Synergetic optimization of thermoelectric properties in SnSe film via manipulating Se vacancies

Thin film thermoelectric materials have attracted much attention due to the application prospect in nanoscale devices. However, the commonly used optimization strategy in bulk materials, such as defect engineering, is hard to be realized in films. In this work, we synthesized SnSe films by the magnetron sputtering method and manipulated the Se vacancy concentration simply by controlling the sputtering time. The carrier transport properties were systematically investigated by combining theoretical simulation and experimental measurement. In the 130 nm-thick ultrathin film, Se vacancies were formed due to the difference in atomic mass between Sn and Se. The electrical conductivity and carrier mobility are improved by the weak electron-phonon coupling strength and small indirect band gap. Meanwhile, the thermal conductivity of the vacancy structure has an average reduction of 33.8% compared with the pristine SnSe films due to the strong vacancy scattering. The Seebeck coefficient maintains above 300 μV K^-1 due to the relatively large direct band gap. A high ZT of 0.6 was finally achieved at 700 K in the film sample with Sn:Se ratio of 1.08, 50% improvement over the pristine SnSe film. However, a transition to the metallic characteristic occurs when Se vacancy concentration further increases, resulting in deterioration of ZT at high temperature. The present work demonstrates a guiding principle of manipulating vacancy concentration for high thermoelectric performance and reveals how vacancies influence energy transfer.