Stretchable and ultraflexible organic electronics
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lastic electronics: Back to basics Five years ago, MRS Bulletin published a theme issue on the then—and still—burgeoning field of stretchable and flexible electronics.1 The authors in that issue explored the mechanics of stretchable thin films,2 approaches to generating stretchable devices from otherwise rigid materials,3 and a range of stretchable device components, from batteries to electronic eye cameras.4 The topics in that issue reflected the state of the art in the field then, which was dominated by devices based on inorganic materials (e.g., metallic films and semiconductor nanoribbons). A complementary approach to extreme deformability is based on organic conductors and semiconductors.5 While the conductivity and transport properties of inorganic materials are superior to those of organics, the advantages of these molecular materials remain—oxide-free interfaces, fabrication by printing in a roll-to-roll manner, low cost, tailorability by synthesis, and intrinsic mechanical compliance.5 Deformability was one of the original goals of organic electronics,6 but some of the best performing materials today remain stiff, and they crack at relatively low strains. Organic devices can be rendered stretchable in many of the same ways as can devices based on inorganic thin films (e.g., using wavy, fractal, or relief features that direct strain away from the sensitive components).7 Other approaches that are applicable only to organics, however, give them a distinct advantage, such as the formation of fibers to form elastic mats and the synthesis of materials whose
molecular structure or solid-state packing structure permits extreme deformation.8 A suite of tools incorporating metrology, synthesis, and fabrication has recently emerged whose goal is to achieve the original dream of organic electronics—to combine state-of-the-art electronic performance with high deformability. This research is, at present, somewhat distinct from efforts in the field to understand and to improve the electronic properties of organic semiconductors, though ultimately, the results of both spheres of inquiry must be merged. The authors of the articles in this issue of MRS Bulletin have made key contributions to developing stretchable new materials or stretchable forms of old ones, new techniques for measuring the mechanical properties of fragile thin films, and new devices that exhibit unprecedented deformability.
New materials and stretchable forms of old ones Stretchable electronics had its beginnings in the 1990s with the work of Wagner, Suo, and others, who examined the mechanics of materials—principally metals—on flexible and stretchable substrates.9 These materials adopted buckled, wavy, or fractured morphologies that accommodated tensile strains while retaining electronic functionality.10 In an example of applying this approach to organic electronic materials, we have made stretchable organic solar cells by buckling the devices on elastic substrates.11 Bettinger and co-workers used a similar approach by producing the first stretchable organic
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