Further analysis of aluminum alloying for the formation of p+ regions in silicon solar cells

We present a detailed study on the formation of full-area screen-printed aluminum-alloyed p+ emitters and back surface fields for n- and p-type silicon solar cells, respectively. We have investigated the structural properties of agglomerated Al-doped p+ Si regions which exhibit serious lateral thickness inhomogeneities. We refined an existing simple model based on wetting phenomena for the formation of agglomerated Al-p+ regions. According to the model, the thickness inhomogeneities are caused by the contraction of the Al-Si melt during alloying, leading to enhanced and decelerated Si recrystallization in the regions of melt accumulation and depletion, respectively. Furthermore, we demonstrate that the melt contraction can be delayed by carefully choosing adequate alloying conditions. In particular, agglomerations are avoided by increasing the printed Al paste amount or decreasing the peak temperature time or peak temperature, thus enabling a more flexible fabrication of Si solar cells with Al-alloyed rear side.