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.

Organic electronic devices that use graphene electrodes have received considerable attention because graphene is regarded as an ideal candidate electrode material. Transfer and lithographic processes during fabrication of patterned graphene electrodes typically leave polymer residues on the graphene surfaces. However, the impact of these residues on the organic semiconductor growth mechanism on graphene surface has not been reported yet. Here, we demonstrate that polymer residues remaining on graphene surfaces induce a stand-up orientation of pentacene, thereby controlling pentacene growth such that the molecular assembly is optimal for charge transport. Thus, pentacene field-effect transistors (FETs) using source/drain monolayer graphene electrodes with polymer residues show a high field-effect mobility of 1.2 cm(2)/V s. In contrast, epitaxial growth of pentacene having molecular assembly of lying-down structure is facilitated by π-π interaction between pentacene and the clean graphene electrode without polymer residues, which adversely affects lateral charge transport at the interface between electrode and channel. Our studies provide that the obtained high field-effect mobility in pentacene FETs using monolayer graphene electrodes arises from the extrinsic effects of polymer residues as well as the intrinsic characteristics of the highly conductive, ultrathin two-dimensional monolayer graphene electrodes.

Recent progress in organic field-effect transistor (OFET) printing processes is reviewed, and a perspective on the future of the field is discussed. The principles underlying the OFET printing techniques are introduced according to two categories: direct write printing and transfer printing. A comprehensive overview of the use of printing techniques in OFET production processes is also provided. Considerations for improving OFET device performance using printing processes are explored. Prior to OFET commercialization, the OFET printing techniques must satisfy several requirements, as discussed here.

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.

Organic thin-film transistors (OTFTs) have attracted considerable attention because of their potential applications in large-area, flexible, and printed electronics. To achieve OTFT devices with desirable properties, recent research has primarily focused on molecular design, [1,2] dielectric-semiconductor interfacial engineering, [3][4][5] and device optimization. [6][7][8][9][10][11] The use of conjugated polymer blends as active materials has brought a new way to tune and optimize the electronic properties of devices; for example, ambipolar field-effect charge transport has been reported in binary blends of p-and n-type conjugated polymers or oligomers. [12,13] Semiconducting and insulating polymer blends have also attracted increasing interest, because they can combine the electronic properties of semiconducting polymers with the low cost and excellent mechanical characteristics of insulating polymers. However, the presence of the insulating component tends to degrade the device performance by diluting the current density of the film. [14,15] To the best of our knowledge, the only effective approach to overcome this drawback is controlling the blended films to form vertically phase-separated structures, to keep the connectivity of the semiconducting layer in the presence of insulating components. In recent works, the composites with this structures have been used in OTFTs to fabricate low-voltage-driven devices, to improve environmental stability or reduce semiconductor cost. [16][17][18][19][20][21] However, the phase-separation process in polymer blends is very complicated. The final morphology in the blend films is highly sensitive to many factors, including the solvent evaporation rate, solubility parameters, film-substrate interactions, the surface tension of the components, and the film thickness. Vertical phase separation can only take place under extreme conditions. [22,23] Therefore, to develop a more facile and general method for realizing high-performance, low-semiconductor-cost devices is of great technological and academic significance.In this paper, we show that the percolation threshold of semiconducting/insulating polymer blends can be drastically decreased by depositing them from a marginal solvent with temperature-dependent solubility. Morphology and crystallinestructure studies reveal that the excellent electronic performance of the devices derives from the efficient charge transport and the good connectivity observed in highly crystalline, interconnected nanofibrillar networks of semiconductors embedded in an insulator matrix.Semiconductor/insulator-blend mother solutions were prepared by blending poly(3-hexylthiophene) (P3HT) and amorphous polystyrene (PS) in dichloromethane (CH 2 Cl 2 ), which is a marginal solvent for P3HT.[24] To completely dissolve P3HT, the CH 2 Cl 2 solution was kept at approximately 40 8C. For comparison, chloroform (CHCl 3 ), which is a good solvent for P3HT, was used as a reference. Thin films with different P3HT and PS ratios were fabricated on a silicon substrat...

