Dielectric-metal-dielectric Research Articles

Transparent electrodes based on molybdenum–titanium–oxide with increased water stability for use as hole-transport/hole-injection components

Recently, dielectric/metal/dielectric (DMD) transparent electrodes based on MoO3 have been applied in solar cells and organic light emitting diodes, by virtue of the favourable properties of MoO3 as hole-transport/hole-injection material. However, amorphous MoO3 layers are easily dissolved in water, thus inhibiting device processing with water-based solutions. To improve the stability in water, the present study introduces DMD electrodes based on mixed molybdenum–titanium–oxide (MTO), fabricated by DC magnetron sputtering from a conductive oxide target. It is demonstrated that the addition of Ti strongly increases the stability in water, while the desirable electronic properties of MoO3, specifically the high work function and wide bandgap, are maintained. The DMD electrodes, with Ag as metal layer, were fabricated on both rigid and flexible substrates, namely glass and polyethylene terephthalate (PET). The obtained electrodes have low sheet resistance around 5 Ω/sq and high average visible transmittance well above 0.7 (including the substrate). As a result of the MTO stability, processing with water-based solutions takes place without electrode degradation. To demonstrate the process compatibility for large-scale, industrial production, the DMDs were sputter-deposited by a roll-to-roll process on a 300 mm-wide PET foil, achieving similar electrode properties with the laboratory-scale samples.Graphical abstract

Bioinspired Toughening Mechanisms in a Multilayer Transparent Conductor Structure.

With increasing demands and interest in flexible and foldable devices, much effort has been devoted to the development of flexible transparent electrodes. An in-depth understanding of failure mechanisms in nanoscale structure is crucial in developing stable, flexible electronics with long-term durability. The present work investigated the mechanoelectric characteristics of transparent conductive electrodes in the form of dielectric/metal/dielectric (DMD) sandwich structures under bending, including one time and repeated cyclic bending test, and provides an explanation of their failure mechanism. We demonstrate how a thin metallic layer helps to enhance the mechanical robustness of the DMD as compared with that without, tune the mechanical properties of the cohesive layer, and improve the electrode fracture resistance. Abnormal crack propagation and toughening of multilayer DMD structures are analyzed, and its underlying mechanisms are explained. We consider the knowledge of the failure mechanisms of transparent conductive electrodes gained from the present study as a foundation for future design improvements.

Enhancement of Optical Coupling Efficiency of Surface Plasmon Resonance Based Sensors

The optical coupling efficiency of surface plasmon resonance (SPR) based sensors is theoretically investigated from the first principle. Analytical expressions for the instantaneous electric flux lines associated with the TM modes of a three-layer dielectric-metal-dielectric (DMD) waveguide structure are calculated. It is shown that these electric flux lines appear in the form of dis-continuous loops at the metal-dielectric interfaces. A careful investigation of the electric flux loops reveals that the transverse component of the electric flux dominates at the interface, and away from the interface the longitudinal electric flux component takes over. It is hence shown that in the incident <inline-formula> <tex-math notation="LaTeX">$p$ </tex-math></inline-formula>-polarized optical light which is used to excite the surface plasmon (SP) modes in SPR sensors if the transverse component of the electric field is increased then a significant increase in the input optical coupling efficiency is observed. The increased input optical coupling increases the real part of the effective index of the SP mode of the sensor yielding an increased sensitivity of the SPR sensor. Calculations are done to optimize various design parameters of an SPR sensor in a typical prism coupling Kretschmann configuration in an angle interrogation setup. It is shown that there is an increase of 64.33&#x0025; per RIU in optical coupling efficiency with a decrease in the refractive index of the high index input coupling prism and an increase of 58.787&#x0025; per RIU with an increase in the refractive index of the affinity layer. Enhancement of optical coupling efficiency due to the thickness of the metal film as well as the wavelength of the input optical light has also been evaluated. The corresponding improvement in the sensitivity and changes in full width at half maxima (FWHM), the figure of merit (FOM), and signal to noise ratio (SNR) of the SPR sensor are also reported.

