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The new world of soft electronics – Stretchable electronic components


by Prof. Dr.-Ing. Patrick Görrn / goerrn@uni-wuppertal.de

Ongoing miniaturization has boosted the performance of microelectronic components over the last few decades in line with Moore’s law. At the same time we are experiencing at the macroelectronic level – for example in displays and solar cells – a trend to ever larger devices. As a result of these developments the surfaces of many everyday objects are beginning to offer scope, in principle, for electronic functionality. Up to now, however, macroelectronics has generally been based on rigid glass substrates, and many of the objects surrounding us are bent in complex ways, or may even be flexible. Against this background, soft, stretchable electronics opens the way to equipping modern architecture, automobiles, furniture and clothing with large surface area devices such as LEDs, displays, solar cells and sensors.

Over the course of human history the materials used to make implements and machines have tended to become consistently harder, from bronze and iron to modern-day steel. But the hardest objects are not necessarily the most enduring, as can easily be seen by taking a can of beans in one hand and a rubber ball in the other and throwing them down from the top of a high-rise building. The rubber ball will absorb the impact and survive unharmed. On the same principle, safety design in a modern automobile does not put the passengers in a rigid cage but surrounds them with crumple zones and airbags – materials that yield under impact and create a balance between the hard frame and shell of the vehicle and the far softer human bodies inside.

Furniture upholstery serves a similar purpose – to mediate between a wooden or metal frame and the human materials of skin, bone and flesh. Likewise, it is the suppleness of the clothes we wear that makes them comfortable. Flexibility can also be an advantage for production and transport processes: a big carpet would be very difficult to lay if it could not be rolled up.

Toughness, compatibility with biological tissue, plasticity – many properties of soft materials are highly attractive for electronic applications such as solar cells ‘straight off the roll’, or ‘intelligent’ textiles. But classical electronics remains based on hard and brittle materials. The substrate of a chip is a crystalline wafer of, for example, silicon or gallium arsenide. As the electronic processes take place within this crystalline substrate, microelectronics cannot function without wafers. ‘Soft’ microchips are not an option. That electronics is nevertheless on the way to an increasingly soft future is largely thanks to progress made in thin film engineering on the one hand and the development of organic semiconductors on the other.

In contrast to microelectronics, thin film components can in principle be produced on any substrate. High resolution LED/LCD displays became possible with the ability to manufacture thin film transistors (TFTs) on wide area glass substrates – in fact today the term TFT has become a virtual synonym for liquid crystal displays (LCDs). It was advances in these new fields of macroelectronics – above all displays – that have made bendable components on flexible foils possible. Thin film solar cells that can be rolled out like carpet are already on the market, and Samsung has just announced a flexible smartphone.

It is easy to see, however, that there will probably be a lower limit to the radius around which flexible electronic components can be bent. If a foil of thickness d is bent round a radius R, the resultant stretching of the outer surface that carries the thin film structure is approx. ε=d/2R. Bearing in mind that a thin layer of rigid material will fracture when stretched by approx. 1%, it follows that the minimum radius around which a typical 50 µm-thick flexible substrate can be bent is 2.5 mm. This suggests that future mobile telephones might be constructed rather like a ballpoint pen (diameter >5 mm) containing a rolled up display to pull out. The phone would be pocket-sized for transport and still provide a large screen. This simplified description shows that even rigid structures can be bent, so long as they are thin enough. But the deformability of flexible foil is limited: once bent in one direction, it cannot be bent in any other.

Figs. 2 & 3: Natural examples of wrinkled and folded surfaces. Figs. 2 & 3: Natural examples of wrinkled and folded surfaces.

The advantages of even softer, elastically stretchable materials would be manifold. Like flexible foil they could be bent and hence produced and coated inexpensively in a roll-to-roll (R2R) process, which would also facilitate transportation. But unlike classical plastic foils they could then be attached to surfaces of almost any shape. A sheet of glass can only be fitted onto a perfectly flat surface, and flexible foils can only be bent cylindrically; but to cover a complex shape like the wings of an airplane, a solar cell must possess a certain level of elasticity. Curved surfaces of this sort are common not only (for aerodynamic reasons) in planes and cars but also in modern architecture, furniture and many other contexts. With elastically stretchable components almost any of these surfaces will become available for electronic applications.

To get an idea of what this means, one need only ask what role surfaces play in our daily life. Surfaces are what we see and touch; they transmit the sound waves we hear. And as our principal sense perceptions are rooted in the dimensions of the human body, a whole range of typical human-machine interfaces like displays, microphones, loudspeakers, keyboards or other pressure-dependent sensors could profit from the incorporation of (opto-)electronic functionality in any of the surfaces around us. This is particularly the case with pressure sensors, because even very small pressure exerted on a stretchable substrate will be transferred to the functional thin-film layer it carries. A soft, pressure-sensitive ‘electronic skin’ modeled on human skin could, for example, equip robots with entirely new levels of sensitivity.

