Topcon back fine grid paste and preparation method

Abstract

The application discloses a TOPCon back surface fine grid paste and a preparation method thereof, and belongs to the field of photovoltaic cell manufacturing. In the application, specific parameter gas-phase silicon dioxide reacts with lead oxide in a glass phase to generate a lead silicate buffer layer in a sintering process, a thermal diffusion barrier is constructed to inhibit excessive corrosion of the glass to a Poly-Si layer, the Poly layer with a thickness fluctuation of 70-180 nm can be adapted, the open-circuit voltage and the fill factor are simultaneously improved, the Voc is improved by more than 10 mV, the efficiency gain is more than 0.05%, the thixotropy of the paste and the printing aspect ratio are improved, and the process window of the TOPCon back surface fine grid paste is widened.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic cell manufacturing, and in particular to a TOPCon back-side grid paste and its preparation method. Background Technology

[0002] N-type TOPCon cells are one of the mainstream technologies for high-efficiency crystalline silicon solar cells. Their back side employs a tunneling oxide passivation contact structure, sequentially forming an ultrathin silicon oxide layer, a phosphorus-doped polycrystalline silicon layer (Poly-Si layer), and a silicon nitride passivation layer. This significantly reduces interfacial recombination and improves open-circuit voltage. The Poly-Si layer, as the core functional layer of the TOPCon structure, typically has a thickness controlled between 70 and 180 nm, and is crucial for the selective transport of charge carriers. However, in actual mass production processes, the thickness of the Poly-Si layer fluctuates by approximately ±20 nm, posing extremely stringent requirements on the sintering window and corrosion control capabilities of the back-side silver paste.

[0003] During the metallization process on the back of TOPCon cells, the glass powder in the silver paste corrodes and penetrates the silicon nitride layer during the high-temperature sintering stage, forming an ohmic contact with the Poly-Si layer. If the glass phase activity is too high or the sintering temperature is too high, the molten glass will excessively corrode the Poly-Si layer or even penetrate into the silicon substrate, causing the silver electrode to directly contact the crystalline silicon, destroying the passivation effect, resulting in an open-circuit voltage (Voc) loss of more than 5mV, a fill factor (FF) decrease of more than 0.5%, and a significant increase in series resistance. Conversely, if the glass powder content is reduced or the glass activity is weakened to avoid over-corrosion, insufficient contact and high contact resistance are likely to occur, which also restricts cell efficiency. Therefore, how to simultaneously consider Voc and FF within a wide range of Poly thickness fluctuations, and achieve a balance between passivation protection and ohmic contact, is a key technical problem that urgently needs to be solved in the fine grid paste for the back of TOPCon cells.

[0004] In the prior art, patent application CN 113724915 A discloses a high-wear-resistant, low-temperature curing conductive silver paste, which achieves low-temperature curing (≤80℃) through the compounding of epoxy resin system and fumed silica, and is mainly used in electronic communication fields such as mobile phone PDS antennas. However, this solution is a low-temperature curing paste, which relies on the cross-linking curing of organic resin to achieve conductivity. The sintering temperature is far lower than the high-temperature sintering process range of 500~800℃ for photovoltaic cells. There is no corrosion and contact mechanism between glass and silicon wafer. Its fumed silica is only used as a common thickening thixotropic component. It is completely different from the application scenario, working mechanism and technical requirements of the fine grid paste on the back of TOPCon cells, and cannot solve the contradiction between over-corrosion of the Poly layer and contact resistance in TOPCon cells.

[0005] Patent application CN 117844308 A discloses an organic carrier for the back-side paste of TOPCon, in which a thixotropic agent system contains a small amount of fumed silica, which is compounded with polyamide wax and hydrogenated castor oil to adjust the thixotropy and printability of the paste. However, this solution only uses fumed silica as a conventional thixotropic agent component, without specifying or designing parameters such as its original particle size, aggregate size, BET specific surface area, and surface hydrophobicity. The ordinary fumed silica used only plays a role in improving the rheological properties of printing and cannot build a "thermal diffusion barrier layer" during sintering to control the corrosion rate of the glass relative to the Poly-Si layer. Therefore, it does not have the function of selective contact protection for Poly layers of different thicknesses, and it is difficult to fundamentally broaden the process adaptability window of TOPCon back-side paste.

