Typically, materials with high electrical conductivity such as metals are opaque, and materials with high optical transparency such as glass are insulating. Finding materials that are both transparent to visible light and electrically conductive has proven to be a challenge. The need for such materials continues to grow, as many of today’s popular devices such as liquid-crystal displays and organic light-emitting diodes in televisions, touch screens in phones or tablet computers, electrophoretic displays in e-readers, or solar cells on a roof require one or more layers to transmit visible light, while simultaneously applying a voltage or conducting a current. Today, the industry’s need for such a material is serviced by various metal oxides, of which indium tin oxide (ITO) is by far the most common. The opto-electronic properties of ITO satisfy industry need for most devices; however, ITO has several drawbacks (e.g., brittle, expensive, and typically applied via costly sputtering techniques). To address these issues, recent advances in solution-processed nanomaterials have enabled several printable alternatives to sputtered ITO. These nanomaterials include conducting polymers, metallic nanostructures, ITO nanostructures, carbon nanotubes, and graphene. The ability to apply nanomaterials from the liquid phase opens the possibility to print these electronic materials roll-to-roll, greatly reducing cost and increasing yield and throughput, while the nanomaterial topology enables truly flexible devices.

Metals possess the highest conductivity among all room-temperature materials; however, ultrathin metal films demonstrate decent optical transparency but poor sheet conductance due to electron scattering from the surface and grain boundaries. This article discusses engineered metal nanostructures in the form of nanogrids, nanowires, or continuous nanofibers as efficient transparent and conductive electrodes. Metal nanogrids are discussed, as they represent an excellent platform for understanding the fundamental science. Progress toward low-cost, nano-ink-based printed silver nanowire electrodes, including silver nanowire synthesis, film fabrication, wire-wire junction resistance, optoelectronic properties, and stability, are also discussed. Another important factor for low-cost application is to use earth-abundant materials. Copper-based nanowires and nanofibers are discussed in this context. Examples of device integrations of these materials are also given. Such metal nanostructure-based transparent electrodes are particularly attractive for solar cell applications.

Carbon nanotubes (CNTs) are high aspect ratio conducting nanocylinders possessing unprecedented mechanical, thermal, optical, and electronic properties. They are ideal building blocks for use in assembling a randomly oriented, highly connected nanoporous network. When this network is deposited on top of a substrate surface as a thin film with a thickness in the range of 10–100 nm, it becomes a transparent conducting film—an ubiquitous material, currently dominated by tin-doped indium oxide (ITO). This article reviews recent progress in CNT transparent conducting films and discusses fundamental properties of CNTs important for the formation of these films, methods for CNT dispersion and assembling CNTs into transparent conducting films, properties of the CNT transparent conducting films, and issues remaining to be solved in order to make these films a commercially viable alternative to ITO.

Networks of nanoscale conductors such as carbon nanotubes, graphene, and metallic nanowires are promising candidates to replace metal oxides as transparent conductors. However, very few previous reports have described nanostructured thin films that reach the standards required by industry for high-performance transparent electrodes. In this review, we analyze the sheet resistance and transmittance data extracted from published literature for solution processed, nanostructured networks. In the majority of cases, as their thickness is reduced below a critical value, nanoconductor networks undergo a transition from bulk-like to percolative behavior. Such percolative behavior is characteristic of sparse networks with limited connectivity and few continuous conductive paths. This transition tends to occur for films with a transmittance between 50% and 90%, which means that the properties of highly transparent films are predominately limited by percolation. Consequently, to achieve low resistance coupled with high transparency, the networks must be much more conductive than would otherwise be the case. We show that highly conductive networks of metallic nanowires appear to be the most promising candidate to replace traditional transparent electrode materials from a technical standpoint. However, many other factors, including cost, manufacturability, and stability, will have to be addressed before commercialization of these materials.

Indium tin oxide (ITO) is the most widely used transparent electrode material today. Although the material possesses a superior combination of high optical transparency and large conductance, conventional ITO thin-film deposition techniques are incompatible with the mechanical and thermal requirements associated with the emerging class of flexible electronic devices. To address this issue, the status of the development of ITO nanostructures such as nanowires and nanoparticles will be reviewed in this article. Two major achievements to be discussed are the growth of vertically oriented arrays of ITO nanowires and the synthesis of small diameter, monodisperse ITO nanoparticles. In addition, solutions of these materials can be deposited utilizing existing printed electronics manufacturing techniques to realize highly transparent, conductive, and flexible ITO thin films on a diverse range of substrates, including plastics. These nanomaterial-based approaches could one day help realize low-cost, flexible electronics based on transparent thin-film electrodes.

A number of applications such as displays, touch sensors, and ultrathin heating elements contain flexible and optically transparent plastic films covered with highly electrically conductive coatings. In most cases, indium tin oxide (ITO) is used as the conductive material for these coatings due to its additional property of being transparent to visible light. Once deposited onto the foil, ITO has to be patterned before use, which is generally a tricky, time-consuming, and costly process. A newly developed economical roll-to-roll printing process utilizing metallic grids now offers a direct print alternative with better functional characteristics.

This article summarizes current applications and the future potential of highly conductive poly(3,4-ethylenedioxythiophene) (PEDOT). The main focus of the article is a water dispersed complex of PEDOT with poly(styrenesulfonic acid) (PSS) as the counter-ion. The availability of PEDOT:PSS as an ink allows many facile ways of solution processing. The basic chemical and physical properties of the PEDOT:PSS complex are discussed to show the fundamentals that allow for the use of PEDOT in transparent conductive applications. Due to the increase in conductivity and transparency of the PEDOT:PSS complex in recent years, this versatile material now has reached the requirements for current devices such as displays, touch screens, and solar cells, and offers an alternative to inorganic transparent conductive oxides. Further advantages of this polymer are the ductility, use in low-cost production processes such as printing, safe handling, and availability on a large scale.

Dissimilatory metal-reducing bacteria (DMRB) are a fascinating group of microorganisms that inhabit many natural environments. They possess a distinct capability wherein they can acquire energy by coupling oxidation of organic matter with reduction of insoluble oxidants such as mineral deposits. This capability requires that DMRB transfer respired electrons to their outer surface where electron transfer can occur to an insoluble oxidant. This is distinct from the dominant paradigm, wherein soluble oxidants are transported into microbes for reduction during metabolism. This unique extracellular electron transfer (EET) capability of DMRB extends to reduction of electrodes on which they can proliferate and form persistent films (biofilms). This capability makes DMRB useful as anode catalysts in microbial fuel cells for alternative energy generation and for degradation of organic wastes. In the case of Geobacter spp., anode biofilms can grow to be many microbes thick. In such biofilms, individual microbes contribute to a flux of electrons to the underlying electrode surface, which may be many cell lengths away, confounding long-held notions about the inability of microbes to engage in such long-range EET. This article describes the electrode-reducing ability of DMRB and the latest results describing the mechanism of long-range extracellular electron transfer, which appears to involve filamentous appendages termed nanowires.