(US20180240922) FOUR JUNCTION SOLAR CELL AND SOLAR CELL ASSEMBLIES FOR SPACE APPLICATIONS

Application Number: 15873135 Application Date: 17.01.2018
Publication Number: 20180240922 Publication Date: 23.08.2018
Publication Kind : A1
IPC:
H01L 31/041
H01L 31/0687
H01L 31/18
B64G 1/44
CPC:
B64G 1/443
H01L 31/041
H01L 31/0687
H01L 31/1844
H01L 31/1852
Applicants: SolAero Technologies Corp.
Inventors: Daniel Derkacs
Zachary Bittner
Samantha Whipple
Alexander Haas
John Hart
Nathaniel Miller
Pravin Patel
Paul Sharps
Priority Data:
Title: (EN) FOUR JUNCTION SOLAR CELL AND SOLAR CELL ASSEMBLIES FOR SPACE APPLICATIONS
Abstract: front page image

(EN)

A four junction solar cell having an upper first solar subcell composed of a semiconductor material having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; and a fourth solar subcell adjacent to and lattice mismatched from said third solar subcell and composed of a semiconductor material having a fourth band gap smaller than the third band gap; wherein the fourth subcell has a direct bandgap of greater than 0.75 eV, and the average band gap of the solar cell is equal to or greater than 1.35 eV.

REFERENCE TO RELATED APPLICATIONS

      This application is a continuation-in-part of U.S. patent application Ser. No. 14/828,206, filed Aug. 17, 2015, which is incorporated herein by reference in its entirety.
      This application is related to co-pending 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 was a continuation in part of U.S. patent application Ser. No. 12/337,043 filed Dec. 17, 2008.
      This application is also related to co-pending U.S. patent application Ser. No. 13/872,663 filed Apr. 29, 2013, 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.
      All of the above related applications are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Field of the Invention

      The present disclosure relates to solar cells and the fabrication of solar cells, and more particularly the design and specification of the composition and band gaps of each of the four subcells in a four junction solar cell based on III-V semiconductor compounds in order to achieve improved “end-of-life” performance as may be specified for a predetermined space mission and environment.

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 are 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 series connected photovoltaic regions with different band gap energies, and accumulating the voltage at a given 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 (W/kg) and power-to-area (W/m 2) ratios of the solar cell array and the lifetime efficiency of a solar cell becomes increasingly more important. There is increasing interest not only the amount of power provided per gram of weight and spatial area of the solar cell, not only at initial deployment but over the entire service life of the satellite system, or in terms of a design specification, the amount of residual power provided at the specified “end of life” (EOL), which is affected by the radiation exposure of the solar cell over time in the particular space environment of the solar cell array, the period of the EOL being different for different missions and applications.
      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 compared to terrestrial 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., in this disclosure, 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 such as 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. Furthermore, the front side of the solar array may provide a conductive coating or adhesive to the periphery of the cover glass to ground the top surface of the cover glass.
      The radiation hardness of the semiconductor material of the solar cell itself is primarily dependent on a solar cell’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 less than 0.02% 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 less than 0.6% lattice constant difference as defining “lattice mismatched” layers).
      The conventional wisdom for many years has been that in a monolithic tandem solar cell, “ . . . the desired optical transparency and current conductivity between the top and bottom cells . . . would be best achieved by lattice matching the top cell material to the bottom cell material. Mismatches in the lattice constants create defects or dislocations in the crystal lattice where recombination centers can occur to cause the loss of photogenerated minority carriers, thus significantly degrading the photovoltaic quality of the device. More specifically, such effects will decrease the open circuit voltage (V oc), short circuit current (J sc), and fill factor (FF), which represents the relationship or balance between current and voltage for effective output” (Jerry M. Olson, U.S. Pat. No. 4,667,059, “Current and Lattice Matched Tandem Solar Cell”).
      As progress has been made toward higher efficiency multijunction solar cells with four or more subcells, nevertheless, “it is conventionally assumed that substantially lattice-matched designs are desirable because they have proven reliability and because they use less semiconductor material than metamorphic solar cells, which require relatively thick buffer layers to accommodate differences in the lattice constants of the various materials” (Rebecca Elizabeth Jones—Albertus et al., U.S. Pat. No. 8,962,993).
      The present disclosure proposes design features for multijunction solar cells which departs from such conventional wisdom for increasing the efficiency of the multijunction solar cell in converting solar energy (or photons) to electrical energy and optimizing such efficiency at the “end-of-life” period.

SUMMARY OF THE DISCLOSURE

Objects of the Disclosure

      It is an object of the present disclosure to provide increased photoconversion efficiency in a multijunction solar cell for space applications over the operational life of the photovoltaic power system.
      It is another object of the present disclosure to provide in a multijunction solar cell in which the composition of the subcells and their band gaps has been configured to maximize the efficiency of the solar cell at operational conditions of a predetermined high temperature (specifically, in the range of 40 to 70 degrees Centigrade) in deployment in space at AM0 one-sun solar spectrum at a predetermined time after the initial deployment, such time being at least one, five, ten, fifteen or twenty years and not at the time of initial deployment.
      It is another object of the present disclosure to provide in a multijunction solar cell in which the composition of the subcells and their band gaps maximizes the efficiency of the solar cell at a predetermined high temperature (in the range of 40 to 70 degrees Centigrade) in deployment in space at AM0 at a predetermined time after the initial deployment, such time being at least one year.
      It is another object of the present disclosure to provide in a multijunction solar cell in which the selection of the composition of the subcells and their band gaps maximizes the efficiency of the solar cell at a predetermined high temperature (in the range of 40 to 70 degrees Centigrade) in deployment in space at AM0 at a predetermined time after the initial deployment, such time being at least one year.
      It is another object of the present invention to provide a four junction solar cell in which the average band gap of all four subcells is greater than 1.35 eV.
      It is another object of the present invention to provide a four junction solar cell in which the lower two subcells are lattice mismatched and in which the current through the bottom subcell is intentionally designed to be substantially greater than current through the top three subcells when measured at the “beginning-of-life” or time of initial deployment.
      Some implementations of the present disclosure may incorporate or implement fewer of the aspects and features noted in the foregoing objects.

