WO2023172950A2

WO2023172950A2 - Compositionally graded buffer for thermo-photovoltaic systems

Info

Publication number: WO2023172950A2
Application number: PCT/US2023/063926
Authority: WO
Inventors: John REIFENBERG; Jon William STEWART; Adam William WELCH
Original assignee: Sierra Nevada Corporation
Priority date: 2022-03-09
Filing date: 2023-03-08
Publication date: 2023-09-14
Also published as: WO2023172950A3

Abstract

Methods, devices, and systems are described for a compositionally graded buffer structure for a TPV system. The compositionally graded buffer structure includes a substrate having a substrate lattice constant, and a first layered structure configured to be disposed above the substrate, the first layered structure having a first layer lattice constant. The compositionally graded buffer structure includes at least one second layered structure disposed above the first layered structure, the second layered structure having a second layer lattice constant. The compositionally graded buffer structure includes a photovoltaic cell configured to be disposed above the first layered structure and the second layered structure. The photovoltaic cell has a photovoltaic cell lattice constant that is different than the substrate lattice constant. The first layer lattice constant matches the substrate lattice constant more than the second layered structure. The second layer lattice constant matches the photovoltaic cell lattice constant more than the first layered structure.

Description

COMPOSITIONALLY GRADED BUFFER FOR THERMO-PHOTOVOLTAIC SYSTEMS

CROSS-REFERENCE TO APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 63/318,293 entitled “COMPOSITIONALLY GRADED BUFFER FOR THERMO- PHOTOVOLTAIC SYSTEMS” and filed on March 9, 2022, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates generally to energy conversion, and more particularly, to a compositionally graded buffer for thermo-photovoltaic systems.

BACKGROUND

[0003] Thermo-photovoltaic (TPV) power generation systems use the photovoltaic (PV) effect to convert heat into thermal radiation and electrical power. TPV systems comprise an emitter configured to deliver heat and thermal radiation, and a PV cell configured to receive the thermal radiation from the heated emitter. The thermal radiation is the emission of electromagnetic waves from heated material. The PV cell may be configured to absorb thermal radiation at a specific wavelength range. The PV cell converts the thermal radiation or thermally-emitted photons into electrical power by utilizing a semiconductor with an appropriate bandgap. The thermally-emitted photons have a higher energy than the bandgap that excite the electron-hole pairs in the PV cell to generate electricity for an external electrical system. The PV cell is typically grown on a substrate. The PV cell may include a semiconductor material such as silicon. TPV systems have the potential to increase energy efficiency and reduce greenhouse gas emissions compared to traditional energy sources.

SUMMARY

[0004] The present disclosure relates generally to the fields of energy conversion, including systems and methods useful for a compositionally graded buffer for a photovoltaic cell in a TPV system.

[0005] In one aspect, disclosed herein are systems for a compositionally graded buffer structure for a TPV system. The compositionally graded buffer structure includes a first layered structure configured to be disposed above a substrate, the first layered structure having a first layer lattice constant. The compositionally graded buffer structure includes at least one second layered structure disposed above the first layered structure, the second layered structure having a second layer lattice constant. The compositionally graded buffer structure includes a photovoltaic cell configured to be disposed above the first layered structure and the second layered structure. The photovoltaic cell has a photovoltaic cell lattice constant that is different than the substrate lattice constant. The first layer lattice constant matches the substrate lattice constant more than the second layered structure. The second layer lattice constant matches the photovoltaic cell lattice constant more than the first layered structure.

[0006] In some variations, the first layered structure includes a phosphide-compound semiconductor layer including at least one element from group III of the periodic table. Additionally, the at least one second layered structure includes a phosphide-compound semiconductor layer including at least one element from group III of the periodic table. Further, the phosphide-compound semiconductor layer is configured to react strongly with a hydrochloric acid etch. In some variations, the first layered structure includes at least one element from group V of the periodic table and wherein the at least one second layered structure includes at least one element from group V of the periodic table.

[0007] In some variations, the substrate has a lattice constant of approximately 5.65- 5.66 Angstroms and includes at least one of gallium arsenide or germanium. Additionally, the photovoltaic cell has a lattice constant of approximately 5.7 to 5.95 Angstroms and includes In_xGai-_xAs where 0.1<x< 0.8. Further, the photovoltaic cell is associated with a bandgap between 1.279eV and 0.503eV. Additionally, the first layered structure has a thickness of 500 nm to 5 pm thick and the at least one second layered structure has a thickness of 500 nm to 5 pm.

