Microstructure of void formation stages at local rear contacts

Passivated emitter and rear contact (PERC) solar cells will dominate the solar cell production in the next years. The laser contact opening (LCO) process enables local contact formation at the passivated rear side of the cell. But LCO processed cells can show a local void formation which is explained by a material diffusion effect. In this work the potential of microstructural investigations for void analysis will be demonstrated. Adapted preparation methods allow selective preparation of characteristic positions at the Al/Si matrix covered rear side. Thereby it is possible to apply high-resolution imaging techniques like electron microscopy. Based on cross-sections as well as top-down view analyses a classification of void formation stages is possible. Inspections of solidified transition areas between voids and filled contacts allow first direct insights to the formation. The interfacial layer between a void and the Al/Si matrix could be attributed to an AlxOy layer. The results of the work permit systematic microstructure studies of process and material parameters which will advance a profound understanding of void formation for future void free solar cells.