Microstructural Evolution of the Thin Films of a Donor-Acceptor Semiconducting Polymer Deposited by Meniscus-Guided Coating

摘要

Crucial to the development and refinement of organic electronics is a fundamental understanding of how deposition processes affect the active material's resulting microstructure in the thin film. Meniscus-guided coating (MGC) methods are attractive because of their amenability to high-throughput, industrially relevant continuous processes like roll-to-roll deposition, but the mechanism of solid film formation has not been elucidated and is valuable for the precise control of thin-film morphology and thus ultimate device performance. In this work, we investigate the microstructural evolution of thin films of a diketopyrrolopyrrole-terthiophene donor-acceptor polymer semiconductor using both in situ and ex situ X-ray diffraction methods. On the basis of a comparison of disorder between the film bulk and the top surface and a depth profiling of the out-ofplane orientation of crystallites, we find that faster coating speeds introduce more disorder into the resulting films because the stochastic nucleation of disordered crystallites at the meniscus air-liquid interface becomes more dominant than substratemediated nucleation. Our results suggest that there exist three separate deposition regimes-namely the shear-dominate, disorder-dominate, and Landau-Levich-Derjaguin regimes-revealed by observing both polymer alignment via dry film thickness and optical dichroism, a property sensitive to the flow and shear fields. At low coating speeds, the shear strain imparted upon the solution directly induces polymer alignment, causing an increase in dichroism as a function of coating speed. When solvent evaporation becomes too rapid as coating speeds increase, a decrease in the dichroic ratio is observed before the classical Landau-Levich-Derjaguin regime occurs at the highest coating speeds, resulting in isotropic films. The preservation of out-ofplane crystalline texture throughout the thickness of the films is seen only for lower coating speeds, and a study of different depositio