Transparent Thin Film Electrodes Research Articles

Highly flexible and transparent electrodes for high-performance thin-film heaters with nanostructured micromesh Cu–Ag ultrathin films

Herein, we present a facile method to fabricate transparent, flexible, and high-performance thin-film heaters (TTFHs) based on nanostructured micromesh (NSMM) Cu–Ag/indium tin oxide bilayer transparent thin-film electrodes (TTFEs) with both nano- and microstructures. Our approach combines Ar-assisted thermal evaporation and pulsed laser ablation techniques to fabricate NSMM TTFEs with a low sheet resistance (∼2.1 Ω/sq) and high optical transmittance (∼80.1 %). By adjusting the spacing of microholes, we can achieve excellent figure of merit values (∼51.8 × 10−3 Ω−1) for the NSMM TTFEs. These electrodes also show excellent durability and flexibility, as evidenced by a resistance change of only 4 %–18 % in cyclic bending tests with up to 200,000 cycles. Finally, we demonstrate that the NSMM TTFEs can be used as high-performance TTFHs with short response times (3.97 s at a steady temperature of 87.5 °C under low-voltage operation (5 V)) and infrared shielding properties. Our results suggest that the Cu–Ag-alloy-based NSMM TTFEs are promising transparent electrodes for various applications in flexible electronics, smart windows, and wearable devices.

AgNWs-a-TiOx: a scalable wire bar coated core–shell nanocomposite as transparent thin film electrode for flexible electronics applications

AgNWs-a-TiOx: a scalable wire bar coated core–shell nanocomposite as transparent thin film electrode for flexible electronics applications

Highly Transparent Planar Dipole Using Liquid Ionized Salt Water Under Surface Tension Condition for UHD TV Applications

In this article, a feasibility study on an optically transparent planar dipole antenna using liquid salt water at wide ultra-high frequency (UHF) band (470-771 MHz) for application in external antenna of ultra-high-definition television (UHD TV) is presented. The most significant reason behind using liquid salt water for transparent antenna applications is its excellent average optical transparency (OTav) (>95% at salinity of 40 ppt) compared with other typical solid transparent thin-film electrodes such as indium tin oxide (>73%) or multilayer films (>78%). Each conductive arm of the proposed dipole is constructed from a salt-water layer held between two clear planar acrylic layers (ε <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> = 2.61, tan δ = 0.01, OT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">av</sub> > 90%) (acrylic/salt water/acrylic; ASA) due to surface tension. To investigate the electrical and optical properties of the ASA structure, we analyze the surface tension to determine the thickness of the salt-water layer that finalizes its sheet resistance and OT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">av</sub> . The average gain and efficiency of the antenna are 1.72 dBi and 74%, respectively, in the operating UHF band. This makes the proposed antenna a good candidate for application as a transparent external antenna in UHD TVs and proves that it is possible to use salt water for high-frequency devices.

Giant Pockels effect in an electrode-water interface for a “liquid” light modulator

We report the development of a light modulator using the Pockels effect of water in a nanometer-thick electric double layer on an electrode surface. The modulator comprises a transparent-oxide electrode on a glass substrate immersed in an aqueous electrolyte solution. When an optical beam is incident such that it is totally reflected at the electrode-water interface, the light is modulated at a specific wavelength with a near-100% modulation depth synchronized with the frequency of the applied AC voltage. This result was reproduced by a calculation that assumes a change in the refractive index of −0.1 in a 2-nm electric double layer and of −0.0031 in a 30-nm space-charge layer formed at the interface between the electrolyte aqueous solution and the transparent electrode. This is the first report of an optical modulator that uses the interfacial Pockels effect of a material that does not allow for the Pockels effect in the bulk. The principle of giant optical modulation is explained by invoking the large Pockels coefficient of interfacial water and a Fabry–Perot-resonance effect in the transparent thin-film electrode.

Brownian Motion and Large Electric Polarizabilities Facilitate Dielectrophoretic Capture of Sub-200 nm Gold Nanoparticles in Water.

