Rapid advances in materials for stretchable electronics are beginning to give rise to realistic applications in bio-integrated sensors, bio-inspired cameras, conformal communication devices and other systems that demand energy sources with similar mechanical properties. This talk describes work on stretchable batteries and photovoltaics, both of which offer high performance operation and the ability to accommodate strains of up to 200%, with linear elastic mechanical responses. These classes of technologies exploit segmented, open mesh architectures and soft, elastomer supports, in which active elements are joined by deformable interconnects. Designs that incorporate surface relief structures in the elastomers can isolate the most brittle components from applied strains and, in certain configurations, limit the allowable levels of deformation. Examples of these devices in integrated electronic, optoelectronic and sensor systems illustrate current capabilities and highlight opportunities for future work.

Understanding and improving the mechanical stability of organic solar cells is necessary for these devices to fill roles in portable devices for fieldwork, integration with textiles, soft robotics, skin-like sensors, and other applications requiring mechanical compliance. Even for applications in which organic solar panels are to be fixed in place, the devices must withstand the stresses of roll-to-roll coating, diurnal and seasonal thermal expansion, and weathering. While the lifetime of organic solar cells against photochemical damage has been intensely studied and dramatically improved, mechanical measurements, however, have shown that organic semiconductors can be quite brittle, and that the interfaces can be quite weak. Field tests suggest that mechanical failure can be a principal form of device degradation. This seminar describes several approaches to improving the mechanical compliance of transparent electrodes, conjugated polymers, and whole devices. Our efforts to produce stretchable transparent electrodes led us to investigate ways to increase the elasticity of films of transparent conductive polymers and carbon nanotubes. The seminar will also discuss the factors that influence the mechanical properties of semiconducting polymers and their blends with fullerenes. We found that the photovoltaic properties of devices in which the polymer is based on the diketopyrrolopyrrole unit are unusually tolerant to tensile strains up to 20%. Finally, we will discuss two approaches to rendering whole devices stretchable: (i) by using instrinsically stretchable materials and (ii) by forming wavy, accordion-like films of non-stretchable materials on elastic substrates. The seminar will compare these approaches and comment on the future applications and scalability of these ultracompliant photovoltaic materials and devices.

Stretchable transparent conductors are an essential component of next generation optoelectronic devices, such as solar cells and displays, as they will enable mechanical compliance in these devices and also increase their durability. In this work, we explore the use of poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) films as stretchable, transparent conductors. By incorporating a fluorosurfactant additive to PEDOT:PSS solutions, we obtain films with improved conductivity in addition to rendering them amenable for deposition on hydrophobic surfaces, such as poly(dimethylsiloxane) (PDMS) substrates. Four-layer PEDOT:PSS films show a sheet resistance of 46 ohms per square at 82% transmittance (at 550nm). These films deposited on pre-strained PDMS substrates are shown to be reversibly stretchable, with no change in sheet resistance over 5000 cycles form 0 to 10% strain. We also show that the means by which these films are deposited on unstrained PDMS substrates has a significant impact on the change in resistance versus strain. Finally, we demonstrate the utility of these films in stretchable organic solar cells and capacitive touch sensors.

Recently we have presented stretchable circuits, based on printed circuit board technology [1]. These circuits are not very well suited for applications where a high degree of biocompatibility is required (e.g. for implantation purposes), because they use standard Cu as the electrical interconnection material. Moreover fine interconnection pitches (smaller than 100 micrometer) cannot be achieved because patterning resolution is limited by the Cu thickness which usually is 17µm or 35µm in PCB technology. Therefore thin-film versions of the PCB technology based technology for elastic circuits have been developed and will be presented in this contribution. They use sputter deposited TiW/Au thin-film metal layers as metal interconnects. TiW (typically 50nm thick) is used as an adhesion layer, while Au (typical thickness 250nm) is a biocompatible metal with low resistivity. This biocompatible thin-film metal stack is supported by polyimide which is a flexible polymer material. The metal layer, as well as its polyimide support are patterned as meanders, which after embedding in biocompatible elastic PDMS (poly-dimethyl siloxane, silicone rubber) material, allows in plane deformation (stretching) of the meander without loss of its electrical interconnection functionality. The polyimide starting material can either be a spin-on material (e.g. HD Microsystems 2611), or a sheet, as used in standard flexible circuits (e.g. Dupont Kapton®). The metal meanders are formed by lithography and wet etching, for the polyimide meanders dry etching with a hard metal mask is used for patterning. Metal meandering lines with widths as low as 20 micrometer were successfully patterned in this way. Supporting the thin-film metal meanders with a flexible polyimide drastically increases the mechanical reliability, compared to non-supported meanders. Measurements for HD2611 supported TiW/Au meanders, embedded in Dow Corning Silastic® MDX4-4210 PDMS show a minimum lifetime of 500&’000 cycles at a strain of 10%, without any measurable change in meander resistance. Moreover, in the case of use of the spin-on polyimide we have demonstrated for the first time the possibility to integrate an ultra-thin chip package (UTCP, [2]) together with thin-film stretchable interconnections in the same soft PDMS substrate. The chip is thinned down to a thickness of 20 to 30 micron. It is embedded in HD2611 spin-on polymide, which simultaneously serves as the support for the stretchable interconnects. The TiW/Au metal layer is used, not only as stretchable interconnection metallization, but also as contact material to the chip pads, and fan-out to the stretchable interconnects. In this contribution we will describe the different fabrication processes in further detail and show examples of fabricated devices, using these technologies. [1] J. Vanfleteren et al., MRS-B, Vol.37, pp.254-260 , 2012. [2] W. Christiaens et al., IEEE Trans. Comp. Pack. Techn., 33 (4): pp. 754-760, 2010.