[0008] In some variations, the photovoltaic cell includes at least one of an arsenide- based charge extraction layer or an antimonide-based charge extraction layer for etch selectivity. Additionally, wherein the at least one of the arsenide-based charge extraction layer or the antimonide-based charge extraction layer is lattice-matched to an absorber (In_xGai-_xAs) layer. Further, the first layer lattice constant and the second layer lattice constant change monotonically from the substrate lattice constant to the photovoltaic cell lattice constant. In some variations, the first layered structure and the second layered structure are at least one of a single crystalline or a polycrystalline. [0009] In some variations, the second layered structure has a threading dislocation density less than 10⁷ cm'². Additionally, the first layered structure includes an indiumphosphide semiconductor layer and wherein the at least one second layered structure includes an indium-phosphide semiconductor layer.

[0010] In another aspect, disclosed herein are systems for a compositionally graded buffer structure for a TPV system. The compositionally graded buffer structure includes a substrate having a substrate lattice constant, and a first layered structure configured to be disposed above the substrate, the first layered structure having a first layer lattice constant. The compositionally graded buffer structure includes at least one second layered structure disposed above the first layered structure, the second layered structure having a second layer lattice constant. The compositionally graded buffer structure includes a photovoltaic cell configured to be disposed above the first layered structure and the second layered structure. The photovoltaic cell has a photovoltaic cell lattice constant that is different than the substrate lattice constant. The first layer lattice constant matches the substrate lattice constant more than the second layered structure. The second layer lattice constant matches the photovoltaic cell lattice constant more than the first layered structure.

[0011] In some variations, the substrate has a lattice constant of approximately 5.65- 5.66 Angstroms and includes at least one of gallium arsenide or germanium. Additionally, the substrate has a photovoltaic cell has a lattice constant of approximately 5.7 to 5.95 Angstroms and includes In_xGai-_xAs where 0.1<x< 0.8. Further, the photovoltaic cell is associated with a bandgap between 1.279eV and 0.503eV. In some variations, the first layered structure includes a phosphide-compound semiconductor layer including at least one element from group III of the periodic table and wherein the at least one second layered structure includes a phosphide- compound semiconductor layer including at least one element from group III of the periodic table. Additionally, the photovoltaic cell includes at least one of an arsenide-based charge extraction layer or an antimonide-based charge extraction layer for etch selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The embodiments herein may be better understood by referring to the following description in conjunction with the accompanying drawings in which like reference numerals indicate identically or functionally similar elements, of which:

[0013] FIG. 1 depicts an example of a compositionally graded buffer between a substrate and a PV cell;

[0014] FIG. 2 depicts another example of a compositionally graded buffer between a substrate and a PV cell with the lattice constant increasing between the substrate and the PV cell;

[0015] FIG. 3 depicts a graph illustrating the relationship between the band gap and lattice constant for different III-V materials; and

[0016] FIG. 4 depicts an emitter, the PV cell, and a bandpass assembly in a TPV system.

DETAILED DESCRIPTION

[0017] The methods, systems, and apparatuses described herein are for a compositionally graded buffer for a TPV system. In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols generally identify similar components, unless context dictates otherwise. The illustrative alternatives described in the detailed description, drawings, and claims are not meant to be limiting. Other alternatives may be used and other changes may be made without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects, as generally described herein, and illustrated in the Figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this application.

[0018] Distributed electrical generation is becoming increasingly important. Common challenges for conventional generators and distributed electricity generation include efficiency, noise, and maintenance demands. TPV generators are uniquely positioned to address many of these demands due to their scalable efficiency, few moving parts, and near silent operation. TPV generators be used in a variety of applications where heat is produced as a byproduct, such as industrial processes, vehicles, and space-based power systems. Additionally, TPV systems may be relatively fuel agnostic, permitting hydrogen and other carbon-free fuels to be used. In the relevant power regime for TPV systems (~10s of kW and lower), TPV systems can potentially outperform any technology in its class with respect to gravimetric/volumetric energy densities including lithium-ion batteries. TPV systems are commonly incorporated with different high-grade heat sources (with their own ewergy-heat efficiencies) including but not limited to concentrated solar power, radioisotope heaters, electrical heaters, or chemical combustors.

[0019] In TPV systems, the photovoltaic cells with a high quantum efficiency can maximize the efficiency and reduce spectral losses of TPV systems. Spectral loss may also occur due to low-energy photons not being absorbed by the photovoltaic cell. Low energy photons may include photons having an energy level less than the bandgap of the cell. Spectral losses may also occur due to band edge relaxation of charge carriers generated by photons with energy greater than the bandgap of the photovoltaic cell. To achieve high quantum efficiency and reduce spectral losses, PV cells for TPV systems are grown with a very small concentration of material defects. Material defects provide recombination sites for photo-generated charge carriers. Threading dislocation densities less than ~10⁶ - 10⁷ cm'² are important to ensure high quantum efficiency.

