Observation of mechanisms of screen printed contact formation during heat treatment on multicrystalline silicon solar cells by transmission electron microscopy

The aim of this study is to understand chemical and metallurgical mechanisms that occur in front side metallisation during heat treatments. Desoxidation of the paste, during firing, leads to the formation of a crystalline system in the upper part of the metallisation. At interface, the authors observe the creation of a new glass thanks to highly reactive titanium. EDS analysis shows that titanium migrates to the paste whereas metallic elements of the glass diffuse, by convection, close to crystalline silicon. This firing allows contact adherence to the cell but it needs an annealing to induce metallic elements diffusion in crystal. This step should be done under reducing gas for oxides dissociation. Contact ohmicity is not realized by the active metal but by metallic elements coming from glass oxides.