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The use of graphene as transparent conducting layer in devices that require high temperature processing is proposed. The material shows stability upon thermal treatments up to 1100 °C if capped with a sacrificial silicon layer. The use of Cu foil or evaporated Cu as catalysts in Catalytic-Chemical Vapor Deposition growth gives rise to graphene of similar properties, which represents a promising result in view of its direct integration in microelectronic devices. Photovoltaic p–i–n thin film devices were fabricated on the as-deposited or annealed graphene membranes and compared with similar devices that incorporate as-deposited Indium Tin Oxide. No degradation in series resistance is observed for the annealed device. A 3.7% and 2.8% photovoltaic conversion efficiency is observed on the devices fabricated on as-transferred and on annealed graphene respectively. The major limitation derives from the high sheet resistance of the as-transferred state-of-the-art material. The results opens the way to the use of graphene in applications that require transparent conducting layers resistant to high temperature processing.

Boron doped multilayers based on silicon carbide/silicon rich carbide, aimed at the formation of silicon nanodots for photovoltaic applications, are studied. X-ray diffraction confirms the formation of crystallized Si and 3C-SiC nanodomains. Fourier Transform Infrared spectroscopy indicates the occurrence of remarkable interdiffusion between adjacent layers. However, the investigated material retains memory of the initial dopant distribution. Electrical measurements suggest the presence of an unintentional dopant impurity in the intrinsic SiC matrix. The overall volume concentration of nanodots is determined by optical simulation and is shown not to contribute to lateral conduction. Remarkable higher room temperature dark conductivity is obtained in the multilayer that includes a boron doped well, rather than boron doped barrier, indicating efficient doping in the former case. Room temperature lateral dark conductivity up to 10-3 S/cm is measured on the multilayer with boron doped barrier and well. The result compares favorably with silicon dioxide and makes SiC encouraging for application in photovoltaic devices. © 2012 Elsevier B.V. All rights reserved.

Silicon carbide/silicon rich carbide multilayers, aimed at the formation of silicon nanodots for photovoltaic applications, have been studied. The electrical properties have been investigated at the nano-scale by conductive Atomic Force Microscopy (c-AFM) and at macro-scale by temperature dependent conductivity measurements. The mixture is composed of highly conductive Si nanoclusters and moderately conductive SiC nanoclusters in a disordered matrix. The conduction mechanism takes place via band states induced by the disorder at the interface between nanodot clusters. Structural properties have been extracted by optical spectroscopy analyses. The results contribute to the understanding of the microscopical electronic mechanisms of the composite material, which is a candidate for third generation photovoltaics.

Silicon nanocrystals (Si NCs) are a promising candidate for the top cell of an all-Si tandem solar cell with a band gap from 1.3-1.7 eV, tuneable by adjusting NC size. They are readily produced within a Si-based dielectric matrix by precipitation from the Si excess in multilayers of alternating stoichiometric and silicon-rich layers. Here we examined the luminescence and transport of Si NCs embedded in SiC. We observed luminescence that redshifts from 2.0 to 1.5 eV with increasing nominal NC size. Upon further investigation, we found that this redshift is to a large extent due to Fabry-Pérot interference. Correction for this effect allows an analysis of the spectrum emitted from within the sample. We also produced p-i-n solar cells and found that the observed I-V curves under illumination could be well-fitted by typical thin-film solar cell models including finite series and parallel resistances, and a voltage-dependent current collection function. A minority carrier mobility-lifetime product on the order of 10-10 cm2/V was deduced, and a maximum open-circuit voltage of 370 mV achieved.