[0020] But one drawback of the PV cells is the potential strain-induced defects of otherwise high-quality PV cell films on mismatched lattice substrates. The high-quality PV cell film may have a lattice constant that does not match the lattice substrate. A high-quality PV cell film may be required to have a lattice constant different from the lattice constant of the substrate. Mismatched lattice substrates in PV cells can lead to a range of performance and efficiency issues. For example, the mismatched lattice constants between the substrate and the high-quality PV cell film may generate lattice defects and misfit dislocations that create structural defects within the PV cell. These lattice defects and misfit dislocations lead to recombination losses and reduce the overall efficiency of the cells. Mismatched lattice constants can also result in an increased rate of degradation and reduced lifetime of the cells. Another issue that can arise from mismatched lattice constants is bandgap variations, which can cause non-uniform performance under different lighting conditions, reducing overall efficiency. Growing a mismatched high-quality PV cell film on a substrate having a different lattice constant causes strains and defects during single crystal, epitaxial growth. As such, a high-quality PV cell film may have strained epitaxial growth on lattice-mismatched substrates.

[0021] The concepts described herein improve the lattice mismatching inefficiencies with a novel compositionally graded buffer structure for TPV systems. Compared to other growth techniques, the novel compositionally graded buffer described herein includes a lower strain due to matching the substrate lattice constant and the photovoltaic cell lattice constant through a gradual transition of lattice constants through two or more layered structures.

[0022] Another drawback of the growing PV cells on substrates is that the substrates need to be re-used for growing additional PV cells. Substrates are re-used to avoid scarcity issues and high costs. An epitaxial lift-off technique may be applied for re-using the substrates to grow the high-quality PV cell films. The epitaxial lift-off technique includes depositing a thin layer (typically <200 nm) of sacrificially etched material in between the substrate and the photovoltaic cells. But many epitaxial lift-off techniques result in bubble formation and deposits on non-etched surfaces, reducing the lifetime of the substrate. Additionally, the substrate may break during time-consuming weight-assisted and other strenuous lift-offs.

[0023] The concepts described herein solve the drawbacks of the epitaxial lift-off technique with a novel compositionally graded buffer structure for TPV systems. The lift-off technique for the compositionally graded buffer described herein minimizes the possibility of bubble formation, deposits on non-etched surfaces, and damaging/time-consuming etches.

[0024] The compositionally graded buffer structure includes a first layered structure having a first layer lattice constant and including a first side and a second side, the first layered structure configured to be disposed above a substrate such that the first side of the first layered structure faces the substrate, the substrate having a substrate lattice constant. The compositionally graded buffer structure includes at least one second layered structure having a second layer lattice constant and disposed on the second side of the first layered structure. The compositionally graded buffer structure includes a photovoltaic cell configured to be disposed above the first layered structure and the second layered structure, the photovoltaic cell having a photovoltaic cell lattice constant, the photovoltaic cell lattice constant being different than the substrate lattice constant. The first layer lattice constant matches the substrate lattice constant more than the second layered structure. The second layer lattice constant matches the photovoltaic cell lattice constant more than the first layered structure. [0025] The first layered structure may include a phosphide-compound semiconductor layer including at least one element from group III of the periodic table. The at least one second layered structure may include a phosphide-compound semiconductor layer including at least one element from group III of the periodic table. The first layered structure may include at least one element from group V of the periodic table. The at least one second layered structure may include at least one element from group V of the periodic table. Group III of the periodic table includes boron (B), aluminum (Al), and gallium (Ga). These elements are characterized by having three valence electrons. Group V of the periodic table includes nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi). These elements have five valence electrons.

[0026] In at least one embodiment, the substrate has a lattice approximately in the range of 5.65 to 5.66 Angstroms and includes at least one of gallium arsenide or germanium. The photovoltaic cell may have a lattice constant approximately in the range of 5.7 to 5.95 Angstroms and includes In_xGai-_xAs where 0.1<x< 0.8, and wherein the photovoltaic cell may have a bandgap between 1.279eV and 0.503eV. The first layered structure and the at least one second layered structure may have a thickness of 500 nm to 5 pm thick. The layered structure enables co-optimization of the lift-off etch characteristics as well as strain-induced dislocations and defects that compromise the performance of the PV cell.

[0027] FIG. 1 depicts an example of a compositionally graded buffer structure 100 between a substrate 110 and a PV cell 150. To increase the thermodynamic efficiency of the TPV system, a PV cell 150 may be grown on a compositionally graded buffer structure 100 to reduce strain, defects, and dislocations from lattice mismatch and to minimize various inefficiencies of epitaxial lift-offs.

