Patent Publication Number: US-2023142352-A1

Title: Inverted metamorphic multijunction solar cells for space applications

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 17/114,158 filed Dec. 7, 2020, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/948,451 filed Dec. 16, 2019. 
     The present application is related to U.S. patent application Ser. No. 15/681,144 filed Aug. 18, 2017, now U.S. Pat. No. 10,700,230, which is a continuation-in-part of Ser. No. 14/828,206 filed Aug. 17, 2015. 
     This application is also related to U.S. patent application Ser. No. 14/660,092 filed Mar. 17, 2015, which is a division of U.S. patent application Ser. No. 12/716,814 filed Mar. 3, 2010, now U.S. Pat. No. 9,018,521; which is a continuation in part of U.S. patent application Ser. No. 12/337,043 filed Dec. 17, 2008. 
     This application is also related to U.S. patent application Ser. No. 16/667,687 filed Oct. 29, 2019, now U.S. Pat. No. 11,063,168, which is a division of Ser. No. 13/872,663 filed Apr. 29, 2012, now U.S. Pat. No. 10,541,349, which was also a continuation-in-part of application Ser. No. 12/337,043, filed Dec. 17, 2008. 
     This application is also related to U.S. patent application Ser. No. 14/828,197 filed Aug. 17, 2015 and Ser. No. 15/352,941 filed Nov. 6, 2016. 
     All of the above related applications are incorporated herein by reference in their entirety. 
    