In this work we present the results we obtained by processing 1 Ωcm p and n-type, multi-crystalline silicon wafers when a double hydrogenated amorphous/crystalline silicon heterojunction is applied to sandwich the substrate, at the sunward surface to form the emitter and at the rear side to act as back surface field. We have investigated the role of the multi-crystalline silicon surface conditioning, on the base of illuminated and dark current-voltage characteristic, quantum efficiency measurements, and overall photovoltaic solar cell performance. In particular, a comparison between an acidic isotexturing and a chemical polishing treatment is presented and discussed in detail. We have found that the morphology of the multi-crystalline silicon surface plays a tough role in the a-Si:H properties when used as surface passivating layer. The commercial acidic surface treatment, commonly used in diffused junction mc-Si solar cell fabrication, requires a level of passivation which is not easily achievable using the very thin a-Si:H film, needed in heterojunction technology. The deposition of a uniform thickness over a whole textured silicon surface of a thin amorphous layers still requires further investigation and improvement. Nevertheless the high Voc obtained on p-type doped mc-Si (625 mV) remarks the effectiveness of a-Si:H/mc-Si technology in achieving high photovoltaic efficiency using low thermal budget manufacturing process. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Scanning Probe Microscopy is a very useful technique that provides a variety of information about the surface of materials. Atomic Force Microscopy (AFM) is used in this work in order to investigate the topography and the local conductivity of a system composed of silicon nano-crystals (Si NCs) embedded in a dielectric matrix (SiC). This system is studied in view of its application as a tunable band-gap absorber in all-silicon multi-junction photovoltaic cells. SiC/Silicon Rich Carbide (SRC) multilayers are produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) and are subsequently annealed in order to obtain NCs formation. The properties of the multilayers have been studied as a function of the NCs size. Macroscopical as well as microscopical characterization are carried out in order to understand the properties of the multilayers. AFM maps are obtained in tapping mode in order to investigate the topography of the layers and the energy dissipated between the tip and the sample. Conductivity measurements as a function of temperature show that the conductivity increases with SRC layer thickness. Local IV measurement are performed with the AFM in contact mode with a conducting probe and the measurements show a similar trend with respect to SRC layer thickness. Conductive-AFM is performed to identify conductive paths at the nano-scale. Regions with different conductivity have been observed, which could be related to compositional variations (Si content) and unintentional doping by contamination with nitrogen and oxygen. The results are correlated with crystalline fraction and structural properties of the layers as obtained by optical analyses and transmission electron microscopy measurements, respectively.

Uterus didelphys, also known as a duplicated uterus, is an embryological abnormality resulting from the failure of fusion of the Mullerian ducts, causing full uterine development to erroneously occur bilaterally. The occurrence of uterus didelphys is very low in the general population and often predisposes women to a variety of gynecological difficulties. There appears to be a high association with obstetrical complications, such as spontaneous abortion, preterm labor, cervical incompetence, and malpresentation. Uterus didelphys is also associated with urinary tract abnormalities. Presented is a case of a woman with a didelphys uterus and a twin pregnancy.

Si NCs embedded in a wide band gap material are promising candidates for all-Si tandem solar cells thanks to the possibility of tuning the NC band gap by varying their size, offered by the NC quantum confinement. The high temperature treatment required for Si crystallization represents a concern for the integration of this material in a tandem solar cell production route. The limiting step for solid phase crystallization is grain nucleation. A high temperature nucleation seeding followed by grain growth at a lower temperature is therefore a promising method to reduce the thermal budget. To this aim, we report on different annealing routes pursued to decrease the thermal budget: in all cases the germs of the c-Si nucleation are created by a high temperature (1150 °C) spike, then the crystallization follows at different temperatures, above and below the 3C-SiC solid phase epitaxial growth temperature (800 °C). The results are discussed in terms of the structural features produced by each process: the time required for complete dehydrogenation, mandatory to avoid disruptive hydrogen evolution during the high temperature spike; Si and SiC crystallization; defect nucleation in c-SiC, correlated to the rate of temperature rise during the annealing; role of the c-Si/SiC interface energy in affecting the growth process. The photovoltaic properties of the resulting structures are estimated based on the measured optical absorption, electronic transitions, and electrical transport.

Silicon nano-crystals (Si NCs) embedded in a dielectric matrix are studied in view of their application as tunable band-gap absorbers in all-silicon multi-junction photovoltaic cells. Although high conductivity and high mobility are requirements for the matrix in order to guarantee an efficient carrier collection, the ideal properties of the nc-Si are more related to the ability of absorbing photons beyond a given energy, and emitting photogenerated carriers into the surrounding matrix. The carrier collection efficiency in this composite system is supposed to depend on the barrier height of the surrounding matrix. In the present case we are dealing with Si NCs embedded in a SiC matrix, which guarantees a lower barrier to the Si NCs with respect to SiO2 matrix and better transport properties. SiC/SiC:Si multilayers produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) have been annealed to obtain Si NCs formation. Various experimental conditions (Si NCs size, doping level, crystallinity) have been considered. The structural parameters were determined by Reflectance and Transmittance spectroscopy analyses. From this the calculated thickness of multilayers is in the range of 131-178 nm. Macroscopical as well as microscopical electrical characterizations were carried out in order to understand the conduction mechanisms. Conductivity measurements as a function of temperature showed that the conductivity ranged from 10-6-10-3S/cm and the activation energy range is 0.19-0.4 eV. Atomic Force Microscopy (AFM) was used to characterize the topography of the sample surface. Roughness values are of the order of 0.6 nm while phase contrast dynamical mode AFM mapping allowed us to obtain an estimated average value of the nano-crystal diameter around 10 nm. Conductive-AFM was performed to identify the conductive path at the nano-crystals level. The Si NCs showed enhanced conductivity with respect to the surrounding matrix. The measurements were correlated with optically determined crystallized fraction and with thermally activated transport properties. The combination of electrical with optical measurements, associated to AFM observation at the nanoscale, gives a fundamental insight in the silicon nano-dots conduction properties.

