Float zone (FZ) silicon: A potential material for advanced commercial solar cells?

Cylindrical rods of multi-crystalline electronic grade FZ silicon with a diameter of 142 mm were grown. Wafers of 100 mm × 100 mm sizes were prepared at different axial positions and characterized by µ-PCD minority carrier life time mapping, residual stress polariscopy and structural etching. Some of these wafers were processed for solar cells. It could be shown that the grown multi-crystalline FZ silicon, surprisingly, shows worse minority carrier lifetimes (between 12 and 19 microseconds) and less efficiency (13.6%) of the solar cells compared with 15.6% for conventional directionally solidified silicon, 17.0% for single-crystalline FZ and 17.3% for single-crystalline CZ silicon. This decrease can be correlated to the stress induced dislocation generation caused by the strongly inhomogeneous temperature gradient during the FZ crystallization. Alternatively, dislocation-free single crystals with a quadratic cross section have been grown in the thermal field of a specially designed RF - inductor without rotation. This facilitates an increased yield of the expensive material. As a consequence, multi-crystalline FZ material does not seem to be suited as basic material for the solar cell production. However, electronic grade single-crystalline FZ material with its high efficiency grown nearly square-shaped with a standard size could become an attractive alternative to commercial PV growth methods. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)