[0006] In summary, the existing technology has not yet provided a back-side fine grid silver paste solution that can precisely control glass corrosion behavior through functional fumed silica to address the thickness fluctuations of the TOPCon cell Poly layer and simultaneously achieve synergistic improvement of Voc and FF. Summary of the Invention

[0007] Technical issues The thickness of the Poly-Si layer in TOPCon cells is subject to process fluctuations. Existing back silver paste cannot simultaneously suppress glass over-corrosion to ensure Voc and ensure sufficient ohmic contact to maintain FF within a wide thickness range, resulting in a narrow sintering process window and limited cell efficiency.

[0008] Technical solution To solve the above-mentioned technical problems, the present invention provides a silver paste for the back-side fine grid of TOPCon, wherein the composition of the silver paste by mass percentage includes: 83-90% silver powder, 1.5-3% glass powder, 7-12% organic phase, and 0.01-1% modified fumed silica.

[0009] Furthermore, the composition of the silver paste, by mass percentage, includes: 85-90% silver powder, 2-3% glass powder, 7-10% organic phase, and 0.05-0.5% modified fumed silica.

[0010] Furthermore, the composition of the silver paste, by mass percentage, includes: 85-90% silver powder, 2-3% glass powder, 7-10% organic phase, and 0.05-0.3% modified fumed silica.

[0011] Furthermore, the silver powder can be selected from conventional silver powder, such as high-density but dispersed silver powder, specifically it can be a compound or a single selection.

[0012] Furthermore, the silver powder can have a particle size of 0.5-3.0 μm and a specific surface area of ​​0.3-2.0 m².2 / g, tap density is 3-7 g / mL.

[0013] Furthermore, the glass powder can be selected from conventional glass powder or Pb-Te-Bi system glass powder.

[0014] Furthermore, the glass powder comprises, by mass percentage: 50-60% PbO, 25-35% TeO2, 5-15% Bi2O3, 1-5% SiO2, and 1-5% Al2O3.

[0015] Furthermore, the glass powder comprises, by mass percentage: 52-57% PbO, 27-32% TeO2, 7-12% Bi2O3, 1-3% SiO2, and 1-3% Al2O3.

[0016] Furthermore, the D50 of the glass powder is 1-3 μm.

[0017] Furthermore, the composition of the organic phase includes organic solvents, resins, thixotropic agents, dispersants, and other additives.

[0018] Furthermore, the composition of the organic phase, by mass percentage, includes 15-20% organic solvent, 35-40% resin, 10-15% thixotropic agent, 20-25% dispersant, and 5-10% other additives.

[0019] Furthermore, the organic solvent includes one or more of diethylene glycol dibutyl ether, diethylene glycol butyl ether, and hexadecyl alcohol ester.

[0020] Furthermore, the resins include ethyl cellulose and thermoplastic resins.

[0021] Furthermore, the thixotropic agent is a polyamide wax, specifically DISPARLON 6500.

[0022] Furthermore, dispersants include TDO and stearic acid.

[0023] Furthermore, other additives include silicone oil.

[0024] Furthermore, the modified fumed silica is hydrophobically treated fumed silica with a contact angle ≥120°, and the modifier is hexamethyldisilazane with a coating rate of 60-90%.

[0025] Furthermore, the primary particle size of the fumed silica is 5 nm to 15 nm, preferably 7 nm to 12 nm; the aggregate size is 80 nm to 200 nm; and the BET specific surface area is 300 m². 2 / g to 400 m 2 / g, preferably 320 m2 / g to 380 m 2 / g.

