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Abstract Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)‐based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox‐potential‐driven. Remarkably, X‐ray scattering shows that despite their large 2‐nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.
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Quantifying Polaron Mole Fractions and Interpreting Spectral Changes in Molecularly Doped Conjugated Polymers
Abstract Molecular doping of conjugated polymers causes bleaching of the neutral absorbance and results in new polaron absorbance transitions in the mid and near infrared. Here, the concentration dependent changes in the spectra for a series of molecularly doped diketopyrrolopyrrole (DPP) co‐polymers with a series of ultra‐high electron affinity cyanotrimethylenecyclopropane‐based dopants is analyzed. With these strong dopants the polaron mole fraction (Θ) reaches saturation. Analysis of the full spectrum enables separation of neutral and polaron signals and quantification of the polaron mole fraction using a simple noninteracting site model. The peak ratios for both neutral and polaron peaks change systematically with increasing polaron mole fraction for all measured polymers. Analysis of the spectral changes indicates that the polaron mole fraction can be quantified to within 5%. While the total change in the absorbance spectrum with increasing polaron mole fraction is linear, the lowest energy polaron peak (P1) grows nonlinearly, which indicates increased polarization/delocalization. Molecular doping of polymers that form either H‐ or J‐aggregates shows systematically different spectral changes in the vibronic peak ratios of the neutral spectra and provides insights into the polymer configuration at undoped sites in the film.
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- Award ID(s):
- 1636385
- PAR ID:
- 10366354
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Electronic Materials
- Volume:
- 8
- Issue:
- 4
- ISSN:
- 2199-160X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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