Patent ID: 12249667

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 Jscthrough a solar subcell through a given planar area, or volume, of semiconductor material constituting the solar subcell.

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

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

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

“Inverted metamorphic multijunction solar cell” or “IMM solar cell” refers to a solar cell in which the subcells are deposited or grown on a substrate in a “reverse” sequence such that the higher band gap subcells, which are to 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, following which the growth substrate is removed leaving the epitaxial structure.

“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.

“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 (′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 electro-optical semiconductor 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.

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

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 non-inverted or “upright” solar cells of the present disclosure. However, more particularly, the present disclosure is directed to the fabrication of a multijunction lattice mis-matched solar cell grown over a metamorphic layer grown on a single growth substrate. More specifically, however, in some embodiments, the present disclosure relates to four or five 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) 1.6 to 1.8 eV, and (ii) 1.41 eV for the middle subcells, and 0.6 to 0.8 eV indirect band gaps, for the bottom subcell, respectively.

The conventional wisdom for many years has been that in a monolithic multijunction 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 (Voc), short circuit current (Jsc, 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).

Even more recently “ . . . current output in each subcell must be the same for optimum efficiency in the series—connected configuration” (Richard R. King et al., U.S. Pat. No. 9,099,595).

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 significant performance improvement in an AM0 spectral environment over that of a traditional three junction solar cell on Ge despite the lattice mismatch and substantial current mismatch between the top three subcells and the bottom Ge subcell. This performance gain is especially realized at high temperature and after high exposure to space radiation as a result of the proposal of the present disclosure to incorporate high band gap semiconductors that are inherently more resistant to radiation and temperature in spite of the disadvantages associated with lattice mismatch and current mismatch.

While the conventional wisdom suggests that the selection of the composition, band gaps, and thickness of each subcell is designed to ensure that the current is matched when operated in an AM0 spectral environment, the present design departs from that teaching and utilizes a design employing higher band gaps than the conventional design, and accordingly, a current mismatch between the upper subcells and the bottom subcell that is substantially greater than current commercial products or other proposed designs.

Although the advance of the present disclosure may be broadly characterized as the deliberate use of lattice mismatching and substantial current mismatching between subcells, in a four junction or five junction solar cell, 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 1.44 eV.

In some embodiments, the growth substrate and 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 0.66 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 provides 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 (i) one year, (ii) five years; (iii) ten years; or (iv) fifteen years.

The disclosure provides 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.

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.

The present disclosure is directed to, in one embodiment, a growth process using a metal organic chemical vapor deposition (MOCVD) process in a standard, commercially available reactor suitable for high volume production. More particularly, 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.

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

There are a multitude of properties that should be considered in specifying and selecting the composition of, inter alia, a specific semiconductor layer, the back metal layer, the adhesive or bonding material, or the composition of the supporting material for mounting a solar cell thereon. For example, some of the properties that should be considered when selecting a particular layer or material are electrical properties (e.g. conductivity), optical properties (e.g., band gap, absorbance and reflectance), structural properties (e.g., thickness, strength, flexibility, Young'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.

Improvement in absorption efficiency is well known to be achieved by a tandem multijunction solar cell in which each subcell absorbs only a narrow energy band spectrum (or range of wavelengths). By connecting an optical series of subcells, each one with continuously decreasing energy gaps, the entire illumination energy will be converted into electricity. Since the subcells are also connected in an electrical series, current flows through each of the subcells, with the voltage associated with each subcell is determined by the material physical characteristics of each subcell.

