Transparent Composite Electrode Research Articles

Transparent Perovskite Light-Emitting Touch-Responsive Device.

A light-emitting touch-responsive device (LETD) for instantaneous visualization of pressure mapping is reported. The LETD integrates an organometal halide perovskite polymer composite emissive layer and a flexible silver nanowire polyurethane composite transparent electrode. The composite emissive layer contains methylammonium lead bromide nanocrystals uniformly dispersed in a poly(ethylene oxide) (PEO) matrix and emits an intense green luminescence that peaks at 529 nm. The PEO matrix promotes the formation of small perovskite grains (∼20 nm) and a pinhole-free composite film with surface roughness of only 2.96 nm. The composite transparent electrode is separated from the emissive layer with a 100 μm thick spacer. When a local pressure is applied, a Schottky contact is formed instantaneously between the metal and the emissive layer, and electroluminescence is produced at voltages as low as 2.5 V and reaches 1030 cd/m2 at 6 V. The transparent LETD has approximately 68% transparency. It can be bent to a 6 mm radius when polyethylene terephthalate is used as the substrate. The perovskite LETD has fast response and can be pixelated to offer potential applications in robotics, motion detection, fingerprint devices, and interactive wallpapers.

The effect of hole transfer layers and anodes on indium-free TiO2/Ag/TiO2 electrode and ITO electrode based P3HT:PCBM organic solar cells

Transparent composite electrodes (TCE), TiO2/Ag/TiO2 (TAgT), have been demonstrated as a promising alternative of indium tin oxide (ITO) in organic solar cell application. TiO2/Ag/TiO2 have been incorporated into bulk heterojunction organic solar cells (OSCs) to replace ITO as the anode. Two different hole transfer layers (HTL), poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) and MoO3, are compared in terms of their compatibility with the TiO2/Ag/TiO2 anode. The MoO3 layer has been shown to be a beneficial HTL for TiO2/Ag/TiO2 based OSC. However, the wettability of MoO3 for polymer blend is not as good as PEDOT:PSS, and results in a relatively thin active layer in OSCs. On the other hand, ITO based OSCs using PEDOT:PSS and MoO3 as the HTL are fabricated as control samples to compare the performance of TAgT and ITO anodes. Results of the investigation shows that the superior electrical property of TAgT anodes contributes to the effective collection of photo-carriers and its lower optical transmittance barely degrades the light absorption in OSCs. In a few words, the higher Haacke figure of merit of TAgT enables better performance of OSCs with MoO3 HTL, in comparison with ITO based OSC.

Surface Engineering of Graphene Composite Transparent Electrodes for High-Performance Flexible Triboelectric Nanogenerators and Self-Powered Sensors.

High-performance transparent and flexible triboelectric nanogenerators (TENGs) based on graphene composite electrodes via surface engineering are proposed and demonstrated. Through modifying the CVD-grown graphene with the conductive polymer poly(3,4-ethylenedioxy-thiophene):polystyrenesulfonate (PEDOT:PSS), composite electrodes with excellent optoelectronic performances were fabricated, which exhibited a high transmittance up to 83.5% and sheet resistance of 85 Ω/□, decreasing from the initial value of 725 Ω/□. As a consequence, the output current density and power of the corresponding TENG were enhanced by 140% to 2.4 μA/cm2 and by 118% to 12 μW, respectively, comparing with the counterpart composed of the pristine graphene electrodes. Furthermore, the composite electrode exhibited an outstanding durability of the physical and electrical characteristics after 10 000 bending cycles and can be readily extended to a large area up to 100 cm2. Such flexible, transparent, stable TENGs pave the way for the application of self-powered body sensors due to their unique characteristics, such as portability, wearability, and human compatibility.

Preparation and investigation of composite transparent electrodes of poly(3, 4-ethylenedioxythiophene) polystyrene sulfonate/single-wall carbon nanotubes

The preparation of composite transparent electrodes of poly(3, 4-ethylenedioxythiophene) polystyrene sulfonate/single-wall carbon nanotubes by the spray method is described. The influence of the successive treatment of each functional layer in acid media with different activities on the optical and electric film characteristics is considered. The composite with the surface resistance of 89 Ω/sq at the transparency of 85.3% (550 nm) on a polymer substrate is obtained.