    
     GOVERNMENT RIGHTS STATEMENT 
     This invention was made with government support under Contract No. FA 9453 19-C-1001 awarded by the U.S. Air Force. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to solar cells and the fabrication of solar cells, and more particularly to the design and specification of both lattice matched and lattice mismatched inverted multijunction solar cells adapted for space missions. 
     DESCRIPTION OF THE RELATED ART 
     Solar power from photovoltaic cells, also called solar cells, has been predominantly provided by silicon semiconductor technology. In the past several years, however, high-volume manufacturing of III-V compound semiconductor multijunction solar cells for space applications has accelerated the development of such technology. Compared to silicon, III-V compound semiconductor multijunction devices have greater energy conversion efficiencies and generally more radiation resistance, although they tend to be more complex to properly specify and manufacture. Typical commercial III-V compound semiconductor multijunction solar cells have energy efficiencies that exceed 29.5% under one sun, air mass 0 (AM0) illumination, whereas even the most efficient silicon technologies generally reach only about 18% efficiency under comparable conditions. The higher conversion efficiency of III-V compound semiconductor solar cells compared to silicon solar cells is in part based on the ability to achieve spectral splitting of the incident radiation through the use of a plurality of photovoltaic regions with different band gap energies, and accumulating the current from each of the regions. 
     In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as payloads use increasing amounts of power as they become more sophisticated, and missions and applications anticipated for five, ten, twenty or more years, the power-to-weight ratio and lifetime efficiency of a solar cell becomes increasingly more important, and there is increasing interest not only the amount of power provided at initial deployment, but over the entire service life of the satellite system, or in terms of a design specification, the amount of power provided at the “end of life” (EOL) which is affected by the radiation exposure of the solar cell over time in a space environment. 
     Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures or stacked sequence of solar subcells, each subcell formed with appropriate semiconductor layers and including a p-n photoactive junction. Each subcell is designed to convert photons over different spectral or wavelength bands to electrical current. After the sunlight impinges on the front of the solar cell, and photons pass through the subcells, with each subcell being designed for photons in a specific wavelength band. After passing through a subcell, the photons that are not absorbed and converted to electrical energy propagate to the next subcells, where such photons are intended to be captured and converted to electrical energy. 
     The individual solar cells or wafers are then disposed in horizontal arrays, with the individual solar cells connected together in an electrical series and/or parallel circuit. The shape and structure of an array, as well as the number of cells it contains, are determined in part by the desired output voltage and current needed by the payload or subcomponents of the payload, the amount of electrical storage capacity (batteries) on the spacecraft, and the power demands of the payloads during different orbital configurations. 
     A solar cell designed for use in a space vehicle (such as a satellite, space station, or an interplanetary mission vehicle), has a sequence of subcells with compositions and band gaps which have been optimized to achieve maximum energy conversion efficiency for the AM0 solar spectrum in space. The AM0 solar spectrum in space is notably different from the AM1.5 solar spectrum at the surface of the earth, and accordingly terrestrial solar cells are designed with subcell band gaps optimized for the AM1.5 solar spectrum. 
     There are substantially more rigorous qualification and acceptance testing protocols used in the manufacture of space solar cells to ensure that space solar cells can operate satisfactorily at the wide range of temperatures and temperature cycles encountered in space. These testing protocols include (i) high-temperature thermal vacuum bake-out; (ii) thermal cycling in vacuum (TVAC) or ambient pressure nitrogen atmosphere (APTC); and in some applications (iii) exposure to radiation equivalent to that which would be experienced in the space mission, and measuring the current and voltage produced by the cell and deriving cell performance data. 
     As used in this disclosure and claims, the term “space-qualified” shall mean that the electronic component (i.e., the solar cell) provides satisfactory operation under the high temperature and thermal cycling test protocols. The exemplary conditions for vacuum bake-out testing include exposure to a temperature of +100° C. to +135° C. (e.g., about +100° C., +110° C., +120° C., +125° C., +135° C.) for 2 hours to 24 hours, 48 hours, 72 hours, or 96 hours; and exemplary conditions for TVAC and/or APTC testing that include cycling between temperature extremes of −180° C. (e.g., about −180° C., −175° C., −170° C., −165° C., −150° C., −140° C., −128° C., −110° C., −100° C., −75° C., or −70° C.) to +145° C. (e.g., about +70° C., +80° C., +90° C., +100° C., +110° C., +120° C., +130° C., +135° C., or +145° C.) for 600 to 32,000 cycles (e.g., about 600, 700, 1500, 2000, 4000, 5000, 7500, 22000, 25000, or 32000 cycles), and in some space missions up to +180° C. See, for example, Fatemi et al., “Qualification and Production of Emcore ZTJ Solar Panels for Space Missions,” Photovoltaic Specialists Conference (PVSC), 2013 IEEE 39th (DOI: 10. 1109/PVSC 2013 6745052). Such rigorous testing and qualifications are not generally applicable to terrestrial solar cells and solar cell arrays. 
     Conventionally, such measurements are made for the AM0 spectrum for “one-sun” illumination, but for PV systems which use optical concentration elements, such measurements may be made under concentrations of 2×, 100×, or 1000× or more. 
     The space solar cells and arrays experience a variety of complex environments in space missions, including the vastly different illumination levels and temperatures seen during normal earth orbiting missions, as well as even more challenging environments for deep space missions, operating at different distances from the sun, such as at 0.7, 1.0 and 3.0 AU (AU meaning astronomical units). The photovoltaic arrays also endure anomalous events from space environmental conditions, and unforeseen environmental interactions during exploration missions. Hence, electron and proton radiation exposure, collisions with space debris, and/or normal aging in the photovoltaic array and other systems could cause suboptimal operating conditions that degrade the overall power system performance, and may result in failures of one or more solar cells or array strings and consequent loss of power. 
