DISTRIBUTED BRAGG REFLECTOR STRUCTURES IN MULTIJUNCTION SOLAR CELLS

Abstract

A multijunction solar cell and its method of fabrication, having an upper first solar subcell composed of a semiconductor material including aluminum and having a first band gap; a second solar subcell adjacent to said first solar subcell and composed of a semiconductor material having a second band gap smaller than the first band gap and being lattice matched with the upper first solar subcell; a third solar subcell adjacent to said second solar subcell and composed of a semiconductor material having a third band gap smaller than the second band gap and being lattice matched with the second solar subcell; a first and second DBR structure adjacent to the third solar subcell; and a fourth solar subcell adjacent to the DBR structures and lattice matched with said third solar subcell and composed of a semiconductor material having a fourth band gap smaller than the third band gap; wherein the fourth subcell has a direct bandgap of greater than 0.75 eV.

Claims

1. A method of forming a multijunction solar cell comprising: forming a first solar subcell comprising an emitter layer and a base layer composed of aluminum gallium arsenide or indium gallium arsenide, the emitter layer and the base layer forming a photoelectric junction; forming a second solar subcell disposed below the first solar subcell and comprising an emitter layer and a base layer forming a photoelectric junction; and forming a combined DBR structure between the first solar subcell and the second solar subcell with no intervening solar subcells, the combined DBR structure comprising a first distributed Bragg reflector (DBR) structure and a second DBR structure; forming a first distributed Bragg reflector (DBR) structure disposed beneath the base layer of the upper solar subcell and composed of a plurality of alternating layers of different semiconductor materials with discontinuities in their respective indices of refraction and arranged so that light can enter and pass through the upper solar subcell and at least a first portion of which light in a first spectral wavelength range can be reflected back into the upper solar subcell by the DBR structure, and a second portion of which light in a second spectral wavelength range can be transmitted through the DBR structure to the layers disposed beneath the first DBR structure, wherein all if the wavelengths in the second spectral wavelength range is greater than all of the wavelengths in the first spectral wavelength range; and forming a second distributed Bragg reflector (DBR) structure disposed beneath the first DBR structure and composed of a plurality of alternating layers of different semiconductor materials with discontinuities in their respective indices of refraction different from the layers of the first DBR structure and arranged so that light can enter and pass through the first DBR structure and at least a portion of which light having a second spectral wavelength range can be reflected back into the upper solar subcell by the second DBR structure, and a third portion of which light in a third spectral wavelength range different from the first and the second spectral wavelength ranges can be transmitted through the second DBR structure to the lower solar subcell disposed beneath the second DBR structure.

2. The method of claim 1 further comprising forming a metamorphic layer between the combined DBR structure and the second solar subcell.

3. The method of claim 2 wherein for the second solar subcell, the emitter layer is composed of germanium and the base layer is composed of germanium.

4. The method of claim 1 further comprising forming tunnel diode layers between the combined DBR structure and the second solar subcell.

5. The method of claim 1 wherein forming the first solar subcell further comprises forming a window layer, tunnel diode layers, and a back surface field (BSF) layer, wherein the base layer is disposed on the BSF layer, the window layer is disposed on the emitter layer, and the tunnel diode layers are disposed on the window layer.

6. The method of claim 1 further comprising forming a first additional solar subcell above the first solar subcell comprising forming an emitter layer and a base layer composed of indium gallium aluminum phosphide, the emitter layer and the base layer forming a photoelectric junction.

7. The method of claim 6 further comprising forming a first tunnel diode layer, a second tunnel diode layer, and a nucleation layer between the combined DBR structure and the second solar subcell, wherein the nucleation layer is composed of indium gallium arsenide and is disposed on the emitter layer of the first additional solar subcell and the first tunnel diode layer composed of gallium arsenide is disposed on the nucleation layer and the second tunnel diode layer composed of aluminum gallium arsenide is disposed on the first tunnel diode layer, wherein the emitter layer of the first solar subcell is composed of indium gallium phosphide or aluminum gallium arsenide.

