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.     

Tunable Decoration of Reduced Graphene Oxide with Au Nanoparticles for the Oxygen Reduction Reaction

Reduced graphene oxide (rGO) films are decorated with non‐overlapping Au nanoparticles using diblock copolymer micelles that provide controllability over the number density as well as the diameter of the nanoparticles. This synthetic process produces a pure Au surface without extra layers. Further­more, the rGO film enables the transferability of the Au nanoparticles without deterioration of their arrays. Thus, the controllability of the Au nanoparticles and their transferability with rGO films allow the effective modification of electrochemical electrodes. With a glassy carbon electrode modified with an rGO film with Au nanoparticles, high electrochemical activity is observed in the oxygen reduction reaction (ORR). Furthermore, it is possible to identify a size‐dependent ORR mechanism, showing that Au nanoparticles with an average diameter of 8.6 nm exhibit a 4‐electron direct reduction of O₂ to H₂O.     

Ultrasmooth Gold Films via Pulsed Laser Deposition

A simple, one step technique for depositing ultrasmooth gold films using pulsed laser deposition is demonstrated by optimizing process para­meters. The smoothest film having a root‐mean‐square roughness of 0.17 nm (including the substrate roughness of 0.11 nm) for a 35 nm thick film on a silicon substrate are obtained by introducing a nitrogen flow in the chamber during deposition. We postulate that the reduction in surface roughness caused by nitrogen gas pressure in the chamber is due to the force of the gas flow acting against the flow of the plasma plume containing Au atoms. The gas acts as a filter that reduces the kinetic energy of the gold adatoms. This is the best result reported so far for a single step deposition of gold. It is a step towards low‐loss planar gold films for surface plasmon applications.     

STM Lithography in an Organic Self‐Assembled Monolayer

We describe the suitability of ultra‐high vacuum scanning tunneling microscopy (UHV‐STM) based nanolithography by using highly ordered monomolecular organic films, called self‐assembled monolayers (SAMs), as ultrathin resists. Organothiol‐type SAMs such as hexadecanethiol (SH–(CH₂)₁₅–CH₃) and N‐biphenylthiol (SH–(C₆H₆)₂–NO₂) monolayers have been prepared by immersion on gold films and Au(111) single crystals. Organosilane‐type SAMs such as octadecyltrichlorosilane (SiCl₃–(CH₂)₁₇–CH₃) monolayers have been prepared on hydroxylated Si(100) surfaces as well as hydroxylated chromium film surfaces. Dense line patterns have been written by UHV‐STM in constant current mode for various tunneling parameters (gap voltage, tunneling current, scan speed, and orientation) and transferred into the underlying substrate by wet etch techniques. The etched structures have been analyzed by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Best resolution has been achieved without etch transfer for a 20 nm × 20 nm square written in hexadecanethiol/Au(111) with an edge definition of about 5 nm. Etch transfer of the STM nanopatterns in Au films resulted in 55 nm dense line patterns (15 nm deep) mainly broadened by the isotropic etch characteristic, while 35 nm wide and 30 nm deep dense line patterns written in octadecyltrichlorosilane/Si(100) and anisotropically etched into Si(100) could be achieved.     

Micro‐/Nanostructured Highly Crystalline Organic Semiconductor Films for Surface‐Enhanced Raman Spectroscopy Applications

The utilization of inorganic semiconductors for surface‐enhanced Raman spectroscopy (SERS) has attracted enormous interest. However, despite the technological relevance of organic semiconductors for enabling inexpensive, large‐area, and flexible devices via solution processing techniques, these π‐conjugated systems have never been investigated for SERS applications. Here for the first time, a simple and versatile approach is demonstrated for the fabrication of novel SERS platforms based on micro‐/nanostructured 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) thin films via an oblique‐angle vapor deposition. The morphology of C8‐BTBT thin films is manipulated by varying the deposition angle, thus achieving highly favorable 3D vertically aligned ribbon‐like micro‐/nanostructures for a 90° deposition angle. By combining C8‐BTBT semiconductor films with a nanoscopic thin Au layer, remarkable SERS responses are achieved in terms of enhancement (≈10⁸), stability (>90 d), and reproducibility (RSD < 0.14), indicating the great promise of Au/C8‐BTBT films as SERS platforms. Our results demonstrate the first example of an organic semiconductor‐based SERS platform with excellent detection characteristics, indicating that π‐conjugated organic semiconductors have a great potential for SERS applications.     

