Imperceptible organic electronics
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ction Electronics and photonics drive innovation in society. Future products are anticipated to form seamless links between living beings and the digital world, and will become ever more indispensable for improving safety and quality of life, without impairing comfort. Only a decade ago, such a vision was not much more than a distant goal, with some tentative approaches toward stretchable electronics and photonics.1 In the last five years, we have seen tremendous progress in the field, with the introduction of epidermal,2 transient,3 and imperceptible4 forms of electronics, being ultrathin, -lightweight, -flexible, and even -conformable. Such extremes in mechanical behavior are achievable with nearly all kind of materials (inorganic and organic, brittle and elastic, hard and soft) by positioning active electronic layers in the neutral mechanical plane and by aggressively scaling the flexural rigidity of the thin-film on foil devices.5 In this article, we review recent developments in organic, large-area imperceptible electronic and photonic devices and their applications, ranging from ultralightweight solar cells for energy supply, to wearable health monitors and biomedical implants.
From early flexible electronics to imperceptible systems that stretch Organic electronics rely on the thin-film field-effect transistor (TFT) architecture. The first functional devices were introduced
as early as 1962 at RCA laboratories as a base for integrated thin-film circuits.6 The thin-film transistor also won the “race” for the first flexible logic and power circuits on paper and plastic foils at the Westinghouse Laboratories in 19677,8 (Figure 1a). The history of the TFT itself is full of setbacks and obstacles before turning into the success story we know today with the ascent of flat-panel displays. The first steps in modern flexible electronics were born out of curiosity, but the large potential of such unusual electronics was not immediately obvious. The apex of space exploration in the mid-1960s necessitated the development of solar cells with improved power per weight. Consequently, light, flexible silicon solar panels attached to plastic foils were introduced in 1967 to maximize rocket payload and minimize cost9 (Figure 1b). “High-tech” flexible electronics were surprisingly well developed in the mid-1960s, then forgotten until renewed interest was sparked in the late 1990s with studies on the mechanics of flexible and rollable electronics. In a milestone contribution published in 1999,10 amorphous silicon thin-film transistors on 3-µm-thick steel substrates were described, comparable to the thinnest plastic substrates used today for ultraflexible electronics, allowing bending around a pencil (Figure 1c). Placing the thin-film electronic layer in the neutral plane of a support encapsulation structure further reduces allowable bending radii.5,11,12
Takao Someya, Department of Electrical and Electronic Engineering, School of Engineering, The University of Tokyo, Japan; and Thin-Film Device Laboratory & Center for Emergent Matter Science, RIKE
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