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)