[0028] The compositionally graded buffer structure 100 corrects problems related to epitaxial lift-offs from substrate 110. Substrate 110 may be re-used due to its high costs. An epitaxial lift-off technique may be applied for re-using the substrate 110 to grow the high- quality films. But common epitaxial lift-off technique has several drawbacks. The epitaxial liftoff technique may include depositing a thin layer (typically <200 nm) of sacrificially etched material in between the substrate 110 and the PV cell 150. The thin layer of sacrificially etched material may be a thin film of aluminum arsenide (AlAs). The thin layer can have a thickness of 5-10 nm and can be used for its high etch selectivity in hydrofluoric acid relative to GaAs and InP substrates. When immersed in hydrofluoric acid, the AlAs layer reacts to form aluminum fluoride and hydrogen bubbles. Aluminum fluoride has very low solubility in aqueous hydrofluoric acid solutions. The presence of aluminum fluoride may impede the progress of the etch by precipitating and redepositing on the non-etched surfaces, potentially damaging the substrate and making substrate removal more difficult.

[0029] Additionally, the exceptionally high aspect ratio of the etch (up to 100,000: 1) limits the ability of fresh reactants to reach the reaction front through diffusion and other processes. Bubble formation at the nanoscale may also impede the ability of the reactants to reach the reaction front. These combined effects limit the lift-off process to rates on the order of 1 mm/hour. Other lift-off techniques, such as weight-assisted lift-off, surface tension- assisted lift-off, and the addition of various solvents to the etch chemistry, may take tens of hours for substrate removal for typical wafer sizes of -150 mm. Furthermore, weight-assisted processes stress the substrate 110 and may cause yield issues due to substrate 110 breakage.

[0030] Even worse, once lift-off has been successfully completed, substrates such as GaAs exhibit enlarged surface roughness and remnant aluminum fluoride particles/deposits from the etch, which must be removed prior to the next growth cycle. This step adds cost through processing equipment, labor, and additional yield losses. It also limits the number of re-uses of the substrate 110 because each successive polishing step removes 10s of microns from the wafer, leaving it thinner and thinner until it is highly likely to break through normal handling. This problem is exacerbated in the case of InP substrates that are more brittle than GaAs substrates.

[0031] To correct these problems associated with substrate lift-off, the compositionally graded buffer structure 100 may have one or more phosphide layers to improve the lift-off layer inefficiencies. In contrast to hydrofluoric acid etches that are both extremely dangerous and suffer from the limitations described above, the compositionally graded buffer structure 100 having the phosphide layer enables hydrochloric acid etches. Hydrochloric acid etches are safer, less expensive, and exhibit near infinite etch selectivity for arsenide compounds. Furthermore, hydrochloric acid reacts strongly with phosphorus compounds, and the reaction products (e.g. indium chloride, gallium chloride, aluminum chloride) are up to lOOOx more soluble in the aqueous etch bath than the fluoride compounds generated during hydrofluoric acid etching, eliminating the problem of solid precipitates fouling the etch process and soiling/roughening the wafer during the lift-off etch. Direct re-use of GaAs substrates without mechanical polishing with a GaAs PV cell was demonstrated using a 100 nm thick InAlP-based lift-off layer with little/no degradation in cell performance.

[0032] Still referring to FIG. 1, the compositionally graded buffer structure 100 includes a first layered structure 120 having a first layer lattice constant and including a first side and a second side, the first layered structure 120 configured to be disposed above a substrate 110 such that the first side of the first layered structure 120 faces the substrate 110, the substrate 110 having a substrate lattice constant. The compositionally graded buffer structure 100 includes at least one second layered structure 130 having a second layer lattice constant and disposed on the second side of the first layered structure 120. The compositionally graded buffer structure 100 includes a PV cell 150 configured to be disposed above the first layered structure 120 and the second layered structure 130, the PV cell 150 having a PV cell lattice constant, the PV cell lattice constant being different than the substrate lattice constant.

[0033] The compositionally graded buffer structure 100 may be single crystalline or polycrystalline. The lattice constants of each successive layer within the compositionally graded buffer structure 100 change monotonically from the lattice constant of the substrate 110 to the lattice constant of the PV cell 150. For example, the first layer lattice constant matches the substrate lattice constant more than the second layered structure 130. In another example, the second layer lattice constant matches the PV cell lattice constant more than the first layered structure 120. More specifically, the first layer lattice constant may be approximately in the range of 5.65 to 5.66 Angstroms and the second layer lattice constant may have a lattice constant approximately in the range of 5.7 to 5.95 Angstroms. The purpose of the compositional gradation is to gradually reduce the strain caused by growing lattice mismatched materials on top of each other, thereby reducing the defect density of the thin film. The total thickness of the compositionally graded buffer structure 100 may depend upon the lattice difference between the substrate 110 and the PV cell 150. Additionally, the total thickness of the compositionally graded buffer structure 100 may be chosen to minimize the influence of defects upon cell performance while retaining a suitably rapid etch rate for lift off. In some embodiments, the larger the lattice difference between the substrate 110 and the PV cell 150, the thicker the compositionally graded buffer structure 100. In some embodiments, the layer or structure of the compositionally graded buffer structure 100 that interfaces with the PV cell 150 may have a threading dislocation density of less than 10⁷ cm'². [0034] The compositionally graded buffer structure 100 may include an indium phosphide layer. For example, the first layered structure 120 may include an indium phosphide semiconductor layer. The at least one second layered structure 130 may include an indium phosphide semiconductor layer.

