HJT solar cells have become a research hotspot owing to their simple process, high efficiency, and low temperature coefficient. In HJT cell manufacturing, the electrical properties and curing process of the low-temperature silver-coated copper paste directly affect the photovoltaic conversion efficiency and production throughput of the cell. 

Because the ITO layer on the HJT cell surface is easily damaged above 200°C, the entire production process must be controlled below 200°C, requiring the silver-coated copper paste to cure rapidly at low temperatures (typically drying at 150°C for 3 min and curing at 200°C for 6 min). 

With its low cost and conductivity approaching that of pure silver paste, silver-coated copper paste has become a key material for improving efficiency and reducing cost in HJT cells. In actual production, silver-coated copper conductive paste is printed onto the silicon wafer surface via screen printing to form the metallized gridline electrodes of the cell.

This study adopted the controlled variable method to systematically investigate the effects of different epoxy curing systems on the electrical properties of the low-temperature silver-coated copper paste. In the experiments, epoxy resins and curing agents were selected to form different curing system combinations, which act as the metallization framework in the electronic paste and directly affect the conductivity and adhesion of the paste. 

The choice of curing system directly determines the curing quality of the low-temperature silver-coated copper paste. A Millennial Solar 3D confocal microscope was used to scan the cured gridline electrodes of the silver-copper paste, enabling an intuitive comparison of the curing performance of different conductive pastes.

Five types of epoxy resin were selected for the experiments: hydrogenated bisphenol A epoxy resin, bisphenol A epoxy resin, trifunctional epoxy resin, tetrafunctional epoxy resin, and phenolic-modified epoxy resin, used to construct different curing systems. 

The paste had a solid content of 92.6% and a silver content of 30%. By adjusting the resin addition amount (1.2%, 1.0%, 0.8%) and the corresponding curing agent content (80%), the volume resistivity, contact resistivity, and adhesion were measured. Combined with 3D microscopy observation of the curing morphology, the optimal main resin for the curing systems was identified.

Table 1: Summary of experimental data of silver-coated copper paste with different main resins

Figure 1: 3D images of cured silver-coated copper conductive paste samples (a–f correspond to different epoxy resin systems)

As the epoxy resin addition amount decreased, both the volume resistivity and the contact resistivity showed a downward trend. For example, in the bisphenol A epoxy resin system, the volume resistivity decreased from 7.6×10⁻⁸ Ω·m to 6.5×10⁻⁸ Ω·m; for the tetrafunctional epoxy resin, it decreased from 5.7×10⁻⁸ Ω·m to 4.8×10⁻⁸ Ω·m. 

This indicates that a lower resin content helps improve the continuity of the conductive pathways, thereby enhancing the electrical properties of the curing system.

l Electrical properties: The tetrafunctional epoxy resin performed best, with the lowest volume resistivity (4.8×10⁻⁸ Ω·m) and the lowest contact resistivity (1.5×10⁻³ Ω·cm²). The hydrogenated bisphenol A epoxy resin exhibited inferior electrical properties because it contains no benzene ring and has low reactivity.

l Adhesion: The tetrafunctional epoxy resin exhibited the highest adhesion (2.2 N), followed by the trifunctional resin (1.8 N), while the bisphenol A resin was the lowest (1.3 N). The higher the functionality, the greater the crosslinking density and the stronger the adhesion.

l Curing morphology: After curing, the paste with the tetrafunctional epoxy resin appeared bright yellow with a distinct metallic luster, indicating complete curing; the pastes with the hydrogenated bisphenol A and phenolic-modified resins appeared grayish, indicating a lower degree of curing.

Based on the preferred tetrafunctional epoxy resin (TT400), five latent curing agents were studied: dicyandiamide (DICY), 2-ethyl-4-methylimidazole (EM124), hexahydrophthalic anhydride (HHPA), boron trifluoride monoethylamine complex (BF₃-MEA), and sebacic dihydrazide (SPH), to construct various low-temperature curing systems. 

By adjusting the curing agent addition ratio and testing the electrical properties and adhesion, the optimal curing system formulation for the low-temperature silver-coated copper paste was identified.

