Probing Polaron Environment in a Doped Polymer via the Photoinduced Stark Effect

Abstract

Conjugated polymers (CPs) constitute a unique class of materials with applications ranging from energy storage to bioelectronics, thanks in part to their ability to be easily doped to produce mobile charge carriers. While methods for doping CPs are well-developed, a deeper understanding of the interplay between the local environment and polaronic properties is currently lacking. Using transient absorption (TA) spectroscopy, we demonstrate that photoexciting polarons in a lightly doped 3,4-propylenedioxythiophene-co-3,4-ethylenedioxythiophene (ProDOT-co-EDOT) polymer film produces a Stark effect that is sensitive to their local environments. The resulting electroabsorption (EA) signal, which originates from a shift in the band gap of nearby polymer chains due to the photoinduced electric field, indicates that polaron excitation spatially separates the hole from its counterion as the hole thermalizes within 200 fs. Changes in the spectral position of the EA signal when tuning the excitation wavelength across the P1 and P2 polaron bands, combined with a dynamic blue shift, suggest a variation in field strength that senses different extents of the local environment. We propose the photoinduced Stark effect in TA experiments as a sensitive probe of the local environment of polarons in doped CPs that can also be used to refine electronic structure models describing their polaronic states and transitions.