MULTIJUNCTION SOLAR CELLS FOR LOW TEMPERATURE OPERATION

Abstract

A multijunction solar cell includes an upper solar subcell, a bottom solar subcell adjacent to the upper solar subcell, a layer of light scattering elements below and directly adjacent to the bottom solar subcell, and a metallic layer disposed below and adjacent to the layer of light scattering elements.

Claims

1. A two junction solar cell comprising: an upper solar subcell composed of InGaP and having an emitter of n-conductivity type with a first band gap and a thickness in the range of 40-150 nm and having a base of p-conductivity type and a thickness in the range of 400-900 nm; a layer of light scattering elements below and adjacent to the bottom solar subcell, wherein the layer of light scattering elements includes at least one of metal, oxide or polymer nanoparticles; a bottom solar subcell adjacent to the upper solar subcell, the bottom solar subcell composed of InGaAs having an emitter of n-conductivity type with a second band gap and a thickness in the range of 40 to 550 nm and having a base of p-conductivity type and a thickness in the range of 300-2500 nm, the base and emitter of the bottom solar subcell forming a p-n junction; and a metallic layer disposed below and adjacent to the layer of light scattering elements.

2. A two junction solar cell as defined in claim 1, wherein the layer of light scattering elements includes discrete periodic or non-periodic arrayed elements having a height of 200-500 nm, a width of 200-500 nm, and a pitch of 200-500 nm.

3. A two junction solar cell as defined in claim 1, wherein a bottom surface of the bottom solar subcell is roughened.

4. A two junction solar cell as defined in claim 3, wherein the layer of light scattering elements includes a surface oxide layer disposed over the roughened surface, and the layer of light scattering elements is configured to redirect incoming light to be totally internally reflected into the bottom solar subcell.

5. A two junction solar cell as defined in claim 1, wherein the emitter of the bottom solar subcell has a thickness of 150 to 550 nm.

6. A two junction solar cell as defined in claim 1, wherein the layer of light scattering elements is composed of oxide nanoparticles.

7. A two junction solar cell as defined in claim 1, wherein the layer of light scattering elements is composed of metal nanoparticles.

8. A two junction solar cell as defined in claim 1, wherein the layer of light scattering elements is composed of polymer nanoparticles.

9. A two junction solar cell as defined in claim 1, wherein the bottom solar subcell is a homojunction solar subcell.

10. A two junction solar cell as defined in claim 1, wherein efficiency of the solar cell is optimized for an operating temperature of 47° C.

11. A two junction solar cell comprising: an upper solar subcell composed of InGaP and having an emitter of n conductivity type with a first band gap; a bottom solar subcell adjacent to the upper solar subcell, the bottom subcell composed of (In)GaAs and having an emitter of n conductivity type with a second band gap less than the first band gap and a base of p conductivity type; a light scattering layer disposed below and directly adjacent to the bottom solar subcell to reflect incoming light into the solar subcell where the light scattering layer includes at least one of metal, oxide, or polymer nanoparticles; and a metallic layer disposed below and directly adjacent to the layer of light scattering elements.

12. A two junction solar cell as defined in claim 11, wherein the light scattering layer includes discrete periodic or non-periodic arrayed elements having a height of 200-500 nm, a width of 200-500 nm, and a pitch of 200-500 nm.

13. A two junction solar cell as defined in claim 11, wherein a bottom surface of the bottom solar subcell is roughened, and the light scattering layer includes a surface oxide layer disposed over the roughened surface.

14. A two junction solar cell as defined in claim 11, wherein the light scattering layer is arranged to redirect incoming light to be totally internally reflected into the bottom solar subcell.

15. A two junction solar cell as defined in claim 11, wherein efficiency of the solar cell is optimized for an operating temperature of 47° C.

16. A two junction solar cell comprising: an upper solar subcell composed of InGaP and having an emitter of n conductivity type with a first band gap and a thickness in the range of 40-150 nm and having a base of p conductivity type and a thickness in the range of 400-900 nm; a bottom solar subcell adjacent to the upper solar subcell, the bottom solar subcell composed of (In)GaAs emitter of n conductivity type with a second band gap and a thickness in the range of 40 to 550 nm and having a base of (Al)(In)GaAs or (Al)InGaP of p conductivity type and a thickness in the range of 300-2500 nm; a layer of light scattering elements below and adjacent to the bottom solar subcell, wherein the layer of light scattering elements includes at least one of metal, oxide or polymer nanoparticles; and a metallic layer disposed below and adjacent to the layer of light scattering elements.

17. A solar cell as defined in claim 16, wherein the bottom solar subcell is a heterojunction.

18. A method of manufacturing a two junction solar cell, the method comprising: providing a semiconductor growth substrate; depositing on the semiconductor growth substrate an etch stop layer; depositing a first sequence of layers of semiconductor material forming a first solar subcell on the etch stop layer; depositing a second sequence of layers of semiconductor material forming a lattice matched second solar subcell over the first solar subcell; forming a layer of light scattering elements over and adjacent to the second solar subcell; mounting and bonding a surrogate substrate on top of the sequence of layers; and removing the semiconductor growth substrate.