The total thickness of the active components of organic electronic devices, including organic solar cells, is typically less than 500 nm. For this reason, the mechanical properties of weight and flexibility are almost entirely determined by the device substrate. Here we demonstrate polymer based photovoltaic devices on plastic foil substrates less than two micron thick, with equal power conversion efficiency to their glass-based counterparts. They can reversibly withstand extreme mechanical deformation and have unprecedented solar cell specific weight. Instead of a single bend, we form a random network of folds within the device area. The extreme flexibility of the device allows them to function as stretchable solar cells when adhered to a prestretched elastomeric support. These ultrathin organic solar cells are over ten times thinner, lighter, and more flexible than any other solar cell of any technology to date.

In the long-term effort to solve the worldwide energy crisis problem, energy harvest and energy storage are two most important techniques, which are generally based on different principles with two separate units, such as solar cell to convert solar energy to electricity, and lithium ion battery to store electric energy as chemical energy. These two types of devices are usually used together as a power system through the physical connection of independent parts. However, such systems are mostly large, heavy, inflexible and inefficient. Here, we present a new approach to fabricate an integrated power pack by hybridizing energy harvest and storage processes. This power pack integrates a series-wound dye-sensitized solar cell (DSSC) and a lithium ion battery (LIB) on two sides of a Ti foil that serves as the common electrode and substrate through double-sided TiO2 nanotube (NTs) arrays on it. The solar cell part is made of two different cosensitized tandem solar cells based on TiO2 nanorod arrays (NRs) and NTs, respectively, which provide an open-circuit voltage of 3.39 V and a short-circuit current density of 1.01 mA/cm2 through harvesting solar energy. On the other side of the Ti foil, the TiO2 NTs act as the anode of a LIB, which stores the electrical energy converted from solar energy by DSSC. Upon the irradiation of a full-sun intensity solar simulator, the power pack can be charged to about 3 V in about 8 min, and the discharge capacity is about 38.89 mu;Ah under the discharge density of 100 mu;A. The total energy conversion and storage efficiency for this device is 0.82%. Thus, through this unique structure of the integrated power pack, we successfully realized the harvest of solar energy and subsequent storage as chemical energy in a single device. Such an integrated power pack with a flexible, ultrathin structure and a lightweight will have a potential application where there are needs for portable and small energy sources, such as for personal electronics. [1] [1] Wenxi Guo, Xinyu Xue, Sihong Wang, Changjian Lin, Zhong Lin Wang. Nano Letters, 12 (2012) 2520-2523.

In this talk we demonstrate a cheap, disposable, alkaline battery where the electrodes is embedded inside a high porous non-woven battery separator paper. The paper provides a support to the composite electrode and enables a bend radii as low as 1mm without any mechanical degradation. We show the mechanical characteristics of the embedded electrode under stress and compare the performance against an unsupported printed battery. Materials used in building conventional electronic devices are generally inflexible and brittle, but in recent years novel vapor and solution based processing of electronics have enabled devices that are thin and flexible. These techniques leverage the trend that semiconductor device performance generally increases as feature size decreases. While battery electrodes can be made flexible to a certain extent by making them thin, thinner batteries have a decreased per area storage density. Beyond this, the performance of most thin film approaches to batteries is altered and degraded by applied external stress. In the approach mentioned above we have achieved the flexibility of a thin film cell with the capacity of a thick film cell in a form factor where flex does not affect battery performance.

In this work we demonstrate a versatile printing technique to deposit inks for battery applications using a commercially available air-brush. This technique allows for rapid deposition of thick slurries on non-conventional substrate with excellent control over the deposition roughness. This method has been implemented to compliment novel fabrication techniques that have enabled traditionally non-compliant, rigid electronic components to be flexible and/or stretchable, leading to devices for sensing, health monitoring and recreation purposes. Introduction of compliant devices has spurred work in power sources such as batteries, supercapacitors and solar cells with similar mechanical properties. Compliant powers sources are generally made from solution based processing of the component where slurries are deposited on compliant substrates such as plastics. We fabricate compliant batteries where the all the component are deposited with spray deposition and we also show a 3D battery fabricated with the spray deposition technique. Moreover, commercially available, mass produced battery grade materials can be used with this approach. The performance of our batteries fabricated through spray coating will be compared to previous compliant batteries we have made, as well as other demonstrated in literature.

Poly(vinylidenefluoride trifluoroethylene), P(VDF-TrFE) is one of the well-known ferroelectric polymers for its high spontaneous polarization. Since it has easy processability of various shapes and adequate flexibility, P(VDF-TrFE) has been in focus as applications in sensors, actuators, and energy harvesters. However, this polymer generates much lower electric power than ferroelectric ceramics. We fabricate a carbon nanotube (CNT)-P(VDF-TrFE) nanocomposite thick film in order to increase the electric output power. Thick film is fabricated by tape casting method on the PEN/ITO substrate. The various amounts of CNTs are added to each polymer film to determine the effects of CNT nanocomposite in the film. Adding CNT leads ferroelectric polymer to have more beta phase crystalline and higher piezoelectric coefficient. CNT dispersed in the ferroelectric polymer film also helps to catch generated electric charges under mechanical stress. For these factors, P(VDF-TrFE) thick film containing CNT generates more electric output powers than film without CNT nanocomposite. This film can work as an advanced energy harvester having flexible and transparent properties as well as higher energy conversion efficiency.