Recently, the use of solution-processable conjugated polymer semiconductors in thin-film transistors (TFTs) has been extensively studied because of their suitability for fabricating large-area devices using established solution-deposition techniques (e.g., spin-coating, screen printing, or inkjet printing). [1][2][3][4][5][6][7][8] An attractive feature of the solution process is that different materials can be easily blended to optimize the electronic and optoelectronic properties for device applications. [9][10][11] Blends of semiconducting and dielectric polymers can combine the optical and electrical properties of semiconductors with the characteristics of dielectric polymers. However, the use of these blends as active layers in TFTs always causes a decrease in the device performance because the dielectric polymer ''dilutes'' the current density. [12,13] Controlling the phase separation in the direction perpendicular to the substrate to form bilayer structures should be an effective way to diminish this effect because it allows retention of the connectivity of the semiconducting layer in the channel region. To this end, organic-semiconductor/dielectric-polymer blends with vertical phase separation have been used to fabricate high-performance TFTs with low operating voltage [14] or with improved environmental stability [15] at high semiconductor concentration (!40%). In a recent publication, [16] Goffri et al. reported that the concentration of semiconductor in crystalline/crystalline bicomponent semiconductor/dielectricpolymer systems can be reduced to a value as low as 3 wt % without any degradation in device performance. However, all structures reported in previous studies are dielectric-top and semiconductor-bottom structures. It would be very interesting to investigate a semiconductor-top and dielectric-bottom bilayer structure, because this structure is identical to the configuration of the semiconductor and dielectric layers in the bottom-gate TFT device. For this reason formation of a semiconductor-top and dielectric-bottom bilayer in a one-step process may provide a simple route for the fabrication of TFT devices.In the present Communication, we report for the first time the fabrication of a semiconductor-top and dielectric-bottom bilayer structure by means of surface-induced vertical phase separation of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) blends. Because the ultrathin and defect-free PMMA dielectric layer can act as blind material, a modifier at the semiconductor/dielectric interface, or a dielectric layer, these bilayer blends have versatile uses in TFTs.Films composed of P3HT/PMMA blends were fabricated by spin-casting chlorobenzene solutions of the polymers onto bare silicon substrates (see Fig. 1a, inset). Since the hydrophilicities of P3HT and PMMA are very different, water contact-angle measurements were carried out to determine qualitatively the changes in composition taking place on the surface of the blended films. The variation of the water contact angle as a ...

We have demonstrated the influence of evaporation‐induced flow in a single droplet on the crystalline microstructure and film morphology of an ink‐jet‐printed organic semiconductor, 6,13‐bis((triisopropylsilylethynyl) pentacene (TIPS_PEN), by varying the composition of the solvent mixture. The ringlike deposits induced by outward convective flow in the droplets have a randomly oriented crystalline structure. The addition of dichlorobenzene as an evaporation control agent results in a homogeneous film morphology due to slow evaporation, but the molecular orientation of the film is undesirable in that it is similar to that of the ring‐deposited films. However, self‐aligned TIPS_PEN crystals with highly ordered crystalline structures were successfully produced when dodecane was added. Dodecane has a high boiling point and a low surface tension, and its addition to the solvent results in a recirculation flow in the droplets that is induced by a Marangoni flow (surface‐tension‐driven flow), which arises during the drying processes in the direction opposite to the convective flow. The field‐effect transistors fabricated with these self‐aligned crystals via ink‐jet printing exhibit significantly improved performance with an average effective field‐effect mobility of 0.12 cm2 V–1 s–1. These results demonstrate that with the choice of appropriate solvent ink‐jet printing is an excellent method for the production of organic semiconductor films with uniform morphology and desired molecular orientation for the direct‐write fabrication of high‐performance organic electronics.

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 cm(2)/(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 × 10(3) Ω/□.