24,771 research outputs found
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Understanding macroscale functionality of metal halide perovskites in terms of nanoscale heterogeneities
Hybrid metal halide perovskites have shown an unprecedented rise as semiconductor building blocks for solar energy conversion and light-emitting applications. Currently, the field moves empirically towards more and more complex chemical compositions, including mixed halide quadruple cation compounds that allow optical properties to be tuned and show promise for better stability. Despite tremendous progress in the field, there is a need for better understanding of mechanisms of efficiency loss and instabilities to facilitate rational optimization of composition. Starting from the device level and then diving into nanoscale properties, we highlight how structural and compositional heterogeneities affect macroscopic optoelectronic characteristics. Furthermore, we provide an overview of some of the advanced spectroscopy and imaging methods that are used to probe disorder and non-uniformities. A unique feature of hybrid halide perovskite compounds is the propensity for these heterogeneities to evolve in space and time under relatively mild illumination and applied electric fields, such as those found within active devices. This introduces an additional challenge for characterization and calls for application of complimentary probes that can aid in correlating the properties of local disorder with macroscopic function, with the ultimate goal of rationally tailoring synthesis towards optimal structures and compositions
How migrating 0.0001% of address space saves 12% of energy in hybrid storage
We present a simple, operating-\ud
system independent method to reduce the num-\ud
ber of seek operations and consequently reduce\ud
the energy consumption of a hybrid storage\ud
device consisting of a hard disk and a flash\ud
memory. Trace-driven simulations show that\ud
migrating a tiny amount of the address space\ud
(0.0001%) from disk to flash already results\ud
in a significant storage energy reduction (12%)\ud
at virtually no extra cost. We show that the\ud
amount of energy saving depends on which part\ud
of the address space is migrated, and we present\ud
two indicators for this, namely sequentiality and\ud
request frequency. Our simulations show that\ud
both are suitable as criterion for energy-saving\ud
file placement methods in hybrid storage. We\ud
address potential wear problems in the flash\ud
subsystem by presenting a simple way to pro-\ud
long its expected lifetime.\u
Spatially controlled electrostatic doping in graphene p-i-n junction for hybrid silicon photodiode
Sufficiently large depletion region for photocarrier generation and
separation is a key factor for two-dimensional material optoelectronic devices,
but few device configurations has been explored for a deterministic control of
a space charge region area in graphene with convincing scalability. Here we
investigate a graphene-silicon p-i-n photodiode defined in a foundry processed
planar photonic crystal waveguide structure, achieving visible - near-infrared,
zero-bias and ultrafast photodetection. Graphene is electrically contacting to
the wide intrinsic region of silicon and extended to the p an n doped region,
functioning as the primary photocarrier conducting channel for electronic gain.
Graphene significantly improves the device speed through ultrafast out-of-plane
interfacial carrier transfer and the following in-plane built-in electric field
assisted carrier collection. More than 50 dB converted signal-to-noise ratio at
40 GHz has been demonstrated under zero bias voltage, with quantum efficiency
could be further amplified by hot carrier gain on graphene-i Si interface and
avalanche process on graphene-doped Si interface. With the device architecture
fully defined by nanomanufactured substrate, this study is the first
demonstration of post-fabrication-free two-dimensional material active silicon
photonic devices.Comment: NPJ 2D materials and applications (2018
Stability of Cubic FAPbI from X-ray Diffraction, Anelastic, and Dielectric Measurements
Among the hybrid metal-organic perovskites for photovoltaic applications
FAPbI_3 (FAPI) has the best performance regarding efficiency and the worst
regarding stability, even though the reports on its stability are highly
contradictory. In particular, since at room temperature the cubic alpha phase,
black and with high photovoltaic efficiency, is metastable against the yellow
hexagonal delta phase, it is believed that alpha-FAPI spontaneously transform
into delta-FAPI within a relatively short time. We performed X-ray diffraction
and thermogravimetric measurements on loose powder of FAPI, and present the
first complete dielectric and anelastic spectra of compacted FAPI samples under
various conditions. We found that alpha-FAPI is perfectly stable for at least
100 days, the duration of the experiments, unless extrinsic factors induce its
degradation. In our tests, degradation was detected after exposure to humidity,
strongly accelerated by grain boundaries and the presence of delta phase, but
it was not noticeable on the loose powder kept in air under normal laboratory
illumination. These findings have strong implications on the strategies for
improving the stability of FAPI without diminishing its photovoltaic efficiency
through modifications of its composition
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Rotating Machine Technologies for Integration of Pulsed and High Power Loads in Naval Electric Power Systems
Advanced electric sensors and weapons are placing increasing demands on the electric power distribution systems of future naval vessels and energy storage is viewed as a critical technology for effective integration of IPS architectures in these platforms. This paper shows that kinetic energy storage, i.e. stored in the angular momentum of a rotating mass, can be applied in differing topologies to address a range of ship power system applications. Rotating machine technologies are presented for UPS and load leveling applications as well as for high cycle rate pulsed power applications.Center for Electromechanic
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