Flexible Transparent Conductive Electrodes Research Articles

Layer‐by‐Layer 2D Ultrathin Conductive Cu3(HHTP)2 Film for High‐Performance Flexible Transparent Supercapacitors

Abstract2D conductive metal–organic frameworks (2D c‐MOFs) naturally possess high active sites, large specific surface areas, and fast ion transport for energy‐storage devices. However, the intelligent exploration of 2D c‐MOFs for the emerging flexible transparent supercapacitors (FTSCs) with excellent photoelectric property and electrochemical activity is rarely implemented. Herein, 2D conductive ultrathin Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11‐hexahydroxytriphenylene) film is synthesized on the indium tin oxide/polyethylene terephthalate (ITO/PET) substrate for flexible transparent conductive electrodes (FTCEs) through the layer‐by‐layer assembly method. The interconnected conductive frameworks possess the cooperative superiority of low internal resistance, fast electrolyte ion diffusion, and quick electron transfer. Cu3(HHTP)2‐15/ITO/PET FTCEs show high photoelectric property of T550 nm = 82.2% and Rs = 49.1 Ω sq−1, and high areal capacitance (CA) of 1700 µF cm−2. The corresponding symmetrical FTSCs (T550 nm = 62.1%) exhibit excellent CA of 939.2 µF cm−2 at the current density of 7 µA cm−2, superior rate capability (63.9% at 150 µA cm−2), long‐term cycling stability (85% after 3000 cycles), and mechanical flexibility under different bending angles. This work provides key insights into the design and synthesis of 2D c‐MOF films for flexible transparent energy‐storage devices.

Optimization of silver nanowire synthesis for flexible transparent conductive electrodes

AbstractIn this work, a range of factors was investigated for the synthesis of silver nanowires. Nickel chloride showed to be a promising controlling agent for the scale‐up synthesis of silver nanowires. Other factors such as reaction temperature, concentrations of surfactant, precursor, controlling agent, and the effect of degassing were studied and optimized. The synthesized silver nanowire sample was applied for flexible electrode fabrication. The fabricated electrode showed to have high transparency (90 %), low sheet resistance (26 Ω/sq), low surface roughness (Rq = 6 nm), and high bending stability (withstand 100 bending cycles at bending radius of 4 mm), which satisfied the requirement for applications in flexible optoelectronic device.

Fabrication of Highly Flat, Flexible Mesh Electrode for Use in Photovoltaics

Metal mesh-based flexible transparent conductive electrodes have attracted much interest as one alternative to conventional indium tin oxide electrodes. In addition to ongoing efforts to develop scalable and cost-effective fabrication processes for high-resolution mesh patterns, the high surface roughness of the mesh which can cause short circuiting and current leakage in optoelectronic devices must be solved. Herein, high-resolution (below ~ 10 μm) mesh patterns with various thicknesses are fabricated by scalable, selective transfer printing and then stably and sufficiently sintered under delicately controlled flash irradiation. A polyethylene terephthalate (PET) film is laminated on the patterns overlaid with a UV-curable resin and then peeled off after UV curing, which produces mesh patterns strongly bonded to the cured resin to be separated in a fully embedded form. The final, highly flat, flexible mesh with a low sheet resistance of 1.7 Ω sq−1, a high optical transparency of 88.6%, excellent mechanical flexibility, and strong adhesion to the substrate is successfully implemented in a flexible perovskite solar cell with a high power conversion efficiency (PCE) of 14.92%.

Flexible transparent conductive electrode of Au/PDMS prepared by electrochemical-assisted peeling

Metal grids have great potential in flexible transparent conductive electrodes (E-TCEs) due to their high conductivity and transparency. Unlike other TCEs, the sheet resistance and optical transmittance of metal grids can be independently adjusted. However, the large surface roughness resulted from thickened metal layer may increase the risk of electrical short-circuiting for optoelectronic devices. In this paper, a TCE of Au-grid embedded in polydimethylsiloxane (PDMS) has been developed by an electrochemical-assisted peeling technique. By a sacrificial etching and transfer process, a smooth surface of Au/PDMS is achieved. The fabricated Au/PDMS TCE shows a surface roughness of 15 nm, a sheet resistance of 8.03 Ω/sq., an optical transmittance of 95.04% and a figure of merit (FoM) of 910. In addition, the mechanical stability of fabricated TCE is also investigated by 10,000 cycles bending with a bending radius of 2 mm. Using the fabricated TCE, a flexible alternating-current-driven electroluminescent (ACEL) device is demonstrated and evaluated, which confirms the use of Au/PDMS TCEs in optoelectronic applications.

