Stretchable Transparent Electrodes Research Articles

Advances in Stretchable Organic Photovoltaics: Flexible Transparent Electrodes and Deformable Active Layer Design.

Stretchable organic photovoltaics (OPVs) have attracted significant attention as promising power sources for wearable electronic systems owing to their superior robustness under repetitive tensile strains and their good compatibility. However, reconciling a high power-conversion efficiency and a reasonable flexibility is a tremendous challenge. In addition, the development of stretchable OPVs must be accelerated to satisfy the increasing requirements of niche markets for mechanical robustness. Stretchable OPV devices can be classified as either structurally or intrinsically stretchable. This paper reviews recent advances in stretchable OPVs, including the design of mechanically robust transparent electrodes, photovoltaic materials, and devices. Initially, we provide an overview of the characteristics and recent research progress in the areas of structurally and intrinsically stretchable OPVs. Subsequently, research into flexible and stretchable transparent electrodes that directly affect the performances of stretchable OPVs is summarized and analyzed. Overall, this review aims to provide an in-depth understanding of the intrinsic properties of highly efficient and deformable active materials, whilst also emphasizing advanced strategies for simultaneously improving the photovoltaic performance and mechanical flexibility of the active layer, including material design, multi-component settings, and structural optimization. This article is protected by copyright. All rights reserved.

Large-area, stretchable, ordered silver nanowires electrode by superwetting-induced transfer of ionic liquid@silver nanowires complex

Stretchable transparent electrodes (STEs) are widely used in electronic skin, implantable biomedical devices and wearable devices, which need to meet metal-like conductivity and high transparency while being prepared in a large area. Silver nanowires are the most widely used material to construct conductive networks because of their excellent optoelectrical properties. However, the silver nanowires-based networks prepared by conventional large-area preparation processes are generally composed of disordered silver nanowires, which suffer both high junction resistance and high diffuse reflection. Herein, a large-area, stretchable, ordered silver nanowires electrode is prepared by superwetting-induced transfer of a self-assembled complex composed of ionic liquid (1-ethyl-3-methylimidazolium (bis-trifluoromethane sulfonyl) imide) and silver nanowires onto stretchable substrates. Then, electroless-welding is used to further reduce the junction resistance between silver nanowires. The generated STE shows a low sheet resistance of 26–46 Ω sq−1 and a high transmittance of 84 %–88 % at 550 nm. The STE is successfully applied for transparent heating devices, exhibiting high heating performance and tensile stability. This study provides a new method for the preparation of large-area high-performance STE for the next generation of stretchable devices.

A Sensing and Stretchable Polymer‐Dispersed Liquid Crystal Device Based on Spiderweb‐Inspired Silver Nanowires‐Micromesh Transparent Electrode

AbstractPolymer‐dispersed liquid crystal (PDLC) devices are truly promising optical modulators for information display, smart window as well as intelligent photoelectronic applications due to their fast switching, large optical modulation as well as cost‐effectiveness. However, realizing highly soft PDLC devices with sensing function remains a grand challenge because of the intrinsic brittleness of traditional transparent conductive electrodes. Here, inspired by spiderweb configuration, a novel type of silver nanowires (AgNWs) micromesh‐based stretchable transparent conductive electrodes (STCEs) is developed to support the realization of soft PDLC device. Benefiting from the embedding design of AgNWs micromesh in polydimethylsiloxane (PDMS), the STCEs can maintain excellent electrical conductivity and transparency even in various extreme conditions such as bending, folding, twisting, stretching as well as multiple chemical corrosion. Further, STCEs with the embedded AgNWs micromesh endow the assembled PDLC device with excellent photoelectrical properties including rapid switching speed (<1 s), large optical modulation (69% at 600 nm), as well as robust mechanical stability (bending over 1000 cycles and stretching to 40%). Moreover, the device displays the pressure sensing function with high sensitivity in response to pressure stimulus. It is conceivable that AgNWs micromesh transparent electrodes will shape the next generation of related soft smart electronics.

