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.     

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.     

Fast Switching ITO Free Electrochromic Devices

Indium‐doped tin oxide free electrochromic devices are prepared by coating electrochromic polymers onto polyethylene terephthalate substrates encompassing two different silver grids as electrodes. One design comprises a flexoprinted highly conductive silver grid electrode, yielding electrochromic devices with a response time of 2 s for an optical contrast of 27%. The other design utilizes an embedded silver grid electrode whereupon response times of 0.5 s for a 30% optical contrast are realized when oxidizing the device. A commercially available conductive poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate acid) formulation (PEDOT:PSS) is coated onto the silver grids as a charge balancing polymer, and is in this setting found to be superior to a polypyrrole previously employed in electrochromic devices. In addition, the PEDOT:PSS layer increases the conductivity in the hexagonal grid structure.     

Stretchable ITO-Free Organic Solar Cells with Intrinsic Anti-Reflection Substrate for High-Efficiency Outdoor and Indoor Energy Harvesting

Flexible photovoltaic devices are promising candidates for triggering the Internet of Things (IoT). However, the power conversion efficiencies (PCEs) of flexible organic photovoltaic (OPV) devices with high conductivity poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes on plastic are lagging behind the rigid devices due to the low transmittance of polyethylene terephthalate (PET)/PEDOT:PSS. Moreover, the poor stretchability of the commonly used plastic substrates largely hinders the practical application of wearable devices. Herein, a novel stretchable indium tin oxide (ITO)-free OPV device with a surface-texturing polydimethylsiloxane (PDMS) substrate for outdoor strong- and indoor dim-light energy harvesting is reported. The high diffuse transmittance and haze effect of the substrate enable stretchable ITO-free devices, yielding a high PCE of 15.3% under 1 sun illumination. More excitingly, the stretchable device based on textured PDMS/PEDOT:PSS maintains a comparable PCE of 20.5% (20.8% for the rigid device) under indoor light illumination. Notably, the stretchable device is much more insensitive to the light direction, maintaining 38.5% of the initial PCE at an extremely small incident angle of 10° (16.3% for glass/ITO-based counterpart). The texturing stretchable substrate provides a new direction for achieving high performance and enhanced light utilization for the stretchable light-harvesting device, suitable for indoor and outdoor applications.     

A Transparent,Flexible, Low‐Temperature,and Solution‐Processible Graphene Composite Electrode

The synthesis and preparation of a new type of graphene composite material suitable for spin‐coating into conductive, transparent, and flexible thin film electrodes in ambient conditions is reported here for the first time. Solution‐processible graphene with diameter up to 50 μm is synthesized by surfactant‐assisted exfoliation of graphite oxide and in situ chemical reduction in a large quantity. Spin‐coating the mixing solution of surfactant‐functionalized graphene and PEDOT:PSS yields the graphene composite electrode (GCE) without the need for high temperature annealing, chemical vapor deposition, or any additional transfer‐printing process. The conductivity and transparency of GCE are at the same level as those of an indium tin oxide (ITO) electrode. Importantly, it exhibits high stability (both mechanical and electrical) in bending tests of at least 1000 cycles. The performance of organic light‐emitting diodes based on a GCE anode is comparable, if not superior, to that of OLEDs made with an ITO anode.     

Enhanced flexibility and stability of PEDOT:PSS electrodes through interfacial crosslinking for flexible organic light-emitting diodes

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films have drawn extensive attention as one of the most promising flexible transparent conductive electrodes to replace traditional indium tin oxide. However, some critical issues, such as weak adhesion, vulnerability to moisture and detrimental acidic property, need to be addressed before the practical application and industrialization. Here, we propose a facile and effective strategy of interfacial crosslinking to further improve the flexibility and stability of PEDOT:PSS electrodes with high transparency and conductivity by introducing polyethyleneimine ethoxylated (PEIE) on the surface. The flexibility and stability of PEDOT:PSS electrodes with PEIE overcoating layer are significantly improved, which can be attributed to the interfacial crosslinking reaction between PEIE and PSS. Finally, flexible organic light-emitting didoes (OLEDs) are constructed based on the PEDOT:PSS electrodes modified by PEIE, and current efficiency is enhanced from 20.5 to 76.4 cd/A with a 2.7-fold enhancement, owning to the improved carrier balance. This study confirms that PEIE is effective in protecting the PEDOT:PSS films from mechanical damage and moisture attack, while maintaining the high conductive and transmittance, and illustrates a promising future in low-cost flexible optoelectronic devices employing PEDOT:PSS electrodes.     

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.     

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.     

Negative mold transfer patterned conductive polymer electrode for flexible organic light-emitting diodes

We demonstrate flexible organic light-emitting diodes (FOLEDs) that use flexible conductive polymer electrodes patterned by negative mold transfer printing (nMTP). Because pristine poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is unsuitable for nMTP owing to problems with wettability, additives are used to improve the surface wetting properties of the polymer on the mold to successfully employ nMTP. Moreover, the additives improve the conductivity of the polymer electrode. FOLEDs fabricated with the modified PEDOT:PSS using nMTP exhibit electrical properties comparable to those of a device having an indium tin oxide (ITO) anode. These results show that the highly conductive PEDOT:PSS patterned by nMTP can be used as transparent high-resolution electrodes in low-cost ITO-free FOLEDs.     

