Bragg reflector with asymmetric reflection and solar cell

Abstract

The invention provides a Bragg reflector with asymmetric reflection and a solar cell, the Bragg reflector comprises a plurality of periodic units which are sequentially arranged along the thickness direction of the Bragg reflector, each periodic unit comprises a plurality of periodic material layers, and the periodic material layers of the same periodic unit have the same thickness; and the periodic material layers of different periodic units have different thicknesses and are used for reflecting photons in different wavelength regions. According to the invention, the reflectivity of the Bragg reflector in a short wave band is improved, the reflectivity of the Bragg reflector in a long wave band is reduced, and the efficiency and the radiation resistance of the solar cell are improved.

Description

Technical Field

[0001] This invention belongs to the field of solar cell technology, and in particular relates to a Bragg reflector and a solar cell with asymmetric reflection. Background Technology

[0002] The space environment contains a large number of charged particles that bombard solar cells, causing damage and degradation of their lifespan. Current technologies address this issue by employing Bragg reflectors. These reflectors typically utilize an AlGa(In)As / AlGa(In)As structure, varying the refractive index through the Al composition to obtain both low- and high-refractive-index materials, thus achieving photon reflection at specific wavelengths and increasing current and radiation resistance. Existing Bragg reflector structures consist of two alternating layers of media with different refractive indices, each layer having the same thickness, achieving high reflection within a relatively short wavelength range. However, when testing reflectivity using this structure, interference fringes appear in the longer wavelengths of the target wavelength range, leading to increased long-wavelength reflection. This increased long-wavelength reflection reduces the short-circuit current of the lower sub-cells, resulting in decreased solar cell efficiency, a phenomenon particularly pronounced in the IMM-3J cell. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides a Bragg reflector and a solar cell with asymmetric reflection, which improves the reflectivity of the Bragg reflector in the short-wavelength band and reduces its reflectivity in the long-wavelength band, thereby enhancing the efficiency and radiation resistance of the solar cell.

[0004] The technical solution adopted in this invention is: a Bragg reflector with asymmetric reflection, comprising a plurality of periodic units arranged sequentially along its thickness direction, each periodic unit comprising several periodic material layers, the periodic material layers of the same periodic unit having the same thickness; the periodic material layers of different periodic units having different thicknesses, for reflecting photons in different wavelength regions.

[0005] Furthermore, along the direction of light incidence, the thickness of the periodic material layer of different periodic units increases sequentially.

[0006] Furthermore, the materials of the periodic material layers in the same periodic unit are the same.

[0007] Furthermore, the periodic material layer includes a high refractive index layer and a low refractive index layer.

[0008] Furthermore, the high refractive index layer is Al. x Gain z As layer, the low refractive index layer is Al y Gainz For layer A, z ≤ 0.3, yx > 0.4.

[0009] Furthermore, along the direction of light incidence, the thicknesses of the high-refractive-index layer and the low-refractive-index layer of the first periodic unit are: ; in, λ 1. λ 2 represents the center wavelength range of the target reflection zone; n is the refractive index of the high-refractive-index layer or the low-refractive-index layer of the periodic material layer.

[0010] Furthermore, along the direction of light incidence, the thicknesses of the high-refractive-index layer and the low-refractive-index layer in the last periodic unit are: ; in, λ 1 represents the minimum value within the center wavelength range of the target reflection zone; n is the refractive index of the high-refractive-index layer or the low-refractive-index layer of the periodic material layer.

[0011] Furthermore, the thickness of the high-refractive-index layer and the low-refractive-index layer of the periodic unit located in the middle is h2, where h1≤h2≤h0.

[0012] Furthermore, the number of periodic material layers is N, where N≥12; the number of periodic units is 2-N / 2.

[0013] A solar cell comprising the aforementioned Bragg reflector with asymmetric reflection.

