Screen‐Printed Poly(3,4‐Ethylenedioxythiophene):Poly(Styrenesulfonate) Grids as ITO‐Free Anodes for Flexible Organic Light‐Emitting Diodes

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) grids have been successfully constructed by roll‐to‐roll compatible screen‐printing techniques and have been used as indium tin oxide (ITO)‐free anodes for flexible organic light‐emitting diodes (OLEDs). The grid‐type transparent conductive electrodes (TCEs) can adopt thicker PEDOT: PSS grid lines to ensure the conductivity, while the mesh‐like grid structure can play an important role to maintain high optical transparency. By adjusting grid periods, grid thickness and treatment of organic additives, PEDOT: PSS TCEs with high optical transparency, low sheet resistance, and excellent mechanical flexibility have been achieved. Using the screen‐printed PEDOT: PSS grids as the anodes, ITO‐free OLEDs achieved peak current efficiency of 3.40 cd A^?1 at the current density of 10 mA cm^?2, which are 1.56 times better than the devices with ITO glass as the anodes. The improved efficiency is attributed to the light extraction effect and improved transparency by the grid structure. The superior optoelectronic performances of OLEDs based on flexible screen‐printed PEDOT: PSS grid anodes suggest their great prospects as ITO‐free anodes for flexible and wearable electronic applications.     

Paper‐Based Electrochromic Devices Enabled by Nanocellulose‐Coated Substrates

As environmental considerations for both the processing and disposal of electronic devices become increasingly important, the ability to replace plastic and glass substrates with bioderived and biodegradable materials remains a major technological goal. Here, the use of cellulose nanofiber‐coated paper is explored as an environmentally benign substrate for preparing low‐resistance (460 Ω sq^?1), colorless (a* = ?2.3, b* = ?2.7) printed poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes. The PEDOT:PSS/paper electrodes support the reversible oxidation of three electrochromic polymers (ECPs) (cyan, magenta, and yellow), affording the possibility for fully printed, color displays on paper. Lateral electrochromic devices (ECDs) incorporating an ion gel electrolyte are demonstrated where a magenta‐to‐colorless device achieves a color contrast (ΔE*) of 56 owing to a highly color‐neutral bleached state of the ECP (a* = ?0.5, b* = 2.9). Black‐to‐colorless devices achieve ΔE* = 29 and are able to retain 86% of their color contrast after 9000 switches. The switching times of these lateral devices are quantified through colorimetric image analysis which shows comparable performance for devices constructed on paper as devices using ITO/glass electrodes (10 Ω sq^?1). The paper ECDs are then combusted in air leaving 3% of the initial mass at 600 °C, highlighting this approach as a promising route toward disposable displays.     

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.     

A highly conductive and smooth AgNW/PEDOT:PSS film treated by hot-pressing as electrode for organic light emitting diode

A highly conductive, smooth and transparent electrode is developed by coating poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) over silver nanowires (AgNWs) followed by a hot-pressing method. The hot-pressed AgNW/PEDOT:PSS film shows a low sheet resistance of 12 Ω/square, a transmittance of 83% at 550 nm and a smooth surface. The improvement of the conductivity and smoothness are ascribed to the fusion of nanowires resulted from the mechanical hot-pressing. The AgNW/PEDOT:PSS film on polyethylene naphthalate (PEN) substrate exhibits higher conductive stability against the bending test than commonly used indium tin oxide (ITO). Using the hot-pressed AgNW/PEDOT:PSS film as the anode, we have fabricated ITO-free organic light emitting diode with a maximum current efficiency of 58.2 cd/A, which is higher than the device with ITO anode. This proves that such AgNW/PEDOT:PSS film treated by hot-pressing is a promising candidate for flexible optoelectronic devices.     

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSS_ADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSS_IMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSS_IMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSS_IMM electrode out-performed the PEDOT:PSS_ADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cm⁻² to 7.15 mA cm⁻². The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.     