In view of the foregoing, it is further evident that the identification or proportion of one particular constituent element (e.g. indium, or aluminum) in a particular subcell, or the thickness, band gap, doping, or other characteristic of the incorporation of that material in a particular subcell, is not a single, independent “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 and its power output. 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 or material variable in a particular layer. The electrical characteristics of a semiconductor layer, such as the short circuit current (Jsc), the open circuit voltage (Voc), and the fill factor (FF), are affected by several factors as the number of subcells, the thickness of each subcell, the composition and doping of each active layer in a subcell. The consequential band structure, electron energy levels, conduction, and absorption of photons of different wavelengths and diffusion lengths in each subcell are not easily mathematically computable as a function of any one, two or small number of distinct single material variables. As an example, the power output may be stipulated to be a product of voltage and current in a subcell, but a simpleminded “result effective variable” approach to change a material variable (such as the amount of an element or doping in the layer), to thereby increase the voltage in a subcell in anticipation that it may result in greater power output, may in fact lead to a decrease in current, or a current mismatch between adjacent subcells in a tandem solar cell, or other interdependent effects (e.g., increased dopants diffusing into other layers and thereby adversely affecting them), with the overall effect of decreasing the power output of the solar cell.

The growth of each of the epitaxial layers of a solar cell in an MOCVD 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.

An illustration of the types of challenges faced by a designer of a photovoltaic power system for a space mission, identifying some of the specific issues faced with respect to the operating environment of the solar cells, a brief excerpt from a paper presented at the 37thPhotovoltaic Specialists Conference in 2011 is presented herein.

The unique space mission described in that paper is the NASA Solar Probe Plus (“SPP”), which is a spacecraft planned for launching in July 2018 to travel in the region of the Sun's corona and withstand temperatures of up to 1370 degrees Centigrade. As described by NASA, the 1350 pound spacecraft with a photovoltaic power system producing over 200 watts, will travel between the Earth and the Sun, specifically between the maximum aphelion at 1.02 AU (or 219 Rs, where “R” is the value of the sun's radius) and the minimum perihelion at 9.5 Rs (or 3.7 million miles), where the solar irradiance levels will vary between 0.97X and 513X. Nevertheless, the requirements for minimum power production and maximum waste heat dissipation from the array remain more or less constant throughout the orbit. Although the present disclosure is not directed to the Solar Probe Plus solar cells, the design issues are illustrative and worthy of note.

The paper observes that, “Array-design modeling for SPP is an iterative process, due to the large number of interdependent unknowns. For example, the cell power conversion efficiency depends on the operating temperature, which in turn depends on the efficiency. The material choices for the array depend on the array operating conditions (most notably, temperature and solar irradiance levels) which in turn depend on the properties of the constituent array materials. And the array geometry (i.e., length/width of the primary and secondary arrays, and the angle between them) necessary to meet the power production requirements of the mission depends on the irradiance, which in turn depends on the array geometry—and so on.”

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

The exemplary solar cell described herein may require the use of aluminum in the semiconductor composition of each of the top two subcells. For example, in some cases, an upper subcell may have an aluminum content in excess of 30% by mole fraction Likewise, in some cases, a middle subcell may have an aluminum content in excess of 25% by mole fraction. Aluminum incorporation is widely known in the III-V compound semiconductor industry to degrade BOL subcell performance due to deep level donor defects, higher doping compensation, shorter minority carrier lifetimes, and lower cell voltage and an increased BOL Eg/q−Vocmetric. In short, increased BOL Eg/q−Vocmay be the most problematic shortcoming of aluminum containing subcells; the other limitations can be mitigated by modifying the doping schedule or thinning base thicknesses.

One aspect of the present disclosure relates to the use of substantial amounts 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 subcell or junction is the difference between the band gap of the semiconductor material in that subcell or junction and the Voc, or open circuit voltage, of that same junction. The smaller the difference, the higher the Vocof the solar cell junction relative to the band gap, and the better the performance of the device. Vocis very sensitive to semiconductor material quality, so the smaller the Eg/q−Vocof 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 Voc−Eg/q difference, indicating that the material quality of the junction decreases with increasing Al content.FIG.1shows this effect. The three compositions cited in the Figure are all lattice matched to GaAs, but have differing Al composition. As seen by the different compositions represented, with increasing amount of aluminum represented by the x-axis, adding more Al to the semiconductor composition increases the band gap of the junction, but in so doing also increases Voc−Eg/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.2is a cross-sectional view of an embodiment of a four junction solar cell200after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer425.

