CN210272385U

CN210272385U - Lattice gradient buffer layer applied to epitaxial growth of lattice-mismatched solar cell

Info

Publication number: CN210272385U
Application number: CN201921181179.XU
Authority: CN
Inventors: 刘建庆; 高熙隆; 文宏; 刘雪珍; 刘恒昌
Original assignee: Zhongshan Dehua Chip Technology Co ltd
Current assignee: Zhongshan Dehua Chip Technology Co ltd
Priority date: 2019-07-25
Filing date: 2019-07-25
Publication date: 2020-04-07
Anticipated expiration: 2029-07-25

Abstract

The utility model discloses a be applied to lattice mismatch solar cell epitaxial growth's lattice gradual change buffer layer, be located between lattice mismatch epitaxial material and the substrate (or the epitaxial layer the same with substrate lattice constant) of lattice mismatch solar cell, compound DBR superpose that has different reflection wave bands by many sets forms, every set of compound DBR is to constituting by many pairs of DBR pairs that the superpose is in the same place, and the lattice constant between adjacent DBR pair is gradient change, every pair of DBR contains two-layer semiconductor material layer, the different nevertheless lattice constant of the refractive index of this two-layer semiconductor material layer is the same or exist and can reach strain compensation's mismatch. The utility model discloses in the compound DBR that will have the broad spectrum reflection fuses into lattice gradual change buffer layer organically, both can reduce the defect density such as a large amount of dislocations that lattice mismatch epitaxial material grows to introduce by a wide margin, the effect that again can full play speculum, reduce simultaneously process steps, grow long and raw and other material losses, be favorable to reduce cost.

Description

Lattice gradient buffer layer applied to epitaxial growth of lattice-mismatched solar cell

Technical Field

The utility model belongs to the technical field of solar photovoltaic power generation's technique and specifically relates to indicate a lattice gradual change buffer layer of being applied to lattice mismatch solar cell epitaxial growth.

Background

With the development of modern industrial technology, energy has become the most important material basis for determining social progress in the survival and development of human society, and with the development of society, coal, petroleum, natural gas and the like have serious environmental pollution and the non-renewable energy is gradually reduced, so that the development of green new energy is imminent. Solar energy is an inexhaustible clean energy source, so the research and development of solar cell technology can alleviate the shortage of future energy sources to a certain extent. From the development process of photovoltaic power generation technology, solar cells can be roughly classified into three major categories: the solar cell comprises a first generation crystalline silicon solar cell, a second generation thin film solar cell and a third generation gallium arsenide multi-junction solar cell. The GaInP/GaInAs/Ge triple-junction solar cell composed of GaInP, GaInAs and Ge sub-cells is used as the mainstream structure of the traditional gallium arsenide multi-junction cell, the conversion efficiency under 500 times of condensation is over 41 percent, the conversion efficiency is far higher than that of a crystalline silicon cell, and the space for further improvement is provided.

The conventional triple-junction cell structurally and integrally keeps lattice matching, and the band gap combination is 1.85/1.40/0.67 eV. However, for the spectrum of sunlight, the band gap of the solar cell is not the optimal combination, because of the larger band gap difference between the GaInAs sub-cell and the Ge sub-cell, the current of the bottom cell is far larger than that of the middle cell and the top cell, because the bottom, middle and top three-junction sub-cells are connected in series, according to the current mechanism of the series structure, the current is determined by the minimum current of the three sub-cells, and the structure causes a large part of solar energy loss, and limits the improvement of the cell performance.

