Reduced Graphene Oxide Micromesh Electrodes for Large Area,Flexible, Organic Photovoltaic Devices

A laser‐based patterning technique—compatible with flexible, temperature‐sensitive substrates—for the production of large area reduced graphene oxide micromesh (rGOMM) electrodes is presented. The mesh patterning can be accurately controlled in order to significantly enhance the electrode transparency, with a subsequent slight increase in the sheet resistance, and therefore improve the tradeoff between transparency and conductivity of reduced graphene oxide (rGO) layers. In particular, rGO films with an initial transparency of ≈20% are patterned, resulting in rGOMMs films with a ≈59% transmittance and a sheet resistance of ≈565 Ω sq^?1, that is significantly lower than the resistance of ≈780 Ω sq^?1, exhibited by the pristine rGO films at the same transparency. As a proof‐of‐concept application, rGOMMs are used as the transparent electrodes in flexible organic photovoltaic (OPV) devices, achieving power conversion efficiency of 3.05%, the highest ever reported for flexible OPV devices incorporating solution‐processed graphene‐based electrodes. The controllable and highly reproducible laser‐induced patterning of rGO hold enormous promise for both rigid and flexible large‐scale organic electronic devices, eliminating the lag between graphene‐based and indium–tin oxide electrodes, while providing conductivity and transparency tunability for next generation flexible electronics.     

Flexible and Transparent Nanocomposite of Reduced Graphene Oxide and P(VDF‐TrFE) Copolymer for High Thermal Responsivity in a Field‐Effect Transistor

A new class of temperature‐sensing materials is demonstrated along with their integration into transparent and flexible field‐effect transistor (FET) temperature sensors with high thermal responsivity, stability, and reproducibility. The novelty of this particular type of temperature sensor is the incorporation of an R‐GO/P(VDF‐TrFE) nanocomposite channel as a sensing layer that is highly responsive to temperature, and is optically transparent and mechanically flexible. Furthermore, the nanocomposite sensing layer is easily coated onto flexible substrates for the fabrication of transparent and flexible FETs using a simple spin‐coating method. The transparent and flexible nanocomposite FETs are capable of detecting an extremely small temperature change as small as 0.1 °C and are highly responsive to human body temperature. Temperature responsivity and optical transmittance of transparent nanocomposite FETs were adjustable and tuneable by changing the thickness and R‐GO concentration of the nanocomposite.     

π‐Conjugated Molecules Crosslinked Graphene‐Based Ultrathin Films and Their Tunable Performances in Organic Nanoelectronics

Graphene‐based ultrathin films with tunable performances, controlled thickness, and high stability are crucial for their uses. The currently existing protocols, however, could hardly simultaneously meet these requirements. Using amino‐substituted π‐conjugated compounds, including 1,4‐diaminobenzene (DABNH2), benzidine (BZDNH2), and 5,10,15,20‐tetrakis (4‐aminophenyl)‐21H,23H‐porphine (TPPNH2), as cross‐linkages, a new protocol through which graphene oxide (GO) nanosheets can be anchored on solid supports with a high stability and controlled thickness via a layer‐by‐layer method is presented. A thermal annealing leads to the reduction of the films, and the qualities of the samples can be inherited by the as‐produced reduced GO films (RGO). When RGO films are integrated as source/drain electrodes in OFETs, tunable performances can be realized. The devices based on the BZDNH2‐crosslinked RGO electrodes exhibit similar electrical behaviors as those based on the non‐π‐conjugated compound crosslinked electrodes, while improved performances can be gained when those crosslinked by DABNH2 are used. The performances can be further improved when RGO films crosslinked by TPPNH2 are employed. This work likely paves a new avenue for graphene‐based films of tunable performances, controlled thickness, and high stability.     

