Broad-Spectrum Ultrathin-Metal-Based Oxide/Metal/Oxide Transparent Conductive Films for Optoelectronic Devices.

High-quality transparent conductive materials are beneficial to improve the charge transfer and light transmittance and reduce the interface defects as well as the production cost of optoelectronic devices. A high threshold thickness of metal layer in oxide/metal/oxide (OMO) compound thin films leads to strong reflectance, especially in the near-infrared region, limiting the broad-spectrum device applications. Here, we propose a novel Zn doping strategy using the low-cost single-target sputtering technology to achieve the growth of Ag-Zn thin films (i.e., Zn-doped Ag) and introduce a trace amount of O2 to further obtain ultrathin Ag-Zn(O) films (thin-film thickness d ≤ 5 nm), which greatly improves the broad-spectrum characteristics of OMO films. Heterogeneous metal and gas doping technology effectively promotes the formation of two-dimensional continuous film growth. By combining the ultrathin Ag-Zn(O) layer with the MGZO (i.e., Mg- and Ga co-doped ZnO) oxide film grown by reactive plasma deposition, a typical broad-spectrum MGZO/Ag-Zn(O)/MGZO (50/5/50 nm)-OMO compound thin film exhibits an average transmittance of 91.6% in the wavelength range of 400-1200 nm and low sheet resistance. The broad-spectrum organic solar cells based on MGZO/Ag-Zn(O)/MGZO electrodes present a high power conversion efficiency of 15.35%, superior to those devices based on single-layer oxide electrodes. The distinguished performances are attributed to the ultrathin Ag-Zn(O) films in OMO, paving the way for applications in broad-spectrum optoelectronic and flexible electronic devices.

Ultrathin-metal-film-based transparent electrodes with relative transmittance surpassing 100.

Flexible transparent electrodes are in significant demand in applications including solar cells, light-emitting diodes, and touch panels. The combination of high optical transparency and high electrical conductivity, however, sets a stringent requirement on electrodes based on metallic materials. To obtain practical sheet resistances, the visible transmittance of the electrodes in previous studies is typically lower than the transparent substrates the electrode structures are built on, namely, the transmittance relative to the substrate is <100%. Here, we demonstrate a flexible dielectric-metal-dielectric-based electrode with ~88.4% absolute transmittance, even higher than the ~88.1% transmittance of the polymer substrate, which results in a relative transmittance of ~100.3%. This non-trivial performance is achieved by leveraging an optimized dielectric-metal-dielectric structure guided by analytical and quantitative principles described in this work, and is attributed to an ultra-thin and ultra-smooth copper-doped silver film with low optical loss and low sheet resistance.

Transparent Perfect Microwave Absorber Employing Asymmetric Resonance Cavity.

The demand for high-performance absorbers in the microwave frequencies, which can reduce undesirable radiation that interferes with electronic system operation, has attracted increasing interest in recent years. However, most devices implemented so far are opaque, limiting their use in optical applications that require high visible transparency. Here, a scheme is demonstrated for microwave absorbers featuring high transparency in the visible range, near-unity absorption (≈99.5% absorption at 13.75 GHz with 3.6 GHz effective bandwidth) in the Ku-band, and hence excellent electromagnetic interference shielding performance (≈26 dB). The device is based on an asymmetric Fabry-Pérot cavity, which incorporates a monolayer graphene and a transparent ultrathin (8 nm) doped silver layer as absorber and reflector, and fused silica as the middle dielectric layer. Guided by derived formulism, this asymmetric cavity is demonstrated with microwaves near-perfectly and exclusively absorbs in the ultrathin graphene film. The peak absorption frequency of the cavity can be readily tuned by simply changing the thickness of the dielectric spacer. The approach provides a viable solution for a new type of microwave absorber with high visible transmittance, paving the way towards applications in the area of optics.

Optical Device Based on a Nanopillar Array by the Pattern Transfer of an Anodic Aluminum Oxide Membrane.

A simple and convenient nanofabrication method is proposed to achieve nanopillar arrays by the pattern transfer of an anodic aluminum oxide membrane, profiting from the rapid and efficient preparation process and regular hexagonal lattice patterns of the anodic aluminum oxide template. The taper angle of the nanopillar is affected by the distribution of the vapor particles during the deposition process, which is highly dependent on the material and deposition power. Based on this method, a novel scheme employing aluminum nanopillar arrays is demonstrated to realize the color tuning feature by simply varying the thickness of the top dielectric layer within a large range. The nanopillar arrays are completely covered by the thick dielectric layer atop due to the great conformality of the atomic layer deposition method that is used for the dielectric deposition. In addition, the color devices present good angular insensitivity up to 45°, resulting from the excited localized surface plasmon resonance within the metallic patches. The simple fabrication method is of great advantage to produce periodic nanostructures over large areas, which are widely used in designs and verifications of optical metasurfaces for various applications, including optical communication, imaging, sensing, and so forth.