Theoretical analysis shows that in order to improve the photoelectric conversion efficiency of the triple-junction solar cell, the band gaps of absorption regions of the middle cell and the top cell need to be reduced, and the middle cell and the top cell absorb more light, so that the current of the middle sub-cell and the current of the top sub-cell are improved, the current of the bottom cell is reduced, and finally the current-matched triple-junction solar cell can be realized. According to the analysis, the MM (Metamorphic material) structure solar cell with the lattice mismatch structure is provided, the MM structure triple-junction cell is firstly applied to the CPV market, and the conversion efficiency can reach more than 42%; in recent years, the MM structure triple-junction cell product is applied to a space cell, the conversion efficiency can reach more than 32%, the attenuation of the MM structure triple-junction cell product after irradiation can reach the level of a conventional triple-junction cell, namely, the efficiency attenuation is lower than 18%, and the MM structure triple-junction cell product has the advantage of being much higher than other photovoltaic cell products. However, when the lattice mismatch material is epitaxial, if the thickness of the mismatch epitaxial layer is smaller than the critical thickness, the lattice constant is consistent with that of the substrate under the action of deformation energy, and once the thickness exceeds the critical thickness, the lattice constant is restored to an inherent value, so that a large amount of mismatch dislocation is generated, and the quality of the material is reduced. In general, the magnitude of the critical thickness is related to the degree of mismatch, with the larger the lattice mismatch, the smaller the critical thickness. For a heteroepitaxial material with a larger mismatch degree with a crystal lattice, the improvement of the epitaxial quality of the material becomes a bottleneck for the application of the heteroepitaxial material in wider fields and further improving the performance of devices.

To solve this problem, a common method is to connect the lattice-mismatched Ge substrate and InGaAs material with a graded buffer layer (typically InGaAs or GaInP material) that can relieve stress. For example, Ga of x ═ 0.1 is to be grown_0.9In_0.1As material, can be in Ge substrate and Ga_0.9In_0.1Growing a series of Ga with gradually changed components between As materials_1-xIn_xAn As buffer layer with a composition x continuously changed from 0.01 lattice-matched with the Ge substrate to 0.1, Ga_1-xIn_xAs bufferThe buffer layer can release the stress generated by lattice mismatch and reduce Ga_0.9In_0.1Dislocations and the like generated in the As material.

In order to ensure that the lattice of the mismatched material is completely relaxed and avoid the increase of lattice dislocation, the component change rate of the component gradual change buffer layer cannot be too fast, so that the thickness of the buffer layer is thicker. But this buffer layer does not have special action in the aspect of photoelectricity, therefore people have made relevant research in the aspect of the attenuate buffer layer on the basis of guaranteeing the buffer layer effect, and different from this, the utility model discloses the switching thinking combines together lattice gradual change buffer layer and Distributed Bragg Reflector (DBR), aims at developing the potential advantage of lattice gradual change buffer layer and makes it have optical action simultaneously.

The DBR structure has been applied to semiconductor devices (including light emitting diodes, LEDs, and solar cells), and its characteristic photon reflection capability greatly improves the performance of the semiconductor devices. In three junction GaAs solar cells, for example, on the one hand, the DBR can significantly reduce the absorption coefficient. Reflecting off a portion of the excess photons that reach the bottom cell prevents their conversion to heat that is released into the battery system, which is important to improve battery stability and extend battery life. In order to further reduce the absorption coefficient, the utility model discloses a photon reflection scope is widened to many sets of DBRs, increases reflection photon quantity. On the other hand, the DBR has obvious effect on improving the anti-radiation performance. As a large number of irradiation experiment results show that the irradiation resistance of the GaInAs sub-battery is much poorer than that of the GaInP sub-battery, the reason is considered by analysis to be that the As atom radius is larger, and the position of the high-energy particle is not easy to recover after irradiation. The DBR is arranged below the neutron cell, and the photons which are not absorbed by the GaInAs material for the first time are reflected back to be absorbed for the second time by adjusting the sunlight of the corresponding wave band reflected by the DBR structure, so that the effective absorption thickness of the GaInAs is increased in a phase-changing manner, the design thickness of the GaInAs subcell can be effectively reduced, and the improvement of the anti-irradiation performance is facilitated. In the field of LEDs, the light extraction efficiency is enhanced through DBRs, which is very important for improving the brightness.

