Barbara Paci

Enhancement of photo/thermal stability of organic bulk heterojunction photovoltaic devices via gold nanoparticles doping of the active layer

This study focuses on the crucial problem of the stability of organic photovoltaic (OPV) devices, aiming to shed light on the photo and thermal degradation mechanisms during prolonged irradiation under ambient conditions. For this purpose, the stability enhancement of bulk heterojunction OPV devices upon embedding surfactant free Au nanoparticles (NPs) into the photoactive layer is investigated by in situ time-resolved energy dispersive X-ray reflectometry (EDXR), photoluminescence (PL) and Raman spectroscopy as well as device degradation electrical measurements. It is shown that besides the improved cell efficiency attributed to plasmon absorption and scattering effects, the embedded NPs act as performance stabilizers, giving rise to enhanced structural stability and, in turn, to reduced photodegradation rate of the respective OPV devices. It is particularly clarified that, in addition to further stabilization of the polymer-fullerene blend, the observed improvement can be ascribed to a NP-mediated mitigation of the photooxidation effect at the cathode-active layer interface. Our work suggests the exploitation of surfactant free NPs to be a successful approach to address aging effects in OPV devices.

24 h stability of thick multilayer silicene in air

Thick epitaxial multilayer silicene films with a root 3 x root 3R(30 degrees) surface structure show only mild surface oxidation after 24 h in air, as measured by Auger electron spectroscopy. X-ray diffraction and Raman spectroscopy measurements performed in air without any protective capping, as well as, for comparison, with a thin Al2O3 cap, showed the (002) reflection and the G, D and 2D Raman structures, which are unique fingerprints of thick multilayer silicene.

In situ energy dispersive x-ray reflectometry measurements on organic solar cells upon working

The change in the morphology of plastic solar cells was studied by means of time-resolved energy dispersive x-ray reflectivity (XRR). This unconventional application of the XRR technique allowed the follow up of in situ morphological evolution of an organic photovoltaic device upon working. The study consisted of three steps: A preliminary set of XRR measurements on various samples representing the intermediate stages of cell construction, which provided accurate data regarding the electronic densities of the different layers; the verification of the morphological stability of the device under ambient condition; a real-time collection of XRR patterns, both in the dark and during 15h in artificial light conditions which allowed the changes in the system morphology at the electrode-active layer interface to be monitored. In this way, a progressive thickening of this interface, responsible for a reduction in the performances of the device, was observed directly.

Controlling photoinduced degradation in plastic photovoltaic cells: A time-resolved energy dispersive x-ray reflectometry study

The electrode-active layer interface of organic photovoltaic cells, a critical point in the development of organic devices, was studied by the energy dispersive x-ray reflectivity (EDXR) technique applied in situ. An EDXR-based protocol allowing discrimination between the possible mechanisms that produce the aging process at the interface was established. The study detects photoinduced oxidation of the electrode at the buried interface, to which fading of the device performances could be attributed. This conclusion was further confirmed by results obtained on a new cell, of selectively modified architecture, whose performances turned out to be stable in time.

Energy dispersive x-ray reflectometry as a unique laboratory tool for investigating morphological properties of layered systems and devices

The principles and technical aspects of the laboratory energy dispersive x-ray reflectometry technique (EDXR) are reviewed. X-ray reflectometry enables us to retrieve information on the morphological parameters (film thickness, surface and interface roughness) of thin films, layered materials and devices at the A resolution. In the energy dispersive mode it makes use of a polychromatic beam and the energy scan is carried out by means of an energy sensitive detector, allowing us to keep the experimental geometry unchanged during data collection. The advantages offered by the technique, in particular when accurate time-resolved studies are to be performed, are outlined. An overview of the most significant works in the field is also reported, in order to present the wide range of its possible applications. The paper was written taking into account the necessity to provide a comprehensive description of this unusual laboratory technique. The first part of the manuscript is, therefore, devoted to the general aspects of EDXR: theoretical bases, experimental characteristics, advantages and drawbacks in comparison with alternative techniques, instrumental effects that may occur and information that can be gained by its application. In the second part of the paper, we resume the main literature results obtained by applying it to a variety of systems, to show examples of the possibilities it offers. Among the experiments reported, in particular much emphasis is given to the time-resolved in situ ones (for example, devices upon working and systems undergoing morphological modifications), which probably represents the most original use of this method.

Titanium and Ruthenium Phthalocyanines for NO2 Sensors: A Mini-Review

This review presents studies devoted to the description and comprehension of phenomena connected with the sensing behaviour towards NO2 of films of two phthalocyanines, titanium bis-phthalocyanine and ruthenium phthalocyanine. Spectroscopic, conductometric, and morphological features recorded during exposure to the gas are explained and the mechanisms of gas-molecule interaction are also elucidated. The review also shows how X-ray reflectivity can be a useful tool for monitoring morphological parameters such as thickness and roughness that are demonstrated to be sensitive variables for monitoring the exposure of thin films of sensor materials to NO2 gas.

Spatially‐Resolved In‐Situ Structural Study of Organic Electronic Devices with Nanoscale Resolution: The Plasmonic Photovoltaic Case Study

A novel high spatial resolution synchrotron X-ray diffraction stratigraphy technique has been applied in-situ to an integrated plasmonic nanoparticle-based organic photovoltaic device. This original approach allows for the disclosure of structure-property relations linking large scale organic devices to length scales of local nano/hetero structures and interfaces between the different components.

