Renormalized thermoelectric figure of merit in a band-convergent Sb2Te2Se monolayer: full electron–phonon interactions and selection rules

Abstract

The band-convergent strategy by improving density of states effective masses has been proposed and successfully used in the past few decades to optimize thermoelectric performance in materials or discover new thermoelectric materials. Unfortunately, when performing the band-convergent strategy in materials with multi-peak (valleys) band structures, the effects of interpeak (intervalley) scatterings and couplings between carriers and phonons other than the longitudinal acoustic phonon mode are generally neglected in the thermoelectric community. Here, based on ab initio calculations, we investigate in detail the full electron–phonon couplings in a Sb₂Te₂Se monolayer with degenerate valence-band peaks, and their influences on electronic transport and thermoelectric properties. We find that, the optical phonon modes dominate the intravalley scatterings for non-degenerate conduction-band electrons, and interpeak scatterings dominate the electron–phonon scatterings for degenerate valence-band holes. The widely used deformation potential approximation method severely misestimates carrier mobilities in band-convergent systems such as Sb₂Te₂Se monolayer. According to the group-theory argument, the selection rules for the full electron–phonon interactions are derived. Finally, we calculate the momentum and energy-dependent relaxation times considering the full electron–phonon couplings, and the thermoelectric figure-of-merit values at 300/500/700 K are then accurately predicted to be 1.28/2.28/2.98 for p-type doping and 1.86/2.99/3.75 for n-type doping, respectively, which shows that Sb₂Te₂Se monolayer are promising for both p- and n-type thermoelectric applications.