MULTIJUNCTION METAMORPHIC SOLAR CELLS
20220190181 · 2022-06-16
Assignee
Inventors
- Daniel Derkacs (Albuquerque, NM, US)
- Zachary Bitter (Albuquerque, NM, US)
- John Hart (Albuquerque, NM, US)
Cpc classification
H01L31/03046
ELECTRICITY
H01L31/1852
ELECTRICITY
H01L31/074
ELECTRICITY
International classification
H01L31/074
ELECTRICITY
Abstract
A multijunction solar cell including a growth substrate; a graded interlayer disposed over the growth substrate, a plurality of subcells disposed over the graded interlayer including a second solar subcell disposed over and lattice mismatched with respect to the growth substrate, and at least a third solar subcell disposed over the second subcell; the grading interlayer including a plurality of N step-graded sublayers (where N is an integer and the value of N is 1<N<10), wherein each successive sublayer has an incrementally greater lattice constant than the sublayer below it and grown in such a manner that each sublayer is fully relaxed, a distributed Bragg reflector (DBR) layer over the grading interlayer and an upper solar subcell disposed over the third solar subcell, a band gap in the range of 1.95 to 2.20 eV, and composed of a semiconductor compound including at least indium, aluminum and phosphorus.
Claims
1-20. (canceled)
21. A multijunction solar cell comprising: a growth substrate; a first solar subcell disposed over or in the growth substrate; a tunnel diode disposed over the first solar subcell; a grading interlayer directly disposed over the tunnel diode; a sequence of layers of semiconductor material forming a solar cell disposed over the grading interlayer comprising a plurality of subcells including a second solar subcell disposed over and lattice mismatched with respect to the growth substrate, and at least a third solar subcell composed of a semiconductor compound including at least indium, gallium, arsenic and phosphorus and disposed over the second solar subcell; wherein the grading interlayer has a band gap equal to or greater than that of the second subcell and is compositionally graded to lattice match the growth substrate on one side and the second subcell on the other side; and being 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 in each of the sublayers of the grading interlayer throughout its thickness being greater than or equal to the lattice constant of the growth substrate; and a fourth or upper solar subcell disposed over the third solar subcell compound of a semiconductor compound including at least aluminum, indium, and having a band gap in the range of 1.95 eV to 2.20 eV, wherein either the third solar subcell or the fourth solar subcell, or both has an aluminum content in excess of 17.5% by mole fraction,
22. A multijunction solar cell as defined in claim 21, wherein the third solar subcell has an aluminium content in excess of 20% by mole fraction.
23. A multijunction solar cell as defined in claim 22, wherein the fourth solar subcell has an aluminium content in excess of 17.5% by mole fraction.
24. A multijunction solar cell as defined in claim 21, wherein the third and fourth solar subcells are lattice matched to the second solar subcell.
25. A multijunction solar cell as defined in claim 21, wherein the second solar subcell has a band gap in the range of 1.0 eV to 1.41 eV; and the third solar subcell has a band gap in the range of approximately 1.35 eV to 1.73 eV, and greater than the second solar subcell.
26. A multijunction solar cell as defined in claim 21, wherein: the fourth or upper subcell is composed of a semiconductor compound including at least aluminum, indium and phosphorus, or the compound indium gallium aluminum phosphide: the third solar subcell is composed of a semiconductor compound including at least indium, gallium, arsenic, and phosphorus, or the compound (aluminum) indium gallium arsenide; the first solar subcell is composed of germanium; and the graded interlayer is composed of N step-graded (In.sub.xGa.sub.1-x).sub.yAl.sub.1-yAs sublayers, with 0<x<1, 0<y<1, where N is an integer and the value of N is 1≤N<10, wherein each successive sublayer has an incrementally greater lattice constant than the sublayer below it and grown in such a manner that each sublayer is fully relaxed (i.e., not in tension or compression).
27. A multijunction solar cell as defined in claim 21, wherein the tunnel diode is directly grown over the growth substrate, with the graded interlayer directly grown over the tunnel diode.
