High Energy Ultraviolet Photons Research Articles

Ultraviolet radiation impact on the efficiency of commercial crystalline silicon-based photovoltaics: a theoretical thermal-electrical study in realistic device architectures

Ultraviolet (UV) radiation has been identified as one of the most critical factors for the degradation of photovoltaics (PVs). Besides that, the UV spectral regime (∼0.28-0.4 µm) is less efficient for silicon-based PVs owing to the excess of the energy of the incident UV photons relative to the semiconductor’s bandgap; thus, a large part of the UV photon energy is transformed into heat, increasing the PV temperature and decreasing its efficiency. Therefore, it is crucial to investigate in detail and evaluate the UV radiation impact on the temperature and efficiency of realistic photovoltaic modules. Here we perform this investigation for crystalline silicon-based photovoltaics that operate outdoors. The investigation is performed by employing a thermal-electrical modeling approach, which takes into account all the major intrinsic processes affected by the temperature variation in the photovoltaic devices. We show that effectively reflecting UV radiation, i.e., up to a cut-off wavelength, which depends on the environmental conditions, results in a reduction of the overall operating temperature and enhancement of the PV cell’s efficiency. Additionally, blocking the high energy UV photons prolongs the lifetime of the PV and its performance in the long term.

Irradiation of pyrimidine in pure H2O ice with high-energy ultraviolet photons.

The detection of nucleobases, the informational subunits of DNA and RNA, in several meteorites suggests that these compounds of biological interest were formed via astrophysical, abiotic processes. This hypothesis is in agreement with recent laboratory studies of irradiation of pyrimidine in H2O-rich ices with vacuum UV photons emitted by an H2-discharge lamp in the 6.9-11.3 eV (110-180 nm) range at low temperature, shown to lead to the abiotic formation of several compounds including the nucleobases uracil, cytosine, and thymine. In this work, we irradiated H2O:pyrimidine ice mixtures under astrophysically relevant conditions (14 K, ≤10(-9) torr) with high-energy UV photons provided by a synchrotron source in three different ranges: the 0(th) order light (4.1-49.6 eV, 25-300 nm), the He i line (21.2 eV, 58.4 nm), and the He ii line (40.8 eV, 30.4 nm). The photodestruction of pyrimidine was monitored with IR spectroscopy, and the samples recovered at room temperature were analyzed with liquid and gas chromatographies. Uracil and its precursor 4(3H)-pyrimidone were found in all samples, with absolute and relative abundances varying significantly from one sample to another. These results support a scenario in which compounds of biological interest can be formed and survive in environments subjected to high-energy UV radiation fields.

Enhancing NIR emission of Yb3+ by silver nanoclusters in oxyfluoride glass

Oxyfluoride glass comprising Yb3+ ions and Ag nanoclusters was prepared via a melt-quenching method. The molecule-like Ag clusters (ML-Ag) in the glass displayed broad absorption and emission bands in the ultraviolet (UV) and visible spectral regions, respectively. The luminescence quantum yield (QY) was determined to be higher than 90%. We observed energy transfer from ML-Ag nanoclusters to Yb3+ ions as evidenced by time resolved luminescence spectra, which resulted in a four-fold enhancement in NIR emission of Yb3+. The luminescence enhancement effect described here would be applied to boost the efficiency of solar cells by converting high energy UV photons into NIR photons that better match the absorption of photovoltaic devices.

Advanced UV Lasers Enable Precision Processing

AbstractLasers with ultraviolet (UV) output offer unique benefits for processing a wide range of materials. In particular, the short wavelength, high energy UV photons experience less diffraction than longer visible or infrared wavelengths, which enables materials processing at higher spatial resolution. In addition, UV lasers micromachine primarily by non‐thermal means, thus minimizing the heat affected zone due to processing. As a result, UV lasers are now employed for many demanding, high precision tasks in semiconductor fabrication, display manufacture, medical device production, therapeutic medical procedures and scientific research. This article explores some of these applications as well as recent advances in the laser technology that has been developed to service them.

