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  • Journal article
    Nguyen N, Maier SA, Hong M, Oulton RFet al., 2016,

    Recovering parity-time symmetry in highly dispersive coupled optical waveguides

    , New Journal of Physics, Vol: 18, ISSN: 1367-2630

    Coupled photonic systems satisfying parity-time symmetry (PTS) provideexibility to engineer the ow of light including non-reciprocal propagation, perfectlaser-absorbers, and ultra-fast switching. Achieving the required index pro le foran optical system with ideal PTS, i.e. n(x) =n(-x)*, has proven to be difficult due to the challenge of controlling gain, loss and material dispersion simultaneously. Consequently, most research has focused on dilute or low gain optical systems where material dispersion is minimal. In this paper, we study a model system of coupled inorganic semiconductor waveguides with potentially high gain (>1,500 cm-1) and dispersion. Our analysis makes use of coupled mode theory's parameters to quantify smooth transitions between PTS phases under imperfect conditions. We find that the detrimental influence of gain-induced dispersion is counteracted and the key features of parity-time symmetric optical systems are recovered by working with non-identical waveguides and bias pumping of the optical waveguides. Our coupled mode theory results show excellent agreement with numerical solutions, proving the robustness of coupled mode theory in describing various degrees of imperfection in systems with PTS.

  • Journal article
    Calado P, Telford AM, Bryant D, Li X, Nelson J, Barnes PRFet al., 2016,

    Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis

    , Nature Communications, Vol: 7, ISSN: 2041-1723

    Ionic migration has been proposed as a possible cause of photovoltaic current-voltage hysteresis in hybrid perovskite solar cells. A major objection to this hypothesis is that hysteresis can be reduced by changing the interfacial contact materials; this is unlikely to significantly influence the behaviour of mobile ionic charge within the perovskite phase. Here we show that the primary effects of ionic migration can in fact be observed regardless of whether the contacts were changed to give devices with or without significant hysteresis. Transient optoelectronic measurements combined with device simulations indicate that electric-field screening, consistent with ionic migration, is similar in both high and low hysteresis CH3NH3PbI3 cells. Simulation of the photovoltage and photocurrent transients shows that hysteresis requires the combination of both mobile ionic charge and recombination near the perovskite-contact interfaces. Passivating contact recombination results in higher photogenerated charge concentrations at forward bias which screen the ionic charge, reducing hysteresis.

  • Journal article
    Belisle RA, Nguyen WH, Bowring AR, Calado P, Li X, McGehee MD, Barnes PRF, O'Regan BCet al., 2016,

    Interpretation of inverted photocurrent transients in organic lead halide perovskite solar cells; proof of the field screening by mobile ions and determination of the space charge layer widths

    , Energy & Environmental Science, Vol: 10, Pages: 192-204, ISSN: 1754-5706

    In Methyl Ammonium Lead Iodide (MAPI) perovskite solar cells, screening of the built in field by mobile ions has been proposed as part of the cause of the large hysteresis observed in the current/voltage scans in many cells. We show that photocurrent transients measured immediately (e.g. 100 μs) after a voltage step can provide direct evidence that this field screening exists. Just after a step to forward bias, the photocurrent transients are reversed in sign (i.e. inverted), and the magnitude of the inverted transients can be used to find an upper bound on the width of the space charge layers adjacent to the electrodes. This in turn provides a lower bound on the mobile charge concentration, which we find to be 1 x 1017/cm3. Using a new photocurrent transient experiment, we show that the space charge layer thickness remains approximately constant as a function of bias, as expected for mobile ions in a solid electrolyte. We also discuss additional characteristics of the inverted photocurrent transients that imply either an unusually stable deep trapping, or a photo effect on the mobile ion conductivity.

