Understanding the interaction between organic semiconductors (OSCs) and dopants in thin films is critical for device optimization. The proclivity of a doped OSC to form free charges is predicated on the chemical and electronic interactions that occur between dopant and host. To date, doping has been assumed to occur via one of two mechanistic pathways: an integer charge transfer (ICT) between the OSC and dopant or hybridization of the frontier orbitals of both molecules to form a partial charge transfer complex (CPX). Using a combination of spectroscopies, we demonstrate that CPX and ICT states are present simultaneously in F4TCNQ-doped P3HT films and that the nature of the charge transfer interaction is strongly dependent on the local energetic environment. Our results suggest a multiphase model, where the local charge transfer mechanism is defined by the electronic driving force, governed by local microstructure in regioregular and regiorandom P3HT.

Printable electronic devices from organic semiconductors are strongly desired but limited by their low conductivity and stability relative to those of their inorganic counterparts. p-Doping of poly­(3-hexyl)­thiophene (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane increases conductivity through integer charge transfer (ICT) to form mobile carriers in P3HT. An alternate undesired reaction pathway is formation of a partial charge transfer complex (CPX), which results in a localized, traplike state for the hole on P3HT. This effort addresses the stability of the free carrier states, once formed. Herein, we demonstrate that, while the ICT state may be kinetically preferred, the CPX state is thermodynamically more stable. Conversion of the ICT state to the CPX state is monitored here over time using a combination of infrared and photoelectron spectroscopies and supported by a complete loss of film conductivity with an increased CPX state concentration. Both the fraction and the rate of conversion to the CPX state are influenced by polymer molecular weight, dopant concentration, and storage conditions, with ambient storage conditions accelerating the conversion. This work suggests that a renewed focus on dopant–matrix reaction chemistry should be considered in the context of both kinetic and thermodynamic considerations.

Organic semiconductors are increasingly employed in electrochemical devices for energy conversion and storage and chemical sensing. In these systems, the conductivity can be modulated with electrochemical doping with substantial variation in electronic charge densities (10 16 to 10 21 cm −3 ) stabilized by electromigration of counterions from the electrolyte phase. Herein, we focus on the model system of regioregular poly(3hexylthiophene) to determine the structural evolution at the onset of conductivity arising from electrochemical doping, specifically targeting elucidation of structural relaxation that precedes volumetric swelling. Using spectroelectrochemical methods, a 20% electrochemical active fraction of the film volume comprised of a nanocrystallite subpopulation serves as a high doping efficiency charge nucleation site with an increase from 10 16 to 10 20 carriers/cm −3 . A small carrier density window is observed where structural reversion of J-to-H aggregates occurs due to electrostatic repulsion of neighboring charges (bipolarons) on the nanocrystallites. After this conformational change, further increase in doping leads to generation of free volume for counterion diffusion in the nanocrystallites along with doping of the amorphous fraction and J-aggregate recovery. This result advances the structural knowledge of conductive polymer electrodes for electrochemical devices beyond what has been reported using X-ray scattering and provides a benchmark for synthetic structural changes to control hybrid electrical−ionic transport, emphasizing the need to control structural conformation relaxations in addition to volumetric swelling.

Controlling interfacial electron-transfer rates is fundamental to maximizing device efficiencies in electrochemical technologies including redoxflow batteries, chemical sensors, bioelectronics, and photo-electrochemical devices. Conductive polymer electrodes offer the possibility to control redox properties through synthesis and processing, if critical structure−property relationships governing charge transfer are understood. In this work, we show that the rate and symmetry of electron transfer at conductive polymer electrodes are directly connected to the microstructure and the density of states (DOS) using the model system of poly(3-hexylthiophene) (P3HT) and ferrocene/ ferrocenium (Fc/Fc + ), as predicted by the Marcus−Gerischer model. Experimentally, crystalline P3HT exhibits a sufficient overlap between the polymer DOS and the DOS of both Fc and Fc + , resulting in a reversible electron transfer. Conversely, the DOS of amorphous electrodeposited P3HT does not overlap with that of Fc + , inhibiting reduction (i.e., kinetic selectivity for oxidation). This proofof-concept work offers a paradigm to predict and control the kinetics at the polymer/liquid interface for applications from biology to energy.