Biomimic Vein-Like Transparent Conducting Electrodes with Low Sheet Resistance and Metal Consumption

Abstract: In this contribution, inspired by the excellent resource management and material transport function of leaf veins, the electrical transport function of metallized leaf veins is mimicked from the material transport function of the vein networks. By electroless copper plating on real leaf vein networks with copper thickness of only several hundred nanometre up to several micrometre, certain leaf veins can be converted to transparent conductive electrodes with an ultralow sheet resistance 100 times lower than that of state-of-the-art indium tin oxide thin films, combined with a broadband optical transmission of above 80% in the UV–VIS–IR range. Additionally, the resource efficiency of the vein-like electrode is characterized by the small amount of material needed to build up the networks and the low copper consumption during metallization. In particular, the high current density transport capability of the electrode of > 6000 A cm−2 was demonstrated. These superior properties of the vein-like structures inspire the design of high-performance transparent conductive electrodes without using critical materials and may significantly reduce the Ag consumption down to < 10% of the current level for mass production of solar cells and will contribute greatly to the electrode for high power density concentrator solar cells, high power density Li-ion batteries, and supercapacitors.[Figure not available: see fulltext.]. © 2020, © 2020, The Author(s)

New Designs for Wearable Technologies: Stretchable e-Textiles and e-Skin

This dissertation comprises research efforts in addressing the challenges of integration of different materials with mechanical mismatches in stretchable e-textiles and e-skin, with a major focus on the design and fabrication of stretchable e-textiles. Chapter 2 describes the solution-based metallization of a knitted textile that conformally coats individual fibers with gold, leaving the void structure intact. The resulting gold-coated textile is highly conductive, with a sheet resistance of 1.07 ohm/sq in the course direction. The resistance decreases by 80% when the fabric is stretched to 15% strain and remains at this value to 160% strain. This outstanding combination of stretchability and conductivity is accompanied by durability to wearing, sweating, and washing. Low-cost screen printing of a wax resist is demonstrated to produce patterned gold textiles suitable for electrically connecting discrete devices in clothing. The fabrication of electroluminescent fabric by depositing layers of device materials onto the gold-coated textile is furthermore demonstrated, intimately merging device functionality with textiles for imperceptible wearable devices. Chapter 3 presents a new textile-centric design paradigm in which we use the textile structure as an integral part of wearable device design. Coating the open framework structure of an ultrasheer knitted textile with a conformal gold film using solution-based metallization forms gold-coated ultrasheer electrodes that are highly conductive (3.6 ± 0.9 ohm/sq) and retain conductivity to 200% strain with R/R0 \u3c 2. The ultrasheer electrodes produce wearable, highly stretchable light-emitting e-textiles that function to 200% strain. Stencil printing a wax resist provides patterned electrodes for patterned light emission; furthermore, incorporating soft-contact lamination produces light-emitting textiles that exhibit, for the first time, readily changeable patterns of illumination. Chapter 4 demonstrates the strategic use of a warp-knitted velour fabric in an “island-bridge” architectural strain-engineering design to prepare stretchable textile-based lithium ion battery (LIB) electrodes. The velour fabric consists of a warp-knitted framework and a cut pile. We integrate the LIB electrode into this fabric by solution-based metallization to create the warp-knitted framework current collector “bridges”, followed by selectively deposition of the brittle electroactive material CuS on the cut pile “islands”. As the textile electrode is stretched, the warp-knitted framework current collector elongates, while the electroactive cut pile fibers simply ride along at their anchor points on the framework, protecting the brittle CuS coating from strain and subsequent damage. The textile-based stretchable LIB electrode exhibited excellent electrical and electrochemical performance with a current collector sheet resistance of 0.85 ± 0.06 ohm/sq and a specific capacity of 400 mAh/g at 0.5 C for 300 charging-discharging cycles, as well as outstanding rate capability. The electrical performance and charge-discharge cycling stability of the electrode persisted even after 1000 repetitive stretching-releasing cycles, demonstrating the protective functionality of the textile-based island-bridge architectural strain-engineering design. Chapter 5 demonstrates the engineering of metal cracking patterns using the topography from acid-oxidized PDMS. Oxidizing the surface of PDMS with aqueous acid mixture created hierarchical topographies. Coating the surface of acid-oxidized PDMS with copper using electroless deposition produced stretchable conductors with a sheet resistance of ~1.2 ohm/sq. The cracking patterns of copper films with strain were tuned by simply adjusting the composition of acid mixture to change the topography of PDMS, which affects the resistance change of copper films with strain. The Cu films with an optimal cracking pattern on acid-treated PDMS remain conductive to 85% strain with R/R0 less than 20

