Recent progresses on solution-processed silver nanowire based transparent conducting electrodes for organic solar cells

Organic photovoltaics (OPVs) are considered as a future alternative for conventional silicon based solar cells, owing to their low cost, ease of production and high-throughput. The transparent conducting electrode (TCE) is a fundamental component of OPVs. Traditionally, indium tin oxide (ITO) has been mainly utilized as a TCE in OPV applications due to its relatively high transparency and low sheet resistance. However, increasing demand for the optoelectronic devices has led to large fluctuations in ITO prices in the past decade and ITO is known to account more than 50% of the total cost of OPV devices. Thus, it is believed that development of solution-processable alternative materials is of great importance in reducing the cost of OPVs. Numerous materials, including silver nanowires, carbon nanotubes, graphene and conducting polymers, have been offered as replacements for ITO. This article reviews recent progress on fabrication of TCE via solution based coating techniques of silver nanowires (Ag NWs). In addition, performance of the Ag NWs based TCE in OPVs is summarized. Finally, we explore the future outlook for Ag NWs based TCE at the end of the review.     

Transparent electrodes fabricated via the self-assembly of silver nanowires using a bubble template

To shore up the demand of transparent electrodes for wide applications such as organic light emitting diodes and solar cells, transparent electrodes are required as an alternative for indium tin oxide electrodes. Herein the self-assembly method with a bubble template paves the way for cost-effective fabrication of transparent electrodes with high conductivity and transparency using self-assembly of silver nanowires (AgNWs) in a bubble template. AgNWs were first dispersed in water that was bubbled with a surfactant and a thickening agent. Furthermore, these AgNWs were assembled by lining along the bubble ridges. When the bubbles containing the AgNWs were sandwiched between two glass substrates, the bubble ridges including the AgNWs formed continuous polygonal structures. Mesh structures were formed on both glass substrates after air-drying. The mesh structures evolved into mesh transparent electrodes following heat-treatment. The AgNW mesh structure exhibited a low sheet resistance of 6.2 Ω/square with a transparency of 84% after heat treatment at 200 °C for 20 min. The performance is higher than that of transparent electrodes with random networks of AgNWs. Furthermore, the conductivity and transparency of the mesh transparent electrodes can be adjusted by changing the amount of the AgNW suspension and the space between the two glass substrates.     

纳米碳材料在可穿戴柔性导电材料中的应用研究进展

可穿戴设备的兴起使得对柔性器件的需求日益提高,柔性导电材料作为可穿戴器件的重要组成部分而成为研究的热点。传统的电极材料主要是金属,因金属材料本身不具有柔性,一般通过降低金属层厚度以及设计波纹结构等策略实现其在柔性器件中的应用,其加工程序复杂,成本较高。以碳纳米管和石墨烯为代表的纳米碳材料兼具良好的柔性和优异的导电性,且具有化学稳定、热稳定、光学透明性等优点,在柔性导电材料领域展现了极大的应用潜力。本文简要综述了近年来纳米碳材料在柔性导电材料领域的研究进展,首先介绍了碳纳米管基柔性导电材料,分别包括基于碳纳米管水平阵列、碳纳米管垂直阵列、碳纳米管薄膜、碳纳米管纤维的柔性导电材料;继而介绍了石墨烯基柔性导电材料,包括基于剥离法制备的石墨烯和化学气相沉积法制备的石墨烯以及石墨烯纤维基柔性导电材料;并简述了碳纳米管/石墨烯复合柔性导电材料;最后论述了纳米碳材料基柔性导电材料所面临的挑战并展望了其未来发展方向。     

Nitrogen‐Doped Carbon Nanotube‐Based Bilayer Thin Film as Transparent Counter Electrode for Dye‐Sensitized Solar Cells (DSSCs)

Carbon nanotubes (CNTs) have been widely considered as one of the promising candidates for replacing fluorine‐doped tin oxide (FTO)/platinum (Pt) electrodes to reduce the fabrication cost of dye‐sensitized solar cells (DSSCs). Here, we report that a bilayer transparent film containing N‐doped CNTs (which are highly catalytic) and normal CNTs (which are highly conductive) as a counter electrode in DSSCs results in efficiencies up to 2.18 %, yet still maintains a good transparency with a transmittance of approximately 57 % at 550 nm.     