As shown in the illustrated example ofFIG.2, the growth substrate or bottom subcell D includes a substrate400formed of p-type germanium (“Ge”) which also serves as a base layer. A back metal contact pad450formed on the bottom of base layer400provides one electrical contact of a positive polarity to the multijunction solar cell500. The bottom subcell D, further includes, for example, a highly doped n-type Ge emitter layer401, and an n-type indium gallium arsenide (“InGaAs”) nucleation layer402. The nucleation layer is deposited over the base layer, and the emitter layer is formed in the substrate by diffusion of deposits into the Ge substrate, thereby forming the n-type Ge layer401. Heavily doped p-type aluminum gallium arsenide (“AlGaAs”) and heavily doped n-type gallium arsenide (“GaAs”) tunneling junction layers403,404may be deposited over the nucleation layer to provide a low resistance pathway between the bottom and middle subcells.

A first alpha layer405, preferably composed of p-type InGaP or other suitable material, is deposited over the tunnel diode403/404, to a thickness of between 0.25 and 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.).

A metamorphic layer (or graded interlayer)406is deposited over the alpha layer405using a surfactant. Layer406is preferably a compositionally step-graded series of p-type 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. The band gap of layer406is constant throughout its thickness, in the range of 1.22 to 1.54 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell C. One embodiment of the graded interlayer may also be expressed as being composed of InxGa1-xAs, 0<x<1, 0<y<1, with x and y selected such that the band gap of the interlayer remains constant at approximately 1.22 to 1.54 eV or other appropriate band gap.

In the surfactant assisted growth of the metamorphic layer406, a suitable chemical element is introduced into the reactor during the growth of layer406to improve the surface characteristics of the layer. In one 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 layer406, 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 layer406.

In one embodiment of the present disclosure, the layer406is composed of a plurality of layers of p type InGaAs, with monotonically changing lattice constant, each layer having the same band gap, approximately in the range of 1.22 to 1.54 eV. In some embodiments, the constant band gap is in the range of 1.27 to 1.31 eV. 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 one embodiment of the present disclosure utilizes a plurality of layers of InGaAs for the metamorphic layer406for 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 greater than or equal to that of the bottom solar cell and less than or equal to that of the third solar cell, and having a bandgap energy greater than that of the third solar cell.

A second alpha layer407, preferably composed of p-type InGaP with a different composition than the first alpha layer405, is deposited over metamorphic buffer layer406, to a thickness from 0.25 to 1.0 micron. Such an alpha layer is intended to prevent threading dislocations from propagating, either opposite to the direction of growth into the 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.).

Distributed Bragg reflector (DBR) layers408are then grown adjacent to and between the second alpha layer407and the third solar subcell C. The DBR layers408are 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 layers408. In the embodiment depicted inFIG.2, the distributed Bragg reflector (DBR) layers408are specifically located between the third solar subcell C and alpha layers407; in other embodiments, the distributed Bragg reflector (DBR) layers may be located above the tunnel diode layers, as depicted inFIG.3.

For some embodiments, distributed Bragg reflector (DBR) layers408can be composed of a plurality of alternating layers408athrough408zof 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) layers408athrough408zincludes a first DBR layer composed of a plurality of n type or p type AlxGa1-xAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of n type or p type AlyGa1-yAs layers, where 0<x<1, 0<y<1, and y is greater than x.

In the illustrated example ofFIG.2, the subcell C includes a highly doped p-type aluminum gallium arsenide (“AlGaAs”) back surface field (“BSF”) layer409, a p-type InGaAs base layer410, a highly doped n-type indium gallium phosphide (“InGaP2”) emitter layer411and a highly doped n-type indium aluminum phosphide (“AlInP2”) window layer412. The InGaAs base layer410of the subcell C can include, for example, approximately 1.5% In. Other compositions may be used as well. The base layer410is formed over the BSF layer409after the BSF layer is deposited over the DBR layers408athrough408z.

