Flexible Organic Photovoltaic Cells with In Situ Nonthermal Photoreduction of Spin‐Coated Graphene Oxide Electrodes

The first reduction methodology, compatible with flexible, temperature‐sensitive substrates, for the production of reduced spin‐coated graphene oxide (GO) electrodes is reported. It is based on the use of a laser beam for the in situ, non‐thermal, reduction of spin‐coated GO films on flexible substrates over a large area. The photoreduction process is one‐step, facile, and is rapidly carried out at room temperature in air without affecting the integrity of the graphene lattice or the flexibility of the underlying substrate. Conductive graphene films with a sheet resistance of as low as 700 Ω sq^?1 and transmittance of 44% can be obtained, much higher than can be achieved for flexible layers reduced by chemical means. As a proof of concept of our technique, laser‐reduced GO (LrGO) films are utilized as transparent electrodes in flexible, bulk heterojunction, organic photovoltaic (OPV) devices, replacing the traditional ITO. The devices displayed a power‐conversion efficiency of 1.1%, which is the highest reported so far for OPV device incorporating reduced GO as the transparent electrode. The in situ non‐thermal photoreduction of spin‐coated GO films creates a new way to produce flexible functional graphene electrodes for a variety of electronic applications in a process that carries substantial promise for industrial implementation.     

Highly Conductive and Environmentally Stable Organic Transparent Electrodes Laminated with Graphene

Improving the lifetime and the operational and thermal stability of organic thin‐film materials while maintaining high conductivity and mechanical flexibility is critical for flexible electronics applications. Here, it is reported that highly conductive and environmentally stable organic transparent electrodes (TEs) can be fabricated by mechanically laminating poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) films containing dimethylsulfoxide and Zonyl fluorosurfactant (PDZ films) with a monolayer graphene barrier. The proposed lamination process allows graphene to be coated onto the PDZ films uniformly and conformally with tight interfacial binding, free of wrinkles and air gaps. The laminated films exhibit an outstanding room‐temperature hole mobility of ≈85.1 cm² V^?1 s^?1 since the graphene can serve as an effective bypass for charge carriers. The significantly improved stability of the graphene‐laminated TEs against high mechanical/thermal stress, humidity, and ultraviolet irradiation is particularly promising. Furthermore, the incorporation of the graphene barrier increases the expected lifetime of the TEs by more than two orders of magnitude.     

Wearable Force Touch Sensor Array Using a Flexible and Transparent Electrode

Transparent electrodes have been widely used for various electronics and optoelectronics, including flexible ones. Many nanomaterial‐based electrodes, in particular 1D and 2D nanomaterials, have been proposed as next‐generation transparent and flexible electrodes. However, their transparency, conductivity, large‐area uniformity, and sometimes cost are not yet sufficient to replace indium tin oxide (ITO). Furthermore, the conventional ITO is quite rigid and susceptible to mechanical fractures under deformations (e.g., bending, folding). In this study, the authors report new advances in the design, fabrication, and integration of wearable and transparent force touch (touch and pressure) sensors by exploiting the previous efforts in stretchable electronics as well as novel ideas in the transparent and flexible electrode. The optical and mechanical experiment, along with simulation results, exhibit the excellent transparency, conductivity, uniformity, and flexibility of the proposed epoxy‐copper‐ITO (ECI) multilayer electrode. By using this multi‐layered ECI electrode, the authors present a wearable and transparent force touch sensor array, which is multiplexed by Si nanomembrane p‐i‐n junction‐type (PIN) diodes and integrated on the skin‐mounted quantum dot light‐emitting diodes. This novel integrated system is successfully applied as a wearable human–machine interface (HMI) to control a drone wirelessly. These advances in novel material structures and system‐level integration strategies create new opportunities in wearable smart displays.     

