Three-junction flexible solar cell and preparation method thereof

Abstract

The invention discloses a three-junction flexible solar cell and a preparation method thereof, and the method comprises the steps: growing an epitaxial layer of an inverted three-junction solar cell on a GaAs substrate, and enabling the epitaxial layer to sequentially comprise a GaInP top cell, a GaAs middle cell and an InGaAs bottom cell from the substrate; then, an AZO film is evaporated on the surface of the InGaAs bottom cell, a plurality of pairs of Ta2O5 layers and SiO2 layers are alternately deposited on the AZO film, and a high-reflection film is formed; permanently bonding the flexible substrate and the surface, with the high-reflection film, of the epitaxial layer together to obtain a bonding sheet; and finally, the GaAs substrate is removed, the front electrode and the antireflection film are prepared, a part of AZO film is etched and exposed to serve as a back electrode, and preparation of the solar cell is completed. According to the invention, the back reflection film structure composed of the AZO film and the high reflection film is designed, so that the problem that an InGaAs bottom cell is poor in near infrared spectrum absorption is solved.

Description

TECHNICAL FIELD

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

[0002] The flexible multi-junction solar cell, particularly the GaInP / GaAs / InGaAs three-junction cell in an inverted structure, is a high-efficiency photovoltaic device for space and special field applications. Each sub-cell absorbs different bands of the solar spectrum: the GaInP top cell absorbs photons in the 300-670 nm band, the GaAs middle cell absorbs photons in the 670-873 nm band, and the InGaAs bottom cell absorbs photons in the 873-1400 nm band. Based on the AM1.5G spectrum, the power absorption of each sub-cell accounts for about 45%, 21% and 17% of the total solar power, respectively. The 17% of the solar power in the band above 1400 nm is not utilized. Although the GaInP top cell absorbs the most energy, it has high voltage and limited current. Therefore, the InGaAs bottom cell is crucial for improving the total current. However, the InGaAs material has a low absorption coefficient for long-wave photons in the 900-1400 nm band and strong penetration ability. The photons passing through the absorption layer once cannot be completely absorbed, which leads to the bottleneck of the photoelectric conversion efficiency in this band and hinders the improvement of the overall performance of the cell.

[0003] The existing technical solutions mainly focus on the optimization of the anti-reflective film (ARC) on the front surface. For example, patent 201711441256.6 uses a method combining electron beam evaporation and ion source assisted deposition to prepare an anti-reflective film on the front surface of the cell at room temperature. The anti-reflective film meets the required refractive index requirements and has sufficient adhesion. However, the optimized band is mainly concentrated in the 400-900 nm band, and the anti-reflective effect for the 900-1400 nm band is limited, which does not effectively solve the fundamental problem of insufficient absorption of near-infrared light by the InGaAs bottom cell. Patent CN202110681570.1 uses ZnO and SiO2 to prepare a wide-spectrum anti-reflective film for GaAs solar cells. Although it achieves a low reflectivity in the 300-1400 nm range, the optimal process requires high-temperature annealing at about 600℃. This temperature can cause thermal damage (such as component diffusion and arsenic evaporation) to the GaAs-based epitaxial layer. More importantly, the flexible substrates commonly used in flexible solar cells, such as polyimide (PI), cannot withstand temperatures above 400℃. High-temperature processes will cause the substrate to glassify and become brittle, making the device lose flexibility and be damaged.

[0004] Therefore, there is a lack of a back reflector solution in the current flexible solar cell field that can efficiently recycle near-infrared light and is fully compatible with low-temperature flexible preparation processes. SUMMARY

[0005] The application aims to provide a triple-junction flexible solar cell and a preparation method thereof, and solve the problem of poor absorption of near-infrared spectrum by an InGaAs bottom cell.

