  Two-dimensional layered materials have attracted tremendous attentions 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 boron phosphide (BP) bilayer, where the
carrier relaxation time is treated within the framework of electron-phonon
coupling. It is 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 ZT value of ~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 ZT of ~1.6
along the y-direction, which is very desirable for fabrication of
thermoelectric modules with comparative efficiencies. Collectively, these
findings demonstrate great advantages of the layered structures containing
earth-abundant elements for environment-friendly thermoelectric applications.
