Excellent average diffusion lenghts of 600 µm of n-Type multicrystalline silicon wafers after the full solar cell process including boron diffusion
In this paper we investigate the material quality of n- and p-type multicrystalline silicon wafers after different high- temperature steps, as applied during cell processing. Both materials start with a high initial bulk diffusion length of around 440 mm (harmonic mean of the whole wafer) which is further improved by the solar cell process. A diffusion length of 510 mm was measured after phosphorus and boron diffusion and firing in the n-type material. The p-type wafers showed diffusion lengths of 540 mm after phosphorus diffusion and firing. These diffusion lengths were measured at a generation rate of 1/20 sun close to maximum power point injection conditions of a solar cell. At higher injection levels both materials reach 600 mm diffusion length. The high material quality of n-type material maintained after the high temperature boron diffusion is remarkable. An efficiency analysis shows that these excellent diffusion lengths allow for high efficiency devices exceeding 20% efficiency.