Study Of Thermoelectric Performance Of Polymer Semiconductor Materials Based On Thiophene Unit

Thermoelectricity is a kind of new and clean energy conversion technology,for which the energy conversion process is based on the charge carrier transport within the materials.Thermoelectric device has no moving parts,and is featured with small size,long service life and no emission.Among thermoelectric matrials,it is the inorganic thermoelectric materials thar are most widely studied.While the toxicity,brittleness,and limited resource impede their development and application.Compared to the inorganic counterparts,organic thermoelectric materials feature flexibility,large-scale production,light weight and abundant raw materials,and become new focus in the research of thermoelectric materials.Up to now,the thermoelectric properties of organic thermoelectric materials are universally inferior to inorganic materials,and need to be promoted.The rather complex microstructure of organic materials makes it challengable to apply the well-established semiconductor physics theories to the guidance for optimized thermoelectric performance of organic materials.It will be of great significance to study the effects of molecular structure,energy level alignment,aggregate structure,polarons state and doping process on the thermoelectric performance of organic materials based on proper materials systems.The universal structure-property-performance relationship will be a powful tool for conducting the optimization of thermoelectric performance of organic materials.Polymer semiconductors based on thiophene unit are chosen for the study to achieve optimized power factors.Both composite materials pellet and materials in thin film are studied.For composite materials pellet,the addition of electrical conductive nanophase and annealing process lead to enhanced thermoelectric performance.For materials in thin film,based on the molecular structure design and chemical doping,the thermoelectric properties are studied and optimized.The main contents and findings are as follow:Based on natural graphite powder and 3,4-ethylenedioxythiophene?EDOT?monomer,the reduced graphene oxide?rGO?was obtained by chemical oxidation process followed by chemical reduction process,and the PEDOT/rGO composite powder was further prepared by in situ micro-emulsion polymerization.The control of morphology in nanoscale,the formation process for pellet,the effects of additive amount of rGO and annealing process on the microstructure and thermoelectric performance of PEDOT pellets are systematically studied.The findings are:?1?Few-layer graphene oxide?GO?with less defects can be obtained by adjusting the mass ratio of graphite powder to oxidant KMnO₄ to 1:1;?2?For the reduction process of GO,hydrazine hydrate is superior reductant compared with NaBH₄ since the former keeps the matrix structure and planarity of graphene preferably;?3?PEDOT nano-powder and PEDOT/rGO composited powder can be prepared by micro-emulsion polymerization,and the optimized annealing temperature for pellets compressed from powder is 200^oC;?4?PEDOT nanoparticles distribute on the surface of rGO,and proper additive amount of rGO can promote the electrical conductivity and Seebeck coefficient of PEDOT pellet,with 0.44 wt%rGO addition enhancing the power factor of PEDOT pellet by 4 times;?5?Annealing process increases the power factor of PEDOT pellet by 16 times,reaching 0.29?W m^-1 K^-2.From the perspective of molecular structure design,a series of poly?3-alkylthiophene??P3AT?are designed and synthesized on the basis of poly?3-hexylthiophene??P3HT?,with carbon atom in side-chain of P3HT sequentially replaced by oxygen atom.The effects of side-chain modification on the molecular regioregulairty,photophysical properties,energy level,electrochemical properties and aggregation structure in thin film state of P3ATs were systematically studied.The results show that as oxygen atom moves to the end of side chain,the variation within P3ATs are:?1?The effect of electro-withdrawing inductive effect subjected to thiophene ring decrease,the E_ox^onset of P3ATs decrease,and the HOMO level increases,making the polymer easier to be oxidized;?2?The out of plane?OOP?lamellar stacking distance shrinks while the in plane?IP??-?stacking distance decreases first and then expandes with P3POET almost displaying the longest OOP lamellar stacking distance and the shortest IP?-?stacking distance;?3?All the polymers feature edge-on orientation manner,and the edge-on orientation crystallinity of P3POET is the strongest.With varied chemical doping levels,the film morphology,solid-state ordering,charge carrier state,backbone structure,and thermoelectric performance of P3ATs were systematically studied.The findings are:?1?As doping level increases,sulphur element with higher chemical value appears,indicating the nature of thiohene ring losing electrons;?2?Within the doping levels studied,only polarons form in P3BOMT and P3EOPT films,while bipolarons form after polarons in P3POET,P3HT and P3MOBT films;?3?At the doping level corresponding to the highest power factor,P3POET film shows the strongest edge-on orientation crystallinity,closer edge-on oriented?-?stacking distance and weakest face-on orientatation crystallinity,which are all in favor for in-plane charge carrier transport;?4?At the optimizaed doping level,P3POET film shows relatively high Seebeck coefficient and also high electrical conductivity with the highest optimized power factor as 19?W m^-1 K^-2.Two-dimensional donor-acceptor copolymer PTB7-Th with low band gap was chosen to study the effects of doping level on film morphology,aggregation structure,charge carrier state,energy level alignment,backbone structure,and thermoelectric performance.The electron loss behavior of copolymer under doping process was also investigated.The findings are:?1?Within the doping levels studied,only polarons form in PTB7-Th films and the polarons level,Feimi level stay unchanged as doping level increases;?2?During the oxidation process,the donor part of PTB7-Th gets oxidized earlier than the acceptor part;?3?PTB7-Th film presents preferable face-on oriented molecular packing,the in plane lamellar stacking distance decreases first and then expandes while the out of plane?-?stacking distance increases first and then decreases;?4?As a comprehensive result of morphology degradation,molecular packing variation and increased charge carrier concentration,PTB7-Th film immersed in 0.01 M FeCl₃/CH₃NO₂ solution for 10 min presents the optimizaed power factor at room temperature as 33.9?W m^-1 K^-2 and 38.3?W m^-1 K^-2 at 348 K,which are the highest reported power factors for donor-acceptor copolymers at room temperature and at high temperature,respectively.