The field of organic electronics has been developed vastly in the past two decades due to its promise for low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, and ease of processing. The performance and lifetime of these devices, such as organic light-emitting diodes (OLEDs), photovoltaics (OPVs), and field-effect transistors (OFETs), are critically dependent on the properties of both active materials and their interfaces. Interfacial properties can be controlled ranging from simple wettability or adhesion between different materials to direct modifications of the electronic structure of the materials. In this Feature Article, the strategies of utilizing surfactant-modified cathodes, hole-transporting buffer layers, and self-assembled monolayer (SAM)-modified anodes are highlighted. In addition to enabling the production of high-efficiency OLEDs, control of interfaces in both conventional and inverted polymer solar cells is shown to enhance their efficiency and stability; and the tailoring of source–drain electrode–semiconductor interfaces, dielectric–semiconductor interfaces, and ultrathin dielectrics is shown to allow for high-performance OFETs.

Interface Engineering for Organic Electronics

Carbon nanotubes – Production and industrial applications

There are several advantages of growing carbon nanotubes (CNTs) directly on bulk metals, for example in the formation of robust CNT-metal contacts during growth. Usually, aligned CNTs are grown either by using thin catalyst layers predeposited on substrates or through vapour-phase catalyst delivery. The latter method, although flexible, is unsuitable for growing CNTs directly on metallic substrates. Here we report on the growth of aligned multiwalled CNTs on a metallic alloy, Inconel 600 (Inconel), using vapour-phase catalyst delivery. The CNTs are well anchored to the substrate and show excellent electrical contact with it. These CNT-metal structures were then used to fabricate double-layer capacitors and field-emitter devices, which demonstrated improved performance over previously designed CNT structures. Inconel coatings can also be used to grow CNTs on other metallic substrates. This finding overcomes the substrate limitation for nanotube growth which should assist the development of future CNT-related technologies.

Direct growth of aligned carbon nanotubes on bulk metals

Single molecule electronic devices in which individual molecules are utilized as active electronic components constitute a promising approach for the ultimate miniaturization and integration of electronic devices in nanotechnology through the bottom-up strategy. Thus, the ability to understand, control, and exploit charge transport at the level of single molecules has become a long-standing desire of scientists and engineers from different disciplines for various potential device applications. Indeed, a study on charge transport through single molecules attached to metallic electrodes is a very challenging task, but rapid advances have been made in recent years. This review article focuses on experimental aspects of electronic devices made with single molecules, with a primary focus on the characterization and manipulation of charge transport in this regime.

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Single Molecule Electronic Devices

A high-mobility organic semiconductor employed as the active material in a field-effect transistor does not guarantee per se that expectations of high performance are fulfilled. This is even truer if a downscaled, short channel is adopted. Only if contacts are able to provide the device with as much charge as it needs, with a negligible voltage drop across them, then high expectations can turn into high performances. It is a fact that this is not always the case in the field of organic electronics. In this review, we aim to offer a comprehensive overview on the subject of current injection in organic thin film transistors: physical principles concerning energy level (mis)alignment at interfaces, models describing charge injection, technologies for interface tuning, and techniques for characterizing devices. Finally, a survey of the most recent accomplishments in the field is given. Principles are described in general, but the technologies and survey emphasis is on solution processed transistors, because it is our opinion that scalable, roll-to-roll printing processing is one, if not the brightest, possible scenario for the future of organic electronics. With the exception of electrolyte-gated organic transistors, where impressively low width normalized resistances were reported (in the range of 10 Ω·cm), to date the lowest values reported for devices where the semiconductor is solution-processed and where the most common architectures are adopted, are ∼10 kΩ·cm for transistors with a field effect mobility in the 0.1-1 cm(2)/Vs range. Although these values represent the best case, they still pose a severe limitation for downscaling the channel lengths below a few micrometers, necessary for increasing the device switching speed. Moreover, techniques to lower contact resistances have been often developed on a case-by-case basis, depending on the materials, architecture and processing techniques. The lack of a standard strategy has hampered the progress of the field for a long time. Only recently, as the understanding of the rather complex physical processes at the metal/semiconductor interfaces has improved, more general approaches, with a validity that extends to several materials, are being proposed and successfully tested in the literature. Only a combined scientific and technological effort, on the one side to fully understand contact phenomena and on the other to completely master the tailoring of interfaces, will enable the development of advanced organic electronics applications and their widespread adoption in low-cost, large-area printed circuits.

Charge injection in solution-processed organic field-effect transistors: physics, models and characterization methods.