Piezoelectric nanogenerator has been studied for energy harvesting device converting mechanical energy into electrical energy with high effective efficiency. Especially, polymer piezoelectric material of PVDF is applied to the energy harvesting.[1] In this study, we explored β phase fraction and generating electrical power of electrospun PVDF in different electrospinning conditions such as supplied voltages and viscosities of polymer solution. The maximum β phase fraction and generating electrical power of the electrospun nanofibers were obtained by electrospinning from low viscosity solutions or under higher applied voltage for high viscosity solutions. These analyzed through piezo d33 meter, X-ray diffraction, Differential scanning calorimetry and Fourier Transform Infrared Spectrum investigations. The piezoelectric electrospun PVDF nanofibers are applicable as transforming a body in motion into electricity to portable devices for energy harvesters. Reference [1] C. Li, P. Wu, S. Lee, A. Gorton, M. J. Schulz, and C. H. Ahn, J. Microelectromech. Syst., vol. 17, no. 2, pp. 334-341, Apr. 2008.

Piezoelectric materials may arguably be the most versatile and widely used materials for a wide range of applications including mechanical sensors, actuators, energy storage and mechanical energy harvesting devices. The successful employment of polymeric piezoelectric materials, such as polyvinylidene fluoride (PVDF) and its related co-polymers, although light weight, flexible, optically transparent and cost effective, are limited by relatively low piezoelectric coefficients, thermal stability, and durability. Recently, efforts have been made theoretically and experimentally to develop a mechanical energy harvester with both single and multilayered PVDF films to achieve either high voltage or high current output. While the obtained results are promising, the device is still fall far behind the inorganic. Materials and technical breakthrough, therefore, is needed to achieve high β phase crystallinity and lower the binding energy. Co-extrusion technology developed by our collaborators allows fabricating multilayer films from a few layers up to thousand layers with 2 or 3 polymers. Layer thickness may range from microns down to nanometers, which has opened the door to study bulk polymeric properties and physical and interfacial confinement effects when the layer thickness is in nanometer range. This has provided a unique opportunity to explore whether the physical and interfacial confinement effects will lead to β phase formation, control of the dipole and domain orientation, and stabilize any preferred crystalline phases. The main objective of this poster presentation will be to provide a comparative study of single and multilayered PVDF with co-extruded PC/PVDF multilayered systems with equivalent PVDF materials processed in the same conditions. Mechanical energy conversion efficiency of the films will be evaluated with a driven sinusoidal mechanical cantilever setup. Both thermal stability and crystallinity will also be measured as functions of number of layers and layer thickness. The effects of physical confinement and interfacial interactions will be demonstrated.

Thick films were prepared using a two-step sol gel spin coating process and were deposited onto a flexible stainless steel substrate followed by annealing of the dried composite. The two-step process comprised of spin coating at 500rpm for 30 seconds and then at 1000rpm for 1 minute. In these composites a third phase inclusion was introduced in the form of Al powders to enhance the overall polarizability of the composite by increasing the dielectric constant. The piezoelectric strain coefficient d31, was studied at increasing volume fractions of Al. It was noted that beyond a critical value the Al particles form conductive pathways within the composite where it prohibits the internal charge separation, this is known as percolation. Percolation in these composites were observed to occur at 20%, 63% and 17% volume fractions of BaTiO3, Epoxy and Al respectively, where the percolation threshold lies between 13% and 17% volume fraction of Al. The morphological studies of the various phases within the composite were also done via SEM and TEM.

Piezoelectric energy harvesting (PEH) device refers to a power device for acquiring mechanical energy from the environment surrounding us which would otherwise be wasted and for converting it into usable electrical energy. PEH is the key technology for making self-powered electronics, such as the wireless sensor network, an implanted biomedical nanodevice and a battery-less mobile device. While much work has been done on developing ZnO nanogenerator (NG) with nanowire arrays, there are some issues of not only scaling up its output power but also optimizing structure for operating feasibly in various conditions. Efficiency of NG is highly dependent on its orientation. The conventional device works along a specific direction because nanowires are 1D structure aligned along its axis. But in many cases, it is not easy to predict where the pressure and vibration may come from. Furthermore, the direction of the applied mechanical stress is usually non-stationary and can be random in various practical applications. This limitation could be a serious bottleneck for commercializing NG. In this work, we investigate an omnidirectional PEH device consisting of the laterally integrated ZnO nanowire arrays. We deposit two helical patterned lines of Au electrode with Cr layer for retarding nanowire growth and pattern ZnO seed layer on a flexible substrate using maskless lithography. ZnO nanowires have been grown from the ZnO seed layer from one electrode to other electrode on the opposite side. After completing the growth of nanowires, PDMS is poured and cured on the surface of NG to protect and to improve flexibility of the device. Consequently, each nanowire has different directions to each other. This isotropic design can lead to the omnidirectional power generation. The morphology of NG is characterized with FESEM and the individual nanowire is analyzed with a Cs-corrected STEM. Maximum output power of this device is smaller than a conventional ZnO NG where all the nanowires are aligned along the optimal direction. But multi-stacked spiral devices provide a potential to scale up its output power independent of direction. In addition, we evaluate the critical condition for the mechanical failure of ZnO nanowires due to the tensile and compressive stress conditions along various angles.

