Francesca Brunetti

Towards Tactile Sensing System on Chip for Robotic Applications

This paper presents the research on tactile sensing system on chip. The tactile sensing chips comprise of 5 × 5 array of Piezoelectric Oxide Semiconductor Field Effect Transistor (POSFET) devices and temperature sensors. The POSFET devices are obtained by spin coating piezoelectric polymer, poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), films directly on to the gate area of Metal Oxide Semiconductor (MOS) transistors. The tactile sensing chips are able to measure dynamic contact forces and temperatures. The readout and the data acquisition system to acquire the tactile signals are also presented. The chips have been extensively tested over wide range of dynamic contact forces and temperatures and the experimental results are presented. The paper also reports the research on tactile sensing chips with POSFET array and the integrated electronics.

Time-Resolved Response of Polymer Bulk-Heterojunction Photodetectors

We have fabricated a polymer bulk-heterojunction photodetector from a (1:0.7) blend solution of poly(3-hexyl-thiophene) (P3HT) and 1-(3-methoxy-carbonyl)propyl-1-phenyl-(6, 6)C-61 (PCBM) in o-dichlorobenzene. Absorption spectrum, external quantum efficiency and current-voltage (I-V) curves, both in dark condition and under illumination of a narrowband green light-emitting diode (green-LED) with λem,peak center at 525 nm, were measured to characterize the photodiode behavior. Moreover, the time-resolved response under the green-LED stimulus modulated at 200 KHz has been measured. This result suggests the application of our device as organic photodetector for all-organic optical data link in low-cost plastic-optical-fiber-based systems.

Study of the Influence of Transverse Velocity on the Design of Cold Cathode-Based Electron Guns for Terahertz Devices

This paper presents the design of an electron gun for terahertz vacuum tubes realized with a nanowire or nanotube cold cathode. This technology is particularly interesting in this field since it allows the reduction of gun dimensions and of power consumption with respect to the thermionic guns. We found that focusing is a rather critical aspect since cold cathodes show a relevant transverse velocity, which increases the divergence of the beam and typically induces high current losses on the anode. An analysis of the parameters that influence the transverse velocity has been performed together with the identification of a proper design procedure that reduces the transverse velocity effects. Different electron guns have been designed that deliver a circular beam in the range of 1-4 mA with a radius in the range of 30 μm.

Photonic Flash Sintering of Ink-Jet-Printed Back Electrodes for Organic Photovoltaic Applications.

A study of the photonic flash sintering of a silver nanoparticle ink printed as the back electrode for organic solar cells is presented. A number of sintering settings with different intensities and pulse durations have been tested on both full-area and grid-based silver electrodes, using the complete emission spectrum of the flash lamps from UV-A to NIR. However, none of these settings was able to produce functional devices with performances comparable to those of reference cells prepared using thermally sintered ink. Different degradation mechanisms were detected in the devices with a flash-sintered back electrode. The P3HT:PCBM photoactive layer appears to be highly heat-sensitive and turned out to be severely damaged by the high temperatures generated in the silver layer during the sintering. In addition, UV-induced photochemical degradation of the functional materials was identified as another possible source of performance deterioration in the devices with grid-based electrodes. Reducing the light intensity does not provide a proper solution because in this case the Ag electrode is not sintered sufficiently. For both types of devices, with full-area and grid-based electrodes, these problems could be solved by excluding the short wavelength contribution from the flash light spectrum using a filter. Optimized sintering parameters allowed manufacture of OPV devices with performance equal to those of the reference devices. Photonic flash sintering of the top electrode in organic solar cells was demonstrated for the first time. It reveals the great potential of this sintering method for the future roll-to-roll manufacturing of organic solar cells from solution.

3-D Simulation and Optimization of Organic Solar Cell With Periodic Back Contact Grating Electrode

In this paper, we report an investigation of the optical and electrical properties of an organic solar cell (OSC) with a back contact grating architecture through 3-D numerical simulations. By using finite-element methods for both optical and transport properties, we have modeled the behavior of OSC with a grating architecture and compared with a conventional planar structure. Based on these optoelectrical simulations, we optimized the back contact grating, obtaining an increment of up to 17.5% in power conversion efficiency with respect to a planar structured OSC. This enhancement is the result of an increase of both short-circuit current and fill factor.

The European project OPTHER for the development of a THz tube amplifier

The European project OPTHER - Optically Driven THz Amplifiers, is focused on the realization of a THz amplifier. This device aims to overcome the actual limitations of THz sources in terms of emitted power, opening the way to the use of THz in monitoring security systems. Two alternative schemes are under study for amplification. In the first one (THz drive signal amplifier) the continuous wave electron beam emitted by the CNT cathode is modulated by the electric field in the interaction structure driven by a THz signal (generated by QCL or laser mixing). In the second one (optically modulated beam THz amplifier), the CNT cathode emits bunches of electrons at the THz rate. Those bunches are transmitted to the interaction structure and generate the amplified output signal.

Airbrush Spray Coating of Amorphous Titanium Dioxide for Inverted Polymer Solar Cells

One of the main topics of organic photovoltaics manufacturing is the need for simple, low cost, and large area compatible techniques. Solution-based processes are the best candidates to achieve this aim. Among these, airbrush spray coating has successfully applied to deposit both active and PEDOT layers of bulk-heterojunction solar cells. However, this technique is not yet sufficiently studied for interfacial layers (electron and hole transporting layers or optical spacers). In this paper, we show that amorphous titanium dioxide () films, obtained with an airbrush from a solution of titanium (IV) isopropoxide diluted in isopropanol, are successfully deposited on glass and PET substrates. Good surface covering results from the coalescence of droplets after optimizing the spray coating system. Simple inverted polymer solar cells are fabricated using as electron transporting layer obtaining encouraging electrical performances (% on glass/FTO and 0.7% on PET/ITO substrates).

Hybrid thermal-field emission of ZnO nanowires

The electron emission properties of an array of ZnO nanowires were studied in the temperature range of 300-473 K. An almost doubling of the current density at 473 K under an electric field of 8 V/μm (j(T=473 K) = 190 μA/cm2, j(T=300 K) = 114 μA/cm2) was observed together with a reduction of the turn-on field from 552 V/μm to 482 V/μm. Theoretical model that combines the thermal-field emission for high electric field and the Schottky emission for the low field can satisfactorily account for temperature dependence of current at low as well as at high applied bias. The obtained effect is particularly appealing for the application in micro-gun for THz vacuum tubes.

Electronic excitations in solution-processed oligothiophene small-molecules for organic solar cells

First principles calculations based on density functional theory and many body perturbation theory have been employed to study the optical absorption properties of a newly synthesized oligo-thiophene molecule, with a quaterthiophene central unit, that has been designed for solution-processed bulk-heterojunction solar cells. To this aim we have employed the GW approach to obtain quasiparticle energies as a pre-requisite to solve the Bethe-Salpeter equation for the excitonic Hamiltonian. We show that the experimental absorption spectrum can be explained only by taking into account the inter-molecular transitions among the π-stacked poly-conjugated molecules that are typically obtained in solid-state organic samples.

Cross-Bar Design of Nano-Vacuum Triode for High-Frequency Applications

In this letter, a new nano-vacuum triode based on carbon nanotubes (CNTs) has been designed. The use of CNTs as emitters with their extremely high aspect ratio and their characteristics to be patterned in specific emitting areas allowed the realization of a cross-bar geometry for which the transconductance is maximized and the grid-cathode capacitance is reduced. This allowed us to achieve a device cutoff frequency of 156 GHz, which is well beyond the state of the art.