Electrodes In Organic Light-emitting Diodes Research Articles

Improvement of electrical characteristics and wet etching procedures for InGaTiO electrodes in organic light-emitting diodes through hydrogen doping

We investigated the effect of hydrogen doping on characteristics of InGaTiO (IGTO) electrode with high conductivity and transmittance during sputtering process. After hydrogen doping, the 300 nm-thick IGTO:H electrode showed lower sheet resistance of 13.79 Ω/square compared to sheet resistance (69.2 Ω/square) of pure IGTO electrode without change of optical transmittance. In addition, wet etching reaction rate of IGTO:H electrode for patterning process was significantly accelerated after hydrogen doping. Possible explanation for hydrogen doping effect on electrical properties and wet etching process of the IGTO:H electrode was suggested. The doped hydrogen influenced the electrical characteristics through the passivation of dangling bonds and oxygen vacancies. Moreover, it significantly impacts crystalline structure, consequently altering the etching rate. Using patterned IGTO:H electrodes, we fabricated red organic light emitting diodes (OLEDs) to show feasibility of the IGTO:H electrodes. External quantum efficiency and current efficiency of the red OLED with IGTO:H (10%) were higher than IGTO electrode without hydrogen doping, indicating that hydrogen doping is an effective method to improve the film properties and IGTO:H electrode can serve as a prospective transparent anode for both flexible and rigid OLEDs.

Investigation of nitrogen-doped carbon dot/ZnO nanocomposites and their application as interlayer in solution-processed organic light emitting diodes

Nitrogen-doped carbon dots (N-CD) were synthesized and added to ZnO dispersions. The resulting composites were deposited between an emissive poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) layer and the top metal electrode of organic light-emitting diodes (OLEDs). The structural, morphological and optical properties of N-CD sample and composite films were investigated. In solid-state, the N-CD presented a highly anisotropic behaviour, with significant size change, as demonstrated in microscopic images. Nevertheless, it was found that small amounts of N-CD reduced the surface roughness of the ZnO films and facilitated electron injection from the electron transport layer (ETL) to the polymeric light-emitting layer, thus demonstrating that ZnO/N-CD composites with appropriate ratios can be successfully used as ETL to boost the performance of OLEDs. All the OLEDs investigated emitted the blue colour characteristic of PFO, with minor shifts in the chromaticity coordinates, suggesting that the N-CD did not play a significant role as light-emitters in these devices.

Charge Transfer Dynamics of Doped Graphene Electrodes for Organic Light-Emitting Diodes.

Atomically thin graphene has attracted immense attention as a future transparent electrode for flat-panel displays owing to its excellent conductivity, optical transparency, and flexibility. In particular, a graphene doping process is essential for implementing graphene-based high-performance devices, and the development of a transparent cathode with a low work function is required to simplify the integration process of thin-film transistors and organic light-emitting diodes (OLEDs) into active matrix displays. In this study, a transparent n-doped graphene cathode is proposed for implementing inverted OLEDs through two types of cesium (Cs)-based doping techniques: a dipping method using wet chemicals and an evaporation method under a vacuum atmosphere. The changes in the chemical structures and work functions of the n-doped graphene electrodes, as well as their surface morphologies and transmittances, were systematically investigated. The n-type doping mechanism of graphene was investigated, and a close relationship between the electrical charge transfer characteristics of graphene transistors and the formation of C-O-Cs complexes was revealed. Finally, an effective Cs-doped graphene electrode was developed, exhibiting a dramatically decreased work function while maintaining high transmittance; therefore, the Cs-doped graphene cathode was successfully integrated with inverted OLEDs with a bottom-light emission structure that exhibited enhanced external quantum efficiency of graphene cathode-based OLEDs. Thus, our findings provide a better understanding of the doping strategies and potential of n-doped graphene as a transparent cathode for developing high-performance future displays.

Overcoming the Limitations of MXene Electrodes for Solution-Processed Optoelectronic Devices.

