MULTIJUNCTION SOLAR CELL AND SOLAR CELL ASSEMBLIES FOR SPACE APPLICATIONS

Abstract

A multijunction solar cell having an upper first solar subcell composed of a semiconductor material having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; a fourth solar subcell adjacent to and lattice mismatched from said third solar subcell and composed of germanium grown on a growth substrate. In some embodiments of a five junction solar cell, the growth substrate forms a bottom solar subcell and is composed of germanium.

Claims

1. A multijunction, space-qualified solar cell comprising: an upper first solar subcell composed of indium gallium aluminum phosphide and having a first band gap in the range of 2.0 to 2.2 eV; a second solar subcell adjacent to said first solar subcell and including an emitter layer composed of indium gallium phosphide or aluminum indium gallium arsenide, and a base layer composed of aluminum indium gallium arsenide and having a second band gap in the range of approximately 1.55 to 1.8 eV and being lattice matched with the upper first solar subcell, wherein the emitter and base layers of the second solar subcell form a photoelectric junction; a third solar subcell adjacent to said second solar subcell and composed of indium gallium arsenide and having a third band gap less than that of the second solar subcell and being lattice matched with the second solar subcell; and a fourth solar subcell adjacent to said third solar subcell and composed of germanium and having a fourth band gap of approximately 0.67 eV; and a growth substrate adjacent to said fourth solar subcell.

2. A multijunction solar cell as defined in claim 1, wherein the fourth solar subcell is at least 3 microns in thickness, and the growth substrate is composed of n-type germanium.

3. A multijunction solar cell as defined in claim 1, further comprising a fifth solar subcell adjacent to said fourth solar subcell and composed of germanium and having a thickness greater than that of the fourth solar subcell.

4. A multijunction solar cell as defined in claim 3, wherein the thickness of the fifth solar subcell is at least five times greater than that of the fourth solar subcell.

5. A multijunction solar cell as defined in claim 1, further comprising a nucleation layer disposed over the growth substrate, wherein a junction is formed in the growth substrate by diffusion from the nucleation layer, forming an additional subcell.

6. The multijunction solar cell as defined in claim 1, wherein the upper first solar subcell has a band gap of less than 2.15, the second solar subcell has a band gap of less than 1.73 eV; and the third solar subcell has a band gap in the range of 1.15 to 1.4 eV.

7. The multijunction solar cell as defined in claim 1, the first solar subcell has a band gap of 2.05 eV.

8. The multijunction solar cell as defined in claim 1, wherein the band gap of the third solar subcell is less than 1.41 eV, and greater than that of the fourth subcell.

9. The multijunction solar cell as defined in claim 2, wherein the multijunction solar cell is a four junction solar cell with the fourth solar subcell being the bottom subcell.

10. The multijunction solar cell as defined in claim 1, wherein the top subcell is composed of a base layer of (In.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is 0.142, corresponding to a band gap of 2.10 eV, and an emitter layer of (In.sub.xGa.sub.1-x).sub.1-yAl.sub.yP where x is 0.505, and y is 0.107, corresponding to a band gap of 2.05 eV.

11. The multijunction solar cell as defined in claim 1, further comprising a tunnel diode disposed over the fourth subcell, and intermediate layer disposed between the third subcell and the tunnel diode wherein the intermediate layer is compositionally graded to lattice match the third solar subcell on one side and the tunnel diode on the other side and is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater than or equal to that of the third solar subcell and different than that of the tunnel diode, and having a band gap energy greater than that of the fourth solar subcell.

12. The multijunction solar cell as defined in claim 1, further comprising an intermediate layer disposed between the third subcell and the fourth subcell wherein the intermediate layer is compositionally step-graded with between one and four steps to lattice match the fourth solar subcell on one side and composed of In.sub.xGa.sub.1-xAs or (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs with 0<x<1, 0<y<1, and x and y selected such that the band gap is in the range of 1.15 to 1.41 eV throughout its thickness.

13. The multijunction solar cell as defined in claim 12, wherein the intermediate layer has a graded band gap in the range of 1.15 to 1.41 eV, or 1.2 to 1.35 eV, or 1.25 to 1.30 eV.

14. The multijunction solar cell as defined in claim 1, wherein either (i) the emitter layer; or (ii) the base layer and emitter layer, of the upper first subcell have different lattice constants from the lattice constant of the second subcell.

15. The multijunction solar cell as defined in claim 1, further comprising: a distributed Bragg reflector (DBR) layer adjacent to and beneath the third solar subcell and arranged so that light can enter and pass through the third solar subcell and at least a portion of which can be reflected back into the third solar subcell by the DBR layer, wherein the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction, wherein the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and wherein the DBR layer includes a first DBR layer composed of a plurality of p type In.sub.zAl.sub.xGa.sub.1-x-zAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type In.sub.wAl.sub.yGa.sub.1-y-wAs layers, where 0<w<1, O<x<1, 0<y<1, 0<z<1 and y is greater than x; and an intermediate layer disposed between the DBR layer and the fourth solar subcell, wherein the intermediate layer is compositionally step-graded to lattice match the DBR layer on one side and the fourth solar subcell on the other side, and is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater than or equal to that of the DBR layer and less than or equal to that of the lower fourth solar subcell, and having a band gap energy greater than that of the fourth solar subcell.

16. A method of fabricating a multijunction, space-qualified solar cell, comprising: providing a growth substrate; forming an upper first solar subcell on the growth substrate composed of indium gallium phosphide and having a first band gap in the range of 2.0 to 2.2 eV; growing a second solar subcell adjacent to said first solar subcell and including an emitter layer composed of indium gallium phosphide or aluminum indium arsenide, and a base layer composed of aluminum indium gallium arsenide and having a second band gap in the range of approximately 1.55 to 1.8 eV and being lattice matched with the upper first solar subcell, wherein the emitter and base layers of the second solar subcell form a photoelectric junction; growing a third solar subcell adjacent to said second solar subcell and composed of indium gallium arsenide and having a third band gap less than that of the second solar subcell and being lattice matched with the second solar subcell; and growing a fourth solar subcell adjacent to said third solar subcell and composed of germanium and having a fourth band gap of approximately 0.67 eV.

17. A method as defined in claim 16, wherein the growth substrate is lattice mismatched from the upper first solar sucbcell.

18. A method as defined in claim 16, wherein the growth substrate and all the solar subcells are lattice matched.

19. A method as defined in claim 16, wherein the third and fourth subcells are lattice mismatched.

20. The method as defined in claim 1, wherein the solar cell has a bonding pad of first and second polarity, and further comprising: (a) a ceria doped borosilicate glass supporting member that is 3 to 6 mils in thickness attached to the upper first solar subcell by a transparent adhesive; (b) providing a plurality of interconnects each composed of a silver-plated nickel-cobalt ferrous alloy material, each interconnect welded to a respective bonding pad on each solar cell to electrically connect the adjacent solar cells in a series electrical circuit; and (c) attaching the bottom of the solar cell to an aluminum honeycomb panel having a carbon composite face sheet, the panel having a coefficient of thermal expansion (CTE) that substantially matches the germanium of the fourth solar subcell.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0107] The invention will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

[0108] FIG. 1 is a graph representing the BOL value of the parameter E.sub.g/qV.sub.oc at 28 C. plotted against the band gap of certain binary materials defined along the x-axis;

[0109] FIG. 2 is a cross-sectional view of the solar cell of a first embodiment of a four junction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;

[0110] FIG. 3 is a graph of the doping profile in the base and emitter layers of a subcell in the solar cell according to the present disclosure;

[0111] FIG. 4A is a cross-sectional view of a first embodiment of an upright metamorphic five junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on a p-type growth substrate up to the contact layer, according to the present disclosure;

[0112] FIG. 4B is a cross-sectional view of a second embodiment of an upright metamorphic five junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on an n-type growth substrate up to the contact layer, according to the present disclosure;

[0113] FIG. 5A is a cross-sectional view of a first embodiment of an inverted metamorphic five junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;

[0114] FIG. 5B is a cross-sectional view of a second embodiment of an inverted metamorphic five junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;

[0115] FIG. 5C is a cross sectional view of a third embodiment of an inverted metamorphic five junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;

[0116] FIG. 6 is a cross-sectional view of a first embodiment of an inverted metamorphic five junction solar cell of FIG. 5A according to the present disclosure after removal of the growth substrate, and the solar cell being depicted with the top subcell A being at the top of the Figure;

[0117] FIG. 7 is a cross-sectional view of the solar cell of the present disclosure as implemented in a CIC and mounted on a panel;

[0118] FIG. 8 is a graph representing the band gap of certain binary materials and their lattice constants; and

[0119] FIG. 9 is an enlargement of a portion of the graph of FIG. 8 illustrating different compounds of GaInAs and GaInP with different proportions of gallium and indium, and the location of specific compounds of the graph.

