Seed-Layer-Free Electroplated Copper Metallization of SHJ Cells Enabled by Chemical Modification of Transparent Conductive Oxides

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Abstract
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.

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