[0006] To achieve the above-mentioned purpose, the application provides a preparation method of a triple-junction flexible solar cell, comprising the following steps: providing a GaAs substrate, growing an epitaxial layer of a flip-chip triple-junction solar cell on the GaAs substrate, wherein the epitaxial layer comprises a GaInP top cell, a GaAs middle cell and an InGaAs bottom cell in sequence from the substrate; evaporating an AZO film on the surface of the InGaAs bottom cell, wherein the temperature during the evaporation of the AZO film is 150-180 DEG C; alternately depositing multiple pairs of Ta2O5 layers and SiO2 layers on the AZO film to form a high-reflection film, wherein the temperature during the deposition of the Ta2O5 layers and the SiO2 layers is 150-180 DEG C; providing a flexible substrate, permanently bonding the flexible substrate and the epitaxial layer with the high-reflection film on one side to obtain a bonded piece, wherein the temperature of the bonding is 150-180 DEG C; removing the GaAs substrate, preparing a front electrode and an anti-reflection film, and etching to expose part of the AZO film as a back electrode from the bonded piece to complete the preparation of the solar cell.

[0007] Further, the refractive index of the Ta2O5 layer is 2.1, and the thickness is 140-150 nm; the refractive index of the SiO2 layer is 1.46, and the thickness is 200-210 nm; and the number of periods of the alternately stacked Ta2O5 layers and SiO2 layers is 6-8 pairs.

[0008] Further, the thickness of the AZO film is 80-120 nm.

[0009] Further, the bonding of the flexible substrate and the epitaxial layer with the high-reflection film on one side comprises the following steps: evaporating a Cr / Ti layer on the flexible substrate; depositing a SiO2 bonding layer on the Cr / Ti layer; attaching the flexible substrate with the SiO2 bonding layer on one side to the surface of the outermost SiO2 layer of the high-reflection film, and permanently bonding under vacuum environment by applying temperature and pressure to obtain a composite bonded piece.

[0010] Further, in the Cr / Ti layer, the thickness of Cr is 30-60 nm, and the thickness of Ti is 50-100 nm; and the thickness of the SiO2 bonding layer is 500-1000 nm.

[0011] Further, the flexible substrate is a PI film, and the thickness of the PI film is 50 microns.

[0012] Further, the bonding pressure is 1-5 MPa and the bonding time is 0.5-1 h.

[0013] The application also provides a three-junction flexible solar cell prepared by the above preparation method.

[0014] After the above scheme, the application has the following advantages: 1. The application introduces a back reflection film structure composed of an AZO film and a high reflection film on the back of the solar cell. The AZO film has high transmittance in the near infrared, a wide spectral response of up to 1800 nm, and good bending stability, and has high reliability of the back reflection. In the high reflection film, the Ta2O5 layer (refractive index n=2.1) and the SiO2 layer (refractive index n=1.46) have high refractive index contrast in the near infrared. Multiple pairs of Ta2O5 layers / SiO2 layers can achieve a peak reflectivity of >99% when designing the center wavelength (such as 1100-1200 nm) of the InGaAs bottom cell, a high reflection bandwidth of about 350-400 nm, covering the 1000-1400 nm band, and perfectly matching the absorption spectrum of the InGaAs bottom cell. The unabsorbed near-infrared photons are efficiently reflected back to the absorption layer of the InGaAs bottom cell, allowing the InGaAs bottom cell to perform secondary absorption on the near-infrared spectrum, promoting the improvement of the short-circuit current of the entire device. Moreover, the Ta2O5 layer and the SiO2 layer are low-absorption materials in the 300-2000 nm range, do not introduce parasitic loss, and can ensure that unabsorbed photons are efficiently returned to the InGaAs bottom cell for secondary use.

[0015] 2. The entire back reflection film preparation and bonding process of the application is controlled at a core temperature below 180℃, i.e. the growth temperature of the AZO film, the Ta2O5 layer, and the SiO2 layer, and the bonding temperature are all 150-180℃. This low-temperature system completely avoids the risk of thermal damage to GaAs-based epitaxial materials at high temperatures. More importantly, it ensures that the PI film and other flexible substrates do not undergo glass transition, maintaining their flexibility and mechanical strength, and providing a key process foundation for realizing high-performance and high-reliability flexible solar cells.