GLOSSARY OF TERMS

[0120] III-V compound semiconductor refers to a compound semiconductor formed using at least one elements from group III of the periodic table and at least one element from group V of the periodic table. III-V compound semiconductors include binary, tertiary and quaternary compounds. Group III includes boron (B), aluminum (Al), gallium (Ga), indium (In) and thallium (T). Group V includes nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi).

[0121] Band gap refers to an energy difference (e.g., in electron volts (eV)) separating the top of the valence band and the bottom of the conduction band of a semiconductor material.

[0122] Beginning of Life (BOL) refers to the time at which a photovoltaic power system is initially deployed in operation.

[0123] Bottom subcell refers to the subcell in a multijunction solar cell which is furthest from the primary light source for the solar cell.

[0124] Compound semiconductor refers to a semiconductor formed using two or more chemical elements.

[0125] Current density refers to the short circuit current density J.sub.sc through a solar subcell through a given planar area, or volume, of semiconductor material constituting the solar subcell.

[0126] Deposited, with respect to a layer of semiconductor material, refers to a layer of material which is epitaxially grown over another semiconductor layer.

[0127] Dopant refers to a trace impurity element that is contained within a semiconductor material to affect the electrical or optical characteristics of that material. As used in the context of the present disclosure, typical dopant levels in semiconductor materials are in the 1016 to 1019 atoms per cubic centimeter range. The standard notation or nomenclature, when a particular identified dopant is proscribed, is to use, for example, the expression GaAs:Se or GaAs:C for selenium or carbon doped gallium arsenide respectively. Whenever a ternary or quaternary compound semiconductor is expressed as AlGaAs or GaInAsP, it is understood that all three or four of the constituent elements are much higher in mole concentration, say on the 1% level or above, which is in the 1021 atoms/cm-3 or larger range. Such constituent elements are not considered dopants by those skilled in the art since the atoms of the constituent element form part of the crystal structure of the compound semiconductor. In addition, a further distinction is that a dopant has a different valence number than the constituent component elements. In a commonly implemented III-V compound semiconductor such as AlGaInAs, none of the individual elements Al, Ga, In, or As are considered to be dopants since they have the same valence as the component atoms that make up the crystal lattice.

[0128] End of Life (EOL) refers to a predetermined time or times after the Beginning of Life, during which the photovoltaic power system has been deployed and has been operational. The EOL time or times may, for example, be specified by the customer as part of the required technical performance specifications of the photovoltaic power system to allow the solar cell designer to define the solar cell subcells and sublayer compositions of the solar cell to meet the technical performance requirement at the specified time or times, in addition to other design objectives. The terminology EOL is not meant to suggest that the photovoltaic power system is not operational or does not produce power after the EOL time.

[0129] Graded interlayer (or grading interlayer)see metamorphic layer.

[0130] Inverted metamorphic multijunction solar cell or IMM solar cell refers to a solar cell in which the subcells are deposited or grown on a substrate in a reverse sequence such that the higher band gap subcells, which would normally be the top subcells facing the solar radiation in the final deployment configuration, are deposited or grown on a growth substrate prior to depositing or growing the lower band gap subcells.

[0131] Layer refers to a relatively planar sheet or thickness of semiconductor or other material. The layer may be deposited or grown, e.g., by epitaxial or other techniques.

[0132] Lattice mismatched refers to two adjacently disposed materials or layers (with thicknesses of greater than 100 nm) having in-plane lattice constants of the materials in their fully relaxed state differing from one another by less than 0.02% in lattice constant. (Applicant expressly adopts this definition for the purpose of this disclosure, and notes that this definition is considerably more stringent than that proposed, for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6% lattice constant difference).

[0133] Metamorphic layer or graded interlayer refers to a layer that achieves a gradual transition in lattice constant generally throughout its thickness in a semiconductor structure.

[0134] Middle subcell refers to a subcell in a multijunction solar cell which is neither a Top Subcell (as defined herein) nor a Bottom Subcell (as defined herein).

[0135] Short circuit current (I.sub.sc) refers to the amount of electrical current through a solar cell or solar subcell when the voltage across the solar cell is zero volts, as represented and measured, for example, in units of milliamps.

[0136] Short circuit current densitysee current density.

[0137] Solar cell refers to an electronic device operable to convert the energy of light directly into electricity by the photovoltaic effect.

[0138] Solar cell assembly refers to two or more solar cell subassemblies interconnected electrically with one another.

[0139] Solar cell subassembly refers to a stacked sequence of layers including one or more solar sub cells.

[0140] Solar subcell refers to a stacked sequence of layers including a p-n photoactive junction composed of semiconductor materials. A solar subcell is designed to convert photons over different spectral or wavelength bands to electrical current.

[0141] Substantially current matched refers to the short circuit current through adjacent solar subcells being substantially identical (i.e. within plus or minus 1%).

[0142] Top subcell or upper subcell refers to the subcell in a multijunction solar cell which is closest to the primary light source for the solar cell.

[0143] ZTJ refers to the product designation of a commercially available SolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0144] Details of the present invention will now be described including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of the actual embodiment nor the relative dimensions of the depicted elements, and are not drawn to scale.

[0145] A variety of different features of multijunction solar cells (as well as inverted metamorphic multijunction solar cells) are disclosed in the related applications noted above. Some, many or all of such features may be included in the structures and processes associated with the upright solar cells of the present disclosure. However, more particularly, the present disclosure is directed to the fabrication of a multijunction solar cell grown on a single growth substrate, including in one embodiment the two middle subcells (e.g., the second and third subcells) being lattice mismatched. More specifically, however, in some embodiments, the present disclosure relates to four or five junction solar cells, with two vertically arranged subcells composed of germanium, with direct band gaps in the range of 2.0 to 2.15 eV (or higher) for the top subcell, and (i) 1.65 to 1.8 eV, and (ii) 1.41 eV or less, for the middle subcells, and (iii) 0.6 to 0.8 eV indirect bandgaps for the two bottom subcells, respectively.

[0146] The present disclosure provides an unconventional four or five junction design (with three grown lattice matched subcells, which are lattice mismatched to a first Ge subcell overlying the Ge substrate) that leads to a surprising significant performance improvement over that of traditional three or four junction solar cell on Ge despite the substantial current mismatch present between the top three junctions and the bottom Ge junction. This performance gain is especially realized at high temperature and after high exposure to space radiation by the proposal of incorporating high band gap semiconductors that are inherently more resistant to radiation and temperature, thus specifically addressing the problem of ensuring continues adequate efficiency and power output at the end-of-life.

[0147] In some embodiments, the fourth subcell is germanium, while in other embodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN or other III-V or II-VI compound semiconductor material.

[0148] Another descriptive aspect of the present disclosure is to characterize the fourth subcell as having a direct band gap of greater than 0.75 eV.

[0149] The indirect band gap of germanium at room temperature is about 0.66 eV, while the direct band gap of germanium at room temperature is 0.8 eV. Those skilled in the art will normally refer to the band gap of germanium as 0.67 eV, since it is lower than the direct band gap value of 0.8 eV.

[0150] The recitation that the fourth subcell has a direct band gap of greater than 0.75 eV is therefore expressly meant to include germanium as a possible semiconductor for the fourth subcell, although other semiconductor material can be used as well.

[0151] More specifically, the present disclosure intends to provide a relatively simple and reproducible technique that does not employ inverted processing associated with inverted metamorphic multijunction solar cells, and is suitable for use in a high volume production environment in which various semiconductor layers are grown on a growth substrate in an MOCVD reactor, and subsequent processing steps are defined and selected to minimize any physical damage to the quality of the deposited layers, thereby ensuring a relatively high yield of operable solar cells meeting specifications at the conclusion of the fabrication processes.

