WO2011118902A1 - Method for fabricating metamorphic compound semiconductor solar cell - Google Patents

Method for fabricating metamorphic compound semiconductor solar cell Download PDF

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Publication number
WO2011118902A1
WO2011118902A1 PCT/KR2010/009041 KR2010009041W WO2011118902A1 WO 2011118902 A1 WO2011118902 A1 WO 2011118902A1 KR 2010009041 W KR2010009041 W KR 2010009041W WO 2011118902 A1 WO2011118902 A1 WO 2011118902A1
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solar cell
layer
buffer layer
cell structure
graded buffer
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French (fr)
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Yon Kil Jeong
Seung-Hyeon Moon
Dong-Won Park
Hee-Sang Shim
Seung-Hwan Oh
Jae-Kwang Lee
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Gwangju Institute of Science and Technology
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Gwangju Institute of Science and Technology
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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F71/00—Manufacture or treatment of devices covered by this subclass
    • H10F71/139—Manufacture or treatment of devices covered by this subclass using temporary substrates
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F10/00—Individual photovoltaic cells, e.g. solar cells
    • H10F10/10—Individual photovoltaic cells, e.g. solar cells having potential barriers
    • H10F10/14—Photovoltaic cells having only PN homojunction potential barriers
    • H10F10/142—Photovoltaic cells having only PN homojunction potential barriers comprising multiple PN homojunctions, e.g. tandem cells
    • H10F10/1425—Inverted metamorphic multi-junction [IMM] photovoltaic cells
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F71/00—Manufacture or treatment of devices covered by this subclass
    • H10F71/127—The active layers comprising only Group III-V materials, e.g. GaAs or InP
    • H10F71/1272—The active layers comprising only Group III-V materials, e.g. GaAs or InP comprising at least three elements, e.g. GaAlAs or InGaAsP
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
    • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
    • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
    • H10P14/29—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by the substrates
    • H10P14/2901—Materials
    • H10P14/2907—Materials being Group IIIA-VA materials
    • H10P14/2911—Arsenides
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
    • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
    • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
    • H10P14/32—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by intermediate layers between substrates and deposited layers
    • H10P14/3202—Materials thereof
    • H10P14/3214—Materials thereof being Group IIIA-VA semiconductors
    • H10P14/3218—Phosphides
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
    • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
    • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
    • H10P14/32—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by intermediate layers between substrates and deposited layers
    • H10P14/3202—Materials thereof
    • H10P14/3214—Materials thereof being Group IIIA-VA semiconductors
    • H10P14/3221—Arsenides
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
    • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
    • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
    • H10P14/32—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by intermediate layers between substrates and deposited layers
    • H10P14/3242—Structure
    • H10P14/3244—Layer structure
    • H10P14/3248—Layer structure consisting of two layers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
    • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
    • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
    • H10P14/32—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials characterised by intermediate layers between substrates and deposited layers
    • H10P14/3242—Structure
    • H10P14/3244—Layer structure
    • H10P14/3254—Graded layers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10P—GENERIC PROCESSES OR APPARATUS FOR THE MANUFACTURE OR TREATMENT OF DEVICES COVERED BY CLASS H10
    • H10P14/00—Formation of materials, e.g. in the shape of layers or pillars
    • H10P14/20—Formation of materials, e.g. in the shape of layers or pillars of semiconductor materials
    • H10P14/34—Deposited materials, e.g. layers
    • H10P14/3402—Deposited materials, e.g. layers characterised by the chemical composition
    • H10P14/3414—Deposited materials, e.g. layers characterised by the chemical composition being group IIIA-VIA materials
    • H10P14/3418—Phosphides
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/544—Solar cells from Group III-V materials

Definitions

  • the present invention relates to a method of fabricating a metamorphic compound semiconductor solar cell, and more particularly, to a method of fabricating a metamorphic compound semiconductor solar cell using a lattice-mismatched growth technique.
  • a solar cell using a compound semiconductor particularly, a Group III-V compound semiconductor, which has a direct transition bandgap, exhibits a higher light absorbance and is more capable of absorbing solar energy incident on the solar cell than a silicon solar cell having an indirect transition bandgap.
  • the compound semiconductor solar cell is under restrictions on the conditions of lattice-matching with a substrate.
  • the restrictions on the lattice-matching conditions mainly occur in stack-type solar cells, such as tandem solar cells.
  • tandem solar cell may theoretically achieve an efficiency of about 40% or lower when an upper solar cell and a lower solar cell have energy bandgaps of about 1.5 eV and 0.9 eV, respectively, a variation in energy bandgap leads to a variation in lattice constant due to the characteristics of the material, thereby causing crystalline defects.
  • the tandem solar cell gives rise to a lattice mismatch with a typical GaAs-based substrate, thereby causing a reliability drop due to the destruction of a solar cell device.
  • a method of fabricating a metamorphic compound semiconductor solar cell includes forming a graded buffer layer having a graded lattice constant from a bottom surface thereof toward a top surface thereof on a support substrate.
  • a sacrificial layer is formed on the graded buffer layer.
  • a solar cell structure is formed on the sacrificial layer.
  • a support structure is formed on the solar cell structure.
  • the sacrificial layer is selectively etched to separate the solar cell structure from the graded buffer layer. The separated solar cell structure is bonded to a host substrate.
  • the lowermost surface of the graded buffer layer may have a lattice match with the support substrate, and the uppermost surface of the graded buffer layer may have a lattice match with a bottom surface of the solar cell structure.
  • the graded buffer layer may include one selected from the group consisting of indium gallium arsenide (InGaAs), indium aluminum arsenide (InAlAs), and indium aluminum phosphide (InAlP).
  • the graded buffer layer may have a graded In content from the bottom surface thereof toward the top surface thereof.
  • a target buffer layer may be further formed on the graded buffer layer.
  • the target buffer layer may have a lattice match with the uppermost surface of the graded buffer layer.