28. A multijunction solar cell as defined in claim 21, wherein 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 50 to 100 degrees Centigrade) in deployment in space at a specific predetermined time after initial deployment (referred to as the beginning of life or BOL), such predetermined time being referred to as the end-of-life (EOL), and being at least five years after the BOL, such selection being designed not to maximize the efficiency at BOL but to increase the solar cell efficiency at the EOL while disregarding the solar cell efficiency achieved at the BOL, such that the solar cell efficiency designed at the BOL is less than the solar cell efficiency at the BOL that would be achieved if the selection were designed to maximize the solar cell efficiency at the BOL.
29. A multijunction solar cell as defined in claim 21, further comprising: a distributed Bragg reflector (DBR) structure disposed between one of the solar subcells and another of the solar subcells and is composed of a plurality of alternating layers of lattice mismatched materials with discontinuities in their respective indices of refraction and arranged so that light can enter and pass through the third solar subcell and at least a first portion of which light having a first spectral width wavelength range including the band gap of the one solar subcell can be reflected back into the one solar subcell by the DBR structure, and a second portion of which light in a second spectral width wavelength range corresponding to longer wavelengths than the first spectral width wavelength range can be transmitted through the DBR structure to the solar subcell disposed beneath the DBR structure, and wherein the difference in refractive indices between the alternating layers in the DBR structure 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 of the DBR structure determines the stop and its limiting wavelength, and wherein the DBR structure includes a first DBR sublayer composed of a plurality of n type or p type Al.sub.x(In)Ga.sub.1-xAs layers, and a second DBR sublayer disposed over the first DBR sublayer and composed of a plurality of n type or p type Al.sub.y(In)Ga.sub.1-yAs layers, where 0<x<1, 0<y<1, and y is greater than x and the designation (In) represents an optional amount of indium in the compound so that the DBR layers are lattice matched to the one solar subcell.
30. A multijunction solar cell as defined in claim 21, wherein the band gap of the first solar subcell is in the range of 1.0 eV to 1.2 eV; the band gap of the second solar subcell is in the range of approximately 1.35 eV to 1.53 eV; and the band gap of the fourth or upper solar subcell is in the range of 1.85 eV to 2.07 eV.
31. A multijunction solar cell as defined in claim 21, wherein the multijunction solar cell is a four junction solar cell and 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.
32. A multijunction solar cell as defined in claim 21, wherein the upper solar subcell has a band gap in the range of approximately 1.9 eV to 2.05 eV, the third solar subcell has a band gap in the range of approximately 1.41 eV to 1.73 eV; and the second solar subcell has a band gap in the range of approximately 1.04 eV to 1.35 eV.
33. A multijunction solar cell as defined in claim 21, wherein the grading interlayer is composed of InGaAs with the indium content in the range of 0 to 25% per mole fraction and a thickness in the range of 100 nm to 500 nm, and a band gap in the range of 1.15 eV to 1.41 eV.
34. A multijunction solar cell as defined in claim 21, wherein one or more of the solar subcells have a base region having a gradation in doping that increases exponentially from a value in the range of 1×10.sup.15 to 1×10.sup.18 free carriers per cubic centimeter adjacent the p-n junction to a value in the range of 1×10.sup.16 to 4×10.sup.18 free carriers per cubic centimeter adjacent to the adjoining layer at the rear of the base, and an emitter region having a gradation in doping that decreases from a value in the range of approximately 5×10.sup.18 to 1×10.sup.17 free carriers per cubic centimeter in the region immediately adjacent the adjoining layer to a value in the range of 5×10.sup.15 to 1×10.sup.18 free carriers per cubic centimeter in the region adjacent to the p-n junction.
35. A multijunction solar cell as defined in claim 34, wherein the doped portion of one or more of the solar subcells may be preceded by an undoped intrinsic portion including quantum wells.
36. A multijunction solar cell as defined in claim 21, wherein the current through the first solar 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.
37. A multijunction solar cell as defined in claim 21, wherein the band gap of the graded interlayer remains at a constant value in the range of 1.22 eV to 1.75 eV throughout its thickness.
