Theoretical Investigations On The Organic Thermoelectric Materials

Searching for high-efficiency innovative organic thermoelectric materials is highly desirable.Recently,unprecedented experimental progresses have been made in optimizing the figure of merit and design of organic thermoelectric materials.Theorical understanding of organic thermoelectric conversion is essential for materials design.In organic solids,the weak van der Waals forces govern the molecular intractions,leading to complex microstructure.Accordingly,the macroscopic transport properties display complicated and versatile characteristics.In this dissertation,thermoelectric properties for the state-of-the-art organic materials were investigated based on density functional theory and classical molecular dynamics simulations.Some pivotal issues,such as doping effect and heat transport were also studied theoretically.Firstly,we predicted the thermoelectric performance of organic small molecular semiconductor with prominent hole mobility,2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene derivatives?C_n-BTBTs?,under the band transport model,using Boltzmann transport theory and deformation potential model.The lattice thermal conductivity was estimated through classical non-equilibrium molecular dynamics simulations.We validate that crystalline C_n-BTBT possesses high mobility and low lattice thermal conductivity,which is in favor of decent thermoelectric properties.There is no doubt that introducing alkyl side chain in organic molecular crystals is an effective strategy to realize high mobility and low lattice thermal conductivity,thus improve figure of merit.The calculated Seebeck coefficient and mobility have been confirmed by the recent experimental measurement.Tuning carrier concentration via chemical or electrochemical doping is one of the most successful strategies to optimize the figure of merit.We took poly?3,4-ethylenedioxythiophene??PEDOT?as an example to unravel the doping effects on geometry configuration,electronic structure and thermoelectric transport property.We corroborate that the PEDOT exhibits a distinct transition from the aromatic to quinoidic-like structure of backbone,and a semiconductor-to-metal transition with increasing the level of doping.By incorporating both acoustic phonon and ionized impurity scattering,we uncover that the ionized impurity scattering is predominant in the doped PEDOT:Tos.The calculated mobility,Seebeck coefficient and power factors of doped PEDOT:Tos are consistent with the experimental data.The lightly doped PEDOT:Tos exhibits power factor superior to the heavily doped one.Hence,we suggest that the carrier concentration ought to be controlled around that of lightly doped one.Lattice thermal conductivity is a critical parameter for thermoelectric materials.We conducted classical non-equlibrium molecular dynamics simulations to explore the lattice heat transport of PEDOT.We find that the crystalline PEDOT manifests outstanding heat transport in the polymer backbone direction,yet poor in the interchain directions.The intrachain figure of merit of crystalline PEDOT is therefore very poor in spite of its excellent power factor.We put forward to design and tailoring the chain length and crystallinity reasonably to engineer thermal transport without degrading the electrical transport properties in the polymer backbone direction,thus enhance figure of merit.The designed chain-oriented amorphous PEDOT fibres possesses low lattice thermal conductivity of 0.97 W m^-1 K^-1,and high figure of merit of 0.48.