Significant Enhancement in the Thermoelectric Properties of PEDOT: PSS Films Through Lone Pair-π Coupling with...

Organic TE materials have gained considerable attention due to their advantages, including low cost, high mechanical flexibility, and low thermal conductivity. However, their TE performance, particularly the Seebeck coefficient, remains notably lower than that of their inorganic counterparts. In this study, the TE properties of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS), which is the most popular TE polymer, are significantly enhanced by depositing a thin layer of polyethylenimine ethoxylated (PEIE). This can enhance the Seebeck coefficient of acid-treated PEDOT: PSS from 18.0 to 59.5 µV K^{- 1} while lowering the electrical conductivity from 3080 S cm^{- 1} to 1443 S cm^{- 1} when the PEIE layer thickness is counted. The power factor is enhanced from 100 µW m^{- 1} K^{- 2} to 511 µW m^{- 1} K^{- 2}. Without the consideration of the PEIE layer thickness, the electrical conductivity and the power factor are 2004 S cm^-1 and 709 µW m^-1 K^-2, respectively. The significant enhancement in the Seebeck coefficient is attributed mainly to the polaron level splitting, which is caused by the coupling between the lone pair electrons on the nitrogen atoms of PEIE and the conjugated PEDOT chains. This interaction can raise the Fermi level and thus enhance the Seebeck coefficient while preserving relatively high electrical conductivity.