AlthoughFIG.2depicts a four junction solar cell, the present disclosure also contemplates an embodiment of a five junction solar cell with middle subcells C1and C2(not shown). As described above, the lower subcell C1would have a band gap of 1.06 eV to 1.41 eV and the subcell C2disposed above subcell C1would have a band gap in the range of 1.35 to 1.66 eV, but greater than that that of subcell C1. As noted above, in the embodiment of a five junction solar cell would, the bottom subcell or growth substrate is composed of germanium, the first middle subcell is composed of indium gallium arsenide; the second middle 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 middle solar subcell is composed of (aluminum) indium gallium phosphide; and the upper subcell is composed of aluminum indium gallium phosphide.

The window layer412is deposited on the emitter layer411of the subcell C. The window layer412in 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 layers413,414may be deposited over the subcell C.

The middle subcell B includes a highly doped p-type aluminum gallium arsenide (“AlGaAs”) back surface field (“BSF”) layer415, a p-type AlGaAs base layer416, a highly doped n-type indium gallium phosphide (“InGaP2”) or AlGaAs layer417and a highly doped n-type indium gallium aluminum phosphide (“AlGaAlP”) window layer418. The InGaP emitter layer417of the subcell B can include, for example, approximately 50% In. Other compositions may be used as well.

Before depositing the layers of the top cell A, heavily doped n-type InGaP and p-type AlGaAs tunneling junction layers419,420may be deposited over the subcell B.

In the illustrated example, the top subcell A includes a highly doped p-type indium aluminum phosphide (“InAlP”) BSF layer421, a p-type InGaAlP base layer422, a highly doped n-type InGaAlP emitter layer423and a highly doped n-type InAlP2 window layer424. The base layer422of the top subcell A is deposited over the BSF layer421after the BSF layer421is formed over the tunneling junction layers419,420of the subcell B. The window layer424is deposited over the emitter layer423of the top subcell A after the emitter layer423is formed over the base layer422.

A cap or contact layer425may be deposited and patterned into separate contact regions over the window layer424of the top subcell A. The cap or contact layer425serves as an electrical contact from the top subcell A to metal grid layer (not shown). The doped cap or contact layer425can be a semiconductor layer such as, for example, a GaAs or InGaAs layer.

After the cap or contact layer425is deposited, the grid lines are formed via evaporation and lithographically patterned and deposited over the cap or contact layer425.

FIG.3is a cross-sectional view of a second embodiment of a four junction solar cell201after 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.

The second embodiment depicted inFIG.3is similar to that of the first embodiment depicted inFIG.2except that the metamorphic buffer layer604is disposed before the tunnel diode layers606,607are deposited, and thus is n-type rather than p-type, to have the same polarity of the buffer layer604.

InFIG.3, layers650,600,601, and602are substantially identical to that of layers450,400,401, and402inFIG.2, and layers608a,608b, . . .625are substantially identical except for the polarity sequence, to that of layers408a,408b, . . .425inFIG.2, so such description will not be repeated here for brevity.

Similarly, except for the polarity the first alpha layer603, the metamorphic buffer layer604, and second alpha layer605are similar to that of layers405,406and407ofFIG.2, and the tunnel diode layer606,607are similar to that of layer403,404ofFIG.2.

FIG.4is a cross-sectional view of a third embodiment of a four junction solar cell202after several steps of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer, according to the present disclosure.

The third embodiment depicted inFIG.4is similar to that of the first embodiment depicted inFIG.2except that a quantum well structure430is included in subcell C. InFIG.4, in one embodiment quantum well layers430athrough430nare disposed between the base layer410and the emitter layer411of subcell C. Quantum well structures in multijunction solar cells are known from U.S. patent application Ser. No. 11/788,315, filed Apr. 18, 2007 hereby incorporated by reference.

In some embodiments, the plurality of quantum layers are “strained balanced” by incorporating alternating lower band gap (or larger lattice constant) compressively strained InGaAs and higher band gap (or smaller lattice constant) tensionally strained GaAsP layers so that the larger/smaller atomic lattices/layers of epitaxy balance the strain to keep the quantum well layers lattice matched to the substrate.

More generally, the lower band gap well material may be any of the following: GaAs, SiGe, SiGeSn, GaAsN, InGaAsN, InGaAsNSb, AlGaAs, AlAs, InGaAs, AlInGaAs, GaAsP, InGaP, GaP, AlInP, AlInGaP, AlP, AlGaP, Si, or Ge.

