Power at Your Fingertips

Battery lifetime remains a crucial factor in the success of any portable electronic device. While battery technology continues to evolve, new features are constantly being added to portable devices, maintaining maximum pressure on power demands. The role of energy harvesting can only become more important, whether to enhance battery function or, ultimately, to give power autonomy.

The ability to generate power from ambient sources, such as mechanical switching, vibration, heat or light, convert it, store it and use it at will, has the potential to make a huge impact on the usefulness of portable devices. Touchscreens are the next target.

Research is underway worldwide into various piezo-electric energy harvesting techniques and new nano-materials that could result in self-powered touchscreens. Meanwhile, solar cells are arguably the most practical, readily available energy harvesting solution today that can be used to power a touchscreen.

Figure 1: Solar powered touchscreen smartphones have been around since 2009, but are not yet a standard feature on new designs today. (Photo Source: Samsung.)

This article will take a look ahead, projecting the outcome of some of the research currently underway, as well as review some of the solar cells currently available for charging batteries and/or powering touchscreen activated portable devices. These will include the Ixolar family from IXYS (KXOB22-01X8, KXOB22-12X1) and the Amorton range (AM-1456CA, AM-5412CAR) from Sanyo Energy.

Flexible power

The ability to harvest piezo-electric energy from a mechanical switch or keyboard actuation is well known. Specialist company enOcean, a Siemens spin-off, has developed and commercialized the technology, together with an associated standard protocol (in the public domain), which now supports an entire ecosystem of vendors (enOcean Alliance). More than 100 companies are now on board supplying devices, software and tools for a number of applications, primarily targeted at the smart buildings sector, from wireless light switches to remote controls and including wireless sensor networks.

However, in many portable devices today, keypads are being replaced by touchscreens. Although haptic feedback of a key press is desirable, the positive mechanical switching effect has all but disappeared. Yet researchers in Asia have found that tiny amounts of energy can be harvested when a touchscreen flexes under a user’s touch. Korean giant Samsung has been working with its local Sungkyunkwan University to perfect an energy scavenging material, laced with flexible and transparent electrodes that ultimately can be used to overlay a touchscreen and provide power.

Although some years away from commercial reality, the researchers have demonstrated a power output of about 1 µW/cm², which is theoretically sufficient to power a touch sensor. While initially such technology may only provide supplementary power, the vision of self-powered portable devices may yet be forthcoming.

In a paper published in Advanced Materials¹, the researchers describe the technology as nanoscale piezoelectronics. In construction, piezoelectric zinc oxide nanorods are sandwiched between highly conductive graphene electrodes, on top of flexible plastic sheets. The ability to keep the material flexible opens up the opportunity of developing foldable or roll-up screens and keyboards, as well as ‘wearable’ devices, which will charge themselves as they flex.

Importantly, the researchers claim, the technology lends itself, eventually, to large-scale roll-to-roll manufacturing processes. Subsequent academic papers have highlighted further experiments with different materials to develop additional useful properties, and extensions of the research have moved into topics such as wearable nanogenerators.

Fractional charge from friction

Moving from Asia to the US, researchers have discovered how to harvest the energy generated when two flexible polymer materials are rubbed together. Called a triboelectric generator, the material could provide alternating current from activities such as walking. Developed at the School of Materials Science and Engineering at the Georgia Institute of Technology², the researchers believe that because the triboelectric generators can be made nearly transparent, they have the potential to be used in active sensors that might replace the technology currently used for touch-sensitive device displays.

In operation, the triboelectric generator could supplement power produced by nanogenerators that use a piezoelectric effect to generate current from the flexing of zinc oxide nanowires. The key, according to Professor Zhong Lin Wang, is the use of a gap separation technique between two dissimilar polymer sheets that produces a voltage drop (see Figure 2 below). If an electrical load is then connected between the two surfaces, a small current will flow to equalize the charge potential.

