INVERTED METAMORPHIC MULTIJUNCTION SOLAR CELLS FOR SPACE APPLICATIONS

Abstract

An inverted metamorphic multijunction solar cell including an upper first solar subcell, a second solar subcell and a third solar subcell. The upper first solar subcell has a first band gap and positioned for receiving an incoming light beam. The second solar subcell is disposed below and adjacent to, and is lattice matched with, the upper first solar subcell, and has a second band gap smaller than the first band gap. The third solar subcell is disposed below the second solar subcell, and is composed of a GaAs base and emitter layer so as to optimize the efficiency of the solar cell after exposure to radiation. In some implementations, at least one of the solar subcells has a graded band gap throughout its thickness.

Claims

1. A multijunction solar cell comprising: an upper first solar subcell having a first band gap; a second solar subcell adjacent to said upper first solar subcell and having a second band gap less than the first band gap; a third solar subcell adjacent to said second solar subcell and having a third band gap less than the first band gap; a first graded interlayer adjacent to said third solar subcell, said first graded interlayer having a fourth band gap greater than said third band gap; and a fourth solar subcell adjacent to said first graded interlayer, said fourth subcell having a fifth band gap less than said third band gap and such that said fourth subcell is lattice mismatched with respect to said third subcell; a second graded interlayer adjacent to said fourth solar subcell, said second graded interlayer having a sixth band gap greater than said fifth band gap; and a lower fifth solar subcell adjacent to said second graded interlayer, said lower fifth subcell having a seventh band gap less than said fifth band gap and such that said fifth subcell is lattice mismatched with respect to said fourth subcell, wherein the first graded interlayer is compositionally graded to lattice match the third solar subcell on one side and the lower fourth solar subcell on the other side, and is composed of (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 remains at a constant value in the range of 1.42 to 1.60 eV throughout the thickness of the first graded layer; and wherein the second graded interlayer is compositionally graded to lattice match the fourth solar subcell on one side and the lower fifth solar subcell on the other side, and is composed of (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 remains at a constant value in the range of 1.2 eV to 1.6 eV throughout the thickness of the second graded layer, and wherein the seventh band gap is in the range of approximately 0.83 to 0.85 eV, the fifth band gap is approximately 1.10 eV, the third band gap is in the range of 1.40 to 1.42 eV, the second band gap is of approximately 1.73 eV and the first band gap is of approximately 2.10 eV; and wherein the third solar subcell is composed of a GaAs emitter layer and a GaAs base layer so as to optimize energy conversion efficiency of the solar cell for a time that occurs after exposure to radiation at 1 MeV electron equivalent fluence area 5×10.sup.14 electrons/cm.sup.2 or more, rather than to optimize the energy conversion efficiency for a time coinciding with initial deployment of the solar cell in outer space.

2. A multijunction solar cell as defined in any of claim 1, wherein each solar subcell includes an emitter layer and a base layer, the emitter layer and the base layer forming a p-n photovoltaic junction and one or more of the solar subcells including a base layer having a gradation in doping that increases exponentially from 1×10.sup.15 free carriers per cubic centimeter adjacent the p-n photovoltaic junction to 4×10.sup.18 free carriers per cubic centimeter adjacent to an adjoining layer at a rear of the base layer, and an emitter layer having a gradation in doping that decreases from approximately 5×10.sup.18 free carriers per cubic centimeter in a region immediately adjacent the adjoining layer to 5×10.sup.17 free carriers per cubic centimeter in a region adjacent to the p-n photovoltaic junction.

3. A multijunction solar cell as defined in claim 1, wherein the upper first solar subcell is composed of AlGaInP, the second solar subcell is composed of an InGaP emitter layer and an AlGaAs base layer, and the fourth and fifth solar subcells are composed of InGaAs.

4. A multijunction solar cell as defined in of claim 1, further comprising: a distributed Bragg reflector (DBR) layer adjacent to and between the second and the third solar subcells and arranged so that light can enter and pass through the second solar subcell and at least a portion of the light can be reflected back into the second solar subcell by the DBR layer.

