High thermoelectric performance in the hexagonal bilayer structure consisting of light boron and phosphorus elements

Abstract

Two-dimensional layered materials have attracted tremendous attention due to their extraordinary physical and chemical properties. Using first-principles calculations and Boltzmann transport theory, we give an accurate prediction of the thermoelectric properties of hexagonal boron phosphide (BP) bilayer, where the carrier relaxation time is treated within the framework of electron-phonon coupling. It was found that the lattice thermal conductivity of BP bilayer is much lower than that of its monolayer structure, which can be attributed to the presence of van der Waals interactions. On the other hand, the graphene-like BP bilayer shows very high carrier mobility with a moderate band gap of 0.88 eV. As a consequence, a maximum $p$-type $\mathit{ZT}$ value of \ensuremath{\sim}1.8 can be realized along the $x$ direction at 1200 K, which is amazingly high for systems consisting of light elements only. Moreover, we obtain almost identical $p$- and $n$-type $\mathit{ZT}$ of \ensuremath{\sim}1.6 along the $y$ direction, which is very desirable for fabrication of thermoelectric modules with comparative efficiencies. Collectively, these findings demonstrate the great advantages of the layered structures containing earth-abundant elements for environmentally friendly thermoelectric applications.