Impact of Si surface topography on the glass layer resulting from screen printed Ag-paste solar cell contacts

For the current transport mechanism between the n+ Si emitter and the screen printed silver contact, there is strong experimental evidence that the largest current contribution flows through Ag crystallites directly connected with the silver finger, which are preferably concentrated in some tips of the Si pyramid surface. For this purpose, we focused on the origin of these Ag crystallites and we analyzed the contact formation on different surface topographies such as pyramid height and rounding degree variations, and flat smooth surfaces, with and without phosphorus doped emitter and not only on mono- but also on multicrystalline Si material with its dislocations, grain boundaries and impurities. Combining contact resistance measurements with SEM investigations, we discovered that smaller pyramids are more capable of creating better contact than rounded pyramids, even though in our case they cover only a few percent of the area on the Si surfaces. This can be explained with the wetting behavior of the glass, which leads to some pyramid tips being glass-free and thus in direct contact with the Ag-bulk, as long as the pyramid heights exceed the thickness of the glass layer. In the case of surfaces which are smoothly polished or with strongly rounded pyramidal surfaces, a continuous glass-layer separates the Ag crystallites from the Ag-finger preventing good contact. Moreover, without a P doped emitter it is possible not only to create Ag crystallites underneath the glass, but also Ag crystallites in direct contact with the silver finger.