Heterojunction (HJT) solar cells have attracted considerable research interest because of their relatively simple process flow, high efficiency, and low temperature coefficient. In HJT manufacturing, the electrical properties and curing process of low-temperature silver-coated copper paste directly affect conversion efficiency and production efficiency. Because the indium tin oxide (ITO) layer on HJT cells is susceptible to damage above 200°C, processing temperatures must remain below 200°C. Accordingly, the paste must cure rapidly at low temperatures (typically drying at 150°C for 3 minutes and curing at 200°C for 6 minutes). Silver-coated copper paste offers lower cost and conductivity approaching that of pure silver paste, making it an important material for improving HJT cell performance while reducing costs.

Epoxy resins and curing agents form the binder network in conductive pastes and directly affect conductivity and adhesion. Using a controlled-variable approach, this study systematically investigates the effects of epoxy resins and curing agents on the electrical properties of low-temperature silver-coated copper paste. 

Millennial Solar 3D confocal microscope was used to scan the cured electrode gridlines, allowing visual comparison of curing morphology among different conductive pastes.

Five epoxy resins were investigated: hydrogenated bisphenol A epoxy resin, bisphenol A epoxy resin, trifunctional epoxy resin, tetrafunctional epoxy resin, and phenolic-modified epoxy resin. The paste had a solids content of 92.6% and a silver content of 30%. Resin loading was varied (1.2%, 1.0%, and 0.8%), together with the corresponding curing-agent content (80%). Volume resistivity, specific contact resistivity, and adhesion were measured, and the cured morphology was examined using 3D microscopy.

Summary of experimental data for silver-coated copper pastes formulated with different base resins

3D images of cured silver-coated copper conductive paste samples

Effect of resin loading: As the epoxy resin content decreased, both volume resistivity and specific contact resistivity tended to decrease. For example, in the bisphenol A epoxy system, volume resistivity decreased from 7.6 × 10⁻⁸ to 6.5 × 10⁻⁸ Ω·m; in the tetrafunctional epoxy system, it decreased from 5.7 × 10⁻⁸ to 4.8 × 10⁻⁸ Ω·m. This indicates that a lower resin content can improve the continuity of conductive pathways.

Effect of resin type:

Electrical properties: The tetrafunctional epoxy resin performed best, with the lowest volume resistivity (4.8 × 10⁻⁸ Ω·m) and specific contact resistivity (1.5 × 10⁻³ Ω·cm²). Hydrogenated bisphenol A epoxy resin showed poorer electrical properties because it lacks aromatic rings and has relatively low reactivity.

Adhesion: The tetrafunctional epoxy resin achieved the highest adhesion (2.2 N), followed by the trifunctional epoxy resin (1.8 N), while the bisphenol A epoxy resin had the lowest value (1.3 N). Higher functionality increases crosslink density and strengthens adhesion.

Cured morphology: The paste based on tetrafunctional epoxy resin appeared bright yellow with a pronounced metallic luster after curing, indicating sufficient curing. Pastes based on hydrogenated bisphenol A and phenolic-modified epoxy resins appeared grayer, suggesting a lower degree of curing.

Using the selected tetrafunctional epoxy resin (TT400), five latent curing agents were evaluated: dicyandiamide (DICY), 2-ethyl-4-methylimidazole (EM124), hexahydrophthalic anhydride (HHPA), boron trifluoride–monoethylamine complex (BF₃-MEA), and sebacic acid dihydrazide (SPH). Their concentrations were varied, and the resulting electrical properties and adhesion were tested.

Summary of experimental data for silver-coated copper pastes with different curing agents

3D images of cured silver-coated copper conductive paste gridlines

Comparison of electrical properties:

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

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

The boron trifluoride–monoethylamine complex has a relatively low curing onset temperature (approximately 120°C) and high reactivity, making it suitable for rapid-curing processes.

Comparison of adhesion:

The boron trifluoride–monoethylamine complex provided the highest adhesion (2.6 N). DICY, HHPA, and SPH followed, while the imidazole-based curing agent showed the lowest adhesion.

Increasing the curing-agent content moderately can improve adhesion, but the benefit tends to level off at higher loadings.

Observations of cured morphology:

The paste cured with the boron trifluoride–monoethylamine complex appeared bright silver-white, indicating a high degree of metallization;

Samples cured with dicyandiamide appeared dull gray, indicating insufficient curing;

Samples cured with hexahydrophthalic anhydride appeared silver-white, indicating good curing.

This study systematically evaluated the effects of epoxy resins and curing agents on low-temperature silver-coated copper pastes for HJT solar cells. Tetrafunctional epoxy resin outperformed the other resins in electrical properties and adhesion, owing to its high reactivity and crosslink density. Among the curing agents, the boron trifluoride–monoethylamine complex showed the most favorable low-temperature, rapid-curing characteristics. Together, the TT400 tetrafunctional epoxy resin and boron trifluoride–monoethylamine complex curing system enabled sufficient paste curing under a 200°C/6 min process, yielding low volume resistivity (4.4 × 10⁻⁸ Ω·m), low specific contact resistivity (1.4 × 10⁻³ Ω·cm²), and high adhesion (2.6 N). This provides a practical material-system option for developing high-performance, lower-cost conductive pastes for HJT solar cells.

Lower epoxy resin content generally reduced both volume and contact resistivity, indicating more continuous conductive pathways.

The tetrafunctional epoxy resin showed the lowest volume resistivity (4.8 × 10⁻⁸ Ω·m), low contact resistivity (1.5 × 10⁻³ Ω·cm²), and the highest adhesion (2.2 N).

TT400 epoxy combined with BF₃-MEA achieved volume resistivity of 4.4 × 10⁻⁸ Ω·m, contact resistivity of 1.4 × 10⁻³ Ω·cm², and adhesion of 2.6 N under 200°C/6 min curing.

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The Millennial Solar ME-PT3000 3D Confocal Microscope is designed to characterize surfaces of precision components and materials across diverse measurement applications. It provides efficient, precise measurements of submicron-scale topography and surface roughness, delivering reliable, high-quality data.

● Accurate and reliable 3D measurement with real-time confocal microscopy images

● High-resolution confocal optics with Z-axis display resolution down to 1 nm

● Combined magnification from 198× to 39,966× for precise submicron topography measurement

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● Comprehensive feedback for cleaning, texturing, and screen-printing processes

With in-situ observation and 3D imaging capabilities, the Millennial Solar 3D Confocal Microscope supports precision characterization and surface roughness analysis, providing an optical measurement tool for process evaluation across multiple fields.

Original Reference: Effects of Epoxy Curing Systems on the Electrical Properties of Low-Temperature Silver-Coated Copper Paste for Heterojunction Solar Cells