Epitaxial structure of three-junction solar cell and preparation method thereof

Abstract

The invention 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 stacked in sequence, the InGaAs sub-cell comprises a plurality of groups of quantum well layers, and each group of quantum well layers is formed by alternately stacking InGaAs well layers and GaAsP barrier layers; in the multiple groups of quantum well layers, In components of the InGaAs well layers are gradually reduced in the direction deviating from the Ge sub-cell, solar spectrums which cannot be absorbed by the InGaAs base region are absorbed step by step, the absorption range of the InGaAs sub-cell to long-wave photons is effectively widened, 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 Ge sub-cell generates an excessive photocurrent. 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 excess 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, it is necessary to reduce the bandgap difference between the sub-cells. Since Ge is a single-element semiconductor, its bandgap cannot be adjusted, and the current mainstream technical path mainly includes the following two kinds: 1. By increasing the proportion of In component in InGaAs, the bandgap of InGaAs is reduced, so as to increase the current density of the InGaAs sub-cell and match it with 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.

[0004] 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 as to achieve stress balance while increasing the In component of InGaAs, maintain lattice matching, and avoid the generation of mismatch dislocations. 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 performance deviation of the whole, which is also not conducive to obtaining good battery performance. SUMMARY

[0005] 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.

[0006] 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 component of the InGaAs well layer gradually decreases in the direction away from the Ge sub-cell, and the P component of the GaAsP barrier layer 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. In the first group of quantum well layers, the In component of the InGaAs well layer is 10%, and the thickness is 8.5nm, and the photoluminescence wavelength of the first group of quantum well layers is 930nm. In the second group of quantum well layers, the In component of the InGaAs well layer is 7.8%, and the thickness is 8.5nm, and the photoluminescence wavelength of the second group of quantum well layers is 910nm. In the third group of quantum well layers, the In component of the InGaAs well layer is 5.5%, and the thickness is 8.5nm, and the photoluminescence wavelength of the third group of quantum well layers is 890nm. 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.

[0007] Optionally, in the first group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5nm, and the P component is 20%; in the second group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5nm, and the P component is 15.5%; in the third group of quantum well layers, the thickness of the GaAsP barrier layer is 8.5nm, and the P component is 11%.

[0008] 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.

[0009] 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 in sequence, the first DBR layer has a reflection wavelength of 830nm-930nm, the second DBR layer has a reflection wavelength of 745nm-835nm, and the third DBR layer has a reflection wavelength of 665nm-745nm.

[0010] 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 first DBR layer has a center wavelength of 880nm and a reflection spectrum width of 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 second DBR layer has a center wavelength of 790nm and a reflection spectrum width of 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 third DBR layer has a center wavelength of 705nm and a reflection spectrum width of 80nm.

[0011] Optionally, the structure of the InGaAs sub-cell includes, in the direction away from the Ge sub-cell, an AlGaAs back field layer, an InGaAs base region, a plurality of quantum well layers, an InGaAs emission region and an AlInP window layer grown in sequence, wherein the InGaAs base region has a thickness of 2000nm-2500nm.

[0012] The application further provides a preparation method of an epitaxial structure of a three-junction solar cell, for preparing the epitaxial structure, comprising: providing a substrate, and forming a Ge sub-cell on the substrate; growing a first tunnel junction on the Ge sub-cell; 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, and the InGaAs subcell comprises three groups of quantum well layers, which are 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 component of the InGaAs well layer is 10%, and the thickness is 8.5 nm, and the photoluminescence wavelength of the first group of quantum well layers is 930 nm; 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, and the photoluminescence wavelength of the second group of quantum well layers is 910 nm; 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, and 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 after combination covers a wavelength band of 665 nm-930 nm; The second tunnel junction is grown on the InGaAs subcell; The GaInP subcell is grown on the second tunnel junction.

[0013] After the above scheme is adopted, the application has the following advantages: 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, so that the photoluminescence wavelength of the quantum well layer gradually increases in the direction facing the Ge subcell, the InGaAs-based region cannot absorb the solar spectrum, the absorption range of the InGaAs subcell to the 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; and compared with a single group of quantum well layers, the In component in the multilayer quantum well layer gradually decreases, which moderates the sharp component mutation at the interface, effectively suppresses the atomic diffusion phenomenon of the heterojunction interface, and ensures the crystal quality of the quantum well layer.

