Stoichiometric anion exchange by a low-dielectric-constant solvent for highly-doped conjugated polymers with enhanced environmental stability

Abstract

High-degree doping of conjugated polymers often employs a strong redox agent, which facilitates polymer ionization but results in poor environmental stability for the counter-ion. Here, we demonstrate an anion-exchange doping using a model study that systematically investigates the effect of the solvent dielectric constant on both doping and anion exchange. The dielectric constant significantly affects the initial doping of poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) films using FeCl₃, as well as in the subsequent anion exchange of [FeCl₄]⁻ to dodecylbenzenesulfonate ([DBS]⁻). A solvent with a higher dielectric constant improves the FeCl₃ doping efficiency but hinders the subsequent anion exchange. Such conflicting effects can be resolved by stepwise immersion in separate solutions of FeCl₃ and dodecylbenzenesulfonic acid (DBSA). Stepwise anion-exchange doping achieves high electrical conductivity with improved environmental stability, while also allowing for the application of desired anions that require extended time for the direct doping method, such as in Brønsted acid doping.