[0152] As suggested above, incremental improvements in the design of multijunction solar cells are made in view of a variety of new space missions and application requirements. Moreover, although such improvements may be relatively minute quantitative modifications in the composition or band gap of certain subcells, as we noted above, such minute parametric changes (such as in the specific band gaps of the upper first subcell, or of the third subcell) provide substantial improvements in efficiency that specifically address the problems that have been identified associated with the existing current commercial multijunction solar cells, and provide a solution that represents an inventive step in the design process.

[0153] Thus, in addition to the characterizing feature that the third and fourth solar subcells are not necessarily lattice matched, we explicitly recite that the fourth solar subcell is lattice mismatched from the third solar subcell, and provide a variety of different band gap specifications associated with different embodiments of the solar cell of the present disclosure. Additional characterizing features are set forth in the claims hereunder.

[0154] Prior to discussing the specific embodiments of the present disclosure, a brief discussion of some of the issues associated with the design of multijunction solar cells in the context of the composition or deposition of various specific layers in embodiments of the product as specified and defined by Applicant is in order.

[0155] There are a multitude of properties that should be considered in specifying and selecting the composition of, inter alia, a specific semiconductor layer, the back metal layer, the adhesive or bonding material, or the composition of the supporting material for mounting a solar cell thereon. For example, some of the properties that should be considered when selecting a particular layer or material are electrical properties (e.g. conductivity), optical properties (e.g., band gap, absorbance and reflectance), structural properties (e.g., thickness, strength, flexibility, Young's modulus, etc.), chemical properties (e.g., growth rates, the sticking coefficient or ability of one layer to adhere to another, stability of dopants and constituent materials with respect to adjacent layers and subsequent processes, etc.), thermal properties (e.g., thermal stability under temperature changes, coefficient of thermal expansion), and manufacturability (e.g., availability of materials, process complexity, process variability and tolerances, reproducibility of results over high volume, reliability and quality control issues).

[0156] In view of the trade-offs among these properties, it is not always evident that the selection of a material based on one of its characteristic properties is always or typically the best or optimum from a commercial standpoint or for Applicant's purposes. For example, theoretical studies may suggest the use of a quaternary material with a certain band gap for a particular subcell would be the optimum choice for that subcell layer based on fundamental semiconductor physics. As an example, the teachings of academic papers and related proposals for the design of very high efficiency (over 40%) solar cells may therefore suggest that a solar cell designer specify the use of a quaternary material (e.g., InGaAsP) for the active layer of a subcell. A few such devices may actually be fabricated by other researchers, efficiency measurements made, and the results published as an example of the ability of such researchers to advance the progress of science by increasing the demonstrated efficiency of a compound semiconductor multijunction solar cell. Although such experiments and publications are of academic interest, from the practical perspective of the Applicants in designing a compound semiconductor multijunction solar cell to be produced in high volume at reasonable cost and subject to manufacturing tolerances and variability inherent in the production processes, such an optimum design from an academic perspective is not necessarily the most desirable design in practice, and the teachings of such studies more likely than not point in the wrong direction and lead away from the proper design direction. Stated another way, such references may actually teach away from Applicant's research efforts and the ultimate solar cell design proposed by the Applicants.

[0157] In view of the foregoing, it is further evident that the identification of one particular constituent element (e.g. indium, or aluminum) in a particular subcell, or the thickness, band gap, doping, or other characteristic of the incorporation of that material in a particular subcell, is not a result effective variable that one skilled in the art can simply specify and incrementally adjust to a particular level and thereby increase the efficiency of a solar cell. The efficiency of a solar cell is not a simple linear algebraic equation as a function of the amount of gallium or aluminum or other element in a particular layer. The growth of each of the epitaxial layers of a solar cell in a reactor is a non-equilibrium thermodynamic process with dynamically changing spatial and temporal boundary conditions that is not readily or predictably modeled. The formulation and solution of the relevant simultaneous partial differential equations covering such processes are not within the ambit of those of ordinary skill in the art in the field of solar cell design.

[0158] Even when it is known that particular variables have an impact on electrical, optical, chemical, thermal or other characteristics, the nature of the impact often cannot be predicted with much accuracy, particularly when the variables interact in complex ways, leading to unexpected results and unintended consequences. Thus, significant trial and error, which may include the fabrication and evaluative testing of many prototype devices, often over a period of time of months if not years, is required to determine whether a proposed structure with layers of particular compositions, actually will operate as intended, let alone whether it can be fabricated in a reproducible high volume manner within the manufacturing tolerances and variability inherent in the production process, and necessary for the design of a commercially viable device.

[0159] Furthermore, as in the case here, where multiple variables interact in unpredictable ways, the proper choice of the combination of variables can produce new and unexpected results, and constitute an inventive step.

[0160] The lattice constants and electrical properties of the layers in the semiconductor structure are preferably controlled by specification of appropriate reactor growth temperatures and times, and by use of appropriate chemical composition and dopants. The use of a deposition method, such as Molecular Beam Epitaxy (MBE), Organo Metallic Vapor Phase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD), or other vapor deposition methods for the growth may enable the layers in the monolithic semiconductor structure forming the cell to be grown with the required thickness, elemental composition, dopant concentration and grading and conductivity type, and are within the scope of the present disclosure.

[0161] The present disclosure is in one embodiment directed to a growth process using a metal organic chemical vapor deposition (MOCVD) process in a standard, commercially available reactor suitable for high volume production. Other embodiments may use other growth technique, such as MBE. More particularly, regardless of the growth technique, the present disclosure is directed to the materials and fabrication steps that are particularly suitable for producing commercially viable multijunction solar cells or inverted metamorphic multijunction solar cells using commercially available equipment and established high-volume fabrication processes, as contrasted with merely academic expositions of laboratory or experimental results.

[0162] Some comments about MOCVD processes used in one embodiment are in order here.

[0163] It should be noted that the layers of a certain target composition in a semiconductor structure grown in an MOCVD process are inherently physically different than the layers of an identical target composition grown by another process, e.g. Molecular Beam Epitaxy (MBE). The material quality (i.e., morphology, stoichiometry, number and location of lattice traps, impurities, and other lattice defects) of an epitaxial layer in a semiconductor structure is different depending upon the process used to grow the layer, as well as the process parameters associated with the growth. MOCVD is inherently a chemical reaction process, while MBE is a physical deposition process. The chemicals used in the MOCVD process are present in the MOCVD reactor and interact with the wafers in the reactor, and affect the composition, doping, and other physical, optical and electrical characteristics of the material. For example, the precursor gases used in an MOCVD reactor (e.g. hydrogen) are incorporated into the resulting processed wafer material, and have certain identifiable electro-optical consequences which are more advantageous in certain specific applications of the semiconductor structure, such as in photoelectric conversion in structures designed as solar cells. Such high order effects of processing technology do result in relatively minute but actually observable differences in the material quality grown or deposited according to one process technique compared to another. Thus, devices fabricated at least in part using an MOCVD reactor or using a MOCVD process have inherent different physical material characteristics, which may have an advantageous effect over the identical target material deposited using alternative processes.

[0164] One aspect of the present disclosure relates to the use of aluminum in the active layers of the upper subcells in a multijunction solar cell. The effects of increasing amounts of aluminum as a constituent element in an active layer of a subcell affects the photovoltaic device performance. One measure of the quality or goodness of a solar cell junction is the difference between the band gap of the semiconductor material in that subcell or junction and the V.sub.oc, or open circuit voltage, of that same junction. The smaller the difference, the higher the V.sub.oc of the solar cell junction relative to the band gap, and the better the performance of the device. V.sub.oc is very sensitive to semiconductor material quality, so the smaller the E.sub.g/qV.sub.oc of a device, the higher the quality of the material in that device. There is a theoretical limit to this difference, known as the Shockley-Queisser limit. That is the best that a solar cell junction can be under a given concentration of light at a given temperature.

[0165] The experimental data obtained for single junction (Al)GaInP solar cells indicates that increasing the Al content of the junction leads to a larger V.sub.ocE.sub.g/q difference, indicating that the material quality of the junction decreases with increasing Al content. FIG. 1 shows this effect. The three compositions cited in the Figure are all lattice matched to GaAs, but have differing Al composition. Adding Al increases the band gap of the junction, but in so doing also increases V.sub.ocE.sub.g/q. Hence, we draw the conclusion that adding Al to a semiconductor material degrades that material such that a solar cell device made out of that material does not perform relatively as well as a junction with less Al.