[0035] Still referring to FIG. 1, the compositionally graded buffer structure 100 may include a phosphide layer to enable a hydrochloric etch. For example, the first layered structure 120 may include a phosphide-compound semiconductor layer including at least one element from group III of the periodic table. The at least one second layered structure 130 may include a phosphide-compound semiconductor layer including at least one element from group III of the periodic table. Group III of the periodic table includes boron (B), aluminum (Al), and gallium (Ga). These elements are characterized by having three valence electrons. The first layered structure 120 may include at least one element from group V of the periodic table. The at least one second layered structure 130 may include at least one element from group V of the periodic table. Group V of the periodic table includes nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi). These elements have five valence electrons.

[0036] In at least one embodiment, the substrate 110 may have a lattice constant of approximately 5.65-5.66 Angstroms and may include at least one of gallium arsenide or germanium. The PV cell 150 may have a lattice constant of approximately 5.7 to 5.95 Angstroms and includes In_xGai-_xAs where 0.1<x< 0.8, and wherein the PV cell 150 is associated with a bandgap between approximately 1.279eV and 0.503eV. The first layered structure 120 has a thickness of 500 nm to 5 pm, and the at least one second layered structure 130 has a thickness of 500 nm to 5 pm thick.

[0037] In at least one embodiment, the first layered structure 120 and the second layered structure 130 may include InGaAlP. In at least one embodiment, the substrate 110 may include GaAs or Ge. In some embodiments, an N-layered structure 140 may be situated between the PV cell 150 and the second layered structure 130. In at least one embodiment, the N-layered structure 140 may include InP. In some embodiments, the PV cell 150 may include InGaAs and may be a multistack layer. In some embodiments, the lattice constant may increase between the substrate 110 and the PV cell 150. In some embodiments, the lattice constant may increase between the first layered structure 120 to the second layered structure 130. In some embodiments, the lattice constant may increase between the second layered structure 130 and the N-layered structure 140. In some embodiments, the lattice constant may increase between the N-layered structure 140 and the PV cell 150. In some embodiments, the substrate lattice constant may be approximately 5.65 Angstroms and the PV cell lattice constant may be 5.87 Angstroms. The total thickness between the substrate 110 and the PV cell 150 may be between 2-3 pm.

[0038] FIG. 2 depicts an example of a compositionally graded buffer structure 100 between a substrate 110 and a PV cell 150 with the lattice constant increasing between the substrate and the PV cell. The compositionally graded buffer structure 100 may be configured for a PV cell 150 utilizing arsenide and/or antimonide-based charge extraction layers and/or window layers (e.g., GaAsSb, InAlAs, AlAsSb, AlGaAsSb) that are lattice-matched to the absorber (e.g., In_xGai-_xAs) layer for etch selectivity. For example, the terminal layer of the sacrificial compositionally graded buffer structure 100 may be lattice matched to the absorber (e.g., In_xGai-_xAs) layer.

[0039] The compositionally graded buffer structure 100 includes a first layered structure 120 having a first layer lattice constant and including a first side and a second side, the first layered structure 120 configured to be disposed above a substrate 110 such that the first side of the first layered structure 120 faces the substrate 110, the substrate 110 having a substrate lattice constant. The compositionally graded buffer structure 100 includes at least one second layered structure 130 having a second layer lattice constant and disposed on the second side of the first layered structure 120. The compositionally graded buffer structure 100 includes a PV cell 150 configured to be disposed above the first layered structure 120 and the second layered structure 130, the PV cell 150 having a PV cell lattice constant, the PV cell lattice constant being different than the substrate lattice constant. The arsenide and/or antimonide- based chemistry of the PV cell 150 may minimize the probability that functional layers of the PV cell 150 are etched during the epitaxial lift-off process.

[0040] In some embodiments, the compositionally graded buffer structure 100 includes a substrate 110 having a substrate lattice constant, and a first layered structure 120 configured to be disposed above the substrate 110, the first layered structure 120 having a first layer lattice constant. The compositionally graded buffer structure 100 includes at least one second layered structure 130 disposed above the first layered structure 120, the second layered structure 130 having a second layer lattice constant. The compositionally graded buffer structure 100 includes a PV cell 150 configured to be disposed above the first layered structure 120 and the second layered structure 130. The PV cell 150 has a PV cell lattice constant that is different than the substrate lattice constant. The first layer lattice constant matches the substrate lattice constant more than the second layered structure 130. The second layer lattice constant matches the PV cell lattice constant more than the first layered structure 120.

