Flexible Transparent Conductors Research Articles

A review of graphene oxide, graphene buckypaper, and polymer/graphene composites: Properties and fabrication techniques

In this critical review, the preparation methods, unique properties, and graphene oxide structure as well as graphene have been interpreted. Various aspects for the graphene oxide reactivity were also discussed. Graphene oxide, usually obtained by exfoliating graphite, provides a way towards flexible, low-cost, bulk production paper-like materials known as buckypapers. Recently, graphene oxide and graphene buckypaper have been used as fillers for preparing composites. The buckypapers are freestanding thin porous network of carbon fillers bound by van der Waals interactions. Several techniques have been used to fabricate graphene oxide and graphene-based paper-like materials such as vacuum filtration method; spin coating technique, resin infiltration technique, evaporation-induced, and self-assembly technique. Among these, the most suitable technique is Langmuir-Blodgett assembly, where the graphene oxide paper formed is usually highly uniform. Free standing polymer/graphene oxide and polymer/graphene paper materials show remarkable mechanical stiffness and excellent flexibility combined with outstanding electrical conductivity. Incorporating polymer into graphene oxide and graphene buckypaper may modify the polymer and filler properties producing high performance materials. The buckypaper made from functionalized filler may form networks that facilitate penetrating matrix chains and result in composites with improved mechanical performance. Due to their outstanding physical, thermal, and electrical properties, these papers have promising applications such as; body armour, aerospace structure improvement, armoured vehicles, flexible energy storage devices, sensors, memory devices, transparent conductors, and photovoltaics.

Additive roll printing activated cold welding of 2D crystals and 1D nanowires layers for flexible transparent conductor and planer energy storage

Large scale integration of 2D crystals and metal nanowires (NWs) with low junction resistance has been a long time challenge for functional structures. Herein, roll to roll printing technique was utilized to directly produce graphene oxide (GO) nanoplatelets stack laminated with metal NWs. This 2D/1D laminated structure activates high pressure between AgNWs, enables localized fusion, and opens a new way to tackle with local nanojunction resistance. Molecular dynamic simulations reveal that this cold nanojoining originates from high localized pressure activated isolated metal atoms, which possess high mobility, flow over nanojunction and recrystallize to form nanojoints. Compared with bare AgNWs network, the GO laminated counterparts achieve excellent performance (17Ω/sq, 93%@λ=550nm), which enlightens optoelectronics applications. The cross nanowire junction resistance decrease is verified by classical percolation theory and experimental characterizations. Haze measurements show that the scattering is significantly reduced. Laminated structure has been engineered layer by layer to accomplish quasi-3D hybrid film, which demonstrated application in flexible supercapacitors with high specific capacitance of 109 F g−1. This hybrid 2D/1D nanomaterial thin film also demonstrated excellent stability during cyclic foldability, stretchability, high temperature and oxidation tests. These investigations provide insight for layered manufacturing of 2D crystals/1D NWs structure and quasi-3D hybrid film for energy storage applications.

Highly Stable Transparent Conductive Silver Grid/PEDOT:PSS Electrodes for Integrated Bifunctional Flexible Electrochromic Supercapacitors

Silver grids are attractive for replacing indium tin oxide as flexible transparent conductors. This work aims to improve the electrochemical stability of silver‐based transparent conductors. A silver grid/PEDOT:PSS hybrid film with high conductivity and excellent stability is successfully fabricated. Its functionality for flexible electrochromic applications is demonstrated by coating one layer of WO3 nanoparticles on the silver grid/PEDOT:PSS hybrid film. This hybrid structure presents a large optical modulation of 81.9% at 633 nm, fast switching, and high coloration efficiency (124.5 cm2 C−1). More importantly, an excellent electrochemical cycling stability (sustaining 79.1% of their initial transmittance modulation after 1000 cycles) and remarkable mechanical flexibility (optical modulation decay of only 7.5% after 1200 compressive bending cycles) is achieved. A novel smart supercapacitor is presented that functions as a regular energy‐storage device and simultaneously monitors the level of stored energy by a rapid and reversible color variation even at high current charge/discharge conditions. The film sustains an optical modulation of 87.7% and a specific capacitance of 67.2% at 10 A g−1 compared to their initial value at a current density of 1 A g−1. The high‐performance silver grid/PEDOT:PSS hybrid transparent films exhibit promising features for various emerging flexible electronics and optoelectronic devices.