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 therein. 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 1.2, or 1.3 to 1.4, or 1.5 to 1.9 eV.
      Also, as used in this disclosure, the expression “band gap” of a solar subcell which internally has layers of different band gap shall be defined to mean the band gap of the layer of the solar subcell in which the majority of the charge carriers are generated (such sublayer typically being the p-type base semiconductor layer of the base/emitter photovoltaic junction of such subcell). In the event such layer in turn has sublayers with different band gaps (such as the case of a base layer having a graded composition and more particularly a graded band gap), the sublayer of that solar subcell with the lowest band gap shall be taken as defining the “band gap” of such a subcell. Apart from a solar subcell, and more generally in the case of a specifically designated semiconductor region (such as a metamorphic layer), in which that semiconductor region has sublayers or subregions with different band gaps (such as the case of a semiconductor region having a graded composition and more particularly a graded band gap), the sublayer or subregion of that semiconductor region with the lowest band gap shall be taken as defining the “band gap” of that semiconductor region.
      Briefly, and in general terms, the present disclosure provides a solar cell comprising an upper first solar subcell composed of a semiconductor material having a first band gap; a second solar subcell adjacent to said first solar subcell composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; and a fourth solar subcell adjacent to said third solar subcell and composed of a semiconductor material having a fourth band gap smaller than the third band gap and lattice mismatched from the third solar subcell; wherein the fourth subcell has a direct bandgap of greater than 0.75 eV.
      In some embodiments, the average band gap of all four subcells (i.e., the sum of the four band gaps of each subcell divided by four) is greater than 1.35 eV.
      In some embodiments, the fourth subcell is germanium.
      In some embodiments, the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN.
      In some embodiments, the fourth subcell has a band gap of approximately 0.67 eV, the third subcell has a band gap of 1.41 eV or less, the second subcell has a band gap of approximately 1.55 to 1.8 eV and the upper first subcell has a band gap in the range of 2.0 to 2.2 eV.
      In some embodiments, the second subcell has a band gap of approximately 1.73 eV and the upper first subcell has a band gap of approximately 2.10 eV.
      In some embodiments, the upper first solar subcell has a band gap of approximately 2.05 to 2.10 eV, the second solar subcell has a band gap in the range of 1.55 to 1.73 eV; and the third solar subcell has a band gap in the range of 1.15 to 1.41 eV.
      In some embodiments, the upper first solar subcell has a band gap of approximately 2.10, the second solar subcell has a band gap of approximately 1.73 eV; and the third solar subcell has a band gap in the range of 1.41 eV.
      In some embodiments, the upper first solar subcell has a band gap of approximately 2.10, the second solar subcell has a band gap of approximately 1.65 eV; and the third solar subcell has a band gap of 1.3 eV.
      In some embodiments, the upper first solar subcell has a band gap of approximately 2.05, the second solar subcell has a band gap of approximately 1.55 eV; and the third solar subcell has a band gap of 1.2 eV.
      In some embodiments, the first solar subcell has a band gap of 2.05 eV.
      In some embodiments, the band gap of the third solar subcell is less than 1.41 eV, and greater than that of the fourth subcell.
      In some embodiments, the third solar subcell has a band gap of 1.41 eV.
      In some embodiments, the third solar subcell has a band gap in the range of 1.15 to 1.35 eV.
      In some embodiments, the third solar subcell has a band gap in the range of 1.1 to 1.2 eV.
      In some embodiments, the third solar subcell has a band gap of approximately 1.2 eV.
      In some embodiments, the upper first subcell is composed of indium gallium aluminum phosphide; the second solar subcell includes an emitter layer composed of indium gallium phosphide or aluminum indium gallium arsenide, and a base layer composed of aluminum indium gallium arsenide; the third solar subcell is composed of indium gallium arsenide; and the fourth subcell is composed of germanium.
      In some embodiments, there further comprises a distributed Bragg reflector (DBR) layer adjacent to and between the third and the fourth solar subcells and arranged so that light can enter and pass through the third solar subcell and at least a portion of which can be reflected back into the third solar subcell by the DBR layer.
      In some embodiments, the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction.
      In some embodiments, the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.
      In some embodiments, the DBR layer includes a first DBR layer composed of a plurality of p type In zAl xGa 1-x-zAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type In wAl yGa 1-y-xAs layers, where 0<x<1, 0<y<1, 0<z<1 and y is greater than x.
      In some embodiments, the selection of the composition of the subcells and their band gaps maximizes the efficiency at high temperature (in the range of 40 to 70 degrees Centigrade) in deployment in space at a predetermined time after the initial deployment (referred to as the beginning-of-life or (BOL), such predetermined time being referred to as the end-of-life (EOL) and being at least five years, and the average band gap (i.e., the numerical average of the lowest band gap material in each subcell) of all four subcells is greater than 1.35 eV.
      In another aspect, the present disclosure provides a four junction solar cell comprising an upper first solar subcell composed of a semiconductor material having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; and a fourth solar subcell adjacent to said third solar subcell and composed of a semiconductor material having a fourth band gap smaller than the third band gap; wherein the average band gap of all four subcells (i.e., the sum of the four band gaps of each subcell divided by four) is greater than 1.35 eV.
      In another aspect, the present disclosure provides a four-junction space-qualified solar cell designed for operation at AM0 and at a 1 MeV electron equivalent fluence of at least 1×10 14 e/cm 2, the solar cell comprising subcells, wherein a combination of compositions and band gaps of the subcells is designed to maximize efficiency of the solar cell at a predetermined time, after initial deployment, when the solar cell is deployed in space at AM0 and at an operational temperature in the range of 40 to 70 degrees Centigrade, the predetermined time being at least five years and referred to as the end-of-life (EOL), the solar cell comprising: an upper first solar subcell composed of indium gallium aluminum phosphide and having a first band gap in the range of 2.0 to 2.15 eV attached to the glass supporting member with a transparent adhesive; a second solar subcell adjacent to said first solar subcell and including an emitter layer composed of indium gallium phosphide or aluminum indium gallium arsenide, and a base layer composed of aluminum indium gallium arsenide and having a second band gap in the range of approximately 1.55 to 1.8 eV and being lattice matched with the upper first solar subcell, wherein the emitter and base layers of the second solar subcell form a photoelectric junction; a third solar subcell adjacent to said second solar subcell and composed of indium gallium arsenide and having a third band gap of 1.41 eV or less and being lattice matched with the second solar subcell; and a fourth solar subcell adjacent to said third solar subcell and lattice mismatched therefrom and composed of germanium and having a fourth band gap of approximately 0.67 eV; wherein the average band gap of all four subcells is greater than 1.425 eV.
      In another aspect, the present disclosure provides a four-junction space-qualified solar cell assembly designed for operation at AM0 and at a 1 MeV electron equivalent fluence of at least 1×10 14 e/cm 2, the solar cell comprising subcells, wherein a combination of compositions and band gaps of the subcells is designed to maximize efficiency of the solar cell at a predetermined time, after initial deployment, when the solar cell is deployed in space at AM0 and at an operational temperature in the range of 40 to 70 degrees Centigrade, the predetermined time being at least five years and referred to as the end-of-life (EOL), the solar cell assembly comprising: a ceria doped borosilicate glass supporting member that is 3 to 6 mils in thickness; an upper first solar subcell composed of indium gallium aluminum phosphide and having a first band gap in the range of 2.0 to 2.15 eV attached to the glass supporting member with a transparent adhesive; a second solar subcell adjacent to said first solar subcell and including an emitter layer composed of indium gallium phosphide or aluminum indium gallium arsenide, and a base layer composed of aluminum indium gallium arsenide and having a second band gap in the range of approximately 1.55 to 1.8 eV and being lattice matched with the upper first solar subcell, wherein the emitter and base layers of the second solar subcell form a photoelectric junction; a third solar subcell adjacent to said second solar subcell and composed of indium gallium arsenide and having a third band gap in the range of 1.2 to 1.41 eV and being lattice matched with the second solar subcell; and a fourth solar subcell adjacent to said third solar subcell and composed of germanium and having a fourth band gap of approximately 0.67 eV; wherein the average band gap of all four subcells is greater than 1.35 eV.