Silicon nano-crystals (Si NCs) embedded in a dielectric matrix are presently studied in view of their application as tunable band-gap absorbers in all-silicon multi-junction photovoltaic cells. Although high conductivity and high mobility are requirements for the matrix in order to guarantee an efficient carrier collection, the ideal properties of the nc-Si are more related to the ability of absorbing photons beyond a given energy, and emitting photogenerated carriers into the surrounding matrix. The carrier collection efficiency in this composite system is supposed to depend on the barrier height of the surrounding matrix. It is therefore very important to clarify the electrical behavior of the Si NCs when inserted in the specific matrix that will be used for the final device. In the present case we are dealing with Si NCs embedded in a silicon carbide matrix, which guarantees a lower barrier to the Si NCs with respect to SiOx matrix and better transport properties. This work deals with the study of the conduction mechanisms in the system composed by silicon nano-crystals within the SiC matrix. SiC/SiC:Si multilayers produced by Plasma Enhanced Chemical Vapor Deposition (PECVD) have been subsequently annealed to obtain Si nano-crystal formation. Various experimental conditions (silicon nano-crystal size, doping level, crystallinity) were considered. The structural parameters were determined by Reflectance and Transmittance spectroscopy analyses. From this the calculated thickness of multilayers is in the range of 131-178 nm. Macroscopical as well as microscopical electrical characterizations were carried out in order to understand the conduction mechanisms. Conductivity measurements as a function of temperature showed that the conductivity ranged from 10-6-10-3S/cm and the activation energy range is 0.19-0.4 eV. Atomic Force Microscopy (AFM) was used to characterize the topography of the sample surface. Roughness values are of the order of 0.6 nm while phase contrast dynamical mode AFM mapping allowed us to obtain an average value of the nano-crystal diameter around 10 nm. Conductive-AFM was performed to identify the conductive path at the nano-crystals level. The Si NCs showed to have higher conductivity with respect to the surrounding matrix. The measurements were correlated with optically determined crystallized fraction and with thermally activated transport properties. The combination of electrical with optical measurements, associated to AFM observation at the nanoscale, gives a fundamental insight in the silicon nano-dots conduction properties.

Silicon nanocrystals, ncSi, embedded in a dielectric matrix are presently studied in view of their application as tunable band-gap absorbers in all-silicon multijunction photovoltaicsa. Althoug high conductivity and high mobility are requirements for the matrix in order to guarantee an efficient carrier collection, the ideal properties of the ncSi are more related to the ability of absorbing photons beyond a given energy, and then emit the photogenerated carrier into the extended states of the surrounding matrix. At present, the role of doping in this context has not fully established, also because doping of nanocrystals suffers from limitations due to segregation at the surface and formation of electrically inactive complexes with dangling bondb, as well as size dependent activation efficiencyd. At the nanoscale, higher conductivity has also been observed either on the ncSi surface or within the ncSi itseld, depending on the doped of intrinsic nature of the ncSi.4. Finally, overlapping of the wavefunction, and therefore carrier collection efficiency, is also predicted to depend on the barirer height of the surrounding matrix. It is therefore very important to clarify the electrical behaviour of the ncSi when inserted in the specific matrix that will be used for the final device. In our case we focus on silicon carbide, which quarantees lower barrier to the ncSi, and better transport properties.

Ordered silicon nanocrystals in silicon carbide are produced by Plasma Enhanced Chemical Vapor Deposition by means of the multilayer approach followed by annealing at 1100 °C. The crystallization is verified by Raman scattering, X-ray diffraction, Transmission Electron Microscopy, and UV–vis spectroscopy. The conditions for the periodic structure to survive the high temperature annealing and for the SiC barrier to confine the Si crystal growth are examined by energy-filtered transmission electron microscopy and X-ray reflection. The final layout appears to be strongly influenced by the structural features of the as-deposited multilayer. Threshold values of Si-rich carbide sublayer thickness and Si-to-C ratio are identified in order to preserve the ordered structure. The crystallized fraction is observed to be correlated with the total silicon volume fraction. The constraints are examined through the use of ab-initio calculations of matrix-embedded silicon nanocrystals, as well as in terms of existing models for nanocrystal formation, in order to establish the role played by the interface energy on nanocrystal outgrowth, residual amorphous fraction, and continuous crystallization. A parameter space of formation of ordered Si nanocrystals is proposed. The diffusivity of carbon in the crystallized material is evaluated, and estimated to be around 10–16 cm2/s at 1100 °C.