The characteristics of organic field-effect transistors (OFETs) were dramatically improved by chemically modifying the surface of the bottom-contact Ag or Cu source−drain (D−S) electrodes with a simple solution method. The contact resistance and energetic mismatch typically observed with Ag D−S electrodes in pentacene bottom-contact OFETs can be properly eliminated when modified by the Ag-TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane). The pentacene transistors with low-cost Ag-TCNQ-modified Ag bottom-contact electrodes exhibit outstanding electrical properties, which are comparable with that of the Au top-contact devices. It thus provides a novel way toward high-performance low-cost bottom-contact OFETs.

High-performance low-cost organic field-effect transistors with chemically modified bottom electrodes.

Patterns and alignment of carbon nanotubes (CNTs) are particularly important for fabricating functional devices such as field emitters, scanning probes, sensors, and nanoelectronics. In this paper, the effects of various metal substrates on CNT growth are studied. Depending on the different growth mechanisms associated with the various substrate surfaces, two- or three-dimensional micropatterns of aligned carbon nanotubes have been prepared via pyrolysis of iron(II) phthalocyanine. Patterns created on silicon substrates were obtained by photolithographic techniques. The prepared CNT patterns have resolutions down to the micrometer scale. Possible growth mechanisms of aligned carbon nanotubes on different metal surfaces are also discussed.

Controllable preparation of patterns of aligned carbon nanotubes on metals and metal-coated silicon substrates

We present a method of fabricating noncoplanar channel organic field-effect transistors (OFETs) by a conventional photolithographic technique. Using this method, OFETs with micrometer critical features in slanting configurations and submicrometer critical features in vertical configurations were fabricated. The critical channel length over 1μm was controlled by the patterning technique, while the one of 0.5μm was defined by the thickness of an insulating layer between the drain and source electrodes. Also, we demonstrate that the OFETs containing two different metals as source and drain electrodes, respectively, are easily realized. All the OFETs based on copper phthalocyanine exhibit a high performance.

Noncoplanar organic field-effect transistor based on copper phthalocyanine

Molecular electronics are considered one of the most promising ways to meet the challenge of micro-electronics facing its scaling down pathway. Molecular devices, especially molecular scale field-effect transistors (MSFET), are key building blocks for molecular electronics. Three major hurdles to device fabrication are yet to be overcome: electrode pairs must be fabricated with a controllable gap size commensurate with the functional molecule size of interest; the molecules of interest must be arranged between the electrodes with precise location and orientation control; and stable, conducting contacts must be made between the molecules and the electrodes. We have combined “top-down” and “bottom-up” approaches to solve these problems. Using photolithography and molecular lithography with self-assembled mono/multiple molecule layer(s) as a resist, we fabricated electrode structures with a controllable molecular-scale gap between source and drain electrodes and a third terminal of a buried gate. For our device, we synthesized a thiolated phthalocyanine derivative molecule, {di-[1-(S-acetylthio)-4-ethynylphenyl]-di-(tert-butyl)phthalocyanato}copper(II), with acetylthio groups on both ends, conjugated with ethynylphenyl groups. The synthesized end-thiolated molecules were assembled between the tailored molecular gap of the as-fabricated FET electrode structures in solution via Au–S bonding, forming stable contacts between the electrodes and the molecules, and a 3 terminal MSFET device was formed. Electrical measurements show that the device has characteristics of a typical FET device. The field-effect mobility of the as-fabricated MS-FET is 0.16 cm2 V−1 s−1.

Fabrication and characterization of molecular scale field-effect transistors

We fabricate single-walled carbon nanotube field-effect transistors (SWNT FETs) with a simple, low-cost, high-efficiency, and solution-based orientational self-assembly method. The SWNT was first functionalized with thiol groups, then suspended in an N,N-dimethylformamide solution, and finally self-assembled on predefined gold contact pads by an orientational N2 blow. A relatively high mobility of 9.2×102cm2∕Vs and an on∕off ratio greater than 105 at a bias voltage of −1V have been achieved as a result of directly covalent interaction between end-thiolated SWNT and gold contacts, which favors the fabrication of SWNT FETs as compared to the SWNT FETs bearing the similar geometry but relies on random deposition. Therefore, this orientational self-assembly approach should be a valuable tool in the mass fabrication of high-performance SWNT FETs and SWNT-based molecular-scale electronic devices.

Xinyu Liu

Papers

High-performance low-cost organic field-effect transistors with chemically modified bottom electrodes.

Controllable preparation of patterns of aligned carbon nanotubes on metals and metal-coated silicon substrates

Noncoplanar organic field-effect transistor based on copper phthalocyanine

Fabrication and characterization of molecular scale field-effect transistors

Orientational self-assembled field-effect transistors based on a single-walled carbon nanotube