Epitaxial structure of a triple-junction solar cell and method for manufacturing the same

Abstract

The application provides an epitaxial structure of a three-junction solar cell and a preparation method thereof. The epitaxial structure comprises a Ge sub-cell, a first tunnel junction, an InGaAs sub-cell, a second tunnel junction and a GaInP sub-cell which are sequentially stacked, wherein the InGaAs sub-cell comprises a plurality of groups of quantum well layers, each group of quantum well layers is composed of InGaAs well layers and GaAsP barrier layers which are alternately stacked, in the plurality of groups of quantum well layers, the In component of the InGaAs well layers gradually decreases in the direction away from the Ge sub-cell, the absorption range of the InGaAs sub-cell to long-wave photons is effectively widened by gradually absorbing the sunlight spectrum that cannot be absorbed by the InGaAs base region, atomic diffusion of the quantum well layers is slowed down, and the photoelectric conversion efficiency of the solar cell is effectively improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of semiconductor devices, and particularly relates to an epitaxial structure of a three-junction solar cell and a preparation method thereof. BACKGROUND

[0002] Solar energy as a clean and sustainable energy has attracted extensive attention in recent years. The lattice-matched GaInP / InGaAs / Ge three-junction solar cell is widely used in satellites and spacecrafts due to its high photoelectric conversion efficiency and excellent anti-radiation performance. At present, the photoelectric conversion efficiency of the mature process type of the battery is generally about 31%, and the bandgaps of the sub-cells are 0.67 eV (Ge), 1.41 eV (InGaAs) and 1.90 eV (GaInP) respectively. Since the bandgap difference between Ge and InGaAs reaches 0.74 eV, which is much larger than the bandgap difference (0.49 eV) between GaInP and InGaAs, the excessive photo-generated current is generated in the Ge sub-cell. Since the three-junction cell adopts a series structure, the overall short-circuit current is limited by the sub-cell with the smallest current, and the current density of the Ge sub-cell is greater than that of the other two sub-cells, so that the Ge sub-cell can only output a small current density, and the excessive current generated thereby cannot be utilized, resulting in a waste of excessive current and limiting the photoelectric conversion efficiency of the GaInP / InGaAs / Ge three-junction solar cell.

[0003] In order to improve the photoelectric conversion efficiency of the GaInP / InGaAs / Ge three-junction solar cell, the bandgap difference between the sub-cells needs to be reduced. Since Ge is a single-element semiconductor, its bandgap cannot be adjusted, and the current mainstream technical path mainly includes the following two kinds:

[0004] 1. By increasing the proportion of In component in InGaAs, the bandgap of InGaAs is reduced, so that the current density of the InGaAs sub-cell is increased to match the Ge sub-cell. However, increasing the In component will lead to lattice mismatch between the InGaAs material and the Ge substrate, introducing dislocations and other crystal defects, reducing the material quality, and further affecting the battery performance.

[0005] 2. An InGaAs / GaAsP multi-quantum well layer is inserted in the InGaAs sub-cell, and the tensile strain introduced by GaAsP is used to offset the compressive strain of InGaAs, so that the In component of InGaAs is increased while the stress balance is achieved, the lattice matching is maintained, and the mismatch dislocations are avoided. However, there is a large bandgap difference between InGaAs and GaAsP in this method, and serious atomic diffusion will occur at the interface, resulting in poor InGaAs material quality and further leading to deviation of the overall performance, which is also not conducive to obtaining good battery performance. SUMMARY

[0006] The present application aims to provide a three-junction solar cell epitaxial structure and a preparation method thereof, expand the absorption spectrum of the InGaAs sub-cell, slow down the atomic diffusion of the quantum well layer, and effectively improve the photoelectric conversion efficiency of the solar cell.

[0007] To achieve the above-mentioned purpose, the present application provides a three-junction solar cell epitaxial structure, comprising a Ge sub-cell, a first tunnel junction, an InGaAs sub-cell, a second tunnel junction and a GaInP sub-cell which are sequentially stacked, wherein the InGaAs sub-cell comprises a plurality of groups of quantum well layers, each group of quantum well layers is composed of InGaAs well layers and GaAsP barrier layers which are alternately stacked; in the plurality of groups of quantum well layers, the In content of the InGaAs well layers gradually decreases in the direction away from the Ge sub-cell, and the P content of the GaAsP barrier layers gradually decreases in the direction away from the Ge sub-cell, the InGaAs sub-cell comprises three groups of quantum well layers, which are sequentially a first group of quantum well layers, a second group of quantum well layers and a third group of quantum well layers in the direction away from the Ge sub-cell.

[0008] In the first group of quantum well layers, the In content of the InGaAs well layers is 10%, and the thickness is 8.5nm, and the photoluminescence wavelength of the first group of quantum well layers is 930nm.

[0009] In the second group of quantum well layers, the In content of the InGaAs well layers is 7.8%, and the thickness is 8.5nm, and the photoluminescence wavelength of the second group of quantum well layers is 910nm.

[0010] In the third group of quantum well layers, the In content of the InGaAs well layers is 5.5%, and the thickness is 8.5nm, and the photoluminescence wavelength of the third group of quantum well layers is 890nm.

