A three-junction flexible solar cell and a method for manufacturing the same

Abstract

The application discloses a kind of three-junction flexible solar cell and preparation method thereof, including growing epitaxial layer of flip-chip three-junction solar cell on GaAs substrate, the epitaxial layer includes GaInP top cell, GaAs middle cell and InGaAs bottom cell from substrate in sequence;Then evaporate AZO film on InGaAs bottom cell surface, deposit multiple pairs of Ta2O5 layer and SiO2 layer on AZO film alternately, form high reflection film;Then the flexible substrate is permanently bonded with the epitaxial layer with high reflection film on one side to obtain bonded sheet;Finally, remove GaAs substrate, prepare front electrode and anti-reflection film, and etch to expose part of AZO film as back electrode, complete the preparation of solar cell, the application is through the design of AZO film and high reflection film composition back reflection module structure, solve the problem that InGaAs bottom cell is not good at absorbing near-infrared spectrum.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor device technology, and specifically relates to a triple-junction flexible solar cell and its preparation method. Background Technology

[0002] Flexible multi-junction solar cells, especially flip-chip GaInP / GaAs / InGaAs triple-junction cells, are high-efficiency photovoltaic devices for space and special applications. Each sub-cell absorbs different wavelengths of the solar spectrum: the GaInP top cell absorbs photons in the 300-670nm band, the GaAs middle cell absorbs photons in the 670nm-873nm band, and the InGaAs bottom cell absorbs photons in the 873nm-1400nm band. Based on the AM1.5G spectrum, the percentages of solar energy absorbed by each sub-cell are approximately 45%, 21%, and 17%, respectively, leaving 17% of solar energy above 1400nm unutilized. While the GaInP top cell absorbs the most energy, its high voltage limits its current. Therefore, the InGaAs bottom cell is crucial for increasing the total current. However, InGaAs material has a low absorption coefficient for long-wavelength photons (900-1400nm) and strong penetrating power; photons passing through the absorption layer in a single pass cannot be completely absorbed, making the photoelectric conversion efficiency in this band a bottleneck for improving the overall performance of the cell.

[0003] Existing technical solutions mainly focus on optimizing the anti-reflective coating (ARC) on the front side. For example, patent 201711441256.6 uses a combination of electron beam evaporation and ion source-assisted deposition to prepare an anti-reflective coating on the front side of the battery at room temperature. This method ensures that the anti-reflective coating meets the required refractive index and has sufficient adhesion. However, its optimized wavelength range is mainly concentrated in the 400nm-900nm range, and its anti-reflective effect in the 900nm-1400nm range is limited. It fails to effectively solve the fundamental problem of insufficient absorption of near-infrared light by InGaAs bottom cells. Another patent, CN202110681570.1, uses alternating stacking of ZnO and SiO2 to prepare a broadband antireflection film for GaAs solar cells. Although it achieves a low reflectivity in the 300nm-1400nm range, its optimal process requires high-temperature annealing at about 600℃. This temperature will cause thermal damage to the GaAs-based epitaxial layer (such as component diffusion and arsenic volatilization). More importantly, the flexible substrates such as polyimide (PI) commonly used in flexible solar cells cannot withstand temperatures exceeding 400℃. High-temperature processes will cause the substrate to vitrify and become brittle, causing the device to lose its flexibility and become damaged.

[0004] Therefore, the current field of flexible solar cells lacks a back reflective film solution that can both efficiently recycle near-infrared spectra and be fully compatible with low-temperature flexible fabrication processes. Summary of the Invention

[0005] The purpose of this invention is to provide a triple-junction flexible solar cell and its fabrication method, solving the problem of poor near-infrared spectrum absorption by InGaAs bottom cells.

[0006] To achieve the above objectives, the present invention provides a method for fabricating a triple-junction flexible solar cell, comprising:

[0007] A GaAs substrate is provided, and an epitaxial layer for a flip-chip triple junction solar cell is grown on the GaAs substrate. The epitaxial layer comprises, from the substrate, a GaInP top cell, a GaAs middle cell, and an InGaAs bottom cell.

[0008] An AZO film is deposited on the surface of an InGaAs bottom cell, and the AZO film is deposited at a temperature of 150℃-180℃.

[0009] Multiple pairs of Ta2O5 and SiO2 layers are alternately deposited on the AZO film to form a high-reflectivity film. The temperature during the deposition of the Ta2O5 and SiO2 layers is 150℃-180℃.

[0010] A flexible substrate is provided, and the flexible substrate is permanently bonded to one side of the epitaxial layer with a high-reflectivity film to obtain a bonded sheet. The bonding temperature is 150℃-180℃.

