Thermoelectric Properties of Granular Materials*

I will present our recent studies of thermoelectric properties of mono-phasic nanocrystalline semiconductors and metals in the weak coupling regime. The focus is in particular on the thermopower and figure of merit for temperatures less than the charging energy. I will show that the dimensionless figure of merit $ZT$, which is a measure for the performance of thermoelectric materials, has a maximum at certain temperatures and grain sizes which can be in the range of technological relevant values $ZT>3$. The talk is based on: Phys. Rev. B {\bf 80}, 245440 (2009) and EuroPhys. Lett. {\bf 87}, 57009 (2009).