Ion-Bound Polaronic Excitations in Doped P3HT

Charge carrier doping of conjugated polymers holds great promise for enhancing electronic and optoelectronic properties, yet, a comprehensive understanding of the electronic structure of doped polymers remains elusive. Herein, we employ transient absorption spectroscopy to investigate a prototypical conjugated polymer, defect-free poly-3-hexylthiophene (DF-P3HT), at varying degrees of hole-doping across a range of wavelengths to account for the representative model of its electronic structure. Our findings elucidate an underlying dynamic dopant–polymer interaction, revealing the formation and evolution of ion-bound charge carriers on femtosecond-to-nanosecond time scales. At low dopant concentrations, efficient charge separation forms free polarons, which contribute to an enhanced current. However, as dopant concentrations increase, we observe a marked transition from free polarons to bound polaronic states, identified for the first time by an unequivocal absorption spectrum, which is ∼1400 cm–1 red-shifted with respect to the free polaron transient absorption feature. This shift is attributed to the saturation of doping sites leading to substantial Coulombic interactions, Stark effect diminished charge mobility due to localization, and increased recombination losses. Our work in identifying ion-bound polaron signature offers a pathway to optimize doping strategies for improved performance in organic electronic devices, and hints at a band-bending model for polaronic transitions.