Homogeneity analysis of multicrystalline silicon ingots with columnar grain structure

The aim of our work is the quality improvement of cast multicrystalline silicon ingots fabricated in a scaled down copy of an industrial production system (ingot size 220×220×180 mm³). Standard ingots show an inhomogeneous grain structure due to a strongly concave solidification interface. In order to improve the quality, the thermal conditions and the mould configuration of the research system were changed in such a way that ingots with a homogeneous columnar grain structure are produced now. Although the overall lateral and vertical homogeneity is excellent, there are still small areas distributed all over the ingot where the electrical quality and particularly the solar cell efficiency are reduced. These areas are found to be correlated with small grain sizes, high defect densities and an increased minority carrier recombination. In spite of these low quality areas test solar cells showed efficiencies over 15% with standard solar cell processes