[0026] Furthermore, the method for preparing the modified fumed silica is as follows: Fumed silica and water were mixed, and hexamethyldisilazane was added and heated to react for a period of time. After high-temperature aging, the mixture was dried to obtain modified fumed silica.

[0027] Furthermore, the amount of hexamethyldisilazane added is 10-20 wt% of the mass of fumed silica.

[0028] Furthermore, the heating reaction is carried out at a temperature of 40~60℃ for a time of 50~100 min.

[0029] Furthermore, the high-temperature curing temperature is 100~120℃, and the time is 50~100 min.

[0030] This invention provides a method for preparing the above-mentioned silver paste, the method comprising: After adding modified fumed silica to the organic phase and dispersing it, glass powder and silver powder are added and stirred, then centrifuged and dispersed, and finally ground until the fineness of the slurry reaches below 5μm.

[0031] The present invention also provides the application of the above-mentioned silver paste in the field of solar cells.

[0032] Furthermore, the application includes printing the silver paste onto a solar cell and sintering it to obtain a solar cell.

[0033] Beneficial effects This invention introduces fumed silica with a native particle size of 5-15 nm, modified with HMDS for hydrophobicity. At high sintering temperatures, this silica preferentially reacts with PbO in the glass to form a lead silicate phase, constructing a localized "thermal diffusion barrier layer" on the poly layer surface. This precisely consumes the glass etching active components, suppressing excessive corrosion of thin poly regions by the glass melt and reducing over-contact rates, while not hindering normal ohmic contact in thick poly regions. This adapts to a wide poly thickness range of 70-180 nm, achieving a Voc increase of over 10 mV with no FF loss and an absolute battery efficiency increase of over 0.05%. Simultaneously, the modified fumed silica, with its three-dimensional branched network structure, imparts excellent thixotropic properties and suspension stability to the paste, improving the aspect ratio and clarity of the printed grid lines, avoiding silver powder sedimentation and solvent precipitation, thus achieving synergistic optimization of printing process performance and electrical performance. Detailed Implementation

[0034] In the following examples and comparative examples, % refers to mass percentage.

[0035] The silver powder is a spherical powder with a D50 of 1.5 μm.

[0036] The glass powder is a Pb-Te-Bi system, specifically a mixed powder of PbO, TeO2, Bi2O3, SiO2, and Al2O3 in a mass ratio of 55%, 30%, 10%, 3%, and 2% (D50 = 1.5μm).

[0037] The organic phase contains the following components: resin, thixotropic agent, dispersant, silicone oil, and organic solvent.

[0038] The resin is composed of ethyl cellulose and thermoplastic resin.

[0039] The thixotropic agent is a polyamide wax, specifically DISPARLON 6500.

[0040] The dispersant consists of TDO and stearic acid; the TDO is Duomeen TDO, an ionic surfactant.

[0041] The organic solvent is one or more of diethylene glycol dibutyl ether, diethylene glycol butyl ether, and hexadecyl alcohol ester.

[0042] The organic phase (8.75%) contains 1.85% ethyl cellulose, 1.5% thermoplastic resin, 1.15% thixotropic agent, 1.5% TDO, 0.5% stearic acid, 0.75% silicone oil and 1.5% organic solvent.

[0043] All fumed silica was purchased from Hubei Huifu Nanomaterials Co., Ltd.

[0044] Example 1 This embodiment provides a fine grid silver paste adapted to different poly thicknesses of the fine grid on the back of TOPCon. The paste composition is as follows: silver powder (spherical, D50-1.5μm) 88.5%, glass powder (Pb-Te-Bi system, D50=1.5μm) 2.6%, organic phase: 8.75%, modified fumed silica: 0.15wt%.

[0045] The native particle size of fumed silica is 8 nm (TEM verified), with a purity of over 99.8%, aggregate size of 110 nm (laser diffraction), and a BET specific gravity of 350 μm. 2 / g, contact angle = 130°.