Time/Space-Resolved Studies of the Nafion Membrane Hydration Profile in a Running Fuel Cell

2009 WILEY-VCH Verlag Gm The determination of the amount and spatial distribution of water in a polymeric membrane of a proton-exchange-membrane fuel cell (PEMFC) under working conditions is a fundamental task to address in PEMFC technology. Indeed, since proton transfer in such polymeric materials is known to be assisted by water, the fuel-cell (FC) performances depend on the proton-exchange membrane (PEM) hydration degree. However, the hydration degree is influenced not only by the electrochemical conditions the FC is submitted to, but also by many other independent parameters, such as the constriction exerted on the membrane by the other FC components, the electrical current flowing across it, the actual temperature, aging effects, etc, which are hard to take into account in theoretical calculations. In this work, an original method based on very-high-energy synchrotron-radiation X-ray diffraction is applied to carry out the first space/time-resolved measurements of the PEM hydration profile in a running FC. Due to their capability of effectively converting chemical into electrical energy, PEMFCs play a major role in the development of future environmentally friendly hydrogen-based technologies. Indeed, PEMFCs are considered promising candidates for automotive propulsion and for stationary applications. To optimize the performance and lifetime of a PEMFC, one major problem that must be solved is the water management, because the PEM proton conductivity is highly dependent upon its water content. On the other hand, an excess of water is detrimental, as it may produce cathode flooding, and a consequent reduction of gas supply. During the running of the cell, the membrane both absorbs water, which is produced by oxygen reduction at the cathode or carried by the humidified gas stream, and releases it, because of the evaporation induced by the gas flow and by heating occurring under operative conditions. Water transport through the membrane is caused mainly by the electro-osmotic drag of water by protons moving from the anode to the cathode, and by back-diffusion of the water produced at the cathode, towards the anode, as a consequence of the concentration gradients that build up upon operation. In steady conditions, equilibrium among these competitive mechanisms is reached. However, when the operative parameters are changed, complex water dynamics are observed. Several theoretical investigations have been carried out to describe water intake, release, and transport through Nafion membranes. Nevertheless, uncertainties are also present in calculations, due to several experimental effects and constraints, which further complicate the water dynamics and are difficult to model. On the other hand, only a few experimental techniques aimed at measuring the water distribution in the membrane are available, due to the intrinsic difficulty of isolating the signal coming from water molecules inside a relatively thick membrane assembled in a working cell (as required by an in situ investigation). X-ray techniques, small-angle neutron scattering (SANS), magnetic-resonance imaging andmicro-Raman, neutron radiography, electrical-resistance measurements, infrared absorption and fluorescence spectroscopy have been used for this purpose. Unfortunately, if one excludes micro-Raman measurements, all of these techniques exhibit rather low spatial resolutions (if any). Moreover, they have other severe limitations, such as slow response to the hydration-degree variations, weak signals (resulting in poor accuracy), and only indirect dependence on the quantity of interest. Here, we propose an alternative approach, based on veryhigh-energy (about 90 keV) X-ray diffraction (VHEXD), to measure the hydration degree of PEMs in a working cell in situ. The method consists of vertical stratigraphy of the membrane from one electrode to the other, corresponding to ideally ‘‘slicing’’ the membrane itself in a stack of layers. As a result, the time-dependence of the hydration degree in each layer has been determined at the highest accuracy ever achieved, in all the experimental conditions in which PEMFCs may operate, and in the presence of all the concomitant effects mentioned above. To apply the method discussed in the experimental section, preliminary tests were required to identify the right inclination of the cell (parallelism between the PEM plane and the X-ray beam) and the height at which the beam intersects only the membrane. With these tests, spurious contributions from the other components of the cell to the diffraction patterns can be prevented. The parallelism condition was met by taking a sequence of radiographies of the cell during a scan of the rocking angle, carried out to visualize its inner parts. Figure 1a shows the first of these radiographies, collected after the cell was placed in the beam trajectory. The components of the cell around the membrane can be easily distinguished. The

Time-resolved energy dispersive x-ray reflectometry measurements on ruthenium phthalocyanine gas sensing films

The energy dispersive (ED) variant of the conventional x-ray reflectivity (XR) provides an atomic scale determination of the morphological characteristics of thin films, such as their thickness and surface roughness. We report on the in situ EDXR measurements of the (minimal) morphological changes of ruthenium phthalocyanine gas sensing thin films. A series of reflectivity spectra have been collected, during the exposure of the films to a gas flux of nitrogen oxides (NOx) molecules. The measurements allowed a very high density time sampling of the evolution of the two morphological parameters, providing important information on the gas-film interaction.

Experimental evidence of a two-step reversible absorption/desorption process in ruthenium phtalocyanine gas sensing films by in situ energy dispersive x-ray reflectometry

An in situ energy dispersive x-ray reflectivity technique was used to study the morphological changes of gas sensing thin films of ruthenium phtalocyanine (RuPc)2 induced by gas absorption/desorption processes. The time-resolved collection of reflectivity spectra during the exposure of each film to a gas flux of nitrogen oxides provided the evolution of the morphological parameters (thickness and roughness). The gas absorption process develops in two stages: The first induces morphological changes characteristic of a surface (adsorption) process, while the second is dominated by a bulk effect. This two-step behavior is also observed in the desorption process: When the thermal treatment is performed at 130°C, the gas is released from the bulk only. Conversely, at higher temperatures, the gas is fully released, i.e., also from the surface, and the initial film thickness is regained. Finally, a further in situ study upon a second absorption treatment was carried out: In this case, only the film bulk diffusio...