Optimal thermoelectric figure of merit in Bi2Te3/Sb2Te3quantum dot nanocomposites

Abstract

Following the idea that there exists an optimal bandwidth for maximizing the thermoelectric figure of merit ($ZT$), we conduct detailed calculations in this paper to search for the optimal $ZT$ in Bi${}_{2}$Te${}_{3}$/Sb${}_{2}$Te${}_{3}$ quantum dot (QD) nanocomposites (NCs) with Bi${}_{2}$Te${}_{3}$ QDs uniformly embedded in Sb${}_{2}$Te${}_{3}$ matrix where electron minibands are formed. The two-channel transport model, which considers both the miniband transport by the quantum-confined carriers and the background transport by the bulklike carriers, is used for electrical transport, while the lattice thermal conductivity is modeled using the modified effective medium approximation. Simultaneous decrease of the lattice thermal conductivity and the Lorenz number leads to an enhanced $ZT$ in QD NCs when the Seebeck coefficient is not dramatically decreased. The optimal structural parameters that result in optimal electronic structure for maximizing $ZT$ are found, with the consideration of realistic carrier scattering physics including phonon bottleneck effect. The optimal QD size is found to be \ensuremath{\sim}$6\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, and the optimal interdot distance depends on the QD size and the doping concentration. For a given QD size, the maximum $ZT$ is determined by the minimum of Lorenz number, which occurs when the quantum-confined carrier transport overwhelms the bulklike carrier transport.