[0046] The aforementioned fumed silica requires surface treatment with hexamethyldisilazane (HMDS) modification. The specific process is as follows: Fumed silica powder was placed in a drying oven and dried at 150°C for 2 hours to remove moisture. The dried fumed silica was then added to a high-speed centrifugal mixer with stirring, and 1 wt% deionized water was added dropwise. HMDS was slowly added while stirring continuously, at a rate of 15 wt% of the fumed silica. The mixture was stirred continuously at 50°C for 60 minutes. The homogeneous mixture was transferred to a sealed container and cured in an oven at 100°C for 1 hour to ensure complete reaction between the HMDS and the hydroxyl groups on the surface of the fumed silica. The cured mixture was then dried at 80°C for 30 minutes to remove volatiles. Finally, the modified fumed silica was sieved through a 400-mesh vibrating sieve to break up any soft agglomerates that might form, thus improving the dispersibility during slurry preparation.

[0047] The slurry preparation process is as follows: Modified fumed silica is added to the organic phase and dispersed by high-speed shearing (6000 rpm, 30 min) and ultrasound (40 kHz, 10 min). Glass powder and silver powder are added and manually stirred until the powder is completely wetted. The stirred slurry is centrifuged and dispersed (3 min) to ensure that there is no powder sedimentation or particles. Finally, it is ground with a three-roll mill. The grinding gap is gradually reduced until the fineness of the slurry reaches below 5 μm. The temperature of the three-roll mill is controlled not to exceed 30℃ to form the slurry.

[0048] Testing Procedure: 90mm thick poly solar cells were selected and printed using a standard screen printing plate. After drying, photoinjection, and sintering, the cells were ready for use. The cells were then placed in an IV (Inductance Testing) machine for IV electrical performance testing, obtaining Eta, Voc, and FF data. The cells were then laser-cut into uniform widths suitable for contact resistance meters to obtain strip-shaped cells, which were then placed on the contact resistance meter to obtain contact resistance data.

[0049] Comparative Example 1 The formulation and steps were followed as described in Example 1, except that the addition of modified fumed silica was omitted. The resulting slurry was also tested according to the testing procedure described in Example 1.

[0050] Table 1

[0051] The test results of Example 1 and Comparative Example 1 are shown in Table 1 above. It can be seen that the pressure opening and contact of the slurry are improved after the addition of modified fumed silica.

[0052] Example 2 Using the slurry prepared in Example 1, a solar cell with a Poly thickness of 150 was selected, and IV and contact resistance data were obtained using the IV contact resistance test method in Example 1.

[0053] Comparative Example 2 The formulation and steps were followed as described in Example 2, except that the addition of modified fumed silica was omitted. The resulting slurry was also tested according to the testing procedure described in Example 1.

[0054] Table 2

[0055] The detection results of Example 2 and Comparative Example 2 are shown in Table 1 above. It can be seen that after adding silicon micropowder, the ρc of the thick Poly region (150 nm) is stably ≤1.6 mΩ·cm. 2 (Traditional slurry > 3.5 mΩ·cm) 2 This proves that it does not hinder normal ohmic contact.

[0056] Example 3 Following the preparation process of fumed silica described in Example 1, only fumed silica with different primary particle sizes (5 nm, 15 nm, and 25 nm) was selected, and the fumed silica was modified with HMDS. The resulting slurry was also tested according to the testing process described in Example 1.

[0057] Table 3

[0058] The three fumed silica slurries obtained above were composed as follows: silver powder (spherical, D50-1.5μm) 88.5%, glass powder (Pb-Te-Bi system, D50=1.5μm) 2.6%, organic phase 8.75%, and modified fumed silica 0.15wt%. Then, select a battery cell with a Poly thickness of 100 from the above three slurries, and use the IV contact resistance test method in Example 1 to obtain IV and contact resistance data.

[0059] Table 4

[0060] The test results are shown in Table 4 above. It can be found that when the original particle size is >15 nm (Example 3C), the aggregates are too large (500 nm), resulting in uneven dispersion, which cannot effectively form a thermal diffusion barrier, causing a surge in over-contact rate and significant Voc loss.

