Evolution and Morphology of Thin Films Formed by Solvent Evaporation: An Organic Semiconductor Case Study

Abstract

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.

Probing Molecular Assembly of Small Organic Molecules during Meniscus-Guided Coating Using Experimental and Molecular Dynamics Approaches

“…Recent studies in microfluidic systems have shown that fluid flow and shear can affect the final crystal morphology. , Similar to microfluidic geometries, length scales on the order of micrometers are present during MGC film deposition, indicating that fluid flow and shear could also be highly relevant to crystallization and solid thin-film formation, thus prompting a study of the significance of coupled evaporation and fluid flow during MGC. Thus far, there is no study that provides a comprehensive molecular length scale understanding of the combined effects of fluid flow and evaporation on molecular assembly. , …”

“…Thus far, there is no study that provides a comprehensive molecular length scale understanding of the combined effects of fluid flow and evaporation on molecular assembly. 16,17 It is assumed that the evaporation rate, coupled with fluid shear, influences the supersaturation and molecular orientations, which then impacts the crystallization behavior of the small molecules. However, the system is difficult to simulate with simple models due to the simultaneous momentum, mass, and thermal gradients, as well as the solidification process of solute-phase transformation into a solid (amorphous or crystalline thin films).…”

Regulation of Molecular Orientations of A–D–A Nonfullerene Acceptors for Organic Photovoltaics: The Role of End‐Group π–π Stacking

“…[32] MD Simulation Procedure of Solvent Evaporation: Although experimental studies have shown that the interfaces can modulate molecular orientations, [46] there are very few attempts including the interfaces in the simulations of the solvent evaporation process for organic semiconductor systems. [47,48] In most cases, solvent evaporation was modeled by deleting solvent molecules randomly from a bulk solution under 3D periodic boundary conditions, [49][50][51] which will lead to an isotropic morphology and cannot reflect the molecular orientations with respect to the substrate. Here, amorphous SiO 2 was used as substrate when simulating the solution processing of thin films for the A-D-A SMAs.…”

A simple predictor of interface orientation of fluids of disk-like anisotropic particles and its implications for organic semiconductors

“…Although the importance of controlling interface orientation in OSC devices is generally well understood, and there is no shortage of experimental 16–18,25 or computational 26–31 examples of preferential alignment of anisotropic molecules at interfaces, there remain few general rules for predicting alignment at either the solid or vapour interface from the chemical structure of OSC molecules. We have recently extensively reviewed the subject in the context of OSCs, 32 and despite a number of factors being implicated in controlling molecular orientation, in many cases an understanding of the general physical principles that dictate this behaviour was lacking, especially at the solid interface.…”