  • the sacrificial layer may have a lattice mismatch with the uppermost surface of the graded buffer layer.
  • the sacrificial layer may have a thickness of about 5 nm or less.
  • the sacrificial layer may be an Al x Ga 1 - x As(0.8 ⁇ x ⁇ 1) layer.
  • the solar cell structure may be a multi-junction solar cell.
  • the solar cell structure may include a first solar cell, a tunnel junction layer, and a second solar cell, which are sequentially stacked, the first solar cell may absorb an energy of about 0.93 eV to 1.2 eV, and the second solar cell may absorb an energy of about 1.4 eV to 1.7 eV.
  • the solar cell structure may include a single solar cell that absorbs an energy of about 0.36 eV to 1.43 eV.
  • the support structure may include an adhesion layer and a support layer, which are sequentially formed on the solar cell structure.
  • the separated solar cell structure and the host substrate may be bonded to each other by Van der Waals force.
  • the support structure After bonding the separated solar cell structure to the host substrate, the support structure may be removed.
  • the present invention provides a method of fabricating a metamorphic compound semiconductor solar cell in which a hetero-substrate can be bonded to a solar cell structure by adopting a thin-film separation technique using a metamorphic buffer layer.
  • an energy absorption layer having a desired combination can be selectively applied, and a volume occupied by a compound semiconductor in the solar cell can be minimized.
  • FIGS. 1 through 5 are cross-sectional views illustrating a method of fabricating a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention.
  • FIG. 6 is a cross-sectional view of a solar cell structure used for fabrication of a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention.
  • FIGS. 1 through 5 are cross-sectional views illustrating a method of fabricating a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention
  • FIG. 6 is a cross-sectional view of a solar cell structure used for fabrication of a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention.
  • a metamorphic compound semiconductor solar cell may include a solar cell structure 130 disposed on an host substrate 500.
  • the solar cell structure 130 may include a single solar cell or a stack-type multi-junction solar cell.
  • an absorption energy bandgap of the solar cell structure 130 may range from 0.36 to 1.43 eV or from 0.36 to 1.34 eV depending on the type of a substrate that is initially used for the fabrication of the compound semiconductor solar cell according to the embodiment of the present invention under the restrictions on the conditions of lattice-matching.
  • the initially used substrate is formed of gallium arsenide (GaAs)
  • the absorption energy bandgap of the solar cell structure 130 may range from 0.36 eV to 1.43 eV
  • the initially used substrate is formed of indium phosphide (InP)
  • the solar cell structure 130 may range from 0.36 eV to 1.34 eV.
  • the solar cell structure 130 when the solar cell structure 130 includes a stack-type multi-junction solar cell, the solar cell structure 130 refers to a stack-type solar cell, such as one of a double-junction solar cell, a triple-junction solar cell, and a quadruple-junction solar cell.
  • the number of stacked solar cells is not limited.
  • the solar cell structure 130 applied to the compound semiconductor solar cell according to the present embodiment may include a double junction solar cell, which includes a first solar cell 131 disposed on the host substrate 500, a tunnel junction layer 133 disposed on the first solar cell 131, and a second solar cell 135 disposed on the tunnel junction layer 133.
  • the first and second solar cells 131 and 135 may have the same lattice constant. Also, energy absorbable by the second solar cell 135 may be higher than energy absorbable by the first solar cell 131.
  • the absorption energy of the first solar cell 131 may range from 0.93 eV to 1.2 eV
  • the absorption energy of the second solar cell 135 may range from 1.4 eV to 1.7 eV.
  • FIGS. 1 through 5 a method of fabricating a compound semiconductor solar cell according to an exemplary embodiment of the present invention will be described with reference to FIGS. 1 through 5.
  • a buffer layer 110 is formed on a substrate 100.
  • the substrate 100 may be a single crystalline substrate and formed of various compound semiconductor materials.
  • the substrate 100 may be formed of one of GaAs and InP, which may be selected according to a solar cell structure 130 to be formed later.
  • the first solar cell 131 of the solar cell structure 130 may be formed of indium gallium arsenide (InGaAs).
  • the first solar cell 131 of the solar cell structure 130 may be formed of indium arsenic phosphide (InAsP).
  • the buffer layer 110 may be a metamorphic buffer layer, which may have a stacked structure. That is, the buffer layer 110 may include a graded buffer layer 110a serving as a first buffer layer and a target buffer layer 110b serving as a second buffer layer disposed on the graded buffer layer 110a.
  • a lowermost portion of the graded buffer layer 110a may have a lattice match with the substrate 100. Specifically, the lowermost portion of the graded buffer layer 110a may have substantially the same lattice constant as the substrate 100. Also, an uppermost portion of the graded buffer layer 110a may have a lattice match with a lowermost portion of the solar cell structure 130. Specifically, the uppermost portion of the graded buffer layer 110a may have substantially the same lattice constant as the lowermost portion of the solar cell structure 130.
  • the target buffer layer 110b may have a lattice match with the uppermost portion of the graded buffer layer 110a.
  • the target buffer layer 110b may have the same lattice constant as the uppermost portion of the graded buffer layer 110a.
  • the uppermost portion of the graded buffer layer 110a has a lattice mismatch with the substrate 100.
  • the uppermost portion of the graded buffer layer 110a may have a higher lattice constant than the lowermost portion thereof.
  • the buffer layer 110 may include an In-containing ternary compound, for example, one selected from the group consisting of InGaAs, indium aluminum arsenide (InAlAs), indium aluminum phosphide (InAlP), and an equivalent thereof.
  • the graded buffer layer 110a may be grown while varying an In content. The growth of the graded buffer layer 110a may be started in a low In-content state and stopped at a time point when the In content reaches a desired content, that is, at a time point when the graded buffer layer 110a has substantially the same lattice constant with that of the lowermost portion of the solar cell structure 130.
