Boosting thermoelectric performance in Bi₂Te₃ crystals by Ge/Se co-doping and melt-growth synthesis

This study investigates the structural and thermoelectric properties of intrinsic p-type Bi₂Te₃ and (Bi_{1- x} Ge _x )₂T_2·7Se_0.3 single crystals, which were synthesized using a modified vertical Bridgman technique. X-ray diffraction analysis confirmed the formation of single-phase Bi₂Te₃ and validated the optimized substitution of Ge and Se. The doped samples exhibited reduced thermal conductivity compared to the undoped counterpart, primarily due to enhanced phonon scattering mechanisms. Electrical transport measurements revealed that the enhancement in the thermoelectric figure of merit ( ZT ) is driven by reduced thermal conductivity, resulting from increased phonon scattering at grain boundaries and defects. In the sample with x = 0.04, a 2.3-fold reduction in thermal conductivity was observed, which led to approximately a 3-fold enhancement in the power factor (5394 μW/m·K²) and a 3.9-fold increase in ZT value (0.67) compared to the pristine sample at 350 K. Consequently, a fourfold enhancement in the ZT value of ∼0.7 at 350 K was achieved for (Bi_0.94Ge_0.06)₂Te_2·7Se_0.3, due to a 5.6-fold reduction in electrical resistivity compared to Bi₂Te₃. These findings highlight the potential of Ge/Se co-doping and melt-growth synthesis in developing advanced thermoelectric materials.