     A further distinctive difference between space solar cell arrays and terrestrial solar cell arrays is that a space solar cell array utilizes welding and not soldering to provide robust electrical interconnections between the solar cells, while terrestrial solar cell arrays typically utilize solder for electrical interconnections. Welding is required in space solar cell arrays to provide the very robust electrical connections that can withstand the wide temperature ranges and temperature cycles encountered in space such as from −175° C. to +180° C. In contrast, solder joints are typically sufficient to survive the rather narrow temperature ranges (e.g., about −40° C. to about +50° C.) encountered with terrestrial solar cell arrays. 
     A further distinctive difference between space solar cell arrays and terrestrial solar cell arrays is that a space solar cell array utilizes silver-plated metal material for interconnection members, while terrestrial solar cells typically utilize copper wire for interconnects. In some embodiments, the interconnection member can be, for example, a metal plate. Useful metals include, for example, molybdenum; a nickel-cobalt ferrous alloy material designed to be compatible with the thermal expansion characteristics of borosilicate glass such as that available under the trade designation KOVAR from Carpenter Technology Corporation; a nickel iron alloy material having a uniquely low coefficient of thermal expansion available under the trade designation Invar, FeNi36, or 64FeNi; or the like. 
     An additional distinctive difference between space solar cell arrays and terrestrial solar cell arrays is that space solar cell arrays typically utilize an aluminum honeycomb panel for a substrate or mounting platform. In some embodiments, the aluminum honeycomb panel may include a carbon composite face sheet adjoining the solar cell array. In some embodiments, the face sheet may have a coefficient of thermal expansion (CTE) that substantially matches the CTE of the bottom germanium (Ge) layer of the solar cell that is attached to the face sheet. Substantially matching the CTE of the face sheet with the CTE of the Ge layer of the solar cell can enable the array to withstand the wide temperature ranges encountered in space without the solar cells cracking, delaminating, or experiencing other defects. Such precautions are generally unnecessary in terrestrial applications. 
     Thus, a further distinctive difference of a space solar cell from a terrestrial solar cell is that the space solar cell must include a cover glass over the semiconductor device to provide radiation resistant shielding from particles in the space environment which could damage the semiconductor material. The cover glass is typically a ceria doped borosilicate glass which is typically from three to six mils in thickness and attached by a transparent adhesive to the solar cell. 
     In summary, it is evident that the differences in design, materials, and configurations between a space-qualified III-V compound semiconductor solar cell and subassemblies and arrays of such solar cells, on the one hand, and silicon solar cells or other photovoltaic devices used in terrestrial applications, on the other hand, are so substantial that prior teachings associated with silicon or other terrestrial photovoltaic system are simply unsuitable and have no applicability to the design configuration of space-qualified solar cells and arrays. Indeed, the design and configuration of components adapted for terrestrial use with its modest temperature ranges and cycle times often teach away from the highly demanding design requirements for space-qualified solar cells and arrays and their associated components. 
     The assembly of individual solar cells together with electrical interconnects and the cover glass form a so-called “CIC” (Cell-Interconnected-Cover glass) assembly, which are then typically electrically connected to form an array of series-connected solar cells. The solar cells used in many arrays often have a substantial size; for example, in the case of the single standard substantially “square” solar cell trimmed from a 100 mm wafer with cropped corners, the solar cell can have a side length of seven cm or more. 
     The radiation hardness of a solar cell is defined as how well the cell performs after exposure to the electron or proton particle radiation which is a characteristic of the space environment. A standard metric is the ratio of the end of life performance (or efficiency) divided by the beginning of life performance (EOL/BOL) of the solar cell. The EOL performance is the cell performance parameter after exposure of that test solar cell to a given fluence of electrons or protons (which may be different for different space missions or orbits). The BOL performance is the performance parameter prior to exposure to the particle radiation. 
     Charged particles in space could lead to damage to solar cell structures, and in some cases, dangerously high voltage being established across individual devices or conductors in the solar array. These large voltages can lead to catastrophic electrostatic discharging (ESD) events. Traditionally for ESD protection the backside of a solar array may be painted with a conductive coating layer to ground the array to the space plasma, or one may use a honeycomb patterned metal panel which mounts the solar cells and incidentally protects the solar cells from backside radiation. 
     The radiation hardness of the semiconductor material of the solar cell itself is primarily dependent on a solar cell&#39;s minority carrier diffusion length (L min ) in the base region of the solar cell (the term “base” region referring to the p-type base semiconductor region disposed directly adjacent to an n-type “emitter” semiconductor region, the boundary of which establishes the p-n photovoltaic junction). The less degraded the parameter L min  is after exposure to particle radiation, the less the solar cell performance will be reduced. A number of strategies have been used to either improve L min , or make the solar cell less sensitive to L min  reductions. Improving L min  has largely involved including a gradation in dopant elements in the semiconductor base layer of the subcells so as to create an electric field to direct minority carriers to the junction of the subcell, thereby effectively increasing L min . The effectively longer L min  will improve the cell performance, even after the particle radiation exposure. Making the cell less sensitive to L min  reductions has involved increasing the optical absorption of the base layer such that thinner layers of the base can be used to absorb the same amount of incoming optical radiation. 
     Another consideration in connection with the manufacture of space solar cell arrays is that conventionally, solar cells have been arranged on a support and interconnected using a substantial amount of manual labor. For example, first individual CICs are produced with each interconnect individually welded to the solar cell, and each cover glass individually mounted. Then, these CICs are connected in series to form strings, generally in a substantially manual manner, including the welding steps from CIC to CIC. Then, these strings are applied to a panel substrate and electrically interconnected in a process that includes the application of adhesive, wiring, etc. All of this has traditionally been carried out in a manual and substantially artisanal manner. 