[0061] Example 4 Referring to the preparation process of fumed silica described in Example 1, only fumed silica powder with a native particle size of 10 nm was selected. The resulting fumed silica was then subjected to different modifications, specifically: Hexamethyldisilazane (HMDS) modification: Fumed silica powder was placed in a drying oven and dried at 150°C for 2 hours to remove moisture. The dried fumed silica was then added to a high-speed centrifugal mixer with stirring, and 1 wt% deionized water was added dropwise. HMDS was slowly added while stirring continuously, at a rate of 15 wt% of the fumed silica. The mixture was stirred continuously at 50°C for 60 minutes. The homogeneous mixture was transferred to a sealed container and cured in an oven at 100°C for 1 hour to ensure complete reaction between the HMDS and the hydroxyl groups on the surface of the fumed silica. The cured mixture was then dried at 80°C for 30 minutes to remove volatiles. Finally, the modified fumed silica was sieved through a 400-mesh vibrating sieve to break up any soft agglomerates that might form, thus improving the dispersibility during slurry preparation.

[0062] Dimethyldichlorosilane (DM-DCS) modification: Fumed silica powder was placed in a drying oven and dried at 150°C for 2 hours to remove moisture. The dried fumed silica and 15 times its mass of toluene were added to a dried three-necked flask, and stirring was started under inert gas protection to form a slurry. DM-DCS was dissolved in anhydrous toluene to prepare a 10wt% DM-DCS solution. The reaction apparatus was equipped with a constant-pressure dropping funnel, a reflux condenser, and a drying tube. Dry nitrogen gas was introduced into the third neck of the three-necked flask to remove air and moisture. The slurry was heated to 90°C and refluxed. Fumed silica was slowly added dropwise over 60 minutes through the constant-pressure dropping funnel. A DM-DCS solution containing 25 wt% silica was added dropwise and kept at a constant temperature for 5 hours. After the reaction, the mixture was filtered to obtain a filter cake. The filter cake was first rapidly washed with a small amount of hot anhydrous ethanol, and then washed with hot ethanol at 70°C until the pH of the filtrate was weakly acidic. The filter cake was then transferred to a vacuum drying oven and dried at 80°C for 10 hours for later use. Finally, the modified fumed silica was sieved through a 400-mesh vibrating sieve to break up any possible soft agglomerates and improve the dispersibility when preparing the slurry.

[0063] Table 5

[0064] The above-obtained 3 types of fumed silica slurries are composed of the following: silver powder (spherical, D50-1.5μm) 88.5%, glass powder (Pb-Te-Bi system, D50=1.5μm) 2.6%, organic phase: 8.75%, and fumed silica: 0.15wt%.

[0065] Then, select a battery cell with a Poly thickness of 100 from the above three slurries, and use the IV contact resistance test method in Example 1 to obtain IV and contact resistance data.

[0066] Table 6

[0067] The test results are shown in Table 6 above. It can be seen that the surface hydroxyl groups (-OH) of the hydrophilic silica powder (Example 4B) have too strong hydrogen bonding with the organic phase, which causes viscosity drift. HMDS modification (Example 4A) reduces polarity and improves dispersion stability through -Si-CH3 groups.

[0068] Example 5 The proportion of fumed silica with a native particle size of 10 nm modified by HMDS, as described in Example 4, in the slurry was adjusted as follows: Example 5A: Silver powder (spherical, D50-1.5μm) 89.41%, glass powder (Pb-Te-Bi system, D50=1.5μm) 2.6%, organic phase 8.75%, modified fumed silica 0.05%; Example 5B: Silver powder (spherical, D50-1.5μm) 88.35%, glass powder (Pb-Te-Bi system, D50=1.5μm) 2.6%, organic phase 8.75%, modified fumed silica 0.30%; Example 5C: Silver powder (spherical, D50-1.5μm) 88.15%, glass powder (Pb-Te-Bi system, D50=1.5μm) 2.6%, organic phase 8.75%, modified fumed silica 0.50%; Example 5D: Silver powder (spherical, D50-1.5μm) 88.65%, glass powder (Pb-Te-Bi system, D50=1.5μm) 2.6%, organic phase 8.75%, modified fumed silica 0%.

