Thermoelectric enhancement for n-type PbS via synergistic effect of Ti doping and Cu2S compositing

PbS is identified as a potential alternative candidate material for thermoelectric power generation and refrigeration, owing to the resemblance with PbTe in crystal and band structures. However, the thermoelectric performance has reached a bottleneck because of its inferior electronic structure and high lattice thermal conductivity. This work focuses on optimizing the electron-phonon transport by the synergistic effect of Ti and Cu2S in n-type PbS. The first-principles calculation, single Kane band model, and Debye model reveal the physical origin of thermoelectric enhancement. The Ti doping introduces a donor-defect state, leading to a high electrical conductivity and a suppression of bipolar diffusion. However, the band structure of PbS is not ideally optimized due to the localization effect of the Ti resonant impurity states. Furthermore, the co-added Cu2S induces additional point defects, multiscale secondary phases, and Cu-rich precipitates at grain boundaries, which significantly scatter phonons in a wide frequency range and reduce the lattice thermal conductivity. As a result, the maximum zT of similar to 0.8 at 823 K and the average zT(ave) of similar to 0.46 from 300 to 623 K are achieved in n-type Pb0.99Ti0.01S-2%Cu2S, demonstrating the important roles of Ti and Cu2S on improving thermoelectrics in n-type PbS.