Polaron-Induced Mode-Selective Vibrational Stark Effect in Molecularly Doped Organic Semiconductors

Doping organic semiconductors with strong electron acceptors generates positively charged carriers (polarons) and counterions.While most carriers in doped polymer films are Coulombically bound to counterions, the influence of the resulting local electric field on molecular vibrations (Stark effect) is rarely reported.Here, we use nanoscale vibrational spectroscopy and imaging to map a mode-selective vibrational Stark effect in poly(3-hexylthiophene-2,5-diyl) (P3HT) doped with 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ).The sub-wavelength spatiospectral capability of our approach enables us to identify charge transfer and local electric field-sensitive molecular vibrations within the F4TCNQ ion.We find that the C=C and C-F vibrational bands are blueshifted, while the C-CN stretching mode is red-shifted in F4TCNQ/P3HT blends compared to pristine F4TCNQ nano-FTIR spectra.Density functional theory calculations indicate that electron transfer alone produces only a red shift.Thus, the observed blue shifts and spectral broadening are attributed to the polaron-induced vibrational Stark effect.In contrast, the C-C stretching mode of the F4TCNQ ion remains unchanged in both pristine and blend films, suggesting an unfavorable dipole orientation relative to the local electric field.Moreover, simultaneous AFM imaging reveals significant location-dependent peak shifts in the asymmetric C=C mode, underscoring the heterogeneous nature of Coulombic interactions between counterions and polarons in the F4TCNQ/P3HT film.This work provides the first direct nanoscale evidence of a mode-selective vibrational Stark effect in doped organic semiconductors, highlighting how local electric fields from Coulombically bound polarons influence specific molecular vibrations.