Large‐Area Fabrication of Periodic Arrays of Nanoholes in Metal Films and Their Application in Biosensing and Plasmonic‐Enhanced Photovoltaics

Plasmonics is a fast developing research area with a great potential for practical applications. However, the implementation of plasmonic devices requires low cost methodologies for the fabrication of organized metallic nanostructures that covers a relative large area (～1 cm²). Here the patterning of periodic arrays of nanoholes (PANHs) in gold films by using a combination of interference lithography, metal deposition, and lift off is reported. The setup allows the fabrication of periodic nanostructures with hole diameters ranging from 110 to 1000 nm, for 450 and 1800 nm of periodicity, respectively. The large areas plasmonic substrates consist of 2 cm × 2 cm gold films homogeneously covered by nanoholes and gold films patterned with a regular microarray of 200 μm diameter circular patches of PANHs. The microarray format is used for surface plasmon resonance (SPR) imaging and its potential for applications in multiplex biosensing is demonstrated. The gold films homogeneously covered by nanoholes are useful as electrodes in a thin layer organic photovoltaic. This is first example of a large area plasmonic solar cell with organized nanostructures. The fabrication approach reported here is a good candidate for the industrial‐scale production of metallic substrates for plasmonic applications in photovoltaics and biosensing.     

Organic Light‐Emitting Transistors Containing a Laterally Arranged Heterojunction

An approach to produce organic light‐emitting transistors (OLETs) containing a laterally arranged heterojunction structure, which minimizes exciton quenching at the metal electrodes, is described. This device configuration provides an organic light‐emitting diode (OLED) structure where the anode (source) electrode, hole‐transport material (field‐effect material), light‐emitting material, and cathode (drain) electrode are laterally arranged, thus offering a chance to control the electroluminescent intensity by changing the gate bias. Pentacene and tris(8‐quinolinolato)aluminum (Alq₃) are employed as the field‐effect and light‐emitting materials, respectively. The laterally arranged heterojunction structures are achieved by successively inclined deposition of the field‐effect and light‐emitting materials. After deposition of pentacene, a narrow gap of about 10–20 nm between the drain electrode and pentacene was obtained, thereby creating an opportunity to fabricate a laterally arranged heterojunction. In the OLETs, unsymmetrical source and drain electrodes, that is, Au and LiF/Al ones, are used to ensure efficient injection of holes and electrons. Visible‐light emission from OLETs is observed under ambient atmosphere. This result is ascribed to efficient carrier injection and transport, formation of a heterojunction, as well as good luminescence from the organic emissive layer. The device structure serves as an excellent model system for OLETs and demonstrates a general concept of adjusting the charge‐carrier injection and transport, as well as the electroluminescent properties, by forming laterally arranged heterojunctions.     

Soft Imprinting: Creating Highly Ordered Porous Anodic Alumina Templates on Substrates for Nanofabrication

We demonstrate a “soft‐imprinting” method for the fabrication of highly ordered porous anodic alumina (HOPAA) templates on different substrates (such as Si, glass slides, and flexible polyimide films) over large areas (> 1.5 cm²). In this process, Ar plasma etching is employed to soft imprint an evaporated Al film on the substrates using a free‐standing HOPAA template as a mask, thus creating ordered nanoindentations on the Al surface. The ordered nanoindentations in turn guide the subsequent anodization of Al to generate HOPAA templates on the substrates (HOPAA–substrates), which inherit the pattern of the free‐standing HOPAA mask. This soft‐imprinting technique is also applicable to the fabrication of HOPAA on flexible polymer films. To demonstrate the potential uses of the HOPAA–substrates in nanofabrication, highly ordered Au nanowire arrays are fabricated on a Si substrate and TiO₂ nanotube arrays are prepared on a glass substrate via solution‐ and vapor‐based fabrication processes, respectively.     

Large‐Scale Highly Ordered Chitosan‐Core Au‐Shell Nanopatterns with Plasmonic Tunability: A Top‐Down Approach to Fabricate Core–Shell Nanostructures

A new strategy for fabricating highly ordered chitosan–Au core–shell nano­patterns with tunable surface plasmon resonance (SPR) properties is developed. This strategy combines fabrication of a chitosan nanopattern by using a soft‐nanoimprint technique with selective deposition of Au nanoparticles onto the patterned chitosan surface. The SPR response can be tuned by controlling the features of the resulting Au shell/polymer hybrid pattern, which makes these materials potentially useful in ultrasensitive optical sensors for molecular detection.     