19. A method as defined in claim 18, wherein the layer of light scattering elements is formed by: semiconductor growth conditions that produce a rough semiconductor surface.

20. A method as defined in claim 18, wherein the first solar subcell is composed of InGaP and has an emitter of n conductivity type with a first band gap; and the second solar subcell is composed of (In)GaAs and has an emitter of n conductivity type with a second band gap less than the first band gap and a base of p conductivity type.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] The present disclosure will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

[0065] FIG. 1A is a cross-sectional view of a two junction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate, and removal of the growth substrate, according to a first embodiment of the present disclosure;

[0066] FIG. 1B is a cross-sectional view of the two junction solar cell of FIG. 1A depicting the internal reflection of an incoming light beam; and

[0067] FIG. 2 is a graph depicting the alpha or AM0 efficiency as a function of the bottom subcell band gap.

GLOSSARY OF TERMS

[0068] “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).

[0069] “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.

[0070] “Beginning of Life (BOL)” refers to the time at which a photovoltaic power system is initially deployed in operation.

[0071] “Bottom subcell” refers to the subcell in a multijunction solar cell which is furthest from the primary light source for the solar cell.

[0072] “Compound semiconductor” refers to a semiconductor formed using two or more chemical elements.

[0073] “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.

[0074] “Deposited”, with respect to a layer of semiconductor material, refers to a layer of material which is epitaxially grown over another semiconductor layer.

[0075] “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.

[0076] “Graded interlayer” (or “grading interlayer”)—see “metamorphic layer”.

[0077] “Inverted metamorphic multijunction solar cell” or “IMM solar cell” refers to a solar cell in which the subcells are deposited or grown on a substrate in a “reverse” sequence such that the higher band gap subcells, which would normally be the “top” subcells facing the solar radiation in the final deployment configuration, are deposited or grown on a growth substrate prior to depositing or growing the lower band gap subcells.

[0078] “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.

[0079] “Lattice mismatched” refers to two adjacently disposed materials or layers (with thicknesses of greater than 100 nm) having in-plane lattice constants of the materials in their fully relaxed state differing from one another by less than 0.02% in lattice constant. (Applicant expressly adopts this definition for the purpose of this disclosure, and notes that this definition is considerably more stringent than that proposed, for example, in U.S. Pat. No. 8,962,993, which suggests less than 0.6% lattice constant difference).

[0080] “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.

[0081] “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).

[0082] “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.

[0083] “Short circuit current density”—see “current density”.

[0084] “Solar cell” refers to an electronic device operable to convert the energy of light directly into electricity by the photovoltaic effect.

[0085] “Solar cell assembly” refers to two or more solar cell subassemblies interconnected electrically with one another.

[0086] “Solar cell subassembly” refers to a stacked sequence of layers including one or more solar subcells.

[0087] “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.

[0088] “Substantially current matched” refers to the short circuit current through adjacent solar subcells being substantially identical (i.e. within plus or minus 1%).

[0089] “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.

[0090] “ZTJ” refers to the product designation of a commercially available SolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0091] 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.

[0092] A variety of different features of multijunction solar cells (as well as inverted metamorphic multijunction solar cells) are disclosed in the related applications noted above. Some, many or all of such features may be included in the structures and processes associated with the inverted multijunction solar cells of the present disclosure.

[0093] Prior to discussing the specific embodiments of the present disclosure, a brief discussion of some of the issues associated with the design of multijunction solar cells, and the context of the composition or deposition of various specific layers in embodiments of the product as specified and defined by Applicant is in order.

[0094] 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).

[0095] 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 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.

[0096] Furthermore, as in the case here, where multiple variables interact in unpredictable ways, the proper choice of the combination of variables can produce new and unexpected results, and constitute an “inventive step”.

[0097] The efficiency of a solar cell is not a simple linear algebraic equation as a function of the amount of gallium or aluminum or other element in a particular layer. The growth of each of the epitaxial layers of a solar cell in a reactor is a non-equilibrium thermodynamic process with dynamically changing spatial and temporal boundary conditions that is not readily or predictably modeled. The formulation and solution of the relevant simultaneous partial differential equations covering such processes are not within the ambit of those of ordinary skill in the art in the field of solar cell design.

[0098] More specifically, the present disclosure intends to provide a relatively simple and reproducible technique that is suitable for use in a high volume production environment in which various semiconductor layers are grown on a growth substrate in an MOCVD reactor, and subsequent processing steps are defined and selected to minimize any physical damage to the quality of the deposited layers, thereby ensuring a relatively high yield of operable solar cells meeting specifications at the conclusion of the fabrication processes.