Highly Conductive Inkjet-Printed PEDOT:PSS Film under Cyclic Stretching.

Inkjet-printed conductive polymer PEDOT:PSS films have provided a new developing direction for realizing the stretchable transparent electrodes in optoelectronic devices. However, their conductivity and stretchability are limited as the presence of insulating PSS chains, rigid PEDOT conjugated backbone, and stronger inter-chain interactions in the pristine polymer, respectively. Here, we report a PEDOT:PSS film with preferable electrical and mechanical performances by inkjet-printing the formulated printable ink containing PEDOT:PSS, formamide (FA), d-sorbitol (SOR), sodium dodecyl benzene sulfonate (DBSS), and ethylene glycol (EG). The inkjet-printed uniform PEDOT:PSS film exhibits a high conductivity of 1050 S/cm and sheet resistance of less than 145 Ω/sq on both rigid and flexible substrates. Moreover, the resistance can remain stable after 200 cycles of stretching at 55% strain. The film also presents good stability during repetitive stretching-releasing cycles. The significantly enhanced conductivity of the film lies on the conformational transition of the backbone by secondary doping and post-treatment with FA as well as removing the excess PSS components after phase separation between PEDOT and PSS. Meanwhile, SOR serves as a plasticizer to break the original hydrogen bonds between PSSH chains and provides larger free volume for polymer chain extension, which gives the PEDOT:PSS film the ability to tolerant cyclic tension. This is one of the optimal performances currently reported for inkjet-printed stretchable PEDOT:PSS films. The inkjet-printed PEDOT:PSS film with high conductivity, stretching properties, as well as good biocompatibility exhibits promising prospects as anodes on optoelectronic devices.

Stretchable and Biocompatible Transparent Electrodes for Multimodal Biosignal Sensing from Exposed Skin

AbstractReal‐time health monitoring technology in daily life requires mechanically robust and transparent electrodes for multimodal biosignal sensing from exposed human epidermis. Here, highly stretchable transparent electrodes comprising a water‐dispersed conductive polymer, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and a protic ionic liquid (IL), 3‐methylimidazolium:bis(trifluoromethylsulfonyl)amide (p‐MIM:TFSI) are reported. Owing to the high water miscibility of p‐MIM:TFSI and its favorable ion exchange capability with PEDOT:PSS, PEDOT:PSS/p‐MIM:TFSI transparent electrodes show enhanced electrical conductivity (σ = 450 S cm−1) and thin‐film stretchability represented by crack onset strain (εc) exceeding 50%. These electrodes outperform other PEDOT:PSS electrodes processed with an aprotic counterpart, 1‐ethyl‐3‐methylimidazolium(EMIM):TFSI, or a traditional ionic salt, Li:TFSI. The PEDOT:PSS/p‐MIM:TFSI thin‐film electrodes are also biocompatible and conformally adhere to human skin; therefore, multimodal biosignals including electrocardiogram, electrooculogram, and electromyogram with high signal‐to‐noise ratios from exposed epidermis on human faces and arms under various measurement conditions mimicking daily activities are collected. Considering the importance of light penetration through human skin for stable biological activity during biosignal monitoring, the results can broaden the applicability of daily‐use wearable biosignal sensors by applying them to exposed human skin.

12.2: Highly Conductive Omnidirectionally Stretchable 2D Transparent Copper mesh electrodes and Applications in Optoelectronic devices

12.2: Highly Conductive Omnidirectionally Stretchable 2D Transparent Copper mesh electrodes and Applications in Optoelectronic devices

Sequentially Coated Wavy Nanowire Composite Transparent Electrode for Stretchable Solar Cells.