An analytical function for the imaginary part of the dielectic constant of microcrystalline silicon is proposed. The function consists of a sum of Lorentz oscillators with Gaussian broadening, and is based on literature data. The real part is numerically obtained by Kramers-Kronig integration. The physical significance of the results is discussed. It is shown that the choice of the minimization procedure may affect the spectral region (low or high absorption) where the accuracy of the fit is optimized. The impact on computed Transmittance data shows the limits of acceptability for the errors in the fit.

In this work we present a structural, optical and electrical characterization of Si x C 1− x /SiC multilayer systems with different silicon content. After the deposition process, an annealing treatment was carried out in order to induce the silicon nanocrystals formation. By means of energy-filtered transmission electron microscopy (EFTEM) we observed the structural morphology of the multilayers and the presence of crystallized silicon nanoprecipitates for samples annealed up to 1100 °C. We discuss the suitability of optical techniques such as Raman scattering and reflectance and transmittance ( R&T ) for the evaluation of the crystalline fraction of our samples at different silicon excess ranges. In addition, the combination of R&T measurements with simulation has proved to be a useful instrument to confirm the structural properties observed by EFTEM. Finally, we explore the origin of the extremely high current density revealed by electrical measurements, probably due to the presence of an undesired defective SiC y O z ternary compound layer, already supported by the structural and optical results. Nevertheless, the variation of the electrical measurements with the silicon amount indicates a small but significant contribution from the multilayers.

27th European Photovoltaic Solar Energy Conference and Exhibition; 4523-4528, It is a diffused opinion that the possibility for PV energy to gain a prominent role in the scenario of energy sources at a worldwide level is strictly linked, among other factors, to the increase of module efficiency, as this is believed to directly result in a net reduction of costs. To verify this hypothesis, we analyzed the costs of smallmedium PV plant installations, by separating the costs in power-related, area-related, and fixed costs, noting that the efficiency of modules only enters in the determination of area-related costs. We obtained that high efficiency is convenient only in the optimistic scenario of very moderate cost increase with efficiency. The results are valid for the case of residential, roof installed PV plants.

Single-polished c-Si (1 0 0) wafers were textured in aqueous solutions with varying concentrations of tetra-methyl ammonium hydroxide (TMAH). The resulting surface reflectance and morphology were examined as a function of etching time and temperature, TMAH concentration, and addition of isopropyl alcohol to the solution. The lowest reflectance, 9.8% at a 600-nm wavelength with 0.3% scattering over a 4″ wafer surface, was obtained after 40 min of etching in a 2% TMAH solution at 80 °C under 700 rpm magnetic stirring. Upon adding isopropyl alcohol to the solution, the resulting pyramids were round-edged, and 12% sample reflectance was obtained. The results are interpreted in terms of micro-masking formation and temperature-dependent crystallographic selectivity. The compatibility of the treatment with photovoltaic applications was evaluated by studying the performance of heterojunction solar cells, which are particularly sensitive to surface quality. A degradation of the open circuit voltage was observed in devices fabricated on surfaces featuring crooked pyramid sides. Optimised process conditions led to smooth pyramid sides and no degradation of the open circuit voltage, which indicates no sign of increased surface recombination-centre concentration. The reduced reflectance resulted in a 16% increase of the short circuit current of the solar cell device.

