Reinhard Schwödiauer

Flexible high power-per-weight perovskite solar cells with chromium oxide–metal contacts for improved stability in air

Photovoltaic technology requires light-absorbing materials that are highly efficient, lightweight, low cost and stable during operation. Organolead halide perovskites constitute a highly promising class of materials, but suffer limited stability under ambient conditions without heavy and costly encapsulation. Here, we report ultrathin (3 μm), highly flexible perovskite solar cells with stabilized 12% efficiency and a power-per-weight as high as 23 W g(-1). To facilitate air-stable operation, we introduce a chromium oxide-chromium interlayer that effectively protects the metal top contacts from reactions with the perovskite. The use of a transparent polymer electrode treated with dimethylsulphoxide as the bottom layer allows the deposition-from solution at low temperature-of pinhole-free perovskite films at high yield on arbitrary substrates, including thin plastic foils. These ultra-lightweight solar cells are successfully used to power aviation models. Potential future applications include unmanned aerial vehicles-from airplanes to quadcopters and weather balloons-for environmental and industrial monitoring, rescue and emergency response, and tactical security applications.

An ultra-lightweight design for imperceptible plastic electronics

Electronic devices have advanced from their heavy, bulky origins to become smart, mobile appliances. Nevertheless, they remain rigid, which precludes their intimate integration into everyday life. Flexible, textile and stretchable electronics are emerging research areas and may yield mainstream technologies. Rollable and unbreakable backplanes with amorphous silicon field-effect transistors on steel substrates only 3 μm thick have been demonstrated. On polymer substrates, bending radii of 0.1 mm have been achieved in flexible electronic devices. Concurrently, the need for compliant electronics that can not only be flexed but also conform to three-dimensional shapes has emerged. Approaches include the transfer of ultrathin polyimide layers encapsulating silicon CMOS circuits onto pre-stretched elastomers, the use of conductive elastomers integrated with organic field-effect transistors (OFETs) on polyimide islands, and fabrication of OFETs and gold interconnects on elastic substrates to realize pressure, temperature and optical sensors. Here we present a platform that makes electronics both virtually unbreakable and imperceptible. Fabricated directly on ultrathin (1 μm) polymer foils, our electronic circuits are light (3 g m−2) and ultraflexible and conform to their ambient, dynamic environment. Organic transistors with an ultra-dense oxide gate dielectric a few nanometres thick formed at room temperature enable sophisticated large-area electronic foils with unprecedented mechanical and environmental stability: they withstand repeated bending to radii of 5 μm and less, can be crumpled like paper, accommodate stretching up to 230% on prestrained elastomers, and can be operated at high temperatures and in aqueous environments. Because manufacturing costs of organic electronics are potentially low, imperceptible electronic foils may be as common in the future as plastic wrap is today. Applications include matrix-addressed tactile sensor foils for health care and monitoring, thin-film heaters, temperature and infrared sensors, displays, and organic solar cells.

25th Anniversary Article: A Soft Future: From Robots and Sensor Skin to Energy Harvesters

Scientists are exploring elastic and soft forms of robots, electronic skin and energy harvesters, dreaming to mimic nature and to enable novel applications in wide fields, from consumer and mobile appliances to biomedical systems, sports and healthcare. All conceivable classes of materials with a wide range of mechanical, physical and chemical properties are employed, from liquids and gels to organic and inorganic solids. Functionalities never seen before are achieved. In this review we discuss soft robots which allow actuation with several degrees of freedom. We show that different actuation mechanisms lead to similar actuators, capable of complex and smooth movements in 3d space. We introduce latest research examples in sensor skin development and discuss ultraflexible electronic circuits, light emitting diodes and solar cells as examples. Additional functionalities of sensor skin, such as visual sensors inspired by animal eyes, camouflage, self-cleaning and healing and on-skin energy storage and generation are briefly reviewed. Finally, we discuss a paradigm change in energy harvesting, away from hard energy generators to soft ones based on dielectric elastomers. Such systems are shown to work with high energy of conversion, making them potentially interesting for harvesting mechanical energy from human gait, winds and ocean waves.

