Strategy for Fabricating Ultrathin Au Film Electrodes with Ultralow Optoelectrical Losses and High Stability.

A vital objective in the wetting of Au deposited on chemically heterogeneous oxides is to synthesize a completely continuous, highly crystalline, ultrathin-layered geometry with minimized electrical and optical losses. However, no effective solution has been proposed for synthesizing an ideal Au-layered structure. This study presents evidence for the effectiveness of atomic oxygen-mediated growth of such an ideal Au layer by improving Au wetting on ZnO substrates with a substantial reduction in free energy. The unexpected outcome of the atomic oxygen-mediated Au growth can be attributed to the unconventional segregation and incorporation of atomic oxygen along the outermost boundaries of Au nanostructures evolving in the clustering and layering stages. Moreover, the experimental and numerical investigations revealed the spontaneous migration of atomic oxygen from an interstitial oxygen surplus ZnO bulk to the Au-ZnO interface, as well as the segregation (float-out) of the atomic oxygen toward the top Au surfaces. Thus, the implementation of a 4-nm-thick, two-dimensional, quasi-single-crystalline Au layer with a nearly complete crystalline realignment at a mild temperature (570 K) enabled exceptional optoelectrical performance with record-low resistivity (<7.5 × 10-8 Ω·m) and minimal optical loss (∼3.5%) at a wavelength of 700 nm.

An unexpected surfactant role of immiscible nitrogen in the structural development of silver nanoparticles: an experimental and numerical investigation.

Artificially designing the crystal orientation and facets of noble metal nanoparticles is important to realize unique chemical and physical features that are very different from those of noble metals in bulk geometries. However, relative to their counterparts synthesized in wet-chemical processes, vapor-depositing noble metal nanoparticles with the desired crystallographic features while avoiding any notable impurities is quite challenging because this task requires breaking away from the thermodynamically favorable geometry of nanoparticles. We used plasma-generated N atoms as a surface-active agent, a so-called surfactant, to control the structural development of Ag nanoparticles supported on a chemically heterogeneous ZnO substrate. The N-surfactant-facilitated sputter deposition provided strong selectivity for crystalline orientation and facets, leading to a highly flattened nanoparticle shape that clearly deviated from the energetically favorable spherical polyhedra, due to the drastic decreases in the surface free energies of Ag nanoparticles in the presence of the N surfactant. The Ag nanoparticles successively developed a nearly unidirectional (111) orientation aligned by stimulating the crystalline coupling of Ag along the orientation of the ZnO substrate. The experimental and simulation results not only offer new insights into the advantages of N as a surfactant for the orientation and shape-controlled synthesis of Ag nanoparticles via sputter deposition but also provide the first solid evidence validating that immiscible, nonresidual gaseous surfactants can be used in the vapor deposition processes of noble metal nanoparticles to manipulate their surface free energies.

Nitrogen-Mediated Growth of Silver Nanocrystals to Form UltraThin, High-Purity Silver-Film Electrodes with Broad band Transparency for Solar Cells.

Controlling the shape and crystallography of nanocrystals during the early growth stages of a noble metal layer is important because of its correlation with the final layer morphology and optoelectrical features, but this task is unattainable in vapor deposition processes dominated by artificially uncontrollable thermodynamic free energies. We report on experimental evidence for the controllable evolution of Ag nanocrystals as induced by the addition of nitrogen, presumed to be nonresidual in the Ag lattice given its strong float-out behavior. This atypical formation of energetically stable Ag nanocrystals with significantly improved wetting abilities on a chemically heterogeneous substrate promotes the development of an atomically flat, ultrathin, high-purity Ag layer with a thickness of only 5 nm. This facilitates the fabrication of Ag thin-film electrodes exhibiting highly enhanced optical transparency over a broad spectral range in the visible and near-infrared spectral range. An Ag thin-film electrode with a ZnO/Ag/ZnO configuration exhibits an average transmittance of about 95% in the spectral range of 400-800 nm with a maximum transmittance of over 98% at 580 nm, which is comparable with the best transparency values so far reported for transparent electrodes. This degree of optical transparency provides an excellent chance to improve the photon absorption of photovoltaic devices employing an Ag thin film as their window electrode. This is clearly confirmed by the superior performance of a flexible organic solar cell with a power conversion efficiency of 8.0%, which is far superior to that of the same solar cell using a conventional amorphous indium tin oxide electrode (6.4%).

