Adaptive forecasting of phytoplankton communities

The global proliferation of harmful algal blooms poses an increasing threat to water resources, recreation and ecosystems. Predicting the occurrence of these blooms is therefore needed to assist water managers in making management decisions to mitigate their impact. Evaluation of the potential for forecasting of algal blooms using the phytoplankton community model PROTECH was undertaken in pseudo-real-time. This was achieved within a data assimilation scheme using the Ensemble Kalman Filter to allow uncertainties and model nonlinearities to be propagated to forecast outputs. Tests were made on two mesotrophic lakes in the English Lake District, which differ in depth and nutrient regime. Some forecasting success was shown for chlorophyll a, but not all forecasts were able to perform better than a persistence forecast. There was a general reduction in forecast skill with increasing forecasting period but forecasts for up to four or five days showed noticeably greater promise than those for longer periods. Associated forecasts of phytoplankton community structure were broadly consistent with observations but their translation to cyanobacteria forecasts was challenging owing to the interchangeability of simulated functional species

Atomistic origins of high-performance in hybrid halide perovskite solar cells

The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitised and organic photovoltaics. High power conversion efficiency can be realised in both mesoporous and thin-film device architectures. We address the origin of this success in the context of the materials chemistry and physics of the bulk perovskite as described by electronic structure calculations. In addition to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses), the materials are structurally and compositionally flexible. As we show, hybrid perovskites exhibit spontaneous electric polarisation; we also suggest ways in which this can be tuned through judicious choice of the organic cation. The presence of ferroelectric domains will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and reduction of recombination through segregation of charge carriers. The combination of high dielectric constant and low effective mass promotes both Wannier-Mott exciton separation and effective ionisation of donor and acceptor defects. The photoferroic effect could be exploited in nanostructured films to generate a higher open circuit voltage and may contribute to the current-voltage hysteresis observed in perovskite solar cells.Comment: 6 pages, 5 figure

Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites.

The introduction of a mobile and polarized organic moiety as a cation in 3D lead-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the time scales of the orientational mobility of the organic cation and the molecular mechanism behind its motion remain unclear, with different experimental and computational approaches providing very different qualitative and quantitative description of the molecular dynamics. Here we use ultrafast 2D vibrational spectroscopy of methylammonium (MA) lead iodide to directly resolve the rotation of the organic cations within the MAPbI3 lattice. Our results reveal two characteristic time constants of motion. Using ab initio molecular dynamics simulations, we identify these as a fast (∼300 fs) "wobbling-in-a-cone" motion around the crystal axis and a relatively slow (∼3 ps) jump-like reorientation of the molecular dipole with respect to the iodide lattice. The observed dynamics are essential for understanding the electronic properties of perovskite materials.This work was supported by The Netherlands Organization for Scientific Research (NWO) through the “Stichting voor Fundamenteel Onderzoek der Materie” (FOM) research program. A.A.B. also acknowledges a VENI grant from the NWO. A.A.B. is currently a Royal Society University Research Fellow. Z.S. and Z.C. acknowledge the ANR-2011-JS09-004-01-PvCoNano project and the EU Marie Curie Career Integration Grant (303824). A.A.B., Z.S., and Z.C. thank Dutch-French Academy for the support through van Gogh grant.This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry Letters, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.5b0155

Grain rotation and lattice deformation during perovskite spray coating and annealing probed in situ by GI-WAXS

We report for the first time on grain rotation in CH3NH3PbI3 perovskite films for ∼12% efficient planar solar cells and present a new method for investigating their texture evolution during thermal annealing. Our technique is based on in situ 2D grazing incidence wide-angle X-ray scattering (GI-WAXS) and employs a 10 keV wide-focussed X-ray beam to simultaneously probe a large number of grains. The ability to track the texture dynamics from a statistically relevant number of spots diffracting from single grains during thermal annealing and in grazing incidence geometry can have applications understanding the processing dynamics of a range of new materials

Constraining uncertainty and process-representation in an algal community lake model using high frequency in-lake observations

Excessive algal blooms, some of which can be toxic, are the most obvious symptoms of nutrient enrichment and can be exacerbated by climate change. They cause numerous ecological problems and also economic costs to water companies. The process-representation of the algal community model PROTECH was tested within the extended Generalised Likelihood Uncertainty Estimation framework which includes pre-defined Limits of Acceptability for simulations. Testing was a precursor to modification of the model for real-time forecasting of algal communities that will place different demands on the model in terms of a) the simulation accuracy required, b) the computational burden associated with the inclusion of forecast uncertainties and c) data assimilation. We found that the systematic differences between the model’s representation of underwater light compared to the real lake systems studied and the uncertainties associated with nutrient fluxes will be the greatest challenges when forecasting algal blooms

Clean thermal decomposition of tertiary-alkyl metal thiolates to metal sulfides: Environmentally-benign, non-polar inks for solution-processed chalcopyrite solar cells

We report the preparation of Cu2S, In2S3, CuInS2 and Cu(In,Ga)S2 semiconducting films via the spin coating and annealing of soluble tertiary-alkyl thiolate complexes. The thiolate compounds are readily prepared via the reaction of metal bases and tertiary-alkyl thiols. The thiolate complexes are soluble in common organic solvents and can be solution processed by spin coating to yield thin films. Upon thermal annealing in the range of 200-400 ??C, the tertiary-alkyl thiolates decompose cleanly to yield volatile dialkyl sulfides and metal sulfide films which are free of organic residue. Analysis of the reaction byproducts strongly suggests that the decomposition proceeds via an SN1 mechanism. The composition of the films can be controlled by adjusting the amount of each metal thiolate used in the precursor solution yielding bandgaps in the range of 1.2 to 3.3 eV. The films form functioning p-n junctions when deposited in contact with CdS films prepared by the same method. Functioning solar cells are observed when such p-n junctions are prepared on transparent conducting substrates and finished by depositing electrodes with appropriate work functions. This method enables the fabrication of metal chalcogenide films on a large scale via a simple and chemically clear process.ope

Importance of Spin-Orbit Coupling in Hybrid Organic/Inorganic Perovskites for Photovoltaic Applications

International audienceThree-dimensional (3D) hybrid perovskites CH3NH3PbX3 (X = Br, I) have recently been suggested as new key materials for dye-sensitized solar cells (DSSC) leading to a new class of hybrid semiconductor photovoltaic cells (HSPC). Thanks to density functional theory calculations, we show that the band gap of these compounds is dominated by a giant spin-orbit coupling (SOC) in the conduction-band (CB). At room temperature, direct and isotropic optical transitions are associated to a spin-orbit split-off band related to the triply degenerated CB of the cubic lattice without SOC. Due to the strong SOC, the electronic states involved in the optical absorption are only slightly perturbed by local distortions of the lattice. In addition, band offset calculations confirm that CH3NH3PbX3/TiO2 is a reference material for driving electrons toward the electrode in HSPC. Two-dimensional (2D) hybrids are also suggested to reach further flexibility for light conversion efficiency. Our study affords the basic concepts to reach the level of knowledge already attained for optoelectronic properties of conventional semiconductors