Polymer blend based photovoltaic devices.

Solar cells on thin conformable substrates require conventional plastics such asPS and PMMA that provide better mechanical and environmental stability with cost reduction. We can also tune charge transfer between PPV derivatives and fullerene derivatives via morphology control of the plastics in the solar cells. Our group has conducted morphology evolution studies in nano- and microscale light emitting domains in poly (2-methoxy, 5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and poly (methyl methacrylate) (PMMA) blends. Our current research has been focused on tricomponent-photoactive solar cells which comprise MEH-PPV, PMMA, and [6,6]-phenyl C61-butyric acid methyl ester (PCBM, Figure 1) in the photoactive layer. Morphology control of the photoactive materials and fine tuning of photovoltaic properties for the solar cells are our primary interest. Similar work has been done by the Sariciftci research group. Additionally, a study on inter- and intramolecular photoinduced charge transfer using MEH-PPV derivatives that have different conjugation lengths (Figure 1, n=1 and 0.85) has been performed.

Reactive power optimization with different objectives in large power systems including HVDC systems and FACTS devices

Present day power systems are growing in size and complexity of operation with inter connections to neighboring systems, introduction of large generating units, EHV 400/765 kV AC transmission systems, HVDC systems and more sophisticated control devices such as FACTS. For planning and operational studies, it requires suitable modeling of all components in the power system, as the number of HVDC systems and FACTS devices of different type are incorporated in the system. This paper presents reactive power optimization with three objectives to minimize the sum of the squares of the voltage deviations (ve) of the load buses, minimization of sum of squares of voltage stability L-indices of load buses (¿L2), and also the system real power loss (Ploss) minimization. The proposed methods have been tested on typical sample system. Results for Indian 96-bus equivalent system including HVDC terminal and UPFC under normal and contingency conditions are presented.

Electrical breakdown of carbon nanotube devices and the predictability of breakdown position

We have investigated electrical transport properties of long (>10 mu m) multiwalled carbon nanotubes (NTs) by dividing individuals into several segments of identical length. Each segment has different resistance because of the random distribution of defect density in an NT and is corroborated by Raman studies. Higher is the resistance, lower is the current required to break the segments indicating that breakdown occurs at the highly resistive segment/site and not necessarily at the middle. This is consistent with the one-dimensional thermal transport model. We have demonstrated the healing of defects by annealing at moderate temperatures or by current annealing. To strengthen our mechanism, we have carried out electrical breakdown of nitrogen doped NTs (NNTs) with diameter variation from one end to the other. It reveals that the electrical breakdown occurs selectively at the narrower diameter region. Overall, we believe that our results will help to predict the breakdown position of both semiconducting and metallic NTs. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. http://dx.doi.org/10.1063/1.4720426]

Substrate temperature influenced physical properties of silicon MOS devices with TiO2 gate dielectric

DC reactive magnetron sputtering technique was employed for deposition of titanium dioxide (TiO2) films. The films were formed on Corning glass and p-Si (100) substrates by sputtering of titanium target in an oxygen partial pressure of 6x10-2 Pa and at different substrate temperatures in the range 303 673 K. The films formed at 303 K were X-ray amorphous whereas those deposited at substrate temperatures?=?473 K were transformed into polycrystalline nature with anatase phase of TiO2. Fourier transform infrared spectroscopic studies confirmed the presence of characteristic bonding configuration of TiO2. The surface morphology of the films was significantly influenced by the substrate temperature. MOS capacitor with Al/TiO2/p-Si sandwich structure was fabricated and performed currentvoltage and capacitancevoltage characteristics. At an applied gate voltage of 1.5 V, the leakage current density of the device decreased from 1.8?x?10-6 to 5.4?x?10-8 A/cm2 with the increase of substrate temperature from 303 to 673 K. The electrical conduction in the MOS structure was more predominant with Schottky emission and Fowler-Nordheim conduction. The dielectric constant (at 1 MHz) of the films increased from 6 to 20 with increase of substrate temperature. The optical band gap of the films increased from 3.50 to 3.56 eV and refractive index from 2.20 to 2.37 with the increase of substrate temperature from 303 to 673 K. Copyright (c) 2012 John Wiley & Sons, Ltd.

A generalized geo-hazard warning system

An automated geo-hazard warning system is the need of the hour. It is integration of automation in hazard evaluation and warning communication. The primary objective of this paper is to explain a geo-hazard warning system based on Internet-resident concept and available cellular mobile infrastructure that makes use of geo-spatial data. The functionality of the system is modular in architecture having input, understanding, expert, output and warning modules. Thus, the system provides flexibility in integration between different types of hazard evaluation and communication systems leading to a generalized hazard warning system. The developed system has been validated for landslide hazard in Indian conditions. It has been realized through utilization of landslide causative factors, rainfall forecast from NASA's TRMM (Tropical Rainfall Measuring Mission) and knowledge base of landslide hazard intensity map and invokes the warning as warranted. The system evaluated hazard commensurate with expert evaluation within 5-6 % variability, and the warning message permeability has been found to be virtually instantaneous, with a maximum time lag recorded as 50 s, minimum of 10 s. So it could be concluded that a novel and stand-alone system for dynamic hazard warning has been developed and implemented. Such a handy system could be very useful in a densely populated country where people are unaware of the impending hazard.

