Electrical,optical, and structural properties of thin films with tri-layers of AZO/ZnMgO/AZO grown by filtered vacuum arc deposition

Transparent conductive oxides (TCO) are indispensable as front electrode for most of thin film electronic devices such as transparent electrodes for flat panel displays, photovoltaic cells, windshield defrosters, transparent thin film transistors, and low emissivity windows. Thin films of aluminum-doped zinc oxide (AZO) have shown to be one of the most promising TCOs. In this study, three layered Al-doped ZnO (AZO)/ZnMgO/AZO heterostructures were prepared by filtered cathodic arc deposition (FCAD) on glass substrates. The objective is to find a set of parameters that will allow for improved optical and electrical properties of the films such as low resistivity, high mobility, high number of charge carriers, and high transmittance. We have investigated the effect of modifications in thickness and doping of the ZnMgO inner layer on the structural, electrical, and optical characteristics of the stacked heterostructures.     

Efficient Welding of Silver Nanowire Networks without Post‐Processing

Silver nanowire (AgNW) random meshes have attracted considerable attention as flexible and high‐performance transparent electrodes. Notably, post‐treatment of the AgNW random meshes, such as thermal annealing, is usually required to guarantee comparable optical transparency and electrical conductivity to commercial indium tin oxide (ITO). Here, the integral elements of preparing a high‐performance, large‐area AgNW random mesh network are discussed. High‐performance nanostructured transparent electrodes can be obtained without any post‐treatment, thereby relieving the restrictions related to the substrate. Solvent washing and a large‐area spray‐coating method effectively reduce the wire–wire contact resistances, thus reducing or eliminating the requirement for post‐treatment.     

Characterization of transparent conducting oxide thin films deposited on ceramic substrates

In this work, we investigate the optical and electrical properties of various transparent conductive oxide (TCO) thin films deposited on insulating ceramics for emerging optoelectronic applications. Thin films investigated include indium tin oxide (ITO), ruthenium oxide (RuO₂), and iridium oxide (IrO₂) on Al₂O₃ ceramic substrates. The conducting films have been deposited by various techniques including RF magnetron sputtering and low-cost spray pyrolysis. The morphological characteristics of the films were carried out using high magnification optical microscopy and atomic force microscopy (AFM). Optical and electrical characterization was carried out by optical absorbance/transmittance, van der Pauw, current-voltage (I-V), and Hall effect measurements. The results are presented in this paper.     

Roll‐to‐Roll Production of Transparent Silver‐Nanofiber‐Network Electrodes for Flexible Electrochromic Smart Windows

Electrochromic smart windows (ECSWs) are considered as the most promising alternative to traditional dimming devices. However, the electrode technology in ECSWs remains stagnant, wherein inflexible indium tin oxide and fluorine‐doped tin oxide are the main materials being used. Although various complicated production methods, such as high‐temperature calcination and sputtering, have been reported, the mass production of flexible and transparent electrodes remains challenging. Here, a nonheated roll‐to‐roll process is developed for the continuous production of flexible, extralarge, and transparent silver nanofiber (AgNF) network electrodes. The optical and mechanical properties, as well as the electrical conductivity of these products (i.e., 12 Ω sq^?1 at 95% transmittance) are comparable with those AgNF networks produced via high‐temperature sintering. Moreover, the as‐prepared AgNF network is successfully assembled into an A4‐sized ECSW with short switching time, good coloration efficiency, and flexibility.     

TCO/metal/TCO structures for energy and flexible electronics

There is increasing attention paid to improving transparent conductive electrodes for applications in large area photovoltaic devices and displays that are being developed for energy and electronics. To date, transparent and conductive oxides (TCO) based on In₂O₃, ZnO, or SnO₂ are commonly used, but advanced devices require new electrodes with lower resistivities than previously achieved and with optical properties superior to those of the present generation. TCO/metal/TCO multilayer structures have emerged as an interesting alternative because they provide optical and electrical characteristics globally superior to those attainable with a single-layer TCO or metal electrode and can be deposited at low temperatures onto inexpensive plastic substrates. Indeed, the fabrication of thin film devices on flexible substrates has substantial interest for application to lightweight products and implementation of roll-to-roll deposition processes that can significantly reduce production costs. In this sense, organic electronics that require low deposition temperatures have the best chance to be the first transferred from conventional glass to inexpensive plastic substrates. The present critical review summarizes current TCO/metal/TCO research results, first analyzed for materials and thickness selection as a function of the optical transmittance and electrical resistance parameters, and then analyzed according to other important properties such as mechanical reliability and thermal and humidity stability. The review concludes with a brief discussion of the results obtained for TCO/metal/TCO structures applied as electrodes in several organic electronic devices.     

