Semitransparent Electrode Research Articles

Easily Prepared Transparent Electrodes for Low-Cost Semitransparent Inverted Polymer Solar Cells

The continuous thin film of silver (Ag) film is important for semitransparent electrodes in polymer solar cells, while the Ag atoms form as non-continuous below a critical thickness. Here, semitransparent inverted polymer solar cells were fabricated using thermally evaporated Ag/germanium (Ge)/Ag as highly transparent electrodes. An ultra-thin Ge film was introduced to modify the growth mode of Ag. The dependence of the device performance and the thickness of the outer Ag film was investigated. Ag/Ge/Ag electrodes exhibited excellent optical and electrical properties, which were proved by the transmittance and reflectance spectra. A champion efficiency of 5.1% was achieved with an open-circuit voltage level of 0.703 V, a short current density of 11.63 mA/cm2, and a fill factor of 63%. The average visible transmittance (300–800 nm) of devices with Ag/Ge/Ag was calculated as 25%.

Investigation of the threshold voltage instability in normally-off p-GaN/AlGaN/GaN HEMTs by optical analysis

A normally-off p-GaN/AlGaN/GaN high-electron-mobility transistor (p-GaN HEMTs) with a semitransparent gate electrode was investigated. Under forward gate bias, electroluminescence (EL) emission was observed which demonstrated the hole injection in the gate heterostructure. The hole injection emerged at a gate bias of 3.5 V and was significantly enhanced when the gate bias was larger than 6 V, the corresponding EL emission intensity was increased by approximately 100 times. It was also found that the electron injection and trapping in the p-GaN resulted in the threshold voltage (V TH) instability of the p-GaN HEMTs, while the hole injection resulted in the electron–hole recombination and the suppression of electron trapping. The carrier injection dynamics in the p-GaN gate region were quantitatively identified at different temperatures. Finally, the GaN band-edge ultraviolet emission with an energy of 3.4 eV was demonstrated as the most effective component in EL emission for V TH instability suppression in the p-GaN HEMTs.

Quantum Tunneling Induced Optical Rectification and Plasmon-Enhanced Photocurrent in Nanocavity Molecular Junctions.

Molecular junctions offer the opportunity for downscaling optoelectronic devices. Separating two electrodes with a single layer of molecules accesses the quantum-tunneling regime at low voltages (<1 V), where tunneling currents become highly sensitive to local nanometer-scale geometric features of the electrodes. These features generate asymmetries in the electrical response of the junction which combine with the incident oscillating optical fields to produce optical rectification and photocurrents. Maximizing photocurrents requires accurate control of the overall junction geometry and a large confined optical field in the optimal location. Plasmonic nanostructures such as metallic nanoparticles are prime candidates for this application, because their size and shape dictate a consistent junction geometry while strongly enhancing the optical field from incident light. Here we demonstrate a robust lithography-free molecular optoelectronic device geometry, where a metallic nanoparticle on a self-assembled molecular monolayer is sandwiched between planar bottom and semitransparent top electrodes, to create molecular junctions with reproducible morphology and electrical response. The well-defined geometry enables predictable and intense plasmonic localization, which we show creates optical-frequency voltages ∼ 30 mV in the molecular junction from 100 μW incident light, generating photocurrent by optical rectification (>10 μA/W) from only a few hundred molecules. Quantitative agreement is thus obtained between DC- and optical-frequency quantum-tunneling currents, predicted by a simple analytic equation. By measuring the degree of junction asymmetry for different molecular monolayers, we find that molecules with a large DC rectification ratio also boost zero-bias electrical asymmetry, making them good candidates for sensing and energy harvesting applications in combination with plasmonic nanomaterials.

