Thermoelectric properties of n-type Cu₄Sn₇S₁₆-based compounds

Copper-based chalcogenides have ultralow thermal conductivity and ultrahigh thermoelectric performance, but most of them are p-type semiconductors. It is urgent to develop n-type counterparts for high efficiency thermoelectric modules based on these copper based-chalcogenides. Cu₄Sn₇S₁₆ is an intrinsically n-type semiconductor with complex crystal structure and low thermal conductivity. However, its thermoelectric properties have not been well studied when compared to the well-known n-type CuFeS₂. In this work, high-quality Cu₄Sn₇S₁₆-based compounds are fabricated and their thermoelectric properties are systematically studied. Using Ag and Sb as dopants, the carrier concentration is tuned over a wide range. The electrical transport properties can be well described by the single parabolic band model with carrier acoustic phonons scattering. It is revealed that Cu₄Sn₇S₁₆ exhibits a low effective mass and relatively high mobility. The thermal conductivity is lower than 0.8 W m^-1 K^-1 from 300 to 700 K and shows a weak dependence on temperature. A maximum zT of 0.27 is obtained in Cu_3.97Ag_0.03Sn₇S₁₆ at 700 K. Further enhancement of thermoelectric performance is possible when a more efficient n-type dopant is used.