Reactive Atomistic Simulations of Doping-Driven Co-Aggregation in P3HT/F4TCNQ

The performance of doped organic semiconductors (OSCs) is governed by a strong coupling between the doping equilibrium and the film morphology, yet capturing both simultaneously in a single atomistic model has remained difficult. Here we use the reactive Monte Carlo molecular dynamics (RMCMD) method to study poly(3-hexylthiophene) (P3HT) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), a prototypical OSC-dopant pair, and show that dopant aggregation and doping efficiency are mutually determined. We find that ionization converts a preaggregated dopant cluster into a mixed P3HT/F4TCNQ "coaggregate" whose extent is set by the reaction free energy; as dopants ionize, Coulomb repulsion drives them apart and pulls in ionized P3HT so that higher doping efficiency and greater OSC-dopant mixing are self-reinforcing. Introducing explicit solvent, we observe ionization-driven P3HT/F4TCNQ coaggregation and displacement of solvent from the dopant coordination shell, consistent with experimentally reported poor solubility of ionized P3HT/F4TCNQ complexes. We also observe a higher doping efficiency in the more polar solvent acetonitrile than in chlorobenzene.