Influence of Grain Boundaries on Strength of Multicrystalline Silicon Wafers

© Fraunhofer CSP

Fig. 1: Fracture toughnesses in the vicinity of a grain boundary, which is located between two highly asymmetrically oriented crystallites (a); Exemplary micro impression directly on a grain boundary (b).

With a market share of nearly 66%, multicrystalline silicon (mc-Si) wafers are widely used substrates for the production of solar cells. However, besides the fact, that their production costs are lower than those for monocrystalline wafers, they also tend to have a lower mechanical strength. That reduced strength, which is substantially determined by cracks induced in the surface, leads to an earlier breakage and, consequently, to a reduced economic efficiency in manufacturing solar cells.

Grain boundaries are suspected as a possible reason for that, because they represent crystal defects and, thus, potential mechanical weak spots. Researchers of the Fraunhofer CSP have now experimentally investigated the mechanical influence of grain boundaries on crack propagation in mc-Si wafers using the micro indentation technique. Thereby, the investigation has focused on crack shapes that are typically caused during wafer processing through multi-wire sawing. Statistical studies of more than 280 indentation events in various regions (on, near and far away from grain boundaries, see Fig. 1a) showed that neither the energetically more favorable symmetrical twin boundaries nor grain boundaries between highly asymmetrically to each other oriented grains have an influence on the crack propagation in mc-Si wafers. Even cracks, which were positioned directly in grain boundaries, propagated in an unimpeded manner next to and parallel to grain boundaries or even crossed them. They did not show any affinity to propagate in grain boundaries (see Fig. 1b). Instead, the grains themselves with their anisotropic mechanical properties and crack distributions seem to reduce the strength. Thus, these results provide a further contribution in acquiring a profound mechanical knowledge of multicrystalline silicon wafers that are getting thinner and thinner.