A spatial three-junction solar cell with DBR structure and a preparation method thereof

Abstract

This invention discloses a spatial triple-junction solar cell with a DBR structure and its fabrication method, comprising a buffer layer, a first tunnel junction, a DBR structure, an InGaAs middle cell, a second tunnel junction, a GaInP top cell, and a cap layer sequentially disposed on a Ge substrate. The DBR structure includes several pairs of Al cells stacked alternately. x Ga 1‑x As / Al y Ga 1‑y As composite layer, the Al x Ga 1‑x As / Al y Ga 1‑y The thickness of the As composite layers decreases monotonically according to the stacking order, with each layer's thickness decreasing by 0.5% to 2% of the thickness of the first layer relative to the previous layer. This increases the effective reflection bandwidth of the DBR, reflecting photons that were not absorbed by the middle cell but had energy higher than the middle cell's bandgap back to the middle cell for reabsorption. This improves the problem of long-wavelength EQE decline caused by the thinning of the middle cell, achieving both good irradiance characteristics and high light absorption efficiency, thus improving the overall performance of the solar cell.

Description

Technical Field

[0001] This invention belongs to the technical field of solar cells, specifically relating to a spatial triple-junction solar cell with a DBR structure and its preparation method. Background Technology

[0002] Space triple-junction solar cells possess advantages such as high photoelectric conversion efficiency, strong radiation resistance, and good stability in on-orbit operating environments, making them widely used in the aerospace field and gradually becoming the mainstream space battery. During their on-orbit service, spacecraft are exposed to radiation from high-energy particles in space. These particles can penetrate solar cells, damaging the solar cell materials and causing a decline in the cell's electrical performance, thus affecting the spacecraft's power supply system and consequently its lifespan and reliability. Therefore, research on the radiation degradation of triple-junction gallium arsenide solar cells is of great significance.

[0003] Under electron irradiation, the lattice atoms inside the battery undergo elastic scattering with the incident electrons. When the energy of the incident electrons is higher than the threshold energy value for the displacement of the lattice atoms, the lattice atoms will be displaced. The displacement damage defect is a trap for photogenerated minority carriers. A higher trap concentration causes the minority carrier diffusion length to be shortened more significantly, thus the decay of collection efficiency is more obvious.

[0004] Studies have found that the middle cell exhibits the most severe degradation in triple-junction solar cells. The main reason is that, due to the need for solar energy absorption, the base region thickness of the middle cell is significantly greater than that of the top cell. In the middle cell, the diffusion length of photogenerated carriers in the base region is roughly equivalent to the base region thickness. After irradiation, the diffusion length of photogenerated carriers is greatly reduced, resulting in some photogenerated carriers failing to reach the PN junction space charge region for collection.

[0005] To address the irradiance degradation problem of space triple-junction solar cells, the most mainstream solution is to reduce the thickness of the base region of the middle cell, thereby reducing the diffusion length of photogenerated carriers to the space charge region and improving the collection efficiency of photogenerated carriers. However, reducing the thickness of the base region will lead to a decrease in the absorption capacity of low-energy photons. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing a spatial triple-junction solar cell with a DBR structure and its fabrication method.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] A spatial triple-junction solar cell with a DBR structure includes a buffer layer, a first tunnel junction, a DBR structure, an InGaAs middle cell, a second tunnel junction, a GaInP top cell, and a cap layer, sequentially disposed on a Ge substrate. The DBR structure comprises several pairs of Al cells stacked alternately. x Ga 1-x As / Aly Ga 1-y As composite layer, the Al x Ga 1-x As / Al y Ga 1-y The thickness of the As composite layers decreases monotonically according to the stacking order, and the thickness of each Al group decreases monotonically. x Ga 1-x As layer and Al y Ga 1-y The thickness reduction of the As layer relative to the previous group is as follows: (The percentage reduction in thickness is not specified in the original text, but can be left as is.) x Ga 1-x As layer and Al y Ga 1-y The thickness of the As layer is 0.5% to 2%.

[0009] Optionally, the Al x Ga 1-x As / Al y Ga 1-y In the As composite layer, 0.85≤x<1, 0<y≤0.15.

[0010] Optionally, the number of pairs of the DBR structure is 10 to 25.

[0011] Optionally, the DBR structure is used to reflect photons in the absorption band of the InGaAs cell.

[0012] Optionally, the effective reflection bandwidth of the DBR structure is 130–150 nm.

[0013] Optionally, from bottom to top, Al x Ga 1-x As / Al y Ga 1-y The thickness of the As composite layer decreases monotonically and uniformly, and the center reflection wavelength of the first pair of DBRs is 880 to 900 nm.

