RECOMBINATION LIFETIME STUDIES OF SILICON SPHERES

The fabrication and properties of solar cells made from silicon sphere s recently have been described in the literature. Using inexpensive ma terial as feed stock, single crystal 1-mm-diameter spheres are grown b y melting silicon particles. Here we will describe the minority-carrie r properties of spheres made from both metallurgical grade silicon (MG S) and electronic grade silicon (EGS). Repeated melting and recrystall ization of spheres getters impurities at the surface. The surface regi on is chemically or mechanically etched following each of these meltin g 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, contactl ess technique with sensitivity such that the sample size varied from a single sphere to about 100 spheres. The injection level was varied fr om low-level to high-level injection using a Q-switched YAG laser as t he excitation source. The RFPCD decay consisted of a ''fast'' componen t 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 ad dition, some spheres had a ''slow'' component that was attributed to s hallow traps. The electrical performance of solar cells made from a gi ven sample set correlated with recombination lifetimes of the componen t spheres, as might be expected. The measurements are a fast and conve nient method of characterizing the various materials used in sphere fa brication.