     The energy conversion efficiency of multijunction solar cells is affected by such factors as the number of subcells, the thickness of each subcell, the composition and doping of each active layer in a subcell, and the consequential band structure, electron energy levels, conduction, and absorption of each subcell, as well as the effect of its exposure to radiation in the ambient environment over time. The identification and specification of such design parameters is a non-trivial engineering undertaking, and would vary depending upon the specific space mission and customer design requirements. Since the power output is a function of both the voltage and the current produced by a subcell, a simplistic view may seek to maximize both parameters in a subcell by increasing a constituent element, or the doping level, to achieve that effect. However, in reality, changing a material parameter that increases the voltage may result in a decrease in current, and therefore a lower power output. Such material design parameters are interdependent and interact in complex and often unpredictable ways, and for that reason are not “result effective” variables that those skilled in the art confronted with complex design specifications and practical operational considerations can easily adjust to optimize performance. 
     Moreover, the current (or more precisely, the short circuit current density J sc ) and the voltage (or more precisely, the open circuit voltage V oc ) are not the only factors that determine the power output of a solar cell. In addition to the power being a function of the short circuit density (J sc ), and the open circuit voltage (V oc ), the output power is actually computed as the product of V oc  and J sc , and a Fill Factor (FF). As might be anticipated, the Fill Factor parameter is not a constant, but in fact may vary at a value between 0.5 and somewhat over 0.85 for different arrangements of elemental compositions, subcell thickness, and the dopant level and profile. Although the various electrical contributions to the Fill Factor such as series resistance, shunt resistance, and ideality (a measure of how closely the semiconductor diode follows the ideal diode equation) may be theoretically understood, from a practical perspective the actual Fill Factor of a given subcell cannot always be predicted, and the effect of making an incremental change in composition or band gap of a layer may have unanticipated consequences and effects on the solar subcell semiconductor material, and therefore an unrecognized or unappreciated effect on the Fill Factor. Stated another way, an attempt to maximize power by varying a composition of a subcell layer to increase the V oc  or J sc  or both of that subcell, may in fact not result in high power, since although the product V oc  and J sc  may increase, the FF may decrease and the resulting power also decrease. Thus, the V oc  and J sc  parameters, either alone or in combination, are not necessarily “result effective” variables that those skilled in the art confronted with complex design specifications and practical operational considerations can easily adjust to optimize performance. 
     Furthermore, the fact that the short circuit current density (J sc ), the open circuit voltage (V oc ), and the fill factor (FF), are affected by the slightest change in such design variables, the purity or quality of the chemical pre-cursors, or the specific process flow and fabrication equipment used, and such considerations further complicates the proper specification of design parameters and predicting the efficiency of a proposed design which may appear “on paper” to be advantageous. 
     It must be further emphasized that in addition to process and equipment variability, the “fine tuning” of minute changes in the composition, band gaps, thickness, and doping of every layer in the arrangement has critical effect on electrical properties such as the open circuit voltage (V oc ) and ultimately on the power output and efficiency of the solar cell. 
     To illustrate the practical effect, consider a design change that results in a small change in the V oc  of an active layer in the amount of 0.01 volts, for example changing the V oc  from 2.72 to 2.73 volts. Assuming all else is equal and does not change, such a relatively small incremental increase in voltage would typically result in an increase of solar cell efficiency from 29.73% to 29.84% for a triple junction solar cell, which would be regarded as a substantial and significant improvement that would justify implementation of such design change. 
     For a single junction GaAs subcell in a triple junction device, a change in V oc  from 1.00 to 1.01 volts (everything else being the same) would increase the efficiency of that junction from 10.29% to 10.39%, about a 1% relative increase. If it were a single junction stand-alone solar cell, the efficiency would go from 20.58% to 20.78%, still about a 1% relative improvement in efficiency. 
     Present day commercial production processes are able to define and establish band gap values of epitaxially deposited layers as precisely as 0.01 eV, so such “fine tuning” of compositions and consequential open circuit voltage results are well within the range of operational production specifications for commercial products. 
     Another important mechanical or structural consideration in the choice of semiconductor layers for a solar cell is the desirability of the adjacent layers of semiconductor materials in the solar cell, i.e. each layer of crystalline semiconductor material that is deposited and grown to form a solar subcell, have similar or substantially similar crystal lattice constants or parameters. 
     Here again there are trade-offs between including specific elements in the composition of a layer which may result in improved voltage associated with such subcell and therefore potentially a greater power output, and deviation from exact crystal lattice matching with adjoining layers as a consequence of including such elements in the layer which may result in a higher probability of defects, and therefore lower manufacturing yield. 
     In that connection, it should be noted that there is no strict definition of what is understood to mean two adjacent layers are “lattice matched” or “lattice mismatched”. For purposes in this disclosure, “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 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 as defining “lattice mismatched” layers). 
     In satellite and other space related applications, the size, mass and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as payloads use increasing amounts of power as they become more sophisticated, and missions and applications anticipated for five, ten, twenty or more years, the power-to-weight ratio and lifetime efficiency of a solar cell becomes increasingly more important, and there is increasing interest not only the amount of power provided at initial deployment, but over the entire service life of the satellite system, or in terms of a design specification, the amount of power provided at the “end of life” (EOL) which is affected by the radiation exposure of the solar cell over time in a space environment. 
