Higher conductivity in doped ethylenedioxythiophene (EDOT) dimers with chalcogen-substituted end groups

Abstract

Conductive polymer, doped poly(3,4-ethylenedioxythiophene) (PEDOT) is a commonly employed material owing to its high conductivity, flexibility, and low-energy fabrication process. However, the broad dispersity poses challenges for understanding the conduction mechanism and designing molecules based on structure–conductivity relationships. To address this, we have focused on monodisperse oligomer conductors that provide detailed structural information owing to their crystallinity and structure control parameters involving the type, number, and sequence of monomer units, which enable the control of their electronic structures and physical properties. In this study, we investigate the effect of end groups, as another structure control parameter, on doped 3,4-ethylenedioxythiophene (EDOT) dimers, the simplest oligomers, by comparing methylthio and methylseleno groups. Substituting heavier chalcogens, from S to Se atoms, at the end groups effectively improves the electrical conductivity of doped EDOT oligomer conductors. This is due to the expansion of molecular orbitals and extension of the conjugate area of the donor, which strengthen the intermolecular interactions and bandwidth while weakening the effective Coulomb repulsion between conducting carriers. Additionally, Se-substituted end groups can induce a multiorbital system in the band structure, where the highest occupied molecular orbital (HOMO) and second HOMO (HOMO−1) bands contribute to electronic properties. Therefore, the electrical conductivities of Se-substituted oligomer salts at room temperature improve by one order of magnitude in relation to those of S-substituted oligomer salts. This approach offers guidance for designing conductors based on end groups, resulting in diverse crystal structures and electronic states that enhance conductivity.