Epoxy lignin modified low temperature curing conductive silver paste, preparation method and application thereof

Abstract

The application belongs to the technical field of silver paste for batteries, and relates to an epoxidized lignin modified low-temperature curing conductive silver paste as well as a preparation method and application thereof.The epoxidized lignin modified low-temperature curing conductive silver paste comprises the following components in percentage by weight: 92-93% of silver powder; 3-3.1% of a resin composition; 0.28-0.32% of a curing agent; and the balance of organic reagents; the resin composition comprises the following components in percentage by weight: 2.1-2.3% of aliphatic epoxy resin; 0.25-0.4% of bisphenol F type epoxy resin; 0.5-0.6% of chlorovinyl resin; and 0.08-0.18% of epoxidized lignin; the epoxidized lignin is obtained by reacting ethyl acetate extracted alkali lignin with gamma-glycidyl ether oxygen propyl trimethoxysilane in an organic solution.The application improves the conductivity and rheological properties of the low-temperature curing conductive silver paste by adding the epoxidized lignin, and improves the photoelectric conversion efficiency of the battery.

Description

Technical Field

[0001] This invention relates to the field of silver paste technology for batteries, and more specifically, to an epoxidized lignin-modified low-temperature curing conductive silver paste, its preparation method, and its application. Background Technology

[0002] The core feature of a heterojunction solar cell (HJT) structure is the growth of an ultrathin intrinsic and undoped amorphous silicon layer on the surface of a high-quality crystalline silicon substrate using plasma-enhanced chemical vapor deposition (PECVD), thus forming a unique heterojunction interface structure. This design cleverly utilizes the excellent passivation properties of amorphous silicon, effectively suppressing carrier recombination on the crystalline silicon surface and significantly improving the cell's open-circuit voltage and fill factor. The TCO layer is one of the key functional layers of an HJT cell, typically deposited as a 20–80 nm thick transparent conductive film on the amorphous silicon layer using PVD. The outermost metal electrode of a heterojunction solar cell is usually prepared by screen printing from low-temperature cured conductive silver paste.

[0003] The manufacturing process of solar cells typically requires curing at relatively low temperatures to reduce potential thermal damage to sensitive materials such as silicon substrates. Low-temperature curing conductive silver paste can better adhere to the surface materials of solar cells (such as silicon wafers), reducing contact resistance problems caused by incomplete curing, and thus potentially improving the overall photoelectric conversion efficiency of solar cells.

[0004] Existing low-temperature curing conductive silver pastes generally consist of silver powder, resin, curing agent, solvent, and other additives. The silver powder is the conductive phase. The resin acts as a binder for the silver powder and provides adhesion of the electrode to the TCO. The curing agent reacts with the resin, promoting resin polymerization. The solvent typically consists of low-boiling-point short-chain hydrocarbons to adjust the rheological properties and wettability of the paste.

[0005] However, traditional epoxy resin systems face significant technical challenges under low-temperature curing conditions: on the one hand, the low-temperature environment significantly reduces the reactivity of epoxy groups, leading to incomplete curing and insufficient crosslinking density, which in turn affects the conductivity and mechanical strength of the silver paste; on the other hand, low-temperature curing often results in insufficient interfacial bonding between silver particles and the resin matrix, increasing contact resistance and reducing the conductivity of the electrode. These technical bottlenecks severely limit the application range of epoxy resin-based conductive silver paste under low-temperature processing conditions. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects and deficiencies of the above-mentioned technical problems and provide an epoxidized lignin-modified low-temperature curing conductive silver paste. By adding epoxidized lignin to the low-temperature curing conductive silver paste, the curing characteristics of the resin are improved, making the curing reaction more efficient and enhancing the dispersibility of silver particles, thereby improving the conductivity and rheological properties of the low-temperature curing conductive silver paste, and thus improving the photoelectric conversion efficiency of the battery.