[0011] The bonding sheet is subjected to GaAs substrate removal, front electrode and antireflection film fabrication, and partial AZO film etching to be used as back electrode, thus completing the fabrication of the solar cell.

[0012] Furthermore, the refractive index of the Ta2O5 layer is 2.1, and the thickness is 140nm-150nm; the refractive index of the SiO2 layer is 1.46, and the thickness is 200nm-210nm; the number of alternating stacked Ta2O5 and SiO2 layers is 6-8 pairs.

[0013] Furthermore, the thickness of the AZO film is 80-120 nm.

[0014] Furthermore, bonding the flexible substrate to one side of the epitaxial layer with the high-reflectivity film includes:

[0015] A Cr / Ti layer is deposited on a flexible substrate by vapor deposition;

[0016] A SiO2 bonding layer is deposited on the Cr / Ti layer;

[0017] The flexible substrate with the SiO2 bonding layer is bonded to the surface of the outermost SiO2 layer of the high-reflectivity film, and permanent bonding is performed by applying temperature and pressure in a vacuum environment to obtain a composite bonded sheet.

[0018] Furthermore, in the Cr / Ti layer, the thickness of Cr is 30nm-60nm, and the thickness of Ti is 50nm-100nm; the thickness of the SiO2 bonding layer is 500nm-1000nm.

[0019] Furthermore, the flexible substrate is a PI film with a thickness of 50 μm.

[0020] Furthermore, during permanent bonding, the bonding pressure is 1MPa-5MPa and the time is 0.5h-1h.

[0021] This application also provides a triple-junction flexible solar cell, which is prepared by the above-described preparation method.

[0022] After adopting the above solution, the beneficial effects of the present invention are as follows:

[0023] 1. This application introduces a back reflection film structure composed of an AZO film and a high-reflectivity film on the back of a solar cell. The AZO film has high transmittance in the near-infrared region, a wide spectral response of up to 1800 nm, good bending stability, and high reliability of back reflection. In the high-reflectivity 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 region. Multiple pairs of Ta2O5 / SiO2 layers can achieve a peak reflectivity of >99% when designing the center wavelength of the InGaAs bottom cell (e.g., 1100-1200nm). The high-reflectivity bandwidth is about 350nm-400nm, covering the 1000nm-1400nm band, which perfectly matches the absorption spectrum of the InGaAs bottom cell. This allows unabsorbed near-infrared photons to be efficiently reflected back to the absorption layer of the InGaAs bottom cell, enabling the InGaAs bottom cell to absorb the near-infrared spectrum again and promoting the improvement of the short-circuit current of the entire device. Moreover, both the Ta2O5 layer and the SiO2 layer are low-absorption materials in the 300nm-2000nm range, which do not introduce parasitic losses and can ensure that unabsorbed photons are efficiently returned to the InGaAs bottom cell for secondary utilization.

[0024] 2. The core temperatures of the entire back reflective film fabrication and bonding process in this application are controlled below 180℃, specifically the growth and bonding temperatures of the AZO film, Ta2O5 layer, and SiO2 layer are all between 150℃ and 180℃. This low-temperature system completely avoids the risk of thermal damage to GaAs-based epitaxial materials caused by high temperatures. More importantly, it ensures that flexible substrates such as the PI film do not undergo glass transition, thus maintaining their flexibility and mechanical strength. This provides a crucial technological foundation for achieving high-performance, high-reliability flexible solar cells.

[0025] 3. In the design of the high-reflectivity film, the Ta2O5 layer exhibits compressive stress, while the SiO2 layer exhibits tensile stress. The alternating deposition of these two layers partially cancels out stress, making it suitable for flexible device fabrication. Furthermore, the SiO2 bonding layer and the Cr / Ti layer can complement each other's stresses, effectively reducing the warpage and internal stress of the entire film stack. This makes the film layer less prone to cracking or detachment during repeated bending of the battery with the integrated back reflective film, greatly improving the long-term operational reliability of flexible devices under complex deformation.

[0026] 4. This application uses SiO2 as the bonding medium. SiO2 is extremely stable under humid heat, ultraviolet light, and atomic oxygen conditions, which can prevent water vapor and ions from diffusing into the sensitive epitaxial layer. Furthermore, the SiO2-SiO2 direct bonding technology replaces the traditional precious metal (such as Au-Au) hot-press bonding commonly used in flexible solar cells, which not only eliminates expensive gold materials and simplifies the process but also avoids the risk of metal ion contamination. At the same time, AZO, Ta2O5, and SiO2 are all common and environmentally friendly thin film materials, enabling this back reflective film structure to possess high performance while also having the advantages of low cost and green production, making it very suitable for large-scale industrial applications. Attached Figure Description

[0027] Figure 1 This is a structural diagram of the epitaxial layer of the flip-chip triple-junction solar cell after growth according to the present invention.

