Indium Tin Oxide (ITO) is widely used in solar cell devices for its excellent electrical and optical characteristics, such as high transparency in the Ultraviolet-Visible range and good conductivity (around 10 4 W -1 cm -1 ). In this work we have compared thin (70-150 nm) ITO layers deposited by Direct Current or Radio Frequency sputtering. We have used different substrate temperatures during film growth and have afterwards thermally annealed the samples at different temperatures up to 300 °C to investigate the effects on the electrical and optical properties of the material. We have found out that the different growth/annealing conditions induce changes in the optical properties of the samples as well as in the conductivity and carrier concentration.

Hole-collecting and buffer layers more transparent than amorphous silicon layers are regarded as one of the possible improvements needed in amorphous/crystalline silicon heterojunctions to help reach the limit efficiency of silicon-based solar cells. Recent studies have demonstrated the suitability of sub-stoichiometric molybdenum oxide (MoOx) as hole-extracting layer in heterojunction solar cells based on n-type crystalline silicon. However, up to now MoOx has mainly been used in combination with amorphous silicon, so limiting its potential as a transparent emitter. This work examines a hole-selective contact based on the combination of a passivating hydrogenated amorphous silicon oxide (a-SiOx:H) and MoOx. The use of these two high-band gap materials (1.9 and 3.5 eV, respectively) should lead to best cell performances if compared to heterojunction devices based only on amorphous silicon. Solar cells were produced and optimized in terms of layers thicknesses. The widely reported problem of s-shape in the light current–voltage characteristics, due to undesired barrier formation against photo-carrier collection, was addressed and solved. The temperature response of the devices was studied and the cell stability under thermal treatment up to 130 °C was demonstrated.

Addiction of ceramic nanoparticles to acrylic polymers provides a simple and effective means to produce paints with important properties, such as mechanical resistance and tailored wettability, even though for optimal performances, an engineered nanoparticle distribution would be desirable. In this paper we report on the realization and on the morphological and functional characterization of nanocomposites where the nanophase is distributed on the surface of acrylic polymer films, in order to enhance the expression of surface-related properties. To this aim, commercial titanium oxide and silicon oxide nanopowders were dispersed in water and the suspensions were air-sprayed on polymeric films prepared by paint brushing, thus producing a nanostructured ceramic surface coating. Control of the pH of suspensions and acrylic acid functionalization of the surface of titania were used together with high power ultrasonic treatments in order to control dimension of the aggregates in the sprayed suspensions. Optical microscopy, mechanical profilometry, and atomic force microscopy were used to characterize the nanocomposite surface morphology and correlate it to the coating functional properties, evaluated through mechanical abrasion tests and contact angle measurements; also, colorimetry on coated stones was performed in order to test the impact of the coatings on the aesthetical appearance and their photostability under UV irradiation. Results show that the nanostructured ceramic layer slightly improves the resistance of coatings to mechanical abrasion in case of polymer films prepared from latexes. The nanocomposite surface layer does not affect the wettability of the polymer, which remained slightly hydrophilic; this behavior is likely due to inadequate distribution of the nanophase. On the other hand UV-induced superhy drophilicity was observed when the concentration of surface titania nanoparticles is about 0.6 mg/cm(2). Colorimetric analysis on historical and Carrara marbles before and after coating evidenced the good transparency of the nanocomposites. Accelerated aging tests permitted to demonstrate that, on the historical marbles, the presence of the nanoparticles has a protective action against UV-induced damage of the underlying polymer film, preventing photodegradation.