[0016] 3. In the design of the film layers of the high reflection film, the Ta2O5 layer is under compressive stress, and the SiO2 layer is under tensile stress. The alternating deposition of these two layers of structures can achieve partial stress offsetting, which is suitable for flexible device preparation. Moreover, the SiO2 bonding layer and the Cr / Ti layer can form stress complementation, effectively reducing the warping and internal stress of the entire film stack. This makes the battery integrated with the back reflection film less prone to cracking or falling off when repeatedly bent, greatly improving the long-term working reliability of the flexible device under complex deformation.

[0017] 4、The application uses SiO2 as a bonding medium, SiO2 is extremely stable under heat and humidity, ultraviolet, atomic oxygen, can block the diffusion of water vapor and ions to the sensitive epitaxial layer, and the SiO2-SiO2 direct bonding technology is used instead of the traditional flexible solar cell commonly used noble metal (such as Au-Au) hot-press bonding, not only saves expensive gold materials, simplifies the process, but also avoids the risk of metal ion pollution. At the same time, AZO, Ta2O5, SiO2 and other common, environmentally friendly thin film materials make the back reflector structure not only have high performance, but also have the advantages of low cost and green production, which is very suitable for large-scale industrial application. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The structure diagram after the epitaxial layer of the inverted three-junction solar cell is grown.

[0019] Figure 2 The structure diagram after the AZO film is evaporated.

[0020] Figure 3 The structure diagram after the high-reflective film is deposited.

[0021] Figure 4 The structure diagram after the Cr / Ti layer is evaporated on the flexible substrate.

[0022] Figure 5 The structure diagram after the SiO2 bonding layer is deposited.

[0023] Figure 6 The structure diagram after permanent bonding.

[0024] Figure 7 The structure diagram of the flexible solar cell.

[0025] Figure 8 The flow chart of the preparation method.

[0026] REFERENCE NUMERALS: 1, GaAs substrate; 2, epitaxial layer; 3, AZO film; 4, high-reflective film; 5, flexible substrate; 6, Cr / Ti layer; 7, SiO2 bonding layer; 8, positive electrode; 9, back electrode; 10, antireflection film. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, not all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the application, and the scope of the application includes end point values.

[0028] As Figure 8 shown, the application provides a preparation method of a three-junction flexible solar cell, comprising the following steps: S1, growing an inverted epitaxial layer: providing a GaAs substrate 1, growing an epitaxial layer 2 of an inverted three-junction solar cell on the GaAs substrate 1 by a MOCVD device, the epitaxial layer 2 comprises a GaInP top cell, a GaAs middle cell and an InGaAs bottom cell in sequence from the substrate, and the structure of the completed epitaxial layer is as shown in Figure 1 .

[0029] S2, evaporating an AZO film: the epitaxial wafer prepared in step S1 is subjected to organic cleaning, and then an AZO film 3 is evaporated on the surface of the InGaAs bottom cell by electron beam evaporation, the temperature during evaporation is 150-180°C, preferably 150°C, the thickness of the AZO film 3 is 80-120 nm, preferably 100 nm, and the structure after evaporating the AZO film 3 is as shown in Figure 2 .

[0030] Optionally, the organic cleaning of the epitaxial wafer comprises: immersing the epitaxial wafer in 3 acetones and 1 isopropyl alcohol for 10 minutes in sequence, followed by deionized water washing and spin drying; S3, depositing a high-reflection film: a plurality of pairs of Ta2O5 layers and SiO2 layers are alternately stacked and deposited on the surface of the AZO film 3 by electron beam evaporation to form a high-reflection film 4, the temperature during deposition is 150-180°C, preferably 150°C; wherein the refractive index of the Ta2O5 layer is 2.1, and the thickness is 140-150 nm, preferably 143 nm; the refractive index of the SiO2 layer is 1.46, and the thickness is 200-210 nm, preferably 205 nm; the number of periods of alternately stacking the Ta2O5 layers and the SiO2 layers is 6-8 pairs, preferably 6 pairs, and the structure after depositing the high-reflection film 4 is as shown in Figure 3 .