     Typical III-V compound semiconductor solar cells are fabricated on a semiconductor wafer in vertical, multijunction structures or stacked sequence of solar subcells, each subcell formed with appropriate semiconductor layers and including a p-n photoactive junction. Each subcell is designed to convert photons over different spectral or wavelength bands to electrical current. After the sunlight impinges on the front of the solar cell, and photons pass through the subcells, with each subcell being designed for photons in a specific wavelength band. After passing through a subcell, the photons that are not absorbed and converted to electrical energy propagate to the next subcells, where such photons are intended to be captured and converted to electrical energy. 
     The energy conversion efficiency of multijunction solar cells is affected by such factors as the number of subcells, the thickness of each subcell, the composition and doping of each active layer in a subcell, and the consequential band structure, electron energy levels, conduction, and absorption of each subcell, as well as the effect of its exposure to radiation in the ambient environment over time. The identification and specification of such design parameters is a non-trivial engineering undertaking, and would vary depending upon the specific space mission and customer design requirements. Since the power output is a function of both the voltage and the current produced by a subcell, a simplistic view may seek to maximize both parameters in a subcell by increasing a constituent element, or the doping level, to achieve that effect. However, in reality, changing a material parameter that increases the voltage may result in a decrease in current, and therefore a lower power output. Such material design parameters are interdependent and interact in complex and often unpredictable ways, and for that reason are not “result effective” variables that those skilled in the art confronted with complex design specifications and practical operational considerations can easily adjust to optimize performance. Electrical properties such as the short circuit current density (J sc ), the open circuit voltage (V oc ), and the fill factor (FF), which determine the efficiency and power output of the solar cell, are affected by the slightest change in such design variables, and as noted above, to further complicate the calculus, such variables and resulting properties also vary, in a non-uniform manner, over time (i.e. during the operational life of the system) due to exposure to radiation during space missions. 
     Another important mechanical or structural consideration in the choice of semiconductor layers for a solar cell is the desirability of the adjacent layers of semiconductor materials in the solar cell, i.e. each layer of crystalline semiconductor material that is deposited and grown to form a solar subcell, have similar crystal lattice constants or parameters. 
     SUMMARY OF THE DISCLOSURE 
     Objects of the Disclosure 
     It is an object of the present disclosure to provide increased photoconversion efficiency in an inverted metamorphic multijunction solar cell for space applications by providing a specific composition in the critical subcells that makes such subcells less sensitive to radiation exposure. 
     It is another object of the present disclosure to increase the current collection in at least one subcell of an inverted metamorphic multijunction solar cell by providing a graded band gap in the emitter layer, the base layer, or both, in that at least one subcell. 
     It is another object of the present disclosure to provide an inverted metamorphic multijunction solar cell in which the subcell current is increased per unit area to enable a greater amount of power output from the solar cell by providing a gradation or variation in band gap in the range of 0.05 to 1.0 eV in the emitter layer, the base layer, or both in at least one subcell of the solar cell. 
     Some implementations of the present disclosure may incorporate or implement fewer of the aspects and features noted in the foregoing objects. 
     A technical objective provided by examples in the present disclosure may be regarded as increasing the efficiency and performance of an inverted metamorphic multijunction solar cell, in particular, at the “end-of-life” after the solar cell has subjected to radiation at the level of 1E15 at 70° C. or more. 
     More specifically, in one embodiment, the present disclosure is directed to an improved design of an inverted metamorphic multijunction solar cell in which the third solar subcell (hereinafter, referred to as solar subcell “C”) from the top or light-facing surface of the solar cell after deployment, with a bandgap in the range of 1.04 eV to 1.35 eV and composition of InGaAs, has been subject to radiation at a level of 1E15 at 70° C., with the design paradigm of utilizing relatively high band gaps in the top or upper subcells by using a substantial amount of aluminum in their composition to increase the bandgaps. 
     Another technical problem to be addressed by the present disclosure may be sated as providing a design paradigm to increase the efficiency and performance of an inverted metamorphic multijunction solar cell after subjection to radiation as experienced in a variety of earth orbits that has a deleterious effect on the current produced by certain solar subcells, and therefore results in lower power produced by the cell. Once such orbit or other mission requirements are specified, the design paradigm can identify the specific compositions and band gaps to achieve the maximum power and efficiency at the EOL time period. 
     Features of the Invention 
     All ranges of numerical parameters set forth in this disclosure are to be understood to encompass any and all subranges or “intermediate generalizations” subsumed herein. For example, a stated range of “1.0 to 2.0 eV” for a band gap value should be considered to include any and all subranges beginning with a minimum value of 1.0 eV or more and ending with a maximum value of 2.0 eV or less, e.g., 1.0 to 2.0, or 1.3 to 1.4, or 1.5 to 1.9 eV. 
     Briefly, and in general terms, the present disclosure provides a multijunction solar cell comprising: an upper first solar subcell having a first band gap; a second solar subcell adjacent to said 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 (InxGa 1-x ) y Al 1-y As with 0&lt;x&lt;1, 0&lt;y&lt;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 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 x Ga 1-x ) y Al 1-y As with 0&lt;x&lt;1, 0&lt;y&lt;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 14  electrons/cm 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. 
     In some embodiments, at least one solar subcell has a constant band gap throughout its thickness; and at least one solar subcell has a graded band gap throughout at least a portion of its thickness. 
     In some embodiments, the band gap in one solar subcell decreases from the top surface of the one solar subcell to the junction of the one solar subcell. 
     In some embodiments, the band gap increases from the junction of the one solar subcell to the bottom surface of the one solar subcell. 
     In some embodiments, the band gap is graded in (i) the emitter; (ii) the base; or (iii) both the base and emitter of a solar subcell. 
     In some embodiments, the change in band gap is in the range of 0.05 eV to 2.0 eV. 
     In some embodiments, the change in band gap is approximately 1.0 eV. 
