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.     

ITO-free organic solar cells using highly conductive phenol-treated PEDOT:PSS anodes

Phenol as one of the most polar solvent was used to enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films. The conductivity of PEDOT:PSS films improved to 1193 S/cm after treatment with phenol vapor and 1054 S/cm after treatment with phenol drop. The treated films also showed high transmittance in the visible region which is one of the crucial factors for optoelectronic devices such as organic solar cells and light emitting diodes. The mechanism of conductivity enhancement of treated thin PEDOT:PSS films was investigated by atomic force microscopy (AFM) and UV/Vis spectrophotometer. The AFM images showed that the ratio of PEDOT to PSS at top most of the surface was increased for treated film. Rearrangement of PEDOT segment throughout the film and hence conformational changes are the reasons for enhancement of conductivity. The modified PEDOT:PSS films were used as electrode for ITO-free organic solar cells (OSCs). These ITO-free OSCs showed almost equal operation to those for ITO electrodes.     

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.     

PEDOT:PSS:sulfonium salt composite hole injection layers for efficient organic light emitting diodes

In this work, we propose a simple and effective approach to modify the optoelectronic properties of the commonly used poly(3,4-ethylenedioxylthiophene):poly(styrene sulfonate) (PEDOT:PSS) and consequently to improve hole injection and transport in organic light emitting diodes (OLEDs) using emissive layers based on a fluorescence copolymer. In particular, two triphenylsulfonium (TPS) salts that consist of the same TPS cation and two different counter anions, in particular, hexafluoroantimonate (SbF₆) and trifluoromethane sulfonate (Triflate) were added in the PEDOT:PSS solution in various concentrations and the composite films were fully characterized for surface and optoelectronic properties and subsequently we employed as hole injection layers (HILs) in OLEDs. It is demonstrated that both, the counter anion and the concentration of TPS-salts in the PEDOT:PSS matrix play significant role in the optoelectronic properties of the composite and thus in the device performance. Although all TPS-salt modified PEDOT:PSS films exhibited higher work function (W_F) values relative to the undoped one thus resulting in more efficient hole injection than pristine PEDOT:PSS, the PEDOT:PSS:TPS-Triflate with the lower concentration (10:1 v/v) showed the highest luminous (LE) and power efficiency (PE) values of 27.04 cd A⁻¹ and 6.26 lm W⁻¹, respectively. This extraordinary performance was ascribed to a significant increase in the conductivity of the composite film combined with the formation of an interface exciplex between the TPS-Triflate (acceptor) and the emissive copolymer (donor). This interfacial electroplex strongly confines the generated excitons and prevents their diffusion towards aluminum cathode which acts as exciton quencher.     

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.     

Stable organic photovoltaic with PEDOT:PSS and MoOX mixture anode interfacial layer without encapsulation

Herein, we report about an efficient and stable organic photovoltaic that uses a poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) and molybdenum oxide (MoO_X) mixture for the anode interfacial layer, and that can reach 4.43% power conversion efficiency (PCE) under AM1.5 conditions. Utilizing PEDOT:PSS:MoO_X (1:1), the shelf lifetime of poly(3-hexylthiophene) (P3HT), and indene-C₆₀ bisadduct (ICBA)-based solar cells without encapsulation, can be realized with only a 25% deterioration after 672 h of storage in air. Furthermore, we compare the photovoltaic performance of the P3HT:ICBA-based organic photovoltaic with PEDOT:PSS, and PEDOT:PSS:MoO_X, in which PEDOT:PSS:MoO_X has outperformed the others. In addition, the water vapor transmission rate of PEDOT:PSS:MoO_X is 0.17 gm/(m² day), which is much less than that of PEDOT:PSS.     