Characterization of Indium‐Tin‐Oxide Thin Film Microelectrodes for Biomedical Use

Selected properties of indium‐tin‐oxide (ITO) films prepared by r.f. diode sputtering have been investigated in consideration of surface morphology, optical properties, crystal structure and phase formation, electrical resistivity and chemical resistance. The ITO films showed low electrical resistivity (6·10^‐5 Ωm), high optical transmittance (> 80 %) and suitable chemical resistance against selected chemicals during 1 hour.     

Properties of multilayer transparent conducting oxide films

Multilayered transparent conducting oxide (TCO) film structures have been designed and fabricated to achieve both high conductivity and high transmittance in the visible spectrum. Double-layered TCO structures consisting of Sn-doped CdO and Sn-doped CdIn₂O₄, Cd-rich Cd₂SnO₄, or Ga-doped ZnO are discussed. By optimizing the thickness of the individual layers and the doping levels within those layers, an effective conductivity of 20 600 S/cm and an average transmittance larger than 85% in the 400-700 nm range have been achieved for films epitaxially grown on MgO substrates. Bi-layer films consisting of Sn-doped CdO and Ga-doped ZnO have also been deposited on plastic substrates at room temperature with resistivities of ∼1×10⁻⁴ Ω cm and an average transmittance of 80-85% in the visible range. These properties are attractive for future TCO applications.     

Reverse‐Offset Printed Ultrathin Ag Mesh for Robust Conformal Transparent Electrodes for High‐Performance Organic Photovoltaics

Mechanically durable transparent electrodes are needed in flexible optoelectronic devices to realize their long‐term stable functioning, for applications in various fields such as energy, healthcare, and soft robotics. Several promising transparent electrodes based on nanomaterials have been previously reported to replace the conventional and fragile indium‐tin oxide (ITO); however, obtaining feasible printed transparent electrodes for ultraflexible devices with a multistack structure is still a great challenge. Here, a printed ultrathin (uniform thickness of 100 nm) Ag mesh transparent electrode is demonstrated, simultaneously achieving high conductance, high transparency, and good mechanical properties. It shows a 17 Ω sq^?1 sheet resistance (R_sh) with 93.2% transmittance, which surpasses the performance of sputtered ITO electrodes and other ultrathin Ag mesh transparent electrodes. The conductance is stable after 500 cycles of 100% stretch/release deformation, with an insignificant increase (10.6%) in R_sh by adopting a buckling structure. Furthermore, organic photovoltaics (OPVs) using our Ag mesh transparent electrodes achieve a power conversion efficiency of 8.3%, which is comparable to the performance of ITO‐based OPVs.     

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

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.     

Electrical and optical performance of transparent conducting oxide films deposited by electrostatic spray assisted vapour deposition

Transparent conducting oxide (TCO) films have the remarkable combination of high electrical conductivity and optical transparency. There is always a strong motivation to produce TCO films with good performance at low cost. Electrostatic Spray Assisted Vapor Deposition (ESAVD), as a variant of chemical vapour deposition (CVD), is a non-vacuum and low-cost deposition method. Several types of TCO films have been deposited using ESAVD process, including indium tin oxide (ITO), antimony-doped tin oxide (ATO), and fluorine doped tin oxide (FTO). This paper reports the electrical and optical properties of TCO films produced by ESAVD methods, as well as the effects of post treatment by plasma hydrogenation on these TCO films. The possible mechanisms involved during plasma hydrogenation of TCO films are also discussed. Reduction and etching effect during plasma hydrogenation are the most important factors which determine the optical and electrical performance of TCO films.     

Highly Deformable and See‐Through Polymer Light‐Emitting Diodes with All‐Conducting‐Polymer Electrodes

Despite the high expectation of deformable and see‐through displays for future ubiquitous society, current light‐emitting diodes (LEDs) fail to meet the desired mechanical and optical properties, mainly because of the fragile transparent conducting oxides and opaque metal electrodes. Here, by introducing a highly conductive nanofibrillated conducting polymer (CP) as both deformable transparent anode and cathode, ultraflexible and see‐through polymer LEDs (PLEDs) are demonstrated. The CP‐based PLEDs exhibit outstanding dual‐side light‐outcoupling performance with a high optical transmittance of 75% at a wavelength of 550 nm and with an excellent mechanical durability of 9% bending strain. Moreover, the CP‐based PLEDs fabricated on 4 µm thick plastic foils with all‐solution processing have extremely deformable and foldable light‐emitting functionality. This approach is expected to open a new avenue for developing wearable and attachable transparent displays.     

