Investigation on thermoelectric performance degradation of In-doped Cu_0.008Bi₂Te₃ induced by bandgap and carrier mobility reduction

In this study, thermoelectric transport properties of In-doped Cu_0.008Bi₂Te₃ (Cu_0.008Bi_2-xIn_xTe₃, x = 0, 0.01, 0.03, 0.05, 0.07, 0.10, and 0.15) polycrystalline alloys were investigated. The carrier concentration was gradually increased by the In doping from 4.3 × 10¹⁹ to 8.5 × 10¹⁹ cm⁻³ at 300 K, and the carrier mobility was also significantly decreased from 224 to 74 cm²/Vs at 300 K. It was also revealed that the measured optical bandgap of Cu_0.008Bi₂Te₃ gradually reduced from 0.43 to 0.14 eV by the in doping. As a result, Seebeck coefficient and electrical conductivity of the In-doped samples decreased simultaneously, and thus the power factor decreased significantly from 2.63 to 0.55 mW/mK² at 300 K. Nevertheless, the lattice thermal conductivity decreased from 1.1 to 0.73 W/mK by 35% at 300 K owing to the additional point defect scattering by the In doping. As a result, the thermoelectric figure-of-merit zT of Cu_0.008Bi₂Te₃ is decreased from 0.41 to 0.12 at 300 K due to rather significant power factor decrease originated by the bandgap and carrier mobility reduction by the in doping. In addition, the degradation of the power factor and zT is further analyzed by single parabolic band model.