Transparent Electrodes In Optoelectronic Devices Research Articles

Enhanced Microstructural Uniformity in Sulfuric-Acid-Treated Poly(3,4-Ethylenedioxythiophene):Poly(Styrene Sulfonate) Films Using Raman Map Analysis.

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films have emerged as potential alternatives to indium-tin oxide as transparent electrodes in optoelectronic devices because of their superior transparency, flexibility, and chemical doping stability. However, pristine PEDOT:PSS films show low conductivities because the insulating PSS-rich domains isolate the conductive PEDOT-rich domains. In this study, the conductivities and corresponding spatially resolved Raman properties of PEDOT:PSS thin films treated with various concentrations of H2SO4 are presented. After the PEDOT:PSS films are treated with the H2SO4 solutions, their electrical conductivities are significantly improved from 0.5 (nontreated) to 4358 S cm-1 (100% v/v). Raman heat maps of the peak shifts and widths of the Cα═Cβ stretching mode are constructed. A blueshift and width decrease of the Cα═Cβ Raman mode in PEDOT are uniformly observed in the entire measurement area (20 × 20 µm2), indicating that microstructural transitions are successfully accomplished across the area from the coiled to linear conformation and high crystallinity upon H2SO4 treatment. Thus, it is proved that comprehensive Raman map analysis can be easily utilized to clarify microstructural properties distributed in large areas induced by various dopants. These results also offer valuable insights for evaluating and optimizing the performance of other conductive thin films.

Highly Conductive Inkjet-Printed PEDOT:PSS Film under Cyclic Stretching.

Inkjet-printed conductive polymer PEDOT:PSS films have provided a new developing direction for realizing the stretchable transparent electrodes in optoelectronic devices. However, their conductivity and stretchability are limited as the presence of insulating PSS chains, rigid PEDOT conjugated backbone, and stronger inter-chain interactions in the pristine polymer, respectively. Here, we report a PEDOT:PSS film with preferable electrical and mechanical performances by inkjet-printing the formulated printable ink containing PEDOT:PSS, formamide (FA), d-sorbitol (SOR), sodium dodecyl benzene sulfonate (DBSS), and ethylene glycol (EG). The inkjet-printed uniform PEDOT:PSS film exhibits a high conductivity of 1050 S/cm and sheet resistance of less than 145 Ω/sq on both rigid and flexible substrates. Moreover, the resistance can remain stable after 200 cycles of stretching at 55% strain. The film also presents good stability during repetitive stretching-releasing cycles. The significantly enhanced conductivity of the film lies on the conformational transition of the backbone by secondary doping and post-treatment with FA as well as removing the excess PSS components after phase separation between PEDOT and PSS. Meanwhile, SOR serves as a plasticizer to break the original hydrogen bonds between PSSH chains and provides larger free volume for polymer chain extension, which gives the PEDOT:PSS film the ability to tolerant cyclic tension. This is one of the optimal performances currently reported for inkjet-printed stretchable PEDOT:PSS films. The inkjet-printed PEDOT:PSS film with high conductivity, stretching properties, as well as good biocompatibility exhibits promising prospects as anodes on optoelectronic devices.

Transferrable, Large‐area and Highly Conductive PEDOT : PSS Film and Its Application for Flexible Thermoelectric Generators

AbstractA facile method was demonstrated to prepare large‐area (∼100 cm2), conductive poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT : PSS) dense film via post‐treatment with sulfuric acid at 160 °C on a glass substrate. This PEDOT : PSS dense films exhibited a high electrical conductivity of up to 1800 S cm−1 without significantly compromising with the high humidity level, and could be easily transferred onto various flexible substrates such as plastics and paper with excellent adhesion. The conductive PEDOT : PSS dense films can be made a flexible and foldable thermoelectric module, showing an output voltage of 2.2 mV at ΔT=26 K. On the other hand, PEDOT : PSS thin films, which were prepared using a similar method, could be transferred onto the polyethylene terephthalate (PET) substrate and suitable to serve as flexible transparent electrodes for optoelectronic devices. Our simple method would hold great promise for scalability and mass production of large‐area, freestanding and conductive PEDOT : PSS polymer film for a variety of applications.