Al-doped ZnO/Ag-nanowire Composite Electrodes for Flexible 3-Dimensional Nanowire Solar Cells

Silver nanowires in conjunction with sputter-coated Al-doped ZnO (AZO) thin films were used as a composite transparent top electrode for hybrid radial-junction ZnO nanowire/a-Si:H p-i-n thin-film solar cells. Solar cells with the composite nanowire top contacts attained a short-circuit current density (Jsc) of 13.9 mA/cm2 and a fill factor (FF) of 62% on glass substrates while a Jsc of 13.0 mA/cm2 and FF of 62% was achieved on plastic substrates. The power conversion efficiency (PCE) of the 3-dimensional solar cells improved by up to 60% compared to using AZO electrodes alone due to enhanced coverage of the top electrode over the 3-D structures, decreasing the series resistance of the device by 5×. The composite layer also showed a 10× reduction in sheet resistance compared to the AZO thin-film contact under applied mechanical strain.

Highly Conductive and Uniform Alginate/Silver Nanowire Composite Transparent Electrode by Room Temperature Solution Processing for Organic Light Emitting Diode.

A novel transparent electrode composed of alginate/silver nanowire (AgNW) with high conductivity and low roughness is fabricated via a solution process at room temperature. The sol-gel transition of the alginate triggered by CaCl2 solution bonds the AgNWs to the substrate tightly. Meanwhile, Cl- in the solution can renovate the cracks on the AgNW surfaces created during the mechanical pressing, resulting in a great increase of the electrical conductivity. The alginate/AgNW composite film can reach a sheet resistance of 2.3 Ω/sq with a transmittance of 83% at 550 nm. The conductivity of the composite film remains stable after bending and tape tests, demonstrating excellent flexibility and great adhesion of AgNWs to the substrate. Moreover, the composite film shows better stability to resist longtime storage than conventional annealed-AgNW film. The organic light emitting diode using such alginate/AgNW composite film as anode presents current densities and luminances comparable to those of indium tin oxide (ITO) anode, and higher efficiencies are obtained due to the better charge balance.

Elastomeric Light Emitting Polymer Enhanced by Interpenetrating Networks.

Conjugated polymers containing long-chain alkyl side groups for solubility are generally unstretchable: large strain induces crack formation, fracture, or plastic deformation. When the polymers are stretched to reorient the conjugated chains along the stretching direction, high dichroic ratio is observed both for absorption of the ground state and radiative decay of the excited state. Here an interpenetrating polymer network (IPN) approach is reported to impart elastomeric deformability to a conjugated polymer. A soluble alkyloxy phenyl substituted poly(1,4-phenylenevinylene) (SY-PPV) with bright yellow fluorescent emission was admixed with an ionically conductive medium containing poly(ethylene oxide), exoxylated trimethylolpropanetriacrylate, and lithium trifluoromethanesulfonate. The spin-cast blend film formed an IPN morphology wherein SY-PPV forms a porous network with pores filled by the ionic medium. PeakForce quantitative nanomechanical mapping showed that the local Young's modulus was high in the SY-PPV phase, while the ionic phase was two-times softer. No global polarization of the SY-PPV chains was observed at strains up to 100% as the dichroic ratio remains close to 1. Light-emitting devices based on the blend sandwiched between two stretchable transparent composite electrodes could be stretched by up to 140% strain. No electroluminescence polarization was observed.

Solution-processed Ag-nanowire/ZnO-nanoparticle composite transparent electrode for flexible organic solar cells

This paper demonstrates a hybrid transparent electrode composed of a solution-processed silver-nanowire (AgNW) film coated by zinc oxide nanoparticles (ZnO-NPs) acting as a modified buffer layer. The effect of the ZnO-NPs’ coating ratio on the performances of indium tin oxide (ITO)-free organic solar cells (OSCs) has been systematically investigated. The optimized ITO-free OSCs achieved a power conversion efficiency (PCE) of 2.85%, while flexible OSCs using the AgNW/ZnO-NP composite transparent electrode grown on a polyethylene terephthalate (PET) substrate showed a PCE of 2.2%.