Helical 3D‐Printed Metal Electrodes as Custom‐Shaped 3D Platform for Electrochemical Devices

3D‐printing represents an emerging technology that can revolutionize the way object and functional devices are fabricated. Here the use of metal 3D printing is demonstrated to fabricate bespoke electrochemical stainless steel electrodes that can be used as platform for different electrochemical applications ranging from electrochemical capacitors, oxygen evolution catalyst, and pH sensor by means of an effective and controlled deposition of IrO₂ films. The electrodes have been characterized by scanning electrode microscopy and energy dispersive X‐ray spectroscopy before the electrochemical testing. Excellent pseudocapacitive as well as catalytic properties have been achieved with these 3D printed steel‐IrO₂ electrodes in alkaline solutions. These electrodes also demonstrate Nernstian behavior as pH sensor. This work represents a breakthrough in on‐site prototyping and fabrication of highly tailored electrochemical devices with complex 3D shapes which facilitate specific functions and properties.     

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.     

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.     

High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing

We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

Cover Picture: High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing (Adv. Funct. Mater. 15/2007)

A new method for direct patterning of organic optoelectronic/electronic devices using a reconfigurable and scalable printing method is reported by Vladimir Bulovic and co‐workers on p. 2722. The printing technique is applied to the fabrication of high‐resolution printed organic light emitting devices (OLEDs) and organic field effect transistors (OFETs). Remarkably, the final print‐deposited films are evaporated onto the substrate (rather than solvent printed), giving high‐quality, solvent‐free, molecularly flat structures that match the performance of comparable high‐performance unpatterned films. We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8‐hydroxyquinoline) (Alq₃) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet‐printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop‐on‐demand” in Inkjet printing technology, MoJet printing is a “flux‐on‐demand” process and we show it capable of fabricating multi‐layer stacked film structures, as needed for engineered organic devices.     

High rate (~7 nm/s), atmospheric pressure deposition of ZnO front electrode for Cu(In,Ga)Se2 thin‐film solar cells with efficiency beyond 15%

Undoped zinc oxide (ZnO) films have been grown on a moving glass substrate by plasma‐enhanced chemical vapor deposition at atmospheric pressure. High deposition rates of ~7 nm/s are achieved at low temperature (200 °C) for a substrate speed from 20 to 60 mm/min. ZnO films are highly transparent in the visible range (90%). By a short (~minute) post‐deposition exposure to near‐ultraviolet light, a very low resistivity value of 1.6·10⁻³ Ω cm for undoped ZnO is achieved, which is independent on the film thickness in the range from 180 to 1200 nm. The photo‐enhanced conductivity is stable in time at room temperature when ZnO is coated by an Al₂O₃ barrier film, deposited by the industrially scalable spatial atomic layer deposition technique. ZnO and Al₂O₃ films have been used as front electrode and barrier, respectively, in Cu(In,Ga)Se2 (CIGS) solar cells. An average efficiency of 15.4 ± 0.2% (15 cells) is obtained that is similar to the efficiency of CIGS reference cells in which sputtered ZnO:Al is used as electrode. Copyright © 2013 John Wiley & Sons, Ltd.     

Highly Stable Graphene‐Based Multilayer Films Immobilized via Covalent Bonds and Their Applications in Organic Field‐Effect Transistors

Highly stable graphene oxide (GO)‐based multilayered ultrathin films can be covalently immobilized on solid supports through a covalent‐based method. It is demonstrated that when (3‐aminopropyl) trimethoxysilane (APTMS), which works as a covalent cross‐linking agent, and GO nanosheets are assembled in an layer‐by‐layer (LBL) manner, GO nanosheets can be covalently grafted on the solid substrate successfully to produce uniform multilayered (APTMS/GO)N films over large‐area surfaces. Compared with conventional noncovalent LBL films constructed by electrostatic interactions, those assembled using this covalent‐based method display much higher stability and reproducibility. Upon thermal annealing‐induced reduction of the covalent (APTMS/GO)N films, the obtained reduced GO (RGO) films, (APTMS/RGO)N, preserve their basic structural characteristics. It is also shown that the as‐prepared covalent (APTMS/RGO)N multilayer films can be used as highly stable source/drain electrodes in organic field‐effect transistors (OFETs). When the number of bilayers of the (APTMS/RGO)N film exceeds 2 (ca. 2.7 nm), the OFETs based on (APTMS/RGO)N electrodes display much better electrical performance than devices based on 40 nm Au electrodes. The covalent protocol proposed may open up new opportunities for the construction of graphene‐based ultrathin films with excellent stability and reproducibility, which are desired for practical applications that require withstanding of multistep post‐production processes.     

