell of the Fig. 12 after the next processing step, in which the mesa 501 is etched down to the metal layer 122 using phosphide and arsenide etchants. The cross section in the figure is shown as seen from the AA plane according to Fig. 7. One or more silver electrodes are then welded to the contact surface or surfaces.

[0051] Fig. 14 is a cross-section of the solar cell of the Fig. 13 after the next processing step, after the surrogate substrate 124 and the adhesive 123 have been removed by EKC 922. The preferred perforations provided in the surrogate substrate have a diameter of 0.8382 mm and are spaced 3.8608 mm apart.

[0052] Fig. 15 is a cross-section of the solar cell of the Fig. 14 after the next processing step in which an adhesive is applied over the ARC layer 130 and a cover glass is attached thereto.

[0053] An experimental indication of the effectiveness of the present invention is given by Fig. 16 to 18. A structure of the type shown in Fig. 1, but without barrier layers 116a and 116b, was grown and in 4 cm 2 -cells. External quantum efficiency (EQE) measurements were performed and the results, shown in Fig. 16, indicate that the long-wavelength response of the middle subcell B was lower than expected. This observation suggests that threading dislocation propagation opposite to the growth direction may be responsible for the degradation of the middle cell's efficiency. Normarski microscopy reveals unexpected cross-striation (a type of strain relief) on the initial epitaxial layer of the lattice-matched subcell A. Photoluminescence map imaging further showed that the luminescence of the middle subcell B was lower than expected. Cathodoluminescence measurements indicated that the threading dislocation density in the middle subcell B was high, but the threading dislocations did not penetrate the upper subcell A. These measurements were consistent with the results obtained in Fig. 16 EQE measurements shown are consistent.

[0054] Fig. Figure 17 illustrates a comparison of EQE measurements of a middle subcell in a triple-junction solar cell with and without the addition of the barrier layer 116a according to the present invention. The graph of subcell B (without the barrier layer) has an integrated current (AMO) of 15.6 mA / cm 2 and a lower EQE than this for subcell D (with the barrier layer), with an integrated current AMO of 17.4 mA / cm 2 .

[0055] The efficiency of using a barrier layer in the solar cell of the present invention can be seen from a comparison of the EQE graphs of the Fig. 16 and Fig. 18. Fig. 16 is an EQE graph for the solar cell of the Fig. 1 without barrier layer, and Fig. Figure 18 is an EQE graph for the solar cell with barrier layer. The current of the middle subcell B of the solar cell of the Fig. 18 (17.4 mA / cm 2) is only slightly below the current for the upper subcell C (18.4 mA / cm 2 ). Such close current matching of the middle subcell and the upper subcell demonstrates the efficiency of the present invention.

[0056] It will be appreciated that any of the elements described above, or two or more together, may also find useful application in other types of constructions that differ from the types of construction described above.

[0057] Although the preferred embodiment of the present invention utilizes a vertical stack of subcells with top and bottom electrical contacts, the subcells may alternatively be contacted by metal contacts with laterally conducting semiconductor layers between the subcells. Such arrangements can be used to form 3-terminal, 4-terminal, and generally n-terminal devices. The subcells can be connected in circuits using these additional terminals such that most of the available photogenerated current density in each subcell can be effectively utilized, resulting in high efficiency for the multijunction cell, even though the photogenerated current densities typically vary between different subcells.

[0058] As noted above, the invention may utilize one or more homojunction cells or subcells, i.e., a cell or subcell in which a pn junction is formed between a p-type semiconductor and an n-type semiconductor, both having the same chemical composition and band gap and differing only in doping species and types. Subcell A with p-type and n-type InGaP is an example of a homojunction subcell. Alternatively, the invention may utilize one or more heterojunction cells or subcells, i.e.,a cell or subcell in which the pn junction is formed between a p-type semiconductor and an n-type semiconductor having different chemical compositions of the semiconductor material in the n-type and n-type zones or regions and / or different band gap energies in the p-type regions, in addition to using different doping species or dopant types in the p-type and n-type regions or zones forming the pn junction.

[0059] The composition of the windows or BSF layers may use other semiconductor compounds, taking into account the lattice constants and bandgap requirements, and these may include: AlInP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AlN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe and similar materials, all of which are within the scope of the invention.