In addition, if will some other optimization applications be in the utility model discloses on, can further promote the utility model discloses improve the advantage in the aspect of the device performance. For example, in the material selection, a dilute nitrogen material with good plasticity and the effect of hardening the film is adopted, so that the defects such as threading dislocation and the like are changed into transverse propagation, and the stress is released, thereby having excellent effect on filtering dislocation. In addition, the DBR is designed to be a periodic structure with strain compensation, so that the collection of minority carriers is improved, and the function of a dislocation blocking layer can be well played.

In conclusion, the lattice gradual change buffer layer with the wide-spectrum reflection function is introduced into the lattice mismatch structure multi-junction solar cell, so that the requirement of reducing the absorption coefficient can be met, the anti-irradiation performance of the cell can be improved, the problem of poor epitaxial crystal quality caused by threading dislocation can be solved, meanwhile, the process steps, the growth time and the raw material loss are reduced, and the cost is reduced. In a word, the advantage of the multi-junction battery with the MM structure can be exerted to the maximum extent, and the battery efficiency is improved.

SUMMERY OF THE UTILITY MODEL

The utility model aims to overcome prior art's shortcoming and not enough, the utility model provides a be applied to lattice mismatch solar cell epitaxial growth's lattice gradual change buffer layer (also can be called lattice gradual change wide-spectrum reflector), wide-spectrum reflection effect has, function through with gradual change buffer layer and compound DBR unites into one, both can reduce the defect density in the epitaxial layer that the mismatch introduced, improve material quality, promote GaInAs subcell irradiation resistance, reflection spectrum wavelength range can be widened again, reduce the absorption coefficient, and then improve battery stability, especially be favorable to the application on the aviation power, and simultaneously, reduce the technology step, long and the raw and other materials loss when growing, be favorable to reduce cost.

In order to achieve the above object, the present invention provides a technical solution: the lattice gradient buffer layer is positioned between a lattice mismatch epitaxial material and a substrate of the lattice mismatch solar cell or between the lattice mismatch epitaxial material and an epitaxial layer with the same lattice constant of the substrate, and is formed by superposing a plurality of sets of composite DBRs with different reflection wave bands, each set of composite DBR is formed by a plurality of pairs of DBR pairs which are superposed together, the lattice constant between adjacent DBR pairs is in gradient change, each pair of DBR pairs comprises two semiconductor material layers, and the two semiconductor material layers have different refractive indexes but the same lattice constant or have mismatch capable of achieving strain compensation.

Further, a first pair of DBR pairs adjacent to the substrate has a lattice constant matched to the substrate, a last pair of DBR pairs is lattice matched to the lattice mismatched epitaxial material, and the lattice constant of the DBR pairs between them increases in a gradient direction from the first to the last DBR pairs.

Furthermore, the composite DBR has at least two sets, and each set of DBR reflects a wave band (lambda)₁－Δλ)～(λ₁+ Δ λ) to (λ)_n－Δλ)～(λ_n+ Δ λ), wherein the combined range (λ)₁－Δλ)～(λ_n+ Δ λ) is determined by the adjacent subcell band gap, λ₁Is the central reflection wavelength, lambda, of the first set of composite DBRs_nThe central reflection wavelength of the nth set of composite DBR is shown, n is an integer, Delta lambda is 1/2 of the reflection range of a certain set of composite DBR, the interval of the central reflection wavelength lambda is within the range of 10-80 nm, and each set of period is 3-20 pairs.

Further, the two layers of semiconductor material of each DBR pair need to have different refractive indices, and the As-system material or the P-system material, such As GaInNAs, AlGaInAs, GaInP or AlGaInP, is selected, and the thickness thereof is designed to follow the formula:

wherein d is the thickness, λ is the central reflection wavelength of the expected reflection band, and n is the refractive index of the corresponding semiconductor material; wherein the As material has a refractive index higher than that of the P material, and the higher the aluminum content in the homologous material, the lower the refractive index.