And the high band gap barrier material may be any of the following: InGaAs, GaAsN, InGaAsN, InGaAsNSb, InP, InGaAsP, SiGe, SiGeSn, InAs, InAsP, GaSb, AlGaSb, InGaPSb, AlGaInPSb, AlGaInPSb, Si, or Ge.

In some embodiments, the number of quantum well layers are between 10 and 300, which each layer being between 10 and 5000 angstroms in thickness.

In some embodiments, the number of quantum well layers are between 10 and 20, which each layer being between 10 and 5000 angstroms in thickness.

In some embodiments, the second middle subcell includes a plurality of quantum well layers that are “strain balanced” by incorporating alternating lower band gap (or larger lattice constant) compressively strained in InGaAs and higher band gap (or smaller lattice constant) tensionally strained GaAsP, GaAs, InGaAs, or GaAsN layers so that the larger/smaller atomic lattices/layers of epitaxy balance the strain to keep the quantum well layers lattice matched to the substrate.

In some embodiments, the total thickness of the quantum well layers are between 200 nm and 2 um; or between 2 and 4 um.

In some embodiments, the quantum well layers form an intermediate band gap layer between the emitter layer and the base layer of the second middle subcell.

Another feature of the present disclosure relates to the doping levels in the subcells of the solar cell. 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 inFIG.5, which depicts ranges of 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 some embodiments, the doped portion of the p-type base may be preceded by an undoped intrinsic portion. In some embodiments, the undoped region may include all quantum wells, a portion of the quantum wells, or no quantum well.

In the example ofFIG.5, 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 1×1015to 1×1018free carriers per cubic centimeter adjacent the p-n junction to a value in the range of 1×1016to 4×1018free 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 5×1018to 1×1017free carriers per cubic centimeter in the region immediately adjacent the adjoining layer to a value in the range of 5×1015to 1×1018free carriers per cubic centimeter in the region adjacent to the p-n junction.

The heavy lines602and603shown inFIG.5illustrates one embodiment of the base doping having an exponential gradation, and the emitter doping being linear.

Thus, in one embodiment, the doping level throughout the thickness of the base layer may be exponentially graded from approximately 1×1016free carriers per cubic centimeter to 1×1018free carriers per cubic centimeter, as represented by the curve603depicted in the Figure.

Similarly, in one embodiment, the doping level throughout the thickness of the emitter layer may decline linearly from approximately 5×1018free carriers per cubic centimeter to 5×1017free carriers per cubic centimeter, as represented by the curve602depicted in the Figure.

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

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

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 electrical 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.6is a highly simplified perspective illustration of an exemplary space vehicle including a deployable panels or flexible sheets including an array of transducer elements and solar cells according to the present disclosure.

As shown with respect toFIG.6, a portion of a space vehicle or spacecraft having a housing or frame10and elongated panel or sheet14,16which may be wound in a stowed state in the housing, the panels or sheets including an assembly or array of transducer elements, photovoltaic devices or solar cells15according to the present disclosure. One end of the panel or sheet is permanently attached to a actuator or mounting bar101or102in or on the housing10.

In some embodiments, the transducer elements include one or more of: semiconductor environmental sensors, antennas, or phased array antenna elements, piezoelectric transducers and thermal transfer elements.

In some embodiments, the space vehicle may be a CubeSat or a type of miniaturized satellite. A typical CubeSat is a 10 cm×10 cm×10 cm cube, thus having a volume of one liter; other dimensions are possible as well. In some cases, CubeSats can be attached to one another in strings or blocks to provide functionalities and capabilities that would not otherwise be practically available in a single CubeSat, and are referred to as a 2U, 3U. etc. CubeSat depending on the number of units (U). For example, one CubeSat can be used as a power source to supply power necessary for other attached CubeSats to perform their functions, such as propulsion, communications, sensing or data collection, altitude determining and control, or command and data processing. For simplicity, we refer to the housing10as a “CubeSat” is this description, although it may be a housing portion of any nano or micro-satellite space vehicle.

