Grain boundary and dislocation effects on the PV performance of high-purity silicon

To quantify the effects of grain size and dislocation defects on the minority charge carrier lifetime τ and photovoltaic (PV) efficiency of silicon, the authors grew high-purity, float-zoned (FZ) ingots with a range of grain sizes from single crystalline (dislocated and dislocation-free) down to 4×10^-4 cm². In situ ingot cooling rates of 18° and 89°C min^-1 were used. Bulk ingot τ ranged from less than 30 μs for the multicrystalline ingots to 2,500 μs for the dislocation-free crystals. Wafers from different positions in the ingots were used for τ measurements and the fabrication of mesa-isolated, 0.04-cm² diagnostic PV device structures. They found that τ decreased to 4 μs and normalized solar cell efficiency decreased to 0.6 for the smallest average grain areas (4×10^-4 cm²)