Abstract:In this work we report on photochromism in transparent thin film samples of oxygencontaining yttrium hydride. Exposure to visible and ultraviolet (UV) light at moderate intensity triggers a decrease in the optical transmission of visible and infrared (IR) light. The photo-darkening is colour-neutral. We show that the optical transmission of samples of 500 nm thickness can be reduced by up to 50% after one hour of illumination with light of moderate intensity. The reaction is reversible and samples that are left in the dark return to the initial transparent state. The relaxation time in the dark depends on the temperature of the sample and the duration of the light exposure. The photochromic reaction takes place under ambient conditions in the as-deposited state of the thin-film samples. Graphical abstract:Research highlights:-Photochromic response in the as-deposited state at ambient conditions -Sensitive to visible and ultraviolet light -Color-neutral photo-darkening -Persistent photoconductivity accompanies the photochromic darkening -Reversible reaction, material relaxes back to transparent state when left in dark

Efficient anti-reflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. By simple electrochemical etching of the Si wafer, porous silicon (PS) layers with excellent broadband anti-reflection properties can be fabricated. In this work, ageing of graded PS has been studied using Spectroscopic Ellipsometry, Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy. During oxidation of PS elements such as pure Si (Si 0 ), Si2O (Si + ), SiO (Si 2+ ), Si2O3 (Si 3+ ), and SiO2 (Si 4+ ) are present. In addition both hydrogen and carbon is introduced to the PS in the form of Si3SiH and CO. The oxide grows almost linearly with time when exposed to oxygen, from an average thickness of 0 -3.8 nm for the surface PS. The oxidation is then correlated to the optical stability of multi-layered PS ARCs. It is found that even after extensive oxidation, the changes in the optical properties of the PS structures are small. PACS numbers:

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...

Material costs may be reduced and electrical properties improved by utilizing thinner solar cells. Light trapping makes it possible to reduce wafer thickness without compromising optical absorption in a silicon solar cell. In this work we present a comprehensive comparison of the light-trapping properties of various bi-periodic structures with a square lattice. The geometries that we have investigated are cylinders, cones, inverted pyramids, dimples (half-spheres), and three more advanced structures, which we have called the roof mosaic, rose, and zigzag structure. Through simulations performed with a 20 μm thick Si cell, we have optimized the geometry of each structure for light trapping, investigated the performance at oblique angles of incidence, and computed efficiencies for the different diffraction orders for the optimized structures. We find that the lattice periods that give optimal light trapping are comparable for all structures, but that the light-trapping ability varies considerably between the structures. A far-field analysis reveals that the superior light-trapping structures exhibit a lower symmetry in their diffraction patterns. The best result is obtained for the zigzag structure with a simulated photo-generated current Jph of 37.3 mA/cm2, a light-trapping efficiency comparable to that of Lambertian light-trapping.

No abstract