The electronic properties of interfaces between two different solids can differ strikingly from those of the constituent materials. For instance, metallic conductivity-and even superconductivity-have recently been discovered at interfaces formed by insulating transition-metal oxides. Here, we investigate interfaces between crystals of conjugated organic molecules, which are large-gap undoped semiconductors, that is, essentially insulators. We find that highly conducting interfaces can be realized with resistivity ranging from 1 to 30 kohms per square, and that, for the best samples, the temperature dependence of the conductivity is metallic. The observed electrical conduction originates from a large transfer of charge between the two crystals that takes place at the interface, on a molecular scale. As the interface assembly process is simple and can be applied to crystals of virtually any conjugated molecule, the conducting interfaces described here represent the first examples of a new class of electronic systems.

Single-crystal field-effect transistors (FETs) based on a fluorocarbon-substituted dicyanoperylene-3,4:9,10-bis(dicarboximide) [PDIF-CN(2)] were fabricated by lamination of the semiconductor crystal on Si-SiO(2)/PMMA-Au gate-dielectric-contact substrates. These devices were characterized both in vacuum and in the air, and they exhibit electron mobilities of ca. 6-3 and ca. 3-1 cm(2) V(-1) s(-1), respectively, I(on):I(off) > 10(3), and near-zero threshold voltage.

We report the observation of ambipolar transport in field-effect transistors fabricated on single crystals of copper-and iron-phthalocyanine, using gold as a high work-function metal for the fabrication of source and drain electrodes. In these devices, the room-temperature mobility of holes reaches 0.3 cm 2 / V s in both materials. The highest mobility for electrons is observed for iron-phthalocyanines and is approximately one order of magnitude lower. Our measurements indicate that these values are limited by extrinsic contact effects due to the transistor fabrication and suggest that considerably higher values for the electron and hole mobility can be achieved in these materials.

We investigated transport properties of organic heterointerfaces formed by single-crystals of two organic donor-acceptor molecules, tetramethyltetraselenafulvalene and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Whereas the individual crystals have unmeasurably high resistance, the interface exhibits a resistivity of few tens of megohm with a temperature dependence characteristic of a small gap semiconductor. We analyze the transport properties based on a simple band diagram that naturally accounts for our observations in terms of charge transfer between two crystals. Together with the recently discovered tetrathiafulvalene–TCNQ interfaces, these results indicate that single-crystal organic heterostructures create functional electronic systems with properties relevant to both fundamental and applied fields

We have investigated the contact resistance of rubrene single-crystal field-effect transistors ͑FETs͒ with nickel electrodes by performing scaling experiments on devices with channel length ranging from 200 nm up to 300 m. We find that the contact resistance can be as low as 100 ⍀ cm with narrowly spread fluctuations. For comparison, we have also performed scaling experiments on similar gold-contacted devices, and found that the reproducibility of FETs with nickel electrodes is largely superior. These results indicate that nickel is a very promising electrode material for the reproducible fabrication of low resistance contacts in organic FETs. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2185632͔The possibility of downscaling organic field-effect transistors ͑FETs͒ is currently hindered by the high contact resistance present at the interface between the metal electrodes and the organic semiconductor.1 One of the main experimental problems in the study and optimization of the contact resistance originates from the observed irreproducibility. In spite of the large effort put in the investigation of contact effects, 1-6 the reason for both the high values and the irreproducibility of the contact resistance are not currently understood. Many different phenomena are likely to play an important role, including the presence of grain boundaries at the metal-organic interfaces, the interface fabrication process ͑e.g., metal diffusion into the organic semiconductors and extrinsic damage introduced during the device assembly process͒, fluctuations in the work function of the metal electrodes, etc. Currently, the problem seems to be particularly severe for oligomer-based devices. Whereas for FETs based on a number of different polymers it has been found that the contact resistance scales linearly with the carrier mobility, 7 for transistors based on oligomers a very broad range of contact resistance values has been measured on identically prepared devices, and no systematic behavior has been observed.