Fast printing of thin,large area,ITO free electrochromics on flexible barrier foil

Processing of large area, indium tin oxide (ITO) free electrochromic (EC) devices has been carried out using roll‐to‐roll (R2R) processing. By use of very fine high‐conductive silver grids with a hexagonal structure, it is possible to achieve good transparency of the electrode covered substrates and when used in EC devices switching times are similar to corresponding ITO devices. This is obtained without the uneven switching of larger areas, which is generally observed when using ITO because of its high‐sheet resistance. The silver electrode structures for 18 × 18 cm² devices can be processed at high speed (10 m/min) on PET by flexographic printing and the EC polymer ECP‐Magenta as well as a minimal color changing polymer MCCP by slot‐die coating, showing the potential for fast fabrication of large volumes of low‐priced flexible EC devices by use of R2R processing techniques. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Transparent Carbon Nanotube Network for Efficient Electrochemiluminescence Devices

A carbon nanotube‐based electrode that combines transparency and good conductivity was used for the first time to develop an electrochemiluminescence (ECL) device. It resulted in an excellent material for ECL applications thanks to the very favorable overpotential of amine oxidation that represents the rate‐determining step for the signal generation in both research systems and commercial instrumentation. The use of carbon nanotubes resulted in a ten times higher emission efficiency compared with commercial transparent indium tin oxide (ITO) electrodes. Moreover, application of this material for proof‐of‐principle ECL imaging was demonstrated, in which micro‐beads were used to mimic a real biological sample in order to prove the possibility of obtaining single cell visualization.     

High density germanium nanowire assemblies: contact challenges and electrical characterization

The conductive properties of vertically aligned germanium nanowires, with mean diameters of 50 and 100 nm, within anodized aluminum oxide (AAO) templates have been characterized by conductive atomic force microscopy (C-AFM) and macrocontact measurements. C-AFM was used to determine the electrical transport properties of individual nanowires within the arrays, while macrocontacts were used to measure the mean current-voltage characteristics of groups of nanowires. Contact resistance between the nanowires and metal macrocontacts was minimized by polishing and gradual etching of the AAO surface, to expose the nanowires, prior to deposition of the contacts. Impedance measurements were used to analyze the importance of defects on the charge transport properties of the germanium nanowire arrays. Conductivity data from C-AFM and macrocontact measurements were found to be comparable suggesting that both methods are inherently suitable for evaluating the electrical transport properties of encapsulated nanowires within a matrix. These results are significant as the ability to make good ohmic contacts to nanowires, within well-defined arrays, is key for the future "bottom-up" fabrication of multilayered device architectures for future electronic and optoelectronic devices.     

Transparent Electrodes Printed with Nanocrystal Inks for Flexible Smart Devices

Transparent electrodes (TEs) are crucial in a wide range of modern electronic and optoelectronic devices. However, traditional TEs cannot meet the requirements of smart devices under development in unique fields, such as electronic skins, wearable electronics, robotic skins, flexible and stretchable displays, and solar cells. Emerging TEs printed with nanocrystal (NC) inks are inexpensive and compatible with solution processes, and have huge potential in flexible, stretchable, and wearable devices. Every development in ink‐based electrodes makes them more competitive for practical applications in various smart devices. Herein, we provide an overview of emergent ink‐based electrodes, such as transparent conducting oxides, metal nanowires, graphene, and carbon nanotubes, and their application in solution‐based flexible and stretchable devices.     

