DOI: https://doi.org/10.29363/nanoge.inform.2019.034
Publication date: 8th January 2019
The ultrafast optical processes in a self-doped conjugated polyelectrolyte, where any complications related to dopant diffusion, formation of partial charge transfer complexes or structural disruption by external dopant molecules are absent, are presented. Conjugated polyelectrolytes have conjugated backbones with ionic side chains. Their numerous applications include biosensing, cell imaging and interlayers in electronic devices. The ability to conduct both electronic and ionic charge, as well as their favorable doping, make conjugated polyelectrolytes particularly interesting. CPE-K is a narrow-bandgap conjugated polyelectrolyte, which becomes self-doped upon dialysis treatment. The doping is directly evident in the absorption spectrum, where the P2 polaron band appears around 1200 nm. By carefully determining the extinction coefficient of this band, we estimate the doping density in the polyelectrolyte doped at different levels. We carried out transient absorption (TA) spectroscopy in CPE-K solutions and thin films, with pumping in either the neutral S0-S1 or the polaronic P2 transition. We show that there is electronic coupling of polarons to nearby neutral sites, which share the same ground state for their optical transitions (both are depleted in the TA experiments, no matter which transition is excited). Similar, correlated and very short-lived dynamics are observed at both excitation wavelengths. This result contrasts with the conventional picture of localized intragap polaron states and warrants a revised model for the electronic structure and optical transitions in doped organic systems. We also show that inter-site Coulomb interactions are present (i.e. the positive polarons cause a Stark shift in the transitions of nearby neutral sites). The electronic coupling and electrostatic effects of the polarons occur independently on doping concentration and on whether the self-doped material forms a thin film or is in solution. Finally, we present the terahertz (THz) conductivity of those doped thin films, establishing the local carrier mobility.
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