The effect of solvent treatment on the buried PEDOT:PSS layer

Solvent treatment has been widely used to improve the device performance of both Organic Light Emitting Diodes (OLEDs) and Polymer Solar Cells (PSCs). One of the proposed mechanisms is the modification of the buried PEDOT:PSS layer underneath the organic active layer by the permeating solvent. By measuring the lateral electric conductivity of the PEDOT:PSS layer, the 3 orders of magnitude's enhancement on the conductivity after solvent treatment confirms that the solvent permeates through the top organic active layer and modifies the PEDOT:PSS layer. Using a “peel-off” method, the buried PEDOT:PSS layer is fully exposed and studied by UV–vis spectra, XPS spectra, and c-AFM images. The data suggest that the permeating solvent dissolves PSS, changes PEDOT:PSS′ core-shell structure into a linear/coiled structure, and moves PSS from the bulk to the surface. As a result, PEDOT becomes more continuous in the bulk. The continuous conducting PEDOT-rich domains create percolating pathways for the current which significantly improve electric conductivity.     

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.     

The application of single-layer graphene modified with solution-processed TiOx and PEDOT:PSS as a transparent conductive anode in organic light-emitting diodes

There are many challenges for a direct application of graphene as the electrodes in organic electronics due to its hydrophobic surfaces, low work function (WF) and poor conductance. The authors demonstrate a modified single-layer graphene (SLG) as the anode in organic light-emitting diodes (OLEDs). The SLG, doped with the solution-processed titanium suboxide (TiO_x) and poly(3,4-ethylenedio-xythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS), exhibits excellent optoelectronic characteristics with reduced sheet resistance (R_sq), increased work function, as well as over 92% transmittance in the visible region. It is notable that the R_sq of graphene decreased by ∼86% from 628 Ω/sq to 86 Ω/sq and the WF of graphene increased about 0.82 eV from 4.30 eV to 5.12 eV after a modification by using the TiO_x–PEDOT:PSS double interlayers. In addition, the existence of additional TiO_x and PEDOT:PSS layers offers a good coverage to the PMMA residuals on SLG, which are often introduced during graphene transfer processes. As a result, the electrical shorting due to the PMMA residues in the device can be effectively suppressed. By using the modified SLG as a bottom anode in OLEDs, the device exhibited comparable current efficiency and power efficiency to those of the ITO based reference OLEDs. The approach demonstrated in this work could potentially provide a viable way to fabricate highly efficient and flexible OLEDs based on graphene anode.     

A facile micropatterning method for a highly flexible PEDOT:PSS on SU-8

We report the micropatterning of conducting polymer on the epoxy-based photoresist to demonstrate fully organic, conducting and flexible electrodes. We show that polystyrene sulfonic acid can be covalently linked to the surface of the photoresist (SU-8) by forming sulfonyl ester at the interfaces. We also present an application of the patterned PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate)/SU-8 to the electroplating of metal electrodes.     

Flexible rewritable organic memory devices using nitrogen-doped CNTs/PEDOT:PSS composites

Nonvolatile rewritable organic memory devices based on poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and nitrogen doped multi-walled carbon nanotube (NCNT) nanocomposites were fabricated on glass and PET substrates.Organic memory devices with bistable resistive switching were obtained using very low NCTN concentration (∼0.002 wt%) in the polymeric matrix. The memory devices exhibited a good ON/OFF ratio of approximately three orders of magnitude, a good retention time of 10⁴ s under operating voltages ≤ |4V| and a few hundredths of write-read-erase-read cycles. The bistable resistive switching is mainly attributed to the creation of oxygen vacancies. These defects are introduced into the thin native Al oxide (AlOx) layer on the bottom electrode during the first voltage sweep. The well-dispersed NCNTs immersed in PEDOT:PSS play a key role as conductive channels for the electronic transport, hindering the electron trapping at the AlOx-polymer interface and inducing a soft dielectric breakdown of the AlOx layer. These PEDOT:PSS + NCNTs memory devices are to easy to apply in flexible low-cost technology and provide the possibility of large-scale integration.     