The adoption of a-SiOx:H films obtained by PECVD in heterojunction solar cells is a key to further increase their efficiency, because of its transparency in the UV with respect to the commonly used a-Si:H. At the same time this layer must guarantee high surface passivation of the c-Si to be suitable in high efficiency solar cell manufacturing. On the other hand the application of amorphous materials like a-Si: H and SiNx on the cell frontside expose them to the mostly energetic part of the sun spectrum, leading to a metastability of their passivation properties. Moreover as for amorphous silicon, thermal annealing procedures are considered as valuable steps to enhance and stabilize thin film properties, when performed at opportune temperature. In this work we explored the reliability of a-SiOx:H thin film layers surface passivation on c-Si substrates under UV exposition, in combination with thermal annealing steps. Both p- and n-type doped c-Si substrates were considered. To understand the effect of UV light soaking we monitored the minority carriers lifetime and Si-H and Si-O bonding, by FTIR spectra, after different exposure times to light coming from a deuterium lamp, filtered to UV-A region, and focused on the sample to obtain a power density of 50 mu W/cm(2). We found a certain lifetime decrease after UV light soaking in both p- and n-type c-Si passivated wafers according to a a-SiOx :H structure. The role of a thermal annealing, which usually enhances the as-deposited SiOx passivation properties, was furthermore considered. In particular we monitored the UV light soaking effect on c-Si wafers after a-SiOx:H coating by PECVD and after a thermal annealing treatment at 300 degrees C for 30 min, having selected these conditions on the basis of the study of the effect due to different temperatures and durations. We correlated the lifetime evolution and the metastability effect of thermal annealing to the a-SiOx :H/c-Si interface considering the evolution of hydrogen in the film revealed by FTIR spectra, and we developed a model for the effect of both treatments on the Si H bonding and the metastability shown in the lifetime of a-SiOx:H/c-Si/a-SiOx :H structure. We found that, after UV exposure, thermal annealing steps can be used as a tool for the c-Si passivation recovery and enhancement. (C) 2016 Elsevier B.V. All rights reserved.

The effect of the order-disorder transition on the band gap of kesterite Cu2ZnSnS4, an interesting material for solar cells, has been investigated by optical spectroscopy. The band gap energy (Eg) decreases continuously with increasing annealing temperature, Ta, and reaches its minimum at Ta ∼ 273 °C. Eg is about 200 meV higher in the most ordered state, than in the fully disordered state. Its value and the transition kinetic depend on the sample stoichiometry. A simplified model able to explain the order degree and stoichiometry effects on the Eg value is developed. Ordering results in narrower Raman peaks without affecting the shape of the photoluminescence spectrum—except for the change in Eg—or the characteristic energy of the exponential tail below the fundamental absorption edge. Although a prolonged annealing increases the order degree, the material properties are still influenced by residual disorder as well as by defects related to the off-stoichiometry composition.

Highly conductive and transparent NiOx films can be very useful as buffer layers for the optimization of the p-type contacts of optoelectronic devices. Thin NiOx films were fabricated by reactive radio frequency (RF) sputtering at room temperature starting from a Ni target. A systematic study of the influence of oxygen partial pressure, RF power and sputtering gas pressure on the films' properties was carried out. The structural, microstructural, optical and electrical properties were affected differently by the sputtering parameters. Resistivity decreased by increasing the oxygen partial pressure and the sputtering total pressure and by decreasing the RF power, while transmittance increased by decreasing the oxygen partial pressure and by increasing the RF power and sputtering pressure. Minimum resistivity of 1.6 × 10−2 Ωcm and a visible transmittance of 40% were achieved for a film grown in a pure oxygen atmosphere, while a higher transmittance of 54% and a resistivity of ρ = 1.1 × 10−1 Ωcm were obtained for a film grown at 30% oxygen partial pressure. The trends of transmittance and resistivity as a function of the oxygen pressure during the sputtering process can be explained in terms of the amount of Ni3+ defects deduced by x-ray photoelectron spectroscopy (XPS) measurements. The full interpretation of the other results is less straightforward and highlights the relevance of the samples' structural properties.

Polyvinylpyrrolidone (PVP)/nickel(II) acetate precursor fibers were deposited by electrospinning directly on radio frequency sputtered thin Ni and NiO films grown on quartz substrate, starting from Ni(II) acetate and PVP solution in ethanol. The samples were calcined in air in the temperature range 400-500 degrees C to obtain transparent and conductive p-type NiO nanofibers on NiO films. A higher density of nanofibers was obtained on Ni/quartz substrates, as compared to NiO/quartz ones, demonstrating the feasibility of fiber adhesion directly to an insulating substrate previously coated by a thin Ni layer.Samples were characterized by field emission-scanning electron microscopy, X-ray diffraction, spectrophotometric and resistance measurements. (C) 2015 Elsevier B.V. All rights reserved.

