Charge transport and injection in high mobility, solution-processed organic semiconductors / Plastic electronics (1,2).
The technological interest for solution-processed organic field-effect transistors (OFETs) has been steadily increasing in the recent past because of the demonstration of devices with mobilities higher than amorphous silicon. Yet fundamental aspects of the charge injection and transport mechanisms have to be fully understood, a mandatory requirement for the further development of this field. Research projects 1,2 are in this area.
Organic opto-electronic devices (3,4).
Molecular carbon based semiconductors have established a solid technology in the field of OLED, for display and lighting, as well in industrial coatings and bio-sensing. Coupled to non conventional techniques such as ink jet printing they offer alternative solution to a variety of technological challenges. The development of new opportunities for application rest on extensive fundamental research and some development, aimed at inventing, exploring and exploiting new phenomena and properties. Research projects 3,4 are in this area.
Photophysics of nanostructures for photovoltaic conversion and other photonic applications (5,7,8). The quantum confinement of electronic states leads to large oscillator strength for optical transitions which enhance light-matter interaction and offer a handle for controlling linear and non linear optical properties. The study of photophysical phenomena in nanostructure requires advanced and broad characterization techniques, including time resolved spectroscopy from fs to ms, space resolved spectroscopy and opto-electronic probing such as photoconductivity and STARK spectroscopy. Research projects 5,7,8 are in this area.
Growth of nanostructured inorganic substrate and active layers for photovoltaic applications (9).
This activity aims at developing an efficient, large scale deposition process for nanostructured thin films and nanoparticles of oxides and semiconductors.
The quest for ohmic contacts in OFETs
Ref.: Mario Caironi and Dario Natali. Contact resistance effects are one of the opened issues for the downscaling of OFETs and the widespread development of plastic electronics. Although several approaches have been proposed to minimize them, a general solution to achieve ohmic contacts has not been discovered yet. Local doping, which is commonly adopted in silicon transistors, could be the answer, but given the difficulty of its implementation in organic semiconductors, it has been regarded as the holy-grail. This challenging project is aimed at investigating viable approaches for the realization of ohmic contacts in OFETs. Local doping achieved by the insertion of non diffusive dopants, as well as organic or hybrid interlayers and the exploitation of advantageous architectures will be pursued.
High resolution mapping of charge density in organic field-effect transistors by means of confocal charge-modulation spectroscopy Ref.: Mario Caironi and Calogero Sciascia Charge transport mechanism in organic semiconductors is still subject of debate. A direct observation of charge density within the channel of a transistor at different bias voltage and its correlation to materials morphology and electronic properties could help to shine light on the subject. Charge carriers in organic semiconductors show typical polaronic optical transitions that can be detected in charge modulation-spectroscopy (CMS) experiments. By combining CMS with a confocal setup, a high resolution mapping of the charge density in OFETs can be achieved. This project aims to test and develop this powerful technique on different organic systems with the goal to obtain important information on the physics of charge transport in organic semiconductors and provide strong experimental evidence for the development of suitable models.
Printed organic active matrix for X-rays imaging application
Ref.: Dario Natali and Mario Caironi The goal of the activities in this area is to develop a digital X-ray image sensor array, with the aim of replacing the slow, conventional film-based X-ray process with an instantly digitized X-ray image. Printed organic semiconductors nicely fit the requirement for light-weight and large-area devices, as needed for bio-medical applications (e.g. mammography). The strategy is to develop an organic photodetector, one of the main building blocks, with suitable sensitivity range, coupled to a scintillator. To do this and to fulfil the requirement for a cost-effective patterning of arrays on large-areas, additive printing techniques, such as inkjet printing have to be adopted and developed. The realization of an active matrix requires also the development of suitable addressing switches, such as transistors. Therefore effective architectures and techniques for the printing of OFETs have to be investigated with the final scope to integrate the detector and the switch in an active pixel to be uniformly replicated in a matrix.
Bio-inspired organic photodetectors array
Ref. Maria Rosa Antognazza and Mario Caironi. The project concerns the realization of an organic photodetectors array, characterized by a not-uniform distribution of the pixels, closely resembling the arrangement of human photoreceptors inside the retina. This should ensure a high visual acuity and at the same time a huge field of view, with interesting applications in robotics, remote surveillance, image processing, color appearance science and biological sensing. For the realization of the variable geometry state-of-the art lithographic and ink-jet technologies will be considered; the device will be then extensively characterized in terms of its optoelectronic properties. Finally, the implementation of the visual sensor in a humanoid robot can be envisaged.
Photophysics of nanostructures for photovoltaic conversion
Ref. Annamaria Petrozza. The PhD research project aims to investigate energy and charge transfer processes at organic/inorganic and fully organic interfaces in nanostructures for photovoltaic applications based on colloidal semiconductor nanocrystals, conjugated polymers and metal oxides of different nature. Combining the use of both advanced spectroscopic techniques such as time-resolved Photoluminescence and Photo-induced Absorption spectroscopy and electrical characterizations, light has to be shed on the fundamental working mechanisms of light harvesting and charge generation; devices structures will be designed, fabricated and optimized consequentially.”
Bio-organic functional interfaces
Ref. Maria Rosa Antognazza. The project concerns the demonstration of new methods for recording and stimulating the activity of in-vitro neural networks, and it will be carried out in strong collaboration with the Neuroscience and Brain Technologies Dept. of IIT (main quarter in Genova). Organic semiconductors will be directly interfaced to living tissues, serving as active materials for signal transduction, from the artificial to the biological world and vice versa. The employment of organic technology might help to overcome the limitations of existing devices, in terms of biocompatibility, electrode invasiveness, mechanical stiffness and spatial-temporal stimulation patterns. Innovative techniques for optical stimulation and recording will be explored as well.