[0014] Optionally, the InGaAs cell comprises, from bottom to top, a p-AlGaAs back field layer, a p-InGaAs base region, an n-InGaAs emitter region, and an n-GaInP window layer; the thickness of the p-InGaAs base region is less than 2 μm.

[0015] Optionally, the center reflection wavelength of the first pair of DBRs from bottom to top is 890nm, the effective reflection bandwidth of the DBR structure is 140nm, and the thickness of the p-InGaAs base region is 1.5μm.

[0016] Optionally, the DBR structure is p-type doped with a doping concentration of 1E18-4E18 cm-3.

[0017] Each group of Al has a different reflectivity x Ga 1-x As / Al y Ga 1-y As composite layers form a pair of DBRs, and by stacking them, a DBR structure with several pairs of DBRs is formed. The central reflection wavelength of each pair of DBRs varies with its thickness. By decreasing the thickness of each pair from bottom to top, a pattern is formed in which the central reflection wavelength gradually changes from long wavelength to short wavelength, and it has a large effective reflection bandwidth. It can effectively reflect photons of a wider range of wavelengths into the InGaAs cell, thereby improving the light absorption efficiency.

[0018] The above-mentioned method for fabricating a space triple-junction solar cell with a DBR structure includes the following steps:

[0019] 1) A buffer layer and a first tunnel junction are sequentially grown on a Ge substrate;

[0020] 2) A DBR structure is grown on the first tunnel junction, wherein the epitaxial growth time of each group of Al is changed. x Ga 1-x As layer and Al y Ga 1-y The thickness of the As layer decreases monotonically according to the growth sequence, with a variation range of 0.5% to 2% of the thickness of the previous layer;

[0021] 3) InGaAs middle cell, second tunnel junction, GaInP top cell and cap layer are grown sequentially on the DBR structure.

[0022] The beneficial effects of this invention are as follows:

[0023] By adding a DBR (Diffraction Bragg Mirror) structure between the middle cell and the bottom cell, and by monotonically varying the thickness of each group of combined layers within the DBR structure, the effective reflection bandwidth of the DBR is increased. This allows photons that are not absorbed by the middle cell but have energy higher than the middle cell's bandgap to be reflected back to the middle cell for reabsorption. This improves the problem of long-wavelength EQE reduction caused by the thinning of the middle cell, achieving both good irradiance characteristics and high light absorption efficiency, thus improving the overall performance of the solar cell. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the spatial triple-junction solar cell structure containing a DBR structure in Example 1;

[0025] Figure 2 This is a schematic diagram of the DBR structure in Example 1. Detailed Implementation

[0026] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments. The accompanying drawings are merely illustrative to facilitate understanding of the invention, and their specific proportions can be adjusted according to design requirements. The vertical relationships of relative elements and the definitions of front / back in the graphics described herein should be understood by those skilled in the art to refer to the relative positions of the components; therefore, they can all be flipped to present the same component, and all of this should fall within the scope disclosed in this specification.

[0027] Example 1

[0028] refer to Figure 1 The embodiment of the space triple-junction solar cell with DBR structure includes, from bottom to top, a Ge substrate 1, an n-GaInP nucleation layer 2, an n-InGaAs buffer layer 3, an n++-GaAs / P++-GaAs first tunnel junction 4, a DBR structure 5, and an In... 0.01 Ga 0.99 The As-based cell 6, n++-GaInP / P++-AlGaAs second tunnel junction 7, GaInP top cell 8, and n-GaAs cap layer 9. The DBR structure 5 comprises 16 pairs of alternately stacked DBRs, each pair being p-Al. 0.9 Ga 0.1 As / p-Al 0.1 Ga 0.9 As composite layer. Starting with the first pair of DBRs, each group of Al 0.9 Ga 0.1 As layer and Al 0.1 Ga 0.9 The thickness gradient of the As layer decreases by the same amount, and the Al layer of the first group is reduced based on the thickness gradient of the previous group. 0.9 Ga 0.1 As layer and Al 0.1 Ga 0.9 The As layer thickness is 1%, the center reflection wavelength of the first pair of DBRs is 890nm, and the effective reflection bandwidth of the formed DBR structure 5 is about 140nm, which covers the 750nm to 890nm band. The DBR structure 5 is p-type doped with a doping concentration of 1E18-4E18 cm-3.

