Role of Metallic Impurities

In this section we discuss the effect of metallic impurities on both the material and device properties. Impurities in silicon wafers are incorporated as foreign elements in the crystalline structure. They can be present as point defects, i. e., substitutional or interstitial in the lattice, as precipitates or at the surfaces of silicon wafers. Their chemical state and distribution depends mainly on the solubility and the diffusivity that are a specific characteristic of each element and essentially related to their atomic size and their affinity to form bonds with silicon atoms in the lattice. Impurities cause defect energy levels within the silicon bandgap, which increase recombination of charge carriers, and therefore strongly affect the electronic material properties. Last but not least, high-temperature processes change the state, aggregation and distribution of impurities. Therefore, the thermal history of ingots, wafers and solar cells determines the material properties under which the cells will operate.