Dielectrophoresis can move small particles using the force resulting from their polarization in a divergent electric field. In liquids, it has most often been applied to micrometric objects such as biological cells or latex microspheres. For smaller particles, the dielectrophoretic force becomes very small and the phenomenon is furthermore perturbed by Brownian motion. Whereas dielectrophoresis has been used for assembly of superstructures of nanoparticles and for the detection of proteins and nucleic acids, the mechanisms underlying DEP of such small objects require further study. This work presents measurements of the alternating-current (AC) dielectrophoretic response of gold nanoparticles of less than 200 nm diameter in water. An original dark-field digital video-microscopic method was developed and used in combination with a microfluidic device containing transparent thin-film electrodes. It was found that the dielectrophoretic force is only effective in a small zone very close to the tip of the electrodes, and that Brownian motion actually facilitates transport of particles towards this zone. Moreover, the fact that particles as small as 80 nm are still efficiently captured in our device is not only due to Brownian transport but also to an effective polarizability that is larger than what would be expected on basis of current theory for a sphere in a dielectric medium.

Electrical and optical characteristics of boron doped nanocrystalline diamond films

Boron doped diamond is a prospective material which can be used as a conductive and optically transparent thin-film electrode in a variety of optoelectronic applications. In this work, we present the results of the temperature resolved electrical conductivity, optical reflection, transmission and absorption of thin boron doped nanocrystalline diamond films grown by a microwave plasma enhanced chemical vapor deposition. Optical transmittance, reflectance and absorptance properties of layers were studied by a photo-thermal deflection spectroscopy analysis. Raman spectroscopy with various excitation wavelengths was employed for the analysis of nanocrystalline diamond layers. The measured position, shift and broadening of the characteristic boron doped diamond Raman lines were used for the determination of the boron concentration. Correlation between the results of the atomic boron concentration estimated via the Raman analysis and measured electrical conductivity values is presented.

Transparent Patch Antenna Using Metal Mesh

This communication presents the design, fabrication, and measurement of transparent antennas with metal mesh (MM) for wireless transparent applications such as smart windows, transparent mobile devices, and transparent Internet of Things. The MM was proposed as a transparent electrode with high conductivity in electrical performance, with proper transparency in optical performance, and low fabrication cost compared to a traditional transparent thin film electrode. To analyze the performance of a transparent antenna fabricated by MM, the antennas were designed to operate in the 2.4–2.5 GHz WLAN band (802.11b). The conductive metal of transparent MM patch antennas was designed using a square lattice structure and was fabricated using wired MM with copper wire (0.2–0.5 mm). In addition, micro-MM film consisting of thin copper wire ( $20~\mu \text{m}$ ) was used to fabricate transparent patch antennas. The realized gain, efficiency, front-to-back ratio, and radiation patterns of the transparent MM patch antennas are discussed for various combinations of radiator and ground planes. Overall, the transparent MM patch antennas offer optical transparency, maintain sufficient performance, and could possibly be used in wireless transparent applications.

Optically Transparent Thin-Film Electrode Chip for Spectroelectrochemical Sensing.

A novel microfabricated optically transparent thin-film electrode chip for fluorescence and absorption spectroelectrochemistry has been developed. The working electrode was composed of indium tin oxide (ITO); the quasi-reference and auxiliary electrodes were composed of platinum. The stability of the platinum quasi-reference electrode was improved by coating it with a planar, solid state Ag/AgCl layer. The Ag/AgCl reference was characterized with scanning electron microscopy and energy-dispersive X-ray spectroscopy. Cyclic voltammetry measurements showed that the electrode chip was comparable to a standard electrochemical cell. Randles-Sevcik analysis of 10 mM K3[Fe(CN)6] in 0.1 M KCl using the electrode chip gave a diffusion coefficient of 1.59 × 10-6 cm2/s, in comparison to the value of 2.38 × 10-6 cm2/s using a standard electrochemical cell. By using the electrode chip in an optically transparent thin-layer electrode (OTTLE), the absorption based spectroelectrochemical modulation of [Fe(CN)6]3-/4- was demonstrated, as well as the fluorescence based modulation of [Ru(bpy)3]2+/3+. For the fluorescence spectroelectrochemical determination of [Ru(bpy)3]2+, a detection limit of 36 nM was observed.

