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During the last few years, the perylene tetracarboxylic diimide derivative PDIF-CN2 has been identified as one of the most interesting compounds to achieve air-stable n-type organic thin-film transistors. In this work, a detailed investigation on the properties of PDIF-CN2 thin films deposited by supersonic molecular beam deposition technique on HMDS-treated Si/SiO2 surfaces is reported. Our efforts have been addressed to analyze deeply the role played by the growth rate (R) on the thin film properties, working with the substrates kept at room temperature and using a kinetic energy of the impinging molecules of about 17 eV. In this way, we could observe that while, just after the deposition, the layers were not compact, being composed of small and mostly separated islands with a rate-dependent density, an unconventional spreading effect of the condensate took place over time until the formation of a continuous film. According to our results, this spontaneous wetting process occurs over very long time scales (i.e., from days to months) typical of a viscous phase and exhibits a dynamics also related to the initial growth rate. These remarkable morphological transitions are coherently accompanied by a several orders of magnitude increase in the field-effect charge carrier mobility, achieving maximum values of about 0.1 cm(2)V(-1)s(-1) in a specific R range. We have also found that a 1 h postannealing procedure at only 50 degrees C guarantees the rapid complete substrate wetting and the structural reassembly of the investigated films, providing comparable high mobility values.

In this work we report on fully operational sub-micrometric low voltage OFETs by using graphene as the source-drain electrodes pair and a high- κ ultra-thin dielectric in a local gate architecture. The impact of the graphene electrodes on the miniaturization of the organic devices has been assessed, with particular attention to the influence of the contact resistances as well as the parasitic overlap gate capacitance on the device bandwidth. By the use of a modified Transmission-Line-Method, contact resistances have been analyzed as function of the applied voltages, revealing characteristic functional trends that follow the doping state of graphene electrodes. Through impedance spectroscopy of the electrodes, cut-off frequencies as high as 105 Hz have been estimated, highlighting the peculiar role of quantum capacitance of graphene in such architectures.

Core-cyanated perylene diimide (PDI_CY) derivatives are molecular compounds exhibiting an uncommon combination of appealing properties, including remarkable oxidative stability, high electron affinities, and excellent self-assembling properties. Such features made these compounds the subject of study for several research groups aimed at developing electron-transporting (n-type) devices with superior charge transport performances. After about fifteen years since the first report, field-effect transistors based on PDI_CY thin films are still intensely investigated by the scientific community for the attainment of n-type devices that are able to balance the performances of the best p-type ones. In this review, we summarize the main results achieved by our group in the fabrication and characterization of transistors based on PDI8-CN2 and PDIF-CN2 molecules, undoubtedly the most renowned compounds of the PDI_CY family. Our attention was mainly focused on the electrical properties, both at the micro and nanoscale, of PDI8-CN2 and PDIF-CN2 films deposited using different evaporation techniques. Specific topics, such as the contact resistance phenomenon, the bias stress effect, and the operation in liquid environment, have been also analyzed.

We discuss the formation and post-deposition instability of nanodrop-like structures in thin films of PDIF-CN2 (a perylene derivative) deposited via supersonic molecular beam deposition technique on highly hydrophobic substrates at room temperature. The role of the deposition rate on the characteristic lengths of the organic nanodrops has been investigated by a systematic analysis of atomic force microscope images of the thin films and through the use of the height-height correlation function. The nanodrops appear to be a metastable configuration for the freshly-deposited films. For this reason, post-deposition wetting effect has been examined with unprecedented accuracy throughout a year of experimental observations. The observed time scales, from few hours to months, are related to the growth rate, and characterize the thin films morphological reordering from three-dimensional nanodrops to a well-connected terraced film. While the interplay between adhesion and cohesion energies favors the formation of 3D-mounted structures during the growth, wetting phenomenon following the switching off of the molecular flux is found to be driven by an instability. A slow rate downhill process survives at the molecular flux shutdown and it is accompanied and maybe favored by the formation of a precursor layer composed of more lying molecules. These results are supported by simulations based on a non-linear stochastic model. The instability has been simulated, for both the growth and the post-growth evolution. To better reproduce the experimental data it is needed to introduce a surface equalizer term characterized by a relaxation time taking into account the presence of a local mechanism of molecular correlation.

In this work we test graphene electrodes in nano-metric channel n-type Organic Field EffectTransistors (OFETs) based on thermally evaporated thin films of perylene-3,4,9,10-tetracarboxylic acid diimide derivative (PDIF-CN2). By a thorough comparison with short channel transistors made with reference gold electrodes, we found that the output characteristics of the graphene-based devices respond linearly to the applied biases, in contrast with the supra-linear trend of gold-based transistors. Moreover, short channel effects are considerably suppressed in graphene electrodes devices. More specifically, current on/off ratios independent of the channel length (L) and enhanced response for high longitudinal biases are demonstrated for L down to ~140 nm. These results are rationalized taking into account the morphological and electronic characteristics of graphene, showing that the use of graphene electrodes may help to overcome the problem of Space Charge Limited Current (SCLC) in short channel OFETs.

In this paper, Scanning Kelvin Probe Microscopy (SKPM) was applied in combination with standard current-voltage measurements to assess the contact resistance (RC) effects and to estimate the contact-free charge carrier mobility in n-type transistors based on evaporated films of PDIF-CN2. This analysis was carried out in particular as a function of the applied polarization voltages. The separate contributions to the RC phenomenon coming from the injecting (source) and extracting (drain) electrodes were investigated with the possibility to separately analyze the specific electrical behaviour of these two regions. In agreement with previous results, the experimental data here discussed show that the devices under analysis are not simply injected limited. Voltage drops at both the injecting and extracting contacts must be taken into consideration. However, it is worth to mention that for the bottom-contact devices here investigated, a minimum RC of 5.5 kΩ cm was found in linear regime. These values are significantly lower (i.e. by a factor 5×) than those previously reported for other perylene dimiide thin-film devices with bottom-contact configuration.

In this paper, nanoscale organic two-terminal devices, based on evaporated films of a perylene diimide derivative (PDI8-CN2) and with channel lengths ranging from 1 μm down to 50 nm, have been fabricated by using Focused Ion Beam technique. Devices have been characterized in terms of I–V curves at several temperatures and perylene diimide nanochannels displayed a measurable electrical conduction even at temperatures lower than 50 K. The recorded experimental data stressed the electrical response according to the channel size. Investigation of nanoscale devices allowed us to study the nature of the charge injection and to estimate the energy barrier and the factors influencing it.