25,556 research outputs found
Multiscale Modeling and Simulation of Organic Solar Cells
In this article, we continue our mathematical study of organic solar cells
(OSCs) and propose a two-scale (micro- and macro-scale) model of heterojunction
OSCs with interface geometries characterized by an arbitrarily complex
morphology. The microscale model consists of a system of partial and ordinary
differential equations in an heterogeneous domain, that provides a full
description of excitation/transport phenomena occurring in the bulk regions and
dissociation/recombination processes occurring in a thin material slab across
the interface. The macroscale model is obtained by a micro-to-macro scale
transition that consists of averaging the mass balance equations in the normal
direction across the interface thickness, giving rise to nonlinear transmission
conditions that are parametrized by the interfacial width. These conditions
account in a lumped manner for the volumetric dissociation/recombination
phenomena occurring in the thin slab and depend locally on the electric field
magnitude and orientation. Using the macroscale model in two spatial
dimensions, device structures with complex interface morphologies, for which
existing data are available, are numerically investigated showing that, if the
electric field orientation relative to the interface is taken into due account,
the device performance is determined not only by the total interface length but
also by its shape
Theory and simulation of quantum photovoltaic devices based on the non-equilibrium Green's function formalism
This article reviews the application of the non-equilibrium Green's function
formalism to the simulation of novel photovoltaic devices utilizing quantum
confinement effects in low dimensional absorber structures. It covers
well-known aspects of the fundamental NEGF theory for a system of interacting
electrons, photons and phonons with relevance for the simulation of
optoelectronic devices and introduces at the same time new approaches to the
theoretical description of the elementary processes of photovoltaic device
operation, such as photogeneration via coherent excitonic absorption,
phonon-mediated indirect optical transitions or non-radiative recombination via
defect states. While the description of the theoretical framework is kept as
general as possible, two specific prototypical quantum photovoltaic devices, a
single quantum well photodiode and a silicon-oxide based superlattice absorber,
are used to illustrated the kind of unique insight that numerical simulations
based on the theory are able to provide.Comment: 20 pages, 10 figures; invited review pape
Quantum-kinetic perspective on photovoltaic device operation in nanostructure-based solar cells
The implementation of a wide range of novel concepts for next-generation
high-efficiency solar cells is based on nanostructures with
configuration-tunable optoelectronic properties. On the other hand, effective
nano-optical light-trapping concepts enable the use of ultra-thin absorber
architectures. In both cases, the local density of electronic and optical
states deviates strongly from that in a homogeneous bulk material. At the same
time, non-local and coherent phenomena like tunneling or ballistic transport
become increasingly relevant. As a consequence, the semi-classical, diffusive
bulk picture conventionally assumed may no longer be appropriate to describe
the physical processes of generation, transport, and recombination governing
the photovoltaic operation of such devices. In this review, we provide a
quantum-kinetic perspective on photovoltaic device operation that reaches
beyond the limits of the standard simulation models for bulk solar cells.
Deviations from bulk physics are assessed in ultra-thin film and
nanostructure-based solar cell architectures by comparing the predictions of
the semi-classical models for key physical quantities such as absorption
coefficients, emission spectra, generation and recombination rates as well as
potentials, densities and currents with the corresponding properties as given
by a more fundamental description based on non-equilibrium quantum statistical
mechanics. This advanced approach, while paving the way to a comprehensive
quantum theory of photovoltaics, bridges simulations at microscopic material
and macroscopic device levels by providing the charge carrier dynamics at the
mesoscale.Comment: 22 pages, 8 figures; review article based on an invited talk at the
MRS Spring Meeting 2017 in Phoeni
PV Parameter Identification using Reduced I-V Data
In this paper, possibility and accuracy of using reduced I-V data in PV parameter identification are discussed. Based on the linear identification method proposed in [1], six I-V points are used instead of the whole I-V curve to identify the PV parameters. The maximum power point (MPP) is then estimated using the identified I-V and P-V characteristics. Validation is done by using different sets of six points on the I-V curve. Experiment results show that the accurate curve fitting (with low RMSE and MPE) and good estimation of MPP can be achieved
Impact of different time series aggregation methods on optimal energy system design
Modelling renewable energy systems is a computationally-demanding task due to
the high fluctuation of supply and demand time series. To reduce the scale of
these, this paper discusses different methods for their aggregation into
typical periods. Each aggregation method is applied to a different type of
energy system model, making the methods fairly incomparable. To overcome this,
the different aggregation methods are first extended so that they can be
applied to all types of multidimensional time series and then compared by
applying them to different energy system configurations and analyzing their
impact on the cost optimal design. It was found that regardless of the method,
time series aggregation allows for significantly reduced computational
resources. Nevertheless, averaged values lead to underestimation of the real
system cost in comparison to the use of representative periods from the
original time series. The aggregation method itself, e.g. k means clustering,
plays a minor role. More significant is the system considered: Energy systems
utilizing centralized resources require fewer typical periods for a feasible
system design in comparison to systems with a higher share of renewable
feed-in. Furthermore, for energy systems based on seasonal storage, currently
existing models integration of typical periods is not suitable
Analytical and Numerical Study of Photocurrent Transients in Organic Polymer Solar Cells
This article is an attempt to provide a self consistent picture, including
existence analysis and numerical solution algorithms, of the mathematical
problems arising from modeling photocurrent transients in Organic-polymer Solar
Cells (OSCs). The mathematical model for OSCs consists of a system of nonlinear
diffusion-reaction partial differential equations (PDEs) with electrostatic
convection, coupled to a kinetic ordinary differential equation (ODE). We
propose a suitable reformulation of the model that allows us to prove the
existence of a solution in both stationary and transient conditions and to
better highlight the role of exciton dynamics in determining the device turn-on
time. For the numerical treatment of the problem, we carry out a temporal
semi-discretization using an implicit adaptive method, and the resulting
sequence of differential subproblems is linearized using the Newton-Raphson
method with inexact evaluation of the Jacobian. Then, we use exponentially
fitted finite elements for the spatial discretization, and we carry out a
thorough validation of the computational model by extensively investigating the
impact of the model parameters on photocurrent transient times.Comment: 20 pages, 11 figure
Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells
We report a model describing the molecular orientation disorder in
CH3NH3PbI3, solving a classical Hamiltonian parametrised with electronic
structure calculations, with the nature of the motions informed by ab-initio
molecular dynamics. We investigate the temperature and static electric field
dependence of the equilibrium ferroelectric (molecular) domain structure and
resulting polarisability. A rich domain structure of twinned molecular dipoles
is observed, strongly varying as a function of temperature and applied electric
field. We propose that the internal electrical fields associated with
microscopic polarisation domains contribute to hysteretic anomalies in the
current--voltage response of hybrid organic-inorganic perovskite solar cells
due to variations in electron-hole recombination in the bulk.Comment: 10 pages; 4 figures, 2 SI figure
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