Furthermore, the lattice constants of the two semiconductor material layers of each DBR pair can be the same, the mismatch capable of achieving strain compensation is allowed to exist, and the mismatch degree is in the range of 0.01% -5%.

Compared with the prior art, the utility model, have following advantage and beneficial effect:

1. the utility model discloses can connect two constitutional units that the lattice mismatch is great, reduce defect density such as dislocation when realizing the lattice transition.

2. A plurality of sets of composite DBRs with different reflection wave bands are introduced, the comprehensive reflection half-width of the DBR can be improved from (100 +/-30) nm to (150 +/-30) nm, the reflection range of the DBR can be effectively widened, the absorption coefficient is reduced, and the stability of the battery is improved.

3. Adopt the utility model discloses a lattice gradual change buffer layer (wide spectrum speculum) has improved the narrow problem of single set DBR reflection spectrum photon reflection scope, and reflection photon quantity increases and can further reduce the effective thickness of subcell, promotes the anti-irradiation performance.

4. The composite DBR with wide-spectrum reflection is organically fused into the lattice gradient buffer layer, so that the purposes of improving the performance and stability of the cell are achieved, meanwhile, the process steps, the growth time and the raw material loss are reduced, and the cost of the solar cell is reduced.

5. The semiconductor materials of the existing mature epitaxial process, such as GaInNAs, AlGaInAs, GaInP and AlGaInP, can be selected for selection. Due to the limitation of specific application positions and adjacent cell band gaps, if materials with rigid characteristics such as GaInNAs or AlGaInP with lattice strengthening effect can be adopted, the DBR is combined and applied to the MM-structure multi-junction solar cell, photon absorption is improved, defects can be filtered, and the function of a dislocation blocking layer is better exerted.

6. Will constitute the high refractive index (n) of the DBR_H) And low refractive index (n)_L) The layer is designed as a strain compensation structure, and as shown in fig. 1, the stress induced by lattice mismatch can be released, and the interface under the action of tensile strain and compressive strain contributes to lateral slip of threading dislocation, so that the dislocation density is further reduced.

Through the experiment, adopt Vecco company's MOCVD preparation the utility model discloses a three knot MM structure solar cell, longer than traditional technology shorten about 46min when growing, has promoted production efficiency and saves raw and other materials in a large number by a wide margin. The X-ray diffractometer (XRD) test analysis shows that the full width at half maximum of the top cell in the triple-junction MM structure solar cell epitaxial wafer manufactured by the method has no obvious difference with the traditional structure, and the defect density caused by lattice mismatch is reduced to a lower level. A white light reflection spectrum test shows that the reflection wave range of the lattice gradual change buffer layer is widened from (840-940) nm to (840-1020) nm, and the integral reflection intensity is not obviously reduced. Through to reflectivity test analysis, compare the triple junction MM structure battery that has single set DBR structure, the utility model discloses the MM triple junction solar cell absorption coefficient of preparation reduces to 0.91 by 0.927, and thermal behavior improves, and the stability of battery is showing the reinforcing.

Through the analysis contrast, adopt the utility model discloses the triple junction MM solar cell of preparation, more traditional DBR, the anti-irradiation performance also has obvious improvement, and conversion efficiency lifting range under the AM0 spectrum can reach 2.1% (as following table 1), can improve space power's output by a wide margin.

TABLE 1 MM STRUCTURE TRI-JUNCTION BATTERY USING A TRADIONAL DBR, GRAPHICAL BUFFER LAYER (WIDE SPECTRUM REFLECTOR)

Analysis of radiation resistance under AM0 space Spectroscopy

Drawings

Fig. 1 is a schematic structural diagram of a lattice-graded buffer layer (also referred to as a lattice-graded wide-spectrum mirror) according to the present invention.

Fig. 2 is a schematic diagram of a lattice-mismatched triple-junction solar cell structure using the lattice-graded buffer layer (also referred to as a lattice-graded wide-spectrum mirror) according to the present invention.

