Solution-processed Graphene Research Articles

Simple synthesis of solution-processable oxygen-enriched graphene as anode buffer layer for efficient organic solar cells

Interface material is a must for highly efficient and stable organic solar cells (OSCs) and has become a significant part of OSC research today. Here, low-cost and oxygen functionalized graphene (FG) was synthesized via a simple two-step method for applications in OSCs as anode buffer layer. The FG shows excellent dispersion in aqueous solution and great process compatibility with spin coating process. The introduction of work-function-tunable FG can effectively improve short current density of the devices. The power conversion efficiency of FG-based devices (4.13%, 4.49%, and 7.11% for P3HT:PC61BM, P3HT:PC71BM and PBDTTT-C:PC71BM, respectively) outperforms PEDOT:PSS-based devices (3.67%, 4.17%, and 6.46%, respectively). Moreover, the stability of the devices was improved with FG as anode buffer layer compared to PEDOT:PSS. The results indicate that simple synthesized FG is a promising solution-processed anode buffer layer material for high-efficiency and stable OSCs.

Impact of synthesis routes on the chemical, optical, and electrical properties of graphene oxides and its derivatives

Solution-processed graphene oxides in their reduced forms are prominent prospective functional materials for organic optoelectronics. For graphene oxide synthesis, several methods have been developed, which induce varying properties in their products. However, the dependence of the graphene oxide properties on their synthesis methods has rarely been studied, hindering the selection of the optimum synthesis route for a target application. In this study, we report our study results on the properties of synthesized graphene oxides and their reduced forms created using several synthesis methods, such as the modified Hummers' method, the improved method, and the Staudenmaier's method as well as from two commercial sources, Angstron Material, Inc. and Graphos, Inc. Focusing on the transparent electrode application, the properties of thin films were investigated using UV–visible spectroscopy, Hall measurements, atomic force microscopy, Raman spectroscopy, work function measurements, and X-ray photoelectron spectroscopy. Our results reveal significant morphological, elemental, structural, and optoelectrical property variations among the as-prepared and reduced thin films of graphene oxides by their synthesis methods. In addition, the results show that the graphene oxides synthesized using the modified Hummers' method and the product from Angstron Material, Inc. are the most suitable materials for the transparent electrode application.

Solution Processable Holey Graphene Oxide and Its Derived Macrostructures for High-Performance Supercapacitors.

Scalable preparation of solution processable graphene and its bulk materials with high specific surface areas and designed porosities is essential for many practical applications. Herein, we report a scalable approach to produce aqueous dispersions of holey graphene oxide with abundant in-plane nanopores via a convenient mild defect-etching reaction and demonstrate that the holey graphene oxide can function as a versatile building block for the assembly of macrostructures including holey graphene hydrogels with a three-dimensional hierarchical porosity and holey graphene papers with a compact but porous layered structure. These holey graphene macrostructures exhibit significantly improved specific surface area and ion diffusion rate compared to the nonholey counterparts and can be directly used as binder-free supercapacitor electrodes with ultrahigh specific capacitances of 283 F/g and 234 F/cm(3), excellent rate capabilities, and superior cycling stabilities. Our study defines a scalable pathway to solution processable holey graphene materials and will greatly impact the applications of graphene in diverse technological areas.

A corrosion-protective coating based on a solution-processable polymer-grafted graphene oxide nanocomposite

A new type of solution-processable graphene coating has been synthesized by grafting polymethylmethacrylate (PMMA) brushes on graphene oxide (GO) via surface-initiated atom transfer radical polymerization (ATRP). One major finding is that the PMMA-grafted GO nanocomposite exhibits synergistic properties of both building blocks, i.e., permeation inhibition of GO and solubility of PMMA in a variety of solvents, which makes it compatible with commonly used coating methods to form uniform coatings with controlled thickness. Our results demonstrate that PMMA-grafted GO coating can effectively block charge transfer at the metal–electrolyte interface and prevent corrosion of the copper substrate under aggressive saline conditions.

In-situ investigation of graphene oxide under UV irradiation: Evolution of work function

Using in-situ Kelvin probe force microscopy (KPFM) to measure surface potential, we investigated the time-dependent work function evolution of solution-processed graphene oxide (GO) under ultraviolet (UV) irradiation. We found that the work function of GO exposed in UV shows a notable decrease with increasing irradiation time, which is proposed to be attributed to the gradual disappearance of oxygen-containing functional groups in GO during the UV-induced reduction reaction process. Fourier transform infrared spectrum and Raman spectrum were used to confirm the reduction of GO under UV irradiation. Our study would give an insight into understanding the transformation of GO’s electronic structures during the reduction process.