Direct Thermal Growth of Large Scale Cl-doped CdTe Film for Low Voltage High Resolution X-ray Image Sensor.

Polycrystalline cadmium telluride (CdTe) X-ray photodetector with advanced performance was fabricated in a Schottky diode form by direct thermal deposition (evaporation) on pixelized complementary metal oxide semiconductor (CMOS) readout panel. Our CdTe X-ray detector shows such a variety of benefits as relatively low process temperature, low cost, low operation voltage less than 40 V, and higher sensitivity and spatial resolution than those of commercial a-Se detectors. CdTe has cubic Zinc Blende structure and maintains p-type conduction after growth in general. For low voltage operation, we succeeded in Cl doping at all stage of CdTe film deposition, and as a result, hole concentration of p-type CdTe was reduced to ~1012 cm-3 from ~1015 cm-3, and such concentration reduction could enable our Schottky diode with Ti electrode to operate at a reverse bias of less than 40 V. Our CdTe Schottky diode/CMOS pixel array as a direct conversion type imager demonstrates much higher resolution X-ray imaging in 7 × 9 cm2 large scale than that of CsI/CMOS array, an indirect conversion imager. To our limited knowledge, our results on polycrystalline CdTe Schottky diode/CMOS array would be very novel as a first demonstration of active pixel sensor system equipped with directly deposited large scale X-ray detector.

Ultrathin Silver Film Electrodes with Ultralow Optical and Electrical Losses for Flexible Organic Photovoltaics.

Improving the wetting ability of Ag on chemically heterogeneous oxides is technically important to fabricate ultrathin, continuous films that would facilitate the minimization of optical and electrical losses to develop qualified transparent Ag film electrodes in the state-of-the-art optoelectronic devices. This goal has yet to be attained, however, because conventional techniques to improve wetting of Ag based on heterogeneous metallic wetting layers are restricted by serious optical losses from wetting layers. Herein, we report on a simple and effective technique based on the partial oxidation of Ag nanoclusters in the early stages of Ag growth. This promotes the rapid evolution of the subsequently deposited pure Ag into a completely continuous layer on the ZnO substrate, as verified by experimental and numerical evidence. The improvement in the Ag wetting ability allows the development of a highly transparent, ultrathin (6 nm) Ag continuous film, exhibiting an average optical transmittance of 94% in the spectral range 400-800 nm and a sheet resistance of 12.5 Ω sq-1, which would be well-suited for application to an efficient front window electrode for flexible solar cell devices fabricated on polymer substrates.

Fabrication of graphene oxide/montmorillonite nanocomposite flexible thin films with improved gas-barrier properties.

Nanocomposites are potential substitutes for inorganic materials in fabricating flexible gas-barrier thin films. In this study, two nanocomposites are used to form a flexible gas-barrier film that shows improved flexibility and a decreased water vapor transmission rate (WVTR), thereby extending the diffusion path length for gas molecules. The nanoclay materials used for the flexible gas-barrier thin film are Na+-montmorillonite (MMT) and graphene oxide (GO). A flexible gas-barrier thin film was fabricated using a layer-by-layer (LBL) deposition method, exploiting electronic bonding under non-vacuum conditions. The WVTR of the film, in which each layer was laminated by LBL assembly, was analyzed by Ca-test and the oxygen transmission rate (OTR) was analyzed by MOCON. When GO and MMT are used together, they fill each other's vacancies and form a gas-barrier film with high optical transmittance and the improved WVTR of 3.1 × 10-3 g per m2 per day without a large increase in thickness compared to barrier films produced with GO or MMT alone. Thus, this film has potential applicability as a barrier film in flexible electronic devices.

Optical Transmittance Enhancement of Flexible Copper Film Electrodes with a Wetting Layer for Organic Solar Cells.

