Design And Synthesis Of Thiophene-type Polymer Semiconductors And Their Thermoelectric Performance Optimization

Thermoelectric materials are a kind of green and clean energy materials,which have received extensive attention from academia and industry in recent years.Organic thermoelectric materials have attracted the interests of researchers due to their ability of low cost,solution processability,and low thermal conductivity.Polymer thermoelectric materials show promising potentials in wearable self-powered devices due to their low toxicity,light weight,flexibility,and large-area processability.The research of polymer thermoelectric materials has been an important direction in organic thermoelectric materials.However,compared with traditional inorganic thermoelectric materials,the performance of polymer thermoelectric materials cannot meet the requirement of practical applications.The main reasons include that the lack of guidelines for the structural design of polymer thermoelectric materials,the unclear doping mechanism,and the unclear relationship between structure and performance.The key point to improve the thermoelectric properties of polymers is to optimize the electrical transport properties which is closely related to the chemical structure and morphology of polymer.Therefore,developing new polymer thermoelectric materials to reveal the effect of chemical structure on the arrangement of polymers and charge transport,and establishing the intrinsic relationship between material structure and thermoelectric properties are still important challenging for organic thermoelectric materials.Based on the above problems,this thesis will focus on the design and synthesis of thiophene-based polymers,and study the effect of polymer chemical structure on molecular arrangement,and then analyze the effects of doping method,solvent,and annealing temperature on the microstructure and electrical property of polymer films.These works will provide ideas for the structural design and performance optimization of new polymer thermoelectric materials.The main research contents are as follows:1.A series of thiophene-based polymer semiconductors were designed and synthesized.A new doping method named of sequential-light-doping is proposed.The polymers are lightly doped twice successively.The GIWAXS results show that,compared with the traditional heavy doping method,sequential-light-doping can avoid self-aggregation of dopants and reduce the impact on the polymer packing.Therefore,the conductivity of the doped film is higher than that of the traditional doping method.The polymer PODTT-4T with the largestπ-conjugation length exhibits the highest conductivity(408 S cm^-1)due to its high mobility and high carrier concentration.The experimental results show that the first step of light doping（pre-doping）makes the dopants intercalating in the amorphous region,while the second step of light doping（post-doping）does not interfere with the morphology and crystallinity of the pre-doped films and effectively dope the crystalline region.The efficient doping of the crystalline and amorphous regions of the thin film can reduce the charge transport barrier and realize the synergistic optimization of doping efficiency and carrier concentration,and thus significantly improve the electrical conductivity.This study demonstrates that the sequential-light-doping method is an effective strategy to optimize the doping efficiency and maintain good crystallinity of polymer films.2.Further,we investigated the effect of solvent on the morphology of polymer films and how the solvent-dependent morphology affects the chemical doping and charge transport.We chose two solvents,chloroform（CF）and chlorobenzene solvent（CB）,to dissolve polymer PODTT-4T and then drop-cast the solutions into thin films.The results show that the film prepared by chloroform(PODTT-4T_CF)mainly has edge-on orientation,while the film prepared by chlorobenzene(PODTT-4T_CB)has both edge-on and face-on orientations.Doping the films with Fe Cl₃,the doping degree is independent on the solvent,but the PODTT-4T_CF film shows a larger crystal size upon doping,which is beneficial to carrier transport and thus a higher conductivity(444 S cm^-1).This study demonstrates that the conductivity can be optimized through proper solvent selection by adjusting molecular orientation.of polymers3.Temperature annealing treatment is also an important method to optimize the microstructure of polymers.In order to explore the effect of temperature on the electrical transport properties of doped films,we designed and synthesized two bithiophene polymers,PODTT-TT and PODTT-TVT.The difference between these two polymers is that the latter one contains vinylidene in the backbone.The experimental results show that the polymer PODTT-TVT with vinylidene structure is easier to be doped and the PODTT-TVT film shows a higher doping level under the same doping conditions.It was found that the conductivity of the doped film increased with the increase of the annealing temperature below the melting temperature while the conductivity decreased with the increase of the temperature above the melting temperature.The highest conductivity of doped PODTT-TVT film is 376 S cm^-1,which is 1.3 times that of the PODTT-TT film(280 S cm^-1).That means the appropriate annealing temperature of doped films can be selected according to the melting temperature of the polymer.Hall experiments show that the carrier concentration can be reduced while the mobility can be improved by temperature-annealing treatment.As a result,the electrical conductivity will be optimized.