The arrays of solar cells can be used, for example, as a power source to supply power to payloads in one or more additional CubeSats attached to the CubeSet10. For example, in some implementations, each panel or sheet14,16is suitable for providing a small amount of power (e.g., less than 50 watts). Each panel or sheet can include, for example, a plurality of discrete photovoltaic devices or solar cells15connected in a serial and/or parallel configuration. In some implementations, each panel or sheet14,16of photovoltaic devices15may be cover-glass-interconnected solar cells (CICs) subassemblies mounted on a flexible polyimide carrier by a pressure sensitive adhesive.

In some embodiments, there are a plurality of substantially identical space vehicles such as that shown inFIG.6in one or more LEO orbits forming a satellite constellation in communication with a network of ground stations on the earth for providing a communications network for users on the earth. Such LEO satellites orbit the earth in about 100 minutes, so one satellite is overhead and able to communicate with a user in a fixed location on the earth only for about 10 minutes at a time before orbiting out of view. Payloads of such satellites include communications protocols to manage handoffs between successively orbiting satellites over the region of the user or communications between satellites so that the user may be continuously connected to the communications network by the successively orbiting satellites.

In addition to use on satellites, the present disclosure also contemplates the use of solar cell arrays on the non-planar surface of a variety of different vehicles used in planetary space missions or on other structures (e.g., building materials)

Exemplary space vehicles for such planetary missions include aerostats (which are lighter than the planet's atmosphere), and aerodynes (which are heavier than the planet's atmosphere). Exemplary aerostats can include, for example, unpowered vessels (e.g., ballons such as helium balloons and hydrogen balloons) and powered vessels (e.g., airships or dirigibles). Exemplary aerodynes can include, for example, unpowered vessels (e.g., kites and gliders) and powered vessels (e.g., airplanes and helicopters). Exemplary aerodynes can be fixed wing vessels (e.g., airplanes and gliders) or rotocraft (e.g., helicopters and autogyros).

Exemplary space vehicles having non-planar surfaces can be motorized or non-motorized, and can be propelled or tethered. Exemplary land vehicles having non-planar surfaces can be motorized (e.g., rovers) or non-motorized (e.g., fixed installations).

FIG.7is a perspective view of an exemplary embodiment of an aerostat900. A first solar cell assembly901is disposed on the bottom non-planar surface of the aerostat900to capture the reflected sunlight from the surface903of the celestial body. A second solar cell assembly902is disposed on the top surface of the aerostat900to capture the direct sunlight. Electrical current generated from the solar cell assemblies can be provided to a propulsion unit904with propeller which drives the aerostat through the atmosphere.

FIG.8is a perspective view of an exemplary embodiment of an unmanned aircraft. Aircraft1000has a non-planar surface and is a fixed wing vessel. The solar cell assembly1001is attached to a non-planar surface of the wing of the aircraft1000. In certain embodiments, when light impinges on the solar cell assembly1001of aircraft1000, electrical current generated from the solar cell assembly1001can be provided for operation of systems (e.g., navigational systems, propulsion systems, and the like) of aircraft1000.

FIG.9is a perspective view of an exemplary embodiment of a land vehicle. Land vehicle3000has a non-planar surface and is a rover that can be used for land navigation and/or exploration on earth or other planets. The solar cell assembly3001is attached to a non-planar surface of the rover3000. In certain embodiments, when light impinges on the solar cell assembly3001of rover3000, electrical current generated from solar cell assembly3001can be provided for operation of systems (e.g., navigational systems, propulsion systems, and the like) of rover3000. In certain embodiments, rover3000is a hybrid or electric powered land vehicle.

FIG.10is a perspective view of an exemplary embodiment of another aerodyne in the form of a rotocraft4000designed for operation in an extraterrestrial atmosphere (such as the NASA “Mars Helicopter” for operation in the atmosphere of Mars). The solar cell assembly4002is attached to a planar platform or panel4001which is situated above the rotor blades4003of the rotocraft. In certain embodiments, when light impinges on the solar cell assembly4002of rotocraft4000, electrical current generated from the solar cell assembly4002can be provided for operation of systems (e.g., navigational systems, propulsion systems and the like). In the payload4004of the rotocraft4000.