      In some embodiments, the top or upper first subcell is composed of a base layer of (In xGa 1-x1-yAl yP where x is 0.505, and y is 0.142, corresponding to a band gap of 2.10 eV, and an emitter layer of (In xGa 1-x-yAl yP where x is 0.505, and y is 0.107, corresponding to a band gap of 2.05 eV.
      In some embodiments, there further comprises an intermediate layer disposed between the third subcell and the fourth subcell wherein the intermediate layer is compositionally graded to lattice match the third solar subcell on one side and the fourth solar subcell on the other side and is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater than or equal to that of the third solar subcell and less than or equal to that of the lower fourth solar subcell, and having a band gap energy greater than that of the fourth solar subcell.
      In some embodiments, there further comprises an intermediate layer disposed between the third subcell and the fourth subcell wherein the intermediate layer is compositionally step-graded with between one and four steps to lattice match the fourth solar subcell on one side and composed of In xGa 1-xAs or (In xGa 1-xyAl 1-yAs with 0<x<1, 0<y<1, and x and y selected such that the band gap remains in the range of 1.15 to 1.41 eV throughout its thickness.
      In some embodiments, the intermediate layer has a constant band gap in the range of 1.15 to 1.41 eV, or 1.2 to 1.35 eV, or 1.25 to 1.30 eV.
      In some embodiments, either (i) the emitter layer; or (ii) the base layer and emitter layer of the upper first subcell have a different lattice constant from the lattice constant of the second subcell.
      In some embodiments, there further comprises a distributed Bragg reflector (DBR) layer adjacent to and beneath the third solar subcell and arranged so that light can enter and pass through the third solar subcell and at least a portion of which can be reflected back into the third solar subcell by the DBR layer, wherein the distributed Bragg reflector layer is composed of a plurality of alternating layers of lattice matched materials with discontinuities in their respective indices of refraction, wherein the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength, and wherein the DBR layer includes a first DBR layer composed of a plurality of p type In zAl xGa 1-x-zAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type In wAl yGa 1-x-wAs layers, where 0<w<1, 0<x<1, 0<y<1, 0<z<1 and y is greater than x; and an intermediate layer disposed between the DBR layer and the fourth solar subcell, wherein the intermediate layer is compositionally step-graded to lattice match the DBR layer on one side and the fourth solar subcell on the other side, and is composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter greater than or equal to that of the DBR layer and less than or equal to that of the lower fourth solar subcell, and having a band gap energy greater than that of the fourth solar subcell.
      In some embodiments, each subcell includes an emitter region and a base region, and one or more of the first, second or third subcells have a base region having a gradation in doping that increases exponentially from 1×10 15 atoms per cubic centimeter adjacent the p-n junction to 4×10 18 atoms per cubic centimeter adjacent to the adjoining layer at the rear of the base, and an emitter region having a gradation in doping that decreases from approximately 5×10 18 per cubic centimeter in the region immediately adjacent the adjoining layer to 5×10 17 per cubic centimeter in the region adjacent to the p-n junction.
      In some embodiments, at least one of the upper sublayers of the graded interlayer has a larger lattice constant than the adjacent layers to the upper sublayer disposed above the grading interlayer.
      In some embodiments, the difference in lattice constant between the adjacent third and fourth subcells is in the range of 0.1 to 0.2 Angstroms.
      In some embodiments, there further comprises at least a first threading dislocation inhibition layer having a thickness in the range of 0.10 to 1.0 microns and disposed over said second solar subcell.
      In some embodiments, there further comprises at least a second threading dislocation inhibition layer having a thickness in the range of 0.10 to 1.0 micron and composed of InGa(Al)P, the second threading dislocation inhibition layer being disposed over and directly adjacent to said grading interlayer for reducing the propagation of threading dislocations, said second threading dislocation inhibition layer having a composition different from a composition of the first threading dislocation inhibition layer.
      In some embodiments, there further comprises: (a) a ceria doped borosilicate glass supporting member that is 3 to 6 mils in thickness attached to the upper first solar subcell by a transparent adhesive; (b) a plurality of interconnects each composed of a silver-plated nickel-cobalt ferrous alloy material, each interconnect welded to a respective bonding pad on each solar cell assembly to electrically connect the adjacent solar cell assemblies of the array in a series electrical circuit; and (c) an aluminum honeycomb panel having a carbon composite face sheet and having a coefficient of thermal expansion (CTE) that substantially matches the Ge layer of the fourth solar subcell in each solar cell assembly mounted thereon.
      In another aspect, the present disclosure provides a method for fabricating a four-junction space-qualified solar cell designed for operation at AM0 and at a 1 MeV electron equivalent fluence of at least 1×10 14 e/cm 2, the solar cell comprising subcells, wherein a combination of compositions and band gaps of the subcells is designed to maximize efficiency of the solar cell as a predetermined time, after initial deployment, when the solar cell is deployed in space at AM0 and at an operational temperature in the range of 40 to 70 degrees Centigrade, the predetermined time being at least five years and referred to as the end-of-life (EOL), comprising: providing a growth substrate; forming a first solar subcell over or in the growth substrate; growing a sequence of layers of semiconductor material using a disposition process to form a solar cell comprising a plurality of subcells including a first middle subcell disposed over the growth substrate and having a band gap in the range of 1.15 to 1.41 eV, a second middle subcell disposed over and lattice matched to the third subcell and having a band gap in the range of approximately 1.55 to 1.8 eV and an upper subcell disposed over and lattice matched to the second middle subcell and having a band gap in the range of 2.05 to 2.10 eV, wherein the average band gap of all four subcells is greater than 1.35 eV.
      In another aspect, the present disclosure provides a method of manufacturing a four junction solar cell comprising providing a germanium substrate; growing on the germanium substrate a sequence of layers of semiconductor material using a semiconductor deposition process to form a solar cell comprising a plurality of subcells including a third subcell disposed over the germanium substrate and having a band gap of 1.41 eV or less, a second subcell disposed over the third subcell and having a band gap in the range of approximately 1.55 to 1.8 eV and an upper first subcell disposed over the second subcell and having a band gap in the range of 2.0 to 2.15 eV.
      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.
      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 disclosure. 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.
      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 invention 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 is a graph representing the BOL value of the parameter E g/q−V oc at 28° C. plotted against the band gap of certain binary materials defined along the x-axis;
       FIG. 2 is a cross-sectional view of the solar cell of a first embodiment of a four junction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;
       FIG. 3 is a graph of the doping profile in the base and emitter layers of a subcell in the solar cell according to the present disclosure;
       FIG. 4A is a cross-sectional view of a second embodiment of a four junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;
       FIG. 4B is a cross-sectional view of a third embodiment of a four junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;
       FIG. 5 is a cross-sectional view of a fourth embodiment of a four junction solar cell after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure;
       FIG. 6 is a cross-sectional view of the solar cell of the present disclosure as implemented in a CIC and mounted on a panel;
       FIG. 7 is a graph representing the band gap of certain binary materials and their lattice constants; and
       FIG. 8 is an enlargement of a portion of the graph of FIG. 7 illustrating different compounds of GaInAs and GaInP with different proportions of gallium and indium, and the location of specific compounds of the graph.