8. The method of claim 6, further comprising: forming a second additional solar subcell disposed between the first solar subcell and the first additional solar subcell, the second additional solar subcell comprising an emitter layer composed of indium gallium arsenide or aluminum gallium arsenide and a base layer composed of aluminum gallium arsenide, wherein the base layer of the first solar subcell comprises InGaAs.

9. The method according to claim 6, further comprising forming a back surface field (BSF) layer composed of p-type aluminum gallium arsenide disposed on the combined DBR structure and a window layer composed of n-type indium gallium aluminum phosphide disposed on the emitter layer of the second solar subcell, wherein the base layer is disposed on the BSF layer.

10. The method of claim 1, wherein one or more of the solar subcells have a gradation in doping in the base layer that increases approximately exponentially from approximately 110.sup.15 free carriers per cubic centimeter in a region adjacent the photoelectric junction to approximately 410.sup.18 free carriers per cubic centimeter in a region adjacent an adjoining layer and a gradation in doping in the emitter layer that increases from approximately 510.sup.17 free carriers per cubic centimeter in a region adjacent the photoelectric junction to approximately 510.sup.18 free carriers per cubic centimeter in a region immediately adjacent an adjoining layer.

11. The method of claim 1, wherein the emitter layer of the first solar subcell comprises highly doped n-type indium gallium phosphide (InGaP).

12. The method of claim 1, wherein the half width value of reflection of the first DBR structure and the second DBR structure is in a range between 250 and 350 nm.

13. The method of claim 1, wherein the combined DBR structure includes forming alternating layers of lattice mismatched materials, the combined DBR structure includes a first DBR layer composed of a plurality of n type or 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.

14. The method of claim 1, wherein the combined DBR structure comprises forming a sequence of alternating first and second different semiconductor layers, and wherein an average lattice constant of the sequence of alternating first and second semiconductor layers is approximately equal to a lattice constant of a substrate.

15. The method of claim 1, wherein the first solar subcell comprises forming a highly doped n-type indium gallium arsenide emitter layer and a highly doped n-type indium gallium aluminum phosphide window layer.

16. The method of claim 1, wherein the first solar subcell comprises a BSF layer comprising highly doped p-type aluminum gallium arsenide (AlGaAs).

17. The method of claim 1, wherein first spectral wavelength rangeof approximately 780 to 860 nm.

18. The method of claim 1, wherein the first solar subcell has a band gap in the range of 1.65 eV to 1.8 eV.

19. The method of claim 1, wherein the first spectral wavelength range overlaps the second spectral wavelength range by less than 10 nm.

20. The method of claim 1, wherein the first spectral wavelength range and the second spectral wavelength range correspond to the spectral absorption band of the first solar subcell.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The invention will be better and more fully appreciated by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

[0043] FIG. 1A is a graph representing the band gap of certain binary materials and their lattice constants;

[0044] FIG. 1B is a graph representing the efficiency of two tandem subcells as a function of the band gap of the two subcells;

[0045] FIG. 2A is a graph of the band gap versus lattice constant of certain binary and ternary III-V semiconductors;

[0046] FIG. 2B is a graph of the band gap versus lattice constant of certain binary and ternary III-V semiconductors;

[0047] FIG. 3A is a cross-sectional view of a three junction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate, according to a first embodiment of the present disclosure;

[0048] FIG. 3B is a cross-sectional view of a four junction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate, according to a second embodiment of the present disclosure;

[0049] FIG. 3C is a cross-sectional view of the solar cell of a four junction solar cell after several stages of fabrication including the deposition of certain semiconductor layers on the growth substrate, according to a third embodiment of the present disclosure;

[0050] FIG. 4A is a graph of the current density per unit energy versus the photon energy of the incoming light in a solar cell;

[0051] FIG. 4B is a schematic representation of photons of different wavelengths being absorbed by, or being transmitted through, different subcells in a three junction tandem solar cell with a single DBR structure;

