Felix Hinkel

Electrochemically Exfoliated Graphene as Solution-Processable, Highly Conductive Electrodes for Organic Electronics

Solution-processable thin layer graphene is an intriguing nanomaterial with tremendous potential for electronic applications. In this work, we demonstrate that electrochemical exfoliation of graphite furnishes graphene sheets of high quality. The electrochemically exfoliated graphene (EG) contains a high yield (>80%) of one- to three-layer graphene flakes with high C/O ratio of 12.3 and low sheet resistance (4.8 kΩ/□ for a single EG sheet). Due to the solution processability of EG, a vacuum filtration method in association with dry transfer is introduced to produce large-area and highly conductive graphene films on various substrates. Moreover, we demonstrate that the patterned EG can serve as high-performance source/drain electrodes for organic field-effect transistors.

Bioinspired Wafer-Scale Production of Highly Stretchable Carbon Films for Transparent Conductive Electrodes†

Flexible transparent and conductive films (TCFs) are essential elements of the next-generation flexible devices, including touch screens and displays, organic light-emitting diodes (OLEDs), solar cells (SCs), organic field-effect transistors (OFETs), and sensors. Indium tin oxide (ITO) has been the most widely used TCF in optoelectronic devices for almost four decades, owing to its low sheet resistance (Rs 10 W/&) coupled with high transmittance (T 80%). However, ITO suffers from inherent brittleness, which makes it unsuitable for flexible devices. In the race to replace ITO, networks of metal nanowires and carbon nanotubes are leading the pack; however, their high costs of synthesis and high surface roughness hamper their commercial applications. 4] Graphene and graphene-based thin films have been recognized as attractive alternatives because of their outstanding electronic, optoelectronic, and mechanical properties. Although sheet resistance as low as 30 W/& (at T= 90 %) has been obtained for graphene grown on metallic substrates, the multitransfer process associated with the additional chemical doping increases the costs dramatically. The establishment of facile, yet controllable methods for the large-area production of flexible TCFs at low costs remains a challenging task. The molecular precursor approach, namely the production of carbon-based TCFs using aromatic-rich precursors as the carbon source seems to be much less explored. Through the thermal treatment of properly selected organic precursors, this synthetic method offers an intriguing means to lowcosts processing and structural control of highly conductive carbon/graphene films, which are difficult to achieve using methods such as chemical vapor deposition (CVD), epitaxial growth, liquid exfoliation, and reduction of graphene oxide (GO) film. Nevertheless, the bottleneck associated with this method lies in the relatively low electrical conductivity of the produced TCFs (e.g., 206 Scm 1 for a pyrolyzed film of a giant polycyclic aromatic hydrocarbon). Recently, inspired by the adhesive proteins secreted by marine mussels, polydopamine (PDA) has proven to be a material for facile and universal surface coating. By selfpolymerization of dopamine, PDA that sticks to the surface of virtually all types of solid materials regardless of their chemical nature, can be produced. 14] The self-polymerization reaction is so mild that simple immersion of substrates in an aqueous solution of dopamine results in the spontaneous deposition of PDA film. 14] The film is layer-structured and the thickness can be tailored at the nanometer scale. PDA is mainly composed of cross-linked indolequinone units (Figure 1, the definitive atomic scale structure of

Transition Between Band and Hopping Transport in Polymer Field‐Effect Transistors

Hall effect and slightly negative temperature dependence of the mobility in polymeric transistors are demonstrated. The semiconductor channel is based on a polycyclopentadithiophene-benzothiadiazole (CDT-BTZ) donor-acceptor copolymer film whose chain direction is oriented by mechanical compression at the surface of an ionic liquid. The mobility is 5.6 cm(2) V(-1) s(-1) at room temperature, and is further improved to 6.7 cm(2) V(-1) s(-1) at 260 K.

Hexasubstituted Benzenes with Ultrastrong Dipole Moments.

Hexasubstituted benzenes have been synthesized with the highest known dipole moments, as determined by dielectric spectroscopy and DFT methods. Based on the preparation of 4,5-diamino-3,6-dibromophthalonitrile, combined with a novel method to synthesize dihydrobenzimidazoles, these benzene derivatives have dipole moments in excess of 10 debye. Such dipole moments are desirable in ferroelectrics, nonlinear optics, and in organic photovoltaics. Structure determination was achieved through single-crystal X-ray crystallography, and the optical properties were determined by UV/Vis absorption and fluorescence spectroscopy.

