Transparent electrodes are a necessary component in many modern devices such as touch screens, LCDs, OLEDs, and solar cells, all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, the most common of which is indium tin oxide, or ITO. Recently, advances in nano-materials research have opened the door for other transparent conductive materials, each with unique properties. These include CNTs, graphene, metal nanowires, and printable metal grids. This review will explore the materials properties of transparent conductors, covering traditional metal oxides and conductive polymers initially, but with a focus on current developments in nano-material coatings. Electronic, optical, and mechanical properties of each material will be discussed, as well as suitability for various applications.

Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures

Carbon Nanotube Thin Films: Fabrication, Properties, and Applications

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

Transparent electrodes attract intense attention in many technological fields, including optoelectronic devices, transparent film heaters and electromagnetic applications. New generation transparent electrodes are expected to have three main physical properties: high electrical conductivity, high transparency and mechanical flexibility. The most efficient and widely used transparent conducting material is currently indium tin oxide (ITO). However the scarcity of indium associated with ITO's lack of flexibility and the relatively high manufacturing costs have a prompted search into alternative materials. With their outstanding physical properties, metallic nanowire (MNW)-based percolating networks appear to be one of the most promising alternatives to ITO. They also have several other advantages, such as solution-based processing, and are compatible with large area deposition techniques. Estimations of cost of the technology are lower, in particular thanks to the small quantities of nanomaterials needed to reach industrial performance criteria. The present review investigates recent progress on the main applications reported for MNW networks of any sort (silver, copper, gold, core-shell nanowires) and points out some of the most impressive outcomes. Insights into processing MNW into high-performance transparent conducting thin films are also discussed according to each specific application. Finally, strategies for improving both their stability and integration into real devices are presented.

Metallic Nanowire-Based Transparent Electrodes for Next Generation Flexible Devices: a Review

We demonstrate high-performance, flexible, transparent heaters based on large-scale graphene films synthesized by chemical vapor deposition on Cu foils. After multiple transfers and chemical doping processes, the graphene films show sheet resistance as low as ∼43 Ohm/sq with ∼89% optical transmittance, which are ideal as low-voltage transparent heaters. Time-dependent temperature profiles and heat distribution analyses show that the performance of graphene-based heaters is superior to that of conventional transparent heaters based on indium tin oxide. In addition, we confirmed that mechanical strain as high as ∼4% did not substantially affect heater performance. Therefore, graphene-based, flexible, transparent heaters are expected to find uses in a broad range of applications, including automobile defogging/deicing systems and heatable smart windows.

High-performance graphene-based transparent flexible heaters.

Transparent Film Heater Using Single‐Walled Carbon Nanotubes

A high-performing UV photodetector that uses large energy bandgap materials of p-type NiO and n-type ZnO without an opaque metal electrode is reported. A quality heterojunction is formed by large-area applicable sputtering deposition method that has an extremely low saturation current density of 0.1 μA cm−2. This abrupt p-NiO/n-ZnO heterojunction device is visible-light transparent and shows the fastest photoresponse time of 24 ms among NiO-based UV photodetectors, along with the highest responsivity (3.85 A W−1) and excellent detectivity (9.6 × 1013 Jones) properties. Structural, physical, optical, and electrical properties of nanocrystalline NiO are systematically investigated. Mott–Schottky analyses are applied to develop the interface of NiO and ZnO by establishing energy diagrams. Defects existing inside the nanocrystalline NiO film enhance the UV detection performance by defect-assisted carrier transportation. The results provide a solid scheme of manipulation of NiO defects for functional photoelectric device applications.

All Transparent Metal Oxide Ultraviolet Photodetector

In view of the worldwide energy challenge in the 21st century, the technology of semiconductor-based photoelectrochemical (PEC) water splitting has received considerable attention as an alternative approach for solar energy harvesting and storage. Two-dimensional (2D) structures such as nanosheets have the potential to tap the solar energy by unlocking the functional properties at the nanoscale. Tin(II) sulfide is a fascinating solar energy material due to its anisotropic material properties. In this manuscript, we report on exploiting the 2D structure modulated optical properties of nanocrystalline SnS thin film synthesized by chemical spray pyrolysis using ambient transport in the harvesting of solar energy. We obtained the nanostructured SnS with well-preserved dimensions and morphologies with one step processing. The work demonstrates that the intrinsically ordered SnS nanostructure on FTO coated glass can tap the incident radiation in an efficient manner. The structure–property relationship to explain the photo-response in nanocrystalline-SnS is verified experimentally and theoretically. The novel design scheme for antireflection coating along with the anisotropic properties of SnS is conceived for realizing a PEC cell. The developed PEC cell consists of a SnS photoanode which shows considerably high photocurrent density of 7 mA cm−2 with aqueous media under AM 1.5G, 100 mW cm−2 exposure with notably stable operation. Electrochemical impedance spectroscopy revealed that a non-ideal capacitive behavior as well as drift assisted transport across the solid-state interface is responsible for such a high photo-current density in the nanocrystalline-SnS photoanode.

Nanostructured SnS with inherent anisotropic optical properties for high photoactivity

A nociceptor is an essential element in the human body, alerting us to potential damage from extremes in temperature, pressure, etc. Realizing nociceptive behavior in an electronics device remains a central issue for researchers, designing neuromorphic devices. This study proposes and demonstrates an all-oxide-based highly transparent ultraviolet-triggered artificial nociceptor, which responds in a very similar way to the human eye. The device shows a high transmittance (>65%) and very low absorbance in the visible region. The current-voltage characteristics show loop opening, which is attributed to the charge trapping/detrapping. Further, the ultraviolet-stimuli-induced versatile criteria of a nociceptor such as a threshold, relaxation, allodynia, and hyperalgesia are demonstrated under self-biased condition, providing an energy-efficient approach for the neuromorphic device operation. The reported optically controlled features open a new avenue for the development of transparent optoelectronic nociceptors, artificial eyes, and memory storage applications.

Joondong Kim

Papers

Transparent Film Heater Using Single‐Walled Carbon Nanotubes

All Transparent Metal Oxide Ultraviolet Photodetector

Nanostructured SnS with inherent anisotropic optical properties for high photoactivity

A Highly Transparent Artificial Photonic Nociceptor

Excitonic metal oxide heterojunction (NiO/ZnO) solar cells for all-transparent module integration