A back electrode structure of a TOPCon solar cell and its fabrication method

Abstract

This invention discloses a back electrode structure and its fabrication method for a TOPCon solar cell, relating to the field of solar cell technology. The back electrode, from the inside out, comprises an interface bonding layer, a stress buffer layer, and a conductive substrate layer. The interface bonding layer has a silver content of 70-85 wt% and contains active glass frit and nano-nucleating agents. The stress buffer layer has a silver content of 50-70 wt% and contains a cordierite and indium tin alloy composite modifier. The conductive substrate layer has a silver content greater than 95 wt%. This invention achieves functional separation of interface bonding, stress buffering, and bulk conductivity through a three-layer functional decoupling design, eliminating stress concentration and simultaneously forming strong interfacial chemical bonds, thus resolving the technical contradiction of simultaneously achieving low contact resistance, high adhesion, and high reliability. The fabrication process employs a one-time co-sintering process, compatible with mass production lines. This invention significantly improves electrode adhesion, reduces contact resistance, avoids microcracks in the battery, and enhances long-term reliability.

Description

Technical Field

[0001] This invention belongs to the field of solar cell technology, specifically relating to a back electrode structure of a TOPCon solar cell and its fabrication method. Background Technology

[0002] In TOPCon solar cell manufacturing, the back electrode is typically formed by screen-printing silver paste and then sintering it at high temperatures. Currently, the industry mainstream uses a single-layer homogeneous silver electrode, but this structure suffers from a long-standing and unresolved "contradiction triangle" problem: First, thermal stress mismatch leads to poor adhesion and microcracks: the high thermal expansion coefficient of silver differs greatly from that of the silicon substrate, generating interfacial stress during thermal cycling, which can cause electrode warping, detachment, or even microcracks in the cell.

[0003] Second, it is difficult to achieve both low contact resistance and high adhesion: to achieve low-resistance ohmic contact, glass frit is required to etch the polycrystalline silicon layer; however, excessive etching will damage the interface and weaken the adhesion, so the two are at odds.

[0004] Third, the long-term stability of the interface is insufficient: the interface of the homogeneous electrode is mainly mechanically interlocked and the chemical bonding is weak, which makes it prone to degradation under humid and hot, thermal cycling and other environments.

[0005] The long-standing improvement efforts of those skilled in the art have mainly focused on optimizing the glass frit composition to adjust its corrosivity, using combinations of silver powders with different particle sizes, or adjusting the sintering process. These improvements all modify the material without changing the basic structure of "single-layer homogeneity" or simple "double-layer stacking," and have consistently failed to systematically and synergistically resolve the aforementioned "contradiction triangle." In essence, this has led to a technological bias: that adjusting the composition or process of a single homogeneous layer is the only or primary way to solve the problem.

[0006] The closest prior art, AU2007289892, discloses a multilayer electrode that teaches the formation of electrodes by using pastes with different silver contents in different layers. However, the design logic of this patent is essentially still pursuing gradient optimization of conductivity, such as a lower silver content in the lower layer to reduce costs or improve initial contact, and a higher silver content in the upper layer to ensure conductivity, which cannot fundamentally solve the aforementioned technical pain points. Summary of the Invention

[0007] To address the problems mentioned in the background art, the present invention provides a back electrode structure of a TOPCon solar cell and its fabrication method, thereby solving the problems in the prior art such as poor adhesion and microcracks caused by thermal stress mismatch, difficulty in achieving both low contact resistance and high adhesion, and insufficient long-term interface stability.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A back electrode structure for a TOPCon solar cell includes an interface bonding layer, a stress buffer layer, and a conductive substrate layer, which are stacked sequentially from the inside to the outside, with the side closest to the silicon substrate of the TOPCon solar cell as the inner side. The silver content of the interface bonding layer is 70-85 wt%, the silver content of the stress buffer layer is 50-70 wt%, and the silver content of the conductive substrate layer is greater than 95 wt%. The stress buffer layer also contains 30-50 wt% of a composite functional modifier, which includes a mixture of ceramic powder and ductile metal powder.