  • a method of growing the graded buffer layer 110a may be classified into a linear method and a nonlinear method.
  • the linear method refers to forming the graded buffer layer 110a such that a degree of lattice mismatch with the substrate 100 is gradually changed (e.g., increased). That is, a variation in the degree of lattice mismatch may be constantly changed over a growth time of the graded buffer layer 110a.
  • the nonlinear method may be categorized as either a convex method or a concave method.
  • the convex method is characterized by sharply varying a lattice constant in an initial stage of the growth of the graded buffer layer 110a.
  • a variation in the degree of lattice mismatch with the substrate 100 is in the form of a square root over time.
  • the concave method is characterized by sharply varying a lattice constant in a final stage of the growth of the graded buffer layer 110a.
  • a variation in the degree of lattice mismatch with the substrate 100 is in the form of a parabola over time.
  • the lattice constant of the graded buffer layer 110a and the degree of lattice mismatch with the substrate 100 may be changed by gradually varying the In content of the graded buffer layer 110a.
  • the target buffer layer 110b may be formed on the graded buffer layer 110a so that the entire target buffer layer 110b can maintain substantially the same composition and lattice constant as the uppermost portion of the graded buffer layer 110a. In this case, the target buffer layer 110b may have a lattice mismatch relationship with the substrate 100.
  • the target buffer layer 110b may be formed to a thickness of about 1 ⁇ m or less.
  • a sacrificial layer 120 is formed on the buffer layer 110.
  • the sacrificial layer 120 may have a lattice mismatch relationship with the target buffer layer 110b.
  • the sacrificial layer 120 having the lattice mismatch relationship with the target buffer layer 110b may prevent crystalline defects, such as dislocation which may occur in the buffer layer 110, from spreading into the solar cell structure 130.
  • the sacrificial layer 120 may be formed to a thickness of about 5 nm or less to suppress the occurrence of the crystalline defects therein. In this case, the spreading of the crystalline defects into the solar cell structure 130 may be prevented once more.
  • the sacrificial layer 120 may be an Al-containing compound layer, for example, an Al-containing binary or ternary compound layer.
  • the sacrificial layer 120 may be formed of Al x Ga 1 - x As(1 ⁇ x ⁇ 0.8). This is because a solution obtained by diluting hydrofluoric (HF) acid with deionized (DI) water used as a typical etch solution may effectively etch Al.
  • the sacrificial layer 120 may be formed of a material having a high Al content. In this case, the selective etching of the sacrificial layer 120 may be further facilitated.
  • a solar cell structure 130 having a thin-film shape is formed on the sacrificial layer 120.
  • a material forming the solar cell structure 130 may have substantially the same lattice constant as the uppermost portion of the graded buffer layer 110a or the target buffer layer 110b. Meanwhile, the lattice constant of the material forming the solar cell structure 130 may have a degree of lattice mismatch of about 3% or less with that of the substrate 100.
  • the solar cell structure 130 may include a single solar cell formed of a compound semiconductor or a multi-junction solar cell formed of a plurality of compound semiconductors.
  • the multi-junction solar cell refers to a stack-type solar cell, such as a double-junction solar cell, a triple-junction solar cell, and a quadruple-junction solar cell, according to the number of stacked thin-film solar cells, but the present invention is not limited in the number of the stacked thin-film solar cells.
  • the thin-film solar cell structure 130 may include a double-junction solar cell obtained by stacking two solar cells.
  • the double-junction solar cell may include a first solar cell 131 disposed on the sacrificial layer 120, a tunnel junction layer 133 disposed on the first solar cell 131, and a second solar cell 135 disposed on the tunnel junction layer 133.
  • the multi-junction solar cell when used as the solar cell structure 130, an energy absorption layer of each solar cell may be selectively used.
  • the first solar cell 131 may include an energy absorption layer formed using a material with an absorption energy bandgap of about 0.93 eV to 1.2 eV
  • the second solar cell 135 may include an energy absorption layer formed using a material with an absorption energy bandgap of about 1.4 eV to 1.7 eV.
  • the first solar cell 131 may include a first-type lower back-surface-field (BSF) layer 131a, a first-type lower base layer 131b, a second-type lower emitter layer 131c, and a second lower front-surface-field (FSF) layer 131d.
  • BSF back-surface-field
  • FSF second lower front-surface-field
  • the present invention is not limited thereto, and the first-type lower BSF layer 131a and the second-type lower FSF layer 131d may be omitted.
  • the first-type lower BSF layer 131a may have a greater bandgap than the first-type lower base layer 131b, and the second-type lower FSF layer 131d may have a greater bandgap than the second-type lower emitter layer 131c.
  • a first-type dopant doped into the first-type lower BSF layer 131a may have a higher concentration than a first-type dopant doped into the first-type lower base layer 131b
  • a second-type dopant doped into the second-type lower FSF layer 131d may have a higher concentration than a second-type dopant doped into the second-type lower emitter layer 131c.
  • the second solar cell 135 may include a first-type upper BSF layer 135a, a first-type upper base layer 135b, a second-type upper emitter layer 135c, and a second-type upper FSF layer 135d, which are sequentially stacked.
  • the present invention is not limited thereto, and the first-type upper BSF layer 135a and the second-type upper FSF layer 135d may be omitted.
  • the first-type upper BSF layer 135a may have a greater bandgap than the first-type upper base layer 135b, and the second-type upper FSF layer 135d may have a greater bandgap than the second-type upper emitter layer 135c.
  • a first-type dopant doped into the first-type upper BSF layer 135a may have a higher concentration than a first-type dopant doped into the first-type upper base layer 135b.
  • a second dopant doped into the second-type upper FSF layer 135d may have a higher concentration than a second-type dopant doped into the second-type upper emitter layer 135c.
  • the first type and the second type may be a p type and an n type, respectively, or be an n type and a p type, respectively.
  • the tunnel junction layer 133 may include a second-type tunnel junction layer 133a and a first-type tunnel junction layer 133b.