Recent progress in transparent oxide semiconductors: Materials and device application

This paper reviews our recent research progress on new transparent conductive oxide (TCO) materials and electronic and optoelectronic devices based on these materials. First, described are the materials including p-type materials, deep-UV transparent TCO(β-Ga₂O₃), epitaxially grown ITO with atomically flat surface, transparent electrochromic oxide (NbO₂F), amorphous TCOs, and nanoporous semiconductor 12CaO · 7Al₂O₃. Second, presented are TCO-based electronic/optoelectronic devices realized to date, UV/blue LED and UV-sensors based on transparent pn junction and high performance transparent TFT using n-type TCO as an n-channel. Finally, unique optoelectronic properties (p-type degenerate conduction, transfer doping of carriers, RT-stable exciton, and large optical nonlinearity) originating from 2D-electronic nature in p-type layered oxychalcogenides are summarized along with the fabrication method of epitaxial thin films of these materials.     

Printed transparent electrodes containing carbon nanotubes for elastic circuits applications with enhanced electrical durability under severe conditions

Organic composites filled with nanostructures are new group of materials with unique physical properties. Carbon nanotubes (CNTs) are demonstrating good electrical and mechanical properties. This enables to produce conductive polymer-CNT thick films optically transparent, which are highly useful in production of printed electronic paper. Currently used indium tin oxide (ITO) and antimony tin oxide (ATO) films exhibit high optical transmittance with reasonable electrical conductivity, but very low resilience to mechanical stresses. This is one of the key problems in fabrication of flexible electronic displays. Current authors’ achievements include fabrication of transparent electrodes obtained by screen printing technique, used for production of fully functional thick film electroluminescent structures.     

Highly Efficient (>10%) Flexible Organic Solar Cells on PEDOT‐Free and ITO‐Free Transparent Electrodes

A novel approach to fabricate flexible organic solar cells is proposed without indium tin oxide (ITO) and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) using junction‐free metal nanonetworks (NNs) as transparent electrodes. The metal NNs are monolithically etched using nanoscale shadow masks, and they exhibit excellent optoelectronic performance. Furthermore, the optoelectrical properties of the NNs can be controlled by both the initial metal layer thickness and NN density. Hence, with an extremely thin silver layer, the appropriate density control of the networks can lead to high transmittance and low sheet resistance. Such NNs can be utilized for thin‐film devices without planarization by conductive materials such as PEDOT:PSS. A highly efficient flexible organic solar cell with a power conversion efficiency (PCE) of 10.6% and high device yield (93.8%) is fabricated on PEDOT‐free and ITO‐free transparent electrodes. Furthermore, the flexible solar cell retains 94.3% of the initial PCE even after 3000 bending stress tests (strain: 3.13%).     

IZO/Al/GZO multilayer films to replace ITO films

Multilayer transparent conducting oxide (TCO) film structures have been designed and fabricated to achieve both high conductivity and high transmittance. In this article we report a buffering method and introduction of an aluminum (Al) interlayer to enhance the electrical conductivity of the IZO/Al/GZO/ZnO multilayer film on glass. Hall measurement results show that this multilayer film has a remarkable increase in mobility compared to those without using an Al interlayer. The surface morphology shows a decrease in surface roughness as the Al layer thickness increases. We have shown that the use of a thin Al interlayer enhances the electrical conductivity without sacrificing its optical transmittance much. By optimizing the thickness of the Al layer, the lowest resistivity of 2.2 × 10⁻⁴ Ω cm and an average transmittance higher than 75% in a range from 400 to 800 nm have been achieved. These properties are acceptable for future TCO applications.     

High quality transparent conductive ZnO/Ag/ZnO multilayer films deposited at room temperature

We have fabricated, by simultaneous DC and RF magnetron sputtering, multilayer transparent electrodes having much lower electrical resistance than the widely used transparent conductive oxide electrodes. The multilayer structure consists of three layers (ZnO/Ag/ZnO). Ag films with different film thickness were used as metallic layers. Optimum thicknesses of Ag and ZnO films were determined for high optical transmittance and good electrical conductivity. Several analytical tools such as spectrophotometer, atomic force microscopy, scanning electron microscopy and four-point probe were used to explore the possible changes in electrical and optical properties. A high quality transparent electrode, having resistance as low as 3 Ω/sq and high optical transmittance of 90% was obtained at room temperature and could be reproduced by controlling the preparation process parameters. The electrical and optical properties of ZnO/Ag/ZnO multilayers were determined mainly by the Ag film properties. The performance of the multilayers as transparent conducting materials was also compared using a figure of merit.     