[0007] The above-mentioned objective of this invention is achieved through the following technical solution:

[0008] An epoxidized lignin-modified low-temperature curing conductive silver paste, comprising the following components by weight percentage:

[0009] Silver powder 92-93%; resin composition 3-3.1%; curing agent 0.28-0.32%; balance organic reagents;

[0010] The resin composition comprises, by weight percentage, the following components:

[0011] Aliphatic epoxy resin 2.1-2.3%;

[0012] Bisphenol F type epoxy resin 0.25-0.4%;

[0013] Chlorinated vinyl acetate resin 0.5-0.6%;

[0014] Epoxidized lignin: 0.08–0.18%;

[0015] The epoxidized lignin was obtained by reacting alkali lignin extracted with ethyl acetate with γ-glycidoxypropyltrimethoxysilane in an organic solution.

[0016] This invention utilizes ethyl acetate as an organic reagent to extract alkali lignin, which significantly improves the compatibility of epoxidized lignin with epoxy resin. This is because ethyl acetate extraction effectively removes low-molecular-weight components and impurities such as ash from lignin, resulting in a narrower molecular weight distribution and higher structural uniformity in ethyl acetate-extracted alkali lignin. Furthermore, the ethyl acetate extraction process exposes more active functional groups in lignin, including phenolic hydroxyl groups, aliphatic hydroxyl groups, and carboxyl groups. The increased content of these functional groups significantly enhances the chemical reactivity of lignin.

[0017] This invention modifies alkali lignin extracted with ethyl acetate and γ-glycidoxypropyltrimethoxysilane through an epoxidation reaction, thereby converting active groups such as phenolic hydroxyl groups in lignin molecules into epoxy groups, thus obtaining reaction characteristics similar to epoxy resins.

[0018] In the epoxidized lignin of this invention, the abundant aromatic ring structure in the molecular structure helps to improve the heat resistance and mechanical strength of the material; the polar groups in the lignin molecule can enhance the interfacial interaction with silver particles and improve the stability of the conductive network; the epoxidized lignin can form a good compatibility system with traditional epoxy resin. When the conductive silver paste is cured at low temperature, under the action of isocyanate curing agent, the epoxy groups of the epoxidized lignin undergo a full cross-linking reaction with the resin to form a three-dimensional network with a gradient cross-linking density, and its curing shrinkage rate is significantly improved. This shrinkage behavior, through the shrinkage stress generated, promotes the reduction of the spacing between silver powder particles; forms a continuous network with increased silver powder contact point density; and increases the contact area between silver powder and silicon wafer substrate, optimizing the conductive path. These structural features work together to not only reduce the interfacial barrier but also significantly improve the carrier mobility, thereby exhibiting lower contact resistance and bulk resistance.

[0019] This invention obtains epoxidized lignin by reacting alkali lignin extracted with ethyl acetate with γ-glycidoxypropyltrimethoxysilane. The epoxidized lignin is then applied to conductive silver paste for heterojunction (HJT) solar cells. The lignin improves the conductivity and rheological properties of the silver paste through its unique chemical structure, and enhances the photoelectric conversion efficiency of the cell by regulating the interaction between the resin and the silver paste, thus promoting the development of high-efficiency solar cell technology.

[0020] Utilizing lignin, a renewable biomass resource, to partially replace petroleum-based epoxy resin reduces dependence on non-renewable resources, aligning with the principles of green chemistry and sustainable development, and providing an environmentally friendly solution for the field of solar cell materials. Furthermore, as a low-cost industrial byproduct, the introduction of lignin significantly reduces material costs while improving silver paste performance, offering an economically feasible solution for the large-scale production of high-performance, low-temperature curing conductive silver paste.

[0021] Vinyl acetate copolymer (VCE) has excellent film-forming properties that help improve the printability of the paste, prevent silver powder settling, and enhance adhesion to the substrate. Appropriate use can improve flexibility, making the silver layer less prone to cracking when bent, and enhance weather resistance and chemical stability.

