The processing characteristics of organic semiconductors make them potentially useful for electronic applications where low-cost, large area coverage and structural flexibility are required. This critical review gives a general introduction about the current standing in the area of OFETs focusing on the new processable small molecules that have been recently reported for their use as organic semiconductors. A general description of the OFETs device operation and the transport mechanisms that dominate organic semiconductors is provided, followed by an overview of the strategies and materials employed to fabricate p-type, n-type and ambipolar OFETs. Some new tendencies and applications that are currently being developed employing OFETs are also described, such as the preparation of electronic paper, sensors or light emitting transistors (85 references).

A growing interest in organic field effect transistors (OFETs) has emerged in past years due to their potential applications in electronics where low-cost, large area coverage, and structural flexibility are required. 1 OFET single crystals are found to give the highest mobilities largely due to their regular molecular ordering that permits extensive intermolecular orbital overlap to occur. Crystalline pentacene is probably the most widely studied organic semiconductor and, because of its high performance (hole mobility of 1.5 cm 2 /(V‚s)), 2 has been the benchmark by which other OFETs are measured. In this paper, we report on a theoretical study to understand the high mobility found in dithiophene-tetrathiafulvalene (DT-TTF) transistors, 3 with respect to that known for pentacene, using an extended measure of the reorganization energy. We demonstrate that the molecular packing is a key factor in assessing hopping mobilities. The relationship between crystal structure and transport properties of a material, crucial to understand for the rational design of new OFET materials, is thus elucidated.At room temperature, the charge mobility of organic materials is often determined by a hopping transport process, which can be depicted as an electron or hole transfer reaction in which an electron or hole is transferred from one molecule to the neighboring one. The localization of charge on a molecule for a sufficient time allows the nuclei to adopt the optimal geometry of the charged state, 4 coupling molecular relaxation with the charge mobility. Two major parameters determine self-exchange rates and, thus, the charge mobility: 5 (i) the electronic coupling between adjacent molecules (transfer integral), 6 which needs to be maximized, and (ii) the reorganization energy (λ reorg ), which needs to be small for efficient charge transport. Neglecting the contributions due to the medium polarization and molecular vibrations, in a hole-hopping material, λ reorg corresponds to the sum of two relaxation energies, λ rel (1,2) , for the transformation of one molecule from the neutral state to the +1 charged state, and, for a neighboring molecule, the transformation from the charged state to the neutral molecular state. These two portions are typically nearly identical (λ reorg ≈ 2λ rel ). 7 The reorganization energy gives a measure of the energy loss (or hopping efficiency) of a charge carrier passing through a single molecule. To explain the high mobility of pentacene transistors, previous studies have focused on the reorganization energy of the isolated pentacene molecule. 5 Interestingly, the reorganization energy calculated for the pentacene molecule is extremely low (0.098 eV), 5 providing persuasive evidence for its high hole mobility.Most attention for improving the mobility of OFETs has been placed on the development of improved device fabrication techniques. 8 Other feasible strategies have attempted to increase the relatively small intermolecular orbital overlap found in pentacene by directed functionalization, 9 or by ...

Organic radicals are neutral, purely organic molecules exhibiting an intrinsic magnetic moment due to the presence of an unpaired electron in the molecule in its ground state. This property, added to the low spin-orbit coupling and weak hyperfine interactions, make neutral organic radicals good candidates for molecular spintronics insofar as the radical character is stable in solid state electronic devices. Here we show that the paramagnetism of the polychlorotriphenylmethyl radical molecule in the form of a Kondo anomaly is preserved in two- and three-terminal solid-state devices, regardless of mechanical and electrostatic changes. Indeed, our results demonstrate that the Kondo anomaly is robust under electrodes displacement and changes of the electrostatic environment, pointing to a localized orbital in the radical as the source of magnetism. Strong support to this picture is provided by density functional calculations and measurements of the corresponding nonradical species. These results pave the way toward the use of all-organic neutral radical molecules in spintronics devices and open the door to further investigations into Kondo physics.

We report on the fabrication and characterization of field-effect transistors based on single crystals of the organic semiconductor dibenzo-tetrathiafulvalene (DB-TTF). We demonstrate that it is possible to prepare very-good-quality DB-TTF crystals from solution. These devices show high field-effect mobilities typically in the range 0.1-1 cm 2 / V s. The temperature dependence was also studied revealing an initial increase of the mobility when lowering the temperature until it reached a maximum, after which the mobility decreased following a thermally activated behavior with activation energies between 50 and 60 meV. Calculations of the molecular reorganization energy and intermolecular transfer integrals for this material were also performed and are in agreement with the high mobility observed in this material. The improved electronic performance of organic fieldeffect transistors (OFETs) over the last few years has shown great potential for a wide range of functional applications where low-cost, light-weight, flexibility, and large-area coverage are required.1 Although most previous studies on OFETs have focused on the fabrication and improving the quality of organic thin films, 2-5 currently, a few groups are devoting their efforts to the preparation of single-crystal OFETs as they typically show higher charge carrier mobilities because of their high molecular order. Crystals of oligoacene 6-10 and thiophene 11 derivatives have been studied and OFET mobilities of up to 15 cm 2 / V s have been reported for rubrene crystals.7 In all these cases, the samples were prepared from the vapor phase in order to obtain high purity materials and/or because of the low solubility of these materials in common organic solvents. In this letter, we report on the preparation of single crystal OFETs based on an organic semiconductor, dibenzo-tetrathiafulvalene [DB-TTF, Fig. 1(a)]. Moreover, we show that it is possible to prepare good quality DB-TTF crystals with very high mobilities from solution, which makes this material very interesting for potential applications in low-cost electronics.Very recently, we studied the correlation between crystal structure and mobility in single-crystal OFETs based on tetrathiafulvalene derivatives.12 A correlation between the mobilities and the different investigated crystal structures was observed. This was corroborated by density functional (DF) calculations of the molecular reorganization energies ͑ reorg ͒ 13 and the maximum intermolecular transfer integrals.14 It was observed that the molecules showing the best performance for OFETs crystallize forming uniform stacks of almost planar molecules in a herringbone pattern.In addition, these molecules were the ones showing higher intermolecular transfer integrals and lower reorganization energies. In particular, dithiophene-tetrathiafulvalene [DT-TTF, Fig. 1(a)] exhibited a mobility of up to 1.4 cm 2 /V s. 15DB-TTF seems, a priori, a very promising candidate molecule to study for the preparation of OFETs as, like DT-TTF, it is symmetric and com...