Electronic devices conformable to non-developable surfaces support applications in artificial electronic muscles, conformable mobile electronic appliances, electronic skin, textile electronics, and electronic bio-interfaces. While there has been tremendous progress in the development of such stretchable electronic devices, research on sources for power are still in an infant state, providing opportunities for exciting new developments at the latest frontier in stretchable electronics. For stand-alone applications of stretchable electronics it is necessary to develop suitable means for power supply. Wireless configurations employing inductive energy transfer or radiofrequency transmission are widely used in commercial devices, for example to charge surgically implanted devices, electrical toothbrushes used in wet environments, and wireless headphones. Such wireless transmission of power is very attractive for stretchable electronics, and solutions were developed recently. Direct generation of power in stretchable electronics is feasible by dielectric elastomer and piezoelectric energy generators, as well as stretchable solar cells. We recently developed ultrathin and lightweight organic solar cells that can withstand extreme mechanical deformation. When placed on a pre-stretched elastomeric support, they can be stretched by 400%. Finally, power should be also stored in stretchable electronic units. Supercapacitors offer short term energy storage, and rechargeable electrochemical batteries can store and supply energy for long-term autonomous operation of stretchable electronic devices.

A wide variety of different technologies exist nowadays to realize conformable electronic systems, where each technology has its own particular characteristics and targeted application fields. Each technology has its advantages and disadvantages in terms of processability, reliability, scalability and cost. In this contribution, results on technology developments are presented aiming to realize conformable electronic systems based on plastic electronics technologies. Substrate materials like PET/PEN in combination with printed (e.g. silver paste) or etched (e.g. from laminated copper foil) meandering metal structures are used as starting point. The foils obtain their conformability by structuring them using laser technology or die cutting processes. Electronic functionality is achieved by assembling electronic components (e.g. LEDs, microcontrollers, passives) by means of conductive pastes (ICAs/ACAs). An encapsulation in a stretchable polymer by liquid injection moulding PDMS or laminating PU foils completes the process. Focus of the developments is on low cost with an acceptable reliability in function of the end-application. This contribution discusses the cyclic endurance reliability of these plastic electronics based technologies, in comparison with earlier reported printed circuit board based elastic microsystems. A study is presented investigating lifetime of stretchable interconnects in relation with design and technology parameters: conductor material choice (etched copper/printed silver), meander support material (PET/PEN/PI) and encapsulation material (PDMS/PU). Cyclic endurance tests performed on testsamples containing stretchable interconnects, show that for elongations upto 10%, depending on the design and technology, the interconnections can survive upto 100.000 cycles, without diminishing in electrical conductivity.

Flexible pressure sensors are essential components in future mobile applications like rollable touch displays, bio monitoring and electronic skin. We report on highly pressure sensitive flexible thin film transistors combining Polyisoindigobithiophenesiloxane (PiI2TSi) [1] as the semicondutor and microstructured Polydimethylsiloxane (PDMS) as the dielectric [2]. PiI2TSi has been recently shown to exhibit highly ordered polycrystalline thin films with an excellent field effect hole mobility >2 sqcm/Vs. By letting the device operate in the subthreshold regime, we show that we are able to enhance the pressure sensitivity by more than one order of magnitude as compared to a bare capacitive sensor based on microstructured PDMS resulting in an unprecendented sensitivity of 8.2/kPa. Moreover, the device also shows high bend sensitivity at small curvatures, thus having great potential for future mobile bio monitoring and electronic skin applications. [1] J. Mei et al., J. Am. Chem. Soc. 133, 20130 (2011). [2] S. C. B. Mannsfeld et al., Nat. Mater. 9, 859(2010).

Compliant energy systems rely on compliant electrodes to connect stretchable energy-harvesting/storage devices. To minimize system stiffness and to reduce strain in inorganic electrodes, metals and ITO can both be patterned into thin film serpentines on polymer substrates. When the polymer substrate is stretched, thin film serpentines can rotate and twist to accommodate the applied deformation, resulting in greatly reduced system-level stiffness and intrinsic strain in the electrode materials. This study reveals the fundamental deformation and failure mechanisms of polymer-supported metallic and ceramic serpentines through both analytical and experimental means. Results from this study may provide guidelines of designing future serpentine-based stretchable electronics.

Electrically small antennas used for radio frequency applications have a physical size that is smaller than the application wavelength. They often have complex electrode patterns that pack a long length into a small volume in order to maximize bandwidth. These designs include spirals, meander lines, or fractal patterns. Furthermore, 3D antennas are known to have better performance than 1 or 2D structures. However, a method is needed to rapidly prototype antennas for laboratory testing and deploying in the field. In our approach, direct-write assembly is used to rapidly produce complex designs on a 2D surface; this 2D structure is assembled into a 3D electrically small antenna through origami-based bending and folding. A silver particle-based ink tailored for low temperature processing is deposited in a complex pattern on a polymer substrate. These features are encapsulated with a polymer coating to prevent delamination during bending. A conventional desktop laser cutter is used to cut and score the device in preparation for deploying. We plan to use this system to test the effects of linewidth, electrode layout, and thermal annealing on the performance characteristics of antennas. Rapid testing of broadband or multimode antennas may also be possible.