Improved efficiency of indium-tin-oxide-free organic light-emitting devices using PEDOT:PSS/graphene oxide composite anode

We have demonstrated an indium-tin-oxide free organic light-emitting device (OLED) with improved efficiency by doping poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) with graphene oxide (GO) as a composite anode. In comparison with a pure PEDOT:PSS anode, 55% enhancement in efficiency has been obtained for the OLEDs based on the PEDOT:PSS/GO composite anode at an optimal condition. The PEDOT:PSS/GO composite anode shows a lower hole-injection barrier, which contributes to the improved device efficiency. Moreover, both high transmittance and good surface morphology similar to that of the pure PEDOT:PSS film also contribute to the enhanced efficiency. It is obvious that composite anode will generally be applicable in organic optoelectronic devices which require smooth and transparent anode.     

Aging process of PEDOT:PSS dispersion and robust recovery of aged PEDOT:PSS as a hole transport layer for organic solar cells

Conducting p-type polymer of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been widely used for organic optoelectronics, particularly as a hole transport layer for organic solar cells. While the aged PEDOT:PSS dispersion impacts device performance, the aging of PEDOT:PSS dispersion have not been well investigated. Moreover, the recovery process of aged (two-year-old) PEDOT:PSS dispersion has not been demonstrated yet. Herein, it is found that aqueous PEDOT:PSS dispersion undergoes extensive phase separation during the aging process, resulting in both nanoscale and macroscale hydrophobic PEDOT-rich agglomerates. When the aged PEDOT:PSS thin film is integrated into P3HT:PCBM organic solar cells, the PEDOT-rich agglomerates trap the photogenerated holes at the PEDOT:PSS/P3HT interface, resulting in poor extraction efficiency in organic solar cells. To recover a hole transport functionality from aged PEDOT:PSS, three different solvents such as isopropyl alcohol (C₃H₇OH), ethanol (C₂H₅OH) and methanol (CH₃OH) are investigated. Among them, it is found that isopropyl alcohol (IPA) yielded very uniform PEDOT:PSS thin film layer. This is because hydrophobic functional groups of IPA solvent facilitated the preferential solvation of phase separated hydrophobic PEDOT-rich agglomerates. However, when non-optimal concentration of IPA solvents was added into the aged PEDOT:PSS dispersion, the size of PEDOT-rich agglomerates was adversely enlarged. When organic solar cells were fabricated using more than a two-year-old PEDOT:PSS that was treated with IPA solvent, the resulting device performance of organic solar cells was fully recovered and became comparable or better than that of organic solar cells fabricated with fresh PEDOT:PSS.     

Highly conductive PEDOT:PSS films prepared through a treatment with geminal diols or amphiphilic fluoro compounds

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films with high conductivity can have important application as the transparent electrode of optoelectronic devices. In this paper, we report the significant conductivity enhancement of PEDOT:PSS through a treatment with germinal diols which have two hydroxyl groups connected to one carbon atom or amphiphilic fluoro compounds which have hydrophobic fluorocarbon groups and hydrophilic hydroxyl or carboxylic groups. Several compounds, including hexafluoroacetone, cyclohexanehexone, formaldehyde, acetaldehyde, and perfluorobenzophenone, which could convert into geminal diols, were used to treat PEDOT:PSS films. The conductivity enhancements are generally consistent with the equilibrium constants for the conversion of these compounds into geminal diols. PEDOT:PSS films were also treated with several amphiphilic fluoro compounds. The conductivity was significantly enhanced when PEDOT:PSS films were treated with hexafluoroisopropanol, trifluoroacetic acid and heptafluorobutyric acid, while it hardly changed when they were treated with 2,2,2-trifluoroethanol. Conductivities of more than 1000 S cm⁻¹ were observed on the treated PEDOT:PSS films. The mechanism for the conductivity enhancement of PEDOT:PSS through the treatment with geminal diols or amphiphilic fluoro compounds is attributed to the phase segregation of PSSH from PEDOT:PSS and conformational change of the PEDOT chains as the results of the compounds-induced reduction in the Coulombic attraction between the positively charged PEDOT and negatively charged PSS chains.     