GLOSSARY OF TERMS

      “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.
      “Dopant” refers to a trace impurity element that is contained within a semiconductor material to affect the electrical or optical characteristics of that material. As used in the context of the present disclosure, typical dopant levels in semiconductor materials are in the 10 16 to 10 19 atoms per cubic centimeter range. The standard notation or nomenclature, when a particular identified dopant is proscribed, is to use, for example, the expression “GaAs:Se” or “GaAs:C” for selenium or carbon doped gallium arsenide respectively. Whenever a ternary or quaternary compound semiconductor is expressed as “AlGaAs” or “GaInAsP”, it is understood that all three or four of the constituent elements are much higher in mole concentration, say on the 1% level or above, which is in the 10 21 atoms/cm −3 or larger range. Such constituent elements are not considered “dopants” by those skilled in the art since the atoms of the constituent element form part of the crystal structure of the compound semiconductor. In addition, a further distinction is that a dopant has a different valence number than the constituent component elements. In a commonly implemented III-V compound semiconductor such as AlGaInAs, none of the individual elements Al, Ga, In, or As are considered to be dopants since they have the same valence as the component atoms that make up the crystal lattice.
      “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 less than 0.02% in lattice constant. (Applicant expressly adopts this definition for the purpose of this disclosure, and notes that this definition is considerably more stringent than that proposed, for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6% lattice constant difference).
      “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.
      “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 “upright” solar cells of the present disclosure. However, more particularly, the present disclosure is directed to the fabrication of a multijunction solar cell grown on a single growth substrate, including in one embodiment the two lower subcells (e.g., the third and fourth subcells) being lattice mismatched. More specifically, however, in some embodiments, the present disclosure relates to four junction solar cells with direct band gaps in the range of 2.0 to 2.15 eV (or higher) for the top subcell, and (i) 1.65 to 1.8 eV, and (ii) 1.41 eV or less, for the middle subcells, and (iii) 0.6 to 0.8 eV indirect bandgaps for the bottom subcell, respectively.
      The present disclosure provides an unconventional four junction design (with three grown lattice matched subcells, which are lattice mismatched to the Ge substrate) that leads to a surprising significant performance improvement over that of traditional three junction solar cell on Ge despite the substantial current mismatch present between the top three junctions and the bottom Ge junction. This performance gain is especially realized at high temperature and after high exposure to space radiation by the proposal of incorporating high band gap semiconductors that are inherently more resistant to radiation and temperature, thus specifically addressing the problem of ensuring continues adequate efficiency and power output at the “end-of-life”.
      Another way of characterizing the present disclosure is that in some embodiments of a four junction solar cell, the average band gap of all four subcells (i.e., the sum of the four band gaps of each subcell divided by four) is greater than 1.35 eV.
      In some embodiments, the fourth subcell is germanium, while in other embodiments the fourth subcell is InGaAs, GaAsSb, InAsP, InAlAs, or SiGeSn, InGaAsN, InGaAsNSb, InGaAsNBi, InGaAsNSbBi, InGaSbN, InGaBiN. InGaSbBiN or other III-V or II-VI compound semiconductor material.
      Another descriptive aspect of the present disclosure is to characterize the fourth subcell as having a direct band gap of greater than 0.75 eV.
      The indirect band gap of germanium at room temperature is about 0.66 eV, while the direct band gap of germanium at room temperature is 0.8 eV. Those skilled in the art will normally refer to the “band gap” of germanium as 0.67 eV, since it is lower than the direct band gap value of 0.8 eV.
      The recitation that “the fourth subcell has a direct band gap of greater than 0.75 eV” is therefore expressly meant to include germanium as a possible semiconductor for the fourth subcell, although other semiconductor material can be used as well.
      More specifically, the present disclosure intends to provide a relatively simple and reproducible technique that does not employ inverted processing associated with inverted metamorphic multijunction solar cells, and is suitable for use in a high volume production environment in which various semiconductor layers are grown on a growth substrate in an MOCVD reactor, and subsequent processing steps are defined and selected to minimize any physical damage to the quality of the deposited layers, thereby ensuring a relatively high yield of operable solar cells meeting specifications at the conclusion of the fabrication processes.
      As suggested above, incremental improvements in the design of multijunction solar cells are made in view of a variety of new space missions and application requirements. Moreover, although such improvements may be relatively minute quantitative modifications in the composition or band gap of certain subcells, as we noted above, such minute parametric changes (such as in the specific band gaps of the upper first subcell, or of the third subcell) provide substantial improvements in efficiency that specifically address the “problems” that have been identified associated with the existing current commercial multijunction solar cells, and provide a “solution” that represents an “inventive step” in the design process.
      Thus, in addition to the characterizing feature that the third and fourth solar subcells are not necessarily lattice matched, we explicitly recite in paragraph [48] above that the fourth solar subcell is “lattice mismatched” from the third solar subcell, and in paragraph [52] through [63] a variety of different band gap specifications associated with different embodiments of the solar cell of the present disclosure. Additional characterizing features are set forth in the claims hereunder.
      Prior to discussing the specific embodiments of the present disclosure, a brief discussion of some of the issues associated with the design of multijunction solar cells in the context of the composition or deposition of various specific layers in embodiments of the product as specified and defined by Applicant is in order.
      There are a multitude of properties that should be considered in specifying and selecting the composition of, inter alia, a specific semiconductor layer, the back metal layer, the adhesive or bonding material, or the composition of the supporting material for mounting a solar cell thereon. For example, some of the properties that should be considered when selecting a particular layer or material are electrical properties (e.g. conductivity), optical properties (e.g., band gap, absorbance and reflectance), structural properties (e.g., thickness, strength, flexibility, Young’s modulus, etc.), chemical properties (e.g., growth rates, the “sticking coefficient” or ability of one layer to adhere to another, stability of dopants and constituent materials with respect to adjacent layers and subsequent processes, etc.), thermal properties (e.g., thermal stability under temperature changes, coefficient of thermal expansion), and manufacturability (e.g., availability of materials, process complexity, process variability and tolerances, reproducibility of results over high volume, reliability and quality control issues).
      In view of the trade-offs among these properties, it is not always evident that the selection of a material based on one of its characteristic properties is always or typically “the best” or “optimum” from a commercial standpoint or for Applicant’s purposes. For example, theoretical studies may suggest the use of a quaternary material with a certain band gap for a particular subcell would be the optimum choice for that subcell layer based on fundamental semiconductor physics. As an example, the teachings of academic papers and related proposals for the design of very high efficiency (over 40%) solar cells may therefore suggest that a solar cell designer specify the use of a quaternary material (e.g., InGaAsP) for the active layer of a subcell. A few such devices may actually be fabricated by other researchers, efficiency measurements made, and the results published as an example of the ability of such researchers to advance the progress of science by increasing the demonstrated efficiency of a compound semiconductor multijunction solar cell. Although such experiments and publications are of “academic” interest, from the practical perspective of the Applicants in designing a compound semiconductor multijunction solar cell to be produced in high volume at reasonable cost and subject to manufacturing tolerances and variability inherent in the production processes, such an “optimum” design from an academic perspective is not necessarily the most desirable design in practice, and the teachings of such studies more likely than not point in the wrong direction and lead away from the proper design direction. Stated another way, such references may actually “teach away” from Applicant’s research efforts and the ultimate solar cell design proposed by the Applicants.
      In view of the foregoing, it is further evident that the identification of one particular constituent element (e.g. indium, or aluminum) in a particular subcell, or the thickness, band gap, doping, or other characteristic of the incorporation of that material in a particular subcell, is not a “result effective variable” that one skilled in the art can simply specify and incrementally adjust to a particular level and thereby increase the efficiency of a solar cell. The efficiency of a solar cell is not a simple linear algebraic equation as a function of the amount of gallium or aluminum or other element in a particular layer. The growth of each of the epitaxial layers of a solar cell in a reactor is a non-equilibrium thermodynamic process with dynamically changing spatial and temporal boundary conditions that is not readily or predictably modeled. The formulation and solution of the relevant simultaneous partial differential equations covering such processes are not within the ambit of those of ordinary skill in the art in the field of solar cell design.
      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 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.
      One aspect of the present disclosure relates to the use of aluminum in the active layers of the upper subcells in a multijunction solar cell. The effects of increasing amounts of aluminum as a constituent element in an active layer of a subcell affects the photovoltaic device performance. One measure of the “quality” or “goodness” of a solar cell junction is the difference between the band gap of the semiconductor material in that subcell or junction and the V oc, or open circuit voltage, of that same junction. The smaller the difference, the higher the V oc of the solar cell junction relative to the band gap, and the better the performance of the device. V oc is very sensitive to semiconductor material quality, so the smaller the E g/q−V oc of a device, the higher the quality of the material in that device. There is a theoretical limit to this difference, known as the Shockley-Queisser limit. That is the best that a solar cell junction can be under a given concentration of light at a given temperature.