UV-treated graphene oxide as anode interfacial layers for P3HT : PCBM solar cells

Solution-processable graphene oxide (GO) ultrathin films were introduced as anode interfacial layers (AILs) for polymer solar cells (PSCs). The photovoltaic performance of PSCs containing thermal- and UV-treated GO was comparable to that of PSCs with conventional poly(3,4-ethyledioxythiphene):poly(styrenesulfonate) AILs. UV treatment induced the surface activation of GO; an increase in the work function of UV-treated GO improved the energy band alignment at the GO/poly(3-hexylthiophene) interface, which accounted for the efficient hole collection and photovoltaic performance of PSCs with treated GO.

(Invited) Microwave Chemistry Enabled Controlled Fabrication of Graphene with Tailored Structures for Designed Applications

Due to its excellent electronic, optical, thermal and mechanical properties, and its large surface area and low mass, graphene holds great potential for a wide range of applications. It seems that the research in graphene has now proceeded from the initial phase of developing myriad strategies for the synthesis of graphene sheets to the use of graphene in various research fields. Mass production of reduced graphene oxide and graphene nanoplatets has recently been achieved. However, it is still challengingto controllably produce solution processable highly conductive graphene sheets in large quantity, at low cost, via eco-friendly and energy saving process, with optimal sheet size, layer thickness, defective (vacancies and holes) and molecular structures (oxygen containing groups and non-defective graphene domains). All these structural parameters determine their electronic, thermal and mechanical properties of graphene, which are the key warrants for their practical application in various devices. As examples, fundamental studies and high-frequency electronics require pristine graphene. However, “bulk” applications such as flexible macroelectronics, and mechanically and electronically reinforced composites, require large quantities of solution-processible highly conductive large graphene sheets manufactured at low cost. On the other hand, holey graphene, referred to graphene with nanoholes in their basal plane, demonstrates much better performance in their application as metal free catalysts and in energy storage. Finally, there is a surge of interests in nanosized graphene sheets for solar cells and various biological applications due to their unique size effects, edge effects, and even quantum confinement effects. Most importantly, most of the commercial available solution processible graphene sheets were fabricated via a long multiple- step process (several hours to 5 days). By exploiting the unique thermal and kinetic effects induced by microwave heating and the right selection of graphite oxidation chemistry, we developed series of quick and scalable strategies to produce a family of “clean” graphene sheets (without surfactants and without metal ions and/or toxic residues) with controlled porous structures, and controlled sizes from a few nanometers to tens of micrometers. By choosing a series of cheap biomass molecules, also with the help of microwave heating, bottom-up fabrication approaches are developed to produce heteroatom (N, B and/or P) doped graphene sheets and graphene quantum dots with bright emission. The applications of these graphene sheets and graphene quantum dots in Lithium battery, metal free-catalysts, and multifunctional drug delivery will be discussed.

Solution-processed graphene quantum dot deep-UV photodetectors.

Fast-response and high-sensitivity deep-ultraviolet (DUV) photodetectors with detection wavelength shorter than 320 nm are in high demand due to their potential applications in diverse fields. However, the fabrication processes of DUV detectors based on traditional semiconductor thin films are complicated and costly. Here we report a high-performance DUV photodetector based on graphene quantum dots (GQDs) fabricated via a facile solution process. The devices are capable of detecting DUV light with wavelength as short as 254 nm. With the aid of an asymmetric electrode structure, the device performance could be significantly improved. An on/off ratio of ∼6000 under 254 nm illumination at a relatively weak light intensity of 42 μW cm(-2) is achieved. The devices also exhibit excellent stability and reproducibility with a fast response speed. Given the solution-processing capability of the devices and extraordinary properties of GQDs, the use of GQDs will open up unique opportunities for future high-performance, low-cost DUV photodetectors.

Highly tunable interfacial adhesion of glass fiber by hybrid multilayers of graphene oxide and aramid nanofiber.

The performance of fiber-reinforced composites is governed not only by the nature of each individual component comprising the composite but also by the interfacial properties between the fiber and the matrix. We present a novel layer-by-layer (LbL) assembly for the surface modification of a glass fiber to enhance the interfacial properties between the glass fiber and the epoxy matrix. Solution-processable graphene oxide (GO) and an aramid nanofiber (ANF) were employed as active components for the LbL assembly onto the glass fiber, owing to their abundant functional groups and mechanical properties. We found that the interfacial properties of the glass fibers uniformly coated with GO and ANF multilayers, such as surface free energy and interfacial shear strength, were improved by 23.6% and 39.2%, respectively, compared with those of the bare glass fiber. In addition, the interfacial adhesion interactions between the glass fiber and the epoxy matrix were highly tunable simply by changing the composition and the architecture of layers, taking advantage of the versatility of the LbL assembly.