[0011] A plurality of groups of InGaAs / AlGaAs DBR layers are arranged between the first tunnel junction and the InGaAs sub-cell, the reflection wavelengths of the plurality of groups of DBR layers gradually decrease in the direction away from the Ge sub-cell, and the effective reflection wavelength of the plurality of groups of DBR layers after compounding covers the wavelength band of 665nm-930nm.

[0012] Optionally, in the first group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5nm, and the P content is 20%; in the second group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5nm, and the P content is 15.5%; in the third group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5nm, and the P content is 11%.

[0013] Optionally, in the first group of quantum well layers, the second group of quantum well layers and the third group of quantum well layers, the number of pairs of the alternately stacked InGaAs well layers and GaAsP barrier layers is 30 pairs.

[0014] Optionally, the first tunnel junction and the InGaAs sub-cell are provided with three groups of DBR layers, in the direction away from the Ge sub-cell, the first DBR layer, the second DBR layer and the third DBR layer are sequentially arranged, the reflection wavelength of the first DBR layer is 830nm-930nm, the reflection wavelength of the second DBR layer is 745nm-835nm, and the reflection wavelength of the third DBR layer is 665nm-745nm.

[0015] Optionally, the first DBR layer is grown by alternating In z1 Ga 1-z1 As and Al z2 Ga 1-z 2As two materials, wherein z1=0.01, 0.6≤z2≤1, and the center wavelength of the first DBR layer is 880nm, and the reflection spectrum width is 100nm; the second DBR layer is grown by alternating Al y1 Ga 1-y1 As and Al y2 Ga 1-y2 As two materials, wherein 0.3≤y1≤0.5, 0.8≤y2≤1, and the center wavelength of the second DBR layer is 790nm, and the reflection spectrum width is 90nm; the third DBR layer is grown by alternating Al x1 Ga 1-x1 As and Al x2 Ga 1- x2 As two materials, wherein 0.3≤x1≤0.5, 0.8≤x2≤1, and the center wavelength of the third DBR layer is 705nm, and the reflection spectrum width is 80nm.

[0016] Optionally, the structure of the InGaAs sub-cell includes AlGaAs back field layer, InGaAs base region, multiple groups of quantum well layers, InGaAs emission region and AlInP window layer, which are sequentially grown in the direction away from the Ge sub-cell, wherein the thickness of the InGaAs base region is 2000nm-2500nm.

[0017] The application further provides a preparation method of an epitaxial structure of a three-junction solar cell, for preparing the epitaxial structure, comprising:

[0018] providing a substrate, and forming a Ge sub-cell on the substrate;

[0019] growing a first tunnel junction on the Ge sub-cell;

[0020] The InGaAs subcell is grown on the first tunnel junction, and the InGaAs subcell comprises a plurality of groups of quantum well layers, each group of quantum well layers being alternately grown by InGaAs well layers and GaAsP barrier layers; in the plurality of groups of quantum well layers, the In component of the InGaAs well layers gradually decreases in a direction away from the Ge subcell, and the P component of the GaAsP barrier layers gradually decreases in the direction away from the Ge subcell; the InGaAs subcell comprises three groups of quantum well layers, and the three groups of quantum well layers are sequentially a first group of quantum well layers, a second group of quantum well layers, and a third group of quantum well layers in the direction away from the Ge subcell;

[0021] In the first group of quantum well layers, the In component of the InGaAs well layer is 10%, and the thickness is 8.5 nm; the photoluminescence wavelength of the first group of quantum well layers is 930 nm;

[0022] In the second group of quantum well layers, the In component of the InGaAs well layer is 7.8%, and the thickness is 8.5 nm; the photoluminescence wavelength of the second group of quantum well layers is 910 nm;

[0023] In the third group of quantum well layers, the In component of the InGaAs well layer is 5.5%, and the thickness is 8.5 nm; the photoluminescence wavelength of the third group of quantum well layers is 890 nm;

[0024] A plurality of groups of InGaAs / AlGaAs DBR layers are arranged between the first tunnel junction and the InGaAs subcell, the reflection wavelengths of the plurality of groups of DBR layers gradually decrease in the direction away from the Ge subcell, and the effective reflection wavelength of the plurality of groups of DBR layers after combination covers a wavelength band of 665 nm-930 nm;

[0025] The second tunnel junction is grown on the InGaAs subcell;

[0026] The GaInP subcell is grown on the second tunnel junction.

[0027] After the above scheme is adopted, the application has the following advantages:

[0028] 1. In the InGaAs subcell, a plurality of groups of InGaAs / GaAsP quantum well layers are inserted, and the In component of the InGaAs well layer gradually decreases in the direction away from the Ge subcell; therefore, the photoluminescence wavelength of the quantum well layer gradually increases in the direction facing the Ge subcell, the InGaAs-based region gradually absorbs sunlight that cannot be absorbed, the absorption range of the InGaAs subcell to long-wave photons is effectively widened, the absorption efficiency of the InGaAs subcell is increased, and the photoelectric conversion efficiency of the solar cell is further improved; compared with a single group of quantum well layers, the In component gradually decreases in the plurality of groups of quantum well layers, which moderates the sharp component mutation at the interface, effectively suppresses the atomic diffusion phenomenon at the heterojunction interface, and ensures the crystal quality of the quantum well layer.