     In some embodiments, the band gap at the top surface of the at least one solar subcell is equal to the band gap at the bottom surface of the at least one solar subcell. 
     In some embodiments, the band gap decreases linearly from the top surface of the one solar subcell to the junction of the one solar subcell. 
     In some embodiments, the band gap increases linearly from the junction of the one solar subcell to the bottom surface of the one solar subcell. 
     In some embodiments, the band gap is linearly or non-linearly graded in (i) the emitter; (ii) the base; or (iii) both the base and the emitter of a solar subcell. 
     In some embodiments, the generation of hole-electron pairs is enhanced in certain regions of the active layer due to a change in band gap in regions of the active layer. 
     In some embodiments, the band gap decrease from the top surface of the one solar subcell to the junction of the one solar subcell results in greater collection at the junction. 
     In some embodiments, the band gap increase from the junction of the one solar subcell to the bottom surface of the one solar subcell results in improved current collection in the active layer of that solar subcell. 
     In some embodiments, the gradation in band gap in the n type semiconductor region is greater than the gradation in band gap in the p type semiconductor region of the one solar subcell. 
     In some embodiments, at least one solar subcell is the second solar subcell (from the top or light-facing surface of the solar subcell) with a band gap of 1.7 eV at the top and bottom surface of the subcell. 
     In some embodiments, the second solar subcell has a band gap of 1.6 eV at the junction of that solar subcell. 
     In some embodiments, the solar cell is an inverted metamorphic multijunction solar cell with the first or second junction having a graded band gap. Optionally, the first, second or third junction each has a graded band gap; or each of the first, second, third, and optionally the fourth junction has a graded band gap; or each of the first, second, third, fourth and optionally the fifth junction has a graded band gap. 
     In some embodiments, additional layer(s) may be added or deleted in the cell structure without departing from the scope of the present disclosure. 
     Some implementations of the present disclosure may incorporate or implement fewer of the aspects and features noted in the foregoing summaries. 
     Additional aspects, advantages, and novel features of the present disclosure will become apparent to those skilled in the art from this disclosure, including the following detailed description as well as by practice of the disclosure. While the disclosure is described below with reference to preferred embodiments, it should be understood that the disclosure is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and embodiments in other fields, which are within the scope of the disclosure as disclosed and claimed herein and with respect to which the disclosure could be of utility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG.  1 A  is a graph of the band gap throughout the thickness of a solar subcell, according to a first embodiment of the present disclosure; 
         FIG.  1 B  is a cross-sectional view of the solar subcell of  FIG.  1 A  depicting the movement of electrons and holes throughout the thickness of the layer due to the internal electric field produced by the graded doping; 
         FIG.  2 A  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; 
         FIG.  2 B  is a cross-sectional view of the solar cell of  FIG.  2 A  after removal of the growth substrate, and with the first-grown subcell A depicted at the top of the Figures; 
         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 
         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 
     “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). 
     “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. 
     “Beginning of Life (BOL)” refers to the time at which a photovoltaic power system is initially deployed in operation. 
     “Bottom subcell” refers to the subcell in a multijunction solar cell which is furthest from the primary light source for the solar cell. 
     “Compound semiconductor” refers to a semiconductor formed using two or more chemical elements. 
     “Current density” refers to the short circuit current density J sc  through a solar subcell through a given planar area, or volume, of semiconductor material constituting the solar subcell. 
     “Deposited”, with respect to a layer of semiconductor material, refers to a layer of material which is epitaxially grown over another semiconductor layer. 
     “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. 
     “Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”. 
     “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. 
     “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. 
     “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). 
     “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. 
     “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). 
     “Short circuit current (I 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. 
     “Short circuit current density”—see “current density”. 
     “Solar cell” refers to an electronic device operable to convert the energy of light directly into electricity by the photovoltaic effect. 
     “Solar cell assembly” refers to two or more solar cell subassemblies interconnected electrically with one another. 
     “Solar cell subassembly” refers to a stacked sequence of layers including one or more solar subcells. 
     “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. 
     “Substantially current matched” refers to the short circuit current through adjacent solar subcells being substantially identical (i.e. within plus or minus 1%). 
     “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. 
     “Top surface” of a subcell refers to the surface of the subcell which is closest to the primary light source for the solar cell. 
     “ZTJ” refers to the product designation of a commercially available SolAero Technologies Corp. triple junction solar cell. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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. 
     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. 
     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. 
     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&#39;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). 
     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&#39;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&#39;s research efforts and direction and the ultimate solar cell design proposed by the Applicants. 
     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. 
     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. 
     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”. 
     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. 
     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. 
     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. 
     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. 
     Some comments about MOCVD processes used in one embodiment are in order here. 
     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. 
     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. 
       FIG.  1 A  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. 
