Comprehensive Study of Sub-grain Boundaries in Si Bulk Multicrystals

Over the past decade, Si bulk multicrystal has emerged as the substrate material for commercial solar cells. To accelerate the adoption of solar cells to the world, ongoing research targets further reductions in energy costs by improving material performance, while reducing manufacturing costs and raw material costs. We approached this issue from deep understanding of crystal physics of Si multicrystals, namely formation mechanism of multicrystalline structure and generation mechanism and properties of defects. The former study lead us to proposal of the dendritic casting method [1]. This article presents the latter study. Among the defects in the Si bulk multicrystals, we focused on sub-grain boundaries (sub-GBs) because they are known to perform as serious carrier recombination sites for solar cells. To improve the crystal quality of Si multicrystals, we must comprehensively clarify the physics of sub-GBs, i.e. 1) electrical properties, 2) interaction with impurities and strain, 3) generation, propagation and vanishing mechanisms and 4) influence of grain boundary structure and crystal orientations, as illustrated in Fig.1. We firstly applied spatially-resolved x-ray diffraction to defects characterization of Si bulk multicrystals to precisely analyze structure and distribution of sub-GBs. Coordinating their high angular resolution with crystallographic multicrystalline structure, we succeeded in detecting the subGBs with small angle difference, as shown in Fig.2. We found that sub-GBs distribute locally and tend to propagate to the growth direction of the ingot across some grain boundaries. Following the structural characterization, we revealed their electrical properties they are significantly electrically active and work as current leakage sites for solar cells. This fundamental study gave common recognition of sub-GBs as dominant defects of the solar cells. Although we were able to investigate structure, distribution and electrical properties of sub-GBs in the practical growth method, it is difficult to clarify the generation mechanisms due to the random and complicated multicrystalline structures. We therefore developed model crystal growth using multi seed crystals with controlled configurations, which provide simplification and systematical control of multicrystalline structure. We found that sub-GBs are generated from grain boundaries and their amount depends on the grain boundary structure. Furthermore, finite element stress analysis modeling the experimented multicrystalline structure revealed that local shear stress around the grain boundary is the key driving force of the sub-GBs generation. This research insist that controlling the grain boundary structure in the practical growth method is a reliable way to suppress the sub-GBs generation. These fundamental knowledge of sub-GBs should be applicable to improve the material performance of Si multicrystals and promise high efficiency solar cells, although interaction with impurities and vanishing mechanism of subGBs remain to be investigated.