  • a second-type dopant doped into the second-type tunnel junction layer 133a may have a higher concentration than a second-type dopant doped into the second-type lower FSF layer 131d disposed adjacent thereto or have a higher concentration than a second-type dopant doped into the second-type lower emitter layer 131c when the second-type lower FSF layer 131d is omitted.
  • a first-type dopant doped into the first-type tunnel junction layer 133b may have a higher concentration than a second-type dopant doped into the first-type upper BSF layer 135a disposed adjacent thereto or have a higher concentration than a second-type dopant doped into the first-type upper base layer 135b when the first-type upper BSF layer 135a is omitted.
  • the tunnel junction layer 133 may prevent the generation and diffusion of a depletion layer due to a p-n junction at an interface between the first and second solar cells 131 and 135, thereby further facilitating transport of electrons and holes.
  • a heavily-doped p-n junction may be formed between the first and second solar cells 131 and 135 so that an energy barrier can be reduced to facilitate the transport of electrons and holes.
  • the first-type lower BSF layer 131a, the first-type lower base layer 131b, the second-type lower emitter layer 131c, and the second-type lower FSF layer 131d may be a p+-type InGaP layer, a p-type InGaAs layer, an n-type InGaAs layer, and an n+-type InGaP layer, respectively.
  • the first-type lower base layer 131b and the second-type lower emitter layer 131c may have a bandgap of about 0.93 eV
  • each of the first-type lower BSF layer 131a and the second-type lower FSF layer 131d may have a bandgap of about 1.41 eV.
  • the first-type upper BSF layer 135a, the first-type upper base layer 135b, the second-type upper emitter layer 135c, and the second-type upper FSF layer 135d may be a p+-type InAlP layer, a p-type InGaP layer, an n-type InGaP layer, and an n+-type InAlP layer, respectively.
  • the p+-type or n+-type layer may have a heavy doping level of ⁇ 10 18 /cm 3
  • the p-type or n-type layer may have a moderate doping level of ⁇ 10 17 /cm 3 .
  • each of the first-type upper base layer 135b and the second-type upper emitter layer 135c may have a bandgap of about 1.41 eV
  • each of the first-type upper BSF layer 135a and the second-type upper FSF layer 135d may have a bandgap of about 1.78 eV
  • the second-type tunnel junction layer 133a may be an n++-type InGaAs layer
  • the first-type tunnel junction layer 133b may be a p++-type InGaAs layer.
  • Materials forming the first solar cell 131, the tunnel junction layer 133, and the second solar cell 135 may have a lattice constant of about 0.5799 nm. When the substrate 100 is formed of GaAs, a degree of lattice mismatch with the substrate 100 may be about 2.6%.
  • a support structure 300 is formed on the solar cell structure 130 to support the solar cell structure 130 and prevent damage to the solar cell structure 130 due to deformation that may occur during a thin-film separation process of the solar cell structure 130.
  • the support structure 300 may be a flexible layer and include an adhesion layer 310 and a support layer 320 disposed on the adhesion layer 310.
  • the adhesion layer 310 may have a thickness of several hundreds of nm and include an adhesive polymer, for example, polydimethylglutarimide, to facilitate the adhesion of the adhesion layer 310 to the surface of the solar cell structure 130.
  • the support layer 320 may have a thickness of several ⁇ m so as to support the separated solar cell structure 130 and be formed of a polymer, such as rubber and/or bisazide.
  • the adhesion layer 310 and/or the support layer 320 are polymer layers, the adhesion layer 310 and/or the support layer 320 may be cured through thermal treatment.
  • the sacrificial layer 120 may be etched, thereby separating the solar cell structure 130 from the buffer layer 110 and the substrate 100.
  • the sacrificial layer 120 when the sacrificial layer 120 is an Al-containing compound layer, the sacrificial layer 120 may be selectively etched using a solution obtained by diluting 12.5wt% HF acid with DI water so that the solar cell structure 130 can be separated from the buffer layer 110 and the substrate 100.
  • the present invention is not limited thereto, and another solution capable of selectively etching the sacrificial layer 120 may be selected according to the kind of the sacrificial layer 120.
  • the substrate 100 on which the buffer layer 110 is formed may be reused to fabricate another solar cell structure 130.
  • a cleaning solution may be DI water.
  • the solar cell structure 130 may be dipped in DI water so that the material of the sacrificial layer 120 remaining on the surface of the solar cell structure 130 can be removed to clean the surface of the solar cell structure 130.
  • the solar cell structure 130 may be bonded to a host substrate 500.
  • the solar cell structure 130 and the host substrate 500 may be bonded to each other by Van der Waals bonding.
  • the solar cell structure 130 and the host substrate 500 may be appropriately processed.
  • the surface of the solar cell structure 130 is coated with the DI water during the cleaning of the solar cell structure 130 and the surface of the host substrate 500 is processed with plasma to have a hydrophilic property, the solar cell structure 130 and the host substrate 500 may be easily bonded to each other.
  • the cleaning solution i.e., DI water
  • the support structure 300 may be removed, thereby completing the fabrication of the compound semiconductor solar cell to which a hetero-host substrate 500 is bonded.
  • the removal of the support structure 300 may be performed using a plasma processing method.
  • the host substrate 500 may be formed of a material, such as silicon, a ceramic material, a plastic material, or a flexible material, which is different from an initial substrate required for the formation of the solar cell structure 130. Meanwhile, in the present invention, the host substrate 500 is not limited to the above-described materials and may be formed of any material different from the substrate 100.

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Abstract

A method of fabricating a metamorphic compound semiconductor solar cell is provided. The method includes forming a graded buffer layer on a support substrate, the graded buffer layer having a graded lattice constant from a bottom surface thereof toward a top surface thereof. A sacrificial layer is formed on the graded buffer layer. A solar cell structure is formed on the sacrificial layer. A support structure is formed on the solar cell structure. The sacrificial layer is selectively etched to separate the solar cell structure from the graded buffer layer. The separated solar cell structure is bonded to a host substrate.

