Wilfried Favre

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.

Les cellules à hétérojonctions de silicium fabriquées par croissance de couches minces de silicium amorphe hydrogéné (a-Si :H) à basse température sur des substrats de silicium cristallin (c-Si) peuvent atteindre des rendements de conversion photovoltaïque élevés (η=23 % démontré). Les efforts de recherche ayant principalement été orientés vers le cristallin de type p jusqu'à présent en France, ce travail s'attache à l'étude du type n pour d'une part déterminer les performances auxquelles s'attendre avec cette nouvelle filière et d'autre part les améliorer. Pour cela, nous avons mis en œuvre des techniques de caractérisation des matériaux composant la structure et de l’interface (a-Si :H/c-Si) couplées à des outils de simulations numériques afin mieux comprendre les phénomènes de transport électronique. Nous nous sommes également intéressés aux cellules à hétérojonctions avec substrats de silicium multicristallin de type n, le silicium multicristallin étant le matériau le plus répandu actuellement dans la fabrication des cellules photovoltaïques.

Photovoltaics, a mature technology, is set to play a vital role in achieving a carbon-free energy system. This article examines the pivotal role of optics in advancing photovoltaics. We identify key scientific research areas where the optics community can make significant contributions. We are guided by the central question: How can optics facilitate the large-scale deployment of photovoltaics necessary for decarbonizing our societies?