Three-junction gallium arsenide solar cell with quantum dot structure and manufacturing method of three-junction gallium arsenide solar cell

Abstract

The invention discloses a three-junction gallium arsenide solar cell with a quantum dot structure and a manufacturing method of the three-junction gallium arsenide solar cell, belongs to the technical field of semiconductor photovoltaics, and aims to solve the problems that a quantum dot cell is large in stress and has many defects, and voltage and current are difficult to consider at the same time. According to the core technical scheme, in a middle battery base region, a GaAsP stress compensation layer is grown above an InGaAs quantum dot base region with gradient decreasing In components. The gradient quantum dots are used for enhancing long-wave absorption to improve short-circuit current; the open-circuit voltage of the GaAsP layer is increased by utilizing the wide forbidden band characteristic, the thickness of the GaAsP layer is accurately controlled, and the compressive stress of the quantum dot region is balanced by the introduced tensile stress. According to the stress balance design, material defects are greatly reduced, the crystal quality is improved, synchronous boosting of short-circuit current and open-circuit voltage is finally achieved on the basis of high reliability, and the overall efficiency of the battery is remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor photovoltaic technology, and in particular relates to a high-efficiency and high-reliability triple-junction gallium arsenide (GaAs) solar cell, specifically a triple-junction GaAs solar cell with a quantum dot stress compensation structure and a manufacturing method thereof. Background Art

[0002] Gallium arsenide (GaAs) solar cells have been widely used in space power systems in aerospace and other fields due to their excellent photoelectric conversion efficiency and outstanding irradiation stability. However, the performance of traditional GaAs solar cells is limited by their inherent material band gap (approximately 1.42 eV), which restricts their ability to absorb photons in the long-wavelength range of the solar spectrum.

[0003] To overcome this limitation, researchers generally use the method of introducing indium arsenide (InAs) quantum dot structures in the base region of the cell, using the quantum size effect of quantum dots to absorb lower-energy long-wavelength photons, thereby increasing the short-circuit current density of the cell. However, traditional quantum dot solar cell designs still face the following technical challenges: 1. Severe stress accumulation: There is a lattice mismatch of up to 7% between InAs and GaAs. During heteroepitaxial growth, this huge mismatch leads to severe strain energy accumulation, which easily generates crystal defects such as dislocations and transgranular defects in the material. This, in turn, severely reduces the minority carrier lifetime and diffusion length, affecting battery performance.

[0004] 2. Enhanced carrier recombination: Quantum dots form potential wells in the band structure. Although they can capture photons, they are also prone to capture and retain photogenerated carriers, increasing the probability of carrier recombination, thereby reducing the current collection efficiency and fill factor.

[0005] 3. Structural instability: Due to the lack of an effective stress compensation mechanism, the internal stress of batteries containing quantum dot structures may be further released or changed when they undergo harsh environmental tests such as thermal cycling or space radiation, leading to structural instability and performance degradation.

[0006] 4. It is difficult to balance voltage and current: Although the introduction of quantum dots with narrow band gaps can increase the current, it often leads to a significant decrease in the open circuit voltage (Voc) of the battery, which limits the improvement of the overall conversion efficiency.

[0007] Therefore, this field urgently needs a new type of solar cell structure that can effectively solve the stress mismatch problem and suppress the drop in open-circuit voltage while enhancing long-wave absorption and increasing short-circuit current, thereby achieving a comprehensive improvement in the overall performance of the cell. Summary of the Invention

[0008] The main purpose of this invention is to overcome the shortcomings of existing technologies by providing a triple-junction gallium arsenide solar cell with a stress-compensated quantum dot structure and a method for its fabrication. This invention aims to address the technical challenges of traditional quantum dot solar cells, such as high lattice mismatch stress, high defect density, and the difficulty in balancing open-circuit voltage and short-circuit current.

[0009] The above technical objectives of the present invention are achieved through the following technical solutions: a triple-junction gallium arsenide solar cell with a quantum dot structure, wherein a bottom cell, a middle cell, and a top cell are sequentially stacked, wherein the middle cell and the bottom cell are connected via a middle-bottom tunnel junction, and the top cell and the middle cell are connected via a middle-top tunnel junction, wherein the base region of the middle cell comprises: An InGaAs quantum dot base region; a GaAsP bulk material base region disposed above the InGaAs quantum dot base region; The In component of the InGaAs quantum dot base region decreases gradually along the epitaxial growth direction, and the GaAsP bulk material base region is used to perform stress compensation on the InGaAs quantum dot base region.