ITO-free silver-doped DMD structures: HiPIMS transparent-conductive nano-composite coatings for electrochromic applications

Nanocomposites of dielectric/metal/dielectric (DMD) materials that are both optically transparent and electrically conductive are of great interest for an array of modern optoelectronic applications, including electrochromic and energy storage devices. Here we report for the first time the creation of ITO-free DMD nanocomposite structures that are composed of only three layers: (Ag-doped WO3)/(Ag)/(WO3). We employed high power impulse magnetron sputtering (HiPIMS) as a new technology to deposit the internal WO3 layer and the external Ag/WO3 nanocomposite. High rates of silver ionization in the HiPIMS process and energetic arrival of silver ions on the negatively biased DMD structure enabled their penetration into the external tungsten oxide layer, forming a nanocomposite structure in a single-step process. We provided evidence that this unique structure pushes the performance of transparent-conductive DMD materials beyond their intrinsic limits. To optimize the surface plasmon resonance effect and thus the electrochromic performance of the structures, we changed the size of silver nanoclusters through vacuum post-annealing in the same sputtering chamber at varied temperatures. The optimized Ag-WO3/Ag/WO3 structure was transparent (average transmittance = 75.89 ± 0.05% in the wavelength range of 300–700 nm) and electrically conductive (sheet resistance = 23.6 ± 0.8 Ω/□). Moreover, it showed favorable electrochromic characteristics with high coloration efficiency (32.1 cm2 C−1), fast switching speed, and excellent long-term stability for at least 2000 cycles. The mechanisms of improvement in the electrochromic performance of such bifunctional structures underpinned by surface plasmon resonance, as modulated by the size and average distance of silver nanoclusters, are elucidated. The versatile new approach using HiPIMS for the fabrication of transparent-conductive nanocomposite coatings on ITO-free substrates holds great potential to manufacture next-generation optoelectronic materials such as electrochromic devices.

Design and fabrication of a semi-transparent solar cell considering the effect of the layer thickness of MoO3/Ag/MoO3 transparent top contact on optical and electrical properties

We conducted the present study to design and manufacture a semi-transparent organic solar cell (ST-OSC). First, we formed a transparent top contact as MoO3/Ag/MoO3 in a dielectric/metal/dielectric (DMD) structure. We performed the production of an FTO/ZnO/P3HT:PCBM/MoO3/Ag/MoO3 ST-OSC by integrating MoO3/Ag/MoO3 (10/d_{m}/d_{{od}} nm) instead of an Ag electrode in an opaque FTO/ZnO/P3HT:PCBM/MoO3/Ag (–/40/130/10/100 nm) OSC, after theoretically achieving optimal values of optical and electrical parameters depending on Ag layer thickness. The transparency decreased with the increase of d_{m} values for current DMD. Meanwhile, maximum transmittance and average visible transmittance (AVT) indicated the maximum values of over 92% for d_{m} ~ = 4 and 8 nm, respectively. For ST-OSCs, the absorption and reflectance increased in the visible region by a wavelength of longer than 560 nm and in the whole near-infrared region by increasing d_{m} up to 16 nm. Moreover, in the CIE chromaticity diagram, we reported a shift towards the D65 Planckian locus for colour coordinates of current ST-OSCs. Electrical analysis indicated the photogenerated current density and AVT values for d_{m} = 6 nm as 63.30 mA/cm2 and 38.52%, respectively. Thus, the theoretical and experimental comparison of optical and electrical characteristics confirmed that the manufactured structure is potentially conducive for a high-performance ST-OSC.

23‐4: Ultrathin Foldable Organic Light‐Emitting Diodes with High Efficiency

We demonstrated ultrathin foldable organic light‐emitting diodes (OLEDs) by using dielectric‐metal‐dielectric (DMD) electrode and polyvinyl alcohol (PVA) substrate. The DMD containing a thin metal layer suppresses waveguide mode formation and help efficiency improvement, and the spin‐coated PVA film forms an ultrathin substrate for OLEDs ~ 1 μm.

