Transparent Conducting Electrode Applications Research Articles

Influence of Er substitution on the properties of ZnO: A comprehensive study

Er-doped ZnO (Zn(1-x)Er(x)O) films with (002) preferential orientated wurtzite structure were fabricated on a c-plane sapphire substrate by pulsed laser deposition (PLD). The effects of Er composition, oxygen pressure during growth, and post-growth annealing on the films' structural, electrical, and optical properties were studied by employing a comprehensive spectroscopic approach. Er-doped ZnO films with a resistivity of 4 × 10−4 Ωcm, electron conductivity n+∼1021 cm−3, and high optical transmittance (>90%) were disclosed to be appropriate candidates for transparent conducting electrode applications. A shallow donor is reported to be associated with the ErZn defect, which is optically inactive. Er-doped ZnO samples grown with high Er composition and oxygen pressure don't show n+ conductivity because of the formation of an O-rich ErZn-related compensating defect. Annealing at 750 °C converts the ErZn shallow donor to another defect configuration, which is optically active for intra-4f-shell transition emissions, but no longer a shallow donor. The growth condition for maximizing the intra-4f-shell transition emission was also obtained.

Fabrication of junction-free Cu nanowire networks via Ru-catalyzed electroless deposition and their application to transparent conducting electrodes

Over the past few years, metal nanowire networks have attracted attention as an alternative to transparent conducting oxide materials such as indium tin oxide for transparent conducting electrode applications. Recently, electrodeposition of metal on nanoscale template is widely used for formation of metal network. In the present work, junctionless Cu nanowire networks were simply fabricated on a substrate by forming a nanostructured Ru with 80 nm width as a seed layer, followed by direct electroless deposition of Cu. By controlling the density of Ru nanowires or the electroless deposition time, we readily achieve desired transmittance and sheet resistance values ranging from ∼1 kΩ sq−1 at 99% to 9 Ω sq−1 at 89%. After being transferred to flexible substrates, the nanowire networks exhibited no obvious increase in resistance during 8000 cycles of a bending test to a radius of 2.5 mm. The durability was verified by evaluation of its heating performance. The maximum temperature was greater than 180 °C at 3 V and remained constant after three repeated cycles and for 10 min. Transmission electron microscopy and x-ray diffraction studies revealed that the adhesion between the electrolessly deposited Cu and the seed Ru nanowires strongly influenced the durability of the core–shell structured nanowire-based heaters.

Surface morphology and optical properties of Ca and Mn doped TiO2 nano-structured thin films

This study was conducted to investigate the impact of Calcium (Ca) and Manganese (Mn) dopants on the optical and surface properties of Titanium dioxide (TiO2) nanomaterial. Ca and Mn doped TiO2 thin films were fabricated by sol–gel spin-coating method. By keeping the doping ratio of Ca constant, Mn was doped at the ratios of 1, 3, and 5%. The morphological, structural and optical properties of the fabricated samples were taken by Atomic Force Microscope (AFM), Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Analysis (EDX), X-Ray Diffraction Method (XRD) and UV–VIS spectroscopy. AFM and SEM analysis revealed that the morphological properties of TiO2 changed depending on the increase in doping ratio of Mn. XRD values obtained from the samples showed that the increases of crystallite size with manganese ions doping into the TiO2 the optical band gap of the samples increased with Ca and Mn dopants. As a result of the investigations, it was determined that Mn doping has significant effects on the morphological and optical properties of TiO2 films and the produced samples can be used in transparent conductive electrode applications, optoelectronic devices, and sensor production.