[0010] Furthermore, in the InGaAs quantum dot base region, the thickness along the epitaxial growth direction is In molar components at The following relationship is satisfied:

[0011] in, is the total thickness of the quantum dot region, is the initial In molar composition of the InGaAs material, is the curvature adjustment factor.

[0012] Further, the initial 1 mol component The range is 5% to 15%, and the curvature adjustment factor The value range is 2 to 4.

[0013] Furthermore, the GaAsP bulk material base region is GaAs (1-y) P y , wherein the molar composition y of P ranges from 5% to 25%.

[0014] Furthermore, the thickness of the GaAsP bulk material base region is Determined by the following relationship:

[0015] in, is the total thickness of the InGaAs quantum dot base region, The thickness of the quantum dot region The lattice mismatch at is the lattice mismatch of the GaAsP bulk material base.

[0016] Furthermore, the lattice mismatch and They are defined by the following relations:

[0017]

[0018] in, 、 and are the lattice constants of the corresponding materials.

[0019] Furthermore, a plurality of distributed Bragg reflectors (DBRs) are provided between the middle bottom tunnel junction and the middle battery.

[0020] A method for manufacturing a triple-junction gallium arsenide solar cell with a quantum dot structure, the method comprising the following steps: Epitaxially growing the structural layers of the bottom cell, middle cell and top cell in sequence; Wherein, when growing the base region of the intermediate battery, the method includes: Grow an InGaAs quantum dot base region, and control the growth parameters to make the In composition of the region decrease gradually along the epitaxial growth direction; A GaAsP bulk material base region is continuously grown on the InGaAs quantum dot base region to perform stress compensation on the InGaAs quantum dot base region.

[0021] Furthermore, in the step of growing the InGaAs quantum dot base region, the thickness along the epitaxial growth direction is controlled to be In molar components at The following relationship is satisfied:

[0022] in, is the total thickness of the quantum dot region, is the initial In molar composition of the InGaAs material, is the curvature adjustment factor.

[0023] Furthermore, in the step of growing the GaAsP bulk material base region, the thickness Determine and control according to the following relationship:

[0024] in, is the total thickness of the InGaAs quantum dot base region, The thickness of the quantum dot region The lattice mismatch at is the lattice mismatch of the GaAsP bulk material base.

[0025] Compared with the prior art, the present invention has the following beneficial effects: 1. Accurate stress compensation and improved crystal quality: This invention achieves precise stress compensation by precisely designing and growing a tensile-stressed GaAsP compensation layer above the compressive-stressed InGaAs quantum dot layer. This allows the two stresses to offset each other numerically, achieving stress balance in the overall structure. This significantly reduces the density of defects such as dislocations caused by lattice mismatch, significantly improving the crystal quality and minority carrier lifetime of the epitaxial material.

[0026] 2. Achieve compatibility between long-wave absorption and high-voltage output: This invention utilizes an InGaAs gradient quantum dot structure to effectively expand the cell's absorption of the long-wave spectrum, increasing short-circuit current (Jsc). Furthermore, the upper GaAsP layer has a wider bandgap, acting as a potential barrier to effectively raise the open-circuit voltage (Voc), perfectly resolving the "increase current while reducing voltage" dilemma inherent in traditional quantum dot cells.

[0027] 3. Optimized carrier transport and suppressed recombination losses: The present invention adds a quantum dot region near the BSF to improve electron confinement, further reducing interfacial carrier recombination and increasing current density and open-circuit voltage. Furthermore, the gradient design of the In component creates a moderately ascending energy band step between the quantum dot region and subsequent layers, which helps establish a built-in electric field and effectively drives the migration of photogenerated carriers toward the PN junction, suppressing their recombination losses in the quantum dot potential well, thereby improving fill factor (FF) and quantum efficiency.

[0028] 4. Improved the radiation stability of the battery: The present invention introduces a wide-bandgap GaAsP base material, which not only has a higher bandgap, but also the addition of phosphorus atoms changes the energy level distribution of crystal point defects, moves the trap energy level away from the neutral zone, and increases the minority carrier lifetime, thereby effectively improving the radiation stability of the solar cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 1 is a schematic diagram of the overall structure of a triple-junction GaAs solar cell according to an embodiment of the present invention; Figure 2 It is a detailed structural diagram of the battery part in an embodiment of the present invention. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the following embodiments are only exemplary descriptions of the present invention and should not be regarded as limiting the scope of protection of the present invention.

