Organic semiconductors based on small conjugated molecules generally behave as insulators when undoped, but the heterointerfaces of two such materials can show electrical conductivity as large as in a metal. Although charge transfer is commonly invoked to explain the phenomenon, the details of the process and the nature of the interfacial charge carriers remain largely unexplored. Here we use Schottky-gated heterostructures to probe the conducting layer at the interface between rubrene and PDIF-CN(2) single crystals. Gate-modulated conductivity measurements demonstrate that interfacial transport is due to electrons, whose mobility exhibits band-like behaviour from room temperature to ~150 K, and remains as high as ~1 cm(2) V(-1) s(-1) at 30 K for the best devices. The electron density decreases linearly with decreasing temperature, an observation that can be explained quantitatively on the basis of the heterostructure band diagram. These results elucidate the electronic structure of rubrene/PDIF-CN(2) interfaces and show the potential of Schottky-gated organic heterostructures for the investigation of transport in molecular semiconductors.

N-type organic field-effect transistors (OFETs), based on two perylene diimide semiconductors (PDI-8 and PDI-8CN2) exhibiting very different air sensitivities, have been fabricated on Si/SiO2 substrates. These OFETs have been electrically characterized in vacuum both in the dark and under white-light illumination by dc transfer and output curves, bias stress experiments and variable temperature measurements. In particular, the combination of variable temperature and light illumination experiments is shown to be a powerful tool to clarify the influence of charge trapping on the device operation. Even if, in vacuum, the air-sensitive PDI-8 devices display slightly better performances in terms of field-effect mobility and maximum current values, according to our results, charge transport in PDI-8 films is much more affected by charge trap states compared to PDI8-CN2 devices. These trapping centers are mainly active above 180 K, and their physical nature can be basically ascribed to the interaction between silanol groups and water molecules absorbed on SiO2 surface that is more active above the H2O supercooled transition temperature.