Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Electroluminescence in conjugated polymers

Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.

Organic Thin Film Transistors for Large Area Electronics

Transparent, conductive, and ultrathin graphene films, as an alternative to the ubiquitously employed metal oxides window electrodes for solid-state dye-sensitized solar cells, are demonstrated. These graphene films are fabricated from exfoliated graphite oxide, followed by thermal reduction. The obtained films exhibit a high conductivity of 550 S/cm and a transparency of more than 70% over 1000−3000 nm. Furthermore, they show high chemical and thermal stabilities as well as an ultrasmooth surface with tunable wettability.

Transparent, Conductive Graphene Electrodes for Dye-Sensitized Solar Cells

Self-organization in many solution-processed, semiconducting conjugated polymers results in complex microstructures, in which ordered microcrystalline domains are embedded in an amorphous matrix1. This has important consequences for electrical properties of these materials: charge transport is usually limited by the most difficult hopping processes and is therefore dominated by the disordered matrix, resulting in low charge-carrier mobilities2 (⩽10-5 cm2 V-1 s-1). Here we use thin-film, field-effect transistor structures to probe the transport properties of the ordered microcrystalline domains in the conjugated polymer poly(3-hexylthiophene), P3HT. Self-organization in P3HT results in a lamella structure with two-dimensional conjugated sheets formed by interchain stacking. We find that, depending on processing conditions, the lamellae can adopt two different orientations—parallel and normal to the substrate—the mobilities of which differ by more than a factor of 100, and can reach values as high as 0.1 cm2 V-1 s-1 (refs 3, 4). Optical spectroscopy of the field-induced charge, combined with the mobility anisotropy, reveals the two-dimensional interchain character of the polaronic charge carriers, which exhibit lower relaxation energies than the corresponding radical cations on isolated one-dimensional chains. The possibility of achieving high mobilities via two-dimensional transport in self-organized conjugated lamellae is important for applications of polymer transistors in logic circuits5 and active-matrix displays4,6.

Two-dimensional charge transport in self-organized, high-mobility conjugated polymers

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Large enhancement of the carrier mobility in semiconducting α‐sexithienylene thin films by pulsed electrochemical inclusion of metal

2010 WILEY-VCH Verlag Gm While some applications are becoming a commercial reality, the basics of organic electronic devices still present many unclear aspects. Understanding their mode of operation is not straightforward, because the high density of defects in the active layer generally obscures the intrinsic mechanisms of the devices. However, as in the inorganic case, single crystals of conjugated molecules can be easily grown and efficient, flexible and large-scale devices can be fabricated and characterized. Many of the effects linked to the non-periodicity of the elemental structure and presence of defects can be worked around and issues regarding the intrinsic properties of a device elucidated. Therefore, in order to clarify the operating mechanism of the organic metal–semiconductor field effect transistor (OMESFET), we used rubrene, a well-know organic semiconductor that forms single crystals with an extremely low defect density. An OMESFET is a non-conventional organic field-effect device, already proposed for use as a low-operating-voltage variable resistor, which differs from the conventional organic field-effect transistor (OFET) by the absence of a gate dielectric. Instead, the gate is directly deposited on the semiconductor layer, on which it forms a blocking contact. This simple architecture requires fewer fabrication steps than a traditional insulated-gate OFET. The device performance depends on the nature of the non-injecting gate metal electrode. Even if its practical integration into complex electronic circuits has still to be demonstrated, its importance for fundamental investigations is obvious, as underlined in a recent paper published during the course of this work. The formation of a metal–organic interface and the injection of electrical charges from a ‘‘non-ohmic’’ contact into an organic active layer involve complex and still unresolved processes, and the analysis of these devices, combined with the characterization of single crystal asymmetric diodes, should help in clarifying some of the aspects underlying organic electronics. Besides its importance in many optoelectronic applications, such as solar cells, light emitting diodes, and logic circuits, the metal– organic interface is also the basic ingredient of OMESFETs; its analysis is therefore a required preliminary step. From the study of asymmetric metal–semiconductor–metal structures, we did not find any evidence for the formation of a depletion layer in the rubrene single crystal close to the non-ohmic contact. Instead, the dependence of the impedance on the voltage, combined with the analysis of the current-voltage (I–V) characteristics, reveals the effect of a built-in potential, arising from the difference between the work function of the two metal electrodes at both sides of the structure. As long as the diode is under reverse bias, that is, when a negative bias is applied to the high work function electrode, no current flow is registered, while when the system is polarized in forward bias, injection of holes occurs from the high work function electrode. The small-signal impedance response of the diode clearly reveals the contribution of a second RC element in series with the expected high resonance geometrical one. On this basis, efficient single crystal metal–semiconductor field effect transistors have been fabricated and characterized. Thanks to the high resistance of an Au/rubrene/Al structure under reverse bias, these voltage-controlled resistors show negligible leakage current and a high ON/OFF ratio, comparable to that found in other low-voltage field effect devices. The absence of a depletion layer in an organic single crystal diode suggests that the physical principles underlying the device operation must be revisited. A first simple attempt in this direction is given here, suggesting that the current modulation is an interface process in which charge injection is controlled by the gate potential. By laying a thin single crystal on a metal substrate and evaporating a top electrode on the opposite side of the crystal, we fabricated asymmetric metal–semiconductor–metal structures, through which the fundamentals of organic diodes can be investigated. Figure 1 shows the semi-logarithmic and linear plots of a typical I–V response of an ITO/rubrene/Al system. These single crystal diodes are hole-only devices with no detectable current in the reverse direction. A measurable current only flows when a positive bias is applied to the anode (high work function electrode, ITO or Au), in which case positive carriers are injected into the active layer thanks to a suitable energy level alignment between the metal Fermi level and the rubrene highest occupied molecular orbital (HOMO) level, while practically no electrons can pass over the high injection barrier between the aluminum and the lowest unoccupied molecular orbital (LUMO) level of the semiconductor. The curve in Figure 1 is qualitatively similar to that of an inorganic diode. However, the principles governing the operation of both devices are quite different. In the organic diode, the rectifying effect is not controlled by a single metal–semiconductor barrier; instead, it is a consequence of the work function difference between the metal electrodes at both sides of the rubrene single crystal and of the resulting built-in potential established in the organic semiconductor layer. Figure 2 shows a close view of the current onset of

Organic metal-semiconductor field-effect transistor (OMESFET) fabricated on a rubrene single crystal.

IR Spectroscopy Evidence for a Substrate-Dependent Organization of Sexithiophene Thin Films Vacuum Evaporated onto SiH/Si and SiO2/Si

Solution-processed organic field-effect transistors (OFETs) using chemically modified copper electrodes are reported. The purpose of this study is to shed light on the use of inexpensive copper electrodes in bottom-contact OFETs, which is consistent with the major goal of organic electronics: the realization of low-cost electronics. 6,13-Bis(triisopropylsilylethynyl)pentacene was used for solution-processed hole-transporting molecular films and pentafluorobenzenethiol was used to form self-assembled monolayers (SAMs) on the contact metals. We conducted a comparative study on copper and gold contacts and realized that, under the same performance improvement schemes, via SAM treatment and controlled crystal growth, the copper electrode device experienced a more significant enhancement than the gold electrode device. We attribute the beneficial effects of SAMs to the improved charge injection and transport properties, which are critical double effects from the fluorinated aromatic SAM structure. Grazing-inci...

Gilles Horowitz

Papers

Large enhancement of the carrier mobility in semiconducting α‐sexithienylene thin films by pulsed electrochemical inclusion of metal

Organic metal-semiconductor field-effect transistor (OMESFET) fabricated on a rubrene single crystal.

IR Spectroscopy Evidence for a Substrate-Dependent Organization of Sexithiophene Thin Films Vacuum Evaporated onto SiH/Si and SiO2/Si

Templating and charge injection from copper electrodes into solution-processed organic field-effect transistors.

Semiconducting conjugated oligomers for molecular electronics