38. A multijunction solar cell as defined in claim 29, wherein the DBR layer is disposed over the grading interlayer.
39. A multijunction solar cell comprising: a growth substrate; a first solar subcell disposed over or in the growth substrate; a grading interlayer directly disposed over the growth substrate; a sequence of layers of semiconductor material forming a solar cell disposed over the grading interlayer comprising a plurality of subcells including: a second solar subcell directly disposed over and lattice mismatched with respect to the growth substrate, and a third solar subcell disposed over and lattice matched with the second solar subcell; and an upper solar subcell disposed over and lattice matched to the third solar subcell; wherein the grading interlayer has a band gap equal to or greater than that of the second subcell and is compositionally graded to lattice match the growth substrate on one side and the second subcell on the other side; and being composed of any of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice constant in each of the sublayers throughout its thickness being greater than or equal to the lattice constant of the growth substrate, and includes N step-graded sublayers where N is an integer and 1≤N≤10, wherein each successive sublayer has an incrementally greater lattice constant than the sublayer below it and grown in such a manner that the graded sublayer is fully relaxed (i.e., not in tension or compression); wherein either the upper solar subcell or the third solar subcell has an aluminum content in excess of 17.5% by mole fraction.
40. A method of manufacturing a multijunction solar cell for use in a space vehicle comprising: providing a germanium growth substrate; forming a first solar subcell over or in the growth substrate; growing a graded interlayer over the growth substrate, followed by a sequence of layers of semiconductor material using a deposition process to form a solar cell comprising a plurality of subcells including: a second solar subcell disposed over and lattice mismatched with respect to the growth substrate and having a band gap in the range of 1.0 eV to 1.41 eV; a third solar subcell disposed over the second subcell and having a band gap in the range of approximately 1.35 eV to 1 73 eV; a fourth or upper subcell disposed over the third solar subcell and a band gap in the range of 1.95 eV to 2.20 eV; wherein the graded interlayer is compositionally graded to lattice match the growth substrate on one side and the second solar subcell on the other side, and is composed of the As, P, N, Sb based III-V compound semiconductors subject to the constraints of having the in-plane lattice parameter throughout its thickness being greater than or equal to that of the growth substrate; and wherein either the third solar subcell or the fourth solar subcell, or both, has an aluminum content in excess of 17.5% by mole fraction.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0102] The disclosure will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:
[0103]
[0104]
[0105]
[0106]
[0107]
GLOSSARY OF TERMS
[0108] “III-V compound semiconductor” refers to a compound semiconductor formed using at least one element 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).
[0109] “Average band gap” of a multijunction solar cell is the numerical average of the lowest band gap material in each subcell of the multifunction solar cell.
[0110] “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.
[0111] “Beginning of Life (BOL)” refers to the time at which a photovoltaic power system is initially deployed in operation.
[0112] “Bottom subcell” refers to the subcell in a multijunction solar cell which is furthest from the primary light source for the solar cell.
[0113] “Compound semiconductor” refers to a semiconductor formed using two or more chemical elements.
[0114] “Current density” refers to the short circuit current density J.sub.sc through a solar subcell through a given planar area, or volume, of semiconductor material constituting the solar subcell.
[0115] “Deposited”, with respect to a layer of semiconductor material, refers to a layer of material which is epitaxially grown over another semiconductor layer.
[0116] “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.
[0117] “Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.
[0118] “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.
[0119] “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.
[0120] “Lattice mismatched” refers to two adjacently disposed materials or layers (with thicknesses of greater than 100 nm) having in-plane lattice constants of the materials in their fully relaxed state differing from one another by more than 0.1% in lattice constant. (Applicant expressly adopts this definition for the purpose of this disclosure, and notes that this definition is considerably more stringent than that proposed, for example, in U.S. Pat. No. 8,962,993, which suggests more than 0.6% lattice constant difference).
[0121] “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.
[0122] “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).
[0123] “Short circuit current (I.sub.sc)” refers to the amount of electrical current through a solar cell or solar subcell when the voltage across the solar cell is zero volts, as represented and measured, for example, in units of milliamps.