Electrical energy storage plays a key role in mobile electronic devices, stationary power systems, and hybrid electrical vehicles. High energy density capacitors based on dielectric polymers are a focus of increasing research effort motivated by the possibility to realize compact and flexible energy storage devices, taking advantage of light weight and facile processability of organic materials. In addition, dielectric polymers enjoy inherent advantages of self-healing mechanism and high breakdown strength, leading to capacitors with great reliability and high energy density. It is the focus of this group to develop a multilayered ferroelectric PVDF system for improved energy storage efficiency. These systems are fabricated using enabling technology in co-extrusion which allows more cost effective and large area device production as opposed to more conventional layer-by-layer techniques. Many efforts have been made by the team to fabricate these micro- and nano-layered systems resulting in much improved device performance. A three-time improvement of capacitive electrical energy density has been demonstrated. The focus of this research is to understand the physics of why these multilayered systems perform better than a single layer by developing a characterization technique using both confocal second harmonic generation (SHG) and electric field induced second harmonic (EFISH) laser spectroscopy. Our results have shown that SHG is a very sensitive, non-destructive and versatile technique that can be used to study the ferroelectric and structural properties of layered systems. When combined with EFISH this technique allows the interrogation of structural and dielectric properties within the individual layers and at the interfaces between the layers. Further, the proposed techniques can be readily employed in-situ which can provide information in real time during sample processing with static and time-resolved spectroscopic measurements.

Many favorable properties of ionic liquids as electrolytes, including nonvolatility, thermal stability, and wide electrochemical windows have spurred a wide range of investigations in recent years. The creation of a solid thin film electrolyte from an ionic liquid base may enable a new class of safe and high performance optoelectronic devices, including electrochemical double layer capacitors (EDLCs) for electrical energy storage. Film rigidity from an ionic liquid starting material can be achieved through the in situ formation of a supporting matrix, such as an entangled polymer network. The resulting ionic liquid-based gel is referred to as an ionogel. Here, compression testing and AC impedance spectroscopy have been used to characterize the mechanical and electrical responses of ionogels containing between 4.9 and 44.7 wt % cross-linked poly(ethylene glycol) diacrylate. Differential scanning calorimetry and variable temperature impedance measurements were performed in order to elucidate the thermal behavior of this class of ionogels. The elastic modulus of these solid electrolyte materials is observed to vary by more than four orders of magnitude within the composition range studied; corresponding changes in gel ionic conductivity and double layer capacitance were much less dramatic. This work lends insight into the potential trade-offs inherent to maximizing gel electrical performance while maintaining an acceptable level of mechanical rigidity, and can guide the development of ideal ionogel formulations for future flexible energy storage devices, such as EDLCs.

We synthesized flexible supercapacitor electrode materials by carbonizing a common livestock biowaste in the form of chicken eggshell membranes. The carbonized eggshell membrane (CESM) is a three-dimensional macroporous carbon film, composed of interwoven connected carbon fibers containing around 10 wt% oxygen and 8 wt% nitrogen. The macroscopic structure of the CESM resembles that of construction paper. Despite relatively low surface area of 221 m2 g-1, exceptional specific capacitances of 297 F g-1 and 284 F g-1 are achieved in basic and acidic electrolytes in 3-electrode system, respectively. This yields an unusually high volumetric energy density in the range of 600 F cm-3. Furthermore the electrodes demonstrate excellent cycling stability: only 3% capacitance fading is observed after 10,000 cycles at a current density of 4 A g-1. These very attractive electrochemical properties are discussed in the context of the unique structure and chemistry of the material.

Portable electronics have become a big part of everyday life, from smart phones to medical devices they have revolutionized the way we live. One bane of the portable electronics era is the need to recharge batteries. This talk will discuss a technology called the dielectric elastomer, which could harvest electrical energy from human movements eliminating the need to recharge batteries. Dielectric elastomer generators are highly suited to harvesting from human motions because they are soft, light weight, highly stretchable, and can efficiently harvest from the low frequency motions associated with human movement. Dielectric elastomer generators have struggled to realize their full potential for flexibility, simplicity and low mass at a system level because they require rigid and bulky external circuitry. This is because a typical generation cycle requires high voltage charge to be supplied or drained from the generator as it is mechanically deformed. Our recent work focusing on producing generator circuitry that is portable and well suited to small scale wearable generators will be discussed. We will describe how the generator&’s circuitry can be integrated directly onto soft dielectric elastomer membranes using a highly stretchable electronics technology called dielectric elastomer switches, resulting in a generator simply fabricated from an acrylic membrane and carbon grease mounted in a frame. This generator achieved a competitive energy density of 9.5mJ/gram.

Harvesting unexploited energy in the living environment to power small electronic devices and systems is attracting increasing massive attention. We have been working on such kind of technology since 2005 by using piezoelectric nanomaterials to convert mechanical energy into electricity. We called this technology as “piezoelectric nanogenerator (NG)”. Mechanical energy is a very conventional energy source in our living environment, with sources including the vibration of a bridge, friction in mechanical transmission systems, deformation in the tires of moving automobiles, etc., all of which are normally wasted. This form of energy is particularly important when other sources of energy, such as sun light or thermal gradients, are not available. Based on a totally new designed device structure, the NG&’s output performance has been promoted to a new level. The measured output voltage reached 10 V, and the output current exceeded 0.6 mu;A (corresponding power density 10 mW/cm3), when it was strained to 0.12% at a strain rate of 3.56% S-1. The first self-powered system driven by a NG was built up by integrating a nanogenerator, rectification circuit, capacitor for energy storage, sensor, and RF data transmitter. It works wirelessly and independently for long distance data transmission. Furthermore, the NG showed the potential for the transportation monitoring to work as a self-powered tire-pressure sensor and speed by scavenging mechanical energy from deformation of the tire during its motion.

