In situ observation of heat-induced degradation of perovskite solar cells

The lack of thermal stability of perovskite solar cells is hindering the progress of this technology towards adoption in the consumer market. Different pathways of thermal degradation are activated at different temperatures in these complex nanostructured hybrid composites. Thus, it is essential to explore the thermal response of the mesosuperstructured composite device to engineer materials and operating protocols. Here we produce devices according to four well-established recipes, and characterize their photovoltaic performance as they are heated within the operational range. The devices are analysed using transmission electron microscopy as they are further heated in situ, to monitor changes in morphology and chemical composition. We identify mechanisms for structural and chemical changes, such as iodine and lead migration, which appear to be correlated to the synthesis conditions. In particular, we identify a correlation between exposure of the perovskite layer to air during processing and elemental diffusion during thermal treatment. Solar cells based on lead halide perovskite composites have become increasingly popular in the past few years owing to a combination of low synthesis cost and high power conversion efficiency, with certified values in excess of 20% (refs 1,2,3,4,5). However, the stability of such devices is a concern—it is well known that heating at or above around 85 ∘C, a temperature close to those reached during normal operation in full sunlight, performance degrades rapidly, and such instability is exacerbated by exposure to moisture; systematic thermal and ageing studies are required to understand such degradation processes. Changes happen in both the organic and inorganic components of the cells; the resilience of the perovskite layer, in particular, is expected to become a limiting factor once different hole-conducting materials (or hole-conductor-free cells) are developed. To overcome this limitation, it is vital to understand the degradation pathways of the structures involved, which here are observed at nanometre-scale spatial resolution in situ, inside an analytical scanning transmission electron microscope (STEM), while the composition is monitored with elemental mapping through energy-dispersive X-ray analysis (EDX). The analysis of such devices is challenging owing to several factors. The spatial dimensions relevant to the fabrication and the operation of the cells are in the 1–100 nm range, and the materials are easily damaged by exposure to an electron beam in a TEM, requiring careful tuning of the electron dose. The system also includes organic and inorganic components in an assembly with complex chemistry and morphology. Finally, the rapid changes to the devices in air and the low degradation temperatures pose an additional challenge to the experiment, which needs to be timed appropriately and carefully executed. The monitoring of this process is made possible by combining several recent advances in TEM technology. The use of high-brightness electron guns and detectors with large collection areas allows the fast acquisition of high-quality EDX maps with limited electron dose on the sample; the signal-to-noise ratio of the maps can be further increased by applying denoising algorithms (PCA, principal components analysis) within an open-source software suite. The development of novel in situ heating holders for TEM, based on micro-heaters and featuring high stability and fast response, was also crucial—in particular, the holder used here allows very precise control (sub-degree) at values just above room temperature, as well as providing fast heating and cooling (a few seconds for the temperatures in use in this paper). The good spatial stability of the holder is crucial in acquiring EDX maps.G.D., S.C., and C.D. acknowledge funding from ERC under grant number 259619 PHOTO EM. C.D. acknowledges financial support from the EU under grant number 312483 ESTEEM2. F.M., L.C. and A.D.C. acknowledge funding from “Polo Solare Organico” Regione Lazio, the “DSSCX” MIURPRIN2010 and FP7 ITN “Destiny”. G.D and S.C. thank Dr. Francisco de la Peña and Dr. Pierre Burdet for assistance with PCA analysis.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nenergy.2015.1

Quasi-1D hyperbranched WO<inf>3</inf> nanostructures for low-voltage photoelectrochemical water splitting

Arrays of hyperbranched mesostructures self-assembled from the gas phase display a decreased overpotential for the water oxidation reaction.M.B. and A.M. and F.D.F acknowledge financial support from European Union through projects PHOCS, ENERGY 2012- 10.2.1, Future Emerging Technologies Collaborative Project, grant N. 309223. G.D. and C.D. acknowledge funding from the ERC under grant number 259619 PHOTO EM.This is the accepted manuscript. The final version is available at http://pubs.rsc.org/en/Content/ArticleLanding/2015/TA/c4ta06786j#!divAbstract

Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces.