[0124] “Short circuit current density”—see “current density”.
[0125] “Solar cell” refers to an electro-optical semiconductor device operable to convert the energy of light directly into electricity by the photovoltaic effect.
[0126] “Solar cell assembly” refers to two or more solar cell subassemblies interconnected electrically with one another.
[0127] “Solar cell subassembly” refers to a stacked sequence of layers including one or more solar subcells.
[0128] “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.
[0129] “Substantially current matched” refers to the short circuit current through adjacent solar subcells being substantially identical (i.e. within plus or minus 1%).
[0130] “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.
[0131] “ZTJ” refers to the product designation of a commercially available SolAero Technologies Corp. triple junction solar cell.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0132] Details of the present disclosure 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.
[0133] 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.
[0134] 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 which is 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 eV to 2.15 eV (or higher) for the top subcell with an aluminum content in excess of 17.5% by mole fraction, and (i) 1.6 eV to 1.8 eV, and (ii) 1.41 eV for the middle subcells, and 0.6 eV to 0.8 eV indirect band gap for the bottom subcell, respectively.
[0135] 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 (V.sub.oc), short circuit current (J.sub.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”).
[0136] 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).
[0137] 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).
[0138] The present disclosure provides an unconventional four junction design (with three grown lattices matched subcells, which in turn 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, and the use of substantial amounts of aluminum in at least the upper subcell.
[0139] 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.
[0140] Although the advance of the present disclosure may be broadly characterized as the deliberate use of lattice mismatching and substantial current mismatching between subcells and high aluminum content in at least the upper solar subcell, in a four junction or five junction solar cell, instead of simply specifying the amount of aluminum (by a mole fraction percentage) in a given solar subcell, another way of characterizing the present disclosure is that in some embodiments of an upright metamorphic 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 a specific value, e.g. 1.44 eV, or optionally 1.30 eV, or optionally 1.25 eV.
[0141] 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.
[0142] 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 0.67 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.
[0143] 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.
[0144] 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.
[0145] 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.
[0146] 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.
[0147] 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 using commercially available equipment and established high-volume fabrication processes, as contrasted with merely academic expositions of laboratory or experimental results.
[0148] 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.
[0149] 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).
[0150] 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.
[0151] 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.
[0152] 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 (J.sub.sc), the open circuit voltage (V.sub.oc) 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.
[0153] 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.
[0154] 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.
[0155] 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 37.sup.th Photovoltaic Specialists Conference in 2011 is presented herein.
[0156] 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 0.97× and 513×. 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.
[0157] 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.”
[0158] 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”.
[0159] The exemplary solar cell described herein may require the use of aluminum in the semiconductor composition in the upper solar subcell, or in each of the top two subcells. For example, in some cases, the upper solar subcell may have an aluminum content in different embodiments, (i) in excess of 30% by mole fraction, or (ii) in excess of 25% by mole fraction, or (iii) in excess of 20% by mole fraction, or (iv) in excess of 17.5% by mole fraction. Likewise, in some cases, the second solar subcell adjacent to the upper solar subcell may have an aluminum content in excess of 25% by mole fraction, or (ii) in excess of 20% by mole fraction, or (iii) in excess of 17.5% by mole fraction.
[0160] Aluminum incorporation is widely known in the III-V compound semiconductor industry to degrade BOL subcell performance due to deep level donor defects, higher doping compensation, shorter minority carrier lifetimes, and lower cell voltage and an increased BOL E.sub.g/q−V.sub.oc metric. In short, increased BOL E.sub.g/q−V.sub.oc may be the most problematic shortcoming of aluminum containing subcells; the other limitations can be mitigated by modifying the doping schedule or thinning base thicknesses.
[0161] One aspect of the present disclosure relates generally 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 V.sub.oc, or open circuit voltage, of that same junction. The smaller the difference, the higher the V.sub.oc of the solar cell junction relative to the band gap, and the better the performance of the device. V.sub.oc is very sensitive to semiconductor material quality, so the smaller the E.sub.g/q−V.sub.oc of a device, the higher the quality of the material in that device. There is a theoretical limit to this difference, known as the Shockley-Queisser limit. That is the best that a solar cell junction can be under a given concentration of light at a given temperature.