Dielectric elastomer (DE) is a kind of new smart material that can provide efficient electrical-mechanical energy transduction. Dielectric elastomer generators (DEG) and dielectric elastomer actuators (DEA) are two main applications that are of great interest to researchers. Current DE materials available on the market include 3M VHB acrylic elastomers and various silicone elastomers. Acrylic elastomers have a higher energy density as well as electromechanical strain, but suffer from significant viscoelasticity and poor processability. Silicone elastomers are superior in processability, have less viscoelasticity, but are limited in strain. Both types of materials will need prestretching to achieve their peak performance. We report a new category of dielectric elastomers that is synthesized from an easy UV radiation polymerization process. The new materials can survive a >100% electromechanical strain, provide a >2 MJ/m3 energy density, have a lower viscoelasticity, while no prestretching is needed. Further improvement of this type of material will allow the demonstration of high performance DEA/DEG devices.

Electrical energy can be generated from mechanical deformations using dielectric elastomers but currently achieved conversion efficiencies and energy densities are still small. In this presentation, we demonstrate that significant improvements, up to 10% in efficiency with an energy density over 500 mJ/g, can be produced using VHB elastomers by altering the mechanical loading geometry and the electrical circuitry. A major limitation is viscous losses in the VHB elastomer indicating that higher efficiencies with other elastomers will be attainable. The basic concept of mechanical energy harvesting with a dielectric elastomer sheet is a straightforward electromechanical cycle leading to a voltage step-up: a sheet is stretched, electrical charge at low voltage is placed on either side using compliant electrodes, the circuit disconnected, the stretch is released causing the sheet&’s initial thickness to be recovered separating the charges which can then be drawn off at higher voltage. Integral to maximizing the energy conversion is the amount of mechanical energy that can be stored elastically in the elastomer sheet during stretching. We show that this can be maximized by equi-biaxial loading. Furthermore, as the electrical capacity varies with the fourth power of the mechanical stretch, the design of the mechanical loading system is key to enhanced performance. Details of our dielectric elastomer generator will be described as well as the procedures we use for quantifying its performance. Designs based on the same optimization of the mechanical loading will be described for harvesting energy from ocean waves and wind.

Vibration-based energy harvesting is an attractive solution for powering autonomous microsystems, because of numerous opportunities for vibration sources in ambient environments. Composite piezoelectric thick films have shown promise as energy harvesting devices due to their flexibility and applicability over a wide range of frequencies. In the present work, three phase flexible piezoelectric PZT-Epoxy-Multiwalled Carbon Nanotube (MWCNT) composite thick films with a thickness of ~ 150 µm have been fabricated. MWCNTs were dispersed in ethanol by ultrasonication and then PZT and epoxy were added to this solution. This mixture was again sonicated and then desiccated. The volume fraction of the MWCNT phase was varied from 1% to 6%. The three phase mixture was then spin coated over a stainless steel substrate of thickness 20 µm by a two step spin coating procedure to achieve a relatively uniform coating. The thick films were poled at a voltage of 2.2 kV/mm to align the dipoles of the piezoelectric phase. The measured values for the piezoelectric strain coefficients, d31, d33, dielectric constant, ε and tan(δ) were found to increase with the volume fraction of the MWCNTs due to the effect of an increase in polarization of the Zr/Ti dipoles with higher MWCNT volume fraction. Both the d31 and d33 values drop after the percolation threshold is reached. The percolation threshold with constant PZT volume fraction and varying MWCNT volume fraction was seen at 30%, 65% and 5% volume fractions of PZT, Epoxy and MWCNT respectively. The electromechanical properties of the thick films depend on the microstructure , clustering of PZT, contact resistance between MWCNT and the different phases, and the surface morphology of the thick films. SEM, TEM and AFM images of the films were used to investigate the effect of the microstructure on the electromechanical response of the thick films.

Carbon nanotubes have been investigated as promising fillers in polymer matrix composites because of their high aspect ratio, which enhance the mechanical and electrical properties of the composite. In the present work, the matrix of two phase PZT-Epoxy composites was reinforced with multiwalled carbon nanotube (MWCNT) inclusions with aspect ratios that varied from 20 - 30. The composites were fabricated by using the sol gel process. MWCNT inclusions were functionalized with ethanol and subsequently added to a solution that was comprised of PZT and epoxy. The composites were poled at a high voltage of 2.2 kV/mm to align the Zr/Ti dipoles in the PZT phase. The measured values for the piezoelectric strain coefficient, d33, dielectric constant, ε and tan(δ) were found to increase with the volume fraction of the MWCNTs due to the effect of an increase in polarization of the Zr/Ti dipoles with higher MWCNT volume fraction. With the increase in the volume fraction of the MWCNT inclusions, conductive pathways are formed that hinder charge separation, which leads to percolation. The composites show a sudden drop in d33 values with a considerable increase in the dielectric constant at the onset of the percolation threshold. The percolation threshold with constant PZT volume fraction (30%) and varying MWCNT volume fraction (1-10%) was seen when the volume fraction of MWCNT was 5%. The percolation threshold is an important parameter that helps us in optimizing the electromechanical characteristics of the piezoelectric composites. Microstructural properties such as agglomeration of MWCNT inclusions and clustering of PZT inclusions in the Epoxy matrix and the contact resistance between the different phases have significant effects on the electromechanical properties of the composite. The present work also includes the analysis of the microstructure of the composites and also the individual phases through SEM and TEM imaging to determine and effects of the aforementioned factors on the electromechanical properties of the composite.

