First-principles Study Of The Effect Of Strain And Defect On The Thermoelectric Properties Of Ge₂Sb₂Te₅

With the increasing demand for fossil fuels in modern society,a series of issues related to human sustainable development such as energy crisis and environmental pollution have gradually emerged,causing widespread concern worldwide.Therefore,thermoelectric materials that can directly convert thermal energy into electrical energy through the migration of charge carriers have become functional materials with great research value and application potential.Among them,Ge₂Sb₂Te₅has become one of the candidates for high thermoelectric performance due to its excellent electrical transport properties and very low lattice thermal conductivity.Theoretical and experimental studies have found that Ge₂Sb₂Te₅has a layered crystal structure and significant anisotropy,and there are a large number of intrinsic point defects in the actual crystal.Therefore,this thesis mainly explores its higher thermoelectric performance through first-principles research on strain and point defects.The research content is as follows:（1）The impact of uniaxial strain on the electronic structure and thermoelectric properties of Ge₂Sb₂Te₅was studied in this research.When a 3%strain is applied along the（001）direction,Ge₂Sb₂Te₅reaches its optimal bandgap at 700 K,and multiple conduction band minima converge,leading to increased participation of multiple conduction bands in charge transport and improved carrier mobility.The tensile strain also increases the bandgap and electron effective mass,resulting in an enhanced Seebeck coefficient and a decoupling of electrical properties.However,the strain negatively affects the power factor of p-type Ge₂Sb₂Te₅due to its single valence band.Both p-type and n-type Ge₂Sb₂Te₅exhibit a significant reduction in lattice thermal conductivity at 3%strain due to enhanced optical phonon scattering and reduced acoustic phonon frequency.At 700 K,a 3%strain produces the maximum enhancement in the thermoelectric performance of Ge₂Sb₂Te₅,with p-type and n-type Ge₂Sb₂Te₅exhibiting a 30%and 424%improvement in their thermoelectric figure of merit,respectively.Therefore,strain is a promising approach to optimizing the thermoelectric performance of systems with multi-valley conduction or valence bands.The influence of intrinsic point defects on the electronic structure and thermoelectric properties of Ge₂Sb₂Te₅was investigated in this research.Vacancies in Ge and Sb,and antisite defects of Sb _Ge and Te _Ge are prone to spontaneous generation.The formation energies of V_Te,Ge _Sb,Ge _Te,and Te _Sb are relatively high,making them difficult to form,while Sb _Te has a formation energy ranging from-1 to 0.3 e V and has a tendency to form spontaneously.V_Ge,V_Sb,Ge _Sb,Ge _Te,and Sb _Te cause the Fermi level to enter the valence band,resulting in p-type Ge₂Sb₂Te₅.Sb _Ge,Te _Ge,and Te _Sb cause the Fermi level to enter the conduction band,resulting in n-type Ge₂Sb₂Te₅.All defects except V_Ge and V_Sb cause a reduction in the bandgap.Low concentrations of V_Ge and V_Sb are beneficial for obtaining better thermoelectric properties.Ge _Sb has no significant effect on the electronic structure but can improve the thermoelectric performance of n-type Ge₂Sb₂Te₅due to the reduction in effective mass caused by the decrease in bandgap.V_Te,Ge _Te,Te _Ge,and Te _Sb result in extremely low Seebeck coefficients and high carrier mobility.At room temperature and low temperature,Ge _Te is advantageous for the power factor of n-type Ge₂Sb₂Te₅,while Te _Ge and Te _Sb are advantageous for the power factor of p-type.However,V_Te is not conducive to thermoelectric performance.At high temperatures,the thermal conductivity of electrons increases,resulting in a significant decrease in the thermoelectric figure of merit of V_Te,Ge _Te,Te _Ge,and Te _Sb.At room temperature,p-type Sb _Geand n-type Sb _Te exhibit better thermoelectric properties,while at high temperatures,they are detrimental to thermoelectric performance.For the preparation of practical crystals,vacancies in Ge and Sb should be controlled within a low concentration range to avoid the significant reduction in ZT caused by the generation of Te vacancies during Te _Ge and n-type doping.Additionally,Sb _Ge and Sb _Te antisite defects should be reasonably controlled to enhance the role of defects in improving thermoelectric performance.