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The aim of this thesis is the elucidation of structure-properties relationship of molecular semiconductors for electronic devices. This involves the use of a comprehensive set of simulation techniques, ranging from quantum-mechanical to numerical stochastic methods, and also the development of ad-hoc computational tools. In more detail, the research activity regarded two main topics: the study of electronic properties and structural behaviour of liquid crystalline (LC) materials based on functionalised oligo(p-phenyleneethynylene) (OPE), and the investigation on the electric field effect associated to OFET operation on pentacene thin film stability. In this dissertation, a novel family of substituted OPE liquid crystals with applications in stimuli-responsive materials is presented. In more detail, simulations can not only provide evidence for the characterization of the liquid crystalline phases of different OPEs, but elucidate the role of charge transfer states in donor-acceptor LCs containing an endohedral metallofullerene moiety. Such systems can be regarded as promising candidates for organic photovoltaics. Furthermore, exciton dynamics simulations are performed as a way to obtain additional information about the degree of order in OPE columnar phases. Finally, ab initio and molecular mechanics simulations are used to investigate the influence of an applied electric field on pentacene reactivity and stability. The reaction path of pentacene thermal dimerization in the presence of an external electric field is investigated; the results can be related to the fatigue effect observed in OFETs, that show significant performance degradation even in the absence of external agents. In addition to this, the effect of the gate voltage on a pentacene monolayer are simulated, and the results are then compared to X-ray diffraction measurements performed for the first time on operating OFETs.

none 10 si Organic semiconductors are intensively studied as promising materials for the realisation of low-cost flexible electronic devices. The flexibility requirement implies either performance stability towards deformation, or conversely, detectable response to the deformation itself. The knowledge of the electromechanical response of organic semiconductors to external stresses is therefore not only interesting from a fundamental point of view, but also necessary for the development of real world applications. To this end, in this work we predict and measure the variation of charge carrier mobility in rubrene single crystals as a function of mechanical strain, applied selectively along the crystal axes. We find that strain induces simultaneous mobility changes along all three axes, and that in some cases the response is higher along directions orthogonal to the mechanical deformation. These variations cannot be explained by the modulation of intermolecular distances, but only by a more complex molecular reorganisation, which is particularly enhanced, in terms of response, by π-stacking and herringbone stacking. This microscopic knowledge of the relation between structural and mobility variations is essential for the interpretation of electromechanical measurements for crystalline organic semiconductors, and for the rational design of electronic devices. mixed Matta, Micaela; Pereira, Marco José; Gali, Sai Manoj; Thuau, Damien; Olivier, Yoann; Briseno, Alejandro; Dufour, Isabelle; Ayela, Cedric; Wantz, Guillaume; Muccioli, Luca Matta, Micaela; Pereira, Marco José; Gali, Sai Manoj; Thuau, Damien; Olivier, Yoann; Briseno, Alejandro; Dufour, Isabelle; Ayela, Cedric; Wantz, Guillaume; Muccioli, Luca

none 3 no Electric field; Density functional theory; organic field effect transistors; dimerization process Density functional theory (DFT) calculations were used to assess the effect of electric fields of varying magnitudes and directions on the molecular structure of pentacene and other acenes. The aim is to understand the response of acenes in organic field effect transistors, specifically the structure of the first monolayer(s) deposited on the gate dielectric, where the transversal electric field and the charge carrier density are largest and charge transport occurs. Pentacene cycloaddition can be enhanced by the application of electric fields oriented along the direction of the forming bonds. Dimerization is likely to occur in low-density, disordered domains, such as grain boundaries or terrace edges. Together with other factors, dimerization could affect device performance leading to an irreversible decrease of mobility due to the creation of new trap states. Matta, Micaela; Biscarini, Fabio; Zerbetto, Francesco Matta, Micaela; Biscarini, Fabio; Zerbetto, Francesco