Achieving High Efficiency and Improved Stability in ITO‐Free Transparent Organic Light‐Emitting Diodes with Conductive Polymer Electrodes

Efficient transparent organic light‐emitting diodes (OLEDs) with improved stability based on conductive, transparent poly(3,4‐ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes are reported. Based on optical simulations, the device structures are carefully optimized by tuning the thickness of doped transport layers and electrodes. As a result, the performance of PEDOT:PSS‐based OLEDs reaches that of indium tin oxide (ITO)‐based reference devices. The efficiency and the long‐term stability of PEDOT:PSS‐based OLEDs are significantly improved. The structure engineering demonstrated in this study greatly enhances the overall performances of ITO‐free transparent OLEDs in terms of efficiency, lifetime, and transmittance. These results indicate that PEDOT:PSS‐based OLEDs have a promising future for practical applications in low‐cost and flexible device manufacturing.     

Conductive PEDOT:PSS on surface-functionalized chitosan biopolymers for stretchable skin-like electronics

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a promising alternative transparent electrode to replace conventional indium tix oxide (ITO) for flexible and stretchable electronics. For their applications in optoelectronic devices, realizing both high conductivity and transmittance for the films is of great necessity as a suitable high performance transparent electrode. Here, we demonstrate simultaneously enhanced electrical and optical properties of PEDOT:PSS films prepared on chitosan bio-substrates by using an organic surface modifier, 11-aminoundecanoic acid (11-AA). The sheet resistance of PEDOT:PSS films decreases from 1120.8 to 292.8 Ω/sq with an increase in a transmittance from 75.9 to 80.4% by 11-AA treatment on the chitosan films. The functional groups of 11-AA effectively enhance the adhesion property at the interface between the chitosan substrate and PEDOT:PSS by forming strong interfacial bondings and decrease insulating PSS from PEDOT:PSS films. The wearable heater devices and on-skin sensors based on the 11-AA-treated PEDOT:PSS on the chitosan bio-substrates are successfully fabricated, showing the excellent thermal and sensing performances. The 11-AA surface-modification approach for highly conductive PEDOT:PSS on chitosan bio-substrates presents a great potential for applications toward transparent, flexible and stretchable electronics.     

Highly Conductive and Transparent PEDOT:PSS Films with a Fluorosurfactant for Stretchable and Flexible Transparent Electrodes

Highly conductive and transparent poly‐(3,4‐ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) films, incorporating a fluorosurfactant as an additive, have been prepared for stretchable and transparent electrodes. The fluorosurfactant‐treated PEDOT:PSS films show a 35% improvement in sheet resistance (R_s) compared to untreated films. In addition, the fluorosurfactant renders PEDOT:PSS solutions amenable for deposition on hydrophobic surfaces, including pre‐deposited, annealed films of PEDOT:PSS (enabling the deposition of thick, highly conductive, multilayer films) and stretchable poly(dimethylsiloxane) (PDMS) substrates (enabling stretchable electronics). Four‐layer PEDOT:PSS films have an R_s of 46 Ω per square with 82% transmittance (at 550 nm). These films, deposited on a pre‐strained PDMS substrate and buckled, are shown to be reversibly stretchable, with no change to R_s, during the course of over 5000 cycles of 0 to 10% strain. Using the multilayer PEDOT:PSS films as anodes, indium tin oxide (ITO)‐free organic photovoltaics are prepared and shown to have power conversion efficiencies comparable to that of devices with ITO as the anode. These results show that these highly conductive PEDOT:PSS films can not only be used as transparent electrodes in novel devices (where ITO cannot be used), such as stretchable OPVs, but also have the potential to replace ITO in conventional devices.     

