High-performance Transparent Conductors Research Articles

Facile Fabrication of Transparent Conductor of Cu Nanowire-PEDOT:PSS

Facile Fabrication of Transparent Conductor of Cu Nanowire-PEDOT:PSS

Flexible, transparent and conductive wearable electronic skin based on 2D titanium carbide (MXene) ink

Flexible and transparent conductors are essentially required in epidermal wearable devices such as electronic-skins (E-skins) that facilitate human interaction and biofeedback. However, due to the “trade-off” effect between high electrical conductivity and optical transparency, it is difficult to obtain transparent E-skins with high electrical conductivity. Furthermore, traditional transparent conductor is bulky and brittle which is not suitable for flexible and wearable applications. Herein, ultrathin two-dimensional (2D) titanium carbide (MXene) nanosheets with metallic conductivity, hydrophilic surface and excellent mechanical flexibility was proposed as a novel high-performance transparent conductor, which was further developed as wearable E-skins. Solution-processable MXene conductive ink was obtained by facilely etching Al from MAX phase for large-scale spray coating to obtain flexible transparent conductors. The flexible, transparent, and conductive MXene-based electrodes own a sheet resistance as low as 200 Ω/sq, while with high optical transparency of 60% @550 nm, which is sufficient for the applications of E-skins. Moreover, the transparent MXene-based soft E-skins also exhibits outstanding mechanical flexibility which could maintain its high opto-electrical performance even after 1000 cyclic bending tests. In addition, the robust flexible transparent MXene-based E-skins also could accurately identify the movements of human body including approaching, touching and pressing, which is essentially required in various application scenarios such as human-machine interactions, touch panels and healthcare sensors. The solution-processable flexible, transparent and conductive MXene-based E-skins holds tremendous potential in emerging wearable applications.

Influence of buffer layer on optoelectronic properties of B-Mg co-doped ZnO ultrathin film for high-performance transparent conductor application

In order to develop transparent conductors for optoelectronic devices, B-Mg co-doped ZnO (BMZO) thin films and ZnO/BMZO bilayer films were prepared by using a magnetron sputtering system in this work. The properties of the films with varying operating pressure and buffer layer thickness were measured and investigated systematically. It is found that all films have high transparency, which are almost independent of the buffer layer thickness and the working pressure. Working pressure affects the chance of collision between sputtered particles and gas molecules, which impacts on the average free range of sputtered particles and the energy of the particles. Thus, the BMZO films with high crystallinity and low resistivity can be obtained at appropriate working pressure. The introduction of ZnO buffer layer makes the BMZO films more suitable for TCO applications compared to monolayer BMZO. The results showed that the thickness of the buffer layer with the introduction of ZnO buffer layer decreased the surface roughness and internal stress of the films, which increased the electrical and optical properties of the film. In particular, the ZnO/BMZO film prepared at a working pressure of 3 Pa, exhibits the optimized optoelectrical properties when the buffer layer thickness is 12 nm.

Transparent Conductors Printed from Grids of Highly Conductive Silver Nanosheets.

Transparent conductors (TCs) represent key components in many applications from optoelectronic devices to electromagnetic shielding. While commercial applications typically use thin films of indium tin oxide, this material is brittle and increasingly scarce, meaning higher performing and cheaper alternatives are sought after. Solution-processible metals would be ideal owing to their high conductivities and printability. However, due to their opacity to visible light, such films need to be very thin to achieve transparency, thus limiting the minimum resistance achievable. One solution is to print metallic particles in a grid structure, which has the advantages of high tunable transparency and resistance at the cost of uniformity. Here, we report silver nanosheets that have been aerosol jet printed into grids as high-performance transparent conductors. We first investigate the effect of annealing on the silver nanosheets where we observe the onset of junction sintering at 160 °C after which the silver network becomes continuous. We then investigate the effect of line width and thickness on the electrical performance and the effect of varying the aperture dimensions on the optical performance. Using these data, we develop simple models, which allow us to optimize the grid and demonstrate a printed transparent conductor with a transmittance of 91% at a sheet resistance of 4.6 Ω/sq.