The present disclosure, like related U.S. patent application Ser. No. 14/828,206, 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 either room temperature (28° C.) or high temperature (i.e., 50° to 70° C.) 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.

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

As an example of different mission requirements, a low earth orbit (LEO) satellite will typically experience radiation of protons equivalent to an electron fluence per square centimeter in the range of 1×1012e/cm2to 2×1014e/cm2(hereinafter may be written as “2E10 e/cm2or 2E14”) over a five year lifetime. A geosynchronous earth orbit (GEO) satellite will typically experience radiation in the range of 5×1014e/cm2to 1×1015e/cm2over a fifteen year lifetime.

As a baseline comparison, 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 (the ZTJ of SolAero Technologies Corp., of Albuquerque, New Mexico), is as follows:

ConditionEfficiencyBOL 28° C.29.1%BOL 70° C.26.4%EOL 70° C.23.4%after 5E14 e/cm2radiationEOL 70° C.22.0%after 1E14 e/cm2radiation
Jscat EOL 70° after 1E15 e/cm2radiation: 17.2 mA
Ratio J1-2/J3: 139%

The Jscof the upper two subcells at EOL under 1E15 conditions is 17.2 mA, and the ratio of the upper junction Jscto the bottom subcell Jscis 139%.

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 configured and described in the present disclosure, four embodiments and their corresponding efficiency data at the end-of-life (EOL) is as follows:

Embodiment 1Band GapCompositionSubcell A2.1AlInGaPSubcell B1.73InGaP orAlGaAs/AlGaAsor InGaAsSubcell C1.41(In)GaAsSubcell D0.67Ge

Efficiency at 70° C. after 5E14 e/cm2radiation: 24.9%

Efficiency at 70° C. after 1E15 e/cm2radiation: 24.4%

Embodiment 2Band GapCompositionSubcell A2.1AlInGaPSubcell B1.88Subcell C1.265InGaAsSubcell D0.67GeEfficiency: 24.6%

Efficiency at 70° C. after 1E15 e/cm2radiation:

Embodiment 3Band GapCompositionSubcell A2.1AlInGaPSubcell B1.73InGaP orAlGaAs/AlGaAsor InGaAsSubcell C1.242(In)GaAsSubcell D0.67GeEfficiency: 24.7%

Efficiency at 70° C. after 1E15 e/cm2radiation:

Embodiment 4Band GapCompositionSubcell A2.1AlInGaPSubcell B1.73InGaP orAlGaAs/AlGaAsor InGaAsSubcell C1.219(In)GaAsSubcell D0.67GeEfficiency: 24.8%

Efficiency at 70° C. after 1E15 e/cm2radiation:

For a five junction solar cell configured and described in the present disclosure, some embodiments and corresponding efficiency data at the end-of-life (EOL) computed at 28° C. is as follows:

Embodiment 5Band GapCompositionSubcell A2.1AlInGaPSubcell B1.78InGaPSubcell C1.48AlGaAs or InGaAsPSubcell D1.2InGaAsSubcell E0.67GeEfficiency: 27.9%

Embodiment 6Band GapCompositionSubcell A2.1AlInGaPSubcell B1.69AlGaAsSubcell C1.38InGaAsSubcell D1.10InGaAsSubcell E0.67GeEfficiency: 28.4%

Embodiment 7Band GapCompositionSubcell A2.1AlInGaPSubcell B1.665AlGaAsSubcell C1.35InGaAsSubcell D1.064InGaAsSubcell E0.67GeEfficiency: 28.4%

The four junction solar cell of the present disclosure has a slightly higher cell efficiency than the standard commercial solar cell (ZTJ) at BOL at 70° C. However, the solar cell in some embodiments described in the present disclosure exhibits substantially improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 5×1014e/cm2, and dramatically improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 1×1015e/cm2.

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]. New cell structures eventually need new RDC measurements as different materials can be more or less damage resistant than materials used in conventional solar cells. A widely accepted total mission equivalent fluence for a geosynchronous satellite mission of 15 year duration is 1 MeV 1×1015electrons/cm2.