Semitransparent Thin-Film Dual-Metal Anode for Red-Phosphorescent Organic Light-Emitting Diodes

Semitransparent dual-metal electrodes comprising several thin layers of metals, such as Ni, Ag, Cu, and Al, were developed for designing flexible red-phosphorescent organic light-emitting diodes (OLEDs). The said diodes were fabricated by first depositing a Ni layer on four glass and polyethylene terephthalate (PET) substrates each to facilitate adhesion with glass and a flexible PET substrate. Subsequently, a conductive layer of Ni, Ag, Cu, and Al was stacked atop the first Ni layer on the four glass and PET substrates each, respectively. The proof of principles has been employed to demonstrate the performance potential via optical, physical, and electrical analyses of dual-as well as single-metal layers prior to device realization. In addition, their electrical and optical characteristics were compared against those of In-Sn-oxide-based OLEDs to demonstrate their potential with regard to application flexibility.

Bifacial Color-Tunable Electroluminescent Devices.

Alternate current electroluminescent (ACEL) devices provide a range of interesting properties, such as facile large-area processability, mechanical flexibility, and outstanding resilience, when compared with other large-area light-emitting technologies. To widen the scope of possible applications for ACEL devices, color tunability and white light emission are desirable. Here, we introduce a novel three-terminal device architecture based on two monolithically stacked ACEL devices (e.g., orange and blue) that allows for color tunability via independent operation of the subdevices. The tandem devices comprise semitransparent bottom and top electrodes based on networks of silver nanowires, which endow the tandem ACEL device with bifacial Janus-type emission. We provide a detailed analysis of the sources of optical losses in single and tandem ACEL devices. Our novel device concept enables novel facets of applications for ACEL in signage and lighting.

Temporary Tattoo Approach for a Transferable Printed Organic Photodiode

Generation of ultrathin, transferable, and imperceptible electronic devices [e.g., organic photodiode (OPD)] for multiple applications, such as personalized health monitors and wearables, is emerging due to the continuous development of materials and manufacturing processes. For such devices, the choice of a suitable substrate is of utmost importance. A water decal transfer from a temporary tattoo paper is adopted here as a substrate for ultrathin and conformable organic components because of easy and reliable transfer of a ≈600 nm robust and transparent polymer nanofilm of ethyl cellulose. Strategies for the fabrication of a transferable OPD on a temporary tattoo are investigated. A device with an overall thickness <1 μm and its performance after transfer are demonstrated. Then, efforts are put into fabricating an OPD by inkjet printing with a water-soluble active layer consisting of polythiophene and fullerene derivatives to aid cost- and material-efficient, large-scale production possibilities. Additionally, a second semitransparent electrode made of printed aluminum-doped zinc oxide and silver nanowires is used to allow usage from both sides to enhance the application potential. Both OPD examples presented here need improvement of the device performance but permitted us to highlight the versatility and application potential of temporary tattoos for transferable components. Target surfaces for the final application after transfer include artificial (flat and smooth, e.g., glass, or even complex and rough, e.g., concrete, paper, and so forth) as well as natural ones.

Introduction of a Novel Figure of Merit for the Assessment of Transparent Conductive Electrodes in Photovoltaics: Exact and Approximate Form

AbstractTransparent conductive electrodes (TCEs) are key components of photovoltaic devices. Being transparent, they allow light to enter the device, and being conductive, they allow the photocurrent generated to be drawn into the outer electric circuit. Ideally, TCEs exhibit maximum light transmission and conductivity at the same time. However, both properties have to be balanced. Depending on the photovoltaic material system, the selection of the most suitable TCE is crucial and is assessed by so‐called figures‐of‐merit (FOM). Here, a novel and exact FOM that explicitly considers the impact on photovoltaic performance is proposed. This novel FOM exhibits several useful attributes, among them: i) proportionality to the potential power output of the photovoltaic device, ii) normalization with regard to the theoretically ultimately attainable photovoltaic performance and, thus, it provides above all iii) meaningful guidance for the development of advanced TCEs. Based on the exact FOM, the transition sheet resistance is defined as the parameter, which separates the so‐far unidentified two regimes of TCE operation: transmittance versus conductance limited. An overview of realized state‐of‐the‐art semitransparent electrodes is reassessed and compared herein. Furthermore, the TCE requirements for various photovoltaic material systems are assessed in dependence on the spectral range of their operation.

