Chemical Pressure-Induced Unconventional Band Convergence Leads to High Thermoelectric Performance in SnTe

Band convergence is considered a net benefit to thermoelectric performance as it decouples the density of states effective mass (md & lowast;$m_{\mathrm{d}}<^>{\mathrm{*}}$) and carrier mobility (mu) by increasing valley degeneracy. Unlike conventional methods that typically prioritize md & lowast;$m_{\mathrm{d}}<^>{\mathrm{*}}$ at the expense of mu, this study theoretically demonstrates an unconventional band convergence strategy to enhance both md & lowast;$m_{\mathrm{d}}<^>{\mathrm{*}}$ and mu in SnTe under pressure. Density functional theory calculations reveal that increasing pressure from 0 to 5 GPa moves the Sigma-band of SnTe upward, reducing the energy offset between L- and Sigma-band from 0.35 to 0.2 eV while preserving the light band feature of the L-band. Consequently, a high power factor (PF) of 119.2 mu W cm-1 K-2 at 300 K is achieved for p-type SnTe under 5 GPa. Chemical pressure also induces conduction band convergence, significantly enhancing the PF of n-type SnTe. Additionally, the interplay between pressure-induced phonon modes leads to a moderate increase in lattice thermal conductivity of SnTe below 3 GPa, which combined with the significantly enhanced PF, contributes to a large enhancement in ZT. Consequently, predicted ZT values of 2.12 at 650 K and 2.55 at 850 K are obtained for p- and n-type SnTe, respectively, showcasing substantial performance enhancements.