Silver Nanowire Network Hybridized with Silver Nanoparticle-Anchored Ruthenium Oxide Nanosheets for Foldable Transparent Conductive Electrodes.

Facile strategies in flexible transparent conductive electrode materials that can sustain their electrical conductivities under 1 mm-scale radius of curvature are required for wider applications such as foldable devices. We propose a rational design as well as a fabrication process for a silver nanowire-based transparent conductive electrode with low sheet resistance and high transmittance even after prolonged cyclic bending. The electrode is fabricated on a poly(ethylene terephthalate) film through the hybridization of silver nanowires with silver nanoparticles-anchored RuO2 nanosheets. This hybridization significantly improves the performance of the silver nanowire network under severe bending strain and creates an electrically percolative structure between silver nanowires and RuO2 nanosheets in the presence of anchored silver nanoparticles on the surface of RuO2 nanosheets. The resistance change of this hybrid transparent conductive electrode is 8.8% after 200,000 bending cycles at a curvature radius of 1 mm, making it feasible for use in foldable devices.

Scalable, All‐Printed Photocapacitor Fibers and Modules based on Metal‐Embedded Flexible Transparent Conductive Electrodes for Self‐Charging Wearable Applications

AbstractThe popularity of wearable smart electronic gadgets, such as smartphones, smartwatches, and medical sensors, is inhibited by their limited operation lifetime due to the lack of a sustainable self‐charging power supply. This constraint can be overcome by developing a flexible, self‐charging photocapacitor that can synchronously harvest and store energy. Here, ultrathin, all‐printed, and metal‐embedded transparent conducting electrodes (ME‐TCEs) are designed for the fabrication of large‐area, flexible organic solar cells (F‐OSCs) and flexible supercapacitors (F‐SCs). Stripe‐shaped F‐OSCs (SF‐OSCs) and F‐SCs (SF‐SCs) are obtained via slitting the as‐fabricated F‐OSCs and F‐SCs with a surgical scalpel, respectively. The SF‐OSCs and SF‐SCs fully retain their performance after slitting, achieving a power conversion efficiency of ≈6.43% and areal capacitance of ≈52 mF cm−2, respectively. Furthermore, photocapacitor fibers are obtained by vertically stacking one SF‐OSC and seven SF‐SCs. Each fiber is fully encapsulated using UV‐curable resin. When woven into a textile, the photocapacitor module (2 series × 4 parallel connections) is able to charge up to a voltage of 3.2 V in 5 min under one‐sun illumination. The photoelectric‐conversion‐and‐storage efficiency (η) of the photocapacitor module is 4.94%. The highly tailorable, mechanically robust photocapacitor demonstrated herein can be a secondary, self‐sustainable power supply for futuristic wearable applications.

A one-step synthesis of ultra-long silver nanowires with ultra-high aspect ratio above 2000 and its application in flexible transparent conductive electrodes

Silver nanowires (AgNWs), appear as an extremely promising candidate for the next generation of flexible transparent conductive electrodes (FTCEs). However, the performance of AgNWs-FTCEs was severely limited by the aspect ratio of AgNWs, while it was still a big challenge to fabricate AgNWs with high aspect ratio nowadays. To improve the aspect ratio of AgNWs, bromide ion (Br−), cupric ion (Cu2+) and polyvinylpyrrolidone (PVP, Mw ≈ 1300 000) which are beneficial for the synthesis of high aspect ratio AgNWs, were introduced in this article. The high quality and uniform AgNWs with the average diameter of 77.6 nm and the aspect ratio above 2000 were fabricated via a one-step solvothermal method. The effects of reaction time, molar ratio of AgNO3 to PVP and the concentration of CuBr2 on the aspect ratio of AgNWs were discussed. The mechanism of the synthesis of high aspect ratio AgNWs was explored. After that, the prepared AgNWs were spin-coated on the surface of PET film, the FTCEs based on the ultra-high aspect ratio AgNWs without any post-treatments exhibits relatively high transmittance, low haze and low sheet resistance, and the AgNWs have little effect on the optical performance of pristine PET film. The outstanding performance of the prepared FTCEs indicated that the ultra-high aspect ratio AgNWs are ideal materials in the application of FTCEs, and the method of fabricating AgNWs could provide a direction to the high aspect ratio AgNWs.