[0022] Preferably, the epoxidized lignin-modified low-temperature curing conductive silver paste comprises, by weight percentage, the following components:

[0023] Silver powder 92-93%; resin composition 3-3.1%; curing agent 0.28-0.32%; balance organic reagents;

[0024] The resin composition comprises, by weight percentage, the following components:

[0025] Aliphatic epoxy resin 2.1-2.3%;

[0026] Bisphenol F type epoxy resin 0.33-0.36%;

[0027] Chlorinated vinyl acetate resin 0.5-0.6%;

[0028] Epoxidized lignin: 0.08–0.12%.

[0029] Preferably, the preparation method of the epoxidized lignin includes the following steps: mixing and stirring the alkali lignin and ethyl acetate for 1-3 hours to obtain ethyl acetate-extracted alkali lignin; reacting the ethyl acetate-extracted alkali lignin with γ-glycidoxypropyltrimethoxysilane in an organic solution to obtain epoxidized lignin; the reaction temperature is 40-60℃. In a specific embodiment, the stirring speed can be 300-400 rpm / min. The mixing reaction time can be 1-2 hours. The organic solution can be a mixture of aqueous acetic acid and acetone.

[0030] Preferably, the mass ratio of ethyl acetate extract of alkali lignin to γ-glycidoxypropyltrimethoxysilane is 1:(2-3).

[0031] Preferably, the alkali lignin contains 1.8~2.0 mmol / g of phenolic hydroxyl groups, 1.8~2.0 mmol / g of alcoholic hydroxyl groups, and 0.8~1.0 mmol / g of carboxyl groups.

[0032] Preferably, the weight-average molecular weight (Mw) of the alkali lignin is 4000~4500 g / mol, and the number-average molecular weight (Mn) is 1000~1300 g / mol.

[0033] Preferably, in the alkali lignin, based on a single C9 phenylpropane structural unit, the content of guaiacol (G-lignin) units is 76-80%, the content of syringyl (S-lignin) units is 15-18%, and the remainder is p-hydroxyl (H-lignin) units.

[0034] Preferably, the organic reagent includes a dispersant, a chain extender, and an organic solvent.

[0035] In specific embodiments, the curing agent can be an isocyanate curing agent. For example, caprolactam-blocked hexamethylene diisocyanate trimer, or dimethyl phthalate (dimethylpyrazole)-blocked isophorone diisocyanate / hexamethylene diisocyanate.

[0036] In a specific embodiment, the dispersant can be an oil-based dispersant, such as oleic acid, stearic acid, or ricinoleic acid.

[0037] In a specific embodiment, the chain extender can be an epoxy chain extender. Examples include epoxy resins, epoxy vinyl resins prepared from methacrylic acid and styrene, bisphenol A diglycidyl ether, or ethylene glycol / butanediol diglycidyl ether.

[0038] Preferably, the organic solvent is one or more of dimethyl phthalate, dibutylcarbidol, or diethylene glycol butyl ether acetate.

[0039] The present invention also protects the preparation method of the epoxidized lignin modified low-temperature curing conductive silver paste according to any one of the above claims, comprising the following steps: dispersing epoxidized lignin in isoflurane solvent to obtain a lignin solution, and then mixing it with the remaining components to obtain the epoxidized lignin modified low-temperature curing conductive silver paste.

[0040] Using isoflurone as a solvent is beneficial for improving the dispersibility of lignin in silver paste.

[0041] Preferably, the solid content of the lignin solution is 35-45%.

[0042] The present invention also protects a silver electrode, which is prepared by printing the epoxidized lignin-modified low-temperature curing conductive silver paste described in any of the above claims onto the surface of a silicon wafer substrate and then curing it at a low temperature of 170–190°C.

[0043] The present invention also protects a heterojunction solar cell comprising the silver electrode described above.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention discloses an epoxidized lignin-modified low-temperature curing conductive silver paste. By adding epoxidized lignin to the low-temperature curing conductive silver paste, the curing characteristics of the resin are improved, making the curing reaction more efficient and enhancing the dispersibility of silver particles, thereby improving the conductivity and rheological properties of the low-temperature curing conductive silver paste, and thus improving the photoelectric conversion efficiency of the battery. Attached Figure Description

[0045] Figure 1 This is a high-resolution Fourier transform infrared (FTIR) spectrum of epoxidized lignin according to an embodiment of the present invention.

