| Currently, lead telluride is one of the most important mid-temperature thermoelectric (TE) materials, and doping is the most common and effective way to improve its TE performance. In the present thesis, first-principles calculations were carried out to investigate the influences of single Pb-cation site and Pb-cation and Te-anion sites dually substitutional doping on the band structure and TE properties of PbTe. Besides, the (Sn, Se) dually doped PbTe were synthesized by melting and the rapid induction hot pressing to verify the calculated results.Results reveal that the reconstructed surface structure shows significant impacts on the doping stability and electronic structure of Ag and Sb co-doped (100) surface of PbTe. Intralayer rumpling and interlayer fluctuation occurs and their amplitudes decrease in the deeper layer as the symmetry reduces at the surface. The calculated formation energy of Ag and Sb indicates that the dopants tend to get close forming Ag-Sb nanodots and distribute towards the outer surface. However, the undulant surface provides additional energy barrier and thus stabilizes the doping configuration. Moreover, the changing regularity of surface properties with layer depth is just opposite in doped odd and even layers due to the opposite structural relaxation behavior. The doping effect of Ag and Sb in PbTe (100) surface is mainly originated from the local bonding conditions with neighboring Te atoms. The Ag-Te and Sb-Te bonds are quite different due to the elemental differences between Ag and Sb, and thus, the configuration of Ag-Sb nanodot shows vital impact on properties of the PbTe (100) surface.A clustering tendency is also observed for (M, N)(M={K, Ag, Ge, Sn, Sb, Bi}, N={S, Se, I}) dual dopants in bulk PbTe. The stable doping configuration is determined by the dopant atomic radii and interactions between dual dopants and the PbTe matrix. There exist two types of structural relaxation behaviors including overall lattice distortion and local atomic relaxation in the dually doped PbTe. The former decreases and the latter increases the band gap, respectively. The originally degenerated bands at band edge split off due to dual doping induced symmetry reduction, dopant-dopant and dopant-matrix interactions. Furthermore, owing to the interactions between dual dopants, dually doped PbTe is quite different from solely doped conditions. For instance, the alkali K re-enlarges the nearly closed band gap in S-or Se-doped PbTe. Calculations on larger supercells for the Ag-S doping configuration indicate that the single (Ag, S) dual-dopant pair behaves in a similar way to that in the small supercells with lower local atomic relaxation, weak band splitting and reduced band gap. The formation of dualdopant nanoclusters can widen the band gap due to the introduction of large local strain in the vicinity of clusters. The impurity cluster size exerts a prominent effect on the band-edge states of PbTe. Most importantly,(Ag, S),(Ge, Se),(Sn, S) and (Sn, Se) co-doping produce camel’s-back-like structures with multiple extrema due to abnormal bending of bands near the band gap.These bended bands can be characterized by two independent parameters,△k and△E. The Seebeck coefficient (S) and electrical conductivity (σ) of PbTe with bended bands at band edge were calculated based on the Boltzmann transport theory. Results imply that Ak has significant impact on S and σ while the effect of△E is much less pronounced. For both n and p-type PbTe, as carrier concentration increases from a low level to higher one, S decreases first and then increases with the increasing of△k at a fixed temperature lower than the intrinsic excitation value (Ti). However, opposite changing behavior of S is observed when temperature is higher than Ti and σ changes with Ak in an opposite way. For p-type valence band maxima bended and n-type conduction band minima bended PbTe, S is significantly enhanced and a almost does not change at low temperature and high carrier concentration range as Ak takes a relatively larger value. However, for small Ak, a trivial decline of S and slight increment of σ occur at the same temperature and carrier concentration range. Owing to the variation of S and σ, the power factor (PF) of PbTe can be remarkably increased by the bending of bands. The changing behaviors of S,σ and PF with band bending degree are mainly derived from the competition effect of the increased number of carrier pocket and the intervalley scattering. The experimental results of (Sn, Se) dually doped PbTe reveals that the distribution behavior of dual dopant and the variation regularity of S, a and PF with doping concentration are in a quite well agreement with the calculated results. This gives credible indirect proof of the veracity of the calculated results.In the present thesis,(I) the distribution behavior of the cation site dopants, Ag and Sb, in PbTe was revealed, and the influence of the doping configuration and surface structure on the surface properties were demonstrated;(Ⅱ) both the distribution condition of dual dopants in PbTe matrix and the dual doping induced band revolution of PbTe were discussed in detail;(Ⅲ) it is found that particular dual dopants can induce band bending near the band edge of PbTe, which is beneficial to enhancing TE performance of PbTe in the low temperature and high carrier concentration range;(Ⅳ) a series of bulk (Sn, Se) dually doped PbTe with very high relative dencity was synthesised via melting and rapid induction hot pressing, and the experimental results provide proofs for the validity of previously calculated results. These results in this thesis provide suggestions in the optimal doping and TE performance enhancement of PbTe and similar materials with the same crystal structure. |
