Ladder-Type Silsesquioxane Copolymer Gate Dielectrics for High-Performance Organic Transistors and Inverters

A ladder-type poly­(phenyl-<i>co</i>-methacryl silsesquioxane) (PPMSQ) copolymer was developed for use as a gate dielectric in high-performance organic field-effect transistors (OFETs). The ladder-type PPMSQ copolymer was synthesized via the hydrolysis of two types of monomers, methacryloxypropyltrimethoxysilane and phenyltrimethoxysilane, followed by a condensation polymerization. The phenyl groups in one monomer were introduced to enhance the structural ordering of the overlying organic semiconductors, whereas the methacryloxypropyl groups in the other monomer were introduced to cross-link the polymer chains via thermal- or photocuring. The curing process enhanced the electrical strength of the gate dielectric layer due to the formation of a network structure with a reduced free volume. Thermal curing reduced the surface energy of the gate dielectrics, which improved the structural order of the overlying organic semiconductors and promoted the formation of large grains. The ladder-type PPMSQ was used as a gate dielectric to produce benchmark p- and n-channel OFETs based on pentacene and <i>N</i>,<i>N</i>′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C₈), respectively. The resulting OFETs exhibited excellent electrical performances, including a high carrier mobility (0.53 cm² V^–1s^–1 for the p-type pentacene OFET and 0.17 cm² V^–1s^–1 for the n-type PTCDI-C₈ OFET) and a high ON/OFF current ratio exceeding 10⁴. The photocured patterned PPMSQ film was successfully used to fabricate complementary OFET-based inverters that yielded high gains. The use of the ladder-type PPMSQ gate dielectrics provides a novel approach to realizing next-generation organic electronics

Low-Temperature Chemical Vapor Deposition Growth of Graphene from Toluene on Electropolished Copper Foils

A two-step CVD route with toluene as the carbon precursor was used to grow continuous large-area monolayer graphene films on a very flat, electropolished Cu foil surface at 600 °C, lower than any temperature reported to date for growing continuous monolayer graphene. Graphene coverage is higher on the surface of electropolished Cu foil than that on the unelectropolished one under the same growth conditions. The measured hole and electron mobilities of the monolayer graphene grown at 600 °C were 811 and 190 cm²/(V·s), respectively, and the shift of the Dirac point was 18 V. The asymmetry in carrier mobilities can be attributed to extrinsic doping during the growth or transfer. The optical transmittance of graphene at 550 nm was 97.33%, confirming it was a monolayer, and the sheet resistance was ∼8.02 × 10³ Ω/□

Boosting Photon Harvesting in Organic Solar Cells with Highly Oriented Molecular Crystals <i>via</i> Graphene–Organic Heterointerface

Photon harvesting in organic solar cells is highly dependent on the anisotropic nature of the optoelectronic properties of photoactive materials. Here, we demonstrate an efficient approach to dramatically enhance photon harvesting in planar heterojunction solar cells by using a graphene–organic heterointerface. A large area, residue-free monolayer graphene is inserted at anode interface to serve as an atomically thin epitaxial template for growing highly orientated pentacene crystals with lying-down orientation. This anisotropic orientation enhances the overall optoelectronic properties, including light absorption, charge carrier lifetime, interfacial energetics, and especially the exciton diffusion length. Spectroscopic and crystallographic analysis reveal that the lying-down orientation persists until a thickness of 110 nm, which, along with increased exciton diffusion length up to nearly 100 nm, allows the device optimum thickness to be doubled to yield significantly enhanced light absorption within the photoactive layers. The resultant photovoltaic performance shows simultaneous increment in <i>V</i>_oc, <i>J</i>_sc, and FF, and consequently a 5 times increment in the maximum power conversion efficiency than the equivalent devices without a graphene layer. The present findings indicate that controlling organic–graphene heterointerface could provide a design strategy of organic solar cell architecture for boosting photon harvesting

Enhancement of the Electrical Properties of Graphene Grown by Chemical Vapor Deposition via Controlling the Effects of Polymer Residue

Residual polymer (here, poly­(methyl methacrylate), PMMA) left on graphene from transfer from metals or device fabrication processes affects its electrical and thermal properties. We have found that the amount of polymer residue left after the transfer of chemical vapor deposited (CVD) graphene varies depending on the initial concentration of the polymer solution, and this residue influences the electrical performance of graphene field-effect transistors fabricated on SiO₂/Si. A PMMA solution with lower concentration gave less residue after exposure to acetone, resulting in less p-type doping in graphene and higher charge carrier mobility. The electrical properties of the weakly p-doped graphene could be further enhanced by exposure to formamide with the Dirac point at nearly zero gate voltage and a more than 50% increase of the room-temperature charge carrier mobility in air. This can be attributed to electron donation to graphene by the −NH₂ functional group in formamide that is absorbed in the polymer residue. This work provides a route to enhancing the electrical properties of CVD-grown graphene even when it has a thin polymer coating

Clean Transfer of Wafer-Scale Graphene <i>via</i> Liquid Phase Removal of Polycyclic Aromatic Hydrocarbons

Pentacene (C₂₂H₁₄), a polycyclic aromatic hydrocarbon, was used as both supporting and sacrificing layers for the clean and doping-free graphene transfer. After successful transfer of graphene to a target substrate, the pentacene layer was physically removed from the graphene surface by using intercalating organic solvent. This solvent-mediated removal of pentacene from graphene surface was investigated by both theoretical calculation and experimental studies with various solvents. The uses of pentacene and appropriate intercalation solvent enabled graphene transfer without forming a residue from the supporting layer. Such residues tend to cause charged impurity scattering and unintentional graphene doping effects. As a result, this clean graphene exhibited extremely homogeneous surface potential profiles over a large area. A field-effect transistor fabricated using this graphene displayed a high hole (electron) mobility of 8050 cm²/V·s (9940 cm²/V·s) with a nearly zero Dirac point voltage

