CN114649437A - Germanium multi-junction solar cell and preparation method thereof - Google Patents

Germanium multi-junction solar cell and preparation method thereof Download PDF

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CN114649437A
CN114649437A CN202011500335.1A CN202011500335A CN114649437A CN 114649437 A CN114649437 A CN 114649437A CN 202011500335 A CN202011500335 A CN 202011500335A CN 114649437 A CN114649437 A CN 114649437A
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cell
subcell
layer
germanium
solar cell
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王俊
占荣
李华
王伟明
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Jiangsu Yixing Derong Technology Co ltd
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    • H01L31/04—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices
    • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier
    • H01L31/068—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by at least one potential-jump barrier or surface barrier the potential barriers being only of the PN homojunction type, e.g. bulk silicon PN homojunction solar cells or thin film polycrystalline silicon PN homojunction solar cells
    • H01L31/0687—Multiple junction or tandem solar cells
    • H—ELECTRICITY
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    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
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    • H01L31/0264—Inorganic materials
    • H01L31/0328—Inorganic materials including, apart from doping materials or other impurities, semiconductor materials provided for in two or more of groups H01L31/0272 - H01L31/032
    • H01L31/0336—Inorganic materials including, apart from doping materials or other impurities, semiconductor materials provided for in two or more of groups H01L31/0272 - H01L31/032 in different semiconductor regions, e.g. Cu2X/CdX hetero-junctions, X being an element of Group VI of the Periodic System
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    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/036—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes
    • H01L31/0392—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate
    • H01L31/03926—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including thin films deposited on metallic or insulating substrates ; characterised by specific substrate materials or substrate features or by the presence of intermediate layers, e.g. barrier layers, on the substrate comprising a flexible substrate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
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    • 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

Abstract

A germanium multijunction solar cell and a method of fabricating the same are disclosed. The germanium multijunction solar cell includes: and the Ge sub-cell, the InGaAs sub-cell, the InGaAsP sub-cell and the InAlGaP sub-cell are sequentially stacked. According to the germanium multijunction solar cell and the manufacturing method thereof, the manufacturing cost of the germanium multijunction solar cell can be reduced, the photoelectric conversion efficiency of the germanium multijunction solar cell is improved, and the thinning and the flexibility of the germanium multijunction solar cell are realized.

Description

Germanium multi-junction solar cell and preparation method thereof
Technical Field
The invention belongs to the field of solar cells, and particularly relates to a germanium multi-junction solar cell and a preparation method thereof.
Background
Germanium multijunction solar cells are receiving more and more attention due to the advantages of high efficiency, high stability, high radiation resistance and the like. However, the cost of germanium multijunction solar cell products has been high and is currently used substantially only in the aerospace field. The existing germanium multi-junction solar cell products all use a germanium substrate, and after a germanium junction is obtained by phosphorus diffusion on the germanium substrate, other multi-junction cell structures are extended. However, the global reserve of germanium element is only about 8600 tons of metal, the global reserve of gold is also 8.6 ten thousand tons, and germanium element is more scarce than gold in terms of the rare reserve. Therefore, the substrate cost is always one of the most important factors for further development and civil use of the germanium multi-junction solar cell.
There are now some cases of epitaxial III-V cells with Si as the substrate. For example: in patent No. CN102779865A, a silicon-based triple-junction solar cell with germanium as a tunneling junction is described, which is provided with a Si bottom cell, a GaAs middle cell and a GaInP top cell, and reduces the substrate cost, but the forbidden band width of each junction is not the result of optimization. In patent No. CN103077981B, a flexible substrate silicon-based multi-junction stacked thin-film solar cell is described, which employs a multi-junction stacked solar cell deposited on a flexible substrate at one time, and thermal damage and ion damage are inevitably caused by using plasma assistance. Patent number US20060021565a1 describes an InGaP/GaAs/Si triple junction cell, which is formed by bonding an InGaP/GaAs double junction peeled off on a GaAs substrate to a Si cell to form an InGaP/GaAs/Si triple junction cell, thereby realizing effective reduction of substrate cost, and the cell conversion efficiency is higher than that of a Si cell but lower than that of a general GaAs triple junction cell structure. These three patents use Si-based substrates, but still use Si junction cells as junction cells to achieve cost reduction, but the photoelectric conversion efficiency is not optimal.
Among the current multi-junction solar cells, Ge/GaInAs/InGaP triple-junction solar cells are increasingly applied due to high photoelectric conversion efficiency, and particularly widely applied in the space efficient cell industry. However, the currents of the Ge/GaInAs/InGaP three-junction solar cell are not matched, and the current of the Ge bottom cell is about twice that of the other two-junction cell, so that great waste is caused, and the further improvement of the cell efficiency is restricted. Therefore, the development from three junctions to four, five or even higher junctions is an important direction. Compared with a three-junction cell, the four-junction cell can reduce heat loss, improve the utilization rate of the solar cell to solar spectrum, and improve the open voltage and fill factor to obtain higher conversion efficiency.
The Ge sub-cell in the existing Ge-based three-junction cell Ge/GaInAs/InGaP forms an n-type Ge layer by diffusing phosphorus or arsenic to the surface of a p-type Ge substrate, and forms a pn junction together with the p-type Ge substrate, thereby generating a photovoltaic effect. The disadvantage of this process is that the Ge sub-cell has no back electric field layer, which results in a low open circuit voltage of the Ge sub-cell.
The existing germanium four-junction solar cell is generally obtained by positive epitaxial growth on a germanium substrate, wherein the thickness of the germanium substrate is about 140 microns, and the germanium substrate is rigid. In the prior art, a substrate is ground to reduce the thickness of the whole battery, so as to realize the thinning of the battery. In the process of thinning the germanium-based multi-junction solar cell by using a substrate grinding process, the back of the germanium substrate is pasted on foaming glue or paraffin for grinding by a common means, but the thickness uniformity difference of the thinned germanium substrate is larger than 30 micrometers due to the poor thickness uniformity of the foaming glue or paraffin and the poor thickness uniformity of the interface between the germanium substrate and the foaming glue, so that the thinned ground germanium-based multi-junction solar cell has poor uniformity and can have problems when a cell module is packaged.