Nonvolatile organic field-effect transistor memory element with a polymeric gate electret

Organic field-effect transistors with a polymeric electret as gate insulator and fullerenes as a molecular semiconductor were fabricated. We observed an amplification of the drain–source current Ids on the order of 104 upon applying a gate voltage Vg. Reversing the gate voltage Vg features large metastable hysteresis in the transfer characteristics Ids(Vg) with a long retention time. The observation of a switchable channel current Ids is proposed to originate from charge storage in the organic electret. As such, this device is a demonstration of an organic nonvolatile memory element switchable with the gate voltage.

Flexible ferroelectret field-effect transistor for large-area sensor skins and microphones

Ferroelectrets generate an electric field large enough to modulate the conductance of the source-drain channel of a thin-film field-effect transistor. Integrating a ferroelectret with a thin-film transistor produces a ferroelectret field-effect transistor. The authors made such transistors by laminating cellular polypropylene films and amorphous silicon thin-film transistors on polyimide substrates. They show that these ferrroelectret field-effect transistors respond in a static capacitive or dynamic piezoelectric mode. A touch sensor, a pressure-activated switch, and a microphone are demonstrated. The structure can be scaled up to large-area flexible transducer arrays, such as roll-up steerable compliant sensor skin.

Large and broadband piezoelectricity in smart polymer-foam space-charge electrets

Charged closed-cell microporous polypropylene foams are shown to exhibit piezoelectric resonance modes in the dielectric function, coupled with a large anisotropy in the electromechanical and elastic material properties. Strong direct and converse dynamic piezoelectricity with a piezoelectric d33 coefficient of 140 pC/N at 600 kHz is identified. The piezoelectric d33 coefficient exceeds that of the ferroelectric polymer polyvinylidene fluoride by a factor of 5 and compares favorably with ferroelectric ceramics. Applications of similar concepts should provide a broad class of easily fabricated “soft” piezoelectric materials.

Fabrication and characterization of solution-processed methanofullerene-based organic field-effect transistors

The fabrication and characterization of high-mobility, n-channel organic field-effect transistors (OFET) based on methanofullerene [6,6]-phenyl C61-butyric acid methyl ester using various organic insulators as gate dielectrics is presented. Gate dielectrics not only influence the morphology of the active semiconductor, but also the distribution of the localized states at the semiconductor-dielectric interface. Spin-coated organic dielectrics with very smooth surfaces provide a well-defined interface for the formation of high quality organic semiconductor films. The charge transport and mobility in these OFET devices strongly depend on the choice of the gate dielectric. The electron mobilities obtained are in the range of 0.05-0.2 cm2 V-1 s-1. Most of the OFETs fabricated using organic dielectrics exhibit an inherent hysteresis due to charge trapping at the semiconductor-dielectric interface. Devices with a polymeric electret as gate dielectric show a very large and metastable hysteresis in its transfer characteristics. The observed hysteresis is found to be temperature dependent and has been used to develop a bistable memory element.

Controlled inflation of voids in cellular polymer ferroelectrets : optimizing electromechanical transducer properties

When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation.

Arrays of Ultracompliant Electrochemical Dry Gel Cells for Stretchable Electronics

