The deposition of semiconducting polymer thin-films from meniscus-guided coating [electronic resource]
Keywords: Chemical Engineering.

Electronic devices permeate almost every aspect of our daily life, and much of their capabilities stem from the electronic properties of conventional inorganic semiconductors like silicon. However, a new generation of "organic" electronics is filling the application spaces beyond the reach of silicon—areas like flexible electronics, biodegradable sensors, stretchable devices, and electronic skin. Semiconducting polymers specifically facilitate the development of these carbon-based devices because their chemical synthesis can imbue them with specific desired properties. These materials can be made stretchable, flexible, translucent, biodegradable, implantable, bioresorbable, and even self-healing. An impediment to the large-scale deployment of organic electronics, though, is the ability to fabricate these devices at scales economically viable for industrial manufacture. Luckily, because semiconducting polymers can be made soluble, they can be processed into thin films using a deposition method that has been in use since the beginning of the Common Era: printing. Printing methods like blade coating constitute a category of solution-phase deposition techniques characterized by the unidirectional passage of a liquid meniscus over a target substrate. These "meniscus-guided coating" (MGC) techniques are attractive for industrial deployment because they are amenable to high-throughput, continuous deposition. However, much of the underlying fluid mechanical phenomena and their effects on the dissolved conjugated polymer itself is poorly understood, and the achievement of high-performance devices requires a fundamental understanding of deposition. In our work, we use solution shearing, a model MGC method, to deposit a diketopyrrolopyrrole-based polymer—PDPP3T—as a representative material to reveal the fundamental processes that occur during deposition. Specifically, solution shearing is capable of uniaxially aligning PDPP3T in the deposited solid film. This alignment does not require any pre- or post-deposition treatments or processing, and the optical dichroic ratios of these films—a metric of alignment—can be as high as 7. The degree to which the polymer is aligned can be controlled solely by changing the coating speed, and we examine the effects of a variety of other material properties and deposition parameters. Taking advantage of its strong ability to align, we further probe the thin-film microstructure of deposited films using various in-situ and ex-situ X-ray diffraction methods to understand the deposition mechanism. Although higher shear strain is imparted to the solution at higher coating speeds, the dichroic ratios of the films peak at a relatively low coating speed rather than increasing to a plateau value. We find that the drop in optical dichroism at higher coating speeds is accompanied by both an increase in crystalline disorder and a lack of crystallographic texture at the top interface of the thin-films. The nucleation of disordered crystallites during coating likely disrupts alignment, leading to the peak. Furthermore, we postulate that the decrease in alignment marks the existence of a deposition regime—at intermediate coating speeds and characterized by enhanced surface nucleation—inherently distinct from the regime at low speeds where increasing shear strain induces increasing alignment. Curiously, high alignment of PDPP3T is only possible when certain solvents are used. Whereas tetralin can yield high dichroism, p-xylene does not. We use solution-phase small-angle X-ray scattering to characterize polymer solutions of various solvents and find that certain ones induce the formation of pseudo-lamella crystalline enough to diffract X-rays. When we attempt to determine the material's Hansen solubility parameters, we find that a single solubility sphere is not sufficient to describe the polymer likely because a mean-field approach to solvent quality does not apply. Furthermore, the solvents that induce pseudo-lamella in solution primarily lie in the solubility sphere with the higher dispersion parameter. Given that conjugated polymers are quite heterogeneous composites of flexible alkyl side-chains with electron-rich conjugated backbones, we believe some solvents preferentially interact with certain polymer substituents over others, giving rise to anisotropically shaped aggregates that can be aligned under the flow field during coating. Such interactions come into play when mixtures of solvents are used during deposition and can lead to unexpected behaviors. Polymer films coated from solutions of 1:1 vol. ratios of m-dichlorobenzene and dipentene exhibit remarkable in-plane alignment behavior as a function of coating speed. We believe a variety of phenomenon related to both differential solvation and differences in fluid flow during coating can contribute to unexpected phenomenon. Lastly, we use fluid dynamical simulations to model the fluid flow upstream of the meniscus under the influence of the coating blade. We establish mathematical metrics to describe and compare the velocity fields and the rate-of-strain tensor across coating blades patterned with arrays of pillars. Different array dimensions are chosen to determine the influence of different pillar spacings and densities. We find a correlation between the enhanced shear strain rates induced by the presence of pillars patterned on the coating blade and experimentally observed differences in the degree of crystallinity in the thin-film, suggesting that this simplified model is a good starting point for the development of more sophisticated simulations models to predict the influence of various pillar geometries. These models can guide the development of coating blades that enhance the thin-film microstructure of coated polymer films to optimize their charge transport properties and realize high-performance devices. In summary, our work examines the fluid mechanics and crystallization kinetics during MGC in order to elucidate the fundamental processes that occur during polymer thin-film solidification. We uncover the effects of deposition-related factors influencing both conjugated polymer alignment and microstructure evolution. With well-understood and well-controlled deposition, devices with optimized charge transport properties can be obtained and used for the next generation of electronics.