Is graphene a good transparent electrode for photovoltaics and display applications?

The current standard material used for transparent electrodes in displays, touch screens and solar cells is indium tin oxide (ITO) which has low sheet resistance (10 Ω/□), high optical transmission in the visible wavelength (85%) and does not suffer of optical haze. However, ITO is mechanically rigid and incompatible with future demands for flexible applications. Graphene materials share many of the properties desirable for flexible transparent conductors, including high optical transparency, high mechanical flexibility and strength. Whilst pristine graphene is not a good transparent conductor, functionalised graphene is at least 1000 times a better conductor than its pristine counterpart and it outperforms ITO. Here the authors review recent work on a novel graphene-based conductor with sheet resistance as low as 8.8 Ω/□ and 84% optical transmission. This material is obtained by ferric chloride (FeCl3) intercalation into few-layer-graphene (FLG), giving rise to a new system which is the best known flexible and transparent electricity conductor. FeCl3-FLG shows no significant changes in the electrical and structural properties for a long exposure to air, to high levels of humidity and at temperatures of up to 150°C in atmosphere. These properties position FeCl3-FLG as a viable and attractive replacement to ITO.

Display Materials and Processes

In addition to three major categories of new and evolving display materials – display glass, flexible transparent conductors, and quantum dots – a potentially disruptive material‐and‐process combination appeared at Display Week this year.

"Nano Necklace" Formed by Electroless Deposition of Metal on Polymer Nanofiber for Flexible Transparent Conducting Films

Flexible transparent thin films are strongly required for electronic devices, display or electric equipment. To obtain both transparency and flexibility, electroless deposition on the polymer nanofiber was carried out. As the result, we successfully obtained uniformly metalized nanofiber with 300nm in diameter by all wet process without using any vacuum process.

27.5L: Late‐News Paper: Hybrid Printing of High Resolution Metal Mesh as A Transparent Conductor for Touch Panels and OLED Displays

A hybrid printing technique has been developed to manufacture high resolution metal mesh as flexible transparent conductors. The metal mesh structures are made by embedding silver nanoparticles inks into trenches which are patterned by high resolution nanoimprinting technology. Compared to other nanosilver type or metal mesh type of transparent conductors, the new technology can make much finer metal mesh (<3µm) with much higher conductivity (<1Ω/□) while still maintaining high transparency (>88%). The transparent conductor sheets can be roll‐to‐roll printed, which offers significant cost advantage over ITO electrodes. The new technology has been successfully implemented in high volume manufacturing of touch panel sensors for displays, as well as adapted to making flexible OLED.

Nano Necklace Formed By Electroless Deposition of Metal on Polymer Nanofiber for Flexible Transparent Conducting Films

Flexible transparent thin films are strongly required for electronic devices, display or electric equipment. To obtain both transparency and flexibility, electroless deposition on the polymer nanofiber was carried out. As the result, we successfully obtained uniformly metalized nanofiber with 300nm in diameter by all wet process without using any vacuum process. First, we fabricated Nylon 6 nanofiber with nanoweb structure with 50nm in diameter by electrospinning method1. Then, NiP was metalized by electroless deposition on the nylon 6 nanofiber at the relatively lower temperature of 60 to 65 oC for 40 seconds2. By the Scanning Electron Microscope observation, it was found that numerous spherical shape of NiP beeds with the diameter of 300nm were tightly connected and arranged on the surface of nylon 6 nanofiber with the diameter of 50nm. It seems that same size of numerous metal beeds were formed just like “Nano Necklace” surrounding the nylon 6 nanofiber. The electrical characteristics and the mechanical characteristics were evaluated and the possibility of the future application for the flexible transparent conductive electrodes were suggested.