Micro-electromechanical systems (MEMS) made of organic materials have attracted efforts for the development a new generation of physical, chemical, and biological sensors, for which the electromechanical sensitivity is the current major concern. Here, we present an organic MEMS made of a rubrene single-crystal air-gap transistor. Applying mechanical pressure on the semiconductor results in high variations in drain current: an unparalleled gauge factor above 4000 has been measured experimentally. Such a high sensitivity is induced by the modulation of charge injection at the interface between the gold electrode and the rubrene semiconductor as an unusual transducing effect. Applying these devices to the detection of acoustic pressure shows that force down to 230 nN can be measured with a resolution of 40 nN. This study demonstrates that MEMS based on rubrene air-gap transistors constitute a step forward in the development of high-performance flexible sensors.

The charge transport properties of ten group 14 phthalocyanine crystals are investigated by means of kinetic Monte Carlo simulations based on experimental X-ray structures. All investigated materials are predicted to exhibit an ambipolar semiconducting behavior, with hole and electron mobilities lying in the range 0.1–1 cm2 V–1 s–1, showing their potential for organic electronic devices. The simulations also provide evidence that the dimensionality of charge transport in these materials can be finely tuned by substituting the phenoxy axial groups with fluorine atoms and by varying their number and positions; a complete substitution gives rise to two-dimensional transport for both electrons and holes. Most remarkably, one of the investigated compounds that incorporates iodine-phenoxy groups as axial substituents exhibits the largest mobilities for both electrons and holes, owing to large intermolecular couplings and low reorganization energies, and thus emerges as a highly promising one-dimensional semiconductor.

In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (100) facet of the semiconductor organic oligomer ( 2 Z , 2 ′ Z ) − 3 , 3 ′ -(1,4-phenylene)bis(2-(4-butoxyphenyl)acrylonitrile) ( β -DBDCS) crystal in water at room temperature. Stick-slip contrast, lateral contact stiffness, and friction forces are found to depend strongly on the sliding direction due to the anisotropic packing of the molecular chains forming the crystal surface along the [010] and [001] directions. The anisotropy also causes the maximum value of the normal force applicable before wearing to increase by a factor of 3 when the scan is performed along the [001] direction on the (100) face. Altogether, our results contribute to achieving a better understanding of the molecular origin of friction anisotropy on soft crystalline surfaces, which has been often hypothesized but rarely investigated in the literature., This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) (Projects No. MAT2012-38810 and No. CTQ2014-58801), by the Comunidad de Madrid (project Mad2D, Grant No. S2013/MIT- 3007), by the Campus of International Excellence (CEI) UAM+CSIC, and by theNational Research Foundation ofKorea (NRF) through a grant funded by the Korean government (MSIP; Grant No. 2009-0081571).

© 2015 American Chemical Society. Thin films of organic semiconductors have been widely studied at different length scales for improving the electrical response of devices based on them. Hitherto, a lot of knowledge has been gained about how molecular packing, morphology, grain boundaries, and defects affect the charge transport in organic thin film transistors. However, little is known about the impact of an electric field on the organic semiconductor microstructure and the consequent effect on the device performances. To fill this gap, we investigated the evolution of the structure of pentacene thin film transistors during device operation by in situ real time X-ray diffraction measurements and theoretical calculations. We observed for the first time the occurrence of a reversible structural strain taking place during the bias application mainly due to reorientation at the terrace edges of monolayer islands under the effect of electrical field. Strain exhibits the same trend of the threshold voltage hinting to the existence of a direct correlation between the phenomenon of bias stress and the structural modification., The authors acknowledge A. Shehu for his collaboration at the early stage of the project, F. Borgatti for the fruitful discussion, J. R. Plaisier, G. Zerauschek and A. Lausi for optimizing the experimental setup at the MCX-ELETTRA beamline, and V. L.R. Jacques and P. Evans for technical support at the ID01-ESRF beamline. Financial support for this research was by the National Project N−CHEM, Flagship NANOMAX, by the DGI (Spain) with Project BE-WELL CTQ2013-40480-R, Generalitat de Catalunya (2014-SGR-17), the ERC StG 2012-306826, and by the Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBERBBN).