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Scientific Highlights

Quantum-Chemical Insights into the Prediction of Charge Transport Parameters for an Enhanced Electron Mobility Copolymer

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The synthesis of high-performance n-type polymers - electron transport (ET) materials - is paving the way for the development of innovative all-organic photovoltaic (OPV) cells, n-channel organic field-effect transistors (OFETs), and organic complementary logic circuits, which require both p- and n-type components. We recently investigated the charge transport processes in a high-mobility n-channel organic field-effect transistors (OFETs) based on poly{[N,N̕-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5̕-(2,2̕-bithiophene)} (P(NDI2ODT2). For this study a theoretical modeling by density functional theory (DFT) and time dependent (TD)-DFT, has been applied. The study focuses on the main parameters involved in the charge transfer processes, such as the reorganization energy and the charge transfer integrals. Both hole and electron charge transfer properties have been worked out. Our theoretical investigation fully support the parallel experimental study (e.g. variable-temperature electrical measurements and charge-modulation spectroscopy), thus revealing an unusually uniform energetic landscape of sites for charge-carrier transport along the channel of the transistor and the importance of the reorganization energy in determining the activation energy for the electron carrier mobility.

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Hierarchical TiO2 photoanode for Dye-Sensitized Solar Cells

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We exploit the self assembly of elemental building blocks from the gas-phase onto a substrate, to develop novel nanostructured materials with controlled properties from the atomic to the meso-scale. The prototypical structure we study resembles a forest of micron-sized quasi-1D nanostructures with a hierarchical organization at different scales, i.e. nanocrystalline domains (and grain boundaries) assembled in a dendritic superstructure with micrometer long 'trees' showing submicrometer organization (size, spacing) and a multirange porosity. Composition, doping, criystallinity, functionalization, size and spacing of the nanoforest can be tailored in order to obtain a desired property: charge transport, optical properties, surface area, etc.

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Microspectroscopy techniques for mapping the opto-electronic properties of organic thin-film transistors

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Solution processable organic field-effect transistors (OFETs) have recently reached a mature stage that preludes their adoption in a variety of commercial applications from light-weight, stretchable, large-area sensors to low-cost, flexible electronic circuits. The optimization of performances required for such applications demands a deep understanding of the device physics: with the aim to investigate the relationship between thin films micro-structure and charge transport properties, we developed a probing technique capable of providing local information regarding field and charge distribution along the channel of a working device. By coupling modulation spectroscopy techniques to a confocal microscope, we are able to image the opto-electronic properties of organic semiconductors with a lateral resolution of about 500 nm.

As we recently reported, exploiting Charge Modulation Microscopy it is possible to image the charge distribution inside the channel of an n-type organic transistor based on the high-mobility polymer poly{[ N , N' -bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (Polyera's ActivInk™ N2200). Our method allows to directly probe buried layers in a transparent device and does not require highly controlled test conditions (e.g. ultrahigh-vacuum). Interestingly, we unveiled a sub-micrometer texture of charge modulation features along the channel of the transistor, which were attributed to the packing motif of the polymer chains in the nanometer-thick accumulation layer of the device. These important evidences provide us with a deeper insight in charge transport properties in a relevant class of semiconducting materials.

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Organic-based artificial retina

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Nowadays the total loss of vision capability, related to photoreceptors malfunctioning or degeneration (due to aging effects or genetic origins), affects more than 30 million people all over the "developed" countries, and this number is expected to raise in the next 10-20 years because of the population aging trend. Currently, there is no mean to restore vision in blind patients; existing artificial visual prosthesis are affected by serious limitations, especially in terms of biocompatibility and achievable visual acuity.

We propose opto-neural interfaces based on light-sensitive organic semiconductors that enable the transduction of the information carried by light into specific patterns of electrical activity in a neuronal network. This would allow the realization of an implantable organic prosthetic device for the treatment of genetic degenerative diseases in patients affected by photoreceptor degeneration, such as retinitis pigmentosa and macular degeneration. First step of this highly multidisciplinary project was the characterization of the organic polymer-liquid interface; the second step consisted in the in vitro proof of concept experiment demonstrating the possibility of photostimulation in a network of primary neurons grown on top of the organic semiconductor.

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