[0166] Turning to the multijunction solar cell device of the present disclosure, FIG. 2 is a cross-sectional view of a first embodiment of a four junction solar cell 100 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322 as presented in to the disclosure of parent application Ser. No. 15/873,135 filed Jan. 17, 2018.

[0167] As shown in the illustrated example of FIG. 2, the bottom or fourth subcell D includes a growth substrate 300 formed of p-type germanium (Ge) which also serves as a base layer. A back metal contact pad 350 formed on the bottom of base layer 300 provides the bottom p type polarity electrical contact to the multijunction solar cell 100. The bottom subcell D, further includes, for example, a highly doped n-type Ge emitter layer 301, and an n-type indium gallium arsenide (InGaAs) nucleation layer 302. The nucleation layer is deposited over the base layer, and the emitter layer 301 is formed in the substrate 300 by diffusion of dopants into the Ge substrate 300, thereby forming the n-type Ge layer 301. Heavily doped p-type aluminum indium gallium arsenide (AlGaAs) and heavily doped n-type gallium arsenide (GaAs) tunneling junction layers 304, 303 may be deposited over the nucleation layer to provide a low resistance pathway between the bottom and middle subcells.

[0168] In some embodiments, Distributed Bragg reflector (DBR) layers 305 are then grown adjacent to and between the tunnel diode 303, 304 of the bottom subcell D and the third solar subcell C. The DBR layers 305 are arranged so that light can enter and pass through the third solar subcell C and at least a portion of which can be reflected back into the third solar subcell C by the DBR layers 305. In the embodiment depicted in FIG. 3, the distributed Bragg reflector (DBR) layers 305 are specifically located between the third solar subcell C and tunnel diode layers 304, 303; in other embodiments, the distributed Bragg reflector (DBR) layers may be located between tunnel diode layers 304/303 and buffer layer 302.

[0169] For some embodiments, distributed Bragg reflector (DBR) layers 305 can be composed of a plurality of alternating layers 305a through 305z of lattice matched materials with discontinuities in their respective indices of refraction. For certain embodiments, the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.

[0170] For some embodiments, distributed Bragg reflector (DBR) layers 305a through 305z includes a first DBR layer composed of a plurality of p type In.sub.zAl.sub.xGa.sub.1-x-zAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type In.sub.wAl.sub.yGa.sub.1-y-wAs layers, 0<w<1, 0<x<1, 0<y<1, 0<z<1, and where y is greater than x.

[0171] Although the present disclosure depicts the DBR layer 305 situated between the third and the fourth subcell, in other embodiments, DBR layers may be situated between the first and second subcells, and/or between the second and the third subcells, and/or between the third and the fourth subcells.

[0172] In the illustrated example of FIG. 2, the third subcell C includes a highly doped p-type aluminum indium gallium arsenide (AlInGaAs) back surface field (BSF) layer 306, a p-type InGaAs base layer 307, a highly doped n-type indium gallium arsenide (InGaAs) emitter layer 308 and a highly doped n-type indium aluminum phosphide (AInP2) or indium gallium phosphide (GaInP) window layer 309. The InGaAs base layer 307 of the subcell C can include, for example, approximately 1.5% In. Other compositions may be used as well. The base layer 307 is formed over the BSF layer 306 after the BSF layer is deposited over the DBR layers 305.

[0173] The window layer 309 is deposited on the emitter layer 308 of the subcell C. The window layer 309 in the subcell C also helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions. Before depositing the layers of the subcell B, heavily doped n-type InGaP and p-type AlGaAs (or other suitable compositions) tunneling junction layers 310, 311 may be deposited over the subcell C.

[0174] The second subcell B includes a highly doped p-type aluminum indium gallium arsenide (AlInGaAs) back surface field (BSF) layer 312, a p-type AlInGaAs base layer 313, a highly doped n-type indium gallium phosphide (InGaP.sub.2) or AlInGaAs layer 314 and a highly doped n-type indium gallium aluminum phosphide (AlGaAlP) window layer 315. The InGaP emitter layer 314 of the subcell B can include, for example, approximately 50% In. Other compositions may be used as well.

[0175] Before depositing the layers of the top or upper first cell A, heavily doped n-type InGaP and p-type AlGaAs tunneling junction layers 316, 317 may be deposited over the subcell B.

[0176] In the illustrated example, the top subcell A includes a highly doped p-type indium aluminum phosphide (InAlP.sub.2) BSF layer 318, a p-type InGaAlP base layer 319, a highly doped n-type InGaAlP emitter layer 320 and a highly doped n-type InAlP.sub.2 window layer 321. The base layer 319 of the top subcell A is deposited over the BSF layer 318 after the BSF layer 318 is formed.

[0177] After the cap or contact layer 322 is deposited, the grid lines are formed via evaporation and lithographically patterned and deposited over the cap or contact layer 322.

[0178] In some embodiments, at least the base of at least one of the first, second or third solar subcells has a graded doping, i.e., the level of doping varies from one surface to the other throughout the thickness of the base layer. In some embodiments, the gradation in doping is exponential. In some embodiments, the gradation in doping is incremental and monotonic.

[0179] In some embodiments, the emitter of at least one of the first, second or third solar subcells also has a graded doping, i.e., the level of doping varies from one surface to the other throughout the thickness of the emitter layer. In some embodiments, the gradation in doping is linear or monotonically decreasing.

[0180] As a specific example, the doping profile of the emitter and base layers may be illustrated in FIG. 3, which depicts the amount of doping in the emitter region and the base region of a subcell. N-type dopants include silicon, selenium, sulfur, germanium or tin. P-type dopants include silicon, zinc, chromium, or germanium.

[0181] In the example of FIG. 3, in some embodiments, one or more of the subcells have a base region having a gradation in doping that increases from a value in the range of 110.sup.15 to 110.sup.18 free carriers per cubic centimeter adjacent the p-n junction to a value in the range of 110.sup.16 to 410.sup.18 free carriers per cubic centimeter adjacent to the adjoining layer at the rear of the base, and an emitter region having a gradation in doping that decreases from a value in the range of approximately 510.sup.18 to 110.sup.17 free carriers per cubic centimeter in the region immediately adjacent the adjoining layer to a value in the range of 510.sup.15 to 110.sup.18 free carriers per cubic centimeter in the region adjacent to the p-n junction.

[0182] The heavy lines 612 and 613 shown in FIG. 3 illustrates one embodiment of the base doping having an exponential gradation, and the emitter doping being linear.

[0183] Thus, the doping level throughout the thickness of the base layer may be exponentially graded from the range of 110.sup.16 free carriers per cubic centimeter to 110.sup.18 free carriers per cubic centimeter, as represented by the curve 613 depicted in the Figure.

[0184] Similarly, the doping level throughout the thickness of the emitter layer may decline linearly from 510.sup.18 free carriers per cubic centimeter to 510.sup.17 free carriers per cubic centimeter as represented by the curve 612 depicted in the Figure.

[0185] The absolute value of the collection field generated by an exponential doping gradient exp [x/k] is given by the constant electric field of magnitude E=kT/q(1/))(exp[x.sub.b/]), where k is the Boltzman constant, T is the absolute temperature in degrees Kelvin, q is the absolute value of electronic change, and is a parameter characteristic of the doping decay.

[0186] The efficacy of an embodiment of the doping arrangement present disclosure has been demonstrated in a test solar cell which incorporated an exponential doping profile in the three micron thick base layer a subcell, according to one embodiment.

[0187] The exponential doping profile taught by one embodiment of the present disclosure produces a constant field in the doped region. In the particular multijunction solar cell materials and structure of the present disclosure, the bottom subcell has the smallest short circuit current among all the subcells. Since in a multijunction solar cell, the individual subcells are stacked and form a series circuit, the total current flow in the entire solar cell is therefore limited by the smallest current produced in any of the subcells. Thus, by increasing the short circuit current in the bottom cell, the current more closely approximates that of the higher subcells, and the overall efficiency of the solar cell is increased as well. In a multijunction solar cell with approximately efficiency, the implementation of the present doping arrangement would thereby increase efficiency. In addition to an increase in efficiency, the collection field created by the exponential doping profile will enhance the radiation hardness of the solar cell, which is important for spacecraft applications.