[0031] See also Figure 1 and Figure 2 , Figure 1 Schematic diagram of the overall structure of a triple-junction gallium arsenide solar cell according to an embodiment of the present invention. Figure 2 This embodiment provides a triple-junction gallium arsenide solar cell with a quantum dot structure and a method for manufacturing the same. By introducing a gradient-composition InGaAs quantum dot region into the mid-cell base region and growing a GaAsP layer of a specific thickness above it, precise stress compensation is achieved. This improves long-wave absorption while also maintaining a high open-circuit voltage, significantly enhancing the overall performance of the cell.

[0032] Example 1 The fabrication method of this embodiment aims to address the technical challenges of conventional quantum dot solar cells, such as large lattice mismatch stress, high defect density, and difficulty balancing open-circuit voltage and short-circuit current. The detailed steps and technical results are analyzed below: 1. Preparation of substrate and bottom cell: On a P-type doped germanium (Ge) substrate, a 0.1μm-deep N-type emitter region is formed on the surface of the Ge substrate through high-temperature diffusion of phosphine (PH3), thus forming the PN junction of the bottom cell. Next, a 0.015μm-thick gallium indium phosphide (GaInP) nucleation layer is epitaxially grown, which also serves as the window layer of the bottom cell.

[0033] 2. Growth of buffer layer: On top of the bottom cell, epitaxial growth continues with an N-type doping concentration of 1×10 18 / cm 3 GaAs / In 0.01 GaAs buffer layer, where the GaAs layer is 0.2 μm thick, In 0.01 The GaAs layer thickness is 0.3 μm.

[0034] 3. Growth of mid-bottom tunnel junction: Growth N ++ GaAs / P ++ The tunnel junction of GaAs structure is used to connect the bottom battery and the middle battery in series. ++ GaAs layer thickness 0.02μm, P ++ The GaAs layer thickness is 0.02 μm, and the doping concentration is 5×10 18 / cm 3 and 1×1019 / cm 3 .

[0035] 4. Growth of multiple sets of distributed Bragg reflectors (DBRs): Atop the tunneling junction, multiple DBRs are grown to enhance the light absorption of the middle cell. This section consists of three sets of reflector stacks for different wavelengths, effectively reflecting long-wavelength photons not absorbed by the middle cell back to the middle cell's absorption region, thereby improving current response.

[0036] Specifically, the multiple DBR groups are divided into three parts, the first part of which consists of 7 pairs of Al 0.2 GaAs / InGaAs structure, each pair of AlGaAs / InGaAs structure has Al 0.2 The thickness of the GaAs layer is 0.066μm, and the thickness of the InGaAs layer is 0.064μm; the second part consists of 10 pairs of Al 0.5 GaAs / InGaAs structure, each pair of Al 0.5 Al in GaAs / InGaAs structure 0.5 The thickness of the GaAs layer is 0.065 μm, and the thickness of the InGaAs layer is 0.060 μm; the third part consists of 15 pairs of Al 0.9 GaAs / InAl 0.2 GaAs structure, each pair of Al 0.9 GaAs / InAl 0.2 Al in GaAs structure 0.9 The thickness of GaAs layer is 0.064μm, InAl 0.2 The thickness of the GaAs layer is 0.055 μm; the DBR is P-type doped, and the doping concentration of each layer is 1×10 18 / cm 3 .

[0037] 5. Growth of mid-cell batteries: The middle cell is the key part of the present invention to solve the core technical problem. Its structure from bottom to top is the back electric field, base region, emitter region and window layer.

[0038] Back surface field (BSF): grow a layer of Al with a thickness of 0.05 μm 0.4 GaAs back electric field.

[0039] Base: The base adopts an innovative composite structure and is the core to achieve all the beneficial effects of the present invention. Its growth sequence and design ideas are as follows: First, grow a 0.08 μm thick In 0.01 GaAs bulk material base region 1.

[0040] Then, the InGaAs quantum dot base region as the core functional region is grown. The total thickness of this region 0.02μm.

[0041] During the growth process of this region, the In component is adjusted from the initial value to the =5% and gradually decreases to 1% along the epitaxial growth direction. The change relationship satisfies the formula:

[0042] The curvature adjustment factor Set to 2.

[0043] This gradient design directly addresses the "enhanced carrier recombination" issue in existing technologies. The gradual decrease in the In component creates a moderately rising energy step in the band structure, effectively establishing a built-in electric field directed toward the PN junction. This electric field effectively drives photogenerated carriers toward the emission region, preventing them from being trapped and recombining in the potential well formed by the quantum dots. This significantly improves charge collection efficiency and increases the cell's fill factor (FF). Furthermore, the InGaAs quantum dot structure itself extends its absorption of sunlight with wavelengths greater than 870nm, laying the foundation for improving short-circuit current (Jsc).