[0014] 2、The application adopts multiple DBR composite structures, and the reflection spectrum of the multiple 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 sunlight that is not completely absorbed can be reflected back to the InGaAs sub-cell for re-absorption, the absorption efficiency of the InGaAs sub-cell is improved, and meanwhile, the thickness of the base region and the quantum well structure of the InGaAs sub-cell can be reduced, so that the anti-radiation capability of the solar cell is effectively increased.

[0015] 3、In the traditional three-junction solar cell without DBR, the thickness of the base region of the InGaAs sub-cell needs to be designed to be more than 3 microns to achieve the target current density, and the application can reduce the thickness of the base region of the InGaAs sub-cell to 2000-2500 nm by designing multiple DBR layers to reflect the light spectrum that is not completely absorbed back to the middle cell for re-absorption, so that 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, so that the anti-radiation capability of the solar cell is further increased.

[0016] 4、All the epitaxial layer materials in the application are carefully designed to be lattice-matched with the Ge substrate, which not only avoids the problem of directly introducing mismatch dislocations due to the increase of the In component, but also uses the InGaAs / GaAsP quantum well layer which is a strain compensation structure to further offset the stress and inhibit the generation of dislocations. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is an epitaxial structure diagram of the solar cell of the application.

[0018] Figure 2 It is a structure diagram of the quantum well layer of the application.

[0019] Figure 3 It is a growth method flowchart of the application.

[0020] REFERENCE NUMERALS: 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 Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. The scope value described in this invention includes two endpoint values.

[0022] like Figure 1 As shown, this application provides an epitaxial structure for a triple-junction solar cell, comprising a Ge sub-cell 1, a first tunneling junction 2, an InGaAs sub-cell 4, a second tunneling junction 5, and a GaInP sub-cell 6 stacked sequentially from bottom to top. The InGaAs sub-cell 4 includes multiple sets of quantum well layers 43, such as... Figure 2 As shown, each quantum well layer 43 is composed of alternating layers of InGaAs well layer 431 and GaAsP barrier layer 432. In the multiple quantum well layers 43, the In composition of the InGaAs well layer 431 decreases stepwise along the direction away from the Ge sub-cell 1 (i.e., from bottom to top), so that the photoluminescence wavelength of the quantum well layer increases stepwise along the direction facing the Ge sub-cell (i.e., from top to bottom), realizing the stepwise absorption of the solar spectrum that the InGaAs base region cannot absorb, effectively broadening the absorption range of the InGaAs sub-cell for long-wavelength photons, and increasing the absorption efficiency of the InGaAs sub-cell; and compared with a single quantum well layer, the stepwise decrease in In composition design in the multilayer quantum well of this application mitigates the drastic compositional abrupt change at the interface, effectively suppresses the atomic diffusion phenomenon at the heterojunction interface, and ensures the crystal quality of the quantum well layer.

[0023] Preferably, in the multiple quantum well layers 43, the P composition of the GaAsP barrier layer 432 decreases stepwise along the direction away from the Ge subcell 1, which can offset the strain caused by the change in In composition, thereby avoiding the generation of mismatch dislocations, ensuring the high crystal quality of the entire epitaxial structure, and the barrier band gap brought by the P composition can better confine the charge carriers in the well, ensuring that the target emission wavelength set by the In composition can be achieved efficiently and stably.

[0024] Specifically, the InGaAs sub-cell 4 includes three sets of quantum well layers, which are arranged sequentially in the direction away from the Ge sub-cell 1 as the first set of quantum well layers, the second set of quantum well layers, and the third set of quantum well layers.

[0025] In the first set of quantum well layers, the InGaAs well layer 431 has an In composition of 10% and a thickness of 8.5 nm, the GaAsP barrier layer 432 has a P composition of 20% and a thickness of 8.5 nm, and the photoluminescence wavelength of the first set of quantum well layers is 930 nm, which can absorb the spectrum below 930 nm.

[0026] In the second group of quantum well layers, the In content of the InGaAs well layer is 7.8%, the thickness is 8.5nm, the P content of the GaAsP barrier layer is 15.5%, the thickness is 8.5nm, the photoluminescence wavelength of the second group of quantum well layers is 910nm, and the spectrum below 910nm can be absorbed.