[0052] FIG. 5A is a graph of the reflectance of a single distributed Bragg reflector (DBR) structure as a function of wavelength;

[0053] FIG. 5B is a graph of the reflectance of a first distributed Bragg reflector (DBR) structure according to the present disclosure compared with that of the structure of FIG. 5A;

[0054] FIG. 5C is a graph of the reflectance of a first distributed Bragg reflector (DBR) structure according to the present disclosure compared with that of the structure of FIG. 5A;

[0055] FIG. 6 is a schematic representation of photons of different wavelengths being absorbed by, or being transmitted through, different subcells in a solar cell that includes two distributed Bragg reflector (DBR) structures according to the present disclosure;

[0056] FIG. 7A is a graph of the quantum efficiency versus wavelength in a three junction solar cell;

[0057] FIG. 7B is a graph of the quantum efficiency versus wavelength in a three junction solar cell after incorporation of a structure in the solar cell according to the present disclosure; and

[0058] FIG. 8 is a graph of the doping profile in the base and emitter layers of a subcell in the solar cell according to the present disclosure.

GLOSSARY OF TERMS

[0059] 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).

[0060] 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.

[0061] Beginning of Life (BOL) refers to the time at which a photovoltaic power system is initially deployed in operation.

[0062] Bottom subcell refers to the subcell in a multijunction solar cell which is furthest from the primary light source for the solar cell.

[0063] Compound semiconductor refers to a semiconductor formed using two or more chemical elements.

[0064] 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.

[0065] Deposited, with respect to a layer of semiconductor material, refers to a layer of material which is epitaxially grown over another semiconductor layer.

[0066] 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.

[0067] Graded interlayer (or grading interlayer)see metamorphic layer.

[0068] 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.

[0069] 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.

[0070] 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).

[0071] 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.

[0072] 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).

[0073] 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.

[0074] Short circuit current densitysee current density.

[0075] Solar cell refers to an electronic device operable to convert the energy of light directly into electricity by the photovoltaic effect.

[0076] Solar cell assembly refers to two or more solar cell subassemblies interconnected electrically with one another.

[0077] Solar cell subassembly refers to a stacked sequence of layers including one or more solar subcells.

[0078] 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.

[0079] Substantially current matched refers to the short circuit current through adjacent solar subcells being substantially identical (i.e. within plus or minus 1%).

[0080] 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.

[0081] ZTJ refers to the product designation of a commercially available SolAero Technologies Corp. triple junction solar cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0082] 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.

[0083] 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 lattice matched or upright solar cells of the present disclosure. However, more particularly, the present disclosure is directed to the fabrication of a multijunction lattice matched solar cell with specific DBR structures grown between subcells.

[0084] 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.

[0085] 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).

[0086] 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 direction and the ultimate solar cell design proposed by the Applicants.

[0087] In view of the foregoing, it is further evident that the identification of one particular constituent element (e.g. indium, or aluminum) in a particular subcell, or the thickness, band gap, doping, or other characteristic of the incorporation of that material in a particular subcell, is not a single result effective variable that one skilled in the art can simply specify and incrementally adjust to a particular level and thereby increase the power output and efficiency of a solar cell.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] More specifically, the present disclosure intends to provide a relatively simple and reproducible technique that does not employ inverted processing associated with inverted metamorphic multijunction solar cells, and is suitable for use in a high volume production environment in which various semiconductor layers are grown on a growth substrate in an MOCVD reactor, and subsequent processing steps are defined and selected to minimize any physical damage to the quality of the deposited layers, thereby ensuring a relatively high yield of operable solar cells meeting specifications at the conclusion of the fabrication processes.

[0092] 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.

[0093] 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.

[0094] Some comments about MOCVD processes used in one embodiment are in order here.

[0095] 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).

[0096] 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.

[0097] 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.

[0098] FIG. 1A is a graph representing the band gap of certain binary materials and their lattice constants. The band gap and lattice constants of ternary materials are located on the lines drawn between typical associated binary materials (such as the ternary material AlGaAs being located between the GaAs and AlAs points on the graph, with the band gap of the ternary material lying between 1.42 eV for GaAs and 2.16 eV for AlAs depending upon the relative amount of the individual constituents). Thus, depending upon the desired band gap, the material constituents of ternary materials can be appropriately selected for growth.

