30 inch Roll-Based Production of High-Quality Graphene Films for Flexible Transparent Electrodes

We report that 30-inch scale multiple roll-to-roll transfer and wet chemical doping considerably enhance the electrical properties of the graphene films grown on roll-type Cu substrates by chemical vapor deposition. The resulting graphene films shows a sheet resistance as low as ~30 Ohm/sq at ~90 % transparency which is superior to commercial transparent electrodes such as indium tin oxides (ITO). The monolayer of graphene shows sheet resistances as low as ~125 Ohm/sq with 97.4% optical transmittance and half-integer quantum Hall effect, indicating the high-quality of these graphene films. As a practical application, we also fabricated a touch screen panel device based on the graphene transparent electrodes, showing extraordinary mechanical and electrical performances

Degradation studies of flexible optoelectronic device electrodes

Flexible transparent electrodes offer significant advantages, such as low cost, large area, light weight, conformability, robustness, and ease of roll-to-roll manufacturing and processing. They are routinely used as anodes in organic light emitting diodes, liquid crystal displays, touch panels, solar cells, solid state lightings, energy harvesting, and biomedical applications to name a few examples. However, the electromechanical and corrosion issues involved when the device is stressed and/or in contact with acid containing components both during manufacturing and/or in service conditions have to be investigated in order to improve, and predict reliability.;The primary objective of this research is to investigate the degradation behavior of two types of flexible transparent conducting layers, indium tin oxide (ITO) and carbon nanotubes (CNT) on polymer substrates, under electromechanical and corrosion conditions. Changes in electrical resistance and morphological features of these thin film electrodes are investigated using experimental methods such as corrosion, bending, fatigue, bending-corrosion, fatigue-corrosion, and tribo-corrosion. Such methods attempt to simulate induced stresses during manufacturing and/or in-service conditions. Studies on both patterned and non-patterned surfaces are performed.;Furthermore, finite element modeling is used to simulate the stress/strain distribution of the electrodes under various deformation modes. The effects and synergies of corrosion, applied strain, film thickness, and number of bending cycles on the electrical and structural integrity of the electrodes are investigated using design of experiment methods.;During this project it was found that CNT-based electrodes outperform their ITO counterparts under fatigue in corrosive environments. However, for most high current electronic devices ITO still needs to be utilized. During combined fatigue corrosion experiments of ITO-coated polymer electrodes externally applied strain was found to be the most critical factor for degradation. Experimental analysis and modeling of thin film electrodes for flexible optoelectronics will aid towards the design of more reliable devices in the future

Mechanical properties and characterisation of substrates for flexible displays

This project is concerned with the electro-mechanical reliability and characterisation of ITO coated polyester substrates for use in touch screen and flexible display applications. Flexible display anode components such as ITO coated polyesters are common in almost all flexible display technologies. However, these hybrid thin systems are unusual in mechanical terms. There is a mismatch between the mechanical properties of the inorganic coating and the organic substrate. It is therefore important to investigate the electromechanical response of such flexible anodes under various stress states and deformation modes. It is also important to develop new mechanical testing techniques for flexible displays. Numerous experimental techniques were used in order to characterise and test the available ITO coated/uncoated PET and PEN substrates. Also, the development of new experimental mechanical testing methods, such as the biaxial 'bulge' apparatus, was undertaken. During this work, various ITO coated polyester substrates were mechanically tested under uniaxial tension, controlled buckling and biaxial tension. In-situ electrical resistance monitoring and ex-situ atomic force microscopy, were used in order to detect and characterise ITO failure mechanisms. Tribological investigation of bare polyester substrates was undertaken. Preliminary nanoscratch and nanoindentation studies were also conducted on coated and uncoated systems. Overall it was shown that ITO coated polyester flexible display electrodes can properly function up to relatively low strains. Electrical resistance generally does not recover during unloading in cyclic experiments. These factors currently limit the use of such components to slightly curbed displays. Various ITO failure modes were observed, depending on the applied deformation mode. It was also shown, that the ITO adhesive failure is as critical as cohesive failure

Hybrid Organic/Inorganic Flexible Structures: Low Temperature Deposition of Silver Conductive Tracks on Flexible Substrates

