Transmission electron microscopy and EDS analysis of screen-printed contacts formation on multicrystalline silicon solar cells

In photovoltaic industry, contacts fabrication is an important step for solar cell efficiency improvement. In order to reduce the cost of the device, the metallizations are realized by screen-printing a paste. In this work, we have analyzed large area industrial solar cells with pn+ junction, SiO2 passivation layer and TiO2 antireflection coating (ARC) on front side. During the process, front contacts are screen-printed after TiO2 ARC deposition. The metallization paste is composed of a glass powder, an active metal (Ag for front paste and Ag/Al for rear paste) and a binding agent. Its deposition is followed by a rapid firing step at high temperature (firing through the oxide layers) and an annealing at lower temperature under reducing ambient. The aim of this study is to understand chemical and metallurgical mechanisms that occur in front and backside metallizations during heat treatments necessary for contacts formation. Cross-section samples are realized by the tripod method. Observations are realized with a 200 kV field effect gun-transmission electron microscope (TEM) associated to an EDS analysis. A software allows to pilot the beam, scanning along a surface or a line to obtain, respectively, an X-ray cartography or concentration profiles by elements. The main result of this analysis is that front contact ohmicity is not realized by the silver active metal of the paste but by metallic species coming from glass oxides.