[0046] Figure 2 The pattern is printed using silver paste according to an embodiment of the present invention. Detailed Implementation

[0047] To more clearly and completely describe the technical solution of the present invention, the present invention will be further described in detail below through specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention. Various changes can be made within the scope of the claims of the present invention.

[0048] The raw materials used in the various embodiments and comparative examples of the present invention are shown in Table 1 below.

[0049] Table 1

[0050]

[0051] The alkali lignin contained 1.9 mmol / g of phenolic hydroxyl groups, 1.96 mmol / g of alcoholic hydroxyl groups, and 0.84 mmol / g of carboxyl groups. Its weight-average molecular weight (Mw) was 4207 g / mol, and its number-average molecular weight (Mn) was 1186 g / mol. Based on a single C9 phenylpropane structural unit, the content of guaiacol (G-lignin) units was 78.8%, the content of syringyl (S-lignin) units was 16.82%, and the remainder was p-hydroxyl (H-lignin) units.

[0052] The preparation method of epoxidized lignin is as follows: 100 mL of ethyl acetate is added to 10 g of alkali lignin, and the mixture is stirred at 400 rpm for 2 h in a 35 ℃ water bath. After filtration, most of the solvent is evaporated from the filtrate, and the filtrate is dried in a 60 ℃ oven. After grinding, ethyl acetate-extracted alkali lignin (AL-EtOAc) is obtained. An aqueous solution of acetic acid with pH ~3 is prepared and mixed with acetone at a mass ratio of 1:7. γ-glycidyl etheroxypropyltrimethoxysilane (KH-560) is mixed with the mixed solution at a mass ratio of 1:5.33 and dissolved for 48 h. The ethyl acetate-extracted alkali lignin is added to KH-560 at a mass ratio of 1:2.34, and the mixture is stirred for 1 h in a 50 ℃ oil bath. The product after reaction is precipitated in water, the precipitate is removed by centrifugation, and dried under vacuum at 50 ℃ for one day to obtain brown epoxidized lignin (LEP) powder.

[0053] Examples 1-4

[0054] An epoxy lignin-modified low-temperature curing conductive silver paste comprises, by weight percentage: silver powder; resin composition; curing agent; and the balance being organic reagents. The resin composition, by weight percentage, comprises: aliphatic epoxy resin; bisphenol F type epoxy resin; vinyl chloride resin; and epoxy lignin. The organic reagents include dispersants, chain extenders, and organic solvents.

[0055] The dosage of each component is shown in Table 2 below:

[0056] Table 2

[0057]

[0058] The preparation method of epoxidized lignin-modified low-temperature curing conductive silver paste includes the following steps:

[0059] Epoxidized lignin was dispersed in isoflurane solvent to obtain a lignin solution with a solid content of 40%, and then mixed with the remaining...

[0060] Each component was added to a 100 mL black plastic container according to Table 2 above. The conductive silver paste was first stirred manually with a glass rod until it was uniform. The silver powder was then dispersed in the organic carrier using a non-invasive material homogenizer (Planetary mixer). In a cleanroom, the silver paste, which had been mixed into a paste, was poured from the 100 mL plastic container and subjected to a three-roll mill (EXAKT 80S) for five grinding processes with different roller spacings. The mixture was mechanically sheared and mixed uniformly to obtain the corresponding conductive silver paste.

[0061] Comparative Examples 1-3

[0062] A low-temperature curing conductive silver paste, by weight percentage, comprises the following components: silver powder; a resin composition; a curing agent; and the balance being organic reagents. The resin composition, by weight percentage, comprises the following components: aliphatic epoxy resin; bisphenol F type epoxy resin; vinyl chloride resin; lignin; and the lignin is alkali lignin. The organic reagents include dispersants, chain extenders, and organic solvents.

[0063] The dosage of each component is shown in Table 3 below.