Electrodeposition of epitaxial metal thin films on silicon for energy conversion and flexible electronics

This research focuses on epitaxial electrodeposition of two coinage metals: Au and Ag thin films on the silicon surface and their applications in flexible electronics and energy conversion and storage. The first paper: Photoelectrochemistry of ultrathin, semi-transparent, and catalytic gold films electrodeposited epitaxially onto n-silicon (111) describes the epitaxial electrodeposition of Au thin films on n-type Si using a simple HAuCl4 bath and the photoelectrochemical properties of the Au-Si junction barrier. The effect of the Au layer on the interfacial energetics as well as the stability of the photoelectrode as a function of the Au coverage/thickness is determined in a regenerative cell. The second paper: Epitaxial lift-off of electrodeposited single-crystal gold foils for flexible electronics shows a technique for epitaxial lift-off of Au foils as semi-transparent, flexible and single crystal-like substrates for flexible electronics. A Au thin film is first deposited on Si and then photooxidation is performed followed by a lift-off process. The Au foils exhibit a low sheet resistance down to 7 ohms per square and show only a 4% increase in resistance after 4000 bending cycles. A flexible organic light-emitting diode that was spin-coated on a Au foil exploited the transmittance and flexibility of the foil. The third paper: Electrodeposition of nanometer-thick epitaxial films of silver onto single-crystal silicon wafers introduces the electrodeposition of epitaxial Ag thin films on n-type Si of three different low-index orientations from an acetate bath. A comparison of silver acetate electrolyte and cyanide electrolyte was also performed, showing advantages of the acetate bath over the cyanide bath for growth of epitaxial films of Ag on Si surfaces --Abstract, page iv

Fabrication and Applications of Flexible Transparent Electrodes Based on Silver Nanowires

There has been an explosion of interests in using flexible transparent electrodes for next-generation flexible electronics, such as touch panels, flexible lighting, flexible solar cells, and wearable sensors. Silver nanowires (AgNWs) are a promising material for flexible transparent electrodes due to high electrical conductivity, optical transparency and mechanical flexibility. Despite many efforts in this field, the optoelectronic performance of AgNW networks is still not sufficient to replace the present material, indium tin oxide (ITO), due to the high junction resistance. Also, the environmental stability and the mechanical properties need enhancement for future commercialization. Many studies have attempted to overcome such problems by tuning the AgNW synthesis and optimizing the film-forming process. In this chapter, we survey recent progresses of AgNWs in flexible electronics by describing both fabrication and applications of flexible transparent AgNW electrodes. The synthesis of AgNWs and the fabrication of AgNW electrodes will be demonstrated, and the performance enhanced by various methods to suit different applications will be also discussed. Finally, technical challenges and future trends are presented for the application of transparent electrodes in flexible electronics

Development and Integration of Stretchable Electronic Components into Light-Emitting Devices.pdf