      The experimental data obtained for single junction (Al)GaInP solar cells indicates that increasing the Al content of the junction leads to a larger V oc−Eg/q difference, indicating that the material quality of the junction decreases with increasing Al content. FIG. 1 shows this effect. The three compositions cited in the Figure are all lattice matched to GaAs, but have differing Al composition. Adding Al increases the band gap of the junction, but in so doing also increases V oc−E g/q. Hence, we draw the conclusion that adding Al to a semiconductor material degrades that material such that a solar cell device made out of that material does not perform relatively as well as a junction with less Al.
      Turning to the multijunction solar cell device of the present disclosure, FIG. 2 is a cross-sectional view of a first embodiment of a four junction solar cell400 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322 according to the present disclosure.
      As shown in the illustrated example of FIG. 2, the bottom or fourth subcell D includes a growth substrate 300 formed of p-type germanium (“Ge”) which also serves as a base layer. A back metal contact pad 350 formed on the bottom of base layer 300 provides electrical contact to the multijunction solar cell200. The bottom subcell D, further includes, for example, a highly doped n-type Ge emitter layer 301, and an n-type indium gallium arsenide (“InGaAs”) nucleation layer 302. The nucleation layer is deposited over the base layer, and the emitter layer is formed in the substrate by diffusion of dopants into the Ge substrate, thereby forming the n-type Ge layer 301. Heavily doped p-type aluminum indium gallium arsenide (“AlGaAs”) and heavily doped n-type gallium arsenide (“GaAs”) tunneling junction layers 304303 may be deposited over the nucleation layer to provide a low resistance pathway between the bottom and middle subcells.
      In some embodiments, Distributed Bragg reflector (DBR) layers 305 are then grown adjacent to and between the tunnel diode 303304 of the bottom subcell D and the third solar subcell C. The DBR layers 305 are arranged so that light can enter and pass through the third solar subcell C and at least a portion of which can be reflected back into the third solar subcell C by the DBR layers 305. In the embodiment depicted in FIG. 3, the distributed Bragg reflector (DBR) layers 305 are specifically located between the third solar subcell C and tunnel diode layers 304303; in other embodiments, the distributed Bragg reflector (DBR) layers may be located between tunnel diode layers 304303 and buffer layer 302.
      For some embodiments, distributed Bragg reflector (DBR) layers 305 can be composed of a plurality of alternating layers 305 through 305 of lattice matched materials with discontinuities in their respective indices of refraction. For certain embodiments, the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.
      For some embodiments, distributed Bragg reflector (DBR) layers 305 through 305 includes a first DBR layer composed of a plurality of p type In zAlxGa 1-x-zAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type In wAl yGa 1-y-wAs layers, 0<w<1, 0<x<1, 0<y<1, 0<z<1, and where y is greater than x.
      Although the present disclosure depicts the DBR layer 305 situated between the third and the fourth subcell, in other embodiments, DBR layers may be situated between the first and second subcells, and/or between the second and the third subcells, and/or between the third and the fourth subcells.
      In the illustrated example of FIG. 2, the third subcell C includes a highly doped p-type aluminum indium gallium arsenide (“AlInGaAs”) back surface field (“BSF”) layer 306, a p-type InGaAs base layer 307, a highly doped n-type indium gallium arsenide (“InGaAs”) emitter layer 308 and a highly doped n-type indium aluminum phosphide (“AlInP 2”) or indium gallium phosphide (“GaInP”) window layer 309. The InGaAs base layer 307 of the subcell C can include, for example, approximately 1.5% In. Other compositions may be used as well. The base layer 307 is formed over the BSF layer 306 after the BSF layer is deposited over the DBR layers 305.
      The window layer 309 is deposited on the emitter layer 308 of the subcell C. The window layer 309 in the subcell C also helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions. Before depositing the layers of the subcell B, heavily doped n-type InGaP and p-type AlGaAs (or other suitable compositions) tunneling junction layers 310311 may be deposited over the subcell C.
      The second subcell B includes a highly doped p-type aluminum indium gallium arsenide (“AlInGaAs”) back surface field (“BSF”) layer 312, a p-type AlInGaAs base layer 313, a highly doped n-type indium gallium phosphide (“InGaP 2”) or AlInGaAs layer 314 and a highly doped n-type indium gallium aluminum phosphide (“AlGaAlP”) window layer 315. The InGaP emitter layer 314 of the subcell B can include, for example, approximately 50% In. Other compositions may be used as well.
      Before depositing the layers of the top or upper first cell A, heavily doped n-type InGaP and p-type AlGaAs tunneling junction layers 316317 may be deposited over the subcell B.
      In the illustrated example, the top subcell A includes a highly doped p-type indium aluminum phosphide (“InAlP 2”) BSF layer 318, a p-type InGaAlP base layer 319, a highly doped n-type InGaAlP emitter layer 320 and a highly doped n-type InAlP window layer 321. The base layer 319 of the top subcell A is deposited over the BSF layer 318 after the BSF layer 318 is formed.
      After the cap or contact layer 322 is deposited, the grid lines are formed via evaporation and lithographically patterned and deposited over the cap or contact layer 322.
      In some embodiments, at least the base of at least one of the first, second or third solar subcells has a graded doping, i.e., the level of doping varies from one surface to the other throughout the thickness of the base layer. In some embodiments, the gradation in doping is exponential. In some embodiments, the gradation in doping is incremental and monotonic.
      In some embodiments, the emitter of at least one of the first, second or third solar subcells also has a graded doping, i.e., the level of doping varies from one surface to the other throughout the thickness of the emitter layer. In some embodiments, the gradation in doping is linear or monotonically decreasing.
      As a specific example, the doping profile of the emitter and base layers may be illustrated in FIG. 3, which depicts the amount of doping in the emitter region and the base region of a subcell. N-type dopants include silicon, selenium, sulfur, germanium or tin. P-type dopants include silicon, zinc, chromium, or germanium.
      In the example of FIG. 3, in some embodiments, one or more of the subcells have a base region having a gradation in doping that increases from a value in the range of 1×10 15 to 1×10 18 free carriers per cubic centimeter adjacent the p-n junction to a value in the range of 1×10 16 to 4×10 18 free carriers per cubic centimeter adjacent to the adjoining layer at the rear of the base, and an emitter region having a gradation in doping that decreases from a value in the range of approximately 5×10 18 to 1×10 17 free carriers per cubic centimeter in the region immediately adjacent the adjoining layer to a value in the range of 5×10 15 to 1×10 18 free carriers per cubic centimeter in the region adjacent to the p-n junction.
      The heavy lines 612 and 613 shown in FIG. 3 illustrates one embodiment of the base doping having an exponential gradation, and the emitter doping being linear.
      Thus, the doping level throughout the thickness of the base layer may be exponentially graded from the range of 1×10 16 free carriers per cubic centimeter to 1×10 18 free carriers per cubic centimeter, as represented by the curve 613 depicted in the Figure.
      Similarly, the doping level throughout the thickness of the emitter layer may decline linearly from 5×10 18 free carriers per cubic centimeter to 5×10 17free carriers per cubic centimeter as represented by the curve 612 depicted in the Figure.
      The absolute value of the collection field generated by an exponential doping gradient exp [−x/k] is given by the constant electric field of magnitude E=kT/q(1/λ)(exp[−x b/λ]), where k is the Boltzman constant, T is the absolute temperature in degrees Kelvin, q is the absolute value of electronic change, and λ is a parameter characteristic of the doping decay.
      The efficacy of an embodiment of the doping arrangement present disclosure has been demonstrated in a test solar cell which incorporated an exponential doping profile in the three micron thick base layer a subcell, according to one embodiment.
      The exponential doping profile taught by one embodiment of the present disclosure produces a constant field in the doped region. In the particular multijunction solar cell materials and structure of the present disclosure, the bottom subcell has the smallest short circuit current among all the subcells. Since in a multijunction solar cell, the individual subcells are stacked and form a series circuit, the total current flow in the entire solar cell is therefore limited by the smallest current produced in any of the subcells. Thus, by increasing the short circuit current in the bottom cell, the current more closely approximates that of the higher subcells, and the overall efficiency of the solar cell is increased as well. In a multijunction solar cell with approximately efficiency, the implementation of the present doping arrangement would thereby increase efficiency. In addition to an increase in efficiency, the collection field created by the exponential doping profile will enhance the radiation hardness of the solar cell, which is important for spacecraft applications.
      Although the exponentially doped profile is the doping design which has been implemented and verified, other doping profiles may give rise to a linear varying collection field which may offer yet other advantages. For example, another doping profile may produce a linear field in the doped region which would be advantageous for both minority carrier collection and for radiation hardness at the end-of-life (EOL) of the solar cell. Such other doping profiles in one or more base layers are within the scope of the present disclosure.
      The doping profile depicted herein are merely illustrative, and other more complex profiles may be utilized as would be apparent to those skilled in the art without departing from the scope of the present invention.
       FIG. 4A is a cross-sectional view of a second embodiment of a four junction solar cell 400 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322, with various subcells being similar to the structure described and depicted in FIG. 2. In the interest of brevity, the description of layers 350300 to 304, and 306 through 322 will not be repeated here.
      In the embodiment depicted in FIG. 4A, an intermediate graded interlayer 505, comprising in one embodiment step-graded sublayers 505 through 505z, is disposed over the tunnel diode layer 304. In particular, the graded interlayer provides a transition in the in-plane lattice constant from the lattice constant of the substrate to the larger lattice constant of the middle and upper subcells.