High transparent, low surface resistance ZTO/Ag/ZTO multilayer thin film electrodes on glass and polymer substrates

Zinc tin oxide (ZTO)/Ag/ZTO multilayer thin films were grown by direct current (DC) magnetron sputtering technique at room temperature on soda lime glass (SLG) and different polymer substrates such as polycarbonate (PC) and polyethylene terephthalate (PET) for transparent conductive electrode (TCE) applications. The effect of substrate on the structural, optical and electrical characteristics of ZTO/Ag/ZTO multilayers was investigated. All prepared ZTO/Ag/ZTO films presented amorphous structure as expected from room temperature deposition process and smooth surface quality with very low surface roughness. We found that ZTO/Ag/ZTO multilayer films grown on SLG, PET and PC substrates have very high optical transmission and low surface resistance. Moreover, after ZTO/Ag/ZTO multilayer thin film deposition on polymer substrates, the optical transmission was found to be enhanced because the higher absorption due to Ag layer is compensated by lower reflectance. Our results suggest that ZTO/Ag/ZTO multilayer thin films on any substrate can be a promising alternative to indium tin oxide (ITO) films as a cost-effective, indium-free, flexible and transparent electrode for various applications.

A one-step synthesis of ultra-long silver nanowires with ultra-high aspect ratio above 2000 and its application in flexible transparent conductive electrodes

Silver nanowires (AgNWs), appear as an extremely promising candidate for the next generation of flexible transparent conductive electrodes (FTCEs). However, the performance of AgNWs-FTCEs was severely limited by the aspect ratio of AgNWs, while it was still a big challenge to fabricate AgNWs with high aspect ratio nowadays. To improve the aspect ratio of AgNWs, bromide ion (Br−), cupric ion (Cu2+) and polyvinylpyrrolidone (PVP, Mw ≈ 1300 000) which are beneficial for the synthesis of high aspect ratio AgNWs, were introduced in this article. The high quality and uniform AgNWs with the average diameter of 77.6 nm and the aspect ratio above 2000 were fabricated via a one-step solvothermal method. The effects of reaction time, molar ratio of AgNO3 to PVP and the concentration of CuBr2 on the aspect ratio of AgNWs were discussed. The mechanism of the synthesis of high aspect ratio AgNWs was explored. After that, the prepared AgNWs were spin-coated on the surface of PET film, the FTCEs based on the ultra-high aspect ratio AgNWs without any post-treatments exhibits relatively high transmittance, low haze and low sheet resistance, and the AgNWs have little effect on the optical performance of pristine PET film. The outstanding performance of the prepared FTCEs indicated that the ultra-high aspect ratio AgNWs are ideal materials in the application of FTCEs, and the method of fabricating AgNWs could provide a direction to the high aspect ratio AgNWs.

Correction to: Aspect Ratio Control of Copper Nanowire via Solution Process and Its Flexible Transparent Conductive Electrode Applications

In the original publication of the article, the Acknowledgements section was published incorrectly. The correct Acknowledgements section is given in this Correction.

Aspect Ratio Control of Copper Nanowire via Solution Process and Its Flexible Transparent Conductive Electrode Applications

Controlling aspect ratio of copper (Cu) nanowire (NW) is a vital role to determine the conductivity at a high transmittance for the transparent conductive electrode application, but systemic studies on aspect ratio control of Cu NW have been still in early stages. Herein, we systemically explore the aspect ratio of Cu NW by varying solution process parameters including reaction time and the concentration of additives (Cu precursor, reducing agent, and capping agent). From optimized process parameters, we successfully synthesize Cu NWs showing the high aspect ratio of ~ 1570, which is relatively comparable than that of previous reports. We characterize the sheet resistance with different transmittances, and consequently achieve excellent sheet resistance of 46 Ω sq−1 at transmittance = 93%, similar to that of indium tin oxide-based electrodes. Obtained Cu NW films also show stable bending reliability with applying bending radius of 6.5 mm. Such remarkable performances are attributed to high aspect ratio of our Cu NWs, which lead to reduced junction resistance in films.