Electrical and optical properties of (In1-xSnx)2O3(1+δ) films (0.03 ≤ x ≤ 0.40, δ ≈ 0.28) grown on Si substrates using co-sputtering of In2O3 and SnO2

Sn-doped In2O3 (ITO) films are in high demand for use as transparent electrodes in optoelectronic devices. Technological developments have improved the quality of ITO films, and a detailed investigation of their properties is desired. Therefore, we investigated the composition dependence of the electrical and optical properties of (In1-xSnx)2O3(1+δ) films (with 0.03 ≤ x ≤ 0.40 and δ ≈ 0.28) films grown on Si and glass substrates utilizing the co-sputtering of In2O3 and SnO2 targets. Using X-ray diffraction, we found that the In2O3-like (222) phase was the dominant phase for x = 0.03. Further, the In2Sn2O7+x (400) phase became the dominant phase as the Sn concentration increased. This work demonstrates that ITO films exhibit excellent transmittance properties (T ≈ 88.3%) in the visible range as well as very low resistivities (ρ ≈ 2 × 10−4 Ω cm), regardless of the Sn composition. However, ITO films with Sn = 0.11, the commercially used composition, exhibited the lowest transmittance in the near-infrared range, 73.7%, whereas other ITO films showed large transmittances in the near-infrared range: T = 90.9% at x = 0.03 and T = 85.7% at x = 0.40.

Efficient Film Fabrication and Characterization of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) (PEDOT:PSS)-Metalloporphine Nanocomposite and Its Application as Semiconductor Material.

The use of composite films with semiconductor behavior is an alternative to enhance the efficiency of optoelectronic devices. Composite films of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and metalloporphines (MPs; M = Co, Cu, Pd) have been prepared by spin-coating. The PEDOT:PSS-MP films were treated with isopropanol (IPA) vapor to modify the polymer structure from benzoid to quinoid. The quinoid structure promotes improvements in the optical and electrical behavior of films. The composite films’ morphology and structure were characterized using infrared and Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Composite films were analyzed for their optical behavior by ultraviolet-visible spectroscopy: at λ < 450 nm, the films become transparent, indicating the capacity to be used as transparent electrodes in optoelectronic devices. At λ ≥ 450 nm, the absorbance in the films increased significantly. The CoP showed an 8 times larger current density compared to the CuP. A light induced change in the J-V curves was observed, and it is larger for the CoP. The conductivity values yielded between 1.23 × 102 and 1.92 × 103 Scm−1 and were higher in forward bias.

Application of intrinsically conducting polymers in flexible electronics

AbstractIntrinsically conducting polymers (ICPs), such as polyacetylene, polyaniline, polypyrrole, polythiophene, and poly(3,4‐ethylenedioxythiophene) (PEDOT), can have important application in flexible electronics owing to their unique merits including high conductivity, high mechanical flexibility, low cost, and good biocompatibility. The requirements for their application in flexible electronics include high conductivity and appropriate mechanical properties. The conductivity of some ICPs can be enhanced through a postpolymerization treatment, the so‐called “secondary doping.” A conducting polymer film with high conductivity can be used as flexible electrode and even as flexible transparent electrode of optoelectronic devices. The application of ICPs as stretchable electrode requires high mechanical stretchability. The mechanical stretchability of ICPs can be improved through blending with a soft polymer or plasticization. Because of their good biocompatibility, ICPs can be modified as dry electrode for biopotential monitoring and neural interface. In addition, ICPs can be used as the active material of strain sensors for healthcare monitoring, and they can be adopted to monitor food processing, such as the fermentation, steaming, storage, and refreshing of starch‐based food because of the resistance variation caused by the food volume change. All these applications of ICPs are covered in this review article.

Silver Nanowire Synthesis and Strategies for Fabricating Transparent Conducting Electrodes.

One-dimensional metal nanowires, with novel functionalities like electrical conductivity, optical transparency and high mechanical stiffness, have attracted widespread interest for use in applications such as transparent electrodes in optoelectronic devices and active components in nanoelectronics and nanophotonics. In particular, silver nanowires (AgNWs) have been widely researched owing to the superlative thermal and electrical conductivity of bulk silver. Herein, we present a detailed review of the synthesis of AgNWs and their utilization in fabricating improved transparent conducting electrodes (TCE). We discuss a range of AgNW synthesis protocols, including template assisted and wet chemical techniques, and their ability to control the morphology of the synthesized nanowires. Furthermore, the use of scalable and cost-effective solution deposition methods to fabricate AgNW based TCE, along with the numerous treatments used for enhancing their optoelectronic properties, are also discussed.

