Enhanced Thermoelectric Properties of Codoped Cr2Se3: The Distinct Roles of Transition Metals and S

Pristine Cr 2 Se 3 is a narrow-band gap semiconductor but with an inferior ZT value of 0.22 obtained at 623 K. In this paper, we improve the thermoelectric performance of the Cr 2 Se 3 material by optimizing carrier concentration, suppressing the bipolar thermal conductivity, and reducing the lattice thermal conductivity simultaneously. First, the effect of different dopants (Nb, Ni, and Mn) on the phase composition and thermoelectric transport properties of M 2 x Cr 2–2 x Se 3 (M = Nb, Ni, and Mn; x = 0–0.02) compounds are systematically investigated. The roles of those dopants are distinct. Mn-doped samples show superior thermoelectric properties in comparison with those of other-element-doped samples. Doping with Mn significantly increases the carrier concentration, accompanied with a suppression of the intrinsic excitation and a reduction of both the bipolar thermal conductivity and the lattice thermal conductivity of Cr 2 Se 3 . To further reduce the thermal conductivity, we have synthesized Mn and S codoped Mn 0.04 Cr 1.96 Se 3–3 x S 3 x ( x = 0–0.1) samples. Alloying with S significantly decreases the lattice thermal conductivity and enlarges the band gap, boosting the Seebeck coefficient. The maximum ZT value of Mn 0.04 Cr 1.96 Se 2.7 S 0.3 reaches 0.33 at 823 K. Compared with the pristine Cr 2 Se 3 sample, the maximum ZT value is increased by 50% and the temperature corresponding to the peak value shifts toward higher temperatures by 200 K.