Synthesis of Gallium-Doped Zinc Oxide (GZO) Nanoparticles for GZO/Silver Nanowire Nanocomposite Transparent Conductive Electrodes

Gallium (Ga)-doped zinc oxide nanoparticles (GZO-NPs) have been synthesized using a heating-up method. The Ga doping concentration strongly affected the particle size, distribution, and bandgap energy of the GZO-NPs. When the Ga concentration was increased from 0% to 5%, the average size of the GZO-NPs decreased from 57 nm to 16 nm while the bandgap increased from 3.14 eV to 3.26 eV. On further increase of the Ga content to 7% and 9%, the particle size increased while the bandgap narrowed. The GZO-NPs synthesized with 5% Ga showed the best uniformity and smallest average diameter of approximately 16 nm. The GZO-NPs with 5% Ga were applied to reduce the mechanical contact between the AgNWs in GZO/silver nanowire (AgNW) composite for application as a transparent conductive electrode, yielding RSH = 18.1 Ω/□, T = 77.8% at 550 nm, and σDC/σOp = 77.53. These results indicate that such GZO-NPs are a very promising nanocrystalline ink precursor for printing thin films for application in nanocomposite transparent conductive electrodes.

Sol-gel synthesized spin coated GO: ZnO composite thin films: optical, structural and electrical studies

In the present work, we report on the structural, optical and electrical properties of solution-processed transparent conducting Graphene Oxide and Zinc oxide (GO: ZnO) composite thin films. Eco-friendly top-down modified Hummer’s method was adopted for the synthesis of GO. The ZnO and GO:ZnO thin films were grown on the glass substrates employing a simple and cost-effective spin coating technique. To get a stable solution for spin coating, an efficient solvent, 2-methoxyethanol is used to disperse GO. The structural properties of the GO: ZnO thin films were analyzed using XRD pattern wherein the average crystallite size decreases with the increase in GO% in ZnO. UV–vis spectroscopy studies showed that the optical band gap and the transmittance (>80%) of thin films are decreased with the increment of GO% in ZnO. FESEM and XRD data revealed a decrease in the particle size. GO: ZnO composite thin films exhibit an ohmic nature when silver is used as a contact with increase in current in composite thin films compared to ZnO thin film. The increase in current is due to the removal of oxygen moieties in GO and partial restoration of the graphene structure occurred during the pre and post-annealing of thin films. Thus, the reduced graphene oxide (rGO) acts as a conductive frame for ZnO. The optical and electrical parameters procured for GO: ZnO composite thin films propel the possibility of using composite thin films for transparent conducting electrode applications.

The effects of thermal annealing on the structural and electrical properties of zinc tin oxide thin films for transparent conducting electrode applications

The as-deposited and annealed Zinc tin oxide (Zn2SnO4) thin films, deposited by electron beam evaporation technique, were characterized for the structural, optical and electrical properties. The x-ray diffraction technique revealed the amorphous nature of as-deposited thin film while the films annealed at 400, 500 and 600 °C in air were found to be polycrystalline in nature. The phase change from amorphous to crystalline Zn2SnO4 resulted in the higher resistance as revealed by resistance versus temperature measurements. From the Hall effect measurements, the as-deposited film shows the electron mobility and carrier concentrations (electron) equal to 33 cm2/V.s and 8.361 × 1017cm−3 respectively. The agglomeration of grains in annealed thin films are observed by Atomic Force Microscopy (AFM) technique. The peaks in Optical transmission spectra, observed by using the UV–Vis spectroscopy confirm the creation of sub levels between conduction band minimum and valence band maximum after annealing. The band gaps calculated by Tauc plot explore the possibility of shifting the Fermi level towards valence band maximum after thermal annealing.

Bioinspired Metal Mesh Structure with Significant Electrical and Optical Properties

Abstract1D metal network is one of the promising alternatives of the conventional transparent conducting electrode (TCE) viz ITO/FTO. Bioinspired fabrication of junction resistance free metal grid structures is explored for their TCE applications. The leaf‐based metal grid electrodes exhibit low DC resistivity (10−5–10−4 Ω cm) and high optical transparency (80–90%) in the visible spectrum. The leaf vein structures are varied to obtain better optoelectronic properties. Finally, a multilayered stacked structure (SnO2‐Ag‐Au‐SnO2) is deposited on a poly ethyl terephthalate substrate and low‐ to high‐temperature electrical stability is demonstrated. X‐ray photoelectron spectroscopy results are used to investigate the structural study of the leaf‐based 1D structure. A minimal variation in Rs values is seen even after 1000 times bending for the flexible transparent structure. It is demonstrated that the specific leaf vein–based metal structures can attain good optical, electrical, and mechanical performance.