MXenes constitute a rapidly growing family of 2D materials that are promising for optoelectronic applications because of numerous attractive properties, including high electrical conductivity. However, the most widely used titanium carbide (Ti3 C2 Tx ) MXene transparent conductive electrode exhibits insufficient environmental stability and work function (WF), which impede practical applications Ti3 C2 Tx electrodes in solution-processed optoelectronics. Herein, Ti3 C2 Tx MXene film with a compact structure and a perfluorosulfonic acid (PFSA) barrier layer is presented as a promising electrode for organic light-emitting diodes (OLEDs). The electrode shows excellent environmental stability, high WF of 5.84eV, and low sheet resistance RS of 97.4Ωsq-1 . The compact Ti3 C2 Tx structure after thermal annealing resists intercalation of moisture and environmental contaminants. In addition, the PFSA surface modification passivates interflake defects and modulates the WF. Thus, changes in the WF and RS are negligible even after 22days of exposure to ambient air. The Ti3 C2 Tx MXene is applied for large-area, 10×10passive matrix flexible OLEDs on substrates measuring 6×6cm. This work provides a simple but efficient strategy to overcome both the limited environmental stability and low WF of MXene electrodes for solution-processable optoelectronics.

Electroplated core\u2013shell nanowire network electrodes for highly efficient organic light-emitting diodes

In this study, we performed metal (Ag, Ni, Cu, or Pd) electroplating of core–shell metallic Ag nanowire (AgNW) networks intended for use as the anode electrode in organic light-emitting diodes (OLEDs) to modify the work function (WF) and conductivity of the AgNW networks. This low-cost and facile electroplating method enabled the precise deposition of metal onto the AgNW surface and at the nanowire (NW) junctions. AgNWs coated onto a transparent glass substrate were immersed in four different metal electroplating baths: those containing AgNO3 for Ag electroplating, NiSO4 for Ni electroplating, Cu2P2O7 for Cu electroplating, and PdCl2 for Pd electroplating. The solvated metal ions (Ag+, Ni2+, Cu2+, and Pd2+) in the respective electroplating baths were reduced to the corresponding metals on the AgNW surface in the galvanostatic mode under a constant electric current achieved by linear sweep voltammetry via an external circuit between the AgNW networks (cathode) and a Pt mesh (anode). The amount of electroplated metal was systematically controlled by varying the electroplating time. Scanning electron microscopy images showed that the four different metals (shells) were successfully electroplated on the AgNWs (core), and the nanosize-controlled electroplating process produced metal NWs with varying diameters, conductivities, optical transmittances, and WFs. The metal-electroplated AgNWs were successfully employed as the anode electrodes of the OLEDs. This facile and low-cost method of metal electroplating of AgNWs to increase their WFs and conductivities is a promising development for the fabrication of next-generation OLEDs.

Green ink formulation for inkjet printed transparent electrodes in OLEDs on biodegradable substrates

In this manuscript we present a green ink formulation for inkjet-printed and blade coated highly conductive PEDOT:PSS thin film. By the addition of a biodegradable surfactant we reduced the surface tension of a water-based PEDOT:PSS formulation enabling the deposition of high quality layers. Finally, we demonstrate the use of inkjet printed PEDOT:PSS films on the basis of these formulation as bottom transparent electrode of organic light-emitting diodes deposited on a biodegradable cellulose acetate (CA) substrate. Our devices outperformed a reference fabricated with a commercially available low-ST PEDOT:PSS that includes unknown additives.

Work function-tunable ZnO/Ag/ZnO film as an effective hole injection electrode prepared via nickel doping for thermally activated delayed fluorescence-based flexible blue organic light-emitting diodes