Flexible Transparent Supercapacitors: Materials and Devices

AbstractThe progressive development of flexible transparent portable electronic devices is in urgent need of matching power sources. Flexible transparent supercapacitors (FTSCs) are the core resources due to their high optical transmittance, endurable mechanical flexibility, excellent electrochemical performance, and facilely accessible device configuration. This review organizes the rational design of nanostructured electrode materials toward FTSCs. First, the structure, mechanism, and property of FTSCs are introduced. Then, the design principles of diverse electrode materials are discussed to achieve flexible transparent conductive electrodes (FTCEs) with different figure of merits (both electrical FoMe and capacitive FoMc), mechanical strength, and environmental stability. Following the achievements in multifunctional FTSCs focusing on film‐supercapacitors, micro‐supercapacitors, electrochromic supercapacitors, photo‐supercapacitors, and battery‐like supercapacitors are also highlighted. Finally, the current challenges and future perspectives on viable materials in the construction of FTSCs to power portable electronics are outlined.

Enhanced flexibility and stability of PEDOT:PSS electrodes through interfacial crosslinking for flexible organic light-emitting diodes

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films have drawn extensive attention as one of the most promising flexible transparent conductive electrodes to replace traditional indium tin oxide. However, some critical issues, such as weak adhesion, vulnerability to moisture and detrimental acidic property, need to be addressed before the practical application and industrialization. Here, we propose a facile and effective strategy of interfacial crosslinking to further improve the flexibility and stability of PEDOT:PSS electrodes with high transparency and conductivity by introducing polyethyleneimine ethoxylated (PEIE) on the surface. The flexibility and stability of PEDOT:PSS electrodes with PEIE overcoating layer are significantly improved, which can be attributed to the interfacial crosslinking reaction between PEIE and PSS. Finally, flexible organic light-emitting didoes (OLEDs) are constructed based on the PEDOT:PSS electrodes modified by PEIE, and current efficiency is enhanced from 20.5 to 76.4 cd/A with a 2.7-fold enhancement, owning to the improved carrier balance. This study confirms that PEIE is effective in protecting the PEDOT:PSS films from mechanical damage and moisture attack, while maintaining the high conductive and transmittance, and illustrates a promising future in low-cost flexible optoelectronic devices employing PEDOT:PSS electrodes. Polyethyleneimine ethoxylated (PEIE) is introduced on the surface of PEDOT:PSS, which improves the flexibility and stability of the highly transparent and conductive PEDOT:PSS electrodes due to the interfacial crosslinking reaction between PEIE and PSS. This approach enhances the luminous efficiency from 20.5 to 76.4 cd/A for flexible organic light-emitting diodes, illustrating a promising future in low-cost flexible optoelectronic devices. • Polyethyleneimine ethoxylated (PEIE) is introduced on the surface of PEDOT:PSS. • The interfacial crosslinking reaction between PEIE and PSS enhanced the flexibility and stability of PEDOT:PSS electrodes. • This approach enhances the luminous efficiency from 20.5 to 76.4 cd/A of flexible organic light-emitting diodes.

Benefits, Problems, and Solutions of Silver Nanowire Transparent Conductive Electrodes in Indium Tin Oxide (ITO)‐Free Flexible Solar Cells