5. A multijunction solar cell as defined in claim 1, further comprising: a distributed Bragg reflector (DBR) layer adjacent to and between the third solar subcell and the first graded interlayer and arranged so that light can enter and pass through the third solar subcell and at least a portion of the light 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 and wherein the difference in refractive indices between alternating layers is optimized in order to reduce 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; and wherein the DBR layer includes a first DBR layer composed of a plurality of p type Al.sub.xGa.sub.1-xAs layers, 0<x<1, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type Al.sub.yGa.sub.1-yAs layers, 0<y<1 and where y is greater than x.

6. A multijunction solar cell as defined in claim 1, wherein the percentage of aluminum in the upper first solar subcell, or the second solar subcell or the third solar subcell, is between 17.5% and 25% by mole fraction.

7. A multijunction solar cell as defined in claim 1, wherein each solar subcell includes an emitter layer and a base layer, wherein at least a particular one of the solar subcells has a graded band gap throughout at least a portion of a thickness of its emitter layer or base layer.

8. A multijunction solar cell as defined in claim 7, wherein the band gap of the particular solar subcell decreases from a top surface of the particular solar subcell to a p-n junction of the particular solar subcell.

9. A multijunction solar cell as defined in claim 7, wherein the band gap of the particular solar subcell increases from a p-n junction of the particular solar subcell to a bottom surface of the particular solar subcell.

10. A multijunction solar cell as defined in claim 7, wherein a change in the band gap in the particular solar subcell is in the range of 0.05 eV to 1.0 eV.

11. A multijunction solar cell as defined in claim 7, wherein the band gap at a top surface of the particular solar subcell is equal to the band gap at a bottom surface of the particular solar subcell.

12. A multijunction solar cell as defined in claim 7, wherein a gradation in the band gap in an n type semiconductor region of the particular solar subcell is greater than a gradation in the band gap in a p type semiconductor region of the particular solar subcell.

13. A multijunction solar cell as defined in claim 8, wherein the particular solar subcell is the second solar subcell with a band gap of 1.7 eV at the top surface and a bottom surface of the particular solar subcell.

14. A multijunction solar cell as defined in claim 13, wherein the second solar subcell has a band gap of 1.6 eV at the p-n junction of that solar subcell.

15. A multijunction solar cell as defined in claim 7, wherein the band gap of (i) two or more of the solar subcells are graded; and (ii) at least one of the solar subcells is not graded.

16. A multijunction solar cell as defined in claim 7, wherein the band gap at a top surface of the particular solar subcell is equal to the band gap at a bottom surface of the particular solar subcell, and the particular solar subcell has a lattice constant that is constant throughout its thickness.

17. A multijunction solar cell as defined in claim 7, wherein a gradation in the band gap in an n type semiconductor region of the particular solar subcell is different from a gradation in the band gap in a p type semiconductor region of the particular solar subcell.

18. A multijunction solar cell as defined in claim 1, wherein composition of the third solar subcell and its band gap optimize solar cell efficiency at high temperature (in the range of 50 to 100 degrees Centigrade) in deployment in space at a specific predetermined time after the initial deployment (referred to as the beginning of life or BOL), such predetermined time being referred to as the end-of-life (EOL), and being at least five years after the BOL, such composition and band gap being designed not to maximize the solar cell efficiency at BOL but to increase the solar cell efficiency at the EOL while disregarding the solar cell efficiency achieved at the BOL, such that the solar cell efficiency designed at the BOL is less than the solar cell efficiency at the BOL that would be achieved if the selection were designed to maximize the solar cell efficiency at the BOL.

19. A multijunction solar cell comprising: an upper first solar subcell having a first band gap; a second solar subcell adjacent to said upper first solar subcell and having a second band gap; a third solar subcell adjacent to said second solar subcell and having a third band gap; a fourth solar subcell disposed below said third solar subcell, said fourth subcell having a fourth band gap less than the third band gap of said third solar subcell; and a fifth solar subcell disposed below said fourth solar subcell and forming a bottom solar subcell of the multijunction solar cell, said fifth solar subcell having a fifth band gap less than the fourth band gap and composed of GaInAs, wherein the third solar subcell is composed of a GaAs emitter layer and a GaAs base layer so as to optimize energy conversion efficiency of the solar cell after exposure to radiation a 1 MeV electron equivalent fluence over 5×10.sup.14 electrons/cm.sup.2 or more, and wherein composition of the third solar subcell is designed to increase solar cell efficiency at EOL while disregarding the solar cell efficiency at BOL, such that the solar cell efficiency at the BOL is less than the solar cell efficiency at the BOL that would be achieved if the composition were designed to maximize the solar cell efficiency at the BOL.