Figure 2: Triboelectric generator schematic, developed by Georgia Institute of Technology.

The technology could also be used to create highly sensitive, self-powered active pressure sensors, potentially for use in organic electronic or opto-electronic systems. Such sensors could detect pressure as low as 13 mPascals, and would not require power from a battery. The material could also be used in conjunction with other energy harvesting mechanisms, such as existing zinc oxide nanogenerators. Experiments have shown that the triboelectric generators are robust, and can be used in continuous operation, for more than 100,000 cycles.

The researchers maintain that the manufacturing process is simple, low cost, and can be scaled up for high volume production and practical applications. The next stage of the research is to create systems that include storage for the energy generated.

Solar fabrics

Light is arguably a more viable source of ambient energy that could be captured to power touchscreen devices today. Miniature solar cells powering low power devices such as calculators, watches, and the like, have been around for decades. However, advanced academic research into solar power technology is progressing dramatically.

Photonics researchers in the US and Europe have been working on the development of silicon based optical fibers with solar-cell capabilities, according to a paper published by Advanced Materials.³ Solar cell fibers that can be woven into a flexible fabric offer the potential of powering devices such as touchscreens as well as charging batteries. However, the wearable battery charger, although within the realms of possibility, is likely to be a few years away yet.

The core of the research, combining silicon and optical fibers, has a wide range of potential applications, not only in optoelectronics and opto signal processing for telecommunications, but also in non-linear photonics for medical, imaging and sensing, as well as solar energy generation. Led by a team at Penn State University⁴ in the US, the work is shared with the Optoelectronics Research Centre at Southampton University⁵ in the UK.

High-pressure chemistry techniques are at the heart of the innovative technology, which the researchers say is an under-utilized process, particularly at the micro and nanoscale, that can be used to create interesting materials properties and behavior.

Figure 3: Silica microstructured optical fiber templates drawn at Southampton University’s Optoelectronics Research Centre.

Applying these techniques to silica microstructured optical fiber nanotemplates produces arrays of pores that can be arranged in virtually any pattern. The pores have diameters as small as 10 nm, but can be meters in length. Layer by layer deposition of the pores forms uniform doped semiconductor homo and heterojunctions, according to the researchers. The resulting excellent optical and electronic properties, combined with the wide variety of sophisticated structures that can be generated, is what makes this combination of nanoscale wires and junctions of interest to so many applications.

The novelty of this technology for battery-powered, touchscreen-operated devices, is the ability to create long, bendable photovoltaic fibers, which can be woven into a deformable fabric. The flexible, foldable, lightweight material could be used to replace rigid glass or plastic based solar cells to provide a battery-boosting capability in conventional solar cell applications. Researchers envision its use in clothing. There is already interest from the military, for soldiers in the field, as well as for sports applications. A major advantage of such as material is the possibility of collecting light at various angles, rather than a single flat surface.

Graphene: Material benefits

Graphene is cited as one of the most exciting new materials under development worldwide. Not only has it been heralded as a potential replacement for conventional semiconductor logic circuits, but it could also provide benefits in touchscreen technology, as well as in batteries, supercapacitors and displays. Stronger than steel and more conductive than copper, researchers around the world are investigating its electrical, thermal and mechanical properties.

At least five universities in the UK will benefit from European funding to aid the development of graphene. The University of Cambridge, in particular is looking at its potential to create thinner and lighter touchscreens and computer displays. Canadian researchers are investigating its use in rechargeable batteries. Stanford and Kansas Universities in the US see it as an alternative material for use in solar cells and photovoltaic devices.

Meanwhile, industrial research continues apace into solar cells to power ever more power-hungry touchscreen operated portable devices, including smartphones, tablets and even TVs. Samsung, for example, demonstrated a solar powered 46 inch LCD TV back in 2011, which is not yet commercially available. Some past LG and Samsung touchscreen phones incorporated large photovoltaic panels, taking four hours to recharge the onboard battery. Apple is understood to be investigating solar cells incorporated into iPad covers and touch-sensitive displays.

