A comparative study on the thermoelectric performance of layered β- and ε-GaSe

Due to the weak interlayer interactions, the binary III-VI chalcogenides GaSe can exist in several distinct polymorphs. Among them, the so-called β- and ε-phase simultaneously exhibit favorable total energies and moderate band gaps, which offer a good platform to explore their thermoelectric properties. Here, we demonstrate by first-principles calculations that the two systems have very similar band structures and phonon dispersions, despite with different stacking sequences between adjacent layers. Interestingly, the lattice thermal conductivity of ε-GaSe is obviously lower than that of the β-GaSe, which is inherently tied with stronger lattice anharmonicity caused by bonding heterogeneity. Besides, both systems exhibit higher p-type power factors due to doubly degenerate bands with weaker dispersions around the valence band maximum. As a consequence, a significantly enhanced p-type ZT of 2.1 can be realized at 700 K along the out-of-plane direction of ε-phase.