The meniscus-guided deposition of semiconducting polymers

Abstract

The electronic devices that play a vital role in our daily life are primarily based on silicon and are thus rigid, opaque, and relatively heavy. However, new electronics relying on polymer semiconductors are opening up new application spaces like stretchable and self-healing sensors and devices, and these can facilitate the integration of such devices into our homes, our clothing, and even our bodies. While there has been tremendous interest in such technologies, the widespread adoption of these organic electronics requires low-cost manufacturing techniques. Fortunately, the realization of organic electronics can take inspiration from a technology developed since the beginning of the Common Era: printing. This review addresses the critical issues and considerations in the printing methods for organic electronics, outlines the fundamental fluid mechanics, polymer physics, and deposition parameters involved in the fabrication process, and provides future research directions for the next generation of printed polymer electronics.

Fig. 1

Nucleation induced by deposition. a Pretreatment of polymer solutions under shear flow can nucleate pseudo-stable P3HT aggregates from polymer molecules (red dots) in solution, which can subsequently be cast. The aggregates are “shish-kebab” structures with π-stacked polymer molecules (blue) attached along the long axis (orange) of the aggregate core. Figure adapted from ref. (copyright 2015 American Chemical Society). b Enhancement of the nucleation of a polymer component in the matrix of an amorphous, co-deposited polymer can result from the use of patterned coating blades that locally increase shear strain and introduce extensional flow. Fluid dynamical simulations of the pillar array (c) indicate variations in fluid velocity (denoted with color) that both contribute to higher shear strain and cause extensional flow (d) as fluid upstream of two adjacent pillars converges between them. Although this method is shown for a binary-component mixture of two polymers, the principle is likely general for semicrystalline polymers. Figure adapted from ref. (copyright 2015 Nature Publishing Group)

Fig. 2

In-plane alignment induced by deposition. a Off-center spin coating of only a pre-aggregated polymer solution can induce uniaxial alignment in the resulting film, attesting to the role of aggregates in the in-plane orientation of the dry film. Figure adapted from ref. (copyright 2015 American Chemical Society). b Bar-coating of naphthalene-dicarboximide-bithiophene polymer solutions yields highly aligned polymer fibers likely because of the high shear strains imposed by the coating bar. Figure adapted from ref. (copyright 2015 Nature Publishing Group). c Blade coating of a diketopyrryolopyrrole (DPP)-based polymer with a flexible blade similarly results in aligned films because the upper liquid–solid interface provided by the blade induces shear strain greater than what would be possible with a free interface (an air–liquid interface like in dip coating). Figure adapted from ref. (copyright 2015 John Wiley and Sons). d In another example, the in-plane alignment of DPP-terthiophene polymer thin films as measured by the optical dichroic ratio can be directly tuned by changing the coating speed and thus the effective imposed shear stress. Figure adapted from ref. (copyright 2016 American Chemical Society)

Fig. 3

Phase separation of multicomponent systems. a Diagram of the OPV operation, which consists of (i) excitation by a photon, (ii) migration of an exciton to a donor–acceptor interface, (iii) exciton dissociation by charge transfer, (iv) charge separation, (v) migration to the electrode, and (vi) charge extraction. One key to effective energy harvesting is the effective dissociation of coulombically bound excitons at the domain boundaries separating the donor and acceptor before the exciton recombines. The exciton diffuses on average about 10 nm before it recombines, thus implying that the optimal domain size should be around the same length scale. The thermodynamic phase diagrams describing a simple two-component polymer system (b) and a ternary diagram addressing the role of the solvent (c). Multiple processes can occur as the non-equilibrium state of the drying film’s microstructure evolves, and therefore overall film evolution is likely a combination of both kinetically limited and thermodynamically limited processes. Figures adapted from ref. (copyright 2010 IOP Science), ref. (copyright 2014 MDPI), and ref. (copyright 2013 American Chemical Society)

Fig. 4

Film evolution of multicomponent systems involving phase separation. a The evolution of the interface between two polymer components in a dry film as a function of thermal annealing as determined by their thermodynamic mixing parameters. Increasing the temperature above the glass transition point post deposition reduces the size of the intermixed region separating donor-rich and acceptor-rich domains. Figure adapted from ref. (copyright 2017 American Chemical Society). b Manipulation of domain size and nucleation density caused by the use of a patterned coating blade to induce extensional flow and increased shear strain. Within the amorphous electron-acceptor polymer matrix (blue), red domains indicate amorphous electron-donor polymer while domains with red bars indicate semicrystalline regions. Higher coating speeds using a blade patterned with pillar structures that causes extensional flow induce the formation of smaller, more numerous nuclei, which become small crystalline domains in the solid film. Figure adapted from ref. (copyright 2015 Nature Publishing Group)