[0028] Figure 2 This is a structural diagram of the AZO film after vapor deposition according to the present invention.

[0029] Figure 3 This is a structural diagram of the high-reflectivity film deposited according to the present invention.

[0030] Figure 4 This is a structural diagram of the present invention after Cr / Ti layer is deposited on a flexible substrate.

[0031] Figure 5 This is a structural diagram of the SiO2 bonding layer after deposition in this invention.

[0032] Figure 6 This is a structural diagram of the present invention after permanent bonding.

[0033] Figure 7 This is a structural diagram of the flexible solar cell of the present invention.

[0034] Figure 8 This is a flowchart of the preparation method of the present invention.

[0035] Label Explanation:

[0036] 1. GaAs substrate; 2. Epitaxial layer; 3. AZO film; 4. High-reflection film; 5. Flexible substrate; 6. Cr / Ti layer; 7. SiO2 bonding layer; 8. Positive electrode; 9. Back electrode; 10. Anti-reflection film. Detailed Implementation

[0037] 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 of ordinary skill in the art without creative effort are within the scope of protection of this application, and the range values ​​mentioned in this application all include endpoint values.

[0038] like Figure 8 As shown, this application provides a method for fabricating a triple-junction flexible solar cell, comprising the following steps:

[0039] S1. Growth of a flip-chip epitaxial layer: A GaAs substrate 1 is provided. Using an MOCVD device, a flip-chip triple-junction solar cell epitaxial layer 2 is grown on the GaAs substrate 1. The epitaxial layer 2 sequentially includes a GaInP top cell, a GaAs middle cell, and an InGaAs bottom cell from the substrate. The structure of the grown epitaxial layer is as follows: Figure 1 As shown.

[0040] S2. Deposition of AZO film: The epitaxial wafer prepared in step S1 is organically cleaned, and then an AZO film 3 is deposited on the surface of the InGaAs base cell by electron beam evaporation. The deposition temperature is 150℃-180℃, preferably 150℃, and the thickness of the AZO film 3 is 80nm-120nm, preferably 100nm. The structure after AZO film 3 deposition is as follows. Figure 2 As shown.

[0041] Optionally, organic cleaning of the epitaxial wafer includes immersing the epitaxial wafer in acetone for 10 minutes each in 3 passes and isopropanol for 1 pass, followed by rinsing with deionized water and evaporating.

[0042] S3. Deposition of a high-reflectivity film: Multiple pairs of Ta2O5 and SiO2 layers are alternately stacked and deposited on the surface of AZO film 3 by electron beam evaporation to form a high-reflectivity film 4. The deposition temperature is 150℃-180℃, preferably 150℃. The Ta2O5 layer has a refractive index of 2.1 and a thickness of 140nm-150nm, preferably 143nm; the SiO2 layer has a refractive index of 1.46 and a thickness of 200nm-210nm, preferably 205nm; the number of alternating stacked Ta2O5 and SiO2 layers is 6-8 pairs, preferably 6 pairs. The structure after deposition of the high-reflectivity film 4 is as follows: Figure 3 As shown.

[0043] S4. Depositing a Cr / Ti layer on a flexible substrate: A flexible substrate 5 is provided, preferably a PI film with a thickness of 50 μm; the flexible substrate 5 is organically cleaned, and then a Cr / Ti layer 6 is deposited on the flexible substrate 5 by electron beam evaporation at room temperature. The Cr / Ti layer 6 is a composite metal layer of Cr and Ti, wherein the thickness of Cr is 30 nm-60 nm and the thickness of Ti is 50 nm-100 nm. The structure after depositing the Cr / Ti layer 6 is as follows. Figure 4 As shown.

[0044] Optionally, the organic cleaning process for the flexible substrate 5 includes immersing the flexible substrate 5 in acetone for 10 minutes in each of the three immersions and isopropanol immersions, and then baking it in an oven at 100°C for 2 hours to dry it.

[0045] S5. Deposition of SiO2 bonding layer: A SiO2 bonding layer 7 is deposited on the Cr / Ti layer 6 using a PECVD device. The thickness of the SiO2 bonding layer 7 is 500nm-1000nm. The structure after deposition of the SiO2 bonding layer 7 is as follows. Figure 5 As shown.