AbstractIn this review, silver nanowires (AgNWs) are introduced, as the primary material to replace indium tin oxide for fabricating cost‐effective flexible organic solar cells (FOSCs), because of their remarkable solution‐processing, flexibility, transparency, and conductivity, along with their enhanced properties in terms of light‐scattering, plasmonic effects, and transmittance in the near infrared region. The drawbacks of AgNWs, particularly their high roughness, low adhesion to substrates, atmospheric corrosion, degradation under UV and visible light, and poor contact at wire‐wire junctions, must be resolved prior to their use in commercial FOSCs applications. Herein, comparisons among all candidates (e.g., graphene, carbon nanotubes, metal grids, and conducting polymers), along with a report of all recent progress in addressing these issues for using AgNWs as flexible transparent conductive electrodes (TCEs), are discussed. In addition, recent publications on the fabrication of highly efficient FOSCs based on AgNWs are summarized. The discussed issues regarding AgNWs‐TCEs apply not only to FOSCs, but can be generalized for other third‐generation solar cells, such as perovskite solar cells and dye‐sensitized solar cells; additionally, they provide insight for other optoelectronic applications, such as organic light‐emitting diodes, liquid crystal displays, smart windows, touch panels, and heaters.

Electrically stable Ag nanowire network anodes densely passivated by a conductive amorphous InSnTiO layer for flexible organic photovoltaics

An electrically stable Ag nanowire (Ag NW) network was fabricated by passivating with a conductive amorphous InSnTiO (ISTO) layer. It could be used in flexible transparent conductive electrodes for flexible organic photovoltaics. By plasma damage-free sputtering of amorphous ISTO on Ag NWs, sulfurization of Ag NWs was effectively prevented without sacrificing sheet resistance (44.5 Ω/sq.), optical transmittance (87.2%), or mechanical flexibility. Flexible organic solar cells (OSCs) fabricated on ISTO covered Ag NW network anodes showed slightly higher power conversion efficiency (5.94%) than reference OSCs with ITO anodes (5.79%). These results demonstrate that passivation of Ag NWs by amorphous conductive ISTO is a useful approach to improving the reliability of Ag NW network electrodes for high-performance flexible OSCs because the amorphous compact structure can effectively prevent the diffusion of sulfur from ambient air.

Near-infrared nanosecond pulsed laser ablation of silver nanowire in aqueous media for low-power and low-debris laser processing

Silver nanowire (Ag NW) is used as an attractive alternative material to fabricate flexible transparent conductive electrodes (TCE) instead of indium tin oxide, which has brittle characteristics and unstable processing expenses. For practical applications of Ag NW TCE, a proper patterning process is essential because the Ag NW layer is usually formed from the solution process. Laser ablation is one of the promising patterning processes for fabricating Ag NW-based optoelectronic devices. Among the laser processes, the nanosecond near-infrared pulsed laser ablation (NIR PLA) is considered advantageous because it is cheaper than ultrashort or ultraviolet pulsed lasers and causes little damage to the flexible substrate. However, since the Ag NW absorbance in this wavelength is very low, laser processing of Ag NW can deteriorate processing quality due to the remaining debris that resulted from incomplete evaporation of Ag NW. In this study, we report NIR PLA of Ag NW in aqueous media that improved the processing results of Ag NW laser patterning. We found that effective debris removal with a low fluence was a result of advantages such as the reduced heat-affected zone and dispersion of silver nanoparticle debris in the liquid media. Under different processing parameters, the performance of patterned Ag NW improved in liquid media compared to the ambient condition. Significant improvements in the processing quality were also observed in the laser patterning of Ag NW on the flexible substrate without any degradation. Consequently, the patterned flexible Ag NW TCE was successfully fabricated via the proposed process.

Effect of layer number on the properties of stable and flexible perovskite solar cells using two dimensional material

Recently, the flexibility and stability of perovskite solar cells (PSCs) with high power-conversion efficiency (PCE) have attracted the interest of many research. Reliable flexible transparent conductive electrodes (TCEs) and high stability at the perovskite/transport layers interface are vital for increasing the performance of flexible- and stable-PSCs. Herein, we develop PSCs with two-dimensional graphene (Gr) TCEs and MoS2 electron transport layers (ETLs). By modulating the number of layers (L) of Gr (LGr) and MoS2 (LMoS2), the maximum PCE of PSCs were 14.42 and 12.92% was achieved for PSCs fabricated on glass and polyethylene terephthalate substrates, respectively, at LGr = 2 and LMoS2 = 1. PSCs showed excellent stability as it lost only 18% of the initial PCE after 500 h. This is attributed to the improved ETL/perovskite interface. Moreover, the flexible PSCs retained 84% and 63% of the original PCE value after 1000 inner- and outer-bending cycles at a radius of curvature of 2 mm; thus, it is suitable for flexible devices.