Flexible Organic Photovoltaics with Colorful Semi-Transparent Metal/Dielectric/Metal Top Electrode

Organic solar cells (OSCs) have the advantages of a semi-transparent thin film, and of changing colors through controlling optical energy bandgap of organic semiconducting materials. In addition, the flexible OSCs can also be fabricated to be applied in multiple applications such as film-type or foldable solar cells. Here, we have fabricated flexible semi-transparent organic solar cells (STOSCs) with high transparency accompanying highly vivid red, green, and blue colors. To gain high transparency of STOSCs, we employed Fabry-Pérot etalon-type electrodes. An antimony oxide (Sb2O3) cavity layer was introduced between two thin silver layers. By adjusting the thickness of Sb2O3 from 45 nm to 100 nm, vivid colors of red, green, and blue semi-transparent top electrodes were achieved. Besides, selecting well-matched photoactive layers for each color, a less drop of short-circuit current density (Jsc) has been observed. Finally, we obtained high maximum transmittance of 22.52%, 30.10%, and 22.13%, with a power conversion efficiency of 9.16%, 9.85%, and 12.64% for red, green, and blue STOSCs, respectively. Likewise, a power conversion efficiency of 5.75% was achieved with a green-colored large area sub-module.

Electrochromic Inorganic Nanostructures with High Chromaticity and Superior Brightness.

The possibility of actively controlling structural colors has recently attracted a lot of attention, in particular for new types of reflective displays (electronic paper). However, it has proven challenging to achieve good image quality in such devices, mainly because many subpixels are necessary and the semitransparent counter electrodes lower the total reflectance. Here we present an inorganic electrochromic nanostructure based on tungsten trioxide, gold, and a thin platinum mirror. The platinum reflector provides a wide color range and makes it possible to “reverse” the device design so that electrolyte and counter electrode can be placed behind the nanostructures with respect to the viewer. Importantly, this makes it possible to maintain high reflectance regardless of how the electrochemical cell is constructed. We show that our nanostructures clearly outperform the latest commercial color e-reader in terms of both color range and brightness.

Exploiting Ternary Blends to Accurately Control the Coloration of Semitransparent, Non‐Fullerene, Organic Solar Cells

Semitransparent organic solar cells (STOSCs) have received increasing attention due to promising applications such as building‐integrated photovoltaics. Successful commercialization requires that STOSCs are aesthetically pleasing as well as having balanced power conversion efficiencies (PCEs) and average visible transmittances (AVTs). Non‐fullerene acceptors, which possess excellent electrical/chemical properties, have helped STOSCs to achieve high PCE and AVT; however, research related to modulating color and appearance of STOSCs has lagged behind. Herein, narrow bandgap donor and acceptor (PTB7‐Th and IEICO‐4F) and ultra‐wide bandgap acceptors (T2‐ORH and T2‐OEHRH) are used to achieve semitransparency and controllable device coloration. Blend films with controllable colors including cyan → blue → purple → reddish purple colors are successfully demonstrated, which are controlled by ratios of IEICO‐4F:T2‐ORH or IEICO‐4F:T2‐OEHRH with PTB7‐Th. By incorporating semitransparent electrodes (comprising Sb2O3/Ag/Sb2O3), STOSCs with PCEs of 6–7% are achieved for cyan, aqua, indigo, and purple and ≈4% PCEs for reddish‐purple colors, with AVTs in the range of 23–35%. Moreover, optical properties of blend films are studied via absorption and transmission measurements, whereas the range of colors achieved is quantified using commission internationale de l'éclairage (CIE) chromaticity and CIE L * a * b* color space then represented as RGB color models.

