Luigi Salamandra

Fluoro-functionalization of vinylene units in a polyarylenevinylene for polymer solar cells

A low band-gap copolymer PDTBTFV alternating bis-thienyl-(bis-alkoxy)-benzothiadiazole blocks with difluorovinylene units and its non-fluorinated counterpart PDTBTV have been synthesized and characterized as donor materials in bulk heterojunction (BHJ) solar cells with PCBM as the acceptor. The solar cells with the fluorinated polymer show better photovoltaic performances than those recorded with the non-fluorinated material. Comparative spectroscopic and computational studies, together with morphological, electrical and optical characterization of thin films, have been carried out to shed light on the reasons for the improvement of performances as induced by the double bond fluorination. Our study introduces the fluorinated double bond as a new conjugated unit in donor polymers for BHJ solar cells.

Atomistic Modeling of Gate-All-Around Si-Nanowire Field-Effect Transistors

We report atomistic simulations of the transport properties of Si-nanowire (SiNW) field-effect transistors. Results have been obtained within a self-consistent approach based on the nonequilibrium Greens function (NEGF) scheme in the density functional theory framework. We analyze in detail the operation of an ultrascaled SiNW channel device and study the characteristics and transfer characteristics behavior of the device while varying several parameters including doping, gate and oxide lengths, and temperature. We focus our attention to the quantum capacitance of the SiNW and show that a well-tempered device design can be accomplished in this regime by choosing suitable doping profiles and gate contact parameters.

Low-temperature synthesis of carbon nanotubes on indium tin oxide electrodes for organic solar cells

Summary The electrical performance of indium tin oxide (ITO) coated glass was improved by including a controlled layer of carbon nanotubes directly on top of the ITO film. Multiwall carbon nanotubes (MWCNTs) were synthesized by chemical vapor deposition, using ultrathin Fe layers as catalyst. The process parameters (temperature, gas flow and duration) were carefully refined to obtain the appropriate size and density of MWCNTs with a minimum decrease of the light harvesting in the cell. When used as anodes for organic solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), the MWCNT-enhanced electrodes are found to improve the charge-carrier extraction from the photoactive blend, thanks to the additional percolation paths provided by the CNTs. The work function of as-modified ITO surfaces was measured by the Kelvin probe method to be 4.95 eV, resulting in an improved matching to the highest occupied molecular orbital level of the P3HT. This is in turn expected to increase the hole transport and collection at the anode, contributing to the significant increase of current density and open-circuit voltage observed in test cells created with such MWCNT-enhanced electrodes.

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.

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).

Chemical Vapor Deposited Graphene-Based Derivative As High-Performance Hole Transport Material for Organic Photovoltaics

UNLABELLED The development of efficient charge transport layers is a key requirement for the fabrication of efficient and stable organic solar cells. A graphene-based derivative with planar resistivity exceeding 10(5) Ω/□ and work function of 4.9 eV is here produced by finely tuning the parameters of the chemical vapor deposition process on copper. After the growth, the film is transferred to glass/indium tin oxide and used as hole transport layer in organic solar cells based on a PBDTTT-C-T:[70]PCBM blend. The cells attained a maximum power conversion efficiency of 5%, matching reference cells made with state-of-the-art PEDOT PSS as the hole transport layer. Our results indicate that functionalized graphene could represent an effective alternative to PEDOT PSS as hole transport/electron blocking layer in solution-processed organic photovoltaics.

Preliminary investigations of the effects of air turbulences on the performance of an indoor optical wireless link

Photonic technologies are becoming more and more necessary for data centre (DC) networks to meet the requirements of data transfer speed and energy saving. The use of optical hybrid wired/wireless solutions, where fibre links are coupled to free space optical links within the same DC facility can contribute to reduce the complexity of the network architecture and increase the efficiency of cooling. Although indoor optical wireless is immune by detrimental weather conditions on the free space optical link, the specific environmental conditions of a data centre room, such as strong local thermal variations and high intensities air flows, can introduce effects over the optical channel that can affect the link performance. The aim of this work is to provide a preliminary evaluation of the effects of controlled turbulences in terms of temperature and air flow speed on the power of the free space optical signal, with the purpose of enabling an initial understanding of the involved phenomena. Finally, the effects of the hot air flow-perturbed FSO link on the quality of a video service delivered on a photonic network are compared to a fully fiber-based link.

Electron-collecting oxide layers in inverted polymer solar cells via oxidation of thermally evaporated titanium

A simple and intuitive deposition technique is discussed to obtain titanium oxide used as an electron collecting layer in polymer solar cells based on the thermal evaporation of pristine titanium and further thermal treatment to convert the metal in oxide. Since the degradation of indium-doped tin oxide at high temperatures is an issue, we demonstrate that the combination of glass/fluorine tin oxide and high temperatures represents a promising approach in the fabrication of inverted polymer solar cells with such a titanium oxide electron collecting layer.