[0029] 2、The application adopts multiple groups of DBR composite structures, and the reflection spectrum of the multiple groups of DBR layers after being combined lasts from 665 nm to 930 nm, covering the entire absorption spectrum of the InGaAs sub-cell, so that the unabsorbed sunlight can be reflected back to the InGaAs sub-cell for reabsorption, the absorption efficiency of the InGaAs sub-cell is improved, meanwhile, the thickness of the base region and the quantum well structure of the InGaAs sub-cell can be reduced, and the anti-radiation capability of the solar cell is effectively increased.

[0030] 3、In the traditional three-junction solar cell without DBR design, the thickness of the base region of the InGaAs sub-cell needs to reach 3 microns or above to achieve the target current density, and the application has multiple DBR layers, so that the unabsorbed light spectrum can be reflected back to the middle cell for reabsorption, the thickness of the base region of the InGaAs sub-cell can be reduced to 2000-2500 nm, the radiation damage of the base region of the cell in outer space is reduced, and the anti-radiation capability of the cell is improved; similarly, the thickness of the quantum well structure can also be reduced, further increasing the anti-radiation capability of the solar cell.

[0031] 4、All epitaxial layer materials in the application are carefully designed to maintain lattice matching with the Ge substrate. This not only avoids the problem of directly introducing mismatch dislocations due to increasing the In component, but also the InGaAs / GaAsP quantum well layer used is a strain compensation structure, which can further offset stress and inhibit dislocation generation. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 It is an epitaxial structure diagram of the solar cell of the application.

[0033] Figure 2 It is a structure diagram of the quantum well layer of the application.

[0034] Figure 3 It is a growth method flowchart of the application.

[0035] REFERENCE NUMERALS:

[0036] 1, Ge sub-cell; 11, substrate; 12, GaInP nucleation layer; 13, GaAs buffer layer; 2, first tunnel junction; 3, DBR layer; 31, first DBR layer; 32, second DBR layer; 33, third DBR layer; 4, InGaAs sub-cell; 41, AlGaAs back field layer; 42, InGaAs base region; 43, quantum well layer; 431, InGaAs well layer; 432, GaAsP barrier layer; 44, InGaAs emission region; 45, AlInP window layer; 5, second tunnel junction; 6, GaInP sub-cell; 61, AlGaInP back field layer; 62, GaInP base region; 63, GaInP emission region; 64, AlInP window layer; 7, GaAs ohmic contact layer. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application, and the range of the present application includes two end values.

[0038] As shown in Figure 1 , the present application provides an epitaxial structure of a triple-junction solar cell, comprising a Ge sub-cell 1, a first tunnel junction 2, an InGaAs sub-cell 4, a second tunnel junction 5 and a GaInP sub-cell 6 which are sequentially stacked from bottom to top, wherein the InGaAs sub-cell 4 contains a plurality of groups of quantum well layers 43, as shown in Figure 2 , each group of quantum well layers 43 is composed of InGaAs well layers 431 and GaAsP barrier layers 432 which are alternately stacked. In the plurality of groups of quantum well layers 43, the In component of the InGaAs well layers 431 gradually decreases in the direction away from the Ge sub-cell 1 (i.e. from bottom to top), so that the photoluminescence wavelength of the quantum well layers gradually increases in the direction facing the Ge sub-cell (i.e. from top to bottom), thereby realizing gradual absorption of solar spectrum that cannot be absorbed by the InGaAs base region, effectively widening the absorption range of long-wave photons by the InGaAs sub-cell, and increasing the absorption efficiency of the InGaAs sub-cell; and compared with a single group of quantum well layers, the gradual decrease of the In component in the plurality of quantum well layers of the present application moderates the sharp composition mutation at the interface, effectively inhibits the atomic diffusion phenomenon at the heterojunction interface, and ensures the crystal quality of the quantum well layer.

[0039] Preferably, in the plurality of quantum well layers 43, the P component of the GaAsP barrier layer 432 gradually decreases in the direction away from the Ge sub-cell 1, which can offset the strain caused by the change of In component, thereby avoiding the generation of mismatch dislocations and ensuring the high crystal quality of the entire epitaxial structure. The band gap of the barrier material caused by the P component can better confine the carriers in the well, ensuring that the target emission wavelength set by the In component can be efficiently and stably achieved.

[0040] Specifically, the InGaAs sub-cell 4 comprises three groups of quantum well layers, which are the first group of quantum well layers, the second group of quantum well layers and the third group of quantum well layers in the direction away from the Ge sub-cell 1.

[0041] In the first group of quantum well layers, the In component of the InGaAs well layer 431 is 10%, the thickness is 8.5 nm, the P component of the GaAsP barrier layer 432 is 20%, and the thickness is 8.5 nm. The photoluminescence wavelength of the first group of quantum well layers is 930 nm, which can absorb light spectrum below 930 nm.

[0042] In the second group of quantum well layers, the In component of the InGaAs well layer is 7.8%, the thickness is 8.5 nm, the P component of the GaAsP barrier layer is 15.5%, and the thickness is 8.5 nm. The photoluminescence wavelength of the second group of quantum well layers is 910 nm, which can absorb light spectrum below 910 nm.

[0043] In the third group of quantum well layers, the In component of the InGaAs well layer is 5.5%, the thickness is 8.5 nm, the P component of the GaAsP barrier layer is 11%, and the thickness is 8.5 nm. The photoluminescence wavelength of the third group of quantum well layers is 890 nm, which can absorb light spectrum below 890 nm.