Thermally treated silicon rich oxides (SRO) used as starting material for the fabrication of silicon nanodots represent the basis of tunable bandgap nanostructured materials for optoelectronic and photonic applications. The optical modelization of such materials is of great interest, as it allows the simulation of reflectance and transmittance (R&T) spectra, which is a powerful non destructive tool in the determination of phase modifications (clustering, precipitation of new phases, crystallization) upon thermal treatments. In this paper, we study the optical properties of a variety of as-deposited and furnace annealed SRO materials. The different phases are treated by means of the effective medium approximation. Upon annealing at low temperature, R&T spectra show the precipitation of amorphous silicon nanoparticles, while the crystallization occurring at temperatures higher than 1000 °C is also clearly identified, in agreement with structural results. The existing literature on the optical properties of the silicon nanocrystals is reviewed, with attention on the specificity of the compositional and structural characteristics of the involved material. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Riassunto Viene presentata la carta della vegetazione naturale del Bacino di Gubbio. Questo e costituito da una depressione limitata da colline marnoso-arenacee, tranne che nel settore Nord-orientale dove si sviluppa una piccola dorsale calcarea. Il fondo del bacino, derivante dal prosciugamento di un lago Villafranchiano, e occupato da formazioni fluvio-lacustri che affiorano in alcune zone, mentre in altre sono ricoperte da detriti calcarei provenienti dalla dorsale. La vegetazione si distribuisce in base alle caratteristiche litologiche dei substrati per i quali e stato possibile evidenziare la presenza di distinte serie di vegetazione che caratterizzano altret- tanti paesaggi. Sulle formazioni marnoso-arenacee si sviluppa la serie planiziale del bosco di rovere (Hieracio racemosi- Quercetum petraeae sigmetum), sulla dorsale calcarea la serie del carpino nero (Scutellario-Ostryetum sigmetum) e sulle colline marnoso-arenacee la serie del cerro (Aceri obtusati-Quercetum cerris sigmetum). Altri tipi di ve...

The development of vegetation analysis in the Apennines has lead to the recognition of vegetation associations related to each other by characteristic dynamic systems linked to geomorphological factors. Techniques which we have recently developed for mapping such systems suggest that there is a direct relationship between the vegetation series and the geomorphological factors. Mapping techniques are now available to provide a simple representation of the vegetation landscape for any area characterised by uniform geomorphology.

Indium tin oxide (ITO) thin films were deposited by magnetron sputtering from a ceramic target, and annealed at temperatures up to 1000 °C in N2 atmosphere. Some samples were capped in a-Si:H or spin-on glass to prevent residual oxygen contamination from the annealing ambient, and annihilation of oxygen vacancies. The electrical and optical properties were measured before and after annealing. It is found that the protecting layer effectively limits the ITO degradation up to 900 °C, whereas high transparency is preserved in all cases. The results indicate the applicability of the procedure to high temperature devices, and opens the way to the application of a variety of nanostructured materials in advanced photovoltaic devices.

Questions: Does the fairy ring fungus Agaricus campestris affects spatial distribution of co-existing plant species? Is ring development related to changes of soil physical, chemical, enzymatic and microbiological properties? What are the causes of weakening and subsequent luxuriance of vegetation during fairy ring dynamics in the soil?

Semi‐natural calcareous grasslands are of great interest in conservation because of their high species richness, but presently they are threatened by both land abandonment and nitrogen eutrophication. The paper reports on a three‐year field experiment conducted in a species‐rich Mediterranean grassland to study the effects of nitrogen application coupled with removal of plant litter and artificial cutting on plant species diversity. Vegetation treatments were applied both to abandoned and adjacent long‐term mowed grassland, to evaluate their restoration value and impact of management abandonment. Cutting significantly increased species diversity in the abandoned area by positively affecting the cover of almost all annual and biennial species and several perennial forbs. Cutting strongly reduced the dominance of perennial grasses. Litter removal had similar effects to cutting, although it was far less effective in increasing species diversity. In the mowed area, cutting abandonment did not reduce ...

Semi-natural grassland communities are of great interest in conservation because of their high species richness. These communities are being threatened by both land abandonment and nitrogen eutrophication, and their continued existence will depend upon correct management. However, there is a distinct lack of studies of the ecological mechanisms that regulate species diversity and productivity in Mediterranean grasslands. We have conducted a 3-year field experiment in a species-poor grassland in central Italy to investigate the effects of nitrogen fertilization coupled with removal of plant litter and artificial cutting on species diversity and community productivity. Vegetation cutting reduced living biomass but increased species diversity. In fact, cutting had positive effects on the cover of almost all of the annual and biennial species, while it had a negative effect on the dominant perennial grasses Brachypodium rupestre and Dactylis glomerata. Litter removal had similar effects to cutting, although it was far less effective in increasing species diversity. In contrast, nitrogen enrichment strongly increased the living biomass while maintaining very low species diversity. Our results have indicated that semi-natural Mediterranean grasslands need specific management regimes for maintenance and restoration of species diversity. In the management of these grasslands, attention should be paid to the potential threat from nitrogen enrichment, especially when coupled with land abandonment.

This study provides an update of phytosociological knowledge of the vegetation of Apennine grasslands that belongs to Brometalia ercti, and it includes a new syntaxonic revision * * * * * * * * * * * * * * * *