Flexible and stretchable electronics are the new format of electronics that remain functional with mechanical bending, twisting, and stretching. These new kinds of devices are expected to open up new opportunities and uses by reforming the way we interact with electronics and fundamentally change our life. To reach these goals, we must move beyond conventional hard, inorganic materials such as glass and silicon and find ways to incorporate electrical function into soft materials that are flexible or even stretchable. This thesis focuses on the development of compliant electronic components including transparent conductive electrodes, light-emitting materials, and metallic electrodes, and their integration into soft light-emitting devices. Chapter 2 reports a new and simple method using shadow masks to produce flexible and stretchable patterned silver nanowire (AgNW) coatings. We easily obtain a variety of geometries and resolutions of the patterns using different shadow masks. These coatings are highly conductive and transparent and exhibit high flexibility, stretchability, and mechanical robustness. We demonstrate their use as electrodes in light-emitting electrochemical cells (LEECs) and show that these devices function during bending. However, due to the high permeability of PDMS substrate, water and air in ambient condition easily penetrate through the substrate and corrode AgNW network to form less conductive particles or rods, making it not suitable for long-term stable applications. To solve this challenge, Chapter 3 reported the fabrication of a chemical stable AgNW composite by simply replacing the highly permeable PDMS substrate with a new airtight material—transparent butyl rubber. The resulting coatings very well maintain their optical, electrical, and mechanical properties when exposing to extremely harsh conditions such as underwater or acidic vapor. Chapter 4 investigates a feasible method to fabricate a stretchable light-emitting material with an improved optical performance by mixing an ionic transition metal complex with an elastic graft copolymer and an ionic conductor. The graft copolymer not only provides the stretchability by its elastic backbone but also acts as ion hosting materials due to its ion trapping side chains. We demonstrate that devices made from this material emit bright yellow light and keep emitting light under repetitive strain cycles. Chapter 5 describes a new, simple, low-cost solution-based scalable method to produce patterned gold film with microcontact printing on elastomeric polydimethylsiloxane (PDMS). This solution-based method enables the metal deposition on not only flat surfaces but also any other irregular shapes. Additionally, the patterning method is also compatible with uneven surface due to the high comfortability of PDMS. Unlike traditional physical vapor deposited gold films that experience electrical failure at very low strain (~1%), our gold films still remain highly conductive at 90% elongations

Development and Integration of Stretchable Electronic Components into Light-Emitting Devices.pdf

Flexible and stretchable electronics are the new format of electronics that remain functional with mechanical bending, twisting, and stretching. These new kinds of devices are expected to open up new opportunities and uses by reforming the way we interact with electronics and fundamentally change our life. To reach these goals, we must move beyond conventional hard, inorganic materials such as glass and silicon and find ways to incorporate electrical function into soft materials that are flexible or even stretchable. This thesis focuses on the development of compliant electronic components including transparent conductive electrodes, light-emitting materials, and metallic electrodes, and their integration into soft light-emitting devices. Chapter 2 reports a new and simple method using shadow masks to produce flexible and stretchable patterned silver nanowire (AgNW) coatings. We easily obtain a variety of geometries and resolutions of the patterns using different shadow masks. These coatings are highly conductive and transparent and exhibit high flexibility, stretchability, and mechanical robustness. We demonstrate their use as electrodes in light-emitting electrochemical cells (LEECs) and show that these devices function during bending. However, due to the high permeability of PDMS substrate, water and air in ambient condition easily penetrate through the substrate and corrode AgNW network to form less conductive particles or rods, making it not suitable for long-term stable applications. To solve this challenge, Chapter 3 reported the fabrication of a chemical stable AgNW composite by simply replacing the highly permeable PDMS substrate with a new airtight material—transparent butyl rubber. The resulting coatings very well maintain their optical, electrical, and mechanical properties when exposing to extremely harsh conditions such as underwater or acidic vapor. Chapter 4 investigates a feasible method to fabricate a stretchable light-emitting material with an improved optical performance by mixing an ionic transition metal complex with an elastic graft copolymer and an ionic conductor. The graft copolymer not only provides the stretchability by its elastic backbone but also acts as ion hosting materials due to its ion trapping side chains. We demonstrate that devices made from this material emit bright yellow light and keep emitting light under repetitive strain cycles. Chapter 5 describes a new, simple, low-cost solution-based scalable method to produce patterned gold film with microcontact printing on elastomeric polydimethylsiloxane (PDMS). This solution-based method enables the metal deposition on not only flat surfaces but also any other irregular shapes. Additionally, the patterning method is also compatible with uneven surface due to the high comfortability of PDMS. Unlike traditional physical vapor deposited gold films that experience electrical failure at very low strain (~1%), our gold films still remain highly conductive at 90% elongations

Electrodeposited semiconductor nanostructures & epitaxial thin films for flexible electronics

