Research on efficiency limiting defects and defect engineering in silicon solar cells ‐ results of the German research cluster SolarFocus

Abstract

AbstractDefects in multicrystalline silicon for photovoltaic applications and their impact on solar cell parameters have been investigated in the material research network project SolarFocus. A series of multicrystalline silicon ingots of ultrapure feedstock material were cast with intentional addition of typical transition metal impurities (Fe, Cu, Cr) and Ge as doping elements. The results of lifetime measurements, NAA and FTIR analysis, solar cell processing and microscopic investigations are presented in this study. For ingots intentionally contaminated with transition metals, the combined analysis reveals that despite the overall high impurity content, good solar cell efficiencies can be reached. A strong influence of the in‐diffusion of metal impurities from the crucible as well as the back‐diffusion from the top region of the ingot ca still be observed. All metals show a strong precipitation behaviour throughout the whole ingot. The solar cell efficiency is both limited by multiple recombination active defects and shunts, induced by a high metal contamination via indirect mechanisms. Solar cells with efficiencies up to 15.2% for material contaminated with 20 ppma Fe in the melt, 15.7% for 20 ppma Cu and 15.1% for 20 ppma Cr were processed. A positive effect of Cu added to the feedstock could not be found. Ge‐rich ingots showed strong effects of increasing silicon carbide and silicon nitride formation with increasing Ge content larger than 0.5 wt.% thus reducing solar cell efficiency. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)