In recent years the study of polymer based piezoelectric materials have gained considerable attention due to their flexible nature and application over wide frequency ranges. The current work includes fabrication of epoxy-PZT composite flexible thick films with micron sized Al inclusions. A mixture of PZT, epoxy and micron sized Al were suspended in ethanol. This mixture was spin coated over a stainless steel substrate followed by curing on a hot plate for 8 hours at a temperature of 75 deg. C. Post curing the films were poled at a high temperature and voltage followed by testing for the piezoelectric strain coefficients, d33 and d31, dielectric constant ε and dielectric loss tan (δ). These coefficients were enhanced with increasing volume fraction of the Al inclusions; however beyond a certain critical value of Al volume fraction the percolation threshold of the composite is reached. After the percolation limit the values of d33 and d31 decrease considerably accompanied by a sharp increase in the dielectric constant. The electromechanical properties of the composite are influenced by the percolation threshold and also the morphology of the microstructure. Furthermore a study of the percolation of the composite and the microstructure based on techniques such as the SEM and AFM is also performed.

The performance of piezoelectric power generation with a thick and rigid template are limited, since they are usually operated in a low magnitude and frequency state with very minute and irregular mechanical energy sources from the living environment, such as body movement, air flow, hydraulic pressure and acoustic vibrations. Therefore, developing piezoelectric energy harvesters as a free-standing type with high performance and stretchability is particularly important. The atomically layered structure of two-dimensional graphene sheets with high mechanical elasticity (elastic modulus of about 1 TPa) and high transparency can be used to fabricate fully flexible transparent piezoelectric power generators. In this work, we fabricated a stretchable, multi-shape transformable, mechanically durable and transparent free-standing power generator with a very high performance by sandwiching a thin film of polymeric piezoelectric material poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] into graphene electrodes which is able to convert low frequency and low magnitude environmental energies. We investigated the mobility modified graphene electrodes with ferroelectric P(VDF-TrFE) remnant polarization to the use of graphene as the electrode material for the fabrication of the free-standing power generator, and a mechanism of switching the carriers mobility with the ferroelectric remnant polarization is proposed. We demonstrate that upon their exposure to the same input sound pressure, the measured output performance of the free-standing power generator with a thin polydimethylsiloxane stretchable rubber template is up to 30 times that of a normal power generator with a plastic substrate.

The focus of our work is the development of fiber-based batteries for seamless integration into smart textile platforms. In this report we present fabrication strategies of flexible and stretchable batteries composed of Li nanoparticle-based (LiFePO4, Li4Ti5O12, etc.) cathode and anode, and a solid poly ethylene oxide (PEO) electrolyte as a separator layer. Featuring solid thermoplastic electrolyte as a key enabling element this battery is potentially extrudable or drawable into fibers or thin stripes which are directly compatible with the weaving process used in smart textile fabrication. In our talk we first present the choice of materials, fabrication and characterisation of electrodes and a separator layer. Then we describe battery assembly and characterization. Finally, we detail a large battery sample made of several long strips woven into a textile, connectorized with conductive threads, and tested for flexible energy storage.

Wasted heat is a rich source of energy that could be harvested. In 2010, for example, more than 50% of the energy generated from all sources in the U.S. was lost mainly in the form of wasted heat, which presents us with a great opportunity to harvest this type of energy using nanotechnology. Harvesting thermoelectric energy mainly relies on the Seebeck effect that utilizes a temperature difference between the two ends of the device for driving the diffusion of charge carriers. The presence of a temperature gradient is a must for the conventional thermoelectric cell. However, in an environment that the temperature is spatially uniform without a gradient, such as in outdoor in our daily life, the Seebeck effect is hardly useful for harvesting thermal energy arising from a time-dependent temperature fluctuation. In this case, the pyroelectric effect is the choice, which is about the spontaneous polarization in certain anisotropic solids as a result of temperature fluctuation, but there are few studies about using pyroelectric effect for harvesting thermal energy. Using this effect, we experimentally demonstrate the first application of pyroelectric ZnO nanowire arrays for converting heat energy into electricity. The coupling of the pyroelectric and semiconducting properties in ZnO creates a polarization electric field and charge separation along the ZnO nanowire as a result of the time-dependent change in temperature. The fabricated nanogenerator has a good stability, and the characteristic coefficient of heat flow conversion into electricity is estimated to be sim;0.05minus;0.08 Vm2/W. Our study has the potential of using pyroelectric nanowires to convert wasted energy into electricity for powering nanodevices. Ref: Ya Yang, Zhong Lin Wang et al. Nano Letters, 2012, dx.doi.org/10.1021/nl3003039.