[0162] Another way of characterizing the present disclosure is that in some embodiments of a four junction cell, aluminum is added to the top subcell in an amount of greater than 20% by mole fraction, and to the middle subcells, so that the resulting 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, or optionally 1.30 eV, or optionally 1.25 eV.
[0163] In some embodiments, 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 4) is greater than 1.44 eV, and each subcell has a lattice constant of 5.653 Angstroms.
[0164] Some comments about MOCVD processes used in one embodiment are in order here.
[0165] 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.
[0166] One aspect of the present disclosure relates to the use of aluminum in the active layers of the upper subcells in a multijunction solar cell. The effects of increasing amounts of aluminum as a constituent element in an active layer of a subcell affects the photovoltaic device performance. One measure of the “quality” or “goodness” of a solar cell junction is the difference between the band gap of the semiconductor material in that subcell or junction and the V.sub.oc, or open circuit voltage, of that same junction. The smaller the difference, the higher the V.sub.oc of the solar cell junction relative to the band gap, and the better the performance of the device. V.sub.oc is very sensitive to semiconductor material quality, so the smaller the E.sub.g/q−V.sub.oc of a device, the higher the quality of the material in that device. There is a theoretical limit to this difference, known as the Shockley-Queisser limit. That is the best that a solar cell junction can be under a given concentration of light at a given temperature.
[0167] The experimental data obtained for single junction (Al)GaInP solar cells indicates that increasing the Al content of the junction leads to a larger V.sub.oc−E.sub.g/q difference, indicating that the material quality of the junction decreases with increasing Al content.
[0168]
[0169] As shown in the illustrated example of
[0170] Distributed Bragg reflector (DBR) layers 305 are then grown adjacent to and between the tunnel diode 303, 304 of the bottom subcell D and the third solar subcell C. The DBR layers 305 are arranged so that light can enter and pass through the third solar subcell C and at least a portion of which can be reflected back into the third solar subcell C by the DBR layers 305. In the embodiment depicted in
[0171] For some embodiments, distributed Bragg reflector (DBR) layers 305 can be composed of a plurality of alternating layers 305a through 305z of lattice matched materials with discontinuities in their respective indices of refraction. For certain embodiments, the difference in refractive indices between alternating layers is maximized in order to minimize the number of periods required to achieve a given reflectivity, and the thickness and refractive index of each period determines the stop band and its limiting wavelength.
[0172] For some embodiments, distributed Bragg reflector (DBR) layers 305a through 305z includes a first DBR layer composed of a plurality of p type Al.sub.xGa.sub.1-xAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of n or p type Al.sub.yGa.sub.1-yAs layers, where 0<x<1, 0<y<1, and y is greater than x.
[0173] In the illustrated example of
[0174] The window layer 309 is deposited on the emitter layer 308 of the subcell C. The window layer 309 in the subcell C also helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions. Before depositing the layers of the subcell B, heavily doped n-type InGaP and p-type AlGaAs (or other suitable compositions) tunneling junction layers 310, 311 may be deposited over the subcell C.
[0175] The middle subcell B includes a highly doped p-type aluminum gallium arsenide (“AlGaAs”) back surface field (“BSF”) layer 312, a p-type AlInGaAs base layer 313, a highly doped n-type indium gallium phosphide (“InGaP.sub.2”) or AlInGaAs emitter 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.
[0176] In some embodiments, the middle subcell B may be composed of InGaAsO.
[0177] Before depositing the layers of the top cell A, heavily doped n-type InGaP and p-type AlGaAs tunneling junction layers 316, 317 may be deposited over the subcell B.
[0178] In the illustrated example, the top subcell A includes a highly doped p-type indium aluminum phosphide (“InAlP.sub.2”) BSF layer 318, a p-type InGaAIP base layer 319, a highly doped n-type InGaAlP emitter layer 320 and a highly doped n-type InAlP.sub.2 window layer 321. The base layer 319 of the top subcell A is deposited over the BSF layer 318 after the BSF layer 318 is formed.