  • Journal article
    Barnes PRF, Vaissier V, Garcia Sakai V, Li X, cabral J, nelson Jet al., 2016,

    How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study

    , Scientific Reports, Vol: 6, ISSN: 2045-2322

    Motions of molecules adsorbed to surfaces may control the rate of charge transport within monolayers in systems such as dye sensitized solar cells. We used quasi-elastic neutron scattering (QENS) to evaluate the possible dynamics of two small dye moieties, isonicotinic acid (INA) and bis-isonicotinic acid (BINA), attached to TiO2 nanoparticles via carboxylate groups. The scattering data indicate that moieties are immobile and do not rotate around the anchoring groups on timescales between around 10 ps and a few ns (corresponding to the instrumental range). This gives an upper limit for the rate at which conformational fluctuations can assist charge transport between anchored molecules. Our observations suggest that if the conformation of larger dye molecules varies with time, it does so on longer timescales and/or in parts of the molecule which are not directly connected to the anchoring group. The QENS measurements also indicate that several layers of acetonitrile solvent molecules are immobilized at the interface with the TiO2 on the measurement time scale, in reasonable agreement with recent classical molecular dynamics results.

  • Journal article
    Mellor AV, Hylton N, Maier S, Ekins-Daukes Net al., 2016,

    Interstitial light-trapping design for multi-junction solar cells

    , Solar Energy Materials and Solar Cells, Vol: 159, Pages: 212-218, ISSN: 0927-0248

    We present a light-trapping design capable of significantly enhancing the photon absorption inany subcell of a multi-junction solar cell. The design works by coupling incident light intowaveguide modes in one of the subcells via a diffraction grating, and preventing these modesfrom leaking into lower subcells via a low-index layer and a distributed Bragg reflector, whichtogether form an omnidirectional mirror. This allows the thickness of the target subcell to bereduced without compromising photon absorption, which improves carrier collection, andtherefore photocurrent. The paper focuses on using the composite structure to improve theradiation hardness of a InGaP/Ga(In)As/Ge space solar cell. In this context, it is shown viasimulation that the Ga(In)As middle-cell thickness can be reduced from 3500 to 700 nm,whilst maintaining strong photon absorption, and that this leads to a significantly improvedend-of-life photocurrent in the Ga(In)As middle cell. However, the design can in general beapplied to a wide range of multi-junction solar cell types. We discuss the principles ofoperation of the design, as well as possible methods of its fabrication and integration intomulti-junction solar cells.

  • Journal article
    Petrus ML, Hu Y, Moia D, Calado P, Leguy AMA, Barnes PRF, Docampo Pet al., 2016,

    The Influence of Water Vapor on the Stability and Processing of Hybrid Perovskite Solar Cells Made from Non-Stoichiometric Precursor Mixtures

    , Chemsuschem, Vol: 9, Pages: 2699-2707, ISSN: 1864-564X

    We investigated the influence of moisture on methylammonium lead iodide perovskite (MAPbI3) films and solar cells derived from non-stoichiometric precursor mixtures. We followed both the structural changes under controlled air humidity through in situ X-ray diffraction, and the electronic behavior of devices prepared from these films. A small PbI2 excess in the films improved the stability of the perovskite compared to stoichiometric samples. We assign this to excess PbI2 layers at the perovskite grain boundaries or to the termination of the perovskite crystals with Pb and I. In contrast, the MAI-excess films composed of smaller perovskite crystals showed increased electronic disorder and reduced device performance owing to poor charge collection. Upon exposure to moisture followed by dehydration (so-called solvent annealing), these films recrystallized to form larger, highly oriented crystals with fewer electronic defects and a remarkable improvement in photocurrent and photovoltaic efficiency.

  • Journal article
    Carter-Gartside J, Burn DM, Cohen LF, Branford WRet al., 2016,

    A Novel Method for the Injection and Manipulation of Magnetic Charge States in Nanostructures

    , Scientific Reports, Vol: 6, ISSN: 2045-2322

    Realising the promise of next-generation magnetic nanotechnologies is contingent on the development of novel methods for controlling magnetic states at the nanoscale. There is currently demand for simple and flexible techniques to access exotic magnetisation states without convoluted fabrication and application processes. 360° domain walls (metastable twists in magnetisation separating two domains with parallel magnetisation) are one such state, which is currently of great interest in data storage and magnonics. Here, we demonstrate a straightforward and powerful process whereby a moving magnetic charge, provided experimentally by a magnetic force microscope tip, can write and manipulate magnetic charge states in ferromagnetic nanowires. The method is applicable to a wide range of nanowire architectures with considerable benefits over existing techniques. We confirm the method’s efficacy via the injection and spatial manipulation of 360° domain walls in Py and Co nanowires. Experimental results are supported by micromagnetic simulations of the tip-nanowire interaction.