Tuning PEDOT:PSS conductivity with iron oxidants

Controlling the electrical properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is crucial for its use in a wide range of energy and sensing applications. We have polymerized PEDOT:PSS using a new iron oxidant, hemin, and compared the resulting polymer to PEDOT:PSS polymerized with the iron oxidant, FeCl₃. We characterize these polymers with five different techniques: visible and near IR spectroscopy, Fourier transform infrared spectroscopy, electron spin resonance spectroscopy, four point probe conductivity measurements, and X-ray photoelectron spectroscopy. Although the elemental composition of both polymers is nearly identical, hemin-oxidized PEDOT:PSS is six orders of magnitude more conductive than FeCl₃-oxidized PEDOT:PSS. This difference is associated with a change in oxidation state of the polymer. In hemin-oxidized PEDOT:PSS, bipolarons are the dominant charge carrier species. In FeCl₃-oxidized PEDOT:PSS, polarons dominate. These results demonstrate that the properties of PEDOT:PSS can be controlled in a single step aqueous reaction by the choice of iron oxidant used for polymerization.     

The enhancement of electrical and optical properties of PEDOT:PSS using one-step dynamic etching for flexible application

The conductivity enhancement of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by dynamic etching process was investigated to introduce the outstanding and simplest method for soft electronics. Four different samples which were pristine PEDOT:PSS, PEDOT:PSS doped with 5 wt.% DMSO, PEDOT:PSS with dipping process, and PEDOT:PSS with dynamic etching process were prepared to compare the properties such as conductivity, morphology, relative atomic percentage, and topography. All samples were characterized by four point probe, current atomic force microscopy (C-AFM), X-ray photoelectron spectroscopy (XPS), and UV–visible spectroscopy. The conductivity of the sample with dynamic etching process showed the highest value as 1299 S/cm among four samples. We proved that the dynamic etching process is superior to remove PSS phase from PEDOT:PSS film, to flow strong current through entire surface of PEDOT:PSS, and to show the smoothest surface (RMS 2.28 nm). XPS analysis was conducted for accurate chemical and structural surface environments of four samples and the relative atomic percentage of PEDOT in the sample with dynamic etching was the highest as 29.5%. The device performance of the sample with the dynamic etching process was outstanding as 10.31 mA/cm² of J_sc, 0.75 eV of V_oc, 0.46 of FF, and 3.53% of PCE. All properties and the device performance for PEDOT:PSS film by dynamic etching process were the most excellent among the samples.     

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.     

Improvement of electrical conductivity of PEDOT:PSS films by 2-Methylimidazole post treatment

The electrical conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films was significantly improved without losing the optical transparency by treating the films with solution of 2-Methylimidazole in ethanol. The maximum electrical conductivity of such a thin film reached 930 S cm⁻¹, more than 1150 order of magnitude higher than that of pure PEDOT:PSS film. The mechanism of conductivity enhancement of treated thin PEDOT:PSS films was explored by atomic force microscopy (AFM) and UV/VIS spectrophotometer. The AFM scans show that the surface of the 2-Methylimidazole treated PEDOT:PSS layer is smoother than that of the pristine PEDOT:PSS thin film. Improvement in the morphology, electrical and optical properties of PEDOT:PSS films makes them highly suitable for numerous applications in optoelectronic devices.     

Metal salt modified PEDOT:PSS as anode buffer layer and its effect on power conversion efficiency of organic solar cells

The effects of metal chlorides such as LiCl, NaCl, CdCl₂ and CuCl₂ on optical transmittance, electrical conductivity as well as morphology of PEDOT:PSS films have been investigated. Transmittance spectra of spun PEDOT:PSS layers were improved by more than 6% to a maximum of 94% in LiCl doped PEDOT:PSS film. The surface of the PEDOT:PSS films has exhibited higher roughness associated with an increase in the electrical conductivity after doping with metal salts. The improvement in the physical properties of PEDOT:PSS as the hole transport layer proved to be key factors towards enhancing the P3HT:PCBM bulk heterojunction (BHJ) solar cells. These improvements include significantly improved power conversion efficiency with values as high as 6.82% associated with high fill factor (61%) and larger short circuit current density (∼18 mA cm⁻²).