Johann Jourdan

The interconnection technology for solar cells using electrically conductive adhesives (ECA) is a promising low-temperature, low-stress, and lead- and bismuth-free solution. The ECA ensures the bonding between either ribbons/wires and solar cells or cell tiles for shingle configurations providing excellent reliability during accelerated ageing tests and in outdoor conditions. The use of ECA has proven its value in recent years, particularly for interconnecting temperature-sensitive heterojunction (HJT) cells. Consequently, interest in the ECA interconnection process is expected to grow in the coming years, especially due to its compatibility with new Si/Perovskite tandem cells. For this technology, solder pads and even busbars are not necessary, allowing for a significant reduction in the amount of paste deposited at the cell level. However, the quantity of ECA used for interconnection remains substantial, and it is at this level that research must focus to reduce silver consumption. It has been shown that lowering the quantity of ECA deposited on the cell can hinder module reliability. Thus, the preferred approach to reduce silver in the interconnection process without affecting reliability is to use uncoated copper ribbons and to reduce the silver content within the ECAs. To achieve this, we used commercial ECAs with low silver content and also developed ECAs based on copper particles coated with a thin layer of silver to prevent oxidation. With the introduction of full copper metallization on the backside of HJT cells, the total amount of silver in the module can be lowered to 7 mg/Wp. This article presents the latest results obtained on the interconnection of HJT solar cells with ECA bonding material, focusing on reducing silver consumption by using low-silver ECAs combined with pure copper ribbons, Cu-Ag and full Cu pastes for cell metallization.

This work presents a seed-layer-free wet-chemical surface treatment enabling direct copper (Cu) electroplating on indium-based transparent conductive oxides (TCOs), particularly indium tin oxide (ITO), for silicon heterojunction (SHJ) solar cells, without the need for a seed or barrier layer. The process involves two sequential chemical baths: an ammoniacal copper complex bath (B1) promoting Cu(OH)₂ chemisorption, followed by a borohydride-based reducing bath (B2) that converts Cu²⁺ to Cu⁰ and catalyzes the formation of metallic indium (In⁰) particles at the surface, enabling adhesion at the TCO interface. Structural and chemical analyses (SEM, EDX, XPS) confirmed the formation of Cu and In species, while electrical (dark I–V) and mechanical (shear strength) tests showed reduced contact resistance and strong adhesion for optimized treatment durations. Interfacial TEM-$ \mu$EDX analysis further reveals that the chemical treatment induces localized modification of the TCO near the surface, while a continuous oxide layer remains beneath the plated copper. The proposed method offers a cost-effective, low-temperature, and easy-to-implement alternative to conventional metallization processes for SHJ technologies.

For the current PV production about 2000 tons of silver are consumed per year, 10% of the entire world annual silver supply. The PV production is expected to grow significantly in the next decades in order to enable the energy transition to 100% renewables. Annual production volumes in the terawatt range are predicted already for 2030 and this may lead to higher silver price and put more pressure on the industry to replace silver by copper. Within the Ameliz project several patterning approaches for copper electrodeposition on heterojunction cells are developed: firstly, patterning by printing a metal seed grid with a dielectric layer as plating mask and secondly masking by a monolayer of self-assembling molecules. These molecules consist of a phosphonic acid group and a hydrocarbon chain. They are bonded to the ITO surface and form a monolayer of well-ordered, densely packed hydrophobic chains, which protects the ITO surface against plating solutions. Furthermore, methods for selective formation of a copper seed layer by electrografting and for improved adhesion by ITO reduction are being investigated.