[0064] Table 3

[0065]

[0066] The preparation method of the above-mentioned low-temperature curing conductive silver paste is the same as that in Example 1.

[0067] Performance testing

[0068] The epoxidized lignin was tested by high-resolution Fourier transform infrared spectroscopy (FTIR), and the test results are as follows: Figure 1 As shown. From Figure 1 It can be seen that at 910 cm -1 The characteristic peak appearing at 1022 cm⁻¹ corresponds to the ring vibration mode of the epoxy group. -1 The strong absorption peak at 1189 cm⁻¹ is attributed to the stretching vibration of the Si-O-Si bond, while the peak at 1189 cm⁻¹ is attributed to the stretching vibration of the Si-O-Si bond. -1 The characteristic peaks appearing at the specified locations clearly indicate the formation of Si-OC bonds. The synergistic appearance of these three characteristic peaks constitutes the successful grafting of epoxy groups onto lignin, confirming the successful preparation of epoxidized lignin (LEP).

[0069] The low-temperature curing conductive silver pastes of each embodiment and comparative example were printed onto the surface of a silicon wafer substrate. The silicon wafers used for screen printing were standard Zhongwei single-wafer wafers (158 mm × 158 mm). Before printing, the screen and silicon wafer underwent rigorous cleaning and surface treatment to remove surface contaminants and improve the adhesion between the silver paste and the substrate. During printing, the squeegee pressure was controlled at 65 N and the printing speed at 200 mm / s to obtain uniform conductive lines. After printing, the samples were placed in an oven at 180 ℃ for low-temperature curing for 30 min. The silver paste printing pattern is shown below. Figure 2 As shown.

[0070] (1) Rheological test:

[0071] All rheological tests of the experimental slurries were performed on an Anton Paar MCR102 rotational rheometer, using a conical rotor CP-25 (D=25 mm, Angel=1°) with a plate spacing of 0.048 mm to ensure that the plate spacing was 10 times the fineness of the slurry.

[0072] Dynamic amplitude strain scanning test involves setting the rheometer to a constant angular frequency of 10 rad / s and the strain (γ) range to 0.01%-100%, measuring the strain-modulus change curve of the conductive paste, and thus obtaining the flow point value of the sample paste. When the storage modulus (G') > loss modulus (G"), the conductive paste macroscopically exhibits mainly solid elastic properties. As the external stress continues to increase, the internal structure of the paste is destroyed, and when the storage modulus (G') = loss modulus (G"), the strain value (flowing point) of the silver paste at this point is recorded as the flow point, which serves as an indicator for judging the paste's flow properties.

[0073] The 3ITT (3-interval thixotropy test) oscillating strain test mode involves setting the rheometer to a constant angular frequency of 10 rad / s and applying different strains to the paste at different test stages to simulate the screen printing process of conductive paste. This study investigates the changes in the viscoelastic properties of the conductive paste under oscillating mode, using the modulus change curve of the conductive paste as the judgment standard. The first stage involves applying a strain of a certain amplitude within the linear viscoelastic region, set at 0.02% (within the LVR range of the conductive paste), to simulate the state of the conductive paste resting on the screen before printing. The second stage involves high strain measurement at γ=1000%, where the phase angle (δ) of the paste is close to 90°, and the macroscopic viscoelasticity of the conductive paste approaches a fluid state, simulating the process of the conductive paste being printed onto the silicon wafer substrate by the action of a squeegee. The third stage has the same test parameters as the first stage and is used to simulate the recovery of the structure after the conductive paste is printed onto the silicon wafer.

[0074] The 3ITT rotational shear test, similar to the 3ITT oscillation mode, is used to simulate viscosity (η) changes during the screen printing process, including screen laying, printing formation, and structural thixotropic recovery. The shear rate is set to 0.1 s⁻¹. -1 -100 S -1 -0.1 S -1 In this test mode, the zero-shear viscosity, shear-thinning viscosity, and thixotropic recovery viscosity of the conductive paste can be obtained, thus yielding the thixotropic recovery rate. The relationship between the paste's viscosity shear-thinning properties and its printing passability, as well as the correlation between grid line shape and viscosity recovery, are investigated.

