Roles of Polyethylenimine Ethoxylated in Efficiently Tuning the Thermoelectric Performance of Poly(3,4-ethylenedioxythiophene)-Rich Nanocrystal Films

The regulation of oxidation levels is of great importance as an efficient way to optimize the thermoelectric (TE) performance of conducting polymers. Many efforts have been devoted to the acquisition of a high TE performance for poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by oxidation/reduction post treatment to achieve an effective compromise. However, a strong oxidant/reductant is usually employed to tune the TE performance of PEDOT:PSS with high electrical conductivity (σ) and Seebeck coefficient (S), and it also presents a number of operational challenges depending on a fast reaction rate. Herein, nontoxic polyethylenimine ethoxylated (PEIE) served as a reductant to successfully realize an enhanced S for PEDOT:PSS, besides playing a significant anion-blocking role in enabling the efficient modulation of the oxidation level by sulfuric acid (H2SO4) with a longer operating time. Eventually, a good PEDOT-rich nanocrystal is achieved by a sequential dipping process in PEIE/ethylene glycol and H2SO4 solutions. The large TE power factor of 133 μW m–1 K–2 can be ascribed to the good formation of PEDOT-rich nanocrystals and an effective compromise between σ and S of PEDOT:PSS films. A mechanism was elucidated for the efficient regulation of σ and S enabling high performance of organic TE materials.