20. A method of manufacturing a solar cell comprising: providing a first substrate; depositing on the first substrate a first sequence of layers of semiconductor material forming an upper first solar subcell, a second solar subcell, and a third solar subcell; depositing on said third solar subcell a first graded interlayer; depositing on said first graded interlayer a second sequence of layers of semiconductor material forming a fourth solar subcell, the fourth solar subcell being lattice mismatched to the third solar subcell; depositing on said fourth solar subcell a second graded interlayer; depositing on said second graded interlayer a third sequence of layers of semiconductor material forming a fifth solar subcell, the fifth solar subcell being lattice mismatched to the fourth solar subcell; mounting and bonding a surrogate substrate on top of the third sequence of layers; and removing the first substrate; wherein the first graded interlayer is compositionally graded to lattice matched the third solar subcell on one side and the fourth solar subcell on the other side, and is composed of one or more As, P, N, Sb based III-V compound semiconductors subject to constraints of having an in-plane lattice parameter greater than or equal to that of the third solar subcell and less than or equal to that of the lower fourth solar subcell, and having a band gap energy greater than that of the third solar subcell and the fourth solar subcell; wherein the second graded interlayer is compositionally graded to lattice match the fourth solar subcell on one side and the lower fifth solar subcell on the other side, and is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to constraints of having an in-plane lattice parameter greater than or equal to that of the fourth solar subcell and less than or equal to that of the fifth solar subcell, and having a band gap energy greater than that of the fourth solar subcell; wherein the fifth solar subcell has a band gap in the range of approximately 0.83 to 0.85 eV, the fourth solar subcell has a band gap of approximately 1.10 eV, the third solar subcell has a band gap in the range of 1.40 to 1.42 eV, the second solar subcell has a band gap of approximately 1.73 eV and the upper first solar subcell has a band gap of approximately 2.10 eV, and the first graded interlayer is compositionally graded to lattice match the third solar subcell on one side and the lower fourth solar subcell on the other side, and is composed of (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 of the first graded interlayer is in a range of 1.42 to 1.60 eV throughout its thickness, and the second graded interlayer is compositionally graded to lattice match the fourth solar subcell on one side and the fifth solar subcell on the other side, and is composed of (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 of the second graded interlayer is in a range of 1.20 to 1.40 eV throughout its thickness; and wherein the upper first solar subcell is composed of AlGaInP, the second solar subcell is composed of an InGaP emitter layer and an AlGaAs base layer, the third solar subcell is composed of GaAs or In.sub.xGa.sub.1-xAs (with 0<x<0.01), and the fourth solar subcell is composed of InGaAs, and the fifth solar subcell is composed of GaInAs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The present disclosure will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

[0072] FIG. 1A is a graph of the band gap throughout the thickness of a solar subcell, according to a first embodiment of the present disclosure;

[0073] FIG. 1B is a cross-sectional view of the solar subcell of FIG. 1A depicting the movement of electrons and holes throughout the thickness of the layer due to the internal electric field produced by the graded doping;

[0074] FIG. 2A is a cross-sectional view of a first embodiment of an inverted metamorphic solar cell according to the present disclosure after an initial stage of fabrication including the deposition of certain semiconductor layers on the growth substrate;

[0075] FIG. 2B is a cross-sectional view of the solar cell of FIG. 2A after removal of the growth substrate, and with the first-grown subcell A depicted at the top of the Figures;

[0076] FIG. 3 is a cross-sectional view of a second embodiment of an inverted metamorphic multijunction solar cell after several stages of fabrication including the growth of certain semiconductor layers and removal of the growth substrate, according to the present disclosure; and

[0077] FIG. 4 is a graph depicting the comparison of the external quantum efficiency of a AlGaAs subcell as a function of wavelength for a subcell with a non-graded band gap and a graded band gap according to the present disclosure.

GLOSSARY OF TERMS

[0078] “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).

[0079] “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.

[0080] “Beginning of Life (BOL)” refers to the time at which a photovoltaic power system is initially deployed in operation.