Synthesis of Pd:ZnO nanofibers and their optical characterization dependent on modified morphological properties

In this study, Pd:ZnO thin films were grown on microscope glass by the sol-gel spin coating method to investigate and improve interaction between surface properties and optical properties. Morphology of the obtained nano-structured thin films was examined using Atomic Force Microscopy (AFM). AFM images showed that the samples were formed in the nanogranule and fiber form. The size and shapes of the fibers varied depending on the Pd concentration. The effect of Pd doping on the optical properties of ZnO films was investigated by spectrophotometric measurements. Optical constants (refractive index, n, and absorption index, k) of the samples were obtained in the wavelength range of 300–800 nm. Dispersion parameters were determined and discussed according to the single oscillator model. As a result of the investigations, it was determined that Pd doping has significant effects on the morphological and optical properties of ZnO films and the produced samples can be used in transparent conductive electrode applications, optoelectronic devices, and sensor production.

Influence of deposition rate on the structural, optical and electrical properties of electron beam evaporated SnO2 thin films for transparent conducting electrode applications

The role of deposition rate in the structural, optical and electrical properties of SnO2 thin films deposited by electron beam evaporation method is investigated by varying the deposition powers viz. 50, 75, and 100 W. The structural characterization of the films is done by X-ray diffraction (XRD) technique. The surface morphology of the films is studied by scanning electron microscopy (SEM). Rutherford back scattering (RBS) measurements revealed the thickness of the films ranging from 200 nm to 400 and also a change in the concentration of oxygen vacancies which is found to be the maximum in the film deposited at the lowest deposition rate. Optical absorption spectrum is recorded using the UV–V is spectroscopy and the films are found to be transparent in nature. A shift in the absorption edge is observed and is attributed to a different level of allowed energy states in conduction band minimum. The Hall effect and electrical measurements show a variation in the carrier concentrations, mobility and resistivity of the films. In order to explore a better compromise in electrical and optical properties for transparent electrode applications, skin depths calculations are also done to find the optimized values of carrier concentration and mobility.

Effect of Ag incorporation on the electrical and optical properties of AZO/Ag/AZO multilayer transparent conducting electrode

Silver (Ag) incorporated aluminium doped zinc oxide (AZO) based multilayer films were fabricated for transparent conducting electrode applications. AZO and Ag thin film layers were deposited using direct current magnetron sputtering system without breaking the vacuum. By carefully optimizing the thickness of the metal layer, the minimum sheet resistance of 6 Ω/square was achieved with 70% of transmission in the visible wavelength region. The surface morphology of the film was studied with atomic force microscope mapping in 5 µm × 5 µm area of the multilayer structure. The significance of the Ag layer thickness in determining the electrical and optical properties of the multilayer film was investigated.

A highly flexible transparent conductive electrode based on nanomaterials

The electrical, optical, thermal, chemical, mechanical and tribological characteristics of a highly flexible transparent conductive electrode (HFTCE) coating based on reduced graphene oxide (rGO), carbon nanotubes (CNTs) and silver nanowires (AgNWs) were investigated under various conditions. The motivation was to develop a highly durable and flexible film for transparent conductive electrode applications. The overall characteristics of multilayers based on rGO, CNTs and AgNWs were found to be much better than those of the single-layer AgNW coating. The rGO and CNT layers served to protect the AgNW layer from damage due to bending and contact sliding motions. The contact pressure and bending stress were effectively distributed by the CNT layer deposited on top of the AgNW layer due to its spring-like behavior. In addition, the shear force from the friction force was reduced by the rGO top layer, which acted as a solid lubricant. Furthermore, the excellent performance of an HFTCE heater based on the rGO/CNT/AgNW coating was demonstrated by the results of a defrosting test.