Fast sputter deposition of MoOx/metal/MoOx transparent electrodes on glass and PET substrates

Dielectric/metal/dielectric (DMD) transparent electrodes emerged as a compelling alternative to the widely used indium-tin-oxide (ITO) for solar cells and optoelectronic devices. DMD electrodes are especially attractive for flexible substrates, as, in contrast to ITO, they retain their low electrical resistance upon substrate bending and they do not require deposition at elevated temperatures. In a DMD, the choice of the dielectric is mainly dictated by the device architecture. Owing to its high work function, MoO3 is a commonly used hole-selective dielectric layer. The present work investigates MoOx/metal/MoOx (with 2 < x < 3) DMD electrodes, with Ag and Au as metals, fabricated by direct current, magnetron sputtering, at industry-relevant, high deposition rates. This was possible with a properly engineered MoOx target, providing high electrical conductivity and compactness. The sputtered electrodes on polyethylene terephthalate (PET) substrates show higher figure-of-merit than similar, evaporated electrodes in the literature. It is shown that the DMD electrodes with amorphous MoOx layers have low stability in water, but they are stable to other solvents, such as toluene, dimethylsulfoxide (DMSO), dimethylformamide (DMF), chlorobenzene or chloroform, allowing their implementation in devices like organic light-emitting diodes or perovskite solar cells. Further, it is shown that the electrodes show dramatically enhanced mechanical stability compared to ITO, when subjected to tensile bending tests.

Influence of Co-Sputtered Ag:Al Ultra-Thin Layers in Transparent V2O5/Ag:Al/AZO Hole-Selective Electrodes for Silicon Solar Cells.

As optoelectronic devices continue to improve, control over film thickness has become crucial, especially in applications that require ultra-thin films. A variety of undesired effects may arise depending on the specific growth mechanism of each material, for instance a percolation threshold thickness is present in Volmer-Webber growth of materials such as silver. In this paper, we explore the introduction of aluminum in silver films as a mechanism to grow ultrathin metallic films of high transparency and low sheet resistance, suitable for many optoelectronic applications. Furthermore, we implemented such ultra-thin metallic films in Dielectric/Metal/Dielectric (DMD) structures based on Aluminum-doped Zinc Oxide (AZO) as the dielectric with an ultra-thin silver aluminum (Ag:Al) metallic interlayer. The multilayer structures were deposited by magnetron sputtering, which offers an industrial advantage and superior reliability over thermally evaporated DMDs. Finally, we tested the optimized DMD structures as a front contact for n-type silicon solar cells by introducing a hole-selective vanadium pentoxide (V2O5) dielectric layer.

Plasmonic logic gates based on dielectric-metal-dielectric design with two optical communication bands

This paper presents a novel structure to simulate and achieve the all-optical plasmonic logic gates. We show how the use of dielectric-metal-dielectric (DMD) plasmonic waveguides with four nanorings and three slots can realize the complete family of logic gates; including NOT, OR, AND, NOT-OR, NAND, exclusive-OR, and exclusive-NOR using the constructive and destructive optical interferences between a control signal and input signal(s). We have adopted two optical wavelengths, namely 900 nm and 1330 nm, which are used for optical communication systems. Performance has been measured using the transmission level which registered more than 100 % for all of the seven gates of both wavelengths except NOT and NOT-OR logic gates. In addition, high extinction ratio of more than 20 dB in NOT and exclusive-NOR gates has been achieved using our novel structure.

Design of a microstructured surface for infrared radiation regulation based on structural combinations

There has been an increased interest on the development of infrared radiation (IR) regulation based on microstructured surfaces with desired radiative properties. Here design of a microstructured surface is proposed in terms of structural combinations to mediate the IR characteristics of targets. The designed microstructured surface is a combined structure, which is a two-dimension periodic composite grating obtained by combining two periodic simple gratings with different parameters and taking the union. Remarkably, the simple grating is also a combined structure obtained by combining and taking the intersection of two different single structures, a dielectric/metal/dielectric (DMD) structure and a metal grating structure. The results show that on the premise of having the structural characteristics of both constituent structures, the designed combined structures can not only inherit the spectral characteristics of both constituent structures but also have some new effects. For the adjusted composite grating, it is found that three absorption peaks with the maximal absorbance up to 89% are achieved in 5–8 μm range. Additionally, the average reflectance is greater than 80% in 3–5 μm and 8–14 μm bands. The adjusted composite grating fulfills the requirement for multiband regulation and the method of structural combinations suggests a feasible means to regulate the IR characteristics of targets.