(Invited) Characterization of an Optically Transparent Thin-Film Electrode Chip for Spectroelectrochemical Sensors

In this work, an optically transparent thin-film electrode chip is investigated for applications in electrochemical and spectroelectrochemical sensing. Compared to other electrode chips that are commercially available, this chip is designed for use with a miniature, portable system to take quick, accurate point of care measurements as well as long-term usage, such as environmental monitoring. The electrode chip is fabricated from standard photolithography and thin-film deposition methods over 1.1 mm thick boro-aluminosilicate glass substrates. The use of glass as a substrate minimizes fluorescence background interferences that would be seen with plastic substrates. The chip has three electrodes: an indium tin oxide (ITO) working electrode (WE, 200 nm), a platinum quasi-reference electrode (RE, 200 nm), and a platinum auxiliary electrode (AE, 200 nm), as shown in Fig. 1a. The optically transparent electrode (OTE) ITO allows for spectroscopic detection using both absorption and fluorescence. The surface of the electrode area is defined by applying a protective photoresist over the chip, leaving the three-electrode area and the electrode leads exposed. The dimensions of the electrode area are 8 mm × 40 mm. The surface area of the working electrode is 7 mm2; the reference is 1 µm2; and the auxiliary is 39 mm2. To improve the stability of the quasi-reference electrode, the platinum reference surface was coated with a planar Ag/AgCl layer. This was done by electroplating silver over the platinum reference electrode using 0.3 M AgNO3 in 1 M NH3­. The electroplated surface was then covered with a solution of 50 mM FeCl3 to chemically oxidize AgCl on top of the silver. Open circuit measurements against a commercial Ag/AgCl reference indicate that the potential of the planar Ag/AgCl reference on the electrode chip varies a maximum 1 mV within the same time span for measurements of the platinum reference electrode to vary nearly 1 V. Cyclic voltammetry measurements of 0.1 mM [Fe(CN)6]3- (0.1 M KCl) using the electrode chip with the planar Ag/AgCl reference shows stable voltammograms over 180 cycles. The formal reduction potential (Eo′) of [Fe(CN)6]3- using the planar reference is Eo′ = 102 mV (0.1 M Cl-). This is 83 mV less than the potential using the commercial reference (Eo′ = 185 mV, 3 M Cl-), as is expected due the difference in chloride ion concentration that the Ag/AgCl electrode is exposed to. To demonstrate that the electrode chip is comparable with a standard electrochemical cell (ITO slide WE, liquid junction Ag/AgCl RE, Pt wire AE), Randles-Sevcik analysis of 10 mM [Fe(CN)6]3- (0.1 M KCl) was executed using both electrochemical systems. The diffusion coefficient of [Fe(CN)6]3- using the electrode chip and standard cell was 1.59 × 10-6 cm2/s and 2.38 × 10-6 cm2/s, respectively. Due to the values being in the same order of magnitude, the electrode chip with the planar Ag/AgCl reference electrode is able to function similarly to the standard three-electrode cell. To demonstrate the spectroelectrochemical applications of the electrode chip, the chip is used in an optically transparent thin-layer electrode (OTTLE) cell for the modulation of [Ru(bpy)3]2+ (1 M KCl) fluorescence. The OTTLE cell was constructed by laying a 15 × 8 × 1 mm quartz slide over the electrode area of the chip. The chip and the quartz slide are separated by 177.9 nm thick silicone spacers. The spectroscopic signal of [Ru(bpy)3]2+ is modulated using potentiostatic techniques. A potential of 0.8 V is applied to hold the analyte in the emissive Ru2+ oxidation state. The potential was then stepped to 1.3 V to convert the analyte to the non-emissive Ru3+ oxidation state. The result of the spectroelectrochemical modulation of 2.0 µM [Ru(bpy)3]2+ (1 M KCl) is shown in Fig. 1b. For calibration, the Δemission is plotted against the concentration of [Ru(bpy)3]2+, from 0 – 5 µM, to yield a detection limit of 36 nM. In conclusion, the electrochemical applications of the electrode chip are determined to be comparable to a standard electrochemical cell. The performance of the chip is improved laying a planar Ag/AgCl electrode over the platinum quasi-reference. The OTE electrode on the chip allows for spectroelectrochemical detection of target species. This low-cost, yet robust electrode chip is an ideal candidate for short-term measurements and extended monitoring. Acknowledgments This work was supported by the Department of Energy STTR Grant DE-SC0009718. Figure 1