Description

METHOD FOR FABRICATING METAMORPHIC COMPOUND SEMICONDUCTOR SOLAR CELL
The present invention relates to a method of fabricating a metamorphic compound semiconductor solar cell, and more particularly, to a method of fabricating a metamorphic compound semiconductor solar cell using a lattice-mismatched growth technique.
In general, a solar cell using a compound semiconductor, particularly, a Group III-V compound semiconductor, which has a direct transition bandgap, exhibits a higher light absorbance and is more capable of absorbing solar energy incident on the solar cell than a silicon solar cell having an indirect transition bandgap.
However, the compound semiconductor solar cell is under restrictions on the conditions of lattice-matching with a substrate. In particular, the restrictions on the lattice-matching conditions mainly occur in stack-type solar cells, such as tandem solar cells.
Although the tandem solar cell may theoretically achieve an efficiency of about 40% or lower when an upper solar cell and a lower solar cell have energy bandgaps of about 1.5 eV and 0.9 eV, respectively, a variation in energy bandgap leads to a variation in lattice constant due to the characteristics of the material, thereby causing crystalline defects. In particular, the tandem solar cell gives rise to a lattice mismatch with a typical GaAs-based substrate, thereby causing a reliability drop due to the destruction of a solar cell device.
In order to solve the foregoing and/or other problems, it is an object of the present invention to provide a method of fabricating a metamorphic compound semiconductor solar cell using a lattice-mismatched growth technique.
According to an exemplary embodiment of the present invention, a method of fabricating a metamorphic compound semiconductor solar cell is disclosed. The method includes forming a graded buffer layer having a graded lattice constant from a bottom surface thereof toward a top surface thereof on a support substrate. A sacrificial layer is formed on the graded buffer layer. A solar cell structure is formed on the sacrificial layer. A support structure is formed on the solar cell structure. The sacrificial layer is selectively etched to separate the solar cell structure from the graded buffer layer. The separated solar cell structure is bonded to a host substrate.
The lowermost surface of the graded buffer layer may have a lattice match with the support substrate, and the uppermost surface of the graded buffer layer may have a lattice match with a bottom surface of the solar cell structure. The graded buffer layer may include one selected from the group consisting of indium gallium arsenide (InGaAs), indium aluminum arsenide (InAlAs), and indium aluminum phosphide (InAlP). The graded buffer layer may have a graded In content from the bottom surface thereof toward the top surface thereof.
Before forming the sacrificial layer, a target buffer layer may be further formed on the graded buffer layer. The target buffer layer may have a lattice match with the uppermost surface of the graded buffer layer.
The sacrificial layer may have a lattice mismatch with the uppermost surface of the graded buffer layer. The sacrificial layer may have a thickness of about 5 nm or less. The sacrificial layer may be an AlxGa1 - xAs(0.8≤x≤1) layer.
The solar cell structure may be a multi-junction solar cell. The solar cell structure may include a first solar cell, a tunnel junction layer, and a second solar cell, which are sequentially stacked, the first solar cell may absorb an energy of about 0.93 eV to 1.2 eV, and the second solar cell may absorb an energy of about 1.4 eV to 1.7 eV.
In another case, the solar cell structure may include a single solar cell that absorbs an energy of about 0.36 eV to 1.43 eV.
The support structure may include an adhesion layer and a support layer, which are sequentially formed on the solar cell structure.
The separated solar cell structure and the host substrate may be bonded to each other by Van der Waals force.
After bonding the separated solar cell structure to the host substrate, the support structure may be removed.
According to the present invention as described above, the present invention provides a method of fabricating a metamorphic compound semiconductor solar cell in which a hetero-substrate can be bonded to a solar cell structure by adopting a thin-film separation technique using a metamorphic buffer layer.
Accordingly, in the present invention, an energy absorption layer having a desired combination can be selectively applied, and a volume occupied by a compound semiconductor in the solar cell can be minimized.
FIGS. 1 through 5 are cross-sectional views illustrating a method of fabricating a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention.
FIG. 6 is a cross-sectional view of a solar cell structure used for fabrication of a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention.
The detailed features and functions of the present invention will be more clearly understood with reference to the drawings and exemplary embodiments thereof.
The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of the invention, and is not intended to represent the only forms in which the present invention may be constructed or utilized. It is to be understood that the same or equivalent functions of the invention set forth herein that may be accomplished by different embodiments are intended to be encompassed within the spirit and scope of this invention. Also, it is to be understood by those skilled in the art that some features shown in the drawings are exaggerated for brevity and the drawings and components are not necessarily illustrated in appropriate proportions. Like reference numerals in the drawings denote like elements, and thus a description thereof will be omitted.
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the appended drawings.
FIGS. 1 through 5 are cross-sectional views illustrating a method of fabricating a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention, and FIG. 6 is a cross-sectional view of a solar cell structure used for fabrication of a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention.
Referring to FIGS. 5 and 6, a metamorphic compound semiconductor solar cell according to an exemplary embodiment of the present invention may include a solar cell structure 130 disposed on an host substrate 500. In this case, the solar cell structure 130 may include a single solar cell or a stack-type multi-junction solar cell.
When the solar cell structure 130 includes a single solar cell, an absorption energy bandgap of the solar cell structure 130 may range from 0.36 to 1.43 eV or from 0.36 to 1.34 eV depending on the type of a substrate that is initially used for the fabrication of the compound semiconductor solar cell according to the embodiment of the present invention under the restrictions on the conditions of lattice-matching. When the initially used substrate is formed of gallium arsenide (GaAs), the absorption energy bandgap of the solar cell structure 130 may range from 0.36 eV to 1.43 eV, and when the initially used substrate is formed of indium phosphide (InP), the solar cell structure 130 may range from 0.36 eV to 1.34 eV.