An Optimized Dielectric-Metal-Dielectric Refractive Index Nanosensor

In this article, a nanoscale refractive index sensor with double rectangular silver slots perpendicularly coupled with a pair of horizontal dielectric-metal-dielectric (DMD) waveguides is proposed. The simulation of the transmission spectra, deploying the finite integration scheme, shows a linear relationship with the refractive index. Parameters of the suggested sensor have been thoroughly optimized to upgrade its sensing competency. The results achieve a maximum sensitivity of 1228.67 nm/RIU and a high resolution of 8.72×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> RIU, making the schematic an attractive option for commercial and research purposes. Additionally, the capability of the nanosensor as a temperature detector is tested and found to be operable for a freezing point as low as -100° C. Also, the device has undergone simulations of undesirable manufacturing defects, which can occur in real-time and has demonstrated a high degree of tolerance. The presented sensor, therefore, can be treated as a notable solution for on-chip gadgets with all these numerous features and shows promise for future development in the plasmonic refractive index sensor domain.

Cross-overlapped flat-silver/hexagonal boron nitride for translucent heat-reflective coatings

Multifunctional heat-reflective coatings are increasingly in demand for use in energy-efficient buildings and vehicles, as well as for optoelectronic devices. Conventional heat-reflective coatings (e.g. dielectric/metal/dielectric (DMD)) exhibit poor structural flexibility and combine complex fabrication process of multilayered structures, which hinders their scalable production. Herein, a low-cost translucent heat reflective mechanism is presented using single-coat composite film of flat-silver (flat-Ag) and hexagonal boron nitride (hBN) as an alternative to the multilayered DMD structure. The conversion of grey Ag-nanoparticles into shiny flat-Ag results in significant enhancement (~7 times) in infrared (IR) reflection. The dielectric-hBN layers integrated with flat-Ag improves the visible transmittance and heat-dissipating properties of coatings. The flat-Ag/hBN ink sprayed onto a flexible, transparent, and adhesive substrate of polydimethylsiloxane (PDMS) exhibits excellent IR-reflection (~70%) with ~57% visible transmittance. When irradiated by an IR heat source, the film displays 8.9 ± 0.5 °C lower temperature than those of uncoated surfaces, that results in reduction of the interior air-temperature by 5.4 ± 0.5 °C compared to the uncoated control. This newfabrication process of heat-reflective coatings with structural flexibility and optical transparency has great potentials to be used for thermal management in building and transportation sectors.

Dichroic Sb2O3/Ag/Sb2O3 Electrodes for Colorful Semitransparent Organic Solar Cells

The pace of advancement and increasing power conversion efficiencies (PCEs) have provided the possibility for organic solar cells (OSCs) to be commercialized in a variety of applications, including semitransparent photovoltaics. The application of dielectric–metal–dielectric (DMD) transparent electrodes to OSCs is an effective way to achieve semitransparent OSCs with different colors. Herein, a DMD multilayer structure based on two Sb2O3 layers and silver (Ag) thin films as the top electrode is introduced. An ultrathin Sb2O3 layer is deposited between the electron transport layer and the Ag film as a bottom layer for the Sb2O3/Ag/Sb2O3 electrode; this layer inhibits Ag atom diffusion and aggregation, which leads to uniform formation of ultrathin Ag films. The thickness of the middle metal layer influences fill factor and short‐circuit current density values, which correlate well with device resistance and light reflection. For the top Sb2O3 layer, the thickness allows the selective transmittance of specific wavelengths of visible light while reflecting wavelengths that are not transmitted, creating a dichroic effect. OSCs are successfully fabricated using three kinds of colorful active layers in conjunction with Sb2O3/Ag/Sb2O3 electrodes, resulting in vividly colored devices with PCE of 6.33–7.88% and average visible transmittances of 23–30%.

