Progress with multi-step metallization processes featuring copper as conducting layer at Fraunhofer ISE
The present work gives an overview of the activities in multi-step front side metallization for crystalline silicon solar cells featuring copper as conducting layer at Fraunhofer ISE. Different approaches are being followed, the two most important being the deposition of nickel and copper onto printed and fired silver seed layers, or directly onto silicon. The first mentioned technique has been found to exhibit an important potential to reduce the silver consumption per cell. Below 40 mg of printing paste per wafer could be deposited by screen printing, as little as 8 mg by aerosol jet printing and metal inkjet printing. Progress with the challenge of adhesion after plating is presented, although further improvement is still needed. The highest peel force that could be achieved was >1.5 N/mm busbar width. For the second concept, detailed investigations of the contact interface have been done, to tackle the challenge of adhesion. An adaptation of the surface geometry is found to raise adhesion, but only 0.6 N/mmBB have been achieved, which is still insufficient. A silicidation process seems to be necessary to obtain sufficient adhesion.