Unusually High Optical Transparency in Hexagonal Nanopatterned Graphene with Enhanced Conductivity by Chemical Doping.

Graphene has received appreciable attention for its potential applications in flexible conducting film due to its exceptional optical, mechanical, and electrical properties. However increasing transmittance of graphene without sacrificing the electrical conductivity has been difficult. The fabrication of optically highly transparent (≈98%) graphene layer with a reasonable electrical conductivity is demonstrated here by nanopatterning and doping. Anodized aluminium oxide nanomask prepared by facile and simple self-assembly technique is utilized to produce an essentially hexagonally nanopatterned graphene. The electrical resistance of the graphene increases significantly by a factor of ≈15 by removal of substantial graphene regions via nanopatterning into hexagonal array pores. However, the use of chemical doping on the nanopatterned graphene almost completely recovers the lost electrical conductivity, thus leading to a desirably much more optically transparent conductor having ≈6.9 times reduced light blockage by graphene material without much loss of electrical conductivity. It is likely that the availability of large number of edges created in the nanopatterned graphene provides ideal sites for chemical dopant attachment, leading to a significant reduction of the sheet resistance. The results indicate that the nanopatterned graphene approach can be a promising route for simultaneously tuning the optical and electrical properties of graphene to make it more light-transmissible and suitable as a flexible transparent conductor.

CuCl2-Mediated Synthesis of Silver Nanowires for Flexible Transparent Conducting Films

Silver nanowires (Ag) were successfully synthesized by a copper chloride (CuCl2)-mediated polyol method at 155oC. Scanning electron microscopy (SEM) and X-Ray diffraction were employed to investigate the morphology and structure of the Ag products, respectively. Without CuCl2, mostly nanoparticles were formed. On the other hand, Ag nanowires with an average length of about 18.8 m and an average diameter of about 96.3 nm were formed after addition of 1.6 M of CuCl2. This suggests that the morphology of the Ag nanowire can be tailored by varying the amount of CuCl2. Ag nanowire suspension in ethanol was evenly coated on cellulose acetate substrates using Meyer rod coating. The electrodes were annealed at 150°C for 30 min to remove the organic materials on the surface of the nanowires. A sheet resistance of <50 ohms/sq with an optical transmittance of 80% at 550nm was obtained for an electrode with a nanowire density of 0.1 mg/cm2.

High-performance flexible organic light-emitting diodes using embedded silver network transparent electrodes.

Because of their mechanical flexibility, organic light-emitting diodes (OLEDs) hold great promise as a leading technology for display and lighting applications in wearable electronics. The development of flexible OLEDs requires high-quality transparent conductive electrodes with superior bendability and roll-to-roll manufacturing compatibility to replace indium tin oxide (ITO) anodes. Here, we present a flexible transparent conductor on plastic with embedded silver networks which is used to achieve flexible, highly power-efficient large-area green and white OLEDs. By combining an improved outcoupling structure for simultaneously extracting light in waveguide and substrate modes and reducing the surface plasmonic losses, flexible white OLEDs exhibit a power efficiency of 106 lm W(-1) at 1000 cd m(-2) with angular color stability, which is significantly higher than all other reports of flexible white OLEDs. These results represent an exciting step toward the realization of ITO-free, high-efficiency OLEDs for use in a wide variety of high-performance flexible applications.

Building complex hybrid carbon architectures by covalent interconnections: graphene-nanotube hybrids and more.