[0179] 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.
[0180] Turning to a second embodiment of the multijunction solar cell device of the present disclosure,
[0181] As shown in the illustrated example of
[0182] A first alpha layer 405, preferably composed of n-type AlGaInAsP, is deposited over the tunnel diode 403/404, to a thickness of from 0.25 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 (Comfeld et al.).
[0183] A metamorphic layer (or graded interlayer) 406 is deposited over the alpha layer 405 using a surfactant. Layer 406 is preferably a compositionally step-graded series of InGaAlAs layers, preferably with monotonically changing lattice constant, so as to achieve a gradual transition in lattice constant in the semiconductor structure from subcell D to subcell C while minimizing threading dislocations from occurring. The band gap of layer 406 is constant throughout its thickness, preferably approximately equal to 1.22 eV to 1.34 eV, or otherwise consistent with a value slightly greater than the band gap of the middle subcell C. One embodiment of the graded interlayer may also be expressed as being composed of In.sub.xGa.sub.1-xAs, with x and y selected such that the band gap of the interlayer remains constant at approximately 1.22 eV to 1.34 eV or other appropriate band gap.
[0184] In some embodiments, in the surfactant assisted growth of the metamorphic layer 406, a suitable chemical element is introduced into the reactor during the growth of layer 406 to improve the surface characteristics of the layer. In the preferred embodiment, such element may be a dopant or donor atom such as selenium (Se) or tellurium (Te). Small amounts of Se or Te are therefore incorporated in the metamorphic layer 406, and remain in the finished solar cell. Although Se or Te are the preferred n-type dopant atoms, other non-isoelectronic surfactants may be used as well.
[0185] 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.
[0186] 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 406.
[0187] In one embodiment of the present disclosure, the layer 406 is composed of a plurality of layers of InGaAs, with monotonically changing lattice constant, each layer having the same band gap, approximately in the range of 1.22 eV to 1.34 eV. In some embodiments, the constant band gap is in the range of 1.27 eV to 1.31 eV. In some embodiments, the constant band gap is in the range of 1.28 eV to 1.29 eV.
[0188] 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.
[0189] Although the preferred embodiment of the present disclosure utilizes a plurality of layers of InGaAs for the metamorphic layer 406 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 greater than or equal to that of the second solar cell and less than or equal to that of the third solar cell, and having a bandgap energy greater than that of the second solar cell.
[0190] In some embodiments, a second alpha layer 407, preferably composed of n+ type GaInP, is deposited over metamorphic buffer layer 406, to a thickness of from 0.25 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 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 (Comfeld et al.).
[0191] Distributed Bragg reflector (DBR) layers 408 are then grown adjacent to and between the alpha layer 407 and the third solar subcell C. The DBR layers 408 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 408. In the embodiment depicted in
[0192] For some embodiments, distributed Bragg reflector (DBR) layers 408 can be composed of a plurality of alternating layers 408a through 408z 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.
[0193] For some embodiments, distributed Bragg reflector (DBR) layers 408a through 408z includes a first DBR layer composed of a plurality of p type Al.sub.xGa.sub.1-xAs layers, and a second DBR layer disposed over the first DBR layer and composed of a plurality of p type Al.sub.yGa.sub.1-yAs layers, where y is greater than x.
[0194] In the illustrated example of
[0195] The window layer 412 is deposited on the emitter layer 411 of the subcell C. The window layer 412 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 413, 414 may be deposited over the subcell C.
[0196] The middle subcell B includes a highly doped p-type aluminum gallium arsenide (“AlGaAs”) back surface field (“BSF”) layer 415, a p-type AlInGaAs base layer 416, a highly doped n-type indium gallium phosphide (“InGaP2”) or AlInGaAs layer 417 and a highly doped n-type indium gallium aluminum phosphide (“AlGaAlP”) window layer 418. The InGaP emitter layer 417 of the subcell B can include, for example, approximately 50% In. Other compositions may be used as well.