[0014] The advantages and positive effects of this invention are as follows: By dividing the periodic material layer into different periodic units, making the thickness of the periodic material layer in the same periodic unit the same, and the thickness of the periodic material layer in different periodic units different, and increasing the thickness of the periodic material layer in different periodic units along the direction of light incidence, different periodic units are used to reflect photons in different wavelength regions, thereby improving its reflectivity in the short-wavelength band, offsetting the interference effect in the long-wavelength band, reducing its reflectivity in the long-wavelength band, reducing reflection loss, and thus improving the efficiency and radiation resistance of the battery. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a Bragg reflector structure with asymmetric reflection in an embodiment of the present invention; Figure 2 This is a schematic diagram of the reflectivity curve of the solar cell in the embodiment; Figure 3 This is a schematic diagram of the reflectivity curve of a solar cell in the comparative example.

[0016] In the picture: 1. First periodic unit; 2. Second periodic unit; 4. Fourth periodic unit; 5. Fifth periodic unit; 6. Periodic material layer. Detailed Implementation

[0017] The embodiments of the present invention will now be described with reference to the accompanying drawings.

[0018] like Figure 1 As shown, this embodiment of the invention proposes a Bragg reflector with asymmetric reflection, comprising a plurality of periodic units arranged sequentially along its thickness direction. Each periodic unit includes several periodic material layers. The periodic material layers in the same periodic unit have the same thickness, while the periodic material layers in different periodic units have different thicknesses, for reflecting photons in different wavelength regions.

[0019] By designing the thickness of the periodic material layers in a Bragg reflector, the optical path difference can be precisely controlled, achieving constructive interference and efficient reflection of light at specific wavelengths while suppressing the transmission of other wavelengths, thus forming narrowband high reflectivity. In existing technologies, Bragg reflectors consist of multiple periodic material layers, each with the same thickness. This structure can achieve high reflection within a relatively short wavelength range; however, it inevitably forms a series of secondary reflection peaks at both ends of the main reflection peak due to interference effects. These secondary reflection peaks have a certain reflectivity for long-wavelength photons, leading to increased long-wavelength reflection and consequently short-circuit current in the underlying sub-cell, reducing cell efficiency. This application addresses this by dividing the periodic material layer into different periodic units. By rationally setting the thickness of the periodic material layer within the same periodic unit and the thickness of the periodic material layer in different periodic units, different periodic units are used to reflect photons in different wavelength regions, canceling the interference effect in the long-wavelength range. This reduces the reflectivity of the Bragg reflector in the long-wavelength range, thereby reducing reflection loss and improving cell efficiency and radiation resistance.

[0020] Furthermore, along the direction of light incidence, the thickness of the periodic material layer of different periodic units increases sequentially.

[0021] In one specific embodiment, an asymmetric Bragg reflector is used in a triple-junction solar cell. The triple-junction solar cell has a top cell and a bottom cell. The top cell is the uppermost layer of the triple-junction cell, and sunlight directly enters the top cell, which directly absorbs photons in the short-wavelength region of sunlight, converting light energy into electrical energy. The bottom cell is the uppermost and lowest layer of the triple-junction cell, absorbing photons in the medium and long-wavelength regions that are not utilized by the top cell. The asymmetric Bragg reflector proposed in this embodiment is placed between the top and bottom cells. Along the direction of light incidence, the periodic material layer of the periodic unit near the top cell is thinner, used to reflect photons in the medium and short-wavelength regions that are not absorbed by the top cell, thereby reducing the transmission loss of medium and short-wavelength photons and improving the absorption efficiency of the top cell. The periodic material layer of the periodic unit near the bottom cell is thicker, used to reflect photons in the central region (whether corresponding to the medium or long-wavelength region). The superposition of the effects of different periodic units cancels out the interference effects of medium and long wavelengths, thereby reducing the reflectivity in the long-wavelength band. Therefore, the asymmetric Bragg reflector structure proposed in this application improves its reflectivity in the short-wavelength band, resulting in a better gain effect for the battery; at the same time, it reduces its reflectivity in the long-wavelength band, reducing reflection loss, thereby improving the battery efficiency and radiation resistance.