Graphene Oxide/Hexylamine Superlattice Field-Effect Biochemical Sensors

The bulky assembly of 2D materials is highly desired for macroscopic applications, but it is still challenging to access the exceptional properties of 2D materials owing to their spontaneous aggregation. Here, to facilitate the access of the field-effect sensing abilities of semiconducting 2D materials in bulk quantities, 3D hybrid superlattices of alternating graphene oxide (GO) and hexylamine molecular layers are constructed. Strikingly, the fabricated flexible GO/hexylamine superlattice exhibits a “V”-shaped ambipolar field-effect transfer characteristic under electrolyte gating, with exceptional sensing ability to pH value and DNA molecules in buffer solutions. In contrast, GO paper prepared by annealing the hybrid superlattices exhibits neither field-effect nor sensing responses to various analytes, highlighting the essential role that hexylamine plays in modulating the GO/hexylamine superlattice to enable a wider interlayer spacing for biomolecules transportation in the internal graphene layer. These achievements, along with stable electrical and sensing performance under mechanical stress of the 3D hybrid superlattices, highlight the unique potential of flexible graphene 3D hybrid superlattices for biochemical applications by overcoming multilayer aggregation.     

An Indium‐Free Anode for Large‐Area Flexible OLEDs: Defect‐Free Transparent Conductive Zinc Tin Oxide

Flexible large‐area organic light‐emitting diodes (OLEDs) require highly conductive and transparent anodes for efficient and uniform light emission. Tin‐doped indium oxide (ITO) is the standard anode in industry. However, due to the scarcity of indium, alternative anodes that eliminate its use are highly desired. Here an indium‐free anode is developed by a combinatorial study of zinc oxide (ZnO) and tin oxide (SnO₂), both composed of earth‐abundant elements. The optimized Zn–Sn–O (ZTO) films have electron mobilities of up to 21 cm² V^?1 s^?1, a conductivity of 245 S cm^?1, and <5% absorptance in the visible range of the spectrum. The high electron mobilities and low surface roughness (<0.2 nm) are achieved by producing dense and void‐free amorphous layers as confirmed by transmission electron microscopy. These ZTO layers are evaluated for OLEDs in two anode configurations: i) 10 cm² devices with ZTO/Ag/ZTO and ii) 41 cm² devices with ZTO plus a metal grid. The ZTO layers are compatible with OLED processing steps and large‐area white OLEDs fabricated with the ZTO/grid anode show better performance than those with ITO/grid anodes. These results confirm that ZTO has the potential as an In‐free and Earth‐abundant alternative to ITO for large‐area flexible OLEDs.     

Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

Photovoltaic Devices: Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells (Adv. Funct. Mater. 6/2011)

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

Ultrathin Metal films for Transparent Electrodes of Flexible Optoelectronic Devices

The need for the development of transparent conductive electrodes (TCEs) supported on flexible polymer substrates has explosively increased in response to flexible polymer‐based photovoltaic and display technologies; these TCEs replace conventional indium tin oxide (ITO) that exhibits poor performance on heat‐sensitive polymers. An efficient, flexible TCE is required to exhibit high electrical conductance and high optical transmittance, as well as excellent mechanical flexibility and long‐term stability, simultaneously. Recent advances in technologies utilizing an ultrathin noble‐metal film in a dielectric/metal/dielectric structure, or its derivatives, have attracted attention as promising alternatives that can satisfy the requirements of flexible TCEs. This review will survey the background knowledge and recent updates of synthetic strategies and design rules toward highly efficient, flexible TCEs based on ultrathin metal films, with a special focus on the principal features and available methodologies involved in the fabrication of highly transparent, conductive, ultrathin noble‐metal films. This survey will also cover the practical applications of TCEs to flexible organic solar cells and light‐emitting diodes.     

High‐Performance Polypyrrole/Graphene/SnCl2 Modified Polyester Textile Electrodes and Yarn Electrodes for Wearable Energy Storage

Flexible supercapacitors have potential for wearable energy storage due to their high energy/power densities and long operating lifetimes. High electrochemical performance with robust mechanical properties is highly desired for flexible supercapacitor electrodes. Usually, the mechanical properties are improved by choosing high flexible textile substrates but at the much expense of electrochemical performance due to the nonideal contact between conductive materials and textile substrates. Herein, the authors present an efficient, scalable, and general strategy for the simultaneous fabrication of high‐performance textile electrodes and yarn electrodes. It is interesting to find that the conformal reduced graphene oxide (RGO) layer is uniformly and successively painted on the surface of SnCl₂ modified polyester fibers (M‐PEF) via a repeated “dyeing and drying” strategy. The large‐area textile electrodes and ultralong yarn electrodes are fabricated by using RGO/M‐PEF as substrate with subsequent deposition of polypyrrole. This work provides new opportunities for developing high flexible textile electrodes and yarn electrodes with further increased electrochemical performance and scalable production.     

Flexible Transparent Bifunctional Capacitive Sensors with Superior Areal Capacitance and Sensing Capability based on PEDOT:PSS/MXene/Ag Grid Hybrid Electrodes

Flexible transparent supercapacitors (FTSs) have aroused considerable attention. Nonetheless, balancing energy storage capability and transparency remains challenging. Herein, a new type of FTSs with both excellent energy storage and superior transparency is developed based on PEDOT:PSS/MXene/Ag grid ternary hybrid electrodes. The hybrid electrodes can synergistically utilize the high optoelectronic properties of Ag grids, the excellent capacitive performance of MXenes, and the superior chemical stability of PEDOT:PSS, thus, simultaneously demonstrating excellent optoelectronic properties (T: ≈89%, R_s: ≈39 Ω sq⁻¹), high areal specific capacitance, superior mechanical softness, and excellent anti-oxidation capability. Due to the excellent comprehensive performances of the hybrid electrodes, the resulting FTSs exhibit both high optical transparency (≈71% and ≈60%) and large areal specific capacitance (≈3.7 and ≈12 mF cm⁻²) besides superior energy storage capacity (P: 200.93, E: 0.24 µWh cm⁻²). Notably, the FTSs show not only excellent energy storage but also exceptional sensing capability, viable for human activity recognition. This is the first time to achieve FTSs that combine high transparency, excellent energy storage and good sensing all-in-one, which make them stand out from conventional flexible supercapacitors and promising for next-generation smart flexible energy storage devices.     

Inverted Layer‐By‐Layer Fabrication of an Ultraflexible and Transparent Ag Nanowire/Conductive Polymer Composite Electrode for Use in High‐Performance Organic Solar Cells

A highly flexible and transparent conductive electrode based on consecutively stacked layers of conductive polymer (CP) and silver nanowires (AgNWs) fully embedded in a colorless polyimide (cPI) is achieved by utilizing an inverted layer‐by‐layer processing method. This CP‐AgNW composite electrode exhibits a high transparency of >92% at wavelengths of 450–700 nm and a low resistivity of 7.7 Ω ?^?1, while its ultrasmooth surface provides a large contact area for conductive pathways. Furthermore, it demonstrates an unprecedentedly high flexibility and good mechanical durability during both outward and inward bending to a radius of 40 μm. Subsequent application of this composite electrode in organic solar cells achieves power conversion efficiencies as high as 7.42%, which represents a significant improvement over simply embedding AgNWs in cPI. This is attributed to a reduction in bimolecular recombination and an increased charge collection efficiency, resulting in performance comparable to that of indium tin oxide‐based devices. More importantly, the high mechanical stability means that only a very slight reduction in efficiency is observed with bending (<5%) to a radius of 40 μm. This newly developed composite electrode is therefore expected to be directly applicable to a wide range of high‐performance, low‐cost flexible electronic devices.     

Reduced Graphene Oxide Micromesh Electrodes for Large Area,Flexible, Organic Photovoltaic Devices

A laser‐based patterning technique—compatible with flexible, temperature‐sensitive substrates—for the production of large area reduced graphene oxide micromesh (rGOMM) electrodes is presented. The mesh patterning can be accurately controlled in order to significantly enhance the electrode transparency, with a subsequent slight increase in the sheet resistance, and therefore improve the tradeoff between transparency and conductivity of reduced graphene oxide (rGO) layers. In particular, rGO films with an initial transparency of ≈20% are patterned, resulting in rGOMMs films with a ≈59% transmittance and a sheet resistance of ≈565 Ω sq^?1, that is significantly lower than the resistance of ≈780 Ω sq^?1, exhibited by the pristine rGO films at the same transparency. As a proof‐of‐concept application, rGOMMs are used as the transparent electrodes in flexible organic photovoltaic (OPV) devices, achieving power conversion efficiency of 3.05%, the highest ever reported for flexible OPV devices incorporating solution‐processed graphene‐based electrodes. The controllable and highly reproducible laser‐induced patterning of rGO hold enormous promise for both rigid and flexible large‐scale organic electronic devices, eliminating the lag between graphene‐based and indium–tin oxide electrodes, while providing conductivity and transparency tunability for next generation flexible electronics.     

Production and Patterning of Liquid Phase–Exfoliated 2D Sheets for Applications in Optoelectronics

2D materials (2DMs), which can be produced by exfoliating bulk crystals of layered materials, display unique optical and electrical properties, making them attractive components for a wide range of technological applications. This review describes the most recent developments in the production of high‐quality 2DMs based inks using liquid‐phase exfoliation (LPE), combined with the patterning approaches, highlighting convenient and effective methods for generating materials and films with controlled thicknesses down to the atomic scale. Different processing strategies that can be employed to deposit the produced inks as patterns and functional thin‐films are introduced, by focusing on those that can be easily translated to the industrial scale such as coating, spraying, and various printing technologies. By providing insight into the multiscale analyses of numerous physical and chemical properties of these functional films and patterns, with a specific focus on their extraordinary electronic characteristics, this review offers the readers crucial information for a profound understanding of the fundamental properties of these patterned surfaces as the millstone toward the generation of novel multifunctional devices. Finally, the challenges and opportunities associated to the 2DMs' integration into working opto‐electronic (nano)devices is discussed.     

Biodegradable and Highly Deformable Temperature Sensors for the Internet of Things

Recent advances in biomaterials, thin film processing, and nanofabrication offer the opportunity to design electronics with novel and unique capabilities, including high mechanical stability and biodegradation, which are relevant in medical implants, environmental sensors, and wearable and disposable devices. Combining reliable electrical performance with high mechanical deformation and chemical degradation remains still challenging. This work reports temperature sensors whose material composition enables full biodegradation while the layout and ultrathin format ensure a response time of 10 ms and stable operation demonstrated by a resistance variation of less than 0.7% when the devices are crumpled, folded, and stretched up to 10%. Magnesium microstructures are encapsulated by a compostable‐certified flexible polymer which exhibits small swelling rate and a Young's modulus of about 500 MPa which approximates that of muscles and cartilage. The extension of the design from a single sensor to an array and its integration onto a fluidic device, made of the same polymer, provides routes for a smart biodegradable system for flow mapping. Proper packaging of the sensors tunes the dissolution dynamics to a few days in water while the connection to a Bluetooth module demonstrates wireless operation with 200 mK resolution prospecting application in food tracking and in medical postsurgery monitoring.     

Integrating Flexible Ultralight 3D Ni Micromesh Current Collector with NiCo Bimetallic Hydroxide for Smart Hybrid Supercapacitors

Flexible and lightweight supercapacitors with superior mechanical flexibility and outstanding capacity are regarded as an ideal power source for wearable electronic devices. Meanwhile, incorporating additional novel characters such as transparency and electrochromism can further benefit the development of smart supercapacitors. Nevertheless, the application of the commonly used planar-structural current collectors is seriously restricted by their intrinsic properties such as poor rigidity, large thickness, and limited loading surface area. Flexible and ultralight current collectors with 3D architecture, high conductivity, and easy integration are believed to be the most appropriate alternatives to build high-performance supercapacitors. In this study, a novel and scalable manufacturing technique is developed to produce a flexible and ultralight 3D Ni micromesh (3D NM) current collector for supercapacitor. Flexible smart supercapacitor integrated by 3D NM and high active Ni–Co bimetallic hydroxide (3D NM@NiCo BH) delivers a considerable rate performance (60.6% capacity retention from 1 to 50 mA cm⁻²). Furthermore, the fabricated hybrid supercapacitor device integrated with electrochromic functionality can visually indicate the energy level by a color display. This flexible electrochromic supercapacitor based on ultralight 3D Ni micromesh provides a novel insight into multifunctional energy storage systems for smart wearable electronic devices.     