[0046] S6. Permanent Bonding: Align and bond the side of the flexible substrate 5 with the SiO2 bonding layer 7 to the surface of the outermost SiO2 layer of the high-reflectivity film 4, and then place it in a wafer bonding machine for permanent bonding to obtain a composite bonded sheet. The structure after permanent bonding is as follows. Figure 6 As shown.

[0047] Specifically, permanent bonding is performed by applying temperature and pressure in a vacuum environment. The bonding temperature is 150℃-180℃, the pressure is 1MPa-5MPa, and the time is 0.5h-1h. The vacuum level inside the wafer bonding machine cavity reaches the Pa level.

[0048] S7. Subsequent Processes: After bonding, the bonded wafer is removed after cooling and voltage reduction. Then, the bonded wafer undergoes a series of processes including wet etching to remove the GaAs substrate (1), photolithography and deposition of the positive electrode (8) on the front side, deposition of an anti-reflection film (10), etching to expose the AZO film as the back electrode (9), and annealing and dicing (these are standard processes and will not be detailed here). This completes the fabrication of the solar cell, resulting in a flexible solar cell structure as shown. Figure 7 As shown.

[0049] Furthermore, during permanent bonding, the bonding temperature is 150℃-180℃, the pressure is 1MPa-5MPa, and the time is 0.5h-1h.

[0050] Therefore, to solve the problem of poor near-infrared spectrum absorption by InGaAs bottom cells, this application introduces a back-reflection film structure on the back of the solar cell, such as... Figure 7As shown, the back reflective film consists of an AZO film 3 and a high reflective film 4, and is disposed on the back side of the epitaxial layer 2 of the solar cell. The AZO (Aluminum-Doped Zinc Oxide) film is aluminum-doped zinc oxide, essentially an N-type transparent conductive oxide. AZO has high near-infrared transmittance, a broad spectral response up to 1800 nm, good bending stability, low-temperature processing compatible with PI films, high-reliability back reflection, no metal contamination, low mass production cost, and no rare metals, making it environmentally friendly and sustainable. The thickness of the AZO film 3 is 80 nm-120 nm, preferably 100 nm.

[0051] The high-reflectivity film 4 is composed of multiple pairs of alternating stacked Ta2O5 and SiO2 layers, forming a distributed Bragg reflector structure and employing the 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 significant difference in refractive indices between the Ta2O5 and SiO2 layers results in a wider reflection bandwidth. Furthermore, the more pairs of Ta2O5 and SiO2 layers there are, the higher the reflectivity, exhibiting high refractive index contrast in the near-infrared region. When designing the InGaAs substrate at its center wavelength (e.g., 1100-1200 nm), multiple pairs of Ta2O5 and SiO2 layers can achieve a peak reflectivity >99%, with a high-reflectivity bandwidth of approximately 350nm-400nm, covering the 1000nm-1400nm band. This perfectly matches the absorption spectrum of the InGaAs substrate, efficiently reflecting unabsorbed near-infrared photons back to the absorption layer of the InGaAs substrate.

[0052] Specifically, in order to efficiently reflect unabsorbed near-infrared photons back to the InGaAs bottom cell, the thickness of the Ta2O5 layer is designed to be 140nm-150nm, and the thickness of the SiO2 layer is designed to be 200nm-210nm. The number of alternating stacking cycles of the Ta2O5 layer and the SiO2 layer is 6-8 pairs. If the number of cycles is set too few, the optimal effect of high reflectivity will not be achieved. If it is set too many, the limit of high reflectivity will be reached, and the gain will be limited. Moreover, if the film is too thick, it will be easy to fall off when the flexible cell is bent.

[0053] Taking a Ta₂O₅ layer with a refractive index of 2.1 and a SiO₂ layer with a refractive index of 1.46 as an example, to achieve a maximum reflectivity of >99% near the center wavelength, the number of alternating stacked Ta₂O₅ and SiO₂ layers should be 6 pairs. If the center wavelength of the InGaAs bottom cell is around 1200nm, the thickness of the Ta₂O₅ layer should be 143nm and the thickness of the SiO₂ layer should be 205nm. Therefore, the preferred number of alternating stacked Ta₂O₅ and SiO₂ layers is 6 pairs, the preferred thickness of the Ta₂O₅ layer is 143nm, and the preferred thickness of the SiO₂ layer is 205nm.

[0054] Therefore, this application adopts a back reflective film structure that combines AZO film 3 and multiple pairs of stacked Ta2O5 / SiO2 high reflective films 4, realizing the secondary absorption of the near-infrared spectrum in the 900nm-1400nm band by the InGaAs bottom cell. At the same time, the bending stability of the AZO film and the internal stress of the Ta2O5 / SiO2 high reflective film self-compensate and adjust the warping, which is very suitable for the fabrication of flexible solar cells with PI film as flexible substrate.