       FIG.  1 B  is a cross-sectional view of the solar subcell of  FIG.  1 A  depicting the movement of electrons and holes throughout the thickness of the layer due to the internal electric field. The conduction E c  and the valence E 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. 
     To illustrate an embodiment of a multijunction solar cell device of the present disclosure,  FIG.  2 A  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. 
       FIG.  2 A  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.). 
     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 . 
     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). 
     In one embodiment, the emitter layer  106  is composed of InGa(Al)P 2  and the base layer  107  is composed of InGa(Al)P 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%. 
     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. 
     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. 
     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. 
     On top of the BSF layer  108  is deposited a sequence of heavily doped p-type and n-type layers  109   a  and  109   b  that forms a tunnel diode, i.e., an ohmic circuit element that connects subcell A to subcell B. Layer  109   a  is preferably composed of p++ AlGaAs, and layer  109   b  is preferably composed of n++ InGaP. 
     A window layer  110  is deposited on top of the tunnel diode layers  109   a / 109   b , 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. 
     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. 
     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. 
     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  114   a  and  114   b  respectively are deposited over the BSF layer  113 , similar to the layers  109   a  and  109   b , forming an ohmic circuit element to connect subcell B to subcell C. The layer  114   a  is preferably composed of p++ AlGaAs, and layer  114   b  is preferably composed of n++ InGaP. 
     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. 
     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. 
     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%&lt;In&lt;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. 
     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. 
     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 . 
     The p++/n++ tunnel diode layers  122   a  and  122   b  respectively are deposited over the BSF layer  121 , similar to the layers  114   a  and  114   b , forming an ohmic circuit element to connect subcell C to subcell D. The layer  122   a  is preferably composed of p++ GaAs, and layer  122   b  is preferably composed of n++ GaAs. 
     An alpha layer  123 , preferably composed of n-type GaInP, is deposited over the tunnel diode  122   a / 122   b , 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.). 
     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 x Ga 1-x ) y  Al 1-y As, 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. 
     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. 
     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. 
     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 . 
     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. 
     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. 
     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. 
     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.). 
     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. 
     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. 
     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 . 
     The p++/n++ tunnel diode layers  130   a  and  130   b  respectively are deposited over the BSF layer  129 , similar to the layers  122   a / 122   b  and  109   a / 109   b , forming an ohmic circuit element to connect subcell D to subcell E. The layer  130   a  is preferably composed of p++ AlGaInAs, and layer  130   b  is preferably composed of n++ GaInP. 
     In some embodiments an alpha layer  131 , preferably composed of n-type GaInP, is deposited over the tunnel diode  130   a / 130   b , 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. 
     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 x Ga 1-x ) y  Al 1-y As, with 0&lt;x&lt;1, 0&lt;y&lt;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. 
     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. 
     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. 
     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. 
     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 . 
     Finally a high band gap contact layer  138 , preferably composed of p++ type AlGaInAs, is deposited on the BSF layer  137 . 
     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&#39;t have to be selectively etched off, to prevent absorption. 
     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. 
     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. 
     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&#39;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. 
     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 . 
     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. 
       FIG.  2 B  is a cross-sectional view of the first embodiment of a solar cell of  FIG.  2 A , 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. 
     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 x Ga 1-x ) y Al 1-y As with 0&lt;x&lt;1, 0&lt;y&lt;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 1-x ) y Al 1-y As with 0&lt;x&lt;1, 0&lt;y&lt;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 14  electrons/cm 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. 
     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. 
       FIG.  3    is a cross-sectional view of a second embodiment of a solar cell similar to that of  FIGS.  2 A and  2 B  that includes distributed Bragg reflector (DBR) layers  122   c  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  122   c . In  FIG.  3   , the distributed Bragg reflector (DBR) layers  122   c  are specifically located between the third solar subcell C and tunnel diode layers  122   a / 122   b.    
       FIG.  3    also includes distributed Bragg reflector (DBR) layers  114   c  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  114   c . In  FIG.  6   , the distributed Bragg reflector (DBR) layers  114   c  are specifically located between subcell B and tunnel diode layers  114   a / 114   b.    
     For some embodiments, distributed Bragg reflector (DBR) layers  114   c  and/or  122   c  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. 
     For some embodiments, distributed Bragg reflector (DBR) layers  114   c  and/or  122   c  includes a first DBR layer composed of a plurality of p type Al x Ga 1-x As layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type Al y Ga 1-y As layers, where y is greater than x, and 0&lt;x&lt;1, 0&lt;y&lt;1. 
       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.  1 A . 
     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 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. 
     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. 
     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: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 BOL 28° C. 
                 29.1% 
                   