Broad-band light-emitting radiation-induced F2 and F3+ electronic point defects, which are stable and laser-active at room temperature in lithium fluoride crystals and films, are used in dosimeters, tuneable color-center lasers, broad-band miniaturized light sources and novel radiation imaging detectors. A brief review of their photoemission properties is presented, and their behavior at liquid nitrogen temperatures is discussed. Some experimental data from optical spectroscopy and fluorescence microscopy of these radiation-induced point defects in LiF crystals and thin films are used to obtain information about the coloration curves, the efficiency of point defect formation, the effects of photo-bleaching processes, etc. Control of the local formation, stabilization, and transformation of radiation-induced light-emitting defect centers is crucial for the development of optically active micro-components and nanostructures. Some of the advantages of low temperature measurements for novel confocal laser scanning fluorescence microscopy techniques, widely used for spatial mapping of these point defects through the optical reading of their visible photoluminescence, are highlighted.

The behaviour of interference optical filters for space applications has been investigated under low- and high-energy proton irradiation. Low-energy protons are expected to be necessary to prove the effects on the coating, whereas the high-energy proton tests shall verify mainly the substrate susceptibility to induced damage. The expected interaction of protons with coating and substrate was simulated by software, to identify the most appropriate conditions for the irradiation experiments. Two different accelerator facilities were used for low- and high- energy protons: 60 keV protons with an integrated fluence of 1012 p+/cm2 and 30 MeV protons with an integrated fluence of 108 p+/cm2. The spectral transmittance of the filters was measured before and after irradiation and, according to simulations, no significant effects were detected in the visible-near infrared spectrum, while some variations appeared at short wavelengths with low-energy irradiation.

We experimentally demonstrate propagation of surface plasmon polaritons in the near-IR window lambda (1.45 microm,1.59 microm) at the interface of indium-tin-oxide films with different thicknesses deposited on glass. Dispersion of such polaritons is strongly dependent on the film thickness, putting into evidence a regime in which polaritons at both films's interfaces are coupled in surface supermodes. The experimental data are shown to be in good agreement with the analytical model for thin and absorbing conducting films. Measurements on aluminum-doped zinc oxide, characterized by a redshifted plasma resonance, do not show any surface plasmon polariton excitation in the same wavelength window.

The authors report on the measurement of the second order nonlinear optical properties of ZnO films deposited by low temperature radio frequency sputtering on ZnO:Al coated glass substrates. They show that ZnO:Al plays a crucial role as a buffer layer for the successive growth of ZnO. The effect of aluminum doping on the relaxation of the stresses acts as a template for crystallization of ZnO, allowing us to obtain partially oriented ZnO films with relatively large nonlinear coefficients. The measurements of the second harmonic tensor by the Maker fringes method are in good agreement with results for the electro-optic coefficient obtained by nonlinear ellipsometry.

In this work we investigated the second-order nonlinear optical properties of a group of Disperse-Red-1-based electro-optic fluorinated copolymers, synthesized with two fluorinated monomers: the hexafluoroisopropyl alphafluoroacrylate monomer and the alphafluoroacrylate monomer bearing the Disperse Red 1 chromophore. Copolymers, with a concentration of chromophore substituted groups ranging from 46% to 75% molar, were synthesized and deposited as single films on glass substrates, for second-harmonic generation, and as sandwich structures with electrodes, for nonlinear ellipsometry measurements. For large concentrations of substituted groups, cracking of the films was observed and did not allow for any electro-optic characterization by nonlinear ellipsometry. The second-harmonic generation tensor main component d33 was retrieved by means of second-harmonic generation at λ=1064nm for each concentration. The results obtained for the copolymer with the lowest concentration are compared with those obtained by nonlinear ellipsometry at λ=1550nm and used as a calibration to infer information on the electro-optic properties at large concentrations. Stability of the poling-induced second-order nonlinear properties was checked by means of nonlinear ellipsometry. By using the temperature scanning technique, a depolarization temperature Tdep=124°C was measured, while the use of the isothermal relaxation technique allowed to observe two different decay regimes, a Vogel-Fulcher-Taman-Hesse one for temperatures above 110°C and an Arrhenius one for temperatures below 110°C, with activation energy EA≈58kcal∕mol.