The ability to atmospherically deposit conductive patterns on flexible polymeric substrates has recently gained considerable interest as an alternative to vacuum processes. This is because it can potentially lead to highly efficient and reliable transparent electrode components that can be used in lightweight and flexible optoelectronic devices such as sensors, solar panels, touch screens, displays and solid state lighting. Common approaches to fabricate transparent conductors include the physical deposition of transparent conductive oxide (TCO) films on polymer substrates. This is leading to mechanically brittle components which are fabricated using costly vacuum coating equipment. It is therefore important to research an alternate route for depositing mechanically reliable conductive structures on unheated flexible substrates.;Metallo-organic decomposition (MOD) inks formulate a metal-organic precipitate beginning with the reduction of metal salt precursor which is then combined with an organic solvent. The ink can then be deposited using various processes such as syringe writing, spray masking, screen printing and ink-jet printing. Low-temperature curing can be achieved without compromising the functionality of the polymer substrate leading to a ductile conductive pattern.;In this work, a MOD ink was formulated by the reduction of silver nitrate (AgNO3) to yield silver octanoate (AgC8H15O 2) precipitate which is then combined with xylene (C8H 10) solvent. Deposition on polymer base substrates was performed using masking techniques and involving a small gauge needle tip luer-lock syringe or a spray gun. Relatively uniform Ag track surface geometries were obtained. Contact angle measurements (\u3c17°) showed good adhesion of the Ag ink on the PEN substrate. Curing of the Ag patterns was performed via radiation-conduction-convection heating at temperatures as low as 150°C yielding electrical resistivities as low as 4.13x10-6O·m, with higher temperatures offering electrical resistivities as low as 3.01x10 -7O·m. Monotonic tensile testing of the cured samples was performed at a cross-head speed of 1 mm/min resulting in a marginal change in resistance up to 10% strain. Cyclic mandrel testing was conducted in order to assess the fatigue characteristics of the flexible components. Nanoindentation testing was performed to analyze mechanical properties of the cured ink in relation to curing temperature. Finally, nanoscratch testing showed good adhesion of the cured ink to the PEN substrate. Surface porosity was related to all mechanical and electromechanical testing.;Hybrid organic/inorganic flexible structures allow for the atmospheric deposition of ductile conductive components unlike current applications requiring vacuum deposition of films that need further patterning

Silver nanowire-based transparent, flexible, and conductive thin film

The fabrication of transparent, conductive, and uniform silver nanowire films using the scalable rod-coating technique is described in this study. Properties of the transparent conductive thin films are investigated, as well as the approaches to improve the performance of transparent silver nanowire electrodes. It is found that silver nanowires are oxidized during the coating process. Incubation in hydrogen chloride (HCl) vapor can eliminate oxidized surface, and consequently, reduce largely the resistivity of silver nanowire thin films. After HCl treatment, 175 Ω/sq and approximately 75% transmittance are achieved. The sheet resistivity drops remarkably with the rise of the film thickness or with the decrease of transparency. The thin film electrodes also demonstrated excellent flexible stability, showing < 2% resistance change after over 100 bending cycles

Tribo-Mechanical Investigation of the Functional Components used in Flexible Energy Harvesting Devices

During the previous decade, the development of energy harvesting devices based on piezoelectric materials has garnered great interest. The ability to capture ambient mechanical energy and convert it to useable electricity is a potential solution to the ever-growing energy crisis. One of the most attractive functional materials used in these devices is zinc oxide (ZnO). This material\u27s relative low cost and ease of large-area processing has spurred numerous device designs based around it. The ability to grow ZnO nanostructures of various geometries with low-temperature chemical methods makes this material even more attractive for flexible devices. Although numerous device architectures have been developed, the long-term mechanical reliability has not been addressed.;This work focuses on the fabrication and mechanical failure analysis of the flexible components typically used in piezoelectric energy harvesting devices. A three-phase iterative design process was used to fabricate prototypical piezoelectric nanogenerators, based on ZnO nanowires. An output of several millivolts was achieved under normal contact and microtensile loading, but device failure occurred after only a few loading cycles, in all cases. Ex situ failure analysis confirmed the primary sources of failure, which became the focus of further, component-level studies. Failure was primarily seen in the flexible electrodes of the nanogenerating devices, but was also observed in the functional piezoelectric layer itself.;Flexible electrodes comprised of polyester substrates with transparent conductive oxide (TCO) coatings were extensively investigated under various loading scenarios to mimic tribo-mechanical stresses applied during fabrication and use in flexible contact-based devices. The durability of these films was explored using microtensile testing, spherical nanoindentation, controlled mechanical buckling, stress corrosion cracking, and shear-contact reciprocating wear. The electro-mechanical performance and reliability of functional ZnO films and nanostructures were also studied. ZnO was deposited on rigid and flexible substrates for investigations including controlled buckling, and contact-based rolling/sliding scenarios. Numerous in situ and ex situ analytical techniques were used to characterize component-level failure mechanisms, including two-probe electrical resistance, optical microscopy, SEM, AFM, and stylus profilometry.;Experimental results show that there is a strong relation between crack onset strain values, during microtensile and controlled bucking loading, and coating thickness. Relatively high crack onset values were observed for both thinner coatings and those patterned using photolithography and wet chemical etching techniques. Tribological experiments show that although piezoelectric ZnO films produce a measurable electrical output during combined rolling/sliding contact, cohesive wear of the oxide and adhesive wear between oxide and substrate is present and detrimental to sustained film functionality

