206 research outputs found
Directed transient long-range transport in a slowly driven Hamiltonian system of interacting particles
We study the Hamiltonian dynamics of a one-dimensional chain of linearly
coupled particles in a spatially periodic potential which is subjected to a
time-periodic mono-frequency external field. The average over time and space of
the related force vanishes and hence, the system is effectively without bias
which excludes any ratchet effect. We pay special attention to the escape of
the entire chain when initially all of its units are distributed in a potential
well. Moreover for an escaping chain we explore the possibility of the
successive generation of a directed flow based on large accelerations. We find
that for adiabatic slope-modulations due to the ac-field transient long-range
transport dynamics arises whose direction is governed by the initial phase of
the modulation. Most strikingly, that for the driven many particle Hamiltonian
system directed collective motion is observed provides evidence for the
existence of families of transporting invariant tori confining orbits in
ballistic channels in the high dimensional phase spaces
Charge transport and trapping in Cs-doped poly(dialkoxy-p-phenylene vinylene) light-emitting diodes
Al/Cs/MDMO-PPV/ITO (where MDMO-PPV stands for poly[2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4phenylene vinylene] and ITO is indium tin oxide) light-emitting diode (LED) structures, made by physical vapor deposition of Cs on the emissive polymer layer, have been characterized by electroluminescence, current-voltage, and admittance spectroscopy. Deposition of Cs is found to improve the balance between electron and hole currents, enhancing the external electroluminescence efficiency from 0.01 cd A-1 for the bare Al cathode to a maximum of 1.3 cd A-1 for a Cs coverage of only 1.5×1014 atoms/cm2. By combining I-V and admittance spectra with model calculations, in which Cs diffusion profiles are explicitly taken into account, this effect could be attributed to a potential drop at the cathode interface due to a Cs-induced electron donor level 0.61 eV below the lowest unoccupied molecular orbital. In addition, the admittance spectra in the hole-dominated regime are shown to result from space-charge-limited conduction combined with charge relaxation in trap levels. This description allows us to directly determine the carrier mobility, even in the presence of traps. In contrast to recent literature, we demonstrate that there is no need to include dispersive transport in the description of the carrier mobility to explain the excess capacitance that is typically observed in admittance spectra of p-conjugated materials
Sub-nanometre resolution imaging of polymer-fullerene photovoltaic blends using energy-filtered scanning electron microscopy
The resolution capability of the scanning electron microscope has increased immensely in recent years, and is now within the sub-nanometre range, at least for inorganic materials. An equivalent advance has not yet been achieved for imaging the morphologies of nanostructured organic materials, such as organic photovoltaic blends. Here we show that energy-selective secondary electron detection can be used to obtain high-contrast, material-specific images of an organic photovoltaic blend. We also find that we can differentiate mixed phases from pure material phases in our data. The lateral resolution demonstrated is twice that previously reported from secondary electron imaging. Our results suggest that our energy-filtered scanning electron microscopy approach will be able to make major inroads into the understanding of complex, nano-structured organic materials
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