A GaInP/GaAs/Ge quadruple junction solar cell is described in patent No. CN102790119A, where the current matching problem is optimized by 2 Ge cells. An AlGaInP/InGaAs/InGaAs/Ge forward mismatched four-junction solar cell is described in patent No. CN107871799A, and both patents provide a four-junction solar cell which can effectively improve the photoelectric conversion efficiency. However, there are many limitations to the bandgap designs in these two patents, especially under the requirement of the optimal bandgap combination of 1.90eV, 1.4eV, 1.00eV, and 0.67eV for the four-junction cell, which is difficult to be realized.
Patent No. CN102790117B describes a GaInP/GaAs/InGaNAs/Ge four-junction solar cell and a method for manufacturing the same, which is bonded to a silicon support substrate by peeling the cell after flip-chip extension of GaInP/GaAs/InGaNAs/Ge on the GaAs substrate. The four-junction battery does not achieve flexible thinning. Although the conversion efficiency of the four-junction battery is improved compared with that of a three-junction battery in design, the low mobility of the InGaNAs material can restrict IQE to be more than 95%, so that the conversion efficiency is difficult to be improved significantly. Meanwhile, the cost of the GaAs substrate used for epitaxy is higher.
In patent No. CN108054231A, a Si substrate-based four-junction solar cell is described, which is a four-junction cell that realizes InGaP/GaAs/InGaAs/Si epitaxially on a Si substrate, and the four-junction cell is not combined with the optimal band gap, and has an improved efficiency compared with the three-junction cell, but is lower than the optimal band gap. Moreover, the epitaxial layer is thick, and a flexible substrate is not used, so that the flexible thinning of the germanium four-junction battery cannot be realized.
Disclosure of Invention
It is an object of the present application to provide a germanium multijunction solar cell that solves and ameliorates at least one of the problems of the prior art.
Specifically, one of the objectives of the present application is to provide a germanium multijunction solar cell to realize an optimal forbidden bandwidth combination between junctions of the cell, thereby improving the photoelectric conversion efficiency of the germanium multijunction solar cell.
Another object of the present application is to reduce the production cost of the germanium multijunction solar cell to facilitate the wide application of the germanium multijunction solar cell.
The application further aims to realize the thinning and flexibility of the germanium multi-junction solar cell so as to meet the flexible application scene of the germanium multi-junction solar cell.
To achieve the above object, one aspect of the present invention provides a germanium multijunction solar cell, comprising: and the Ge sub-cell, the InGaAs sub-cell, the InGaAsP sub-cell and the InAlGaP sub-cell are sequentially stacked.
According to one embodiment, the Ge sub-cell comprises: InGaP, InGaAs or InAlGaAS back electric field layers, Ge base regions, Ge emitter regions, and InGaP or AlInP window layers.
According to one embodiment, the thickness of the Ge emitter region is greater than the thickness of the Ge base region, and the doping concentration of the Ge base region is 3-5 × 1018m-3The Ge emitting region has a doping concentration of 3-5 × 1018m-3。
According to one embodiment, the InGaAs subcell comprises: an InAlGaP or InAlGaAs back electric field layer, an InGaAs base region, an InGaAs or InGaP emitter region, and an AlInP window layer.
According to one embodiment, the InGaAsP subcell comprises: InAlGaP or InAlGaAs back electric field layer, InGaAsP base region, InAlGaAs or InGaP emitter region and AlInP window layer.
According to one embodiment, the InAlGaP subcell comprises: InAlGaP or InAlGaAs back electric field layer, InAlGaP base region, InAlGaP emitter region and AlInP window layer.
According to one embodiment, the Ge subcell, the InGaAs subcell, the InGaAsP subcell, and the InAlGaP subcell have a tunnel junction therebetween.
According to one embodiment, the Ge subcell has a first compositionally graded buffer layer between the InGaAs subcells.
According to one embodiment, a second compositionally graded buffer layer is provided between the InGaAsP subcell and the InAlGaP subcell.
According to one embodiment, the germanium multijunction solar cell further comprises a Si substrate, and the Ge subcell, the InGaAs subcell, the InGaAsP subcell, and the InAlGaP subcell are sequentially disposed on the Si substrate.
According to one embodiment, a SiGe alloy buffer layer is provided between the Si substrate and the Ge subcell.
According to one embodiment, the germanium multijunction solar cell further comprises a flexible substrate, and the Ge subcell, the InGaAs subcell, the InGaAsP subcell, and the InAlGaP subcell are sequentially disposed on the flexible substrate.
According to one embodiment, the flexible substrate is a thin film metal substrate or a polyimide substrate.
Another aspect of the present invention provides a method for fabricating a germanium multijunction solar cell, comprising:
providing a Si substrate; and
and epitaxially growing a Ge sub-cell, an InGaAs sub-cell, an InGaAsP sub-cell and an InAlGaP sub-cell on the Si substrate in sequence.
According to one embodiment, a sacrificial layer is epitaxially grown on a Si substrate prior to epitaxially growing a Ge subcell on the Si substrate, and,
the method further comprises the following steps: and stripping the Si substrate by corroding the sacrificial layer, and connecting the flexible substrate on the Ge sub-cell stripped from the Si substrate.