Single-crystal Si is the bedrock of semiconductor devices due to the high crystalline perfection which minimizes electron-hole recombination, and the dense native silicon oxide which minimizes surface states. To expand the palette of electronic materials beyond planar Si, an inexpensive source of highly ordered material is needed that can serve as an inert substrate for the epitaxial growth of grain boundary-free semiconductors, photonic materials, and superconductors. There is also a need for a simple, inexpensive, and scalable fabrication technique for the growth of semiconductor nanostructures and thin films. This dissertation focuses on the fabrication of semiconducting nanowires (polycrystalline Ge & epitaxial ZnO) and epitaxial thin films (Au & Cu₂O) using electrodeposition from an aqueous solution at ambient conditions as a simple benchtop process. Paper I describes a simple one-step electrodeposition of Ge nanowires on an indium-tin oxide substrate decorated with In nanoparticles. An In metal acts both as a catalyst for electrodeposition and as a solvent for recrystallization of the nanowires at ambient conditions. Ge nanowires are an attractive anode material for Li-ion batteries, due to their larger theoretical capacity compared to graphite. Paper II presents a scheme for epitaxial electrodeposition of ultrathin Au films on Si as an inexpensive proxy for single crystal Au for the electrodeposition of epitaxial Cu₂O thin films. A detailed study of the epitaxial growth, morphology, junction characteristics, and crystallinity is performed for both the Au and Cu₂O thin films. Paper III describes a technique for epitaxial lift-off of wafer-scale Au foils as transparent, single-crystal and flexible substrates for flexible electronics. The Au foils offer the order of traditional single-crystal semiconductors without the constraint of a rigid substrate. An organic light emitting diode is presented to evaluate the flexibility and transparency of Au foils. To study the single crystal nature of Au foil an epitaxial Cu₂O thin film inorganic diode with an improved diode quality factor is demonstrated --Abstract, page iv

Silver Nanowire Coated Threads for Electrically Conductive Textiles

The emerging area of e-textiles requires electrically conductive threads. We demonstrate that nylon, polyester, and cotton threads can be made conductive by coating their surfaces with random networks of solution-synthesized silver nanowires. A resistance per unit length of 0.8 O cm1 was achieved and can be varied through the density of the nanowire coating. Because the nanowires are 35 nm in diameter, and the mesh structure does not cover the entire surface like a thin-film, less metal is used compared to conventional silver-coated conductive threads. This leads to a much lower weight and mechanically flexible coating. The functionality of the thread as a heater and the fabrication of stretchable conductive thread are also demonstrated.4 month

Atomic-Scale Insights into Light Emitting Diode

In solid-state lightning, GaN-based vertical LED technology has attracted tremendous attention because its luminous efficacy has surpassed the traditional lightning technologies, even the 2014 Nobel Prize in Physics was awarded for the invention of efficient blue LEDs, which enabled eco-friendly and energy-saving white lighting sources. Despite today’s GaN-based blue VLEDs can produce IQE of 90% and EQE of 70-80%, still there exist a major challenge of efficiency droop. Nonetheless, state-of-the-art material characterization and failure analysis tools are inevitable to address that issue. In this context, although LEDs have been characterized by different microscopy techniques, they are still limited to either its semiconductor or active layer, which mainly contributes towards the IQE. This is also one of the reason that today’s LEDs IQE exceeded above 80% but EQE of 70-80% remains. Therefore, to scrutinize the efficiency droop issue, this work focused on developing a novel strategy to investigate key layers of the LED structure, which play the critical role in enhancing the EQE = IQE x LEE factors. Based on that strategy, wafer bonding, reflection, GaN-Ag interface, MQWs and top-textured layers have been systematically investigated under the powerful advanced microscopy techniques of SEM-based TKD/EDX/EBSD, AC-STEM, AFM, Raman spectroscopy, XRD, and PL. Further, based on these correlative microscopy results, optimization suggestions are given for performance enhancement in the LEDs. The objective of this doctoral research is to perform atomic-scale characterization on the VLED layers/interfaces to scrutinize their surface topography, grain morphology, chemical composition, interfacial diffusion, atomic structure and carrier localization mechanism in quest of efficiency droop and reliability issues. The outcome of this research advances in understanding LED device physics, which will facilitate standardization in its design for better smart optoelectronics products