      The graph on the left side of FIG. 4A depicts the in-plane lattice constant being incrementally monotonically increased from sublayer 505 through sublayer 505 z, such sublayers being fully relaxed.
      At least a first “alpha” or threading dislocation inhibition layer 504, preferably composed of p-type InGaP, is deposited over the tunnel diode 303304, to a thickness of from 0.10 to 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 bottom subcell D, or in the direction of growth into the subcell C, and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.). More generally, the alpha layer has a different composition than the adjacent layers above and below it.
      A metamorphic layer (or graded interlayer) 505 is deposited over the alpha layer 504 using a surfactant. Layer 505 is preferably a compositionally step-graded series of p-type InGaAs or InGaAlAs layers, preferably with monotonically changing lattice constant, so as to achieve a gradual transition in lattice constant in the semiconductor structure from subcell D to subcell C while minimizing threading dislocations from occurring. In one embodiment, the band gap of layer 505 is constant throughout its thickness, preferably approximately equal to 1.22 to 1.34 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell C. In another embodiment, the band gap of the sublayers of layer 505 vary in the range of 1.22 to 1.34 eV, with the first layer having a relatively high band gap, and subsequent layers incrementally lower band gaps. One embodiment of the graded interlayer may also be expressed as being composed of In xGa 1-xAs, with 0<x<1, 0<y<1, and x and y selected such that the band gap of the interlayer remains constant at approximately 1.22 to 1.34 eV or other appropriate band gap.
      In one embodiment, aluminum is added to one sublayer to make one particular sublayer harder than another, thereby forcing dislocations in the softer material.
      In the surfactant assisted growth of the metamorphic layer 505, a suitable chemical element is introduced into the reactor during the growth of layer 505to improve the surface characteristics of the layer. In the preferred embodiment, such element may be a dopant or donor atom such as selenium (Se) or tellurium (Te). Small amounts of Se or Te are therefore incorporated in the metamorphic layer 406, and remain in the finished solar cell. Although Se or Te are the preferred n-type dopant atoms, other non-isoelectronic surfactants may be used as well.
      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 505.
      In one embodiment of the present disclosure, the layer 505 is composed of a plurality of layers of InGaAs, with monotonically changing lattice constant, each layer having a band gap in the range of 1.22 to 1.34 eV. In some embodiments, the band gap is constant in the range of 1.27 to 1.31 eV through the thickness of layer 505. In some embodiments, the constant band gap is in the range of 1.28 to 1.29 eV.
      The advantage of utilizing a constant bandgap material such as InGaAs is that arsenide-based semiconductor material is much easier to process in standard commercial MOCVD reactors.
      Although the described embodiment of the present disclosure utilizes a plurality of layers of InGaAs for the metamorphic layer 505 for reasons of manufacturability and radiation transparency, other embodiments of the present disclosure may utilize different material systems to achieve a change in lattice constant from subcell C to subcell D. Other embodiments of the present disclosure may utilize continuously graded, as opposed to step graded, materials. More generally, the graded interlayer may be composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter less than or equal to that of the third solar subcell C and greater than or equal to that of the fourth solar subcell D. In some embodiments, the layer 505 has a band gap energy greater than that of the third solar subcell C, and in other embodiments has a band gap energy level less than that of the third solar subcell C.
      In some embodiments, a second “alpha” or threading dislocation inhibition layer 507, preferably composed of p type GaInP, is deposited over metamorphic buffer layer 505, to a thickness of from 0.10 to 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 fourth subcell D, or in the direction of growth into the third subcell C, and is more particularly described in U.S. Patent Application Pub. No. 2009/0078309 A1 (Cornfeld et al.).
      In the embodiment depicted in FIG. 4B, an intermediate graded interlayer 506, comprising in one embodiment step-graded sublayers 505 through 505zz, is disposed over the tunnel diode layer 304. In particular, the graded interlayer provides a transition in lattice constant from the lattice constant of the substrate to the larger lattice constant of the middle and upper subcells, and differs from that of the embodiment of FIG. 4A only in that the top or uppermost sublayer 505 zz of the graded interlayer 506 is strained or only partially relaxed, and has a lattice constant which is greater than that of the layer above it, i.e., the alpha layer 507 (should there be a second alpha layer) or the BSF layer 306. In short, in this embodiment, there is an “overshoot” of the last one sublayer 505 zz of the grading sublayers, as depicted on the left hand side of FIG. 4B, which shows the step-grading of the lattice constant becoming larger from layer 505 to 505 zz, and then decreasing back to the lattice constant of the upper layers 507 through 322.
       FIG. 5 is a cross-sectional view of a fourth embodiment of a four junction solar cell 500 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322, with various subcells being similar to the structure described and depicted in FIG. 3.
      In this embodiment, both a grading interlayer 505 and a DBR layer 305 are disposed between subcell C and subcell D. The layers 450400 to 404504to 507 and 306 through 322 are substantially similar to that of FIG. 2 and FIG. 4A or 4B and their description need not be repeated here.
      In this embodiment, Distributed Bragg reflector (DBR) layers 305 are then grown adjacent to and over the alpha layer 507 (or the metamorphic buffer layer 505 if layer 507 is not present). The DBR layers 305 are arranged so that light can enter and pass through the third solar subcell C and at least a portion of which can be reflected back into the third solar subcell C by the DBR layers 305. In the embodiment depicted in FIG. 5, the distributed Bragg reflector (DBR) layers 305 are specifically located between the third solar subcell C and metamorphic layer 505.
      For some embodiments, distributed Bragg reflector (DBR) layers 305 can be composed of a plurality of alternating layers 305 through 305 of lattice matched materials with discontinuities in their respective indices of refraction. For certain embodiments, the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.
      For some embodiments, distributed Bragg reflector (DBR) layers 305 through 305 include a first DBR sublayer composed of a p type In zAl xGa 1-x-zAs layer, and a second DBR sublayer disposed over the first DBR sublayer and composed of a p type In wAl yGa 1-y-wAs layer, where 0<w<1, 0<x<1, 0<y<1, 0<z<1 and y is greater than x.
       FIG. 6 is a cross-sectional view of a portion of the solar cell assembly according to the present disclosure as mounted on a panel or supporting substrate, with the Figure depicting two adjacent solar cells 601 and 701 and corresponding CICs 600 and 700 respectively.
      As previously noted, for space applications, the solar cell 601701 includes a coverglass 603703 respectively over the semiconductor device to provide radiation resistant shielding from particles in the space environment which could damage the semiconductor material. The cover glass 603703 is typically a ceria doped borosilicate glass which is typically from three to six mils in thickness and attached by a transparent adhesive 602702 respectively to the corresponding solar cell 601701.
      Bonding pads of a first and second polarity type are provided on each solar cell. In one embodiment, a back metal 604 and 704 respectively form contacts of a first polarity type. On the top surface of each solar cell, a metal contact 705 is provided along one edge of the solar cell form a contact of the second polarity.
      A plurality of electrical interconnects 607 each composed of a strip of silver-plated nickel-cobalt ferrous alloy material are provided, each interconnect being welded to a respective bonding pad 612 and 705 on each solar cell assembly to electrically connect the adjacent solar cell assemblies of the array in a series electrical circuit.
      In some embodiments, an aluminum honeycomb panel 606 having a carbon composite face sheet 605 with a coefficient of thermal expansion (CTE) that substantially matches the germanium of the fourth solar subcell in each solar cell is provided with each CIC 600700 or solar cell assembly mounted thereon.
      Another feature of the solar cell assembly in the embodiment illustrated in FIG. 6 is that the cover glasses 603703 each have a metal wrap-around clip608 depicted in CIC 600 which makes contact with the surface of the cover glass 603, and extends down the gap or space along the side of the solar cell assembly 600 between CICs 600 and 700 to make electrical contact with the metal bonding pad 612 on the back surface of CIC 600, which in turn makes contact with electrical ground. Thus the clip 608 grounds the electrical charge build-up on the surface of the cover glass 603 to the ground of the panel or spacecraft. Other configurations of grounding techniques for the surface(s) of the coverglass 603703 are within the scope of this disclosure.
       FIG. 7 is a graph representing the band gap of certain binary materials and their lattice constants. The band gap and lattice constants of ternary materials are located on the lines drawn between typical associated binary materials (such as the ternary material AlGaAs being located between the GaAs and AlAs points on the graph, with the band gap of the ternary material lying between 1.42 eV for GaAs and 2.16 eV for AlAs depending upon the relative amount of the individual constituents). Thus, depending upon the desired band gap, the material constituents of ternary materials can be appropriately selected for growth.
       FIG. 8 is an enlargement of a portion of the graph of FIG. 7 illustrating different compounds of GaInAs and GaInP with different proportions of gallium and indium, and the location of specific compounds on the graph.
      The present disclosure provides a multijunction solar cell that follows a design rule that one should incorporate as many high band gap subcells as possible to achieve the goal to increase the efficiency at high temperature EOL. For example, high band gap subcells may retain a greater percentage of cell voltage as temperature increases, thereby offering lower power loss as temperature increases. As a result, both HT-BOL and HT-EOL performance of the exemplary multijunction solar cell, according to the present disclosure, may be expected to be greater than traditional cells.
      For example, the cell efficiency (%) measured at room temperature (RT) 28° C. and high temperature (HT) 70° C., at beginning of life (BOL) and end of life (EOL), for a standard three junction commercial solar cell (ZTJ), is shown in Table 1:
[TABLE-US-00001]