[0031] S4, evaporating a Cr / Ti layer on a flexible substrate: a flexible substrate 5 is provided, which is preferably a PI film, and the thickness of the PI film is 50 μm; the flexible substrate 5 is subjected to organic cleaning, and then a Cr / Ti layer 6 is evaporated on the flexible substrate 5 by electron beam evaporation at room temperature, the Cr / Ti layer 6 is a composite metal layer composed of Cr and Ti, wherein the thickness of Cr is 30-60 nm, and the thickness of Ti is 50-100 nm, and the structure after evaporating the Cr / Ti layer 6 is as shown in Figure 4 .

[0032] Optionally, the organic cleaning of the flexible substrate 5 comprises: immersing the flexible substrate 5 in 3 acetones and 1 isopropyl alcohol for 10 minutes in sequence, and then baking in an oven at 100°C for 2 hours to dry.

[0033] S5, depositing a SiO2 bonding layer: depositing a SiO2 bonding layer 7 on the Cr / Ti layer 6 by a PECVD device, the thickness of the SiO2 bonding layer 7 is 500nm-1000nm, the structure after depositing the SiO2 bonding layer 7 is as shown in Figure 5 .

[0034] S6, permanent bonding: aligning and pasting the side of the flexible substrate 5 with the SiO2 bonding layer 7 with the surface of the SiO2 layer of the outermost layer of the high reflection film 4, and then placing it in a wafer bonding machine to do permanent bonding, to obtain a composite bonding sheet, the structure after permanent bonding is as shown in Figure 6 .

[0035] Specifically, the temperature and pressure are applied in a vacuum environment for permanent bonding, the bonding temperature is 150℃-180℃, the pressure is 1MPa-5MPa, the time is 0.5h-1h, and the vacuum degree in the cavity of the wafer bonding machine reaches Pa level.

[0036] S7, subsequent process: after bonding, the bonding sheet is taken out after cooling and decompression, and then the bonding sheet is sequentially subjected to GaAs substrate 1 removal by wet etching, front side photolithography and positive electrode 8 evaporation, deposition of anti-reflection film 10, etching to expose AZO film as back electrode 9, annealing and dicing, etc. (this is a conventional process, which will not be described here), to complete the preparation of the solar cell, and the flexible solar cell structure is as shown in Figure 7 .

[0037] Further, the bonding temperature is 150℃-180℃, the pressure is 1MPa-5MPa, and the time is 0.5h-1h.

[0038] Therefore, to solve the problem that the InGaAs bottom cell does not absorb near-infrared spectrum well, a back reflection film structure is introduced on the back of the solar cell, as shown in Figure 7 , the back reflection film is composed of an AZO film 3 and a high reflection film 4, and is arranged on the back of the solar cell epitaxial layer 2; wherein the AZO (Aluminum-Doped Zinc Oxide) film is an aluminum-doped zinc oxide, which is essentially an N-type transparent conductive oxide, the transmittance of AZO in the near-infrared is very high, the wide spectrum response can reach 1800nm, and the bending stability is good, the low-temperature process is compatible with the PI film, the back reflection has high reliability, there is no metal pollution, and the mass production cost is low, there is no rare metal, and it has environmental protection and sustainability. The thickness of the AZO film 3 is 80nm-120nm, preferably 100nm.

[0039] The high-reflection film 4 is composed of multiple pairs of alternately stacked Ta2O5 layers and SiO2 layers, is in a distributed Bragg reflector structure, and adopts a quarter-wavelength optical thickness principle. The refractive index of the Ta2O5 layer is 2.1, and the refractive index of the SiO2 layer is 1.46. The refractive indices of the Ta2O5 layer and the SiO2 layer are greatly different, which can make the reflection bandwidth larger. The more pairs of the Ta2O5 layer and the SiO2 layer, the higher the reflectivity. In the near-infrared, the high-reflection film has a high refractive index contrast. When the multiple pairs of the Ta2O5 layer and the SiO2 layer are designed for the center wavelength (for example, 1100-1200 nm) of the InGaAs bottom cell, the peak reflectivity can be greater than 99%, the high-reflection bandwidth is about 350-400 nm, the wavelength range covered is 1000-1400 nm, and the high-reflection film is perfectly matched with the absorption spectrum of the InGaAs bottom cell. The near-infrared photons that are not absorbed can be efficiently reflected back to the absorption layer of the InGaAs bottom cell.