[0081] “Bottom subcell” refers to the subcell in a multijunction solar cell which is furthest from the primary light source for the solar cell.

[0082] “Compound semiconductor” refers to a semiconductor formed using two or more chemical elements.

[0083] “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.

[0084] “Deposited”, with respect to a layer of semiconductor material, refers to a layer of material which is epitaxially grown over another semiconductor layer.

[0085] “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.

[0086] “Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

[0087] “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.

[0088] “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.

[0089] “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 more than 0.1% 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 more than 0.6% lattice constant difference).

[0090] “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.

[0091] “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).

[0092] “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.

[0093] “Short circuit current density”—see “current density”.

[0094] “Solar cell” refers to an electronic device operable to convert the energy of light directly into electricity by the photovoltaic effect.

[0095] “Solar cell assembly” refers to two or more solar cell subassemblies interconnected electrically with one another.

[0096] “Solar cell subassembly” refers to a stacked sequence of layers including one or more solar subcells.

[0097] “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.

[0098] “Substantially current matched” refers to the short circuit current through adjacent solar subcells being substantially identical (i.e. within plus or minus 1%).

[0099] “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.

[0100] “Top surface” of a subcell refers to the surface of the subcell which is closest to the primary light source for the solar cell.

[0101] “ZTJ” refers to the product designation of a commercially available SolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0102] 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.

[0103] 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 inverted multijunction solar cells of the present disclosure.

[0104] 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, and 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.

[0105] 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).

[0106] 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 direction and the ultimate solar cell design proposed by the Applicants.

[0107] 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 single “result effective variable” that one skilled in the art can simply specify and incrementally adjust to a particular level and thereby increase the power output and efficiency of a solar cell.

[0108] 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.

[0109] 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”.

[0110] 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.

[0111] More specifically, the present disclosure intends to provide a relatively simple and reproducible technique that 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.

[0112] 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.

[0113] 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.

[0114] Some comments about MOCVD processes used in one embodiment are in order here.

[0115] 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.

[0116] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0117] FIG. 1A is a graph of the band gap throughout the thickness of a solar subcell, according to one embodiment of the present disclosure. In particular, there is depicted a AiGaAs solar subcell in which the band gap at the window/emitter surface is 1.7 eV. The band gap then decreases linearly to a band gap of 1.6 eV at the junction, shown by the dashed and dotted line. The band gap then gradually increases linearly through the base region until it reaches the value of 1.7 eV at the base/BSF surface.

[0118] FIG. 1B is a cross-sectional view of the solar subcell of FIG. 1A depicting the movement of electrons and holes throughout the thickness of the layer due to the internal electric field. The conduction E.sub.c and the valence E.sub.v bands are illustrated, as well as the emitter and base regions of the solar subcell, the junction, the depletion region, and graded doping throughout the thickness of the subcell.

[0119] To illustrate an embodiment of a multijunction solar cell device of the present disclosure, FIG. 2A is a cross-sectional view of an embodiment of an inverted metamorphic multijunction solar cell 600 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate 101 up to the contact layer 138 according to the present disclosure.

[0120] FIG. 2A depicts the multijunction solar cell according to a first embodiment of 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), but may also be germanium (Ge) or other suitable material. For GaAs, the substrate is preferably a 15° off-cut substrate, that is to say, its surface is orientated 15° off the (100) plane towards the (111)A plane, as more fully described in U.S. Patent Application Pub. No. 2009/0229662 A1 (Stan et al.).

[0121] In the case of a Ge substrate, a nucleation layer (not shown) is deposited directly on the substrate 101. On the substrate, or over the nucleation layer (in the case of a Ge substrate), a buffer layer 102 and an etch stop layer 103 are further deposited. In the case of GaAs substrate, the buffer layer 102 is preferably GaAs. In the case of Ge substrate, the buffer layer 102 is preferably InGaAs. A contact layer 104 of 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. The subcell A is generally latticed matched to the growth substrate 101.

[0122] 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).

[0123] In one embodiment, the emitter layer 106 is composed of InGa(Al)P.sub.2 and the base layer 107 is composed of InGa(Al)P.sub.2. 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%.

[0124] Subcell A will ultimately become the “top” subcell of the inverted metamorphic structure after completion of the process steps according to the present disclosure to be described hereinafter.

