The nature of singlet exciton fission in carotenoid aggregates.

Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure-property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B(u) photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.This work was supported by the EPSRC (UK) (EP/G060738/ 1), the European Community (LASERLAB-EUROPE, grant agreement no. 284464, EC’s Seventh Framework Programme; and Marie-Curie ITN-SUPERIOR, PITN-GA-2009-238177), and the Winton Programme for the Physics of Sustainability. G.C. acknowledges support by the European Research Council Advanced Grant STRATUS (ERC-2011-AdG No. 291198). J.C. acknowledges support by the Royal Society Dorothy Hodgkin Fellowship and The University of Sheffield’s Vice- Chancellor’s Fellowship scheme.This is the final published version. It was first made available by ACS at http://pubs.acs.org/doi/abs/10.1021/jacs.5b01130

Singlet exciton fission in solution.

Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.B.J.W. was supported by a Herchel Smith Research Fellowship. A.J.M. received funding from a Marie Curie Scholarship. D.B. is a FNRS Research Director. Both A.J.M and D.B. acknowledge support from the European Community’s Initial Training Network SUPERIOR (PITN-GA-2009-238177). Further funding for this project came from the Engineering and Physical Sciences Research Council (EPSRC) and a pump-prime grant from the Winton Programme for the Physics of Sustainability.This is the accepted version of an article originally published in Nature Chemistry 5, 1019–1024 and available online at http://www.nature.com/nchem/journal/v5/n12/full/nchem.1801.html. Nature Publishing Group's conditions for reuse are detailed at http://www.nature.com/authors/policies/license.html

AI et al.: Machines Are About to Change Scientific Publishing Forever

Artificial intelligence (AI)-powered text generation will change scientific publishing fundamentally. In the past year, multiple AI systems have showcased production of visual and textual content increasingly indistinguishable from human-generated work, creating almost overnight new possibilities for intellectual workers, and at the same time raising similarly potent concerns. While artists and journalists are more evidently at the forefront of this incipient revolution, it is not hard to imagine a researcher looking away from the frustratingly sparse draft of a research article and wondering: “Could a machine write it for me?

How Many Mobile Ions Can Electrical Measurements Detect in Perovskite Solar Cells?

In recent years, mobile ions have been assigned to various degradation mechanisms in perovskite solar cells. Some of these include nonreversible degradation, like migration into charge transport layers (CTLs) (1) or reaction with electrodes. (2) Others focus on the electrostatic effects due to mobile ions. Most importantly, the accumulation of a large density of mobile ions at the interface between perovskite and charge transport layers can lead to screening of the built-in potential, which can result in enhanced interface and bulk recombination, reducing the short-circuit current density and fill-factor. (3) The accumulation of mobile ions has also been connected to a decrease in open-circuit voltage. (4) To obtain a comprehensive understanding of the impact of mobile ions on the device physics of perovskite solar cells, accurately determining the density and diffusion coefficient of mobile ions in perovskites is of utmost importance. However, measured ion densities cover multiple orders of magnitude from 1015 cm–3 to 1019 cm–3. (3,5−7) To determine ion densities, electrical measurements like transient current measurements, also known as bias-assisted charge extraction, (3) capacitance frequency, also known as impedance spectroscopy, (8,9) transient capacitance measurements, also known as transient ion drift measurements, (9) and low-frequency Mott–Schottky measurements (5) have been applied. Here, we illustrate that it becomes impossible to determine the ion density if it is high enough to screen a significant portion of the built-in field. To illustrate the difficulty of extracting high ion densities from the different electrical measurements, we carried out drift-diffusion simulations. For the transport layers, we chose parameters resembling thin organic transport layers 2PACz and C60. We assume that ionic transport is mediated by halide vacancies, (10−12) and their charge is compensated by nonmobile negatively charged ions. (13) We carried out the simulations for different mobile ion densities ranging from 1016 to 1020 cm–3, and a typical ionic conductivity σion = eμionNion of 1.6 · 10–10S/cm, where e is the elementary charge, μion is the ionic mobility, and Nion is the density of mobile ions. The complete simulation parameters are listed in the Supporting Information. We emphasize that the absolute values of the presented results are only valid for the parameter set studied in this work

Integrated artificial neurons from metal halide perovskites

Hardware neural networks could perform certain computational tasks orders of magnitude more energy-efficiently than conventional computers. Artificial neurons are a key component of these networks and are currently implemented with electronic circuits based on capacitors and transistors. However, artificial neurons based on memristive devices are a promising alternative, owing to their potentially smaller size and inherent stochasticity. But despite their promise, demonstrations of memristive artificial neurons have so far been limited. Here we demonstrate a fully on-chip artificial neuron based on microscale electrodes and halide perovskite semiconductors as the active layer. By connecting a halide perovskite memristive device in series with a capacitor, the device demonstrates stochastic leaky integrate-and-fire behavior, with an energy consumption of 20 to 60 pJ per spike, lower than that of a biological neuron. We simulate populations of our neuron and show that the stochastic firing allows the detection of sub-threshold inputs. The neuron can easily be integrated with previously-demonstrated halide perovskite artificial synapses in energy-efficient neural networks