Thermoelectric Transport Properties from the Boltzmann Equation and Beyond

In this talk we will discuss our efforts for predicting the transport properties of materials from first principles simulations, which span over both the development of novel methods for studying materials transport properties and the development of software for increasing the speed and accuracy of transport simulations.<br/>First, we will discuss the thermoelectric transport properties of Bi₂Se₃. The transport properties of this narrow gap semiconductor are not precisely captured by semiclassical transport models. Not only is Bi₂Se₃ characterized by bipolar transport, given that the carrier’s gap is comparable to thermal energy, transport is also dominated at low doping concentrations by Zener tunneling. This mechanism allows carriers to tunnel between valence and conduction band and thus carrying charge by hopping between states, rather than charge diffusion under the action of an electric field. Here we show how to describe this phenomenon using a novel first-principles model based on the Wigner distribution. Surprisingly, we find that Zener tunneling doesn’t just involve low-energy carriers, but occurs also between band subvalleys of energy larger than the band gap, thanks to a combination of strong dipole interactions and large density of states.<br/>Next, we will discuss the latest developments of Phoebe, an open-source software for computing thermoelectric properties, capable of solving both phonon and electron Boltzmann equations. Phoebe can compute electron-phonon and phonon-phonon scattering properties from first-principles simulations. In a single package, it is now possible to fully characterize the thermoelectric transport properties of a material from ab-initio simulations. The software has been designed to take advantage of the latest high-performance computing architectures, and implemented an efficient mix of MPI, OpenMP and GPU acceleration strategies, allowing us to simulate more complex materials.