Recent advances in fabricating stretchable and transparent electrodes have led to various techniques for establishing next-generation form-factor optoelectronic devices. Wavy Ag nanowire networks with large curvature radii are promising platforms as stretchable and transparent electrodes due to their high electrical conductivity and stretchability even at very high transparency. However, there are disadvantages such as intrinsic nonregular conductivity, large surface roughness, and nanowire oxidation in air. Here, we introduce electrically synergistic but mechanically independent composite electrodes by sequentially introducing conducting polymers and ionic liquids into the wavy Ag nanowire network to maintain the superior performance of the stretchable transparent electrode while ensuring overall conductivity, lower roughness, and long-term stability. In particular, plenty of ionic liquids can be incorporated into the uniformly coated conducting polymer so that the elastic modulus can be significantly lowered and sliding can occur at the nanowire interface, thereby obtaining the high mechanical stretchability of the composite electrode. Finally, as a result of applying the composite film as the stretchable transparent electrode of stretchable organic solar cells, the organic solar cell exhibits a high power conversion efficiency of 11.3% and 89% compared to the initial efficiency even at 20% tensile strain, demonstrating excellent stretching stability.

Highly Conductive Omnidirectionally Stretchable 2D Transparent Copper Mesh Electrodes and Applications in Optoelectronic Devices

Abstract2D stretchable transparent electrodes (STE) are required to conform to non‐flat complex 3D surfaces for next‐generation stretchable optoelectronic devices. However, it is a great challenge to simultaneously maintain omnidirectional stretchability, high transmittance, and the least change in conductivity at large strain. In this work, omnidirectionally stretchable 2D transparent electrode is achieved by hybrid printed copper mesh embedded in an elastic material. The electrode displays exceedingly low sheet resistance down to 0.12 Ω sq−1 while still maintaining 80% of optical transparency. Two types of mesh geometries, the horseshoe‐like and sinusoid‐like shapes, are thoroughly investigated by simulations and experiments. By optimizing the key geometrical parameters of mesh structure, the copper mesh can endure stretching up to 130% with no fractures and no resistivity change. After 1000 stretch‐and‐release cycles at 10% strain, the copper mesh remains intact with negligible resistance variation. For 2D stretchability, the copper mesh is stretched in eight directions simultaneously and can maintain its initial conductivity up to 50% tensile deformation. As a demonstration of application, the copper mesh STE is employed in a stretchable electroluminescent device that maintained uniform lighting up to 120% stretching and 100 stretch‐release cycles at 30% strain, showing its potential in stretchable and wearable optoelectronic applications.

Strain‐Tolerant, High‐Detectivity, and Intrinsically Stretchable All‐Polymer Photodiodes

AbstractA skin‐like photodiode (PD) that is stretchable and skin‐conformable is crucial to opening the next‐generation wearable electronics for optical biometric monitoring, biomedical imaging, and others. To achieve reliable PD characteristics under large deformation, stretchable PDs with high detectivity and high mechanical stretchability must be developed. Herein, intrinsically stretchable polymer‐based PDs (is‐PPDs) comprising all‐polymeric constituent layers are demonstrated. In particular, elastomeric photoactive layers consisting of an elastomer with p‐/n‐type semiconducting polymers and conducting polymer‐based stretchable transparent electrodes with modulated work functions improve both the mechanical stability and the detectivity (D*) of is‐PPDs. Accordingly, is‐PPDs show excellent D* over 1013 Jones with a suppressed dark current density of 0.1 nA cm−2 before and after 100% stretching. The proposed is‐PPDs record high‐quality and stable photoplethysmography signals at the wrist with outward extension.

Intrinsically stretchable, semi-transparent organic photovoltaics with high efficiency and mechanical robustness via a full-solution process

AZO@Ag nanowire@AZO (AAA) composite stretchable transparent electrode enables full-solution-processed semi-transparent intrinsically stretchable OPV achieving record 10.90% PCE, with excellent mechanical robustness against stretch-release cycles.

