Thermoelectric properties of Bi2O2Se single crystals

Abstract

Bismuth oxyselenide (Bi${}_2$O${}_2$Se) attracts great interest as a potential n-type complement to p-type thermoelectric oxides in practical applications. Previous investigations were generally focused on polycrystals. Here, we performed a study on the thermoelectric properties of Bi${}_2$O${}_2$Se single crystals. Our samples exhibit electron mobility as high as 250 cm${}^2.$V${}^{-1}$.s${}^{-1}$ and thermal conductivity as low as $2$ W.m${}^{-1}$.K${}^{-1}$ near room temperature. The maximized figure of merit is yielded to be 0.188 at 390 K, higher than that of polycrystals. Consequently, a rough estimation of the phonon mean free path (${\ell }_{\text{ph}}$) from the kinetic model amounts to 12 $\tilde{A}$ at 390 K and follows a $T^{-1}$ behavior. An extrapolation of ${\ell }_{\text{ph}}$ to higher temperatures indicates that this system approaches the Ioffe-Regel limit at about 1100 K. In light of the phonon dispersions, we argue that the ultralow ${\ell }_{\text{ph}}$ is attributed to intense anharmonic phonon-phonon scattering, including Umklapp process and acoustic to optical phonon scattering. Our results suggest that single crystals provide a further improvement of thermoelectric performance of Bi${}_2$O${}_2$Se.

6 pages, 4 figures