Current distribution in conducting nanowire networks

Conducting nanowire networks find diverse applications in solar cells, touch-screens, transparent heaters, sensors, and various related transparent conducting electrode (TCE) devices. The performances of these devices depend on effective resistance, transmittance, and local current distribution in these networks. Although, there have been rigorous studies addressing resistance and transmittance in TCE, not much attention is paid on studying the distribution of current. Present work addresses this compelling issue of understanding current distribution in TCE networks using analytical as well as Monte-Carlo approaches. We quantified the current carrying backbone region against isolated and dangling regions as a function of wire density (ranging from percolation threshold to many multiples of threshold) and compared the wired connectivity with those obtained from template-based methods. Further, the current distribution in the obtained backbone is studied using Kirchhoff's law, which reveals that a significant fraction of the backbone (which is believed to be an active current component) may not be active for end-to-end current transport due to the formation of intervening circular loops. The study shows that conducting wire based networks possess hot spots (extremely high current carrying regions) which can be potential sources of failure. The fraction of these hot spots is found to decrease with increase in wire density, while they are completely absent in template based networks. Thus, the present work discusses unexplored issues related to current distribution in conducting networks, which are necessary to choose the optimum network for best TCE applications.

The effect of Cu on the properties of CdO/Cu/CdO multilayer films for transparent conductive electrode applications

Transparent conductive CdO/Cu/CdO multilayer films were prepared using rf plasma magnetron sputtering and electron beam evaporation techniques. The CdO layers were prepared using rf plasma magnetron sputtering, while the Cu interlayer was prepared by electron beam evaporation technique. The Cu layer thickness was varied between 1 and 10 nm. The structural and optical properties as well as the sheet resistance of the multilayer films were studied. X-ray diffraction measurements revealed the presence of cubic CdO structure and the Cu peak was only observed for the multilayers prepared with 10 nm of Cu. It has been observed that the Cu interlayer thickness has a great influence on the optical and electrical properties of the multilayers. The transmittance of the multilayer films decreased while the reflectance increased with increasing Cu interlayer thickness. The refractive index and the extinction coefficient of the multilayer films were calculated. The estimated optical band gap values were found to be decreased from 2.75 ± 0.02 to 2.40 ± 0.02 eV as the Cu interlayer thickness increased from 1 to 10 nm. The sheet resistance was sensitive to the Cu interlayer thickness and it decreased with increasing Cu interlayer thickness. A sheet resistSSance of 21.7 Ω/sq, an average transmittance (between 700 and 1000 nm) of 77%, and an optical band gap of 2.5 ± 0.02 eV were estimated for the multilayer film with 2 nm Cu layer. The multilayer film with 2 nm Cu layer has the highest figure of merit value of 3.2 × 10−3 Ω−1. This indicates that the properties of this multilayer film are suitable for transparent conductive electrode applications.

Electrical Properties of the Multilayered Graphene in Terms of Carrier Density and Mobility

