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.

The tetrafluorinated derivative of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is of interest for charge transfer complex formation and as a p-dopant in organic electronic materials. Fourier transform infrared (FTIR) spectroscopy is commonly employed to understand the redox properties of F4TCNQ in the matrix of interest; specifically, the ν­(CN) region of the F4TCNQ spectrum is exquisitely sensitive to the nature of the charge transfer between F4TCNQ and its matrix. However, little work has been done to understand how these vibrational modes change in the presence of possible acid/base chemistry. Here, FTIR spectroelectrochemistry is coupled with density functional theory spectral simulation for study of the electrochemically generated F4TCNQ radical anion and dianion species and their protonation products with acids. Vibrational modes of HF4TCNQ–, formed by proton-coupled electron transfer, are identified, and we demonstrate that this species is readily formed by strong acids, such as trifluoroacetic acid, and to a lesser extent, by weak acids, such as water. The implications of this chemistry for use of F4TCNQ as a p-dopant in organic electronic materials is discussed.

It has often been observed anecdotally and implied through experimentation that acrylic emulsion paintings accumulate and entrain soils over time due to the inherent mechanical softness in artist's acrylic paint films, through the presence of hydrophilic film components, and by virtue of the ubiquitous presence of surfactant moieties on these film surfaces once they dry. In the present study, it has been this last effect that we have sought to describe more fully in terms of surfactant responsiveness to both temperature and relative humidity (RH). Surfactant hydration and dehydration under varying temperature and RH conditions affects the ultimate partitioning of the surfactant at the paint-air interface, as well as the inherent size, aggregation tendencies, and solubility of surfactant in the bulk paint materials which contain components that are highly responsive to changes in temperature and RH (e.g. polyacrylic or polymaleic anhydride-type dispersal materials). In this work, analytical techniques including three-dimensional microscopy and quartz crystal microbalance with dissipation were used to add to and reinforce current understanding of the physical and mechanical changes to acrylic paint films with temperature and RH. The migration of surfactant at the film surface was studied using desorption electrospray ionization-mass spectrometry and attenuated total reflectance Fourier transform infrared microscopy.

The prototypical system for understanding doping in solution-processed organic electronics has been poly(3-hexylthiophene) (P3HT) p-doped with 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ). Multiple charge transfer states, defined by the fraction of electron transfer to F4TCNQ, are known to coexist and are dependent on polymer molecular weight, crystallinity, and processing. Less well understood is the loss of conductivity after thermal annealing of these materials. Specifically, in thermoelectrics, F4TCNQ-doped regioregular (rr) P3HT exhibits significant conductivity losses at temperatures lower than other thiophene-based polymers. Through detailed spectroscopic investigation of progressively heated P3HT films coprocessed with F4TCNQ, we demonstrate that this diminished conductivity is due to formation of the non-chromophoric, weak dopant HF4TCNQ -. This species is likely formed through hydrogen abstraction from the alpha aliphatic carbon of the hexyl chain at the 3-position of thiophene rings of rr-P3HT. This reaction is eliminated for polymers with ethylene glycol-containing side chains, which retain conductivity at higher operating temperatures. In total, these results provide a critical materials design guideline for organic electronics.

DESI-MS is effective in monitoring binding media within an intact painting cross-section via mass spectrometric methods. This includes distinguishing between lipid-containing and modern binding materials present in a known mock-up cross-section matrix as well as identifying lipid-binding media in a 17th century baroque era painting.

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Degradation pathways of small molecule donors for organic photovoltaics are shown to be dependent on chemical traits and not just redox properties.