Solution-Processed Highly Conductive PEDOT:PSS/AgNW/GO Transparent Film for Efficient Organic-Si Hybrid Solar Cells

Hybrid solar cells based on n-Si/poly(3,4-ethylenedioxythiophene):poly(styrene- sulfonate) (PEDOT:PSS) heterojunction promise to be a low cost photovoltaic technology by using simple device structure and easy fabrication process. However, due to the low conductivity of PEDOT:PSS, a metal grid deposited by vacuum evaporation method is still required to enhance the charge collection efficiency, which complicates the device fabrication process. Here, a solution-processed graphene oxide (GO)-welded silver nanowires (AgNWs) transparent conductive electrode (TCE) was employed to replace the vacuum deposited metal grid. A unique "sandwich" structure was developed by embedding an AgNW network between PEDOT:PSS and GO with a figure-of-merit of 8.6×10(-3) Ω(-1), which was even higher than that of sputtered indium tin oxide electrode (6.6×10(-3) Ω(-1)). A champion power conversion efficiency of 13.3% was achieved, because of the decreased series resistance of the TCEs as well as the enhanced built-in potential (Vbi) in the hybrid solar cells. The TCEs were obtained by facile low-temperature solution process method, which was compatible with cost-effective mass production technology.

Solution processable graphene oxide hole transport layers and their application in P3HT:HHPER active layer based BHJSC

Graphene oxide (GO) material was synthesized by an improved Hummers method and characterized by FT-IR, XPS, XRD, AFM, SEM, and UV-VIS analyses. The thickness of the GO layer was measured as 1.5 nm. Solution processed bulk heterojunction solar cells comprising poly(3-hexylthiophene) (P3HT) as the electron donor and N,N'-bis-2-(1-hydoxyhexyl)-3,4,9,10-perylenebis(dicarboximide) (HHPER) as the electron acceptor component of the active layer were produced with and without the GO doped PEDOT-PSS hole transport layers. The optical investigations of the active layer were performed by ground state absorption and photoluminescence measurements. Optimized blend w/w was determined as P3HT:HHPER, 3:1. It was found that the presence of GO in PEDOT:PSS by 0.05 w/w reduces the charge transfer resistance and enhances not only the $J_{sc}$, but also V$_{oc}$ values. However, it cannot inhibit V$_{oc}$ losses obtained through annealing the active layer at temperatures higher than 120 $^{\circ}$C.

塗布グラフェン電極を用いた有機／無機ハイブリッドCMOS素子の形成

塗布グラフェン電極を用いた有機／無機ハイブリッドCMOS素子の形成

The importance of spinning speed in fabrication of spin-coated organic thin film transistors: Film morphology and field effect mobility

We have investigated the film morphology and the field effect mobility of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) thin films which were formed by spin coating on the SiO2 substrate with solution-processed graphene electrodes. The domain size and the density of aggregates in the C8-BTBT film showed the same dependence on the spinning speed. These competitive two factors (domain size and density of aggregates) give an optimum spinning speed, at which the field effect mobility of C8-BTBT transistor showed a maximum (2.6 cm2/V s). This result indicates the importance of spinning speed in the fabrication of solution processed organic thin film transistors by spin coating.

High‐Performance Semitransparent Tandem Solar Cell of 8.02% Conversion Efficiency with Solution‐Processed Graphene Mesh and Laminated Ag Nanowire Top Electrodes

A high‐performance semitransparent tandem solar cell that uses solution‐processed graphene mesh and laminated Ag NW as a transparent anode and cathode, respectively, is demonstrated. The laminated top electrode can be deposited without causing any damage to the underneath organic solar cells. Power conversion efficiencies of 8.02% and 6.47% are obtained when the light is projected from the solution‐processed graphene mesh and laminated AgNW, respectively. The performance of the tandem cell is found to be comparable to a tandem solar cell fabricated using commercially available indium tin oxide. These findings offer a high‐performance device and open a new pathway in searching for a potential replacement to the frequently used transparent conducting electrodes.

Relationship between Material Properties and Transparent Heater Performance for Both Bulk-like and Percolative Nanostructured Networks

Transparent heaters are important for many applications and in the future are likely to be fabricated from thin, conducting, nanostructured networks. However, the electrical properties of such networks are almost always controlled by percolative effects. The impact of percolation on heating effects has not been considered, and the material parameter combinations that lead to efficient performance are not known. In fact, figures of merit for transparent heaters have not been elucidated, either in bulk-like or percolative systems. Here, we develop a simple yet comprehensive model describing the operation of transparent heaters. By considering the balance of Joule heating versus power dissipated by both convection and radiation, we derive an expression for the time-dependent heater temperature as a function of both electrical and thermal parameters. This equation can be modified to describe the relationship between temperature, optical transmittance, and electrical/thermal parameters in both bulk-like and percolative systems. By performing experiments on silver nanowire networks, systems known to display both bulk-like and percolative regimes, we show the model to describe real systems extremely well. This work shows the performance of transparent heaters in the percolative regime to be significantly less efficient compared to the bulk-like regime, implying the diameter of the nanowires making up the network to be critical. The model allows the identification of figures of merit for networks in both bulk-like and percolative regimes. We show that metallic nanowire networks are most promising, closely followed by CVD graphene, with networks of solution-processed graphene and carbon nanotubes being much less efficient.