Influence of the sputtering system''s vacuum level on the properties of indium tin oxide films

The influence of the chamber residual pressure level in the radio frequency magnetron sputtering process on the electrical, optical and structural properties of indium thin oxide (ITO) is investigated. Several ITO films were deposited at various residual pressure levels on Corning glass using In₂O₃:SnO₂ target in argon atmosphere and without the addition of oxygen partial pressure. It is found that a very good vacuum is associated to metallic films and results in less transparent ITO films, with some powder formation on the surface. On the contrary highly transparent and conducting films are produced at a higher residual pressure. The best deposition conditions are addressed for ITO films as transparent conducting oxide layers in silicon heterojunction solar cells. Using the optimal vacuum level for ITO fabrication, a maximum short circuit current of 36.6 mA/cm² and a fill-factor of 0.78 are obtained for solar cells on textured substrates with a device conversion efficiency of 16.2%.     

Robust and Aligned Carbon Nanotube/Titania Core/Shell Films for Flexible TCO‐Free Photoelectrodes

Carbon nanotube (CNT)/semiconducting oxide hybrids are an ideal architecture for light‐harvesting devices, in which the CNTs are expected to not only act as a scaffold but also provide fast transport paths for photogenerated charges in the oxide. However, the current potential of CNTs for charge transport is largely suppressed due to the nanotubes not being interconnected but isolated by the low conductive oxide coatings. Herein, a flexible and conductive CNT/TiO₂ core/shell heterostructure film is reported, with aligned and interconnected CNTs wrapped in a continuous TiO₂ coating. Without using additional transparent conducting oxide (TCO) substrates, this unique feature of the film boosts the incident photon‐to‐electron conversion efficiency to 32%, outperforming TiO₂ nanoparticle electrodes fabricated on TCO substrates. Moreover, the film shows high structural stability and can generate a stable photocurrent even after being bent hundreds of times.     

Role of substrate temperature on the structural,optoelectronic and morphological properties of (400) oriented indium tin oxide thin films deposited using RF sputtering technique

RF sputtering process has been used to deposit highly transparent and conducting films of tin-doped indium oxide onto quartz substrates keeping the RF power constant at 250 W. The electrical, optical and structural properties have been investigated as a function of substrate temperature. XRD has shown that deposited films are polycrystalline and have (400) preferred orientation. Indium tin oxide layers with low resistivity values and high transmittance in the visible region have been deposited. Detailed Analyses based on X-ray diffraction, optical and electrical results are attempted to gain more insight into the factors that are governed by the influence of varying substrate temperature in this investigation. AFM pictures showed uniform surface morphology with very low surface roughness values. It has been observed that ITO films deposited in this study, keeping the substrate temperature at 150 °C, can provide the required optimum electrical and optical properties rendering them useful for developing many optoelectronic devices at a moderate temperature.     

The Itinerant 2D Electron Gas of the Indium Oxide (111) Surface: Implications for Carbon‐ and Energy‐Conversion Applications

Transparent conducting oxides (TCO) have integral and emerging roles in photovoltaic, thermoelectric energy conversion, and more recently, photocatalytic systems. The functional properties of TCOs, and thus their role in these applications, are often mediated by the bulk electronic band structure but are also strongly influenced by the electronic structure of the native surface 2D electron gas (2DEG), particularly under operating conditions. This study investigates the 2DEG, and its response to changes in chemistry, at the (111) surface of the model TCO In₂O₃, through angle resolved and core level X‐ray photoemission spectroscopy. It is found that the itinerant charge carriers of the 2DEG reside in two quantum well subbands penetrating up to 65 Å below the surface. The charge carrier concentration of this 2DEG, and thus the high surface n‐type conductivity, emerges from donor‐type oxygen vacancies of surface character and proves to be remarkably robust against surface absorbents and contamination. The optical transparency, however, may rely on the presence of ubiquitous surface adsorbed oxygen groups and hydrogen defect states that passivate localized oxygen vacancy states in the bandgap of In₂O₃.