[0041] Still referring to FIG. 2, the compositionally graded buffer structure 100 may be single crystalline or polycrystalline. The lattice constants of each successive layer within the compositionally graded buffer structure 100 change monotonically from the lattice constant of the substrate 110 to the lattice constant of the PV cell 150. For example, the first layer lattice constant matches the substrate lattice constant more than the second layered structure 130. The second layer lattice constant matches the PV cell lattice constant more than the first layered structure 120. More specifically, the first layer lattice constant may be approximately in the range of 5.65 to 5.66 Angstroms and the second layer lattice constant may have a lattice constant approximately in the range of 5.7 to 5.95 Angstroms. The purpose of the compositional gradation is to gradually reduce the strain caused by growing lattice-mismatched materials on top of each other, thereby reducing the defect density of the thin film. The total thickness of the compositionally graded buffer structure 100 may depend upon the lattice difference between the substrate 110 and the PV cell 150, where the larger the lattice difference the thicker the compositionally graded buffer structure 100 and vice versa. Additionally, the total thickness of the compositionally graded buffer structure 100 may be chosen to minimize the influence of defects upon cell performance while retaining a suitably rapid etch rate for lift off. The layer of the compositionally graded buffer structure 100 that interfaces with the PV cell 150 may have a threading dislocation density less than 10⁷ cm'².

[0042] The compositionally graded buffer structure 100 may include a phosphide layer to enable a hydrochloric etch. For example, the first layered structure 120 may include a phosphide-compound semiconductor layer including at least one element from group III of the periodic table. The at least one second layered structure 130 may include a phosphide- compound semiconductor layer including at least one element from group III of the periodic table. Group III of the periodic table includes boron (B), aluminum (Al), and gallium (Ga). These elements are characterized by having three valence electrons. The first layered structure 120 may include at least one element from group V of the periodic table. The at least one second layered structure 130 may include at least one element from group V of the periodic table. Group V of the periodic table includes nitrogen (N), phosphorous (P), arsenic (As), antimony (Sb), and bismuth (Bi). These elements have five valence electrons. [0043] In at least one embodiment, the substrate 110 may have a lattice constant approximately in the range of 5.65-5.66 Angstroms and includes at least one of gallium arsenide or germanium. The PV cell 150 may have a lattice constant approximately in the range of 5.7 to 5.95 Angstroms and includes In_xGai-_xAs where 0.1<x<0.8. The PV cell 150 may be associated with a bandgap between 1.279eV and 0.503eV. The first layered structure 120 may have a thickness of 500 nm to 5 pm, and the at least one second layered structure 130 may have a thickness of 500 nm to 5 pm thick. The thickness of the graded buffer layer may minimize the formation of threading dislocation defects. The thickness of the graded buffer layer affords a lower aspect ratio lateral etch, which results in faster reaction limited etch rates as opposed to the diffusion-limited etch regime of previous HF and HC1 efforts. The enhanced etch rate will reduce the cost of the overall process and reduce susceptibility of the substrate 110 to contamination and undesired etching.

[0044] Still referring to FIG. 2, the first layered structure 120 and the second layered structure 130 may include a phosphide material. In at least one embodiment, the substrate 110 may include GaAs or Ge. In some embodiments, an N-layered structure 140 may be situated between the PV cell 150 and the second layered structure 130. In at least one embodiment, the N-layered structure 140 may include of phosphide material. In some embodiments, the N- layered structure 140 may be lattice matched to the PV cell 150 where the PV cell 150 includes In_xGai-_xAs. In some embodiments, the second layered structure 130 or the N-layered structure may have a threading dislocation density less than ~10⁶ - 10⁷ cm'² to ensure a high quantum efficiency. In some embodiments, the lattice constant may increase between the substrate 110 and the PV cell 150. In some embodiments, the lattice constant may increase between the first layered structure 120 to the second layered structure 130. In some embodiments, the lattice constant may increase between the second layered structure 130 and the N-layered structure 140. In some embodiments, the lattice constant may increase between the N-layered structure 140 and the PV cell 150. In some embodiments, the substrate lattice constant may be approximately 5.65 Angstroms and the PV cell lattice constant may be 5.87 Angstroms. The total thickness between the substrate 110 and the PV cell 150 may be between 2-3 pm.

[0045] FIG. 3 depicts a graph illustrating the relationship between the bandgap and lattice constant for different III-V materials. The compositionally graded buffer structure 100 layers are comprised of phosphide based materials with at least one group III and one group V element. In some embodiments, the PV cell 150 may be comprised of arsenide or antimonide based materials to enable epitaxial lift off using hydrochloric acid resulting in high etch selectivity between the PV cell 150/substrate HO tothe compositionally graded buffer structure 100 layers. Advantages of the compositionally graded buffer structure 100 include the hydrochloric acid epitaxial lift off enables faster and cleaner etches, which minimizes wafer re-preparation complexity compared to hydrofluoric acid based lift offs.