Alternating Donor–Acceptor Arrays from Hexa‐peri‐hexabenzocoronene and Benzothiadiazole: Synthesis, Optical Properties, and Self‐Assembly

Donor-acceptor (D-A) structures were obtained by alternating arrays of hexa-peri-hexabenzocoronene (HBC) and benzo[c][1,2,5]thiadiazole (BTZ). Optoelectronic investigations revealed a charge transfer due to strong push-pull interactions. 2 D wide-angle X-ray scattering (WAXS) data indicated an arrangement in liquid-crystalline columnar assemblies, in which the π-stacking distances and molecular orientation depend on the number of HBC units in the molecules.

Reverse Engineering of Conjugated Microporous Polymers: Defect Structures of Tetrakis(4‐ethynylphenyl)stannane Networks

Two different conjugated microporous polymers (CMPs) based on tetrakis(4-ethynylphenyl)stannane as the repeating unit were synthesized and their BET surfaces and thermal properties were investigated. The first direct method to elucidate the molecular structure of the organic linkers between the tin centers by digestion of the CMP is described. Selective cleavage of the tin-carbon bonds with chloroacetic acid afforded the isolated bridging units and provided insight into the surprisingly varied chemical composition of these networks.

A New Solution to an Old Problem: Synthesis of Unsubstituted Poly(para-phenylene)

Unsubstituted and structurally well-defined poly(para-phenylene) (PPP) has been long-desired as an organic semiconductor prototype of conjugated polymers. To date, several attempts to synthesize unsubstituted, pristine, high-molecular-weight PPP have failed. Here we solved this synthetic problem by a versatile precursor route. Suzuki polymerization of kinked disubstituted 1,4-dimethoxycyclohexadienylene monomers yielded a well-soluble, nonaromatic precursor polymer. Its solubility allowed processing by spin-coating into nanometer-thick films. Subsequent additive-free thermal treatment induced aromatization and led to exclusively para-connected, highly fluorescent PPP with a length of about 75 phenylene units.

A Stable π-Conjugated Singlet Biradical.

A large polycyclic hydrocarbon with a singlet biradical ground state has been obtained by the combined effects of topology and electron-accepting aromatic substituents, which stabilize its open-shell singlet state.

Chemical Tongues and Noses Based upon Conjugated Polymers

Abstract We report the uses of conjugated polymers in multisensory applications and in chemical and optoelectronic tongues. We look at the potential of single polymers to discriminate multiple analytes and into small libraries of conjugated polymers that represent sensors. These small libraries combine several barely selective, promiscuous sensor elements and react with the analytes in a fairly non-selective fashion by change of color, emission wavelength, or emission intensity. In such optoelectronic noses and tongues, response of a single element is not specific or particularly useful at all, but the response pattern after the combination of several sensor elements is often specific for an analyte and allows discrimination and identification without any problem. These types of tongues and noses are well suited for quality control of foodstuff, beverages, and biological species such as proteins or cells. The discriminative process is often not well understood but it is powerful, particularly if the obtained data are analyzed by sophisticated statistical methods, i.e., linear discriminant analysis and/or principal component analysis. This added layer of analysis extracts the hidden information/patterns out of the data and allows visualization of the results.

Immobilization Strategies for Organic Semiconducting Conjugated Polymers

This Review details synthetic routes toward and properties of insoluble polymeric organic semiconductors obtained through desolubilization strategies. Typical applications include fixation of donor-acceptor bulk-heterojunction morphologies in organic photovoltaic cells, cross-linking of charge transport materials and active emitters in light emitting diodes or similar devices, and immobilization of morphologies in field effect transistors. A second important application is the structuring of organic semiconductors, using them as photoresists. After desolubilization, removal of the nonirradiated resist leads to elevated, micron-sized features of the semiconductor. In this Review, different strategies for desolubilization are covered. By photochemical or thermal cleavage of solubility-mediating groups such as esters, sulfonium salts, amides, ethers, and acetals or by retro-Diels-Alder reactions, volatile elimination products and the insoluble semiconductor are formed. In another case, desolubilization is achieved by cross-linking via functional groups present in the polymer side chains including vinyl, halide, silicone, boronic acid, and azide functionalities, which polymerize thermally or photochemically. Alternatively, small molecular additives such as photoacids, oligothiols, or oligoazides result in network formation in combination with compatible functional groups present in the immobilizable polymers. Advantages and disadvantages of the respective methods are discussed.