[0099] FIG. 1B is a graph representing the efficiency of two tandem subcells as a function of the band gap of the two subcells. In particular, it is depicted to demonstrate that the maximum efficiency of tandem combination of subcells is not a simple linear function of the band gap of either the top or high band gap subcell, or the lower or low band gap subcell.

[0100] FIG. 2A is an enlargement of a portion of the graph of FIG. 1A illustrating different compounds of GaInAs and GaInP with different proportions of gallium and indium, and the location of specific compounds on the graph.

[0101] FIG. 2B is a representation of the theoretical efficiency of a tandem solar cell in which the band gap of the top solar subcell is plotted along the y-axis, and the band gap of the adjacent middle solar subcell is plotted along the x-axis graph, with the two ternary compounds Ga.sub.xIn.sub.1-xAs and Ga.sub.yIn.sub.1-yP having identical lattice constants being plotted as a straight line.

[0102] FIG. 3A illustrates a particular example of an embodiment of a three junction solar cell 3000 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322 as provided by the present disclosure.

[0103] As shown in the illustrated example of FIG. 3A, the bottom subcell C includes a substrate 300 formed of p-type germanium (Ge) which also serves as a base layer. A back metal contact pad 350 formed on the bottom of base layer 300 provides electrical contact to the multijunction solar cell 400. The bottom subcell C, further includes, for example, a highly doped n-type Ge emitter layer 301, and an n-type indium gallium arsenide (InGaAs) nucleation layer 302. The nucleation layer is deposited over the base layer, and the emitter layer is formed in the substrate by diffusion of deposits into the Ge substrate, thereby forming the n-type Ge layer 301. Heavily doped p-type aluminum gallium arsenide (AlGaAs) and heavily doped n-type gallium arsenide (GaAs) tunneling junction layers 303, 304 may be deposited over the nucleation layer to provide a low resistance pathway between the bottom and middle subcells.

[0104] A first Distributed Bragg reflector (DBR) structure (DBR-2) consisting of layers 305 (specifically, individual layers 305a through 305z) are then grown adjacent to and between the tunnel diode 303, 304 of the bottom subcell C and the second solar subcell B. The DBR layers 305 are arranged so that light can enter and pass through the third solar subcell B and DBR structure 306 and at least a portion of which can be reflected back into the second solar subcell B by the DBR layers 305. In the embodiment depicted in FIG. 3A, the distributed Bragg reflector (DBR) layers 305 are specifically located between the second solar subcell B/DBR structure 306 and tunnel diode layers 304, 303; in other embodiments, the distributed Bragg reflector (DBR) layers may be located between tunnel diode layers 304/303 and buffer layer 302.

[0105] A second Distributed Bragg reflector (DBR) structure (DBR-1) consisting of layers 306 (specifically, 306a through 306z) being compositionally and optically different from DBR structure DBR-1, are then grown adjacent to and between the DBR-2 structure and the second solar subcell B. The DBR layers 306 are arranged so that light can enter and pass through the third solar subcell B and at least a portion of which can be reflected back into the third solar subcell B by the DBR layers 306. In the embodiment depicted in FIG. 3A, the distributed Bragg reflector (DBR) layers 306 are specifically located between the second solar subcell B and tunnel diode layers 304, 303; in other embodiments, the distributed Bragg reflector (DBR) layers 306 may be located between tunnel diode layers 304/303 and DBR-2 structure.

[0106] For some embodiments, distributed Bragg reflector (DBR) layers 305 and 306 can be composed of a plurality of alternating layers 305a through 305z and 306a through 306z, respectively, 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.

[0107] For some embodiments, distributed Bragg reflector (DBR) layers 305a through 305z, and 306a through 306z 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.