Graphene: Highly Stretchable Piezoresistive Graphene–Nanocellulose Nanopaper for Strain Sensors (Adv. Mater. 13/2014)

A strategy based on graphene and nanocellulose is developed by P. S. Lee and co-workers on page 2022, for the fabrication of highly stretchable strain sensors for human-motion detection. The image shows the 3D macroporous structure of the composite film from intertwined nano cellulose fibrils and crumpled graphene. The stretchable sensors made from low-cost and solutionprocessable graphene and nano cellulose hold promising applications in emerging human-interactive devices.

Highly soluble polyetheramine-functionalized graphene oxide and reduced graphene oxide both in aqueous and non-aqueous solvents

Among many methods to synthesize graphene, solution-based processing provides many advantages owing to its low cost, high productivity, chemical versatility, and scalability. In particular, graphene oxide (GO) is one of the most promising nanocarbons that enable the incorporation of graphene and related materials into bulk materials and nanocomposites. GO has hydrophilic nature that enables straightforward dispersion in aqueous solution by sonication, but GO show poor dispersibility in common organic solvents, which prevent much wider applications such as solution-mixing polymer nanocomposites. Here we prepared highly soluble, functionalized GO in both aqueous and non-aqueous solvents. This was achieved by reacting polyetheramine consisting of amphiphilic components, e.g., polypropylene oxide and polyethylene oxide, with carboxylic acid groups at GO edges. Moreover, the reduced GO (rGO) was also highly dispersible in aqueous solution as well as non-aqueous solutions. These functionalized GO and rGO can be used for many solution-processed graphene composites.

Thermally reduced solution-processed graphene oxide thin film: An efficient infrared photodetector

A sharp response of infrared light detecting capability is observed in reduced graphene oxide based photodetector. The photoresponse increased with increasing solution concentration and annealing duration, at a low percolation. The photocurrent increases considerably before saturation and persists for a maximum time of 26min. This Letter also reports on superiority of device performance with continuous annealing over annealing with an interval. A maximum photoresponsivity of 0.55A/W and external quantum efficiency of 57% was achieved at low power light intensity (0.8mW/cm2). This Letter presents an efficient graphene oxide thin film infrared photodetector processed via a modified synthesis protocol.

A solution-processable polymer-grafted graphene oxide derivative for nonvolatile rewritable memory

A new conjugated polymer, poly{[9,9-di(triphenylamine) fluorene]-[9,9-dihexylfluorene]-[4,4′-(9H-fluorene-9,9-diyl) dibenzenamine]}(PTHF), was synthesized by the Suzuki coupling reaction and used to react with the graphene oxide (GO) containing surface-bonded acyl chloride moieties to give a solution-processable GO–PTHF. Covalent attachment of PTHF onto GO resulted in the partial fluorescence quenching of the parent polymer, suggesting energy and/or electron transfer between the excited singlet states of the PTHF moiety and the GO moiety. The absolute photoluminescence (PL) quantum yields (ΦPL) of the samples were changed from 0.59 for PTHF to 0.31 for GO–PTHF. A light-induced electron paramagnetic resonance (LEPR) technique was employed to study the photoinduced spins in the conjugated GO–PTHF. Upon irradiation with a 432 nm laser, both the polaron signals of the polymer chain at g = 2.1132, 2.0665 and 2.0197, and the signals of radical anions on GO at g// = 1.9726 and g⊥ = 1.9747 were kept unchanged, while the signal at g = 2.0002 almost completely disappeared due to a typical photoinduced selective quenching of the triplet state or a photoinduced spin polarization transfer process acting with a radical–triplet pair mechanism. A memory device based on GO–PTHF, in which the GO–PTHF film was sandwiched between the indium–tin oxide and Al electrodes, can be switched to the ON state under a negative electrical sweep, and can also be reset to the initial OFF state by a reverse (positive) electrical sweep.

Advancement in liquid exfoliation of graphite through simultaneously oxidizing and ultrasonicating

Simultaneously oxidizing and ultrasonicating graphite for 60 min can create high-structural integrity yet solution-processable graphene for a great many applications.