Band anisotropy and quartic anharmonicity cooperate to drive p -type thermoelectricity in the ternary diamondlike semiconductor Cu 2 SiSe 3

Diamondlike semiconductors (DLSs) with abundant and eco-friendly constituents are highly valued in energy technology. Those with innately low thermal conduction are uncommon yet helpful for thermoelectric (TE) applications. We present the impact of lattice and electron dynamics on the TE performance of a ternary DLS ${\mathrm{Cu}}_{2}{\mathrm{SiSe}}_{3}$. The structural and mechanical analyses uncover the ionic-covalent bonding and ductile nature of ${\mathrm{Cu}}_{2}{\mathrm{SiSe}}_{3}$. Interestingly, the ${\mathrm{CuSe}}_{3}$ and ${\mathrm{SiSe}}_{3}$ tetrahedral geometries have been identified to contribute independently to the valence and conduction band edges, respectively, offering an opportunity to engineer the band structure. The bonds with different chemical natures within these geometries suggest the possibility of anharmonicity. With a band gap value of \ensuremath{\sim}1.42 eV, the valence band is featured with band anisotropy, providing a large Seebeck coefficient. The relatively weaker interaction between Cu-Se and their considerable vibration belonging to the 100 $\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ region decides most of the thermal conduction. On accounting for quartic anharmonicity-based phonon renormalization along with three- and four-phonon scattering, a lower value of average lattice thermal conductivity ${\ensuremath{\kappa}}_{l}^{\mathrm{avg}}\ensuremath{\sim}2.02\phantom{\rule{0.16em}{0ex}}(0.99)\phantom{\rule{4pt}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 300 (700) K is noticed, with particlelike contribution ${\ensuremath{\kappa}}_{p}^{\mathrm{avg}}\ensuremath{\sim}1.9\phantom{\rule{0.16em}{0ex}}(0.83)\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ and wavelike contribution ${\ensuremath{\kappa}}_{c}^{\mathrm{avg}}\ensuremath{\sim}0.12\phantom{\rule{0.16em}{0ex}}(0.16)\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$. Ionized impurity-scattering-dominated band edges give an overall hole lifetime of \ensuremath{\sim}52 fs at room temperature, and at the higher temperature of 700 K, higher mobility is maintained at \ensuremath{\sim}26.0 $\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ with $\ensuremath{\sim}{10}^{19}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ doping of carriers. A higher power factor for $p$-type ${\mathrm{Cu}}_{2}{\mathrm{SiSe}}_{3}$ is related to the high density of states effective mass and band geometry near the valence band edge. Average ZT \ensuremath{\sim} 0.37 at 300 K and >1.0 for the intermediate temperature range (500--700 K) is reported within ${10}^{19}\text{--}{10}^{20}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ carrier doping. These excellent $p$-type TE signatures would provide an avenue to flexible ternary DLSs as a green energy solution.