High-pressure sintering driven defect modulation for enhanced thermoelectric performance in Li+/S2− co-doped Cu2Se

As a typical ionic crystal exhibiting liquid-like behavior, p-type semiconductor Cu2Se has been a research hotspot in thermoelectric materials due to its low thermal conductivity. In this work, by varying the temperature and pressure conditions during synthesis, different morphologies and quantities of microstructural defects were introduced in Li+ and S2− co-doped copper selenide samples. The influence of these defects on the thermoelectric performance of the samples was subsequently investigated. The synthesized samples were characterized for their phase composition, microstructure, electrical transport properties, thermal transport properties, and thermoelectric performance. The results indicate that cracks are the primary defects in samples sintered at high temperature and atmospheric pressure, while impurity phases and micron-sized pores are the main defects in samples sintered under high pressure and low temperature. These defects significantly affect the electrical and thermal transport properties of the doped Cu2Se samples. The presence of residual Cu3Se2 and elemental Cu in the sample sintered at 5 GPa/room temperature leads to anomalous electrical transport behavior. When the pressure and temperature exceed 5 GPa and 1200 °C, the shape of pore defects is markedly altered, their number is reduced, and the propagation of crack defects is simultaneously suppressed. By employing Li+ and S2− co-doping combined with high-temperature and high-pressure sintering, the ZT value of the doped Cu2Se material was enhanced from 2.1 to a maximum of approximately 2.4.