[0125] On top of the base layer 107 a back surface field (“BSF”) layer 108 preferably p+ AlGaInP is deposited and used to reduce recombination loss.

[0126] 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.

[0127] 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.

[0128] A window layer 110 is deposited on top of the tunnel diode layers 109a/109b, and is preferably n+ InGaP. The advantage of utilizing InGaP 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.

[0129] 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 and AlinGaAs respectively (for a Ge substrate or growth template), or InGaP and AlGaAs respectively (for a GaAs substrate), although any other suitable materials consistent with lattice constant and bandgap requirements may be used as well. Thus, 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.

[0130] In previously disclosed implementations of an inverted metamorphic solar cell, the second subcell or subcell B or 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.

[0131] 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.

[0132] A window layer 118 preferably composed of n+ type GaInP is then deposited over the tunnel diode layer 114. 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.

[0133] 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 GaAs and n+ type 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.

[0134] In some embodiments, subcell C may be (In)GaAs with 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 i.e., 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.

[0135] 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.

[0136] A BSF layer 121, preferably composed of InGaAlAs, is then deposited on top of the cell C, the BSF layer performing the same function as the BSF layers 108 and 113.

[0137] 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++ GaAs, and layer 122b is preferably composed of n++ GaAs.

[0138] An alpha layer 123, preferably composed of n-type GaInP, is deposited over the tunnel diode 122a/122b, to a thickness in the range of 1.1 to 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the top and middle subcells A, B and C, or in the direction of growth into the subcell D, and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).

[0139] A metamorphic layer (or graded interlayer) 124 is deposited over the alpha layer 123 using a surfactant. Layer 124 is preferably a compositionally step-graded series of InGaAlAs layers, preferably with monotonically changing lattice constant, so as to achieve a gradual transition in lattice constant in the semiconductor structure from subcell C to subcell D while minimizing threading dislocations from occurring. The band gap of layer 124 is constant throughout its thickness, preferably approximately equal to 1.5 to 1.6 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell C. One embodiment of the graded interlayer may also be expressed as being composed of (In.sub.xGa.sub.1-x).sub.y Al.sub.1-yAs, with x and y selected such that the band gap of the interlayer remains constant at approximately 1.5 to 1.6 eV or other appropriate band gap.

[0140] In the surfactant assisted growth of the metamorphic layer 124, a suitable chemical element is introduced into the reactor during the growth of layer 124 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 124, 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.

[0141] 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.

[0142] 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 124.

[0143] In the inverted metamorphic structure described in the Wanlass et al. paper cited above, the metamorphic layer consists of nine compositionally graded InGaP steps, with each step layer having a thickness of 0.25 micron. As a result, each layer of Wanlass et al. has a different band gap. In one of the embodiments of the present disclosure, the layer 124 is composed of a plurality of layers of InGaAlAs, with monotonically changing lattice constant, each layer having the same band gap, approximately in the range of 1.5 to 1.6 eV.

[0144] The advantage of utilizing a constant bandgap material such as InGaAlAs is that arsenide-based semiconductor material is much easier to process in standard commercial MOCVD reactors, while the small amount of aluminum assures radiation transparency of the metamorphic layers.

[0145] Although the preferred embodiment of the present disclosure utilizes a plurality of layers of InGaAlAs for the metamorphic layer 124 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. Thus, the system of Wanlass using compositionally graded InGaP is a second embodiment of the present disclosure. 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 greater than or equal to that of the second solar cell and less than or equal to that of the third solar cell, and having a bandgap energy greater than that of the second solar cell.

[0146] An alpha layer 125, preferably composed of n+ type AlGaInAsP, is deposited over metamorphic buffer layer 124, to a thickness of about 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the top and middle subcells A, B and C, or in the direction of growth into the subcell D, and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).

[0147] A window layer 126 preferably composed of n+ type InGaAlAs is then deposited over alpha layer 125. 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.

[0148] 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 InGaAs and p type InGaAs respectively, or n+ type InGaP and p type InGaAs for a heterojunction subcell, although another suitable materials consistent with lattice constant and bandgap requirements may be used as well.

[0149] 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.

[0150] 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.