Expandable, serpentine Au nanotrough networks for skin-like electrodes with superior thermal and moisture stability

Flexible and stretchable electrodes as the cornerstone of bioelectronics are bridging human with biomedical devices in an unprecedentedly intimate and safer way. However, maintaining high electrical properties while adapting to moist, complex and dynamic biosurfaces for a prolonged period of usage remains a great challenge. Herein, we demonstrate an omnidirectionally stretchable transparent electrode based on serpentine Au nanotrough (AuNT) networks with highly tunable optoelectronic performances (sheet resistance of down to ∼13 Ω sq−1 at transmittance of 90%). Owing to the junction-free nanoweb structure with an ultralow percolation density, the AuNT networks demonstrate excellent thermal stability as an efficient Joule heater with a wide operating temperature range up to 390 °C. The in-plane buckled AuNT electrode exhibits controllable heat generation upon repetitive tensile strains of 60%, enabling wearable thermotherapy at highly strained joints. Such highly stretchable electrodes can be directly laminated on the balloon catheter as expandable, high-density nanomesh ablators, showing excellent thermal and moisture stability during 15 days of storage under conditions of relative humidity of 85% at 70 °C and over 1000 heating cycles to 60 °C, offering strong potential for biomedical therapeutics, such as thermal ablation and electrochemotherapy.

Customizable Stretchable Transparent Electrodes Based on AgNW/CNT Hybrids via Tailoring Sizes of Building Blocks

Stretchable transparent electrodes (STEs) based on silver nanowires (AgNWs) have received considerable attention for a variety of flexible and wearable electronic/optoelectronic devices. Up to now, most efforts have focused on optimizing the STEs composed by a single AgNW conductive network. On the contrary, the structure–performance correlations of STEs formed by a hybrid percolative network which comprises the AgNW and a second conductive nanomaterial have rarely been discussed. In this work, we fabricated hybrid-type STEs by selecting three kinds of carbon nanotubes (CNTs) with different diameters to pair with three types of AgNWs with variable length-to-diameter ratios. The size effect of building blocks of the nine combinations on the optical, electrical, and mechanical properties of resultant STEs was thoroughly investigated. The results reveal that AgNWs and CNTs with smaller diameters are beneficial to achieve hybrid electrodes with a high transmittance and low haze. AgNWs with larger length-to-diameter ratios contribute hybrid STEs with lower sheet resistance by adding a suitable amount of CNTs. Importantly, the smaller differences in diameters of AgNWs and CNTs lead to more effective capillary-force-induced welding, which boosts both the conductivity and stretchability of STEs. An optimized AgNW/CNT hybrid electrode demonstrated a transmittance of 66.4% and a haze of 11.0% at a sheet resistance of 8.70 Ω sq.–1 which could endure a uniaxial tensile strain as large as 490%, while its resistance increased only 46.9% after experiencing 1000 cycles of 50% tensile strain. Alternating current electroluminescent devices based on such AgNW/CNT hybrid STEs were also successfully developed, showing uniform and stable patterned luminescence.

Structures and Materials in Stretchable Electroluminescent Devices.

Stretchable electroluminescent (EL) devices are obtained by partitioning a large emission area into areas specifically for stretching and light-emission (island-bridge structure). Buckled and textile structures are also shown effective to combine the conventional light emitting diode fabrication with elastic substrates for structure-enabled stretchable EL devices. Meanwhile, intrinsically stretchable EL devices which are characterized with uniform stretchability down to microscopic scale are relatively less developed but promise simpler device structure and higher impact resistance. The challenges in fabricating intrinsically stretchable EL devices with high and robust performance are in many facets, including stretchable conductors, emissive materials, and compatible processes. For the stretchable transparent electrode, ionically conductive gel, conductive polymer coating, and conductor network in surface of elastomer are all proven useful. The stretchable EL materials are currently limited to conjugated polymers, conjugated polymers with surfactants and ionic conductors added to boost stretchability, and phosphor particles embedded in elastomer matrices. These emissive materials operate under different mechanisms, require different electrode materials and fabrication processes, and the corresponding EL devices face distinctive challenges. This review aims to provide a basic understanding of the materials meeting both the mechanical and electronic requirements and important techniques to fabricate the stretchable EL devices.