[0044] The sunlight passes through the third group of quantum well layers first, and then is absorbed by the second group of quantum well layers and the first group of quantum well layers in turn, thereby realizing the step-by-step absorption of sunlight spectrum that cannot be absorbed by the InGaAs base region and effectively widening the absorption range of the InGaAs sub-cell to long-wave photons.

[0045] Optionally, in the first group of quantum well layers, the second group of quantum well layers and the third group of quantum well layers, the alternating layering of the InGaAs well layer 431 and the GaAsP barrier layer 432 is 30 pairs.

[0046] Preferably, a composite structure of a plurality of DBR layers 3 is arranged between the first tunnel junction 2 and the InGaAs sub-cell 4. DBR (distributed Bragg reflective) is a kind of mirror structure, which comprises an adjustable multi-layer structure composed of two kinds of optical materials, and the optical thickness of a single layer of material is one quarter of the center wavelength of the reflection spectrum.

[0047] The reflection wavelengths of the multiple sets of DBR layers gradually decrease in a direction away from the Ge sub-cell 1. The effective reflection wavelengths of the multiple sets of DBR layers after being combined cover a wavelength band of 665 nm-930 nm. The reflection spectrum band covers the entire absorption spectrum of the InGaAs sub-cell 4. The unabsorbed sunlight can be reflected back to the InGaAs sub-cell for reabsorption, thereby improving the absorption efficiency of the InGaAs sub-cell and the photoelectric conversion efficiency of the solar cell. Meanwhile, the thickness of the base region and the quantum well structure of the InGaAs sub-cell can be reduced, thereby effectively increasing the anti-radiation capability of the solar cell. Moreover, in a conventional three-junction solar cell without DBR, the thickness of the base region of the InGaAs sub-cell needs to be designed to be greater than 3 μm to achieve the target current density. In the present application, the multiple DBR layers can reflect the unabsorbed light spectrum back to the middle cell for reabsorption. The thickness of the base region of the InGaAs sub-cell can be reduced to 2000 nm-2500 nm, thereby reducing the radiation damage to the base region of the cell when working in outer space and improving the anti-radiation capability of the cell. Similarly, the thickness of the quantum well structure can also be reduced, thereby further increasing the anti-radiation capability of the solar cell.

[0048] Specifically, three sets of DBR layers 3 are arranged between the first tunnel junction 2 and the InGaAs sub-cell 4, and are sequentially the first DBR layer 31, the second DBR layer 32 and the third DBR layer 33 in a direction away from the Ge sub-cell 1.

[0049] The first DBR layer 31 is grown by alternately growing In z1 Ga 1-z1 As and Al z2 Ga 1-z2 As, wherein z1=0.01, 0.6≤z2≤1, the center wavelength of the first DBR layer 31 is 880 nm, the reflection spectrum width is 100 nm, and the reflection wavelength is 830 nm-930 nm.

[0050] The second DBR layer 32 is grown by alternately growing Al y1 Ga 1-y1 As and Al y2 Ga 1-y2 As, wherein 0.3≤y1≤0.5, 0.8≤y2≤1, the center wavelength of the second DBR layer 32 is 790 nm, the reflection spectrum width is 90 nm, and the reflection wavelength is 745 nm-835 nm.

[0051] The third DBR layer 33 is grown by alternately growing Al x1 Ga 1-x1 As and Al x2 Ga 1-x2As two materials grow alternately, wherein 0.3≤x1≤0.5, 0.8≤x2≤1, and the center wavelength of the third DBR layer 33 is 705 nm, the reflection spectrum width is 80 nm, and the reflection wavelength is 665-745 nm.

[0052] Optionally, the number of pairs of the alternating DBR layers in each group is 15-20, and the doping elements are both P-type doping element Zn, and the doping amounts are both 1E 18 cm -3 -4E 18 cm -3 .

[0053] Optionally, the Ge sub-cell 1 is formed on a substrate 11, preferably, N-type phosphorus diffusion is performed on a 9-degree P-type Ge substrate to diffuse the most surface layer of Ge into N-type to obtain the pn junction of the Ge sub-cell; then a GaInP nucleation layer 12 and a GaAs buffer layer 13 matched with the Ge lattice are sequentially grown, and the two layers are used as the window layer of the Ge sub-cell and the connection layer of the Ge substrate and the subsequent epitaxial layer.

[0054] Optionally, the thickness of the GaInP nucleation layer 12 is 20-50 nm, and the thickness of the GaAs buffer layer 13 is 200-300 nm.

[0055] Specifically, the structure of the InGaAs sub-cell 4 comprises an AlGaAs back field layer 41, an InGaAs base region 42, a plurality of quantum well layers 43, an InGaAs emission region 44, and an AlInP window layer 45 sequentially grown in the direction away from the Ge sub-cell 1.

[0056] Optionally, the thickness of the AlGaAs back field layer 41 is 80-100 nm, and the Al component is 60-90%; the thickness of the InGaAs base region 42 is 2000-2500 nm, and the InGaAs base region is specifically an In x2 Ga 1-x2 As base region, wherein x2=0.01, the photoluminescence wavelength of the InGaAs base region is 880 nm, the doping element is P-type doping element Zn, and the doping amount gradually changes from 5E 17 cm -3 to 1E 16 cm -3 ; the thickness of the InGaAs emission region 44 is 50-200 nm, and the InGaAs emission region is specifically an In x3 Ga 1-x3 As emission region, wherein x3=0.01, the doping element is N-type doping element Si, and the doping amount is 1E 18 cm -3 -3E 18cm -3 ; the thickness of the AlInP window layer 45 is 50-200 nm, and the AlInP window layer is specifically Al x4 In 1-x4 P window layer, wherein 0.5≤x4≤0.6, the doping element is an N-type doping element Si, and the doping amount is 1E 18 cm -3 -3E 18 cm -3 .

