The Simplest Model for Doped Poly(3,4‐ethylenedioxythiophene) (PEDOT): Single‐crystalline EDOT Dimer Radical Cation Salts

Abstract

Although doped poly(3,4‐ethylenedioxythiophene) (PEDOT) is extensively used in electronic devices, their molecular‐weight distributions and inadequately defined structures have hindered the elucidation of their underlying conduction mechanism. In this study, we introduce the simplest discrete oligomer models: EDOT dimer radical cation salts. Single‐crystal structural analyses revealed their one‐dimensional (1D) columnar structures, in which the donors were uniformly stacked. Band calculations identified 1D metallic band structures with a strong intracolumnar orbital interaction (band width W≈1 eV), implying the origin of the high conductivity of doped PEDOT. Interestingly, the salts exhibited semiconducting behavior reminiscent of genuine Mott states as a result of electron–electron repulsion (U) dominant over W. This study realized basic models with tunable W and U to understand the conduction mechanism of doped PEDOT through structural modification in oligomers, including the conjugation length.

Band-filling effects in single-crystalline oligomer models for doped PEDOT: 3,4-ethylenedioxythiophene (EDOT) dimer salt with hydrogen-bonded infinite sulfate anion chains

“…S9e, ESI †). The strong antiferromagnetic interaction and the observation of the spin-Peierls-like transition, as detected in the half-filled 2OÁA, 18 support the subtle deviation of the band-filling from the half-filled state, which was consistent with the results of the single-crystal structure and Raman spectroscopy analyses. The band-filling modulation may originate from the charge doping by either the protons shared between the hydrogensulfate anions or the inclusion of charged molecules such as oxonium cations and hydroxide anions hidden in the crystal voids.…”

“…2(b)) that has not been previously observed in doped oligoEDOT systems. 18,19 Considering the donor charge (d E 1) estimated from bond length analyses and Raman spectroscopy analyses, the hydrogen-bonded chains are likely to be mainly composed of HSO 4…”

“…Next, we calculated the band structure of 2O-HSO 4 to estimate W using first-principles calculations (OpenMX software 28 ) with a generalized gradient approximation of Perdew-Berke-Ernzerhof functional (GGA-PBE) 29,30 based on the single-crystal structures, wherein the contribution of U eff was not considered. The band calculation 31 Head-to-tail 18 Head-to-tail 18 Head-to-tail 18 Head-to-head Donors' interplanar distance a 3.462 Å 18 3.471 Å 18 3.481 Å 18…”

“…1). 18,19 The single-crystal XRD structural analyses revealed the 1 : 1 composition of donors and counter anions, where the donors were uniformly p-stacked with large W (E1 eV) comparable to that of typical molecular conductors (e.g., TTFÁtetracyanoquinodimethane (TCNQ) with W E 0.5 eV) and superconductors, 20,21 implying the origin of the excellent conductivity of doped PEDOT. However, the electronic properties of 2OÁA were still dominated by the much larger U eff compared to W (i.e., U eff /W 4 1), that leads to ''Mott insulating states'' where the carriers are localized by strong electron correlation with large U eff in the half-filling states, resulting in semiconducting behavior.…”

“…Nevertheless, such a band-filling controlling has scarcely been demonstrated in half-filled materials, 8 ). 18 Combination of the atomic-level structural information obtained through their XRD structural analyses and the results of the electrical and optical reflectivity measurements suggested that conductivity enhancement originates from the deviation of the band-filling from the half-filled state.…”