[0022] Furthermore, in the embodiments of this application, the periodic material layers of the same periodic unit are made of the same material, so as to accurately control the thickness of the periodic material layer.

[0023] In one specific embodiment, the periodic material layer includes staggered high-refractive-index layers and low-refractive-index layers, which form a refractive index difference. Optical path difference control and constructive interference are achieved through the refractive index difference. The larger the refractive index difference or the more layers there are, the narrower the bandwidth and the higher the reflectivity. Furthermore, the high refractive index layer is Al x Gain z The As layer and the low-refractive-index layer are Al. y Gain z As layer, Al x Gain z As and Al y Gain z As are all materials capable of forming high refractive index differences. By using these materials to form high-refractive-index and low-refractive-index layers, the required number of periodic material layers can be reduced, simplifying the manufacturing process. In a specific embodiment, the aforementioned Bragg reflector with asymmetric reflection is used in a triple-junction solar cell, where z ≤ 0.3 and yx > 0.4. The In content in this cell structure does not exceed 30%; by setting yx > 0.4, its reflectivity can be ensured to meet the usage requirements.

[0024] Furthermore, along the direction of light incidence, the thicknesses of the high-refractive-index layer and the low-refractive-index layer of the first periodic unit are: ; in, λ 1. λ 2 represents the center wavelength range of the target reflection zone; n represents the refractive index of either the high-refractive-index layer or the low-refractive-index layer of the periodic material layer.

[0025] It is understandable that, due to the different refractive indices of the high-refractive-index layer and the low-refractive-index layer, the thicknesses of the high-refractive-index layer and the low-refractive-index layer obtained by the above calculation are also different; within the same periodic unit, all high-refractive-index layers have the same thickness, and all low-refractive-index layers have the same thickness, so that the periodic material layers formed by the two have the same thickness.

[0026] This setting allows the thickness of the first periodic cell along the direction of light incidence to determine the position of the main reflection peak.

[0027] Furthermore, along the direction of light incidence, the thicknesses of the high-refractive-index layer and the low-refractive-index layer in the last periodic unit are: ; in, λ 1 represents the minimum value within the center wavelength range of the target reflection zone; n is the refractive index of the high refractive index layer or the low refractive index layer.

[0028] This setting ensures that the thickness limitation of the last periodic cell along the light incident direction guarantees sufficient intensity for the central reflection peak of the DBR. If the thickness is less than this value, the reflectivity at the center wavelength will decrease; if the thickness is greater, the asymmetric reflection objective of this invention cannot be achieved.

[0029] Furthermore, the thickness of the high-refractive-index layer and the low-refractive-index layer in the central periodic unit is h2, where h1≤h2≤h0. This arrangement ensures that the central periodic unit guarantees the reflection intensity of the reflection center peak along the direction of light incidence.

[0030] In a preferred embodiment, there are N periodic material layers, where N ≥ 12, and 2-N / 2 periodic units. The more periodic material layers there are, the higher the reflectivity. When there are 12 or more periodic material layers and 2 or more periodic units, the reflectivity of the reflector can be ensured to meet the battery's requirements. Specifically, the number of periodic material layers can be 12, 13, 14, 15, 16, etc., as long as it is not less than 12; the number of periodic units can be 2, 3, 4, 5, 6, etc.; each periodic unit can include one periodic material layer, or two or more periodic material layers, which can be set according to requirements.

[0031] This application also proposes a solar cell structure that includes the Bragg reflector with asymmetric reflection proposed in this application.

[0032] Specifically, the solar cell includes a top cell, a tunnel junction, a spacer layer, a Bragg reflector with asymmetric reflection, a spacer layer, and a bottom cell arranged in sequence.

[0033] The technical solution of this application will be described in detail below through specific embodiments. Example

[0034] A solar cell includes a top cell, a tunnel junction, a spacer layer, a Bragg reflector with asymmetric reflection, a spacer layer, and a bottom cell arranged in sequence.

