Progress of Strategies for Improving Thermoelectric Performance of Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate)

In the recent decade, due to their versatile advantages of solution processability, light-in-weight, super flexibility, low thermal conductivity and highly adjustable molecular structures or components, organic and organic/inorganic composite thermoelectric materials as well as their flexible devices have made great progress. A variety of judicious strategies of both material preparation and flexible device assembly have been developed to improve the corresponding thermoelectric performances. Hence, they are very promising for wide application fields, including harvesting of waste heat, flexible electronics, soft robotics and internet-of-things. So far, in all of the reported organic polymer and composite thermoelectric materials, poly(3,4-ethylene dioxyethiophene):poly (styrene sulfonate) (PEDOT:PSS) is probably the most frequently studied and most successful system with the highest thermoelectric performance. Nevertheless, compared with inorganic thermoelectric materials, the thermoelectric properties of PEDOT:PSS is distinctly lower than the maximum value of the inorganic counterparts. Considering the fact of few special reviews about the strategies to effectively enhance the thermoelectric performances for PEDOT:PSS to date, it is timely and urgent to publish a relevant review. Here, we present a summarization of the recent progress in the development of strategies to significantly enhance the thermoelectric properties of PEDOT:PSS developed so far. First, the advances of the doping/de-doping strategies, such as second doping/de-doping, acid or alkali treatment, and treating by ionic liquid, are focused in details. The corresponding mechanism is discussed as well. Then, the recent developments of three types of boosting strategies, i.e. tuning of aggregation structures (including crystalline and orientation microstructures), construction of PEDOT nano-micro structures (for example, nanospheres, nanorods, micro- or nano-tubes and nanofibers), and fabrication of thermoelectric composites with inorganic particles such as carbon nanotubes and graphene nanoplatelets, are concentrated. The relevant thermoelectric properties are compared. Finally, we conclude that some strategies to effectively enhance the thermoelectric properties for PEDOT:PSS have been successfully developed, and there is still a long way to go for the actual applications. In particular, three tentative suggestions to the future investigations are proposed, i.e. in-depth investigation of the mechanism between microstructure and thermoelectric performances for the neat PEDOT:PSS, developing novel fabrication procedures with strong interfacial interaction like covalent-bonding and the relevant mechanism study, and exploiting of precise measurement techniques for the thermoelectric parameters of films, hydrogels and aerogels. Finally, the prospects of the future work are outlooked.