Role of the Anion on the Transport and Structure of Organic Mixed Conductors

Abstract

Organic mixed conductors are increasingly employed in electrochemical devices operating in aqueous solutions that leverage simultaneous transport of ions and electrons. Indeed, their mode of operation relies on changing their doping (oxidation) state by the migration of ions to compensate for electronic charges. Nevertheless, the structural and morphological changes that organic mixed conductors experience when ions and water penetrate the material are not fully understood. Through a combination of electrochemical, gravimetric, and structural characterization, the effects of water and anions with a hydrophilic conjugated polymer are elucidated. Using a series of sodium‐ion aqueous salts of varying anion size, hydration shells, and acidity, the links between the nature of the anion and the transport and structural properties of the polymer are systematically studied. Upon doping, ions intercalate in the crystallites, permanently modifying the lattice spacings, and residual water swells the film. The polymer, however, maintains electrochemical reversibility. The performance of electrochemical transistors reveals that doping with larger, less hydrated, anions increases their transconductance but decreases switching speed. This study highlights the complexity of electrolyte‐mixed conductor interactions and advances materials design, emphasizing the coupled role of polymer and electrolyte (solvent and ion) in device performance.

Balancing Ionic and Electronic Conduction for High‐Performance Organic Electrochemical Transistors

“…Considering that the onset of oxidation potential is approaching to 0 V versus Ag/AgCl for the polymers with high EG content (Figure S4, Supporting Information), we expect that these polymers are already oxidized in the absence of an applied bias. Cendra et al showed that due to its low ionization potential, g‐100% can become oxidized by side reactions in ambient conditions . It might be also possible that the longer EG hydrophilic chains present in the 2g polymer chelate ions, contributing to the capacitance in the unbiased condition.…”

“…Our results demonstrate that conjugated polymers operating in aqueous electrolytes experience significant changes in their structure and morphology during electrochemical doping. Previous work with p(g2T‐TT), i.e., g‐100%, evidenced that during doping, anions cause a lamellar expansion in the crystallites as they infiltrate and accumulate between the hydrophilic side chains . The majority of water uptake, on the other hand, occurs in the amorphous regions of the film .…”

“…Cendra et al showed that due to its low ionization potential, g-100% can become oxidized by side reactions in ambient conditions. [33] It might be also possible that the longer EG hydrophilic chains present in the 2g polymer chelate ions, contributing to the capacitance in the unbiased condition.…”

“…2020, 30,1907657 chains. [32,33] The majority of water uptake, on the other hand, occurs in the amorphous regions of the film. [21] Semicrystalline conjugated polymers have a complex microstructure comprised of crystalline regions embedded in an amorphous matrix.…”

“…When dopant ions migrate from the electrolyte into the polymer bulk, the film experiences a large degree of deformation . The volumetric expansion/deformation depends on the size and hydration shell of the dopant ion, as well as on the water molecules that follow the injected ions due to osmotic forces . Although a hydrated channel is important for unobstructed ion conduction, we have recently reported that it also negatively influences the electron transport and, consequently, the performance of OECTs .…”