The First Record of Calvolia summersi (Mostafa, 1970) (Acari: Winterschmidtiidae) from the Oriental Region and a new record of host association with Xylocopa (Ctenoxylocopa) fenestrata (Fabricius, 1798) with a review on Xylocopa-mite associations in India

Mites have long been associated with bees, often showing a close relationship with particular taxa, probably due to a co-evolutionary process. The present study is the first confirmation on the occurrence of the mite species Calvolia summersi (Mostafa, 1970) in India and its association with Xylocopa fenestrata (Fabricius, 1798), a large carpenter bee species. The mite species was previously reported from Brazil in association with Zethus, a neotropical potter wasp genus. C. summersi is isolated and studied from different populations of X. fenestrata in Sagar Islands, West Bengal, the biggest island of mangrove Sundarban deltaic complex. A literature-based review of the Xylocopa-mite associations from India is provided herewith to understand the diversity and pattern of mite species on Xylocopa host selection in the country. Instigating a baseline study of Xylocopa-mite association is recommended to demark the status of dynamism for better conservation and protection

Good Vibrations:Locking of Octahedral Tilting in Mixed-Cation Iodide Perovskites for Solar Cells

Metal halide perovskite solar cells have rapidly emerged as leading contenders in photovoltaic technology. Compositions with a mixture of cation species on the A-site show the best performance and have higher stability. However, the underlying fundamentals of such an enhancement are not fully understood. Here, we investigate the local structures and dynamics of mixed A-cation compositions. We show that substitution of low concentrations of smaller cations on the A-site in formamidimium lead iodide (CH(NH2)2PbI3) results in a global "locking" of the PbI6 octahedra tilting. In the locked structure the octahedra tilt at a larger angle but undergo a much reduced amplitude of rocking motion. A key impact of this feature is that the rotational or tumbling motion of the CH(NH2)2+ molecular ion in a locked cage is severely restricted. We discuss the impact of locking on the photovoltaic performance and stability.</p

Mixed A-Cation Perovskites for Solar Cells:Atomic-Scale Insights into Structural Distortion, Hydrogen Bonding, and Electronic Properties

Hybrid lead halide perovskites containing a mixture of A-site cations such as the formamidinium (CH­(NH₂)₂⁺, FA) and the smaller cesium (Cs⁺) cations have attracted considerable interest due to their improved stability and solar cell performance. However, the structural changes at the atomic scale and modifications to the optoelectronic properties of these mixed cation perovskites are not fully understood. Here, we investigate the FA_1–<i>x</i>Cs_<i>x</i>PbI₃ (<i>x</i> ≤ 0.25) system using a combination of static and dynamic <i>ab initio</i> computational methods. We find that the incorporation of Cs⁺ cations into the parent FAPbI₃ structure induces a chemical pressure or lattice strain effect through Cs/FA ion size mismatch resulting in structural distortion and stronger FA-iodide (N–H···I) hydrogen bonding interactions. The dynamic tilting of PbI₆ octahedra and the rotational motion of FA cations are also suppressed, which leads to symmetry-breaking of the lattice. Such symmetry-breaking distortions of the Pb/I lattice give rise to a Rashba-type effect, which spin-splits the frontier electronic bands making the band gap indirect. Our results suggest that the direct–indirect band gap transition may be a factor in the reduced charge-carrier recombination rate in these mixed cation perovskites

Good Vibrations:Locking of Octahedral Tilting in Mixed-Cation Iodide Perovskites for Solar Cells

Metal halide perovskite solar cells have rapidly emerged as leading contenders in photovoltaic technology. Compositions with a mixture of cation species on the A-site show the best performance and have higher stability. However, the underlying fundamentals of such an enhancement are not fully understood. Here, we investigate the local structures and dynamics of mixed A-cation compositions. We show that substitution of low concentrations of smaller cations on the A-site in formamidimium lead iodide (CH­(NH₂)₂PbI₃) results in a global “locking” of the PbI₆ octahedra tilting. In the locked structure the octahedra tilt at a larger angle but undergo a much reduced amplitude of rocking motion. A key impact of this feature is that the rotational or tumbling motion of the CH­(NH₂)₂⁺ molecular ion in a locked cage is severely restricted. We discuss the impact of locking on the photovoltaic performance and stability

Substitution of lead with tin suppresses ionic transport in halide perovskite optoelectronics.

Despite the rapid rise in the performance of a variety of perovskite optoelectronic devices with vertical charge transport, the effects of ion migration remain a common and longstanding Achilles' heel limiting the long-term operational stability of lead halide perovskite devices. However, there is still limited understanding of the impact of tin (Sn) substitution on the ion dynamics of lead (Pb) halide perovskites. Here, we employ scan-rate-dependent current-voltage measurements on Pb and mixed Pb-Sn perovskite solar cells to show that short circuit current losses at lower scan rates, which can be traced to the presence of mobile ions, are present in both kinds of perovskites. To understand the kinetics of ion migration, we carry out scan-rate-dependent hysteresis analyses and temperature-dependent impedance spectroscopy measurements, which demonstrate suppressed ion migration in Pb-Sn devices compared to their Pb-only analogues. By linking these experimental observations to first-principles calculations on mixed Pb-Sn perovskites, we reveal the key role played by Sn vacancies in increasing the iodide ion migration barrier due to local structural distortions. These results highlight the beneficial effect of Sn substitution in mitigating undesirable ion migration in halide perovskites, with potential implications for future device development

Lead-Free Perovskite Semiconductors Based on Germanium-Tin Solid Solutions:Structural and Optoelectronic Properties

Solar cells and optoelectronics based on lead halide perovskites are generating considerable interest but face challenges with the use of toxic lead. In this study, we fabricate and characterize lead-free perovskites based on germanium and tin solid solutions, CH₃NH₃Sn_{(1–<i>x</i>)}Ge_<i>x</i>I₃ (0 ≤ <i>x</i> ≤ 1). We show that these perovskite compounds possess band gaps from 1.3 to 2.0 eV, which are suitable for a range of optoelectronic applications, from single junction devices and top cells for tandems to light-emitting layers. Their thermodynamic stability and electronic properties are calculated for all compositions and agree well with our experimental measurements. Our findings demonstrate an attractive family of lead-free perovskite semiconductors with a favorable band-gap range for efficient single-junction solar cells

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

A semantic context model for managing privacy on smartphones

In this work we describe an approach to protecting the privacy and security of user data on mobile devices using a richer semantic model of a user's context. Mobile OS frameworks like Android always lacked mechanisms for dynamic privacy control, while recent advances in context modeling, tracking and collaborative localization has led to the emergence of a new class of smartphone applications that can access and share embedded sensor data. Existing literature on context based privacy and security has predominantly focused on device user's context for probing privacy vulnerability and enforcing security at runtime. We bring into picture the most important component of privacy vulnerability on smartphones, the resident applications themselves and we introduce the novel idea of application provenance. Our context model is realized as a dynamic knowledge base of RDF [67] triples grounded in an ontology in the semantic web language OWL. Policies in the form of rules over this knowledge base monitor and control application access to sensitive information and sensor data. The policies filter data flowing from sensor resources to applications to reduce disclosure by generalizing or obfuscating data. Our ontology includes the ability to represent application provenance and other metadata that can be used by the policies. The resulting system provides fine-grained, context-dependent control to sensitive user data

Early Modern London Theatres (EMLoT)

This is a review of Early Modern London Theatres (EMLoT)