Characterization of tri-crystalline silicon for photovoltaic applications

Tri-crystalline silicon is being developed as a high quality wafer material because of its increased mechanical stability allowing thin slicing with higher mechanical yields. Using a tri-crystal seed, poly-free tri-crystal ingots are grown in commercial Cz pullers using multiple recharging. Due to the ⟨110⟩ pulling axis, dislocations often cannot be completely eliminated. Wafer mappings of the stress induced optical birefringence correlate well with the dislocation patterns. With a simple model for the thermal stress in the cooling crystal the resolved shear stresses on all glide systems are calculated. The obtained accumulated shear stress distributions correlate well with the etch pit images. The defect analysis reveals a structural stability despite the presence of dislocations. The carrier diffusion length is correlated with the dislocation density, too. On untextured, 250 μm thin wafers best efficiency values 15.5% were achieved with SiN front surface coating