Correction to: Aspect Ratio Control of Copper Nanowire via Solution Process and Its Flexible Transparent Conductive Electrode Applications

In the original publication of the article, the Acknowledgements section was published incorrectly. The correct Acknowledgements section is given in this Correction.

Sorting purified silver nanowires using supercritical carbon dioxide elutriation coupled with membrane for fabricating flexible and transparent conductive electrodes

Silver nanowires (AgNWs) are promising materials to fabricate flexible transparent conductive electrodes (TCEs) widely used in next-generation electronic devices. However, the impurity silver particles generated during synthesis severely deteriorated the optical transmission of AgNWs meshes. Isolating AgNWs from its reacted mixture is a vexing and challenging issue but highly desirable. In this work, a facile approach of using supercritical carbon dioxide elutriation coupled with a membrane was developed to purify the AgNWs from its synthesized mixture. Almost 100 % of purified AgNWs with the yield of 56 % was obtained, from which the AgNWs-based electrodes with excellent flexibility, conductivity, and transparency were fabricated. The AgNWs-based TCEs displayed a transmittance of 99.6 % with the haze of 0.56 % at the sheet resistance of 248 Ω·sq−1 or transmittance of 96.7 % with the haze of 1.36 % at the sheet resistance of 32.5 Ω·sq−1. The sheet resistance remained almost constant after bending in 1000 cycles.

Aspect Ratio Control of Copper Nanowire via Solution Process and Its Flexible Transparent Conductive Electrode Applications

Controlling aspect ratio of copper (Cu) nanowire (NW) is a vital role to determine the conductivity at a high transmittance for the transparent conductive electrode application, but systemic studies on aspect ratio control of Cu NW have been still in early stages. Herein, we systemically explore the aspect ratio of Cu NW by varying solution process parameters including reaction time and the concentration of additives (Cu precursor, reducing agent, and capping agent). From optimized process parameters, we successfully synthesize Cu NWs showing the high aspect ratio of ~ 1570, which is relatively comparable than that of previous reports. We characterize the sheet resistance with different transmittances, and consequently achieve excellent sheet resistance of 46 Ω sq−1 at transmittance = 93%, similar to that of indium tin oxide-based electrodes. Obtained Cu NW films also show stable bending reliability with applying bending radius of 6.5 mm. Such remarkable performances are attributed to high aspect ratio of our Cu NWs, which lead to reduced junction resistance in films.

Highly Efficient n-Type Doping of Graphene by Vacuum Annealed Amine-Rich Macromolecules.

Flexible transparent conducting electrodes (FTCE) are an essential component of next-generation flexible optoelectronic devices. Graphene is expected to be a promising material for the FTCE, because of its high transparency, large charge carrier mobilities, and outstanding chemical and mechanical stability. However, the electrical conductivity of graphene is still not good enough to be used as the electrode of an FTCE, which hinders its practical application. In this study, graphene was heavily n-type doped while maintaining high transmittance by adsorbing amine-rich macromolecules to graphene. The n-type charge-transfer doping of graphene was maximized by increasing the density of free amine in the macromolecule through a vacuum annealing process. The graphene adsorbed with the n-type dopants was stacked twice, resulting in a graphene FTCE with a sheet resistance of 38 ohm/sq and optical transmittance of 94.1%. The figure of merit (FoM) of the graphene electrode is as high as 158, which is significantly higher than the minimum standard for commercially available transparent electrodes (FoM = 35) as well as graphene electrodes doped with previously reported chemical doping methods. Furthermore, the n-doped graphene electrodes not only show outstanding flexibility but also maintain the doping effect even in high temperature (500 K) and high vacuum (~10−6 torr) conditions. These results show that the graphene doping proposed in this study is a promising approach for graphene-based next-generation FTCEs.

Geometrical optimization for silver nanowire mesh as a flexible transparent conductive electrode.