Optical and Chromaticity Properties of Metal-Dielectric Composite-Based Multilayer Thin-Film Structures Prepared by RF Magnetron Sputtering

Coated glass products, and especially the low-emissivity coatings, have become a common building material used in modern architectural projects. More recently, these material systems became common in specialized glazing systems featuring solar energy harvesting. Apart from achieving the stability of optical parameters in multilayer coatings, it is also important to have improved control over the design of visual color properties of the coated glass. We prepare metal-dielectric composite (MDC)-based multilayer thin-film structures using the radio frequency (RF)-magnetron sputtering deposition and report on their optical and chromaticity properties in comparison with these obtained using pure metal-based Dielectric/Metal/Dielectric (DMD) trilayer structures of similar compositions. Experimentally achieved Hunter L, a, b values of MDC-based multilayer building blocks of coatings provide a new outlook on the engineering of future-generation optical coatings with better color consistency and developing approaches to broaden the range of achievable color coordinates and better environmental stability.

Semitransparent Perovskite Solar Cells with Dielectric/Metal/Dielectric Top Electrodes

A semitransparent perovskite solar cell (PSC) with a dielectric/metal/dielectric (DMD) multilayer film as the top transparent electrode is investigated. Through adjusting the thickness and the deposition rate of Ag and WO3 layers, a transparent electrode with a low sheet resistance of 7 Ω sq−1 and high average visible transmittance (AVT) of 73% in the visible wavelength range of 400–800 nm is obtained. Using the resultant DMD film as the top transparent electrode and different bandgap perovskites of CH3NH3PbI3 (MAPbI3), CH(NH2)2PbI3 (FAPbI3), and FA0.5MA0.38Cs0.12PbI2.04Br0.96 as the optical active layer, a solar cell with a device architecture of ITO/SnO2/perovskite/spiro‐OMeTAD/MoO3/Ag/WO3 is fabricated. A series of efficient semitransparent PSCs with high transmittance are achieved.

Optical properties of ultra-thin silver films deposited by thermal evaporation and its application in optical filters

The ultra-thin silver films with different thickness are deposited by resistance thermal evaporation at room temperature, and the optical constants n and k are obtained by using variable angle spectroscopic ellipsometer (VASE) in the spectral range 380–1600 nm. The thickness range of ultra-thin silver film is 5.3–24.9 nm. SEM images on the surface morphology of ultra-thin silver films show that the ultrathin silver films have discontinuous island structure. The volume fraction of solid silver materials in ultra-thin silver films are simulated by Brugeman approximation theory. Dielectric-metal-dielectric (D-M-D) sandwich structure ZnS/Ag/ZnS optical filter is designed with the optical constant of the modified ultra-thin silver films. The average transmittance of the fabricated D-M-D optical filter is as high as 88.5% and peak transmittance is above 90.5% in the wavelength range between 400 and 700 nm. The maximum reflectance was 94.0% in the region of 2000–12000 nm.

Low-Temperature and Corrosion-Resistant Gas Diffusion Multibarrier with UV and Heat Rejection Capability-A Strategy to Ensure Reliability of Organic Electronics.