Reduced graphene oxide prepared at low temperature thermal treatment as transparent conductors for organic electronic applications

In this work, we obtain transparent conducting thin films of both chemically and thermally reduced graphene oxide. High-quality films are normally obtained with thermal treatments at temperatures about 1000 °C, while the highest temperatures employed during the thermal treatment in this work were as low as 400 °C, which is a mandatory condition when dealing with organic electronic devices on glass substrates. To reach such a low thermal treatment, a two-step oxidation process was employed in order to allow the formation of carbonyl chemical groups rather than epoxy functionalization. Each GO sample was structurally and chemically analysed by Infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), Ultraviolet–visible absorption spectroscopy (UV-VIS), Thermogravimetric analysis (TGA), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The GO conducting thin films exhibited a sheet resistance of 3.2 × 10³ Ω/sq as well as a high transmittance: up to 80% at 550 nm. Furthermore, Raman spectroscopy, X-ray diffraction and AFM show that the thermally reduced thin films are mainly composed of single and bilayer GO sheets with a very low average roughness. Also, these GO thin films, with such surprising quality, have been employed as non-doped and metal free electrodes in organic light emitting diodes.     

Highly Conductive and Environmentally Stable Organic Transparent Electrodes Laminated with Graphene

Improving the lifetime and the operational and thermal stability of organic thin‐film materials while maintaining high conductivity and mechanical flexibility is critical for flexible electronics applications. Here, it is reported that highly conductive and environmentally stable organic transparent electrodes (TEs) can be fabricated by mechanically laminating poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) films containing dimethylsulfoxide and Zonyl fluorosurfactant (PDZ films) with a monolayer graphene barrier. The proposed lamination process allows graphene to be coated onto the PDZ films uniformly and conformally with tight interfacial binding, free of wrinkles and air gaps. The laminated films exhibit an outstanding room‐temperature hole mobility of ≈85.1 cm² V^?1 s^?1 since the graphene can serve as an effective bypass for charge carriers. The significantly improved stability of the graphene‐laminated TEs against high mechanical/thermal stress, humidity, and ultraviolet irradiation is particularly promising. Furthermore, the incorporation of the graphene barrier increases the expected lifetime of the TEs by more than two orders of magnitude.     

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.     

Infrared Detection Using Transparent and Flexible Field‐Effect Transistor Array with Solution Processable Nanocomposite Channel of Reduced Graphene Oxide and P(VDF‐TrFE)

Photodetectors using optically responsive graphene (Gr) or reduced graphene oxide (R‐GO) on rigid substrates have showed promising results for detection of broad band light including infrared (IR). However, there have been only a few reports on Gr or R‐GO photodetectors with new functionalities such as optical transparency and/or flexibility. Herein, a new kind of transparent and flexible IR photodetector is presented using a field‐effect transistor (FET) structure in which an IR‐responsive nanocomposite layer of R‐GO and poly(vinylidenefluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)) is employed as the channel. The IR photodetector exhibits high IR responsivity, stability, and reproducibility under mechanical strain and ambient conditions. In addition, the capability of measuring the distribution of responses from each device in the transparent and flexible nanocomposite FET array under IR radiation from the human body is also demonstrated. Therefore, the development of a flexible IR photodetector with high responsivity, transparency, ease of integration, and stability in an ambient environment is a suitable alternative approach for achieving the stable monitoring of IR in many flexible and transparent electronic systems.     

Novel Film‐Casting Method for High‐Performance Flexible Polymer Electrodes

A new film‐casting method for polymer electrodes is reported, in which thickness‐controlled drop‐casting (TCDC), using polyaniline doped with camphorsulfonic acid (PANI:CSA) is used. By combining the advantages of conventional spin‐casting and drop‐casting methods, and by rigorously controlling the film formation parameters, flexible polymer electrodes with high conductivity and excellent transmittance can be produced. The PANI:CSA electrodes cast by the TCDC method exhibited constant thickness‐independent conductivities of ～600 S cm^?1 down to a film thickness of 0.2 μm, and a high optical transmittance of about 85% at 550 nm. Furthermore, the new casting method significantly reduced the sheet resistance (～90 Ω/square) of the PANI:CSA electrodes compared with the conventional spin‐cast films, enhancing the performance of the devices deposited on plastic substrates. The flexible polymer light‐emitting diode produced a brightness of 6000 cd m^?2, and the flexible polymer solar cell exhibited a power conversion efficiency of 2%, both of which were much higher than those of the devices fabricated by the conventional spin‐casting method.     

