Manipulating the phonon transport towards reducing thermal conductivity via replacement of Cu by Mn in Cu₂SnSe₃ thermoelectric system

The substitution of Cu by Mn in the Cu_2-xMn_xSnSe₃ (0 ​≤ ​x ​≤ ​0.20) system is presented with an objective to optimize the thermal transport and analyse thermoelectric behaviour in the low and near room temperature regime (10–350 ​K). The existence of hole-like small polarons as thermally activated carriers, mediating the p-type electrical transport at high temperatures (>80 ​K), is experimentally validated. Temperature dependence of Seebeck coefficient and electrical transport at low temperatures reveals that the variable range hopping (VRH) mechanism is responsible for conduction for temperatures (<80 ​K). Mn doping resulted in the improvement of the Seebeck coefficient, attaining the highest value of 228.3 ​μV/K at 350 ​K for the x ​= ​0.20 sample. A reduction in thermal conductivity is achieved in all the Mn-doped samples, presumably due to strong point defect scattering of high-frequency phonons. The x ​= ​0.20 sample has the lowest thermal conductivity of 1.68 ​W/mK at 350 ​K. Even though the ZT value is observed to decrease with Mn doping, enhancement in thermoelectric quality factor is seen for the sample with x ​= ​0.05, which is attributed to the reduction in lattice thermal conductivity.