Modelling ion motion in perovskite films

There are two main methods for modelling the electronic properties of perovskite devices. Drift-diffusion modelling [1,2,3] works by solving differential equations, while equivalent circuit modelling [4,5] works by treating different parts of the device as electrical components. This work focuses on drift-diffusion modelling.Most drift-diffusion models of perovskite use finite-difference methods [1,3], which solve the differential equations at equally-spaced grid points. These models have achieved a valuable qualitative description of the influence of ion motion on the electrical characteristics of perovskite solar cells. However, in order to relate the model predictions to microscopic material properties, a more quantitative model is required.We use a hybrid model [2] where an asymptotic approximation is used to model the accumulation of ionic charge at the interfaces between materials. The charge accumulation, which is a function of time, is then input into a second model, in which the electron and hole concentrations across the film are expressed as a sum of Chebyshev polynomials using the MATLAB add-on module Chebfun. [6]In this work, the ability of different drift-diffusion modelling methods to reproduce experimental current-voltage measurements quantitatively is compared. References[1] S. van Reenen, M. Kemerink and H. J. Snaith, J. Phys. Chem. Lett. (2015), Vol. 6, 1511-1515[2] G. Richardson et al, Energy Environ. Sci. (2016), Vol 9, 1476-1485[3] P. Calado et al., arXiv (2016), https://arxiv.org/abs/1606.00818 [4] A. Pockett et al., J. Phys. Chem. C. (2015), Vol. 119, 3456-3465[5] L. Cojocaru et al., Chem. Lett. (2015), Vol. 44, 1750-1752[6] T. A. Driscoll, N. Hale and L. N. Trefethen, Chebfun Guide (2015)<br/

The Role of Surface Recombination on the Performance of Perovskite Solar Cells:Effect of Morphology and Crystalline Phase of TiO₂ Contact

Herein, the preparation of 1D TiO2 nanocolumnar films grown by plasma-enhanced chemical vapor deposition is reported as the electron selective layer (ESL) for perovskite solar devices. The impact of the ESL architecture (1D and 3D morphologies) and the nanocrystalline phase (anatase and amorphous) is analyzed. For anatase structures, similar power conversion efficiencies are achieved using an ESL either the 1D nanocolumns or the classical 3D nanoparticle film. However, lower power conversion efficiencies and different optoelectronic properties are found for perovskite devices based on amorphous 1D films. The use of amorphous TiO2 as electron selective contact produces a bump in the reverse scan of the current–voltage curve as well as an additional electronic signal, detected by impedance spectroscopy measurements. The dependence of this additional signal on the optical excitation wavelength used in the IS experiments suggests that it stems from an interfacial process. Calculations using a drift-diffusion model which explicitly considers the selective contacts reproduces qualitatively the main features observed experimentally. These results demonstrate that for a solar cell in which the contact is working properly the open-circuit photovoltage is mainly determined by bulk recombination, whereas the introduction of a “bad contact” shifts the balance to surface recombination.</p

Temperature and plant genotype alter alkaloid concentrations in ryegrass infected with an epichloё endophyte and this affects an insect herbivore.

Asexual Epichloё endophytes colonise agricultural forage grasses in a relationship which is mutually beneficial and provides the host plant with protection against herbivorous insects. The endophyte strain AR37 (Epichloё festucae var. lolii) produces epoxy-janthitrem alkaloids and is the only endophyte known to provide ryegrass with resistance against porina larvae (Wiseana cervinata (Walker)), a major pasture pest in cooler areas of New Zealand. This study examined the effect of temperature on concentrations of epoxy-janthitrems in AR37-infected ryegrass and determined how the resulting variations in concentration affected consumption, growth and survival of porina larvae. Twenty replicate pairs of perennial (Lolium perenne L.) and Italian ryegrass (Lolium multiflorum Lam.) plants with and without endophyte were prepared by cloning, with one of each pair grown at either high (20°C) or low (7°C) temperature. After 10 weeks, herbage on each plant was harvested, divided into leaf and pseudostem, then freeze dried and ground. Leaf and pseudostem material was then incorporated separately into semi-synthetic diets which were fed to porina larvae in a bioassay over 3 weeks. Epoxy-janthitrem concentrations within the plant materials and the semi-synthetic diets were analysed by HPLC. AR37-infected ryegrass grown at high temperature contained high in planta concentrations of epoxy-janthitrem (30.6 µg/g in leaves and 83.9 µg/g in pseudostems) that had a strong anti-feedant effect on porina larvae when incorporated into their diets, reducing their survival by 25-42% on pseudostems. In comparison, in planta epoxy-janthitrem concentrations in AR37-infected ryegrass grown at low temperature were very low (0.67 µg/g in leaves and 7.4 µg/g in pseudostems) resulting in a small anti-feedant effect in perennial but not in Italian ryegrass. Although alkaloid concentrations were greatly reduced by low temperature this reduction did not occur until after 4 weeks of exposure. Alkaloid concentrations were slightly lower in Italian than in perennial ryegrass and concentrations were higher in the pseudostems when compared with the leaves. In conclusion, epoxy-janthitrems expressed by the AR37 endophyte show strong activity against porina larvae. However, when ryegrass plants are grown at a constant low temperature for an extended period of time in planta epoxy-janthitrem concentrations are greatly reduced and are les

