Full-solution processed flexible organic solar cells using low-cost printable copper electrodes

Photogalvanics of copper and brass working electrodes in the NaOH-Allura Red-d-galactose-DDAC electrolyte for solar power generation

Solar energy is a limitless energy resource that can be used to produce electricity forever. Photogalvanic cells can convert solar energy into electricity with inherent power storage. The electrolyte(s) and a combination of two electrodes are the main materials required for fabrication of these cells. So far, platinum has been used as the working electrode in photogalvanic cells. Platinum is an extremely rare and expensive metal. Copper and its alloy (brass) have been identified as alternative working electrodes to substitute the platinum working electrode in photogalvanic cells. In addition, copper and brass utilization is identified to be an effective, user-friendly, and safe approach for high-power generation. Therefore, in the present work, cheap and easily obtainable copper and brass (alloy of copper and zinc) working electrodes have been exploited with the twin aim of high-power generation with less input cost. In the present study, the observed power, current, potential, and efficiency for the copper electrode are 552.3 µW, 4030 µA, 713 mV, and 8.54%, respectively, and those for the brass electrode are 546.4 µW, 5320 µA, 739 mV, and 6.12%, respectively. The observed electrical performance is greatly enhanced compared to most of the already reported photogalvanics with platinum electrode. Copper and brass are slightly and slowly corroded by the alkali, but despite this electrode loss, both materials are promising to produce the highest power. In the future, this electrode loss can be checked by using inhibitors.

Efficient Flexible Fabric-Based Top-Emitting Polymer Light-Emitting Devices for Wearable Electronics

Low-cost fabric-based top-emitting polymer light-emitting devices (Fa-TPLEDs) have aroused increasing attention due to their remarkable potential applications in wearable displays. However, it is still challenging to realize efficient all-solution-processed devices from bottom electrodes to top electrodes with large-scale fabrication. Here, a smooth reflective Ag cathode integrated on fabric by one-step silver mirror reaction and a composite transparent anode of polydimethylsiloxane/silver nanowires/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) via a water-assisted peeling method are presented, both of which possess excellent optoelectrical properties and robust mechanical flexibility. The Fa-TPLEDs are constructed by spin-coating functional layers on the bottom reflective cathodes and laminating the top transparent anodes. The Fa-TPLEDs show a current efficiency of 16.3 cd A-1 , an external quantum efficiency of 4.9% and angle-independent electroluminescence spectra. In addition, the Fa-TPLEDs possess excellent mechanical stability, maintaining a current efficiency of 14.3 cd A-1 after 200 bending cycles at a radius of 4 mm. The results demonstrate that the integration of solution-processed reflective cathodes and transparent anodes sheds light on a new avenue to construct low-cost and efficient fabric-based devices, showing great potential applications in emerging smart flexible/wearable electronics.

Silver Nanoparticle-Decorated Silica Nanospheres and Arrays as Potential Substrates for Surface-Enhanced Raman Scattering

Poly(vinylpyrrolidone) (PVP) was used as both a modifier and reductant to in situ deposit silver nanoparticles (denoted Ag NPs) on the surface of silica nanospheres (nanosilica or nano-SiO₂), affording Ag-decorated nanosilica (denoted SiO₂@Ag). The as-obtained SiO₂@Ag composite can form silver nanoparticle-decorated silica nanosphere arrays (denoted SiO₂@Ag arrays) via evaporation-induced self-assembly. The as-prepared SiO₂@Ag composite and SiO₂@Ag array were used as the SERS substrates to measure the Raman signals of the dilute solutions of rhodamine 6G (denoted R6G), an organic dye that is a potential pollutant to the environment. The findings indicate that the as-prepared SiO₂@Ag composite and SiO₂@Ag array as potential SERS substrates simultaneously exhibit a high degree of metal coverage and small size of Ag NPs as well as good stability and abundant "hot spots", which contributes to their desired Raman enhancement capacities. For the detection of trace R6G, they provide a limit of detection of as low as 10^-9-10^-11 M as well as good reproducibility, showing promising potential for monitoring chemical and biological molecules.

Review: Sensors for Biosignal/Health Monitoring in Electronic Skin

Skin is the largest sensory organ and receives information from external stimuli. Human body signals have been monitored using wearable devices, which are gradually being replaced by electronic skin (E-skin). We assessed the basic technologies from two points of view: sensing mechanism and material. Firstly, E-skins were fabricated using a tactile sensor. Secondly, E-skin sensors were composed of an active component performing actual functions and a flexible component that served as a substrate. Based on the above fabrication processes, the technologies that need more development were introduced. All of these techniques, which achieve high performance in different ways, are covered briefly in this paper. We expect that patients' quality of life can be improved by the application of E-skin devices, which represent an applied advanced technology for real-time bio- and health signal monitoring. The advanced E-skins are convenient and suitable to be applied in the fields of medicine, military and environmental monitoring.

Tissue adhesive hydrogel bioelectronics

Flexible bioelectronics have promising applications in electronic skin, wearable devices, biomedical electronics, etc. Hydrogels have unique advantages for bioelectronics due to their tissue-like mechanical properties and excellent biocompatibility. Particularly, conductive and tissue adhesive hydrogels can self-adhere to bio-tissues and have great potential in implantable wearable bioelectronics. This review focuses on the recent progress in tissue adhesive hydrogel bioelectronics, including the mechanism and preparation of tissue adhesive hydrogels, the fabrication strategies of conductive hydrogels, and tissue adhesive hydrogel bioelectronics and applications. Some perspectives on tissue adhesive hydrogel bioelectronics are provided at the end of the review.

Recrystallized ice-templated electroless plating for fabricating flexible transparent copper meshes

Flexible transparent conductors as a replacement for indium tin oxide (ITO) have been urgently pursued due to the inherent drawbacks of ITO films. Here, we report the fabrication of flexible transparent copper meshes with recrystallized ice-crystal templates. Completely different to conventional approaches, this novel method needs neither the fabrication of mesh patterns via micro/nanofabrication technologies nor the deposition of copper through evaporation or sputtering. The linewidth and mesh size of the prepared copper meshes can be regulated, as the ice recrystallization process is controllable. Therefore, the formed copper meshes have tailorable conductivity and transparency, which are critical for optoelectronic devices. Remarkably, the electrical performance of the copper meshes is maintained even after storing for 60 days in ambient conditions or bending for 1000 cycles. This strategy is modular and can also be employed to prepare other metal meshes, such as silver meshes, offering versatile substitutes for ITO in electronic devices.

Herein, we report the fabrication of flexible copper meshes using recrystallized ice-crystal templates. The linewidth and mean size of the copper meshes can be tuned by adjusting the ice grains.

Polymer-Assisted Metal Deposition (PAMD) for Flexible and Wearable Electronics: Principle, Materials, Printing, and Devices

The rapid development of flexible and wearable electronics favors low-cost, solution-processing, and high-throughput techniques for fabricating metal contacts, interconnects, and electrodes on flexible substrates of different natures. Conventional top-down printing strategies with metal-nanoparticle-formulated inks based on the thermal sintering mechanism often suffer from overheating, rough film surface, low adhesion, and poor metal quality, which are not desirable for most flexible electronic applications. In recent years, a bottom-up strategy termed as polymer-assisted metal deposition (PAMD) shows great promise in addressing the abovementioned challenges. Here, a detailed review of the development of PAMD in the past decade is provided, covering the fundamental chemical mechanism, the preparation of various soft and conductive metallic materials, the compatibility to different printing technologies, and the applications for a wide variety of flexible and wearable electronic devices. Finally, the attributes of PAMD in comparison with conventional nanoparticle strategies are summarized and future technological and application potentials are elaborated.

High-Adhesion Stretchable Electrode via Cross-Linking Intensified Electroless Deposition on a Biomimetic Elastomeric Micropore Film

For the stretchable electrode, strong interface adhesion is the primary guarantee for long service life, and the maximization of the tensile limit with remarkable electrical stability can expand the scope of its use. Here, a cost-effective strategy is proposed to fabricate a high-adhesion stretchable electrode. By modifying dopamine and functionalized silane on a polydimethylsiloxane (PDMS) substrate in sequence before the electroless deposition process, super-high adhesion up to 3.1 MPa is achieved between the PDMS substrate and silver layer, and the electrode exhibits extraordinary conductivity of 4.0 × 10⁷ S/m. This process is also suitable for other common flexible substrates and metals. Moreover, inspired by the micro-/nanostructure on the surface of lotus leaf, a biomimetic elastomeric micropore film with a uniformly distributed micropore is fabricated by the one-step soft lithography replication process. The electrode exhibits a large tensile limit exceeding 70% uniaxial tensile and superior electrical stability from 6.3 to 11.5 Ω under 20% uniaxial tensile for more than 10 000 cycles. This study seeks a promising method to manufacture stretchable electrodes with high adhesion, large tensile limit, and excellent electrical stability, showing great potential to detect various biological signals including joint movement, surface electromyography, and so forth.

One-step selective laser patterning of copper/graphene flexible electrodes

Flexible electrodes have attracted much attention in consumer electronic applications. In this work, laser direct writing is used to fabricate copper/graphene composite electrodes on a flexible substrate in one step. This direct writing process with a low power laser can reduce copper ions in thin films to form copper nanomaterials and spontaneously interconnect them to gain good conductivity, while the laser also induces the growth of multi-layer graphene that coats on copper to improve the oxidation resistance of electrodes. The electrical performance and chemical composition of flexible electrodes can be tuned by laser power, scanning speed, and defocus distance. A mechanism of in situ reduction and interconnection of copper nanomaterials during laser direct writing has been proposed. This method could largely reduce the oxidation issue by avoiding synthesis and sintering processes of copper nanomaterials. These as-written copper electrodes have good stability and have potential applications in flexible electronics, such as flexible heaters or antennas as demonstrated.

Chemical formation of soft metal electrodes for flexible and wearable electronics

Efficient chemical approaches to fabricating soft metal electrodes aiming at wearable electronics are summarized and reviewed.

Room Temperature Electrochemical Sintering of Zn Microparticles and Its Use in Printable Conducting Inks for Bioresorbable Electronics

This study describes a conductive ink formulation that exploits electrochemical sintering of Zn microparticles in aqueous solutions at room temperature. This material system has relevance to emerging classes of biologically and environmentally degradable electronic devices. The sintering process involves dissolution of a surface passivation layer of zinc oxide in CH₃ COOH/H₂ O and subsequent self-exchange of Zn and Zn²⁺ at the Zn/H₂ O interface. The chemical specificity associated with the Zn metal and the CH₃ COOH/H₂ O solution is critically important, as revealed by studies of other material combinations. The resulting electrochemistry establishes the basis for a remarkably simple procedure for printing highly conductive (3 × 10⁵ S m^-1 ) features in degradable materials at ambient conditions over large areas, with key advantages over strategies based on liquid phase (fusion) sintering that requires both oxide-free metal surfaces and high temperature conditions. Demonstrations include printed magnetic loop antennas for near-field communication devices.

Low-Temperature Soft-Cover Deposition of Uniform Large-Scale Perovskite Films for High-Performance Solar Cells

Large-scale high-quality perovskite thin films are crucial to produce high-performance perovskite solar cells. However, for perovskite films fabricated by solvent-rich processes, film uniformity can be prevented by convection during thermal evaporation of the solvent. Here, a scalable low-temperature soft-cover deposition (LT-SCD) method is presented, where the thermal convection-induced defects in perovskite films are eliminated through a strategy of surface tension relaxation. Compact, homogeneous, and convection-induced-defects-free perovskite films are obtained on an area of 12 cm² , which enables a power conversion efficiency (PCE) of 15.5% on a solar cell with an area of 5 cm² . This is the highest efficiency at this large cell area. A PCE of 15.3% is also obtained on a flexible perovskite solar cell deposited on the polyethylene terephthalate substrate owing to the advantage of presented low-temperature processing. Hence, the present LT-SCD technology provides a new non-spin-coating route to the deposition of large-area uniform perovskite films for both rigid and flexible perovskite devices.

Waterproof, Ultrahigh Areal-Capacitance, Wearable Supercapacitor Fabrics

High-performance supercapacitors (SCs) are promising energy storage devices to meet the pressing demand for future wearable applications. Because the surface area of a human body is limited to 2 m² , the key challenge in this field is how to realize a high areal capacitance for SCs, while achieving rapid charging, good capacitive retention, flexibility, and waterproofing. To address this challenge, low-cost materials are used including multiwall carbon nanotube (MWCNT), reduced graphene oxide (RGO), and metallic textiles to fabricate composite fabric electrodes, in which MWCNT and RGO are alternatively vacuum-filtrated directly onto Ni-coated cotton fabrics. The composite fabric electrodes display typical electrical double layer capacitor behavior, and reach an ultrahigh areal capacitance up to 6.2 F cm^-2 at a high areal current density of 20 mA cm^-2 . All-solid-state fabric-type SC devices made with the composite fabric electrodes and water-repellent treatment can reach record-breaking performance of 2.7 F cm^-2 at 20 mA cm^-2 at the first charge-discharge cycle, 3.2 F cm^-2 after 10 000 charge-discharge cycles, zero capacitive decay after 10 000 bending tests, and 10 h continuous underwater operation. The SC devices are easy to assemble into tandem structures and integrate into garments by simple sewing.

Microfluidic Patterning of Metal Structures for Flexible Conductors by In Situ Polymer-Assisted Electroless Deposition

A low-cost, solution-processed, versatile, microfluidic approach is developed for patterning structures of highly conductive metals (e.g., copper, silver, and nickel) on chemically modified flexible polyethylene terephthalate thin films by in situ polymer-assisted electroless metal deposition. This method has significantly lowered the consumption of catalyst as well as the metal plating solution.

Towards a conceptual framework of OSH risk management in smart working environments based on smart PPE, ambient intelligence and the Internet of Things technologies

Recent developments in domains of ambient intelligence (AmI), Internet of Things, cyber-physical systems (CPS), ubiquitous/pervasive computing, etc., have led to numerous attempts to apply ICT solutions in the occupational safety and health (OSH) area. A literature review reveals a wide range of examples of smart materials, smart personal protective equipment and other AmI applications that have been developed to improve workers' safety and health. Because the use of these solutions modifies work methods, increases complexity of production processes and introduces high dynamism into thus created smart working environments (SWE), a new conceptual framework for dynamic OSH management in SWE is called for. A proposed framework is based on a new paradigm of OSH risk management consisting of real-time risk assessment and the capacity to monitor the risk level of each worker individually. A rationale for context-based reasoning in SWE and a respective model of the SWE-dedicated CPS are also proposed.

Photoreactive and Metal-Platable Copolymer Inks for High-Throughput, Room-Temperature Printing of Flexible Metal Electrodes for Thin-Film Electronics

Photoreactive and metal-platable copolymer inks are reported for the first time to allow high-throughput printing of high-performance flexible electrodes at room temperature. This new copolymer ink accommodates various types of printing technologies, such as soft lithography molding, screen printing, and inkjet printing. Electronic devices including resistors, sensors, solar cells, and thin-film transistors fabricated with these printed electrodes show excellent electrical performance and mechanical flexibility.

Polyethylenimine-Mediated Electrostatic Assembly of MnO2 Nanorods on Graphene Oxides for Use as Anodes in Lithium-Ion Batteries

In recent years, the development of electrochemically active materials with excellent lithium storage capacity has attracted tremendous attention for application in high-performance lithium-ion batteries. MnO2-based composite materials have been recognized as one of promising candidates owing to their high theoretical capacity and cost-effectiveness. In this study, a previously unrecognized chemical method is proposed to induce intra-stacked assembly from MnO2 nanorods and graphene oxide (GO), which is incorporated as an electrically conductive medium and a structural template, through polyethylenimine (PEI)-derived electrostatic modulation between both constituent materials. It is revealed that PEI, a cationic polyelectrolyte, is capable of effectively forming hierarchical, two-dimensional MnO2-RGO composites, enabling highly reversible capacities of 880, 770, 630, and 460 mA·h/g at current densities of 0.1, 1, 3, and 5 A/g, respectively. The role of PEI in electrostatically assembled composite materials is clarified through electrochemical impedance spectroscopy-based comparative analysis.

