Crystal defects and their impact on ribbon growth on substrate (RGS) silicon solar cells

Ribbon Growth on Substrate (RGS) silicon wafers are casted directly from the silicon melt onto reusable substrates. Material losses by wafer sawing are omitted and high production speeds can be achieved. However, multicrystalline RGS silicon as it is produced today incorporates high densities of crystal defects and impurities limiting the efficiency of the corresponding solar cells. The local impact of crystal defects on material quality is estimated via models developed by Donolato and Micard et al.. By theoretically negating the impact of grain boundaries and dislocations, charge carrier diffusion lengths are still limited to values <100 µm. In addition to crystal defects which are common in other multicrystalline silicon materials, we found current collecting structures within grain boundaries. These structures can be associated with carbon and oxygen precipitation and are the cause for shunting phenomena. We conclude that high impurity concentrations are the dominant factor for limiting the performance of RGS silicon solar cells.