TABLE 1
CONDITION EFFICIENCY
BOL 28° C. 29.1%
BOL 70° C. 26.4%
EOL 70° C. 23.4% After 5E14 e/cmradiation
EOL 70° C. 22.0% After 1E15 e/cmradiation
      For the solar cell initially described in the parent application, U.S. patent application Ser. No. 14/828,206 filed Aug. 17, 2015 (and corresponding published European Patent Application EP 3 133 65 Al) the corresponding data is shown in Table 2:
[TABLE-US-00002]

TABLE 2
CONDITION EFFICIENCY
BOL 28° C. 29.1%
BOL 70° C. 26.5%
EOL 70° C. 24.5% After 5E14 e/cmradiation
EOL 70° C. 23.5% After 1E15 e/cmradiation

The solar cell described in the earliest applications of Applicant has a slightly higher cell efficiency than the standard commercial solar cell (ZTJ) at BOL at 70° C. However, the solar cell described in one embodiment of the disclosure exhibits substantially improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 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.

      In view of different satellite and space vehicle requirements in terms of operating environmental temperature, radiation exposure, and operational life, a range of subcell designs using the design principles of the present disclosure may be provided satisfying specific defined customer and mission requirements, and several illustrative embodiments are set forth hereunder, along with the computation of their efficiency at the end-of-life for comparison purposes. As described in greater detail below, solar cell performance after radiation exposure is experimentally measured using 1 MeV electron fluence per square centimeter (abbreviated in the text that follows as e/cm 2), so that a comparison can be made between the current commercial devices and embodiments of solar cells discussed in the present disclosure.
      As an example of different mission requirements, a low earth orbit (LEO) satellite will typically experience radiation equivalent to 5×10 14 electron fluence per square centimeter (hereinafter written as “5E14 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 to 1×10 15 e/cm over a fifteen year lifetime.
      The simplest manner to express the different embodiments of the present disclosure and their efficiency compared to that of the standard solar cell noted above is to list the embodiments with the specification of the composition of each successive subcell and their respective band gap, and then the computed efficiency.
      Thus, for a four junction solar cell as configured and described in the present disclosure, four embodiments and their corresponding efficiency data at the end-of-life (EOL) is as follows:
[TABLE-US-00003]

Band
Embodiment 1 Gap Composition
Subcell A 2.1 AlInGaP
Subcell B 1.73 InGaP/AlInGaAs or
AlinGaAs/AlInGaAs
Subcell C 1.41 (In)GaAs
Subcell D 0.67 Ge

Efficiency at 70° C. after 5E14 e/cmradiation: 24.5%

Efficiency at 70° C. after 1E15 e/cmradiation: 23.5%

Efficiency at 70° C. after 5E14 e/cmradiation: 24.5%

Efficiency at 70° C. after 1E15 e/cmradiation: 23.5%

[TABLE-US-00004]

Embodiment 2 Band Gap Composition
Subcell A 2.1 AlInGaP
Subcell B 1.67 InGaP/AlInGaAs or
AlinGaAs/AlInGaAs
Subcell C 1.34 InGaAs
Subcell D 0.67 Ge

Efficiency at 70° C. after 1E15 e/cm radiation: 24.9%

[TABLE-US-00005]

Embodiment 3 Band Gap Composition
Subcell A 2.1 AlInGaP
Subcell B 1.65 InGaP/AlInGaAs or
AlinGaAs/AlInGaAs
Subcell C 1.30 (In)GaAs
Subcell D 0.67 Ge

Efficiency at 70° C. after 1E15 e/cm radiation: 25.3%

[TABLE-US-00006]

Embodiment 4 Band Gap Composition
Subcell A 2.03 AlInGaP
Subcell B 1.55 InGaP/AlInGaAs or
AlinGaAs/AlInGaAs
Subcell C 1.2 (In)GaAs
Subcell D 0.67 Ge

Efficiency at 70° C. after 1E15 e/cm radiation: 25.7%

      Although the differences in band gap among the various embodiments described above, i.e., of the order of 0.1 to 0.2 eV, may seem relatively small, it is evident that such adjustments result in an increase in the EOL solar cell efficiency from 24.4% as reported in the parent application U.S. patent application Ser. No. 14/828,206 filed Aug. 17, 2015 (and corresponding published European Patent Application EP 3 133 650 Al) to 25.7% for the solar cell of embodiment 4 described above, which is certainly a surprising and unexpected improvement that would constitute an “inventive step” over the related configuration described in the parent application and European patent application publication.
      The wide range of electron and proton energies present in the space environment necessitates a method of describing the effects of various types of radiation in terms of a radiation environment which can be produced under laboratory conditions. The methods for estimating solar cell degradation in space are based on the techniques described by Brown et al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of the Telstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963] and Tada [Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G. Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication 82-69, 1982]. In summary, the omnidirectional space radiation is converted to a damage equivalent unidirectional fluence at a normalised energy and in terms of a specific radiation particle. This equivalent fluence will produce the same damage as that produced by omnidirectional space radiation considered when the relative damage coefficient (RDC) is properly defined to allow the conversion. The relative damage coefficients (RDCs) of a particular solar cell structure are measured a priori under many energy and fluence levels in addition to different coverglass thickness values. When the equivalent fluence is determined for a given space environment, the parameter degradation can be evaluated in the laboratory by irradiating the solar cell with the calculated fluence level of unidirectional normally incident flux. The equivalent fluence is normally expressed in terms of 1 MeV electrons or 10 MeV protons.
      The software package Spenvis (www.spenvis.oma.be) is used to calculate the specific electron and proton fluence that a solar cell is exposed to during a specific satellite mission as defined by the duration, altitude, azimuth, etc. Spenvis employs the EQFLUX program, developed by the Jet Propulsion Laboratory (JPL) to calculate 1 MeV and 10 MeV damage equivalent electron and proton fluences, respectively, for exposure to the fluences predicted by the trapped radiation and solar proton models for a specified mission environment duration. The conversion to damage equivalent fluences is based on the relative damage coefficients determined for multijunction cells [Marvin, D. C., Assessment of Multijunction Solar Cell Performance in Radiation Environments, Aerospace Report No. TOR-2000 (1210)-1, 2000]. A widely accepted total mission equivalent fluence for a geosynchronous satellite mission of 15 year duration is 1 MeV 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 subcells. Aluminum incorporation is widely known in the III-V compound semiconductor industry to degrade BOL subcell performance due to deep level donor defects, higher doping compensation, shorter minority carrier lifetimes, and lower cell voltage and an increased BOL E g/q−V oc metric. In short, increased BOL E g/q−V ocmay be the most problematic shortcoming of aluminum containing subcells; the other limitations can be mitigated by modifying the doping schedule or thinning base thicknesses.
      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 four subcells, various aspects and features of the present disclosure can apply to stacks with fewer or greater number of subcells, i.e. two junction cells, three junction cells, five, six, seven junction cells, etc.
      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 C, with p-type and n-type InGaAs 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.