Stable ultrathin partially oxidized copper film electrode for highly efficient flexible solar cells

Advances in flexible optoelectronic devices have led to an increasing need for developing highly efficient, low-cost, flexible transparent conducting electrodes. Copper-based electrodes have been unattainable due to the relatively low optical transmission and poor oxidation resistance of copper. Here, we report the synthesis of a completely continuous, smooth copper ultra-thin film via limited copper oxidation with a trace amount of oxygen. The weakly oxidized copper thin film sandwiched between zinc oxide films exhibits good optoelectrical performance (an average transmittance of 83% over the visible spectral range of 400–800 nm and a sheet resistance of 9 Ω sq⁻¹) and strong oxidation resistance. These values surpass those previously reported for copper-based electrodes; further, the record power conversion efficiency of 7.5% makes it clear that the use of an oxidized copper-based transparent electrode on a polymer substrate can provide an effective solution for the fabrication of flexible organic solar cells.

Light enters and exits optoelectronic devices through transparent conductive electrodes, which are one of their most expensive components. Here, the authors develop stable transparent conductive electrodes based on copper and oxide layers that lead to efficient flexible organic solar cells.

Enhancing the energy density of asymmetric stretchable supercapacitor based on wrinkled CNT@MnO2 cathode and CNT@polypyrrole anode

With the advantages of high stickiness and stretchability of the hydrogel electrolyte as well as the resilient properties of film electrodes, the facile "prestrain-stick-release" strategy can be utilized for the assembly of a stretchable supercapacitor. Two major issues of concern are the relatively low mechanical strength of the hydrogel electrolyte and the low energy density of the assembled device. Herein, vinyl group grafted silica (CH2═CH-SiO2) nanoparticles were used as a nanoparticle cross-linker for polyacrylamide (PAAM), enhancing the tensile strength of 844 kPa at the strain of 3400% for the KCl-CH2═CH-SiO2/PAAM hydrogel electrolyte. Besides, carbon nanotube supported polypyrrole (CNT@PPy) and manganese dioxide (CNT@MnO2) film electrodes are prepared to assemble the stretchable asymmetric CNT@MnO2//KCl-CH2═CH-SiO2/PAAM//CNT@PPy supercapacitor, significantly enhancing the potential window to 0-2.0 V and achieving a high energy density of 40 Wh kg(-1) at the power density of 519 kW kg(-1) with the strain of 100%, which is the best known for the reported stretchable supercapacitors.

Organic transistor memory with a charge storage molecular double-floating-gate monolayer

A flexible, low-voltage, and nonvolatile memory device was fabricated by implanting a functional monolayer on an aluminum oxide dielectric surface in a pentacene-based organic transistor. The monolayer-forming molecule contains a phosphonic acid group as the anchoring moiety and a charge-trapping core group flanked between two alkyl chain spacers as the charge trapping site. The memory characteristics strongly depend on the monolayer used due to the localized charge-trapping capability for different core groups, including the diacetylenic (DA) unit as the hole carrier trap, the naphthalenetetracarboxyldiimide (ND) unit as the electron carrier trap, and the one with both DA and ND units present, respectively. The device with the monolayer carrying both DA and ND groups has a larger memory window than that for the one containing DA only and a longer retention time than that for the one containing DA or ND only, giving a memory window of 1.4 V and a retention time around 10(9) s. This device with hybrid organic monolayer/inorganic dielectrics also exhibited rather stable device characteristics upon bending of the polymeric substrate.

Biomimicking Topographic Elastomeric Petals (E-Petals) for Omnidirectional Stretchable and Printable Electronics

Elastomeric petals directly replicated from natural rose petal are new versatile substrates for stretchable and printable electronics. Compared with conventional flat polydimethylsiloxane substrates, elastomeric petals have biomimicking topographic surfaces that can effectively inhibit the propagation of microcracks formed in the conducting layer, which is deposited on top, regardless of the type of conductive materials and the deposition methods.