EPIC STAR: A Reliable and Efficient Approach for Phonon- and Impurity-limited Charge Transport Calculations

IHPC, NUS and IMRE have been collaborating in the five-year Hybrid Thermoelectric Materials for Ambient Applications Pharos programme that started on 1 Jan 16; this programme aims to pre-position Singapore at the forefront of renewable energy research by proposing novel high-efficiency thermoelectric materials.  EPIC STAR is a milestone and the culmination of IHPC's work within this Pharos programme to predict novel high-performance thermoelectric materials.  

EPIC STAR – the acronym for "Energy-dependent Phonon- and Impurity-limited Carrier Scattering Time AppRoximation" – is a computational code developed in-house within IHPC capable of fast and reliable prediction of carrier mobility and thermoelectric properties for inorganic materials based on first-principles.  To date, there are about half a million known inorganic compounds, but only a few dozen were found to possess good thermoelectric performance.  Due to the lack of any reliable theoretical guidelines, current searches for novel/high performance thermoelectric materials are based on experimental trial-and-error, seriously hampering progress in this area.  This lack of reliable theoretical guidelines arose because theoretical predictions of thermoelectric behaviour require calculations of electron-phonon couplings which cost too much computational time to allow high-throughput computational materials discovery.  While various approximations have been proposed to reduce computational time, none of them provide generally reliable thermoelectric predictions applicable across different inorganic materials.

By developing a generalised first-principles-based approximation to account for the effect of all types of phonons on short-range interactions, and analytical expressions with parameters computed from first-principles to describe long-range interactions, IHPC successfully reduced the computational cost of first-principles calculations within EPIC STAR by at least one order of magnitude relative to other accurate first-principles-based methods, while retaining the capability to account for a broad range of physical phenomena from first-principles. EPIC STAR was also validated against, and achieved quantitative agreement with, both experimentally measured and numerically computed properties for a representative set of high-performance electronic and thermoelectric materials (i.e. Si, GaAs, Mg2Si and NbFeSb) which includes both polar and non-polar, and isotropic and anisotropic materials.  

EPIC STAR, being both fast and reliable, is suitable for adoption into automated, unsupervised systems for high-throughput computational discovery of high-performance materials, such as high mobility semiconductors, and high-performance photovoltaic and thermoelectric materials.  The applications of high-performance materials discovered using EPIC STAR are myriad, including renewable energy generation, light-emitting diodes, field-effect transistors, and solar cells.  The IHPC team is working with our experimental partners to identify about a dozen high performance structures for new thermoelectric materials, transparent conductors and light-emitting materials, and subsequently to promote them to industry.  Future development of EPIC STAR could include extension to materials with reduced dimensionality, inclusion of more complicated mechanisms, and further efficiency and usability improvements.

The article titled "EPIC STAR: a reliable and efficient approach for phonon- and impurity-limited charge transport calculations" by researchers from IHPC, IMRE and NUS has been published in npj Computational Materials (impact factor of 9.341).  The IHPC researchers involved in this work are Dr Deng Tianqi, Dr Wu Gang, Dr Michael Sullivan, Dr Marvin Wong, and Dr Yang Shuo Wang.