[0108] The scope of the compositional and optical difference in the structures 305 and 306 will be described and specified in more detail subsequent to the discussion of other embodiments.

[0109] In the illustrated example of FIG. 3A, the subcell B includes a highly doped p-type aluminum gallium arsenide (AlGaAs) back surface field (BSF) layer 312, a p-type AlGaAs base layer 313, a highly doped n-type indium gallium phosphide (InGaP) or AlGaAs emitter layer 314 and a highly doped n-type indium gallium aluminum phosphide (InGaAlP) window layer 315. Other compositions may be used as well. The base layer 313 is formed over the BSF layer 312 after the BSF layer 312 is deposited over the DBR layers 306.

[0110] The window layer 315 helps reduce the recombination loss and improves passivation of the cell surface of the underlying junctions.

[0111] 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.

[0112] 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 InGaAlP base layer 319, a highly doped n-type InGaAlP emitter layer 320 and a highly doped n-type InAlP.sub.2 window layer 321.

[0113] A cap or contact layer 322 of GaAs is deposited over the window layer 321 and the grid lines are formed via evaporation and lithographically patterned and deposited over the cap or contact layer 322.

[0114] Turning to another embodiment of the multijunction solar cell device of the present disclosure, FIG. 3B is a cross-sectional view of an embodiment of a four junction solar cell 4000 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322, with various layers and subcells being similar to the structure described and depicted in FIG. 3A.

[0115] The second embodiment depicted in FIG. 3B is similar to that of the first embodiment depicted in FIG. 3A except that an additional middle subcell, subcell C, including layers 307 through 311 is now included, and since the other layers in FIG. 3B are substantially identical to that of layers in FIG. 3A, the description of such layers will not be repeated here for brevity.

[0116] In the illustrated example of FIG. 3B, the subcell C includes a highly doped p-type aluminum gallium arsenide (AlGaAs) back surface field (BSF) layer 307, a p-type InGaAs base layer 308a, a highly doped n-type indium gallium arsenide (InGaAs) emitter layer 308b and a highly doped n-type indium aluminum phosphide (AlInP.sub.2) or indium gallium phosphide (GaInP) window layer 309. The InGaAs base layer 308a of the subcell C can include, for example, approximately 1.5% In. Other compositions may be used as well. The base layer 308a is formed over the BSF layer 307 after the BSF layer 307 is deposited over the DBR layers 306.

[0117] 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 C, 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.

[0118] Turning to another embodiment of the multijunction solar cell device of the present disclosure, FIG. 3C is a cross-sectional view of an embodiment of a four junction solar cell 600 after several stages of fabrication including the growth of certain semiconductor layers on the growth substrate up to the contact layer 322, with various subcells being similar to the structure described and depicted in FIG. 3B.

[0119] In a first embodiment of the present invention, shown in FIG. 3C, an intrinsic layer constituted by a strain-balanced multi-quantum well structure 500 is formed between base layer 410b and emitter layer 411 of middle subcell C. The strain-balanced quantum well structure 500 includes a sequence of quantum well layers formed from alternating layers of compressively strained InGaAs and tensionally strained gallium arsenide phosphide (GaAsP). Strain-balanced quantum well structures are known from the paper of Chao-Gang Lou et al., Current-Enhanced Quantum Well Solar Cells, Chinese Physics Letters, Vol. 23, No. 1 (2006), and M. Mazzer et al., Progress in Quantum Well Solar Cells, Thin Solid Films, Volumes 511-512 (26 Jul. 2006).

[0120] In an alternative example, the strain-balanced quantum well structure 500, comprising compressively strained InGaAs and tensionally strained gallium arsenide, may be provided as either the base layer 410b or the emitter layer 411.