The application provides a germanium multijunction solar cell and a preparation method thereof, which have the following advantages:
1. compared with the Ge-based four-junction battery in the prior art, the Ge-based multi-junction solar battery comprises a Ge sub-battery, an InGaAs sub-battery, an InGaAsP sub-battery and an InAlGaP sub-battery, wherein the Ge sub-battery, the InGaAs sub-battery, the InGaAsP sub-battery and the InAlGaP sub-battery have the optimal forbidden bandwidth combination, solar spectral energy can be fully utilized, current mismatch among the sub-batteries and heat energy loss in a photoelectric conversion process are reduced, optimal current matching among the sub-batteries is achieved, and therefore higher photoelectric conversion efficiency can be obtained.
2. Compared with a conventional triple-junction cell structure, the germanium multi-junction solar cell increases an InGaAs sub-cell with a junction forbidden band width of about 1.0eV, the open-circuit voltage of the InGaAs sub-cell can be increased by 0.6V, and the photoelectric conversion efficiency of the cell can be improved.
3. The germanium multijunction solar cell adopts the Si substrate to replace the germanium substrate as the substrate, so that the production cost of the germanium multijunction solar cell can be greatly reduced.
4. According to the germanium multijunction solar cell, the Ge sub cell is obtained through epitaxial growth on the Si substrate, and a back electric field layer and a window layer of the Ge sub cell can be epitaxially grown, so that the open-circuit voltage of the Ge sub cell is improved;
5. the germanium multi-junction solar cell has the advantages that InGaAs, InAlGaAs or InGaP buffer layer materials exist between the Ge sub-cell and the InGaAs sub-cell and/or between the InGaAsP sub-cell and the InAlGaP sub-cell, epitaxial growth of materials with different lattice constants can be achieved, the forbidden bandwidth of each sub-cell can be expanded, each sub-cell is enabled to have wider forbidden bandwidth, and photoelectric conversion efficiency of the cell is improved.
6. According to the germanium multi-junction solar cell, the doping concentration and the pn junction structure of the Ge sub-cell are designed, and the thin base region with high doping concentration and the thick emitter region structure with high doping concentration are adopted, so that the thin film of the Ge sub-cell is realized.
7. According to the method, the separation of the germanium four-junction solar cell and the Si substrate is realized by adopting a sacrificial layer stripping technology, and then the Si substrate is replaced by the flexible substrate, so that the thin film and the flexibility of a cell device are realized.
Drawings
Figure 1 is a schematic diagram of the basic structure of a germanium four-junction solar cell according to one embodiment of the present invention;
fig. 2 is a schematic structural diagram of a germanium four-junction solar cell according to another embodiment of the present invention.
Figure 3 is a schematic diagram of a germanium four-junction solar cell in preparation according to another embodiment of the present invention.
Fig. 4 is a schematic structural diagram of the completed ge-quadruple junction solar cell of fig. 3.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the drawings of the embodiments of the present invention. In the embodiments of the present invention and the drawings, the same reference numerals refer to the same meanings unless otherwise defined. In the drawings used to describe embodiments of the invention, the thickness of a layer or region is exaggerated for clarity; in the drawings of some embodiments of the present invention, only the structures related to the inventive concept are shown, and other structures may refer to general designs. In addition, some drawings only illustrate the basic structure of the embodiments of the present invention, and the detailed parts are omitted.
Unless defined otherwise, technical or scientific terms used herein shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention belongs. The use of "first," "second," and similar terms in the present application do not denote any order, quantity, or importance, but rather the terms are used to distinguish one element from another. The word "comprising" or "comprises", and the like, is intended in an open-ended sense, and does not exclude the presence of other elements, components, portions or items than those listed. The terms "connected" or "coupled" and the like are not restricted to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "upper", "lower", "left", "right", and the like are used merely to indicate relative positional relationships, and when the absolute position of the object being described is changed, the relative positional relationships may also be changed accordingly. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being "on" or "under" another element, it can be "directly on" or "under" the other element or intervening elements may be present.
Fig. 1 shows a basic structural schematic of a germanium four-junction solar cell 100 according to an exemplary embodiment of the present invention. As shown in fig. 1, from the backlight side B to the light incident side a, the ge-quadruple junction solar cell 100 sequentially includes: ge subcell 110, InGaAs subcell 120, InGaAsP subcell 130, InAlGaP subcell 140. The Ge subcell 110, the InGaAs subcell 120, the InGaAsP subcell 130, and the InAlGaP subcell 140 may be obtained by sequentially epitaxially growing on a Si substrate.
Specifically, the Ge sub-cell 110 may include an InGaP or InGaAs back electric field layer 111, a Ge base region 112, a Ge emitter region 113, and an InGaP window layer 114.
The InGaAs subcell 120 may include an InGaP or inalgas back electric field layer 121, an InGaP emitter region 122, an InGaAs base region 123, and an InGaP window layer 124. The PN junction formed between the InGaAs base region 123 and the InGaP emitter region 122 is a heterojunction.
The InGaAsP subcell 130 may include an inalgas back electric field layer 131, an inalgas emitter region 132, an InGaAsP base region 133, and an AlInP window layer 134.
The InAlGaP subcell 140 may include an InAlGaP back electric field layer 141, an InAlGaP base region 142, an InAlGaP emitter region 143, and an AlInP window layer 144. The PN junction formed between the InAlGaP base region 142 and the InAlGaP emitter region 143 is a homojunction.
The Ge sub-cell 110 and the InGaAs sub-cell 120 can be connected by a first tunneling junction 115, and the In isThe GaAs subcell 120 and the InGaAsP subcell 130 may be connected by a second tunnel junction 125, and the InGaAsP subcell 130 and the InAlGaP subcell 140 may be connected by a third tunnel junction 135. The first tunnel junction 115 may be made of GaAs, InGaP, or other semiconductor materials, and has a doping concentration of 1 × 1019cm-3As described above, the second tunnel junction 125 may be InAlGaAs, InGaP or other semiconductor materials, and the doping concentration may reach 1 × 1019cm-3As above, the third tunnel junction 135 may be inalgas or other semiconductor materials, and the doping concentration may reach 1 × 1019cm-3The above.