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.

The terms “substantially”, “essentially”, “approximately”, “about”, or any other similar expression relating to particular parametric numerical value are defined as being close to that value as understood by one of ordinary skill in the art in the context of that parameter, and in one non-limiting embodiment the term is defined to be within 10% of that value, in another embodiment within 5% of that value, in another embodiment within 1% of that value, and in another embodiment within 0.5% of that value.

The terminology used in this disclosure is for the purpose of describing specific identified embodiments only and is not intended to be limiting of different examples or embodiments.

The term “coupled” as used herein is defined as connected, although not necessarily directly or physically adjoining, and not necessarily structurally or mechanically. A device or structure that is “configured” in a certain way is arranged or configured in at least that described way, but may also be arranged or configured in ways that are not described or depicted.

It is to be noted that the terms “front”, “back”, “side”, “top”, “bottom”, “over”, “on”, “above”, “beneath”, “below”, “under”, and the like in the description and the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the disclosure described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. For example, if the assembly in the figures is inverted or turned over, elements of the assembly described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The assembly may be otherwise oriented (rotated by a number of degrees through an axis).

In the drawings, the position, relative distance, lengths, widths, and thicknesses of supports, substrates, layers, regions, films, etc., may be exaggerated for presentation simplicity or clarity. Like reference numerals designate like elements throughout the specification. It will be understood that when an element such as an element layer, film, region, or feature is referred to as being “on” another element, it can be disposed directly on the other element or the presence of intervening elements may also be possible. In contrast, when an element is referred to as being disposed “directly on” another element, there are no intervening elements present.

Furthermore, those skilled in the art will recognize that boundaries and spacings between the above described units/operations are merely illustrative. The multiple units/operations may be combined into a single unit/operation, a single unit/operation may be distributed in additional units/operations, and units/operations may be operated at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular unit/operation, and the order of operations may be altered in various other embodiments.

The terms “front side” and “backside” refer to the final arrangement of the panel, integrated cell structure or of the individual solar cells with respect to the illumination or incoming light incidence.

In the claims, the word ‘comprising’ or ‘having’ does not exclude the presence of other elements or steps than those listed in a claim. It is understood that the terms “comprise”, “comprising”, “includes”, and “including” if used herein, specify the presence of stated components, elements, features, steps, or operations, components but do not preclude the presence or addition of one or more other components, elements, features, steps, or operations, or combinations and permutations thereof.

The terms “a” or “an”, as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to disclosures containing only one such element, even when the claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an”. The same holds true for the use of definite articles.

Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. It will be understood that, although the terms “first”, “second”, etc. may be used herein to describe various elements, devices, components, regions, layers, areas, and/or sections, these elements, devices, components, regions, layers, areas, and/or sections should not be limited by these terms. These terms are only used to distinguish one device, element, component, region, layer, area, or section from another device, element, component, region, layer area or section. Thus, a first device, element, component, region, layer, area, or section discussed below could be termed a second device, element, component, region, layer, area or section without departing from the teachings of example embodiments

The present disclosure can be embodied in various ways. To the extent a sequence of steps are described, the above described orders of the steps for the methods are only intended to be illustrative, and the steps of the methods of the present disclosure are not limited to the above specifically described orders unless otherwise specifically stated. Note that the embodiments of the present disclosure can be freely combined with each other without departing from the spirit and scope of the disclosure.

Although some specific embodiments of the present disclosure have been demonstrated in detail with examples, it should be understood by a person skilled in the art that the above examples are only intended to be illustrative but not to limit the scope and spirit of the present disclosure. The above embodiments can be modified without departing from the scope and spirit of the present disclosure which are to be defined by the attached claims. Accordingly, other implementations are within the scope of the claims.

Although described embodiments of the present disclosure utilizes a vertical stack of a certain illustrated number of 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, four, 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 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 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, AnnAs, AlInPAs, GaAsSb, GaAsSb, 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 forgoing others can, by applying current knowledge, readily adapt the present for various applications. Such adaptions should and are intended to be comprehended within the meaning and range of equivalence of the following claims.