[0121] In the illustrated example, the strain-balanced quantum well structure 500 is formed in the depletion region of the middle subcell C and has a total thickness of about 3 microns (mm). Different thicknesses may be used as well. Alternatively, as noted above, the middle subcell C can incorporate the strain-balanced quantum well structure 500 as either the base layer 410 or the emitter layer 411 without an intervening layer between the base layer 410b and emitter layer 411. A strain-balanced quantum well structure can include one or more quantum wells. The quantum wells may be formed from alternating layers of compressively strained InGaAs and tensionally strained GaAsP. An individual quantum well within the structure includes a well layer of InGaAs provided between two barrier layers of GaAsP, each having a wider energy band gap than InGaAs. The InGaAs layer is compressively strained due to its larger lattice constant with respect to the lattice constant of the substrate 400. The GaAsP layer is tensionally strained due to its smaller lattice constant with respect to the substrate 400. The strain-balanced condition occurs when the average strain of the quantum well structure is approximately equal to zero. Strain-balancing ensures that there is almost no stress in the quantum well structure when the multijunction solar cell layers are grown epitaxially. The absence of stress between layers can help prevent the formation of dislocations in the crystal structure, which would otherwise negatively affect device performance. For example, the compressively strained InGaAs well layers of the quantum well structure 500 may be strain-balanced by the tensile strained GaAsP barrier layers.

[0122] The quantum well structure 500 may also be lattice matched to the substrate 400. In other words, the quantum well structure may possess an average lattice constant that is approximately equal to a lattice constant of the substrate 400. In other embodiments, lattice matching the quantum well structure 500 to the substrate 400 may further reduce the formation of dislocations and improve device performance. Alternatively, the average lattice constant of the quantum well structure 500 may be designed so that it maintains the lattice constant of the parent material in the middle subcell C. For example, the quantum well structure 500 may be fabricated to have an average lattice constant that maintains the lattice constant of the AlGaAs BSF layer 410a. In this way, dislocations are not introduced relative to the middle cell C. However, the overall device 600 is lattice mismatched if the lattice constant of the middle cell C is not matched to the substrate 400. The thickness and composition of each individual InGaAs or GaAsP layer within the quantum well structure 500 may be adjusted to achieve strain-balance and minimize the formation of crystal dislocations. For example, the InGaAs and GaAsP layers may be formed having respective thicknesses about 100 to 300 angstroms. Between 100 and 300 total InGaAs/GaAsP quantum wells may be formed in the strain-balanced quantum well structure 500. More or fewer quantum wells may be used as well. Additionally, the concentration of indium in the InGaAs layers may vary between 10 and 30%.

[0123] Furthermore, the quantum well structure 500 can extend the range of wavelengths absorbed by the middle subcell C. An example of approximate quantum efficiency curves for the multijunction solar cell of FIG. 3C is illustrated in FIG. 7A. As shown in the example of FIG. 7A, the absorption spectrum for the bottom subcell 603, subcell D, extends between 890-1600 nm; the absorption spectrum of the middle subcell 602 extends between 660-1000 nm, overlapping the absorption spectrum of the bottom subcell; and the absorption spectrum of the top subcell 601, subcell A, extends between 300-660 nm. Incident photons having wavelengths located within the overlapping portion of the middle and bottom subcell absorption spectrums may be absorbed by the middle subcell 602 prior to reaching the bottom subcell 603. As a result, the photocurrent produced by middle subcell 602 may increase by taking some of the current that would otherwise be excess current in the bottom subcell 603. In other words, the photo-generated current density produced by the middle subcell 602 may increase. Depending on the total number of layers and thickness of each layer within the quantum well structure 500, the photo-generated current density of the middle subcell 602 may be increased to match the photo-generated current density of the bottom subcell 603.

[0124] The overall current produced by the multijunction cell solar cell then may be raised by increasing the current produced by top subcell 601. Additional current can be produced by top subcell 601 by increasing the thickness of the p-type InGaAlP2 base layer 422 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 601 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.

[0125] As a result, both the introduction of strain-balanced quantum wells in the middle subcell 602 and the increase in thickness of top subcell A provide an increase in overall multijunction solar cell current generation and enable an improvement in overall photon conversion efficiency. Furthermore, the increase in current may be achieved without significantly reducing the voltage across the multijunction solar cell.

