Investigation Of Thermoelectric Properties Of Two-Dimensional Triphosphides

Global warming and fossil fuel shortage are very serious,and the development and utilization of renewable clean energy have attracted extensive attention.It is estimated that over 60%of fossil energy is consumed in the form of waste heat during utilization.Thermoelectric materials can convert waste heat generated by thermal power plants and nuclear power plants into electric energy,which is expected to realize clean energy conversion.In recent years,with the extensive research on the excellent properties of graphene,lots of new two-dimensional(2D)materials emerged,such as black phosphorus,silicene,transition metal dichalcogenides,etc.Triphosphide is a new star in two-dimensional thermoelectric materials because of its semiconductor properties and excellent electronic transport properties,which can be widely used in thermoelectric devices.In this paper,combined with first-principles calculation and Boltzmann transport theory,the thermoelectric properties of new 2D materials triphosphide are studied,the influencing factors of thermoelectric properties of 2D materials are explored,the microscopic mechanism of phononic and electronic transport in materials is described,and the optimization method of thermoelectric transport of 2D materials is proposed.The research contents are as follows:(1)The thermoelectric properties of monolayer triphosphides are studied.It is proved that triphosphide is a dynamically stable indirect band gap semiconductor.At room temperature,the power factors of n-type doped AlP3,GaP3,and InP3 are 35.51 mW m-1 K-2,12.47 mW m-1 K-2,and 18.41 mW m-1 K-2,respectively.The lattice thermal conductivities of AlP3,GaP3,and InP3 at room temperature are 0.60 W m-1 K-1,4.47 W m-1 K-1,and 1.93 W m-1 K-1,respectively.The lattice thermal conductivities are lower than other 2D thermoelectric materials,which are attributed to the small phonon group velocity and phonon lifetime.At 700 K,the ZT values of n-doped AlP3,GaP3,and InP3 are 8.33,2.09,and 4.22,respectively.This work demonstrates the outstanding thermoelectric properties of the new 2D material triphosphide and promotes the development of thermoelectric technology.(2)The effect of biaxial tensile strain on the thermoelectric properties of monolayer AIP3 is studied.By calculating phonon dispersion curves,it is proved that the material is still stable under biaxial tensile strain.The electronic transport properties are calculated,and it is found that the electron mobility increases first and then decreases with the increase of tensile strain.At a 4%tensile strain,the power factor of monolayer AIP3 increases significantly,indicating that strain can improve the power factor of monolayer AlP3.Furthermore,strain increases lattice thermal conductivity of AlP3 due to higher phonon lifetime compared to unstrained AlP3.The maximum ZT values of monolayer AlP3 under 0%,2%,4%,6%,and 8%tensile strains at 300 K are 4.26,3.15,3.52,0.48,and 0.24,respectively.Under 4%tensile strain,the maximum energy conversion efficiency of n-type doped monolayer AlP3 can reach 29.4%at 700 K.The results show that monolayer AlP3 has great potential in the application of flexible thermoelectric devices.(3)The effect of doping on the thermoelectric properties of triphosphide is studied.Three doping structures Al0.5Ga0.5P3,Ga0.5In0.5P3,and Al0.5In0.5P3 are constructed,and the stabilities of the doping structures are proved by phonon dispersion curves.It is found that the power factor of Al0.5Ga0.5P3 and Al0.5In0.5P3 is significantly higher than that of AlP3,which is attributed to the improvement of electrical conductivity by doping.In addition,the electronic thermal conductivity of Al0.5In0.5P3 is one order of magnitude higher than that of the other two doped structures.The ZT values of the three doped materials are obtained by combining the lattice thermal conductivity.At 300 K,the maximum ZT values of Al0.5Ga0.5P3,Ga0.5In0.5P3,and Al0.5In0.5P3 are 4.82,0.95,and 8.65,respectively,which are increased by 2.03,7.65,and 5.73 times after construction of the doping structure.It is proved that doping can further optimize the thermoelectric properties of triphosphides.