[0044] Next, a 0.3 μm thick In 0.01 GaAs bulk material base region 2.

[0045] Finally, a layer of GaAs is grown on top 0.85 P 0.15 The bulk material base region is a stress compensation layer with a thickness of 0.13 μm.

[0046] The material (GaAsP) and thickness of this layer are key to achieving stress balance and high voltage in this invention. Its P content is 15% and its thickness is 0.13μm, which is derived from the following stress balance equation:

[0047] The InGaAs quantum dot layer introduces compressive stress due to its large lattice constant, while the GaAsP layer, with its smaller lattice constant, introduces tensile stress. This solution precisely controls the thickness of the GaAsP layer so that the tensile stress it introduces and the compressive stress accumulated in the underlying InGaAs quantum dot region offset each other numerically, achieving stress balance across the entire base region and ultimately the entire cell. This significantly reduces the density of dislocations and transgranular defects caused by lattice mismatch, improves epitaxial crystal quality and carrier lifetime, and resolves the fundamental problem of "severe stress accumulation and numerous defects" in existing technologies.

[0048] Traditional quantum dot batteries increase current while also inevitably reducing voltage. The GaAsP layer in this invention has a wider bandgap than GaAs (>1.45eV). Acting as a potential barrier, it effectively raises the battery's open-circuit voltage (Voc), compensating for the voltage loss caused by the introduction of narrow-bandgap quantum dots. This perfectly resolves the technical dilemma of balancing voltage and current, achieving simultaneous increases in both short-circuit current (Jsc) and open-circuit voltage (Voc).

[0049] Emitting region and window layer: A 0.08 μm GaInP emitter region and a 0.05 μm AlInP window layer are grown in sequence.

[0050] 6. Growth of top tunnel junction and top cell: Continue to grow N ++ GaInP / P ++ AlGaAs top tunnel junction, and Al 0.5 The top cell consists of GaInP back field, GaInP base / emitter, and AlInP window layer. ++ The thickness of GaInP is 0.02 μm and the doping concentration is 3×10 18 / cm 3 ;P ++ The thickness of AlGaAs is 0.02 μm and the doping concentration is 5×10 19 / cm 3 ; Back electric field Al 0.5 The thickness of GaInP is 0.1 μm, the thickness of GaInP base region is 0.6 μm, and the doping concentration is 3×10 16 / cm 3 The thickness of the emitter region is 0.15 μm and the doping concentration is 2×10 18 / cm 3 The thickness of the AlInP window layer is 0.05 μm, and the doping concentration is 2×10 18 / cm 3 .

[0051] 7. Growth of contact layer: Finally, a heavily doped GaAs contact layer with a thickness of 0.5 μm is grown to form a good ohmic contact.

[0052] In summary, this embodiment systematically solves the core problems in the prior art through a set of precise and coordinated designs. Its technical essence and beneficial effects are concentrated in the composite structure of the middle battery base area: The present invention does not regard the InGaAs quantum dot layer as an isolated functional layer, but solves the huge compressive stress problem introduced by it by growing a layer of GaAs with a smaller lattice constant on top of it. 0.85 P0.15 The GaAsP layer introduces tensile stress in opposite directions. Crucially, the thickness of the GaAsP layer (0.13μm) is precisely calculated using an integral formula based on the cumulative strain of the underlying quantum dot layer (0.02μm, In composition 5% -> 1%). This tailored design allows the two stresses to offset each other on a macroscopic scale, achieving stress balance in the overall structure. This has the direct effect of significantly reducing the density of defects such as dislocations caused by lattice mismatch, significantly improving the crystal quality and minority carrier lifetime of the epitaxial material, and laying a solid physical foundation for the high performance and reliability of the battery.

[0053] Furthermore, the present invention utilizes the energy band characteristics of the two functional layers to achieve a simultaneous increase in both short-circuit current and open-circuit voltage. On the one hand, the InGaAs quantum dots expand their absorption of long-wavelength (>870nm) photons, effectively increasing the short-circuit current (Jsc). On the other hand, the GaAsP layer above them has a wider bandgap (>1.45eV), forming an effective electron barrier in the band structure. This barrier not only does not hinder the normal transport of photogenerated electrons, but also effectively raises the open-circuit voltage (Voc) of the entire mid-cell, thereby perfectly compensating for the voltage loss caused by the introduction of narrow-bandgap quantum dots. This collaborative design of "one to increase current, the other to compensate for voltage" successfully overcomes the technical bottleneck of traditional quantum dot batteries, which have difficulty balancing Jsc and Voc.

