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Concept: Flexible electronics


Wearable sensor technologies are essential to the realization of personalized medicine through continuously monitoring an individual’s state of health. Sampling human sweat, which is rich in physiological information, could enable non-invasive monitoring. Previously reported sweat-based and other non-invasive biosensors either can only monitor a single analyte at a time or lack on-site signal processing circuitry and sensor calibration mechanisms for accurate analysis of the physiological state. Given the complexity of sweat secretion, simultaneous and multiplexed screening of target biomarkers is critical and requires full system integration to ensure the accuracy of measurements. Here we present a mechanically flexible and fully integrated (that is, no external analysis is needed) sensor array for multiplexed in situ perspiration analysis, which simultaneously and selectively measures sweat metabolites (such as glucose and lactate) and electrolytes (such as sodium and potassium ions), as well as the skin temperature (to calibrate the response of the sensors). Our work bridges the technological gap between signal transduction, conditioning (amplification and filtering), processing and wireless transmission in wearable biosensors by merging plastic-based sensors that interface with the skin with silicon integrated circuits consolidated on a flexible circuit board for complex signal processing. This application could not have been realized using either of these technologies alone owing to their respective inherent limitations. The wearable system is used to measure the detailed sweat profile of human subjects engaged in prolonged indoor and outdoor physical activities, and to make a real-time assessment of the physiological state of the subjects. This platform enables a wide range of personalized diagnostic and physiological monitoring applications.

Concepts: Electrical engineering, Integrated circuit, Sensors, Sensor, Measurement, Flexible electronics, Potassium, Signal processing


Few-layered transition metal dichalcogenides (TMDs) are known as true two-dimensional materials, with excellent semiconducting properties and strong light-matter interaction. Thus, TMDs are attractive materials for semitransparent and flexible solar cells for use in various applications. Hoewver, despite the recent progress, the development of a scalable method to fabricate semitransparent and flexible solar cells with mono- or few-layered TMDs remains a crucial challenge. Here, we show easy and scalable fabrication of a few-layered TMD solar cell using a Schottky-type configuration to obtain a power conversion efficiency (PCE) of approximately 0.7%, which is the highest value reported with few-layered TMDs. Clear power generation was also observed for a device fabricated on a large SiO2 and flexible substrate, demonstrating that our method has high potential for scalable production. In addition, systematic investigation revealed that the PCE and external quantum efficiency (EQE) strongly depended on the type of photogenerated excitons (A, B, and C) because of different carrier dynamics. Because high solar cell performance along with excellent scalability can be achieved through the proposed process, our fabrication method will contribute to accelerating the industrial use of TMDs as semitransparent and flexible solar cells.

Concepts: Cadmium, Diode, Flexible electronics, IBM, Wafer, Energy conversion, Energy conversion efficiency, Solar cell


Planar perovskite solar cells made entirely via solution-processing at low temperatures (<150°C) offer promise for simple manufacturing, compatibility with flexible substrates, and perovskite-based tandem devices; however, they require an electron-selective layer that performs well with similar processing. We report a contact passivation strategy using chlorine-capped TiO2 colloidal nanocrystal (NC) film that mitigates interfacial recombination and improves interface binding in low-temperature planar solar cells. We fabricated solar cells with certified efficiencies of 20.1% and 19.5% for active areas of 0.049 and 1.1 square centimeters, respectively, achieved via low-temperature solution processing. Solar cells with efficiency >20% retained 90% (97% after dark recovery) of their initial performance after 500 hours continuous room-temperature operation at their maximum power point under one-sun illumination.

Concepts: Solar cells, Oxygen, Flexible electronics, Titanium dioxide, Quantum dot, Light, Titanium, Solar cell


We fabricated flexible, transparent, and conductive metal grids as Transparent Conductive Materials (TCM) with adjustable properties by direct nanoimprinting of self-assembling colloidal metal nanowires. Ultrathin gold nanowires (diameter below 2 nm) with high mechanical flexibility were confined in a stamp and readily adapted to its features. During drying, the wires self-assembled into dense bundles that percolated throughout the stamp. The high aspect ratio and the bundling yielded continuous, hierarchical superstructures that connected the entire mesh even at low gold contents. A soft sintering step removed the ligand barriers but retained the imprinted structure. The material exhibited high conductivities (sheet resistances down to 29 Ω/sq) and transparencies that could be tuned by changing wire concentration and stamp geometry. We obtained TCMs that are suitable for applications such as touch screens. Mechanical bending tests showed a much higher bending resistance than commercial ITO: conductivity dropped by only 5.6 % after 450 bending cycles at a bending radius of 5 mm.

Concepts: Wire, Flexible electronics, Ratio, Copper, Solar cell, Aspect ratio, Electrical conductivity, Flexible


Solution processing is a promising route for the realization of low-cost, large-area, flexible and lightweight photovoltaic devices with short energy payback time and high specific power. However, solar cells based on solution-processed organic, inorganic and hybrid materials reported thus far generally suffer from poor air stability, require an inert-atmosphere processing environment or necessitate high-temperature processing, all of which increase manufacturing complexities and costs. Simultaneously fulfilling the goals of high efficiency, low-temperature fabrication conditions and good atmospheric stability remains a major technical challenge, which may be addressed, as we demonstrate here, with the development of room-temperature solution-processed ZnO/PbS quantum dot solar cells. By engineering the band alignment of the quantum dot layers through the use of different ligand treatments, a certified efficiency of 8.55% has been reached. Furthermore, the performance of unencapsulated devices remains unchanged for over 150 days of storage in air. This material system introduces a new approach towards the goal of high-performance air-stable solar cells compatible with simple solution processes and deposition on flexible substrates.

Concepts: Oxygen, Cadmium, Photovoltaic module, Flexible electronics, Band gap, Photovoltaics, Quantum dot, Solar cell


Large-area Cu nanosheets are synthesized by a strategy of Cu nanocrystal self-assembly, and then aqueous conductive Cu-nanosheet ink is successfully prepared for direct writing of the conductive circuits on flexible electronics. The Cu nanocrystals, as building blocks, are self-assembled along <111> direction, and grow into large-area nanosheets about 30 to 100 μm in diameter and a few hundreds nm in thickness. The laminar stackable patterns of the Cu-nanosheet circuits increase contact area of the Cu nanosheets and improve the stability of the conductor under stress, with the result that the Cu-nanosheet circuits display excellent conductive performance under folding and unfolding repeatedly. Moreover, heterostructures of Ag nanoparticles coated Cu nanosheets are achieved for improving the thermal stability of the nanosheet circuits at high temperature.

Concepts: Improve, Synthesis, Flexible electronics, Electronic engineering, Electrical conductivity, Copper, Nanocrystal, Electricity


Pristine graphene is the strongest material ever measured. However, large-area graphene films produced by means of chemical vapor deposition (CVD) are polycrystalline and thus contain grain boundaries that can potentially weaken the material. We combined structural characterization by means of transmission electron microscopy with nanoindentation in order to study the mechanical properties of CVD-graphene films with different grain sizes. We show that the elastic stiffness of CVD-graphene is identical to that of pristine graphene if postprocessing steps avoid damage or rippling. Its strength is only slightly reduced despite the existence of grain boundaries. Indentation tests directly on grain boundaries confirm that they are almost as strong as pristine. Graphene films consisting entirely of well-stitched grain boundaries can retain ultrahigh strength, which is critical for a large variety of applications, such as flexible electronics and strengthening components.

Concepts: Flexible electronics, Scanning electron microscope, Carbon nanotube, Transmission electron microscopy, Stiffness, Electron, Chemical vapor deposition, Materials science


Fast flexible electronics operating at radio frequencies (>1 GHz) are more attractive than traditional flexible electronics because of their versatile capabilities, dramatic power savings when operating at reduced speed and broader spectrum of applications. Transferrable single-crystalline Si nanomembranes (SiNMs) are preferred to other materials for flexible electronics owing to their unique advantages. Further improvement of Si-based device speed implies significant technical and economic advantages. While the mobility of bulk Si can be enhanced using strain techniques, implementing these techniques into transferrable single-crystalline SiNMs has been challenging and not demonstrated. The past approach presents severe challenges to achieve effective doping and desired material topology. Here we demonstrate the combination of strained- NM-compatible doping techniques with self-sustained-strain sharing by applying a strain-sharing scheme between Si and SiGe multiple epitaxial layers, to create strained print-transferrable SiNMs. We demonstrate a new speed record of Si-based flexible electronics without using aggressively scaled critical device dimensions.

Concepts: Flexible electronics, Radio, Hertz, Germanium, Electronic engineering, Amorphous silicon, Semiconductor materials, Silicon


Solution-phase printing of exfoliated graphene flakes is emerging as a low-cost means to create flexible electronics for numerous applications. The electrical conductivity and electrochemical reactivity of printed graphene has been shown to improve with post-print processing methods such as thermal, photonic, and laser annealing. However, to date no reports have shown the manipulation of surface wettability via post-print processing of printed graphene. Herein, we demonstrate how the energy density of a direct-pulsed laser writing (DPLW) technique can be varied to tune the hydrophobicity and electrical conductivity of the inkjet-printed graphene (IPG). Experimental results demonstrate that the DPLW process can convert the IPG surface from one that is initially hydrophilic (contact angle ∼47.7°) and electrically resistive (sheet resistance ∼21 MΩ □(-1)) to one that is superhydrophobic (CA ∼157.2°) and electrically conductive (sheet resistance ∼1.1 kΩ □(-1)). Molecular dynamic (MD) simulations reveal that both the nanoscale graphene flake orientation and surface chemistry of the IPG after DPLW processing induce these changes in surface wettability. Moreover, DPLW can be performed with IPG printed on thermally and chemically sensitive substrates such as flexible paper and polymers. Hence, the developed, flexible IPG electrodes treated with DPLW could be useful for a wide range of applications such as self-cleaning, wearable, or washable electronics.

Concepts: Flexible electronics, Electric current, Electrical conductivity, Semiconductor, Electrochemistry, Electricity, Electrical conductor, Resistivity


Conductive paper owns low-cost, lightweight, sustainable, easy to scale-up and tailorable advantages, allowing for its promising potential in flexible electronics, such as bendable supercapacitors, solar cells, electromagnetic shields and actuators. Ionic gels, exhibiting a lower Young’s modulus together with facile manufacturing, can fully serve as the conductive component to prepare conductive paper. Herein we report a low-cost (~1.3 dollars/m(2)), continuous and high-throughput (up to ~ 30 m/min) fabrication of reliable and long-term (stable for more than two months) conductive paper. As-prepared conductive paper shows a high electrical durability with negligible bending-recovering signal changes over 5,000 cycles. Using this ionic gel paper (IGP) as a key component, we build a variety of proof-of-principle demonstrations to show the capacity of IGP in constructing flexible electroluminescent devices with diverse patterns, including a square, an alphabetic string and a laughing face. Our methodology has the potential to open up a new powerful route to fabricate bendable conductive paper for a myriad of applications in future flexible electronics.

Concepts: Electricity, Young's modulus, Flexible electronics, Solar cell