Enhanced thermoelectric performance by lone-pair electrons and bond anharmonicity in the two-dimensional Ge2Y2 family of materials with Y = N, P, As, or Sb

Using density functional theory combined with the Boltzmann transport equation, the charge, thermal transport, and thermoelectric properties in two-dimensional (2D) Ge2Y2 (Y = N, P, As, or Sb) monolayers characterized by two structural phases, i.e., alpha-Ge2Y2 and beta-Ge2Y2, have been studied systematically. Our theoretical results demonstrate that the lone-pair electrons have remarkable influences on their lattice thermal conductivity. By performing comparative studies on the two different structures of Ge2Sb2, we uncover that the above influences not only originate from the interactions between the lone-pair electrons around Sb atoms and the bonding electrons of the adjacent Ge atom, but also from the interlayer Coulomb repulsive forces of lone-pair electrons distributed in different layers. The latter leads to a strong anharmonicity, which greatly suppresses the lattice thermal conductivity. Thus, alpha-Ge2Sb2 monolayer has an ultralow thermal conductivity with 0.19 W/mK, while beta-Ge2Sb2 monolayer with 5.1 W/mK at the temperature 300 K. Owing to the ultralow lattice thermal conductivity induced by lone-pair electrons, the predicted maximum value of the thermoelectric figure of merit (ZT) reaches 1.2 for p-type and 1.18 for n-type doping alpha-Ge2Sb2. Our theoretical results put forward another effective mechanism to design and optimize 2D thermoelectric materials with high thermoelectric conversion efficiency.