When placed in an outdoor environment, organic electronic devices (OEDs) can degrade on exposure to moisture, UV light, and heat, owing to the chemical sensitivity and decomposition of the organic materials. Therefore, to protect OEDs from outdoor environments, thin-film passivation, which can block harmful elements from reaching organic materials, is required. To meet the demands and trends in encapsulation technologies, in this study, we developed a low-temperature, simple, and effective gas diffusion multibarrier (GDM), which is UV and heat reflective as well as corrosion resistant. The designed UV- and heat-reflective GDM (UHGDM) has a multistacked structure in the form of a UV filter/Ag/gas diffusion barrier (GDB)/polymer based on a dielectric/metal/dielectric (DMD) configuration. First, the DMD structure was used as a heat mirror for infrared reflectance. Second, the bottom dielectric layer of the DMD structure was used as the UV filter, and it consisted of a ZnS/LiF multistacked structure with large differences in refractive indexes. Third, a nanolaminate-based GDB barrier with multi-interfacial and defect-decoupling systems, which achieved a water vapor transmission rate of 1.58 × 10-5 g/m2/day at a thickness of 60 nm, was used as the top dielectric layer of the DMD structure. Finally, an inorganic/organic hybrid polymer layer was coated on the DMD structure to provide corrosion-resistance and waterproofing properties. The fabricated UHGDM showed high transparency in the visible region and excellent reflectance in the UV and IR regions, resulting in excellent UV and heat rejection capability in practical UV and heat reflection tests. In addition to optical functionalities, the UHGDM maintained its functionality against harsh environmental conditions because of the GDB/polymer structure. Finally, the feasibility of the UHGDM was demonstrated using organic solar cells through water immersion and shelf lifetime tests.

Improved photovoltaic effect in graphene/silicon solar cell using MoO3/Ag/MoO3 multilayer coating

Molybdenum trioxide (MoO3) is one of the proven transition metal oxides to boost up the work function of graphene. In this letter, we report for the first time the use of MoO3 based dielectric/metal/dielectric (DMD) multilayer coating to improve the photovoltaic performance of graphene/silicon solar cells. Solar cells employing transparent conductive MoO3/Ag/MoO3 on graphene offered improved power conversion efficiency (PCE) by a factor of 4 compared to graphene only devices. This strategy appears advantageous not only to modulate the work function of graphene but also to reduce the device series resistance for better charge carrier extraction.

Nonprecious Copper‐Based Transparent Top Electrode via Seed Layer–Assisted Thermal Evaporation for High‐Performance Semitransparent n‐i‐p Perovskite Solar Cells

AbstractSemitransparent perovskite solar cells (ST‐PSCs) are highly attractive for applications in building‐integrated photovoltaics as well as in multijunction tandem devices. To fabricate high‐performance ST‐PSCs, suitable transparent top electrodes are strongly needed. Dielectric/metal/dielectric (DMD) multilayer structures have been shown to be promising candidates, though generally based on high‐value metals such as gold or silver, the latter causing also stability issues by reacting with perovskite. Here, a novel DMD transparent electrode based on nonprecious, less‐reactive copper is developed via thermal evaporation and used as a top anode in the fabrication of high‐performance semitransparent n‐i‐p perovskite solar cells, the best device yielding a power conversion efficiency as high as 12.5%. The DMD architecture consists of a gold‐seeded Cu thin film sandwiched between two MoO x dielectric layers. It is demonstrated that Cu self‐aggregation and diffusion into MoOx can be substantially limited by introducing an ultrathin (1.5 nm) Au seed layer, and conductive Cu films as thin as 9.5 nm can be achieved. A fine tuning of the perovskite layer thickness is also carried out to further enhance the device transparency up to a maximum average visible transmittance approaching 25%.

Polymer interlayers on flexible PET substrates enabling ultra-high performance, ITO-free dielectric/metal/dielectric transparent electrode

This work reports on the fabrication and properties of a dielectric/metal/dielectric (DMD) transparent electrode on polyethylene terephthalate (PET) flexible substrate with polymer interlayers. The sputtered TiOx/Ag/AZO (AZO: Al-doped ZnO) DMD electrode yields performance beyond the current state-of-the-art for transparent electrodes on PET, with an average transmittance larger than 85% (including the substrate) in the range 400–700 nm and sheet resistance below 6 Ω/square. Within a broad visible spectral range, the transmittance of the flexible electrode surpasses the transmittance of the bare PET. This performance was enabled by the use of a polymer layer between the PET and the DMD electrode. The interlayer modifies the growth of the TiOx on the flexible substrate, rendering it similar to its growth on glass. This markedly increases the transmittance of the DMD electrode and decreases its sheet resistance. The presented electrode has a large potential for photovoltaic and optoelectronic applications, outperforming the widely-used indium-tin-oxide (ITO) on flexible substrates.