[0035] The Bragg reflector with asymmetric reflection comprises 16 periodic material layers, forming a first periodic unit, a second periodic unit, a third periodic unit, a fourth periodic unit, and a fifth periodic unit along the direction from the top cell to the bottom cell. The first periodic unit includes two periodic material layers, each with a thickness of 140 nm; the second periodic unit includes three periodic material layers, each with a thickness of 142 nm; the third periodic unit includes three periodic material layers, each with a thickness of 144 nm; the fourth periodic unit includes three periodic material layers, each with a thickness of 146 nm; and the fifth periodic unit includes five periodic material layers, each with a thickness of 150 nm.

[0036] Specifically, the method for fabricating this solar cell includes the following steps: The method for fabricating this solar cell is as follows: S1. GaInP, tunnel junction, and GaAs top cell are sequentially epitaxially grown on the substrate; S2, epitaxial AlGaAs / GaAs tunnel junction; S3, an epitaxial n-type AlGaAs spacer layer, with Si or Mg as the dopant, a doping concentration of 5E17-5E18, and a thickness of 200-500 nm; (in specific embodiments, range values ​​are avoided as much as possible, unless only range values ​​can be achieved in actual operation, the same below) S4, two extensional Al 0.8 Gain 0.05 As / Al 0.1 Gain 0.05 As periodic material layer, n-type doped, with Si or Mg as dopants, doping concentration of 5E17-5E18, and thickness of 140nm.

[0037] S5, three extensional Al 0.7 Gain 0.15 As / Al0.1 Gain 0.1 The 5As periodic material layer is n-type doped with Si or Mg as the dopant, and the doping concentration is 5E17-5E18. The thickness of the periodic material layer is 42nm.

[0038] S6, three extensional Al 0.65 Gain 0.2 As / Al 0.1 Gain 0.2 As periodic material layer, n-type doped, with Si or Mg as dopants, doping concentration of 5E17-5E18, and thickness of 144nm.

[0039] S7, three extensional Al 0.6 Gain 0.25 As / Al 0.1 Gain 0.25 As periodic material layer, n-type doped, with Si or Mg as dopants, doping concentration of 5E17~5E18, and thickness of periodic material layer of 146nm.

[0040] S8, five extended Al 0.5 Gain 0.35 As / Al 0.1 Gain 0.35 As periodic material layer, n-type doped, with Si or Mg as dopants, doping concentration of 5E17~5E18, and thickness of periodic material layer of 150nm.

[0041] S9, epitaxial n-type Al 0.5 Gain 0.28 As spacer layer, doped with Si or Mg, with a doping concentration of 5E17~5E18 and a thickness of 200-500nm.

[0042] S10, Epitaxial GaIn 0.28 As-based batteries.

[0043] In the embodiments described above, the solar cells are prepared using a reverse epitaxial method; in some other embodiments of this application, they can also be prepared using a forward epitaxial method, both of which can achieve the structural and functional effects of the solar cells.

[0044] Comparative Example A solar cell in the prior art includes a top cell, a tunnel junction, a spacer layer, a Bragg reflector, a spacer layer, and a bottom cell arranged in sequence.

[0045] The Bragg reflector uses a structure found in existing technology, consisting of 16 periodic material layers, each with a thickness of 140 nm.

[0046] The method for fabricating this solar cell is as follows: S1. GaInP, tunnel junction, and GaAs top cell are sequentially epitaxially grown on the substrate; S2, epitaxial AlGaAs / GaAs tunnel junction; S3, an epitaxial n-type AlGaAs spacer layer, with Si or Mg as the dopant, a doping concentration of 5E17-5E18, and a thickness of 200-500 nm; S4, sequentially extend two Al... 0.8 Gain 0.05 As / Al 0.1 Gain 0.05 As periodic material layer, n-type doped, with Si or Mg as dopants, doping concentration of 5E17-5E18, and thickness of 140nm.

[0047] S5, epitaxial n-type Al 0.5 Gain 0.28 As spacer layer, doped with Si or Mg, with a doping concentration of 5E17~5E18 and a thickness of 200-500nm.