Claims

[1] A method of forming a multijunction solar cell having an upper subcell, a middle subcell, and a lower subcell, the method comprising: Providing a first substrate (101) for the epitaxial growth of the semiconductor material; forming a first solar subcell (A) on the first substrate (101) having a first band gap; forming a second solar subcell (B) over the first solar subcell (A) having a second bandgap smaller than the first bandgap; forming a barrier layer (116a) composed of InGa(Al)P over the second subcell (B); forming a grading intermediate layer (116) composed of InGaAlAs above and directly adjacent to the barrier layer (116a), the grading intermediate layer (116) having a third band gap that is greater than the second band gap; and forming a third solar subcell (C) over the grading interlayer (116) having a fourth bandgap smaller than the second bandgap such that the third subcell (C) is lattice mismatched with respect to the second subcell (B), wherein the barrier layer is configured to prevent the propagation of threading dislocations into a solar subcell. [2] The method of claim 1, wherein the barrier layer has a thickness of approximately 1.0 µm and prevents the propagation of threading dislocations, either propagating opposite to a growth direction or in the growth direction. [3] The method of claim 1, further comprising: forming a second barrier layer directly over the grading interlayer prior to forming the third solar subcell. [4] The method of claim 3, wherein the second barrier layer is composed of any of the following semiconductors: As, P, N or Sb-based III-V compound semiconductors having a band gap energy greater than or equal to that of the grading interlayer. [5] The method of claim 1, wherein the first substrate is selected from the group consisting of germanium or GaAs. [6] The method of claim 1, wherein the first solar subcell is constructed from an InGa(Al)P emitter region and an InGa(Al)P base region. [7] The method of claim 6, wherein the second solar subcell is constructed from a GaInP, GaInAs, GaAsSb or GaInAsN emitter region and a GaInAs, GaAsSb or GaInAsN base region. [8] The method of claim 1, wherein the grading interlayer has a planar lattice parameter greater than or equal to that of the second solar cell and less than or equal to that of the third solar cell, and wherein the grading interlayer has a band gap energy greater than that of the second solar subcell. [9] The method of claim 6, wherein the second solar subcell consists of an InGaP emitter region and a GaAs base region. [10] The method of claim 8, wherein the intermediate grading layer consists of nine levels of layers with monotonically varying lattice constants.

11. The method of claim 1, further comprising: depositing a contact layer over the third solar subcell and making electrical contact therewith. [12] The method of claim 11, further comprising: applying a surrogate substrate over the contact layer and removing the first substrate. [13] The method of claim 12, further comprising: patterning the contact layer into a grid; etching a well around the perimeter of the solar cell to form a mesa structure on the surrogate substrate. [14] A multijunction solar cell comprising: a substrate; a lower solar subcell on the substrate having a first band gap; a grading interlayer of InGaAlAs disposed over the lower subcell and having a second band gap of 1.5 eV, which is larger than the first band gap; a barrier layer of InGaAlAs, arranged above and directly adjacent to the grading interlayer to reduce the propagation of threading dislocations; a middle solar subcell disposed above the barrier layer and having a third bandgap smaller than the second bandgap and greater than the first bandgap, the middle solar subcell being lattice mismatched with respect to the lower subcell; and an upper solar subcell arranged above the middle solar subcell and having a fourth bandgap that is larger than the third bandgap. [15] The solar cell of claim 14, wherein the barrier layer has a band gap greater than or equal to that of the grading interlayer. [16] The solar cell of claim 14, further comprising a second barrier layer disposed directly adjacent to the grading interlayer between the latter and the lower subcell. [17] The solar cell of claim 16, wherein the second barrier layer is made of any As, P, N, or Sb-based III-V compound semiconductors having a band gap greater than or equal to that of the grading interlayer. [18] The solar cell according to claim 14, wherein the substrate is formed from the group consisting of germanium or GaAs. [19] The solar cell of claim 14, wherein the lower solar subcell is constructed of InGa(Al)P, and the middle solar subcell consists of: [20] The solar cell of claim 14, wherein the middle solar subcell consists of: a GaInP, GaInAs, GaAsSb or GaInAsN emitter region and a GaInAs, GaAsSb or GaInAsN base region. [21] The solar cell of claim 14, wherein the upper solar subcell is constructed of InGaAs.

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