In addition, when the solar cell structure 130 includes a stack-type multi-junction solar cell, the solar cell structure 130 refers to a stack-type solar cell, such as one of a double-junction solar cell, a triple-junction solar cell, and a quadruple-junction solar cell. However, in the present invention, the number of stacked solar cells is not limited.
For example, the solar cell structure 130 applied to the compound semiconductor solar cell according to the present embodiment may include a double junction solar cell, which includes a first solar cell 131 disposed on the host substrate 500, a tunnel junction layer 133 disposed on the first solar cell 131, and a second solar cell 135 disposed on the tunnel junction layer 133.
In this case, the first and second solar cells 131 and 135 may have the same lattice constant. Also, energy absorbable by the second solar cell 135 may be higher than energy absorbable by the first solar cell 131. For example, the absorption energy of the first solar cell 131 may range from 0.93 eV to 1.2 eV, and the absorption energy of the second solar cell 135 may range from 1.4 eV to 1.7 eV.
Hereinafter, a method of fabricating a compound semiconductor solar cell according to an exemplary embodiment of the present invention will be described with reference to FIGS. 1 through 5.
Referring to FIG. 1, a buffer layer 110 is formed on a substrate 100. The substrate 100 may be a single crystalline substrate and formed of various compound semiconductor materials. Specifically, the substrate 100 may be formed of one of GaAs and InP, which may be selected according to a solar cell structure 130 to be formed later. For example, when the substrate 100 is formed of GaAs, the first solar cell 131 of the solar cell structure 130 may be formed of indium gallium arsenide (InGaAs). Also, when the substrate 100 is formed of InP, the first solar cell 131 of the solar cell structure 130 may be formed of indium arsenic phosphide (InAsP).
The buffer layer 110 may be a metamorphic buffer layer, which may have a stacked structure. That is, the buffer layer 110 may include a graded buffer layer 110a serving as a first buffer layer and a target buffer layer 110b serving as a second buffer layer disposed on the graded buffer layer 110a.
In this case, a lowermost portion of the graded buffer layer 110a may have a lattice match with the substrate 100. Specifically, the lowermost portion of the graded buffer layer 110a may have substantially the same lattice constant as the substrate 100. Also, an uppermost portion of the graded buffer layer 110a may have a lattice match with a lowermost portion of the solar cell structure 130. Specifically, the uppermost portion of the graded buffer layer 110a may have substantially the same lattice constant as the lowermost portion of the solar cell structure 130. The target buffer layer 110b may have a lattice match with the uppermost portion of the graded buffer layer 110a. Specifically, the target buffer layer 110b may have the same lattice constant as the uppermost portion of the graded buffer layer 110a. In this case, the uppermost portion of the graded buffer layer 110a has a lattice mismatch with the substrate 100. For example, the uppermost portion of the graded buffer layer 110a may have a higher lattice constant than the lowermost portion thereof.
The buffer layer 110 may include an In-containing ternary compound, for example, one selected from the group consisting of InGaAs, indium aluminum arsenide (InAlAs), indium aluminum phosphide (InAlP), and an equivalent thereof. In this case, the graded buffer layer 110a may be grown while varying an In content. The growth of the graded buffer layer 110a may be started in a low In-content state and stopped at a time point when the In content reaches a desired content, that is, at a time point when the graded buffer layer 110a has substantially the same lattice constant with that of the lowermost portion of the solar cell structure 130.
A method of growing the graded buffer layer 110a may be classified into a linear method and a nonlinear method. The linear method refers to forming the graded buffer layer 110a such that a degree of lattice mismatch with the substrate 100 is gradually changed (e.g., increased). That is, a variation in the degree of lattice mismatch may be constantly changed over a growth time of the graded buffer layer 110a. The nonlinear method may be categorized as either a convex method or a concave method. The convex method is characterized by sharply varying a lattice constant in an initial stage of the growth of the graded buffer layer 110a. A variation in the degree of lattice mismatch with the substrate 100 is in the form of a square root over time. Unlike the convex method, the concave method is characterized by sharply varying a lattice constant in a final stage of the growth of the graded buffer layer 110a. A variation in the degree of lattice mismatch with the substrate 100 is in the form of a parabola over time. As described above, when the graded buffer layer 110a is formed of an In-containing ternary compound, the lattice constant of the graded buffer layer 110a and the degree of lattice mismatch with the substrate 100 may be changed by gradually varying the In content of the graded buffer layer 110a.
The target buffer layer 110b may be formed on the graded buffer layer 110a so that the entire target buffer layer 110b can maintain substantially the same composition and lattice constant as the uppermost portion of the graded buffer layer 110a. In this case, the target buffer layer 110b may have a lattice mismatch relationship with the substrate 100. The target buffer layer 110b may be formed to a thickness of about 1 ㎛ or less.
A sacrificial layer 120 is formed on the buffer layer 110. The sacrificial layer 120 may have a lattice mismatch relationship with the target buffer layer 110b. The sacrificial layer 120 having the lattice mismatch relationship with the target buffer layer 110b may prevent crystalline defects, such as dislocation which may occur in the buffer layer 110, from spreading into the solar cell structure 130. The sacrificial layer 120 may be formed to a thickness of about 5 nm or less to suppress the occurrence of the crystalline defects therein. In this case, the spreading of the crystalline defects into the solar cell structure 130 may be prevented once more.
The sacrificial layer 120 may be an Al-containing compound layer, for example, an Al-containing binary or ternary compound layer. For example, the sacrificial layer 120 may be formed of AlxGa1 - xAs(1≥x≥0.8). This is because a solution obtained by diluting hydrofluoric (HF) acid with deionized (DI) water used as a typical etch solution may effectively etch Al. To this end, the sacrificial layer 120 may be formed of a material having a high Al content. In this case, the selective etching of the sacrificial layer 120 may be further facilitated.