[0188] Although the exponentially doped profile is the doping design which has been implemented and verified, other doping profiles may give rise to a linear varying collection field which may offer yet other advantages. For example, another doping profile may produce a linear field in the doped region which would be advantageous for both minority carrier collection and for radiation hardness at the end-of-life (EOL) of the solar cell. Such other doping profiles in one or more base layers are within the scope of the present disclosure.

[0189] The doping profile depicted herein are merely illustrative, and other more complex profiles may be utilized as would be apparent to those skilled in the art without departing from the scope of the present invention.

[0190] FIG. 4A is a cross-sectional view of a first embodiment of a multijunction solar cell 200 after several stages of fabrication including the growth of certain semiconductor layers on a p-type growth substrate of p type germanium up to the contact layer 322, with various subcells being similar to the structure described and depicted in FIG. 2. In the interest of brevity, the description of layers 350, 300 to 304, and 306 through 322 will not be repeated here.

[0191] Although the depicted embodiment is a five junction solar cell 200 with three lattice matched upper subcells A, B, C which are lattice mismatched from lower germanium subcells D and E, in other embodiments, there may be two lattice matched upper subcells, and/or one lower germanium subcell. The lattice constant graph on the left-hand side of the Figure depicts the change in lattice constant through the thickness of the solar cell.

[0192] In the five-junction embodiment, illustrated in FIG. 4A, the bottom subcell 300/301 is labelled Subcell E. Furthermore, on top of the p++ tunnel diode layer 304 is a highly doped p-type back surface field (BSF) layer 405, an epitaxially grown p-type Ge base layer 406, and an epitaxially grown highly doped n-type germanium emitter layer 407, forming Subcell D.

[0193] On top of the emitter layer 407, heavily doped n++ type and p++ type tunneling junction layers 408 and 409 are deposited.

[0194] In the embodiment depicted in FIG. 4A, an intermediate graded interlayer 505, comprising in one embodiment step-graded sublayers 505a through 505z, is disposed over the tunnel diode layer 409. In particular, the graded interlayer provides a transition in the in-plane lattice constant from the lattice constant of the substrate and subcell D to the larger lattice constant of the middle and upper subcells C, B and A.

[0195] The graph on the left side of FIG. 4A depicts the in-plane lattice constant being incrementally monotonically increased from sublayer 505a through sublayer 505z, such sublayers being fully relaxed.

[0196] A metamorphic layer (or graded interlayer) 505 is preferably a compositionally step-graded series of p-type InGaAs or InGaAlAs layers, preferably with monotonically changing lattice constant, so as to achieve a gradual transition in lattice constant in the semiconductor structure from subcell D to subcell C while minimizing threading dislocations from occurring. In one embodiment, the band gap of layer 505 is constant throughout its thickness, preferably approximately equal to 1.22 to 1.34 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell C. In another embodiment, the band gap of the sublayers of layer 505 vary in the range of 1.22 to 1.34 eV, with the first layer having a relatively high band gap, and subsequent layers incrementally lower band gaps. One embodiment of the graded interlayer may also be expressed as being composed of In.sub.xGa.sub.1-xAs, with 0<x<1, 0<y<1, and x and y selected such that the band gap of the interlayer remains constant at approximately 1.22 to 1.34 eV or other appropriate band gap.

[0197] In one embodiment, aluminum is added to one sublayer to make one particular sublayer harder than another, thereby forcing dislocations in the softer material.

[0198] In the surfactant assisted growth of the metamorphic layer 505, a suitable chemical element is introduced into the reactor during the growth of layer 505 to improve the surface characteristics of the layer. In the preferred embodiment, such element may be a dopant or donor atom such as selenium (Se) or tellurium (Te). Small amounts of Se or Te are therefore incorporated in the metamorphic layer 406, and remain in the finished solar cell. Although Se or Te are the preferred n-type dopant atoms, other non-isoelectronic surfactants may be used as well.

[0199] Surfactant assisted growth results in a much smoother or planarized surface. Since the surface topography affects the bulk properties of the semiconductor material as it grows and the layer becomes thicker, the use of the surfactants minimizes threading dislocations in the active regions, and therefore improves overall solar cell efficiency.

[0200] As an alternative to the use of non-isoelectronic one may use an isoelectronic surfactant. The term isoelectronic refers to surfactants such as antimony (Sb) or bismuth (Bi), since such elements have the same number of valence electrons as the P atom of InGaP, or the As atom in InGaAlAs, in the metamorphic buffer layer. Such Sb or Bi surfactants will not typically be incorporated into the metamorphic layer 505.

[0201] In one embodiment of the present disclosure, the layer 505 is composed of a plurality of layers of InGaAs, with monotonically changing lattice constant, each layer having a band gap in the range of 1.22 to 1.34 eV. In some embodiments, the band gap is constant in the range of 1.27 to 1.31 eV through the thickness of layer 505. In some embodiments, the constant band gap is in the range of 1.28 to 1.29 eV.

[0202] The advantage of utilizing a constant bandgap material such as InGaAs is that arsenide-based semiconductor material is much easier to process in standard commercial MOCVD reactors.

[0203] Although the described embodiment of the present disclosure utilizes a plurality of layers of InGaAs for the metamorphic layer 505 for reasons of manufacturability and radiation transparency, other embodiments of the present disclosure may utilize different material systems to achieve a change in lattice constant from subcell C to subcell D. Other embodiments of the present disclosure may utilize continuously graded, as opposed to step graded, materials. More generally, the graded interlayer may be composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter less than or equal to that of the third solar subcell C and greater than or equal to that of the fourth solar subcell D. In some embodiments, the layer 505 has a band gap energy greater than that of the third solar subcell C, and in other embodiments has a band gap energy level less than that of the third solar subcell C.

[0204] FIG. 4B depicts a second embodiment of an upright metamorphic multijunction solar cell 400 grown on an n type germanium substrate 390. Although the depicted embodiment is a five junction solar cell with three lattice matched upper subcells A, B, C which are lattice mismatched from lower germanium subcells D and E, in other embodiments there may be two lattice matched upper subcells, and/or one lower germanium subcell. The lattice constant graph on the left-hand side of the Figure depicts the change in lattice constant through the thickness of the solar cell.

[0205] In FIG. 4B various subcells are similar to the structure described and depicted in FIG. 4A and in the interest of brevity, the description of layers 405 to 409, and 306 through 322 will not be repeated here.

[0206] Since in this embodiment, the growth substrate 390 does not include a photovoltaic junction, a tunnel diode consisting of an n++ layer 391 is grown directly over the growth substrate 390, and a p++ layer 392 of the tunnel diode is grown over the n++ layer 391.

[0207] A BSF layer 393 is then grown over the p++ layer 392. Subcell E, consisting of a p type germanium base layer 401 and an n+ type germanium emitter layer 402 is then grown over the BSF layer 393.

[0208] In the embodiment depicted in FIG. 4B, similar to that of FIG. 4A, an intermediate graded interlayer 505, comprising in one embodiment step-graded sublayers 505a through 505z, is disposed over the tunnel diode layer 409. In particular, the graded interlayer provides a transition in lattice constant from the lattice constant of the substrate to the larger lattice constant of the middle and upper subcells. In some embodiments, the top or uppermost sublayer of the graded interlayer 506 is strained or only partially relaxed, and has a lattice constant which is greater than that of the layer above it, i.e., the alpha layer 507 (should there be a second alpha layer) or the BSF layer 306. In short, in this embodiment, there is an overshoot of the last one sublayer 505z of the grading sublayers, so that the step-grading of the lattice constant becoming larger from layer 505a to 505y, and then decreasing back to the lattice constant of the upper layers 507 through 322 in layer 505z.

[0209] FIG. 5A depicts a cross-sectional view of a first embodiment 500 of an inverted metamorphic multijunction solar cell according to the present disclosure after the sequential formation of the five subcells A, B, C, D, and E on a GaAs growth substrate. More particularly, there is shown a growth substrate 101, which is preferably gallium arsenide (GaAs) or other suitable material.

[0210] In the case of a GaAs substrate, a metamorphic layer 150 is deposited directly on the substrate 101. Over the metamorphic layer, an etch stop layer 103 is further deposited. In the case of GaAs substrate, the metamorphic layer 150 is preferably InGaAs. A contact layer 104 of n++ GaAs is then deposited on layer 103, and a window layer 105 of AlInP is deposited on the contact layer. The subcell A, consisting of an n+ emitter layer 106 and a p-type base layer 107, is then epitaxially deposited on the window layer 105, which will form the top subcell of the solar cell after removal of the growth substrate.