[0057] Specifically, the structure of the GaInP sub-cell 6 is composed of an AlGaInP back field layer 61, a GaInP base region 62, a GaInP emission region 63 and an AlInP window layer 64 grown in the direction away from the Ge sub-cell 1 in sequence.

[0058] Optionally, the thickness of the AlGaInP back field layer 61 is 50-100 nm, and the AlGaInP back field layer is specifically (Al x6 Ga 1-x6 ) y3 In 1-y3 P back field layer, wherein 0.5≤x6≤0.8, y3=0.5, the doping element is a P-type doping element Zn, and the doping amount is 1E 18 cm -3 -4E 18 cm -3 ; the thickness of the GaInP base region 62 is 600-800 nm, and the GaInP base region is specifically Ga x7 In 1-x7 P base region, wherein 0.4≤x7≤0.6, the doping element is a P-type doping element Zn, and the doping amount is gradually changed from 1E 18 cm -3 to 1E 17 cm -3 ; the thickness of the GaInP emission region 63 is 50-150 nm, and the GaInP emission region is specifically Ga x8 In 1-x8 P emission region, wherein 0.4≤x8≤0.6, the doping element is an N-type doping element Si, and the doping amount is 1E 18 cm -3 -2E 18 cm -3 ; the thickness of the AlInP window layer 64 is 20-50 nm, and the AlInP window layer is specifically Al x9 In 1-x9 P window layer, wherein 0.5≤x9≤0.6, the doping element is an N-type doping element Si, and the doping amount is 1E 18 cm -3-5E 18 cm -3 .

[0059] Optionally, the first tunnel junction 2 has a thickness of 10-30 nm and is composed of N-type heavily doped GaAs and P-type heavily doped GaAs, wherein the N-type heavily doped GaAs is doped with Te at a doping amount of 1E 19 cm -3 -2E 19 cm -3 ; the P-type heavily doped GaAs is doped with C at a doping amount of 1E 20 cm -3 -2E 20 cm -3 The first tunnel junction connects the Ge sub-cell and the InGaAs sub-cell by using a tunneling effect.

[0060] Optionally, the second tunnel junction 5 has a thickness of 10-30 nm and is composed of N-type heavily doped Ga y2 In 1-y2 P and P-type heavily doped Al x5 Ga 1-x5 As, wherein 0.5≤y2≤0.6, 0.4≤x5≤0.6, and the N-type heavily doped Ga y2 In 1-y2 P is doped with Si at a doping amount of 1E 19 cm -3 -2E 19 cm -3 The P-type heavily doped Al x5 Ga 1-x5 As is doped with C at a doping amount of 1E 20 cm -3 -2E 20 cm -3 The second tunnel junction also connects the InGaAs sub-cell and the GaInP sub-cell by using a tunneling effect.

[0061] Optionally, the GaInP sub-cell 6 further has a GaAs ohmic contact layer 7 stacked thereon, the GaAs ohmic contact layer has a thickness of 500-800 nm and is doped with N-type doping element Si at a doping amount of 3E 18 cm -3 -5E 18 cm -3 .

[0062] The application further provides a preparation method of an epitaxial structure of a three-junction solar cell, which is used for preparing the epitaxial structure and is specifically grown by using an organic chemical vapor phase epitaxy (MOCVD) method, with reference to Figure 3 The preparation method comprises the following steps:

[0063] S1, providing a substrate 11, the substrate is selected from a 9-degree P-type Ge substrate, N-type phosphorus diffusion is carried out on the 9-degree P-type Ge substrate, the Ge in the topmost layer is diffused into N-type, and the pn junction of the Ge sub-cell 1 is obtained; then a GaInP nucleation layer 12 and a GaAs buffer layer 13 matched with the Ge lattice are sequentially grown, and the growth thicknesses are 20-50 nm and 200-300 nm respectively, the two layers are used as the window layer of the Ge sub-cell 1 and also used as the connecting layer of the Ge substrate and the subsequent epitaxial layer, and the preparation of the Ge sub-cell is completed.

[0064] S2, growing the first tunnel junction 2 on the Ge sub-cell 1, that is, growing the first tunnel junction 2 on the GaAs buffer layer 13.

[0065] Specifically, the growth thickness of the first tunnel junction 2 is 10-30 nm, and the first tunnel junction 2 is composed of N-type heavily doped GaAs and P-type heavily doped GaAs, wherein the N-type heavily doped GaAs is doped with Te, and the doping amount is 1E 19 cm -3 -2E 19 cm -3 ; the P-type heavily doped GaAs is doped with C, and the doping amount is 1E 20 cm -3 -2E 20 cm -3 , and the first tunnel junction connects the Ge sub-cell 1 and the InGaAs sub-cell 4 by using the tunneling effect.