Highly conductive PEDOT:PSS film by doping p-toluenesulfonic acid and post-treatment with dimethyl sulfoxide for ITO-free polymer dispersed liquid crystal device

In this paper, the highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film realized by applying the doping technique and the post-treatment process is demonstrated. The conductivity of the spin coated PEDOT:PSS film enhanced greatly from 0.7 S/cm to 736 S/cm after 1.25% of p-toluenesulfonic acid solution (50 wt%) was doped into the PEDOT:PSS aqueous dispersion. The post-treatment using dimethyl sulfoxide further improved the conductivity to 1549 S/cm. The highly conductive PEDOT:PSS film was used as transparent electrode to fabricate ITO-free polymer dispersed liquid crystal (PDLC) cell. The experimental results showed that the electro-optical properties of the PDLC cell fabricated by the highly conductive PEDOT:PSS film were comparable to those of the PDLC cell constructed by ITO. This study reveals that the highly conductive PEDOT:PSS film is a prospective material for manufacturing ITO-free liquid crystal devices.     

Efficient ITO-free organic light-emitting diodes comprising PEDOT:PSS transparent electrodes optimized with 2-ethoxyethanol and post treatment

We demonstrate highly conductive poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) films introduced with a newly investigated solvent 2-ethoxyethanol. The films are optimized by simple solvent post treatment and show enhanced conductivities and reduced sheet resistances. Solvent post treatment for 2-ethoxyethanol added PEDOT:PSS films reduces insulating PSS and forms conductive PEDOT networks in conductive films, resulting in improved electrical properties. ITO-free white OLEDs are fabricated with post-treated PEDOT:PSS electrodes and show almost equal performance to ITO-based OLEDs. Our work demonstrate that the conductive PEDOT:PSS electrode optimized by 2-ethoxyethanol and post treatment promises its potential as alternative transparent electrode in flexible, low-cost, high-performance ITO-free OLEDs.     

Photolithographic patterning of PEDOT:PSS with a silver interlayer and its application in organic light emitting diodes

A patterning scheme for poly(3,4-ethylenedioxythio-phene):poly(styrenesulfonate) (PEDOT:PSS) is reported. With a silver interlayer, the conductive PEDOT:PSS film can be patterned down to micrometer scales by traditional photolithography, and this patterning scheme can be applied on large-area flexible substrates. Through systematical investigations, the patterning processes have no obvious influence on both the bulk and surface properties of PEDOT:PSS films. Efficient organic light emitting diodes (OLEDs) are realized based on this patterned PEDOT:PSS anode, and they show comparable performance to those devices with an indium tin oxide (ITO) anode. High-resolution OLED pixel arrays are also demonstrated. Our interlayer approach here has an advantage of patterning PEDOT:PSS with high resolution and large scale, and it is also compatible with traditional photolithographic processes which substantially save the capital cost. Results indicate that the photographically patterned conductive PEDOT:PSS film becomes a promising candidate for eletrical eletrode material in organic electronic applications.     

Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

Photovoltaic Devices: Highly Conductive PEDOT:PSS Electrode with Optimized Solvent and Thermal Post‐Treatment for ITO‐Free Organic Solar Cells (Adv. Funct. Mater. 6/2011)

Highly conductive poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films as stand‐alone electrodes for organic solar cells have been optimized using a solvent post‐treatment method. The treated PEDOT:PSS films show enhanced conductivities up to 1418 S cm^?1, accompanied by structural and chemical changes. The effect of the solvent treatment on PEDOT:PSS has been investigated in detail and is shown to cause a reduction of insulating PSS in the conductive polymer layer. Using these optimized electrodes, ITO‐free, small molecule organic solar cells with a zinc phthalocyanine (ZnPc):fullerene C₆₀ bulk heterojunction have been produced on glass and PET substrates. The system was further improved by pre‐heating the PEDOT:PSS electrodes, which enhanced the power conversion efficiency to the values obtained for solar cells on ITO electrodes. The results show that optimized PEDOT:PSS with solvent and thermal post‐treatment can be a very promising electrode material for highly efficient flexible ITO‐free organic solar cells.     