Optical outcoupling in monochromatic top-emitting organic light-emitting diodes with an Au/Ag semi-transparent electrode

Top-emitting organic light-emitting diodes can achieve high efficiencies due to the strong cavity effect resulting from the relatively thick semi-transparent metallic top electrode. The strong cavity resonance, however, simultaneously brings along negative side effects such as pronounced angular-dependent emission and spectral narrowing. In this work, through numerical simulations, we demonstrate that top-emitting organic light-emitting diodes using a thin Au(2 nm)/Ag(7 nm) top electrode can achieve light-outcoupling efficiency comparable to a thick silver electrode, while reducing spectral narrowing. This can be realized by tuning the organic capping layer thickness without affecting the electrical properties, which can be applied to diodes based on either intrinsic or efficiently doped charge transport layers.

Utilization of Trapped Optical Modes for White Perovskite Light-Emitting Diodes with Efficiency over 12%

<h2>Summary</h2> The inferior light extraction efficiency (LEE), which is generally less than 20%, based on optical modeling, and the difficulty in achieving white emission are the two main challenges in the metal-halide-perovskite light-emitting diode (PeLED) field. Herein, we report a simple and efficient approach to construct high-performance white PeLEDs with much-enhanced LEE by coupling a blue PeLED with a layer of red perovskite nanocrystal (PeNC) down-converter through a rationally designed multilayer semitransparent electrode (LiF/Al/Ag/LiF). The red PeNC layer allows the extraction of the trapped waveguide mode and surface plasmon polariton mode in a blue PeLED and converts them to red emission, resulting in over 50% LEE improvement. Simultaneously, the complementary emission spectrum of blue photons and down-converting red photons contributes to a white PeLED with a high external quantum efficiency and luminance of more than 12% and approximately 2,000 cd m⁻², respectively, which represent state-of-the-art results in this field.

Surface plasmonic resonance tunable nanocomposite thin films applicable to color filters, heat mirrors, semi-transparent electrodes, and electromagnetic-shields.

This study proposes a plasmonic resonance-tunable nanocomposite thin film, which applies to a color filter, heat mirror, semi-transparent color electrode, and electromagnetic shield, given that the size and structure of nanoclusters can be controlled by a sputtering power density. The structural and functional properties of silver/plasma-polymer-fluorocarbon (Ag/PPFC) nanocomposite thin films, which were sputtered by ternary composite targets, were investigated with various compositions and sputtering power densities. The growth of Ag nanoclusters of the thin film was suppressed as the sputtering power density increased due to the rich functional group of -CFx- fluorine. As a result, a continuous color change from blue to yellow could be expressed on films given the precise control of the surface plasmonic resonance phenomenon. Grazing-incidence small-angle scattering (GISAXS) analysis indicated that the sputtering power density had a significant effect on the size, distribution, and orientation of the Ag nanoclusters in the thin film. For low sputtering power densities, Ag nanoclusters were forming aggregations along the out-of-plane direction, but as the sputtering power density increased, the nanoclusters showed random distribution instead of large aggregates. We also demonstrated applications of Ag/PPFC nanocomposite thin films to a color filter, heat mirror, semi-transparent electrode, and electromagnetic shield. In addition, the fabrication of a large-area film (400 × 700 mm2) showed that the approach applies highly to industries.

Contact photolithography-free integration of patterned and semi-transparent indium tin oxide stimulation electrodes into polydimethylsiloxane-based heart-on-a-chip devices for streamlining physiological recordings.