[0151] In some embodiments an alpha layer 131, preferably composed of n-type GaInP, is deposited over the tunnel diode 130a/130b, to a thickness in the range of 0.1 to 0.5 micron. Such alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the middle subcells C and D, or in the direction of growth into the subcell E, and is more particularly described in copending U.S. patent application Ser. No. 11/860,183, filed Sep. 24, 2007.

[0152] A second metamorphic layer (or graded interlayer) 132 is deposited over the barrier layer 131. Layer 132 is preferably a compositionally step-graded series of AlGaInAs 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 E while minimizing threading dislocations from occurring. In some embodiments the band gap of layer 132 is constant throughout its thickness, preferably approximately equal to 1.1 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell D. One embodiment of the graded interlayer may also be expressed as being composed of (In.sub.xGa.sub.1-x).sub.y Al.sub.1-yAs, with 0<x<1, 0<y<1, and x and y selected such that the band gap of the interlayer remains constant at approximately 1.1 eV or other appropriate band gap.

[0153] In one embodiment of the present disclosure, an optional second barrier layer 133 may be deposited over the AlGaInAs metamorphic layer 132. The second barrier layer 133 performs essentially the same function as the first barrier layer 131 of preventing threading dislocations from propagating. In one embodiment, barrier layer 133 has not the same composition than that of barrier layer 131, i.e. n+ type GaInP.

[0154] A window layer 134 preferably composed of n+ type GaInP is then deposited over the barrier layer 133. 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.

[0155] 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 GaInAs and p type GaInAs respectively, although other suitable materials consistent with lattice constant and band gap requirements may be used as well.

[0156] 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.

[0157] Finally a high band gap contact layer 138, preferably composed of p++ type AlGaInAs, is deposited on the BSF layer 137.

[0158] 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.

[0159] 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.

[0160] 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.

[0161] 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.

[0162] 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.

[0163] Optionally, the original substrate can be removed by a sequence of lapping and/or etching steps in which the substrate 101, and the buffer layer 102 are removed. The choice of a particular etchant is growth substrate dependent.

[0164] FIG. 2B is a cross-sectional view of the first embodiment of a solar cell of FIG. 2A, with the orientation with the metal contact layer 139 being at the bottom of the Figure and with the original substrate having been removed. In addition, the etch stop layer 103 has been removed, for example, by using a HCl/H2O solution.

[0165] To summarize, the present disclosure provides a multijunction solar cell comprising: an upper first solar subcell (106, 107) having a first band gap; a second solar subcell (111, 112) adjacent to said first solar subcell (106, 107) and having a second band gap less than the first band gap; a third solar subcell (119,120) adjacent to said second solar subcell (111, 112) and having a third band gap less than the first band gap; a first graded interlayer (124) adjacent to said third solar subcell (119, 120); said first graded interlayer (124) having a fourth band gap greater than said third band gap; and a fourth solar subcell (127, 128) adjacent to said first graded interlayer (124), said fourth subcell (127, 128) having a fifth band gap less than said third band gap and such that said fourth subcell (127, 128) is lattice mismatched with respect to said third subcell (119, 120); a second graded interlayer (132) adjacent to said fourth solar subcell (127, 128); said second graded interlayer (132) having a sixth band gap greater than said fifth band gap; and a lower fifth solar subcell (135, 136) adjacent to said second graded interlayer (132), said lower fifth subcell (135, 136) having a seventh band gap less than said fifth band gap and such that said fifth subcell (135, 136) is lattice mismatched with respect to said fourth subcell (127, 128), wherein the first graded interlayer (124) is compositionally graded to lattice match the third solar subcell (119, 120) on one side and the lower fourth solar subcell (127, 128) on the other side, and is composed of (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 remains at a constant value in the range of 1.42 to 1.60 eV throughout its thickness; and wherein the second graded interlayer (132) is compositionally graded to lattice match the fourth solar subcell (127, 128) on one side and the lower fifth solar subcell (135, 136) on the other side, and is composed of (InxGa.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 remains at a constant value in the range of 1.2 eV to 1.6 eV throughout its thickness, and wherein the seventh band gap is in the range of approximately 0.83 to 0.85 eV, the fifth band gap is approximately 1.10 eV, the third band gap is in the range of 1.40 to 1.42 eV, the second band gap is of approximately 1.73 eV and the first band gap is of approximately 2.10 eV; and wherein the third solar subcell is composed of a GaAs emitter layer and a GaAs base layer so as to optimize energy conversion efficiency of the solar cell after exposure to radiation a 1 MeV electron equivalent fluence area 5×10.sup.14 electrons/cm.sup.2 or more rather than to optimize the energy conversion efficiency for a time coinciding with initial deployment of the solar cell in outer space.

[0166] 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. For example, one or more distributed Bragg reflector (DBR) layers can be added for various embodiments of the present invention.

[0167] FIG. 3 is a cross-sectional view of a second embodiment of a solar cell similar to that of FIGS. 2A and 2B that includes distributed Bragg reflector (DBR) layers 122c adjacent to and between the third solar subcell C and the graded interlayer 124 and 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 122c. In FIG. 3, the distributed Bragg reflector (DBR) layers 122c are specifically located between the third solar subcell C and tunnel diode layers 122a/122b.

[0168] FIG. 3 also includes distributed Bragg reflector (DBR) layers 114c adjacent to and between the second solar subcell B and the subcell C and arranged so that light can enter and pass through the second solar subcell B and at least a portion of which can be reflected back into the second solar subcell B by the DBR layers 114c. In FIG. 6, the distributed Bragg reflector (DBR) layers 114c are specifically located between subcell B and tunnel diode layers 114a/114b.

[0169] For some embodiments, distributed Bragg reflector (DBR) layers 114c and/or 122c can be composed of a plurality of alternating layers 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 or otherwise optimized in order to minimize or otherwise reduce 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 114c and/or 122c includes a first DBR layer composed of a plurality of p type Al.sub.xGa.sub.1-xAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type Al.sub.yGa.sub.1-yAs layers, where y is greater than x, and 0<x<1, 0<y<1.

[0171] FIG. 4 is a graph depicting the comparison of the external quantum efficiency of a AlGaAs subcell as a function of wavelength for a subcell with a non-graded band gap and a graded band gap according to the present disclosure. Diffusion lengths shown in the legend are fitted to the experimental QE data for this J2 AlGaAs example. In this case the bandgap is 1.63 eV for the non-graded, and 1.70 to 1.60 for the graded cell as represented in FIG. 1A.

[0172] The Figure indicates that an AlGaAs subcell (which may typically be of composition of the second, third, or lower subcell in a multijunction solar cell) has minority carrier diffusion length L.sub.min of 3.5 μm for a solar subcell subject to radiation exposure and damage, compared to 3.0 μm for a similar solar subcell with a constant or non-graded band gap, demonstrating the efficiency of the use of a graded band gap as taught by the present disclosure.

[0173] For some embodiments, the present disclosure provides an inverted metamorphic multijunction solar cell that follows a design rule that one should incorporate as many high bandgap subcells as possible to achieve the goal to increase high temperature EOL performance. For example, high bandgap 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 inverted metamorphic multijunction solar cell may be expected to be greater than traditional cells.

[0174] 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 as follows:

TABLE-US-00001 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

[0175] For the four junction IMM solar cell described in the parent application, the corresponding data is as follows:

TABLE-US-00002 Condition Efficiency BOL 28° C. 29.5% BOL 70° C. 26.6% EOL 70° C. 24.7% After 5E14 e/cm.sup.2 radiation EOL 70° C. 24.2% After 1E15 e/cm.sup.2 radiation

[0176] One should note the slightly higher cell efficiency of the IMM solar cell than the standard commercial solar cell (ZTJ) at BOL both at 28° C. and 70° C. However, the IMM solar cell described above exhibits substantially improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 5×10.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 1×10.sup.15 e/cm.sup.2.

[0177] For one embodiment of a five junction IMM solar cell described in the present application, the corresponding data is as follows:

TABLE-US-00003 Condition Efficiency BOL 28° C. 32.1% BOL 70° C. 30.4% EOL 70° C. 27.1% After 5E14 e/cm.sup.2 radiation EOL 70° C. 25.8% After 1E15 e/cm.sup.2 radiation

[0178] In some embodiments, the solar cell according to the present disclosure is also applicable to low intensity (LI) and/or low temperature (LT) environments, such as might be experienced in space vehicle missions to Mars, Jupiter, and beyond. A “low intensity” environment refers to a light intensity being less than 0.1 suns, and a “low temperature” environment refers to temperatures being in the range of less than minus 100 degrees Centigrade.