[0040] Specifically, to efficiently reflect the near-infrared photons that are not absorbed back to the InGaAs bottom cell, the thickness of the Ta2O5 layer is designed to be 140-150 nm, the thickness of the SiO2 layer is designed to be 200-210 nm, and the number of periods of the alternately stacked Ta2O5 layer and SiO2 layer is designed to be 6-8. If the number of periods is too small, the best high-reflection effect cannot be achieved. If the number of periods is too large, the high-reflection rate has reached the limit, and the gain is limited. Moreover, the film layer is too thick, and is easy to fall off when the flexible cell is bent.

[0041] Taking the refractive index of the Ta2O5 layer as 2.1 and the refractive index of the SiO2 layer as 1.46 as an example, to achieve a maximum reflectivity greater than 99% near the center wavelength, the number of periods of the alternately stacked Ta2O5 layer and SiO2 layer is designed to be 6. If the center wavelength of the InGaAs bottom cell is designed to be about 1200 nm, the thickness of the Ta2O5 layer is 143 nm, and the thickness of the SiO2 layer is 205 nm. Therefore, the number of periods of the alternately stacked Ta2O5 layer and SiO2 layer is preferably 6, the thickness of the Ta2O5 layer is preferably 143 nm, and the thickness of the SiO2 layer is preferably 205 nm.

[0042] Therefore, the back-reflection film structure combining the AZO film 3 and the multiple pairs of stacked Ta2O5 / SiO2 high-reflection film 4 is adopted, the InGaAs bottom cell realizes secondary absorption of the near-infrared spectrum in the wavelength range of 900-1400 nm, the bending stability of the AZO film and the internal stress self-compensation and adjustment of the Ta2O5 / SiO2 high-reflection film can adjust the warping, and the structure is very suitable for the production of the flexible solar cell with the PI film as the flexible substrate.

[0043] In addition, the growth temperature of the AZO film 3 and the high reflection film 4 is 150-180℃, the low temperature growth process is compatible with the PI film, and is suitable for the solar cell with the PI film as the flexible substrate.

[0044] In addition, the application further provides a three-junction flexible solar cell prepared by the preparation method, which has a structure as shown in the drawing. Figure 7 The three-junction flexible solar cell comprises, from the back light surface to the light receiving surface, a flexible substrate 5, a high reflection film 4, an AZO film 3, an epitaxial layer 2 of a flip-chip three-junction solar cell, a positive electrode 8 and an anti-reflection film 10; wherein the epitaxial layer 2 comprises, from the substrate, a GaInP top cell, a GaAs middle cell and an InGaAs bottom cell, the AZO film 3 is etched to expose a part as a back electrode 9; and the high reflection film 4 is composed of a plurality of pairs of alternately stacked Ta2O5 layers and SiO2 layers.

[0045] Further, the flexible substrate 5 is a PI film, and the flexible substrate 5 and the high reflection film 4 further comprise, in sequence, a Cr / Ti layer 6 and a SiO2 bonding layer 7. The SiO2 bonding layer 7 and the Cr / Ti layer 6 can form stress complementation, effectively reducing the warping and internal stress of the whole film stack; the use of SiO2 as the bonding medium is stable under the conditions of humidity, ultraviolet light and atomic oxygen, can block the diffusion of water vapor and ions to the sensitive epitaxial layer, and the use of SiO2-SiO2 direct bonding technology instead of the traditional noble metal (such as Au-Au) hot-press bonding commonly used in flexible solar cells not only saves the expensive gold material, simplifies the process, but also avoids the risk of metal ion pollution.

