Effect Of Side Chain On Electronic Structure And Thermoelectric Properties Of D-A Conjugated Polymer

Thermoelectric materials can convert heat energy directly into electricity through the seebeck effect.Compared with inorganic thermoelectric materials,organic thermoelectric materials have advantages of mechanical flexibility,light weight and non-toxic,achieve the optimal thermoelectric performance at lower temperatures which has a good development prospect in smart wearable and other fields.Doping high mobility with narrow band gap donor and acceptor（D-A）conjugated polymers is an effective method to develop high performance organic thermoelectric materials.P-type polymer materials lose electrons and are oxidized when doped,high HOMO is beneficial to improve the doping efficiency and the good miscibility with dopant provides a basis for the subsequent processing and further optimization of the performance.Therefore,in this paper,different series of D-A conjugated polymers were designed and synthesized using thienoisoindigo（TIIG）as the electron acceptor and donor unit with different side chains to regulate the electronic structure of the material.These polymers were doped by two common dopants,their thermoelectric properties were optimized and the effect of side chain structure was investigated.The main work of this paper is as follows:（1）Three narrow band gap D-A conjugated polymers were synthesized using bithiophene structure with different side chains and TIIG via Stille coupling alternate copolymerization.They were then doped with Fe Cl3 and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane（F4TCNQ）in different doping concentrations and were characterized.The results showed that HOMO level increased with the introduction of alkyl oxygen and ether oxygen side chain.After doping with Fe Cl3,the conductivity of the two polymers containing oxygen in the side chain increased faster and achieved the optimal thermoelectric performance at lower doping concentration.The two polymers containing oxygen side chain exhibited excellent miscibility with dopant and smoother surface morphology.Finally,ether oxygen side chain substituted polymer PTIIG-BT2EGOMe acquired the best power factor of 5.0μW m^-1 K^-2 at 40%molar ratio F4TCNQ doping concentration in this research.（2）Benzodithiophene（BDT）derivatives containing different conjugated length side chains were synthesized,and two novel two-dimensional D-A conjugated polymers（PTIIG-BDT-TO and PTIIG-BDT-T-TO）were obtained via alternate copolymerization with TIIG.They were also doped and characterized by Fe Cl3 and F4TCNQ respectively.Although both two polymers possessed strong plane rigidity structure,they showed good solubility in many solvents,the optimal power factors were 0.22 and 0.18μW m^-1 K^-2 after 12 m M Fe Cl₃ doped,respectively.