Formation of Ag-Al Alloy in context of PERC solar cell metallization

Abstract

The Ag-Al spike formation at the transition between silver pad and aluminum metallization of the rear side of industrial PERC solar cells was investigated. Silver and aluminum layers were deposited by thermal evaporation on top of silicon nitride layers forming a system of reduced complexity. The absence of glass frit and other usual components of metallization paste leads to a focused study on interaction between the metal alloy and passivation layer. The samples were tempered stepwise from 400°C to 1000°C using a rapid thermal processing tool. X-ray diffraction measurement reveals the expected formation of Ag2Al phase, starting at 400°C. Furthermore, silicon was detected which is accompanied with the occurrence of square shaped indentations in the metal layer, which are the base area of pyramidal metal spikes penetrating the silicon. The formation of these spikes occur for anneal temperatures above 700°C, whereas the silicon nitride shows a high resistance against the molten alloy. The silicon nitride has to contain pinholes to enable a contact between metal and silicon substrate to form these spikes. Electrical characterization of these unintended contacts show an ohmic behavior, similar to the Al-Si system. In combination with the previous observations, a model for the spike formation at the rear side of industrial solar cells is presented.