Fabrication of high aspect ratio Si nanogratings with smooth sidewalls for a deep UV-blocking particle filter

To measure space plasmas and neutral particles one must filter out high-energy ultraviolet photons that would increase background count or damage sensors. To enable sensitive neutral particle measurements, a photon-to-particle rejection rate of 1014 is desired, far exceeding the requirements of prior filters. The authors propose a high-aspect ratio Si grating with densely packed, sub-100 nm slits. In this article, the authors report the development of a new technique for fabricating sturdy, self-supported transmission gratings in silicon using nanoimprint lithography and deep reactive ion etching, resulting in grating slits with scalloping under 7 nm and high (8.5:1) aspect ratios.

Phosphor coatings to enhance Si photovoltaic cell performance

Polycrystalline Si photovoltaic cells were coated with photoluminescent phosphors to increase the power conversion efficiency by down-conversion of high energy ultraviolet photons to less energetic visible photons, for which the Si solar cell conversion efficiency is greater. Phosphor coatings consisting of Y2O3:Eu3+, Y2O2S:Eu3+, 3,9-perylenedicarboxylic acid bis(2-methylpropyl) ester, and tetraphenylporphyrin dispersed in either polyvinyl alcohol or polymethylmethacrylate were tested. Cells coated with the Y2O3:Eu3+ or Y2O2S:Eu3+ showed an increased power conversion efficiency by up to a factor of 14 under UV illumination. The increased power conversion efficiency was shown to depend on the quantum efficiency of the down-conversion phosphor, the overlap between the photoluminescence excitation and the illumination spectra, and the overlap of the emission spectrum and the solar cell spectral response function.

Ultraviolet-assisted pulsed laser deposition of thin oxide films

The properties of thin Y2O3, ZnO, and Ba0.5Sr0.5TiO3 films grown using an in situ ultraviolet (UV)-assisted pulsed laser deposition (UVPLD) technique have been studied. With respect to films grown by conventional PLD under similar conditions but without UV illumination, the UVPLD grown films exhibited better structural, optical, and electrical properties, especially for lower substrate temperatures. These improvements can be explained by the combined action of several processes. First, high energy UV photons and ozone ensure a better in situ cleaning of the substrate prior to the deposition. Second, the presence of ozone and atomic oxygen formed by photodissociation of molecular O2 promotes the growth of more oxygenated films. Finally, absorption of UV photons by adatoms could result in an increase of their surface mobility. All these factors have a beneficial effect upon crystalline growth.

