Atomistic Simulations of P(NDI2OD-T2) Morphologies: From Single Chain to Condensed Phases

We
investigate theoretically the structure, crystallinity, and
solubility of a high-mobility n-type semiconducting copolymer, P­(NDI2OD-T2),
and we propose a set of new force field parameters. The force field
is reparametrized against density functional theory (DFT) calculations,
with the aim to reproduce the correct torsional angles that govern
the polymer chain flexibility and morphology. We simulate P­(NDI2OD-T2)
oligomers in different environments, namely, in vacuo, in the bulk
phase, and in liquid toluene and chloronaphthalene solution. The choice
of these solvents is motivated by the fact that they induce different
kinds of molecular preaggregates during the casting procedures, resulting
in variable device performances. Our results are in good agreement
with the available experimental data; the polymer bulk structure,
in which the chains are quite planar, is correcly reproduced, yet
the isolated chains are flexible enough to fold in vacuo. We also
calculate the solubility of P­(NDI2OD-T2) in toluene and chloronaphthalene,
predicting a much better solubility of the polymer in the latter,
also in accordance to experimental observations. Different morphologies
and dynamics of the oligomers in the two solvents have been observed.
The proposed parameters make it possible to obtain the description
of P­(NDI2OD-T2) in different environments and can serve as a basis
for extensive studies of this polymer semiconductor, such as, for
example, the dynamics of aggregation in solvent.