The First-principles Study On The Thermoelectric Performance Of The SiPb,AlBiTe₃,Y₂GeI₂ Monolayers

Thermoelectric generator（TEG）converts heat into electricity to give the energy to devices with lower power by Seebeck effect,it not only is the one of the application in renewable energy technology,but can convert heat from metabolism of human body into electricity to provide energy for lower-power equipment operating at room temperature,such as wearable devices,medical or health care equipment and communications module in sensors and so on,so that it can eliminate many effects from traditional batteries used in these devices.Ordinary thermoelectric chips of TEGs are made up of polymers,whose figure of merit（ZT）value is usually lower,so that the output power is not enough,consequently,it is remarkably meaningful to find new thermoelectric materials with higher ZT value at room temperature to apply to TEGs.Based on first-principles and density function theory with semi classical Boltzmann transport theory,this thesis establish relationship between ZT value and carrier concentration at different temperature of the Si Pb,Al Bi Te₃ and Y₂Ge I₂ monolayer,and confirm more precise ZT value of materials by Fermi-Dirac distribution function to calculate carrier concentrations and bipolar effect,and provides research about experiment with theoretical basis.The main content contains:1.The geometrical structure of Si Pb monolayer material is optimized by HSE06hybrid functional.The thermal and dynamic stability can be confirmed by ab initio molecular dynamic simulation and phonon dispersion carve.The lattice thermal conductivity can be acquired by Sheng BTE code with second and third force constants calculated by first-principle.Based on the energy bands calculated by HSE06 and dense k points（20×40×1）,the transport coefficients（Seebeck coefficient,electrical conductivity,electronic thermal conductivity）can be obtained through solving electronic semi-Boltzmann transport equations,and the relationships among figure of merit ZT,carrier concentrations and temperature are established.By using Fermi-Dirac distribution functions,energy bands calculated by HSE06 and density of state,the two types of carriers and corresponding ZT value are calculated.Then the reduced ZT_bp value can be confirmed according to the bipolar effect.The results show:ZT_bpof Si Pb monolayer reach 0.666/0.907 along x/y direction at 310 K,which is larger than common polymers as the thermoelectric material applied to wearable devices and implantable bioelectronics,this denotes that Si Pb monolayer is pretty potential in the thermoelectric chips of wearable devices and implantable bioelectronics.2.The intrinsic carrier concentration by calculation is not the one that the ZT peak corresponds,but it can be close to maximum by adding the dope element to adjust carrier concentration,at the same time,appropriate dopes can reduce lattice thermal conductivity.Consequently,we study the thermoelectric performance of Al Bi Te₃ monolayer adding Al to the Bi₂Te₃.Firstly,thermoelectric performance of Bi₂Te₃monolayer are calculated,After calculating intrinsic carrier concentration and considering bipolar effect,the results is that the ZT_bp=0.23（x direction）and 0.26（y direction）at 300 K,this results conform to reported experiment data,which denotes the calculated methods is reliable.The transport coefficients and lattice thermal conductivity of doping Al Bi Te₃ monolayer use the same calculated method,the results show that the lattice thermal conductivity decrease clearly,especially the x direction decline remarkably,it’s almost 10 times.The power factor of Al Bi Te₃ improve according to the result of transport coefficient.ZT_bpreach 1.650 along x direction and 1.926 for y direction,which are larger than those of Bi₂Te₃,so it is definite that adding Al element to Bi₂Te₃ can enhance thermoelectric performance.3.Considering deformation potential theory often overestimate relaxation time though calculated amounts are more less,we adopt more precise method of electron-phonon coupling to calculate relaxation time of carriers.Therefore we use these two methods to calculate relaxation time of Y₂Ge I₂ monolayer.The results suggest the relaxation time calculated by deformation potential theory is apparently higher than those of electron-phonon coupling,which denotes deformation potential theory definitely overrate relaxation time.The result of ZT relative to temperature demonstrates,at higher temperature,ZT_bpare very lower as a result of significant bipolar effect,but luckily bipolar effect is not clear at lower temperature.At 300 K,ZT_bpreach 1.061/0.932 along x/y direction（the results corresponding deformation potential theory reach 1.673/1.435）,which demonstrate that Y₂Ge I₂ monolayer is promising as the thermoelectric material of wearable devices and implantable bioelectronics.Besides,Y₂Ge I₂ monolayer is not like Si Pb monolayer which possesses toxic element,do not required special treatment,so the Y₂Ge I₂ monolayer has more application prospect.