Optimization of SnO2/Ag nanowire transparent hybrid electrodes for optoelectronic applications

Silver nanowire (Ag NW) networks are gaining more interest as promising candidates for the substitution of indium tin oxide (ITO) for top electrodes in optoelectronic devices. In this work we investigated the electrical, optical, structural, and morphological properties of SnO2/Ag NW hybrid film deposited by spray pyrolysis root. We showed that annealing at appropriate temperature improves optoelectronic and morphological properties of the SnO2/AgNWs electrodes; the optimal annealing temperature was 180 °C for 20 min. These annealing conditions allow better homogenization of the nanowires and their welding at the intersection nodes ensuring conduction of the charge carriers along the conductive grid formed of nanowires. The optimized SnO2/AgNWs electrodes have a large optical window covering the near-UV, Vis and IR range, with an average transparency of 85% and a sheet resistance of 6.1 Ω/sq. These optoelectronic performances have led to a merit factor of 2.5 × 10−2 Ω−1 being a competitive performance among the currently developed electrodes that can be promising candidates for applications as a transparent electrodes in optoelectronic devices.

(Invited) Highly Conductive Polymers and Their Applications for Energy Conversion

Although intrinsically conductive polymers were discovered in 1970s, their application is quite limited mainly due to their poor processability and low conductivity. Most of conductive polymers are insoluble in any solvent and cannot melt. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) has gained great attention and considered as the most successful conductive polymer, because it can be dispersed in water and some polar organic solvents. But it has a problem of low conductivity. The as-prepared PEDOT:PSS films from its aqueous solution usually has a conductivity of less than 1 S/cm. Here, I will present several novel methods to significantly enhance the conductivity of PEDOT:PSS. The conductivity can be enhanced to be more than 3000 S/cm. Thus, highly conductive PEDOT:PSS can be used as the transparent electrode of optoelectronic devices like solar cells because it can have high transparency in the visible range. In addition, they can exhibit high thermoelectric properties.

Formation and Stability of Phenylphosphonic Acid Monolayers on ZnO: Comparison of In Situ and Ex Situ SAM Preparation.

Self-assembled monolayers (SAMs) enable an electronic interface tailoring of conductive metal oxides and offer an alternative to common transparent electrodes in optoelectronic devices. Here, the influence of surface orientation and pretreatment on the formation and stability of SAMs has been studied for the case of phenylphosphonic acid (PPA) on ZnO single crystals. Using thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray adsorption fine structure spectroscopy (NEXAFS) and density-functional theory (DFT) calculations, the thermal stability and orientational ordering of PPA-SAMs on the polar and mixed-terminated ZnO surfaces were analyzed. On all surfaces, PPA-SAMs remain stable up to 550 K, while at higher temperatures a C-P bond cleavage and dissociative desorption takes place yielding two distinct desorption peaks. Based on DFT calculations, these desorption channels are attributed to protonated and deprotonated chemisorbed PPA molecules, which can be related to tri- and bidentate species, hence allowing to determine their relative abundance from the intensity ratio. Beside immersion, an alternative monolayer preparation based on vacuum deposition in combination with controlled desorption of excess multilayers is demonstrated. This enables a SAM preparation on bare ZnO surfaces without any precoating due to exposure to ambient air, which is further compared with SAM formation on intentionally hydroxylated substrates. Corresponding TDS data indicate that initial hydroxylation favors the formation of tridentate and deprotonated bidentate, while the OMBD preparation on bare surfaces yields a larger fraction of protonated bidentate species. The orientation of PPA molecules adopted in the SAMs was determined from the dichroism of K-edge NEXAFS measurements and reveals an almost upright orientation for the deprotonated species, while a slight tilting is obtained for monolayer films with a large fraction of protonated bidentate molecules.

Novel Patterning Method for Silver Nanowire Electrodes for Thermal-Evaporated Organic Light Emitting Diodes.

PSS) composite electrode. A thermal-evaporated organic light-emitting diode (OLED) is directly fabricated onto the patterned AgNWs/ PSS composite electrode. The device shows well-defined pixel edges and a uniformly lighted pixel area. A uniform OLED with very low leakage current is realized. The enhanced efficiency compared to the controlled device with prepatterned indium tin oxide (ITO) electrode is attributed to the scattering effects of the AgNWs electrode.