Stretchable transparent electrodes for conformable wearable organic photovoltaic devices

To achieve adhesive and conformable wearable electronics, improving stretchable transparent electrode (STE) becomes an indispensable bottleneck needed to be addressed. Here, we adopt a nonuniform Young’s modulus structure with silver nanowire (AgNW) and fabricate a STE layer. This layer possesses transparency of >88% over a wide spectrum range of 400–1000 nm, sheet resistance below 20 Ω sq−1, stretchability of up to 100%, enhanced mechanical robustness, low surface roughness, and good interfacial wettability for solution process. As a result of all these properties, the STE enables the fabrication of a highly efficient ultraflexible wearable device comprising of both organic photovoltaic (OPV) and organic photodetector (OPD) parts with high mechanical durability and conformability, for energy-harvesting and biomedical-sensing applications, respectively. This demonstrates the great potential of the integration of OPVs and OPDs, capable of harvesting energy independently for biomedical applications, paving the way to a future of independent conformable wearable OPV/OPDs for different applications.

Stretchable Transparent Electrode via Wettability Self-Assembly in Mechanically Induced Self-Cracking.

Stretchable and transparent electrodes (STEs) are indispensable components in numerous emerging applications such as optoelectrical devices and wearable devices used in health monitoring, human-machine interaction, and artificial intelligence. However, STEs have limitations in conductivity, robustness, and transmittance owing to the exposure of the substrate and fatigue deformation of nanomaterials under strain. In this study, an STE consisting of conductive materials embedded in in situ self-cracking strain-spread channels by wettability self-assembly is fabricated. Finite element analysis is used to simulate the crevice growth using the representative unit cell network and strain deformation using a random network. The embedded conductive materials are partly protected by the strain-opening crevice channel, and network dissociation is avoided under stretching, showing a maximum strain of 125%, a transmittance of approximately 89.66% (excluding the substrate) with a square resistance of 9.8 Ω sq-1, and high stability in an environment with high temperature and moisture. The wettability self-assembly coating process is verified and expanded to several kinds of hydrophilic inks and hydrophobic coating materials. The fabricated STE can be employed as a strain sensor in motion sensing, vital sign and posture feedback, and mimicking bioelectronic spiderweb with spatial gravity induction.

UV‐Curable Adhesive Tape‐Assisted Patterning of Metal Nanowires for Ultrasimple Fabrication of Stretchable Pressure Sensor

AbstractAg nanowires (AgNWs) have been highlighted as a flexible or stretchable transparent electrode material. Contrary to general perception, AgNWs have not been used in earnest for product development with a flexible form factor because the patterning of an AgNW‐based transparent electrode is complicated and costly, and its reproducibility and the possible pattern specifications are poor. Herein, a highly reproducible, practical patterning approach is developed that not only enables fine patterning down to 30 µm line width, but also enables complicated patterns. When the UV‐curable adhesive is irradiated with UV light, the oligomers cross‐link, resulting in 1) a decrease of adhesion, and 2) curing and shrinking. After attaching this adhesive tape to the temporary substrate coated with AgNWs, through the selective irradiation of UV and peel‐off of the tape, the AgNWs impregnated with the cured adhesive are removed from the temporary substrate, leaving finely defined AgNW patterns. The AgNW patterns thus fabricated are impregnated under the polydimethylsiloxane surface by an inverted layer processing, and could be used to precisely detect whether a gloved finger is touched, or the magnitude of pressure applied to the electrode surface, maintaining high sensitivity even in the stretched state.