The optimal TiO2/Ag/TiO2 electrode for organic solar cell application with high device-specific Haacke figure of merit

As a promising indium-free electrode, transparent composite electrode (TCE) TiO2/Ag/TiO2 is an attractive alternative to indium tin oxide (ITO). The multilayer structure of TiO2/Ag/TiO2 is deposited on glass substrate by room-temperature sputtering. The effects of deposition rate, thickness of Ag and TiO2 layers on the electrical and optical properties of this TCE are presented. Results show that the Ag films tend to have fewer voids to form, if the Ag deposition rate is sufficiently high. The presence of fewer voids in the Ag layer results in higher transmittance and lower sheet resistance of the corresponding TiO2/Ag/TiO2 structure. In order to increase the solar input to the P3HT:PC61BM organic solar cell, the layer thicknesses of TiO2/Ag/TiO2 electrodes are theoretically and then experimentally optimized, such that its optical transmittances are maximized across the wavelength range where the light absorption of P3HT:PC61BM is highest. In this study, the device-specific Haacke figure of merit (FOM) of the TiO2/Ag/TiO2 electrode is optimal with the use of 42nm TiO2 layers and 10nm Ag layer. This optimal Haacke FOM is two times higher than that of ITO electrodes. The conventional bulk heterojunction organic solar cell (OSC) fabricated on optimized TiO2/Ag/TiO2 electrode shows high power conversion efficiency (PCE), almost 100% higher than PCE of OSC fabricated on ITO. Our finding indicates that TiO2/Ag/TiO2 electrode can be specifically tailored to particular applications. It can be implemented in organic solar cell and has considerable potential to replace ITO in solar cell applications.

Optical properties of transparent electrodes based on carbon nanotubes and graphene platelets

Composite transparent electrodes based on carbon nanostructures such as multiwalled carbon nanotubes and graphene platelets were spray coated onto glass substrates and characterized by spectrophotometry and spectroscopic ellipsometry measurements. The dispersion relations of the ellipsometric angle rate, i.e. Ψ and Δ versus wavelength λ were measured in spectral range from 190 to 1700 nm. On the basis of these results, it was possible to estimate the value of the refractive index and extinction coefficient. Effective medium approximation model was chosen to calculate the optical constants of a mixed material. The average surface roughness and the average thickness of spray coated transparent resistive layers were also determined. The materials have a heterogeneous structure as confirmed by scanning electron microscopy and optical measurements (changes of depolarisation). From the Tauc plot it was possible to determine the energy gap. The influence of the coating process and the paint preparation on the optical properties was observed.

Fabrication of silver nanowires and metal oxide composite transparent electrodes and their application in UV light-emitting diodes

In this paper, we prepared the silver nanowires (AgNWs)/aluminum-doped zinc oxide (AZO) composite transparent conducting electrodes for n-ZnO/p-GaN heterojunction light emitting-diodes (LEDs) by drop casting AgNW networks and subsequent atomic layer deposition (ALD) of AZO at 150 °C. The contact resistances between AgNWs were dramatically reduced by pre-annealing in the vacuum chamber before the ALD of AZO. In this case, AZO works not only as the conformal passivation layer that protects AgNWs from oxidation, but also as the binding material that improves AgNWs adhesion to substrates. Due to the localized surface plasmons (LSPs) of the AgNWs resonant coupling with the ultraviolet (UV) light emission from the LEDs, a higher UV light extracting efficiency is achieved from LEDs with the AgNWs/AZO composite electrodes in comparison with the conventional AZO electrodes. Additionally, the antireflective nature of random AgNW networks in the composite electrodes caused a broad output light angular distribution, which could be of benefit to certain optoelectronic devices like LEDs and solar cells.

