Despite the ubiquity of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in applications demanding mechanical flexibility, the effect on the mechanical properties of common additives—i.e., dimethylsulfoxide (DMSO), Zonyl fluorosurfactant (Zonyl), and poly(ethyleneimine) (PEI)—has not been reported. This paper describes these effects and uses plasticized films in solar cells and mechanical sensors for the detection of human motion. The tensile moduli of films spin‐coated from solutions containing 0%, 5%, and 10% DMSO and 0.1%, 1%, and 10% Zonyl (nine samples total) are measured using the buckling technique, and the ductility is inferred from measurements of the strain at which cracks form on elastic substrates. Elasticity and ductility are maximized in films deposited from solutions containing 5% DMSO and 10% Zonyl, but the conductivity is greatest for samples containing 0.1% Zonyl. These experiments reveal enlargement of presumably PEDOT‐rich grains, visible by atomic force microscopy, when the amount of DMSO is increased from 0% to 5%. PEI—which is used to lower the work function of PEDOT:PSS—has a detrimental effect on the mechanical properties of the PEDOT:PSS/PEI bilayer films. Wearable electronic sensors employing PEDOT:PSS films containing 5% DMSO and 10% Zonyl are fabricated, which exhibit detectable responses at 20% strain and high mechanical robustness through elastic deformation.
mechanical deformability and charge transport [ 3 ] are pairs of physical properties that derive their mutual exclusivity either from fundamental physics or practical diffi culty. In the fi eld of organic electronics, solubility of π-conjugated polymers necessitates alkyl side chains that in some cases reduce the effi ciency of charge transport, and in all cases limit the mechanical strength. [ 4,5 ] For fl exible, stretchable, and durable applications of these materials, they should have a high elasticity (low modulus), elastic limit (yield point), strain at fracture, and toughness. [ 6,7 ] The ability to accommodate mechanical deformation without fracture is a prerequisite for using organic electronic devices in fl exible, stretchable, and portable applications, as in the outdoor environment (where they are subjected to a range of thermomechanical stresses) and for fabrication by roll-toroll processing. [ 8 ] Despite the interest in using these materials for applications demanding mechanical deformability, the Increasing the fl exibility of polymer chains is a common method of increasing the deformability of solid polymeric materials. Here, the effects of "conjugation-break spacers" (CBSs)-aliphatic units that interrupt the sp 2 -hybridized backbone of semiconducting polymers-on the mechanical and photovoltaic properties of a diketopyrrolopyrrole-based polymer are described. Unexpectedly, the tensile moduli and cracking behavior of a series of polymers with repeat units bearing 0%, 30%, 50%, 70%, and 100% of the CBS are not directly related to the percent incorporation of the fl exible unit. Rather, the mechanical properties are a strong function of the order present in the fi lm as determined by grazing-incidence x-ray diffraction. The effect of the CBSs on the photovoltaic performance of these materials, on the other hand, is more intuitive: it decreases with increasing fraction of the fl exible units. These studies highlight the importance of solid-state packing structureas opposed to only the fl exibility of the individual moleculesin determining the mechanical properties of a conjugated polymer fi lm for stretchable, ultrafl exible, and mechanically robust electronics.
This paper describes the fabrication of transparent electrodes based on grids of copper microwires using a non-photolithographic process. The process—“abrasion lithography”—takes two forms. In the first implementation (Method I), a water-soluble commodity polymer film is abraded with a sharp tool, coated with a conductive film, and developed by immersion in water. Water dissolves the polymer film and lifts off the conductive film in the unabraded areas. In the second implementation (Method II), the substrate is abraded directly by scratching with a sharp tool (i.e., no polymer film necessary). The abraded regions of the substrate are recessed and roughened. Following deposition of a conductive film, the lower profile and roughened topography in the abraded regions prevents mechanical exfoliation of the conductive film using adhesive tape, and thus the conductive film remains only where the substrate is scratched. As an application, conductive grids exhibit average sheet resistances of 17 Ω sq–1 and transparencies of 86% are fabricated and used as the anode in organic photovoltaic cells in concert with the conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Compared to devices in which PEDOT:PSS alone serves as an anode, devices comprising grids of copper/nickel microwires and PEDOT:PSS exhibit lowered series resistance, which manifests in greater fill factor and power conversion efficiency. This simple method of forming micropatterns could find use in applications where cost and environmental impact should be minimized, especially as a potential replacement for the transparent electrode indium tin oxide (ITO) in thin-film electronics over large areas (i.e., solar cells) or as a method of rapid prototyping for laboratory-scale devices.
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