[0029] refer to Figure 2 The first group of p-Al 0.9 Ga 0.1 As layer 51A and p-Al 0.1 Ga 0.9 If the thicknesses of As layer 51B are X1 and Y1 respectively, then the second group of p-Al on top of it... 0.9 Ga 0.1 As layer 52A and p-Al 0.1 Ga 0.9The thicknesses of the As layer 52B are X2 = 0.99X1 and Y2 = 0.99Y1, respectively. The third group of p-Al is located above the second group. 0.9 Ga 0.1 As layer 53A and p-Al 0.1 Ga 0.9 The thicknesses of As layer 53B are X3 = 0.98X1, Y3 = 0.98Y1, and so on, until the target logarithm is reached.

[0030] Specifically, in 0.01 Ga 0.99 In As, battery 6, from bottom to top, includes p-Al 0.5 Ga 0.5 As back field layer 61, p-In 0.01 Ga 0.99 As base region 62, n-In 0.01 Ga 0.99 As emission region 63, n-GaInP window layer 64; where p-In 0.01 Ga 0.99 The As base region 62 has a thickness of 1.5 μm. The GaInP top cell 8 consists of, from bottom to top, a p-AlGaInP back field layer 81, a p-Ga... 0.5 In 0.5 P-based region 82, n-Ga 0.5 In 0.5 P-emitting region 83 and n-AlInP window layer 84; wherein p-Ga 0.5 In 0.5 The thickness of the P-based region 82 is 0.6 μm. This forms a space Ga... 0.5I n 0.5 P / In 0.01 Ga 0.99 As / Ge triple-junction solar cells have bandgap combinations of 1.9eV / 1.4eV / 0.67eV, which are used to absorb different wavelength bands to fully utilize the solar energy spectrum and achieve high photoelectric conversion efficiency.

[0031] EQE tests revealed the absorption of different photons and the collection of photogenerated carriers. The top cell absorbed photons below 700 nm, while the Ge bottom cell absorbed photons above 900 nm. For the middle cell, its absorption capacity for photons of different wavelengths was affected by the thickness of the base region. When the base region thickness was 3 μm, the middle cell absorbed photons with energies greater than its bandgap quite well. When the thickness was reduced to 2 μm, the absorption capacity of the middle cell for photons with wavelengths above 800 nm was significantly weakened. When the base region thickness of the middle cell was reduced to 1.5 μm, the absorption attenuation of the middle cell began at a wavelength of 750 nm. Reducing the thickness of the base region can decrease the diffusion length of photogenerated carriers to the space charge region, thereby improving the collection efficiency of photogenerated carriers and obtaining better irradiation characteristics. When the thickness of the base region of the middle cell is expected to be reduced to 1.5 μm, this embodiment uses a DBR structure with gradually varying thickness to achieve an effective reflection bandwidth of about 140 nm, covering the long wavelength band of 900 nm. This will enable photons that are not absorbed by the middle cell and whose energy is higher than the band gap of the middle cell to be reflected back to the middle cell for reabsorption, thus improving the problem of long-wavelength EQE reduction caused by the reduction in thickness.

[0032] The fabrication method of the above-mentioned DBR structure space triple junction solar cell adopts an epitaxial growth process, on which an n-GaInP nucleation layer 2, an n-InGaAs buffer layer 3, and an n++-GaAs / P++-GaAs first tunnel junction 4, a DBR structure 5, and an In layer are sequentially grown on a Ge substrate 1. 0.01 Ga 0.99 The structure consists of an As-based cell 6, an n++-GaInP / P++-AlGaAs second tunnel junction 7, a GaInP top cell 8, and an n-GaAs cap layer 9. The DBR structure 5 is constructed using metal-organic chemical vapor deposition (MOCVD) with alternating p-Al layers. 0.9 Ga 0.1 As layer and p-Al 0.1 Ga 0.9 The thickness of the As layer is adjusted by controlling the growth time, so that each group of p-Al 0.9 Ga 0.1 As layer and p-Al 0.1 Ga 0.9 The thickness of the As layer is reduced by 1%X1 and 1%Y1 respectively compared to the previous group.

[0033] Comparative Example 1

[0034] The difference between Comparative Example 1 and Example 1 is that its DBR structure is composed of Al 0.9 Ga 0.1 As and Al 0.1 Ga 0.9As is composed of two materials grown alternately, with a reflection center wavelength of 870nm, 15 DBR pairs, and a constant thickness for each layer. The thickness of the base region in the middle cell is 2.1μm. All other details are the same as in Example 1.

[0035] Comparative Example 2

[0036] The difference between Comparative Example 2 and Example 1 is that the DBR structure is composed of Al 0.9 Ga 0.1 As and Al 0.1 Ga 0.9 As the two materials are grown alternately, this DBR contains two sets of DBRs with different reflection center wavelengths. During epitaxial growth, 15 pairs of DBRs with a reflection center wavelength of 870nm are grown first, and then another set of 15 pairs of DBRs with a reflection center wavelength of 790nm are grown. The thickness of the cell substrate region is 1.5μm. All other details are the same as in Example 1.