Enhanced Light Utilization in Semitransparent Organic Photovoltaics Using an Optical Outcoupling Architecture.

Building-integrated photovoltaics employing transparent photovoltaic cells on window panes provide an opportunity to convert solar energy to electricity rather than generating waste heat. Semitransparent organic photovoltaic cells (ST-OPVs) that utilize a nonfullerene acceptor-based near-infrared (NIR) absorbing ternary cell combined with a thin, semitransparent, high conductivity Cu-Ag alloy electrode are demonstrated. A combination of optical outcoupling and antireflection coatings leads to enhanced visible transmission, while reflecting the NIR back into the cell where it is absorbed. This combination of coatings results in doubling of the light utilization efficiency (LUE), which is equal to the product of the power conversion efficiency (PCE) and the average photopic transparency, compared with a conventional semitransparent cell lacking these coatings. A maximum LUE = 3.56 ± 0.11% is achieved for an ST-OPV with a PCE = 8.0 ± 0.2% at 1 sun, reference AM1.5G spectrum. Moreover, neutral colored ST-OPVs are also demonstrated, with LUE = 2.56 ± 0.2%, along with Commission Internationale d'Eclairage chromaticity coordinates of CIE = (0.337, 0.349) and a color rendering index of CRI = 87.

Electrodeposition of Large Area, Angle-Insensitive Multilayered Structural Colors.

We demonstrate structural colors produced by a simple, inexpensive, and nontoxic electrodeposition process. Asymmetric metal-dielectric-metal (MDM) multilayered structures were achieved by sequential electrodeposition of smooth gold, thin cuprous oxide, and finally thin gold on conductive substrates, forming an effective optical cavity with angle-insensitive characteristics. Different colors of high brightness were achieved by simply tuning the thickness of the electrodeposited middle cavity layer. This process is compatible with highly nonplanar substrates of arbitrary shape, size, and roughness. This work is the first demonstration of solution-processed, electrodeposited, MDM film stacks that are uniform over large areas and highlights the clear advantages of this approach over traditional deposition or assembly methods for preparing colored films.

High-Performance Large-Scale Flexible Optoelectronics Using Ultrathin Silver Films with Tunable Properties.

One key obstacle in fabricating efficient flexible and printable optoelectronic devices is the absence of ideal flexible transparent conductors with superior optical, electrical, and mechanical properties. Here, high-performance flexible transparent conductors are demonstrated using ultrathin (<10 nm) doped silver films, which exhibit an averaged visible transmittance of 80% without any antireflection coating, sheet resistance less than 20 Ω sq-1, and mechanical stability over 1000 bending cycles. The conductor is prepared by doping silver with an additive metal (e.g., nickel, copper, titanium, chromium), and its various properties can be readily tuned by either using different doping metal species or controlling the dopant concentration. Centimeter-size, flexible polymer light-emitting diodes are fabricated using a nickel-doped silver-based electrode, and the devices exhibit 30% enhanced current efficiencies compared to their indium tin oxide counterparts, invariant emission spectra at large viewing angles, and operational stability for over 1200 bending circles. In addition, 6 in. flexible low-emissivity coatings are demonstrated using a copper-doped silver-based conductor, which transmit 85.2% of the visible light while rejecting over 90% of the infrared radiation beyond 1250 nm.

Highly Transparent and Broadband Electromagnetic Interference Shielding Based on Ultrathin Doped Ag and Conducting Oxides Hybrid Film Structures.

Reducing electromagnetic interference (EMI) across a broad radio frequency band is crucial to eliminate adverse effects of increasingly complex electromagnetic environment. Current shielding materials or methods suffer from trade-offs between optical transmittance and EMI shielding capability. Moreover, poor mechanical flexibility and fabrication complexity significantly limit their further applications in flexible electronics. In this work, an ultrathin (8 nm) and continuous doped silver (Ag) film was obtained by introducing a small amount of copper during the sputtering deposition of Ag and investigated as transparent EMI shielding components. The electromagnetic Ag shielding (EMAGS) film was realized in the form of conductive dielectric-metal-dielectric design to relieve the electro-optical trade-offs, which transmits 96.5% visible light relative to the substrate and shows an excellent average EMI shielding effectiveness (SE) of ∼26 dB, over a broad bandwidth of 32 GHz, covering the entire X, Ku, Ka, and K bands. EMI SE >30 dB was obtained by simply stacking two layers of EMAGS films together and can be further improved up to 50 dB by separating two layers with a quarter-wavelength space. The flexible EMAGS film shows a stable EMI shielding performance under repeated mechanical bending. In addition, large-area EMAGS films were demonstrated by a roll-to-roll sputtering system, proving the feasibility for mass production. The high-performance EMAGS film holds great potential for various applications in wearable electronics, healthcare devices, and electronic safety areas.