[0069] Using the above four slurries, select a solar cell with a Poly thickness of 85nm and obtain IV and contact resistance data using the IV contact resistance test method described in Example 1.

[0070] Table 7

[0071] The test results are shown in Table 7 above. It can be seen that the optimal addition amount is 0.1-0.3 wt%, at which point the Voc increase is >10 mV and there is no loss of FF. However, when the fumed silica is added in excess (0.5 wt%), the silica powder agglomerates, leading to grid breakage in the printing process.

[0072] Comparative Example 3 Refer to the slurry formulation and steps in Example 1, except that fumed silica is replaced with ordinary silica powder (commercially available product, obtained by ordinary precipitation method).

[0073] Comparative Example 4 Refer to the slurry formulation and steps in Example 1, except that the fumed silica is replaced with spherical nano silica (commercially available product with a particle size of 1 μm).

[0074] Table 8

[0075] The test results are shown in Table 8 above. The fumed silica slurry is uniform, has good printability, and exhibits no precipitation, resulting in superior efficiency. Ordinary silica printing results in more grid breaks, significantly reduced efficiency, and severe solvent precipitation. Although nano-silica exhibits less solvent precipitation, its printability is still poor, which has a significant impact on efficiency.

[0076] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A silver paste for the fine grid on the back of a TOPCon, characterized in that, The silver paste, by mass percentage, comprises: 85-90% silver powder, 2-3% glass powder, 7-10% organic phase, and 0.05-0.5% modified fumed silica; The modified fumed silica is prepared by mixing fumed silica and water, adding hexamethyldisilazane, heating and reacting for a period of time, then aging at high temperature, and finally drying to obtain modified fumed silica; the amount of hexamethyldisilazane added is 10~20wt% of the mass of fumed silica.

2. The silver paste according to claim 1, characterized in that, The composition of the silver paste, by mass percentage, includes: 85-90% silver powder, 2-3% glass powder, 7-10% organic phase, and 0.05-0.3% modified fumed silica.

3. The silver paste according to claim 1, characterized in that, The silver powder has a particle size of 0.5-3.0 μm and a specific surface area of ​​0.3-2.0 m². 2 / g, tap density is 3-7 g / mL.

4. The silver paste according to claim 1, characterized in that, The glass powder comprises, by mass percentage: 50-60% PbO, 25-35% TeO2, 5-15% Bi2O3, 1-5% SiO2, and 1-5% Al2O3.

5. The silver paste according to claim 1, characterized in that, The composition of the organic phase, by mass percentage, includes 15-20% organic solvent, 35-40% resin, 10-15% thixotropic agent, 20-25% dispersant and 5-10% other additives. Organic solvents include one or more of diethylene glycol dibutyl ether, diethylene glycol butyl ether, and hexadecyl alcohol ester; other additives include silicone oil.

6. The silver paste according to claim 1, characterized in that, The fumed silica has a primary particle size of 5 nm to 15 nm; an aggregate size of 80 nm to 200 nm; and a BET specific surface area of ​​300 m². 2 / g to 400 m 2 / g.

7. The silver paste according to claim 1, characterized in that, The heating reaction temperature is 40~60℃ and the time is 50~100 min; the high-temperature aging temperature is 100~120℃ and the time is 50~100 min.

8. The method for preparing silver paste according to any one of claims 1 to 7, characterized in that, The preparation method includes: After adding modified fumed silica to the organic phase and dispersing it, glass powder and silver powder are added and stirred, then centrifuged and dispersed, and finally ground until the fineness of the slurry reaches below 5μm.

9. The application of the silver paste according to any one of claims 1 to 7 in the field of solar cells.

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