[0197] Before depositing the layers of the top cell A, heavily doped n-type InGaP and p-type GaAs tunneling junction layers 419, 420 may be deposited over the subcell B.
[0198] In the illustrated example, the top subcell A includes a highly doped p-type indium aluminum phosphide (“InAlP”) BSF layer 421, a p-type InGaAlP base layer 422, a highly doped n-type InGaAlP emitter layer 423 and a highly doped n-type InAlP2 window layer 424. The base layer 422 of the top subcell A is deposited over the BSF layer 421 after the BSF layer 421 is formed over the tunneling junction layers 419, 420 of the subcell B. The window layer 424 is deposited over the emitter layer 423 of the top subcell A after the emitter layer 423 is formed over the base layer 422.
[0199] A cap or contact layer 425 may be deposited and patterned into separate contact regions over the window layer 424 of the top subcell A. The cap or contact layer 425 serves as an electrical contact from the top subcell A to metal grid layer (not shown). The doped cap or contact layer 425 can be a semiconductor layer such as, for example, a GaAs or InGaAs layer.
[0200] After the cap or contact layer 425 is deposited, the grid lines are formed via evaporation and lithographically patterned and deposited over the cap or contact layer 425.
[0201]
[0202] The third embodiment depicted in
[0203] 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.
[0204] 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.
[0205] As a specific example, the doping profile of the emitter and base layers may be illustrated in
[0206] In the example of
[0207] In the example of
[0208] The heavy lines 602 and 603 shown in
[0209] Thus, in one embodiment, the doping level throughout the thickness of the base layer may be exponentially graded from approximately 1×10.sup.16 free carriers per cubic centimeter to 1×10.sup.18 free carriers per cubic centimeter, as represented by the curve 603 depicted in the Figure.
[0210] Similarly, in one embodiment, the doping level throughout the thickness of the emitter layer may decline linearly from approximately 5×10.sup.18 free carriers per cubic centimeter to 5×10.sup.17 free carriers per cubic centimeter, as represented by the curve 602 depicted in the Figure
[0211] 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
[0212] 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
[0213] 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.
[0214] 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.
[0215] 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
[0216] 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.
[0217] The wide range of electron and proton energies present in the space environment necessitates a method of describing the effects of various types of radiation in terms of a radiation environment which can be produced under laboratory conditions. The methods for estimating solar cell degradation in space are based on the techniques described by Brown et al. [Brown, W. L., J. D. Gabbe, and W. Rosenzweig, Results of the Telstar Radiation Experiments, Bell System Technical J., 42, 1505, 1963] and Tada [Tada, H. Y., J. R. Carter, Jr., B. E. Anspaugh, and R. G. Downing, Solar Cell Radiation Handbook, Third Edition, JPL Publication 82-69, 1982]. In summary, the omnidirectional space radiation is converted to a damage equivalent unidirectional fluence at a normalized energy and in terms of a specific radiation particle. This equivalent fluence will produce the same damage as that produced by omnidirectional space radiation considered when the relative damage coefficient (RDC) is properly defined to allow the conversion. The relative damage coefficients (RDCs) of a particular solar cell structure are measured a priori under many energy and fluence levels in addition to different cover glass thickness values. When the equivalent thence 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.
[0218] 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 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.sup.15 electrons/cm.sup.2.
[0219] The exemplary solar cell described herein may require the use of aluminum in the semiconductor composition of each of the top two subcells. Aluminum incorporation is widely known in the III-V compound semiconductor industry to degrade BOL subcell performance due to deep level donor defects, higher doping compensation, shorter minority carrier lifetimes, and lower cell voltage and an increased BOL E.sub.g/q−V.sub.oc metric. In short, increased BOL E.sub.g/q−V.sub.oc may be the most problematic shortcoming of aluminum containing subcells; the other limitations can be mitigated by modifying the doping schedule or thinning base thicknesses.
[0220] 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.sup.2), so that a comparison can be made between the current commercial devices and embodiments of solar cells discussed in the present disclosure.