Metal-doped ZnO, as an alternative to indium tin oxide, has been extensively investigated; however, its widespread use has been limited because of its low conductivity and work function. In this study, ZnO was Ni-doped via co-sputtering to achieve work function tuning and improve its conductivity. The Ni-doped ZnO could be used as an effective hole injection electrode for organic light-emitting diodes (OLEDs). Herein, we prepared a Ni-doped ZnO/Ag/ZnO (Ni:ZAZ) electrode for thermally activated delayed fluorescence-based flexible blue OLEDs. The Ni:ZAZ electrode exhibited outstanding performance with a surface resistivity of 6.33 Ω/sq, high transmittance of 93.3% at 484 nm, and work function of up to 0.6 eV, along with a smooth surface and excellent mechanical durability. In addition, the Ni:ZAZ-based OLEDs exhibited excellent performance with a turn-on voltage of 3.0 V and a maximum current efficiency of 17.2 cd/A owing to its improved hole injection property. Furthermore, on flexible substrates, these OLEDs exhibited improved mechanical stability even after being subjected to 2000 bending cycles at a radius of curvature of 6 mm. Thus, the synthesis approach proposed in this study will pave a new pathway for the development of transparent electrodes for flexible and wearable opto-electronic devices.

Facile preparation of high conductive silver electrodes by dip-coating followed by quick sintering.

With polyol-synthesized silver nanoparticles (AgNPs) as raw materials, the silver electrodes with high conductivity were fabricated via a dip-coating method followed by sintering process, and the effects of the dip-coating and sintering process on the conductivity and surface roughness of silver electrodes were investigated in detail. The silver film with a thickness of 1.97 µm and a roughness of about 2 nm can be prepared after dip-coating at a pulling rate of 500 µm s−1 for 40 coating times. The non-conductive dip-coated silver films are transformed into conductive silver electrodes after conventional sintering in a muffle oven, infrared sintering and microwave sintering, respectively. Compared with high sintering temperature and long sintering time of conventional sintering and infrared sintering, microwave sintering can achieve quick sintering of silver films to fabricate high conductive silver electrodes. The silver electrodes with a sheet resistance of 0.75 Ω sq−1 and a surface roughness of less than 1 nm can be obtained after microwave sintering at 500 W for 50 s. The adjustable dip-coating method followed by quick microware sintering is an appropriate approach to prepare high conductive AgNPs-based electrodes for organic light-emitting diodes or other devices.

High transmittance Er-doped ZnO thin films as electrodes for organic light-emitting diodes

Rare earth element-Erbium (Er) doped ZnO (ErZO) thin films were deposited on glass substrates by pulsed laser deposition (PLD). The effect of the Er doping concentration on photoelectric properties of ErZO thin films was investigated in the range of 0–2.0 wt. %. The Er doping resulted in the improvement of n-type conductivity as compared with intrinsic ZnO thin films. The optimized ErZO thin films present the low resistivity of 3.4 × 10−4 Ω/cm, high carrier concentration of 5.9 × 1020/cm3, and high visible optical transmittance (∼93%) when the Er content is 1.0 wt. %. The ErZO thin films were used as transparent anodes to fabricate organic light-emitting diodes (OLEDs). Impressively, with ErZO as the anode, the current efficiency of the OLED device can reach as high as 86.5 cd/A, which was increased by 14% when compared with the reference OLED device (76.0 cd/A) using indium tin oxide as the anode.

Flexible transparent electrodes for organic light-emitting diodes simply fabricated with AuCl3-modied graphene

Abstract We used chemical doping method (using AuCl3 solution in nitromethane) to incorporate gold into graphene sheets. Gold cations reduced on the surface of the graphene film forming pure gold particles. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) showed that the sizes of gold particles were proportional to the doping time. Gold particles mainly accumulated at the graphene defects and wrinkles, effectively reducing the surface resistance and increasing the carrier concentration without significantly affecting the optical properties of graphene. Resistance of the doped single-layer graphene (SLG) sheet can be reduced from 365Ω/sq to 190Ω/sq. Optical transmittance slightly decreased with the soaking time: the light transmittance at 550 nm gradually decreased from 87.6% (for undoped graphene) to 85.6% (for graphene soaked in 1 mg/ml AuCl3 solution for 10 min). Organic light-emitting device (OLED) prepared based on the doped SLG transparent conductive electrode showed much higher performance than a device made of pure graphene as an anode. The maximum luminance of the OLED with doped graphene anode was increased by 135% and the maximum current efficiency was improved by 25.2%.