In addition, as known to those skilled in the art, the ge-quadruple junction solar cell of the above embodiment may further include an ohmic contact layer 150, upper and lower electrodes, an anti-reflective film, etc., which will not be described in detail herein.
Compared with the existing germanium triple-junction cell, the germanium quadruple-junction solar cell increases an InGaAs sub-cell with a junction forbidden band width of about 1.0eV, the open-circuit voltage of the germanium quadruple-junction solar cell can be increased by 0.6V, and the photoelectric conversion efficiency of the cell can be improved.
According to the germanium four-junction solar cell, the Ge sub-cell, the InGaAs sub-cell, the InGaAsP sub-cell and the InAlGaP sub-cell can achieve the optimal forbidden bandwidth combination of 1.90eV, 1.4eV, 1.00eV and 0.67eV, so that solar spectral energy can be fully utilized, current mismatch among the sub-cells and heat energy loss in the photoelectric conversion process are reduced, the optimal current matching among the sub-cells is achieved, and higher photoelectric conversion efficiency is obtained.
In addition, the Ge sub-cell of the present embodiment is obtained by epitaxial growth on a Si substrate, and the Ge sub-cell has a back electric field layer and a window layer. Compared with the Ge sub-cell formed by diffusing phosphorus or arsenic to the surface of a p-type Ge substrate to form an n-type Ge layer and forming a pn junction together with the p-type Ge substrate in the multi-junction Ge cell in the prior art, the germanium four-junction solar cell of the embodiment can improve the open-circuit voltage of the Ge sub-cell by adding a back electric field layer.
Fig. 2 is a schematic diagram of a germanium four-junction solar cell 200 according to another embodiment of the present invention. The germanium four junction solar cell of this embodiment retains the silicon substrate and adds compositionally graded buffer layers between the Ge and InGaAs subcells and between the InGaAsP and InAlGaP subcells.
Specifically, the germanium quadruple junction solar cell 200 comprises a Ge subcell 210, an InGaAs subcell 220, an InGaAsP subcell 230, an InAlGaP subcell 240 epitaxially grown on a Si substrate 201.
The Si substrate 201 may be a p-type Si single crystal substrate having thereon a p-type nucleation layer 202 and a SiGe alloy layer as a germanium four-junction cell buffer layer 203.
The Ge subcell 210 may include: a p-type InGaP or InGaAs back field layer 211; a p-type Ge layer as a base region 212 of the Ge sub-cell 210; an n-type Ge layer as emitter region 213 of Ge subcell 210; and an n-type InGaP layer as the window layer 214 of the Ge sub-cell 210. The thickness of the emitter region 213 is preferably greater than the thickness of the base region 212, and high doping concentrations are used for both the base region 212 and the emitter region 213 to reduce the Ge subcell thickness.
The first tunneling junction 215 connecting the Ge subcell and the InGaAs subcell may be an n-type GaAs or InGaP layer and a p-type GaAs or Al0.3Ga0.7An As or InGaP layer. Connecting the first tunneling junction 215 and the InGaAs subcell may be a p-type InGaAs compositional graded buffer layer 216. The compositionally graded buffer layer 216 is used to achieve a transition in lattice constant between the Ge subcell and the InGaAs subcell and is capable of extending the range of the forbidden bandwidth of the InGaAs subcell.
The InGaAs sub-cell 220 may include a p-type InAlGaAs back electric field layer 221 with a high doping concentration; a p-type InGaAs layer as the base region 222 of the InGaAs sub-cell; an n-type InGaP layer as an emitter region 223 of the InGaAs sub-cell; and a high doping concentration n-type AlInP window layer 224.
The second tunnel junction 225 connecting the InGaAs subcell 220 and the inalgas subcell 230 includes an n-type InGaP layer and a p-type inalgas layer.
The InGaAsP subcell 230 includes: the p-type InAlGaAs back electric field layer 231 with high doping concentration and the p-type InGaAsP layer are used as the base region 232 of the InGaAsP sub-battery; an n-type InAlGaAs layer as the emitter 233 of the InAlGaAs sub-cell; and a high doping concentration n-type AlInP layer as the window layer 234 of the InGaAsP subcell.
The third tunnel junction 235 connecting the InGaAsP subcell 230 and the InAlGaP subcell 240 includes an n-type InAlGaP layer and a p-type inalgas layer. Optionally, a p-type inalgas composition graded buffer layer 236 is disposed between the third tunnel junction 235 and the InAlGaP subcell. The compositionally graded buffer layer 236 serves to achieve a transition in lattice constant between the InGaAsP subcell 230 and the InAlGaP subcell 240 and to extend the range of forbidden bandwidths of the InGaAsP subcell 230 and the InAlGaP subcell 240.
The InAlGaP subcell 240 includes: the p-type InAlGaP back electric field layer 241 with high doping concentration and the p-type InAlGaP layer are used as the base region 242 of the InAlGaP sub-cell; an n-type InAlGaP layer as the emitter region 243 of the InAlGaP subcell; and a high doping concentration n-type AlInP layer as the window layer 244 of the InAlGaP subcell.
In addition, as known to those skilled in the art, the ge-quadruple junction solar cell of the above embodiment may further include other functional layers such as an ohmic contact layer 250, upper and lower electrodes, an anti-reflective film, etc., and will not be described in detail herein.
The germanium four-junction solar cell 200 of the present embodiment is prepared as follows:
1) entering MOCVD or MBE or other epitaxial growth equipment, using a P-type Si single crystal substrate 201, epitaxially growing a P-type nucleation layer 202 on the P-type Si single crystal substrate, and then growing a SiGe alloy to be used as a buffer layer 203 of the four-junction cell. The nucleation layer 202 may have a thickness of 0.05-0.1 μm and a doping concentration of 0.5-5 × 1017m-3(ii) a The doping concentration of the buffer layer 203 may be in the range of 0.5 to 5 × 1018m-3. Epitaxial growth on Si-based substrates can be achieved using a buffer layer of SiGe alloy, and the use of Si substrates allows for a solution for low cost battery fabrication relative to germanium-based substrates.