Claims

1. A four-junction space-qualified solar cell assembly designed for operation at AM0 and at a 1 MeV electron equivalent fluence of at least 1×10 14e/cm 2, the solar cell comprising subcells, wherein a combination of compositions and band gaps of the subcells is designed to maximize efficiency of the solar cell at a predetermined time, after initial deployment, when the solar cell is deployed in space at AM0 and at an operational temperature in the range of 40 to 70 degrees Centigrade, the predetermined time being at least five years and referred to as the end-of-life (EOL), the solar cell comprising:

an upper first solar subcell composed of indium gallium aluminum phosphide and having a first band gap in the range of 2.0 to 2.2 eV;

a second solar subcell adjacent to said first solar subcell and including an emitter layer composed of indium gallium phosphide or aluminum indium gallium arsenide, and a base layer composed of aluminum indium gallium arsenide and having a second band gap in the range of approximately 1.55 to 1.8 eV and being lattice matched with the upper first solar subcell, wherein the emitter and base layers of the second solar subcell form a photoelectric junction;
a third solar subcell adjacent to said second solar subcell and composed of indium gallium arsenide and having a third band gap less than that of the second solar subcell and being lattice matched with the second solar subcell; and
a fourth solar subcell adjacent to said third solar subcell and composed of germanium and having a fourth band gap of approximately 0.67 eV;
wherein the average band gap of the solar cell (i.e., the average, or numerical sum of the band gaps of each of the four subcells, divided by four) is equal to or greater than 1.35 eV.

2. The four junction solar cell assembly as defined in claim 1, wherein the upper first solar subcell has a band gap of less than 2.15, the second solar subcell has a band gap of less than 1.73 eV; and the third solar subcell has a band gap in the range of 1.15 to 1.2 eV.

3. The four junction solar cell assembly as defined in claim 1, the first solar subcell has a band gap of 2.05 eV.

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

5. The four junction solar cell assembly as defined in claim 1, further comprising:

a distributed Bragg reflector (DBR) layer adjacent to and disposed between the third and the fourth solar subcells and arranged so that light can enter and pass through the third solar subcell and at least a portion of which can be reflected back into the third solar subcell by the DBR layer, and is composed of a plurality of alternating sublayers of lattice matched materials with discontinuities in their respective indices of refraction; and
wherein the difference in refractive indices between alternating sublayers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.

6. The four junction solar cell assembly as defined in claim 5, wherein the DBR layer includes a first DBR layer composed of a plurality of p type InzAl xGa 1-x-zAs sublayers, and a second DBR layer disposed over and adjacent to the first DBR layer and composed of a plurality of p type In wAlyGa 1-y-wAs sublayers, where 0<w<1, 0<x<1, 0<y<1, 0<z<1 and y is greater than x, thereby increasing the reflection bandwidth of the DBR layer.

7. The four junction solar cell assembly as defined in claim 1, wherein the fourth solar subcell is lattice mismatched with respect to the third solar subcell.

8. The four junction solar cell assembly as defined in claim 1, wherein the top subcell is composed of a base layer of (In xGa 1-x1-yAl yP where x is 0.505, and y is 0.142, corresponding to a band gap of 2.10 eV, and an emitter layer of (In xGa 1-x1-yAl yP where x is 0.505, and y is 0.107, corresponding to a band gap of 2.05 eV.

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

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

11. The four junction solar cell assembly as defined in claim 9, wherein the intermediate layer has a graded band gap in the range of 1.15 to 1.41 eV, or 1.2 to 1.35 eV, or 1.25 to 1.30 eV.

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

13. The four junction solar cell assembly as defined in claim 1, further comprising:

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

14. The four junction solar cell assembly as defined in claim 1, wherein each subcell includes an emitter region and a base region, and one or more of the subcells have a base region having a gradation in doping that increases exponentially from 1×10 15 atoms per cubic centimeter adjacent the p-n junction to 4×10 18 atoms per cubic centimeter adjacent to the adjoining layer at the rear of the base, and an emitter region having a gradation in doping that decreases from approximately 5×10 18 atoms per cubic centimeter in the region immediately adjacent the adjoining layer to 5×10 17atoms per cubic centimeter in the region adjacent to the p-n junction.

15. The four junction solar cell assembly as defined in claim 9, wherein at least one of the upper sublayers of the graded interlayer has a larger lattice constant than the adjacent layers to the upper sublayer disposed above the grading interlayer.

16. The four junction solar cell assembly as defined in claim 1, wherein the difference in lattice constant between the adjacent third and fourth subcells is in the range of 0.1 to 0.2 Angstroms.

17. The four junction solar cell assembly as defined in claim 1, further comprising a inactive majority carrier layer (i.e., a window, BSF, or tunnel diode layer) disposed over the second or third solar subcell and having a lattice constant that is greater than that of the third and fourth solar subcells so that the tunnel diode layers are strained in tension.

18. The four junction solar cell assembly as defined in claim 1, further comprising a first threading dislocation inhibition layer having a thickness in the range of 0.10 to 1.0 microns and disposed over said second solar subcell.

19. The four junction solar cell assembly as defined in claim 17, further comprising a second threading dislocation inhibition layer having a thickness in the range of 0.10 to 1.0 micron and composed of InGa(Al)P, the second threading dislocation inhibition layer being disposed over and directly adjacent to said grading interlayer for reducing the propagation of threading dislocations, said second threading dislocation inhibition layer having a composition different from a composition of the first threading dislocation inhibition layer.

20. A method for fabricating a four-junction space-qualified solar cell assembly designed for operation at AM0 and at a 1 MeV electron equivalent fluence of at least 1×10 14 e/cm 2, the solar cell comprising subcells, wherein a combination of compositions and band gaps of the subcells is designed to maximize efficiency of the solar cell as a predetermined time, after initial deployment, when the solar cell is deployed in space at AM0 and at an operational temperature in the range of 40 to 70 degrees Centigrade, the predetermined time being at least five years and referred to as the end-of-life (EOL), comprising:

providing a ceria doped borosilicate glass supporting member that is 3 to 6 mils in thickness;
providing a growth substrate;
forming a first solar subcell over or in the growth substrate;
growing a sequence of layers of semiconductor material using a disposition process to form a solar cell comprising a plurality of subcells including a first middle subcell disposed over the growth substrate and having a band gap in the range of 1.15 to 1.41 eV, a second middle subcell disposed over and lattice matched to the third subcell and having a band gap in the range of approximately 1.55 to 1.8 eV and an upper subcell disposed over and lattice matched to the second middle subcell and having a band gap in the range of 2.05 to 2.20 eV, wherein the average band gap of the solar cell (i.e., the average or numerical sum of the band gaps of each subcell divided by four) is greater than 1.35 eV; and
attaching the upper subcell to the glass supporting member by a transparent adhesive.

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