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

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

Reversible Switching Phenomenon in Diarylethene Molecular Devices with Reduced Graphene Oxide Electrodes on Flexible Substrates

Photoswitching molecular electronic devices with reduced graphene oxide (rGO) top electrodes on flexible substrates are fabricated and characterized. It has been reported previously that diarylethene molecular devices with poly‐(3,4‐ethylenedioxythiophene) stabilized with poly‐(4‐styrenesulfonic acid)/Au top electrodes can hold two stable electrical conductance states when the devices are exposed to UV or visible light during device fabrication. However, those devices fail to show the reversible switching phenomenon in response to illumination after device fabrication. By employing conducting and transparent rGO top electrodes, it is demonstrated that the diarylethene molecular devices show a reversible switching phenomenon, i.e., the fabricated devices change their conductance state in response to the alternating illumination with UV and visible light. Furthermore, the molecular devices with rGO top electrodes also exhibit good longtime stability and reliable electrical characteristics when subjected to various mechanical stresses (bending radius down to 5 mm and bending cycle over 10⁴).     

ITO‐Free and Flexible Organic Photovoltaic Device Based on High Transparent and Conductive Polyaniline/Carbon Nanotube Thin Films

The synthesis and characterization of thin films of polyaniline/carbon nanotubes nanocomposites is reported, as well as their utilization as transparent electrodes in ITO‐free organic photovoltaic devices. These films are generated by interfacial synthesis, which provides them with the unique ability to be deposited onto any substrate as transparent films, thus enabling the production of flexible solar cells using substrates like PET. Very high carbon nanotube loadings can be achieved using these films without significantly affecting their transparency (≈80–90% transmittance at 550 nm). Sheet resistances as low as 300 Ω/□ are obtained using secondary polyaniline doping in the presence of carbon nanotubes. These films present excellent mechanical stability, exhibiting no lack in performance after 100 bend cycles. Flexible and completely ITO‐free organic photovoltaic devices are built using these films as transparent electrodes, and high efficiencies (up to 2.27%) are achieved.     

Maintaining Cytocompatibility of Biopolymers Through a Graphene Layer for Electrical Stimulation of Nerve Cells

Here, the utility of large‐area graphene as a flexible, biocompatible electrode to stimulate cell growth is demonstrated. Chemical vapor deposition allows the production of highly crystalline, single, double, or few‐layered graphene on copper substrates. The subsequent transfer to a biopolymer support, such as polylactic acid (PLA) or polylactic‐co‐glycolic acid (PLGA) copolymers, provides a unique electrode structure retaining the flexibility and surface properties of the underlying materials with a conductive graphene layer sufficient to enable electrical communication with excitable cells. The growth and compatibility of PC‐12 cells on these graphene‐biopolymer (GPB) electrodes is influenced more by the underlying polymer than the presence of graphene, demonstrating that the characteristics influencing biocompatibility have been retained after graphene modification. Differentiation of these cells into neural phenotypes is enhanced using electrical stimulation through the graphene conductive layer, confirming that the conductivity of graphene is sufficient to electrically communicate with cells grown on the surface. The process described herein demonstrates that non‐conducting, flexible biopolymer surfaces can be easily coated with graphene without changing the biocompatibility of the materials. This could be used to create electrodes from non‐conducting materials with optimized cell compatibility with graphene providing electrical properties suitable for stimulation of cells without greatly changing the surface properties.     

Bioinspired Graphene Actuators Prepared by Unilateral UV Irradiation of Graphene Oxide Papers

Inspired by natural autonomous systems that demonstrate controllable shape, appearance, and actuation under external stimuli, a facile preparation of moisture responsive graphene‐based smart actuators by unilateral UV irradiation of graphene oxide (GO) papers is reported. UV irradiation of GO is found to be an effective protocol to trigger the reduction of GO; however, due to the limited light transmittance and thermal relaxation, thick GO paper cannot be fully reduced. Consequently, by tuning the photoreduction gradient, anisotropic GO/reduced GO (RGO) bilayer structure can be easily prepared toward actuation application. To get better control over the responsive properties, GO/RGO bilayer paper with a certain curvature and RGO patterns are successfully prepared for actuator design. As representative examples, smart humidity‐driven graphene actuators that mimic the cilia of respiratory tract and tendril climber plant are successfully developed for controllable objects transport.     