[0126] In some embodiments, the sequence of first 501A and second 501B different semiconductor layers forms the base layer of the second subcell.

[0127] In some embodiments, the sequence of first and second different semiconductor layers comprises compressively strained and tensionally strained layers, respectively.

[0128] In some embodiments, an average strain of the sequence of first and second different semiconductor layers is approximately equal to zero.

[0129] In some embodiments, each of the first and second semiconductor layers is approximately 100 nm to 300 angstroms thick.

[0130] In some embodiments, the first semiconductor layer comprises InGaAs and the second semiconductor layer comprises GaAsP.

[0131] In some embodiments, a percentage of indium in each InGaAs layer is in the range of 10 to 30%.

[0132] In some embodiments, the top subcell comprises InGaP and has a thickness so that it generates approximately 4-5% less current than said first current.

[0133] FIG. 4A is a graph of the current density per unit energy versus the photon energy and the wavelength of the incoming light in a solar cell under the AM0 spectral environment.

[0134] FIG. 4B is a schematic representation of photons of different wavelengths being absorbed by, or being transmitted through, different subcells in a three junction tandem solar cell with a single DBR structure (DBR-1). In this particular representation the light wavelengths .sub.C>.sub.B>.sub.A. The top subcell A is designed to absorb light of wavelength .sub.A, the middle subcell is designed to absorb light of wavelength .sub.B, and the bottom subcell is designed to absorb light of wavelength .sub.C.

[0135] FIG. 5A is a graph 500 of the reflectance of a single distributed Bragg reflector structure as a function of wavelength such as known in the prior art. In this particular example, the wavelengths of light which are most strongly reflected extend over the range 790 to 910 nm. Assuming that subcell B would absorb light in the wavelength range of 790 to 910 nm, employment of this DBR structure below subcell B will reflect any light entering the DBR structure of that wavelength back into subcell B, to allow photons of such wavelength to make a second pass through subcell B, increasing the photocurrent of subcell B.

[0136] FIG. 5B is a graph 501 of the reflectance of a first distributed Bragg reflector (DBR-2) structure according to the present disclosure compared with that of the graph 500 of FIG. 5A. In this particular example, the wavelengths of light which are most strongly reflected by DBR-2 extend over the range 850 to 920 nm. Assuming that subcell B would absorb light in the wavelength range of 790 to 910 nm, employment of DBR-1 below subcell B will reflect any light entering DBR-1 of wavelength range 850 to 920 nm back into subcell B to allow photons of such wavelength to make a second pass through subcell B, increasing the photocurrent of subcell B.

[0137] FIG. 5C is a graph 502 of the reflectance of a second distributed Bragg reflector (DBR-1) structure according to the present disclosure compared with that of graph 500 of FIG. 5A. In this particular example, the wavelengths of light which are most strongly reflected extend over the range 780 to 860 nm. Assuming that subcell B would absorb light in the wavelength range of 780 to 860 nm, employment of DBR-1 below subcell B will reflect any light entering DBR-1 of that wavelength range back into subcell B to allow photons of such wavelength to make a second pass through subcell B, increasing the photocurrent of subcell B.

[0138] The present disclosure contemplates the use of both DBR-2 and DBR-1 to more finely tune or accurately cover the wavelength range to be reflected back into subcell B compared with the single DBR structure of FIG. 5A.

[0139] FIG. 6 is a schematic representation of photons of different wavelengths (.sub.A, .sub.B1, .sub.B2, .sub.C) being absorbed by, or being transmitted through, different subcells in a solar cell that includes two distributed Bragg reflector (DBR) structures DBR-1 and DBR-2 according to the present disclosure.

[0140] FIG. 7A is a graph of the quantum efficiency versus wavelength in a three junction solar cell, represented by the top cell graph 601, the middle cell graph 602, and the bottom cell graph 603.

[0141] FIG. 7B is a graph of the quantum efficiency versus wavelength in a three junction solar cell after incorporation of a structure in the solar cell according to the present disclosure.