[0066] S3, growing a composite structure of multiple DBR layers 3 on the first tunnel junction 2, and the composite structure comprises a first DBR layer 31, a second DBR layer 32 and a third DBR layer 33 grown in turn from bottom to top.

[0067] The first DBR layer 31 is alternately grown from In z1 Ga 1-z1 As and Al z2 Ga 1-z2 As, wherein z1=0.01, 0.6≤z2≤1, the center wavelength of the first DBR layer is 880 nm, the reflection spectrum width is 100 nm, and the reflection wavelength is 830-930 nm; the second DBR layer 32 is alternately grown from Al y1 Ga 1-y1 A and Al y2 Ga 1-y2 As, wherein 0.3≤y1≤0.5, 0.8≤y2≤1, the center wavelength of the second DBR layer is 790 nm, the reflection spectrum width is 90 nm, and the reflection wavelength is 745-835 nm; and the third DBR layer 33 is alternately grown from Alx1 Ga 1-x1 As and Al x2 Ga 1-x2 , wherein 0.3≤x1≤0.5, 0.8≤x2≤1, the center wavelength of the third DBR layer is 705nm, the reflection spectrum width is 80nm, and the reflection wavelength is 665nm-745nm. And the number of pairs of each group of DBR layers is 15 pairs-20 pairs, and the doping elements are all P-type doping elements Zn, and the doping amounts are all 1E 18 cm -3 -4E 18 cm -3 .

[0068] Therefore, the reflection wavelengths of the multiple groups of DBR layers gradually decrease from bottom to top, and the effective reflection wavelength of the multiple groups of DBR layers after compounding covers the wavelength band of 665nm-930nm, which covers the entire absorption spectrum of the InGaAs sub-cell. The unabsorbed sunlight can be reflected back to the InGaAs sub-cell for reabsorption, thereby improving the absorption efficiency of the InGaAs sub-cell. Meanwhile, the thickness of the base region and the quantum well structure of the InGaAs sub-cell can be reduced, thereby effectively increasing the anti-radiation capability of the solar cell.

[0069] S4, growing the InGaAs sub-cell 4 on the compounding structure of the multiple groups of DBR layers 3, i.e. growing the InGaAs sub-cell 4 on the third DBR layer 33, wherein the structure of the InGaAs sub-cell 4 comprises, from bottom to top, an AlGaAs back field layer 41, an InGaAs base region 42, multiple groups of quantum well layers 43, an InGaAs emitter region 44, and an AlInP window layer 45.

[0070] The growth thickness of the AlGaAs back field layer 41 is 80nm-100nm, and the Al component is 60%-90%; the growth thickness of the InGaAs base region 42 is 2000nm-2500nm, and the InGaAs base region is specifically an In x2 Ga 1-x2 As base region, wherein x2=0.01, the photoluminescence wavelength of the InGaAs base region is 880nm, the doping element is a P-type doping element Zn, and the doping amount gradually changes from 5E 17 cm -3 to 1E 16 cm -3 .

[0071] Each of the plurality of quantum well layers 43 is composed of InGaAs well layers 431 and GaAsP barrier layers 432 grown alternately, and the number of pairs of the alternately grown layers in each of the plurality of quantum well layers is 30.

[0072] In the first quantum well layer, the In content of the InGaAs well layer 431 is 10%, the growth thickness is 8.5 nm, the P content of the GaAsP barrier layer 432 is 20%, and the growth thickness is 8.5 nm. The photoluminescence wavelength of the first quantum well layer is 930 nm, and the first quantum well layer can absorb light spectrum below 930 nm. In the second quantum well layer, the In content of the InGaAs well layer is 7.8%, the growth thickness is 8.5 nm, the P content of the GaAsP barrier layer is 15.5%, and the growth thickness is 8.5 nm. The photoluminescence wavelength of the second quantum well layer is 910 nm, and the second quantum well layer can absorb light spectrum below 910 nm. In the third quantum well layer, the In content of the InGaAs well layer is 5.5%, the growth thickness is 8.5 nm, the P content of the GaAsP barrier layer is 11%, and the growth thickness is 8.5 nm. The photoluminescence wavelength of the third quantum well layer is 890 nm, and the third quantum well layer can absorb light spectrum below 890 nm.

[0073] Therefore, the sunlight passes through the third quantum well layer from top to bottom, is absorbed by the third quantum well layer, and then passes through the second quantum well layer and the first quantum well layer, and is absorbed by them in turn, so that the sunlight spectrum that cannot be absorbed by the InGaAs base region is absorbed step by step. Since the photoluminescence wavelength of the quantum well layer increases step by step along the direction facing the Ge sub-cell, the sunlight spectrum that cannot be absorbed by the InGaAs base region is absorbed step by step, the absorption range of the InGaAs sub-cell to the long-wave photons is effectively widened, the absorption efficiency of the InGaAs sub-cell is increased, and the photoelectric conversion efficiency of the solar cell is improved. Compared with a single quantum well layer, the In content in the multi-layer quantum well of the application decreases step by step, which moderates the sharp composition change at the interface, effectively suppresses the atomic diffusion phenomenon at the heterojunction interface, and ensures the crystal quality of the quantum well layer. In addition, the P content of the GaAsP barrier layer decreases step by step in the direction away from the Ge sub-cell, which can offset the strain caused by the change of the In content, thereby avoiding the generation of mismatch dislocations and ensuring the high crystal quality of the entire epitaxial structure. In addition, the barrier material band gap caused by the P content can better confine the carriers in the well, ensuring that the target emission wavelength set by the In content can be efficiently and stably achieved.

