• Energy Research

The optical photoluminescent (PL) emission of Yb3+ ions in the near infrared (NIR) spectral region at about 950–1100 nm has many potential applications, from photovoltaics to lasers and visual devices. However, due to their simple energy-level structure, Yb3+ ions cannot directly absorb UV or visible light, putting serious limits on their use as light emitters. In this paper we describe a broadband and efficient strategy for sensitizing Yb3+ ions by Ag codoping, resulting in a strong 980 nm PL emission under UV and violet-blue light excitation. Yb-doped silica–zirconia–soda glass–ceramic films were synthesized by sol-gel and dip-coating, followed by annealing at 1000 °C. Ag was then introduced by ion-exchange in a molten salt bath for 1 h at 350 °C. Different post-exchange annealing temperatures for 1 h in air at 380 °C and 430 °C were compared to investigate the possibility of migration/aggregation of the metal ions. Studies of composition showed about 1–2 wt% Ag in the exchanged samples, not modified by annealing. Structural analysis reported the stabilization of cubic zirconia by Yb-doping. Optical measurements showed that, in particular for the highest annealing temperature of 430 °C, the potential improvement of the material’s quality, which would increase the PL emission, is less relevant than Ag-aggregation, which decreases the sensitizers number, resulting in a net reduction of the PL intensity. However, all the Ag-exchanged samples showed a broadband Yb3+ sensitization by energy transfer from Ag aggregates, clearly attested by a broad photoluminescence excitation spectra after Ag-exchange, paving the way for applications in various fields, such as solar cells and NIR-emitting devices.

The growth of TiO2 nanotube arrays on plastic flexible substrates is researched. The approach uses anodization of a titanium thick film for obtaining nanotubes directly on poly(ethylene terephthalate) (PET) and Kapton HN substrate. The morphological features of the tubes can be finely tuned by varying the preparation conditions, and tube morphology affects the functional properties of the nanotube array. Crystallization of the anatase phase in nanotubes on Kapton HN substrate is obtained via post growth annealing. The nanotube arrays have been dye-sensitized using the commercial Ru-based N719 dye. The system was tested as photoanode in a flexible dye sensitized solar cell. Photoconversion efficiency of 3.5% was obtained.

In this study, unmodified and graphene (G)-modified TiO2–CuO mixed oxide thin films were synthesized via spray pyrolysis, incorporating varying graphene content (y = 2, 4, 6, and 8 at. %). The modified samples were subjected to photocatalytic (Rhodamine B (RhB), Malachite green (MG), Methylene Blue, and Methyl Orange) and photovoltaic performance evaluations. X-ray diffraction (XRD) confirmed the formation of well-crystalline thin films, while X-ray photoelectron spectroscopy (XPS) (XPS) provided insights into the electronic states of Cu, Ti, C, and O elements. After graphene modification, the Cu 2p and Ti 2p spectra exhibited a negative and positive shift, respectively, indicating Cu reduction and Ti oxidation. Optical absorption analysis revealed an increase in band gap energy with higher graphene concentrations, reaching 1.78 eV at 6 at. % graphene content. The as-prepared samples were tested for photocatalytic degradation of organic dyes in polluted water, including Rhodamine B (RhB), Malachite Green (MG), Methylene Blue (MB), and Methyl Orange (MO). The film dropped at 8 at. % graphene demonstrated remarkable photocatalytic efficiency, achieving degradation rates of 90 %, 85 %, 96 %, and 87 % for RhB, MG, MB, and MO, respectively, within 2 h of solar illumination. Furthermore, the application of G-TiO2-CuO as a secondary absorber layer in CZTS solar cells was optimized using Silvaco TCAD software, resulting in an efficiency enhancement from 10.25 % to 15.31 %. These findings highlight the crucial role of graphene modification in enhancing the physical properties of semiconductor materials, making them promising candidates for advanced optoelectronic applications.

AbstractThe combination of the ability to absorb most of the solar radiation and simultaneously suppress infrared re-radiation allows selective solar absorbers (SSAs) to maximize solar energy to heat conversion, which is critical to several advanced applications. The intrinsic spectral selective materials are rare in nature and only a few demonstrated complete solar absorption. Typically, intrinsic materials exhibit high performances when integrated into complex multilayered solar absorber systems due to their limited spectral selectivity and solar absorption. In this study, we propose CoSbx (2 < x < 3) as a new exceptionally efficient SSA. Here we demonstrate that the low bandgap nature of CoSbx endows broadband solar absorption (0.96) over the solar spectral range and simultaneous low emissivity (0.18) in the mid-infrared region, resulting in a remarkable intrinsic spectral solar selectivity of 5.3. Under 1 sun illumination, the heat concentrates on the surface of the CoSbx thin film, and an impressive temperature of 101.7 °C is reached, demonstrating the highest value among reported intrinsic SSAs. Furthermore, the CoSbx was tested for solar water evaporation achieving an evaporation rate of 1.4 kg m−2 h−1. This study could expand the use of narrow bandgap semiconductors as efficient intrinsic SSAs with high surface temperatures in solar applications.

The NO cycle takes place in the deepest layer of a H-burning core or shell, when the temperature exceeds T {\simeq} 30 {\cdot} 106 K. The O depletion observed in some globular cluster giant stars, always associated with a Na enhancement, may be due to either a deep mixing during the RGB (red giant branch) phase of the star or to the pollution of the primordial gas by an early population of massive AGB (asymptotic giant branch) stars, whose chemical composition was modified by the hot bottom burning. In both cases, the NO cycle is responsible for the O depletion. The activation of this cycle depends on the rate of the 15N(p,��)16O reaction. A precise evaluation of this reaction rate at temperatures as low as experienced in H-burning zones in stellar interiors is mandatory to understand the observed O abundances. We present a new measurement of the 15N(p,��)16O reaction performed at LUNA covering for the first time the center of mass energy range 70-370 keV, which corresponds to stellar temperatures between 65 {\cdot} 106 K and 780 {\cdot}106 K. This range includes the 15N(p,��)16O Gamow-peak energy of explosive H-burning taking place in the external layer of a nova and the one of the hot bottom burning (HBB) nucleosynthesis occurring in massive AGB stars. With the present data, we are also able to confirm the result of the previous R-matrix extrapolation. In particular, in the temperature range of astrophysical interest, the new rate is about a factor of 2 smaller than reported in the widely adopted compilation of reaction rates (NACRE or CF88) and the uncertainty is now reduced down to the 10% level. 6 pages, 5 figures