Composite structure of ZnO films coated with reduced graphene oxide: structural,electrical and electrochemical properties

ZnO films coated with reduced graphene oxide (RGO-ZnO) were prepared by a simple chemical approach.The graphene oxide (GO) films transferred onto ZnO films by spin coating were reduced to RGO films by two steps (exposed to hydrazine vapor for 12 h and annealed at 600 ℃).The crystal structures,electrical and photoluminescence properties of RGO-ZnO films on quartz substrates were systematically studied.The SEM images illustrated that RGO layers have successfully been coated on the ZnO films very tightly.The PL properties of RGO-ZnO were studied.PL spectra show two sharp peaks at 390 nm and a broad visible emission around 490 nm.The resistivity of RGO-ZnO films was measured by a Hall measurement system,RGO as nanofiller considerably decrease the resistivity of ZnO films.An electrode was fabricated,using RGO-ZnO films deposited on Si substrate as active materials,for super capacitor application.By comparison of different results,we conclude that the RGO-ZnO composite material couples possess the properties of super capacitor.     

Engineering of Graphene Layer Orientation to Attain High Rate Capability and Anisotropic Properties in Li‐Ion Battery Electrodes

Novel carbon films with different graphene layer orientations are investigated as electrode materials for Li‐ion batteries. It is demonstrated that engineering the crystallographic orientation with graphene layers oriented perpendicular to the surface substantially alters stress evolution during Li insertion. With this crystallographic orientation the intercalating/de‐intercalating Li‐ions also have direct access to the graphene interlayer spaces, resulting in higher capacity at faster electrochemical cycling, compared to carbon films with graphene layers parallel to the film surface. Electrodes with perpendicular alignment are prepared by supramolecular synthesis using either spin coating or bar coating of chromonic liquid crystal precursors into precursor organic films followed by in situ carbonization. These materials are compared with in situ stress measurements during lithiation/delithiation cycles, and the bar‐coated films exhibit a highly anisotropic stress which is consistent with long‐range alignment of the graphene layers. In contrast, the in‐plane stresses in the spin‐coated films are isotropic, which is consistent with the presence of randomly oriented domains (still with graphene layers oriented perpendicular to the surface). Overall, the use of thin film graphitic materials with controlled crystallographic orientations provides a valuable platform for investigating the impact of graphene structure on the properties of Li‐ion battery electrode materials.     

Efficient flexible organic photovoltaics using silver nanowires and polymer based transparent electrodes

Planarization and filling voids between wires are key issues when using nanowire electrodes in flexible solar cells such as organic photovoltaics (OPV). For this purpose, we use poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) which leads to an electrically well connected silver nanowire (AgNW) network. Furthermore, the use of water based PEDOT: PSS leads to humidity assisted AgNW fusing, resulting in a maximum processing temperature of only 120 °C. OPV cells using this AgNW/PEDOT: PSS transparent electrodes exhibit power conversion efficiencies up to 7.15%. Moreover, OPV devices on PET substrates with an alumina encapsulation and barrier adhesive show excellent mechanical flexibility.     

Self‐Propagating Domino‐like Reactions in Oxidized Graphite

Graphite oxide (GO) has received extensive interest as a precursor for the bulk production of graphene‐based materials. Here, the highly energetic nature of GO, noted from the self‐propagating thermal deoxygenating reaction observed in solid state, is explored. Although the resulting graphene product is quite stable against combustion even in a natural gas flame, its thermal stability is significantly reduced when contaminated with potassium salt by‐products left from GO synthesis. In particular, the contaminated GO becomes highly flammable. A gentle touch with a hot soldering iron can trigger violent, catastrophic, total combustion of such GO films, which poses a serious fire hazard. This highlights the need for efficient sample purification methods. Typically, purification of GO is hindered by its tendency to gelate as the pH value increases during rinsing. A two‐step, acid–acetone washing procedure is found to be effective for suppressing gelation and thus facilitating purification. Salt‐induced flammability is alarming for the fire safety of large‐scale manufacturing, processing, and storage of GO materials. However, the energy released from the deoxygenation of GO can also be harnessed to drive new reactions for creating graphene‐based hybrid materials. Through such domino‐like reactions, graphene sheets decorated with metal and metal oxide particles are synthesized using GO as the in situ power source. Enhanced electrochemical capacitance is observed for graphene sheets loaded with RuO₂ nanoparticles.     