2) Growing p-type InGaP or InGaAs as a back electric field layer 211 of the Ge sub-cell, then growing a p-type Ge layer as a base region 212 of the Ge sub-cell, then growing an n-type Ge layer as an emitter region 213 of the Ge sub-cell, and finally growing an n-type GaInP layer as a window layer 214 of the Ge sub-cell. The growth thickness of the InGaP or InGaAs back electric field layer 211 can be 0.1-0.3 μm, and the doping concentration can be 3-8 × 1018m-3(ii) a The growth thickness of the base region p-type Ge layer 212 can be 0.1-0.2 μm, and the doping concentration can be 3-5 × 1018m-3(ii) a The n-type Ge layer 213 of the emitter region has a growth thickness of 20-30 μm and a doping concentration of 3-5 × 1018m-3(ii) a The growth thickness of the n-type InGaP layer 214 of the window layer can be 0.01-0.03 μm, and the doping concentration can be 3-8 × 1018m-3。
3) A first tunnel junction 215 connecting the Ge and InGaAs subcells is then grown: growing an n-type GaAs or InGaP layer; regrown p-type GaAs or Al0.3Ga0.7An As or InGaP layer forming a first tunneling junction 215;
4) after the first tunnel junction 215 is grown again, an InGaAs or InAlGaAs or InGaP buffer layer 216 is grown, and the ideal lattice is gradually reached by adopting a lattice gradient mode.
5) After the buffer layer 216 is completed, the InGaAs subcell 220 is grown: growing a p-type back electric field layer 221 with high doping concentration; a p-type InGaAs layer is grown again to serve as a base region 222 of the second junction InGaAs sub-cell; then growing an n-type InGaAs layer or an InGaP layer as an emitter region 223 of the second junction InGaAs sub-cell; finally, an n-type AlInP window layer 224 with high doping concentration is grown. The back electric field layer 221 is made of InAlGaAs or AlGaInP, has a thickness of 0.1-0.3 μm, and has a doping concentration of 3-8 × 1018m-3(ii) a The thickness of the InGaAs base region 222 can be 1-1.5 μm, and the doping concentration can be 0.5-1 × 1017m-3(ii) a The thickness of the InGaAs emitter region 223 can be 0.1-0.3 μm, and the doping concentration can be 1-3 × 1018m-3. The n-type AlInP window layer 224 may be grown to a thickness of 0.02-0.03 μm and a doping concentration of 3-8 × 1018m-3。
6) Growing a second tunnel junction 225 connecting the InGaAs subcell and the InGaAsP subcell: firstly growing an n-type InAlGaAs or InGaP layer; and growing a p-type InAlGaAs layer to form a second tunneling junction.
7) Growing the InGaAsP subcell 230: growing a p-type back electric field layer 231 with high doping concentration; a p-type InGaAsP layer is grown again to be used as a base region 232 of the InGaAsP sub-battery; then growing n-type InGaAsP or InGaP layer as the emission of InGaAsP sub-cellRegion 233; and growing an n-type AlInP layer with high doping concentration as a window layer 234 of the InGaAsP sub-cell. The P-type back electric field layer 231 can be made of InAlGaAs or InAlGaP, the thickness can be 0.1-0.3 μm, and the doping concentration can be 3-8 × 1018m-3(ii) a The thickness of the p-type InGaAsP base region 232 is 1.0-1.6 μm, and the doping concentration is 1-3 × 1017m-3(ii) a The n-type InGaAsP or InGaP emitter region 233 may have a thickness of 0.08-0.15 μm and a doping concentration of 0.5-1 × 1018m-3(ii) a The n-type AlInP window layer 234 may be grown to a thickness of 0.02-0.03 μm and a doping concentration of 3-8 × 1018m-3。
8) Growing a third tunnel junction 235 connecting the InGaAsP subcell and the InAlGaP subcell: firstly growing an n-type InAlGaAs layer; and growing a p-type InAlGaAs layer to form a third tunneling junction.
9) After the third tunnel junction is grown, an InAlGaAs or InGaP buffer layer 236 is grown, and the ideal lattice is gradually reached by adopting a lattice gradual change mode.
10) Growth of InAlGaP subcell 240: growing a p-type back electric field layer 241 with high doping concentration; regrowing a p-type InAlGaP layer as the base region 242 of the InAlGaP sub-cell; then, an n-type InAlGaP layer is grown to serve as an emitter region 243 of the InAlGaP sub-cell; an n-type AlInP layer of high doping concentration is grown as the window layer 244 of the InAlGaP subcell. The P-type back electric field layer 241 is made of InAlGaP with a thickness of 0.1-0.3 μm and a doping concentration of 3-8 × 1018m-3(ii) a The thickness of the p-type InAlGaP base region 242 can be 1.0-1.6 μm, and the doping concentration can be 1-3 × 1017m-3(ii) a The thickness of the n-type InAlGaP emitter region 243 is 0.08-0.15 μm, and the doping concentration can be 0.5-1 × 1018m-3(ii) a The n-type AlInP window layer 244 may be grown to a thickness of 0.02-0.03 μm and a doping concentration of 3-8 × 1018m-3。
11) Growing the ohmic contact layer 250: an n-type InGaAs cap layer with high doping concentration is grown as the ohmic contact layer 250.
12) An upper electrode is formed on the ohmic contact layer 250: designing and manufacturing a photoetching layout, gluing and developing, depositing upper electrode metal, removing glue and annealing;
13) a lower electrode was produced on the Si single crystal substrate 201: and depositing metal of the lower electrode.