Table 2: Summary of experimental data of silver-coated copper paste with different curing agents

Figure 2: 3D images of cured silver-coated copper conductive paste under different curing agent systems (a–e correspond to samples T2/T6/T8/T12/T14)

l The boron trifluoride monoethylamine complex performed best, with the lowest volume resistivity (4.4×10⁻⁸ Ω·m) and the lowest contact resistivity (1.4×10⁻³ Ω·cm²).

l Hexahydrophthalic anhydride ranked second, while dicyandiamide exhibited the poorest electrical properties.

l The boron trifluoride monoethylamine complex has a low curing onset temperature (approximately 120°C) and high reactivity, making it suitable for rapid curing processes and enabling complete curing of the silver-coated copper paste at low temperatures.

The boron trifluoride monoethylamine complex exhibited the highest adhesion (2.6 N), followed by dicyandiamide, HHPA, and SPH, while the imidazole-type agent was the lowest. Appropriately increasing the curing agent content can enhance adhesion, but excessive amounts tend to stabilize the effect. This curing system offers the best overall performance.

l After curing with the boron trifluoride monoethylamine complex, the paste appeared bright silver-white with a high degree of metallization;

l The sample cured with dicyandiamide appeared dark gray, indicating poor curing;

l The sample cured with hexahydrophthalic anhydride appeared silver-white, indicating good curing.

This study systematically evaluated the effects of different curing systems on the performance of the low-temperature silver-coated copper paste for HJT solar cells. The results show that the tetrafunctional epoxy resin, owing to its high reactivity and high crosslinking density, outperforms other resins in both electrical properties and adhesion, making it the best choice as the main resin in the epoxy curing system. Among the curing agents, the boron trifluoride monoethylamine complex exhibited the best low-temperature rapid curing characteristics. 

The curing system combining the tetrafunctional epoxy resin TT400 with the boron trifluoride monoethylamine complex enables complete curing of the silver-coated copper paste under process conditions of 200°C/6 min, achieving a low volume resistivity (4.4×10⁻⁸ Ω·m), low contact resistivity (1.4×10⁻³ Ω·cm²), and high adhesion (2.6 N), providing a feasible epoxy curing system solution for the development of high-performance, low-cost conductive pastes for HJT cells.

Q1: Why do HJT solar cells require low-temperature silver-coated copper paste?

The ITO transparent conductive layer of HJT cells is damaged at temperatures above 200°C, so the low-temperature silver-coated copper paste must complete curing below 200°C. 

Compared with pure silver paste, silver-coated copper paste can reduce costs by 30–50% while maintaining comparable conductivity, making it a key material for cost reduction and efficiency improvement in HJT cells. Silver-coated copper conductive paste has become the mainstream choice for HJT cell metallization.

Q2: Why must the curing temperature of silver-coated copper paste not exceed 200°C?

The ITO layer of HJT solar cells is temperature-sensitive: exceeding 200°C increases the sheet resistance and reduces the transmittance of the ITO layer, directly affecting the photovoltaic conversion efficiency of the cell. Therefore, low-temperature curing (≤200°C) is a hard requirement for the HJT cell metallization process, and the epoxy curing system must complete the curing of the silver-coated copper paste below 200°C.

Q3: What advantages does tetrafunctional epoxy resin offer over bisphenol A epoxy resin?

Tetrafunctional epoxy resin offers higher reactivity and crosslinking density, forming a denser metallization framework after curing, which reduces the volume resistivity of the silver-coated copper paste by 37%, the contact resistivity by 50%, and improves the adhesion by 69%. It is the preferred main resin in the epoxy curing system for silver-coated copper paste.

Q4: What are the advantages of the boron trifluoride monoethylamine complex as a curing agent?

The boron trifluoride monoethylamine complex is a typical latent curing agent with a low curing onset temperature (approximately 120°C) and high reactivity. It enables complete curing of the silver-coated copper paste under a rapid curing process of 200°C/6 min, achieving optimal electrical properties and adhesion, and is the best choice of curing agent in the epoxy curing system.

Q5: How are the volume resistivity and contact resistivity of silver-coated copper paste measured?

The volume resistivity is measured using the four-point probe method, and the contact resistivity is measured by TLM (transmission line method). Test samples are cured at 200°C for 6 min, after which a 3D confocal microscope is used to observe the curing morphology and measure the film thickness. The electrical properties of different epoxy curing systems can be compared using this method.

Q6: How can the adhesion of silver-coated copper paste be optimized?