We report the effect of the geometric parameters on transparency and conductivity in a metallic nanowire mesh as a transparent electrode. Today, indium tin oxide and fluorine-doped tin oxide are used as the transparent electrode for displays and solar cells. Still, there is a definite need for their replacement due to drawbacks such as brittleness, scarcity, and adverse environmental effects. Metallic nanowire mesh is likely the best replacement option, but the main issue is how to find the optimal structure and how to get the best performance. Since the interaction of light with nanowire mesh is complicated, there is no straightforward rule with a simple analytical solution. We developed a kit based on wave optics for calculating the optical transmission of metallic nanowire mesh, which, unlike previous works, includes the interaction of light with the nanowire mesh, such as localized surface plasmon resonance (LSPR), surface plasmon polariton (SPP), and Rayleigh anomaly (RA). So, it is possible to accurately predict the effect of these phenomena and the transmission of mesh. Using the mentioned kit, we will be able to investigate the different geometrical structures of meshes to achieve optimal geometry. This kit is based on the classical Maxwell theory and empirical data and uses finite-difference time-domain for solving equations and experiential results for validation. Comparing the results by a redefined figure of merit shows that LSPR has the most significant reduction on transparency, whereas increasing the thickness (t) to width (w) ratio of the nanowire in the metallic mesh can reduce the LSPR effect and/or shifts it to the invisible region. The wire pitch (p) has no tangible impact on LSPR, but p can be chosen higher than 700 or lower than 350 nm to remove the extinction effects of the first-order RA. If p was larger than 150 nm, SPP could appear in the visible region of the spectrum. In small p, lower modes of SPP with higher intensities occur; therefore, there is an optimum value for p around 300 nm. The reduction of t and w reduces the intensity of SPP and causes it to red shift. By comparing the 900 different structures, the highest figure of merit is obtained in a p of 300 nm with a minimum w (10 nm) and maximum t (100 nm).

Intense pulsed light-induced structure-transformed ultrathin Ni shell for improving the chemical, thermal, and electrical reliability of metal nanowire electrodes without transmittance loss

Metal nanowires (NWs) boost the development of flexible transparent conductive electrodes (TCEs), but they are hardly applied in industrial productions due to poor reliability. Although various coating technologies have been used to resolve this issue, most of them deteriorate the transmittance of TCEs due to the thick coating shell. Herein, we propose a facile and highly compatible structural transformation process for fabricating core-shell NW TCEs with highly improved reliability. Instead of increasing the thickness of the coating shell to guarantee sufficient coverage around NWs, an instantaneous and surface heating technology of intense pulsed light is creatively applied to realize the structural transformation of scattered Ni crumbs into an ultrathin and uniform Ni coating shell. Results show that without the decline of the transmittance, the reliability of structure-transformed Ag@Ni (STAN) core-shell NW TCEs has been increased to 10 times higher than that of pristine Ag@Ni NW TCEs, which are much better than most of the state-of-the-art publications on core-shell NW TCEs. Flexible transparent heaters fabricated by the STAN NW TCEs exhibit both rapid response and long-term electrical stability. It is believed that the proposed structure-transformed coating method provides a novel and highly versatile way to generate ultrathin protective shells around NWs, which breaks the contradiction between achieving the high transmittance and obtaining the long-term reliability during the fabrication of NW TCEs.

The Transmittance Modulation of ZnO/Ag/ZnO Flexible Transparent Electrodes Fabricated by Magnetron Sputtering.

In this study, the critical role of the oxide thickness at the top and bottom layers of the ZnO/Ag/ZnO flexible transparent conductive electrodes was investigated. The peak transmittance of an 8 nm thick Ag electrode was observed at a wavelength of 320 nm, while the addition of the top and bottom ZnO layers, which serve as antireflection coating, resulted in peak shifts to the visible light wavelengths with a simultaneous improvement in the transmittance as well as the spectral uniformity. By independently varying the thickness values of the top and bottom ZnO layers, it was determined that the thickness variation of the top ZnO layer had a dominant effect in the transmittance, while a lesser but still significant contribution was obtained by varying the thickness of the bottom ZnO layer. At the optimized values of thickness for the top and bottom ZnO layers, the ZnO/Ag/ZnO electrode exhibits peak and average transmittance of 97.2% and 89.7%, respectively, along with a sheet resistance of 7.8 Ω/sq, with a corresponding Haacke's figure of merit (ΦH = T10ave/Rs) of 0.043, which exceeds that of the conventional indium-tin-oxide electrodes.