  This work describes through semiclassical Boltzmann transport theory and
simulation a novel nanostructured material design that can lead to
unprecedentedly high thermoelectric power factors, with improvements of more
than an order of magnitude compared to optimal bulk material power factors. The
design is based on a specific grain/grain-boundary (potential well/barrier)
engineering such that: i) carrier energy filtering is achieved using potential
barriers, combined with ii) higher than usual doping operating conditions such
that high carrier velocities and mean-free-paths are utilized, iii) minimal
carrier energy relaxation after passing over the barriers to propagate the high
Seebeck coefficient of the barriers into the potential wells, and importantly,
iv) the formation of an intermediate dopant-free (depleted) region. The design
consists thus of a three-region geometry, in which the high doping resides in
the center/core of the potential well, with a dopant-depleted region separating
the doped region from the potential barriers. It is shown that the filtering
barriers are optimal when they mitigate the reduction in conductivity they
introduce, and this can be done primarily when they are clean from dopants
during the process of filtering. The potential wells, on the other hand, are
optimal when they mitigate the reduced Seebeck they introduce by: i) not
allowing carrier energy relaxation, and importantly ii) by mitigating the
reduction in mobility that the high concentration of dopant impurities cause.
Dopant segregation, with clean dopant-depletion regions around the potential
barriers, serves this key purpose of improved mobility towards the
phonon-limited mobility levels in the wells. Using quantum transport
simulations as well as semi-classical Monte Carlo simulations we also verify
the important ingredients and validate this clean-filtering design.