Measurement and modelling of dark current decay transients in perovskite solar cells

The current decay in response to a sudden change of applied bias up to 1 V has been measured on a methylammonium lead triiodide perovskite solar cell with titania and spiro-OMeTAD transport layers, for temperatures between 258 and 308 K. These measurements are highly reproducible, in contrast to most other techniques used to investigate perovskite cells. A drift-diffusion model that accounts for slow moving ions as well as electrons and holes acting as charge carriers was used to predict the current transients. The close fit of the model predictions to the measurements shows that mobile ions in the perovskite layer influence transient behaviour on timescales of up to 50 s. An activation energy of 0.55 eV is inferred from fitting simulations to measurements made at room temperature

Divergent adaptive and innate immunological responses are observed in humans following blunt trauma

<p>Abstract</p> <p>Background</p> <p>The immune response to trauma has traditionally been modeled to consist of the systemic inflammatory response syndrome (SIRS) followed by the compensatory anti-inflammatory response syndrome (CARS). We investigated these responses in a homogenous cohort of male, severe blunt trauma patients admitted to a University Hospital surgical intensive care unit (SICU). After obtaining consent, peripheral blood was drawn up to 96 hours following injury. The enumeration and functionality of both myeloid and lymphocyte cell populations were determined.</p> <p>Results</p> <p>Neutrophil numbers were observed to be elevated in trauma patients as compared to healthy controls. Further, neutrophils isolated from trauma patients had increased raft formation and phospho-Akt. Consistent with this, the neutrophils had increased oxidative burst compared to healthy controls. In direct contrast, blood from trauma patients contained decreased naïve T cell numbers. Upon activation with a T cell specific mitogen, trauma patient T cells produced less IFN-gamma as compared to those from healthy controls. Consistent with these results, upon activation, trauma patient T cells were observed to have decreased T cell receptor mediated signaling.</p> <p>Conclusions</p> <p>These results suggest that following trauma, there are concurrent and divergent immunological responses. These consist of a hyper-inflammatory response by the innate arm of the immune system concurrent with a hypo-inflammatory response by the adaptive arm.</p

GDE6 promotes progenitor identity in the vertebrate neural tube

The generation of neurons in the central nervous system is a complex, stepwise process necessitating the coordinated activity of mitotic progenitors known as radial glia. Following neural tube closure, radial glia undergo a period of active proliferation to rapidly expand their population, creating a densely packed neurepithelium. Simultaneously, radial glia positioned across the neural tube are uniquely specified to produce diverse neuronal sub-types. Although these cellular dynamics are well studied, the molecular mechanisms governing them are poorly understood. The six-transmembrane Glycerophosphodiester Phosphodiesterase proteins (GDE2, GDE3, and GDE6) comprise a family of cell-surface enzymes expressed in the embryonic nervous system. GDE proteins can release Glycosylphosphatidylinositol-anchored proteins from the cell surface via cleavage of their lipid anchor. GDE2 has established roles in motor neuron differentiation and oligodendrocyte maturation, and GDE3 regulates oligodendrocyte precursor cell proliferation. Here, we describe a role for GDE6 in early neural tube development. Using RNAscope, we show that Gde6 mRNA is expressed by ventricular zone progenitors in the caudal neural tube. Utilizing in-ovo electroporation, we show that GDE6 overexpression promotes neural tube hyperplasia and ectopic growths of the neurepithelium. At later stages, electroporated embryos exhibit an expansion of the ventral patterning domains accompanied by reduced cross-repression. Ultimately, electroporated embryos fail to produce the full complement of post-mitotic motor neurons. Our findings indicate that GDE6 overexpression significantly affects radial glia function and positions GDE6 as a complementary factor to GDE2 during neurogenesis

