Clarification of the relation between the grain structure of industrial grown mc-Si and the area fraction of electrical active defects by means of statistical grain structure evaluation

Abstract

This paper presents results of a statistical investigation of the grain size-, grain orientation- and grain boundary type distribution of six industrially grown conventional multicrystalline (mc-Si) silicon bricks and six high performance (hp) mc-Si bricks of different producers. It was found that hp mc-Si wafers have a more uniform initial grain size (coefficient of variation CV < 2.5) and grain orientation (CV < 1.5) distribution, with a smaller mean grain size (<4 mm2) and a much higher length fraction of random grain boundaries (>60%) in comparison to the conventional mc-Si wafers. Despite the totally different initial grain structures between the hp and conventional mc-Si bricks the grain structure of the top wafers is more or less comparable. It was concluded that the grain structure development of the hp mc-Si bricks is driven by an energy minimization due to the surface energy as well as the grain boundary energy. The grain structure investigations show clearly that especially the initial grain structure of the bricks significantly influences the area fraction of electrical active defects as well as the solar cell efficiency. The hp mc-Si bricks show the lowest area fraction of electrical active defects up to 4% and the highest solar cell efficiency of 18.8%, whereas the conventional mc-Si bricks have an area fraction of electrical active defects up to 14% and a maximum solar cell efficiency of about 18.4%.