Multi-type Electronic Interactions in Precursor Solutions of Molecular Doped P3HT Polymer

Spin casting of molecular doped polymer solution mixtures is one of the commonly used methods to obtain conductive organic semiconductor films. In spin-casted films, electronic interaction among the dopant and polymer is one of the crucial factors that dictate the doping efficiency. Here, we investigate excitonic couplings using ultrafast two-dimensional electronic spectroscopy to examine the different types of electronic interaction in ion pairs of the prototype F4TCNQ doped P3HT polymer system in precursor solution mixture for spin-casting. Off-diagonal peaks in 2D spectra clearly establish the excitonic coupling between P3HT+ and F4TCNQ− ions in solution. The observed excitonic coupling is the direct manifestation of Coulombic interaction amongst the ion-pair. The excited state lifetime of F4TCNQ− in ion-pairs shows biexponential decay: 30 fs and 200 fs, which hints towards the presence of heterogeneous population with different interaction strengths. To examine the nature of these different types of interactions in solution mixture, we study the system using molecular dynamics simulations on a fully solvated model employing the generalized Amber force field. We retrieve three dominant interaction modes of F4TCNQ anions with P3HT: side-chain, π-stack and slipped stack. To quantify these interactions, we complement our studies with electronic structure calculations, which reveal the excitonic coupling strengths of: ∼75 cm−1 for side-chain, ∼150 cm−1 for π-π-stack, and ∼69 cm−1 for slipped stack. These various interaction modes provide information on the key geometries of the seed structures in precursor solution mixtures, which may determine the final structures in spin-casted films. The insights gained from our study may guide new strategies to control and ultimately tune the Coulomb interactions in polymer-dopant solutions.