[0074] The growth thickness of the InGaAs emission region 44 is 50 nm-200 nm, and the InGaAs emission region is In x3 Ga 1- x3As emission region, wherein x3=0.01, the doping element is N-type doping element Si, and the doping amount is 1E 18 cm -3 -3E 18 cm -3 ; the growth thickness of the AlInP window layer 45 is 50nm-200nm, and the AlInP window layer is specifically Al x4 In 1-x4 P window layer, wherein 0.5≤x4≤0.6, the doping element is N-type doping element Si, and the doping amount is 1E 18 cm -3 -3E 18 cm -3 .

[0075] S5, growing a second tunnel junction 5 on the InGaAs sub-cell 4, that is, growing the second tunnel junction 5 on the AlInP window layer 45.

[0076] Specifically, the growth thickness of the second tunnel junction 5 is 10nm-30nm, and is composed of N-type heavily doped Ga y2 In 1-y2 P and P-type heavily doped Al x5 Ga 1-x5 As, wherein 0.5≤y2≤0.6, 0.4≤x5≤0.6, and the N-type heavily doped Ga y2 In 1-y2 P dopes Si, and the doping amount is 1E 19 cm -3 -2E 19 cm -3 , the P-type heavily doped Al x5 Ga 1-x5 As dopes C, and the doping amount is 1E 20 cm -3 -2E 20 cm -3 , the second tunnel junction also utilizes the tunneling effect to connect the InGaAs sub-cell 4 and the GaInP sub-cell 6.

[0077] S6, growing a GaInP sub-cell 6 on the second tunnel junction 5, the structure of the GaInP sub-cell 6 comprises AlGaInP back field layer 61, GaInP base region 62, GaInP emission region 63 and AlInP window layer 64 grown in turn from bottom to top.

[0078] Wherein, the growth thickness of the AlGaInP back field layer 61 is 50nm-100nm, and the AlGaInP back field layer is specifically (Al x6 Ga 1-x6 ) y3 In1-y3 P back field layer, wherein 0.5≤x6≤0.8, y3=0.5, the doping element is P-type doping element Zn, and the doping amount is 1E 18 cm -3 -4E 18 cm -3 ; the growth thickness of the GaInP base region 62 is 600nm-800nm, and the GaInP base region is specifically Ga x7 In 1-x7 P base region, wherein 0.4≤x7≤0.6, the doping element is P-type doping element Zn, and the doping amount is 1E 18 cm -3 gradually changes to 1E 17 cm -3 ; the growth thickness of the GaInP emission region 63 is 50-150nm, and the GaInP emission region is specifically Ga x8 In 1-x8 P emission region, wherein 0.4≤x8≤0.6, the doping element is N-type doping element Si, and the doping amount is 1E 18 cm -3 -2E 18 cm -3 ; the growth thickness of the AlInP window layer 64 is 20nm-50nm, and the AlInP window layer is specifically Al x9 In 1-x9 P window layer, wherein 0.5≤x9≤0.6, the doping element is N-type doping element Si, and the doping amount is 1E 18 cm -3 -5E 18 cm -3 .

[0079] S7, growing a GaAs ohmic contact layer 7 on the GaInP sub-cell 6, that is, growing a GaAs ohmic contact layer 7 on the AlInP window layer 64, the growth thickness of the GaAs ohmic contact layer is 500nm-800nm, the doping element is N-type doping element Si, and the doping amount is 3E 18 cm -3 -5E 18 cm -3 .

[0080] The solar cell prepared above is tested, and the photoelectric conversion efficiency obtained by the test is about 32.5%, which is effectively improved compared with the photoelectric conversion efficiency of the solar cell in the prior art.

[0081] It is worth noting that the thickness of the Ge sub-cell 1, the first tunnel junction 2, the DBR layer 3, the InGaAs sub-cell 4, the second tunnel junction 5, the GaInP sub-cell 6 and the GaAs ohmic contact layer 7 shown in the drawings of the present application are only examples and do not represent the true thickness. Also, the true proportions between the Ge sub-cell 1, the first tunnel junction 2, the DBR layer 3, the InGaAs sub-cell 4, the second tunnel junction 5, the GaInP sub-cell 6 and the GaAs ohmic contact layer 7 are not as shown in the drawings, but are only for reference.

[0082] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be mutually referred to.

[0083] The above description of disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An epitaxial structure of a triple-junction solar cell, characterized by: The structure of the InGaAs sub-cell comprises an AlGaAs back field layer, an InGaAs base region, a plurality of quantum well layers, an InGaAs emission region and an AlInP window layer, which are sequentially grown in the direction away from the Ge sub-cell, wherein the thickness of the InGaAs base region is 2000-2500 nm. The first group of quantum well layers comprises an InGaAs well layer with an In content of 10% and a thickness of 8.5 nm, and the photoluminescence wavelength of the first group of quantum well layers is 930 nm. The second group of quantum well layers comprises an InGaAs well layer with an In content of 7.8% and a thickness of 8.5 nm, and the photoluminescence wavelength of the second group of quantum well layers is 910 nm. The third group of quantum well layers comprises an InGaAs well layer with an In content of 5.5% and a thickness of 8.5 nm, and the photoluminescence wavelength of the third group of quantum well layers is 890 nm. The first tunnel junction and the InGaAs sub-cell are provided with a plurality of InGaAs / AlGaAs DBR layers, the reflection wavelengths of the plurality of DBR layers gradually decrease in the direction away from the Ge sub-cell, and the effective reflection wavelength of the plurality of DBR layers covers the wavelength range of 665-930 nm.

