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Berlin, Germany

The Helmholtz-Zentrum Berlin for Materials and Energy is a research centre and part of the Helmholtz Association of German Research Centres. The institute carries out research into the structure and dynamics of novel materials and also investigates solar cell technology.Several large scale facilities are available, the most important of which are the 10 MW BER-II research reactor at the Lise Meitner campus in Wannsee and the 3rd generation BESSY synchrotron in Adlershof. The institute also specialises in research at high magnetic fields and low temperatures and is a world leader in providing sample environment for neutron scattering and physical property measurements.Both the reactor and synchrotron operate as user facilities. Due to the high competition for experiments, beam time is awarded after peer review of two page proposals which state the scientific case for each measurement. User groups are expected to run experiments on a 24-hour basis to maximise the use of the facility. Onsite guesthouses exist at both campuses. Wikipedia.

Stolterfoht N.,Helmholtz Center Berlin
Physical Review A - Atomic, Molecular, and Optical Physics

Simulations of ion guiding through an insulating cylindrical nanocapillary are performed in continuation of a recent theoretical work. The ions are assumed to move on classical trajectories affected by the electric field primarily produced by the charge patch deposited near the capillary entrance. The deposited charges are transported along the capillary wall using a nonlinear conductivity law. Calculations for different capillary tilt angles from 0 to 8 are performed and compared with previous experimental results. The main focus of the analysis is to reveal unknown guiding mechanisms by a detailed investigation of the calculated results. Surprisingly, after reaching a maximum, the field component perpendicular to the capillary axis is found to decrease with increasing charge inserted into the capillary. At equilibrium, this field is nearly constant in all directions of the capillary along the entrance charge patch. The extension of this charge patch increases with increasing tilt angle although a simple picture of undeflected ions predicts the opposite behavior. These unexpected results simplify the theoretical treatment so that analytical expressions could be derived describing essential properties of the ion guiding. In particular, unknown parameters previously introduced in semiempirical models are interpreted. © 2013 American Physical Society. Source

Schulze T.F.,Helmholtz Center Berlin | Schmidt T.W.,University of New South Wales
Energy and Environmental Science

All photovoltaic solar cells transmit photons with energies below the absorption threshold (bandgap) of the absorber material, which are therefore usually lost for the purpose of solar energy conversion. Upconversion (UC) devices can harvest this unused sub-threshold light behind the solar cell, and create one higher energy photon out of (at least) two transmitted photons. This higher energy photon is radiated back towards the solar cell, thus expanding the utilization of the solar spectrum. Key requirements for UC units are a broad absorption and high UC quantum yield under low-intensity incoherent illumination, as relevant to solar energy conversion devices, as well as long term photostability. Upconversion by triplet-triplet annihilation (TTA) in organic chromophores has proven to fulfil the first two basic requirements, and first proof-of-concept applications in photovoltaic conversion as well as photo(electro)chemical energy storage have been demonstrated. Here we review the basic concept of TTA-UC and its application in the field of solar energy harvesting, and assess the challenges and prospects for its large-scale application, including the long term photostability of TTA upconversion materials. © 2015 The Royal Society of Chemistry. Source

Seiffert S.,Helmholtz Center Berlin | Seiffert S.,Free University of Berlin | Sprakel J.,Wageningen University
Chemical Society Reviews

Supramolecular polymer networks are three-dimensional structures of crosslinked macromolecules connected by transient, non-covalent bonds; they are a fascinating class of soft materials, exhibiting properties such as stimuli-responsiveness, self-healing, and shape-memory. This critical review summarizes the current state of the art in the physical-chemical characterization of supramolecular networks and relates this knowledge to that about classical, covalently jointed and crosslinked networks. We present a separate focus on the formation, the structure, the dynamics, and the mechanics of both permanent chemical and transient supramolecular networks. Particular emphasis is placed on features such as the formation and the effect of network inhomogeneities, the manifestation of the crosslink relaxation dynamics in the macroscopic sample behavior, and the applicability of concepts developed for classical polymer melts, solutions, and networks such as the reptation model and the principle of time-temperature superposition (263 references). © 2012 The Royal Society of Chemistry. Source

Transparent conductive electrodes play important roles in information and energy technologies. These materials, particularly transparent conductive oxides, are widely used as transparent electrodes across technical fields such as low-emissivity coatings, flat-panel displays, thin-film solar cells and organic light-emitting diodes. This Review begins by summarizing the properties and applications of transparent conductive oxides such as In 2 O 3, SnO 2, ZnO and TiO 2. Owing to the increasing demand for raw materials - especially indium - scientists are currently searching for alternatives to indium tin oxide. Carbon nanotube and metal nanowire networks, as well as regular metal grids, have been investigated for use as transparent conductive electrodes. This Review compares these materials and the recently 'rediscovered' graphene with today's established transparent conductive oxides. © 2012 Macmillan Publishers Limited. All rights reserved. Source

A solar cell includes a photoactive, semiconductive absorber layer configured to generate excess charge carriers of opposed polarity by light incident on a front of the absorber layer during operation. The absorber layer is configured to separate and move, via at least one electric field formed in the absorber layer, the photogenerated excess charge carriers of opposed polarity over a minimal effective diffusion length L

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