Minority-carrier lifetime studies of silicon spheres [for solar cells]

The fabrication and properties of solar cells made from silicon spheres recently have been described in the literature. Using inexpensive material as feedstock, single crystal 1 mm diameter spheres, are grown by melting silicon particles. Here, the authors describe the minority-carrier properties of spheres made from both metallurgical grade silicon (MGS) and electronic grade silicon (EGS). Repeated melting and recrystallization of spheres getters impurities at the surface. The surface region is chemically or mechanically etched following each of these melting operations. Here, they applied a radiofrequency photoconductive decay technique (RFPCD) for measurement of the minority-carrier lifetime in spheres after each purification step. This is a noninvasive, contactless technique with sensitivity such that the sample size varied from a single sphere to about 100 spheres. The electrical performance of solar cells made from a given sample set correlated with recombination lifetimes of the component spheres, as might be expected. The measurements are a fast and convenient method of characterizing the various materials used in sphere fabrication.