114 research outputs found
Neighbor selection and hitting probability in small-world graphs
Full text linkSmall-world graphs, which combine randomized and structured elements, are
seen as prevalent in nature. Jon Kleinberg showed that in some graphs of this
type it is possible to route, or navigate, between vertices in few steps even
with very little knowledge of the graph itself. In an attempt to understand how
such graphs arise we introduce a different criterion for graphs to be navigable
in this sense, relating the neighbor selection of a vertex to the hitting
probability of routed walks. In several models starting from both discrete and
continuous settings, this can be shown to lead to graphs with the desired
properties. It also leads directly to an evolutionary model for the creation of
similar graphs by the stepwise rewiring of the edges, and we conjecture,
supported by simulations, that these too are navigable.Comment: Published in at http://dx.doi.org/10.1214/07-AAP499 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Extraction Current Transients for Selective Charge-Carrier Mobility Determination in Non-Fullerene and Ternary Bulk Heterojunction Organic Solar Cells
Get PDFThe field of organic solar cells has recently gained broad research interest due to the introduction of non-fullerene small-molecule acceptors. The rapid improvement in solar cell efficiency put increased demand on moving toward scalable device architectures. An essential step toward this is enabling thicker active layers for which the hole and electron mobilities and their ratio become increasingly important. In this work, we demonstrate selective charge-carrier mobility determination using the charge extraction by a linearly increasing voltage (CELIV) method. By tuning the contact properties of the solar cell diodes, the hole and electron mobilities are determined separately using the recently developed metalâintrinsic semiconductorâmetal-CELIV (MIM-CELIV) technique. Balanced mobility is measured both in non-fullerene and in ternary blends with the recently published PBBF11 polymer. The mobility results are confirmed using the well-established metalâinsulatorâsemiconductor (MIS) and photo-CELIV techniques
Decentralized Search with Random Costs
Full text linkA decentralized search algorithm is a method of routing on a random graph
that uses only limited, local, information about the realization of the graph.
In some random graph models it is possible to define such algorithms which
produce short paths when routing from any vertex to any other, while for others
it is not.
We consider random graphs with random costs assigned to the edges. In this
situation, we use the methods of stochastic dynamic programming to create a
decentralized search method which attempts to minimize the total cost, rather
than the number of steps, of each path. We show that it succeeds in doing so
among all decentralized search algorithms which monotonically approach the
destination. Our algorithm depends on knowing the expected cost of routing from
every vertex to any other, but we show that this may be calculated iteratively,
and in practice can be easily estimated from the cost of previous routes and
compressed into a small routing table. The methods applied here can also be
applied directly in other situations, such as efficient searching in graphs
with varying vertex degrees
Quantifying QuasiâFermi Level Splitting and OpenâCircuit Voltage Losses in Highly Efficient Nonfullerene Organic Solar Cells
Get PDFAdaptive Dynamics of Realistic Small-World Networks
Full text linkContinuing in the steps of Jon Kleinberg's and others celebrated work on
decentralized search in small-world networks, we conduct an experimental
analysis of a dynamic algorithm that produces small-world networks. We find
that the algorithm adapts robustly to a wide variety of situations in realistic
geographic networks with synthetic test data and with real world data, even
when vertices are uneven and non-homogeneously distributed.
We investigate the same algorithm in the case where some vertices are more
popular destinations for searches than others, for example obeying power-laws.
We find that the algorithm adapts and adjusts the networks according to the
distributions, leading to improved performance. The ability of the dynamic
process to adapt and create small worlds in such diverse settings suggests a
possible mechanism by which such networks appear in nature
Energetics and Kinetics Requirements for Organic Solar Cells to Break the 20% Power Conversion Efficiency Barrier
Get PDFThe thermodynamic limit for the efficiency of solar cells is predominantly
defined by the energy bandgap of the used semiconductor. In case of organic
solar cells both energetics and kinetics of three different species play role:
excitons, charge transfer states and charge separated states. In this work, we
clarify the effect of the relative energetics and kinetics of these species on
the recombination and generation dynamics. Making use of detailed balance, we
develop an analytical framework describing how the intricate interplay between
the different species influence the photocurrent generation, the recombination,
and the open-circuit voltage in organic solar cells. Furthermore, we clarify
the essential requirements for equilibrium between excitons, CT states and
charge carriers to occur. Finally, we find that the photovoltaic parameters are
not only determined by the relative energy level between the different states
but also by the kinetic rate constants. These findings provide vital insights
into the operation of state-of-art non-fullerene organic solar cells with low
offsets
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