The First-principles Study On Thermoelectric Properties Of BiCuSeO-series Materials

Thermoelectric（TE）material can realize the direct conversion of heat and electricity.This characteristic is significant to deal with the energy crisis and environmental pollution.As an environment-friendly function material,TE material attracted much attention of researchers in recent decades.However,the conversion efficiency of TE material isn’t high enough,which limits its application.The theoretical calculation can help us predict new materials,and understand the electrons and phonons’behavior.BiCuSeO has attracted much attention due to its low intrinsic thermal conductivity,there are few reports about its superlattice structures and low dimensional BiCuSeO.This thesis is based on the first-principles and Boltzmann transport theory.The transport properties of electrons and phonons of BiCuSeO-derived materials are calculated.The physics mechanisms are analyzed and provide some theoretical reference to improve the TE performance.The main work of this thesis is as the following:1.The electronic structures and transport properties of superlattices of BiCuSeO are studied.Due to the low symmetry of superlattice,the electrical conductivity and electronic thermal conductivity decreased.The low symmetry also leads to strong anharmonic scattering and low lattice thermal conductivity.At 700 K,the power factor of Bi Cu Te O/BiCuSeO increased by 15%compared with BiCuSeO.Its lattice thermal conductivity reduced to 0.07 W/m K,which is a quarter of that of BiCuSeO.It can be predicted that Bi Cu Te O/BiCuSeO should have a much higher TE performance than BiCuSeO.2.The electronic structures and transport properties of monolayer BiCuSeO are studied.The fermi level passes through the conduction band of[Bi₂O₂]²⁺layer and valence band of[Cu₂Se₂]^2-layer,this leads to the monolayer BiCuSeO showing no band gap.First-principles calculation shows that negative hydrostatic pressure could weaken the intra-layer bonding and enhance the inter-layer coupling.Thus,the Fermi level would gradually get out of conduction and valence bands along with the increased pressure.To improve the TE performance of monolayer BiCuSeO,negative hydrostatic pressures are applied and successfully increased its band gap values from 0 e V to 0.85 e V.At the same time,an obvious band convergence near the conduction band minimum（CBM）is found under the-2.22 GPa pressure.Besides,the band convergence near the Fermi level under negative pressure will greatly improve the TE performance of n-and p-type monolayer BiCuSeO.3.We inserted 2 Cu atoms in the[Cu₂Se₂]^2-layer of monolayer BiCuSeO and investigated its electronic structures and transport properties under strain.The[Cu₂Se₂]^2-layer is lack 2 electrons,which leads to its fermi level crosses through the valence band.We inserted 2 Cu atoms in the[Cu₂Se₂]^2-layer to form monolayer Cu₂Se.The band gap value of monolayer Cu₂Se is 0.9 e V.The band structure is worth engineering to improve the power factor.The biaxial tensile strains are applied and induced band convergence near the fermi level.With the increase of tensile strains,the Cu-Se bonding shortened,and the anti-bonding states enhanced.The band gap value increased along with the increased strains.The large band gap under strain leads to a smallσ/τ,so the power factor of n-type monolayer Cu₂Se decreased.The enhanced Seebeck coefficient of p-type monolayer Cu₂Se increased its power factor by 15%compared with that without strain.