皮秒激光直写还原石墨烯氧化物薄膜的研究

利用皮秒激光直写还原绝缘石墨烯氧化物(GO) 薄膜,成功制备了图案化的导电石墨烯。先 通过旋涂法制备GO薄膜,再使用皮秒激光进行直写扫描,可以同步实现GO的还原和图案化 两个关键步 骤。光学显微镜成像显示,还原前后GO颜色发生明显变化,图案结构清晰,分辨率较高。结 合拉曼光谱和 X光电子能谱(XPS)进行表征分析的结果表明,激光直写区域石墨层缺陷程度和 含氧量均明显降低,GO还 原程度较高。图案化还原后GO(RP-GO)薄膜电学测试的结果表明,可以通过 改变皮秒激光 的输出功率对RP-GO的导电性能进行调控。本文技术一次性解决了石墨烯的大规模制备、图 案化成形和电学 性能调控三大难题,开辟了石墨烯基微电子器件生产制造的新道路。     

Flash‐Assisted Processing of Highly Conductive Zinc Oxide Electrodes from Water

Fabricating high‐quality transparent conductors using inexpensive and industrially viable techniques is a major challenge toward developing low cost optoelectronic devices such as solar cells, light emitting diodes, and touch panel displays. In this work, highly transparent and conductive ZnO thin films are prepared from a low‐temperature, aqueous deposition method through the careful control of the reaction chemistry. A robotic synthetic platform is used to explore the wide parameter space of a chemical bath system that uses only cheap and earth abundant chemicals for thin film deposition. As‐deposited films are found to be highly resistive, however, through exposure to several millisecond pulses of high‐intensity, broadband light, intrinsically doped ZnO films with sheet resistances as low as 40 Ω □^?1 can be readily prepared. Such values are comparable with state‐of‐the‐art‐doped transparent conducting oxides. The mild processing conditions (<150 °C) of the ZnO electrodes also enable their deposition on temperature sensitive substrates such as PET, paving the way for their use in various flexible optoelectronic devices. Proof‐of‐concept light emitting devices employing ZnO as a transparent electrode are presented.     

Enhanced gas barrier properties of graphene-TiO2 nanocomposites on plastic substrates assisted by UV photoreduction of graphene oxide

Reduced graphene oxide (rGO) sheets have received great attention as a key element for thin barrier films that block the permeation of water vapor and other gases. However, it remains a challenge to prepare the rGO-based barrier films on plastic substrates through a chemically benign and low temperature fabrication route. Toxic chemicals or high temperature thermal treatments that are widely used for preparing rGO need to be avoided because they can damage the underlying plastic substrates. In this study, we report the fabrication of rGO/TiO₂ composite films via an eco-friendly and low temperature ultraviolet (UV) photoreduction process and demonstrate their enhanced gas barrier properties by measuring water vapor transmission rates (WVTRs). When photocatalytic TiO₂ nanoparticles are employed, UV exposure reduces the GO/TiO₂ composite solution to form rGO/TiO₂, which is subsequently deposited on plastic substrates. The rGO/TiO₂ composites become resistant to water absorption because the UV photoreduction of GO/TiO₂ effectively removes most polar groups on the GO sheets. We confirmed that rGO/TiO₂ composites were successfully deposited onto the plastic substrate through a solution process and the barrier films led to a substantial reduction in WVTRs of the substrate. Our strategy for preparing graphene-based thin barrier films by using a UV photoreduction process enables the fabrication of solution-processed graphene-based encapsulation layers on plastic substrates with an eco-friendly and low temperature fabrication method.