[0142] FIG. 8 is a graph of the doping profile in the base and emitter layers of a subcell in the solar cell according to the present disclosure. In some embodiments, at least the base of at least one of the first A, second B or third C 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.

[0143] In some embodiments, the emitter of at least one of the first A, second B or third C 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.

[0144] As a specific example, the doping profile of the emitter and base layers may be illustrated in FIG. 8, which depicts the amount of doping in the emitter region and the base region of a subcell. N-type dopants include silicon, selenium, sulfur, germanium or tin. P-type dopants include silicon, zinc, chromium, or germanium.

[0145] In the example of FIG. 8, the emitter doping decreases from anywhere in the range of approximately 510.sup.18 to 110.sup.17 free carriers per cubic centimeter in the region immediately adjacent the adjoining layer to anywhere in the range of 110.sup.18 to 110.sup.15 free carriers per cubic centimeter in the region adjacent to the p-n junction which is shown by the dotted line in the referenced Figure.

[0146] The base doping increases from anywhere in the range of 110.sup.15 to 110.sup.18 free carriers per cubic centimeter adjacent the p-n junction to anywhere in the range of 110.sup.16 to 110.sup.19 free carriers per cubic centimeter adjacent to the adjoining layer at the rear of the base.

[0147] In some embodiments, the doping level throughout the thickness of the base layer may be exponentially graded from the range of 110.sup.16 free carriers per cubic centimeter to 110.sup.18 free carriers per cubic centimeter, as represented by the curve 603 depicted in the Figure.

[0148] In some embodiments, the doping level throughout the thickness of the emitter layer may decline linearly from 510.sup.18 free carriers per cubic centimeter to 510.sup.17 free carriers per cubic centimeter as represented by the curve 602 depicted in the Figure.

[0149] The absolute value of the collection field generated by an exponential doping gradient exp [x/] is given by the constant electric field of magnitude E=kT/q(1/))(exp[x.sub.b/]), where k is the Boltzman constant, T is the absolute temperature in degrees Kelvin, q is the absolute value of electronic change, and is a parameter characteristic of the doping decay.

[0150] The efficacy of an embodiment 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 of a subcell, according to one embodiment of the present disclosure. Following measurements of the electrical parameters of the test cell, there was observed a 6.7% increase in current collection. The measurements indicated an open circuit voltage (V.sub.oc) equal to at least 3.014 volts, a short circuit current density (J.sub.sc) of at least 16.55 mA/cm.sup.2, and a fill factor (FF) of at least 0.86 at AM0.

[0151] The exponential doping profile taught by the present disclosure produces a constant field in the doped region. In the particular multijunction solar cell materials and structure of the present disclosure, the bottom subcell has the smallest short circuit current among all the subcells. Since in a multijunction solar cell, the individual subcells are stacked and form a series circuit, the total current flow in the entire solar cell is therefore limited by the smallest current produced in any of the subcells. Thus, by increasing the short circuit current in the bottom cell, the current more closely approximates that of the higher subcells, and the overall efficiency of the solar cell is increased as well. In 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.

[0152] 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, including 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.

[0153] 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.

[0154] In some embodiments, the composition of the window layer is linearly graded so that the concentration of Al in the window layer linearly increases from the bottom surface of the window layer to the top surface of the window layer.

[0155] In some embodiments, the window layer is composed of InAlP or InGaP and the Al content at the bottom surface of the window layer is between 40.0 and 48.5% by mole fraction.

[0156] In some embodiments, the composition of the window layer is graded so that the lattice constant in the window layer is in compression at the bottom surface of the window layer, and increases to the top surface of the window layer so that the lattice constant in the window layer is in compression at the top surface.

[0157] 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.

[0158] 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.

[0159] 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.

[0160] 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.

[0161] 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.

[0162] 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, GalnAs, GaInPAs, AlGaAs, AlInAs, AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GalnSb, AlGaInSb, AIN, GaN, InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, ZnSSe, CdSSe, and similar materials, and still fall within the spirit of the present invention.

[0163] 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.

[0164] 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.

[0165] 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.