Composition Waves in Solution-Processed Organic Films and Its Propagations from Kinetically Frozen Surface Mesophases

Organic thin films deposited from solution attract wide interest for next-generation (opto-)electronic and energy applications. During solvent evaporation, the phase evolution dynamics for different components at different locations are not synchronic within the incrementally concentrated liquid films, determining the final anisotropic morphology and performance. Herein, by examining tens of widely investigated optoelectronic organic films, the general existence of composition wave propagating along the surface-normal direction upon solidification is identified. The composition wave is initiated by a few nanometers thick surface mesophase kinetically forming at the foremost stage of phase transition, and afterward propagates toward the substrate during solvent evaporation. The composition waves exhibit well-defined wave properties, including spatial wavelength, period, amplitude, and propagation velocity. These wave properties are closely correlated with the evaporation rate and the diffusion rate of organic molecules, which determines the dynamically varied local composition gradient along the surface-normal direction. Such composition waves are commonly found for more than 80% of randomly examined solution-processed thin films for high-performance organic electronic devices including photovoltaic cells and field-effect transistors.