Passivation quality of wet oxides grown from purified steam

An effective passivation of the rear surface is a prerequisite to considerably improve the conversion efficiency of industrial solar cells. An excellent level of surface passivation on lowly doped p-type surfaces is provided by thermal grown silicon oxides. However, dry thermal oxidation processes require relatively high temperatures (~1000°C) and, due to the low growth rate, long process times. To decrease both oxidation temperature and process time the dry oxidation process can be replaced by a wet oxidation. The most common way to introduce high purity water vapor into the oxidation tube is the pyrolytic generation from high purity gases (H2 and O2). A more easy and cost effective option for the supply of water vapour that was applied within this work is the direct purification of steam. The passivation quality of dry and wet oxides, the latter grown from pyrolytic generated water vapor and purified steam was compared within this work. The passivation quality obtained for the wet oxides grown from purified steam was found to be comparable to those oxides grown from pyrolytic generated water vapor. On LFC solar cells conversion efficiencies well above 20% could be reached independent of the oxide that was applied for the rear side passivation.