First-principles electronic transport approach: Efficiency, robustness, and flexibility

Transport parameters are crucial for novel material deployment in a variety of technological applications, including solar cells, solid-state batteries, light-emitting diodes (LED), photocatalysis, thermoelectrics, and many more. One of the earliest and most common approaches is to calculate transport is solving the Boltzmann transport equation (BTE) in the constant relaxation time (CRT) approximation. DFT and DFPT have enabled calculations of electron–phonon interactions from the first principles. This procedure can be accelerated within the EPW code. However, this method is still highly resource-intensive for materials with larger unit cells (containing more atoms and basis functions) and lower symmetry (featuring larger non-equivalent k-space regions). Dr Zhen Li, Dr Patrizio Graziosi and Prof Neophytos Neophytou from School of Engineering, University of Warwick, UK, combined the DFPT + Wannier method with the deformation potential theory, offering an alternative direction to calculate transport properties which provides efficiency, robustness, and flexibility. Acoustic, optical, and inter-valley deformation potentials are calculated from e–ph matrix elements using first-principles calculations. Overall scattering rates is completed by computing polar optical-phonon and ionized impurity scattering rates. Using ElecTra, they validate the approach by performing an in-depth investigation for the promising TE material n-type Mg3Sb2, chosen for its band structure complexity, unit cell size, and degree of symmetry. Excellent agreement with the DFPT + Wannier method is achieved while utilizing no more than 10% of its computational cost. Applying the same approach to Si, a simpler material, once again that ab initio accuracy is attained, this time at less than 1% of the corresponding ab initio computational cost. This method belongs to the category of methods that compute and process matrix elements. However, it distinguishes itself through advancements in accuracy and flexibility. Firstly, accuracy is ensured by selectively computing crucial matrix elements at specific energies and wavevectors, focusing on regions responsible for electronic transitions. This allows to afford dense grids around these significant areas. Secondly, this approach provides explicit information on individual scattering processes (acoustic, optical, intra- and inter-valley), offering valuable insights and capabilities that are particularly advantageous for designing materials with optimal multi-valley electronic structures. This approach offers an alternative that combines efficiency, robustness, and flexibility beyond the commonly employed constant relaxation time approximation with the accuracy of fully first-principles calculations.  

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