An Investigation into Fracture of Multi-Crystalline Silicon

As the thickness of multi-crystalline silicon solar cells continues to reduce, understanding the mechanical properties of the material is of increasing importance. In this study, a variety of techniques are used to study multi-crystalline silicon. Fracture tests are performed using four- and three-point bending. The fracture stress of as-sawn material reduces with increasing beam width and is increased in beams with a polished front surface. This indicates that fracture initiates from surface flaws. Modifications to standard fracture testing, including testing under liquid, are made so that beams fracture into just two pieces. By determining the crystallography either side of the location of fracture, multi-crystalline silicon was found to fail by transgranular fracture in the samples studied. Further evidence for this is gained from indentation experiments at grain boundaries. In order to understand the relative strength of grain boundaries, new approaches need to be considered. Therefore, a novel micromechanical technique, which enables individual grain boundaries to be studied, has started to be applied to multi-crystalline silicon. A focused ion beam is used to mill micron-scale cantilevers across notched grain boundaries, which are then loaded to fracture using the tip of a nanoindenter. The technique is shown to reproduce the known fracture toughness of {110} planes in single-crystal silicon, giving a value of 0.7 ± 0.3MPam1/2. Preliminary results are presented for fracture of multi-crystalline silicon.