Here and now

Already available, however, is a promising technology from French start-up, Wysips: a transparent photovoltaic film that can be incorporated into (over or under) a conventional display or touchscreen. The technology, ‘what you see is photovoltaic surface’, bonds miniaturized photovoltaic materials to a lens network. In combination, they generate an optical effect that masks the photovoltaic cells and captures energy from artificial light or the sun. The photovoltaic element is connected to dedicated circuitry to convert and manage the electrical energy produced, using it to top up the device’s batteries.

The material can be integrated into any type of display or touchscreen, the company states, providing 90% transparency and even helping to widen the angle of vision on small screens. The power generated by current devices can reach 5.8 mW/cm² in good sunlight. Continuing research and development into next generation, organic semiconductor polymers, is expected to yield 10 mW/cm² peak power by 2014.

The company claims that even when a smartphone is completely discharged, holding a Wysips-enabled phone up to the light will generate enough energy to search for a network and make remote payments, for example, or an emergency call. While the Wysips technology has the potential to replace charging units for low power devices such as e-book readers and low end phones, it is expected to allow higher power devices to be designed with reduced size batteries. The company is expected to announce licensing deals with manufacturers in the near future.

Solar cells

OEMs looking for solar cells to incorporate into touchscreen driven, battery powered products today, can find a number of options from a variety of manufacturers. For example, one might consider the Ixolar SolarBIT range from IXYS. Although designed for a broader range of applications than just touchscreen displays, these high efficiency monocrystalline devices are compact enough to be easily incorporated into many types of battery-powered products. The smallest devices in the range are the KXOB22-01X8, which measures just 22 x 7 x 1.6 mm, and the KXOB22-12X1, measuring 27 x 7 x 1.8 mm. Typical voltage ratings are 0.5 V/44.6 mA and 3.4 V/3.8 mA respectively, and efficiency is quoted at 22% measured at the wafer level.

Another popular vendor choice is Sanyo Energy, offering a wide range of amorphous solar cells. Amorphous solar cells, in contrast to crystal silicon, feature irregular atomic arrangements, which the manufacturer claims allows more light to be absorbed. In addition, using metal or plastic for the substrate, flexible solar cells can be produced. Sanyo’s Amorton range uses three amorphous silicon layers on a glass substrate. Cells can be connected in series on a substrate allowing any desired voltage to be obtained, depending on the application requirement.

Solar cells are available for both indoor and outdoor use. One of the smallest models available for indoor use, in devices such as touchscreen calculators, for example, is the AM-1456CA. Measuring just 10 x 25 mm, and weighing 700 mg, typical operating characteristics include 1.5 V/5.3 A operation with a 200 lux luminance. For outdoor use, in devices such as smartphones, the Amorton devices can generate typically 100 mW/cm². A mid-size device, such as the AM-5412CAR, with external dimensions of 50.1 x 33.1 mm, and weighing 7.3 g, is specified at 93 mW maximum output (2.6 V/35.8 mA typical).

Summary

This article illustrates that energy harvesting has great potential application in touchscreen devices, whether powering them directly, or boosting battery power. International research findings show that there is much technology on the way that will make touchscreen devices a yet more viable and efficient proposition for energy harvesting. Both solar power and piezo-electric generated energy techniques are under development. Yet, today, there are already solutions that can be, and are already, in use. These are based on miniature solar cells which themselves are improving in efficiency.

References:

1. Advanced Materials journal: Samsung/Sunghunkwan University paper on nanoscale piezo-electric energy harvesting
2. Triboelectric generators: Georgia Institute of Technology, School of Materials Science & Engineering
3. Advanced Materials journal: Penn State University paper on flexible silicon/fibre optic cores
4. Penn State University: nanotemplates for photonics