14) Manufacturing an anti-reflection film on the ohmic contact layer 250: entering an evaporator to deposit an optical film.
15) And (6) scribing and testing.
According to the germanium four-junction solar cell, the Si base is adopted to replace the germanium substrate, the high-quality germanium multi-junction cell is formed by extending on the Si base, the germanium consumption can be greatly reduced, the production cost is reduced by orders of magnitude, and meanwhile, the sustainable development of the germanium multi-junction solar cell industry can be promoted.
Meanwhile, according to the germanium four-junction solar cell of the embodiment, by adding the InGaAs, InAlGaAs or InGaP buffer layer material between the Ge sub-cell and the InGaAs sub-cell and/or between the InGaAsP sub-cell and the InAlGaP sub-cell, the theoretical optimal forbidden bandwidth combination between the sub-cells is realized, meanwhile, the epitaxial growth of materials with different lattice constants can be realized through the component gradual change layer, the forbidden bandwidth of each sub-cell can be expanded, so that each sub-cell has a wider forbidden bandwidth, and the photoelectric conversion efficiency of the cell is improved. For example, in the present embodiment, the band gap of InGaAs can be between 1.0-1.2 eV, the band gap of InGaAsP can be between 1.3-1.5 eV, and the band gap of InAlGaP can be between 1.8-2.2 eV.
In addition, according to the germanium four-junction solar cell of the embodiment, the doping and pn junction structures of the Ge sub-cell are redesigned, and the thin base region with high doping concentration and the thick emitter region structure with high doping concentration are adopted, so that the thickness of the Ge sub-cell can be greatly reduced, and the thinning of the Ge sub-cell is realized.
Specifically, because the Ge material is an indirect band gap material, the indirect band gap is 0.67eV, and the direct band gap is 0.8eV, the photon absorption coefficient of the Ge material to the energy of 0.67eV-0.8eV is lower and is lower than 100cm-1And the Ge material has an absorption coefficient greater than 1000cm for photons having an energy greater than 0.8eV-1Therefore, the thickness of the germanium sub-cell in the germanium-based four-junction cell in the conventional design needs to reach 300 um. Compared with the prior art, in the embodiment, the doping concentration of the base region of the Ge sub-cell is increased, and the thickness of the emitter region is larger than that of the base regionThe absorption coefficient of the germanium junction to the photon with the energy of 0.67eV-0.8eV is from lower than 100cm-1Increased to over 1000cm-1Therefore, the thickness of the Ge sub-battery can be lower than 30um, and the thinning is realized. The thinning of the germanium multi-junction battery has very important practical significance, the efficiency-weight ratio of the battery can be improved, the higher the efficiency-weight ratio of the battery is, and for the field of aerospace, the weight of the battery with the same area of a spacecraft is smaller, and more loads can be carried.
Figure 3 is a schematic diagram of a germanium four-junction solar cell in preparation according to another embodiment of the present invention. Fig. 4 is a schematic structural diagram of the completed ge-quadruple junction solar cell of fig. 3. This embodiment has substantially the same structure as the four-junction solar cell 200 of fig. 2, except that a sacrificial layer structure is added, and the four-junction solar cell can be peeled off from the rigid Si substrate by etching the sacrificial layer, and then the flexible substrate is attached. The flexible substrate can be thin film metal such as copper and aluminum or polyimide, so that the thinning and flexibility of the germanium four-junction solar cell are realized.
Specifically, as shown in fig. 3, the germanium quadruple junction solar cell 300 includes a Ge subcell 310, an InGaAs subcell 320, an InGaAsP subcell 330, and an InAlGaP subcell 340 epitaxially grown on a Si substrate 301.
For example, the substrate 301 is a p-type Si single crystal substrate having thereon a p-type nucleation layer 302, a SiGe alloy layer as a germanium four-junction cell buffer layer 303, a sacrificial layer 304, and a p-type ohmic contact layer 305.
The Ge sub-cell 310 may include: a p-type InGaP or InGaAs back electric field layer 311; a p-type Ge layer 312 as a base region of the Ge sub-cell 310; an n-type Ge layer 313 as an emitter region of the Ge sub-cell 310; and an n-type InGaP layer 314 as a window layer of the Ge sub-cell 310.
The first tunneling junction 315 connecting the Ge subcell 310 and the InGaAs subcell 320 may be an n-type GaAs or InGaP layer and a p-type GaAs or Al0.3Ga0.7An As or InGaP layer. Connecting the first tunneling junction 315 and the InGaAs subcell 320 may be a p-type InGaAs compositionally graded buffer layer 316.
The InGaAs subcell 320 may include a high doping concentration p-type inalgas back electric field layer 321; a p-type InGaAs layer as a base region 322 of the InGaAs sub-cell; an n-type InGaP layer as an emitter region 323 of the InGaAs subcell; and a high doping concentration n-type AlInP window layer 324.
The second tunnel junction 325 connecting the InGaAs subcell 320 and the InGaAsP subcell 330 may include an n-type InGaP layer and a p-type InAlGaAs layer.
The InGaAsP subcell 330 may include: the p-type back electric field layer 331 with high doping concentration and the p-type InGaAsP layer are used as the base region 332 of the InGaAsP sub-cell; an n-type InAlGaAs layer as an emitter region 333 of the InAlGaAs sub-cell; and a high doping concentration n-type AlInP layer as the window layer 334 of the InGaAsP subcell.
The third tunnel junction 335 connecting the InGaAsP subcell 330 and the InAlGaP subcell 340 includes a layer of n-type InAlGaP and a layer of p-type inalgas. A p-type inalgas composition graded buffer layer 336 may be disposed between the third tunnel junction 335 and the InAlGaP subcell 340.
The InAlGaP subcell 340 may include: the p-type InAlGaP back electric field layer 341 and the p-type InAlGaP layer with high doping concentration are used as the base region 342 of the InAlGaP sub-cell; an n-type InAlGaP layer as the emitter region 343 of the InAlGaP sub-cell; and a high doping concentration n-type AlInP layer as the window layer 344 of the InAlGaP subcell.