               
               
                   
                 BOL 70° C. 
                 26.4% 
               
               
                   
                 EOL 70° C. 
                 23.4% 
                 After 5E14 e/cm 2  radiation 
               
               
                   
                 EOL 70° C. 
                 22.0% 
                 After 1E15 e/cm 2  radiation 
               
               
                   
                   
               
            
           
         
       
     
     For the four junction IMM solar cell described in the parent application, the corresponding data is as follows: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Condition 
                 Efficiency 
               
               
                   
                   
               
             
            
               
                   
                 BOL 28° C. 
                 29.5% 
                   
               
               
                   
                 BOL 70° C. 
                 26.6% 
               
               
                   
                 EOL 70° C. 
                 24.7% 
                 After 5E14 e/cm 2  radiation 
               
               
                   
                 EOL 70° C. 
                 24.2% 
                 After 1E15 e/cm 2  radiation 
               
               
                   
                   
               
            
           
         
       
     
     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 14  e/cm 2 , and dramatically improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 1×10 15  e/cm 2 . 
     For one embodiment of a five junction IMM solar cell described in the present application, the corresponding data is as follows: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Condition 
                 Efficiency 
               
               
                   
                   
               
             
            
               
                   
                 BOL 28° C. 
                 32.1% 
                   
               
               
                   
                 BOL 70° C. 
                 30.4% 
               
               
                   
                 EOL 70° C. 
                 27.1% 
                 After 5E14 e/cm 2  radiation 
               
               
                   
                 EOL 70° C. 
                 25.8% 
                 After 1E15 e/cm 2  radiation 
               
               
                   
                   
               
            
           
         
       
     
     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. 
     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). 
     A low earth orbit (LEO) satellite will typically experience radiation equivalent to 5×10 14  e/cm 2  over a five year lifetime. A geosynchronous earth orbit (GEO) satellite will typically experience radiation in the range of 5×10 14  e/cm 2  to 1×10 15  e/cm 2  over a fifteen year lifetime. 
     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. 
     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 15  electrons/cm 2 . 
     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 g -V 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. 
     Furthermore, at BOL, it is widely accepted that great subcells have a room temperature E g -V oc  of approximately 0.40. A wide variation in BOL E g -V oc  may exist for subcells of interest to IMMX cells. However, Applicants have found that inspecting E g -V 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 g -V oc  of approximately 0.6 at room temperature 28° C. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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.