[0009] Preferably, the interfacial bonding layer also contains 5-15 wt% active glass material and 0.5-2 wt% nano-sized α-Al2O3 nucleating agent.

[0010] Preferably, the active glass material is a TeO2-V2O5-Bi2O3 system glass material.

[0011] Preferably, the composite functional modifier is a mixture of cordierite powder and indium tin alloy powder in a mass ratio of 7:3.

[0012] Preferably, the cordierite powder has a particle size of 1-5 μm, and the indium tin alloy powder is an In-30wt%Sn alloy powder.

[0013] Preferably, the conductive substrate layer is made of a mixture of spherical silver powder and flake silver powder, with a mass ratio of spherical silver powder to flake silver powder of 1:1; the volume resistivity of the conductive substrate layer is less than 5 × 10⁻⁶. -6 Ω・cm.

[0014] A preparation method for preparing a back electrode structure includes the following steps: sequentially screen printing an interface bonding layer paste, a stress buffer layer paste, and a conductive host layer paste on the back of a TOPCon solar cell; after printing, performing a co-sintering to form the back electrode structure.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention achieves functional decoupling of “interface bonding-stress buffering-bulk conductivity” through a three-layer functional partition gradient electrode structure. Each layer can be specifically optimized for the core function, completely solving the “contradictory triangle” in the prior art where low contact resistance, high adhesion and high resistance to microcracks cannot be achieved at the same time.

[0016] 2. This invention uses a "high-low-high" silver content gradient design to actively reduce the silver content in the intermediate stress buffer layer and incorporate composite functional modifiers, thereby constructing a gradient of thermal expansion coefficients from the silicon substrate to the metal electrode. This fundamentally eliminates interfacial stress concentration during thermal cycling, avoids microcracks in the battery cell, and significantly improves electrode adhesion.

[0017] 3. The interface bonding layer of the present invention can be controlled by active glass material to form a stable ohmic contact, and at the same time form a strong chemical bond with the silicon substrate. The bonding force is far greater than that of traditional physical interlocking, which greatly improves the long-term stability of the electrode interface.

[0018] 4. The preparation method of the present invention adopts a one-time co-sintering process, which does not require additional process steps, is compatible with existing TOPCon battery mass production lines, and is easy to promote and apply in industrial applications. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the back electrode structure; The diagram is marked as follows: 1-Interface bonding layer; 2-Stress buffer layer; 3-Conductive host layer. Detailed Implementation

[0020] To facilitate understanding of the technical content of this invention by those skilled in the art, the invention will be further described in detail below with reference to the accompanying drawings and specific examples. It should be understood that the specific examples described herein are merely illustrative and not intended to limit the scope of the invention.

[0021] A back electrode structure for a TOPCon solar cell, with the side closest to the silicon substrate of the TOPCon solar cell as the inner side, consists of an interface bonding layer 1, a stress buffer layer 2, and a conductive main body layer 3 stacked sequentially from the inside out. All three layers are formed by screen printing corresponding pastes and then co-sintering them in a single process. Specifically: Interface bonding layer 1: Located on the innermost side, it is in direct contact with the doped polycrystalline silicon layer of the TOPCon battery. The silver content is 85wt%, the active glass material content of the TeO2-V2O5-Bi2O3 system is 14wt%, and the nano-scale α-Al2O3 nucleating agent content is 1wt%. The softening point of the active glass material is 780-850℃, which matches the dopant activation temperature of the polycrystalline silicon layer.

[0022] Stress buffer layer 2: Located between the interfacial bonding layer 1 and the conductive host layer 3, with a silver content of 60wt% and a composite functional modifier content of 40wt%; the composite functional modifier is a mixture of cordierite powder and indium tin alloy powder in a mass ratio of 7:3, wherein the cordierite powder has a particle size of 1-5μm and a coefficient of thermal expansion of ~2.5×10 -6 / K, the indium-tin alloy powder is an In-30wt%Sn alloy powder with a melting point of approximately 120℃.