Tailoring Mg:Ag functionalities for organic light-emitting diodes

Abstract Magnesium (Mg) and silver (Ag) alloys are staple electrodes for organic light-emitting diodes (OLEDs). In this study, the Mg:Ag binary phase diagram is used to establish an electrode materials selection guide for both bottom- and top-emitting OLEDs. To achieve low turn-on voltage and high efficiency in bottom-emitting OLEDs, it is found that Mg:Ag cathodes must be selected from AgMg3 Mg phase region which provides the best figure of merits in low work function and high optical reflectance. For top-emitting OLEDs, however, Mg:Ag cathodes in the AgMg3 Mg region show large imaginary refractive index, and thus a stacked Mg:Ag/Ag is found essential to achieve optimal device performance. Furthermore, insertion of a thin LiF layer is found to create an effective charge injection cathode. In order to theoretically compute the devices’ optical characteristics, a database of complex optical constants of various materials including the metals is obtained by using a spectroscopic ellipsometer. This new database is found essential for accurate device optical design and engineering.

Flexible and Transparent Graphene Electrode Architecture with Selective Defect Decoration for Organic Light‐Emitting Diodes

AbstractGraphene produced by chemical vapor deposition (CVD) has attracted great interest as a transparent conducting material, due to its extraordinary characteristics such as flexibility, optical transparency, and high conductivity, especially in next‐generation displays. Graphene‐based novel electrodes have the potential to satisfy the important factors for high‐performance flexible organic light‐emitting diodes (OLEDs) in terms of sheet resistance, transmittance, work function, and surface roughness. In this study, flexible and transparent graphene electrode architecture is proposed by adopting a selective defect healing technique for CVD‐grown graphene, which results in several benefits that produce high‐performance devices with excellent stabilities. The proposed architecture, which has a multi‐layer graphene structure treated by a layer‐by‐layer healing process, exhibits significant improvement in sheet resistance with high optical transparency. For improving the charge transport property and mechanical robustness, various defect sites of the CVD‐grown graphene are successfully decorated with gold nanoparticles through a simple electroplating (EP) method. Further, a graphene‐based OLED device that integrates the proposed electrode architecture on flexible substrates is demonstrated. Therefore, this architecture provides a new strategy to fabricate graphene electrode in OLEDs, extending graphene's immense potential as an advanced conductor toward high‐performance, flexible, and transparent displays.

Recycling indium tin oxide (ITO) anodes for use in organic light-emitting diodes (OLEDs)

Electrodes in many thin-film optoelectronic devices are made of layers of indium tin oxide (ITO), which are suitably conductive, can be deposited by established methods, and are nearly transparent and colorless. Regrettably, indium is an uncommon element, its price continues to rise steeply, and producing ITO electrodes consumes a large fraction of the total energy needed to make thin-layer devices. Nevertheless, ITO electrodes are widely used in devices, both in research and industrial production. The growing commercial importance of these devices creates an urgent need for ways to ensure sustainability, such as by recovering ITO from devices no longer needed or even by recycling intact ITO electrodes. Recent work has shown that patterned ITO electrodes can be recovered from organic thin-film photovoltaic devices (OPVs) after simple exposure to water or dilute aqueous base, and then reused in new devices with essentially no decrease in performance. We now report that similar approaches can be used to recover and reuse ITO electrodes in organic light-emitting diodes (OLEDs), which are currently manufactured in much greater numbers than OPVs. Use of atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to study the surfaces of ITO electrodes recovered from OLEDs revealed minor changes in morphology, but the performance of the recycled electrodes in new OLEDs proved to be virtually unchanged, even after multiple cycles of recovery and reuse.

Ag Nanowire Films Coated with Graphene as Transparent Conductive Electrodes for Organic Light-Emitting Diodes

Ag Nanowire Films Coated with Graphene as Transparent Conductive Electrodes for Organic Light-Emitting Diodes

Silver Nanowire-IZO-Conducting Polymer Hybrids for Flexible and Transparent Conductive Electrodes for Organic Light-Emitting Diodes.