Stable Delocalized Singlet Biradical Hydrocarbon for Organic Field‐Effect Transistors

Delocalized singlet biradical hydrocarbons hold promise as new semiconducting materials for high‐performance organic devices. However, to date biradical organic molecules have attracted little attention as a material for organic electronic devices. Here, this work shows that films of a crystallized diphenyl derivative of s‐indacenodiphenalene (Ph₂‐IDPL) exhibit high ambipolar mobilities in organic field‐effect transistors (OFETs). Furthermore, OFETs fabricated using Ph₂‐IDPL single crystals show high hole mobility (μ_h = 7.2 × 10^?1 cm² V^?1 s^?1) comparable to that of amorphous Si. Additionally, high on/off ratios are achieved for Ph₂‐IDPL by inserting self‐assembled mono­layer of alkanethiol between the semiconducting layer and the Au electrodes. These findings open a door to the application of ambipolar OFETs to organic electronics such as complementary metal oxide semiconductor logic circuits.     

Trilayer Nanomesh Films with Tunable Wettability as Highly Transparent,Flexible, and Recyclable Electrodes

Metallic mesh materials are promising candidates to replace traditional transparent conductive oxides such as indium tin oxide (ITO) that is restricted by the limited indium resource and its brittle nature. The challenge of metal based transparent conductive networks is to achieve high transmittance, low sheet resistance, and small perforation size simultaneously, all of which significantly relate to device performances in optoelectronics. In this work, trilayer dielectric/metal/dielectric (D/M/D) nanomesh electrodes are reported with precisely controlled perforation size, wire width, and uniform hole distribution employing the nanosphere lithography technique. TiO₂/Au/TiO₂ nanomesh films with small hole diameter (≤700 nm) and low thickness (≤50 nm) are shown to yield high transmittance (>90%), low sheet resistance (≤70 Ω sq^?1), as well as outstanding flexural endurance and feasibility for large area patterning. Further, by tuning the surface wettability, these films are applied as easily recyclable flexible electrodes for electrochromic devices. The simple and cost‐effective fabrication of diverse D/M/D nanomesh transparent conductive films with tunable optoelectronic properties paves a way for the design and realization of specialized transparent electrodes in optoelectronics.     

Top‐Emission Organic Light‐Emitting Diode with a Novel Copper/Graphene Composite Anode

The relatively high sheet resistance of graphene compared with indium tin oxide (ITO) blocks the applications of graphene as transparent electrodes in organic light‐emitting diodes. A novel copper (Cu)/graphene composite electrode is presented and employed as the anode of a top‐emission organic light‐emitting diode with the structure of Cu/graphene/V₂O₅/NPB/Alq₃/Alq₃: C545T/Bphen: Cs₂CO₃/Sm/Au. The Cu/graphene composite electrodes are fabricated by growing graphene directly on Cu substrates via the chemical vapor deposition method without any transfer process. The maxima of current efficiency and power efficiency of a typical Cu/graphene composite anode device reach 6.1 cd/A and 7.6 lm/W, respectively, which are markedly higher than those of the control devices with a graphene anode, a Cu anode or an ITO anode. The low sheet resistance of the composite electrode, the high quality of graphene without any transfer process and the avoidance of wave guiding loss in glass or polyethylene terephthalate substrates result in the improvements of light emission efficiencies.     

Surface Control of Planarization Layer on Embossed Glass for Light Extraction in OLEDs

We developed a highly refractive index planarization layer showing a very smooth surface for organic light‐emitting diode (OLED) light extraction, and we successfully prepared a highly efficient white OLED device with an embossed nano‐structure and highly refractive index planarization layers. White OLEDs act as an internal out‐coupling layer. We used a spin‐coating method and two types of TiO₂ solutions for a planarization of the embossed nano‐structure on a glass substrate. The first TiO₂ solution was TiO₂ sol, which consists of TiO₂ colloidal particles in an acidic aqueous solution and several organic additives. The second solution was an organic and inorganic hybrid solution of TiO₂. The surface roughness (R_a) and refractive index of the TiO₂ planarization films on a flat glass were 0.4 nm and 2.0 at 550 nm, respectively. The J–V characteristics of the OLED including the embossed nano‐structure and the TiO₂ planarization film were almost the same as those of an OLED with a flat glass, and the luminous efficacy of the aforementioned OLED was enhanced by 34% compared to that of an OLED with a flat glass.