Thermoelectric Transport Mechanism And Performance Adjustment In Organic Molecular-scale Materials

With the development of miniaturization of electronic devices,the accumulated heat needed to be dissipated by other devices with the same size.Meanwhile,with the aggravation of the energy crisis,people are eager to find ways to promote energy efficiency at low cost.Organic molecular-scale thermoelectric materials can meet these two needs at the same time,so people pay more and more attention to them.In this dissertation,the mechanism of thermoelectric transport,thermoelectric properties and performance adjustment on organic molecular-scale materials are investigated.For the organic molecular devices,the effects of the contact geometry,the change of coupling between the electrode and the central molecule,the replacement of the bridging atom,the doping of the electrode and the destructive quantum interference on the electronic,phonon transport and thermoelectric properties of the device are studied.Meanwhile,the effects of the position of Boron-Nitrogen dimer,the angle of benzene ring and the change of dimension,the external pressure on the thermoelectric properties transport and thermoelectric conversion efficiency of organic conjugated framework and molecular crystal materials were examined,respectively.In addition,we also explored the feasibility and the enhancement of the thermoelectric efficiency for different types of organic molecular-scale materials by various regulatory means.In detail,we have made the following research works:First of all,we investigated the thermoelectric properties of phenalenyl-based molecular devices by using the non-equilibrium Green's function method combined with density function theory.The results show that the thermoelectric performance of molecular device can be significantly improved by different contact geometries.The ZT value of the device can reach 1.2 at room temperature,which is two orders of magnitude higher than that of graphene.Moreover,the change of the coupling between molecule and electrodes can also enhance the ZT value.The ZT value can be further optimized to 1.4 at 300 K and 5.9 at 100 K owing to the decrease of electronic thermal conductance and almost unchanged power factor.Then,by using the same method of density function theory combined with the non-equilibrium Green's function,the thermoelectric properties of para-Xylene-based molecular devices are investigated.It is found that the destructive quantum interference can be triggered in n-type of para-connected para-Xylene-based molecular device and can obviously enhance the thermoelectric performance of the devices.Moreover,bridge atom electrophilic substitution can significantly improve the thermoelectric properties of p-type monolayer molecular device.The ZT value of p-type monolayer molecular device with doped electrodes can be optimized to 2.2 at 300 K and 2.8 at 500 K,and n-type bilayer molecular device can achieve the value of 1.2 at 300 K and 2.0 at 500 K.These results offer the information to design the complete molecular thermoelectric device with p-type and n-type of components and to promote the thermoelectric properties of bilayer molecular junctions by employing destructive quantum interference effects.Next,by applying the method of density functional theory combined with Boltzmann transport equation,the thermoelectric properties of graphphenyl with Boron-Nitrogen dimer are studied.It is found that the adjustment of band gap and the improvement of thermoelectric performance can be realized by controlling the position of Boron-Nitrogen dimer.Moreover,the optimal thermoelectric performance direction can be switched by the Boron-Nitrogen dimer.The ZT values can reach to 0.6 in p-type and n-type thermoelectric materials at room temperature.In addition,the thermoelectric properties can be further controlled by rotating the benzene ring in these materials.This work provides a theoretical support for the design of electrodes and thermoelectric devices with the same material,and a new way to control the performance of such planar thermoelectric devices by twisting the benzene ring.Finally,the thermoelectric properties of two-dimensional molecular crystal of dibenzo-thieno-dithiophene are investigated by using a combination of density functional theory,deformation potential theory and Boltzmann transport equation.The results show that the toggle between optimal p-type and n-type thermoelectric materials can be realized by the change of doping type and work direction,showing the convenience of the same molecular crystal material to assemble the complete organic thermoelectric devices.The thermoelectric properties of the material at room temperature can reach to 0.5 for p-type and 0.4 for n-type,and can continue to be promoted as the temperature increases.In addition,it is found that the thermoelectric properties of this material can be further enhanced by external pressure,and the thermoelectric efficiency at room temperature can be improved by 5%.