[0037] The irradiation performance of the space triple-junction solar cells of Example 1 and Comparative Examples 1-2 was tested under the following conditions:

[0038] Irradiation conditions: Electron energy: 1 MeV; Electron flux: 1 × 10¹¹ e cm⁻² s⁻¹; Electron injection rate: 1 E¹⁵ e cm⁻²;

[0039] Irradiation test environment: temperature 18℃, relative humidity 45%, normal pressure;

[0040] Test conditions: AM0@25℃.

[0041] The test results are shown in the table below:

[0042]

[0043] As can be seen from the table:

[0044] Due to the reduction in the thickness of the intermediate cell, Example 1 and Comparative Example 2 showed a reduction of approximately 3% in EFF attenuation after irradiation compared to Comparative Example 1.

[0045] The series resistance of Comparative Example 2 is greater than that of Example 1 and Comparative Example 1, resulting in a lower fill factor (FF) of approximately 2% and an EFF below 30%. The DBR with a phased change in center absorption wavelength in Comparative Example 2 requires a higher number of DBR pairs to achieve the same DBR reflection bandwidth as the DBR with a gradually changing center absorption wavelength in this invention. For example, Comparative Example 2 requires 30 DBR pairs to achieve similar performance as the 16 DBR pairs in Example 1. The interface between the two layers of the DBR structure exhibits a potential barrier peak, which significantly impacts the series resistance of the battery; the more DBR pairs, the greater the series resistance. Increased series resistance leads to a significant decrease in the battery's fill factor, affecting battery performance.

[0046] Therefore, through the structural design of Example 1, comprehensive performance that cannot be achieved in Comparative Example 1 and Comparative Example 2 can be realized.

[0047] The above embodiments are only used to further illustrate a spatial triple-junction solar cell with DBR structure and its preparation method according to the present invention. However, the present invention is not limited to the embodiments. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A spatial triple-junction solar cell with a DBR structure, characterized in that: The device comprises, in sequence, a buffer layer, a first tunneling junction, a DBR structure, an InGaAs inner cell, a second tunneling junction, a GaInP top cell, and a cap layer disposed on a Ge substrate. The DBR structure is used to reflect photons in the absorption band of the InGaAs inner cell. The InGaAs inner cell, from bottom to top, comprises a p-AlGaAs back field layer, a p-InGaAs base region, an n-InGaAs emitter region, and an n-GaInP window layer. The thickness of the p-InGaAs base region is less than 2 μm. The DBR structure includes several groups of Al alternating stacks. x Ga 1-x As / Al y Ga 1-y As composite layer, 0.85≤x<1, 0<y≤0.15; each group Al x Ga 1-x As / Al y Ga 1-y The As composite layer forms a pair of DBRs, and the number of pairs of DBR structures is 10 to 16; the DBR structures are p-type doped with a doping concentration of 1E18-4E18 cm⁻¹. -3 The Al x Ga 1-x As / Al y Ga 1-y As the thickness of the composite layers decreases monotonically and uniformly according to the stacking sequence, and the Al of each group... x Ga 1-x As layer and Al y Ga 1-y The thickness reduction of the As layer relative to the previous group is as follows: (The percentage reduction in thickness is not specified in the original text, but can be left as is.) x Ga 1-x As layer and Al y Ga 1-y The thickness of the As layer is 0.5% to 2%; the center reflection wavelength of the first pair of DBRs is 880 to 900 nm; the effective reflection bandwidth of the DBR structure is 130 to 150 nm.

2. The spatial triple-junction solar cell with DBR structure according to claim 1, characterized in that: The center reflection wavelength of the first pair of DBRs from bottom to top is 890nm, the effective reflection bandwidth of the DBR structure is 140nm, and the thickness of the p-InGaAs base region is 1.5μm.

3. The method for preparing a spatial triple-junction solar cell with a DBR structure according to any one of claims 1 to 2, characterized in that, Includes the following steps: 1) A buffer layer and a first tunnel junction are sequentially grown on a Ge substrate; 2) A DBR structure is grown on the first tunnel junction, wherein the epitaxial growth time is changed to make the Al of each group... x Ga 1-x As layer and Al y Ga 1-y The thickness of the As layer decreases monotonically according to the growth sequence, with a variation range of 0.5% to 2% of the thickness of the previous layer; 3) InGaAs middle cell, second tunnel junction, GaInP top cell and cap layer are grown sequentially on the DBR structure.

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