Controlled electrical stimulation is essential for evaluating the physiology of cardiac tissues engineered in heart-on-a-chip devices. However, existing stimulation techniques, such as external platinum electrodes or opaque microelectrode arrays patterned on glass substrates, have limited throughput, reproducibility, or compatibility with other desirable features of heart-on-a-chip systems, such as the use of tunable culture substrates, imaging accessibility, or enclosure in a microfluidic device. In this study, indium tin oxide (ITO), a conductive, semi-transparent, and biocompatible material, was deposited onto glass and polydimethylsiloxane (PDMS)-coated coverslips as parallel or point stimulation electrodes using laser-cut tape masks. ITO caused substrate discoloration but did not prevent brightfield imaging. ITO-patterned substrates were microcontact printed with arrayed lines of fibronectin and seeded with neonatal rat ventricular myocytes, which assembled into aligned cardiac tissues. ITO deposited as parallel or point electrodes was connected to an external stimulator and used to successfully stimulate micropatterned cardiac tissues to generate calcium transients or propagating calcium waves, respectively. ITO electrodes were also integrated into the cantilever-based muscular thin film (MTF) assay to stimulate and quantify the contraction of micropatterned cardiac tissues. To demonstrate the potential for multiple ITO electrodes to be integrated into larger, multiplexed systems, two sets of ITO electrodes were deposited onto a single substrate and used to stimulate the contraction of distinct micropatterned cardiac tissues independently. Collectively, these approaches for integrating ITO electrodes into heart-on-a-chip devices are relatively facile, modular, and scalable and could have diverse applications in microphysiological systems of excitable tissues.

A critical review on semitransparent organic solar cells

Semitransparent solar cells not only can generate electricity but also exhibit promising application in facades and roofs of building, as well as the coatings of vehicle. Ideal semitransparent solar cells should have a high utilization efficiency of ultraviolet and near infrared photons, and maintain an appropriate balance between absorption and transparency in visible light range. Organic semiconducting materials with adjustable optical band gap own great potential in preparing highly efficient semitransparent organic solar cells (OSCs). There are great challenges in achieving semitransparent electrodes with good conductivity and high transmittance in visible light range, as well as high reflectance in ultraviolet and near infrared range. In this critical review, we summarize the recent progress of semitransparent OSCs, concentrating on organic semiconducting materials, semitransparent top electrode and device engineering. Semitransparent OSCs have attracted more and more attention due to its great potential in energy source in door, electricity-generating windows, building-integrated photovoltaics and agricultural greenhouse. Semitransparent OSCs have attracted more and more attention due to its great potential in electricity-generating windows, building-integrated photovoltaics and agricultural greenhouse. This review comprehensively summarizes recent progresses of organic material, semitransparent top electrode and device engineering, the challenges on achieving semitransparent OSCs are highlighted to promote the development of this field. • Recent development of organic materials with strong ultraviolet or near infrared absorption were summarized. • The key parameters for evaluating the performance of semitransparent OSCs were discussed. • This review comprehensively summarized recent progress of transparent top electrodes. • The light management methods and device designs are firstly comprehensively reported.

Top-Illuminated Flexible Organic Photodetectors Integrated With Hole Extraction Layers Synthesized With Solution-Processed NiOₓ Films at Room Temperature

Inverted, top-illuminated flexible organic photodetectors (OPDs) are demonstrated using solution-processed hole extraction layer (HEL) at room temperature and semitransparent electrodes. The simplified fabrication process OPD yields a low dark current density of 80.9 nA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at −5 V and a high sensitivity, as confirmed by a record high photo-to-dark-current ratio in excess of 7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$ </tex-math></inline-formula> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> in the cases of light illuminations at 525 nm. Furthermore, the top-illuminated OPD also exhibits a high responsivity and a detection gain of 0.35A/W and 2.15 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$ </tex-math></inline-formula> 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> Jones, respectively. This flexible OPD exhibits an outstanding mechanical flexibility following tests with 60 000 bends. It is envisaged that this research will have a meaningful contribution to the next-generation OPD applications.