Work-Function Engineering of Graphene Electrodes by Self-Assembled Monolayers for High-Performance Organic Field-Effect Transistors

We have devised a method to optimize the performance of organic field-effect transistors (OFETs) by controlling the work functions of graphene electrodes by functionalizing the surface of SiO₂ substrates with self-assembled monolayers (SAMs). The electron-donating NH₂-terminated SAMs induce strong n-doping in graphene, whereas the CH₃-terminated SAMs neutralize the p-doping induced by SiO₂ substrates, resulting in considerable changes in the work functions of graphene electrodes. This approach was successfully utilized to optimize electrical properties of graphene field-effect transistors and organic electronic devices using graphene electrodes. Considering the patternability and robustness of SAMs, this method would find numerous applications in graphene-based organic electronics and optoelectronic devices such as organic light-emitting diodes and organic photovoltaic devices

Selective-Area Fluorination of Graphene with Fluoropolymer and Laser Irradiation

We have devised a method to selectively fluorinate graphene by irradiating fluoropolymer-covered graphene with a laser. This fluoropolymer produces active fluorine radicals under laser irradiation that react with graphene but only in the laser-irradiated region. The kinetics of C–F bond formation is dependent on both the laser power and fluoropolymer thickness, proving that fluorination occurs by the decomposition of the fluoropolymer. Fluorination leads to a dramatic increase in the resistance of the graphene while the basic skeletal structure of the carbon bonding network is maintained. Considering the simplicity of the fluorination process and that it allows patterning with a nontoxic fluoropolymer as a solid source, this method could find application to generate fluorinated graphene in graphene-based electronic devices such as for the electrical isolation of graphene

Flexible and Transparent Dielectric Film with a High Dielectric Constant Using Chemical Vapor Deposition-Grown Graphene Interlayer

We have devised a dielectric film with a chemical vapor deposited graphene interlayer and studied the effect of the graphene interlayer on the dielectric performance. The highly transparent and flexible film was a polymer/graphene/polymer ‘sandwich-structure’ fabricated by a one-step transfer method that had a dielectric constant of 51, with a dielectric loss of 0.05 at 1 kHz. The graphene interlayer in the film forms a space charge layer, <i>i.e.</i>, an accumulation of polarized charge carriers near the graphene, resulting in an induced space charge polarization and enhanced dielectric constant. The characteristic of the space charge layer for the graphene dielectric film, the sheet resistance of the graphene interlayer, was adjusted through thermal annealing that caused partial oxidation. The dielectric film with higher sheet resistance due to the oxidized graphene interlayer had a significantly lower dielectric constant compared to that with the graphene with lower interlayer sheet resistance. Oxidizing the graphene interlayer yields a smaller and thinner space charge density in the dielectric film, ultimately leading to decreased capacitance. Considering the simplicity of the fabrication process and high dielectric performance, as well as the high transparency and flexibility, this film is promising for applications in plastic electronics

Design of a Polymer–Carbon Nanohybrid Junction by Interface Modeling for Efficient Printed Transistors

Molecularly hybridized materials composed of polymer semiconductors (PSCs) and single-walled carbon nanotubes (SWNTs) may provide a new way to exploit an advantageous combination of semiconductors, which yields electrical properties that are not available in a single-component system. We demonstrate for the first time high-performance inkjet-printed hybrid thin film transistors with an electrically engineered heterostructure by using specially designed PSCs and semiconducting SWNTs (sc-SWNTs) whose system achieved a high mobility of 0.23 cm² V^–1 s^–1, no <i>V</i>_on shift, and a low off-current. PSCs were designed by calculation of the density of states of the backbone structure, which was related to charge transfer. The sc-SWNTs were prepared by a single cascade of the density-induced separation method. We also revealed that the binding energy between PSCs and sc-SWNTs was strongly affected by the side-chain length of PSCs, leading to the formation of a homogeneous nanohybrid film. The understanding of electrostatic interactions in the heterostructure and experimental results suggests criteria for the design of nanohybrid heterostructures

Manipulation of Chain Conformation for Optimum Charge-Transport Pathways in Conjugated Polymers

A pair of different diketopyrrolopyrrole-based conjugated polymers (CPs) were designed and synthesized to investigate the effect of chain conformation on their molecular assembly. Conformation management was achieved by the incorporation of different linkers during polymerization. Through the use of computational calculations and UV–vis absorption measurements, the resulting CPs (PDPP-T and PDPP-BT) were found to exhibit partly modulated chain geometry. Grazing incident X-ray diffraction experiments with a two-dimensional detector revealed that PDPP-T having a planar chain conformation exhibited an edge-on type molecular arrangement, which evolved to a face-on type chain assembly when the planar geometry was altered to a slightly twisted one as in PDPP-BT. In addition, it was verified that the directional electric carrier mobility of CPs was critically distinguished by the distinctive chain arrangement in spite of their similar chemical structure. Concentration-dependent absorption measurements could provide an improved understanding of the assembly mechanism of CP chains: the planar conformation of PDPP-T facilitates the formation of preassembled chains in a concentrated solution and further directs the edge-on stacking, while the twisted dihedral angle along the benzothiophene in PDPP-BT prevents chain assembly, resulting in the face-on stacking. Because CP chain conformation is inevitably connected with the generation of preassembled chains, manipulating CP geometry could be an efficient tool for extracting an optimum chain assembly that is connected with the principal charge-transport pathway in CPs