[0046] It is worth noting that the thicknesses of the GaAs substrate 1, the epitaxial layer 2, the AZO film 3, the high reflection film 4, the flexible substrate 5, the Cr / Ti layer 6, the SiO2 bonding layer 7, the positive electrode 8, the back electrode 9 and the anti-reflection film 10 shown in the drawings of the application are only examples and do not represent the actual thicknesses. In addition, the actual proportions between the GaAs substrate 1, the epitaxial layer 2, the AZO film 3, the high reflection film 4, the flexible substrate 5, the Cr / Ti layer 6, the SiO2 bonding layer 7, the positive electrode 8, the back electrode 9 and the anti-reflection film 10 are not as shown in the drawings, but only for reference.

[0047] 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 referred to each other.

[0048] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Therefore, 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 herein.

Claims

1. A method for producing a triple-junction flexible solar cell, characterized by, The application relates to a preparation method of a three-junction flexible solar cell. Providing a GaAs substrate, growing epitaxial layers of an inverted three-junction solar cell on the GaAs substrate, wherein the epitaxial layers comprise a GaInP top cell, a GaAs middle cell and an InGaAs bottom cell in sequence from the substrate; Evaporating an AZO film on the surface of the InGaAs bottom cell, wherein the temperature during evaporation of the AZO film is 150-180 DEG C; Depositing a plurality of pairs of Ta2O5 layers and SiO2 layers on the AZO film in alternation to form a high-reflection film, wherein the temperature during deposition of the Ta2O5 layers and the SiO2 layers is 150-180 DEG C; Providing a flexible substrate, permanently bonding the flexible substrate to one side of the epitaxial layers with the high-reflection film to obtain a bonded sheet, wherein the temperature during bonding is 150-180 DEG C; Removing the GaAs substrate, preparing a front electrode and an anti-reflection film, and etching to expose part of the AZO film as a back electrode from the bonded sheet to complete preparation of the solar cell.

2. The method of claim 1, wherein the method further comprises: The refractive index of the Ta2O5 layer is 2.1, and the thickness is 140-150 nm; the refractive index of the SiO2 layer is 1.46, and the thickness is 200-210 nm; and the number of periods of the alternately stacked Ta2O5 layers and SiO2 layers is 6-8. ​ 3. The method for fabricating a triple-junction flexible solar cell as described in claim 1, characterized in that: The thickness of the AZO film is 80-120 nm.

4. The method of claim 1, wherein the method further comprises: depositing a first layer of a first material on the substrate; depositing a second layer of a second material on the first layer; and depositing a third layer of a third material on the second layer. The bonding of the flexible substrate to one side of the epitaxial layers with the high-reflection film comprises the following steps: Evaporating a Cr / Ti layer on the flexible substrate; Depositing a SiO2 bonding layer on the Cr / Ti layer; Bonding one side of the flexible substrate with the SiO2 bonding layer to the surface of the outermost SiO2 layer of the high-reflection film, and permanently bonding under vacuum environment by applying temperature and pressure to obtain a composite bonded sheet.

5. The method of claim 4, wherein the method further comprises: depositing a third layer of p-type semiconductor material on the second layer of p-type semiconductor material; and depositing a third layer of n-type semiconductor material on the second layer of n-type semiconductor material. In the Cr / Ti layer, the thickness of Cr is 30-60 nm, and the thickness of Ti is 50-100 nm; and the thickness of the SiO2 bonding layer is 500-1000 nm.

6. The method of claim 4, wherein the method further comprises: depositing a third layer of p-type semiconductor on the second layer of p-type semiconductor; and depositing a third layer of n-type semiconductor on the second layer of n-type semiconductor. The flexible substrate is a PI film, and the thickness of the PI film is 50 mu m.

7. The method of claim 4, wherein the method further comprises: depositing a third layer of p-type semiconductor on the second layer of p-type semiconductor; and depositing a third layer of n-type semiconductor on the second layer of n-type semiconductor. During permanent bonding, the bonding pressure is 1-5 MPa, and the bonding time is 0.5-1 h.

8. A triple-junction flexible solar cell, characterized by: The three-junction flexible solar cell is prepared by the preparation method of any one of claims 1-7.

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