[0054] Furthermore, this invention addresses the problem of quantum dot potential wells easily trapping carriers and increasing the probability of recombination. By designing the In composition of the InGaAs quantum dot region to decrease gradually along the growth direction (from 5% to 1%), a gentle built-in electric field (or gradient potential field) is formed within the base region, directed toward the PN junction. This built-in electric field acts like a "downhill slope," effectively pushing photogenerated electrons toward the emission region for collection, significantly suppressing their retention and recombination in the quantum dot region. This significantly improves charge collection efficiency and the battery's fill factor (FF).

[0055] This specific embodiment is merely an explanation of the present invention and is not intended to limit the present invention. After reading this specification, those skilled in the art may make non-creative modifications to this embodiment as needed. However, as long as such modifications are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A triple-junction gallium arsenide solar cell with a quantum dot structure, comprising a bottom cell, a middle cell, and a top cell stacked in sequence, wherein the middle cell and the bottom cell are connected via a middle-bottom tunnel junction, and the top cell and the middle cell are connected via a middle-top tunnel junction, characterized in that: The base region of the intermediate battery comprises: An InGaAs quantum dot base region; a GaAsP bulk material base region disposed above the InGaAs quantum dot base region; The In component of the InGaAs quantum dot base region decreases gradually along the epitaxial growth direction, and the GaAsP bulk material base region is used to perform stress compensation on the InGaAs quantum dot base region.

2. The triple-junction gallium arsenide solar cell with a quantum dot structure according to claim 1, characterized in that: In the InGaAs quantum dot base region, the thickness along the epitaxial growth direction is In molar components at The following relationship is satisfied: ； in, is the total thickness of the quantum dot region, is the initial In molar composition of the InGaAs material, is the curvature adjustment factor.

3. The triple-junction gallium arsenide solar cell with a quantum dot structure according to claim 2, characterized in that: The initial 1 mol composition The range is 5% to 15%, and the curvature adjustment factor The value range is 2 to 4.

4. The triple-junction gallium arsenide solar cell with a quantum dot structure according to claim 1, characterized in that: The GaAsP body material base region is GaAs (1-y) P y , wherein the molar composition y of P ranges from 5% to 25%.

5. The triple-junction gallium arsenide solar cell with a quantum dot structure according to claim 4, characterized in that: The thickness of the GaAsP bulk material base region Determined by the following relationship: ； in, is the total thickness of the InGaAs quantum dot base region, The thickness of the quantum dot region The lattice mismatch at is the lattice mismatch of the GaAsP bulk material base.

6. The triple-junction gallium arsenide solar cell with a quantum dot structure according to claim 5, characterized in that: The lattice mismatch and They are defined by the following relations: ； ； in, 、 and are the lattice constants of the corresponding materials.

7. The triple-junction gallium arsenide solar cell with a quantum dot structure according to claim 1, characterized in that: A plurality of groups of distributed Bragg reflectors are further arranged between the middle bottom tunnel junction and the middle battery.

8. A method for manufacturing a triple-junction gallium arsenide solar cell with a quantum dot structure, characterized in that: The method comprises the following steps: Epitaxially growing the structural layers of the bottom cell, middle cell and top cell in sequence; Wherein, when growing the base region of the intermediate battery, the method includes: Grow an InGaAs quantum dot base region, and control the growth parameters to make the In composition of the region decrease gradually along the epitaxial growth direction; A GaAsP bulk material base region is continuously grown on the InGaAs quantum dot base region to perform stress compensation on the InGaAs quantum dot base region.

9. The method for manufacturing a triple-junction gallium arsenide solar cell with a quantum dot structure according to claim 8, characterized in that: In the step of growing the InGaAs quantum dot base region, the thickness along the epitaxial growth direction is controlled to be In molar components at The following relationship is satisfied: ； in, is the total thickness of the quantum dot region, is the initial In molar composition of the InGaAs material, is the curvature adjustment factor.

10. The method for manufacturing a triple-junction gallium arsenide solar cell with a quantum dot structure according to claim 8, characterized in that: In the step of growing the GaAsP bulk material base region, the thickness Determine and control according to the following relationship: ； in, is the total thickness of the InGaAs quantum dot base region, The thickness of the quantum dot region The lattice mismatch at is the lattice mismatch of the GaAsP bulk material base.

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