The theory of superconductivity developed by Bardeen, Cooper and Schrieffer (BCS) explains the stabilization of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap within which the density of states is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted, in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. Here we report scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb subgap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results prove odd frequency spin-triplet superconductivity at superconductor/inhomogeneous magnet interfaces.Engineering and Physical Sciences Research Council (Grant ID: NanoDTC EP/G037221/1)This is the final version of the article. It first appeared from Nature Publishing Group via http://dx.doi.org/10.1038/ncomms905

Hyperbranched Quasi-1D TiO2 Nanostructure for Hybrid Organic-Inorganic Solar Cells

The performance of hybrid solar cells is strongly affected by the device morphology. In this work we demonstrate a Poly(3-hexylthiophene-2,5-diyl)/TiO2 hybrid solar cell where the TiO2 photoanode comprises an array of tree-like hyperbranched quasi-1D nanostructures self-assembled from the gas phase. This advanced architecture enables us to increase the power conversion efficiency to over 1%, doubling the efficiency with respect to state of the art devices employing standard mesoporous titania photoanodes. This improvement is attributed to several peculiar features of this array of nanostructures: high interfacial area; increased optical density thanks to the enhanced light scattering; and enhanced crystallization of Poly(3-hexylthiophene-2,5-diyl) inside the quasi-1D nanostructure

Structural Characterization of Mesoporous Thin Film Architectures: A Tutorial Overview

Mesoporous thin film architectures are an important class of materials that exhibit unique properties, which include high surface area, versatile surface functionalization, and bicontinuous percolation paths through a broad library of pore arrangements on the 10 nm length scale. Although porosimetry of bulk materials via sorption techniques is common practice, the characterization of thin mesoporous films with small sample volumes remains a challenge. A range of techniques are geared toward providing information over pore morphology, pore size distribution, surface area and overall porosity, but none of them offers a holistic evaluation and results are at times inconsistent. In this work, we present a tutorial overview for the reliable structural characterization of mesoporous films. Three model samples with variable pore size and porosity prepared by block copolymer (BCP) coassembly serve for a rational comparison. Various techniques are assessed side-by-side, including scanning electron microscopy (SEM), atomic force microscopy (AFM), grazing incidence small-angle X-ray scattering (GISAXS), and ellipsometric porosimetry (EP). We critically discuss advantages and limitations of each technique and provide guidelines for reliable implementation

Meeting the Australian 24-Hour Movement Guidelines for the Early Years is associated with better social-emotional development in preschool boys

24-hour Movement Guidelines for the Early Years promote that achieving all three-movement behaviour (sleep, sedentary behaviour and physical activity) recommendations is important for child health and development. We examined the association between meeting all, none and combinations of the Australian 24-Hour Movement Guidelines for the Early Years and social-emotional development in 1363 preschool (2–5 years) boys (52%) and girls. The PLAYCE study (Perth, Western Australia) parent survey collected data on children's social-emotional development (Strengths &amp; Difficulties Questionnaire), screen time, sleep and socio-demographic factors. Physical activity was measured using seven-day accelerometry. Only 8% of preschoolers met all three guidelines (5% met none). A higher proportion of boys than girls met physical activity-related guideline combinations (physical activity only, physical activity plus screen, physical activity plus sleep, all), while more girls than boys met sleep only guidelines (all p &lt; 0.05). In boys, meeting all guidelines, compared with none, was associated with a lower total difficulties score (adjusted difference in means −1.90; 95%CI: −3.88, −0.10). Meeting the screen only guideline or the screen plus sleep guidelines, compared with none, were associated with lower total difficulties, conduct problems and hyperactivity scores in boys (all p &lt; 0.05). Meeting the physical activity plus sleep guidelines, compared with none, were associated with lower total difficulties and conduct problems scores in boys (all p &lt; 0.05). No significant associations were found for girls. These findings highlight the positive impact for boys social-emotional development in meeting all guidelines. Future guideline development should consider dose–response evidence to identify guideline thresholds for specific health and developmental outcomes for boys and girls.</p

Gold and iodine diffusion in large area perovskite solar cells under illumination.