In addition, as known to those skilled in the art, the four-junction solar cell of the above embodiment may further include ohmic contact layers 305 and 350, upper and lower electrodes, an anti-reflective film, and the like, which will not be described in detail herein.
The sacrificial layer 304 of the embodiment can be removed by corrosion in the subsequent manufacturing process, so that the epitaxial structure of the ge-quadruple junction solar cell is peeled off from the hard Si substrate and connected with the flexible substrate, thereby realizing the flexible thinning of the ge-quadruple junction solar cell. Fig. 4 is a schematic structural diagram of the completed four-junction solar cell of fig. 3, in which the portion below the sacrificial layer 304 in fig. 3 is stripped and the cell structure above the sacrificial layer 304 is connected to a flexible substrate 401. The flexible substrate 401 may be a thin film metal such as copper or aluminum, or polyimide. The preparation process of the germanium four-junction solar cell of the embodiment is as follows:
1) entering MOCVD or MBE or other epitaxial growth equipment, a P-type Si single crystal substrate 301 is adopted, a P-type nucleation layer 302 is epitaxially grown on the P-type Si single crystal substrate, and then SiGe alloy is grown to serve as a buffer layer 303 of the four-junction battery. The nucleation layer 302 may have a thickness of 0.05-0.1 μm and a doping concentration of 0.5-5 × 1017m-3(ii) a The doping concentration of the buffer layer 303 may be 0.5-5 × 1018m-3. The use of a buffer layer of SiGe alloy enables epitaxial growth of germanium on a Si-based substrate, which allows a solution for low-cost cell fabrication to be achieved relative to germanium-based substrates. Next AlGaAs or AlAs is grown as a sacrificial layer 304. Next, p-type InGaAs is grown on the AlAs sacrificial layer 304 as a buffer layer ohmic contact layer 305 of the four junction cell. In the subsequent step, the epitaxially grown cell structure can be stripped from the Si substrate by corroding the sacrificial layer, so that the thinning of the cell is realized.
2) Growing p-type InGaP as a back electric field layer 311 of the Ge sub-cell 310, then growing a p-type Ge layer as a base region 312 of the Ge sub-cell, then growing an n-type Ge layer as an emitter region 313 of the Ge sub-cell, and finally growing an n-type InGaP layer as a window layer 314 of the Ge sub-cell. The growth thickness of the InGaP back electric field layer 311 can be 0.1-0.3 μm, and the doping concentration can be 3-8 × 1018m-3(ii) a The growth thickness of the base region p-type Ge layer 312 can be 0.1-0.2 μm, and the doping concentration can be 3-5 × 1018m-3(ii) a The n-type Ge layer 313 of the emitter region is grown to a thickness of 20-30 μm and has a doping concentration of 3-5 × 1018m-3(ii) a The growth thickness of the n-type InGaP layer 314 of the window layer can be 0.01-0.03 μm, and the doping concentration can be 3-8 × 1018m-3。
3) Next, a first tunnel junction 315 connecting the Ge subcell 310 and the InGaAs subcell 320 is grown: growing an n-type GaAs or InGaP layer; regrown p-type GaAs or Al0.3Ga0.7An As or InGaP layer forming a first tunneling junction 315;
4) after the first tunnel junction 315 is grown, an InGaAs or InAlGaAs or InGaP buffer layer 316 is grown, and the ideal lattice is gradually reached by adopting a lattice grading mode.
5) After the buffer layer 316 is completed, the InGaAs subcell 320 is grown: growth high dopingA p-type back electric field layer 321 of impurity concentration; regrowing a p-type InGaAs layer as a base region 322 of a second junction InGaAs sub-cell; then growing an n-type InGaAs layer or an InGaP layer as an emitter region 323 of the second junction InGaAs sub-cell; finally, an n-type AlInP window layer 324 of high doping concentration is grown. The back electric field layer 321 is InAlGaAs or InAlGaP with a thickness of 0.1-0.3 μm and a doping concentration of 3-8 × 1018m-3(ii) a The thickness of the InGaAs base region 322 can be 1-1.5 μm, and the doping concentration can be 0.5-1 × 1017m-3(ii) a The thickness of the InGaAs emitter region 323 can be 0.1-0.3 μm, and the doping concentration can be 1-3 × 1018m-3。
6) Growing a second tunnel junction 325 connecting the InGaAs subcell 320 and the InGaAsP subcell 330: firstly growing an n-type InAlGaAs or InGaP layer; a p-type inalgas layer is regrown forming a second tunnel junction 325.
7) Growing the InGaAsP subcell 330: growing a p-type back electric field layer 331 with high doping concentration; a p-type InGaAsP layer is regrown to be used as a base region 332 of the InGaAsP sub-battery; then growing an n-type InAlGaAs or InGaP layer as an emitting region 333 of the InAlGaAs sub-cell; an n-type AlInP layer with high doping concentration is grown as the window layer 334 of the InGaAsP subcell. The P-type back electric field layer 331 can be InAlGaAs or InAlGaP, and has a thickness of 0.1-0.3 μm and a doping concentration of 3-8 × 1018m-3(ii) a The thickness of the p-type InGaAsP base region 332 is 1.0-1.6 μm, and the doping concentration is 1-3 × 1017m-3(ii) a The thickness of the n-type InAlGaAs or InGaP emitter region 333 can be 0.08-0.15 μm, and the doping concentration can be 0.5-1 × 1018m-3(ii) a The n-type AlInP window layer 334 may be grown to a thickness of 0.02-0.03 μm and a doping concentration of 3-8 × 1018m-3。
8) Growing a third tunnel junction 335 connecting the InGaAsP subcell 330 and the InAlGaP 340 subcell: firstly growing an n-type InAlGaAs layer; and growing a p-type InAlGaAs layer to form a third tunneling junction.
