Structure influence on charge transport in naphthalenediimide-thiophene copolymers

Reported here is a characterisation of a series of NDI-thiophene copolymers with 1, 2, 3 and 4 thiophene units synthesized using Stille polycondensation of dibromo-naphthalene diimide and the trimethylstannylthiophene monomers. The effect of extension of the thiophene donor group is studied in terms of structure-charge transport correlation. The influence of side chains located on the thiophene units of copolymers with 2 and 4 thiophene units per monomer is also investigated. Charge transport of both signs is studied experimentally in field-effect transistors. Microstructural data obtained by near-edge x-ray absorption fine structure (NEXAFS) and grazing incidence wide-angle x-ray scattering (GIWAXS) is supported by AFM topography scans. UPS and UV-Vis spectroscopy data are employed in measurement of energy levels and changes with annealing temperature are also discussed. Most of the polymers reach excellent electron as well as hole mobility with one copolymer (NDI-T4) exhibiting an especially balanced ambipolar charge transport of 0.03 cm2V-1s-1. An odd-even effect in hole mobility is observed with higher values for polymers with even number of thiophene units. The reported findings indicate that the final charge transport properties are a result of the interplay of many factors, including crystallinity, planarity and linearity of chain, spacing between acceptor units and packing of solubilizing branched side chains.CRM acknowledges support from the Australian Research Council (FT100100275, DP130102616) and thanks Dr. Nigel Kirby and Dr. Steven Mudie of the Australian Synchrotron for technical support. The work in Mons is supported by the European Commission / Région Wallonne (FEDER – Smartfilm RF project), the Interuni-versity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 7/05) and the Programme d’Excellence de la Région Wallonne (OPTI2MAT project). MMS acknowledges support from the Winton Programme for the Physics of Sustainability. MiS acknowledges funding from the Fonds der chemischen In-dustrie.This is the accepted manuscript. The final published version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/cm5033578

Understanding charge transport in lead iodide perovskite thin-film field-effect transistors

Fundamental understanding of the charge transport physics of hybrid lead halide perovskite semiconductors is important for advancing their use in high-performance optoelectronics. We use field-effect transistors (FETs) to probe the charge transport mechanism in thin films of methylammonium lead iodide (MAPbI$_{3}$). We show that through optimization of thin-film microstructure and source-drain contact modifications, it is possible to significantly minimize instability and hysteresis in FET characteristics and demonstrate an electron field-effect mobility (μ$_{FET}$) of 0.5 cm$^{2}$/Vs at room temperature. Temperature-dependent transport studies revealed a negative coefficient of mobility with three different temperature regimes. On the basis of electrical and spectroscopic studies, we attribute the three different regimes to transport limited by ion migration due to point defects associated with grain boundaries, polarization disorder of the MA$^{+}$ cations, and thermal vibrations of the lead halide inorganic cages.S.P.S. acknowledges funding from the Royal Society London for a Newton Fellowship. B.Y. acknowledges support from China Council Scholarship and Cambridge Overseas Trust. A.S. and R.H.F. acknowledge funding and support from the Engineering and Physical Sciences Research Council (EPSRC) through the India-U.K. APEX project. P.D. acknowledges support from the European Union through the award of a Marie Curie Intra-European Fellowship. X.M. is grateful for the support from the Royal Society. B.N. is grateful for the support from Gates Cambridge and the Winton Program for the Physics of Sustainability. We acknowledge funding from the EPSRC through a program grant (EP/M005143/1). We acknowledge funding from the German Federal Ministry of Education and Research under agreement number 01162525/1. This work was performed in part on the SAXS/WAXS beamline of the Australian Synchrotron, Victoria, Australia (55, 56). C.R.M. acknowledges support from the Australian Research Council (DP13012616)

Simpson's Paradox, Lord's Paradox, and Suppression Effects are the same phenomenon – the reversal paradox

This article discusses three statistical paradoxes that pervade epidemiological research: Simpson's paradox, Lord's paradox, and suppression. These paradoxes have important implications for the interpretation of evidence from observational studies. This article uses hypothetical scenarios to illustrate how the three paradoxes are different manifestations of one phenomenon – the reversal paradox – depending on whether the outcome and explanatory variables are categorical, continuous or a combination of both; this renders the issues and remedies for any one to be similar for all three. Although the three statistical paradoxes occur in different types of variables, they share the same characteristic: the association between two variables can be reversed, diminished, or enhanced when another variable is statistically controlled for. Understanding the concepts and theory behind these paradoxes provides insights into some controversial or contradictory research findings. These paradoxes show that prior knowledge and underlying causal theory play an important role in the statistical modelling of epidemiological data, where incorrect use of statistical models might produce consistent, replicable, yet erroneous results

Aggregation-Induced Emission of Naphthalene Diimides: Effect of Chain Length on Liquid and Solid-Phase Emissive Properties.