2D Nanomaterial Arrays for Electronics and Optoelectronics

Two‐dimensional (2D) materials, benefitting from their unique planar structure and various appealing electronic properties, have attracted much attention for novel electronic and optoelectronic applications. As a basis for practical devices, the study of micro/nano‐2D material arrays based on coupling effects and synergistic effects is critical to the functionalization and integration of 2D materials. Moreover, micro/nano‐2D material arrays are compatible with traditional complementary metal oxide semiconductor (CMOS) electronics, catering well to high‐integration, high‐sensitivity, and low‐cost sensing and imaging systems. This review presents some recent studies on 2D material arrays in sequence from their novel preparations to high‐integration applications as well as explorations on dimension tuning. A first focus is on various typical fabrication methods for 2D material arrays, including photolithography, 2D printing, seeded growth, van der Waals epitaxial growth, and self‐assembly. Then, the applications of 2D material arrays, such as field effect transistors, photodetectors, pressure sensors, as well as flexible electronic devices of photodetectors and strain sensors, are elaborately introduced. Furthermore, the recent burgeoning exploration of mixed‐dimensional heterostructure arrays including 0D/2D, 1D/2D, and 3D/2D is discussed. Ultimately, conclusions and an outlook based on the current developments in this promising field are presented.     

Energy Storing Plant Stem with Cytocompatibility for Supercapacitor Electrode

Endowing natural biological objects, such as plants or their parts, with non-native and targeted electroactivity is an appealing challenge in nano-biotechnology, which if addressed will circumvent the biomass carbonization and promote the emission-free transformation of raw bioresources for green energy. Here, a general concept to produce supercapacitor electrodes from plant stems is reported, which exhibit promising electroactivity and cytocompatibility. The core innovation is to apply a conformal electroactive coating onto the surfaces of hyperbranched channels of plant stem by interfacial self-assembly. The electroactive plant stem can be employed as a functional supercapacitor electrode, the capacitance of which is up to 2.35 F. Furthermore, more than 98% cell viability is well maintained after the plant stem electrode is incubated with MCF-7 cells for 72 h, and even after 100 discharge–recharge cycles, suggesting this plant stem electrode has excellent cytocompatibility and cycling stability. This energy storing plant stem gives rise to new opportunities for integrating natural bioresources with artificial electroactive nanomaterials for cytocompatible energy storage devices.     

Hexagonal Boron Nitride–Enhanced Optically Transparent Polymer Dielectric Inks for Printable Electronics

Solution‐processable thin‐film dielectrics represent an important material family for large‐area, fully‐printed electronics. Yet, in recent years, it has seen only limited development, and has mostly remained confined to pure polymers. Although it is possible to achieve excellent printability, these polymers have low (≈2–5) dielectric constants (ε_r). There have been recent attempts to use solution‐processed 2D hexagonal boron nitride (h‐BN) as an alternative. However, the deposited h‐BN flakes create porous thin‐films, compromising their mechanical integrity, substrate adhesion, and susceptibility to moisture. These challenges are addressed by developing a “one‐pot” formulation of polyurethane (PU)‐based inks with h‐BN nano‐fillers. The approach enables coating of pinhole‐free, flexible PU+h‐BN dielectric thin‐films. The h‐BN dispersion concentration is optimized with respect to exfoliation yield, optical transparency, and thin‐film uniformity. A maximum ε_r ≈ 7.57 is achieved, a two‐fold increase over pure PU, with only 0.7 vol% h‐BN in the dielectric thin‐film. A high optical transparency of ≈78.0% (≈0.65% variation) is measured across a 25 cm² area for a 10 μm thick dielectric. The dielectric property of the composite is also consistent, with a measured areal capacitance variation of <8% across 64 printed capacitors. The formulation represents an optically transparent, flexible thin‐film, with enhanced dielectric constant for printed electronics.