Review of Geohazard Warning Systems toward Development of a Popular Usage Geohazard Warning Communication System

The advent and evolution of geohazard warning systems is a very interesting study. The two broad fields that are immediately visible are that of geohazard evaluation and subsequent warning dissemination. Evidently, the latter field lacks any systematic study or standards. Arbitrarily organized and vague data and information on warning techniques create confusion and indecision. The purpose of this review is to try and systematize the available bulk of information on warning systems so that meaningful insights can be derived through decidable flowcharts, and a developmental process can be undertaken. Hence, the methods and technologies for numerous geohazard warning systems have been assessed by putting them into suitable categories for better understanding of possible ways to analyze their efficacy as well as shortcomings. By establishing a classification scheme based on extent, control, time period, and advancements in technology, the geohazard warning systems available in any literature could be comprehensively analyzed and evaluated. Although major advancements have taken place in geohazard warning systems in recent times, they have been lacking a complete purpose. Some systems just assess the hazard and wait for other means to communicate, and some are designed only for communication and wait for the hazard information to be provided, which usually is after the mishap. Primarily, systems are left at the mercy of administrators and service providers and are not in real time. An integrated hazard evaluation and warning dissemination system could solve this problem. Warning systems have also suffered from complexity of nature, requirement of expert-level monitoring, extensive and dedicated infrastructural setups, and so on. The user community, which would greatly appreciate having a convenient, fast, and generalized warning methodology, is surveyed in this review. The review concludes with the future scope of research in the field of hazard warning systems and some suggestions for developing an efficient mechanism toward the development of an automated integrated geohazard warning system. DOI: 10.1061/(ASCE)NH.1527-6996.0000078. (C) 2012 American Society of Civil Engineers.

Analysing modifica-tions in the synthesis of multiple state mechanical devices using configuration space and topology graphs

Automated synthesis of mechanical designs is an important step towards the development of an intelligent CAD system. Research into methods for supporting conceptual design using automated synthesis has attracted much attention in the past decades. In our research, ten experimental studies are conducted to find out how designers synthesize solution concepts for multi-state mechanical devices. The designers are asked to think aloud, while carrying out the synthesis. These design synthesis processes are video recorded. It has been found that modification of kinematic pairs and mechanisms is the major activity carried out by all the designers. This paper presents an analysis of these synthesis processes using configuration space and topology graph to identify and classify the types of modifications that take place. Understanding of these modification processes and the context in which they happened is crucial to develop a system for supporting design synthesis of multiple state mechanical devices that is capable of creating a comprehensive variety of solution alternatives.

Flexible poly(vinyl alcohol-co-ethylene)/modified MMT moisture barrier composite for encapsulating organic devices

Flexible, nano-composite moisture barrier films of poly(vinyl alcohol-co-ethylene) with surface modified montmorillonite fabricated by solution casting were used to encapsulate organic devices. The composite films were characterized by FTIR, UV-visible spectroscopy and SEM imaging. Thermal and mechanical properties of the composite films were studied by DSC and UTM. Calcium degradation test was used to determine the transmission rate of water vapour through the composite films, which showed a gradual reduction from similar to 0.1 g m(-2) day(-1) to 0.0001 g m(-2) day(-1) with increasing modified montmorillonite loading in the neat copolymer. The increase in moisture barrier performance is attributed to the decreased water vapour diffusivity due to matrix-filler interactions in the composite. The accelerated aging test was carried out for non-encapsulated and encapsulated devices to evaluate the efficiency of the encapsulants. The encapsulated devices exhibited longer lifetimes indicating the efficacy of the encapsulant.

Robustness of early warning signals of regime shifts in time-delayed ecological models

Various ecological and other complex dynamical systems may exhibit abrupt regime shifts or critical transitions, wherein they reorganize from one stable state to another over relatively short time scales. Because of potential losses to ecosystem services, forecasting such unexpected shifts would be valuable. Using mathematical models of regime shifts, ecologists have proposed various early warning signals of imminent shifts. However, their generality and applicability to real ecosystems remain unclear because these mathematical models are considered too simplistic. Here, we investigate the robustness of recently proposed early warning signals of regime shifts in two well-studied ecological models, but with the inclusion of time-delayed processes. We find that the average variance may either increase or decrease prior to a regime shift and, thus, may not be a robust leading indicator in time-delayed ecological systems. In contrast, changing average skewness, increasing autocorrelation at short time lags, and reddening power spectra of time series of the ecological state variable all show trends consistent with those of models with no time delays. Our results provide insights into the robustness of early warning signals of regime shifts in a broader class of ecological systems.