A solar cell structure 130 having a thin-film shape is formed on the sacrificial layer 120. A material forming the solar cell structure 130 may have substantially the same lattice constant as the uppermost portion of the graded buffer layer 110a or the target buffer layer 110b. Meanwhile, the lattice constant of the material forming the solar cell structure 130 may have a degree of lattice mismatch of about 3% or less with that of the substrate 100.
The solar cell structure 130 may include a single solar cell formed of a compound semiconductor or a multi-junction solar cell formed of a plurality of compound semiconductors. The multi-junction solar cell refers to a stack-type solar cell, such as a double-junction solar cell, a triple-junction solar cell, and a quadruple-junction solar cell, according to the number of stacked thin-film solar cells, but the present invention is not limited in the number of the stacked thin-film solar cells.
For example, as shown in FIG. 6, the thin-film solar cell structure 130 may include a double-junction solar cell obtained by stacking two solar cells.
The double-junction solar cell may include a first solar cell 131 disposed on the sacrificial layer 120, a tunnel junction layer 133 disposed on the first solar cell 131, and a second solar cell 135 disposed on the tunnel junction layer 133.
In addition, when the multi-junction solar cell is used as the solar cell structure 130, an energy absorption layer of each solar cell may be selectively used. For example, in the case of the double-junction solar cell, the first solar cell 131 may include an energy absorption layer formed using a material with an absorption energy bandgap of about 0.93 eV to 1.2 eV, and the second solar cell 135 may include an energy absorption layer formed using a material with an absorption energy bandgap of about 1.4 eV to 1.7 eV.
The above-described solar cell structure 130 including the double-junction solar cell will now be described in detail. To begin with, the first solar cell 131 may include a first-type lower back-surface-field (BSF) layer 131a, a first-type lower base layer 131b, a second-type lower emitter layer 131c, and a second lower front-surface-field (FSF) layer 131d. However, the present invention is not limited thereto, and the first-type lower BSF layer 131a and the second-type lower FSF layer 131d may be omitted. The first-type lower BSF layer 131a may have a greater bandgap than the first-type lower base layer 131b, and the second-type lower FSF layer 131d may have a greater bandgap than the second-type lower emitter layer 131c. Also, a first-type dopant doped into the first-type lower BSF layer 131a may have a higher concentration than a first-type dopant doped into the first-type lower base layer 131b, and a second-type dopant doped into the second-type lower FSF layer 131d may have a higher concentration than a second-type dopant doped into the second-type lower emitter layer 131c.
The second solar cell 135 may include a first-type upper BSF layer 135a, a first-type upper base layer 135b, a second-type upper emitter layer 135c, and a second-type upper FSF layer 135d, which are sequentially stacked. However, the present invention is not limited thereto, and the first-type upper BSF layer 135a and the second-type upper FSF layer 135d may be omitted. The first-type upper BSF layer 135a may have a greater bandgap than the first-type upper base layer 135b, and the second-type upper FSF layer 135d may have a greater bandgap than the second-type upper emitter layer 135c. Also, a first-type dopant doped into the first-type upper BSF layer 135a may have a higher concentration than a first-type dopant doped into the first-type upper base layer 135b. A second dopant doped into the second-type upper FSF layer 135d may have a higher concentration than a second-type dopant doped into the second-type upper emitter layer 135c.
The first type and the second type may be a p type and an n type, respectively, or be an n type and a p type, respectively.
The tunnel junction layer 133 may include a second-type tunnel junction layer 133a and a first-type tunnel junction layer 133b. A second-type dopant doped into the second-type tunnel junction layer 133a may have a higher concentration than a second-type dopant doped into the second-type lower FSF layer 131d disposed adjacent thereto or have a higher concentration than a second-type dopant doped into the second-type lower emitter layer 131c when the second-type lower FSF layer 131d is omitted. Also, a first-type dopant doped into the first-type tunnel junction layer 133b may have a higher concentration than a second-type dopant doped into the first-type upper BSF layer 135a disposed adjacent thereto or have a higher concentration than a second-type dopant doped into the first-type upper base layer 135b when the first-type upper BSF layer 135a is omitted. The tunnel junction layer 133 may prevent the generation and diffusion of a depletion layer due to a p-n junction at an interface between the first and second solar cells 131 and 135, thereby further facilitating transport of electrons and holes. In other words, since the second-type tunnel junction layer 133a has a high second-type dopant concentration and the first-type tunnel junction layer 133b has a high first-type dopant concentration, a heavily-doped p-n junction may be formed between the first and second solar cells 131 and 135 so that an energy barrier can be reduced to facilitate the transport of electrons and holes.
For example, the first-type lower BSF layer 131a, the first-type lower base layer 131b, the second-type lower emitter layer 131c, and the second-type lower FSF layer 131d may be a p+-type InGaP layer, a p-type InGaAs layer, an n-type InGaAs layer, and an n+-type InGaP layer, respectively. In this case, the first-type lower base layer 131b and the second-type lower emitter layer 131c may have a bandgap of about 0.93 eV, and each of the first-type lower BSF layer 131a and the second-type lower FSF layer 131d may have a bandgap of about 1.41 eV. The first-type upper BSF layer 135a, the first-type upper base layer 135b, the second-type upper emitter layer 135c, and the second-type upper FSF layer 135d may be a p+-type InAlP layer, a p-type InGaP layer, an n-type InGaP layer, and an n+-type InAlP layer, respectively. In this case, the p+-type or n+-type layer may have a heavy doping level of ~1018/cm3, while the p-type or n-type layer may have a moderate doping level of ~1017/cm3. In this case, each of the first-type upper base layer 135b and the second-type upper emitter layer 135c may have a bandgap of about 1.41 eV, and each of the first-type upper BSF layer 135a and the second-type upper FSF layer 135d may have a bandgap of about 1.78 eV. Also, the second-type tunnel junction layer 133a may be an n++-type InGaAs layer, and the first-type tunnel junction layer 133b may be a p++-type InGaAs layer. Materials forming the first solar cell 131, the tunnel junction layer 133, and the second solar cell 135 may have a lattice constant of about 0.5799 nm. When the substrate 100 is formed of GaAs, a degree of lattice mismatch with the substrate 100 may be about 2.6%.
