Ultrahigh thermoelectric performance in RbGeI3/CsSnI3 superlattices

By first-principles methods combined with Boltzmann transport theory, we perform a systematic investigation of the thermoelectric properties of $\mathrm{RbGe}{\mathrm{I}}_{3}/\mathrm{CsSn}{\mathrm{I}}_{3}$ superlattices. $\mathrm{RbGe}{\mathrm{I}}_{3}/\mathrm{CsSn}{\mathrm{I}}_{3}$ superlattices construct a quantum well with valence and conduction states near the Fermi level located in $\mathrm{CsSn}{\mathrm{I}}_{3}$ layers, which yields a periodic repetition of two-dimensional transport channels. The quantum-confinement effects and the ultralow lattice thermal conductivity of the superlattices greatly enhance the in-plane thermoelectric power factor and figure of merit ($\mathit{ZT}$), which further increase when decreasing the thickness of $\mathrm{CsSn}{\mathrm{I}}_{3}$ layers in the superlattices. In ${(\mathrm{RbGe}{\mathrm{I}}_{3})}_{5}/{(\mathrm{CsSn}{\mathrm{I}}_{3})}_{1}$ superlattice, the power factor increases by two times, and $\mathit{ZT}$ is enhanced by nine times, compared to the parent $\mathrm{CsSn}{\mathrm{I}}_{3}$. The $\mathit{ZT}$ of this superlattice is 1.34 at 300 K and 3.78 at 550 K, providing a potential candidate for room-temperature thermoelectric applications. These results also confirm a way to improve the thermoelectric performance of $\mathrm{CsSn}{\mathrm{I}}_{3}$.