The development of highly efficient flexible transparent electrodes (FTEs) supported on polymer substrates is of great importance to the realization of portable and bendable photovoltaic devices. Highly conductive, low-cost Cu has attracted attention as a promising alternative for replacing expensive indium tin oxide (ITO) and Ag. However, highly efficient, Cu-based FTEs are currently unavailable because of the absence of an efficient means of attaining an atomically thin, completely continuous Cu film that simultaneously exhibits enhanced optical transmittance and electrical conductivity. Here, strong two-dimensional (2D) epitaxy of Cu on ZnO is reported by applying an atomically thin (around 1 nm) oxygen-doped Cu wetting layer. Analyses of transmission electron microscopy images and X-ray diffraction patterns, combined with first-principles density functional theory calculations, reveal that the reduction in the surface and interface free energies of the wetting layers with a trace amount (1-2 atom %) of oxygen are largely responsible for the two-dimensional epitaxial growth of the Cu on ZnO. The ultrathin 2D Cu layer, embedded between ZnO films, exhibits a highly desirable optical transmittance of over 85% in a wavelength range of 400-800 nm and a sheet resistance of 11 Ω sq-1. The validity of this innovative approach is verified with a Cu-based FTE that contributes to the light-to-electron conversion efficiency of a flexible organic solar cell that incorporates the transparent electrode (7.7%), which far surpasses that of a solar cell with conventional ITO (6.4%).

Stable ultrathin partially oxidized copper film electrode for highly efficient flexible solar cells.

Advances in flexible optoelectronic devices have led to an increasing need for developing highly efficient, low-cost, flexible transparent conducting electrodes. Copper-based electrodes have been unattainable due to the relatively low optical transmission and poor oxidation resistance of copper. Here, we report the synthesis of a completely continuous, smooth copper ultra-thin film via limited copper oxidation with a trace amount of oxygen. The weakly oxidized copper thin film sandwiched between zinc oxide films exhibits good optoelectrical performance (an average transmittance of 83% over the visible spectral range of 400-800 nm and a sheet resistance of 9 Ω sq(-1)) and strong oxidation resistance. These values surpass those previously reported for copper-based electrodes; further, the record power conversion efficiency of 7.5% makes it clear that the use of an oxidized copper-based transparent electrode on a polymer substrate can provide an effective solution for the fabrication of flexible organic solar cells.

Highly efficient and bendable organic solar cells using a three-dimensional transparent conducting electrode.

A three-dimensional (3D) transparent conducting electrode, consisting of a quasi-periodic array of discrete indium-tin-oxide (ITO) nanoparticles superimposed on a highly conducting oxide-metal-oxide multilayer using ITO and silver oxide (AgOx) as oxide and metal layers, respectively, is synthesized on a polymer substrate and used as an anode in highly flexible organic solar cells (OSCs). The 3D electrode is fabricated using vacuum sputtering sequences to achieve self-assembly of distinct ITO nanoparticles on a continuous ITO-AgOx-ITO multilayer at room-temperature without applying conventional high-temperature vapour-liquid-solid growth, solution-based nanoparticle coating, or complicated nanopatterning techniques. Since the 3D electrode enhances the hole-extraction rate in OSCs owing to its high surface area and low effective series resistance for hole transport, OSCs based on this 3D electrode exhibit a power conversion efficiency that is 11-22% higher than that achievable in OSCs by means of conventional planar ITO film-type electrodes. A record high efficiency of 6.74% can be achieved in a bendable OSC fabricated on a poly(ethylene terephthalate) substrate.

Silver nanowire network transparent electrodes with highly enhanced flexibility by welding for application in flexible organic light-emitting diodes.

