Band Structure And Thermoelectric Performance Optimization Of GeTe-based Materials

Thermoelectric materials(TE)realizes the direct conversion of electricity and heat.?-? compounds have been focused due to their excellent thermoelectric performance which originated from their special L and?band structures.Pb Te reaches the level of devices,but the toxicity of Pb has limited its application.GeTe-based thermoelectric materials with a similar band structure have become a research focus due to their excellent thermoelectric properties.However,because of the high carrier concentration,high electrical and thermal conductivity and phase transition of pure GeTe at 720 K,the thermoelectric performance of GeTe is limited.In this paper,GeTe-based materials are used as based materials.Through band engineering,carrier engineering,phonon engineering and crystal structure engineering,experiments and theoretical calculations are used to improve thermoelectric performance.The main conclusions are as follows:First,high carrier concentration of GeTe-based thermoelectric materials was optimized.The electrical mechanism of Cu¹⁺-Sb³⁺pair replaced by Ge²⁺-Ge²⁺pair was found,which effectively tuned the high carrier concentration.;It is proved that Cu/Sb doping can reduce the Ge vacancy formation energy of GeTe by using first-principles calculation.Cu/Sb alloyed GeTe forms a special structure.The special twin grain boundaries were found by using the high-resolution transmission electron microscope(HRTEM).Meanwhile the Debye-Callaway model was used to analyze the effects of multiple scatterings such as point defects,grain boundaries,and mesoscale scattering on the lattice thermal conductivity.Band engineering was investigated to opitimize bands degeneracy on the GeTe-based materials.It found that Ti doping GeTe could reduce the separation energy between light and heavy bands increasing band degeneracy and density of state near Fermi level to enhance the Seebeck coefficient by using first principle calculation;the carrier concentration of Ti doped GeTe sample increase and adversely affect the thermoelectric promance.Vacancy formation energy increase after Ti/Sb doping.The Sb³⁺effectively reduces the carrier concentration,combined with the band degeneracy originate from Ti doping improves the electrical performance;dislocations,grain boundaries and second phases in the Ti/Sb-doped GeTe sample were found by using high-resolution transmission electron microscopy(HRTEM),combined with the point defects by doping and the mesoscale scattering originating from spark plasma sintering.Multiscale structure is formed,which scattering multiple phonons reduces the lattice thermal conductivity.Resonant states and phonon engineering were using to improve TE properties of GeTe-based materials.Resonant energy states successfully introduced by In doping,but the carrier concentration of In doped samples increases.In addition,the low solid solubility of In limits TE properties.Se as a dopant have no obviously effect on the electronic structure of GeTe,but point defect scatter and thermal scatter reduce the electronic conductivity after Se doping,which leads to decrease lattice thermal conductivity consistent with the Debye-Callaway model.In addition,the high thermal and electrical conductivity can be optimized by doping a large amount of Se due to high solid solubility of GeTe-Ge Se,which provides some new ideas for tuning TE properties of GeTe materials.Study the influence of crystal structure on the thermoelectric properties of GeTe materials.The results show that the phase transition temperature and the axis angle were changed by alloying Cu₂Se.Crystal structure engineering was successfully introduced and improves the power factor.But the influence on crystal structure is little due to the limitation of doping content;the Cu₂Se doping samples forms a layered structure reducing the lattice thermal conductivity.However,the abnormal increase in electrical conductivity leads to an increase in electronic thermal conductivity,which limits thermoelectric performance.The carrier concentration of high doping sample almost unchange,that one Cu ion replaces Ge ion and the other Cu ion fills the interstitial site.Increase of electric conductivity originates formd directional movement Cu ions at high-temperature.Finally,the formation mechanism of lattice defects in GeTe materials and the effect on phonon scattering through more advanced analysis techniques,n-type GeTe and the development and application of thermoelectric devices were briefly discussed.