[0023] Conductive substrate layer 3: Located on the outermost side, with a silver content of 96wt%, it is a mixture of spherical and flake silver powders in a 1:1 mass ratio. After sintering, the volume resistivity of conductive substrate layer 3 is less than 5×10⁻⁶. -6 Ω・cm.

[0024] In this embodiment, the effective thermal expansion coefficient of the interface bonding layer 1 is ~12×10⁻⁶. −6 / K, the effective thermal expansion coefficient of stress buffer layer 2 is ~10×10 −6 / K, the effective thermal expansion coefficient of the conductive host layer 3 is ~19×10. −6 / K. By introducing a thermal expansion coefficient between that of silicon substrate (~2.6×10⁻⁶ K). −6 The interfacial bonding layer 1 and stress buffer layer 2 between the silicon substrate ( / K) and the metal electrode form a multi-level transition structure from a low-expansion silicon substrate to a high-expansion conductive host layer 3. This design effectively avoids the abrupt change in the coefficient of thermal expansion in traditional single-layer electrodes, significantly reduces interfacial thermal mismatch stress, and improves the reliability of the electrode structure.

[0025] The fabrication method of the back electrode structure in this embodiment includes the following steps: Slurry preparation: Interface bonding layer slurry, stress buffer layer slurry, and conductive host layer slurry are prepared separately. The solid phase composition of each slurry is consistent with the composition described in the back electrode structure above. Organic carrier is added to formulate a slurry system suitable for screen printing.

[0026] Screen printing: On the back of the TOPCon solar cell after the tunneling oxide layer and doped polycrystalline silicon layer have been prepared, the interface bonding layer paste, stress buffer layer paste, and conductive host layer paste are screen printed in sequence. After each layer is printed, the solvent is removed by low-temperature drying.

[0027] Co-sintering: The solar cell with three layers of paste printing is placed in a sintering furnace and co-sintered once at a peak temperature of 780-850℃. After holding at the temperature, it is cooled to obtain the back electrode structure of the TOPCon solar cell.

[0028] The back electrode structure in this embodiment is based on a functional decoupling design of "interface bonding - stress buffering - bulk conductivity", with each layer working together. The core working principle is as follows: The working principle of interface bonding layer 1: During sintering, the TeO2-V2O5-Bi2O3 system of active glass frit controllably etches the passivation layer on the polycrystalline silicon surface, forming locally highly doped silicon regions, enabling silver particles to form low-resistance ohmic contacts with these regions. By controlling the ratio of Bi2O3 to V2O5 in the glass frit, the etching depth is precisely controlled between 5-20nm, achieving a contact effect that is "good enough but not damaging." Simultaneously, the Bi in the glass frit... 3+ and V 5+ During the sintering reaction, ions form stable compounds such as Bi2SiO5 and vanadium silicate with SiO2 and Si, and form strong covalent / ionic bonds with the silicon matrix. The bonding force far exceeds the physical interlocking of traditional homogeneous electrodes, fundamentally improving the interfacial bonding strength and long-term stability.

[0029] Working principle of stress buffer layer 2: As the core functional layer of this invention, through the low silver content formulation design, combined with the compound modification of cordierite ceramic powder and indium tin plastic metal powder, the hard cordierite ceramic particles can hinder crack propagation and disperse stress, while the soft low melting point indium tin alloy phase undergoes plastic deformation during sintering cooling and subsequent thermal cycling, absorbing and dissipating stress energy, making this layer a "sacrificial layer" and "plastic hinge" for stress digestion, avoiding stress transmission to the brittle silicon wafer, and fundamentally solving the problem of microcracks in solar cells.

[0030] Working principle of conductive main body layer 3: Under the stress protection of the stress buffer layer 2 below, this layer does not need to make performance compromises for interface stress issues. It can achieve high packing density and continuous conductive path purely through the formulation design of high silver content and spherical and flake silver powder, providing extremely low volume resistivity and excellent macroscopic mechanical support performance, ensuring the current collection efficiency and structural stability of the battery.