Solution-processed silver nanowire (AgNW) has been considered as a promising material for next-generation flexible transparent conductive electrodes. However, despite the advantages of AgNWs, some of their intrinsic drawbacks, such as large surface roughness and poor interconnection between wires, limit their practical application in organic light-emitting diodes (OLEDs). Herein, we report a high-performance AgNW-based hybrid electrode composed of indium-doped zinc oxide (IZO) and poly (3,4-ethylenediowythiophene):poly(styrenesulfonate) [PEDOT:PSS]. The IZO layer protects the underlying AgNWs from oxidation and corrosion and tightly fuses the wires together and to the substrate. The PEDOT:PSS effectively reduces surface roughness and increases the hybrid films’ transmittance. The fabricated electrodes exhibited a low sheet resistance of 5.9 Ωsq−1 with high transmittance of 86% at 550 nm. The optical, electrical, and mechanical properties of the AgNW-based hybrid films were investigated in detail to determine the structure-property relations, and whether optical or electrical properties could be controlled with variation in each layer’s thickness to satisfy different requirements for different applications. Flexible OLEDs (f-OLEDs) were successfully fabricated on the hybrid electrodes to prove their applicability; their performance was even better than those on commercial indium doped tin oxide (ITO) electrodes.

Flexible and Mechanically Robust Organic Light-Emitting Diodes Based on Photopatternable Silver Nanowire Electrodes

We developed a simple methodology for fabricating silver nanowire (AgNW) micropatterns on a plastic substrate using a photocurable polymer. The patterning method began with the lamination of a UV-curable prepolymer film onto the AgNW-coated rigid glass substrate. Selective UV exposure of the UV-curable prepolymer film through a photomask solidified the exposed regions, and the unexposed regions were simply removed by the solvent. AgNW micropatterns of various sizes and shapes could be readily formed across the entire plastic substrate. Importantly, this photopatterning process enabled the embedding of the AgNW structures into the polymer matrix, which dramatically reduced the surface roughness and enhanced the mechanical stability of the AgNW film. The AgNW structures served as transparent anode electrodes in organic light-emitting diodes (OLEDs) that performed well compared to OLEDs fabricated using conventional indium tin oxide (ITO) or conducting polymer electrodes. This simple, inexpensive, and scalab...

(Invited) Advanced Transparent Conductive Top Electrode for Transparent and Top-Emitting Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) have successfully entered mobile-phone display market and large-size OLED TVs have also been available by overcomming many technical obstacles. However, novel and innovative technologies are still required for flexible, transparent, and high-resolution displays. For example, augmented reaility (AR) and virtual reality (VR) have recently received peculiar attention and they need transparent and high-resolution displays with thousands of pixels per inch (PPI) to realize immersive displays. Top-emitting OLEDs (TEOLEDs) have an advantage for high-resolution displays due to their large aperture ratio. Top electrode is one of crutial factors to decide the performance of TEOLEDs as well as transparent OLEDs (TOLEDs) and should have high transmittance, low reflectance, and high conductivity. There are many candidate materials for transparent conductive electrodes but the top electrode choices are limited, because top electrode materials should not damage the organic layers of OLEDs during the deposition process. Most TEOLEDs and TOLEDs that have been reported so far utilize semitransparent thin metal top electrode owing to their compatibility with OLED fabrication process, but thin metal top electrodes without sophisticated optical design degrade the efficiency and color quality [1]. Indium-tin-oxide (ITO) has optically good properties, but it needs sputtering and high temperature precess which leads to damage of organic layers in OLEDs [2]. Carbon-based materials suchs as carbon nanotubes (CNT), graphene, and conducting polymers are lately used as transparent conductive top electrodes in OLEDs and polymer solar cells because of their high trasmittance and low reflectance [3-6]. Most devices show optically good but electrical poor properties due to low conductivity of carbon-based non-metal transparent conductive materials. To apply carbon-based electrodes to large-size devices, their electrical conductivity should be improved. In this work, we make carbon-based transparent conductive electrode films and enhace the conductivity of the film by embedding silver mesh electrodes. The film showed high transmittance and low reflectance in visible spectral region in spite of existence of metal auxiliary wire. We have fabricated small molecule-based transparent OLEDs with these electrode films as top electrodes using a dry lamination technique and inverted structure which has cathodes on the substrate is employed because the conducting polymer has high work function that is suitable for an anode instead of a cathode. The device exhibited low driving voltage with high transmittance. In addition, we have demonstrated a 4 cm x 4 cm transparent segment panel with silver mesh embedded conducting polymer top electrodes. We also investigated color shift of white TEOLEDs with conducting polymer top electrodes depending on viewing angle. AcknowledgmentThis work was supported by Institute for Information & communications Technology Promotion(IITP) grant funded by the Korea government(MSIP) (B0101-16-0133, The core technology development of light and space adaptable energy-saving I/O platform for future advertising service)