Lithium Ion Thin-Film Electrodes for All-Solid-State and Electrochromic Applications

Establishing thin-film technologies for lithium ion batteries is an important step towards all-solid-state batteries. Solid thin-film electrodes expose significantly higher charge and discharge rates than standard composite electrodes, thus have the potential to increase the power density of thin-film batteries. Sol-gel procedures are established in battery research for synthesizing active material powders with controlled particle sizes. Here, we prepared single- and multi-layer films of active materials viadip-coating method. Both electrode single-layer systems exhibit excellent electrochemical properties such as high reversibility and low polarization in long-term CV measurements and show almost full capacity retention at charge and discharge currents up to 100 C. Furthermore, applied on transparent conducting oxides, the electrodes were of high optical quality and showed intense color changes upon charging and discharging which positions them as interesting electrochromic compounds. Both materials are well known for their high reversibility and low cost compared to commonly known electrochromic metal oxides such as tungsten oxide. The combination of their highly stable and very fast cycling capabilities with their color switching properties, make the presented transparent thin film electrodes promising candidates for high-power all-solid-state and electrochromic applications. This paper shows transparent Li4Ti5O12 and LiMn2O4 thin film electrodes characterized by the above mentioned electrochemical measurements and their optical properties with the help of UV-vis spectroscopy. Additionally, material characterization is demonstrated via X-ray diffraction, XPS and SEM imaging. Figure 1

Stretchable and transparent electrodes based on in-plane structures.

Stretchable electronics has attracted great interest with compelling potential applications that require reliable operation under mechanical deformation. Achieving stretchability in devices, however, requires a deeper understanding of nanoscale materials and mechanics beyond the success of flexible electronics. In this regard, tremendous research efforts have been dedicated toward developing stretchable electrodes, which are one of the most important building blocks for stretchable electronics. Stretchable transparent thin-film electrodes, which retain their electrical conductivity and optical transparency under mechanical deformation, are particularly important for the favourable application of stretchable devices. This minireview summarizes recent advances in stretchable transparent thin-film electrodes, especially employing strategies based on in-plane structures. Various approaches using metal nanomaterials, carbon nanomaterials, and their hybrids are described in terms of preparation processes and their optoelectronic/mechanical properties. Some challenges and perspectives for further advances in stretchable transparent electrodes are also discussed.

Inkjet-printed, flexible, and photoactive thin film strain sensors

In an effort to improve the resilience and reliability of infrastructure systems, many advanced sensors have been developed for structural health monitoring. However, these sensors often suffer from high energy demand, large form factors, and have difficulties conforming to complex structural surfaces. Recently, a self-sensing poly(3-hexylthiophene)–based thin film sensor was proposed, and it was shown that light illumination generates an electrical current that was also proportional to applied strains. However, its strain sensing range was limited largely due to the brittleness of the electrodes. In this study, inkjet printing was employed for printing flexible polymer electrodes that enhanced fabrication scalability and its strain sensing range. First, a commercial off-the-shelf Canon inkjet printer was customized for electrode fabrication. Second, a conductive ink solution was prepared for printing the conductive and transparent thin film electrodes. Then, the printed thin film electrodes were characterized for their electrical conductivity, optical transmittance, profile (i.e. roughness and thickness), and piezoresistivity. Finally, poly(3-hexylthiophene)–based strain sensors were fabricated using these electrodes, and strain sensing was validated to 2% tensile strains.