[0221] 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 1×10.sup.12 e/cm.sup.2 to 2×10.sup.14 e/cm.sup.2 (hereinafter may be written as “2E10 e/cm.sup.2 or 2E14”) over a five year lifetime. A geosynchronous earth orbit (GEO) satellite will typically experience radiation in the range of 5×10.sup.14 e/cm.sup.2 to 1×10.sup.15 e/cm.sup.2 over a fifteen year lifetime.
[0222] 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, N.M.), is as follows:
TABLE-US-00001 Condition Efficiency BOL 28° C. 29.1% BOL 70° C. 26.4% EOL 70° C. 23.4% after 5E14 e/cm.sup.2 radiation EOL 70° C. 22.0% after 1E15 e/cm.sup.2 radiation
[0223] The J.sub.sc of the upper two subcells at EOL under 1E15 conditions is 17.2 mA, and the ratio of the upper junction J.sub.sc to the bottom subcell J.sub.sc is 139%.
[0224] 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.
[0225] Thus, for a four junction solar cell configured and described in the parent application and the present disclosure, four embodiments and their corresponding efficiency data at the end-of-life (EOL) is as follows:
TABLE-US-00002 Embodiment 1 Band Gap Composition Subcell A 2.1 AlInGaP Subcell B 1.73 InGaP or AlInGaAs/AlInGaAs or InGaAs Subcell C 1.41 (In)GaAs Subcell D 0.67 Ge Efficiency at 70° C. after 1E15 e/cm.sup.2 radiation: 23.4%
TABLE-US-00003 Embodiment 2 Band Gap Composition Subcell A 2.1 AlInGaP Subcell B 1.67 InGaP or AlInGaAs/AlInGaAs or InGaAs Subcell C 1.34 InGaAs Subcell D 0.67 Ge Efficiency at 70° C. after 1E15 e/cm.sup.2 radiation: 23.8%
TABLE-US-00004 Embodiment 3 Band Gap Composition Subcell A 2.5 AlInGaP Subcell B 1.63 InGaP or AlInGaAs/AlInGaAs or InGaAs Subcell C 1.30 (In)GaAs Subcell D 0.67 Ge Efficiency at 70° C. after 1E15 e/cm.sup.2 radiation: 24.2%
TABLE-US-00005 Embodiment 4 Band Gap Composition Subcell A 2.02 AlInGaP Subcell B 1.58 InGaP or AlGaAs/AlInGaAs or InGaAs Subcell C 1.25 (In)GaAs Subcell D 0.67 Ge Efficiency at 70° C. after 1E15 e/cm.sup.2 radiation: 25.2%
[0226] For a five junction solar cell configured and described in the parent application and the present disclosure, some embodiments and corresponding efficiency data at the end-of-life (EOL) computed at 28° C. is as follows:
TABLE-US-00006 Embodiment 5 Band Gap Composition Subcell A 1.98 AlInGaP Subcell B 1.78 InGaP or AlInGaAs Subcell C 1.48 AlInGaAs Subcell D 1.2 InGaAs Subcell E 0.67 Ge Efficiency: 27.9%
TABLE-US-00007 Embodiment 6 Band Gap Composition Subcell A 1.95 AlInGaP Subcell B 1.69 InGaP or AlInGaAs Subcell C 1.38 AlInGaAs Subcell D 1.10 InGaAs Subcell E 0.67 Ge Efficiency: 28.4%
[0227] The four junction solar cell of the present disclosure has a 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 5×10.sup.14 e/cm.sup.2, and dramatically improved cell efficiency (%) over the standard commercial solar cell (ZTJ) at 1 MeV electron equivalent fluence of 1×10.sup.15 e/cm.sup.2.
[0228] 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.
[0229] 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.
[0230] 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.
[0231] 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.
[0232] 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.
[0233] 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).
[0234] 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.
[0235] 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.
[0236] 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.
[0237] 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.
[0238] 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.
[0239] 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 team “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
[0240] 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.
[0241] 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.
[0242] 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.
[0243] 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.
[0244] 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.
[0245] 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.
[0246] 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, 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.
[0247] 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 disclosure.
[0248] 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 band, 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.
[0249] 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.