Molecular Dynamics Study of the Thermodynamics of Integer Charge Transfer vs Charge-Transfer Complex Formation in Doped Conjugated Polymers

Abstract

Molecular dopants such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) can interact with conjugated polymers such as poly­(3-hexylthiophene-2,5-diyl) (P3HT) in two different ways: they can undergo integer charge transfer (ICT) or they can form a partial-charge-transfer complex (CTC). Both are seen experimentally, but the CTC has been challenging to characterize, making it difficult to answer questions such as the following. Which polymorph is more stable? Do they have similar barriers for formation? Is there a thermodynamic route to convert one to the other? Here, we study the structure and the thermodynamics of bulk F4TCNQ-doped P3HT with all-atom molecular dynamics simulations, using thermodynamic integration to calculate the relative free energies. We find that the ICT and CTC polymorphs have similar thermodynamic stabilities. The barrier to create the ICT polymorph, however, is lower than that to make the CTC polymorph, because the ICT polymorph has a small critical nucleus, but the critical nucleus for the CTC polymorph is larger than what we can simulate. Moreover, simulated thermal annealing shows that the activation barrier for converting the CTC polymorph to the ICT polymorph is relatively modest. Overall, the simulations explain both the observed structures and the thermodynamics of F4TCNQ-doped P3HT and offer guidelines for targeting the production of a desired polymorph for different applications.

Critical dopant concentrations govern integer and fractional charge-transfer phases in doped P3HT

“…They concluded that there exists, indeed, a kinetically unstable, crystalline CPX polymorph, as found by Jacobs et al in addition to a stable CPX phase in the amorphous regions. These findings were followed up by Wu et al [23] employing all-atom molecular dynamics simulations which demonstrated that the IPA and CPX crystalline phases in this system are respectively entropy-and enthalpy-driven. There, however, the key mechanism favoring IPA formation was suggested to be a nucleation process starting with as few as 2-3 dopant radical anions as critical nucleus among the simulated 1728 P3HT monomer units while, in contrast, no critical nucleus was found for the crystalline CPX phase up to the maximum dopant loading that was investigated (144 CPX-forming dopants).…”

“…In the following, we discuss the nucleation effect for IPA formation, postulated by Wu et al [23], as a potential way to explain the differences between the doping phenomenology of FTCNQ and TCNQ. When assigning a host HOMO-DOS width σ to the nucleation threshold-concentration χ IPA ≈ 0.1% of ionized dopants for each total dopant molar ratio χ, we observe less broadening for higher χ, suggesting a higher probability for the occurrence of the crystalline IPA phase, all while our experimental data shows increased CPX formation.…”

“…In our previous study [24] we hypothesized that an initially small percentage of IPA content causes σ broadening and, subsequently, ionization of further dopants. In the same vein, we also consider the nucleation effect for IPA formation at its threshold-concentration χ IPA ≈ 0.1% (excluding non-ionized dopants from the total dopant molar ratio χ), as postulated in the aforementioned study by Wu et al [23]. As χ IPA regards only ionized dopants versus P3HT, it best corresponds to our data when we only consider the ionized portion at the dopant molar ratio χ, which we can calculate since the respective p calc values provide us with the fraction of ionized dopants:…”

“…Therefore, the host HOMO-DOS broadening as a driving force behind IPA formation will be underestimated. An additional CPX driving force would be required to be modeled such as the aforementioned work by Wu et al [23], where they suggest the formation of crystalline IPA and CPX phases being subject to nucleation effects and the overall change in free energy, which however goes beyond the scope of our work. Consequently, in our discussion of the drivers behind IPA and CPX formation, we essentially employ the results of our DOS modeling only in IPA-rich cases and lean more firmly on our experimental findings when significant CPX formation occurs.…”

“…Here, we now aim to determine the extent to which the nucleation process put forward by Wu et al [23] can be consistent with a process of initial ionization that triggers further IPA formation. For χ > 8%, that is for dopant loadings higher than those investigated by Wu et al [23], we study whether crystalline CPX-phase nucleation-sites may occur. This possibility has been suggested by the authors and appears likely because CPX-formation in rr-P3HT-F4TCNQ is typically found for high χ values [14][15][16].…”