[0048] S6, Epitaxial GaIn 0.28 As-based batteries.

[0049] Reflectance measurements were performed on the solar cells in Example 1 and Comparative Example 1 using a UV-Vis-NIR spectrophotometer with an integrating sphere attachment. The test range was 400 nm to 1200 nm, and the scan step size was 1 nm. The reflectance curves of the solar cells were obtained; where the integrating sphere attachment... Figure 2 The attached diagram shows the solar cell reflectivity curve in the embodiment. Figure 3 This is a reflectance curve of the solar cell in the comparative example. In the figure, the horizontal axis represents wavelength, and the vertical axis represents reflectance. By comparison, it can be concluded that the solar cell in the embodiment has a significantly higher reflectance for short wavelengths than the solar cell in the comparative example; at the same time, the solar cell in the embodiment has a significantly lower reflectance for long wavelengths than the solar cell in the comparative example.

[0050] This application divides the periodic material layer into different periodic units. By making the periodic material layer within the same periodic unit the same thickness, and the periodic material layer in different periodic units different thicknesses, and increasing the thickness of the periodic material layer in different periodic units along the direction of light incidence, different periodic units are used to reflect photons in different wavelength regions. This improves the reflectivity in the short-wavelength band, cancels the interference effect in the long-wavelength band, reduces the reflectivity in the long-wavelength band, reduces reflection loss, and thus improves the efficiency and radiation resistance of the battery.

[0051] The embodiments of the present invention have been described in detail above, but the content described is only a preferred embodiment of the present invention and should not be considered as limiting the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the patent coverage of the present invention.

Claims

1. A Bragg reflector with asymmetric reflection, characterized in that: It includes multiple periodic units arranged sequentially along its thickness direction. Each periodic unit includes several periodic material layers. The periodic material layers in the same periodic unit have the same thickness. The periodic material layers in different periodic units have different thicknesses, which are used to reflect photons in different wavelength regions.

2. The Bragg reflector with asymmetric reflection according to claim 1, characterized in that: Along the direction of light incidence, the thickness of the periodic material layer of different periodic units increases sequentially.

3. The Bragg reflector with asymmetric reflection according to claim 2, characterized in that: The materials of the periodic material layers in the same periodic unit are the same.

4. The Bragg reflector with asymmetric reflection according to claim 3, characterized in that: The periodic material layer includes a high refractive index layer and a low refractive index layer.

5. The Bragg reflector with asymmetric reflection according to claim 4, characterized in that: The high refractive index layer is Al. x Gain z As layer, the low refractive index layer is Al y Gain z For layer A, z ≤ 0.3, yx > 0.

4.

6. The Bragg reflector with asymmetric reflection according to claim 4, characterized in that: Along the direction of light incidence, the thicknesses of the high-refractive-index layer and the low-refractive-index layer of the first periodic unit are: ； in, λ 1. λ 2 represents the center wavelength range of the target reflection zone; n is the refractive index of the high-refractive-index layer or the low-refractive-index layer of the periodic material layer.

7. The Bragg reflector with asymmetric reflection according to claim 6, characterized in that: Along the direction of light incidence, the thicknesses of the high-refractive-index layer and the low-refractive-index layer in the last periodic unit are: ； in, λ 1 represents the minimum value within the center wavelength range of the target reflection zone; n is the refractive index of the high-refractive-index layer or the low-refractive-index layer of the periodic material layer.

8. The Bragg reflector with asymmetric reflection according to claim 7, characterized in that: The thickness of the high-refractive-index layer and the low-refractive-index layer of the periodic unit located in the middle is h2, where h1≤h2≤h0.

9. The Bragg reflector with asymmetric reflection according to claim 1, characterized in that: The number of periodic material layers is N, where N≥12; the number of periodic units is 2-N / 2.

10. A solar cell, characterized in that: Includes the Bragg reflector with asymmetric reflection as described in any one of claims 1-9.

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