Multilayer optical devices generally suffer from two main losses sources: absorption of the materials and scattering losses, due both to volume and surface effects. The exact estimation of this latter contribution is of extreme importance for the final assessment and optimization of efficient devices. In particular, when intrinsic absorption of the materials cannot be further reduced, scattering measurements may provide useful information for improving optical device performance. In this work we investigated single SiO2, Al2O3 and HfO2 layers deposited by r.f. sputtering under different deposition conditions. These materials are being studied for implementation in multilayer dichroic mirrors for laser applications in the range from 260 to 350 nm. To avoid radiation damage, such devices need to be loss-free in the pumping and lasing region; hence, an insightful knowledge of all losses sources is fundamental.

Highly stable F-2 color centers are very efficiently produced in lithium fluoride (LiF) by electron beam irradiation at room temperature. We have fabricated optical microcavities in which the active medium is a low-energy electron beam irradiated LiF film, whose optical thickness is comparable with the peak wavelength (similar to 668 nm) of the F-2 broad photoluminescence band. By selecting the proper electron beam energy, one can control the F-2 color center depth distribution. This distribution influences the photoemission angular distribution of the microcavity, whose resonance properties arc determined by the coupling of the depth profile of the defects with the pump electromagnetic field and microcavity modes.

Optical coatings for the use in free electron laser systems have to withstand high power laser radiation and the intense energetic background radiation of the synchrotron radiation source. In general, the bombardment with high energetic photons leads to irreversible changes and a discoloration of the specimen. For the development of appropriate optical coatings, the degradation mechanisms of available optical materials have to be characterized. In this contribution the degradation mechanisms of single layer coatings (fluoride and oxide materials) and multilayer systems will be presented. Fluoride and oxide single layers were produced by thermal evaporation and high energetic ion beam sputter deposition. The same methods were employed for the deposition of multilayer systems. High reflecting coatings for the wavelength region around 180 nm were chosen for the irradiation tests. All samples were characterized after production by spectrophotometry covering the VUV, VIS, and MIR spectral range. Mechanical coating stress was evaluated with interferometric methods. Synchrotron irradiation tests were performed at ELETTRA, using a standardized irradiation cycle for all tests. Ambient pressure and possible contamination in the vacuum environment were monitored by mass spectrometry. For comparison, the optical coatings were investigated again in the VUV, VIS, and MIR spectral range after irradiation. On selected samples XRD measurements were performed. The observed degradation mechanisms comprise severe damages like coating and substrate surface ablation. Color centre formation in the VIS spectral range and an increase of VUV absorption were found as a major origin for a severe degradation of VUV transmittance On the basis of the performed investigations, a selection of coating materials and coating systems is possible in respect to the damage effects caused by synchrotron radiation.

The relative emission efficiency of F-3(+) and F-2 centers as a function of their concentration has been investigated in LiF crystals colored at room temperature and 260degreesC with 3 MeV electrons. From a careful study of their absorption and emission spectra, we discovered that the emission efficiency h of both F-3(+) and F-2 defects is constant up to similar to10(16) centers/cm(3). When increasing the concentration up to 10(18) centers/cm(3), h decreases slightly for the F-2 centers and considerably for the F-3(+) ones, a difference which is bigger at room temperature with respect to liquid nitrogen temperature. A thermally activated process is taking place, where the F-2 center and other aggregated defects play an important role. (C) 2003 The Electrochemical Society.

The stable formation of several colour centres (CCs) has been investigated in lithium fluoride (LiF) single crystals irradiated at room temperature (RT) and at 213 K by 5 MeV electrons with doses from 1019 to 1023 eV/cm3. The temperature during irradiation influences the production of aggregate defects, in particular the ratio between the F3+ and F2 laser active centres and the amount of parasitic complex defects. Optical absorption and photoluminescence spectra allow clarifying the role of different aggregate defects on the emission properties of the F3+ and F2 centres at concentrations up to 1018 cm−3.