Referring to FIG. 2, a support structure 300 is formed on the solar cell structure 130 to support the solar cell structure 130 and prevent damage to the solar cell structure 130 due to deformation that may occur during a thin-film separation process of the solar cell structure 130.
In this case, the support structure 300 may be a flexible layer and include an adhesion layer 310 and a support layer 320 disposed on the adhesion layer 310. The adhesion layer 310 may have a thickness of several hundreds of nm and include an adhesive polymer, for example, polydimethylglutarimide, to facilitate the adhesion of the adhesion layer 310 to the surface of the solar cell structure 130. Also, the support layer 320 may have a thickness of several ㎛ so as to support the separated solar cell structure 130 and be formed of a polymer, such as rubber and/or bisazide. When the adhesion layer 310 and/or the support layer 320 are polymer layers, the adhesion layer 310 and/or the support layer 320 may be cured through thermal treatment.
Referring to FIG. 3, the sacrificial layer 120 may be etched, thereby separating the solar cell structure 130 from the buffer layer 110 and the substrate 100.
For example, when the sacrificial layer 120 is an Al-containing compound layer, the sacrificial layer 120 may be selectively etched using a solution obtained by diluting 12.5wt% HF acid with DI water so that the solar cell structure 130 can be separated from the buffer layer 110 and the substrate 100. However, the present invention is not limited thereto, and another solution capable of selectively etching the sacrificial layer 120 may be selected according to the kind of the sacrificial layer 120.
After the solar cell structure 130 is separated from the buffer layer 110 and the substrate 100, the substrate 100 on which the buffer layer 110 is formed may be reused to fabricate another solar cell structure 130.
By cleaning the separated solar cell structure 130, the remaining material of the sacrificial layer 120 may be removed. In this case, a cleaning solution may be DI water. During the cleaning process, the solar cell structure 130 may be dipped in DI water so that the material of the sacrificial layer 120 remaining on the surface of the solar cell structure 130 can be removed to clean the surface of the solar cell structure 130.
Referring to FIG. 4, the solar cell structure 130 may be bonded to a host substrate 500. The solar cell structure 130 and the host substrate 500 may be bonded to each other by Van der Waals bonding. To this end, the solar cell structure 130 and the host substrate 500 may be appropriately processed. When the surface of the solar cell structure 130 is coated with the DI water during the cleaning of the solar cell structure 130 and the surface of the host substrate 500 is processed with plasma to have a hydrophilic property, the solar cell structure 130 and the host substrate 500 may be easily bonded to each other.
Referring to FIG. 5, the cleaning solution (i.e., DI water) remaining around the solar cell structure 130 may be evaporated and the support structure 300 may be removed, thereby completing the fabrication of the compound semiconductor solar cell to which a hetero-host substrate 500 is bonded. The removal of the support structure 300 may be performed using a plasma processing method.
In this case, the host substrate 500 may be formed of a material, such as silicon, a ceramic material, a plastic material, or a flexible material, which is different from an initial substrate required for the formation of the solar cell structure 130. Meanwhile, in the present invention, the host substrate 500 is not limited to the above-described materials and may be formed of any material different from the substrate 100.
While the invention has been shown and described with reference to m certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (14)

  1. A method of fabricating a compound semiconductor solar cell, comprising:
    forming a graded buffer layer on a support substrate, the graded buffer layer having a graded lattice constant from a bottom surface thereof toward a top surface thereof;
    forming a sacrificial layer on the graded buffer layer;
    forming a solar cell structure on the sacrificial layer;
    forming a support structure on the solar cell structure;
    separating the solar cell structure from the graded buffer layer by selectively etching the sacrificial layer; and
    bonding the separated solar cell structure to a host substrate.
  2. The method of claim 1, wherein a lowermost surface of the graded buffer layer has a lattice match with the support substrate, and an uppermost surface of the graded buffer layer has a lattice match with a lowermost surface of the solar cell structure.
  3. The method of claim 1, wherein the graded buffer layer includes one selected from the group consisting of indium gallium arsenide (InGaAs), indium aluminum arsenide (InAlAs), and indium aluminum phosphide (InAlP).
  4. The method of claim 3, wherein the graded buffer layer has a graded indium (In) content from the bottom surface thereof toward the top surface thereof.
  5. The method of claim 1, further comprising, before forming the sacrificial layer, forming a target buffer layer on the graded buffer layer.
  6. The method of claim 1 or 2, wherein the sacrificial layer has a lattice mismatch with the uppermost surface of the graded buffer layer.
  7. The method of claim 1, wherein the sacrificial layer has a thickness of about 5 nm or less.
  8. The method of claim 1, wherein the sacrificial layer is an AlxGa1 - xAs (0.8≤x≤1) layer.
  9. The method of claim 1, wherein the solar cell structure is a multi-junction solar cell.
  10. The method of claim 9, wherein the solar cell structure includes a first solar cell, a tunnel junction layer, and a second solar cell, which are sequentially stacked,
    wherein the first solar cell absorbs an energy of about 0.93 eV to 1.2 eV, and
    the second solar cell absorbs an energy of about 1.4 eV to 1.7 eV.
  11. The method of claim 1, wherein the solar cell structure includes a single solar cell that absorbs an energy of about 0.36 eV to 1.43 eV.
  12. The method of claim 1, wherein the support structure includes an adhesion layer and a support layer, which are sequentially formed on the solar cell structure.
  13. The method of claim 1, wherein the separated solar cell structure and the host substrate are bonded to each other by Van der Waals force.
  14. The method of claim 1, further comprising, after bonding the separated solar cell structure to the host substrate, removing the support structure.
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