From New Phosphor, a Double Crop of Photons

MATERIALS SCIENCE Cheap, efficient, and cool, fluorescent tubes are so common that it is hard to imagine a public or office building without them. But they are not ideal. They contain mercury, which is poisonous and has to vaporize each time the power goes on, resulting in a lag that keeps them from being used as brake lights for cars and in fax and photocopy machines. In this issue, a team of Dutch researchers reports a step toward a better fluorescent tube, which could end up making these lights even more ubiquitous. Replacing the mercury inside the tubes with a noble gas like xenon would eliminate the hazard and speed start-up. The stumbling block is the phosphor: the coating that absorbs ultraviolet (UV) light given off by the vapor inside the tube and reemits white light. Although current phosphors absorb UV photons from the mercury and reemit them as visible photons with about 90% efficiency, that's not good enough for xenon. Xenon generates UV photons at much higher energies than mercury, so one-to-one conversion to low-energy visible photons means that much of the energy going into the xenon is wasted as heat. For practical xenon tubes, “it became necessary to develop phosphors that give more than one [visible] photon for every absorbed UV photon,” says Alok Srivastava, a chemist at General Electric in New York. On page [663][1], chemist Andries Meijerink and his colleagues at Utrecht University in the Netherlands report an experimental phosphor that does just that. The quest for a photon-doubling phosphor started almost 30 years ago. Researchers began looking for the effect in materials containing lanthanides—the heavy elements that are a standard ingredient in ordinary phosphors. Some early formulations did manage the two-for-one conversion, but they emitted lots of ultraviolet or infrared photons, and not enough visible light. “By the 1990s, people gave up,” says Meijerink. Meijerink, however, was one of the few who persisted, studying how lanthanide ions lose energy after UV light has excited their electrons to high energy levels. He hoped to find that a lanthanide boosted to a high energy level could, say, lose energy in two steps, emitting a visible photon each time. But it turned out, he says, “that using only one ion would never work.” Then Meijerink's colleague Harry Donker thought of distributing the energy from each absorbed UV photon between two different types of lanthanide ions. Meijerink and his colleagues tested a pair of lanthanides, gadolinium and europium, in the form of crystals of lithium gadolinium fluoride, doped with europium. They bombarded the crystals with high-energy UV photons, which bumped the gadolinium into an excited state. The gadolinium got rid of its excess energy by transferring it to two nearby europium ions through a complex interaction of electric fields. The europium ions in turn shed the energy by each emitting a single red photon, giving a total of two red photons emitted for each UV photon. To make sure that each UV photon really was generating two red photons, the researchers bombarded the crystals with two different levels of UV radiation, one energetic enough to drive the two-step process, the other able only to drive a one-step process that produced just one red photon. The red fluorescence sparked by the higher energy UV was twice as intense, indicating that double the number of photons was being emitted. Because plain red light isn't useful in most settings, Meijerink now wants to hunt for other phosphors that will use the same principle to emit green and blue photons, which in combination with red photons would yield white light. He also needs to find a way to make his phosphor absorb more UV photons, so that it can emit brighter light. “There are engineering issues, like sample purity and stability, that have to be addressed,” says Kevin Bray, a chemist at Washington State University in Pullman, “but they have shown that it is possible to take a high-energy UV photon and extract from it two visible photons. That is very promising.” Srivastava agrees: “This is the first experiment to demonstrate that you can get a practical system out of these ideas.” [1]: /lookup/doi/10.1126/science.283.5402.663

Chemical studies of (001)-oriented Er 1Ba 2Cu 3O 7−δ,Er 5Ba 7Cu 12O y, and Er 4Ba 5Cu 9O y high-temperature superconductor thin films by photoemission spectroscopy

Clean surfaces of (001)-oriented Er 1Ba 2Cu 3O 7−δ, Er 5Ba 7Cu 12O y , and Er 4Ba 5Cu 9O y high-temperature superconductor thin films were studied by photoemission spectroscopy using high-energy ultraviolet photons ( hν in the 50 to 200 eV range). The surfaces were cleaned by heating in an ultrahigh-vacuum electron spectrometer at 400°C (or 350°C) in 1 × 10 −3 Torr oxygen. Valence bands and outer core levels were studied. Ba 4 d and Ba 5 p doublet photopeaks showed spin-orbit splitting of 2.6 and 2.2 eV, respectively, as expected. Er, Ba, Cu, and O features in the valence band were identified.

Effects of Ultraviolet Radiation on the Characteristics of Hydrogenated Amorphous Silicon Thin Film Transistors

ABSTRACTThe experimental results regarding to the effects of ultraviolet (UV) light illumination on the characteristics of hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFT's) have been presented. The device parameters of a-Si:H TFT, such as threshold voltage, field-effect mobility, and subthreshold slope, have been degraded by electrical stress and visible light illumination, but substantially improved by UV radiation. This may be attributed to an annealing effect on the dangling-bond defects, involving a number of phonons generated by absorption of high energy UV photons in the a-Si:H TFT channel. It has been also observed that the off-current of a-Si:H TFT decreases remarkably while the on-current changes very little. From the experimental results, we report that the improved on/off current ratio of a-Si:H TFT may be achieved by UV radiation.