  • Journal article
    Moia D, Szumska A, Vaissier V, Planells M, Robertson N, O'Regan BC, Nelson J, Barnes PRet al., 2016,

    Interdye Hole Transport Accelerates Recombination in Dye Sensitized Mesoporous Films

    , Journal of the American Chemical Society, Vol: 138, Pages: 13197-13206, ISSN: 1520-5126

    Charge recombination between oxidized dyes attached to mesoporous TiO2 and electrons in the TiO2 was studied in inert electrolytes using transient absorption spectroscopy. Simultaneously, hole transport within the dye monolayers was monitored by transient absorption anisotropy. The rate of recombination decreased when hole transport was inhibited selectively, either by decreasing the dye surface coverage or by changing the electrolyte environment. From Monte Carlo simulations of electron and hole diffusion in a particle, modeled as a cubic structure, we identify the conditions under which hole lifetime depends on the hole diffusion coefficient for the case of normal (disorder free) diffusion. From simulations of transient absorption and transient absorption anisotropy, we find that the rate and the dispersive character of hole transport in the dye monolayer observed spectroscopically can be explained by incomplete coverage and disorder in the monolayer. We show that dispersive transport in the dye monolayer combined with inhomogeneity in the TiO2 surface reactivity can contribute to the observed stretched electron-hole recombination dynamics and electron density dependence of hole lifetimes. Our experimental and computational analysis of lateral processes at interfaces can be applied to investigate and optimize charge transport and recombination in solar energy conversion devices using electrodes functionalized with molecular light absorbers and catalysts.

  • Journal article
    Albella Echave P, Shibanuma T, Maier S, 2016,

    Unidirectional light scattering with high efficiency at optical frequencies based on low-loss dielectric nanoantennas

    , Nanoscale, Vol: 8, Pages: 14184-14192, ISSN: 2040-3372

    Dielectric nanoparticles offer low optical losses and access to both electric and magnetic Mie resonances. This enables unidirectional scattering along the incident axis of light, owing to the interference between these two resonances. Here we theoretically and experimentally demonstrate that an asymmetric dimer of dielectric nanoparticles can provide unidirectional forward scattering with high efficiency. Theoretical analyses reveal that the dimer configuration can satisfy the first Kerker condition at the resonant peaks of electric and magnetic dipolar modes, therefore showing highly efficient directional forward scattering. The unidirectional forward scattering with high efficiency is confirmed in our experiments using a silicon nanodisk dimer on a transparent substrate. This study will boost the realization of practical applications using low-loss and efficient subwavelength all-dielectric nanoantennas.

  • Journal article
    Alonso Alvarez D, Ekins-Daukes N, 2016,

    SPICE modelling of photoluminescence and electroluminescence based current-voltage curves of solar cells for concentration applications

    , Journal of Green Engineering, Vol: 5, Pages: 33-48, ISSN: 2245-4586

    Quantitative photoluminescence (PL) or electroluminescence (EL) experiments can be usedto probe fast and in a non-destructive way the current-voltage (IV) characteristics ofindividual subcells in a multi-junction device, information that is, otherwise, not available.PL-based IV has the advantage that it is contactless and can be performed even in partlyfinished devices, allowing for an early diagnosis of the expected performance of the solarcells in the production environment. In this work we simulate the PL- and EL-based IVcurves of single junction solar cells to assess their validity compared with the true IV curveand identify injection regimes where artefacts might appear due to the limited in-planecarrier transport in the solar cell layers. We model the whole photovoltaic device as anetwork of sub-circuits, each of them describing the solar cell behaviour using the two diodemodel. The sub-circuits are connected to the neighbouring ones with a resistor, representingthe in-plane transport in the cell. The resulting circuit, involving several thousand subcircuits,is solved using SPICE.

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