Graphene/Silver Nanowires/Graphene Sandwich Composite for Stretchable Transparent Electrodes and Its Fracture Mechanism.

Polycrystalline graphene grown by chemical vapor deposition (CVD) is characterized by line defects and disruptions at the grain boundaries and nucleation sites. This adversely affects the stretchability and conductivity of graphene, which limits its applications in the field of flexible, stretchable, and transparent electrodes. We demonstrate a composite electrode comprised of a graphene/silver nanowires (AgNWs)/graphene sandwich structure on a polydimethylsiloxane substrate to overcome this limitation. The sandwich structure exhibits high transparency (>90%) and excellent conductivity improvement of the graphene layers. The use of AgNWs significantly suppresses the conductivity loss resulting from stretching. The mechanism of the suppression of the conductivity loss was investigated using scanning electron microscopy, atomic force microscopy, and lateral force microscopy. The results suggest that the high surface friction of the sandwich structure causes a sliding effect between the graphene layers would produce low crack or hole formation to maintain the conductivity. In addition to acting as conductive layers, the top and bottom graphene layers can also protect the AgNWs from oxidation, thereby enabling maintenance of the electrical performance of the electrodes over a prolonged period. We also confirmed the applicability of the sandwich structure electrode to the human body, such as on the wrist, finger, and elbow.

Macroscopic properties of single-crystalline and polycrystalline graphene on soft substrate for transparent electrode applications

Large-area graphene synthesized by a chemical vapor deposition (CVD) method is promising for a flexible, stretchable transparent electrode but suffers from structural defects such as grain boundaries, which causes to degrade its physical properties. Recently, preparation methods of large-scale growth templates for single-crystalline graphene (SCG) were reported, but the macroscopic properties of SCG have not been adequately examined yet. Here, an SCG-based flexible, stretchable graphene transparent electrode is successfully demonstrated, which shows superior optoelectrical, electromechanical, and barrier properties compared to polycrystalline graphene (PCG)-based transparent electrodes. The result exhibits the grain boundary effect of graphene on soft substrates on a macroscopic scale and a great promise toward realizing a tremendous performance improvement of graphene-based flexible transparent electrodes on a large scale.

Multifunctional Stretchable Organic–Inorganic Hybrid Electronics with Transparent Conductive Silver Nanowire/Biopolymer Hybrid Films

AbstractStretchable organic electronics open opportunities for novel applications in next‐generation intelligent electronics, such as human‐friendly, skin‐attachable, and implantable devices. Stretchable transparent electrodes with appropriate electrical, mechanical, and optical properties as well as high conformability to curved surfaces and human skin are key components of wearable and implantable devices. Herein, a silver nanowire (AgNW)‐embedded chitosan biopolymer hybrid film is presented as a high‐performance stretchable transparent electrode. The introduction of an organic surface modifier and embedding the AgNW network results in remarkably improved adhesion between the nanowires and substrate; consequently, the electrical properties and mechanical performance of the stretchable films are improved simultaneously. The fabricated hybrid films show high transmittance (89.0% at 550 nm), low sheet resistance (8.4 ohm sq−1), and limited resistance change under tensile strains. Moreover, these stretchable transparent electrodes with versatile applicability are integrated into multifunctional stretchable electronic devices such as heaters, strain sensors, and light‐emitting devices, which show excellent thermal, strain sensing, and light‐emitting properties, respectively. In addition, they conform to curved surfaces or human skin and maintain their mechanical robustness. The proposed strategy offers new opportunities for stretchable and biocompatible organic electronics, particularly for skin‐attachable, visually imperceptible thermal management devices, strain sensors, and light‐emitting devices.

M13 bacteriophage-templated gold nanowires as stretchable electrodes in perovskite solar cells

This work reports stretchable gold nanowires grown on M13 bacteriophage in perovskite solar cells, which is the first demonstration of the M13 virus as a stretchable transparent electrode.