Influence of thermal annealing on the optical and electrical properties of Ag-Al ultrathin film-based sandwich-like transparent conductor

Heating process is always needed and sometimes inevitable in fabricating high-performance transparent conductors. In this work, the thermal stabilities of the optoelectronic properties of the annealed Ag-Al ultrathin film and the sandwich-like structure of TiO2/Ag-Al/aluminum-doped zinc oxide (AZO) are systematically investigated under different annealing temperatures and durations. After annealing at 200 °C for 20 min, the pure Ag layer dewets into isolated islands completely, but the Ag-Al alloy ultrathin film retains continuous film morphology with low sheet resistance, demonstrating alloying with Al element can effectively improve the thermal stability of the Ag layer. However, increasing the annealing duration further upon Ag-Agl alloy film will lead to serious dewetting, agglomeration, and increased resistance. The TiO2/Ag-Al/AZO sample has a high optical transmittance, which can be maintained after annealing at 200 °C for 60 min. Nevertheless, when annealing at 300 °C its sandwiched structure is destroyed as a result of the agglomeration of the Ag-Al layer. The annealed TiO2/Ag-Al/AZO structure has an average transmittance of about 85.4% in the range of 400–700 nm and a sheet resistance of 12.5 Ω.sq−1, indicating the potential for the high-performance transparent conductor application.

Iodine ion modification enables Ag nanowire film with improved carrier transport properties and stability as high-performance transparent conductor

Iodine ion modification enables Ag nanowire film with improved carrier transport properties and stability as high-performance transparent conductor

SrNbO3 as a transparent conductor in the visible and ultraviolet spectra

Few materials have been identified as high-performance transparent conductors in the visible regime (400–700 nm). Even fewer conductors are known to be transparent in ultraviolet (UV) spectrum, especially at wavelengths below 320 nm. Doped wide-bandgap semiconductors employed currently as UV transparent conductors have insufficient electrical conductivities, posing a significant challenge for achieving low resistance electrodes. Here, we propose SrNbO3 as an alternative transparent conductor material with excellent performance not only in the visible, but also in the UV spectrum. The high transparency to UV light originates from energetic isolation of the conduction band, which shifts the absorption edge into the UV regime. The standard figure of merit measured for SrNbO3 in the UV spectral range of 260–320 nm is on par with indium tin oxide in the visible, making SrNbO3 an ideal electrode material in high-performance UV light emitting diodes relevant in sanitation application, food packaging, UV photochemotherapy, and biomolecule sensing.

Synthesis of Ultrathin Metal Nanowires with Chemically Exfoliated Tungsten Disulfide Nanosheets

Transition metal dichalcogenides (TMDs) have attracted great interest owing to their fascinating properties with atomically thin nature. Although TMDs have been exploited for diverse applications, the effective role of TMDs in the synthesis of metal nanowires has not been explored. Here, we propose a new approach to synthesize ultrathin metal nanowires using TMDs for the first time. High-quality ultrathin nanowires with an average diameter of 11.3 nm are successfully synthesized for realizing high-performance transparent conductors that exhibit excellent conductivity and transparency with low haze. The growth mechanism is carefully investigated using high-resolution transmission electron microscopy, and growth of nanowires with tunable diameters is achieved by controlling the nanosheet dimension. Finally, we unravel the important role of TMDs acting as both reducing and nucleating agents. Therefore, our work provides a new strategy of the TMD as an innovative material for the growth of metal nanowires as a promising building block in next-generation optoelectronics.

Mixed polyvinyl pyrrolidone hydrogel-mediated synthesis of high-quality Ag nanowires for high-performance transparent conductors

A novel mixed PVP hydrogel-mediated polyol reduction method was developed for the synthesis of high-quality Ag nanowires with negligible Ag nanoparticles produced, high aspect ratio (exceeding 1200), and a thin (∼1 nm) capping layer.