A Facile Solution-Phase Approach to Transparent and Conducting ITO Nanocrystal Assemblies

Monodisperse 11 nm indium tin oxide (ITO) nanocrystals (NCs) were synthesized by thermal decomposition of indium acetylacetonate, In(acac)(3), and tin bis(acetylacetonate)dichloride, Sn(acac)(2)Cl(2), at 270 °C in 1-octadecene with oleylamine and oleic acid as surfactants. Dispersed in hexane, these ITO NCs were spin-cast on centimeter-wide glass substrates, forming uniform ITO NC assemblies with root-mean-square roughness of 2.9 nm. The assembly thickness was controlled by ITO NC concentrations in hexane and rotation speeds of the spin coater. Via controlled thermal annealing at 300 °C for 6 h under Ar and 5% H(2), the ITO NC assemblies became conductive and transparent with the 146 nm-thick assembly showing 5.2 × 10(-3) Ω·cm (R(s) = 356 Ω/sq) resistivity and 93% transparency in the visible spectral range-the best values ever reported for ITO NC assemblies prepared from solution phase processes. The stable hexane dispersion of ITO NCs was also readily spin-cast on polyimide (T(g) ～360 °C), and the resultant ITO assembly exhibited a comparable conductivity and transparency to the assembly on a glass substrate. The reported synthesis and assembly provide a promising solution to the fabrication of transparent and conducting ITO NCs on flexible substrates for optoelectronic applications.     

石墨烯透明导电薄膜

石墨烯是一种新型二维晶体材料,它独特的单原子层结构显示出许多优异的物理化学性质。以石墨烯为原料制备的透明导电薄膜继承了石墨烯的优点,与氧化铟锡(ITO)薄膜相比,具有更好的力学强度、透光性以及化学稳定性,已逐渐成为全世界范围内的研究热点。本文首先介绍了石墨烯的光电性能,然后分别从石墨烯透明导电薄膜的前驱体和制备方法两个不同的角度,归纳总结了最近几年石墨烯透明导电薄膜的研究进展,就目前所面临的问题进行了讨论,并展望了石墨烯透明导电薄膜的未来发展。     

ITO-free transparent conductive films based on carbon nanomaterials with metal grid for liquid crystal displays

Carbon nanomaterials with metal grids were used as transparent conductive electrodes for liquid crystal displays (LCDs) to develop an indium tin oxide (ITO)-free device. We prepared LCDs with CNTs and graphene electrodes; however, the working voltage of the device with the CNT electrodes was high. The device with graphene electrodes had good performance, but not as good as devices with ITO electrodes. To improve the device performance, we applied a metal grid to the carbon nanomaterial to create low sheet-resistance transparent electrodes. The device with the graphene and metal grid transparent electrodes had a threshold voltage as low as 0.23 V/µm, which is similar to that of typical LCDs with ITO electrodes. The results show that a hybrid transparent conductive film with graphene and metal grid could be an alternative to ITO for developing ITO-free LCDs.     

Nanotubes and nanowires

Synthesis and characterization of nanotubes and nanowires constitute an important part of nanoscience since these materials are essential building units for several devices. We have prepared aligned carbon nanotube bundles and Y-junction nanotubes by the pyrolysis of appropriate organic precursors. The aligned bundles are useful for field emission display while the Y-junction nanotubes are likely to be useful as nanochips since they exhibit diode properties at the junction. By making use of carbon nanotubes, nanowires of metals, metal oxides and GaN have been obtained. Both the oxide and GaN nanowires are single crystalline. Gold nanowires exhibit plasmon bands varying markedly with the aspect ratio. GaN nanowires show excellent photoluminescence characteristics. It has been possible to synthesise nanotubes and nanowires of metal chalcogenides by employing different strategies.     

Nanoparticle films as a conducting layer for anodic aluminum oxide template-assisted nanorod synthesis

A simple, inexpensive, and robust methodology was developed to fabricate conductive film substrates by mechanically packing nanoparticles (NPs) on one side of anodic aluminum oxide (AAO). Gold, silver NPs, and carbon nanotubes were used as building blocks in the synthesis of conductive film substrates, upon which perpendicular nanorod arrays and colloidal free-standing nanorods were easily constructed. Characterizations by field emission scanning electron microscopy (FE-SEM) and optical dark-field microscopy confirmed the validity of the conductive NP film substrates on the AAO template. This contribution could provide a convenient and low-cost means for the fabrication of various conductive substrates on AAO.     

Pinning of single-walled carbon nanotubes by selective electrodeposition

A maskless method for the fabrication of electrical or mechanical contacts to the single-walled carbon nanotubes (SWNTs) by selective electrodeposition is reported. Both semiconducting SWNTs and metallic SWNTs can be pinned on prepatterned electrodes by the locally deposited metal, leaving the section of SWNTs between the electrodes clean. The distribution of deposited metal on the SWNTs is mainly determined by the covering power of the plating bath and the plating potential. This research provides a parallel method for the large-scale integration of SWNTs into electronic, optoelectronic, and sensing systems.     