Probing disorder and transport properties in polypyrrole thin-film devices by impedance and Raman spectroscopy

In the present study, impedance and Raman spectroscopy are adopted to probe the nature and extent of disorder to correlate with transport properties in doped polypyrrole (PPy) thin-film devices, synthesized electrochemically at different temperatures. A comparative study of the impedance spectroscopy is performed on PPy devices by both experimental and simulation approach with varying extent of disorder. The impedance measurements of PPy devices are well described by introducing a constant phase element (CPE) (Q) in modified RQ circuit, which accounts for frequency dependence of dielectric response. However, for the PPy grown at lower temperature, an equivalent circuit consisting of two such RQ elements in series is used for successful modelling of the impedance results, which accounts for the depletion region near the electrode. Raman spectroscopy and the de-convoluted spectra are successfully studied to probe the variation in C=C bond stretching and distribution of conjugation length, which relates to disorder in PPy films and the interpretation is well correlated to the impedance results.

Carbon nanotube-ZnO nanowire hybrid architectures as multifunctional devices

We report on multifunctional devices based on CNT arrays-ZnO nanowires hybrid architectures. The hybrid structure exhibit excellent high current Schottky like behavior with ZnO as p-type and an ideality factor close to the ideal value. Further the CNT-ZnO hybrid structures can be used as high current p-type field effect transistors that can deliver currents of the order of milliamperes and also can be used as ultraviolet detectors with controllable current on-off ratio and response time. The p-type nature of ZnO and possible mechanism for the rectifying characteristics of CNT-ZnO has been presented.

Facile synthesis of Cu2CoSnS4 nanoparticles exhibiting red-edge-effect: Application in hybrid photonic devices

Cu2CoSnS4 (CCTS) quaternary semiconducting nanoparticles with size distribution from 20 nm to 60 nm were synthesized by one-pot low temperature time and surfactant dependent hydrothermal route. Nanoparticles were characterized structurally and optically. Excitation dependent fluorescence exhibited a dynamic stoke shift referring to the Red-Edge-Effect with peak shifting by a greater magnitude (>100 nm) towards red side, in all the samples. Hybrid devices, fabricated from CCTS nanoparticle inorganic counterparts benefitting from the conjugation of organic P3HT polymer matrix, were demonstrated for photodetection under infra-red and A. M 1.5 solar light illuminations. Faster rise and decay constants of 37 ms and 166 ms, with one order photocurrent amplification from 1.6 x 10(-6) A in the dark to 6.55 x 10(-5) A, upon the 18.50 mW cm(-2) IR lamp illumination, make CCTS a potential candidate for photodetector and photovoltaic applications. (C) 2013 AIP Publishing LLC.

Graphene-MoS2 hybrid structures for multifunctional photoresponsive memory devices

Combining the electronic properties of graphene(1,2) and molybdenum disulphide (MoS2)(3-6) in hybrid heterostructures offers the possibility to create devices with various functionalities. Electronic logic and memory devices have already been constructed from graphene-MoS2 hybrids(7,8), but they do not make use of the photosensitivity of MoS2, which arises from its optical-range bandgap(9). Here, we demonstrate that graphene-on-MoS2 binary heterostructures display remarkable dual optoelectronic functionality, including highly sensitive photodetection and gate-tunable persistent photoconductivity. The responsivity of the hybrids was found to be nearly 1 x 10(10) A W-1 at 130 K and 5 x 10(8) A W-1 at room temperature, making them the most sensitive graphene-based photodetectors. When subjected to time-dependent photoillumination, the hybrids could also function as a rewritable optoelectronic switch or memory, where the persistent state shows almost no relaxation or decay within experimental timescales, indicating near-perfect charge retention. These effects can be quantitatively explained by gate-tunable charge exchange between the graphene and MoS2 layers, and may lead to new graphene-based optoelectronic devices that are naturally scalable for large-area applications at room temperature.

ABOUT FORMS OF ELECTRONIC AND IONIC DEVICES WITH THERMIONIC CATHODES.

The miniaturization of electronic and ionic devices with thermionic cathodes and thc improvement of their vacuum properties are questions of very great interest to the electronic engineer. However there have bcen no proposals so far to analyse the problem of miniaturization of such devices In a fundamental way. The present work suggests a choice of the geometrical shape of the cathode, the anode and the envelope of the device, that may help towards such a fundamcnlal approach.It is shown that a design, in which the cathode and the envelope of the tube are made of thm prismatic shape and the anode coincides with the cnvclope, offers a slriknrg advantage over the conventional cylindrical design, in respect of over-all size. The use of the prismatic shape will lead to considerable economy in msterials and may facilitate simpler prodoct~ont echn~ques. I n respect of the miin criteria of vacuum, namely the grade of vacuum, the internal volume occupied by residual gases, the evolution of gases in the internal space and the diffusion of gases from outside into the devicc, it is shown that the prismatic form is at least as good as, if not somewhat superior lo, the cylindrical form.In the actual construction of thin prismatic tubes, manv practical problems will arise, the most important being the mechanical strength and stablity of the structure. But the changeover from the conventional cylindrical to the new prirmaiic form, with its basic advantages, is a development that merits close attention.