[0046] FIG. 4 depicts an emitter 410, the PV cell 150, and a bandpass assembly 425 in a TPV system 400. The PV cell 150 may be removed from the substrate 110 and placed into the TPV system 400. The TPV system 400 may be configured to convert the thermal radiation emitted from an emitter 410 into electrical energy via a PV cell 150. TPV power systems require no moving parts, which allows them to be inherently quiet and reduces maintenance requirements relative to engines in the same design space.

[0047] The emitter 410, PV cell 150, and the bandpass assembly 425 may be placed into a housing 405. The bandpass assembly 425 may be interposed between emitter 410 and the PV cell 150 in the housing 405. The housing 405 may enclose the PV cell 150 and the bandpass assembly 425. In some embodiments, the PV cell 150 may be positioned in the bottom portion of the housing 405 and configured to only receive light emitted from the emitter 410. In some embodiments, the bandpass assembly 425 may be situated within the housing 405 such that only light from the emitter 410 within a specific wavelength range may pass through the bandpass filter. Vacuum 430 may exist between the PV cell 150 and the bandpass assembly 425 and between the emitter 410 and the bandpass assembly 425. Vacuum 430 may be created by the housing 405 being airtight to the external environment. Insulation 450 may be interposed along the edges and crevices of the housing 405 to make the housing 405 airtight or to reduce parasitic thermal and optical losses. Additionally, the insulation 450 may include a porous or semi-porous material to make the TPV system 400 more efficient.

[0048] The emitter 410 may be a broadband thermal emitter. The emitter 410 may be configured to have an operating temperature of at least 1250K. The emitter 410 may be a gray body emitter, a black body emitter, or a spectrally- selective emitter. The spectrally-selective emitter may be configured to radiate energy at the bandgap determined by the PV cell 150. For example, the emitter 410 may be configured to emit a majority of thermal radiation within a wavelength range between 0.7 pm and 10 pm. In some embodiments, the emitter 410 may emit a minimal amount of thermal radiation outside the wavelength range between 0.7 pm and 10 pm. In some embodiments, the emitter 410 may radiate more thermal energy than what can be efficiently absorbed at the PV cell 150.

[0049] Still referring to FIG. 4, the PV cell 150 may be configured to absorb the thermal radiation from the emitter 410 and convert the thermal radiation to electricity. The PV cell 150 may have a myriad of specifications to effectively absorb incident photons from the emitter 410 and convert them into electrical energy. These specifications of the emitter 410 should match the specifications of the PV cell 150. For example, spectral losses may be minimized as the thermally-emitted photons from the emitter 410 match the PV cells bandgap energy. Otherwise, thermally-emitted photons with energy less than the PV cell bandgap cannot be converted into electrical energy due to the photovoltaic effect and thermally-emitted photons with energy greater than the bandgap of the PV cell 150 may also lose energy due to band edge relaxation and cause overheating of the PV cell 150. In another example, the conversion efficiency of the PV cell 150 may be maximized when the emitter 410 matches the highest quantum efficiency of the PV cell 150. A high quantum efficiency results from most or all photons incident to the PV cell 150 generating an electron that is extracted externally from the PV cell 150.

[0050] The bandgap assembly may be configured to control the spectrum incident on the PV cell 150 by passing a range of wavelengths of light and reflecting or absorbing light outside the wavelength range. That is, the bandpass assembly 425 may filter and/or reflect light outside of the wavelength range required by the PV cell 150. Additionally, the bandpass assembly 425 may recycle photons by reflecting them back at the emitter 410 to be absorbed.

[0051] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the embodiments. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. [0052] Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” may be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about.”

[0053] The many features and advantages of the disclosure are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the disclosure which fall within the true spirit and scope of the disclosure. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the disclosure to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the disclosure.

[0054] In the descriptions above and in the claims, phrases such as “at least one of’ or “one or more of’ may occur followed by a conjunctive list of elements or features. The term “and/or” may also occur in a list of two or more elements or features. Unless otherwise implicitly or explicitly contradicted by the context in which it is used, such a phrase is intended to mean any of the listed elements or features individually or any of the recited elements or features in combination with any of the other recited elements or features. For example, the phrases “at least one of A and B;” “one or more of A and B;” and “A and/or B” are each intended to mean “A alone, B alone, or A and B together.” A similar interpretation is also intended for lists including three or more items. For example, the phrases “at least one of A, B, and C;” “one or more of A, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, B alone, C alone, A and B together, A and C together, B and C together, or A and B and C together.” Use of the term “based on,” above and in the claims is intended to mean, “based at least in part on,” such that an unrecited feature or element is also permissible.

[0055] The implementations set forth in the foregoing description do not represent all implementations consistent with the subject matter described herein. Instead, they are merely some examples consistent with aspects related to the described subject matter. Although a few variations have been described in detail herein, other modifications or additions are possible. In particular, further features and/or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and sub-combinations of the disclosed features and/or combinations and sub- combinations of one or more features further to those disclosed herein. In addition, the logic flows depicted in the accompanying figures and/or described herein do not necessarily require the particular order shown, or sequential order, to achieve desirable results. The scope of the following claims may include other implementations or embodiments.

