High Electron Mobility in Vacuum and Ambient for PDIF-CN2 Single-Crystal Transistors

We have investigated the electron mobility on field-effect transistors based on PDIF-CN$_{2}$ single crystals. The family of the small molecules PDI8-CN$_{2}$ has been chosen for the promising results obtained for vapour-deposited thin film FETs. We used as gate dielectric a layer of PMMA (spinned on top of the SiO$_{2}$), to reduce the possibility of electron trapping by hydroxyl groups present at surface of the oxide. For these devices we obtained a room temperature mobility of 6 cm$^{2}$/Vs in vacuum and 3 cm$^{2}$/Vs in air. Our measurements demonstrate the possibility to obtain n-type OFETs with performances comparable to those of p-type devices.Comment: published online in JAC

Influence of the dielectric and of the active layer doping on the FET mobility in PPV-based devices

We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on Poly-phenylenevinylene derivatives. We observe a direct influence of the dielectric surface on the field-effect mobility as well as on the charge injection at the source electrode, despite the fact that we used a top contact transistor structure. We find that the presence of traps at the dielectric surface, decreases the hole mobility and increases the threshold voltages. By treating the silicon dioxide dielectric surface with gas phase molecules such as octadecyltrichlorosilane (OTS) and hexamethyldisilazane (HMDS) the hole mobility improves and the threshold voltage slightly increases. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related to the density of the oxydryl groups (-OH), which are the most efficient traps for the charges flowing in the device. We use different polymer species such as polyvinylalchol (PVA), polymethylmetacrilate (PMMA) and a cyclotene derivative (B-staged bisbenzocyclobutene or BCB). The elimination of the -OH groups and of other traps, produces the same effect observed with HMDS coupled to a more pronounced enhancement of the threshold voltage, with the exception of PMMA. The electrical characteristics obtained with HMDS and PMMA polymer dielectrics are the highest reported to date for PPV-based field-effect transistors. We confirm that the purification of the active material is crucial to enhance the device performances and to achieve a better device to device reproducibility. We also investigated the effect of the dispersion of a phosphorescent dye into the active polymeric material. The electrical characteristics of OFETs with HMDS or PMMA dielectric with and without dye doping are compared

Correlation between dielectric/organic interface properties and key electrical parameters in PPV-based OFETs

none8We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on polyphenylenevinylene derivatives. Through a systematic investigation of the most common dielectric surface treatments, a direct correlation of their effect on the field-effect electrical parameters, such as charge carrier mobility, On/Off current ratio, threshold voltage, and current hysteresis, has been established. It is found that the presence of OH groups at the dielectric surface, already known to act as carrier traps for electrons, decreases the hole mobility whereas it does not substantially affect the other electrical characteristics. The treatment of silicon dioxide surfaces with gas phase molecules such as octadecyltrichlorosilane and hexamethyldisilazane leads to an improvement in hole mobility as well as to a decrease in current hysteresis. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related not only to the OH groups density but also to the interaction between the dielectric and the semiconductor molecules. Specifically, the elimination of the OH groups produces the same effect observed with hexamethyldisilazane. The hole mobility values obtained with hexamethyldisilazane and polymer dielectrics are the highest reported to date for PPV-based field-effect transistors.noneTODESCATO F; CAPELLI R; DINELLI F; MURGIA M; CAMAIONI N; YANG MJ; BOZIO R.; MUCCINI MTodescato, Francesco; Capelli, R; Dinelli, F; Murgia, M; Camaioni, N; Yang, Mj; Bozio, Renato; Muccini, M

Dimers of polar chromophores in solution: role of excitonic interactions in one- and two-photon absorption properties

Abstract: The possibility to exploit a bottom-up approach to design and synthesize multichromophoric structures from a single molecular unit is strategic for the targeted synthesis of molecular compounds with well defined linear and nonlinear absorption properties. In this view, it is important to be able to predict the properties of multichromophoric units, based on the knowledge of the properties of the individual chromophores and their mutual arrangement. To this end, we present a combined experimental and theoretical study on 4-(para-di-n-butylaminostyryl)-pyridine, a push-pull molecule, and its dimer, 4,4'-bis(para-di-n-butylaminostyryl)-2,2'-bipyridine, formed by connecting the two pyridine groups into a bipyridine structure. One photon absorption and fluorescence spectra are measured in solvents of different polarity, and two-photon absorption spectra are recorded in dichloromethane. Experimental results are compared with results of TDDFT (Time-Dependent Density Functional Theory) and CIS (Configuration Interaction with Single excitation) methods implemented in the Gaussian03 program suite. An essential-state analysis of optical spectra is used to rationalize the observed behavior

Synthesis, characterization, and transistor response of tetrathia-[7]-helicene precursors and derivatives

In this paper we report polycyclic, fully-conjugated, heteroaromatic compounds 1-5, two of them having a helical shape (tetrathia-[7]-helicenes 1 and 2), and explore their potential use as p-type semiconductors for organic thin-film transistors (OTFTs). Furthermore, we describe their optical absorption, emission, and electrochemical properties. In addition, the single crystal X-ray diffraction structure for compound 3 is reported. Thin films of all compounds were fabricated on several substrates at different temperatures by physical vapor deposition, and were characterized by optical spectroscopy, wide-angle X-ray diffraction, atomic force microscopy, and TFT measurements. The AFM images of compound 1 exhibit an interesting tubular morphology for films deposited at room temperature. This is the first observation of a compound with a helical shape leading to a nanowire-like morphology. In agreement with the molecular structure/morphology, only compound 4 is a p-type semiconductor with good carrier mobility

Villes et aires de route dans le processus de formation de l'\uc9tat territorial (Toscane, XIIIe-XVIe si\ue8cles)

In a densely populated region like Tuscany, at least up to the first decades of the 14th century, the control and the organization of a stable and coherent road network was one of the main stakes in the development process of a first state structure. The hamlets and the big centers of the Tuscany countrysides where thus integrated into a communication system that had to be funcional to the cities in the strictes sense of the word (civitates) from a political, economic and demographic point of view. The originated a policy of road management that, first of all, led the subject communities to the maintenance, the supervision and the stabilization of a stable road network, whic was defined by the juridical and political legitimateness of the leading communal villas; it emphasized even more, on the one hand, the asymmetry of the connection between the cities and the hinterland and, on the other hand, the reactions of the communities of tuscans "minor centers".Dans une r\ue9gion \ue0 haute densit\ue9 d\ue9mographique, aumoins jusqu\u2019aux premi\ue8res d\ue9c\ue9nnies du XIVe si\ue8cle, le contr\uf4le et l\u2019organisation d\u2019un reseau routier stable et coherent fut l\u2019un des enjeux principaux dans le processus de formation d\u2019une premi\ue8re structure \ue9tatique. Les bourgades et les grands centres des campagnes toscanes furent ainsi integr\ue9s dans un sist\ue8me de comunications qui, d\u2019un point de vue politique, \ue9conomique et d\ue9mographique \ue9tait fonctionnel aux villes proprement dites (civitates). Cela fut \ue0 l\u2019origine d\u2019une politique en mati\ue8re de gestion des routes qui, en premier lieu, entraina les communaut\ue9s sujettes dans l\u2019entretien, la surveillance et la consolidation d\u2019un reseau stable et defini par la l\ue9gitimation juridique et politique des villes communales dominantes faisant ressortir encore plus d\u2019un cot\ue8 l\u2019asym\ue9trie du rapport entre centre et arri\ue8re pays et, de l\u2019autre, les r\ue9actions des communaut\ue9s des \uabcentri minori\ubb toscans