Enhanced thermoelectric efficiency of monolayer InP₃under strain: a first-principles study

We study the thermoelectric properties of monolayer indium triphosphide (InP₃) under uniaxial compressive and tensile strains using density functional theory in conjunction with Boltzmann transport formalism. InP₃is a recently predicted two-dimensional (2D) material with a host of interesting multi-functional properties. Though InP₃is a low lattice thermal conductivity material, its thermoelectric figure of merit,ZTis found to be low. We thoroughly examined how its thermoelectric transport properties evolve under external strain. We find that the tensile (t) and compressive (c) strains have contrasting effects on the transport coefficients, both leading to the same effect of enhancing theZTvalue strongly. Whilet-strain enhances the power factor dramatically,c-strain gives rise to an ultra-low lattice thermal conductivity. Both these effects lead to an enhancement ofZTvalue at high temperatures by an order of magnitude compared to the corresponding value for free InP₃. The maximumZTvalue of InP₃at 800 K is found to be ∼0.4 undert-strain and ∼0.32 underc-strain, values which are comparable to those observed for some of the leading 2D thermoelectric materials. Another finding relevant to optoelectronic properties is that underc-strain the material shows a transition from an indirect to a direct band gap semiconductor with an accompanying increase in the valley degeneracy. The structural, electronic, and thermal properties of the material are thoroughly analyzed and discussed.