Molecular dopants with aromatic rings like 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) can interact with conjugated polymers like P3HT in two different ways: they can π-stack with the polymer backbone and form partial charge-transfer complexes (CTCs) or they can reside in the polymer lamellae and undergo integer charge transfer (ICT). ICT is generally preferred as it produces mobile carriers needed for applications. However, even though the ratio of the ICT to CTC polymorphs can be controlled by both the dopant infiltration conditions and the crystallinity of the P3HT, both polymorphs are always present simultaneously. Moreover, the CTC polymorph has been challenging to characterize, which makes it hard to answer questions such as: Which polymorph is thermodynamically more stable? Is there a barrier for doping? And what is the activation barrier for converting one doped polymorph into the other? Here, we answer these questions by studying the structure and the thermodynamics of bulk F4TCNQ-doped P3HT with classical molecular dynamics simulations. We obtain free energies from thermodynamic integration and use nonequilibrium simulations to explore the kinetics of annealing. Two different forcefields were used in the simulations, but both forcefields gave the same qualitative conclusions. We find that the ICT and CTC polymorphs have a similar thermodynamic stability, which explains why both are always present simultaneously. However, the barrier for creating the ICT polymorph from the neutral polymer is lower than that for the CTC polymorph. Moreover, the ICT polymorph has a critical nucleation size of only 2 dopants, while there always appears to be a significant cost to create the CTC polymorph, explaining why the ICT polymorph is more prevalent. Finally, simulated thermal annealing shows that the barrier for converting the CTC polymorph to the ICT polymorph is relatively modest, and that the ICT polymorph becomes thermodynamically more stable at higher temperatures. This explains why annealing F4TCNQ-doped P3HT films converts the CTC polymorph into the ICT polymorph. Overall, our simulations are able to explain the observed structures and the thermodynamics of doping P3HT with F4TCNQ and thus offer guidelines for how to make doped films with the more desirable ICT polymorph.

Molecular dynamics study of the formation of integer charge transfer vs. charge-transfer complexes in F4TCNQ-doped P3HT

ACS Spring 2022

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