Investigation of aluminum–gallium co-doped zinc oxide targets for sputtering thin film and photovoltaic application

Aluminum (Al) and gallium (Ga) co-doped ZnO (AGZ) powder was synthesized by a chemical co-precipitation method and compacted to form targets. In order to demonstrate the effects of Ga additive and analyze the discrepancy between AGZ and Al doped ZnO (AZO), the microstructure, electrical properties and densification of sintered targets were investigated. Further research was carried out on the properties of AGZ thin film and its application on nano-crystalline Si (nc-Si) solar cells. Results showed that Al and Ga co-doped ZnO ceramic targets have higher densities and lower resistivities than Al doped ZnO ceramic targets with the same doping concentration. Similarly, AGZ thin films have lower resistivities than AZO thin films deposited under the same conditions. In addition, Al–Ga co-doping leads to higher carrier concentration and broader optical band gap. The nc-Si solar cell coated with AGZ transparent thin film electrode exhibits higher conversion efficiency.

ITO nanowires and nanoparticles for transparent films

Abstract

Aligned TiO2 nanocolumnar layers prepared by PVD-GLAD for transparent dye sensitized solar cells

Transparent thin film electrodes made of vertically aligned nanocolumns of TiO2 with well-controlled oblique angles were grown by physical vapor deposition at glancing incidence (PVD-GLAD). For an electrode thickness of 500 nm, we report a 40% variation on solar cell efficiency (from 0.6% to 1.04%) when the deposition angle was modified between 60° and 85°. Transparent thicker films with higher surface area deposited at the optimal angle of 70° were grown with a zigzag morphology which confers high mechanical strength to the thin films. Using this topology, the application of an electrode thickness of 3 µm in a DSC resulted in a power conversion efficiency of 2.78% maintaining electrode transparency.

High-energy, efficient and transparent electrode for lithium batteries

In this work, we describe for the first time a high-energy and transparent thin film electrode made from nanosized LiFeO2 grown on an indium tin oxide (ITO) substrate and in situ mixed with submicronic grains of Ag homogeneously distributed to improve the conductivity. An accurate description of its texture and structure was accomplished by combining various techniques such as XPS, SEM, TEM, EDX and AFM. The electrode has the ability to deliver capacity values above 160 Ah kg−1 upon extensive cycling with capacity retention near to 98%, an average voltage of 3.0 V vs. Li+/Li and a specific energy close to 410 W h kg−1 as a result.

THE DEVELOPMENT OF ITO FILM ELECTRODES FOR OPTOELECTRONIC DEVICES

For creation of transparent thin film electrodes the influence of target composition, specific magnetron power and substrate temperatures on the crystalline structure, electrical, optical properties and the quality factor ITO films have been investigated. The ITO films were obtained by the nonreactive magnetron deposition on direct current. The maximal quality factor F = 4.79∙10-2 Ohm-1 of ITO films is observed at the sputtering of target consisted of 90 weight. % In2 O3 and 10 weight. % SnO2 : the substrate temperature was 300 î Ñ and specific magnetron power – 0.4 W/sm2 .

Functionalized Mesoporous Silica Films as a Matrix for Anchoring Electrochemically Active Guests

Mesoporous silica thin films were shown to be an appropriate matrix for immobilization of discrete electroactive moieties, yielding uniform transparent thin film electrodes with defined texture and enhanced electrochemical activity. The mesoporous silica films prepared on conducting FTO-coated glass substrate were postsynthetically functionalized. Alkoxysilanes were used as precursors for subsequent grafting via ionic or covalent bonds of representative electroactive species, such as polyoxometalate PMo12O(40)3-, hexacyanoferrate(III), and ferrocene. The electrochemically active concentration within the silica-based composite electrodes achieves 90, 260, and 60 micromol cm(-3) for polyoxometalate, hexacyanoferrate(III), and ferrocene, respectively. The amount of molecules involved in the charge-transfer sequence is proportional to the film thickness and comparable to the total amount of embedded guests. Thus, eventually the whole bulk volume of the modified silica films is electrochemically accessible. Immobilization in the chemically modified silica matrix alters the redox potential of the electroactive molecules. Electron exchange between the adjacent redox centers (electron hopping) is proposed as a possible charge propagation pathway through the insulating silica matrix, which is supported by the fact that the high charge uptake is observed also for the hybrid electrodes with the covalently anchored redox guests.