Accompanying Structural Transformations in Polarity Switching of Heavily Doped Conjugated Polymers.

Abstract

Despite significant recent advancements in highly functional organic semiconductors (OSCs), the n-type OSCs reported to date lag behind their p-type counterparts in terms of long-term environmental stability. As an alternative approach to n-type materials, a few p-type polymers have been shown to undergo dramatic transitions in their charge carrier polarity to n-type through transition metal-incorporated Lewis acid doping. Although the concept of polarity switching is promising, its unclear chemical origin-particularly from a materials science perspective-limits its potential as an n-type counterpart. In this work, the chemical and structural mechanisms underlying the p-to-n polarity switching in a heavily doped conjugated polymer are elucidated. Using gold(III) chloride-doped indacenodithiophene-co-benzothiadiazole (IDTBT) as a model system, doping-induced thin-film structural changes are investigated. Quantitative X-ray photoelectron spectroscopy analysis of doped IDTBT films provides direct evidence of oxidation state changes in Au and Cl ions and confirms the covalent chlorination of the polymer backbone, establishing a direct correlation between the chemical doping mechanism and polarity switching. Finally, leveraging this polarity switching behavior, a p-n homojunction organic diode is demonstrated with a rectification ratio of 10⁴-10⁵, highlighting the versatility and potential of this excessively p-doped n-type OSC system for tailoring charge transport properties.