Newly qualified doctors' views about whether their medical school had trained them well: questionnaire surveys

A survey of newly qualified doctors in the UK in 2000/2001 found that 42% of them felt unprepared for their first year of employment in clinical posts. We report on how UK qualifiers' preparedness has changed since then, and on the impact of course changes upon preparedness. Methods Postal questionnaires were sent to all doctors who qualified from UK medical schools, in their first year of clinical work, in 2003 (n = 4257) and 2005 (n = 4784); and findings were compared with those in 2000/2001 (n = 5330). The response rates were 67% in 2000/2001, 65% in 2003, and 43% in 2005. The outcome measure was the percentage of doctors agreeing with the statement "My experience at medical school has prepared me well for the jobs I have undertaken so far". Results In the 2000/2001 survey 36.3% strongly agreed or agreed with the statement, as did 50.3% in the 2003 survey and 58.5% in 2005 (chi-squared test for linear trend: χ2 = 259.5; df = 1; p < 0.001). Substantial variation in preparedness between doctors from different medical schools, reported in the first survey, was still present in 2003 and 2005. Between 1998 and 2006 all UK medical schools updated their courses. Within each cohort a significantly higher percentage of the respondents from schools with updated courses felt well prepared. Conclusion UK medical schools are now training doctors who feel better prepared for work than in the past. Some of the improvement may be attributable to curricular change

Deducing transport properties of mobile vacancies from perovskite solar cell characteristics

The absorber layers in perovskite solar cells possess a high concentration of mobile ion vacancies. These vacancies undertake thermally activated hops between neighboring lattice sites. The mobile vacancy concentration N 0 is much higher and the activation energy E A for ion hops is much lower than is seen in most other semiconductors due to the inherent softness of perovskite materials. The timescale at which the internal electric field changes due to ion motion is determined by the vacancy diffusion coefficient D v and is similar to the timescale on which the external bias changes by a significant fraction of the open-circuit voltage at typical scan rates. Therefore, hysteresis is often observed in which the shape of the current-voltage, J-V, characteristic depends on the direction of the voltage sweep. There is also evidence that this defect migration plays a role in degradation. By employing a charge transport model of coupled ion-electron conduction in a perovskite solar cell, we show that E A for the ion species responsible for hysteresis can be obtained directly from measurements of the temperature variation of the scan-rate dependence of the short-circuit current and of the hysteresis factor H. This argument is validated by comparing E A deduced from measured J-V curves for four solar cell structures with density functional theory calculations. In two of these structures, the perovskite is MAPbI 3, where MA is methylammonium, CH 3 NH 3; the hole transport layer (HTL) is spiro (spiro-OMeTAD, 2,2 ′,7,7 ′- tetrakis[N,N-di(4-methoxyphenyl) amino]-9,9 ′-spirobifluorene) and the electron transport layer (ETL) is TiO 2 or SnO 2. For the third and fourth structures, the perovskite layer is FAPbI 3, where FA is formamidinium, HC (NH 2) 2, or MAPbBr 3, and in both cases, the HTL is spiro and the ETL is SnO 2. For all four structures, the hole and electron extracting electrodes are Au and fluorine doped tin oxide, respectively. We also use our model to predict how the scan rate dependence of the power conversion efficiency varies with E A, N 0, and parameters determining free charge recombination. </p

Net neutrality discourses: comparing advocacy and regulatory arguments in the United States and the United Kingdom

Telecommunications policy issues rarely make news, much less mobilize thousands of people. Yet this has been occurring in the United States around efforts to introduce "Net neutrality" regulation. A similar grassroots mobilization has not developed in the United Kingdom or elsewhere in Europe. We develop a comparative analysis of U.S. and UK Net neutrality debates with an eye toward identifying the arguments for and against regulation, how those arguments differ between the countries, and what the implications of those differences are for the Internet. Drawing on mass media, advocacy, and regulatory discourses, we find that local regulatory precedents as well as cultural factors contribute to both agenda setting and framing of Net neutrality. The differences between national discourses provide a way to understand both the structural differences between regulatory cultures and the substantive differences between policy interpretations, both of which must be reconciled for the Internet to continue to thrive as a global medium