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Cu 5 Sn 2 Se 7 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 Cu 5 Sn 2 Se 7 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 Cu 5−x Zn x Sn 2 Se 7 (x = 0–1) samples with a large effective mass of 2.5 m 0, 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 Cu 4.1 Zn 0.9 Sn 2 Se 7 , which is 325% higher over the control sample Cu 5 Sn 2 Se 7 (ZT = 0.12 @ 750 K). • The thermoelectric properties of Cu 5−x Zn x Sn 2 Se 7 were systematically investigated. • Overcome the intrinsically high hole concentration of Cu 5 Sn 2 Se 7 and enhance its TE performance by using Zn as doping agent. • The electrical transport properties of Cu 5 Sn 2 Se 7 are explored through a SPB model. • A maximum ZT of 0.51 is achieved at 750 K for Cu 4.1 Zn 0.9 Sn 2 Se 7 , which is 325% higher than that of pristine Cu 5 Sn 2 Se 7. [ABSTRACT FROM AUTHOR]