Aligned carbon nanotube/polymer composite films with robust flexibility, high transparency, and excellent conductivity

This work reports a novel and general synthesis of aligned carbon nanotube/polymer composite films with high optical transparency, robust flexibility, and excellent conductivity. These composite films show many potential applications such as flexible conductors for optoelectronic devices.     

A facile route to fabrication of inorganic-small organic molecule cable-like nanocomposite arrays

A novel and facile method is reported for the preparation of silver iodide-small organic molecule (SOM) cable-like nanocomposites arrays, which involved first the fabrication of SOM nanotubes inside an anodic aluminium oxide (AAO) membrane, and then using the SOM nanotubes in AAO as secondary template to prepare the AgI nanowires in aqueous solution at room temperature.     

Well-dispersed SnO_2 nanocrystals on N-doped carbon nanowires as efficient electrocatalysts for carbon dioxide reduction

The conversion of carbon dioxide into valuable organic compounds is a highly promising approach to address the energy issues and environmental problems(e.g., global warming). Herein, we presents a facile and efficient method to prepare highly dense and well-dispersed SnO_2 nanocrystals on 1 D N-doped carbon nanowires as advanced catalysts for the efficient electroreduction of CO_2 to formate. The ultrasmall SnO_2 coated on the N-doped carbon nanowires(SnO_2@N-CNW) has been synthesized via the simple hydrothermal treatment coupled with a pyrolysis process. The unique structure enables to expose the active tin oxide and also provides the facile pathways for rapid transfer of electron and electrolyte along with the highly porous carbon foam composed with interconnected carbon nanowires. Therefore, SnO_2@NCNW electrocatalyst exhibits good durability and high selectivity for formate formation with a Faradaic efficiency of ca. 90%. This work demonstrates a simple method to rationally design high-dense tin oxide nanocrystals on the conductive carbon support as advanced catalysts for CO_2 electroreduction.     

Seed layer effect on morphological,structural, and optical properties of electrochemically grown ZnO nanowires over different SnO2:F/glass substrates

Journal of Solid State Electrochemistry - Transparent conductive oxide electrodes and specifically SnO2:F/glass are widely employed substrates in the preparation of optoelectronic devices. This...     

Surfactant-free water-processable photoconductive all-carbon composite

Heterojunctions between different graphitic nanostructures, including fullerenes, carbon nanotubes and graphene-based sheets, have attracted significant interest for light to electrical energy conversion. Because of their poor solubility, fabrication of such all-carbon nanocomposites typically involves covalently linking the individual constituents or the extensive surface functionalization to improve their solvent processability for mixing. However, such strategies often deteriorate or contaminate the functional carbon surfaces. Here we report that fullerenes, pristine single walled carbon nanotubes, and graphene oxide sheets can be conveniently coassembled in water to yield a stable colloidal dispersion for thin film processing. After thermal reduction of graphene oxide, a solvent-resistant photoconductive hybrid of fullerene-nanotube-graphene was obtained with on-off ratio of nearly 6 orders of magnitude. Photovoltaic devices made with the all-carbon hybrid as the active layer and an additional fullerene block layer showed unprecedented photovoltaic responses among all known all-carbon-based materials with an open circuit voltage of 0.59 V and a power conversion efficiency of 0.21%. The ease of making such surfactant-free, water-processed, carbon thin films could lead to their wide applications in organic optoelectronic devices.     

Metallic single-walled carbon nanotubes for conductive nanocomposites

This article reports an unambiguous demonstration that bulk-separated metallic single-walled carbon nanotubes offer superior performance (consistently and substantially better than the as-produced nanotube sample) in conductive composites with poly(3-hexylthiophene) and also in transparent conductive coatings based on PEDOT:PSS. The results serve as a validation on the widely held view that the carbon nanotubes are competitive in various technologies currently dominated by conductive inorganic materials (such as indium tin oxide).