Stretchable electronics is the building or embedding of electronic circuits and devices in compliant material. Substrate and interconnects should be made stretchable rather than flexible or rigid (as is the case in flexible electronics or printed circuit boards). Whereas application of flexible electronics is limited to flat substrates, stretchable electronics can cover moving parts, such as joints in robotic elements, and also curved substrates or unusual materials such as silk, paper, leather etc. Despite extensive efforts to advance stretchable electronics, including the integration of active components like diodes, transistors and integrated circuits, as well as sensors and actuators, surprisingly no solution for integrating a power supply into such electronic products has so far been found. Here we demonstrate dry gel cells that withstand stretch ratios up to 100% before failure; deliver open circuit voltages close to 1.5 V and short circuit currents up to 30mA, a lifetime of more than 1000 h and capacities of 3.5 mAh cm 2 active area. The fabrication process allows for mass production with roll-to-roll techniques based on printing and laminating. With compliant interconnects, the dry cells can be connected in series or in parallel to form arrays. Such arrays of gel cells allow the construction of self-powered stretchable electronic items. The future in electronics is flexible and stretchable, electronic items are thought to be used in settings were to date electronic functionalities are currently not available. In flexible electronics, an astonishing variety of devices have been demonstrated, including solar cells, active matrices of field-effect transistors and memories, integrated circuits, actuators, displays, and transponders. Flexible power supplies are also available, including printed batteries and supercapacitors. However, a similar maturity has not been achieved in stretchable electronics; there are no stand alone applications possible due to the lack of concepts for powering such items. Batteries are electrochemical cells that are used to convert stored chemical energy into electrical energy. Dry cells are common power sources in many household and industrial applications, being a multimillion dollar market. Zinc carbon, alkali manganese, or lithium ion cells are among the best known elements. Flexible batteries have been shown for a variety of basic cells; making these systems ultra-compliant is, however, a nontrivial task, since the batteries are not allowed to be internally short-circuited upon mechanical stretching. Therefore, designs currently available in printed batteries will not work for compliant ones. Supercapacitors may be an alternative to batteries in powering conformable electronics; first attempts have been reported with a maximum current below 1mA at a voltage of 1V, too low for practical purposes. Our concept for ultra-compliant and mechanically robust dry cells that can deliver power to stretchable electronics is based on the integration of highly elastic carbon black silicon oil paste electrodes into a stretchable acrylic elastomer (VHB 4910 from 3M, introduced for dielectric elastomer actuators). A plotted cell of zinc, carbon, and xanthan forms the cathode, whereas the printed anode consists of manganese dioxide, carbon, and electrolyte (NH4Cl, ZnCl2) pastes. The resulting gel cell is depicted in the scheme of Figure 1. To avoid intermixing of the chemicals upon stretching (which would cause internal short circuits and damage of the battery) it is crucial to laterally separate the two 1 cm Zn andMnO2 containing electrodes by a distance of 0.3 cm. Thus, high stretch ratios are achieved as documented below. A printed xanthan electrolyte gel is closing the power cell circuit. Finally, the stretchable power supply is sealed by lamination of another layer of the acrylic elastomer, resulting in a total thickness of 2mm for the whole element. Such batteries with a lateral separation of anode and cathode can easily be arranged in arrays; with elastic conductors they can be connected in parallel to enhance the output current or in series to enhance the voltage. Thereby, distributed power sources are easily generated to drive stretchable electronic devices. Different power levels are common in nowadays devices, flash memories for example require at least three power levels, one for reading and two for writing and erasing the memories. Charge pumps provide an interesting means for delivering different power levels from one supply, but they are currently unavailable in stretchable electronics. The concept of arrays of batteries in a single elastomer matrix is illustrated with the experiment depicted in Figure 2. Figure 2a shows a sketch of two gel cells connected in series to enhance the voltage. The two batteries power an SMD light emitting diode (green emitting SMD LED with an operating voltage of 2–2.6 V and a current consumption between 3 and 20mA), following the stiff island elastic interconnect approach. The whole circuit is subject to uniaxial stretching in Figures 2b–e and biaxial stretching in Figure 2f. Figure 2b shows a photo of the device before stretching, Figure 2c shows the dry gel cells stretched to 25% of their initial length, and Figure 2d proves the mechanical

Piezo- and pyroelectricity of a polymer-foam space-charge electret

Charged closed-cell polypropylene polymer foams are highly sensitive and broadband piezoelectric materials with a quasistatic piezoelectric d33 coefficient about 250 pC/N and a dynamic d33 coefficient of 140 pC/N at 600 kHz. The piezoelectric coefficient is much larger than that of ferroelectric polymers, like polyvinylidene fluoride, and compares favorably with ferroelectric ceramics, such as lead zirconate titanate. The pyroelectric coefficient p3=0.25 μC/m2 K is small in comparison to ferroelectric polymers and ferroelectric ceramics. The low density, small pyroelectric coefficient and high piezoelectric sensitivity make charged polymer foams attractive for a wide range of sensor and transducer applications in acoustics, air-borne ultrasound, medical diagnostics, and nondestructive testing.