[0211] It should be noted that the multijunction solar cell structure could be formed by any suitable combination of group III to V elements listed in the periodic table subject to lattice constant and bandgap requirements, wherein the group III includes boron (B), aluminum (Al), gallium (Ga), indium (In), and thallium (T). The group IV includes carbon (C), silicon (Si), germanium (Ge), and tin (Sn). The group V includes nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi).

[0212] In one embodiment, the emitter layer 106 is composed of InGa(Al)P2 and the base layer 107 is composed of InGa(Al)P2. The aluminum or Al term in parenthesis in the preceding formula means that Al is an optional constituent, and in this instance may be used in an amount ranging from 0% to 40%.

[0213] On top of the base layer 107 a back surface field (BSF) layer 108 preferably p+ InAlP or AlGaInP is deposited and used to reduce recombination loss.

[0214] The BSF layer 108 drives minority carriers from the region near the base/BSF interface surface to minimize the effect of recombination loss. In other words, a BSF layer 108 reduces recombination loss at the backside of the solar subcell A and thereby reduces the recombination in the base.

[0215] On top of the BSF layer 108 is deposited a sequence of heavily doped p-type and n-type layers 109a and 109b that forms a tunnel diode, i.e., an ohmic circuit element that connects subcell A to subcell B. Layer 109a is preferably composed of p++ AlGaAs, and layer 109b is preferably composed of n++ InGaP.

[0216] A window layer 110 is deposited on top of the tunnel diode layers 109a/109b, and is preferably n+ AlInGaP. The advantage of utilizing AlInGaP as the material constituent of the window layer 110 is that it has an index of refraction that closely matches the adjacent emitter layer 111, as more fully described in U.S. Patent Application Pub. No. 2009/0272430 A1 (Cornfeld et al.). The window layer 110 used in the subcell B also operates to reduce the interface recombination loss. It should be apparent to one skilled in the art, that additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present disclosure.

[0217] On top of the window layer 110 the layers of subcell B are deposited: the n-type emitter layer 111 and the p-type base layer 112. These layers are preferably composed of InGaP or AlGaAs and AlInGaAs respectively, although any other suitable materials consistent with lattice constant and bandgap requirements may be used as well. Thus, in other embodiments, subcell B may be composed of a GaAs, InGaP, AlGaInAs, AlGaAsSb, GaInAsP, or AlGaInAsP, emitter region and a GaAs, InGaP, AlGaInAs, AlGaAsSb, GaInAsP, or AlGaInAsP base region.

[0218] In previously disclosed implementations of an inverted metamorphic solar cell, the second subcell or subcell B was a homostructure. In the present disclosure, similarly to the structure disclosed in U.S. Patent Application Pub. No. 2009/0078310 A1 (Stan et al.), the second subcell or subcell B becomes a heterostructure with an InGaP emitter and its window is converted from InAlP to AlInGaP. This modification reduces the refractive index discontinuity at the window/emitter interface of the second subcell, as more fully described in U.S. Patent Application Pub. No. 2009/0272430 A1 (Cornfeld et al.). Moreover, the window layer 110 is preferably is doped three times that of the emitter 111 to move the Fermi level up closer to the conduction band and therefore create band bending at the window/emitter interface which results in constraining the minority carriers to the emitter layer.

[0219] On top of the cell B is deposited a BSF layer 113 which performs the same function as the BSF layer 109. The p++/n++ tunnel diode layers 114a and 114b respectively are deposited over the BSF layer 113, similar to the layers 109a and 109b, forming an ohmic circuit element to connect subcell B to subcell C. The layer 114a is preferably composed of p++ AlGaAs, and layer 114b is preferably composed of n++ InGaP.

[0220] A window layer 118 preferably composed of n+ type GaInP is then deposited over the tunnel diode layer 114b. This window layer operates to reduce the recombination loss in subcell C. It should be apparent to one skilled in the art that additional layers may be added or deleted in the cell structure without departing from the scope of the present disclosure.

[0221] On top of the window layer 118, the layers of cell C are deposited: the n+ emitter layer 119, and the p-type base layer 120. These layers are preferably composed of n+ type (In)GaAs and p type (In)GaAs respectively, or n+ type InGaP and p type GaAs for a heterojunction subcell, although another suitable materials consistent with lattice constant and bandgap requirements may be used as well.

[0222] In some embodiments, subcell C may have a band gap between 1.40 eV and 1.42 eV. Grown in this manner, the cell has the same lattice constant as GaAs but has a low percentage of Indium 0%<In<1% to slightly lower the band gap of the subcell without causing it to relax and create dislocations. In this case, the subcell remains lattice matched, albeit strained, and has a lower band gap than GaAs. This helps improve the subcell short circuit current slightly and improve the efficiency of the overall solar cell.

[0223] In some embodiments, the third subcell or subcell C may have quantum wells or quantum dots that effectively lower the band gap of the subcell to approximately 1.3 eV. All other band gap ranges of the other subcells described above remain the same. In such embodiment, the third subcell is still lattice matched to the GaAs substrate. Quantum wells are typically strain balanced by incorporating lower band gap or larger lattice constant InGaAs (e.g. a band gap of 1.3 eV) and higher band gap or smaller lattice constant GaAsP. The larger/smaller atomic lattices/layers of epitaxy balance the strain and keep the material lattice matched.

[0224] A BSF layer 121, preferably composed of (In)GaAlAs, is then deposited on top of the cell C, the BSF layer performing the same function as the BSF layers 108 and 113.

[0225] The p++/n++ tunnel diode layers 122a and 122b respectively are deposited over the BSF layer 121, similar to the layers 114a and 114b, forming an ohmic circuit element to connect subcell C to subcell D. The layer 122a is preferably composed of p++(In)GaAs, and layer 122b is preferably composed of n++(In)GaAs.

[0226] A window layer 126 preferably composed of n+ type InGaAlAs is then deposited over the tunnel diode layer 122b. This window layer operates to reduce the recombination loss in the fourth subcell D. It should be apparent to one skilled in the art that additional layers may be added or deleted in the cell structure without departing from the scope of the present disclosure.

[0227] On top of the window layer 126, the layers of cell D are deposited: the n+ emitter layer 127, and the p-type base layer 128. These layers are preferably composed of n+ type Ge and p type Ge, InGaAs or InGaP respectively, although another suitable materials consistent with lattice constant and bandgap requirements may be used as well.

[0228] A BSF layer 129, preferably composed of p+ type InGaAlAs, is then deposited on top of the cell D, the BSF layer performing the same function as the BSF layers 108, 113 and 121.

[0229] The p++/n++ tunnel diode layers 130a and 130b respectively are deposited over the BSF layer 129, similar to the layers 122a/122b and 109a/109b, forming an ohmic circuit element to connect subcell D to subcell E. The layer 130a is preferably composed of p++ AlGaInAs, and layer 130b is preferably composed of n++ GaInP.

[0230] A window layer 134 preferably composed of n+ type GaInP is then deposited over the tunnel diode layer 130b. This window layer operates to reduce the recombination loss in the fifth subcell E. It should be apparent to one skilled in the art that additional layers may be added or deleted in the cell structure without departing from the scope of the present invention.

[0231] On top of the window layer 134, the layers of cell E are deposited: the n+ emitter layer 135, and the p-type base layer 136. These layers are preferably composed of n+ type Ge and p type Ge, InGaAs, or InGaP respectively, although other suitable materials consistent with lattice constant and band gap requirements may be used as well.

[0232] A BSF layer 137, preferably composed of p+ type AlGaInAs, is then deposited on top of the cell E, the BSF layer performing the same function as the BSF layers 108, 113, 121 and 129.

[0233] Finally, a high band gap contact layer 138, preferably composed of p++ type AlGaInAs, deposited on the BSF layer 137.

[0234] The composition of this contact layer 138 located at the bottom (non-illuminated) side of the lowest band gap photovoltaic cell (i.e., subcell E in the depicted embodiment) in a multijunction photovoltaic cell, can be formulated to reduce absorption of the light that passes through the cell, so that (i) the backside ohmic metal contact layer below it (on the non-illuminated side) will also act as a mirror layer, and (ii) the contact layer doesn't have to be selectively etched off, to prevent absorption.

[0235] It should be apparent to one skilled in the art, that additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present invention.

