Characterization of Laser Doped Silicon and Overcoming Adhesion Challenges of Solar Cells with Nickel-Copper Plated Contacts

The combination of localized laser patterning and metal plating allows to replace conventional silver screen printing with nickel-copper plating to form inexpensive front contacts for crystalline silicon solar cells. Within this work the fundamental physical and chemical processes are explained, problems arising with mass production are pointed out, and solutions to achieve electrically and mechanical stable solar cells are developed.

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The combination of localized laser patterning and metal plating allows to replace conventional silver screen printing with nickel-copper plating to form inexpensive front contacts for crystalline silicon solar cells. In this work, a focus is put on effects that could cause inhomogeneous metal deposition and low metal contact adhesion. A descriptive model of the silicon nitride ablation mechanism is derived from SEM imaging and a precise recombination analysis using QSSPC measurements. Surface sensitive XPS measurements are conducted to prove the existence of a parasitic surface layer, identified as SiOxNy. The dense SiOxNy layer is an effective diffusion barrier, hindering the formation of a nickel silicide interlayer. After removal of the SiOxNy layer, cells show severe degradation caused by metal-induced shunting. These shunts are imaged using reverse biased electroluminescence imaging. A shunting mechanism is proposed and experimentally verified. New laser process sequences are devised and proven to produce cells with adhering Ni-Cu contacts. Conclusively the developed processes are assessed based on their industrial feasibility as well as on their efficiency potential.