Fermi-surface dynamics and high thermoelectric performance along the out-of-plane direction in n-type SnSe crystals

The layered chalcogenide SnSe has attracted great interest for its remarkable ZT records. Here, we report the excellent thermoelectric performance of n-type SnSe single crystals by alloying with PbBr 2 , where a maximum ZT of 2.1 at 770 K is achieved along the out-of-plane direction. Interestingly, we observed a sharp drop of the carrier mobility at PbBr 2 content x = 3%, which terminates the continuous increase of ZT from x = 0.5% to x = 2%. It is shown that the anomalous decrease is due to the evolution of the Fermi surface driven by the enlargement of the out-of-plane axis. When 1% Sn is substituted by Ge, the restoration of the Fermi surface increases the ZT for the x = 3% sample by 200%. This study reveals the crucial role of the Fermi-surface dynamics in n-type SnSe, and the consequent strategy is confirmed to be highly efficient in the optimization of thermoelectric performance. Fermi-surface dynamics drives the thermoelectric performance of n-type SnSe along the out-of-plane direction.