Elucidating Design Rules toward Enhanced Solid-State Charge Transport in Oligoether-Functionalized Dioxythiophene-Based Alternating Copolymers

Abstract

This study investigates the solid-state charge transport properties of the oxidized forms of dioxythiophene-based alternating copolymers consisting of an oligoether-functionalized 3,4-propylenedioxythiophene (ProDOT) copolymerized with different aryl groups, dimethyl ProDOT (DMP), 3,4-ethylenedioxythiophene (EDOT), and 3,4-phenylenedioxythiophene (PheDOT), respectively, to yield copolymers P(OE3)-D, P(OE3)-E, and P(OE3)-Ph. At a dopant concentration of 5 mM FeTos₃, the electrical conductivities of these copolymers vary significantly (ranging between 9 and 195 S cm^-1) with the EDOT copolymer, P(OE3)-E, achieving the highest electrical conductivity. UV-vis-NIR and X-ray spectroscopies show differences in both susceptibility to oxidative doping and extent of oxidation for the P(OE3) series, with P(OE3)-E being the most doped. Wide-angle X-ray scattering measurements indicate that P(OE3)-E generally demonstrates the lowest paracrystallinity values in the series, as well as relatively small π-π stacking distances. The significant (i.e., order of magnitude) increase in electrical conductivity of doped P(OE3)-E films versus doped P(OE3)-D or P(OE3)-Ph films can therefore be attributed to P(OE3)-E exhibiting both the highest carrier ratios in the P(OE3) series, along with good π-π overlap and local ordering (low paracrystallinity values). Furthermore, these trends in the extent of doping and paracrystallinity are consistent with the reduced Fermi energy level and transport function prefactor parameters calculated using the semilocalized transport (SLoT) model. Observed differences in carrier ratios at the transport edge (c_t) and reduced Fermi energies [η(c)] suggest a broader electronic band (better overlap and more delocalization) for the EDOT-incorporating P(OE3)-E polymer relative to P(OE3)-D and P(OE3)-Ph. Ultimately, we rationalize improvements in electrical conductivity due to microstructural and doping enhancements caused by EDOT incorporation, a structure-property relationship worth considering in the future design of highly electrically conductive systems.

Keywords: charge transport; dioxythiophene polymers; oligoether side chains; solid-state electrical conductivity.

Figure 1

(a) Structures of the P(OE3)-D, P(OE3)-E, and P(OE3)-Ph with conductivity values (σ) and Seebeck coefficients (S) measured at a doping concentration of 5 mM FeTos₃ in acetonitrile (ACN). (b) Top-view and side-view visualization of XDOT units (left to right: DMP, EDOT, and PheDOT) illustrate differences in XDOT unit planarity and oxygen lone-pair p-orbital orientations. Figure is reproduced with permission from ref (39).

Figure 2

Measured electrical conductivities of blade-coated P(OE3) films doped with FeTos₃ on glass substrates.

Figure 3

Composite UV–vis–NIR spectra for (a) P(OE3)-D, (b) P(OE3)-E, (c) P(OE3)-Ph films doped with different concentrations of FeTos₃/ACN for 1 min. Note: as the as-cast blade-coated P(OE3)-E films demonstrated significant oxidation in air (consistent with our previous study), P(OE3)-E films were treated with hydrazine vapors to achieve a similarly reduced state to the as-cast blade-coated P(OE3)-D and P(OE3)-Ph films, prior to being chemically doped with FeTos₃.

Figure 4

Charge carrier ratio as a function of FeTos₃ dopant concentration for films of the P(OE3) copolymer series.

Figure 5

Radially integrated GIWAXS profiles of pristine films and 0.125, 0.5, 5, and 50 mM FeTos₃-doped films of (a) P(OE3)-D, (b) P(OE3)-E, and (c) P(OE3)-Ph. Shaded areas show regions of the (100) and (020) features. (d) π–π spacings calculated from the GIWAXS profiles as a function of dopant concentration.

Figure 6

Charge transport analysis of the P(OE3) series. (a) Thermoelectric values of the P(OE3) series depicted with an S–σ plot. Points left to right indicate increasing doping concentrations with FeTos₃ (0.125, 0.25, 0.375, 0.5, 1, 5, 50 mM). Error bars represent ± one standard deviation from at least three unique films at that doping level. (b) Graphical illustration of the SLoT transport parameters. η, the reduced Fermi energy level, is dependent on the density of electronic states, g(E), indicated by the blue curve. As c increases, η and σ increase, while absolute Seebeck, |S|, decreases. σ₀, the doping- and energy-independent transport function prefactor, is suggested to be constant for a polymer-dopant-processing system. (c) SLoT model analysis of the reduced Fermi energy as a function of carrier ratio, η(c). The inset depicts the η(c) values near the transport edge, E_t. (d) Average σ₀ values for each polymer system. The nominal σ₀ value represents the polymer average across multiple doping levels and films, and the error bars represent ± one standard deviation.