We present highly flexible Ag nanowire (AgNW) networks welded with transparent conductive oxide (TCO) for use in electrical interconnects in flexible and wearable electronic devices. The hybrid transparent conductive electrodes (TCEs) produced on polymer substrates consist of AgNW networks and TCO that is deposited atop the AgNWs. The TCO firmly welds the AgNWs together at the junctions and the AgNWs to the polymer substrates. Transmission electron microscopy (TEM) analysis show that TCO atop and near AgNWs grows as crystalline because AgNWs act as crystalline seeds, but the crystallinity of the matrix TCO can be controlled by sputtering conditions. The sheet resistances (Rs) of hybrid TCEs are less than the AgNW networks because junction resistance is significantly reduced due to welding by TCO. The effect of welding on decreasing Rs is enhanced with increasing matrix crystallinity, as the adhesion between AgNWs and TCO is improved. Furthermore, the bending stability of the hybrid TCEs are almost equivalent to and better than AgNW networks in static and cyclic bending tests, respectively. Flexible organic light-emitting diodes (f-OLEDs) are successfully fabricated on the hybrid TCEs without top-coats and the performances of f-OLEDs on hybrid TCEs are almost equivalent to those on commercial TCO, which supports replacing indium tin oxide (ITO) with the hybrid TCEs in flexible electronics applications.

Preparation of flexible organic solar cells with highly conductive and transparent metal-oxide multilayer electrodes based on silver oxide.

We report that significantly more transparent yet comparably conductive AgOx films, when compared to Ag films, are synthesized by the inclusion of a remarkably small amount of oxygen (i.e., 2 or 3 atom %) in thin Ag films. An 8 nm thick AgOx (O/Ag=2.4 atom %) film embedded between 30 nm thick ITO films (ITO/AgOx/ITO) achieves a transmittance improvement of 30% when compared to a conventional ITO/Ag/ITO electrode with the same configuration by retaining the sheet resistance in the range of 10-20 Ω sq(-1). The high transmittance provides an excellent opportunity to improve the power-conversion efficiency of organic solar cells (OSCs) by successfully matching the transmittance spectral range of the electrode to the optimal absorption region of low band gap photoactive polymers, which is highly limited in OSCs utilizing conventional ITO/Ag/ITO electrodes. An improvement of the power-conversion efficiency from 4.72 to 5.88% is achieved from highly flexible organic solar cells (OSCs) fabricated on poly(ethylene terephthalate) polymer substrates by replacing the conventional ITO/Ag/ITO electrode with the ITO/AgOx/ITO electrode. This novel transparent electrode can facilitate a cost-effective, high-throughput, room-temperature fabrication solution for producing large-area flexible OSCs on heat-sensitive polymer substrates with excellent power-conversion efficiencies.

Fabrication of a completely transparent and highly flexible ITO nanoparticle electrode at room temperature.

We report the fabrication of a highly flexible indium tin oxide (ITO) electrode that is completely transparent to light in the visible spectrum. The electrode was fabricated via the formation of a novel ITO nanoarray structure, consisting of discrete globular ITO nanoparticles superimposed on an agglomerated ITO layer, on a heat-sensitive polymer substrate. The ITO nanoarray spontaneously assembled on the surface of the polymer substrate by a simple sputter coating at room temperature, without nanolithographic or solution-based assembly processes being required. The ITO nanoarray exhibited a resistivity of approximately 2.3 × 10(-3) Ω cm and a specular transmission of about 99% at 550 nm, surpassing all previously reported values of these parameters in the case of transparent porous ITO electrodes synthesized via solution-based processes at elevated temperatures. This novel nanoarray structure and its fabrication methodology can be used for coating large-area transparent electrodes on heat-sensitive polymer substrates, a goal unrealizable through currently available solution-based fabrication methods.

Antireflective silica nanoparticle array directly deposited on flexible polymer substrates by chemical vapor deposition.

We report the direct coating of a novel antireflective (AR) nanoarray structure of silica nanoparticles on highly flexible polymer substrates by a conventional vacuum coating method using plasma-enhanced chemical vapor deposition. Globular-shaped silica nanoparticles are found to be self-arranged in a periodic pattern on subwavelength scales without the use of artificial assemblies that typically require complicated nanolithography or solution-based nanoparticle fabrication approaches. Highly efficient AR characteristics in the visible spectral range are obtained at optimized refractive indices by controlling the dimensions and average distances of the silica nanoparticle arrays in a level accuracy of tens of nanometers. The AR nanoarrays exhibit sufficient structural durability against the very high strain levels that arise from the flexibility of polymer substrates. This simple coating process provides a cost-effective, high-throughput, room-temperature fabrication solution for producing large-area polymer substrates with AR characteristics.