[0236] A metal contact layer 139 is deposited over the p++ semiconductor contact layer 138. The metal is the sequence of metal layers Ti/Au/Ag/Au in some embodiments.

[0237] The metal contact scheme chosen is one that has a planar interface with the semiconductor, after heat treatment to activate the ohmic contact. This is done so that (1) a dielectric layer separating the metal from the semiconductor doesn't have to be deposited and selectively etched in the metal contact areas; and (2) the contact layer is specularly reflective over the wavelength range of interest.

[0238] Optionally, an adhesive layer (e.g., Wafer Bond, manufactured by Brewer Science, Inc. of Rolla, Mo.) can be deposited over the metal layer 131, and a surrogate substrate can be attached. In some embodiments, the surrogate substrate may be sapphire. In other embodiments, the surrogate substrate may be GaAs, Ge or Si, or other suitable material. The surrogate substrate can be about 40 mils in thickness, and can be perforated with holes about 1 mm in diameter, spaced 4 mm apart, to aid in subsequent removal of the adhesive and the substrate. As an alternative to using an adhesive layer, a suitable substrate (e.g., GaAs) may be eutectically or permanently bonded to the metal layer 131.

[0239] Following attachment of the surrogate substrate, the growth substrate 101 can be removed by a sequence of lapping and/or etching steps in which the substrate 101, and the metamorphic layer 102 are removed. The choice of a particular etchant is growth substrate dependent.

[0240] FIG. 5B depicts a cross-sectional view of a second embodiment 600 of an inverted multijunction solar cell 600 according to the present disclosure after the sequential formation of five subcells A, B, C, D and E on a Ge growth substrate. In this embodiment, all of the subcells are substantially lattice matched to the substrate, as represented by the lattice constant graph depicted on the left side of the Figure.

[0241] Over the Ge substrate 201 an etch stop layer 103 is deposited. The formation and composition of layers 103 through 139 are substantially similar to that described in connection with FIG. 5A and therefore will not be repeated here for brevity.

[0242] FIG. 5C depicts a cross-sectional view of a third embodiment 700 of an inverted multijunction solar cell 700 according to the present disclosure after the sequential formation of five subcells A, B, C, D and E on a GaAs growth substrate. In this embodiment, the subcells A, B, and C are substantially lattice matched to the substrate, as represented by the lattice constant graph depicted on the left side of the Figure, but subcells D and E are lattice mismatched from the substrate.

[0243] Over the GaAs substrate 101 a buffer layer 102 of InGaAs is provided, and an etch stop layer 103 is deposited over the buffer layer 102. The formation and composition of layers 103 through 122 are substantially similar to that described in connection with FIG. 5B and therefore will not be repeated here for brevity.

[0244] FIG. 6 is a cross-sectional view of the embodiment of the solar cell of FIG. 5A with the top subcell A now depicted at the top of the drawing, and the metal contact layer 131 being at the bottom of the Figure, with the original substrate having been removed. In addition, the etch stop layer 103 has been removed, for example, by using a HCl/H.sub.2O solution. Similar depictions of the solar cells of FIGS. 5B and 5C are omitted for brevity.

[0245] It should be apparent to one skilled in the art, that additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present disclosure.

[0246] FIG. 7 is a cross-sectional view of a portion of the solar cell assembly according to the present disclosure as mounted on a panel or supporting substrate, with the Figure depicting two adjacent solar cells 601 and 701 and corresponding CICs 600 and 700 respectively.

[0247] As previously noted, for space applications, the solar cell 601, 701 includes a coverglass 603, 703 respectively over the semiconductor device to provide radiation resistant shielding from particles in the space environment which could damage the semiconductor material. The cover glass 603, 703 is typically a ceria doped borosilicate glass which is typically from three to six mils in thickness and attached by a transparent adhesive 602, 702 respectively to the corresponding solar cell 601, 701.

[0248] Bonding pads of a first and second polarity type are provided on each solar cell. In one embodiment, a back metal 604 and 704 respectively form contacts of a first polarity type. On the top surface of each solar cell, a metal contact 705 is provided along one edge of the solar cell form a contact of the second polarity.

[0249] A plurality of electrical interconnects 607 each composed of a strip of silver-plated nickel-cobalt ferrous alloy material are provided, each interconnect being welded to a respective bonding pad 612 and 705 on each solar cell assembly to electrically connect the adjacent solar cell assemblies of the array in a series electrical circuit.

[0250] In some embodiments, an aluminum honeycomb panel 606 having a carbon composite face sheet 605 with a coefficient of thermal expansion (CTE) that substantially matches the germanium of the fourth solar subcell in each solar cell is provided with each CIC 600, 700 or solar cell assembly mounted thereon.

[0251] Another feature of the solar cell assembly in the embodiment illustrated in FIG. 6 is that the cover glasses 603, 703 each have a metal wrap-around clip 608 depicted in CIC 600 which makes contact with the surface of the cover glass 603, and extends down the gap or space along the side of the solar cell assembly 600 between CICs 600 and 700 to make electrical contact with the metal bonding pad 612 on the back surface of CIC 600, which in turn makes contact with electrical ground. Thus the clip 608 grounds the electrical charge build-up on the surface of the cover glass 603 to the ground of the panel or spacecraft. Other configurations of grounding techniques for the surface(s) of the coverglass 603, 703 are within the scope of this disclosure.

[0252] FIG. 8 is a graph representing the band gap of certain binary materials and their lattice constants. The band gap and lattice constants of ternary materials are located on the lines drawn between typical associated binary materials (such as the ternary material AlGaAs being located between the GaAs and AlAs points on the graph, with the band gap of the ternary material lying between 1.42 eV for GaAs and 2.16 eV for AlAs depending upon the relative amount of the individual constituents). Thus, depending upon the desired band gap, the material constituents of ternary materials can be appropriately selected for growth.

[0253] FIG. 9 is an enlargement of a portion of the graph of FIG. 8 illustrating different compounds of GaInAs and GaInP with different proportions of gallium and indium, and the location of specific compounds on the graph.

[0254] The present disclosure provides a multijunction solar cell that follows a design rule that one should incorporate as many high band gap subcells as possible to achieve the goal to increase the efficiency at high temperature EOL. For example, high band gap subcells may retain a greater percentage of cell voltage as temperature increases, thereby offering lower power loss as temperature increases. As a result, both HT-BOL and HT-EOL performance of the exemplary multijunction solar cell, according to the present disclosure, may be expected to be greater than traditional cells.

[0255] For example, the cell efficiency (%) measured at room temperature (RT) 28 C. and high temperature (HT) 70 C., at beginning of life (BOL) and end of life (EOL), for a standard three junction commercial solar cell (ZTJ), is shown in Table 1:

TABLE-US-00001 TABLE 1 CONDITION EFFICIENCY BOL 28 C. 29.1% BOL 70 C. 26.4% EOL 70 C. 23.4% After 5E14 e/cm.sup.2 radiation EOL 70 C. 22.0% After 1E15 e/cm.sup.2 radiation

[0256] For the solar cell initially described in the parent application, U.S. patent application Ser. No. 14/828,206 filed Aug. 17, 2015 (and corresponding published European Patent Application EP 3 133 65 A1) the corresponding data is shown in Table 2:

TABLE-US-00002 TABLE 2 CONDITION EFFICIENCY BOL 28 C. 29.1% BOL 70 C. 26.5% EOL 70 C. 24.5% After 5E14 e/cm.sup.2 radiation EOL 70 C. 23.5% After 1E15 e/cm.sup.2 radiation
The solar cell described in the earliest applications of Applicant has a slightly higher cell efficiency than the standard commercial solar cell (ZTJ) at BOL at 70 C. However, the solar cell described in one embodiment of the disclosure exhibits substantially improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 510.sup.14 e/cm.sup.2, and dramatically improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 110.sup.15 e/cm.sup.2.

[0257] In view of different satellite and space vehicle requirements in terms of operating environmental temperature, radiation exposure, and operational life, a range of subcell designs using the design principles of the present disclosure may be provided satisfying specific defined customer and mission requirements, and several illustrative embodiments are set forth hereunder, along with the computation of their efficiency at the end-of-life for comparison purposes. As described in greater detail below, solar cell performance after radiation exposure is experimentally measured using 1 MeV electron fluence per square centimeter (abbreviated in the text that follows as e/cm2), so that a comparison can be made between the current commercial devices and embodiments of solar cells discussed in the present disclosure.