Toward Stretchable Self‐Powered Sensors Based on the Thermoelectric Response of PEDOT:PSS/Polyurethane Blends

The development of new flexible and stretchable sensors addresses the demands of upcoming application fields like internet‐of‐things, soft robotics, and health/structure monitoring. However, finding a reliable and robust power source to operate these devices, particularly in off‐the‐grid, maintenance‐free applications, still poses a great challenge. The exploitation of ubiquitous temperature gradients, as the source of energy, can become a practical solution, since the recent discovery of the outstanding thermoelectric properties of a conductive polymer, poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS). Unfortunately the use of PEDOT:PSS is currently constrained by its brittleness and limited processability. Herein, PEDOT:PSS is blended with a commercial elastomeric polyurethane (Lycra), to obtain tough and processable self‐standing films. A remarkable strain‐at‐break of ≈700% is achieved for blends with 90 wt% Lycra, after ethylene glycol treatment, without affecting the Seebeck voltage. For the first time the viability of these novel blends as stretchable self‐powered sensors is demonstrated.     

Roll‐to‐Roll Printed Silver Nanowire Semitransparent Electrodes for Fully Ambient Solution‐Processed Tandem Polymer Solar Cells

Silver nanowires (AgNWs) and zinc oxide (ZnO) are deposited on flexible substrates using fast roll‐to‐roll (R2R) processing. The AgNW film on polyethylene terephthalate (PET) shows >80% uniform optical transmission in the range of 550–900 nm. This electrode is compared to the previously reported and currently widely produced indium‐tin‐oxide (ITO) replacement comprising polyethylene terephthalate (PET)|silver grid|poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)|ZnO known as Flextrode. The AgNW/ZnO electrode shows higher transmission than Flextrode above 490 nm in the electromagnetic spectrum reaching up to 40% increased transmission at 750 nm in comparison to Flextrode. The functionality of AgNW electrodes is demonstrated in single and tandem polymer solar cells and compared with parallel devices on traditional Flextrode. All layers, apart from the semitransparent electrodes which are large‐scale R2R produced, are fabricated in ambient conditions on a laboratory roll‐coater using printing and coating methods which are directly transferrable to large‐scale R2R processing upon availability of materials. In a single cell structure, Flextrode is preferable with active layers based on poly‐3‐hexylthiophene(P3HT):phenyl‐C61‐butyric acid methylester (PCBM) and donor polymers of similar absorption characteristics while AgNW/ZnO electrodes are more compatible with low band gap polymer‐based single cells. In tandem devices, AgNW/ZnO is more preferable resulting in up to 80% improvement in PCE compared to parallel devices on Flextrode.     

Effects of the ink concentration on multi-layer gravure-printed PEDOT:PSS

To date, highly conductive PEDOT:PSS is the most promising transparent electrode for printing-based flexible organic electronics. Spin-coating and slot-die coating have been commonly used for printing this material. Among the roll-to-roll printing processes, gravure is the most promising for manufacturing large area electronics offering the advantages of high speed and high printing definition. However, gravure printing highly conductive PEDOT encounters some technological limitations such as low thickness, layer inhomogeneity and high surface roughness resulting in a layer not suitable as electrode in electronic devices. In order to realize an electrode of highly conductive PEDOT by gravure printing, a multilayer approach with variable ink concentration was tried using IPA as process solvent. Variable solvent amount of overlapped printed layers was found to play an important role in the spreading of the PEDOT ink onto the pre-printed layers and in the smoothing of its existent peaks. In particular, adopting increasing ink dilution with increasing of the overlapped layers, multi-layer gravure-printed highly conductive PEDOT was successfully realized with characteristics suitable as transparent electrode for organic electronic devices (sheet resistance lower than 130 Ω/sq, conductivity higher than 450 S/cm and optical transmittance over 80%). This is the first time that such results were reached by gravure printing technique thanks to the easy proposed approach.     

