Temperature dependence of void formation in PERC cells and their spatially resolved detection by combining scanning acoustic microscopy and electroluminescence measurements

Key aspect of this work is the investigation of local Al contacts with regard to void formation under a non-uniform temperature of the solar cell during the firing process in the belt furnace. The impact on electrical properties is determined by IV measurements and electroluminescence imaging (EL). Scanning acoustic microscope (SAM) measurements on full cell area are used for a spatially resolved localization of voids. Combining all these characterization techniques, the impact of thermal non-uniformity on contact formation can be determined spatially resolved. Furthermore, a detailed investigation of BSF thickness of two different Al-pastes shows a clear correlation with temperature distribution on the wafer during the co-firing process. All results demonstrate the importance of a well optimized Al-paste and an absolutely uniform temperature on the Si wafer for an excellent formation of local rear contacts