The crucial role played by the solution–vapor
interface in determining the growth and morphology of an organic semiconductor
thin film formed by solvent evaporation has been examined in atomic
detail. Specifically, how the loss of individual solvent molecules
from the surface of the solution induces solute assembly has been
studied using molecular dynamics simulations. The system consisted
of bis(2-phenylpyridine) (acetylacetonate)iridium(III)
[Ir(ppy)2(acac)] and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) in chloroform at
310 K. The simulations clearly indicate that (a) the system does not
undergo uniform phase separation (spinodal decomposition), (b) solute
aggregation initiates at the solution–vapor interface, (c)
the distribution of solvent in the film is nonhomogeneous, (d) this
nonhomogeneous distribution can induce preferential alignment of host
molecules, and (e) a portion of the solvent likely remains trapped
within the film. The work not only demonstrates the ability to directly
model evaporation in atomic detail on the relevant length scales but
also shows that atomistic simulations have the potential to shed new
light on morphological properties of a wide range of organic semiconductor
devices manufactured using solution-processing methods.