Microfluidic fabrication with silver nanowires for optofluidic structures with three-dimensional operation

With the current design of modern optical lens technology, the optical parameters and field-of-view are restricted, preventing maximum functionality. Such restrictions can be mitigated with the implementation of a three-dimensional compound-eye lens, however, the fabrication of this three-dimensional compound-eye lens is associated with several challenges. Such challenges include the development of a semi-transparent and flexible electrode material and the operation and adherence of out-of-plane optofluidic lenses to oblique interfaces. This work addresses these challenges through two experimental analyses. The first experimental analysis is a study of silver nanowires with feature size of approximately 90 nm for use as a flexible and semi-transparent electrode material, and the effects of filtering and processing methods on the electrode sheet resistance. With optimal preparation of the silver nanowires, the fabricated electrode is seen to have favourable properties for this application. The second experimental analysis is a study of the optimal hydrophobic coating for minimal hysteresis and the optical operation of out-of-plane optofluidic lenses situated on oblique interfaces, as is required for peripheral out-of-plane optofluidic lenses in a three-dimensional compound-eye lens. The out-of-plane optofluidic lenses are seen to have favourable operation and voltage control with minimal hysteresis. The results show promise towards development of wide field-of-view systems with tunable optical parameters.

Contactless electroreflectance spectroscopy with a semitransparent capacitor made of a silver mesh of ultrathin lines

Semitransparent electrodes operating in the broad spectral region are needed for many applications and one of them is contactless electroreflectance (CER) spectroscopy. In this work, the technology of printed electronics has been applied to fabricate mesh-like semitransparent electrodes composed of ultrathin (10 µm) silver lines of different densities. These electrodes have been printed on borosilicate glass and used in CER measurements. It has been shown that the CER signal is linearly proportional to the applied voltage and inversely proportional to the distance. In addition, a shadow effect has been observed for some distances because of non-transparent silver paths. Finally, a phenomenological formula has been proposed to describe the intensity of the CER signal in the capacitor with the semitransparent electrode. Moreover, the mesh-like printed electrodes have been applied to measure CER spectra for van der Waals crystals and perspectives for further development of semitransparent electrodes for CER measurements have been discussed.

A Comparison of the Structure and Properties of Opaque and Semi-Transparent NIP/PIN-Type Scalable Perovskite Solar Cells

For over a decade, single-junction perovskite solar cells (PSCs) have experienced an unprecedent increase in efficiencies and even offer opportunities to surpass the Shockley–Queisser limit in multijunction configuration. There is consequently an intense need for easily processable semi-transparent PSCs as a basis of affordable tandems. The current study reports the comparison of negative-intrinsic-positive (NIP) and positive-intrinsic-negative (PIN) architectures based on CH3NH3PbI3{Cl}-based perovskite. Both devices could be prepared with the same N-type (SnO2 nanoparticles) and P-type (poly-triarylamine (PTAA) polymer) materials. Each layer (except for electrodes) was deposited using solvent-based low temperature processes, contrasting with other literature studies, especially SnO2 for PIN-type purposes. A thorough experimental comparison of the two architectures reveals rather similar optical and structural properties for perovskites, whether deposited on an N- or P-type underlayer, with also comparable efficiencies in the final devices. A compatible deposition process for sputtered indium tin oxide (ITO) as a semi-transparent electrode was then performed for both architectures. Upon varying the illuminated devices’ side, the semi-transparent cells exhibited different photocurrent behaviors, the magnitude of which depended on the device’s architecture. In conclusion, despite slightly better efficiencies for the semi-transparent NIP-type devices, the semi-transparent PIN-type counterparts also appear to be optically attractive for (two-terminal) tandem applications.

High-Performance 4H-SiC Schottky Photodiode With Semitransparent Grid-Electrode for EUV Detection

In this work, a 2.5 × 2.5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> large-area 4H-SiC Schottky barrier photodiode with a grid-shaped semitransparent metal electrode is designed and fabricated for extreme ultraviolet (EUV) detection. The photodiode shows an ultra-low leakage current of ~3 pA under 20 V reverse bias at room temperature. Based on a synchrotron radiation source, the photo-response characteristics of the photodiode is measured between 5 nm and 140 nm, which show considerably higher responsivity than that of the control whole electrode device. The higher quantum efficiency of the grid-electrode device benefits from considerable lateral depletion of the SiC surface light absorption layer between adjacent electrode strips, which occurs even under zero bias. The device further exhibits good potential for high temperature operation.