Graphene is theoretically a robust two-dimensional (2D) sp(2)-hybridized carbon material with high electrical conductivity and optical transparency. However, due to the existence of grain boundaries and defects, experimentally synthesized large-area polycrystalline graphene sheets are easily broken and can exhibit high sheet resistances; thus, they are not suitable as flexible transparent conductors. As described in this issue of ACS Nano, Tour et al. circumvented this problem by proposing and synthesizing a novel hybrid structure that they have named "rebar graphene", which is composed of covalently interconnected carbon nanotubes (CNTs) with graphene sheets. In this particular configuration, CNTs act as "reinforcing bars" that not only improve the mechanical strength of polycrystalline graphene sheets but also bridge different crystalline domains so as to enhance the electrical conductivity. This report seems to be only the tip of the iceberg since it is also possible to construct novel and unprecedented hybrid carbon architectures by establishing covalent interconnections between CNTs with graphene, thus yielding graphene-CNT hybrids, three-dimensional (3D) covalent CNT networks, 3D graphene networks, etc. In this Perspective, we review the progress of these carbon hybrid systems and describe the challenges that need to be overcome in the near future.

Electrically robust metal nanowire network formation by in-situ interconnection with single-walled carbon nanotubes.

Modulation of the junction resistance between metallic nanowires is a crucial factor for high performance of the network-structured conducting film. Here, we show that under current flow, silver nanowire (AgNW) network films can be stabilised by minimizing the Joule heating at the NW-NW junction assisted by in-situ interconnection with a small amount (less than 3 wt%) of single-walled carbon nanotubes (SWCNTs). This was achieved by direct deposition of AgNW suspension containing SWCNTs functionalised with quadruple hydrogen bonding moieties excluding dispersant molecules. The electrical stabilisation mechanism of AgNW networks involves the modulation of the electrical transportation pathway by the SWCNTs through the SWCNT-AgNW junctions, which results in a relatively lower junction resistance than the NW-NW junction in the network film. In addition, we propose that good contact and Fermi level matching between AgNWs and modified SWCNTs lead to the modulation of the current pathway. The SWCNT-induced stabilisation of the AgNW networks was also demonstrated by irradiating the film with microwaves. The development of the high-throughput fabrication technology provides a robust and scalable strategy for realizing high-performance flexible transparent conductor films.

Real-time resistance, transmission and figure-of-merit analysis for transparent conductors under stretching-mode strain

In this paper, we describe an apparatus capable of measuring electrical resistance and light transmission through a flexible transparent conductor (TC) as it is actively deformed. The measurements are performed in situ with time resolution on the order of ~100ms, and the resistance/transmission data is further used to calculate TC figures-of-merit in real-time. With this instrument, we are able to track and compare the evolution of figures-of-merit for different TCs as they are actively deformed. To demonstrate the tool, we evaluate several common TCs including indium tin oxide, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and silver nanowires, as well as a more unconventional nanowire/PEDOT:PSS hybrid. As expected, indium tin oxide degrades rapidly with strain, while the more flexible PEDOT:PSS is far more strain tolerant. The nanowire/PEDOT:PSS composite is found to produce an advantageous effect, as the silver nanowires generate large figures-of-merit at low strain, and the addition of a PEDOT:PSS component vastly improves strain tolerance.

Polymer Embedded Silver-Nanowire Network Structures – A Platform for the Facile Fabrication of Flexible Transparent Conductors

Solution-processed silver nanowire (Ag-NW) network structures are candidate materials for application as flexible transparent conductors in a wide range of flexible electronics. However, challenges remain in fabricating these materials in an efficient and scalable manner in addition to making them mechanically robust. Here, a method based on the direct processing of Ag-NW network structures from aqueous polymer solutions is presented that significantly increases the uniformity of the NW networks and lowers the percolation threshold, thus resulting in an increased figure of merit (percolative FoM Π > 40) of Ag-NW based transparent conductors. Direct polymer solution processing also reduces the batch-to-batch variability of optical and electrical properties of the NW networks to <5% of the mean (as compared to 10–20% variability for pristine Ag-NW networks), and significantly improves their resistance to mechanical deformation. Our results further indicate that semicrystalline polymers such as poly (vinyl alcohol) (PVA) could serve as viable and benign alternatives to conducting polymer matrices (such as PEDOT:PSS) that pose a challenge to device fabrication due to their corrosive properties.

Controllable fabrication of a flexible transparent metallic grid conductor based on the coffee ring effect

Flexible transparent metallic grid-based conductors were successfully fabricated and sintered at room temperature based on the coffee ring effect.