[0099] The lattice constants and electrical properties of the layers in the semiconductor structure are preferably controlled by specification of appropriate reactor growth temperatures and times, and by use of appropriate chemical composition and dopants. The use of a deposition method, such as Molecular Beam Epitaxy (MBE), Organo Metallic Vapor Phase Epitaxy (OMVPE), Metal Organic Chemical Vapor Deposition (MOCVD), or other vapor deposition methods for the growth may enable the layers in the monolithic semiconductor structure forming the cell to be grown with the required thickness, elemental composition, dopant concentration and grading and conductivity type, and are within the scope of the present disclosure.

[0100] The present disclosure is in one embodiment directed to a growth process using a metal organic chemical vapor deposition (MOCVD) process in a standard, commercially available reactor suitable for high volume production. Other embodiments may use other growth technique, such as MBE. More particularly, regardless of the growth technique, the present disclosure is directed to the materials and fabrication steps that are particularly suitable for producing commercially viable multijunction solar cells or inverted metamorphic multijunction solar cells using commercially available equipment and established high-volume fabrication processes, as contrasted with merely academic expositions of laboratory or experimental results.

[0101] Some comments about MOCVD processes used in one embodiment are in order here.

[0102] 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.

[0103] 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.

[0104] FIG. 1A illustrates a particular example of an embodiment of a two junction solar cell 100 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate (not shown) up to the top layer 101 of the semiconductor body as provided by the present disclosure.

[0105] As shown in the illustrated example of FIG. 1A, the solar cell 100 includes a bottom subcell B includes a layer 104 formed of p-type (In)GaAs which serves as a base layer. A back metal contact 106 is formed on the bottom of base layer 104 provides electrical contact to the multijunction solar cell 100. The bottom subcell B, further includes, for example, a highly doped n-type (In)GaAs emitter layer 103.

[0106] In the illustrated example, the top subcell A includes a p-type (In)GaAs base layer 102, a highly doped n-type (In)GaAs emitter layer 101 and a highly doped n-type InAlP.sub.2 window layer.

[0107] A cap or contact layer 216 of GaAs is deposited over the window layer 215.

[0108] The overall current produced by the multijunction cell solar cell may be raised by increasing the current produced by top subcell. Additional current can be produced by top subcell by increasing the thickness of the p-type InGaAlP.sub.2 base layer in that cell. The increase in thickness allows additional photons to be absorbed, which results in additional current generation. Preferably, for space or AM0 applications, the increase in thickness of the top subcell maintains the approximately 4 to 5% difference in current generation between the top subcell A and middle subcell C. For AM1 or terrestrial applications, the current generation of the top cell and the middle cell may be chosen to be equalized.

[0109] FIG. 1B is a cross-sectional view of the two junction solar cell of FIG. 1A depicting the internal reflection of an incoming light beam.

[0110] FIG. 2 is a graph depicting the alpha or AM0 efficiency as a function of the bottom subcell band gap.

[0111] 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.

[0112] Although described embodiments of the present disclosure utilizes a vertical stack of three subcells, various aspects and features of the present disclosure can apply to stacks with fewer or greater number of subcells, i.e. two junction cells, three junction cells, five, six, seven junction cells, etc.

[0113] In addition, although the disclosed embodiments are configured with top and bottom electrical contacts, the subcells may alternatively be contacted by means of metal contacts to laterally conductive semiconductor layers between the subcells. Such arrangements may be used to form 3-terminal, 4-terminal, and in general, n-terminal devices. The subcells can be interconnected in circuits using these additional terminals such that most of the available photogenerated current density in each subcell can be used effectively, leading to high efficiency for the multijunction cell, notwithstanding that the photogenerated current densities are typically different in the various subcells.

[0114] As noted above, the solar cell described in the present disclosure may utilize an arrangement of one or more, or all, homojunction cells or subcells, i.e., a cell or subcell in which the p-n junction is formed between a p-type semiconductor and an n-type semiconductor both of which have the same chemical composition and the same band gap, differing only in the dopant species and types, and one or more heterojunction cells or subcells. Subcell 309, with p-type and n-type InGaP is one example of a homojunction subcell.

[0115] In some cells, a thin so-called “intrinsic layer” may be placed between the emitter layer and base layer, with the same or different composition from either the emitter or the base layer. The intrinsic layer may function to suppress minority-carrier recombination in the space-charge region. Similarly, either the base layer or the emitter layer may also be intrinsic or not-intentionally-doped (“NID”) over part or all of its thickness.

[0116] The composition of the window or BSF layers may utilize other semiconductor compounds, subject to lattice constant and band gap requirements, and may include AlInP, AlAs, AlP, AlGaInP, AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AlN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present invention.

[0117] 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.

[0118] 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.

[0119] Without further analysis, from the foregoing others can, by applying current knowledge, readily adapt the present invention for various applications. Such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.