Large effective mass and low lattice thermal conductivity contributing to high thermoelectric performance of Zn-doped Cu5Sn2Se7

Abstract

Cu5Sn2Se7 is a new family member of ternary Cu-based chalcogenides that consists of abundant and environmental friendly elements. It shows a great potential for thermoelectric application, but its metal-like electronic transport behavior leads to overall low thermoelectric performance. In this work, we successfully lower the intrinsically high hole concentration of Cu5Sn2Se7 via an aliovalent doping approach, i.e. replacing the monovalent Cu by divalent Zn. It is shown that one single parabolic band model reasonably fits the room temperature carrier concentration dependent Seebeck coefficients of Cu5−xZnxSn2Se7 (x = 0–1) samples with a large effective mass of 2.5 m0, and their power factors are significantly optimized by charge carrier concentration regulation. Moreover, these materials display ultralow lattice thermal conductivity close to the theoretical minimum (0.55 Wm−1K−1) at high temperatures because of its large Grüneisen parameter of ∼2 estimated by sound velocity data. Altogether, a maximum ZT of 0.51 is achieved at 750 K for Cu4.1Zn0.9Sn2Se7, which is 325% higher over the control sample Cu5Sn2Se7 (ZT = 0.12 @ 750 K).