| As a green energy material that can directly convert between thermal energy and electrical energy,thermoelectric material is indispensable in the future to achieve sustainable development.However,due to the complex coupling relationship between the electrical and thermal parameters in the material,the improvement of its thermoelectric conversion efficiency is greatly limited.Therefore,it is particularly important to explore high-performance,inexpensive thermoelectric materials and to find strategies for collaboratively optimizing the thermoelectric parameters of devices.Organic molecular thin films with flexible,inexpensive and low thermal conductivity properties are a potential thermoelectric material.However,due to the complex transport characteristics of carriers in interchain and intrachain,it is extremely challenging to greatly improve the thermoelectric properties of organic molecular films.Attempts to determine the thermoelectric properties of individual molecules by constructing a composite system of nano-electrodes and molecules,which provides design rules for the development of efficient organic thermoelectric materials,with a view to subsequent translation of their enhanced functionality to self-assembled molecular layers.In this thesis,we use density functional theory combined with the non-equilibrium Green's function method to study the effect of molecular length,molecular-electrode coupling,neutral side-groups on backbone and moleculars magnetism on thermoelectric properties in composite system of nano-electrodes and molecules,and several potential strategies for simultaneously optimizing its thermoelectric performance are proposed.Firstly,we study the thermoelectric properties of graphene-finite length carbon chains-graphene junction.The results show that the phonon thermal conductance at room temperature is odd-even dependent with the change of carbon chain length.Because the covalent bond between carbon chain and electrode is conducive to phonon transmission,it is difficult to improve the thermoelectric properties of graphene junction by changing the length of carbon chain.Therefore,we adopt the ?-? coupling style between molecule and electrode.The results show that the thermal conductance of graphene junction is reduced by more than one order of magnitude.This is mainly due to the existence of weak van der Waals interactions which greatly inhibits the transmission of phonons.In addition,the power factor of ?-? coupling graphene junction is regulated by introducing the electrochemical gate or electrode doping,and finally the thermoelectric figure of merit is close to 4 near the Fermi level.Secondly,we studied the thermoelectric properties of graphene junctions based on different lengths of naphthacene and rubrene.The results show that the conductance of the two molecular junctions is almost the same,and there is no exponential decay as the period length increasing.However,the thermal conductance of the naphthacenebased graphene junction shows the “V” shape with the increase of the length,which may be due to the phonon wave interference changing from coherent to incoherent characteristics.Due to the presence of neutral phenyl-groups at the edge of the rubrene molecule backbone,the thermal conductance of rubrene-based graphene junction decreases exponentially with the increase of the length.Compared with that of the former,the thermal conductance of latter has reduced by nearly half,which is mainly due to the local vibration of the phenyl-groups blocking the phonon transmission along the backbone.It indicates that the introduction of neutral side-groups onto the backbone can not only maintain the charge transmission along the main chain,but also inhibit the phonon transmission,which provides a new strategy for cooperative optimization of thermoelectric performance.When people study the thermoelectric properties of organic molecular devices,people always neglect a kind of molecules with intrintic magnetism,such as metallocene compounds containing transition metal elements or nano-graphenes with triangular topology.However,such molecules often have large Seebeck coefficients due to the presence of spin polarization.Therefore,we first studied the thermally induced spin current and thermoelectric properties of a metallocene-dimer molecular junction containing chromium,manganese,iron and cobalt,respectively.The results show that the thermally induced spin polarization current is generated in the molecular junctions containing chromium,manganese and cobalt,respectively.In addition,the thermally induced spin filtering effect and negative differential thermoelectric resistance phenomenon appear in the molecular junctions containing chromium and manganese.An interesting phenomenon is that the thermally induced pure spin current is obtained in the molecular junction containing cobalt,and the spin-dependent seebeck coefficient is much larger than that of the molecular junction containing chromium and manganese.This indicates that large spin thermopower can be generated by adjusting the generation of pure spin current in the molecular junction,which is a potential thermoelectric conversion mechanism.In addition,we also studied the thermoelectric properties of a class of nano graphene junctions with topological frustration.The results show that the dimer composed of two ferromagnetic trangulene molecules by ?-? stacking style appears antiferromagnetic properties.When two molecules in the ?-stacked dimers rigidly translate and rotate with respect to each other,the magnetism property of ?-stacked dimers changes from antiferromagnetism to ferromagnetism.This is because the overlap of charge density between layers is weakened,resulting in unequal numbers of different spin directions electrons occupied in the space orbit.More importantly,we found that the molecular magnetic transition induced by translation and rotation enhances the thermoelectric properties of the ?-stacked dimer molecule.This provides a new way to improve the thermoelectric performance of organic molecular devices. |
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