A Simple Cost-Effective Approach to Enhance Performance of Bifacial Dye-Sensitized Solar Cells

A simple cost-effective approach was demonstrated to increase the rear efficiency and ratio of rear-to-front efficiencies for bifacial dye-sensitized solar cells (DSCs). This was achieved by enhancing the transparency of the counter electrode of the DSC using a composite of platinum- and aluminum-doped zinc oxide (Pt/AZO). The thermally deposited composite Pt/AZO counter electrodes exhibited transmittance T550 nm = 0.70–0.74 versus Pt with 0.63. The comparable front power conversion efficiency (PCEfront) of fabricated DSCs using the composite Pt/AZO counter electrodes suggested that the presence of AZO in the composite had minimal effect on the catalytic activity of the platinum. DSCs with composite Pt/AZO counter electrode presented superior rear PCEs compared with platinum counterparts and was attributed to higher short-circuit current density $J_{{\rm sc}}$ resulting from the larger transmittance of the composite counter electrode. The higher PCErear / PCEfront of the composite-based DSCs compared with Pt-based DSC suggested that such transparent composite counter electrodes could be employed in bifacial DSCs.

Fabrication of sintering-free flexible copper nanowire/polymer composite transparent electrodes with enhanced chemical and mechanical stability

The thermal decomposition synthesis of long copper nanowires (CuNWs) was achieved by controlling the synthesis parameters. A detailed study was performed to determine the effect of the molar ratio of copper chloride to nickel acetylacetonate, temperature, and stirring rate on the final shape of the products. Transparent electrodes (TEs) were fabricated by wet treatment with acetic acid (AA), without using a sintering process. The low oxidation stability and high surface roughness are the main disadvantages of the CuNW TEs, which limit their applications. In order to overcome these issues, we prepared CuNW/polymer composite TEs by partial embedding of the CuNWs into poly(methyl methacrylate) (PMMA) on poly(ethylene terephthalate) (PET) substrates. The CuNW/PMMA composite TEs exhibit excellent optoelectronic performance (91.3% at 100.7 Ω/sq), low surface roughness (4.6 nm in height), and good mechanical and chemical stability as compared with CuNW TEs. On the basis of these properties, we believe that CuNW-based composite TEs could serve as low-cost materials for a wide range of new optoelectronic devices.

24-3:Invited Paper: Light Out-Coupling of OLEDs: the Transparent Electrode Effects

Judicious use of low-index transparent conductors in transparent electrodes of OLEDs effectively enhances out-coupling both into substrate and directly into air. 39%, 57%, and 62% EQEs have been demonstrated for OLEDs with simple planar structures, planar transparent electrode plus external extraction, and nanostructured composite transparent electrode plus external extraction, respectively.

Analyses of optical out-coupling of organic light-emitting devices having micromesh indium tin oxide and conducting polymer as composite transparent electrode.

PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)], that are fabricated by the facile and convenient microsphere lithography and are useful for enhancing light extraction. The rigorous electromagnetic simulation based on the three-dimensional finite-difference time-domain (FDTD) method was conducted to study optical properties and mechanisms in such devices. It provides a different but consistent viewpoint/insight of how this microstructured electrode enhances optical out-coupling of OLEDs, compared to that provided by ray optics simulation in previous works. Both experimental and simulation studies indicate such a microstructured electrode effectively enhances coupling of internal radiation into the substrate, compared to devices with the typical planar ITO electrode. By combining this internal extraction structure and the external extraction scheme (e.g. by attaching extraction lens) to further extract radiation into the substrate, a rather high external quantum efficiency of 46.8% was achieved with green phosphorescent OLEDs, clearly manifesting its high potential.

Preparation and investigation of nano-thick BaSnO3/Cu/BaSnO3 multilayer structures for transparent electrodes

The nano-thick BaSnO3/Cu/BaSnO3 multilayer structures have been designed and deposited onto glass substrates by magnetron sputtering at room temperature to develop an indium free transparent composite electrode. The effects of Cu mid–layer on structural, electrical and optical properties of BaSnO3/Cu/BaSnO3 multilayer composite structures were investigated. As the Cu layer thickness increases, the resistivity decreases. The lowest resistivity value of 7.9×10−5Ωcm with a carrier concentration of 1.2×1022cm−3 was obtained in the Cu layer with optimum thickness (9nm). The multilayer film has been optimized to obtain an average optical transmittance of ~81% in the visible range of wavelengths, while the figure of merit (FOM) reached a maximum of 14.6×10−3Ω−1.