(Invited) Critical Parameters for GW Scale Perovskite PV Manufacturing

A recent NREL white paper proposing a new US Perovskite Energy Consortium states “Fulfilling the world’s demand for PV requires continued exponential growth of production capacity without exponential growth in capital expenditure and remains a major challenge due to the inadequate capital efficiency of PV using existing technologies. There is an opportunity and a need for disruptive and fundamentally profitable technologies to supplement the ongoing growth of the PV manufacturing industry”. Perovskite technology promises high conversion efficiency, low manufacturing cost, and low capital expense (CapEx) to build manufacturing capacity. Fulfilling that promise requires a manufacturing process with a very high throughput. The ability to fabricate perovskite films using low-temperature, solution based methods has been touted as a path to low cost and low CapEx manufacturing, however fast deposition of the perovskite layer alone is not sufficient to create the throughput that transforms the PV manufacturing landscape; an in-line process that integrates the deposition of all the functional layers of the device, as well as cell interconnects and module lamination, is required. Any slow vacuum deposition step (e.g., sputtering ITO for the transparent conductor) or long thermal annealing step (e.g., forming a charge transport barrier by forming a compact TiO2 layer) commonly used in lab-scale perovskite device fabrication must be avoided. Our cost modeling shows that the throughput of a single line needed to get to disruptive CapEx and manufacturing cost will be on the order of 45 m2/min. That results in over 3GW/year production capacity at conservative uptime and yield values. An integrated in-line roll-to-roll (RtR) process with a 1.5m wide web would require all deposition steps to run at 30 m/min; much faster than processes cited in the academic literature for lab scale devices. That line speed is not taxing to most commercially available RtR systems, but practical limitations on oven size means a tight window for drying and annealing steps of all solution coated layers. The challenges of creating high performance transparent conductors, inorganic charge transport and barrier layers, perovskite layers and cell interconnects at those speeds will be discussed. The implied TAKT time is less than 3 seconds for 2m2 modules coming off of a single production line. The final assembly and test operations take much longer for each module, so methods of dealing with the mismatch must be implemented. We will also describe the practical considerations, including material sourcing and process requirements for production at GW scale.

Direct Room Temperature Welding and Chemical Protection of Silver Nanowire Thin Films for High Performance Transparent Conductors.

Silver nanowire (Ag-NW) thin films have emerged as a promising next-generation transparent electrode. However, the current Ag-NW thin films are often plagued by high NW-NW contact resistance and poor long-term stability, which can be largely attributed to the ill-defined polyvinylpyrrolidone (PVP) surface ligands and nonideal Ag-PVP-Ag contact at NW-NW junctions. Herein, we report a room temperature direct welding and chemical protection strategy to greatly improve the conductivity and stability of the Ag-NW thin films. Specifically, we use a sodium borohydride (NaBH4) treatment process to thoroughly remove the PVP ligands and produce a clean Ag-Ag interface that allows direct welding of NW-NW junctions at room temperature, thus greatly improving the conductivity of the Ag-NW films, outperforming those obtained by thermal or plasmonic thermal treatment. We further show that, by decorating the as-formed Ag-NW thin film with a dense, hydrophobic dodecanethiol layer, the stability of the Ag-NW film can be greatly improved by 150-times compared with that of PVP-wrapped ones. Our studies demonstrate that a proper surface ligand design can effectively improve the conductivity and stability of Ag-NW thin films, marking an important step toward their applications in electronic and optoelectronic devices.