Highly Conductive PEDOT:PSS Films with 1,3-Dimethyl-2-Imidazolidinone as Transparent Electrodes for Organic Light-Emitting Diodes.

PSS) films as transparent electrodes for organic light-emitting diodes (OLEDs) are doped with a new solvent 1,3-dimethyl-2-imidazolidinone (DMI) and are optimized using solvent post-treatment. The DMI doped PSS films show significantly enhanced conductivities up to 812.1 S cm(-1) . The sheet resistance of the PSS films doped with DMI is further reduced by various solvent post-treatment. The effect of solvent post-treatment on DMI doped PSS films is investigated and is shown to reduce insulating PSS in the conductive films. The solvent posttreated PSS films are successfully employed as transparent electrodes in white OLEDs. It is shown that the efficiency of OLEDs with the optimized DMI doped PSS films is higher than that of reference OLEDs doped with a conventional solvent (ethylene glycol). The results present that the optimized PSS films with the new solvent of DMI can be a promising transparent electrode for low-cost, efficient ITO-free white OLEDs.

Application of patterned Ag-nanowire networks to transparent thin-film heaters and electrodes for organic light-emitting diodes

We present patterned Ag-nanowire (AgNW) networks for their application to transparent electrodes in flexible devices. Using capillary-force-based soft lithography (CFL), we formed 25- to 30-µm-wide line patterns of AgNWs on flexible polymer substrates. Organic light-emitting diodes (OLEDs) and transparent thin-film heaters (TFHs) were successfully fabricated on the patterned substrates, which verified the potential of AgNW patterns formed by CFL as interconnects in flexible devices.

Challenge beyond Graphene: Metal Oxide/Graphene/Metal Oxide Electrodes for Optoelectronic Devices.

Graphene has shown strong potential to occupy transparent electrodes, replacing indium tin oxide (ITO). However, the commercialization of graphene is still limited because of its poor chemical and electrical stability from reaction with environmental factors or essential materials such as poly[3,4-(ethylenedioxy)thiophene]:poly(styrenesulfonate) ( PSS). Here, we have demonstrated a multilayered electrode in which graphene is sandwiched between metal oxides (MOs) that have high stability and optical properties. The MOs overcoated graphene, and thereby protected it from desorption of chemical dopants. Because of the resulting chemical and electrical stability, the electrodes maintain low sheet resistance 2.4 times longer than bare graphene and 36 times longer than PSS-coated graphene. On the basis of optical simulations, we derive the design rules for highly transparent MO/graphene/MO stacks and demonstrate an optimized structure with a TiO2 and WO3 electrode that has high transmittance (96%) which exceeds those of ITO (87%) and graphene (90%). Using a TiO2/graphene/WO3 electrode in organic light-emitting diodes (λ = 520 nm) instead of ITO or graphene anodes increases the cavity resonance and thereby increases power efficiencies by up to 30%. The MO/graphene/MO stacks designed will provide opportunities for commercialization of flexible electronics with graphene electrodes.