Textile‐Based Flexible Electroluminescent Devices

Flexible and transparent textile‐based conductors are developed by inkjet printing poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) onto polyethylene terephthalate (PET) mesh fabrics. The conductivity–transparency relationship is determined for textile‐based conductors with different thicknesses of the printed PEDOT:PSS film. The function of these textile‐based conductors is studied in the alternating current powder electroluminescent (ACPEL) devices and compared with indium tin oxide (ITO) glass in an ACPEL device of the same configuration. Textiles coated with conducting polymers are a potential alternative to coated polymer films for flexible, transparent conductors.     

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.     

Stretching‐Induced Growth of PEDOT‐Rich Cores: A New Mechanism for Strain‐Dependent Resistivity Change in PEDOT:PSS Films

It remains a fundamental challenge in the development of stretchable electronics to understand how mechanical strain changes the electrical properties of materials. Although the piezoresistive behavior of poly(3,4‐ethylene‐ dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been observed, its intrinsic origin is not yet fully understood because there are many extrinsic contributing factors and an experimental platform with which to assess such behavior has not been established. Here, systematic analysis shows that the matching Poisson's ratio and elastic modulus between PEDOT:PSS films and their underlying substrates is important in decoupling the factors that affect the material's piezoresistivity, allowing for tunable resistivity. Based on such a fundamental understanding, the conductivity of PEDOT:PSS can be controlled to be invariant and decrease as a function of applied tensile stress. Furthermore, a linear response of the resistivity with respect to mechanical strains of up to 60%, which has never before been realized, is shown. The irreversible conductivity enhancement is primarily caused by the coalescence‐induced growth of conductive PEDOT‐rich cores.     

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.     

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.     

Stability of Thin Film Neuromodulation Electrodes under Accelerated Aging Conditions

Thin film electrodes are becoming increasingly common for interfacing with tissue. However, their long-term stability has yet to be proven in neuromodulation applications where electrical stimulation over months to years is desired. Here, the stability of pristine and PEDOT:PSS-coated Au, as well as pristine PEDOT:PSS microelectrodes are examined over a period of 3 months in an accelerated aging setup where they are exposed to current stimulation, hydrogen peroxide, mechanical agitation, and high temperature. Pristine PEDOT:PSS electrodes show the highest stability, while pristine Au electrodes show the lowest stability. Failure mode analysis reveals that delamination and Au corrosion are the key drivers of electrode degradation. The PEDOT:PSS coating slows down Au corrosion to a degree that depends on the overlap between the two films. The results demonstrate that pristine PEDOT:PSS electrodes represent a promising way forward toward thin film devices for long-term in vivo neuromodulation applications.     

Doped PEDOT:PSS electrodes,patterned through wettability control,and their effects on the electrical properties of polymer thin film transistors

The water−based conductive polymer, poly(3,4−ethylenedioxythiophene), doped with poly(styrene sulfonate) (PEDOT:PSS), has received much attention for its utility as a printable electrode due to its transparency, thermal stability, and processability; however, the electrical properties of devices prepared with printed PEDOT:PSS electrodes are generally inferior to those of devices fabricated with evaporated metal electrodes or their inorganic counterparts. Here, we show that the electrical performances of polymer thin film transistors could be improved by doping the PEDOT:PSS chains used as source and drain electrodes. The addition of HAuCl₄ to the PEDOT:PSS solution increased the electrical conductivity and work function of the electrodes. The PEDOT:PSS film doped with 10 mM HAuCl₄ provided a field effect mobility exceeding 0.01 cm²V⁻¹s⁻¹, a factor of 7 greater than the value obtained from the device prepared with pristine PEDOT electrodes.     

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.     

Graphene‐Conducting Polymer Hybrid Transparent Electrodes for Efficient Organic Optoelectronic Devices

To achieve the broad utilization of the full functionality of graphene (GR) in devices, a transfer method should be developed that can simplify the process without leaving residue of the insulating supporting layer on the surface of GR. Furthermore, stable GR doping without the use of an insulating polymer is required. Here, a new GR transfer method that uses a popular conducting polymer, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), is reported as a new supporting layer for the transfer of GR films that are synthesized by chemical vapor deposition. The GR/PEDOT:PSS bilayer can be directly utilized without the removal process. Therefore, this transfer method simplifies the transfer process and solves the residue problem of conventional transfer methods. The stable doping of GR films is simultaneously achieved by using the PEDOT:PSS layer. The new GR/PEDOT:PSS hybrid electrodes are fully functional in polymer solar cells and polymer light‐emitting diodes, outperforming the conventionally transferred GR electrodes and indium tin oxide electrodes.     

Optimization of electrohydrodynamic-printed organic electrodes for bottom-contact organic thin film transistors

In this study, we investigate the optimization of printed (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as source/drain electrodes for organic thin film transistors (OTFTs) through electrohydrodynamic (EHD) printing process. The EHD-printed PEDOT:PSS electrodes should fulfill the prerequisites of not only high conductivity but also optimum surface tension for successful jetting. The conductivity of PEDOT:PSS was dramatically enhanced from 0.07 to 352 S/cm by the addition of dimethylsulfoxide (DMSO). To use the DMSO-treated PEDOT:PSS solution in the EHD printing process, its surface tension was optimized by the addition of surfactant (Triton X-100), which was found to enable various jetting modes. In the stable cone-jet mode, the patterning of the modified PEDOT:PSS solution was realized on the surface-functionalized SiO₂ substrates; the printed line widths were in the range 384 to 81 μm with a line resistance of 8.3 × 10³ Ω/mm. In addition, pentacene-based OTFTs employing the EHD-printed PEDOT:PSS as source and drain electrodes were found to exhibit electrical performances superior to an equivalent vacuum-deposited Au-based device.