Energy storage is a critical parameter to the operation of electronic devices, as is highly efficient, light-weight and wearable energy storage to the field of smart and electronic textiles. Previously reported fabric electric double layer capacitors (EDLCs or supercapacitors) focused primarily on technologies which are either not directly applicable for full device integration (e.g., nonwoven or electrospun textiles) or use expensive carbon nanomaterials with limited energy and power density (e.g., carbon nanotubes). EDLCs, based on non-toxic and non-flammable materials are attractive for developing textile supercapacitors for wearable electronics. Such textile supercapacitors can be used to power wearable sensors and antennas or harvest energy from wearable solar panels or piezoelectric materials. We have previously shown [1] that screen printing is an excellent technique for impregnating commonly worn textiles with capacitive carbon materials, and achieved electrodes made of cotton and activated carbon having ~0.5 F/cm2 areal capacitance and 85 F/g gravimetric capacitances with a sufficiently low series resistance of 4 Omega;-cm2, results that are comparable to conventional film carbon electrodes. This research demonstrates how using established textile technologies with conventional carbon materials enables the direct implementation of energy storage devices into textiles. In this study, we present the fabrication techniques and electrochemical results of two all-textile EDLCs. The first device is comprised of a custom 3D knitted carbon fiber current collector that was impregnated through screen printing with activated carbon paint. These devices yield ~0.75 F/cm2, a 50% improvement over our previous work [1] and the highest capacitance to date for all-carbon textile EDLCs. This work was recently a winner in the NSF-IGERT Poster-Video Competition. The second device is fabricated via the partial-electrochemical-activation of the same carbon fibers, to create fibers that have an activated outer shell and highly conductive core. This technique eliminates the need for binders (traditionally used in EDLCs) that often increase the device resistance; partial activation of the fibers also makes the material more durable to stretching and strain. Both systems are tested using a solid “no leak” electrolyte that acts as both the electrolyte and separator. We will present the electrochemical performance of these devices from cyclic voltammetry, galvanostatic cycling, and impedance spectroscopy and compare the performances to conventional carbon film electrodes. 1. K. Jost, C. R. Perez, J. McDonough, V. Presser, M. Heon, G. Dion, Y. Gogotsi, Carbon Coated Textiles for Flexible Energy Storage, Energy and Environmental Science, 4, 5060-5067 (2011)

Charges induced in triboelectric process are usually referred as a negative effect either in scientific research or technological applications, and they are wasted energy in many cases. Here, we demonstrate a simple, low cost and effective approach of using the charging process in friction to convert mechanical energy into electric power for driving small electronics. The triboelectric generator (TEG) is fabricated by stacking two polymer sheets made of materials having distinctly different triboelectric characteristics, with metal films deposited on the top and bottom of the assembled structure [1]. Once subjected to mechanical deformation, a friction between the two films, owing to the nano-scale surface roughness, generates equal amount but opposite signs of charges at two sides, respectively. Thus, a triboelectric potential layer is formed at the interface region if the generated triboelectric charges are separated; the electrons in the external load are driven to flow for generating an induced potential for screening the triboelectric charges. This is the mechanism of the trioboelectric generator. An TEG gives an output voltage of up to 18 V at a current density of ~0.13 uA/cm2 [2]. Furthermore, the as-prepared nanogenerator can be applied as a self-powered pressure sensor for sensing a water droplet (8 mg, ~3.6 Pa in contact pressure) and a falling feather (20 mg, ~0.4 Pa in contact pressure) with a low-end detection limit of ~13 mPa. TEGs have the potential of harvesting energy from human activities, rotating tires, ocean waves, mechanical vibration and more, with great applications in self-powered systems for personal electronics, environmental monitoring, medical science and even large-scale power. [1] F.R. Fan, Z.Q. Tian and Z.L. Wang “Flexible triboelectric generator”, Nano Energy, 1 (2012) 328-324. [2] F.R. Fan, L. Lin, G. Zhu, W.Z. Wu, R. Zhang, Z.L. Wang “Transparent Triboelectric Nanogenerators and Self-powered Pressure Sensors Based on Micro-patterned Plastic Films”, Nano Letters, dx.doi.org/10.1021/nl300988z

We demonstrate the design and fabrication of a composite electrode by coating ultrathin films of MnO₂ on high surface area Au nanowires (NWs) grown on flexible plastic substrates via electrochemical deposition processes. The electrode demonstrates high specific capacitance, high-energy density, high-power density, and long-term life stability. In Na₂SO₄(aq) (1 M), the maximum specific capacitance is determined to be 1130 F/g by cyclic voltammetry (CV) at a scan rate 2 mV/s. By chronopotentiometry, a capacitance 1080 F/g is measured at a current density 1 A/g. The flexible composite electrode also exhibits a maximum specific energy 100 Wh/kg, and a specific power 45 kW/kg. The cycling performance with 95% capacitance retention can be achieved over 500 cycles. These results suggest that the nanostructured Au NWs/MnO₂ composite material is promising for next generation high-performance flexible supercapacitor applications.

Graphene nanosheets offer possibility for easy fabrication of free standing paper electrodes for flexible energy storage device applications. However, the unique properties of individual graphene sheet are greatly compromised in the macroscopic assembly due to the inevitable aggregation and restacking resulting from the interlayer van der Waals attraction. Effective mitigation of the self-restack issue is thus of great important for developing graphene-based components for energy storage. Here, we demonstrated a facile route to prevent the restacking by minimizing the surface tension through different drying techniques. The resulting paper shaped graphene aerogel maintained excellent flexibility and open structure, where the thickness of the paper was expanded to 30 micron as compared to 2 micron in the air-dried film. The paper shape graphene aerogel can be used as both active materials and substrates in supercapacitor and lithium-ion battery applications. As a potential anode material, the graphene aerogel delivered a specific initial discharge of 3050 mAh/g and maintained 600 mAh/g after 50 cycles at a charge/discharge rate of 100 mA/g. Moreover, a unique 3-D nano-architecture was achieved with nano-sized TiO2 intercalated on the open surface of the graphene aerogel paper through atomic layer deposition (ALD), and the composite paper displayed significantly enhanced Li-ion insertion/extraction rate. Other metal oxides e.g. SnO2, V2O5, ZnO, have also been successfully incorporated into the flexible graphene paper with further enhanced energy storage capability. The promising performance of paper shaped graphene aerogel and graphene-metal oxide aerogel in supercapacitor application will be discussed as well.