[0258] As an example of different mission requirements, a low earth orbit (LEO) satellite will typically experience radiation equivalent to 510.sup.14 electron fluence per square centimeter (hereinafter written as 5E14 e/cm.sup.2) over a five year lifetime. A geosynchronous earth orbit (GEO) satellite will typically experience radiation in the range of 510.sup.14 e/cm.sup.2 to 110.sup.15 e/cm.sup.2 over a fifteen year lifetime.

[0259] The simplest manner to express the different embodiments of the present disclosure and their efficiency compared to that of the standard solar cell noted above is to list the embodiments with the specification of the composition of each successive subcell and their respective band gap, and then the computed efficiency.

[0260] Thus, for a four junction solar cell as configured and described in the present disclosure, and Ser. No. 15/873,135 filed Jan. 17, 2018, four embodiments and their corresponding efficiency data at the end-of-life (EOL) is as follows:

TABLE-US-00003 Embodiment 1 BandGap Composition Subcell A 2.1 AlInGaP Subcell B 1.73 InGaP/AlInGaAs or AlinGaAs/AlInGaAs Subcell C 1.41 InGaAs Subcell D 0.67 Ge [0261] Efficiency at 70 C. after 5E14 e/cm.sup.2 radiation: 24.5% [0262] Efficiency at 70 C. after 1E15 e/cm.sup.2 radiation: 23.5%

TABLE-US-00004 Embodiment2 BandGap Composition Subcell A 2.1 AlInGaP Subcell B 1.67 InGaP/AlInGaAs or AlinGaAs/AlInGaAs Subcell C 1.34 InGaAs Subcell D 0.67 Ge [0263] Efficiency at 70 C. after 1E15 e/cm.sup.2 radiation: 24.9%

TABLE-US-00005 Embodiment 3 Band Gap Composition Subcell A 2.1 AlInGaP Subcell B 1.65 InGaP/AlInGaAs or AlinGaAs/AlInGaAs Subcell C 1.30 (In)GaAs Subcell D 0.67 Ge [0264] Efficiency at 70 C. after 1E15 e/cm.sup.2 radiation: 25.3%

TABLE-US-00006 Embodiment4 BandGap Composition Subcell A 2.03 AlInGaP Subcell B 1.55 InGaP/AlInGaAs or AlinGaAs/AlInGaAs Subcell C 1.2 (In)GaAs Subcell D 0.67 Ge [0265] Efficiency at 70 C. after 1E15 e/cm.sup.2 radiation: 25.7%

[0266] Although the differences in band gap among the various embodiments described above, i.e., of the order of 0.1 to 0.2 eV, may seem relatively small, it is evident that such adjustments result in an increase in the EOL solar cell efficiency from 24.4% as reported in the parent application U.S. patent application Ser. No. 14/828,206 filed Aug. 17, 2015 (and corresponding published European Patent Application EP 3 133 650 A1) to 25.7% for the solar cell of embodiment 4 described above, which is certainly a surprising and unexpected improvement that would constitute an inventive step over the related configuration described in the parent application and European patent application publication.

[0267] The wide range of electron and proton energies present in the space environment necessitates a method of describing the effects of various types of radiation in terms of a radiation environment which can be produced under laboratory conditions. The methods for estimating solar cell degradation in space are based on the techniques described by Brown et al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of the Telstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963] and Tada [Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G. Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication 82-69, 1982]. In summary, the omnidirectional space radiation is converted to a damage equivalent unidirectional fluence at a normalised energy and in terms of a specific radiation particle. This equivalent fluence will produce the same damage as that produced by omnidirectional space radiation considered when the relative damage coefficient (RDC) is properly defined to allow the conversion. The relative damage coefficients (RDCs) of a particular solar cell structure are measured a priori under many energy and fluence levels in addition to different coverglass thickness values. When the equivalent fluence is determined for a given space environment, the parameter degradation can be evaluated in the laboratory by irradiating the solar cell with the calculated fluence level of unidirectional normally incident flux. The equivalent fluence is normally expressed in terms of 1 MeV electrons or 10 MeV protons.

[0268] The software package Spenvis (www.spenvis.oma.be) is used to calculate the specific electron and proton fluence that a solar cell is exposed to during a specific satellite mission as defined by the duration, altitude, azimuth, etc. Spenvis employs the EQFLUX program, developed by the Jet Propulsion Laboratory (JPL) to calculate 1 MeV and 10 MeV damage equivalent electron and proton fluences, respectively, for exposure to the fluences predicted by the trapped radiation and solar proton models for a specified mission environment duration. The conversion to damage equivalent fluences is based on the relative damage coefficients determined for multijunction cells [Marvin, D.C., Assessment of Multijunction Solar Cell Performance in Radiation Environments, Aerospace Report No. TOR-2000 (1210)-1, 2000]. A widely accepted total mission equivalent fluence for a geosynchronous satellite mission of 15 year duration is 1 MeV 110.sup.15 electrons/cm.sup.2.

[0269] The exemplary solar cell described herein may require the use of aluminum in the semiconductor composition of each of the top two subcells. Aluminum incorporation is widely known in the III-V compound semiconductor industry to degrade BOL subcell performance due to deep level donor defects, higher doping compensation, shorter minority carrier lifetimes, and lower cell voltage and an increased BOL E.sub.g/qV.sub.oc metric. In short, increased BOL E.sub.g/qV.sub.oc may be the most problematic shortcoming of aluminum containing subcells; the other limitations can be mitigated by modifying the doping schedule or thinning base thicknesses.

[0270] It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of structures or constructions differing from the types of structures or constructions described above.

[0271] Although described embodiments of the present disclosure utilizes a vertical stack of four subcells, various aspects and features of the present disclosure can apply to stacks with fewer or greater number of subcells, i.e. two junction cells, three junction cells, five, six, seven junction cells, etc.

[0272] In addition, although the disclosed embodiments are configured with top and bottom electrical contacts, the subcells may alternatively be contacted by means of metal contacts to laterally conductive semiconductor layers between the subcells. Such arrangements may be used to form 3-terminal, 4-terminal, and in general, n-terminal devices. The subcells can be interconnected in circuits using these additional terminals such that most of the available photogenerated current density in each subcell can be used effectively, leading to high efficiency for the multijunction cell, notwithstanding that the photogenerated current densities are typically different in the various subcells.

[0273] As noted above, the solar cell described in the present disclosure may utilize an arrangement of one or more, or all, homojunction cells or subcells, i.e., a cell or subcell in which the p-n junction is formed between a p-type semiconductor and an n-type semiconductor both of which have the same chemical composition and the same band gap, differing only in the dopant species and types, and one or more heterojunction cells or subcells. Subcell C, with p-type and n-type InGaAs is one example of a homojunction subcell.

[0274] In some cells, a thin so-called intrinsic layer may be placed between the emitter layer and base layer, with the same or different composition from either the emitter or the base layer. The intrinsic layer may function to suppress minority-carrier recombination in the space-charge region. Similarly, either the base layer or the emitter layer may also be intrinsic or not-intentionally-doped (NID) over part or all of its thickness.

[0275] The composition of the window or BSF layers may utilize other semiconductor compounds, subject to lattice constant and band gap requirements, and may include AIlnP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AIN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present invention.

[0276] While the solar cell described in the present disclosure has been illustrated and described as embodied in a conventional multijunction solar cell, it is not intended to be limited to the details shown, since it is also applicable to inverted metamorphic solar cells, and various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

[0277] Thus, while the description of the semiconductor device described in the present disclosure has focused primarily on solar cells or photovoltaic devices, persons skilled in the art know that other optoelectronic devices, such as thermophotovoltaic (TPV) cells, photodetectors and light-emitting diodes (LEDS), are very similar in structure, physics, and materials to photovoltaic devices with some minor variations in doping and the minority carrier lifetime. For example, photodetectors can be the same materials and structures as the photovoltaic devices described above, but perhaps more lightly-doped for sensitivity rather than power production. On the other hand LEDs can also be made with similar structures and materials, but perhaps more heavily-doped to shorten recombination time, thus radiative lifetime to produce light instead of power. Therefore, this invention also applies to photodetectors and LEDs with structures, compositions of matter, articles of manufacture, and improvements as described above for photovoltaic cells.

[0278] Without further analysis, from the foregoing others can, by applying current knowledge, readily adapt the present invention for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.