Oxygen-containing yttrium hydride thin films exhibit photochromic behavior: Transparent thin films reversibly switch from a transparent state to a photodarkened state after being illuminated with UV or blue light. From optical spectrophotometry and ellipsometry measurements of the transparent state and photodarkened state, it is concluded that the photochromic effect can be explained by the gradual growth, under illumination, of metallic domains within the initial wide-band-gap semiconducting lattice. This conclusion is supported by Raman measurements. DOI: 10.1103/PhysRevB.95.201301 Oxygen-containing yttrium hydride films exhibit photochromic (PC) behavior, i.e., the optical properties of the films change reversibly when illuminated by light of adequate energy (wavelengths in the blue or UV range). Early works by Hoekstra et al. [1] reported photoconductivity in yttrium hydrides at low temperature, and Ohmura et al. [2,3] accidentally discovered PC behavior in yttrium hydride films subjected to high pressure. In addition, Huiberts et al. observed for the first time the gasochromic behavior of yttrium hydride thin films [4]. Later, Mongstad et al. [5,6] reported PC behavior in transparent oxygen-rich yttrium hydride films under atmospheric conditions and at room temperature. In the latter case, however, the yttrium hydride films were directly obtained by reactive magnetron sputtering rather than by the subsequent hydrogenation of a predeposited metallic Y layer. Oxygen-rich yttrium hydride is not the only oxygen-containing hydride which exhibits interesting physical properties. For instance, Miniotas et al. have reported gigantic resistivity and band-gap changes in oxygen-containing gadolinium hydride [7].The mechanism of the PC behavior in oxygen-rich yttrium hydride is still unclear and seems to have no relation to the PC mechanism reported for transition-metal oxides [8]. In the present Rapid Communication, the properties of oxygen-rich transparent semiconducting yttrium hydride thin films-hereafter referred to as YH x O sc w , where the superscript sc refers to their semiconducting character-and of opaque metallic yttrium hydride thin films-from now on referred to as YH y O m z , where y < x and where the superscript m refers to their metallic character-have been investigated by grazing incidence x-ray diffraction (GIXRD), Raman spectroscopy, ellipsometry, and spectrophotometry. Both sets of films, YH x O sc w and YH y O m z , were deposited onto soda-lime glass substrates by sputter deposition at a hydrogen/argon ratio = 0.18 and 0.13, respectively, and then exposed to air where they oxidize. A detailed description of the deposition method can be found in our previous work [9] Chandran et al. [8], which reports changes of the hydrogen species in oxygen-containing yttrium hydride after illumination, suggesting the release of electrons and the formation of a metallic phase.Figure 1(a) shows GIXRD patterns-obtained by using Cu Kα radiation at a fixed angle of incidence of 2• in a Bruker Siemens D5000 diffr...

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Vertically aligned radial-junction solar cell designs offer potential improvements over planar geometries, as carrier generation occurs close to the junction for all absorption depths, but most production methods still require a single crystal substrate. Here, we report on the fabrication of such solar cells from polycrystalline, low purity (99.98%) p-type silicon starting material, formed into silicon core, silica sheath fibres using bulk glass draw techniques. Short segments were cut from the fibres, and the silica was etched from one side, which exposed the core and formed a conical cavity around it. We then used vapour deposition techniques to create p-i-n junction solar cells. Prototype cells formed from single fibres have shown conversion efficiencies up to 3.6%, despite the low purity of the starting material. This fabrication method has the potential to reduce the energy cost and the silicon volume required for solar cell production. Simulations were performed to investigate the potential of the conical cavity around the silicon core for light collection. Absorption of over 90% of the incident light was predicted, over a wide range of wavelengths, using these structures in combination with a 10% volume fraction of silicon.

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