Recombination lifetime studies of silicon spheres

The fabrication and properties of solar cells made from silicon spheres recently have been described in the literature. Using inexpensive material as feed stock, single crystal 1-mm-diameter spheres are grown by melting silicon particles. Here we will 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 we applied a radio-frequency photoconductive decay technique (RFPCD) for measurement of the minority-carrier lifetime in spheres after each purification step. This is a non-invasive, contactless technique with sensitivity such that the sample size varied from a single sphere to about 100 spheres. The injection level was varied from low-level to high-level injection using a Q-switched YAG laser as the excitation source. The RFPCD decay consisted of a ''fast'' component in all cases, with a lifetime that varied from about 20 ns to 1 or 2 mu s. The latter was attributed to electron-hole recombination. In addition, some spheres had a ''slow'' component that was attributed to shallow traps. 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.