[0179] For such applications, depending upon the specific intensity and temperature ranges of interest, the band gaps of the subcells may be adjusted or “tuned” to maximize or otherwise optimize the solar cell efficiency, or otherwise optimize performance (e.g. at EOL or over the operational working life period).

[0180] A low earth orbit (LEO) satellite will typically experience radiation equivalent to 5×10.sup.14 e/cm.sup.2 over a five year lifetime. A geosynchronous earth orbit (GEO) satellite will typically experience radiation in the range of 5×10.sup.14 e/cm.sup.2 to 1×10.sup.15 e/cm.sup.2 over a fifteen year lifetime.

[0181] 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 normalized 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.

[0182] 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]. New cell structures eventually need new RDC measurements as different materials can be more or less damage resistant than materials used in conventional solar cells. A widely accepted total mission equivalent fluence for a geosynchronous satellite mission of 15 year duration is 1 MeV 1×10.sup.15 electrons/cm.sup.2.

[0183] The exemplary solar cell described herein may require the use of aluminum in the semiconductor composition of each of the top two or three 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 Eg-Voc metric. In short, increased BOL E.sub.g-V.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.

[0184] Furthermore, at BOL, it is widely accepted that great subcells have a room temperature E.sub.g-V.sub.oc of approximately 0.40. A wide variation in BOL E.sub.g-V.sub.oc may exist for subcells of interest to IMMX cells. However, Applicants have found that inspecting E.sub.g-V.sub.oc at HT-EOL may reveal that aluminum containing subcells perform no worse than other materials used in III-V solar cells. For example, all of the subcells at EOL, regardless of aluminum concentration or degree of lattice-mismatch, have been shown to display a nearly-fixed E.sub.g-V.sub.oc of approximately 0.6 at room temperature 28° C.

[0185] The exemplary inverted metamorphic multijunction solar cell design philosophy may be described as opposing conventional cell efficiency improvement paths that employ infrared subcells that increase in expense as the bandgap of the materials decreases. For example, proper current matching among all subcells that span the entire solar spectrum is often a normal design goal. Further, known approaches—including dilute nitrides grown by MBE, upright metamorphic, and inverted metamorphic multijunction solar cell designs—may add significant cost to the cell and only marginally improve HT-EOL performance. Still further, lower HT-EOL $/W may be achieved when inexpensive high bandgap subcells are incorporated into the cell architecture, rather than more expensive infrared subcells. The key to enabling the exemplary solar cell design philosophy described herein is the observation that aluminum containing subcells perform well at HT-EOL.

[0186] 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 constructions differing from the types of constructions described above.

[0187] Although the preferred embodiment of the present disclosure utilizes a vertical stack of five subcells, the present disclosure can apply to stacks with fewer or greater number of subcells, i.e. two junction cells, three junction cells, four junction cells, six junction cells, etc.

[0188] In addition, although the present embodiment is 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.

[0189] As noted above, 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 A, with p-type and n-type AlInGaP is one example of a homojunction subcell. Alternatively, as more particularly described in U.S. Patent Application Pub. No. 2009/0078310 A1 (Stan et al.), the present disclosure may utilize one or more, or all, heterojunction 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 having different chemical compositions of the semiconductor material in the n-type regions, and/or different band gap energies in the p-type regions, in addition to utilizing different dopant species and type in the p-type and n-type regions that form the p-n junction.

[0190] The composition of the window or BSF layers may utilize other semiconductor compounds, subject to lattice constant and band gap requirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present disclosure.

[0191] While the disclosure has been illustrated and described as embodied in an inverted metamorphic multijunction solar cell, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present disclosure.

[0192] 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.

[0193] Although described embodiments of the present disclosure utilizes a vertical stack of three 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.

[0194] 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.

[0195] 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 309, with p-type and n-type InGaP is one example of a homojunction subcell.

[0196] 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.

[0197] The composition of the window or BSF layers may utilize other semiconductor compounds, subject to lattice constant and band gap requirements, and 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, and still fall within the spirit of the present invention.

[0198] 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.

[0199] 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.

[0200] 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.