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

Because the conductivity of organic semiconductors is very low, a useful model for the organic diode consists of treating the organic layer as an insulator, an approximation often referred to as the metal-insulator-metal (MIM) model. Moreover, the dominant charge carrier injection process is diffusion, so that a modified Schottky theory can be used to derive a simple analytical equation for the current voltage curve of the diode. Here, we carried out a full analysis of the MIM model for the organic diode. We show that Schottky theory is only valid when charge injection is poor, that is, for high injection barriers. When the injection barrier is lowered, the current given by Schottky theory is still valid in the weak injection regime, when the applied potential is lower than the diffusion potential. However, it becomes largely overestimated in the strong injection regime. We also show that in the strong injection regime, the current given by the MIM model merges with Mott-Gurnery space-charge-limited regime.

Drift-diffusion current in organic diodes

Organic Diodes with Highly Improved Charge Injection and Transport via One-Pot Treatment Process

Nonlinear transport is intensively explained through Poole-Frenkel (PF) transport mechanism in organic thin film transistors with solution-processed small molecules, which is, 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene. We outline a detailed electrical study that identifies the source to drain field dependent mobility. Devices with diverse channel lengths enable the extensive exhibition of field dependent mobility due to thermal activation of carriers among traps.

Nonlinear Transport in Organic Thin Film Transistors with Soluble Small Molecule Semiconductor.

Prepn. of oligomers of aromatic heterocycles, partic. thiophone, opt. substd. in alpha- and/or alpha1-position, of formula (I) comprises either: (a) reacting an oligomer of aromatic heterocycle, partic. thiphene, of formula (II) with an oligomer of aromatic heterocyclic, partic. thiophene, of the formula (III) according to the reaction scheme given below, in the presence of a catalyst or (b) reacting an oligomer of aromatic heterocyclic, partic. oligothiphene, of formula (IV) with an oligomer of aromatic heterocyclic, partic. oligothiphene, of formula (V) according to the reaction scheme given below, X = O, S, Se, Te or N-R, pref. S; Z = a non-reactive substituent; R = H or opt. substd. hydrocarbon gp. R1 and R2 = H, electron-accepting residue A or electron-donating residue D; n = 2-20; m = 1 or 2 or 0 and the Z's can be same or different when n = 2. M1 = magnesium residue (MgHal) or zinc residue (ZnHal), Hal = Cl, F or Br; n1 + m2 = n and M2 = H or Li and is identical. When M2 = Li, the reaction is effected electrochemically and when M2 = H, the reaction is effected in presence of an aprotic polar solvent, and in both cases reaction is effected in presence of a catalyst. Also claimed are the prepns. of (II) and (III), the obtd. oligomers and their use in electronic and optoelectronic equipment esp. in the form of thin layers.

Prepn. of aromatic heterocyclic, esp. thiophene, oligomers - used in electronic and opto-electronic equipment, partic. as thin layer

Temperature dependent steady state and transient photoluminescence (PL) measurements were performed on sexithiophene single crystals. As the temperature is lowered, three different sets of equally spaced peaks appear on the steady state PL spectrum. This behavior is interpreted in the frame of the molecular exciton theory. The lowest energy set is attributed to transitions from the lower A u excited state, resulting from the Davydov splitting of the 1B u excited state of the isolated molecules. The highest energy set of peaks is ascribed to an excited A g level. At low excitation levels, the transient PL follows a single exponential decay, with a time constant of 1.8 ns. When the excitation energy is increased beyond to 35 (mu) J/cm 2 , a second much faster transient is superimposed, and the PL spectrum narrows to a single line, centered at the 0-1 transition of the low energy set of the steady PL spectrum. The width of the emission line is 13 cm -1 . These observations are explained in terms of a stimulated emission process.

Gilles Horowitz

Papers

Drift-diffusion current in organic diodes

Organic Diodes with Highly Improved Charge Injection and Transport via One-Pot Treatment Process

Nonlinear Transport in Organic Thin Film Transistors with Soluble Small Molecule Semiconductor.

Prepn. of aromatic heterocyclic, esp. thiophene, oligomers - used in electronic and opto-electronic equipment, partic. as thin layer

Stimulated emission in sexithiophene single crystals