[0031] The back electrode structure of this embodiment achieves the following beneficial effects compared to the prior art: It completely solves the "contradictory triangle" problem of existing technologies. Through functional decoupling design, it simultaneously achieves low contact resistance, high electrode adhesion, excellent anti-microcrack performance and long-term operational stability, solving a technical pain point that has long been unresolved in this field.

[0032] Breaking away from the technical bias in this field that "electrode problems are solved by adjusting the composition or process of a single homogeneous layer," this innovative three-layer structure design achieves synergistic optimization of various functions, rather than the gradient optimization of conductivity that is only targeted at existing technologies.

[0033] A gradient of thermal expansion coefficients was constructed, which fundamentally eliminated the interfacial stress problem caused by the mismatch of thermal expansion coefficients between the silver electrode and the silicon substrate, avoiding electrode warping, detachment and microcracks in the cell, and significantly improving product yield and reliability.

[0034] To verify the technical effect of this embodiment, this embodiment was set as the experimental group, and three comparative groups were set up for parallel comparison experiments. Except for the electrode structure, all other battery manufacturing processes, raw materials, and testing conditions were completely identical. The test results are shown in Table 1: Table 1

[0035] Experimental data description: Compared with Comparative Example 1, the experimental group of the present invention showed significantly improved adhesion, significantly reduced contact resistance, no microcracks, and significantly reduced power attenuation, and was significantly superior to the traditional single-layer homogeneous electrode in all key performance aspects.

[0036] Compared with Comparative Example 2, it is shown that omitting the dedicated stress buffer layer 2 significantly reduces electrode adhesion, long-term reliability, and resistance to microcracks, highlighting the indispensable core role of the stress buffer layer 2 in the three-layer structure.

[0037] Compared with Comparative Example 3, this demonstrates that the specific silver content gradient of "high-low-high" in this invention is key, and that a simple three-layer stack cannot effectively buffer stress, and its effect is far inferior to that of this invention.

[0038] Those skilled in the art should understand that the above embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention. Any reasonable modifications, equivalent substitutions, or adaptive improvements made based on the technical concepts disclosed in this invention without departing from the essential spirit of the invention should be considered to fall within the scope of protection defined by the claims of this invention.

Claims

1. A back electrode structure of a TOPCon solar cell, characterized by, The structure includes an interface bonding layer (1), a stress buffer layer (2), and a conductive substrate layer (3) stacked sequentially from the inside to the outside, with the side closest to the silicon substrate of the TOPCon solar cell as the inside. The silver content of the interface bonding layer (1) is 70-85wt%, the silver content of the stress buffer layer (2) is 50-70wt%, and the silver content of the conductive substrate layer (3) is greater than 95wt%. The stress buffer layer (2) also contains 30-50wt% of a composite functional modifier, which includes a mixture of ceramic powder and plastic metal powder.

2. The back electrode structure according to claim 1, wherein The interface bonding layer (1) also contains 5-15wt% active glass material and 0.5-2wt% nano-sized α-Al2O3 nucleating agent.

3. The back electrode structure according to claim 2, wherein The active glass material is a TeO2-V2O5-Bi2O3 system glass material.

4. The back electrode structure according to claim 1, wherein The composite functional modifier is a mixture of cordierite powder and indium tin alloy powder in a mass ratio of 7:

3.

5. The back electrode structure according to claim 4, wherein The particle size of cordierite powder is 1-5μm, and the indium-tin alloy powder is an In-30wt%Sn alloy powder.

6. The back electrode structure according to claim 1, wherein The conductive main body layer (3) is made of mixed powder of spherical silver powder and flaky silver powder, and the mass ratio of the spherical silver powder to the flaky silver powder is 1:1; the volume resistivity of the conductive main body layer (3) is less than 5×10 -6 Ω·cm.

7. A method of manufacturing a back electrode structure according to any one of claims 1 to 6, c h a r a c t e r i s e d in that Includes the following steps: The interface bonding layer paste, stress buffer layer paste, and conductive substrate paste are sequentially screen-printed on the back of the TOPCon solar cell. After printing, a co-sintering process is performed to form the back electrode structure.

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