Fabrication of ultrathin, free-standing, transparent and conductive graphene/multiwalled carbon nanotube film with superior optoelectronic properties

Here we report a wet chemical technique to obtain transparent electrodes for optoelectronic devices. This technique is simple, facile, low cost and an effective way to prepare ultrathin free standing hybrid graphene/multiwalled carbon nanotubes (MWCNT) films. The graphene/MWCNT films, up to 36mm in diameter, with controllable thickness and root mean square surface roughness of the order of 12.6nm are prepared. The ratio of graphene/MWCNT is optimized to make them free standing and easily transferrable on various substrates. The ratio of direct current conductivity to the optical conductivity (σdc/σopt) which is considered as figure of merit for transparent conductors, is enhanced to 10.27 for graphene/MWCNT hybrid films. The prepared films showed outstanding transmittance up to 87.3±1% at 550nm, 87.9±1% at 800nm and sheet resistance of 136±22.4Ω/sq.

Multifunctionality of Giant and Long-Lasting Persistent Photoconductivity: Semiconductor–Conductor Transition in Graphene Nanosheets and Amorphous InGaZnO Hybrids

Composite materials can play a decisive role to reveal novel physical properties and enable to advance new generation technologies. Here, we discover that phototransistors based on the integration of two-dimensional graphene nanosheets (GNSs) and amorphous indium–gallium–zinc–oxide (a-IGZO) semiconductors exhibit a giant photo-to-dark current ratio and long-lasting persistent photoconductivity (PPC). Under the illumination of UV light (350 nm) at 50 mW/cm2, a photo-to-dark current ratio up to 2.0 × 107 was obtained, which is about 3 orders of magnitude higher than its pure a-IGZO device counterpart. Moreover, the GNSs/a-IGZO phototransistor possesses an enduring lifetime up to years for the recovery of the transfer characteristics after switching off the UV light. The giant and long-lasting PPC leads GNSs/a-IGZO to become an excellent conductor with conductivity much better than indium tin oxide. The observed unique features represent a semiconductor–conductor transition. In addition to next generation flat, flexible, and display, it can open up a wide variety of application, such as transparent electrodes for optoelectronic devices, optical memory, and light harvesting for energy storage. As an example, we demonstrated the operation of optical memory devices, which may lead to the novel application of holographic storage. Our results shown here therefore provide an outstanding new route for the future development of solution-processable semiconducting optoelectronic devices.

Investigation of Electrical and Optical Properties of Highly Transparent TCO/Ag/TCO Multilaye

Transparent conductive oxides (TCOs) have been widely used as transparent electrodes for opto-electronic devices, such as solar cells, flat-panel displays, and light-emitting diodes, because of their unique characteristics of high optical transmittance and low electrical resistivity. Among various TCO materials, zinc oxide based films have recently received much attention because they have advantages over commonly used indium and tin-based oxide films. Most TCO films, however, exhibit valleys of transmittance in the wavelength range of 550-700 nm, lowering the average transmittance in the visible region and decreasing short-circuit current (Isc) of solar cells. A TCO/Ag/TCO multi-layer structure has emerged as an attractive alternative because it provides optical characteristics without the valley of transmittance compared with a 100-nm-thick single-layer TCO. In this article, we report the electrical, optical and surface properties of TCO/Ag/TCO. These multi-layers were deposited at room temperature with various Ag film thicknesses from 5 to 15 nm while the thickness of TCO thin film was fixed at 40 nm. The TCO/Ag/TCO multi-layer with a 10-nm-thick Ag film showed optimum transmittance in the visible (400-800 nm) wavelength region. These multi-layer structures have advantages over TCO layers of the same thickness.

Investigation of light transmission and scattering properties in silver nanowire mesh transparent electrodes

Silver nanowire mesh is a promising replacement for indium tin oxide (ITO) transparent electrodes for optoelectronic devices. In this article, effective light scattering by Ag nanowires (Ag NWs) are investigated. Compared with commercial FTO and ITO, we demonstrated that Ag nanowires exhibit better optical transmission and light-scattering properties in all wavelength regions. Combined with the ease of preparation and low cost, our results indicated that Ag NWs are a promising candidate for use as a transparent conductive material for thin film solar cells.

Transparent metallic fractal electrodes for semiconductor devices.

Nanostructured metallic films have the potential to replace metal oxide films as transparent electrodes in optoelectronic devices. An ideal transparent electrode should possess a high, broadband, and polarization-independent transmittance. Conventional metallic gratings and grids with wavelength-scale periodicities, however, do not have all of these qualities. Furthermore, the transmission properties of a nanostructured electrode need to be assessed in the actual dielectric environment provided by a device, where a high-index semiconductor layer can reflect a substantial fraction of the incident light. Here we propose nanostructured aluminum electrodes with space-filling fractal geometries as alternatives to gratings and grids and experimentally demonstrate their superior optoelectronic performance through integration with Si photodetectors. As shown by polarization and spectrally resolved photocurrent measurements, devices with fractal electrodes exhibit both a broadband transmission and a flat polarization response that outperforms both square grids and linear gratings. Finally, we show the benefits of adding a thin silicon nitride film to the nanostructured electrodes to further reduce reflection.

