From a viewpoint of cost effectiveness, more than 50% of commercial solar cells are fabricated with multicrystalline silicon (mc-Si) crystals with lower purity and perfectness, grown via high-throughput directional solidification at a low cost . Many dislocation clusters acting as recombination centers are frequently generated during the crystal growth, and they degrade the macroscopic electric properties around them, such as the carrier recombination velocity, which is much inferior to that in monocrystalline Si solar cells. One important issue to fabricate high quality mc-Si solar cells is, therefore, to control the generation of dislocation clusters during the crystal growth. Although it is known that the degree of dislocation generation in mc-Si depends on the type of grain boundaries (GBs) , the generation sites are hardly determined in commercial mc-Si ingots with complicated grain structure. Recently, three-dimensional (3D) distribution of dislocation clusters is visualized in a high-performance mc-Si (hp-Si) ingot used for commercial solar cells by photoluminescence (PL) image processing , and it is proposed that some triple junctions of GBs would be related to dislocation generation . In the present work, we have systematically examined the atomistic structure around the triple junctions to discuss the relationship between the macroscopic electric properties (within a spatial resolution of sub-millimeter) and nanoscopic structural properties including the nature and distribution of GBs and dislocation clusters, by using PL image processing and transmission electron microscopy (TEM) combined with electron back scattering diffraction (EBSD) and etch-pit techniques (Fig. 1).