High performance transparent conductor of graphene wrapped copper/nickel microgrids

A high performance, highly stable transparent conducting structure based on microscale copper/nickel grids wrapped with graphene is presented. Graphene is selectively deposited on the surfaces of the microgrids by atmospheric pressure chemical vapor deposition method. The optical transmittance of the copper/nickel microgrid sample is ∼80% over the visible and near-infrared spectra with a very small sheet resistance of ∼0.58 Ω/sq. After the high temperature deposition of graphene, the sample's transmittance increases to be ∼90% due to the line width reduction of the microgrids while the sheet resistance also increases to ∼5 Ω/sq. The graphene layer is deposited to keep the copper/nickel surfaces from being oxidized in the air. Both stability testing and composition spectra results confirm the long-term stability of the copper/nickel microgrids wrapped with graphene.

Site-Selective Growth of Patterned Silver Grid Networks as Flexible Transparent Conductive Film by Using Poly(dopamine) at Room Temperature

Metal transparent conductive films (TCFs) have received increasing attention in various flexible electronics. However, there are two crucial issues that need to be addressed: (1) strong adhesion between metal TCFs and the flexible substrates and (2) high conductivity with short treatment time and low process temperature, simultaneous with high transparency. In this paper, a site-selective electroless plating combination with poly(dopamine) modification is demonstrated to fabricate a new high performance transparent conductor composed of a periodic two-dimensional silver network on a heat sensitive flexible substrate at room temperature. The TCF reveals an extremely high ratio of DC to optical conductivity (σ(DC)/σ(Op)) value in the range of 350-1000 for various fabricated silver grid films. It also exhibits particularly strong adhesion, which can resist ultrasonic treatment in water or organic solvent for several hours. Its reliability (stable for at least 1440 h during 85 °C/85% RH aging) meets the essential requirements for microelectronic applications. Using this method, we obtain silver grid film on a flexible polyethylene terephthalate substrate with optical transmittance of 91% and sheet resistance of 8 Ohm sq(-1), which is comparable to or better than the commercially available indium tin oxide.

A broadband plasmonic enhanced transparent conductor

We have designed a high performance transparent conductor using silver and aluminium stacked films perforated with hexagonally periodic subwavelength holes which are close to the lattice period. A broadband (λ of 400-800 nm) enhanced transmission is achieved by strong coupling of surface plasmons and propagating modes through the holes. Optical and electrical modeling predict a good combination of transparency and sheet resistance (e.g. ∼85% T and ∼2.4 Ω sq(-1)). Preliminary characterization results of the samples made via nanosphere lithography will also be presented.

Reconfigurable Topography for Rapid Solution Processing of Transparent Conductors

Cost-effective materials for transparent conducting electrodes are essential for many devices used in clean energy production and consumer electronics. Here we report a technique for non-lithographic patterning of silver nanowires on flexible substrates from solution via microcontact transfer printing using donor substrates with reconfigurable topography. This approach is a highly scalable fabrication strategy for high performance transparent conductors.

Integrating Simulations and Experiments To Predict Sheet Resistance and Optical Transmittance in Nanowire Films for Transparent Conductors

Metal nanowire films are among the most promising alternatives for next-generation flexible, solution-processed transparent conductors. Breakthroughs in nanowire synthesis and processing have reported low sheet resistance (Rs ≤ 100 Ω/sq) and high optical transparency (%T > 90%). Comparing the merits of the various nanowires and fabrication methods is inexact, because Rs and %T depend on a variety of independent parameters including nanowire length, nanowire diameter, areal density of the nanowires and contact resistance between nanowires. In an effort to account for these fundamental parameters of nanowire thin films, this paper integrates simulations and experimental results to build a quantitatively predictive model. First, by fitting the results from simulations of quasi-2D rod networks to experimental data from well-defined nanowire films, we obtain an effective average contact resistance, which is indicative of the nanowire chemistry and processing methods. Second, this effective contact resistance is used to simulate how the sheet resistance depends on the aspect ratio (L/D) and areal density of monodisperse rods, as well as the effect of mixtures of short and long nanowires on the sheet resistance. Third, by combining our simulations of sheet resistance and an empirical diameter-dependent expression for the optical transmittance, we produced a fully calculated plot of optical transmittance versus sheet resistance. Our predictions for silver nanowires are validated by experimental results for silver nanowire films, where nanowires of L/D > 400 are required for high performance transparent conductors. In contrast to a widely used approach that employs a single percolative figure of merit, our method integrates simulation and experimental results to enable researchers to independently explore the importance of contact resistance between nanowires, as well as nanowire area fraction and arbitrary distributions in nanowire sizes. To become competitive, metal nanowire systems require a predictive tool to accelerate their design and adoption for specific applications.

