Theoretical Study On Thermoelectric Properties Of Two-dimensional CSe

Thermoelectric materials have the ability to convert heat energy into electric energy,and the equipment made of thermoelectric materials can effectively use the waste heat emitted by automobiles and factories.This is one of the ways to alleviate the current energy crisis and reduce the environmental pollution caused by fossil fuels,so it is widely concerned in the field of renewable green energy.The conversion ability and efficiency of thermoelectric materials can be measured by power factorand thermoelectric figure of merit(5.High conversion efficiency depends on the high thermoelectric figure of merit,that is,high power factor and low thermal conductivity.Therefore,in order to make the thermoelectric equipment with high conversion efficiency,it is necessary to find new thermoelectric materials with high thermoelectric figure of merit,or to use energy band engineering and doping methods to improve the thermoelectric figure of merit in known materials.At present,there have been a lot of studies on layered group IV-VI compounds.Because of the large anharmonic effect,PdTe and SnSe bulk have good thermoelectric properties.The corresponding two-dimensional materials can be prepared by exfoliating,and have lower thermal conductivity,which can further improve the thermoelectric properties.However,these studies are concentrated in the rare heavy elements region instead of the earth-abundant elements.Based on the first-principles calculations and Boltzmann transport theory,the geometric structure,electron transport parameters,phonon transport parameters and thermoelectric properties of CSe sheet are studied.The CSe sheet is an indirect band gap semiconductor with optimized lattice constant of 3.07（?）and band gap of 1.51 e V.The calculation of phonon spectrum and ab-initio molecular dynamics simulations show that the dynamic and thermal stability of the CSe sheet.Based on the band structure calculated by first-principles calculations,the effective mass,elastic constant and deformation potential constant of electron and hole can be obtained by using the deformation potential theory,and then the carrier relaxation time and mobility can be obtained.Based on the semi-classical Boltzmann theory,the thermoelectric parameters such as Seebeck coefficient,electrical conductivity,power factor and electronic thermal conductivity at different temperatures are calculated.The absolute values of Seebeck coefficients of n-type and p-type CSe sheets are relatively small in the low temperature region.When the carrier concentration increases,the absolute values of Seebeck coefficients decrease,but when the carrier concentration is constant,the absolute values of Seebeck coefficients increase with the increase of temperature.The power factor first increases with the increase of carrier concentration.After reaching a peak,the power factor decreases with the increase of carrier concentration.In the whole carrier concentration range,the power factor decreases with the increase of temperature.When the electron concentration is about 1.17×10¹³ cm^-2,the maximum power factor of n-type CSe sheet is 618 to 483 m W m^-1 K^-2 in the range of 300 to 700 K.When the hole concentration is about 1.24×10¹³ cm^-2,the maximum power factor of p-type CSe sheet is9.87 to 7.42 m W m^-1 K^-2.The lattice thermal conductivity of the CSe sheet is 40.12 to17.26 W m^-1 K^-1 in the selected temperature range,which decreases with the increase of temperature.The contribution of different acoustic branches to the lattice thermal conductivity and the influence of phonon mean free path on the lattice thermal conductivity are further explored.When the phonon mean free path is in the range of 10to 1000 nm,the lattice thermal conductivity will change significantly.The thermoelectric figure of merit of n-type CSe sheet is 0.55 to 0.78 in the selected temperature range,and that of p-type CSe sheet is 0.07 to 0.20 at the appropriate carrier concentration.These high thermoelectric properties indicate that the CSe sheet made of earth-abundant elements may be a potential n-type two-dimensional thermoelectric material.