Due to the graphene superior electrical mechanical and optical properties, graphene is particularly advantageous for flexible transparent conductive electrode applications. However, its sheet resistance still higher than indium tin oxide has been a critical problem. Increasing of graphene layers is well known and basic method for sheet resistance reduction. But, this enhancement was not fully explained in terms of carrier density and mobility which are the elements of the sheet resistance. Because of the graphene electrical property is strongly influenced by surface condition, each graphene layer in the multilayer might be has different electrical property. Therefore, simple measuring the sheet resistance gives insufficient information about the origin of sheet resistance decreasing. By understanding the number of layer and surface condition effect on carrier density and mobility, we can get the potential pathway to reduce the sheet resistance. So, separated measurement of electrical property is required. In this work, enhancement of electrical properties with increasing the number of graphene layer is systemically investigated by Hall measurement which can divide sheet resistance into the carrier mobility and density. In order to get multilayer graphene with the different interface condition, two different layer stacking methods were introduced. Single layer graphenes were synthesized by chemical vapor deposition and stacked the graphene with and without benzimidazole which strongly p-dopes the graphene. 1 to 4 layers of graphene were stacked and analyzed. The sheet resistance of inter-layer undoped and doped graphene are reduced from ~700 to ~220 Ω/sq and ~280 to ~80 Ω/sq, respectively. Both sheet resistances are inversely proportional to the number of layers. However, carrier mobility and density show clearly different characteristics. Regardless of the number of graphene layers, carrier density of inter-layer undoped graphene is maintained in the range of ~8 x 1012 /cm2, but it is linearly increased from ~1.8 x 1013 to ~6.2 x 1013 /cm2 when inter-layer was chemically doped. The carrier mobility of inter-layer doped graphene remained same in 1200 cm2/Vs but, that of undoped graphene is increased from 1000 to 3000 cm2/Vs. Because the major carrier generation of undoped graphene occurred only on the top of the layer, it has constant carrier density regardless of the number of graphene layer. Compare to the top layer, the layers inside undoped graphene have a much smaller carrier density it leads less charge carrier interaction and high carrier mobility. On the other hands, inter-layer doped graphene has uniform carrier density in inside each layer. So, it result the uniform carrier mobility and linearly increasing carrier density depending on the number of graphene layers. In addition, by stacking the various coverage of graphene which are the 16, 38, 84 and 100 % onto the full coverage monolayer graphene, we checked the partial coverage additional layer effect even without the second layer conducting path. The sheet resistance is gradually decreases from ~700 to ~350 Ω/sq by increasing the second layer coverage. The carrier density show uniform value of ~8 x 1012 /cm2 in all coverage, but mobility linearly increases from 1040 to 2288 cm2/Vs. Increased mobility leads the sheet resistance decreases. We find out that the origin of sheet resistance reduction in the multilayer graphene is different depending on the surface condition. This demonstration would be very useful for the use of graphene as a flexible transparent conductive electrode, and in other electrical applications.

Orthogonally weaved silver nanowire networks for very efficient organic optoelectronic devices

We demonstrate a simple but effective method to control the orientation of silver nanowires (AgNWs). Shear-flow-induced AgNW preferable orientation is realized by judiciously controlling the process parameters in the bar-coating method. This controllability of the NW direction enables the formation of AgNW cross-linking networks for transparent conductive electrode (TCE) applications. We experimentally demonstrate that the orthogonally weaved AgNW networks possess predominant advantages of lower percolation limit, higher transmission, and lower sheet resistance compared with the randomly orientated AgNW counterparts. The phenomenon is also confirmed with theoretical calculation by the Monte Carlo method. These high-quality AgNW TCEs exhibit a high transmittance of ∼94% with a sheet resistance of ∼20 Ω/sq, which meet the requirements of modern optoelectronic devices. Very efficient organic light-emitting diodes (OLEDs) and organic solar cells (OSCs) prepared by these AgNW TCEs are demonstrated. The OLED exhibits exceptionally high luminance efficiency, power efficacy, and external quantum efficiency of 92 cd/A, 111 lm/W, and 26.8%, respectively. The OSCs also deliver a high power conversion efficiency of up to 7.5%.

Direct Chemical-Vapor-Deposition-Fabricated, Large-Scale Graphene Glass with High Carrier Mobility and Uniformity for Touch Panel Applications

In this work, we report the transfer-free measurement of carrier dynamics and transport of direct chemical vapor deposition (CVD) grown graphene on glass with the aid of ultrafast transient absorption microscopy (TAM) and demonstrate the use of such graphene glass for high-performance touch panel applications. The 4.5 in.-sized graphene glass was produced by an optimized CVD procedure, which can readily serve as transparent conducting electrode (TCE) without further treatment. The graphene glass exhibited an intriguing optical transmittance and electrical conductance concurrently, presenting a sheet resistance of 370-510 Ω·sq-1 at a transmittance of 82%, much improved from our previous achievements. Moreover, direct measurement of graphene carrier dynamics and transport by TAM revealed the similar biexponential decay behavior to that of CVD graphene grown on Cu, along with a carrier mobility as high as 4820 cm2·V-1·s-1. Such large-area, highly uniform, transparent conducting graphene glass was assembled to integrate resistive touch panels that demonstrated a high device performance. Briefly, this work aims to present the great feasibility of good quality graphene glass toward scalable and practical TCE applications.