High-Performance Transparent Conductors from Networks of Gold Nanowires

We report the deposition and characterization of thin networks of gold nanowires on plastic substrates. The average nanowire diameter was 47 nm, while the networks had mean thicknesses in the range of 35–750 nm. The conductivity of networks with mean thicknesses below 100 nm was controlled by percolation, as characterized by the percolation exponent, n = 0.8, and the percolative figure of merit, Π = 28. However, networks with thicknesses above 100 nm had thickness-independent direct current conductivity of σDC,B = 5.4 × 105 S/m. The conductivity was limited by junction resistances, which we estimate at ∼3 kΩ. Networks of all thicknesses were described by an optical conductivity of σOp = 1.0 × 104 S/m. These electrical and optical properties result in networks with sheet resistance and transmittance very close to industry requirements, that is, 49 Ω/□ @ 83%. These values are superior to almost all reported carbon nanotube networks and are competitive with silver nanowire networks. We measured the work func...

Electrospun Composite Nanofiber Transparent Conductor Layer for Solar Cells

ABSTRACTDeveloping a durable and scalable transparent conductor (TC) as an electrode with high optical transmission and low sheet resistance is a significant opportunity for enabling next generation solar cell devices. High performance fibrous composite materials based on a carrier polymer with embedded functional nanostructures have the potential to serve as a TC with high surface area that can be deposited by the novel and scalable process of electrospinning. This work presents the development of a fibrous TC, where polyacrylonitrile (PAN) is used as a carrier polymer for multi-walled carbon nanotubes (MWCNT) to create electroactive nanofibers 200-500nm in diameter. Once carbonized, thin layers of this material have a low sheet resistance and high optical transmission. It is shown that in a two stage carbonization process, the second stage temperature of above 700C is the primary factor in establishing a highly conductive material and single layers of nanofibers are typically destabilized at high temperatures. A high performance TC has been developed through optimizing carbonization rates and temperatures to allow for single nanofiber layers fabricated by electrospinning MWCNT/PAN solutions onto quartz. These TCs have been optimized for concentrations of MWCNTs less than 20% volume fraction with well above 90% transmissivity and sheet resistances of between .5-1kohm/square. The required MWCNT loading is well below that for TCs based on random networks of MWCNTs.

The Remarkable Thermal Stability of Amorphous In‐Zn‐O Transparent Conductors

AbstractTransparent conducting oxides (TCOs) are increasingly critical components in photovoltaic cells, low‐e windows, flat panel displays, electrochromic devices, and flexible electronics. The conventional TCOs, such as Sn‐doped In2O3, are crystalline single phase materials. Here, we report on In‐Zn‐O (IZO), a compositionally tunable amorphous TCO with some significantly improved properties. Compositionally graded thin film samples were deposited by co‐sputtering from separate In2O3 and ZnO targets onto glass substrates at 100 °C. For the metals composition range of 55–84 cation% indium, the as‐deposited IZO thin films are amorphous, smooth (RRMS < 0.4 nm), conductive (σ ∼ 3000 Ω−1 · cm−1), and transparent in the visible (TVis > 90%). Furthermore, the amorphous IZO thin films demonstrate remarkable functional and structural stability with respect to heating up to 600 °C in either air or argon. Hence, though not completely understood at present, these amorphous materials constitute a new class of fundamentally interesting and technologically important high performance transparent conductors.