Excited state dynamics and non linear optics in nanostructures
Ref. Krishna Vishunubhatla, Guglielmo Lanzani, Moreno Meneghetti. Due to quantum confinement of the electronic states both molecular materials and nanostructures have large non linear optical responses that can be exploited in optical gain, optical switching, saturable absorption and lasing applications. Principal investigation techniques use ultrashort light pulses for generating and probing excited state dynamics. Excited state non linear properties are enhanced in comparison to ground state properties, suggesting a new paradigm for photonics based on photo-induced non-linearity to be investigated by a combination of nanosecond and femtosecond excitation in solution, possibly through micro-fluidic channels and lab on chip technology.
Excited state dynamics in nano-crystals
Ref. Francesco Tassone. In collaboration with Liberato Manna, at IIT Genoa, state of art multi size and multi shaped nanocrytals of inorganic semiconductors are available for through characterization by optical spectroscopy. The PhD work will focus on the study of the properties of the hybrid interface, and on its influence on the optical and electronic properties of the single nanocrystals and of their hybrid assemblies. Characterization will be carried out using several instrumental techniques available at the Institute, combining the study of the ultra-fast dynamics of the optically injected carriers in the nanocrystals (either isolated, organically capped, or in hybrid assembly), with other optical techniques focusing on the charge carrier recombination on long times. Interpretation of results of charge transfer dynamics, carrier localization and influence of defect states will be supplemented by modeling of the optical and electronic properties of these systems, which may be extended from simple phenomenological models up to more detailed molecular-like approaches based on electronic structure calculations.
Production of thin films and nanostructures of oxides and semiconductors by means of a supersonic reactive plasma jet.
Ref. Fabio Di Fonzo. The novel, patented, process is based on the segmentation of the gas phase material synthesis in two separate steps: chemistry control in a reactive cold plasma environment; nucleation and assembling control by means of a supersonic inseminated jet over a substrate. By tuning deposition parameters it is possible to obtain either dense or mesoporous films. The overall structure at the nanoscale is controlled too by the extraction parameters. Particular emphasis will be put on the control of the nanoscale architecture of the materials produced (i.e. hierarchical quasi-1D structures). A thorough understanding of the fundamental physical properties of the so produced materials is an integral part of the proposed studies.
Modeling and optimization of the plasma jet source (plasma chemistry, flow dynamics, cluster nucleation, thin film growth
Mesoporous hierarchical structures of metal-oxides (i.e. TiO2, ZnO, SnO) for Organic, Hybrid and Dye Sensitized solar cells.
Quantum dots and nanostructured films of group IV semiconductors (i.e. Ge and Si).
Growth of nanostructured surfaces and interfaces for energy harvesting applications. Ref. Paola Bruno. The PhD activity aims at developing an efficient, large scale deposition process for quantum dots and nanostructured films of group IV semiconductors (i.e. Si). The work will focus on the development of a series of composite nanostructured architectures that sees layered semiconductors materials and low dimension (QD) systems assembled together by physical deposition growth techniques (i.e. Pulsed Laser Deposition (PLD), thermal evaporation, magnetron sputtering and so on) in a structure that would exhibit a combined multitude of potential bulk and surface properties and makes them inherently multifunctional and attractive in a number of energy-related technologies (solar cells, thermoelectrics and so on). The choice of the method should open a promising route for the simultaneous or multi-step synthesis of QDs within a layered structure with controlled size, compositional control and properties tailoring. An important part of the PhD program will be dedicated to the material characterization using several instrumental techniques available, combining the use of Scanning Electron Microscopy also equipped with energy dispersive X-ray (EDX) and backscattering electron analyzers, X-ray Diffraction analysis, inelastic light scattering techniques (Raman spectroscopy), Photoluminescence and electrical characterizations for an elucidative and complete understanding of the morphological, optical, electrical and transport properties of these nanostructured systems. Initially efforts will be made for obtaining materials with characteristic surface and interfaces properties that make them excellent candidates for the development of high efficient PV devices.
(1) The PhD program will follow the organization of the School of Doctoral Programs of Politecnico di Milano (see http://www.ricerca.polimi.it/index.php?id=2467 or http://pcsiwa.rett.polimi.it/~phdweb/eng/ for details).
Duration: 3 years, starting from January 1st, 2012
for Phd admission : Applicants that hold a “Vecchio ordinamento” Laurea (Master of Science, MSc, previous to DM 509/99), Postgraduate Degree/MA or MSc or any similar academic qualification from Universities abroad, can be admitted to the PhD without any limits of age and citizenship. Admission is selective, subject to skills and inclination to research. The large majority of the PhD candidates are supported by scholarships but admission is also possible without any financial support, conditioned on admission and on position availability.
Laurea (MSc) degree “Vecchio ordinamento” or “specialistica / magistrale“ obtained from an Italian University, or equivalent academic title obtained from foreign University.
Knowledge of the English language at B2-level or higher
How to apply: Interested candidates will apply to the Politecnico di Milano PhD school (instructions can be found on the website) and if successfully, be approved by the Politecnico committee.
- Engineering oxides at the nanoscale: from isolated clusters to 3-D assemblies;
For further info, please check the application deadline on the Polimi website