[0027] In the third group of quantum well layers, the In content of the InGaAs well layer is 5.5%, the thickness is 8.5nm, the P content of the GaAsP barrier layer is 11%, the thickness is 8.5nm, the photoluminescence wavelength of the third group of quantum well layers is 890nm, and the spectrum below 890nm can be absorbed.

[0028] The sunlight passes through the third group of quantum well layers first from top to bottom, and then passes through the second group of quantum well layers and the first group of quantum well layers in turn, and is absorbed by them, so as to realize step-by-step absorption of the sunlight spectrum that cannot be absorbed by the InGaAs base area, and effectively widen the absorption range of the InGaAs sub-cell to long-wave photons.

[0029] 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 number of the InGaAs well layer 431 and the GaAsP barrier layer 432 is 30 pairs.

[0030] Preferably, a composite structure of multiple groups of DBR layers 3 is arranged between the first tunnel junction 2 and the InGaAs sub-cell 4. The DBR (distributed Bragg reflective, distributed Bragg reflector) is a kind of mirror structure, which includes an adjustable multilayer structure composed of two kinds of optical materials, and the optical thickness of a single layer of material is one fourth of the center wavelength of the reflection spectrum.

[0031] The reflection wavelengths of the multiple DBR layers gradually decrease in the direction away from the Ge sub-cell 1, and the effective reflection wavelength of the multiple DBR layers covers the wavelength range of 665nm-930nm, which covers the entire absorption spectrum of the InGaAs sub-cell 4. The reflected sunlight that is not completely absorbed can be reabsorbed by the InGaAs sub-cell, thereby improving the absorption efficiency of the InGaAs sub-cell and the photoelectric conversion efficiency of the solar cell. At the same time, the thickness of the base region and the quantum well structure of the InGaAs sub-cell can be reduced, thereby effectively improving the anti-radiation capability of the solar cell. Moreover, in the conventional three-junction solar cell without DBR, the thickness of the base region of the InGaAs sub-cell needs to be designed to be more than 3μm to achieve the target current density. In the present application, the multiple DBR layers can reflect the light spectrum that is not completely absorbed back to the middle cell for reabsorption, thereby reducing the thickness of the base region of the InGaAs sub-cell to 2000nm-2500nm, 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 improving the anti-radiation capability of the solar cell.

[0032] Specifically, three groups 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 the direction away from the Ge sub-cell 1.

[0033] 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 880nm, the reflection spectrum width is 100nm, and the reflection wavelength is 830nm-930nm.

[0034] 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 790nm, the reflection spectrum width is 90nm, and the reflection wavelength is 745nm-835nm.

[0035] 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.

[0036] Optionally, the number of pairs of the alternating DBR layers in each group is 15-20, and the doping elements are P-type doping element Zn, and the doping amount is 1E 18 cm -3 -4E 18 cm -3 .

[0037] 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 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.

[0038] 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.

[0039] 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.

[0040] 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 .

[0041] 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.

[0042] 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 gradually changes 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 .

[0043] 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.

[0044] 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.

[0045] 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 .

[0046] The application further provides a preparation method of the epitaxial structure of the 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: S1, providing a substrate 11, the substrate is selected from a 9-degree P-type Ge substrate, N-type phosphorus diffusion is performed on the 9-degree P-type Ge substrate, the Ge in the topmost layer is diffused into N-type, and a pn junction of a 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 thicknesses of the two layers are 20 nm-50 nm and 200 nm-300 nm respectively, the two layers are used as a window layer of the Ge sub-cell 1 and also used as a connecting layer of the Ge substrate and a subsequent epitaxial layer, and the preparation of the Ge sub-cell is completed.

[0047] S2, growing a first tunnel junction 2 on the Ge sub-cell 1, that is, growing the first tunnel junction 2 on the GaAs buffer layer 13.

[0048] Specifically, the growth thickness of the first tunnel junction 2 is 10 nm-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 The first tunnel junction connects the Ge sub-cell 1 and an InGaAs sub-cell 4 by using a tunneling effect.

[0049] S3, growing a composite structure of multiple DBR layers 3 on the first tunnel junction 2, the composite structure includes a first DBR layer 31, a second DBR layer 32 and a third DBR layer 33 grown in sequence from bottom to top.

[0050] 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 nm-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 nm-835 nm; and the third DBR layer 33 is alternately grown from Al x1 Ga1-x1 As and Al x2 Ga 1-x2 As is grown by alternating layers of two materials, where 0.3 ≤ x1 ≤ 0.5 and 0.8 ≤ x2 ≤ 1, and the center wavelength of the third DBR layer is 705 nm, the reflection spectral width is 80 nm, and the reflection wavelength is 665 nm-745 nm. Furthermore, each set of DBR layers has 15-20 alternating pairs, and all are doped with p-type Zn at a doping level of 1E. 18 cm -3 -4E 18 cm -3 .

[0051] Therefore, the reflection wavelength of multiple DBR layers decreases gradually from bottom to top. The effective reflection wavelength of the composite DBR layers covers the band of 665nm-930nm. This reflection spectrum band covers the entire absorption spectrum of the InGaAs sub-cell, which can reflect incompletely absorbed sunlight back to the InGaAs sub-cell for reabsorption, thereby improving the absorption efficiency of the InGaAs sub-cell. At the same time, it can also reduce the thickness of the base region and quantum well structure of the InGaAs sub-cell, effectively increasing the radiation resistance of the solar cell.

[0052] S4. Grow an InGaAs sub-cell 4 on a composite structure of multiple DBR layers 3, that is, grow an InGaAs sub-cell 4 on the third DBR layer 33. The structure of the InGaAs sub-cell 4 includes an AlGaAs back field layer 41, an InGaAs base region 42, multiple quantum well layers 43, an InGaAs emitter region 44 and an AlInP window layer 45 grown sequentially from bottom to top.

[0053] The AlGaAs backfield layer 41 has a growth thickness of 80nm-100nm and an Al composition of 60%-90%; the InGaAs base region 42 has a growth thickness of 2000nm-2500nm, and the InGaAs base region is specifically composed of In... x2 Ga 1-x2 In the As-based region, where x2 = 0.01, the photoluminescence wavelength of the InGaAs-based region is 880 nm, and the doping element is the p-type dopant Zn, with a doping amount of 5E. 17 cm -3 Gradient to 1E 16 cm -3 .

[0054] Each set of quantum well layers 43 consists of alternating InGaAs well layers 431 and GaAsP barrier layers 432, with 30 alternation pairs in each set. Specifically, three sets of quantum well layers are grown, from bottom to top: the first set of quantum well layers, the second set of quantum well layers, and the third set of quantum well layers.

[0055] In the first group of quantum well layers, the InGaAs well layer 431 has an In composition of 10% and a growth thickness of 8.5 nm, and the GaAsP barrier layer 432 has a P composition of 20% and a growth thickness of 8.5 nm. The photoluminescence wavelength of the first group of quantum well layers is 930 nm, which can absorb the spectrum below 930 nm. In the second group of quantum well layers, the InGaAs well layer has an In composition of 7.8% and a growth thickness of 8.5 nm, and the GaAsP barrier layer has a P composition of 15.5% and a growth thickness of 8.5 nm. The photoluminescence wavelength of the second group of quantum well layers is 910 nm, which can absorb the spectrum below 910 nm. In the third group of quantum well layers, the InGaAs well layer has an In composition of 5.5% and a growth thickness of 8.5 nm, and the GaAsP barrier layer has a P composition of 11% and a growth thickness of 8.5 nm. The photoluminescence wavelength of the third group of quantum well layers is 890 nm, which can absorb the spectrum below 890 nm.

[0056] Thus, sunlight, from top to bottom, first passes through the third set of quantum well layers, where it is absorbed, and then passes through the second and first sets of quantum well layers, being absorbed sequentially by them, achieving a stepwise absorption of the solar spectrum that the InGaAs base region cannot absorb. Because the photoluminescence wavelength of the quantum well layers increases progressively along the direction facing the Ge sub-cell, achieving stepwise absorption of the solar spectrum that the InGaAs base region cannot absorb effectively broadens the absorption range of long-wavelength photons for the InGaAs sub-cell, increasing its absorption efficiency and thus improving the photoelectric conversion efficiency of the solar cell. Furthermore, compared to a single set of quantum well layers, the progressively decreasing In composition design in this application's multi-layer quantum wells mitigates the drastic compositional abrupt changes at the interface, effectively suppressing atomic diffusion at the heterojunction interface and ensuring the crystal quality of the quantum well layers. Furthermore, the P composition of the GaAsP barrier layer gradually decreases along the direction away from the Ge subcell, which can offset the strain caused by the change in In composition, thereby avoiding the generation of mismatch dislocations and ensuring the high crystal quality of the entire epitaxial structure. Moreover, the barrier band gap brought by the P composition can better confine the charge carriers in the well, ensuring that the target emission wavelength set by the In composition can be achieved efficiently and stably.

[0057] The growth thickness of the InGaAs emitter region 44 is 50nm-200nm, and the InGaAs emitter region is specifically In... x3 Ga 1- x3 As the emitter region, where x3 = 0.01, the doping element is N-type 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 Alx4 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 .

[0058] S5, growing a second tunnel junction 5 on the InGaAs sub-cell 4, i.e. growing the second tunnel junction 5 on the AlInP window layer 45.

[0059] Specifically, the growth thickness of the second tunnel junction 5 is 10nm-30nm, and the second tunnel junction 5 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 N-type heavily doped Ga y2 In 1-y2 P doped with Si, and the doping amount is 1E 19 cm -3 -2E 19 cm -3 , P-type heavily doped Al x5 Ga 1-x5 As doped with 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.

[0060] S6, growing a GaInP sub-cell 6 on the second tunnel junction 5, and the structure of the GaInP sub-cell 6 comprises an AlGaInP back field layer 61, a GaInP base region 62, a GaInP emission region 63 and an AlInP window layer 64 grown in turn from bottom to top.

[0061] 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 In 1-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 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 1E 18 cm -3 gradually changes to 1E 17 cm -3 ; the growth 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 growth 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 .

[0062] S7, growing a GaAs ohmic contact layer 7 on the GaInP sub-cell 6, i.e. growing a GaAs ohmic contact layer 7 on the AlInP window layer 64, the growth thickness of the GaAs ohmic contact layer is 500-800 nm, the doping element is an N-type doping element Si, and the doping amount is 3E 18 cm -3 -5E 18 cm -3 .

[0063] 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.

[0064] It is worth noting that the thicknesses 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 thicknesses thereof. Moreover, 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.

[0065] The various embodiments described in this specification are presented by way of example, and each embodiment is not necessarily composed of all features described with respect to other embodiments. Each embodiment described in this specification can be combined with one or more other embodiments to produce new embodiments that are not explicitly described in this specification.

[0066] The previous description of the disclosed embodiments is not intended to limit the scope of the application, but is merely intended to describe the ways in which they can be practiced. The scope of the application is shown by the appended claims, rather than the description and examples. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed in this patent.

Claims

1. An epitaxial structure for a triple-junction solar cell, characterized in that: The device includes a Ge sub-cell, a first tunneling junction, an InGaAs sub-cell, a second tunneling junction, and a GaInP sub-cell, which are stacked sequentially. The InGaAs sub-cell contains multiple sets of quantum well layers, each set of which is composed of alternating InGaAs well layers and GaAsP barrier layers. In the multiple sets of quantum well layers, the In component of the InGaAs well layers decreases progressively in the direction away from the Ge sub-cell, and the P component of the GaAsP barrier layers decreases progressively in the direction away from the Ge sub-cell. The InGaAs sub-cell contains three sets of quantum well layers, which are the first set of quantum well layers, the second set of quantum well layers, and the third set of quantum well layers in the direction away from the Ge sub-cell. In the first set of quantum well layers, the In composition of the InGaAs well layer is 10%, the thickness is 8.5 nm, and the photoluminescence wavelength of the first set of quantum well layers is 930 nm. In the second set of quantum well layers, the In composition of the InGaAs well layer is 7.8%, the thickness is 8.5 nm, and the photoluminescence wavelength of the second set of quantum well layers is 910 nm. In the third set of quantum well layers, the In composition of the InGaAs well layer is 5.5%, the thickness is 8.5 nm, and the photoluminescence wavelength of the third set of quantum well layers is 890 nm. Multiple InGaAs / AlGaAs DBR layers are provided between the first tunnel junction and the InGaAs subcell. The reflection wavelength of the multiple DBR layers decreases stepwise along the direction away from the Ge subcell. The effective reflection wavelength of the multiple DBR layers after recombination covers the band of 665nm-930nm.

2. The epitaxial structure of a triple-junction solar cell as described in claim 1, characterized in that: In the first group of quantum well layers, the GaAsP barrier layer has a thickness of 8.5 nm and a P composition of 20%; in the second group of quantum well layers, the GaAsP barrier layer has a thickness of 8.5 nm and a P composition of 15.5%; in the third group of quantum well layers, the GaAsP barrier layer has a thickness of 8.5 nm and a P composition of 11%.

3. The epitaxial structure of a triple-junction solar cell as described in claim 1, characterized in that: 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 alternating stacked pairs of InGaAs well layers and GaAsP barrier layers is 30 pairs each.

4. The epitaxial structure of a triple-junction solar cell as described in claim 1, characterized in that: Three DBR layers are provided between the first tunnel junction and the InGaAs subcell. The layers are arranged in sequence along the direction away from the Ge subcell: the first DBR layer, the second DBR layer, and the third DBR layer. 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.

5. The epitaxial structure of a triple-junction solar cell as described in claim 4, characterized in that: The first DBR layer consists of In z1 Ga 1-z1 As and Al z2 Ga 1-z2 The first DBR layer is formed by alternating growth of two materials, z1=0.01, 0.6≤z2≤1, and the center wavelength of the first DBR layer is 880nm with a reflection spectral width of 100nm; the second DBR layer is composed of Al. y1 Ga 1-y1 As and Al y2 Ga 1-y2 The second DBR layer is formed by alternating growth of two materials, wherein 0.3≤y1≤0.5 and 0.8≤y2≤1, and the center wavelength of the second DBR layer is 790nm with a reflection spectral width of 90nm; the third DBR layer is composed of Al. x1 Ga 1-x1 As and Al x2 Ga 1-x2 As is formed by alternating growth of 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.

6. The epitaxial structure of a triple-junction solar cell as described in claim 1, characterized in that: The InGaAs sub-cell structure includes an AlGaAs back field layer, an InGaAs base region, multiple quantum well layers, an InGaAs emitter region, and an AlInP window layer, which are grown sequentially along the direction away from the Ge sub-cell. The thickness of the InGaAs base region is 2000nm-2500nm.

7. A method for fabricating an epitaxial structure of a triple-junction solar cell, used to fabricate the epitaxial structure of a triple-junction solar cell as described in any one of claims 1-6, characterized in that, include: A substrate is provided, and a Ge sub-cell is formed on the substrate; The first tunnel junction is grown on a Ge subcell; An InGaAs subcell is grown on a first tunnel junction. The InGaAs subcell contains multiple sets of quantum well layers, each set of quantum well layers being formed by alternating growth of InGaAs well layers and GaAsP barrier layers. In the multiple sets of quantum well layers, the In composition of the InGaAs well layers decreases progressively along the direction away from the Ge subcell, and the P composition of the GaAsP barrier layers decreases progressively along the direction away from the Ge subcell. The InGaAs subcell contains three sets of quantum well layers, which are, in order, the first set of quantum well layers, the second set of quantum well layers, and the third set of quantum well layers along the direction away from the Ge subcell. In the first set of quantum well layers, the In composition of the InGaAs well layer is 10%, the thickness is 8.5 nm, and the photoluminescence wavelength of the first set of quantum well layers is 930 nm. In the second set of quantum well layers, the In composition of the InGaAs well layer is 7.8%, the thickness is 8.5 nm, and the photoluminescence wavelength of the second set of quantum well layers is 910 nm. In the third set of quantum well layers, the In composition of the InGaAs well layer is 5.5%, the thickness is 8.5 nm, and the photoluminescence wavelength of the third set of quantum well layers is 890 nm. Multiple sets of InGaAs / AlGaAs DBR layers are provided between the first tunnel junction and the InGaAs subcell. The reflection wavelength of the multiple sets of DBR layers decreases step by step along the direction away from the Ge subcell. The effective reflection wavelength of the multiple sets of DBR layers after recombination covers the band of 665nm-930nm. A second tunnel junction is grown on an InGaAs sub-cell; GaInP subcells are grown on the second tunnel junction.

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