Detailed Description

To further illustrate the present invention, the following detailed description is given with reference to the accompanying drawings and the specific embodiments.

As shown in fig. 1 and fig. 2, the lattice-mismatched triple-junction solar cell provided in this embodiment includes a Ge substrate 1, where the Ge substrate 1 is a single-side polished 004-oriented p-type Ge single crystal wafer; sequentially growing a GaInP nucleating layer 2, a GaInAs buffer layer 3, a first tunneling junction 4, a lattice gradient buffer layer 5 (also called as a lattice gradient wide spectrum reflector), a GaInAs sub-cell 6, a second tunneling junction 7 and a GaInP sub-cell 8 on the polished surface of the Ge substrate 1 from bottom to top according to a layered superposed structure; wherein, the lattice gradual change buffer layer 5 comprises two setsThe two sets of composite DBRs 51 and 52 with different reflection wave bands are formed by superposing a plurality of pairs of DBR pairs, the lattice constants between the adjacent DBR pairs are in gradient change, each DBR pair comprises two semiconductor material layers, the refractive indexes of the two semiconductor material layers are different, but the lattice constants are the same or mismatch capable of achieving strain compensation is allowed to exist, and the mismatch degree is in the range of 0.01% -5%. Additionally, the lattice constant of the first DBR pair adjacent to the substrate is matched to the substrate, the last DBR pair is lattice matched to the lattice mismatched epitaxial material, and the lattice constant of the DBR pair between them increases in a gradient direction from the first DBR pair to the last DBR pair; each DBR reflection band (lambda)₁－Δλ)～(λ₁+ Δ λ) to (λ)_n－Δλ)～(λ_n+ Δ λ), wherein the combined range (λ)₁－Δλ)～(λ_n+ Δ λ) is determined by the adjacent subcell band gap, λ₁Is the central reflection wavelength, lambda, of the first set of composite DBRs_nThe central reflection wavelength of the nth set of composite DBR is set, n is an integer, delta lambda is 1/2 of the reflection range of a certain set of composite DBR, the interval of the central reflection wavelength lambda is within the range of 10-80 nm, and each set of period is 3-20 pairs; the two semiconductor material layers of each DBR pair need to have different refractive indexes, and the As system material or the P system material can be selected, and alloy materials such As GaInNAs, AlGaInAs, GaInP, AlGaInP and the like are adopted, and the thickness design of the semiconductor material layers follows the formula:

In this embodiment, the lattice-graded buffer layer 5 is composed of a GaInAs/AlGaInAs DBR and a GaInNAs/AlGaInAs DBR In which the In component increases In a pair-by-pair gradient, and the lattice constant of the first pair of GaInAs/AlGaInAs DBR is the same as that of the substrateThe final pair of GaInNAs/AlGaInAs DBRs are both p-type doped layers with hole concentration of 1 × 10, as same as GaInAs subcells¹⁸/cm³. The reflecting wavelength range of the GaInAs/AlGaInAs DBR is 840-940 nm, wherein the logarithm of the GaInAs/AlGaInAs combination layer is 12 pairs; the GaInAs material and the AlGaInAs material are designed to be strain compensation structures, and the lattice mismatch degree is 0.3%. The reflection wavelength range of the GaInNAs/AlGaInAs DBR is 930-1020 nm, wherein the logarithm of the GaInNAs/AlGaInAs combination layer is 4 pairs; the GaInNAs material and the AlGaInAs material are designed to be strain compensation structures, and the lattice mismatch degree is 0.3%.

The GaInP nucleating layer 2, the GaInAs buffer layer 3 and the first tunneling junction 4 are in lattice matching with the Ge substrate 1.

The GaInP nucleation layer 2 is an n-type doped layer with an electron concentration of 2 × 10¹⁸/cm³And the thickness is 5 nm.

The GaInAs buffer layer 3 is an n-type doped layer with electron concentration of 4 multiplied by 10¹⁸/cm³The thickness is 500 nm.