Neutral-pH PEDOT:PSS as over-coating layer for stable silver nanowire flexible transparent conductive films

The silver nanowire (AgNW) mesh film with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the over-coating layer is a promising flexible transparent conductive film technology. In this work, experimental studies show that the hygroscopic and acid properties of the common PEDOT:PSS lead to poor stabilities of the composite films, due to the conductivity degradation of PEDOT:PSS by the water absorption and the acid corrosion of AgNWs by PEDOT:PSS. By using the modified PEDOT:PSS of neutral pH as the over-coating layer, the long term shelf-life time, thermal and current stressing stabilities are all significantly improved without sacrifice of transparency, electrical conductivity and mechanical flexibility. Under both cases of thermal aging test at 210 °C for 20 min and 12 h continuous current stressing at a current density of 30 mA/cm², no obvious change of the conductivity is observed. The results clearly demonstrate that using the neutral-pH PEDOT:PSS as an over-coating layer can help to achieve flexible AgNW transparent conductive films with superior stability for flexible optoelectronic devices.     

Improving the color switch contrast in PEDOT:PSS-based electrochromic displays

Poly(3,4-ethylenedioxythiophene) chemically doped with poly(styrene sulfonic acid) (PEDOT:PSS) is a material system commonly used as a conductive and transparent coating in several important electronic applications. The material is also electrochemically active and exhibits electrochromic (EC) properties making it suitable as the active element in EC display applications. In this work uniformly coated PEDOT:PSS layers were used both as the pixel electrode and as the counter electrode in EC display components. The pixel and counter electrodes were separated by a whitish opaque and water-based polyelectrolyte and the thicknesses of the two EC layers were varied independently in order to optimize the color contrast of the display element. A color contrast (ΔE^∗, CIE L^∗a^∗b^∗ color space) exceeding 40 was obtained with maintained relatively short switching time at an operational voltage less than 2 V.     

Selectively modulated inkjet printing of highly conductive and transparent foldable polymer electrodes for flexible polymer light-emitting diode applications

High efficient flexible polymer light-emitting devices which composed with highly conductive and transparent foldable polymer electrodes were fabricated. New doping materials, n-methyl-2-pyrrolidone (NMP) and n-methylformamide (NMF), have much improved the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The selectively modulated inkjet-printing method facilitated the PEDOT:PSS’s application to both transparent anodes and highly conductive bus line electrodes. Multiple-time printed PEDOT:PSS electrodes showed a similar performance to Ag bus lines while one-time printed anode showed a much better figure of merit than that of ITO on plastic. Due to the flexible property, high transparency and high work function of the polymeric anode, ITO-free PLEDs showed a high performance with foldable characteristics.     

High‐Conductivity Poly(3,4‐ethylenedioxythiophene):Poly(styrene sulfonate) Film and Its Application in Polymer Optoelectronic Devices

The conductivity of a poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film can be enhanced by more than two orders of magnitude by adding a compound with two or more polar groups, such as ethylene glycol, meso‐erythritol (1,2,3,4‐tetrahydroxybutane), or 2‐nitroenthanol, to an aqueous solution of PEDOT:PSS. The mechanism for this conductivity enhancement is studied, and a new mechanism proposed. Raman spectroscopy indicates an effect of the liquid additive on the chemical structure of the PEDOT chains, which suggests a conformational change of PEDOT chains in the film. Both coil and linear conformations or an expanded‐coil conformation of the PEDOT chains may be present in the untreated PEDOT:PSS film, and the linear or expanded‐coil conformations may become dominant in the treated PEDOT:PSS film. This conformational change results in the enhancement of charge‐carrier mobility in the film and leads to an enhanced conductivity. The high‐conductivity PEDOT:PSS film is ideal as an electrode for polymer optoelectronic devices. Polymer light‐emitting diodes and photovoltaic cells fabricated using such high‐conductivity PEDOT:PSS films as the anode exhibit a high performance, close to that obtained using indium tin oxide as the anode.