Solution-processed flexible transparent conductors based on carbon nanotubes and silver grid hybrid films

In a simple, cost-effective, and solution-based process, a thin-film of single-walled carbon nanotubes is hybridized on a PET film which has been patterned with solution self-assembled Ag nanoparticles. Such a flexible and transparent electrode exhibits a sheet resistance down to ∼5.8 Ω sq(-1) at ∼83.7% optical transmittance. The hybrid films are stable under ambient conditions and offer excellent bendability.

Cu(In,Ga)Se2 Thin Film Solar Cells with Solution Processed Metal Nanowire Based Transparent Conductor

Since the traditional sputtered metal oxide thin films are not suited for using as transparent electrodes of flexible thin film solar cells and their increasing prices limit development of solar cells industry, high performance, low cost and flexible transparent conductor technology has aroused researchers’ great interests. In this paper, we successfully apply a solution processed silver nanowire composite film as the transparent electrode for Cu(In,Ga)Se2 thin film solar cells with ZnS buffer layers. Compared with traditional sputtered window layer, the solution processed transparent electrode causes no physical damage to the adjacent device layers and junction region, which can greatly decrease the required thickness of the ZnS buffer layer. Properties of the silver nanowire composite transparent conductor and its effects on performance of Cu(In,Ga)Se2/ZnS thin film solar cells are studied.

Nanostructured Electrodes and Photoactive Layers for Efficient, Stable and Flexible Organic Photovoltaic Devices

This work presents our latest results for conventional and inverted organic photovoltaic (OPV) devices based on polymers such as poly(3-hexylthiophene) (P3HT) and poly[N-9”-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole) (PCDTBT) blended with fullerene derivatives such as [6,6]-phenyl C61-butyric acid methyl ester (PC61BM) and PC71BM. It is observed that by controlling the morphology of acceptor and donor domains in the BHJ photoactive films higher power conversion efficiency (PCE) and performance for OPV devices can be achieved, with the best efficiencies demonstrated for the P3HT:PC61BM and PCDTBT:PC71BM devices exceeding 4% and 7%, respectively. Here, the interplay between fabrication processes and the performance of the devices is investigated to achieve optimum PCE, short-circuit current and open-circuit voltage for the OPV devices. In addition to contact and BHJ films, a flexible solar cell requires a flexible transparent conductor (TC). We report our results for conductive electrospun nanofiber based TCs as replacements for the brittle indium tin oxide (ITO) used for solar cells on glass substrates. These novel nanofiber TC web and their associated flexible substrates act as new platforms for fabrication of low cost, flexible solar cells. Performance of the TC is compared to other novel TCs in terms of transparency, sheet resistance and flexibility. The performance of the nanofiber TC webs is manifested in OPV devices that employ these TCs as the transparent electrode and demonstrate comparable performance to the devices using ITO electrodes. This work demonstrates the significance of novel materials and deposition technologies for enabling efficient and stable OPV devices for roll-to-roll manufacturing on flexible substrates.

Efficient and bright organic light-emitting diodes on single-layer graphene electrodes

Organic light-emitting diodes are emerging as leading technologies for both high quality display and lighting. However, the transparent conductive electrode used in the current organic light-emitting diode technologies increases the overall cost and has limited bendability for future flexible applications. Here we use single-layer graphene as an alternative flexible transparent conductor, yielding white organic light-emitting diodes with brightness and efficiency sufficient for general lighting. The performance improvement is attributed to the device structure, which allows direct hole injection from the single-layer graphene anode into the light-emitting layers, reducing carrier trapping induced efficiency roll-off. By employing a light out-coupling structure, phosphorescent green organic light-emitting diodes exhibit external quantum efficiency >60%, while phosphorescent white organic light-emitting diodes exhibit external quantum efficiency >45% at 10,000 cd m(-2) with colour rendering index of 85. The power efficiency of white organic light-emitting diodes reaches 80 lm W(-1) at 3,000 cd m(-2), comparable to the most efficient lighting technologies.