Two-dimensional electronic structure for high thermoelectric performance in halide perovskite Cs₂Au(i)Au(iii)I₆

Pb- or Sn-based halide perovskites usually exhibit poor thermoelectric performance, arising from their low electrical conductivity or oxidation state instability. It is highly desired to search for new halide perovskites with good thermoelectric properties. In this work, the thermally stable mixed-valence halide perovskite Cs₂Au(i)Au(iii)I₆ is revealed to be a highly promising thermoelectric material with high in-plane power factor and ultralow lattice thermal conductivity using first-principles calculations. The high in-plane power factor is achieved due to the novel two-dimensional electronic structure near the Fermi level driven by the weak interaction between Au^I-5d and I-p orbitals. In addition, the small group velocities and short phonon lifetimes give rise to ultralow lattice thermal conductivity in Cs₂Au(i)Au(iii)I₆. These excellent electronic and thermal properties lead to a high ZT value, which is close to 1 at 300 K and ∼4 at 800 K. Our results suggest that the 2D electronic structure from the weak interaction between d and p crystal orbitals is a promising route to design high-efficiency halide double perovskite thermoelectric materials.