High Mobility of Dithiophene-Tetrathiafulvalene Single-Crystal Organic Field Effect Transistors

Abstract

The processing characteristics of organic semiconductors make them potentially useful for electronic applications where low-cost, large area coverage, and structural flexibility are required. 1 Charge carrier mobility is a measure of the quality of organic semiconductors and is a primary factor that determines the performance of organic field effect transistors (OFETs). The highest mobilities are found in single crystals due to molecular ordering that permits good overlapping of the π-π orbitals. A mobility of 1.5 cm 2 /Vs was reported for pentacene, 2 and recently a mobility of 8 cm 2 /Vs was reported for rubrene. 3 These organic crystals were grown from the vapor phase in long and complex experiments. To progress in this field toward device applications, it is very important to search for new molecules that are able to act as organic semiconductors 4 and also to develop easy methods to grow single crystals or films. In this communication, we report on the clear observation of field effect hole mobility in single crystals of the organic semiconductor dithiophene-tetrathiafulvalene (DT-TTF, Figure 1a) grown by drop casting, a very simple method. The maximum mobility observed in these crystals was 1.4 cm 2 /Vs.Tetrathiafulvalene (TTF) and its derivatives are successfully used as building blocks for charge-transfer salts, giving rise to a multitude of organic conductors and superconductors. These conducting salts are prepared mainly as single crystals but also as thin films of nanocrystals supported on dielectric polymers. [5][6][7] The driving force in the crystallization of the salts is the π-π stacking, which permits, together with the S‚‚‚S interactions, an intermolecular electronic transfer responsible for their transport properties. The transport properties of a few single crystals of neutral TTF derivatives have been measured, showing semiconductor character with a roomtemperature conductivity on the order of 10 -5 S/cm. 8-10 Additionally, organic thin films of TTF derivatives have been grown by vacuum deposition techniques, but only their morphology and nanomechanical properties have been studied. 11 However, TTF derivatives have never been used for the preparation of OFETs despite the fact that they are soluble in various solvents, easily chemically modified, and good electron donors.DT-TTF is a particularly promising molecule because it is rigid and completely conjugated, having a low conformational freedom that favors stronger intermolecular interactions and, thus, higher carrier mobilities. Cyclic voltammetry measurements of DT-TTF show two separate reversible one-electron oxidations with E 1/2 1 and E 1/2 2 of 0.78 and 0.96 V, respectively (in DMF/0.1 M TBAPF 6 vs SCE). X-ray studies reveal that the DT-TTF molecules stack uniformly along the b axis in a herringbone pattern ( Figure 1a). 12 Moreover, it is seen that the sulfur atoms also promote interactions between the different DT-TTF stacks within the layer which can lead to electronic dimensionality enhancing 5 (d(S‚‚‚S) ) 3.55 and 3.61 Å). This cr...

Push-pull effect on the geometrical, optical and charge transfer properties of disubstituted derivatives of mer-tris(4-hydroxy-1,5-naphthyridinato) aluminum (mer-AlND3)

“…In these calculations only the structural modification of the molecules are considered neglecting the polarization effect in the surrounding medium. The electron/hole transport is predictable from the electron (λ ele )/hole (λ hole ) reorganization energies and in general has good agreement with the experimental observations [27,71,82,[85][86][87][88][89][90][91][92][93][94][95]. Previously the reorganization energies for hole/ electron of mer-Alq3 and its derivatives [56,57,59,61,63] and mer-AlND3 [64] has been calculated at B3LYP/6-31G(D) level.…”

“…Using this model the charge transport that is calculated here is the intermolecular process in which the charge hops between two molecules. The hole and electron transport process at the molecular level in the electroluminescent layer can then be portrayed as the electron transfer/hole transfer reactions between the neighboring molecules as [27,[84][85][86][87][88][89][90][91][92][93][94][95]:…”

“…The λ hole and λ ele can be used to estimate approximately the charge transport and balance between the holes and electrons. The reorganization energy for both the hole (λ hole ) and electron (λ ele ) transport are evaluated using the following formulas [64,[90][91][92].…”

Solution-processed, high-performance n-channel organic microwire transistors

“…These unique materials combine the high-performance of singlecrystalline structures with solution-processability by dispersion (17,18). Several p-channel (hole-transporting) organic wires prepared by solution-processing have exhibited Ͼ 0.1 cm 2 /Vs for singlewire transistors (19)(20)(21)(22), whereas only a few solution-synthesized n-channel organic wires have been reported, typically with low performance ( Ϸ10 Ϫ3 Ϫ 10 Ϫ2 cm 2 /Vs) (23,24).Despite the intrinsic high mobility of single-crystalline wires, precise wire placement and wire-to-wire performance variation, due to the difference in the contact quality at the wire/insulator and wire/electrode interfaces, substantially hinder successful device integration (10, 25). Therefore, the technology to achieve network films of organic MWs deposited from a dispersion with controlled alignment and density is acutely desired.…”

“…These unique materials combine the high-performance of singlecrystalline structures with solution-processability by dispersion (17,18). Several p-channel (hole-transporting) organic wires prepared by solution-processing have exhibited Ͼ 0.1 cm 2 /Vs for singlewire transistors (19)(20)(21)(22), whereas only a few solution-synthesized n-channel organic wires have been reported, typically with low performance ( Ϸ10 Ϫ3 Ϫ 10 Ϫ2 cm 2 /Vs) (23,24).…”