Aggregate structure evolution induced by annealing and subsequent solvent post-treatment for thermoelectric property enhancement of PEDOT:PSS films

Abstract

• Effect of aggregation structure evolution on thermoelectric performance is studied. • Thermal annealing induces notably increased PEDOT crystallinity. • Solvent treatment affects nanofibril formation, edge-on stacking, phase separation. • Power factor of the treated film is 216 times of that of the pristine. Organic polymer thermoelectric (TE) materials have developed rapidly in recent years in terms of strategies to improve their TE performance. However, the effect of molecular mechanisms on their TE performance remains yet to be unveiled. Insights into the evolution of crystalline microstructures of conjugated polymers are critical in determining their aggregate structure and hence their TE performance. Herein, an effective approach has been developed to modify the aggregate structures of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). During annealing and subsequent solvent post-treatment, phase separation between the conductive PEDOT and the insulating PSS occurs and the excess PSS in the skin layer is effectively removed. As a result, carrier mobility and carrier concentration are synergistically boosted via the formation of well-crystallized molecular morphology and the increase of PEDOT doping level, leading to significantly enhanced TE performance with an unprecedented power factor that is 216 times of that of the pristine samples. The corresponding molecular mechanisms are discussed in detail and the aggregate structure evolution route is thoroughly proposed. It is believed that the aggregate structure evolution proposed in this work and its influential mechanisms on TE performance can be extended to other polymers and potentially direct the future design of high-performance polymer-based TE materials.