High-color-purity, angle-invariant, and bidirectional structural colors based on higher-order resonances.

Structural colors with high color purity and low fabrication cost are highly desired in a wide variety of applications including displays, light emitting diodes, decorations, and optical detections. Here, we demonstrate a semitransparent pentalayer structure for creating angle-insensitive, high-purity reflective colors that exploit a higher-order cavity resonance. Moreover, the designed structure in a symmetric configuration presents bright and saturated colors from both directions with a high efficiency up to 85% and a high angular tolerance up to ±60°. The described scheme involves one deposition run, thereby providing a significant step toward large-area applications in various areas.

Visualizing Mie Resonances in Low-Index Dielectric Nanoparticles.

Resonant light scattering by metallic and high-index dielectric nanoparticles has received enormous attention and found many great applications. However, low-index dielectric nanoparticles typically do not show resonant scattering behaviors due to poor light confinement caused by small index contrast. This Letter describes a simple and effective approach to drastically enhance the resonance effect of the low-index particles by partial metal dressing. Mie resonances of low-index nanoparticles can now be easily visualized by scattered light. This scattering peak depends on sphere size and has a reasonable linewidth. A size difference as small as 8 nm was resolved by a peak shift or even by color change. The scattering peak is attributed to the enhanced TE_{11} Mie resonance of the low-index nanospheres. The metal dress not only provides a high-reflection boundary, but also functions as an antenna to couple the confined light power to the far field, leading to scattering maxima in the spectra. Additionally, the enhanced TE_{11} Mie resonance in low-index nanoparticles features a considerable magnetic response due to the strong circulating displacement currents induced by the intensified E field despite of a low permittivity (hence low index) of the particles. The enhanced Mie resonances could be used to sense minute changes in size or refractive index of low-index nanoparticles and benefit a wide range of applications.

Robust Extraction of Hyperbolic Metamaterial Permittivity using Total Internal Reflection Ellipsometry.

Hyperbolic metamaterials are optical materials characterized by highly anisotropic effective permittivity tensor components having opposite signs along orthogonal directions. The techniques currently employed for characterizing the optical properties of hyperbolic metamaterials are limited in their capability for robust extraction of the complex permittivity tensor. Here we demonstrate how an ellipsometry technique based on total internal reflection can be leveraged to extract the permittivity of hyperbolic metamaterials with improved robustness and accuracy. By enhancing the interaction of light with the metamaterial stacks, improved ellipsometric sensitivity for subsequent permittivity extraction is obtained. The technique does not require any modification of the hyperbolic metamaterial sample or sophisticated ellipsometry set-up, and could therefore serve as a reliable and easy-to-adopt technique for characterization of a broad class of anisotropic metamaterials.

High-Performance Doped Silver Films: Overcoming Fundamental Material Limits for Nanophotonic Applications.

The field of nanophotonics has ushered in a new paradigm of light manipulation by enabling deep subdiffraction confinement assisted by metallic nanostructures. However, a key limitation which has stunted a full development of high-performance nanophotonic devices is the typical large losses associated with the constituent metals. Although silver has long been known as the highest quality plasmonic material for visible and near infrared applications, its usage has been limited due to practical issues of continuous thin film formation, stability, adhesion, and surface roughness. Recently, a solution is proposed to the above issues by doping a proper amount of aluminum during silver deposition. In this work, the potential of doped silver for nanophotonic applications is presented by demonstrating several high-performance key nanophotonic devices. First, long-range surface plasmon polariton waveguides show propagation distances of a few centimeters. Second, hyperbolic metamaterials consisting of ultrathin Al-doped Ag films are attained having a homogeneous and low-loss response, and supporting a broad range of high-k modes. Finally, transparent conductors based on Al-doped Ag possess both a high and flat transmittance over the visible and near-IR range.

Invisibility cloak with image projection capability.

Investigations of invisibility cloaks have been led by rigorous theories and such cloak structures, in general, require extreme material parameters. Consequently, it is challenging to realize them, particularly in the full visible region. Due to the insensitivity of human eyes to the polarization and phase of light, cloaking a large object in the full visible region has been recently realized by a simplified theory. Here, we experimentally demonstrate a device concept where a large object can be concealed in a cloak structure and at the same time any images can be projected through it by utilizing a distinctively different approach; the cloaking via one polarization and the image projection via the other orthogonal polarization. Our device structure consists of commercially available optical components such as polarizers and mirrors, and therefore, provides a significant further step towards practical application scenarios such as transparent devices and see-through displays.