[0075]

[0076] The test results for each embodiment and comparative example are shown in Table 4 below:

[0077] Table 4

[0078]

[0079] The viscosity of the silver pastes in each embodiment and comparative example is in the range of 25~45 Pa·s at 20 1 / s.

[0080] As can be seen from Table 4 above, compared with Comparative Example 1, the silver paste of the present invention has a phase angle (δ) approaching 90°, which reflects the weakening of the elastic characteristics of the paste. This is beneficial for the paste to pass smoothly through the micro-pores of the screen and maintain clear pattern edges. The increase in flow point significantly improves the transfer efficiency of the paste under the shearing action of the squeegee. The enhancement of thixotropic recovery rate ensures that the paste can quickly reconstruct the three-dimensional network structure after printing, maintaining the aspect ratio and morphological stability of the grid lines.

[0081] (2) 3D microscope test:

[0082] The microstructure of the cured silver paste sample was characterized using a 3D microscope (LY-WN-YH600H). The linewidth and lineheight of the silver paste grid lines were precisely measured at 50x magnification to evaluate its printing performance.

[0083] The test results of the cross-sectional line height, cross-sectional line width, and aspect ratio of the silver paste grid lines in each embodiment and comparative example are shown in Table 5 below:

[0084] Table 5

[0085]

[0086] As can be seen from Table 5 above, compared with the comparative example, the silver paste grid lines obtained after curing the silver paste in the embodiment of the present invention have a higher aspect ratio, which can reach 0.5831~0.7124. The higher line height can ensure good lateral conductivity, and the narrower width can minimize the light-blocking area.

[0087] This is likely because the polar groups on the epoxidized lignin molecular chain interact strongly with the silver powder surface, significantly reducing the migration rate of silver powder particles in organic solvents; optimized rheological properties allow the paste to quickly recover its structural strength after printing, maintaining the stability of the grid line shape; and a moderate cross-linking network density ensures both the printability of the paste and provides sufficient mechanical support. The three-dimensional network framework and abundant active functional groups of epoxidized lignin form a stable spatial support structure during curing, effectively suppressing the lateral diffusion and longitudinal collapse of the grid lines, thereby improving the aspect ratio of the grid lines.

[0088] (3) Resistance test:

[0089] The cured silver paste silicon wafer substrate was cut into strips with a width of 1 cm using a laser slicing machine, and the contact resistance and bulk resistance of the silver paste were systematically tested using a BrightSpot TLM resistance tester.

[0090] The test results of the silver pastes in each embodiment and comparative example are shown in Table 6 below:

[0091] Table 6

[0092]

[0093] As can be seen from Table 6 above, compared with the comparative example, the silver paste of the present invention has lower contact resistance and volume resistance, with the contact resistance as low as 2.117~2.437 mΩ·cm. 2 The volume resistivity can be as low as 0.92~0.94×10⁻⁶. -4 The mΩ·cm reading indicates that the silver paste of this embodiment exhibits excellent conductivity. This is likely because, under the action of the isocyanate curing agent, the epoxy groups of epoxidized lignin undergo a cross-linking reaction with the resin, forming a three-dimensional network with a gradient cross-linking density, resulting in a significantly higher curing shrinkage rate than conventional systems. This shrinkage behavior optimizes the conductive pathway through the following mechanisms: the generated shrinkage stress reduces the spacing between silver powder particles; a continuous network with increased silver powder contact point density is formed; and the contact area between the silver powder and the silicon wafer substrate is increased. These structural features work together to not only lower the interfacial barrier but also significantly improve carrier mobility, thereby resulting in lower contact resistance and bulk resistance.

[0094] (4) Photoelectric conversion efficiency test (IV):

[0095] A 158mm×158mm silicon wafer was used as the substrate. Silver paste with the number B721X (provided by Ruina New Materials Technology Co., Ltd.) was printed on the front side, and silver pastes for each embodiment and comparative example were printed on the back side. After curing, the photoelectric conversion performance of the silicon wafer was tested using an IV tester.