High performance all-carbon composite transparent electrodes containing uniform carbon nanotube networks

Indium tin oxide-free, flexible transparent electrodes (TEs) are crucial for the future commercialization of flexible and wearable electronics. While carbon-based TEs containing carbon nanotube (CNT) networks show promise, they usually exhibit poor dispersion properties, limiting their performance and practicality. In this study, we report a highly efficient and bending durable all-carbon composite TE (ac-TE) that employs uniform CNT networks on a monolayer graphene/polyethylene terephthalate (PET) substrate via a simple air spray deposition method. The air-sprayed CNT/graphene assembly was free-standing on solution, making a polymer-free transfer of carbon composites to target substrates possible. The excellent performance of the ac-TEs was attributed to the uniformly networked CNTs on the polycrystalline graphene with a well-controlled density, effectively bridging the line defects and filling the tears/voids or folds necessarily existing in the as-processed graphene. The sheet resistance of the ac-TEs was increased only 6% from its original value at a bending radius of 2.7 mm, while that of the pristine graphene/PET assembly increased 237%. Mechanical bending of the ac-TEs worsened the electrical performance by only ∼1.7% after 2000 bending cycles at a bending radius of 2.5 mm. Degradation of the performance by the bending was the result of line defects formation in the graphene, demonstrating the potential of the uniform CNT networks to achieve more efficient and flexible carbon-based TEs. Furthermore, the chemically-doped ac-TEs showed commercially suitable electronic and optical properties with much enhanced thermal stability, closer to practical TEs in flexible devices.

Working from Both Sides: Composite Metallic Semitransparent Top Electrode for High Performance Perovskite Solar Cells.

We report herein perovskite solar cells using solution-processed silver nanowires (AgNWs) as transparent top electrode with markedly enhanced device performance, as well as stability by evaporating an ultrathin transparent Au (UTA) layer beneath the spin-coated AgNWs forming a composite transparent metallic electrode. The interlayer serves as a physical separation sandwiched in between the perovskite/hole transporting material (HTM) active layer and the halide-reactive AgNWs top-electrode to prevent undesired electrode degradation and simultaneously functions to significantly promote ohmic contact. The as-fabricated semitransparent PSCs feature a Voc of 0.96 V, a Jsc of 20.47 mA cm(-2), with an overall PCE of over 11% when measured with front illumination and a Voc of 0.92 V, a Jsc of 14.29 mA cm(-2), and an overall PCE of 7.53% with back illumination, corresponding to approximately 70% of the value under normal illumination conditions. The devices also demonstrate exceptional fabrication repeatability and air stability.

Ultrasmooth, highly conductive and transparent PEDOT:PSS/silver nanowire composite electrode for flexible organic light-emitting devices

The next generation of optoelectronic devices requires transparent conductive electrodes to be flexible, inexpensive and compatible with large scale manufacturing processes. We report an ultrasmooth, highly conductive and transparent composite electrode on a flexible photopolymer substrate by employing a template stripping method. A random silver nanowire (AgNW) network buried in poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film constituted the composite electrode. Besides the effectively decreased surface roughness, its sheet resistance and transmittance are comparable to those of conventional PEDOT:PSS electrode. As a result, the efficiency of the OLEDs based on the composite electrode exhibited 25% enhancement compared to the OLEDs with conventional PEDOT:PSS electrode. Moreover, the performance of the flexible OLEDs remains stable after over one hundred bending cycles.

Prediction of transmittance spectra for transparent composite electrodes with ultra-thin metal layers

Recent interest in indium-free transparent composite-electrodes (TCEs) has motivated theoretical and experimental efforts to better understand and enhance their electrical and optical properties. Various tools have been developed to calculate the optical transmittance of multilayer thin-film structures based on the transfer-matrix method. However, the factors that affect the accuracy of these calculations have not been investigated very much. In this study, two sets of TCEs, TiO2/Au/TiO2 and TiO2/Ag/TiO2, were fabricated to study the factors that affect the accuracy of transmittance predictions. We found that the predicted transmittance can deviate significantly from measured transmittance for TCEs that have ultra-thin plasmonic metal layers. The ultrathin metal layer in the TCE is typically discontinuous. When light interacts with the metallic islands in this discontinuous layer, localized surface plasmons are generated. This causes extra light absorption, which then leads to the actual transmittance being lower than the predicted transmittance.