Metallic Nanowire Percolating Network: From Main Properties to Applications

There has been lately a growing interest into flexible, efficient and low-cost transparent electrodes which can be integrated for many applications. This includes several applications related to energy technologies (photovoltaics, lighting, supercapacitor, electrochromism, etc.) or displays (touch screens, transparent heaters, etc.) as well as Internet of Things (IoT) linked with renewable energy and autonomous devices. This associated industrial demand for low-cost and flexible industrial devices is rapidly increasing, creating a need for a new generation of transparent electrodes (TEs). Indium tin oxide has so far dominated the field of TE, but indium’s scarcity and brittleness have prompted a search into alternatives. Metallic nanowire (MNW) networks appear to be one of the most promising emerging TEs. Randomly deposited MNW networks, for instance, can present sheet resistance values below 10 Ω/sq., optical transparency of 90% and high mechanical stability under bending tests. AgNW or CuNW networks are destined to address a large variety of emerging applications. The main properties of MNW networks, their stability and their integration in energy devices are discussed in this contribution

Chapter Metallic nanowire percolating networks: from main properties to applications

There has been lately a growing interest into flexible, efficient and low-cost transparent electrodes which can be integrated for many applications. This includes several applications related to energy technologies (photovoltaics, lighting, supercapacitor, electrochromism, etc.) or displays (touch screens, transparent heaters, etc.) as well as Internet of Things (IoT) linked with renewable energy and autonomous devices. This associated industrial demand for low-cost and flexible industrial devices is rapidly increasing, creating a need for a new generation of transparent electrodes (TEs). Indium tin oxide has so far dominated the field of TE, but indium’s scarcity and brittleness have prompted a search into alternatives. Metallic nanowire (MNW) networks appear to be one of the most promising emerging TEs. Randomly deposited MNW networks, for instance, can present sheet resistance values below 10 Ω/sq., optical transparency of 90% and high mechanical stability under bending tests. AgNW or CuNW networks are destined to address a large variety of emerging applications. The main properties of MNW networks, their stability and their integration in energy devices are discussed in this contribution

Single-Walled Carbon Nanotube electrodes for all-plastic, electronic device applications

In this thesis, new mechanically robust, high performance transparent conducting films of commercially sourced arc-made Single-Walled Carbon Nanotubes (SWCNTs) on both glass and flexible substrates were produced using spin-coating or spray deposition, interlayer or stencil patterning methods and used for fabricating efficient, flexible polymer-fullerene bulk hetero-junction solar cells. After carefully optimizing the dispersion process of SWCNTs with H2O:SDS (up to 0.03 wt.%) and developing and efficient surfactant removal/p-doping procedure with nitric acid, highly conductive and smooth SWCNT thin films (ca. 30 nm) were obtained with more than 6,500 Scm-1 at > 69 % transmittance and 7 nm (r.m.s.) roughness. In particular, SWCNT films spray coated from H2O:SDS exhibited electrical conductivities of up to 7694 ± 800 Scm-1. To our knowledge, these values are the highest so far reported for SWCNT electrodes. Peak values for the ratio of the dc conductivity to the optical conductivity (σdc/σop) were obtained as up to 24, which is quite similar to state of the art SWCNT films so far reported. In addition, two patterning methods were developed to define electrode patterns of SWCNT thin films for electronic device applications. Interlayer lithography provided a fast and high resolution patterning procedure for SWCNT thin films at micron and sub-micron length scales, which is important for the fabrication of high-speed transistors requiring short channel lengths, and offers an attractive route to fabricating high-density integrated circuits. In addition, stencil patterning provides a simple and fast method, which is well suited for low resolution electronic device applications such as organic solar cells. The patterned highly conductive SWCNT electrodes were incorporated into P3HT:PCBM bulk heterojunction solar cell applications, obtaining the best device performance of 3.6 %, which is the best result so far reported in the literature. Finally, to break through the limited performance (σdc/σop < 25) of SWCNT thin films, layered hybrid thin films of SWCNTs on reduced Graphene-Oxide were fabricated by a simple spray coating method and the optimised hybrid films were incorporated into relatively efficient organic solar cells (2 % efficiency)