Operational stability is the main issue hindering the commercialisation of perovskite solar cells. Here, a long term light soaking test was performed on large area hybrid halide perovskite solar cells to investigate the morphological and chemical changes associated with the degradation of photovoltaic performance occurring within the devices. Using Scanning Transmission Electron Microscopy (STEM) in conjunction with EDX analysis on device cross sections, we observe the formation of gold clusters in the perovskite active layer as well as in the TiO2 mesoporous layer, and a severe degradation of the perovskite due to iodine migration into the hole transporter. All these phenomena are associated with a drastic drop of all the photovoltaic parameters. The use of advanced electron microscopy techniques and data processing provides new insights on the degradation pathways, directly correlating the nanoscale structure and chemistry to the macroscopic properties of hybrid perovskite devices.European Research Council (291522), European Research Council (259619

Copper Single Atoms Chelated on Ligand-Modified Carbon for Ullmann-type C−O Coupling

Cross-coupling reactions are of great importance in chemistry due to their ability to facilitate the construction of complex organic molecules. Among these reactions, the Ullmann-type C−O coupling between phenols and aryl halides is particularly noteworthy and useful for preparing diarylethers. However, this reaction typically relies on homogeneous catalysts that rapidly deactivate under harsh reaction conditions. In this study, we introduce a novel heterogeneous catalyst for the Ullmann-type C−O coupling reaction, comprised of isolated Cu atoms chelated to a tetraethylenepentamine-pyrrole ligand that is immobilized on graphite nanoplatelets. The catalytic study reveals the recyclability of the material, and demonstrates the crucial role of the pyrrole linker in stabilizing the Cu sites. The work expands the potential of single-atom catalyst nanoarchitectures and underscores the significance of ligands in stabilizing metals in cationic forms, providing a novel, tailored catalyst for cross-coupling chemistries

Structural properties of thin-film ferromagnetic topological insulators

We present a comprehensive study of the crystal structure of the thin-film, ferromagnetic topological insulator (Bi, Sb)2 xVxTe3 .The dissipationless quantum anomalous Hall edge states it manifests are of particular interest for spintronics, as a natural spin filter or pure spin source, and as qubits for topological quantum computing. For ranges typically used in experiments, we investigate the effect of doping, substrate choice and film thickness on the (Bi, Sb)2Te3 unit cell using high-resolution X-ray diffractometry. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements provide local structural and interfacial information. We find that the unit cell is unaffected in-plane by vanadium doping changes, and remains unchanged over a thickness range of 4–10 quintuple layers (1 QL 1 nm). The in-plane lattice parameter (a) also remains the same in films grown on different substrate materials. However, out-of-plane the c-axis increases with the doping level and thicknesses >10 QL, and is potentially reduced in films grown on Si (1 1 1).This work was financially supported by the Leverhulme Trust (RPG-2013-337), the European Commission through a Marie Curie Grant (MSCA-IFEF-ST No. 656485-Spin3), the Royal Society, and the Engineering and Physical Sciences Research Council (EP/P026311/1).C.-Z.C. and J.S.M. acknowledge support from from the NSF (DMR-1207469, DMR-1700137), ONR (N00014-13-1-0301, N00014-16-1-2657), and the STC Center for Integrated Quantum Materials under NSF grant DMR-1231319

All-Optical Detection of Neuronal Membrane Depolarization in Live Cells Using Colloidal Quantum Dots

Luminescent semiconductor quantum dots (QDs) have recently been suggested as novel probes for imaging and sensing cell membrane voltages. However, a key bottleneck for their development is a lack of techniques to assess QD responses to voltages generated in the aqueous electrolytic environments typical of biological systems. Even more generally, there have been relatively few efforts to assess the response of QDs to voltage changes in live cells. Here, we develop a platform for monitoring the photoluminescence (PL) response of QDs under AC and DC voltage changes within aqueous ionic environments. We evaluate both traditional CdSe/CdS and more biologically compatible InP/ZnS QDs at a range of ion concentrations to establish their PL/voltage characteristics on chip. Wide-field, few-particle PL measurements with neuronal cells show the QDs can be used to track local voltage changes with greater sensitivity (ΔPL up to twice as large) than state-of-the-art calcium imaging dyes, making them particularly appealing for tracking subthreshold events. Additional physiological observation studies showed that while CdSe/CdS dots have greater PL responses on membrane depolarization, their lower cytotoxicity makes InP/ZnS far more suitable for voltage sensing in living systems. Our results provide a methodology for the rational development of QD voltage sensors and highlight their potential for imaging changes in cell membrane voltage