[0055] Furthermore, the growth temperature of the AZO film 3 and the high-reflectivity film 4 is 150℃-180℃. This low-temperature growth process is compatible with PI film and is suitable for solar cells with PI film as a flexible substrate.

[0056] Furthermore, this application also provides a triple-junction flexible solar cell prepared by the above-described method, with the structure as follows: Figure 7 As shown, from the back surface to the light-receiving surface of the solar cell, it sequentially includes a flexible substrate 5, a high-reflectivity film 4, an AZO film 3, an epitaxial layer 2 of a flip-chip triple-junction solar cell, a positive electrode 8, and an anti-reflection film 10; wherein, the epitaxial layer 2 sequentially includes a GaInP top cell, a GaAs middle cell, and an InGaAs bottom cell from the substrate, and the AZO film 3 is etched to expose a portion to serve as the back electrode 9; the high-reflectivity film 4 is composed of multiple pairs of alternately stacked Ta2O5 layers and SiO2 layers.

[0057] Furthermore, the flexible substrate 5 is a PI film, and a Cr / Ti layer 6 and a SiO2 bonding layer 7 are sequentially stacked between the flexible substrate 5 and the high-reflectivity film 4. The SiO2 bonding layer 7 and the Cr / Ti layer 6 can form stress complementarity, effectively reducing the warpage and internal stress of the entire film stack. Using SiO2 as the bonding medium, SiO2 is very stable under humid heat, ultraviolet light, and atomic oxygen conditions, which can prevent water vapor and ions from diffusing to the sensitive epitaxial layer. Moreover, the use of SiO2-SiO2 direct bonding technology replaces the traditional precious metal (such as Au-Au) hot-press bonding commonly used in flexible solar cells, which not only saves expensive gold materials and simplifies the process, but also avoids the risk of metal ion contamination.

[0058] It is worth noting that the thicknesses of the GaAs substrate 1, epitaxial layer 2, AZO film 3, high-reflection film 4, flexible substrate 5, Cr / Ti layer 6, SiO2 bonding layer 7, positive electrode 8, back electrode 9, and anti-reflection film 10 shown in the accompanying drawings are merely examples and do not represent their actual thicknesses. Furthermore, the actual proportions between the GaAs substrate 1, epitaxial layer 2, AZO film 3, high-reflection film 4, flexible substrate 5, Cr / Ti layer 6, SiO2 bonding layer 7, positive electrode 8, back electrode 9, and anti-reflection film 10 are not as shown in the drawings and are for reference only.

[0059] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0060] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not 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 three-junction flexible solar cell, characterized by, The application relates to a preparation method of a three-junction flexible solar cell. A GaAs substrate is provided, and epitaxial layers of an inverted three-junction solar cell are grown 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; An AZO film is evaporated on the surface of the InGaAs bottom cell, and the temperature during evaporation of the AZO film is 150-180 DEG C; A plurality of pairs of Ta2O5 layers and SiO2 layers are alternately deposited on the AZO film to form a high-reflection film, and the AZO film and the high-reflection film combine to form a back-reflection film structure; the temperature during deposition of the Ta2O5 layers and the SiO2 layers is 150-180 DEG C, the refractive index of the Ta2O5 layers is 2.1, and the thickness of the Ta2O5 layers is 140-150 nm; the refractive index of the SiO2 layers is 1.46, and the thickness of the SiO2 layers is 200-210 nm; and the number of periods of the alternately stacked Ta2O5 layers and SiO2 layers is 6-8 pairs; A flexible substrate is provided, and a Cr / Ti layer is evaporated on the flexible substrate; An SiO2 bonding layer is deposited on the Cr / Ti layer; One side of the flexible substrate with the SiO2 bonding layer is attached to the surface of the outermost SiO2 layer of the high-reflection film, and permanent bonding is performed under vacuum environment by applying temperature and pressure to obtain a composite bonding sheet, and the bonding temperature is 150-180 DEG C; The GaAs substrate is removed, a front electrode and an anti-reflection film are prepared, and part of the AZO film is etched to expose the AZO film as a back electrode to complete preparation of the solar cell.

2. 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 thickness of the AZO film is 80-120 nm.

3. The method for fabricating a triple-junction flexible solar cell as described in claim 1, characterized in that: 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.

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 thickness of the PI film is 50 microns.

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

6. A triple-junction flexible solar cell, characterized by: The three-junction flexible solar cell is prepared by the preparation method. The application relates to a preparation method of a three-junction flexible solar cell.

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