To achieve stretchable electronics, the thin film components must first be deposited on a soft, pliable substrate, which is generally a silicone. In contrast to flexible substrates, which can be bent and rolled but not elastically stretched, this type of substrate allows stretching by tens of percent, which is immediately transferred to the thin film itself. But, as mentioned above, rigid materials tear when they are stretched by only 1%. So this will result in fracturing of the film as shown in Fig. 1b. Compression, on the other hand, leads not to fracturing of the film but to the formation of wrinkles (Fig. 1c) of the sort that are very common in nature wherever a relatively firm skin has to cover softer tissue (see Figs. 2 & 3). It is only by wrinkling that the skin of an aging capsicum, for instance, can continue to cover and protect the diminishing volume of the fruit inside without breaking (Fig. 2).

This mechanism suggests a way of making rigid surfaces stretchable that could be very useful in electronics. If the elastic substrate is stretched and its surface then coated with a harder material, this new ‘skin’ will form wrinkles when the tension is released and the substrate allowed to return to its original form. Wrinkled thin-film surfaces of this sort behave like the pleats of a concertina, unfolding as the substrate stretches to its preset limit, and folding back again as it shrinks (see Fig. 4).

Together with my former colleagues at Princeton University I was able to show that microscopically folded gold film 20 nanometers thick (Fig. 4) can be stretched up to 40% in any direction without fracturing, whereas smooth film will tear when stretched by only 1% (Fig. 1b). Film of this sort retains the levels of transverse conductivity required to construct efficient light emitting diodes, solar cells and other such components. Because of its softness in comparison with other metals, gold is particularly suited to the formation of stretchable structures. On this basis, as gold electrodes with good conductivity become available, the vision of components that can be stretched in all directions becomes increasingly attainable. All other functional layers can be made of considerably softer organic materials.

Research into organic semiconductors, especially polymers, has a long tradition at the University of Wuppertal. Under the leadership of Prof. Dr. Ullrich Scherf, the Institute of Polymer Technology links research groups from various faculties investigating different aspects of these materials, ranging from synthesis, through physical properties, to thin film engineering – an area in which printing technology plays a significant role.

In 2010, working together with UW’s Department of Electronic Components (led by Prof. Dr. Thomas Riedl), I succeeded in developing and demonstrating a stretchable organic laser made completely of polymers. The laser dye was in this case also made stretchable by wrinkling, but a special procedure was used to make the resultant folds assume a self-organized parallel structure exactly 320 nanometers apart. The resulting grid formed a distributed feedback (DFB) resonator determining the wavelength (i.e. color) emitted by the organic laser. This wavelength is proportional to the distance between the folds, so that if the component is stretched, the color emitted shifts toward longer (red) wavelengths. This makes it possible, by observing the color emitted by the laser, to measure – continuously and without physical contact – the precise mechanical force exerted at any point on a large surface. In other words the laser constitutes a sort of ‘optical skin’ that could be effectively used in applications like construction site security.

Lasers, however, are not the only application for optical grids. These can raise the efficiency of numerous optoelectronic thin-film components, on the one hand by increasing the light extraction of LEDs and displays, and on the other by significantly boosting the absorption rates of very thin solar cells. Thus the efficiency of the best currently available organic solar cells could be enhanced up to 100% by grid-based plasmon excitation. The production of large-area sub-micrometer sized structures like optical grids on hard substrates remains slow, and is only possible with considerable technological input. But soft substrates allow the formation of optical grids on almost any surface in a matter of seconds through self-organized wrinkling. The choice of soft substrates thus opens the door to new technological possibilities – even for applications that do not involve stretching, like flat solar modules on house roofs.

Since March 2012 my research into stretchable solar cells has been funded by the German Research Foundation’s Emmy Noether Program. While the first structures described above consisted of single metal or polymer layers deposited on silicone foil, the current challenge is to create technologically, as well as physically, more complex structures consisting of several layers. Here the research group can draw on the experience of the Department of Electronic Components, of which it is part. Since 2009 the department has been developing organic solar cells on glass substrates. The aim of the present research is to develop inexpensive elastically stretchable solar cells that can be manufactured as single large area units and fitted to any surface, and will at the same time be more efficient than their rigid or (merely) bendable predecessors.

Solar cells are an ideal first step in stretchable electronics, because they do not require complex structuring of the thin-film layers. This becomes necessary once one embarks on elastically stretchable applications that require more elaborate circuitry, like displays, sensors etc. Here printing technology will play a decisive role. So there is still a long way to go before a smartphone, for example, can be fully integrated into intelligent clothing. But one thing is already clear: electronics is becoming softer!

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