Rational solvent selection strategies to combat striation formation during spin coating of thin films

Abstract

Striation defects in spin-coated thin films are a result of unfavorable capillary forces that develop due to the physical processes commonly involved in the spin-coating technique. Solvent evaporation during spinning causes depletion at the surface of the more volatile solution components while simultaneous viscous out-flow occurs providing the main source of solution thickness reduction during any typical spinning run. The composition changes in the surface layer can either stabilize or destabilize the surface with respect to convective motions within the coating solution. Destabilization (and therefore possible striation formation) happens when the surface composition changes so that a larger surface tension will develop. Thus, a careful cross-referencing of solvent volatility with surface tension effects can help establish solution conditions that will prevent this instability from arising. A plot of solvent vapor pressure (Pv) versus solvent surface tension (σ) is introduced and utilized to help discuss the impact of solvent choice when making coatings via spin coating. One important result is that when desiring to deposit a coating having a surface tension of σsolid, then it is favorable to use a fully miscible solvent that has a higher surface tension (i.e., σliquid > σsolid). More complicated solution mixtures were also examined, including dual-solvent systems and water-containing systems.

Influence of Solvent Mixing on the Morphology and Performance of Solar Cells Based on Polyfluorene Copolymer/Fullerene Blends

“…These lines are known in the literature as striations. [27] Imaging the reflection and the photocurrents of the diodes using a light-pulse technique confirmed the presence of these variations in thickness and in the photocurrent (see Fig. 4): thicker films gave larger photocurrents.…”

Nanoperforations in poly(lactic acid) free-standing nanomembranes to promote interactions with cell filopodia

“…On the other hand, observation of the micrographs displayed in Figure 7 evidences that ultra-thin films frequently exhibit micrometric striation flaws. These defects have been attributed to unfavorable capillary forces provoked by changes in surface tension during solvent evaporation [12,27]. This phenomenon, which is called "Marangoni instability", has been largely appreciated in ultra-thin films prepared by spin-coating.…”

In Situ Studies of Phase Separation and Crystallization Directed by Marangoni Instabilities During Spin‐Coating

“…[ 6,7 ] Meanwhile, evaporative thinning is dependent upon the vapor pressure of the solvent. [ 5,8 ] As the fi lm thins complex non-equilibrium processes such as phase separation and crystallization occur, resulting in a rich variety of accessible morphologies, from a relatively straightforward processing route and solution composition.…”