The first tunneling junction 4 is of a p-AlGaInAs/n-GaInP structure, wherein the thickness of the p-AlGaInAs/n-GaInP structure is 12 nm.

The total cell thickness of the GaInAs sub-cell 6 is 1800nm, and the optical band gap of the GaInAs material is 1.3 eV.

The second tunneling junction 7 is of a p-AlGaInAs/n-GaInP structure, wherein the thickness of the p-AlGaInAs/n-GaInP structure is 12 nm.

The total cell thickness of the GaInP sub-cell 8 is 600nm, and the optical band gap of the GaInP sub-cell material is 1.85 eV.

To sum up, the utility model discloses combine the characteristics of lattice gradual change buffer layer and DBR (Distributed Bragg Reflector, Distributed Brag Reflector), the function with gradual change buffer layer and DBR unites and introduces MM structure multijunction solar cell as an organic whole, both can play the effect of lattice gradual change buffer layer, reduce the defect density in the epitaxial layer that the mismatch introduced, improve material quality, reflection spectrum wavelength range can be widened again, promote GaInAs subcell irradiation resistance, reduce the absorption coefficient, solve and get into the too much photon of end battery and change the problem that heat energy influences battery stability into, and simultaneously, reduction process steps, length and raw and material loss when growing, be favorable to reduce cost. In a word, the utility model discloses can exert lattice mismatch solar cell's photoelectric conversion efficiency more fully, be worth promoting.

The above-mentioned embodiments are only preferred embodiments of the present invention, and the scope of the present invention is not limited thereto, so that all the changes made according to the shape and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. The lattice gradual change buffer layer applied to the epitaxial growth of the lattice mismatch solar cell is characterized in that: the lattice gradient buffer layer is positioned between a lattice mismatch epitaxial material and a substrate of the lattice mismatch solar cell or between an epitaxial layer with the same lattice constant of the lattice mismatch epitaxial material and the substrate, and is formed by overlapping a plurality of sets of composite DBRs with different reflection wave bands, each set of composite DBR is formed by overlapping a plurality of DBR pairs, the lattice constant between the adjacent DBR pairs is in gradient change, each DBR pair comprises two semiconductor material layers, and the two semiconductor material layers have different refractive indexes but the same lattice constant or have mismatch capable of achieving strain compensation.

2. The lattice graded buffer layer applied to epitaxial growth of lattice mismatched solar cells according to claim 1, wherein: the lattice constant of the first DBR pair adjacent to the substrate is matched to the substrate, the last DBR pair is lattice matched to the lattice mismatched epitaxial material, and the lattice constant of the DBR pair between them increases in a gradient from the first DBR pair to the last DBR pair.

3. The lattice graded buffer layer applied to epitaxial growth of lattice mismatched solar cells according to claim 1, wherein: the composite DBR has at least two sets, and each set of DBR reflects wave band (lambda)₁－Δλ)～(λ₁+ Δ λ) to (λ)_n－Δλ)～(λ_n+ Δ λ), wherein the combined range (λ)₁－Δλ)～(λ_n+ Δ λ) is determined by the adjacent subcell band gap, λ₁Is the central reflection wavelength, lambda, of the first set of composite DBRs_nThe central reflection wavelength of the nth set of composite DBR is shown, n is an integer, Delta lambda is 1/2 of the reflection range of a certain set of composite DBR, the interval of the central reflection wavelength lambda is within the range of 10-80 nm, and each set of period is 3-20 pairs.

4. The lattice graded buffer layer applied to epitaxial growth of lattice mismatched solar cells according to claim 1, wherein: the two semiconductor material layers of each DBR pair have different refractive indexes, and are As material layers or P material layers, and the thickness design follows the formula:

5. The lattice graded buffer layer applied to epitaxial growth of lattice mismatched solar cells according to claim 1, wherein: the lattice constants of the two semiconductor material layers of each DBR pair can be the same, mismatch capable of achieving strain compensation is allowed to exist, and the mismatch degree is in the range of 0.01% -5%.