[0096] The test results for each embodiment and comparative example are shown in Table 7 below:

[0097] Table 7

[0098]

[0099] As can be seen from the above data, the silver paste of the present invention can improve the photoelectric conversion efficiency, which can reach 25.96%.

[0100] In summary, this invention discloses an epoxidized lignin-modified low-temperature curing conductive silver paste. By adding epoxidized lignin to the low-temperature curing conductive silver paste, the curing characteristics of the resin are improved, making the curing reaction more efficient and enhancing the dispersibility of silver particles, thereby improving the conductivity and rheological properties of the low-temperature curing conductive silver paste, and thus improving the photoelectric conversion efficiency of the battery.

[0101] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A low-temperature curing conductive silver paste modified with epoxidized lignin, characterized in that, By weight percentage, it includes the following components: Silver powder 92-93%; resin composition 3-3.1%; curing agent 0.28-0.32%; balance organic reagents; The resin composition comprises, by weight percentage, the following components: Aliphatic epoxy resin 2.1-2.3%; Bisphenol F type epoxy resin 0.25-0.4%; Chlorinated vinyl acetate resin 0.5-0.6%; Epoxidized lignin: 0.08–0.18%; The epoxidized lignin was obtained by reacting alkali lignin extracted with ethyl acetate with γ-glycidoxypropyltrimethoxysilane in an organic solution.

2. The epoxidized lignin-modified low-temperature curing conductive silver paste according to claim 1, characterized in that, By weight percentage, it includes the following components: Silver powder 92-93%; resin composition 3-3.1%; curing agent 0.28-0.32%; balance organic reagents; The resin composition comprises, by weight percentage, the following components: Aliphatic epoxy resin 2.1-2.3%; Bisphenol F type epoxy resin 0.33-0.36%; Chlorinated vinyl acetate resin 0.5-0.6%; Epoxidized lignin: 0.08–0.12%.

3. The epoxidized lignin-modified low-temperature curing conductive silver paste according to claim 1, characterized in that, The preparation method of the epoxidized lignin includes the following steps: mixing the alkali lignin and ethyl acetate and stirring for 1-3 hours to obtain ethyl acetate-extracted alkali lignin; mixing the ethyl acetate-extracted alkali lignin with γ-glycidoxypropyltrimethoxysilane in an organic solution to obtain epoxidized lignin; the reaction temperature is 40-60℃.

4. The epoxidized lignin-modified low-temperature curing conductive silver paste according to claim 3, characterized in that, The mass ratio of ethyl acetate extract of alkali lignin to γ-glycidoxypropyltrimethoxysilane was 1:(2-3).

5. The epoxidized lignin-modified low-temperature curing conductive silver paste according to claim 3, characterized in that, The alkali lignin contains 1.8~2.0 mmol / g of phenolic hydroxyl groups, 1.8~2.0 mmol / g of alcoholic hydroxyl groups, and 0.8~1.0 mmol / g of carboxyl groups.

6. The epoxidized lignin-modified low-temperature curing conductive silver paste according to claim 1, characterized in that, The organic reagents include dispersants, chain extenders, and organic solvents.

7. The method for preparing the epoxidized lignin-modified low-temperature curing conductive silver paste according to any one of claims 1 to 6, characterized in that, The process includes the following steps: dispersing epoxidized lignin in isoflurane solvent to obtain a lignin solution, and then mixing it with the remaining components to obtain the epoxidized lignin-modified low-temperature curing conductive silver paste.

8. The preparation method of the epoxidized lignin-modified low-temperature curing conductive silver paste according to claim 7, characterized in that, The solid content of the lignin solution is 35-45%.

9. A silver electrode, characterized in that, The conductive silver paste modified by epoxidized lignin as described in any one of claims 1 to 6 is printed onto the surface of a silicon wafer substrate and then cured at a low temperature of 170 to 190°C.

10. A heterojunction solar cell, characterized in that, Includes the silver electrode as described in claim 9.

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