Two-dimensional metamaterial transparent metal electrodes for infrared optoelectronics.

We examine the optical properties of two-dimensionally nanostructured metals in the metamaterial regime for infrared applications. Compared with straight nanowires and nanogrids, serpentine structures exhibit much lower optical losses of less than 7% even at a large metal area fraction of 0.3. The low loss is primarily due to a small effective conductivity of the meandering structures, and self-inductance plays a modest role in reducing losses in these structures. The high transparency at a large metal area coverage would be useful for transparent electrodes in optoelectronic devices.

Fabrication , Structural and Optical Characterization of in Doped ZnO Thin Films Prepared by the Colloidal Method

Regarding to the excellent conductivity and high transparency in the visible range, the zinc oxide (ZnO) films have been widely used as transparent electrodes in optoelectronic devices, ZnO is a direct wide band-gap (3.37 eV) semiconductor. The conductivity of ZnO will be largely enhanced by doping little In, but it still keeps high transparency. So, IZO film has been widely investigated and is considered to be a promising possible alternative to ITO films. This work consist to the (...)

Comparison of properties of polymer organic solar cells prepared using highly conductive modified PEDOT:PSS films by spin- and spray-coating methods

Bulk heterojunction (BHJ) solar cells have made great progress over the past decade and consequently are now attracting extensive academic and commercial interest because of their potential advantages: lightweight, flexible, low cost, and high-throughput production. Polymer conductivity is a key factor for improving the performance of electronic and photonic devices. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is promising for use as a next-generation transparent electrode of optoelectronic devices. In this research, we compare the effect of nanomorphology on conductivity, and power conversion efficiency of polymer organic solar cells prepared by the spin- and spray-coating methods. To improve the conductivity of spray-deposited PEDOT:PSS, we modified the PEDOT:PSS films by simple UV irradiation and by UV irradiation with treatment using various solvents such as methanol, ethanol, acetone, acetonitrile, hydrochloric acid, and sulfuric acid to form a hole transport layer (HTL). The active layer of PTB7:PC70BM is spray-coated on top of the PEDOT:PSS layer. The films were examined by optical spectroscopy, micro-Raman spectroscopy, and conductivity measurements. The surface morphology of the deposited films was examined by atomic force microscopy (AFM). The current density–voltage (J–V) characteristics were measured under illumination with simulated solar light at 100 mW/cm2 (AM 1.5G) using an oriel 1000 W solar simulator. The obtained results are expected to have a considerable impact and suggest a bright future for organic polymer solar cells.

“Secondary doping” methods to significantly enhance the conductivity of PEDOT:PSS for its application as transparent electrode of optoelectronic devices

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the most successful conducting polymer in terms of the practical application. It can be dispersed in water and some polar organic solvents, and high-quality PEDOT:PSS films can be readily prepared through solution processing. In addition, PEDOT:PSS is highly transparent in the visible range and has excellent thermal stability. Nevertheless, PEDOT:PSS has a problem of low conductivity. The as-prepared PEDOT:PSS films from its aqueous solution have a conductivity of lower than 1Scm−1, which severely impedes the application of PEDOT:PSS in various aspects. It has been discovered that the conductivity of as-prepared PEDOT:PSS from its aqueous solution can be significantly enhanced by adding organic compounds like high-boiling point polar organic solvents, ionic liquids and surfactants or through a post-treatment of PEDOT:PSS films with organic compounds, including high-boiling point polar solvents, salts, zwitterions, cosolvents, organic and inorganic acids. Conductivity of more than 3000Scm−1 was recently observed on PEDOT:PSS films after treated with sulfuric acid. This conductivity is comparable to that of indium tin oxide (ITO), the conventional transparent electrode material of optoelectronic devices. In addition, PEDOT:PSS has high mechanical flexibility while ITO is a brittle material. Thus, PEDOT:PSS is very promising to be the next-generation transparent electrode material. This article reviews the methods to enhance the conductivity of PEDOT:PSS, the mechanisms for the conductivity enhancements and the application of the highly conductive PEDOT:PSS films in polymer light-emitting diodes and polymer solar cells.