2. An epitaxial structure for a triple-junction solar cell as recited in claim 1, wherein: In the first group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5 nm, and the P content is 20%; in the second group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5 nm, and the P content is 15.5%; and in the third group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5 nm, and the P content is 11%.

3. An epitaxial structure for a triple-junction solar cell as recited in claim 1, wherein: In the first group of quantum well layers, the second group of quantum well layers and the third group of quantum well layers, the number of pairs of the alternately stacked InGaAs well layers and GaAsP barrier layers is 30 pairs.

4. An epitaxial structure for a triple-junction solar cell as described in claim 1, wherein: The first tunnel junction and the InGaAs sub-cell are provided with three DBR layers, which are sequentially a first DBR layer, a second DBR layer and a third DBR layer in the direction away from the Ge sub-cell, the reflection wavelength of the first DBR layer is 830-930 nm, the reflection wavelength of the second DBR layer is 745-835 nm, and the reflection wavelength of the third DBR layer is 665-745 nm.

5. An epitaxial structure for a triple-junction solar cell as described in claim 4, wherein: The first DBR layer is grown alternately from In z1 Ga 1-z1 As and Al z2 Ga 1-z2 As, wherein z1=0.01, 0.6≤z2≤1, and the center wavelength of the first DBR layer is 880 nm and the reflection spectrum width is 100 nm; the second DBR layer is grown alternately from Al y1 Ga 1-y1 As and Al y2 Ga 1-y2 As, wherein 0.3≤y1≤0.5, 0.8≤y2≤1, and the center wavelength of the second DBR layer is 790 nm and the reflection spectrum width is 90 nm; and the third DBR layer is grown alternately from Al x1 Ga 1-x1 As and Al x2 Ga 1-x2 As, wherein 0.3≤x1≤0.5, 0.8≤x2≤1, and the center wavelength of the third DBR layer is 705 nm and the reflection spectrum width is 80 nm.

6. An epitaxial structure for a triple-junction solar cell as described in claim 1, wherein: the first, second, and third subcells are arranged in the order of the first subcell, the second subcell, and the third subcell. The structure of the InGaAs sub-cell comprises an AlGaAs back field layer, an InGaAs base region, a plurality of quantum well layers, an InGaAs emission region and an AlInP window layer, which are sequentially grown in the direction away from the Ge sub-cell, wherein the thickness of the InGaAs base region is 2000-2500 nm.

7. A method for producing an epitaxial structure of a triple-junction solar cell, for producing an epitaxial structure of a triple-junction solar cell as claimed in any one of claims 1 to 6, characterized in that The structure of the InGaAs sub-cell comprises an AlGaAs back field layer, an InGaAs base region, a plurality of quantum well layers, an InGaAs emission region and an AlInP window layer, which are sequentially grown in the direction away from the Ge sub-cell, wherein the thickness of the InGaAs base region is 2000-2500 nm. The structure of the InGaAs sub-cell comprises an AlGaAs back field layer, an InGaAs base region, a plurality of quantum well layers, an InGaAs emission region and an AlInP window layer, which are sequentially grown in the direction away from the Ge sub-cell, wherein the thickness of the InGaAs base region is 2000-2500 nm. ​ An InGaAs subcell is grown on the first tunnel junction, and the InGaAs subcell comprises a plurality of groups of quantum well layers, each group of quantum well layers being alternately grown from InGaAs well layers and GaAsP barrier layers; in the plurality of groups of quantum well layers, the In content of the InGaAs well layers gradually decreases in a direction away from the Ge subcell, and the P content of the GaAsP barrier layers gradually decreases in the direction away from the Ge subcell; the InGaAs subcell comprises three groups of quantum well layers, and the three groups of quantum well layers are sequentially a first group of quantum well layers, a second group of quantum well layers, and a third group of quantum well layers in the direction away from the Ge subcell; In the first group of quantum well layers, the In content of the InGaAs well layers is 10%, and the thickness of the InGaAs well layers is 8.5 nm; the photoluminescence wavelength of the first group of quantum well layers is 930 nm; In the second group of quantum well layers, the In content of the InGaAs well layers is 7.8%, and the thickness of the InGaAs well layers is 8.5 nm; the photoluminescence wavelength of the second group of quantum well layers is 910 nm; In the third group of quantum well layers, the In content of the InGaAs well layers is 5.5%, and the thickness of the InGaAs well layers is 8.5 nm; the photoluminescence wavelength of the third group of quantum well layers is 890 nm; A plurality of groups of InGaAs / AlGaAs DBR layers are arranged between the first tunnel junction and the InGaAs subcell, the reflection wavelengths of the plurality of groups of DBR layers gradually decrease in the direction away from the Ge subcell, and the effective reflection wavelength of the plurality of groups of DBR layers in combination covers a wavelength range of 665 nm-930 nm; A second tunnel junction is grown on the InGaAs subcell; A GaInP subcell is grown on the second tunnel junction.

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