Research on the function and conductive mechanism of nano-silver powder in high-performance silver-coated copper paste

In the field of solar cell technology, the conversion efficiency of silicon heterojunction (SHJ) solar cells has reached 27.08%. Meanwhile, perovskite/SHJ tandem solar cells based on this structure have achieved an efficiency of 34.85%, surpassing the 33.7% theoretical limit for single-junction devices. As the industry shifts from single-junction to tandem configurations, SHJ cells—benefiting from their distinctive structure and low-temperature fabrication process—offer superior compatibility with perovskite layers. This positions SHJ technology to play a critical role in the development of perovskite/tandem solar cells.<br>The application of high-performance silver-coated copper (Ag@Cu) paste for electrode metallization provides a viable approach to reduce the cost and improve the performance of SHJ cells. However, the micron-scale particle size of Ag@Cu powder (typically several micrometers) limits the packing density of the electrode layer. To address this, nano-silver powder (~100 nm) is commonly introduced as an additive, enhancing both the packed density of the powder and the electrical conductivity through nano-effects. Although many studies focus on isolated aspects such as paste conductivity, a systematic evaluation covering contact resistivity, printed and cured electrode morphology, overall cell performance, and long-term stability remains scarce. Potential adverse effects of nano-silver addition have also been overlooked. Therefore, a thorough investigation into the role of nano-silver in low-temperature Ag@Cu pastes is necessary.<br>Highly conductive low-temperature curing pastes typically employ binary or ternary composite powders with well-separated particle sizes to achieve high packing density according to the dense packing theory. In this work, we systematically adjusted the proportions of three conductive powders: micro-sized Ag@Cu (3-5 μm), sub-micron silver (500 nm), and nano-silver (100 nm), to study the effect of nano-silver on key properties of Ag@Cu paste. These include: curing temperature and sintering behavior, microstructure of cured electrodes, interface structure between electrodes and the silicon wafer, electrical resistivity, and the overall conversion efficiency of SHJ solar cells. The aim is to clarify the underlying mechanisms and optimize the nano-silver content.<br>This research reveals several significant impacts of nano-silver addition on Ag@Cu paste properties: (1) It markedly reduces the resistivity of the cured electrode. Compared to sub-micron silver, nano-silver facilitates improved lateral conductivity at lower sintering temperatures. (2) It introduces additional pores at the contact interface with the silicon wafer, increasing contact resistivity. A thickened organic layer at the interface also forms, which reduces the open-circuit voltage of the cell. (3) It enhances paste thixotropy, leading to narrower printed electrode lines that reduce shading loss and increase short-circuit current density. Concurrently, it raises electrode height and cross-sectional area, which helps improve the fill factor. (4) With nano-silver content controlled at 15%, the efficiency of SHJ cells matches or approaches that of reference cells with pure silver electrodes, mainly due to enhanced fill factor and short-circuit current density.<br>In summary, an optimized amount of nano-silver powder (e.g., 15%) enables simultaneous improvement in electrode conductivity, printability, and opto-electrical performance, yielding SHJ cells with efficiency comparable to those using pure silver electrodes. This demonstrates the potential of Ag@Cu pastes as a cost-effective alternative without compromising performance. Future studies should focus on the long-term reliability of such paste systems and their scalability, supporting the mass adoption of this technology in perovskite/SHJ tandem solar cells.