[0056] The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.

[0057] While the foregoing is directed to implementations of the present disclosure, other and further implementations of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

WHAT IS CLAIMED IS:

1. An apparatus comprising: a first layered structure having a first side and a second side opposing the first side, the first layered structure having a first layer lattice constant, the first layered structure configured to be disposed above a substrate such that the first side of the first layered structure faces the substrate, the substrate having a substrate lattice constant different than the first layer lattice constant; at least one second layered structure disposed on the second side of the first layered structure, the at least one second layered structure having a second layer lattice constant; and a photovoltaic cell configured to be disposed above the first layered structure and the second layered structure, the photovoltaic cell having a photovoltaic cell lattice constant, the photovoltaic cell lattice constant being different than the substrate lattice constant, wherein the first layer lattice constant matches the substrate lattice constant more than the second layered structure, and wherein the second layer lattice constant matches the photovoltaic cell lattice constant more than the first layered structure.

2. The apparatus of claim 1, wherein the first layered structure includes a phosphide- compound semiconductor layer including at least one element from group III of the periodic table and wherein the at least one second layered structure includes a phosphide-compound semiconductor layer including at least one element from group III of the periodic table.

3. The apparatus of claim 2, wherein the phosphide-compound semiconductor layer is configured to react strongly with a hydrochloric acid etch.

4. The apparatus of claim 1, wherein the first layered structure includes at least one element from group V of the periodic table and wherein the at least one second layered structure includes at least one element from group V of the periodic table.

5. The apparatus of claim 1, wherein the substrate has a lattice constant of approximately 5.65-5.66 Angstroms and includes at least one of gallium arsenide or germanium.

6. The apparatus of claim 1 wherein the photovoltaic cell has a lattice constant of approximately 5.7 to 5.95 Angstroms and includes In_xGai-_xAs where 0.1<x< 0.8.

7. The apparatus of claim 1, wherein the photovoltaic cell is associated with a bandgap between 1.279eV and 0.503eV.

8. The apparatus of claim 1, wherein the first layered structure has a thickness of 500 nm to 5 pm thick and the at least one second layered structure has a thickness of 500 nm to 5 pm.

9. The apparatus of claim 1, wherein the photovoltaic cell includes at least one of an arsenide-based charge extraction layer or an antimonide-based charge extraction layer for etch selectivity.

10. The apparatus of claim 9, wherein the at least one of the arsenide-based charge extraction layer or the antimonide-based charge extraction layer is lattice-matched to an absorber (In_xGai-_xAs) layer.

11. The apparatus of claim 1, wherein the first layer lattice constant and the second layer lattice constant change monotonically from the substrate lattice constant to the photovoltaic cell lattice constant.

12. The apparatus of claim 1, wherein the first layered structure and the second layered structure are at least one of a single crystalline or a polycrystalline.

13. The apparatus of claim 1, wherein the second layered structure has a threading dislocation density less than 10⁷ cm'².

14. The apparatus of claim 1, wherein the first layered structure includes an indiumphosphide semiconductor layer and wherein the at least one second layered structure includes an indium-phosphide semiconductor layer.

15. An apparatus comprising: a substrate having a substrate lattice constant; a first layered structure disposed above the substrate, the first layered structure having a first layer lattice constant; a second layered structure disposed above the first layered structure, the second layered structure having a second layer lattice constant; and a photovoltaic cell configured to be disposed above the first layered structure and the second layered structure, the photovoltaic cell having a photovoltaic cell lattice constant, the photovoltaic cell lattice constant being different than the substrate lattice constant, wherein the first layer lattice constant matches the substrate lattice constant more than the second layered structure, and wherein the second layer lattice constant matches the photovoltaic cell lattice constant more than the first layered structure.

16. The apparatus of claim 1, wherein the substrate has a lattice constant of approximately 5.65-5.66 Angstroms and includes at least one of gallium arsenide or germanium.

17. The apparatus of claim 1 wherein the photovoltaic cell has a lattice constant of approximately 5.7 to 5.95 Angstroms and includes In_xGai-_xAs where 0.1<x< 0.8.

18. The apparatus of claim 1, wherein the photovoltaic cell is associated with a bandgap between 1.279eV and 0.503eV.

19. The apparatus of claim 1, wherein the first layered structure includes a phosphide- compound semiconductor layer including at least one element from group III of the periodic table and wherein the at least one second layered structure includes a phosphide-compound semiconductor layer including at least one element from group III of the periodic table.

20. The apparatus of claim 1, wherein the photovoltaic cell includes at least one of an arsenide-based charge extraction layer or an antimonide-based charge extraction layer for etch selectivity.