Copper as conducting layer in advanced front side metallization processes for crystalline silicon solar cells, exceeding 20% on printed seed layers

Our work deals with the creation of copper-containing stack systems for the front side metallization of silicon solar cells. In this contribution, we give an overview of different approaches from our labs. We have developed processes to apply nickel diffusion barriers onto seed layers and directly onto silicon with both electrolytic and electroless processes. These are reinforced by a light-induced copper plating process. On aerosol-printed seed layers, cell efficiencies equal to those of reference cells with advanced silver metallization have been achieved with a nickel/copper/tin stack system (16.8% on 5×5cm² industrial Cz-material, 20.3% on FZ high-efficiency substrates, 2×2cm²). As the long term stability of the resulting cells is a critical factor, there is need for a method to characterize this aspect. We developed a thermally accelerated ageing procedure, mirroring the total copper diffusion during a typical cell life cycle. Solar cells with advanced metal stack systems have shown no significant decrease in performance during this thermal stress test.