Improving Thermoelectric Properties Of P-type Polymers Based On Simple-structure Thiophene Groups

Thermoelectric（TE）conversion technology can directly realize the conversion of thermal energy and electric energy,and has the unique advantages of no moving parts,no noise,and the ability to achieve thermal and electrical conversion even under small temperature differences.Compared with inorganic materials,organic semiconductors（OSCs）have the advantages of good mechanical tensile properties and solution processability,and are considered to be TE materials with great development potential and application prospects,which are more suitable for the preparation of flexible electronic devices.The TE properties of P-type conjugated polymers have achieved an order of magnitude improvement after years of development.However,such materials are still complex in synthesis,difficult to innovating high-performance structural units and polymers,and the relationship between structure and performance is urgently needed in-depth understanding.To further promote the development of organic TE materials,the development and research of novel OSCs structures is the key.The properties of new organic TE materials are inseparable from their molecular structures and thin film preparation methods.We use a simple and easy-to-prepare random copolymerization strategy to modify the side chain of the polymer,and use simple structural unit copolymerization to synthesize a variety of polymers.At the same time,in the film preparation process,the method of blade coating is used to obtain an orderly and tightly stacked polymer film by adjusting the substrate temperature and coating speed.Finally,the ferric chloride（FeCl₃）as the P-type dopant,the performance differences of the polymers were analyzed and optimized through various testing methods to obtain high-performance thermoelectric properties.The specific work is as follows:Primarily,we introduced a polar side chain-containing thiophene unit（g₃2T）into a copolymer of pyrrolopyrrolone and bis（5-（trimethyltinyl）thiophen-2-yl）ethylene（PDPP-TVT）by a simple random copolymerization method,a series of random copolymers PTVT-g₃2Tx were synthesized.The random copolymerization method can realize the gradual introduction of polar side chains to avoid the problem of poor solubility caused by the complete replacement of alkyl side chains by polar side chains;at the same time,the third unit（g₃2T）with polar side chains can improve the polarity of the polymer and the miscibility between host and dopants;the gradually introduction of g₃2T increased the highest occupied orbital（HOMO）energy level of the polymer because of the strong electron-donating ability of g₃2T,the higher HOMO energy is beneficial for P-type doping.It was found that the four polymers showed different chemical and physical behaviors after being doped with FeCl₃.The results of TE test showed that the conductivity and power factor of polymer PTVT-g₃2T_0.2 were significantly improved,and the power factor was 144.1μWm^-1K^-2.This work exhibited that the gradual introduction of polar side chains through random copolymerization can synergistically regulate the crystallinity and polarity of polymers,which is an effective method to improve doping efficiency.In the second work,we designd the structure of polymer based on the classical simple unit--thiophene.The thiophene unit structure was easy to adjust and the synthesis.We introduced ester functional groups into its alkyl side chain to make the polymer backbone smoother through the interaction between the unit structures.Then,two kinds of polymers with different conjugated backbones were designed and synthesized by introducing thiophene（T）units and thiophene（TT）units with different electron donating abilities and volumes.It was found that the polymer PTOTT-TVT with stronger electron donating ability and larger volume had higher HOMO energy level,significantly improved molecular chain coplanarity and tighter intermolecular stacking.The OFET device showed that the hole carrier mobility(μ_h=0.54 cm²V^-1S^-1)of the polymer PTOTT-TVT was 6 times higher than that of PTOT-TVT(μ_h=0.09 cm²V^-1S^-1).After being doped with FeCl₃,the polymer PTOTT-TVT also exhibited better TE properties,with an electrical conductivity of302.6 S cm^-1 and a power factor of 189.9μWm^-1K^-2,which was a characteristic of such a simple structure.The results indicated that the TE properties of polymers can be significantly improved by coordinating the intermolecular ordering of the backbone and side chains.