9) After the third tunnel junction 335 is grown, an InAlGaAs or InGaP buffer layer 336 is grown, and the ideal lattice is gradually reached by adopting a lattice gradual change mode.
10) Growth of InAlGaP subcell 340: growing a p-type back electric field layer 341 with high doping concentration; regrowing a p-type InAlGaP layer as a base region 342 of the InAlGaP sub-cell; then growing an n-type InAlGaP layer as an emitting region 343 of the InAlGaP sub-battery; an n-type AlInP layer of high doping concentration is grown as the window layer 344 of the InAlGaP subcell 340. The P-type back electric field layer 241 is made of AlGaInP, has a thickness of 0.1-0.3 μm, and has a doping concentration of 3-8 × 1018m-3(ii) a The thickness of the p-type InAlGaP base region 342 can be 1.0-1.6 μm, and the doping concentration can be 1-3 × 1017m-3(ii) a The thickness of the n-type InAlGaP emitter region 343 is 0.08-0.15 μm, and the doping concentration can be 0.5-1 × 1018m-3(ii) a The n-type AlInP window layer 344 may be grown to a thickness of 0.02-0.03 μm and a doping concentration of 3-8 × 1018m-3。
11) And (3) growing the ohmic contact layer 350: an n-type InGaAs cap layer with high doping concentration is grown as the ohmic contact layer 350.
13) Etching the sacrificial layer 304 to realize epitaxial layer stripping;
14) a lower electrode is fabricated on the ohmic contact layer 305: depositing and annealing the lower electrode metal;
15) attaching the stripped cell epitaxial layer structure to a flexible substrate 401;
12) an upper electrode is fabricated on the ohmic contact layer 350: designing and manufacturing a photoetching layout, gluing and developing, depositing upper electrode metal, removing glue and annealing;
16) an anti-reflection film is formed on the ohmic contact layer 350: entering an evaporation machine to deposit an optical film;
17) and (6) scribing and testing.
In addition to the advantages of the foregoing embodiments, the ge-based four-junction solar cell of this embodiment may be separated from the substrate by extending the sacrificial layer between the Si substrate and the cell structure, so as to connect the ge-based four-junction solar cell to the flexible substrate, thereby implementing a flexible thin film of the ge-based four-junction solar cell.
Although the above embodiments illustrate the concept of the present invention by taking a germanium four-junction solar cell as an example, it should be understood by those skilled in the art that any multi-junction solar cell including the four-junction sub-cell structure of the above embodiments can also achieve the technical effects of the present invention.
Although a few embodiments of the present general inventive concept have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the claims and their equivalents.

Claims (15)

1. A germanium multijunction solar cell, comprising: and the Ge sub-cell, the InGaAs sub-cell, the InGaAsP sub-cell and the InAlGaP sub-cell are sequentially stacked.
2. The germanium multijunction solar cell of claim 1, wherein the Ge subcell comprises: InGaP, InGaAs or InAlGaAS back electric field layers, Ge base regions, Ge emitter regions, and InGaP or AlInP window layers.
3. The germanium multijunction solar cell of claim 2, wherein the thickness of the Ge emitter region is greater than the thickness of the Ge base region, and the doping concentration of the Ge base region is 3-5 x 1018m-3The doping concentration of the Ge emitting region is 3-5 multiplied by 1018m-3。
4. The germanium multijunction solar cell of claim 1, wherein the InGaAs subcell comprises: an InAlGaP or InAlGaAs back electric field layer, an InGaAs base region, an InGaAs or InGaP emitter region, and an AlInP window layer.
5. The germanium multijunction solar cell of claim 1, wherein said InGaAsP subcell comprises: InAlGaP or InAlGaAs back electric field layer, InGaAsP base region, InAlGaAs or InGaP emitter region and AlInP window layer.
6. The germanium multijunction solar cell of claim 1, wherein said InAlGaP subcell comprises: InAlGaP or InAlGaAs back electric field layer, InAlGaP base region, InAlGaP emitter region and AlInP window layer.
7. The germanium multijunction solar cell of claim 1, wherein said Ge, InGaAs, InGaAsP, and InAlGaP subcells have a tunneling junction therebetween.
8. The germanium multijunction solar cell of any one of claims 1-7, wherein a first compositionally graded buffer layer is between said Ge and InGaAs subcells.
9. The germanium multijunction solar cell of claim 8, wherein a second compositionally graded buffer layer is between said InGaAsP subcell and said InAlGaP subcell.
10. The germanium multijunction solar cell of any one of claims 1-7, further comprising a Si substrate on which said Ge subcell, InGaAs subcell, InGaAsP subcell, and InAlGaP subcell are sequentially disposed.
11. The germanium multijunction solar cell of claim 10, wherein a SiGe alloy buffer layer is between the Si substrate and the Ge subcell.
12. The germanium multijunction solar cell of any one of claims 1-7, further comprising a flexible substrate, said Ge subcell, InGaAs subcell, InGaAsP subcell, and InAlGaP subcell being sequentially disposed on said flexible substrate.
13. The germanium multijunction solar cell of claim 12, wherein said flexible substrate is a thin film metal substrate or a polyimide substrate.
14. A method of fabricating a germanium multijunction solar cell, comprising:
providing a Si substrate; and
and epitaxially growing a Ge sub-cell, an InGaAs sub-cell, an InGaAsP sub-cell and an InAlGaP sub-cell on the Si substrate in sequence.
15. The method of claim 14, wherein prior to epitaxially growing the Ge subcell on the Si substrate, a sacrificial layer is epitaxially grown on the Si substrate, and,
the method further comprises the following steps: and stripping the Si substrate by corroding the sacrificial layer, and connecting the flexible substrate on the Ge sub-cell stripped from the Si substrate.
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