Grain boundary effect on lifetime in high performance multicrystalline silicon during solar cell processing

High performance multicrystalline silicon wafers used in solar cell processing have been investigated with focus on quantification of the grain boundary effect on lifetime. The lifetime of a set of 16 wafers from different positions along the ingot and after different process steps – phosphorus gettering, SiNx:H layer deposition and firing – is measured by µPCD and compared with microstructural information from EBSD. This allows for analysis of the behaviour of grain boundaries and their influence on lifetime during solar cell processing. The minority carrier lifetime of HPMC-Si wafers is not increased after the gettering step, but even reduced for some samples. It is shown that the lifetime in areas close to grain boundaries is reduced during the gettering step and this has a stronger effect on the average value than the improvement within the grains. Only wafers after both gettering and hydrogenation show an overall improvement in carrier lifetimes. However, in the regions close to the bottom of the ingot, wafers show lifetime degradation after the hydrogenation process. The results are used to obtain quantitative information on recombination velocity of grain boundaries.