The aggregation-induced emission (AIE) properties of a systematic series of naphthalene diimides (NDIs) varying the chain length at the imide positions have been studied. A solvophobic collapse of NDI units through the flash injection of THF NDI solutions in sonicating water triggers the formation of stable suspensions with enhanced fluorescence emissions. Shorter chains favor the π-π stacking of NDI units through H-aggregation producing a strong AIE effect showing remarkably high quantum yields that have not been observed for non core-substitued NDIs previously. On the other hand, NDIs functionalized with longer chains lead to more disordered domains where π-π stacking between NDI units is mainly given by J-aggregation unfavoring the AIE effect

Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors

Recent demonstrations of inverted thermal activation of charge mobility in polymer field- effect transistors have excited the interest in transport regimes not limited by thermal bar- riers. However, rationalization of the limiting factors to access such regimes is still lacking. An improved understanding in this area is critical for development of new materials, establishing processing guidelines, and broadening of the range of applications. Here we show that precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copo- lymer results in single crystal-like and voltage-independent mobility with vanishing activation energy above 280 K. Key factors are uniaxial chain alignment and thermal annealing at temperatures within the melting endotherm of films. Experimental and computational evi- dences converge toward a picture of electrons being delocalized within crystalline domains of increased size. Residual energy barriers introduced by disordered regions are bypassed in the direction of molecular alignment by a more efficient interconnection of the ordered domains following the annealing process

An alkylated indacenodithieno[3,2-b]thiophene-based nonfullerene acceptor with high crystallinity exhibiting single junction solar cell efficiencies greater than 13% with low voltage losses

A new synthetic route, to prepare an alkylated indacenodithieno[3,2-b]thiophene-based nonfullerene acceptor (C8-ITIC), is reported. Compared to the reported ITIC with phenylalkyl side chains, the new acceptor C8-ITIC exhibits a reduction in the optical band gap, higher absorptivity, and an increased propensity to crystallize. Accordingly, blends with the donor polymer PBDB-T exhibit a power conversion efficiency (PCE) up to 12.4%. Further improvements in efficiency are found upon backbone fluorination of the donor polymer to afford the novel material PFBDB-T. The resulting blend with C8-ITIC shows an impressive PCE up to 13.2% as a result of the higher open-circuit voltage. Electroluminescence studies demonstrate that backbone fluorination reduces the energy loss of the blends, with PFBDB-T/C8-ITIC-based cells exhibiting a small energy loss of 0.6 eV combined with a high JSCof 19.6 mA cm-2

Investigating International Time Trends in the Incidence and Prevalence of Atopic Eczema 1990-2010: A Systematic Review of Epidemiological Studies

The prevalence of atopic eczema has been found to have increased greatly in some parts of the world. Building on a systematic review of global disease trends in asthma, our objective was to study trends in incidence and prevalence of atopic eczema. Disease trends are important for health service planning and for generating hypotheses regarding the aetiology of chronic disorders. We conducted a systematic search for high quality reports of cohort, repeated cross-sectional and routine healthcare database-based studies in seven electronic databases. Studies were required to report on at least two measures of the incidence and/or prevalence of atopic eczema between 1990 and 2010 and needed to use comparable methods at all assessment points. We retrieved 2,464 citations, from which we included 69 reports. Assessing global trends was complicated by the use of a range of outcome measures across studies and possible changes in diagnostic criteria over time. Notwithstanding these difficulties, there was evidence suggesting that the prevalence of atopic eczema was increasing in Africa, eastern Asia, western Europe and parts of northern Europe (i.e. the UK). No clear trends were identified in other regions. There was inadequate study coverage worldwide, particularly for repeated measures of atopic eczema incidence. Further epidemiological work is needed to investigate trends in what is now one of the most common long-term disorders globally. A range of relevant measures of incidence and prevalence, careful use of definitions and description of diagnostic criteria, improved study design, more comprehensive reporting and appropriate interpretation of these data are all essential to ensure that this important field of epidemiological enquiry progresses in a scientifically robust manner

Resolving the backbone tilt of crystalline poly(3-hexylthiophene) with resonant tender X-ray diffraction

The way in which conjugated polymers pack in the solid state strongly affects the performance of polymer-based optoelectronic devices. However, even for the most crystalline conjugated polymers the precise packing of chains within the unit cell is not well established. Here we show that by performing resonant X-ray diffraction experiments at the sulfur K-edge we are able to resolve the tilting of the planar backbones of crystalline poly(3-hexylthiophene) (P3HT) within the unit cell. This approach exploits the anisotropic nature of the X-ray optical properties of conjugated polymers, enabling us to discern between different proposed crystal structures. By comparing our data with simulations based on different orientations, a tilting of the planar conjugated backbone with respect to the side chain stacking direction of 30 ± 5° is determined

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta